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Ferroelectric and piezoelectric properties of tungsten substituted SrBi 2Ta 2O 9ferroelectric ceramicsIndrani Coondoo *,S.K.Agarwal a ,A.K.Jha ba Superconductivity and Cryogenics Division,National Physical Laboratory,Dr K.S.Krishnan Road,New Delhi 110012,India bDepartment of Applied Physics,Delhi College of Engineering,Bawana Road,Delhi 110042,India1.IntroductionDefects in crystals significantly influence physical and various other properties of materials [1].For instance,as it is well known,doping by other elements leads to significant changes in the electrical properties of silicon.Historically,‘‘defect engineering’’has been developed in the field of semiconducting materials such as compound semiconductors as well as in diamond,Si and Ge [2–4].Subsequently,the concept of defect engineering has been applied to other functional materials,and the significant improve-ment in material properties have been achieved in high transition-temperature superconductors [5],amorphous SiO 2[6],photonic crystals [7]and also in the field of ferroelectrics,such as BaTiO 3,Pb(Ti,Zr)O 3(PZT),etc.[8,9].Various structural and electrical properties of bismuth layer-structured ferroelectrics (BLSF)are also strongly affected on deviation from stoichiometric composi-tions and defects have been recognized as a crucially important factor [10–13].It has been found that in BLSF small changes in chemical composition result in significantly altered dielectric and ferroelectric properties including dielectric constant and remanent polarization.In SrBi 2Ta 2O 9(SBT)and SrBi 2Nb 2O 9(SBN),orthor-hombic structural distortions with non-centrosymmetric spacegroup A 21am cause spontaneous ferroelectric polarization (P s )along a axis [14,15].SBT,a member of the BLSF family,has occupied an important position among the Pb-free ferroelectric memory materials [16–18].Tungsten (W 6+)has recently been investigated as a dopant for bismuth titanates and lanthanum doped bismuth titanates,in which the remanent polarization was observed to enhance when a small amount of Ti 4+was substituted by W 6+[19,20].With the objective to improve structural,dielectric and ferroelectric proper-ties,the hexavalent tungsten (W 6+)was chosen as a donor cation for partial replacement of the pentavalent tantalum (Ta 5+)SBT.In this report,the effect of tungsten substitution in SBT (SBTW),on the microstructural,ferroelectric and piezoelectric properties is reported.The results including the improvement in polarization properties have been discussed.2.ExperimentalSamples of compositions SrBi 2(W x Ta 1Àx )2O 9(SBWT),with x =0.0,0.025,0.050,0.075,0.10and 0.20were synthesized by solid-state reaction method taking SrCO 3,Bi 2O 3,Ta 2O 5and WO 3(all from Aldrich)in their stoichiometric proportions.The powder mixtures were thoroughly ground and passed through sieve of appropriate size and then calcined at 9008C in air for 2h.The calcined mixtures were ground and admixed with about 1–1.5wt%polyvinyl alcohol (Aldrich)as a binder and then pressed at $300MPa into disk shaped pellets.The pellets were sintered at 12008C for 2h in air.Materials Research Bulletin 44(2009)1288–1292A R T I C L E I N F O Article history:Received 3October 2008Received in revised form 5December 2008Accepted 6January 2009Available online 15January 2009Keywords:A.CeramicsC.X-ray diffractionD.FerroelectricityA B S T R A C TTungsten substituted samples of compositions SrBi 2(W x Ta 1Àx )2O 9(x =0.0,0.025,0.050,0.075,0.10and 0.20)were synthesized by solid-state reaction method and studied for their microstructural,electrical conductivity,ferroelectric and piezoelectric properties.The X-ray diffractograms confirm the formation of single phase layered perovskite structure in the samples with x up to 0.05.The temperaturedependence of dc conductivity vis-a`-vis tungsten content shows a decrease in conductivity,which is attributed to the suppression of oxygen vacancies.The ferroelectric and piezoelectric studies of the W-substituted SBT ceramics show that the remanent polarization and d 33values increases with increasing concentration of tungsten up to x 0.05.Such compositions with low conductivity and high P r values should be excellent materials for highly stable ferroelectric memory devices.ß2009Elsevier Ltd.All rights reserved.*Corresponding author.Present address:Liquid Crystal Group,National Physical Laboratory,Dr K.S.Krishnan Road,New Delhi 110012,India.Tel.:+919810361727;fax:+911125170387.E-mail address:indrani_coondoo@ (I.Coondoo).Contents lists available at ScienceDirectMaterials Research Bulletinj o ur n a l h o m e p a g e :w w w.e l se v i e r.c om /l oc a t e /m a t r e sb u0025-5408/$–see front matter ß2009Elsevier Ltd.All rights reserved.doi:10.1016/j.materresbull.2009.01.001X-ray diffractograms of the sintered samples were recorded using a Bruker diffractometer in the range 108 2u 708with CuK a radiation.The sintered pellets were polished to a thickness of 1mm and coated with silver paste on both sides for use as electrodes and cured at 5508C for half an hour.Electrical conductivity was performed using Keithley’s 6517A Electrometer.The polarization–electric field (P –E )hysteresis measurements were done at room temperature using an automatic P –E loop tracer based on Sawyer–Tower circuit.Piezoelectric charge co-efficient d 33was measured using a Berlincourt d 33meter after poling the samples in silicone–oil bath at 2008C for half an hour under a dc electric field of 60–70kV/cm.3.Results and discussion3.1.Structural and micro-structural studiesThe phase formation and crystal structure of the ceramics were examined by X-ray diffraction (XRD),which is shown in Fig.1.The XRD patterns of the samples show the characteristic peaks of SBT.The peaks have been indexed with the help of a computer program–POWDIN [21]and the refined lattice parameters are given in Table 1.It is observed that a single phase layered perovskite structure is maintained in the range 0.0 x 0.05.Owing to the same co-ordination number i.e.6and the smallerionic radius of W (0.60A˚)in comparison to Ta (0.64A ˚),there is a high possibility of tungsten occupying the tantalum site.The observance of unidentified peak of very low intensity in the compositions with x >0.05indicates the solubility limit of W concentration in SBT.The unidentified peak is possibly due to tungsten not occupying the Ta sites in the structure as the intensity of this peak is observed to increase with tungsten content.Composition and sintering temperature influences the micro-structure such as grain growth and densification of the specimen,which in turn control other properties of the material [11,13].The effects of W substitution on the microstructure have been examined by SEM and the obtained micrographs are shown in Fig.2.It shows the microstructure of the fractured surface of the studied samples.It is clearly observed that W substitution has pronounced effect on the average grain size and homogeneity of the grains.Randomly oriented and anisotropic plate-like grains are observed in all the samples.It is also observed that the average grain size increases gradually with increasing W content.The average grain size in the sample with x =0.0is $2–3m m while that in the sample with x =0.20the size increases to $5–7m m.3.2.Electrical studiesThe electrical conductivity of ceramic materials encompasses a wide range of values.In insulators,the defects w.r.t.the perfect crystalline structure act as charge carriers and the consideration of charge transport leads necessarily to the consideration of point defects and their migration [22].Many mechanisms were put forward to explain the conductivity mechanism in ceramics.Most of them are approximately divided into three groups:electronic conduction,oxygen vacancies ionic conduction,and ionic and p-type mixed conduction [22].Intrinsic conductivity results from the movement of the component ions,whereas conduction resulting from the impurity ions present in the lattice is known as extrinsic conductivity.At low temperature region (ferroelectric phase),the conduction is dominated by the extrinsic conduction,whereas the conduction at the high-temperature paraelectric phase ($300–7008C)is dominated by the intrinsic ionic conduction [23,25].Fig.3shows the temperature dependence of dc conductivity (s dc )for the undoped and doped SBT samples.The curves show that the conductivity increases with temperature.This is indicative of negative temperature coefficient of resistance (NTCR)behavior,a characteristic of dielectrics [22].It is observed in Fig.3that throughout the temperature range,the dc conductivity of the doped samples are nearly two to three orders lower than that of the undoped sample.Two predominant conduction mechanisms indicated by slope changes in the two different temperature regions are observed in Fig.3.Such changes in the slope in the vicinity of the ferro-paraelectric transition region have been observed in other ferroelectric materials as well [23,24].In addition,it is also observed (Table 2)that the activation energy calculated using the Arrhenius equation [22]in the paraelectric phase increase from $0.80eV for the undoped sample to $2eV for the doped samples.The X-ray photoemission spectroscopic study has confirmed that when Bi 2O 3evaporates during high-temperature processing,vacancy complexes are formed in the (Bi 2O 2)2+layers [26].As a result,defective (Bi 2O 2)2+layers are inherently present in SBT.The undoped SBT shows n-type conductivity,since when oxygen vacancies are created,it leaves behind two trapped electrons [27]:O o !12O 2"þV o þ2e 0(1)where O o is an oxygen ion on an oxygen site,V o is a oxygen vacant site and e 0represents electron.The conductivity in the perovskites can be described as an ordered diffusion of oxygen vacancies [28].Their motion is manifested by enhanced ionic conductivity associated with an activation energy value of $1eV [26].These oxygen vacancies can be suppressed by addition of donors,since the donor oxide contains more oxygen per cation than the host oxide it replaces [29].It has been reported that conductivity in Bi 4Ti 3O 12(BIT)can be significantly decreased,up to three orders of magnitude with the addition of donors,such as Nb 5+and Ta 5+at the Ti 4+sites [23,30].A few other studies on layered perovskites have also reported a decrease inconductivityFig.1.XRD patterns of SrBi 2(W x Ta 1Àx )2O 9samples sintered at 12008C.Table 1Lattice parameters of SrBi 2(W x Ta 1Àx )2O 9samples.Concentration of W a (A ˚)b (A ˚)c (A ˚)0.0 5.5212 5.513924.92230.025 5.5214 5.520225.10790.05 5.5217 5.519925.05850.075 5.5191 5.504525.05670.10 5.5142 5.506125.0850.205.51335.493925.0861I.Coondoo et al./Materials Research Bulletin 44(2009)1288–12921289with addition of donors [23,24,31].In the present study,the Ta 5+-site substitution by W 6+in SBT can be formulated using a defect chemistry expression as WO 3þV o!Ta 2O 512W Ta þ3O o (2)It shows that the oxygen vacancies are reduced upon the substitution of donor W 6+ions for Ta 5+ions.Hence,it is reasonable to believe that the conductivity in SBT is suppressed by donor addition.As per the above discussion,the high s dc observed in the undoped SBT (Fig.3)can be attributed to the motion of oxygen vacancies.As already discussed,the doped samples show reduced conductivity because the transport phenomena involving oxygen vacancies are greatly reduced.The high E a value of $1.75–2eVcorresponding to the high-temperature region in the doped ceramics is consistent with the fact that in the donor-doped materials,the ionic conduction reduces [32].The activation energy E a in the low temperature ferroelectric region (Table 2)corre-sponds to extrinsic conduction.At lower temperatures the extrinsic conductivity results from the migration of impurity ions in the lattice.Some of these impurities may also be associated with lattice defects.Pure SBT has large number of Schottky defects (oxygen vacancies)in addition to impurity ions whereas in the doped samples,due to charge neutrality,there is relatively less content of oxygen vacancies.Thus,in the doped samples the conductivity in the low temperature region is largely due to the impurity ions only.This explains the high activation energy in pure SBT in the low temperature region compared to doped samples (Table 2).In the high-temperature region,the value of E a in the doped samples is observed to increase with W concentration up to x =0.05but beyond that,it decreases (Table 2).The decrease in the activation energy for samples with x >0.05suggests an increase in the concentration of mobile charge carriers [33].This observation can be ascribed to the existence of multiple valence states of tungsten.Since tungsten is a transitional metal element,the valence state of W ions in a solid solution most likely varies from W 6+to W 4+depending on the surrounding chemical environment [34].When W 4+are substituted for the Ta 5+sites,oxygen vacancies would be created,i.e.one oxygen vacancy would be created for every two tetravalent W ions entering the crystal structure,whichFig.3.Variation of dc conductivity with temperature in SrBi 2(W x Ta 1Àx )2O 9samples.Fig.2.SEM micrographs of fractured surfaces of SrBi 2(W x Ta 1Àx )2O 9samples with (a)x =0.0,(b)x =0.025,(c)x =0.050,(d)x =0.075,(e)x =0.10and (f)x =0.20Table 2Activation energy (E a )in the high-temperature paraelectric region and low temperature ferroelectric region;Curie temperature (T c )in SrBi 2(W x Ta 1Àx )2O 9samples.Concentration of W E a (high temp.)(eV)E a (low temp.)(eV)T c (8C)0.00.790.893110.025 1.920.593080.05 1.960.543250.075 1.940.543380.10 1.860.573680.201.740.54390I.Coondoo et al./Materials Research Bulletin 44(2009)1288–12921290explains the increase in the concentration of mobile charge carriers which ultimately results in an decrease in the E a beyond x>0.05. Hence it is reasonable to conclude that W ions in the SBWT exists as a varying valency state,i.e.at lower doping concentration they exist in hexavalent state(W6+)and at a higher doping concentra-tion,they tend to exist in lower valency states[8].The P–E loops of SrBi2(Ta1Àx W x)2O9are shown in Fig.4.It is observed that W-doping results in formation of well-defined hysteresis loops.Fig.5shows the compositional dependence of remanent polarization(2P r)and the coercivefield(2E c)of SrBi2(Ta1Àx W x)2O9samples.Both the parameters depend on W content of the samples.It is observed that2P rfirst increases with x and then decreases while2E cfirst decreases with x and then increases(Fig.5).The optimum tungsten content for maximum2P r ($25m C/cm2)is observed to be x=0.075.It is known that ferroelectric properties are affected by compositional modification,microstructural variation and lattice defects like oxygen vacancies[10,35,36].In hard ferroelectrics, with lower valent substituents,the associated oxide vacancies are likely to assemble in the vicinity of domain walls[37,38].These domains are locked by the defects and their polarization switching is difficult,leading to an increase in E c and decrease in P r[38]. On the other hand,in soft ferroelectrics,with higher valent substituents,the defects are cation vacancies whose generation in the structure generally increases P r.Similar observations have been made in many reports[38–41].Watanabe et al.[42]reported a remarkable improvement in ferroelectric properties in the Bi4Ti3O12ceramic by adding higher valent cation,V5+at the Ti4+ site.It has also been reported that cation vacancies generated by donor doping make domain motion easier and enhance the ferroelectric properties[43].Further,it is known that domain walls are relatively free in large grains and are inhibited in their movement as the grain size decreases[44].In the larger grains, domain motion is easier which results in larger P r.Also for the SBT-based system,it is known that with increase in the grain size the remanent polarization also increases[45,46].Based on the obtained results and above discussion,it can be understood that in the undoped SBT,the oxygen vacancies assemble at sites near domain boundaries leading to a strong domain pinning.Hence,as observed,well-saturated P–E loop for pure SBT is not obtained.But in the doped samples,the suppression of the oxygen vacancies reduces the pinning effect on the domain walls,leading to enhanced remanent polarization and lower coercivefield.Also,the increase in grain size in tungsten added SBT,as observed in SEM micrographs(Fig.2)contribute to the increase in polarization values.In the present study,the grain size is observed to increase with increasing W concentration.However, the2P r values do not monotonously increase and neither the E c decreases continuously with increasing W concentration(Fig.5). The variation of P r and E c beyond x>0.05,seems possibly affected by the presence of secondary phases(observed in XRD diffracto-grams),which hampers the switching process of polarization [47–50].Also,beyond x>0.05the increase in the number of charge carriers in the form of oxygen vacancies leads to pinning of domain walls and thus a reduction in the values of P r and increase in E c is observed.Fig.6shows the variation of piezoelectric charge coefficient d33 with x in the SrBi2(Ta1Àx W x)2O9.The d33values increases with increase in W content up to x=0.05.A decrease in d33values is observed in the samples with x!0.075.The piezoelectric coefficient,d33,increases from13pC/N in the sample with x=0.0to23pC/N in the sample with x=0.05.It is known that the major drawback of SBT is its relatively higher conductivity,which hinders proper poling[51].High resistivity is therefore important for maintenance of poling efficiency at high-temperature[52,53].The W-doped SBT samples show an electrical conductivity value up to three orders of magnitude lower than that of undoped sample(Fig.3).The positional variation of2P r and2E c in SrBi2(W x Ta1Àx)2O9samples.Fig.6.Variation of d33in SrBi2(W x Ta1Àx)2O9samples.Fig. 4.P–E hysteresis loops in SrBi2(W x Ta1Àx)2O9samples recorded at roomtemperature.I.Coondoo et al./Materials Research Bulletin44(2009)1288–12921291decrease in conductivity upon donor doping improve the poling efficiency resulting in the observed higher d33values.Moreover, since the grain size increases with W content in SBT,it is reasonable to believe that the increase in grain size will also contribute to the increase in d33values[54].The decrease in the value of d33for samples with x!0.075is possibly due to the presence of secondary phases as observed in diffractograms[1,51,55]and the increase in oxygen vacancies for samples with x>0.05.4.ConclusionsX-ray diffractograms of the samples reveal that the single phase layered perovskite structure is maintained in the samples with tungsten content x0.05.SEM micrographs reveal that the average grain size increases with increase in W concentration. The temperature dependence of the electrical conductivity shows that tungsten doping results in the decrease of conductivity by up to three order of magnitude compared to W free SBT.All the tungsten-doped ceramics have higher2P r than that of the undoped sample.The maximum2P r($25m C/cm2)is obtained in the composition with x=0.075.The reduced conductivity allows high-temperature poling of the doped samples.Such compositions with low loss and high P r values should be excellent materials for highly stable ferroelectric memory devices.The d33value is observed to increase with increasing W content up to x0.05.The value of d33 in the composition with x=0.05is$23pC/N as compared to$13 pC/N in the undoped sample.AcknowledgmentsThe authors sincerely thank Prof.P.B.Sharma,Dean,Delhi College of Engineering,India for his generous support and providing ample research infrastructure to carry out the research work.The authors are thankful to Dr.S.K.Singhal,Scientist, National Physical 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《La-N、Gd-N掺杂量对ZnO电子结构及吸收光谱影响的研究》一、引言随着科技的发展，半导体材料的研究与应用越来越受到人们的关注。
氧化锌（ZnO）作为一种宽禁带半导体材料，具有优异的物理和化学性质，其应用领域涵盖了光电子器件、太阳能电池等。
近年来，通过掺杂不同的元素来调控ZnO的电子结构和光学性能成为研究热点。
本文着重研究La-N、Gd-N掺杂量对ZnO电子结构及吸收光谱的影响。
二、La-N、Gd-N掺杂ZnO的制备与实验方法本实验采用溶胶凝胶法，通过控制掺杂元素的含量，制备了不同La-N、Gd-N掺杂量的ZnO样品。
在实验过程中，通过X射线衍射（XRD）、扫描电子显微镜（SEM）、紫外可见光谱等手段对样品进行表征和性能分析。
三、La-N、Gd-N掺杂对ZnO电子结构的影响1. 能带结构变化La-N、Gd-N掺杂ZnO后，由于掺杂元素的引入，使得ZnO 的能带结构发生改变。
随着掺杂量的增加，ZnO的禁带宽度可能发生变化，导致其电子结构发生调整。
通过计算分析，我们发现La-N、Gd-N的掺入使得ZnO的能带结构变得更加复杂，出现了新的能级。
2. 载流子浓度变化La-N、Gd-N的掺入会改变ZnO中的载流子浓度。
随着掺杂量的增加，载流子浓度呈现先增加后减小的趋势。
这主要是由于掺杂元素在ZnO中的替代作用和杂质能级的形成所导致的。
四、La-N、Gd-N掺杂对ZnO吸收光谱的影响1. 吸收边移动La-N、Gd-N掺杂ZnO后，其吸收光谱发生明显变化。
随着掺杂量的增加，吸收边出现红移或蓝移现象。
这主要是由于掺杂元素引入的杂质能级与ZnO的能级之间的相互作用所导致的。
2. 吸收峰变化除了吸收边的移动，La-N、Gd-N掺杂还会在ZnO的吸收光谱中引入新的吸收峰。
这些新峰的出现与掺杂元素在ZnO中的能级分布和电子跃迁有关。
通过分析这些新峰的位置和强度，可以进一步了解掺杂元素对ZnO光学性能的影响。
五、结论本文通过研究La-N、Gd-N掺杂量对ZnO电子结构及吸收光谱的影响，发现掺杂元素的引入可以改变ZnO的能带结构和载流子浓度，同时还会导致其吸收光谱发生明显变化。
Excited States of Phosphorescent Platinum(II)Complexes Containing N∧C∧N-Coordinating Tridentate Ligands:Spectroscopic Investigations and Time-Dependent Density Functional Theory CalculationsWataru Sotoyama,*,†Tasuku Satoh,†Hiroyuki Sato,Azuma Matsuura,and Norio SawatariFujitsu Laboratories Ltd.,10-1Morinosato-Wakamiya,Atsugi243-0197,JapanRecei V ed:June22,2005;In Final Form:September4,2005The absorption and emission spectra of the Pt(II)complexes containing N∧C∧N-coordinating tridentate ligands,platinum(II)1,3-di(2-pyridyl)benzene chloride[Pt(dpb)Cl]and platinum(II)3,5-di(2-pyridyl)toluene chloride[Pt(dpt)Cl],together with their corresponding free ligands,1,3-di(2-pyridyl)benzene(dpbH)and3,5-di(2-pyridyl)toluene(dptH),have been analyzed by density functional theory(DFT)for the ground state and time-dependent DFT(TDDFT)for the excited states.T1(A1)and S1(B2)of the complexes(in C2V symmetry)wereassigned on the basis of the calculated excitation energies as well as comparison of the experimentalspectroscopic properties and the calculated states’characteristics.The calculated excitation energies for T1and S1of the complexes as well as those for T1of the free ligands were in good agreement with their observedvalues within600cm-1.The d-π*characters of the excited states were evaluated from the change in electrondensities between the ground and excited states by Mulliken population analysis;values of25%for T1and32%for S1were obtained for both complexes.The calculated values of d-π*character were found to beconsistent with the reported emission lifetimes as well as the observed emission energy shifts from thecorresponding free ligands.Most spectroscopic properties of the complexes and the free ligands,which includesolvatochromic shift,Stokes shifts,methyl substitution shifts,and emission spectra profiles,were well explainedfrom the calculation results.1.IntroductionStudies on phosphorescent transition-metal complexes are ofparticular interest not only from a scientific viewpoint but alsoin development of emissive dopants for phosphorescent organiclight-emitting diodes(OLEDs).1,2The rapid growth of interestin phosphorescent OLEDs has prompted syntheses and photo-physical investigations of a wide variety of phosphorescentcomplexes.3-7Recently,a new class of platinum(II)complexesconsisting of platinum,chloride,and N∧C∧N-coordinatingtridentate ligands[platinum(II)1,3-di(2-pyridyl)benzene chlo-ride,Pt(dpb)Cl,and platinum(II)3,5-di(2-pyridyl)toluene chlo-ride,Pt(dpt)Cl,Figure1]have been synthesized,8,9and highphosphorescence quantum yields ranging from0.58to0.68insolution at room temperature have been reported in these complexes.9Pt(dpt)Cl and its derivative with a phenoxide group that replaces the chloride have been reported to exhibit remark-able performance as emissive dopants of OLEDs.10The lowest excited triplet states(T1)of Pt(dpb)Cl and Pt(dpt)Cl have been assigned to primarily ligand-centered(LC) 3π-π*character from their highly structured emission profile with very small Stokes shifts and relatively long emission lifetimes[Pt(dpb)Cl7.2µs and Pt(dpt)Cl7.8µs in solution at room temperature].9However,these emission lifetimes are not much longer than those of the complexes,which have the triplet-emitting states with metal-to-ligand charge transfer(MLCT) character.For example,in the case of Ir(ppy)3,where ppy represents2-phenylpyridine ligand,the emitting states have been assigned to be mainly3MLCT character by theoretical studies11 and spectroscopic measurements.12The emission lifetimes of this complex were found to be2µs and5µs at room temperature and77K,respectively,in solution.13Taking into account the possible admixture ofπ-π*/d-π*(MLCT)character,which is common for T1of many phosphorescent transition-metal complexes,14-20it is expected that the d-π*character would participate in T1of Pt(dpb)Cl and Pt(dpt)Cl.The d-π* admixture to an excited state,especially to T1,is important even though the state character is primaryπ-π*,since a small amount of d-π*character can dominate the kinetics of the deactivation processes of the excited state by its strong spin-orbit coupling ability to the states with different spin multi-plicities.16,17,21-23In this paper we describe detailed analysis of the excited states of Pt(dpb)Cl and Pt(dpt)Cl by correlating the spectroscopic data with the calculation results based on density functional theory (DFT).The main objectives of this study are assignment of the*To whom correspondence should be addressed:e-mail wataru_sotoyama@fujifilm.co.jp.†Present address:Advanced Core Technology Laboratories,Fuji Photo Film Co.,Ltd.,210Nakanuma,Minamiashigara250-0193,Japan.Figure1.Schematic structure of the two complexes[Pt(dpb)Cl and Pt(dpt)Cl]and the two free ligands[dpbH and dptH]along with the molecular axis.The numbers in the structures designate each carbon atom.9760J.Phys.Chem.A2005,109,9760-976610.1021/jp053366c CCC:$30.25©2005American Chemical SocietyPublished on Web10/11/2005low-lying excited states of the complexes and quantitative evaluation of the d-π*character at the complexes.The free tridentate ligands[1,3-di(2-pyridyl)benzene(dpbH)and3,5-di-(2-pyridyl)toluene(dptH),Figure1]are also investigated to assist in analysis of the excited states of the complexes.Several aspects of the spectroscopic features,such as effect of methyl substitution on the excitation energies and the difference in emission profile between the complexes and the free ligands, will be discussed in light of the calculation results by DFT and time-dependent DFT(TDDFT).2.Experimental Section2.1.Materials and Spectroscopic Measurements.Pt(dpb)-Cl and Pt(dpt)Cl were synthesized from dpbH and dptH, respectively,according to the literature.8,9Absorption and emission measurements in solution were conducted in1cm path length quartz cuvettes at room temperature or in5mm diameter quartz tubes at77K.Absorption spectra were recorded on a Hitachi U-4100spectrophotometer.Emission spectra of Pt(dpb)-Cl and Pt(dpt)Cl were measured on a Minolta CS-1000 spectroradiometer,equipped with an ultra-high-pressure mercury lamp and a visible blocking filter,as the excitation source.The solutions of free ligands(dpbH and dptH)showed phosphores-cence with lifetimes of a few seconds at77K;phosphorescence spectra of these solutions were measured with a phosphoroscope in addition to the above apparatus.putational Details.The ground-state DFT and the excited-state time-dependent DFT(TDDFT)24-26calculations were carried out with the Gaussian98program package.27 TDDFT has been successfully applied to excited-state calcula-tions for many transition-metal complexes.11,28-34All calcula-tions were carried out with the LANL2DZ basis set with the relativistic effective core potential(ECP)for Pt35and the6-31G* basis set for the other elements.The calculation results(orbitals and densities)were plotted with the visualization program(xmo 4.0)in MOPAC2002.36Calculations on the electronic ground state(S0)of the complexes[Pt(dpb)Cl and Pt(dpt)Cl]and the free ligands(dpbH and dptH)were carried out by use of B3LYP density functional theory.37,38The S0geometries were optimized under symmetry constraints of C2V for Pt(dpb)Cl and C s(zx symmetry plane,Figure1)for Pt(dpt)Cl.The optimized planar geometries of these complexes were confirmed by the vibrational frequencies calculations with no imaginary frequencies.The optimized geometry of Pt(dpb)Cl was found to be in good agreement with the X-ray crystallographic data(Table1).8For dpbH and dptH,the geometries optimized within symmetry constraints similar to the complexes(planar forms)were different from those fully optimized without symmetry con-straints(twisted forms).Although the twisted forms have lower S0total energy than the planar forms for both dpbH and dptH, the differences in the S0total energy between the planar and twisted forms were found to be negligible(158cm-1for dpbH and179cm-1for dpbH).On the basis of the minor energy deviations from the twisted forms,we present calculation results at the planar forms of the free ligands in this study for convenience of comparison with the results for the complexes. The geometries for the lowest excited triplet states(T1)of Pt(dpb)Cl and dpbH were optimized at the restricted open-shell B3LYP level.At the respective optimized geometries,TDDFT calculations with the B3LYP functional were carried out.Ten lowest roots for triplet states as well as singlet states were calculated for the complexes and the ligands.In addition to the vertical excitation energies for all calculated roots,the one-particle densities and the dipole moments for the three lowest triplet and singlet roots were obtained for the complexes.3.Results3.1.Absorption and Emission Spectra at Room Temper-ature.Figure2shows absorption and emission spectra of Pt-(dpb)Cl(a)and Pt(dpt)Cl(b)together with the absorption spectra of dpbH(a)and dptH(b)in toluene at room temperature.The spectra of the complexes in Figure2replicate the results reported by Williams et al.9The lowest energy weak peaks in Pt(dpb)Cl and Pt(dpt)Cl absorption spectra are assigned as S0-T1absorp-tion by their mirror-symmetry relation with the emission peak. The strong absorption envelopes at22000-30000cm-1are assigned to the absorptions to the singlet excited states with significant MLCT character involving Pt and Cl orbitals from the comparison with the absorption spectra of the free ligands. The lowest energy peak among these absorptions is considered as the S1peak of the each complex.(Strictly speaking,the absorption spectra do not exclude the possibility of the existence of one or more singlet excited states with very small absorption cross sections at the lower energy side of the S1peak assigned above.However,even if there are such dark states,the discussion about the orbital assignment of the S1peak based on the absorption intensities in section4.1still remains unchanged.)TABLE1:Calculated and Observed X-ray Crystallographic Data for Representative Bond Lengths(Å)and Angles (degree)for Pt(dpb)Cl abond b calcd obsd c Pt-Cl 2.471 2.417Pt-C(1) 1.926 1.907Pt-N 2.065 2.037dC(1)-Pt-N80.4380.5dPt-C(1)-C(2)118.62119.2dPt-N-C(5)113.95114.6da Bond lengths are given in angstroms;bond angles are given in degrees.b See Figure1for numbering of the carbon atoms.c Data from ref8.d Average of two equivalent datapoints.Figure2.Absorption and emission spectra of the complexes alongwith absorption spectra of the free ligands in toluene at roomtemperature:(a)Pt(dpb)Cl and dpbH;(b)Pt(dpt)Cl and dptH.Solidline(green),emission of the complexes;dashed line(purple),absorptionof the complexes;dotted line(red),absorption of the complexes in anexpanded scale(134times);dashed-dotted line(blue),absorption ofthe free ligands.Phosphorescent Platinum(II)Complexes J.Phys.Chem.A,Vol.109,No.43,20059761We observed negative solvatochromic behavior (peak blue shift with increasing solvent polarity)reported in ref 9for S 1and T 1peaks of Pt(dpb)Cl and Pt(dpt)Cl in several solvents.The absorption peak energies in toluene and acetonitrile are presented in Table 2.(See ref 9for further information about spectral data in various solvents including toluene and aceto-nitrile.)According to the theoretical study on solvent effect by Marcus,39,40negative solvatochromic behavior of an absorption peak has been interpreted to be associated with the decrease in dipole moments by excitation.It is observed that the solvato-chromic shifts are fairly larger for S 1peaks than T 1peaks,which would suggest a lager decrease in dipole moment by the S 0-S 1excitation than that of the S 0-T 1excitations of these complexes.3.2.Phosphorescence Spectra at 77K.Figure 3shows the phosphorescence spectra of dpbH and Pt(dpb)Cl (a)and of dptH and Pt(dpt)Cl (b)in toluene at 77K.The highest energy peak of each spectrum (presented in Tables 3and 4)is attributed to the vibrational 0-0origin of the T 1-S 0transition.The phosphorescence origins of the Pt(dpb)Cl and Pt(dpt)Cl have red-shifted 2200and 2580cm -1from those of the corresponding free ligands,respectively.These shifts are much larger than those for complexes with ligand-centered emitting states in the literature,such as [Pt(bpy)2]2+(950cm -1)or [Rh(bpy)3]3+(600cm -1),41where bpy is 2,2′-bipyridine,suggesting significant admixture of the metal character in the emitting state of the complexes.The relative intensity of the origin peak,compared to the peaks of vibrational satellites with frequencies of about 1300cm -1,would become notably larger for the complexes than for the free ligands;these features imply closer equilibrium geometries between S 0and T 1for these complexes compared to those for the free ligands.Similar trends have been observed for complexes having T 1with significant MLCT character,such as [Ru(bpy)3]2+or Pt(thpy)2,where thpy is 2-(2-thienyl)pyridine,in comparison with their corresponding free ligands.17,18,42The characteristics such as the position and profile of the spectraindicate a certain amount of metal d-orbital admixture to T 1of Pt(dpb)Cl and Pt(dpt)Cl.The methyl substitution in place of the hydrogen atom at C(4)of the phenyl ring causes somewhat different effects to the emission spectra between the complexes and the free ligands.Both the methyl-substituted complex [Pt(dpt)Cl]and free ligand (dptH)displayed red-shifted emission from their unsubstituted counterparts [Pt(dpb)Cl and dpbH].However,the spectral shift was found to be substantially greater between the complexes (540cm -1)than between the free ligands (160cm -1).This large difference in methyl-substitution effect would suggest the orbital nature difference of T 1between the complexes and the free ligands.4.Theoretical and Experimental Data Comparison 4.1.Assignments and Excitation Energies of Calculated Roots.Figure 4depicts the four highest occupied and the two lowest unoccupied molecular orbitals (HOMOs and LUMOs)of Pt(dpb)Cl,as well as the HOMO and LUMO of dpbH obtained from the ground-state DFT calculations.The HOMOs and LUMOs for Pt(dpt)Cl and dptH are similar in shape with the corresponding orbitals presented in Figure 4.Every fourTABLE 2:T 1and S 1Absorption Peak Energies for Pt(dpb)Cl and Pt(dpt)Cl in Toluene and Acetonitrile at Room Temperatureabsorption peak energy/cm -1complex state toluene acetonitrile Pt(dpb)Cl T 12040020700Pt(dpb)Cl S 12400025200Pt(dpt)Cl T 12000020300Pt(dpt)ClS 12340024500TABLE 3:Parameters a from Ground-State DFT and TDDFT Calculations for the Three Lowest Excited Triplet and Singlet Roots of Pt(dpb)Cl and Pt(dpt)Cl at the Ground-State GeometriesPt(dpb)ClPt(dpt)Cl root (assignment b )symmetry cdominant excitation c ,d E /cm -1µz (∆µz )/D f E /cm -1µz (∆µz )/D fground state S 01A 1 6.647.07triplet root 1(T 1)3A 1b 1f b 1*20421 5.51(-1.13)20059 6.69(-0.38)2(T 2)3B 2b 1f a 2*20892 1.03(-5.61)20270 2.66(-4.41)3(T 3)3A 1a 2f a 2*22993 4.41(-2.23)22886 5.09(-1.98)T 1(expt e )2058020040singlet root 1(S 1)1B2b 1f a 2*23489-2.43(-9.07)0.07423120-1.04(-8.10)0.0772(S 2)1A 1b 1f b 1*23707 3.06(-3.58)0.00723342 4.64(-2.43)0.0063(S 3)1A 2b 2f b 1*25928-4.95(-11.58)0.00025899-4.47(-11.54)0.000S 1(expt f )2400023400aE ,vertical excitation energy;µz ,z component of dipole moment;∆µz ,dipole moment differences between ground and excited states;f ,oscillatorstrength.Experimental excitation energies are also listed.b See text.c Symmetry of states and orbitals are designated for Pt(dpb)Cl (C 2V ,see Figure 1).d For designation of the orbitals,see Figure 4.e Phosphorescence 0-0peak energy in toluene at 77K.f Absorption peak energy in toluene at roomtemperature.Figure 3.Emission spectra of the complexes and the free ligands in toluene at 77K:(a)Pt(dpb)Cl and dpbH;(b)Pt(dpt)Cl and dptH.Solid line (green),emission of the complexes;dotted line (blue),emission of the free ligands measured with a phosphoroscope.9762J.Phys.Chem.A,Vol.109,No.43,2005Sotoyama et al.HOMOs of the complexes are characterized by significant admixtures by the Pt d-orbitals.Two of them [b 1(HOMO)and a 2(HOMO-3)]are represented by combinations of d orbitals of the Pt and πorbitals of the tridentate ligand and Cl,while the remainder [b 2(HOMO-1)and a 1(HOMO-2)]have σcharacters that are symmetrical about the molecular plane.On the other hand,the LUMOs correspond to the π*orbitals almost localized in the ligand.Table 3summarizes the TDDFT calculation results of the properties that include dominant orbital excitations,vertical excitation energies (E ),dipole moments (z component,µz ),dipole moment differences from S 0(∆µz ),and oscillatorstrengths (f )of the lowest three triplet and the lowest three singlet roots for Pt(dpb)Cl and Pt(dpt)Cl.Table 3also includes the observed excitation energies for T 1(the origins in the low-temperature phosphorescence spectra)and S 1(the absorption peaks in the room-temperature spectra).The lowest three triplet roots as well as the lowest two singlet roots are represented by dominant excitations from mixed d πorbitals to π*orbitals.The third lowest singlet root is represented by a d σ-π*excitation.Excitations with d σ-π*character appear at the fifth lowest and higher roots for triplet states that are not specified in Table 3.The lowest three triplet as well as singlet roots for Pt(dpt)Cl are identical in characteristics of dominant excitations to the counterparts of Pt(dpb)Cl.In succeeding sections,we discuss only about Pt(dpb)Cl and dpbH,unless the methyl substitution effect is explored.We begin the discussion on the comparison of the calculated and experimental data by assignment of T 1and S 1from the calculated triplet and singlet roots for Pt(dpb)Cl.Experimental evidence is needed for the T 1and S 1assignments because both the calculated energy differences between the two lowest roots of triplet as well as singlet are very small.On the other hand,the third lowest roots for triplet and singlet are excluded from the T 1or S 1assignment because they are calculated to be energetically well above the lowest two roots.Considering that the excitation to S 1has a substantial extinction coefficient in the absorption spectrum (section 3.1),the lowest calculated singlet root (1B 2;f )0.074)is assigned to be S 1since the second lowest root (1A 1;f )0.007)is predicted to have 1order smaller absorption intensity.The calculated S 1has a large ∆µz (-9.07D)that is consistent with the experimentally obtained large negative solvatochromic shift of the S 1peak.39,40On the other hand,T 1of Pt(dpb)Cl has different spectral characteristics from S 1as discussed in section 3.1.Therefore,we assign the lowest calculated triplet root (3A 1)to T 1on the basis of the different dominant orbital excitation from S 1(1B 2)and the small ∆µz (-1.13D)that explain the small negative solvatochromic shift along with the small Stokes shift of the T 1peak.39The second and third lowest calculated triplet (or singlet)roots are assigned to be T 2and T 3(or S 2and S 3),respectively.The calculated T 1excitation energies of Pt(dpb)Cl and Pt(dpt)Cl show excellent agreements with the respective ex-perimental phosphorescence origins at 77K;the differences between the calculated and experimental values are found to be less than 200cm -1.In these comparisons,the calculated energies represent the vertical excitation energies at S 0geom-etries,while the experimental energies correspond to the 0-0transitions.However,these comparisons may be justified by the phosphorescence spectra,which indicate close equilibrium geometries between S 0and T 1of these complexes,as shown in section 3.2.(Geometries of T 1together with the adiabatic excitation energies will be discussed in section 4.3.)The calculated S 1excitation energies of the complexes also show good conformity with the experimental absorption peak energies within 600cm -1.Although TDDFT is known to have irregulari-ties in description of long-range charge-transfer excited states,43-45the above calculation results for excitation energies ensure the applicability of the TDDFT calculations for T 1and S 1of the concerned molecules.Hence we further investigate the charac-teristics of T 1and S 1of the complexes by use of the TDDFT calculation results in the following sections.TDDFT calculations were also carried out for the free ligands.Calculated vertical excitation energies and excitation types for the two lowest excited triplet roots of dpbH and dptH are summarized in Table 4together with the experimental energiesTABLE 4:Vertical Excitation Energies (E )from TDDFT Calculations for Triplet Excited States of DpbH and DptH with Experimental Excitation EnergiesE /cm -1root (assignment a )symmetry bdominant excitation b ,c dpbH dptH 1(T 1)3A 1a 2f a 2*23344232172(T 2)3B 2b 1f a 2*2730427077T 1(expt d )2278022620aSee text.b Symmetry of states and orbitals are designated for dpbH (C 2V ,see Figure 1).c For designation of the orbitals,see Figure 4.dPhosphorescence 0-0peak energy in toluene at 77K.Figure 4.Contour plots of the two lowest unoccupied and the four highest occupied molecular orbitals of Pt(dpb)Cl,along with the LUMO and HOMO of dpbH.Phosphorescent Platinum(II)ComplexesJ.Phys.Chem.A,Vol.109,No.43,20059763of the phosphorescence origin.For dpbH,the lowest calculated triplet root (3A 1)was assigned to be T 1on the basis of the large energy separation between the first and second lowest roots.The small differences between the calculated and experimental T 1excitation energies of dpbH and dptH (560-600cm -1)evidenced the accuracy of the TDDFT calculations.4.2.Evaluation of d -π*Character in the Excited States.Evaluation of the degree of d -π*participation for low-lying excited states of mixed π-π*/d -π*character would be very helpful to understand the photophysical properties of the phosphorescent complexes.The zero-field splittings (ZFS)of sublevels of T 1are shown to be a valuable experimental parameter to measure the metal participation in T 1of com-plexes.17-19Theoretical calculation is considered to be an alternative for the evaluation of the metal participation that can be applied to a variety of complexes.The theoretical studies reported so far treat the evaluation of d -π*participation by several methods:the population analysis of orbitals involved in dominant excitations,11the analysis of distribution of spin densities and atomic charges by SCF calculations in T 1,30and the population analysis of metal atoms 46or the metal d-orbitals for the ground and excited states.47We employed the population analysis of metal orbitals in combination with TDDFT calcula-tions as discussed in the following section.We have noted that T 1and S 1of the concerned complexes are each represented by a dominant excitation from a mixed d πorbital to a π*orbital.More detailed pictures of the excited states depicting the changes in electron density 48are provided in Figure 5,for T 1,S 1,and T 3from S 0of Pt(dpb)Cl along with that for T 1from S 0of dpbH.(The changes for T 2and S 2of thecomplex bear resemblances to those for S 1and T 1,respectively.)T 1and S 1of Pt(dpb)Cl are characterized by d-electron density decrease on Pt and π-electron densities on Cl and some of the carbons in the tridentate ligand and by increases of π-electron densities of the tridentate ligand with respect to S 0,which indicate an admixture of d -π*and π-π*characters in each excited states.Considering that TDDFT is a single excitation theory,49the decrease in d-electron densities calculated by TDDFT ap-proximately gives the percentage of d -π*character in the excited state;that is,one electron decrease in d-electron densities represents 100%d -π*character.The decreases in d-electron densities for T 1and S 1of Pt(dpb)Cl are evaluated by Mulliken population analysis.46,47Table 5summarizes the orbital popula-tions of the Pt consisting of atomic orbitals s,p π,p σ,d π,and d σ(πand σdenote antisymmetric and symmetric orbitals about the molecular plane,respectively)for S 0,T 1,and S 1of Pt(dpb)-Cl and Pt(dpt)Cl.The differences between the two complexes are insignificant,and the s,p σ,and d σdensities remain unchanged by the T 1and S 1excitations.The d πdensities decrease in T 1by 0.25e and in S 1by 0.32e from S 0for both of the complexes,suggesting the percentages of d -π*character in T 1and S 1as 25%and 32%,respectively.On the other hand,the p πdensities on Pt increase in T 1by 0.06e [Pt(dpb)Cl]and 0.05e [Pt(dpt)Cl]from S 0,which indicates some delocalization of the spreading of the π*orbitals over the Pt atom.The emitting states (T 1)of Pt(dpb)Cl and Pt(dpt)Cl with 25%d -π*character seem to fall into the intermediate category of significant LC/MLCT admixture in the sequence of complexes ordered by metal -d character (from pure LC to pure MLCT)of the emitting states.18,19The d -π*character in the emitting state of such a complex dominates the kinetics of the deactiva-tion processes by effective spin -orbit coupling with singlet excited states.The evaluated d -π*characters in T 1of Pt(dpb)-Cl and Pt(dpt)Cl are considered to be consistent with the reported emission lifetimes (7-8µs 9)as well as the emission energy shifts from the corresponding free ligands (2200-2600cm -1).It should be mentioned that the d -π*character in T 1does not lead to large ∆µz ,the difference in dipole moment between the ground and excited state,for the objective complexes despite its charge-transfer nature.The d -π*character combined with the small ∆µz in T 1is clearly shown in Figure 5,which depicts the dominant charge flow from Pt,Cl,and phenyl ring to both pyridine rings in the tridentate ligand that cancel each other in the y -axis,thus exhibiting little change along the dipole moment in the z -axis.On the other hand,S 1of Pt(dpb)Cl was calculated to have a considerably larger ∆µz (-9.07D)than T 1,in contrast to the small differences in the d -π*character between T 1(25%)and S 1(32%).The charge flow in S 1is almost“straightforward”Figure 5.Contour plots of changes in electron densities for T 1,S 1,and T 3from S 0of Pt(dpb)Cl,along with that for T 1from S 0of dpbH.TABLE 5:Orbital Populations on Pt for the Ground and Excited States from Mulliken Population Analysis aorbital population (difference from S 0)state s p πb p σb d πbd σb Pt(dpb)Cl S 0 2.712.094.123.784.74T 1 2.71(0.00) 2.15(0.06) 4.12(0.00) 3.53(-0.25) 4.74(0.00)S 12.71(0.00) 2.09(0.00) 4.12(0.00)3.46(-0.32)4.74(0.00)Pt(dpt)Cl S 0 2.712.094.123.834.69T 1 2.71(0.00) 2.14(0.05) 4.12(0.00) 3.58(-0.25) 4.69(0.00)S 12.71(0.00)2.09(0.00)4.12(0.00)3.51(-0.32)4.70(0.00)aOrbital populations are given in unit electron charge.Inner core electrons (1s through 4f)are not included because they are replaced by the effective core potential (ECP).b σand πrepresent symmetrical and antisymmetrical orbitals about the molecular plane,respectively.9764J.Phys.Chem.A,Vol.109,No.43,2005Sotoyama et al.from the Pt and Cl to the phenyl ring that causes a large dipole moment change parallel to the z-axis.parison of Phosphorescence Spectra of the Complexes and Free Ligands:Methyl Substitution Effects and Spectral Features.The methyl substitution causes a substantial decrease in phosphorescence0-0energy from Pt(dpb)Cl to Pt(dpt)Cl(540cm-1),while the difference between the free ligands(dpbH and dptH)is small(160cm-1),as explained in section3.2.These calculations reproduced the differences in the T1excitation energies(362cm-1between the complexes and127cm-1between the free ligands).The difference in the methyl substitution effect between the com-plexes and the free ligands can be assigned to the different orbital nature of T1in respective species.Although T1of Pt(dpb)Cl and dpbH have identical symmetry(A1),they differ in dominant orbital excitations[b1f b1*for Pt(dpb)Cl;a2f a2*for dpbH].Theπorbitals of a2symmetry in dpbH[as well as Pt(dpb)Cl]have no component at the atoms on the symmetry axis,including C(4),as shown in the MO plots(Figure4).Themethyl substitution at C(4)is expected to have a minimal effect on the orbitals of a2symmetry as well as the a2f a2* excitations.On the other hand,the b1orbital(HOMO)of Pt(dpb)Cl spreads over C(4);hence,it resulted in a substantial difference in T1excitation energies between Pt(dpb)Cl and Pt(dpt)Cl.The difference in the dominant orbital excitations between the complexes and the free ligands stated above gives valuable information about the phosphorescence spectra of these species. The phosphorescence spectra of the free ligands(Figure3)show increased vibrational progressions compared to those of the complexes.These spectral features can be explained from the plots of the changes of electron density(Figure5)that show the difference in T1character between Pt(dpb)Cl and dpbH.The changes of the electron density are found on the atoms for Pt-(dpb)Cl,whereas those for dpbH,which show some resemblance to those for T3of Pt(dpb)Cl,are found mainly on the bonds in the phenyl ring and between the phenyl and pyridine rings. These electron-density changes on the bonds,especially in the phenyl ring,are expected to induce significant geometry changes for the free ligands between T1and S0.These geometry changes appear as strong vibrational progressions in the phosphorescence spectra with frequencies of aromatic ring stretching about1300 cm-1.The geometry changes expected by the electron density calculations were confirmed by T1geometry optimizations for Pt(dpb)Cl and dpbH at the restricted open-shell B3LYP level. Geometries were optimized in C2V symmetry and then compared to their S0structures.Particular attention was paid to selection of the initial-guess orbitals for the SCF calculations in T1 geometry optimizations to obtain coherent results with the TDDFT calculations about nature of the singly occupied orbitals. Table6summarizes the changes in bond length between S0and T1.For dpbH,the length of the C(2)-C(3)bond shows a notable increase(0.085Å)by excitation to T1that is consistent with the electron density plot(Figure5),while changes in bond length for Pt(dpb)Cl are rather small compared to those for dpbH,and mainly observed for the bonds between Pt and its neighboring atoms[Cl and C(1)].Additional TDDFT calculations were carried out for both Pt(dpb)Cl and dpbH at the T1optimized geometry to obtain the relaxation energy in T1(the difference between the T1 energies at the S0and T1optimized geometries)and the adiabatic T1excitation energy(the vertical T1excitation energy at the S0 optimized geometry minus the relaxation energy in T1).The obtained T1relaxation energy for Pt(dpb)Cl(631cm-1)is notably smaller than that of dpbH(2070cm-1);this result isconsistent with the above discussion on the phosphorescenceprofiles of Pt(dpb)Cl and dpbH.The adiabatic T1excitationenergies are figured out to be19790cm-1for Pt(dpb)Cl and21274cm-1for dpbH;these values are in good agreement withthe experimental0-0excitation energies in S0-T1transitions.5.ConclusionThe lowest triplet and singlet excited states(T1and S1)ofthe objective complexes Pt(dpb)Cl and Pt(dpt)Cl have beenassigned to3A1(b1f b1*)and1B2(b1f a2*),respectively,from the TDDFT calculations with the assistance of the observedspectroscopic properties.The calculated excitation energies forT1and S1of the complexes show good agreement with theirrespective observed values within600cm-1.These states arepredicted to have mixed characters of d-π*andπ-π*excitations.On the basis of the single excitation nature of theTDDFT approach,the d-π*character in excited states isevaluated from the decrease in the d-electron densities on Ptby excitation;values of25%for T1and32%for S1have beenobtained for both complexes.The emitting states of the freeligands(dpbH and dptH)are predicted to be different in orbitalnature from those of the complexes;this difference in orbitalnature explains the observed difference in methyl-substitutioneffect as well as different emission profiles between thecomplexes and the free ligands.Other spectroscopic featuresof the complexes,such as the negative solvatochromic shift andsmall Stokes shift,are also successfully explained from thecalculation results based on the above assignment.The above results have proved that the TDDFT approach isrational for analysis of the excited-state properties of theobjective complexes.The presented procedure to evaluate thed-π*character may be a useful tool to design highly phos-phorescent complexes.We consider that it is very important toconduct further studies to evaluate d-π*character in the excitedstates of a variety of phosphorescent complexes and then toinvestigate the relationship between the d-π*character andphosphorescence properties of these complexes in order to getan in-depth understanding of the photophysics of the complexes. Acknowledgment.We thank Dr.N.F.Cooray for helpful suggestions and discussions.References and Notes(1)Baldo,M.A.;O’Brien,D.F.;You,Y.;Shoustikov,A.;Sibley,S.; Thompson,M.E.;Forrest,S.R.Nature(London)1998,395,151. TABLE6:Calculated Bond Lengths and Differences between S0and T1of Pt(dpb)Cl and DpbH aPt(dpb)Cl b dpbH b bond c S0T1difference S0T1difference Pt-Cl 2.471 2.444-0.027Pt-C(1) 1.926 1.899-0.027Pt-N 2.065 2.063-0.002C(1)-C(2) 1.403 1.4340.031 1.402 1.397-0.005 C(2)-C(3) 1.402 1.390-0.012 1.405 1.4900.085 C(3)-C(4) 1.401 1.4120.011 1.393 1.386-0.007 C(2)-C(5) 1.469 1.451-0.018 1.492 1.442-0.050 C(5)-C(6) 1.397 1.3970.000 1.406 1.4240.018 C(6)-C(7) 1.392 1.390-0.002 1.392 1.388-0.004 C(7)-C(8) 1.395 1.4080.013 1.393 1.3950.002 C(8)-C(9) 1.391 1.385-0.006 1.396 1.4080.012 C(9)-N 1.343 1.3510.008 1.333 1.321-0.012 C(5)-N 1.379 1.3980.019 1.347 1.3690.022a Bond lengths are given in angstroms.b Geometries are optimized within the symmetry constraint of C2V.c See Figure1for numbering of the carbon atoms.Phosphorescent Platinum(II)Complexes J.Phys.Chem.A,Vol.109,No.43,20059765。
Electrochimica Acta 92 (2013) 248–256Contents lists available at SciVerse ScienceDirectElectrochimicaActaj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /e l e c t a c taGel-combustion synthesis of LiFePO 4/C composite with improved capacity retention in aerated aqueous electrolyte solutionMilica Vujkovi´c a ,Ivana Stojkovi´c a ,Nikola Cvjeti´canin a ,Slavko Mentus a ,b ,∗,1a University of Belgrade,Faculty of Physical Chemistry,P.O.Box 137,Studentski trg 12-16,11158Belgrade,Serbia bThe Serbian Academy of Sciences and Arts,Kenz Mihajlova 35,11158Belgrade,Serbiaa r t i c l ei n f oArticle history:Received 2October 2012Received in revised form 3January 2013Accepted 5January 2013Available online 11 January 2013Keywords:Aqueous rechargeable Li-ion battery Galvanostatic cycling Gel-combustion Olivine LiFePO 4LiFePeO 4/C compositea b s t r a c tThe LiFePO 4/C composite containing 13.4wt.%of carbon was synthesized by combustion of a metal salt–(glycine +malonic acid)gel,followed by an isothermal heat-treatment of combustion product at 750◦C in reducing atmosphere.By a brief test in 1M LiClO 4–propylene carbonate solution at a rate of C/10,the discharge capacity was proven to be equal to the theoretical one.In aqueous LiNO 3solu-tion equilibrated with air,at a rate C/3,initial discharge capacity of 106mAh g −1was measured,being among the highest ones observed for various Li-ion intercalation materials in aqueous solutions.In addition,significant prolongation of cycle life was achieved,illustrated by the fact that upon 120charg-ing/discharging cycles at various rates,the capacity remained as high as 80%of initial value.The chemical diffusion coefficient of lithium in this composite was measured by cyclic voltammetry.The obtained val-ues were compared to the existing literature data,and the reasons of high scatter of reported values were considered.© 2013 Elsevier Ltd. All rights reserved.1.IntroductionThanks to its high theoretical Coulombic capacity (170mAh g −1)and environmental friendliness,LiFePO 4olivine became a desir-able cathodic material of Li-ion batteries [1,2],competitive to other commercially used cathodic materials (LiMnO 4,LiCoO 2).As evidenced in non-aqueous electrolyte solutions,a small vol-ume change (6.81%)that accompanies the phase transition LiFePO 4 FePO 4enables Li +ion insertion/deinsertion reactions to be quite reversible [1–3].The problem of low rate capability,caused by low electronic conductivity [4,5],was shown to be solv-able to some extent by reduction of mean particle size [6].Further improvements in both conductivity and electrochemical perform-ances were achieved by forming composite LiFePO 4/C,where in situ produced carbon served as an electronically conducting con-stituent [5,7–27].Ordinarily,both in situ formed carbon and carbon black additive,became unavoidable constituent of the LiFePO 4-based electrode materials [28–37].Zhao et al.[27]reported that Fe 2P may arise as an undesirable product during the synthesis of LiFePO 4/C composite under reducing conditions,however,other authors found later that this compound may contribute positively∗Corresponding author at:University of Belgrade,Faculty of Physical Chemistry,P.O.Box 137,Studentski trg 12-16,11158Belgrade,Serbia.Tel.:+381112187133;fax:+381112187133.E-mail address:slavko@ffh.bg.ac.rs (S.Mentus).1ISE member.to the electronic conductivity and improve the electrochemical per-formance of the composite [28–30].Severe improvement in rate capability and capacity retention was achieved by partial replace-ment of iron by metals supervalent relative to lithium [31–37].Thus one may conclude that the main aspects of practical applica-bility of LiFePO 4in Li-ion batteries with organic electrolytes were successively resolved.After the pioneering studies by Li and Dahn [38,39],recharge-able Li-ion batteries with aqueous electrolytes (ARLB)attracted considerable attention [40–50].The first versions of ARLB’s,suf-fered of very low Coulombic utilization and significantly more pronounced capacity fade relative to the batteries with organic electrolyte,regardless on the type of electrode materials [43].For the first time,LiFePO 4was considered as a cathode material in ARLB’s by Manickam et al.in 2006[44].He et al.[46],in an aqueous 0.5M Li 2SO 4solution,found that LiFePO 4displayed both a surprisingly high initial capacity of 140mAh g −1at a rate 1C and recognizable voltage plateau at a rate as high as 20C,which was superior relative to the other electrode materials in ARLB’s.Recently,the same authors reported a high capacity decay in aer-ated electrolyte solution,amounting to 37%after only 10cycles [48].In the same study,they demonstrated qualitatively by a brief cyclovoltammetric test,that a carbon layer deposited from a vapor phase over LiFePO 4particles,suppressed the capacity fade [48].Inspired by the recent discoveries about excellent rate capa-bility [46]but short cycle life [48]of LiFePO 4in aerated aqueous solution,we attempted to prolong the cycle life by means of protecting carbon layer over the LiFePO 4particles.Therefore we0013-4686/$–see front matter © 2013 Elsevier Ltd. All rights reserved./10.1016/j.electacta.2013.01.030M.Vujkovi´c et al./Electrochimica Acta92 (2013) 248–256249synthesized LiFePO4/C composite by a fast and simple glycine-nitrate gel-combustion technique.This method,although simpler than a classic solid state reaction method combined with ball milling[44,48],was rarely used for LiFePO4synthesis[19,27].It yielded a porous,foamy LiFePO4/C composite,easily accessible to the electrolyte.Upon the fair charging/discharging performance was confirmed by a brief test in organic electrolyte,we examined in detail the electrochemical behavior of this material in aqueous electrolyte,by cyclic voltammetry,complex impedance and cyclic galvanostatic charging/discharging methods.In comparison to pure LiFePO4studied in Ref.[48],this composite displayed markedly longer cycle life in aerated aqueous solutions.The chemical dif-fusion coefficient of lithium was also determined,and the reasons of its remarkable scatter in the existing literature were considered.2.ExperimentalThe LiFePO4/C composite was synthesized using lithium nitrate, ammonium dihydrogen phosphate(Merck)and iron(II)oxalate dihydrate(synthesized according to the procedure described else-where[51])as raw materials.Our group acquired the experience in this synthesis technique on the examples of spinels LiMn2O4 [52]and LiCr0.15Mn1.85O4[53],where glycine served as both fuel and complexing/gelling agent to the metal ions.A stoichiometric amount of each material was dissolved in deionized water and mixed at80◦C using a magnetic stirrer.Then,first glycine was added into the reaction mixture to provide the mole ratio of glycine: nitrate of2:1,and additionally,malonic acid(Merck)was added in an amount of60wt.%of the expected mass of LiFePO4.The role of malonic acid was to decelerate combustion and provide con-trollable excess of carbon[14].After removing majority of water by evaporation,the gelled precursor was heated to initiate the auto-combustion,resulting in aflocculent product.The combustion product was heated in a quartz tube furnacefirst at400◦C for3h in Ar stream,and then at750◦C for6h,under a stream of5vol.%H2in Ar.This treatment consolidated the olivine structure and enabled to complete the carbonization of residual organic matter.The VO2powder prepared by hydrothermal method was used as an active component of the counter electrode in the galvanostatic experiments in aqueous electrolyte solution.The details of the syn-thesis and electrochemical behavior of VO2are described elsewhere [54,55].The considerable stoichiometric excess of VO2was used,to provide that the LiFePO4/C composite only presents the main resis-tive element,i.e.,determines the behavior of the assembled cell on the whole.The XRD experiment was performed using Philips1050diffrac-tometer.The Cu K␣1,2radiation in15–70◦2Ârange,with0.05◦C step and2s exposition time was used.The carbon content in the composite was determined by its com-bustion in theflowing air atmosphere,by means of thermobalance TA SDT Model2090,at a heating rate of10◦C min−1.The morphology of the synthesized compounds was observed using the scanning electron microscope JSM-6610LV.For electrochemical investigations,the working electrode was made from LiFePO4/C composite(75%),carbon black-Vulcan XC72 (Cabot Corp.)(20%),poly(vinylidenefluoride)(PVDF)binder(5%) and a N-methyl-2-pyrrolidone solvent.The resulting suspension was homogenized in an ultrasonic bath and deposited on electron-ically conducting support.The electrode was dried at120◦C for 4h.Somewhat modified weight ratio,85:10:5,and the same drying procedure,were used to prepare VO2electrode.The non-aqueous electrolyte was1M LiClO4(Lithium Corpo-ration of America)dissolved in propylene carbonate(PC)(Fluka). Before than dissolved,LiClO4was dried over night at140◦C under vacuum.The aqueous electrolyte solution was saturated LiNO3solution.The cyclic voltammetry and complex impedance experiments were carried out only for aqueous electrolyte solutions,by means of the device Gamry PCI4/300Potentiostat/Galvanostat.The three electrode cell consisted of a working electrode,a wide platinum foil as a counter electrode,and a saturated calomel electrode(SCE) as a reference one.The experiments were carried out in air atmo-sphere.The impedance was measured in open-circuit conditions, at various stages of charging and discharging,within the frequency range10−2−105Hz,with7points per decade.Galvanostatic charging/discharging experiments were carried out in a two-electrode arrangement,by means of the battery testing device Arbin BT-2042,with two-terminal connectors only.In the galvanostatic tests in non-aqueous solution,working electrode was a2×2cm2platinum foil carrying2.3mg of compos-ite electrode material(1.5mg of olivine),while counter electrode was a2×2cm2lithium foil.The cell was assembled in an argon-filled glove box and cycled galvanostatically within a voltage range 2.1–4.2V.The galvanostatic tests in the aqueous electrolyte solution were carried out in a two-electrode arrangement,involving3mg of cathodic material,as a working electrode,and VO2in a multi-ple stoichiometric excess,as a counter electrode.According to its reversible potential of lithiation/delithiation reaction[55],VO2per-formed as an anode in this cell.The4cm2stainless steel plates were used as the current collectors for both positive and negative electrode.The cell was assembled in room atmosphere,and cycled within the voltage window between0.01and1.4V.3.Result and discussion3.1.The XRD,SEM and TG analysis of the LiFePO4/C compositeFig.1shows the XRD patterns of the composite LiFePO4/C pre-pared according to the procedure described in the Experimental Section.As visible,the diffractogram agrees completely with the one of pure LiFePO4olivine,found in the JCPDS card No.725-19. The narrow diffraction lines indicate complete crystallization and relatively large particle dimensions.On the basis of absence of diffraction lines of carbon,we may conclude that the carbonized product was amorphous one.Fig.2shows the SEM images of the LiFePO4/C composite at two different magnifications.Theflaky agglomerates,Fig.2left,with apparently smooth surface and low tap density,are due to a partial liquefaction and evolution of gas bubbles during gel-combustion procedure.These agglomerates consist of small LiFePO4/CFig.1.XRD patterns of LiFePO4/C composite in comparison to standard crystallo-graphic data.250M.Vujkovi´c et al./Electrochimica Acta 92 (2013) 248–256Fig.2.SEM images of LiFePO 4/C composite at two different magnification,20000×and 100000×.composite particles visible better at higher magnification,Fig.2,ly at the magnification of 100,000×,one may see that the size of majority of composite particles was in the range 50–100nm.The mean particle diameter,2r,as per SEM microphotograph amounted to 75nm.This analysis evidences that the gel-combustion method may provide nanodisprsed particles,desirable from the point of view of rate capability.For instance,Fey et al.[16]demonstrated that particle size reduction from 476to 205nm improved the rate capa-bility of LiFePO 4/C composite in organic electrolyte,illustrated by the increase of discharge capacity from 80mAh g −1to 140mAh g −1at discharging rate 1C.Also,carbon matrix prevented particles from agglomeration providing narrow size distribution,contrary to often used solid state reaction method of synthesis,when sintering of ini-tially nanometer sized particles caused the appearance of micron sized agglomerates [22].The SEM microphotograph (Fig.2)alone did not permit to rec-ognize carbon constituent of the LiFePO 4/C composite.However,carbonized product was evidenced,and its content measured,by means of thermogravimetry,as described elsewhere [9].The dia-gram of simultaneous thermogravimetry and differential thermal analysis (TG/DTA)of the LiFePO 4/C composite performed in air is presented in Fig.3.The process of moisture release,causing a slight mass loss of 1%,terminated at 150◦C.In the temperature range 350–500◦C carbon combustion took place,visible as a drop of the TG curve and an accompanying exothermic peak of the DTA curve.However,the early stage of olivine oxidation merged to some extent with the late stage of carbon combustion,and therefore,the minimum of the TG curve,appearing at nearly 500◦C,was not so low as to enable to read directly the carbon content.Fortunately,as proven by XRD analysis,the oxidation of LiFePO 4at tempera-ture exceeding 600◦C,yielded only Li 3Fe 2(PO 4)3and Fe 2O 3,whatFig.3.TGA/DTA curve of LiFePO 4/C under air flow at heating rate of 10C min−1.corresponded to the relative gain in mass of exactly 5.07%[9].Therefore,the weight percentage of carbonaceous fraction in the LiFePO 4/C composite was determined as equal to the difference between the TG plateaus at temperatures 300and 650◦C,aug-mented for 5.07%.According to this calculation the carbon fraction amounted to 13.4wt.%,and by means of this value,the electro-chemical parameters discussed in the next sections were correlated to pure LiFePO 4.Specific surface area of LiFePO 4,required for the measurement of diffusion constant,was determined from SEM image (Fig.2).Assuming a spherical particle shape and accepting mean particle radius r =37.5nm,the specific surface area was estimated on the basis of equation [17,22,45,46]:S =3rd(1)where the bulk density d =3.6g cm −3was used .This calculation resulted in the value S =22.2m 2g −1.In this calculation the contri-bution of carbon to the mean particle radius was ignored,however the error introduced in such way is more acceptable than the error which may arise if standard BET method were applied to the com-posite with significant carbon ly,due to a usually very developed surface area of carbon,the measured specific sur-face may exceed many times the actual surface area of LiFePO 4.3.2.Electrochemical measurements3.2.1.Non-aqueous electrolyte solutionIn order to compare the behavior of the synthesized LiFePO 4/C composite to the existing literature data,available predominantly for non-aqueous solutions,a brief test was performed in non-aqueous 1M LiClO 4+propylene carbonate solution by galvano-static experiments only.The results for the rates C/10,C/3and C,within the voltage limits 2.1–4.2V,were presented in Fig.4.The polarizability of the lithium electrode was estimated on the basis of the study by Churikov [56–67],who measured the current–voltage curves of pure lithium electrode in LiClO 4/propylene carbon-ate solutions at various temperatures.To the highest rate of 1C =170mA g −1in nonaqueous electrolyte,the corresponding cur-rent amounted to 0.25mA,which was equal to the current density of 0.064mA cm −2through the Li counter electrode.According to Fig.2in Ref.[67],for room temperature,the corresponding over-voltage amounted to only 6mV.Since lithium electrode is thus practically non-polarizable in this system,the voltages presented on the ordinate of the left diagram are the potentials of the olivine electrode expressed versus Li/Li +reference electrode.The clear charge and discharge plateaus at about 3.49V and 3.40V,respec-tively,correspond to the LiFePO 4 FePO 4phase equilibria [5].At discharging rate of C/10,the initial discharge capacity,within the limits of experimental error,was close to a full theoreticalM.Vujkovi´c et al./Electrochimica Acta 92 (2013) 248–256251Fig.4.The initial charge/discharge curves (a)and cyclic performance (b)of LiFePO 4/C composite in 1M LiClO 4+PC at different rates within a common cut-off voltage of2.1–4.2V.Fig.5.Charge/discharge profile and corresponding cyclic behavior of LiFePO 4/C in 1M LiClO 4+PC at the rate of 1C.capacity of LiFePO 4(170mAh g −1).This value is higher than that for LiFePO 4/C composite obtained by glycine [19],malonic acid [14]and adipic acid/ball milling [15]assisted methods.As usual,the discharge capacity decreased with increasing discharging rate (Fig.4b),and amounted to 127mAh g −1at C/3,and 109mAh g −1at 1C.For practical application of Li-ion batteries,a satisfactory rate capability and long cycle life are of primary importance.The charge/discharge profiles and dependence of capacity on the cycle number at the rate 1C are presented in Fig.5.The capacity was almost independent on the number of cycles,similarly to theearlier reports by Fey et al.[37–39].For comparison,Kalaiselvi et al.[19],by a glycine assisted gel-combustion procedure,with an additional amount (2wt.%)of carbon black,produced a similar nanoporous LiFePO 4/C composite displaying somewhat poorer per-formance,i.e.,smaller discharge capacity of 160mAh g −1at smaller discharging rate of C/20.On the other hand,better rate capability of LiFePO 4/C com-posite,containing only 1.1–1.8wt.%of carbon,in a non-aqueous solution,was reported by Liu et al.[21].For instance they mea-sured 160mAh g −1at the rate 1C,and 110at even 30C [21].This may be due to a thinner carbon layer around the LiFePO 4olivine particles.However the advantage of here applied thicker carbon layer exposed itself in aqueous electrolyte solutions,as described in the next section.3.2.2.Aqueous electrolyte solution3.2.2.1.Cyclic voltammetry.By the cyclic voltammetry method (CV)the electrochemical behavior of LiFePO 4/C composite in satu-rated aqueous LiNO 3solution was preliminary tested in the voltage range 0.4–1V versus SCE.The cyclic voltammograms are pre-sented in Fig.6.The highest scan rate of 100mV s −1,tolerated by this material,was much higher than the ones (0.01–5mV s −1)used in previous studies in both organic [13,24,25]and aqueous electrolyte solutions [47,48].Since one deals here with the thin layer solid redox electrode,limited in both charge consumption and diffusion length,the voltammogram is more complicated for interpretation comparing with the classic case of electroactive species in a liquid solution.A sharp,almost linear rise of current upon achieving reversible potential,with overlapped rising parts at various scan rates,similar to ones reported elsewhere [21,25],resembles closely the voltammogram of anodic dissolution ofaFig.6.Cyclic voltammograms of LiFePO 4/C in saturated LiNO 3aqueous electrolyte with a scan rate of 1mV s −1(left)and at various scan rates in the range 1–100mV s −1.252M.Vujkovi´c et al./Electrochimica Acta 92 (2013) 248–256Fig.7.Anodic and cathodic peak current versus square root of scan rate forLiFePO 4/C composite in aqueous LiNO 3electrolyte solution.thin metal layer [56],which proceeds under constant reactant activity.Since the solid/solid phase transitions LiFePO 4 FePO 4accompanies the redox processes in this system [5,8,57,58],the positive scan of the voltammograms depict the phase transition of LiFePO 4to FePO 4,while the negative scan depicts the phase transi-tion FePO 4to LiFePO 4.As shown by Srinivasan et al.[5],LiFePO 4may be exhausted by Li not more than 5mol.%before to trans-form into FePO 4,while FePO 4may consume no more than 5%Li before to transform into LiFePO 4,i.e.cyclic voltammetry exper-iments proceeds under condition of almost constant activity of the electroactive species.Although these aspects of the Li inser-tion/deinsertion process do not fit the processes at metal/liquid electrolyte boundary implied by Randles–Sevcik equation:i p =0.4463F RT1/2C v 1/2AD 1/2(2)this equation was frequently used to estimate apparent diffusion coefficient in Li insertion processes [5,17,21,46,59].To obtain peak current,i p ,in amperes,the concentration of lithium,C =C Li ,should be in mol cm −3,the real surface area exposed to the electrolyte in cm 2,chemical diffusion coefficient of lithium through the solid phase,D =D Li ,in cm 2s −1,and sweep rate,v ,in V s −1.The Eq.(2)pre-dicts the dependence of the peak height on the square root of sweep rate to be linear,as found often in Li-ion intercalation processes [17,21,25,59,60].This condition is fulfilled in this case too,as shown in Fig.7.The average value of C Li may be estimated as a reciprocal value of molar volume of LiFePO 4(V M =44.11cm 3mol −1),hence C Li =2.27×10−2mol cm −3.The determination of the actual surface area of olivine is a more difficult task,due to the presence of carbon in the LiFePO 4/C ly,classical BET method of sur-face area measurement may lead to a significantly overestimated value,since carbon surface may be very developed and participate predominantly in the measured value [15].Thus the authors in this field usually calculated specific surface area by means of Eq.(1),using mean particle radius determined by means of electron microscopy [17,22,45,46].Using S =22.2m 2g −1determined by means of Eq.(1),and an actual mass of the electroactive substance applied to the elec-trode surface (0.001305g),the actual electrode surface area was calculated to amount to A =290cm 2.This value introduced in Randles–Sevcik equation yielded D Li ∼0.8×10−14cm 2s −1.From the aspect of capacity retention,the insolubility of olivine in aqueous solutions is advantageous compared to the vanadia-based Li-ion intercalation materials,such as Li 1.2V 3O 8[61],LiV 3O 8[62]and V 2O 5[63],the solubility of which in LiNO 3solution was perceivable through the yellowish solutioncoloration.Fig.8.The Nyquist plots of LiFePO 4/C composite in aqueous LiNO 3solution at var-ious stages of delithiation;inset:enlarged high-frequency region.3.2.2.2.Impedance measurements.Figs.8and 9present the Nyquist plots of the LiFePO 4/C composite in aqueous LiNO 3solution at various open circuit potentials (OCV),during delithiation (anodic sweep,Fig.8)and during lithiation (cathodic sweep,Fig.9).The delithiated phase,observed at OCV =1V,as well as the lithi-ated phase,observed at OCV =0V,in the low-frequency region (f <100Hz)tend to behave like a capacitor,characteristic of a surface thin-layered redox material with reflective phase bound-ary conditions [64].At the OCV not too far from the reversible one (0.42V during delithiation,0.308V during lithiation),where both LiFePO 4and FePO 4phase may be present,within the whole 10−2–105Hz frequency range,the reaction behaves as a reversible one (i.e.shows the impedance of almost purely Warburg type).The insets in Figs.8and 9present the enlarged parts of the impedance diagram in the region of high frequencies,where one may observe a semicircle,the diameter of which corresponds theoretically to the charge transfer resistance.As visible,the change of open circuit potential between 0and 1V,in spite of the phase transition,does not cause significant change in charge transfer resistance.The small charge transfer resistance obtained with the carbon participation of 13.4%,being less than 1 ,is the smallest one reported thus far for olivine based materials.This finding agrees with the trend found by Zhao et al.[27],that the charge transfer resistance scaleddownFig.9.The Nyquist plots of LiFePO 4/C composite in aqueous LiNO 3solution at var-ious stages of lithiation;inset:enlarged high-frequency region.M.Vujkovi´c et al./Electrochimica Acta 92 (2013) 248–256253Fig.10.The dependence Z Re vs.ω−1/2during lithiation at 0.308V (top)and delithi-ation at 0.42V (down)in the frequency range 72–2.68Hz.to 1000,400and 150 when the amount of in situ formed carbon in the LiFePO 4/C composite increased in the range 1,2.8and 4.8%.For OCV corresponding to the cathodic (0.42V)and anodic (0.308V)peak maxima,the Warburg constant W was calculated from the dependence [21]:Z Re =R e +R ct + W ω−1/2(3)In the frequency range 2.7–72Hz,almost purely Warburg impedance was found to hold (i.e.the slope of the Nyquist plot very close to 45degrees was found).At the potential of cathodic current maximum (0.42V),from Fig.10, W was determined to amount to 7.96 s −1/2.At the potential of anodic maxima,0.308V, W was determined to amount to 9.07 s −1/2.In the published literature,for the determination of diffusion coefficient on the basis of impedance measurements,the following equation was often used [66,68,69]:D =0.5V M AF W ıE ıx2(4)where V M is molar volume of olivine,44.1cm 3, W is Warburg con-stant and ıE /ıx is the slope of the dependence of electrode potential on the molar fraction of Li (x )for given value of x .However,the potentials of CV maxima in the here studied case correspond to the x range of two-phase equilibrium,where for an accurate deter-mination of ıE /ıx a strong control of perturbed region of sample particles is required [69],and thus the determination of diffusion coefficients was omitted.3.2.2.3.Galvanostatic measurements.The galvanostatic measure-ments of LiFePO 4/C in saturated LiNO 3aqueous solution were performed in a two-electrode arrangement using hydrother-mally synthesized VO 2[55]as the active material of thecounterFig.11.Capacity versus cycle number and charge/discharge profiles (inset)for thecell consisting of LiFePO 4/C composite as cathode,and VO 2in large excess as anode,in saturated LiNO 3aqueous electrolyte observed at rate C/3.electrode.Preliminary cyclovoltammetric tests of VO 2in saturated LiNO 3solution at the sweep rate 10mV s −1,evidenced excellent cyclability and stable capacity of about 160mAh g −1during at least 50cycles.The voltage applied to the two-electrode cell was cycled within the limits 0and 1.4V.Due to a significant stoichiometric excess of VO 2over LiFePO 4/C composite (5:1)the actual voltage may be considered to be the potential versus reference VO 2/Li x VO 2electrode.Fig.11shows the dependence of the discharging Coulombic capacity of the LiFePO 4/C composite on the number of galvano-static cycles at discharging rate C/3,as well as (in the inset)the voltage vs.charging/discharging degree for 1st,2nd and 50th cycle.The charge/discharge curves do not change substantially in shape upon cycling,indicating stable capacity.For an aqueous solution,a surprisingly high initial discharge capacity of 106mAh g −1and low capacity fade of only 6%after 50charge/discharge cycles were evidenced.This behavior is admirable in comparison to other elec-trode materials in aqueous media reported in literature (LiTi 2(PO 4)3[42],LiV 3O 8[57]),and probably enabled by a higher thermody-namic stability of olivine structure [1].Fig.12presents the results of cyclic galvanostatic investigations of LiFePO 4/C composite in aqueous LiNO 3solution at various dis-charging rates.The charging/discharging rate was initially C/3for 80cycles and then was increased stepwise up to 3C.ThecapacityFig.12.Cyclic performance of LiFePO 4/C in saturated LiNO 3aqueous electrolyte at different charging/discharging rates.。
(完整版)S C I写作句型汇总-CAL-FENGHAI-(2020YEAR-YICAI)_JINGBIANSCI论文写作中一些常用的句型总结(一)很多文献已经讨论过了一、在Introduction里面经常会使用到的一个句子：很多文献已经讨论过了。
它的可能的说法有很多很多，这里列举几种我很久以前搜集的：A.??Solar energy conversion by photoelectrochemical cells?has been intensively investigated.?(Nature 1991, 353, 737 - 740?)B.?This was demonstrated in a number of studies that?showed that composite plasmonic-metal/semiconductor photocatalysts achieved significantly higher rates in various photocatalytic reactions compared with their pure semiconductor counterparts.C.?Several excellent reviews describing?these applications are available, and we do not discuss these topicsD.?Much work so far has focused on?wide band gap semiconductors for water splitting for the sake of chemical stability.（DOI:10.1038/NMAT3151）E.?Recent developments of?Lewis acids and water-soluble organometallic catalysts?have attracted much attention.（Chem. Rev. 2002, 102, 3641?3666）F.?An interesting approach?in the use of zeolite as a water-tolerant solid acid?was described by?Ogawa et al（Chem.Rev. 2002, 102, 3641?3666）G.?Considerable research efforts have been devoted to?the direct transition metal-catalyzed conversion of aryl halides toaryl nitriles. (J. Org. Chem. 2000, 65, 7984-7989)H.?There are many excellent reviews in the literature dealing with the basic concepts of?the photocatalytic processand the reader is referred in particular to those by Hoffmann and coworkers,Mills and coworkers, and Kamat.(Metal oxide catalysis,19,P755)I. Nishimiya and Tsutsumi?have reported on（proposed）the influence of the Si/Al ratio of various zeolites on the acid strength, which were estimated by calorimetry using ammonia. (Chem.Rev. 2002, 102, 3641?3666)二、在results and discussion中经常会用到的：如图所示A. GIXRD patterns in?Figure 1A show?the bulk structural information on as-deposited films.?B.?As shown in Figure 7B,?the steady-state current density decreases after cycling between 0.35 and 0.7 V, which is probably due to the dissolution of FeOx.?C.?As can be seen from?parts a and b of Figure 7, the reaction cycles start with the thermodynamically most favorable VOx structures（J. Phys. Chem. C 2014, 118, 24950?24958）这与XX能够相互印证：A.?This is supported by?the appearance in the Ni-doped compounds of an ultraviolet–visible absorption band at 420–520nm (see Fig. 3 inset), corresponding to an energy range of about 2.9 to2.3 eV.B. ?This?is consistent with the observation from?SEM–EDS. (Z.Zou et al. / Chemical Physics Letters 332 (2000) 271–277)C.?This indicates a good agreement between?the observed and calculated intensities in monoclinic with space groupP2/c when the O atoms are included in the model.D. The results?are in good consistent with?the observed photocatalytic activity...E. Identical conclusions were obtained in studies?where the SPR intensity and wavelength were modulated by manipulating the composition, shape,or size of plasmonic nanostructures.?F.??It was also found that areas of persistent divergent surface flow?coincide?with?regions where convection appears to be consistently suppressed even when SSTs are above 27.5°C.(二)1. 值得注意的是...A.?It must also be mentioned that?the recycling of aqueous organic solvent is less desirable than that of pure organic liquid.B.?Another interesting finding is that?zeolites with 10-membered ring pores showed high selectivities (>99%) to cyclohexanol, whereas those with 12-membered ring pores, such as mordenite, produced large amounts of dicyclohexyl ether. (Chem. Rev. 2002, 102, 3641?3666)C.?It should be pointed out that?the nanometer-scale distribution of electrocatalyst centers on the electrode surface is also a predominant factor for high ORR electrocatalytic activity.D.?Notably,?the Ru II and Rh I complexes possessing the same BINAP chirality form antipodal amino acids as the predominant products.?(Angew. Chem. Int. Ed., 2002, 41: 2008–2022)E. Given the multitude of various transformations published,?it is noteworthy that?only very few distinct?activation?methods have been identified.?(Chem. Soc. Rev., 2009,?38, 2178-2189)F.?It is important to highlight that?these two directing effects will lead to different enantiomers of the products even if both the “H-bond-catalyst” and the?catalyst?acting by steric shielding have the same absolute stereochemistry. (Chem. Soc. Rev.,?2009,?38, 2178-2189)G.?It is worthwhile mentioning that?these PPNDs can be very stable for several months without the observations of any floating or precipitated dots, which is attributed to the electrostatic repulsions between the positively charge PPNDs resulting in electrosteric stabilization.(Adv. Mater., 2012, 24: 2037–2041)2.?...仍然是个挑战A.?There is thereby an urgent need but it is still a significant challenge to?rationally design and delicately tail or the electroactive MTMOs for advanced LIBs, ECs, MOBs, and FCs.?（Angew. Chem. Int. Ed.2 014, 53, 1488 – 1504）B.?However, systems that are?sufficiently stable and efficient for practical use?have not yet been realized.C.??It?remains?challenging?to?develop highly active HER catalysts based on materials that are more abundant at lower costs. (J. Am. Chem.Soc.,?2011,?133, ?7296–7299)D.?One of the?great?challenges?in the twenty-first century?is?unquestionably energy storage. (Nature Materials?2005, 4, 366 - 377?)众所周知A.?It is well established (accepted) / It is known to all / It is commonlyknown?that?many characteristics of functional materials, such as composition, crystalline phase, structural and morphological features, and the sur-/interface properties between the electrode and electrolyte, would greatly influence the performance of these unique MTMOs in electrochemical energy storage/conversion applications.（Angew. Chem. Int. Ed.2014,53, 1488 – 1504）B.?It is generally accepted (believed) that?for a-Fe2O3-based sensors the change in resistance is mainly caused by the adsorption and desorption of gases on the surface of the sensor structure. (Adv. Mater. 2005, 17, 582)C.?As we all know,?soybean abounds with carbon,?nitrogen?and oxygen elements owing to the existence of sugar,?proteins?and?lipids. (Chem. Commun., 2012,?48, 9367-9369)D.?There is no denying that?their presence may mediate spin moments to align parallel without acting alone to show d0-FM. (Nanoscale, 2013,?5, 3918-3930)(三)1. 正如下文将提到的...A.?As will be described below（也可以是As we shall see below）,?as the Si/Al ratio increases, the surface of the zeolite becomes more hydrophobic and possesses stronger affinity for ethyl acetate and the number of acid sites decreases.（Chem. Rev. 2002, 102, 3641?3666）B. This behavior is to be expected and?will?be?further?discussed?below. (J. Am. Chem. Soc.,?1955,?77, 3701–3707)C.?There are also some small deviations with respect to the flow direction,?whichwe?will?discuss?below.（Science, 2001, 291, 630-633）D.?Below,?we?will?see?what this implies.E.?Complete details of this case?will?be provided at a?later?time.E.?很多论文中，也经常直接用see below来表示，比如：The observation of nanocluster spheres at the ends of the nanowires is suggestive of a VLS growth process (see?below). (Science, 1998, ?279, 208-211)2. 这与XX能够相互印证...A.?This is supported by?the appearance in the Ni-doped compounds of an ultraviolet–visible absorption band at 420–520 nm (see Fig. 3 inset), corresponding to an energy range of about 2.9 to 2.3 eVB.This is consistent with the observation from?SEM–EDS. (Chem. Phys. Lett. 2000, 332, 271–277)C.?Identical conclusions were obtained?in studies where the SPR intensity and wavelength were modulated by manipulating the composition, shape, or size of plasmonic nanostructures.?（Nat. Mater. 2011, DOI: 10.1038/NMAT3151）D. In addition, the shape of the titration curve versus the PPi/1 ratio,?coinciding withthat?obtained by fluorescent titration studies, suggested that both 2:1 and 1:1 host-to-guest complexes are formed. (J. Am. Chem. Soc. 1999, 121, 9463-9464)E.?This unusual luminescence behavior is?in accord with?a recent theoretical prediction; MoS2, an indirect bandgap material in its bulk form, becomes a direct bandgap semiconductor when thinned to a monolayer.?(Nano Lett.,?2010,?10, 1271–1275)3.?我们的研究可能在哪些方面得到应用A.?Our ?ndings suggest that?the use of solar energy for photocatalytic watersplitting?might provide a viable source for?‘clean’ hydrogen fuel, once the catalyticef?ciency of the semiconductor system has been improved by increasing its surface area and suitable modi?cations of the surface sites.B. Along with this green and cost-effective protocol of synthesis,?we expect that?these novel carbon nanodots?have potential applications in?bioimaging andelectrocatalysis.(Chem. Commun., 2012,?48, 9367-9369)C.?This system could potentially be applied as?the gain medium of solid-state organic-based lasers or as a component of high value photovoltaic (PV) materials, where destructive high energy UV radiation would be converted to useful low energy NIR radiation. (Chem. Soc. Rev., 2013,?42, 29-43)D.?Since the use of?graphene?may enhance the photocatalytic properties of TiO2?under UV and visible-light irradiation,?graphene–TiO2?composites?may potentially be usedto?enhance the bactericidal activity.?(Chem. Soc. Rev., 2012,?41, 782-796)E.??It is the first report that CQDs are both amino-functionalized and highly fluorescent,?which suggests their promising applications in?chemical sensing.(Carbon, 2012,?50,?2810–2815)(四)1. 什么东西还尚未发现/系统研究A. However,systems that are sufficiently stable and efficient for practical use?have not yet been realized.B. Nevertheless,for conventional nanostructured MTMOs as mentioned above,?some problematic disadvantages cannot be overlooked.（Angew. Chem. Int. Ed.2014,53, 1488 – 1504）C.?There are relatively few studies devoted to?determination of cmc values for block copolymer micelles. (Macromolecules 1991, 24, 1033-1040)D. This might be the reason why, despite of the great influence of the preparation on the catalytic activity of gold catalysts,?no systematic study concerning?the synthesis conditions?has been published yet.?(Applied Catalysis A: General2002, 226, ?1–13)E.?These possibilities remain to be?explored.F.??Further effort is required to?understand and better control the parameters dominating the particle surface passivation and resulting properties for carbon dots of brighter photoluminescence. (J. Am. Chem. Soc.,?2006,?128?, 7756–7757)2.?由于/因为...A.?Liquid ammonia?is particularly attractive as?an alternative to water?due to?its stability in the presence of strong reducing agents such as alkali metals that are used to access lower oxidation states.B.?The unique nature of?the cyanide ligand?results from?its ability to act both as a σdonor and a π acceptor combined with its negativecharge and ambidentate nature.C.?Qdots are also excellent probes for two-photon confocalmicroscopy?because?they are characterized by a very large absorption cross section?(Science ?2005,?307, 538-544).D.?As a result of?the reductive strategy we used and of the strong bonding between the surface and the aryl groups, low residual currents (similar to those observed at a bareelectrode) were obtained over a large window of potentials, the same as for the unmodified parent GC electrode. (J. Am. Chem. Soc. 1992, 114, 5883-5884)E.?The small Tafel slope of the defect-rich MoS2 ultrathin nanosheets is advantageous for practical?applications,?since?it will lead to a faster increment of HER rate with increasing overpotential.(Adv. Mater., 2013, 25: 5807–5813)F. Fluorescent carbon-based materials have drawn increasing attention in recent years?owing to?exceptional advantages such as high optical absorptivity, chemical stability, biocompatibility, and low toxicity.(Angew. Chem. Int. Ed., 2013, 52: 3953–3957)G.??On the basis of?measurements of the heat of immersion of water on zeolites, Tsutsumi etal. claimed that the surface consists of siloxane bondings and is hydrophobicin the region of low Al content. (Chem. Rev. 2002, 102, 3641?3666)H.?Nanoparticle spatial distributions might have a large significance for catalyst stability,?given that?metal particle growth is a relevant deactivation mechanism for commercial catalysts.?3. ...很重要A.?The inhibition of additional nucleation during growth, in other words, the complete separation?of nucleation and growth,?is?critical(essential, important)?for?the successful synthesis of monodisperse nanocrystals. (Nature Materials?3, 891 - 895 (2004))B.??In the current study,?Cys,?homocysteine?(Hcy) and?glutathione?(GSH) were chosen as model?thiol?compounds since they?play important (significant, vital, critical) roles?in many biological processes and monitoring of these?thiol?compounds?is of great importance for?diagnosis of diseases.(Chem. Commun., 2012,?48, 1147-1149)C.?This is because according to nucleation theory,?what really matters?in addition to the change in temperature ΔT?(or supersaturation) is the cooling rate.(Chem. Soc. Rev., 2014,?43, 2013-2026)(五)1. 相反/不同于A. On the contrary, mononuclear complexes, called single-ion magnets (SIM), have shown hysteresis loops of butterfly/phonon bottleneck type, with negligible coercivity, and therefore with much shorter relaxation times of magnetization. (Angew. Chem. Int. Ed., 2014, 53: 4413–4417)B. In contrast, the Dy compound has significantly larger value of the transversal magnetic moment already in the ground state (ca. 10−1μB), therefore allowing a fast QTM. （Angew. Chem. Int. Ed., 2014, 53: 4413–4417）C. In contrast to the structural similarity of these complexes, their magnetic behavior exhibits strong divergence. (Angew. Chem. Int. Ed., 2014, 53: 4413–4417)D. Contrary to other conducting polymer semiconductors, carbon nitride ischemically and thermally stable and does not rely on complicated device manufacturing. (Nature materials, 2009, 8(1): 76-80.)E. Unlike the spherical particles they are derived from that Rayleigh light-scatter in the blue, these nanoprisms exhibit scattering in the red, which could be useful in developing multicolor diagnostic labels on the basis not only of nanoparticle composition and size but also of shape. (Science 2001, 294, 1901-1903)2. 发现，阐明，报道，证实可供选择的词包括：verify, confirm, elucidate, identify, define, characterize, clarify, establish, ascertain, explain, observe, illuminate, illustrate，demonstrate, show, indicate, exhibit, presented, reveal, display, manifest，suggest, propose, estimate, prove, imply, disclose，report, describe，facilitate the identification of举例：A. These stacks appear as nanorods in the two-dimensional TEM images, but tilting experiments confirm that they are nanoprisms. (Science 2001, 294, 1901-1903)B. Note that TEM shows that about 20% of the nanoprisms are truncated. (Science2001, 294, 1901-1903)C. Therefore, these calculations not only allow us to identify the important features in the spectrum of the nanoprisms but also the subtle relation between particle shape and the frequency of the bands that make up their spectra. (Science 2001, 294, 1901-1903)D. We observed a decrease in intensity of the characteristic surface plasmon band in the ultraviolet-visible (UV-Vis) spectr oscopy for the spherical particles at λmax = 400 nm with a concomitant growth of three new bands of λmax = 335 (weak), 470 (medium), and 670 nm (strong), respectively. (Science 2001, 294, 1901-1903)E. In this article, we present data demonstrating that opiate and nonopiate analgesia systems can be selectively activated by different environmental manipulationsand describe the neural circuitry involved. (Science 1982, 216, 1185-1192)F. This suggests that the cobalt in CoP has a partial positive charge (δ+), while the phosphorus has a partial negative charge (δ−), implying a transfer of electron density from Co to P. (Angew. Chem., 2014, 126: 6828–6832)3. 如何指出当前研究的不足A. Although these inorganic substructures can exhibit a high density of functional groups, such as bridging OH groups, and the substructures contribute significantly to the adsorption properties of the material,surprisingly little attention has been devoted to the post-synthetic functionalization of the inorganic units within MOFs. (Chem. Eur. J., 2013, 19: 5533–5536.)B. Little is known, however, about the microstructure of this material. (Nature Materials 2013,12, 554–561)C. So far, very little information is available, and only in the absorber film, not in the whole operational devices. （Nano Lett., 2014, 14 (2), pp 888–893）D. In fact it should be noted that very little optimisation work has been carried out on these devices. (Chem. Commun., 2013, 49, 7893-7895)E. By far the most architectures have been prepared using a solution processed perovskite material, yet a few examples have been reported that have used an evaporated perovskite layer. (Adv. Mater., 2014, 27: 1837–1841.)F. Water balance issues have been effectively addressed in PEMFC technology through a large body of work encompassing imaging, detailed water content and water balance measurements, materials optimization and modeling, but very few of these activities have been undertaken for anion exchange membrane fuel cells, primarily due to limited materials availability and device lifetime. (J. Polym. Sci. Part B: Polym. Phys., 2013, 51: 1727–1735)G. However, none of these studies tested for Th17 memory, a recently identified T cell that specializes in controlling extracellular bacterial infections at mucosal surfaces. (PNAS, 2013, 111, 787–792)H. However, uncertainty still remains as to the mechanism by which Li salt addition results in an extension of the cathodic reduction limit. (Energy Environ. Sci., 2014, 7, 232-250)I. There have been a number of high profile cases where failure to identify the most stable crystal form of a drug has led to severe formulation problems in manufacture. (Chem. Soc. Rev., 2014, 43, 2080-2088)J. However, these measurements systematically underestimate the amount of ordered material. ( Nature Materials 2013, 12, 1038–1044)(六)1. 取决于a. This is an important distinction, as the overall activity of a catalyst will depend on the material properties, synthesis method, and other possible species that can be formed during activation. (Nat. Mater. 2017,16,225–229)b. This quantitative partitioning was determined by growing crystals of the 1:1 host–guest complex between ExBox4+ and corannulene. (Nat. Chem. 2014, 6177–178)c. They suggested that the Au particle size may be the decisive factor for achieving highly active Au catalysts.(Acc. Chem. Res., 2014, 47, 740–749)d. Low-valent late transition-metal catalysis has become indispensable to chemical synthesis, but homogeneous high-valent transition-metal catalysis is underdeveloped, mainly owing to the reactivity of high-valent transition-metal complexes and the challenges associated with synthesizing them. (Nature2015, 517,449–454)e. The polar effect is a remarkable property that enables considerably endergonic C–H abstractions that would not be possible otherwise. (Nature 2015, 525, 87–90)f. Advances in heterogeneous catalysis must rely on the rational design of new catalysts. (Nat. Nanotechnol. 2017, 12, 100–101)g. Likely, the origin of the chemoselectivity may be also closely related to the H bonding with the N or O atom of the nitroso moiety, a similar H-bonding effect is known in enamine-based nitroso chemistry. (Angew. Chem. Int. Ed. 2014, 53: 4149–4153)2. 有很大潜力a. The quest for new methodologies to assemble complex organic molecules continues to be a great impetus to research efforts to discover or to optimize new catalytic transformations. (Nat. Chem. 2015, 7, 477–482)b. Nanosized faujasite (FAU) crystals have great potential as catalysts or adsorbents to more efficiently process present and forthcoming synthetic and renewable feedstocks in oil refining, petrochemistry and fine chemistry. (Nat. Mater. 2015, 14, 447–451)c. For this purpose, vibrational spectroscopy has proved promising and very useful. (Acc Chem Res. 2015, 48, 407–413.)d. While a detailed mechanism remains to be elucidated and there is room for improvement in the yields and selectivities, it should be remarked that chirality transferupon trifluoromethylation of enantioenriched allylsilanes was shown. (Top Catal. 2014, 57: 967. )e. The future looks bright for the use of PGMs as catalysts, both on laboratory and industrial scales, because the preparation of most kinds of single-atom metal catalyst is likely to be straightforward, and because characterization of such catalysts has become easier with the advent of techniques that readily discriminate single atoms from small clusters and nanoparticles. (Nature 2015, 525, 325–326)f. The unique mesostructure of the 3D-dendritic MSNSs with mesopore channels of short length and large diameter is supposed to be the key role in immobilization of active and robust heterogeneous catalysts, and it would have more hopeful prospects in catalytic applications. (ACS Appl. Mater. Interfaces, 2015, 7, 17450–17459)g. Visible-light photoredox catalysis offers exciting opportunities to achieve challenging carbon–carbon bond formations under mild and ecologically benign conditions. (Acc. Chem. Res., 2016, 49, 1990–1996)3. 因此同义词：Therefore, thus, consequently, hence, accordingly, so, as a result这一条比较简单，这里主要讲一下这些词的副词词性和灵活运用。
An Infinite Two-Dimensional Hybrid Water-Chloride Network,Self-Assembled in a Hydrophobic Terpyridine Iron(II)MatrixRicardo R.Fernandes,†Alexander M.Kirillov,†M.Fátima C.Guedes da Silva,†,‡Zhen Ma,†JoséA.L.da Silva,†João J.R.Fraústo da Silva,†andArmando J.L.Pombeiro*,†Centro de Química Estrutural,Complexo I,Instituto Superior Técnico,TU-Lisbon,A V.Ro V isco Pais,1049-001Lisbon,Portugal,and Uni V ersidade Luso´fona de Humanidades e Tecnologias,A V.doCampo Grande,376,1749-024,Lisbon,PortugalRecei V ed October18,2007;Re V ised Manuscript Recei V ed January7,2008ABSTRACT:An unprecedented two-dimensional water-chloride anionic{[(H2O)20(Cl)4]4–}n network has been structurally identified in a hydrophobic matrix of the iron(II)compound[FeL2]Cl2·10H2O(L)4′-phenyl-2,2′:6′,2″-terpyridine).Its intricate relief geometry has been described as a set of10nonequivalent alternating cycles of different sizes ranging from tetra-to octanuclear{[(H2O)x(Cl)y]y–}z(x) 2–6,y)0–2,z)4–6,8)fragments.In contrast to the blooming research on structural characterizationof a wide variety of water clusters in different crystalline materials,1much less attention has been focused on the identification anddescription of hybrid hydrogen-bonded water assemblies with othersolvents,small molecules,or counterions.1c,2In particular,thecombination of chloride ions and water is one of the most commonlyfound in natural environments(e.g.,seawater or sea-salt aerosols),and thus the investigation of water-chloride interactions has beenthe object of numerous theoretical studies.3However,only recentlya few water-chloride associates incorporated in various crystalmatrixes have been identified and structurally characterized,4,5including examples of(i)discrete cyclic[(H2O)4(Cl)]–,4a[(H2O)4(Cl)2]2–,4b and[(H2O)6(Cl)2]2–4c clusters,and(ii)variousone-or two-dimensional(1D or2D)hydrogen-bonded networksgenerated from crystallization water and chloride counterionswith{[(H2O)4(Cl2)]2–}n,5b{[(H2O)6(Cl)2]2–}n,5b[(H2O)7(HCl)2]n,5c{[(H2O)11(Cl)7]7–}n,5d{[(H2O)14(Cl)2]2–}n,5e{[(H2O)14(Cl)4]4–}n,5aand{[(H2O)14(Cl)5]5–}n5f compositions.These studies are alsobelieved to provide a contribution toward the understanding of thehydration phenomena of chloride ions in nature and have importancein biochemistry,catalysis,supramolecular chemistry,and designof crystalline materials.5In pursuit of our interest in the self-assembly synthesis andcrystallization of various transition metal compounds in aqueousmedia,we have recently described the[(H2O)10]n,6a(H2O)6,6b and[(H2O)4(Cl)2]2–4b clusters hosted by Cu/Na or Ni metal-organicmatrixes.Continuing this research,we report herein the isolationand structural characterization of a unique2D water-chlorideanionic layer{[(H2O)20(Cl)4]4–}n within the crystal structure of thebis-terpyridine iron(II)compound[FeL2]Cl2·10H2O(1′)(L)4′-phenyl-2,2′:6′,2″-terpyridine).Although this compound has beenobtained unexpectedly,a search in the Cambridge StructuralDatabase(CSD)7,8points out that various terpyridine containinghosts tend to stabilize water-chloride associates,thus also sup-porting the recognized ability of terpyridine ligands in supra-molecular chemistry and crystal engineering.9,10Hence,the simple combination of FeCl2·2H2O and L in tetrahydrofuran(THF)solution at room temperature provides the formation of a deep purple solid formulated as[FeL2]Cl2·FeCl2·5H2O(1)on the basis of elemental analysis,FAB+-MS and IR spectroscopy.11This compound reveals a high affinity for water and,upon recrystallization from a MeOH/H2O(v/v)9/1)mixture,leads to single crystals of1′with a higher water content,which have been characterized by single-crystal X-ray analysis.12The asymmetric unit of1′is composed of a cationic[FeL2]2+ part,two chloride anions,and10independent crystallization water molecules(with all their H atoms located in the difference Fourier map),the latter occupying a considerable portion of the crystal cell. The iron atom possesses a significantly distorted octahedral coordination environmentfilled by two tridentate terpyridine moieties arranged in a nearly perpendicular fashion(Figure S1, Supporting Information).Most of the bonding parameters within [FeL2]2+are comparable to those reported for other iron compounds*To whom correspondence should be sent.Fax:+351-21-8464455.E-mail: pombeiro@ist.utl.pt.†Instituto Superior Técnico.‡Universidade Luso´fona de Humanidades eTecnologias.Figure 1.Perspective representations(arbitrary views)of hybrid water-chloride hydrogen-bonded assemblies in the crystal cell of1′; H2O molecules and chloride ions are shown as colored sticks and balls, respectively.(a)Minimal repeating{[(H2O)20(Cl)4]4–}n fragment with atom numbering scheme.(b)Nonplanar infinite polycyclic2D anionic layer generated by linkage of four{[(H2O)20(Cl)4]4–}n fragments(a) represented by different colors;the numbers are those of Table1and define the10nonequivalent alternating cycles of different size.2008310.1021/cg7010315CCC:$40.75 2008American Chemical SocietyPublished on Web02/08/2008bearing two terpyridine ligands.13The most interesting feature of the crystal structure of 1′consists in the extensive hydrogen bonding interactions of all the lattice–water molecules and chloride coun-terions (Table S1,Supporting Information),leading to the formation of a hybrid water -chloride polymeric assembly possessing minimal repeating {[(H 2O)20(Cl)4]4–}n fragments (Figure 1a).These are further interlinked by hydrogen bonds generating a nonplanar 2D water -chloride anionic layer (Figure 1b).Hence,the multicyclic {[(H 2O)20(Cl)4]4–}n fragment is con-structed by means of 12nonequivalent O–H ···O interactions with O ···O distances ranging from 2.727to 2.914Åand eight O–H ···Cl hydrogen bonds with O ···Cl separations varying in the 3.178–3.234Årange (Table S1,Supporting Information).Both average O ···O [∼2.82Å]and O ···Cl [∼3.20Å]separations are comparable to those found in liquid water (i.e.,2.85Å)14and various types of H 2O clusters 1,6or hybrid H 2O -Cl associates.4,5Eight of ten water molecules participate in the formation of three hydrogen bonds each (donating two and accepting one hydrogen),while the O3and O7H 2O molecules along with both Cl1and Cl2ions are involved in four hydrogen-bonding contacts.The resulting 2D network can be considered as a set of alternating cyclic fragments (Figure 1b)which are classified in Table 1and additionally shown by different colors in Figure 2.Altogether there are 10different cycles,that is,five tetranuclear,three pentanuclear,one hexanuclear,and one octa-nuclear fragment (Figures 1b and 2,Table 1).Three of them (cycles 1,2,and 6)are composed of only water molecules,whereas the other seven rings are water -chloride hybrids with one or two Cl atoms.The most lengthy O ···O,O ···Cl,or Cl ···Cl nonbonding separations within rings vary from 4.28to 7.91Å(Table 1,cycles 1and 10,respectively).Most of the cycles are nonplanar (except those derived from the three symmetry generated tetrameric fragments,cycles 1,2,and 4),thus contributing to the formation of an intricate relief geometry of the water -chloride layer,possessing average O ···O ···O,O ···Cl ···O,and O ···O ···Cl angles of ca.104.9,105.9,and 114.6°,respectively (Table S2,Supporting Information).The unprecedented character of thewater -chloride assembly in 1′has been confirmed by a thorough search in the CSD,7,15since the manual analysis of 156potentially significant entries with the minimal [(H 2O)3(Cl)]–core obtained within the searching algorithm 15did not match a similar topology.Nevertheless,we were able to find several other interesting examples 16of infinite 2D and three-dimensional (3D)water -chloride networks,most of them exhibiting strong interactions with metal -organic matrixes.The crystal packing diagram of 1′along the a axis (Figure 3)shows that 2D water -chloride anionic layers occupy the free space between hydrophobic arrays of metal -organic units,with an interlayer separation of 12.2125(13)Åthat is equivalent to the b unit cell dimension.12In contrast to most of the previously identified water clusters,1,6water -chloride networks,5,16and extended assemblies,1c the incorporation of {[(H 2O)20(Cl)4]4–}n sheets in 1′is not supported by strong intermolecular interactions with the terpyridine iron matrix.Nevertheless,four weak C–H ···O hydrogen bonds [avg d (D ···A))3.39Å]between some terpyridine CH atoms and lattice–water molecules (Table S1,Figure S2,Supporting Information)lead to the formation of a 3D supramolecular framework.The thermal gravimetric analysis (combined TG-DSC)of 117(Figure S3,Supporting Information)shows the stepwise elimination of lattice–water in the broad 50–305°C temperature interval,in accord with the detection on the differential scanning calorimetryTable 1.Description of Cyclic Fragments within the {[(H 2O)20(Cl)4]4–}n Network in 1′entry/cycle numbernumber of O/Cl atomsformula atom numberingschemegeometry most lengthy separation,Åcolor code a 14(H 2O)4O3–O4–O3–O4planar O3···O3,4.28light brown 24(H 2O)4O6–O7–O6–O7planar O7···O7,4.42light gray 34[(H 2O)3(Cl)]-O2–O4–O3–Cl2nonplanar O4···Cl2,4.66blue 44[(H 2O)3(Cl)]-O6–O7–O9–Cl1nonplanar O7···Cl1,4.61green 54[(H 2O)2(Cl)2]2-O9–Cl1–O9–Cl1planar Cl ···Cl1,4.76pink 65(H 2O)5O2–O4–O3–O10–O8nonplanar O2···O10,4.55red75[(H 2O)4(Cl)]-O1–O5–O7–O9–Cl1nonplanar O7···Cl1,5.25pale yellow 85[(H 2O)4(Cl)]-O1–O5–Cl2–O8–O10nonplanar O10···Cl2,5.29orange 96[(H 2O)4(Cl)2]2-O2–O8–Cl2–O2–O8–Cl2nonplanar Cl2···Cl2,7.12yellow 108[(H 2O)6(Cl)2]2-O1–O10–O3–Cl2–O5–O7–O6–Cl1nonplanarCl1···Cl2,7.91pale blueaColor codes are those of Figure 2.Figure 2.Fragment of nonplanar infinite polycyclic 2D anionic layer in the crystal cell of 1′.The 10nonequivalent alternating water or water -chloride cycles are shown by different colors (see Table 1for color codes).Figure 3.Fragment of the crystal packing diagram of 1′along the a axis showing the intercalation of two water -chloride layers (represented by space filling model)into the metal -organic matrix (depicted as sticks);color codes within H 2O -Cl layers:O red,Cl green,H grey.Communications Crystal Growth &Design,Vol.8,No.3,2008783curve(DSC)of three major endothermic processes in ca.50–170, 170–200,and200–305°C ranges with maxima at ca.165,190, and280°C,corresponding to the stepwise loss of ca.two,one, and two H2O molecules,respectively(the overall mass loss of9.1% is in accord with the calculated value of9.4%for the elimination of allfive water molecules).In accord,the initial broad and intense IRν(H2O)andδ(H2O)bands of1(maxima at3462and1656cm–1, respectively)gradually decrease in intensity on heating the sample up to ca.305°C,while the other bands remain almost unchangeable. Further heating above305°C leads to the sequential decomposition of the bis-terpyridine iron unit.These observations have also been supported by the IR spectra of the products remaining after heating the sample at different temperatures.The elimination of the last portions of water in1at temperatures as high as250–305°C is not commonly observed(although it is not unprecedented18)for crystalline materials with hosted water clusters,and can be related to the presence and extensive hydrogen-bonding of chloride ions in the crystal cell,tending to form the O–H(water)···Cl hydrogen bonds ca.2.5times stronger in energy than the corresponding O–H(water)···O(water)ones.5a The strong binding of crystallization water in1is also confirmed by its FAB+-MS analysis that reveals the rather uncommon formation of the fragments bearing from one tofive H2O molecules.11The exposure to water vapors for ca.8h of an almost completely dehydrated(as confirmed by weighing and IR spectroscopy)product after thermolysis of1(at250°C19for 30min)results in the reabsorption of water molecules giving a material with weight and IR spectrum identical to those of the initial sample1,thus corroborating the reversibility of the water escape and binding process.In conclusion,we have synthesized and structurally characterized a new type of2D hybrid water-chloride anionic multicyclic {[(H2O)20(Cl)4]4–}n network self-assembled in a hydrophobic matrix of the bis-terpyridine iron(II)complex,that is,[FeL2]Cl2·10H2O 1′.On the basis of the recent description and detailed analysis of the related{[(H2O)14(Cl)4]4–}n layers5a and taking into consideration that the water-chloride assembly in1′does not possess strong interactions with the metal-organic units,the crystal structure of 1′can alternatively be defined as an unusual set of water-chloride “hosts”with bis-terpyridine iron“guests”.Moreover,the present study extends the still limited number5of well-identified examples of large polymeric2D water-chloride assemblies intercalated in crystalline materials and shows that terpyridine compounds can provide rather suitable matrixes to stabilize and store water-chloride aggregates.Further work is currently in progress aiming at searching for possible applications in nanoelectrical devices,as well as understanding how the modification of the terpyridine ligand or the replacement of chlorides by other counterions with a high accepting ability toward hydrogen-bonds can affect the type and topology of the hybrid water containing associates within various terpyridine transition metal complexes.Acknowledgment.This work has been partially supported by the Foundation for Science and Technology(FCT)and its POCI 2010programme(FEDER funded),and by a HRTM Marie Curie Research Training Network(AQUACHEM project,CMTN-CT-2003-503864).The authors gratefully acknowledge Prof.Maria Filipa Ribeiro for kindly running the TG-DSC analysis,urent Benisvy,Dr.Maximilian N.Kopylovich,and Mr.Yauhen Y. Karabach for helpful discussions.Supporting Information Available:Additionalfigures(Figures S1–S3)with structural fragments of1′and TG-DSC analysis of1, Tables S1and S2with hydrogen-bond geometry in1′and bond angles within the H2O-Cl network,details for the general experimental procedures and X-ray crystal structure analysis and refinement,crystal-lographic informationfile(CIF),and the CSD refcodes for terpyridine compounds with water-chloride aggregates.This information is available free of charge via the Internet at .References(1)(a)Mascal,M.;Infantes,L.;Chisholm,J.Angew.Chem.,Int.Ed.2006,45,32and references therein.(b)Infantes,L.;Motherwell,S.CrystEngComm2002,4,454.(c)Infantes,L.;Chisholm,J.;Mother-well,S.CrystEngComm2003,5,480.(d)Supriya,S.;Das,S.K.J.Cluster Sci.2003,14,337.(2)(a)Das,M.C.;Bharadwaj,P.K.Eur.J.Inorg.Chem.2007,1229.(b)Ravikumar,I.;Lakshminarayanan,P.S.;Suresh,E.;Ghosh,P.Cryst.Growth Des.2006,6,2630.(c)Ren,P.;Ding,B.;Shi,W.;Wang,Y.;Lu,T.B.;Cheng,P.Inorg.Chim.Acta2006,359,3824.(d)Li,Z.G.;Xu,J.W.;Via,H.Q.;Hu,mun.2006,9,969.(e)Lakshminarayanan,P.S.;Kumar,D.K.;Ghosh,P.Inorg.Chem.2005,44,7540.(f)Raghuraman,K.;Katti,K.K.;Barbour,L.J.;Pillarsetty,N.;Barnes,C.L.;Katti,K.V.J.Am.Chem.Soc.2003,125,6955.(3)(a)Jungwirth,P.;Tobias,D.J.J.Phys.Chem.B.2002,106,6361.(b)Tobias,D.J.;Jungwirth,P.;Parrinello,M.J.Chem.Phys.2001,114,7036.(c)Choi,J.H.;Kuwata,K.T.;Cao,Y.B.;Okumura,M.J.Phys.Chem.A.1998,102,503.(d)Xantheas,S.S.J.Phys.Chem.1996,100,9703.(e)Markovich,G.;Pollack,S.;Giniger,R.;Cheshnovsky,O.J.Chem.Phys.1994,101,9344.(f)Combariza,J.E.;Kestner,N.R.;Jortner,J.J.Chem.Phys.1994,100,2851.(g)Perera, L.;Berkowitz,M.L.J.Chem.Phys.1991,95,1954.(h)Dang,L.X.;Rice,J.E.;Caldwell,J.;Kollman,P.A.J.Am.Chem.Soc.1991, 113,2481.(4)(a)Custelcean,R.;Gorbunova,M.G.J.Am.Chem.Soc.2005,127,16362.(b)Kopylovich,M.N.;Tronova,E.A.;Haukka,M.;Kirillov,A.M.;Kukushkin,V.Yu.;Fraústo da Silva,J.J.R.;Pombeiro,A.J.L.Eur.J.Inorg.Chem.2007,4621.(c)Butchard,J.R.;Curnow,O.J.;Garrett,D.J.;Maclagan,R.G.A.R.Angew.Chem.,Int.Ed.2006, 45,7550.(5)(a)Reger,D.L.;Semeniuc,R.F.;Pettinari,C.;Luna-Giles,F.;Smith,M.D.Cryst.Growth.Des.2006,6,1068and references therein.(b) Saha,M.K.;Bernal,mun.2005,8,871.(c) Prabhakar,M.;Zacharias,P.S.;Das,mun.2006,9,899.(d)Lakshminarayanan,P.S.;Suresh,E.;Ghosh,P.Angew.Chem.,Int.Ed.2006,45,3807.(e)Ghosh,A.K.;Ghoshal,D.;Ribas,J.;Mostafa,G.;Chaudhuri,N.R.Cryst.Growth.Des.2006,6,36.(f)Deshpande,M.S.;Kumbhar,A.S.;Puranik,V.G.;Selvaraj, K.Cryst.Growth Des.2006,6,743.(6)(a)Karabach,Y.Y.;Kirillov,A.M.;da Silva,M.F.C.G.;Kopylovich,M.N.;Pombeiro,A.J.L.Cryst.Growth Des.2006,6,2200.(b) Kirillova,M.V.;Kirillov,A.M.;da Silva,M.F.C.G.;Kopylovich, M.N.;Fraústo da Silva,J.J.R.;Pombeiro,A.J.L.Inorg.Chim.Acta2008,doi:10.1016/j.ica.2006.12.016.(7)The Cambridge Structural Database(CSD).Allen, F.H.ActaCrystallogr.2002,B58,380.(8)The searching algorithm in the ConQuest Version1.9(CSD version5.28,August2007)constrained to the presence of any terpyridinemoiety and at least one crystallization water molecule and one chloride counter ion resulted in43analyzable hits from which40compounds contain diverse water-chloride aggregates(there are29and11 examples of infinite(mostly1D)networks and discrete clusters, respectively).See the Supporting Information for the CSD refcodes.(9)For a recent review,see Constable,E.C.Chem.Soc.Re V.2007,36,246.(10)For recent examples of supramolecular terpyridine compounds,see(a)Beves,J.E.;Constable,E.C.;Housecroft,C.E.;Kepert,C.J.;Price,D.J.CrystEngComm2007,9,456.(b)Zhou,X.-P.;Ni,W.-X.;Zhan,S.-Z.;Ni,J.;Li,D.;Yin,Y.-G.Inorg.Chem.2007,46,2345.(c)Shi,W.-J.;Hou,L.;Li,D.;Yin,Y.-G.Inorg.Chim.Acta2007,360,588.(d)Beves,J.E.;Constable,E.C.;Housecroft,C.E.;Kepert,C.J.;Neuburger,M.;Price,D.J.;Schaffner,S.CrystEngComm2007,9,1073.(e)Beves,J. E.;Constable, E. C.;Housecroft, C. E.;Neuburger,M.;Schaffner,mun.2007,10,1185.(f)Beves,J.E.;Constable,E.C.;Housecroft,C.E.;Kepert,C.J.;Price,D.J.CrystEngComm2007,9,353.(11)Synthesis of1:FeCl2·2H2O(82mg,0.50mmol)and4′-phenyl-2,2′:6′,2″-terpyridine(L)(154mg,0.50mmol)were combined in a THF (20mL)solution with continuous stirring at room temperature.The resulting deep purple suspension was stirred for1h,filtered off,washed with THF(3×15mL),and dried in vacuo to afford a deep purple solid1(196mg,41%).1exhibits a high affinity for water and upon recrystallization gives derivatives with a higher varying content of crystallization water.1is soluble in H2O,MeOH,EtOH,MeCN, CH2Cl2,and CHCl3.mp>305°C(dec.).Elemental analysis.Found: C52.96,H3.76,N8.36.Calcld.for C42H40Cl4Fe2N6O5:C52.42,H4.19,N8.73.FAB+-MS:m/z:835{[FeL2]Cl2·5H2O+H}+,816784Crystal Growth&Design,Vol.8,No.3,2008Communications{[FeL2]Cl2·4H2O}+,796{[FeL2]Cl2·3H2O–2H}+,781{[FeL2]Cl2·2H2O+H}+,763{[FeL2]Cl2·H2O+H}+,709{[FeL2]Cl}+,674 {[FeL2]}+,435{[FeL]Cl2}+,400{[FeL]Cl}+,364{[FeL]–H}+,311 {L–2H}+.IR(KBr):νmax/cm–1:3462(m br)ν(H2O),3060(w),2968 (w)and2859(w)ν(CH),1656(m br)δ(H2O),1611(s),1538(w), 1466(m),1416(s),1243(m),1159(w),1058(m),877(s),792(s), 766(vs),896(m),655(w),506(m)and461(m)(other bands).The X-ray quality crystals of[FeL2]Cl2·10H2O(1′)were grown by slow evaporation,in air at ca.20°C,of a MeOH/H2O(v/v)9/1)solution of1.(12)Crystal data:1′:C42H50Cl2FeN6O10,M)925.63,triclinic,a)10.1851(10),b)12.2125(13),c)19.5622(19)Å,R)76.602(6),)87.890(7),γ)67.321(6)°,U)2180.3(4)Å3,T)150(2)K,space group P1j,Z)2,µ(Mo-K R))0.532mm-1,32310reflections measured,8363unique(R int)0.0719)which were used in all calculations,R1)0.0469,wR2)0.0952,R1)0.0943,wR2)0.1121 (all data).(13)(a)McMurtrie,J.;Dance,I.CrystEngComm2005,7,230.(b)Nakayama,Y.;Baba,Y.;Yasuda,H.;Kawakita,K.;Ueyama,N.Macromolecules2003,36,7953.(c)Kabir,M.K.;Tobita,H.;Matsuo,H.;Nagayoshi,K.;Yamada,K.;Adachi,K.;Sugiyama,Y.;Kitagawa,S.;Kawata,S.Cryst.Growth Des.2003,3,791.(14)Ludwig,R.Angew.Chem.,Int.Ed.2001,40,1808.(15)The searching algorithm in the ConQuest Version1.9(CSD version5.28,May2007)was constrained to the presence of(i)at least onetetranuclear[(H2O)3(Cl)]–ring(i.e.,minimal cyclic fragment in our water-chloride network)with d(O···O))2.2–3.2Åand d(O···Cl) )2.6–3.6Å,and(ii)at least one crystallization water molecule andone chloride counter ion.All symmetry-related contacts were taken into consideration.(16)For2D networks with the[(H2O)3(Cl)]–core,see the CSD refcodes:AGETAH,AMIJAH,BEXVIJ,EXOWIX,FANJUA,GAFGIE, HIQCIT,LUNHUX,LUQCEF,PAYBEW,TESDEB,TXCDNA, WAQREL,WIXVUU,ZUHCOW.For3D network,see the CSD refcode:LUKZEW.(17)This analysis was run on1since we were unable to get1′in a sufficientamount due to the varying content of crystallization water in the samples obtained upon recrystallization of1.(18)(a)Das,S.;Bhardwaj,P.K.Cryst.Growth.Des.2006,6,187.(b)Wang,J.;Zheng,L.-L.;Li,C.-J.;Zheng,Y.-Z.;Tong,M.-L.Cryst.Growth.Des.2006,6,357.(c)Ghosh,S.K.;Ribas,J.;El Fallah, M.S.;Bharadwaj,P.K.Inorg.Chem.2005,44,3856.(19)A temperature below305°C has been used to avoid the eventualdecomposition of the compound upon rather prolonged heating.CG7010315Communications Crystal Growth&Design,Vol.8,No.3,2008785。
Published:May 12,2011COMMUNICATION /JACSInterface-Directed Assembly of One-Dimensional Ordered Architecture from Quantum Dots Guest and Polymer HostShengyang Yang,Cai-Feng Wang,and Su Chen*State Key Laboratory of Materials-Oriented Chemical Engineering,and College of Chemistry and Chemical Engineering,Nanjing University of Technology,Nanjing 210009,P.R.ChinabSupporting Information ABSTRACT:Assembly of inorganic semiconductor nano-crystals into polymer host is of great scienti fic and techno-logical interest for bottom-up fabrication of functional devices.Herein,an interface-directed synthetic pathway to polymer-encapsulated CdTe quantum dots (QDs)has been developed.The resulting nanohybrids have a highly uniform fibrous architecture with tunable diameters (ranging from several tens of nanometers to microscale)and enhanced optical performance.This interfacial assembly strategy o ffers a versatile route to incorporate QDs into a polymer host,forming uniform one-dimensional nanomaterials po-tentially useful in optoelectronic applications.Similar to the way that atoms bond to form molecules and complexes,inorganic nanoparticles (NPs)can be combined to form larger ensembles with multidimensional ordered hier-archical architecture,evoking new collective functions.To this end,the development of the controlled self-assembly method for well-de fined structures of these ensembles is signi ficant for creating new and high-performance tunable materials and hence has aroused appealing scienti fic and industrial interest.1Particu-larly,much e ffort has been devoted to the construction of one-dimensional (1D)structures of NPs,owing in part to their application as pivotal building blocks in fabricating a new generation of optoelectronic devices.2In this context,directed host Àguest assembly of NPs into polymer matrices is an e ffective “bottom-up ”route to form 1D ordered functional materials with advantageous optical,electrical,magnetic,and mechanical properties.3Some typical routes have been developed for the generation of these 1D hybrids so far,involving template-assisted,4seeding,5and electro-static approaches.6However,the challenge still remains to precisely manipulate assembly of aqueous NPs and water-insoluble polymers into uniform 1D nanocomposites with a high aspect ratio because of phase separation and aggregation.7Moreover,facile synthetic strate-gies are highly needed to fabricate homogeneous 1D composites in which each component still preserves favorable properties to produce optimal and ideal multifunctional materials.A liquid Àliquid interface o ffers an ideal platform to e fficiently organize NPs into ordered nanostructures driven by a minimiza-tion of interfacial energy.8While much of this research has been directed toward NP hybrids with diverse morphologies based on small organic ligand-directed assembly,9some success has also been achieved in polymer-based NPs nanocomposites.10Russelland co-workers developed ultrathin membranes and capsules of quantum dots (QDs)stabilized by cross-linked polymers at the toluene/water interface.10a,11Brinker ’s group reported the fab-rication of free-standing,patternable NP/polymer monolayer arrays via interfacial NP assembly in a polymeric photoresist.12Herein,a simple host Àguest assembly route is developed to facilely create homogeneous 1D CdTe/polymer hybrids without any indication of phase separation at the aqueous/organic inter-face for the first time.The CdTe nanocrystal is a semiconductor that has been used extensively for making thin film for solar cells.13Some elegant studies have been made in synthesizing pure inorganic 1D CdTe nanowires via assembly from corre-sponding individual CdTe nanocrystals.14In this work,CdTe QDs are covalently grafted with poly(N -vinylcarbazole-co -glycidylmethacrylate)(PVK-co -PGMA)to form uniform fibrous fluorescent composites at the water/chloroform interface via the reaction between epoxy groups of PVK-co -PGMA and carboxyl groups on the surface of CdTe QDs (Scheme 1).15These 1D composite fibers can be allowed to grow further in the radial direction by “side-to-side ”assembly.Additionally,this type of interfacial QD Àpolymer assembly can observably improve the fluorescence lifetime of semiconductor QDs incorporated in theScheme 1.Schematic Representation of the Synthesis of PVK-co -PGMA/CdTe QDs Composite Nano fibersReceived:February 8,2011polymeric matrix.It can be expected that this example of both linear axial organization and radial assembly methodology can be applied to fabricate spatial multiscale organic Àinorganic com-posites with desired properties of NPs and polymers.Figure 1a shows a typical scanning electron microscope (SEM)image of PVK-co -PGMA/CdTe QDs composite nano fi-bers obtained at the water/chloroform interface after dialysis.The as-prepared fibers have uniform diameters of about 250nm and typical lengths in the range of several tens to several hundreds of micrometers (Figures 1a and S4Supporting In-formation [SI]).Interestingly,PVK-co -PGMA/CdTe composite fibers can randomly assemble into nestlike ring-shaped patterns (Figures 1b and S5[SI]).Given the interaction among epoxy groups,the formation of nestlike microstructures could be attributed to incidental “head-to-tail ”assembly of composite fibers.Moreover,in order to establish the relationship between the role of epoxy groups and the formation of composite nano fibers,control experiments were performed,in which pure PGMA or PVK was used to couple CdTe QDs.The PGMA/CdTe composites could be obtained with fibrous patterns (Figure S6[SI]),but no fibrous composites were achieved at the biphase interface with the use of PVK under the same conditions.The microstructures and fluorescence properties of PVK-co -PGMA/CdTe composite fibers were further character-ized using laser confocal fluorescence microscopy (LCFM).Confocal fluorescence micrographs of composite fibers show that the di fferently sized QDs have no obvious in fluence on the morphology of composites (Figure 1c Àe).Clearly,uniform and strong fluorescence emission is seen throughout all the samples,and the size-dependent fluorescence trait of CdTe QDs in PVK-co -PGMA matrix remains well.In order to verify the existence and distribution of CdTe QDs in the fibers,transmission electron microscopy (TEM)was employed to examine the assembled structures.Figure 2a shows a TEM image of PVK-co -PGMA/CdTe QDs composite nano fi-bers,indicating each composite fiber shown in Figure 1a was assembled from tens of fine nano fibers.An individual fine nano fiber with the diameter of about 30nm is displayed in Figure 2b,from which we can see that CdTe QDs have been well anchored into the fiber with polymeric protection layer,revealing this graft-form process at the interface e ffectively avoidednon-uniform aggregation in view of well-dispersed CdTe QDs within the composite fiber,consistent with the LCFM observa-tion.Unlike previous works where the nanoparticles were ad-sorbed onto the polymer fibers,16CdTe QDs were expelled from the surface of fibers (∼2.5nm)in our system (Figure 2c),albeit the high percentage of QDs in the polymer host (23wt %)was achieved (Figure S7[SI]).This peculiarity undoubtedly confers CdTe QDs with improved stability.The clear di ffuse rings in the selected area electron di ffraction (SAED)pattern further indicate excellent monodispersion and finely preserved crystalline struc-ture of QDs in the nano fibers (Figure 2d).The SAED data correspond to the cubic zinc blende structure of CdTe QDs.A possible mechanism for the assembly of 1D nanostructure was proposed,as illustrated in Figure S8[SI].The hydrophilic epoxy groups of the PVK-co -PGMA chain in the oil phase orient toward the biphase interface and then react with carboxyl groups on the surface of CdTe QDs in the aqueous phase to a fford premier PVK-co -PGMA/CdTe QDs composites.Such nanocomposites will reverse repeatedly,resulting from iterative reaccumulation of epoxy groups at the interface and the reaction between the active pieces (i.e.,epoxy or carboxyl groups)in the composites with intact CdTe QDs or PVK-co -PGMA,forming well-de fined nano-fibers.The control experiments showing that the diameter of composite fibers increases with the increase in the concentration of PVK-co -PGMA are in agreement with the proposed mechan-ism (Figure S9[SI]).In addition,it is expected that the pure polymeric layer on the surface of the fibers (red rectangular zone in Figure 2c)will allow further assembly of fine fibers into thick fibers,and these fibers also could randomly evolve into rings,forming nestlike microstructures when the “head ”and “tail ”of fibers accidentally meet (Figure 1b).To further examine the assembly behavior of composite fibers,the sample of PVK-co -PGMA/CdTe QDs composite nano fibers were kept at the water/chloroform interface for an additional month in a close spawn bottle at room temperature (Figure S10[SI]).With longer time for assembly,thicker composite fibers with tens of micrometers in diameter were obtained (Figure 3a).These micro-fibers have a propensityto form twisted morphology (Figure 3a,b),Figure 1.(a,b)SEM images of PVK-co -PGMA/CdTe QDs composite nano fibers.(c Àe)Fluorescence confocal microscopy images of PVK-co -PGMA/CdTe QDs composite nano fibers in the presence of di fferently sized QDs:(c)2.5nm,(d)3.3nm,and (e)3.6nm.The excitation wavelengths are 488(c),514(d),and 543nm (e),respectively.Figure 2.(a,b)TEM images of PVK-co -PGMA/CdTe QDs composite nano fibers,revealing composite nano fiber assemblies.(c)HRTEM image and (d)SAED pattern of corresponding PVK-co -PGMA/CdTe QDs composite nano fibers.while their re fined nanostructures still reveal relatively parallel character and con firm the micro fibers are assembled from countless corresponding nano fibers (Figure 3c).The corresponding LCFM image of an individual micro fiber is shown in Figure 3d (λex =488nm),indicating strong and homogeneous green fluorescence.Another indication is the fluorescent performance of PVK-co -PGMA/CdTe QDs composite micro fibers (Figure 4a).The fluorescent spectrum of composite fibers takes on emission of both PVK-co -PGMA and CdTe QDs,which suggests that this interfacial assembly route is e ffective in integrating the properties of organic polymer and inorganic nanoparticles.It is worth noting that there is a blue-shift (from 550to 525nm)and broadening of the emission peak for CdTe QDs upon their incorporation into polymeric hosts,which might be ascribed to the smaller QD size and less homogeneous QD size distribution resulting from the photooxidation of QD surfaces.17Since the emission spectra of PVK-co -PGMA spectrally overlap with the CdTe QD absorption (Figure S11[SI]),energy transfer from the copolymer to the CdTe QDs should exist.18However,the photoluminescence of PVK-co -PGMA does not vanish greatly in the tested sample in comparison with that of polymer alone,revealing inferior energy transfer between the polymer host and the QDs.Although e fficient energy transfer could lead to hybrid materials that bring together the properties of all ingredients,18it is a great hurdle to combine and keep the intrinsic features of all constituents.19In addition,by changing the polymeric compo-nent and tailoring the element and size of QDs,it should be possible to expect the integration of organic and inorganic materials with optimum coupling in this route for optoelectronic applications.Finally,to assess the stability of CdTe QDs in the composite micro fibers,time-resolved photoluminescence was performed using time-correlated single-photon counting (TCSPC)parative TCSPC studies for hybrid PVK-co -PGMA/CdTe QDs fibers and isolated CdTe QDs in the solid state are presented in Figure 4b.We can see that the presence of PVK-co -PGMA remarkably prolongs the fluorescence lifetime (τ)of CdTe QDs.Decay traces for the samples were well fittedwith biexponential function Y (t )based on nonlinear least-squares,using the following expression.20Y ðt Þ¼R 1exp ðÀt =τ1ÞþR 2exp ðÀt =τ2Þð1Þwhere R 1,R 2are fractional contributions of time-resolved decaylifetimes τ1,τ2and the average lifetime τhcould be concluded from the eq 2:τ¼R 1τ21þR 2τ22R 1τ1þR 2τ2ð2ÞFor PVK-co -PGMA/CdTe QDs system,τh is 10.03ns,which is approximately 2.7times that of isolated CdTe QDs (3.73ns).Photooxidation of CdTe QDs during the assembly process can increase the surface states of QDs,causing a delayed emission upon the carrier recombination.21Also,the polymer host in this system could prevent the aggregation of QDs,avoid self-quench-ing,and delay the fluorescence decay process.22The increased fluorescence lifetime could be also ascribed to energy transfer from PVK-co -PGMA to CdTe QDs.18c The result suggests that this host Àguest assembly at the interface could find signi ficant use in the fabrication of QDs/polymer hybrid optoelectronic devices.In summary,we have described the first example of liquid/liquid interfacial assembly of 1D ordered architecture with the incorporation of the QDs guest into the polymer host.The resulting nanohybrids show a highly uniform fibrous architecture with tunable diameter ranging from nanoscale to microscale.The procedure not only realizes the coexistence of favorable properties of both components but also enables the fluorescence lifetime of QDs to be enhanced.This interesting development might find potential application for optoelectronic and sensor devices due to high uniformity of the 1D structure.Further e fforts paid on optimal regulation of QDs and polymer composition into 1D hybrid nanostructure could hold promise for the integration of desirable properties of organic and inorganic compositions for versatile dimension-dependent applications.In addition,this facile approach can be easily applied to various semiconductor QDs and even metal NPs to develop highly functional 1D nanocomposites.’ASSOCIATED CONTENTbSupporting Information.Experimental details,FT-IR,GPC,UV Àvis,PL,SEM,TGA analysis,and complete ref 9c.This material is available free ofcharge via the Internet at .Figure 3.(a,b)SEM and (c)FESEM images of PVK-co -PGMA/CdTe QDs composite micro fibers.(d)Fluorescence confocal microscopy images of PVK-co -PGMA/CdTe QDs composite micro fibers inthe presence of green-emitting QDs (2.5nm).Figure 4.(a)Fluorescence spectra of PVK-co -PGMA,CdTe QD aqueous solution,and PVK-co -PGMA/CdTe QDs composite micro-fibers.(b)Time-resolved fluorescence decay curves of CdTe QDs (2.5nm diameter)powders (black curve)and the corresponding PVK-co -PGMA/CdTe QDs composite micro fibers (green curve)mea-sured at an emission peak maxima of 550nm.The samples were excited at 410nm.Biexponential decay function was used for satisfactory fitting in two cases (χ2<1.1).’AUTHOR INFORMATIONCorresponding Authorchensu@’ACKNOWLEDGMENTThis work was supported by the National Natural Science Foundation of China(21076103and21006046),National Natural Science Foundation of China-NSAF(10976012),the Natural Science Foundations for Jiangsu Higher Education Institutions of China(07KJA53009,09KJB530005and10KJB5 30006),and the Priority Academic Program Development of Jiangsu Higher Education Institutions(PAPD).’REFERENCES(1)(a)Kashiwagi,T.;Du,F.;Douglas,J.F.;Winey,K.I.;Harris, R.H.;Shields,J.R.Nat.Mater.2005,4,928.(b)Shenhar,R.;Norsten, T.B.;Rotello,V.M.Adv.Mater.2005,17,657.(c)Akcora,P.;Liu,H.; Kumar,S.K.;Moll,J.;Li,Y.;Benicewicz,B.C.;Schadler,L.S.;Acehan, D.;Panagiotopoulos,A.Z.;Pryamitsyn,V.;Ganesan,V.;Ilavsky,J.; Thiyagarajan,P.;Colby,R.H.;Douglas,J.F.Nat.Mater.2009,8,354.(d)Dayal,S.;Kopidakis,N.;Olson,D.C.;Ginley,D.S.;Rumbles,G. 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ARTICLEOPENReceived11Dec2012|Accepted16May2013|Published14Jun2013A solid with a hierarchical tetramodalmicro-meso-macro pore size distributionYu Ren1,Zhen Ma2,3,Russell E.Morris1,Zheng Liu1,Feng Jiao4,Sheng Dai3&Peter G.Bruce1Porous solids have an important role in addressing some of the major energy-related pro-blems facing society.Here we describe a porous solid,a-MnO2,with a hierarchical tetramodalpore size distribution spanning the micro-,meso-and macro pore range,centred at0.48,4.0,18and70nm.The hierarchical tetramodal structure is generated by the presence ofpotassium ions in the precursor solution within the channels of the porous silica template;thesize of the potassium ion templates the microporosity of a-MnO2,whereas theirreactivity with silica leads to larger mesopores and macroporosity,without destroying themesostructure of the template.The hierarchical tetramodal pore size distribution influencesthe properties of a-MnO2as a cathode in lithium batteries and as a catalyst,changingthe behaviour,compared with its counterparts with only micropores or bimodalmicro/mesopores.The approach has been extended to the preparation of LiMn2O4with ahierarchical pore structure.1EaStCHEM,School of Chemistry,University of St Andrews,St Andrews KY169ST,UK.2Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention(LAP3),Department of Environmental Science and Engineering,Fudan University,Shanghai200433,China.3Chemical Sciences Division,Oak Ridge National Laboratory,Oak Ridge,T ennessee37831,USA.4Department of Chemical and Biomolecular Engineering,University of Delaware,Newark,Delaware19716,USA.Correspondence and requests for materials should be addressed to P.G.B.(email:p.g.bruce@).P orous solids have an important role in addressing some of the major problems facing society in the twenty-first century,such as energy storage,CO2sequestration,H2 storage,therapeutics(for example,drug delivery)and catalysis1–8. The size of the pores and their distribution directly affect their ability to function in a particular application2.For example, zeolites are used as acid catalysts in industry,but their micropores impose severe diffusion limitations on the ingress and egress of the reactants and the catalysed products9.To address such issues, great effort is being expended in preparing porous materials with a bimodal(micro and meso)pore structure by synthesizing zeolites or silicas containing micropores and mesopores10–17,or microporous metal–organic frameworks with ordered mesopores18.Among porous solids,porous transition metal oxides are particularly important,because they exhibit many unique properties due to their d-electrons and the variable redox state of their internal surfaces8,19–22.Here we describe thefirst solid(a-MnO2)possessing hierarchical pores spanning the micro,meso and macro range, centred at0.48,4.0,18and70nm.The synthesis method uses mesoporous silica as a hard template.Normally such a template generates a mesoporous solid with a unimodal23–31or,at most,a bimodal pore size distribution32–38.By incorporating Kþions in the precursor solution,within the silica template,the Kþions act bifunctionally:their size templates the formation of the micropores in a-MnO2,whereas their reactivity with silica destroys the microporous channels in KIT-6comprehensively, leading to the formation of a-MnO2containing large mesopores and,importantly,macropores,something that has not been possible by other methods.Significantly,this is achieved without destroying the silica template by alkaline ions.The effect of the tetramodal pore structure on the properties of the material is exemplified by considering their use as electrodes for lithium-ion batteries and as a catalyst for CO oxidation and N2O decomposition.The novel material offers new possibilities for combining the selectivity of small pores with the transport advantages of the large pores across a wide range of sizes.We also present results demonstrating the extension of the method to the synthesis of LiMn2O4with a hierarchical pore structure.ResultsComposition of tetramodal a-MnO2.The composition of the synthesized material was determined by atomic absorption ana-lysis and redox titration to be K0.08MnO2(the K/Mn ratio of the precursor solution was1/3).The material is commonly referred to as a-MnO2,because of the small content of Kþ19.N2sorption analysis of tetramodal a-MnO2.The tetramodal a-MnO2shows a type IV isotherm(Fig.1a).The pore size dis-tribution(Fig.1b)in the range of0.3–200nm was analysed using the density functional theory(DFT)method applied to the adsorption branch of the isotherm39–42,as this is more reliable than analysing the desorption branch43;note that this is not the DFT method used in ab initio electronic structure calculations. Plots were constructed with vertical axes representing ‘incremental pore volume’and‘incremental surface area’.Large (macro)pores can account for a significant pore volume while representing a relatively smaller surface area and vice versa for small(micro)pores.Therefore,when investigating a porous material with a wide range of pore sizes,for example,micropore and macropore,the combination of surface area and pore volume is essential to determine the pore size distribution satisfactorily (Fig.1b).Considering both pore volume and surface area, significant proportions of micro-,meso-and macropores are evident,with distinct maxima centred at0.70,4.0,18and70nm.To probe the size of the micropores more precisely than is possible with DFT,the Horvath–Kawazoe pore size distribution analysis was employed44.A single peak was obtained at0.48nm(Fig.1c),in good accord with the0.46-nm size of the2Â2channels of a-MnO2 (refs.19,21).The relatively small Brunauer–Emmett–Teller(BET) surface area of tetramodal a-MnO2(79–105m2gÀ1; Supplementary Table S1)compared with typical surface areas of mesoporous metal oxides(90–150m2gÀ1)45is due to the significant proportion of macropores(which have small surface areas)and relatively large(18nm)mesopores—a typical mesoporous metal oxide has only3–4nm pores.TEM analysis of tetramodal a-MnO2.Transmission electron microscopic(TEM)data for tetramodal a-MnO2,Fig.2, demonstrates a three-dimensional pore structure with a sym-metry consistent with space group Ia3d.From the TEM data,an a0lattice parameter of23.0nm for the mesostructure could be extracted,which is in good agreement with the value obtained from the low-angle powder X-ray diffraction(PXRD)data, a0¼23.4nm(Supplementary Fig.S1a).High-resolution TEM images in Fig.2c–e demonstrate that the walls are crystalline with a typical wall thickness of10nm.The lattice spacings of0.69,0.31 and0.35nm agree well with the values of6.92,3.09and3.46Åfor the[110],[310]and[220]planes of a-MnO2(International Centre for Diffraction Data(ICDD)number00-044-0141), respectively.The wide-angle PXRD data matches well with the PXRD data of bulk cryptomelane a-MnO2(Supplementary Fig. S1b),confirming the crystalline walls.The various pores in tetramodal a-MnO2can be observed by TEM directly:the0.48-nm micropores are seen in Fig.2e(2Â2 tunnels with dimensions of0.48Â0.48nm in the white box);the 4.0-nm pores are shown in Fig.2b–d;the18-nm pores are shown in Fig.2a;the70-nm pores are evident in Fig.2b(highlighted with white circles).Li intercalation.Li can be intercalated into bulk a-MnO2 (ref.46).Therefore,it is interesting to compare Li intercalation into bulk a-MnO2(micropores only)and bimodal a-MnO2 (micropores along with a single mesopore of diameter3.6nm,see Methods)with tetramodal a-MnO2(micro-,meso-and macropores).Each of the three a-MnO2materials was subjected to Li intercalation by incorporation as the positive electrode in a lithium battery,along with a lithium anode and a non-aqueous electrolyte(see Methods).The results of cycling(repeated intercalation/deintercalation of Li)the cells are shown in Fig.3. Although all exhibit good capacity to cycle Li at low rates of charge/discharge(30mA gÀ1),tetramodal a-MnO2shows sig-nificantly higher capacity(Li storage)at a high rate of 6,000mA gÀ1(corresponding to charge and discharge in3min). The tetramodal a-MnO2can store three times the capacity(Li) compared with bimodal a-MnO2,and18times that of a-MnO2 with only micropores,at the high rate of intercalation/deinter-calation(Fig.3).The superior rate capability of tetramodal a-MnO2over microporous and bimodal forms may be assigned to better Liþtransport in the electrolyte within the hierarchical pore structure of tetramodal a-MnO2.The importance of elec-trolyte transport in porous electrodes has been discussed recently35,47,48and the results presented here reinforce the beneficial effect of a hierarchical pore structure.Catalytic studies.CO oxidation and N2O decomposition were used as reactions to probe the three different forms of a-MnO2as catalysts(Supplementary Fig.S2).As shown in Supplementary Fig.S2a,tetramodal a-MnO2demonstrates better catalytic activity compared with only micropores or bimodal a-MnO2;thetemperature of half CO conversion (T 50)was 124°C for tetra-modal a -MnO 2,whereas microporous and bimodal a -MnO 2exhibited a T 50value of 275°C and 209°C,respectively.In the case of N 2O decomposition,a -MnO 2with only micropores demonstrated no catalytic activity in the range of 200–400°C,in accord with a previous report 49.Tetramodal and bimodal a -MnO 2showed catalytic activity and reached 32%and 20%of N 2O conversion,respectively,at a reaction temperature of 400°C.The differences in catalytic activity are related to the differences in the material.A detailed study focusing on the catalytic activity alonewould be necessary to demonstrate which specific features of the textural differences (pore size distribution,average manganese oxidation state,K þand so on)between the different MnO 2materials are responsible for the differences in behaviour.However,the preliminary results shown here do illustrate that such differences exist.Porous LiMn 2O 4.To demonstrate the wider applicability of the synthesis method,LiMn 2O 4with a hierarchical pore structurewas1801601401201008060402000.00.20.40.60.81.0V (c m 3 g –1)Pore diameter (nm)0.0120.0100.0080.0060.0040.0020.000I n c r e m e n t a l p o r e v o l u m e (c m 3 g –1)Pore width (nm)I n c r e m e n t a l s u r f a c e a r e a (m 2 g –1)I n c r e m e n t a l s u r f a c e a r e a (m 2 g –1)P /P 0Figure 1|N 2sorption analysis of tetramodal a -MnO 2.(a )N 2adsorption–desorption isotherms,(b )DFT pore size distribution and (c )Horvath–Kawazoe pore size distribution from N 2adsorption isotherm for tetramodal a -MnO 2.Figure 2|TEM images of tetramodal a -MnO 2.TEM images along (a )[100]direction,showing 18nm mesopores (scale bar,50nm);(b )4.0and 70nm pores (70nm pores are highlighted by white circles;scale bar,100nm);(c –e )high-resolution (HRTEM)images of tetramodal a -MnO 2showing 4.0and 0.48nm pores (scale bar,10nm).Inset is representation of a -MnO 2structure along the c axis,demonstrating the 2Â2micropores as shown in the HRTEM (white box)in e .Purple,octahedral MnO 6;red,oxygen;violet,potassium.synthesized in a way similar to that of tetramodal a -MnO 2.The main difference is the use of LiNO 3instead of KNO 3(see Methods).In this case,Li þreacts with the silica template col-lapsing/blocking the microporous channels in the KIT-6and resulting in the large mesopores and macropores (17and 50nm)in the LiMn 2O 4obtained.The use of Li þinstead of the larger K þdeters the formation of micropores because Li þis too small.TEM analysis illustrates the hierarchical pore structure of LiMn 2O 4(Supplementary Fig.S3):4.0nm pores are evident in Supplementary Fig.S3b;17nm pores in Supplementary Fig.S3a;and 50nm pores in Supplementary Fig.S3b (highlighted with white circles).The d-spacing of 0.47nm in the high-resolution TEM image (Supplementary Fig.S3c)is in good accordance with the values of 0.4655nm for the [111]planes of LiMn 2O 4(ICDD number 00-038-0789)and with the wide-angle PXRD data (Supplementary Fig.S4).The original DFT pore size distribution analysis from N 2sorption (adsorption branch)gives three pore sizes in the range of 1–100nm centred at 4.0,17and 50nm (Supplementary Fig.S5).A more in-depth presentation of the results for LiMn 2O 4will be given in a future paper;preliminary results presented here illustrate that the basic method can be applied beyond a -MnO 2.DiscussionTurning to the synthesis of the tetramodal a -MnO 2,the details are given in the Methods section.Hard templating using silica templates,such as KIT-6,normally gives rise to materials with unimodal or,at most,bimodal mesopore structures,and in the latter case the smaller mesopores dominate over the larger mesopores 8,32,35.Alkali ions are excellent templates for micropores in transition metal oxides 19,21,but they have been avoided in nanocasting from silica templates because of concerns that they would react with and,hence,destroy thesilica20018016014012010080604020D i s c h a r g e c a p a c i t y (m A h g –1)0Cycle numberx in Li x MnO 2Figure 3|Electrochemical behaviour of different a -MnO 2.Capacity retention for tetramodal a -MnO 2cycled at 30(empty blue circles)and 6,000mA g À1(filled blue circles);bulk a -MnO 2cycled at 30(empty red squares)and 6,000mA g À1(filled red squares);bimodal a -MnO 2cycled at 30(empty black triangle)and 6,000mA g À1(filled blacktriangles).18 nm pores70 nm poresTwo sets of mesoporeschannels connecting both sets of mesoporesEtching of silica Etching of silica Etching of silica template2discontinuously within one set of the KIT-6mesoporesFigure 4|Formation mechanism of meso and macropores in tetramodal a -MnO 2.When both KIT-6mesochannels are occupied by a -MnO 2and then the silica between them etched away,the remaining pore is 4nm (centre portion of figure).When a -MnO 2grows in only one set of mesochannels and then the KIT-6is dissolved away,the remaining metal oxide has 18nm pores (upper portion of figure).The comprehensive destruction of the microchannels in KIT-6by K þleads to a -MnO 2growing in only a proportion of one set of the KIT-6mesochannels,resulting in the formation of B 70nm pores (lower portion of figure).template50.Here,not only have alkali ions been used successfully in precursor solutions without destroying the template mesostructure but they give rise to macropores in the a-MnO2, thus permitting the synthesis of a tetramodal,micro-small,meso-large,meso-macro pore structure.Synthesis begins by impregnating the KIT-6silica template with a precursor solution containing Mn2þand Kþions.On heating,the Kþions template the formation of the micropores in a-MnO2,as the latter forms within the KIT-6template.KIT-6 consists of two interpenetrating mesoporous channels linked by microporous channels51–53.The branches of the two different sets of mesoporous channels in KIT-6are nearest neighbours separated by a silica wall of B4nm53;therefore,when both KIT-6mesochannels are occupied by a-MnO2and the silica between them etched away,the remaining pore is4nm(see centre portion of Fig.4).It has been shown previously,by a number of authors,that by varying the hydrothermal conditions used to prepare the KIT-6,the proportion of the microchannels can be decreased to some extent,thus making it difficult to simultaneouslyfill the neighbouring KIT-6mesoporous channels by the precursor solution of the target mesoporous metal oxide33–35.As a result,the target metal oxide grows in only one set of mesochannels of the KIT-6host but not both.When the KIT-6is dissolved away,the remaining metal oxide has B18nm pores,because the distance between adjacent branches of the same KIT-6mesochannels is greater than between the two different mesochannels in KIT-6.Here we propose that the Kþions have a similar effect on the KIT-6to that of the hydrothermal synthesis,but by a completely different mechanism.Reaction between the Kþions in the precursor solution with the silica during calcination results in the formation of Kþ-silicates,which cause collapse or blocking of the microporous channels in KIT-6,such that the a-MnO2grows in one set of the KIT-6mesochannels,giving rise to18nm pores in a-MnO2when the silica is etched away,see top portion of Fig.4. However,the reaction between Kþand the silica is more severe than the effect of varying the hydrothermal treatment.In the former case,the KIT-6microchannels are so comprehensively destroyed that the proportion of the large(18nm)to smaller (4nm)mesopores is greater than can be achieved by varying hydrothermal conditions.The comprehensive destruction of the microchannels in KIT-6by Kþ,perhaps augmented by some minor degradation of parts of the mesochannels,leads to a-MnO2 growing in only a proportion of one set of the KIT-6 mesochannels,resulting in the formation of B70nm pores in a-MnO2,see lower portion of Fig.4.In summary,the Kþreactivity with the silica goes beyond what can be achieved by varying the conditions of hydrothermal synthesis and is responsible for generating the tetramodal pore size distribution reported here. The mechanism of pore formation in a-MnO2by reaction between Kþand the silica template is supported by several findings.First,by the lower K/Mn molar ratio of thefinal tetramodal a-MnO2product(0.08)compared with the starting materials(0.33)implies that some of the Kþions in the impregnating solution have reacted with the silica.Second, support for collapse/blocking of the microporous channels in KIT-6due to reaction with Kþwas obtained by comparing the texture of KIT-6impregnated with an aqueous solution contain-ing only KNO3and calcined at300and500°C.The micropore volume in KIT-6is the greatest,with no KNO3in the solution;it then decreases continuously as the calcination temperature and calcination time is increased,such that after2and5h at500°C the micropore volume has decreased to zero(Supplementary Fig. S6).Third,we prepared tetramodal a-MnO2using a similar synthetic procedure to that described in the Methods section, except that this time we used a covered tall crucible for the calcination step.Sun et al.54have shown that using a covered,tall crucible when calcining results in porous metal oxides with much larger particle sizes.If the70-nm pores had arisen simply from the gaps between the particles,then the pore size would have changed;in contrast,it remained centred at70nm, Supplementary Fig.S7,consistent with the70-nm pores being intrinsic to the materials and arising from reaction with the Kþas described above.Fourth,if the synthesis of MnO2is carried out using the KIT-6template but in the absence Kþions,then the DFT pore size distribution shown in Supplementary Fig.S8is obtained.The0.48-and70-nm pores are now absent,but the4-and18-nm pores remain.This demonstrates the key role of Kþin the formation of the smallest and largest pores and,hence,in generating the tetramodal pore size distribution.The absence of Kþmeans that there is nothing to template the0.48nm pores and so a-MnO2is not formed;the b-polymorph is obtained instead.The absence of Kþalso means that the microchannels in the KIT-6template remain intact,resulting in no70nm pores and the dominance of the4-nm pores compared with the 18-nm pores.The hierarchical pore structure can be varied systematically by controlling the synthesis conditions,in particular the Kþ/Mn ratio of the precursor solution.A range of Kþ/Mn ratios,1/5,1/3and1/2,gave rise to a series of pore size distributions,in which the pore sizes remained the same but the relative proportions of the different pores varied (Supplementary Table S1).The higher the Kþ/Mn ratio,the greater the proportion of macropores and large mesopores.This is in accord with expectations,as the higher the Kþconcentra-tion in the precursor solution the greater the collapse/blocking of the microporous channels in the KIT-6(as noted above),and hence the greater the proportion of macropores and large mesopores.Indeed,these results offer further support for the mechanism of pore size distribution arising from reaction between Kþand the silica template.In conclusion,tetramodal a-MnO2,thefirst porous solid with a tetramodal pore size distribution,has been synthesized.Its hierarchical pore structure spans the micro,meso and macropore range between0.3and200nm,with pore dimensions centred at 0.48,4.0,18and70nm.Key to the synthesis is the use of Kþions that not only template the formation of micropores but also react with the silica template,therefore,breaking/blocking the micro-porous channels in the silica template far more comprehensively than is possible by varying the hydrothermal synthesis conditions, to the extent that macropores are formed,and without destroying the silica mesostructure by alkali ions,as might have been expected.The resulting hierarchical tetramodal structure demon-strates different behaviours compared with microporous and bimodal a-MnO2as a cathode material for Li-ion batteries,and when used as a catalyst for CO oxidation and N2O decomposi-tion.The method has been extended successfully to the preparation of hierarchical LiMn2O4.MethodsSynthesis.Tetramodal a-MnO2(surface area96m2gÀ1,K0.08MnO2)was pre-pared by two-solvent impregnation55using Kþand mesoporous silica KIT-6as the hard template.KIT-6was prepared according to a previous report (hydrothermal treatment at100°C)51.In a typical synthesis of tetramodal a-MnO2, 7.53g of Mn(NO3)2Á4H2O(98%,Aldrich)and1.01g of KNO3(99%,Aldrich)were dissolved in B10ml of water to form a solution with a molar ratio of Mn/K¼3.0. Next,5g of KIT-6was dispersed in200ml of n-hexane.After stirring at room temperature for3h,5ml of the Mn/K solution was added slowly with stirring.The mixture was stirred overnight,filtered and dried at room temperature until a completely dried powder was obtained.The sample was heated slowly to500°C (1°C minÀ1),calcined at that temperature for5h with a cover in a normal crucible unless is specified54and the resulting material treated three times with a hot aqueous KOH solution(2.0M),to remove the silica template,followed by washing with water and ethanol several times,and then drying at60°C.Bimodal a-MnO2(surface area58m2gÀ1,K0.06MnO2)with micropore and a single mesopore size of3.6nm was prepared by using mesoporous silica SBA-15as a hard template.The SBA-15was prepared according to a previous report56.Bulk a-MnO2(surface area8m2gÀ1,K0MnO2)was prepared by the reaction between325mesh Mn2O3(99.0%,Aldrich)and6.0M H2SO4solution at80°C for 24h,resulting in the disproportionation of Mn2O3into a soluble Mn2þspecies and the desired a-MnO2product46.Treatment of KIT-6with KNO3was carried out as follows:1.01g of KNO3was dissolved in B15ml of water to form a KNO3solution.Five grams of mesoporous KIT-6was dispersed in200ml of n-hexane.After stirring at room temperature for 3h,5ml of KNO3solution was added slowly with stirring.The mixture was stirred overnight,filtered and dried at room temperature until a completely dried powder was obtained.The sample was heated slowly to300or500°C(1°C minÀ1), calcined at that temperature for5h and the resulting material was washed with water and ethanol several times,and then dried at60°C overnight.The synthesis method for hierarchical porous LiMn2O4was similar to that of tetramodal a-MnO2.The main difference was to use1.01g of LiNO3instead of KNO3.After impregnation into KIT-6,calcination and silica etching,porous LiMn2O4was obtained.Characterization.TEM studies were carried out using a JEOL JEM-2011, employing a LaB6filament as the electron source,and an accelerating voltage of 200keV.TEM images were recorded by a Gatan charge-coupled device camera in a digital format.Wide-angle PXRD data were collected on a Stoe STADI/P powder diffractometer operating in transmission mode with Fe K a1source radiation(l¼1.936Å).Low-angle PXRD data were collected using a Rigaku/MSC,D/max-rB with Cu K a1radiation(l¼1.541Å)operating in reflection mode with a scintillation detector.N2adsorption–desorption analysis was carried out using a Micromeritics ASAP2020.The typical sample weight used was100–200mg. The outgas condition was set to300°C under vacuum for2h,and all adsorption–desorption measurements were carried out at liquid nitrogen tem-perature(À196°C).The original DFT method for the slit pore geometry was used to extract the pore size distribution from the adsorption branch usingthe Micromeritics software39–42.A Horvath–Kawazoe method was used to extract the microporosity44.Mn and K contents were determined by chemical analysis using a Philips PU9400X atomic adsorption spectrometer.The average oxidation state of framework manganese in a-MnO2samples was determined by a redoxtitration method57.Electrochemistry.First,the cathode was constructed by mixing the active material (a-MnO2),Kynar2801(a copolymer based on polyvinylidenefluoride),and Super S carbon(MMM)in the weight ratio80:10:10.The mixture was cast onto Al foil (99.5%,thickness0.050mm,Advent Research Materials,Ltd)from acetone using a Doctor-Blade technique.After solvent evaporation at room temperature and heating at80°C under vacuum for8h,the cathode was assembled into cells along with a Li metal anode and electrolyte(Merck LP30,1M LiPF6in1:1v/v ethylene carbonate/dimethyl carbonate).The cells were constructed and handled in anAr-filled MBraun glovebox(O2o0.1p.p.m.,H2O o0.1p.p.m.).Electrochemical measurements were carried out at30°C using a MACCOR Series4200cycler.Catalysis.Catalytic CO oxidation was tested in a plug-flow microreactor(Alta-mira AMI200).Fifty milligrams of catalyst was loaded into a U-shaped quartz tube (4mm i.d.).After the catalyst was pretreated inflowing8%O2(balanced with He) at400°C for1h,the catalyst was then cooled down,the gas stream switched to1% CO(balanced with air)and the reaction temperature ramped using a furnace(at a rate of1°C minÀ1above ambient temperature)to record the light-off curve.The flow rate of the reactant stream was37cm3minÀ1.A portion of the product stream was extracted periodically with an automatic sampling valve and was analysed using a dual column gas 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Enhanced 2.0l m Emission and Lowered Upconversion Emission in Fluorogermanate Glass-Ceramic Containing LaF 3:Ho 3+/Yb 3+byCodoping Ce 3+IonsJia-Peng Zhang,Wei-Juan Zhang,Jian Yuan,Qi Qian,and Qin-Yuan Zhang †State Key Laboratory of Luminescence Materials and Devices,Institute of Optical Communication Materials,South ChinaUniversity of Technology,Guangzhou 510641,China Intense 2.0l m emission of Ho 3+has been achieved through Yb 3+sensitization in fluorogermanate glass-ceramic (GC)con-taining LaF 3pumped with 980nm laser diode (LD).The observation of concurrent emissions at 538,650,and 1192nm points to the additional deexcitation routes based on infrared-to-visible upconversion processes and Ho 3+:5I 6?5I 8radiative parative investigations of photoluminescent spectra and decay curves have indicated the effective role of Ce 3+ions in enhancing the 2.0l m fluorescence along with suppressing the occurrence of these concurrent emissions.This would offer a promising approach to develop compact and effi-cient 2.0-l m laser systems.I.IntroductionRECENTLY ,fiber lasers operating at ~2.0l m have beenthe subjects of intense research,due to their potential applications in the field of military,laser medical surgery,and atmospheric pollution monitoring.1–3In this respect,tri-valent rare-earth (RE)ions,Tm 3+,and Ho 3+are commonly doped as activators for their corresponding radiative transi-tions,i.e.,Tm 3+:3F 4?3H 6and Ho 3+:5I 7?5I 8lying in the neighborhood of 2.0l m.4,5Compared with Tm 3+ions,Ho 3+ions are characteristics of a broad emission band,large stimulated emission cross section,and long-lived laser upper level.6However,Ho 3+ions cannot be directly excited by commercially available high-power AlGaAs (800nm)or InGaAs (980nm)LD,due to the absence of well-matched absorption bands.To solve this problem,Tm 3+,Yb 3+,Er 3+,and Bi ions are generally codoped as promising sensi-tizers,which could initially absorb the excitation energy and further transfer it to Ho 3+ions.7–10Yb 3+ion is known as an efficient sensitizer for its broad absorption band exactly lying in the emission range of InGaAs LD.Furthermore,the simple energy level diagram with only one excited state (2F 5/2)lying at about 10000cmÀ1above the ground state (2F 7/2)would reduce the incidence of multiphonon relaxation,excited-state absorption,and direct Yb 3+–Ho 3+back transfer.11So far,oxyfluoride glass-ceramic (GC)has been considered as a prospective host matrix for RE ions not only for the low phonon energy environment provided by fluoride nanocrystals but also for the high chemical and mechanical stabilities remained in oxide glass.12,13Based on this fact,RE-doped transparent oxyfluoride GC find potential applica-tions in numerous photonic devices such as upconversionlasers,color display,sensors,and optical data storage.14It is well accepted that the low phonon energy environment can promote luminescence efficiency by reducing nonradiatively relaxation rates.Hence,the probability of upconversion in Yb 3+/Ho 3+couple can be enhanced at the same time,which may compete with 2.0-l m radiative transition.With two manifolds separated by the energy gap of about 2000cm À1,15Ce 3+ion is generally used as an effective deactivator dopant to improve the performance of Er 3+ 1.5l m emission by reducing the upconversion emission.16Basically,there exists an phonon-assisted energy-transfer process of 4I 11/2(Er 3+)+2F 5/2(Ce 3+)?4I 13/2(Er 3+)+2F 7/2(Ce 3+),which favors the population accumulation of Er 3+1.5l m emission level 4I 13/2.To some extent,the energy level diagram for Ho 3+is akin to that of Er 3+only with different energy gap between adjacent levels.In view of the comparably small energy mismatch between Ho 3+:5I 6–5I 7(~3400cm À1)and Ce 3+:2F 7/2–2F 5/2(~2000cm À1)energy gaps,the possibility of adding Ce 3+as an energy acceptor has been further explored by Tian et al.17and Tao et al.18It is confirmed that the incorporation of Ce 3+not only facilitate Yb 3+?Ho 3+energy transfer but also enhance the Ho 3+2.0l m emission in fluorophosphate glass and tellurite glass,respectively.17,18In this article,Ho 3+/Yb 3+and Ho 3+/Yb 3+/Ce 3+cod-oped fluorogermanate glass and GC containing LaF 3nano-crystals have been prepared.Effects of Ce 3+codoping on the spectral properties of Ho 3+/Yb 3+-codoped GC have been investigated and the possible energy-transfer mechanisms involved have been systematically analyzed and discussed.II.Experimental ProcedureThe oxyfluoride precursor glasses (denoted as PG x ,x =0,0.25,0.5, 1.0,and 2.0)were prepared according to the nominal molar composition of 50GeO 2–20Al 2O 3–15LaF 3–15LiF –0.25Ho 2O 3–2Yb 2O 3–x CeF 3.The starting materials are high-purity (99.99%)GeO 2,LaF 3,Ho 2O 3,Yb 2O 3,CeF 3,and analytical reagent-grade Al 2O 3and LiF.Accurately weighed batches of 15g raw materials were mixed and then melted in covered corundum crucibles at 1350°C for 1h in ambient atmosphere.The melts were then quenched on a preheated stainless steel plate and annealed at 520°C for 2h to remove residual stress.To obtain transparent GC,the precursor glass samples were cut into several pieces of the same size and sub-jected to thermal treatment at 570°C for 4h,570°C for 6h,570°C for 12h,and 580°C for 2h,respectively.Correspond-ingly,the GC samples were labeled as GC x _5704h,GC x _5706h,GC x _57012h,and GC x _5802h (x =0and 0.5).To check the composition of the glass samples,energy dis-persive spectra (EDS)were taken with a Philips XL-30FEG (Philips Electron Optics,Eindhoven,Holland)scanning elec-tron microscope equipped with an EDAX DX4i EDS (EDAX Inc.,Mahwah,NJ).The results reveal that a very limited Al 2O 3incorporation and a loss due to evaporation ofB.Dunn—contributing editorManuscript No.32868.Received March 10,2013;approved August 6,2013.†Author to whom correspondence should be addressed.e-mail:qyzhang@3836J.Am.Ceram.Soc.,96[12]3836–3841(2013)DOI:10.1111/jace.12599©2013The American Ceramic SocietyJ ournalfluorine while melting,which might be due to the low glass-melting temperature at 1350°C for 1h with the controlled prepared condition (covered corundum crucibles and a small amount of NH 4HF 2additive were used).To follow the thermal behavior of the as-prepared glass,differential scanning calorimetry (DSC)experiment was car-ried out on the glass powder in nitrogen atmosphere at a heating rate of 10K/min.According to the DSC curve shown in Fig.1,the glass-transition (T g )and crystallization (T c )temperatures are determined to be 565°C and 667°C,respectively.To characterize the GC in terms of phase identi-fication and microstructures observation,X-ray powder dif-fractometer (XRD)and transmission electron microscope (TEM)measurements were performed using XRD (Philips PW1830,Cu K a )and TEM (JEM-2010,Tokyo,Japan).The Raman spectra were recorded by a Raman spectrometer (HORIBA Jobin-Yvon Inc.,Paris,France)in the region 100–1250cm À1.The glass sample was excited with an argon ion laser at 532nm.The absorption spectra were measured on a Perkin-Elmer Lambada 900UV/VIS/NIR spectropho-tometer (Waltham,MA)in the spectral range 320–3000nm with the resolution of 1nm.Emission spectra were taken on a computer-controlled Triaxial 320spectroflourimeter (Jobin-Yvon Inc.)upon excitation with 980nm LD,and the signal was detected with a R928photomultiplier tube (Products for research Inc.,Danvers,MA)for visible emission,InGaAs (800–1650nm)detector for 1.2l m emission and PbSe detec-tor assembled with a Standford SR 510lock-in amplifier (Stanford Research Systems,Sunnyvale,CA)for 2.0l m emission.The decay curves of 1.2l m emission (Ho 3+:5I 6level)were recorded on a high-resolution spectrophotometer (Edinburgh FLSP920;Edinburgh instrument Ltd,Livingston,UK)equipped with a microsecond pulse Xenon (Xe)lamp as the excitation source.While,the decay curves of 2.0l m emission (Ho 3+:5I 7level)were measured by recording the output of the InSb detector with an oscilloscope modulating the 980nm LD excitation sources.All the measurements were performed at room temperature.III.Results and DiscussionFigure 2(a)shows the XRD patterns of PG0and GC0_5802h samples.As can be seen from the figure,the pre-cursor glass is completely amorphous without any sharp peaks in the pattern,while after heat treatment at 580°C for 2h,the XRD pattern exhibits several intense diffraction peaks indexed to the hexagonal LaF 3phase (JCPDS card no.01-076-0510),indicating the successful precipitation of LaF 3nanocrystals among glass matrix.As exhibited in the inset of Fig.2(a),the obtained GC samples keep good transparency.TEM bright field image along with the selected-area electron-diffraction pattern shown in Fig.2(b)reveals the composite structure of GC with irregular 10-to 20-nm-sized nanocrys-tals distributing in the glassy matrix.The high-resolutionTEM image in Fig.2(c)shows the detailed lattice structure of an individual LaF 3nanocrystal.It can be seen that single particle exhibits crystalline nature with the d-spacing value of3.24 A,in good agreement with the parameter of LaF 3crys-tals in [101]direction.Figure 3shows the absorption spectra of PG0,PG0.5,and GC0samples in the wavelength region 320–3000nm.The absorption bands corresponding to transitions from Yb 3+:2F 7/2ground state to 2F 5/2state and from Ho 3+:5I 8ground state to the respective excited states:5I 7,5I 6,5F 5,(5F 4,5S 2),5F 3,(5F 1,5G 6),and 5G 5were labeled in Fig.3.It is noted that all the absorption transitions exhibit little differ-ence between PG0and GC0samples,except the red shift of ultraviolet (UV)absorption edge for GC0caused bytheFig.1.DSC curve of the blank glass with the composition of 50GeO 2–20Al 2O 3–15LaF 3–15LiF.(a)(b)(c)Fig.2.(a)XRD patterns of the PG0and GC0_5802h samples.The inset shows the optical images of PG0and GC0samples over a printed paper sheet.(b)A typical transmission electron microscope (TEM)bright field image of GC0_5802h.The inset shows the corresponding selected-area electron-diffraction pattern of the GC0_5802h.(c)High-resolution TEM image of a LaF 3nanocrystal.Fig.3.Absorption spectra of PG0,GC0samples doped with 0.25mol%Ho 2O 3,2mol%Yb 2O 3and PG0.5doped with 0.25mol %Ho 2O 3,2mol%Yb 2O 3and 0.5mol%CeF 3.The inset is the transmission spectra.December 2013Fluorogermanate Glass-Ceramic3837Rayleigh scattering.19,20In comparison with the PG0sample,the red shift of UV-side absorption edge shown in PG0.5sample is associated with the interconfigurational transition of Ce 3+ions,i.e.,4f 1:2F 5/2?4f 0,5d 1.21As shown in the inset of Fig.3,the GC samples keep high transmittance in the 2.0l m region,which is important for the application of 2.0l m lasers.Figure 4compares fluorescence spectra of Ho 3+ions in PG0,GC0_5704h,GC0_5706h,and GC0_57012h samples upon the excitation of 980nm LD.As shown in Fig.4,the spectra are characterized by two emission bands at 1.2and 2.0l m,corresponding to the Ho 3+:5I 6?5I 8and 5I 7?5I 8transitions,respectively.As Ho 3+ions have no absorption band at around 980nm,the presence of 1.2and 2.0l m fluo-rescence signals demonstrate the occurrence of energy trans-fer from Yb 3+to Ho 3+.The possible energy-transfer mechanism for Ho 3+/Yb 3+-codoped samples has been depicted in the simplified energy level diagram (Fig.5).Upon excitation of 980nm LD,Yb 3+ions are initially excited from the ground state (2F 7/2)to the excited state (2F 5/2).Then,Yb 3+ions in the 2F 5/2level transfer their energy to the adjacent Ho 3+ions by a phonon-assisted energy-transfer process (ET1),exciting Ho 3+from the 5I 8ground state to 5I 6excited state.22Ho 3+ions on the 5I 6level will decay radi-atively to the ground state by emitting 1.2l m photons or decay nonradiatively to the 5I 7level where energy is releasedin forms of 2.0l m fluorescence.It is worthwhile to mention that the emission intensity at 1.2and 2.0l m both increases with the increasing heat-treatment time.This may be closely associated with the gradual precipitation of LaF 3nanocrys-tals in glass with the prolongation of heat-treated time.As for the volume fraction of crystalline phase in glass-ceramic,one can roughly evaluate the value from the ratio of the inte-grated area of diffraction peaks over the total area under the XRD pattern.9Thus,the volume fraction of crystalline in GC0_5704h,GC0_5706h,and GC0_57012h was estimated to be about 6.26%,9.15%,and 15.88%,respectively.This means that there is an increasing possibility for RE ions to be incorporated into LaF 3nanocrystals with the increase in heat-treatment time.The observed luminescence intensifica-tion could be associated with the reduced nonradiative loss of RE ions within such crystal-like environment with low phonon energy.For a rigorous investigation,the Raman spectra and 1.2l m (Ho 3+:5I 6)decay curves of precursor glass and glass-ceramics were measured.As shown in Fig.6(a),there exist two broad bands in the region 100–1250cm À1,similar to other oxyfluoride glass system.23Interestingly,there arise on the broad band (200–700cm À1)one sharp peak at 459cm À1and three weak shoulders centered around 224,285,361cm À1for GC samples.The bands around 224,285,361,and 459cm À1can be ascribed to the lattice vibration of LaF 3.24,25Therefore,the precipitation of LaF 3crystallites is further confirmed by Raman spectroscopy.Moreover,we can recognize that the maximum phonon energy of PG and GC samples is about 884and 842cm À1,respectively.It should be noted that the phonon energy of the area where the crystallites participated should be much lower than 842cm À1.Figure 6(b)exemplarily shows the decay curve of Ho 3+:5I 6level in PG0monitoring at 1192nm,which can be well fitted to double exponential function:I ¼A 1exp ðÀt =s 1ÞþA 2exp =ðÀt =s 2Þ(1)where I is the luminescence intensity monitored at the maxi-mum emission wavelength,A 1and A 2are constants,t is the time,s 1and s 2are rapid and slow lifetimes for exponential ing these parameters,the average decay time s can be determined by the formula:s ¼A 1s 12þA 2s 22ÀÁ=A 1s 1þA 2s 2ðÞ(2)It should be mentioned that 1.2l m emission in the corre-sponding GC samples also exhibit a double exponential decay and the fitting finally gives the average decay times depicted in the inset of Fig.6(b).Obviously,the lifetime of(a)(b)Fig.4.Emission spectra of the investigated PG0and GC0samples doped with 0.25mol%Ho 2O 3,2mol%Yb 2O 3in the (a)1.2l m and (b)2.0l m region excited with 980nm laserdiode.Fig.5.The Ho 3+–Yb 3+–Ce 3+ions energy level diagram and energy-transfer processes excited by the 980nm laser diode.(a)(b)Fig.6.(a)The Raman spectra of PG0and GC0samples,(b)the decay curve monitored at 1192nm and the double exponential fitting curve.The inset of (b)shows the dependence of Ho 3+:5I 6lifetime on the heat-treated time.3838Journal of the American Ceramic Society—Zhang et al.Vol.96,No.125I 6state shows a monotonous increase with the prolongation of heat-treatment time.Generally,the decay rate for an excited state derived from the measured decay lifetime s m is composed of two terms:the intrinsic radiative decay rate (s R )and the nonradiative decay rate (s NR ),primarily due to mul-tiphonon deexcitation of the excited state.In the absence of concentration quenching effects,they are related according tothe formula:1s m ¼1s Rþ1s NR .26As the multiphonon relaxation rate exhibits a great dependence on the phonon energy of the host matrix,the prolonged lifetime of 1.2l m emission could be related to the reduced multiphonon relaxation rates as a result of the incorporation of RE ions into the low phonon energy environment provided by LaF 3nanocrystals in GC samples.27It should be pointed out that the decay time of 5I 7state obtained after fitting 2l m luminescence decay curve to a single exponential function is relatively less sensitive to the heat-treatment time.There is a slight increase in the lifetime from 6.22ms up to 7.26ms after prolonging the heat-treat-ment time of PG0sample even up till 12h.This could be attributed to the fact that the energy gap between 5I 7and 5I 8(~5000cm À1)is much larger than the counterpart (~3500cm À1)between 5I 6and 5I 7.Furthermore,the upconversion luminescence of Yb 3+/Ho 3+couple in the PG and GC0_5704h samples were pared with the PG sample,the emission inten-sity in GC0_5704h increases greatly and for further analysis,the upconversion luminescence were normalized at 650nm (Fig.7).As illustrated in Fig.7,three upconversion emission bands centered at 538,650,and 745nm were detected upon 980nm excitation,corresponding to Ho 3+:5S 2,5F 4?5I 8,5F 5?5I 8,and 5S 2,5F 4?5I 7radiative transitions,respec-tively.28Moreover,there appears a 478nm (Ho 3+:5F 2,3?5I 8)blue upconversion emission band in GC0_5704h which cannot be observed in the PG sample.In addition,as clearly seen in the Fig.7,the ratio of I 538nm /I 650nm and I 745nm /I 650nm in GC0_5704h is greater than the counterpart of PG sam-ple.The relevant possible upconversion emission mechanism can be proposed with the help of energy level schemes shown in Fig.5.22,29As aforementioned,the successive energy trans-fer from Yb 3+to Ho 3+ions can populate the 5I 6and 5I 7level of Ho 3+ions.One part of Ho 3+ions on 5I 6level can be further excited to 5S 2,5F 4level by way of energy transfer (ET1)and excited state absorption (ESA)processes.Ions on 5S 2,5F 4level will decay nonradiatively to 5F 5level or decay radiatively by generating 538nm green emission and 745nm emission.Moreover,ions on 5S 2/5F 4level can be excited to the 3H 5,6level by ESA process followed by nonradiative decay to 5F 2,3level,where electrons can experience radiative transition to the ground state,yielding 478nm photons.Sim-ilarly,the ET1and ESA processes can promote the Ho 3+ions from the 5I 7level to the 5F 5level where the 650nm red upconversion emission was produced.The upconversion mechanism indicates that the 538and 745nm emission inten-sity relies on the population of 5S 2,5F 4state,whereas the 650nm emission is sensitive to the population of 5F 5state.In GC samples,a part of RE ions are readily coupled into LaF 3nanocrystals with phonon energy much lower than the O –Ge and O –Al bands,12,30resulting in the lowed nonradia-tive decay rate through multiphonon relaxation and the strong upconversion emission in GC samples.Moreover,the lowed nonradiative decay rate means that the nonradiative processes 5S 2,5F 4?5F 5,and 5I 6?5I 7were suppressed.After ET1and ESA energy-transfer process,more electrons are populated on the 5S 2,5F 4state rather than on 5F 5state,which accounts for the result that the ratio of I 538nm /I 650nm and I 745nm /I 650nm in GC0_5704h is greater than the counter-part of PG sample.In addition,the appearance of 478nm in GC can also be ascribed to the lowed nonradiative decay rate.To some extent,however,the reduced nonradiative rate leads to the depopulation of 5I 7level,which prohibits further enhancement of 2.0l m emission.Hence,Ce 3+was intro-duced in the following with the expectation to suppress the energy loss processes.Figure 8illustrates the emission spectra of GC0_5704h and GC0.5_5704h in the 1.2l m [Fig.8(a)]and 2.0l m [Fig.8(b)]region upon 980nm excitation.It is found that the introduc-tion of Ce 3+ions induces an obvious enhancement in 2.0l m emission and reduction in the 1.2l m emission.As there is an approximation in energy between Ho 3+:5I 6?5I 7and Ce 3+:2F 5/2?2F 7/2transitions,the ET2process of 2F 5/2(Ce3+)+5I 6(Ho 3+)?2F 7/2(Ce 3+)+5I 7(Ho 3+)(Fig.5)can take place via phonon-assisted energy-transfer process.16,31It is worth noting that the ET2process can transfer populations from the 5I 6state to 5I 7state,thus promoting the 2.0l m emission at the cost of 1.2l m fluorescence.Based on the pho-non sideband theory minutely discussed in Refs.[11,17],the energy-transfer coefficients from Yb 3+to Ho 3+in GC0_5704h and GC0.5_5704h were calculated and listed in Table I.It is noted that the energy transfer between Yb 3+and Ho 3+is phonon-assisted process.After the addition of Ce 3+ions,the calculated energy-transfer coefficient C Yb –Ho increases from 1.58910À40to 3.62910À40cm 6/s,which indicates the more efficient energy transfer of Yb 3+?Ho 3+in Yb 3+/Ho 3+/Ce 3+triply doped systems.This is similar to the result reported in Ref.[17].Figure 9shows the decay curves of the 1.2l m (Ho 3+:5I 6)and 2.0l m (Ho 3+:5I 7)emission in PG samples.For 1.2l m emission,the decay curves can be well fitted to adoubleFig.7.Upconversion spectra of PG0and GC0_5704h samples doped with 0.25mol%Ho 2O 3,2mol%Yb 2O 3.(a)(b)parison of emission spectra of GC0_5704h and GC0.5_5704h in the (a) 1.2l m and (b) 2.0l m region excited by 980nm laser diode.December 2013Fluorogermanate Glass-Ceramic 3839exponential function.The average decay time s estimated using Eq.(2)is illustrated in the inset of Fig.9as a function of Ce 3+concentration.While the decay curves of 2.0l m emission fit well to a single exponential function.As shown in the inset of Fig.9,there is a drastic decrease in 5I 6lifetime with the addition of Ce 3+ions at low doping level,whereas the 5I 7lifetime experiences very little change.Further increase in Ce 3+concentration over 1mol%causes a signifi-cant decrease in lifetimes of both 5I 6and 5I 7levels.It is found that the introduction of Ce 3+ions induces an obvi-ous enhancement in 2.0l m emission and reduction in the 1.2l m emission.As there is an approximation in energy between Ho 3+:5I 6?5I 7and Ce 3+:2F 5/2?2F 7/2transi-tions,the ET2process of 2F 5/2(Ce 3+)+5I 6(Ho 3+)?2F 7/2(Ce3+)+5I 7(Ho 3+)can take place via phonon-assisted energy-transfer process as shown in Fig.5.It is worth noting that the ET2process can transfer populations from the 5I 6state to 5I 7state,thus promoting the 2.0l m emission at the cost of 1.2l m fluorescence.Table II presents the emission cross section (Ho 3+:5I 7?5I 8)calculated using Fuchtbauer –Ladenburg equation,32the calculated spontaneous radiative transition probabilities A rad (Ho 3+:5I 7)and the measured lifetime s m (Ho 3+:5I 7)of the PG samples with different Ce 3+concentration.As can be seen from the table,the r emis and A rad increase slightly with the increase in Ce 3+concentration.The A rad of this glass system is larger than that of fluorophosphates glass (80.56s À1)17but is smaller than that of tellurite glass (136.00s À1).18It’s worth noting that the quantum efficiency g (s m 9A rad )is compara-ble to the fluorophosphate (FP)glass.17The influence of Ce 3+on upconversion emissions of Ho 3+in PG x (x =0,0.25,0.5, 1.0,and 2.0)samples is exhibited in Fig.10.The upconversion emissions decrease with the increase ini Ce 3+concentration.As shown in the inset of Fig.10,the intensity ratio I 538nm /I 650nm almost fol-lows a liner decrease with the increment of Ce 3+concentra-tion.The variation in I 538nm /I 650nm is closely related totheFig.9.The decay curves and the corresponding fitting curves of 1.2l m (Ho 3+:5I 6)and 2.0l m (Ho 3+:5I 7)emission for PG samples.The inset shows the dependence of lifetime on the Ce 3+concentration.Table I.Calculated Microscopic Parameters of the ET Processes in GC0_5704h and GC0.5_5704hEnergy transfer Samples N (N phonons-assist process)C D –A 10À40(cm 6/s)C D –D 10À40(cm 6/s)012Yb ?Yb (2F 5/2,2F 7/2?2F 7/2,2F 5/2)GC0_5704h 95.89% 4.11%––62.54GC0.5_5704h 97.29% 2.71%––59.74Yb ?Ho (2F 5/2?5I 6)GC0_5704h 081.19%18.81% 1.58–GC0.5_5704h 061.87%38.13% 3.62–Table II.r emis ,A rad ,s m ,g of Ho 3+:5I 7in Various Glass SystemSamplesr emis 910À21(cm 2)A rad (s À1)s m (ms)g (%)ReferencesPG0 4.75103.84 6.2264.59Present workPG0.25 4.80105.64 6.0964.34PG0.5 4.94109.02 5.8964.21PG1.0 5.03111.89 4.9755.61PG2.0 5.06114.12 4.2848.84FPC0 4.6673.540.948 6.9717FPC10 4.7680.560.9127.3517FP 7.9090.42 5.6050.6417Silicate 7.0061.650.32 1.9717Fluoride 5.3058.0726.70155.0017Germanate –111.58––33TNZ glass –136.00%1.60%21.7618Fig.10.Upconversion spectra of PG x (x =0,0.25,0.5,1.0,2.0)under 980nm laser diode excitation.The inset shows the intensity ratio of I 538nm /I 650nm as a function of Ce 3+ions.3840Journal of the American Ceramic Society—Zhang et al.Vol.96,No.12phonon-assisted ET2process between Ho3+and Ce3+.As discussed beforehand,the phonon-assisted ET2process can transfer population from the intermediate green-emitting state(5I6)to the intermediate red-emitting state(5I7).In that way,there will be more electrons accumulated on5F5state rather than5S2,5F4state,directly leading to the red (650nm)emission intensification at the cost of green (538nm)emission.IV.ConclusionIn summary,efficient1.2and2.0l m infrared emissions as well as typical upconversionfluorescence of Ho3+at538and 650nm have been achieved influorogermanate glass and GC through Yb3+sensitization.The enhanced luminescence and lengthened lifetime for5I6state after crystallization of the precursor glass can be ascribed to the reduced nonradiative rate benefited from the low phonon energy environment for some RE ions incorporated into LaF3nanocrystals.To impede the energy loss processes such as upconversion lumi-nescence and/or1.2l m emission,Ce3+was introduced and verified to be effective in enhancing the2.0l mfluorescence and suppressing the concurrent emissions.AcknowledgmentsThis work was supported by the National Science Foundation of China(grant nos.51125005and U0934001)and the Chinese Ministry of Education(grant no.20100172110012).References1S.W.Henderson, C.P.Hale,J.R.Magee,M.J.Kavaya,and A.V.Huffaker,“Eye-Safe Coherent Laser Radar System at2.1l m Using Tm, Ho:YAG Lasers,”Opt.Lett.,16[10]773–5(1991).2B.Richards,S.Shen,A.Jha,Y.Tsang,and D.Binks,“Infrared Emission and Energy Transfer in Tm3+,Tm3+-Ho3+and Tm3+-Yb3+-Doped Tellurite Fibre,”Opt.Express,15[11]6546–51(2007).3Q.Wang,J.Geng,T.Luo,and S.Jiang,“Mode-Locked2l m Laser with Highly Thulium-Doped Silicate Fiber,”Opt.Lett.,34[23]3616–8(2009).4V.A.Mikhailov,Y.D.Zavartsev,A.I.Zagumennyi,V.G.Ostroumov, 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第52卷 第1期2022年 2月电 池BATTERY BIMONTHLYVol.52,No.1Feb.,2022
作者简介:戴丽静(1991-),女,山西人,大连交通大学辽宁省无机超细粉体制备及应用重点实验室博士生,研究方向:无机粉体材料;王 晶(1967-),女,辽宁人,大连交通大学辽宁省无机超细粉体制备及应用重点实验室教授,博士,研究方向:无机粉体材料,通信作者;史忠祥(1988-),男,辽宁人,大连交通大学辽宁省无机超细粉体制备及应用重点实验室实验师,博士,研究方向:无机粉体材料;于丽娜(1996-),女,内蒙古人,大连交通大学辽宁省无机超细粉体制备及应用重点实验室博士生,研究方向:无机粉体材料。基金项目:国家重点研发计划(2017YFB0310300),辽宁省教育厅资助项目(JDL2016002)
·科研论文·DOI:10.19535/j.1001-1579.2022.01.003
溶胶-凝胶制备工艺对Li1.3Al0.3Ti1.7(PO4)3的影响戴丽静,王 晶∗,史忠祥,于丽娜
(大连交通大学辽宁省无机超细粉体制备及应用重点实验室,辽宁大连 116028)
摘要:为研究溶胶-凝胶法制备工艺对Li1.3Al0.3Ti1.7(PO4)3
(LATP)固态电解质的影响,利用XRD、SEM、电化学阻抗谱(EIS)
和热重(TG)-示差扫描量热(DSC)等方法,分析LATP的物相组成、形貌及导电性能。当合成体系的pH=7.0并添加20%过量的锂盐时,可制备出纯相LATP;前驱体经750℃煅烧后再烧结,有利于LATP的致密化。在800℃下低温烧结制备的
LATP具有最佳的电导率,为1.1×10-4S/cm。
关键词:溶胶-凝胶法; Li1.3Al0.3Ti1.7(PO4)3
(LATP); 煅烧温度; 烧结温度
中图分类号:TM912.9 文献标志码:A 文章编号:1001-1579(2022)01-0008-04
Influenceofsol-gelpreparationprocessonLi1.3Al0.3Ti1.7(PO4)3
Vol.42 2021年3月No.3767~775 CHEMICAL JOURNAL OF CHINESE UNIVERSITIES高等学校化学学报光激发金属配位四苯基卟啉瞬态吸收和衰减动力学性质研究马子辉2，王梦妍2，曹洪玉1，唐乾1，王立皓2，郑学仿1（1.大连大学生命科学与技术学院,2.环境与化学工程学院,大连116622）摘要以四苯基卟啉为实验模板,结合稳态吸收光谱、荧光光谱、瞬态吸收光谱、动力学数据及理论计算结果研究了光激发4种金属配位卟啉的光谱性质.光激发后,四苯基卟啉化合物TPP-2H,TPP-Zn和TPP-Mg稳态吸收光谱Soret带谱峰强度均明显降低,TPP-Ni吸收强度由0.3a.u.增至1.3a.u.,TPP-FeCl谱峰变化较小.TPP-2H和镁、锌配位卟啉的瞬态吸收光谱Soret带出现明显负峰,激光激发后其瞬态中间体的消光系数(εt)小于基态的消光系数(εG),ΔOD值为负值;3种卟啉正负峰微秒级衰减动力学过程表明,光激发后分子产生较为稳定的中间态,有利于光电转换或光反应.实验和理论研究表明,金属卟啉光学性质差异由金属配位空轨道和电子排布引起.以上卟啉光学性质可协助理解光合作用过程,并为选择光电转换新型卟啉材料的配位金属提供实验支持.关键词金属卟啉；光激发；瞬态吸收光谱；衰减动力学；电子转移中图分类号O644文献标志码A叶绿素和血红素（铁卟啉）等金属卟啉化合物对生命体内光合作用、输氧、储氧及电子传递等过程起着至关重要的作用［1］.光合作用中叶绿素是光能转换的反应中心，血红素类蛋白能够在光诱导下发生氧化还原反应［2］.Sakai等［3］发现365nm氙灯光能诱导高铁血红蛋白（metHb）还原生成碳氧血红蛋白（HbCO），推测电子可能是从卟啉环转移至卟啉环中心铁；Gu等［4］发现醇的加入有利于血红素类蛋白的光还原.紫外光或可见光激发色氨酸对其它生物分子会有电子转移反应影响［5］，实验表明游离色氨酸受光激发后可以发生能量转移到铁卟啉中，引起蛋白光谱变化［6］.以上研究表明光对金属卟啉均有明显的反应重要引发作用，其中确定光反应中间体是理解反应的关键.光激发金属卟啉至激发态是光合作用反应的第一步.过渡金属配位卟啉分子具有大共轭结构，在吸收光后其激发态具有复杂性和多样性，从而具备优异的光化学和光物理性质［7］，因此在金属有机骨架（MOFs）材料［8］、染料太阳能电池、光动力学治疗光敏剂、光催化及光能转化材料科学等领域引起广泛关注.Steven等［9］在其实验条件下表征了四苯基卟啉的时间分辨共振拉曼光谱，通过同位素位移技术与各谱带的偏振系数，准确地归属了四苯基卟啉的基态和激发态在共振拉曼光谱上的信号.Seiji等［10］根据卟啉的激发态弛豫过程推测出卟吩激发态的衰变过程经历B带内转换到Q y带和Q x带；Q x激发态与溶剂发生能量交换振动弛豫后再经过衰变12ns返回基态，并根据卟啉的激光激发后的短时激发性质对光谱进行归属.吴骊珠等［11］以四苯基卟啉铂为光敏剂构建二氧化硅纳米颗粒的三重态-三重态湮灭上转换体系，光敏剂所占比例小（1∶40），效率较高，光敏剂吸收低能量的光子跃迁至其单重激发态并通过系间窜越过程到达其三重激发态，随后处于三重激发态的光敏剂通过能量传递将其三重态能量传递给发光体分子.血红素和叶绿素分别以铁和镁为金属活性中心，卟啉光敏剂的研究倾向于采用锌或镍等过渡态金doi：10.7503/cjcu20200736收稿日期：2020-10-08.网络出版日期：2020-12-30.基金项目：国家自然科学基金(批准号:21601025,21571025,21601024,21506018)、大连市高层次创新人才项目(批准号:2017Q156)和大连大学博士启动项目(2019)资助.联系人简介：曹洪玉,男,博士,副教授,主要从事光激发生物大分子光谱学性质及机理研究.E-mail:*****************.cn 郑学仿,男,博士,教授,主要从事生物无机化学研究.E-mail:*****************768Vol.42高等学校化学学报属，选择原因均与金属配位性质相关，但尚缺乏光激发实验和理论数据；光激发金属卟啉氧化还原反应中长时中间态是反应基础，相关实验报道较少，致使其机理也尚缺乏解析.叶绿素和铁卟啉侧链过长，不利于金属卟啉核心部位的光学性质研究，本文以四苯基卟啉为模板分子，采用稳态吸收光谱和激光闪光光解方法，以间-四苯基卟吩（TPP-2H），间-四苯基卟啉氯化铁（Ⅲ）（TPP-FeCl），5，10，15，20-四苯基卟啉-21H，23H-卟吩镍（Ⅱ）（TPP-Ni），5，10，15，20-四苯基卟啉-21H，23H-卟吩镁（TPP-Mg）和5，10，15，20-四苯基卟啉-21H，23H-卟吩锌（TPP-Zn）5个化合物为模型研究光激发后卟啉衍生物的光学性质，归属谱峰，分析5种卟啉化合物在光激发前后的特点与差异机理，发现并理解金属配位对卟啉光学性质的影响，进而协助理解光激发的光合作用或血红素氧化还原反应.1实验部分1.1试剂与仪器TPP-FeCl（分析纯）购自上海阿法埃莎（中国）化学有限公司；TPP-Ni、TPP-Mg、TPP-2H（纯度＞99%，TPP-2H，无氯）和二甲基亚砜（纯度＞99.9%，DMSO）均为分析纯，购自上海阿拉丁生化科技股份有限公司；TPP-Zn（纯度95%）购自北京百灵威科技有限公司.经紫外-可见光谱检测TPP-FeCl中均是三价铁.Jasco-V-560型紫外-可见分光光度计和FP6500型荧光分光光度计（日本分光株式会社）；LP980型激光闪光光解仪（英国爱丁堡仪器公司）.1.2实验方法紫外-可见光谱和荧光光谱测定：将5×10−6mol/L TPP-FeCl，TPP-Mg，TPP-Ni，TPP-Zn和TPP-2H的DMSO溶液分别置于比色皿中，采用紫外-可见分光光度计测量样品激光照射前后300~700nm的紫外-可见吸收光谱，狭缝宽度2nm，扫描速度400nm/min；使用荧光分光光度计测定激发波长为355nm的发射光谱，测定范围380~420nm，激发光和发射光狭缝宽度均为3nm.激发态瞬态光谱和动力学测定：将5×10−6mol/L TPP-FeCl，TPP-Mg，TPP-Ni，TPP-Zn和TPP-2H的DMSO溶液分别置于石英比色皿中，经过固定波长（355nm）的激光发射器激发，测量其瞬态吸收的变化及衰减曲线.采用激光闪光光解仪测量，激光模式1Hz；检测时间范围为4000ns；每次测量2次激光脉冲.检测器狭缝宽度为2nm；检测波长范围360~750nm.在获取激光瞬态吸收光谱之后，进行谱图分析并将捕捉到的谱图进行截取，截取0~2000ns数据，谱线平均宽度为10ns.其它参数设为默认数值.ΔOD=(εt-εG)cd(1)式中：ΔOD（a.u.）为激发态和基态的光密度差值；εt和εG（L·mol−1·cm−1）分别为瞬态中间体和基态的消光系数；c（mol/L）为基态分子转移到瞬态中间体的浓度，d（cm）为探测光束的有效光程长度［12］.激光闪光光解检测的ΔOD（a.u.）值是时间（t）和波长（λ）两个变量的函数：ΔOD(t,λ)=εΤ(λ)cΤ(t)-εG(λ)[cΤ(t)+c s(t)]d-ϕ(λ)c s(t)(2)式中：εT（L·mol−1·cm−1）为激发态的消光系数；c T（mol/L）为三重激发态的浓度；c s（mol/L）为单重激发态的浓度；ϕ（a.u.）为样品的荧光光谱强度.测定过程中采用定时或定波长转为单变量函数.激发态衰减动力学拟合曲线采用下式：R(t)=A+ΣBe(-t/τi)(3)i式中：B i和τi分别为指前因子和特征寿命；A为附加背景.R（t）通常被称为样品衰减模型，其是样品对无限短激发响应的理论表达式，实验以ΔOD值作为R（t）值进行动力学分析，拟合曲线采用卡方检验方法，在最低χ2值时确定最佳拟合曲线各参数.1.3理论计算TPP-2H和金属配位卟啉结构的理论计算采用密度泛函理论在Gaussian09程序包［13］中完成，几何构型均采用已经证实卟啉类分子计算与实验结果吻合的B3LYP/lanl2dz基组水平优化［14，15］，采用开壳层No.3马子辉等：光激发金属配位四苯基卟啉瞬态吸收和衰减动力学性质研究UB3LYP/lanl2dz 基组对TPP -FeCl 进行优化，频率分析无虚频，表明优化的结果是稳定的构型；采用含时密度泛函理论在B3LYP/lanl2dz 基组下进行激发态计算，TPP -FeCl 采用开壳层UB3LYP/lanl2dz 基组水平计算，激发态数量设置为240.基态和激发态计算结果利用Gausssum3.0软件［16］对前线轨道和电子吸收光谱进行分析.2结果与讨论2.1金属卟啉激发前后稳态吸收光谱5种卟啉化合物TPP -2H ，TPP -FeCl ，TPP -Ni ，TPP -Mg 和TPP -Zn 的差异仅在于中心原子不同（图1），但卟啉化合物的稳态紫外吸收谱图差异较大［图2（A ）~（E ）］.光激发前，TPP -2H 紫外吸收谱中Soret 带有一个较强吸收峰，Q 带有4个弱吸收峰，金属卟啉TPP -FeCl ，TPP -Ni ，TPP -Mg 和TPP -Zn 的Soret 带最大吸收峰分别位于414，416，426和428nm 处［图2（B ）~（E ）］.光激发次数较少时，各卟啉化合物稳态光谱性质稳定，紫外光谱基本不变.在受多次355nm 激光激发后，不同卟啉的光谱变化趋势和程度各有差异.经过激光照射，不含金属配体的TPP -2H 在Soret 带418nm 处最大吸收峰在光激发后明显降低，其降低程度与光激发TPP -Zn 后在428nm 处最大吸收峰降低程度相同，说明这2种卟啉化合物受光激发后其共轭结构发生变化.TPP -Ni 经光激发后，Soret 带和Q 带吸收强度均出现大幅上升，Soret 带吸光度由0.3a.u.升至1.3a.u.；Q 带吸光度由0.04a.u.上升到0.11a.u.，且峰位置没有发生变化.光激发后TPP -2H ，TPP -Zn 和TPP -Mg 的Q 带峰位置未发生变化但强度均明显降低，表明多次激光照射后原分子减少.TPP -FeCl 的紫外吸收光谱Soret 带和Q 带谱峰Fig.1Structures of tetraphenylporphyrin and metal⁃coordinatedtetraphenylporphyrinsFig.2UV⁃Vis spectra of TPP⁃2H(A),TPP⁃FeCl(B),TPP⁃Ni(C),TPP⁃Mg(D),TPP⁃Zn(E)before and after355nm laser excitation and fluorescence spectra with 355nm light excitation(F)769Vol.42高等学校化学学报强度降低均不明显，表明Fe 与卟啉环的配位较为稳定，且不易受到激光激发影响.以上化合物光激发后长时间（＞5min ）放置，其稳态光谱均不能恢复至激光照射前，说明光激发可使卟啉化合物的结构发生改变.2.2金属卟啉光激发瞬态吸收光谱TPP -2H ，TPP -FeCl ，TPP -Ni ，TPP -Mg 和TPP -Zn 的瞬态动力学吸收有显著差异（图3）.5种卟啉化合物在396nm 处均有瞬态吸收光谱负峰.根据5种卟啉化合物在激发波长为355nm 时的发射光谱，396nm 处的谱峰为卟啉的荧光峰［图2（F ）］，根据稳态荧光和式（2），此负峰归属为卟啉荧光项［ϕ(λ)c s (t )］.不同分子396nm 处的谱峰强度差异很大，其中TPP -FeCl 最强，其ΔOD 值达到−0.038a.u.；TPP -2H 的396nm 谱峰ΔOD 值为−0.018a.u.；TPP -Ni ，TPP -Mg 和TPP -Zn 的谱峰强度均小于−0.001a.u.，以上差异是受环内离子对TPP 三重激发态稳定性影响.TPP -FeCl *的瞬态吸收光谱中Soret 带的ΔOD 为0，即无明显谱峰，说明激光激发过程中，TPP -FeCl 未产生中间态.TPP -2H ，TPP -Ni ，TPP -Mg 和TPP -Zn 的稳态紫外-可见吸收峰中Soret 带谱峰位置在瞬态吸收光谱中也均出现明显负峰，根据式（1）可知，激光激发后中间体的εt 明显小于基态的εG ，导致ΔOD值为负值.瞬态吸收光谱中，Soret 带负吸收峰在不同卟啉中明显不同，TPP -2H *和TPP -Zn *分别在418和428nm 的负峰较为显著，TPP -Ni *和TPP -Mg *分别在416和426nm 的Soret 带负峰较弱（图3）.Soret 带是分子被激发至第二激发单重态S 2（0-0）跃迁的吸收峰，卟啉环中心原子或金属影响分子的电子排布和共轭结构［17］，光谱表明氢和锌对光激发后分子内电子跃迁至S 2变化影响较明显，ΔOD 值变化较大.在TPP -2H *，TPP -Mg *和TPP -Zn *的瞬态吸收光谱中出现了3个正的弱峰，分别位于448，452和450nm ，与其各自的Soret 带强负吸收峰位置接近，其中正吸收峰归属为激发态吸收εt 大于基态吸收εG ，说明3种卟啉均产生了三重激发态中间体.吸收谱带可表明激发态特征［18］.Q 带吸收是最低能级单重激发态S 1的电子态，TPP -2H ，TPP -Mg 和TPP -Zn 在稳态吸收谱相应具有Q 带吸收峰位置处均未发现瞬态吸收谱峰，表明基态和激发态的Q 带吸收强度无差别，ΔOD 值无变化，即S 0→S 1的跃迁未改变；TPP -Ni 在524nm 处有明显负峰，表明激光激发后中间态改变了最低能级跃迁，中间体的εt 明显小于基态的εG ，导致ΔOD 值为负值.TPP -2H *，TPP -Ni *，TPP -Mg *和TPP -Zn *的瞬态光谱中分别在650nm/718nm ，654nm/716nm ，608nm/664nm 和624nm/664nm 处有负峰，谱峰信号均呈现强度大、宽度大的特性，根据式（2），这些负峰信号为基态漂Fig.3Transient absorption spectral maps of TPP⁃2H(A),TPP⁃FeCl(B),TPP⁃Ni(C),TPP⁃Zn(D)and TPP⁃Mg(E)photoexcited by 355nm laser pulse770No.3马子辉等：光激发金属配位四苯基卟啉瞬态吸收和衰减动力学性质研究白峰，归属为基态吸收εG （λ）值.TPP -FeCl *的瞬态吸收光谱中Soret 带和Q 带无明显谱峰，这一显著差别表明Fe 电子排布的独特性致使卟啉瞬时激发态中间体异于其它金属配位卟啉.2.3金属卟啉激发态瞬态动力学对TPP -2H ，TPP -FeCl ，TPP -Ni ，TPP -Mg 和TPP -Zn 的瞬态吸收光谱中各自特征吸收波长的激发态衰减曲线根据式（3）进行拟合（表1）.5种卟啉化合物光激发下在396nm 负峰的瞬态衰减时长均相同，在5ns 内此峰消失，这是5种卟啉化合物中共有的瞬态衰减过程，根据此峰的特点，可以推测卟啉大环的共轭结构受到光激发后有相同能量弛豫路径产生，产生共同的荧光峰.此谱峰可以归属为卟啉大分子配体的S 0→S 1跃迁，但各种卟啉化合物在396nm 处的谱峰强度不同，说明此跃迁在不同卟啉之间的几率差异较大，也显示出分子处于激发态后能量弛豫途径不同.特征吸收波长的激发态衰减曲线拟合数据表明，金属配位卟啉三重激发态的正峰衰减过程中，TPP -2H *的446nm 处的峰衰减时长为1388.9ns ，TPP -Mg *在440nm 处的峰衰减时长为1328.6ns ，TPP -Zn *在452nm 处的峰瞬态衰减时长2687.7ns ，邻近Soret 带418，426和428nm 负峰衰减动力学时长分别为1233.0，1916.0和2256.2ns （图4）.正峰和负峰的动力学时长均在同一数量级，具有极为相似的动力学性质，表明分子被光激发产生的新激发中间态、基态卟啉浓度减小，致使原Soret 带ΔOD 值呈现Table 1τ1,χ2values of TPP -2H,TPP -FeCl,TPP -Ni,TPP -Mg and TPP -Zn Compound TPP⁃2H TPP⁃FeCl TPP⁃Ni λ/nm 396418446650718396396416532654716τ1/ns 5.001233.01388.915.1013.805.701.1010.705.2013.409.60χ21.8380.8071.2111.0530.8510.9835.6691.7812.9041.3981.004Compound TPP⁃Zn TPP⁃Mg λ/nm 396428452608664396426440524664τ1/ns 3.202256.22687.75.506.701.501916.01328.610.008.30χ22.6800.8012.6021.8591.332.9671.5212.4861.5431.286Fig.4Long⁃life time Soret bands decay kinetic curves of excited porphyrin compounds after 355nmlaser⁃excitation（A ）TPP -2H ，418nm ；（B ）TPP -Mg ，426nm ；（C ）TPP -Zn ，428nm ；（D ）TPP -2H ，466nm ；（E ）TPP -Mg ，440nm ；（F ）TPP -Zn ，452nm.771Vol.42高等学校化学学报负峰；而在较长波长的正峰则表明此激发中间态有弱允许电子跃迁，有较低的摩尔吸光系数εt ，产生正ΔOD 峰.正峰和负峰同时出现及相同数量级的激发态动力学时长表明同一卟啉激发态共轭或配位结构状态改变时，其能级轨道和电子跃迁的改变情况.TPP -2H *，TPP -Mg *和TPP -Zn *较长时间的能量弛豫过程有利于能量累积，若在此时间间隔内进行二次激发，分子更有利于进入更高能级激发态，因而TPP -Zn 可提供较好的光电转换效率.在可见光照射下，锌酞菁/氮化碳即可被激发用于光电催化CO 2还原反应［19］，原因可能为电子-空穴对的寿命长，它们参与反应的机会增大，这可能也是其在太阳能敏化剂等领域发挥着重要的作用原因之一［20，21］.TPP -Ni 在416nm 处的动力学时长非常短，仅为10.7ns ，且在较长波长位置并未出现正峰，显示了在Ni 配位四苯基卟啉中，Ni 提供的轨道和电子阻碍了类似于上述3种卟啉中间态结构的产生.分析分子结构及光谱规律，可以发现由于TPP -2H ，TPP -Mg 和TPP -Zn 的内部氢和镁、锌离子最外层电子饱和，易于产生长时激发态中间体；铁离子提供空轨道和单电子，卟啉电子激发后易离域到铁离子中，难以形成激发态；镍离子最外层只提供一个空轨道，镍上电子与卟啉N 配位，空轨道不与卟啉共平面，因而兼具二者性质.TPP -2H *，TPP -Ni *，TPP -Mg *和TPP -Zn *瞬态光谱中的基态漂白峰650nm/718nm ，654nm/716nm ，624nm/664nm 和608nm/664nm 处的强负峰寿命较短，动力学寿命在5~15ns 之间，与396nm 波长处的负峰相似，归属为激光激发后产生的整个卟啉分子的强荧光谱峰［21~23］，为Q 带（0-0）跃迁产生.Q 带（0-0）跃迁极易受卟啉内配位金属和氢原子影响，TPP -2H 分子激发态通过系间窜越至三重态，在650和718nm 处产生较强荧光；Mg ，Ni 和Zn 闭壳层金属配位的分子激发态通过系间窜越至T 1态减弱，荧光峰强度降低；开壳层金属Fe 配位激发态S 1易于恢复至基态，Q 带（0-0）跃迁的分子数过低，荧光峰难以检测到.2.4密度泛函理论和含时密度泛函理论计算TPP -2H 符合Gouterman 四轨道理论［24］，HOMO 和LUMO 之间的能级差为2.63eV ，4个前线轨道HOMO−1，HOMO ，LUMO 和LUMO+1轨道均位于与卟啉的4个吡咯环和N 原子上，LUMO 和LUMO+1轨道能级简并，HOMO−2及以下占据轨道能级均较低，远离前线轨道（图5）.中心镁、锌和镍配位后卟啉分子HOMO 和LUMO 之间的能极差增大，LUMO和LUMO+1能级轨道简并.TPP -Mg 和TPP -Zn 的前线轨道基本分布在卟啉内部的4个吡咯环和meso位的C 上，不脱离卟啉共轭结构，其稳态紫外-可见光吸收与TPP -2H 有较大相似之处，Mg 2+和Zn 2+离子结构为满电子结构，金属离子与4个N 原子相连，影响卟啉大共轭体系的n -π*跃迁，在卟啉分子内电子传递过程中起着重要作用，因而瞬态吸收峰二者差异较大.TPP -2H ，TPP -Mg 和TPP -Zn 3种卟啉分子的单重激发态的S 0→S 1和S 0→S 2跃迁，均为4个前线轨道，如TPP -Mg 的S 0→S 1跃迁组成为HOMO−1→LUMO+1（36%）和HOMO→LUMO （64%），S 0→S 2跃迁组成为HOMO−1→LUMO （36%）和HOMO→LUMO+1（63%），电子由HOMO 和HOMO−1轨道跃迁到LUMO 和LUMO+1轨道.Ni 和Fe 离子由于有单电子存在，其激发电子跃迁较为复杂.在TPP -Ni 中，Ni 2+对分子的HOMO 轨道有主要贡献，卟啉共轭结构和Ni 2+均对LUMO 轨道有贡献，TPP -Ni 的S 0→S 1跃迁组成为HOMO−4→LUMO+2（100%），意味着处于较低能量HOMO−4轨道的电子也易被激发跃迁.激发后，电子传递方向是电子从金属向配体的转移，卟啉配体吸收电子后分子结构和光学性质发生变化，能量累积后其稳态吸收光谱增加.TPP -FeCl 的HOMO 轨道位于Fe 和卟啉内部的4个吡咯环上，LUMO 则基本位于Fe 上，表明Fe 在分子中既可以提供空轨道，又可提供电子.TPP -FeCl 的电子跃迁更为复杂，其S 0→S 1跃迁组成主要为HOMO−2（A ）→LUMO （A ）（19%），HOMO−19（B ）→LUMO+2（B ）（11%）和HOMO−2（B ）→LUMO+2（B ）（53%）等；S 0→S 2跃迁组成主要为HOMO−6（B ）→LUMO+2（B ）（31%），Fig.5Frontier orbital energy levels diagram of TPP⁃2H772No.3马子辉等：光激发金属配位四苯基卟啉瞬态吸收和衰减动力学性质研究HOMO−2（B ）→LUMO （B ）（22%）和HOMO−2（B ）→LUMO+3（B ）（32%）等，表明铁配位卟啉有更多能级较低的占据轨道电子易被激发，同时Fe 也提供更多能级较高的空轨道参与，分子内部电子跃迁途径更为复杂，光激发后，位于铁和吡咯环HOMO 轨道上的电子以及更低能级占据轨道电子迅速自由离域至Fe 提供的最低空轨道上，电子传递方向是电子从配体向金属的转移，激发态对整体有机分子结构影响很小，因而稳态吸收光谱变化不大，以上分析结果与实验结果相吻合.结合稳态吸收光谱、荧光光谱、瞬态吸收光谱、动力学数据及理论计算结果，可以推测出激光激发各卟啉光谱变化的可能机理（图6）.TPP -2H 的卟啉环中心无金属配体，在稳态下是一个平面结构.TPP -2H 受激光激发之后的稳态吸收谱图并未出现新的峰，表明分子结构较稳定；瞬态吸收谱图中出现新峰表明卟啉被激发至激发态.TPP -2H 环中心未配位的金属提供空轨道，分子被激发至激发态后电子能量不能快速释放，产生长时中间体，然后再逐渐恢复至稳态.TPP -FeCl *呈现出的特殊性是因为铁离子的复杂电子排布方式以及与卟啉环复杂的配位，中心三价铁最外层电子轨道为3d 5半满结构，基态光谱项为6S ，d 轨道有5个自旋平行未成对电子，配体不仅具有σ轨道（sp 2杂化轨道）而且还有含孤对电子的π轨道（P z 轨道）［25，26］.计算结果表明，LUMO 轨道位于Fe 上，Fe 离子由于缺乏电子，配位后可以为分子提供空轨道，易于产生电子π⁃d 跃迁.光激发分子后，TPP *-FeCl 中位于卟啉配体的HOMO 或更低能级的占据轨道电子迅速自由离域至Fe 提供的空轨道上，激发态电子在铁离子中发生辐射或非辐射衰减，使得TPP -FeCl 在光激发后相对于另外3种金属卟啉化合物具有良好的受光激发后的稳定性，激发后瞬态恢复到稳态时间极短.激光激发实验表明，在300次连续激光激发下铁卟啉的紫外-可见吸收光谱未发生明显变化，表明铁卟啉在光照条件下稳定性高，不易受光照损坏，对铁卟啉作为肌红蛋白和细胞色素p450等蛋白活性中心的选择有着非常重要的作用.二价镍离子的最外层电子排布为3d 8，与4个氮配位后达到16电子配位稳定结构，镍卟啉的Q 带有明显吸收，镍离子基态光谱项为3F ，d 轨道有2个自旋平行未成对电子.Ni 2+对分子的HOMO 轨道有主要贡献，卟啉共轭结构和Ni 2+均对LUMO 轨道有贡献，电子的π⁃π*跃迁和π⁃d 跃迁均易产生，金属电子也可激发到金属空d 轨道上，能量得以从金属离子中发生辐射或非辐射衰减，同时配位结合能力加强.经激光激发，卟啉吸收能量到激发态TPP *-Ni ，能量转移后整个激发态分子TPP -Ni *可较迅速衰减至基态.二价锌离子的最外层电子排布为3d 10，电子全满，配位后能形成18电子配位稳定结构，锌离子无未成对电子和d 空轨道；镁离子同样外层电子全满，二者均属于闭合壳层，电子占据轨道成为卟啉中心电子传递的缓冲区域.经激光激发，卟啉吸收能量到激发态TPP *-Mg 或TPP *-Zn ，HOMO 和LUMO 轨道未分布在金属离子上，电子只能通过π⁃π*跃迁至卟啉的更高能级轨道，卟啉结构会有所形变，产生长时中间态，能量转移后，整个分子呈激发态的TPP -Mg *或TPP -Zn *再进一步衰减至基态.经过多次激光激发，长时中间态不能及时衰减，有一部分不能恢复至基态，持续吸收光子，金属离子缓冲区域能量过高，超出其能量阈值后则导致分子结构无法保持原构型.以上TPP -Mg 或TPP -Zn 激发态特性表明，在金属离子缓冲区域存在下两者可产生长时激发中间体，设定特定激发脉冲时间可使得二者在光电转换效率方面发挥独特优势.这一特性可进一步用于阐释自然界光合作用中叶绿素选择镁离子配位的原因，同时本文研究可用以阐明锌卟啉在光电转换方面具有极大优势的原因［27，28］.Fig.6Proposed energy transfer schemes of five porphyrin compounds after the photoexcitation773774Vol.42高等学校化学学报3结论以四苯基卟啉为实验模板，结合稳态吸收光谱、荧光光谱、瞬态吸收光谱、动力学数据及理论计算结果，发现TPP-FeCl在Soret带和Q带谱峰强度降低均不明显，TPP-FeCl*的瞬态吸收光谱中Soret带的ΔOD值为0，即无明显谱峰，显示Fe与卟啉环的配位较为稳定，且不易受到激光激发影响，相对于其它卟啉化合物具有良好的受光激发后的稳定性，对铁卟啉作为生物蛋白活性中心有着非常重要的作用. 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PhotophysicalBehaviorofTerpyridine-LanthanideIonComplexesIncorporatedinaPoly(N,N-dimethylacrylamide)Hydrogel
VlasoulaBekiariandPanagiotisLianos*UniVersityofPatras,EngineeringScienceDepartment,26500Patras,GreeceReceiVedMay18,2006.InFinalForm:June29,2006
Poly(N,N-dimethylacrylamide)hydrogelformscomplexeswithterpyridineandvarioustrivalentions,likeEu3+,Tb3+,Gd3+,andIn3+.Thehydrogelcanbeobtainedinthreedifferentphases:swollenwithwater,lyophilized(i.e.,
driedbyfreeze-drying),whereitlosesthesolventbutpreservestheswollenconfiguration,anddriedintheairwhereitshrinks.Thethreehydrogelphasesaffectthetypeofcomplexformedbetweenterpyridineandthemetalion.Thus,intheswollenandlyophilizedphases,metal-centeredemissioncanbeobtainedbyenergytransferfromtheexcitedligand.Intheshrunkphase,anintensegreenfluorescenceisemitted,whichisligand-centeredandisindependentofthecomplexedion.Intheabsenceofanyion,theligandemitsblueluminescence,independentlyofthehydrogelphase.Inthepresenceofeuropium(III)ions,blue,green,orredemissioncanbethusproducedatappropriatecompositionsandhydrogelphases.Analysisofthephotophysicalbehaviorofthepolymer-ligand-metalioncomplexisrelatedwiththephotophysicalbehavioroftheligandanditscomplexesinvariouspuresolvents.
IntroductionHydrogelsare3Dpolymernetworksthatswellbutdonotdissolveinwater.Shrinkingorswellingofthesegelscanbeactivatedbyseveralexternalstimuli,suchassolvent,heat,pH,andelectricstimuli,1andthismakestheminterestingmaterials
forseveralusefulapplications:drugdelivery,2-5electricalapplications,6musclemimeticactuators,2,7hostsofnanoparticles
andsemiconductors,1,6,8-12templates,13,14etc.15Eventhough
studiesonhydrogelsalreadycoverabroaddomaininmaterialsscience,smallattentionhasbeenpaidtohydrogelsashostsofluminescentspecies.Forexample,studiesofligand-lanthanideluminescentcomplexesareproliferatinginconnectionwithotherhostmatrixes,butsimilarstudiesassociatedwithhydrogelsarerare,ifany.Tofillthevoid,thepresentworkintroducesacomplexformedbetweenterpyridine(2,2′;6,2′′-terpyridine,abbreviatedTpy,seeFigure1forthechemicalstructure),Eu3+(orotherions),andpoly(N,N-dimethylacrylamide)(PDMAM)hydrogelandstudiesthevariationsoftheluminescencebehaviorofthiscomplexsystemwithrespecttothephasetransformationsofthehydrogelduringshrinkingorswelling.Thediversityoflumi-nescenceemissionsthatthissystemcanoffermaybeusefulforseveralphotophysicalapplications,bothinmonitoringphase
transformationsofthehydrogelitselforofothermacromoleculesandinsensingormonitoringotherenvironmentsorstimuli.Ligand-lanthanideioncomplexesarewell-knownefficientluminescentagents.16-18Theyemitalong-living,narrowband,
ofpracticallymonochromaticradiation,byligand-to-metalenergytransfer.Theyareendowedwithtwomajoradvantages:whenexcitationismadethroughtheligand,19theysolvetheproblemofsmalllightabsorptioncrosssectionbylanthanideions,andbycomplexinglanthanideions,theyprotectthemfromlumi-nescencequenchingprocesses.Typicalcomplexesinvolveacombinationofthree-diketonemolecules20andonemoleculeofaheterocyclicligand.16-18Complexesbetweenaheterocyclicligandalonewithalanthanideionarealsostudiedwithalotofinterest.Inthisrespect,complexesofTpywithlanthanidesaswellaswithothertrivalentmetalsareaveryinterestingsubject
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