硅烯与锗烯英文文献报告

Cite this:DOI:10.1039/c1cs15193b Chemistry and physics of a single atomic layer:strategies and challenges for functionalization of graphene and graphene-based materialsLiang Yan,ab Yue Bing Zheng,c Feng Zhao,ab Shoujian Li,d Xingfa Gao,ab Bingqian Xu,e Paul S.Weiss*c and Yuliang Zhao*abdReceived 18th July 2011DOI:10.1039/c1cs15193bGraphene has attracted great interest for its superior physical,chemical,mechanical,and electrical properties that enable a wide range of applications from electronics tonanoelectromechanical systems.Functionalization is among the significant vectors that drive graphene towards technological applications.While the physical properties of graphene have been at the center of attention,we still lack the knowledge framework for targeted graphenefunctionalization.In this critical review ,we describe some of the important chemical and physical processes for graphene functionalization.We also identify six major challenges in graphene research and give perspectives and practical strategies for both fundamental studies and applications of graphene (315references).1.IntroductionSince its conception,1–18graphene has attracted enormous interest due to its unique physical properties,such as novel magneto transport,2electromechanical modulation,5extremely high carrier mobility,6,7tunable band gap,8quantum Hall effect,2,9–11,19Klein tunneling nature,12and electron confinement effects.14These properties make graphene a promising candidate for a broad range of applications in next-generation nanotechnologies where current materials are limited in functionality.For example,current chemical separation materials cannot efficiently remove residual actinide fuels from radioactive fission fragments.20This costs more than $2billion per year for treating the large amount of radioactive wastes generated from nuclear power facilities.21–23Graphene exhibits superior properties as separation materials due to its selective adsorption of heavy metals (e.g.,lanthanides and actinides)with super loading and in situ rebirth capacity.For example,we may use graphene-nanoparticle multilayers for separation column materials in nuclear fuel recycling.Other fields in which graphene may have potential applications include electronics,photonics,optoelectronics,and mechanics (Fig.1).1–12,24,25Despite the wide range of possible applications,there are still many challenges for graphene to reach its full potential.For example,graphene has an intrinsic zero band-gap energy.We have to manage to open up the band gap for semiconductor applications,possibly even as a successor of silicon in the post-Moore’s law electronics era,26,27as a single-molecule gas or bio-sensor,28–30and as a stretchable transparent electrode or electron devices.31–38In addition,graphene is insoluble in organic solvents and susceptible to aggregation in aqueous solu-tions.Even for relatively simple uses of graphene,e.g.,filler in aChinese Academy of Sciences Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety,National Center for Nanoscience and Technology of China,Beijing 100190,China.E-mail:zhaoyuliang@ bInstitute of High Energy Physics,Chinese Academy of Sciences,Beijing 100049,China cCalifornia NanoSystems Institute,and Departments of Chemistry &Biochemistry and Materials Science &Engineering,University of California,Los Angeles,Los Angeles,California 90095,USA.E-mail:psw@ dThe College of Chemistry,Sichuan University,Chengdu 610064,China eFaculty of Engineering &Nanoscale Science and Engineering Center,University of Georgia,Athens,Georgia 30602,USALiang Yan is a graduate student of Chinese Academy of Sciences Key Laboratory for Biomedical Effects of Nanomaterials &Nanosafety.He received his BSc in Chemistry from Sichuan University in 2009.He is now studying on the chemical function-alization of graphenes as well their biomedical effects.Yue Bing Zheng is a postdoctoral scholar with Prof.Weiss at the University of California,Los Angeles.He received his PhD in Engineering Science and Mechanics from The Pennsylvania State University in 2010,his MS in Physics from the National University of Singapore in 2004,and his BS in Physics from Nankai University,China,in 2001.From 2004to 2006,he was a Research Fellow in the Institute of Materials Research and Engineering,Singapore.His research centers on designing,measuring,and controlling molecules and light at the nanoscale.Chem Soc RevDynamic Article Links/csrCRITICAL REVIEWD o w n l o a d e d b y H e n a n N o r m a l U n i v e r s i t y o n 23 N o v e m b e r 2011P u b l i s h e d o n 16 N o v e m b e r 2011 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 1C S 15193BView Online / Journal Homepagepolymer matrices,the surface properties of graphene sheets must be altered through further functionalization to obtain stable dispersions in solvents.Most recently,it was found that a supercapacitor fabricated by chemical activation of graphene with KOH possessed high values of gravimetric capacitance and energy density,39higher than the supercapacitor produced by the chemically reduced graphene.40Therefore,functionalization approaches that can modify the structural,electronic,and chemical properties of graphene are critical for applications.41–47In principle,graphene can be functionalized at two classes of locations:the basal plane and the edges.On the basal plane,sp 2hybridization of carbon leads to a strong covalent bonding,as well as to delocalization of the p electrons.The interaction of the basal plane with guest atoms or molecules leads to modi-fication of the p –p conjugation and thus the electron density distribution and the physical and chemical properties.The dangling bonds at edge sites of graphene are highly reactive to guest atoms or molecules.Typically,functionalization approaches used for fullerenes and carbon nanotubes (CNTs)can be applied to graphene.There are significant chemical differences due to the single atomic layer of graphene sheets.Much of this chemistry remains to be explored.The rehybridization from sp 2to sp 3via covalent reaction occurs both at the edges and on the basal plane.Before discussing functionalization methods,we sum-marize six critical issues to consider regarding functionaliza-tion of graphene and related materials.1.1.Multiple methods for graphene productionSo far,several methods have been successfully established for graphene preparation,such as peeling-offgraphite,1liquid-phase exfoliation,48–50chemical vapor deposition,39,51–53graphiti-zation of silicon carbide,26,54templating,55,56reduction of graphene oxide,57,58unzipping carbon nanotubes,59,60organic synthesis,61,62and anodic bonding.63These different methods produce graphene with different size,shape,chemical composition,and environ-ment (Fig.2),all of which have different requirements for functionalization.1.2.DefectsAll current methods cannot produce structurally and morpho-logically perfect graphene sheets because of the defects created unintentionally and unavoidably.64,65These defects,including structural imperfections and chemical impurities,complicate graphene functionalization.1.3.EdgesCombining the two basic types of edge configurations (i.e.,armchair and zigzag)leads to a variety of edges in graphene.This makes it difficult to characterize the structural and electronic properties and thus to control the functionalization processes at the edges.Feng Zhao is currently a research fellow at the Chinese Academy of Sciences Key Laboratory for Biomedical Effects of Nano-materials and Nanosafety,Institute of High Energy Physics,Chinese Academy of Sciences.She received her BSc in Chemistry from the College of Chemical Engineering,Tsinghua University in 2002,and MSc in Biology from the College of Biology,Tsinghua University in 2005.Her current research interests include nanotoxicology,and studies on nanoparticles and their interactions with cells or biomacromolecules.Shoujian Li received his PhD in Radiochemistry at Sichuan University.He is the head of the research group Functional Materials for Radionuclides Separation of the Chemistry College of Sichuan University.His research interests include the design and synthesis of new functional carbonaceous materials,advanced materials and technologies for spent fuel reprocessing,and treat-ment and disposal of nuclear wastes,including the use of carbon nanomaterials.Xingfa Gao received his PhD from the Institute of High Energy Physics,Chinese Academy of Sciences in 2006.After working as a postdoctoral fellow (2006–2011)at the Institute for Mole-cular Science (Japan)and Rensselaer Polytechnic Institute (USA),he returned to the Institute of High Energy Physics as a professor.His research interests lie in applying computa-tional methods to p -conjugated systems including fullerenes,carbon nanotubes,graphene and their hybrids.He has great interest in developing efficient theoretical methods to investigate novel p -conjugated molecules and reactions through close inter-play between theoretical predictions and experimental tests.Bingqian Xu received his PhD degree in Materials Science and Engineering from Arizona State University (ASU),Tempe,in 2004.Then,he was with the Electrical Engineering Department,ASU,as a Faculty Research Associate in molecular electronics.In 2006,he moved to the University of Georgia (UGA),Athens,and now is an Associate Professor,directing the Molecular Nanoelectronics Laboratory.His research interests include molecular electronics and single-molecule studies of biomolecular assemblies and systems.Dr Xu is a member of the American Chemical Society,Materials Research Society,and American Physical Society.Paul S.Weiss received his PhD in Chemistry in 1986from UC Berkeley.He was a postdoctoral fellow at AT&T Bell Laboratories and IBM Almaden Research Center.He began his academic career at Penn State,becoming Distinguished Professor of Chemistry and Physics before moving to UCLA in 2009.At UCLA,he is the Fred Kavli Chair in NanoSystems Sciences,the Director of the California NanoSystems Institute,and Distinguished Professor of Chemistry &Biochemistry and Materials Science &Engineering.He is the founding Editor-in-Chief of ACS Nano.His research interests are in single-molecule/assembly function,chemical pattern-ing,self-assembly,and nanoscale analyses.Yuliang Zhao’s research interests include Nanotoxicological Chemistry (nanotoxicology,cancer nanotechnology,and nano-chemistry),Nanobioanalytical Sciences,and Molecular Dynamics Simulations of biochemical processes at the nano/bio interface.He serves as editorial board member for eight international journals in the United States and Europe.He is Professor and Director,Chinese Academy of Sciences Key Laboratory for Biomedical Effects of Nanomaterials &Nanosafety,and also serves as Deputy Director-General of National Center for Nanoscience and Technology of China,and a member of National Steering Council for Nanosciences and Technology of China.D o w n l o a d e d b y H e n a n N o r m a l U n i v e r s i t y o n 23 N o v e m b e r 2011P u b l i s h e d o n 16 N o v e m b e r 2011 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 1C S 15193B1.4.StoichiometryThe functionalized graphene is non-stoichiometric in its chemical composition.This makes it difficult to control its properties.1.5.Low reactivityGraphene has a much lower chemical reactivity than fullerenes and CNTs.66,67The low reactivity limits the effective approaches for functionalizing graphene.411.6.Low solubilityGraphene has low solubility in both aqueous and organic solutions.This makes it difficult to manipulate graphene in solutions in which many functionalization processes occur.Thus,pre-treatment is sometimes required to increase the solubility of graphene for further functionalization.The electronic structure of graphene underlies its chemical properties.Ideal graphene is an infinite-scale two-dimensional sheet without edges and basal plane flposed of sp 2carbon,ideal graphene is chemically unsaturated (Fig.3A).Carbon atoms can form an extra covalent bond,which converts sp 2to sp 3hybridization and makes carbon reach its saturated state.Thus,this ‘‘unsaturation’’is regarded as the origin of graphene’s reactivity in covalent addition reactions.However,graphene is chemically inert (or stable)because all its p z atomic orbitals are strongly coupled and stabilized in a giant,deloca-lized p bonding system (Fig.3A).On one hand,this p system usually precludes graphene from covalent addition.On the other hand,as a kind of p ligand,it renders versatile com-plexation reactions for graphene, e.g.,with organic com-pounds and transition metals through p –p ,H–p and metal–p interactions.The associated anti-bonding p *molecular orbitals can accommodate electrons,which facilitates favorable adsorp-tion between graphene and electron-rich particles such as ions and alkali metals.In contrast to ideal graphene,practical graphene unavoidably contains edges,basal plane fluctuations,vacancies,andotherFig.1Application examples of functionalized graphene in electronics and single-molecule gas sensors:(a)schematic exploded illustration of a graphene mixer circuit.The critical design aspects include a top-gated graphene transistor and two inductors connected to the gate and the drain of the graphene field-effect transistor (GFET).24(b)Concentration,D n ,of chemically induced charge carriers in a single-layer graphene exposed to different concentrations,C ,of NO 2.Upper inset:scanning electron micro-graph of this device.Lower inset:characterization of the graphene device by using the electric-field effect.28Reproduced from ref.24.Copyright 2011American Association for the Advancement of Science.Reproduced from ref.28.Copyright 2007Nature PublishingGroup.Fig.2Graphene prepared by different methods:(a)Large graphene crystal prepared on an oxidized Si wafer by the scotch-tape technique.(b)Left panel:suspension of microcrystals obtained by ultrasound cleavage of graphite in chloroform.Right panel:such suspensions can be printed on various substrates.(c)The first graphene wafers are now available as polycrystalline one-to five-layer films grown on Ni and transferred onto a Si wafer.(d)State-of-the-art SiC wafer with atomic terraces covered by a graphitic monolayer (indicated by ‘‘1’’).Double and triple layers (‘‘2’’and ‘‘3’’)grow at the steps.7(e)Representation of the gradual unzipping of one wall of a carbon nanotube to form a nanoribbon.60Reproduced from ref.7.Copyright 2009American Association for the Advancement of Science.Reproduced from ref.60.Copyright 2009Nature Publishing Group.D o w n l o a d e d b y H e n a n N o r m a l U n i v e r s i t y o n 23 N o v e m b e r 2011P u b l i s h e d o n 16 N o v e m b e r 2011 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 1C S 15193Bchemical impurities.By altering the electronic structure,these structural ‘‘imperfections’’can alter the chemical properties and reactions of graphene.During covalent addition,inner carbons that are strictly constrained in the basal plane have to protrude outward from the plane to adopt a tetrahedral sp 3geometry,causing strain in the plane.Edge carbon atoms are usually terminated by chemical groups such as hydrogen.Unlike inner carbon atoms,edge carbon atoms can adopt tetrahedral geo-metries more freely without causing extra strain.Therefore,edge carbons are preferred in covalent addition.Because fron-tier molecular orbitals are mainly localized at zigzag edges,68,69zigzag edges are particularly reactive in graphene (Fig.3B).67For the same reason,vacancies like edges created inside the graphene basal plane,are also very reactive (Fig.3C).Basal plane fluctuations cause curvature of graphene sheets.The curvature reduces the overlap of the p z atomic orbital of one carbon with p z orbitals of the three surrounding carbons (Fig.3D).Thus,the curvature can lead to localized states with higher energies,which enhances the reactivity of the carbon.70This paper reviews recent progress on graphene functiona-lization based on the different types of reactivity discussed above.We discuss the functionalization of graphene by basal plane covalent addition (Section 2).Because edges are parti-cularly reactive sites in graphene,the covalent functionaliza-tion of graphene edges is discussed separately (Section 3).Unlike covalent addition,both complexation and charge-transfer adsorption do not destroy graphene’s p bonding network and are discussed in Section 4.Finally,we review perspectives and challenges for graphene functionalization (Section 5).It is worth noting that practical reactions of graphene are usually compli-cated.Different types of reactivity may be involved simulta-neously or at different stages of reactions.It is usually difficult toattribute a reaction solely to one or two origins of reactivity.Therefore,the above classification of the origins of reactivity,as well as the settings of Sections 2through 4,are only attempted to emphasize the main driving force for the reactions.2.Functionalization of graphene targeting basal plane covalent additionComposed of sp 2carbon,graphene is chemically unsaturated.Intrinsically,it is possible to undergo covalent addition to change the carbons from sp 2to sp 3hydridization.However,carbon atoms in the graphene basal plane are protected by their p -conjugation system,whose motion is constrained by surrounding carbon atoms.Therefore,basal plane covalent addition usually encounters large energy barriers,and reactive chemical groups,such as atomic hydrogen,fluorine,and pre-cursors of other chemical radicals,are usually needed as the reactants.So far,the chemical modification of graphene cannot be fully controlled.Therefore,most of the reactions discussed in this section can also take place on graphene edges.2.1.Hydrogenation and dehydrogenationHydrogenation of the free-standing graphene and of graphene located on top of oxidized Si substrates has been investigated both experimentally and theoretically.66,71–75The supported graphene displays different structures and electronic properties before and after hydrogenation.Hydrogenation changes the hybridization of carbon atoms from sp 2to sp 3,resulting in elongated C–C bonds in the H-modified graphene.Hydrogen atoms tend to react with both surfaces of the plane of pristine graphene (Fig.4).If only one side is hydrogenated,it can then be rolled to form CNTs because of the unbalanced external stress.76Fig.3Origin of chemical reactivity of graphene.(A)Intrinsic reactivity arising from the delocalized p -bonding system.(B)Zigzag and armchair edges.(C)Monovacancy.(D)Local structure of a curved graphene sheet.In (A)and (D),the p z atomic orbitals are shown;the dashed lines represent overlap between p z orbitals.In (C),the dangling s bonds are shown.D o w n l o a d e d b y H e n a n N o r m a l U n i v e r s i t y o n 23 N o v e m b e r 2011P u b l i s h e d o n 16 N o v e m b e r 2011 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 1C S 15193BThe semi-hydrogenated graphene possesses ferromagnetic semi-conductor properties because the partial hydrogenation can destroy the delocalized p bonding network of graphene.77The fully hydrogenated graphene is called ‘‘graphane’’,which is prepared under hydrogen plasma atmosphere and has also been the subject of a number of studies.75In addition,it is worth mentioning that plasma activation supplies an alternative way to prepare the precursors of graphene derivatives,such as graphenol,graphenoic acid,and graphenamine.Alternatively,several research groups also studied the chemi-sorptions of hydrogen on the graphene surface,in which case only small subareas of the graphene basal plane are hydro-genated unintentionally.78–81There are four types of configura-tions that hydrogen pairs form on the basal plane from thermal graphitization of SiC (0001)after exposing to a 1600K D-atom beam.79The adsorbate forms and morphologies depend on various conditions,such as temperature,pressure,and hydrogen coverage.And hydrogens can be desorbed by annealing without influencing the electronic properties of graphene.Moreover,an energy band-gap can be induced via patterned hydrogen chemisorption.78The dehydrogenation of graphene can proceed via an annealing process through which the properties of graphene can be partially restored.82The reactivity of annealed graphene is higher than that of pristine graphene 66largely because defects form during annealing.Hydrogenation and dehydrogenation are useful processes for graphene functionalization,particularly by which ferromagnetism can be introduced,the band gap can be opened,and the p -conjugated skeleton of graphene can be altered.Additionally,the dehydrogenated graphene displays higher chemical reactivity than pristine graphene.This opens up new means for generating multifunctional graphene-based materials.Hydrogenation also offers a practical means to control electro-nic structure,which is the basis for fabricating novel devices.In addition,graphane,(CH)n ,if produced,may be an excellent new material for hydrogen storage.722.2.Fluorination reactionsAb initio pseudopotential calculations indicate that the electronicstructure of fluorinated graphene sheets has a ffiffiffi3p Âffiffiffi3p periodic structure in charge distribution near the Fermi energy.83It is known that fluorine on the sidewall of fluoro-CNTs can be replaced by alkyl groups in the presence of alkyl lithium,84alkylidene amino,85or Grignard reagents.86The replacement reactions can be extended to fluoro-graphene functionaliza-tion.The fluorination of graphene is also useful for further substitution and functionalization.For example,fluorinated graphene sheets successfully reacted in situ with butylamine 87to obtain the alkylated graphene.Alkylated graphenes are important graphene-based materials because they are readily dispersed in common organic solvents such as dichlorobenzene,dichloromethane,and THF.Moreover,they can be completely dealkylated by annealing to recover the original properties of pristine graphene sheets.88The fluorinated graphene has been prepared by plasma treat-ment of chemically converted graphene (CCG)at room tempera-ture followed by subsequent reaction with butylamine.87If the initial graphene sheets are prepared by thermally exfoliating graphene oxide or CCG,they have some residual oxygen-containing species on the basal plane.The presence of oxygen-containing species may prevent fluorine atoms from attach-ing to the graphene sheet in the fluorination process,which undoubtedly reduces the degree of fluorination.2.3.Oxidation reactionsThe oxidized graphene sheet is one of the most important forms of graphene,in which graphene is heavily oxygenated with a wide variety of oxygen species such as carbonyl,carboxyl,and hydroxyl groups.89Because of the existence of oxygen-containing functional groups,graphene oxides react easily with soluble moieties.This enables changing the hydrophilicity,hydrophobicity,or organophilicity of graphene,as required for many applications.For example,many modified graphene sheets are readily dispersed in organic solvents for further functionalization,or for mixing with organic matrices to form new nanocomposite materials.Oxidation can also generate a monotype of oxygen-containing functional group,such as graphene with only hydroxyls (graphenol),or only carboxyls (graphenic acid)on the basal plane.Three experimental chemical routes have been developed for graphene oxidation.The first is a one-step process,and is achieved through direct oxidization of graphene with strong oxidants such as concentrated sulfuric acid,concentrated nitric acid,or potassium permanganate.90The second is a two-step process,in which graphite is oxidized through Hummers’,91Brodies’,92Staudenmaiers’,93or modified Hummer’s methods,94or electrochemic oxidation,95,96followed by exfoliating or thermally expanding the graphene oxide obtained.97,98The third isaFig.4The most favorable conformations of graphene after hydro-genation,the chair (a)and boat (b)conformations.Carbon is shown as black and hydrogen is shown as red.In functionalization,hydrogen tends to attach to both sides of the graphene plane.D o w n l o a d e d b y H e n a n N o r m a l U n i v e r s i t y o n 23 N o v e m b e r 2011P u b l i s h e d o n 16 N o v e m b e r 2011 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 1C S 15193Bphysicochemical process:graphene oxide nanoribbons are created through lengthwise cutting and unraveling the side walls of multiwalled carbon nanotubes (MWCNTs)by oxida-tive processes.59,60During the oxidation process,graphitic structures break down into smaller fragments.The formation mechanism,electronic properties,and conformations of these fragments were studied both experimentally and theoretically.99–102It is worth noting that direct oxidization is not usually used to prepare graphene oxide but has the potential to control the size,shape,number of layers,and electronic properties of graphene.For example,experimentally,thermal oxidation of few-layer graphene with O 2can remove 2–3layers of graphene and eventually yield a single layer,half-metallic graphene.1032.4.DiazotizationGraphene has an electron-rich surface due to its p electrons.When electron-accepting moieties such as aryl diazonium salts react with graphene,electrons can transfer from the basal plane to the reactant.Because of the increase of pyramidalization of the deformed plane,diazonium salts easily react with graphene.Diazotization could be used to regulate electrical conductivity of the graphene since it can modulate the surface potential of graphene via regiofunctionalization.104–108Both CCG and epitaxial graphene (EG)have been success-fully modified by diazotization at room temperature.98,109When nitrophenyl groups were attached to the EG surface,the resultant diazonium-functionalized graphene sheets were easily dispersed in polar aprotic solvents such as dimethylformamide (DMF),dimethylacetamide (DMAc),and N -methylpyrrolidone (NMP).Nitro groups on the surface of diazotized graphene can be reduced further to amine.The amine groups make sub-sequent graphene functionalization possible because amine can react with many other groups,such as hydroxyl radicals,carboxyl groups,and acyl chlorides.Interestingly,nonvolatile memory devices were fabricated by attachment of gold nanoparticles (AuNPs)-4-mercapto-benzenediazonium tetrafluoroborate salt (MBDT)conjugates (AuNPs-MBDTs)through diazotization reactions onto the p -conjugated skeleton of CCG.110Different graphene preparation methods lead to graphene with different properties after diazotization.For EG on a sub-strate,there is a preferential adjustment of the density distribu-tion of electrons in response to the regiofunctionalization of the surface.The diazotization process can form a defect-free and oxygen-free surface for EG that can be used to fabricate sensors,detectors,and other electronic devices.For CCG,both surfaces of the basal plane can be diazotized,but with defects and oxygen-ated groups.As a result,the conductivity of the diazotized CCG is significantly lower than that of the diazotized EG.2.5.Other cycloaddition reactionsThe thermal functionalization of fullerenes 111and carbon nanotubes 112,113by nitrenes via [2+1]cycloaddition reactions has been studied extensively.These reactions have also been applied to graphene to form aziridine adducts and to immo-bilize graphene on silicon wafers.114The first step of the reaction protocol is the thermal or photochemical decomposition of per-fluorophenylazides (PFPA)via nitrogen elimination,giving rise to the highly reactive single perfluorophenylnitrene.The second stepconsists of [2+1]cycloaddition of the nitrene to the surface of graphene.Alkyne-terminated molecules have been coupled to graphene sheets via ‘‘click’’reactions to form stable hetero-cyclic linkages under mild reaction conditions.115Cyclo-addition reactions of fullerenes and CNTs such as the Bingel [2+1]cyclopropanation reaction,116can also be applied to graphene functionalization.And 1,3-dipolar cycloaddition has been successfully carried out not just at the edge but also at the internal C Q C bonds of graphene.117Claisen rearrangement was also used to functionalize graphene oxide,yielding a water-soluble derivative.118The modular zwitterion-mediated transformations that have been used to functionalize CNTs and fullerenes 119can also be applied to graphene.These processes introduce a variety of functional groups for further reaction of graphene to expand the library of graphene-based materials for practical applications.For example,if graphene is coupled with chelating ligands,it can complex metal ions,and hence can be used to detect and to remove toxic heavy metal ions from the environment,or can be used as a novel filler to fabricate the columns for the efficient separation/treatment of radioactive wastes of nuclear power stations.2.6.Reverse reactions of covalent additionChemical and thermal reduction of graphene oxide provide alternative approaches for the production of CCG 120,121and graphene-based composites.122The oxygen-containing func-tional groups on the basal plane of graphene can be removed by thermal treatment or chemical reaction with reductants such as hydroquinone,reducing sugar,L -glutathione,N 2H 4,and their derivatives.57–58,121–134Heating graphene oxide in ultrahigh vacuum is effective in controlling the percentage of oxygen-containing groups being removed.124Solvothermal reduction of graphene oxide occurs at a much lower reaction temperature than pyrolysis and provides a means to reduce the defects in graphene.135–137Photothermal heating has also been used to reduce graphene oxide.In contrast to chemical and thermal treatments,a photothermal heating process is rapid and does not require other reactants.In addition,heated atomic force microscope (AFM)tips can be used to reduce graphene oxide locally.138However,none of these methods are capable of achieving complete reduction of graphene oxide.As a result,the reduced graphene oxide exhibits non-metallic behavior due to defects related to residual oxygen functional groups.57,58,124,129,130A combination of several different reduction methods may enhance the degree of reduction and improve the quality of graphene.139For example,hydrogen reduction,combined with thermal treatment,can remove oxygen species effectively with little contamination on the basal plane.Nonetheless,the reduction process makes graphene hydrophobic and easy to aggregate.140One way to prevent aggregation is to add poly-mers into the graphene oxide solution during the reduction process.However,the polymers also introduce extra impurities into the graphene,which are undesirable for some applications because the impurities can modify both surface and electronic properties.An alternative solution to the aggregation issue is to graft a charged unit such as sulfonate (–SO 3H)to the graphene plane so that it can maintain the stability of grapheneD o w n l o a d e d b y H e n a n N o r m a l U n i v e r s i t y o n 23 N o v e m b e r 2011P u b l i s h e d o n 16 N o v e m b e r 2011 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 1C S 15193B。

SiO 2/graphene composite for highly selective adsorption of Pb(II)ionLiying Hao,Hongjie Song,Lichun Zhang,Xiangyu Wan,Yurong Tang,Yi Lv ⇑Key Laboratory of Green Chemistry &Technology,Ministry of Education,College of Chemistry,Sichuan University,Chengdu,Sichuan 610064,Chinaa r t i c l e i n f o Article history:Received 24October 2011Accepted 8December 2011Available online 16December 2011Keywords:Pb(II)ionSiO 2/graphene composite Adsorptiona b s t r a c tSiO 2/graphene composite was prepared through a simple two-step reaction,including the preparation of SiO 2/graphene oxide and the reduction of graphene oxide (GO).The composite was characterized by UV–Vis spectroscopy,Fourier transform infrared spectroscopy,scanning electron microscope,and X-ray pho-toelectron spectroscopy,and what is more,the adsorption behavior of as-synthesized SiO 2/graphene composite was investigated.It was interestingly found that the composite shows high efficiency and high selectivity toward Pb(II)ion.The maximum adsorption capacity of SiO 2/graphene composite for Pb(II)ion was found to be 113.6mg g À1,which was much higher than that of bare SiO 2nanoparticles.The results indicated that SiO 2/graphene composite with high adsorption efficiency and fast adsorption equilibrium can be used as a practical adsorbent for Pb(II)ion.Ó2011Elsevier Inc.All rights reserved.1.IntroductionGraphene (G),discovered in 2004[1],has been attempted in many applications due to its excellent characteristics,such as mobil-ity of charge carriers,mechanical flexibility,thermal and chemical stability,and large surface area [2–4].Significantly,graphene,as ideal two-dimensional ultrathin material with large surface area,is a promising building block material for composites [5];further-more,decoration of the graphene nanosheets with metal/metal oxide/nonmetallic oxide nanomaterials can bring about an impor-tant kind of graphene-based composites [6–10].The decoration of nanomaterials onto graphene nanosheets is also helpful to over-come the aggregation of individual graphene nanosheets [11]and nanomaterials themselves.Besides,the composites with larger sur-face area show superior properties,compared with bare nanomate-rials [12],due to the synergistic effect between graphene nanosheets and nanomaterials.Therefore,in recent years,many endeavors have been poured on the synthesis of graphene-based nanocomposites,e.g.,graphene/metal oxide and graphene/metal composites,and these composite materials have been explored as adsorbents [13,14],catalysts [15],and lithium ion batteries [16]along with an excellent application potential.Considering the inexpensive cost,innocuity,reliable and chemical stability,biocompatibility,and ver-satility of SiO 2[17],graphene/silica composite would be one of the greatly popular and interest topics in the field of nanomaterial and nanotechnology.On the other hand,there has been a long-time concern on the pollution of heavy metals to the aquatic environment because oftheir toxicity and detriment to living species including humans.Among all of the heavy metal ions,lead ion,which commonly exists in industrial and agricultural wastewater and in acidic leach-ate from landfill sites [18],is ubiquitous in the environment and severely hazardous to human and living things.Long-term drink-ing water containing high level of lead ion would cause serious dis-orders,such as anemia,kidney disease,nausea,convulsions,coma,renal failure,and cancer,along with subtly negative effects on metabolism and intelligence [19].Up to now,many techniques have been applied to remove Pb(II)ion from waste water,such as ion exchange [20],cloud point extraction [21],coprecipitation [22],flocculation [23],membrane filtration [24],reverse osmosis [25],adsorption [26],and so forth.Among these methods,adsorption-based methodology is greatly popular thanks to its high efficiency,cost-effectiveness,simple operation,and environmental friendliness [27].Especially,adsorptive removal of aqueous Pb(II)ion has been widely investigated by using various materials,such as activated carbon,ash,zeolites,metal oxides,chitosan,and agri-cultural by-products [28].It is also worth mentioning here that graphene/nanomaterials composites are also considered to be a highly effective adsorbent due to the peculiar properties and large surface area.Particularly,the research about the application of graphene/nanomaterials composites in the adsorption of heavy metal ions is important for environment and human.In this work,SiO 2/graphene composite was prepared via a two-step procedure route that contains the preparation of silica nanoparticles in the presence of graphene oxide solution and the reduction of graphene oxide in the presence of silica nanoparticles.Then,the resulting composite was chosen as an adsorbent toward Pb(II)ion and the adsorption behaviors were investigated in de-tails.Meanwhile,the influence of experimental conditions,includ-ing pH value,ionic strength and contact time,adsorbability,and0021-9797/$-see front matter Ó2011Elsevier Inc.All rights reserved.doi:10.1016/j.jcis.2011.12.023Corresponding author.Fax:+862885412798.E-mail address:lvy@ (Y.Lv).adsorption capacity,was also discussed.Interestingly,the SiO 2/graphene composite was found to be highly effective adsorbent with high selectivity and fast adsorption equilibrium toward Pb(II)ion.2.Materials and methods2.1.Chemical reagents and materialsGraphite powder was of SpecPure grade and was purchased from Tianjin Guangfu Fine Chemical Research Institute.Other reagents were of analytical grade and were used without further purification.Deionized (DI)water from ULUPURE Water Purification System (Chengdu,China)was used to prepare all solutions.Lead nitrate (Pb(NO 3)2,P 99.0%),ethanol,sodium hydroxide (NaOH),hydrazine hydrate (H 2NNH 2ÁH 2O,P 50.06%),and hydrochloric acid (HCl,P 36.46%)were obtained from Chengdu Kelong Chemical Reagent Company (China).Stock standard solution of lead (1000mg L À1)was prepared from analytical grade lead nitrate.2.2.Preparation and characterization of SiO 2/graphene composite The soluble graphene oxide–based sheets were produced by complete exfoliation of graphite oxide as an entry into SiO 2/graph-ene composite.Graphite oxide was synthesized according to the Hummers method through the oxidation of natural graphite pow-der [12].After that,graphite oxide (100mg)was exfoliated in 400mL of distilled alcohol–water (7:1,v/v)solution by ultrasonic treatment for 2h to form a colloidal suspension approximately.Then,the collected colloidal suspension was separated by centrifu-gation at 4000rpm,and the supernatant was obtained in order to prepare the followed composite.The well-known hydrolysis of tet-ramethyl orthosilicate (TEOS)was used for the fabrication of the composite.Briefly,the pH of the reaction mixture was adjusted to 9.00with ammonia solution and then added TEOS (2.1mL)into this dispersion,resulting graphene oxide–containing sol [29].The obtained mixture was stirred magnetically and reduced with hydrazine hydrate (P 50.06%)at 95°C for 24h.was collected through 0.45l m filter and water to remove the excess hydrazine hydrate,thesized composite was dried at 323K overnight 2.3.Characterization and apparatusThe UV–Vis spectra of GO and SiO 2/GO 200–500nm were recorded by U-2910UV–Vis The surface properties and composition of silica nanoparticles were investigated by Fourier IR)spectroscopy using Thermo Nicolet IS10FT-IR KBr pellets in the range 500–4000cm À1.X-ray troscopy (XPS)was performed with a XSAM 800eter (Kratos)using medium resolution and radiation to analyze the surface composition and of products.The binding energies were calibrated tainment carbon (284.8eV).Also,the wide-angle 35mA)powder X-ray diffraction (XRD)using a X’Pert Pro X-ray diffractometer (Philips)tion (k =1.5406Å).The surface morphology of the examined by SEM (Hitachi,S3400).The (BET)surface area and the pore size distribution were measured using N 2adsorption and desorption SI,Quantachrome,USA)at 77K over a relative 0.0955to 0.993.2.4.Batch adsorption experimentBatch adsorption tests were carried out at room temperature (25°C)and used to investigate the effects of various parameters on the adsorption of Pb(II)ion by SiO 2/graphene.For adsorption experiments,3mg of adsorbents was dispersed into a 20mg L À1Pb(II)ion solution (10mL)and was shaken with a magnetic stirrer for 60min to reach equilibrium except kinetic experiments.The SiO 2/graphene solution mixtures were filtered with a 0.45l m fil-ter,and the equilibrium concentrations of Pb(II)ion in the solution were quantified by flame atomic absorption spectroscopy (FAAS,Zeeman GGX-6,China).According to the above procedure,the impact of the pH value,ionic strength,and contact time on adsorp-tion was investigated.The adsorption capacity (q e ,mg g À1)and the adsorption efficiency (E ,%)were calculated according to Eqs.(1)and (2):q e ¼ðC 0ÀC e ÞÁVWð1ÞE ¼C 0ÀC eC 0Â100%ð2Þwhere C 0and C e (mg L À1)are the initial and equilibrium concentra-tions of Pb(II)ion in aqueous phase,and V is the volume of the solu-tion (L ),and W is the mass of dry adsorbent used (g ),respectively.3.Results and discussion 3.1.Characterizations of compositeUV–Vis spectrogram (Fig.S1)shows that GO nanosheets pres-ent a clear characteristic absorption in aqueous solution with a maximum wavelength at 228nm.On the other hand,SiO 2/GO composite exhibits a weak absorption at 226nm,which is due to the assembly of the GO nanosheets.The phenomenon of blueshift of the maximum wavelength is attributed to the change of the environment around the GO nanosheets,which preliminarily indi-cates that SiO 2/GO composite is successfully prepared.Similar 382L.Hao et al./Journal of Colloid and Interface Science 369(2012)381–387the A OH bending vibration of the adsorbed water molecules[29]. This suggests that SiO2nanoparticles are successfully prepared through the above pared with SiO2nanoparticles, the minor and weak peaks are observed at2980and2930cmÀ1, which are attributed to the C A H stretching vibration[33],which indicate the effective attachment of graphene and the successful preparation of SiO2/graphene composite.The surface composition and the element characterization of the composite were analyzed using XPS spectra of composite, which was conducted in the region of0–1100eV.As shown in Fig.2a,there are three elements in the XPS spectra of the compos-ite,namely carbon,oxygen,and silicon,without other elements. The spectra of XPS(Fig.2a)exist the characteristic peaks of Si2s (150eV),Si2p(104.5eV),which is indicative of the formation of the SiO2phase in composite.Moreover,the presence of SiO2can be further confirmed by the O1s XPS peak at532.8eV(Fig.2c), which is regarded as the oxygen species in the SiO2[34,35].In addition,there are at least three types of oxygen species about the O1s peak(Fig.2c),that is,the contribution of the anionic oxy-gen in SiO2at about532.8eV,the oxygen-containing functional groups at around532.3eV,and water at higher binding energies. The C1s XPS spectra,as shown in Fig.2b,contain four components corresponding to carbon atoms in different oxygen-containing functional groups[36]:(a)the non-oxygenated ring C at 284.8eV,(b)the carbon in C A O at285.9eV,(c)the carbonyl car-bon(C@O)at287.0eV,and(d)the carboxylate carbon(O A C@O) at288.2eV.The C1s spectrum of SiO2/graphene shows mainly the nonoxygenated carbon(284.8eV)and the carbon in C A O (285.9eV).Moreover,nonoxygenated carbon is more than the car-bon in C A O,which indicates that deoxygenation has appeared. Meanwhile,XRD was used to further verify the deoxygenation (Supplementary data,Fig.S2).The peak at10.4°corresponding to the diffraction peak of GO was disappeared and the newly obtuse peak at23.0°was observed,which confirm that GO was reduced with hydrazine hydrate and amorphous SiO2nanoparticles were formed[29,32].However,small amount residual oxygenated groups are still left,which are verified by the O1s XPS peaks at 532.3eV(C@O)and533.6eV(C A O)and also indicated that GO has not been completely reduced by hydrazine hydrate.Besides,silica nanoparticles to form a composite in nanoscale.In the images of SiO2/graphene(Fig.3c and d),the layer structure of graphene is well observed at high magnification and SiO2nanoparticles are tightly covered by the corrugated grapheneflakes,which is differ-ent from previous report[30]that graphene nanosheets are immo-bilized onto SiO2nanoparticles through surface assembly.In addition,N2adsorption–desorption isotherms were also em-ployed to investigate the specific surface area and the pore struc-tures of prepared samples(the chemical analysis reveals that the weight percentage of graphene is about12.5wt.%)(Fig.4).The BET surface area and pore volume estimated from Barret–Joyner–Halenda(BJH)analysis of the isotherms were determined to be 252.5m2gÀ1and0.3771cm3gÀ1,respectively.Also,the average of the pore size distribution is2.987nm,which was calculated from the absorption branch by the BJH method.As Fig.4shows, a slight adsorption is observed in the low pressure region(<0.6P/ P0),followed by a sharp adsorption at0.8P/P0,which suggests that this adsorption step occurs on its surface and the interlayer of restacking graphene layers[37].Also,a hysteresis loop can be seen in desorption branch.The shape of adsorption isotherms may be considered to be reversible type V isotherms,which is considered that there is weak interaction between materials and nitrogen. The shape of desorption branch is a typical H3type,indicating that the slit holes in the composite may be formed by the aggregation of various platelike particles.Thus,SiO2/graphene could be a good candidate as a kind of adsorption material.3.2.Adsorption performance3.2.1.Effect of pH on the adsorptionThe pH of the solution usually exerts a great effect on the adsorption of metal ions.According to the solubility-product con-stant of Pb(OH)2(Ksp=1.43Â10À15)and the initial concentration of Pb(II)ion of20mg LÀ1,the pH value of appearance of metal ion hydroxides precipitation is calculated as8.59.In order to investi-gate the effect of pH on the adsorption of Pb(II)ion onto SiO2/ graphene,10mL Pb(II)ion solution with the concentration of 20mg LÀ1was adjusted to a pH range of2.00–7.00with different concentrations of NaOH and HCl solutions,during which noL.Hao et al./Journal of Colloid and Interface Science369(2012)381–387383results in low adsorption.As the pH increased,more binding sites were released and there were less competition of active sites between hydrogen ion and lead(II)ion,resulting in better adsorp-tion behavior.In addition,the surface charge of SiO2/graphene with more negative charge density at higher pH causes more electro-static attractions of Pb(II)ion,which serves as another reason for the better adsorption behavior.3.2.2.Effect of ionic strength on the adsorptionThe different ionic strengths,such as0.001M,0.005M,0.01M, 0.05M,0.1M KNO3,and without KNO3,were chosen to investigate their effect on Pb(II)ion adsorption by SiO2/graphene.Fig.5b shows that Pb(II)ion adsorption decreases with increasing ionic strength.This phenomenon could be attributed to following reasons:(1)the Pb(II)ion forms outer-spherethe adsorbent sites,which favor the adsorptiontration of the competing salt is decreased.adsorption between the adsorbent andmainly of ionic interaction nature;(2)ionicinfluences the activity coefficient of metaltransfer to the composite surfaces[38].3.2.3.Effect of contact time on the adsorptionTime course of Pb(II)ion adsorption ontoexecuted under Pb(II)ion solution with concentrationat pH=6.00and I=0.001M KNO3.Fig.6contact time on the adsorption of Pb(II)composite.It can be seen that the adsorptionsharply,with about95%of total Pb(II)ion10min,then the adsorption reaches equilibriumfast adsorption rate is attributed to the laminatedlarge external surface of SiO2/graphene.Furthertime does not enhance the adsorption percentage2value industrial applications.The kinetics of Pb(II)ion adsorption was determined in order to understand the adsorption behavior of the SiO2/graphene compos-ite.The adsorption data of Pb(II)ion at different time intervals are fit for a pseudo-second-order kinetic model.The calculated curve corresponding to Pb(II)ion sorption was plotted in Fig.6(inset). The kinetic rate equation is expressed asdqtdt¼k2Áðq eÀq tÞ2ð3ÞBy integrating Eq.(3)with the boundary conditions of q t=0at t=0and q t=q t at t=t,the following linear equation can be obtained:tt¼12eþteð4ÞFig.3.SEM image of SiO2(a)and the different magnification of SiO2/graphene composite(b–d). (black)–desorption(red)isotherms and pore sizeV0¼k2Áq2eð5Þwhere q t and q e are the amounts of Pb(II)ion adsorbed at time t and at equilibrium(mg gÀ1),respectively.The k2(g mgÀ1minÀ1)repre-sents the pseudo-second-order rate constant for the kinetic model, which can be obtained by a plot of t/q t against t.V0(mg gÀ1minÀ1) is the initial sorption rate.As shown in Table S1,the comparison be-tween the experimental adsorption capacity(q exp)value and the calculated adsorption capacity(q cal)value shows that q cal value is very close to q exp value for the pseudo-second-order kinetics. Moreover,the adsorbent system can be well described by pseudo-second-order kinetic model,which also is confirmed according to the correlation coefficient value for pseudo-second-order model, equal to1.000,higher than that of pseudo-first-order,suggesting that the adsorption may be the rate-limiting step involving valence forces through sharing or exchange of electrons between the adsor-bent and the adsorbate.3.3.Adsorption isothermsIn addition to adsorption kinetics,we measured the absorption isotherms of Pb(II)ion onto SiO2/graphene to explore the adsorp-tion mechanism much deeply.As shown in Fig.7,at low initial Pb(II)ion concentration,the composite exhibits high adsorption percentage as98.82%.Although the adsorptivity decreases with increasing initial Pb(II)ion concentration,Pb(II)ion adsorption capacity steadily rises.The Langmuir and Freundlich models are the most frequently used models among the abundant isothermal models.The Lang-muir isotherm,which assumes monolayer coverage on adsorbent [39]and no subsequent interaction among adsorbed molecules, is expressed as[40]:1qe¼1qmþ1K LÁq mÁC eThe Freundlich isotherm is derived to model multilayer adsorp-tion on adsorbent.It can be described as[40]:ln qe¼ln K Fþ1nÁln C ewhere q e and C e are the adsorption capacity(mg gÀ1)and the equi-librium concentration of the adsorbate(mg LÀ1),respectively.K L is the constant related to the free energy of adsorption(L mgÀ1),and q m is the maximum adsorption capacity(mg gÀ1).K F and n are the Freundlich constants,which represent the adsorption capacity (mg gÀ1)and the adsorption strength,respectively.The values of q m and K L are calculated from the slope and intercept of the linear plot of1/q e against1/C e.ln K F and1/n can be obtained from the intercept and the slope of the linear plot of ln q e versus ln C e.The adsorption isotherms of Pb(II)ion on the SiO2/graphene com-posite as a function of Pb(II)concentration(pH=6.00,30min adsorption time)are shown in Fig.7(inset),and the Langmuir and Freundlich constants are presented in Table1.The adsorption data(a)and ionic strength(b)for the adsorption percentage and capacity of Pb(II)ion at room temperature(25°C):adsorption time, concentration,20mg LÀ1;and ionic strength,0.001M KNO3for a;pH,6.00for b.arefit for Langmuir model,and it shows the maximum adsorption capacity of113.6mg gÀ1for the SiO2/graphene composite.Also, the higher correlation coefficients indicate that the Langmuir model fits the adsorption data better than the Freundlich model.In other words,this adsorption process took place by monolayer on the homogeneous sites of the surface of SiO2/graphene.The adsorption capacities of other absorbents toward Pb(II)ion are listed in Table S2,and the comparative results show that the adsorption capacity of SiO2/graphene is higher.Therefore,it can be concluded that SiO2/graphene has much superior adsorption capacity for removing Pb(II)ion.3.4.Selective adsorption experimentThere are mainly six different heavy metals in the waste water: Cu2+,Pb2+,Ni2+,Co2+,Cd2+,and Cr3+.We chose a mixed solution of metal ions,which was prepared by diluting1000mg LÀ1of Cu2+, Pb2+,Ni2+,Cd2+,Co2+,and Cr3+to20mg LÀ1in25mLflask volumet-ric for a selective adsorption experiment;6.0mg adsorbent was dispersed in20mL of solution and the mixture was stirred for 30min at room temperature.In order to avoid to produce Cu(OH)2 (pH=5.92)and Cr(OH)3(pH=5.07)at the optimum condition (pH=6.00),the pH value of solution was chosen as4.80,at which the adsorption efficiency of Pb(II)ion would be little lower.Under this condition,the uptake of Pb(II)ion from this mixed metal ion solution on the SiO2/graphene composite is as high as84.23%, while other ions show only slight/negligible adsorption.The exper-iment data demonstrate highly selective adsorption of Pb(II)ion on the SiO2/graphene composite.3.5.Adsorption mechanismGenerally speaking,the adsorption of metal ions is based on the three adsorption mechanisms:electrostatic interactions,ion ex-change,and complex formation[41].In our study,the pH value of the solution increased after adsorption of Pb(II)ion(Table2), and the adsorption efficiency of Pb(II)ion increased with increas-ing the pH value until the optimum pH,which is in accordance with the related literature[27].As we all know,graphene sheets, containing delocalized p electrons,and lead ion/hydrogen ion act as electron donor and acceptor,respectively,which can form the electron donor–acceptor complexes.In this system,the complex is formed by a coordination bond(or dative bond or dipolar bond) between the unshared electron pair of the composites and an elec-tron-deficient atom of lead ion and hydrogen ion.So,it suggests that lead ion and hydrogen ion simultaneously adsorbed onto graphene and form composites,and results in an increase in the pH value.This phenomenon indicates that ion exchange is not the main cause.For our study,the surface charge is regarded as negative at high pH,which provides the ability of binding cations through electrostatic interaction.Besides,according to the previ-ous report,the basic sites as C p electrons on graphene sheets are considered as the important adsorption sites[42].Conse-quently,electrostatic interaction between Pb(II)cations and nega-tive surface charge and/or C p electrons of the composite is regarded as the main interaction for the adsorption of Pb(II)ion onto the composite.In addition,the specific surface area (252.5m2gÀ1)according to BET measure is another course,which provides more active sites for Pb(II)ion adsorption.However,re-search is needed for the clear mechanism in further investigations.4.ConclusionsIn summary,the SiO2/graphene composite was synthesized via a facile,fast,and low-cost process and further was developed to be highly efficient adsorbent for Pb(II)ion in aqueous solution.The SiO2/graphene composite reduces the serious stacking of graphene sheets and prevents the agglomeration of SiO2nanoparticles,and also produces a high surface area,which enables the composite to show high binding capability and excellent adsorption proper-ties for Pb(II)ion.This adsorbent is stable,low-cost,and environ-mentally friendly and shows potential application in the removal of Pb(II)ion from agricultural and industrial waste water.In addi-tion,successful preparation of SiO2/graphene composite was very helpful to understand the fundamental properties of graphene-based composites and some practical applications.AcknowledgmentsThis work was supported by the National Nature Science Foun-dation of China(21075084)and the Sichuan Youth Science&Tech-nology Foundation(No.2009-18-409).The authors also would like to show gratitude for Dr.Jiqiu Wen and Dr.Hong Chen of Analytical &Testing Center at Sichuan University for their assistance in the XRD and XPS analysis.Appendix A.Supplementary materialSupplementary data associated with this article can be found,in the online version,at 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ARTICLEReceived1Apr2014|Accepted9Jan2015|Published24Feb2015Observation of long-lived interlayer excitonsin monolayer MoSe2–WSe2heterostructuresPasqual Rivera1,John R.Schaibley1,Aaron M.Jones1,Jason S.Ross2,Sanfeng Wu1,Grant Aivazian1,Philip Klement1,Kyle Seyler1,Genevieve Clark2,Nirmal J.Ghimire3,4,Jiaqiang Yan4,5,D.G.Mandrus3,4,5, Wang Yao6&Xiaodong Xu1,2Van der Waals bound heterostructures constructed with two-dimensional materials,such asgraphene,boron nitride and transition metal dichalcogenides,have sparked wide interest indevice physics and technologies at the two-dimensional limit.One highly coveted hetero-structure is that of differing monolayer transition metal dichalcogenides with type-II bandalignment,with bound electrons and holes localized in individual monolayers,that is,interlayer excitons.Here,we report the observation of interlayer excitons in monolayerMoSe2–WSe2heterostructures by photoluminescence and photoluminescence excitationspectroscopy.Wefind that their energy and luminescence intensity are highly tunable by anapplied vertical gate voltage.Moreover,we measure an interlayer exciton lifetime of B1.8ns,an order of magnitude longer than intralayer excitons in monolayers.Our work demonstratesoptical pumping of interlayer electric polarization,which may provoke further explorationof interlayer exciton condensation,as well as new applications in two-dimensional lasers,light-emitting diodes and photovoltaic devices.1Department of Physics,University of Washington,Seattle,Washington98195,USA.2Department of Materials Science and Engineering,University of Washington,Seattle,Washington98195,USA.3Department of Physics and Astronomy,University of T ennessee,Knoxville,T ennessee37996,USA.4Materials Science and T echnology Division,Oak Ridge National Laboratory,Oak Ridge,T ennessee37831,USA.5Department of Materials Science and Engineering,University of T ennessee,Knoxville,T ennessee37996,USA.6Department of Physics and Center of Theoretical and Computational Physics, University of Hong Kong,Hong Kong,China.Correspondence and requests for materials should be addressed to P.R.(email:pasqual@)or to X.X. (email:xuxd@).T he recently developed ability to vertically assemble different two-dimensional(2D)materials heralds a newrealm of device physics based on van der Waals heterostructures(HSs)1.The most successful example to date is the vertical integration of graphene on boron nitride.Such novel HSs not only markedly enhance graphene’s electronic properties2, but also give rise to superlattice structures demonstrating exotic physical phenomena3–5.A fascinating counterpart to gapless graphene is a class of monolayer direct bandgap semiconductors, namely transition metal dichalcogenides(TMDs)6–8.Due to the large binding energy in these2D semiconductors,excitons dominate the optical response,exhibiting strong light–matter interactions that are electrically tunable9,10.The discovery of excitonic valley physics11–15and strongly coupled spin and pseudospin physics16,17in2D TMDs opens up new possibilities for device concepts not possible in other material systems. Monolayer TMDs have the chemical formula MX2where the M is tungsten(W)or molybdenum(Mo),and the X is sulfur(S) or selenium(Se).Although these TMDs share the same crystalline structure,their physical properties,such as bandgap,exciton resonance and spin–orbit coupling strength,can vary signifi-cantly.Therefore,an intriguing possibility is to stack different TMD monolayers on top of one another to form2D HSs.First-principle calculations show that heterojunctions formed between monolayer tungsten and molybdenum dichalcogenides have type-II band alignment18–20.Recently,this has been confirmed by X-ray photoelectron spectroscopy and scanning tunnelling spectroscopy21.Since the Coulomb binding energy in2D TMDs is much stronger than in conventional semiconductors, it is possible to realize interlayer excitonic states in van der Waals bound heterobilayers,that is,bound electrons and holes that are localized in different layers.Such interlayer excitons have been intensely pursued in bilayer graphene for possible exciton condensation22,but direct optical observation demonstrating the existence of such excitons is challenging owing to the lack of a sizable bandgap in graphene.Monolayer TMDs with bandgaps in the visible range provide the opportunity to optically pump interlayer excitons,which can be directly observed through photoluminescence(PL)measurements.In this report,we present direct observation of interlayer excitons in vertically stacked monolayer MoSe2–WSe2HSs.We show that interlayer exciton PL is enhanced under optical excitation resonant with the intralayer excitons in isolated monolayers,consistent with the interlayer charge transfer resulting from the underlying type-II band structure.We demonstrate the tuning of the interlayer exciton energy by applying a vertical gate voltage,which is consistent with the permanent out-of-plane electric dipole nature of interlayer excitons.Moreover,wefind a blue shift in PL energy at increasing excitation power,a hallmark of repulsive dipole–dipole interac-tions between spatially indirect excitons.Finally,time-resolved PL measurements yield a lifetime of1.8ns,which is at least an order of magnitude longer than that of intralayer excitons.Our work shows that monolayer semiconducting HSs are a promising platform for exploring new optoelectronic phenomena.ResultsMoSe2–WSe2HS photoluminescence.HSs are prepared by standard polymethyl methacrylate(PMMA)transfer techniques using mechanically exfoliated monolayers of WSe2and MoSe2(see Methods).Since there is no effort made to match the crystal lattices of the two monolayers,the obtained HSs are considered incom-mensurate.An idealized depiction of the vertical MoSe2–WSe2HS is shown in Fig.1a.We have fabricated six devices that all show similar results as those reported below.The data presented here are from two independent MoSe2–WSe2HSs,labelled device1and device2.Figure1b shows an optical micrograph of device1,which has individual monolayers,as well as a large area of vertically stacked HS.This device architecture allows for the comparison of the excitonic spectrum of individual monolayers with that of the HS region,allowing for a controlled identification of spectral changes resulting from interlayer coupling.We characterize the MoSe2–WSe2monolayers and HS using PL measurements.Inspection of the PL from the HS at room temperature reveals three dominant spectral features(Fig.1c). The emission at1.65and1.57eV corresponds to the excitonic states from monolayer WSe2and MoSe2(refs10,15),respectively. PL from the HS region,outlined by the dashed white line in Fig.1a,reveals a distinct spectral feature at1.35eV(X I).Two-dimensional mapping of the spectrally integrated PL from X I shows that it is isolated entirely to the HS region(inset,Fig.1c), with highly uniform peak intensity and spectral position (Supplementary Materials1).Low-temperature characterization of the HS is performed with 1.88eV laser excitation at20K.PL from individual monolayer WSe2(top),MoSe2(bottom)and the HS area(middle)are shown with the same scale in Fig.1d.At low temperature,the intralayer neutral(X M o)and charged(X MÀ)excitons are resolved10,15,where M labels either W or parison of the three spectra shows that both intralayer X M o and X MÀexist in the HS with emission at the same energy as from isolated monolayers,demonstrating the preservation of intralayer excitons in the HS region.PL from X I becomes more pronounced and is comparable to the intralayer excitons at low temperature.We note that the X I energy position has variation across the pool of HS samples we have studied (Supplementary Fig.1),which we attribute to differences in the interlayer separation,possibly due to imperfect transfer and a different twisting angle between monolayers.We further perform PL excitation(PLE)spectroscopy to investigate the correlation between X I and intralayer excitons.A narrow bandwidth(o50kHz)frequency tunable laser is swept across the energy resonances of intralayer excitons(from1.6to 1.75eV)while monitoring X I PL response.Figure2a shows an intensity plot of X I emission as a function of photoexcitation energy from device2.We clearly observe the enhancement of X I emission when the excitation energy is resonant with intralayer exciton states(Fig.2b).Now we discuss the origin of X I.Since X I has never been observed in our exfoliated monolayer and bilayer samples,if its origin were related to defects,they must be introduced by the fabrication process.This would result in sample-dependent X I properties with non-uniform spatial dependence.However,our data show that key physical properties of X I,such as the resonance energy and intensity,are spatially uniform and isolated to the HS region(inset of Fig.1c and Supplementary Fig.2).In addition,X I has not been observed in WSe2–WSe2homo-structures constructed from exfoliated or physical vapor deposi-tion(PVD)grown monolayers(Supplementary Fig.3).All these facts suggest that X I is not a defect-related exciton.Instead,the experimental results support the observation of an interlayer exciton.Due to the type-II band alignment of the MoSe2–WSe2HS18–20,as shown in Fig.2c,photoexcited electrons and holes will relax(dashed lines)to the conduction band edge of MoSe2and the valence band edge of WSe2,respectively.The Coulomb attraction between electrons in the MoSe2and holes in the WSe2gives rise to an interlayer exciton,X I,analogous to spatially indirect excitons in coupled quantum wells.The interlayer coupling yields the lowest energy bright exciton in the HS,which is consistent with the temperature dependence of X I PL,that is,it increases as temperature decreases (Supplementary Fig.4).From the intralayer and interlayer exciton spectral positions,we can infer the band offsets between the WSe 2and MoSe 2monolayers (Fig.2c).The energy difference between X W and X I at room temperature is 310meV.Considering the smaller binding energy of interlayer than intralayer excitons,this sets a lower bound on the conduction band offset.The energy difference between X M and X I then provides a lower bound on the valence band offset of 230meV.This value is consistent with the valence band offset of 228meV found in MoS 2–WSe 2HSs by micro X-ray photoelectron spectroscopy and scanning tunnelling spectro-scopy measurements 21.This experimental evidence strongly corroborates X I as an interlayer exciton.The observation of bright interlayer excitons in monolayer semiconducting HSs is of central importance,and the remainder of this paper will focus on their physical properties resulting from their spatially indirect nature and the underlying type-II band alignment.WSe 2HSMoSe 2W M SeIn te n s i t y (a .u .)1.31.51.7Energy (eV)MoSe 2HeterostructureWSe 2W0WX X X X −0MoMo−e hehe h1.3 1.41.51.6 1.7I n t e n s i t y (a .u .)Energy (eV)5μm 0123×104Y (μm )246X (μm)0246Figure 1|Intralayer and interlayer excitons of a monolayer MoSe 2–WSe 2vertical heterostructure.(a )Cartoon depiction of a MoSe 2–WSe 2heterostructure (HS).(b )Microscope image of a MoSe 2–WSe 2HS (device 1)with a white dashed line outlining the HS region.(c )Room-temperature photoluminescence of the heterostructure under 20m W laser excitation at 2.33eV.Inset:spatial map of integrated PL intensity from the low-energy peak (1.273–1.400eV),which is only appreciable in the heterostructure area,outlined by the dashed black line.(d )Photoluminescence of individual monolayers and the HS at 20K under 20m W excitation at 1.88eV (plotted on the samescale).Energy (eV)WSe MoSe PL energy (eV)E x c i t a t i o n e n e r g y (e V )1.28 1.3 1.32 1.34 1.36 1.381.61.651.71.754,0006,0008,00010,000IntensityFigure 2|Photoluminescence excitation spectroscopy of the interlayer exciton at 20K.(a )PLE intensity plot of the heterostructure region with an excitation power of 30m W and 5s charge-coupled device CCD integration time.(b )Spectrally integrated PLE response (red dots)overlaid on PL (black line)with 100m W excitation at 1.88eV.(c )Type-II semiconductor band alignment diagram for the 2D MoSe 2–WSe 2heterojunction.interlayer exciton .Applying vertical energy of Figure 3a contact stacked insu-Electrostatic contact shows the 100to about analogue of reversed,varied expected for from reduces device 2,conduction 3b,c.of the in the on top band-offset at X I PL energy of basis of would should have X I PL This effect,intensity.further Power dependence and lifetime of interlayer exciton PL .The interlayer exciton PLE spectrum as a function of laser power with excitation energy in resonance with X W o reveals several properties of the X I .Inspection of the normalized PLE intensity (Fig.4a)shows the evolution of a doublet in the interlayer excitonspectrum,highlighted by the red and Both peaks of the doublet display a consistent increased laser intensity,shown by the dashed which are included as a guide to the eye.intensity of X I also exhibits a strong saturation laser power,as shown in Fig.4b (absolute Supplementary Fig.6).The sublinear power excitation powers above 0.5m W is distinctly the intralayer excitons in isolated monolayers,saturation power threshold of about Fig.7).The low power saturation of X I PL lifetime than that of intralayer excitons.the intralayer exciton is substantially reduced interlayer charge hopping 23,which is quenching of intralayer exciton PL (Fig.Fig.8).Moreover,the lifetime of the interlayer because it is the lowest energy configuration indirect nature leads to a reduced optical long lifetime is confirmed by time-resolved Fig.4c.A fit to a single exponential decay exciton lifetime of 1.8±0.3ns.This timescale the intralayer exciton lifetime,which is ps 24–27.By modelling the saturation behaviour three-level diagram,the calculated saturation interlayer exciton is about 180times (Supplementary Fig.7;Supplementary with our observation of low saturation intensity DiscussionWe attribute the observed doublet feature splitting of the monolayer MoSe 2conduction assignment is mainly based on the fact difference between the doublet is B 25with MoSe 2conduction band splitting predicted calculations 28.This explanation is also supported by the evolution of the relative strength of the two peaks with increasing excitation power,as shown in Fig.4a (similar results in device 1with 1.88eV excitation shown in Supplementary Fig.9).At low power,the lowest energy configuration of interlayer excitons,with the electron in the lower spin-split band of MoSe 2,is populated first.Due to phase space filling effects,the interlayer excitonSiO 2n + Si2MoSe 2e –h +e –h +P Ee –h +V g < 0WSe 2MoSe 2WSe 2MoSe 2h ωV g = 0Photon energy (eV)1.321.361.41.444080e –h +h +PL intensity (a.u.) -hω’-the interlayer exciton and band alignment.(a )Device 2geometry.The interlayer exciton has a out-of-plane electric polarization.(b )Electrostatic control of the band alignment and the interlayer exciton photoluminescence as a function of applied gate voltage under 70m W excitation at 1.744eV,1s integrationconfiguration with the electron in the higher energy spin-split band starts to be filled at higher laser power.Consequently,the higher energy peak of the doublet becomes more prominent at higher excitation powers.The observed blue shift of X I as the excitation power increases,indicated by the dashed arrows in Fig.4a,is a signature of the repulsive interaction between the dipole-aligned interlayer excitons (cf.Fig.3a).This is a hallmark of spatially indirect excitons in gallium arsenide (GaAs)coupled quantum wells,which have been intensely studied for exciton Bose-Einstein condensation (BEC)phenomena 29.The observation of spatially indirect interlayer excitons in a type-II semiconducting 2D HS provides an intriguing platform to explore exciton BEC,where the observed extended lifetimes and repulsive interactions are two key ingredients towards the realization of this exotic state of matter.Moreover,the extraordinarily high binding energy for excitons in this truly 2D system may provide for degenerate exciton gases at elevated temperatures compared with other material systems 30.The long-lived interlayer exciton may also lead to new optoelectronic applications,such as photovoltaics 31–34and 2D HS nanolasers.MethodsDevice fabrication .Monolayers of MoSe 2are mechanically exfoliated onto 300nm SiO 2on heavily doped Si wafers and monolayers of WSe 2onto a layer of PMMA atop polyvinyl alcohol on Si.Both monolayers are identified with an opticalmicroscope and confirmed by their PL spectra.Polyvinyl alcohol is dissolved in H 2O and the PMMA layer is then placed on a transfer loop or thin layer of poly-dimethylsiloxane (PDMS).The top monolayer is then placed in contact with the bottom monolayer with the aid of an optical microscope and micromanipulators.The substrate is then heated to cause the PMMA layer to release from the transfer media.The PMMA is subsequently dissolved in acetone for B 30min and then rinsed with isopropyl alcohol.Low-temperature PL measurements .Low-temperature measurements are con-ducted in a temperature-controlled Janis cold finger cryostat (sample in vacuum)with a diffraction-limited excitation beam diameter of B 1m m.PL is spectrally filtered through a 0.5-m monochromator (Andor–Shamrock)and detected on a charge-coupled device (Andor—Newton).Spatial PL mapping is performed using a Mad City Labs Nano-T555nanopositioning system.For PLE measurements,a continuous wave Ti:sapphire laser (MSquared—SolsTiS)is used for excitation and filtered 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chalcogenides.Proc.Natl A111,6198–6202 (2014).AcknowledgementsThis work is mainly supported by the US DoE,BES,Materials Sciences and Engineering Division(DE-SC0008145).N.J.G.,J.Y.and D.G.M.are supported by US DoE,BES, Materials Sciences and Engineering Division.W.Y.is supported by the Research Grant Council of Hong Kong(HKU17305914P,HKU9/CRF/13G),and the Croucher Foun-dation under the Croucher Innovation Award.X.X.thanks the support of the Cottrell Scholar Award.P.R.thanks the UW GO-MAP program for their support.A.M.J.is partially supported by the NSF(DGE-0718124).J.S.R.is partially supported by the NSF (DGE-1256082).S.W.and G.C.are partially supported by the State of Washington through the UW Clean Energy Institute.Device fabrication was performed at the Washington Nanofabrication Facility and NSF-funded Nanotech User Facility. Author contributionsX.X.and P.R.conceived the experiments.P.R.and P.K.fabricated the devices,assisted by J.S.R.P.R.performed the measurements,assisted by J.R.S.,A.M.J.,J.S.R.,S.W.and G.A. P.R.and X.X.performed data analysis,with input from W.Y.N.J.G.,J.Y.and D.G.M. synthesized and characterized the bulk WSe2crystals.X.X.,P.R.,J.R.S.and W.Y.wrote the paper.All authors discussed the results.Additional informationSupplementary Information accompanies this paper at / naturecommunicationsCompetingfinancial interests:The authors declare no competingfinancial interests. Reprints and permission information is available online at / reprintsandpermissions/How to cite this article:Rivera,P.et al.Observation of long-lived interlayer excitons in monolayer MoSe2–mun.6:6242doi:10.1038/ncomms7242(2015).。

不饱和类锗烯H 2C ‗GeLiCl 的DFT 研究李文佐1,*谭海娜1肖翠平1宫宝安1程建波1,2(1烟台大学化学生物理工学院,山东烟台264005;2吉林大学超分子结构与材料教育部重点实验室,长春130012)摘要:采用密度泛函理论方法,在B3LYP/6⁃311G(d,p )水平上研究了不饱和类锗烯H 2C ‗GeLiCl 的结构及异构化反应.结果表明,不饱和类锗烯H 2C ‗GeLiCl 有三种平衡构型,其中非平面的p ⁃配合物型构型能量最低,是其存在的主要构型.对平衡构型间异构化反应的过渡态进行了计算,求得了转化势垒.计算预言了最稳定构型的振动频率和红外吸收强度.关键词:不饱和类锗烯H 2C ‗GeLiCl ；DFT ；异构化中图分类号:O641.12DFT Study on the Unsaturated Germylenoid H 2C ‗GeLiClLI Wen ⁃Zuo 1,*TAN Hai ⁃Na 1XIAO Cui ⁃Ping 1GONG Bao ⁃An 1CHENG Jian ⁃Bo 1,2(1Science and Engineering College of Chemistry and Biology,Yantai University,Yantai 264005,Shandong Province,P.R.China;2KeyLaboratory for Supramolecular Structure and Materials of Ministry of Education,Jilin University,Changchun130012,P.R.China )Abstract:The unsaturated germylenoid H 2C ‗GeLiCl was studied by using the DFT method at the B3LYP/6⁃311G(d,p )level of theory.Geometry optimization calculations indicated that H 2C ‗GeLiCl had three equilibrium configurations,in which the non ⁃planar p ⁃complex was lowest in energy and was the most stable structure.The transition states for isomerization reactions of H 2C ‗GeLiCl were located and the energy barriers were calculated.For the most stable structure,the vibrational frequencies and infrared intensities had been predicted.Key Words:Unsaturated germylenoid H 2C ‗GeLiCl;DFT;Isomerization锗烯(germylene)与卡宾和硅烯类似,是一种重要的有机反应活性中间体[1-3].由于有机锗类化合物具有某些生物活性[4-10],对锗烯及其衍生物的研究日益增多[11-16].类锗烯(germylenoid)与类卡宾与类硅烯类似,可看作由锗烯与碱金属卤化物形成,用通式R 1R 2GeMX 来表示.自Gaspar 等[17]提出类锗烯是某些化学反应的中间体以来,对类锗烯的研究逐渐增多.Qiu 等[18]首次对最简单的类锗烯H 2GeLiF 进行了ab initio 计算;Tan 等[19,20]对H 2GeMF(M=Li,Na)与R —H (R=F,OH,NH 2,CH 3)的插入反应进行了研究;Ma 等[21]报道了对H 2GeLiCl 的理论研究结果.H 2C ‗GeMX 是一类与不饱和类碳烯H 2C ‗CMX [22]和不饱和类硅烯H 2C ‗SiMX [23]类似的不饱和类锗烯[24,25],对其结构、性质及稳定性的研究具有重要意义.为进一步丰富类锗烯的内容,深入了解不饱和类锗烯的结构和性质,本文采用密度泛函理论(DFT)方法对不饱和类锗烯H 2C ‗GeLiCl 进行了理论研究,以期为实验工作者提供更多的理论依据,并对进一步研究类锗烯有所裨益.1计算方法对不饱和类锗烯H 2C ‗GeLiCl 的平衡构型及构型间异构化反应的过渡态均采用DFT B3LYP [26,27]方法进行全参数优化,并进行振动频率分析及IRC [28]Received ：May 21,2007;Revised ：June 6,2007;Published on Web:August 6,2007.*Corresponding author.Email:liwenzuo@;Tel:+86535⁃6902063.烟台大学博士科研基金(HY05B30)资助项目ⒸEditorial office of Acta Physico ⁃Chimica Sinica[Note]物理化学学报(Wuli Huaxue Xuebao )Acta Phys.鄄Chim.Sin .,2007,23(11)：1811-1814November 1811Acta Phys.鄄Chim.Sin.,2007Vol.23计算,以确证过渡态.计算采用Gaussian 03[29]程序的6⁃311G(d,p )基组[30].2结果和讨论与不饱和卡宾H 2C ‗C:[22]和不饱和硅烯H 2C ‗Si:[23]类似,不饱和锗烯H 2C ‗Ge:的基态是单重态[31].在该分子中,Ge 原子采取sp 杂化,一个sp 杂化轨道与碳原子形成σ键,两个电子占据另一个sp 杂化轨道.两个相互垂直的p 轨道中,一个p 轨道与另一个碳原子的对应p 轨道形成π键,另一个p 轨道是不占电子的空轨道.不饱和锗烯H 2C ‗Ge:的结构如图1所示.不饱和锗烯中的p 轨道是空轨道,具有亲电性,sp 杂化轨道是占据轨道具有亲核性.这种具有双重性的H 2C ‗Ge:与强极性的LiCl 分子结合,从而构成不饱和类锗烯H 2C ‗GeLiCl 在构型上的复杂性.2.1平衡构型B3LYP/6⁃311G(d,p )构型优化与频率计算结果表明,H 2C ‗GeLiCl 有三种平衡构型,见图2中1-3所示.图2中4和5为H 2C ‗GeLiCl 异构化反应的过渡态构型.图2中还给出自然电荷布居结果.各构型的总能量、相对能量、偶极矩及虚频数目列于表1.图2中的构型1,是缺电子带正电荷的Li 端与Ge 原子占据两个电子的sp 杂化轨道结合,富电子带负电荷的Cl 端与Ge 原子的空p 轨道结合形成的三元环状结构.这一构型中,所有原子在同一平面内,属C s 点群.三元环中Li —Cl 的键长(0.2141nm)比LiCl 分子中Li —Cl 的键长(0.2025nm)长0.0116nm.1中C —Ge 键长(0.1759nm)比H 2C ‗Ge 中C —Ge 键长(0.1798nm)略短.通过比较构型1、H 2C ‗Ge 及LiCl 中Ge 、Li 和Cl 原子上的自然电荷布居大小变化(见图2),可以看出,计算得到构型1中存在Cl →Ge →Li →Cl 的电子流向,这应该是构型1能够稳定存在的主要原因.计算得到构型1的解离能(1→H 2C ‗Ge(1A 1)+LiCl)为74.44kJ ·mol -1.从表1可以看出,H 2C ‗GeLiCl 的构型1比能量最低构型2的能量高54.09kJ ·mol -1.构型2可看作带负电的Cl 端进攻Ge 的p 空图2H 2C ‗GeLiCl 的平衡构型及异构化反应的过渡态Fig.2The equilibrium configurations and the transition states for isomerization reactions of H 2C ‗GeLiClcalculated at B3LYP/6⁃311G(d,p )level;bond lengths are given in nm and angles in degrees,values in parentheses are the natural charges图1不饱和锗烯H 2C ‗Ge:的结构示意图Fig.1Schematic diagram of unsaturated germyleneH 2C ‗Ge:C 2vC 2v1812No.11李文佐等：不饱和类锗烯H 2C ‗GeLiCl 的DFT 研究轨道产生的构型,在该构型中,Cl 上的电子向Ge 的p 空轨道迁移,构型2可称之为p ⁃配合物型构型.p ⁃配合物型构型是非平面结构,可看作[H 2C —Ge —Cl]-Li +结构,其中Li 原子不仅与Cl 有相互作用,与C 和Ge 也有相互作用.2中C ‗Ge 键长(0.1860nm)比H 2C ‗Ge 中的C ‗Ge 键长(0.1798nm)长0.0062nm,Li —Cl 键长(0.2189nm)比LiCl 分子中的Li —Cl 键长长0.0164nm.在该结构中存在一个三原子(C 、Ge 、Cl)四电子的弯曲的离域π键,使得2的能量较低,从表1可以看出,构型2在三个平衡构型中能量最低.构型2的解离能(2→H 2C ‗Ge(1A 1)+LiCl)为128.53kJ ·mol -1.构型3可以称为σ⁃配合物型构型,它可看作LiCl 分子以电正性的Li 端接近Ge 原子的σ占据轨道形成Ge →Li 授受键而得到的一种配合物.结果表明构型3属C 2v 点群.构型3中,C ‗Ge 键长(0.1780nm)比H 2C ‗Ge 中的C ‗Ge 键长(0.1798nm)短,而Li —Cl 键长(0.2040nm)比LiCl 分子中Li —Cl 键长稍长.从表1可以看出,构型3是三个平衡构型中能量最高的构型.构型3的解离能(3→H 2C ‗Ge(1A 1)+LiCl)为28.86kJ ·mol -1.综上所述,不饱和类锗烯H 2C ‗GeLiCl 共有三种平衡构型,三种平衡构型均可视为不饱和锗烯H 2C ‗Ge:与LiCl 的加成物.从各平衡构型能量上分析(见表1),H 2C ‗GeLiCl 三种平衡构型的热力学稳定次序为:2>1>3.2.2构型间的异构化反应及动力学稳定性图2中构型4和5为H 2C ‗GeLiCl 势能面上的两个过渡态,由它们的反应矢量(能量二阶导数的唯一负本征值对应的本征矢)及IRC 计算结果分析可知,构型4为1与2异构化的过渡态,构型5为1与3异构化的过渡态.4和5的反应矢量(e )分别为e (4)=-0.003L CGe -0.002L ClGe -1.089θH1CGe +9.454θClGeC -2.521θLiClGe -2.578θClGeCH1+56.207θLiClGeC -3.323θH2CGeH1e (5)=0.002L LiGe +7.219θLiGeC -34.779θClLiGe式中,键长(L )和键角(θ)的单位分别为nm 和(°).键长系数小于0.0005和键角系数小于0.5的全部忽略.振动分析表明,构型4和5均存在唯一的虚频.在B3LYP/6⁃311G(d,p )水平上,4和5的虚频分别为84.4i 和27.1i,从而确证为真正的过渡态.为了确证过渡态与稳定几何构型的连接,以得到的过渡态为起始点,沿着反应途径分别向前和向后进行了IRC 计算,结果表明过渡态结构与稳定几何构型的连接正确.一般说来,各平衡构型的稳定性取决于它们自身能量的高低及相互异构化的活化能.由图3可以比较直观地看到,构型1异构为3的活化势垒为45.83kJ ·mol -1,而构型3异构为1的活化势垒仅为0.25kJ ·mol -1,所以构型3很容易异构为构型1.构型1异构为2的活化势垒为6.10kJ ·mol -1,而构型2异构为1的活化势垒为60.19kJ ·mol -1,所以构型1很容易异构为构型2.构型2应该是H 2C ‗GeLiCl 存在的主要结构.2.3最稳定构型的振动频率和红外吸收强度由上述计算结果可知,构型2是不饱和类锗烯H 2C ‗GeLiCl 存在的主要结构.为了给不饱和类锗烯H 2C ‗GeLiCl 的结构分析提供参考,将在B3LYP/6⁃311G(d,p )水平上计算得到的构型2的振动频率及红外吸收强度列于表2.图3H 2C ‗GeLiCl 的势能面沿构型异构化反应通道的剖面图Fig.3The potential profile for the isomerizationreactions of H 2C ‗GeLiClGeometriesE tot (a.u.)E rel /(kJ ·mol -1)1030μ/(C ·m)N imag 1-2584.1260019018.9902-2584.1466034-54.0913.0903-2584.108641945.5829.8104-2584.1236796 6.1017.7915-2584.108544545.8328.6816-2116.2645558-0.6907-467.8330933-23.94表1B3LYP/6⁃311G(d,p )水平上计算的H 2C ‗GeLiCl的总能量(E tot ),相对能量(E rel ),偶极矩(μ)及虚频个数(N imag )Table 1The B3LYP/6⁃311G(d,p )calculated total energies(E tot ),relative energies(E rel ),dipole moments (μ),and number of imaginary frequency (N imag )of thegeometries for H 2C ‗GeLiCl1813Acta Phys.⁃Chim.Sin.,2007Vol.233结论应用密度泛函理论DFT 方法研究了不饱和类锗烯H 2C ‗GeLiCl 的结构及异构化反应.B3LYP/6⁃311G(d,p )计算结果表明,不饱和类锗烯H 2C ‗GeLiCl 有3种可能的平衡构型,其中p 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03,Revision B.03.Pittsburgh,PA,200330Hehre,W.J.;Radom,L.;Schleyer,P.V.R;Pople,J.A.Ab initio molecular orbital theory.New York:Wiley,198631He,S.G.;Tackett,B.S.;Clouthier,D.J.J.Chem.Phys.,2004,121:257ν/cm-1I /(km ·mol -1)ν/cm -1I /(km ·mol -1)172.230.61625.725.35202.445.15678.852.18252.7 6.43744.7158.12442.960.781381.010.81451.068.843021.614.69566.421.663140.37.78表2图2中不饱和类锗烯H 2C ‗GeLiCl 的构型2的振动频率(ν)及红外吸收强度(I )Table 2Calculated vibrational frequencies(ν)and infrared intensities(I )of configuration 2in Fig.2forH 2C ‗GeLiCl1814。

附录附录A:外文资料翻译—原文部分SemiconductorA semiconductor is a solid material that has electrical conductivity between those of a conductor and an insulator; it can vary over that wide range either permanently or dynamically.[1]Semiconductors are important in electronic technology. Semiconductor devices, electronic components made of semiconductor materials, are essential in modern consumer electronics, including computers, mobile phones, and digital audio players. Silicon is used to create most semiconductors commercially, but dozens of other materials are used.Bragg reflection in a diffuse latticeA second way starts with free electrons waves. When fading in an electrostatic potential due to the cores, due to Bragg reflection some waves are reflected and cannot penetrate the bulk, that is a band gap opens. In this description it is not clear, while the number of electrons fills up exactly all states below the gap.Energy level splitting due to spin state Pauli exclusionA third description starts with two atoms. The split states form a covalent bond where two electrons with spin up and spin down are mostly in between the two atoms. Adding more atoms now is supposed not to lead to splitting, but to more bonds. This is the way silicon is typically drawn. The band gap is now formed by lifting one electron from the lower electron level into the upper level. This level is known to be anti-bonding, but bulk silicon has not been seen to lose atoms as easy as electrons are wandering through it. Also this model is most unsuitable to explain how in graded hetero-junction the band gap can vary smoothly.Energy bands and electrical conductionLike in other solids, the electrons in semiconductors can have energies only within certain bands (ie. ranges of levels of energy) between the energy of the ground state, corresponding to electrons tightly bound to the atomic nuclei of the material, and the free electron energy, which is the energy required for an electron to escape entirely from the material. The energy bands each correspond to a large number of discrete quantum states of the electrons, and most of the states with low energy (closer to the nucleus) are full, up to a particular band called the valence band. Semiconductors and insulators are distinguished from metals because the valence band in the semiconductor materials is very nearly full under usual operating conditions, thus causing more electrons to be available in the conduction band.The ease with which electrons in a semiconductor can be excited from the valence band to the conduction band depends on the band gap between the bands, and it is the size of this energybandgap that serves as an arbitrary dividing line (roughly 4 eV) between semiconductors and insulators.In the picture of covalent bonds, an electron moves by hopping to a neighboring bond. Because of the Pauli exclusion principle it has to be lifted into the higher anti-bonding state of that bond. In the picture of delocalized states, for example in one dimension that is in a wire, for every energy there is a state with electrons flowing in one direction and one state for the electrons flowing in the other. For a net current to flow some more states for one direction than for the other direction have to be occupied and for this energy is needed. For a metal this can be a very small energy in the semiconductor the next higher states lie above the band gap. Often this is stated as: full bands do not contribute to the electrical conductivity. However, as the temperature of a semiconductor rises above absolute zero, there is more energy in the semiconductor to spend on lattice vibration and — more importantly for us — on lifting some electrons into an energy states of the conduction band, which is the band immediately above the valence band. The current-carrying electrons in the conduction band are known as "free electrons", although they are often simply called "electrons" if context allows this usage to be clear.Electrons excited to the conduction band also leave behind electron holes, or unoccupied states in the valence band. Both the conduction band electrons and the valence band holes contribute to electrical conductivity. The holes themselves don't actually move, but a neighboring electron can move to fill the hole, leaving a hole at the place it has just come from, and in this way the holes appear to move, and the holes behave as if they were actual positively charged particles.One covalent bond between neighboring atoms in the solid is ten times stronger than the binding of the single electron to the atom, so freeing the electron does not imply destruction of the crystal structure.Holes: electron absence as a charge carrierThe notion of holes, which was introduced for semiconductors, can also be applied to metals, where the Fermi level lies within the conduction band. With most metals the Hall effect reveals electrons to be the charge carriers, but some metals have a mostly filled conduction band, and the Hall effect reveals positive charge carriers, which are not the ion-cores, but holes. Contrast this to some conductors like solutions of salts, or plasma. In the case of a metal, only a small amount of energy is needed for the electrons to find other unoccupied states to move into, and hence for current to flow. Sometimes even in this case it may be said that a hole was left behind, to explain why the electron does not fall back to lower energies: It cannot find a hole. In the end in both materials electron-phonon scattering and defects are the dominant causes for resistance.Fermi-Dirac distribution. States with energy εbelow the Fermi energy, here μ, have higher probability n to be occupied, and those above are less likely to be occupied. Smearing of the distribution increases with temperature.The energy distribution of the electrons determines which of the states are filled and which are empty. This distribution is described by Fermi-Dirac statistics. The distribution is characterized bythe temperature of the electrons, and the Fermi energy or Fermi level. Under absolute zero conditions the Fermi energy can be thought of as the energy up to which available electron states are occupied. At higher temperatures, the Fermi energy is the energy at which the probability of a state being occupied has fallen to 0.5.The dependence of the electron energy distribution on temperature also explains why the conductivity of a semiconductor has a strong temperature dependency, as a semiconductor operating at lower temperatures will have fewer available free electrons and holes able to do the work.Energy–momentum dispersionIn the preceding description an important fact is ignored for the sake of simplicity: the dispersion of the energy. The reason that the energies of the states are broadened into a band is that the energy depends on the value of the wave vector, or k-vector, of the electron. The k-vector, in quantum mechanics, is the representation of the momentum of a particle.The dispersion relationship determines the effective mass, m* , of electrons or holes in the semiconductor, according to the formula:The effective mass is important as it affects many of the electrical properties of the semiconductor, such as the electron or hole mobility, which in turn influences the diffusivity of the charge carriers and the electrical conductivity of the semiconductor.Typically the effective mass of electrons and holes are different. This affects the relative performance of p-channel and n-channel IGFETs, for example (Muller & Kamins 1986:427).The top of the valence band and the bottom of the conduction band might not occur at that same value of k. Materials with this situation, such as silicon and germanium, are known as indirect bandgap materials. Materials in which the band extrema are aligned in k, for example gallium arsenide, are called direct bandgap semiconductors. Direct gap semiconductors are particularly important in optoelectronics because they are much more efficient as light emitters than indirect gap materials.Carrier generation and recombinationWhen ionizing radiation strikes a semiconductor, it may excite an electron out of its energy level and consequently leave a hole. This process is known as electron–hole pair generation.Electron-hole pairs are constantly generated from thermal energy as well, in the absence of any external energy source.Electron-hole pairs are also apt to recombine. Conservation of energy demands that these recombination events, in which an electron loses an amount of energy larger than the band gap, beaccompanied by the emission of thermal energy (in the form of phonons) or radiation (in the form of photons).In some states, the generation and recombination of electron–hole pairs are in equipoise. The number of electron-hole pairs in the steady state at a given temperature is determined by quantum statistical mechanics. The precise quantum mechanical mechanisms of generation and recombination are governed by conservation of energy and conservation of momentum.As the probability that electrons and holes meet together is proportional to the product of their amounts, the product is in steady state nearly constant at a given temperature, providing that there is no significant electric field (which might "flush" carriers of both types, or move them from neighbour regions containing more of them to meet together) or externally driven pair generation. The product is a function of the temperature, as the probability of getting enough thermal energy to produce a pair increases with temperature, being approximately 1×exp(−E G / kT), where k is Boltzmann's constant, T is absolute temperature and E G is band gap.The probability of meeting is increased by carrier traps – impurities or dislocations which can trap an electron or hole and hold it until a pair is completed. Such carrier traps are sometimes purposely added to reduce the time needed to reach the steady state.DopingThe property of semiconductors that makes them most useful for constructing electronic devices is that their conductivity may easily be modified by introducing impurities into their crystal lattice. The process of adding controlled impurities to a semiconductor is known as doping. The amount of impurity, or dopant, added to an intrinsic (pure) semiconductor varies its level of conductivity. Doped semiconductors are often referred to as extrinsic.DopantsThe materials chosen as suitable dopants depend on the atomic properties of both the dopant and the material to be doped. In general, dopants that produce the desired controlled changes are classified as either electron acceptors or donors. A donor atom that activates (that is, becomes incorporated into the crystal lattice) donates weakly-bound valence electrons to the material, creating excess negative charge carriers. These weakly-bound electrons can move about in the crystal lattice relatively freely and can facilitate conduction in the presence of an electric field. (The donor atoms introduce some states under, but very close to the conduction band edge. Electrons at these states can be easily excited to conduction band, becoming free electrons, at room temperature.) Conversely, an activated acceptor produces a hole. Semiconductors doped with donor impurities are called n-type, while those doped with acceptor impurities are known as p-type. The n and p type designations indicate which charge carrier acts as the material's majority carrier. The opposite carrier is called the minority carrier, which exists due to thermal excitation at a much lower concentration compared to the majority carrier.For example, the pure semiconductor silicon has four valence electrons. In silicon, the most common dopants are IUPAC group 13 (commonly known as group III) and group 15 (commonly known as group V) elements. Group 13 elements all contain three valence electrons, causing them to function as acceptors when used to dope silicon. Group 15 elements have five valence electrons, which allows them to act as a donor. Therefore, a silicon crystal doped with boron creates a p-type semiconductor whereas one doped with phosphorus results in ann-type material.Carrier concentrationThe concentration of dopant introduced to an intrinsic semiconductor determines its concentration and indirectly affects many of its electrical properties. The most important factor that doping directly affects is the material's carrier concentration. In an intrinsic semiconductor under thermal equilibrium, the concentration of electrons and holes is equivalent. That is,n = p = n iIf we have a non-intrinsic semiconductor in thermal equilibrium the relation becomes:n0 * p0 = (n i)2Where n is the concentration of conducting electrons, p is the electron hole concentration, and n i is the material's intrinsic carrier concentration. Intrinsic carrier concentration varies between materials and is dependent on temperature. Silicon's n i, for example, is roughly 1.6×1010 cm-3 at 300 kelvin (room temperature).In general, an increase in doping concentration affords an increase in conductivity due to the higher concentration of carriers available for conduction. Degenerately (very highly) doped semiconductors have conductivity levels comparable to metals and are often used in modern integrated circuits as a replacement for metal. Often superscript plus and minus symbols are used to denote relative doping concentration in semiconductors. For example, n+ denotes an n-type semiconductor with a high, often degenerate, doping concentration. Similarly, p−would indicate a very lightly doped p-type material. It is useful to note that even degenerate levels of doping imply low concentrations of impurities with respect to the base semiconductor. In crystalline intrinsic silicon, there are approximately 5×1022 atoms/cm³. Doping concentration for silicon semiconductors may range anywhere from 1013 cm-3 to 1018 cm-3. Doping concentration above about 1018 cm-3 is considered degenerate at room temperature. Degenerately doped silicon contains a proportion of impurity to silicon in the order of parts per thousand. This proportion may be reduced to parts per billion in very lightly doped silicon. Typical concentration values fall somewhere in this range and are tailored to produce the desired properties in the device that the semiconductor is intended for.Effect on band structureDoping a semiconductor crystal introduces allowed energy states within the band gap but very close to the energy band that corresponds with the dopant type. In other words, donor impurities create states near the conduction band while acceptors create states near the valence band. The gap between these energy states and the nearest energy band is usually referred to as dopant-sitebonding energy or E B and is relatively small. For example, the E B for boron in silicon bulk is0.045 eV, compared with silicon's band gap of about 1.12 eV. Because E B is so small, it takes little energy to ionize the dopant atoms and create free carriers in the conduction or valence bands. Usually the thermal energy available at room temperature is sufficient to ionize most of the dopant.Dopants also have the important effect of shifting the material's Fermi level towards the energy band that corresponds with the dopant with the greatest concentration. Since the Fermi level must remain constant in a system in thermodynamic equilibrium, stacking layers of materials with different properties leads to many useful electrical properties. For example, the p-n junction's properties are due to the energy band bending that happens as a result of lining up the Fermi levels in contacting regions of p-type and n-type material.This effect is shown in a band diagram. The band diagram typically indicates the variation in the valence band and conduction band edges versus some spatial dimension, often denoted x. The Fermi energy is also usually indicated in the diagram. Sometimes the intrinsic Fermi energy, E i, which is the Fermi level in the absence of doping, is shown. These diagrams are useful in explaining the operation of many kinds of semiconductor devices.Preparation of semiconductor materialsSemiconductors with predictable, reliable electronic properties are necessary for mass production. The level of chemical purity needed is extremely high because the presence of impurities even in very small proportions can have large effects on the properties of the material. A high degree of crystalline perfection is also required, since faults in crystal structure (such as dislocations, twins, and stacking faults) interfere with the semiconducting properties of the material. Crystalline faults are a major cause of defective semiconductor devices. The larger the crystal, the more difficult it is to achieve the necessary perfection. Current mass production processes use crystal ingots between four and twelve inches (300 mm) in diameter which are grown as cylinders and sliced into wafers.Because of the required level of chemical purity and the perfection of the crystal structure which are needed to make semiconductor devices, special methods have been developed to produce the initial semiconductor material. A technique for achieving high purity includes growing the crystal using the Czochralski process. An additional step that can be used to further increase purity is known as zone refining. In zone refining, part of a solid crystal is melted. The impurities tend to concentrate in the melted region, while the desired material recrystalizes leaving the solid material more pure and with fewer crystalline faults.In manufacturing semiconductor devices involving heterojunctions between different semiconductor materials, the lattice constant, which is the length of the repeating element of the crystal structure, is important for determining the compatibility of materials.附录B:外文资料翻译—译文部分半导体半导体是一种导电性能介于导体与绝缘体之间的固体材料。

外文文献检索报告检索关键词：the Triazene Reagents检索过程：打开校园网，点击“图书馆电子资源”，进入界面后点击“外文电子资源”，并进入“Elsevier Science 电子期刊”，点击“Search”，并在其中输入“the Triazene Reagents”，检索后即可得到大约15页有关氮烯化合物的研究论文，从中选取几篇论文列举如下，检索结果：【1】Synthesis of a new triazene reagent and its application for the determination of silver(I) by the Rayleigh light-scattering ；Analytica Chimica Acta；Received 6 August 1998; accepted 10December 1998，page,45-50;Yunkun Zhaoa, Qiu-E Caoa, Zhide Hua, Qiheng XubISSN: 0003-2670【2】The synthesis and application of1-(o-nitrophenyl)-3-(2-thiazolyl)triazene for the determination of palladium(II) by the resonance enhanced Rayleighlight-scattering technique；Spectrochimica Acta Part A； Molecular and Biomolecular Spectroscopy；Received 29 June 1999; accepted 30 September 1999；page45-50; Yunkun Zhaoa, Qiu-E Caoa, Zhide Hua, Qiheng XubISSN: 1386-1425【3】A facile route to the new triazene dyes based on substituted pyrazolidin-3,5-dione derivatives；Dyes and Pigments；Received 14 April 2011 Received in revised form 28 July 2011 Accepted 29 July 2011 Available online 9 August 2011；page45-50,Jalal Isaad，Fouad Malek, Ahmida El Achari；ISSN: 0143-7208【4】Novel platinum(II) selective membrane electrode based on 1,3-bis(2-cyanobenzene)triazene；Talanta；Received 29 October 2008 Received in revised form December 2008 Accepted 30 December 2008 Available online 22 January 2009；page121-125,Mohammad BagherGholivand, Moslem Mohammadi,Mehdi Khodadadian, Mohammad KazemRofouei；ISSN: 0039-9140【4】Mercury(II) selective membrane electrode based on1,3-bis(2-methoxybenzene)triazene；Materials Science andEngineering: C；Received 10 February 2009 Received in revised form18 April 2009 Accepted 27 April 2009 Available online 5 May 2009；page87-92,Mohammad Kazem Rofouei , Moslem Mohammadi , Mohammad Bagher Gholivand；ISSN: 0928-4931【5】Structural and solution studies of a novel tetranuclear silver(I) cluster of [1,3-di(2-methoxy)benzene]triazene；Inorganica ChimicaActa；Received 19 May 2006; received in revised form 17 September 2006; accepted 20 September 2006，Available online 29 September 2006；page2015-2019,Mahmood Payehghadr, Mohammad Kazem Rofouei, Ali Morsali Mojtaba Shamsipur；ISSN: 0020-1693二次检索：下面进行二次检索，在检索结果的左上方对文章进行筛选，在“Content Type”中选择“Journal”,在“Journal”中选择“Tetrahedron”，在“Year”中选择“2012(13)”，点击“Limit to”,检索得到一篇符合要求的文献，列举如下：Synthesis of 1,3,5-trisubstituted-1,2,4-triazoles bymicrowave-assisted N-acylation of amide derivatives and theconsecutive reaction with hydrazine hydrochlorides；Tetrahedron；Received 30 December 2011 Accepted 4 January 2012 Available online8 January 2012；page2045-2051; Jongbok Lee, Myengchan Hong, YoonchulJung, Eun Jin Cho , Hakjune Rhee;ISSN: 0040-4020对文章内容分析如下：Background:1,2,4-Triazole analogues have been attracting attentionover the last decade due to their biological activities, such as antiviral, antitumor, and anti-inflammatory activities.1e3 1,2,4-Triazoles can be prepared by the reaction of hydrazonyl chlorides with aromatic or aliphatic nitriles in the presence of AlCl3 or Yb(OTf)3.4,5 Its synthesis can also be accomplished by cyclization of 1,2,4-triazene from hydrazonyl chlorides with a primary amine in the presence of triethylamine followed by oxidation using various oxidizing agents, such as Ag2CO3, NaOCl, Ca(OCl)2, DesseMartin periodinane, Ley’s TPAP/NMO, and H2O2/KOH.These methods, however, have limited functional group tolerance and require more than stoichiometric amounts of reagents. Thus it is desirable to develop more efficient and general method for the preparation of 1,3,5- trisubstituted-1,2,4-triazoles under mild reaction conditions. Over recent years, a great number of publications have reported.Study significance: Facile and efficient procedures for theN-acylation reaction of amide derivatives with various acid anhydrides and the cyclization reaction of N-acylated amide derivatives with various hydrazine hydrochlorides were described. The reactions were carried out under microwave irradiation to give products in good yields in a few minutes. The synthesis of 1,3,5-trisubstituted-1,2,4-triazoles from benzamides can also be accomplished in a simple one-pot sequential reaction.Research institution： Department of Bionanotechnology, HanyangUniversity, Sa 3-Dong 1271, Ansan, Kyunggi-Do 426-791, Republic of Korea Department of Chemistry and Applied Chemistry, Hanyang University, Sa 3-Dong 1271, Ansan, Kyunggi-Do 426-791, Republic of Korea.References and notes:【1】. Al-Soud, Y. A.; Al-Dweri, M. N.; Al-Masoudi, N. A. Farmaco 2004, 59, 775.【2】. Demirbas, N.; Ugurluoglu, R.; Demirbas, A. Bioorg. Med. Chem. 2002, 10, 3717.【3】. Tozkoparan, B.; Gokhan, N.; Aktay, G.; Yesilada, E.; Ertan, M. Eur. J. Med. Chem.2000, 35, 743.【4】. Conde, S.; Corral, C.; Madronero, R. Synthesis 1974, 28. 【5】. Su, W.; Yang, D.; Li, J. Synth. Commun. 2005, 35, 1435. 【6】. Paulvannan, K.; Chen, T.; Hale, R. Tetrahedron 2000, 56, 8071. 【7】. Paulvannan, K.; Hale, R.; Sedehi, D.; Chen, T. Tetrahedron 2001, 57, 9677.【8】. Buzykin, B. I.; Bredikhina, Z. A. Synthesis 1993, 59.【9】. Caddik, S. Tetrahedron 1995, 51, 10403.【10】. Lidstrom, P.; Tierney, J.; Wathey, B.; Westman, J. Tetrahedron 2001, 57, 9225.【11】. Jagerovic, N.; Hernandez-Folgado, L.; Alkorta, I.; Goya, P.; Navarro, M.; Serrano,A.; Fonseca, F. R.; Dannert, M. T.; Alsasua, A.; Suardiaz, M.; Pascual, D.; Martin,M. I. J. Med. Chem. 2004, 47, 2939. 【12】. Czollner, L.; Szilagyi, G.; Lango, J.; Janaky, J. Arch. Pharm. 1990, 323, 225.【13】. Yoon, U. C.; Cho, S. J.; Lee, Y. J.; Mancheno, M. J.; Mariano, P. S. J. Org. Chem. 1995,60, 2353.。

硅材料作文英文回答：Silicon: A Versatile Material with Countless Applications。

Silicon is the second most abundant element on Earth, after oxygen. It is a metalloid that has properties of both metals and non-metals. Silicon is used in a wide variety of applications, including electronics, solar cells, and glass.Electronics: Silicon is the primary material used in semiconductors, which are essential for electronic devices such as computers, smartphones, and digital cameras.Silicon's ability to conduct electricity makes it ideal for use in these devices.Solar Cells: Silicon is also used in solar cells,which convert sunlight into electricity. Silicon is a good absorber of light, and it has a high efficiency inconverting light into electricity.Glass: Silicon dioxide (SiO2) is the main component of glass. Glass is used in a variety of applications,including windows, bottles, and containers. Silicon dioxide is also used as a coating for other materials, such as metals and plastics.In addition to these applications, silicon is also used in a variety of other products, including:Ceramics: Silicon is used in ceramics, which are used in a variety of applications, including cookware, tiles, and pottery.Rubber: Silicon is used in rubber, which is used in a variety of applications, including tires, hoses, and seals.Medical Devices: Silicon is used in medical devices, such as implants, prosthetics, and catheters.Silicon is a versatile material with countlessapplications. It is essential for the electronics industry, and it is also used in a wide range of other products. Silicon is a key material for the 21st century and will continue to be used in a variety of applications for many years to come.中文回答：硅材料，用途广泛的多功能材料。

Adsorption of Organic Acids on Polyaminated Highly Porous Chitosan:EquilibriaWataru Takatsuji*,†Industrial Technology Center of Wakayama Prefecture,60Ogura,Wakayama649-6261,JapanHiroyuki YoshidaDepartment of Chemical Engineering,Osaka Prefecture University,1-1Gakuen-Cho,Sakai599-8531,JapanThe adsorption of organic acids on a new weakly basic ion exchanger,highly porous polyaminatedchitosan(Chitopearl CCS),which has the primary amino group of chitosan and the primary,secondary,and tertiary amino groups of poly(ethylene imine),appeared feasible technically.Three different organic acids,acetic acid(R′-COOH),malic acid(R′′-(COOH)2)and citric acid(R′′′-(COOH)3)were used in this experimental study.These organic acids were adsorbed on theresin by an acid/base neutralization reaction.The adsorption isotherms were independent ofthe initial concentrations of organic acids.The theoretical equations for the adsorption isothermswere derived by considering the dissociation of organic acids in the solution and the adsorptionon each functional group and by applying the mass action law.They correlated the experimentaladsorption isotherms and titration curves well.Chitopearl CCS was observed to be a feasiblemedium for the adsorption of organic acids.Especially in high pH region(low concentrationregion),Chitopearl CCS could adsorb more organic acids than a commercial ion exchanger,DIAION WA30.1.IntroductionOrganic acids have been used in a number of chemical industries.Recently,attention has been focused on the main components of biodegradable plastics.Almost all organic acids have been produced by decomposition of organics using a microorganism or fermentation.Waste sludge may become important organics that produce organic acids,and this may give one of the solutions to solve the big problem of waste sludge.After producing organic acids by the above methods,separation and purification are important from the standpoint of cost and quality of products.Although public information is limited,organic acids have been separated by means of esterification and calcium precipitation in almost all commercial plants.These methods do not offer highly purified organic acids.In addition,they consume a lot of energy and also the capital costs are high.Recently, new purification technologies,such as electrodialysis and adsorption,have been investigated to overcome the above problems.Weakly basic resins are commonly used for removal and/or recovery of acids from industrial aqueous streams. Helfferich(1965)proposed a model that HCl was ad-sorbed on weakly basic resins by ion exchange ac-companied by an acid/base neutralization reaction. After his review,Adams et al.(1969),Warner and Kennedy(1970),Hubner and Kadlec(1978),Rao and Gupta(1982a,b),Helfferich and Hwang(1985)and Bhandari et al.(1992,1993)presented papers dealing with the adsorption of acids on weakly basic resins. However,few studies have been reported on the adsorp-tion of weak acids on weakly basic resins.Takatsuji and Yoshida(1994)reported that DIAION WA30(Mit-subishi Chemical Co.,Japan,a comercial weakly basic resin)and Chitopearl CCS(Fuji Spinning Co.,Japan, polyaminated highly porous chitosan bead)were excel-lent adsorbents for organic acids from wine,which contained ethanol,glucose,and various organic acids. They also presented the theoretical equations for the adsorption isotherms of the organic acids on DIAION WA30,which has a tertiary amino group as a fixed functional group(Takatsuji and Yoshida,1997).As standard weakly basic ion exchangers such as Dowex WGR,DIAION WA10and Russian AN have two to three different fixed amino groups in the resin phase,a more general approach is necessary for under-standing the adsorption of organic acids on weakly basic resins.In the present work,we have investigated the adsorp-tion isotherms of organic acids on Chitopearl CCS.It is a new weakly basic ion exchanger that was fabricated by introducing poly(ethylene imine)into the macropore of highly porous cross-linked chitosan.Chitosan is produced by the deacetylation of chitin,which is a natural biopolymer extracted from the shell of arthro-pods such as lobsters,shrimp,and crabs.Since such arthropods are abundantly available,chitosan may be produced from them very cheaply,and as chitosan is harmless to human,it may be utilized for ion exchang-ers and adsorbents in the food and pharmaceutical industries.Chitopearl CCS may have at least four different fixed groups,the primary amino group of chitosan and the primary,secondary,and tertiary amino groups of poly(ethylene imine).The derivation of the theoretical equations on the adsorption of organic acids is more complicated than DIAION WA30.Yoshida et al.(1994)derived theoretical equations for adsorption of HCl on the polyaminated highly porous chitosan bead. In this work,to make clear the effect of the number of carboxylic group of organic acid on the adsorption†Telephone:INT+81-734-77-1271.Fax:INT+81-734-77-2880.1300Ind.Eng.Chem.Res.1998,37,1300-1309S0888-5885(97)00567-8CCC:$15.00©1998American Chemical SocietyPublished on Web02/18/1998isotherm,acetic acid(R′-COOH),malic acid(R′′-(COOH)2),and citric acid(R′′′-(COOH)3)were used in the experimental study.Assuming that these organic acids are adsorbed by the neutralization reaction with each functional group on the weakly basic resin and applying the mass action law,we derived theoretical equations for adsorption isotherms.The experimental adsorption isotherms and titration curves are compared with those theoretical equations.Further,the results obtained in the present work are compared with the results for adsorption of organic acids on DIAION WA30,which has one kind of functional group.2.Experimental SectionBefore measuring the adsorption isotherms,about10 cm3of the ion exchange particles were placed in acolumn,and0.5kmol/m3of NaOH aqueous solution was allowed to flow through the bed at a flow rate of7×10-5m3/h for3days.Thereafter the bed was washed with distilled and deionized water thoroughly.Finally, the resin particles were kept in pure water. Adsorption isotherms were measured by the batch method.Before the resin particles were weighed,the water around the particles was removed using a cen-trifugal filter(Sanyou Rikagaku-kiki Seisakusho)ro-tated at5000rpm for3min.Thereafter,they were put in contact with the HCl solution or each organic acid solution and well mixed.The amounts of HCl,acetic acid,malic acid,and citric acid adsorbed on the resin were measured after5,7,and11days.Since there was no difference between the results for5and7days with respect to HCl,the resin particles and HCl solution were put in contact for5days.Since the equilibrium for acetic acid,malic acid and citric acid were fully reached after7days,the resin particles and each organic acid solution were put in contact for7days.The pH of the solution was analyzed with a Horiba pH meter F-7AD. The concentration of organic acid was analyzed with a Shimadzu high performance liquid chromatography organic acid analysis system.The adsorbed phase concentration was calculated according to eq1,where C0and C are the initial concentration and the equilib-rium concentration in the liquid phase(kmol/m3), respectively.q denotes the adsorbent-phase concentra-tion(kmol/m3of wet resin).V and W are the volume of the solution(m3)and the weight of the wet resin particles(kg),respectively.F is the apparent density (kg of wet resin/m3of wet resin).All experiments were carried out at293K.3.Ion ExchangerThe ion exchanger used in this experimental study was a new weakly basic resin,Chitopearl CCS(Fuji Spinning Co.).Chitopearl CCS was fabricated by introducing poly(ethylene imine)(hereafter called PEI) with a molecular weight of10000into the cross-linked chitosan,and its functional groups consisted of four different amino groups,the primary amino group of chitosan and the primary,secondary and tertiary amino groups of PEI.The experimental physical properties of Chitopearl CCS are compared with those of DIAION WA30(Mitsubishi Chemical Co.),which is a commerial weakly basic resin,in Table1.The concentration of each amino group in the resin was determined by measuring the equilibrium isotherm for adsorption of HCl.The experimental equilibrium coefficients K and saturation capacities Q were determined according to the procedure presented by Yoshida et al.(1994).They are listed in Table2.Table1.Experimental Physical Properties of Chitopearl CCS and DIAION WA30resin functional group diameter(cm)apparent density(kg/m3)porositychitosan derivative(spherical particles)Chitopearl CCS a R′,-NHR;R′′,-NHCH2C(OH)HCH2-PEI c0.0625411360.831 styreneDIAION WA30b–CH–CH2–CH–CH2N(CH3)20.0650511310.486a Fuji Spinning Co.,Ltd.b Mitsubishi Chemical Co.,Ltd.c PEI:poly(ethylene imine)of which the molecular weight is10000.Structure of Chitosan resin:q)(C-C)F VW(1)Table2.Experimental Coefficients of HCl and AceticAcid(AH)ai)C i)P1i)P2i)P3tot.Adsorption of HClChitopearl CCSK i,HCl(m3/kmol)8.2×10576 1.4×103 2.9×104Q i(kmol/m3)0.5980.6140.9300.692 2.83DIAION WA30K i,HCl(m3/kmol) 3.8×104Q i(kmol/m3) 2.80 2.80Adsorption of AHChitopearl CCSK i,A(m3/kmol) 5.5×1051934 2.8×103Q i,A(kmol/m3)0.6300.4630.8680.690 2.65DIAION WA30K i,A(m3/kmol) 1.5×102Q i,A(kmol/m3) 2.94 2.94a AH:CH3COOH.Ind.Eng.Chem.Res.,Vol.37,No.4,199813014.Results and Discussion4.1.Adsorption of Acetic Acid.Figure 1a shows the experimental adsorption isotherm for the Chitopearl CCS -acetic acid system.The adsorption isotherm is not affected by the initial liquid-phase concentration of acetic acid C A0.Since acetic acid molecule (AH)has one carboxylic group,it may be adsorbed on each functional group of the resin by the following reactions:The elementary reactions are shown in Appendix 1.Applying the mass action law to eqs 2-6,eqs 7-10are derived,where q C,A ,q P3,A ,q P2,A ,and q P1,A denote theequilibrium concentrations of acetic acid adsorbed on R C -NH 2,R P3-N,R P2-NH,and R P1-NH 2,respectively (kmol/m 3wet resin).Q C,A ,Q P3,A ,Q P2,A ,and Q P1,A show the saturation capacities on R C -NH 2,R P3-N,R P2-NH,and R P1-NH 2,respectively (kmol/m 3wet resin).[AH]and [A -]represent the equilibrium concentrations of CH 3COOH and CH3COO -in the liquid phase (kmol/m 3),respectively.[AH]can be calculated using the Henderson -Hasselbalch equation (Glasstone and Lewis,1960).The equilibrium concentration of acetic acid in the liquid phase is given by eq 11,where p K a is 4.56at293K (Martell and Smith,1974,1975,1977).The totalamount of acetic acid adsorbed on Chitopearl CCS is given by eq 12.The values of K and Q in eqs 7-10were determined by the same procedure as the adsorption for HCl shown by Yoshida et al.(1994).Since R C -NH 2shows the strongest basicity,only eq 3occurs in the first step.Theadsorption isotherm is expressed by eq 7,and it is transformed to eq 13.Figure 2shows the plots of the data for pH >4.8based on eq 13.The straight line was determined using the least-squares method in pH >4.8.The correlation coefficient was 0.969.K C,A and Q C,A were determined from the intercept and slope of the straight line,respectively.They are listed in Table 2.Next,acetic acid is adsorbed on R C -NH 2and R P3-N,which shows the second strongest basicity,simulta-neously in the second step.Therefore,eqs 3and 4occur simultaneously,and eq 14is valid.Equation 14is transformed to eq 15using eq 7,in which the values ofAH y \z K aH ++A -(2)R C -NH 2+AH y\z K C,AR C -NH 3+A -(3)R P3-N +AH y\z K P3,A R P3-NH +A -(4)R P2-NH +AH y\z K P2,A R P2-NH 2+A -(5)R P1-NH 2+AH y\z K P1,A R P1-NH 3+A -(6)q C,A )K C,A Q C,A [AH]1+K C,A [AH](7)q P3,A )K P3,A Q P3,A [AH]1+K P3,A [AH](8)q P2,A )K P2,A Q P2,A [AH]1+K P2,A [AH](9)q P1,A )K P1,A Q P1,A [AH]1+K P1,A [AH](10)C A )[AH]+[A -])10-pH (1+10pH -p K a )10pH -p K a(11)q A )q C,A +q P3,A +q P2,A +q P1,A(12)Figure 1.Experimental equilibrium isotherms for adsorption of acetic,malic,and citric acids on Chitopearl CCS.Key:(O )C 0)0.6kmol/m 3;(4)C 0)0.52kmol/m 3;(0)C 0)0.5kmol/m 3;(3)C 0)0.42kmol/m 3;(])C 0)0.4kmol/m 3;(y )C 0)0.3kmol/m 3;(5)C 0)0.26kmol/m 3;(!)C 0)0.25kmol/m 3;(8)C 0)0.2kmol/m 3;(*)C 0)0.12kmol/m 3;(Y )C 0)0.1kmol/m 3;(6)C 0)0.05kmol/m 3;(@)C 0)0.03kmol/m 3;(7)C 0)0.02kmol/m 3,(&)C 0)0.01kmol/m 3;(b )C 0)0.005kmol/m 3;(2)C 0)0.002kmol/m 3;(9)C 0)0.001kmol/m 3.Figure 2.Plots of data for pH >4.8for adsorption of acetic acid on Chitopearl CCS based on eq 13.[AH])-1K C,A +Q C,A[AH]q A(13)1302Ind.Eng.Chem.Res.,Vol.37,No.4,1998K C,A and Q C,A were determined above (Table 2).Figure3shows the plots of the data for 3.5<pH <4.8based on eq 15.The data were correlated by a straight line.The correlation coefficient was 0.973.K P3,A and Q P3,A determined from the straight line are listed in Table 2.Similarly,assuming that eqs 3-5occur simulta-neously in the third step,eqs 16and 17are obtained,where the values of K C,A ,Q C,A ,K P3,A ,and Q P3,A areknown (Table 2).Figure 4shows the plots of the data for 2.7<pH <4.8based on eq 17.The data were correlated by a straight line.The correlation coefficient was 0.865.K P2,A and Q P2,A obtained from the straight line are listed in Table 2.Finally,assuming that all reactions of eqs 3-6occur simultaneously in the whole pH region (in the fourthstep),eq 18is derived,where K P1,A and Q P1,A areunknown and other values of K and Q are known (Table 2).Figure 4shows the plots of the data for 2.3<pH <4.8based on eq 18.The data are correlated by a straight line for pH >2.3.The correlation coefficient was 0.912.K P1,A and Q P1,A determined from the straight line are listed in Table 2.Figure 6shows the titration curves for adsorption of acetic acid on chitopearl CCS and DIAION WA30.The solid lines in Figures 1a and 6show the theoretical ones calculated from eqs 7-12.They correlate the data reasonably well.As can be seen from Table 2the saturation capacities of each amino group for adsorption of acetic acid coincide nearly with those for adsorption of HCl.The dashed line in Figure 6shows the theoreti-cal one for the adsorption of the acetic acid on DIAION WA30calculated from eq 8with K and Q listed in Table 2(Takatsuji and Yoshida,1997).The two theoretical lines in Figure 6intersect at about pH 3.5.For pH <3.5DIAION WA30was able to adsorb the more acetic acid than Chitopearl CCS,but for pH >3.5the amount of acetic acid adsorbed on Chitopearl CCS surpassed DIAION WA30.4.2.Adsorption of Malic Acid.Figure 1b shows the experimental equilibrium isotherm for adsorption of malic acid on Chitopearl CCS.The adsorption isotherm is not affected by the initialliquid-phaseFigure 3.Plots of data for 3.5<pH <4.8for adsorption of acetic acid on Chitopearl CCS based on eq15.Figure 4.Plots of data for 2.7<pH <4.8for adsorption of acetic acid on Chitopearl CCS based on eq 17.q A )K C,A Q C,A [AH]1+K C,A [AH]+K P3,A Q P3,A [AH]1+K P3,A [AH](14)[AH])-1K P3,A +Q P3,A[AH]q A -q C,A(15)q A )K C,A Q C,A [AH]1+K C,A [AH]+K P3,A Q P3,A [AH]1+K P3,A [AH]+K P2,A Q P2,A [AH]1+K P2,A [AH](16)[AH])-1K P2,A +Q P2,A [AH]q A -(q C,A +q P3,A )(17)Figure 5.Plots of data for 2.3<pH <4.8for adsorption of acetic acid on Chitopearl CCS based on eq18.Figure 6.Titration curve for adsorption of acetic acid on Chitopearl CCS and DIAION WA30.Key:(sbs )Chitopearl CCS;(--O --)DIAION WA30.[AH])-1K P1,A +Q P1,A[AH]q A -(q C,A +q P3,A +q P2,A )(18)Ind.Eng.Chem.Res.,Vol.37,No.4,19981303concentration of malic acid,C M0.Since malic acid (MH 2)has two carboxylic groups,it may be adsorbed on the resin by the following reactions:The elementary reactions are shown in Appendix 1.Applying the mass action law to eqs 19-28,and using the procedure of the previous paper (Takatsuji and Yoshida,1997),eqs 29-32were obtained.Eqs 21and 22:Eqs 23and 24:Eqs 25and 26:Eqs 27and 28:Here [MH 2],[MH -],and [M 2-]represent the equilibriumconcentrations of C 4H 6O 5,C 4H 5O 5-,and C 4H 4O 52-in the liquid phase (kmol/m 3),respectively.q P3,M ,q P3,M ,q P2,M ,and q P1,M denote the equilibrium amounts of malic acid adsorbed on R C -NH 2,R P3-N,R P2-NH,and R P1-NH 2,respectively (kmol/m 3wet resin).Q C,M ,Q P3,M ,Q P2,M ,and Q P1,M show the saturation capacities on R C -NH 2,R P3-N,R P2-NH,and R P1-NH 2,respectively (kmol/m 3wet resin).The total amount of malic acid adsorbed on Chitopearl CCS is given by eq 33.[MH 2]can becalculated by using the Henderson -Hasselbalch equa-tion (Glasstone and Lewis,1960).The equilibrium concentration of malic acid in the liquid phase C M (kmol/m 3)is given by eq 34,where p K ma1and p K ma2are 3.24and 4.71at 293K,respectively (Martell and Smith,1974,1975,1977).Resin has multiple sites of amino groups.There is a possibility for the carboxylic groups of malic acid to adsorb on different types of amino groups of the resin.However since the basicities of different types of amino groups are different,two carboxylic groups of malic acid may adsorb on the same type of amino group at a given pH value.In addition the distance between carboxylic groups in one molecule of malic acid is long enough to associate with the two amino groups in chitosan and with the two amino groups of the same type in PEI.Therefore the values of K in eqs 29-32were determined according to the procedures presented by Yoshida et al.(1994),and Takatsuji and Yoshida (1997).We also assumed that Q C,M )Q C ,Q P3,M )Q P3,Q P2,M )Q P2,and Q P1,M )Q P1.When the concentration of malic acid in the liquid phase is very low,eqs 21and 22occur since R C -NH 2shows the strongest basicity.The adsorption isotherm for malic acid in the first step is expressed by eq 29.When q C,M )Q C /2is substituted in eq 29,eq 35isobtained.The value of K C,M1can be determined from the value of [MH 2]at q M )Q C /2on the experimental adsorption isotherm.The value of K C,M2is determined from eq 36,to which eq 29is transformed.Q C +2(q M-Q C /2)(1+K C,M1[MH 2])/(1-K C,M1[MH 2])was plotted against 8(q M -Q C /2)2[MH 2]/(1-K C,M1[MH 2])2using the data in pH >6.3.The plots were correlated well by a straight line.The correlation coefficient was 0.936.The value of K C,M2was determined from the slope of the straight line.K C,M1and K C,M2are listed in Table 3.MH 2y \z K ma1H ++MH -(19)MH -y\z K ma2H ++M 2-(20)R C -NH 2+MH 2y \z K C,M1R C -NH 3+MH -(21)2R C -NH 2+MH 2y\z K C,M2(R C -NH 3+)2M 2-(22)R P3-N +MH 2y \z K P3,M1R P3-NH +MH -(23)2R P3-N +MH 2y\z K P3,M2(R P3-NH +)2M 2-(24)R P2-NH +MH 2y\z K P2,M1R P2-NH 2+MH -(25)2R P2-NH +MH 2y \z K P2,M2(R P2-NH 2+)2M 2-(26)R P1-NH 2+MH 2y\z K P1,M1R P1-NH 3+MH -(27)2R P1-NH 2+MH 2y\z K P1,M2(R P1-NH 3+)2M 2-(28)q C,M )Q C,M /2+(1-K C,M1[MH 2]){(1+K C,M1[MH 2])-(1+K C,M1[MH 2])2+8K C,M2[MH 2]Q C,M }/8K C,M2[MH 2](29)q P3,M )Q P3,M /2+(1-K P3,M1[MH 2]){(1+K P3,M1[MH 2])-(1+K P3,M1[MH 2])2+8K P3,M2[MH 2]Q P3,M }/8K P3,M2[MH 2](30)q P2,M )Q P2,M /2+(1-K P2,M1[MH 2]){(1+K P2,M1[MH 2])-(1+K P2,M1[MH 2])2+8K P2,M2[MH 2]Q P2,M }/8K P2,M2[MH 2](31)q P1,M )Q P1,M /2+(1-K P1,M1[MH 2]){(1+K P1,M1[MH 2])-(1+K P1,M1[MH 2])2+8K P1,M2[MH 2]Q P1,M }/8K P1,M2[MH 2](32)q M )q C,M +q P3,M +q P2,M +q P1,M(33)C M )[MH 2]+[MH -]+[M 2-])10-pH (1+10pH -p K ma1+10pH -p K ma1×10pH -p K ma2)10pH -p K ma1+2×10pH -p K ma1×10pH -p K ma2(34)K C,M1)[1[MH 2]]q M)Q C /2(35)Q C +2(q M -Q C2)(1+K C,M1[MH 2])1-K C,M1[MH 2])K C,M28(q M -Q C 2)2[MH 2](1-K C,M1[MH 2])2(36)1304Ind.Eng.Chem.Res.,Vol.37,No.4,1998Next,malic acid is adsorbed on R C-NH2and R P3-N simultaneously in the second step.Therefore,eqs21-24occur simultaneously and eq37is valid.When q M-q C,M)Q P3/2is substituted in eq37,eq 38is obtained.The value of K P3,M1can be determined from the value of[MH2]at q M-q C,M)Q P3/2on the experimental adsorption isotherm.The value of K P3,M2 is determined from eq39,to which eq37is transformed.Q P3+2(q M-q C,M-Q P3/2)(1+K P3,M1[MH2])/(1-K P3,M1[MH2])was plotted against8(q M-q C,M-Q P3/ 2)2[MH2]/(1-K P3,M1[MH2])2using the data for6.3>pH >5.4based on eq39.The plots were correlated wellby a straight line for6.3>pH>5.4.The correlation coefficient was0.921.K P3,M1and K P3,M2are listed in Table3.In the third step,eqs21-26occur simultaneously and eq40is obtained.When q M-q C,M-q P3,M)Q P2/2issubstituted into eq40,eq41is obtained.The value of K P2,M1can be determined from the value of[MH2]at q M-qC,M-qP3,M)QP2/2on the experimental adsorption isotherm.The value of K P2,M2is determined from eq42to whicheq40is transformed.Q P2+2(q M-q C,M-q P3,M-Q P2/ 2)(1+K P2,M1[MH2])/(1-K P2,M1[MH2])was plotted against8(q M-q C,M-q P3,M-Q P2/2)2[MH2]/(1-K P2,M1[MH2])2using the data for6.3>pH>2.6based on eq42.The plots were correlated by a straight line. The correlation coefficient was0.847.K P2,M1and K P2,M2 are listed in Table3.Finally,eqs21-28occur simultaneously in the fourth step,and eq43is obtained.The value of K P1,M1can bedetermined from the value of[MH2]at q M-q C,M-q P3,M-qP2,M)QP1/2on the experimental adsorption isotherm as shown by eq44.The value of K P1,M2is determinedfrom eq45to which eq43is transformed.Q P1+2(q M-qC,M-qP3,M-qP2,M-QP1/2)(1+K P1,M1[MH2])/(1-K P1,M1[MH2])was plotted against8(q M-q C,M-q P3,M-q P2,M-Q P1/2)2[MH2]/(1-K P1,M1[MH2])2using the data for6.3>pH>1.8based on eq45.The plots were correlated by a straight line.The correlation coefficient was0.770.K P1,M1and K P1,M2are listed in Table3. Figure7shows the titration curves for adsorption of malic acid on Chitopearl CCS and DIAION WA30.The solid lines in Figures1b and7show the theoretical ones for Chitopearl CCS calculated from eqs29-34using theTable3.Experimental Coefficients of Malic Acid(MH2) and Citric Acid(CH3)ai)C i)P1i)P2i)P3Adsorption of MH2Chitopearl CCSK i,M1(m3/kmol) 5.6×1010 3.6258.1×104 K i,M2(m3/kmol)28.4×101011350 3.9×108 DIAION WA30K i,M1(m3/kmol) 3.5×104 K i,M2(m3/kmol)2 2.1×107Adsorption of C H3Chitopearl CCSK i,C1(m3/kmol) 1.0×1011 6.841 1.4×105 K i,C2(m3/kmol)28.0×1015 1.554 6.0×108 K i,C3(m3/kmol)3 1.0×10170.015 3.3 1.0×1011 DIAION WA30K i,C1(m3/kmol) 1.0×105 K i,C2(m3/kmol)28.0×108 K i,C3(m3/kmol)3 2.0×1010a Key:MH2,C(CH2COOH)H(OH)COOH;C H3,C(CH2COOH)2-(OH)COOH.q M )qC,M+QP3/2+(1-KP3,M1[MH2]){(1+KP3,M1[MH2])-(1+K P3,M1[MH2])2+8K P3,M2[MH2]Q P3}/8KP3,M2[MH2](37)KP3,M1)[1[MH2]]q M-q C,M)Q P3/2(38)QP3+2(q M-q C,M-Q P32)(1+K P3,M1[MH2])1-KP3,M1[MH2])KP3,M28(q M-q C,M-Q P32)2[MH2](1-KP3,M1[MH2])2(39)q M )qC,M+qP3,M+QP2/2+(1-KP2,M1[MH2]){(1+KP2,M1[MH2])-(1+K P2,M1[MH2])2+8K P2,M2[MH2]Q P2}/8KP2,M2[MH2](40)KP2,M1)[1[MH2]]q M-q C,M-q P3,M)Q P2/2(41)QP2+2(q M-q C,M-q P3,M-Q P22)(1+K P2,M1[MH2])1-KP2,M1[MH2])KP2,M28(q M-q C,M-q P3,M-Q P22)2[MH2](1-KP2,M1[MH2])2(42)qM)qC,M+qP3,M+qP2,M+QP1/2+(1-KP1,M1[MH2]){(1+KP1,M1[MH2])-(1+K P1,M1[MH2])2+8K P1,M2[MH2]Q P1}/8KP1,M2[MH2](43)KP1,M1)[1[MH2]]q M-q C,M-q P3,M-q P2,M)Q P1/2(44)QP1+2(q M-q C,M-q P3,M-q P2,M-Q P12)(1+K P1,M1[MH2])1-KP1,M1[MH2])KP1,M28(q M-q C,M-q P3,M-q P2,M-Q P12)2[MH2](1-KP1,M1[MH2])2(45)Ind.Eng.Chem.Res.,Vol.37,No.4,19981305values of K given in Table 3.They correlate the data reasonably well.The saturation capacity for ad-sorption of malic acid is total concentration of amino groups fixed in the resin.The dashed line in Figure 7shows the theoretical one for DIAION WA30calcu-lated from eq 30with K listed in Table 3(Takatsuji and Yoshida,1997).For pH >5Chitopearl CCS could adsorb more malic acid than DIAION WA30,be-cause only R C -NH 2could react with malic acid for pH > 6.For pH <5DIAION WA30with a greater concentration of R P3-N,which shows the second strong-est basicity,adsorbed more malic acid than Chitopearl CCS.4.3.Adsorption of Citric Acid.Figure 1c shows the experimental equilibrium isotherm for adsorption of citric acid on Chitopearl CCS.The adsorption isotherm is not affected by the initial liquid-phase concentration of citric acid C C0.Since citric acid (C H 3)has three carboxylic groups,it may be adsorbed on the resin by the following reactions:The elementary reactions are shown in Appendix 1.Applying the mass action law to eqs 46-60,and using the procedure of the previous paper (Takatsuji and Yoshida,1997),eq 61was obtainedwherewhere [C H 3],[C H 2-],[C H 2-],and [C 3-]denote the equilibrium concentrations of C 6H 8O 7,C 6H 7O 7-,C 6H 6O 72-,and C 6H 5O 73-in the liquid phase (kmol/m 3),respectively.q C,C ,q P3,C ,q P2,C ,and q P1,C are the equilib-rium amounts of citric acid adsorbed on R C -NH 2,R P3-N,R P2-NH,and R P1-NH 2,respectively (kmol/m 3wetFigure 7.Titration curve for adsorption of malic acid on Chito-pearl CCS and DIAION WA30.Key:(sbs )Chitopearl CCS;(--O --)DIAION WA30.C H 3y \z K ca1H ++C H 2-(46)C H 2-y\z K ca2H ++C H 2-(47)C H 2-y\z K ca3H ++C 3-(48)R C -NH 2+C H 3y \z K C,C1R C -NH 3+C H 2-(49)2R C -NH 2+C H 3y\z K C,C2(R C -NH 3+)2C H 2-(50)3R C -NH 2+C H 3y\z K C,C3(R C -NH 3+)3C 3-(51)R P3-N +C H 3y\z K P3,C1R P3-NH +C H 2-(52)2R P3-N +C H 3y\z K P3,C2(R P3-NH +)2C H 2-(53)3R P3-N +C H 3y \z K P3,C3(R P3-NH +)3C 3-(54)R P2-NH +C H 3y\z K P2,C1R P2-NH 2+C H 2-(55)2R P2-NH +C H 3y\z K P2,C2(R P2-NH 2+)2C H 2-(56)3R P2-NH +C H 3y\z K P2,C3(R P2-NH 2+)3C 3-(57)R P1-NH 2+C H 3y \z K P1,C1R P1-NH 3+C H 2-(58)2R P1-NH 2+C H 3y\z K P1,C2(R P1-NH 3+)2C H 2-(59)3R P1-NH 2+C H 3y\z K P1,C3(R P1-NH 3+)3C 3-(60)q i,C )Q i,C3+2K i,C227K i,C3(1-2K i,C1[C H 3]+2K i,C22[C H 3]9K i,C3)+(23-4K i,C227K i,C1K i,C3-13K i,C1[C H 3])A i+K i,C23K i,C12[C H 3]A i 2(i )C,P3,P2,P1)(61)A i )(-b i2+ R i)1/3+(-b i2- R i)1/3(R i >0)(62)A i )2 -a i cosθi 3(R i <0)(63)R i )b i24+a i3(64)cos θi )-b i 2 -a i3(65)a i )K i,C1232K i,C3(K i,C1-22K i,C2232K i,C3)[C H 3]2+K i,C1232K i,C3[C H 3](66)b i )2K i,C13K i,C233K i,C32(23K i,C2233K i,C3-K i,C1)[C H 3]3-K i,C133K i,C3(2K i,C232K i,C3+Q i,C )[C H 3]2(67)1306Ind.Eng.Chem.Res.,Vol.37,No.4,1998resin).Q C,C ,Q P3,C ,Q P2,C ,and Q P1,C show the saturation capacities on R C -NH 2,R P3-N,R P2-NH,and R P1-NH 2,respectively (kmol/m 3wet resin).The total amount of citric acid adsorbed on Chitopearl CCS is given by eq 68.[C H 3]can be calculated by using the Henderson -Hasselbalch equation (Glasstone and Lewis,1960).The equilibrium concentration of citric acid in the liquid phase C C (kmol/m 3)is given aswhere p K ca1,p K ca2,and p K ca3are 2.87,4.35,and 5.69at 293K,respectively (Martell and Smith,1974,1975,1977).Since the distances among three carboxylic groups in one molecule of citric acid are long enough to associate with the three amino groups in chitosan and with the three amino groups of the same type in PEI and since the basicities of different types of amino groups are different,three carboxylic groups of citric acid may adsorb on the same type of amino group at a given pH value.We also assumed that Q C,C )Q C ,Q P3,C )Q P3,Q P2,C )Q P2,and Q P1,C )Q P1,and the values of K in eq 61were evaluated by fitting q C with experimen-tal values in turn from high pH to low pH.They are listed in Table 3.Figure 8shows the titration curves for adsorption of citric acid on Chitopearl CCS and DIAION WA30.The solid lines in Figures 1c and 8show the theoretical ones for Chitopearl CCS calculated from eqs 61-69,and they correlate the data reasonably well.The saturation capacity for adsorption of citric acid is also total concentration of amino groups fixed in the resin.The dashed line in Figure 8shows the theoretical one for DIAION WA30(Takatsuji and Yoshida,1997).Similar to the adsorption of malic acid,Chitopearl CCS couldadsorb more citric acid than DIAION WA30for pH >5.This is because only R C -NH 2on Chitopearl CCS,which shows the strongest basicity,could react with citric acid in the low concentration for pH >6,but since DIAION WA30has a higher concentration of R P3-N than Chitopearl CCS,it could adsorb more citric acid for pH <5.5.ConclusionThe adsorption of organic acids (acetic acid,malic acid,and citric acid)on Chitopearl CCS,which is polyaminated highly porous chitosan,appeared feasible technically.1.Chitopearl CCS could adsorb organic acids as well as DIAION WA30which was a commerial weakly basic resin.In high pH region (low concentration region),Chitopearl CCS could adsorb more organic acids than DIAION WA30.2.The adsorption isotherms were not affected by the initial concentration of organic acids.3.The theoretical equations for the adsorption iso-therms were derived by assumig that organic acids were adsorbed by an acid/base neutralization reaction on each fixed functional group (four different amino groups)and by considering the dissociation of organic acids.In an organic acid with one carboxylic group in one molecule,such as acetic acid,the adsorption isotherm was ex-pressed by eqs 7-12.In an organic acid with two carboxylic groups in one molecule,such as malic acid,the adsorption isotherm was given by eqs 29-34.In an organic acid with three carboxylic groups in one molecule,such as citric acid,the adsorption isotherm was expressed by eqs 61-69.The equilibrium coef-ficients for adsorption on each fixed functional group were determined from the experimental titration curve in turn from high pH to low pH.The theoretical lines agreed resonably well with the experimental adsorption isotherms and titration curves.NomenclatureC j )equilibrium concentration of organic acid (j )A;acetic acid,M;malic acid,C;citric acid)in liquid phase,kmol/m 3C j0)initial concentration of organic acid (j )A;acetic acid,M;malic acid,C;citric acid)in liquid phase,kmol/m 3K i,HCl )equilibrium constant for adsorption of HCl on the functional group (i )C;the primary amino group of chitosan,P1;the primary amino group of PEI,P2;the secondary amino group of PEI,P3;the tertiary amino group of PEI)fixed in adsorbent phase,m 3/kmol K a )equilibrium constant (eq 2),kmol/m 3K C,A )equilibrium constant (eq 3),m 3/kmol K P1,A )equilibrium constant (eq 6),m 3/kmol K P2,A )equilibrium constant (eq 5),m 3/kmol K P3,A )equilibrium constant (eq 4),m 3/kmol K ma1)equilibrium constant (eq 19),kmol/m 3K ma2)equilibrium constant (eq 20),kmol/m 3K C,M1)equilibrium constant (eq 21),m 3/kmol K P1,M1)equilibrium constant (eq 27),m 3/kmol K P2,M1)equilibrium constant (eq 25),m 3/kmol K P3,M1)equilibrium constant (eq 23),m 3/kmol K C,M2)equilibrium constant (eq 22),(m 3/kmol)2Figure 8.Titration curve for adsorption of ciric acid on Chitopearl CCS and DIAION WA30.Key:(sbs )Chitopearl CCS;(--O --)DIAION WA30.q C )q C,C +q P3,C +q P2,C +q P1,C(68)C C )[C H 3]+[C H 2-]+[C H 2-]+[C 3-])10-pH (1+10pH -p K ca1+10pH -p K ca1×10pH -p K ca2+10pH -p K ca1×10pH -p K ca2×10pH -p K ca3)/[10pH -p K ca1+2×10pH -p K ca1×10pH -p K ca1+3×10pH -p K ca1×10pH -p K ca2×10pH -p K ca3](69)Ind.Eng.Chem.Res.,Vol.37,No.4,19981307。

Direct Synthesis and Characterization of Hydrophobic Aluminum-Free Ti-Beta Zeolite T.Blasco,†M.A.Camblor,*,†A.Corma,†P.Esteve,†J.M.Guil,‡A.Martı´nez,†J.A.Perdigo´n-Melo´n,‡and S.Valencia†Instituto de Tecnologı´a Quı´mica,UPV-CSIC,A V da.Los Naranjos s/n,46071Valencia,Spain,and Instituto de Quı´mica Fı´sica“Rocasolano”,CSIC,Serrano117,28006Madrid,SpainRecei V ed:October9,1997XIncorporation of Ti into the framework of aluminium-free zeolite Beta has been achieved in F-medium andhas produced hydrophobic selective oxidation catalysts.The Ti-Beta(F)materials have been characterizedby X ray diffraction,infrared,Raman,ultraviolet,XANES,EXAFS,29Si MAS NMR,and1H f29Si CP MASNMR spectroscopies,adsorption microcalorimetry,and catalytic testing.At near neutral pH the incorporationof Ti into the framework appears to present an upper limit of ca.2.3Ti/uc,beyond which anatase is detectedin the calcined materials.However,at higher pH(ca.11)larger amounts of Ti can be incorporated withoutanatase formation.After calcination,Ti incorporation in the framework is characterized by an increase inthe unit cell volume,the appearance of one Raman band and three infrared bands in the region near960cm-1and the presence of a strong absorption band in the205-220nm ultraviolet spectrum.By29Si MAS NMR,1H f29Si CP MAS NMR,and infrared spectroscopies it is concluded that upon contact with ambient humiditythere is no hydrolysis of Si-O-Ti bonds in Ti-Beta zeolites prepared by the fluoride route,while it isprobably a major feature of those synthesized in OH-medium.XANES and EXAFS spectroscopies of calcineddehydrated Ti-Beta zeolites unambiguously demonstrate the tetrahedral coordination of Ti with a Ti-Obond length of ca.1.80Å.Upon hydration,the changes in the XANES and EXAFS spectra are consistentwith a change in the coordination of Ti to reach a state which depends on the composition and synthesisroute and which ranges from a5-fold coordination for Al-free Ti-Beta synthesized by the F-method to ahighly distorted6-fold coordination in Ti,Al-Beta synthesized in OH-medium.Adsorption microcalorimetryexperiments show the strict hydrophobic nature of pure SiO2zeolite Beta synthesized in F-medium whileevidencing a slight increase in the hydrophilicity of the material upon incorporation of Ti to the framework.This is due to the relatively strong adsorption of precisely one H2O molecule per Ti site.On the contrary,the materials synthesized in OH-medium show an enhanced hydrophilicity.Finally,Ti-Beta(F)is an activeand selective catalyst for oxidation of organic substrates with H2O2.A comparison of the activities andselectivities of Ti-Beta(F),Ti-Beta(OH)and TS-1in the epoxidation of1-hexene using acetonitrile andmethanol as solvents demonstrates that the major differences between Ti-Beta and TS-1catalysts are intrinsicto each zeolitic structure.Because of its high hydrophobicity,Ti-Beta(F)catalyst can advantageously replaceTi-Beta(OH)in the epoxidation of substrates,like unsaturated fatty acids or esters,which contain a polarmoiety.IntroductionSelective catalytic oxidation of organic compounds by hydrogen peroxide or organic hydroperoxides over Ti-substituted zeolites has received much attention in the past decade mainly because of two reasons.First,their potential industrial applica-tions as oxidation catalysts could replace current stoichiometric or homogeneously catalyzed processes that generate large amounts of waste residues and/or need to be worked out under severe astringent conditions.Secondly,from a fundamental point of view Ti-containing zeolites show the remarkable property of being highly active catalysts in the epoxidation of olefins with aqueous H2O2in the presence of polar solvents.In contrast,the activity of most homogeneous or heterogeneous catalysts based on transition metal compounds is severely retarded by water and polar solvents,and alkylhydroperoxides are the oxidants of choice.1Consequently,much attention has been paid to the study of this new class of materials,but little is known about what gives Ti-zeolites their remarkable proper-ties,which are loosely attributed to the isolation of Ti species in the zeolitic framework.One of the challenges in this field is to understand why different Ti-zeolites seem to have different catalytic properties, beyond those that can be explained by mere shape selectivity constraints imposed by the size and shape of their channels, even if isolated Ti species were incorporated in all cases into the framework.Notably,the most interesting Ti-zeolites,TS-12and Ti-Beta,3exhibit a markedly different behavior, exemplified by their different activity and selectivity dependence on the solvent.4,5Differences between the“intrinsic”activity and selectivity of both materials were early attributed to the presence of Al in Ti-Beta,6but once the synthesis of Ti-Beta was made possible in the absence of Al,7,8the differences between the catalytic behavior of these zeolites persisted.We have very recently developed new methods to synthesize Ti-Beta zeolites with a wide range of chemical compositions and†Instituto de Tecnologı´a Quı´mica.‡Instituto de Quı´mica Fı´sica“Rocasolano”.X Abstract published in Ad V ance ACS Abstracts,December1,1997.75J.Phys.Chem.B1998,102,75-88S1089-5647(97)03288-4CCC:$15.00©1998American Chemical SocietyPublished on Web01/01/1998physical properties.7-9Here we report on the state of Ti in Al-free Ti-Beta zeolite prepared by different routes,the different physicochemical properties of these materials,and the implications of these differences on their catalytic behavior for alkenes epoxidation.We conclude that the main differences observed in the activity and selectivity of TS-1and Ti-Beta in the epoxidation of alkenes are essentially related to their different crystalline structures and cannot be solely attributed to other parameters as previously suggested(presence of Al,presence of defects,hydrophilicity).Experimental SectionSynthesis of Ti-Beta.Tetraethylammonium cations(TEA+) were used as the organic structure-directing agent in all the syntheses.The syntheses were done using either F-or OH-anions as mineralizers and in the presence or in the complete absence of Al.Four types of Ti-Beta zeolites were thus synthesized and will be denoted according to its chemical composition(Ti-or Ti,Al-Beta)and synthetic method used (Beta(F)or Beta(OH)).Unless noted otherwise the crystalliza-tion temperature was always413K,and PTFE lined stainless steel60mL autoclaves were used under tumbling(60rpm). The autoclaves were removed at different time intervals,and the contents were filtered or centrifuged and extensively washed with distillate water.Beta(F).Al-free Ti-Beta zeolites were synthesized in F-medium at near neutral pH8-9from gels of composition x TiO2:25SiO2:14TEAOH:8.6H2O2:189H2O:14HF,with x in the range0-2.5.The products obtained had an average crystal size in the range1-5µm.Smaller crystal sizes(below1µm) were obtained by seeding the gels with the dealuminated zeolite Beta,whose preparation is described below.For pure silica Beta(F)(x)0)an unseeded synthesis was done also at448K without rotation,to get a material with a crystal size of ca.10µm.To incorporate Al in the zeolite framework by this method (Ti,Al-Beta(F)sample)a starting gel of composition TiO2:50 SiO2:0.21Al2O3:28TEAOH:16.8H2O2:361H2O:28.4HF was used.Beta(OH).Aluminum-free Ti-Beta was synthesized in basic medium(pH ca.12)from gels of composition x TiO2:40SiO2: 22TEAOH:13.5H2O2:265H2O,with x in the range0.33-2. Dealuminated zeolite Beta crystals(see below for preparation) were used as seeds,and crystals smaller than0.5µm were obtained.The aluminum-containing material(Ti,Al-Beta(OH) sample)was synthesized from a gel of composition TiO2:60 SiO2:0.077Al2O3:32.4TEAOH:613H2O.Experimental details of the synthesis procedures have been already described elsewhere,3,7-10except for the Al-containing Beta(F)series.In this case,the same experimental procedure as in the Al-free syntheses9was followed,but metal Al was dissolved in TEAOH and then added to the synthesis gel at the end of the preparation.Dealuminated Zeolite Beta Seeds.A gel of composition SiO2: 0.04Al:0.56TEAOH:6.5H2O was crystallized at413K for72 h,yielding nanocrystalline zeolite Beta with ca.50nm average crystal size and a Si/Al ratio of ca.21.Further details of the synthesis were given elsewhere.11The zeolite was dealuminated by treatment at80°C during24h with HNO3(60%)in a liquid to solid ratio of60.12The final Si/Al ratio of the seeds was higher than1000.A mass ratio seeds/SiO2of0.029was used in the syntheses.Characterization.Phase purity was determined from powder X-ray diffraction(XRD)data recorded in a Philips X’Pert MPD diffractometer equipped with a PW3050goniometer(Cu K R radiation,graphite monochromator),provided with a variable divergence slit and working in the fixed irradiated area mode. 29Si MAS NMR spectra were recorded at a29Si frequency of 79.459MHz and a spinning rate of5kHz on a Varian VXR 400S WB spectrometer.Bloch decay(BD)spectra were acquired with a38.6°pulse length of3.0µs and a recycle delay of20s.1H f29Si CP MAS NMR spectra were acquired with a90°pulse length of7µs,contact times of500,1500,and 2500µs,and a recycle delay of3s.The29Si chemical shifts are reported relative to TMS.Infrared spectra in the region of framework vibrations(1900-300cm-1)were obtained in a Nicolet710FTIR spectrometer using the KBr pellet technique.Spectra in the hydroxyl stretching region(4000-3000cm-1)were recorded in a BioRad STS40A FTIR spectrometer using self-supported wafers of10 mg cm-2outgassed overnight at673K and10-3Pa.Raman spectra in the1400-150cm-1region were recorded in a FT-Raman II Bio-Rad spectrometer using a Nd:YAG laser beam (1064nm).For the detection of anatase(main band at144 cm-1)additional Raman spectra down to about50cm-1were recorded using a DILOR XY spectrometer operating in the micro-Raman mode with the514nm line of an Ar laser beam. Diffuse reflectance ultraviolet-visible(DRUV)spectra were recorded in a Cary5Varian spectrometer equipped with a “Praying Mantis”cell from Harrick.X-ray absorption experiments were performed on stations XAS-2and XAS-3at LURE(Orsay,France).The X-ray beam was emitted by the DCI ring with a stored current between228 and320mA.The data were collected using Si(311)and Si-(111)two-crystal monochromators for XANES and EXAFS, respectively.At the XAS-2station the beamline was equipped with two parallel mirrors for harmonic rejection.At the XAS-3 station harmonics were rejected by detuning the monochromator by50%from the maximum intensity.The spectra were recorded at room temperature in the transmission mode,and the detection was carried out by using two ionization chambers. The samples and the anatase were used as self-supported wafers. The calcined samples were dehydrated at623K for1h, transferred into a special chamber under Ar atmosphere,and kept under vacuum during the spectra acquisition.Experimental data were analyzed with a set of programs developed by Michalowicz.13XANES spectra were treated by subtracting the linear background determined by least-squares fitting of the preedge region and normalized with respect to the beginning of the EXAFS oscillations.The standard procedures for background removal,extraction of EXAFS oscillations,and normalization of the edge absorption were applied.The K3 weighted Fourier transform between2.80and11.50Å-1was calculated using a Kaiser window.Filtering of the first peak between1.09and1.96Åfor calcined dehydrated Ti-Beta and between1.10and1.86Åfor rehydrated samples was applied to analyze the first coordination shell of Ti.The EXAFS spectrum of anatase was used as a reference to obtain the phase shift and backscattering amplitude for Ti-O.These functions were calculated considering six oxygen atoms at a distance of 1.95Åand a Debye-Waller factorσ2)2.9×10-3for anatase. The later value was obtained from the simulation of the experimental spectrum using the theoretical curves calculated by Mckale et al.14For adsorption microcalorimetry experiments toluene,n-hexane,and water(purity>99%)were used as adsorbates after purification by successive freeze-thaw cycles inside the adsorp-tion apparatus.Adsorption measurements were performed in a conventional volumetric apparatus.15A heat-flow microcalo-76J.Phys.Chem.B,Vol.102,No.1,1998Blasco et al.rimeter of the Tian-Calvet type(model BT,Setaram,France) was used to determine differential heats of adsorption by measuring the heat evolved in the adsorption of a given amount of adsorbate.For this purpose the calorimeter cells are part of the volumetric apparatus.The heat/voltage proportionality constant of the microcalorimeter was calibrated by the Joule effect.The correction for the heat evolved in the gas compres-sion associated with the gas entrance in the cell was determined by previous experiments with helium.Before each adsorption experiment the sample was heated in oxygen flow,ca.30cm3/ min,increasing the temperature at4K/min from room temper-ature up to723K,kept at this temperature for2h,and outgassed overnight at the same temperature in a vacuum of more than1 mPa.Volumetric and calorimetric isotherms were determined simultaneously in the usual way by measuring amounts adsorbed in successive doses at increasing pressures.Experiments were carried out at315K.In a different set of experiments water was adsorbed following toluene adsorption after outgassing for 30min at the temperature of the experiment.The amount of adsorbate removed in the intermediate outgassing was calculated from the desorption calorimetric peak.Amounts adsorbed are expressed as millimole of adsorbate per gram of sample dried under vacuum at723K.Catalytic Activity.The experiments of epoxidation of 1-hexene were performed in a25mL round-bottom flask immersed in a thermostated bath and equipped with a condenser, a thermometer,and a magnetic stirrer.Typically,17mmol of alkene,11.8g of solvent(either methanol or acetonitrile),and 0.4g of diluted H2O2(35wt%in water)were homogenized in the flask under stirring,and the mixture was heated at323K. Then,0.1g of catalyst was added at once to the reaction mixture (time zero).Small aliquots were carefully withdrawn from the mixture at time intervals to follow the kinetics of the reaction. For the epoxidation of oleic acid,1mmol of alkene,2mL of acetonitrile(solvent),24mg of H2O2(35wt%in water),and 30mg of catalyst were mixed in a5mL flask and heated up to 323K under continuous stirring.In both cases the reaction products were analyzed by gas chromatography in a Varian3400 GC equipped with a capillary column(5%methylphenylsilicone, 25m length)and a FID.Product identification was performed by GC-MS and available standard compounds.Unreactedhydrogen peroxide was determined by iodometric titration. Results and DiscussionSynthesis in Fluoride Medium.In the synthesis of Ti-Beta(F)and Ti,Al-Beta(F)under the conditions described above,no competition of other crystalline phases was detected by powder XRD.However,there is spectroscopic evidence of the presence of anatase in the calcined solids for Ti contents higher than5wt%as TiO2and final pH of the reaction smaller than ca.11(see below).The introduction of Ti in a nearly neutral or slightly basic pure silica reaction mixture in fluoride medium results in a significant increase of the crystallization time and only50%of the Ti initially present in the reaction mixture is incorporated into the final material,contrarily to the observed behavior in OH-medium,in which total incorporation is achieved(Figure1).This holds until a Ti/(Ti+Si)molar fraction in the gel of about0.06is reached;beyond this limit, which corresponds in the zeolite to about a0.037molar ratio (2.3Ti/uc),anatase coprecipitates(see below)and the amount of Ti in the solid largely increases(Figure1).It is interesting that the yield(and the apparent upper limit) of Ti incorporation into the framework of zeolite Beta in fluoride medium at nearly neutral pH is much lower than that obtained when OH-anions are used as mineralizers(Figure1).Obvi-ously,the differences between both types of synthesis(F-V s OH-)are the pH and the presence of F-.To check the influence of both parameters on Ti incorporation,we performed a series of experiments in which F-/Si ratio and pH were varied by using different amounts of HF,NH4F,TEAOH,and HCl.The results are shown in Figure2.The amount of Ti in the final solids strongly depends on the pH of the reaction mixture.At a pH beyond10all Ti initially present in the reaction mixture is found in the final solid,but anatase is generally detected in the calcined product.However,a further pH increase to about11avoided anatase precipitation,preserving the complete incorporation of Ti.This is also the common observation in the synthesis of Ti-Beta in OH-medium(pH ca.12).The effect of F-on the final yield of Ti is also clear,though less marked:an increase in F-concentration at a given pH causes a decrease in the amount of Ti recovered.These results suggest that,in the absence of alkali cations,the formation of soluble titanosilicate species is favored at high pH,affording total Ti incorporation into the framework of zeolite Beta.At low pH soluble complexes with F-exist that maintain a significant portion of Ti in solution,precluding total incorporation to the zeolite. Apparently,such complexes are less stable as pH increases and, Figure1.Efficiency of the incorporation of Ti to the zeolite Beta structure for synthesis in OH-medium(b)and in F-medium at pH) 7-9(9).The open square corresponds to a calcined material in which a large amount of anatase was detected.Figure2.Incorporation of Ti to the solids as a function of synthesis pH and F-concentration.F-/SiO2)0.40(b),0.54(9),and1.08(2). Open symbols correspond to materials in which anatase was detected.Hydrophobic Aluminum-Free Ti-Beta Zeolite J.Phys.Chem.B,Vol.102,No.1,199877as a consequence,at an intermediate pH total recovery of Ti in the solids(or almost total,depending on F-concentration)but only partial incorporation in the framework occurs.Figure2 demonstrates that the incorporation of Ti in the framework of zeolite Beta in F-medium strongly depends on the pH.For this reason,we have not found so far an“intrinsic”upper limit for the isomorphous substitution of Si by Ti.Evidence for the Isomorphous Substitution.The isomor-phous substitution of Si by Ti in zeolites can be ascertained by a combination of physicochemical techniques and catalytic tests. One of the most widely used techniques is powder X-ray diffraction,where the incorporation of Ti in the framework is, in principle,expected to cause a linear increase in the unit cell volume,due to the longer Ti-O bond distance(1.79-1.92Å, depending on the actual coordination number of Ti)comparedto the Si-O bond distance(typically1.60-1.65Åin zeolites). Although it has been recently shown that isomorphous substitu-tions in zeolites do not necessarily bear the expected trend in unit cell expansions or contractions,16all reports on Ti-zeolites have shown so far the expected increase in unit cell volume. For zeolite Beta this technique is much limited by the structural nature of the material,which is an intergrowth of at least two polymorphs.17Probably due to that,but maybe also due to possible symmetry changes originated by the isomorphous substitution,we were able to index the XRD patterns of the Ti-Beta(F)calcined materials only with the indices of the tetragonal polymorph A.Actually,the XRD patterns of zeolite Beta samples synthesized by the fluoride route show a gradual change in the shape of the first low angle peak as the Ti content increases(Figure3).The shape of this peak is sensitive to changes in the relative proportion of different polymorphs.17 Thus,this could indicate a decreased presence of polymorph A as the Ti content increases but symmetry changes could also be responsible for that effect.As shown in Figure4,there is a linear increase in the unit cell volume of polymorph A as the Ti content of the material synthesized at pH)7-9.5(see above)increases up to a Ti/ (Ti+Si)mole fraction in the zeolite of0.037.Beyond this value no further increase of the unit cell volume occurs and anatase starts to show up in the DRUV and Raman spectra(see below).As shown in Figure5,the expansion of the unit cell upon the isomorphous substitution is markedly anisotropic, occurring in a larger extent for parameter c,whereas parameter a shows a minimum at low Ti content.The incorporation of Ti in the zeolite Beta framework in F-medium is also evidenced by spectroscopic techniques.First, all calcined Beta(F)samples show a sharp absorption in the 205-220nm ultraviolet region(Figure6),with no absorptions attributable to anatase when the Ti molar fraction in the material is below0.037(2.3Ti/uc).Beyond that concentration,anatase starts to show up in the materials synthesized at nearly neutral pH,as evidenced by the presence of a broad absorption band around330nm.For materials synthesized at higher pH(above ca.11)no anatase is detected by DRUV despite its high Ti content.Secondly,Ti-Beta(F)samples also show absorptions in the950-980cm-1region of the infrared(Figure7)and Raman (Figure8)spectra which are generally attributed to the presence of Ti in the framework,although the assignment of such bands is still a matter of debate(see below).Finally,the Raman spectra in the low Raman shift region(Figure9)show that no anatase(characterized by a strong adsorption at ca.144cm-1) is formed when the Ti molar fraction in the material synthesized at nearly neutral pH is below the upper limit mentioned above. Additional spectroscopic and catalytic proofs of the isomorphous substitution of Si by Ti are discussed below.Figure3.X-ray diffraction patterns of aluminum-free zeolite Beta samples synthesized by the fluoride route with different Ti contents.A gradual change of the shape of the low-angle peak as the Ti content increases,possibly indicating symmetry changes or variations in the relative proportions of polymorphs,17is shown in the inset.Figure4.Variation of the unit cell volume of Ti-Beta(F)calcined materials synthesized at near neutral pH,indexed according to the tetragonal polymorph A,17as a function of their Ti content.(The open square corresponds to a sample containing a large amount of anatase.) Figure5.Change in the tetragonal unit cell parameters(b,a;9,c) of Ti-Beta(F)calcined materials as a function of Ti content(open symbols correspond to a material containing a large amount of anatase).78J.Phys.Chem.B,Vol.102,No.1,1998Blasco et al.29SiMAS NMR Spectroscopy.We have previously shown that pure silica zeolite Beta synthesized in fluoride medium in the presence of TEA +is free of connectivity defects,within the detection limits of BD and CP 29Si MAS NMR spec-troscopies.18By the CP technique,however,it is possible to detect Si -OH groups in the external surface,provided that the crystallites are small enough (<ca.10µm,see Figure 10).The absence of connectivity defects brings about a high local order in pure silica Beta and a lack of dipolar coupling to protons that allow sharp lines and a remarkable high resolution of Si-(4Si)crystallographic sites in the 29Si MAS NMR spectrum of the calcined sample.When Ti is incorporated to Al-free zeolite Beta by the route described here,a broadening of the peaks is apparent in the 29Si BD MAS NMR spectrum (Figure 11).This broadening,which increases as the Ti content increases,causes a major loss of resolution,and only three broad resonances are evident at ca.-112,-113,and -116ppm.A similar broadening was observed in MFI materials when Si is substitutedby either Ti 19or Ge.20The reason for this is most likely a decreased local order due to the distortion of the framework caused by the incorporation of Ti atoms and the presence of overlapping Si(OSi)4and Si(OSi)3OTi resonances unspecifically occupying different crystallographic sites.Another plausible explanation for this loss of resolution could be the presence of a high concentration of connectivity defects,which could arise from the hydrolysis of Si -O -Ti bonds.However,this is not the case for Ti -Beta(F),as no Q 3bands are detected in the BD 29Si NMR (Figure 11).More conclusively,1H f 29Si cross polarization experiments,which are more sensitive to Si -OH groups,reveal the presence of only a small concentration of such moieties in these materials,and this concentration appears to be dependent on crystal size but not on Ti concentration (Figure 10).Therefore,we assign these bands to Si -OH moieties at the external surface of the crystallites.Finally,the increase in water content of the zeolites as Ti content increases (due to the increase in hydrophilicity,see below)could also contribute to the loss of resolution by broadening of the 29Si MAS NMR lines through dipolar coupling with water protons.However,1H decoupled 29Si MAS NMR experiments (not shown)demonstrate this has only a minor broadening effect.Contrarily to these observations,the 29Si MAS NMR spectrum of Ti -Beta(OH)clearly evidences the presence of a high concentration of Si -OH defect groups.In the BDexperimentsFigure 6.Diffuse reflectance ultraviolet spectra of calcined Ti -Beta samples synthesized in F -medium at near neutral pH (from bottom to top:0.37,0.68,1.05,2.35,1.38,4.77Ti/uc)and at pH ≈11(2.87Ti/uc,vertically offset for clarity).Figure 7.Infrared spectra of calcined zeolite Beta materials:(bottom to top)pure silica(F),Ti -Beta(F,1.03Ti/uc),high silica(OH,Si/Al >700),and Ti -Beta(OH,1.63Ti/uc).Figure 8.Raman spectra of calcined materials:(bottom to top)pure silica Beta(F),Ti -Beta(F,1.05Ti/uc),Ti -Beta(OH,1.63Ti/uc),and Ti,Al -Beta(OH,2.32Ti/uc,0.50Al/uc).Figure 9.Raman spectra in the low Raman shift region of (bottom to top)silica Beta(F)and Ti -Beta(F)(0.37,0.68,1.02,1.38,2.35,and 4.77Ti/uc)calcined samples and anatase.Hydrophobic Aluminum-Free Ti -Beta Zeolite J.Phys.Chem.B,Vol.102,No.1,199879(Figure 12)an increased broadening of the Si(4Si)resonances,with a concomitant higher loss of resolution when compared to Ti -Beta(F)materials is apparent.Furthermore,the presenceof Si(3Si,1OH)at ca.-102ppm,that is,Q 3species in connectivity defects,already seen in the Bloch decay spectrum,is clearly demonstrated under 1H f 29Si cross polarization conditions (Figure 10)which evidence even the presence of some Si(2Si,2OH)Q 2signals at ca.-92ppm.Apparently,the concentration of such moieties does depend on the Ti content in Ti -Beta(OH)materials and can thus be related in part to the hydrolysis of Si -O -Ti bonds.FTIR Spectroscopy.OH Stretching Region (3200-4000cm -1).The investigation of this region of the spectrum is limited to some extent by the necessity to grind the powders to make self-supported wafers.This causes some breaking of the crystallites and therefore the formation of terminal Si -OH groups in all the samples,thus diminishing the differences between them and precluding the extraction of quantitative information.Notwithstanding we can see that pure silica Beta-(F)samples display only small and relatively narrow bands in the 3650-3750cm -1region (Figure 13).Because of their narrowness and their position,and based on the 1H f 29Si CP MAS NMR mentioned commented above,all three bands (3690,3736,3746cm -1)are assigned to terminal Si -OH groups.The difference in wavenumbers is primarily related to the silanol groups being involved or not in weak hydrogen bonding.Thus,the band at 3746cm -1is assigned to free Si -OH groups,21that at 3736cm -1is assigned to those Si -OH groups in which the O is weakly hydrogen bonded to an adjacent OH moiety,which itself is considered to be responsible of the band at around 3690cm -1(Scheme 1).It is known that hydrogen bonding causes the OH stretching bands to shift to lower wavenumbers and to broaden,the magnitude of both effects being related to the strength of the hydrogen bond.Thus,the relatively narrow band at 3690cm -1cannot be due to an OH involved in averyFigure 10.29Si CP MAS NMR spectra of calcined Beta samples (contact time:1500µs).Bottom to top:pure silica Beta(F)with approximate crystal size of 10,3,and 0.5µm;Ti -Beta(F)of about 3µm (0.57and 1.03Ti/uc);Ti -Beta(F)of less than 1µm (0.66and 2.35Ti/uc);Ti -Beta(OH)of less than 0.5µm (0.69,1.35,and 1.63Ti/uc).All the spectra are plotted in absolute intensityscale.Figure 11.29Si BD MAS NMR spectra of Al-free zeolite Beta materials synthesized in F -medium:(bottom to top)pure silica Beta and Ti -Beta with 0.37,0.66,1.03,1.05,1.38,and 2.35Ti/uc.Note the absence of any resonance at fields below -109ppm.Figure 12.29Si BD MAS NMR spectra of Al-free zeolite Beta materials synthesized in OH -medium:(bottom to top)0.69,1.35,and 1.63Ti/uc.Note the presence of a band at ca.-102ppm,assigned to Q 3species,and a small band at ca.-92ppm,assigned to Q 2species.SCHEME 180J.Phys.Chem.B,Vol.102,No.1,1998Blasco et al.。

siliconeSiliconeIntroduction:Silicone, also known as polydimethylsiloxane, is a versatile and widely used synthetic material. It is composed of silicon, oxygen, and hydrocarbon groups, making it a unique compound with various applications. Silicones are commonly found in many industries due to their excellent thermal stability, chemical resistance, and low toxicity. In this document, we will explore the properties, uses, and manufacturing process of silicone.Properties:1. Thermal Stability:Silicone exhibits remarkable thermal stability, allowing it to withstand extreme temperature conditions. It can remain stable across a wide range of temperatures, from as low as -100°C to as high as 300°C. This property makes silicone an ideal material for applications in industries such as automotive, aerospace, and electrical.2. Chemical Resistance:Silicone is highly resistant to various chemicals, including acids, bases, solvents, and oils. This exceptional resistance makes it a preferred choice for manufacturing gaskets, seals, and O-rings. Silicone can maintain its chemical integrity even when exposed to harsh environments, ensuring long-lasting performance.3. Low Toxicity:Silicone possesses low toxicity and is considered safe for many applications, including food and medical industries. It is biocompatible, non-allergenic, and non-carcinogenic. These properties make silicone suitable for use in medical implants, baby products, and food-grade containers.Uses:1. Sealants and Adhesives:Silicone-based sealants and adhesives are extensively used in construction, automotive, and household applications. Silicone sealants provide excellent water resistance, flexibility, and durability, making them ideal for sealing gaps and joints.Silicone adhesives offer high bonding strength and can adhere to various surfaces, including glass, metal, and plastic.2. Lubricants and Greases:Silicone lubricants are valued for their exceptional temperature stability and compatibility with various materials. They offer low friction and provide long-lasting lubrication. Silicone greases are commonly used in automotive, electrical, and mechanical applications to reduce wear and protect against corrosion.3. Medical and Personal Care Products:Silicone's biocompatible and non-reactive nature makes it a preferred material for medical and personal care products. It is used in a wide range of applications, including medical implants, prosthetics, contact lenses, and skincare products. Silicone is hypoallergenic, easy to clean, and comfortable to wear, ensuring its popularity in healthcare and personal care industries.Manufacturing Process:The manufacturing process of silicone involves several steps:1. Raw Material Preparation:Silicone is produced by hydrolyzing and polymerizing silanes (compounds containing silicon and hydrogen). The silane compounds are mixed with catalysts and other additives to form a silicone base.2. Polymerization:The silicone base is heated and subjected to a controlled polymerization process. This process allows the polymer chains to grow, resulting in the formation of the desired silicone polymer.3. Vulcanization:To enhance the properties of silicone, such as tear resistance and elasticity, vulcanization is performed. The silicone polymer is mixed with crosslinking agents and heated to promote crosslinking between polymer chains. This step greatly improves the mechanical properties of silicone.4. Fabrication:After vulcanization, the silicone material can be shaped using various fabrication techniques such as molding, extrusion, or calendaring. These techniques enable the production ofdifferent silicone products, including sheets, tubes, gaskets, and custom parts.Conclusion:Silicone is a highly versatile material that has revolutionized various industries. Its exceptional properties, including thermal stability, chemical resistance, and low toxicity, have made it indispensable in sectors such as construction, automotive, healthcare, and personal care. The manufacturing process of silicone involves several steps, including raw material preparation, polymerization, vulcanization, and fabrication. As technology advances, the potential applications of silicone continue to expand, and its importance in our daily lives cannot be overstated.。

doi:10.1017/S1431927614002268© Microscopy Society of America 2014 Ex-situ and In-situ Analysis of MoVTeNb Oxide by Aberration-Corrected Scanning Transmission Electron MicroscopyPinghong Xu1, Maricruz Sanchez-Sanchez2, Andre C. Van Veen2, Nigel D. Browning3, Johannes A. Lercher2,31 Department of Chemical Engineering and Materials Science, University of California, Davis, Davis, USA.2 Department of Chemistry and Catalysis Research Center, Technische Universitat Munchen, 85748 Munich, Germany.3 Physical Sciences Laboratory, Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, WA 99352, USA.Short chain olefins, especially ethylene and propylene are very important industrial raw materials and are of increasing demand worldwide. The abundance of C2-C3 light alkanes in shale gas makes production of these olefins from oxidative dehydrogenation (ODH) one of the most attractive alternatives to industrial processes [1]. Among all the catalysts for C2-C3 light alkane ODH reactions, the two-phase MoVTeNb oxide system has shown great promise [1]. Much attention has been devoted to the analysis of the crystallography structure of catalytic active M1 phase. However, the nature of the surfaces where the reactions take place and their roles in catalysis have not been resolved. Additionally, it has been reported that surfaces of the M1 phase undergo changes during ODH reactions, which would affect the catalytic activity of the system [2]. Therefore, it is also very important to perform the in situ experimental observations under relatively realistic conditions for a fundamental understanding of the outstanding catalytic performance of this material.Here, we report an identification of the nature of crystalline termination of the M1 phase at atomic scale using an aberration-corrected scanning transmission electron microscope (STEM). Figure 1 shows a typical M1 phase particle viewed in the crystal growth direction (the [001] orientation). Based on the analysis of over 50 particles, it is shown that the lateral surfaces of these rods are faceted and the most preferential lateral facets have been determined. The configuration of these facets means it is possible to quantify amount of ODH active sites exposed per area on each facet. Moreover, statistical analysis of the proposed active sites exposure in the M1 phase particles with different morphologies agrees very well with experimental data, showing a ~30% higher activity of the small rounded M1 phase rods compared to the large flattened ones. These results demonstrate that morphology has a large impact on catalytic activity of the MoVTeNb oxide system. Simulations are underway to determine the energies of those surface facets with slightly different configurations, which is essential for fundamental understanding of termination mechanism of material of such complex structures.Direct imaging of structural changes in the M1 phase was performed under an oxidative atmosphere by in-situ TEM technique using a Protochip heating holder in an environmental TEM. An M1 phase particles was heated to 350 °C in 10 mbar oxygen/argon (23%/77%) showed that tellurium units disappeared from hexagonal channels while the crystal framework remained unaffected. Control experiments with sample heated to the same temperature without oxygen showed no such effects. The crystalline structure was damaged within seconds. These results indicate that oxygen stabilizes the MoVTeNb oxide structure at elevated temperature. Further in-situ experiments using a gas stage holderwhich allows much higher partial pressure (above 1 bar) are underway to investigate the gas pressure effects [3].References:[1] F. Cavani et al., Catalysis Today 127 (2007), 113-131.[2] M. Hävecker et al.,Journal of Catalysis 285 (2012) 48–60[3] This work is supported by the U.S. Department of Energy Grant No. DE-FG02-03ER46057. The research described in this paper is part of the Chemical Imaging Initiative at PNNL under Contract DE-AC05-76RL01830 operated for DOE by Battelle. It was conducted under the Laboratory Directed Research and Development Program at PNNL, a multiprogram national laboratory operated by Battelle for the U.S. Department of Energy. A portion of the research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.Figure 1. (A) Unprocessed STEM image showing nicely crystalline and faceted surface of M1 phase. (B-D) Magnified view of rectangular areas 1-3 in (A) respectively, showing different facets configurations.。

10.1149/1.3104058 © The Electrochemical SocietyUV-vis Spectroscopy and Cyclic Voltammetry Investigations of Tubular J-Aggregates of Amphiphilic Cyanine DyesJ. L. Lyon a,c, D. M. Eisele b, S. Kirstein b, J. P. Rabe b,D. A. Vanden Bout a, and K. J. Stevenson aa Department of Chemistry, University of Texas at Austin, Austin, TX 78712, USAb Department of Physics, Humboldt University, D-12489 Berlin, Germanyc Current Affiliation: Office of Biotechnology, The University of Texas Health ScienceCenter at Houston, Houston, TX 77030, USAThe redox and light absorption properties of immobilizedaggregates of the cyanine dye 3,3'-bis(2-sulfopropyl)-5,5',6,6'-tetrachloro-1,1'-dioctylbenzimidacarbocyanine (C8S3) ontransparent, conductive indium tin oxide (ITO) electrodes havebeen studied directly using cyclic voltammetry (CV) inconjunction with UV-vis spectroscopy to elucidate uniquemechanistic features of J-aggregate oxidation. C8S3 self-assemblesin aqueous solution to form double-walled, tubular J-aggregateswith ~13 nm diameter and lengths up to several hundrednanometers. Spectroelectrochemical measurements indicate thatirreversible J-aggregate oxidation occurs primarily along the outerwall of the tubular structure as evidenced by the potential-inducedirreversible bleaching of J-band absorption. Additionally, J-aggregate oxidation involves both electrochemical and chemicalsteps in which dimerization and subsequent dehydrogenation of theJ-aggregate leads to the formation of a new dehydrogenated dimeroxidation product.IntroductionTubular J-aggregates of amphiphilic carbocyanine dyes have drawn interest as a model system to study the efficient excitonic energy migration properties similar to those in Light Harvesting Systems (LHS) of plants and bacteria (1, 2). In particular, the double-walled tubular structure of the cyanine dye 3,3'-bis(2-sulfopropyl)-5,5',6,6'-tetrachloro-1,1'-dioctylbenzimidacarbocyanine (C8S3) J-aggregates is an extremely attractive supramolecular model for an artificial LH S due to its ordered, quasi-one-dimensional structure that allows investigation of energy migration and charge transfer processes.H owever, a general problem of aqueous based J-aggregate systems is oxidation, especially in the presence of metal cations such as Ag+ where selective oxidation of the J-aggregate has been observed to occur along with the formation of Ag nanoparticles along the outer wall of the tube (3). Still, the exact mechanism of particle nucleation at the J-aggregate, and its effect on the intrinsic energy migration properties of the aggregate, is poorly understood. For future applications in the context of artificial LH S, more knowledge about the oxidation process is necessary to prevent or control the destructive oxidation of the light harvesting antenna system.H erein, we report electrochemical and spectroscopic properties of immobilized J-aggregates on transparent, conductive indium tin oxide (ITO) electrodes using cyclic voltammetry (CV) in conjunction with UV-vis spectroscopy to elucidate uniquemechanistic features of J-aggregate oxidation. This work contributes to the limited number of electrochemical studies of immobilized cyanine dye J-aggregates in aqueous solution (4,5).ExperimentalC8S3 J-aggregate PreparationThe amphiphilic cyanine dye 3,3’ bis(2 sulfopropyl) 5,5’,6,6’ tetrachloro 1,1’ dioctylbenzimidacarbocyanine (C8S3) is available as a sodium salt from FEW Chemicals (Wolfen, Germany) and was used as received. A 3.00 mM stock solution of monomeric C8S3 was prepared by dissolving an appropriate amount of C8S3 (MW = 902.8 g/mol) in pure methanol (Fisher Scientific) by stirring to form a clear red solution. To prepare C8S3 J aggregates, 130 ȝL of the C8S3 stock solution was added to 500 ȝL of ultrapure H2O (>18.2 Mȍ cm, Barnstead) and agitated to ensure even mixing. An immediate color change from clear red to opalescent, bright pink was observed by eye, indicating the formation of double-walled tubular C8S3 aggregates (6). The solution was stored in the dark for 24 h before adding an additional 500 ȝL H2O to stabilize the aggregation process, resulting in a final dye concentration of 3.36 x 10-4 M. Solutions of J-aggregates were typically used for experiments within five days of preparation, and stored in the dark when not in use. Absorption measurements of C8S3 monomer and J-aggregate solutions were collected using an Agilent Instruments 8453 UV-Visible spectrometer with a photodiode array detector. The measurement cell consisted of two quartz slides with a fixed Teflon gasket (0.1 mm path length) containing 45 ȝL of either monomer or J-aggregate solution.Electrochemical MeasurementsElectrochemical experiments were conducted on J-aggregates immobilized on transparent, conductive indium tin oxide (ITO)-coated glass electrodes (Delta Technologies, Ltd.; 15 ȍ/Ƒ) immersed in an aqueous solution containing 1 M KNO3 supporting electrolyte (Fisher Scientific). The pH of this supporting electrolyte was 5.78 ± 0.03 as prepared. Prior to use, the ITO working electrode substrates were cleaned by immersion in 30% (v/v) aqueous ethanolamine (Aldrich) at 80 °C for 20 min, followed by rinsing with methanol and sonicating in ultrapure H2O for 30 min. The substrates were then dried under a stream of nitrogen. Films of J aggregates were prepared on ITO by drop casting 10 ȝL of J aggregate solution on a 0.5 cm2 area of ITO followed by drying in the dark for ~ 2 h. Once dried, they remained at the ITO surface when immersed in supporting electrolyte. ITO working electrodes, either bare or with dried J aggregate films, were fitted in a 20 mL glass cell containing ~5 mL supporting electrolyte, Au wire counter electrode, and a Hg/Hg2SO4 (sat’d. K2SO4) reference electrode (CH Instruments; E° = +0.640 V vs. NHE) with the ITO surface oriented perpendicular to the bottom of the cell. Electrochemical measurements were performed at room temperature (23 ± 2 °C) using an Autolab PGSTAT30 potentiostat interfaced with Autolab GPES version 4.9 software. Prior to each experiment, the cell was purged with Ar for at least 5 min. All experiments were conducted under flowing Ar.Spectroelectrochemical MeasurementsSpectroelectrochemical experiments were performed using a CH700 bipotentiostat (CH Instruments) interfaced to an Agilent Instruments 8453 UV-Visible spectrometer with a photodiode array detector. A homemade electrochemical cell with a fixed working electrode area of 0.45 cm2 and ~1 mL 1 M KNO3 supporting electrolyte was placed in the spectrometer sample holder with the J-aggregate film-coated ITO working electrode in the beam path. The cell also contained a Pt wire counter electrode and H g/H g2SO4 (sat’d. K2SO4) reference electrode. The potential was cycled between -0.7 and +0.45 V for three consecutive cycles while absorption spectra between 350 and 800 nm were collected every 4.0 s with a 0.5 s integration time. A slow scan rate of 0.01 V/s was used, permitting spectral acquisition every 45 mV.Results and DiscussionC8S3 self-assembles in aqueous solution in the presence of 10 wt % methanol to form double-walled, tubular J-aggregates with ~8 nm inner diameter and lengths up to several hundred nanometers (6). The aggregation of the dyes produces new visible absorption bands that result from the dipolar coupling of the cyanine chromophores. As shown in Figure 1, the key features of the spectrum are two sharp J-aggregate bands at 590 and 600 nm that have been assigned to absorption by the outer and inner wall, respectively. Since each of the tubes has a distinct absorption feature, it is possible to follow the redox chemistry of the inner and outer wall independently. This double walled tubular structure has been confirmed by cryogenic TEM, polarized spectroscopy and theory (6). The morphology of the tubules is strongly dependent on the solvent conditions and a variety of other structures can be obtained by altering the chemical composition of the solvent (7).Figure 1. UV-Vis spectrum of the C8S3 monomer and tubular aggregate showing the inner and outer wall J-bands that form upon aggregation.Figure 2 depicts cyclic voltammograms (CVs) showing the irreversible oxidation of immobilized J-aggregates at ITO in aqueous pH 5.78, 1 M KNO3. The J-aggregates are significantly easier to oxidize as evidenced by the appearance of an irreversible J-aggregate oxidation (labeled I, black trace) as the potential is scanned positive. A single oxidation peak for the J-aggregate appears at +0.22 V compared to that reported for the monomer at +1.04 V (8) and consistent with those reported for other J-aggregate systems (4), in which a lower oxidation potential is reflective of a lower activation energy for J-aggregate oxidation relative to the cyanine monomer due to a shift in HOMO-LUMOenergy levels upon aggregation. Subsequent scans between positive and negative potential extremes reveal a second, reversible set of peaks centered at E1/2= -0.378 V, labeled II/II’. Note that the current response for this new reduction/oxidation process continues to increase until a steady state response is obtained after ~15 consecutive cycles. Additionally, voltammetric studies were conducted in which the potential window of the immobilized J-aggregate ITO electrode was cycled 15 times without inducing J-aggregate oxidation (i.e. from the same negative extreme as that shown in Figure 2 to +0.025 V, just before the onset of J-aggregate oxidation). In this case, we do not see the appearance of the redox response for II/II’. However, if the potential was then scanned positive (anodically) to +0.45 V to induce J-aggregate oxidation, the reduction peak II’ appears on the return sweep. This data indicates that peaks II/II’ are indeed related to the formation of a new species as a result of J-aggregate oxidation; this species is most likely a J-aggregate oxidation product whose redox activity behaves in accordance with a previously proposed electrohydrodimerization (EHD) mechanism (9).Figure 2. Cyclic voltammograms (15 cycles) displaying the initial irreversible oxidation of the J-aggregate (peak I) and the formation of a second, reversible couple, (peaks II/II’), during potential cycling of a J-aggregate/ ITO electrode in pH 5.78, 1 M KNO3. Scan rate 0.1 V/s.In a previous study (8) we conducted several other pH-dependent measurements to understand this complicated, proton-coupled electron transfer process. Scheme 1 depicts our proposed potential-dependent mechanism of C8S3 J-aggregate oxidation and subsequent formation of other redox active products, which support an electrochemical-chemical-electrochemical (ECE) process that consists of a net 4-electron transfer per dye molecule to yield a dehydrogenated dimer, labeled as DHD ox4+.Scheme 1. Depiction of the overall EH D mechanism involving ECE steps for C8S3 J-aggregate oxidation and subsequent formation of a dehydrogenated dimer product (DHD).We note that while the DH D ox4+ formation mechanism is derived from studies of cyanine dye monomers, it may be even more probable in the case of J-aggregates, in which monomer units of dye are spatially arranged near each other, promoting dimer associations of oxidized dye and the subsequent formation of DH D ox4+. If the species responsible for II/II’ in Figure 2 is a DH D ox4+product formed from neighboring dye molecules in the immobilized J-aggregates at ITO, then its voltammetric behavior should follow that predicted for a surface-confined redox species. As reported previously (8), the peak currents for both II and II’ at steady state do, in fact, scale linearly with increasing scan rate between 0.01 and 3 V/s, as predicted for an immobilized system.To correlate the electrochemical oxidation of C8S3 J-aggregates with their well-defined absorption spectrum, spectroelectrochemical experiments were carried out in which the potential of an ITO working electrode containing a J-aggregate film was scanned concurrently with the acquisition of absorbance spectra in the visible region from 350-800 nm. Similar studies have been undertaken previously to investigate J-aggregates organized on modified Au(111) (5). To maximize the number of spectra acquired for correlation to the CV, a slow scan rate of 0.01 V/s was used, permitting spectral acquisition every 45 mV. Figure 3 depicts the decrease in absorbance observed upon electrochemical oxidation of the J-aggregate film. As more anodic potentials are applied, the absorbance at 590 nm corresponding to the outer wall of the J-aggregate tubular structure decreases dramatically. The inner wall absorbance at 600 nm also decreases, though not as substantially as that of the outer wall; the relative intensity of the inner wall in fact surpasses that of the outer wall as a result of electrochemical oxidation. This change in relative absorbance most likely reflects a more facile oxidation of the outer wall, due to outer wall dye molecules being in closer electrical contact with the electrode, along with a more hindered oxidation of the inner wall dye molecules due to their occlusion by the C8 chains, and a possible charge compensation hindrance due to slower ion transport within the interior of the tube.Figure 3. Spectroelectrochemical data for the oxidation of immobilized C8S3 J-aggregates on ITO: (a) absorption spectra versus applied potential, (b) cyclic voltammogram at 0.01 V/s in pH 5.78, 1 M KNO3, (c) differential absorbance (įA/įV) plot for outer wall and inner wall J-aggregate absorbance bands centered at 590 and 600 nm, respectively.Importantly, the absorption maxima do not shift as a function of applied potential, but rather remain fixed at 590 and 600 nm, respectively. Previous spectroscopic studies by Lenhard and H ein correlated wavelength shifts with disruption of the J-aggregate dyeupon chemical oxidation (10); however, these results suggest that the J-aggregates’ morphology is not disturbed as a result of electrochemical oxidation. The differential absorbance (įA/įV) plots for both inner and outer wall oxidation correlate well with the voltammetric response shown in Figure 3c. The įA/įV plots nearly perfectly overlay the CV data, showing the direct correlation between electrochemical oxidation of the J-aggregates and changes in the J-band absorbance. At this slower scan rate, the J-aggregate oxidation peak in Figure 3b features a shoulder near +0.230 V in addition to the sharp oxidation at +0.166 V. Closer inspection of Figure 3c reveals that the inner wall of the J-aggregate structure passes through a maximum in differential absorbance a few seconds after the outer wall does, and that the resulting differential absorbance plot very closely resembles the shouldered peak in Figure 3b. On the return sweep of the voltammogram shown in Figure 3b, the reduction peak II’ corresponding to the reduction of DH D ox4+occurs at -0.378 V, and a response in the įA/įV plots is observed concurrently. Assuming that only the absorbance bands centered at 590 and 600 nm are present in spectra during the course of this experiment, this data suggests that the J-band absorbance is increasing as DH D ox4+is reduced. H owever, examination of the full absorption spectrum reveals a new broad absorption generated upon reduction of II’ that overlaps with the aggregate peaks. This absorption correlates to the formation of DHD red2+.Our spectroelectrochemical studies are also consistent with spectral changes observed in the reaction of J-aggregates in the presence of Ag+. The kinetics of the chemistry can be directly monitored because the effects of the silver growth can be clearly seen in the absorption spectra before and after particle formation, Figure 4.Figure 4. UV-Vis spectrum of a C8S3 aggregate solution after the addition of Ag(NO3). Initial spectrum (black), spectrum with Ag(NO3) after 3 hours in the dark (gray), spectrum with Ag(NO3) after 15 minutes with 5 minutes of illumination (orange). Inset: red arrow indicates the presence of Ag plasmon absorbance near 410 nm.The formation of the silver particles results in the loss of absorption from the aggregates and an increase in absorption due to a silver plasmon. The decay of the inner and outer wall absorptions differ from one another with the outer wall decaying nearly to zero while some portion of the inner wall remains after the chemistry is complete. However, at this stage the resulting effect of photo-oxidation upon the aggregates’ morphology remains unclear, as well as the mechanistic details of particle nucleation and growth.Last, we also performed additional experiments in which a solution of J-aggregates had been illuminated with UV-light in the presence of Ag+. This solution was then evaporated on an ITO electrode. Cyclic voltammetry was subsequently conducted on the J-aggregate film in pH 5.78, 1 M KNO3. As shown in Figure 5, a distinct anodic response associated with the oxidation of Ag0 is observed at +0.03 V, as well as a diminished oxidative response at +0.2 V associated with the residual oxidation of un-reacted J-aggregates. Integration of the Ag oxidation peak gives 9.2x10-7 C, which correlates to ~95 pmol Ag. We also tried to illuminate a dried J-aggregate film in the presence of Ag+ while in the spectroelectrochemical cell, but the UV-Vis spectrum showed no decrease in absorbance at 590 or 600 nm. However, these preliminary experiments indicate that the J-aggregate acts as a reducing agent in the presence of Ag+ which leads to nucleation and growth of Ag nanoparticles at outer wall sites along the J-aggregate. More studies are necessary to understand how the J-aggregates act as a localized photoreductant to facilitate and confine the reduction of metal ions.Figure 5. Cyclic voltammogram of immobilized J-aggregates on an ITO electrode prepared from UV illumination of a J-aggregate solution containing Ag+.SummaryThe spectroelectrochemical studies of tubular J-aggregates immobilized at ITO electrodes has been described. The electrochemical behavior of C8S3 J-aggregates is comparable to that observed in previous studies performed on solution-phase J-aggregates and cyanine dye monomers. An electrohydrodimerization mechanism was proposed to explain the unique potential dependent redox response. Both irreversible dimerization and dehydrogenation chemical steps follow the initial J-aggregate oxidation, which leads to the formation of a new dehydrogenated dimer oxidation product that is most likely confined within the J-aggregate itself. UV-vis spectroscopy studies demonstrate that the J-aggregate oxidation is mostly confined to the outer wall of the tubular structure, and that the inner wall is oxidized to a lesser extent relative to the outer wall. Future studies involve the development of a model of all the species involved in the redox chemistry and verify this model with spectral measurements acquired in the absence and presence of metal salts (chemical oxidants) and with and without illumination to UV light.AcknowledgmentsFinancial support for this work was provided by the R.A. Welch Foundation (grants F-1377 and F-1529), the National Science Foundation (CH E-0809770), and Deutsche Forschungsgemeinschaft [Sfb 448: Mesoscopically Organized Composites].ReferencesMcDermott et al.,Nature 374, 517 (1995).1. G.2. H. v. Amerongen, L. Valkunas, R. v. Grondelle, in Photosynthetic Excitons.(World Scientific, Singapore, 2000).3. S. Kirstein, H. v. Berlepsch, C. Böttcher, Int. J. Photoener. 2006 (47917), 1(2006).4. M. Kawasaki, T. Sato, J. Phys. Chem. B 105, 796 (2001).5. M. Kawasaki, D. Yoshidome, T. Sato, M. Iwasaki, J. Electroanal. Chem. 543, 1(2003).Didraga et al.,J. Phys. Chem. B 108, 14976 (2004).6. C.7. H. von Berlepsch, S. Kirstein, R. Hania, A. Pugzlys, C. Böttcher, J. Phys. Chem.B 111, 1701 (2007).8. J. L. Lyon et al.,J. Phys. Chem. C 112, 1260 (2008).9. J. R. Lenhard, A. D. Cameron, J. Phys. Chem. 97, 4916 (1993).10. J. R. Lenhard, B. R. Hein, J. Phys. Chem. 100, 17287 (1996).。