当前位置:文档之家 > Biomolecular-Induced Synthesis of LaNanostructures and Its Morphology-Heldtoward La2O3 and La(OH)3

Biomolecular-Induced Synthesis of LaNanostructures and Its Morphology-Heldtoward La2O3 and La(OH)3

DOI:

Biomolecular-Induced Synthesis of LaNanostructures and Its Morphology-Heldtoward La2O3 and La(OH)3

10.1021/cg801053p

Biomolecular-Induced Synthesis of Self-Assembled Hierarchical

La(OH)CO3One-Dimensional Nanostructures and Its Morphology-Held

Conversion toward La2O3and La(OH)3

Jinsong Xie,?Qingsheng Wu,*,?,§Da Zhang,?and Yaping Ding?

?Department of Chemistry,Tongji University,Shanghai,200092,China,?Department of Chemistry,

Shanghai University,Shanghai200444,China,and§Shanghai Key Laboratory of Molecular Catalysis

and Innovative Materials,Fudan University,Shanghai200433,China

Received September19,2008;Revised Manuscript Received June30,2009

ABSTRACT:Novel hierarchical layer-by-layer self-assembled one-dimensional(1D)La(OH)CO3nanostructures,with a diameter of around700nm and lengths in the range of6-8μm,were synthesized by a developed hydrothermal method using La2O3and glycine as the starting materials.Various experimental conditions,such as the reaction time,temperature,and the molar ratios of the starting reagents,were studied.The obtained1D La(OH)CO3nanostructures can be successfully converted to La2O3 and La(OH)3nanorods via calcination under appropriate conditions.Analytical methods such as X-ray diffraction,scanning electron microscopy,transmission electron microscopy,and selected area electron microscopy were employed to characterize these products,and the possible growth mechanism of1D La(OH)CO3nanostructures was explored.The UV-visible diffuse reflectance absorbance spectra indicate that the1D nanostructures have enhanced UV-light absorbance properties in contrast to the bulk materials.The electrochemical studies show that1D La(OH)CO3nanostructures have a stronger ability to promote electron transfer between ascorbic acid(H2A)and the glass-carbon(GC)electrode than the bulk La(OH)CO3.These layer-by-layer self-assembled hierarchical products have possible application as an efficient support matrix for the immobilization of enzymes and some biomolecules.This one-pot method is likely to be useful in the preparation of many other layered structures.

1.Introduction

Nowadays,rare-earth compounds have been an attractive subject due to their unique optical,magnetic,and catalytic properties.These compounds have been widely used in various fields,1such as high-performance luminescent devices,high-quality phosphors,up-conversion materials,magnets,cata-lysts,time-resolved fluorescence labels for biological detection, and so on.So far,there have been some reports on preparing La(OH)CO3,La2O3,and La(OH)3nanomaterials,including nanowires2and microspheres3of La(OH)CO3;nanoplates,4 nanorods,5nanowires,6nanobelts,7macropores,8and hollow trapezohedron9of La2O3;nanorods,6nanowires,10nanobelts,7 and nanospheres11of La(OH)3,etc.However,to the best of our knowledge,the preparation of layer-by-layer La(OH)CO3 nanomaterials self-assembled by single nanoplates through a facile one-pot hydrothermal method and the structure and morphology conversion from La(OH)CO3to La(OH)3and La2O3have not been found in the scientific literature.

In recent years,many synthesis efforts have been focused on utilizing biomolecules with special structures and unique self-assembling properties12as templates to prepare novel structure materials.Some small biomolecules such as amino acids and their derivatives have been employed for the fabrication of nanostructured materials.For instance,with the aid of L-cysteine,lead chalcogenide(PbE,E=S,Se,Te) nanotubes,13porous spongelike Ni3S2nanostructures,14 CoS nanowires,15and Sb2S3nanowires16were synthesized; in the presence of a small biomolecule of glycine,CdS dendrites17and R-Fe2O3nanowires18were fabricated.Nickel nanocrystals with controlled size and shape in the presence of peptide nanotubes,19highly ordered snowflake-like structures of Bi2S3nanorods with the assistance of glutathione,20one-dimensional Ag nanostructures21and porous MgO nanomaterials22using dextran as an effective directing reagent were obtained.Furthermore,Zhao et al.23 manipulated the crystal growth of CeOHCO3and NdOH-CO3using amino acids as additives.The above examples verify the effectiveness of biomolecules to tailor the struc-tures of various materials,which inspire us to explore a simpler and more economical method to prepare rare earth hydroxycarbonates by means of some small biomolecules. In this paper,1D La(OH)CO3nanomaterials,self-as-sembled by single-crystal nanoplates,are obtained for the first time using a facile oxide-biomolecule-hydrothermal (OBHT)method,24in which oxide and biomolecule are directly used as raw materials without any additives.Through careful investigation of various influencing factors,the rela-tionship between reaction conditions and morphology is discussed and the growth mechanism of the La(OH)CO3is presented.The obtained1D La(OH)CO3nanostructures can be successfully converted to porous La(OH)3and La2O3 nanorods.This work is significant for preparing and exploit-ing potential applications of hierarchical1D La(OH)CO3 structures in many fields:efficient support matrix for the immobilization of enzymes and some biomolecules,and the process in the work can also be utilized in preparation of many other layered structures.

2.Experimental Section

Materials.Bulk-phase NH2CH2COOH(99.0%)and La2O3 (>99.99%)powders were purchased from Shanghai Chemical Reagent Ltd.,(Shanghai,China)and were used without further

*Corresponding author.E-mail:qswu@http://www.doczj.com/doc/0dc275abaeaad1f346933f53.html.Tel.:t86-21-

65982620.Fax:t86-21-65981097.

http://www.doczj.com/doc/0dc275abaeaad1f346933f53.html/crystal

Published on Web08/06/2009

r2009American Chemical Society

3890Crystal Growth&Design,Vol.9,No.9,2009Xie et al.

purification.Deionized water,obtained by means of a water-purification system,was used in the experiments.Ascorbic acid solutions(H2A)were prepared using phosphate buffer solution (pH7.50).The preparation of La(OH)CO3nanomaterials was per-formed by an OBHT route.The reactions were carried out in a Teflon-lined stainless steel autoclave with a capacity of20mL. Taking a typical example,1?10-3mol of La2O3,3?10-2mol of NH2CH2COOH,and10mL of deionized water were mixed in an autoclave and sufficiently dispersed by an ultrasonic generator. The autoclave was sealed and kept still in a digital-type temperature-controlled oven at200°C for32h and then cooled to room temperature naturally.Sequentially,the precipitation was sepa-rated by centrifugation and washed with deionized water and absolute ethanol several times,respectively.Finally,the as-obtained products were dried in air at60°C for further characterization.

Electrochemical responses were performed on a CHI-820elec-trochemical workstation with a three-electrode system including a bare or modified GC electrode as the working electrode,a platinum wire as the counter electrode,and a saturated calomel electrode (SCE)as the reference electrode,employing a scanning rate of 60mV/s and a rest time of2s.To prepare GC electrodes modified by the products,5mg of bulk La(OH)CO3,1D La(OH)CO3 nanomaterials respectively was dispersed into5mL of deionized water under ultrasonic.Then the solution(20μL)was dropped onto the surface of the GC electrode using a microsyringe,which then dried in air at room temperature.The different relative humidities of air were adjusted by a humidity controller(YBCK-906,Weiming, Inc.China).

Characterization.X-ray diffraction(XRD)patterns of samples were measured on a Bruker D8-advance X-ray diffractometer with Cu K R radiation(λ=0.154056nm)(Germany),using a voltage of 40kV,a current of40mA,and a scanning rate of0.02°/s,in2θranges from10°to70°.The cell lattice constants of samples were calculated and corrected by MDI Jade(5.0edition)software. Thermal stability of La(OH)CO3was investigated by a simulta-neous thermogravimetric and differential scanning calorimetric system(TG/DSC,Netzsch STA409PC,Selb,Germany)with Al2O3powder as the reference.Approximately10mg of the sample was loaded into a standard Al2O3crucible,which was heated from room temperature to1000°C at a heating rate of10°C/min in a flowing N2atmosphere.The morphologies and structure of the products were examined with scanning electron microscope(SEM, Philips XL30,Holand)at a accelerating voltage of20kV,transmis-sion electron microscopy(TEM),and high-resolution transmission electron microscopy(HRTEM,JEM-2010,JEOL,Tokyo,Japan) with selected area electron diffraction(SAED)at an accelerating voltage of200kV.UV-visible diffuse reflectance absorbance spectra(DRS)were obtained with a UV-vis spectrometer (BWS003,Newark,DE).A CHI-820electrochemical workstation (Shanghai,China)was used to test the electrocatalytic activity of the 1D La(OH)CO3nanostructures-modified GC electrodes toward H2A oxidation.IR spectra were measured on a Nexus FT-IR spectrophotometer using KBr pellets.The chromatograms were recorded using the Agilent1100Series(Agilent Technologies) chromatographic system with DAD detector.Mass detection was performed on a Varian310LC-MS/MS triple quadrupole mass spectrometer equipped with electrospray ion source(Varian,Inc. America).

3.Results and Discussion

1D La(OH)CO3,La2O3,and La(OH)3nanostructures with similar morphologies were in situ synthesized by a facile method.The1D layer-by-layer self-assembled hierarchical La(OH)CO3,with a diameter of around700nm and lengths in the range of6-8μm(Figure1b,c),were obtained by La2O3 reacting with NH2CH2COOH(molar ratio1:30)at200°C for32h.It belongs to hexagonal phase La(OH)CO3(JCPDS 26-0815,Figure1a).The1D La(OH)CO3can be converted to La2O3by calcination at900°C for1h in a vacuum.The XRD pattern reveals the presence of hexagonal La2O3 (JCPDS05-0602,Figure1d).Scanning electron microscopy (SEM)images show that the morphology of the samples retains the original1D shape(Figure1e,f),except that the underlying layer-by-layer self-assembled structures disap-peared.The SEM images show that the diameters and the lengths of the La2O3porous nanorods are about600-700

Biomolecular-Induced Synthesis of LaNanostructures and Its Morphology-Heldtoward La2O3 and La(OH)3

nm Figure1.(a-i)Representative XRD patterns and SEM images of La(OH)CO3,La2O3,and La(OH)3,respectively.

Article Crystal Growth &Design,Vol.9,No.9,20093891

Biomolecular-Induced Synthesis of LaNanostructures and Its Morphology-Heldtoward La2O3 and La(OH)3

Biomolecular-Induced Synthesis of LaNanostructures and Its Morphology-Heldtoward La2O3 and La(OH)3

and 3-5μm,http://www.doczj.com/doc/0dc275abaeaad1f346933f53.html(OH)CO 3was first transformed into La 2O 3through calcination at 900°C for 1h,then cooled with a high relative humidity of 95%for 3h,and the as-obtained La 2O 3would further change into La(OH)3via a hydrolysis process.The products belong to the hexagonal phase La(OH)3(JCPDS 83-2034,Figure 1g)according to the XRD pattern.SEM images indicate the morphologies of La(OH)3are porous nanorods nearly the same as La 2O 3(Figure 1h,i).

A series of experiments have been performed to examine the effect of the humidity of air on the final products during the cooling http://www.doczj.com/doc/0dc275abaeaad1f346933f53.html(OH)CO 3was first transformed into La 2O 3through calcination at 900°C for 1h,then cooled under different relative humidities of 20%,70%,95%for 3h,respectively,and the final products were pure La 2O 3,a mixture La 2O 3of La(OH)3and pure La(OH)3,which were characterized by XRD shown in Figure 2a -c,respectively.However,under a relative humidity of 95%for 1h,the La 2O 3has not been completely hydrolyzed and the products are also mixture of La 2O 3and La(OH)3(Figure 2d).In theory,during the heating process,the La(OH)3may be fabricated as a middle product.However,Our thermogravimetric/differential scanning calorimetry (TG/DSC)measurements (see Figure S1,Supporting Information)and the correlative literature 25indicate that La(OH)3could not be directly achieved by heating La(OH)CO 3in ambient atmosphere.So we can draw a conclusion as follows:when heated in air,the hierarchical La(OH)CO 3structures first can be converted to porous La 2O 3nanorods;during the cooling process,the La 2O 3nanomaterials readily absorb the water in moist air and finally turn into La(OH)3nanomaterials.

Figure 3shows typical XRD patterns of samples prepared at different reaction times at 200°C.Figure 3b -e shows that the obtained products belong to pure hexagonal La(OH)CO 3when the reaction is performed over 15h.In each XRD pattern,all of the reflections can be readily indexed to the hexagonal phase (space group P 6h (No.174))of La(OH)CO 3with lattice constants a =12.6291and c =

10.0144A

comparable with the values given in JCPDS (26-0815).No other peaks were observed in the patterns,showing the high purity of the samples.However,Figure 3a indicates that the products prepared belong to the mixture of orthorhombic La(OH)CO 3(JCPDS 49-0981)and hexagonal La(OH)CO 3when the reaction is carried out around 8h.The results indicate that orthorhombic La(OH)CO 3as a kind of intermediate product appears first.The intermediate products can completely transform into hexagonal phase La(OH)CO 3crystal when the reaction time is beyond 15h.It can be seen from Figure 3that the intensities of all the crystal peaks were heightened,implying the improved crystallinity of the pro-ducts with extending the reaction time.Furthermore,the relative intensities of peaks between (300)and (302)were evidently changed along with the treatment time,suggesting that the (300)crystal facets group were preferentially grown.The shapes of the samples should be associated with the crystal growth change,which were further proved by micro-graphs characterized SEM and TEM.

The morphologies of the products obtained under differ-ent conditions were carefully investigated by SEM.Figure 4shows SEM micrographs of the growth process of layer-by-layer self-assembled 1D nanostructures obtained at 200°C for 8,15,24,32,and 48h.Time-dependent experi-ments indicate that the reaction time exerts a strong influ-ence on the diameters and lengths of the 1D La(OH)CO 3nanostructures.Quasi-spheres with a diameter around 200nm formed with a reaction time of 8h (Figure 4a),While the reaction time increased to 15h,the diameters increased slightly,and the lengths grew up to 1-2μm,and the quasi-sphere structures gradually grew into 1D nanos-tructures (Figure 4b).When the reaction time lasted for 24h,both the diameters and the lengths of the self-assembled 1D nanostructure increased quickly.As shown in Figure 4c,the morphologies of the 1D nanostructures are irregular.The lengths and the diameters of them are not uniform.After extending the reaction time to 32h,very regular 1D nanostructures with lengths of up to 6-8μm and diameters of about 770nm were obtained,as presented in Figure 4d.However,it should be pointed out that when the reaction time was up to 48h,the lengths of the 1D nanostructures started to decrease (Figure 4e).There are two possible reasons for this phenomenon.Owing to the presence of a large amount of -OH groups in La(OH)CO 3molecules,there are some weak interaction forces,such as weak van der Waals intermolecular interaction

and

Figure 2.XRD patterns of the products by calcination at 900°C for 1h and cooled in air with various reaction humidities and time:(a)20%;(b)70%;(c)95%for 3h;and (d)95%for 1

h.

Figure 3.XRD patterns of as-obtained products at 200°C with a molar ratio of La 2O 3to glycine of 1:30under different reaction times:(a)8h;(b)15h;(c)24h;(d)32h;(e)48h.

3892Crystal Growth &Design,Vol.9,No.9,2009Xie et al.

Biomolecular-Induced Synthesis of LaNanostructures and Its Morphology-Heldtoward La2O3 and La(OH)3

Biomolecular-Induced Synthesis of LaNanostructures and Its Morphology-Heldtoward La2O3 and La(OH)3

hydrogen bonding,between nanodisks in the layer-by-layer self-assembled 1D nanostructure.With the increase of the reaction time,these weak forces would be further weakened and may cause the 1D nanostructure to be shortened and disassembled.On the other hand,the constant collision among the assemblies probably is likely to shorten the 1D nanostructure with the prolonged reaction time.The reaction temperature also plays a crucial role in the crystallization and morphology control of La(OH)CO 3.Figure 5a shows that the La(OH)CO 3sample prepared at 160°C displays a disk-like with the mean diameter of 380nm,some of which have already self-assembled into 1D nano-structure with the length around 1μm.When the reaction temperature increased to 180°C,the mean diameter of

the

Figure 4.SEM images of as-obtained products at 200°C with a molar ratio of La 2O 3to glycine of 1:30under different reaction times:(a)8h;(b)15h;(c)24h;(d)32h;(e)48

h.

Figure 5.SEM images of as-obtained products at different reaction temperatures for 32h with a molar ratio of La 2O 3to glycine of 1:30:(a)160°C;(b)180°C;(c)200°C;(d)220°C.

Article Crystal Growth &Design,Vol.9,No.9,20093893

Biomolecular-Induced Synthesis of LaNanostructures and Its Morphology-Heldtoward La2O3 and La(OH)3

Biomolecular-Induced Synthesis of LaNanostructures and Its Morphology-Heldtoward La2O3 and La(OH)3

disk-like products has increased to around 460nm and the self-assembled 1D nanostructures are widely http://www.doczj.com/doc/0dc275abaeaad1f346933f53.html-pared with the sample prepared at 180°C (Figure 5b),the lengths of the 1D nanostructures were longer and the morphologies of the products were more regular when the reaction temperature increased to 200°C (Figure 5c).How-ever,the mophologies of the products became irregular when the reaction temperature reached 220°C (Figure 5d).

The effect of the reaction temperature on the morphology of the products was further investigated,as shown in Figure 6.The products were composed of dispersed single nanoplates with the diameters less than 1μm,and no assembly appeared at 160°C for 8h (Figure 6a).When the reaction temperature is increased up to 200°C,the diameters of nanoplates decreased instead of a continuous increase and most of them self-assembled into a quasi -sphere-like shape (Figure 6b).The possible reason for the phenomenon is as follows:at lower temperature,these nanodisks were able to freely grow in short chainlike polypeptide molecules.As the temperature increases,more and more polypeptide molecules were formed in the reaction system,which would prevent the free growth of the nanodisks and induce them to assemble a 1D nano-structure.

Variety in the molar ratio of La 2O 3to glycine also has a great impact on the morphologies of the as-obtained products as shown in Figure 7.When the molar ratio of La 2O 3to glycine was 1:20,the products were composed of nonerratic nanodisks with a diameter of 300nm and a thickness of 100nm and some self-assembled 1D nanostructure with a length about 1μm (Figure 7a).However,when the molar ratio decreased to 1:25,the shapes of part of the samples were in a mess with various lengths (Figure 7b).The diameters and lengths of the products were almost uniform after the molar ratio was changed to 1:30(Figure 7c).While the molar ratio was reduced to 1:40,the morphologies of the products became irregular again (Figure 7d).

To study the function of the glycine in the formation of the uniform 1D La(OH)CO 3nanostructures,a series of control experiments were performed.When adding some surfactants,such as polyethylene glycol 4000(PEG 4000),polyvinylprrolidone (PVP),and cetyltrimethylammonium bromide (CTAB),into the former system,the morphologies of the products were thoroughly changed compared with no surfactants in the system (see Figure S2,Supporting In-formation).While the Na 2CO 3and NaOH were substituted for glycine,the 1D nanostructues also completely http://www.doczj.com/doc/0dc275abaeaad1f346933f53.htmlparing with glycine,the above-mentioned sur-factants might have a stronger control ability and undermine the primal 1D nanostructures.The facts confirm that the glycine does play a role in inducing the self-assembly of nanoplates into 1D nanostructures.

On the basis of the above experimental results,the pre-ferable experimental parameters for the obtained uniform 1D nanostructure are a reaction temperature of 200°

C,

Figure 6.SEM images of as-obtained products at different reaction temperatures for 8h with a molar ratio of La 2O 3to glycine of 1:30:(a)160°C;(b)200°

C.

Figure 7.SEM images of as-obtained products at 200°C for 32h with different molar ratios of La 2O 3to glycine:(a)1:20;(b)1:25;(c)1:30;(d)1:40.

3894Crystal Growth &Design,Vol.9,No.9,2009Xie et al.

Biomolecular-Induced Synthesis of LaNanostructures and Its Morphology-Heldtoward La2O3 and La(OH)3

a reaction time of 30h,and a molar ratio of La 2O 3to glycine of 1:30.

To further investigate crystal status and growth process of layer-by-layer self-assembled nanostructures,the products were characterized by TEM,HRTEM,and SAED.The TEM images further show the products at 160°C are regular,and most of them are nanodisks with the diameter around 380nm (Figure 8a 1),some of which are self-assembled short 1D nanostructures (Figure 8a 2).The TEM images shown in Figure 8b 1further affirm that the as-prepared La(OH)CO 3at 200°C are multilayer 1D structures composed of numerous delicate thin nanodisks.The SAED pattern (inset in Figure 8b 2)of the flank of the 1D nanostructures show successive bright dots for the (002)plane,which indicate the thin nanodisks aggregate into thick nanodisks with a highly oriented [001]crystallographic axis.This observation is supported by HRTEM (Figure 8b 2).The TEM images of the cross section of the 1D nanostructures further show the 1D nanostructures are made up of thick nanodisks (Figure 8b 3).Furthermore,these nanodisks show a highly single-crystal nature,determined by HRTEM images and SAED patterns,as shown in Figure 8b http://www.doczj.com/doc/0dc275abaeaad1f346933f53.htmlttice spacings about 0.362and 0.211nm correspond to the (300)and (330)planes of a unit cell hexagonal phase La(OH)CO 3,respectively.The spatial ar-rangement of the spots in the ED images reveal the set of lattice planes derived from a single hexagonal crystal with its [0001]direction being oriented toward the direction of the electron beam.The La(OH)CO 3multilayer self-assembled nanostructures with the advantages of high ratio surface area and analogy-graphite layer structure are possibly favorable for potential application in optics,catalysis,intercalation chemistry,etc.

The UV -visible diffuse reflectance absorbance spectra of bulk La(OH)CO 3and its 1D nanostructures at 200°C for 32h are shown in Figure 9.The 1D nanostructures have more intense absorbance of UV -visible light than that of bulk materials from the curves.Especially in the UV region,the absorbance intensity of the nanomaterials is as twice as the bulk materials.The enhanced UV-light absorbance properties could be ascribed to the reduced size of materials.As the size decreased,the sharply increased surface areas and energies of the samples would result in enhancement

of

Figure 8.TEM images of as-obtained products at different reaction temperatures for 32h with a molar ratio La 2O 3to glycine of 1:30:(a 1,a 2)160°C;(b 1,b 3)200°C,(b 2,b 4)the HRTEM (inset ED)images of (b 1,b 3),respectively.

Article Crystal Growth &Design,Vol.9,No.9,20093895

Biomolecular-Induced Synthesis of LaNanostructures and Its Morphology-Heldtoward La2O3 and La(OH)3

Biomolecular-Induced Synthesis of LaNanostructures and Its Morphology-Heldtoward La2O3 and La(OH)3

Biomolecular-Induced Synthesis of LaNanostructures and Its Morphology-Heldtoward La2O3 and La(OH)3

absorbance of light.These results show that the 1D nanoma-terials are possibly applicable to prepare photosensors and photocatalysis.

Figure 10shows the cyclic voltammograms of different electrodes in the 0.1M PBS solution with 1mM H 2A.When the bare GC electrode was used as the working electrode,the oxidation potential of H 2A was 717mV and the anodic peak current was 8.6μA.After the GC electrode was modified by the bulk La(OH)CO 3,the oxidation potential of H 2A was exhibited at 696mV and the anodic peak current was 10.4μA.When the 1D La(OH)CO 3nanomaterial-modified GC elec-trode was employed,the oxidation potential of H 2A appeared at 611mV and the anodic peak current changed to 13μA.The above experimental data indicate that 1D La(OH)CO 3nanomaterials have better electrocatalytic properties and can greatly promote electron transfer between H 2A and the GC electrode compared with the bulk La(OH)CO 3,This phenomenon can be explained by the morphology and special

surface of the La(OH)CO 3nanostructures.The particular 1D layer-by-layer self-assembled nanostructures,possibly bear-ing a special surface structure and a high surface-to-volume ratio,would be more active sites to absorb more ascorbic acid molecules on the surface of the GC electrode and accelerate the electron transfer between ascorbic acid and the GC electrode.26So there was an enhancement of the anodic current when the 1D La(OH)CO 3nanomaterial-modified GC electrode was used.When a La 2O 3-modified GC electrode and a La(OH)3-modified GC electrode were used,respec-tively,the oxidation peaks and the anodic currents were obviously enhanced in contrast to the bare GC electrode as the work electrode,indicating that the 1D La 2O 3and La(OH)3nanomaterials both could improve the electron transfer between ascorbic acid and the GC electrode.By comparison with curve c,the oxidation peak sites have a small shift in curve d and curve e,which may be associated with different structures,morphologies,and sizes of the La(OH)-CO 3,La 2O 3,and La(OH)3nanomaterials.On the other hand,due to the different intermolecular forces between La(OH)-CO 3,La 2O 3,or La(OH)3and ascorbic acid molecules,the varying in oxidation peaks sites would be quite predictable and reasonable when different modified electrodes were used.It is probable that a hydrogen-bond existed between La(OH)3and ascorbic acid molecules owing to the presence of a large amount of -OH groups in La(OH)3molecules.So the oxida-tion peak was strongly enhanced when the La(OH)3/GC electrode was used compared with La(OH)CO 3or La 2O 3/GC electrodes.

On the basis of our experiment results,the possible forma-tion processes of the 1D La(OH)CO 3nanostructures are explored and elucidated as follows:First,under hydrothermal conditions,La 2O 3oxide first transform into lanthanum hy-droxide according to eq 1(Scheme 1).It is stated that Ln 2O 3(Ln=La,Nd)can react with H 2O and produce corresponding Ln(OH)3in some related works.24Furthermore,a series of experiments were carried out to prove that La 2O 3can first react with H 2O and give rise to La(OH)3in our experimental system (see Figure S3-S4,Supporting Information).And some of La(OH)3is ionized to La 3tion as represented in eq 2.Glycine acts not only as the carbon source but also as a ligand to form a La 3t-glycine complex (eq 3),due to its possessing functional groups of -NH 2and -COOH,which has been discussed in detail.27Meanwhile,CO 2and CH 3NH 2(eq 4)are generated through the decarboxylation of some of the glycine molecules and further react with water to form CO 32-and OH -in solution,respectively.The increase of concentrations of as-produced CO 32-and OH -would defi-nitely weakened the coordination power between La 3tand glycine;therefore,the NH 2CH 2COO -ion in La 3t-glycine would be gradually replaced by CO 32-and OH -and

finally

Figure 9.UV -visible diffuse reflectance absorbance spectra of bulk (a)and as-prepared 1D nanostructures at 200°C for 32h with a molar ratio of La 2O 3to glycine of 1:30

(b).

Figure 10.Cyclic voltammograms of different electrodes in 0.1M phosphate buffer solution (PBS)(pH =7.5)with 1mM ascorbic acid (H 2A).(a)Bare GC electrode,(b)La(OH)CO 3-bulk-modified GC electrode,(c)1D La(OH)CO 3nanomaterial-modified GC electrode,(d)1D La 2O 3nanomaterial-modified GC electrode,and (e)1D La(OH)3nanomaterial-modified GC electrode.

Scheme 1.The Possible Reaction Equations between La 2O 3and

Glycine under Hydrothermal

Conditions

3896Crystal Growth &Design,Vol.9,No.9,2009Xie et al.

Biomolecular-Induced Synthesis of LaNanostructures and Its Morphology-Heldtoward La2O3 and La(OH)3

La(OH)CO 3nuclei appear (eq 6).With the nucleation and its follow-up growth,La(OH)CO 3tends to reach disk morphol-ogies due to isotropically property of La 3t-glycine complex,which make the growth of La(OH)CO 3nanocrystals along all the directions.Finally,some of glycine molecules are poly-merized through repeated hydration -dehydration cycles to form the strong peptide linkage (-CO-NH-)among them and produce polypeptide (up to 13units)(eq 7).It has indeed been experimentally demonstrated that amino acids or their deri-vatives can form polypeptide when subjected to repeated hydration -dehydration cycles under suitable conditions.28The residual reacting solution of La 2O 3with glycine was separated by reversed-phase high performance liquid chro-matography (RP-HPLC).Many products with different retention times were observed and their retention times are longer than that of glycine.It can be concluded they have lower polarity and longer chains in contrast with glycine (see Figure S5a,b,Supporting Information),which indicates some glycine molecules probably have dehydrated and poly-merized to form polypeptide by peptide linkage.On the other hand,the liquid chromatography-electrospray ioniza-tion mass spectrometer (LC-ESI/MS)further confirmed that the produced solution were comprised of a great diversity of glycine polymers (see Figure S6a,b,Supporting In-formation),which have been detected including the dimers,trimers,and tetramers of glycine molecules.The above-mentioned dimers,trimers,and tetramers could further be decarboxylated and produce the corresponding peptide frag-ment fingerprinting respectively in the LC-ESI/MS/MS spec-tra (see Figure S6c -e,Supporting Information).There are some reports 29that peptides can provide a new class of molecular template for organizing metal or inorganic nano-crystals to fabricate devices at the nanometer scale in that they are able to self-assemble into nanotubes or vesicles.So it is possible that the obtained polypeptide are able to self-assemble into chain-like structures,and the La(OH)CO 3nanodisks are inclined to self-assembly into 1D hierarchical nanostructure with inducement of these self-assembled chain-like polypeptide.The whole possible growth patterns of the layer-by-layer self-assembly of La(OH)CO 3nanoplates are shown in Figure 11.

4.Conclusions

In summary,a simple hydrothermal method is introduced for synthesizing a high-crystalline hexagonal layer-by-layer self-assembled La(OH)CO 31D nanostructure around 700nm in diameter and in the range of 6-8μm in length under mild conditions.By controlling the experimental parameters,such as the reaction time,temperature,and the molar ratios of the starting reagents,a series of morphologies of La(OH)CO 3were obtained and their structures were investigated.In this system,the growth process can be attributed to a nucleation -growth-assembly process.The structures and morphologies conversion among La(OH)CO 3,La 2O 3,and La(OH)3have been first discussed.The as-produced hierarchical 1D nanos-tructures show superior optical and electrical properties to the bulk materials.These porous nanorod-like products have possible application in the fields of catalysis,gas storage,biosensors,biofiltration,and heat dissipation.The biomole-cule-assisted synthesis method demonstrated herein may also be extended to synthesize a variety of nanostructures assembled from nanocrystals or nanodisks.

Acknowledgment.We are grateful for the financial support of the National Natural Science Foundation (no.50772074)of China,the State Major Research Plan (973)of China (no.2006CB932302)and the Nano-Foundation of Shanghai in China (No.0852nm01200),and the Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials (No.2009KF04).

Supporting Information Available:TG and DSC curves of La-(OH)CO 3(Figure S1);SEM images of as-obtained products with some surfactants (Figure S2);XRD patterns and SEM images of as-obtained products at 200°C:(a)a molar ratio of La 2O 3to glycine of 1:5for 32h;(b)a molar ratio of La 2O 3to glycine of 1:10for 12h (Figure S3-S4);RP-HPLC (Figure S5)and ESI-MS (Figure S6)spectra of the pure glycine solution and the residual aqueous solution of La 2O 3react with glycine;the process of synthesis bulk La(OH)CO 3and its XRD pattern (Figure S7);FT-IR spectra of La(OH)CO 3(Figure S8).This information is available free of charge via the Internet at http://www.doczj.com/doc/0dc275abaeaad1f346933f53.html.

References

(1)(a)Xu,A.W.;Gao,Y.;Liu,H.Q.J.Catal.2002,207,151.

(b)Cheng,B.;Samulski,E.T.J.Mater.Chem.2001,11,

2901.

Figure 11.The schematic diagram of the possible growth pattern of the 1D La(OH)CO 3nanostructures by layer-by-layer self-assembly nanoplates.

Article Crystal Growth&Design,Vol.9,No.9,20093897

(c)Palmer,M.S.;Neurock,M.;Olken,M.M.J.Am.Chem.Soc.2002,

124,8452.(d)Peruski,A.H.;Johnson,L.H.;Peruski,L.F.J.Immunol.

Methods2002,263,35.(e)Capobianco,J.A.;Boyer,J.C.;Vetrone,F.;

Speghini,A.;Bettinelli,M.Chem.Mater.2002,14,2915.(f)Cotter,J.

P.;Fitzmaurice,J.C.;Parkin,I.P.J.Mater.Chem.1994,4,1603.

(2)Li,Z.H.;Zhang,J.L.;Du,J.M.;Gao,H.X.;Gao,Y.N.;Mu,T.

C.;Han,B.X.Mater.Lett.2005,59,963.

(3)Zhang,Y.J.;Han,K.D.;Cheng,T.;Fang,Z.Y.Inorg.Chem.

2007,46,4713.

(4)Si,R.;Zhang,Y.W.;You,L.P.;Yan,C.H.Angew.Chem.,Int.Ed.

2005,44,3256.

(5)Wu,Q.Z.;Shen,Y.;Liao,J.F.;Li,Y.G.Mater.Lett.2004,58,

2688.

(6)Wang,X.;Li,Y.D.Chem.;Eur.J.2003,9,5627.

(7)Hu,C.G.;Liu,H.;Dong,W.T.;Zhang,Y.Y.;Bao,G.;Lao,C.S.;

Wang,Z.L.Adv.Mater.2007,19,470.

(8)Tang,B.;Ge,J.C.;Wu,C.J.;Zhuo,L.H.;Niu,J.Y.;Chen,Z.Z.;

Shi,Z.Q.;Dong,Y.B.Nanotechnology2004,15,1273.

(9)Niu,F.;Cao,A.M.;Song,W.G.;Wan,L.J.J.Phys.Chem.C2008,

112,17988.

(10)Tang,B.;Ge,J.C.;Zhuo,L.H.Nanotechnology2004,15,1749.

(11)Wang,X.;Li,Y.D.Angew.Chem.,Int.Ed.2002,41,4790.

(12)(a)Bronstein,L.M.;Linton,C.;Karlinsey,R.;Stein,B.;Svergun,

D.I.;Zwanziger,J.W.;Spontak,R.J.Nano Lett.2002,2,873.(b)

Liu,Y.;Meyer-Zaika,W.;Franzka,S.;Schmid,G.;Tsoli,M.;Kuhn,H.

Angew.Chem.,Int.Ed.2003,42,2853.(c)Monson,C.F.;Woolley,A.

T.Nano Lett.2003,3,359.(d)Kanehara,M.;Oumi,Y.;Sano,T.;

Teranishi,T.J.Am.Chem.Soc.2003,125,8708.

(13)Tong,H.;Zhu,Y.J.;Yang,L.X.;Li,L.;Zhang,L.Angew.Chem.,

Int.Ed.2006,45,7739.

(14)Zhang,B.;Ye,X.C.;Dai,W.;Hou,W.Y.;Xie,Y.Chem.;Eur.J.

2006,12,2337.(15)Bao,S.J.;Li,C.M.;Guo,C.X.;Qiao,Y.J.Power Sources2008,

180,676.

(16)Chen,X.Y.;Zhang,X.F.;Shi,C.W.;Li,X.L.;Qian,Y.T.Solid

State Commun.2005,134,613.

(17)Niu,H.X.;Yang,Q.;Tang,K.B.;Xie,Y.;Zhu,Y.C.J.Nanosci.

Nanotechnol.2006,6,162.

(18)Chen,X.Y.;Zhang,Z.J.;Qiu,Z.G.;Shi,C.W.;Li,X.L.Solid

State Commun.2006,140,267.

(19)Yu,L.;Banerjee,I.A.;Shima,M.;Rajan,K.;Matsui,H.Adv.

Mater.2004,16,709.

(20)Lu,Q.Y.;Gao,F.;Komarneni,S.J.Am.Chem.Soc.2004,126,54.

(21)Kong,R.;Yang,Q.;Tang,K.B.Chem.Lett.2006,35,402.

(22)Niu,H.X.;Yang,Q.;Tang,K.B.;Xie,Y.Scripta Mater.2006,54,

1791.

(23)Zhao,D.L.;Yang,Q.;Han,Z.H.;Zhou,J.;Xu,S.B.;Sun,F.Y.

Solid State Sci.2008,10,31.

(24)(a)Ma,J.;Wu,Q.S.;Ding,Y.P.;Chen,Y.Cryst.Growth Des.

2007,7,153.(b)Ma,J.;Wu,Q.S.J.Am.Ceram.Soc.2007,90,3890.

(25)Nikol’skaya,O.K.;Dem’yanets,L.N.Inorg.Mater.2005,41,

1366.

(26)(a)Chen,P.;Wu,Q.S.;Ding,Y.P.Small2007,3,644.(b)Ni,Y.H.;

Cao,X.F.;Hu,G.Z.;Yang,Z.S.;Wei,X.W.;Chen,Y.H.;Xu,J.Cryst.

Growth Des.2007,7,280.

(27)(a)Kuang,D.B.;Xu,A.W.;Fang,Y.P.;Liu,H.Q.;Frommen,C.;

Fenske,D.Adv.Mater.2003,15,1747.(b)Zhang,X.J.;Zhao,Q.R.;

Tian,X.B.;Xie,Y.Cryst.Growth Des.2004,4,355.

(28)(a)Yanagawa,H.;Ogawa,Y.;Kojima,K.;Ito,M.Orig.Life Evol.

Biospheres1988,18,179.(b)White,D.H.;Kennedy,R.M.;Macklin, J.Origins of Life;D.Reidel Publishing Company:New York,1984.(c) Cox,J.S.;Seward,T.M.Geochim.Cosmochim.Acta2007,71,2264.

(29)Santoso,S.;Hwang,W.;Hartman,H.;Zhang,S.G.Nano Lett

2002,2,687.