CO3 One-Dimensional Nanostructures and Its Morphology

Home Screen Newspaper. No matches found. No publications added. Journals. Books. Book Series. Working Papers. If it takes too long to load the home pa...
2 downloads 0 Views 4MB Size
Download PDF
DOI: 10.1021/cg801053p

Biomolecular-Induced Synthesis of Self-Assembled Hierarchical La(OH)CO3 One-Dimensional Nanostructures and Its Morphology-Held Conversion toward La2O3 and La(OH)3

2009, Vol. 9 3889–3897

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

Department of Chemistry, Tongji University, Shanghai, 200092, China, ‡Department of Chemistry, Shanghai University, Shanghai 200444, China, and §Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, China Received September 19, 2008; Revised Manuscript Received June 30, 2009

ABSTRACT: Novel hierarchical layer-by-layer self-assembled one-dimensional (1D) La(OH)CO3 nanostructures, with a diameter of around 700 nm and lengths in the range of 6-8 μm, were synthesized by a developed hydrothermal method using La2O3 and 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 obtained 1D La(OH)CO3 nanostructures can be successfully converted to La2O3 and La(OH)3 nanorods 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 of 1D La(OH)CO3 nanostructures was explored. The UV-visible diffuse reflectance absorbance spectra indicate that the 1D nanostructures have enhanced UV-light absorbance properties in contrast to the bulk materials. The electrochemical studies show that 1D La(OH)CO3 nanostructures 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 selfassembled 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,1 such as high-performance luminescent devices, highquality phosphors, up-conversion materials, magnets, catalysts, 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)3 nanomaterials, including nanowires2 and microspheres3 of La(OH)CO3; nanoplates,4 nanorods,5 nanowires,6 nanobelts,7 macropores,8 and hollow trapezohedron9 of La2O3; nanorods,6 nanowires,10 nanobelts,7 and nanospheres11 of 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)CO3 to La(OH)3 and La2O3 have 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 properties12 as 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,13 porous spongelike Ni3S2 nanostructures,14 CoS nanowires,15 and Sb2S3 nanowires16 were synthesized; in the presence of a small biomolecule of glycine, CdS dendrites17 and R-Fe2O3 nanowires18 were fabricated.

Nickel nanocrystals with controlled size and shape in the presence of peptide nanotubes,19 highly ordered snowflakelike structures of Bi2S3 nanorods with the assistance of glutathione,20 one-dimensional Ag nanostructures21 and porous MgO nanomaterials22 using dextran as an effective directing reagent were obtained. Furthermore, Zhao et al.23 manipulated the crystal growth of CeOHCO3 and NdOHCO3 using amino acids as additives. The above examples verify the effectiveness of biomolecules to tailor the structures 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)CO3 nanomaterials, self-assembled by single-crystal nanoplates, are obtained for the first time using a facile oxide-biomolecule-hydrothermal (OBHT) method,24 in which oxide and biomolecule are directly used as raw materials without any additives. Through careful investigation of various influencing factors, the relationship between reaction conditions and morphology is discussed and the growth mechanism of the La(OH)CO3 is presented. The obtained 1D La(OH)CO3 nanostructures can be successfully converted to porous La(OH)3 and La2O3 nanorods. This work is significant for preparing and exploiting potential applications of hierarchical 1D 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

*Corresponding author. E-mail: [email protected]. Tel.: þ86-2165982620. Fax: þ86-21-65981097.

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

r 2009 American Chemical Society

Published on Web 08/06/2009

pubs.acs.org/crystal

3890

Crystal Growth & Design, Vol. 9, No. 9, 2009

Xie et al.

Figure 1. (a-i) Representative XRD patterns and SEM images of La(OH)CO3, La2O3, and La (OH)3, respectively. purification. Deionized water, obtained by means of a waterpurification system, was used in the experiments. Ascorbic acid solutions (H2A) were prepared using phosphate buffer solution (pH 7.50).The preparation of La(OH)CO3 nanomaterials was performed by an OBHT route. The reactions were carried out in a Teflon-lined stainless steel autoclave with a capacity of 20 mL. Taking a typical example, 1  10-3 mol of La2O3, 3  10-2 mol of NH2CH2COOH, and 10 mL 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 temperaturecontrolled oven at 200 °C for 32 h and then cooled to room temperature naturally. Sequentially, the precipitation was separated by centrifugation and washed with deionized water and absolute ethanol several times, respectively. Finally, the as-obtained products were dried in air at 60 °C for further characterization. Electrochemical responses were performed on a CHI-820 electrochemical 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 60 mV/s and a rest time of 2 s. To prepare GC electrodes modified by the products, 5 mg of bulk La(OH)CO3, 1D La(OH)CO3 nanomaterials respectively was dispersed into 5 mL of deionized water under ultrasonic. Then the solution (20 μL) 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 KR radiation (λ=0.154056 nm) (Germany), using a voltage of 40 kV, a current of 40 mA, and a scanning rate of 0.02°/s, in 2θ ranges from 10° to 70°. The cell lattice constants of samples were calculated and corrected by MDI Jade (5.0 edition) software. Thermal stability of La(OH)CO3 was investigated by a simultaneous thermogravimetric and differential scanning calorimetric system (TG/DSC, Netzsch STA 409PC, Selb, Germany) with Al2O3 powder as the reference. Approximately 10 mg of the sample was loaded into a standard Al2O3 crucible, which was heated from room temperature to 1000 °C at a heating rate of 10 °C/min in

a flowing N2 atmosphere. The morphologies and structure of the products were examined with scanning electron microscope (SEM, Philips XL30, Holand) at a accelerating voltage of 20 kV, transmission 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 of 200 kV. UV-visible diffuse reflectance absorbance spectra (DRS) were obtained with a UV-vis spectrometer (BWS003, Newark, DE). A CHI-820 electrochemical workstation (Shanghai, China) was used to test the electrocatalytic activity of the 1D La(OH)CO3 nanostructures-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 Agilent 1100 Series (Agilent Technologies) chromatographic system with DAD detector. Mass detection was performed on a Varian 310 LC-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)3 nanostructures with similar morphologies were in situ synthesized by a facile method. The 1D layer-by-layer self-assembled hierarchical La(OH)CO3, with a diameter of around 700 nm and lengths in the range of 6-8 μm (Figure 1b,c), were obtained by La2O3 reacting with NH2CH2COOH (molar ratio 1:30) at 200 °C for 32 h. It belongs to hexagonal phase La(OH)CO3 (JCPDS 26-0815, Figure 1a). The 1D La(OH)CO3 can be converted to La2O3 by calcination at 900 °C for 1 h in a vacuum. The XRD pattern reveals the presence of hexagonal La2O3 (JCPDS 05-0602, Figure 1d). Scanning electron microscopy (SEM) images show that the morphology of the samples retains the original 1D shape (Figure 1e,f), except that the underlying layer-by-layer self-assembled structures disappeared. The SEM images show that the diameters and the lengths of the La2O3 porous nanorods are about 600-700 nm

Article

Crystal Growth & Design, Vol. 9, No. 9, 2009

3891

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

Figure 3. XRD patterns of as-obtained products at 200 °C with a molar ratio of La2O3 to glycine of 1:30 under different reaction times: (a) 8 h; (b) 15 h; (c) 24 h; (d) 32 h; (e) 48 h.

and 3-5 μm, respectively. La(OH)CO3 was first transformed into La2O3 through calcination at 900 °C for 1 h, then cooled with a high relative humidity of 95% for 3 h, and the asobtained La2O3 would further change into La(OH)3 via 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)3 are porous nanorods nearly the same as La2O3 (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 process. La(OH)CO3 was first transformed into La2O3 through calcination at 900 °C for 1 h, then cooled under different relative humidities of 20%, 70%, 95% for 3 h, respectively, and the final products were pure La2O3, a mixture La2O3 of La(OH)3 and pure La(OH)3, which were characterized by XRD shown in Figure 2a-c, respectively. However, under a relative humidity of 95% for 1 h, the La2O3 has not been completely hydrolyzed and the products are also mixture of La2O3 and La(OH)3 (Figure 2d). In theory, during the heating process, the La(OH)3 may be fabricated as a middle product. However, Our thermogravimetric/ differential scanning calorimetry (TG/DSC) measurements (see Figure S1, Supporting Information) and the correlative literature25 indicate that La(OH)3 could not be directly achieved by heating La(OH)CO3 in ambient atmosphere. So we can draw a conclusion as follows: when heated in air, the hierarchical La(OH)CO3 structures first can be converted to porous La2O3 nanorods; during the cooling process, the La2O3 nanomaterials readily absorb the water in moist air and finally turn into La(OH)3 nanomaterials. Figure 3 shows 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)CO3 when the reaction is performed over 15 h. In each XRD pattern, all of the reflections can be readily indexed to the hexagonal phase (space group P6h (No. 174)) of La(OH)CO3 with lattice constants a = 12.6291 and c = 10.0144 A˚ 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)CO3 (JCPDS 49-0981) and hexagonal La(OH)CO3 when the reaction is carried out around 8 h. The results indicate that orthorhombic La(OH)CO3 as a kind of intermediate product appears first. The intermediate products can completely transform into hexagonal phase La(OH)CO3 crystal when the reaction time is beyond 15 h. It can be seen from Figure 3 that the intensities of all the crystal peaks were heightened, implying the improved crystallinity of the products 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 micrographs characterized SEM and TEM. The morphologies of the products obtained under different conditions were carefully investigated by SEM. Figure 4 shows SEM micrographs of the growth process of layerby-layer self-assembled 1D nanostructures obtained at 200 °C for 8, 15, 24, 32, and 48 h. Time-dependent experiments indicate that the reaction time exerts a strong influence on the diameters and lengths of the 1D La(OH)CO3 nanostructures. Quasi-spheres with a diameter around 200 nm formed with a reaction time of 8 h (Figure 4a), While the reaction time increased to 15 h, the diameters increased slightly, and the lengths grew up to 1-2 μm, and the quasi-sphere structures gradually grew into 1D nanostructures (Figure 4b). When the reaction time lasted for 24 h, both the diameters and the lengths of the selfassembled 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 32 h, very regular 1D nanostructures with lengths of up to 6-8 μm and diameters of about 770 nm were obtained, as presented in Figure 4d. However, it should be pointed out that when the reaction time was up to 48 h, 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)CO3 molecules, there are some weak interaction forces, such as weak van der Waals intermolecular interaction and

3892

Crystal Growth & Design, Vol. 9, No. 9, 2009

Xie et al.

Figure 4. SEM images of as-obtained products at 200 °C with a molar ratio of La2O3 to glycine of 1:30 under different reaction times: (a) 8 h; (b) 15 h; (c) 24 h; (d) 32 h; (e) 48 h.

Figure 5. SEM images of as-obtained products at different reaction temperatures for 32 h with a molar ratio of La2O3 to glycine of 1:30: (a) 160 °C; (b) 180 °C; (c) 200 °C; (d) 220 °C.

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)CO3. Figure 5a shows that the La(OH)CO3 sample prepared at 160 °C displays a disk-like with the mean diameter of 380 nm, some of which have already self-assembled into 1D nanostructure with the length around 1 μm. When the reaction temperature increased to 180 °C, the mean diameter of the

Article

Crystal Growth & Design, Vol. 9, No. 9, 2009

3893

Figure 6. SEM images of as-obtained products at different reaction temperatures for 8 h with a molar ratio of La2O3 to glycine of 1:30: (a) 160 °C; (b) 200 °C.

Figure 7. SEM images of as-obtained products at 200 °C for 32 h with different molar ratios of La2O3 to glycine: (a) 1:20; (b) 1:25; (c) 1:30; (d) 1:40.

disk-like products has increased to around 460 nm and the self-assembled 1D nanostructures are widely observed. Compared 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). However, 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 1 μm, and no assembly appeared at 160 °C for 8 h (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 nanostructure. Variety in the molar ratio of La2O3 to glycine also has a great impact on the morphologies of the as-obtained products as shown in Figure 7. When the molar ratio of La2O3 to glycine was 1:20, the products were composed of nonerratic

nanodisks with a diameter of 300 nm and a thickness of 100 nm and some self-assembled 1D nanostructure with a length about 1 μm (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)CO3 nanostructures, 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 Information). While the Na2CO3 and NaOH were substituted for glycine, the 1D nanostructues also completely disappeared. Comparing with glycine, the above-mentioned surfactants 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 preferable experimental parameters for the obtained uniform 1D nanostructure are a reaction temperature of 200 °C,

3894

Crystal Growth & Design, Vol. 9, No. 9, 2009

Xie et al.

Figure 8. TEM images of as-obtained products at different reaction temperatures for 32 h with a molar ratio La2O3 to glycine of 1:30: (a1, a2) 160 °C; (b1, b3) 200 °C, (b2, b4) the HRTEM (inset ED) images of (b1, b3), respectively.

a reaction time of 30 h, and a molar ratio of La2O3 to 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 380 nm (Figure 8a1), some of which are self-assembled short 1D nanostructures (Figure 8a2). The TEM images shown in Figure 8b1 further affirm that the as-prepared La(OH)CO3 at 200 °C are multilayer 1D structures composed of numerous delicate thin nanodisks. The SAED pattern (inset in Figure 8b2) 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 8b2). The TEM images of the cross section of the 1D nanostructures further show the 1D nanostructures are made up of thick nanodisks (Figure 8b3). Furthermore, these nanodisks show a highly single-crystal nature, determined by HRTEM images and SAED patterns,

as shown in Figure 8b4. Lattice spacings about 0.362 and 0.211 nm correspond to the (300) and (330) planes of a unit cell hexagonal phase La(OH)CO3, respectively. The spatial arrangement 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)CO3 multilayer 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)CO3 and its 1D nanostructures at 200 °C for 32 h 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

Article

Crystal Growth & Design, Vol. 9, No. 9, 2009

3895

Scheme 1. The Possible Reaction Equations between La2O3 and Glycine under Hydrothermal Conditions

Figure 9. UV-visible diffuse reflectance absorbance spectra of bulk (a) and as-prepared 1D nanostructures at 200 °C for 32 h with a molar ratio of La2O3 to glycine of 1:30 (b).

Figure 10. Cyclic voltammograms of different electrodes in 0.1 M phosphate buffer solution (PBS) (pH = 7.5) with 1 mM ascorbic acid (H2A). (a) Bare GC electrode, (b) La(OH)CO3 -bulk-modified GC electrode, (c) 1D La(OH)CO3 nanomaterial-modified GC electrode, (d) 1D La2O3 nanomaterial-modified GC electrode, and (e) 1D La(OH)3 nanomaterial-modified GC electrode.

absorbance of light. These results show that the 1D nanomaterials are possibly applicable to prepare photosensors and photocatalysis. Figure 10 shows the cyclic voltammograms of different electrodes in the 0.1 M PBS solution with 1 mM H2A. When the bare GC electrode was used as the working electrode, the oxidation potential of H2A was 717 mV and the anodic peak current was 8.6 μA. After the GC electrode was modified by the bulk La(OH)CO3, the oxidation potential of H2A was exhibited at 696 mV and the anodic peak current was 10.4 μA. When the 1D La(OH)CO3 nanomaterial-modified GC electrode was employed, the oxidation potential of H2A appeared at 611 mV and the anodic peak current changed to 13 μA. The above experimental data indicate that 1D La(OH)CO3 nanomaterials have better electrocatalytic properties and can greatly promote electron transfer between H2A and the GC electrode compared with the bulk La(OH)CO3, This phenomenon can be explained by the morphology and special

surface of the La(OH)CO3 nanostructures. The particular 1D layer-by-layer self-assembled nanostructures, possibly bearing 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.26 So there was an enhancement of the anodic current when the 1D La(OH)CO3 nanomaterial-modified GC electrode was used. When a La2O3-modified GC electrode and a La(OH)3-modified GC electrode were used, respectively, 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 La2O3 and La(OH)3 nanomaterials 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)CO3, La2O3, and La(OH)3 nanomaterials. On the other hand, due to the different intermolecular forces between La(OH)CO3, La2O3, or La(OH)3 and 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)3 and ascorbic acid molecules owing to the presence of a large amount of -OH groups in La(OH)3 molecules. So the oxidation peak was strongly enhanced when the La(OH)3/GC electrode was used compared with La(OH)CO3 or La2O3/ GC electrodes. On the basis of our experiment results, the possible formation processes of the 1D La(OH)CO3 nanostructures are explored and elucidated as follows: First, under hydrothermal conditions, La2O3 oxide first transform into lanthanum hydroxide according to eq 1 (Scheme 1). It is stated that Ln2O3 (Ln=La, Nd) can react with H2O and produce corresponding Ln(OH)3 in some related works.24 Furthermore, a series of experiments were carried out to prove that La2O3 can first react with H2O and give rise to La(OH)3 in our experimental system (see Figure S3-S4, Supporting Information). And some of La(OH)3 is ionized to La3þ ion as represented in eq 2. Glycine acts not only as the carbon source but also as a ligand to form a La3þ-glycine complex (eq 3), due to its possessing functional groups of -NH2 and -COOH, which has been discussed in detail.27 Meanwhile, CO2 and CH3NH2 (eq 4) are generated through the decarboxylation of some of the glycine molecules and further react with water to form CO32- and OH- in solution, respectively. The increase of concentrations of as-produced CO32- and OH- would definitely weakened the coordination power between La3þ and glycine; therefore, the NH2CH2COO- ion in La3þ-glycine would be gradually replaced by CO32- and OH- and finally

3896

Crystal Growth & Design, Vol. 9, No. 9, 2009

Xie et al.

Figure 11. The schematic diagram of the possible growth pattern of the 1D La(OH)CO3 nanostructures by layer-by-layer self-assembly nanoplates.

La(OH)CO3 nuclei appear (eq 6). With the nucleation and its follow-up growth, La(OH)CO3 tends to reach disk morphologies due to isotropically property of La3þ-glycine complex, which make the growth of La(OH)CO3 nanocrystals along all the directions. Finally, some of glycine molecules are polymerized through repeated hydration-dehydration cycles to form the strong peptide linkage (-CO-NH-) among them and produce polypeptide (up to 13 units) (eq 7). It has indeed been experimentally demonstrated that amino acids or their derivatives can form polypeptide when subjected to repeated hydration-dehydration cycles under suitable conditions.28 The residual reacting solution of La2O3 with glycine was separated by reversed-phase high performance liquid chromatography (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 polymerized to form polypeptide by peptide linkage. On the other hand, the liquid chromatography-electrospray ionization 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 Information), which have been detected including the dimers, trimers, and tetramers of glycine molecules. The abovementioned dimers, trimers, and tetramers could further be decarboxylated and produce the corresponding peptide fragment fingerprinting respectively in the LC-ESI/MS/MS spectra (see Figure S6c-e, Supporting Information). There are some reports29 that peptides can provide a new class of molecular template for organizing metal or inorganic nanocrystals 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 selfassemble into chain-like structures, and the La(OH)CO3 nanodisks are inclined to self-assembly into 1D hierarchical nanostructure with inducement of these self-assembled chainlike polypeptide. The whole possible growth patterns of the layer-by-layer self-assembly of La(OH)CO3 nanoplates 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)CO3 1D nanostructure around 700 nm in diameter and in the range of 6-8 μm 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)CO3 were obtained and their structures were investigated. In this system, the growth process can be attributed to a nucleationgrowth-assembly process. The structures and morphologies conversion among La(OH)CO3, La2O3, and La(OH)3 have been first discussed. The as-produced hierarchical 1D nanostructures 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 biomolecule-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)CO3 (Figure S1); SEM images of as-obtained products with some surfactants (Figure S2); XRD patterns and SEM images of asobtained products at 200 °C: (a) a molar ratio of La2O3 to glycine of 1:5 for 32 h; (b) a molar ratio of La2O3 to glycine of 1:10 for 12 h (Figure S3-S4); RP-HPLC (Figure S5) and ESI-MS (Figure S6) spectra of the pure glycine solution and the residual aqueous solution of La2O3 react with glycine; the process of synthesis bulk La(OH)CO3 and its XRD pattern (Figure S7); FT-IR spectra of La(OH)CO3 (Figure S8). This information is available free of charge via the Internet at http://pubs.acs.org.

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.

Article

(2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)

(13) (14)

(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. Methods 2002, 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. 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. Zhang, Y. J.; Han, K. D.; Cheng, T.; Fang, Z. Y. Inorg. Chem. 2007, 46, 4713. Si, R.; Zhang, Y. W.; You, L. P.; Yan, C. H. Angew. Chem., Int. Ed. 2005, 44, 3256. Wu, Q. Z.; Shen, Y.; Liao, J. F.; Li, Y. G. Mater. Lett. 2004, 58, 2688. Wang, X.; Li, Y. D. Chem.;Eur. J. 2003, 9, 5627. Hu, C. G.; Liu, H.; Dong, W. T.; Zhang, Y. Y.; Bao, G.; Lao, C. S.; Wang, Z. L. Adv. Mater. 2007, 19, 470. Tang, B.; Ge, J. C.; Wu, C. J.; Zhuo, L. H.; Niu, J. Y.; Chen, Z. Z.; Shi, Z. Q.; Dong, Y. B. Nanotechnology 2004, 15, 1273. Niu, F.; Cao, A. M.; Song, W. G.; Wan, L. J. J. Phys. Chem. C 2008, 112, 17988. Tang, B.; Ge, J. C.; Zhuo, L. H. Nanotechnology 2004, 15, 1749. Wang, X.; Li, Y. D. Angew. Chem., Int. Ed. 2002, 41, 4790. (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. Tong, H.; Zhu, Y. J.; Yang, L. X.; Li, L.; Zhang, L. Angew. Chem., Int. Ed. 2006, 45, 7739. Zhang, B.; Ye, X. C.; Dai, W.; Hou, W. Y.; Xie, Y. Chem.;Eur. J. 2006, 12, 2337.

Crystal Growth & Design, Vol. 9, No. 9, 2009

3897

(15) Bao, S. J.; Li, C. M.; Guo, C. X.; Qiao, Y. J. Power Sources 2008, 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. Small 2007, 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. Biospheres 1988, 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. Acta 2007, 71, 2264. (29) Santoso, S.; Hwang, W.; Hartman, H.; Zhang, S. G. Nano Lett 2002, 2, 687.