Synthesis of 3-D Hierarchical Dendrites of Lead Chalcogenides in

Jan 13, 2007 - microcrystals with different morphologies (cubes, six-pods, bugle-like dendrites, and six-armed dendrites) could be simply fabricated...
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Synthesis of 3-D Hierarchical Dendrites of Lead Chalcogenides in Large Scale via Microwave-Assistant Method Hongliang Cao, Qiang Gong, Xuefeng Qian,* Huili Wang, Jiantao Zai, and Zikang Zhu School of Chemistry and Chemical Technology, Shanghai Jiao Tong UniVersity, Shanghai 200240, P.R. China

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 2 425-429

ReceiVed July 2, 2006; ReVised Manuscript ReceiVed December 3, 2006

ABSTRACT: Hierarchical superstructures of lead chalcogenides (PbS, PbSe, and PbTe) have been fabricated in large scale via microwave-assistant method. The FESEM and HRTEM images showed that dendritic and six-armed PbS superstructures were constructed with prism-shaped branches which grew epitaxially from the backbone of PbS. Detailed experiments revealed that PbS microcrystals with different morphologies (cubes, six-pods, bugle-like dendrites, and six-armed dendrites) could be simply fabricated by adjusting the reaction conditions (initial concentration of precursors or sulfur source). The results implied that the formation of 3-D hierarchical structures was controlled by the kinetic conditions. In addition, the dendrites of PbSe and PbTe have also been fabricated by the assistance of appropriate reductive reagent in a similar method. 1. Introduction Materials with different size and architecture may open new opportunities for exploring their physical and chemical properties, and great efforts have been devoted to controlling materials’ size, shape, and dimension. In the past few years, the selfassembly of three-dimensional (3-D) superstructure has attracted much attention because of the complexity of possible arrangement, such as multipods,1 snowflakes,2 hyperbranches,3 and dendrites.4,5 On the other hand, 3-D nanostructured architectures also have great potential applications in the new generation of advanced devices (e.g., sensors, batteries, and fuel cells) dependent on their rapid electrochemical reactions owing to the extremely large specific surface areas for charge, mass, or gas transport.6-11 Generally, 3-D superstructures were synthesized through electrochemical method, CVD method, solution method, etc. In these methods, the solution method was one of the most promising methods due to its variability and simplicity, and great progress has been achieved. For example, dandelion-like ZnO12 and CuO,13 hierarchical and core-shell MnO2,14,15 and hierarchical nanoporous TiO216 have been fabricated via a hydrothermal method, and hierarchical BaMoO4 dendrites17 etc. have been achieved in reverse-micelle solution. Lead chalcogenides (PbS, PbSe, and PbTe) are very promising materials for thermoelectric (TE) applications.18,19 Especially, PbS, as an important binary π-π semiconductor with small band gap (0.41 eV) and large exciton Bohr radius (18 nm), has attracted more and more attention due to its wide potential applications in optical devices as optical switches, near-IR (NIR) communication, thermal and biologic images, and photovoltaic solar cells,20-24 etc. Conventionally, metal chalcogenides were synthesized by the reaction of elements at elevated temperature. The methods included gas-phase reaction between elements and gaseous H2S, H2Se, and H2Te, solid reaction, and pyrolysis of single-source precursors. However, high-temperature (500 °C) and toxic reaction reagents or highly sensitive precursors are required. Recently, with the development of synthetic technology of nanomaterials, some wet-chemical methods have been explored to synthesize lead chalcogenides. For example, nanoparticles, nanocubes, nano-octahedra, dendrites, and star-shaped architectures of lead chalcogenide have been successfully * Towhomcorrespondenceshouldbeaddressed.E-mail: [email protected].

prepared through hydrothermal or reflux method in the presence (or absence) of surfactants.24-30 On the other hand, lead chalcogenide colloids with rich morphologies also have been prepared in a high boiling point solvent in the presence of a protecting reagent at high temperature.31-34 After the microwave radiation method was introduced by Kerner et al. to prepare PbSe and PbTe nanoparticles by the so-called “polyol process”,35 PbS particles with rich morphologies such as spherical shape, star shape, and dendritic shape were synthesized via microwave radiation method.36-38 However, the preparation of PbS with 3-D six-armed dendrites and the formation mechanism of dendrites were less studied in this system. In addition, 3-D dendrites of PbSe and PbTe have not been obtained yet. In this paper, hierarchical PbS dendrites with prism-shaped branches were successfully prepared in large scale by the modified microwave radiation method, and the superstructures of PbSe and PbTe were first synthesized with the assistance of appropriate reductive reagent in a similar method. In our cases, ethylene glycol (EG) has been selected as solvent based on its high permanent dipole (2.45 GHz)36 and higher boiling point (197 °C), and it served as an excellent susceptor of the microwave radiation. 2. Experimental Section All reagents were analytically pure and used as received. In a typical synthesis of PbS, 0.025 mol of Pb(OAc)2‚3H2O and 0.05 mol of thiourea (Tu) were dissolved into 100 mL of ethylene glycol with the assistance of ultrasonic treatment in a single-neck flask, then the mixture was placed in the microwave oven (650 W, 2.45 GHz) with reflux exchanger equipment, and the microwave reactor was kept at 40% of the total power for 2-10 min. The synthesis of PbSe and PbTe was carried out by adding stoichiometric quantities of Se and Te powders into lead acetate ethylene glycol solution containing 0.5-1 mL of 85 wt % hydrazine monohydrate (N2H4‚H2O), and then the mixed solution was performed in a microwave oven with 40% of the total power for 1 h. As-prepared black or blue black precipitates were collected and washed with distilled water and ethanol several times and dried in a vacuum oven at 60 °C. The phases of as-prepared products were characterized using powder X-ray diffraction (XRD, Shimadzu XRD-6000) with Cu KR radiation (λ ) 1.5406 Å) from 20° to 70° at a scanning rate of 6°/min. X-ray tube voltage and current were set at 40 kV and 30 mA, respectively. The morphologies of samples were characterized by field-emission scanning electron microscopy (FESEM, FEI SIRION 200, with an accelerating voltage of 5 kV), transmission electron microscopy (TEM,

10.1021/cg060415h CCC: $37.00 © 2007 American Chemical Society Published on Web 01/13/2007

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Figure 1. XRD pattern of PbS formed at [Pb2+] ) 0.25 M for 10 min. JEM 100CX, with an accelerating voltage of 100 kV), and highresolution transmission electron microscopy (HRTEM, JEOL, 2100F, with an accelerating voltage of 200 kV).

3. Results and Discussion 3.1. Hierarchical Dendrites of PbS. In the formation of hierarchical dendrites of PbS, Pb(OAc)2 and Tu were used as precursors and EG as solvent; no surfactants were used. Transparent solution was first formed because Pb2+ could be chelated with an excess amount of Tu before microwave reaction. Once the system was heated by microwave, Tu was hydrolyzed to release S2- and reacted with Pb2+ to form black PbS. The black precipitate of PbS could be observed in 2 min. To complete the reaction, the process was prolonged to 10 min. Figure 1 shows the X-ray diffraction pattern of the product. All peaks match well with cubic PbS phase with a fcc rock-salt structure (JCPDS file No.5-992) and not any impurities can be

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detected. The narrow sharp peaks suggest that the sample of PbS is well crystalline. The morphologies of samples were determined by fieldemission scanning electron microscopy (FESEM). Figure 2a is the typical low-magnification SEM image of PbS prepared at [Pb2+] ) 0.25 M for 10 min, from which we find that hierarchical PbS particles in dendritic structure are the exclusive products, which means that the dendrites of PbS can be prepared in large scale. From the SEM image we can also find that the individual dendrite of PbS is made up of trunks and branches. Interestingly, many hierarchical structures with 3-D six symmetric trunks are observed from Figure 2a. The profile and top view of individual dendrite shown in high-magnification SEM images (Figures 2b and 2c) clearly reveal that the individual PbS dendrite has six trunks distributed along 3-D symmetric axis directions, the length of each trunk is about 700-1500 nm, and 4 rows of nanorods (branches) with different lengths symmetrically stand on each trunk. The branches in each trunk are parallel to each other and in the same plane (Figures 2b and 2c). Careful observation further reveals that the branches (nanorods) are in prism shape with quadrangled cross section, and the diameter of the rod is about 100 nm, as demonstrated in Figure 2d. To the best of our knowledge, the morphology of PbS branches with quadrangled cross section is confirmed for the first time. Besides the hierarchical dendritic structures with 3-D six symmetric trunks (or arms), dendritic crystals with two arms, three arms, or four arms can also be observed by carefully examining several SEM images of the sample, but these dendritic trunks are still distributed along a 3-D symmetric axis. So it is reasonably believed that these hierarchical structures may originate from the broken 3-D six-armed symmetric dendrite of PbS crystals due to the vigorous movement and impingement under microwave heating condition. The asprepared hierarchical dendritic structures of PbS were further characterized by transmission electron microscopy (TEM). The representative TEM image of an individual four-armed dendritic

Figure 2. FESEM images of PbS at [Pb2+] ) 0.25 M for 10 min. (a) Low-magnification SEM image; (b) high-magnification SEM image of 3-D dendrites from side view; (c) high-magnification SEM image of 3-D dendrites from top view; (d) high-magnification SEM image of branches.

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Figure 3. (a) TEM image of individual PbS dendrite; (b) and (c) SAED patterns recorded are of the trunk and branch, respectively; (d) and (e) HRTEM images taken from trunk and branch, respectively.

crystal which lies against the carbon-coated copper grid (shown in Figure 3a) further indicates that the dendrite indeed possesses symmetric structure. Selective-area electron diffractions (SAED) patterns (Figures 3b and 3c), recorded from different areas (tips of the trunk and branch) in individual dendrites of PbS, are identical and can be attributed to the [010] zone axis diffraction, suggesting that the hierarchical dendrite is a single crystal. A representative HRTEM image recorded on the trunk (Figure 3d) shows that the 2-D lattice fringes are structurally uniform with the fringe space of 0.306 nm, which is close to the space between two (200) planes of bulk cubic-phase PbS (d200). These results indicate that the crystal growth direction of the trunk is

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preferred along the 〈100〉 direction and the trunk is in a good single crystalline structure. The typical HRTEM image (shown in Figure 3e) taken from the branch of the dendrite also reveals the lattice spacing of 0.306 nm. Both SAED patterns and HRTEM images demonstrate that the individual PbS dendrite is a single crystal, and the trunks and branches of the PbS dendrite have identical crystal orientation. Similar results have also been observed by Kuang and co-workers.24 Further studies revealed that the initial concentration of precursors had great influence on the evolution of PbS morphologies. For example, when [Pb2+] was 0.01 M and kept [Pb2+]:[Tu] ) 1:2 and other reaction conditions constant, cubes and square plates with the edge length of about 100-250 nm were obtained (Figure 4a). Increasing [Pb2+] concentration to 0.025 M, 3-D PbS symmetric structures with six short arms were formed (Figure 4b). Further increasing [Pb2+] concentration to 0.05 M, 3-D PbS structures with six symmetric smooth arms were also held except that the arms grew longer (Figure 4c). From the SEM images (Figures 4a-4c), it is clearly found that the arms are prisms with a square cross section and are grown from the six {100} planes of the PbS cube. However, once the concentration of [Pb2+] was increased to 0.1 M, the arms would become a spindle-like structure in shape and four rows of prominent parts symmetrically grew on the surface of the trunk along the long axis of the arm (Figure 4d) even though the reaction time was set to 2 min. Further prolonging of the reaction time to 5 min in the same reaction system, the prominent parts became more prominent and branches began to appear on the tips of the arms (Figure 4e). More interestingly, the branches of trunks’ tips would spontaneously expand outward to fabricate a 3-D bugle-like dendrite (Figure 4f) if the reaction time was extended to 10 min. The above results indicate that the branches on the arms of PbS tend to grow in higher precursor concentration, and the 3-D six-armed dendrites of PbS may originate from the evolution of the six (100) crystal faces of cube owing to the preferential growth along the six 〈100〉 direction. According to a previous report,39 the shape of fcc nanocrystal was mainly determined by the ratio (γ) of the growth rate in the 〈100〉 directions to that in the 〈111〉 directions, and cubes bounded by the six {100} planes would be formed if the γ value

Figure 4. FESEM images of PbS prepared at different initial concentrations of Pb2+ (a) [Pb2+] ) 0.01 M, 10 min; (b) [Pb2+] ) 0.025 M, 10 min; (c) [Pb2+] ) 0.05 M, 10 min; (d) [Pb2+] ) 0.1 M, 2 min; (e) [Pb2+] ) 0.1 M, 5 min; (f) [Pb2+] ) 0.1 M, 10 min.

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Figure 5. XRD patterns of (a) PbSe and (b) PbTe prepared from [Pb2+] ) 0.1 M for 1 h under microwave heating condition.

was relatively low (e.g., γ ) 0.58). In our cases, cubes obtained in lower initial precursor concentration indicated a lower growth rate in 〈100〉 directions than that in 〈111〉 directions. 3-D sixarmed dendrites or more complex structures of PbS were formed in higher initial concentration, which exactly originated from the six 〈100〉 directions of the cubic lattice of PbS owing to the higher growth rate in 〈100〉 directions than in 〈111〉 directions. On the other hand, Cheon and co-workers had also successfully prepared multipods PbS structure through kinetic control of temperature when the temperature was lower than 230 °C, and they thought that the growth of pods on {100} faces were favored when temperature was enhanced.32 From the above results, we reasonably believed that the formation of complex morphologies of PbS were also kinetically controlled by the concentration in our case. Increasing the concentration of Pb2+

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(e.g., >0.01 M), the growth rate was accelerated, and the enhanced growth rate on the {100} face induced the shrinking of the six {100} face into the six prominent prisms which would result in a six-armed structure (Figures 4b and 4c). When the Pb2+ concentration was further increased (e.g., >0.05 M), the growth rate of {100} faces became quicker, which further induced the shrinking of the {100} faces of arms to form prominent parts, and a large mass of supersaturating precursors grew along the four 〈100〉 directions of the arms and finally lead to the growth of symmetric branches. To explore the influence of sulfur sources on the morphologies of final PbS products under similar reaction conditions, thioacetamine and Na2S2O3 were used as sulfur sources instead of thiourea. When Na2S2O3 was used and kept [Pb2+] ) 0.1 M, cuboid or truncated cube particles with about 250-300 nm in size were obtained (Figure s1a, Supporting Information), while smaller particles (60-100 nm) were obtained (Figure s1b, Supporting Informaiton) with thioacetamine as sulfur source, and truncated cube particles and plate-like particles were mainly products under a similar condition (Figure s1c, Supporting Informaiton). For different [Pb2+] concentrations (from 0.01 to 0.25 M), similar results (including size and shape) were also found. These results demonstrated that thiourea as sulfur source plays a key role in the formation of 3-D hierarchical dendrites of PbS under our experimental conditions. When Na2S2O3 or thioacetamine was used as sulfur source, transparent solution was formed only under lower concentration of Pb2+ (0.025 M), and only cubelike morphologies were obtained, which was similar with thiourea as sulfur source at low

Figure 6. TEM images of PbSe and PbTe prepared from [Pb2+] ) 0.1 M for 1 h. (a) Low-magnification image of PbSe; (b) high-magnification image of PbSe; (c) low-magnification image of PbTe; (d) high-magnification image of PbTe.

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concentration. The results also further confirmed that the formation of complex morphologies of PbS was kinetically controlled by the concentration. 3.2. Hierarchical Dendrites of PbSe and PbTe. Hierarchical dendrites of PbSe and PbTe were prepared with Se and Te powders as selenium and tellurium sources. Because of the weak reductive ability of EG to Se and Te, other reductive reagents (e.g., appropriate amount of hydrazine) were added into the reaction system. Figure 5 shows the XRD patterns of as-obtained PbSe and PbTe when the initial concentration of [Pb2+] was 0.1 M and reaction time was set to 1 h. All of the diffraction peaks in Figure 5a,b can be well-assigned to the cubic phase of PbSe (JCPDS Card No. 6-354, space group Fm3m, a ) 0.6124 nm) and cubic phase of PbTe (altaite, JCPDS Card No.38-1435, space group Fm3m, a ) 0.6459 nm), respectively. No Se or Te and other purities were detected. The results demonstrated that pure-phase PbSe or PbTe could be synthesized by adding another stronger reductive reagent. If no hydrazine was used, a small amount of Te and Se powders were detected besides PbSe and PbTe, even though the reaction time was prolonged to 2 h. The TEM images of as-prepared PbSe and PbTe under the above experimental conditions were shown in Figure 6. From the TEM images of PbSe (Figures 6a and 6b), we find that 3-D hierarchical dendrites are obtained, which possess a longer needle-like trunk and many branches composed of nanorods. Careful observation reveals that the branches grow on the surface of the trunk in order and form nanorods arrays. Interestingly, more complex nanowire architectures of PbSe with sub-branches and hyperbranches are found in Figure 6b. The architectures are comprise of backbone, first-order branches, second-order branches, and third-order branches. From the TEM image, it is clearly found that first-order branches grow almost perpendicularly on the backbone and second-order branches are almost perpendicular on first-order branches, etc. The results are similar with PbS dendrites, where branches are perpendicular on trunks. From the TEM images of PbTe, we can also find that snowflakelike dendrites are the main products. Careful observation of an individual dendrite (Figure 6d) reveals that the dendrite possesses four arms, and the branches grow perpendicularly on each arm. The structure is very similar to the dendrites of PbS from previous discussion (Figure 3a). Though the exact formation mechanism for the complex morphologies of PbSe and PbTe is not clear, we rationally infer that the formation of PbSe and PbTe dendrites is similar to that of PbS dendrites because they have similar crystal structure (cubic-phase) and morphology compared with PbS. Because of preferential growth along the 〈100〉 direction, the backbone was first formed, and the branches followed to grow on the backbone via an epitaxial growth process to form the dendrites of PbSe and PbTe as the final products. 4. Conclusion In summary, microwave-assistant heating method was used to synthesize hierarchical architectures of PbS with EG as solution, and different morphologies, e.g., cubic particles, six arms, and bugle-like dendrites, could be simply fabricated by adjusting the [Pb2+] concentration and sulfur sources. The results indicated that the growth of 3-D hierarchical structure was controlled by the kinetic conditions. In a similar reaction system, dendrites of PbSe and PbTe could be synthesized by adding another reductive reagent. This promising method could be expected to fabricate other 3-D metal chalcogenides. Acknowledgment. This work is supported by the National Science Foundation of China (No. 20671061).

Crystal Growth & Design, Vol. 7, No. 2, 2007 429 Supporting Information Available: TEM images of PbS prepared with different sulfur sources. This material is available free of charge via the Internet at http://pubs.acs.org.

References (1) Mokari, T.; Rothenberg, E.; Popov, I.; Costi, R.; Banin U. Science 2004, 304, 787. (2) Liu, Q.; Gao, F.; Komarneni, S. J. Am. Chem. Soc. 2004, 126, 54. (3) Wang, D. L.; Qian, F.; Yang, C.; Zhong, Z. H.; Lieber, C. M. Nano Lett. 2004, 4, 871. (4) Wen, J. G.; Lao, J. Y.; Wang, D. Z.; Kyaw, T. M.; Foo, Y. L.; Ren, Z. F. Chem. Phys. Lett. 2003, 372, 717. (5) Yan, H. Q.; He, R. R.; Johnson, J.; Law, M.; Saykally, R. J.; Yang, P. D. J. Am. Chem. Soc. 2003, 125, 4728. (6) Weissman, J. M.; Sunkara, H. B.; Tse, A. S.; Asher, S. A. Science 1996, 274, 959. (7) Holtz, J. H.; Holtz, J. S. W.; Munro, C. H.; Asher, S. A. Anal. Chem. 1998, 70, 780. (8) Ghosh, S.; Inganas, O. AdV. Mater. 1999, 11, 1214. (9) Rolison, D. R.; Dunn, B. J. Mater. Chem. 2001, 11, 963. (10) Liu, Y.; Zha, S.; Liu, M. AdV. Mater. 2004, 16, 256. (11) Shin, H. C.; Liu, M. L. Chem. Mater. 2004, 16, 5460. (12) Liu, B.; Zen, H. C. J. Am. Chem. Soc. 2004, 126, 16744. (13) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 8124. (14) Ding, Y. S.; Shen, X. F.; Gomez, S.; Luo, H.; Aindow, M.; Suib, S. L. AdV. Funct. Mater. 2006, 16, 549. (15) Li, Z.; Ding, Y.; Xiong, Y.; Yang, Q.; Xie, Y. Chem. Commun. 2005, 918. (16) Ho, W. K.; Yu, J. C. Chem. Commun. 2006, 1115. (17) Shi, H. T.; Qi, L. M.; Ma, J. M.; Wu, N. Z. AdV. Funct. Mater. 2005, 15, 442. (18) Shchennikov, V. V.; Ovsyannikov, S. V. Solid State Commun. 2004, 126, 373. (19) Ovsyannikov, S. V.; Shchennikov, V. V.; Ponosov, Y. S.; Gudina, S. V.; Guk, V. G.; Skipetrov, E. P.; Mogilenskikh, V. E. J. Phys. D: Appl. Phys. 2004, 37, 1151. (20) McDonald, S. A.; Konstantatos, G.; Zhang, S.; Cyr, P. W.; Klem, E. J. D.; Levina, L.; Sargent, E. H. Nat. Mater. 2005, 4, 138. (21) Levina, L.; Sukhovatkin, V.; Musikhin, S.; Cauchi, S.; Nisman, R.; Bazett-Jones, D. P.; Sargent, E. H. AdV. Mater. 2005, 17, 1854. (22) Choudhury, K. R.; Sahoo, Y.; Jang, S.; Prasad, P. N. AdV. Funct. Mater. 2005, 15, 751. (23) Ge, J.-P.; Wang, J.; Zhang, H.-X.; Wang, X.; Peng, Q.; Li, Y.-D. Chem. Eur. J. 2005, 11, 1889. (24) Kuang, D. B.; Xu, A. W.; Fang, Y. P.; Liu, H. Q.; Frommen, C.; Fenske, D. AdV. Mater. 2003, 15, 1747. (25) Zhao, N. N.; Qi, L. M. AdV. Mater. 2006, 18, 359-362. (26) Ma, Y. R.; Qi, L. M.; Ma, J. M.; Cheng, H. M. Cryst. Growth Des. 2004, 4, 351. (27) Zhou, G.; Lu, M.; Xiu, Z.; Wang, S.; Zhang, H.; Zhou, Y.; Wang, S. J. Phys. Chem. B 2006, 110, 6543. (28) Zhang, C.; Kang, Z.; Shen, E.; Wang, E.; Gao, L.; Luo, F.; Tian, C.; Wang, C.; Lan, Y.; Li, J.; Cao, X. J. Phys. Chem. B 2006, 110, 184. (29) Wang, D. B.; Yu, D. B.; Shao, M. W.; Liu, X. M.; Yu, W. C.; Qian, Y. T. J. Cryst. Growth 2003, 257, 384. (30) Huang, Q.; Gao, L. Chem. Lett. 2004, 33, 1338. (31) Cho, K. S.; Talapin, D. V.; Gaschler, W.; Murray, C. B. J. Am. Chem. Soc. 2005, 127, 7140. (32) Lee, S. M.; Jun, Y. W.; Cho, S. N.; Cheon, J. J. Am. Chem. Soc. 2002, 124, 11244. (33) Joo, J.; Na, H. B.; Yu, T.; Yu, J. H.; Kim, Y. W.; Wu, F. X.; Zhang, J. Z.; Hyeon T. J. Am. Chem. Soc. 2003, 125, 11100. (34) Lu, W. G.; Fang, J.; Stokes, K. L.; Lin, J. J. Am. Chem. Soc. 2004, 126, 11798. (35) Kerner, R.; Palchik, O.; Gedanken, A. Chem. Mater. 2001, 13, 1413. (36) Ni, Y. H.; Liu, H. J.; Wang, F.; Liang, Y. Y.; Hong, J. M.; Ma, X.; Xu, Z. Cryst. Growth Des. 2004, 4, 759. (37) Ding, T.; Zhang, J. R.; Long, S.; Zhu, J. J. Microelectron. Eng. 2003, 66, 46. (38) Ni, Y. H.; Wang, F.; Liu, H. J.; Yin, G.; Hong, J. M.; Ma, X.; Xu, Z. J. Cryst. Growth 2004, 262, 399. (39) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153.

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