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Synthesis of Submicrometer-Sized Cu2O Crystals with Morphological Evolution from Cubic to Hexapod Structures and Their Comparative Photocatalytic Activity Jin-Yi Ho and Michael H. Huang* Department of Chemistry, National Tsing Hua UniVersity, Hsinchu 30013, Taiwan ReceiVed: April 28, 2009; ReVised Manuscript ReceiVed: June 11, 2009
We report a facile method for the synthesis of cuprous oxide nanocrystals with systematic morphological evolution. Cubic, truncated cubic, cuboctahedral, truncated octahedral, octahedral, and short hexapod structures have been synthesized in an aqueous solution of CuCl2, NaOH, sodium dodecyl sulfate (SDS) surfactant, and hydroxylamine (NH2OH · HCl) reductant by simply varying the volume of hydroxylamine added to the reaction mixture. A slight modification in the volume of some reagents produced the extended hexapods. The order of the introduction of the reagents is important to the formation of these crystals with distinct morphologies and sharp faces. The sizes of these particles fall mostly in the range of 400-700 nm. Clear transition in the relative intensities of the (111) and the (200) reflection peaks in their XRD patterns was observed. Scattering bands dominate the UV-vis absorption spectra of these crystals. Crystal model analysis revealed that the {111} face contains surface copper atoms with dangling bonds, and is expected to interact more strongly with negatively charged molecules. Tests of photodegradation of negatively charged methyl orange showed that octahedra and the extended hexapods were catalytically active. The cubes with only the {100} faces were not active. On the contrary, both cubes and octahedra were not effective at photodecomposing positively charged methylene blue molecules. Surprisingly, octahedra and hexapods cannot be well suspended in the methylene blue solution; a significant amount of the crystals gradually moved to the surface of the solution with increasing stirring time. The results clearly demonstrate the dramatic differences in the catalytic activities of the {111} and {100} faces of Cu2O crystals for the first time. Introduction Cuprous oxide (Cu2O) is a p-type semiconductor with a direct band gap of 2.17 eV. Cu2O nanostructures possess properties that may find applications in gas sensing,1,2 CO oxidation,3 photocatalysis,4-6 photochemical evolution of H2 from water,7,8 photocurrent generation,9 and organic synthesis.10,11 Numerous Cu2O nano- and microstructures have been synthesized in the past few years, including nanocubes,4,5,12-14 octahedra,5,15-17 nanocages,18-22 and multipods.22-25 Recently, micrometer-sized rhombic dodecahedral Cu2O crystals have been made.26 Among the various Cu2O crystal morphologies, the controlled syntheses of cubic and octahedral structures are perhaps the most important. This is because many other structural forms of Cu2O crystals have been reported to derive from these two basic geometric shapes. One of the most successful methods to prepare Cu2O crystals with systematic shape evolution from cubic to truncated octahedral and then to octahedral structure is by a cathodic deposition approach, in which crystals are grown directly on the electrode surface.15,16,27 The experimental setup seems costly with the use of precious metal-coated electrode surfaces. Although some studies have described the formation of Cu2O microcrystals with this structural variety, the growth conditions are less systematic and require inconsistent changes of reaction conditions to achieve this morphology variation.23,28 Previously, we have developed a facile room temperature procedure for the synthesis of Cu2O nanocrystals capable of this systematic shape evolution from cubic to octahedral structures by simply varying the amount of reductant added to the reaction mixture.5 However, the surfaces of these nano* To whom correspondence should be addressed. E-mail: hyhuang@ mx.nthu.edu.tw.
crystals are not very sharp, even for particles with sizes in the range of 400-600 nm. They have smooth edges and not so perfectly flat faces. The ability to systematically synthesize these Cu2O nanocrystals with sharp faces is highly desirable because their facet-specific properties can be examined with greater distinction and certainty. In this study, we have successfully synthesized sharp-faced Cu2O nanocrystals with morphological evolution from cubic to octahedral and then to hexapod structures by modifying our previous preparation procedure. The crystals are generally submicrometer-sized. Their crystal structures have been carefully analyzed by electron microscopy and X-ray diffraction techniques. The formation process of the hexapods was also examined. The successful syntheses of cubic-, octahedral-, and hexapod-shaped Cu2O nanocrystals with sharp surfaces are particularly useful for the examination of their comparative photocatalytic activity. Cu2O hexapods have been reported before but their catalytic performance has not been measured. Here we consider the atomic structures of the {100} and {111} surfaces of Cu2O to understand the different photocatalytic behavior of cubic-, truncated octahedral-, octahedral-, and hexapod-shaped Cu2O nanocrystals. The analysis suggests that these two crystal faces should interact differently to molecules carrying positive or negative charges. We examined the extent of photodegradation of negatively charged methyl orange and positively charged methylene blue molecules by these different Cu2O nanocrystals (see the Supporting Information for their molecular structures) and found very different behavior of the crystals toward these molecules and their photocatalytic activity. The observation of such distinctly different surface properties is possible only with the synthesis of Cu2O crystals having
10.1021/jp903928p CCC: $40.75 2009 American Chemical Society Published on Web 07/01/2009
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SCHEME 1: Schematic Illustration of the Procedure Used To Grow Cu2O Crystals with Various Shapesa
a
The corresponding SEM images of the samples (a-h) are shown in Figure 1.
homogeneous shapes, particularly the successful preparation of sharp-faced cubes. Experimental Section Anhydrous copper(II) chloride (CuCl2, 97%) and hydroxylamine hydrochloride (NH2OH · HCl, 99%) were purchased from Aldrich. Sodium hydroxide (98.2%) and sodium dodecyl sulfate (SDS, 100%) were acquired from Mallinckrodt. All chemicals were used without further purification. For the syntheses of submicrometer-sized Cu2O crystals with various morphologies from cubic to hexapod structures, 9.55 to 8.75 mL of deionized water (18.3 MΩ) was respectively added to seven sample vials labeled a to g (see Scheme 1). The amount of water added to each vial depends on the volume of NH2OH · HCl introduced later, such that the final solution volume is 10 mL for all the samples. Then 0.1 mL of 0.1 M CuCl2 and 0.2 mL of 1.0 M NaOH solution were added and the vials were shaken for 10 s. Threadlike light blue Cu(OH)2 precipitate was formed. Next, 0.087 g of SDS powder was added with vigorous stirring of the vials until dissolution of the powder. Finally, 0.15 to 0.95 mL of 0.2 M NH2OH · HCl was mixed with the solutions in vials a to g. For the preparation of extended Cu2O hexapods, 0.75 mL of 0.2 M NH2OH · HCl, titrated to a solution pH of 7.0 with 1.0 M NaOH, was added to the same mixture of CuCl2, NaOH, and SDS as before but with the use of 9.10 mL of water. The sample was labeled as h. The concentrations of Cu2+ ions and SDS surfactant in the final solution are 1.0 × 10-3 and 3.0 × 10-2 M, respectively. The solution turned from light blue to yellow within seconds after the addition of NH2OH · HCl. We aged the solution for 2 h to obtain the desired products. After aging, the samples were centrifuged at 3500 rpm for 2 min (Hermle Z323 centrifuge) and the top solution was withdrawn. The precipitates were centrifuged twice more in ethanol to remove unreacted chemicals and the SDS surfactant, and were finally dispersed in 0.5 mL of ethanol. A drop of the precipitate solution was transferred to either a carbon-coated copper grid or a clean silicon substrate for electron microscopy analysis. For the examination of the photocatalytic activity of these Cu2O crystals, the amount of crystals needed is much more than that produced by following the above procedure. Instead of performing the same experiment many times, we used 10 times the reagent amounts to make each Cu2O crystal sample here (i.e., a total volume of 100 mL). The entire centrifuged Cu2O crystals were dispersed in 90 mL of 15 mg/L aqueous methyl orange solution or 90 mL of 10 mg/L aqueous methylene blue solution. The entire solution was transferred to a homemade cubic quartz cell with an inner cell edge length of about 4.5 cm and a small capped opening at the top (see the Supporting Information for its photographs). The cell was constantly stirred and irradiated with light from a 200 W mercury lamp placed
Figure 1. SEM images of the Cu2O nanocrystals with various morphologies: (a) cubes, (b) truncated cubes, (c) cuboctahedra, (d) type I truncated octahedra, (e) type II truncated octahedra, (f) octahedra, (g) short hexapods, and (h) extended hexapods. Scale bar ) 1 µm.
25 cm away. Air conditioning of the room was employed to minimize the heat effect. The light intensity reaching the cell was measured with a power meter to be 486 mW/cm2. UV-vis absorption spectra of these samples were taken before and after every 60 min of irradiation for up to 240 min (and every 20 min for up to 120 min in the methylene blue case) by removing the cap to withdraw the solution. Scanning electron microscopy (SEM) images of the synthesized Cu2O crystals were obtained with a JEOL JSM-7000F scanning electron microscope. Transmission electron microscope (TEM) characterization was performed on a JEOL JEM-2100 electron microscope operating at 200 kV. Powder X-ray diffraction (XRD) patterns were collected with a Shimadzu XRD-6000 diffractometer with Cu KR radiation (λ ) 1.5418 Å). UV-vis absorption spectra were acquired with the use of a JASCO V-570 spectrophotometer. The crystal structure of Cu2O was drawn by using the Diamond 3.0 program.
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TABLE 1: Average Particle Sizes and Standard Deviations of the Cu2O Crystals Synthesized in Samples a-h sample
morphology
av particle size (nm)
standard deviation (%)
a b c d e f g h
cubes truncated cubes cuboctahedra truncated octahedra (type I) truncated octahedra (type II) octahedra short hexapods extended hexapods
583 ( 104 489 ( 69 540 ( 67 585 ( 75 545 ( 66 460 ( 67 492 ( 64 998 ( 168
18 14 12 13 12 15 13 17
Results and Discussion For this study, the major difference in the prepration process of Cu2O nanocrystals from our previous procedure is the sequence of the introduction of the reagents (see Scheme 1).5 Here NaOH was the second reagent added. Previously NaOH was added last. The idea here is to have Cu(OH)2 and Cu(OH)42species present and well-mixed in the solution before the introduction of NH2OH · HCl reductant, since the crystal growth rate is fast and the initial crystal morphology determines the final product structure. We discovered that this simple change of procedure to make Cu(OH)2 precipitate first promotes the formation of Cu2O nanocrystals with sharp faces. A slightly smaller amount of NaOH was used in this work (0.20 mL vs. 0.25 mL previously used). Figure 1 presents the SEM images of the Cu2O nanocrystals made in this study. By simply varying the amount of NH2OH · HCl reductant added from 0.15 to 0.95 mL, cubic-, truncated cubic-, cuboctahedral-, truncated octahedral-, octahedral-, and short hexapod-shaped Cu2O nanocrystals were synthesized (samples a-g). Two types of truncated octahedral nanocrystals can be discerned. Nanocrystals in sample d have 8 regular hexagonal faces and 6 square faces. They are called type I truncated octahedra here. Nanocrystals in sample e have 8 truncated triangular faces and 6 square faces resulting from an increase in the area of the {111} faces. They are called type II truncated octahedra here. Interestingly, perfect cubes can be easily synthesized following this new procedure. Previously, nanocubes with truncated edges and corners were obtained.4,5 The truncated cubes formed in sample b show only truncated corners. The cuboctahedra collected in sample c have 8 triangular faces and 6 square faces. The short hexapods represent a progressive structural evolution beyond the octahedral structure with each corner of an octahedron developing into a short square pyramidal branch enclosed by the {111} surfaces. Thus, a structure with 24 {111} facets is formed. Addition of 0.75 and 0.85 mL of NH2OH · HCl also produced short hexapods, but with a larger distribution of particle sizes. As seen from Figure 1, it is evident that the gradual and systematic morphological evolution of the Cu2O nanocrystals with good separation of each distinct geometric shape can be achieved. The average particle sizes and their standard deviations are provided in Table 1. The sizes of these particles fall mostly in the range of 400-700 nm, so they are submicrometer-sized particles. Particle size histograms can be found in the Supporting Information. To grow the extended hexapods (sample h), slightly different amounts of NH2OH · HCl, NaOH, and H2O were used. This is because a further increase in the amount of NH2OH · HCl added to the reaction mixture can make the solution pH drop below 7 with the production of roughly spherical Cu2O particles. Solution pH decreases from 12.00 for the nanocube sample to 8.70 for the short hexapod sample with increasing volumes of NH2OH · HCl introduced. With the adjustments made, the solution pH can
Figure 2. Powder X-ray diffraction patterns of the different morphologies of Cu2O nanocrystals. A standard diffraction pattern of Cu2O is also given. The intensity scale for each sample is the same.
remain basic at 10.61 for sample h. The extended hexapods have longer branches and they have grown to about 1 µm in size. Solution pH values for the other samples are available in the Supporting Information. The high morphological uniformity of these Cu2O crystals is reflected by their XRD patterns. Figure 2 gives the XRD patterns of the different morphologies of Cu2O nanocrystals. The diffraction patterns show clearly a transition in the relative intensities of the (111) and the (200) peaks with morphology change. As expected, nanocubes show an exceptionally strong (200) reflection peak and an extremely weak (111) reflection peak. The intensity of the (111) peak increases progressively as nanocrystals with more {111} surfaces are formed. The two peaks become more comparable in intensity for the type I truncated octahedra. The (111) peak then dominates for octahedra and hexapods. The observation of this distinct transition results from the uniformity of the particle morphology and their preferred orientation of deposition on substrates. Figure 3 presents the TEM images, their corresponding SAED patterns, and representative SEM images of individual Cu2O nanocrystals of various morphologies. The SEM images are provided to show the exact viewing directions of the respective TEM images and SAED patterns. TEM images of the Cu2O crystals viewed along (a1) the [100] direction of a perfect nanocube, (b1 and c1) the [100] and [111] directions of a cuboctahedron, (d1) the [111] direction of an octahedron, (e1) the [111] direction of a short hexapod, and (f1) the [111] direction of an extended hexapod are shown. The single crystalline nature and sharp faces of these particles are evident. The extended hexapods show a smooth curvature for their branches, although the central portions of these hexapods can be less well-defined. Because the hexapods are a new type of Cu2O nanostructure not previously synthesized by using our original procedure, we also took TEM images of the intermediate products formed in the growth of the short hexapods.5 Figure 4 is a TEM image of the intermediate product obtained after just 1 min of reaction following the addition of NH2OH · HCl in the formation of short hexapods. Interestingly, hexapods have already been formed in such a short reaction time. The particle
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Figure 3. TEM images, their corresponding SAED patterns, and representative SEM images of the various morphologies of Cu2O nanocrystals synthesized. TEM images of the Cu2O crystals viewed along (a1) the [100] direction of a nanocube, (b1 and c1) the [100] and [111] directions of a cuboctahedron, (d1) the [111] direction of an octahedron, (e1) the [111] direction of a short hexapod, and (f1) the [111] direction of an extended hexapod are shown. Their corresponding SAED patterns were recorded along the (a2 and b2) [200] and (c2-f2) [111] zone axes of the cuprite crystal. For samples d1, e1, and f1, the SAED patterns displayed were taken from the square regions of the respective particles. Because of the large size of the nanocube shown in panel a1, the SAED pattern given in panel a2 was taken from a smaller nanocube.
Figure 4. TEM image of the intermediate product obtained after just 1 min of reaction following the addition of NH2OH · HCl in the formation of short hexapods.
size is smaller than a fully developed one collected after 2 h of reaction. It appears to be formed by an aggregation of tiny particles. Through the continuous growth and surface reconstruction, larger hexapods with smooth faces were produced. The optical properties of these Cu2O nanostructures were examined by taking their UV-vis absorption spectra (see the Supporting Information). The spectral features are similar to those observed before.5 Because of the large particle sizes, broad scattering bands dominate in the absorption spectra, covering a wide range from before 600 nm to the near-infrared region beyond 1300 nm for all the samples. Intrinsic band gap absorption can still be observed for some samples in the 400 to 580 nm range, but the bands are generally weak. The synthesized Cu2O nanocrystals with well-defined structures and sharp faces are most useful for the examination of their comparative photocatalytic activity. Our previous work has demonstrated that octahedral Cu2O crystals with entirely {111} faces are photocatalytically more active than truncated cubic crystals with mostly {100} facets.4,5 To try to understand this difference, we constructed a model of the Cu2O crystal structure to reveal the surface atomic arrangements of these two crystal
faces. Cu2O has a cuprite crystal structure, which is composed of a body-centered cubic packing of oxygen atoms with copper atoms occupying half of the tetrahedral sites. Figure 5 displays the cubic crystal structure of Cu2O oriented to show the (100) and the (111) planes. The crystal model shows that the (100) planes contain oxygen atoms as they do in the unit cell. However, a cut of the unit cell over one of its (111) planes reveals the presence of surface Cu atoms with dangling bonds. Thus, this simple comparison should indicate that the {111} faces are higher in surface energy and expected to be more catalytically active than the {100} faces. Furthermore, Cu2O crystals bounded by the {111} faces contain positively charged copper atoms at the surfaces, whereas those bounded by the {100} faces such as the cubes are electrically neutral. This observation may be significant, because it suggests that Cu2O octahedra should interact more strongly with negatively charged molecules and photodegradation of these molecules is more effective. Use of a positively charged molecule can result in a poor photodecomposition performance. On the other hand, cubic Cu2O crystals are less sensitive to the charge of the adsorbed molecules and are simply not photocatalytically active. To see the relative photocatalytic activities of some of the Cu2O nanostructures synthesized, methyl orange, a negatively charged molecule, was first used for the photodecomposition experiments. Figure 6a is a plot of the extent of photodegradation of methyl orange vs. time for the various Cu2O nanostructures used. The corresponding UV-vis absorption spectra for the cubes, truncated octahedra, octahedra, and extended hexapods are available in the Supporting Information. After irradiation for 4 h, the fraction of remaining absorption of methyl orange measured at 462 nm was 0.96 for the blank sample, 0.92 for cubes, 0.74 for truncated octahedra, 0.64 for octahedra, and 0.32 for extended hexapods. As expected, octahedra with entirely {111} facets are much more photocatalytically active than cubes. In fact, the cubes were practically not effective at photodecomposing methyl orange. Such a large difference is only observable with the use of perfect cubes; previous tests with truncated nanocubes with {111} corners and {110} edges showed a lower but appreciable degree of photodegradation than that of octahedra.4,5 The lower extent of photodegradation of methyl
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Figure 5. The crystal structure of Cu2O oriented to show (a) the (100) planes and (b) the (111) planes. A unit cell is drawn in white lines for each crystal orientation. For panel b, the viewing angle is along the [110] direction. The (111) plane shown is the plane formed by the three red spheres located at the upper right vertex and the two lower middle vertices (one in front and one at the back). Surface Cu atoms on the {111} faces (yellow circles in panel b) may contain dangling bonds, making these faces catalytically more active.
Figure 6. A plot of the extent of photodegradation of methyl orange vs. time for the various Cu2O nanostructures is shown. Here type II truncated octahedra were used. The blank sample did not contain Cu2O crystals but only the methyl orange solution. The temperature change of the solution over this time period is also given.
Figure 7. A plot of the extent of photodegradation of methylene blue vs. time for the Cu2O cubes and octahedra is shown. The blank sample did not contain Cu2O crystals but only the methylene blue solution. Lower cell temperatures were recorded because the room temperature was lower here.
orange by truncated octahedra may be attributed to the presence of the {100} corners. A previous study comparing the photocatalytic activity of cubic and octahedral Cu2O particles also found octahedra were much more photochemically active than the cubes, but a drastic drop in absorbance was recorded before starting the photodegradation experiment.29 Another notable finding from this series of experiments is the exceptionally high photocatalytic performance of the extended hexapods. This result suggests that Cu2O nanocrystals with more {111} facets can serve as more efficient photocatalysts. The presence of more sharp edges between the {111} facets in the hexapods may also enhance their catalytic activity. During the irradiation period, the cell was kept cool to minimize the effect of heat. Only minor temperature increase was recorded in the first hour of irradiation, and constant temperature was measured after the first hour (see Figure 6b). Methylene blue, a positively charged molecule, was selected to see how it would behave to photodegradation by the Cu2O crystals. Figure 7 shows a plot of the extent of photodegradation of methylene blue vs. time for the Cu2O cubes and octahedra synthesized. The corresponding UV-vis absorption spectra for the cubes, octahedra, and the blank sample are
available in the Supporting Information. Both the cubes and octahedra did not cause photodegradation of methylene blue, as the degree of degradation after 2 h of irradiation for these crystals was the same as that of the blank sample. Amazingly, although the cubes can be dispersed in the methylene blue solution as usual, the octahedra cannot mix well with the solution. After the solution was stirred for minutes to over an hour, the octahedra gradually moved to the surface of the solution or adhered to the inner top wall of the cell. Extended hexapods showed a similar effect with a significant amount of the particles moving to the surface of the solution in minutes. UV-vis spectra of the methylene blue solution with the extended hexapods revealed a gradual decrease of the absorption and scattering bands from the hexapods with increasing stirring time (see the Supporting Information). The electrostatic repulsion force is believed to cause this effect for the octahedra and hexapods, a result consistent with the crystal model analysis. The cubes, as predicted, were insensitive to the molecular charge and can stay in the solution. However, they were not photocatalytically active. When methyl violet, another positively charged molecule, was used to examine its response toward
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photodegradation by the extended hexapods, the hexapods similarly did not mix well with the solution, but moved to the surface of the solution and adhered to the cell wall. Rhodamine B, a zwitterionic molecule, should have its carboxylic acid group in the deprotonated form at the solution pH used in our previous studies.4,5,30 The presence of this negative charge kept the Cu2O particles in the solution. To the best of our knowledge, the observation of this dramatic difference in the photocatalytic activity of Cu2O crystals bounded by entirely {111} and {100} facets has not been reported before. This study also shows that hexapods, and possibly other multipods with more {111} facets, are catalytically more active and effective than the octahedra, and should be considered in future catalysis studies with Cu2O crystals. Conclusion We have adopted a modified synthetic procedure for the preparation of various Cu2O nanostructures with systematic shape evolution from cubic to octahedral and then to the hexapod structures. Eight distinct morphologies have been obtained. The crystals are submicrometer-sized except for the extended hexapods, and they possess sharp faces. The crystals have been characterized by SEM, TEM, and XRD techniques. Clear transition in the relative intensities of the (111) and the (200) reflection peaks in their XRD patterns was observed. UV-vis absorption spectra of the particles were also taken. Octahedra and the extended hexapods exhibited good photocatalytic activity toward the photodegradation of methyl orange, while perfect cubes were practically not active. The observation of this dramatic difference in the photocatalytic activity of Cu2O crystals is only possible with the successful synthesis of sharp cubes and octahedra. The catalytic activity of the {111} face is attributed to the presence of dangling bonds from the surface copper atoms. The electrically neutral {100} face cannot interact well with charged molecules and is catalytically inactive. Negatively charged molecules can be more effectively adsorbed on the {111} surfaces of the Cu2O crystals for the photodegradation of the molecules to occur. On the contrary, solutions containing positively charged molecules can repel the crystals with {111} surfaces and make a significant amount of the crystals float to the top surface of the solution. Thus, no catalytic activity was measured. It is expected that more insight into the catalytic properties of Cu2O can be provided by using these crystals with well-defined structures. Acknowledgment. Financial support for this work is provided by the National Science Council of Taiwan (Grant NSC95-2113M-007-031-MY3). Supporting Information Available: Molecular structures of methyl orange and methylene blue, particle size histograms,
Ho and Huang solution pH values of the samples, UV-vis absorption spectra of the various morphologies of Cu2O crystals synthesized, UV-vis absorption spectra of the photodegradation experiments, photographs of the methylene blue solution with the extended hexapods, and UV-vis absorption spectra of the methylene blue solution with the extended hexapods as a function of stirring time. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Zhang, J.; Liu, J.; Peng, Q.; Wang, X.; Li, Y. Chem. Mater. 2006, 18, 867–871. (2) Zhang, H.; Zhu, Q.; Zhang, Y.; Wang, Y.; Zhao, L.; Yu, B. AdV. Funct. Mater. 2007, 17, 2766–2771. (3) White, B.; Yin, M.; Hall, A.; Le, D.; Stolbov, S.; Rahman, T.; Turro, N.; O’Brien, S. Nano Lett. 2006, 6, 2095–2098. (4) Kuo, C.-H.; Chen, C.-H.; Huang, M. H. AdV. Funct. Mater. 2007, 17, 3773–3780. (5) Kuo, C.-H.; Huang, M. H. J. Phys. Chem. C 2008, 112, 18355– 18360. (6) Yu, H.; Yu, J.; Liu, S.; Mann, S. Chem. Mater. 2007, 19, 4327– 4334. (7) Hara, M.; Kondo, T.; Komoda, M.; Ikeda, S.; Shinohara, K.; Tanaka, A.; Kondo, J. N.; Domen, K. Chem. Commun. 1998, 357–358. (8) Nian, J.-N.; Hu, C.-C.; Teng, H. Int. J. Hydrogen Energy 2008, 33, 2897–2903. (9) Yang, Z.; Chiang, C.-K.; Chang, H.-T. Nanotechnology 2008, 025604. (10) Tang, B.-X.; Wang, F.; Li, J.-H.; Xie, Y.-X.; Zhang, M.-B. J. Org. Chem. 2007, 72, 6294–6297. (11) Altman, R. A.; Koval, E. D.; Buchwald, S. L. J. Org. Chem. 2007, 72, 6190–6199. (12) Gou, L.; Murphy, C. J. Nano Lett. 2003, 3, 231–234. (13) Kim, M. H.; Lim, B.; Lee, E. P.; Xia, Y. J. Mater. Chem. 2008, 18, 4069–4073. (14) Zhao, H. Y.; Wang, Y. F.; Zeng, J. H. Cryst. Growth Des. 2008, 8, 3731–3734. (15) Siegfried, M. J.; Choi, K.-S. J. Am. Chem. Soc. 2006, 128, 10356– 10357. (16) Siegfried, M. J.; Choi, K.-S. AdV. Mater. 2004, 16, 1743–1746. (17) Guo, S.; Fang, Y.; Dong, S.; Wang, E. Inorg. Chem. 2007, 46, 9537–9539. (18) Kuo, C.-H.; Huang, M. H. J. Am. Chem. Soc. 2008, 130, 12815– 12820. (19) Lu, C.; Qi, L.; Yang, J.; Wang, X.; Zhang, D.; Xie, J.; Ma, J. AdV. Mater. 2005, 17, 2562–2567. (20) Teo, J. J.; Chang, Y.; Zeng, H. C. Langmuir 2006, 22, 7369–7377. (21) Chang, Y.; Teo, J. J.; Zeng, H. C. Langmuir 2005, 21, 1074–1079. (22) Xu, H.; Wang, W. Angew. Chem., Int. Ed. 2007, 46, 1489–1492. (23) Pang, H.; Gao, F.; Lu, Q. Chem. Commun. 2009, 1076–1078. (24) Chang, Y.; Zeng, H. C. Cryst. Growth Des. 2004, 4, 273–278. (25) Xu, J.; Xue, D. Acta Mater. 2007, 55, 2397–2406. (26) Liang, X.; Gao, L.; Yang, S.; Sun, J. AdV. Mater. 2009, 21, 2068– 2071. (27) Li, H.; Liu, R.; Zhao, R.; Zheng, Y.; Chen, W.; Xu, Z. Cryst. Growth Des. 2006, 6, 2795–2798. (28) Liu, H.; Miao, W.; Yang, S.; Zhang, Z.; Chen, J. Cryst. Growth Des. 2009, 9, 1733–1740. (29) Xu, H.; Wang, W.; Zhu, W. J. Phys. Chem. B 2006, 110, 13829– 13834. (30) Moreno-Villoslada, I.; Jofre´, M.; Miranda, V.; Gonza´lez, R.; Sotelo, T.; Hess, S.; Rivas, B. L. J. Phys. Chem. B 2006, 110, 11809–11812.
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