ARTICLE pubs.acs.org/crystal
Axis-Oriented, Anatase TiO2 Single Crystals with Dominant {001} and {100} Facets Cuong Ky Nguyen, Hyun Gil Cha, and Young Soo Kang* Korea Center for Artificial Photosynthesis and Department of Chemistry, Sogang University, Shinsu-dong #1, Mapo-gu, Seoul 121-854, Korea
bS Supporting Information ABSTRACT: Owing to wide-ranging industrial applications and fundamental importance, tailored synthesis of well-faceted single crystals of anatase TiO2 with a high percentage of photoactive facets has attracted much research interest. In this work, single crystalline anatase TiO2 has been prepared using hydrothermal conditions with precursors such as TiF4 and TiCl3. Fluorine species (herein HF) are introduced as a stabilizing agent to reduce surface energies of anatase TiO2, especially {001} facets, which leads to a large percentage of the reactive {001} facets exposed in final assemblies or films of titania with the largest exposure area. Anatase TiO2 single crystals with various degrees of truncation are prepared as a result of differences in relative concentration ratios of HF to TiF4. The “rice-like” anatase single crystals with exposed {100} facets were prepared in the system using TiCl3 precursor. Carboxymethyl cellulose (CMC), used as a dispersing agent, helps to control the reaction rate and suppress the agglomeration of “truncated” and “rice-like” crystals. The effects of fluorine species and precursors on the quality, morphology, and crystal structure of anatase TiO2 single crystals have been investigated.
’ INTRODUCTION Titanium dioxide has wide applications in pigments, sensors, dielectric ceramics, catalysts, and electrodes in solar cells as it exhibits outstanding ability for deodorization, air purification, and sterilization as a strong photocatalyst.16 Its performance depends to a large extent on its physical and chemical nature, which are related to its crystal structure, grain (crystal) size, morphology, and even surface structure.5 Therefore, there is considerable interest in the controlled preparation of TiO2 with a specific crystal structure for the largest exposure of high photoactive facets.717 Surface structure of the photocatalysts has attracted considerable attention because the properties of a photocatalyst material are strongly dependent on its crystal surface and surface energy by its electronic density functions.5,18,19 Usually, different facets of a single crystal exhibit distinctive physical and chemical properties. For anatase TiO2, it has been found that the {001} facet is one of the most photoactive.5,20 The order of the average surface energies is γ{110} (1.09 J m2) > γ{001} (0.90 J m2) > γ{100} (0.53 J m2) > γ{101} (0.44 J m2).5,20 Generally, the facets with high surface energies diminish quickly during a crystal growth process for the minimization of the total surface energy. Therefore, anatase TiO2 single crystals with exposed {001} and {100} facets are rarely observed, especially in the same structure. Most of the anatase TiO2 micro- and nanostructures reported to date are dominated by {101} facets because of the minimization of the surface energy of the crystals.5 r 2011 American Chemical Society
Because the {001} facets are more reactive for dissociative adsorption of reactant molecules compared with {101} facets,2123 high photocatalytic efficiency is expected for anatase TiO2 particles with {001} and {100} facets.24 There has been recent increasing interest in the controlled synthesis of TiO2 particles with exposed {001} facets. Important progress in the preparation of anatase TiO2 single crystals with exposed {001} facets was achieved by Lu and co-workers,25 who used TiF4 as raw material. Since then, several research groups have prepared anatase TiO2 single crystals with exposed {001} facets from different raw materials such as titanium fluoride, titanium chloride, titanium tetrabutoxide, titanium tetraisopropoxide, and so on.2634 However, these materials have high hydrolytic reaction rates which make it difficult to control their reaction processes. Most of these works are focused on the {001} facets only, and the development of facile and reproducible synthesis of shape-controlled anatase TiO2 single crystals with different reactive facets such as {100} or {110} ones still remains a great challenge for the preparation of the largest area exposure of the most photoactive facets. Herein, we report on the preparation and characterization of micrometer-sized TiO2 single crystals with predominant {001} and {100} facets via a conventional hydrothermal approach. Received: April 28, 2011 Revised: July 27, 2011 Published: August 03, 2011 3947
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Figure 1. SEM images of anatase single crystals prepared by the solution at a small additional amount (0.2 mL) of HF (10 wt %), with (a) 2.67 mM, (b) 5.33 mM, (c) 8.0 mM, (d) 10.67 mM, and (e) 13.33 mM TiF4. Scale bar is 2.5 μm.
Scheme 1. Schematic Diagram Showing Anatase Crystal Formation
’ EXPERIMENTAL SECTION Synthesis of Anatase TiO2 Single Crystals. Synthesis of Anatase Single Crystals with {001} Facets. Hydrochloric acid (1.5 M) was used to adjust the pH of deionized water to around 2.1. Titanium tetrafluoride (TiF4, Aldrich Chemical) was dissolved in the solution up to a concentration of 0.04 M, which changed the pH to 1.8. Finally, deionized water was used to adjust the concentration of TiF4 aqueous solution to a concentration of 2.67 to 13.33 mM. In a typical synthesis, 30 mL of TiF4 aqueous solution and a small amount of hydrofluoric acid (10 wt %) were added to a Teflon-lined autoclave and a transparent mixture was formed; the mixture was kept at 180 °C for 10 h in an oven. After completion of the reaction, the solid products obtained were washed three times with deionized water and then dried at 80 °C or redispersed in deionized water for further characterization. Synthesis of Anatase Single Crystals with {100} Facets. In a typical synthesis procedure, 30 mL aqueous solution of titanium trichloride (TiCl3, 5.33 mM), carboxymethyl cellulose, sodium salt (carboxymethyl cellulose (CMC), 2.8 g/L, Mw = 90 000) and 0.4 mL of hydrofluoric acid (10 wt %) were added to a Teflon-lined autoclave and a green transparent mixture was formed. The solution was then put into the oven at 180 °C for 10 h. The assynthesized product was collected by centrifugation and washed with deionized water several times to remove dissolvable ionic impurities. The samples were then dried at 80 °C in air. Fluorine Removal from the Surface of Anatase TiO2 Single Crystals. Typically, the powder samples of as-prepared {001} anatase TiO2 single crystals were heat treated in static air in a muffle furnace at temperatures of 600 °C for 90 min with a ramping rate of 5 °C/min.
With the sample of {100} anatase TiO2 single crystals, fluorine and organic contaminants were removed using the same process. The samples were heat-treated at 600 °C for 3 h and then cooled to room temperature in the muffle furnace for further characterization. Materials Characterization. Crystallographic information of anatase TiO2 single crystals was obtained with X-ray diffraction (XRD) (Rigaku miniFlex-II desktop X-ray diffractometer, Cu KR radiation with λ = 0.154056 nm). Morphology and crystal structure of anatase TiO2 single crystals were examined using scanning electron microscopy (SEM) (Hitachi S-4300 FE-SEM) and transmission electron microscopy (TEM) (JEM 2100F). Samples of anatase TiO2 single crystals were dispersed in deionized water and dropped on a conductive SEM sample holder or a carbon-coated copper grid with irregular holes for electron microscopy TEM analysis. X-ray photoelectron spectroscopy and X-ray diffraction samples were prepared by drying the sedimented particles overnight at 80 °C.
’ RESULTS AND DISCUSSION Effect of Titanium Tetrafluoride Concentration. The synthesis of truncated bipyramids ({001} anatase crystals) is based on the method previously described by Lu et al.25 The morphology and structure of TiO2 product were similarly obtained. In this work, fluorine played a role in reducing the surface energies of anatase crystal, especially {001} facets, and resulted in “truncated” octahedral anatase TiO2 crystals with a high percentage of {001} facets. To test the effect of the titanium source, the TiF4 concentration was 3948
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Crystal Growth & Design varied while fixing the volume of HF (10 wt %) at 0.2 mL. Increasing the TiF4 concentration above 5.33 mM changed the shape from “truncated” octahedral crystals to polycrystalline agglomerates (see Figure 1). When initial F-surrounded seeds were prepared by hydrothermal reaction, F atoms decreased the aggregation of titania nuclei due to its low (lower than of OO and HH in case of O- or H-terminated anatase surfaces) FF bonding energy (D0 = 158.8 kJ mol1). In other words, high bonding energies (D0) of HH (436.0 kJ mol1) and OO (498.4 kJ mol1) contributed to high surface energies of anatase crystals, restricting the formation of large anatase single crystals. At low concentration of TiF4, small amounts of formed small seed particles were completely covered by adsorbed fluorine. But at an increased concentration, large numbers of small seed particles were formed, resulting in a lower fluorine density on the surface, causing small seed particles to diffuse to form larger crystals to reduce the surface area (or surface energy). Bridges between crystals were then formed and gave rise to polycrystalline agglomerates (Scheme 1).36 These results gave us initial evidence that the relative ratio of TiF4/HF in the reaction system is a very critical factor for the formation of different shapes of the TiO2 crystals. Effect of Added Amount of Hydrofluoric Acid Solution. To further illustrate the importance of the relative ratio of TiF4/HF in the reaction system, the amount of HF was varied while fixing the concentration of titanium source precursor at 2.67 mM. Again,
Figure 2. SEM images of anatase single crystals at different additional amounts of HF (10 wt %) (a) 0.2 mL, (b) 0.3 mL, (c) 0.4 mL, and (d) 0.5 mL. Experimental conditions: titanium tetrafluoride (TiF4) aqueous solution (2.67 mM) at 180 °C for 10 h in autoclave. Scale bar is 2.5 μm.
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the aggregation of anatase crystals was easily observed at the low amount of acid (Figure 2a). Increasing the acid volume suppressed the agglomeration (Figure 2b,c) and decreased the diameter of anatase crystals along the z-axis. A continuous increase of the amount of hydrofluoric acid led to destruction of anatase TiO2 crystals as pictured in Figure 2d. This effect can be attributed to the etching by fluorine species. Therefore, the relative ratio of TiF4/HF is very critical on the morphology and size distribution of titania single crystals. A small amount of HF would cause the lack of protection from the chemisorbed fluorine species, resulting in the formation of polycrystalline agglomerates. A large amount of HF (>0.5 mL) could possibly induce etching by the fluorine species, thereby destroying the crystal structure. The effect of the relative ratio of TiF4/HF in the reaction system was studied by increasing the titanium tetrafluoride concentration and the amount of acid, respectively, to find the optimized conditions for synthesis of “truncated” anatase crystals with a narrow size distribution. Typical SEM images of single crystals with different percentages of {001} facets based on the different reactant concentrations are shown in Figure 3. Anatase single crystals formed with a large amount of HF exhibited a high degree of truncation. This is attributed to the higher fluorine density on the surface, which makes the isotropic growth more obvious. This is remarkably consistent with theoretical prediction25 and can be understood from the viewpoint of shapecontrol chemistry.37,38 The etching effect was suppressed by increasing the concentration of TiF4 from 2.67 mM (Figure 2d) to 5.33 mM (Figure 3b) at a fixed amount of HF, 0.5 mL. This phenomenon can be attributed by changing relative ratio of TiF4/ HF. Higher concentration of TiF4 (or higher ratio of TiF4/HF) helped to suppress the etching effect of fluorine species. Figure 4 shows the morphology of anatase single crystals prepared by large-scale production. In this image, Figure 4a, it is clearly observed that TiO2 single crystals with narrow size distribution were plate-like in shape. This plate-like structure indicates that our titania single crystals have a high percentage of {001} reactive facets. Crystal structure of titanium dioxide product was also defined through the X-ray diffraction (XRD) pattern shown in Figure 4b; all diffraction peaks matched well with the crystal structure of the anatase TiO2 phase (JCPDS No. 21-1272), and no residual Ti phase could be detected. The lower XRD pattern, Figure 4d, shows the crystal structure of anatase “monolayer” prepared by a manual assembly method from titania single crystals on the glass substrate. This method helped to arrange and orient “truncated” crystals on the pretreated substrates by sticky polymeric linkers. Principle and critical factors of the manual assembly method can be found in elsewhere.39 The dominance of the {001} peak and the small peak at the {101}
Figure 3. SEM images of anatase single crystals at different percentages of {001} facets based on different reactant concentrations. Experimental conditions: titanium trichloride (TiCl3) aqueous solutions (5.33 mM) with (a) 0.4, (b) 0.5, and (c) 0.7 mL of HF at 180 °C for 10 h in autoclave. Scale bar is 1 μm. 3949
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Figure 4. SEM image (a) and XRD pattern (b) of typically as-synthesized anatase single crystals at large-scale preparation, (c) image of oriented monolayer of single crystals, and (d) XRD pattern of the monolayer on glass substrate.
Figure 5. Controlled experiments for the synthesis of anatase crystals with adding polyvinylpyrrolidone (PVP, Mw = 10 000) instead of additional HF. SEM images of polycrystalline particles at (a) low and (b) high magnification. Experimental conditions: 8.0 mM titanium tetrafluoride (TiF4) aqueous solution, at 180 °C for 10 h.
position indicate that all single crystals were aligned with their z-axes (or [001] axes) perpendicular to the substrate plane as in Figure 4c. From this result, further photocatalytic applications of single crystal layers with selectively exposed reactive facets would be really promising. As further confirmation, no crystal facet control was observed in the absence of hydrofluoric acid, and only spherical polycrystalline anatase particles were formed (Figure 5). Although the polymeric surfactant was used as a dispersing agent, the spherical polycrystalline aggregates of anatase TiO2 (inset XRD pattern) were prepared instead of octahedral single crystals.40 Hydrofluoric acid is believed to have dual roles here: to retard hydrolysis of the titanium precursor and to reduce surface energy to promote the isotropic growth along the [010] and [100] axes.25 Effect of Different Kinds of Titanium Precursors. When changing the titanium source precursor from titanium tetrafluoride to titanium trichloride, the morphology of anatase single crystals was changed. Instead of the “truncated” octahedral structure a truncated
Figure 6. Typical SEM images of “truncated” anatase single crystals. (a) The side view and (b) top-view morphology of anatase single crystals synthesized with 5.33 mM TiF4 aqueous solution and 0.4 mL of HF (10 wt %) at 180 °C for 8 h. Panels (c) and (d), as in (a) and (b), but for TiCl3 precursor instead.
“rice-like” structure was obtained. The SEM images in Figure 6 show the side- and top-view of anatase single crystals from different precursors, TiF4 (Figure 6a,b) and TiCl3 (Figure 6c,d). The top-view of both types of anatase single crystals do not differ much, with the flat square surface and isosceles trapezoidal surfaces indicating {001} and {101} facets, respectively. Compared to the side-view, it is clearly observed that the truncated tetragonal bipyramids are changed due to the disappearance of the sharp edge, distinguishing between {101} and {101} facets. The disappearance of this edge is caused by formation of an 3950
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Crystal Growth & Design additional quartet column at the center of a particle, separating the truncated bipyramid into two parts located at the ends of the column. The shape of these particles is consistent with the predicted morphology enclosed by {001}, {101}, and {100} facets of anatase as shown in Scheme 2.38 Figure 7a,b shows the bright field TEM image of a truncated “rice-like” TiO2 single crystal and the corresponding selectedarea electron diffraction (SAED) pattern, confirming that the free-standing crystal shows single-crystalline characteristics. As shown in Figure 7a,b, the angles labeled in the corresponding fast-Fourier transform (FFT) calculation are 68.3 ( 0.3° and 21.7 ( 0.3°, which are identical to the theoretical values for the angles between the {101} and {001}, as well as {101} and {100} facets, respectively. The high-resolution TEM (HRTEM) image Scheme 2. Schematic Morphology of Anatase Single Crystals with Different Titanium Precursors
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(Figure 7c) shows the (101), (101), (002), and (100) atomic planes with lattice spacing of 0.35, 0.35, 0.48, and 0.38 nm, respectively. Moreover, the axis of the particle was parallel to the [001] direction, indicating that the elongated particle grows along the [001] direction. The angle between [100] and [001] directions is 90°, in good agreement with the model of truncated “rice-like” anatase TiO2 enclosed by {100} facets projected along the [010] direction (Figure 7d). On the basis of the SEM and TEM images as well as the crystallographic symmetries of anatase, we can easily confirm that the anatase truncated “rice-like” TiO2 single crystals have eight {101} and four {100} exposed facets as well as {001} facets at top/bottom surfaces. The formation of anatase single crystals with exposed {100} facets occurs by the following mechanism. As mentioned above, the theoretical calculations indicate that fluoride ions can markedly reduce the surface energy of the {001} surface to a level lower than that of {101} surfaces. This effect comes from the establishment of a new balance between OO/FO repulsions and TiO/TiF attractions, which stabilize Ti and O atoms on the surfaces.25 This phenomenon is also supposed to have the same effect on other surfaces of anatase TiO2 based on the interaction between fluorine species and the outermost unsaturated Ti and O atoms. The {100} facets with surface energy (0.53 J m2) between those of {001} (0.90 J m2) and {101} (0.44 J m2) facets can be affected by this phenomenon and be exposed. A slow hydrolysis
Figure 7. (a) Typical TEM image of truncated “rice-like” anatase single crystals viewed along the [010] direction; (b) the corresponding SAED pattern confirms the single crystalline character; (c) HRTEM image show crystalline lattice fringes of the {101}, {100}, and {002} planes; (d) projected schematic model of an ideal “rice-like” crystal along the [010] direction; red O2, blue Ti4+. 3951
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Scheme 3. Growing Particles in the Micro “Channels” of CMC Salt
Figure 8. Typical SEM images of “truncated” “rice-like” TiO2 product. Low, high magnification images of sample (a, b) without and (c, d) with addition of CMC. Experimental conditions: 5.33 mM titanium trichloride (TiCl3) aqueous solution, at 180 °C for 10 h. Residual product of CMC was removed at 600 oC for 3 h.
rate is required to provide sufficient time for better packing of the TiOTi bonds to grow well-formed anatase single crystals and more fully adsorb fluoride ions on the surfaces of the TiO2 crystals, reducing the surface energies. In this study, a slow reaction rate was maintained by using TiCl3 as a titanium source. Ti(III) must be oxidized to Ti(IV) in order to construct the TiO2 crystal. The slow oxidation by dissolved oxygen results in the preferential formation of large anatase single crystals. The slow oxidation process from Ti(III) to Ti(IV) occurs via dissolved oxygen:41 Ti3þ þ H2 O f TiOH2þ þ Hþ TiOH2þ þ O2 f TiðIVÞ oxospecies þ O2 f TiO2 The slow oxidation steps from TiOH2+ (III) to Ti (IV) oxo complexes occurred at the rate k3 = 4.25 ( 0.13 M1 s1 (25 °C, I = 1.0, LiCI) much less than the rates of transformations between Ti(IV) oxo complexes themselves.42 In the oxidation process, TiOH2+ is the only reactive species, so the ratedetermining factor is the TiOH2+ (or Ti3+) concentration. Furthermore, when adding HF into the aqueous TiCl3 solution, the change in solution color (from purple to green) suggests that Ti3+ complex of fluoride and possibly chloride has been formed. It is neither complex titanium with chloride only (deep blue compound) nor the colorless complex TiF62. This green complex may further retard the hydrolysis of the titanium precursor. However, the true structure and effect of the green complex need to be further investigated. The final important point is the use of CMC. As previously reported, the CMC molecules have many carboxymethyl side groups which prevent CMC backbones from getting close to each other.43 The solution can be divided into numerous “channels” by the CMC molecules in the reaction system as in Scheme 3 to confine the growth of anatase TiO2 single crystals, leading to the formation of TiO2 single crystals with a narrower size distribution as well as monodispersion (Figure 8). Using an organic compound such as CMC leads to residual products in the autoclaving conditions (black carbon beads), although it can
make TiO2 single crystals more monodispersed. These organic residual compounds can be easily removed at 600 °C for 3 h without altering the morphology. Therefore, CMC is also believed to play a role in suppressing the agglomeration through micro “channels” to obtain monodispersed single crystals, retarding hydrolysis and the diffusion rate of titanium precursor by high viscosity, and promoting the exposure of stabilized {100} facets. It is well-known that the {001} facets are more reactive than stable {101} facets. This high reactivity comes from unsaturated bonding of the atoms on the outermost surface, 5-fold Ti and 2-fold O atoms. In contrast to {101} facets with only 50% five-coordinate Ti (Ti5c) atoms, {001} facets with 100% Ti5c atoms were once considered more reactive in heterogeneous reactions.2024 Beside the reactive {001} facets, the {100} facets also have 100% Ti5c atoms. These facets are presumed to be as reactive as {001} facets in spite of their lower surface energy. Recent theoretical studies have given surprising results that {100} facets are even more reactive than {001} facets in the heterogeneous reaction between carbon dioxide and anatase TiO2 surfaces.44,45 This result leads to great interest in the further preparation, theoretical study, and photocatalytic applications of these {100} facets. Finally, not only low-index facets as {001} or {101} but also high-index ones can be present in the anatase catalytic particles.35 These facets could possibly have promising photocatalytic activities when they are under controlled preparation.
’ SUMMARY AND CONCLUSIONS In summary, a variety of hydrothermal synthesis methods of anatase TiO2 single crystals have been comparatively studied by changing the relative ratio of titanium precursor and HF for different degrees of truncation. Increasing TiF4 results in an increase in agglomeration and size, while increasing HF results in smaller diameter (z-axis) single crystals with greater monodispersity. Consequently, overincreasing HF causes the etching effect of fluorine species. Elongated anatase single crystals with additionally exposed {100} facets were prepared using a TiCl3 precursor. The exposure of {100} facets can be attributed to the slow reaction rate of TiCl3 in the presence of CMC as a dispersing agent. Relevant studies on {100} facets for photocatalytic applications are now in progress with the aim of improving the photocatalytic properties. ’ ASSOCIATED CONTENT
bS
Supporting Information. The surface area calculation of difference facets, additional schematic drawing and the average percentages of different facets in single crystals. This information is available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the Korea Center for Artificial Photosynthesis (KCAP) located at Sogang University funded by the Ministry of Education, Science, and Technology (MEST) through the National Research Foundation of Korea (NRF-2009C1AAA001-2009-0093879). ’ REFERENCES (1) O’Regan, B.; Gr€atzel, B. M. Nature 1991, 353, 737. (2) Karch, J.; Birriger, R.; Gleiter, H. Nature 1987, 330, 556. (3) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (4) Shang, J.; Chai, M.; Zhu, Y.-F. J. Solid State Chem. 2003, 174, 104. (5) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (6) Lao, C.-F.; Chuai, Y.-T.; Su, L.; Liu, X.; Huang, L.; Cheng, H.-M.; Zou, D.-C. Sol. Energy Mater. Sol. Cells 2005, 85, 457. (7) Tian, Z.-R.; Voigt, J. A.; Liu, J.; McKenzie, B.; Xu, H.-F. J. Am. Chem. Soc. 2003, 125, 12384. (8) Chemseddine, A.; Moritz, T. Eur. J. Inorg. Chem. 1999, 2, 235. (9) Li, Y.; White, T. J.; Lim, S. H. J. Solid State Chem. 2004, 177, 1372. (10) Wang, W.; Gu, B.-H.; Liang, L.-Y.; Hamilton, W. A.; Wesolowski, D. J. J. Phys. Chem. B 2004, 108, 14789. (11) Jiang, X.-C.; Herricks, T.; Xia, Y.-N. Adv. Mater. 2003, 15, 1205. (12) Cozzoli, P. D.; Kornowski, A.; Weller, H. J. Am. Chem. Soc. 2003, 125, 14539. (13) Wen, B.-M.; Liu, C.-Y.; Liu, Y. Chem. Lett. 2005, 34, 396. (14) Wen, B.-M.; Liu, C.-Y.; Liu, Y. New J. Chem. 2005, 29, 969. (15) Yang, S.-W.; Gao, L. Chem. Lett. 2005, 34, 964. (16) Yang, S.-W.; Gao, L. Chem. Lett. 2005, 34, 972. (17) Macak, J. M.; Tsuchiya, H.; Schmuki, P. Angew. Chem., Int. Ed. 2005, 44, 2100. (18) Somorjai, G. A. Chem. Rev. 1996, 96, 1223. (19) Seker, F.; Meeker, K.; Kuech, T. F.; Ellis, A. B. Chem. Rev. 2000, 100, 2505. (20) Lazzeri, M.; Vittadini, A.; Selloni, A. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 65, 119901. (21) Vittadini, A.; Selloni, A.; Rotzinger, F. P.; Gratzel, M. Phys. Rev. Lett. 1998, 81, 2954. (22) Herman, G. S.; Dohnalek, Z.; Ruzycki, N.; Diebold, U. J. Phys. Chem. B 2003, 107, 2788. (23) Tilocca, A.; Selloni, A. J. Phys. Chem. B 2004, 108, 19314. (24) Selloni, A. Nat. Mater. 2008, 7, 613. (25) Yang, H. G.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H. M.; Lu, G. Q. Nature 2008, 453, 638. (26) Yang, H. G.; Liu, G.; Qiao, S. Z.; Sun, C. H.; Jin, Y. G.; Smith, S. C.; Zou, J.; Cheng, H. M.; Lu, G. Q. J. Am. Chem. Soc. 2009, 131, 4078. (27) Han, X.; Kuang, Q.; Jin, M.; Xie, Z.; Zheng, L. J. Am. Chem. Soc. 2009, 131, 3152. (28) Dai, Y.; Cobley, C. M.; Zeng, J.; Sun, Y. M.; Xia, Y. Nano Lett. 2009, 9, 2455. (29) Amano, F.; Prieto-Mahaney, O.; Terada, Y.; Yasumoto, T.; Shibayama, T.; Ohtani, B. Chem. Mater. 2009, 21, 2601. (30) Zhang, D.; Li, G.; Yang, X.; Yu, J. C. Chem. Commun. 2009, 4381. (31) Liu, M.; Piao, L.; Zhao, L.; Ju, S.; Yan, Z.; He, T.; Zhou, C.; Wang, J. Chem. Commun. 2010, 46, 1664. (32) Zhang, D.; Li, G.; Wang, H.; Chan, K. M.; Yu, J. C. Cryst. Growth Des 2010, 10, 1130. (33) Ma, X. Y.; Chen, Z. G.; Hartono, S. B.; Jiang, H. B.; Zou, J.; Qiao, S. Z.; Yang, H. G. Chem. Commun. 2010, 46, 6608.
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