J. Phys. Chem. C 2007, 111, 18563-18567
18563
A Crucial Step to Platinum Nanocrystals with Special Surfaces: Control of Aquo/Chloro Ligand Exchange in Aqueous PtCl62- Solution Ying-Tao Yu,*,†,‡ Jing Wang,† Jun-Hai Zhang,† Hai-Jun Yang,‡ Bo-Qing Xu,*,‡ and Jun-Cai Sun† College of EnVironmental Science and Engineering, Dalian Maritime UniVersity, Dalian 116026, China, and Key Lab of Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua UniVersity, Beijing 100084, China ReceiVed: February 12, 2007; In Final Form: August 13, 2007
Platinum (Pt) nanocrystals with special surfaces attract much attention in the field of chemistry because the surface structure plays a vital role for the activity and selectivity of Pt catalyst. The surface evolution of Pt nanocrystals is closely related with facet growth, which depends on not only the oxidizing property of Pt precursor but also the competitiveness of precursor adsorption on the nanocrystal surface. Besides precursor/ protector ratio, the charge of precursor also influences its competitive adsorption on Pt nanocrystals since metal nanocrystals smaller than 10 nm have a strong tendency to keep electroneutrality. By using blue or violet light, the control of aquo/chloro ligand exchange of PtCl62- is attempted with a 1.5 × 10-4 mol‚dm-3 K2PtCl6 aqueous solution at room temperature. First derivative spectra of UV absorption, X-ray photoelectron spectra (XPS), and 195Pt nuclear magnetic resonance (NMR) spectra reveal that the aquo/chloro ligand exchange between PtCl62- and H2O is dominant; hydroxyl/chloro exchange and the reduction is not significant under the investigated conditions. The influence of visible light on the stability of PtCl62- in aqueous solution is remarkably dependent on the wavelength, which is associated with the spin-forbidden (465 nm) or spinallowed (483 nm) d-d electron transition of PtCl62-. The measurements of open-circuit potential (OCP) and di/dE-E plots suggest that the oxidizing property of the studied K2PtCl6 solution is enhanced due to the aquo/chloro ligand exchange of PtCl62-. The electroneutral product of the aquo/chloro ligand exchange, PtCl4(H2O)2, can be easily adsorbed on Pt nanocrystals and probably be more reducible under H2 reduction than the parent PtCl62- ions, which effects a faster growth of Pt{111} and results in a selective formation of cubic Pt nanocrystals enclosed with {100} facets.
Introduction Specially shaped Pt nanocrystals enclosed with particular facets attract much attention in the field of chemistry because their catalytic activities are significantly dependent on the surface structure.1 These Pt nanocrystals are mainly synthesized by H2 reduction of Pt precursors in aqueous solutions containing protective agents.2 Hitherto, factors that affect the surface evolution and shape development of Pt nanocrystals have been documented.2 According to the overlapping model, a faster growing facet gradually disappears along with the size increase of the crystal because interfacial angles of external facets tend to keep constant; eventually, the crystal is enclosed with slow growing facets and exhibits a special shape.3 For instance, Pt{111} is more efficient than Pt{100} in catalyzing the H2 reduction of the Pt precursor and therefore has a tendency for faster growth, which benefits the formation of cubic Pt nanocrystals enclosed with {100}.2b On the other hand, the formation of a {111} surface was observed on freezing when a melt of fcc solid was slowly cooled; in contrast, a rapid cooling resulted in the formation of a {100} surface.4 Further, tetrahedral clusters were generated by laser evaporation of pure Au in a He atmosphere, during which neither reductant nor protective agent was used.5 These observations seem to hint that, for a nanocrystal with fcc structure (e.g., Pt, Au, Ag, Cu, Ni, Pd, and Rh), a slow facet growth might be favorable to the formation of a minimum free energy surface, {111}. * Corresponding authors. E-mail:
[email protected]; bqxu@ mail.tsinghua.edu.cn. † Dalian Maritime University. ‡ Tsinghua University.
Although the surface evolution of Pt nanocrystals has not been entirely understood, it is generally agreed that a fine control of the facet growth would be a crucial step. K2PtCl4 and K2PtCl6 are common and cheap Pt precursors, and H2 has been a preferred reductant because its oxidation product (H2O) brings little effect on crystal growth in aqueous solution.2 As abovementioned, Pt facets are known to catalyze the H2 reduction of the precursor ions adsorbed on the nanocrystal surface.2b Meanwhile, the competitive adsorption on the nanocrystal surface between the precursor and polymer protector is believed to greatly influence the facet growth and shape development process. For instance, Ahmadi and co-workers observed that a higher ratio of protector/precursor seemed to benefit the synthesis of tetrahedral Pt nanocrystals in H2 reduction of K2PtCl4 solution using NaPA for the protector while a lower protector/precursor ratio favors the formation of cubic ones.2a Recently, we also reported the effect of PVP/K2PtCl6 ratio on the shape evolution of the Pt nanocrystal in H2 reduction.2e,h On the other hand, the charges of species can also be expected to remarkably influence the competitive adsorption on the Pt nanocrystal surface because metal nanocrystals smaller than 10 nm have a strong tendency to keep electroneutrality (Kubo effect).6 For this reason, the aquo/chloro ligand exchange of PtCl42- or PtCl62-, as leading to the variation of both chemical composition and precursor charge, might affect not only the oxidizing character of the precursor but also the competitive adsorption on the preformed Pt nanocrystal. Nevertheless, the hydrolysis of PtCl42- is extensive and difficult to manage in aqueous solution while that of PtCl62- is very slow under otherwise equal conditions.7
10.1021/jp071216r CCC: $37.00 © 2007 American Chemical Society Published on Web 11/29/2007
18564 J. Phys. Chem. C, Vol. 111, No. 50, 2007
Yu et al.
To the best of our knowledge, fine control of aquo/chloro ligand exchange of PtCl62- precursor on the formation of specially shaped Pt nanocrystals has not been investigated in the open literature. Rich and Taube8 have observed that the ligand exchange between Cl- in PtCl62- and free Cl- in solution can be accelerated by visible light. In this paper, the control of aquo/chloro ligand exchange of the PtCl62- ions is attempted by using violet, blue, green, and yellow light, respectively. It is found that either violet or blue light can efficiently weaken the characteristic LMCT absorption of PtCl62-. First derivative spectra of UV absorption, X-ray photoelectron spectra (XPS), and 195Pt nuclear magnetic resonance (NMR) spectra are used to characterize the aquo/chloro or hydroxyl/chloro ligand exchange performance and to show whether PtCl62- is reduced or not under the investigated conditions. Open-circuit potential (OCP) and di/dE-E plots are employed to trace the variation of oxidizing character of the Pt precursor caused by the aquo/ chloro ligand exchange, whose influence on the facet growth of Pt nanocrystals is also discussed. Experimental Methods K2PtCl6 and Na2PtCl6‚6H2O were of analytical grade and purchased from Shanghai Reagent Co. and were used without further purification. D2O (99.8%) was purchased from Beijing Hanweishi Reagent Co. Sodium polyacrylate (NaPA, Mw ) 2100, Fluka) was of analytical grade. Milli-Q pure water (18.2 MΩ‚cm) was used in solution preparation. Ultraviolet (UV) spectra were obtained from a JASCO-V550 UV-vis spectrometer (step: 0.2 nm), and photoluminescence (PL) spectra were taken by using a Beijing Hanguang 500 optical spectrograph (step: 0.5 nm). The violet (415 nm, peak half-width 12 nm, 11 mW), blue (477 nm, peak half-width 22 nm, 22 mW), green (528 nm, peak half-width 32 nm, 12 mW), and yellow light (610 nm, peak half-width 16 nm, 20 mW) were obtained from four LED light sources, respectively. NMR spectra were measured by using a JOEL JNM ECA-600 instrument, during which D2O was used to lock the field. XPS spectra were obtained by using a PHI-5300 ESCA spectrometer. Open circuit potential (OCP) and di/dE-E plots were measured by an IM6e electrochemical working station in a three-electrode system without exposure to light. Glass carbon, Pt wire, and Ag/AgCl were employed as working, auxiliary, and reference electrode, respectively. No supporting electrolyte was added during the measurements. The potential of working electrode was linearly decreased, at a rate of dE/dt ) -20 mV/s. Before each measurement, the glass carbon electrode (diameter ) 3 mm) was polished with chamois and then washed ultrasonically with ethanol (5 min) and pure water (15 min) in turn; highpurity N2 was bubbled through the solution for 10 min to remove the dissolved oxygen, and then the electrochemical cell was sealed for the measurement. For the synthesis of Pt nanocrystals, the solution containing 1.05 g‚dm-3 NaPA and 1.0 × 10-4 mol‚dm-3 K2PtCl6 was bubbled with high-purity Ar (20 min) and H2 (10 min) in turn and then sealed for 18 h. The NaPA was added to the K2PtCl6 solution just before the synthesis of Pt nanocrystals, and pH was adjusted to 7.0 with dilute HCl. All these operations were prevented from exposure to light by enclosing the glassware with Al foil. Fresh K2PtCl6 solution was used to prepare truncated-octahedral Pt nanocrystals.7b In the synthesis of cubic Pt nanocrystals, the aquo/chloro ligand exchange of PtCl62- had occurred before H2 reduction and the characteristic absorption of PtCl62- at 263 nm was hardly observable.7b The Pt colloid was dropped onto a carbon-coated Cu grid and then dried in
Figure 1. (a) UV and (b) dA/dλ spectra of a 1.5 × 10-4 mol‚dm-3 K2PtCl6 solution irradiated by the blue light (477 nm) for different periods.
air without exposure to light. The morphology of Pt nanocrystals was examined with a transmission electron microscope (TEM) at 200 kV on a JEM-200CX. High-resolution TEM (HRTEM) images of Pt nanocrystals were obtained by using a JEM-2010F equipped with a Link ISIS-300 detector. Results and Discussion The aqueous solution of 1.5 × 10-4 mol‚dm-3 K2PtCl6 proved fairly stable, and no change of UV spectra was detected after several days of aging in dark. To study the effect of light on the stability of this solution, four light sources (violet, blue, green, and yellow) were used to irradiate a 1.5 × 10-4 mol‚dm-3 K2PtCl6 solution, respectively. The photoluminescence spectra are given as Figure 5 in the SI (Supporting Information). No change of UV absorption occurred in the solution irradiated by the yellow light despite a prolonged irradiation time. On the contrary, the violet and blue lights both remarkably weakened the characteristic LMCT absorption (263 nm) of PtCl62- in 4 or 5 h. Comparably, only a very slight weakening of the UV absorption was detected when the solution was irradiated by the green light for 48 h. These results indicate that the effect of visible light on the PtCl62- ions in aqueous solution is greatly dependent on the wavelength, which might be related with the spin-forbidden (465 nm) or spin-allowed (483 nm) d-d electron transition9 of the PtCl62- ions. Figure 1a presents the UV absorption spectra of 1.5 × 10-4 mol‚dm-3 K2PtCl6 solution irradiated by the blue light for different periods while Figure 1b presents the first derivative
Platinum Nanocrystals with Special Surfaces
Figure 2. 195Pt NMR spectra of a 0.12 mol‚dm-3 Na2PtCl6 solution and that irradiated by the violet light for (a) 0, (b) 8, and (c) 56 h.
(dA/dλ) spectra. As shown in Figure 1a, the characteristic absorption peak at 263 nm was gradually weakened along with the irradiation. There may be two possibilities for this observed variation in UV spectra: (1) PtCl62- is reduced to a trivalent or bivalent Pt complex; (2) H2O and/or OH- in the solution exchanges with the Cl- in PtCl62- ions. On the other hand, a yellowish floc was observed to form in 1.0 × 10-4 mol‚dm-3 K2PtCl6 aqueous solution that had been kept in glass bottle and exposed to ambient light for 2 months. XPS measurement (Figure 6 in the SI) showed that the ratio of Cl/Pt in the floc was much lower than that in K2PtCl6 powder. Also, no K element was detected in the XPS spectra of the floc. Meanwhile, the binding energy of Pt element in the floc was similar to that in K2PtCl6, as greatly different from that of K2PtCl4. Because trivalent Pt complex (PtCl52-) would be short-lived in aqueous solution and, if formed, can immediately transfer into PtII and PtIV complexes via disproportionation,10 the formation of either trivalent or bivalent Pt complex seems to be of little possibility in the present system, which will be further checked by NMR measurement as presented in Figure 2. Consequently, the variation of UV spectra in Figure 1a is mainly attributed to the aquo/chloro or OH-/Cl ligand exchange of PtCl62-. Since the O atom in H2O has a stronger attractive force to electrons than Cl-, the aquo/chloro ligand exchange of PtCl62- is expected to induce an energy increase for the electron transition from the ligand orbital to that of Pt4+, which is consistent with the known fact that PtCl62-, PtCl5(H2O)-, and PtCl4(H2O)2 complexes show absorption peaks at 263, 255, and 235 nm, respectively.11 On the contrary, because the attractive force of OH- to electons is weaker than that of Cl-, the OH-/ Cl- ligand exchange of PtCl62- would lead to a shift of LMCT absorption toward longer wavelength, which is supported by the fact that the absorption peak wavelength of PtCl4(OH)22(274 nm) is longer than that of PtCl62- (263 nm).11 For this reason, the variation of absorption spectra at wavelength longer than 263 nm can be used to judge whether OH-/Cl exchange occurs during the irradiation. The first derivative spectra in Figure 1b, which is more efficient to distinguish overlapping absorptions, show that the negative peak of LMCT absorption appeared at 276 nm before the irradiation (0 h) and gradually shifted to shorter wavelength (271.8 nm, 24 h) during the irradiation. This observation suggests that the aquo/chloro (i.e., H2O/Cl-) ligand exchange of PtCl62- should dominate the chemistry under the conditions of this study. On the other hand, the 195Pt chemical shift in NMR spectra is remarkably influenced by the composition of coordination sphere and the valence of central platinum ion. Both H2O/Cland OH-/Cl- ligand exchange of PtCl62- can cause the 195Pt
J. Phys. Chem. C, Vol. 111, No. 50, 2007 18565 chemical shift toward lower field, but the latter leads to much larger shift than the former.12 Alternatively, the reduction of PtCl62- to PtCl42- results in a significant shift toward higher field. Typically, the characteristic signals of PtCl62-, PtCl5(H2O)-, PtCl4(H2O)2, PtCl5(OH)2-, PtCl4(OH)22-, and PtCl42- appear at 0, 504, 1029, 666, 1263, and -1664 ppm, respectively.12 We prepared a D2O solution of 0.12 mol‚dm-3 Na2PtCl6 and then exposed this solution to the violet light. Figure 2a-c presents the 195Pt NMR spectra of the solution after it was irradiated for 0, 8, and 56 h, respectively. As shown, only the signal of PtCl62(0 ppm) appeared in the measurement of fresh solution (0 h); in addition to the signal of PtCl62- (0 ppm), a second signal was detected at 508 ppm in the solution irradiated for 8 h (Figure 2b), and the area ratio of this signal to that at 0 ppm for the parent PtCl62- ions. Due to the effect of ligand exchange between D2O and Cl- on 195Pt chemical shift being similar to that of H2O/Cl- exchange, the appearance of the second signal is ascribed to the formation of PtCl5(D2O)-. Further, when the solution was irradiated for a period as long as 56 h (Figure 2c), it showed a 195NMR spectrum very similar to that of the solution irradiated for 8 h (Figure 2b); No signal for PtCl4(D2O)2 was detected in the range 700-1300 ppm (not shown). Thereafter, the 0.12 mol‚dm-3 Na2PtCl6 solution irradiated for 8 and 56 h were diluted to 2.0 × 10-4 mol‚dm-3, respectively. UV spectra (Figure 7 in SI) showed that the characteristic absorptions of PtCl62- were maintained in the diluted solutions. These results indicate that the ligand exchange reaction has reached equilibrium among PtCl62-, D2O, PtCl5(D2O)-, and Cl- in the concentrated solution of Na2PtCl6. And, the first equilibrium constant, K1, can be calculated as follows: K1 ) [PtCl5(D2O)-] × [Cl-]/[PtCl62-] ) 0.004, where the brackets represent concentrations of the bracketed species. Because of their charge difference, PtCl5(D2O)- has a weaker repulsive force to Cl- than PtCl62-. Hence, the secondary equilibrium constant of the H2O/Cl- ligand exchange (K2 ) [PtCl4(D2O)2] × [Cl-]/[PtCl5(D2O)-]) should be lower than K1. And the following equations can be obtained:
[PtCl5(D2O)-]/[PtCl62-] ) K1/[Cl-]
(1)
[PtCl4(D2O)2]/[PtCl62-] ) K1K2/[Cl-]2
(2)
Herein, the high concentration (0.02 mol‚dm-3) of Clproduced during the first H2O/Cl- ligand exchange is believed to inhibit further ligand exchange, which accounts for that no signal of PtCl4(D2O)2 was detected in the 195Pt NMR spectra (Figure 2: b and c). Alternatively, when the initial concentration of PtCl62- is decreased to 1.5 × 10-4 mol‚dm-3, both [PtCl5(D2O)-]/[PtCl62-] and [PtCl4(D2O)2]/[PtCl62-] can remarkably increase at the equilibrium of ligand exchange because the concentration of Cl- produced is greatly decreased. In addition, no signal was observed in the range 600-700 ppm (Figure 2b,c), indicating little formation of PtCl5(OD)2-. Further, no signal was detected between -700 and -2200 ppm (not shown), suggesting that neither PtCl42- nor its hydrolysis products existed in the solution. Therefore, the irradiation by violet light should mainly result in the H2O/Cl- ligand exchange of PtCl62-. Since the wavelength of violet light is the shortest in this study, we conclude that the other visible lights of lower energies or longer wavelengths employed in this work should also not cause a reduction of the PtCl62- ions. If we recall that Zou et al.13 have observed that PtCl62- can be reduced by UV light, the influence of light on PtCl62- in aqueous solution is believed to greatly depend on the wavelength.
18566 J. Phys. Chem. C, Vol. 111, No. 50, 2007
Figure 3. di/dE-E plots of 1.5 × 10-4 mol‚dm-3 K2PtCl6 solutions after exposure to violet light irradiation for different periods.
Due to the Kubo effect,6 the electroneutral product of H2O/ ligand exchange, PtCl4(H2O)2, should more easily adsorb on Pt nanocrystals than PtCl62-. Meanwhile, the change of coordination structure of the precursor might also influence the oxidizing character of the precursor. Electrochemical measurement shows that the open circuit potentials (OCP) of the solutions exposed to the violet light for 0, 3, 6, 9, 12, and 15 h were 0.596, 0.660, 0.735, 0.740, 0.737, and 0.737 V (vs NHE), respectively. Such an increase of OCP suggests a formation of species with oxidizing character stronger than [PtCl6]2-. If we recall that OCP is more influenced by the species with a high exchange current,14 the exchange current of the species with Cl-
Yu et al. stronger oxidizing character should be much higher than that of [PtCl6]2-, which explains why the OCP increased rapidly during the first 6 h exposure to the violet light while a thereafter prolonged exposure had a little consequence on the OCP. On the other hand, UV spectra (Figure 8 in the SI) indicate that the increase of UV absorption at 235 nm was also remarkable during the first 6 h exposure to the violet light and then became insigificant with the prolonged exposure. Considering that the absorption peak of PtCl4(H2O)2 is at 235 nm, it is reasonable to assume that the species associated with the higher exchange current and stronger oxidizing character is PtCl4(H2O)2. Figure 3 presents the di/dE-E plots of 1.5 × 10-4 mol‚dm-3 K2PtCl6 solutions after exposure to the violet light for different times. As shown, the di/dE of the fresh solution increased slightly along with the decrease of potential and exhibited a broad peak at ca. -0.2 V (vs NHE). In contrast, a new but also broad peak appeared at ca. 0.3 V (vs NHE) in the di/dE-E plots of the solution irradiated for 3 h. And, this newly developed peak became stronger when the irradiation time was extended to 6 h or longer. During a linear potential scan, both electrochemical reaction and double layer charging would contribute to the cathode current. To judge whether the new peak at 0.3 V (vs NHE) was caused by any electrochemical reaction, the di/ dE-E plots of 5.0 × 10-2 mol‚dm-3 KClO4 was examined under the otherwise indentical conditions. As shown in Figure 9 in the SI, the above-mentioned new peak did not appear. Therefore, the new peak at 0.3 V (vs NHE) should result from an electrochemical reaction of the precursor ions, which hints at a facile reduction of the product of H2O/Cl- ligand exchange. The difficulty in reduction of PtCl62- might be partly caused by its charge (-2), which inhibits the acceptance of electrons
Figure 4. TEM images and size distributions of Pt nanocrystals synthesized by H2 reduction of K2PtCl6 solution in the presence of NaPA: (a) fresh K2PtCl6 solution; (b) K2PtCl6 solution hydrolyzed to such a degree that the absorption of PtCl62- had almost disappeared. Scale bar: 25 nm.
Platinum Nanocrystals with Special Surfaces from the cathode; if one recalls the increase of OCP as a consequence of increasing the irradiation time under the violet light, another possibility could be that the oxidizing character of PtCl62- is relatively weak. Although the electrochemical reduction of Pt precursors cannot entirely simulate their reduction by H2, these results seem to partly support the assumption that the product of H2O/Cl- ligand exchange could be more easily reduced than PtCl62-. Figure 4a shows a representative TEM image of Pt nanocrystals produced by H2 reduction of a hydrolyzed 1.0 × 10-4 mol‚dm-3 K2PtCl6 solution in the presence of 1.05 g‚dm-3 NaPA protector. The hydrolysis of the K2PtCl6 solution went to such an extent that the absorption peak of PtCl62- at 263 nm had almost disappeared before the H2 reduction. It can be found that cubic Pt nanocrystals enclosed with {100} facets were selectively produced (Figure 10 in the SI shows the HRTEM pattern and the EDS spectra of a cubic Pt nanocrystals). For comparison, Figure 4b presents the result of H2 reduction of a fresh K2PtCl6 solution under the otherwise identical conditions, in which truncated-octahedral Pt nanocrystals were dominant. These results suggest that, although Pt{111} facet is more efficient in catalyzing H2 reduction of Pt precursors, the charge of the parent PtCl62- ions might remarkably inhibit its adsorption on Pt nanocrystal surface; in addition, the low reactivity of PtCl62- might also prevent it from reduction. Therefore, the difference of growth rate between {111} and {100} facet is dwindled, which leads to the formation of truncated-octahedral Pt nanocrystals enclosed by both {111} and {100}. Alternatively, the electroneutral hydrolysis product, PtCl4(H2O)2, can more easily adsorb on Pt nanocrystal and probably be easier to reduce, which might benefit to a faster growth of Pt{111} in H2 reduction and promote the evolution to cubic shape. As shown, the truncated-octahedrons in Figure 4b are generally smaller than the cubes in Figure 4a. Also, the sizes of truncated octahedral crystals were clearly smaller than those of the cubic ones in the earlier reports.2a,2b These data are at least partly in support of the difference in crystal growth of the Pt{111} and {100} facets. In recent years, the syntheses of tetrahedral, cubic, rodlike, prismatic, belt, and disklike nanocrystals have been reported for metals in subgroups VIIIB (Pt, Pd, Ni, Rh, Co, Fe) and IB (Au, Ag, Cu).15 Except for Co and Fe, most of these transition metals are thermodynamically stable with a face-centered cubic (fcc) structure. It is believed that the control of the aquo/ligand exchange of the metal precursors can also be important for the facet growth and shape development of these fcc metal nanocrystals. Conclusions Besides the ratio of precursor/protector, the control of aquo/ chloro ligand exchange of PtCl62- plays an important role in effecting the shape evolution of Pt nanocrystals from the H2 reduction of PtCl62- in aqueous solution, which can be achieved by proper irradiation with visible light (blue or violet). The influence of visible light on stability of PtCl62- in aqueous solution remarkably depends on the light wavelength, which is associated with the spin-forbidden (465 nm) or spin-allowed (483 nm) d-d electron transition of PtCl62-. The hydroxy/chloro exchange of PtCl62- is insignificant, and no reduction of PtCl62-
J. Phys. Chem. C, Vol. 111, No. 50, 2007 18567 is detected after the light irradiation. The aquo/chloro ligand exchange of PtCl62- can lead to the formation of PtCl5(H2O)and PtCl4(H2O)2 that would be easily adsorbed on Pt nanocrystals and probably have also stronger oxidizing characters, both of which are believed to accelerate the growth of Pt{111} facet and promote a selective formation of Pt nanocrystals enclosed with {100} facets. Acknowledgment. This work is partly supported by the NSF China(Grant: 20573062),MOSTofChina(Grant: 2006AA03Z225), and the BSRF National Lab Fund (Grant: Sr-06001). Supporting Information Available: Photoluminescence (PL), XPS, and UV spectra, di/dE-E plots, an HRTEM pattern, and EDS spectra (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Lyuksyutov, I.; Naumovets, A.-G.; Pokrovsky, V. TwoDimensional Crystal; Academic: Boston, MA, 1992; pp 349-379. (b) Stamenkovic´, V.; Markovic´, N. M. Langmuir 2001, 17, 2388-2394. (c) Balint, I.; Miyazaki, A.; Aika, K. A. Appl. Catal., B 2002, 37, 217-229. (d) Desai, S. K.; Neurock, M.; Kourtakis, K. J. Phys. Chem. B 2002, 106, 2559-2568. (d) Vidal, F.; Busson, B.; Six, C.; et al. Surf. Sci. 2002, 502503, 485-489. (2) (a) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; et al. Science 1996, 272, 1924-1926. (b) Petroski, J. M.; Wang, Z. L.; Green, T. C.; et al. J. Phys. Chem. B 1998, 102, 3316-3320. (c) Miyazaki, A.; Nakano, Y. Langmuir, 2000, 16, 7109-7111. (d) Fu, X.; Wang, Y.; Wu, N.; et al. Langmuir 2002, 18, 4619-4624. (e) Yu, Y. T.; Xu, B. Q. Chinese Sci. Bull. 2003, 48, 2589-2593. (f) Yu, Y. T.; Xu, B. Q. Chem. J. Chinese UniV. 2004, 25, 2384-2386. (g) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 12663-12676. (h) Yu, Y. T.; Xu, B. Q. Appl. Organomet. Chem. 2006, 20, 638-647. (3) (a) Mullin, J.-W. Crystallization, 3rd ed.; Butterworth: Oxford, U.K., 1993; pp 172-263. (b) Yu, Y. T.; Zhang, Q. H.; Xu, B. Q. Prog. Chem. 2004, 16, 520-527. (4) Goodman, R. M.; Somorjai, G. A. J. Chem. Phys. 1970, 52, 63256331. (5) Li, J.; Li, X.; Zhai, H. J.; et al. Science 2003, 299, 864-867. (6) Kubo, R. J. Phys. Soc. Jpn. 1962, 17, 975-986. (7) (a) Cotton, F. A.; Wilkinson, G.; Murilto, C. A.; et al. AdVanced Inorganic Chemistry; New York: Wiley, 1999; p 1071. (b) Yu, Y. T.; Xu, B. Q. Acta Chim. Sin. 2003, 61 (11), 1758-1764. (8) Rich, R. L.; Taube, H. J. Am. Chem. Soc. 1954, 76, 2609-2611. (9) Swihart, D. L.; Mason, W. R. Inorg. Chem. 1970, 9, 17491757. (10) Znakovskaya, I. V.; Sosedova, Y. A.; Glebov, E. M.; et. al. Photochem. Photobiol. Sci. 2005, 4, 897-902. (11) (a) Mang, T.; Breitscheidel, B.; Polanek, P.; et al. Appl. Catal., A 1993, 106, 259-273. (b) Shelimov, B.; Lambert, J. F.; Che, M. J. Catal. 1999, 185, 462-478. (12) Shelimov, B.; Lambert, J. F.; Che, M.; et. al. J. Am. Chem. Soc. 1999, 121, 545-556. (13) Zou, Z.; Arakawa, H. J. Photochem. Photobiol., A 2003, 158, 145162. (14) (a) Zha, Q. X. An Introduction to Kinetics on Electrode, 3rd ed.; Science Press: Beijing, 2002; pp 141-142. (b) Bard, A. J.; Faulkner, L. R. (Sao, Y. H.; Zhu G. Y.; Dong X. D.; et. al. (Translators)). Electrochemical methods, 2nd ed.; Chemical Industry Press: Beijing, 2005; pp 394-395. (15) (a) Selven, S. T.; Hayakawa, T.; Nogami, M. et al. J. Phys. Chem. B 1999, 103, 7441-7448. (b) Bradley, J. S.; Tesche, B.; Busser, W.; et al. J. Am. Chem. Soc. 2000, 122, 4631-4636. (c) Jana, N. R.; Wang, Z. L.; Sau, T. K.; et al. Res. Commun. 2000, 79, 1367-1370. (d) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115-2117. (e) Sun, Y.; Xia, Y. Science 2002, 298, 2176-2179. (f) Chen, J.; Herricks, T.; Geissler, M.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 10854-10855. (g) Herricks, T.; Chen, J.; Xia, Y. Nano Lett. 2004, 4 (12), 2367-2371. (h) Dumestre, F.; Chaudret, B.; Amiens, C.; et al. Science 2004, 303, 821823. (i) Hoefelmeyer, J. D.; Niesz, K.; Somorjai, G. A.; et al. Nano Lett. 2005, 5, 435-438.
