Letter pubs.acs.org/NanoLett
Ligand-Exchange Assisted Formation of Au/TiO2 Schottky Contact for Visible-Light Photocatalysis Dawei Ding,† Kai Liu,† Shengnan He,† Chuanbo Gao,*,† and Yadong Yin*,‡ †
Center for Materials Chemistry, Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an, Shaanxi 710054, China ‡ Department of Chemistry, University of California, Riverside, California 92521, United States S Supporting Information *
ABSTRACT: Plasmonic noble metal nanoparticles have emerged as a promising material in sensitizing wide-bandgap semiconductors for visible-light photocatalysis. Conventional methods in constructing such heterocatalysts suffer from either poor control over the size of the metal nanoparticles or inefficient charge transfer through the metal/semiconductor interface, which limit their photocatalytic activity. To solve this problem, in this work we construct Au/TiO2 photocatalysts by depositing presynthesized colloidal Au nanoparticles with wellcontrolled sizes to TiO2 nanocrystals and then removing capping ligands on the Au surface through a delicately designed ligand-exchange method, which leads to close Au/ TiO2 Schottky contact after a mild annealing process. Benefiting from this unique synthesis strategy, the obtained photocatalysts show superior activity to conventionally prepared photocatalysts in dye decomposition and water-reduction hydrogen production under visible-light illumination. This study not only opens up new opportunities in designing photoactive materials with high stability and enhanced performance for solar energy conversion but also provides a potential solution for the well-recognized challenge in cleaning capping ligands from the surface of colloidal catalyst nanoparticles. KEYWORDS: Schottky junction, gold nanoparticle, photocatalysis, visible light, hydrogen production
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The absence of organic capping ligands facilitates formation of metal/semiconductor Schottky contact and thus favorable electron transfer through this interface for photocatalytic reactions. However, these methods often appear to be ineffective in controlling the size and morphology of the noble metal nanoparticles, which are critical factors for their photocatalytic activities. However, recent advances in colloidal synthesis have enabled precise control over the shape and size of noble metal nanoparticles and thus systematic tuning of their SPR properties.22−27 Unfortunately, deposition of these preformed colloidal nanoparticles onto semiconductor nanoparticles to form an effective heterogeneous photocatalyst has not been extensively explored. This can be largely attributed to the detrimental effect of capping ligands to the efficient transfer of photoelectrons and holes. These organic ligands are required in colloidal synthesis for stabilizing noble metal nanoparticles, but they remain at the metal/semiconductor interface after deposition and form an insulating layer that prevents the formation of effective Schottky contact.28,29 Calcination is a reliable pathway to completely remove the organic ligands, which, however, requires a high temperature and thus leads to a
emiconductor-based solar energy conversion has attracted increasing interest in the past decades due to its great potential in renewable energy creation and environmental remedies. However, conventional processes generally suffer from limited absorption of solar energy in the visible range, which suppresses the overall photocatalytic efficiency. Substantial efforts have been made to expand the energy utilization to the visible range of the solar spectrum, including dyesensitization, band engineering of wide-bandgap semiconductors, and development of narrow-bandgap semiconductors.1−7 Recently, it was discovered that plasmonic noble metal nanoparticles, Au and Ag, for example, are capable of sensitizing wide-bandgap semiconductors such as titania (TiO2), exhibiting much enhanced visible-light response.8−13 In brief, strong surface plasmon resonance (SPR) is resulted from visible-light illumination, and during its dephasing, hot electrons are injected via a Schottky junction into the conduction band of the semiconductor for reduction reactions, leaving holes in the noble metal nanoparticles where oxidation reaction occurs.14−16 This novel mechanism opens up new opportunities for the development of visible-light active photocatalysts with high efficiency and stability. Conventionally, plasmonic noble metal nanoparticles can be deposited onto semiconductor nanoparticles by deposition− precipitation (DP) or photodeposition (PD) methods.17−21 © XXXX American Chemical Society
Received: September 17, 2014 Revised: October 14, 2014
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AuNPs and their loading amount on the photocatalytic activity of the Au/TiO2 nanocomposites. For convenience, the Au/ TiO2 nanocomposites synthesized from this ligand-exchange process were denoted as TiO2−Au(x)-y, where x is the loading percentage of Au by weight, and y represents the size of the AuNPs in nanometer. Figure 1a shows a transmission electron
loss of dispersity and morphology of the noble metal nanoparticles.30,31 Chemical oxidation and solvent extraction represent another two typical methods developed to date.29,32 The former only deals with highly stable metals such as Au because strong oxidants are often involved, and the latter is typically not efficient enough to remove all the ligands. Therefore, it is highly desirable to develop alternative strategies for efficient ligand removal under mild conditions, which makes it possible to take full advantage of the well-developed colloidal noble metal nanoparticles and produce metal/semiconductor nanocomposites with optimal visible-light photocatalytic activity. In this work, we take Au (Figure S1, Supporting Information) and TiO2 (Degussa P25) nanoparticles as an example and report a ligand-exchange strategy for the efficient ligand removal from the colloidal Au nanoparticles (AuNPs) to form effective Au/TiO2 Schottky contact (Scheme 1). The key Scheme 1. Synthesis of the Au/TiO2 Nanocomposites by the Ligand-Exchange Method and the Conventional Deposition−Precipitation (DP) Method; the Original AuNP in the Ligand-Exchange Process Is Covered with PVP
Figure 1. TEM images of the a typical Au/TiO2 nanocomposite, TiO2−Au(6)-10, obtained by the ligand-exchange method. (a) A lowresolution TEM image. (b−d) HRTEM images showing close contact and lattice correlations between the AuNPs and the P25 nanocrystals.
microscopy (TEM) image of a typical nanocomposite TiO2− Au(6)-10. Uniformly distributed AuNPs were observed on the surface of TiO2 nanoparticles without aggregations. A close investigation of the Au/TiO2 interface by highresolution TEM (HRTEM) (Figure 1b−d) reveals that the lattices of Au and TiO2 are in close contact, and the interfacial part of the AuNP has been reshaped into a hemisphere partially buried in a thin layer of TiO2, indicating that strong interactions between Au and TiO2 have been established during the mild annealing process. A clear relationship between the lattices of the Au and TiO2 nanocrystals can be further observed: in many cases the AuNPs were found to be typically aligned with their ⟨111⟩ direction parallel to the ⟨101⟩ direction of the anatase nanocrystals. These observations undoubtedly suggest strong metal−support interactions during the unique ligand exchange/annealing processes, which lead to the formation of close Schottky contact between Au and TiO2 nanoparticles. The effectiveness of the ligand-exchange method has been further examined by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) (Figure 2a−c). The FTIR spectrum of the as-synthesized AuNPs after washing showed clear vibrational bands of PVP, for example, at 1660 (CO), 2850 (−CH2−), and 2917 cm−1 (−CH2−), suggesting that PVP as a ligand can strongly adhere to Au surface. After ligand exchange with MPA and vacuum evaporation, the vibrational peaks of PVP completely disappeared and no peaks
of this strategy is to find an intermediate ligand, which can first replace the original ligand on the AuNPs and then be easily removed from this system. In this work, we chose a small thiolbased molecule, 3-mercaptopropionic acid (MPA), as the intermediate ligand. MPA readily replaces the original ligand of the AuNPs (e.g., polyvinylpyrrolidone, PVP) due to its high affinity to Au at ambient temperature33,34 and is volatile in nature to enable its subsequent removal by vacuum evaporation. MPA dually provides driving force for the efficient loading of the AuNPs onto the TiO2 nanocrystals through its interactions with both Au and TiO2 by its thiol and carboxylic groups, respectively. We further discovered that the formation of close Au/TiO2 Schottky contact requires not only complete removal of the ligand but also annealing at a temperature of 200 °C, much lower than that required in the conventional DP method, which produces even less efficient visible-light photocatalysts. As a result, we were able to construct reliable and effective Au/TiO2 Schottky junctions while retaining the original size and morphology of the noble metal nanoparticles and reveal the independent effects of the properties of the B
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Figure 2. Spectral characterizations. (a) FTIR spectra of the AuNPs with PVP as the ligand (AuNPs) and those after ligand exchange and vacuum evaporation (AuNPs-ligand exchange). Spectra of pure PVP and MPA were listed for comparison. (b) TGA of the Au/TiO2 nanocomposites obtained by the salting-out method (TiO2−Au(6)-SO; loading amount of Au, 6 wt %) and the ligand-exchange method (TiO2−Au(6)-10), compared with that of pure TiO2 (P25). (c) XPS of the Au/TiO2 nanocomposites obtained after ligand exchange and annealing at 200 °C. (d) UV− vis diffuse reflectance spectra of the Au/TiO2 nanocomposites obtained by DP (DP-400, annealed at 400 °C) and the ligand-exchange method (TiO2−Au(x)-y) with varying sizes of the AuNPs. Loading amount of Au, 6 wt %.
corresponding to MPA (for example, 1711 cm−1 from −COOH) emerged and therefore confirmed that PVP has been completely removed in the ligand-exchange process and so did MPA in the evaporation process. In addition, TGA results shown in Figure 2b suggested that the Au/TiO2 nanocomposite prepared by the ligand-exchange method showed weight loss of less than 2% during heating to 800 °C, which is very close to that of pure TiO2, confirming the absence of PVP. For comparison, the Au/TiO2 nanocomposite obtained by an alternative salting-out method showed a significant weight loss starting from ∼250 °C, corresponding to the oxidative decomposition of PVP in air, which indicates the presence of PVP in this Au/TiO2 nanocomposite. The absence of MPA in the final Au/TiO2 nanocomposite after evaporation and annealing processes can be further evidenced by XPS measurements shown in Figure 1c. No S (2p) peaks can be detected in its XPS spectrum, which otherwise should be found in the range of 150−180 eV of binding energy. The peak of Au (4f7/2) appeared at 83.1 eV, which was lower than that of free metallic Au (∼83.8 eV). This difference indicates significant charge transfer from TiO2 to Au and thus confirms the strong Au/TiO2 interaction.35,36 The resultant Au/TiO2 nanocomposites exhibit strong absorption of visible light as demonstrated in the UV−vis diffuse reflectance spectroscopy (Figure 2d). The spectra of the materials prepared by the ligand-exchange method show a narrow absorption band centered at ∼550 nm due to the uniform size of the AuNPs from a colloidal synthesis (Figure S2, Supporting Information). By contrast, the spectrum of the Au/TiO2 nanocomposite obtained by a conventional DP
method shows a much broader absorption band with low intensity as a result of the broad size distribution of the AuNPs (Figure S3, Supporting Information). Photocatalytic activities of the materials were further investigated by decomposition of methylene orange (MO) (Figure 3). Only AuNPs were excited under visible-light illumination (λ > 410 nm; 34 mW cm−2), which produced hot electrons to be transferred to titania, leaving holes in the AuNPs for MO oxidation. Figure 3a shows the photocatalytic activity of pure P25 and Au/TiO2 nanocomposites synthesized by different methods. Neither P25 nor the Au/TiO2 nanocomposite synthesized by the salting-out method showed noticeable activity in MO decomposition during a period of 4 h, which can be attributed to the wide bandgap of the material and the presence of insulating ligand in the Au/TiO2 interface (TGA, see Figure 2b), respectively. By contrast, the Au/TiO2 nanocomposites synthesized by the DP and the ligandexchange methods are visible-light active, indicating the formation of Au/TiO2 Schottky contact for hot electron transfer. The photocatalyst synthesized by ligand-exchange method was particularly active, which can be attributed to (i) the close Schottky contact resulting from the unconventional deposition strategy and (ii) uniform size of the AuNPs obtained from a colloidal synthesis so that the photocatalytic activity can be optimized, which will be discussed below. Therefore, our ligand-exchange method showed obvious superiority in fabricating highly active visible-light photocatalysts. It is important to point out that the ligand exchange process itself does not necessarily leads to formation of a close Au/ TiO2 Schottky contact: the subsequent annealing process C
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Figure 3. Photocatalytic MO decomposition by the TiO2−Au(x)-y nanocomposites under visible-light illumination (λ > 410 nm; 34 mW cm−2). Au/TiO2 nanocomposites obtained by the salting-out method (TiO2−Au(6)-SO) and DP method (DP-x; x, annealing temperature, °C) and pure P25 were employed as control materials. Loading amount of Au, 6 wt %. (a) Decomposition of MO by P25 and Au/TiO2 nanocomposites synthesized by different methods. (b) Decomposition of MO by the TiO2−Au(6)-10 nanocomposites, which were annealed at different temperatures. (c) Decomposition of MO by the TiO2−Au(6)-y nanocomposites with different sizes of the AuNPs. (d) A summary of the photocatalytic activity of the TiO2−Au(x)-y nanocomposites as a function of particle size and loading amount of Au.
annealing temperature was raised to 400 °C, which confirmed that the annealing process helps to establish more close Au/ TiO2 contact. However, this high temperature easily causes aggregations of the AuNPs, giving rise to limited photocatalytic activity (Figure S3, Supporting Information). Clearly, our ligand-exchange method produces Au/TiO2 photocatalysts with much effective Schottky junctions under mild conditions, which favors high photocatalytic activities and retention of the original size and morphology of the AuNPs (see Figure S6, Supporting Information for another example of Au nanorods). One of the advantages of our strategy is that it enables reliable evaluation of the size effect of the AuNPs and thus further optimization of the photocatalytic activity because they can be presynthesized with precisely controlled sizes and then deposited to the TiO2 surface without experiencing apparent changes. In a typical evaluation (Figure 3c), the loading amount of the AuNPs was fixed at 6 wt %, and their size varied from 7 to 20 nm. The resultant TiO2−Au(6)-y nanocomposites showed the best activity on MO decomposition when the size of the AuNPs was 10 nm. The TEM image of TiO2− Au(6)-7 with smaller AuNPs showed higher number density of the AuNPs and presence of TiO2 nanoparticles attaching multiple AuNPs (Figure S2, Supporting Information), which led to low photocatalytic activity because the AuNPs can dually serve as recombination centers for photoelectrons and holes. For nanocomposites with large AuNPs, the number density of the AuNPs is drastically decreased (Figure S2, Supporting Information), which accounts for the low overall efficiency of the AuNPs in producing photoelectrons and holes for photocatalysis.
appears to be equally important. As shown in Figure 3b, a ligand-free Au/TiO2 nanocomposite obtained after the ligand exchange process was annealed at different temperatures (150, 200, 300, and 400 °C) for 2 h. The one annealed at 150 °C showed negligible photocatalytic activity on MO decomposition under visible-light illumination. However, the one annealed at 200 °C exhibited pronounced activity, indicating that close Schottky contact has been established. Further raising the annealing temperature led to sintering of the AuNPs (Figure S4, Supporting Information) and thus decreased photocatalytic activity. It suggests that in the absence of capping ligands, strong interactions required to form Au/TiO2 Schottky contact could be established by simply annealing at a considerably low temperature (200 °C). The UV−vis diffuse reflectance spectroscopy showed that after annealing at 150 and 200 °C, the absorption band of the AuNPs shifted to the red by ∼3 and 16 nm, respectively (Figure S5, Supporting Information). As the red-shift can be primarily attributed to the presence of high dielectric-constant materials in the vicinity of the AuNPs, the spectroscopic observation confirmed that close Au/TiO2 contact starts to form at 200 °C. It is reasonable that this elevated annealing temperature enables modulations of both Au and TiO2 lattices for establishing maximal interfaces with proper orientations and thus close Schottky contact. By contrast, as shown in Figure 3a, the Au/TiO2 nanocomposites obtained by the conventional DP method showed considerably low photocatalytic activity when the annealing temperature was 200 °C, due to the broad size distribution of the AuNPs such that the photocatalytic acitivity was not optimized (Scheme 1). The photocatalytic activity increased slightly when the D
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Figure 4. Photocatalytic hydrogen generation from the TiO2−Au(x)-y nanocomposites under visible-light illumination in a period of 6 h. Bare P25 and Au/TiO2 nanocomposites obtained by the DP method (DP-x; x, annealing temperature, °C; Au loading, 6 wt %) were employed as the control materials. (a) Illumination conditions: λ > 515 nm; , 100 mW cm−2. (b) Illumination conditions: λ > 390 nm; 100 mW cm−2. (c) Photocatalytic hydrogen production from the photocatalyst TiO2−Au(6)-10 in 4 cycles; illumination conditions: λ > 515 nm; 100 mW cm−2.
attributed to the electron sink effect of the AuNPs. In addition, the AuNPs may enhance the light absorption of the P25 nanocrystals by the electric near-field amplification effect, which further contributes to the photocatalytic activity of the nanocomposite.18 Intensively excited photoelectrons transferred from TiO2 to the AuNPs, with the recombination events of the electron−hole pairs greatly suppressed. The dependence of the photocatalytic activity on the illumination wavelength (Figure S10, Supporting Information) unambiguously confirmed this mechanism under λ > 390 illumination. As the flow of electrons from the semiconductor to the metal through a Schottky barrier is much easier than conduction in the opposite direction,37 the hydrogen production rates in this experiment were highly accelerated, and the photocatalytic activity of the catalysts became less dependent on the Au/TiO2 contact, compared with that conducted under long wavelength illumination (Figure 4a). Nevertheless, TiO2−Au(6)-10 still exhibited the highest activity, confirming that the ligandexchange method leads to more close Schottky junctions for efficient electron transfer for photocatalysis. Additionally, these catalysts showed high stability in photocatalysis, which was exemplified by TiO2−Au(6)-10 in water reduction reaction (Figure 4c). During each cycle of 6 h, almost the same amount of hydrogen was generated, and no significant decay in the photocatalytic activity was observed in 24 h of our investigation. The high stability can be attributed to the intrinsic erosion-resistance of the Au and TiO2 nanoparticles and their robust Schottky contact constructed by the method of ligand-exchange and mild annealing. In summary, we report that close Schottky contact could be established in Au/TiO2 heterostructures through a ligand exchange process followed by mild calcination, producing efficient visible-light photocatalysis. Different from conventional methods such as the widely used deposition-precipitation process, this route takes advantage of preformed colloidal Au NPs with defined sizes and morphologies. The key of our strategy is the complete removal of the ligands from the AuNPs achieved by first ligand-exchange with MPA and then MPA evaporation. The clean Au surface makes it possible to produce close Schottky contact of Au/TiO2 by annealing at a considerably low temperature (200 °C) without causing sintering of the AuNPs. The photocatalysts synthesized by this method show superior activity in both dye decomposition and water-reduction hydrogen production. This unique synthesis route also allows us to reveal the independent effects of
Figure 3d summarizes the effects of the size and loading amount of the AuNPs on the photocatalytic activity of the Au/ TiO2 nanocomposites under visible-light illumination (additional plots shown in Figure S7, Supporting Information). As a general trend, for all photocatalysts with AuNPs of a fixed size, the photocatalytic activity initially arises and eventually drops when the loading amount of the AuNPs increases. The drop in the photocatalytic activity can be attributed to the increasing recombination events of the photoelectrons and holes as a result of the high number density of the AuNPs. In addition, the optimal loading amount of the AuNPs readily shifts to high values when the size of the AuNPs increases. This is because for AuNPs of a large size, significantly higher loading amount is required to achieve considerably the same number density of the AuNPs , which has proven to be critical to their photocatalytic activities. These Au/TiO2 nanocomposites were then applied to visible-light water reduction for hydrogen production in the presence of methanol, a sacrificial hole scavenger. A filter with cutoff wavelengths of λ > 515 nm was applied to the light source so that only the AuNPs were excited and hot electrons flew from Au to TiO2 (Figure 4a). As a control sample, P25 did not show any photocatalytic activity. TiO2−Au(6)-10 exhibited the highest activity, and a clear size effect of the AuNPs on the photocatalytic activity of the TiO2−Au(6)-y catalysts can be observed. By contrast, the photocatalysts obtained by the conventional DP method showed much lower activity in hydrogen production, and even no hydrogen was detected from the photocatalyst DP-200 annealed at a low temperature, which indicates that inefficient Au/TiO2 Schottky contact has been established by this method. All these observations are in good agreement with those in MO decomposition (Figure 3c), confirming the superiority of the ligand-exchange method in constructing effective Au/TiO2 Schottky contact for visiblelight photocatalysis. When a commonly used nominal λ > 400 nm filter (practical λ > 390 nm) was applied, the photocatalysts showed much enhanced photocatalytic activity in hydrogen production (Figure 4b). Bare P25 showed low but detectable hydrogen, which can be attributed to its slight absorption “tail” extending to ∼410 nm of wavelength (Figure S9, Supporting Information). Therefore, P25 was excited in this case with electron−hole pairs generated for photocatalysis. Compared to bare P25, the Au/TiO2 nanocomposites showed dramatically enhanced photocatalytic activity, which can be primarily E
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(16) Mubeen, S.; Lee, J.; Singh, N.; Kramer, S.; Stucky, G. D.; Moskovits, M. Nat. Nanotechnol. 2013, 8, 247−251. (17) Murdoch, M.; Waterhouse, G. I. N.; Nadeem, M. A.; Metson, J. B.; Keane, M. A.; Howe, R. F.; Llorca, J.; Idriss, H. Nat. Chem. 2011, 3, 489−492. (18) Pu, Y.-C.; Wang, G.; Chang, K.-D.; Ling, Y.; Lin, Y.-K.; Fitzmorris, B. C.; Liu, C.-M.; Lu, X.; Tong, Y.; Zhang, J. Z.; Hsu, Y.-J.; Li, Y. Nano Lett. 2013, 13, 3817−3823. (19) Lu, Q.; Lu, Z.; Lu, Y.; Lv, L.; Ning, Y.; Yu, H.; Hou, Y.; Yin, Y. Nano Lett. 2013, 13, 5698−5702. (20) Jovic, V.; Chen, W.-T.; Sun-Waterhouse, D.; Blackford, M. G.; Idriss, H.; Waterhouse, G. I. N. J. Catal. 2013, 305, 307−317. (21) Qian, K.; Sweeny, B. C.; Johnston-Peck, A. C.; Niu, W.; Graham, J. O.; DuChene, J. S.; Qiu, J.; Wang, Y.-C.; Engelhard, M. H.; Su, D.; Stach, E. A.; Wei, W. D. J. Am. Chem. Soc. 2014, 136, 9842− 9845. (22) Millstone, J. E.; Hurst, S. J.; Metraux, G. S.; Cutler, J. I.; Mirkin, C. A. Small 2009, 5, 646−664. (23) Lu, X.; Rycenga, M.; Skrabalak, S. E.; Wiley, B.; Xia, Y. Annu. Rev. Phys. Chem. 2009, 60, 167−192. (24) Ryceng, M.; Cobley, C. M.; Zeng, J.; Li, W.; Moran, C. H.; Zhang, Q.; Qin, D.; Xia, Y. Chem. Rev. 2011, 111, 3669−3712. (25) Gao, C.; John, V.; Zhang, Q.; Liu, Y.; Yin, Y. Nanoscale 2012, 4, 2875−2878. (26) Liu, X.; Yin, Y.; Gao, C. Langmuir 2013, 29, 10559−10565. (27) Liu, X.; Li, L.; Yang, Y.; Yin, Y.; Gao, C. Nanoscale 2014, 6, 4513−4516. (28) Costa, N. J. S.; Rossi, L. M. Nanoscale 2012, 4, 5826−5834. (29) Liu, L.; Ouyang, S.; Ye, J. Angew. Chem., Int. Ed. 2013, 52, 6689−6693. (30) Menard, L. D.; Xu, F.; Nuzzo, R. G.; Yang, J. C. J. Catal. 2006, 243, 64−73. (31) Comotti, M.; Li, W. C.; Spliethoff, B.; Schuth, F. J. Am. Chem. Soc. 2006, 128, 917−924. (32) Lopez-Sanchez, J. A.; Dimitratos, N.; Hammond, C.; Brett, G. L.; Kesavan, L.; White, S.; Miedziak, P.; Tiruvalam, R.; Jenkins, R. L.; Carley, A. F.; Knight, D.; Kiely, C. J.; Hutchings, G. J. Nat. Chem. 2011, 3, 551−556. (33) Li, J.; Zeng, H. C. Chem. Mater. 2006, 18, 4270−4277. (34) Lu, Z.; Gao, C.; Zhang, Q.; Chi, M.; Howe, J. Y.; Yin, Y. Nano Lett. 2011, 11, 3404−3412. (35) Tsukamoto, D.; Shiraishi, Y.; Sugano, Y.; Ichikawa, S.; Tanaka, S.; Hirai, T. J. Am. Chem. Soc. 2012, 134, 6309−6315. (36) Zwijnenburg, A.; Goossens, A.; Sloof, W. G.; Craje, M. W. J.; van der Kraan, A. M.; de Jongh, L. J.; Makkee, M.; Moulijn, J. A. J. Phys. Chem. B 2002, 106, 9853−9862. (37) Tung, R. T. Appl. Phys. Rev. 2014, 1, 011304.
the size and loading amount of the AuNPs and subsequently enable convenient optimization of the photocatalysts. We believe the strategy reported in this work can be extended to other metal/semiconductor systems such as Ag/TiO2 and therefore provides new opportunities in designing advanced photoactive materials with high stability and enhanced performance for solar energy conversion. In a broader perspective, our method may also represent a potential solution for the well-recognized challenge in cleaning capping ligands from the surface of colloidal catalyst nanoparticles.
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ASSOCIATED CONTENT
* Supporting Information S
Detailed synthesis procedures, additional TEM, UV−vis, and photocatalysis results. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*(C.G.) E-mail:
[email protected]. *(Y.Y.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS C.G. acknowledges support by the National Natural Science Foundation of China (Grant No. 21301138), the Fundamental Research Funds for the Central Universities (Grant No. XJJ2013033), the startup fund, and operational fund for the Center for Materials Chemistry from Xi’an Jiaotong University. D.D. acknowledges support by the China Postdoctoral Science Foundation (Grant No. 2014M552424). Y.Y. acknowledges the support from the U.S. Department of Energy (DE-FG0209ER16096).
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REFERENCES
(1) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269−271. (2) Chen, X.; Mao, S. S. Chem. Rev. 2007, 107, 2891−2959. (3) Bao, N. Z.; Shen, L. M.; Takata, T.; Domen, K. Chem. Mater. 2008, 20, 110−117. (4) Yi, Z.; Ye, J.; Kikugawa, N.; Kako, T.; Ouyang, S.; StuartWilliams, H.; Yang, H.; Cao, J.; Luo, W.; Li, Z.; Liu, Y.; Withers, R. L. Nat. Mater. 2010, 9, 559−564. (5) Chen, X.; Liu, L.; Yu, P. Y.; Mao, S. S. Science 2011, 331, 746− 750. (6) Xia, T.; Zhang, Y.; Murowchick, J.; Chen, X. Catal. Today 2014, 225, 2−9. (7) Tong, H.; Ouyang, S.; Bi, Y.; Umezawa, N.; Oshikiri, M.; Ye, J. Adv. Mater. 2012, 24, 229−251. (8) Thomann, I.; Pinaud, B. A.; Chen, Z.; Clemens, B. M.; Jaramillo, T. F.; Brongersma, M. L. Nano Lett. 2011, 11, 3440−3446. (9) Linic, S.; Christopher, P.; Ingram, D. B. Nat. Mater. 2011, 10, 911−921. (10) Ingram, D. B.; Linic, S. J. Am. Chem. Soc. 2011, 133, 5202−5205. (11) Seh, Z. W.; Liu, S.; Low, M.; Zhang, S. Y.; Liu, Z.; Mlayah, A.; Han, M. Y. Adv. Mater. 2012, 24, 2310−2314. (12) Bian, Z.; Tachikawa, T.; Zhang, P.; Fujitsuka, M.; Majima, T. J. Am. Chem. Soc. 2014, 136, 458−465. (13) Jiang, R.; Li, B.; Fang, C.; Wang, J. Adv. Mater. 2014, 26, 5274− 5309. (14) Gomes Silva, C.; Juarez, R.; Marino, T.; Molinari, R.; Garcia, H. J. Am. Chem. Soc. 2011, 133, 595−602. (15) Warren, S. C.; Thimsen, E. Energy Environ. Sci. 2012, 5, 5133− 5146. F
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