Efficiently Enhancing Visible Light Photocatalytic Activity of Faceted

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Efficiently Enhancing Visible light Photocatalytic Activity of Faceted TiO Nanocrystals by Synergistic Effects of Core-shell structured Au@CdS Nanoparticles and their Selective Deposition 2

Ruifeng Tong, Chang Liu, Zhenkai Xu, Qin Kuang, Zhaoxiong Xie, and Lan-Sun Zheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05563 • Publication Date (Web): 01 Aug 2016 Downloaded from http://pubs.acs.org on August 10, 2016

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ACS Applied Materials & Interfaces

Efficiently Enhancing Visible light Photocatalytic Activity of Faceted TiO2 Nanocrystals by Synergistic Effects of Core-shell structured Au@CdS Nanoparticles and their Selective Deposition Ruifeng Tong, Chang Liu, Zhenkai Xu, Qin Kuang,* Zhaoxiong Xie, and Lansun Zheng

State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005. Email: [email protected].

KEYWORDS. visible light photocatalytic activity, H2 evolution, Au@CdS nanoparticles, TiO2, selective deposition, charge separation

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ABSTRACT.

Integrating

wide

bandgap

semiconductor

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photocatalysts

with

visible-light-active inorganic nanoparticles (such as Au and CdS) as sensitizers is one of the most efficient methods to improve their photocatalytic activity in the visible light region. However, as for all such composite photocatalysts, a rational design and precise control over their architecture is often required to achieve optimal performance. Herein, a new TiO2-based ternary composite photocatalyst with superior visible light activity was designed and synthesized. In this composite photocatalyst, the location of the visible light sensitizers was engineered according to the intrinsic facet-induced effect of well-faceted TiO2 nanocrystals on the spatial separation of photogenerated carriers. Experimentally, core-shell structured Au@CdS nanoparticles acting as visible light sensitizers were selectively deposited onto photoreductive {101} facets of well-faceted anatase TiO2 nanocrystals through a two-step in situ photodeposition route. Due to the fact that the combination of Au@CdS and specific {101} facets of TiO2 nanocrystals facilitates the transport of charges photogenerated under visible light irradiation, this well-designed ternary composite photocatalyst exhibited superior activity in visible-light-driven photocatalytic H2 evolution, as expected.

1. Introduction Semiconductor-based photocatalysis has been long regarded as the most optimal method to simultaneously solve the problems of energy shortage and environmental 2 ACS Paragon Plus Environment

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protection. Therefore, considerable efforts have been made over the past decades in the development of photocatalysts with the purpose of acquiring high energy conversion efficiencies.1-3 Since semiconductor-based water splitting was discovered, TiO2 has been regarded as the most promising photocatalyst, being intensively investigated due to its excellent optical and electric properties, low cost, non-toxicity, and availability.4 However, the application of TiO2 in photocatalysis continues to face two major obstacles, namely (i) poor response to visible light, owing to its wide bandgap, and (ii) low utilization of light absorbed, due to the high recombination of photogenerated carriers.5 Regardless of efforts made to improve the photocatalytic properties of TiO2, the existing modification strategies, including morphology optimization, foreign element doping, dye sensitization, and metal deposition, among others, still fall short of achieving these.6,7 Among the various photocatalyst modification methods available, integrating wide bandgap semiconductors like TiO2 with visible-light-active inorganic nanoparticles (NPs) has been heralded due to its advantages in strengthening charge transport and reducing recombination while simultaneously extending the light absorption range.8,9 CdS, with a bandgap of 2.4 eV, is one of the most common visible light sensitizers, since its band-edge levels are capable of driving both the reduction and oxidation of water under visible light irradiation.10-14 In addition, recent studies have experimented with plasmonic metal (such as Au) NPs as visible light sensitizers able to arouse intense absorption in the visible light region due to the surface plasmon resonance (SPR) effect.15-19 However, both CdS and plasmonic metal NPs have their own 3 ACS Paragon Plus Environment


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shortcomings as visible light sensitizers: CdS NPs are subject to activity loss due to the occurrence of photocorrosion, while the photocatalytic efficiency caused by the SPR effect of Au NPs is very low, far from practical application.20 To acquire optimal photocatalytic activity under visible light, greater attention has been given to ternary composite photocatalysts integrating TiO2, CdS, and Au.21-26 Of note, the intrinsic surface effect of TiO2 in photocatalysis is often ignored in studies focusing on the incorporation of CdS and/or Au sensitizers. In fact, the different facets of well-faceted semiconductor nanocrystals (NCs) usually exhibit distinct reactivities in photocatalysis.27-29 For example, the {101} and {001} facets of anatase TiO2 have proven to facilitate reductive and oxidative reaction in photocatalysis,

respectively.30-34

This

unique

facet-dependent

photochemical

reactivity of semiconductor NCs hints us that rationally engineering co-catalysts or sensitizers at desired locations on semiconductor photocatalyst surfaces may be an effective strategy to optimize the photocatalytic activities of composite photocatalysts. By means of selective deposition of co-catalysts or sensitizers, the charge separation of semiconductor photocatalyst would be significantly enhanced due to the synergy between the facet-induced effect and heterojunction induced effect, which have been well demonstrated in many semiconductor cases including TiO2, BiVO4, etc.35-39 Herein, the design of a special ternary composite photocatalyst in which core-shell structured Au@CdS NPs were selectively deposited onto photoreductive {101} facets of anatase TiO2 NCs is presented. The architectural design of Au@CdS-TiO2 is based on the following considerations. First, for Au@CdS, the photogenerated electrons will 4 ACS Paragon Plus Environment

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transfer from the excited CdS shell to Au NPs owing to the lower Fermi level of Au when the CdS shell is irradiated by visible light.40,41 Second, not only can Au NPs directly deposited on TiO2 NCs act as plasmonic photosensitizers to enhance the response to visible light, but they also can facilitate the electron transfer from CdS to TiO2 caused by the lower conduction band level of TiO2. Further, although anatase TiO2 is inactive under visible light, the {101} facets intrinsically exhibit photoreductive activity and can thus facilitate the gathering of more electrons from the core-shell structured Au@CdS NPs.42 Encouragingly, this photocatalyst modification strategy seemed to have worked in H2 evolution experiments. The visible-light-driven photocatalytic activity of truncated octahedral anatase TiO2 (TOAT) NCs in H2 evolution was remarkably enhanced by selective deposition of Au@CdS NPs onto {101} facets due to their synergistic effect.

2. Experimental Section 2.1 Chemicals Potassium hydroxide (KOH, 90%), hexamethylenetetramine (HMTA, 99%), sodium borohydride (NaBH4, AR), methanol (99.5%), and ethanol (99.9%) were purchased

from

Sinopharm

Chemical

Reagent

Co.,

Ltd.

Hydrogen

hexachloroplatinate hydrate (H2PtCl6·6H2O, 99.95%), hydrogen tetrachloroaurate hydrate (HAuCl4·4H2O, 99.95%) and sulfur powder (S8, AR) were purchased from Alfar Aesar. Cadmium nitrate tetrahydrate (Cd(NO3)2·4H2O, AR) was purchased from Aladdin Industrial Corporation. The commercial photocatalyst Degussa P25 was 5 ACS Paragon Plus Environment


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purchased from Shanghai Haiyi Co. All reagents were used as received without further purification. 2.2 Synthesis of samples 2.2.1 Synthesis of truncated octahedral anatase TiO2 NCs TOAT NCs used in this work were prepared via a facile hydrothermal route according to our previous study, in which potassium titanate nanowires (KTNWs) acted as the precursor and HMTA as a shape regulator.43 First, precursor KTNWs were prepared via a hydrothermal reaction of 2 g Degussa P25 in 10 mol·L–1 KOH solution at 200 °C for 24 h. Then, KTNWs (10 mg) and HMTA (0.70 g, 5 mmol) were ultrasonically dispersed in 6 mL of distilled water. The resulting suspension was transferred into a 25-mL Teflon-lined stainless-steel autoclave, sealed, and maintained at 200 °C for 12 h. After reaction, the white precipitate in the autoclave was collected by centrifugation, washed with distilled ethanol three times, and finally dried in an oven. 2.2.2 Synthesis of TOAT NCs with Au@CdS NPs selectively deposited on {101} facets The designed ternary composite photocatalyst, i.e. TOAT NCs with Au@CdS NPs selectively deposited on {101} facets, was synthesized via a two-step photodeposition process involving the deposition of Au NPs onto {101} facets of TOAT NCs and the growth of CdS shells on Au NPs. The selective deposition of Au NPs on {101} facets of TOAT NCs was achieved via a photoreduction process with HAuCl4 as the metal precursor. Typically, 50 mg of TOAT NCs prepared as above were ultrasonically 6 ACS Paragon Plus Environment

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dispersed in 50 mL of deionized water, followed by the addition of 2.538 mL HAuCl4 aqueous solution (1 mmol·L–1) according to 1 wt% of Au/TiO2. The resulting suspension was magnetically stirred for 5 minutes, and irradiated under a 300 W Xe lamp for 1 h at room temperature. The product was collected by centrifugation and washed with water and ethanol three times. The growth of the CdS shell on Au NPs deposited on TOAT NCs was achieved through a photochemical process using Cd(NO3)2 and S8 powder as sources and ethanol as a hole scavenger. Typically, 30 mg of Au-deposited TOAT NCs were dispersed in a mixed solution of 40 mL distilled water and 10 mL ethanol, followed by successive addition of 0.247 g Cd(NO3)2·4H2O and 8.4 mg S8 powder under ultrasonic dispersion. The resulting suspension was irradiated for 3 h under a 300 W Xe lamp with continuous stirring. Following the reaction, the product was collected by centrifugation, and washed with water three times. For convenience, the as-prepared composite photocatalyst with Au NPs and Au@CdS NPs selectively deposited on {101} facets were denoted as p-Au-TOAT and p-Au@CdS-TOAT, respectively. In addition, a similar deposition process of CdS, in which blank TiO2 NCs (i.e., TOAT) was used as the support of CdS, was conducted while keeping other conditions constant, and the as-prepared product was denoted as p-CdS-TOAT. 2.2.3 Synthesis of TOAT NCs with Au@CdS NPs randomly deposited TOAT NCs with Au@CdS NPs randomly deposited were synthesized via a similar two-step process in which the first deposition procedure, i.e., deposition of Au NPs, 7 ACS Paragon Plus Environment


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was achieved by chemical reduction with NaBH4 as reductant, instead of through photoreduction. Typically, the pH of the suspension containing 50 mg TiO2 NCs in 50 mL of deionized water was adjusted to 1 through dropwise addition of HCl prior to the addition of HAuCl4 aqueous solution (2.538 mL, 1 mmol·L–1). A NaBH4 solution (25 mL, 0.1 mol·L–1) was then added dropwise into the suspension in an ice water bath under magnetic stirring. The reaction lasted for 2 h in the ice water bath. Finally, the product was collected by centrifugation, and washed with water and ethanol three times. The second deposition procedure, i.e., growth of the CdS shell on Au NPs, was identical to that for p-Au@CdS-TOAT. For convenience, the photocatalysts synthesized on the first (chemical reduction) and second (photodeposition) steps were denoted c-Au-TOAT and c-Au@CdS-TOAT, respectively. 2.2.4

Probing

transfer

direction

of

photogenerated

electrons

in

p-Au@CdS-TOAT under visible light To probe the transfer direction of photogenerated electrons within the ternary composite photocatalyst p-Au@CdS-TOAT NCs under visible light, the deposition of Pt NPs on the surface of p-Au@CdS-TOAT photoreduction was conducted. Typically, 10 mg p-Au@CdS-TOAT NCs were ultrasonically suspended in a mixed solution of 40 mL distilled water and 10 mL ethanol, and then 50 µL H2PtCl6 solution (1 mmol·L–1) were added. Following stirring in the dark, the resulting suspension was irradiated under a 300 W Xe lamp with a UV light filter for 1 h. The deposition location of Pt NPs was determined by using transmission electron microscopy (TEM). 2.3 Characterization of samples 8 ACS Paragon Plus Environment

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The composition of samples was determined by powder X-ray diffraction (XRD) patterns on a Panalytical X'pert PRO X-ray diffractometer (Cu Kα radiation, 0.15418 nm). The morphology and structure of samples were observed using a scanning electron microscope (Hitachi S-4800) at a 10 kV accelerating voltage and a transmission electron microscope (JEOL JEM-2100) at a 200 kV accelerating voltage. High-angle

annular dark

field

scanning

transmission electron microscopy

(HAADF-STEM) and element mapping were acquired on a field emission TEM (Philips, FEI TECNAI F30) operated at a 300 kV accelerating voltage, which was equipped with energy dispersive X-ray (EDX) spectroscopy. The UV-vis diffuse reflectance spectra (DRS) of samples were measured by on Varian Cary-5000 UV-vis spectrophotometer. The oxidation states of Ti, O, Au, Cd, and S atoms were determined by X-ray photoelectron spectroscopy (XPS) using a Thermo Scientific K-alpha photoelectron spectrometer (PHI Quantum-2000) with monochromatic Al Kα radiation as the excitation source. All peak positions were calibrated with the C1s peak (284.5 eV) as an internal standard. The effective loading amounts of Au on TiO2 NCs were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES, Baird PS-4). 2.4 Photocatalytic activity measurement in H2 evolution The photocatalytic H2 evolution experiments were conducted in a Labsolar-III (AG) photocatalytic system (Perfectlight Co. Ltd). Typically, 25 mg of photocatalysts were dispersed by sonication in 100 mL aqueous solution containing 0.1 mol·L–1 Na2S and 0.1 mol·L–1 Na2SO3 under magnetic stirring, both of which acted as hole scavengers. 9 ACS Paragon Plus Environment


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The closed system was degassed thoroughly to remove air and then irradiated under the 300 W Xe lamp (PLS-SXE-300UV, Beijing Trusttech Co. Ltd). A UV cutoff filter (UVCUT400) was employed to achieve visible light irradiation (>400 nm). The photocatalytic H2 evolution rate was analyzed using an online gas chromatograph (Tianmei GC7900). During the photocatalytic H2 evolution experiments, constant magnetic stirring was used and the reaction temperature was maintained at 10 °C through a cooling water circulation system.

3. Results and discussion

Scheme 1 Schematic illustration of the synthetic route of TiO2-based ternary photocatalysts with core-shell structured Au@CdS nanoparticles selectively deposited. The ternary composite photocatalyst with core-shell-structured Au@CdS NPs selectively deposited onto the {101} facets was synthesized through a two-step photodeposition process as illustrated in Scheme 1. For TOAT NCs, the top/bottom surfaces are enclosed by {001} facets and the side surfaces are enclosed by {101} facets (Figure S1). Upon UV light irradiation, the generated electrons and holes prefer to spontaneously transfer to the {101} and {001} facets, respectively, due to the various energy levels of the conduction and valence bands associated with the different facets.31 Owing to this unique facet-induced effect of anatase TiO2 on 10 ACS Paragon Plus Environment

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photogenerated carriers, Au NPs could be selectively deposited on the side surfaces (i.e., {101} facets) of TOAT NCs in the first photodeposition step. The establishment of a Schottky junction between TiO2 and Au trapped the photogenerated electrons in TiO2 NCs into the Au NPs deposited on the {101} facets in the second photodeposition step. Because of their good affinity with Au, S8 molecules prefer to be adsorbed on the surface of Au NPs and reduced to S2– ions by the electrons concentrated there, thus leading to the formation of a CdS shell (step 2).9 In this synthetic process, the unique facet-induced photochemical behavior of TiO2 NCs plays a key role in controlling the location of Au@CdS NPs. The results herein demonstrated that, once chemical reduction (rather than photoreduction) was applied in the first deposition step, Au NPs would be deposited on all facets of TOAT NCs.

Figure 1. Typical SEM images of a, b) p-Au-TOAT, and c, d) c-Au-TOAT.

Figure 1 shows typical scanning electron microscopy (SEM) images of and Au-deposited TOAT NCs synthesized by photoreduction and chemical reduction in 11 ACS Paragon Plus Environment


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the first deposition step. Prior to Au deposition, the top/bottom and side surfaces of TOAT NCs were smooth (Figure S1a). For Au-deposited TOAT NCs synthesized by photoreduction (i.e., p-Au-TOAT), a large number of NPs of 5–15 nm diameter were deposited on the side surfaces ({101} facets) of TOAT NCs, while the bottom/top surface ({001} facets) remained smooth (Figure 1b). However, for Au-deposited TOAT NCs synthesized by chemical reduction (i.e., c-Au-TOAT), all TOAT NCs surfaces were randomly deposited with NPs of similar sizes. EDX and ICP-AES analysis revealed the deposited NPs to be Au, with a real loading percentage of approximately 1.1 wt% for p-Au-TOAT and 1.3 wt% for c-Au-TOAT, which are comparable to the raw material ratio of 1 wt% (Figure S2). However, the characteristic peaks of Au were not detected in the XRD patterns of either sample due to the low loading amounts (Figure S3).

Figure 2. SEM and TEM images of a,b) p-Au@CdS-TOAT and c,d) c-Au@CdS-TOAT. 12 ACS Paragon Plus Environment

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Figures 2a and c show the typical SEM images of TOAT NCs with Au NPs selectively and non-selectively deposited after the second deposition step, i.e. p-Au@CdS-TOAT and c-Au@CdS-TOAT. In both cases, NPs deposited on the surface of TOAT NCs grew larger than in the first deposition, reaching sizes of 15‒25 nm. Although CdS was not detected in the samples (Figure S4), the corresponding TEM images revealed that all the deposited NPs presented a typical core-shell structure, as indicated by the darker Au cores (due to their stronger scattering ability) and the lighter CdS shells (Figure 2b, d). The core-shell structure of Au@CdS NPs deposited on TOAT NCs was more clearly observed in the HAADF-STEM image (Figure 3a) and STEM-EDX elemental mapping (Figure 3c) recorded from p-Au@CdS-TOAT. Further, high resolution TEM (HR-TEM) image revealed that Au NPs were indeed covered by a crystalline layer 3‒5 nm thick. The lattice fringe spacing in the shell was approximately 3.52 Å, which is in close agreement with that of the {100} planes of cubic CdS. Therefore, the crystalline layer on Au NPs is CdS.



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Figure 3. a) HAADF-STEM image of p-Au@CdS-TOAT. b) HR-TEM image recorded from the interface region between Au@CdS and TiO2 marked with a blue rectangle in a). c) STEM-EDX elementary mapping of Ti, O, Au, Cd, and S obtained from the region marked with a red rectangle in a).

It should be noted that the quantity of Au@CdS NPs anchored on TOAT in p-Au@CdS-TOAT seemed lower than that in c-Au@CdS-TOAT. This is because Au atoms prefer to be reduced on the originally formed Au NPs in the first photoreduction process, which results in the increase of Au NPs in size and the decrease in deposition density compared to those in the case of chemical reduction. In fact, the real loading amounts of Au and CdS in the samples prepared by two methods are almost same (around 3 wt%) according to the results of EDS and ICP-AES analysis (Figure S2 and S5). In addition, in both p-Au@CdS-TOAT and c-Au@CdS-TOAT, there were no additional CdS particles directly deposited on the surface of TOAT NCs, indicating that the formation of CdS would prefer to proceed on the Au NPs due to the good affinity of S8 with Au. To identify the origin of the electrons reducing the S8 molecules to S2– ions in Au NPs, the CdS deposition procedure was conducted under different irradiation conditions (Figure S6). The results show that the CdS shell could not form on the surface of Au NPs under visible light irradiation, indicating that CdS shell formation occurs independently to the hot electrons of Au NPs, and that the electrons involved in S8 to S2– reduction are those photogenerated by UV light in TiO2 and which were trapped in Au NPs due to the lower Fermi level of Au. 14 ACS Paragon Plus Environment

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Figure 4. a) Survey XPS spectrum and high-resolution XPS spectra of b) Cd 3d and c) S 2p for p-Au@CdS-TOAT. d) Comparison of high-resolution XPS spectra of Au 4f for p-Au@CdS-TOAT and p-Au-TOAT.

Figure 4a shows the XPS spectrum for p-Au@CdS-TOAT, confirming the presence of Ti, O, Cd, S, and Au in the as-prepared ternary composite photocatalyst. The chemical valence state of each element in this photocatalyst was further confirmed by high-resolution XPS analysis. As shown in Figure 4b, the high resolution XPS spectrum of Cd 3d displayed doublet peaks located at 405.2 eV and 412.0 eV with a spin-orbit separation of 6.8 eV, corresponding to 3d5/2 and 3d3/2 of Cd2+ in CdS, respectively.25 Two peaks, located at 161.4 eV and 169.0 eV, respectively, were also observed in the high-resolution XPS spectrum of S 2p. The peak at 161.4 eV was assigned to S2–, which is formed by photoreduction of S8. The additional peak located at 169.0 eV was attributed to SO42– generated by the photo-oxidation of S8, simultaneously occurring on the {001} oxidative facets of TOAT NCs.25 In the 15 ACS Paragon Plus Environment


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high-resolution XPS spectrum of Au 4f, the peaks at 87.45 eV and 83.75 eV were attributed to Au 4f 5/2 and Au 4f 7/2 of metallic Au. Due to the encapsulation of Au by CdS, the intensity of the Au 4f peaks for p-Au@CdS-TOAT was much lower in comparison with that for p-Au-TOAT.25

Figure 5. UV-visible DRS of TOAT, p-Au-TOAT, p-Au@CdS-TOAT, c-Au-TOAT, and c-Au@CdS-TOAT.

The above results clearly indicate that the selective deposition of core-shell structured Au@CdS NPs onto the {101} facets of TOAT NCs can be achieved through a two-step photodeposition process. It is further noted that the TiO2 photocatalyst color changed significantly following each photodeposition. As shown in inset of Figure 5, Au-deposited TOAT NCs were purple, while Au@CdS-deposited TOAT NCs turned green. The UV-visible DRS clearly reflected the influence of Au and Au@CdS NPs on the light absorption of TOAT NCs. As shown in Figure 5, all samples exhibited high light absorption below 400 nm, corresponding to the band-edge absorption of anatase TiO2. Compared to TOAT, the Au-deposited TOAT NCs (either p-Au-TOAT or c-Au-TOAT) showed an additional strong absorption 16 ACS Paragon Plus Environment

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band around 540 nm, coinciding with the plasmon resonance absorption of Au NPs.44 For Au@CdS-deposited TOAT NCs (p-Au@CdS-TOAT and c-Au@CdS-TOAT), the plasmon resonance absorption peaks were red shifted due to the strong electronic couple interaction between Au and CdS.25 Owing to the larger Au NPs and thicker CdS shells, p-Au@CdS-TOAT showed higher absorption in the visible light range than c-Au@CdS-TOAT.

Figure 6. a) time-dependent H2 evolution curves and b) H2 evolution efficiencies of TOAT, c-Au-TOAT, p-Au-TOAT, p-Au@CdS-TOAT, c-Au@CdS-TOAT, and CdS-TOAT under visible light (λ >400 nm). Reaction condition: 25 mg catalyst in 100 ml solution containing 0.1 M Na2S and 0.1 M Na2SO3 aqueous solution. Clearly, the loading of Au or Au@CdS greatly extended the light absorption range of TOAT NCs from the UV region to the visible region, which would stimulate their photocatalytic activity under visible light irradiation. Thus, photocatalytic H2 evolution experiments were performed under visible light (λ >400 nm) with 0.1 M Na2S and 0.1 M Na2SO3 aqueous solutions as hole scavengers (Figure 6). Under visible light irradiation, TOAT NCs showed no activity in photocatalytic H2 evolution, since TiO2 could not absorb visible light due to its wide band gap. For c-Au-TOAT 17 ACS Paragon Plus Environment


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and p-Au-TOAT, an extremely weak but stable H2 evolution was promoted by the loading of Au NPs, attributed to the hot electrons produced by surface plasmon resonance of Au NPs under visible light irradiation (Figure S7). Of note, H2 evolution efficiency over p-Au-TOAT (0.76 µmol/h/g) was slightly higher than that over c-Au-TOAT NCs (0.609 µmol/h/g) (Figure 6b). This difference in photocatalytic activity between the two Au-deposited TiO2 photocatalysts is likely related to several factors, including the actual loading amounts of Au NPs as well as their location and contact strength on TOAT. Unfortunately,

the

visible-light-driven

activities

of

Au-deposited

TiO2

photocatalysts was too low to meet practical demand. However, as expected, the activity was remarkably enhanced by the coating of CdS on Au NPs deposited on TOAT. Figure 6b shows that the H2 evolution efficiency over p-Au@CdS-TOAT and c-Au@CdS-TOAT reached 3.56 and 1.44 mmol/h/g, respectively, corresponding to a 4,500- and 2,400-fold increase to that prior to coating (i.e., p-Au-TOAT and c-Au-TOAT). Of note, the H2 evolution rate over c-Au-TOAT was only half that of p-Au@CdS-TOAT, indicating that the visible-light-driven activity of ternary composite photocatalysts was significantly influenced by the location of Au@CdS NPs on TOAT NCs. To investigate the role of CdS in H2 evolution, a photocatalyst (CdS-TOAT) where CdS NPs was directly deposited on the electron-rich {101} facets of TOAT NCs by photoreduction was specifically synthesized (Figure S8). The photocatalytic measurement revealed that CdS-TOAT was shown to have an H2 evolution efficiency of 0.487 mmol/h/g, being just 1/8 that of the p-Au@CdS-TOAT. 18 ACS Paragon Plus Environment

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Overall, it can be deduced that the enhanced visible-light-driven photocatalytic activity of p-Au@CdS-TOAT is associated with 1) the plasmon photosensitization of Au NPs; 2) the construction of a Au@CdS core-shell heterojunction; and 3) the selective deposition of Au NPs on TOAT {101} facets. The synergistic effect between the core-shell structured Au@CdS NPs and their location on TOAT NCs is the main driving force, as the direct contribution of photocatalytic activity from the Au NP SPR was extremely low in this case.

Figure 7. a) Repeated runs of H2 evolution under visible light (λ >400 nm) over p-Au@CdS-TOAT. Reaction condition: 25 mg catalyst in 100 ml solution containing 0.1 M Na2S and 0.1 M Na2SO3 aqueous solution. b) TEM image of p-Au@CdS-TOAT after four photocatalytic H2 evolution cycles. Inset is a HR-TEM image recorded from the zone marked by the dotted rectangle in b). As previously mentioned, CdS is likely to corrode upon irradiation due to S2− ion oxidation to elementary sulfur or other oxidized sulfur species by the photogenerated holes. In the H2 evolution measurements, Na2S/Na2SO3 were added as hole scavenger agents to suppress the photocorrosion of CdS. As shown in Figure 7a, during the first 19 ACS Paragon Plus Environment


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cycle, the H2 evolution amount over p-Au@CdS-TOAT was steady. However, the photocatalytic activity of p-Au@CdS-TOAT gradually decreased and the H2 evolution amount after 4 h was reduced by half after four cycles, as shown by the hollow dotted curves in Figure 7a. TEM images (Figure 7b) revealed that the CdS shell on Au NPs became thicker, in contrast to before the cycle experiment, and partially displayed 0.372 nm fringes, which can be assigned to the {221} planes of monoclinic sulfur. It is worth mentioning that there were Cd2+ ions of 0.3 µg/mL in the reaction solution after four runs of H2 evolution according to ICP-AES analysis. These results indicated that photocorrosion still occurred on the CdS shell of Au@CdS NPs anchored on TOAT NCs in p-Au@CdS-TOAT under visible light irradiation. This may have been due to the concentration of S2−/SO32− gradually reducing as the reaction progressed, finally being unable to efficiently scavenge the photogenerated holes in CdS. To confirm this, a run where the Na2S/Na2SO3 solutions were supplemented in each cycle was conducted. As shown by the solid dot curve in Figure 7a, p-Au@CdS-TOAT photocatalyst stability was significantly improved in the presence of sufficient hole scavengers, and the activity being well preserved after four cycles. Over the past decade, studies assessing the loading of Au and/or CdS NPs as co-catalysts or sensitizers have demonstrated the technique to be highly effective in improving the photocatalytic activity of wide bandgap semiconductor photocatalysts like TiO2, especially in the visible light region.21-26,45-47 Interestingly, for these binary or ternary composite photocatalysts, the electron transfer pathway under visible light 20 ACS Paragon Plus Environment

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is often different from that under ultraviolet light. For example, in Au/TiO2,44, 48-49 in a photocatalytic process under ultraviolet light, the photogenerated electrons transfer from TiO2 to Au and the reduction reaction occurs around the surface of the deposited Au NPs. Nevertheless, the electron transfer pathway would be reversed under visible light irradiation, i.e., from Au to TiO2.48 To experimentally assess the transfer direction of photogenerated electrons, photoreduction deposition of Pt under visible light irradiation (λ >400 nm) was conducted on p-Au@CdS-TOAT in the present study. As shown in Figure S9, Pt NPs formed by photoreduction were mainly located on the surface of truncated octahedral TiO2 NPs instead of the CdS shell, indicating that the electrons generated by visible light actually transferred from CdS to TiO2 with the aid of Au NPs in [email protected] Clearly, this electron transfer pathway is in marked contrast to the Z scheme mechanism under UV light irradiation.21-22 Interestingly, previous studies demonstrated that the transfer of visible-light-generated electrons would follow the pathway CdS⟶TiO2⟶Au in the case of Au@TiO2-CdS ternary composite photocatalysts in which CdS NPs were deposited on the surface of the core-shell-structured Au@TiO2 NCs.23 Thus, the spatial separation between photogenerated carriers in composite photocatalysts is greatly influenced by their structure architecture.



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Scheme 2. Schematic illustration of photocatalytic H2 evolution mechanism for p-Au@CdS-TOAT under visible light (λ >400 nm) irradiation.

Scheme 2 illustrates the mediating role of Au NPs in storing and shuttling photogenerated electrons from the CdS shell to the TiO2 support in a photocatalytic process under visible light. Firstly, the CdS shell absorbs visible light and the electron/hole pairs are formed in the conduction/valence bands. The photogenerated electrons are then trapped in the Au core. Given that the Au core is completely encased by CdS, the electrons transfer from the Au core to TiO2, where they react directly with H2O to produce H2. On the other hand, the holes left in the valence band of CdS are scavenged by the S2−/SO32− ions in solution. Through this mechanism, the CdS shell acts as a visible light sensitizer, while the Au core acts as a bridge for the transfer of photogenerated electrons from CdS to TiO2.41 Since the H2 evolution reaction actually occurs on the surface of TiO2, the visible-light-driven photocatalytic activity of this composite photocatalyst is closely related to the location of Au@CdS on TiO2. For anatase TiO2, the photogenerated electrons prefer to accumulate on the {101} facets in contrast to the {001} facets, because {001} facets exhibit more negative conduction band potential than {101} facets.50-52 Thus, it is reasonable to 22 ACS Paragon Plus Environment

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presume that the separation efficiency of photogenerated electrons and holes are relatively higher when Au@CdS NPs are selectively deposited on the {101} facets of TOAT NCs than on the {001} facets. This explains why the visible-light-driven H2 evolution

efficiency

over

p-Au@CdS-TOAT

was

higher

than

that

over

c-Au@CdS-TOAT despite similar loading amounts.

4. Conclusions A new ternary composite photocatalyst, p-Au@CdS-TOAT, with core-shell structured Au@CdS NPs selectively loaded onto the {101} facets of TOAT NCs was successfully synthesized via a two-step photodeposition process based on the facet-dependent

photochemical

reactivity

of

anatase

TiO2.

Compared

to

randomly-loaded Au@CdS NPs and with only Au- or CdS-loaded photocatalysts, this well-designed ternary photocatalyst was demonstrated to exhibit a much superior photocatalytic activity in H2 evolution under visible light. The remarkably enhanced photocatalytic activity of p-Au@CdS-TOAT can be attributed to the synergistic effect between the core shell structure of Au@CdS and the specific facet-induced effect of TiO2 NCs. On one hand, the introduction of Au@CdS NPs improves the poor light absorption of TiO2 in the visible region. On the other, the selective deposition of Au@CdS NPs on the {101} facets of TOAT NCs greatly strengthens the separation between photogenerated carriers with help from the Au core. The authors believe that the strategy demonstrated herein, which was based on engineering the location of



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co-catalysts, could be potentially applied in the development of other wide bandgap high performance, semiconductor-based composite photocatalysts.

ASSOCIATED CONTENT

Supporting Information. SEM and TEM of TOAT NCs; XRD pattern, EDX spectra and ICP-AES results of p-Au-TOAT and c-Au-TOAT; TEM images of p-Au-TOAT NCs after undergoing the deposition of CdS under different irradiation conditions; TEM image of CdS-TOAT; TEM image of p-Au@CdS-TOAT after Pt photoreduction under visible light. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Email: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Basic Research Program of China (2015CB932301), the National Natural Science Foundation of China (21473146 and 21333008) and the Fundamental Research Funds for the Central Universities (20720160026). 24 ACS Paragon Plus Environment

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REFERENCES (1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69‒96. (2) Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting.

Chem. Soc. Rev. 2009, 38, 253‒278. (3) Chen, X.; Shen, S.; Guo, L.; Mao, S. S. Semiconductor-based Photocatalytic Hydrogen Generation. Chem. Rev. 2010, 110, 6503‒6570. (4) Hashimoto, K.; Irie, H.; Fujishima, A. TiO2 Photocatalysis: a Historical Overview and Future Prospects. Jpn. J. Appl. Phys. 2005, 44, 8269. (5) Schneider, J.; Matsuoka, M.; Takeuchi, M.; Zhang, J.; Horiuchi, Y.; Anpo, M.; Bahnemann, D. W. Understanding TiO2 Photocatalysis: Mechanisms and Materials.

Chem. Rev. 2014, 114, 9919‒9986. (6) Linsebigler, A. L.; Lu, G.; Yates Jr, J. T. Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results. Chem. Rev. 1995, 95, 735‒758. (7) Chen, X.; Mao, S. S. Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications. Chem. Rev. 2007, 107, 2891‒2959. (8) Tachikawa, T.; Fujitsuka, M.; Majima, T. Mechanistic Insight into the TiO2 Photocatalytic Reactions: Design of New Photocatalysts. J. Phys. Chem. C 2007, 111, 5259‒5275. (9) Yu, S.; Kim, Y. H.; Lee, S. Y.; Song, H. D.; Yi, J. Hot-Electron-Transfer Enhancement for the Efficient Energy Conversion of Visible Light. Angew. Chem.

Int. Ed. 2014, 126, 11385‒11389. 25 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(10)

Page 26 of 33

Zang, L.; Macyk, W.; Lange, C.; Maier, W. F.; Antonius, C.; Meissner, D.;

Kisch, H. Visible-light Detoxification and Charge Generation by Transition Metal Chloride Modified Titania. Chem. - Eur. J. 2000, 6, 379‒384. (11)

Banerjee, S.; Mohapatra, S. K.; Das, P. P.; Misra, M. Synthesis of Coupled

Semiconductor by Filling 1D TiO2 Nanotubes with CdS. Chem. Mater. 2008, 20, 6784‒6791. (12)

Sun, W.-T.; Yu, Y.; Pan, H.-Y.; Gao, X.-F.; Chen, Q.; Peng, L.-M. CdS

Quantum Dots Sensitized TiO2 Nanotube-array Photoelectrodes. J. Am. Chem. Soc. 2008, 130, 1124‒1125. (13)

Li, G.-S.; Zhang, D.-Q.; Yu, J. C. A New Visible-light Photocatalyst: CdS

Quantum Dots Embedded Mesoporous TiO2. Environ. Sci. Technol. 2009, 43, 7079‒ 7085. (14)

Huo, Y.; Yang, X.; Zhu, J.; Li, H. Highly Active and Stable CdS–TiO2

Visible Photocatalyst Prepared by in Situ Sulfurization under Supercritical Conditions. Appl. Catal., B 2011, 106, 69‒75. (15)

Subramanian, V.; Wolf, E. E.; Kamat, P. V. Catalysis with TiO2/gold

Nanocomposites. Effect of Metal Particle Size on the Fermi Level Equilibration. J.

Am. Chem. Soc. 2004, 126, 4943‒4950. (16)

Primo, A.; Corma, A.; García, H. Titania Supported Gold Nanoparticles as

Photocatalyst. Appl. Catal., B 2011, 13, 886‒910. (17)

Zheng, Z.; Huang, B.; Qin, X.; Zhang, X.; Dai, Y.; Whangbo, M.-H. Facile in

Situ Synthesis of Visible-light Plasmonic Photocatalysts M@TiO2 (M= Au, Pt, Ag) 26 ACS Paragon Plus Environment

Page 27 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and Evaluation of Their Photocatalytic Oxidation of Benzene to Phenol. J. Mater.

Chem. 2011, 21, 9079‒9087. (18)

Seh, Z. W.; Liu, S.; Low, M.; Zhang, S. Y.; Liu, Z.; Mlayah, A.; Han, M. Y.

Janus Au‐TiO2 Photocatalysts with Strong Localization of Plasmonic Near‐Fields for Efficient Visible-Light Hydrogen Generation. Adv. Mater. 2012, 24, 2310‒2314. (19)

Mukherjee, S.; Libisch, F.; Large, N.; Neumann, O.; Brown, L. V.; Cheng, J.;

Lassiter, J. B.; Carter, E. A.; Nordlander, P.; Halas, N. J. Hot Electrons Do the Impossible: Plasmon-Induced Dissociation of H2 on Au. Nano Lett. 2013, 13, 240‒ 247. (20)

Bian, Z.; Tachikawa, T.; Zhang, P.; Fujitsuka, M.; Majima, T. Au/TiO2

Superstructure-Based

Plasmonic

Photocatalysts

Exhibiting

Efficient

Charge

Separation and Unprecedented Activity. J. Am. Chem. Soc. 2014, 136, 458‒465. (21)

Tada, H.; Mitsui, T.; Kiyonaga, T.; Akita, T.; Tanaka, K. All-solid-state

Z-scheme in CdS–Au–TiO2 Three-component Nanojunction System. Nat. Mater. 2006, 5, 782‒786. (22)

Zhu, H.; Yang, B.; Xu, J.; Fu, Z.; Wen, M.; Guo, T.; Fu, S.; Zuo, J.; Zhang,

S. Construction of Z-scheme Type CdS–Au–TiO2 Hollow Nanorod Arrays with Enhanced Photocatalytic Activity. Appl. Catal., B 2009, 90, 463‒469. (23)

Fang, J.; Xu, L.; Zhang, Z.; Yuan, Y.; Cao, S.; Wang, Z.; Yin, L.; Liao, Y.;

Xue, C. Au@TiO2–CdS Ternary Nanostructures for Efficient Visible-Light-Driven Hydrogen Generation. ACS Appl. Mater. Interfaces 2013, 5, 8088‒8092.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(24)

Page 28 of 33

Zhou, H.; Ding, L.; Fan, T.; Ding, J.; Zhang, D.; Guo, Q. Leaf-inspired

Hierarchical Porous CdS/Au/N-TiO2 Heterostructures for Visible Light Photocatalytic Hydrogen Evolution. Appl. Catal., B 2014, 147, 221‒228. (25)

Kim, M.; Kim, Y. K.; Lim, S. K.; Kim, S.; In, S.-I. Efficient Visible

Light-induced H2 Production by Au@CdS/TiO2 Nanofibers: Synergistic Effect of Core–shell Structured Au@CdS and Densely Packed TiO2 Nanoparticles. Appl.

Catal., B 2015, 166, 423‒431. (26)

Zhao, H.; Wu, M.; Liu, J.; Deng, Z.; Li, Y.; Su, B.-L. Synergistic Promotion

of Solar-driven H2 Generation by Three-dimensionally Ordered Macroporous Structured TiO2-Au-CdS Ternary Photocatalyst. Appl. Catal., B 2016, 184, 182‒190. (27)

Li, R.; Zhang, F.; Wang, D.; Yang, J.; Li, M.; Zhu, J.; Zhou, X.; Han, H.; Li,

C. Spatial Separation of Photogenerated Electrons and Holes Among {010} and {110} Crystal Facets of BiVO4. Nat. Commun. 2013, 4, 1432. (28)

Chen, W.; Kuang, Q.; Wang, Q. X.; Xie, Z. X. Engineering a High Energy

Surface of Anatase TiO2 Crystals towards Enhanced Performance for Energy Conversion and Environmental Applications. RSC Adv. 2015, 5, 20396‒20409. (29)

Zhang, L.; Wang, W.; Sun, S.; Jiang, D.; Gao, E. Selective Transport of

Electron and Hole Among {001} and {110} Facets of BiOCl for Pure Water Splitting.

Appl. Catal., B 2015, 162, 470‒474. (30)

Ohno, T.; Sarukawa, K.; Matsumura, M. Crystal Faces of Rutile and Anatase

TiO2 Particles and Their Roles in Photocatalytic Reactions. New J. Chem. 2002, 26, 1167‒1170. 28 ACS Paragon Plus Environment

Page 29 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(31)

D’Arienzo, M.; Carbajo, J.; Bahamonde, A.; Crippa, M.; Polizzi, S.; Scotti,

R.; Wahba, L.; Morazzoni, F. Photogenerated Defects in Shape-Controlled TiO2 Anatase Nanocrystals: A Probe to Evaluate the Role of Crystal Facets in Photocatalytic Processes. J. Am. Chem. Soc. 2011, 133, 17652‒17661. (32)

Tachikawa,

T.;

Yamashita,

S.;

Majima,

T.

Evidence

for

Crystal-face-Dependent TiO2 Photocatalysis from Single-Molecule Imaging and Kinetic Analysis. J. Am. Chem. Soc. 2011, 133, 7197‒7204. (33)

Roy, N.; Sohn, Y.; Pradhan, D. Synergy of Low-Energy {101} and

High-Energy {001} TiO2 Crystal Facets for Enhanced Photocatalysis. ACS Nano 2013, 7, 2532‒2540. (34)

He, Z.; Cai, Q.; Wu, M.; Shi, Y.; Fang, H.; Li, L.; Chen, J.; Chen, J.; Song, S.

Photocatalytic Reduction of Cr(VI) in an Aqueous Suspension of Surface-Fluorinated Anatase TiO2 Nanosheets with Exposed {001} Facets. Ind. Eng. Chem. Res. 2013, 52, 9556‒9565. (35)

Sun, D.; Yang, W.; Zhou, L.; Sun, W.; Li, Q.; Shang, J. K. The selective

deposition of silver nanoparticles onto {101} facets of TiO2 nanocrystals with co-exposed {001}/{101} facets, and their enhanced photocatalytic reduction of aqueous nitrate under simulated solar illumination. Appl. Catal., B 2016, 182, 85‒93. (36)

Lin, F.; Wang, D.; Jiang, Z.; Ma, Y.; Li, J.; Li, R.; Li, C. Photocatalytic

Oxidation of Thiophene on BiVO4 with Dual Co-catalysts Pt and RuO2 under Visible Light Irradiation Using Molecular Oxygen as Oxidant. Energy Environ. Sci. 2012, 5, 6400‒6406. 29 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(37)

Page 30 of 33

Liu, C.; Han, X. G.; Xie, S. F.; Kuang, Q.; Wang, X.; Jin, M. S.; Xie, Z. X.;

Zheng, L. S. Enhancing the Photocatalytic Activity of Anatase TiO2 by Improving the Specific Facet‐Induced Spontaneous Separation of Photogenerated Electrons and Holes. Chem. -Asian J. 2013, 8, 282‒289. (38)

Wang, L.; Ge, J.; Wang, A.; Deng, M.; Wang, X.; Bai, S.; Li, R.; Jiang, J.;

Zhang, Q.; Luo, Y. Designing p-Type Semiconductor–Metal Hybrid Structures for Improved Photocatalysis. Angew. Chem. Int. Ed. 2014, 126, 5207‒5211. (39)

Liu, C.; Tong, R. F.; Xu, Z. K.; Kuang, Q.; Xie, Z. X.; Zheng, L. S.

Efficiently Enhancing the Photocatalytic Activity of Faceted TiO2 Nanocrystals by Selectively Loading α-Fe2O3 and Pt co-catalysts. RSC Adv. 2016, 6, 29794‒29801. (40)

Yang, T.-T.; Chen, W.-T.; Hsu, Y.-J.; Wei, K.-H.; Lin, T.-Y.; Lin, T.-W.

Interfacial Charge Carrier Dynamics in Core−Shell Au-CdS Nanocrystals. J. Phys.

Chem. C 2010, 114, 11414‒11420. (41)

Ha, J. W.; Ruberu, T. P. A.; Han, R.; Dong, B.; Vela, J.; Fang, N.

Super-Resolution Mapping of Photogenerated Electron and Hole Separation in Single Metal–Semiconductor Nanocatalysts. J. Am. Chem. Soc. 2014, 136, 1398‒1408. (42)

Batzill, M., Fundamental Aspects of Surface Engineering of Transition Metal

Oxide Photocatalysts. Energy Environ. Sci. 2011, 4, 3275‒3286. (43)

Han, X. G.; Zheng, B. J.; Ouyang, J. J.; Wang, X.; Kuang, Q.; Jiang, Y. Q.;

Xie, Z. X.; Zheng, L. S. Control of Anatase TiO2 Nanocrystals with a Series of High-Energy Crystal Facets via a Fuorine-Free Strategy. Chem. - Asian J. 2012, 7, 2538‒2542. 30 ACS Paragon Plus Environment

Page 31 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(44)

Linic, S.; Christopher, P.; Ingram, D. B. Plasmonic-Metal Nanostructures for

Efficient Conversion of Solar to Chemical Energy. Nat. Mater. 2011, 10, 911‒921. (45)

Tian, Y.; Tatsuma, T. Mechanisms and Applications of Plasmon-Induced

Charge Separation at TiO2 Films Loaded with Gold Nanoparticles. J. Am. Chem. Soc. 2005, 127, 7632‒7637. (46)

Li, H.; Bian, Z.; Zhu, J.; Huo, Y.; Li, H.; Lu, Y. Mesoporous Au/TiO2

Nanocomposites with Enhanced Photocatalytic Activity. J. Am. Chem. Soc. 2007,

129, 4538‒4539. (47)

Li, J.; Cushing, S. K.; Zheng, P.; Senty, T.; Meng, F.; Bristow, A. D.;

Manivannan, A.; Wu, N. Solar Hydrogen Generation by a CdS-Au-TiO2 Sandwich Nanorod Array Enhanced with Au Nanoparticle as Electron Relay and Plasmonic Photosensitizer. J. Am. Chem. Soc. 2014, 136, 8438‒8449. (48)

Gomes Silva, C.; Juárez, R.; Marino, T.; Molinari, R.; García, H. Influence of

Excitation Wavelength (UV or Visible Light) on the Photocatalytic Activity of Titania Containing Gold Nanoparticles for the Generation of Hydrogen or Oxygen from Water. J. Am. Chem. Soc. 2011, 133, 595‒602. (49)

Tan, T. H.; Scott, J.; Ng, Y. H.; Taylor, R. A.; Aguey-Zinsou, K.-F.; Amal,

R. Understanding Plasmon and Band Gap Photoexcitation Effects on the Thermal-Catalytic Oxidation of Ethanol by TiO2-Supported Gold. ACS Catal. 2016,

6, 1870‒1879.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(50)

Page 32 of 33

Gupta, B.; Melvin, A. A.; Matthews, T.; Dash, S.; Tyagi, A. K. TiO2

Modification by Gold (Au) for Photocatalytic Hydrogen (H2) Production. Renewable

Sustainable Energy Rev. 2016, 58, 1366‒1375. (51)

Hengerer, R.; Kavan, L,; Krtil, P.; Grätzel, M. Orientation Dependence of

Charge-Transfer Processes on TiO2 (Anatase) Single Crystals. J. Electrochem. Soc. 2000, 147, 1467‒1472.

(52)

Angelis, F.; Vitillaro, G.; Kavan, L.; Nazeeruddin, M. K.; Grätzel, M.

Modeling Ruthenium-Dye-Sensitized TiO2 Surfaces Exposing the (001) or (101) Faces: A First-Principles Investigation. J. Phys. Chem. C 2012, 116, 18124−18131

(53)

Kullgren, J.; Aradi, B.; Frauenheim, T.; Kavan, L.; Deák, P. Resolving the

Controversy about the Band Alignment between Rutile and Anatase: The Role of OH−/H+ Adsorption. J. phys. Chem. C 2015, 119, 21952‒21958


Page 33 of 33

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Efficiently Enhancing Visible Light Photocatalytic Activity of Faceted

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