TiO2 Using Plasmonic Au Nanoparticles for

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Sensitization of Pt/TiO2 using Plasmonic Au Nanoparticles for Hydrogen Evolution under Visible Light Irradiation Fenglong Wang, Roong Jien Wong, Jie Hui Ho, Yijiao Jiang, and Rose Amal ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06265 • Publication Date (Web): 22 Aug 2017 Downloaded from http://pubs.acs.org on August 22, 2017

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ACS Applied Materials & Interfaces

Sensitization of Pt/TiO2 using Plasmonic Au Nanoparticles for Hydrogen Evolution under Visible Light Irradiation

Fenglong Wang,1,*,# Roong Jien Wong,1,# Jie Hui Ho,1 Yijiao Jiang, 2,* and Rose Amal1,*

1

School of Chemical Engineering, UNSW Australia, Sydney, NSW 2052, Australia.

Tel: +612-9385-4361;Fax: +612-9385-5966; E-mail: [email protected]; [email protected]. 2

Department of Engineering, Macquarie University, Sydney, NSW 2109, Australia.

Email: [email protected].

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ABSTRACT Au nanoparticles with different sizes (10 nm, 20 nm, 30 nm and 50 nm) were synthesized using a seed-assisted approach and anchored onto Pt/TiO2 employing 3-mercaptopropionic acid as the organic linker. The sizes of the Au nanoparticles were controlled within a narrow range so that the size-dependent surface plasmonic resonance effect on sensitizing Pt/TiO2 can be thoroughly studied. We found that 20 nm Au nanoparticles (Au20) gave the best performance in sensitizing Pt/TiO2 to generate H2 under visible light illumination. Photoelectrochemical measurements indicated that Au20-Pt/TiO2 exhibited the most efficient “hot” electrons separation among the studied catalysts, correlating well with the photocatalytic activity. The superior performance of Au supported Pt/TiO2 (Au20-Pt/TiO2) compared with Au anchored to TiO2 (Au20/TiO2) revealed the important role of Pt as a cocatalyst for proton reduction. To elucidate how the visible light excited hot electrons in Au nanoparticles involved into the proton reduction reaction process, Au20/TiO2 was irradiated by visible light (λ>420 nm) with the presence of Pt precursor (H2PtCl6) in a methanol aqueous solution under deaerated condition. Energy-dispersive X-ray spectroscopy mapping analysis on the recovered sample showed that Pt ions could be reduced on the surfaces of both Au nanoparticles and TiO2 support. This observation indicated that the generated “hot” electrons on Au nanoparticles were injected into TiO2 conduction band, which were then subsequently transferred to Pt nanoparticles where proton reduction proceeded. Besides, the excited hot electrons could also participate into the proton reduction on Au nanoparticles surface. KEY WORDS: Surface plasmonic resonance effect; photocatalytic hydrogen production; gold nanoparticles; hot electron transfer; visible-light photocatalysis


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INTRODUCTION Solar-to-chemical energy conversion by photocatalysis is an alluring process that has potential to alleviate our dependency on the non-renewable fossil fuels and solve the environmental problems caused by fossil fuel combustion.1-7 After the initial report on the photoelectrochemical water splitting in 1972, TiO2-based photocatalysts have been extensively studied due to their availability, high photo-stability and low toxicity.8-15 However, their wide applications are still hindered due to the low energy conversion efficiency caused by high electron-hole recombination rate and weak response to the visible light.8 A considerable number of studies have been performed to improve the separation process of the photo-induced charge carriers and extend the absorption edge of TiO2-based materials to the visible light spectrum.16-19 Metal nanoparticles with larger work functions relative to TiO2 are widely explored as the electron sinks to trap the photo-excited electrons from the conduction band of TiO2, and thus the life time of the photo-generated charge carriers can be prolonged.1 As such, enhancement on the photocatalytic activity would be expected. Besides, the deposited nanoparticles also act as co-catalysts offering the active sites for proton reduction reactions.20 In order to shift the absorption edge of TiO2-based catalysts to visible light region for a better solar energy conversion efficiency, except for doping with metal and non-metal elements into the lattice of TiO2 to create the intermediate bands, coupling TiO2 with narrower band gap materials, metal clusters and nanoparticles possessing suitable band alignment with TiO2 has emerged as a promising route to achieve this goal.21-24

Recently, Au and Ag nanoparticles supported on metal oxide semiconductors have been explored as new efficient media for solar-to-chemical energy conversion due to their surface plasmonic resonance (SPR) properties.25 Induced by the incident electromagnetic irradiation, the oscillations of the conduction electrons in these metal nanostructures show strong visible



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light absorption. When nano Au or Ag deposited on TiO2 supports are illuminated, the generated “hot” electrons of these metal nanoparticles are injected into the conduction band of TiO2 due to the potential gradient at the interfaces and consequently the visible-light induced activity is anticipated.26 The charge transfer process from the noble metal nanoparticles to TiO2 has been detected by surface photocurrent, transient photovoltage and single-particle fluorescence spectroscopies.25,

27

In addition it is found that particle size,

particle shape and supporting materials governs the surface plasmonic resonance effect.28 For example, the local field strength can be greatly enhanced at the sharp tip of the noble metal particles.29 For spherical nanoparticles, the induced field strength is uniformly distributed and the plasmonic resonance effect on photocatalytic activity is governed by the nanoparticle size.30 Qian et al. showed that to some extent larger Au nanoparticles could be a better sensitizer for TiO2 in visible light photocatalysis.30 Furthermore, Yin’s group prepared the Au nanoparticles with a narrow size distribution to sensitize TiO2 materials for hydrogen generation under visible light irradiation. In their study, Au nanoparticles with size of 10 nm acted as efficient sensitizers for TiO2 for visible-light-driven photocatalysis.31 It is important to further improve the efficiency of such plasmonic photocatalysts and get better understanding on the plasmonic-induced photocatalytic process especially with the presence of a co-catalyst on TiO2 surface.

Herein, with the aim to understand the hydrogen production performance on such plasmonic photocatalysts, we synthesized Au nanoparticles of different sizes to sensitize Pt/TiO2 nanocomposites for hydrogen generation under visible light irradiation. The sizes of the Au nanoparticles were controlled from 10 nm to 50 nm to study the effect of size on the surface plasmonic resonance induced photocatalytic activity. The enhanced activity observed for Au20-Pt/TiO2 compared to Au20/TiO2 demonstrated an important role of Pt as a co-catalyst for


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proton reduction. The non-activity observed for Pt/TiO2 under visible illumination confirmed the indispensable role of “hot” electrons from plasmonic Au nanoparticles in initiating the reaction. Photoelectrochemical (PEC) characterizations on different sized Au supported catalysts indicated that Au20-Pt/TiO2 showed the most efficient “hot” electron separation. Besides, under visible light irradiation, Pt6+ could be reduced on both TiO2 and Au nanoparticle surfaces when Au20/TiO2 was used, revealing that the proton reduction reaction could occur on both the surfaces of excited Au nanoparticles and TiO2 supports. The approach presented in this study displays a facile way to prepare isolated Au and Pt metals on TiO2 in which Au nanoparticles act as the light absorber and Pt nanoparticles perform as the co-catalysts for proton reduction. Moreover, the mechanism studies would also advance our understanding on the SPR-induced photocatalysis in the presence of co-catalysts.

EXPERIMENTAL SECTION Au nanoparticles synthesis and Pt/TiO2 preparation. Gold nanoparticles with the size range of 10-50 nm were synthesised by a modified seed-assisted method reported previously.32 Firstly, 3 ml of HAuCl4 (5 mM) and 3 ml of sodium citrate (5 mM) were added into 54 ml of Milli-Q H2O. While stirring the solution at 1000 rpm, 1.8 ml of ice-cold NaBH4 solution (0.1 M) was quickly injected into the solution. After stirring for 4 h, the gold seed solution was obtained. The growth solution was subsequently prepared by adding 5 wt.% polyvinylpyrrolidone (PVP) solution (2.5 ml), 0.1 M L-ascorbic acid solution (1.25 ml), 0.2 M KI solution (1 ml), and 0.25 M HAuCl4 solution (0.3 ml) with 10 ml of Milli-Q H2O under stirring. 16.0, 1.0, 0.5, and 0.1 ml of the gold seed solution was injected into the above growth solution under vigorous stirring to obtain 10 nm, 20 nm, 30 nm, and 50 nm Au nanoparticles, respectively. After 10 min reaction, Au nanoparticles with sizes of 10, 20, 30, and 50 nm, denoted as Au10, Au20, Au30, and Au50, respectively, were obtained. 5 ACS Paragon Plus Environment


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Pt/TiO2 nanocomposites with Pt loading of 1 wt.% were prepared through a chemical reduction method using NaBH4 as the reducing agent as reported previously.33-34

Preparation of Au-Pt/TiO2 composite catalysts. 500 mg of the prepared Pt/TiO2 were dispersed into 100 ml of H2O containing 1 ml of 3-mercaptopropionic acid (3-MPA) by sonication for 10 min. After stirring for another 10 min, the solids were recovered by centrifuge to remove the residual 3-MPA and redispersed into 100 ml of H2O by sonication and stirring for 10 min, respectively. Then the appropriate amount of Au NPs sols were injected into the suspension under vigorous stirring to achieve 2 wt.% loading. The suspension was stirred for another 10 min. The mixture was kept static overnight, and then centrifuged and dried at 60 oC. The nanocomposites were annealed at 200 oC for 2 h in Ar atmosphere to obtain Au10-Pt/TiO2, Au20-Pt/TiO2, Au30-Pt/TiO2 and Au50-Pt/TiO2. For the preparation of Au20/TiO2 composite catalysts, the bare TiO2 was used as support. The loading efficiency of the metals was determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES) and energy dispersive X-ray spectra (EDX) under transmission electron microscopy (TEM) (Table S1, Fig. S1-S2). Photocatalyst characterisations. High resolution TEM (HRTEM) images of the prepared catalysts were obtained on Philips CM200 microscope. X-ray photoelectron spectroscopy (XPS) measurements were conducted on an ESCALAB250Xi spectrometer (Thermo Scientific, UK) using a mono-chromated Al K-alpha X-ray radiation at 15.2 kV and 168 W. The spectra deconvolution was carried out by XPS PEAK41 software packages. The highangle annular dark field scanning TEM (HAADF-STEM) images and EDX spectra of photoreduced Pt-Au20/TiO2 were recorded on a JEOL JEM-ARM200F TEM with an EDX detector. The Au-L and Pt-L edges were used to obtain the elemental distribution information in the EDX spectra. 6 ACS Paragon Plus Environment

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Photocatalytic activity evaluation. The photocatalytic hydrogen generation performance of the prepared catalysts was evaluated in a top-irradiation reactor using a 300 W Xenon arc lamp as the light source. In order to study the SPR effect of the metal nanostructures under visible-light irradiation on the photocatalytic process, a cutoff optical filter of λ > 420 nm was used. In a typical procedure, 100 mg of the prepared catalyst was dispersed with an ultrasonic water bath in 150 ml of water/methanol mixture (volume ratio = 9:1). Ar was purged into the system for more than 30 min to completely remove the air. The mixture was then irradiated for 6 h under magnetic stirring and a water jacket was placed on the top of the reactor to remove the thermal catalytic effect. The evolved gas was analysed using a gas chromatography (GC, Shimadzu, 8A) equipped with a thermal conductivity detector (TCD). In order to identify the reduction sites during photocatalytic hydrogen generation, Pt photodeposition was conducted. 30 mg of the Au20/TiO2 catalysts were dispersed into 100 ml of water/methanol mixture (volume ratio = 3:1) in the reactor and then the suspension was purged with Ar for 1 h to completely remove the dissolved oxygen. Subsequently, the desired amount of chloroplatinic acid solution was injected into the suspension under stirring aiming for the Pt loading of 2 wt.% on the catalysts surface. The suspension was then irradiated under visible light (λ > 420 nm) for 3 h and the solids were collected by washing and centrifuging for TEM analysis. The recovered solids were denoted as photo-reduced PtAu20/TiO2. PEC measurements. The photo-electrodes were prepared by drop-casting method. In a typical process, 2 mg of the catalyst was dispersed into 2 ml of ethanol by sonication and then the suspension was dropped onto a confined area on carbon fibre (1 cm×1 cm) through a layer-by-layer self-assembly method to fabricate the working electrode. Photocurrent measurements were undertaken at room temperature in a 0.5 M Na2SO4 aqueous solution as the electrolyte using an Autolab potentiostat (Model PGSTAT302N). A bias of 0.75 V was 7 ACS Paragon Plus Environment


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applied in a three-electrode PEC cell with a Pt plate as the counter electrode and Ag/AgCl electrode as the reference electrode. The catalyst-coated carbon fibres were used as the working electrodes. The electrolyte solution was purged with N2 for 10 min prior to analysis with the purging continued during the photocurrent measurement to remove any dissolved O2 in the cell. The photocurrent was measured using a 300 W Xenon arc lamp with a 420 nm cutoff filter. To minimize the heat effect of the electrolyte solution during illumination, a water jacket was placed between the Xenon arc lamp and the PEC cell. Bandpass filters with >90 % light transmission and 50 nm band width at full width half maximum (FWHM) were obtained from Edmund optics. The filters obtained have maximum light transmission at 400 nm (blue light), 550 nm (green light), and 675 nm (red light).

RESULTS AND DISCUSSION Fig. 1 shows the synthesis strategy to specifically anchor Au nanoparticles onto TiO2 surface of Pt/TiO2 nanocomposite catalysts. In a typical procedure, Pt/TiO2 composites with Pt loading of 1 wt.% were prepared by a chemical reduction method as the support for the Au nanoparticles.33 Upon completion of Pt/TiO2 preparation, the 3-MPA molecules were anchored onto the surface of Pt/TiO2 through the dehydration reaction between the carboxyl groups of 3-MPA and the hydroxyl groups on Pt/TiO2 surface. The residual 3-MPA molecules in the mixture were removed by centrifuge and washing. Subsequently, due to its strong affinity to Au, the thiol group (-SH) on the other head of 3-MPA molecules would readily replace PVP on Au nanoparticles when the PVP protected Au nanoparticles were present, thus forming Pt/TiO2-MPA-Au structure. In the following low-temperature thermal treatment, due to its volatile nature, 3-MPA was easily removed resulting in intimate attachment between Au and Pt/TiO2 without any serious aggregation.31


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Fig. 1 Strategy for synthesis of Au nanoparticles supported on Pt/TiO2 composite catalysts.

Figs.2a-d show that the Au nanoparticles with narrow size distributions can be obtained from the seeded growth method. When the addition of Au seed solution increases from 0.1 to 0.5, 1 and 16 ml, the size of gold nanoparticles decreases from ca. 50 to 30, 20 and 10 nm, respectively. It is also shown that the agglomeration of the Au nanoparticles can be inhibited by using PVP as the protection layer on the surface.31 In the growth solution, the synergistic effect between KI and ascorbic acid leads to the mild nucleation of gold on the seeds and thus the self-nucleation of gold nanoparticles can be avoided.32 Given that the growth solutions are constant for all the preparations, by varying the volume of the seed solution, it would be easy to manipulate the size of the final gold nanoparticles since the numbers of the nucleation sites are varied in each solution. Besides, with the presence of PVP molecules, the capping effect would prevent the inter-particle agglomeration of the Au nanoparticles, affording the monodispersed spherical Au nanostructures. Fig. 2e shows the TEM image of the as-prepared



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Pt/TiO2, and it indicates that the Pt nanoparticles with an average size of around 3 nm are distributed uniformly on the surface of TiO2. Fig. 2f and Fig. S4 (in the Supporting Information) display that, after hybridization with 20 nm gold nanoparticles, Au20-Pt/TiO2 nanocomposites are achieved.

Fig.2 TEM images of the synthesised gold nanoparticles with size of approximately 50 nm (a), 30 nm (b), 20 nm (c), and 10 nm (d), TEM images of Pt/TiO2 showing 3 nm Pt 10 ACS Paragon Plus Environment

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nanoparticles distributed on the surface of TiO2 (e) and Au20-Pt/TiO2 nanocomposites displaying Au20 and Pt nanoparticles supported on the TiO2 surface (f). The XPS core-level spectra of Au20-Pt/TiO2 sample were recorded to study the electronic properties of the loaded metal nanoparticles (Fig. 3). As shown in Fig. 3a, the peaks at 71.4 eV and 74.5 eV can be assigned to the Pt 4f7/2 and Pt 4f5/2 orbits of metallic state Pt, respectively,and the peaks at 71.9 eV and 75.8 eV arise from Pt 4f7/2 and Pt 4f5/2 levels of Pt oxide species, respectively.35 Further analysis of the peak areas indicates that the metallic state Pt is dominant, taking up ca. 88 % of the present Pt element (carried out by the XPSPEAK 41 software package). The presence of the oxidized Pt species probably could be attributed to the oxidation of metallic Pt in the air prior to XPS analysis.33 The gold in the Au20-Pt/TiO2 sample exists in metallic states, as indicated by the Au 4f spectra, which were composed of the Au 4f7/2 and Au 4f5/2 peaks at 83.1 eV and 86.8 eV, respectively (Fig. 3b). Compared to the XPS data taken from bulk gold materials (84.0 eV), the Au 4f7/2 peaks were slightly shifted toward lower binding energy region, indicating a strong interaction between Au and the TiO2 support.36-37 The strong interaction between Au nanoparticles and TiO2 support would be expected to benefit the “hot” electrons transfer from the Au nanoparticles surface to the TiO2 conduction band through the Schottky energy barriers.

(a)

(b)

Au 4f7/2 Au 4f5/2

Intensity (a. u.)

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Pt 4f 5A (metal)

Pt 4f 7B (oxide)

Pt 4f 7A (metal)

Pt 4f 5B (oxide)

78

76

74 72 Binding energy (eV)

70 90

88

86 84 Binding Energy (eV)

82

80

Fig. 3 (a) XPS Pt 4f spectrum and (b) XPS Au 4f spectrum of the Au20-Pt/TiO2 sample.



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The visible-light photocatalytic activity of hydrogen production over the synthesised catalysts was tested, and the size effect on SPR induced photocatalysis was investigated. As shown in Fig. 4, when the size of Au nanoparticles is around 20 nm, the Au20-Pt/TiO2 composite catalyst shows the best activity, revealing that the most photo-excited electrons are available for proton reduction, which gives a hint that the charge injection from Au20 nanoparticles to the conduction band of TiO2 might be the most efficient one among the studied catalysts. A representative TEM image of the recovered Au20-Pt/TiO2 demonstrates that the Au and Pt nanoparticles are well dispersed after reaction, showing the high stability of such catalysts (Fig. S5). In order to verify that the activity is contributed by the SPR effect of the gold nanoparticles but not the TiO2 support, control experiment with the Pt/TiO2 was conducted under the identical condition. The result shows that in the absence of gold nanoparticles, the Pt/TiO2 shows no activity for the hydrogen generation, ascertaining that the activity of the composite catalysts was induced by the SPR effect of gold. Another control experiment with Au20/TiO2 shows that the catalyst exhibits lower activity compared to the Au20-Pt/TiO2, indicating that loading of Pt is necessary to enhance the activity for proton reduction into hydrogen. The observation that the presence of Pt nanoparticles in the Au20-Pt/TiO2 system improves the activity for hydrogen production, but no activity was observed for Pt/TiO2 confirms the electrons flow from Au nanoparticles to TiO2 support and further into Pt nanoparticles, where the proton reduction occurs. With the absence of Pt nanoparticles as cocatalysts, the migrated “hot” electrons in the conduction bands of TiO2 would probably be captured by the Ti4+ ions, which led to less electrons available for proton reduction.38


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Fig. 4 Hydrogen production activity of different catalysts under visible light irradiation (λ > 420 nm) from water/methanol mixture.

To understand the “hot” electron transfer process in various Au supported catalysts, transient photo-current measurements were conducted in a three-electrode PEC cell under visible light illumination (λ > 420 nm). As reported previously, Pt-TiO2 electrode showed no photoresponse under visible irradiation since TiO2 is only ultraviolet responsive.34 For all the Au supported catalysts as shown in Fig. 5, a fast and uniform photocurrent response was observed for each on/off cycle under visible light irradiation, which can be attributed to the generation and transfer of “hot” electrons from the plasmonic Au nanoparticles. It clearly shows that, under visible light illumination, the photocurrent of the Au20-Pt/TiO2 electrode is strongly enhanced compared to the other catalysts, which would probably due to the efficient “hot” electron transfer process from Au20 to TiO2 conduction band. The efficient charge separation process could lead to the enhanced activity for hydrogen generation.



8 Au20/TiO2

6

Au10-Pt/TiO2

on off

Au20-Pt/TiO2 Au30-Pt/TiO2

Current (µA)

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Au50-Pt/TiO2

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80 100 120 140 160 180 Time (s)

Fig. 5 Photocurrent response of Au nanoparticles with different sizes supported on Pt/TiO2 nanocomposites under visible light irradiation (λ > 420 nm). Every 40 s cycle comprises 20 s of irradiation, followed by 20 s of dark current measurement.

The visible light response of Au20Pt/TiO2 is further investigated by performing a wavelength dependence PEC measurement. The photocurrent responses of Au20Pt/TiO2 under blue light (400 nm), green light (550 nm), and red light (675 nm) are presented in Fig. 6. The PEC performance of Au20Pt/TiO2 corresponds to the UV-Vis diffuse reflectance spectrum (Fig. S3). UV-Vis diffuse reflectance spectrum reveals the SPR excitation wavelength of Au nanoparticles peaks at ~550 nm, suggesting a stronger SPR enhancement under green light irradiation than other lights. This wavelength dependent SPR enhancement effect is also observed from the PEC measurement, indicating the reliance of Au20Pt/TiO2 visible light activity on the SPR excitation of Au nanoparticles.


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Fig. 6 Photocurrent responses of Au20Pt/TiO2 under full visible light spectrum (>420 nm), blue light (400 nm), green light (550 nm), and red light (675 nm). All measurements were performed with a 420 nm cutoff filter to prevent the excitation of TiO2 support under UV light. Bandpass filters (50 nm FWHM bandwidth) were used to obtain the 400 nm, 550 nm, and 675 nm light sources.

To gain better understanding on the hydrogen generation mechanism on Au20-Pt/TiO2, Au20/TiO2 was irradiated under visible light for 3 h with the presence of Pt precursor (chloroplatinic acid hydrate) and the product was examined under TEM, as shown in Fig. 7. From the TEM image, it is easy to observe that the Au nanoparticles maintain the size of around 20 nm after visible light irradiation for 3 h, showing the high stability of such plasmonic catalysts. It also shows that Pt nanoparticles with a size of around 2-3 nm are deposited on Au nanoparticle surfaces and TiO2 surfaces, indicating that Pt4+ reduction occurs on both Au and TiO2 surfaces (Fig. 7a). The EDX mapping analysis based on Pt-L and Au-L edges shown in Fig. 7b-d clearly demonstrate that Pt nanoparticles are deposited on both the surface of Au and TiO2, confirming the TEM observations.



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Based on the above results, the proposed reaction mechanism on the surface of such plasmonic catalysts is illustrated in Fig. 7e. The proton reduction could occur on the surface of Au nanoparticles and TiO2 support, revealing that part of the visible light excited “hot” electrons are migrated to the conduction band of TiO2 through the Schottky energy barrier. Without Pt deposits, in Au20/TiO2 some of the excited “hot” electrons will be injected into TiO2 conduction band, which might be captured by the surface defects or proton ions, and the rest of the electrons would oscillate onto the Au nanoparticles surface where they will reduce protons into hydrogen molecules. At the presence of Pt nanoparticles, the electrons injected into the TiO2 conduction band will be captured by Pt nanoparticles and participate into the proton reduction reaction. This would be the reason why Au20-Pt/TiO2 exhibited enhanced hydrogen generation compared to Au20/TiO2.


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Fig. 7 TEM image of the products obtained through photo-deposition of Pt on Au20/TiO2 nanocomposites (a-d) and the proposed photocatalytic hydrogen production process on the Au sensitized Pt/TiO2 under visible light irradiation (e).



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To further elucidate the charge transfer mechanism, PEC measurement was performed in the presence of different sacrificial agents, i.e. methanol and ethanol as hole scavengers. The PEC performance of Au20Pt/TiO2 when methanol was used as the hole scavenger yields approximately 50 % higher photocurrent response compared to ethanol, a lesser hole scavenger (Fig. 8). The hole scavenging dependence of the photocurrent response during SPR excitation indicates the SPR enhancement is primarily a “hot electron-hole” charge transfer mechanism, therefore in agreement with the proposed mechanism in Fig. 7e.

Fig. 8 Photocurrent response of Au20Pt/TiO2 in the presence of (a) methanol and (b) ethanol as hole scavenger. PEC measurement is performed with a 420 nm cutoff filter to prevent the excitation of TiO2. The photocurrent response is baseline corrected to show the SPR enhancement under visible light.

CONCLUSIONS In the current work, Au nanoparticles with sizes from 10 to 50 nm in a narrow size distribution have been successfully prepared through a seeded growth method for the sensitization of Pt/TiO2 as visible-light responsive catalysts for hydrogen production. The TEM characterisations show that in the Au-Pt/TiO2 nanocomposites, both Au and Pt 18 ACS Paragon Plus Environment

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nanoparticles are uniformly distributed on the surface of TiO2. XPS core-level spectra indicate that the Au exists in metallic state and Pt nanoparticles are partially oxidized with the metallic state as the dominance. Photoelectrochemical measurements show that in Au20Pt/TiO2 the “hot” electron transfer from Au nanoparticles to TiO2 is the most efficient one among the studied catalysts. In addition, control experiment with Au20-TiO2 in the absence of Pt nanoparticles shows that the Pt nanoparticles play an important role as the co-catalysts for proton reduction. Meanwhile, it is also deduced that part of the electrons flow from Au nanoparticles to TiO2 and further to Pt nanoparticles in the composite catalysts and the other part of the excited “hot” electrons oscillate to Au nanoparticles surface participating into the reduction reactions. This study advances the understanding of the plasmonic photocatalysis and also provides an efficient route to prepare visible-light active TiO2-based catalysts.

Supporting Information. The UV-Vis spectra of the studied TiO2-based catalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

Author contributions #

These authors contributed equally.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

Financial support through the ARC Discovery Early Career Researcher Award (DE120100329) and ARC Discovery Project (DP140102432) are gratefully acknowledged. We also thank the UNSW Mark Wainwright Analytical Centre at UNSW Australia for use of 19 ACS Paragon Plus Environment


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facilities. The use of JEOL JEM-ARM200F TEM funded by ARC Linkage Infrastructure, Equipment and Facilities grant (LE120100104) located at the UOW Electron Microscopy Centre is also greatly appreciated. We are grateful to the anonymous reviewers for their valuable comments and suggestions.

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TiO2 Using Plasmonic Au Nanoparticles for

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