Enhanced photoelectrochemical performance of tungsten oxide film by bifunctional Au nanoparticles

Abstract

In the present study, Au nanoparticles were used to improve the photoelectrochemical performance of tungsten oxide film. Our study indicated that the surface plasmon resonance (SPR) of the Au nanoparticles at about 560 nm wavelength extended the light adsorption of the WO₃ film. Mott–Schottky measurements and theoretical calculations determined that an inner electric field is built at the Au/WO₃ interface through electronic transfer from Au to WO₃. The measured photocurrent density and quantum efficiency results indicated that the synergistic effect of SPR and the inner electric field improved the PEC performance of the Au/WO₃ photoelectrodes. However, the load of Au nanoparticles should be carefully considered due to the introduction of the surface recombined centers. The present study provides a reasonable understanding of the effects of the Au nanoparticles on the PEC performance of the WO₃ film.

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1. Introduction

Photoelectrochemical (PEC) water splitting for hydrogen and oxygen production has been paid considerable attention as it is a promising technology to alleviate both the energy crisis and environmental issues.^1–5 For the PEC system, the properties of photoelectrode materials play an important role on the system performance. In recent years, the photoelectrode consisting of nanostructured n-type transition metal oxide has attracted increasing research interest due to the advantages of chemical stability, efficient light absorption, and ease of fabrication.^6–11 Herein, tungsten oxide (WO₃) has been seen as a promising photoelectrode material for the PEC water oxidation system due to its 2.6–2.8 eV band gap and photostability in water.^12–15 Furthermore, WO₃ can also provide enough overpotential for the crucial half reaction of oxygen evolution due to its favorable valence position (which is approximately 3 V versus NHE). Recently, Ma and coworkers reported that a mesoporous WO₃ photoanode with a dual oxygen and tungsten vacancies-containing overlayer could greatly enhance the PEC activity for water oxidation, which was attributed to alleviating charge recombination at the electrode/electrolyte interface.¹⁶ Wang's group successfully prepared WO₃ nanoplate arrays on FTO glass substrates by tuning the hydrothermal conditions, which presented the highest photocurrent density for the bare WO₃ photoanode without any cocatalysts or sacrificial reagents.¹⁷

Nevertheless, WO₃ still suffers from some inadequacies, such as the rapid electron–hole recombination, lower reducibility of conduction band electrons, and limited response in most of the visible light region. To overcome these issues, various endeavors have been applied by controlling the nanostructure, doping other elements, constructing heterojunction, and loading cocatalysts.^18–22 Ding et al. prepared one-dimensional nanostructured WO₃ by a flame vapor deposition process, which achieved a maximum IPCE efficiency of 45.8% at 410 nm through controlling the size of the WO₃ nanostructure.¹² Yang and coworkers used Fe-doping to enhance the PEC performance of the WO₃ photoanode, suggesting that this enhancement was related to doping-enhanced charge carrier density.²³ Bard's group demonstrated that the efficient heterojunction formed on the interface of the ZnWO₄/WO₃ composite induced an increased PEC activity for water oxidation under both UV and visible illumination.²⁴ In general, noble metals (such as Pt, Au, etc.) and some metal oxides (such as IrO₂, RuO₂, etc.) can act as the water splitting cocatalysts to improve the PEC performance of semiconductor based materials.^25–30 Pt is usually used as a suitable hydrogen evolution cocatalyst. Bao et al. obtained a considerably high quantum yield (about 60.34%) when 3–5 nm Pt nanocrystals were loaded as cocatalysts onto CdS.²⁶ Yang et al. fabricated a HfO₂ passivation layer on the WO₃ surface using a dip-coating method, which improved the WO₃ PEC performance by suppressing the interfacial recombination of electrons and holes. Among them, Au nanoparticles have been focused on and used to extend the light response region due to their surface plasmon resonance (SPR)^31,32 when coupled with appropriate materials.^33–35 TiO₂ decorated by Au nanoparticles showed an enhanced PEC performance due to the increased optical absorption associated with the SPR effect. Wu's group noted that the Au nanoparticles in the CdS–Au–TiO₂ system not only acted as a photosensitizer extending the light response wavelength from 525 to 725 nm but also promoted the interfacial charge transfer.³³ Two effects of the loaded Au nanoparticles, both the surface catalysis and the optical functions, were also observed on the CeO₂ surface to determine the final photocatalytic performance.³⁶ Therefore, a positive role of the Au nanoparticles on the WO₃ photoelectrode can be expected for improving the PEC performance.

Herein, we report a facile hydrothermal method for in situ preparing WO₃ films, and successfully obtaining a series of Au modified WO₃ photoanodes by an immersion–calcination method. We characterized the PEC performance for all the samples under both simulated sunlight and visible light irradiation. DFT calculations together with experimental studies were employed to determine the optimal loading amount of Au nanoparticles, which revealed that the enhanced PEC performance is attributed to the two effects of the loaded Au nanoparticles. Our study takes the bifunctional roles of Au nanoparticles into consideration simultaneously, and thus can provide a new insight in designing different metal–semiconductor systems for PEC water oxidation.

2. Experimental

2.1 Preparation of WO₃ crystal seeds

All reagents were of analytical grade and used as received without further purification. The WO₃ film was fabricated with pre-seeding. In a typical procedure, 3.299 g of Na₂WO₄·2H₂O was dissolved in 50 mL deionized water, and the abovementioned solution was vigorously stirred for 10 min. Then, 30 mL of 3 M HCl solution was added until no more faint yellow precipitate was formed. The yellow precipitate was collected by centrifugation and washed with deionized water. After that, the precipitate was dissolved into 2 mL H₂O₂ solution to obtain a yellow transparent sol, which was used as the WO₃ crystal seed precursor. The as-prepared WO₃ crystal seed precursor was then spun coated onto fluorine-doped tin oxide (FTO) glasses (with a size of 3 × 3.3 cm² and a resistance of 7 ohm), which were ultrasonically washed beforehand by deionized water, acetone, and isopropyl alcohol in sequence. Finally, all the spin coated substrates were calcined at 523 K for 1 h.

2.2 Preparation of WO₃ film

1.1547 g Na₂WO₄·2H₂O and 2.0453 g NaCl were dissolved into 70 mL deionized water and the abovementioned solution was vigorously stirred for 10 min. Then, the pH value of the solution was adjusted to 1.70 by drop-wise adding of 3 M HCl solution under constant stirring. The as-prepared solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave and was the precursor for WO₃ film growth. The FTO glass substrate coated by WO₃ crystal seeds was placed at an incline against the internal wall of the Teflon-lined stainless-steel autoclave, which was sealed and maintained at 413 K for 4 h. The as-prepared WO₃ film was rinsed with deionized water and alcohol in sequence and dried at 333 K for 1 h. Then, the WO₃ film was treated in atmosphere at 523 K for 2 h.

2.3 Preparation of Au nanoparticles modified WO₃ film

5 M HAuCl₄ solutions were prepared with high purity water at room temperature, and then the pH value of the abovementioned HAuCl₄ solution was adjusted to 4.5 with 5 M NaOH solution. The WO₃ film prepared as 2.2 was immersed into the HAuCl₄ solution (pH 4.5) to form [AuCl(OH)₃]⁻ complex on WO₃ surface, followed by calcination at 523 K for 2 h to grow Au nanoparticles.³⁷ For the purpose of obtaining a series of Au-decorated WO₃ film samples with different loading amounts, the immersion time was increased from 15 min to 4 h. The as-prepared Au modified WO₃ films are denoted as Au/WO₃-X (X represents the immersing time) in this study.

2.4 Characterization

Powder X-ray diffraction (XRD) patterns were obtained on a D8 Advanced X-ray diffractometer (Bruker Germany), using Cu Kα radiation at a scan rate of 0.02° 2θ s⁻¹. Scanning electron microscopy (SEM) was carried out using an S-4800 field emission SEM (FESEM, Hitachi, Japan) with an accelerating voltage of 5 kV. High-resolution transmission electron microscopy (HRTEM) analysis was conducted with the use of a JEM 2100 microscope at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 spectrometer equipped with an Al Kα source. All the binding energies were referenced to the C1s peak at 284.8 eV of the surface adventitious carbon. Ultraviolet visible (UV-vis) diffuse reflectance spectra were recorded using a UV-vis spectrophotometer (Shimadzu UV-2550, Japan).

2.5 Photoelectrochemical measurements

The photoelectrochemical measurements were carried out by a workstation with a standard three electrode system, in which three electrodes were composed of the as-prepared WO₃ film (with and without Au nanoparticles) samples as the working electrode, a Pt foil as the counter electrode and Ag/AgCl (saturated KCl) as the reference electrode. The electrolyte was 0.5 M Na₂SO₄ aqueous solution, and it was bubbled with nitrogen for 30 min under vigorous stirring before measurement. A 300 W Xe lamp (PLS-SXE300C, Beijing Trusttech Co., Ltd) was employed as the light source, and it was equipped with band-pass or cut-off filters to provide the required light irradiation. The photocurrent density with light on and off was measured by I–t curves at open circuit potential. Mott–Schottky curves were measured with an AC potential frequency of 1 kHz under dark conditions. The electrochemical impedance spectroscopy (EIS) was measured in the frequency range from 10⁵ to 10⁻¹ Hz under simulated sunlight irradiation. Moreover, the incident-photon-to-current-conversion efficiency (IPCE) was performed in the same three-electrode system at a zero bias versus Ag/AgCl using an electrochemical workstation.

3. Results and discussion

3.1 Material characterization

The crystalline phases of the as-prepared WO₃ film were determined from the XRD patterns. The XRD pattern of the FTO substrate is given in Fig. 1a, which is assigned to tetragonal SnO₂ (JCPDS no. 41-1445). Fig. 1b gives the XRD patterns of the WO₃ film on the FTO substrate, in which the characteristic peaks are assigned to hexagonal WO₃·0.33H₂O (JCPDS no. 35-1001). As shown in Fig. 1c, no impurity peaks and phase changes were observed after treating the film at 523 K. The strengthened peaks of the WO₃ film indicated that the high-temperature annealing improved the WO₃ crystallinity. The Au/WO₃-X samples were also characterized by XRD, as shown in the ESI (Fig. S1†). All samples presented negligible changes compared with the bare WO₃ film, which we attributed to trace amounts of Au loading.

Fig. 1 X-ray diffraction (XRD) patterns of (a) FTO substrate, as-prepared WO₃ film samples (b) before and (c) after calcination at 523 K for 1 h in air.

Fig. 2 shows the SEM of the as-prepared WO₃ film before and after calcination at 523 K. As shown in Fig. 2a, the WO₃ film mainly consisted of several block-like particles with 0.5–2 μm sizes after the hydrothermal process. Some nanorods were found to be dispersed on the substrate, which is consistent with the nanorods of the WO₃ crystal seed layer. Fig. 2b shows that the particles mainly consisted of bundled nanorods with about 30 nm diameter. The nanorod-like WO₃ morphology is in agreement with the previous results under the same experimental conditions.³⁸ Fig. 2c and d present the morphology of the WO₃ film after calcination, wherein more block-like particles on the substrate can be observed. Furthermore, as shown in Fig. 2e and f, the 560 nm thickness WO₃ film with high-temperature treatment is much thicker than the 360 nm untreated film. The observation indicates that the high-temperature treatment promoted the growth and assembly of WO₃ nanorods, which is in agreement with the improved WO₃ film crystallinity from the XRD analysis.

Fig. 2 Top-view SEM images of WO₃ film before (a and b) and after (c and d) calcination at 523 K, and cross-section SEM images of WO₃ film before (e) and after (f) calcination at 523 K.

Based on the abovementioned analysis, a three-stage growth technique of the WO₃ film is proposed and illustrated in Fig. 3. First, peroxopolytungstic acid (PTA) is prepared by a sol–gel method, and then it is spin-coated onto the clean FTO substrate. The WO₃ crystal seed layer with some tiny nanorods is formed by the decomposition of PTA under high-temperature treatment, as shown in Fig. S2.† Second, the crystal seed layer acts as nucleation sites for the WO₃ particles to grow in the hydrothermal process. Over time, the initial nanorods gradually aggregate to form broccoli-like structures, and then the WO₃ film is formed on the substrate. It is suggested that the precursor condition in the hydrothermal process plays an important role in the WO₃ crystal phase and morphology.³⁹ Third, WO₃ nanorods further grow and aggregate by the high-temperature treatment. In the process, the continuation of the Ostwald ripening induces increased thickness and improved WO₃ film crystallinity. The changes promote their electrical contact to the underlying FTO electrode.

Fig. 3 Schematic of the formation mechanism for the as-prepared WO₃ film.

The WO₃ films are then immersed in the Au precursor solution, followed by calcination at 523 K to load Au nanoparticles. Fig. 4a and b give the SEM images of the Au/WO₃-1 h sample, which show a similar morphology to the bare WO₃ film. It should be noted that some nanoparticles were distributed on the WO₃ film with 15–30 nm size. As shown in Fig. 4c and d, the HRTEM image taken at the interface of the nanoparticle and the WO₃ nanorods clearly reveals two distinct sets of lattice fringes. The nanoparticle presented an interplanar spacing of 0.24 nm, which is indexed to the Au (111) plane. The interplanar spacing of 0.38 nm corresponds to the hexagonal WO₃ (0001) plane, which indicates the 〈0001〉 growth orientation of the WO₃ nanorod. The measured diameters of the Au nanoparticle and the WO₃ nanorods were about 25 nm and 30 nm, respectively. In fact, no noticeable changes were observed for the size and shape of the Au nanoparticles with increased immersion time, as shown in Fig. S3,† which is in agreement with previous study.³⁷ The good interface is in favor of the interaction between the Au nanoparticles and the WO₃ nanorods.

Fig. 4 (a and b) SEM images and (c and d) HRTEM image for the Au/WO₃-1 h sample.

Fig. 5 gives the main surface chemical composition of the bare WO₃ and Au/WO₃-1 h films investigated by the XPS technique. As shown in Fig. 5a and b, the peaks have been divided into two different peaks (W4f_5/2 and W4f_7/2), and each W4f peak contains two oxidation states: W⁶⁺ with binding energies of 35.45 and 37.58 eV, and W⁵⁺ with binding energies of 34.90 and 37.05 eV.^40,41 The calculated W⁵⁺/W⁶⁺ ratios are about 3.24% and 8.39% for the bare WO₃ and Au/WO₃-1 h films, respectively. Fig. 5c and d show the O1s core level spectra, in which the peak at 530.5 eV is ascribed to the lattice oxygen, while the peak at 532.0 eV is ascribed to oxygen vacancy and OH group adsorbed on the vacancy sites.^42–44 Stronger vacancy states for the Au/WO₃-1 h film were observed. Two reasons contribute to this observation: one is the interaction between Au nanoparticles and the surface oxygen of WO₃, and the other is the remnant –OH from [AuCl(OH)₃]⁻ compound. The Au XPS spectrum, as shown in Fig. 5e, gives the characteristic peaks (Au4f_5/2 and Au4f_7/2). The peak with binding energy of 83.7 eV is attributed to Au⁰ states, and the peak with binding energy of 84.4 eV is attributed to Au⁺ states.^41,45 The formation of Au⁺ states suggests that some holes were left at the Au/WO₃ interface through the electronic transfer from Au to WO₃. The process will influence the properties of the Au/WO₃ photoelectrode, as discussed below. Moreover, the ratios of Au/W on the Au/WO₃-X surfaces using XPS analysis are listed in Table 1, which shows an increasing Au amount as the immersion time is increased.

Fig. 5 X-ray photoelectron spectra of region scans of (a and b) W4f, (c and d) O1s, (e) Au4f, and (f) the whole survey scans for the bare WO₃ and Au/WO₃-1 h samples.

Table 1 The ratios of Au/W, W⁵⁺/W⁶⁺ and Au⁺/Au measured from XPS analysis

Au/WO3-X	0	15 min	30 min	1 h	2 h	4 h
Au/W (%)	—	0.52	0.84	1.73	2.45	3.68
W^5+/W^6+ (%)	3.24	4.35	6.14	8.39	9.16	9.98
Au^+/Au (%)	—	0.32	0.57	0.75	0.94	1.21

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3.2 Photoelectrochemical performance

With the three-electrode system mentioned above, we studied the photocurrents of the WO₃ photoelectrodes as a function of Au loading amount and compared them to those of the bare WO₃ film. As shown in Fig. 6a and b, eliminating the effect of dark current, the simulated sun- and visible-light photocurrent densities of the bare WO₃ film were about 5.1 and 0.02 μA cm⁻², respectively. All the Au/WO₃-X photoelectrodes showed a higher photocurrent density, as listed in Table 2. The results indicated that the loaded Au nanoparticles were favorable for improving the PEC performance. The Au/WO₃-1 h photoelectrode presents the highest photocurrent densities of about 21.7 and 1.1 μA cm⁻².

Fig. 6 Photocurrent response of WO₃ and Au/WO₃-X samples (a) under simulated sunlight irradiation, and (b) under visible light (λ > 420 nm) irradiation. (c) UV-Vis diffuse reflectance spectra, and (d) photocurrent response with λ > 520 nm light irradiation of the bare WO₃ and Au/WO₃-1 h samples.

Table 2 The PEC performance for bare WO₃ and Au/WO₃-X electrodes

Au/WO3-X		0	15 min	30 min	1 h	2 h	4 h
Photocurrent densities (μA cm^−2)	Sunlight	5.1	6.6	6.7	21.7	16.5	12.2
	Visible light	0.02	0.2	0.5	1.1	0.8	0.6
Vfb (eV)		0.78	0.57	0.53	0.50	0.46	0.42
Rct (ohm)		5537	5083	4521	2164	3661	5485
IPCE (%)	350 nm	10.81	19.14	24.83	35.79	26.99	21.67
	420 nm	0.39	1.07	2.07	3.51	1.57	1.16
	500 nm	0.08	0.18	0.35	0.47	0.46	0.29

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Fig. 6c shows the UV-vis absorption spectra obtained from the WO₃ films in the absence and presence of Au nanoparticles. The bare WO₃ film displays an absorption edge at around 420 nm, which matches the band gap of hexagonal tungsten trioxide. The broad and weak absorption region beyond 700 nm is assigned to the effect of oxygen vacancy.^46,47 Although there are almost no changes of the host absorption edge, the presence of Au nanoparticles loaded on the WO₃ films extended the absorption range up to 700 nm. The absorption band centered at 560 nm corresponds to the surface plasmon resonance (SPR) of the Au nanoparticles. The SPR peak is correlated with the size, shape, and the surrounding environment of the Au nanoparticle.⁴⁸ Although the center of the SPR peak of 10 nm Au nanospheres in an aqueous solution is typically at 520 nm, the SPR peak would redshift to 560 nm when the Au nanoparticles were deposited on the WO₃ surface.⁴⁹ The redshift was also observed in the Au/TiO₂ system, where the size and shape of the Au nanoparticle are similar to those in the present study. As shown in Fig. 6d, the photocurrent density of the Au/WO₃-1 h photoelectrode was about 0.05 μA cm⁻² under the λ > 520 nm light irradiation.

The results provide that the SPR of the Au nanoparticle plays an important role in improving the PEC performance through harvesting more sunlight.

We next emphasized the shape changes of the photocurrent curve. As shown in Fig. 6a and b, the photocurrent densities of the bare WO₃ photoelectrode increased over time during each circulation. This characteristic is attributed to the continuous accumulation of the photoexcited electrons on the surface of the electrode, which originates from the weak reducibility of the WO₃ semiconductor.⁵⁰ However, for the Au/WO₃-X photoelectrodes, the photocurrent curves became square, and the steady-state photocurrents were quickly reached with increasing cyclic times, where the generating and consuming of the photoexcited electrons levelled off. The result implies that Au nanoparticles improved the reducibility of the WO₃ semiconductor and promoted the reduction reactions on the electrode surface. In addition, a remarkable decay of the photocurrent curve was observed for the initial light-on stage in the Au/WO₃-X photoelectrodes, and the effect was augmented with increasing Au amounts. The observation indicates that the electron–hole recombination process is present at the surface of photoelectrodes before achieving steady-state photocurrents.⁵ One of the reasons for the recombination is due to the reduction of the initially adsorbed O₂ on the WO₃ surface by the photoexcited electrons with improved reducibility; another reason should be the compensation of the initial holes accumulated at the Au/WO₃ interface, as discussed with the XPS analysis. Therefore, although the SPR effect of the Au nanoparticles could expand the light response region of photoelectrodes, the modification should be carefully evaluated due to the losses of surface recombination.

In the present study, Mott–Schottky measurements for the bare WO₃ and Au/WO₃-X electrodes in the dark were conducted to investigate the influence of the Au nanoparticles on the WO₃ electronic properties. As shown in Fig. 7a, all WO₃ electrodes showed a positive slope in the Mott–Schottky plots, as expected for an n-type semiconductor. The flat band potential (V_fb) of the bare WO₃ electrode was 0.26 eV versus Ag/AgCl. For the Au/WO₃-X electrodes, this value was shifted to the negative direction. For comparison, the V_fb values were converted to go against a reversible hydrogen electrode (RHE) according to the Nernst equation⁵¹ as follows:

E_RHE (V) = E_Ag/AgCl + [0.059 (V) × pH] + E⁰_Ag/AgCl (0.194 V)

Fig. 7 (a) Mott–Schottky plot and (b) electrochemical impedance spectroscopy Nyquist plots of WO₃ and Au/WO₃-X samples.

The standardized flat band potentials were 0.78, 0.57, 0.53, 0.50, 0.46 and 0.42 eV for the bare WO₃ and Au/WO₃-X electrodes, respectively. The decreasing of the V_fb is related to the negative shift of the Fermi level of the electrodes, which is due to electronic accumulation on the WO₃.⁵² DFT calculations were employed to analyze the electronic transfer process at the Au/WO₃ interface. Fig. 8a and b present the optimized structures of the bare WO₃ and Au/WO₃ (010) surfaces. The two strong O–Au bonds were formed with 2.147 and 2.139 Å lengths, and the surface O–W bonds were elongated from 1.708 Å to 1.812 Å. The elongated bonds indicate that the surface O were activated by the adjacent Au, which agreed with the changes of O1s core level in the XPS analysis. Fig. 8c gives the electron density difference map at the Au/WO₃ interface. It could be seen that some electrons of the Au are captured by the adjacent surface O ions, which causes a decrease in the work function of the WO₃ surface from 7.32 eV to 6.90 eV. That is, the strong interaction of the Au and the surface O at the Au/WO₃ interface is responsible for the negative shift of the Fermi level observed in the Mott–Schottky measurements. Importantly, the shift tendency with more Au loading was also observed by theoretical calculations, as shown in Fig. S4.† Therefore, we conclude that an inner electric field at the Au/WO₃ interface with direction from Au to WO₃ was built through the electron transfer process. The introduction of the inner electric field will promote the separation of the photoexcited electron–hole at the Au/WO₃ interface, which is one of the reasons for the enhanced photocurrent density. To better understand the dynamics of electronic migration in the Au/WO₃ photoelectrodes, the electrochemical impedance spectroscopy (EIS) measurements were conducted, and the results are shown in Fig. 7b. The semicircle in each Nyquist plot can be fitted using an equivalent circuit containing a series resistance (R_s), a charge transfer resistance (R_ct) and a constant phase element (CPE). The EIS results gave a smaller charge transfer resistance (5083, 4521, 2164, 3661 and 5485 ohm) for the Au/WO₃-X photoelectrodes compared with the 5537 for the bare WO₃ photoelectrode. The result again confirms the present and effect of the inner electric field at the Au/WO₃ interface. Note that further increasing the amount of Au nanoparticles did not further improve electronic transport in the Au/WO₃-2 h and -4 h photoelectrodes. This characteristic is in accordance with the result of the photocurrents, which is attributed to the effect of the surface/interface recombination process induced by the Au nanoparticles.

Fig. 8 The structures and the potentials for (a) the bare WO₃ and (b) the Au/WO₃ surfaces, and (c) the electron different density map at the Au/WO₃ interface. The blue and red dashed lines represent the Fermi and vacuum levels, respectively.

The incident-photon-to-current-conversion efficiency (IPCE) spectra were collected to elucidate the contribution of the loaded Au nanoparticles. The IPCE values at specific wavelengths were calculated as follows:⁶

IPCE = (1240I)/(λJ_light)

where I is the photocurrent density (mA cm⁻²), J_light is the measured incident light irradiance (mW cm⁻²) at a specific wavelength, and λ incident light wavelength (nm). As shown in Fig. 9a, all Au/WO₃-X photoelectrodes exhibited a higher IPCE value compared to the bare WO₃ photoelectrode in the 350–420 nm region. The measured IPCE values are summarized in Table 2, which shows the maximum IPCE of 36.3% for the Au/WO₃-1 h photoelectrode with 350 nm incident light. It is worth noting that Au/WO₃-X photoelectrodes have IPCE peaks at λ > 500 nm, which is in good agreement with the SPR absorption of the Au nanoparticles. Based on the above discussions, two positive effects of the Au nanoparticles are proposed to improve the PEC performance of the WO₃ photoelectrode, as presented in Fig. 9b. One is the SPR effect of the Au nanoparticles to harvest more solar energy and the other is the formation of the inner electric field at the Au/WO₃ interface to promote electron–hole separation. However, increased recombined centers are also introduced by the Au nanoparticles. Therefore, the loaded amount of the Au nanoparticles must be balanced to maximize the quantum efficiency of the Au/WO₃ system.

Fig. 9 (a) IPCE plots of the WO₃ and Au/WO₃-X samples in 0.5 M Na₂SO₄ aqueous solution under illumination, and (b) proposed mechanism of the bifunctional Au nanoparticles under simulated sunlight irradiation.

4. Conclusion

In the present study WO₃ films were prepared by the hydrothermal method, and then the Au nanoparticles were loaded through an immersion method. The 10–20 nm Au nanoparticles showed a significant surface plasmon resonance, which induced the light adsorption at around 560 nm wavelength. The Mott–Schottky measurements and theoretical calculations indicated that an inner electric field is built at the Au/WO₃ interface, which will promote separation of the photo-excited electron–hole pairs. The synergistic effect of SPR and the inner electric field improve the PEC performance of the Au/WO₃ photoelectrodes. However, we also noted that the loading quantity for the Au nanoparticles should be carefully considered due to introduction of the recombined centers. The present study proposes bifunctional Au nanoparticles to improve the PEC performance of the WO₃ photoelectrode, and provides a basic understanding to construct metal/semiconductor systems.

Acknowledgements

This study is supported by the National Natural Science Foundation of China (Grant No. 51402169), and the Shandong Provincial Natural Science Foundation, China (Grant No. ZR2014BP005). The numerical calculations in this study were carried out on the supercomputing system in the Supercomputing Center, Shandong University, Weihai.

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Footnote

† Electronic supplementary information (ESI) available: The XRD patterns of the Au/WO₃-X samples, the top-view images of the bare FTO glass with and without crystal seed, the HRTEM of Au/WO₃-2 h and Au/WO₃-4 h samples, and the surface structures and potentials for the WO₃ (010) faces with two, three and four Au atoms. See DOI: 10.1039/c7ra01426k

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