Effective silicon nanowire arrays/WO3 core/shell photoelectrode for neutral pH water splitting

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Effective silicon nanowire arrays/WO3 core/shell photoelectrode for neutral pH water splitting

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Effective silicon nanowire arrays / WO ₃ core / shell photoelectrode for neutral pH water splitting

Zhen Chen^1,2, Minghui Ning¹, Ge Ma^1,3, Qingguo Meng⁴,

Yongguang Zhang³, Jinwei Gao², Mingliang Jin^1,3, Zhihong Chen^4,6, Mingzhe Yuan⁴, Xin Wang^1,3,6 , Jun-Ming Liu^2,5 and Guofu Zhou^1,3

1Institute of Electronic Paper Displays, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, Guangdong Province, People’s Republic of China

2Institute of Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, Guangdong Province, People’s Republic of China

3International Academy of Optoelectronics at Zhaoqing, South China Normal University, Guangdong Province, People’s Republic of China

4Shenyang Institute of Automation, Guangzhou, Chinese Academy of Sciences, Guangzhou 511458, People’s Republic of China

5Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, People’s Republic of China

E-mail:[email protected]@scnu.edu.cn Received 10 March 2017, revised 21 May 2017

Accepted for publication 22 May 2017 Published 13 June 2017

Abstract

We report theﬁrst demonstration of a high-efﬁciency photoelectrochemical(PEC)water splitting reaction using a novel Si NWs/WO₃core/shell photoanode prepared by a mild and inexpensive metal-catalyzed electroless etching process followed by dip-coating, airing and annealing methods.

The dense and vertically aligned Si NWs/WO₃core/shell nanostructure were characterized by scanning electron microscopy, transmission electron microscopy and x-ray diffraction. In comparison to planar n-Si, Si NWs and planar Si/WO₃, the Si NWs/WO₃samples showed significantly enhanced photocurrent over the entire potential sweep range. More significantly, the Si NWs/WO₃samples have an exceptionally low photocurrent onset potential of−0.6393 V versus reversible hydrogen electrode(RHE), indicating very efficient charge separation and charge transportation processes. The as-prepared electrode also has a photocurrent density of

2.7 mA cm⁻²at 0.6107 V versus RHE in 0.5 M Na₂SO₄solution under simulated solar light irradiation(100 mW cm⁻²from 300 W Xenon lamp coupled with an AM 1.5 Gﬁlter). An optimal solar-to-hydrogen efﬁciency of about 1.9% was achieved at 0.2676 V versus RHE.

Electrochemical impedance spectroscopy was conducted to investigate the properties of the charge transfer process, and the results indicated that the enhanced PEC performance may due to the increased charge separation. The x-ray photoelectron spectroscopy measurements indicated the chemical composition of the Si NWs/WO₃nanostructure. Our work has provided an efﬁcient strategy to improve the energy conversion efﬁciency and photocurrent of water splitting materials.

Supplementary material for this article is availableonline

Keywords: water splitting, photoelectrochemical cells, SiNWs/ WO₃core/shell nanostructures (Someﬁgures may appear in colour only in the online journal)

Nanotechnology28(2017)275401(9pp) https://doi.org/10.1088/1361-6528/aa749d

6 Authors to whom any correspondence should be addressed.

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1. Introduction

Since Fujishima and Honda ﬁrst demonstrated that water could be split into H₂ and O₂ on the TiO₂ electrode in a photoelectrochemical(PEC)cell under light illumination[1], the PEC water splitting process on semiconducting materials has received increasing attention, due to its potential as one of the clean, low-cost, and environmentally friendly strategies to solve the energy crisis[2,3]. Until now, a variety of nanostructured metal oxides, such as TiO₂[4], ZnO[5], Fe₂O₃[6], and WO₃ [7–13] have been investigated as photoanodes for PEC hydrogen generation.

Among these semiconducting materials, tungsten trioxide (WO3) is a promising photoanode material because of its nontoxicity, chemical stability, high electron mobility and positive valence band position [14]. Despite the promising features of WO₃, the conversion efficiency of the reported WO₃photoanodes is still far below its theoretical maximum conversion efficiency(6.3%) [15], thus limiting its applica- tions. In order to enhance the conversion efficiency of WO₃ photoanodes, various strategies such as morphology mod- ification and heterojunction have been employed to increase the PEC performance by improving the charge transport properties [16, 17]. Unfortunately, these strategies have not been able to sufficiently address the major inherent drawback of WO₃ (large band gap Eg2.6 eV) [18], which largely limits its absorbance and photocatalytic activity. Considerable efforts have been made to expand the spectral response of WO3, such as elemental doping(e.g., C, In, and N)[19–21] and sensitizing with narrow band gap semiconductors(e.g., CdS, PbS, and Bi₂S₃)[22–24]. Therefore, developing WO₃- based nanostructured heterojunction photoanodes with superior light absorbance property can greatly promote its application in the PEC water splitting process.

Silicon is potentially an excellent photoelectrode material because it possesses a narrow band gap enabling the absorption of energy from solar radiation ranging from ultraviolet to visible light[25,26]. Furthermore, one-dimen- sional silicon nanowires(Si NWs)exhibit better behavior in the PEC water splitting process compared with bulk Si, since they have faster charge transport pathways, more efficient charge collection, higher contact area with the electrolyte and lower reflectivity [27–30]. However, the Si nanowires photoelectrode is catalytically inactive and has poor chemical stability because it is rapidly oxidized in aqueous solutions under solar illumination or under anodic bias[31]. Recently, various Si NWs/semiconductor core/shell nanostructures were formed by coating the Si nanowires with catalytically active and stable semiconductors. For example Kargar and his co-worker have reported a p-Si/TiO₂/Fe₂O₃ core/shell/ hierarchical nanowire photoelectrode which is able to handle overall solar water splitting in the neutral pH water with a solar-to-hydrogen (STH) efficiency of 0.18% [32]. A Si NWs@MoS₂ heterostructure also have been reported as a photocathodes for the solar-driven PEC production of H₂and the onset potential and current density at 0 V versus reversible hydrogen electrode(RHE)of Si@MoS₂NWs were+0.122 V and −8.41 mA cm⁻² [33]. Other shell materials such as,

InGaN[34], ZnIn₂S₄[27], Ta₃N₅[35], ZnO[36], Fe₂O₃[37], NiO_x[38]and even polymer materials graphic Carbon Nitride (g-C₃N₄) [39]. These coating materials were shown to efﬁ- ciently enhance the catalytic activity and stability of Si nanowires photoelectrodes. On the other hand, the semiconductor shell material also provides sufﬁcient energy from photoelectrons for the water splitting reaction.

In this present work, we report a high-efficiency PEC Si NWs/WO₃ core/shell photoanode, prepared by a simple sequence of metal-assisted etching, dip-coating, airing and annealing methods for thefirst time. The Si NWs/WO₃core/ shell photoanode was fully characterized and its various PEC properties were evaluated and compared against those of planar Si, planar Si/WO3, Si NWs samples. To the best of our knowledge, there is no report until now focusing on highly efficient Si NWs/WO₃ core/shell photoanode. Finally, its excellent activity in the PEC process was attributed to the synergistic effects of Si nanowire and WO₃, as shown in figures1(a)and (b). The detailed results are presented in the following sections.

2. Experimental

2.1. Synthesis of Si NWs/WO3nanoarrays

The Si NWs/WO₃ core/shell nano-arrays were synthesized by a simple metal-assisted etching, dip-coating and annealing methods, as shown inﬁgure1(c). In a typical process, The Si NWs were prepared by a mild metal-assisted chemical etching method using a typical procedure[40–42]. First, the Si NWs were immersed into HF solution(1 ml of HF solution(49%), diluted in 50 ml of deionized water), and then washed several times by deionized water. Then dip-coating and annealing processes were conducted to forming WO₃shell. In details, a certain quantity of ammonium tungstate(APT)was dissolved in a solution consisting of 5 ml 40% H₂O₂and 15 ml deionized water to form a homogenous APT aqueous solution.

Then, 30μl of the APT aqueous solution was poured onto the cleaned and wetted Si NWs, and dried in air naturally. Then, the dried Si NWs/APT was annealed for 3 h at 550°C at the heating rate of 10°C min⁻¹in a vacuum tube furnace to form the Si nanowire/WO₃core/shell. The thickness of WO₃shell can be controlled by pouring different concentrations of APT aqueous solution. It should be note that the vacuum annealing process will convert APT shell to WO₃without any change in morphology.

2.2. Characterization

X-ray diffraction (XRD) measurements of the ﬁlm and powders were performed on an x-ray diffractometer(D/max- IIIA instrument)using Cu Kαradiation at a scanning rate of 0.02°s⁻¹. The reﬂectance spectra of the samples were measured by a UV–vis spectrophotometer(Shimadzu UV-3600, Japan). The morphologies of the samples were studied by a scanning electron microscope(SEM, ZEISS ULTRA 55), and energy-dispersive spectroscopy (EDS) was performed using

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Nanotechnology28(2017)275401 Z Chenet al

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an EDS device from Oxford Instruments. The samples microstructures were analyzed by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) (JEM 2100F, ﬁeld emission electron microscope). X-ray photoelectron spectra were done by XPS(Thermo Scientiﬁc, Escalab250Xi)

2.3. Measurement of PEC properties

The PEC measurements were performed in a custom-made electrochemical cell with a 1.0 cm² quartz window using a CHI660E electrochemical workstation(CHI instrument)with an Ag/AgCl reference electrode and platinum wire coil as the counter electrode. A 300 W Xenon lamp was used to provide AM 1.5 illumination(100 mW cm⁻²)in each experiment. The electrolyte used was 0.5 M Na₂SO₄ aqueous solution (The pH value of 0.5 M Na₂SO₄is about 7.02, which was measured by a pH meter). All the samples were assembled into the as-mentioned electrochemical cell with only a quartz window exposed to the Xe lamp. An indium–gallium alloy (99.99%, Sigma-Aldrich) was applied to the back-side of Si substrate enabling ohmic contact. In linear sweep voltammetry measurements, the scan rate was set at 50 mV s⁻¹from

−1.5 to 1.5 V versus Ag/AgCl. Electrochemical impedance spectra (EIS) were measured using the same workstation under an open-circuit condition with the frequency ranging from 0.05 Hz to 100 kHz.

3. Results and discussion

The linear sweep voltammetry (J–V) curves of the as-prepared photoanodes measured under illumination are shown in figure 2(a). It can be clearly found that the photocurrent densities of Si NWs sample is less than 0.5 mA cm⁻²and the planar Si photoanode shows a much lower photocurrent density at about 1.0 V versus RHE. The significantly enhanced photocurrent density of the Si NWs photoanode was due to the larger specific surface area, less light reflectance, and more efficient charge transport in the Si NWs.

When planar Si and Si NWs were coated by WO₃layer via dip-coating and annealing methods, the onset potentials of planar Si and Si NWs shifted towards negative potentials. At 0.6107 V versus RHE, both the planar Si and Si NWs photoanodes show enhanced photocurrent density with WO₃, indicating that the WO₃ coating is an efﬁcient strategy to enhance the PEC performance of Si-based photoanodes.

Furthermore, the onset potential and photocurrent density of Si NWs/WO₃ core/shell photoanodes were as high as

−0.6393 V versus Ag/AgCl and 2.7 mA cm⁻²at0.6107 V versus RHE, respectively, indicating that the Si NWs/WO3

core/shell photoanodes possessed better photo-induced charge generation, separation, and heterojunction. On the other hand, the remarkable negatively shifted onset potential suggests a larger accumulation of electrons and decreased charge recombination in the heterojunction. In order to investigate the effect of WO3thickness on the photocurrent

Figure 1.Composite Si NWs/WO3photoanode.(a)Schematic and(b)general energy band diagram scheme of n-Si NWs/WO3core/shell nanowire in contact with electrolyte under illumination.(c)Schematic representation of fabrication procedure for the Si NWs/WO3

photoanodes.

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density, APT solutions with different concentrations were used to wet the Si NWs sample. The linear sweep voltammetry (J–V) curves of the resulting samples with varying WO₃shell thicknesses are shown in ﬁgure 2(b). When the mass of APT increases from 1 to 4 g, the current densityﬁrst increases and then drastically decreases. This is likely because the greater thickness of the WO₃shell lowers the absorbance of Si NWs and the photo-generated electrons must travel

longer distances to the interface to recombine with holes generated by Si NWs. Figure 2(c) shows the linear sweep voltammetry curve with chopped AM 1.5 light of 100 mW cm⁻²intensity. From −0.75 to 0 V, there is negli- gible difference between the photocurrent and dark current, but when the potential becomes a positive value, the photocurrent becomes much higher than the dark current, indicating favorable photoanode activity. Figures 2(d)–(f) shows the

Figure 2.(a)–(c)Linear sweepJ–Vmeasurements using a three-electrode setup under 100 mW cm⁻²simulated sunlight illumination in 0.5 M Na2SO4neutral aqueous solution:(a)planar Si, Si NWs, planar Si/WO3, Si NWs/WO3electrodes.(b)Si NWs/WO3nanowire arrays electrodes obtained by different concentrations of precursor(1 g, 2 g, 3 g, 4 g ammonium tungstate dissolved in 15 ml DIW and 5 ml 40%

H₂O₂)Wetting .(c)Si NWs/WO₃nanowire arrays electrodes illuminated with chopped white light.(d)and(e)Showing the STH(solar-to- photoelectron)efﬁciency of planar Si, Si NWs, planar Si/WO3, Si NWs/WO3electrodes at different applied bias.(f)The calculated optimal conversion efﬁciencies as a function of the mass of APT.

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STH conversion efﬁciency (η) calculated by the following equation[43–45]:

j_p 1.23 V I₀, 1

h= ( -∣ ∣) ( )

where,j_pis the photocurrent density,Vis the applied potential versus RHE, andI₀is the power density of incident light. It should be noted that all the PEC tests are conducted by using Ag/AgCl reference electrode and the measured potential versus Ag/AgCl was also converted to the RHE scale according to the Nernst equation

E_RHE=E_{Ag AgCl}+0.059pH +E^o_{Ag AgCl}, ( )2 whereE_RHEis the converted potential versus RHE,E^o_Ag/AgCl=

0.1976 V at 25°C, andE_Ag/AgClis the experimentally measured potential versus the Ag/AgCl reference, the pH of electrolyte is about 7.02. The maximum efﬁciencies of the planar Si wafer, Si NWs, and planar Si/WO₃ photoanodes are 0.0058% at

−0.0884 V, 0.043% at −0.357 V, and 0.87% at 0.0014 V, respectively. For the Si NWs/WO₃heterojunction, the efﬁciency shows a considerable enhancement, reaching 1.9% at

−0.2676 V. Figure4(f)shows the calculated optimal conversion efﬁciencies as a function of the mass of APT, revealing the optimum thickness of WO₃shell. Either too thin or too thick of the WO₃shell will cause the less dense harvested solar energy or blocking the light absorption by Si NWs. To investigate the stability of Si NWs/WO₃photoelectrode, a photocurrent versus time measurement has been taken under the AM 1.5 illumination by giving a bias of 0.8 V versus Ag/AgCl. As shown inﬁgure S1 is available online at stacks.iop.org/NANO/28/275401/ mmedia, the initial photocurrent is extremely high with a value of 4.5 mA cm⁻² and in a period of 400 s it deceased to

0.35 mA cm⁻². This phenomenon could be attribute to the formation of SiO_xlayer. Although the WO₃shell coated the Si NWs uniform as shown inﬁgures3(b)and S2, there are many area in the interior of the Si NWs cluster which could not be covered by APT since the high density of the Si NWs and the nature of the dip-coating process. These areas allow the formation of SiO_x when the Si NWs/WO₃ electrode was added a positive bias versus RHE and the SiO_xis a layer with a very low conductivity, so the photocurrent of Si NWs/WO₃decreased rapidly.

Figures3(a),(b)displays the cross-sectional views of n-Si NWs arrays and n-Si NWs/WO₃core/shell, respectively. It can be clearly seen that the Si NWs grew vertically on the Si substrate, and then they agglomerated when they were coated with WO₃ layer. It should be noted that there still are inter- spaces between the Si NWs/WO₃ core/shell nanostructures (shown infigure3(c)), which benefit the light absorption and improve the contact surface between electrode and electrolyte, leading to higher efficiency of the photocatalytic reaction. The length of Si NWs is about 7.8μm, and the fundamental morphology of the Si NWs did not change when they were coated with the WO₃shell. The WO₃shell is thin enough so that more photo-generated holes from inside the core can reach the interfaces of semiconductor and electrolyte for better performance in the water splitting reaction. The diameter of a single Si NWs/WO₃core/shell is about 200 nm and the entire surface of Si NWs was evenly covered with the WO₃layer with good interfacial contact(as shown infigure3(d)). Furthermore the morphologies of Si NWs/WO₃ samples with different thickness were investigate by SEM as shown infigure S2. The lattice fringe of WO₃and silicon is clearly shown infigure3(e). This HRTEM image also clearly reveals the lattice plane with

Figure 3.Typical cross-sectional view SEM images:(a)pristine Si NWs and(b)Si NWs/WO₃core/shell nanostuctrue.(c)Top-view SEM images of Si NWs/WO3core/shell array.(d)TEM images of WO3nanoshells on a single Si nanowire.(e)HRTEM image of Si NWs/WO3

core/shell interface and insertﬁgure showing selected area electron diffraction(SAED)of the nanowire.(f)EDX spectrum recorded on a small area of Si NWs/WO₃.

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spacing of 0.376 nm, corresponding to the(200)plane of WO₃, which indicates that the WO₃shell has good crystallinity. The insert electron diffraction (SAED) map of selected area also provides clear evidence of the polycrystalline nature of the WO₃ shell. EDX analysis conﬁrmed the presence of W, O, and Si elements(ﬁgure3(f)). The above results indicate that the het- erogeneous photoelectrodes were formed as desired by the dip- coating and annealing methods.

XRD spectrum of the as-prepared WO₃powder sample and Si NWs/WO₃with different thickness of WO₃nanoshell are shown infigure4(a). It can be seen that there are several significant peaks for cubic WO₃in WO₃powder, attributed to the diffractions from (200), (220), (222), (400) and (420) planes[JCPDS, 46-1096]. Comparing to pure WO3powder, the intensity of every peaks of WO₃ in different Si NWs/ WO₃samples are lower. The possible cause of this problem may be that the thickness of the WO3is too thin and as a result the intensity of peaks increased with the quality of APT in precursor increasing. The peak intensity of Si NWs at 38.4° and 44.6°have been shown in different Si NWs/WO₃samples and they decrease with the thickness of WO₃ shell increasing. This may be due to the shielding by the WO₃ layer. The reflectivity spectra of the planar Si, Si NWs, planar Si/WO₃and Si NWs/WO₃core/shell electrodes are shown infigure4(b). It can be seen from the spectra that Si NWs and Si NWs/WO₃show drastically reduced reflectance compared to the planar Si and planar Si/WO₃ samples, indicating its superior anti-reflection properties. This is possibly caused by the strong light trapping and scattering behavior of Si NWs. It should be noted that there is a slight decrease in the reflectance observed after the deposition of WO3 onto Si NWs.

This is attributed to the rough surface of WO₃ shell which also has enhanced light trapping. Furthermore, the excellent broadband light-trapping ability is of great importance for efﬁcient solar energy conversion.

X-ray photoelectron spectroscopy (XPS) measurements were conducted to determine the surface electronic states and chemical composition of the nanocomposites of the prepared

Si NWs/WO₃nanowires. Figure5shows the high resolution XPS spectra of so called Si NWs/WO₃. The characteristic peaks of W, O and Si elements in the 0–1200 eV binding energy range can be seen in figure 5(a). The presence of Si, W, O and C elements further confirms the high purity of the products. In figure 5(b), the peaks at 35.5, 37.6 and 42 eV binding energies correspond to W4f₇_/₂, W4f₅_/₂and W5p₃_/₂, respectively. A careful study shows that the weak peaks at 34.5 and 36.6 eV corresponding to low oxidation state of W, indicating the possible absence of O in WO₃. The peak at 532.5 eV can be attributed to O atoms in WO₃, as shown in figure5(c)[45]. The peaks with binding energies of 530.5 and 531.5 eV are attributed to O in SiO_x/Si and water molecules adsorbed on the surface of the film [46, 47]. The peaks of Si2p have shown in figure 5(d) and the peak at 99.4 and 103.3 eV banding energies corresponding to the Si/SiOx[48]. Combined with the SEM, TEM, XRD and XPS results, it can be assuredly concluded that the Si NWs/WO₃ nanowires were successfully obtained by a simple MCEE, dip-coating and annealing procedure.

Electrochemical impedance spectroscopy (EIS) has been carried out to investigate the properties of the charge transfer process in PEC. The EIS measurements were conducted at 0 V (versus Ag/AgCl) under irradiation covering a frequency interval of 0.05 Hz–100 M Hz. Figure 6(a)shows the Nyquist plots of Si NWs/WO3, planar Si/WO3, Si NWs, and planar Si electrodes. The solid lines represent the experimental data. In typical three electrode experiments, the equivalent circuit (shown in the inset ofﬁgure6(a))consists of two parts[49]:(i) the series resistance(R₁)of the pure electrolyte, circuit wire and contact resistance between work electrode and wire; (ii) the charge transfer impedance at the electrolyte–semiconductor interface which is a parallel connection between a resistance (R₂)and a constant phase element(CPE₁). For Si NWs/WO₃ and planar Si/WO₃electrodes, there is another charge transfer impedance at the Si–WO₃interface, which consists of parallel installed resistance (R₃) and constant phase element (CPE₂). The arc radius of the planar Si wafer, Si NWs, planar Si/WO₃,

Figure 4.(a)XRD patterns of WO3powder and Si NWs/WO3samples obtained by different quality of APT in precursor solution.(b) Reﬂectance spectra of the planar Si, Si NWs, planar Si/WO3and Si NWs/WO3core-shell arrays electrodes.

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and Si NWs/WO₃samples are gradually decreased(shown in ﬁgure6(a)). A smaller arc radius indicates faster charge transfer and more effective separation of the photogenerated electron– hole pairs. The Si NWs/WO₃sample shows the smallest arc radius in the low frequency range, suggesting that it has superior charge transfer and charge separation due to the formation of the heterojunction. In the Bode phase plots shown in ﬁgure6(b), the Si NWs/WO₃sample correspondingly has the lowest characteristic peak frequency below 0.1 Hz, indicating

the longest electron lifetimes. Bode plots inﬁgure6(b)exhibit the efﬁcient electron lifetime in planar Si, Si NWs, planar Si/WO₃ and Si NWs/WO₃ samples. The lifetime (τe) of electron can be calculated byτe=1/(2πf_max)and thef_maxis the frequency at which the frequency peak appears in the Bode plot [13]. The values of the maximum characteristic frequencies are decreased gradually and vary from the Si NWs(10–100 Hz)to the planar Si (1–10 Hz), indicate theτe of Si NWs is shorter than that of planar Si. The possible reason may be the

Figure 5.XPS spectra of Si NWs/WO3:(a)wide scan;(b)W4f of Si NWs/WO3(c)O1s of Si NWs/WO3and(d)Si of Si NWs/WO3.

Figure 6.EIS measured of planar Si, Si NWs, planar Si/WO3, Si NWs/WO3photoanodes under illumination:(a)Nyquist and(b)Bode phase plots.

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nanostructure leading to the recombination of generated hole– electron pair. Comparing to the maximum characteristic frequency of Si NWs (10–100 Hz), the maximum characteristic frequency of Si NWs/WO₃ is below 0.10 Hz represent the longer lifetime of generated electron in Si NWs/WO₃.

4. Conclusions

In conclusion, we have presented a facile method for the preparation of Si NWs/WO₃core/shell photoanodes for application in PEC water splitting for theﬁrst time. The as-prepared NWs/ WO₃core/shell photoanodes showed a remarkable photocurrent density of 2.7 mA cm⁻² at 0.6107 V versus RHE and onset potential of −0.6393 V under solar light irradiation. It is believed that the deposition of WO₃nanoshells on the surface of Si NWs can enhance the separation rate of the photo-generated holes, and further improve the separation of electron–hole pairs.

Moreover, light absorption of the core/shell photoanode is also enhanced due to the complementary absorption of Si NWs and WO₃ in different regions of the solar spectrum. The above characteristics of the Si NWs/WO₃ core/shell photoanode contribute to its enhanced water splitting performance, as evi- denced by a cathodic shift in the photocurrent onset potential.

Overall, the results of this work provide a new approach to development of improved PECmaterials.

Acknowledgments

The authors acknowledge theﬁnancial support from the NSFC (Grant No. 51602111), the National Key R & D Program of China (2016YFB0401502), Guangdong Provincial Grant (2015A030310196,2014B090915005), the Pearl River S&T Nova Program of Guangzhou (201506040045), the Hundred Talent Program of Chinese Academy Sciences (QG Meng), Guangzhou Post-doctoral Initial Funding and the National 111 Project.

ORCID

Xin Wang https://orcid.org/0000-0002-4771-8453

References

[1] Fujishima A and Honda K 1972 Electrochemical photolysis of water at a semiconductor electrodeNature23837–8 [2] Abe R 2010 Recent progress on photocatalytic and

photoelectrochemical water splitting under visible light irradiationJ. Photochem. Photobiol.P11179–209 [3] Li Z, Luo W, Zhang M, Feng J and Zou Z 2013

Photoelectrochemical cells for solar hydrogen production:

current state of promising photoelectrodes, methods to improve their properties, and outlookEnergy Environ. Sci.6 347–70

[4] Li Y, Wang R, Li H, Wei X, Feng J, Liu K, Dang Y and Zhou A 2015 Efﬁcient and stable photoelectrochemical

seawater splitting with TiO₂@g-C₃N₄nanorod arrays decorated by Co–PiJ. Phys. Chem.C11920283–92 [5] Yang X, Wolcott A, Wang G, Sobo A, Fitzmorris R C, Qian F,

Zhang J Z and Li Y 2009 Nitroge ZnO nanowire arrays for photoelectrochemical water splittingNano Lett.92331–6 [6] Sivula K, Le Formal F and Gratzel M 2011 Solar water

splitting: progress using hematite(alpha-Fe2O3) photoelectrodesChem. Sus. Chem.4432–49

[7] Yun G, Balamurugan M, Kim H-S, Ahn K-S and Kang S H 2016 Role of WO3layers electrodeposited on SnO2inverse opal skeletons in photoelectrochemical water splitting J. Phys. Chem.C1205906–15

[8] Li W, Zhan F, Li J, Liu C, Yang Y, Li Y and Chen Q 2015 Enhancing photoelectrochemical water splitting by aluminum-doped plate-like WO3electrodesElectrochim.

Acta16057–63

[9] Liu C, Yang Y, Li W, Li J, Li Y and Chen Q 2016In situ synthesis of Bi2S3sensitized WO3nanoplate arrays with less interfacial defects and enhanced photoelectrochemical performanceSci. Rep.623451

[10] Zhan F, Li J, Li W, Liu Y, Xie R, Yang Y, Li Y and Chen Q 2015In situformation of CuWO₄/WO₃heterojunction plates arrayﬁlms with enhanced photoelectrochemical propertiesInt. J. Hydrog. Energy406512–20

[11] Zhan F, Li J, Li W, Yang Y, Liu W and Li Y 2016In situ synthesis of CdS/CdWO4/WO3heterojunctionﬁlms with enhanced photoelectrochemical propertiesJ. Power Sources 325591–7

[12] Zhan F, Xie R, Li W, Li J, Yang Y, Li Y and Chen Q 2015 In situsynthesis of g-C3N4/WO3heterojunction plates array ﬁlms with enhanced photoelectrochemical performanceRSC Adv.569753–60

[13] Zhan F, Yang Y, Li W, Li J, Liu W, Li Y and Chen Q 2016 Preparation of DyVO4/WO3heterojunction plate arrayﬁlms with enhanced photoelectrochemical activityRSC Adv.6 10393–400

[14] Dias P, Lopes T, Meda L, Andrade L and Mendes A 2016 Photoelectrochemical water splitting using WO₃photoanodes:

the substrate and temperature rolesPhys. Chem. Chem. Phys.

185232–43

[15] Xu F, Yao Y, Bai D, Xu R, Mei J, Wu D, Gao Z and Jiang K 2015 A signiﬁcant cathodic shift in the onset potential and enhanced photoelectrochemical water splitting using Au nanoparticles decorated WO3nanorod arrayJ. Colloid Interface Sci.458194–9

[16] Hong S J, Jun H, Borse P H and Lee J S 2009 Size effects of WO3nanocrystals for photooxidation of water in particulate suspension and photoelectrochemicalﬁlm systemsInt. J.

Hydrog. Energy343234–42

[17] Su J, Guo L, Bao N and Grimes C A 2011 Nanostructured WO3/BiVO4heterojunctionﬁlms for efﬁcient

photoelectrochemical water splittingNano Lett.111928–33 [18] Zheng H, Ou J Z, Strano M S, Kaner R B, Mitchell A and

Kalantar-zadeh K 2011 Nanostructured tungsten oxide- properties, synthesis, and applicationsAdv. Funct. Mater.21 2175–96

[19] Sun Y, Murphy C J, Reyes-Gil K R, Reyes-Garcia E A, Thornton J M, Morris N A and Raftery D 2009 Photoelectrochemical and structural characterization of carbo WO₃ﬁlms prepared via spray pyrolysisInt. J. Hydrog.

Energy348476–84

[20] Kizu T, Aikawa S, Mitoma N, Shimizu M, Gao X, Lin M-F, Nabatame T and Tsukagoshi K 2014 Low-temperature processable amorphous In–W–O thin-ﬁlm transistors with high mobility and stabilityAppl. Phys. Lett.104152103 [21] Takeuchi M, Shimizu Y, Yamagawa H, Nakamuro T and

Anpo M 2011 Preparation of the visible light responsive N³

−-doped WO3photocatalyst by a thermal decomposition of ammonium paratungstateAppl. CatalysisB1101–5

8

(10)

[22] Zhang L J, Li S, Liu B K, Wang D J and Xie T F 2014 Highly efﬁcient CdS/WO₃Photocatalysts: z-scheme photocatalytic mechanism for their enhanced photocatalytic H₂evolution under visible lightACS Catalysis43724–9

[23] Zhong J, Li X, Hu B, Yuan L, Feng Y, Li T, Song T, Hu J and Zhou J 2013 Synthesis and photoelectrochemical property of PbS quantum dots modiﬁed WO3nanoﬂowersECS Trans.

5041–4

[24] He H, Berglund S P, Xiao P, Chemelewski W D, Zhang Y and Mullins C B 2013 Nanostructured Bi2S3/WO3heterojunction ﬁlms exhibiting enhanced photoelectrochemical performance J. Mater. Chem.A112826

[25] Noh S Y, Sun K, Choi C, Niu M, Yang M, Xu K, Jin S and Wang D 2013 Branched TiO2/Si nanostructures for enhanced photoelectrochemical water splittingNano Energy 2351–60

[26] Yu R, Lin Q, Leung S-F and Fan Z 2012 Nanomaterials and nanostructures for efﬁcient light absorption and

photovoltaicsNano Energy157–72

[27] Liu Q, Wu F, Cao F, Chen L, Xie X, Wang W, Tian W and Li L 2015 A multijunction of ZnIn₂S₄nanosheet/TiO₂ﬁlm/ Si nanowire for signiﬁcant performance enhancement of water splittingNano Res.83524–34

[28] Duan C, Wang H, Ou X, Li F and Zhang X 2014 Efﬁcient visible light photocatalyst fabricated by depositing plasmonic Ag nanoparticles on conductive polymer- protected Si nanowire arrays for photoelectrochemical hydrogen generationACS Appl. Mater. Interfaces69742–50 [29] Devarapalli R R, Debgupta J, Pillai V K and Shelke M V 2014

C@SiNW/TiO2core-shell nanoarrays with sandwiched carbon passivation layer as high efﬁciency photoelectrode for water splittingSci. Rep.44897

[30] Wang X, Peng K Q, Wu X L and Lee S T 2010 Single crystalline ordered silicon wire/Pt nanoparticle hybrids for solar energy harvestingElectrochem. Commun.12509–12 [31] Oh I, Kye J and Hwang S 2012 Enhanced

photoelectrochemical hydrogen production from silicon nanowire array photocathodeNano Lett.12298–302 [32] Kargar A, Khamwannah J, Liu C-H, Park N, Wang D,

Dayeh S A and Jin S 2016 Nanowire/nanotube array tandem cells for overall solar neutral water splittingNano Energy19 289–96

[33] Chen C-J, Yang K-C, Liu C-W, Lu Y-R, Dong C-L, Wei D-H and Liu R-S 2017 Silicon microwire arrays decorated with amorphous heterometal-doped molybdenum sulﬁde for water photoelectrolysisNano Energy32422–32 [34] Hwang Y J, Wu C H, Hahn C, Jeong H E and Yang P 2012 Si/

InGaN core/shell hierarchical nanowire arrays and their photoelectrochemical propertiesNano Lett121678–82 [35] Narkeviciute I, Chakthranont P, Mackus A J M, Hahn C,

Pinaud B A, Bent S F and Jaramillo T F 2016 Tandem

core–shell Si–Ta₃N₅photoanodes for photoelectrochemical water splittingNano Lett.127565–72

[36] Kargar Aet al2013 Tailoring n-ZnO/p-Si branched nanowire heterostructures for selective photoelectrochemical water oxidation or reductionNano Lett.133017–22

[37] Wang X, Peng K Q, Hu Y, Zhang F Q, Hu B, Li L, Wang M, Meng X M and Lee S T 2014 Silicon/hematite core/shell nanowire array decorated with gold nanoparticles for unbiased solar water oxidationNano Lett.1418–23 [38] Zhang F-Q, Hu Y, Meng X-M and Peng K-Q 2015 Fabrication

and photoelectrochemical properties of silicon/nickel oxide core/shell nanowire arraysRSC Adv.588209–13

[39] Chen Zet al2017 Fabrication and photoelectrochemical properties of silicon nanowires/g-C3N4core/shell arrays Appl. Surf. Sci.396609–15

[40] Huang Z, Wang C, Chen Z, Meng H, Lv C, Chen Z, Han R and Zhang C 2014 Tungsten sulﬁde enhancing solar-driven hydrogen production from silicon nanowiresACS Appl.

Mater. Interfaces610408–14

[41] Huang Z, Geyer N, Werner P, de Boor J and Gosele U 2011 Metal-assisted chemical etching of silicon: a reviewAdv.

Mater.23285–308

[42] Peng K, Xu Y, Wu Y, Yan Y, Lee S T and Zhu J 2005 Aligned single-crystalline Si nanowire arrays for photovoltaic applicationsSmall11062–7

[43] Li H, Cheng C, Li X, Liu J, Guan C, Tay Y Y and Fan H J 2012 Composition-graded ZnxCd1−xSe@ZnO core–shell nanowire array electrodes for photoelectrochemical hydrogen generationJ. Phys. Chem.C1163802–7 [44] Li Y, Takata T, Cha D, Takanabe K, Minegishi T,

Kubota J and Domen K 2013 Vertically aligned Ta3N5

nanorod arrays for solar-driven photoelectrochemical water splittingAdv. Mater.25125–31

[45] Yu H, Chen S, Fan X, Quan X, Zhao H, Li X and Zhang Y 2010 A structured macroporous silicon/graphene

heterojunction for efﬁcient photoconversionAngew. Chem., Int. Ed. Engl.495106–9

[46] Qiu M, Zhu D, Bao X, Wang J, Wang X and Yang R 2016 WO₃with surface oxygen vacancies as an anode buffer layer for high performance polymer solar cellsJ. Mater. Chem.A 4894–900

[47] Hollinger G 1981 Structures chimique et electronique de l’interface SiO2–SiAppl. Surf. Sci.8318–36

[48] Cardinaud C, Rhounna A, Turban G and Grolleau B 1989 Analyse XPS des surfaces de Si et SiO2exposées aux plasmas de CHF3et CHF3–C2F6. Polymérisation et gravure Rev. Phys. Appliquée24309–21

[49] Huang Z, Zhong P, Wang C, Zhang X and Zhang C 2013 Silicon nanowires/reduced graphene oxide composites for enhanced photoelectrochemical propertiesACS Appl. Mater.

Interfaces51961–6

Effective silicon nanowire arrays/WO3 core/shell photoelectrode for neutral pH water splitting

Effective silicon nanowire arrays/WO3 core/shell photoelectrode for neutral pH water splitting

Effective silicon nanowire arrays / WO 3 core / shell photoelectrode for neutral pH water splitting

Effective silicon nanowire arrays / WO ₃ core / shell photoelectrode for neutral pH water splitting