In situ Sn-doped WO3 films with enhanced

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J Solid State Electrochem DOI 10.1007/s10008-017-3569-4

ORIGINAL PAPER

In situ Sn-doped WO3 films with enhanced photoelectrochemical performance for reducing CO2 into formic acid Yahui Yang 1 & Faqi Zhan 2 & Hang Li 1 & Wenhua Liu 2 & Sha Yu 2

Received: 22 December 2016 / Revised: 2 March 2017 / Accepted: 19 March 2017 # Springer-Verlag Berlin Heidelberg 2017

Abstract We report exploiting effective Sn incorporation to enhance the photoelectrochemical activity of WO3 plate films, applied as photoanodes for photoelectrocatalytic (PEC) CO2 reduction into formic acid (HCOOH). The in situ Sn-doped WO3 films were prepared on a fluorine-doped tin oxide (FTO) substrates by a hydrothermal method with adding Na2SnO3 as the Sn precursor. Sn dopants were confirmed with an X-ray photoelectron spectroscopy (XPS), which enlarged the growth density and crystallinity of WO3 plate films. Comparing the PEC properties, Sn-doped WO3 anode exhibits a photocurrent density of 1.11 mA/cm2 at 1.2 V vs. Ag/AgCl, which is approximately 1.4 times higher than that of the undoped films. The highest IPCE value increased from 28.3% to 45.1% after Sn doping, which is approximately 1.6 times higher than that of the undoped ones. After 3 h for PEC reduction of CO2, the maximum formic acid yield of Sn-doped WO 3 film is 485 nmol/cm 2 , while that of undoped WO 3 film is 206 nmol/cm 2 . Based on the electrochemical and photoelectrochemical analysis, the enhanced PEC performance in Sn-doped WO3 films owes to its improved carriers density, electrical conductivity, and electrons lifetime. This is the first report to investigate the effect of Sn doping on the photoelectrochemical properties of WO3 nanostructures. Electronic supplementary material The online version of this article (doi:10.1007/s10008-017-3569-4) contains supplementary material, which is available to authorized users. * Yahui Yang [email protected]

1

College of Resources and Environment, Hunan Agricultural University, Changsha 410128, China

2

School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China

Keywords WO3 films . In situ Sn doping . Photoelectrochemical . CO2

Introduction The reduction of CO2 into carbon-containing fuels is an ideal artificial technology for reducing the global warming and depletion of fossil fuels [1–4]. Versatile photoanode-driven photoelectrochemical system for CO2 reduction have been widely researched, which combine CO2 reduction with water oxidization simultaneously [5–9]. Tungsten oxide (WO3) recently attracted great attentions and was used as photoanode due to its suitable optical band gap (~2.6 eV) [10–13], excellent chemical stability in acid solution and high resistance against photo corrosion [14]. Besides, its valence band edge owns a sufficient potential drive for oxidizing water [15]. Despite this, the efficiency of WO3 for water oxidation is still low because of its fast photogenerated charge recombination and poor oxygen evolution reaction kinetics. Many attempts have been made to improve the efficiency of WO 3 photoelectrode. One approach to enhance the PEC activity of WO3 is to design nanostructured photoelectrodes. In particular, the low-dimensional nanostructures attracted much attention due to their unique optical and electrical properties including faster electron transport and photoconductivity [16–27]. Additionally, elemental doping plays a critical role in improving the electrical conductivity and optical absorption coefficient and in enhancing the PEC performance of WO3 photoanodes. Such as Al [28], Co [29], Ni [30], Cs [31], Sn [32], and Yb [33] dopants have been investigated. Therefore, doping transition metals into low-dimensional nanostructures is very promising for their application in photoelectrochemical systems: performance enhancements arise from both dopant incorporation and low-dimensional nanostructures.

J Solid State Electrochem

Sivula et al. observed that Sn diffused from the FTO substrates into the hematite film with pronounced photocurrent density when the substrate was sintered at a high temperature (800 °C) [34]. The incorporation of Sn caused a twofold enhancement in the optical absorption coefficient. After that, Ling et al. confirmed the Sn diffusion from the FTO substrate and further investigated the effect of Sn doping on the morphology and electronic properties of hematite nanostructures using photoelectrochemical measurements and ultrafast laser spectroscopy [35]. They found that Sn dopant serves as an electron donor and increases the carrier density of hematite nanostructures. The enhanced photoactivity in Sn-doped hematite nanostructures was due to the improved electrical conductivity and increased surface area. However, when Sn diffused from the FTO substrates into semiconductors at high temperature, there are two decisive factors that may limit the photoactivity of nanostructures films. One is that Sn diffusion would result in severe loss of conductivity in FTO. Another is that the high-temperature annealing would destroy the lowdimensional nanostructures of semiconductors. To overcome these difficulties, in situ doping is a simple process that dopants are incorporated into the semiconductors nanostructures during material synthesis [35, 36]. It uses a relatively low sintering temperature and requires no additional doping steps [37]. Herein, we fabricated in situ Sn-doped two-dimensional (2D) WO3 photoanode on FTO substrates through a hydrothermal method. The photoelectrocatalytic abilities and corresponding photoelectrochemical properties of the Sn-doped WO3 films were investigated in details by PEC CO2 reduction into formic acid. Enhanced photoelectrochemical performance for WO3 films was observed after Sn doping. To the best of our knowledge, this is the first report to investigate the effect of Sn doping on the photoelectrochemical properties of WO3 nanostructures.

Experimental section

5%, and 10% Na2SnO3 (mol%, Na2SnO3/Na2WO4) into the hydrothermal precursor solution. Characterization The crystalline phase analysis was characterized by X-ray diffraction (XRD, D/Max2250, Rigaku Corporation, Japan) with Cu Kα radiation. The microscopic morphologies were investigated by a scanning electron microscope (SEM, Nova NanoSEM 230) equipped with X-ray energy dispersive spectrometer (EDS) and high-resolution transmission electron microscope (HRTEM, G2 F20). The surface chemical states were analyzed by X-ray photoelectron spectroscopy (XPS). The processing of XPS data was performed with an XPS peak-fit program. The UV-vis spectra were obtained using a diffused reflectance ultraviolet and visible spectrophotometer (DR-UVS, Shimadzu 2450 spectrophotometer). Photoelectrocatalytic CO2 reduction measurements The photoelectrocatalytic performances of as-prepared films were characterized using a standard three-electrode configuration for PEC CO2 reduction. A copper foam (1 × 1 cm2) was used as a counter electrode to reduce CO2, Ag/AgCl (saturated KCl) as a reference electrode, and pure WO3 or Sn-WO3 films (1 × 1 cm2) was used as the photoanode. The photoanodedriven PEC CO2 reduction measurement was performed in an airtight photoelectrochemical H-type cell with three electrodes and two compartments separated by a Nafion 117 membrane anion exchange membrane with 0.2 M Na2SO4 electrolyte in anodic chamber and CO2-saturated 0.5 M KHCO3 electrolyte in cathodic chamber. Visible light irradiation was performed under a 150-W Xe lamp (CHF-XM35, Beijing Trusttech Go. Ltd.) with a 400-nm cutoff filter to remove UV light, and the light intensity was adjusted to 50 mW/cm2 by a solar simulator. Considering the limitation of experiment equipment and product-testing equipment. The formic acid (HCOOH) as the main liquid product was collected from the

Synthesis of photoelectrodes 2-D WO3 platelike arrays films on FTO glasses were prepared by a simple hydrothermal method based on our previous report [38]. In a typical experiment, two pieces of cleaned FTO glasses were placed within a 100-mL Teflon-lined stainless autoclave containing a solution consisting of 0.231 g Na2WO4·2H2O and 0.2 g (NH4)2C2O4 (at pH 2.0, adjusted by 3 M HCl). The hydrothermal synthesis process was carried out at 140 °C for 6 h. After cooling to room temperature naturally, the films were taken out and dried in the oven. Then the resulting films were annealed at 500 °C for 1 h in air. In situ Sn-doped WO3 films were prepared by adding 2%,

Fig. 1 XRD patterns of the photoelectrodes films

J Solid State Electrochem Fig. 2 SEM images of WO3 film (a), 2% (b), 5% (c), and 10% SnWO3 films (d)

cathodic chamber after 3 h photoelectrocatalysis with a constant potential of 1.2 V (vs. Ag/AgCl) and quant yield

by headspace method using a gas chromatograph-mass spectrometer (GC-MS, ICS 2000, Dionex, USA).

Fig. 3 EDX images of WO3 (a), 2% Sn (b), 5% Sn (c), and 10% Sn doped WO3 films

J Solid State Electrochem Table 1 by EDS

The final loading amounts of Sn dopants in all films calculated

Samples

Sn (at.%, EDS)

Sn (at.%, actual loading)

WO3 2% Sn-WO3 5% Sn-WO3 10% Sn-WO3

8.80 9.97 13.06 17.12

FTO 1.17 4.26 8.32

Photoelectrochemical measurements The photoelectrochemical cell comprised of as-prepared films as working electrode, Ag/AgCl electrode as reference electrode, Pt sheet as counter electrode immersed in 0.2 M Na2SO4 solution (pH = 7). The photocurrents were measured using linear sweep voltammograms in a potential range from 0 to 1.4 V (vs. Ag/AgCl) with a scan rate of 20 mV/s. The MottSchottky measurements were conducted at the AC frequency of 1 kHz. The electrochemical impedance spectra (EIS) were measured at 0.8 V (vs. Ag/AgCl) with the AC frequency of 10 kHz to 100 mHz. The amplitude of AC voltage was 10 mV in both Mott-Schottky and EIS measurements. Meanwhile, the incident-photon-to-current-efficiency (IPCE) was measured with the system consists of a xenon lamp light source (LSH-X150), a monochromator (Omni-l300), a data collection system (DCS300PA), and a Si detector (QE-B3). The intensity modulated photocurrent spectroscopy (IMPS) were obtained using a Zahner CIMPS-2 system with a blue lightemitting diode (LED) lamp as the light source, which can provide both DC and AC components of illumination. The AC component of the current to drive the LED generated a 10% modulation superimposed on the DC light intensity. In Fig. 4 TEM image (a), HRTEM image (b) and TEM-mapping images (c–f) of in situ 5% Sndoped WO3 film

this work, all potentials mentioned were referred to an Ag/ AgCl electrode (vs. Ag/AgCl).

Results and discussion Characterization of the synthesized photoanodes Figure 1 presents the XRD spectra of the undoped and Sndoped WO3 films which are found to match with monoclinic WO3 phase according to JCPDS data (JCPDS 43-1035). No diffraction peaks corresponding to Sn or other Sn compounds were observed in the XRD spectra. All the as-prepared films displayed similar XRD patterns, except that the peak intensities increase on Sn doping, especially for the (002) and (-202) planes. This indicates improvement in the crystallinity of the doped films, which may be due to the fact that Sn incorporation in the WO3 lattice network reduces density of nucleation centers which in turn favors the growth of crystal grains [39]. Another important observation in the XRD spectra is that after Sn doping, the peak positions shift towards the lower diffraction angles. The location of (200) plane shifts from 24.26° to 24.16°. Such a decrease in 2θ arises due to incorporation of Sn4+ ion into WO3 lattice matrix as that ionic radii of Sn4+ and W6+ are 0.071 and 0.067 nm, respectively [40]. Figure 2 depicts the SEM images comparing pristine and in situ Sn-doped WO3 photoanodes. All samples show very similar platelike morphology and the same film thickness of ~1.4 μm (inset); alternatively, the larger growth density for the in situ Sn-doped WO3 films was observed. The pristine WO3 film reveals a platelike structure grown vertically on the FTO substrates. The average length and thickness of WO3 platelets were about 1.3 μm and 300 nm, respectively.

J Solid State Electrochem

Fig. 5 XPS spectra of the Sn-doped WO3 film. a W 4f. b Sn 3d

Compared with the undoped films, in situ Sn-doped photoanodes have a smaller size with a length of ~1.1 μm and thickness of ~400 nm. This difference originates from the dopant Na2SnO3 in the precursor solution that is utilized for Sn incorporation. As shown in Fig. 3, the EDS results confirm incorporation of Sn ions into WO3 lattice network. In the previous investigation, the ionic radii of Sn4+ and W6+ are well within 15% difference, which shows that Sn ions can substitutionally replace W ions in the WO3 lattice [32]. Apart from the Sn within FTO substrate, the actual loading amounts of the Sn dopants were shown in Table 1. To observe more detailed information about the crystal structures of samples, TEM images of Sn-doped WO3 film are presented in Fig. 4. It can be seen that the in situ Sndoped WO3 has platelike structure (Fig. 4a). Figure 4b shows the high-magnification TEM image of 5% Sn-doped WO3. The lattice d-spacing of 0.384 nm corresponds to the (200) plane of monoclinic WO3, and lattice d-spacing of 0.238 nm (200) for the SnO2. The WO3 lattice planes became slightly blurred, and some defects were observed after Sn-doped, which due to the Sn atom displaces the W atom or occupies interstitial positions. The spatial chemical compositions in

TEM-mapping images (Fig. 4c–f) revealed that Sn dopant was homogeneously distributed through the WO3 plate. In brief, some of the Sn atoms entered into the lattices of WO3, while the other Sn dopants distributed on the grain boundary or on the surface of WO3 plates in the form of SnO2. As a type of material that exhibits no photoresponse to visible light, SnO2 (Eg = 3.6 eV [41, 42]) may acted as a passivation layer to inhibit the recombination of electrons and holes at the interface of electrodes and electrolyte, which resembles HfO2 surface passivation effect on photoelectrochemical water splitting performance was described in our previous study [43]. To investigate the effect of Sn doping on the chemical composition and chemical environment of the WO3, the XPS analysis of 5% Sn-WO3 was conducted (Fig. 5). The C 1s peak at 284.8 eV was assigned to adventitious carbon species from the XPS instrument. Figure 5a exhibits the XPS high-resolution spectrum of W 4f in the samples of undoped and Sn-doped WO3. The peaks of W 4f7/2 and W 4f5/2 for pristine WO3 are located at 35.4 and 37.5 eV, respectively, which is consistent with the W6+ [44]. For Sn-doped WO3, the characteristic peaks of W 4f7/2 and W 4f5/2 were shifted to a higher binding energy by approximately 0.2 eV, indicating

Fig. 6 UV-vis absorption spectra (a) and the band gap determination (b) of WO3 and Sn-doped WO3 films

J Solid State Electrochem

reason is due to the existence of Sn 5s and Sn 4d electron orbits trapped level [52, 53]. We have used a simple DFT calculation to verify the electronic band structure, total density of states (DOS) and partial density of states (PDOS) in Fig. S2, and the Sn electronic states were found to be in the mid-gap region of WO3, which support above mentioned results very well. Photoelectrochemical measurements of as-prepared photoanodes

Fig. 7 Formic acid yields for WO3 and Sn-doped WO3 films after 3 h of PEC CO2 reduction

that the incorporation of Sn ions into WO3 lattice. The XPS high-resolution spectrum of Sn 3d is shown in Fig. 5b. Detailed analysis of the Sn 3d3/2 and Sn 3d5/2 peaks located around 495.4 and 487 eV, confirmed the presence of Sn4+ dopants in Sn-doped WO3 films [45, 46]. Figure 6a shows the optical absorption spectra of the undoped and Sn-doped WO3 films. The band gap energy (Eg) of the samples was calculated by the Tauc formula [47]:  ð1Þ ðαhvÞ1=2 ¼ A hv−Eg where α, h, v, and A are the absorption coefficient, the Plank’s constant, the frequency of the radiation and a constant, respectively. As shown in Fig. 6b, the pure WO3 film has a band gap of about 2.65 eV, consistent with other reports [48–50]. It has been found that the optical band gap decreases to 2.60 eV after Sn doping. These results are consistent with the previous literature [32]. This band narrowing owes to the lattice disorder arising by the dopant ions in the Sn-doped films, as explained in reported literatures [32, 51]. The other

The PEC CO2 reduction experiments were conducted under visible irradiation of a 150 W Xe lamp with a power density of 50 mW/cm2, and the formic acid in electrolyte was analyzed by a gas chromatograph-mass spectrometer (GC-MS). After 3 h of photoelectrocatalysis, the production yields of HCOOH for WO 3 and Sn-doped WO 3 films were shown in Fig. 7. Compared with the pure WO3 film (206 nmol/cm2), the 5% Sn-WO 3 films exhibits a maximum HCOOH yield of 485 noml/cm2, which indicates that the Sn dopant possesses a positive effect on photoelectrocatalytic properties of WO3 films with moderate amount. The possible reasons will be further discussed through photoelectrochemical measurements. To further investigate the possible reasons for the enhanced photoelectrocatalytic activity of Sn-WO3 with different Sndoping ratios, several photoelectrochemical measurements were performed. Linear sweep voltammetry (LSV) curves of as-prepared films are observed in the dark and under visible illuminations, as shown in Fig. 8a. In the dark, all of photoelectrodes display very weak dark currents compared to their photocurrent. It turned out that there are few water splitting reactions that occur. Compared to pure WO3, the doping of element Sn has a great influence on the photocurrent under light illumination. Among of different Sn-doped WO3 films, the 5% Sn-WO3 photoanode exhibits the highest relative PEC performance. Its photocurrent density is 1.11 mA/cm2 compared to the pure WO3 anode (0.80 mA/cm2) at 1.2 V vs.

Fig. 8 Photocurrent densities (a) and IPCE (b) values of WO3 and Sn-doped WO3 films

J Solid State Electrochem

Mott-Schottky curves for all films manifests the n-type character of WO3 and in situ Sn-doped WO3 films. The flat band potential (Vfb) at semiconductor/electrolyte interface was estimated by the Mott-Schottky equation [54]: 1=C 2 ¼ ð2=εε0 qN d Þ½ðV−V fb Þ−kT=q

ð2Þ

By extrapolating the X-intercepts of the liner region in MottSchottky plots, and Vfb of pure WO3 film was ~0.08 V vs. Ag/ AgCl, while that of 5% Sn-doped WO3 films shifted to 0 V. The carrier density in photoanodes can also be calculated based on the Mott-Schottky equation [55]:   −1 N d ¼ ð2=εε0 qÞ d 1=C 2 =dV Fig. 9 Mott-Schottky plots of photoelectrodes

Ag/AgCl. Additionally, the photocurrent decreased to a similar or lower level of pure WO3 plate with higher Sn doping. It turns out that the suitable doping amount of Sn can effectively enhance photoelectrochemical activity. As discussed below, the enhanced PEC performance was due to the improved electronic properties as a result of Sn ion incorporation and the surface passivation effect of SnO2. As shown in Fig. 8b, the incident-photon-to-currentefficiency (IPCE) as a function of incident light wavelength was measured at 1.2 V vs. Ag/AgCl for the pure WO3 and Sndoped WO3 films. In comparison with pure WO3, the Sndoped WO3 anodes exhibit extremely enhanced photoactivity over the whole photo response region (300~460 nm), especially for 5% Sn-doped WO3 films. The highest IPCE value (45.1%) was detected at 350 nm for the 5% Sn-doped WO3 film, which is 1.6 times higher than that of the pure WO3 film (28.3%). This result was consistent with the improvements observed in the photocurrent densities. The Mott-Schottky plots were recorded at a frequency of 1 kHz in 0.2 M Na2SO4 aqueous electrolyte under the visible light illumination, as presented in Fig. 9. The positive slope of

ð3Þ

where ε and ε0 denote the dielectric constant of the semiconductor and the vacuum permittivity, q the electron charge, Nd the carrier density, and V the potential applied at the electrode, respectively. The electron density of the pristine WO3 film was calculated to be 1.8 × 1021 cm−3, which conforms to previous studies [56, 57], while the Nd value of 5% Sndoped WO3 film was 3.7 × 1021 cm−3. This data provides concrete evidence and supports the idea that extrinsic Sn dopants act as electron donors. The bigger electron density in the in situ Sn-doped WO3 film is believed to be another factor contributing to the photocurrent density enhancement. Figure 10 displays the electrochemical impedance spectroscopy (EIS) Nyquist plots and bode plots of pure WO3 and the Sn-doped WO3 photoanodes at the applied potential of 0.8 V (vs. Ag/AgCl) under visible light illumination. Through the study of these plots, it would generate a better understanding to the electron transport properties and the efficient electron lifetime [58]. Seen from the Nyquist plots (Fig. 10a), the circular radius of the 5% Sn-WO3 film is smaller than the other films, indicating lower electron transfer resistance at the interface of the photoanode/electrolyte. The inset of Fig. 10a presents the equivalent circuit, which was composed of solution resistance (R 1 ), charge transfer resistance(R2), and a chemical capacitance (CPE1) [59].

Fig. 10 EIS Nyquist plots (a) and bode plots (b) of WO3 and Sn-doped WO3 films

J Solid State Electrochem

According to the calculate result, the values of the resistance R1 and R2 were 33.2 and 701.2 Ω for the pure WO3 and 29.9 and 447.3 Ω for the 5% Sn-WO3, respectively. This suggests that Sn-doped WO3 film owns lower electron transfer resistance than WO3 film, corresponding to the better PEC performance. Meanwhile, bode plots are transformed by the Z-view software as presented in Fig. 10b. The frequency peak (fp) of WO3 film shifts from 18.6 to 10.0 Hz after 5% Sn doped. From this, the efficient electron lifetime (τe) is calculated by the following formula [60]:   τ e ¼ 1= 2π f p

ð4Þ

and τe for WO3 and 5% Sn-doped WO3 are 8.6 and 15.9 ms, respectively. It means that the Sn doping improved the electron lifetime and separation efficiency of photogenerated electrons and holes in WO3 films. Furthermore, the intensity modulated photocurrent spectroscopy (IMPS) was used to investigate photogenerated electron transport rate within photoelectrodes. The electron transport time (τd) is the average time that photogenerated electrons need to migrate the back contact and can be calculated from the IMPS curves by the following formula: τ d ¼ 1=ð2π f min Þ

ð5Þ

where fmin is the frequency at the minimum IMPS plot. In the results shown in Fig. S1, τd of the pure WO3 and 5% Sn-doped WO3 films are 1.49 and 0.95 ms, respectively. This indicates that the photogenerated electron transport rate in Sn-doped WO3 photoanode is faster than the pure WO3 film, which is consistent with the EIS results. It may be attributed to the lower resistance and the proper impurity level state of Sn dopant, which can be supported by the theoretical calculation.

Acknowledgements This work was supported by the National Nature Science Foundation of China (21471054) and postgraduate research and innovation project of Hunan Province (CX2016B305).

References 1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

Conclusion In summary, we have synthesized in situ Sn-doped WO3 plates films on FTO substrate using a hydrothermal method, followed by annealing in air. In situ Sn doping enlarged the growth density and improved the crystallinity. Some of the Sn atoms entered into the lattices of WO3, while the other Sn dopants distributed on the grain boundary or on the surface of WO3 plates in the form of SnO2 passivation layer. In comparison to undoped WO3 films, these in situ Sn-doped WO3 films exhibited higher photocurrent densities and enhanced photoelectrochemical CO2 reduction properties as a result of improved separation of photogenerated electron/hole pairs, carrier density, electrical conductivity, electron lifetime, and the faster electron transport rate due to the proper impurity level state created by Sn doping.

11.

12. 13.

14.

15.

16.

17.

Gao S, Lin Y, Jiao X, Sun Y, Luo Q, Zhang W, Li D, Yang J, Xie Y (2016) Partially oxidized atomic cobalt layers for carbon dioxide electroreduction to liquid fuel. Nature 529(7584):68–71 Guo S, Zhao S, Gao J, Zhu C, Wu X, Fu Y, Huang H, Liu Y, Kang Z (2016) Cu-CDots nanocorals as electrocatalyst for highly efficient CO2 reduction to formate. Nano. doi:10.1039/C6NR08104E Sato S, Arai T, Morikawa T (2015) Toward solar-driven photocatalytic CO2 reduction using water as an electron donor. Inorg Chem 54(11):5105–5113 Hisatomi T, Domen K (2016) Introductory lecture: sunlight-driven water splitting and carbon dioxide reduction by heterogeneous semiconductor systems as key processes in artificial photosynthesis. Faraday Discuss. doi:10.1039/C6FD00221H Magesh G, Kim ES, Kang HJ, Banu M, Kim JY, Kim JH, Lee JS (2014) A versatile photoanode-driven photoelectrochemical system for conversion of CO2 to fuels with high faradaic efficiencies at low bias potentials. J Mater Chem A 2(7):2044–2050 Kim JH, Magesh G, Kang HJ, Banu M, Kim JH, Lee J, Lee JS (2015) Carbonate-coordinated cobalt co-catalyzed BiVO4/WO3 composite photoanode tailored for CO2 reduction to fuels. Nano Energy 15:153–163 Cheng J, Zhang M, Liu J, Zhou J, Cen K (2015) A Cu foam cathode used as a Pt–RGO catalyst matrix to improve CO2 reduction in a photoelectrocatalytic cell with a TiO2 photoanode. J Mater Chem A 3(24):12947–12957 Zhou X, Liu R, Sun K, Chen Y, Verlage E, Francis SA, Lewis NS, Xiang C (2016) Solar-driven reduction of 1 atm of CO2 to formate at 10% energy-conversion efficiency by use of a TiO2-protected III–V tandem photoanode in conjunction with a bipolar membrane and a Pd/C cathode. ACS Energy Letters 1(4):764–770 Yang Y, Xie R, Li H, Liu C, Liu W, Zhan F (2016) Photoelectrocatalytic reduction of CO2 into formic acid using WO3–x/TiO2 film as novel photoanode. T Nonferr Metal Soc 26(9):2390–2396 Yagi M, Maruyama S, Sone K, Nagai K, Norimatsu T (2008) Preparation and photoelectrocatalytic activity of a nano-structured WO3 platelet film. J Solid State Chem 181(1):175–182 Zheng H, Ou JZ, Strano MS, Kaner RB, Mitchell A, Kalantar-zadeh K (2011) Nanostructured tungsten oxide-properties, synthesis, and applications. Adv Funct Mater 21(12):2175–2196 Zheng H, Tachibana Y, Kalantar-Zadeh K (2010) Dye-sensitized solar cells based on WO3. Langmuir 26(24):19148–19152 Kalantar-zadeh K, Ou JZ, Daeneke T, Mitchell A, Sasaki T, Fuhrer MS (2016) Two dimensional and layered transition metal oxides. Applied Materials Today 5:73–89 Jiao Z, Wang J, Ke L, Sun XW, Demir HV (2011) Morphologytailored synthesis of tungsten trioxide (hydrate) thin films and their photocatalytic properties. ACS Appl Mater Interf 3(2):229–236 Amano F, Li D, Ohtani B (2010) Fabrication and photoelectrochemical property of tungsten(vi) oxide films with a flake-wall structure. Chem Commun 46(16):2769–2771 Sieb NR, Wu N-c, Majidi E, Kukreja R, Branda NR, Gates BD (2009) Hollow metal nanorods with tunable dimensions, porosity, and photonic properties. ACS Nano 3(6):1365–1372 Lu X, Wang G, Zhai T, Yu M, Gan J, Tong Y, Li Y (2012) Hydrogenated TiO2 nanotube arrays for supercapacitors. Nano Lett 12(3):1690–1696

J Solid State Electrochem 18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

29.

30.

31.

32.

33.

34.

35.

36.

37.

Lu X, Zhai T, Zhang X, Shen Y, Yuan L, Hu B, Gong L, Chen J, Gao Y, Zhou J (2012) WO3–x@Au@MnO2 core-shell nanowires on carbon fabric for high-performance flexible supercapacitors. Adv Mater 24(7):938–944 Ng CY, Razak KA, Lockman Z (2014) WO3 nanorods prepared by low-temperature seeded growth hydrothermal reaction. J Alloy Compd 588:585–591 Yong S-M, Nikolay T, Ahn BT, Kim DK (2013) One-dimensional WO3 nanorods as photoelectrodes for dye-sensitized solar cells. J Alloy Compd 547:113–117 Zhan F, Li J, Li W, Liu Y, Xie R, Yang Y, Li Y, Chen Q (2015) In situ formation of CuWO4/WO3 heterojunction plates array films with enhanced photoelectrochemical properties. Int J Hydrogen Energ 40(20):6512–6520 Zhan F, Xie R, Li W, Li J, Yang Y, Li Y, Chen Q (2015) In situ synthesis of g-C3N4/WO3 heterojunction plates array films with enhanced photoelectrochemical performance. RSC Adv 5(85): 69753–69760 Zhan F, Yang YH, Li W, Liu W, Li Y, Chen Q (2016) Preparation of DyVO4/WO3 heterojunction plates array films with enhanced photoelectrochemical activity. RSC Adv 6:10393–10400 Zhan F, Li J, Li W, Yang Y, Liu W, Li Y (2016) In situ synthesis of CdS/CdWO 4 /WO 3 heterojunction films with enhanced photoelectrochemical properties. J Power Sources 325:591–597 Alves SA, Soares LL, Goulart LA, Mascaro LH (2016) Solvent effects on the photoelectrochemical properties of WO3 and its application as dopamine sensor. J Solid State Electr 20(9):2461–2470 Tubtimtae A, Cheng K-Y, Lee M-W (2014) Ag2S quantum dotsensitized WO3 photoelectrodes for solar cells. J Solid State Electr 18(6):1627–1633 Zhuiykov S, Kats E, Carey B, Balendhran S (2014) Proton intercalated two-dimensional WO3 nano-flakes with enhanced chargecarrier mobility at room temperature. Nano 6(24):15029–15036 Li W, Zhan F, Li J, Liu C, Yang Y, Li Y, Chen Q (2015) Enhancing photoelectrochemical water splitting by aluminum-doped plate-like WO3 electrodes. Electrochim Acta 160:57–63 Kumar RD, Karuppuchamy S (2016) Microwave mediated synthesis of nanostructured Co-WO3 and CoWO4 for supercapacitor applications. J Alloy Compd 674:384–391 Kumar RD, Andou Y, Karuppuchamy S (2016) Synthesis and characterization of nanostructured Ni-WO 3 and NiWO 4 for supercapacitor applications. J Alloy Compd 654:349–356 Miseki Y, Kusama H, Sugihara H, Sayama K (2010) Cs-modified WO3 photocatalyst showing efficient solar energy conversion for O2 production and Fe (III) ion reduction under visible light. J Phys Chem Lett 1(8):1196–1200 Upadhyay SB, Mishra RK, Sahay PP (2014) Structural and alcohol response characteristics of Sn-doped WO3 nanosheets. Sensors Actuators B Chem 193:19–27 Liew SL, Subramanian GS, Seng Chua C, Luo H-K (2016) Studies into the Yb-doping effects on photoelectrochemical properties of WO3 photocatalysts. RSC Adv 6(23):19452–19458 Sivula K, Zboril R, Le Formal F, Robert R, Weidenkaff A, Tucek J, Frydrych J, Gratzel M (2010) Photoelectrochemical water splitting with mesoporous hematite prepared by a solution-based colloidal approach. J Am Chem Soc 132(21):7436–7444 Ling Y, Wang G, Wheeler DA, Zhang JZ, Li Y (2011) Sn-doped hematite nanostructures for photoelectrochemical water splitting. Nano Lett 11(5):2119–2125 Chiam SY, Kumar MH, Bassi PS, Seng HL, Barber J, Wong LH (2014) Improving the efficiency of hematite nanorods for photoelectrochemical water splitting by doping with manganese. ACS Appl Mater Interf 6(8):5852–5859 Cai L, Cho IS, Logar M, Mehta A, He J, Lee CH, Rao PM, Feng Y, Wilcox J, Prinz FB (2014) Sol-flame synthesis of cobalt-doped

38.

39.

40.

41.

42.

43.

44.

45.

46.

47.

48.

49.

50.

51.

52.

53.

54.

TiO2 nanowires with enhanced electrocatalytic activity for oxygen evolution reaction. Phys Chem Chem Phys 16(24):12299–12306 Yang J, Li WZ, Li J, Sun DB, Chen QY (2012) Hydrothermal synthesis and photoelectrochemical properties of vertically aligned tungsten trioxide (hydrate) plate-like arrays fabricated directly on FTO substrates. J Mater Chem 22(34):17744–17752 Riad A, Mahmoud S, Ibrahim A (2001) Structural and DC electrical investigations of ZnO thin films prepared by spray pyrolysis technique. Phys B Condens Matter 296(4):319–325 Keskenler EF, Turgut G, Aydın S, Doğan S (2013) W doped SnO2 growth via sol-gel routes and characterization: nanocubes. OptikInternational Journal for Light and Electron Optics 124(21):4827– 4831 Lou XW, Wang Y, Yuan C, Lee JY, Archer LA (2006) Templatefree synthesis of SnO2 hollow nanostructures with high lithium storage capacity. Adv Mater 18(17):2325–2329 Roy A, Arbuj S, Waghadkar Y, Shinde M, Umarji G, Rane S, Patil K, Gosavi S, Chauhan R (2016) Concurrent synthesis of SnO/SnO2 nanocomposites and their enhanced photocatalytic activity. J Solid State Electrochem. doi:10.1007/s10008-016-3328-y Yang Y, Xie R, Liu Y, Li J, Li W (2015) Effect of surface passivation on photoelectrochemical water splitting performance of WO3 vertical plate-like films. Catalysts 5(4):2024–2038 Han S, Li J, Chen X, Huang Y, Liu C, Yang Y, Li W (2012) Enhancing photoelectrochemical activity of nanocrystalline WO3 electrodes by surface tuning with Fe(III). Int J Hydrogen Energ 37(22):16810–16816 Wang L, Lee CY, Mazare A, Lee K, Müller J, Spiecker E, Schmuki P (2014) Enhancing the water splitting efficiency of Sn-doped hematite nanoflakes by flame annealing. Chemistry–a European journal 20(1):77–82 Xi L, Chiam SY, Mak WF, Tran PD, Barber J, Loo SCJ, Wong LH (2013) A novel strategy for surface treatment on hematite photoanode for efficient water oxidation. Chem Sci 4(1):164–169 Subrahmanyam A, Karuppasamy A (2007) Optical and electrochromic properties of oxygen sputtered tungsten oxide (WO3) thin films. Sol Energ Mat Sol C 91(4):266–274 Santato C, Odziemkowski M, Ulmann M, Augustynski J (2001) Crystallographically oriented mesoporous WO3 films: synthesis, characterization, and applications. J Am Chem Soc 123(43): 10639–10649 L i W Z , L i J , Wa n g X , M a J , C h e n Q Y ( 2 0 1 0 ) Photoelectrochemical and physical properties of WO3 films obtained by the polymeric precursor method. Int J Hydrogen Energ 35(24):13137–13145 Li WZ, Li J, Wang X, Ma J, Chen QY (2010) Effect of citric acid on photoelectrochemical properties of tungsten trioxide films prepared by the polymeric precursor method. Appl Surf Sci 256(23):7077– 7082 Chahmat N, Souier T, Mokri A, Bououdina M, Aida MS, Ghers M (2014) Structure, microstructure and optical properties of Sn-doped ZnO thin films. J Alloy Compd 593:148–153 Zhou W, Liu L, Yuan M, Song Q, Wu P (2012) Electronic and optical properties of W-doped SnO2 from first-principles calculations. Comput Mater Sci 54:109–114 Regoutz A, Oropeza FE, Poll CG, Payne DJ, Palgrave RG, Panaccione G, Borgatti F, Agrestini S, Utsumi Y, Tsuei KD, Liao YF, Watson GW, Egdell RG (2016) Identification of metal s states in Sn-doped anatase by polarisation dependent hard X-ray photoelectron spectroscopy. Chem Phys Lett 647: 59–63 Hahn NT, Mullins CB (2010) Photoelectrochemical performance of nanostructured Ti-and Sn-doped α-Fe2O3 photoanodes. Chem Mater 22(23):6474–6482

J Solid State Electrochem 55.

56.

57.

Wang G, Ling Y, Wheeler DA, George KE, Horsley K, Heske C, Zhang JZ, Li Y (2011) Facile synthesis of highly photoactive alpha-Fe2 O 3-based films for water oxidation. Nano Lett 11(8):3503–3509 Sivakumar R, Raj AME, Subramanian B, Jayachandran M, Trivedi D, Sanjeeviraja C (2004) Preparation and characterization of spray deposited n-type WO3 thin films for electrochromic devices. Mater Res Bull 39(10):1479–1489 Su L, Zhang L, Fang J, Xu M, Lu Z (1999) Electrochromic and photoelectrochemical behavior of electrodeposited tungsten trioxide films. Sol Energ Mat Sol C 58(2):133–140

58.

59.

60.

Zhang WD, Jiang LC, Ye JS (2009) Photoelectrochemical study on charge transfer properties of ZnO nanowires promoted by carbon nanotubes. J Phys Chem C 113(36):16247–16253 Pilli SK, Deutsch TG, Furtak TE, Brown LD, Turner JA, Herring AM (2013) BiVO4/CuWO4 heterojunction photoanodes for efficient solar driven water oxidation. Phys Chem Chem Phys 15(9): 3273–3278 Zhou M, Bao J, Xu Y, Zhang J, Xie J, Guan M, Wang C, Wen L, Lei Y, Xie Y (2014) Photoelectrodes based upon Mo: BiVO4 inverse opals for photoelectrochemical water splitting. ACS Nano 8(7): 7088–7098