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Defected ZnWO4-decorated WO3 nanorod arrays for efficient photoelectrochemical water splitting

The utilization of solar energy in photoelectrochemical water splitting is a popular approach to store solar energy and minimize the dependence on fossil fuels. Herein, defected ZnWO4-decorated WO3 nanorod arrays with type II heterojunction structures were synthesized via a two-step solvothermal method. By controlling the amount of Zn precursor, WO3 nanorods were decorated in situ with tunable amounts of ZnWO4 nanoparticles. Characterization confirmed the presence of abundant W5+ species in the defected ZnWO4-decorated WO3 samples, leading to enhanced light absorption and charge-separation efficiency. Therefore, the decorated WO3 nanorod arrays show much higher photoelectrochemical (PEC) activity than pure WO3 nanorod arrays. Specifically, the sample with optimal ZnWO4 decoration and surface defects exhibits a current density of 1.87 mA cm−2 in water splitting at 1.23 V vs. RHE under 1 sun irradiation (almost 2.36 times higher than that of pure WO3), a high incident photon-to-current efficiency of nearly 40% at 350 nm, and a relatively high photostability. However, the decoration of WO3 with too much ZnWO4 blocks the light absorption of WO3, inhibiting the PEC performance, even when many defects are present. This work provides a promising approach to rationally construct defected heterojunctions as highly active PEC anodes for practical applications.


Introduction
As a promising method to convert solar energy into storable chemical energy, photoelectrochemical (PEC) water splitting into H 2 and O 2 has attracted wide scientic and technological interest. [1][2][3][4] Since the rst discovery of PEC water splitting on TiO 2 by Fujishima et al., many studies focused on fabricating efficient photoanodes have been conducted. [5][6][7][8] Among the various semiconductor materials, n-type tungstic oxide (WO 3 ) has emerged as a promising photoanode material for PEC water splitting because of its suitable bandgap (ca. 2.7 eV), which can utilize a portion of visible light, and its valence band (VB) edge (ca. 3 V vs. RHE), which is positive enough to provide a sufficient driving force for oxygen production. 9,10 However, one of the greatest challenges hindering the photoactivity of WO 3 is its high rate of electron-hole recombination. 11 To solve this problem, the construction of heterostructures with matching band structures on WO 3 has been shown to be an efficient strategy. Type II heterojunctions, in which the both VB and conduction band (CB) of one semiconductor are either lower or higher than those of the other, are the most common composites. The formation of the proper band offset on the interface can drive photo-induced holes to the semiconductor with higher VB potential, while the electrons are driven to the semiconductor with lower CB potential. Thus, the recombination of charges is alleviated to some degree, and the spatial separation of charge carriers is enhanced. [12][13][14][15][16] In recent years, several semiconductors have been applied to fabricate type II heterojunctions with WO 3 , such as BiVO 4 , 17 BiOCl, 18 and tungstate materials (e.g., NiWO 4 , 19 CoWO 4 , 14 and ZnWO 4 (ref. 20)). Among them, tungstate materials have drawn considerable attention and been widely studied due to their superior compatibility with WO 3 crystals. Using a modied scanning electrochemical microscope method, Leonard et al. 20 found that the addition of Zn to WO 3 resulted in the best photoperformance among 25 types of metals. Furthermore, several studies on the construction of zinc tungstate and WO 3 have also been carried out. 13,21,22 However, the PEC activity of WO 3 requires further improvement. The introduction of W 5+ species/ oxygen vacancies is a promising approach because the dopant and shallow donors could prevent the recombination of photoinduced electron-hole pairs 23,24 and create defect levels below the CB, inducing new channels for light absorption and thus improving the PEC performance. 25,26 Herein, we develop a hydrothermal method with two steps, the formation of WO 3 nanorod arrays (NAs) and the reaction of Zn precursor with WO 3 NAs to form ZnWO 4 nanoparticles (NPs), to nally synthesize ZnWO 4 -decorated WO 3 NAs (WO 3 / ZnWO 4 , see Scheme 1). Importantly, the insertion of Zn atoms into the WO 3 NA crystals leads to distortion and forms abundant W 5+ species. By controlling the amount of Zn precursor, tunable amounts of ZnWO 4 NPs were crystallized in situ on the surfaces of WO 3 nanorods (named WZ-x, where x reects the content of ZnWO 4 NPs). Characterization conrmed the presence of abundant defects on the surface of ZnWO 4 . Therefore, with its continuous interface, the WO 3 / defected ZnWO 4 NA shows a much higher PEC activity than that of pure WO 3 NA. In particular, sample WZ-2 exhibits a current density as high as 1.87 mA cm À2 in PEC water splitting at 1.23 V vs. RHE under 1 sun irradiation (almost 2.36 times higher than that of pure WO 3 ), a high incident photon-to-current efficiency (IPCE) of nearly 40% at 350 nm, and a relatively high photostability.

Sample preparation
2.2.1 Synthesis of WO 3 nanorod arrays. Glass substrates coated with uorine-doped tin oxide (FTO) were cut into slides with dimensions of 1 cm Â 5 cm and washed sequentially with acetone, absolute ethanol and deionized water for 30 min each while under ultrasonication. In a typical preparation process, 1 g of (NH 4 ) 6 (H 2 W 12 O 40 )$xH 2 O was dissolved in 95 mL ultrapure water followed by the addition of 3 mL hydrochloric acid (36-38%) and stirring for 5 min. Subsequently, 2 mL of H 2 O 2 (30%) was added followed by vigorous stirring for 1 h. The as-prepared solution (20 mL) was then transferred into a Teon-lined autoclave (50 mL) in which the FTO substrate had been placed at an angle against the wall of the Teon liner with the conducting side facing down. Hydrothermal synthesis was conducted at 160 C for 4 h. The resulting WO 3 NA/FTO was rinsed with ultrapure water and dried at 60 C for 12 h. Finally, the obtained sample was annealed at 500 C in air for 1 h at a heating rate of 5 C min À1 .
2.2.2 Synthesis of WO 3 /defected ZnWO 4 nanorod arrays (NAs). WO 3 /defected ZnWO 4 NAs were synthesized from the above WO 3 NA/FTO via a hydrothermal growth process using only Zn precursor. Briey, WO 3 NA/FTO was added to a solution of 20 mL ethanol containing zinc acetate at a concentration of c x mM (c 1 , c 2 , c 3 ). The solution was then solvothermally treated in a Teon-lined autoclave at 180 C for 24 h. Subsequently, the obtained lm was washed with absolute ethanol, dried at 60 C overnight, and annealed at 500 C in air for 1 h (at a heating rate of 5 C min À1 ). The molar concentration of Zn(CH 3 COO) 2 in the prepared solution was xed as c 1 ¼ 3.125 mM, c 2 ¼ 6.25 mM or c 3 ¼ 12.5 mM, and the obtained lms were named WZ-x (WZ-1, WZ-2, or WZ-3, respectively).

Characterization
Crystal structures were characterized by X-ray diffraction (XRD) using a Rigaku D/MAX-2500 diffractometer equipped with Cu Ka radiation at 40 kV and 140 mA at a scanning rate of 5 min À1 . Raman spectra were obtained using a Raman spectrometer (DXR Microscope) with a green semiconductor laser (532 nm) as the excitation source. Scanning electron microscopy (SEM) images were recorded using a eld-emission scanning electron microscope (Hitachi, S-4800). High-resolution transmission electron microscopy (TEM) observations were made with a Tecnai G 2 F-20 transmission electron microscope. X-ray spectroscopy (EDX) elemental maps were obtained using an EDX system attached to the TEM instrument. X-ray photoelectron spectroscopy (XPS) was carried out using a PHI-1600 X-ray photoelectron spectroscope equipped with Al Ka radiation. The binding energy was calibrated by the C1s peak (284.8 eV). Ultravioletvisible (UV-Vis) diffuse reectance spectroscopy (DRS) was conducted with a Shimadzu UV-2600 spectrometer equipped with a 60 mm-diameter integrating sphere using BaSO 4 as the reectance. Steady-state photoluminescence (PL) spectra were recorded using a Hitachi F-4600 instrument with excitation at 325 nm.

Measurement of photoanode PEC performance
The PEC properties of the as-prepared WZ-x samples were evaluated in a typical three-electrode quartz cell with a Pt wire counter electrode and a saturated Ag/AgCl reference electrode in 0.5 M Na 2 SO 4 electrolyte. The illumination source was a 300 W Xe arc lamp (100 mW cm À2 , PLS-SXE300, Beijing Trusttech. Co. Ltd). Linear sweep voltammetry (LSV) was conducted by sweeping the potential to the positive direction at a scan rate of 10 mV s À1 . Based on the Nernst equation, E (vs. RHE) ¼ E (vs. Ag/AgCl) + 0.059 Â pH + 0.197, 27 the potential vs. Ag/AgCl reference electrode was converted to the potential vs. RHE.
The PEC stability (I-t curve) was evaluated at a potential of 0.4 V vs. Ag/AgCl under AM 1.5 G simulated sunlight irradiation. Electrochemical impedance spectroscopy (EIS) measurements were recorded with a sinusoidal ac perturbation of 10 mV applied over the frequency range of 1-10 5 Hz.
The IPCE was calculated according to eqn (1): 28 where l is the wavelength (nm), I is the photocurrent density (mA cm À2 ), and J light is the incident power illumination density of monochromatic light (mW cm À2 ). Supposing 100% faradaic efficiency, the applied bias photonto-current efficiency (ABPE) was calculated by eqn (2): 2 where V bias is the applied potential, and J light is the incident illumination power density (100 mW cm À2 ).

Crystal structure and morphology
As shown in the XRD patterns ( Fig. 1a 31 The crystal structures of the as-prepared photoanodes were also veried by Raman spectroscopy (Fig. 1c). Pure WO 3 exhibits six main Raman peaks located at ca. 74, 136, 274, 328, 718 and 808 cm À1 , which are attributed to the active Raman scattering modes of monoclinic WO 3 . 23,32 The strong Raman peaks at 718 and 808 cm À1 are assigned to the W-O stretching modes, while the two other peaks at 274 and 328 cm À1 are related to bending vibrations (W-O-W). 28 An extra peak at 906 cm À1 is observed for WZ-x samples and can be ascribed to symmetric stretching () O)W/O/), which is contributed to by the newly generated ZnWO 4 crystals. 33 The intensity of the peak at 906 cm À1 increases gradually with increasing Zn amount (from WZ-1 to WZ-3). Furthermore, the synthesized crystals exhibit a relatively broad vibrational mode, indicating short-range structural disorder. 34 The surface morphologies of the WZ-x samples were characterized by SEM and TEM, as shown in Fig. 2. WO 3 exhibits a uniform rod-like morphology. The rods are cuboid shaped with lengths of 500 nm to 1.5 mm, widths of 200-400 nm, and heights of ca. 2.4 mm ( Fig. 2a and b). Aer secondary solvothermal crystallization and thermal calcination (WZ-x samples), the surface layers of the WO 3 arrays are converted to ZnWO 4 NPs, forming a well-contacted heterojunction structure ( Fig. 2c-h). Compared to the pristine WO 3 NAs, the WZ-x samples show much rougher surfaces aer the formation of ZnWO 4 NP layers. Compared to the other WZ-x samples, the lowest concentration of Zn precursor (3.125 mM; WZ-1) resulted in relatively fewer ZnWO 4 NPs with crystal sizes of ca. 20-40 nm. When the Zn precursor concentration was increased to 6.25 mM (WZ-2), the amount of ZnWO 4 NPs increased along with the crystal size (ca. 50 nm). Further increasing the concentration of Fig. 1 XRD patterns (a and b) and Raman spectra (c) of pure WO 3 and WZ-x NAs.
Zn precursor to 12.5 mM (WZ-3) caused the ZnWO 4 NPs to become much larger and denser, almost covering the surface of WO 3 completely. This dense coverage might inhibit optical absorption and mass transfer for the PEC reaction.
The moderate coverage of ZnWO 4 on WO 3 in sample WZ-2 should promote optical absorption and facilitate the separation of generated electron-hole pairs for the constructed heterojunction. Moreover, this hierarchically structured lm can provide sufficient surface active sites to effectively enable reactions at the interface between the photoanode and electrolyte, thus improving the PEC performance. 35 The TEM images (Fig. 2i-k) conrm the formation of WO 3 / ZnWO 4 NAs. The spacing of 0.308 nm is in good agreement with the interplanar spacing of the WO 3 (112) plane, 36 Fig. 2i) clearly reveals that the ZnWO 4 NPs are highly crystalline. It is worth noting that an compact interface in the WO 3 /ZnWO 4 heterojunction was formed (Fig. 2k). This can be attributed to the in situ reaction of the Zn precursor with WO 3 NAs, which is benecial for the migration of photogenerated carriers between the two composites. Meanwhile, the elemental line scanning proles (Fig. 2l) (Fig. 2m-o) also conrm the primary distribution of W (and O) and Zn in the rod core and on the surface, respectively. 11,38

Chemical state
XPS was conducted to detect the surface chemical states of the as-prepared photoanodes. The survey spectra (Fig. S1a, ESI †) show the presence of W, O and Zn in the pure WO 3 and WZ-x samples without any impurities. In the high-resolution O 1s XPS spectra (Fig. S1b, ESI †), the peak with a binding energy of 530.5 eV is attributed to O-W and O-Zn bonds. 39,40 The Zn 2p XPS spectra (Fig. 3a) of the WZ-x samples show similar characteristic Zn 2p 3/2 and Zn 2p 1/2 peaks centered at 1021.9 and 1045.0 eV, respectively. These energies are slightly higher than those reported in the literature (1021.6 and 1044.7 eV, respectively), 40 indicating that fewer electrons locate in the Zn atoms of WZ-x compared to in pristine ZnWO 4 . Because Zn has a lower electronegativity (1.65) than W (2.36), the shi in binding energy conrms the close interaction between WO 3 and ZnWO 4 . As shown in Fig. 3b, the WZ-x samples exhibit obvious lowenergy shis in W 4f binding energy compared to pure WO 3 . For samples WZ-2 and WZ-3, a 0.3 eV negative shi in the W 4f binding energy is observed. This indicates that the formation of WZ-x heterojunctions increases the outer electron cloud density of the W atom, which results from W-O-Zn bonding and/or the presence of surface defects. 41 Pristine WO 3 shows only two symmetrical sharp peaks of W 4f (Fig. 3c), which is typical of W 6+ , while the W 4f peak of WZ-x can be divided into four peaks (Fig. 3d-f). The strong peaks at 35.4 and 37.6 eV correspond to W 6+ , while the weak peaks at 34.5 and 36.7 eV correspond to W 5+ . 26 By tting the XPS peak of W 4f, the contents of W 5+ in samples WZ-1, WZ-2 and WZ-3 are estimated to be 3.9%, 4.9% and 7.2% (atomic percentage), respectively, and the content of W 5+ is positively correlated with the amount of ZnWO 4 .

Optical properties
The UV-Vis DRS spectra of the WO 3 and WZ-x photoanodes are shown in Fig. 4a. For pristine WO 3 , the absorption edge appears at ca. 460 nm, consistent with its intrinsic band-gap absorption. 42 Aer coating with ZnWO 4 NPs, the absorption intensity is obviously enhanced in the ultraviolet region compared to pure WO 3 . Among the WZ-x samples, WZ-2 achieves the best absorption due to the moderate decoration of defected ZnWO 4 on WO 3 in this sample. The enhanced optical absorption should be related to the enhanced light scattering by the rough surface. Moreover, the W 5+ species in ZnWO 4 create defect levels below the CB, giving rise to absorption in the far-visible and/or infrared regions and producing tailing at the onset of absorption. 21 The high-resolution VB XPS spectra of WO 3 and WZ-x were recorded to determine the VB edge. As shown in Fig. 4b, compared to pure WO 3 (2.8 eV), 43 the VB positions of WZ-x are clearly shied toward a lower energy (0.1-0.2 eV), indicating that the ZnWO 4 NPs have a lower VB edge (and lower CB on the basis of bandgap data 16 ) than WO 3 , which benets the formation of type II heterojunctions. 26 The recombination of photogenerated charge carriers was investigated through PL emission measurements. Generally, a higher PL intensity indicates a higher recombination rate of photoexcited electrons and holes, while a lower PL intensity indicates a lower recombination rate. 14 Fig. 4c shows that pure WO 3 and WZ-x exhibit a major PL emission peak centered at around 463 nm due to the intrinsic feature of WO 3 and/or ZnWO 4 . This peak is ascribed to the radiative decay of selftrapped excitons in the crystals. 35,44 However, the PL intensities of WZ-x are clearly lower than that of pristine WO 3 , indicating a much greater charge-separation efficiency of photoexcited electron-hole pairs. For surface ZnWO 4 , the PL peak at ca. 460 nm (Fig. S2, ESI †) is mainly caused by the charge transition between the O 2p orbitals and the empty d orbitals of the central W 6+ ions in the WO 6 2À complex, while the yellow-red emission is extrinsic and may be related to W 5+ . 44 The defects induce the new energy states in the bandgap, which may benet the separation efficiency of e À -h + pairs. 31 Fitting the PL peak indicated that the amount of W 5+ species in WZ-x gradually increased with increasing Zn precursor concentration, consistent with the XPS results. However, for sample WZ-3, the total coverage of ZnWO 4 on the WO 3 surface will block light absorption and signicantly inhibit the activity of WO 3 , even when a large number of defects are present. Thus, WZ-2 with abundant defects and a heterojunction structure exhibits the highest separation efficiency of photogenerated charge carriers. 45

PEC performances of photoanodes
PEC catalysis is emerging as a promising method for solar water splitting (hydrogen and oxygen generation). 9 In PEC water splitting, the oxidation and reduction processes are separated into two half-cell reactions [eqn (3) and (4)], with eqn (5) showing the overall reaction: To identify the PEC performances of the pristine WO 3 and WZ-x photoanodes, PEC measurements were performed under simulated sun light illumination (100 mW cm À2 ) in 0.5 M Na 2 SO 4 electrolyte. As shown in Fig. 5a, no current was detected for pure WO 3 in the dark (WO 3 -dark). Under light illumination, the photocurrent density of pure WO 3 reaches 0.79 mA cm À2 at an applied bias 1.23 V vs. RHE. The ZnWO 4 -coated samples WZ-1 and WZ-3 show enhanced photocurrent densities of 1.44 and 1.24 mA cm À2 , respectively, while WZ-2 exhibits the highest photocurrent density of ca. 1.87 mA cm À2 at 1.23 V vs. RHE, nearly 2.36 times higher than that of pure WO 3 . The PEC performances of recently reported WO 3 -based photoanodes are summarized in Table S2 (ESI †). Notably, WZ-2 shows a higher PEC current than the previously reported photoanodes. Moreover, the on-set potentials of the WZ-x photoanodes (0.70 V vs. RHE for WZ-1, 0.68 V for WZ-2, and 0.75 V for WZ-3) are also lower than that of pure WO 3 (0.77 V vs. RHE). Based on the LSV results in Fig. 5a, WZ-2 achieved the maximum ABPE in this study (0.18% at 1.05 V vs. RHE; Fig. 5b), compared to only 0.07% for pure WO 3 .
The IPCE for PEC water splitting was carried out at 1.2 V vs. RHE, and the results are shown in Fig. 5c. The wavelengths of the initial light response are below 470 nm for the pure WO 3 and WZ-x photoanodes and IPCE increases gradually as the irradiation wavelength decreases. Based on the IPCE values, the PEC decreases in the following order: WZ-2 > WZ-1 > WZ-3 > pure WO 3 . The IPCE value of WZ-2 reaches 40% at 350 nm. Sample WZ-2 also possesses relatively good photostability, with only a slight loss aer long-term continuous illumination (Fig. S3, ESI †). Aer continuous PEC testing (over 3 h), WZ-2 shows nearly the same XRD peaks (Fig. S4a, ESI †) and surface morphology (Fig. S4b, ESI †) as those of a fresh sample, again conrming the relatively good photostability of WZ-2.
For the WO 3 photoanode, the occurrence of hole-capture reactions (e.g., 2H 2 O + 2h + / H 2 O 2 + 2H + ) on the surface is responsible for the corrosion of WO 3 via the formation of peroxotungstates, which hinders charge transfer at the WO 3 /electrolyte interface. In WZ-x samples, the decoration of WO 3 with defected ZnWO 4 forms a type II heterogeneous structure. Since ZnWO 4 possesses higher VB positions than WO 3 , the photoinduced holes will transfer rapidly to the ZnWO 4 NPs, preventing the accumulation of holes on WO 3 and preventing the photocorrosion of WO 3 . In addition, ZnWO 4 has better Paper chemical stability and photocorrosion resistance than WO 3 . 20,31 Therefore, the decoration of WO 3 with ZnWO 4 benets the stability of the photoanode.
To obtain more insight into the charge-transfer kinetics of the photoanodes, EIS measurements were carried out. The arc radii of the Nyquist plots can be used to evaluate the chargetransfer resistance at the semiconductor/electrolyte interface, with a smaller arc radius implying a smaller charge-transfer resistance. 46 Based on the EIS plots (Fig. 5d), the light irradiation signicantly decreases the arc radius of the photoanode, and the effect is more obvious for WZ-x than for pure WO 3 . This suggests that WZ-x exhibits more efficient light absorption and conversion than pure WO 3 . The charge-transfer resistances of the photoanodes were calculated by tting the EIS spectra (see the inset in Fig. 5d), where R s and R ct represent the series resistance and interfacial charge-transfer resistance across the electrode/electrolyte interface, respectively. 47 The tted values of each component are listed in  13,20 As the shallow donors, the surface W 5+ species in ZnWO 4 will introduce a new band level below the CB (ca. 0.30 V, calculated based on the peak emission wavelength in the PL spectrum). Therefore, the matched band structures benet the formation of interface heterojunctions between WO 3 and defected ZnWO 4 in the WZ-x samples. Under light illumination, the electrons are excited from the VBs of WO 3 and ZnWO 4 to their own CBs, leaving positive holes in the VBs. The photogenerated electrons in the CB of the ZnWO 4 NPs would rst migrate to the band of W 5+ (or related oxygen vacancies) and then to the CB of WO 3 . The accumulated electrons will rapidly transfer from WO 3 to the FTO substrate via back contact and nally reach the Pt counter electrode for hydrogen generation. Meanwhile, the photogenerated holes in the VB of WO 3 have a higher probability of transferring to the surface of ZnWO 4 because of the internal electric eld of the heterojunction. Thus, the holes have a higher probability of reacting with water molecules to generate oxygen gas.
The good PEC performance of sample WZ-2 in this study should be closely related to the heterojunction and surface W 5+ species. The heterojunction will force the spatial separation of electrons and holes in WO 3 and ZnWO 4 , respectively, while the surface W 5+ species will inhibit charge recombination.

Conclusions
Using a two-step solvothermal method, we successfully fabricated WO 3 /defected ZnWO 4 as a photoanode for efficient PEC water splitting. By controlling the amount of Zn precursor, tunable amounts of ZnWO 4 nanoparticles were decorated on WO 3 nanorods. The type II heterojunction with abundant defects enhances the light absorption and charge-separation efficiency. Thus, the WZ-x samples show much higher PEC activities than those of pure WO 3 NA. Among the samples, WZ-2 exhibits the highest photocurrent, with a current density of 1.87 mA cm À2 in PEC water splitting at 1.23 V vs. RHE (almost 2.36 times higher than that of pure WO 3 ), a high IPCE of ca. 40% at 350 nm, and a relatively high photostability. This work paves the way for fabricating highly active PEC anodes with defected heterojunction structures for practical applications.

Conflicts of interest
There are no conicts to declare.