WO3-Nanoplate-Array Heterojunction

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Conformal BiVO4‑Layer/WO3‑Nanoplate-Array Heterojunction Photoanode Modified with Cobalt Phosphate Cocatalyst for Significantly Enhanced Photoelectrochemical Performances Xueliang Zhang,†,‡ Xin Wang,†,‡ Defa Wang,*,†,‡ and Jinhua Ye*,†,‡,§

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TJU−NIMS International Collaboration laboratory, Key Lab of Advanced Ceramics and Machining Technology (Ministry of Education), Tianjin Key Lab of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, 92 Weijin Road, Tianjin 300072, China ‡ Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), 92 Weijin Road, Tianjin 300072, China § International Center of Materials Nanoarchitectonics (WPI−MANA), National Institute for Materials Science (NIMS), 1-1Namiki, Tsukuba, Ibaraki 305-0044, Japan S Supporting Information *

ABSTRACT: Constructing semiconductor heterojunctions via surface/interface engineering is an effective way to enhance the charge carrier separation/transport ability and thus the photoelectrochemical (PEC) properties of a photoelectrode. Herein, we report a conformal BiVO4-layer/WO3-nanoplate-array heterojunction photoanode modified with cobalt phosphate (Co-Pi) as oxygen evolution cocatalyst (OEC) for significant enhancement in PEC performances. The BiVO4/ WO3 nanocomposite is fabricated by coating a thin conformal BiVO4 layer on the surface of presynthesized WO3 nanoplate arrays (NPAs) via stepwise spin-coating, and the decoration of Co-Pi OEC is realized by photoassisted electrodeposition method. The optimized Co-Pi@ BiVO4/WO3 heterojunction photoanode shows a maximum photocurrent of 1.8 mA/cm2 at 1.23 V vs RHE in a phosphate buffer electrolyte under an AM1.5G solar simulator, which is 5 and 12 times higher than those of bare WO3 and BiVO4 photoanode, respectively. Measurements of UV−vis absorption spectra, electrochemical impedance spectra (EIS) and photoluminescence (PL) spectra reveal that the enhanced PEC performances can be attributed to the increased charge carrier separation/transport benefited from the type II nature of BiVO4/WO3 heterojunction and the promoted water oxidation kinetics and photostability owing to the decoration of Co-Pi cocatalyst. KEYWORDS: photoelectrochemistry, WO3, BiVO4, cobalt phosphate cocatalyst, heterojunction photoanode, charge carrier separation, water oxidation

1. INTRODUCTION Photoelectrochemical (PEC) water splitting is one of the most promising green technologies for producing renewable clean hydrogen energy.1−5 A typical process of overall water splitting consists of two reactions, that is, water oxidation to evolve O2(H2O/O2) on photoanode and water reduction to evolve H2 (H2O/H2) on photocathode.6 Compared with the hydrogen evolution reaction (HER), oxygen evolution reaction (OER) is considerably more complex, since it suffers from inferior charge transport and slower kinetics of four-electron reaction.7 In this context, it is of great significance to develop efficient photoanode materials for constructing a practical PEC water splitting system. Various semiconductors including TiO2,8 ZnO,9 α-Fe2O3,10 WO3,11 and BiVO412 etc. have been developed as photocatalytic materials, among which WO3 has been widely studied as a promising photoanode candidate for PEC water splitting due to its suitable band gap (2.7 eV),13 moderate hole diffusion length (150 nm),14 and good chemical © XXXX American Chemical Society

stability in acidic environment (pH < 4) under solar illumination.15 Great efforts such as surface passivation,11 nanostructure engineering,15,16 selective doping,17 and oxygen evolution cocatalyst loading18 have been made to improve its PEC performance. However, WO3 photoanodes still showed relatively lower photocurrent density because of the limited theoretical value (4.8 mA/cm2 under AM1.5 G illumination),19 and the significant recombination rate of electron−hole pairs.20 Moreover, WO3 suffers from a gradual loss of photoactivity during long-term reaction due to the accumulation of peroxospecies on the surface in neutral electrolyte.21 Special Issue: Artificial Photosynthesis: Harnessing Materials and Interfaces for Sustainable Fuels Received: April 4, 2018 Accepted: June 28, 2018

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DOI: 10.1021/acsami.8b05477 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Constructing a heterojunction with two or more semiconductors has been regarded as an attractive method to enhance the charge carrier separation and transport. For examples, WO3 is often combined with other semiconductors such as ZnO,22 Fe2O3,23 Cr2O3,24 CaFe2O4,25 CuWO3,26 gC3N 4,27 Bi2S3,28 CdS,29 etc., demonstrating effectively improved PEC performances. Among the various semiconductor partners, BiVO4 seems to be an ideal candidate for coupling with WO3, because the former can absorb more visible light for its narrower band gap (2.4 eV) than WO3. More importantly, both the conduction band edge and valence band edge of BiVO4 are more negative than the corresponding values of WO3, respectively. Thus, the as-formed type II heterojunction allows the injection of photogenerated electrons from BiVO4 to WO3 and the flow of photogenerated holes from WO3 to BiVO4.30 As a result, the charge recombination rate can be considerably reduced. Several kinds of WO3/BiVO4 heterojunction photoanodes grown on conductive glass substrate have been reported for considerably improved PEC performance,31−34 which was greatly influenced by the morphology of under-layer WO3 and the interfacial structure of WO3/BiVO4.31 On the other hand, using highly ordered one-dimensional (1D) nanorod or two-dimensional (2D) nanoplate arrays with high aspect ratio and active areas to maximize light absorption and charge transfer efficiency has been demonstrated as an effective strategy to compensate for the limitations of heterojunction films.28,31,32,35−37 Generally speaking, to construct an ideal WO3/BiVO4 heterojunction photoanode with improved PEC performances, the upper layer of BiVO4 is required to be transparent enough for penetration of photons from BiVO4 to WO3, and the interface should be favorable for the easy transfer of photoinduced electrons/holes. Moreover, creating sufficient reactive sites on semiconductor surface is necessary to accelerate the reaction kinetics at the semiconductor/electrolyte interface. In this Forum Article, we report a conformal BiVO4-layer/ WO3-nanoplate-array heterojunction photoanode modified with cobalt phosphate (Co-Pi) oxygen evolution cocatalyst (OEC), showing significantly enhanced PEC properties. We first fabricate WO3 nanoplate arrays (NPAs) on fluorine-doped tin oxide (FTO) glass substrate by a facile hydrothermal method and then coat a thin conformal BiVO4 layer on WO3 NPAs using a stepwise spin-coating technique, forming a BiVO4-layer/WO3−NPAs (hereinafter denoted as BiVO4-x/ WO3, x represents the amount of BiVO4) nanocomposites. It is believed that the specific structure of WO3 NPAs is favorable for absorbing more incident photons via the light scattering/ trapping effect, and the type II heterojunction nature of asformed BiVO4-x/WO3 enhances the photoinduced charge carrier separation and transport. Moreover, depositing Co-Pi OEC on the surface of BiVO4/WO3 forms the Co-Pi@BiVO4/ WO3 heterojunction photoanode, of which the water oxidation kinetics and stability are further promoted significantly.

Figure 1. Schematic illustration of the fabrication process of Co-Pi@ BiVO4/WO3 heterojunction. performed at 550 °C for 2 h, the WO3 NPAs coated with BiVO4 precursor was allowed in advance to dry at room temperature followed by a brief heat treatment at 450 °C for 5 min in air. Lastly, a photoassistant electrodeposition method was used to deposit Co-Pi OEC on the BiVO4/WO3 nanocomposite,39,40 forming the Co-Pi@ BiVO4/WO3 heterojunction photoanodes. Details for the sample fabrication can be referred to the Supporting Information. 2.2. Materials Characterization. Crystal structure was determined using a powder X-ray diffractometer (XRD; D/MAX-2500, Rigaku, Japan). Raman spectrum was recorded on a confocal microRaman spectrometer (XploRa, HORIBA Scientific, USA) under excitation of a green laser at 532 nm. Morphology and microstructure were observed on a field emission scanning electron microscope (FESEM; S4800, Hitachi, Japan) and a transmission electron microscope (TEM; FEI Tecnai G2 F20, USA), and each was equipped with an energy dispersive X-ray spectrometer (EDX). UV− Vis diffuse reflectance spectra were recorded on a spectrophotometer (UV-1800, Shimadzu, Japan) with an integrating sphere attachment using BaSO4 as the reference. X-ray photoelectron spectroscopy (XPS) measurements were performed on an Escalab 250 (Thermo Scientific, USA) using monochromated Al Kα radiation and C 1s peak (284.8 eV) as the reference. Steady-state fluorescence measurements were carried out on a fluorescence spectrophotometer (Fluorolog-3, HORIBA Scientific, USA) with an excitation wavelength of 360 nm. 2.3. Photoeletrochemical Property Evaluation. PEC performance was tested on a standard three-electrode electrochemical workstation (CHI 660E Instruments) using platinum wire, Ag/AgCl in saturated KCl (3 M), and the prepared photoanode as the counter electrode, reference electrode, and working electrode, respectively. 0.1 M potassium phosphate buffer solution (pH 7) was used as the electrolyte after saturation with Ar gas for 30 min. A 500 W Xe lamp with an AM 1.5G filter was used for the simulated solar illumination. The power intensity of incident light was adjusted to be 100 mW/cm2 by a spectroradiometer (Avantes AvaSpec-ULS2048). Current− voltage (J−V) characteristics were measured by linear sweep voltammetry (LSV) at a scan rate of 10 mV/s in a quartz reactor under unchopped or chopped illumination on an area of 1 cm2. All the PEC performances were conducted using the front illumination mode. The photoconversion efficiency (η) was calculated using the following equation: η = I × (1.23 − VRHE)/Jlight

2. EXPERIMENTAL SECTION

where VRHE is the potential of the working electrode vs the reversible hydrogen electrode (RHE) in the unit of volt, I is the photocurrent density taken from J−V curves at the measured potential, and Jlight is the intensity of incident light from the AM1.5G irradiance (100 mW/ cm2). Amperometric I−t curves were recorded at a constant bias of 1.23 V vs RHE. The incident photon-to-current conversion efficiency (IPCE) was measured at the applied bias of 1.23 V vs RHE using a 300 W Xe lamp coupled with an aligned monochromator in the range of 370−600 nm. IPCE can be defined as a function of wavelength as follows:

2.1. Sample Preparation. As shown in Figure 1, a three-step approach was developed for the fabrication of Co-Pi@BiVO4/WO3 heterojunction photoanodes. First, the WO3 NPAs were synthesized on FTO by a modified hydrothermal method,38 followed by annealing treatment at 500 °C for 1 h. Then, a thin conformal BiVO4 layer was coated on the WO3 NPAs surface by a stepwise spin-coating technique, forming the BiVO4-x/WO3 nanocomposite, in which the x (x = 10, 20, 30, 40 μL) represents the amount of BiVO4 precursor solution used in each coating. Before the final annealing treatment B

DOI: 10.1021/acsami.8b05477 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces IPCE = (1240 × I )/(λ × Jlight ) where I is the photocurrent density, λ represents the incident light wavelength, and Jlight is the measured power density of incident light.41 Electrochemical impedance spectroscopy (EIS) was carried out in potentiostatic mode with the AC amplitude of 10 mV in the frequency range of 0.1 Hz−1 MHz at 1.23 V vs RHE under AM 1.5G illumination. The measured potential vs Ag/AgCl was converted to that vs RHE using the Nernst equation: ERHE = EAg/AgCl + 0.059pH + E0Ag/AgCl, where EAg/AgCl is the experimentally measured potential and E0Ag/AgCl = 0.209 V at 25 °C for an Ag/AgCl electrode in 3 M KCl.42

3. RESULTS AND DISCUSSION 3.1. XRD and Raman Structure Analysis. Figure 2 displays the XRD patterns of WO3 NPAs, BiVO4, and BiVO4-

Figure 3. Raman spectra of the bare WO3 NPAs, BiVO4, and BiVO420/WO3 nanocomposite.

cm−1 could be assigned to the bending vibrations (W−O−W) in monoclinic phase WO3.34,44 All the peaks observed in the Raman spectrum of BiVO4-20/WO3 nanocomposite could be indexed by the characteristic peaks for WO3 and BiVO4, respectively. 3.2. SEM/TEM Microstructure Observation and EDX Composition Measurement. Figure 4a and 4b show the

Figure 2. XRD patterns of the bare WO3 NPAs, BiVO4, and BiVO4x/WO3 (x = 10, 20, 30, 40) nanocomposites.

x/WO3 (x = 10, 20, 30, 40) nanocomposites on FTO substrate. The XRD patterns of all samples can be well indexed according to the monoclinic WO3 (JCPDS Card No. 72-0677), the monoclinic BiVO4 (JCPDS No. 14−0688), and the tetragonal SnO2 (JCPDS Card No. 46-1088) originated from the FTO substrate. No impurity phase was detected. The main diffraction peaks located at 2θ = 22.9°, 23.4°, 24.2°, and 34.0° were correspondent to the (002), (020), (200), and (202) crystal planes of the monoclinic phase WO3, respectively. For the BiVO4-x/WO3 nanocomposites, the peaks at 2θ = 18.9°, 28.7°, 30.5°, and 35.0° could be indexed as the (011), (1̅21), (040), and (002) crystal planes of the coated BiVO4 layer, respectively. It is clear that the intensity of monoclinic BiVO4 phase increases with increasing the amount of BiVO4 in the BiVO4-x/WO3 nanocomposites. The crystal structures of as-prepared samples were also characterized by Raman spectrometry. Figure 3 represents the Raman spectra of WO3, BiVO4, and BiVO4-20/WO3 nanocomposite measured by using a monochromatic laser of 532 nm. A strong peak at 825 cm−1 and a weak peak at 710 cm−1 were observed in bare BiVO4 film, which could be assigned to the asymmetric and symmetric V−O stretching modes in BiVO4, respectively. The peaks at 329 and 367 cm−1 were attributed to the asymmetric and symmetric bending vibrations of VO43−, respectively.43 For pure WO3 NPAs, four Raman peaks locating at 270, 330, 715, and 806 cm−1 were observed, being consistent with the monoclinic phase of WO3. The Raman peaks at 715 and 806 cm−1 could both be assigned to the W−O stretching modes, while the peaks at 270 and 330

Figure 4. Top-view SEM images of (a) WO3 NPAs and (b) BiVO420/WO3 nanocomposite. The insets in panels a and b are the respective magnified images. Low magnification TEM (c) and HRTEM (d) images of BiVO4-20/WO3 nanocomposite.

typical low and high magnification SEM images of the assynthesized WO3 NPAs and BiVO4-20/WO3 nanocomposite, both of which were featured as densely packed nanoplate arrays. The thickness of WO3 plates was around 200 nm (see inset of Figure 4a). From Figure 4b, we can see clearly the conformal coating of BiVO4 on WO3 nanoplates, and the density of BiVO4-20/WO3 was higher than that of WO3 NPAs. The SEM images of other BiVO4-x/WO3 nanocomposites and Co-Pi@BiVO4-20/WO3 show a similar morphology but an increased density with increasing the coated amount of BiVO4 (Figure S1 in Supporting Information). Shown in Figure 4c and 4d are the representative TEM and HRTEM images of BiVO4-20/WO3 nanocomposite. We can see that the BiVO4 layer with a thickness of ∼10 nm was uniformly coated on C

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the block of the relatively thick layer of BiVO4, the composite samples BiVO4-30/WO3 and BiVO4-40/WO3 showed nearly the same absorption edge as that of pure BiVO4. In addition, no obvious difference was observed between the spectra of CoPi@BiVO4-20/WO3 and BiVO4-20/WO3, indicating that coating of Co-Pi almost did not affect light absorption of BiVO4-20/WO3. 3.4. PEC Performances. The LSV curves of BiVO4-x/ WO3 (x = 10, 20, 30, 40) photoanodes were first measured in a three-electrode cell containing a 0.1 M potassium phosphate solution buffered to pH = 7 under AM1.5G illumination. Considering the higher photocurrent and cathodic shift onset potential for all BiVO4-x/WO3 nanocomposite samples obtained by front irradiation than that by back irradiation (Figure S6), all the PEC measurements were conducted under front irradiation. Figure 6a shows the chopped photocurrent density−voltage (J−V) curves of BiVO4-x/WO3 with different amounts of BiVO4. All the photoanodes showed an obvious “photo-switching” effect with fast response. The photocurrent density gradually increased with increasing amount of coated BiVO4 and reached the maximum value of 1.65 mA/cm2 at 1.23 V vs RHE for the sample BiVO4-20/WO3. Further increasing the coated amount of BiVO4 resulted in degradation of the photocurrent density. The reason might be as follows: when the thickness of BiVO4 layer was comparable to the diffusion length of the charge carriers, bulk recombination would prevail, leading to reduced photocurrent.11,17 Figure 6b shows the comparison of chopped linear sweep J− V curves of WO3 NPAs, BiVO4, BiVO4-20/WO3, and Co-Pi@ BiVO4-20/WO3 under an AM1.5G illumination. Clearly, the BiVO4-20/WO3 heterojunction photoanode exhibited a much better PEC performance than either WO3 NPAs or BiVO4 film. For the BiVO4-20/WO3 photoanodes, we obtained a photocurrent density of 1.6 mA/cm2 at 1.23 V vs RHE, which was 4.6 times and 10.7 times higher than those of bare WO3 (0.35 mA/cm2) and bare BiVO4 (0.15 mA/cm2), respectively. Moreover, the onset potential of BiVO4-20/WO3 was measured to be 0.39 V vs RHE, which was a negative shift of 0.28 V in comparison with the bare WO3 NPAs (Figure S7). The cathodic shift of onset potential could be attributed to the more negative conduction band position of BiVO4 compared with WO3. To further improve the PEC performance of the photoanode, Co-Pi OEC was deposited on BiVO4. As clearly evidenced by the significantly increased photocurrent density and cathodic shift of onset potential (see Figure 6b), the decoration of Co-Pi OEC was an effective way to further enhancing the water oxidation ability of Co-Pi@BiVO4-20/ WO3 photoanode. We then measured the LSV curves of WO3, BiVO4-x/WO3, and Co-Pi@BiVO4-x/WO3 in phosphate buffer solution with and without Na2SO3 as the hole-scavenger (Figure S8). The photocurrent density of BiVO4-20/WO3 photoanode for water oxidation was nearly 90% of that for sulphite oxidation at 1.23 V vs RHE, indicating that the majority of holes migrated to semiconductor/electrolyte interface was used to oxidize water. However, the photocurrent for water oxidation was still far lower than sulphite oxidation at low bias, indicating that a great number of the surface-reaching holes were lost to surface recombination. The charger transfer efficiencies of BiVO4-x/WO3 and CoPi@BiVO4-x/WO3 were estimated by comparing the photocurrent densities for water oxidation and sulphite oxidation. Na2SO3 is a typical hole-scavenger with high activity and the

WO3 NPAs. The thicknesses of BiVO4 layer in other BiVO4-x/ WO3 films were measured to be ∼6, ∼22, and ∼35 nm for BiVO4-10/WO3, BiVO4-30/WO3, and BiVO4-40/WO3, respectively (Figure S2 and Table S1). TEM-EDX analysis of BiVO4-20/WO3 gave rise to a atomic ratio of Bi:V:W = 4:4:91 (Figures S3 and S4 and Table S2), which could be converted to 6.13 wt % of BiVO4 in BiVO4-20/WO3. A relatively higher atomic ratio of Bi(V):W was obtained for BiVO4-20/WO3 from the SEM-EDX analysis, which reflected more surface information. For the same reason, the atomic ratio of Bi(V):W for BiVO4-20/WO3 measured from XPS was also higher than that from TEM-EDX analysis (Figure S5 and Table S3). The other BiVO4-x/WO3 composites and Co-Pi@BiVO4-20/WO3 film were also tested by SEM-EDX, and the results were summarized in Table S1. While the above measurements did not detect directly the interdiffusion of elements across the BiVO4/WO3 interface, it was highly likely considering the relatively high annealing temperature after spin-coating. In fact, such a phenomenon was ever observed in the similar system.45 It is worthy to note that in comparison with the dip-coating and drop-coating methods that easily caused severe aggregation in nanostructure film,46 the employed stepwise spin-coating method has been demonstrated the superiority in forming a perfect interfacial structure to secure intimate and uniform contact of the conformal BiVO4 layer on WO3 nanoplate. Such an excellent contact would definitely be favorable for the charge carrier separation/transfer between BiVO4 and WO3 and hence the enhanced PEC property of BiVO4-20/WO3 nanocomposite as will be discussed below. 3.3. UV−Vis Spectra Measurement. Shown in Figure 5 are the UV−vis diffuse reflectance spectra of bare WO3 NPAs,

Figure 5. UV−Vis diffuse reflectance spectra of WO3 NPAs, BiVO4, BiVO4-x/WO3 (x = 10, 20, 30, 40), and Co-Pi@BiVO4-20/WO3.

BiVO4, BiVO4-x/WO3 (x = 10, 20, 30, 40) nanocomposites, and Co-Pi@BiVO4-20/WO3. We can see that the main absorption edges of WO3 NPAs and BiVO4 were around 460 and 520 nm, corresponding to their band gaps of 2.7 and 2.4 eV, respectively. For the BiVO4-x/WO3 nanocomposites, two absorption edges corresponding to WO3 and BiVO4 could be clearly seen. In comparison with WO3, the “mixed” absorption edge of BiVO4-x/WO3 nanocomposites shifted to longer wavelength range with increasing the amount of coated BiVO4. In no doubt, the thin conformal BiVO4 layer greatly enhanced the light absorption ability of the BiVO4-x/WO3 nanocomposite photoanodes. It should be noted that because of D

DOI: 10.1021/acsami.8b05477 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. Chopped linear sweep photocurrent−potential curves of (a) BiVO4-x/WO3 heterojunctions (x = 10, 20, 30, 40), and (b) WO3 NPAs, BiVO4, BiVO4-20/WO3, and Co-Pi@BiVO4-20/WO3. (c)Photoconversion efficiencies of WO3NPAs, BiVO4, BiVO4-20/WO3, and Co-Pi@BiVO420/WO3. (d) Photocurrent densities (at 1.23 V vs RHE) as a function of time for WO3NPAs, BiVO4-20/WO3, and Co-Pi@BiVO4-20/WO3.

further enhanced to 0.6% at 0.75 V vs RHE by deposition of Co-Pi OEC on the BiVO4 surface. The stability of bare WO3 NPAs, BiVO4/WO3, and Co-Pi@ BiVO4/WO3 heterojunction were comparatively evaluated using a potentiostat in a three-electrode configuration at 1.23 V vs RHE under an AM1.5G solar simulator. As shown in Figure 6d, the photocurrent density of bare WO3 NPAs declined from 0.3 mA to 0.12 mA after illumination for 1200 s, because the bare WO3 NPAs suffered from not only a constant photocorrosion upon illumination but also the chemical corrosion from hydrogen peroxide produced by oxygen reduction on WO3 nanoplate surface.21,47,48 When coating a conformal BiVO4 layer on WO3 NPAs, the photostability was greatly improved. This could be evidenced that the photocurrent of BiVO4-20/WO3 was prone to be stable after a sharp decline in the initial 60 s. The photocurrent spike appearing at the moment of light-on was due to the occurrence of a quick charge carriers’ recombination process, which was followed subsequently by relaxation of the photocurrent to a steady state.49 A similar phenomenon was ever observed in previously reported studies.33,50,51 Clearly, the stability of photoanode was further improved significantly after Co-Pi OEC was deposited on BiVO4-20/WO3 heterojunction. The above observations demonstrated that conformal coating of BiVO4 and decoration of Co-Pi cocatalyst could effectively improve the PEC activity and the stability of Co-Pi@BiVO4-20/WO3 heterojunction photoanode. 3.5. IPCE Measurement. Wavelength dependence of photoactivity was investigated by measuring the incident photon-to-current conversion efficiency (IPCE). As shown in Figure 7, compared with the bare WO3 NPAs, the BiVO4-20/ WO3 nanocomposite exhibited a slightly higher IPCE value in

surface charge separation efficiency of sulphite oxidation is nearly 100%. Thus, the surface charge separation and transfer efficiency for water oxidation can be described as the following equation: ηtrans = JH O /Jsulphite 2

The calculated result showed that while all BiVO4-x/WO3 composite samples showed higher surface charge separation efficiency (ηtrans) than bare WO3, the surface charge transfer efficiency decreased with increasing the amount of BiVO4 coated on WO3 (Figure S9), which could be attributed to the severe charge carrier recombination at the interface of WO3 and BiVO4 caused by longer hole transfer path in thicker BiVO4 layer. Since the light absorption increased with increasing the BiVO4 content in BiVO4-x/WO3, the optimized photocurrent density for BiVO4-20/WO3 could be attributed to a balance between the improved light absorption and the charge carrier separation. It is worthy to note that although the Co-Pi deposition did not change the light absorption of BiVO4-20/WO3 photoanode, a significantly enhanced surface charge carrier separation and transfer efficiency was observed on Co-Pi@BiVO4-20/WO3 in comparison with BiVO4-20/ WO3, which accounted for the better PEC performance of CoPi@BiVO4-20/WO3 than BiVO4-20/WO3. The plots of calculated photoconversion efficiency as a function of applied bias are shown in Figure 6c. While the optimal photoconversion efficiencies were only 0.05% at 1.0 V vs RHE and 0.04% at 0.8 V vs RHE for bare WO3 and BiVO4 photoanodes, respectively, a much higher optimal conversion efficiency of 0.4% at 0.8 V vs RHE was achieved for the BiVO420/WO3 nanocomposite photoanode. Moreover, the optimal conversion efficiency of BiVO4-20/WO3 photoanode was E

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transfer resistance (Rct) and corresponding Helmholtz capacitance (CH).54−56 The contact resistance (Rs) was assigned to be in series with two parallel RC circuits. Generally, the charge separation/transfer process in the bulk is faster than that at the semiconductor/electrolyte interface and the diffusion of ions in electrolyte solution. Therefore, the smaller arc radius at medium frequencies corresponds to a lower Rct value, indicating a faster semiconductor/electrolyte interfacial charge transfer and a more effective separation of the photogenerated electron−hole pairs in the BiVO4/WO3 photoanode during light illumination. The equivalent circuit and simulated electrochemical parameters are shown in Table S4. As shown in Figure 8a, compared with bare WO3 or bare BiVO4, lower charge transfer resistance was observed in the BiVO4-x/WO3 (x = 10, 20, 30, 40) nanocomposites, in which the charge transfer process was accelerated. Owing to the appropriate thickness of BiVO4 layer beneficial for photogenerated hole-transfer from electrode to electrolyte, the BiVO4-20/WO3 showed the lowest Rct value, which could account for its highest PEC performance. Moreover, the deposition of Co-Pi OEC on the surface of BiVO4-20/WO3 reduced not only the interfacial charge transfer resistance (Rct) but also the bulk resistance (Rsc). This result implied an increase in collection efficiency for photogenerated holes from bulk to depletion layer, and a decreased electron−hole combination rate at the electrode/electrolyte interface, underlining the important role of Co-Pi OEC in enhancing PEC performance.55,57 The lifetime (τn) of electrons is correlated with the maximum frequency peaks (f max) in the Bode phase plots, which can be described by the following equation: τn = 1/ (2πf max).5,24,27,58 The Bode phase plots of WO3 NPAs, BiVO4, BiVO4-20/WO3, and Co-Pi@BiVO4-20/WO3 in Figure 8b show that the maximum frequency value of BiVO4-20/WO3 was much smaller than that of bare WO3 or BiVO4, and a further significant decrease in the maximum frequency value was observed for Co-Pi@BiVO4-20/WO3. The lifetimes of electrons calculated from the characteristic frequency at the peak of high-frequency semicircle under irradiation were 0.414, 0.500, 2.33, and 74.1 μs for bare WO3, BiVO4, BiVO4-20/ WO3, and Co-Pi@BiVO4-20/WO3, respectively (see Table S2). The much longer lifetime of Co-Pi@BiVO4-20/WO3 indicated a lower recombination rate in the composite electrode, leading to a higher charge collection efficiency and thus improved PEC performance.

Figure 7. IPCE of WO3, BiVO4-20/WO3, and Co-Pi@BiVO4-20/ WO3 measured at 1.23 V vs RHE in the incident wavelength range from 360 to 600 nm.

the range of 370−420 nm, and a much higher value from 420− 500 nm. It is known that the PEC performance is mainly decided by light harvesting efficiency, charge separation and collection yields.52 The absorption-spectrum-dependence of IPCE indicated undoubtedly the photoreaction nature, and the enhanced IPCE of BiVO4-20/WO3 was due to the extended light absorbance from the BiVO4 layer, especially in visible light region. Moreover, the type II heterojunction of BiVO4/ WO3 could effectively improve the charge separation and collection, which further increased the photocurrent. The CoPi@BiVO4-20/WO3 showed a higher IPCE value than that of BiVO4-20/WO3, which could be attributed to the further improved charge carrier separation ability from the Co-Pi OEC. The IPCE results were in good agreement with the J−V measurements as mentioned above. 3.6. EIS Measurement. To further elucidate the kinetics of charge transfer during oxygen evolution reaction, EIS measurement was conducted to get the information about the inner and interfacial charger transfer resistance between semiconductor, oxygen evolution catalyst, and electrolyte.53 Figure 8a presents the EIS Nyquist plots of bare WO3, BiVO4, BiVO4x/WO3, and Co-Pi@BiVO4/WO3 heterojunctions under an AM1.5G solar simulator. We can see that each Nyquist curve consists of two semicircles. An equivalent circuit composed of two “RC” elements could be adopted to interpret the charge transport behaviors: the high frequency response was assigned to a resistance (R sc ) of bulk BiVO 4 -x/WO 3 and its accompanying capacity (Csc), while the low frequency response was assigned to the semiconductor/electrolyte interface charge

Figure 8. (a) EIS curves of bare WO3, bare BiVO4, BiVO4-x/WO3 (x = 10, 20, 30, 40), and Co-Pi@BiVO4-20/WO3. (b) Bode phase plots of bare WO3 NPAs, bare BiVO4, BiVO4-20/WO3, and Co-Pi@BiVO4-20/WO3. The EIS was measured at 1.23 V vs RHE under an AM1.5G solar simulator. F

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ACS Applied Materials & Interfaces 3.7. PL Spectra. To further understand the different charge carrier separation/transport behaviors in WO3 NPAs, BiVO4x/WO3 (x = 10, 20, 30, 40) nanocomposites, and Co-Pi@ BiVO4/WO3 heterojunction, we examined their PL spectra showing in Figure 9. For bare WO3 NPAs, the main emission

hole pairs in WO3/BiVO4 composite under illumination. The photogenerated electrons can easily travel from conduction band of BiVO4 to that of WO3 due to band alignment, and then transfer to the Pt counter electrode through FTO substrate. Meanwhile, the photogenerated holes from WO3 will be injected to the valence band of BiVO4, and then reach to the Co-Pi OEC, catalyzing the water oxidation process with improved reaction kinetics owing to the efficient charge separation and transfer in heterojunction.59,60 It should be mentioned that in the Co-Pi@BiVO4/WO3 heterojunction photoanode, the light scattering/trapping effect of NPAs structure increases the light harvesting efficiency, and the asformed type II BiVO4/WO3 heterojunction with an intimate contact of thin conformal BiVO4 layer on the surface of WO3 NPAs enhances the charge carrier separation/transport ability at the WO3/BiVO4 interface. More importantly, the decoration of Co-Pi OEC further improves the photostability of Co-Pi@ BiVO4/WO3 heterojunction photoanode and promotes the water oxidation kinetics. Thus, the PEC performance of CoPi@BiVO4/WO3 heterojunction photoanode is improved significantly.

Figure 9. PL spectra of WO3 NPAs, BiVO4-x/WO3, and Co-Pi@ BiVO4-20/WO3 heterojunctions. The wavelength of excitation light was 360 nm.

4. CONCLUSIONS In summary, an efficient and stable nanostructured Co-Pi@ BiVO4/WO3 heterojunction photoanode on FTO conductive substrate has been successfully fabricated through a three-step approach involving hydrothermal synthesis of WO3 NPAs, spin-coating of conformal BiVO4 thin layer on WO3 NPAs surface, and photoassistant electrodeposition of Co-Pi OEC on BiVO4 surface, consecutively. Significantly improved PEC performance has been achieved for the optimized Co-Pi@ BiVO4/WO3 heterojunction photoanode with a maximum photocurrent density of 1.8 mA/cm2 at 1.23 V (vs RHE) in 0.1 M potassium phosphate (pH = 7) electrolyte under an AM1.5G solar simulator, which was 5 and 12 times higher than those of bare WO3 and BiVO4 samples, respectively. The excellent PEC performance could be attributed to the enhanced light harvesting by the light scattering/trapping effect of WO 3 NPAs structure, the improved charge separation/transport efficiency from the type II heterojunction nature, and the promoted water oxidation kinetics owing to the deposited Co-Pi OEC. Our work provides an effective strategy for design and fabrication of nanostructured heterojunction photoelectrode with efficient PEC performances.

peak appeared around 440 nm, which was close to the absorption edge of WO3. The PL spectra of BiVO4-x/WO3 (x = 10, 20, 30, 40) showed two major peaks around 430 and 500 nm, which were essentially correspondent to the near bandedge transitions in WO3 and BiVO4, respectively. Interestingly, the PL intensities of BiVO4-x/WO3 heterojunctions around 430 nm were weaker than bare WO3 NPAs, indicating that the recombination of electron−hole pairs has been suppressed deeply. Among all the BiVO4-x/WO3 heterojunctions, BiVO420/WO3 showed the lowest PL intensities around both 430 and 500 nm. Moreover, Co-Pi deposition on the BiVO4-20/ WO3 heterojunction further decreased the PL intensity, indicating the superior charge carrier separation/transfer ability that could account for the significantly enhanced PEC performance of Co-Pi@BiVO4-20/WO3 than the other samples. On the basis of the above experimental results and discussion, we propose in Figure 10 the mechanism of charge carrier separation/transport and subsequent PEC reaction for the Co-Pi@BiVO4/WO3 heterojunction photoanode. Both of WO3 and BiVO4 can be photoexcited to generate electron−

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b05477.

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SEM, SEM-EDX, TEM, and HRTEM images of BiVO4x/WO3; TEM-EDX of BiVO4-20/WO3; XPS of BiVO420/WO3; J−V curves of BiVO4-20/WO3 for sulphite oxidation; simulated electrochemical parameters of WO3, BiVO4, and BiVO4-x/WO3; estimated electron lifetimes of WO3, BiVO4, and BiVO4-20/WO3 heterojunction (PDF)

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Figure 10. Mechanism of charge carrier separation/transport and subsequent PEC reaction over the Co-Pi@BiVO4/WO3 heterojunction photoanode under front illumination.

*E-mail: [email protected]. *E-mail: [email protected]. G

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Xueliang Zhang: 0000-0001-5037-5781 Defa Wang: 0000-0001-7196-6898 Jinhua Ye: 0000-0002-8105-8903 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

Financial support from the National Natural Science Foundation of China (51572191, 21633004) and the National Basic Research Project of China (973, 2014CB239300) is highly appreciated.

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DOI: 10.1021/acsami.8b05477 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX