Trans. Nonferrous Met. Soc. China 31(2021) 533-544

Construction of BiVO₄ nanosheets@WO₃ arrays heterojunction photoanodes by versatile phase transformation strategy

Xin SU¹, Can-jun LIU¹, Yang LIU², Ya-hui YANG³, Xuan LIU¹, Shu CHEN¹

1. Key Laboratory of Theoretical Organic Chemistry and Function Molecule of Ministry of Education, School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China;

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

3. School of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, China

Received 12 March 2020; accepted 21 December 2020

Abstract: A versatile phase transformation strategy was proposed to synthesize novel BiVO₄ nanosheets (NSs)@WO₃ nanorod (NR) and nanoplate (NP) arrays films. The strategy was carried out by following a three-step hydrothermal process (WO₃→WO₃/Bi₂WO₆→WO₃/BiVO₄). According to the characterization results, plenty of BiVO₄ NSs grew well on the surface of WO₃ NR and NP arrays films, thus forming the WO₃/BiVO₄ heterojunction structure. The prepared WO₃/BiVO₄ heterojunction films were used as the photoanodes for the photoelectrochemical (PEC) water splitting. As indicated by the results, the photoanodes exhibited an excellent PEC activity. The photocurrent densities of the WO₃/BiVO₄ NR and NP photoanodes at 1.23 V (vs RHE) without cocatalyst under visible light illumination reached up to about 1.56 and 1.20 mA/cm², respectively.

Key words: photoanode; bismuth vanadate; tungsten oxide; heterojunction

1 Introduction

The renewable generation of the clean fuels plays an essential role in meeting the growing demand for the energy and reversing environmental deterioration [1,2]. Photoelectrochemical (PEC) water splitting is potentially an ideal approach to obtaining hydrogen fuels directly from sunlight [3]. Since the first report on TiO₂ film photoanode in 1972 [4], plenty of efforts have been devoted to developing a high-efficiency photoanode for PEC water splitting over the past decades [5]. Up to now, some semiconductor materials have been applied as photoanodes, such as ZnO [6], Fe₂O₃ [7], WO₃ [8], TiO₂ [9] and BiVO₄ [10].

Especially, WO₃ is widely recognized as a promising photoanode material for its nontoxicity, high electrochemical stability and excellent charge- transferring ability [11]. However, the solar-to- hydrogen efficiency (STHE) of single WO₃ photoanode is low due to its poor ability to absorb visible light and the high recombination rate of photogenerated carriers [12]. Thus, some strategies have been proposed to address these issues, including morphology control [13], element  doping [14], and heterojunction constructing [15], etc. Particularly, the construction of heterojunction with other narrow-gap semiconductors has been confirmed as an effective method in promoting the spatial separation of photogenerated carriers and extending the visible-light response simultaneously.

As a narrow-gap semiconductor, BiVO₄ is considered to be one of the most promising materials fit for the construction of type-II alignment heterojunction with WO₃, due to its excellent performance in absorbing visible light (λ≤520 nm), high chemical stability and desirable conduction band (CB) position [16,17]. Therefore, WO₃/BiVO₄ films as photoanodes have been extensively studied [18]. Applying a solvothermal technique, SU et al [19] prepared a novel WO₃/BiVO₄ nanorod (NR) arrays photoanode that achieved a higher PEC activity than the planar WO₃/BiVO₄ photoanode. SHI et al [20] constructed a WO₃/BiVO₄ helix nanostructure heterojunction that demonstrated such advantages as effective light scattering, large contact surface area as well as high rate of carriers separation and transportation. Besides, its performance was verified through theoretical simulation and analysis. LEE et al [21] prepared a 1D WO₃/BiVO₄ photoanode by depositing dot-like BiVO₄ on the WO₃ NR surface. A systematic analysis was conducted to reveal the vital role played by the optimization of WO₃ NR morphology in achieving high PEC efficiency. According to the aforementioned studies, the morphology and nanostructure of WO₃/BiVO₄ heterojunction play a significant role in improving the PEC efficiency.

Recently, an in-situ transformation method has been widely applied to constructing heterostructure. Taking a anion transformation approach, GAO et al [22] prepared novel BiVO₄/Bi₂S₃ hollow nano- discoidals that exhibited superior photocatalytic activity. CHITRADA et al [23] produced Bi₂O₃- BiO_2-x photoanode using the in-situ photo- conversion method and the formation of photo- converted Bi₂O_4-x phase was conducive to absorbing more visible light, thus leading to high photocurrent density. The in-situ transformation method is considered to be more effortless to  adjust the morphology and nanostructure of heterostructure compared with other methods. Besides, the heterostructure constructed using this method shows a reduced interface defect [24].  Thus, it is necessary to develop an in-situ transformation to prepare WO₃/BiVO₄ hetero- junctions with various morphologies and study their PEC performance.

In this study, a versatile phase transformation strategy was applied to designing and fabricating novel BiVO₄ NS@WO₃ NR and NP arrays photo- anodes. The strategy was performed via a three- step hydrothermal process (WO₃→WO₃/Bi₂WO₆→ WO₃/BiVO₄). The morphology and nanostructure of samples prepared by the strategy were characterized and the corresponding PEC properties were investigated.

2 Experimental

2.1 Synthesis of WO₃ NR and NP arrays on FTO substrate

WO₃ NR arrays were prepared using a hydrothermal method as described in the previous reports [25]. Firstly, 0.15 g of ammonium paratungstate was dissolved in 15 mL of deionized water. Then, 0.6 mL of 12 mol/L HCl and 0.3 mL of H₂O₂ were added in sequence into the above- mentioned aqueous solution by stirring. Then, an FTO substrate was immersed in 20 mL of Teflon- lined autoclave and placed against the wall with the conductive side downward. Subsequently, the aforementioned precursor solution was poured into the autoclave and the hydrothermal process was conducted at 170 °C for 4 h. Finally, the obtained films were annealed at 300 °C for 1 h before further use.

WO₃ NP arrays were synthesized using a hydrothermal method as described in the previous reports [26]. Firstly, 0.25 g of Na₂WO₄·2H₂O was added into 30 mL of deionized water by stirring. Then, 6 mL of 3 mol/L HCl, 30 mL of deionized water and 0.20 g of (NH₄)₂C₂O₄ were added into the aforementioned solution and stirred for 30 min. Then, an FTO substrate was immersed in 100 mL of Teflon-lined autoclave and placed against the wall with the conductive side downward. Afterwards, the above-mentioned precursor solution was poured into the autoclave and the hydrothermal process was conducted at 140 °C for 6 h. Finally, the obtained films were annealed at 300 °C for 1 h before further use.

2.2 Synthesis of WO₃/Bi₂WO₆ NR and NP arrays on FTO substrate

The WO₃/Bi₂WO₆ films were prepared through hydrothermal treatment. Firstly, the hydrothermal reaction solution was obtained by adding 0.6 g of Bi(NO₃)₃·5H₂O as the Bi source into 30 mL of 0.2 mol/L HNO₃ solution. Then, the WO₃ NR or WO₃ NP arrays films were immersed in 50 mL of Teflon-lined autoclave and placed against the wall with the sample side facing down. Then, the aforementioned precursor solution was poured into the autoclave and the hydrothermal process was conducted at 200 °C for 10 h. Finally, the obtained films were annealed at 540 °C for 4 h before further use.

2.3 Synthesis of WO₃/BiVO₄ NR and NP arrays on FTO substrate

The WO₃/BiVO₄ films were prepared through hydrothermal treatment. Firstly, the precursor solution was prepared by adding 4 mg of NH₄VO₃ and 0.2 mL of 3 mol/L HCl into 30 mL of deionized water. Then, the WO₃/Bi₂WO₆ films were immersed in 50 mL of Teflon-lined autoclave and placed against the wall with the sample side facing down. Then, the above-mentioned solution was transferred into the autoclave and placed in an oven at 180 °C for 4 h. Subsequent to the reaction, the formed films were calcined at 500 °C for 1 h before further use.

2.4 Characterization

The crystal, morphological and optical characteristics were examined using X-ray diffractometer (XRD, D8 Advance, AXS), transmission electron microscope (TEM, Titan G2 60-300, FEI), field emission scanning electron microscope (FE-SEM, MIRA3, TESCAN) and UV-vis spectrophotometer (UV-vis, 2450, Shimadzu).

2.5 PEC measurement

The PEC measurements were performed in a three-electrode PEC cell on an electrochemical workstation (Zahner, Zennium, Germany). 0.5 mol/L of KH₂PO₄ (pH≈4.1) was treated as the electrolyte. A 500 W Xe lamp (CHF-XM, Perfectlight) coupled with a 400 nm cutoff filter was applied as the light source and the light intensity at the photoanode position was adjusted to 100 mW/cm², as measured using a light power meter (PL-MW2000, Perfectlight). The electro- chemical impedance spectra (EIS) were recorded at 1.23 V (vs RHE) with a frequency of 10⁴-0.1 Hz.

3 Results and discussion

3.1 Microstructure

WO₃/BiVO₄ arrays heterojunction films were fabricated using a three-step hydrothermal strategy and the fabrication process is illustrated in Fig. 1. The WO₃ NR and NP arrays films were initially prepared according to the previous reports [25,26]. Subsequently, the Bi₂WO₆ NS developed on the surface of WO₃ NR and NP arrays films under hydrothermal conditions as a result of the chemical reaction is shown as

2Bi³⁺+WO₃+3H₂O→Bi₂WO₆+6H⁺                       (1)

Finally, Bi₂WO₆ NS was transformed into BiVO₄ NS on the surface of WO₃ to obtain   BiVO₄ NS@WO₃ arrays films. The potential reaction is expressed as

→         (2)

The microstructure and morphology of the samples were characterized using SEM and TEM. According to the SEM images of the WO₃ NR (Figs. 2(a, b)) and NP (Figs. 2(g, h)) arrays films, both WO₃ NR and NP had vertically grown on the FTO substrate after the hydrothermal treatment and their surface was smooth. As shown in the cross-sectional view images, the length of WO₃ NR is approximately 2.5 μm and WO₃ NP ranges from 0.5 to 1 μm in edge length and from 80 to 200 nm in thickness. After the second-step hydrothermal treatment, the surface of WO₃ NR and NP was rough and plenty of Bi₂WO₆ NS has epitaxially grown on the WO₃ surface, thus leading to the generation of WO₃/Bi₂WO₆ NR (Figs. 2(c, d)) and NP (Figs. 2(i, j)) films, which is attributed to the in-situ formation of Bi₂WO₆ NS as a result of the reaction occurring between WO₃ and Bi³⁺ under hydrothermal conditions. As shown in Figs. 2(e, f) and Figs. 2(k, l), there are some small NSs adhering on the surface of WO₃ NR and NP, because the Bi₂WO₆ NS on the WO₃ surface could be transformed into BiVO₄ NS by the third-step hydrothermal treatment, indicating the formation of WO₃/BiVO₄ NR and NP heterojunction films.

Fig. 1 Schematic illustration of preparation process for WO₃/BiVO₄ NR and NP arrays films

Fig. 2 SEM images of WO₃ NR (a, b), WO₃/Bi₂WO₆ NR (c, d), WO₃/BiVO₄ NR (e, f), WO₃ NP (g, h), WO₃/Bi₂WO₆ NP (i, j) and WO₃/BiVO₄ NP (k, l) arrays films

According to the TEM images of WO₃/BiVO₄ NR (Figs. 3(a-c)) and NP (Figs. 3(d-f)), WO₃ NR has a rod-like nanostructure and WO₃ NP possesses a plate-like nanostructure, which is consistent with the SEM images. As shown in LRTEM image, the surfaces of WO₃ NR and NP are covered with a large number of NSs. According to the HRTEM images, NS exhibits clear lattice fringes with spacings of 0.260 and 0.254 nm, which are consistent with the (200) and (020) crystal planes of monoclinic BiVO₄, respectively. By contrast, the NP shows a lattice spacing of 0.335 nm, corresponding to the (120) crystal plane of the monoclinic WO₃.

Furthermore, as suggested by the elemental mappings of WO₃/BiVO₄ NR (Fig. 4(a)) and NP (Fig. 4(b)), W, V and Bi elements are  distributed uniformly, indicating the formation of BiVO₄ NS@WO₃ arrays heterojunction films.

Fig. 3 TEM images of WO₃/BiVO₄ NR (a-c) and NP (d-f)

Fig. 4 Elemental mapping images of WO₃/BiVO₄ NR (a) and NP (b)

Fig. 5 XRD patterns (a, b) and UV-vis absorption spectra (c, d) of prepared film

The phase and crystallinity of WO₃, WO₃/Bi₂WO₆ and WO₃/BiVO₄ films were characterized by XRD, as shown in Figs. 5(a, b). The obtained WO₃ arrays films exhibit the characteristic diffraction peaks of monoclinic WO₃ (JCPDS 83-0950). As for the WO₃/Bi₂WO₆ films, the observed peaks of characteristic diffraction at 28.31° and 32.93° are assigned to the (113) and (020) crystal planes of orthorhombic Bi₂WO₆ (JCPDS 73-2020), respectively. Corresponding to the (011), (112) and (004) crystal planes of monoclinic BiVO₄, new peaks observed at 18.98°, 28.95° and 30.53° are distinguishable from the XRD patterns of WO₃/BiVO₄ films (JCPDS 75-2480), suggesting the formation of BiVO₄ on the surface of WO₃ films. Figures 5 shows the spectra of UV-vis absorption. The optical absorption of WO₃ films shows similarity to WO₃/Bi₂WO₆ films, with the edges of their absorption being around 470 nm, which corresponds to their bandgap energy (E_g≈2.7 eV). In addition, WO₃/BiVO₄ films show a visible redshift and their absorption edges are about 520 nm, which is consistent with the bandgap energy of BiVO₄ (E_g≈2.4 eV). As a result, the absorption of visible light by the films can be improved significantly by the formation of BiVO₄ on the surface of WO₃. As shown in Figs. 5(c, d), the images of the prepared films also are consistent with the UV-vis absorption results. WO₃ NR and NP films are yellow-green while WO₃/BiVO₄ films are bright yellow.

3.2 PEC performance of WO₃/BiVO₄ photo- anodes

In order to assess the PEC performance, the chopped linear sweep voltammetry (LSV) plots were obtained in 0.5 mol/L KH₂PO₄ electrolyte under visible light. As shown in Figs. 6(a, b), the photocurrent density of the WO₃/BiVO₄ NR and NP photoanodes is significantly improved after the formation of BiVO₄ on the surface of WO₃ arrays. The photocurrent densities of WO₃/BiVO₄ NR and NP photoanodes reach ca. 1.56 and 1.21 mA/cm² at 1.23 V (vs RHE), respectively, which are much higher compared with pure WO₃ NR and NP photoanodes. More importantly, the photocurrent density and onset potential of the prepared WO₃/BiVO₄ photoanodes are comparable to the reported WO₃/BiVO₄ photoanodes without cocatalysts, as indicated in Table 1.

In this study, the incident photon-to-electron conversion efficiency (IPCE) was examined to figure out the relationship between photocurrent density and light absorption. The IPCE plots were recorded at a constant potential of 1.23 V(vs RHE), as shown in Figs. 6(c, d). It can be seen that WO₃/BiVO₄ NR and NP photoanodes achieve the highest IPCE value in the overall wavelength  range due to the type-II heterojunction structure of the WO₃/BiVO₄ photoanodes, which is conducive to the separation and transfer of photogenerated charge carriers. Furthermore, WO₃/BiVO₄ photo- anodes show an extended photo-responsive range (~520 nm) due to the narrower bandgap energy of BiVO₄.

Fig. 6 LSV plots (a, b) and IPCE spectra (c, d) of as-prepared photoanode

Table 1 PEC performance of WO₃/BiVO₄ photoanode without cocatalyst in previous literatures

To evaluate the ability of charge transfer, the EIS of the photoanodes was measured within the frequency range of 10⁴-10^-1 Hz at 1.23 V (vs RHE) under visible light, as shown in Fig. 7. All photoanodes show a single semicircle, suggesting that the charge transfer across the photoelectrode/ electrolyte interface is the only rate-limiting step. Compared with the WO₃ and WO₃/Bi₂WO₆ photoanodes, WO₃/BiVO₄ photoanodes exhibit a smaller arc radius, indicating the better performance achieved by WO₃/BiVO₄ photoanode in interfacial charge transfer.

In addition, the EIS data are fitted by the equivalent circuit consisting of a solution resistance (R_s)_, a charge transfer resistance (R₁) and a constant phase element (CPE₁), as shown in the inset in Fig. 7. As indicated by the fitted results listed in Table 2, WO₃/BiVO₄ photoanodes have the smallest R₁ value, indicating the lowest resistance to charge transfer at the interface between WO₃/BiVO₄ photoanode and electrolyte solution. The EIS results conform to the LSV plots.

Fig. 7 EIS spectra of as-prepared NR (a) and NP (b) arrays photoanodes

Table 2 Fitted values of equivalent circuit in PEC water splitting

Besides, the efficiency of WO₃/BiVO₄ photoanodes in charge transfer was estimated according to η_trans=J_H2O/J_Na2SO3, as shown in Fig. 8. The η_trans values at 1.23 V (vs RHE) of WO₃/BiVO₄ NR and NP photoanodes are 65.6% and 74.9%, respectively. It is speculated that the low values of η_trans result from the absence of cocatalyst on the surface of photoanode.

Fig. 8 Photocurrent density curves (a, c) in 0.5 mol/L Na₂SO₃ and KH₂PO₄ solution and surface charge transfer efficiency (b, d) of BiVO₄/WO₃ NR (a, b) and NP (c, d) arrays photoanodes

The PEC stability of WO₃/BiVO₄ photoanodes was evaluated at 1.23 V (vs RHE) under long-term irradiation, as shown in Fig. 9. The photocurrent density of WO₃/BiVO₄ photoanodes is stable and maintained about 90% after 2 h, indicating their excellent PEC stability in the water-splitting process.

Fig. 9 Photocurrent density-time curve of WO₃/BiVO₄ NR and NP arrays photoanodes

In order to confirm the flat band potentials  (V_fb) of the prepared photoanodes, Mott-Schottky (MS) plots were measured in darkness in a 0.5 mol/L KH₂PO₄ solution with applied frequencies of 1, 2.5 and 5 kHz, respectively. The V_fb and N_d could be estimated using the MS equation [33]:

                (3)

where C represents the specific capacitance, ε₀ indicates the permittivity of vacuum, ε denotes the dielectric constant of the electrode material used, q is the electron charge, V_E stands for the applied potential, N_d means the donor density, and k is the Boltzmann constant. As shown in Fig. 10, all M-S plots exhibit a positive slope, implying that the photoanode materials are n-type semiconductors. V_fb can be obtained from the intercept of the fitted tangent line. Compared with the WO₃ photoanode, WO₃/BiVO₄ photoanodes show a more negative V_fb, suggesting the possibility that the photogenerated electrons are transferred from BiVO₄ to WO₃ due to the more negative CB edge of BiVO₄.

Fig. 10 MS plots of WO₃ NR (a), WO₃/BiVO₄ NR (b), WO₃ NP (c) and WO₃/BiVO₄ NP (d) arrays photoanodes

3.3 Mechanism of charge separation and transfer of WO₃/BiVO₄ photoanode

Figure 11 illustrates the mechanism of charge separation and transfer. With the phase transformation strategy applied, the dense BiVO₄ NS was coated on the WO₃ surface, thus forming a WO₃/BiVO₄ type-II heterojunction. Under irradiation, the electrons in BiVO₄ and WO₃ were excited from their valence band (VB) to the CB. Due to the more negative CB position of BiVO₄, the excited electrons in the CB of BiVO₄ were easily transferred to the CB of WO₃. These electrons were further moved onto the FTO substrate along the WO₃ NR and NP pathway and then to the Pt electrode through the external circuit. Then, the H⁺ in the electrolyte reacted with the electrons to generate H₂ on the surface of Pt electrode. In the meantime, the photoexcited holes shifted from WO₃ to the VB of BiVO₄ and oxidized H₂O to produce oxygen on the BiVO₄ surface. Thus, type-II heterojunction can significantly improve the outcome of charge separation and transfer.

Fig. 11 Schematic illustration for charge separation and transfer of WO₃/BiVO₄ photoanode

4 Conclusions

(1) A versatile strategy of phase transformation was proposed to prepare BiVO₄ NS@WO₃ NR and NP arrays films. The strategy was carried out via a three-step hydrothermal process (WO₃→WO₃/ Bi₂WO₆→WO₃/BiVO₄).

(2) As indicated by the SEM, TEM and XRD results, plenty of BiVO₄ NS successfully formed on the surface of the WO₃ NR and NP arrays films with Bi₂WO₆ NS as the intermediate product.

(3) The prepared WO₃/BiVO₄ photoanodes were applied for PEC water splitting and exhibited a significantly higher PEC activity compared with WO₃ photoanodes. It is believed that the proposed strategy is promising in the construction of various heterojunction systems for the future.

Acknowledgments

The authors are grateful for the financial supports from the National Natural Science Foundation of China (21808051, 51904356, 21703062).

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一种通用的相转换策略构建BiVO₄纳米薄片@WO₃阵列异质结光阳极

苏 昕¹，刘灿军¹，刘 洋²，杨亚辉³，刘 玄¹，陈 述¹

1. 湖南科技大学 化学化工学院 理论有机化学与功能分子教育部重点实验室，湘潭 411201；

2. 中南大学 化学化工学院，长沙 410083；

3. 湖南师范大学 化学化工学院，长沙 410081

摘  要：提出一种制备BiVO₄纳米薄片@WO₃纳米棒和纳米片薄膜的相转换策略。这种策略包括三步水热过程(WO₃→WO₃/Bi₂WO₆→WO₃/BiVO₄)。表征结果表明：大量的BiVO₄纳米薄片原位生长在WO₃纳米棒和纳米片阵列薄膜表面，从而形成WO₃/BiVO₄异质结。制备的WO₃/BiVO₄异质结薄膜作为光阳极被用于光电化学分解水，并展现出优异的光电化学活性。在可见光照射和没有沉积助催化剂的情况下，WO₃/BiVO₄ 纳米棒和纳米片光阳极的光电流密度分别达到约1.56和1.20 mA/cm²(V=1.23 V (vs RHE))。

关键词：光阳极；钒酸铋；氧化钨；异质结

 (Edited by Xiang-qun LI)

Corresponding author: Can-jun LIU, Tel: +86-15073161782, E-mail: liucanjun@hnust.edu.cn; Shu CHEN, E-mail: chenshu@hnust.edu.cn

DOI: 10.1016/S1003-6326(21)65515-2

1003-6326/ 2021 The Nonferrous Metals Society of China. Published by Elsevier Ltd & Science Press