Facet effect on the photoelectrochemical performance of a WO3/BiVO4 heterojunction photoanode
Graphical abstract
Introduction
Photoelectrochemical (PEC) water splitting using semiconductor photoelectrodes is one of the most desirable and environmentally friendly methods to produce hydrogen from water using renewable solar energy, and it offers a promising approach for producing sustainable and renewable hydrogen fuel. Since the discovery of water splitting over TiO2, semiconductor materials such as Fe2O3, WO3, CdS, Bi2WO6, BiOIO3 and BiVO4, have been widely chosen for fabrication into photoelectrodes for PEC water splitting [[1], [2], [3], [4], [5], [6]]. Of these, WO3 is an attractive choice because it is an environmentally-friendly and low-cost material, has a moderate hole diffusion length of 150 nm, and an electron mobility of 12 cm2 V−1 s−1 [7]. However, the photoresponse region of WO3 is narrow because of its wide band gap (2.6–2.8 eV). It limits the maximum photocurrent of WO3 based photoanodes. To extend the photoresponse region and improve photoelectrochemical performance, enormous effort is being devoted to doping WO3 with foreign elements or pairing with another semiconductor to construct a heterojunction.
Compared with other heterojunction photoanodes based on WO3, WO3/BiVO4 is a good candidate for PEC water splitting [[8], [9], [10]]. Since Chatchai et al. [11] demonstrated a WO3/BiVO4 photoanode in 2009, this heterojunction has attracted tremendous attention. Hong et al. [12] fabricated WO3/BiVO4 electrodes using a layer-by-layer deposition method, and they demonstrated an optimal consisting of four layers of WO3 covered by a single layer of BiVO4. To improve the charge transport ability of BiVO4, Mo was used as a foreign element for doping, and the resulting Mo-doped BiVO4/WO3 (WO3/BiV0.95Mo0.05O4) showed 1.5 times the photocurrent compared with the undoped WO3/BiVO4 heterojunction photoanode [13]. W-doped BiVO4 was also used as an interlayer to enhance the PEC performance of WO3/BiVO4 heterojunctions [14]. Meanwhile, Sayama et al. introduced a very thin SnO2 interlayer between WO3 and BiVO4, which improved the intrinsic quantum efficiency of the photocurrent generated from excited electrons in BiVO4 [15]. Constructing suitable nanostructure or morphology is also a common route to obtaining high-performance WO3/BiVO4 heterojunction photoanode. Thus, one-dimensional WO3/BiVO4 electrodes [[16], [17], [18], [19], [20]], butterfly wing-like structures [21], inverse opal structure [22], and hierarchical nano-porous sphere arrays [23] have been fabricated in recent years. To investigate the effect of WO3 morphology, Hwang et al. coated spherical-, rod-like- and plate-like-WO3 via a doctor-blade method and loaded BiVO4 on the WO3 films. Though the spherical WO3 didn’t show the highest photocurrent of these three WO3 photoanodes, it showed the highest photocurrent after loading BiVO4 [24]. Aside from the difference in morphology, the spherical-WO3 samples also had different peak intensity proportions for the (002), (020), and (200) peaks in XRD when compared with rod-like- and plate-like-WO3. We also found different peak proportions when comparing the one-dimensional WO3/BiVO4 electrodes in other papers, all of which showed different PEC performance [16,18,19,25]. Some published papers have shown different water oxidation abilities for WO3 samples with different exposed facets [[26], [27], [28]]. Such a facet adjustment should also affect the interface with other components and the corresponding PEC performance [29]. However, few papers have discussed this facet effect of WO3 as related to the PEC performance of heterojunction electrodes. In one study, Dai et al. deposited silver nanoparticles onto WO3 nanorods with different facets by an in situ photoreduction method [30]. The Ag/WO3-110 catalysts with dominant exposed {001} facets exhibited better photocatalytic activity than Ag/WO3-001 with a high percentage of exposed {100} and {010} facets.
In this paper, we have synthesized four different WO3 films with various crystallographic orientations and exposed facets. We discuss possible reasons for the correlation between the PEC performance of a WO3/BiVO4 heterojunction photoanode and the structural characteristics of WO3 with different crystallographic orientations or facets. X-ray diffraction (XRD) patterns and pole figures are used to determine the crystallographic orientations of these WO3 films. Ultraviolet photoelectron spectroscopy (UPS) and valence band X-ray photoelectron spectroscopy (XPS) measurements were performed to investigate how the electronic structure changes in the heterojunction films.
Section snippets
Chemicals and materials
Tungstic Acid (H2WO4), sodium tungstate dihydrate (Na2WO4·2H2O), poly(vinyl alcohol) (M.W. ≈86,000), bismuth(III) nitrate pentahydrate (Bi(NO)3·5H2O, 98%) and vanadyl acetylacetonate were purchased from Sigma-Aldrich. Hydrogen peroxide (30%), oxalic acid (C2H2O4), urea, acetonitrile, and acetic acid were purchased from Fisher Chemicals. Hydrochloric acid (HCl) was obtained from VWR Inc. All the chemicals were used as received and deionized (DI) water (resistance ≈18.2 MΩ) was used throughout
Results and discussion
The XRD patterns of WO3 samples are shown in Fig. 1. Based on the characteristic peaks, monoclinic WO3 (ICDD PDF# 72-0677) patterns were obtained for all of the WO3 films. The peaks at 2θ values of 23.1°, 23.6°, and 24.2° can be ascribed as the (002), (020), and (200) planes, respectively. The peak intensity profiles of 200-WO3, plate-WO3, 002-WO3, and thinflake-WO3 show different peak proportions. First, the most dominant peak in the XRD patterns of 200-WO3 is the (200) peak. While the plate-WO
Conclusion
In conclusion, we successfully investigated the effect of WO3 facet preference on the performance of WO3/BiVO4 heterojunctions. The WO3 films with varied ratios of exposed facets were synthesized by different hydrothermal methods. The WO3 film with higher ratios of the 002 facet showed the lowest work function, which leads to a higher quasi-fermi level for the heterojunction film. The preferential facet orientation of WO3 also affects the heteroepitaxial growth and facets of BiVO4, which affect
Acknowledgements
The authors acknowledge the generous support of the United States Department of Energy, Basic Energy Sciences (Grant no. DE-FG02-09ER16119) and also the Welch Foundation through grant F-1436. Yang Liu thanks Xiaole Chen, Hugo Celio, Karalee Jarvis, and Raluca Gearba for the characterization assistance. We also acknowledge the China Scholarship Council (CSC) scholarship under the State Scholarship Fund.
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