Elsevier

Catalysis Today

Volume 361, 1 February 2021, Pages 183-190
Catalysis Today

The effects of W/Mo-co-doped BiVO4 photoanodes for improving photoelectrochemical water splitting performance

https://doi.org/10.1016/j.cattod.2020.03.066Get rights and content

Highlights

  • W/Mo-co-doped -BiVO4 was fabricated via dip-coating and heat treatment cycles.

  • PEC performance significantly increased for 0.5W-2Mo-BVO compared to pure BVO.

  • W/Mo co-doping increased ND and decreased WSCL compared with pure BVO.

  • 0.5W-2Mo-BVO generated 19.5 μmol.cm-2 of H2 and 9.4 μmol.cm−2 of O2 after 2 h.

Abstract

The poor electrical conductivity of bismuth vanadate (BVO) can be improved by metal-doping at optimum percentage. Previously, we improved the photoelectrochemical (PEC) properties of BVO photoanodes by optimizing the different atomic percentage of molybdenum (Mo). Here, we co-doped BVO simultaneously with tungsten/molybdenum (W/Mo) to enhance the water splitting properties of BVO photoanodes. At optimum co-doping percentages of 0.5 %W-2%Mo BiVO4 (0.5W-2Mo-BVO) the photocurrent density maximum was ∼ 0.97 mA cm−2 at 0.6 V vs. Ag/AgCl. The water splitting was performed under 100 mW. cm-2 irradiation and 0.6 V vs. Ag/AgCl applied voltage for 0.5W-2Mo-BVO. The H2 production was 19.5 μmol.cm-2 after 2 h at optimized W/Mo co-doping. W/Mo co-doping into BiVO4 greatly improved the electron mobility, as demonstrated by photocurrent, Mott-Schottky, electrochemical impedance, open circuit potential (ΔOCP) and incident photon conversion efficiencies (IPCE) measurements. As compared to pure BVO, 0.5W-2Mo-BVO showed ∼2700-fold increase in the donor concentration (ND) and greatly decreased (1/60) space charge layer thickness (WSCL), which induced efficient charge carrier mobility and enhanced the PEC performance.

Introduction

The production of hydrogen through water splitting using solar-powered technologies has drawn strong research attention in the last decade. Photoelectrochemical (PEC) devices can combine the processes of both light absorption and water splitting into a single component [[1], [2], [3]]. By absorbing sunlight, photocatalytic semiconductor materials can directly convert sunlight into chemical energy by water splitting [4,5]. To date, various photocatalysts have been developed to harvest the sunlight and generate electron-hole pairs for artificial photosynthesis. However, it is difficult to find appropriate photocatalysts with suitable band gap and band position for photocatalytic water splitting. In other words, the photon energy absorption by photocatalysts is determined by band gap and the possibility of artificial water is regulated by band positions. Among n-type semiconductors with visible light activation and valence band maximum more positive than O2/H2O potential level, WO3 and BiVO4 have been extensively studied as oxygen evolution photocatalysts [[6], [7], [8]].

Mature BiVO4 has been adopted as a promising photoanode because of its excellent hole diffusion, and facile kinetics control for oxidation evolution reaction. However, BiVO4 photocatalysts have some drawbacks such as slow electron-transport and poor electroconductivity at the photoanode which can be improved by optimizing the doping donor concentration using appropriate metal-doping procedures [[9], [10], [11], [12]]. Generally, transition metal elements are exchanged with the V5+ sites without changing the monoclinic structure of BiVO4. Appropriate transition metals such as tungsten (W) [[13], [14], [15]] and molybdenum (Mo) [16] have been adopted as promising metal elements for improving the donor concentration of n-type photocatalysts and subsequently their electroconductivity [[17], [18], [19]].

Here, we optimized the simultaneous doping of tungsten and molybdenum into monoclinic BiVO4 photoanode and restored its crystalline structure by an intermediated heat treatment procedure. The PEC properties of co-doped W/Mo-BVO were investigated systematically and the photophysical properties of the photoanodes, including donor concentration (ND), flat band potential (Vfb), τD, incident photon conversion efficiency (IPCE) and space charge layer thickness (WSCL), are discussed based on the metal co-doping concentration. Furthermore, to analyze the durability of the photoanodes after 17 h continuous water splitting illumination, their physiochemical and morphological properties were investigated. Finally, directly testing of hydrogen production under light illumination (100 mW.cm−2) in a 0.5 M aqueous solution of Na2SO4 at pH = 7 as the electrolyte demonstrated that W/Mo-doping improved the PEC performance of BiVO4.

Section snippets

Fabrication of W/Mo co-doped BiVO4 (BVO) films

Photoanodes of W/Mo-co-doped BiVO4 with different tungsten and molybdenum atomic percentages were prepared via the procedure reported in our previously work [20](see supporting information). The films with 0, 0.5 %W/2%Mo, 0.5 %W/3%Mo, 1%W/2%Mo, and 1%W/3%Mo atomic percentages are labeled as BVO, 0.5W-2Mo-BVO, 0.5W-3Mo-BVO, 1W-2Mo-BVO and 1W-3Mo-BVO, respectively.

Photoelectrochemical (PEC) and water splitting measurement

PEC measurements were carried out by a homemade designed cell in a three-electrode configuration. Pt wire was utilized as the counter

Physiochemical and morphological properties

Fig. 1 shows the structure of the photoanodes prepared with different W and Mo doping percentages. The X-ray diffraction (XRD) results indicated BiVO4 with a monoclinic structure was formed (Fig. 1a) with different metal doping concentrations. No secondary phase was observed in the XRD patterns. However, the XRD peaks at 35 and 47∘ were shifted and merged, indicating that W and Mo were well dissolved in the BiVO4 solution [17].

Fig. 1b shows the symmetry stretching Raman modes of Vsingle bondO bonds [21]

Conclusion

In this study, W/Mo-co-doped BiVO4 was successfully fabricated via an easy and low cost preparation method using dip coating. The identified key parameters of the PEC performance (0.5W-2Mo-BVO) of W/Mo-co-doped BVO photoanodes were greatly improved as compared to those of pure BVO. The resulting 0.5W-2Mo-BVO photoanode displayed an excellent improvement in the PEC performance, which was 40-fold higher than that of pure BVO (0.97 mA/cm2 at 0.6 V vs. Ag/AgCl), indicating its remarkable light

Funding

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MIST: Ministry of Science and ICT) (No. 2019R1A2C2085250).

Intellectual property

We confirm that we have given due consideration to the protection of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication, with respect to intellectual property. In so doing we confirm that we have followed the regulations of our institutions concerning intellectual property.

Research ethics

We further confirm that any aspect of the work covered in this manuscript that has involved human patients has been conducted with the ethical approval of all relevant bodies and that such approvals are acknowledged within the manuscript.

IRB approval was obtained (required for studies and series of 3 or more cases)

Written consent to publish potentially identifying information, such as details or the case and photographs, was obtained from the patient(s) or their legal guardian(s).

Authorship

All listed authors meet the ICMJE criteria. 
We attest that all authors contributed significantly to the creation of this manuscript, each having fulfilled criteria as established by the ICMJE

We confirm that the manuscript has been read and approved by all named authors.

We confirm that the order of authors listed in the manuscript has been approved by all named authors.

Declaration of Competing Interest

No conflict of interest exists.

Acknowledgement

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MIST: Ministry of Science and ICT) (No. 2019R1A2C2085250).

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