• Bennett, Robert

Abstract

Solar hydrogen generation by photoelectrochemical (PEC) water splitting allows for direct photon-to-molecule conversion in a standalone system. Although it is one of the most promising approaches to generate renewable hydrogen, it is also the most challenging because of the multitude of processes occurring concurrently and at very short time scales.Bismuth vanadate (BiVO4) is an ideal photoanode material because of its favourable band gap of 2.4 eV [1], which allows it to absorb photons in the visible region of the solar spectrum and has a theoretical solar to hydrogen (STH) efficiency of ~ 9.2%. It has suitable band edges for the water redox process, is durable in aqueous solution, good crystallinity and made up of earth abundant materials. However, BiVO4 photoanodes suffer from poor charge separation in the bulk of the material and at the electrolyte interface during the PEC water splitting process [2]. Doping of BiVO4 is a common strategy used to improve charge separation [3]. Doping by electron donors can improve the electrical conductivity by increasing the electron density of BiVO4 [4]. Doping of BiVO4 with hexavalent Molybdenum (Mo6+) has been shown to enhance photocurrent generation due to more efficient charge separation [5].In this work, we compare the charge separation efficiency of a homogenous doping concentration profile to a gradient doping concentration profile of Mo-doped BiVO4. Gradient doping induces an upward band bending which amplifies charge separation within the bulk of the BiVO4 film. Mo-doped BiVO4 thin films are deposited on FTO-coated glass substrates by ultrasonic spray pyrolysis (USP). Morphological characterisations reveal controlled successive layered films, wherein the number of layers correspond to the number of cycles using our USP deposition process. Gradient doping is achieved by depositing controlled successive Mo-doped BiVO4 layers that contain increasing dopant concentrations using this method. Their improvement in optical absorption and PEC performance is elucidated by material, optical, Mott-Schottky characterisations and electrochemical impedance spectroscopy.References: [1] Liang, Y., Tsubota, T., Mooij, L.P. and van de Krol, R., 2011. Highly improved quantum efficiencies for thin film BiVO4 photoanodes. The Journal of Physical Chemistry C, 115(35), pp.17594-17598.[2] Trześniewski, B.J. and Smith, W.A., 2016. Photocharged BiVO 4 photoanodes for improved solar water splitting. Journal of Materials Chemistry A, 4(8), pp.2919-2926.[3] Zhang, B., Zhang, H., Wang, Z., Zhang, X., Qin, X., Dai, Y., Liu, Y., Wang, P., Li, Y. and Huang, B., 2017. Doping strategy to promote the charge separation in BiVO4 photoanodes. Applied Catalysis B: Environmental, 211, pp.258-265.[4] Zhao, X., Hu, J., Yao, X., Chen, S. and Chen, Z., 2018. Clarifying the roles of oxygen vacancy in W-doped BiVO4 for solar water splitting. ACS Applied Energy Materials, 1(7), pp.3410-3419.[5] Jeong, H.W., Jeon, T.H., Jang, J.S., Choi, W. and Park, H., 2013. Strategic modification of BiVO4 for improving photoelectrochemical water oxidation performance. The Journal of Physical Chemistry C, 117(18), pp.9104-9112.

Topics

• Deposition
• density
• impedance spectroscopy
• molybdenum
• thin film
• Oxygen
• glass
• glass
• layered
• Hydrogen
• ultrasonic
• electrical conductivity
• crystallinity
• vacancy
• Bismuth
• spray pyrolysis