Yan Zhao¹, Qisen Jia¹, Zhenming Tian¹, Yanan Wang¹, Jiashu Li¹, Shixu Song¹, Teng Fu¹, Xuejing Cui¹, Guangbo Liu^1,*, Xin Zhou^2,3,*, Luhua Jiang^1,*

¹ Nanomaterials & Electrocatalysis Laboratory, College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, PR China

² Interdisciplinary Research Center for Biology and Chemistry, Liaoning Normal University, Dalian 116029, PR China

³ College of Environment and Chemical Engineering, Dalian University, Dalian 116622, PR China

* Corresponding authors:liugb@qust.edu.cn; zhouxin@dlu.edu.cn; luhuajiang@qust.edu.cn

Abstract:

Photoelectrochemical water oxidation (PEC-WO) as a green and sustainable route to produce H₂O₂ has attracted extensive attentions. However, water oxidation to H₂O₂ via a 2e^- pathway is thermodynamically more difficult than to O₂ via a 4e^- pathway. Herein, with a series of BiVO₄-based photoanodes, the decisive factors determining the PEC activity and selectivity are elucidated, combining a comprehensive experimental and theoretical investigations. It is discovered that the ZnO/BiVO₄ photoanode (ZnO/BVO) forms a Type-II heterojunction in energy level alignment, which due to the accelerated photogenerated charge separation/transfer dynamics generates denser surface holes and higher surface photovoltage, therefore the activity of water oxidation reaction is promoted. The selectivity of PEC-WO to H₂O₂ is found to be potential-dependent, i.e., at the lower potentials (PEC-dominated) surface hole density determines the selectivity and at the higher potentials (electrochemical-dominated) surface reaction barriers governs the selectivity. For the ZnO/BVO heterojunction photoanode, the higher surface hole density facilitates the generation of OH• and the subsequent OH•/OH• coupling to form H₂O₂, thus rising up with potentials; at the higher potentials, the 2-electron pathway barrier over ZnO/BVO surface is lower than over BVO surface, which benefiting from the electronic structure regulation by the underlying ZnO alleviates the over-strong adsorption of *OH on BVO, thus the two-electron pathway to produce H₂O₂ is more favored than on BVO surface. This work highlights the crucial role of band energy structure of semiconductors on both PEC reaction activity and selectivity, and the knowledge gained is expected to be extended to other photoeletrochemical reactions.

Keywords: Photoelectrochemical water oxidation; reaction selectivity; BiVO₄ photoanode; production of H₂O₂; energy band structure.