Bulk Charge Transfer Characteristics And Its Effect On Photoelectrochemical Performance Of Porous BiVO₄ Photoanode

For centuries,human society has relied on the burning of fossil fuels for energy.However,the rapid consumption of energy and various environmental problems force people to seek green renewable energy.As ideal renewable energy,solar energy utilization is considered to be the most effective way to solve global energy and environmental problems.Photoelectrochemical（PEC）water splitting with solar energy has become one of the most effective and sustainable ways to produce hydrogen.Because the oxygen evolution reaction is a four-electron process,it is thought to be the speed step in the water-splitting reaction.Therefore,reasonable design and preparation of photoanode will become the most critical problem to improve the utilization rate of solar energy.Among various semiconductor photoanodes,as an ideal n-type oxygen evolution reaction photoanode material,BiVO₄ has been widely studied for its medium band gap（～2.4e V）,suitable band position,high solar-hydrogen conversion efficiency（STH:～9.3%）,stable structure,low cost and non-toxic.However,BiVO₄ has some disadvantages such as poor charge separation and transport performance,relatively serious photogenerated charge recombination and slow water oxidation kinetics.Researchers often improve photoelectrochemical performance by employing structural controlling,element doping,heterojunction construction and cocatalyst loading to solve these problems.Although some progress has been made,people usually focus on how to further improve the water oxidation performance of BiVO₄in the whole process of photoelectrochemical water decomposition,while ignoring the internal factors that limit the photoelectrochemical water oxidation performance.Therefore,in this paper,we selected porous BiVO₄ as the base material,and the effects of light absorption coefficient,carrier diffusion length and bulk charge transmission characteristics of porous BiVO₄ were explored by UV-vis absorption spectrum,reflection spectrum,surface photovoltage and photoelectrochemical tests.The relationship between semiconductor structure,photogenerated charge transfer characteristics,and photoelectrochemical water oxidation performance is deeply understood,which provides theoretical guidance for the construction of high-efficient photoanode.1.Optimization calculation method of absorption coefficient and carrier diffusion length of BiVO₄ film:To accurately calculate the absorption coefficient and carrier diffusion length of BiVO₄ film,based on the test results of UV-vis absorption spectrum and reflection spectrum of FTO and FTO/BiVO₄,the measured absorption value is modified and standardized by Min-Max through reflection spectrum.The absorption coefficient of BiVO₄ film at different wavelengths was calculated by using the relationship between absorption coefficient and absorption value.Combining with the test results of surface photovoltage,the carrier diffusion length of BiVO₄ film is obtained,which is 370 nm.2.Bulk charge transfer characteristics and its effect on photoelectrochemical performance of porous BiVO₄ photoanode:Based on the previous work,the thickness of BiVO₄ film was regulated by changing the temperature,and then FeOOH/NiOOH oxygen evolution cocatalyst was loaded on its surface to obtain BiVO₄ and BiVO₄/FeOOH/NiOOH photoanodes with different thicknesses.The fundamental reasons limiting the further improvement of photoelectrochemical water oxidation performance of porous BiVO₄ photoanode were studied.The results show that for BiVO₄ photoanode with porous structure,the transport of photogenerated electrons is an important factor affecting the performance of photoelectrochemical water splitting.