Preparation Of Bismuth Vanadate-based Thin Film Photoanodes And Their Photoelectrochemical Water Splitting Performance

One of the most important ways is to utilize solar energy to convert the non-transportable solar energy with low energy density into clean hydrogen energy,which could be conveniently transported,stored,and directly combusted,by splitting water into hydrogen and oxygen using photochemical technology.Usually,photochemical solar energy conversion could be achieved through photocatalysis or photoelectrocatalysis.The photoelectrocatalytic（PEC）technology has attracted the attention of many researchers,because it not only solves the difficulty to separate and recover the photocatalyst in the photocatalytic reaction,but the existence of an external bias allows the effective separation of photogenerated carriers.Among many photoelectrode catalytic materials,the complex oxide bismuth vanadate（Bi VO₄）,as a new type of semiconductor material,has gradually drawn much attention in the research field of PEC for its many excellent properties,such as corrosion-resistance,low price,and abundant reserves.However,its photoelectric conversion efficiency is far from the theoretical value due to its high recombination rate and low electron mobility.In response to the above problems,this thesis is focused on the modification of Bi VO₄ by doping,building heterojunction with other semiconductors,and co-catalyst modification,in order to improve its PEC photoelectriccatalytic efficiency.The main research contents and results are as follows:（1）Preparation of rhodium oxide modified bismuth vanadate thin film photoanodes and their photoelectric catalytic performance for water splittingThe Rh O₂ cocatalyst was loaded on the Bi VO₄ thin film photoanode by the impregnation-calcination method,therefore the Rh O₂ modified Bi VO₄ photoanode was prepared.The effect of different loading ratios on the PEC performance of the Bi VO₄photoanode and its mechanism were studied.Bi VO₄/0.1-Rh O₂ is the photoanode with the best loading ratio,with visible light photocurrent density of 3.81 m A·cm^-2.The loading of Rh O₂ effectively improved the surface water oxidation kinetics of the Bi VO₄photoanode,thereby significantly improving the PEC performance.In addition,because the photogenerated holes can undergo water oxidation reaction faster after Rh O₂ is loaded,thereby reducing the accumulation on the surface of the photoanode,Bi VO₄/0.1-Rh O₂ can achieve stability for more than 10 hours.（2）Preparation of tungsten oxide/bismuth vanadate heterojunction thin film photoanode and their photoelectric catalytic performance for water splittingThe porous and transparent WO₃ nanoparticle film was prepared on the conducting glass substrate by the surfactant-assisted synthesis method,and then the Bi VO₄ was fabricated on its surface by the metal organic decomposition approach,thereby obtaining the uniform and highly transparent WO₃/Bi VO₄ composite photoanode.The effect of different coupling ratios on the PEC performance of composite photoanode and its mechanism was studied.WO₃/3-Bi VO₄ composite photoanode exhibits the highest PEC performance under visible light,with the photocurrent of 3.25 m A·cm^-2.The composite photoanode of two different semiconductors formed a heterojunction,thereby able to separate photogenerated carriers with higher efficiency,which is an important reason for the increase in photocurrent density.In addition,by analyzing the optical performance and electrochemical impedance properties of different composite ratio photoanodes,it is found that WO₃/Bi VO₄ composite photoanodes made effectively use of the advantages of Bi VO₄ with small band gap,strong light absorption,and WO₃ with low electrochemical impedance and fast charge transfer,thereby achieving higher PEC performance.（3）Preparation of molybdenum-doped bismuth vanadate thin film photoanodes and their photoelectrocatalytic performance for water splittingThe monoclinic Mo-Bi VO₄ thin film photoanode was prepared by a simple metal-organic thermal decomposition method.The diameter of the nanoparticles was about100-250 nm and the thickness was about 400 nm.3 at%Mo-Bi VO₄ photoanode showed the best PEC performance.While giving the external bias voltage at 1.23 V vs.RHE,the visible light photocurrent was 5.28 m A·cm^-2,improved by 8.46 times than the undoped ones,indicating that Mo doping effectively improved the photoelectrocatalytic performance of Bi VO₄ photoanode.By analyzing the photocurrent density of the photoanode with light source illuminating from the back side and front side,it was found that Mo doping effectively improves the conductivity of Bi VO₄,thereby inhibiting the recombination of photogenerated carriers,which was one of the important reasons for the improvement of PEC performance.In addition,electrochemical impedance spectroscopy analysis proved that the donor concentration in the 3 at%Mo-BVO photoanode was increased by 7.3 times compared with bare Bi VO₄,and the flat band potential V_FB was shifted by 50 m V,which significantly improved the photoelectric catalytic efficiency.