Surface Treatments Of Oxide Photoanodes And Their Improvement Mechanisms Of Photoelectrochemical Water Splitting Performance

With the growth of the population,the rapid development of the economy and the accelerating speed of fuel consumption,the time for the use of fossil fuels is becoming limited.In addition,the use of fossil fuel causes serious environment pollution.Therefore,it is imminent to develop a long-term sustainable green energy.Solar energy is an inexhaustible natural energy.The energy from the sun to the earth per second is equal to the energy generated by the combustion of 5 million tons of coal.Hydrogen is a green and pollution-free energy,and the only product of its combustion is water.Therefore,the photoelectrochemical?PEC?water splitting technology that produce hydrogen by using solar energy has attracted widespread attention.The exploration of semiconductor with potential for the preparation of high performance PEC photoanode is an important research direction in this field.At present,the main PEC photoanode include TiO₂,ZnO,Fe₂O₃,CdS,BiVO₄,WO₃,ZnS,SnO₂ and so on show good PEC performances.However,in order to commercialize the PEC water splitting technology,the used semiconductor material needs to be further improved.The strategies for improving the PEC performance of semiconductor include doping element,controling morphology,covering passivation layer,forming heterojunction and so on.These strategies can improve the PEC performance of semiconductor to a certain extent,and a more convenient surface treatment is beneficial to the performance enhancement of photoanode.In this thesis project,Ti:Fe₂O₃/CdSe and W:BiVO₄ photoanodes are prepared,and are treated with different surface treatments.Result shows that the PEC performances of the photoanodes after surface treatments is improved.The improvement mechanisms of the photoanodes are further studied.This paper is divided into three parts:1.First of all,the importance of improving the utilization of solar energy is discussed.Secondly,the basic principle of PEC water splitting and several key conditions to satisfy the PEC water splitting are briefly described,and then the importance of the study of photoanode materials is discussed.Thirdly,the main measurement indexes in the characterization of PEC performance and progresses in the research of several common photoanodes are briefly summarized.Finally,the materials,equipments and characterization methods used in the experiments are introduced.2.The preparation and surface treatment of Ti:Fe₂O₃/CdSe heterostructure and its PEC behavior are studied.Ti:Fe₂O₃ is prepared by bath method and CdSe is electrodeposited on Ti:Fe₂O₃ to form heterojunction,then the effect of different atmosphere annealing on the PEC performance of Ti:Fe₂O₃/CdSe photoanode is studied.The Ti:Fe₂O₃/CdSe photoanode delivers a photocurrent density of 3.25 mA/cm² at 1.2 V vs.RHE,which is about 6.5 times that of the Ti:Fe₂O₃.The improvement of PEC performance is mainly due to the formation of Ti:Fe₂O₃/CdSe heterojunction accelerating the separation and transfer of photogenerated electrons and holes.Annealing in the air makes the Se oxide protective layer form on the surface of the photoanode,which reduces the anode corrosion of the electrode and improves the stability of the electrode.3.The surface treatment of W:BiVO₄ and its PEC behavior are studied.W:BiVO₄and BiVO₄ photoanodes are prepared by a metal organic decomposition method.Then the two photoanodes are treated with a surface treatment using NaOH solution.The photocurrent density of the W:BiVO₄ photoanode reaches 0.65 mA/cm² at 1.23 V vs.RHE,which is about 2.3 times that of the W:BiVO₄ electrode without surface treatment.Interestingly,the photocurrent density of the BiVO₄ photoanode after surface treatment has not been improved.The main reason for surface treatment to enhance the performance of W:BiVO₄ is that W⁶⁺may act as the recombination center of electrons and holes,which affects the charge transfer.NaOH solution can dissolve the recombination center.The investigations above show that the surface treatment of photoanode is an effective method to improve the PEC performance.At the interface between photoanode and electrolyte,there are some key factors that limit the PEC performance.Therefore,surface treatment can promote the separation and transfer of carriers,avoid serious charge recombination,and weaken the surface corrosion of electrode.In the future,the methods for the surface treatment of photoanode can be extended.