Study On The Photogenerated Charge Behavior And Performance Of Inorganic Semiconductor Based Photocatalyst:Insights From Surface And Interface

The serious environmental pollution and increasing energy demand in recent years have been recognized as two main global challenges.Inorganic semiconductor-based photocatalysis systems,which is low-cost and environmental friendly,provide a promising route to overcome the above difficulty.Much effort has been devoted to the development of efficient photocatalysts for solar energy application,such as photocatalytic water splitting,photoelectrochemical（PEC）water splitting,photocatalytic CO2 reduction and photocatalytic degradation of organic pollutants.In spite of considerable process in photocatalysis,the photocatalytic activity of photocatalysts is still too low to meet the requirement of practical applications.Photocatalytic reaction processes on a semiconductor include three main steps:（1）the generation of electron-hole pairs through light absorption;（2）the separation,recombination and migration processes of photogenerated charge carriers.（3）the reduction or oxidation reaction at active sites on the surface of semiconductor.The efficiency of a photocatalyst is primarily determined by the balance of the thermodynamics and kinetics of these processes.It is difficult for a common photocatalyst to possess a broad light absorption range for efficient sunlight utilization,efficient charge transfer from the bulk to the surface and rapid consumption of photogenerated carriers for the surface reaction simultaneously.Various strategies have been proposed to promote the above performance of semiconductor photocatalysts,such as metal/nonmetal doping,increasing the density of surface defects,introducing homo/heterojunctions,cocatalyst loading,etc..Actually,these strategies could be classified as two categories: surface engineering and interface engineering.Surface and interface are the two key elements in the design of inorganic semiconductor based photocatalyst.On one hand,reduction or oxidation reaction occurring at the surface of a semiconductor,which greatly determines the activation abilities for reactant molecules and thus influences the activity and selectivity for photocatalytic reactions.On the other hand,interface will significantly affect the charge separation and injection efficiency of composite photocatalyst,holding the key to restraining the recombination process of photogenerated charge carriers.The theme issues of this thesis are the surface and interface engineering of inorganic semiconductor based photocatalyst.Moreover,we elucidated the relationship between photophysics process and photocatalysis by studying the separation and transfer processes of photogenerated charge carriers.This thesis encompasses four areas of research:1.Study on the influence of Ni doping on the surface states properties of Cd S nanorods: The influence of charge trapping on charge transport in semiconductor is especially complicated and debatable.Cd S nanocrystal was employed as the model photocatalyst due to its abundant surface states densities and we further increased its surface states densities by Ni doping.The effect of the surface states on the photogenerated charge carriers were investigated by surface photovoltage techniques.The results reveal that light Ni doping can increase the densities of shallow surface states,while heavy Ni doping can form deep surface states of Cd S nanorods.Shallow surface could prolong the lifetime of photogenerated charge carriers and promote charge carriers separation,thus resulting in an efficient photocatalytic reaction.2.Study on the influence of different charge trapping states（electron traps and hole traps）on the photocatalytic performance of Cd S nanocrystal: Cd S nanocrystals with different charge trapping states were obtained by changing the reaction conditions with different surfactant.The trapping properties of different surface states on the photogenerated charge carriers were investigated by surface photovoltage techniques.The surface trapping states are revealed to act as electron or hole traps to modulate the generation of radicals and thus influence the photodegradation mechanism of photocatalytic reaction.3.Study on the hole injection between Ni₂ P cocatalyst and electrolyte interface in Ni₂ P modified Ti4+ doped Fe₂O₃（Ti-Fe₂O₃）photoanode system: Ti-Fe₂O₃ was employed as the model photoanode and Ni₂ P was employed as cocatalyst.The transfer of charge carriers across the cocatalyst/electrolyte interface was investigated mainly by photoelectrochemical methods.Surface deposition of Ni₂ P cocatalyst resulted in a three times increase in water oxidation photocurrent density at 1.23 V vs.RHE and the photocurrent onset potential was shifted cathodically by 140 m V.Ni₂ P cocatalyst could significantly promote the hole injection efficiency and suppress the back reaction of the water oxidation reaction over Ti-Fe₂O₃ photoanode.4.Fabrication of metallic charge transfer channel between photoanode Ti-Fe₂O₃ and cocatalyst Co O_x and its influence on the photoelectrochemical performance of water oxidation: The charge transfer process occurring at the semiconductor/cocatalyst interface should be improved.We elaborately design and construct a charge transfer channel of metal Co experimentally between Ti-Fe₂O₃ and Co O_x.The transfer of charge carriers across the photoelectrode/cocatalyst interface was investigated by surface photovoltage techniques and photoelectrochemical methods.The existence of charge transfer channel of metallic Co could allow the photogenerated holes to transfer across photoelectrode/cocatalyst interface rapidly and arrive at the surface of the cocatalyst readily,which resulted in 150 m V photocurrent onset potential cathodic shift and a seven times increase in water oxidation photocurrent density at 1.23 V vs.RHE.