Properties And Charge Transfer Mechanism Of Surface States On TiO₂ Photoanode

Exploring renewable and clean energy is the key factor to realize the sustainable development of human society.Semiconductor based photoelectrochemical?PEC?wa-ter splitting,which is an ideal solar energy storage technology,can convert intermediate solar energy into hydrogen.However,there are many scientific challenges in develop-ing this technology.The solar light absorption efficiency of semiconductor electrode,the separation efficiency of photogenerated charges and the surface hole injection ef-ficiency are the main factors that limit the solar energy conversion efficiency of semi-conductor photoelectrode.The separation of photogenerated charge and the injection of holes mainly occur at the surface of semiconductor electrode,that is,the solid/liquid interface between semiconductor and electrolyte.It is of great significance to study the mechanism of charge separation,transport and transfer at the solid/liquid interface for the development of PEC cellsA space charge layer is formed at the n-type semiconductor side of the solid/liquid interface.There is a built-in electric field in the space charge layer inducing upward band bending,which is conducive to the separation of photogenerated charges.Due to the periodic destruction of the semiconductor surfaces or the absorption of foreign ions,surface states with the energy level between the conduction band and the valence band are formed on the surface of the semiconductor electrode.Photoanode with nano-crystallization semiconductor materials were development in order to increase the vol-ume density of the space charge layer and enhance the PEC activity.The influence of the surface states on the performance of the photoanode and the charge transfer at solid/liquid interface cannot be ignored.However,the role of the surface states in the process of charge transfer at the solid/liquid interface is not well understood.In this thesis,TiO₂ nanotube photoan-ode with large electrochemical active areas and rich surface states is selected as the study electrode.The chemical composition of TiO₂ surface state,the role of TiO₂ sur-face states in the process of charge transfer at solid/liquid and solid/solid interface,and the influence of TiO₂ surface states on the saturated photocurrent density were studied On this basis,the method of electrochemical doping was used to tune the surface states density of TiO₂ to improve the PEC performance of TiO₂ nanotube electrodes.The main research contents of the study are as follows:Back electron transfer mechanism of TiO₂ nanotube photoanode.In general,adding charge scavengers in the electrolyte will contain the recombination of holes and electrons at the electrode surface and,as a result,the observed photocurrent density was enhanced.Here,we detected,on TiO₂ nanotube photoanodes after using hydrogen peroxide?H2O₂?as a hole scavenger,a nearly 40%saturated photocurrent decrease in alkaline electrolyte.We studied the unnormal phenomenon and found that surface states were existence on the TiO₂ nanotube and its capacity for electron storage was large and independent with the pH value of electrolyte.Surface states on TiO₂ can act as a back electron transfer pathway that photogenerated electrons trapped in surface states can back transfer to the electrolyte reducing hydrogen peroxide even applied a very high positive bias on the electrode.Enhanced PEC performance of TiO₂ nanotube photoanode by tuning the sur-face states via Electrochemical doping.Here,we found that the saturated photocur-rent density for TiO₂ nanotube electrode after electrochemical doping increased dra-matically.We first elucidated the physical meaning of Mott-Schottky?M-S?plots of the TiO₂ nanotubes electrode with three linear ranges and confirmed the chemical com-position of TiO₂ surface states Ti-OH.Electrochemically doping the TiO₂ nanotubes electrode at a negative potential will induce the insertion of protons into the lattice of TiO₂ via a reaction of Ti?O₂+e-+H+=Ti?O?OH?increasing the surface states den-sity.Surface states on electrode are usually considered as the charge recombination centers for photogenerated holes and electrons.A theoretical explanation is given for the correlation between current-potential curves and Mott-Schottky plots of TiO₂ nan-otube electrode that the potential at which the photocurrent reached saturation and the Mott-Schottky plots both had a positive shift with the increasing of electrochemical dop-ing level.We found that surface states on TiO₂ nanotube electrode are not the charge recombination centers but an important photogenerated electron transfer route,and can trap photogenerated electrons and detrap it to external circuit significantly increasing charge separation efficiencyEnhanced charge separation efficiency by TiO₂ surface states induced by gal-vanic cell reaction.After NaBH4 treatment,the PEC performance for TiO₂ nanotube photoanode enhanced dramatically.Researchers generally believed that NaBH4 solu-tion chemically react with TiO₂ inducing Ti3+and oxygen vacancies Vo,which were responsible for the improvement of PEC performance.When doing NaBH4 treatment that TiO₂ nanotube electrode was completely immersed in NaBH4 solution and partially immersed in NaBH4 solution,the PEC performance was different.The Ti conductor parts and the TiO₂ semiconductor parts of the TiO₂ nanotube electrode in the NaBH4 electrolyte form a galvanic cell that Ti conductor parts act as the anode and TiO₂ nan-otubes act as the cathode.The anodic reaction was NaBH4+8 OH-=NaBO₂+8 e-+6H2O and the cathodic reaction was Ti?VO₂+e-+H+=Ti?O?OH?.We found that a spontaneous galvanic reaction on the TiO₂ electrode induced a well-known electro-chemical doping reaction during NaBH4 reduction treatmentEnhanced interfacial charge transfer of CdS/TiO₂ heterojunction via TiO₂ surface states.Deposition narrow band gap semiconductor or semiconductor quan-tum dots on wide band gap semiconductor photoanode can increase the absorption ef-ficiency of solar light and the length of space charge layer,thus improving the its PEC performance.CdS quantum dots were growth on the surface of the TiO₂ nanotube array with a sequential chemical bath deposition method?S-CBD?.The role of TiO₂ surface states in the process of solid/solid interfacial charge transfer were studied.The surface states density for TiO₂ nanotube electrode increased after electrochemical doping.For CdS/TiO₂?ED?nanotube photoanodes the saturated photocurrent density increased and the potential at which the photocurrent reached saturation has a positive shift compar-ing to CdS/TiO₂ nanotube electrode.In-situ Raman spectroscopy was used to further study the formation mechanism of surface states?Ti-OH?on TiO₂ during electrochem-ical doping.The surface states on TiO₂ can capture the photogenerated electrons in CdS quantum dots,reducing the recombination rate of the photogenerated electrons and holes in CdS quantum dots,thus the intensity of photoluminescence emission for CdS quantum dots is significantly inhibited.Surface states on TiO₂ nanotube act as an electron transfer pathway,as well as a reservoir for photogenerated electrons,in the process of photogenerated electrons in CdS/TiO₂ heterojunction transferring from CdS to TiO₂,...