Highly-efficient Photoelectrochemical Photoanodes Based On WO₃:fabrication And Performance Study

Photoelectrochemical(PEC)water splitting refers to the process of using photoactive semiconductor as photoelectrodes to split water into hydrogen and oxygen with the assistance of applied voltage as well as solar light.Because of its clean and renewable characteristics,PEC water splitting has been regarded as an important way to achieve hydrogen generation.In order to enhance the efficiency of PEC water splitting,researchers have focused on exploring suitable materials for photoelectrodes.Tungsten trioxide(WO3)is a typical material for PEC photoanode.However,the wide band gap and sluggish oxygen evolution dynamics of WO3 limit its PEC efficiency.To solve these problems,we improved the PEC performance of WO3 photoanode by the means of heterojunction fabrication and surface modification,respectively.We designed and fabricated three composite photoanodes:WO3/Bi2S3,WO3/CdIn2S4 and WO3/NiFe-layered double hydroxide(WO3/NiFe-LDH).Their PEC performance and working mechanism were investigated.The main research contents of this dissertation are as follows;(1)W03/Bi2S3:The plate-like WO3 array was prepared by hydrothermal method.The Bi2S3 seed layer was fabricated onto WO3 surface by successive ionic layer adsorption and reaction.The Bi2S3 nanorods were grown on WO3 by chemical bath deposition.The WO3/Bi2S3 grown with the aid of seed layer obtained photocurrent density of 5.95 mA cm-2 at 0.9 V vs.RHE,which is 35 and 1.4 times higher than those of WO3/Bi2S3 grown without seed layer and pristine WO3,respectively.The type-II heterojunction between WO3 and Bi2S3 promoted the carrier separation.The existence of Bi2S3 seed layer improved the contact between WO3 and Bi2S3.The better light absorption and efficient carrier separation simultaneously contribute to the enhanced PEC performance.(2)WO3/CdIn2S4:The CdIn2S4 nanoplates were grown on WO3 by chemical bath deposition.The photocurrent density of W±3/CdIn2S4 composite photoanode is 4.96 mA cm-2 at 1.23 V vs.RHE,which is 13.7 times higher than that of bare WO3 photoanode.The band positions of WO3 and CdIn2S4 were confirmed through ultraviolet photoemission spectroscopy spectra and UV-visible light absorption spectra.The type-? heterojunction structure between WO3 and CdIn2S4 has been demonstrated,which benefits to the carrier separation and transfer.The better light absorption,smaller charge transfer impedance and higher carrier density lead to the high performance of WO3/CdIn2S4 photoanode.(3)WO3/NiFe-LDH:The NiFe-LDH cocatalyst was prepared on plate-like WO3 array through electrodeposition.Then,the NiFe-LDH further underwent NaBH4 treatment.The WO3/NiFe-LDH treated by NaBH4 achieved a photocurrent of 0.62 mA cm-2,which is 1.19 and 1.88 times of WO3/NiFe-LDH before NaBH4 treatment and bare WO3 photoanode,respectively.Based on the electrochemical impedance spectra of photoanodes,we find that the existence of NiFe-LDH can decrease the carrier transfer impedance at electrode/electrolyte interface.After the treatment of NaBH4,the carrier transfer impedance was further decreased,and the life time of carriers obviously increased.Through the phase characterization of samples,we find that the valence state of metal atoms changes,which could be the factor to the enhanced catalytic activity of photoanode.