Enhanced Charge Separation By Oriented Growth Of Ta₃N₅-Cu₂O Heterojunction

Photoelectrochemical water splitting is currently the focus of research in the energy field as well as one of the best solutions to global warming and energy shortages.The oxygen evolution reaction is a four-electron coupling reaction and the mobility of the holes is significantly lower than that of the electrons,so that the oxygen evolution reaction is slower than the hydrogen evolution reaction which limits the performance of the photoelectrode.Through studying the relationship between the properties of materials and their photoelectrochemical properties,it can enhance the certain properties of materials in a targeted manner and ultimately achieve the purpose of improving performance.Ta3N5 is a very promising photoanode material with the theoretical maximum photocurrent density of 12.9 mA cm-2 and the solar-to-hydrogen efficiency of 15.9%under AM 1.5G sunlight.However,researches confirm that Ta3N5 has the low carrier mobility(1.3?4.4 cm2 V-1 s-1)and the fast carrier recombination(<10 ps),which seriously hinder the performance of Ta3N5 photoanode.Heterojunction construction is considered to be one of the most effective means to improve the photoelectrochemical performance of materials,but the band structure and interface quality of Ta3N5 greatly limit the choice of materials.It is very challenging to construct Type-II heterojunction of Ta3N5.This paper takes Ta3N5 nanorod array photoanode as the research object,and Cu2O is grown on the surface of Ta3N5 nanorod array to build Type-II heterojunction,which improved the charge separation efficiency of Ta3N5 photoanode from the perspective of band structure.Ta3Ns and Cu2O are prepared by molten salt method and electrodeposition method respectively.The thickness of Cu2O in Ta3Ns-Cu2O photoanode is controlled by changing the electrodeposition time to explore the possibility of Ta3N5 and Cu2O forming heterojunctions.The Ta3N5 nanorod array prepared by the molten salt method grows along the a-axis in which the electron effective mass is the smallest(0.21 m0).The electron transport is faster,and at the same time,it has excellent characteristics of the light absorption.Cu2O is one of the best candidate materials for heterojunction construction.It is confirmed by experiments that Cu2O is successfully synthesized and deposited on the surface of Ta3Ns nanorods;and Cu2O could form Type-? heterojunction with Ta3Ns.The heterojunction can form a built-in electric field inside the Ta3N5-Cu2O,enlarging band bending upward and having a larger potential gradient in the photoanode-electrolyte interface.The heterojunction prevents electron-hole recombination from the bulk Ta3N5 effectively and more photogenerated holes reach the surface of the photoanode to participate in the chemical reaction.Compared with the pure Ta3Ns photoanode,the photoelectrochemical performance of Ta3Ns-Cu2O photoanode is greatly improved.