Boron-Doped Carbon Nitride And FeCo-MOF Modified Fe₂O₃ Nanoarray Photoanode For Water Splitting

With the development of the society,energy consumption and environmental pollution are becoming more and more serious.Developing renewable clean energy is so urgent.Photoelectrochemical（PEC）water splitting is an efficient and sustainable technique for H₂production.As one 4-electron transfer reaction process,the oxygen evolution reaction（OER）at photoanode is considered to be the rate-limiting step for PEC water splitting.Hematite（Fe₂O₃）is considered as an ideal photoanode due to the narrow band gap,appropriate valence band position,abandence earth reservesand high theoretical solar-to-hydrogen（STH）conversion efficiency of 16%.However,the inherent defects including low conductivity,serious bulk charge recombination,high surface state density and slow water oxidation kinetic restrict its PEC water oxidation performance.It is resonable to modify Fe₂O₃electrode with other materials to improve its PEC performance.Thus,researchers have modified Fe₂O₃with semiconductor materials,such as oxides（Ti O₂）,hydroxides（Fe OOH）,sulfides（Cd S）and even noble metal oxides（Ir O₂）to improve the PEC performance.However,due to the limited kinds of inorganic non-metallic materials,the energy band and crystal structure of the modified materials are poorly matched with Fe₂O₃,and the band structure of these materials are hard to regulate,so it is difficult to get great performance.Molecular semiconductor owns unique large-area delocalized conjugated orbitals,which are similar to the valence band and conduction band of semiconductor.In this case,molecular semiconductor can respond to the sunlight and form heterojunction with other semiconductors.Compared with inorganic non-metallic semiconductors,the molecular semiconductor can be easily synthesized and its energy band structure can be adjusted by doping or surface modification.Besides,most of the polymer molecules have good flexibility,which is easy to form a fully coated film or nanolayer structure on the surface of Fe₂O₃nanoarray.This can reduce the charge transfer impedance at material interface and thus improve the charge transfer efficiency.In this paper,molecular semiconductors were used to modify the Fe₂O₃photoanode to improve the PEC water oxidation performance.The research content mainly includes the following two parts:（1）Boron doped graphite carbon nitride（B-doped g-C₃N₄,BCN）film and Ni Co（OH）_xcocatalyst were successively loaded on the surface of Fe₂O₃photoanode by simple chemical vapor deposition and electrochemical deposition method,respectively.The sythesized composite photoanode shows high photocurrent density of 2.7 m A cm^–2at 1.23 V vs.RHE.Further experiments show that the growth of BCN film can effectively improve the charge separation efficiency of Fe₂O₃photoanode.As a cocatalyst,Ni Co（OH）_xcan accelerate the surface water oxidation kinetics and improve the charge injection efficiency.The highly improved PEC performance can be assigned the synergistic effect of BCN and Ni Co（OH）_xcocatalyst.（2）Multifunctional Fe Co MOF nanolayers were grown on Fe₂O₃photoanode by an in-situ solvothermal method.As expected,the obtained Fe Co-MOF/Fe₂O₃photoanode ehibits high photocurrent density of 2.8 m A cm^–2at 1.23 V vs.RHE,which is 2.5 times that of Fe₂O₃.It is found that Fe Co MOF nanolayers play vital roles in enhancing PEC performance:1)the p-n heterojunction was constructed between Fe Co MOF and Fe₂O₃,which drives photogenerated electrons and holes transfer to opposite directions under the built-in electric field,improving the carrier separation efficiency;2)Fe Co MOF nanolayers act as passivator can passivate the surface state of Fe₂O₃photoanode,inhibiting the surface charge recombination;3)Fe Co MOF nanolayers provide abundant reaction active sites and improve the water oxidation kinetics.In such a case,the high PEC performance of Fe Co-MOF/Fe₂O₃is achieved.