A Study On The Preparation Of Surface Modified Cuprous Oxide Photocathodes For Photoelectrochemical Water Splitting

In developing sustainable,ecologically benign hydrogen production technologies,solar driven photo-electrochemical（PEC）water splitting has received considerable attention.The activity,stability and availability of the photo-electrode materials are critical issues limiting the practical applications of the PEC systems.So far,a variety of n-type semiconductors such as Ti O₂,BiVO₄,Fe₂O₃ and WO₃ have been used as photoanodes and studied intensively.However,the development of effective and stable photocathode research still faces challenges.The band gap of cuprous oxide（Cu₂O）is about 2.0 e V,and the theoretical photocurrent is estimated to be-14.7 m A/cm².One of the main issue limiting the use of Cu₂O as a photocathode is the severe electron-hole recombination.The electron diffusion length of Cu₂O ranges approximately from 20-100 nm.It is very incommensurate with the light absorption depth,which is at least 1μm to adsorb most of the sunlight.Another issue is its photo-instability in aqueous solution.The redox potentials of Cu⁺are positioned within the band gap of Cu₂O.Cu₂O may be reduced to be Cupper or oxidized to be Cu O by the photo-generated electrons and holes in the electrolyte.Many attempts have been made to improve the PEC performance of Cu₂O,including（i）morphology manipulation,（ii）hetero-junction,construction,（iii）elements doping,（iv）surface modification with protective layers.In this study,gradient Si doping and protective layers deposition were achieved by a facile dip-casting method firstly.The PEC performances have been greatly improved,as evidenced by the increased photocurrent density（3.1 times of bare Cu₂O）and improved stability（decay of 3.8 times of bare Cu₂O）.The structure analysis and PEC characterizations proved that downward band bending from the bulk to the surface region was formed which has greatly promoted the charge separation and transfer,verified by steady state and transit state photoluminescence measurement as well as electrochemical impedance.In addition,the amorphous SiO_x layers served as protection layers and prevented the Cu₂O from direct contact with the electrolyte,which retard the self-reduction and oxidation,maintaining a high photocurrent density over the stability test duration.In the second part of this paper,we used a simple one-step spin coating method to form a dense and uniform hydrophobic organic film on the surface of the cuprous oxide.By considering the amount of spin coating,we have selected the thin film material with the best photoelectrochemical performance,200μL.And the final photocurrent density is 1.74 m A/cm²,which is 1.93 times of the blank and maintain the best stability current of 1.7 m A/cm² for 1200 s,which is 2.4 times of the blank and IPCE reaches 38%at 532 nm.This time,the surface of cuprous oxide was modified by polyimide,so that a uniform,smooth and dense protective film was formed on the surface of the material,which prevented it from contacting the electrolyte.X-ray diffraction proved that the presence of polyimide effectively inhibited the self-oxidation of cuprous oxide.It is effectively reduced,the charge recombination on the surface of the steady-state test surface material is effectively suppressed,and the recombination probability of electrons and holes is reduced,which is of great help to the improvement of photoelectric performance.