Studies On The Construction And Properties Of CuBi₂O₄-based Core-shell Nanosheet Arrays Photocathode For Photoelectrochemical Water Splitting To Produce Hydrogen

The excessive consumption of energy and environmental pollution have seriously limited the development of human society,looking for sustainable development of green energy has become an urgent need.Hydrogen energy is considered as the most promising clean energy because of its high heat,abundant reserves,clean and non-pollution.Among various hydrogen production technologies,photoelectrochemical water splitting for hydrogen production with high catalytic efficiency,due to the coupling of light and electricity,is one of the cutting-edge technologies to solve the current energy crisis and environmental pollution.The core of photoelectrocatalytic technology lies in the development and design of efficient and stable photoelectrode.In photoelectrocatalytic systems,reduction reactions mainly occur at the photocathode,and p-type semiconductors are commonly used photocathode materials because they can produce appropriate band bending directions when contacted with solution.The narrow-band gap（1.5-1.8 e V）p-type CuBi₂O₄semiconductor with a suitable band structure has many advantages,for example,simple preparation and non-toxicity,driving the hydrogen evolution reaction,absorbing and using visible light.So,it is a very suitable photocathode material for the photoelectrochemical water splitting to produce hydrogen.However,the catalytic efficiency of CuBi₂O₄ is low due to the massive recombination of photogenerated electrons and holes.In order to improve this deficiency,we proposed to modify CuBi₂O₄ semiconductor material by regulation of morphology and construction of heterojunction.The nanosheet array can enhance the absorption of visible light,and increase the large specific surface area to generate more active sites,which is conducive to the occurrence of catalytic reactions.The core-shell structure ensures that the components are in close contact with each other,effectively inhibiting photogenerated electron and hole recombination.Therefore,the main research contents of this paper are as follows:（1）Bi OI nanosheet arrays were prepared on FTO by electrodeposition,and carbon quantum dots（CQDs）modified CuBi₂O₄/CuO（CQDs-CuBi₂O₄/CuO）core-shell nanosheet arrays were prepared by using Bi OI as a precursor.XRD,Raman,HRTEM and XPS characterization indicate the successful construction of CQDs-CuBi₂O₄/CuO core-shell nanosheet array photocathode.It can be seen from the UV-visible absorption spectrum that,compared with CuBi₂O₄,the absorption capacity of CQDs-CuBi₂O₄/CuO photocathode is greatly enhanced,which significantly improves the utilization efficiency of visible light.Meanwhile,the results show that CQDs-CuBi₂O₄/CuO photocathodes show the highest reducing activity,with Faraday efficiency of 92%,while CuBi₂O₄ is only 48%,because the carrier separation efficiency is greatly enhanced.The photocurrent density of photocathode CQDs-CuBi₂O₄/CuO at an applied bias of 0.4 V vs.RHE was-0.95 m A cm^-2,which is 1.63 times that of CuBi₂O₄/CuO and 6.3 times that of CuBi₂O₄.Moreover,when the wavelength is 380 nm,the CQDs-CuBi₂O₄/CuO incident light-electron conversion efficiency（IPCE）is as high as 27.13%.Ultravioletphotoelectron spectroscopy（UPS）results prove that CuBi₂O₄ and CuO form a type II heteroconcretion.The interaction between core-shell nanosheet array and type II heterojunction can greatly promote the separation of photogenerated electrons and holes.（2）On the basis of CuBi₂O₄/CuO heterojunction,amorphous n-Ti O₂ was introduced by low temperature calcination method to form p-n junction with p-CuBi₂O₄,thus,CuBi₂O₄/CuO/Ti O₂ double heterojunction photocathode was obtained.Due to the synergistic effect of double heterojunction,CuBi₂O₄/CuO/Ti O₂ has faster electron migration rate and higher carrier separation efficiency than CuBi₂O₄/CuO,and finally improves the catalytic efficiency.SEM,XRD,Raman and XPS characterization indicate that the photocathode of CuBi₂O₄/CuO/Ti O₂ nanosheet array has been successfully prepared.Importantly,the photocurrent density of CuBi₂O₄/CuO/Ti O₂ photocathode is-0.89 m A cm^-2,and the IPCE at380 nm is 14.71%,which is higher than that of CuBi₂O₄/CuO and pure CuBi₂O₄.In addition,the CuBi₂O₄/CuO/Ti O₂ has double layer capacitance and tafel slope of 0.394 m F cm^-2 and 104 m V dec^-1,respectively.Thus,CuBi₂O₄/CuO/Ti O₂ photocathode has larger electrochemically active area and faster kinetic process,compared with CuBi₂O₄/CuO and CuBi₂O₄.UPS reveals the flow path of photogenerated charge.The construction of double heterojunction enables electrons to flow from CuBi₂O₄ to CuO and finally converge on the surface of Ti O₂,which indicates that double heterojunction has a greater contribution to carrier separation than single heterojunction.It provides a new idea to design efficient photoelectrode system by constructing double heterogeneous bond.