Design And Fabrication Of Cu₂O-based Photocathodes And Their Photoelectrochemical Properties

It is an effective way to resolve the crisis and problem of environmental pollution by utilizing the renewable and pollution-free energy rationally in nature.Photocatalytic reaction is a method of solar energy conversion by using semiconductors as photoelectrodes.It is still a challenge to find a low-cost,high-efficiency and stable photoelectrode material.Cuprous oxide（Cu₂O）,an intrinsic p-type semiconductor,has a band gap of about 2.2 e V and can absorb light in the wavelength range of 300-620 nm.With theoretical photocurrent density of-14.7 m A cm^-2,Cu₂O plays an important role in photocatalytic reaction as a photocathode.However,due to the mismatch between the optical absorption depth and the carrier diffusion length of Cu₂O,the photogenerated carrier separation efficiency of Cu₂O is low,and the photocorrosion leads to the poor stability of Cu₂O in the photoelectrochemical system,which makes it difficult to achieve satisfactory photocatalytic performance.Effective charge transfer layer materials are designed on the surface of Cu₂O photocathode to enable rapid carrier transfer at the interface and thus improve the charge separation in its bulk phase.Graphene and its derivatives are a suitable class of charge transfer layer materials because of their good transmittance,conductivity,and corrosion resistance.Among them,a large number of structural defects in graphene oxide（GO）lead to a decrease in its electrical conductivity,but the GO interlayers are rich in oxygen functional groups,which makes it easy to modulate the band gap and enhance its carrier mobility through chemical functionalization.In summary,the core content is to design the charge transport layer of the Cu₂O photocathode to ensure more effective charge transfer and separation and improve the performance in PEC applications.The specific research contents are as follows:1.Adenine,a small biomolecule,is used to functionalize GO（A-GO）to shorten the band gap of GO and enhance its carrier mobility.A-GO is spin-coated on the surface of Cu₂O to form A-GO/Cu₂O photocathode.The electron-donating groups in adenine can reduce GO to a certain extent,decrease its band gap,and increase its carrier mobility.Thus,A-GO promotes electron transfer on the Cu₂O surface and improves the charge separation efficiency of the A-GO/Cu₂O photocathode.In addition,the presence of multiple N active sites in A-GO/Cu₂O can synergize with Cu₂O for PEC CO2 reduction reaction.As a result,compared with GO/Cu₂O,A-GO/Cu₂O exhibits a higher photocurrent density of-2.74 m A cm^-2（0 V vs.RHE）.2.The three-dimensional porous structure of ZIF-67 has a high specific surface area and abundant metal sites.The functionalization of GO using ZIF-67 can reduce the stacking of nanosheets,enhance the conductivity of GO through rapid charge transfer,and provide additional metal active sites for GO.ZIF-67 and GO grow on the surface of Cu₂O by self-assembly to form the ZIF-GO/Cu₂O photocathode.In ZIF-GO,the ZIF-67 nanoparticles are cross-linked with GO nanosheets,which can increase its specific surface area for effective charge transfer and mass transfer.The enhanced interfacial charge transfer ability leads to the effective charge separation of Cu₂O.As a result,the photocurrent density of ZIF-GO/Cu₂O reached-4.18 m A cm^-2（0 V vs.RHE）.3.The redox reaction between the Cu foam substrate and GO dispersion is used to form r GO-wrapped Cu₂O photocathode.The free Cu ions are inserted into the interlayer of r GO through cation-πinteractions during the reaction process to form a Cu-r GO/Cu₂O photocathode.The reduced bandgap of r GO and the functionalization of the Cu ions contribute to the fast charge mobility in the Cu-r GO.Therefore,Cu-r GO can inhibit the charge recombination and the charge accumulation by improving the charge transfer at the interface of Cu₂O.Cu-r GO/Cu₂O photocathode exhibits a high photocurrent density of-7.03 m A cm^-2（0 V vs.RHE）,as well as good photostability to PEC water splitting.