Elucidating Electronic Structure,Optical Properties And Photoelectrochemical Water Splitting Properties Of CuWO₄

Photoelectrochemical(PEC)water splitting is a promising way to convert solar energy into hydrogen energy.As an n-type semiconductor,CuWO4 possess a suitable band position,ideal band gap(?2.2 eV)and high theoretical photocurrent density(10.7 mA/cm2),which exhibits great potential for photoelectrochemical(PEC)conversion of solar energy into chemical fuels.However,the photocurrent density(0.3 mA/cm2)reported so far is much lower than its theoretical value.In addition,there is considerable controversy about the electronic structure of CuWO4(the orbital composition of the conduction band).Based on this,a detailed understanding of the fundamental electronic structure and its correlation with PEC activity is of significant importance for devising strategies for further improvements.In this paper,we studied the electronic structure of CuWO4 and revealed the reasons for its poor PEC performance.Furthermore,the implications of the fundamental electronic structure on its PEC characteristics,as well as strategies for further improvements by adding Co3O4 as a co-catalyst to form a p-n heterojunction,are discussed.The main content includes:(1)We have synthesized CuWO4 thin films showing a record photocurrent density of 0.48 mA/cm2 at+1.23 V vs.RHE.Importantly,we have used a synergistic combination of photoemission spectroscopy,X-ray absorption spectroscopy and density functional theory(DFT)to unravel the electronic structure of CuWO4.Our results show that the valence band(VB)consists of strongly hybridized states of O 2p6 and Cu 3d9,while the bottom of the conduction band(CB)is primarily composed of unoccupied Cu 3d states.The localized nature of the Cu 3d state leads to the low charge carrier mobility and the localization of the photo-excited electrons to the CB.The combined experimental and theoretical results also indicate that CuWO4 is better described as having a direct but d-d forbidden optical bandgap,leading to a low absorption coefficient for visible light.(2)We tried to improve the PEC performance of CuWO4 by adding Co3O4 on its surface.The results show that when the loading thickness of C03O4 is 1.5 nm,the photocurrent density of CuWO4 film can further increased to 0.6 mA/cm2 at +1.23 V vs RHE and the stability of the film has also improved.The results of EIS and XPS show that Co3O4 not only accelerates the oxidation kinetics of water at the interface between CuWO4 film and electrolyte,but also forms a p-n junction with CuWO4,which promotes the separation of photo-generated carriers in CuWO4 surface,thus improving the PEC performanc.In addition,by comparing the XRD and XPS data of the film before and after the reaction,we found that the dissolution of the particles on the surface of the CuWO4 film was the main reason for its poor stability.