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Electronic structure and interface energetics of CuBi2O4 photoelectrodes

DOI: 10.1021/acs.jpcc.0c08455 DOI Help

Authors: Freddy E. Oropeza (Eindhoven University of Technology; IMDEA Energy Institute) , Nelson Y. Dzade (Cardiff University) , Amalia Pons-martí (Eindhoven University of Technology) , Zhenni Yang (Xiamen University) , Kelvin H. L. Zhang (Xiamen University) , Nora H. De Leeuw (Cardiff University; University of Leeds; Utrecht University) , Emiel J. M. Hensen (Eindhoven University of Technology) , Jan P. Hofmann (Eindhoven University of Technology; University of Darmstadt)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: The Journal Of Physical Chemistry C

State: Published (Approved)
Published: September 2020
Diamond Proposal Number(s): 19191

Abstract: CuBi2O4 exhibits significant potential for the photoelectrochemical (PEC) conversion of solar energy into chemical fuels, owing to its extended visible-light absorption and positive flat band potential vs the reversible hydrogen electrode. A detailed understanding of the fundamental electronic structure and its correlation with PEC activity is of significant importance to address limiting factors, such as poor charge carrier mobility and stability under PEC conditions. In this study, the electronic structure of CuBi2O4 has been studied by a combination of hard X-ray photoemission spectroscopy, resonant photoemission spectroscopy, and X-ray absorption spectroscopy (XAS) and compared with density functional theory (DFT) calculations. The photoemission study indicates that there is a strong Bi 6s–O 2p hybrid electronic state at 2.3 eV below the Fermi level, whereas the valence band maximum (VBM) has a predominant Cu 3d–O 2p hybrid character. XAS at the O K-edge supported by DFT calculations provides a good description of the conduction band, indicating that the conduction band minimum is composed of unoccupied Cu 3d–O 2p states. The combined experimental and theoretical results suggest that the low charge carrier mobility for CuBi2O4 derives from an intrinsic charge localization at the VBM. Also, the low-energy visible-light absorption in CuBi2O4 may result from a direct but forbidden Cu d–d electronic transition, leading to a low absorption coefficient. Additionally, the ionization potential of CuBi2O4 is higher than that of the related binary oxide CuO or that of NiO, which is commonly used as a hole transport/extraction layer in photoelectrodes. This work provides a solid electronic basis for topical materials science approaches to increase the charge transport and improve the photoelectrochemical properties of CuBi2O4-based photoelectrodes.

Journal Keywords: X-rays; Photoemission; Oxides; Electrical conductivity; Materials

Subject Areas: Materials, Chemistry, Energy

Instruments: I09-Surface and Interface Structural Analysis

Other Facilities: Laboratorio Nacional de Luz Sincrotron (LNLS)