Electronic Structure and Interface Energetics of CuBi2O4 Photoelectrodes

Freddy E Oropeza; Nelson Y Dzade; Amalia Pons-Martí; Zhenni Yang; Kelvin H L Zhang; Nora H de Leeuw; Emiel J M Hensen; Jan P Hofmann

doi:10.1021/acs.jpcc.0c08455

Electronic Structure and Interface Energetics of CuBi₂O₄ Photoelectrodes

J Phys Chem C Nanomater Interfaces. 2020 Oct 15;124(41):22416-22425. doi: 10.1021/acs.jpcc.0c08455. Epub 2020 Sep 30.

Authors

Freddy E Oropeza^{1

2}, Nelson Y Dzade³, Amalia Pons-Martí¹, Zhenni Yang⁴, Kelvin H L Zhang⁴, Nora H de Leeuw^{3

5

6}, Emiel J M Hensen¹, Jan P Hofmann^{1

7}

Affiliations

¹ Laboratory of Inorganic Materials and Catalysis, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.
² IMDEA Energy Institute, Avenida Ramón de la Sagra, 3, 28935 Móstoles, MadridSpain.
³ School of Chemistry, Cardiff University, Main Building, Park Place, CF10 3AT Cardiff, U.K.
⁴ State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, P.R. China.
⁵ School of Chemistry, University of Leeds, Leeds LS2 9JT, U.K.
⁶ Department of Earth Sciences, Utrecht University, Princetonlaan 8a, 3584 CB Utrecht, The Netherlands.
⁷ Surface Science Laboratory, Department of Materials and Earth Sciences, Technical University of Darmstadt, Otto-Berndt-Strasse 3, 64287 Darmstadt, Germany.

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.