Electronic Structure and Interface Energetics of CuBi2O4 Photoelectrodes.

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

doi:10.1021/acs.jpcc.0c08455

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.

Highlights

The tetragonal copper bismuth oxide CuBi2O4 has a crystal structure that features a three-dimensional array of [CuO4]6− square-planar units, staggered along the c-axis and separated by Bi3+ ions, as shown in Figure S1 of the Supporting Information (SI)
Whereas calculations by Sharma et al.[2] suggested that Bi 6s and Cu 3d have comparable contributions to the valence band maximum (VBM), the results presented by Janson et al.[6] and Di Sante et al.[7] suggest that the Bi contribution to the VBM is negligible, so that an effective one-band model is appropriate for the description of the low-lying excitations in CuBi2O4
We report a detailed study of the electronic structure of CuBi2O4 based on an advanced and comprehensive X-ray spectroscopic approach, combining hard X-ray photoemission spectroscopy (HAXPES), resonant photoemission spectroscopy (ResPES), O K-edge X-ray absorption spectroscopy (XAS), and firstprinciples density functional theory (DFT) calculations

Summary

■ INTRODUCTION

The tetragonal copper bismuth oxide CuBi2O4 has a crystal structure that features a three-dimensional array of [CuO4]6− square-planar units, staggered along the c-axis and separated by Bi3+ ions, as shown in Figure S1 of the Supporting Information (SI). Soft X-ray photoemission spectroscopy preferentially probes Cu 3d valence states, whereas Bi 6s and Bi 6p make up most of the spectral features in HAXPES This distinction allows identification of the Bi and Cu contributions to the valence band by taking valence band spectra at different ionization photon energies. For the nickel oxide hole transport layer, nickel nitrate hexahydrate (Sigma-Aldrich 99.999%) was dissolved in a 5:2 v:v solution of acetic acid−water to yield 5 mL of a 0.5 M solution This solution was further diluted with 2.5 mL of 2metoxyethanol and the resulting solution was used to spin-coat 2.5 × 2.5 cm[2] sized FTO substrates at 3000 rpm. Samples studied by X-ray spectroscopy may be subject to beam damage We addressed this issue by making sure that the chemistry of the material, as probed by coreline spectra (e.g., Cu 2p or Bi 4f), remained unchanged during the experiments. Article emission spectrum will be proportional to the photoionization cross sections of corresponding orbitals in free atoms based on the Gelius model.[28,29]

■ RESULTS AND DISCUSSION

■ CONCLUSION

■ ACKNOWLEDGMENTS

■ REFERENCES