Lab-Based NAP-XPS for Probing Photoelectrochemical Interfaces: The Case of CuBi2O4

Katarina S. Flashar^a^,^b, Miriam Fehrenbach^a^,^b, Gaurav Anand^c, Dominic Rapf^a^,^b, Barbara A.J. Lechner^c, Ian D. Sharp^a^,^b, Verena Streibel^a^,^b

^aWalter Schottky Institute, Technical University of Munich, Am Coulombwall 4, 85748 Garching, Germany

^bPhysics Department, TUM School of Natural Sciences, Technical University of Munich, Am Coulombwall 4, 85748 Garching, Germany

^cChemistry Department, TUM School of Natural Sciences, Technical University of Munich, Lichtenbergstr. 4, 85748 Garching, Germany

Proceedings of MATSUS Spring 2026 Conference (MATSUSSpring26)

G5 In Situ and Operando Characterization Across Disciplines: Advanced Lab-Based Techniques for Energy Conversion Research

Barcelona, Spain, 2026 March 23rd - 27th

Organizers: Johanna Eichhorn and Verena Streibel

Oral, Katarina S. Flashar, presentation 224
Publication date: 15th December 2025

Understanding complex (photo)electrochemical conversion processes at functional interfaces requires in situ and operando characterization. A tool enabling such investigations is near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). While NAP-XPS has traditionally been performed at synchrotrons, the emergence of lab-based systems now enables flexible access, long-term measurements, and rapid feedback during materials development. In this contribution, we present first case studies using our recently installed, lab-based NAP-XPS (SPECS). It features three different excitation energies and is designed for depth-resolved studies across gas|solid, liquid|solid and solid|solid interfaces. Beyond flexible reaction atmospheres, the instrument includes an illumination port to investigate light-induced processes in photoactive materials. The central focus of the talk will be the implementation of an open liquid cell for (photo)electrochemical measurements. We discuss several practical challenges, including bubble formation and electrolyte evaporation, and outline mitigation strategies. As a photoelectrochemical case study, we examine sputter-deposited copper bismuthate (CuBi₂O₄) thin films in contact with water-based electrolytes. CuBi₂O₄ is a native p-type semiconductor, with suitable band alignment for photocathode applications [1]. Previous NAP-XPS studies have indicated light- and gas-dependent changes in surface chemistry [2]. Here, we extend these studies to the solid|liquid interface, examining CuBi₂O₄ under aqueous conditions and illumination. These investigations demonstrate the capability of our lab-based NAP-XPS to probe functional interfaces of energy materials under working conditions, offering an accessible and flexible alternative to synchrotron-based measurements.

References:

[1] Cooper, J. K.; Zhang, Z.; Roychoudhury, S.; Jiang, C.-M.; Gul, S.; Liu,Y.-S.; Dhall, R.; Ceballos, A.; Yano, J.; Prendergast, D.; Reyes-Lillo, S. E. CuBi2O4: Electronic Structure, Optical Properties, and Photoelectrochemical Performance Limitations of the Photocathode. Chem. Mater. 2021, 33, 3, 934–945.

[2] Oropeza, F. E.; Feleki, B. T.; Zhang, K. H. L.; Hensen, E. J. M.; Hofmann, J. P. Influence of Reduced Cu Surface States on the Photoelectrochemical Properties of CuBi2O4. ACS Appl. Energy Mater. 2019, 2, 6866−6874.

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