HAXPES Systems

n situ x-ray spectroscopies offer a powerful way to understand the electronic structure of the electrode–electrolyte interface under operating conditions. However, most x-ray techniques require vacuum, making it necessary to design spectro-electrochemical cells with a delicate interface to the wet electrochemical environment. The design of the cell often dictates what measurements can be done and which electrochemical processes can be studied. Hence, it is important to pick the right spectro-electrochemical cell for the process of interest. To facilitate this choice, and to highlight the challenges in cell design, we critically review four recent, successful cell designs. Using several case studies, we investigate the opportunities and limitations that arise in practical experiments.

J.-J. Velasco-Vélez, L. J. Falling, D. Bernsmeier, M. J Sear, P. C. J. Clark, T.-S. Chan, E. Stotz, M. Hävecker, R. Kraehnert, and A. Knop-Gericke
Juan-Jesús Velasco-Vélez et al 2021 J. Phys. D: Appl. Phys. 54 124003

Bismuth vanadate (BiVO₄) is an established n-type oxide semiconductor for photoelectrochemical oxygen evolution. Direct charge carrier recombination at the solid/liquid interface is a major cause of efficiency loss in BiVO₄-based devices. Intrinsic and extrinsic surface states (SSs) can act as electron and hole traps that enhance the recombination rate and lower the faradaic efficiency. In this study, we investigate the BiVO4/aqueous KPi interface using two types of samples. The samples were prepared at two different deposition and annealing temperatures (450 °C and 500 °C) leading to different morphologies and stoichiometries for the two samples. Both samples exhibit SSs in the dark that are passivated under illumination. In situ ambient pressure hard x-ray photoelectron spectroscopy experiments performed under front illumination conditions reveal the formation of a bismuth phosphate (BiPO₄) surface layer for the sample annealed at 450 °C, whereas the sample annealed at 500 °C exhibits band flattening without the formation of BiPO₄. These results imply that the light-induced formation of BiPO₄ may not be responsible for SS passivation. Our study also suggests that slight differences in the synthesis parameters lead to significant changes in the surface stoichiometry and morphology, with drastic effects on the physical-chemical properties of the BiVO₄/electrolyte interface. These differences may have important consequences for device characteristics such as long-term stability.

M. Favaro, I.Y. Ahmet, P. C. J. Clark, F. F. Abdi, M. J. Sear, R. van de Krol, and D. E. Starr
Marco Favaro et al 2021 J. Phys. D: Appl. Phys. 54 164001

We present a newly developed end-station at BESSY II dedicated to in situ Spectroscopic Analysis with Tender X-rays (SpAnTeX). The core of the end-station is a new SPECS PHOIBOS 150 HV NAP electron spectrometer. First, we show that the system has successfully achieved high electron transmission and detection efficiency under gas pressures up to 30 mbar and photon energies ranging between 200 eV and 10 keV. Second, using two features of this spectrometer (a new lateral resolution lens and a 3D delay line detector), we show that the endstation enables collection of the photoelectron spatial distribution under realistic working conditions (p ≥ 20 mbar) with a resolution better than 30 μm and the possibility to perform time resolved studies using a continuous tender X-ray source. We conclude by reporting an example of the possible experiments that can be performed using this new endstation using the Dip-and-Pull technique.

Although mainly focused on the characterization of solid/liquid interfaces using AP-HAXPES, the end-station can be used at soft X-ray beamlines for more traditional AP-XPS experiments. The Dip-and-Pull module also demonstrates good electrochemical performance. The wide pressure and photon energy range covered by this end-station also enables investigations of solid/solid, solid/gas, liquid/vapor and liquid/liquid interfaces at pressures up to 30 mbar with tender X-rays.

M. Favaro, P. C. J. Clark, M. J. Sear, M. Johansson, S. Maehl, R. van de Krol, and D.E. Starr
Surface Science
Volume 713, November 2021, 121903

Ever since the invention of photoelectron spectroscopy, researchers have attempted to analyze materials under conditions
closely resembling their application environment. Near-Ambient Pressure X-ray Photoelectron Spectroscopy is a logical
development in this quest, since it allows for analyzing non-vacuum compatible samples in general, and phase boundaries,
such as solid|liquid or solid|gas interfaces, in particular. With the development of spectrometer systems compatible with
analysis pressures of up to 100 mbar, many novel experimental geometries have been realized since the early 2000s. Since
then, experimental capability and variety have further progressed through the proliferation of off-synchrotron laboratory
systems, and advanced sample environments to simulate material usage conditions. This progress has, e.g., enabled the
performance of operando spectroscopy during catalytic or electrochemical experiments. The present work gives, from an
instrumental point of view, a short overview over basic system design considerations and recent developments in the field.

M. Weidner and V. Streibel
表面と真空 Vol. 65, No. 3, pp. 133–138, 2022