Ambient Pressure X-Ray Photoelectron Spectroscopy at the IOS (23-ID-2) Beamline at the National Synchrotron Light Source II

Introduction Studying energy materials under realistic operating conditions is necessary to reveal chemical and electronic properties as well as fundamental processes that determine the functional properties of the materials. This has been the driving force for the development of various in-situ and operando experimental techniques. Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) has emerged as one of the most powerful tools for the in-situ investigation of the surfaces and interfaces of such energy materials, on which the critical surface processes and reactions occur, thanks to its inherent surface sensitivity, elemental specificity, and sensitivity to different chemical environments. The ability to perform AP-XPS experiments at pressures ranging from the typical tens of millibars to a few bars [1] has enabled scientists to close the so-called “pressure gap” between real industrial processes and surface science experiments typically performed under ultra-high vacuum (UHV) conditions. As a result, AP-XPS instruments have proliferated around the world in the past two decades, starting at synchrotron light sources, followed by lab-based instruments [2]. As one of the newest and brightest synchrotron light sources in the world, the National Synchrotron Light Source II (NSLS-II), a U.S. Department of Energy (DOE) Office Of Science user facility located at DOE’s Brookhaven National Laboratory (BNL), offers new and exciting opportunities for energy research using in-situ and operando X-ray techniques, including AP-XPS [3]. The In situ and Operando Soft Xray Spectroscopy beamline (IOS, 23-ID-2) [4], formerly called CSX2, was part of the first group of beamlines to open to general users at NSLS-II, where the AP-XPS user program has been thriving since 2016. In this technical report, we present a description of the current state of the IOS beamline and AP-XPS endstation, examples of recent scientific highlights, as well as an overview of future developments.