Understanding solid-gas and solid-liquid interfaces through near ambient pressure X-ray photoelectron spectroscopy

Abstract

The surface of energy material is the direct place where energy storage and conversion reactions occur. Thus, the surface chemistry and the structure of the material under real reaction conditions are the key descriptors to clarify the reaction mechanism. However, such surfaces are usually immersed in gaseous or liquid environments under real reaction conditions, so it is not a simple task to identify the real physical and chemical properties of the interface under in situ conditions. X-ray photoelectron spectroscopy (XPS), as a surface-sensitive technique, is one of the main techniques for studying complex composition and electronic structure of material surfaces. However, due to the limited mean free path of photoelectrons in gas, liquid and solid media, the traditional XPS is confined to vacuum conditions, which poses a significant obstacle for studying solid-gas and solid-liquid interfaces under in situ conditions. With the introduction of differentially pumped analyzers and electrostatic lenses system, this limitation no longer restricts XPS only suitable for ultra-high vacuum conditions. With the active development of synchrotron radiation sources worldwide, near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) offers advanced features combined with the benefits of synchrotron radiation sources. Compared to traditional X-ray source, synchrotron radiation sources have significantly higher photon flux and much smaller spot size, which enables more electrons to escape to the electron analyzer, therefore can effectively improve the signal-to-noise ratio and the maximum working pressure, and the continuous wavelength tunability of synchrotron radiation makes experimental measurements more flexible and provides more information on the surface reaction. Over the years, NAP-XPS has rapidly emerged as an influential tool for investigating various solid-gas and solid-liquid interfaces, reflecting the importance of understanding reaction mechanisms and structure-performance relationship of materials under conditions closer to practical reacting conditions, particularly in heterogeneous catalysis. Information at atomic scale can be delivered with surface and interface sensitivity by NAP-XPS in conjunction with several advanced spectroscopy and microscopy techniques. In this paper, we provide a concise overview of recent notable advancements in NAP-XPS to showcase the novel insights generated by research on solid-gas and solid-liquid interfaces in cutting-edge scientific fields. This demonstrates how the knowledge gained from NAP-XPS studies can contribute to a fundamental understanding of reaction mechanisms at a molecular level. Finally, we discuss new challenges and prospects to ensure a comprehensive understanding of this technique and, hopefully, inspire fresh ideas.