Observation of surface reduction of NiO to Ni by surface-sensitive total reflection X-ray spectroscopy using Kramers–Kronig relations

Abstract

We have developed a promising surface-sensitive X-ray absorption fine structure (XAFS) measurement method. This method is based on total reflection detection and Kramers–Kronig relations, and has been named the KK-XAFS method. Total reflection spectra are transformed via Kramers–Kronig relations to obtain XAFS spectra. KK-XAFS experiments give us surface-sensitive structural parameters, while usual EXAFS analyses yield bulk structural parameters. The total reflection spectra themselves are useful for observing and discussing time evolutions of chemical reactions at surfaces by quick scanning measurements. Chemical species are analyzed to estimate their fractions during reactions. The whole method would be named total reflection X-ray spectroscopy (TREXS). A reduction of the NiO layer at the surface of Ni (30 nm)/Si was observed in a laboratory-built TREXS in situ cell. The method would be applicable to observe chemical reactions starting at surfaces and to study their kinetics and mechanisms.