Real-Time Observation of Surface Chemical Reactions Proceeding in the Depth Direction by Wavelength-Dispersive Soft X-ray Absorption Spectroscopy

Abstract

It has long been a challenging task to observe surface chemical reactions proceeding in the depth direction without stopping the reaction (i.e., real-time observation), with subnanometer depth resolution. X-ray absorption spectroscopy (XAS) in the soft X-ray region is one of the most crucial methods to analyze the chemical states of light elements; however, simultaneous time- and depth-resolved XAS measurements during the reaction have not been realized before. Herein, we show the real-time observation of the time-evolving oxidation process of a Co thin film with a fluorescence-yield wavelength-dispersive XAS method, which also has subnanometer depth resolution. The series of Co L-edge XAS spectra show that the oxide component increases from the surface to deeper regions as the reaction proceeds. The progress of oxidation in the depth direction was interpreted by the reaction with O2 gas at the surface and the interlayer reaction by oxygen migration.

Time- and Depth-Resolved Chemical State Analysis of the Surface-to-Subsurface Oxidation of Cu by X-ray Absorption Spectroscopy at Near Ambient Pressure

“…Figure b shows the experimental setup in side view for the wavelength-dispersive measurements. Wavelength-dispersive soft X-rays, in which the wavelength (photon energy) continuously changes as a function of position, are obtained at the exit slit of the soft X-ray monochromator ,, and are reimaged onto the sample by a toroidal mirror. The soft X-rays are absorbed at each position of the sample, and the fluorescence soft X-rays emitted from the sample are collected by the fluorescence X-ray imaging system consisting of two spherical mirrors, so that X-ray fluorescence emitted from each position on the sample is focused onto the detector separately.…”

“…Figure a shows a photo of the fluorescence-yield wavelength-dispersive XAS apparatus used for analysis in the soft X-ray region, and Figure b shows a schematic of the near ambient-pressure setup used in this study (see the Experimental Methods section for further information). Details of the apparatus have been reported previously. , A two-dimensional detector was used for simultaneous wavelength-dispersive [to acquire the energy spectrum (vertical axis)] and angle-resolved [for the depth-profile analysis (horizontal axis)] measurements. Thus, neither monochromator scanning nor angle changes are required, thus enabling time-resolved observation.…”

“…As a result of these improvements, the progress of reactions occurring from the surface to the bulk with time can be investigated, and we have recently reported our investigation of the surface oxidation of Co thin films using this method in air at ca. 10 Pa. 29 Because Co is very reactive, the Co film is oxidized, even in the presence of a small amount of oxygen.…”

“…Details of the apparatus have been reported previously. 26,29 A two-dimensional detector was used for simultaneous wavelength-dispersive [to acquire the energy spectrum (vertical axis)] and angle-resolved [for the depth-profile analysis (horizontal axis)] measurements. Thus, neither monochromator scanning nor angle changes are required, thus enabling time-resolved observation.…”

Development of Fluorescence-yield Wavelength-dispersive Soft X-ray Absorption Spectroscopy for Real-time Observation of Surface Chemical Reaction

“…Since the photon energy of the fluorescence soft X-rays does not depend on that of the incident beam, the probing depth is kept almost constant in the case of the normal incidence and grazing emission configuration. Moreover, the depth-resolved analysis can be applied during the real-time observation because a set of XAS data with different emission angles, corresponding to the different probing depths, are simultaneously recorded by using the two-dimensional detector as shown in Figure 1 [11].…”