Photon Factory Activity Report 2022 #40 (2023)

BL-16A/2020G595

Time- and Depth-resolved Chemical State Analysis of the Surface-to-subsurface
Oxidation of Cu by Wavelength-dispersive soft X-ray Absorption Spectroscopy
at Near Ambient Pressure
Kaoruho SAKATA 1*, and Kenta AMEMIYA 1
1
Institute of Materials Structure Science,High Energy Accelerator Research Organization,
1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
1 Introduction
The X-ray absorption spectroscopy (XAS) in soft x-ray
region has an advantage in its significant interaction withlight elements in the surface region. It typically requires
high-vacuum conditions, however, recently XAS
measurements at higher pressures have been realized byusing appropriate thin film windows (e.g. Si3N4 windows),
and this could enable the observation of surface chemical
states during reactions at ambient pressure.
In addition, to follow the chemical reaction, real-timemeasurement is required, but the absorption intensity is
obtained at each energy in a step-by-step fashion using a
monochromator, which makes in situ, real-timemeasurements impossible in conventional XAS. To
conquer the drawback of the measurement time, wedeveloped a method to illuminate the sample withdispersed soft x rays and separately collecting thefluorescent x-ray emitted at each position on the sample [1].
In addition, non-destructive depth-resolved measurements[2] have been combined with dispersive XAS, and thisenables depth profiles of chemical states having a
resolution of 1 nm (that is, the near-surface region) in realtime [3].
In this study, we carried out these measurements at
higher pressures using Si3N4 windows to separate thevacuum required for equipment operation and the ambient
air required for sample oxidation. This enabled us to study
the oxidation of less reactive metals. Thus, we report thereal-time observation of the surface oxidation of the Cu
surface using fluorescence-yield wavelength-dispersive
XAS with depth-resolved measurements. We carried out
Cu L3-edge XAS measurementsin real time in air atpressures up to ca. 5000 Pa.

resolved measurements with a resolution of ca. 10 s. The
top-view in Fig. 1a shows the setup for depth-resolvedanalysis. The depth-dependent XAS data are obtained
simultaneously, allowing the reaction occurring in thedepth direction to be monitored with time, as shown in Fig.
1b. Further detail is shown in ref. [3, 4].

2 Experiment
Measurements were carried out using fluorescence-yieldwavelength-dispersive XAS to enable the real-timemeasurement of soft X-ray absorption spectra with depthresolved analysis [2]. Fig. 1 shows the experimental setup
for the wavelength-dispersive measurements. Wavelengthdispersive soft X-rays are illuminated to be absorbed ateach position of the sample, and the fluorescence soft Xrays emitted from the sample are collected by thefluorescence X-ray imaging system consisting of two
spherical mirrors so that X-ray fluorescence emitted fromeach position on the sample is focused onto the detector
separately. The setups used this study realized time-All experiments were performed at beamline BL-16A. A
soft X-ray CCD camera was used as the detector. A 10-mm
diameter Cu(001) single crystal was placed in themeasurement chamber and was cleaned by Ar+
bombardment. Subsequently, the XAS spectra of the Cu
surface were measured continuously while air wasintroduced from pressures of ca. 10-5 to 5000 Pa.

(a)

(b)

Fig. 1: (a) Schematic of the optical configuration in the sideand top views. (b) The spectra around Cu L3-edge wereobtained in real time, and the depth distribution of thechemical species around the surface was obtained at eachtime step.

3 Results and Discussion
In wavelength-dispersive mode, the Cu L3-edge XAS
spectrum of the Cu surface in air at 500 Pa at room
temperature was obtained, as shown in Fig. 2a, andsubsequently spectra were obtained as the pressure wasincreased to 5000 Pa, as shown in Fig. 2b. At 500 Pa, thePhoton Factory Activity Report 2022 #40 (2023)

intensity of the peak at approximately 932 eV increased
markedly with time (Fig. 2a), suggesting oxidation
progress, i.e. the formation of CuO on the Cu surface. At
5000 Pa, the intensity of the peak at approximately 934 eV
increased, as shown in Fig. 2b, suggesting the formation ofCu2O at this pressure.

by the interdiffusion of Cu from inner layer and that ofCu2O to inner layer. The reported technique hasapplications for the analysis of surface phenomena atambient pressure, whose understanding is crucial in many
fields, from chemistry to structural engineering.

(c)Fig. 2: (a) Cu L3-edge XAS spectra of Cu sequentially
recorded (from bottom to top) in wavelength-dispersive
XAS mode in air at (a) 500 Pa and increased to (b) 5000
Pa. (c) Change in chamber pressure with time as ambient
air was introduced.
Next, we examine the depth-resolved spectra using six
probe depths (λ) from 0.6 to 1.6 (Fig. 3a). The depthresolved spectra were obtained at each time step andnumerically fitted to the standard spectra of Cu, CuO, andCu2O (Fig 3b). Please refer ref. [4] for further detail. As
shown, in the initial stages (before 500 s and below ca. 500Pa), Cu was dominant but some Cu2O was present aroundthe surface. Once the air pressure reached 500 Pa, theamount of CuO rapidly increased, reaching a ratio of 0.6–
1. The composition ratio is larger at smaller probing depth,
which directly suggests that CuO was formed mainly
around the surface. From these results, the chemicalcomposition was strongly depth dependent, therefore, weanalyzed the data and quantified the depth-dependent
composition Ci(z) (see ref. [4]). Fig. 4 shows the contour
map of the Ci(z) profile corresponding to exposure time.
From Fig. 4a, Cu is dominant at depths below 1 nm until
the pressure reaches 500 Pa, and during this time, Cu2O
exits around the surface as shown in Fig. 4c. At 500 Pa,
CuO appears, especially at depths above 1 nm, and theCuO-rich region gradually expands into the bulk, as shownin Fig. 4b. Subsequently, the amount of CuO decreases
after the pressure is increased up to 5000 Pa, eventuallyremaining only at the surface. In contrast, Cu2O appears atdepths below 1 nm at 5000 Pa, as shown in Fig. 4c. These
data suggest a unique oxidation mechanism: CuO forms on
the top surface and subsequently Cu2O forms in thesubsurface layers (> 1 nm from the surface), accompaniedFig. 3: (a) Example spectra obtained at probing depths (λ)
of 0.6 (the most surface sensitive) and 1.6 nm (the deepestregion) ca. 1970 s from the beginning of measurement.
CuO and Cu2O ratios were estimated by fitting to thestandard spectra. (b) Ratio of Cu, CuO, and Cu 2O withrespect to time at λ = 0.6 and 1.6 nm.
(a)

(b)

(c)

Fig. 4: The contour map of the Ci(z) profile for (a) Cu, (b)CuO and (c) Cu2O, with respect to exposure time.
References[1] K. Amimia et al., Rev. Sci. Instrum. 91, 093104 (2020).
[2] M. Sakamaki et al., Rev. Sci. Instrum. 88, 083901(2017).
[3] K. Sakata, M. Suzuki-Sakamaki, K. Amemiya, Nano
Lett. 21, 7152 (2021).
[4] K. Sakata, K. Amemiya, J. Phys. Chem. Lett., 13 9573
(2022).
* kaoruho.sakata@kek.jp