Photon Factory Activity Report 2014 #32 (2015) B

BL-27A/2014G553

Chemical-state Analysis of Ultra-trace Amount of Cesium Adsorbed on Oxide
Surface by Total Reflection X-ray Photoelectron Spectroscopy
Yuji Baba*, Iwao Shimoyama, Norie Hirao, Toshinori Izumi
Japan Atomic Energy Agency, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan
1 Introduction
Radioactive cesium (137Cs or 134Cs), once trapped inminerals such as clay and soil, is hard to be released.
Recent spectroscopic measurements have shown that
cesium at a specific site in minerals is strongly bound [1].
It should be noted that the number of cesium atoms inradioactive cesium is extremely small compared with thatobserved by spectroscopic methods. Since the chemical
behavior of such ultra-trace amount of elements issometimes quite different from that of macro-quantity, it
is important to clarify the chemical states of ultra-trace
amount of cesium adsorbed on mineral surfaces in orderto know why radioactive cesium is hard to be released. In
this report, we demonstrate that chemical states of ultratrace amount of cesium on oxides can be measured bytotal reflection X-ray photoelectron spectroscopy (TRXPS) [2].
2 Experiment
The oxide samples used were silicon dioxide, which
was made by oxidizing Si(001) single crystal at 1000ºCfor 2 hours. The SiO2 substrate was immersed in 10 mM
cesium chloride solution for two days, and dried in
atmosphere. Measurements were performed at the BL27A. For normal XPS measurements, the X-rays wereirradiated at 35º from the surface. For TR-XPS, theincident X-rays were irradiated at 0.2º that is lower thanthe critical angle of total reflection.
3 Results and Discussion
Figure 1 shows wide-scan XP spectra for clean surfaceof SiO2 irradiated by two incident angles. For 0.2ºincidence (TR-mode), the background around 720 eV,
where Cs 3d peak is located, reduced less than 1/10
compared with that for 35º incidence. This is because theincident X-rays were totally reflected, so the amount of
inelastically scattered electrons produced by O 1s
photoelectron emission fairly decreased.
Spectrum (a) of Fig. 2 shows the narrow-scan TR-XPSin Cs 3d region for Cs-adsorbed SiO2. Next, the Csadsorbed sample was rinsed by ultrasonic cleaning inwater for 15 min. The spectrum for the sample afterrinsing is shown (b). Before rinsing, the energy of the Cs
3d5/2 peak is located at 725.5 eV, which is close to thereported value for cesium oxide [3]. The 3d5/2 peakenergy was almost constant irrespective of the adsorptioncondition. After rinsing, the intensity of Cs 3d5/2 fairly
decreased but a small peak is still seen. Thickness of thissample estimated from XPS peak intensities was about
4.4 pm, which corresponds to 2.2×10-3 layers. TheFig. 1: Wide-scan XP spectra for clean surface of SiO2
irradiated by two incident angles (35º and 0.2º).
amount of cesium in this layer is only about 205 pg∙cm-2.
This weight corresponds to about 400 Bq 137Cs. Thus it isdemonstrated that ultra-trace amount of cesiumcomparative to radioactive 137Cs level can be detected byTR-XPS.
As to the chemical states, the Cs 3d5/2 peak of spectrum(b) shifted by 0.7 eV to higher binding energy comparedto that of spectrum (a). It was reported that in contrast tomost of metals, the higher the polarization of Cs-X (X:anion) bond is, the lower the Cs binding energy becomes[4]. Thus the Cs-O bonds remaining after rinsing is lesspolarized than those before rinsing. This suggests that theultra-trace amount of cesium is covalently bonded withoxygen rather than ionically bonded.
References

Fig. 2: Narrow-scan TR-XP spectra for SiO2 in Cs 3dregion. (a) Cs was adsorbed by solution
method. (b) Cs-adsorbed sample was rinsed byultrasonic cleaning.
[1] C. Benjamin et al. Environ. Sci. Technol. 36, 2670
(2002).
[2] J. Kawai, J. Electron. Spectrosc. Relat. Phenom. 178179, 268 (2010).
[3] M. Ayyoob, M.S. Hegde, Surf. Sci. 133, 516 (1983).
[4] E.A. Podgornov et al. J. Mol. Catal. A: Chemical 158,
337 (2000).
* baba.yuji@jaea.go.jp