Photon Factory Activity Report 2016 #34 (2017)

BL-27A/2016G005

Chemical States of Trace-level Strontium Adsorbed on Layered Oxide by XPS andXANES under Total Reflection Condition
Yuji Baba1,* and I. Shimoyama2
Japan Atomic Energy Agency, 7-1 Aza-Omachi, Taira, Iwaki-shi, Fukushima 970-8026, Japan
2
Japan Atomic Energy Agency, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan

1

1 Introduction
The behavior of hazardous radionuclides released into
the environment as a result of accident of a nuclear power
plant or a nuclear weapon test is of considerable worldwide
concern. Among these nuclides, 90Sr which emits β-rays
with a half-life of 28.8 years is one of the most hazardous
radionuclides, because it has a property to be strongly
retained in living organisms, mainly in the bones [1]. It has
been elucidated that clay minerals present a high sorption
capability for strontium [2]. In order to clarify the behaviorof Sr in environment, the adsorption states of strontium inclay minerals is of great importance. The chemical statesof Sr in clay minerals have been investigated by EXAFS[3] and X-ray absorption spectroscopy [4]. However, it
should be noted that the number of atoms in radioactive90
Sr is extremely small compared with that in nonradioactive strontium. The chemical property of such tracelevel element is in many cases different from that of macroquantity. In this report, chemical states of Sr adsorbed on alayered oxide (mica), which is the main trapping site of Srin clay, have been investigated by X-ray photoelectron
spectroscopy (XPS) and X-ray absorption near edge
structure (XANES) spectroscopy. In order to observe
trace-level Sr, XPS and XANES were measured under totalreflection condition of incident X-rays.

Fig. 1 shows the XPS wide-scan spectra of Sr-adsorbed
mica for two incident X-ray angles. The background in theTR-XPS around 2000 eV where Sr 2p peak is located isreduced to less than one-tenth of that in normal XPS. Thisis because the inelastically scattered electrons from thebulk due to the Si 1s photoelectron emission disappear inTR mode. Owing to the background reduction and the2 Experimental
The mica used was a single crystal of artificiallysynthesized phlogopite. The chemical composition of themica was KMg3AlSi3O10F2. Sr was adsorbed on mica by
immersing it in 20 mmol•dm-3 strontium nitrate solution.
In order to simulate the decontamination procedure, Sradsorbed mica was washed by various procedures such assplashing water and immersing in water.
The XPS and XANES were measured at the BL-27A
station. For normal XPS, the X-rays were irradiated at 35ºfrom the surface and the take-off direction of thephotoelectrons was surface normal. In the TR-XPS mode,
the incident X-rays were irradiated at 0.2º from the surface
which is smaller than the critical angle of the totalreflection at 3 keV photons (0.65º). The take-off directionof photoelectrons in TR-XPS was surface normal. In theTR-XANES measurements, the angle of the incident Xrays was 0.2º from the surface. TR-XANES spectra weremeasured by plotting a sample drain current as a functionof the photon energy (total-electron-yield mode).

enhancement of the Sr 2p3/2 peak in TR-XPS, we could
observe ultra-thin layer down to 0.004 nm. This thicknesscorresponds to 150 pg•cm-2 (1.03×1012 atoms•cm-2). If allof the Sr is composed of 90Sr, this weight corresponds toabout 300 Bq of 90Sr.
Fig. 2 shows the TR-XPS narrow scans in Sr 2p3/2 region
for Sr-adsorbed mica with various thickness. From theintensity of the Sr 2p3/2 and Si 1s peaks, we can estimate
the averaged thickness of adsorbed layer which is shownin each spectrum. On the right of Fig. 2 displays the Sr 2p3/2
peak energy as a function of the thickness. When thelogarithmic scale is set on the thickness, the Sr 2p3/2 peakenergy decreases almost in proportion to the decrease in
the thickness.
On the left of Fig. 3 shows TR-XANES spectra at the Sr
L3-edge for Sr-adsorbed mica with various thickness. The
thickness of the strontium layer is indicated in eachspectrum. In all spectra, sharp resonance peaks areobserved around 1943 eV. Based on the probabilities of
dipole allowed transitions, the main peaks can be identified
as the resonance excitations from the Sr 2p3/2 ground statesto the valence unoccupied 4d* states [5]. On the right ofFig. 3, the resonance peak energy is plotted as a function

3 Results and Discussion

Fig. 1: XPS wide scan spectra of Sr-adsorbed mica for
two X-ray incidence angles. Photon energy was 3000
eV.

Photon Factory Activity Report 2016 #34 (2017)

Fig. 2: Left figure shows the TR-XPS narrow scan
spectra in Sr 2p3/2 region for Sr-adsorbed mica.
Thickness of the strontium layer is indicated in eachspectrum. Right figure displays the relation between
the layer thickness and the binding energy of Sr 2p3/2line.

Fig. 3: Left figure shows the Sr L3-edge TR-XANES
spectra for Sr-adsorbed mica. Thickness of thestrontium layer is indicated in each spectrum. Right
figure displays the relation between the layer thickness
and the resonance peak energy.

of the thickness. Similar to the Sr 2p3/2 peak in XPS, theresonance peak energy decreases almost in proportion tothe decrease in the thickness.
For XPS, a chemical shift of core-level binding energy
is caused by various factors such as charge transfer withina simple point-charge, the long-range electrostaticinteractions (Madelung potential), and final-state
contributions due to extra-atomic relaxation of the core–hole (screening effect). But the contributions of the latter
two factors are negligible in the present experiments
because thin films less than monolayer were measured(Madelung potential can be ignored) and the insulating
substrates were used (screening effect is negligible). Thus,
we will qualitatively discuss the origin of the observed
binding-energy shifts on the basis of the charge transfer
within a simple point-charge.
When we consider a compound M-X (M: metal, X:anion), the higher the polarization of an M-X bond is, thehigher the core-level energy of M becomes. In other words,
when the  value in M+-X- bond becomes large, the corelevel of M shifts to higher energy side. This tendency is
generally observed in most of the metals. However, thisgeneral tendency does not hold for some of alkali metals
and alkali-earth metals. Strontium compounds show thereverse tendency to the most of the other metal compounds.
Vasquez has measured Sr 3d5/2-XPS spectra for various
strontium compounds, and has shown that the higher thepolarization of a Sr-X bond is, the lower the Sr 3d5/2 energy
becomes [6]. For example, the energy of Sr 3d5/2 core-levelis in the order of SrI2>SrBr2,SrCl2>SrF2 [6]. Also it wasreported that energy of Sr 3d5/2 core-level energy decreases
on going from Sr-O to Sr-F [7]. Considering the differencein the tendency of the energy shifts between general metals
and strontium as discussed above, it is deduced that thechemical bond between trace level strontium and mica ismore polarized than that for thick layer. This means thatthe ultra-trace amount of strontium adsorbed on mica ismore ionically bonded with the substrate compared withthe thicker layer.
These speculations are supported by the peak energy
shift in the XANES spectra (Fig. 3). The main peakassigned as Sr 2p3/2→4d* resonance shifts to lower energy
side with the decrease in the thickness, which is similar to
the Sr 3d5/2 core-level shift in XPS . But it should be notedthat the energy shift of resonance peak in XANES spectra
is caused by those of both Sr 2p3/2 (initial) and 4d* (final)
states. As to this point, we consider the resonance energy
shift in the XANES spectra is mainly caused by the Sr 2p3/2
energy shift, in comparison with the 3d transition-metalcompounds where the direction and absolute value of the2p3/2→3d* resonance energy shift in XANES is similar tothat of 2p3/2 in XPS [8]. Also it was reported that the Ti 2p
level in titanium oxides is more sensitive to the chemical
environment than Ti 3d* levels [9]. On the basis of these
speculations, the energy shift in Fig. 3 similar to that in
XPS supports that the chemical bond between trace level
strontium and mica is more polarized than that for thicklayer.
Although more detailed analysis of the chemicalbonding states is needed from both experimental andtheoretical viewpoints, it is suggested that the ionicallybonded state of ultra-trace amount of strontium with mica
is one of the reasons that the radioactive strontium is hard
to be released from clay minerals.
References[1] P. Froidevaux et al., Sci. Total Environ. 367, 596 (2006).
[2] D. Mallants et al., Nucl. Mater. 298, 125 (2001).
[3] S. Choi et al., Environ. Sci. Technol. 40, 2608 (2006).
[4] N. Sahai et al., J. Colloid Interface Sci. 222, 198 (2000).
[5] T.A. Sasaki, Y. Baba, K. Yoshii, H. Yamamoto, J.
Electron Spectrosc. Relat. Phenom. 79, 229 (1996).
[6] R.P. Vasquez, J. Electron Spectrosc. Relat. Phenom. 56,
217 (1991).
[7] V. Young et al., Appl. Surf. Sci. 20, 228 (1985).
[8] J.G. Chen, Surf. Sci. Rep. 30, 1 (1997).
[9] M. Yoshiya et al., J. Phys.: Condens. Matter 11, 3217
(1999).
* baba.yuji@jaea.go.jp