Supplemental Material
in situ X-ray Photoelectron Spectroscopy
for Electrochemical Reaction
in Ordinary Solutions
Takuya Masuda,1 Hideki Yoshikawa,2 Hidenori Noguchi,1,3,4 Tadahiro Kawasaki,5 Masaaki
Kobata,2 Keisuke Kobayashi,2 and Kohei Uosaki1,3,4*
1
Global Research Center for Environment and Energy based on Nanomaterials Science
(GREEN), National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan
2
Synchrotron X-ray Station at SPring-8, National Institute for Materials Science (NIMS),
Sayo, Hyogo 679-5148, Japan
3
Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo,
Hokkaido 060-0810, Japan
4
International Center for Materials Nanoarchitechtonics (WPI-MANA), National Institute for
Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan
5
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603,
Japan
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Preparation steps of the micro-volume cell.
Figure S1 shows preparation steps of the micro-volume cell filled with water, sealed by a 15
nm-thick Si membrane. Adhesive of the Cu tape was used for a vacuum seal to the water in
the cavity of the Si membrane. This adhesive also served as an insulator layer between frame
and Cu. A bias voltage application was performed as previously reported.1 The potential of the
Si membrane with respect to the Cu tape was controlled by a potentiostat (HSV-110, Hokuto
Denko). Since not only the 15 nm-thick Si membrane but also the frame is in contact with
water, the I-V data (Fig. 2 (A) blue) is representative of the membrane and frame.
Figure S1. Preparation steps of the micro-volume cell filled with water, sealed by a 15
nm-thick Si membrane.
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Derivation of eq. 1.
IA, IB, and IC, photoelectron intensities from the layers A, B, and C, can be described as eqs.
S1, S2 and S3, respectively.


1  exp( d /  )
d
I A  N SiO2  exp  x / SiO2  dx
0
I A  N SiO2  SiO2

eq. S1
SiO2

I B  N Si  exp  d / SiO2  

Da
0

exp  x / Si   dx
I B  N Si  Si  exp  d / SiO2  1  exp( D  a  / Si )


I C  N SiO2  exp  d / SiO2  exp( D  a  / Si )  

d b
0

eq. S2


exp  x / SiO2  dx

I C  N SiO2  SiO2  exp  d / SiO2  exp( D  a  / Si )  1  exp( d  b  / SiO2 )

eq. S3
Since the total thickness of the Si membrane is 15 nm,
D  15  2d
eq. S4
The decrease in thickness of layer B and the increase layer C are a nm and b nm, respectively.
Those can be described as eq. S5 from the Si atom densities of Si, NSi, 5.00 × 1022 atoms/cm3,
and Si oxide, NSiO2, 2.28 × 1022 atoms/cm3.2
1
a / N Si1  b / N SiO
2
b  2.2a
eq. S5
Thus, eq. 1 is obtained.
I SiO2
I Si
I SiO2
I Si


I A  IC
IB




N SiO2  SiO2 1  exp(  d SiO2 )  N SiO2  SiO2 1  exp(  (d  2.2a) SiO2 )  exp(  d SiO2 )  exp(  15  2d  a  Si )
N Si  Si 1  exp(  15  2d  a  Si ) exp(  d SiO2 )
eq. 1
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where SiO2 (12.2 nm), and Si (10.3 nm) are the inelastic mean free paths of the Si 2p
photoelectrons for Si oxide and Si, obtained using the TPP-2M formula.3 The photoelectron
intensities of the Si oxide from the layers A and C, ISiO2, and of Si from layer B, ISi, are
obtained by integrating the Si4+ peak and the doublet peaks corresponding to the Si 2p3/2 and
2p1/2, respectively (Fig. 2 (B)). Thus, the potential and time-dependent thickness of each layer,
reflecting the anodic growth of the oxide can be quantitatively determined, as shown in Fig.
S2 (C).
Figure S2. Schematic models of the Si membrane (A) before and (B) after the oxidation. (C)
Thickness change of the three layers of the Si membrane calculated from eq. (1).
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References
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F. J. Himpsel, F. R. Mcfeely, A. Talebibrahimi, J. A. Yarmoff, and G. Hollinger, Phys. Rev. B
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