Photon Factory Activity Report 2017 #35 (2018)

BL-9A/2016G035, 2017G628

Extracting the surface states of Pt films under electrochemical environments using
total reflection fluorescence X-ray absorption fine structure spectroscopy
Yuki Wakisaka1, Hiromitsu Uehara1, Qiuyi Yuan1, Daiki Kido1, Takahiro Wada2, Yohei Uemura3,Yutaro Kamei4, Seiichi Kuroda4, Akihiro Ohira5, Satoru Takakusagi1, and Kiyotaka Asakura1*
1Institute for Catalysis, Hokkaido University, Hokkaido, 001-0021, Japan
2
Tokyo Medical and Dental University, Tokyo, 113-8549, Japan
3
Institute for Molecular Science, Aichi 444-8585, Japan
4FC-Cubic TRA, Tokyo, 135-0064, Japan
5National Institute of Advanced Industrial Science and Technology, Ibaraki, 305-8568, Japan
1 Introduction
Improvements in catalytic performances of fuel cells isessential for their widespread application and realizing aclean energy society. In fuel cells, Pt nanoparticles
covered with ionomer play a crucial role for the catalytic
reactions. It is, therefore, of great importance to elucidate
the fundamental surface structures of ionomer-coated Pt
under electrochemical environments. In this respect, amodel system of Nafion-coated Pt polycrystalline filmwith 30 nm thickness was investigated by in-situ totalreflection fluorescence X-ray absorption fine structure
spectroscopy (TRF-XAFS). The possibility of extracting
the local adsorption states of Pt films is presented in thisreport.
2 Experiment
A 30-nm-thick Pt polycrystalline film was grown by a
sputter deposition on a 1-inch-diameter Si (100) wafer,followed by ultraviolet-ozone cleaning. This Pt film
sample was coated by a 15-nm-thick Nafion by spin
coating.
TRF-XAFS spectra were measured at BL-9A. Thefluorescence X-ray signal was measured by using a 19element Ge solid-state detector placed perpendicular to
the X-ray beam direction. A 400-µm pinhole was used toregulate the size of the incident X-ray to reduce elastic
scatterings. A 5-axis goniometer was used to adjust thetotal reflection conditions. An X-ray CCD was placed
most downstream from the cell to monitor the X-ray
incident angle which was adjusted as 3.5 mrad,
approximately half of the theoretically-calculated criticalangle of Pt. Transmission spectra of Pt foil were takensimultaneously using the total reflected X-ray for energy
calibration.
The sample was inserted in an in-situ electrochemical
XAFS cell made of polychlorotrifluoroethylene (PCTFE)
with a 2.5-µm-thick polyester membrane as an X-raywindow. The electrolyte solution,the counter andreference electrodes were 0.1 M HClO4,a Pt coil, and areversible hydrogen electrode, respectively. Whilechanging the sample potential, the membrane window
was inflated by applying a high enough pressure of thesolution to assure ion diffusion. During the XAFS
measurements, in contrast, the window was deflated byapplying a slightly negative pressure to make the solution
layer sufficiently thin between the sample surface and themembrane window. Thus, the sample was notmechanically moved between measurements at different
potentials once the total-reflection condition was fixed.
The X-ray window was placed under humid N2 gas using
an X-ray transparent cup. The X-ray reflectivity wasconstantly checked that it only varies within about ±5% at
a certain energy.
3 Results and Discussion

Fig. 1: Normalized XANES spectra at different potentials.
The difference spectra Δμ=μX-μ0.40 V are also shown withvertical shifts.
Figure 1 shows the potential dependence of thenormalized Pt L3 X-ray absorption near edge structure(XANES) spectra at P polarization with their difference
spectra (Δμ=μX-μ0.40 V) overlaid. The potentials wereapplied starting from 0.40 V to 0. 70 V, 0.75 V, 0.40 V,
and 0.15 V. The Pt surface states at 0.15, 0.40 V, and0.70-0.75 V correspond to adsorption of hydrogen,double-layer, and adsorption of oxygen species,
respectively. Compared to the double-layer potential (0.40V), an increase in the spectral intensity at around 11564
eV for 0.70 and 0.75 V, and a slight decrease at around11568 eV for 0.75 V are observe clearly. On the other
hand, a broad increase in the spectral intensity centered at11567 eV is observed for 0.15 V. These characteristics
are consistent to the previous XANES spectra observed inPt nanoparticles [1, 2]. Therefore, it is confirmed that theXAFS information from metal/solution interface in Pt
films having limited surface/bulk ratio can be captured byusing TRF-XAFS. The feasibility of TRF-XAFS for
capturing the surface states of Pt films enables us toPhoton Factory Activity Report 2017 #35 (2018)

directly compare XAFS information with other surfacesensitive measurements, which will help us understandthe surface or interface structures comprehensively.
Acknowledgement
This work was supported by the New Energy andIndustrial Technology Development Organization(NEDO) of the Ministry of Economy, Trade, and Industry(METI), Japan.
References[1] M. Teliska et al., J. Phys. Chem. B 109, 8076 (2005).
[2] K. Asakura et al., J. Synchrotron. Radiat. 6, 439(1999).
* askr@cat.hokuda.ac.jp