Photon Factory Activity Report 2018 #36 (2019)

BL-9C, BL-12C, AR-NW10A /2018G649

Transformation of Silver Site over TiO2 Effectively Used on the Photocathode ofSolar Cell
1Kazuki Urushidate,1 Mao Yoshiba,1 and Yasuo Izumi1,*
Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku,
Chiba 263-8522, Japan

1 Introduction
High-voltage type solar cell (SC) as high as 2.11 V wasrecently reported using photocatalysts on both electrodes[1]. Water mediates the redox reactions on bothelectrodes, and it is essential how to collect light-excitedelectrons and transfer them to O2 molecules on thephotocathode. We applied O vacancy site of BiOCl [2,3]and silver nanoparticle dispersed on TiO2 for the electron
collect & transfer [4]. The transformation of Ag site onTiO2 was tentatively investigated using Ag K-edge
extended X-ray absorption fine structure (EXAFS), but
the sample was as disk [4] not film on electrode. In thisreport, an Ag–TiO2 film was formed on indium tin oxide(ITO)/Pyrex and the Ag site transformation wasmonitored by Ag K-edge EXAFS.

3 Results and Discussion
Ag K-edge EXAFS spectra were measured undervarious conditions related to cathodic conditions in SCand were analyzed by curve fitting (Figure 1). For asprepared Ag–TiO2 film, the Fourier-filtered k3-wighted
EXAFS χ-function was fit with two shells of Ag–O andAg–Ag at 0.2080 and 0.2850 nm. The associated N values
were 2.6 and 0.41, respectively, demonstrating
predominant Ag2O phase. The latter contribution wasregarded as inert metallic Ag nanoparticles formed on
specific part on TiO2 [4]. The ratio of inert Ag0 was
significantly lower for film sample based on the N value(0.41) in comparison to 2.0 for disk sample measured
under similar conditions [4].

2 Experimental Section
Silver K-edge EXAFS spectra were measured at 290 K
in the fluorescene mode in the Photon Factory on
beamline NW10A [4]. A Si(3 1 1) double-crystal
monochromator and a Pt-coated focusing cylindricalmirror were inserted into the X-ray beam path. Piezo
transducer was used to detune the X-ray intensity to twothirds of the maximum to suppress higher harmonics. TheAg K-edge absorption energy was calibrated to 25516.5
eV [5] using the X-ray spectrum of a Ag metal foil.
A film of Ag (3.0 wt %)–TiO2 photocatalyst (2.0 mg)
was formed on an ITO/Pyrex glass plate and wasmeasured using a Lytle detector. First, the sample was inair, then immersed in HCl aqueous solution of pH 2.0
under N2 atmosphere using a sealable thin transparent
plastic bag. Next, the sample in HCl aqueous solution wasirradiated by UV–visible light from a 500-W xenon arc
lamp (Ushio) at the beamline for 40 min. Finally, the light
was turned off and the sample cell was purged with O2,then with air.
Thus obtained XAFS data were analyzed using theXDAP software package. Multiple-shell curve-fit
analyses were performed for the Fourier-filtered k3weighted EXAFS data in k-and R-space using empirical
amplitude and phase-shift parameters extracted from theEXAFS data for Ag metal foil, Ag2O, and AgCl powders.
The interatomic distance (R) and its associatedcoordination number (N) for the Ag–Ag, Ag–O, and Ag–
Cl pairs were set to 0.2889 nm with the N value of 12,0.2044 nm with the N value of 2, and 0.2775 nm with theN value of 6.

Figure 1. The change of coordination number (N) for
Ag–O, Ag–Cl, and Ag–Ag interatomic pair of Ag (3.0wt %)–TiO2 photocatalyst pasted on ITO/Pyrex electrodebased on Ag K-edge EXAFS. Under air, in HCl aqueous
solution and N2, under irradiation of UV–visible light,
and under dark and O2 or air.
Next, the Ag–TiO2 film on ITO/Pyrex plate wasimmersed in HCl solution under N2 atmosphere. The Nvalues based on curve fitting analysis became 0.2, 2.9,
and 0.7 for Ag–O, Ag–Cl, and Ag–Ag at 0.2069, 0.2661,
and 0.2875 nm, respectively. Thus, Ag2O nanoparticles
under air reacted with HCl to form AgCl. This reaction
was confirmed also by cyclic voltammetry for disk
sample [4]. In contrast, for film sample in this study, theN value for Ag–Ag pair increased from 0.41 to 0.7

Photon Factory Activity Report 2018 #36 (2019)(Figure 1) while the value negligibly changed from 2.0 to1.5 for disk sample [4]. We suspect the reduction fromAgICl to Ag0 (0.22 V @standard hydrogen electrode) by
the defect site of TiO2.
Then, the film of Ag–TiO2 photocatalyst was irradiatedby UV–visible light. The N value for Ag–Ag dramaticallyincreased to 9.8–12.4 (Figure 1) demonstrating
photoreduction of AgICl to Ag0 and possibly
corresponding amount of water was oxidized to O2. The
remained Ag–O and/or Ag–Cl coordination suggested
Ag2O and/or AgCl core and reduced metallic Ag0 shellstructure.
After the light was turned off, the N values for Ag–O,
Ag–Cl, and Ag–Ag did not change either under O2 (@82min) or under air (@97 min), indicating the stability ofAg0 nanoparticles as film of Ag–TiO2 over ITO electrode.

Acknowledgements
The authors are grateful for the financial supports fromthe Grant-in-Aid for Scientific Research C (17K05961)
from Japan Society for the Promotion of Science.

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* yizumi@faculty.chiba-u.jp