Photon Factory Activity Report 2019 #37 (2020)

BL-9C, AR-NW10A / 2018G649

Monitoring of active site in BiOCl used on photocathode at pH 1.0–2.0
Keisuke Hara, Kazuki Urushidate, and Yasuo Izumi*
Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan

1 Introduction
Photocatalytic solar cell (PC-SC) that enables theelectromotive force of as high as 2.11 V was recently
reported using photocatalysts on both electrodes [1]. The
cell power of PC-SC is presently limited by the rate ofoxygen reduction reaction that takes place on thephotocathode. The oxygen reduction reaction proceeds incombination with protons and electrons.

Table 1. Curve-Fitting Results of Bi L3-Edge EXAFS for
BiOCl immersed in HCl Solution at pH 2.0–1.0
Bi–O
Entry

Bi–Cl

pH
N

R (nm)NR (nm)O2 + 4H+ + 4e− → 2H2O (1)

a2.00

4.0
± 0.5

0.232
± 0.001

4.0± 1.3

0.323± 0.003

Based on the equation 1, the reaction is advantageous toproceed at lower pH. However, the stability and reactivityof BiOCl at pH lower than 2 are rarely reported.
Therefore, the active site of BiOCl immersed in HCl ofpH at 1–2 was monitored by means of Bi L3-edge EXAFS.

b1.78(15 min)

4.3
± 0.7

0.231
± 0.001

3.8
± 1.4

0.323
± 0.003c

1.78(60 min)

4.1
± 0.7

0.232
± 0.001

2.8
± 0.9

0.324
± 0.017d1.00(15 min)

4.4
± 0.3

0.227
± 0.001

2.6
± 0.8

0.337
± 0.018

BiOCl

40.2316420.3059
0.3500Bi2O3

50.233030.2500

2 Experimental Section
Bismuth L3-edge EXAFS spectra were obtained at 290
K in transmission mode on beamline NW10A and 9C.
The Bi compounds were thoroughly mixed with boron
nitride (BN) and pressed to form disks. Samples wereplaced in Pyrex glass cell equipped with a Kapton film
window (Dupont, Wilmington, DE, USA; 50 µm thick)for X-ray transmission and a polyethene terephthalate
film window (Teijin, Japan, G2; 50 µm thick) for both
UV–visible light and X-ray transmission, filled with HClaqueous solution (pH 2.0–1.0).
Bi L3-edge absorption energy was calibrated to 13 426
eV for the spectrum of a Bi metal powder diluted withBN.
The EXAFS data were analysed using the Athena andArtemis package. Curve-fitting analyses were performed
for the inversely Fourier-filtered angular wavenumber(k)3-weighted EXAFS χ function both in k- andinteratomic distance (R) spaces using amplitude andphase-shift parameters theoretically generated using
FEFF version 6L for the crystal structure of BiOCl. The R
values and its associated coordination numbers (N) for
Bi···Bi, Bi–O, and Bi–Cl pairs were set to 0.3887 nmwith an N value of 4, 0.2316 nm with an N value of 4, and0.3059 nm with an N value of 4 and 0.3500 nm with an N
value of 2. The experimental data was Fourier-filtered in
the k-range between 30 and 130 nm−1. We assumed thatthe many-body reduction factor was identical for thesample and theoretical model.

3 Results and Discussion
The N values of Bi-O and Bi-Cl at pH 1.78 and 15 min
and 60 min and at pH 1.00 and 15 min are listed in Table
1b–d. Figure 1 shows Fourier transform at each condition.

BiCl3

Figure 1. Fourier-transform of Bi L3-edge EXAFS for
BiOCl at pH 1.78 (Top) and pH 1.00 (Bottom).
Based on these results, the following reaction partiallyproceeded.
3BiOCl + HCl → Bi2O3 + H+ + BiCl4−(2)

Photon Factory Activity Report 2019 #37 (2020)

Bi2O3 is poor for photocatalysis and this reaction wasdisadvantageous as on photocathode in SC. In contrast,
the Bi–Cl coordination was retained at pH 1.78 and 15min (3.8, Table 1b). At pH 1.78, the Wmax value of SC
increased by 40% compared to that at pH 2.0 [2]. At pH
1.78 and 60 min and at pH 1.00 and 15 min, the Wmax
value of SC decreased drastically.

References(1) K. Urushidate, S. Matsuzawa, K. Nakatani, J. Li, T.
Kojima, and Y. Izumi, ACS Sustainable Chem. Eng.
6, 11892–11903 (2018).
(2) K. Urushidate, J. Li, K. Hara, T. Kojima, and Y.
Izumi, ACS Sustainable Chem. Eng. 8, 1406–1416(2020).
* yizumi@faculty.chiba-u.jp