Observation of Photoexcited Carrier Transfer in MnOx/SrTiO3 Photoelectrode by In Situ X-ray Absorption Fine Structure

The Mn-oxide/SrTiO₃ photoelectrode for oxygen evolution reaction was investigated by in-situ Mn K-edge XAFS spectroscopy under UV irradiation. The oxidization of the Mn oxide was observed via photoexcited carrier transfer, which results in the positive potential shift of the Mn oxide cocatalyst toward oxygen evolution reaction.

Introduction

   Photoelectrochemical water splitting is a candidate for the sustainable production of hydrogen gas using solar energy.¹ Recently, MnO_x have been shown to remarkably improve the photoelectrochemical activity for water splitting, because the MnO_x on the semiconductors work as an efficient oxygen evolution cocatalyst.² Thus, the surface modification of MnO_x cocatalysts to a semiconductor photoelectrode is presently attracting attention as an efficient photoelectrochemical system for water splitting. However, the photoexcited carrier transfer processes at the photocatalyst/cocatalyst interface are not fully understood. In this study, photoexcited carrier transfer toward the MnO_x cocatalyst from a SrTiO₃ photoelectrode was investigated by in-situ Mn K-edge XAFS spectroscopy under UV irradiation (Fig. 1).

Experimental

   n-Type SrTiO₃ substrates were used after photodeposition of the Mn oxide cocatalysts, according to a previous work.² The surface morphology of the MnO_x cocatalysts was observed by scanning electron microscopy (SEM), confirming that the MnO_x thin film was deposited on the SrTiO₃ surface. A Teflon electrochemical cell was equipped with a Pt wire counter electrode and a Ag/AgCl (saturated KCl) reference electrode in 0.1 M Na₂SO₄aqueous solution with Ar bubbling. The in-situ XAFS measurements were performed at BL-12C of the Photon Factory and BL01B1 of SPring-8 as fluorescence excitation spectra using 19 element Ge detectors. XAFS spectra of the prepared samples were measured at various potentials under dark conditions or under UV irradiation using a Xe lamp.³

Results and discussion

   First,the in-situ XAFS measurements of the MnO_x/SrTiO₃ photoelectrode were tested at 0.5 V under UV irradiation (Fig. 2). The peak position at 6557.8 eV was gradually shifted to 6560.0 eV under irradiation, which suggests that the Mn³⁺ species changed to Mn⁴⁺ due to the migration of photoexcited holes, which is consistent with the photoelectrochemical reaction where the Mn oxide species acts as an oxygen evolution cocatalyst via the transfer of photoexcited holes. On the other hand, when the XAFS spectra were taken for the applied potential at 0.0 V or -0.5 V, the peak position shifted to 6559.2 eV or 6557.8 eV, respectively, indicating that the potential shift of MnO_x via the photoexcited hole transfers from photoelectrode to MnO_x cocatalyst is dependent on the applied electrode potential. These results demonstrate that the photoexcited carrier transfer into the cocatalyst from the photoelectrode can be probed under UV irradiation by in situ electrochemical XAFS measurement.

References

(1) Nocera, D. G. Acc. Chem. Res. 2012, 45, 767-776.

(2) Wang, D.; Li, R.; Zhu, J.; Shi, J.; Han, J.; Zong, X.; Li, C. J. Phys. Chem. C 2012, 116, 5082-5089.

(3) Yoshida, M.; Yomogida, T.; Mineo, T.; Nitta, K.; Kato, K.; Masuda, T.; Nitani, H.; Abe, H.; Takakusagi, S.; Uruga, T.; Asakura, K.; Uosaki, K.; Kondoh, H. Chem. Commun. 2013, 49, 7848-7850.