Opposite Charge Transfer to Cocatalyst Surface in SrTiO₃:Rh,Sb Photocatalyst for Water Splitting Depending on Excitation Wavelength Observed by <i>In Situ</i> Soft X-ray Absorption Spectroscopy

A crucial process in photocatalytic water splitting (PWS) is photoinduced charge transfer to reaction sites. Employing surface-sensitive in situ soft X-ray absorption spectroscopy, we measured the chemical-state evolution in Co under working conditions to track the electron and hole transfer to the CoOOH cocatalyst, which works as the oxygen evolution reaction site in Rh/Cr2O3–CoOOH/SrTiO3:Rh,Sb photocatalyst system. We found that the type of charge carrier transferred to the CoOOH cocatalyst is switched by the excitation-light energy; holes for UV light and electrons for visible light. Remarkably, the photocatalytic activity disappears under visible light irradiation, although the visible light photons are readily absorbed by the photocatalyst. We propose a possible scheme of charge transfer and consumption processes that is likely related to the PWS activity. This study highlights the pivotal role of excitation-light energy in charge transfer during the PWS.