Direct Observation of Two Electron Holes in a Hematite Photoanode during Photoelectrochemical Water Splitting

Visible light active photoelectrodes for hydrogen generation by solar photoelectrochemical water splitting have been under scrutiny for many decades. In particular, the role of electron holes and charge transfer remains controversial. We have investigated the oxygen evolution of hematite in alkaline aqueous electrolyte under a bias potential during visible light illumination in a photoelectrochemical cell operando with soft X-ray (O 1s) spectroscopy. Only under these conditions, two new spectral signatures evolve in the valence band, which we identify as an O 2p hole transition into the charge transfer band and an Fe 3d type hole into the upper Hubbard band. Quantitative analysis of their spectral weight and comparison with the photocurrent reveals that both types of holes, contrary to earlier speculations and common perception, contribute to the photocurrent.