High-Valent Iron Redox-Mediated Photoelectrochemical Water Oxidation

Fe3+ is widely used as a conduction band electron acceptor in the photocatalytic and photoelectrochemical (PEC) oxidation of water and various substrates. However, a question of the possible involvement of Fe3+ as a valence band hole acceptor has been raised. Herein, we demonstrate that the PEC water oxidation using oxide semiconductor (WO3, TiO2, and BiVO4; primarily WO3) films can proceed via the formation of high-valent iron species in the presence of aqueous Fe3+ ions at pH 1–3. The presence of Fe3+ (1–100 mM) enhances the photocurrent generation, O2 evolution, and the Faradaic efficiency (FE) of ∼90% with a biased WO3 electrode (1.23–1.88 VRHE), whereas the formation of Fe2+ is significantly inhibited. An in situ transient absorption spectroscopic analysis reveals the formation of high-valent iron species. The selective oxidation of dimethyl sulfoxide to dimethyl sulfone using in situ high-valent iron species is achieved with an FE of ∼98% in the PEC reaction with Fe3+. The proposed reaction mechanism should call for attention to the conventional role of Fe3+ in the PEC reaction.