Mechanistic insights into the valence state-dependent photogeneration of holes to promote methanol oxidation in Ir-doped BaTiO3.

This work reveals the intriguing role of Ir species (Ir4+, Ir3+ and Ir0) in improving the efficiency of the photocatalytic methanol oxidation reaction (MOR) to form formaldehyde using the Ir-doped BaTiO3 photocatalysts. Ultraviolet (UV) light and visible light are selectively employed to elicit specific electron transitions induced by Ir doping, resulting in the generation of holes in the valence band and/or in-gap states. Under UV irradiation, the photocatalyst with predominant Ir4+ species generated ∼1.6 times lower amount of formaldehyde than the sample with Ir3+ and Ir0. On the contrary, under visible light (λ > 420 nm), the former generated ∼1.7 times higher amount of formaldehyde than the latter. Such wavelength-dependent distinctive performance was rationalized based on the roles of the Ir valence states on the efficiency of photogenerated holes and their energetic positions. The proposed mechanism was validated by probing the Ir-valence state distribution post-MOR using X-ray photoelectron spectroscopy. These insights offer guided principles to realize the concerted utilization of holes and electrons towards the sustainable production of chemicals and H2, respectively, from a single photocatalyst system.