Unveiling Valence State-Dependent Photocatalytic Water Splitting Activity and Photocathodic Behavior in Visible Light-Active Iridium-Doped BaTiO₃

Despite having favorable energetics and tunable optoelectronic properties, utilization of BaTiO₃ (BTO) for photocatalytic reactions is limited by its absorption only in the ultraviolet region. To address this challenge, BTO is doped with iridium (Ir) to induce visible light absorption. The visible light-induced photocatalytic H₂ generation efficiency is enhanced by 2 orders of magnitude on selective conversion of the Ir valence state from Ir⁴⁺ to Ir³⁺. To understand such intriguing behavior, valence state-dependent changes in the optoelectronic, structural, and surface properties and electronic band structure are comprehensively investigated. The effect of electron occupancy change between Ir⁴⁺ (t_2g⁵ e_g⁰) and Ir³⁺ (t_2g⁶ e_g⁰) and their energetic positions within the band gap is found to significantly influence H₂ generation. Besides this, converting Ir⁴⁺ to Ir³⁺ enhanced the photocathodic current and lowered the onset potential. Results aid in designing photocatalysts to efficiently use low-energy photons for enhancing solar H₂ production in these emerging BTO-based photocatalysts. Collectively, the observations made in this work highlight the promising application of Ir³⁺:BTO in z-scheme photocatalysis.