Revisiting surface chemistry in TiO₂: A critical role of ionic passivation for pH-independent and anti-corrosive photoelectrochemical water oxidation

TiO₂ photoanodes suffer from low catalytic activity and poor stability under acidic media for water oxidation. Here, we report pH-independent high-efficiency photoelectrochemical (PEC) water oxidation and prolonged stability via simple ionic passivation of rutile TiO₂ (rTiO₂) surface. The controlled surface passivation by hydroxide ions (OH⁻) onto the rTiO₂ surface (OH-rTiO₂) enables extraordinarily high PEC performance and long-term durability independent of the pH value of electrolyte. The OH-rTiO₂ photoanode shows a charge transfer efficiency of ∼100% at 1.23 V vs RHE over the entire pH range (pH 0 ∼ pH 14) without incorporation of any co-catalysts, hole scavengers or overlayer, which implies that the molecular surface passivation by hydroxide ions successfully promotes the water oxidation reaction pathway. According to our density functional theory (DFT) calculations, the OH-rTiO₂ surface can possess increased hydroxide ion coverage, faster decay of the positive surface charge in acidic environments, and weakened H⁺ adsorption for pH > 7, which enable enhanced water oxidation performance independent of the pH value of the electrolyte. Thus, our report proves that the ionic passivation by hydroxide ions can affect PEC performances and durability of photoanode materials for pH-universal photoelectrochemical water oxidation.