Interlayer Atomic Voids by Partial Cesium Defect in Layered Titanate Activate Photo(electro)catalytic H ₂ and O ₂ Generation

Although many layered oxide semiconductors possess seemingly suitable band gaps for photo(electro)catalysis in the UV–vis range, they often exhibit low or no activity in practice. Bulk layered cesium titanate is one such example of an inactive semiconductor, where the introduction of cesium vacancies with subsequent thermal treatment leads to the activation of its photocatalytic properties. Here, we demonstrate the promising effect of cesium vacancies on the photocatalytic (PC) activity enhancement in the hydrogen evolution reaction (HER) from water in the presence of H 2 O/MeOH (80:20). In a separate experiment, we also further prove that the Cs-vacant (V Cs ) layered titanate sample treated at 700 °C exhibits a remarkably improved photoelectrocatalytic (PEC) activity in the oxygen evolution reaction (OER) of partially exfoliated cesium titanates. In contrast, bulk cesium titanate shows no PC activity toward HER and only minimal PEC activity toward OER. Computational modeling reveals that partial interlayer Cs vacancies can increase the surface area up to 120 Å 2 and generate interlayer voids as large as 10 Å. Furthermore, hybrid density functional theory (DFT) calculations indicate that these Cs vacancy defects lead to the formation of midgap states, which are expected to enhance photogenerated charge carrier separation and stabilization, thereby improving both PC and PEC activities. Our approach results in the development of a cocatalyst-free semiconductor light absorber capable of producing hydrogen with significantly higher efficiency than both bulk cesium titanate and protonated layered titanate.