Defect‐Induced Band Bending Mitigation Enables Cathodic Response in Cu Single Atom Doped Faceted TiO ₂ Thin Film Photoelectrodes

Cu‐doped TiO 2 is a well‐known semiconductor photocatalyst for hydrogen evolution, yet its performance in photoelectrochemical (PEC) systems remains underexplored. Here, we report that copper single atom–doped {111}‐faceted TiO 2 (Cu SA T 111 ) thin films, despite their n‐type nature, exhibit an unusually efficient photocathodic response for PEC water splitting. Fabricated via a scalable spray pyrolysis method, the optimized Cu SA (0.85)T 111 electrode (0.85 at% Cu, 50 µm thick) delivers a photocurrent density of −2.22 mA cm − 2 at −1.0 V versus RHE under AM 1.5G illumination, 2.92 times higher than its {101}‐faceted analogue. It also features a low onset potential (−0.22 V vs. RHE), small overpotential (−0.28 V), and high incident photon‐to‐current efficiency (IPCE) (27.1% at 420 nm). Structural and spectroscopic analyses confirm the presence of Cu 2 + single atoms and a high density of oxygen vacancies, which introduce shallow donor states, enhance visible‐light absorption and promote charge separation via direct interfacial charge transfer. Increased donor density reduces band bending and depletion width, while defect‐induced surface states partially pin the Fermi level. DFT calculations further reveal favorable H 2 O adsorption on Cu SA (0.85)T 111 , supporting the enhanced PEC performance. These results highlight Cu SA T 111 as a low‐cost, efficient platform for solar‐driven hydrogen production.