Cl-adsorption drives band engineering beyond oxygen-vacancy-induced band-gap narrowing in TiO2 photocatalysis: experimental and DFT insights

Although band gap narrowing via defect states can improve light absorption in photocatalysts, it may also impair charge transfer. Here, we show that in anatase TiO₂, HCl treatment and thermal processing generate two distinct surface states: Cl-modified anatase with an E_g = 3.18 eV, and oxygen-vacancy-rich TiO₂ exhibiting visible-light absorption and E_g = 3.02 eV. Despite enhanced light harvesting, the vacancy-rich sample shows only moderate solar-driven H₂ evolution (∼94.7 μmol h⁻¹ g⁻¹) from aqueous methanol, indicating that the vacancy-rich defect state generated here is not the optimal configuration for photocatalytic H₂ evolution. In contrast, Cl-modified anatase achieves the highest activity (∼133.3 μmol h⁻¹ g⁻¹) without a cocatalyst, outperforming pristine TiO₂ (∼7.3 μmol h⁻¹ g⁻¹) and P25 under identical conditions. DFT calculations reveal that band gap narrowing is consistent with contributions from bulk-like oxygen-vacancy states, whereas surface vacancies are expected to influence band-edge alignment more strongly. At the same time, surface-adsorbed Cl preserves the band gap but repositions both band edges to straddle the H⁺/H₂ and H₂O/O₂ levels when referenced on the aqueous pH 7 scale. Reaction-pathway analysis further reveals slightly favorable hydrogen-evolution energetics on Cl-modified surfaces. Separately, the OER was assessed solely by DFT, which predicts a fully downhill reaction pathway on Cl-adsorbed TiO₂ under simulated illumination, unlike oxygen-vacancy-containing TiO₂, which retains non-spontaneous reaction steps.