Hydrogen-doped Brookite TiO₂ Nanobullets Array as a Novel Photoanode for Efficient Solar Water Splitting

As a representative photocatalyst for photoelectrochemical solar water splitting, TiO₂ has been intensively studied but most researches have focused on the rutile and anatsase phases because brookite, another important crystalline polymorph of TiO₂, rarely exists in nature and is difficult to synthesize. In this work, hydrogen doped brookite (H:brookite) nanobullet arrays were synthesized via a well-designed solution reaction for the first time. H:brookite shows highly improved PEC properties with excellent stability, enhanced photocurrent, and significantly high Faradaic efficiency for overall solar water splitting. To support the experimental data, ab initio density functional theory calculations were also conducted. At the interstitial doping site that has minimum formation energy, the hydrogen atoms act as shallow donors and exist as H⁺. which has the minimum formation energy among three states of hydrogen (H⁺. H⁰, and H^-). The calculated density of states of H:brookite shows a narrowed bandgap and an increased electron density compared to the pristine brookite. The combined experimental and theoretical results provide frameworks for the exploration of the PEC properties of doped brookite and extend our knowledge regarding the undiscovered properties of brookite of TiO₂.