N, H Dual-Doped Black Anatase TiO₂ Thin Films toward Significant Self-Activation in Electrocatalytic Hydrogen Evolution Reaction in Alkaline Media

Titanium dioxide (TiO₂) is an important application-worthy oxide semiconductor. Herein, pulsed laser deposited N, H dual-doped black anatase TiO₂ (N–H:TiO₂) films grown under NH₃ are examined for the electrochemical water splitting application. This case is compared with only nitrogen or oxygen vacancy doped films. These dual-doped films are highly conducting (98.77 μΩ cm @ 300 K) with holes as majority carriers. Electrostatic force microscopy also reveals significant differences in the work functions for the cases compared. Interestingly, the doped films not only exhibit impressive hydrogen evolution reaction (HER) activity with an initial overpotential of ≈0.6 V (vs reversible hydrogen electrode [RHE]), but the same reduces significantly down to ≈0.42 V at 10 mA cm⁻² after 5 h chronoamperometry. Concurrently, there is a remarkable increase in current density (from ≈10 to 30 mA cm⁻²) at the overpotential of 0.6 V after 2000 cycles with excellent durability over 15 h. This peculiar self-activation and performance enhancement of the catalysis is examined through X-ray photoelectron spectroscopy (XPS) analysis which reveals the increased percentage of oxygen vacancies and incorporation of a new phase TiO_xN_y on the film surface via electrochemical surface reactions. Density functional theory (DFT) calculations confirm that the N–H:TiO₂ thin films have appropriate hydrogen adsorption Gibbs free energies commensurate with observed high HER activity.