Molecular p–n heterojunction-enhanced visible-light hydrogen evolution over a N-doped TiO2 photocatalyst

Abstract

This study was mainly aimed at designing molecular p–n heterojunctions on the surface of N-doped TiO₂ for visible-light hydrogen evolution. A series of NiO/N-TiO₂ samples were prepared via NH₃-postnitridation of nickel oxide-modified TiO₂. H₂ production from ethanol/water solution was utilized as a model reaction to evaluate the photocatalytic properties of the catalysts. Compared to N-TiO₂, a 90-fold-enhanced H₂ evolution rate was achieved over the optimal NiO/N-TiO₂ catalyst with a 0.5 wt% Ni content. Detailed characterizations clearly showed that loading NiO via wet impregnation leads to high dispersion of Ni²⁺ species on the surface oxygen vacancy (V_o) sites of the anatase TiO₂ nanoparticles, predominantly presenting mononuclear Ti–O–Ni heteroatomic clusters on the surface of TiO₂ in the case of low Ni content. These surface heteroatomic clusters can speed up the transfer and separation of photogenerated carriers of N-TiO₂. It can thus be established that the molecular Ti–O–Ni heterojunctions are the main contributors to the synergistic enhancement of H₂ evolution, whereas NiO nanoclusters are not responsible for the photoactivity. Only one of the N species, N–Ti–V_o, was active and effective for this cooperation with the hydrogen-releasing Ni sites, which can induce ca. 20-fold improvement of hydrogen production over NiO/TiO₂ under visible light irradiation.