Kinetics modulation of Cu‑TiO2/g‑C3N4 via type‑ii heterojunction for enhanced photocatalytic nitrogen fixation

Ammonia synthesis via photocatalytic nitrogen fixation is regarded as a green and sustainable alternative to the conventional Haber-Bosch process. Nevertheless, its practical application is greatly hindered by the chemically inert N N triple bonds and slow reaction kinetics. In this work, a Cu-doped TiO 2 /g-C 3 N 4 composite with a well-defined type-II heterojunction was synthesized through a facile wet-chemical method integrating low-temperature crystallization and in-situ molecular assembly. Characterizations confirm that Cu-TiO 2 nanoparticles are evenly immobilized on g-C 3 N 4 nanosheets, which effectively suppresses nanoparticle aggregation. On the one hand, Cu doping tunes the band structure of TiO 2 and introduces abundant oxygen vacancies, greatly promoting the adsorption and activation of N 2 molecules. On the other hand, the formed type-II heterojunction establishes an internal electric field, which broadens light response and accelerates the separation and migration of photogenerated charge carriers. Benefiting from the synergistic effect of Cu doping, oxygen vacancies and type-II heterojunction, the optimal Cu-TiO 2 /g-C 3 N 4 (CuTCN-2) achieves an outstanding NH 3 evolution rate of 91.14 μmol·g −1 ·h −1 with a quantum yield of 1.75% at 420 nm under simulated solar illumination. The activity is 7.6-, 3.4- and 34.1-fold higher than that of bare TiO 2 , single Cu-TiO 2 and pristine g-C 3 N 4 , respectively. This work reveals that the co-regulation of heteroatom doping, defect engineering and heterojunction construction can remarkably boost photocatalytic kinetics. It also provides a feasible design principle for high-performance photocatalysts toward solar-driven nitrogen fixation.