Metal/TiO2 composite photocatalysts with tunable activity for nitrogen fixation: the impact of metal nanoparticles

Photocatalytic N₂ fixation reactions can harvest solar energy to convert the abundant but inert N₂ into available NH₄⁺. Nanoscale metal/semiconductor composites are among the most developed photocatalysts for the conversion, whereas systematic investigations of the impact of metal species and status on their photocatalytic performance are rarely reported. Herein, metal nanoparticle-modified TiO₂ (M-TiO₂, M = Au, Pd and Cu) samples were selected as model composites for photocatalytic N₂ fixation. By varying the metal species and reductants in the synthesis, the obtained M-TiO₂ exhibited tunable photocatalytic activities for nitrogen fixation. Among them, Cu-TiO₂ prepared using sodium borohydride showed the highest rate of NH₄⁺ production (16.3 mg L^-1 h^-1). Mechanism studies revealed that the high activity of Cu-TiO₂ was attributed to the synergistic effect of the small size of Cu NPs (2-6 nm), the enhanced interfacial interactions, and the high usage of photogenerated electrons. This work may guide the design of efficient metal/semiconductor photocatalysts for nitrogen fixation and other energy conversion reactions.