Light-Driven Nitrogen Fixation to Ammonia over Aqueous-Dispersed Mo-Doped TiO₂ Colloidal Nanocrystals

Photocatalytic nitrogen fixation to ammonia and nitrates holds great promise as a sustainable route powered by solar energy and fed with renewable energy resources (N₂ and H₂O). This technology is currently under deep investigation to overcome the limited efficiency of the process. The rational design of efficient and robust photocatalysts is crucial to boost the photocatalytic performance. Widely used bulk materials generally suffer from charge recombination due to poor interfacial charge transfer and difficult surface diffusion. To overcome this limitation, this work explores the use of aqueous-dispersed colloidal semiconductor nanocrystals (NCs) with precise morphological control, better carrier mobility, and stronger redox ability. Here, the TiO₂ framework has been modified via aliovalent molybdenum doping, and resulting Mo-TiO₂ NCs have been functionalized with charged terminating hydroxyl groups (OH^-) for the simultaneous production of ammonia, nitrites, and nitrates via photocatalytic nitrogen reduction in water, which has not been previously found in the literature. Our results demonstrate the positive effect of Mo-doping and nanostructuration on the overall N₂ fixation performance. Ammonia production rates are found to be dependent on the Mo-doping loading. 5Mo-TiO₂ delivers the highest NH₄⁺ yield rate (ca. 105.3 μmol g^-1 L^-1 h^-1) with an outstanding 90% selectivity, which is almost four times higher than that obtained over bare TiO₂. The wide range of advance characterization techniques used in this work reveals that Mo-doping enhances charge-transfer processes and carriers lifetime as a consequence of the creation of new intra band gap states in Mo-doped TiO₂ NCs.