Ammonia (NH₃) is one of the most important commodity chemicals in today’s chemical industry, representing a key feedstock for the synthesis of urea, ammonium nitrate, nitric acid and various other nitrogen-containing compounds. Industrially, NH₃ is produced by N₂ hydrogenation through the Haber–Bosch process under extreme reaction conditions (200–250 bar, 400–500 ^oC).

The process consumes ~2% of the energy used by humans annually and releases huge amounts of CO₂ into the atmosphere (since the H₂ used to reduce N₂ comes from steam methane reforming, with fossil fuels being combusted to provide reaction heat).

Conversely, in nature, the enzyme nitrogenase reduces N₂ to NH₃ at ambient temperature and pressure (nitrogenase contains cationic Fe and FeMo active sites), inspiring the development of novel photocatalytic and electrocatalytic systems that can drive nitrogen reduction with water under similar low temperature and low pressure conditions.

Recently, photocatalysts containing abundant surface oxygen-vacancies (V_O) and coordinatively unsaturated metal sites have been shown to be capable of activating N₂ reduction under appropriate photoexcitation. However, most of the photocatalysts studied to date for N₂ fixation exhibit weak light absorption above 500 nm, severely handicapping their solar spectrum-utilization efficiency. Thus, it is imperative to develop new semiconductor photocatalysts with abundant V_O and wide light absorption range to allow efficient solar-driven nitrogen fixation under ambient conditions.

In a recent pioneering breakthrough, Tierui Zhang (Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing) and co-workers discovered a new approach for introducing V_O into ultrathin TiO₂ nanosheets.

They have found that the concentration of V_O in ultrathin TiO₂ nanosheets could be precisely controlled by doping with copper ions. This represented a major advancement in semiconductor defect engineering, since previously researchers relied almost exclusively on morphology control as the means of varying V_O concentrations (i.e., by reducing semiconductor dimensions to the nanoscale, a modicum of control over surface V_O is possible).

Zhang’s team found that V_O introduced in ultrathin TiO₂ nanosheets via Cu doping served as active sites for the adsorption and activation of N₂ and H₂O. Defect-rich TiO₂ nanosheets containing 6 mol.% Cu exhibited remarkable and stable performance for the photofixation of N₂ to NH₃ in water under visible light irradiation (NH₃ evolution rate 78.9 µmol g^-1 h^-1 under full spectrum UV-vis irradiation), with the activity extending up to 700 nm (the NH₃ evolution rate was 0.72 µmol g^-1 h^-1 at 700 nm with a quantum yield of 0.05%).

Detailed structural analysis and density functional theory calculations revealed that the excellent activity of the Cu-doped TiO₂ photocatalysts could be attributed to V_O and compressive strain in the TiO₂ lattice created by Jahn-Teller distortions about Cu ions, which acted synergistically to promote photoexcited charge transfer from defect-rich TiO₂ to adsorbed N₂ under light irradiation.

This work demonstrates a promising new strategy for achieving precise control of the concentration of V_O in semiconductor photocatalysts for solar ammonia synthesis (and intuitively other applications), providing a potential stepping stone towards more sustainable industrial-scale NH₃ manufacture.