Scientists have developed defect-rich TiO₂ nanosheets that showed remarkable and stable performance for the photofixation of N₂ to NH₃ in water under visible light irradiation, exhibiting photoactivity up to 700 nm. This work was published in Advanced Materials. 

In a recent pioneering breakthrough, a research team led by Prof. Tierui Zhang (Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing) discovered a new approach for introducing V_O into ultrathin TiO₂ nanosheets. They 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). Previously, 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. 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. 

This work is financially supported by the National Key Projects for Fundamental Research and Development of China (2017YFA0206904, 2017YFA0206900, and 2016YFB0600901), the National Natural Science Foundation of China (51825205, 51772305, 51572270, U1662118, 21871279, and 21802154), the Beijing Natural Science Foundation (2191002, 2182078, and 2194089), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17000000), the Royal Society‐Newton Advanced Fellowship (NA170422), the International Partnership Program of Chinese Academy of Sciences (GJHZ1819), the Beijing Municipal Science and Technology Project (Z181100005118007), the K. C. Wong Education Foundation, the Young Elite Scientist Sponsorship Program by CAST (YESS), and the Youth Innovation Promotion Association of the CAS. G.I.N.W. acknowledges funding support from the Energy Education Trust of New Zealand and the MacDiarmid Institute for Advanced Materials and Nanotechnology.