A novel two-dimensional Janus TiSiGeN₄ monolayer with N vacancies for efficient photocatalytic nitrogen reduction

The photocatalytic nitrogen reduction reaction (pNRR) is a clean technology that converts H₂O and N₂ into NH₃ under environmental conditions using inexhaustible sunlight. Herein, we designed a novel two-dimensional (2D) Janus TiSiGeN₄ structure and evaluated the pNRR performance of the structure with the presence of nitrogen vacancies at different positions using density functional theory (DFT) calculations. The intrinsic dipoles in the Janus TiSiGeN₄ structure generate a built-in electric field, which promotes the migration of photogenerated electrons and holes towards the (001) and (00-1) surfaces, respectively, to achieve efficient charge separation. For the pNRR, the Si atoms exposed after the formation of top N vacancies can realize the efficient activation of N₂ through the "acceptance-donation" mechanism, while the presence of middle N vacancies not only suppresses the hydrogen evolution reaction, a competition reaction, but also lowers the reaction barrier for the protonation of N atoms. The limiting potential of TiSiGeN₄ with the coexistence of both top and middle N vacancies (TiSiGeN₄-V_N-mt) is as low as -0.44 V. In addition, the introduction of N vacancies generates defect levels, narrowing the band gap and improving the light response. This work provides theoretical guidance for the design of efficient pNRR photocatalysts under mild conditions.