Near-Infrared-Responsive Photo-Driven Nitrogen Fixation Enabled by Oxygen Vacancies and Sulfur Doping in Black TiO_2–<i>x</i>S_<i>y</i> Nanoplatelets

Abstract

Solar-driven nitrogen fixation is a promising clean and mild approach for ammonia synthesis beyond the conventional energy-intensive Haber-Bosch process. However, it is still challenging to design highly active, stable, and low-cost photocatalysts for activating inert N₂ molecules. Herein, we report the synthesis of anatase-phase black TiO_2-<i>x</i>S_<i>y</i> nanoplatelets enriched with abundant oxygen vacancies and sulfur anion dopants (<i>V</i>_O-S-rich TiO_2-<i>x</i>S_<i>y</i>) by ion exchange method at gentle conditions. The <i>V</i>_O-S-rich TiO_2-<i>x</i>S_<i>y</i> nanoplatelets display a narrowed bandgap of 1.18 eV and much stronger light absorption that extends to the near-infrared (NIR) region. The co-presence of oxygen vacancies and sulfur dopants facilitates the adsorption of N₂ molecules, promoting the reaction rate of N₂ photofixation. Theoretical calculations reveal the synergistic effect of oxygen vacancies and sulfur dopants on visible-NIR light adsorption and photoexcited carrier transfer/separation. The <i>V</i>_O-S-rich TiO_2-<i>x</i>S_<i>y</i> exhibits improved ammonia yield rates of 114.1 μmol g^-1 h^-1 under full-spectrum irradiation and 86.2 μmol g^-1 h^-1 under visible-NIR irradiation, respectively. Notably, even under only NIR irradiation (800-1100 nm), the <i>V</i>_O-S-rich TiO_2-<i>x</i>S_<i>y</i> can still deliver an ammonia yield rate of 14.1 μmol g^-1 h^-1. This study presents the great potential to regulate the activity of photocatalysts by rationally engineering the defect sites and dopant species for room-temperature N₂ reduction.