Synergism of heterovalent valence state and oxygen vacancy defect engineering in Co/S co-doped TiO2 for nitrogen photoreduction to ammonia

Abstract

Herein, we demonstrate a Co/S doped TiO₂ bimetal sulfur oxide nanoparticle catalyst (labeled CoTiOS) with heterovalent Co states and abundant oxygen vacancy (O_v) defects for the photocatalytic nitrogen reduction reaction (PNRR) in an ambient environment. Co/S co-doping converted TiO₂ into a catalyst for visible light photocatalytic nitrogen fixation. The incorporation of Co endows TiO₂ with better electrical conductivity and electron trapping ability. The introduction of sulfur adjusts the amount of O_v and Co³⁺/Co²⁺ ratio. O_v acts as an active site for trapping H₂O and activating H₂O to generate a proton for the N₂ reduction reaction. Active atoms Co³⁺ were exposed and activated for N₂, and electrons hopping between Co²⁺ and Co³⁺ enhanced electron transfer. CoTiOS-3 with appropriate Co/S doping has abundant O_v, suitable heterovalent Co³⁺/Co²⁺ ratios, higher surface active sites, smaller crystal sizes, higher specific surface area (S_BET), and fast transfer of electrons, and it exhibited a super PNRR activity of 491.56 μmol g⁻¹ h⁻¹, an apparent quantum efficiency (AQE) of 1.89% at 420 nm and a solar-to-ammonia (STA) conversion efficiency of 0.083% in pure water under AM1.5G light irradiation. It also exhibited excellent stability and persistence and shows potential for photocatalytic nitrogen fixation.