Plasmon‐Driven Nitrogen Photoreduction to Ammonia Using Silica‐Encapsulated Au Nanostar/TiO2 Nanohybrids

Photocatalytic N₂ reduction is a sustainable method for ammonia production. Au nanostars (AuNST) with TiO₂ excel, especially when encased in mesoporous silica, enhancing stability and selectivity without scavengers. Electromagnetic fields at nanostar spikes drive hot-electron transfer to TiO₂ for N₂ reduction under visible light, while hot holes oxidize water, maintaining charge balance and enabling efficient, eco-friendly performance.

Abstract

Plasmon-induced photocatalysis has gained traction as a promising means to efficiently drive chemical reactions using light. In particular, photocatalytic N₂ reduction emerges as a sustainable route to produce ammonia, a key starting material in the manufacture of nitrogen-rich fertilizers and a potential energy vector. Here, various Au nanoparticle morphologies combined with a TiO₂ semiconductor are initially screened, and Au nanostar is identified as the most efficient morphology. Encasing this material within a mesoporous silica shell improved their stability and selectivity to ammonia formation, eliminating the need for hole scavengers. Advanced characterization including TEM and operando SERS spectroscopy together with the evaluation of the material in the presence of optical filters and probes reveal that the superior performance originates from the injection of excited “hot” charge carriers from the plasmonic material to the semiconductor, driving N₂ reduction to NH₃ under visible light. The wavelength-dependence experiments demonstrate a synergistic interaction between gold interband transitions and plasmonic effects, combined with the TiO₂ semiconductor, which enhances catalytic performance across the spectrum. Importantly, hot holes generated at the plasmonic sites oxidize water into oxygen and subsequently to nitrates, maintaining charge balance in the photocatalyst. This dual functionality ensures effective charge circulation and sustainable performance across multiple cycles.