Silver-Doped TiO₂ Nanophotocatalytic Coatings for Urban Air Purification: Visible-Light Activation, Environmental Stability, and Mechanistic Insight

Urban air pollution, dominated by nitrogen oxides (NO), volatile organic compounds (VOCs), and humidity-driven particulate precursors, remains a pressing global health challenge. Here, we report the design and evaluation of silver-modified TiO₂ nanophotocatalytic coatings optimized for realistic urban conditions, including fluctuating irradiance, variable humidity, and high pollutant loads. The coatings, prepared via a sol–gel dip-coating process, exhibited anatase-phase TiO₂ with homogeneously dispersed Ag nanoparticles, band gap narrowing from 3.18 eV to 2.72 eV, and a distinct plasmonic absorption band. Photocatalytic tests under simulated solar irradiation demonstrated superior performance, with 78% NO and 65% toluene removal within 120 min—corresponding to a twofold increase in the apparent rate constant compared to pristine TiO₂. Normalized rates reached 0.35 µmol·m⁻²·s⁻¹ with a visible apparent quantum yield of 0.6%. Durability tests confirmed > 90% retention after ten light/dark cycles and negligible Ag leaching (< 10 ppb). Mechanistic studies combining electrochemical impedance spectroscopy, photoluminescence, radical scavenger experiments, and density functional theory revealed that Ag nanoparticles act as electron sinks, suppressing recombination and enabling O₂•⁻-driven oxidation pathways. The coatings also showed improved tolerance to high relative humidity, with only 18% efficiency loss at 80% RH compared to 32% for pristine TiO₂. These findings establish Ag–TiO₂ coatings as a scalable, stable, and environmentally compatible strategy for passive urban air purification, bridging laboratory performance with real-world deployment.