Enhanced CO₂ Gas Sensing Performance of Au/Ag-Decorated TiO₂ Hetero-Nanostructured Thin Films: A Study on Structural,...

This study explores the structural, morphological, optical, and gas-sensing characteristics of titanium dioxide (TiO₂) thin films with thicknesses of 90, 130, and 160 nm, both before and after decoration with gold (Au/TiO₂) and silver (Ag/TiO₂) nanoparticles. XRD confirmed the formation of the pure anatase phase, with crystallite size increasing from 13.7 to 14.7 nm and microstrain decreasing from 12.05 × 10^-3 to 11.18 × 10^-3 as film thickness increased. Scanning electron microscopy revealed a thickness-dependent grain growth (15.4 ± 3-34.7 ± 4.5 nm), while the successful decoration with Au nanoparticles enlarged the particle size to 19 ± 3.5-37.9 ± 5 nm. EDX spectroscopy confirmed stoichiometric TiO₂ composition, diffusion of substrate elements (Na, Ca, Mg, and Si), and controlled metal loading (Au: 5.8-10.3 wt %; Ag: 1.4-2.8 wt %). Spectroscopic ellipsometry results indicated refractive indices of 2.3-3.4 and optical band gaps between 3.44-3.60 eV, suggesting improved crystallinity and reduced defect density with increasing thickness. Transmission spectroscopy under CO₂ and air atmospheres revealed negligible response for bare glass and pure TiO₂, whereas Au/TiO₂ and Ag/TiO₂ films exhibited localized surface plasmon resonance (LSPR) dips at 520-550 nm and 450-500 nm, respectively. Upon CO₂ exposure, Ag/TiO₂ showed red-shifts of 10-15 nm and transmission changes of 5-8%, while Au/TiO₂ exhibited larger shifts (12-20 nm) and 6-10% modulation. Transmission change ratio analysis confirmed the superior sensitivity of Au/TiO₂ (TCR up to -0.50) compared to Ag/TiO₂ (TCR up to 0.20), demonstrating their potential for efficient, label-free optical CO₂ sensing.