Engineering TiO2 nanotubes with single-atom Cu, Ag, and Au decoration for efficient CO and CO2 gas adsorption: Insights from density functional theory.

We present a first-principles investigation of pristine and transition-metal (Cu, Ag, Au) decorated TiO2 nanotubes (TiO2-NTs) for CO and CO2 sensing. Density functional theory calculations show that metal substitution at the O1 site significantly tailors the electronic structure through band-gap modulation, localized states, and spin polarization while preserving structural stability. Natural bond orbital and density-of-states analyses reveal strong charge polarization induced by Cu, moderate electron donation from Ag, and partial back-donation effects associated with Au. Adsorption results indicate that CO2 interacts most strongly with pristine TiO2-NTs, whereas CO exhibits pronounced chemisorption on Au-decorated nanotubes via C-Ti bond formation, supported by orbital hybridization and noncovalent interaction analyses. Recovery-time calculations demonstrate that Ag- and Au-decorated TiO2-NTs enable fast and reversible desorption, making them suitable for reusable sensing applications. Overall, transition-metal decoration emerges as an effective strategy to enhance the sensitivity, selectivity, and reversibility of TiO2-NT-based gas sensors for CO and CO2 detection.