Zinc oxide nanoclusters and their potential application as CH₄ and CO₂ gas sensors: Insight from DFT and TD-DFT

We have investigated the adsorption of CH₄ and CO₂ gases on zinc oxide nanoclusters (ZnO NCs) using density functional theory (DFT). It was found that the CH₄ tends to be physically adsorbed on the surface of all the ZnO NCs with adsorption energy in the range -11 to -14 kcal/mol. Even though, the CO₂ is favorably chemisorbed on the Zn₁₂ O₁₂ and Zn₁₅ O₁₅ NCs, with adsorption energy about -38 kcal/mol at B3LYP/6-311G(d,p) level of theory. When the CH₄ and CO₂ gases are adsorbed on the ZnO NCs, their electrical conductivities are decreased, and thus the studied ZnO NCs do not generate an electrical signal in the presence of CH₄ and CO₂ gases. Interestingly, both pure and gas adsorbed Zn₂₂ O₂₂ NC exhibited more favorable electronic and reactive properties than other NCs. Comparison of the structural, electronic, and optical data predicted by DFT/B3LYP and TD-DFT/CAM-B3LYP calculations with those experimentally obtained show good agreement.