Charge transfer mechanism of CO adsorption on the surface of CeO2 and Mn-Doped CeO2 nanoparticles: an experimental and ab initio study

Cerium oxide (CeO₂) is a widely studied gas-sensing material whose properties can be effectively tailored through transition-metal doping. In this work, pristine CeO₂ and Mn-doped CeO₂ nanoparticles (NPs) were synthesized via a co-precipitation method and characterized using XRD, UV–Vis DRS, XPS and HRTEM analysis. Structural analysis confirmed the formation of a cubic fluorite phase, accompanied by a reduction in crystallite size upon Mn incorporation. Optical studies revealed band-gap energies of 3.14 eV increased to 3.25 eV for pristine and Mn-doped CeO₂ NPs. Gas-sensing measurements toward CO demonstrated that Mn-doped CeO₂ NPs exhibit an enhanced sensing response of 79.26%, compared to 71.21% for pristine CeO₂ NPs, at an operating temperature of 100 °C (50 ppm). The Mn-doped sensor also shows improved selectivity and faster sensing kinetics, as reflected by shorter response and recovery times. Density functional theory (DFT) calculations were employed to elucidate the sensing mechanism by investigating CO adsorption on pristine and Mn-doped CeO₂ (1 1 1) surfaces. For pristine CeO₂, weak orbital interaction between Ce 5d and O 2p states near the Fermi level leads to weak CO adsorption and a longer characteristic time. In contrast, Mn doping introduces Mn 3d states that strongly overlap with O 2p states, enabling enhanced σ-donation and π-back donation. This results in significantly stronger CO chemisorption, reduced characteristic time (τ), and a higher theoretical sensing response. These findings highlight Mn doping as an effective way to enhance the electrical properties and gas-sensing capability of CeO₂ nanoparticles.