Oxygen Vacancy Enhanced Gas-Sensing Performance of CeO₂/Graphene Heterostructure at Room Temperature

Oxygen vacancies (O_v) as the active sites have significant influences on the gas sensing performance of metal oxides, and self-doping of Ce³⁺ in CeO₂ might promote the formation of oxygen vacancies. In this work, hydrothermal process is adopted to fabricate the composites of graphene and CeO₂ nanoparticles, and the influences of oxygen vacancies as well as Ce³⁺ ions on the sensing response to NO₂ are studied. It is found that the sensitivity of the composites to NO₂ increases gradually, as the proportion of Ce³⁺ relative to all of the cerium ions is increased from 14.6% to 50.7% but decreases after that value. First-principles calculations illustrate that CeO₂ becomes metallic at the Ce³⁺ proportion of <50.7%, the chemical potential of electrons on surface decreases, and the Fermi level shifts upward due to the existence of low-electronegativity Ce³⁺ ions, resulting in reduced Schottky barrier height (SBH) at the CeO₂/graphene interface, enhanced interfacial charge transfer, and high gas sensing performance. However, deep energy level will be induced at the Ce³⁺ proportion of >50.7%, and the Fermi level is pinned at the interface. As a result, the density of free electrons is reduced, leading to increased SBH and poor gas sensing response. It demonstrates that an appropriate concentration of oxygen vacancies in CeO₂ is needed to enhance the gas sensing performance to NO₂.