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

Oxygen vacancies (Ov) 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.