Oxygen vacancies (O) as the active sites have significant influences on the gas sensing performance of metal oxides, and self-doping of Ce in CeO might promote the formation of oxygen vacancies. In this work, hydrothermal process is adopted to fabricate the composites of graphene and CeO nanoparticles, and the influences of oxygen vacancies as well as Ce ions on the sensing response to NO are studied. It is found that the sensitivity of the composites to NO increases gradually, as the proportion of Ce 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 CeO becomes metallic at the Ce 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 Ce ions, resulting in reduced Schottky barrier height (SBH) at the CeO/graphene interface, enhanced interfacial charge transfer, and high gas sensing performance. However, deep energy level will be induced at the Ce 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 CeO is needed to enhance the gas sensing performance to NO.

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Although high-energy facets on metal oxides are usually active and preferred for gas sensing, it is difficult to expose them according to thermodynamics. In this work, nanocomposites of SnO2 and graphene are prepared by a hydrothermal method. The SnO2 nanoparticles change from a lance shape to an octahedral shape as the concentration of HCl in the solution is increased gradually from 6.5 to 10 vol %. However, the SnO2 nanoparticles have an elongated octahedral shape if the concentration of HCl is increased further. The octahedral SnO2 nanoparticles are mainly surrounded by high-surface-energy {221} facets, thus facilitating gas sensing. First-principles calculation shows that the surface energy and adsorption energy of the {221} facets are larger than those of the stable {110} facets, and so, the gas adsorption capacity on the {221} facets is better. Furthermore, because the Fermi level of the SnO2{221} facet is higher than that of graphene, the electrons are transferred from SnO2 nanoparticles to graphene sheets, enabling effective electron exchange between the composite and external NO2 gas. The excellent gas-sensing properties of the octahedral SnO2/graphene composites are ascribed to the high-surface-energy {221} facets exposed.