Facet-engineered CeO2/graphene composites for enhanced NO2 gas-sensing

The sensitivity of semiconductor gas sensors depends on the exposed crystal planes. High-energy crystal planes usually have an open surface structure and more active sites which favor the absorption of gas molecules and thus might improve the gas sensing performance. However, it is commonly difficult for the high-energy crystal planes to be exposed. In this paper, the morphology and exposed facets of CeO2 nanoparticles in the CeO2 nanoparticles/graphene composites are tailored simply by changing the volume ratio of ethylene glycol to deionized water (EG/H2O) in the solution during fabrication. CeO2 nanocubes enclosed by the {100} facets are produced with an EG/H2O ratio of 1 : 1, otherwise CeO2 nanograins enclosed by the {111} facets are obtained, which is closely related to the changes in the energetics on the surface. Furthermore, it is found that the CeO2{100}/graphene composites deliver substantially enhanced gas sensing performance for NO2, as compared to CeO2{111}/graphene composites. First-principles calculations illustrate that the electrons flow from graphene to the CeO2{111} facets resulting in electron depletion on graphene. In contrast, the electrons flow from the CeO2{100} surface to graphene resulting in electron accumulation on graphene and the band gap of CeO2{100}/graphene is nearly zero implying a metallic behavior. The rapid charge exchange between NO2 and CeO2{100}/graphene composites leads to the improved gas sensing properties. The results present us a clear physical picture for the tunable facets and morphology as well as the enhanced performances of nanoparticles/graphene composites.