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.