Octahedral SnO₂/Graphene Composites with Enhanced Gas-Sensing Performance at Room Temperature

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.