Attachment of Co₃O₄ layer to SnO₂ nanowires for enhanced gas sensing properties

We prepared nanocomposites of n-SnO₂/p-Co₃O₄ for application in chemical sensors. In order to fabricate p-Co₃O₄-decorated n-SnO₂ nanowires, we sputtered a Co layer and subsequently annealed the material in air ambient. Characterization revealed that crystalline cubic Co₃O₄ with a tubular-like structure was attached to the surface of SnO₂ core nanowires. We carried out sensing tests at 573 K in at NO₂ gas concentrations ranging between 2 and 10 ppm. The sensor response was increased both by adding the Co₃O₄ layer and also by decreasing the thickness of the Co₃O₄ layer from 19.2 to 6.4 nm. We proposed possible mechanisms to explain the enhanced sensor properties obtained by Co₃O₄-functionalization. Co₃O₄-functionalized SnO₂ nanowires exhibited a higher sensor response than pristine nanowires, not only due to the heterostructure-induced depletion of n-SnO₂ region but also due to the surface effects of Co₃O₄. The generation of hole-accumulated Co₃O₄ layer in case of thicker-layered nanowires will decrease the sensor response. We demonstrated that Co₃O₄-functionalized SnO₂ nanowire sensors can be used as gas sensors at very low concentrations.