Attachment of metal nanoparticles to SnO2 nanowires for enhancement of gas sensing properties.

We fabricated SnO2/cobalt (Co) core-shell nanowires by means of a two-step process, for their application as chemical sensors. For Co-functionalization, we synthesized SnO2-Co core-shell nanowires by the sputtering deposition of Co layers on the surface of networked SnO2 nanowires, subsequently transforming the continuous Co-shell layers into crystalline islands by thermal heating. While scanning electron microscopy (SEM) images of annealed core-shell nanowires exhibited a rough surface, transmission electron microscopy (TEM) images revealed that the roughness is related to the agglomeration of the sputtered Co layer. The X-ray diffraction (XRD) pattern and lattice-resolved TEM images coincidentally indicated that the agglomerated particles are comprised of a hexagonal Co phase. The NO2 sensing test revealed that the sensor response was enhanced by decoration with Co nanoparticles. In addition, both response and recovery times tended to decrease as a result of the Co-functionalization. This indicates that the Co-functionalized SnO2 nanowire sensors can be used to sense gases at very low concentrations. We discussed possible mechanisms for enhancing sensor properties by Co-functionalization. The NO2 gas sensing test demonstrated the ability of the Co-functionalization to provide higher sensitivity, shorter response time, and shorter recovery time than would bare SnO2 nanowires.