Fabrication of metal-shelled coaxial nanowires and annealing-induced formation of metal nanoparticles for catalytic applications: Growth and characteristics of SnO2-branched nanostructures

We have fabricated SnO₂ branches on SiO_x stem nanowires, via a novel multi-step process. With the SnO₂ branches having diameters in the range 20–80 nm, X-ray diffraction, transmission electron microscopy and selected area diffraction pattern coincidentally revealed that the branches were crystalline rutile SnO₂ structures. We suggested that a Ag-catalyzed base-growth vapor–liquid–solid growth mechanism was responsible for the growth of SnO₂ branches. Photoluminescence analysis indicated that the Ag-coated SiO_x nanowires exhibited emission bands centered at 2.6 eV and 3.1 eV, presumably from the SiO_x core nanowires. Subsequent annealing induced 2.4-eV band, whereas the growth of SnO₂ branches induced 2.1-eV band. For the branched product, we have investigated the O₂ and NO₂ sensing properties. A linear relationship between sensitivity and the O₂ gas concentration was observed, which demonstrates its potential application to chemical sensors.