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Nanostructures p-CoO x /n-SnO 2 based on tin oxide nanowires have been prepared by two step CVD technique and characterized in detail by XRD, XRF, XPS, HAADF-STEM imaging and EDX-STEM mapping. Depending on the temperature of decomposition of cobalt complex during the second step of CVD synthesis of nanostructures cobalt oxide forms a coating and/or isolated nanoparticles on SnO 2 nanowire surface. It was found that cobalt presents in +2 and +3 oxidation states. The measurements of gas sensor properties have been carried out during exposure to CO (14 ppm), NH 3 (21 ppm), and H 2 S (2 ppm) in dry air. The opposite trends were observed in the effect of cobalt oxide on the SnO 2 gas sensitivity when detecting CO or NH 3 in comparison to H 2 S. The decrease of sensor signal toward CO and NH 3 was attributed to high catalytic activity of Co 3 O 4 in oxidation of these gases. Contrary, the significant increase of sensor signal in the presence of H 2 S was attributed to the formation of metallic cobalt sulfide and removal of the barrier between p-CoO x and n-SnO 2 . This effect provides an excellent selectivity of p-CoO x /n-SnO 2 nanostructures in H 2 S detection.

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Nanocomposites CoO x /SnO 2 based on tin oxide powders with different crystallinity have been prepared by wet chemical synthesis and characterized in detail by ICP-MS, XPS, EPR, XRD, HAADF-STEM imaging and EDX-STEM mapping. It was shown that cobalt is distributed differently between the bulk and surface of SnO 2 nanocrystals, which depends on the crystallinity of the SnO 2 matrix. The measurements of gas sensor properties have been carried out during exposure to CO (10 ppm), and H 2 S (2 ppm) in dry air. The decrease of sensor signal toward CO was attributed to high catalytic activity of Co 3 O 4 leading to oxidation of carbon monoxide entirely on the surface of catalyst particles. The formation of a p-CoO x /n-SnO 2 heterojunction results in high sensitivity of nanocomposites in H 2 S detection. The conductance significantly changed in the presence of H 2 S, which was attributed to the formation of metallic cobalt sulfide and removal of the pn junction.