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Nanocrystalline cobalt oxide Co3O4 has been prepared by precipitation and subsequent thermal decomposition of a carbonate precursor, and has been characterized in detail using XRD, transmission electron microscopy, and FTIR spectroscopy. The sensory characteristics of the material towards carbon monoxide in the concentration range 6.7–20 ppm have been examined in both dry and humid air. A sensor signal is achieved in dry air at sufficiently low temperatures T = 80–120 °C, but the increase in relative humidity results in the disappearance of sensor signal in this temperature range. At temperatures above 200 °C the inversion of the sensor signal in dry air was observed. In the temperature interval 180–200 °C the sensor signal toward CO is nearly the same at 0, 20 and 60% r.h. The obtained results are discussed in relation with the specific features of the adsorption of CO, oxygen, and water molecules on the surface of Co3O4. The independence of the sensor signal from the air humidity combined with a sufficiently short response time at a moderate operating temperature makes Co3O4 a very promising material for CO detection in conditions of variable humidity.

In this work, the hybrids based on nanocrystalline SnO2 or In2O3 semiconductor matrixes and heterocyclic Ru(II) complex are studied as materials for gas sensors operating at room temperature under photoactivation with visible light. Nanocrystalline semiconductor oxides are obtained by chemical precipitation with subsequent thermal annealing and characterized by XRD, SEM and single-point BET methods. The heterocyclic Ru(II) complex is synthesized for the first time and investigated by 1H NMR, 13C NMR APT, MALDI-MS analysis, and UV-Vis spectroscopy. The HOMO and LUMO energies of the Ru(II) complex are calculated from cyclic voltammetry data. The hybrid materials are characterized by TGA-MS analysis and EDX mapping. The optical properties of hybrids are studied by UV-Vis spectroscopy in the diffuse reflection mode. The investigation of spectral dependencies of photoconductivity of hybrid samples demonstrates that the role of organic dye consists in shifting the photosensitivity range towards longer wavelengths. Sensor measurements demonstrate that hybrid materials are able to detect NO2 in the concentration range of 0.25–2 ppm without the use of thermal heating under periodic illumination with even low-energy long-wavelength (red) light.

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.

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.

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