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Development of sensor materials based on metal oxide semiconductors (MOS) for selective gas sensors is challenging for the tasks of air quality monitoring, early fire detection, gas leaks search, breath analysis, etc. An extensive range of sensor materials has been elaborated, but no consistent guidelines can be found for choosing a material composition targeting the selective detection of specific gases. Fundamental relations between material composition and sensing behavior have not been unambiguously established. In the present review, we summarize our recent works on the research of active sites and gas sensing behavior of n-type semiconductor metal oxides with different composition (simple oxides ZnO, In2O3, SnO2, WO3; mixed-metal oxides BaSnO3, Bi2WO6), and functionalized by catalytic noble metals (Ru, Pd, Au). The materials were variously characterized. The composition, metal-oxygen bonding, microstructure, active sites, sensing behavior, and interaction routes with gases (CO, NH3, SO2, VOC, NO2) were examined. The key role of active sites in determining the selectivity of sensor materials is substantiated. It was shown that the metal-oxygen bond energy of the MOS correlates with the surface acidity and the concentration of surface oxygen species and oxygen vacancies, which control the adsorption and redox conversion of analyte gas molecules. The effects of cations in mixed-metal oxides on the sensitivity and selectivity of BaSnO3 and Bi2WO6 to SO2 and VOCs, respectively, are rationalized. The determining role of catalytic noble metals in oxidation of reducing analyte gases and the impact of acid sites of MOS to gas adsorption are demonstrated.

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Tungsten oxide is a renowned material for resistive type gas sensors with high sensitivity to nitrogen oxides. Most studies have been focused on sensing applications of WO3 for the detection of NO2 and a sensing mechanism has been established. However, less is known about NO sensing routes. There is disagreement on whether NO is detected as an oxidizing or reducing gas, due to the ambivalent redox behavior of nitric oxide. In this work, nanocrystalline WO3 with different particle size was synthesized by aqueous deposition of tungstic acid and heat treatment. A high sensitivity to NO2 and NO and low cross-sensitivities to interfering gases were established by DC-resistance measurements of WO3 sensors. Both nitrogen oxides were detected as the oxidizing gases. Sensor signals increased with the decrease of WO3 particle size and had similar dependence on temperature and humidity. By means of in situ infrared (DRIFT) spectroscopy similar interaction routes of NO2 and NO with the surface of tungsten oxide were unveiled. Analysis of the effect of reaction conditions on sensor signals and infrared spectra led to the conclusion that the interaction of WO3 surface with NO was independent of gas-phase oxidation to NO2.

Transformation of the Sn-BEA site structure during the interaction with water has been investigated by means of Fourier transform infrared spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and catalytic experiments. It is shown that the Lewis and Brønsted acid properties of Sn-BEA zeolite before and after the adsorption of water change significantly. New surface OH groups exhibiting different structures are observed after adsorption, whereas tin oxide supported on Si-BEA is inactive in this transformation. It is demonstrated that the formed bridged OH groups possess strong Brønsted acidity, thus enabling the protonation of pyridine. It is suggested that the adsorption of water occurred over tin Lewis acid sites followed by the hydrolysis of the Si–O–Sn bonds and the formation of Si–OH and Sn–OH surface species. In this process, the tin atoms change their coordination number from 4 to 6, possessing different kinetics for the different types of Sn sites observed by NMR spectroscopy. The formation of additional catalytically active acid sites through water adsorption on Sn-BEA is demonstrated in situ in the course of isobutene dimerization reaction.

Active sites of nanocrystalline tin dioxide materials with variable particle size, surface area, and catalytic modifiers were studied. Effect of palladium and ruthenium oxides clusters on the activity and concentration of tin dioxide surface centers was evaluated by temperature-programmed desorption techniques using probe molecules, FTIR spectroscopy, EPR, and thermogravimetric methods. The surface site concentration decrease was observed with an increase of SnO 2 particle size and BET area decrease. The active sites of SnO 2 were found to be selectively promoted by the additives. Accumulation of surface OH groups including hydroxyl spin centers and Broensted acid sites was characteristic for SnO 2 /PdO x nanocomposites as a result of water chemisorption enhancement due to proposed electronic clusters−support interaction. Ruthenium oxide was shown to increase the concentration of chemisorbed oxygen species via oxygen spillover route.

Nanocrystalline perovskite-type BaSnO3 was obtained via microwave-assisted hydrothermal route followed by annealing at variable temperature. The samples composition and microstructure were characterized. Particle size of 18–23 nm was unaffected by heat treatment at 275–700 °C. Materials DC-conduction was measured at variable temperature and oxygen concentration. Barium stannate exhibited n-type semiconductor behavior at 150–450 °C with activation energy being dependent on the materials annealing temperature. Predominant ionosorbed oxygen species types were estimated. They were shown to change from molecular to atomic species on increasing temperature. Comparative test of sensor response to various inorganic target gases was performed using nanocrystalline SnO2-based sensors as reference ones. Despite one order of magnitude smaller surface area, BaSnO3 displayed higher sensitivity to SO2 in comparison with SnO2. DRIFT spectroscopy revealed distinct interaction routes of the oxides surfaces with SO2. Barium-promoted sulfate formation favoring target molecules oxidation was found responsible for the increased BaSnO3 sensitivity to ppm-range concentrations of SO2 in air.