A. V. Marikutsa

Effect of Au and NiO catalysts on the NO2 sensing properties of nanocrystalline SnO2

Nanocrystalline tin dioxide has been synthesized, and its surface has been modified with Au and NiO. Their distributions in the nanocrystalline tin dioxide have been examined by X-ray diffraction and transmission electron microscopy. The NO2 sensing properties of the materials have been studied in the range 100–1000 ppb. Both gold and nickel enhance the NO2 response of SnO2. Codoping with Au and NiO markedly enhances its sensing response and, in addition, lowers the peak response temperature. The observed effect of NO2 concentration in dry air on the sensing response of the SnO2〈Au, NiO〉 nanocomposite can be understood in terms of the sequence of processes that take place on the SnO2 surface upon nitrogen dioxide adsorption in the presence of chemisorbed oxygen.

Nanocrystalline BaSnO3 as an Alternative Gas Sensor Material: Surface Reactivity and High Sensitivity to SO2

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.

Nanocrystalline tin dioxide: Basics in relation with gas sensing phenomena. Part I. Physical and chemical properties and sensor signal formation

The experimental and theoretical concepts of the physical and chemical properties of tin dioxide which favor its use in semiconductor gas sensors are systematized. The interrelations of the band structure, the nature of intrinsic defects, microstructure parameters, and reactivity of nanocrystalline SnO2 during the detection of gases of different chemical nature are considered. The existing concepts which describe the change in electrical properties and the mechanism of the sensor signal formation are analyzed.

A study of the sensitivity of nanocrystalline indium oxide with various sizes of nanocrystals to nitrogen dioxide

The influence of NO2 adsorption on the electric conductance of the nanocrystalline indium oxide with various sizes of nanocrystals has been investigated. When the nanocrystal size decreases, the sensitivity (the ratio of the In2O3 conductance in the air and the conductance after NO2 adsorption) grows at first but then declines. An explanation for the nonmonotonous behavior of the sensitivity is offered.

Charge carrier transport in indium oxide nanocrystals

Nanocrystalline indium oxide samples with various sizes of nanocrystals are synthesized by the sol-gel method. The minimal and maximal average sizes of nanocrystals are 7–8 and 18–20 nm, respectively. An analysis of conductivity measured at dc and ac signals in a wide temperature range (T = 50–300 K) shows that the transport of charge carriers at high temperatures takes place over the conduction band, while in the low-temperature range, the hopping mechanism with a varying jump length over localized states is observed.

Nanocrystalline tin dioxide: Basics in relation with gas sensing phenomena part II. Active centers and sensor behavior

The experimental data and theoretical concepts on the nature and physicochemical properties of the active centers at the surface of tin dioxide are reported, which are involved in detection of toxic and explosive ambient impurities. The active centers formed at the nanocrystal SnO2 surface are classified on the basis of their chemical properties, and their role in the interaction of semiconductor nanocrystal oxides with the gases exhibiting the redox properties is confirmed. The chemical modification of the SnO2 surface aimed at elaborating a controlled amount of specific active centers is shown to be the most efficient method for increasing the selectivity of sensors. Selecting the optimum catalytic modifiers (nanoparticles or clusters of noble metals and their oxides) allows the sensor sensitivity and selectivity of the target gas detection to be increased.

Active sites on the surface of nanocrystalline semiconductor oxides ZnO and SnO 2 and gas sensitivity

The data on active sites on the surface of nanocrystalline semiconductor oxides ZnO and SnO2 are reviewed. Their interrelation to the gas sensitivity of the materials toward the main air pollutants, viz., CO, NO2, NH3, and H2S, is analyzed. The influence of the synthesis conditions, microstructure parameters, content of dopant impurities, and the presence of catalytic modifiers on the concentration of various active sites on the oxide surface is considered. Relationships between the concentration of the surface sites and sensitivity of the oxides to gases with various chemical properties are revealed. The active sites responsible for the formation of a sensory signal upon the selective interaction with molecules of the detected gases are determined.

Effect of Ga and in doping on acid centers and oxygen chemisorption on the surface of nanocrystalline ZnO

Nanocrystalline gallium- and indium-doped zinc oxide samples have been prepared through coprecipitation from aqueous solutions. Acid centers on the surface of the materials have been investigated using temperature-programmed desorption and IR spectroscopy. The results demonstrate that, with increasing dopant concentration, the density of OH groups on the surface of ZnO〈Ga〉 and ZnO〈In〉 increases and the contribution of cation centers to surface acidity decreases. The interaction of the material with oxygen has been studied using in situ electrical conductivity measurements. Doping of zinc oxide with gallium or indium has been shown to increase the percentage of molecular chemisorbed oxygen species on the surface of the material.

Analysis of CO and NH3 Reductive Gases Mixture by Chemically Modified Nanocrystalline Tin Dioxide

The possibility to discriminate two reducing gases CO and NH3 in their mixture by semiconductor gas sensors based on modified nanocrystalline tin dioxide was demonstrated. The selectivity was achieved by the optimization of SnO2 microstructure, surface modifier and operating temperature. Introducing catalytic clusters of palladium or ruthenium oxides increased tin dioxide sensitivity to CO at low temperature and to ammonia at raised temperature, respectively. Cross-sensing tests verified that SnO2/RuO2 sensors are selective to NH3 in comparison with CO, while SnO2/PdO displayed a complex behaviour in the presence of both gases. Possible sensor signal formation routes are discussed regarding the modifiers catalytic action and their distinctive influence on surface active sites.

Interplay between active sites of modified nanocrystalline tin dioxide and selectivity to CO and NH 3 gases

A specific effect of surface modifiers on the active sites of sol-gel synthesized nanocrystalline tin dioxide improving its selectivity to CO and NH₃ gases was demonstrated. Introducing PdO_x clusters led to the increase of OH-groups concentration, while that of RuO_y - to accumulation of active oxygen species on the SnO₂ surface. This interplay contributes to increased low-temperature CO-sensitivity of SnO₂/PdO_x and to high NH₃-sensitivity of SnO₂/RuO_y at raised temperature. Cross-sensitivity tests to CO+NH₃ mixtures in air revealed the possibility to discriminate the reductive gases by modified SnO₂-based sensors operated at appropriate temperature. The impact of different active sites to the selectivity improvement was estimated. In situ DRIFT studies indicated a key role of catalytic clusters in promoting selective SnO₂/PdO_x-CO and SnO₂/RuO_y-NH₃ interactions. Possible sensor signal formation processes were suggested regarding the modifiers catalytic action and their distinctive influence on tin dioxide active sites.