A. M. Gas’kov

The electrical conductivity of polycrystalline SnO2(Cu) films and their sensitivity to hydrogen sulfide

The effect of doping with copper on the sensor properties and the electrical conductivity of polycrystalline SnO2(Cu) films has been investigated. It has been found that at room temperature the residual conductivity is observed after the films are exposed to H2S. This made it possible to determine the character of the low-temperature conductivity of the films for different degrees of saturation with hydrogen sulfide. A comparison of the obtained data with the results of layerwise elemental analysis suggested a model that explains the mechanism of the gas sensitivity of SnO2(Cu) to hydrogen sulfide. In contrast to the mechanisms, which are associated with the work done by the surface and which are standard for gas sensors, in the present case the change in the conductivity is due to the chemical reaction of the electrically active copper with sulfur in the entire volume of the film. This reaction determines the selectivity and high sensitivity of SnO2(Cu) to H2S.

Influence of the microstructure of semiconductor sensor materials on oxygen chemisorption on their surface

The correlations between the microstructure and oxygen chemisorption on the surface of nanocrystalline SnO2 and In2O3 were established. The activation energy of oxygen chemisorption was estimated, and the chemisorbed species dominating in the temperature range 200–400°C (working temperature range of semiconductor gas sensors) was identified. An increase in the crystallite size tends to decrease the effective activation energy of oxygen chemisorption and to increase the surface coverage with chemisorbed oxygen and the contribution from atomic species.

Impedance spectroscopy of ultrafine-grain SnO2 ceramics with a variable grain size

The impedance spectra of nanocrystalline SnO2 ceramics with an average grain size d ranging from 3 to 43 nm were investigated in the frequency range 1–106 Hz at temperatures from 25 to 300°C in a dry oxygen atmosphere. Analysis of the experimental data by the graphoanalytical method made it possible to separate the contributions of grain bulk and grain boundaries to the conductivity. It is shown that the samples investigated can be arbitrarily divided into two groups with respect to the character of their conductivity. For the samples with an average grain size d < 25 nm, the charge transfer processes are almost completely determined by the grain boundaries. In samples with a larger grain size, the contribution of grain bulk to the conductivity is comparable with that of grain boundaries.

Metal-oxide based nanocomposites as materials for gas sensors

Correlations between the composition, structure, and sensor properties of SnO2-MIIO (MIIO = Fe2O3, MoO3, V2O5) nanocomposites prepared by wet chemistry synthesis were elucidated. The elemental and phase compositions of the materials, distribution of components between the bulk and surface, particle size, and specific surface area were examined. Surface modification of semiconductor oxides allows controlling the type and density of surface acid centers and redox properties of materials. The result is an increase in the sensor selectivity.

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.

Microstructure and sensing properties of nanocrystalline indium oxide prepared using hydrothermal treatment

Nanocrystalline indium(III) oxide samples were prepared using soft chemistry. The microstructure (particle size and shape, specific surface area, pore-size distribution) and sensing properties of In2O3 were studied as functions of synthesis parameters. Indium oxide synthesis including hydrothermal treatment produces materials having high sensing sensitivity in NO2 detection.

Conductivity of structures based on doped nanocrystalline SnO2 films with gold contacts

The conductivity of nanocrystalline Pt-, Pd-, and Ni-doped tin dioxide films on insulating SiO2 substrates is investigated in the temperature range 77–400 K. Doping allows variation of the resistance from 104 to 107Ω. It is established that, in contrast with a Au/single-crystal SnO2 contact, the gold contacts for the nanocrystalline material are ohmic in the entire temperature range and their contribution to the conductance of all the structures investigated does not exceed 5%.

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.

Voltage effect on the sensitivity of nanocrystalline indium oxide to nitrogen dioxide under ultraviolet irradiation

The influence of the applied voltage on the sensitivity of nanocrystalline indium oxide to nitrogen dioxide under ultraviolet irradiation at room temperature was studied. The sensitivity of nanocrystalline indium oxide to nitrogen dioxide increases with increasing applied voltage. This effect is presumably explained by the influence of the voltage on the bending of energy levels existing on the sample surface.