L. I. Trakhtenberg

Conductivity of SnO2-In2O3 nanocrystalline composite films

The conductivity of films consisting of a mixture of SnO2 and In2O3 nanocrystals at 200–500°C was studied. Based on the experimental data, it was assumed that in films containing less than 20 wt % In2O3, the current flows along SnO2 nanocrystals. A model of conductivity in these films is presented; it includes an electron transfer from In2O3 to SnO2, which forms positively charged In2O3 nanocrystals that contact the negatively charged SnO2 nanocrystals. In the presence of In2O3 nanocrystals, the activation energy of the electron transfer between SnO2 nanocrystals decreased substantially because of a decrease in the barrier of electron transfer between SnO2 crystals under the action of the negative charge. As a result, a percolation cluster of charged SnO2 crystals formed. At high contents of In2O3 (over 20 wt %), the conductivity increased dramatically. The curve of the temperature dependence of conductivity changed because of the appearance of a percolation cluster of In2O3 nanocrystals, in which the current passed. The conductivity of a mixed film of this kind differed from that of the nanocrystalline film of pure In2O3.

Sensor Effect Theory for the Detection of Reducing Gases

A theory of sensor response to reducing gases in nanostructured semiconducting oxides was developed for the example of SnO2. Donor impurities (oxygen vacancies) provide noticeable electron density in the conduction band. Oxygen atoms, which appear in the adsorption of oxygen on the surface of oxide nanoparticles, are electron traps; they sharply decrease system conductivity. In the adsorption of reducing gases (H2, CO), oxygen atoms react with them, electrons are released, and conductivity increases; this is the sensor effect. A kinetic scheme corresponding to the picture described above was constructed, and the corresponding equations were solved. As a result, the dependences of sensor sensitivity on temperature, hydrogen pressure, and the mean size of oxide nanoparticles were obtained. The dependences satisfactorily described the literature experimental data.

Structure and Physicochemical Properties of Nanostructured Metal Oxide Films for Use as the Sensitive Layer in Gas Sensors

The morphological features of nanostructured films of tin, zinc, indium, and cerium oxides are established. The parameters of electron traps, such as adsorbed oxygen atoms and structural defects, responsible for the sensory effect are determined. An increase in the conductance of indium oxide films upon annealing in vacuum is revealed.

Sensor properties of the nanostructured In2O3-CeO2 system in detection of reducing gases

The sensor properties of nanostructured In2O3-CeO2 composite films with different compositions in hydrogen and carbon monoxide detection in air in the temperature range 280–500°C were studied. The temperature curves of the sensor effect S have a shape typical for metal oxide sensors with maxima Smax at definite temperatures Tmax. The maxima characterize the sensor properties of the films and increased considerably when small amounts of CeO2 were added to In2O3. The highest sensitivity was found in composite films with 3–10 wt % CeO2. When the composite was further enriched with ceric oxide, the sensitivity decreased; at 40 wt % CeO2 it was considerably lower than that of pure In2O3. The introduction of CeO2 in In2O3 also caused a shift of Tmax toward lower temperatures. The mechanism of the sensitivity of the In2O3-CeO2 composite was considered; it includes the promotion of sensor reactions by small CeO2 nanoclusters lying on the surface of In2O3 crystals and an electron transfer from In2O3 to CeO2.

The sensor properties of SnO2 · In2O3 nanocomposite oxides in the detection of hydrogen in air

The sensor properties of In2O3 · SnO2 polycrystalline films having different compositions were studied in the detection of 2% hydrogen in air over the temperature range 330–530°C. Films containing 19% In2O3 were most sensitive to hydrogen. The temperature dependence of the sensitivity of sensors passed a maximum, the position of which depended on the composition of the film. The temperature at which sensor sensitivity was maximum decreased as the content of indium oxide increased. This temperature was 485°C for the SnO2 film and 425°C for the In2O3 film. The response and relaxation times of sensors also decreased as the amount of In2O3 in the composite metal oxide film increased. Possible mechanisms of the sensor sensitivity of the films are discussed.

Small CeO2 clusters on the surface of semiconductor nanoparticles

Nanocomposite sensors containing CeO2 clusters on the surface of In2O3 and SnO2 crystals were synthesized. The structure of these systems was determined by Raman spectroscopy. In the CeO2 nanoclusters deposited on In2O3 crystals, the Ce-O vibration frequency was 462 cm−1 and did not depend on the CeO2 concentration. The Raman spectra of the clusters deposited on SnO2 crystals contained two peaks of Ce-O vibrations with frequencies of 462 and 470 cm−1. It was concluded that the peak at 470 cm−1 showed itself at low CeO2 concentrations in the composite (1–3 wt %) and its intensity quickly decreased as the CeO2 concentration increased; this peak was attributed to the CeO2 clusters that directly contact the SnO2 crystals and contain dissolved Sn+4. It was shown that when CeO2 was deposited on In2O3, the In+3 ions were not transferred into the deposited CeO2 clusters because of the difference between the charges and valences of the metal ions in the substrate and clusters; the mean size of the clusters was 9 nm. The relationship between the structure of the CeO2 nanoclusters and their influence on sensor effects was discussed.

Mechanism of the conductivity and sensor response of nanostructured In 2 O 3 +ZnO films

The conductivity and sensor properties of mixed nanostructured In2O3+ZnO metal oxide systems with different component ratios are investigated. It is found that maximum sensor sensitivity in detecting hydrogen and CO in composite films containing 15 and 80 wt % In2O3 considerably exceeds the sensitivity of individual oxides. A mechanism of the sensor action, which is largely determined by the dependency of the paths of conductivity in a composite metal-oxide film on its composition, is proposed. It is established that the main factors determining the conductivity and sensor sensitivity of In2O3 + ZnO composite are modifications in the electron structure of crystals (mainly by In2O3) during the formation of composites, electron transfer from In2O3 to ZnO, and the catalytic activity of ZnO. It is shown in particular that ZnO effectively catalyzes the reaction of hydrogen dissociation and, in contact with In2O3, favors the chemical sensibilization of the sensor response of such mixed metal oxide systems in detecting H2 and CO.

The sensor properties of Fe2O3• In2O3 films: The detection of low ozone concentrations in air

The detection of low ozone concentrations in air (no higher than 120 ppb) using semiconducting films based on Fe2O3 · In2O3 obtained by laser ablation of the corresponding targets onto alumina substrates was studied. The temperature of the substrate during film deposition influenced their sensor properties. Temperature effects on the sensitivity of the films with respect to ozone were studied over the temperature range 200–380°C. Maximum sensitivity was reached at 250°C irrespective of the temperature of film deposition. The dependence of film sensitivity on the concentration of ozone in air was determined. At equal ratios between In2O3 and Fe2O3, the sensitivity of the sensor films prepared by laser ablation was much higher than that of thick-film sensors obtained from aqueous metal oxide suspensions by the stenciling technique. Possible reasons for the effects observed were considered.

Theory of slightly fluctuating ratchets

We consider a Brownian particle moving in a slightly fluctuating potential. Using the perturbation theory on small potential fluctuations, we derive a general analytical expression for the average particle velocity valid for both flashing and rocking ratchets with arbitrary, stochastic or deterministic, time dependence of potential energy fluctuations. The result is determined by the Green’s function for diffusion in the time-independent part of the potential and by the features of correlations in the fluctuating part of the potential. The generality of the result allows describing complex ratchet systems with competing characteristic times; these systems are exemplified by the model of a Brownian photomotor with relaxation processes of finite duration.

Effect of the composition and structure of metal oxide nanocomposites on the sensor process when detecting reducing gases

The effect the nature of metal oxide components, quantitative and qualitative composition, structure of binary metal oxide nanocomposites, and temperature have on the physicochemical processes that occur during the detection of reducing gases and are responsible for the efficiency and selectivity of sensors based on these composites is considered. The relationship between the mechanisms of the conductivity and sensor effect in composites is determined. The crucial role of electron transfer between metal oxide components with different work functions leading to the mutual charging of these components is noted. The mechanisms of electronic and chemical sensitization of the sensor effect in composite materials consisting of metal oxides with various electronic and chemical properties are discussed. The important role of the way composite materials are obtained is noted. The effect of small clusters of one oxide on the surfaces of nanoparticles of other components, formed during the synthesis of composites via impregnation, is studied. Systems consisting of composite nanofibers of the core–shell type based on metal oxides of different natures are considered. It is shown that by changing the nature of the components and their relative location in the nanofibers, the sensitivity and selectivity of a sensor system can be adjusted for different chemical compounds.