E. Yu. Spiridonova

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 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.

Structural properties of metal oxide nanocomposites: Effect of preparation method

In2O3 + CeO2 and In2O3 + ZnO nanocomposites are prepared by the mixing of commercial nanopowders of respective oxides or the impregnation of indium oxide nanoparticles with cerium or zinc salts and the subsequent transformation of the salts into respective oxides. According to X-ray diffraction analysis, regardless of the preparation method, only two phases—indium oxide and cerium or zinc oxide—are present in the samples. The sizes and structure of the nanoparticles in the nanocomposites are determined by transmission electron microscopy and X-ray diffraction analysis. It is found that the use of the impregnation method leads to the formation of small clusters of cerium or zinc oxides on the surface of the indium oxide nanoparticles. The size of the cerium and zinc oxide nanoparticles in the impregnated samples is 3–15 and 5–25 nm, respectively. The size of nanoparticles of these oxides in the impregnated samples slightly decreases with an increase in their content in the composite.

Sensor Properties of Nanostructured Systems Based on Indium Oxide with Co 3 O 4 or ZrO 2 Additives

The effect of Co3O4 and ZrO2 additives on the sensory response of In2O3-based nanostructured composites to H2 and CO is studied. It is shown that the addition of small amounts of Co3O4 or ZrO2 to In2O3 leads to a sharp increase in the sensory response to hydrogen. The maximum sensory response of the ZrO2−In2O3 composite to 1100 ppm of hydrogen increases from 80 to 270 as the ZrO2 content changes 0 to 20 wt %. The response to CO varies only slightly. For Co3O4−In2O3 composites, the maximum response to H2 and CO increases with the Co3O4 content within 0−10 wt %. A further increase in the Co3O4 content leads to a significant decrease in the response, with composites containing ∼60 wt % Co3O4 being characterized by a very low efficiency. In the Co3O4−In2O3 system with a content of up to 60 wt % Co3O4, electronic conduction is realized, which changes to hole conduction at Co3O4 within 80−100 wt %. In the ZrO2−In2O3 system, electric current flows through In2O3 nanocrystals, i.e., n-type conduction takes place. Possible reasons for the observed effects are discussed.

Conductivity of nanostructured India oxide films containing Co3O4 or ZrO2

The effect of additives of cobalt and zirconium oxides on the conductivity of nanostructured composites based on indium oxide is studied. It is shown that addition of up to 20 wt % ZrO2 to In2O3 leads to a sharp decrease in the conductivity of the composite. For the Co3O4−In2O3 system, the conductivity decreases up to a Co3O4 content of 60 wt %, after which it increases. At a Co3O4 content in the Co3O4−In2O3 system of up to 60 wt %, n-type conduction takes place, changing to p-type at 80 to 100 wt % Co3O4. Zirconium oxide exhibits practically no n-type conduction, so electric current in the ZrO2−In2O3 system flows through In2O3 nanocrystals, i.e., n-type conduction takes place. Possible causes of the observed effects are considered.

Investigating the sensor response of ceria-containing binary metal oxide nanocomposites

The hydrogen sensing performance of ceria-containing nanocrystalline indium and tin oxides is investigated for different concentrations of added ceria. The sensor response of nanocrytsalline In2O3 is considerably enhanced at low CeO2 concentrations. In contrast, low levels of CeO2 cause a substantial drop in the sensor response of SnO2-based composite; at a 3 wt % level of added ceria, its hydrogen sensing ability is almost entirely suppressed. Possible causes of these effects are investigated via X-ray photoelectron spectroscopy (XPS) and X-ray diffraction. XPS data show that additions of CeO2 have different effects on the structure of the base oxides (In2O3 and SnO2), with implications for the hydrogen sensing performance of the composites.

Sensory properties of oxide films with high concentrations of conduction electrons

The dependence of a sensor’s response to hydrogen on the temperature and hydrogen pressure in an indium oxide nanostructured film is measured. A theory of sensor’s response to reducing gases in nanostructured semiconducting oxides with high concentrations of electrons in the conduction band is developed (using the example of In2O3). It is shown that the capture of conduction electrons by adsorbed oxygen redistributes the electrons in nanoparticles and reduces the surface electron density and the conductivity of a system; the conductivity is proportional to the electron density in nanoparticle contacts, i.e., to the surface electron density. It is found that atomic oxygen ions react with reducing gases (H2, CO) during adsorption of the latter: electrons are released and enter the volumes of nanoparticles; the conductivity of the system grows, creating the sensory effect. Using a model developed earlier to describe the distribution of conduction electrons in a semiconductor nanoparticle, a kinetic scheme corresponding to the above scenario is built and corresponding equations are solved. As a result, a theoretical dependence of a sensor’s sensitivity to temperature is found that describes the experimental data well.

The optical and gas-sensitive properties of opal-like structures based on SnO2

Opal-like materials based on tin dioxide were prepared, and their structural and sensor characteristics were studied. The optical transmission spectra of opal-like structures based on SnO2 were recorded, and the volume fraction occupied in them by tin dioxide was estimated. It was shown that structures based on SnO2 contained a photon stop-zone in the visible spectrum range. The sensor properties of the materials toward CO and H2 were studied over the temperature range 375−425°C. The SnO2 samples studied had much higher sensitivity to CO compared with SnO2 materials without opal-like structures.