T. V. Belysheva

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.

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.

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.

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.