M. N. Rumyantseva

CuO/SnO2 thin film heterostructures as chemical sensors to H2S

Abstract CuO/SnO2 heterostructures as well as SnO2(CuO) polycrystalline films have been studied for H2S sensing. Gas sensing properties of these materials have been compared in conditions: 25–300 ppm H2S in N2 at 100–250°C. A shorter response time of the heterostructures as compared to that of the SnO2(CuO) films has been found. It is suggested that the improvement of dynamic sensor properties of SnO2/CuO heterostructures is caused by the localization of electrical barrier between CuO and SnO2 layers.

Effect of combined Pd and Cu doping on microstructure, electrical and gas sensor properties of nanocrystalline tin dioxide

The effect of combined Pd and Cu doping on microstructure, electrical and gas sensor properties of nanocrystalline tin dioxide was studied. SnO2, SnO2(PdO), SnO2(CuO), and SnO2(PdO+CuO) films thickness of 0.8–1 μm with doping metal content 0.5–1.6 at.% were synthesized by aerosol pyrolysis. An average SnO2 grain size decreased with the addition of both Pd and Cu. The resistance measurements at 77–373 K showed that all types of doping induce resistivity increase accompanied by the appearance of conductivity activation process. Conductivity transients in the presence of CO were studied at 323–523 K. For the samples doped with Pd the sensor response to CO was found to be comparable with the resistivity increment induced by Pd incorporation into SnO2 matrix. To reveal the effect of CO on the conductivity the low temperature resistance was measured for the films in non-equilibrium state reached by cooling down the film exposed to CO at T=523 K. Experimental data proved that CO adsorption may be regarded as a factor neutralizing the Pd doping action on the films conductivity. The catalytic effect of Pd clusters was found in the interaction of SnO2(PdO+CuO) films with CO.

Dopants in nanocrystalline tin dioxide

The review surveys studies aimed at constructing new materials for gas sensors based on nanocrystalline tin dioxide. The influence of doping with various impurities (Pt, Pd, Ru, Rh, Cu, Ni, or Fe) on the composition, microstructure, and electrophysical and sensor properties of nanocrystalline SnO2 was discussed. The conditions for the preparation of powders and thick and thin SnO2 films by the wet chemical method and aerosol pyrolysis of organometallic compounds are reported. The mechanism of interaction of pure and doped nanocrystalline SnO2 with a gas phase was analyzed based on the data from Mossbauer, Auger electron, and X-ray photoelectron spectroscopy and the results of in situ Raman spectroscopy, XANES, and conductivity measurements.

Nature of gas sensitivity in nanocrystalline metal oxides

Problems associated with developing gas-sensitive inorganic materials are discussed. The principle of operation of a semiconductor gas sensor of resistive type is considered, and main band structure parameters sensitive to the gas phase composition are determined. Ways to solve the problem of selectivity of semiconducting oxides are discussed. The influence of microstructure, dopants, and analysis temperature is looked into on the basis of experimental results obtained in studying nanocrystalline tin dioxide and zinc oxide. Prospects for use of systems based on two or more nanocrystalline oxides (nanocomposites) are considered.

Microstructure and electrophysical properties of SnO2, ZnO and In2O3 nanocrystalline films prepared by reactive magnetron sputtering

The influence of oxygen concentration in the plasma-forming gas on the microstructure, phase composition and electrical conductivity has been investigated for SnO2, ZnO and In2O3 films grown by reactive magnetron sputtering method. The evolution of the oxide microstructure and resistivity under annealing at 400 8C was also studied. The nanocrystallite size remains unaltered for all the investigated films, while the agglomerate size varies significantly depending on the type of oxide and annealing duration. The agglomerate size growth leads to reduction of film resistance and conductivity activation energy. # 2002 Elsevier Science B.V. All rights reserved.

Effect of interdiffusion on electrical and gas sensor properties of CuO/SnO2 heterostructure

The influence of annealing on the electrical and H2S gas sensor properties of p-CuO/n-SnO2 heterostructures has been investigated. The heterostructures were prepared by magnetron sputtering technique with subsequent oxidation. The depth composition was analyzed by the secondary neutral mass-spectrometry (SNMS) method. The Sn and Cu interdiffusion coefficients at 573 K were estimated as 3×10−14 cm2 s−1 and 1×10−15 cm2 s−1, respectively. Resistance behavior and high H2S gas sensitivity are associated with the formation of a transitional layer at CuO–SnO2 interface due to interdiffusion processes.

Mechanism of sensing CO in nitrogen by nanocrystalline SnO2 and SnO2(Pd) studied by Mössbauer spectroscopy and conductance measurements

The mechanism of CO/N2 sensitivity of undoped and Pd-doped nanocrystalline tin dioxide was studied in situ by coupled electrical measurements and 119Sn Mossbauer spectroscopy. Nanocrystalline SnO2 with a grain size of about 6–8 nm was synthesized by the sol–gel method. SnO2(Pd) ([Pd] = 1 at%) was prepared from SnO2 by the impregnation method. Electrical properties of SnO2 and SnO2(Pd) in the presence of CO/N2 were studied at a fixed temperature in the range 50 ≤ T ≤ 380 °C. Addition of Pd strongly increases the sensor response to CO over the whole temperature range. It was shown by Mossbauer spectroscopy that two different mechanisms of gas sensitivity can exist in the presence of CO/N2. The first “low temperature” mechanism (T < 100 °C) may be attributed to redox reactions between surface oxygen and CO catalyzed by PdOx clusters. The second “high temperature” one (125–380 °C) involves oxygen vacancies diffusing into the bulk of the tin dioxide grains which leads to reversible reduction of Sn(IV) to Sn(II).

Copper and nickel doping effect on interaction of SnO2 films with H2S

The pyrosol spraying deposition technique has been used for the synthesis of SnO 2 , SnO 2 –CuO and SnO 2 –NiO polycrystalline films with grain size of 3–10 nm. The composition, microstructure and electrical properties of the films have been investigated by X-ray diffraction, electron probe microanalysis, Auger electron spectroscopy and X-ray photoelectron spectroscopy. The interaction of SnO 2 , SnO 2 –CuO and SnO 2 –NiO polycrystalline films with the reducing gases: H 2 S, C 2 H 5 OH, CO and CH 4 has been investigated by conductance measurements. It has been found that copper and nickel have a significant effect on the sensitivity of SnO 2 films to H 2 S. The model of interaction of SnO 2 films with H 2 S gas and different sensor properties of tin dioxide films doped with copper and nickel are discussed with regard to the position of these metals in the films.

Influence of copper on sensor properties of tin dioxide films in H2S

Abstract The interaction of SnO 2 (CuO) polycrystalline films with H 2 S + N 2 gas mixtures has been investigated by conductivity measurements, Auger electron spectroscopy, secondary neutral mass spectrometry and X-ray diffraction, A significant effect of film conductivity increase in the presence of H 2 S has been found. It is suggested that the reversibility of the electrical properties of the doped films may be produced by a change in the copper state in the tin dioxide by means of selective chemical reactions with H 2 S and oxygen accordingly.

Materials for solid-state gas sensors

The major problems in the development of inorganic gas-sensor materials are discussed. The general principle of semiconductor gas sensors is considered, and the band-structure parameters sensitive to the gas-phase composition are determined. Ways of improving sensor selectivity are examined. The composition, microstructure, and gas sensitivity of nanocrystalline SnO2 and ZnO are investigated. The dopant content and grain size of Ni-doped SnO2 are optimized for H2S detection. The prospects of employing systems of two or more nanocrystalline oxides (nanocomposites) for gas detection are discussed