G. Delabouglise

CO and NO2 gas sensitivity of nanocrystalline tin dioxide thin films doped with Pd, Ru and Rh

Abstract The effect of Pd, Ru and Rh doping on microstructure, electrical, and gas sensor properties of nanocrystalline tin dioxide films is studied. SnO2 and SnO2(M) (M=Pd, Ru, Rh) films of 0.9–1-μm thickness and a doping metal content of 0.07–1.6 at.% are synthesized by aerosol pyrolysis. Studies at 50–400 °C of the evolution of conductivity of noble metal-doped tin dioxide films put in contact with gas mixtures containing small amounts of CO and NO2 give evidence of strong similarities of the interaction mechanisms. Correlations between the electrical response and oxygen affinity of noble metal clusters were found.

Influence of Pd and Pt additives on the microstructural and electrical properties of SnO2-based sensors

Abstract For gas sensor applications, thin-films (thickness ∼1 μm) of SnO2(Pd, Pt) have been synthesised on oxidized silicon 〈100〉 in the temperature range of 460–560°C using pyrolysis of an aerosol. By doping with Pd or Pt metallic particles (1–12 wt.%) selectivity and sensitivity were improved. Electrical measurements under 300 ppm of CO were performed on thin films in steady-state and dynamic modes. Sensitivity, rate and response time were studied. The optimum synthesis conditions were determined in order to obtain the best microstructure of the films and the best concentration of doping additives for the detection of the carbon monoxide in the range of room temperatures. The mechanism of the sensor response to CO and the role of the additives (Pd, Pt) on the basis of chemical and electronic mechanisms were discussed.

Undoped and Pd-doped SnO2 thin films for gas sensors

Abstract The aim of this work is the study of the structure and temperature dependence on the conductance and sensitivity of undoped and Pd-doped SnO2 thin films (0.5–2 μm). Polycrystalline SnO2 thin films have been synthesized by pyrolysis, on a heated substrate (oxidized silicon 〈100〉), of an aerosol produced by ultra-high frequency spraying of a volatile precursor solution. A mixture of two precursors is used to obtain Pd-doped SnO2. Growth rate and microstructure of the films are particularly well controlled by the deposition temperature (460–560 °C). Conductivity measurements were performed between 50 and 500 °C, alternately under pure air (G0 and polluted air (G) (ethanol or CO), using gold layers as electrodes. The pure SnO2 sensitivity (G  G0)/G0) to ethanol increases when the elaboration temperature decreases, in connection with the specific area increase. Palladium incorporation significantly promotes the sensitivity. CO sensitivity is increased by 30 when palladium is incorporated into the SnO2 thin films and exhibits a marked peak at low temperature. The sensitivity stability is better when the deposition temperature is higher, according with a better stability of the microstructure. A variable frequency electrical study was carried out using complex impedance spectroscopy (1 mHz–20 MHz). Ethanol and CO reactions were observed and the role of grain boundaries studied.

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.

Pd-doped SnO2 thin films deposited by assisted ultrasonic spraying CVD for gas sensing: selectivity and effect of annealing

Polycrystalline Pd-doped SnO2 thin films (0.25–1.75 μm) have been deposited by spray pyrolysis on silicon nitride substrate heated between 460 and 540°C. The gas sensitivity (S=(Ggas−Gair)/Gair) in the steady-state and in a dynamic regime have been evaluated under CO (300 ppm in air), ethanol (100 ppm in air) and methane (1000 ppm in air) as a function of temperature. The thin films synthesised at lower temperature (460–500°C) are the most sensitive to CO according to their smaller crystallite size and higher resistivity. In the steady-state, sensitivities up to 4500 have been obtained for the thinnest films at low working temperature of 100°C. A cross-sensitivity to ethanol and to methane was observed. Annealing under pure air stabilises the microstructure and increases the resistivity in air of the Pd-doped SnO2 thin films, which provides a sensitivity to carbon monoxide 2 to 10 times higher, since the resistance of the films under CO remains unchanged.

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.

Capacitance effects and gaseous adsorption on pure and doped polycrystalline tin oxide

Abstract Conductance and capacitance effects have been studied simultaneously on thin films and ceramics of tin oxide by using d.c. conductance and impedance measurements. Pure SnO2 and SnO2-CuO composite materials were studied. The total resistance of the samples is mainly due to the grain boundaries and the adsorption on these boundaries of gaseous species with high dielectric constant as ethanol leads to a high capacitance value. This capacitance effect is more important in ceramics than in thin films, n-type SnO2-type CuO composite materials exhibit particular capacitance and conductance responses under pollutant gas, due to n–p junctions at the SnO2-CuO interfaces.

Synthesis, structure and gas sensitivity properties of pure and doped SnO2

Polycrystalline SnO2 showing small particle size has been synthesized by means of pyrolysis in a tubular furnace of an aerosol produced by ultrahigh-frequency spraying of a solution. Pd nanoparticles have also been dispersed on the surface of cassiterite grains. In both cases, three solution precursors have been used: chloride, oxalate and sulfate. An SEM study indicates that two kinds of textures are obtained depending on the precursor solution: hollow spherical particles or quasi-spherical grain agglomerates. Cold pressing and annealing modify such morphologies. Conductance G = f(T) curves were studied from 50 to 500 °C under pure air (G = G0), 80 ppm ethanol or 300 ppm CO. G = f(T) curves under polluted air show different behaviors. Most of them show a pronounced maximum and corresponding sensitivity S = (G - G0)/G0 varies between 10 and 390.

Thin films of magnesium oxide by modified CVD : a buffer layer for HTCS films

Abstract Magnesium oxide thin films, as buffer layers for depositing high-T c superconducting thin films, were prepared by a modified CVD process. This process uses pyrolysis of an aerosol produced by ultra-high frequency spraying of a solution. The raw material was magnesium acetate dissolved in ethanol or butanol. The deposition was carried out in air between 450 and 600°C and thickness varies between 100 and 500nm. A highly 〈100〉 oriented polycrystalline film can be obtained on oxydized silicon wafers. The influence of deposition parameters on the film growth was examined by X-ray, SEM. Grain size varies between 60 and 150 nm. Electrical properties were examined by complex impedance spectroscopy and I(V) characteristic: resistivity (>10 12 Ω.cm) and dielectric constant.