M. Labeau

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.

Electrical properties evolution under reducing gaseous mixtures (H2, H2S, CO) of SnO2 thin films doped with Pd/Pt aggregates and used as polluting gas sensors

Abstract An analysis of the electrical properties evolution of a series of SnO2 thin films doped with small amounts of Pd or Pt under pure air and air with 300 ppm CO is presented. As several types of chemical reactions are clearly involved in the solid–gas interactions at the film surface, it has been necessary to simplify the system by favouring interactions in absence of oxygen. Films were consequently also put in contact with N2 or Ar mixed with small amounts of H2, CO, H2S to favour behaviours in absence of oxygen gas. As under air+CO at 300°C, important features of Pd/Pt-doped film conductance closely resemble CO2 production rates of CO oxidation at the surface of Pd aggregates reported in the literature, an attempt of interpretation of the conductance evolution has been made along this line. Under CO and H2 mixed with neutral gas, dynamic (kinetic) electrical conductance measurements show that the dispersions of metallic elements induce a two-step time dependent behaviour. The first step is associated with a reduction of the oxygen molecules adsorbed at the SnO2 grain surface and an increase of electron density in the SnO2 depletion zone. The second step with a sharp conductance increase implies a reduction of the metallic aggregates and an electron transfer from the aggregates to the SnO2 grain conduction band. For H2S the conductance increases smoothly.

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.

Sensor properties of Pt doped SnO2 thin films for detecting CO

Abstract Polycristalline Pt-doped SnO2 thin films have been integrated to silicon substrate by ultrasonic spray deposition. This deposition technique differs from the usual SnO2 deposition methods by using a liquid source. It allows one to obtain a very fine and homogeneous dispersion of Pt aggregates which act as a catalyst for the low temperature CO detection (25–100°C) by conductance change. The influence of synthesis temperature (460–560°C), concentration of Pt additive (0.1–5 at.%) on gas sensitivity has been studied. The realisation of gas sensor includes a gas sensitive highly porous layer (SnO2/Pt, thickness: ∼1 μm). The results of electrical measurements under 300 ppm of CO for thin films in a dynamic and quasistatic regime are discussed. The narrow peak of gas sensitivity in the range of low temperatures (25–100°C) is obtained for about 2 at.% Pt in the SnO2 film.

Electrical properties under polluting gas (CO) of Pt- and Pd-doped polycrystalline SnO2 thin films: analysis of the metal aggregate size effect

Abstract In Pt- or Pd-doped SnO2 thin films prepared by using a submicronic aerosol pyrolysis method where both metal elements are co-deposited, the best sensitivity to CO is obtained from low-concentration Pd/Pt precursor solutions. TEM observations from SnO2 thin films doped with small amounts of platinum have been combined with results obtained from in situ X-ray Absorption Spectroscopy (XAS) to give a schematic description of the metallic particle effect on the electrical properties of the films. It is demonstrated that the lower the concentration, the smaller are the metallic particles. The evolution of the particle size and density as a function of concentration can be understood in terms of nucleation-growth processes. The XAS analysis shows, in particular, that the Pt local environments are modified by metal concentration variations and by the gas in contact with the film surface. The modifications are discussed on the basis of competing bulk and surface contributions to CO reduction processes, the latter being dominant in smaller particles. It is inferred that smaller particles contribute as a whole to the electrical behavior whereas in bigger particles, a surface and a bulk contribution have to be considered.

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.