Tin Dioxide Gas Sensors Research Articles

Detection of the concentration of CO using SnO 2 gas sensors in combustion gases of different fuels

Thick-film tin dioxide gas sensors together with a commercial gas sensor are tested in the laboratory to oberve the influence of water vapour when sensing CO. Sensors are also tested in combustion processes to determine their ability to detect the concentration of CO. Results show that the semiconductor sensors can follow the concentration of CO in gases emitted during the combustion of different fuels. CO can be detected even when there is a high concentration of water vapour in the gas.

Tin dioxide gas sensors. Part 3.—Solid-state electrochemical investigations of reactions taking place at the oxide surface

The use of standard electrochemical measurements effected through solid oxide electrolytes has proved to be a useful method for observing the surface chemistry of semiconducting oxides in gaseous atmospheres. Open-circuit potential measurements across a stabilized zirconia ceramic with electrodes of tin dioxide and platinum, respectively, yield non-zero values over most of the temperature range between ambient and ca. 400 °C. Such measurements are made without the separation of a reference atmosphere and finite potential differences are attributable to non-equilibrium processes by which differences in surface oxygen potential may arise on the two electrode materials. Open-circuit potentials measured under these circumstances are extremely sensitive to the presence of reducing gases in an air ambient.Current–potential measurements with a relatively small contact area between tin dioxide and the electrolyte allow the derivation of exchance current densities at this interface, and these too are sensitively affected by the presence of reducing gases in air. Measurement of the exchange current density over a range of temperatures has enabled the activation energy for the surface oxygen reaction to be derived (1.4 eV) . This value is in tolerable agreement with the activation with the activation energy obtained from conductivity measurements (1.1 eV) as described in part 1 of this series.

The effect of microstructure on the height of potential energy barriers in porous tin dioxide gas sensors

A theoretical study is made on the height of potential energy barriers formed at intergranular contacts in porous tin dioxide. Expressions are derived for the height of barriers in a one-dimensional case when the donor concentration is constant and also when there is a concentration gradient in the surface. Expressions are also derived for the case when the intergranular contact is in the shape of a cylinder. The treatment in this case, together with the application of Fermi–Dirac statistics, leads to a conclusion of the existence of a distribution of the height of barriers. The potential barrier on the center line of the cylinder vanishes above a critical radius. The effect of the distribution of the height of barriers on the activation energy of the conductance of porous tin dioxide is discussed on the basis of a random barrier network model.

Tin dioxide gas sensors. Part 2.—The role of surface additives

It has been shown earlier (J. F. McAleer, P. T. Moseley, J. O. W. Norris and D. E. Williams, J. Chem. Soc., Faraday Trans. 1, 1987, 83, 1323) that the gas response of porous pellets of tin dioxide depends crucially on electronic surface states involving adsorbed oxygen, and on the rates of combustion reactions involving the gases to be detected. It is expected, therefore, that the use of surface additives capable of either pinning the Fermi level of the tin dioxide or altering the rate of combustion (depending on choice of material and temperature) would profoundly affect the gas sensor response of the material. This is found to be the case. Studies of the influence of surface additions of precious metals and of metal oxide particles on the gas response behaviour of tin dioxide are described. The use of precious metals at temperatures near to ambient imposes an oxygen-independent Schottky barrier on the tin dioxide surface and results in a distinct low-temperature response to carbon monoxide, probably by adsorption on the precious metal modifying the surface potential (and hence the Schottky barrier). This particular effect is expected only when the precious metal particles distributed over the oxide surface are extremely small (1–10 nm): the effect disappears if the catalyst particles are aggregated by heating. The distribution of particles of a foreign oxide (here Ag2O or ZnO) on the surface of the tin dioxide also appears to pin the surface-energy levels to those of the additives, and tin dioxide treated in this way exhibits gas-sensing properties that are modified to resemble those of the additive oxide in bulk. At higher temperatures, especially in the presence of a precious-metal catalyst, the gas is completely oxidised within a thin outer shell of the specimen and consequently the response to the gas of the measured resistance of the pellet disappears. Indeed, a response to the reaction products (H2O and CO2) is obtained.

Tin oxide based gas sensors

The key parameters on which the function of resistance-modulating tin dioxide gas sensors depends nave been studied. A model based on the controlling influence of chemisorbed oxygen molecules is supported by electrical conductivity measurements performed both in air and in vacuo and by measurements of surface state energies evaluated by a novel method involving a solid electrolyte. It has been shown that water vapour behaves as an interference in the detection of gases over the whole range between room temperature and 600°C and as a sensor poison at temperatures below around 300°C. The application of fine particles of precious metals to the surface of the tin dioxide introduces the possibility of an oxygen-independent influence on surface energetics and this is found to substantially modify the gas sensing properties of the material.

Tin dioxide gas sensors. Part 1.—Aspects of the surface chemistry revealed by electrical conductance variations

Detailed measurements are reported of the dependence of the conductance of porous, sintered pellets of tin dioxide, on temperature, moisture and oxygen partial pressure. Comparison is made with the behaviour of thin layers prepared by radio-frequency sputtering. The effect of introducing a non-equilibrium mixture of a combustible gas in air, on the conductance and conductance activation energy is described. The theoretical basis for current interpretations of the behaviour of this material is reviewed. The results are discussed using a model in which the conductance of the porous pellets is controlled by different sorts of intergrain contact, represented as necks, necks depleted of conduction electrons, and Schottky barriers. The results are best rationalised by postulating that the conductance is in fact controlled by Schottky barriers separating domains or agglomerates, each comprising a rather large number of crystallites. The effects of temperature, moisture, oxygen partial pressure and combustible gases are discussed in terms of their effect on the Schottky barrier height by way of altrations in the area density, charge and occupancy of surface states (chemisorbed oxygen species). The existence of a surface state level located ≳ 1.1 eV below the conduction band edge is deduced. Adsorbed water strongly affects both the conductance and the response to combustible gases. Loss of water from the surface over the temperature range 280–450 °C results in a sigmoidal conductance-temperature relationship in moist air. The effect is to lower the resistance at temperatures below this range to no more than one-tenth of the values observed for dried pellets. A surface transformation O2–↔ OH– is inferred. In dried air, with dried pellets, inflexions in the conductance-temperature behaviour at temperatures below 230 °C are tentatively attributed to the effect of O2–↔ O–↔ O–2 surface transformations. The onset of a conductance response to the presence of a combustible gas coincides with the onset of a surface-catalysed combustion. At higher temperatures an effect of combustible gases is to lower the conductance activation energy for the porous pellets, but not for the sputtered layers. The effect of moisture on the response to CO was to extend the response to lower temperature; on dried pellets in dried air the response disappeared abruptly when the temperature fell below 350 °C.

Tin dioxide gas sensors: use of the seebeck effect

This paper describes a novel type of gas sensor, which relies on the thermovoltage generated when one region of a porous semiconductor is heated by the reaction of a combustible gas with oxygen. One of the main attractions of this type of sensor is that the power requirements are minimal. Use is made of a voltage measurement, which distinguishes the device from other semiconductor gas detectors that depend upon the measurement of resistance.