The Effects of Residual Chlorides on Gas Sensing Characteristics of Tin Oxide Gas Sensors

Abstract

In an investigation on carbon monoxide sensors, the effect on electrical conductivity of residual chloride content of tin oxide materials have been studied to provide information relating to the sensitivity and selectivity in the detection of carbon monoxide. The starting material was a-stannic acid, precipitated in the reactidn between tin chloride and ammonia, which was calcined at 650~ for 2 h. The lower the chloride content of the original dried precipitate, the greater was the surface area and the greater was the sensitivity to carbon monoxide of the calcined material. Conversely, the higher the chloride content, the smaller was the surface area and lower the sensitivity to carbon monoxide and higher the sensitivity to propane and methane. The residual chloride content in the calcined material, whatever the amount of chloride in the original precipitate, was invariably about 0.2 weight percent.The high conductivity of SnO2 is believed to arise from the presence of native defects, i.e., oxygen vacancies and/or interstitial tin ions associated with the nonstoichiometric composition. The effect of chloride in oxide has been studied mainly in respect of films. In the case of pure SnO2 films, i.e., undoped material but containing residual chlorides, conductivity has been attributed to a combination of chloride ions and oxygen vacancies remaining from the incomplete decomposition of SnC14 and the incomplete oxidation of the film to stoichiometric SnQ (1-4). Most of the investigations concerned with residual chloride deal with the effect of the impurity on electrical properties. In the present work, however, the emphasis has been on the different physical properties of heat-treated materials, differences apparently related to the chloride content of the precipitates.Selectivity to different gases appears to be associated with different physical properties, i.e., the grain size and surface area of the sensing materials which, whatever the chloride content in the original precipitate, invariably contained the same amount of chloride after heat-treatment. The most useful analytical data on chloride in SnO2 was that on a film made by vapor deposition from SnC14 (5). According to this work, the lowest residual chloride content, 0.3 weight percent (w/o), was established after heat-treatment in hydrogen; it was 0.8-1.2 w/o following that treatment in nitrogen or oxygen. This may be compared with the residual content of 0.2 w/o after heat-treatment found in the present work. This chloride content, 0.2 w/o, justified a comparison in the electrical properties between the film and the sintered materials of the present work.The effect of operating temperature, residual chloride contents, and calcination atmosphere on sensitivity to CO are examined. Measurements on propane and methane were also carried out for comparison purposes.

Room-Temperature Hydrogen-Sensitive Pt-SnO2 Composite Nanoceramics: Contrasting Roles of Pt Nano-Catalysts Loaded via Two Different Methods

“…Moreover, for this sintering temperature, the sampl prepared using the solution reduction method still exhibited a much more a ractive room-temperature hydrogen sensing capacity than the other one prepared using th impregnation method. Chlorine-containing precursors are frequently used to load noble metal nano-catalysts in many applications, while chlorine is generally known to have adverse effects on these nano-catalysts [37][38][39]. A post-treatment is mainly adopted to remove chlorine in the precursors effectively, and the thermal removal of chlorine from H 2 PtCl 6 can be expressed explicitly as [37,40,41]:…”

Preparation and Process Chemistry of SnO2 Films Derived from SnC2O4 by the Aqueous Sol–Gel Method

“…prepared SnO 2 films from organic tin tartrate solved in water/ethanol mixed solutions but still added a small amount of inorganic acids for stabilizing the precursor solutions. Apparently, the counterions and impurity ions existing in the sols are harmful for controlling film properties 21–23 . For example, using chloridate as precursor will lead to random, n ‐type doping in the final samples.…”

“…Apparently, the counterions and impurity ions existing in the sols are harmful for controlling film properties. [21][22][23] For example, using chloridate as precursor will lead to random, n-type doping in the final samples. So, it is still a challenge to obtain high-quality SnO 2 films derived from highly pure sols with no counterions and a mild chemical property, i.e., a neutralized pH condition.…”

A portable gas sensor based on In2O3@CuO P–N heterojunction connected via Wi-Fi to a smartphone for real-time carbon monoxide determination

“…In the final sensing materials layers, there has been a minimal presence of impurities. Son et al have previously reported on the negative effect of impurities on the performance of sensing materials 37 . Synthesis methods are equally significant as concentration reagent and organic surfactant, temperature, and solvent media in determining the final crystallographic planes of the P-N heterojunction.…”