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Author: Admin | 2025-04-28
Based on band theory, band theory can be applied to the gas sensor to explain the sensing mechanism. On interaction of the analytes (undetected) with the surface of nanostructures, these analytes react with attached oxygen ions on the surface of nanostructures; a change in the carrier concentrations of the material occurs. Due to the change in carrier concentrations of the material, the electrical resistivity of the materials changes. Decrease in resistivity (increase in conductivity) occurs for n-type metal oxide semiconductor on interaction of reducing gas [25]. So the sensing mechanism of oxide semiconductor is mainly based on the principle of modification in electrical properties (resistivity/conductivity) as a consequence of chemical reaction between gas molecules and the reactive oxygen ions on the surface of MOS nanostructure material. The sensing mechanism can be divided into three sections: (a) adsorption of oxygen at surface, (b) detection of gases by a reaction with adsorbed oxygen, and (c) change in resistance due to charge transfer at the surface.2.4.2 Adsorption of oxygen at surfaceInteractions of oxygen with the surface of a metal oxide semiconductor are of utmost importance in gas sensing mechanism. Oxygen is a strong electron acceptor on the surface of a metal oxide. Since the majority of sensors operate in an air at ambient temperature, therefore the concentration of oxygen on the surface is directly related to the sensor electrical properties. The conversion to O2− or O− at prominent temperatures is useful in gas sensing mechanism, as only a monolayer of oxygen ions are present with these strongly chemisorbed species [26, 27]. Different forms of oxygen ions may be ionosorbed on the surface of metal oxide semiconductor nanostructures [28]. At low temperature ranges (150–200°C), molecules in the form of neutral O2 or charged O− are adsorbed. At higher temperatures ranges above 200°C, atomic form of oxygen as O− ions is adsorbed [29]. It is observed that the reaction kinetics increase with increase in temperature. Sensors based on resistivity/conductivity properties (resistive sensors) work better at temperature of 300°C or above to react with ionosorbed oxygen at the surface. At temperature T E0.4eVO2gas→O2adsE1O2ads+e→O2−adsE2At temperature T > 200–400°C, the following reaction takes place at the surface of sensor (for chemisorptions):E>0.4eVO2−ads+e→2O−adsE3Adsorption energy of oxygen on metals lies in the range of 4–6 eV. Extracted carriers originate from donor sites of the metal oxide surface in the material [30]. Intrinsic oxygen vacancies and other impurity defects give rise to
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