Abstract
The physical properties and the effect of effective surface area (ESA) on the sensing properties of tin dioxide [SnO2] thin films in air and propane [C3H8] atmosphere as a function of operating temperature and gas concentration have been studied in this paper. SnO2 thin films with different estimated thicknesses (50, 100 and 200 nm) were deposited on glass substrates by the chemical spray technique. Besides, they were prepared at two different deposition temperatures (400 and 475 °C). Tin chloride [SnCl4 · 5H2O] with 0.2 M concentration value and ethanol [C2H6O] were used as tin precursor and solvent, respectively. The morphological, and structural properties of the as-prepared films were analyzed by AFM and XRD, respectively. Gas sensing characteristics of SnO2 thin solid films were measured at operating temperatures of 22, 100, 200, and 300 °C, and at propane concentration levels (0, 5, 50, 100, 200, 300, 400, and 500 ppm). ESA values were calculated for each sample. It was found that the ESA increased with the increasing thickness of the films. The results demonstrated the importance of the achieving of a large effective surface area for improving gas sensing performance. SnO2 thin films deposited by spray chemical were chosen to study the ESA effect on gas sensing properties because their very rough surfaces were appropriate for this application.
Highlights
Gas sensors based on semiconductor oxides play an important role in the detection of toxic gases such as carbon monoxide [CO] [1], nitrogen oxide [NOx] [2], carbon dioxide [CO2] [3], hydrogen sulfide [H2S] [4], sulfur dioxide [SO2] [5], and some inflammable gases, for example hydrogen [H2] [6], methane [CH4] [7], and propane [C3H8] [8], among others
The SnO2-based gas sensor is an n-type semiconductor oxide due to the non-stoichiometry associated with the oxygen vacancies and/or tin excesses, which act as donor states providing conduction electrons
For an effective surface area (ESA) less than 7.6 m2, the sensitivity of SnO2 films deposited at 475 °C is larger than that of SnO2 films deposited at 400 °C due to the fact that the morphology of SnO2 films deposited at 475 °C is better than that of SnO2 films deposited at 400 °C
Summary
Gas sensors based on semiconductor oxides play an important role in the detection of toxic gases such as carbon monoxide [CO] [1], nitrogen oxide [NOx] [2], carbon dioxide [CO2] [3], hydrogen sulfide [H2S] [4], sulfur dioxide [SO2] [5], and some inflammable gases, for example hydrogen [H2] [6], methane [CH4] [7], and propane [C3H8] [8], among others. The SnO2-based gas sensor is an n-type semiconductor oxide due to the non-stoichiometry associated with the oxygen vacancies and/or tin excesses, which act as donor states providing conduction electrons. When the surface is placed under an oxidizing atmosphere, the surface states physisorbed atomic and molecular oxygen; those captured conduction electrons produce a blending of the energy bands in this surface region, which leads to a surface electrical resistance increase [9]. On the other hand in the presence of a reducing gas, the electrons trapped by the oxygen species are released due to reaction between the reducing molecules and the physisorbed oxygen species, resulting from this fact a decrease of the surface electrical resistance [10]. Different physical and chemical deposition techniques have been utilized for depositing SnO2 thin films. Among the physical techniques are sputtering [11], chemical vapor deposition [12], and evaporation [13], among others whereas sol-gel [14] and spray pyrolysis [15] are the most used chemical techniques used to deposit
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