Present paper reports the synthesis of SnO2, its characterization and performance as Liquefied Petroleum Gas (LPG) Sensor. XRD pattern revealed the tetragonal crystalline nature of the material. Crystallites sizes were in the range 14 - 30 nm. Tin oxide thick film was prepared by using screen printing technique. After that these were investigated through SEM. SEM image of thick-film surface was spherical in shape and porous. Further at room temperature, the film was exposed to LPG in a controlled gas chamber and variations in resistance with the concentrations of LPG were observed. The maximum value of average sensitivity of thick film was 37 MΩ/min for 5 vol. % of LPG. Sensor responses as a function of exposure and response times were also estimated and maximum sensor response were found 273 and 312 for 4 and 5 vol. % of LPG respectively.
The properties of metal oxides have received a great deal of interest for many years, due to applications in various fields such as solar cells, optical devices and oxidation catalysts. Numerous metal oxide semiconductor materials have been reported to be usable as gas sensor, such as ZnO, SnO2, and TiO2 and so on [1-10]. These materials have non-stoichiometric structure, so free electron, originating from oxygen vacancies contribute to electrical conductivity. Tin oxide (SnO2) is extensively studied because of its interest in both the application and the fundamental research in the last decade due to its remarkable optical and electrical properties. Tin oxide is a wide band gap (3.6 eV) having its conductivity depending upon oxygen vacancies that act as donors [11-14].
Several deposition techniques have been developed to grow undoped and doped SnO2 films such as spray pyrolysis evaporation, chemical vapor deposition, sol gel technique, magnetron sputtering pulsed laser deposition and screen printing technique. The production of sensing layers by screen-printing technology has met the great interest in this field. In fact, screen-printing is a simple and automated manufacturing technique that allows the production of low cost and robust chemical sensors with good reproducibility. Such technique allows the deposition of a controlled amount of paste. Thick films are suitable for such sensors since the gas sensing properties are related to the material surface and the gases are always adsorbed and react with the films surface [15-19]. The development of gas sensors to monitor combustible gases is imperative due to the concern for safety requirements in homes and industries. The liquefied petroleum gas (LPG) is one of extensively used but potentially hazardous gases and its detection is particularly important, because explosion accident may be caused when it leaks out accidentally or by mistake. So the detection of LPG in domestic appliances must be no false or missing alarms during cooking, which requires the equipment to identify LPG [20].
The gas sensing properties of SnO2 semiconductor thick film have been found to depend strongly on the method of processing [21,22]. Among the structural parameters, the crystallite size has prominent effect on the gas sensitivity. It is now recognized that SnO2 semiconductor thick film can have maximum gas sensitivity only if the nanocrystallite size within the film is comparable with its space-charge layer thickness [23,24]. Hence, the major objectives of the present investigation are set to synthesize SnO2 semiconductor thick film, having nanosized crystallites using the screen printing technique. Various analytical techniques such as Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD) and UVvisible absorption analysis (UV-vis) are utilized in the present investigation for determining the surface morphology, crystallite size and band gap within the film. Besides, the present study also focuses on demonstrating the suitability of such film for sensing LPG gas at room temperature.
2. Experimental Procedure
Tin oxide was prepared by chemical co-precipitation method. 12 g stannous chloride [SnCl2·2H2O] was dissolved in 95 mL of isopropyl alcohol to make 1 M solution. Ammonium hydroxide was added drop wise to 1 M stannous chloride dehydrate solution. After vigorous stirring for 6 h, a dispersed solution was formed. This dispersed solution was sonicated for 30 minutes, yielding a precipitate. The precipitated compound was thoroughly washed with distilled water to remove chloride ions. The paste thus formed was screen printed onto an ultrasonically cleaned alumina substrate. Then the thick film was allowed to stabilize at room temperature for 6 h and calcined at 450˚C for 1 h. The silver contacts on both the ends of film were made for signal registration. It was exposed to LPG in a self-designed conventional chamber and corresponding variations in resistances with the time were recorded by using a digital millimeter.
3. Characterization Technique3.1. Scanning Electron Microscopy
Surface morphology of the sensing element was analyzed using Scanning Electron Microscope unit. SEM image is shown in Figure 1(a). It exhibits that the grown samples consist of spherical grains having more space as pores. Each grain is uniformly distributed over the surface. Figure 1(b) shows the SEM of SnO2 after exposition of the LPG. These adsorption sites are disappeared after the adsorption of LPG on the surface of the sensing film. The grains were seemed to be swallowed and as a result their sizes became larger.
3.2. X-Ray Diffraction
The crystal structure and phase of the powdered sample were analyzed by using X-ray Diffractometer with Cu-Kα radiation as source having wavelength 1.546 Å. Crystallite size was estimated using the broadening of XRD peaks by the Debye-Scherer formula which is as follows:
where β is the full width at half maximum (FWHM) of the peak, λ is X-ray wavelength, θ is the Bragg angle and K = 0.94, a dimensionless constant. Figure 2 shows the XRD pattern of SnO2 powder and confirms the structure. The high intensity peak, centered at 2θ = 26˚ is assigned to tetragonal crystalline SnO2 (110) reflection having‘d’ spacing 3.35454 Å and FWHM 0.5353˚. Also the peak with low intensity at 2θ = 71˚ is assigned to SnO2 (202) reflection having “d” spacing and FWHM 1.31985 Å and 0.6912˚ respectively. Other higher angle reflection such as (101), (200), (211) were indicating tetragonal crystalline nature of SnO2. The crystallite size is calculated by using Debye-Scherrer formula. The minimum crystallite size was found to be 14 nm at 2θ = 42.67˚ with “d” spacing 2.118945 Å and FWHM 0.7557˚.
3.3. UV-Visible Absorption Analysis
Optical characterization of the sensing element was done by using UV-visible spectrophotometer (Varian, Carry- 50Bio). Figure 3 shows UV-visible absorption spectra of tin oxide in UV and visible range. Tin oxide nanoparticles reveal a strong change of their optical absorption when their size is reduced. Therefore, absorption spectra of tin oxide nanoparticles obtained in the UV-visible region show blue shift in the absorption edge at 268 nm as compared to bulk. The corresponding band gap was found 4.66 eV respectively. It is evident that tin oxide shows significant blue shift of the absorption peak relative to the bulk absorption. This blue shift is useful for gas sensing applications.
4. Device Assembly
The main goal of this investigation is to develop a high sensitive LPG sensor and at the same time to analyze the effect of parameters on the gas sensing properties of the thick film. The purpose is not only to evaluate the gas sensing response, but also to reproduce as much as possible a real environment for a working LPG sensor. Therefore, following protocol is used to test the gas sensing behavior of the thick film. The schematic diagram of ex-
perimental setup is shown in Figure 4(a). The heart part of the device is a resistance measuring thick film holder. It is well fitted in a glass chamber having inlet and outlet knobs for LPG. Inlet knob is associated with the concentration measuring system along with a thermocouple.
Concentration Measuring System
Figure 4(b) consists of a glass bottle containing double distilled water, which is saturated with LPG, in order to avoid the possibility of dissolution of inserted gas above this bottle, the measuring tube (pipette) is connected by vacuum seal. The cock I is connected to the LPG cylinder and cock II is connected to the inlet of the gas chamber. When the cock I is opened, the LPG from the cylinder is filled in the glass bottle and an equivalent amount of water is displaced in the measuring pipette. When the cock II is opened, a desired amount of gas e.g. 1, 2, 3 vol% and onwards is cast out in the gas chamber.
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