Effect of heat treatment on the characteristics and NH3 Sensing properties of Tin Dioxide SnO2 Thin Film

In this research, Spray pyrolysis technique used to prepared Tin dioxide (SnO2) thin films on glass substrate with thickness about 200± 8 nm by dissolved 2.2563 g of SnCl2.2H2O in 100 ml of ethanol then added 60 drops from concentrated hydrochloric acid (HCl). After that the films were annealed at different temperatures (300, 400 and 400 °C ). X-ray analyses shows that the structure of all SnO2 films is polycrystalline with tetragonal rutile crystalline structure with preferential orientation in the direction (200). The optical measurements show that the optical transition has been direct and the average band gap has been tendency to decreases from 3.98 eV to 3.73 eV with increasing of Ta. The extent and nature of transmittance and optimized band gap of the material assure to utilize it for sensor applications. From sensing measurements for NH3 gas at different operating temperature (100,200,250 and 300)°C and gas concentration 1(5%, 10%, 15%, 20%, 25%, 30%, and 35% ) can be seen that the sensitivity increases with increasing operating temperature and gas concentration, the response time decreases to smallest value (4s) at operating temperature 200 °C while the recovery time (22 s).


Introduction
In the developmh entt of the last few, deck ades and until the present day, SnO 2 , CdO, and ZnO are semi conducting metal oxides with large optical band gaps that are very dynamic materials in study. Because of its greater stability in atmospheric conditions, mechanical hardness, high-temperature resistance, and chemical inertness, SnO2 is the most common material among the accessible TCOs (Transparent conducting oxides). High electrical conductivity tends to be an aspirantt for gas sensors, optoelectronic applications such as window layers for solar cells, light-emitting diode (LED), and phototransistors [1][2][3][4][5] because of its distinct physical characteristic features such as high transparency in the visible region of the spectrum and greater reflectivity in the infrared region. The presence of flammable and harmful gases in the atmosphere can be dangerous to public health and safety [6]. Then there are new applications for gas sensors, such as dangerous and environmental gas detection, wearable electronics, portable gas analyzers, and the Internet of Things (IoT). Fast response/recovery times and low detection limits are anticipated from new sensors lower powerrconsumption, work at a lower.temperature, high sensitivity, good,selectivity, and longterm,stability [7]. Chemical sensors using semiconducting metaleoxides like (Sno 2, Zno, WO, and TIO) are,one of the most intensively investigated,gas-sensing system [6,8], The sensors based on these materials change their conductivity in the presence of reducing and oxidizing gases as electron density, this adsorbed oxygen is trapped at the grain boundary trap states or the absorption and desorption.of O -2 , O -2 or O -, cause that the surface is modified thereby increasing the grain boundary potentiallbarrier so the Small grain,size, which has been shown to increase sensitivity [9] . The sensing mechanism involves a discernible change in the resistance of the metal oxide upon exposure to reduction or oxidation,happens with the detectedegas [6,8]. The change in conductivity is due to the interaction of the surrounding gas with the,sensing layer [10]. This study explains.how gas sensors respond , discussingghow gases.are absorbed on the surface of semiconductor materials and their impact on the properties of semiconductors, also focused on the efficiency of gas sensor madeeof nanostructures in the form of thin SnO 2 film and treated under different annealing temperature to sensing different gaze concentration and different operating temperature.

Experimental procedure.
Structure at the nanostructures The SnO 2 film was made. by spraying a 0.1 M tin salt solution on a glass substrate The solution was prepared at a temperature of 450°C by solving,2.2563 g of SnCl2.2H2O was dissolved in 100 ml of ethanol then added 60 drope from concentrated hydrochloric acid (HCl) by using drop by drop technique the solution put on magnetic stirrer for 30 minits. The addition of HCl rendered the solution transparent, microscope glass slides, cleaned with HCL, distilled water solvents, and then put in uletra sonic cleaner for 15 mints .after that put it in ethanol and distilled water solvent and again put in uletrasonic cleaner for 15 mints ,finaly the substrates put in 100 ml of distilled water and eject it, then wait for it to dry. The solution's spray rate was set to one sprinkling per minute. The sprinkling time was approximately 10 seconds. The spray nozzle's normalized distance from the substrate was 30 cm. SnO 2 films were thermally treated at various temperatures. (300,400, and 500) o C for two hours in air. The thickness,of the films (t) was determined using the weighing-method as shown in the following equation: where Δm reprbb besents the mass difference of slide after and before the deposition, A represents the area of the film and ρ is the SnO 2 density which is 6.85 g/cm 3 at room temperature , the thickness of the films prepared about 200 nm. The structure,of the films was examined,using X-Ray Diffraction (XRD) using a Philips X-ray diffractometer system which records the intensity as a function of Bragg's angle.. The source of radiation was Cu (kα) with wavelength λ=1.5406 Å, the current was 30 mA and the voltage,.was 40 kV. The scanning.angle 2θ was variedd in the range of 20°-60° . The transmittance and absorbance of the films was measured using UV-VIS spectrophotometer Shimadzu UV/ Visible recorder spectrometer model 12600 in the spectral range 200-1100 nm. The intensity of light (I) after crossing thickness of material x in an isotropic medium can be estimated by [11]: where I o is the initial intensity. 3 The whole sensorstest system as figure (1). The voltage of bias is 10 volts, and the chamber,pressure in range of (1 × 10 −2 bar). The mixing. of the gas with air have been controlled by flow meter. The variation.of sensor sensitivity S, as calculated.using the equation: [12] where R is, the electrical, resistance, and  is the electrical, conductance.

Structure Properties
The analyses of XRD for SnO 2 thin film deposit on glass substrate shown in figure (2), which indicates that, the structure of the films polycrystalline. It is well known that tin dioxide SnO 2 has a tetragonal rutile crystalline structure [13]. The major diffraction,peaks of some lattice,planes can be indexed to the tetragonal unit cell structure of SnO 2 with lattice constants a = 4.71 Å and c= 3.19 Å, which are consistent with the standard values for bulk SnO 2 (JCPDS-041-1445, card No. 96-900-9083) [13]. There are six apparent peaks with 2θ values of (26.  where λ is the wavelength of X-ray ( λ = 1.5406A) , θ is the angle of diffraction, and β is the full-width at half-maximum (FWHM) intensity (in radians).

Linear Optical Properties
Tauc equation. has been used to calculated the optical energy gaps for allowed direct transition: [11] which B is Tauc constant and hυ is the energy of incant photon , α is the absorption coefficient and r =1/2 for allowed direct transition .figure (3) (2), Band gap lies in the range of( 3.98-3.72) eV our results. are in good agreement with those reportedd in literature [16]. The values of optical energy gaps as shown decrease with increasing annealing temperatures this may be attributed to the increase in the localizes levels near the band edges.

gas sensor measurements
Sensing measurements are investigated at different NH 3 : air mixing ratio (5%, 10%, 15%, 20%, 25%,30%, and 35%) and different operating temperature (R.T, 100, 200,250, and 300) o C to. Figure (4) shows the variation of the sensitivity with operating temperature which is obvious that the sensitivity,increases with increasing operating temperature .and the best temperature as clearly shown is about ~200 o C due to increase the rate of surface reaction of target -gas. From the figure the high sensitivity obtained at annealing temperature 300 ºC. The sensitivity measurements of SnO 2 film for different ratio of NH 3 : (5%, 10%, 15%, 20%, 25%, 30%, and 35%) have been done it using equation (3). Figure (5) shows that the sensitivity of the sensor increases linearly in the low gas,concentration region less than 30%, which is provide,advantage to detect and estimate the low concentrations of combustible gases. While with higher of gas concentrationnthe sensitivity tends to saturate, which is resulted due to saturation of adsorption of NH 3 atoms at the Al electrode/SnO 2 interface, and lack of adsorbed oxygen ions at the surface [17]. Our result good agreement,with L.Kamble et.al [2].  Figure 6. Variation of response and recovery time with operating Temperature for 30% NH 3 ratio and annealing temperature 300 ºC .

Conclusion.
There are main conclusions that have obtained from this work. From X-ray diffraction results can be concluded that the structured of SnO 2 films is ploys crystalline with tetragonal rutile structure.with preferentialLorientationnnin the (200) direct ion. Annealing process leads to improve,in the crystallization... From the optical properties concluded that the optical transitions in SnO 2 is direct transition and the,optical energy,gap decreases with annealing temperature. From the above study for gas sensor properties can be concluded that the SnO 2 sensors demonstrated high sensitivity to NH 3 gas. Moreover, the annealing temperatures were caused improving of performance in sensitivity. The responserecovery time of SnO 2 sensing element was in range of 4s and 22 s respectively for operating temperature 200 o C and 30% NH3 ratio at annealing temperature 300 ºC, which are considered as workable and appropriate to get fast and sensitive gas sensor capable of detecting toxic and flammable gases . So it can be concluded that the best operating temperature is about 200 o C, the best sensitivity at gas mixing ratio 30 %, and the optimum annealing temperature 300 ºC for SnO 2 materiall which benefits an actuator to enabling it to detect different concentrations of combustible gases.