Volatile organic compounds are considered a major component that participates in the formation of ozone, which are air pollutants, and their percentage increases from weakness in the outside air, due to their presence in many products in the home, as volatile organic compounds are now included in about 90% of the products that enter the home, Its sources include drinking water, carpets, paints, deodorants, cleaning methods, materials used for shoe polishing, cosmetics, dry cleaning clothes, moth repellents, air fresheners, and car exhaust   . One of the volatile organic compounds that must be detected is ethanol. Ethanol is an organic chemical compound that belongs to the alcohol family. It has the chemical formula: C2H5OH and it is called generalized alcohol . Ethanol is a colorless, flammable substance formed from the fermentation of sugar. It is used in alcoholic beverages and in the manufacture of perfumes and is used as a fuel in mechanical engines prepared for ethanol . Whereas, eating small to moderate amounts may lead to symptoms of toxicities, such as inconsistency in muscle work, poor vision, slurred speech … etc. As for eating large amounts, dampening of the bulb reflexes, such as drowsiness, forgetfulness, and memory impairment, amnesia, hypothermia, hypoglycemia, stupor, coma, respiratory depression may occur . Therefore, it is necessary to find a high-efficiency and low-cost sensor material for the detection of ethanol . TiO2, Cr2O3, CuO, ZnO, and SnO2, etc. are semiconducting metal oxides. Defined as high-efficiency and low-cost sensor materials for the detection of toxic and harmful gases     . Recently, different structures of MOS materials with a large specific surface area have been reported to improve the sensing response of sensors, for an example nanowires, nanoparticles, nanorods, hierarchical flower-like structure and hollow microspheres  .
The SnO2 known as n-type semiconductor with band gap 3.6 eV. Also, it is promising for gas sensing materials in order to chemical stability, perfect thermal, excellent mobility of an electron and low cost . To synthesis SnO2 nanocrystal line, Different methods have been used such as chemical vapor deposition, sputtering, hydrothermal and sol-gel, among them hydrothermal method is simple and inexpensive method. Syntheses of pure SnO2 nanoparticles by a hydrothermal method are still limited. In this work, pure SnO2 nanoparticles were successfully synthesized by hydrothermal method without any template. The obtained sample was analyzed by SEM, TEM, EDX, and XRD. Moreover, the sensing performances and gas sensing mechanism were discussed.
In a typical procedure, 1.6 g NaOH, 1.4 g SnCl2·2H2O and 1 g PVP were dissolved in mixed solution contained of 5 ml H2O2 and 35 ml DI under a magnetic stirring at 30˚C for 2 h. Then the above mixed solution was transferred to 50 ml Teflon-lined stainless-steel autoclave and reacted for 10 h at 180˚C, therefore, after the autoclave cooling down to room temperature naturally the precipitate was washed with deionized water and ethanol for serval time, finally the SnO2 powders were obtained after dried in a furnace for 24 h. The crystal structure X-ray diffraction (XRD) of SnO2 nanoparticles was tested by using an X-ray diffractometer (XRD, D/Max-2400) with Cu Kα1 radiation (λ = 0.15406 nm), Elemental composition was tested by (EDX) an energy-dispersive X-ray detector, surface morphological and microstructural of the sample was carried out on a scanning electron microscopy (SEM, S-4800) and transmission electron microscopy (TEM, JEM-2010). The gas sensing properties were evaluated by the WS-30B gas sensing apparatus (Wei Sheng Electronics Science and Technology Co., Ltd., Henan Province, China). The sensor response (R) to gas was defined as Ra/Rg, where Ra and Rg were the initial sensor resistance in air and gas .
3. Results and Discussion
The X-ray diffraction (XRD) analysis was used to examine the crystal structure of the sample, in Figure 1(a). The peak positions of the sample displayed a rutile type tetragonal structure of SnO2, which were matched well with a standard card (JCPDS, 41-1445) with a = b = 4.736 Å and c = 3.185 Å. No impurity phase detected which indicates the high purity of the prepared SnO2 . The crystallite size d is measured using Debye-Scherer’s formula:
β is the Full Width at Half Maximum (FWHM) of the peak, θ is the Braggs angle, and λ is the wavelength of X-ray. After calculating by means of the above equation, he found that it is equal to 2.215 nm  . (the EDX spectroscopy of SnO2 nanoparticles) in Figure 1(b), indicating that our sample composed of Sn and O elements. Figure 2(a), Figure 2(b) showed the SEM images of SnO2 nanoparticles, the nanoparticles are well crystallized, Figure 2(c) displays the TEM image of the as-synthesized product. It can be clearly seen that nanoparticles with rough surfaces, it is matched well with results of SEM, rough surfaces. It is very good to the desorption and adsorption of gas molecules  , the inner figure in Figure 2(c) is SAED pattern illustrated the polycrystalline nature of SnO2 sample, the HRTEM image displayed in Figure 2(d), the lattice distance was calculated to be 0.175 nm. It corresponds to (211) crystallographic orientation, the optimum operating temperature was determined, the response values of
Figure 1. (a) The XRD pattern of SnO2 nanoparticles; (b) showed EDS pattern of the SnO2 nanoparticles.
Figure 2. (a) (b) displayed the morphologies of SnO2 nanoparticles; (c) showed the TEM picture of SnO2 nanoparticles, the inner figure in (c) showed the SAED pattern; (d) displayed HRTEM picture of SnO2 nanoparticles.
SnO2 nanoparticles to 100 ppm ethanol under different operating temperatures in the range of 140˚C → 340˚C are evaluated and depicted in Figure 3(a), when the increase of ethanol concentrations from 5 → 1000 gas response increases progressively, Figure 3(b), when the concentration is above 150 ppm, the gas sensor nearly be stable. The responses of SnO2 nanoparticles to 100 ppm different gasses at 180˚C determined in Figure 3(c). Our sensor exhibits high selectivity to ethanol. The response and recovery times are about 4 s and 2 s, respectively, Figure 3(d). In Figure 3(e) indicated the sensor nearly to be stable. The results indicate that the SnO2 nanoparticles based sensor can successfully differentiate ethanol at 180˚C. Table 1 showed the Comparison between various SnO2 based gas sensors to C2H5OH.
Experimental results of SnO2 nanoparticles sensor have been compared with the results reported by the other workers on C2H5OH sensors and presented in Table 1. It can be seen that SnO2 nanoparticles sensor can reach a relatively higher response toward C2H5OH at lower temperature. The obtained results indicate that the SnO2 nanoparticles sensor is promising for C2H5OH gas sensing. In Figure 4 the gas-sensing mechanism of SnO2 nanoparticles when the sample is exposed to air, oxygen molecules will be absorbed on the surface, trapping the conduction band’s electrons and creating chemisorbed oxygen species (e.g. , and ) through Eqs .
Figure 3. (a) displayed the gas sensor responses of sample to 100 ppm ethanol to different operating temperatures; (b) is gas sensor responses to different concentrations (5 - 1000 ppm) of ethanol; (c) displayed the response to 100 ppm ethanol at different gases; (d) is the response and recover time towards 100 ppm ethanol at 180˚C; (e) showed the stability of SnO2 sample to 100 ppm ethanol at 180˚C.
Figure 4. Demonstrated gas sensing mechanism of SnO2 nanoparticles.
Table 1. The Comparison between various SnO2 based gas sensors to C2H5OH.
Where:EF ≡electrospinning followed; HM≡hydrothermal method; BM ≡ball-milling solid chemical reaction method; ES ≡electrospinning.
which will generate an electron depletion layer on the surface, leading to high resistance of the material. When the sample is exposed to ethanol, the adsorbed ethanol molecules will react with the surface oxygen species. This process releases the trapped electrons back to the conduction band. Thus, the thickness of the electron depletion layer will decrease, resulting in low resistance of the sample. This progress can be described as follows :
In summary, SnO2 nanoparticles have been successfully synthesized through a facile and low-cost hydrothermal method. The sensor exhibits excellent sensitivity about 27, fast response and recovery time (4 s and 2 s), long-term stability and the optimum operating temperate 180˚C. Thus SnO2 nanoparticles can be used as a promising material for ethanol sensors.
The authors would like to thank the associate editor and the anonymous reviewer for their valuable comments and suggestions, which have led to a significant improvement of the whole manuscript. This work was supported by the National Natural Science Foundation of China (Grant No. 11864034 and 11964035), and the Scientific Research Project of Gansu Province (Grant No. 18JR3RA089 and 17JR5RA072).
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