VOCs (Volatile organic compounds) are responsible for the odor, scents, and perfumes as well as pollutants. Some VOCs are very serious matters to human health and cause harm to the environment. Most VOCs are flammable, explosive, and toxic, and are mainly found in the gaseous phase, so they need to be detected to preserve the health of the peoples  . Formaldehyde is a form of a volatile organic compound, so it must be disclosed that it is toxic and odorless, and is considered a cause of environmental pollution  , Formaldehyde is fatal if exposed for short periods of time and exposure to it for a long period’s cause’s difficulty breathing, as it is a carcinogen , It is necessary to find a high-performance sensor to detect formaldehyde gas because it is already internationally classified by the IARC (International Agency for Research on Cancer) . The MOS (metal oxide semiconductor) such as WO3, ZnO, SnO2, NiO, Cu2O, and In2O3 have been used due to their stable properties in chemical transport, so it has been widely used for the detection of different kinds of volatile organic compounds . In detail, SnO2 (n-type) has the best MOS because it has many advantages such as low cost, good stability, ultra-high sensitiveness, also has excellent performance of gas sensors. The flower-like structure can cause vast concern owing to its tailored structure and high surface area, which is applicable for gas adsorption and desorption   .
Gas sensors based on SnO2 materials were developed by many researchers, D. P. Xue, Y. W, et al. synthesis of SnO2 sensor to methane by an impregnation route , Q. W, P. S et al. discussed SnO2 sensor to find out formaldehyde via hydrothermal technique , According to the previous researches, detecting gas sensors to the pure SnO2 didn’t Achieve Gas sensor detection requirements    , but in our work pure SnO2 has good sensing materials for gas sensors detecting. In gas-detection SnO2 sensors have been utilized closely none less, they are usually used at high operating temperatures (above 300˚C). Thus, it is important to create and develop SnO2 sensors for low-temperature .
In this subject, the hydrothermal technique is used to prepare SnO2 flower-shaped. Various analysis techniques were used to test the SnO2 sample. Performance of gas sensing has been discussed systematically, like a response to diverse concentrations, response to various gases and response/recovery time. Optimum operating temperature, and stability, Likewise, we studied the gas sensor mechanism. Consequently, the SnO2 material could be a good new kind of formaldehyde gas sensor in later years.
In this experiment, all chemicals were used without any further purification, the steps of hydrothermal method are as follows; 0.4 g Tin (II) Chloride dehydrates, 2.5 g Sodium Citrate dehydrate, and 0.3 g Polyvinylpyrrolidone (Mw = 1,300,000) was fully dissolved into 30 mL mingled solution contained deionized water (DI) and ethylene glycol under the magnetic stirring for 1 h at 30˚C until the solution became identical, and then ammonia solution was added till PH = 9. The uniform solution got after the magnetic stirring for 1 hour. Then solution transferred to Teflon-lined stainless-steel autoclave, and then heated in a stove at 180˚C for 20 h, and Cooled down to room temperature normally. Next, the Precipitates by centrifugation were washed four times with ethanol and (DI), one by one, next, the sample dried up at 60˚C for 24 h. lastly, SnO2 flower-shaped were obtained through annealing process in a furnace at 400˚C for 2 h at a ramping rate of 10˚C/min.
Morphological analysis was carried out by Field-emission scanning electron microscopy (SEM, S-4800) and transmission electron microscopy (TEM, USA FEI TEVNAI G2 TF20). The specific surface area was analyzed by Brunauer-Emmett-Teller (BET) method through nitrogen adsorption using the BK132F instrument. The crystalline structure of SnO2 flower-shaped was determined by X-ray diffraction (XRD, D/Max-2400) using Cu Kα1 radiation with λ = 1.5406 Å, X-ray Photoelectron Spectroscopy (XPS, ESLALAB 220-XL) were examined the elemental composition and chemical state of SnO2 flower-shaped.
We tested the properties of the gas-sensing using a WS-30A gas sensing measurement system (Wei Sheng Electronics Science and Technology Co., Ltd., Henan Province, China).
The response value (R) was defined as Ra/Rg, where Ra and Rg were the resistance of the sensor in the air and in the test gas, respectively  . The response and recovery times are defined as the time required to reach 90% of the final equilibrium value  .
We put a little of the powder obtained after calcination the process on a glass slide and mixed it with DI water to form clay, next, the slurry uniformly pasted with a toothpick onto a ceramic tube surface. Finally, the sensor was dried naturally at 24 h and was antiquated for 72 h to improve the stability , Ni-Cr heating wire played a vital role to adjust the operating temperature (Figure 1(a)).
Figure 1. Graphic explanation of gas sensing device, ((a), (b)) the gas sensor and (c) the electrical circuit for gas sensing device.
Figure 1(c) showed that load resistor (RL) and ready sensor have been tuned and were connected with DC power at 5 V in a gas sensing device. Then, it was selected (RL) and tuning by (VH) and was controlled the temperature of the Ni-Cr wire, in Figure 1(b), After inserting the sensor into the test room, the liquid was sprayed into the chamber (with 18 L volume) by the syringe according to the target gas concentrations, and there are also two fans for easy gas dispersion in the test room.
3. Results and Discussion
Figure 2 shows XRD patterns of SnO2 flower-shaped, it matches well all the diffraction peaks to the tetragonal rutile SnO2, which were indexed by the standard card (JCPDS, 41-1445) with a = b = 4.736 Å and c = 3.185 Å. That denotes a high crystallinity of the sample after calcining at 500˚C for 3 h. further, there is no impurity phase showed which indicates the prepared SnO2 was in high purity.
The nanosheets are become gathered and uniform by helping of PVP (Figure 3(a)), all the flower-shaped collected look like similar nanosheets with diameters of about 1 µm. Figure 3(b) showed that our sample is 3D nanostructures. Figure 3(c) and Figure 3(d) presented TEM images, the diameter of flower-shaped microstructures is 4 µm, and it is matching with SEM results. Besides, the rim portion of the flower-shaped structures is very clear because of the unique structures of nanosheets, Also uniform nanosheets are rough, which it’s good for desorption and adsorption of gas molecules, subsequently, gas sensing improvement .
Meanwhile, the HRTEM image shown in Figure 3(e) the lattice distances of SnO2 flower-shaped are 0.232 nm and 0.267 nm, it matched well with (200) (101) crystallographic orientation.
What’s more, the inner figure in Figure 3(e) is the SAED pattern detected polycrystalline SnO2 nanostructures.
Figure 2. The XRD pattern of SnO2 flower-shaped.
Figure 3. The morphologies of SnO2 flower-shaped: (a) The low enlargement SEM picture of SnO2 flower-shaped; and (b) The high enlargement SEM picture of SnO2 flower-shaped; ((c), (d)) The TEM picture of SnO2 flower-shaped; (e) The HRTEM picture of SnO2 flower-shaped, the inner figure in Figure 3(e) is SAED pattern.
Figure 4(a) displayed N desorption-adsorption of SnO2 sample, the BJH pore size is 30 nm and the surface area (BET) of SnO2 sample is 23.53 m2/g. Generally, a great pore size is useful to gas disperse, also, a great surface area to supply more active sites, so this leads to improving gas sensing capability  . Figures 4(b)-(d) presented XPS of SnO2 for the purpose of examining the elements and their corresponding valence state. The XPS broad spectrum showed the peaks of Sn, O, and C elements in the sample suggesting Sn has successfully co-operated toward the SnO2.
In Figure 4(c), the binding energies of Sn 3d established to 495.2 and 486.7 eV peaks and are compatible with Sn 3d5/2 and Sn 3d3/2 binding energies, respectively that is pointed out typical oxidation valence phase of Sn4+  (Figure 4(d)). The binding energies of O 1s separated at 530.6 eV, 531.8 peaks. And matched well to crystal lattice oxygen and adsorption oxygen, respectively , when the optimal operating temperature increasing, in Figure 5(a), the response of the gas sensor increases gradually until getting to the max value of 30 at 220˚C and then decrease, and were calculated to 100 ppm HCHO, this incident could be simply explained by the kinetics and thermodynamics of gas adsorption and desorption on the surface of the sensing layer   from Figure 5(a), the optimal operating temperature of the sample is 220˚C Also. As long as, in Figure 5(b), the response of the gas sensors application was very significant, and it was measured with different concentrations of formaldehyde (5, 15, 25, 50, 100, 150, 200, 500 and 1000 ppm), when the formaldehyde concentration increases that means the response gradually going to increase the gas sensor almost be
Figure 4. (a) N2 desorption-adsorption of SnO2 flower-shaped and the inner figure is pore-size distribution curves obtained by BJH method, XPS spectra of the SnO2; (b) a survey spectrum; (c) Sn 3d; (d) O 1s.
stable when the concentration is above 200 ppm, which means It connects the responses with the low concentrations. The dynamic response/recovery times of (3D) SnO2 flower-shaped exhibited in Figure 5(c) for different concentrations (15, 25, 50, 100, and 200) ppm at 220˚C for HCHO, we can obviously see that the response increases gradually with an increasing concentration of HCHO. This shows that our sensor can show a significant response (about 7 for Formaldehyde), while the gas concentration is low (15 ppm). That leads to sensing properties of the sample (3D) SnO2 flower-shaped is good. Figure 5(d) the response/recovery times to 50 ppm formaldehyde about 5 s and 22 s respectively. The rough surface has a significant role in short recovery and response times. the selectivity of the sample is a necessary role for the performance of gas sensing, the selectivity of the sample was examined to different gasses in Figure 5(e) displayed Our sensor is high selectivity to formaldehyde, Figure 5(f) we have tested the stability of the sample in a few days, which clarified that our sample is gradually stable. Also, Table 1 summarized comparison between this work and previous works reported.
The mechanism of the SnO2 sample was also investigated in Figure 6. (Ammonia, Polyvinylpyrrolidone and sodium citrate) are a key factor affecting the morphology growth in good shape, the NH3∙H2O can release OH− as a low alkaline solution that reacts with Sn2+, thus its control the final SnO2 flower-shaped . In the next equations, we explain that:
Figure 5. (a) gas sensor responses of sample to 100 ppm HCHO to different operating temperatures; (b) gas sensor responses to different concentrations (5 - 1000 ppm) of HCHO; (c) The response/recovery time to 15, 25, 50, 100, 200, ppm HCHO; (d) the response and recover time towards 50 ppm HCHO at 220˚C; (e) The responses of SnO2 sample to 100 ppm various test gases; (f) The stability of SnO2 sample to 100 ppm HCHO at 220˚C.
Table 1. Showed a comparison of various SnO2 sensors between published work and this work.
Where: EF = electrospinning followed, HM = hydrothermal method, BM = Ball-milling solid chemical reaction method.
Figure 6. Gas sensing mechanism of SnO2 flower-shaped.
While the nanosheets increase more separately, individually and uniformly due to PVP, the growth of SnO2 nanosheets and accelerate the gathering of nanosheets into flower-shaped by sodium citrate .
The pure SnO2 flower-shaped was fabricated through the hydrothermal method, the sensor has good sensitivity, stability, high response (30 s), and excellent selectivity to formaldehyde at 220˚C, and a high response/recovery time about (5 s and 22 s). Also, the mechanism of SnO2 gas sensors is also discussed. Thus, the SnO2 could become a promising gas sensing material to formaldehyde.
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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