Facile Synthesis of Hierarchical Tin Oxide Nanoflowers with Ultra-High Methanol Gas Sensing at Low Working Temperature
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In this work, the hierarchical tin oxide nanoflowers have been successfully synthesized via a simple hydrothermal method followed by calcination. The as-obtained samples were investigated as a kind of gas sensing material candidate for methanol. A series of examinations has been performed to explore the structure, morphology, element composition, and gas sensing performance of as-synthesized product. The hierarchical tin oxide nanoflowers exhibit sensitivity to 100 ppm methanol and the response is 58, which is ascribed to the hierarchical structure. The response and recovery time are 4 s and 8 s, respectively. Moreover, the as-prepared sensor has a low working temperature of 200 °C which is lower than that for other gas sensors of such type has been reported elsewhere. The excellent sensitivity of the sensor is caused by its complex phase mixture of SnO, SnO2, Sn2O3, and Sn6O4 revealed by XRD analysis. The proposed hierarchical tin oxide nanoflowers gas sensing material is promising for development of methanol gas sensor.
KeywordsHierarchical tin oxide nanoflowers Hydrothermal method Gas sensor Methanol
Hierarchical tin oxide nanoflowers
Scanning electron microscopy
Transmission electron microscopy
Thermal gravimetric analysis
X-ray powder diffraction Instrument
With the development of science and technology, methanol gas sensors have been widely applied in chemical industry. As a kind of colorless gas with a smell of ethanol, methanol can cause extremely serious health problems and environmental safety problems. For example, the central nervous system disorder and explosion would happen when the methanol concentration exceeds a threshold value.
Recently, the methanol gas sensing performance of variety of sensitive materials has been researched by a lot of researchers. For instance, Das et al. studied the gas sensing properties of the Mg+ 2:LaNiO3 thin film and obtained the response of 200–400 ppm methanol at 325 °C . Ji et al. investigated the high-performance methanol sensor based on GaN nanostructures grown on a silicon nanoporous pillar array and gained the high gas sensing performance at the operating temperature of 350 °C . Wang et al. reported methanol sensing properties of honeycomb-like SnO2 grown on a silicon nanoporous pillar array and showed the quick response properties at the operating temperature of 320 °C . Although the above scientists have made an excellent progress, the high working temperature and cost sufficiently restrict the application of sensor, which motivates us to fabricate a high methanol performance gas sensor working at low temperature and prepared via a simple and low-cost synthetic method.
SnO2 gas sensors attract attention of researchers due to their simple and low-cost synthesis method and room temperature operation. The huge improvement of gas sensing performance has been achieved by decreasing crystallite size [4, 5], doping by metal elements [6, 7], fabricating heterostructure of metal oxide [8, 9], and, especially, fabricating hierarchical structure [10, 11]. To date, a variety of gas sensors has been proposed to improve gas sensing to methanol, such as In2O3 porous nanospheres , three-dimensionally macroporous LaFeO3 , and aluminum-mesoporous silicon coplanar . However, the gas sensing properties of most of methanol gas sensors are not enough satisfactory. Therefore, the development of a kind of methanol gas sensor with high sensing properties at low working temperature by a simple method is still relevant.
In this work, we have tried to fabricate hierarchical tin oxide nanoflowers (HTONF) gas sensor to improve the gas sensing properties of sensor. We have performed a series of gas sensing examination of HTONF. And the results indicate the HTONF own excellent gas sensing performance (high sensitivity, good selectivity, rapid response and recover rates, and long stability) to methanol at low working temperature. The high gas sensing performance is caused by the hierarchical structure of the material and its phase composition, and this kind of hierarchical structure could provide many effective sites, which make the detected gas and material contact very well and extremely improve the gas sensing performance of the as-obtained materials.
Fabrication of Sensor
The crystallinity and structure of HTONF were characterized by X-ray diffraction (XRD) ADX-2700D X-ray Powder Diffraction Instrument with CuKα radiation (λ = 1.15406 Å). The morphology of the samples was observed by scanning electron microscopy (FEI SKLSLM Magellan 400) and transmission electron microscopy (TEM, JEOLJEM-200FS). The specific surface area and pore size distribution of the samples were evaluated by Brunauer-Emmett-Teller (BET) model by Beijing JW-BK132F equipment. The mass changes of as-obtained HTONF powders were measured by thermal gravimetric analysis (TGA, SDT Q600, TA Instruments, USA). The CGS-8 system (chemical gas sensing, Beijing Elite Tech. Co. Ltd., China) was used to measure the gas sensing performance of samples.
Results and Discussion
XRD and BET Analysis
In turn, the oxidation of metallic tin by oxygen of air is accompanied with generation of SnO and following repeating of reaction (4 and 5). Analysis of literature revealed that the rate of formation of mixture oxides (Schemes 4–5) and their disproportioning depends on many factors: composition of the initial precursors and conditions of reaction for fabrication of SnO and following thermal annealing regime. As it was shown that the complete oxidation to SnO2 is usually observed at annealing temperatures over 450 °C.
SEM and TEM Analysis
It should be noted that the temperature of the hydrothermal synthesis sufficiently influenced on the morphology of the obtained composites. And SEM image of as-obtained HTONF annealed at different temperatures were shown in Fig. 6a–j. From this picture, it is very clearly seen that the morphologies of tin oxide materials will change when the annealing temperature is increased, and the tin oxide materials annealed at 400 °C shows the morphology similar to magnificent fractal structures. Such morphology should have specific surface area higher than other sintered disordered powder-like materials and it was confirmed by BET measurements (Fig. 5). As it was noted in , the as-obtained composite forms cubic shape with surface covered by nano-sheets at the temperature 140 °C. Thus, the temperature is the factor that obviously effects on the nucleation process and following growth of tin-containing composites. And the morphology of as-prepared materials will be changed when the calcination temperature changes.
Gas Sensing Properties
Comparison of methanol gas sensors based on tin oxide materials with different morphologies
In addition, the response behavior of the gas sensor to different concentrations of methanol at 200 °C is shown in Fig. 10b. It can be easily found that the sensor presents upward tendency when the concentration of methanol is increased. The gas sensitivity depends linearly on the concentrations of methanol vapor varying from 1 to 100 ppm. However, the gas sensitivity saturates when the concentration of methanol exceeds 2000 ppm. The phenomenon can be explained as follows: the methanol molecules are absorbed on the surface of the HTONF and participate in surface reaction, resulting in the rise of gas sensing. The gas sensor is saturated when the concentration of methanol exceeds a threshold value, and the gas sensitivity of the sensor shows slow growth [47, 48].
Gas Sensing Mechanism
The presence of humidity in the atmosphere should decrease the methanol gas sensing performance of sensor by increasing the conductivity of the metal oxide gas sensor as it was described in . It is noticeable that the sensing behavior of the HTONF sensor to formaldehyde having molecular mass less than methanol, the lower sensitivity was observed comparing to methanol. The methanol and other alcohols contain hydroxyl group that allows its easy adsorption on surface of HTONF with chemisorbed oxygen. At the same time, the least molecular mass of the methanol in comparison with other alcohols provides its fast diffusion in pores of the fabricated HTONF increasing sensitivity of the material in accordance with the Knudsen approach (Eq. 12). In the series of studied gases, the noticeable is low sensitivity of the fabricated sensor to formaldehyde (Fig. 11). Although the formaldehyde molecule is even less than methanol one, it contains highly negative-polarized O atom that sufficiently impedes absorption of the molecule on the surface of material with chemisorbed oxygen species and consequently sensitivity of the device to that gas is low.
In light of the results obtained from XRD spectra, thermogravimetric curve, and surface area measurements, the model of sensitivity we proposed also accounts the next two reasons. The material annealed at temperatures below 450 °C includes several phases of tin oxides: SnO2, Sn2O3, and Sn3O4. These phases are responsible, by our opinion, for high sensitivity properties of the obtained structures, as it was reported by different research teams in recent publications.
The second factor is the high specific surface area of the sample 400 °C. Annealing of the samples at temperature 300 °C and 500 °C and higher results in formation of the developed surface with area value less than that of the sample annealed at 400 °C. The two described factors effect onto the sensitivity in opposite ways. And the compromise annealing temperature 400 °C resulted in simultaneously high value of the surface area and the high-sensitive tin oxides Sn2O3 and Sn3O4. It led to the highest sensitivity of the fabricated sensor based on HTONF.
In summary, hierarchical tin oxide nanoflowers for gas sensors have been synthesized via one-type hydrothermal route. A series of results indicates that the HTONF gas sensor shows high gas sensitivity, short response and recovery time, long stability, and high reproducibility. The gas response of the as-obtained gas sensor to 100 ppm methanol is about 58 and response/recovery time is about 4 s and 8 s, respectively. The excellent gas sensing performance of the HTONF sensor is attributed to the original hierarchical structure and phase composition of tin oxides. The HTONF could be a desirable candidate for applications in sensor area.
This work has been supported by the national long-term project [no. WQ20142200205] of Thousand Talents Plan of Bureau of Foreign Experts Affairs of the People’s Republic of China.
LS analyzed the result and wrote the final version of the paper. AL, LL, NIK, HL, and LS organized and performed the experiment, analyzed, and discussed the results of the manuscript. DC carried out BET and SEM measurement. MF and Jun Kai carried out XPS and TEM measurement. DB carried out the XRD studies. Meanwhile, the authors are very grateful to Prof. Igor Zatovskii for sufficient help in explanation of chemical transformations and interpretation of results of the XRD analysis of structures. All authors analyzed and discussed the results. And all authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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