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Nanomaterials 2017, 7(10), 285; doi:10.3390/nano7100285

Article
Synthesis and Enhanced Ethanol Gas Sensing Properties of the g-C3N4 Nanosheets-Decorated Tin Oxide Flower-Like Nanorods Composite
Yan Wang 1,2Orcid, Jianliang Cao 3,*Orcid, Cong Qin 3,*, Bo Zhang 3, Guang Sun 3 and Zhanying Zhang 3
1
The Collaboration Innovation Center of Coal Safety Production of Henan Province, Jiaozuo 454000, China
2
State Key Laboratory Cultivation Bases Gas Geology and Gas Control (Henan Polytechnic University), Jiaozuo 454000, China
3
School of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo 454000, China
*
Correspondence: Tel.: +86-391-398-7440 (J.C. & C.Q.)
Received: 23 August 2017 / Accepted: 18 September 2017 / Published: 22 September 2017

Abstract

:
Flower-like SnO2/g-C3N4 nanocomposites were synthesized via a facile hydrothermal method by using SnCl4·5H2O and urea as the precursor. The structure and morphology of the as-synthesized samples were characterized by using the X-ray powder diffraction (XRD), electron microscopy (FESEM and TEM), and Fourier transform infrared spectrometer (FT-IR) techniques. SnO2 displays the unique 3D flower-like microstructure assembled with many uniform nanorods with the lengths and diameters of about 400–600 nm and 50–100 nm, respectively. For the SnO2/g-C3N4 composites, SnO2 flower-like nanorods were coupled by a lamellar structure 2D g-C3N4. Gas sensing performance test results indicated that the response of the sensor based on 7 wt. % 2D g-C3N4-decorated SnO2 composite to 500 ppm ethanol vapor was 150 at 340 °C, which was 3.5 times higher than that of the pure flower-like SnO2 nanorods-based sensor. The gas sensing mechanism of the g-C3N4nanosheets-decorated SnO2 flower-like nanorods was discussed in relation to the heterojunction structure between g-C3N4 and SnO2.
Keywords:
nanocomposites; microstructure; gas sensor; flower-like SnO2 nanorod; graphitic carbon nitride

1. Introduction

As an n-type metal-oxide semiconductor, tin oxide (SnO2) has wide applications in many fields, such as lithium-ion batteries [1], photocatalysis [2], and gas sensors [3]. SnO2 has been investigated as a typical semiconductor gas sensor to ethanol because of its unique chemical properties and crystal structure [4]. As is known, the gas-sensing performance of SnO2-based sensors can be improved by means of morphology and size control. Hence, diverse shape-controlled SnO2 nanostructures have been synthesized, such as nanoflower [5], nanoarray [6], nanoplate [7], and nanowire [8]. These SnO2-based sensors exhibited good sensing properties, including low-cost and fast response and recovery.
However, there are some limitations which prevent the direct application of these sensors, such as poor electrical characteristics, high work temperature, and a low response [9]. Coupling SnO2 with other semiconductors to construct the heterojunction structure could be an efficient way to overcome these disadvantages. Therefore, many SnO2-based composites such as SnO2/r-GO [10,11,12,13,14,15], SnO2/ZnO [16,17,18,19], SnO2/Fe2O3 [20,21,22,23], and SnO2/NiO [24,25,26] have been synthesized as high-efficiency gas sensors. Graphitic carbon nitride (g-C3N4) is a two-dimensional (2D) semiconductor with a 2.7 eV band gap, which possesses good chemical stability and a large surface area. It is available to form an n/n junction structure with SnO2 [27]. For example, SnO2/g-C3N4nanocomposites with a strong heterojunction structure were designed and fabricated. The photocatalytic activity of the SnO2/g-C3N4 nanocomposites exhibited enhanced catalytic activity and stable cycle property [28]. Zhang et al. prepared the α-Fe2O3/g-C3N4 heterostructural nanocomposites as an ethanol gas sensor, and the composites exhibited a high response value (S = 7.76) to 100 ppm ethanol under a working temperature of 340 °C [29]. Zeng et al. successfully fabricated the α-Fe2O3/g-C3N4 composites for the cataluminescence sensing of H2S [30]. An efficient dielectric barrier discharge (DBD) plasma-assisted method for the fabrication of the g-C3N4-Mn3O4 composite was investigated by Hu et al., which displayed a highly selective, sensitive, and linear cataluminescence (CTL) response towards H2S gas [31]. Sanjay Mathur et al. synthesized SnO2 nanowires via the CVD method, and the SnO2 nanowires exhibited an excellent photoresponse performance [32]. Kuang et al. have synthesized high-yield SnO2 nanowires via an Au catalytic vapor-liquid-solid (VLS) growth process and the SnO2 nanowire-based humidity sensor displayed a fast response and high sensitivity to relative humidity changes at room temperature [33]. The three-dimensional network’s SnO2 nanowire was prepared via a flame-based thermal oxidation process (FTS) and applied for ethanol sensing [34]. Oleg Lupan et al. investigated the hybrid networks of heterogeneous shell-core Ga2O3/GaN:Ox@SnO2 nano- and micro-cables with a shell in mixed phases for improving sensor properties [35]. However, to our best knowledge, there is still no research focused on the design and gas sensing application of the g-C3N4 nanosheets-decorated SnO2 flower-like nanorods.
Herein, the hydrothermal method was utilized for the first time to synthesis the g-C3N4 nanosheets-decorated SnO2 flower-like nanorods for the ethanol sensing application. It was found that the g-C3N4 nanosheets-decorated tin oxide flower-like nanorods composite possesses a much higher response value, repeatability, and stability to ethanol vapor than pure flower-like SnO2 nanorods.

2. Results and Discussion

2.1. Sample Characterization

The pure SnO2 and SnO2/g-C3N4 composites with 5, 7, and 9 wt % g-C3N4 contents were synthesized by a facile hydrothermal method. Also, the as-prepared samples were marked as SnO2/g-C3N4-5, SnO2/g-C3N4-7, and SnO2/g-C3N4-9, respectively. Figure 1 displays the XRD patterns of the synthesized SnO2, g-C3N4, and g-C3N4nanosheets-decorated tin oxide flower-like nanorods (SnO2/g-C3N4) composites with different g-C3N4 contents. One can see from the XRD pattern that two diffraction peaks at 13.1° and 27.5° can be observed for pure g-C3N4;these two peaks were accorded to the (100) plane and (002) plane of g-C3N4,which could be due to the inter-layer structure of the tri-s-triazine unit with interplannar spacing and the conjugated aromatic system, respectively [36]. The XRD patterns of SnO2/g-C3N4 composites show some diffraction peaks at 26.61°, 33.89°, 37.94°, and 51.78°, which could be assigned to the (110), (101), (200), and (211) planes of the tetragonal rutile structure SnO2 (JCPDS Card No. 41-1445). However, the diffraction peaks of g-C3N4 are not observed in the SnO2/g-C3N4 composites. This could be due to the small content of g-C3N4.
The microstructure and morphology of the synthesized samples were verified by using FESEM and TEM. One can see clearly from Figure 2a that the morphology of the as-prepared g-C3N4 possesses many wrinkles with overlaps at the edges, demonstrating the existence of the two dimensional (2D) nano-lamellar structure. It can be observed from Figure 2b that the pure SnO2 product displays the unique 3D flower-like microstructure assembled with many uniform nanorods. The lengths and diameters of a single nanorod are about 400–600 nm and 50–100 nm, respectively. For the g-C3N4 nanosheets-decorated SnO2 flower-like nanorods composite, as shown in Figure 2c, the SnO2 flower-like nanorods were closely coupled by g-C3N4 nanosheets. A proposed growth mechanism of SnO2 flower-like nanorods can be summarized by crystal growth and nucleation theory. The SnO2nanocrystals nucleation points are generated in different orientations. Therefore, the SnO2 nanorods grow in irregular directions and finally formed into 3D flower-like structures. Figure 2f displays the typical TEM image of pure g-C3N4, and g-C3N4possesses two dimensional nanosheets structure with many wrinkles. The TEM images of g-C3N4 nanosheets-decorated tin oxide flower-like nanorods composites are displayed in Figure 2d,e, and SnO2 flower-like nanorods were coupled by a lamellar structure, which is 2D g-C3N4. Thus, we can conclude that g-C3N4 nanosheets-decorated SnO2 flower-like nanorods composites were successfully synthesized by the hydrothermal method combining the above analysis results offered by XRD, FESEM, and TEM.
Figure 3 exhibits the FT-IR spectra of g-C3N4, SnO2, and SnO2/g-C3N4-7 samples. As can be seen from Figure 3b, the broad absorption peaks could be observed at wave-numbers of 570 cm−1 and 660 cm−1, which could be assigned to the Sn–O characteristic peaks. In Figure 3a,c, the peaks in the range of 1240–1637 cm−1 are ascribed to the C–N and C=N stretching vibration modes, and the peak at 808 cm−1 corresponds to the triazine units. These two sets of characteristic vibration peaks are characteristic of g-C3N4. As is shown in Figure 3c, all the characteristic peaks of SnO2 and g-C3N4 can be observed clearly. These results make up for our XRD analysis, in which g-C3N4 and SnO2 are coexisting in the SnO2/g-C3N4 composites. Compared with pure g-C3N4, there is a slight red shift in the bands of g-C3N4 in the composite. This result indicates that there is an interaction between SnO2 and g-C3N4 [27], which is beneficial to the gas sensing application.
TG analysis was investigated by heating up from room temperature to 800 °C at a heating rate of 5 °C·min−1 to reveal the weight change of g-C3N4. It can be seen from Figure 4 that the weight of g-C3N4 is set constant at temperature below 500 °C. When the temperature increases to 510 °C, the weight of g-C3N4 starts to decrease (the combustion of g-C3N4 in air). The weight stays at the same level when the temperature exceeds 655 °C. It can be concluded that g-C3N4 is stable at low temperature and burn at high temperature. This phenomenon demonstrates that g-C3N4 could stably exist in the composite under the operating temperature in the range of 200–400 °C in the gas-sensing test process.

2.2. Sensing Performance Tests

In order to investigate the gas sensing performance of the synthesized samples-based sensor to ethanol, a series of tests were performed. The response values (Ra/Rg) of the g-C3N4nanosheets-decorated tin oxide flower-like nanorods composite and the pure flower-like SnO2-based gas sensors toward 500 ppm ethanol vapor were measured under different operating temperature. With the increase of operating temperature, one can see from Figure 5a that all of the samples exhibited the similar variation tendency. Also, the response values of the SnO2/g-C3N4-based sensors reached the maximum value at 340 °C, while the maximum value of pure SnO2 was 70 at 360 °C. This result shows that the optimum operating temperature of SnO2/g-C3N4 decreased compared with that of pure SnO2. This result may be due to the fact that the chemisorbed oxygen species can achieve the required energy and effectively react with ethanol vapor molecules on sample surface varying the resistance significantly [37]. The response value of the SnO2/g-C3N4 composite-based sensor is much higher than pure SnO2. The response values increased with adding the content of g-C3N4 from 5 wt. % to 7 wt. % and decreased with further increase of g-C3N4 content. The response values of the pure SnO2, SnO2/g-C3N4 composites with g-C3N4 contents of 5, 7, and 9 wt. % to 500 ppm ethanol vapor at 340 °C are 43, 125, 150, and 135, respectively, indicating that the addition of g-C3N4 has a great influence on enhancing the gas sensing performance. When the mass percentage of g-C3N4 in the composites is 7 wt. %, the response reaches the maximum value. A suitable content of g-C3N4 in the composite is beneficial to form the preferable heterojunction structure in the interface region between flower-like nanorods SnO2 and 2D g-C3N4. However, much higher addition of g-C3N4may result in the formation of the connection of bulk. This will further decrease the specific surface area of the sample and reduce the active sites for oxygen and ethanol gas adsorption, further leading to the degradation of sensing performance. Hence, the optimum operating temperature is 340 °C and the optimum g-C3N4 content is 7 wt. % for this g-C3N4 nanosheets-decorated SnO2 flower-like nanorods nanomaterial. Therefore, all of the further research was carried out by using SnO2/g-C3N4-7 composite sensor at 340 °C. Figure 5b displays the response values of the four samples to different ethanol vapors in the concentration range of 100–3000 ppm at 340°C. With the increase of ethanol concentration, the response values increased for all of these four sensors. The curves increased rapidly in the range of 100–500 ppm and increased slowly with the increasing concentrations from 500 ppm to 3000 ppm.
Figure 6a shows the real time response curves of the pure SnO2 and SnO2/g-C3N4-7 to ethanol in the range of 100–3000 ppm at 340 °C. All of the response-recovery cycles were measured about 300 s with a response interval and a recovery interval of 150 s, respectively. We can observe from Figure 6a that the two samples show a similar trend: the response values increase with increasing ethanol concentration. To the same concentration of ethanol, SnO2/g-C3N4-7 exhibited much higher response value than that of pure SnO2. The response value of the SnO2/g-C3N4-7 composite-based sensor towards 500 ppm ethanol vapor was about 150, about four times higher than pure SnO2. As is known, response-recovery time is another very important influential factor on the application of gas sensor. Figure 6b shows the response-recovery time curve of the SnO2/g-C3N4-7 composite toward 500 ppm ethanol vapor. As seen from the curve, when the sensor was exposed and separated to ethanol, the response increased rapidly (31 s) and also decreased rapidly (24 s), respectively.
The repeatability and stability are both crucial influence factors of gas sensing performances. As is shown in Figure 7a, the response values of these four response-recovery cycles of SnO2/g-C3N4-7 sensor stay almost the same (165, 160, 167, and 155) toward 500 ppm ethanol at 340 °C. This result indicated that the synthesized SnO2/g-C3N4-7 composite possesses an admirable repeatability for ethanol vapor detection. Figure 7b displays the stability test result of SnO2/g-C3N4-7 composite sensor toward 500 ppm ethanol vapor at 340 °C, and the response values were kept at a stability of around 155 after 30 days test, indicating that the synthesized g-C3N4 nanosheets-decorated SnO2 flower-like nanorods composite possesses an excellent stability.
As is well known, the selectivity of the sensor for the different gases is one of the most important factors for its practical application. Figure 8 displays the selectivity test results of SnO2 flower-like nanorods and SnO2/g-C3N4-7 composite to methanol, ethanol, toluene, formaldehyde, and acetone with the concentration of 500 ppm. The test results indicated that the sample possesses the superior selectivity to ethanol vapor at the operating temperature of 340 °C. The high selectivity to ethanol maybe come from the fact that when reacted with the absorbed oxygen, ethanol is more likely to lose electrons, and hydroxyl group (–OH) is easy to oxidize under the optimum operating condition.
Table 1 lists the sensing performances of different materials to ethanol vapor. One can observe from Table 1 that the RGO/hollow SnO2 nanoparticles [11], hollow ZnO/SnO2 spheres [16], tubular α-Fe2O3/g-C3N4 [29], and Au/3D SnO2 microstructure [38] samples possess the response values to ethanol of 70.4, 78.2, 7.76, and 30, respectively. In our research, the 7 wt. % 2D g-C3N4 decorated SnO2 flower-like nanorods composite possesses the response value of 85 to 100 ppm ethanol vapor at 340 °C, indicating a great potential application of the synthesized sample to ethanol detection.

2.3. Mechanism Discussion

SnO2 flower-like nanorods have been synthesized via a hydrothermal reaction method, and the schematic diagram of the synthesis of the g-C3N4 nanosheets-decorated SnO2 flower-like nanorods is displayed in Figure 9. Many researchers hold the point that the diameter of the SnO2 nanorods is changed by varying the Sn+/OH ratio in solution [39]. However, Vuong et al. declared that the diameter of SnO2 nanorods decreased with the increase of the stannic chloride amount. A proposed growth mechanism of SnO2 flower-like nanorods can be summarized by crystal growth and nucleation theory. The synthesis process includes two sections of nucleation and crystal growth. In the hydrothermal condition, the Sn(OH)62− nucleus is formed via the following chemical reactions in the process of nucleation stage [40]:
Sn 4 + + 4 OH Sn ( OH ) 4
Sn ( OH ) 4 + 2 OH Sn ( OH ) 6 2
Sn ( OH ) 6 2 SnO 2 + H 2 O + 2 OH
In the process, nucleation plays an important role not merely in the morphology formation but also in the quantity of the final product. At the initial stage of the chemical reaction, the Sn4+ ions start to react with the redundant OH ions and further form the [Sn(OH)6]2− coordination ions. Meanwhile, a small quantity of SnO2nanocrystals can be generated due to the decomposition of the [Sn(OH)6]2− coordination ions. These initial SnO2nanocrystals play an important role as seeds in the next hydrothermal stage. The [Sn(OH)6]2− coordination ions could be accelerated to decompose and form plenty of SnO2nanocrystals in the later hydrothermal reaction condition, which can be aggregated into SnO2 nanoparticles. At the same time, these SnO2 nanocrystals nucleation points around [Sn(OH)6]2− can be oriented to grow into the rod-like structures. The SnO2 nanocrystals nucleation points are generated in different orientations. As a result, the SnO2 nanorods grow in irregular directions and finally form into 3D flower-like structures. This phenomenon can be explained by the fact that the surface-free energy of the rutile structure of crystalline SnO2 faced an increase in the order of (110) < (100) < (101) < (001), which lead to the crystal growth on the faces of (001) or (101). However, the other faces have no exceptions [41,42].
2D g-C3N4 nanosheets-decorated SnO2 flower-like nanorods composites were synthesized via a facile hydrothermal method as a high-property gas sensor for detecting ethanol. In order to understand the gas-sensing process, the schematic diagram of the test gas that reacted with SnO2/g-C3N4 composite was shown in Figure 10a. As is known, the similar principle of gas sensor is the surface-adsorbed oxygen theory. When the sensor was exposed in the air condition, the oxygen molecules were adsorbed on the SnO2 surface and capture electrons from the conduction band of SnO2. Furthermore, the adsorbed oxygen molecules were ionized into O2−, O, and O2 (Equation (4)), and formed a depletion layer with a certain width (Ws) of the hole accumulation (h+) on the SnO2 surface [43,44]. When the sensor was exposed in ethanol gas, the reduced gas ethanol molecules were oxidized into acetaldehyde and finally turned into carbon dioxide and water by these oxygen anions (Equations (5) and (6)) [10]. As a result, the trapped electrons were released back to the SnO2, depletion layer, where the width (Ws) of depletion area of the hole accumulation (h+) became thinner and led to the decrease of the resistance by the transfer of electrons between ethanol molecules and oxygen anions. As is well known, the electrons transfer may affect the great change of the composite resistance. In addition, the improved ethanol-sensing performances of the g-C3N4 nanosheets-decorated flower-like SnO2 nanorods composite could be attributed to the structure of SnO2 nanorods coupled by 2D g-C3N4nanosheets and the heterojunction of interface region between flower-like SnO2 nanorods and 2D g-C3N4.
O 2 + e O 2
2 CH 3 CH 2 OH + O 2 2 CH 3 CHO + 2 H 2 O + e
2 CH 3 CHO + 5 O 2 4 CO 2 + 4 H 2 O + 5 e
In general, the large specific area of 2D g-C3N4 nanosheets can provide more active sites to adsorb oxygen molecules and ethanol molecules. The interconnecting network structure created by SnO2 nanorods and 2D g-C3N4 nanosheets could supply more channels for the gas adsorption and diffusion and further enhance the interaction between SnO2 and ethanol molecules. The energy band model (Figure 10b) was used to explain the energy change of SnO2/g-C3N4 for ethanol detection. Figure 10b shows that the g-C3N4 and SnO2 have the structure of valence band and conduction band (Ev and Ec) and the Fermi level (Ef) is between these two bands. When flower-like SnO2 nanorods and 2D g-C3N4 nanosheetswere combined together, a heterojunction structure was formed. When ethanol molecules pass through the interface between g-C3N4 and SnO2, the electrical property at the heterojunction was changed. SnO2 and g-C3N4 are all n-type semiconductors with band gaps of 3.6 eV and 2.7 eV, respectively. Since the work function of g-C3N4 is smaller than that of SnO2, the electrons will inflow from the conduction band of g-C3N4 to the conduction band of SnO2, leading to a higher potential barrier. The fermi level is aligned when the electronic transmission achieves a new dynamic balance. The electrons may go over the low energy barriers and the schottky barrier is 0.4 eV. As a result, the electrons and holes are separated [27,43]. Meanwhile, the heterojunction structure may suppress the recombination of electron-hole and urge electrons to transfer quickly from ethanol vapour to the surface of SnO2/g-C3N4. Therefore, this leads to a higher response because of the increased conductivity of the heterojunction structure [29].

3. Materials and Methods

3.1. Chemicals

Analytical-grade purity SnCl4·5H2O (99.0%), NaOH, and absolute ethyl alcohol were purchased from Shanghai Macklin Biochemical Co., Ltd, Shanghai, China and were used without further purification.

3.2. Sample Preparation

Graphitic carbon nitride (g-C3N4) was synthesized by our previous reported method [45]. Typically, 7 wt. % g-C3N4 nanosheets-decorated SnO2 flower-like nanorods (SnO2/g-C3N4-7) were synthesized by the hydrothermal method: 0.17 g g-C3N4 powder was dispersed into 200 mL ethanol under ultrasonic treatment for 2 h. 5.259 g SnCl4·5H2O was added into 200 mL of NaOH solution (0.81 M). Subsequently, the g-C3N4 solution was added into this mixture solution with magnetic stirring until it formed a white suspension. Finally, the mixture was transferred into a 500 mL stainless-steel Teflon-lined autoclave, then put into an oven and further heated at 200 °C for 48 h. The final product was washed with deionized water and ethanol several times and dried at 60 °C. According to this method, the SnO2/g-C3N4 composites with 5 and 9 wt. % g-C3N4 content were also synthesized and marked as SnO2/g-C3N4-5 and SnO2/g-C3N4-9, respectively. The pure flower-like SnO2nanorods were also synthesized by the same method.

3.3. Characterizations

X-ray diffraction (XRD) analysis was carried out on Bruker-AXS D8 (Bruker, Madison, WI, USA) with CuKα radiation at 40 kV and 25 mA. Fourier Transform Infrared Spectrometer (FT-IR) was recorded on a Bruker Tensor 27 (Bruker, Madison, WI, USA). Thermogravimetry (TG) analysis was completed on a NETZSCH STA449C Simultaneous Thermal Analyzer (NETZSCH, Selb, Germany) at a heating rate of 10 °C·min−1 under air atmosphere. Field-emission scanning electron microscopy (FESEM) (Quanta™ 250 FEG, FEI, Eindhoven, The Netherlands) was used to observe the structure and morphology of the sample. Transmission electron microscopy (TEM) analysis was done on a JEOL JEM-2100 microscope (JEOL, Tokyo, Japan) operating at 200 kV.

3.4. Gas Sensor Fabrication and Analysis

Gas-sensing performance test of the synthesized sample was carried out on an intelligent gas-sensing analysis system of CGS-4TPS (Beijing Elite Tech. Co., Ltd., Beijing, China). Figure 11 shows the schematic diagram of the system, the sensor structure, and the working principle. In the sensor fabricate process, the synthesized sample was mixed with several drops of distilled water to form a paste. Then, a ceramic substrate (13.4 mm × 7 mm, screen-printed with Ag-Pd comb-like electrodes) was coated onto the paste to obtain the resistance-type sensor. Before the gas sensing test, the sensor was aged at 200 °C for 12 h to improve its stability and repeatability. In the sensing performance test process, the test gas was first injected into the closed 0.018 m3 volume chamber by a microinjector with the relative humidity of 40%. The operating temperature was set in the range of 200 °C to 400 °C. The gas response (S) was defined as the ratio of Ra/Rg, where Ra and Rg were the resistances of sensor in air and in the test gas, respectively. The response and recovery times were defined as the time required for a change in response to reach 90% of the equilibrium value.

4. Conclusions

In summary, the g-C3N4 nanosheets-decorated tin oxide flower-like nanorods (SnO2/g-C3N4) composite was successfully synthesized by using a facile hydrothermal method. The as-prepared sample possesses flower-like nanorods and a lamellar structure. Compared with pure SnO2, the g-C3N4 nanosheets-decorated SnO2 flower-like nanorods exhibited an obvious improvement of gas sensing performance to ethanol, and the response value was 150 to 500 ppm ethanol at 340 °C. The improved sensing properties are mainly attributed to the high surface area of the sample and the heterojunction between SnO2 and g-C3N4. Considering the effective synthesis approach and the high sensing performance, the as-prepared SnO2/g-C3N4 composite could be an ideal candidate for ethanol detection application.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (51404097, 51504083, U1404613), Natural Science Foundation of Henan Province of China (162300410113), Program for Science & Technology Innovation Talents in Universities of Henan Province (17HASTIT029, 18HASTIT010), Project funded by China Postdoctoral Science Foundation (2016M592290), the Research Foundation for Youth Scholars of Higher Education of Henan Province (2016GGJS-040), the Fundamental Research Funds for the Universities of Henan Province (NSFRF1606,NSFRF1614), Program for Innovative Research Team in University of Ministry of Education of China (IRT_16R22), Foundation for Distinguished Young Scientists of Henan Polytechnic University (J2016-2, J2017-3), and the State Key Laboratory Cultivation Base for Gas Geology and Gas Control (Henan Polytechnic University) (WS2017A03).

Author Contributions

Yan Wang conceived and designed the experiments; Bo Zhang, Guang Sun, and Zhanying Zhang performed the experiments and analyzed the data; Jianliang Cao and Cong Qin provided the concept of this research and managed all the experimental and writing process as the corresponding authors; all authors discussed the results and commented on the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. X-ray powder diffraction (XRD) patterns of the synthesized SnO2, g-C3N4, and g-C3N4nanosheets-decorated tin oxide flower-like nanorods composites (SnO2/g-C3N4-5, SnO2/g-C3N4-7, and SnO2/g-C3N4-9).
Figure 1. X-ray powder diffraction (XRD) patterns of the synthesized SnO2, g-C3N4, and g-C3N4nanosheets-decorated tin oxide flower-like nanorods composites (SnO2/g-C3N4-5, SnO2/g-C3N4-7, and SnO2/g-C3N4-9).
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Figure 2. Field-emission scanning electron microscopy (FESEM) images of pure g-C3N4 (a); SnO2 flower-like nanorods (b); SnO2/g-C3N4nanocomposite (c); and transmission electron microscopy (TEM) images of the SnO2/g-C3N4 composite (d,e) and pure g-C3N4 (f).
Figure 2. Field-emission scanning electron microscopy (FESEM) images of pure g-C3N4 (a); SnO2 flower-like nanorods (b); SnO2/g-C3N4nanocomposite (c); and transmission electron microscopy (TEM) images of the SnO2/g-C3N4 composite (d,e) and pure g-C3N4 (f).
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Figure 3. Fourier transform infrared spectrometer (FT-IR) spectra of g-C3N4 (a); SnO2 (b); and SnO2/g-C3N4-7 (c) nanocomposite.
Figure 3. Fourier transform infrared spectrometer (FT-IR) spectra of g-C3N4 (a); SnO2 (b); and SnO2/g-C3N4-7 (c) nanocomposite.
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Figure 4. Thermogravimetry (TG) analysis for heating the g-C3N4from room temperature to 800 °C.
Figure 4. Thermogravimetry (TG) analysis for heating the g-C3N4from room temperature to 800 °C.
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Figure 5. The response values of the SnO2, SnO2/g-C3N4-5, SnO2/g-C3N4-7, and SnO2/g-C3N4-9 to 500 ppm ethanol (a) under different operating temperatures; (b) for different concentrations of ethanol at 340 °C.
Figure 5. The response values of the SnO2, SnO2/g-C3N4-5, SnO2/g-C3N4-7, and SnO2/g-C3N4-9 to 500 ppm ethanol (a) under different operating temperatures; (b) for different concentrations of ethanol at 340 °C.
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Figure 6. (a) The real time response curves of the SnO2 and SnO2/g-C3N4-7 composite sensors toward ethanol vapor; (b) response-recovery time characteristics of the SnO2/g-C3N4-7 based sensor to 500 ppm ethanol vapor at 340 °C.
Figure 6. (a) The real time response curves of the SnO2 and SnO2/g-C3N4-7 composite sensors toward ethanol vapor; (b) response-recovery time characteristics of the SnO2/g-C3N4-7 based sensor to 500 ppm ethanol vapor at 340 °C.
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Figure 7. (a) Repeatability and (b) stability measurements of the SnO2/g-C3N4-7-based sensor to 500 ppm ethanol at 340 °C.
Figure 7. (a) Repeatability and (b) stability measurements of the SnO2/g-C3N4-7-based sensor to 500 ppm ethanol at 340 °C.
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Figure 8. Comparision of the response values of the flower-like SnO2 nanorods and the SnO2/g-C3N4-7 composite toward 500 ppm different gases at 340 °C.
Figure 8. Comparision of the response values of the flower-like SnO2 nanorods and the SnO2/g-C3N4-7 composite toward 500 ppm different gases at 340 °C.
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Figure 9. The schematic diagram of the synthesis of the g-C3N4 nanosheets decorated SnO2 flower-like nanorods.
Figure 9. The schematic diagram of the synthesis of the g-C3N4 nanosheets decorated SnO2 flower-like nanorods.
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Figure 10. (a) The schematic diagram of air and ethanol react with the synthesized composite; and (b) the band diagram of SnO2/g-C3N4 before and after the combination.
Figure 10. (a) The schematic diagram of air and ethanol react with the synthesized composite; and (b) the band diagram of SnO2/g-C3N4 before and after the combination.
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Figure 11. (a) The schematic diagram of the CGS-4TPS gas-sensing test system and (b) the gas sensor structure.
Figure 11. (a) The schematic diagram of the CGS-4TPS gas-sensing test system and (b) the gas sensor structure.
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Table 1. The ethanol sensing performance of the previous reported results and this work.
Table 1. The ethanol sensing performance of the previous reported results and this work.
MaterialsEthanol Vapor Concentration (ppm)Temperature (°C)Response (Ra/Rg)Ref.
RGO/hollow SnO210030070.4[11]
Hollow ZnO/SnO2 spheres10022578.2[16]
α-Fe2O3/g-C3N41003407.76[29]
Au/3D SnO2 microstructure15034030[38]
SnO2/g-C3N410034085this work
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