nanomaterials Article

Calcination Method Synthesis of SnO2/g-C3N4 Composites for a High-Performance Ethanol Gas Sensing Application Jianliang Cao 1 , Cong Qin 1 , Yan Wang 2, *, Bo Zhang 1 , Yuxiao Gong 1 , Huoli Zhang 1 , Guang Sun 1 , Hari Bala 1 and Zhanying Zhang 1, * 1

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School of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo 454000, China; [email protected] (J.C.); [email protected] (C.Q.); [email protected] (B.Z.); [email protected] (Y.G.); [email protected] (H.Z.); [email protected] (G.S.); [email protected] (H.B.) School of Safety Science and Engineering, State Key Laboratory Cultivation Base for Gas Geology and Gas Control, Henan Polytechnic University, Jiaozuo 454000, China Correspondence: [email protected] (Y.W.); [email protected] (Z.Z.); Tel.: +86-391-398-7440 (Y.W. & Z.Z.)

Academic Editor: Guozhen Liu Received: 7 February 2017; Accepted: 12 April 2017; Published: 29 April 2017

Abstract: The SnO2 /g-C3 N4 composites were synthesized via a facile calcination method by using SnCl4 ·5H2 O and urea as the precursor. The structure and morphology of the as-synthesized composites were characterized by the techniques of X-ray diffraction (XRD), the field-emission scanning electron microscopy and transmission electron microscopy (SEM and TEM), energy dispersive spectrometry (EDS), thermal gravity and differential thermal analysis (TG-DTA), and N2 -sorption. The analysis results indicated that the as-synthesized samples possess the two dimensional structure. Additionally, the SnO2 nanoparticles were highly dispersed on the surface of the g-C3 N4 nanosheets. The gas-sensing performance of the as-synthesized composites for different gases was tested. Moreover, the composite with 7 wt % g-C3 N4 content (SnO2 /g-C3 N4 -7) SnO2 /g-C3 N4 -7 exhibits an admirable gas-sensing property to ethanol, which possesses a higher response and better selectivity than that of the pure SnO2 -based sensor. The high surface area of the SnO2 /g-C3 N4 composite and the good electronic characteristics of the two dimensional graphitic carbon nitride are in favor of the elevated gas-sensing property. Keywords: graphitic carbon nitride; SnO2 ; calcination method; SnO2 /g-C3 N4 composite; ethanol gas sensing

1. Introduction In recent years, poisonous and harmful gases of industrial production have frequently leaked. Meanwhile, organic poisonous gases, such as methylbenzene and formaldehyde, volatilize from furniture and newly-decorated houses. As a result, it severely threatens the health of mankind [1–4]. Therefore, there is the need for an effective and necessary method for detecting the vaporable substances. In the past several years, considerable attentions have been dedicated to metal-oxide semiconductor (MOS) material-based gas sensors. Various metal-oxide semiconductors (MOS), such as SnO2 [5], ZnO [6], CuO [7], α-Fe2 O3 [8], Co3 O4 [9], MnO2 [10], WO3 [11], In2 O3 [12], and NiO [13], are widely used as gas sensors. These gas sensors exhibited unique performances, including a low-cost, small size, and fast response and recovery time. SnO2 , a typical n-type metal-oxide semiconductor with a rutile crystalline structure, is widely used to detect different kinds of gases such as ethanol [14], formaldehyde [15], acetone [16], nitrogen dioxide [17], etc. These are all due to their remarkable characteristics such as Nanomaterials 2017, 7, 98; doi:10.3390/nano7050098

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their good chemical and physical stability, non-pollution property, low-energy feature, use of simple preparation, and so on. The most remarkable characteristic of SnO2 is that the resistance varies when it is exposed to different types of target gases. As a rule, the resistance value of SnO2 decreases when it is exposed in reducing gases, and conversely, it increases under the oxidizing gases. As we all know, there are many oxygen vacancies of SnO2 which are in favor of the process of gas adsorption. In general, it has a better gas-sensing performance with a higher concentration of oxygen vacancies. Nonetheless, there are some significant defects which restricts its application in gas sensors. For instance, a high working temperature, long response and recovery time, high resistance, and easy agglomeration draw the attention of researchers. In order to overcome these shortcomings, it is necessary to explore an electroconductive intermediate to support SnO2 nanoparticles, in order to improve the electrical conductivity and dispersity. Therefore, many two-dimensional (2D) carbon materials are widely used as intermediates [18–20]. Graphene, a unilaminar sp2 -hybridized carbon atoms configuration, exhibits an excellent performance, including a large specific surface area, better electronic conductivity, and superior stability. On account of these advantages, graphene and reduced graphene oxide are widely used as gas-sensing materials to detect different kinds of gases. Different metal-oxide decorated graphene nanocomposite gas sensors were reported and exhibited outstanding gas-sensing performances [21–24]. However, the process of preparing GO and r-GO is complicated and high-cost. Hence, it is necessary to explore a novel 2D structure material with graphene. Recently, graphitic carbon nitride (g-C3 N4 ) has attracted increasing attention due to its high photology and chemical stability, facile preparation, high specific surface area, and nontoxicity [25–28]. In general, g-C3 N4 is readily synthesized by calcining abundant nitrogen-rich precursors such as melamine, dicyandiamide, and urea. It has been widely used in fields such as photocatalysis, degradation, and energy storage materials [29–31]. Until now, there are few reports on the application of g-C3 N4 in the field of sensors. Zeng et al. have successfully prepared the α-Fe2 O3 /g-C3 N4 nanocomposite through a facile refluxing method for the cataluminescence sensing of H2 S [32]. Zhang et al. have developed a novel fluorescence sensor based on a g-C3 N4 /MnO2 sandwich nanocomposite for the rapid and selective sensing of glutathione [33]. Moreover, CeO2 /g-C3 N4 composites with high photocatalytic activity have been synthesized successfully [34]. To the best of the authors’ knowledge, SnO2 /g-C3 N4- based sensors have yet not been reported in the literature. In this work, we report a facile calcination approach to synthesize different mass ratios of SnO2 /g-C3 N4 composites for ethanol sensing. The gas-sensing properties, including the selectivity, stability, and sensitivity, of SnO2 /g-C3 N4 to ethanol, were investigated. As a result, the SnO2 /g-C3 N4 composite-based sensor exhibited a higher response value and better selectivity to ethanol than that of the pure SnO2 nanoparticle-based sensor. The mechanism of the as-prepared sample gas-sensing to ethanol was discussed, in detail. 2. Results and Discussion 2.1. Sample Characterization Figure 1 shows the XRD patterns of the as-prepared pure SnO2 particles and SnO2 /g-C3 N4 composites. As demonstrated by the curves, five distinct diffraction peaks around 26.6◦ , 33.8◦ , 37.9◦ , 51.7◦ , and 65.9◦ can be seen, which correspond to (110), (101), (200), (211), and (301) planes of the tetragonal rutile structure SnO2 (JCPDS Card No. 41-1445), respectively. However, the diffraction peaks of g-C3 N4 in the SnO2 /g-C3 N4 composites are not observed in the curves. This may be due to the relatively small content of g-C3 N4 in the composites. Another reason is that the peak around 27.5◦ of g-C3 N4 is overlapped by the peak around 26.6◦ of SnO2 . The SEM images of the g-C3 N4 , SnO2 , and SnO2 /g-C3 N4 -7 composites are displayed in Figure 2. Figure 2a displays the SEM image of the g-C3 N4 sample. We can see many wrinkles on the edge of the thin layers, which represents the nanosheet structure. Figure 2b shows many SnO2 particles

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agglomerated together with different sizes. This phenomenon indicates that pure SnO2 particles agglomerated together with different sizes. This phenomenon indicates that pure SnO2 particles are are easy to agglomerate, which is harmful to the process of gas adsorption. As shown in Figure 2c, easy to agglomerate, which is harmful to the process of gas adsorption. As shown in Figure 2c, Nanomaterials2017, 7, x FOR PEER REVIEW  3 of 13  many nanoparticles are attached to  the g-C3 N4 nanosheet. This could be beneficial to improving many nanoparticles are attached to the g-C3N4 nanosheet. This could be beneficial to improving the the gas-sensing properties. Figure 2d illustrates the typical EDS mappings of the2 particles are  SnO 2 /g-C3 N4 -7 agglomerated together with different sizes. This phenomenon indicates that pure SnO gas-sensing properties. Figure 2d illustrates the typical EDS mappings of the SnO 2/g-C3N4-7 composite recorded from the surface area that is observed in Figure 2c, in which four elements easy to agglomerate, which is harmful to the process of gas adsorption. As shown in Figure 2c, many  composite recorded from the surface area that is observed in Figure 2c, in which four elements of C, of C, N, Sn, and O are concurrently existent. be thatSnO SnO Nimproving  are coexistent in nanoparticles  are concurrently attached  to  the  g‐CIt 3N 4can   nanosheet.  This  that could  be 2 and beneficial  the  2 and 3are 4 coexistent N, Sn, and O are existent. Itcan be concluded concluded g-Cg-C in 3N4to  the composite. gas‐sensing properties. Figure 2d illustrates the typical EDS mappings of the SnO 2/g‐C3N4‐7 composite  the composite. recorded from the surface area that is observed in Figure 2c, in which four elements of C, N, Sn, and  O are concurrently existent. It can be concluded that SnO2 and g‐C3N4 are coexistent in the composite. 

 

Figure 1. XRD patterns of g-C the 3 N g-C -5, SnO SnO22/g-C -7,4 -7, and SnO Figure 1. XRD patterns of the SnO /g-C and SnO 4, SnO 2, SnO 2/g-C 3N 3N 2/g-C 3N4-93 N4 -9 4 ,3N 2 , SnO 2 /g-C 3N 44-5, 34N 2 /g-C Figure 1.  XRD patterns of the g‐C3N4,  SnO2, SnO2/g‐C3N4‐5, SnO2/g‐C3N4‐7, and SnO2/g‐C3N4‐9  composites. composites. composites. 

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  Figure 2. SEM images of (a) g‐C3N4, (b) SnO2 and (c) SnO2/g‐C3N4‐7 composite, (d) the EDS mappings 

Figure 2. SEM images of (a) g-C3 N4 , (b) SnO2 and (c) SnO2 /g-C3 N4 -7 composite, (d) the EDS mappings of C, N, O, and Sn elements related to the selected area in (c).  of C, N, O, and Sn elements related to the selected area in (c).

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Figure 2. SEM images of (a) g-C3N4, (b) SnO2 and (c) SnO2/g-C3N4-7 composite, (d) the EDS mappings of C, N, O, and Sn elements related to the selected area in (c). Nanomaterials 2017, 7, 98 Nanomaterials2017, 7, x FOR PEER REVIEW

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Figure Figure33shows showsthe theTEM TEMimages imagesof ofg-C g-C33N N44 and and the the SnO SnO22/g-C /g-C33N N44-7 -7composite. composite. We We can can clearly clearly see see Figure 33ashows the TEM images of folds g-C3 Nwhich SnOlike We can clearlythe see 4 and the 2 /g-C 3 N4 -7 composite. from Figure that there are a lot of seem floccules, clearly demonstrating from Figure 3a that there are a lot of folds which seem like floccules, clearly demonstrating the from Figure 3a that there are a lot of folds which seem like floccules, clearly demonstrating the existence existence existence of of the the g-C g-C33N N44 nanosheet. nanosheet. The The lines lines represent represent the the stacked stacked rolled rolled edges edges of of the the nanosheet nanosheet of the g-C N nanosheet. The lines represent the stacked rolled edges of the nanosheet structure. 3 4 structure. As seen from Figure 3b, the SnO particles are highly dispersed on the surface 2 structure. As seen from Figure 3b, the SnO2 particles are highly dispersed on the surface of of the the As seen from Figure 3b, the SnO particles are highly dispersed on the surface of the g-C N nanosheet. 2 the disadvantage of an easy agglomeration of SnO23particles 4 g-C N nanosheet. This overcame 3 4 g-C3N4 nanosheet. This overcame the disadvantage of an easy agglomeration of SnO2 particles and and This overcame thethe disadvantage of an easy Therefore, agglomeration of SnO2 particles and further enhanced the further furtherenhanced enhanced thespecific specificsurface surfacearea. area. Therefore, this this test test identifies identifies itit as as an an excellent excellent candidate candidate specific surface materials. area. Therefore, this test identifies it as an excellent candidate for gas-sensing materials. for gas-sensing for gas-sensing materials.

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Figure 3.3.TEM g-C composite. 3NN 4-7 -7 Figure TEM images of (a) N and(b) (b)SnO SnO222/g-C /g-C composite. Figure3. TEMimages imagesof of(a) (a)g-C g-C333N N444and /g-C 3N 3 4-7 4 composite.

TG-DTA was carried the weight change situation of g-C N and SnO /g-C N -7. TG-DTA carried out out to to reveal reveal the theweight weightchange changesituation situationof ofg-C g-C3N 3N44 and SnO22/g-C33N44-7. TG-DTA was was carried out to reveal SnO2 /g-C−1 3 4 and 3N 4 -7. The temperature range is from room temperature to to 700 700◦°C, °C,and andthe theheating heatingrate rate is1010 °C·min . As −1 The temperature range is from room temperature is °C·min . As ◦ − 1 The temperature range is from room temperature to 700 C, and the heating rate is 10 C·min . As is is isshown shown inFigure Figure4,the the redand and bluelines lines stand for weight and heat heat flow curves, respectively. The shown in in Figure 4, 4, thered red and blue blue lines correspond correspondto toweight weightand and heatflow flowcurves, curves,respectively. respectively. first agravity peak is between 100 °C and 300 °C, which is due to the desorption of moisture and the The ◦ C and ◦ C, which The first first agravity agravity peak peak is is between between 100 100 °C and 300 300 °C, which is is due due to to the the desorption desorption of of moisture moisture and and solvent. The second agravity peakpeak is between 400 °C andand 600 600°C, °C, which is due to the combustion of the solvent. The second agravity is between 400°C which is due to the combustion ◦ ◦ the solvent. The second agravity peak is between 400 C and 600 C, which is due to the combustion of g-C N in air. The inset in Figure 4 is the TG-DTA profiles of SnO /g-C N -7. The agravic peak below 3 4 3N4 in air. The inset in Figure 4 is the TG-DTA profiles of 2 SnO 3 2/g-C 4 of 3N4-7. The agravic peak g-Cg-C air. The inset in Figure 4 is the TG-DTA profiles of SnO2 /g-C The agravic peak below 3 Nis 4 in 3 Nthe 4 -7. composite 400 °C due to the desorption of solvent, and the remanent content of is 92%isafter below 400°C is due to the desorption of solvent, and the remanent content of the composite 92% ◦ 400 C is due to the desorption of solvent, and the remanent content of the composite is 92% after the the combustion of g-C N . This result demonstrated that g-C N was not decomposed at the 3 4 3 4 after the combustion of g-C 3N4. This result demonstrated that g-C3N4 was not decomposed at the combustion of g-C3 N4 . This result demonstrated that g-C3 N4 was not decomposed at the optimum optimum temperature of °C in of testing the properties. optimum temperature of300 300°C in the theofprocess process the gas-sensing gas-sensing temperature of 300◦ C in the process testing of thetesting gas-sensing properties.properties. Figure 4. TG-DTA profiles of the g-C3N4 and SnO2/g-C3N4-7 composite. 100

Heat Flow(mW)

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3 Figure 5 depicts the80 N2 adsorption-desorption isotherms and the corresponding pore size distribution of the as-prepared SnO2 and SnO2/g-C3N4-7 samples. It can be seen from Figure 5a that 60 0 the isotherms of the two samples show type IV, which are the typical characteristics of mesoporous 40 -3 materials according to the IUPAC. The well-defined hysteresis loop belonging to the H3-type clearly 20 -6 indicates the existence of an aggregation of the laminated structure with a narrow slit formed by the 100 10 0 99 g-C3N4 and SnO2 nanoparticles. Figure 5b displays the0 corresponding pore-9size distribution of the 98 -10 -20 two samples. It can be clearly seen that the pore diameter of SnO2 and SnO2/g-C3N4 are relatively 97 -20 96 -30 -12 -40 concentrate upon 4.54 nm and 3.79 nm, respectively, according to the small and that the majority 95 -40 94 DFT method. The BET -60 calculated results show that-50the specific surface-15area of the SnO2 and 93 -60 2 −1 −1 92 ·g -70 SnO2/g-C3N4-7 samples are 94.3 m and 132.5 m2·g800 , respectively. The specific surface area of the -80 0 200 400 600 -18 as-prepared composite has been significantly which could be in favor of enhancing the Temperature improved, (oC) -100 200 400 600 gas-sensing properties.

Temperature (C)

Figure /g-C3N composite. 3N 4 -7composite. Figure4.4. TG-DTA TG-DTAprofiles profilesof ofthe theg-C g-C33N N44 and and SnO SnO22/g-C 4-7

Figure 5 depicts the N2 adsorption-desorption isotherms and the corresponding pore size distribution of the as-prepared SnO2 and SnO2/g-C3N4-7 samples. It can be seen from Figure 5a that

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Figure 5 depicts the N2 adsorption-desorption isotherms and the corresponding pore size distribution of the as-prepared SnO2 and SnO2 /g-C3 N4 -7 samples. It can be seen from Figure 5a that the isotherms of the two samples show type IV, which are the typical characteristics of mesoporous  the isotherms of the two samples show type IV, which are the typical characteristics of3‐type clearly  mesoporous materials according to the IUPAC. The well‐defined hysteresis loop belonging to the H materials according to the IUPAC. The well-defined hysteresis loop belonging to the H3 -type clearly indicates the existence of an aggregation of the laminated structure with a narrow slit formed by the  indicates theSnO existence of an aggregation ofdisplays  the laminated structure withpore  a narrow slit formed by the g‐C 3N4  and  2  nanoparticles.  Figure  5b  the  corresponding  size  distribution  of  the  g-C3 N4 and SnO2 nanoparticles. Figure 5b displays the corresponding pore size2/g‐C distribution of the two two samples. It can be clearly seen that the pore diameter of SnO 2 and SnO 3N4 are relatively  samples. Itthat  can the  be clearly seen that the pore diameter of and  SnO3.79  and SnO /g-C N are relatively small small and  majority  concentrate  upon 4.54  nm  nm,  respectively,  according  to  the  2 2 3 4 and that the majority concentrate upon 4.54 nm and 3.79 according DFT method. DFT  method.  The  BET  calculated  results  show  that nm, the respectively, specific  surface  area toof the the  SnO 2  and  2∙gthe −1 and 132.5 m 2∙g−1, respectively. The specific surface area of the  The 2BET results show that specific surface area of the SnO2 and SnO2 /g-C3 N4 -7 samples SnO /g‐Ccalculated 3N4‐7 samples are 94.3 m are 94.3 m2 ·g−1 and 132.5 m2 ·g−1 , respectively. The specific surface area of the as-prepared composite as‐prepared composite has been significantly improved, which could be in favor of enhancing the  has been significantly improved, which could be in favor of enhancing the gas-sensing properties. gas‐sensing properties. 

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Figure 5. (a) N2 adsorption-desorption isotherms and (b) the corresponding pore size distribution Figure  5.  the (a)  N 2  adsorption‐desorption  isotherms  and  (b)  the  corresponding  pore  size  distribution  curves of SnO 2 and SnO2 /g-C3 N4 -7 composite. curves of the SnO2 and SnO2/g‐C3N4‐7 composite. 

2.2. Gas-Sensing Performance 2.2. Gas‐Sensing Performance  The gas-sensing properties of the as-prepared samples in relation ethanol were investigated, The gas‐sensing properties of the as‐prepared samples in relation ethanol were investigated, in  in detail. Figure 6a shows the response values of pure SnO2 and SnO2 /g-C3 N4 -based sensors to detail. Figure 6a shows the response values of pure SnO2 and SnO2/g‐C3N4‐based sensors to 500 ppm  500 ppm ethanol at different operating temperatures. It can be clearly observed from the line chart of ethanol at different operating temperatures. It can be clearly observed from the line chart of the three  the three SnO2 /g-C3 N4 samples that the response values increase with the increase of the operating SnO2/g‐C3N4  samples  that  the  response  values  increase  with  the  increase  of  the  operating  temperature under 300 ◦ C. However, the response values decrease when the temperature is above temperature under 300 °C. However, the response values decrease when the temperature is above  ◦ 300 C. The maximum response of SnO2 /g-C3 N4 -7 is 360 at 300 ◦ C. In stark contrast, the maximum 300 °C. The maximum response of SnO2/g‐C 3N4‐7 is 360 at 300 °C. In stark contrast, the maximum  response of the pure SnO2 is only 95 at 320◦ C. This phenomenon can be explained by the fact that the response of the pure SnO2 is only 95 at 320°C. This phenomenon can be explained by the fact that the  SnO2 /g-C3 N4 -7-based sensor can tend to the balance between the speeds of chemical adsorption and SnO2/g‐C3N4‐7‐based sensor can tend to the balance between the speeds of chemical adsorption and  desorption at a lower temperature, and reach a higher response than that of the pure SnO2 sensor. This desorption at a lower temperature, and reach a higher response than that of the pure SnO2 sensor.  result indicates that the SnO2 /g-C3 N4 -based sensor has a great influence and enhances the gas-sensing This  result  indicates  that  the  SnO2/g‐C3N4‐based  sensor  has  a  great  influence  and  enhances  the  properties for ethanol. It reaches the maximum response when the mass percentage of g-C3 N4 gas‐sensing properties for ethanol. It reaches the maximum response when the mass percentage of  in the composites is 7%. The specific surface area of the SnO2 , SnO2 /g-C3 N4 -5, SnO2 /g-C3 N4 -7, g‐C3N4 in the composites is 7%. The specific surface area of the SnO 2, SnO2/g‐C3N4‐5, SnO2/g‐C3N4‐7,  and SnO2 /g-C3 N4 -9 composites is 94.3 m2 ·g−1 , 113.8 m2 ·g−1 , 132.5 m2 ·g−1 , and 122.2 m2 ·g−1 , and SnO2/g‐C3N4‐9 composites is 94.3 m2∙g−1, 113.8 m2∙g−1, 132.5 m2∙g−1, and 122.2 m2∙g−1, respectively.  respectively. When the g-C3 N4 content in the composites exceeds a certain value (e.g., 7 wt % in When the g‐C3N4 content in the composites exceeds a certain value (e.g., 7 wt % in this work), it may  this work), it may form the connection of bulk. As a result, the specific surface area of the composite form the connection of bulk. As a result, the specific surface area of the composite will decrease and  will decrease and there will be a reduced number of active sites for the adsorption of oxygen and ethanol there will be a reduced number of active sites for the adsorption of oxygen and ethanol gas, leading  gas, leading to the degradation of gas-sensing properties. Consequently, the gas sensor performance to the degradation of gas‐sensing properties. Consequently, the gas sensor performance increases at  increases at first and decreases when the g-C3 N4 content in the composites increases. A suitable first  and  decreases  when  the  g‐C3N4  content  in  the  composites  increases.  A  suitable  amount  of  amount of g-C3 N4 in the composite is beneficial to the dispersity, and a preferable heterojunctional g‐C3N4 in  the  composite  is  beneficial  to  the  dispersity,  and  a  preferable  heterojunctional  structure  structure can be formed in the interface region between 2D g-C3 N4 and SnO2 . A high content of 2D can  be  formed  in  the  interface  region  between  2D  g‐C3N4 and  SnO2.  A  high  content  of  2D  g‐C3N4  g-C3 N4 may lead to the connection of the g-C3 N4 nanosheets, which could form micro electric bridges may  lead  to  the  connection  of  the  g‐C3N4  nanosheets,  which  could  form  micro  electric  bridges  on  the surface. The micro electric bridges may result in the semiconductor’s resistance being reduced,  causing  a  reduction  in  the  gas  sensor  performance.  Figure  6b displays the response values of the  four samples at 300 °C to different concentrations of ethanol. As shown in the line chart, the response 

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on the surface. The micro electric bridges may result in the semiconductor’s resistance being reduced, Nanomaterials2017, 7, x FOR PEER REVIEW    6 of 13  causing a reduction in the gas sensor performance. Figure 6b displays the response values of the four ◦ samples at 300 C to different concentrations of ethanol. As shown in the line chart, the response values increase with the increasing ethanol concentrations. The slope of the curves increases rapidly  values increase with the increasing ethanol concentrations. The slope of the curves increases rapidly when the concentration range of ethanol is from 50 ppm to 500 ppm. However, it increases slowly  when the concentration range of ethanol is from 50 ppm to 500 ppm. However, it increases slowly gradually  with  an  increasing  concentration  in  the  range  of  500–3000  ppm.  Furthermore,  the  gradually with an increasing concentration in the range of 500–3000 ppm. Furthermore, the responses responses of the SnO 2/g‐C3N4 sensors are much higher than that of pure SnO2. It can be concluded  of that the adsorption of ethanol has approached the saturation value when the concentration reaches  the SnO2 /g-C3 N4 sensors are much higher than that of pure SnO2 . It can be concluded that the adsorption of ethanol has approached the saturation value when the concentration reaches 3000 ppm. 3000 ppm.  6 of 13

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(a) values of the sensors based on SnO2 , SnO2 /g-C (b) 3 N4 -5, SnO2 /g-C3 N4 -7, Figure 6. (a) Response Figure  6.  (a)  Response  values  of  the  sensors  based  on  SnO2,  SnO2/g‐C3N4‐5,  SnO2/g‐C3N4‐7,  and  and SnO /g-C N -9 to 500 ppm ethanol as a function of the operating temperature; (b) N the responses 2 6. (a) 3 Response 4 Figure values of the sensors based on SnO2, SnO2/g-C3N4-5, SnO 3 4-7, and 2/g‐C3N4‐9 to 500 ppm ethanol as a function of the operating temperature;  (b) 2/g-C the responses of  SnO of sensors (SnO andoperating SnO2 /g-C 300 ◦ C of versus 2 /g-C 4 -5, SnO 3 N4 -7, 3 N4 -9) operated SnO2/g-C -9, SnO to 500 ppm3 N ethanol as 2a/g-C function of the temperature; (b) the at responses 3N42 2, SnO2/g‐C3N4‐5, SnO2/g‐C3N4‐7, and SnO2/g‐C3N4‐9) operated at 300 °C versus different  sensors (SnO different concentrations of ethanol. sensors (SnO2, SnO2/g-C 3N4-5, SnO2/g-C3N4-7, and SnO2/g-C3N4-9) operated at 300 ° C versus concentrations of ethanol.  different concentrations of ethanol.

Figure 7 7  displays curvesof  ofthe  thepure  pureSnO SnO Figure  displays the the real real time time successive successive  response-recover response‐recover  curves  2  and  2 and Figure 7 displays the real time successive response-recover curves◦ of the pure SnO2 and SnO 2 /g‐C 3 N 4 ‐7 to 500 ppm ethanol in the range of 50–3000 ppm at 300 °C. As shown by the curves,  SnO2 /g-C3 N4 -7 to 500 ppm ethanol in the range of 50–3000 ppm at 300 C. As shown by the curves, SnO2/g-C3N4-7 to 500 ppm ethanol in the range of 50–3000 ppm at 300 °C. As shown by the curves, thethe response values of both sensors increase with the increasing concentration. The response value  response values of both sensors increasingconcentration. concentration. response the response values of both sensorsincrease increase with with the the increasing TheThe response valuevalue the  SnO 2/g‐CN 3N4-7-based ‐7‐based  sensor  is  much  higher  than  that  of  the  pure  SnO 2‐based  sensor  the  of of  theof SnO /g-C sensor is much higher than that of the pure SnO -based sensor the 3 34N4-7-based sensor is much higher than that of the pure SnO2-based 2 the 2SnO2/g-C sensor to to  theto same concentration of ethanol. The gas‐sensing properties of the composites enhanced a lot, which is  samesame concentration of ethanol. The gas-sensing the composites compositesenhanced enhanced a lot, which is concentration of ethanol. The gas-sensingproperties properties of the a lot, which consistent with the expected.  is consistent withexpected. the expected. consistent with the

  Figure 7. Real timetime response curves ofofthe /g-C N -7 to ethanol in the range of Figure 7. Real response curves thepure pureSnO SnO22 and and SnO SnO22/g-C 3N34-7 4to ethanol in the range of Figure 7.  Real time response curves of the pure SnO 2  and SnO 2 /g‐C 3 N 4‐7 to ethanol in the range of  50–3000 ppm. 50–3000 ppm. 50–3000 ppm.  The repeatability and stability are both crucial criteria to measure the gas-sensing properties. Figure 8a reveals the repeatability of the SnO2/g-C3N4-7 sensor to 500 ppm ethanol at 300 °C. As shown by the curves, the response values of the four response-recovery cycles are almost maintained at about 360. It can be concluded that the composite sensor has an admirable repeatability for ethanol gas sensing. A durable response value was measured to explore the stability of the SnO2/g-C3N4-7 sensor. Figure 8b displays the test result every five days, and the

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The The repeatability and stability are both crucial criteria to measure the gas‐sensing properties.  repeatability and stability are both crucial criteria to measure the gas-sensing properties. ◦ C.°C.  Figure  8a  reveals  the  repeatability  the 2SnO 2/g‐C sensor  ppm  ethanol  Figure 8a reveals the repeatability of theof SnO /g-C -74‐7  sensor to to  500500  ppm ethanol at at  300300  AsAs  shown 3 N43N shown by the curves, the response values of the four response‐recovery cycles are almost maintained  by the curves, the response values of the four response-recovery cycles are almost maintained at about  360. be It  concluded can  be  concluded  that  the  composite  has  an  admirable  repeatability  aboutat 360. It can that the composite sensorsensor  has an admirable repeatability for for  ethanol ethanol  gas  sensing.  A  durable  response  value  was  measured  to  explore  the  stability  of  the N -7 gas sensing. A durable response value was measured to explore the stability of the SnO2 /g-C 3 4 SnO2/g‐C3N4‐7 sensor. Figure 8b displays the test result every five days, and the response values to  sensor. Figure 8b displays the test result every five days, and the response values to 500 ppm ethanol 500  ppm  ethanol  7 ofthe  13 Nanomaterials 2017, 7, 98at  300  °C  are  maintained  at  around  360.  Therefore,  we  may  safely  draw  at 300 ◦ C are maintained at around 360. Therefore, we may safely draw the conclusion that the conclusion  that  the  SnO2/g‐C3N4‐7‐based  sensor  has  an  unexceptionable  stability  for  ethanol  gas  SnO2safely /g-C3draw N4 -7-based sensor that has the an unexceptionable stability gas sensing. the conclusion SnO2/g-C3N4-7-based sensor for hasethanol an unexceptionable stability for sensing.  ethanol gas sensing.

(a)  (b) Figure 8. (a) Repeatability (a) and (b) stability measurements of the SnO2 /g-C (b) 3 N4 -7-based sensor to Figure 8.  (a) Repeatability and (b) stability measurements of the SnO2/g‐C3N4‐7‐based sensor to 500  500 ppm ethanol at 300 ◦ C. Figure 8. (a) Repeatability and (b) stability measurements of the SnO2/g-C3N4-7-based sensor to 500 ppm ethanol at 300 °C.  ppm ethanol at 300 °C.

It  is  generally  known  that  selectivity  is  another  key  criterion  of  gas  sensors.  9  It is generally known that selectivity is another key criterion of gas sensors. Figure 9Figure  summarizes It is generally known that selectivity is another key criterion of gas sensors. Figure 9 summarizes the selectivity test results of the pure SnO 2  and SnO 2 /g‐C 3 N 4 ‐7 sensors to five different  the selectivity test results of the pure SnO2 and SnO2 /g-C3 N4 -7 sensors to five different gases of summarizes the selectivity test results of the pure SnO2 and SnO2/g-C3N4-7 sensors to five different gases of 500 ppm, including methanol, ethanol, toluene, formaldehyde, and acetone. It can be seen  500 ppm, including methanol, ethanol, toluene, formaldehyde, and acetone. It can be seen that the gases of 500 2ppm, including methanol, ethanol, toluene, formaldehyde, and acetone. It can be seen that the SnO /g‐C3N 4‐7 sensor exhibits a higher response to ethanol than to other gases compared to  SnO2 /g-C3 N4 -7 sensor exhibits a higher response to ethanol than to other gases compared to the pure that the SnO2/g-C 3N4-7 sensor exhibits a higher response to ethanol than to other gases compared to the pure SnO 2 sensor. The higher responses to ethanol may be because ethanol is more likely to lose  SnO2the sensor. The higher responses to ethanol may be because ethanol is more likely to lose electrons pure SnO 2 sensor. The higher responses to ethanol may be because ethanol is more likely to lose electrons in the process of the redox reaction with the absorbed oxygen and hydroxyl group (−OH)  in theelectrons process of the redox withreaction the absorbed andoxygen hydroxyl (−OH) is much in the process reaction of the redox with theoxygen absorbed and hydroxyl groupand (−OH) and is much easier to oxidize at the optimum operating temperature [35].    group easierand to is oxidize at thetooptimum temperature [35]. much easier oxidize atoperating the optimum operating temperature [35].

  Figure 9. Responses of SnO2 and SnO2/g‐C3N4‐7‐based sensors to 500 ppm different reducing gases at  300 °C. 

The  gas‐sensing  performance  of  different  sensing  materials  when  using  ethanol  is  listed  in  ◦ Figure 9. Responses SnO SnO32N /g-C sensors to 500 ppm differentreducing reducinggases gases at 300 C. Figure 9. Responses of SnO2 of and SnO sensors to 500 ppm different 2 and 3N4-7-based 2 /g-C 4 -7-based Table 1. As can be seen from Table 1, the response values of Fe 2O3 nanoparticles coated with SnO2  at 300 °C.

The The gas-sensing performance sensingmaterials materialswhen when using ethanol is listed gas-sensing performance of of different different sensing using ethanol is listed in in TableTable 1. As1.can be seen from Table 1, 1, the of Fe Fe22OO3 3nanoparticles nanoparticles coated As can be seen from Table theresponse response values values of coated withwith SnO2SnO2 nanowires [14], α-Fe2O3/g-C3N4 [35], RGO-SnO2 [36], and Au/SnO2 [37] were 31.18, 7.76, 70.4, and 18, respectively. In this work, the response value of SnO2/g-C3N4-7 to 100 ppm of ethanol vapor was 85 at 300 °C. Therefore, the SnO2/g-C3N4 composites show an excellent sensing property to ethanol vapor, and thus have a great potential application.

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nanowires [14], α-Fe2 O3 /g-C3 N4 [35], RGO-SnO2 [36], and Au/SnO2 [37] were 31.18, 7.76, 70.4, and 18, respectively. In this work, the response value of SnO2 /g-C3 N4 -7 to 100 ppm of ethanol vapor was 85 at 300 ◦ C. Therefore, the SnO2 /g-C3 N4 composites show an excellent sensing property to ethanol vapor, and thus have a great potential application. Table 1. Gas-sensing performance comparison of various gas sensors toward ethanol. Nanomaterials 2017, 7, 98

Sensing Materials

Ethanol Concentration (ppm)

Temperature (◦ C)

Response (Ra /Rg )

Fe2 O3 -SnO2 Table 1. Gas-sensing 100 300 gas sensors toward 31.18 performance comparison of various ethanol. α-Fe2 O3 /g-C3 N4 100 340 7.76 Sensing Materials Ethanol Concentration (ppm) Temperature (°C) Response (Ra/Rg) RGO-SnO 100 300 70.4 2 Fe2O23-SnO2 300 31.18 Au/SnO 100100 340 18 340 7.76 2O 3/g-C SnO2α-Fe /g-C 100100 300 85 3N 4 3N4 RGO-SnO2 Au/SnO2 SnO2/g-C3in N4 Figure displayed

100 100 100 absorption

300 340 300 SnO 2,

70.4 18 SnO2 /g-C3 N485-7

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Ref.

[14] [35] Ref. [36] [14] [37] [35] this work

[36] [37] g-Cthis 3 Nwork 4 are

As 10a, the edge of and around 377 nm, 441 nm, and 455 nm, respectively. This result can be ascribed to the heterojunction structure As displayed in Figure 10a, the absorption edge of SnO , SnO2/g-C3N4-7 and g-C3N4 are around between SnO2 and g-C3 N4 . The band gap energies of SnO2 2and g-C estimated according to 3 N4 cantobethe 377 nm, 441 nm, and 455 nm, respectively. This result can be ascribed heterojunction n/2 the equation (Ahν = k(hν-Eg) In3Nthis equation, n is determined by the type of optical transition structure between SnO2 and ). g-C 4. The band gap energies of SnO2 and g-C3N4 can be estimated n/2 of a semiconductor. value(Ahv for g-C 4 and 1,nrespectively. k and according to theThe equation = k(hv-Eg) In this is determined A, by ν, theEg, type of h are 3 N4 and). SnO 2 isequation, optical transition of a semiconductor. The Planck value forconstant, g-C3N4 andband SnO2 gap is 4 and A, v, the absorption coefficient, light frequency, and1,arespectively. constant, respectively. Eg, k and h are the absorption light band gap and a constant, respectively. The diagrams are shown in Figurecoefficient, 10b,c. The Egfrequency, of SnO2 /g-C 3 N4 -7 should be roughly calculated The diagrams are shown in Figure 10b,c. The Eg of SnO2/g-C3N4-7 should be roughly calculated according to the equation (Eg = 1240/λ) because of the uncertain type of optical transition. As a result, according to the equation (Eg = 1240/λ) because of the uncertain type of optical transition. As a the Eg of SnO2 , g-C SnO /g-C3 N/g-C 3.49 eV, 2.75 eV, and 2.81 eV, respectively. 3N 4 , and 4 -7 are result, the Eg of SnO , g-C N , 2and SnO N -7 are 3.49 eV, 2.75 eV, and 2.81 eV, respectively. 2

3

4

2

3

4

(a)

(b)

(c)

Figure 10. (a) UV-vis diffuse reflectance spectra of SnO2, g-C3N4, and SnO2/g-C3N4-7 composites, (b)

Figure 10. (a) UV-vis diffuse reflectance spectra of SnO , g-C3 N4 , and SnO2 /g-C composites, 1/2 3 N4 -7 plot of (Ahv)2 versus energy (hv) for the band gap energy2 of SnO versus energy 2, (c) plot of (Ahv) 2 versus energy (hv) for the band gap energy of SnO , (c) plot of (Ahν)1/2 versus energy (b) plot (hv) of (Ahv) 2 for the band gap energy of g-C3N4. (hν) for the band gap energy of g-C3 N4 . The sensing mechanism of the SnO2/g-C3N4 composite towards ethanol gas need to be further investigated. When the sensor was exposed in air, oxygen molecules were adsorbed on the surface of SnO2 and capture electrons from the conduction band of SnO2. Then oxygen molecules were

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The sensing mechanism of the SnO2 /g-C3 N4 composite towards ethanol gas need to be further investigated. When the sensor was exposed in air, oxygen molecules were adsorbed on the surface of SnO2 and capture electrons from the conduction band of SnO2 . Then oxygen molecules were ionized to O2− , O− , and O2 − , and the formation of depletion layers led to the increase in the resistance of the composite sensor. However, when the sensor was exposed to the ethanol gas, the ethanol molecules proceeded oxidation and the reduction reaction with oxygen ions absorbed on the surface of the sensor. Concurrently, the ethanol molecules were oxidized into acetaldehyde and eventually turned into carbon dioxide and water [24]. As a result, the trapped electrons were released back to the depletion layer of the sensing film, resulting in the decrease in the resistance of the composite-based sensor. The SnO2 /g-C3 N4 composite exhibited more preferable gas-sensing properties than that of pure SnO2 . This is mainly due to the high specific surface area and the interaction between g-C3 N4 and SnO2 . The presence of g-C3 N4 can prevent the aggregation of SnO2 particles to form a high surface approachability structure, leading to the promotion of the adsorption and diffusion process of ethanol molecules. In this composite, the s-triazine structure g-C3 N4 sheet substrate can provide more active sites to adsorb O2 molecules. The elevated gas-sensing properties may also be due to the interactions between Sn and g-C3 N4 , and the heterojunction of the interface region between g-C3 N4 and SnO2 . The electrical property at the heterojunction changes when ethanol gas molecules pass through the interface region between g-C3 N4 and SnO2 . Both SnO2 and g-C3 N4 are n-type semiconductors. The band gaps are 3.71 eV and 2.7 eV, respectively. The conduction band level of g-C3 N4 is more negative than SnO2 . When SnO2 and g-C3 N4 were combined, they formed a heterojunction structure. The electrons will thus inflow from the conduction band of g-C3 N4 to the conduction band of SnO2 , leading to a higher potential barrier. As a result, the electrons and holes are separated [38]. Meanwhile, the heterojunction structure may suppress the recombination of the electron-hole and urge electrons to quickly transfer from the ethanol vapor to the surface of SnO2 /g-C3 N4 . Therefore, this leads to a higher response because of the increased conductivity of the heterojunction structure [35]. This co-adjacent retiform structure could provide more accesses for the gas adsorption and diffusion between SnO2 and ethanol molecules. O2 + e− → O2−

(1)

2CH3 CH2 OH + O2− → 2CH3 CHO + 2H2 O + e−

(2)

2CH3 CHO + 5O2− → 4CO2 + 4H2 O + 5e−

(3)

3. Materials and Methods 3.1. Chemicals Urea and Tin (IV) chloride pentahydrate (SnCl4 ·5H2 O, 99.0%) were purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). All chemicals were used as received, without further purification. 3.2. Preparation of g-C3 N4 Graphitic carbon nitride (g-C3 N4 ) wasdirectly synthesized by the pyrolysis of urea in the muffle furnace (Luoyang Shenjia Kiln Co., Ltd., Luoyang, Henan, China). A total of 20 g urea was put into an alumina crucible with a cover and then warmed to a temperature of 250 ◦ C within 110 min, before being kept at 250 ◦ C for 1 h. Then, the temperature was increased to 350 ◦ C within 50 min and kept at 350 ◦ C for 2 h. Finally, temperature rose to 550 ◦ C within 100 min and was kept at 550 ◦ C for another 2 h. The heating rate of the whole reaction was 2 ◦ C·min−1 . The resulting yellow powder was collected. 3.3. Synthesis of the SnO2 /g-C3 N4 Composite SnO2 /g-C3 N4 composites were synthesized through a facile calcination method. In a typical preparation process, a certain amount g-C3 N4 was dissolved in 100 mL H2 O and 2.09 g SnCl4 ·5H2 O

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was added into the dispersed suspension with ultrasonic treatment for 2 h. Then, the mixed solution was transferred into an alumina crucible and put it into the muffle furnace. It was heated to 400 °C was added into the dispersed suspension with ultrasonic treatment for 2 h. Then, the mixed solution for 2 h. Finally, the resulting product was ground to powder. According to this method, the different was transferred into an alumina crucible and put it into the muffle furnace. It was heated to 400 ◦ C mass ratios of the SnO2/g-C3N4 composites were synthesized and marked as SnO2/g-C3N4-5, for 2 h. Finally, the resulting product was ground to powder. According to this method, the different SnO2/g-C3N4-7, and SnO2/g-C3N4-9. For comparison, the same method was used to synthesize the mass ratios of the SnO2 /g-C3 N4 composites were synthesized and marked as SnO2 /g-C3 N4 -5, pure SnO2 particles with the absence of g-C3N4. SnO2 /g-C3 N4 -7, and SnO2 /g-C3 N4 -9. For comparison, the same method was used to synthesize the pure SnO2 particles with the absence of g-C3 N4 . 3.4. Characterizations 3.4. Characterizations The samples were characterized by X-ray diffraction (XRD, Bruker-AXS D8, Bruker, Madison, WI, USA) with Cuwere Kα radiation at 40 kV and 25diffraction mA. The structure and morphology of theMadison, samples The samples characterized by X-ray (XRD, Bruker-AXS D8, Bruker, wereUSA) observed by Kα field-emission electron (FESEM, Quanta™ 250 FEG) (FEI, WI, with Cu radiation atscanning 40 kV and 25 mA.microscopy The structure and morphology of the samples Eindhoven, The Netherlands). Transmission electron microscopy (TEM) analysis was performed on were observed by field-emission scanning electron microscopy (FESEM, Quanta™ 250 FEG) (FEI, aEindhoven, JEOL JEM-2100 microscope (JEOL, Tokyo,electron Japan) operating at (TEM) 200 kV. The thermal gravity and The Netherlands). Transmission microscopy analysis was performed on differential thermal analysis (TG-DTA) was carried on TA-SDT Q600 (TA, New Castle, DE, USA) at a JEOL JEM-2100 microscope (JEOL, Tokyo, Japan) operating at 200 kV. The thermal gravity and −1 under an air atmosphere. Nitrogen adsorption-desorption isotherms were adifferential heating rate 10 °C·min thermal analysis (TG-DTA) was carried on TA-SDT Q600 (TA, New Castle, DE, USA) at a obtained on a Quantachrome Autosorb-iQ sorptionNitrogen analyzeradsorption-desorption (Quantachrome, Boynton Beach,were FL, ◦ C·min− 1 under heating rate 10 an air atmosphere. isotherms USA). Before carrying out the measurement, the samples were degassed for more than 6 h at 300 obtained on a Quantachrome Autosorb-iQ sorption analyzer (Quantachrome, Boynton Beach, °C. FL, USA). Before carrying out the measurement, the samples were degassed for more than 6 h at 300 ◦ C. 3.5. Gas-Sensing Test 3.5. Gas-Sensing Test performance of the as-synthesized samples when using ethanol was tested by The gas-sensing usingThe the gas-sensing intelligent gas-sensing analysis of CGS-4TPS (Beijing Elite Co.,ethanol Ltd., Beijing, China). performance of the system as-synthesized samples when using was tested by Figure 11 shows a brief schematic diagram of the device. The gas sensors were prepared according using the intelligent gas-sensing analysis system of CGS-4TPS (Beijing Elite Co., Ltd., Beijing, China). to the usual way.aAbrief small amount diagram of the as-prepared sample ground agate mortar withto a Figure 11 shows schematic of the device. Thewas gas fully sensors were in prepared according few dropsway. ethanol, which served as the agglomerant to form starchiness. the with pastesa the usual A small amount of the as-prepared sample was fully groundAfterwards, in agate mortar were equably spread on the ceramic substrate (13.4 mm × 7 mm) with interdigitated Ag-Pd few drops ethanol, which served as the agglomerant to form starchiness. Afterwards, the pastes were electrodes to form film.substrate Before carrying out× the test, the substrate was aged at 180 °C for 24to h equably spread onthe thethin ceramic (13.4 mm 7 mm) with interdigitated Ag-Pd electrodes ◦ Csensors to improve thefilm. stability and repeatability the gas sensors. The of the defined form the thin Before carrying out theoftest, the substrate wasresponse aged at 180 for 24 hwas to improve as ratio and of Rrepeatability a/Rg, where R andgas Rg sensors. were theThe resistance in airwas anddefined in the as target gas, thethe stability ofa the responseofofsensor the sensors the ratio respectively. of Ra /Rg , where Ra and Rg were the resistance of sensor in air and in the target gas, respectively.

and thethe structure of Figure 11. The internal internal structure structurediagram diagramofofthe theCGS-4TPS CGS-4TPSgas-sensing gas-sensingtest testsystem system and structure thethe substrate. of substrate.

4. Conclusions 2 ·2g 2/g-C3 3N high surface surface area area (132.5 (132.5 mm ·g−−11)) were In this study, study, the the SnO SnO2 /g-C N44 composites composites with a high synthesized via a facile calcination calcination method method by by using using SnCl SnCl44··5H SnO22 5H22O and urea as the precursor. precursor. The SnO particles were highly highly distributed distributed on on the the g-C g-C33N N44 nanosheets. nanosheets. The gas-sensing properties of The gas-sensing properties of the SnO22/g-C 4-7 sensor exhibited exhibitedpreferable preferable results when compared the SnO pure /g-C3N composite-based sensor results when compared to thetopure 3N 4 -7composite-based 2, SnO 2, including the sensitivity and selectivity. Considering the easy-preparation process, the including the sensitivity and selectivity. Considering the easy-preparation process, the SnO /g-C N 2 3 4 composite could be a promising candidate for high-performance ethanol gas-sensing applications.

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Acknowledgments: This work was supported by the National Natural Science Foundation of China (51404097, 51504083, U1404613), Program for Science&Technology Innovation Talents in Universities of Henan Province (17HASTIT029), Natural Science Foundation of Henan Province of China (162300410113), 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 Science and Technology) in the University of Henan Province (16IRTSTHN005), and Foundation for Distinguished Young Scientists of Henan Polytechnic University (J2016-2, J2017-3). Author Contributions: Jianliang Cao conceived and designed the experiments; Cong Qin, Bo Zhang, Yuxiao Gong, Huoli Zhang, Guang Sun, and Hari Bala performed the experiments and analyzed the data; Yan Wang and Zhanying Zhang 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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