- Email: [email protected]
rGO nanocomposites and their trimethylamine sensing properties
Materials Research Bulletin 114 (2019) 61–67
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
One-step synthesis of Ag/SnO2/rGO nanocomposites and their trimethylamine sensing properties
T
Saisai Zhanga,1, Bowen Zhangc,1, Guang Suna,b, , Yanwei Lia, Bo Zhanga, Yan Wangb, ⁎ Jianliang Caoa,b, Zhanying Zhanga,b, ⁎
a
School of Materials Science and Engineering, Cultivating Base for Key Laboratory of Environment-friendly Inorganic Materials in University of Henan Province, Henan Polytechnic University, Jiaozuo, 454000, China b The Collaboration Innovation Center of Coal Safety Production of Henan Province, Henan Polytechnic University, Jiaozuo, 454000, China c State Key Laboratory of Superhard Materials, Jilin University, Changchun, 130000, China
ARTICLE INFO
ABSTRACT
Keywords: Ag Reduced graphene oxide (rGO) SnO2 Ternary nanocomposites Gas sensor
Herein we present the synthesis of Ag/SnO2/reduced graphene oxide (rGO) ternary nanocomposites (NCs) through one-step route. Many methods were employed to investigate the phase and morphological structure of the prepared composites. The coexisting of SnO2 and Ag can be confirmed by X-ray diffractometry and X-ray photoelectron spectroscopy analysis and a large number of nanoparticles decorated on the surface of rGO are observed from field-emission scanning electron and transmission electron microscopy images. The composites were employed to fabricate gas sensor to detect TEA gas. The Ag/SnO2/rGO sensor exhibits higher response to TEA gas at a low working temperature of 220 °C compared with SnO2 and SnO2/rGO. This markedly enhancement of TEA gas-sensing performance may be related to the well catalytic activity and higher specific surface area of the Ag/SnO2/rGO NCs.
1. Introduction Gas sensor, as a device that can convert fraction volume of gases into corresponding electrical signals, has been used to detect harmful and hazardous gases and plays an important role in ecological management and environment monitoring [1–3]. Over the last decades, various kinds of gas sensors have been fabricated and developed to achieve better gas-sensing properties, such as semiconductor sensor [4,5], electrochemical sensor [6], catalytic combustion sensor [7], thermal conductivity sensor [8], infrared sensor [9], and solid electrolyte sensor [10]. Among them, the gas sensor based on semiconductor, especially metal oxide semiconductor (MOS), has drawn extensive research interest due to their advantages of simple preparation, good stability, and environment-friendly [5,11–13]. Among numerous MOS materials, SnO2 is the one of the earliest and most typical gas sensing material. Many reports indicated that SnO2 based gas sensors will exhibit good performance [14,15]. However, there are many limitations to restrict the development of SnO2 based sensor, such as low response and poor selectivity. Therefore, further improving the gassensing performance of the SnO2 materials is one of the current
research focuses. Graphene is a two-dimensional carbon nanomaterial with hexagon honeycomb lattice composed of carbon atoms through sp2 hybridized orbital and has received wide attention due to its excellent optical, electrical and mechanical properties [16–18]. Recently, Graphene, especially reduced graphene oxide (rGO), has been considered as a revolutionary material in the future and exhibits important application prospects in materials science [19], energy engineering [20,21], gas sensor [22–26], and biomedicine and drug delivery [27]. Therefore, gas sensors based on rGO/MOS composites are expected to achieve improved gas-sensing properties. Various rGO/MOS composite have been synthesized as sensing materials, such as MoS2/rGO composites [23], ZnO quantum-dots/graphene NCs [28], Cu2O/graphene sheets [29], SnO2/graphene [30] and so on. It is reported that the particle size of MOS can be controlled by graphene oxide (GO), and the rGO would also prevent the aggregation of oxides particles which can improve the electron transfer capability [31]. Hence, as compared with pure SnO2, the SnO2/rGO composites always exhibited enhanced gas sensing behaviors. In addition, noble metal nanoparticles are usually added into the SnO2/rGO composites to enhance its sensitivity. Meng et.al
Corresponding authors at: School of Materials Science and Engineering, Cultivating Base for Key Laboratory of Environment-friendly Inorganic Materials in University of Henan Province, Henan Polytechnic University, Jiaozuo, 454000, China. E-mail addresses: [email protected] (G. Sun), [email protected] (Z. Zhang). 1 These authors contributed equally to this work. ⁎
https://doi.org/10.1016/j.materresbull.2019.02.019 Received 9 October 2018; Received in revised form 3 January 2019; Accepted 19 February 2019 Available online 21 February 2019 0025-5408/ © 2019 Elsevier Ltd. All rights reserved.
Materials Research Bulletin 114 (2019) 61–67
S. Zhang, et al.
synthesized Au/SnO2/rGO NCs with high response to ethanol by onestep synthesis method [24]. Su et.al used a one-pot method to fabricate Pd/SnO2/rGO NCs, which exhibited high response to NH3 gas at low working temperature [25]. TEA, as one of toxic and harmful organic amines, can bring serious irritation to the respiratory tract and even cause pulmonary edema and death after inhalation [32]. In addition, as a common industrial raw material, TEA poses a great threat to human health even at concentration as low as 10 ppm. Therefore, the rapid and effective detection of TEA is of great significance for the protection of human health. However, to our best knowledge, there are few reports on the detection of TEA based on ternary Ag/SnO2/rGO composites with good selectivity. In this work, a facile solution strategy was employed to synthesis ternary Ag/SnO2/rGO NCs. And such ternary Ag/SnO2/rGO NCs as well as pure SnO2 and SnO2/rGO have been used to fabricate gas sensors to detect TEA. The measurement results show that the Ag/ SnO2/rGO sensor exhibits a significantly enhancement in sensing properties and optimum selectivity to TEA compared with pure SnO2 or SnO2/rGO sensors. The better gas sensing performance may be attributed to the higher specific surface area and conductivity and catalytic activity of the Ag/SnO2/rGO ternary composites.
microscope).The element analysis was carried out by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI electron spectrometer using Al Kα radiation). 2.4. Measurement of gas-sensing Gas-sensing tests were carried out on an intelligent gas-sensing analysis system of CGS-4TPS (Beijing Elite Tech Co., Ltd., China). The fabricating and testing procedure of gas sensors has been described in our previous report [13]. Then the fabricated sensors would be aged at 220 °C for days to improve their mechanical strength and stability before gas sensing test. The sensor response (S) was defined as Ra/Rg, where Ra and Rg were the sensor resistance in air and target gas, respectively. The sensing performances of the sensors were investigated under laboratory conditions (17% RH. 25 °C). 3. Results and discussion 3.1. Structural and morphological characteristics The phase and crystallinity of the synthesized samples were examined by XRD shown in Fig. 1. From Fig. 1(a), it can be seen that all of diffraction peaks can be correspond to tetragonal rutile SnO2 phase (JCPDS card no.41–1445) and no additional characteristic peaks of other impurities were found, indicating the formation of SnO2 with high purity. Compare with pure SnO2, the SnO2/rGO (Fig. 1(b)) shows a similar XRD pattern but broader peaks, suggesting that the smaller crystal size of SnO2 can be obtained. This can be attributed to the control function of GO on the particle size of MOS [28]. Fig. 1(c) displays the XRD patterns of Ag/SnO2/rGO sample. Besides of the peaks arising from SnO2, five peaks were also appeared in the pattern at 38.118, 44.304, 64.45, 77.407 and 81.55°, which are assigned as Ag (111), (200), (220), (311) and (222), respectively. And these observed diffraction peaks are well matched with Ag (JCPDS card no.04-0783), indicating the coexistence of Ag and SnO2 phase. The FESEM and TEM in Fig. 2 show the microstructure and morphology of the products. A large-scale silk-like sheet with smooth surface was observed in Fig. 2(a), suggesting that high quality GO has been obtained. Fig. 2(b) displays the structure of SnO2 nanoparticles prepared via hydrothermal method. It can be seen that a large number of SnO2 particles agglomerate together to form large blocks. The HRTEM image is shown in Fig. 2(b) inset to further analysis the microstructure and size of particles. The typical zero-dimensional nanomaterials can be found, consisting of many ellipsoidal nanoparticles with an average size of 3.8 nm. The FESEM image of SnO2/rGO samples is displayed in
2. Experimental 2.1. Materials Graphene powder, cubic niter, sulphuric acid, tin chloride pentahydrate, silver nitrate, ammonia were of analytical grade and used without further purification. 2.2. Preparation of Ag/SnO2/rGO NCs The grapheme oxide (GO) was synthesized from graphite powder according to a modified Hummers method [33]. Typically, designed amounts of graphite powder (1.5 g) and NaNO3 (1.5 g) were added slowly into 60 mL of sulphuric acid, and then 9 g of KMnO4 uniform added in mixture solution with magnetic stirring under ice bath. After 90 min, the precursor solution was transferred to a water bath under stirring at 38 °C for 30 min. Next, the solution was warmed up and maintained at 78 °C, and 132 mL of distilled water and a small amount of H2O2 were slowly dropped into the mixture solution. Finally, the suspension was collected by centrifugation and washed with hydrochloric acid solution and water for several times to obtain the GO samples. The rGO synthesis method was as follows: 4 ml of GO suspension and 4 ml of triethylamine were ultrasonicated for 1 h, and the solution was placed in a reaction vessel for hours at 180 °C. The Ag/SnO2/rGO NCs were prepared via a simple one-step method. 5 mL of 10 wt% rGO solution was added into 25 mL of SnCl4·5H2O ethanol solution (2.5 mM/L). Then 5 mL of 2 wt% AgNO3 solution was added into the mixture solution drop by drop and ultrasonicated for 20 min. Finally 12.8 mL of deionized water and 2.2 mL ammonia were added into the above suspension under stirring, and then the as-prepared precursor solution was placed to 50 mL of autoclave and kept 160 °C for 24 h. Then the precipitates were washed for several times with water and ethanol, respectively, and dried at 80 °C to obtain the final products. For comparison, pure SnO2 and SnO2/rGO NCs were synthesized by a similar hydrothermal method without addition of AgNO3 and rGO, AgNO3, respectively. 2.3. Characterization The purity of samples was characterized by using X-ray power diffraction (XRD) on a Bruker/D8Advance diffract meter with Cu Kα radiation. The morphologies and microstructures were examined by fieldemission scanning electron microscopy (FSEM, Quanta 250 FEG) and transmission electron microscopy (TEM, JEOL JEM-2100
Fig. 1. XRD patterns of (a) pure SnO2, (b)SnO2/rGO and (c) Ag/SnO2/rGO samples. 62
Materials Research Bulletin 114 (2019) 61–67
S. Zhang, et al.
Fig. 2. The FESEM images of (a) GO, (b) pure SnO2 nanoparticles, inset is the HRTEM image of nanoparticles, (c) SnO2/rGO NCs films, inset is the high magnification image of the selected area, and (d) the FESEM images and (e,f) TEM images of Ag/SnO2/rGO NCs films.
Fig. 2(c), revealing that the composites consist of massive wrinkled sheets. From the inset of Fig. 2(c), the surface is covered with a large amount of particles and results in a rougher surface compared with GO observed from Fig. 2(a). The Ag/SnO2/rGO sample exhibits a similar morphology compared to SnO2/rGO sample, and no obvious Ag particles can be observed in Fig. 2(d). This is because that the size of Ag nanoparticles is too small to be observed clearly. Fig. 2(e) displays the TEM images of Ag/SnO2/rGO which clearly reveal the existence of massive nanoparticles on the surface of the rGO sheets. The SnO2 and Ag nanoparticles are small and aggregated together. A typical HRTEM image is presented in Fig. 2(f). The measured interplanar distances are about 0.33 nm and 0.24 nm which can be corresponded well to (110) plane for SnO2 and (111) plane for Ag, respectivily. Moreover, the average size of SnO2 nanoparticles is about 2.88 nm, which is smaller than pure SnO2. This can be consistent with the XRD analysis results. XPS analysis was employed to further investigate the chemical states in the Ag/SnO2/rGO NCs. Fig. 3(a) displays the wide-survey XPS spectrum which reveals that the presence of Sn, C and O elements. And the Sn and O peaks are much higher than the C peak, indicating that a
great deal of SnO2 nanoparticles is formed. A high resolution core lever spectrum of Sn3d is shown in Fig. 3(c), displaying two peaks at 487.6 eV and 495.8 eV corresponding to Sn 3d3/2 and Sn 3d5/2, respectively. Fig. 3(d) illuminates the presence of Ag element in the composites due to the two peaks at 368.3 eV and 374.3 eV which are attributed to Ag 3d5/2 and Ag 3d3/2 respectively. The specific surface area and the porosity of the prepared samples were investigated with N2 adsorption-desorption analysis. The N2 adsorption-desorption isotherms (shown in Fig. 4(a)) of all samples display type IV cures with H2-type hysteresis loops according to the IUPAC. The cures of pore size distribution shown in Fig. 4(b) reveal that the samples have relatively small pores with a size of 1˜4 nm, indicating the well homogeneity of the pores. The BET calculated result shows that the specific surface areas of SnO2/rGO and Ag/SnO2/rGO composites are 204.284 and 196.434 m2 g−1, respectively. However, specific surface area of the SnO2 samples is only 171.616 m2 g−1. These results indicate that a higher surface area can be obtained when SnO2 nanoparticles decorated on the surface of rGO, which can provide more surface active sites and available space for gas molecules adsorption 63
Materials Research Bulletin 114 (2019) 61–67
S. Zhang, et al.
Fig. 3. (a)XPS spectra of Ag/SnO2/rGO NCs; high resolution spectra for (b) C 1 s, (c) Sn 3d and (d) Ag 3d of the Ag/SnO2/rGO NCs.
and surface reaction. This is beneficial to improvement of the gas-sensing performance. Although Ag/SnO2/rGO composites exhibit a relatively smaller surface area compared to SnO2/rGO composites, the catalytic activity of Ag will further enhance the gas sensing performance. 3.2. Gas sensing properties The ternary Ag/SnO2/rGO NCs with high special surface area and catalytic activity were expected to improve the gas sensing performance. Therefore, the sensor based Ag/SnO2/rGO NCs was fabricated and its gas sensing properties were investigated and compared with pure SnO2 and SnO2/rGO, in which TEA was chose as a representative reducing gas. For MOS based gas sensors, the working temperature can affect the adsorption-desorption efficiency of gas molecules during the testing process and is an important parameters for gas sensors. Therefore, the sensitivities of three sensors to 100 ppm TEA were tested at different working temperature to obtain the optimum working temperature. It can be found that the sensitivity of the three sensors increased firstly and then decreased with rising the working temperature. The highest response value toward TEA can be observed of Ag/SnO2/ rGO based sensor, and the maximum value was 82.47 at 220 °C. The response curves of SnO2 and SnO2/rGO exhibit analogous trend of "increase-maximum-decrease" and their maximum values are 32.87 and 70.52 at 220 °C, respectively. Thus, 220 °C was chosen for the three sensors as the optimum working temperature. The relationships between the responses and TEA concentrations for all the sensors are described as shown in Fig. 5(b). The response increased with the TEA concentration ranging from 0.5 ppm to 500 ppm, and all the sensors present a rapid increase in response. However, when the TEA concentration exceeds 100 ppm, the rate of increase in response begins to slacken with further increasing the TEA concentration. It can also found that the corresponding sensitivities of all sensors rise with increasing TEA concentration and the Ag/SnO2/rGO based sensor exhibits the highest response value to each concentration of TEA. More details of the response
Fig. 4. a) N2 adsorption-desorption isotherm and (b) pore-size distribution curves of SnO2 nanoparticles, SnO2/rGO and Ag/SnO2/rGO composites. 64
Materials Research Bulletin 114 (2019) 61–67
S. Zhang, et al.
Fig. 6. (a) Response and recovery curves of the three kinds of sensors to different concentration of TEA; (b) response transients of these sensors to 20 ppm TEA.
Ag/SnO2/rGO based sensor to 100 ppm TEA is 82.47, which is about four times of ethanol (23.83), 4.5 times of formaldehyde (18.25), seven times of acetone and methanol (12.69 and 11.89) and eight times of propanediol (10.89), respectively. In contrast, the SnO2 and SnO2/rGO based sensors show similar selective characteristics but lower sensitivity to the same concentration of testing gases. Such phenomenon can be attributed to the high conductivity and catalytic activity of the Ag/ SnO2/rGO ternary composites. In order to further evaluate the longterm stability of the sensor, the response of the sensor based on Ag/ SnO2/rGO to 20 ppm TEA was carried out, as shown in Fig. 7(b). It’s found that the response value of the sensor was almost constant without an obvious changeable within 4 weeks, demonstrating its good longterm stability. On the basis of its good stability, the present Ag/SnO2/ rGO sensor may be the potential candidate for practical application.
Fig. 5. (a) Response of SnO2, SnO2/rGO and Ag/SnO2/rGO sensors to 100 ppm TEA at different temperatures; (b) Concentration-dependent response of the three sensors toward TEA at 220 °C, inset is more details of response variation to different TEA concentration ranges.
variation to gas concentration in the range of 0.5–40 ppm are shown in the inset in Fig. 5(b). The Ag/SnO2/rGO sensor shows an approximately linearity in the concentration rang of 0.5–40 ppm, indicating their great potential for quantitative detection of TEA at ppm level. Fig. 6(a) displays the dynamic response-recovery curves of the three sensors toward different TEA concentration at 220 °C. Obviously, the response amplitudes could gradually increase with the TEA concentration increasing from 0.5 to 500 ppm, indicating our sensors possess good capability to TEA. Furthermore, the response amplitude of Ag/SnO2/rGO is obviously higher than that of SnO2 and SnO2/rGO. The response transient curves of all sensors to 20 ppm TEA are shown in Fig. 6(b). It is clear that the curve of Ag/SnO2/rGO rises sharply upon exposure to TEA, and declines rapidly after the gas is released. From Fig. 6(b), the calculated response times of SnO2 and SnO2/rGO are determined as 30 and 14 s, respectively. In contrast, the Ag/SnO2/rGO sensor shows a shorter response time of 11 s, suggesting its faster response characteristics. The faster response speed of Ag/SnO2/rGO reveals its promising application in real-time detection of TEA. In terms of practical applications, selectivity is a vital parameter of gas sensors. Thus, in order to investigate the selectivity of Ag/SnO2/ rGO NCs, the response of all sensors to 100 ppm TEA were tested by compared with other gases, such as ethanol, formaldehyde, acetone, methanol, and propanediol. From Fig. 7(a), the Ag/SnO2/rGO sensor exhibits the higher response value to TEA than other gases, indicating its good selectivity of TEA over other gases. For example, the value of
3.3. Gas sensing mechanism The gas sensing mechanism of Ag/SnO2/rGO ternary composites can be well illustrated by the surface-controlled type theory [34]. Firstly, as shown in Fig. 8, when the sensors are exposed to ambient air, the oxygen molecules will absorb on the surface of SnO2 to form oxygen ions (O2−, O−, O2−) by capturing free electrons from conduction band, which can reduce the free-charge-carriers concentration and increase the resistance of sensing materials. Once the sensors are exposed to reducing gas, such as TEA, the adsorbed oxygen ions could react with the gas molecules, which can be simply expressed as [35]:
2(C2 H5 )3N + 430
12CO2 + 15H2 O + 2NO2 + 43e
After the reaction, the trapped electrons would be released back into the materials, leading to the electrons-concentration increased and the resistance of materials decreased. 65
Materials Research Bulletin 114 (2019) 61–67
S. Zhang, et al.
Thirdly, it can be attributed to the electronic and chemical sensitization of Ag in the ternary Ag/SnO2/rGO NCs. The presence of Ag additive not only adjusts the electron density on the surface of Ag/SnO2/rGO NCs by electronic sensitization, but also makes chemical sensitization by enhancing the catalytic activity of the composites. In other words, the Ag component may afford more active sites for gas adsorption and enhance the catalytic decomposition of TEA. The catalytic activity of noble metals on MOS has been extensively discussed in many literatures [22,35] and will not be explained in detail here. 4. Conclusions The ternary Ag/SnO2/rGO NCs have been successfully prepared through a facile one-step route as well as the pure SnO2 and SnO2/rGO. XRD, FESEM, TEM, and XPS results reveal that the SnO2 and Ag nanoparticles decorate on the surface of rGO, forming a ternary composite. The gas sensing measurement results show that the Ag/SnO2/rGO NCs based sensor exhibits good response, well selectivity, and stability capacity to TEA at low working temperature of 220 °C in the comparison with pure SnO2 and SnO2/rGO based sensors. Therefore, the Ag/ SnO2/rGO NCs might be the candidate for fabricating TEA sensors because of the low-cost and good TEA sensing performance. Acknowledgements This work is supported by the National Natural Science Foundation of China (U1704255, U1404613), Program for Science & Technology Innovation Talents in Universities of Henan Province (18HASTIT010, 17HASTIT029), Young Core Instructor Project of Colleges and Universities in Henan Province (2015GGJS-063, 2016GGJS-040), Foundation of Henan Scientific and Technology key project (182102310892), and the Education Department Natural Science Foundation of fund Henan province (16A150051).
Fig. 7. (a) Selectivity of three kinds of sensors to different gases of 100 ppm, and (b) the long-term stability measurement of the Ag/SnO2/rGO to 20 ppm TEA at 220 °C.
References [1] A. Mirzaei, J.H. Kim, H.W. Kim, S.K. Sang, Resistive-based gas sensors for detection of benzene, toluene and xylene (BTX) gases: a review, J. Mater. Chem. C 6 (2018) 4342–4370. [2] M.T. Dr, Porous metal oxides as gas sensors, Chem. Eur. J. 13 (2010) 8376–8388. [3] B. Hua, G. Shi, Gas sensors based on conducting polymers, Sensors 7 (2007) 267–307. [4] A. Šutka, K.A. Gross, Spinel ferrite oxide semiconductor gas sensors, Sens. Actuators B Chem. 222 (2016) 95–105. [5] Y. Gong, Y. Wang, G. Sun, T. Jia, L. Jia, F. Zhang, L. Lin, B. Zhang, J. Cao, Z. Zhang, Carbon nitride decorated ball-flower like Co3O4 hybrid composite: hydrothermal synthesis and ethanol gas sensing application, Nanomaterials 8 (2018) 132. [6] X. Pang, M.D. Shaw, S. Gillot, A.C. Lewis, The impacts of water vapour and copollutants on the performance of electrochemical gas sensors used for air quality monitoring, Sens. Actuators B Chem. 266 (2018) 674–684. [7] M. Watanabe, H. Nakatani, S. Tamura, N. Imanaka, Catalytic Combustion-Type C2H4 Gas Sensors Employing Pt-Loaded CeO2-ZrO2-MOxSolid Solutions, Sens. Lett. (2017). [8] I. Simon, M. Arndt, Thermal and gas-sensing properties of a micromachined thermal conductivity sensor for the detection of hydrogen in automotive applications, Sens. Actuators A Phys. 97 (2002) 104–108. [9] D. Gibson, C. Macgregor, A novel solid state non-dispersive infrared CO2 gas sensor compatible with wireless and portable deployment, Sensors 13 (2013) 7079–7103. [10] Y.S. Lin, L.G.J.D. Haart, K.J.D. Vries, A.J. Burggraaf, A kinetic study of the electrochemical vapor deposition of solid oxide electrolyte films on porous substrates, J. Electrochem. Soc. 137 (2017) 3960–3966. [11] S. Peng, W. Chen, J. Liu, Z. Xin, X. Li, X. Hu, G. Lu, Hierarchical assembly of αFe2O3 nanosheets on SnO2 hollow nanospheres with enhanced ethanol sensing properties, ACS Appl. Mater. Interfaces 7 (2015) 19119–19125. [12] B. Zhang, W. Fu, X. Meng, A. Ruan, P. Su, H. Yang, Synthesis and enhanced gas sensing properties of flower-like ZnO/α-Fe2O3 core-shell nanorods, Ceram. Int. 43 (2017) 5934–5940. [13] S. Zhang, G. Sun, Y. Li, B. Zhang, L. Lin, Y. Wang, J. Cao, Z. Zhang, Continuously improved gas-sensing performance of SnO2/Zn2SnO4 porous cubes by structure evolution and further NiO decoration, Sens. Actuators B Chem. 255 (2018) 2936–2943. [14] B. Zhang, W. Fu, H. Li, X. Fu, Y. Wang, H. Bala, X. Wang, G. Sun, J. Cao, Z. Zhang, Synthesis and characterization of hierarchical porous SnO2 for enhancing ethanol sensing properties, Appl. Surf. Sci. 363 (2016) 560–565. [15] G. Sun, H. Chen, Y. Li, Z. Chen, S. Zhang, G. Ma, T. Jia, J. Cao, H. Bala, X. Wang,
Fig. 8. Schematic illustration of the possible sensing mechanism.
The enhanced gas sensing behavior of prepared Ag/SnO2/rGO NCs may put down to the following several factors. First, it is reported that the heterojunctions can exist on the SnO2/rGO NCs, which can effectively improve the sensing performance of the composites [36,37]. SnO2 is an n-type MOSs and rGO has p-type semiconducting properties. Thus, the transfusion of electron will occur at the p-n interface, resulting in an additional electron depletion of SnO2. While exposed to TEA, more change of electrons-concentration is expected to make contribution to improve the sensitivity. Secondly, the introduction of rGO not only can avoids the agglomeration of SnO2 nanoparticles to some extent, but also increase the higher specific surface area of the ternary Ag/SnO2/rGO NCs which will allow more oxygen molecules to absorb on the surface of sensing materials and enhance performance. 66
Materials Research Bulletin 114 (2019) 61–67
S. Zhang, et al.
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
Synthesis and improved gas sensing properties of NiO-decorated SnO2 microflowers assembled with porous nanorods, Sens. Actuators B Chem. 233 (2016) 180–192. S. Park, J. An, R.D. Piner, I. Jung, D. Yang, A. Velamakanni, S.B.T. Nguyen, R.S. Ruoff, Aqueous suspension and characterization of chemically modified graphene sheets, Chem. Mater. 20 (2016) 6592–6594. Y. Cheng, R. Wang, J. Sun, L. Gao, A stretchable and highly sensitive graphene‐based Fiber for sensing tensile strain, bending, and torsion, Adv. Mater. 27 (2016) 7365–7371. P. Liu, Y. Huang, J. Yan, Y. Yang, Y. Zhao, Construction of CuS nanoflakes vertically aligned on magnetically decorated graphene and their enhanced microwave absorption properties, ACS Appl. Mater. Interfaces 8 (2016) 5536–5546. P. Liu, Y. Huang, J. Yan, Y. Zhao, Magnetic graphene@PANI@porous TiO2 ternary composites for high-performance electromagnetic wave absorption, J. Mater. Chem. C 4 (2016) 6362–6370. Y. Yang, C. Han, B. Jiang, J. Iocozzia, C. He, D. Shi, T. Jiang, Z. Lin, Graphene-based materials with tailored nanostructures for energy conversion and storage, Mater. Sci. Eng. R Rep. 102 (2016) 1–72. P. Liu, J. Yan, X. Gao, Y. Huang, Y. Zhang, Construction of layer-by-layer sandwiched graphene/polyaniline nanorods/carbon nanotubes heterostructures for high performance supercapacitors, Electrochim. Acta 272 (2018) 77–87. Y. Li, N. Luo, G. Sun, B. Zhang, L. Lin, H. Jin, Y. Wang, H. Bala, J. Cao, Z. Zhang, In situ decoration of Zn2SnO4 nanoparticles on reduced graphene oxide for high performance ethanol sensor, Ceram. Int. 44 (2018) 6836–6842. W.J. Min, S.M. Kang, K.H. Nam, K.S. An, B.C. Ku, Highly transparent and flexible NO2 gas sensor film based on MoS2/rGO composites using soft lithographic patterning, Appl. Surf. Sci. 456 (2018) 7–12. F. Meng, H. Zheng, Y. Chang, Y. Zhao, M. Li, C. Wang, Y. Sun, J. Liu, One-step synthesis of Au/SnO2/RGO nanocomposites and their VOC sensing properties, IEEE Trans. Nanotechnol. 17 (2018) 212–219. P.G. Su, L.Y. Yang, NH 3 gas sensor based on Pd/SnO2 /RGO ternary composite operated at room-temperature, Sens. Actuators B Chem. 223 (2016) 202–208. Z. Ye, H. Tai, R. Guo, Z. Yuan, C. Liu, Y. Su, Z. Chen, Y. Jiang, Excellent ammonia sensing performance of gas sensor based on graphene/titanium dioxide hybrid with improved morphology, Appl. Surf. Sci. 419 (2017) 84–90.
[27] B. Zhang, Y. Wang, G. Zhai, Biomedical applications of the graphene-based materials, Mater. Sci. Eng. C 61 (2016) 953–964. [28] Q. Huang, D. Zeng, H. Li, C. Xie, Room temperature formaldehyde sensors with enhanced performance, fast response and recovery based on zinc oxide quantum dots/graphene nanocomposites, Nanoscale 4 (2012) 5651–5658. [29] L.P. Mei, J.J. Feng, L. Wu, J.R. Chen, L. Shen, Y. Xie, A.J. Wang, A glassy carbon electrode modified with porous Cu2O nanospheres on reduced graphene oxide support for simultaneous sensing of uric acid and dopamine with high selectivity over ascorbic acid, Microchim. Acta 183 (2016) 2039–2046. [30] A. Zain Ul, P. Jae Young, K. Hyoun Woo, K. Sang Sub, Graphene-loaded tin oxide nanofibers: optimization and sensing performance, Nanotechnology 28 (2017) 035501. [31] F.L. Meng, H.H. Li, L.T. Kong, J.Y. Liu, J. Zhen, L. Wei, J. Yong, J.H. Liu, X.J. Huang, Parts per billion-level detection of benzene using SnO2/graphene nanocomposite composed of sub-6 nm SnO 2 nanoparticles, Anal. Chim. Acta 736 (2012) 100–107. [32] S. Shi, F. Zhang, H. Lin, Q. Wang, E. Shi, F. Qu, Enhanced triethylamine-sensing properties of P-N heterojunction Co3O4/In2O3 hollow microtubes derived from metal–organic frameworks, Sens. Actuators B Chem. 262 (2018) 739–749. [33] J. Chen, B. Yao, C. Li, G. Shi, An improved Hummers method for eco-friendly synthesis of graphene oxide, Carbon 64 (2013) 225–229. [34] W. Li, H. Xu, T. Zhai, H. Yu, Q. Xu, X. Song, J. Wang, B. Cao, High-sensitivity, highselectivity, and fast-recovery-speed triethylamine sensor based on ZnO micropyramids prepared by molten salt growth method, J. Alloys. Compd. 695 (2017) 2930–2936. [35] Y.H. Cho, N.K. You, Y.C. Kang, I.D. Kim, J.H. Lee, Ultraselective and ultrasensitive detection of trimethylamine using MoO3 nanoplates prepared by ultrasonic spray pyrolysis, Sens. Actuators B Chem. 195 (2014) 189–196. [36] H.W. Kim, H.G. Na, Y.J. Kwon, S.Y. Kang, M.S. Choi, J.H. Bang, P. Wu, S.S. Kim, Microwave-assisted synthesis of graphene–SnO2 nanocomposites and their applications in gas sensors, ACS Appl. Mater. Interfaces 9 (2017) 31667–31682. [37] Z.U. Abideen, J.-H. Kim, A. Mirzaei, H.W. Kim, S.S. Kim, Sensing behavior to ppmlevel gases and synergistic sensing mechanism in metal-functionalized rGO-loaded ZnO nanofibers, Sens. Actuators B Chem. 255 (2018) 1884–1896.
67
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
One-step synthesis of Ag/SnO2/rGO nanocomposites and their trimethylamine sensing properties
T
Saisai Zhanga,1, Bowen Zhangc,1, Guang Suna,b, , Yanwei Lia, Bo Zhanga, Yan Wangb, ⁎ Jianliang Caoa,b, Zhanying Zhanga,b, ⁎
a
School of Materials Science and Engineering, Cultivating Base for Key Laboratory of Environment-friendly Inorganic Materials in University of Henan Province, Henan Polytechnic University, Jiaozuo, 454000, China b The Collaboration Innovation Center of Coal Safety Production of Henan Province, Henan Polytechnic University, Jiaozuo, 454000, China c State Key Laboratory of Superhard Materials, Jilin University, Changchun, 130000, China
ARTICLE INFO
ABSTRACT
Keywords: Ag Reduced graphene oxide (rGO) SnO2 Ternary nanocomposites Gas sensor
Herein we present the synthesis of Ag/SnO2/reduced graphene oxide (rGO) ternary nanocomposites (NCs) through one-step route. Many methods were employed to investigate the phase and morphological structure of the prepared composites. The coexisting of SnO2 and Ag can be confirmed by X-ray diffractometry and X-ray photoelectron spectroscopy analysis and a large number of nanoparticles decorated on the surface of rGO are observed from field-emission scanning electron and transmission electron microscopy images. The composites were employed to fabricate gas sensor to detect TEA gas. The Ag/SnO2/rGO sensor exhibits higher response to TEA gas at a low working temperature of 220 °C compared with SnO2 and SnO2/rGO. This markedly enhancement of TEA gas-sensing performance may be related to the well catalytic activity and higher specific surface area of the Ag/SnO2/rGO NCs.
1. Introduction Gas sensor, as a device that can convert fraction volume of gases into corresponding electrical signals, has been used to detect harmful and hazardous gases and plays an important role in ecological management and environment monitoring [1–3]. Over the last decades, various kinds of gas sensors have been fabricated and developed to achieve better gas-sensing properties, such as semiconductor sensor [4,5], electrochemical sensor [6], catalytic combustion sensor [7], thermal conductivity sensor [8], infrared sensor [9], and solid electrolyte sensor [10]. Among them, the gas sensor based on semiconductor, especially metal oxide semiconductor (MOS), has drawn extensive research interest due to their advantages of simple preparation, good stability, and environment-friendly [5,11–13]. Among numerous MOS materials, SnO2 is the one of the earliest and most typical gas sensing material. Many reports indicated that SnO2 based gas sensors will exhibit good performance [14,15]. However, there are many limitations to restrict the development of SnO2 based sensor, such as low response and poor selectivity. Therefore, further improving the gassensing performance of the SnO2 materials is one of the current
research focuses. Graphene is a two-dimensional carbon nanomaterial with hexagon honeycomb lattice composed of carbon atoms through sp2 hybridized orbital and has received wide attention due to its excellent optical, electrical and mechanical properties [16–18]. Recently, Graphene, especially reduced graphene oxide (rGO), has been considered as a revolutionary material in the future and exhibits important application prospects in materials science [19], energy engineering [20,21], gas sensor [22–26], and biomedicine and drug delivery [27]. Therefore, gas sensors based on rGO/MOS composites are expected to achieve improved gas-sensing properties. Various rGO/MOS composite have been synthesized as sensing materials, such as MoS2/rGO composites [23], ZnO quantum-dots/graphene NCs [28], Cu2O/graphene sheets [29], SnO2/graphene [30] and so on. It is reported that the particle size of MOS can be controlled by graphene oxide (GO), and the rGO would also prevent the aggregation of oxides particles which can improve the electron transfer capability [31]. Hence, as compared with pure SnO2, the SnO2/rGO composites always exhibited enhanced gas sensing behaviors. In addition, noble metal nanoparticles are usually added into the SnO2/rGO composites to enhance its sensitivity. Meng et.al
Corresponding authors at: School of Materials Science and Engineering, Cultivating Base for Key Laboratory of Environment-friendly Inorganic Materials in University of Henan Province, Henan Polytechnic University, Jiaozuo, 454000, China. E-mail addresses: [email protected] (G. Sun), [email protected] (Z. Zhang). 1 These authors contributed equally to this work. ⁎
https://doi.org/10.1016/j.materresbull.2019.02.019 Received 9 October 2018; Received in revised form 3 January 2019; Accepted 19 February 2019 Available online 21 February 2019 0025-5408/ © 2019 Elsevier Ltd. All rights reserved.
Materials Research Bulletin 114 (2019) 61–67
S. Zhang, et al.
synthesized Au/SnO2/rGO NCs with high response to ethanol by onestep synthesis method [24]. Su et.al used a one-pot method to fabricate Pd/SnO2/rGO NCs, which exhibited high response to NH3 gas at low working temperature [25]. TEA, as one of toxic and harmful organic amines, can bring serious irritation to the respiratory tract and even cause pulmonary edema and death after inhalation [32]. In addition, as a common industrial raw material, TEA poses a great threat to human health even at concentration as low as 10 ppm. Therefore, the rapid and effective detection of TEA is of great significance for the protection of human health. However, to our best knowledge, there are few reports on the detection of TEA based on ternary Ag/SnO2/rGO composites with good selectivity. In this work, a facile solution strategy was employed to synthesis ternary Ag/SnO2/rGO NCs. And such ternary Ag/SnO2/rGO NCs as well as pure SnO2 and SnO2/rGO have been used to fabricate gas sensors to detect TEA. The measurement results show that the Ag/ SnO2/rGO sensor exhibits a significantly enhancement in sensing properties and optimum selectivity to TEA compared with pure SnO2 or SnO2/rGO sensors. The better gas sensing performance may be attributed to the higher specific surface area and conductivity and catalytic activity of the Ag/SnO2/rGO ternary composites.
microscope).The element analysis was carried out by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI electron spectrometer using Al Kα radiation). 2.4. Measurement of gas-sensing Gas-sensing tests were carried out on an intelligent gas-sensing analysis system of CGS-4TPS (Beijing Elite Tech Co., Ltd., China). The fabricating and testing procedure of gas sensors has been described in our previous report [13]. Then the fabricated sensors would be aged at 220 °C for days to improve their mechanical strength and stability before gas sensing test. The sensor response (S) was defined as Ra/Rg, where Ra and Rg were the sensor resistance in air and target gas, respectively. The sensing performances of the sensors were investigated under laboratory conditions (17% RH. 25 °C). 3. Results and discussion 3.1. Structural and morphological characteristics The phase and crystallinity of the synthesized samples were examined by XRD shown in Fig. 1. From Fig. 1(a), it can be seen that all of diffraction peaks can be correspond to tetragonal rutile SnO2 phase (JCPDS card no.41–1445) and no additional characteristic peaks of other impurities were found, indicating the formation of SnO2 with high purity. Compare with pure SnO2, the SnO2/rGO (Fig. 1(b)) shows a similar XRD pattern but broader peaks, suggesting that the smaller crystal size of SnO2 can be obtained. This can be attributed to the control function of GO on the particle size of MOS [28]. Fig. 1(c) displays the XRD patterns of Ag/SnO2/rGO sample. Besides of the peaks arising from SnO2, five peaks were also appeared in the pattern at 38.118, 44.304, 64.45, 77.407 and 81.55°, which are assigned as Ag (111), (200), (220), (311) and (222), respectively. And these observed diffraction peaks are well matched with Ag (JCPDS card no.04-0783), indicating the coexistence of Ag and SnO2 phase. The FESEM and TEM in Fig. 2 show the microstructure and morphology of the products. A large-scale silk-like sheet with smooth surface was observed in Fig. 2(a), suggesting that high quality GO has been obtained. Fig. 2(b) displays the structure of SnO2 nanoparticles prepared via hydrothermal method. It can be seen that a large number of SnO2 particles agglomerate together to form large blocks. The HRTEM image is shown in Fig. 2(b) inset to further analysis the microstructure and size of particles. The typical zero-dimensional nanomaterials can be found, consisting of many ellipsoidal nanoparticles with an average size of 3.8 nm. The FESEM image of SnO2/rGO samples is displayed in
2. Experimental 2.1. Materials Graphene powder, cubic niter, sulphuric acid, tin chloride pentahydrate, silver nitrate, ammonia were of analytical grade and used without further purification. 2.2. Preparation of Ag/SnO2/rGO NCs The grapheme oxide (GO) was synthesized from graphite powder according to a modified Hummers method [33]. Typically, designed amounts of graphite powder (1.5 g) and NaNO3 (1.5 g) were added slowly into 60 mL of sulphuric acid, and then 9 g of KMnO4 uniform added in mixture solution with magnetic stirring under ice bath. After 90 min, the precursor solution was transferred to a water bath under stirring at 38 °C for 30 min. Next, the solution was warmed up and maintained at 78 °C, and 132 mL of distilled water and a small amount of H2O2 were slowly dropped into the mixture solution. Finally, the suspension was collected by centrifugation and washed with hydrochloric acid solution and water for several times to obtain the GO samples. The rGO synthesis method was as follows: 4 ml of GO suspension and 4 ml of triethylamine were ultrasonicated for 1 h, and the solution was placed in a reaction vessel for hours at 180 °C. The Ag/SnO2/rGO NCs were prepared via a simple one-step method. 5 mL of 10 wt% rGO solution was added into 25 mL of SnCl4·5H2O ethanol solution (2.5 mM/L). Then 5 mL of 2 wt% AgNO3 solution was added into the mixture solution drop by drop and ultrasonicated for 20 min. Finally 12.8 mL of deionized water and 2.2 mL ammonia were added into the above suspension under stirring, and then the as-prepared precursor solution was placed to 50 mL of autoclave and kept 160 °C for 24 h. Then the precipitates were washed for several times with water and ethanol, respectively, and dried at 80 °C to obtain the final products. For comparison, pure SnO2 and SnO2/rGO NCs were synthesized by a similar hydrothermal method without addition of AgNO3 and rGO, AgNO3, respectively. 2.3. Characterization The purity of samples was characterized by using X-ray power diffraction (XRD) on a Bruker/D8Advance diffract meter with Cu Kα radiation. The morphologies and microstructures were examined by fieldemission scanning electron microscopy (FSEM, Quanta 250 FEG) and transmission electron microscopy (TEM, JEOL JEM-2100
Fig. 1. XRD patterns of (a) pure SnO2, (b)SnO2/rGO and (c) Ag/SnO2/rGO samples. 62
Materials Research Bulletin 114 (2019) 61–67
S. Zhang, et al.
Fig. 2. The FESEM images of (a) GO, (b) pure SnO2 nanoparticles, inset is the HRTEM image of nanoparticles, (c) SnO2/rGO NCs films, inset is the high magnification image of the selected area, and (d) the FESEM images and (e,f) TEM images of Ag/SnO2/rGO NCs films.
Fig. 2(c), revealing that the composites consist of massive wrinkled sheets. From the inset of Fig. 2(c), the surface is covered with a large amount of particles and results in a rougher surface compared with GO observed from Fig. 2(a). The Ag/SnO2/rGO sample exhibits a similar morphology compared to SnO2/rGO sample, and no obvious Ag particles can be observed in Fig. 2(d). This is because that the size of Ag nanoparticles is too small to be observed clearly. Fig. 2(e) displays the TEM images of Ag/SnO2/rGO which clearly reveal the existence of massive nanoparticles on the surface of the rGO sheets. The SnO2 and Ag nanoparticles are small and aggregated together. A typical HRTEM image is presented in Fig. 2(f). The measured interplanar distances are about 0.33 nm and 0.24 nm which can be corresponded well to (110) plane for SnO2 and (111) plane for Ag, respectivily. Moreover, the average size of SnO2 nanoparticles is about 2.88 nm, which is smaller than pure SnO2. This can be consistent with the XRD analysis results. XPS analysis was employed to further investigate the chemical states in the Ag/SnO2/rGO NCs. Fig. 3(a) displays the wide-survey XPS spectrum which reveals that the presence of Sn, C and O elements. And the Sn and O peaks are much higher than the C peak, indicating that a
great deal of SnO2 nanoparticles is formed. A high resolution core lever spectrum of Sn3d is shown in Fig. 3(c), displaying two peaks at 487.6 eV and 495.8 eV corresponding to Sn 3d3/2 and Sn 3d5/2, respectively. Fig. 3(d) illuminates the presence of Ag element in the composites due to the two peaks at 368.3 eV and 374.3 eV which are attributed to Ag 3d5/2 and Ag 3d3/2 respectively. The specific surface area and the porosity of the prepared samples were investigated with N2 adsorption-desorption analysis. The N2 adsorption-desorption isotherms (shown in Fig. 4(a)) of all samples display type IV cures with H2-type hysteresis loops according to the IUPAC. The cures of pore size distribution shown in Fig. 4(b) reveal that the samples have relatively small pores with a size of 1˜4 nm, indicating the well homogeneity of the pores. The BET calculated result shows that the specific surface areas of SnO2/rGO and Ag/SnO2/rGO composites are 204.284 and 196.434 m2 g−1, respectively. However, specific surface area of the SnO2 samples is only 171.616 m2 g−1. These results indicate that a higher surface area can be obtained when SnO2 nanoparticles decorated on the surface of rGO, which can provide more surface active sites and available space for gas molecules adsorption 63
Materials Research Bulletin 114 (2019) 61–67
S. Zhang, et al.
Fig. 3. (a)XPS spectra of Ag/SnO2/rGO NCs; high resolution spectra for (b) C 1 s, (c) Sn 3d and (d) Ag 3d of the Ag/SnO2/rGO NCs.
and surface reaction. This is beneficial to improvement of the gas-sensing performance. Although Ag/SnO2/rGO composites exhibit a relatively smaller surface area compared to SnO2/rGO composites, the catalytic activity of Ag will further enhance the gas sensing performance. 3.2. Gas sensing properties The ternary Ag/SnO2/rGO NCs with high special surface area and catalytic activity were expected to improve the gas sensing performance. Therefore, the sensor based Ag/SnO2/rGO NCs was fabricated and its gas sensing properties were investigated and compared with pure SnO2 and SnO2/rGO, in which TEA was chose as a representative reducing gas. For MOS based gas sensors, the working temperature can affect the adsorption-desorption efficiency of gas molecules during the testing process and is an important parameters for gas sensors. Therefore, the sensitivities of three sensors to 100 ppm TEA were tested at different working temperature to obtain the optimum working temperature. It can be found that the sensitivity of the three sensors increased firstly and then decreased with rising the working temperature. The highest response value toward TEA can be observed of Ag/SnO2/ rGO based sensor, and the maximum value was 82.47 at 220 °C. The response curves of SnO2 and SnO2/rGO exhibit analogous trend of "increase-maximum-decrease" and their maximum values are 32.87 and 70.52 at 220 °C, respectively. Thus, 220 °C was chosen for the three sensors as the optimum working temperature. The relationships between the responses and TEA concentrations for all the sensors are described as shown in Fig. 5(b). The response increased with the TEA concentration ranging from 0.5 ppm to 500 ppm, and all the sensors present a rapid increase in response. However, when the TEA concentration exceeds 100 ppm, the rate of increase in response begins to slacken with further increasing the TEA concentration. It can also found that the corresponding sensitivities of all sensors rise with increasing TEA concentration and the Ag/SnO2/rGO based sensor exhibits the highest response value to each concentration of TEA. More details of the response
Fig. 4. a) N2 adsorption-desorption isotherm and (b) pore-size distribution curves of SnO2 nanoparticles, SnO2/rGO and Ag/SnO2/rGO composites. 64
Materials Research Bulletin 114 (2019) 61–67
S. Zhang, et al.
Fig. 6. (a) Response and recovery curves of the three kinds of sensors to different concentration of TEA; (b) response transients of these sensors to 20 ppm TEA.
Ag/SnO2/rGO based sensor to 100 ppm TEA is 82.47, which is about four times of ethanol (23.83), 4.5 times of formaldehyde (18.25), seven times of acetone and methanol (12.69 and 11.89) and eight times of propanediol (10.89), respectively. In contrast, the SnO2 and SnO2/rGO based sensors show similar selective characteristics but lower sensitivity to the same concentration of testing gases. Such phenomenon can be attributed to the high conductivity and catalytic activity of the Ag/ SnO2/rGO ternary composites. In order to further evaluate the longterm stability of the sensor, the response of the sensor based on Ag/ SnO2/rGO to 20 ppm TEA was carried out, as shown in Fig. 7(b). It’s found that the response value of the sensor was almost constant without an obvious changeable within 4 weeks, demonstrating its good longterm stability. On the basis of its good stability, the present Ag/SnO2/ rGO sensor may be the potential candidate for practical application.
Fig. 5. (a) Response of SnO2, SnO2/rGO and Ag/SnO2/rGO sensors to 100 ppm TEA at different temperatures; (b) Concentration-dependent response of the three sensors toward TEA at 220 °C, inset is more details of response variation to different TEA concentration ranges.
variation to gas concentration in the range of 0.5–40 ppm are shown in the inset in Fig. 5(b). The Ag/SnO2/rGO sensor shows an approximately linearity in the concentration rang of 0.5–40 ppm, indicating their great potential for quantitative detection of TEA at ppm level. Fig. 6(a) displays the dynamic response-recovery curves of the three sensors toward different TEA concentration at 220 °C. Obviously, the response amplitudes could gradually increase with the TEA concentration increasing from 0.5 to 500 ppm, indicating our sensors possess good capability to TEA. Furthermore, the response amplitude of Ag/SnO2/rGO is obviously higher than that of SnO2 and SnO2/rGO. The response transient curves of all sensors to 20 ppm TEA are shown in Fig. 6(b). It is clear that the curve of Ag/SnO2/rGO rises sharply upon exposure to TEA, and declines rapidly after the gas is released. From Fig. 6(b), the calculated response times of SnO2 and SnO2/rGO are determined as 30 and 14 s, respectively. In contrast, the Ag/SnO2/rGO sensor shows a shorter response time of 11 s, suggesting its faster response characteristics. The faster response speed of Ag/SnO2/rGO reveals its promising application in real-time detection of TEA. In terms of practical applications, selectivity is a vital parameter of gas sensors. Thus, in order to investigate the selectivity of Ag/SnO2/ rGO NCs, the response of all sensors to 100 ppm TEA were tested by compared with other gases, such as ethanol, formaldehyde, acetone, methanol, and propanediol. From Fig. 7(a), the Ag/SnO2/rGO sensor exhibits the higher response value to TEA than other gases, indicating its good selectivity of TEA over other gases. For example, the value of
3.3. Gas sensing mechanism The gas sensing mechanism of Ag/SnO2/rGO ternary composites can be well illustrated by the surface-controlled type theory [34]. Firstly, as shown in Fig. 8, when the sensors are exposed to ambient air, the oxygen molecules will absorb on the surface of SnO2 to form oxygen ions (O2−, O−, O2−) by capturing free electrons from conduction band, which can reduce the free-charge-carriers concentration and increase the resistance of sensing materials. Once the sensors are exposed to reducing gas, such as TEA, the adsorbed oxygen ions could react with the gas molecules, which can be simply expressed as [35]:
2(C2 H5 )3N + 430
12CO2 + 15H2 O + 2NO2 + 43e
After the reaction, the trapped electrons would be released back into the materials, leading to the electrons-concentration increased and the resistance of materials decreased. 65
Materials Research Bulletin 114 (2019) 61–67
S. Zhang, et al.
Thirdly, it can be attributed to the electronic and chemical sensitization of Ag in the ternary Ag/SnO2/rGO NCs. The presence of Ag additive not only adjusts the electron density on the surface of Ag/SnO2/rGO NCs by electronic sensitization, but also makes chemical sensitization by enhancing the catalytic activity of the composites. In other words, the Ag component may afford more active sites for gas adsorption and enhance the catalytic decomposition of TEA. The catalytic activity of noble metals on MOS has been extensively discussed in many literatures [22,35] and will not be explained in detail here. 4. Conclusions The ternary Ag/SnO2/rGO NCs have been successfully prepared through a facile one-step route as well as the pure SnO2 and SnO2/rGO. XRD, FESEM, TEM, and XPS results reveal that the SnO2 and Ag nanoparticles decorate on the surface of rGO, forming a ternary composite. The gas sensing measurement results show that the Ag/SnO2/rGO NCs based sensor exhibits good response, well selectivity, and stability capacity to TEA at low working temperature of 220 °C in the comparison with pure SnO2 and SnO2/rGO based sensors. Therefore, the Ag/ SnO2/rGO NCs might be the candidate for fabricating TEA sensors because of the low-cost and good TEA sensing performance. Acknowledgements This work is supported by the National Natural Science Foundation of China (U1704255, U1404613), Program for Science & Technology Innovation Talents in Universities of Henan Province (18HASTIT010, 17HASTIT029), Young Core Instructor Project of Colleges and Universities in Henan Province (2015GGJS-063, 2016GGJS-040), Foundation of Henan Scientific and Technology key project (182102310892), and the Education Department Natural Science Foundation of fund Henan province (16A150051).
Fig. 7. (a) Selectivity of three kinds of sensors to different gases of 100 ppm, and (b) the long-term stability measurement of the Ag/SnO2/rGO to 20 ppm TEA at 220 °C.
References [1] A. Mirzaei, J.H. Kim, H.W. Kim, S.K. Sang, Resistive-based gas sensors for detection of benzene, toluene and xylene (BTX) gases: a review, J. Mater. Chem. C 6 (2018) 4342–4370. [2] M.T. Dr, Porous metal oxides as gas sensors, Chem. Eur. J. 13 (2010) 8376–8388. [3] B. Hua, G. Shi, Gas sensors based on conducting polymers, Sensors 7 (2007) 267–307. [4] A. Šutka, K.A. Gross, Spinel ferrite oxide semiconductor gas sensors, Sens. Actuators B Chem. 222 (2016) 95–105. [5] Y. Gong, Y. Wang, G. Sun, T. Jia, L. Jia, F. Zhang, L. Lin, B. Zhang, J. Cao, Z. Zhang, Carbon nitride decorated ball-flower like Co3O4 hybrid composite: hydrothermal synthesis and ethanol gas sensing application, Nanomaterials 8 (2018) 132. [6] X. Pang, M.D. Shaw, S. Gillot, A.C. Lewis, The impacts of water vapour and copollutants on the performance of electrochemical gas sensors used for air quality monitoring, Sens. Actuators B Chem. 266 (2018) 674–684. [7] M. Watanabe, H. Nakatani, S. Tamura, N. Imanaka, Catalytic Combustion-Type C2H4 Gas Sensors Employing Pt-Loaded CeO2-ZrO2-MOxSolid Solutions, Sens. Lett. (2017). [8] I. Simon, M. Arndt, Thermal and gas-sensing properties of a micromachined thermal conductivity sensor for the detection of hydrogen in automotive applications, Sens. Actuators A Phys. 97 (2002) 104–108. [9] D. Gibson, C. Macgregor, A novel solid state non-dispersive infrared CO2 gas sensor compatible with wireless and portable deployment, Sensors 13 (2013) 7079–7103. [10] Y.S. Lin, L.G.J.D. Haart, K.J.D. Vries, A.J. Burggraaf, A kinetic study of the electrochemical vapor deposition of solid oxide electrolyte films on porous substrates, J. Electrochem. Soc. 137 (2017) 3960–3966. [11] S. Peng, W. Chen, J. Liu, Z. Xin, X. Li, X. Hu, G. Lu, Hierarchical assembly of αFe2O3 nanosheets on SnO2 hollow nanospheres with enhanced ethanol sensing properties, ACS Appl. Mater. Interfaces 7 (2015) 19119–19125. [12] B. Zhang, W. Fu, X. Meng, A. Ruan, P. Su, H. Yang, Synthesis and enhanced gas sensing properties of flower-like ZnO/α-Fe2O3 core-shell nanorods, Ceram. Int. 43 (2017) 5934–5940. [13] S. Zhang, G. Sun, Y. Li, B. Zhang, L. Lin, Y. Wang, J. Cao, Z. Zhang, Continuously improved gas-sensing performance of SnO2/Zn2SnO4 porous cubes by structure evolution and further NiO decoration, Sens. Actuators B Chem. 255 (2018) 2936–2943. [14] B. Zhang, W. Fu, H. Li, X. Fu, Y. Wang, H. Bala, X. Wang, G. Sun, J. Cao, Z. Zhang, Synthesis and characterization of hierarchical porous SnO2 for enhancing ethanol sensing properties, Appl. Surf. Sci. 363 (2016) 560–565. [15] G. Sun, H. Chen, Y. Li, Z. Chen, S. Zhang, G. Ma, T. Jia, J. Cao, H. Bala, X. Wang,
Fig. 8. Schematic illustration of the possible sensing mechanism.
The enhanced gas sensing behavior of prepared Ag/SnO2/rGO NCs may put down to the following several factors. First, it is reported that the heterojunctions can exist on the SnO2/rGO NCs, which can effectively improve the sensing performance of the composites [36,37]. SnO2 is an n-type MOSs and rGO has p-type semiconducting properties. Thus, the transfusion of electron will occur at the p-n interface, resulting in an additional electron depletion of SnO2. While exposed to TEA, more change of electrons-concentration is expected to make contribution to improve the sensitivity. Secondly, the introduction of rGO not only can avoids the agglomeration of SnO2 nanoparticles to some extent, but also increase the higher specific surface area of the ternary Ag/SnO2/rGO NCs which will allow more oxygen molecules to absorb on the surface of sensing materials and enhance performance. 66
Materials Research Bulletin 114 (2019) 61–67
S. Zhang, et al.
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
Synthesis and improved gas sensing properties of NiO-decorated SnO2 microflowers assembled with porous nanorods, Sens. Actuators B Chem. 233 (2016) 180–192. S. Park, J. An, R.D. Piner, I. Jung, D. Yang, A. Velamakanni, S.B.T. Nguyen, R.S. Ruoff, Aqueous suspension and characterization of chemically modified graphene sheets, Chem. Mater. 20 (2016) 6592–6594. Y. Cheng, R. Wang, J. Sun, L. Gao, A stretchable and highly sensitive graphene‐based Fiber for sensing tensile strain, bending, and torsion, Adv. Mater. 27 (2016) 7365–7371. P. Liu, Y. Huang, J. Yan, Y. Yang, Y. Zhao, Construction of CuS nanoflakes vertically aligned on magnetically decorated graphene and their enhanced microwave absorption properties, ACS Appl. Mater. Interfaces 8 (2016) 5536–5546. P. Liu, Y. Huang, J. Yan, Y. Zhao, Magnetic graphene@PANI@porous TiO2 ternary composites for high-performance electromagnetic wave absorption, J. Mater. Chem. C 4 (2016) 6362–6370. Y. Yang, C. Han, B. Jiang, J. Iocozzia, C. He, D. Shi, T. Jiang, Z. Lin, Graphene-based materials with tailored nanostructures for energy conversion and storage, Mater. Sci. Eng. R Rep. 102 (2016) 1–72. P. Liu, J. Yan, X. Gao, Y. Huang, Y. Zhang, Construction of layer-by-layer sandwiched graphene/polyaniline nanorods/carbon nanotubes heterostructures for high performance supercapacitors, Electrochim. Acta 272 (2018) 77–87. Y. Li, N. Luo, G. Sun, B. Zhang, L. Lin, H. Jin, Y. Wang, H. Bala, J. Cao, Z. Zhang, In situ decoration of Zn2SnO4 nanoparticles on reduced graphene oxide for high performance ethanol sensor, Ceram. Int. 44 (2018) 6836–6842. W.J. Min, S.M. Kang, K.H. Nam, K.S. An, B.C. Ku, Highly transparent and flexible NO2 gas sensor film based on MoS2/rGO composites using soft lithographic patterning, Appl. Surf. Sci. 456 (2018) 7–12. F. Meng, H. Zheng, Y. Chang, Y. Zhao, M. Li, C. Wang, Y. Sun, J. Liu, One-step synthesis of Au/SnO2/RGO nanocomposites and their VOC sensing properties, IEEE Trans. Nanotechnol. 17 (2018) 212–219. P.G. Su, L.Y. Yang, NH 3 gas sensor based on Pd/SnO2 /RGO ternary composite operated at room-temperature, Sens. Actuators B Chem. 223 (2016) 202–208. Z. Ye, H. Tai, R. Guo, Z. Yuan, C. Liu, Y. Su, Z. Chen, Y. Jiang, Excellent ammonia sensing performance of gas sensor based on graphene/titanium dioxide hybrid with improved morphology, Appl. Surf. Sci. 419 (2017) 84–90.
[27] B. Zhang, Y. Wang, G. Zhai, Biomedical applications of the graphene-based materials, Mater. Sci. Eng. C 61 (2016) 953–964. [28] Q. Huang, D. Zeng, H. Li, C. Xie, Room temperature formaldehyde sensors with enhanced performance, fast response and recovery based on zinc oxide quantum dots/graphene nanocomposites, Nanoscale 4 (2012) 5651–5658. [29] L.P. Mei, J.J. Feng, L. Wu, J.R. Chen, L. Shen, Y. Xie, A.J. Wang, A glassy carbon electrode modified with porous Cu2O nanospheres on reduced graphene oxide support for simultaneous sensing of uric acid and dopamine with high selectivity over ascorbic acid, Microchim. Acta 183 (2016) 2039–2046. [30] A. Zain Ul, P. Jae Young, K. Hyoun Woo, K. Sang Sub, Graphene-loaded tin oxide nanofibers: optimization and sensing performance, Nanotechnology 28 (2017) 035501. [31] F.L. Meng, H.H. Li, L.T. Kong, J.Y. Liu, J. Zhen, L. Wei, J. Yong, J.H. Liu, X.J. Huang, Parts per billion-level detection of benzene using SnO2/graphene nanocomposite composed of sub-6 nm SnO 2 nanoparticles, Anal. Chim. Acta 736 (2012) 100–107. [32] S. Shi, F. Zhang, H. Lin, Q. Wang, E. Shi, F. Qu, Enhanced triethylamine-sensing properties of P-N heterojunction Co3O4/In2O3 hollow microtubes derived from metal–organic frameworks, Sens. Actuators B Chem. 262 (2018) 739–749. [33] J. Chen, B. Yao, C. Li, G. Shi, An improved Hummers method for eco-friendly synthesis of graphene oxide, Carbon 64 (2013) 225–229. [34] W. Li, H. Xu, T. Zhai, H. Yu, Q. Xu, X. Song, J. Wang, B. Cao, High-sensitivity, highselectivity, and fast-recovery-speed triethylamine sensor based on ZnO micropyramids prepared by molten salt growth method, J. Alloys. Compd. 695 (2017) 2930–2936. [35] Y.H. Cho, N.K. You, Y.C. Kang, I.D. Kim, J.H. Lee, Ultraselective and ultrasensitive detection of trimethylamine using MoO3 nanoplates prepared by ultrasonic spray pyrolysis, Sens. Actuators B Chem. 195 (2014) 189–196. [36] H.W. Kim, H.G. Na, Y.J. Kwon, S.Y. Kang, M.S. Choi, J.H. Bang, P. Wu, S.S. Kim, Microwave-assisted synthesis of graphene–SnO2 nanocomposites and their applications in gas sensors, ACS Appl. Mater. Interfaces 9 (2017) 31667–31682. [37] Z.U. Abideen, J.-H. Kim, A. Mirzaei, H.W. Kim, S.S. Kim, Sensing behavior to ppmlevel gases and synergistic sensing mechanism in metal-functionalized rGO-loaded ZnO nanofibers, Sens. Actuators B Chem. 255 (2018) 1884–1896.
67