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Au nanoparticles ternary composites with enhanced formaldehyde sensing performance
Physica E 118 (2020) 113953
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Physica E: Low-dimensional Systems and Nanostructures journal homepage: http://www.elsevier.com/locate/physe
Synthesis of reduced graphene oxide/SnO2 nanosheets/Au nanoparticles ternary composites with enhanced formaldehyde sensing performance Qi Wei, Jing Sun, Peng Song *, Zhongxi Yang, Qi Wang ** School of Material Science and Engineering, University of Jinan, Jinan, 250022, China
A R T I C L E I N F O
A B S T R A C T
Keywords: SnO2 nanosheets Au nanoparticles Reduced graphene oxide Ternary composites Formaldehyde Gas sensor
In this paper, ternary composites composed of reduced graphene oxide (rGO), two dimension (2D) SnO2 nanosheets, and one dimension (0D) Au nanoparticles were successfully synthesized via a facile two-step approach. The rGO/SnO2/Au composites were characterized by X-ray diffraction (XRD), scanning electron mi croscope (SEM), energy dispersive spectrometer (EDS), transmission electron microscope (TEM), and X-ray photoelectron spectroscopy (XPS). Characterization results revealed that unique SnO2 nanosheets decorated with Au nanoparticles were homogeneously attached on the surface of rGO. Gas-sensing test results proved that incorporating SnO2 nanosheets with Au nanoparticles and rGO improved the gas-sensing performance toward formaldehyde (HCHO) in terms of lower operating temperature, high sensor response, and good selectivity. The enhanced sensing properties could mainly be attributed to the synergistic effect of ohmic contact between rGO and SnO2 nanosheets, high surface area and strong gas adsorption capacity of sheet-on-sheet heterostructured architectures, and the catalytic effect of Au nanoparticles. This work suggests that the rational design of 0D noble metal nanoparticles, 2D metal oxide nanosheets and 2D rGO to form ternary composites provides an opportunity for achieving high-performance sensing materials.
1. Introduction Formaldehyde (HCHO), as one of the noxious gas in our life, has given rise to more and more attention in the area of gas monitoring [1–4]. As is known to all, when the concentration of HCHO reaches to a certain extent (12–24 mg/m3), the health of our eye, nose and throat will be damaged with the symptom of coughing, sneezing and even potential death [5]. At the same time, the HCHO always exists in some occasion such as indoors and industries in which people are easy to be infected if ignoring the necessity of gas shield [6,7]. Therefore, it is essential to find a portable and stable device for the monitor of low-concentration HCHO. To detect and decrease the hazard caused by HCHO in envi ronment, the sensors based on semiconductor have mushroom grew in recent years. Among this, several metal oxides such as SnO2, In2O3, ZnO, and Fe2O3 have been researched by many groups. SnO2 is a traditional sensing material and a series of nanostructures have been prepared successfully [8–11]. However, due to the restriction of single metal oxide, there exist some of defects in SnO2, such as high working tem perature and relatively low sensing properties. Therefore, more efforts
should be made to optimize the SnO2 sensing performance. In recent years, reduced graphene oxide (rGO) has been introduced into a number of metal oxides with the purpose of improving materials’ performance in catalytic activity [12], batteries [13], sensors [14], and so on. Expect for the superior mechanical properties, the typical two-dimension plane morphology and high electron mobility [15,16] makes to be a hot substrate material in the field of sensors. For instance, Lu et al. prepared the rGO/CoTiO3 nanocomposite and improved sensing performance of ethanol at low temperature [17]. In view of the gas sensing research mentioned above, we realize that the rGO con tributes not only to decrease operating temperature of sensors based on metal oxide, but also promote sensing response to some extent. Although, after combining with a handful of rGO, the sensors prepared by the nanocomposites could come into play at low temperature, the degree of improving sensing response by rGO is still finite. Therefore, finding an effective method of improving sensing properties in rGO nanocomposites is essential in sensors. As is known to all, the surface modification is an advisable means to enhance sensing performance. Among this, the decoration of noble
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (P. Song), [email protected] (Q. Wang). https://doi.org/10.1016/j.physe.2020.113953 Received 28 September 2019; Received in revised form 31 December 2019; Accepted 3 January 2020 Available online 7 January 2020 1386-9477/© 2020 Elsevier B.V. All rights reserved.
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Fig. 1. FESEM image of (a, b) pure rGO and peony-like SnO2 nanosheets; (c, d) SnO2 nanosheets combined with rGO; (e, f) Au nanoparticles-decorated SnO2/rGO nanocomposites and (g–j) the elemental mapping of SnO2/rGO/Au composites.
metal is the most typical one. From now on, there are several noble metals such as Au [18], Pt [19], and Ag [20] have been used in sensors. The mechanism of noble metal can be ascribed to the catalysis that motivates the surface reaction between targeted gases and semi conductors [21]. With the existence of Au NPs, the conversion of oxygen molecules and reaction between targeted gas and oxygen molecules can be promoted, which enables sensing materials to require superior sensing properties. To further verify the function of Au NPs, several researches about Au are described in detail. Lee et al. explored the impact of Au NPs size on sensing performance of p-CuO nanowires, which contributes to high gas-sensing properties towards NO2 and CO [22]. In addition, Chava and his partners synthesized Au@In2O3 cor e–shell nanostructures by hydrothermal approach and the product
exhibits greater response and more selective to H2 [23]. According to the above investigation, we choose Au NPs as metal additive acts to further enhance gas sensing performance of rGO/SnO2 composites. In this paper, SnO2 nanosheets combined with rGO were successfully synthesized by hydrothermal method under the condition of 160 � C and 24 h. The obtained products were further decorated with Au NPs on SnO2 nanosheets. The samples have effective sensing for HCHO at 200 � C that is lower than the rGO/SnO2 nanocomposites. At the same time, the value of sensing response has promoted from 12.4 to 32.7 when exposing to 100 ppm HCHO. Meanwhile, the gas sensing mechanism for detecting HCHO has been also researched.
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Physica E: Low-dimensional Systems and Nanostructures 118 (2020) 113953
Fig. 3. XRD patterns of pure SnO2 nanosheets, SnO2/rGO nanocomposites and SnO2/rGO/Au composites.
Fig. 4. Raman spectra of rGO and SnO2/rGO/Au composites samples.
2. Experimental 2.1. Chemical reagents and materials All raw ingredients were analytical grade and used without further purification, which were purchased form Sinopharm Chemical Reagent Co., Ltd. 2.2. Preparation of rGO/SnO2 nanocomposites During this synthesized experiment, we dissolved 1 mg GO into deionized water (20 mL) under the condition of stirring and the mixed solution was ultrasonicated for 30 min to make the GO more uniformly. And then SnCl2⋅5H2O (0.35 g) was put into the solution which called A. Furthermore, NaOH (0.33 g) and CTAB (0.74 g) were solved into 20 mL aqueous solution that called B. This solution stirred few minutes until generating plenty of bubbles. Finally, A solution was put into B solution and the mixed solution was transferred into a Teflon-lined stainless-steel autoclave after stirring for 20 min. And the heat condition is 160 � C for 24 h. After the temperature decreases, the grey products could be gathered by washed and centrifugation with deionized water/methanol
Fig. 2. (a, b) Low-magnification TEM images and (c) high-resolution TEM image of SnO2/rGO/Au composites.
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Physica E: Low-dimensional Systems and Nanostructures 118 (2020) 113953
Fig. 5. XPS spectra of SnO2/rGO/Au composites. (a) survey spectrum; (b) Au 4f binding energy spectrum; (c) Sn 3d binding energy spectrum; (d) C 1s binding energy spectrum.
surface morphology of the samples were observed by field-emission scanning electronic microscope (FESEM, FEI Company, QUANTA FEG 250) and the energy-dispersive X-ray spectroscopy (EDS) analysis was analyzed by the FESEM attachment. The more information of the sam ples was obtained by transmission electron microscope (TEM, Hitachi H800). The X-ray photoelectron spectroscopy (XPS) data were recorded on a Thermo Scientific Escalab 250Xi to determine the elements and its valence of the samples. The fabrication and measurement of gas sensors are similar to our previous reported paper [24]. The gas-sensing prop erties were tested using a commercially computer-controlled WS-30A system. The sensor response was defined as Response ¼ Rair/Rgas, where Rair is the resistance of sensor in air, and Rgas is the resistance of sensor in test gas.
Table 1 Results of the EDS analysis for SnO2/rGO/Au composites. Element
Weight percentage
Element percentage
C O Sn Au
0.55 19.52 77.96 1.97
3.37 62.51 33.61 0.51
for several cycles, respectively. Drying in air under the condition of 60 � C for 12 h, the rGO/SnO2 nanocomposites could be successfully synthesized. 2.3. Preparation of hierarchical rGO/SnO2/Au composites
3. Results and discussion
In this detailed preparation, 50 mg as-obtained rGO/SnO2 compos ites, 1 mL of 0.01 M chloroauric acid (HAuCl4) and 1 mL of 0.01 M Llysine (C11H23N3O6) solution were dissolved into 15 mL deionized water under the condition of ultrasonication for 15 min, and L-lysine was used as an adhesives between Au NPs and rGO/SnO2 composites. 0.1 mL of 0.1 M Na3cit solution was added in the above solution dropwise under continually stirring for 30 min. The as-prepared products were washed several times with deionized water and ethanol respectively, and dried at 60 � C for 6 h. Finally the rGO/SnO2/Au nanocomposites could be obtained after annealing at 300 � C for 30 min.
The microstructure and morphology of rGO/SnO2/Au composites were researched by the field emission scanning electron microscope. According to the FESEM of pure rGO and SnO2 nanosheets, the rGO prepared by hydrothermal method possesses thinner thickness with wrinkled surface, which is beneficial the adhesion of other metal oxide (Fig. 1(a)). At the same time, the SnO2 nanosheets exist serious phe nomenon of aggregation (Fig. 1(b)), therefore we hypothesize the introduction of rGO can relieve this aggregation phenomenon. The FESEM images of the rGO/SnO2 composites are illustrated in Fig. 1(c and d). It is obvious that the SnO2 nanosheets are homogeneous grown on the surface of rGO, without any other impurities can be detected in nanocomposites. After cooperated with Au NPs, rGO/SnO2 nano composites have less change in nanostructure and the size of Au NPs is small (Fig. 1(e and f)). At the same time, there is no aggregation per formance that exists in Au NPs. Otherwise, the sample consists of C, Sn, Au and O elements on the basis of EDS spectra (Fig. 1(g-j)), which
2.4. Characterization and measurement of gas sensing performance The elements composition, microcosmic structure and crystalline of as-prepared samples were estimated by powder X-ray diffractometer (XRD, Bruker D8 Advance, λ ¼ 0.15406 nm) using CuKα1 radiation. The 4
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and the size of nanoparticles are about 10 nm by increasing the magnification (Fig. 2(b)), which is consistent with the images of SEM. To further analyze of the nanostructure of rGO/SnO2/Au composites, Fig. 2 (c) exhibits the HRTEM images lattice fringe of SnO2 and Au. The lattice fringe spacing of SnO2 and Au are measured to be 0.326 nm and 0.254 nm, respectively. They can be attributed to the (110) tetragonal rutile structure of SnO2 and the (111) plane of Au. The XRD patterns of rGO/SnO2 and rGO/SnO2/Au have been exhibited in Fig. 3. From the XRD patterns, we can realize that, the SnO2 nanosheets are related to the tetragonal structure of SnO2 (JCPDS No. 41–1445). All the peaks conform well to standard card, which means the SnO2 nanosheets have high purity after the progress of preparation. At the low position (about 10� ), XRD peak curve bread has been perceived that signifies the existence of amorphous rGO. Compared with the rGO/ SnO2 composites pattern, the XRD peak curve of rGO/SnO2/Au com posites is similar with SnO2/rGO because of the additive amount of Au NPs is few. At the same time, four small peaks can be observed in the XRD pattern of rGO/SnO2/Au composites, which can be ascribed to the (111) and (200) planes of face-centered cubic (fcc) Au (JCPDS No. 65–8601). The Raman spectra of rGO/SnO2/Au composites has been referred to verify the state of rGO and illustrated in Fig. 4. In the Raman spectra, there are two characteristic peaks located at 1354 cm 1 and 1591 cm 1, which are corresponding to the D band and G band of rGO, respectively. The intensity ratio of the D and G bands (ID/IG) is an essential factor to judge the number of defects in carbon materials. The D to G intensity ratio of D and G peaks in products is about 0.97, which is very close to the pure rGO (1.03). This phenomenon can be ascribed to the average size of the sp2 domains of the rGO/SnO2/Au. Therefore, the superior conductivity of graphene can be maintained after combining with other metal oxide. To require the element composition and chemical states of rGO/ SnO2/Au composite, the X-ray photoelectron spectroscopy has been adopted and the pictures of measurement results have been shown in Fig. 5. Among this, Fig. 5(a) is the O 1s XPS spectrum which exhibits three states: Olatt (530.7), Oads (532.1) and Osurf (533.4). The most spectra peaks of Au 4f5/2 and Au 4f7/2 correspond to 83.65 eV and 87.65 eV (Fig. 5(b)), respectively, which means the chemical state of metallic Au is zero oxidation [25]. The XPS spectrum in Sn 3d (Fig. 5(c)) exhibits two element states of the Sn 3d3/2 and 3d5/2 that is corresponded to 495.2 eV and 486.7 eV, respectively. The spectrum distance between the two peaks is 8.5 eV, indicating the formation of Sn4þ oxidation state in SnO2 nanosheets [26]. Meanwhile, the binding energy of Sn 3d5/2 is lower than the Sn 3d3/2, which can be attributed to existence of oxygen deficiency in samples [27]. In addition, Fig. 5(d) exhibits the XPS spectra of C 1s and three peaks located at 284.7 eV, 285.2 eV and 288.8 eV corresponding to the groups of C–C, C–O and C ¼ O. It is obvious that the peak intensities of C–O and C ¼ O have decreased greatly compared with the C–C, which can be related to a number of oxygen-containing groups have been removed [28], and the SnO2/rGO/Au nanocomposites are successfully required. Furthermore, from the XPS spectra of O, Sn and Au elements, the peaks areas of them are 69184.13, 398862.09 and 3142.44, respectively. According to the sensitivity of O (0.72), Sn (3.78) and Au (3.11) elements, we calculated the atomic ratio of O: Sn: Au is 131:59:1. Meanwhile, the result of EDS analysis for SnO2/rGO/Au composites has been introduced in order to confirm the content of ele ments and the form is exhibited as follows (Table 1). From the results, the existence of C element can be proved and the atomic ratio can be available in this form (O: Sn: C: Au is 131:59:6:1).
Fig. 6. (a) Sensing response and resistance of pure SnO2 nanosheets; (b) SnO2/ rGO nanocomposites and SnO2/rGO/Au composites to 100 ppm HCHO under different temperature; (c) the sensing response to different gases with the concentration of 100 ppm.
confirms that the Au NPs have successfully adhered on the surface of rGO/SnO2 composites. In the meantime, due to the fewer amount of Au doping, the distribution of Au NPs is inconspicuous than other elements. The texting technologies of TEM and HRTEM have been introduced in order to research the size and lattice fringe of the samples. From Fig. 2 (a), it is obviously that, other than same SnO2 nanosheets, a number of Au NPs were successfully grown on the surface of rGO/SnO2 composites
3.1. Gas sensing performances As is known to all, the working temperature is one of significant aspects and the optimum temperature can promote the sensing response of samples. The sensors based on peony-like SnO2 nanosheets, rGO/ SnO2 composites and rGO/SnO2/Au composites were measured by 100 5
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Physica E: Low-dimensional Systems and Nanostructures 118 (2020) 113953
Fig. 7. (a) Relationships between sensor responses and HCHO concentration from 1 to 100 ppm; (b) the corresponding log (S-1) vs. log (C) of samples; (c) Three periods of response curve based on SnO2/rGO nanocomposites and SnO2/rGO/Au composites and (d) The long-stability in gas sensing response of SnO2/rGO/ Au composites.
consequence, we can realize that the sensing response of rGO/SnO2/Au composites is superior to the SnO2 nanosheets and rGO/SnO2 compos ites, and the operating temperature also decreased prominently. Fig. 6 (b) exhibits the resistance variation of SnO2 nanosheets, rGO/SnO2 composites and rGO/SnO2/Au composites under different atmosphere at their operating temperature. Exposed to the air, the resistance of SnO2 nanosheets drops significantly after combining with rGO, which can be ascribed to the superior electrical conductivity of rGO. With the addition of Au NPs, the resistance of the rGO/SnO2 composites increases slightly because of the active absorption of Au NPs. When the sensors touching with HCHO, the release of electron from oxygen ions contributes to samples’ electrical conductivity recover. The sensors prepared by rGO/SnO2 and rGO/SnO2/Au composites are measured by five other gases comparing with HCHO at the optimum temperature (Fig. 6(c)). Obviously, after the decoration of Au NPs, the sensing performances have improved greatly in HCHO comparing with rGO/SnO2 composites. But the gas sensor prepared by rGO/SnO2/Au composites has the better sensing performance for HCHO than others, which indicates that rGO/SnO2/Au composites have superior properties of selectivity. To further invest the sensing performance of rGO/SnO2 and rGO/ SnO2/Au composites. Fig. 7(a) illustrates the relationship between the sensing performance of rGO/SnO2 and rGO/SnO2/Au composites with the various HCHO concentration ranging from 1 ppm to 100 ppm. This figure exhibits that, with the gas concentration increasing, the sensing properties of the two sensors promotes greatly. In the meantime, rGO/ SnO2/Au composites have a large promotion in sensing response with the HCHO concentration increasing. In addition, the sensing response of the rGO/SnO2/Au composites is even three times superior than rGO/ SnO2 composites to 100 ppm HCHO. Fig. 7(b) is the logarithmic rela tionship between the sensing properties of products and the HCHO concentration. According to the analysis, both of products possess a good linear fitting with the HCHO concentration (1–100 ppm).
Table 2 Gas-sensing properties toward HCHO based on various metal oxide gas sensors. Sensing materials
T (� C)
TEA (ppm)
Response (Ra/Rg)
Res/ Rec (s)
Ref
SnO2 nanosheets Graphene/ZnO nanosheets In2O3@RGO heterostructures Au@In2O3 Ga/ZnO nanocomposites SnO2 nanosheets
240 200
100 100
7 12
8/18 3/5
[30] [31]
225
100
1.8
22/43
[32]
200 400
100 205
17 13
12/66 14/12
[33] [34]
320
100
6.5
2/12
220
100
12.4
2/7
200
100
32.7
2/5
this work this work this work
SnO2/rGO composites SnO2/rGO/Au composites
ppm HCHO. According to Fig. 6(a), the operating temperature ranges from 140 � C to 360 � C. The sensing properties of SnO2 nanosheets, rGO/ SnO2 and rGO/SnO2/Au composites are on the rise by increasing the operating temperature before the optimum one. One reason can explain this phenomenon is that high working temperature provides more en ergy to produce excitation electrons that contributes to the adsorption of the reducing gas and oxygen species form SnO2 surface. When the temperature excels the optimum one, the value response of products decrease a lot, which contributes to the HCHO molecules are much easier to be desorbed from the surface of SnO2 [29]. The operating temperatures of SnO2 nanosheets, rGO/SnO2 and rGO/SnO2/Au com posites are 320 � C, 220 � C and 200 � C, which are referred to the optimal sensitivity 6.5, 12.4 and 32.7, respectively (Fig. 6(a)). From this 6
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Physica E: Low-dimensional Systems and Nanostructures 118 (2020) 113953
Fig. 8. Schematic diagram of possible gas sensing mechanisms on Au nanoparticles-functionalized SnO2/rGO nanocomposites.
Otherwise, the slopes of rGO/SnO2 and rGO/SnO2/Au composites are 0.391 and 0.567, respectively. Although the slope of rGO/SnO2/Au composites is lower than the SnO2/rGO nanocomposites, due to the higher value response (1 ppm, 6.5) of rGO/SnO2/Au composites than the rGO/SnO2 composites (1 ppm, 1.7) at the low concentration, implying that sensing properties in SnO2/rGO/Au composites are superior than the sample without the decoration of Au⋅In Fig. 7(c), the stability per formance of both rGO/SnO2 and rGO/SnO2/Au composites has been exhibited. It is clearly that, the sensing performances of both samples have little obvious variation after 3 cycles and the sensors maintain the fast response and recover to target gas at the optimum temperature. The sensors prepared by rGO/SnO2 and rGO/SnO2/Au composites are measured by 16 days with HCHO at the optimum temperature (Fig. 7 (d)). Obviously, the gas sensing performances of rGO/SnO2/Au com posites has less change during these days, which indicates that rGO/ SnO2/Au composites possess long-stability properties to HCHO. Corre spondingly, comparing the sensing performance with other metal oxide in formaldehyde, the result is summarized in Table 2 [30–34]. It is summarized that rGO/SnO2/Au composites exhibit lower operating temperature and superior sensing properties.
electrons back to conduction band of samples, which result in the thickness of electron depletion decreasing. Hence, the conduction of samples will recover to the original state before touching to the air. The particular reactions of this progress can be represented as follows [37, 38]: O2(ads) þ2e → 2O (ads)
(1)
HCHO þ 2O (ads) → CO2 þ H2Oþ 2e
(2)
After combining with rGO, the rGO/SnO2 composites still belong to the n-type sensing materials because of the SnO2 nanosheets play an importance role in sensing performance. The rGO possesses significant electron conduction [39–41] which enables electrons to spread quickly and can be easily caught by oxygen or texted gases. Therefore, the op timum temperature of rGO/SnO2 composites is lower than the pure SnO2 nanosheets. In addition, the heterojunction between SnO2 and rGO will be other positive factors to promote sensing properties of samples. The depth of depletion between rGO and SnO2 will change when sensors based on rGO/SnO2 composites exposing to different atmosphere (air and targeted gas) and in the heterojunction, the electrons transmission can facilitate the gas detection by changing about electrical conductivity [42]. To further promote the sensing performance of rGO/SnO2 compos ites, noble metal has been introduced by acting as active catalysts. From the result of HCHO sensing measurement, it is apparent that the sensi tivity of samples improved greatly after adding some Au NPs. At the same time, the optimum temperature of rGO/SnO2/Au composites also decreased 20 � C. The contribution of Au NPs can be attributed to two aspects. Firstly, due to the catalytic of Au NPs, the dissociation of mo lecular oxygen can be easy to proceed and the diffusion rate of absorbed species has been enhanced as well [43,44]. Otherwise, from this reason, more oxygen molecules were absorbed easily by the surface of the SnO2 nanosheets and the resistance of rGO/SnO2/Au composites further increased which results in the significant sensing performance [45,46].
3.2. Gas sensing mechanism As is known to all, the pure SnO2 nanosheets are typical n-type semiconductors and the SnO2 sensing material belongs to the surfacecontrolled type. Therefore, the conductivity of SnO2 is greatly influ enced by the surface depletion layer and the resistance of samples will change by exposing to the atmosphere of air and targeted gas. In the condition of air, the sensors based on SnO2 nanosheets will absorbed oxygen from the air and oxygen molecules will extract electrons from condition band to generate oxygen ions (O2 ,O ,O2 ) [35,36]. The electron depletion layer will be formed because of the electrons’ defi ciency at the surface. After exposed to the atmosphere of HCHO, the oxygen ions will react with HCHO molecules accompanied by releasing 7
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Secondly, the work function between SnO2 and Au is another positive aspect for promoting sensing performance of rGO/SnO2/Au composites. As is known to all, the work function of SnO2 and Au is 4.5 eV and 5.1eV [47,48], respectively, and the Schottky junctions would form at the interface. The existence of Schottky junctions enable electrons to transfer from SnO2 nanosheets to Au NPs, just like in Fig. 8, which re sults in the electron depletion areas become much thicker than the samples without Au [49,50] and the resistance of SnO2/rGO/Au com posites (Rair) would further increase in air. Therefore, according to the formula of (Rair/Rgas ¼ response), the sensitivity of rGO/SnO2/Au composites is higher than the SnO2/rGO nanocomposites.
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Xu, PdPt bimetalfunctionalized SnO2 nanosheets: controllable synthesis and its dual selectivity for detection of carbon monoxide and methane, ACS Appl. Mater. Interfaces 11 (2019) 26116–26126. [27] C.C. Li, X.M. Yin, Q.H. Li, Enhanced gas sensing properties of ZnO/SnO2 hierarchical architectures by glucose-induced attachment, CrystEngComm 13 (2011) 1557–1563. [28] J. Zhang, J. Wu, X. Wang, Enhancing room-temperature NO2, sensing properties via forming heterojunction for NiO-rGO composited with SnO2 nanoplates, Sens. Actuators B Chem. 243 (2017) 1010–1019. [29] S. Tian, X. Ding, D. Zeng, Pore-size-dependent property of hierarchical SnO2 mesoporous microfibers as HCHO sensors, Sens. Actuators B Chem. 186 (2013) 640–647. [30] H. Yu, T. Yang, R. Zhao, B. Xiao, Z. Li, M. Zhang, Fast formaldehyde gas sensing response properties of ultrathin SnO2 nanosheets, RSC Adv. 5 (2015) 104574–104581. [31] Z.W. Chen, Y.Y. Hong, Z.D. Lin, L.M. Liu, X.W. Zhang, Enhanced formaldehyde gas sensing properties of ZnO nanosheets modified with graphene, Electron. Mater. Lett. 13 (2017) 270–276. [32] R.K. Mishra, G. Murali, T.H. Kim, J.H. Kim, Y.J. Lim, B.S. Kim, P.P. Sahay, S.H. Lee, Nanocube In2O3@rGO heterostructure based gas sensor for acetone and formaldehyde detection, RSC Adv. 7 (2017) 38714–38724. [33] X. Li, J. Liu, H. Guo, X. Zhou, C. Wang, P. Sun, X. Hu, G. Lu, Au@In2O3 core–shell composites: a metal–semiconductor heterostructure for gas sensing applications, RSC Adv. 5 (2015) 545–551. [34] N. Han, Y. Tian, X. Wu, Improving humidity selectivity in formaldehyde gas sensing by a two-sensor array made of Ga-doped ZnO, Sens. Actuators B Chem. 138 (2009) 228–235.
4. Conclusion In summary, we synthesized the rGO/SnO2/Au composite by a twostep approach (a facile hydrothermal reaction and subsequent in situ reducing process) and researched their morphology through a series of characterizations. According to the figures of SEM and TEM, Au nano particles successfully grew uniformly on the surface of SnO2/rGO nanocomposites with smaller size. And the rGO/SnO2/Au composite exhibits superior sensing properties in HCHO to SnO2/rGO nano composites and pure peony-like SnO2. Meanwhile, the working tem perature of SnO2 nanosheets decreases 100 � C by introducing GO, which verifies that the introduction of grapheme can be an effective method to reduce operating temperature of nanocomposites. At the same time, the sensing response of SnO2/rGO nanocomposites promotes from 12.4 to 32.7 with the addition of Au NPs. The existence of rGO relieves the aggregation phenomenon, and Au NPs decorated on the surface of samples provide a number of active absorption, which makes a huge contribution to sensing properties in HCHO. Author statement No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. In this work, ternary composites composed of reduced graphene oxide, SnO2 nanosheets, and Au nanoparticles were successfully syn thesized via a facile two-step approach. Gas sensors based on ternary composites exhibited enhanced gas-sensing performance toward form aldehyde in terms of lower operating temperature, high sensor response, and good selectivity. The enhanced sensing properties could mainly be attributed to the synergistic effect of ohmic contact between rGO and SnO2 nanosheets, high surface area and strong gas adsorption capacity of sheet-on-sheet heterostructured architectures, and the catalytic effect of Au nanoparticles. Acknowledgement This work was financially supported by National Natural Science Foundation of China (No. 61102006), and Natural Science Foundation of Shandong Province, China (No. ZR2018LE006 and ZR2015EM019). References [1] D. Yan, P.C. Xu, Q. Xiang, H.R. Mou, J.Q. Xu, W.J. Wen, X.X. Li, Y. Zhang, Polydopamine nanotubes: bio-inspired synthesis, formaldehyde sensing properties and thermodynamic investigation, J. Mater. Chem. A 4 (2016) 3487–3493. [2] H. Kudo, Y. Suzuki, T. Gessei, Biochemical gas sensor (bio-sniffer) for ultrahighsensitive gaseous HCHO monitoring, Biosens. Bioelectron. 26 (2010) 854–858. [3] L. He, X. Yang, M. Ren, An ultra-fast illuminating fluorescent probe for monitoring HCHO in living cells, shiitake mushrooms, and indoors, Chem. Can. 52 (2016) 9582–9585. [4] Y. Li, N. Chen, D. Deng, HCHO detection: SnO2, microspheres for HCHO gas sensor with high sensitivity, fast response/recovery and good selectivity, Sens. Actuators B Chem. 238 (2017) 264–273.
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[35] J.M. Walker, S.A. Akbar, P.A. Morris, Synergistic effects in gas sensing semiconducting oxide nano-heterostructures: a review, Sens. Actuators B Chem. 286 (2019) 624–640. [36] P. Karnati, S.A. Akbar, P.A. Morris, Conduction mechanisms in one dimensional core-shell nanostructures for gas sensing: a review, Sens. Actuators B Chem. 295 (2019) 127–143. [37] P. Song, H.W. Qin, L. Zhang, K. An, Z.J. Lin, J.F. Hu, M.H. Jiang, The structure, electrical and ethanol-sensing properties of La1 xPbxFeO3 perovskite ceramics with x�0.3, Sens. Actuators, B 104 (2005) 312–316. [38] F.C. Chung, R.J. Wu, F.C. Cheng, Fabrication of an Au@SnO2 core–shell structure for gaseous HCHO sensing at room temperature, Sens. Actuators B Chem. 190 (2014) 1–7. [39] Y. Chang, Y. Yao, B. Wang, Reduced graphene oxide mediated SnO2 nanocrystals for enhanced gas-sensing properties, J. Mater. Sci. Technol. 29 (2013) 157–160. [40] Y. Li, C. Zhong, J. Liu, Atomically thin mesoporous Co3O4 layers strongly coupled with N-rGO nanosheets as high-performance bifunctional catalysts for 1D knittable zinc-air batteries, Adv. Mater. 30 (2018), https://doi.org/10.1002/ adma.201703657. [41] X. Zhou, Z. Zhang, X. Lv, Facile and rapid synthesis of Sb2O3/CNTs/rGO nanocomposite with excellent sodium storage performances, Mater. Lett. 213 (2018) 201–203. [42] H. Zhang, J. Feng, T. Fei, SnO2 nanoparticles-reduced graphene oxide nanocomposites for NO2 sensing at low operating temperature, Sens. Actuators B Chem. 190 (2014) 472–478.
[43] S. Zhang, P. Song, J. Zhang, H.H. Yan, J. Li, Z.X. Yang, Q. Wang, Highly sensitive detection of acetone using mesoporous In2O3 nanospheres decorated with Au nanoparticles, Sens. Actuators B Chem. 242 (2017) 983–993. [44] X. Li, X. Zhou, H. Guo, C. Wang, J. Liu, P. Sun, F. Liu, G. Lu, Design of Au@ZnO yolk-shell nanospheres with enhanced gas sensing properties, Appl. Mater. Inter. 6 (2014) 18661–18667. [45] F. Li, T. Zhang, X. Gao, Coaxial electrospinning heterojunction SnO2/Au-doped In2O3 core-shell nanofibers for acetone gas sensor, Sens. Actuators B Chem. 252 (2017) 822–830. [46] ] L.L. Wang, Z. Lou, T. Fei, T. Zhang, Templating synthesis of ZnO hollow nanospheres loaded with Au NPs and their enhanced gas sensing properties, J. Mater. Chem. 22 (2012) 4767–4771. [47] X. Wang, S. Qiu, C. He, G. Lu, W. Liu, J. Liu, Synthesis of Au decorated SnO2 mesoporous spheres with enhanced gas sensing performance, RSC Adv. 3 (2013) 19002–19008. [48] L. Wang, H. Dou, Z. Lou, T. Zhang, Encapsuled nanoreactors (Au@SnO2): a newsensing material for chemical sensors, Nanoscale 5 (2013) 2686–2691. [49] J. Guo, Z.J. Hang, H. Gong, Au nanoparticle-functionalized 3D SnO2 microstructures for high performance gas sensor, Sens. Actuators B Chem. 226 (2016) 266–272. [50] T. Maosong, D. Guorui, G. Dingsan, Surface modification of oxide thin film and its gas-sensing properties, Appl. Surf. Sci. 171 (2001) 226–230.
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Contents lists available at ScienceDirect
Physica E: Low-dimensional Systems and Nanostructures journal homepage: http://www.elsevier.com/locate/physe
Synthesis of reduced graphene oxide/SnO2 nanosheets/Au nanoparticles ternary composites with enhanced formaldehyde sensing performance Qi Wei, Jing Sun, Peng Song *, Zhongxi Yang, Qi Wang ** School of Material Science and Engineering, University of Jinan, Jinan, 250022, China
A R T I C L E I N F O
A B S T R A C T
Keywords: SnO2 nanosheets Au nanoparticles Reduced graphene oxide Ternary composites Formaldehyde Gas sensor
In this paper, ternary composites composed of reduced graphene oxide (rGO), two dimension (2D) SnO2 nanosheets, and one dimension (0D) Au nanoparticles were successfully synthesized via a facile two-step approach. The rGO/SnO2/Au composites were characterized by X-ray diffraction (XRD), scanning electron mi croscope (SEM), energy dispersive spectrometer (EDS), transmission electron microscope (TEM), and X-ray photoelectron spectroscopy (XPS). Characterization results revealed that unique SnO2 nanosheets decorated with Au nanoparticles were homogeneously attached on the surface of rGO. Gas-sensing test results proved that incorporating SnO2 nanosheets with Au nanoparticles and rGO improved the gas-sensing performance toward formaldehyde (HCHO) in terms of lower operating temperature, high sensor response, and good selectivity. The enhanced sensing properties could mainly be attributed to the synergistic effect of ohmic contact between rGO and SnO2 nanosheets, high surface area and strong gas adsorption capacity of sheet-on-sheet heterostructured architectures, and the catalytic effect of Au nanoparticles. This work suggests that the rational design of 0D noble metal nanoparticles, 2D metal oxide nanosheets and 2D rGO to form ternary composites provides an opportunity for achieving high-performance sensing materials.
1. Introduction Formaldehyde (HCHO), as one of the noxious gas in our life, has given rise to more and more attention in the area of gas monitoring [1–4]. As is known to all, when the concentration of HCHO reaches to a certain extent (12–24 mg/m3), the health of our eye, nose and throat will be damaged with the symptom of coughing, sneezing and even potential death [5]. At the same time, the HCHO always exists in some occasion such as indoors and industries in which people are easy to be infected if ignoring the necessity of gas shield [6,7]. Therefore, it is essential to find a portable and stable device for the monitor of low-concentration HCHO. To detect and decrease the hazard caused by HCHO in envi ronment, the sensors based on semiconductor have mushroom grew in recent years. Among this, several metal oxides such as SnO2, In2O3, ZnO, and Fe2O3 have been researched by many groups. SnO2 is a traditional sensing material and a series of nanostructures have been prepared successfully [8–11]. However, due to the restriction of single metal oxide, there exist some of defects in SnO2, such as high working tem perature and relatively low sensing properties. Therefore, more efforts
should be made to optimize the SnO2 sensing performance. In recent years, reduced graphene oxide (rGO) has been introduced into a number of metal oxides with the purpose of improving materials’ performance in catalytic activity [12], batteries [13], sensors [14], and so on. Expect for the superior mechanical properties, the typical two-dimension plane morphology and high electron mobility [15,16] makes to be a hot substrate material in the field of sensors. For instance, Lu et al. prepared the rGO/CoTiO3 nanocomposite and improved sensing performance of ethanol at low temperature [17]. In view of the gas sensing research mentioned above, we realize that the rGO con tributes not only to decrease operating temperature of sensors based on metal oxide, but also promote sensing response to some extent. Although, after combining with a handful of rGO, the sensors prepared by the nanocomposites could come into play at low temperature, the degree of improving sensing response by rGO is still finite. Therefore, finding an effective method of improving sensing properties in rGO nanocomposites is essential in sensors. As is known to all, the surface modification is an advisable means to enhance sensing performance. Among this, the decoration of noble
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (P. Song), [email protected] (Q. Wang). https://doi.org/10.1016/j.physe.2020.113953 Received 28 September 2019; Received in revised form 31 December 2019; Accepted 3 January 2020 Available online 7 January 2020 1386-9477/© 2020 Elsevier B.V. All rights reserved.
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Fig. 1. FESEM image of (a, b) pure rGO and peony-like SnO2 nanosheets; (c, d) SnO2 nanosheets combined with rGO; (e, f) Au nanoparticles-decorated SnO2/rGO nanocomposites and (g–j) the elemental mapping of SnO2/rGO/Au composites.
metal is the most typical one. From now on, there are several noble metals such as Au [18], Pt [19], and Ag [20] have been used in sensors. The mechanism of noble metal can be ascribed to the catalysis that motivates the surface reaction between targeted gases and semi conductors [21]. With the existence of Au NPs, the conversion of oxygen molecules and reaction between targeted gas and oxygen molecules can be promoted, which enables sensing materials to require superior sensing properties. To further verify the function of Au NPs, several researches about Au are described in detail. Lee et al. explored the impact of Au NPs size on sensing performance of p-CuO nanowires, which contributes to high gas-sensing properties towards NO2 and CO [22]. In addition, Chava and his partners synthesized Au@In2O3 cor e–shell nanostructures by hydrothermal approach and the product
exhibits greater response and more selective to H2 [23]. According to the above investigation, we choose Au NPs as metal additive acts to further enhance gas sensing performance of rGO/SnO2 composites. In this paper, SnO2 nanosheets combined with rGO were successfully synthesized by hydrothermal method under the condition of 160 � C and 24 h. The obtained products were further decorated with Au NPs on SnO2 nanosheets. The samples have effective sensing for HCHO at 200 � C that is lower than the rGO/SnO2 nanocomposites. At the same time, the value of sensing response has promoted from 12.4 to 32.7 when exposing to 100 ppm HCHO. Meanwhile, the gas sensing mechanism for detecting HCHO has been also researched.
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Fig. 3. XRD patterns of pure SnO2 nanosheets, SnO2/rGO nanocomposites and SnO2/rGO/Au composites.
Fig. 4. Raman spectra of rGO and SnO2/rGO/Au composites samples.
2. Experimental 2.1. Chemical reagents and materials All raw ingredients were analytical grade and used without further purification, which were purchased form Sinopharm Chemical Reagent Co., Ltd. 2.2. Preparation of rGO/SnO2 nanocomposites During this synthesized experiment, we dissolved 1 mg GO into deionized water (20 mL) under the condition of stirring and the mixed solution was ultrasonicated for 30 min to make the GO more uniformly. And then SnCl2⋅5H2O (0.35 g) was put into the solution which called A. Furthermore, NaOH (0.33 g) and CTAB (0.74 g) were solved into 20 mL aqueous solution that called B. This solution stirred few minutes until generating plenty of bubbles. Finally, A solution was put into B solution and the mixed solution was transferred into a Teflon-lined stainless-steel autoclave after stirring for 20 min. And the heat condition is 160 � C for 24 h. After the temperature decreases, the grey products could be gathered by washed and centrifugation with deionized water/methanol
Fig. 2. (a, b) Low-magnification TEM images and (c) high-resolution TEM image of SnO2/rGO/Au composites.
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Fig. 5. XPS spectra of SnO2/rGO/Au composites. (a) survey spectrum; (b) Au 4f binding energy spectrum; (c) Sn 3d binding energy spectrum; (d) C 1s binding energy spectrum.
surface morphology of the samples were observed by field-emission scanning electronic microscope (FESEM, FEI Company, QUANTA FEG 250) and the energy-dispersive X-ray spectroscopy (EDS) analysis was analyzed by the FESEM attachment. The more information of the sam ples was obtained by transmission electron microscope (TEM, Hitachi H800). The X-ray photoelectron spectroscopy (XPS) data were recorded on a Thermo Scientific Escalab 250Xi to determine the elements and its valence of the samples. The fabrication and measurement of gas sensors are similar to our previous reported paper [24]. The gas-sensing prop erties were tested using a commercially computer-controlled WS-30A system. The sensor response was defined as Response ¼ Rair/Rgas, where Rair is the resistance of sensor in air, and Rgas is the resistance of sensor in test gas.
Table 1 Results of the EDS analysis for SnO2/rGO/Au composites. Element
Weight percentage
Element percentage
C O Sn Au
0.55 19.52 77.96 1.97
3.37 62.51 33.61 0.51
for several cycles, respectively. Drying in air under the condition of 60 � C for 12 h, the rGO/SnO2 nanocomposites could be successfully synthesized. 2.3. Preparation of hierarchical rGO/SnO2/Au composites
3. Results and discussion
In this detailed preparation, 50 mg as-obtained rGO/SnO2 compos ites, 1 mL of 0.01 M chloroauric acid (HAuCl4) and 1 mL of 0.01 M Llysine (C11H23N3O6) solution were dissolved into 15 mL deionized water under the condition of ultrasonication for 15 min, and L-lysine was used as an adhesives between Au NPs and rGO/SnO2 composites. 0.1 mL of 0.1 M Na3cit solution was added in the above solution dropwise under continually stirring for 30 min. The as-prepared products were washed several times with deionized water and ethanol respectively, and dried at 60 � C for 6 h. Finally the rGO/SnO2/Au nanocomposites could be obtained after annealing at 300 � C for 30 min.
The microstructure and morphology of rGO/SnO2/Au composites were researched by the field emission scanning electron microscope. According to the FESEM of pure rGO and SnO2 nanosheets, the rGO prepared by hydrothermal method possesses thinner thickness with wrinkled surface, which is beneficial the adhesion of other metal oxide (Fig. 1(a)). At the same time, the SnO2 nanosheets exist serious phe nomenon of aggregation (Fig. 1(b)), therefore we hypothesize the introduction of rGO can relieve this aggregation phenomenon. The FESEM images of the rGO/SnO2 composites are illustrated in Fig. 1(c and d). It is obvious that the SnO2 nanosheets are homogeneous grown on the surface of rGO, without any other impurities can be detected in nanocomposites. After cooperated with Au NPs, rGO/SnO2 nano composites have less change in nanostructure and the size of Au NPs is small (Fig. 1(e and f)). At the same time, there is no aggregation per formance that exists in Au NPs. Otherwise, the sample consists of C, Sn, Au and O elements on the basis of EDS spectra (Fig. 1(g-j)), which
2.4. Characterization and measurement of gas sensing performance The elements composition, microcosmic structure and crystalline of as-prepared samples were estimated by powder X-ray diffractometer (XRD, Bruker D8 Advance, λ ¼ 0.15406 nm) using CuKα1 radiation. The 4
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and the size of nanoparticles are about 10 nm by increasing the magnification (Fig. 2(b)), which is consistent with the images of SEM. To further analyze of the nanostructure of rGO/SnO2/Au composites, Fig. 2 (c) exhibits the HRTEM images lattice fringe of SnO2 and Au. The lattice fringe spacing of SnO2 and Au are measured to be 0.326 nm and 0.254 nm, respectively. They can be attributed to the (110) tetragonal rutile structure of SnO2 and the (111) plane of Au. The XRD patterns of rGO/SnO2 and rGO/SnO2/Au have been exhibited in Fig. 3. From the XRD patterns, we can realize that, the SnO2 nanosheets are related to the tetragonal structure of SnO2 (JCPDS No. 41–1445). All the peaks conform well to standard card, which means the SnO2 nanosheets have high purity after the progress of preparation. At the low position (about 10� ), XRD peak curve bread has been perceived that signifies the existence of amorphous rGO. Compared with the rGO/ SnO2 composites pattern, the XRD peak curve of rGO/SnO2/Au com posites is similar with SnO2/rGO because of the additive amount of Au NPs is few. At the same time, four small peaks can be observed in the XRD pattern of rGO/SnO2/Au composites, which can be ascribed to the (111) and (200) planes of face-centered cubic (fcc) Au (JCPDS No. 65–8601). The Raman spectra of rGO/SnO2/Au composites has been referred to verify the state of rGO and illustrated in Fig. 4. In the Raman spectra, there are two characteristic peaks located at 1354 cm 1 and 1591 cm 1, which are corresponding to the D band and G band of rGO, respectively. The intensity ratio of the D and G bands (ID/IG) is an essential factor to judge the number of defects in carbon materials. The D to G intensity ratio of D and G peaks in products is about 0.97, which is very close to the pure rGO (1.03). This phenomenon can be ascribed to the average size of the sp2 domains of the rGO/SnO2/Au. Therefore, the superior conductivity of graphene can be maintained after combining with other metal oxide. To require the element composition and chemical states of rGO/ SnO2/Au composite, the X-ray photoelectron spectroscopy has been adopted and the pictures of measurement results have been shown in Fig. 5. Among this, Fig. 5(a) is the O 1s XPS spectrum which exhibits three states: Olatt (530.7), Oads (532.1) and Osurf (533.4). The most spectra peaks of Au 4f5/2 and Au 4f7/2 correspond to 83.65 eV and 87.65 eV (Fig. 5(b)), respectively, which means the chemical state of metallic Au is zero oxidation [25]. The XPS spectrum in Sn 3d (Fig. 5(c)) exhibits two element states of the Sn 3d3/2 and 3d5/2 that is corresponded to 495.2 eV and 486.7 eV, respectively. The spectrum distance between the two peaks is 8.5 eV, indicating the formation of Sn4þ oxidation state in SnO2 nanosheets [26]. Meanwhile, the binding energy of Sn 3d5/2 is lower than the Sn 3d3/2, which can be attributed to existence of oxygen deficiency in samples [27]. In addition, Fig. 5(d) exhibits the XPS spectra of C 1s and three peaks located at 284.7 eV, 285.2 eV and 288.8 eV corresponding to the groups of C–C, C–O and C ¼ O. It is obvious that the peak intensities of C–O and C ¼ O have decreased greatly compared with the C–C, which can be related to a number of oxygen-containing groups have been removed [28], and the SnO2/rGO/Au nanocomposites are successfully required. Furthermore, from the XPS spectra of O, Sn and Au elements, the peaks areas of them are 69184.13, 398862.09 and 3142.44, respectively. According to the sensitivity of O (0.72), Sn (3.78) and Au (3.11) elements, we calculated the atomic ratio of O: Sn: Au is 131:59:1. Meanwhile, the result of EDS analysis for SnO2/rGO/Au composites has been introduced in order to confirm the content of ele ments and the form is exhibited as follows (Table 1). From the results, the existence of C element can be proved and the atomic ratio can be available in this form (O: Sn: C: Au is 131:59:6:1).
Fig. 6. (a) Sensing response and resistance of pure SnO2 nanosheets; (b) SnO2/ rGO nanocomposites and SnO2/rGO/Au composites to 100 ppm HCHO under different temperature; (c) the sensing response to different gases with the concentration of 100 ppm.
confirms that the Au NPs have successfully adhered on the surface of rGO/SnO2 composites. In the meantime, due to the fewer amount of Au doping, the distribution of Au NPs is inconspicuous than other elements. The texting technologies of TEM and HRTEM have been introduced in order to research the size and lattice fringe of the samples. From Fig. 2 (a), it is obviously that, other than same SnO2 nanosheets, a number of Au NPs were successfully grown on the surface of rGO/SnO2 composites
3.1. Gas sensing performances As is known to all, the working temperature is one of significant aspects and the optimum temperature can promote the sensing response of samples. The sensors based on peony-like SnO2 nanosheets, rGO/ SnO2 composites and rGO/SnO2/Au composites were measured by 100 5
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Fig. 7. (a) Relationships between sensor responses and HCHO concentration from 1 to 100 ppm; (b) the corresponding log (S-1) vs. log (C) of samples; (c) Three periods of response curve based on SnO2/rGO nanocomposites and SnO2/rGO/Au composites and (d) The long-stability in gas sensing response of SnO2/rGO/ Au composites.
consequence, we can realize that the sensing response of rGO/SnO2/Au composites is superior to the SnO2 nanosheets and rGO/SnO2 compos ites, and the operating temperature also decreased prominently. Fig. 6 (b) exhibits the resistance variation of SnO2 nanosheets, rGO/SnO2 composites and rGO/SnO2/Au composites under different atmosphere at their operating temperature. Exposed to the air, the resistance of SnO2 nanosheets drops significantly after combining with rGO, which can be ascribed to the superior electrical conductivity of rGO. With the addition of Au NPs, the resistance of the rGO/SnO2 composites increases slightly because of the active absorption of Au NPs. When the sensors touching with HCHO, the release of electron from oxygen ions contributes to samples’ electrical conductivity recover. The sensors prepared by rGO/SnO2 and rGO/SnO2/Au composites are measured by five other gases comparing with HCHO at the optimum temperature (Fig. 6(c)). Obviously, after the decoration of Au NPs, the sensing performances have improved greatly in HCHO comparing with rGO/SnO2 composites. But the gas sensor prepared by rGO/SnO2/Au composites has the better sensing performance for HCHO than others, which indicates that rGO/SnO2/Au composites have superior properties of selectivity. To further invest the sensing performance of rGO/SnO2 and rGO/ SnO2/Au composites. Fig. 7(a) illustrates the relationship between the sensing performance of rGO/SnO2 and rGO/SnO2/Au composites with the various HCHO concentration ranging from 1 ppm to 100 ppm. This figure exhibits that, with the gas concentration increasing, the sensing properties of the two sensors promotes greatly. In the meantime, rGO/ SnO2/Au composites have a large promotion in sensing response with the HCHO concentration increasing. In addition, the sensing response of the rGO/SnO2/Au composites is even three times superior than rGO/ SnO2 composites to 100 ppm HCHO. Fig. 7(b) is the logarithmic rela tionship between the sensing properties of products and the HCHO concentration. According to the analysis, both of products possess a good linear fitting with the HCHO concentration (1–100 ppm).
Table 2 Gas-sensing properties toward HCHO based on various metal oxide gas sensors. Sensing materials
T (� C)
TEA (ppm)
Response (Ra/Rg)
Res/ Rec (s)
Ref
SnO2 nanosheets Graphene/ZnO nanosheets In2O3@RGO heterostructures Au@In2O3 Ga/ZnO nanocomposites SnO2 nanosheets
240 200
100 100
7 12
8/18 3/5
[30] [31]
225
100
1.8
22/43
[32]
200 400
100 205
17 13
12/66 14/12
[33] [34]
320
100
6.5
2/12
220
100
12.4
2/7
200
100
32.7
2/5
this work this work this work
SnO2/rGO composites SnO2/rGO/Au composites
ppm HCHO. According to Fig. 6(a), the operating temperature ranges from 140 � C to 360 � C. The sensing properties of SnO2 nanosheets, rGO/ SnO2 and rGO/SnO2/Au composites are on the rise by increasing the operating temperature before the optimum one. One reason can explain this phenomenon is that high working temperature provides more en ergy to produce excitation electrons that contributes to the adsorption of the reducing gas and oxygen species form SnO2 surface. When the temperature excels the optimum one, the value response of products decrease a lot, which contributes to the HCHO molecules are much easier to be desorbed from the surface of SnO2 [29]. The operating temperatures of SnO2 nanosheets, rGO/SnO2 and rGO/SnO2/Au com posites are 320 � C, 220 � C and 200 � C, which are referred to the optimal sensitivity 6.5, 12.4 and 32.7, respectively (Fig. 6(a)). From this 6
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Fig. 8. Schematic diagram of possible gas sensing mechanisms on Au nanoparticles-functionalized SnO2/rGO nanocomposites.
Otherwise, the slopes of rGO/SnO2 and rGO/SnO2/Au composites are 0.391 and 0.567, respectively. Although the slope of rGO/SnO2/Au composites is lower than the SnO2/rGO nanocomposites, due to the higher value response (1 ppm, 6.5) of rGO/SnO2/Au composites than the rGO/SnO2 composites (1 ppm, 1.7) at the low concentration, implying that sensing properties in SnO2/rGO/Au composites are superior than the sample without the decoration of Au⋅In Fig. 7(c), the stability per formance of both rGO/SnO2 and rGO/SnO2/Au composites has been exhibited. It is clearly that, the sensing performances of both samples have little obvious variation after 3 cycles and the sensors maintain the fast response and recover to target gas at the optimum temperature. The sensors prepared by rGO/SnO2 and rGO/SnO2/Au composites are measured by 16 days with HCHO at the optimum temperature (Fig. 7 (d)). Obviously, the gas sensing performances of rGO/SnO2/Au com posites has less change during these days, which indicates that rGO/ SnO2/Au composites possess long-stability properties to HCHO. Corre spondingly, comparing the sensing performance with other metal oxide in formaldehyde, the result is summarized in Table 2 [30–34]. It is summarized that rGO/SnO2/Au composites exhibit lower operating temperature and superior sensing properties.
electrons back to conduction band of samples, which result in the thickness of electron depletion decreasing. Hence, the conduction of samples will recover to the original state before touching to the air. The particular reactions of this progress can be represented as follows [37, 38]: O2(ads) þ2e → 2O (ads)
(1)
HCHO þ 2O (ads) → CO2 þ H2Oþ 2e
(2)
After combining with rGO, the rGO/SnO2 composites still belong to the n-type sensing materials because of the SnO2 nanosheets play an importance role in sensing performance. The rGO possesses significant electron conduction [39–41] which enables electrons to spread quickly and can be easily caught by oxygen or texted gases. Therefore, the op timum temperature of rGO/SnO2 composites is lower than the pure SnO2 nanosheets. In addition, the heterojunction between SnO2 and rGO will be other positive factors to promote sensing properties of samples. The depth of depletion between rGO and SnO2 will change when sensors based on rGO/SnO2 composites exposing to different atmosphere (air and targeted gas) and in the heterojunction, the electrons transmission can facilitate the gas detection by changing about electrical conductivity [42]. To further promote the sensing performance of rGO/SnO2 compos ites, noble metal has been introduced by acting as active catalysts. From the result of HCHO sensing measurement, it is apparent that the sensi tivity of samples improved greatly after adding some Au NPs. At the same time, the optimum temperature of rGO/SnO2/Au composites also decreased 20 � C. The contribution of Au NPs can be attributed to two aspects. Firstly, due to the catalytic of Au NPs, the dissociation of mo lecular oxygen can be easy to proceed and the diffusion rate of absorbed species has been enhanced as well [43,44]. Otherwise, from this reason, more oxygen molecules were absorbed easily by the surface of the SnO2 nanosheets and the resistance of rGO/SnO2/Au composites further increased which results in the significant sensing performance [45,46].
3.2. Gas sensing mechanism As is known to all, the pure SnO2 nanosheets are typical n-type semiconductors and the SnO2 sensing material belongs to the surfacecontrolled type. Therefore, the conductivity of SnO2 is greatly influ enced by the surface depletion layer and the resistance of samples will change by exposing to the atmosphere of air and targeted gas. In the condition of air, the sensors based on SnO2 nanosheets will absorbed oxygen from the air and oxygen molecules will extract electrons from condition band to generate oxygen ions (O2 ,O ,O2 ) [35,36]. The electron depletion layer will be formed because of the electrons’ defi ciency at the surface. After exposed to the atmosphere of HCHO, the oxygen ions will react with HCHO molecules accompanied by releasing 7
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Secondly, the work function between SnO2 and Au is another positive aspect for promoting sensing performance of rGO/SnO2/Au composites. As is known to all, the work function of SnO2 and Au is 4.5 eV and 5.1eV [47,48], respectively, and the Schottky junctions would form at the interface. The existence of Schottky junctions enable electrons to transfer from SnO2 nanosheets to Au NPs, just like in Fig. 8, which re sults in the electron depletion areas become much thicker than the samples without Au [49,50] and the resistance of SnO2/rGO/Au com posites (Rair) would further increase in air. Therefore, according to the formula of (Rair/Rgas ¼ response), the sensitivity of rGO/SnO2/Au composites is higher than the SnO2/rGO nanocomposites.
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4. Conclusion In summary, we synthesized the rGO/SnO2/Au composite by a twostep approach (a facile hydrothermal reaction and subsequent in situ reducing process) and researched their morphology through a series of characterizations. According to the figures of SEM and TEM, Au nano particles successfully grew uniformly on the surface of SnO2/rGO nanocomposites with smaller size. And the rGO/SnO2/Au composite exhibits superior sensing properties in HCHO to SnO2/rGO nano composites and pure peony-like SnO2. Meanwhile, the working tem perature of SnO2 nanosheets decreases 100 � C by introducing GO, which verifies that the introduction of grapheme can be an effective method to reduce operating temperature of nanocomposites. At the same time, the sensing response of SnO2/rGO nanocomposites promotes from 12.4 to 32.7 with the addition of Au NPs. The existence of rGO relieves the aggregation phenomenon, and Au NPs decorated on the surface of samples provide a number of active absorption, which makes a huge contribution to sensing properties in HCHO. Author statement No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. In this work, ternary composites composed of reduced graphene oxide, SnO2 nanosheets, and Au nanoparticles were successfully syn thesized via a facile two-step approach. Gas sensors based on ternary composites exhibited enhanced gas-sensing performance toward form aldehyde in terms of lower operating temperature, high sensor response, and good selectivity. The enhanced sensing properties could mainly be attributed to the synergistic effect of ohmic contact between rGO and SnO2 nanosheets, high surface area and strong gas adsorption capacity of sheet-on-sheet heterostructured architectures, and the catalytic effect of Au nanoparticles. Acknowledgement This work was financially supported by National Natural Science Foundation of China (No. 61102006), and Natural Science Foundation of Shandong Province, China (No. ZR2018LE006 and ZR2015EM019). References [1] D. Yan, P.C. Xu, Q. Xiang, H.R. Mou, J.Q. Xu, W.J. Wen, X.X. Li, Y. Zhang, Polydopamine nanotubes: bio-inspired synthesis, formaldehyde sensing properties and thermodynamic investigation, J. Mater. Chem. A 4 (2016) 3487–3493. [2] H. Kudo, Y. Suzuki, T. Gessei, Biochemical gas sensor (bio-sniffer) for ultrahighsensitive gaseous HCHO monitoring, Biosens. Bioelectron. 26 (2010) 854–858. [3] L. He, X. Yang, M. Ren, An ultra-fast illuminating fluorescent probe for monitoring HCHO in living cells, shiitake mushrooms, and indoors, Chem. Can. 52 (2016) 9582–9585. [4] Y. Li, N. Chen, D. Deng, HCHO detection: SnO2, microspheres for HCHO gas sensor with high sensitivity, fast response/recovery and good selectivity, Sens. Actuators B Chem. 238 (2017) 264–273.
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