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graphene composite fabricated via a hydrothermal method for gas sensors working at room temperature
Sensors and Actuators B 173 (2012) 139–147
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Tin oxide/graphene composite fabricated via a hydrothermal method for gas sensors working at room temperature Qianqian Lin, Yang Li ∗ , Mujie Yang Department of Polymer Science and Engineering, Cyrus Tang Center for Sensor Materials and Applications, Zhejiang University, MOE Key Laboratory of Macromolecule Synthesis and Functionalization, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 30 March 2012 Received in revised form 19 June 2012 Accepted 21 June 2012 Available online 8 August 2012 Keywords: Gas sensor Tin oxide Graphene Composite Hydrothermal method Room temperature sensing
a b s t r a c t SnO2 /graphene (GN) composite was fabricated via a simple one-pot hydrothermal method with graphene oxide (GO) and SnCl2 as the precursors. The composite was characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray diffraction patterns, scanning electron microscopy and high resolution transmittance electron microscopy. It exhibited 3D nanostructure in which flower-like microspheres consisting of SnO2 nanoflakes distributed among GN layers decorated with tiny SnO2 nanoparticles, and was featured with high surface area (94.9 m2 /g). GO is supposed to act as a template in the hydrothermal process, promoting the preferential growth of SnO2 nanocrystals and preventing the agglomeration of SnO2 nanoparticles. NH3 sensing characteristics of the composite at room temperature were investigated, and found to closely relate to its composition and structure. Under optimal conditions, the composite displayed high response magnitude (15.9% for 50 ppm NH3 ), fast response (response and recovery time < 1 min), good reversibility and repeatability. Moreover, it exhibited small temperature coefficients in the range of 15–45 ◦ C. The ability of detecting gas at good room temperature of the composite is proposed to relate to its high specific surface, good conductivity of GN and interactions between GN and SnO2 . © 2012 Elsevier B.V. All rights reserved.
1. Introduction SnO2 is one of the promising inorganic semiconductor gas sensing materials with good gas response to various types of toxic gases and organic vapors [1–5]. At present, SnO2 based gas sensors have found wide applications in process control, environmental monitoring, etc. However, such sensors usually work at high temperatures, which leads to great power consumption and has adverse effect on the long-term stability [6,7]. Moreover, high working temperature may bring about safety problem for measurements in environment where explosive gases might exist. Much effort has been devoted to solving the problem which is common to inorganic gas sensing materials [8–11]. In recent years, there are a few reports on SnO2 sensor with the capability of room temperature sensing, which is usually realized by mixing with noble metals [12] or forming nanocomposite with carbon nanotubes [6,13], inorganic semiconductors [14,15], or conducting polymers [16]. The high specific surface area of nanomaterials and synergetic effects of the constituent components is usually proposed to play an important role in achieving good gas response at room temperature.
∗ Corresponding author. Tel.: +86 571 87952444; fax: +86 571 87952444. E-mail address: [email protected] (Y. Li). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.06.055
Since its discovery in 2004, graphene (GN), which is a 2D nanomaterial with good electronic conductivity and outstanding mechanical properties, received much attention worldwide [17,18], In particular, there have been a number of reports on its nanocomposite with various metal oxides [19–21]. Such a heterostructured nanocomposite not only prevents the irreversible restacking of GN and agglomeration of metal oxide nanoparticles, but also brings about new physical and chemical properties. There have been reports on the composite of GN and SnO2 with improved electrochemical properties and potential applications in lithiumion batteries [21]. GN decorated with SnO2 nanoparticles and SnO2 nanorods aligned on GN nanosheets were employed as cataluminescence and amperometric gas sensors, respectively [22,23]. Both composites showed better gas sensing characteristics than the constituent components at elevated temperatures. A very recent paper revealed that GN and Cu2 O nanocomposite could detect NO2 of low concentration even at room temperature [24]. Inspired by the advanced physical and chemical properties of GN/metal oxide composite, in this paper, we prepared SnO2 /GN composite by a simple one-pot hydrothermal procedure. It was found that the preferential growth of SnO2 nanocrystals along (1 0 1) plane took place not only on the surface of GN nanosheets but also among the GN lamellars, obtaining a 3D nanostructure. The composite was employed to construct a resistive-type gas sensor and its room temperature gas sensing properties to low
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concentration NH3 were investigated. The composite was featured with high response magnitude, fast response and recovery, and good repeatability in room temperature sensing of NH3 . The sensing characteristics are dependent on the composition and structure of the composite, which could be easily modulated by changing the ratio of SnCl2 to graphene oxide (GO) in the precursor solution. The enhanced gas sensing properties of the composite suggests its potential in the development of high performance gas sensors working at room temperature. 2. Experimental 2.1. Materials Graphite flake (45 m) was obtained from Alfa Aesar. H2 SO4 (98%) was purchased from Quzhou Juhua Reagent Co., Ltd. P2 O5 was obtained from Shanghai Lingfeng Chemical Reagent Co., Ltd. KMnO4 was purchased from Hangzhou Xiaoshan Reagent Co., Ltd. Hydrochloric acid (36–38%) was obtained from Hangzhou Chemical Reagent Co., Ltd. H2 O2 (30%) was obtained from Linan Lanling Reagent Co., Ltd. SnCl2 ·2H2 O and urea were purchased from Sinopharm Chemical Reagent Co., Ltd. All the chemicals were used as received. 2.2. Preparation of SnO2 /GN composite Graphene oxide (GO) was prepared by a modified Hummers’ method as described in reference [25]. SnO2 /GN composite was prepared via a hydrothermal method with SnCl2 and GO as the precursors [22]. In a typical process, 50 mg of GO was dispersed in 50 mL of deionized water by ultrasonication for 2 h. Then 2 mL of the GO solution (1 mg/mL) was mixed with 18 mL of deionized water by magnetic stirring for 30 min, followed by addition of 0.45 g of SnCl2 ·H2 O and 0.36 g of urea under continuous stirring (urea decomposed slowly in solution and liberated NH3 , which would neutralize HCl and promote the conversion of SnCl2 to SnO2 in the hydrothermal synthesis [22]). The resulting mixture was treated by ultrasonication for 30 min. Afterwards 30 L of hydrochloric acid (36–38%) was added and magnetically stirred for 30 min, obtaining a translucent brown solution. The solution was then transferred into a 50 mL of Teflon-lined stainless-steel autoclave and the reaction system was kept at 120 ◦ C for 8 h and naturally cooled to room temperature, attaining a suspension of SnO2 /GN nanocomposite with little precipitation. For comparison, SnO2 was prepared using the same method in the absence of GO, and GN was obtained by putting the GO solution in the autoclave at 120 ◦ C for 8 h. 2.3. Characterization Fourier transform infrared spectra (FT-IR) were recorded using a Bruker Vector 22 in the range of 4000 cm−1 to 400 cm−1 . Raman spectra were obtained on RS JDbin-yvon LabRamHRUV system with a 514 nm wavelength laser in the range from 3500 cm−1 to 300 cm−1 . The morphologies of GO, GN, SnO2 and SnO2 /GN nanocomposite were observed using field-emission scanning electron microscopy (FESEM, Hitachi S4800) with an accelerating voltage of 3 kV. X-ray diffraction (XRD) patterns were recorded on a ˚ radiation with PAN analytical X’Pert PRO using CuK␣ ( = 1.5418 A) a 2 scanning range of 5–80◦ . High resolution transmission electron microscopy (HRTEM) images were obtained from a Fecnai F20 with an accelerating voltage of 200 kV. Brunauer–Emmett–Teller (BET) surface area and pore diameter distribution measurements were performed using an ASIC-2 gas adsorption analyzer (N2 as absorbate, operation temperature: −196 ◦ C).
Fig. 1. Photos of (a) GO and (b–e) reduction of GO via a hydrothermal method at 120 ◦ C for different times (b) 2 h, (c) 4 h, (d) 6 h and (e) 8 h. For all the samples, [GO] = 0.1 mg/mL.
2.4. Fabrication of gas sensors and measurements of sensing properties As-prepared SnO2 /GN suspension was drop-coated onto the interdigitated gold electrodes, followed by heating at 100 ◦ C for 2 h to obtain resistive-type NH3 sensors. The size of the interdigitated electrodes with a ceramic substrate was 12 mm × 5 mm × 0.5 mm, and both the width and gaps of the 13 pairs of gold tracks on the electrode were 40 m. For comparison, sensors based on GO, GN and SnO2 were fabricated by depositing GO solution or hydrothermally prepared suspensions of GN and SnO2 on the electrodes and subsequent heat treatment at 120 ◦ C for 2 h. Gas sensing properties were investigated by recording online the impedance of the gas sensors in N2 flow or NH3 gas of different concentrations at room temperature. The sensors were placed in a testing box inside a temperature-humidity cyclic chamber (Shanghai Shangqun Technology Co., Ltd.) and the measurements were performed at constant temperatures from 15 ◦ C to 45 ◦ C. The gas flow rate was maintained at 0.5 L/min by two computer-driven mass flow controllers (MFC CS200A, Beijing Seven Star Electronics Co., Ltd.). NH3 gas of different concentrations were obtained by diluting standard NH3 gas (NH3 in N2 , Hangzhou New Century Gas Co., Hangzhou) with N2 . The impedance of the sensors was obtained using a computer data acquisition system with an applied voltage of 2.5 V (f = 1 kHz). The response magnitude of the sensors is defined as S = ((ZNH3 − Zcarrier gas )/Zcarrier gas ) × 100%, where ZNH3 and Zcarrier gas are the impedance in NH3 gas and carrier gas, respectively. The response time and recovery time of sensors are defined as the time to reach 90% of total impedance change. 3. Results and discussion 3.1. Characterization of SnO2 /GN nanocomposite Hydrothermal procedure is known as an efficient method for preparation of nanostructured metal oxides [26–28]. Recently, it was employed to fabricate nanocomposite of GN with SnO2 , Cu2 O, etc. Depending on the precursors used and the hydrothermal procedures, the obtained nanocomposites displayed different morphology and structures [21–23]. In this work, we employed SnCl2 and GO as precursors to prepare nanocomposites via a hydrothermal procedure at 120 ◦ C for 8 h. Urea was employed in the synthesis, which decomposed slowly in solution and liberated NH3 to neutralize HCl and promote the conversion of SnCl2 to SnO2 in the hydrothermal process [22]. Interestingly it was found that GO could be directly reduced to GN from its aqueous solution under hydrothermal conditions (high pressure and high temperature) without any reducing agent or additives, as indicated by the color changes of the aqueous solutions of GO from light brown to dark black with the extension of reaction time in Fig. 1, which supported the successful reduction of GO to GN by the hydrothermal method
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Fig. 2. FT-IR spectra of GO and GN obtained by reduction of GO under hydrothermal conditions (T = 120 ◦ C, t = 8 h).
Fig. 3. Raman spectra of GN, SnO2 and SnO2 /GN composite (precursor solution: SnCl2 /GO = 0.1 mmol/0.1 mg).
Fig. 5. SEM images of (a) GO, (b) GN reduced by a hydrothermal method (T = 120 ◦ C, t = 8 h) and (c) SnO2 prepared by a hydrothermal method (T = 120 ◦ C, t = 8 h).
Fig. 4. XRD patterns of (a) SnO2 (SnCl2 : 0.1 mmol/L), (b) SnO2 /GN (precursor solution: SnCl2 /GO = 0.1 mmol/0.5 mg), (c) SnO2 /GN (precursor solution: SnCl2 /GO = 0.1 mmol/0.25 mg), (d) SnO2 /GN (precursor solution: SnCl2 /GO = 0.1 mmol/0.15 mg) and (e) SnO2 /GN composite (precursor solution: SnCl2 /GO = 0.1 mmol/0.1 mg); all the samples were prepared by a hydrothermal method at 120 ◦ C for 8 h.
[29]. Moreover, the comparison of FT-IR spectra of GO and GN in Fig. 2 reveals the great decrease in the intensity of characteristic absorption peaks of various oxygen-containing groups such as O H, C O and C O after the hydrothermal treatment, which supported the successful reduction of GO to GN [29]. According to Lv et al., during the hydrothermal process, GO was in situ reduced to GN while SnCl2 was oxidized to obtain SnO2 [22]. However, the fact that GO could be directly reduced to GN without any reducing agent under hydrothermal conditions suggests there may be a different mechanism which is unclear till now. Raman spectra of GN, SnO2 and SnO2 /GN composite are shown in Fig. 3. In the spectrum of GN, two characteristic peaks at
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Fig. 6. SEM images of (a–d) SnO2 /GN and (e) cross-section of electrode coated with SnO2 /GN (the composite was prepared via a hydrothermal method at 120 ◦ C for 8 h; precursor solution: SnCl2 /GO = 0.1 mmol/0.1 mg).
1350 cm−1 and 1580 cm−1 are observed, corresponding to D band and G band, respectively [30]. Meantime, the spectrum of SnO2 displays only some weak peaks in the range of 500–800 cm−1 . Both characteristic peaks of GN and SnO2 are clearly seen in the spectrum of SnO2 /GN composite, which further proves that SnO2 /GN composite has been successfully fabricated via the hydrothermal method. XRD patterns were used to characterize the crystalline structure of SnO2 and SnO2 /GN composite prepared by the hydrothermal method, and the results are depicted in Fig. 4. It is seen that all diffraction peaks of SnO2 indicate a rutile structure (JCPDS card no. 41-1445). For the SnO2 /GN composites, by decreasing ratios of GO to SnCl2 in the precursor solution from 0.1 mmol/0.5 mg to 0.1 mmol/0.1 mg, the XRD patterns of the resulting composite exhibit an ever weakened diffraction peaks of (1 1 0) and (2 1 1) plane, while the diffraction peak of (0 0 2) plane is gradually strengthened. In particular, the diffraction peak attributed to (1 0 1) plane is enhanced significantly. It is known that GO contains some oxygen containing functional groups, such as hydroxyl group and carboxyl group. Thus it is proposed that, under the hydrothermal conditions, there might exist some interactions between the functional groups on GO and SnCl2 , which promotes the preferential growth of SnO2 along (1 0 1) plane on the reduced GO templates. However, the growth of SnO2 in the absence of GO or in the presence of excessive amount of GO could weaken or destroy the orientated growth of SnO2 along (1 0 1) plane. The underlying reasons need further investigation. Morphologies of GO, GN, SnO2 and SnO2 /GN composites were observed by SEM. It is seen in Fig. 5 that both GO and GN obtained by the hydrothermal method were curled nanosheets, while SnO2 prepared via the hydrothermal growth were large square-shaped crystals sizing ranging from several tens to
hundreds of micrometers, which could be observed even by naked eyes. In comparison, hydrothermally fabricated SnO2 /GN composite (SnCl2 /GO = 0.1 mmol/0.1 mg) displayed a much more elaborate structure. It is observed that the composite shows a 3D nanostructure in which radial flower-like SnO2 microspheres distribute among the curled ultrathin GN lamellars (Fig. 6a). A close observation reveals that the SnO2 microspheres are composed of assembled ultrathin SnO2 nanoflakes with length of several tens of nanometers which fill the interplace of GN nanosheets (Fig. 6b and d). Moreover, it is seen from Fig. 6c that all the GN nanosheets were decorated with plenty of tiny SnO2 nanoparticles, which is more apparent in the HRTEM images in Fig. 7. The SEM image of cross-section of gas sensor based on the composite is shown in Fig. 6e. The dense region of the SEM image is the ceramic substrate of the electrode and the lamellar on the substrate is the SnO2 /GN composite. The crystallinity and morphology of SnO2 /GN composite was further characterized by HRTEM as shown in Fig. 7. From Fig. 7a and b, it can be seen that GN nanosheets are decorated with plenty of SnO2 nanocrystals. Fig. 7c displays the magnified HRTEM image of the tiny SnO2 nanocrystals (diameters < 5 nm) with adjacent fringe spacing of 0.26 nm, corresponding to (1 0 1) plane of SnO2 crystal. Selected-area electron diffraction (SAED) pattern of SnO2 crystal in Fig. 7c is illustrated in Fig. 7d, which shows typical diffraction rings of rutile SnO2 crystal. The architecture of the SnO2 /GN composite was explored by nitrogen adsorption and desorption measurements. In the isotherm adsorption–desorption curve shown in Fig. 8, a distinct hysteresis loop can be seen at relative pressures of 0.4–1.0. It suggested the presence of a mesoporous structure and relatively high surface area. The composite exhibited very high Brunauer–Emmett–Teller (BET) surface area of 94.9 m2 /g as determined from the four data points at low relative pressure (Fig. 8, inset). Moreover, it shows an average pore
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Fig. 7. HRTEM images and SAED pattern of SnO2 /GN (the composite was prepared via a hydrothermal method at 120 ◦ C for 8 h; precursor solution: SnCl2 /GO = 0.1 mmol/0.1 mg).
Fig. 8. Isothermal adsorption/desorption curve (N2 at 150 ◦ C) of SnO2 /GN; inset: calculation of BET surface area by four data points in low relative pressure (the composite was prepared via a hydrothermal method at 120 ◦ C for 8 h; precursor solution: SnCl2 /GO = 0.1 mmol/0.1 mg).
Fig. 9. Pore diameter distribution of SnO2 /GN composite fabricated by a hydrothermal method (T = 120 ◦ C, t = 8 h; precursor solution: SnCl2 /GO = 0.1 mmol/0.1 mg).
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Fig. 10. Photos of (a–c) SnO2 /GN gas sensors (precursor solution: (a) (b) SnCl2 /GO = 0.1 mmol/0.1 mg; (c) SnCl2 : SnCl2 /GO = 0.1 mmol/0.5 mg; 0.1 mmol/mL) and (d) blank interdigitated gold electrode.
diameter of 6.11 nm and total pore volume of 0.142 cm3 /g as calculated by the Barrett–Joyner–Halenda method (Fig. 9). The results proves that the formation of 3D nanostructures of the composite has significantly enlarged the surface area and pore volume, which will greatly facilitate the adsorption and transport of gases, thus leading to enhancement in response magnitude. 3.2. Gas sensing properties NH3 gas is a toxic gas widely existing in agricultural, environmental, automotive, as well as medical and industrial chemistry areas [31–34]. Therefore detection of NH3 is urgently needed. SnO2
Fig. 11. Response to 50 ppm of the sensors based on (a) GN, (b) GO, (c) SnO2 /GN (precursor solution: SnCl2 /GO = 0.1 mmol/1 mg), (d) SnO2 /GN (precursor solution: SnCl2 /GO = 0.1 mmol/0.5 mg), (e) SnO2 /GN (precursor solution: SnCl2 /GO = 0.1 mmol/0.25 mg), (f) SnO2 /GN (precursor solution: SnCl2 /GO = 0.1 mmol/0.15 mg) and (g) SnO2 /GN (precursor solution: SnCl2 /GO = 0.1 mmol/0.1 mg) (15 ◦ C, N2 as the carrier gas for all the measurements). All the samples (except GO) were prepared by a hydrothermal method at 120 ◦ C for 8 h.
could be used to detect NH3 gas with good gas response, but mainly at high operation temperatures. In this work, we examined NH3 sensing properties of the as-prepared SnO2 /GN composite at room temperature. It was found that SnO2 alone exhibited poor adherence to the interdigitated gold electrode, while SnO2 /GN could adhere well to the electrode. Moreover, with the increase in the amount of GN, the composite exhibited improved adherence to
Fig. 12. Gas sensing properties to NH3 of the sensor based on SnO2 /GN (precursor solution: SnCl2 /GO = 0.1 mmol/0.1 mg). (a) Dynamic response to NH3 with N2 refreshing after each exposure to NH3 , (b) dynamic response to NH3 without N2 refreshing, (c) response magnitude vs. concentration of NH3 and (d) dynamic response to 50 ppm NH3 and N2 during cyclic tests (15 ◦ C, N2 as the carrier gas for all the measurements).
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Fig. 13. NH3 sensing properties of the sensor based on SnO2 /GN (precursor solution: SnCl2 /GO = 0.1 mmol/0.1 mg) at different temperatures: (a) 15 ◦ C, (b) 25 ◦ C, (c) 35 ◦ C and (d) 45 ◦ C (N2 as the carrier gas for all the measurements).
Fig. 14. Response of SnO2 /GN composite sensor (precursor solution: SnCl2 /GO = 0.1 mmol/0.1 mg) to 50 ppm NH3 in the presence of water vapors (28%RH, 28.0 ◦ C, compressed air with approximately the same humidity and temperature as the carrier gas).
Fig. 15. Response of SnO2 /GN composite sensor (precursor solution: SnCl2 /GO = 0.1 mmol/0.1 mg) to 50 ppm NH3 in the presence of water vapors (25%RH, 23.5 ◦ C, N2 with approximately the same humidity and temperature as the carrier gas).
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the electrode as shown in Fig. 10. Typical gas response curves of resistive-type gas sensors based on GO, GN and SnO2 /GN composites towards NH3 gas of 50 ppm at 15 ◦ C are illustrated in Fig. 11. GO and GN based sensor exhibited impedance of ∼400 k and 15 k, respectively. It is found that their impedance decreased on exposure to NH3 , exhibiting a small negative response of −3.3% and −2.7%, respectively. By contrast, the sensor based on SnO2 crystals hydrothermally grown in the absence of GO showed very high impedance (>10 M) and no detectable response to NH3 gas at room temperature. The SnO2 /GN composites prepared from precursor solutions with different ratios of SnCl2 to GO exhibited impedance ranging from 100 k to 400 k, which increased with decrease in the ratio of GO. Obviously, the inclusion of GN in the composite significantly enhanced the conductivity of the composites. In contrary to sensors based on GO or GN, the composite sensors displayed impedance increase when in contact with NH3 , exhibiting positive responses. Moreover, the response magnitude was enhanced with the decrease in the ratio of GO in the precursor solution. The sensor obtained with a precursor solution of 0.1 mmol SnCl2 /0.1 mg GO achieved the highest response magnitude of 15.9% to NH3 gas of 50 ppm at room temperature, which is employed in following investigations on sensing properties. Further decreasing the ratio of GO in the precursor solution (for example, 0.1 mmol SnCl2 /0.05 mg GO), the composite sensor showed too high impedance (>10 M) and no electrical response to NH3 gas at room temperature. The dynamic response to NH3 gas of different concentrations of the composite sensor was investigated and the results are shown in Fig. 12. The composite sensor was exposed to NH3 gas of 10–50 ppm then refreshed with N2 (Fig. 12a). It can be seen that the impedance of the sensor quickly increases when it is in contact with NH3 . By flushing with N2 , its impedance returns to the initial baseline values in a short time. This suggests good reversibility of the composite in NH3 sensing at room temperature. Moreover, both the response and recovery times are short, less than 1 min. Fig. 12b presents the response of the composite sensor to NH3 gas with step increase or decrease in the gas concentrations. It is clearly seen that the sensor displays fast and reversible responses to NH3 gas of various concentrations even without refreshing with N2 . The response magnitude of the composite sensor is plotted against gas concentration as shown in Fig. 12c, which reveals an exponential trend and indicates that its response magnitude tends to saturate at higher concentration of NH3 . The dynamic response of the composite sensor to periodical changes between NH3 gas of 50 ppm and N2 is illustrated in Fig. 12c. It is seen that the sensor registers good repeatability during the cycle tests. The effect of temperature on the NH3 sensing of the composite was also examined and the results are shown in Fig. 13. It is found that the response magnitude decreases with the increase in the temperature. The temperature coefficient is defined as ˛ = (S2 − S1 )/(T2 − T1 ), where S1 and S2 are the response magnitude at T1 and T2 , respectively. It is determined that ˛ is as small as 0.1%/◦ C and 2.2%/◦ C for NH3 gas of 10 ppm and 50 ppm, respectively. Compared with its good response magnitude at 15 ◦ C (5.9% for 10 ppm NH3 and 15.9% for 50 ppm NH3 ), the composite sensor exhibits a small temperature coefficient in the range of 15–45 ◦ C. It is believed that water molecules could affect the gas response of SnO2 /GN composite, since water molecules could compete with the gas in the adsorption process and the adsorbed water molecules would dissociate and lead to decreased impedance of SnO2 , which can be employed as a humidity sensitive material [35]. We investigated the response of the composite to NH3 in both N2 and air atmosphere in the presence of water vapors, and the results are shown in Figs. 14 and 15. It is found that the composite could detect NH3 in air (Fig. 14). However, water vapors have different effect on the response of the composite to NH3 in air and in N2 . In air, the response was much lower, but both the response and
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Moreover, highly conductive GN enhanced the conductivity of the composite so that the conductivity change can be easily detected, as in the case of the composite of carbon nanotubes and discrete SnO2 [13]. The composite would display higher gas response to NH3 due to much increased surface accessibility and higher conductivity. Furthermore, there must be special interactions between GN and SnO2 which resulted in much improved gas sensing behaviors of the composite at room temperature. Much more work is needed to clarify the sensing mechanism. 4. Conclusions
Fig. 16. Response of SnO2 /GN composite sensor to humidity (21.6 ◦ C, saturated salt solutions are used as the humidity sources: LiCl for 11%RH, MgCl2 for 33%RH, K2 CO3 for 43%RH, NaBr for 57%RH and NaCl for 75%RH). The measurements were carried out in air atmosphere.
recovery processes were much accelerated. The detailed explanations need more systematic investigations in future work. Fig. 16 presents the electrical response to humidity of the composite. Here we define the response of the composite sensor to humidity as SRH = ((ZRH − Z11%RH )/Z11%RH ) × 100%, where ZRH and Z11%RH are the impedance at tested humidity levels and 11%RH, respectively. It is clearly seen that the composite exhibited negative response towards water vapors. Therefore, it is necessary to consider the effect of water vapors while developing the SnO2 /GN composite gas sensors. The above experimental results reveal that the hydrothermally prepared SnO2 /GN composites are featured with good NH3 sensing properties at room temperature, including good response magnitude, high reversibility, fast response and recovery, good repeatability and small temperature coefficient. Obviously, the composite exhibits much enhanced room temperature sensing characteristics than their constituents alone. However, it is found that the sensing behaviors are different from the literature reports. It is known that SnO2 is an n-type semiconductor, and its interactions with oxidizing and reducing gases would lead to decreased and increased conductivity, respectively. Moreover, it was reported that SnO2 /GN composite displayed increased conductivity while in contact with reducing gases such as H2 S and NH3 at high temperature [23]. Wei et al. found that single-walled carbon nanotubes (SWCNTs) and SnO2 exhibited decreased conductivity while detecting oxidizing NO2 at room temperature, and attributed the room temperature gas response to the establishment of p/n junction between p-type SWCNTs and SnO2 [36]. Deng et al. found that the composite of p-type GN and p-type Cu2 O showed higher conductivity while in contact with NO2 [24]. On the contrary, Van Hieu et al. prepared a composite of multi-walled carbon nanotubes (MWCNTs) and SnO2 by simple ultrasonication-assisted solution blending and subsequent heat treatment, and found that it exhibited increased resistance towards NH3 at room temperature. They also attributed the unusual gas response to the establishment of p/n junction [6]. Obviously, the reported results of the SnO2 composite sensing materials were in disagreement with each other. In our case, the exact sensing mechanism of the composite towards NH3 at room temperature is still unclear. However, one thing is certain: GN played a crucial role in bestowing the composite with electrical response to NH3 at room temperature, since SnO2 alone exhibited too low conductivity and did not display detectable response to NH3 at room temperature. The introduction of GN led to the formation of a composite with 3D nanostructure featured with much higher surface accessibility (BET surface area of ∼100 m2 /g), which greatly facilitated the adsorption and diffusion of NH3 molecules.
Composite of SnO2 /GN could be facilely fabricated via a hydrothermal procedure with SnCl2 and GO as the precursor. The resulting composite exhibited a 3D network nanostructure, in which GN lamellas were separated by radial flower-like SnO2 microspheres. Moreover, the GN nanosheets were decorated with plenty of tiny SnO2 nanoparticles. The existence of GN significantly increased the conductivity of the composite, and led to very high specific surface area of ∼100 m2 /g. Meantime, it promoted the preferential growth of SnO2 nanocrystals along (1 0 1) plane. Gas sensors based on the composite exhibited good gas sensing properties to NH3 of 10 to 50 ppm at room temperature, such as high response magnitude, fast response and recovery, good reversibility and repeatability. The much enhanced gas sensing properties of the composite at room temperature is proposed to relate to the high surface area, 3D nanostructure and special interactions between GN and SnO2 . Acknowledgements This work is financially supported by the National Natural Science Foundation of China (Contract no. 51073134), the Fundamental Research Funds for the Central Universities (Contract no. 2011QNA4024), National “863” program of China (Contract no. 2006AA10Z215) and Zhejiang Provincial Natural Science Foundation of China (Grant no. Y4080128). References [1] A. Lassesson, M. Schulze, J. van Lith, S.A. Brown, Tin oxide nanocluster hydrogen and ammonia sensors, Nanotechnology 19 (2008) 015502. [2] F. Gyger, M. Hubner, C. Feldmann, N. Barsan, U. Weimar, Nanoscale SnO2 hollow spheres and their application as a gas-sensing material, Chemistry of Materials 22 (2010) 4821–4827. [3] X.M. Yin, C.C. Li, M. Zhang, Q.Y. Hao, S. Liu, Q.H. Li, L.B. Chen, T.H. Wang, SnO2 monolayer porous hollow spheres as a gas sensor, Nanotechnology 20 (2009) 455503. [4] Z.W. Chen, D.Y. Pan, B. Zhao, G.J. Ding, Z. Jiao, M.H. Wu, C.H. Shek, L.C.M. Wu, J.K.L. Lai, Insight on fractal assessment strategies for tin dioxide thin films, ACS Nano 4 (2010) 1202–1208. [5] G.K. Fan, Y. Wang, M. Hu, Z.Y. Luo, G. Li, Synthesis of flowerlike nano-SnO2 and a study of its gas sensing response, Measurement Science and Technology 22 (2011) 045203. [6] N. Van Hieu, L.T.B. Thuy, N.D. Chien, Highly sensitive thin film NH3 gas sensor operating at room temperature based on SnO2 /MWCNTs composite, Sensors and Actuators B 129 (2008) 888–895. [7] N.V. Quy, V.A. Minh, N.V. Luan, V.N. Hung, N.V. Hieu, Gas sensing properties at room temperature of a quartz crystal microbalance coated with ZnO nanorods, Sensors and Actuators B 153 (2011) 188–193. [8] G. Rao, D.T. Rao, Gas sensitivity of ZnO based thick film sensor to NH3 at room temperature, Sensors and Actuators B 55 (1999) 166–169. [9] K.K. Khun, A. Mahajan, R.K. Bedi, SnO2 thick films for room temperature gas sensing applications, Journal of Applied Physics 106 (2009) 124509. [10] I. Singh, R.K. Bedi, Surfactant-assisted synthesis, characterizations, and room temperature ammonia sensing mechanism of nanocrystalline CuO, Solid State Science 13 (2011) 2011–2018. [11] M.A. Chougule, S.G. Pawar, S.L. Patil, B.T. Raut, P.R. Godse, S. Sen, V.B. Patil, Polypyrrole thin film: room temperature ammonia gas sensor, IEEE Sensors Journal 11 (2011) 2137–2141. [12] G. De, R. Kohn, G. Xomeritakis, C.J. Brinker, Nanocrystalline mesoporous palladium activated tin oxide thin films as room-temperature hydrogen gas sensors, Chemical Communications (2007) 1840–1842.
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Biographies Qianqian Lin is a postgraduate student in the Department of Polymer Science and Engineering, Zhejiang University. His research interests include polymeric chemical sensors. Yang Li received his Ph.D. degree in polymer chemistry and physics from Zhejiang University in 2000. He has been working in Department of Polymer Science and Engineering, Zhejiang University since 2000 and was appointed associate professor in polymer science in 2002. His research interests include polymer materials and organic/inorganic composites for chemical sensors. Mujie Yang graduated from Zhongshan University, China in 1963. She has been working in Zhejiang University since 1963. She was promoted to full professor in polymer science in 1992. Her research interests include functional polymers with optical and electrical characteristics.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Tin oxide/graphene composite fabricated via a hydrothermal method for gas sensors working at room temperature Qianqian Lin, Yang Li ∗ , Mujie Yang Department of Polymer Science and Engineering, Cyrus Tang Center for Sensor Materials and Applications, Zhejiang University, MOE Key Laboratory of Macromolecule Synthesis and Functionalization, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 30 March 2012 Received in revised form 19 June 2012 Accepted 21 June 2012 Available online 8 August 2012 Keywords: Gas sensor Tin oxide Graphene Composite Hydrothermal method Room temperature sensing
a b s t r a c t SnO2 /graphene (GN) composite was fabricated via a simple one-pot hydrothermal method with graphene oxide (GO) and SnCl2 as the precursors. The composite was characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray diffraction patterns, scanning electron microscopy and high resolution transmittance electron microscopy. It exhibited 3D nanostructure in which flower-like microspheres consisting of SnO2 nanoflakes distributed among GN layers decorated with tiny SnO2 nanoparticles, and was featured with high surface area (94.9 m2 /g). GO is supposed to act as a template in the hydrothermal process, promoting the preferential growth of SnO2 nanocrystals and preventing the agglomeration of SnO2 nanoparticles. NH3 sensing characteristics of the composite at room temperature were investigated, and found to closely relate to its composition and structure. Under optimal conditions, the composite displayed high response magnitude (15.9% for 50 ppm NH3 ), fast response (response and recovery time < 1 min), good reversibility and repeatability. Moreover, it exhibited small temperature coefficients in the range of 15–45 ◦ C. The ability of detecting gas at good room temperature of the composite is proposed to relate to its high specific surface, good conductivity of GN and interactions between GN and SnO2 . © 2012 Elsevier B.V. All rights reserved.
1. Introduction SnO2 is one of the promising inorganic semiconductor gas sensing materials with good gas response to various types of toxic gases and organic vapors [1–5]. At present, SnO2 based gas sensors have found wide applications in process control, environmental monitoring, etc. However, such sensors usually work at high temperatures, which leads to great power consumption and has adverse effect on the long-term stability [6,7]. Moreover, high working temperature may bring about safety problem for measurements in environment where explosive gases might exist. Much effort has been devoted to solving the problem which is common to inorganic gas sensing materials [8–11]. In recent years, there are a few reports on SnO2 sensor with the capability of room temperature sensing, which is usually realized by mixing with noble metals [12] or forming nanocomposite with carbon nanotubes [6,13], inorganic semiconductors [14,15], or conducting polymers [16]. The high specific surface area of nanomaterials and synergetic effects of the constituent components is usually proposed to play an important role in achieving good gas response at room temperature.
∗ Corresponding author. Tel.: +86 571 87952444; fax: +86 571 87952444. E-mail address: [email protected] (Y. Li). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.06.055
Since its discovery in 2004, graphene (GN), which is a 2D nanomaterial with good electronic conductivity and outstanding mechanical properties, received much attention worldwide [17,18], In particular, there have been a number of reports on its nanocomposite with various metal oxides [19–21]. Such a heterostructured nanocomposite not only prevents the irreversible restacking of GN and agglomeration of metal oxide nanoparticles, but also brings about new physical and chemical properties. There have been reports on the composite of GN and SnO2 with improved electrochemical properties and potential applications in lithiumion batteries [21]. GN decorated with SnO2 nanoparticles and SnO2 nanorods aligned on GN nanosheets were employed as cataluminescence and amperometric gas sensors, respectively [22,23]. Both composites showed better gas sensing characteristics than the constituent components at elevated temperatures. A very recent paper revealed that GN and Cu2 O nanocomposite could detect NO2 of low concentration even at room temperature [24]. Inspired by the advanced physical and chemical properties of GN/metal oxide composite, in this paper, we prepared SnO2 /GN composite by a simple one-pot hydrothermal procedure. It was found that the preferential growth of SnO2 nanocrystals along (1 0 1) plane took place not only on the surface of GN nanosheets but also among the GN lamellars, obtaining a 3D nanostructure. The composite was employed to construct a resistive-type gas sensor and its room temperature gas sensing properties to low
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concentration NH3 were investigated. The composite was featured with high response magnitude, fast response and recovery, and good repeatability in room temperature sensing of NH3 . The sensing characteristics are dependent on the composition and structure of the composite, which could be easily modulated by changing the ratio of SnCl2 to graphene oxide (GO) in the precursor solution. The enhanced gas sensing properties of the composite suggests its potential in the development of high performance gas sensors working at room temperature. 2. Experimental 2.1. Materials Graphite flake (45 m) was obtained from Alfa Aesar. H2 SO4 (98%) was purchased from Quzhou Juhua Reagent Co., Ltd. P2 O5 was obtained from Shanghai Lingfeng Chemical Reagent Co., Ltd. KMnO4 was purchased from Hangzhou Xiaoshan Reagent Co., Ltd. Hydrochloric acid (36–38%) was obtained from Hangzhou Chemical Reagent Co., Ltd. H2 O2 (30%) was obtained from Linan Lanling Reagent Co., Ltd. SnCl2 ·2H2 O and urea were purchased from Sinopharm Chemical Reagent Co., Ltd. All the chemicals were used as received. 2.2. Preparation of SnO2 /GN composite Graphene oxide (GO) was prepared by a modified Hummers’ method as described in reference [25]. SnO2 /GN composite was prepared via a hydrothermal method with SnCl2 and GO as the precursors [22]. In a typical process, 50 mg of GO was dispersed in 50 mL of deionized water by ultrasonication for 2 h. Then 2 mL of the GO solution (1 mg/mL) was mixed with 18 mL of deionized water by magnetic stirring for 30 min, followed by addition of 0.45 g of SnCl2 ·H2 O and 0.36 g of urea under continuous stirring (urea decomposed slowly in solution and liberated NH3 , which would neutralize HCl and promote the conversion of SnCl2 to SnO2 in the hydrothermal synthesis [22]). The resulting mixture was treated by ultrasonication for 30 min. Afterwards 30 L of hydrochloric acid (36–38%) was added and magnetically stirred for 30 min, obtaining a translucent brown solution. The solution was then transferred into a 50 mL of Teflon-lined stainless-steel autoclave and the reaction system was kept at 120 ◦ C for 8 h and naturally cooled to room temperature, attaining a suspension of SnO2 /GN nanocomposite with little precipitation. For comparison, SnO2 was prepared using the same method in the absence of GO, and GN was obtained by putting the GO solution in the autoclave at 120 ◦ C for 8 h. 2.3. Characterization Fourier transform infrared spectra (FT-IR) were recorded using a Bruker Vector 22 in the range of 4000 cm−1 to 400 cm−1 . Raman spectra were obtained on RS JDbin-yvon LabRamHRUV system with a 514 nm wavelength laser in the range from 3500 cm−1 to 300 cm−1 . The morphologies of GO, GN, SnO2 and SnO2 /GN nanocomposite were observed using field-emission scanning electron microscopy (FESEM, Hitachi S4800) with an accelerating voltage of 3 kV. X-ray diffraction (XRD) patterns were recorded on a ˚ radiation with PAN analytical X’Pert PRO using CuK␣ ( = 1.5418 A) a 2 scanning range of 5–80◦ . High resolution transmission electron microscopy (HRTEM) images were obtained from a Fecnai F20 with an accelerating voltage of 200 kV. Brunauer–Emmett–Teller (BET) surface area and pore diameter distribution measurements were performed using an ASIC-2 gas adsorption analyzer (N2 as absorbate, operation temperature: −196 ◦ C).
Fig. 1. Photos of (a) GO and (b–e) reduction of GO via a hydrothermal method at 120 ◦ C for different times (b) 2 h, (c) 4 h, (d) 6 h and (e) 8 h. For all the samples, [GO] = 0.1 mg/mL.
2.4. Fabrication of gas sensors and measurements of sensing properties As-prepared SnO2 /GN suspension was drop-coated onto the interdigitated gold electrodes, followed by heating at 100 ◦ C for 2 h to obtain resistive-type NH3 sensors. The size of the interdigitated electrodes with a ceramic substrate was 12 mm × 5 mm × 0.5 mm, and both the width and gaps of the 13 pairs of gold tracks on the electrode were 40 m. For comparison, sensors based on GO, GN and SnO2 were fabricated by depositing GO solution or hydrothermally prepared suspensions of GN and SnO2 on the electrodes and subsequent heat treatment at 120 ◦ C for 2 h. Gas sensing properties were investigated by recording online the impedance of the gas sensors in N2 flow or NH3 gas of different concentrations at room temperature. The sensors were placed in a testing box inside a temperature-humidity cyclic chamber (Shanghai Shangqun Technology Co., Ltd.) and the measurements were performed at constant temperatures from 15 ◦ C to 45 ◦ C. The gas flow rate was maintained at 0.5 L/min by two computer-driven mass flow controllers (MFC CS200A, Beijing Seven Star Electronics Co., Ltd.). NH3 gas of different concentrations were obtained by diluting standard NH3 gas (NH3 in N2 , Hangzhou New Century Gas Co., Hangzhou) with N2 . The impedance of the sensors was obtained using a computer data acquisition system with an applied voltage of 2.5 V (f = 1 kHz). The response magnitude of the sensors is defined as S = ((ZNH3 − Zcarrier gas )/Zcarrier gas ) × 100%, where ZNH3 and Zcarrier gas are the impedance in NH3 gas and carrier gas, respectively. The response time and recovery time of sensors are defined as the time to reach 90% of total impedance change. 3. Results and discussion 3.1. Characterization of SnO2 /GN nanocomposite Hydrothermal procedure is known as an efficient method for preparation of nanostructured metal oxides [26–28]. Recently, it was employed to fabricate nanocomposite of GN with SnO2 , Cu2 O, etc. Depending on the precursors used and the hydrothermal procedures, the obtained nanocomposites displayed different morphology and structures [21–23]. In this work, we employed SnCl2 and GO as precursors to prepare nanocomposites via a hydrothermal procedure at 120 ◦ C for 8 h. Urea was employed in the synthesis, which decomposed slowly in solution and liberated NH3 to neutralize HCl and promote the conversion of SnCl2 to SnO2 in the hydrothermal process [22]. Interestingly it was found that GO could be directly reduced to GN from its aqueous solution under hydrothermal conditions (high pressure and high temperature) without any reducing agent or additives, as indicated by the color changes of the aqueous solutions of GO from light brown to dark black with the extension of reaction time in Fig. 1, which supported the successful reduction of GO to GN by the hydrothermal method
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Fig. 2. FT-IR spectra of GO and GN obtained by reduction of GO under hydrothermal conditions (T = 120 ◦ C, t = 8 h).
Fig. 3. Raman spectra of GN, SnO2 and SnO2 /GN composite (precursor solution: SnCl2 /GO = 0.1 mmol/0.1 mg).
Fig. 5. SEM images of (a) GO, (b) GN reduced by a hydrothermal method (T = 120 ◦ C, t = 8 h) and (c) SnO2 prepared by a hydrothermal method (T = 120 ◦ C, t = 8 h).
Fig. 4. XRD patterns of (a) SnO2 (SnCl2 : 0.1 mmol/L), (b) SnO2 /GN (precursor solution: SnCl2 /GO = 0.1 mmol/0.5 mg), (c) SnO2 /GN (precursor solution: SnCl2 /GO = 0.1 mmol/0.25 mg), (d) SnO2 /GN (precursor solution: SnCl2 /GO = 0.1 mmol/0.15 mg) and (e) SnO2 /GN composite (precursor solution: SnCl2 /GO = 0.1 mmol/0.1 mg); all the samples were prepared by a hydrothermal method at 120 ◦ C for 8 h.
[29]. Moreover, the comparison of FT-IR spectra of GO and GN in Fig. 2 reveals the great decrease in the intensity of characteristic absorption peaks of various oxygen-containing groups such as O H, C O and C O after the hydrothermal treatment, which supported the successful reduction of GO to GN [29]. According to Lv et al., during the hydrothermal process, GO was in situ reduced to GN while SnCl2 was oxidized to obtain SnO2 [22]. However, the fact that GO could be directly reduced to GN without any reducing agent under hydrothermal conditions suggests there may be a different mechanism which is unclear till now. Raman spectra of GN, SnO2 and SnO2 /GN composite are shown in Fig. 3. In the spectrum of GN, two characteristic peaks at
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Fig. 6. SEM images of (a–d) SnO2 /GN and (e) cross-section of electrode coated with SnO2 /GN (the composite was prepared via a hydrothermal method at 120 ◦ C for 8 h; precursor solution: SnCl2 /GO = 0.1 mmol/0.1 mg).
1350 cm−1 and 1580 cm−1 are observed, corresponding to D band and G band, respectively [30]. Meantime, the spectrum of SnO2 displays only some weak peaks in the range of 500–800 cm−1 . Both characteristic peaks of GN and SnO2 are clearly seen in the spectrum of SnO2 /GN composite, which further proves that SnO2 /GN composite has been successfully fabricated via the hydrothermal method. XRD patterns were used to characterize the crystalline structure of SnO2 and SnO2 /GN composite prepared by the hydrothermal method, and the results are depicted in Fig. 4. It is seen that all diffraction peaks of SnO2 indicate a rutile structure (JCPDS card no. 41-1445). For the SnO2 /GN composites, by decreasing ratios of GO to SnCl2 in the precursor solution from 0.1 mmol/0.5 mg to 0.1 mmol/0.1 mg, the XRD patterns of the resulting composite exhibit an ever weakened diffraction peaks of (1 1 0) and (2 1 1) plane, while the diffraction peak of (0 0 2) plane is gradually strengthened. In particular, the diffraction peak attributed to (1 0 1) plane is enhanced significantly. It is known that GO contains some oxygen containing functional groups, such as hydroxyl group and carboxyl group. Thus it is proposed that, under the hydrothermal conditions, there might exist some interactions between the functional groups on GO and SnCl2 , which promotes the preferential growth of SnO2 along (1 0 1) plane on the reduced GO templates. However, the growth of SnO2 in the absence of GO or in the presence of excessive amount of GO could weaken or destroy the orientated growth of SnO2 along (1 0 1) plane. The underlying reasons need further investigation. Morphologies of GO, GN, SnO2 and SnO2 /GN composites were observed by SEM. It is seen in Fig. 5 that both GO and GN obtained by the hydrothermal method were curled nanosheets, while SnO2 prepared via the hydrothermal growth were large square-shaped crystals sizing ranging from several tens to
hundreds of micrometers, which could be observed even by naked eyes. In comparison, hydrothermally fabricated SnO2 /GN composite (SnCl2 /GO = 0.1 mmol/0.1 mg) displayed a much more elaborate structure. It is observed that the composite shows a 3D nanostructure in which radial flower-like SnO2 microspheres distribute among the curled ultrathin GN lamellars (Fig. 6a). A close observation reveals that the SnO2 microspheres are composed of assembled ultrathin SnO2 nanoflakes with length of several tens of nanometers which fill the interplace of GN nanosheets (Fig. 6b and d). Moreover, it is seen from Fig. 6c that all the GN nanosheets were decorated with plenty of tiny SnO2 nanoparticles, which is more apparent in the HRTEM images in Fig. 7. The SEM image of cross-section of gas sensor based on the composite is shown in Fig. 6e. The dense region of the SEM image is the ceramic substrate of the electrode and the lamellar on the substrate is the SnO2 /GN composite. The crystallinity and morphology of SnO2 /GN composite was further characterized by HRTEM as shown in Fig. 7. From Fig. 7a and b, it can be seen that GN nanosheets are decorated with plenty of SnO2 nanocrystals. Fig. 7c displays the magnified HRTEM image of the tiny SnO2 nanocrystals (diameters < 5 nm) with adjacent fringe spacing of 0.26 nm, corresponding to (1 0 1) plane of SnO2 crystal. Selected-area electron diffraction (SAED) pattern of SnO2 crystal in Fig. 7c is illustrated in Fig. 7d, which shows typical diffraction rings of rutile SnO2 crystal. The architecture of the SnO2 /GN composite was explored by nitrogen adsorption and desorption measurements. In the isotherm adsorption–desorption curve shown in Fig. 8, a distinct hysteresis loop can be seen at relative pressures of 0.4–1.0. It suggested the presence of a mesoporous structure and relatively high surface area. The composite exhibited very high Brunauer–Emmett–Teller (BET) surface area of 94.9 m2 /g as determined from the four data points at low relative pressure (Fig. 8, inset). Moreover, it shows an average pore
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Fig. 7. HRTEM images and SAED pattern of SnO2 /GN (the composite was prepared via a hydrothermal method at 120 ◦ C for 8 h; precursor solution: SnCl2 /GO = 0.1 mmol/0.1 mg).
Fig. 8. Isothermal adsorption/desorption curve (N2 at 150 ◦ C) of SnO2 /GN; inset: calculation of BET surface area by four data points in low relative pressure (the composite was prepared via a hydrothermal method at 120 ◦ C for 8 h; precursor solution: SnCl2 /GO = 0.1 mmol/0.1 mg).
Fig. 9. Pore diameter distribution of SnO2 /GN composite fabricated by a hydrothermal method (T = 120 ◦ C, t = 8 h; precursor solution: SnCl2 /GO = 0.1 mmol/0.1 mg).
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Fig. 10. Photos of (a–c) SnO2 /GN gas sensors (precursor solution: (a) (b) SnCl2 /GO = 0.1 mmol/0.1 mg; (c) SnCl2 : SnCl2 /GO = 0.1 mmol/0.5 mg; 0.1 mmol/mL) and (d) blank interdigitated gold electrode.
diameter of 6.11 nm and total pore volume of 0.142 cm3 /g as calculated by the Barrett–Joyner–Halenda method (Fig. 9). The results proves that the formation of 3D nanostructures of the composite has significantly enlarged the surface area and pore volume, which will greatly facilitate the adsorption and transport of gases, thus leading to enhancement in response magnitude. 3.2. Gas sensing properties NH3 gas is a toxic gas widely existing in agricultural, environmental, automotive, as well as medical and industrial chemistry areas [31–34]. Therefore detection of NH3 is urgently needed. SnO2
Fig. 11. Response to 50 ppm of the sensors based on (a) GN, (b) GO, (c) SnO2 /GN (precursor solution: SnCl2 /GO = 0.1 mmol/1 mg), (d) SnO2 /GN (precursor solution: SnCl2 /GO = 0.1 mmol/0.5 mg), (e) SnO2 /GN (precursor solution: SnCl2 /GO = 0.1 mmol/0.25 mg), (f) SnO2 /GN (precursor solution: SnCl2 /GO = 0.1 mmol/0.15 mg) and (g) SnO2 /GN (precursor solution: SnCl2 /GO = 0.1 mmol/0.1 mg) (15 ◦ C, N2 as the carrier gas for all the measurements). All the samples (except GO) were prepared by a hydrothermal method at 120 ◦ C for 8 h.
could be used to detect NH3 gas with good gas response, but mainly at high operation temperatures. In this work, we examined NH3 sensing properties of the as-prepared SnO2 /GN composite at room temperature. It was found that SnO2 alone exhibited poor adherence to the interdigitated gold electrode, while SnO2 /GN could adhere well to the electrode. Moreover, with the increase in the amount of GN, the composite exhibited improved adherence to
Fig. 12. Gas sensing properties to NH3 of the sensor based on SnO2 /GN (precursor solution: SnCl2 /GO = 0.1 mmol/0.1 mg). (a) Dynamic response to NH3 with N2 refreshing after each exposure to NH3 , (b) dynamic response to NH3 without N2 refreshing, (c) response magnitude vs. concentration of NH3 and (d) dynamic response to 50 ppm NH3 and N2 during cyclic tests (15 ◦ C, N2 as the carrier gas for all the measurements).
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Fig. 13. NH3 sensing properties of the sensor based on SnO2 /GN (precursor solution: SnCl2 /GO = 0.1 mmol/0.1 mg) at different temperatures: (a) 15 ◦ C, (b) 25 ◦ C, (c) 35 ◦ C and (d) 45 ◦ C (N2 as the carrier gas for all the measurements).
Fig. 14. Response of SnO2 /GN composite sensor (precursor solution: SnCl2 /GO = 0.1 mmol/0.1 mg) to 50 ppm NH3 in the presence of water vapors (28%RH, 28.0 ◦ C, compressed air with approximately the same humidity and temperature as the carrier gas).
Fig. 15. Response of SnO2 /GN composite sensor (precursor solution: SnCl2 /GO = 0.1 mmol/0.1 mg) to 50 ppm NH3 in the presence of water vapors (25%RH, 23.5 ◦ C, N2 with approximately the same humidity and temperature as the carrier gas).
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the electrode as shown in Fig. 10. Typical gas response curves of resistive-type gas sensors based on GO, GN and SnO2 /GN composites towards NH3 gas of 50 ppm at 15 ◦ C are illustrated in Fig. 11. GO and GN based sensor exhibited impedance of ∼400 k and 15 k, respectively. It is found that their impedance decreased on exposure to NH3 , exhibiting a small negative response of −3.3% and −2.7%, respectively. By contrast, the sensor based on SnO2 crystals hydrothermally grown in the absence of GO showed very high impedance (>10 M) and no detectable response to NH3 gas at room temperature. The SnO2 /GN composites prepared from precursor solutions with different ratios of SnCl2 to GO exhibited impedance ranging from 100 k to 400 k, which increased with decrease in the ratio of GO. Obviously, the inclusion of GN in the composite significantly enhanced the conductivity of the composites. In contrary to sensors based on GO or GN, the composite sensors displayed impedance increase when in contact with NH3 , exhibiting positive responses. Moreover, the response magnitude was enhanced with the decrease in the ratio of GO in the precursor solution. The sensor obtained with a precursor solution of 0.1 mmol SnCl2 /0.1 mg GO achieved the highest response magnitude of 15.9% to NH3 gas of 50 ppm at room temperature, which is employed in following investigations on sensing properties. Further decreasing the ratio of GO in the precursor solution (for example, 0.1 mmol SnCl2 /0.05 mg GO), the composite sensor showed too high impedance (>10 M) and no electrical response to NH3 gas at room temperature. The dynamic response to NH3 gas of different concentrations of the composite sensor was investigated and the results are shown in Fig. 12. The composite sensor was exposed to NH3 gas of 10–50 ppm then refreshed with N2 (Fig. 12a). It can be seen that the impedance of the sensor quickly increases when it is in contact with NH3 . By flushing with N2 , its impedance returns to the initial baseline values in a short time. This suggests good reversibility of the composite in NH3 sensing at room temperature. Moreover, both the response and recovery times are short, less than 1 min. Fig. 12b presents the response of the composite sensor to NH3 gas with step increase or decrease in the gas concentrations. It is clearly seen that the sensor displays fast and reversible responses to NH3 gas of various concentrations even without refreshing with N2 . The response magnitude of the composite sensor is plotted against gas concentration as shown in Fig. 12c, which reveals an exponential trend and indicates that its response magnitude tends to saturate at higher concentration of NH3 . The dynamic response of the composite sensor to periodical changes between NH3 gas of 50 ppm and N2 is illustrated in Fig. 12c. It is seen that the sensor registers good repeatability during the cycle tests. The effect of temperature on the NH3 sensing of the composite was also examined and the results are shown in Fig. 13. It is found that the response magnitude decreases with the increase in the temperature. The temperature coefficient is defined as ˛ = (S2 − S1 )/(T2 − T1 ), where S1 and S2 are the response magnitude at T1 and T2 , respectively. It is determined that ˛ is as small as 0.1%/◦ C and 2.2%/◦ C for NH3 gas of 10 ppm and 50 ppm, respectively. Compared with its good response magnitude at 15 ◦ C (5.9% for 10 ppm NH3 and 15.9% for 50 ppm NH3 ), the composite sensor exhibits a small temperature coefficient in the range of 15–45 ◦ C. It is believed that water molecules could affect the gas response of SnO2 /GN composite, since water molecules could compete with the gas in the adsorption process and the adsorbed water molecules would dissociate and lead to decreased impedance of SnO2 , which can be employed as a humidity sensitive material [35]. We investigated the response of the composite to NH3 in both N2 and air atmosphere in the presence of water vapors, and the results are shown in Figs. 14 and 15. It is found that the composite could detect NH3 in air (Fig. 14). However, water vapors have different effect on the response of the composite to NH3 in air and in N2 . In air, the response was much lower, but both the response and
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Moreover, highly conductive GN enhanced the conductivity of the composite so that the conductivity change can be easily detected, as in the case of the composite of carbon nanotubes and discrete SnO2 [13]. The composite would display higher gas response to NH3 due to much increased surface accessibility and higher conductivity. Furthermore, there must be special interactions between GN and SnO2 which resulted in much improved gas sensing behaviors of the composite at room temperature. Much more work is needed to clarify the sensing mechanism. 4. Conclusions
Fig. 16. Response of SnO2 /GN composite sensor to humidity (21.6 ◦ C, saturated salt solutions are used as the humidity sources: LiCl for 11%RH, MgCl2 for 33%RH, K2 CO3 for 43%RH, NaBr for 57%RH and NaCl for 75%RH). The measurements were carried out in air atmosphere.
recovery processes were much accelerated. The detailed explanations need more systematic investigations in future work. Fig. 16 presents the electrical response to humidity of the composite. Here we define the response of the composite sensor to humidity as SRH = ((ZRH − Z11%RH )/Z11%RH ) × 100%, where ZRH and Z11%RH are the impedance at tested humidity levels and 11%RH, respectively. It is clearly seen that the composite exhibited negative response towards water vapors. Therefore, it is necessary to consider the effect of water vapors while developing the SnO2 /GN composite gas sensors. The above experimental results reveal that the hydrothermally prepared SnO2 /GN composites are featured with good NH3 sensing properties at room temperature, including good response magnitude, high reversibility, fast response and recovery, good repeatability and small temperature coefficient. Obviously, the composite exhibits much enhanced room temperature sensing characteristics than their constituents alone. However, it is found that the sensing behaviors are different from the literature reports. It is known that SnO2 is an n-type semiconductor, and its interactions with oxidizing and reducing gases would lead to decreased and increased conductivity, respectively. Moreover, it was reported that SnO2 /GN composite displayed increased conductivity while in contact with reducing gases such as H2 S and NH3 at high temperature [23]. Wei et al. found that single-walled carbon nanotubes (SWCNTs) and SnO2 exhibited decreased conductivity while detecting oxidizing NO2 at room temperature, and attributed the room temperature gas response to the establishment of p/n junction between p-type SWCNTs and SnO2 [36]. Deng et al. found that the composite of p-type GN and p-type Cu2 O showed higher conductivity while in contact with NO2 [24]. On the contrary, Van Hieu et al. prepared a composite of multi-walled carbon nanotubes (MWCNTs) and SnO2 by simple ultrasonication-assisted solution blending and subsequent heat treatment, and found that it exhibited increased resistance towards NH3 at room temperature. They also attributed the unusual gas response to the establishment of p/n junction [6]. Obviously, the reported results of the SnO2 composite sensing materials were in disagreement with each other. In our case, the exact sensing mechanism of the composite towards NH3 at room temperature is still unclear. However, one thing is certain: GN played a crucial role in bestowing the composite with electrical response to NH3 at room temperature, since SnO2 alone exhibited too low conductivity and did not display detectable response to NH3 at room temperature. The introduction of GN led to the formation of a composite with 3D nanostructure featured with much higher surface accessibility (BET surface area of ∼100 m2 /g), which greatly facilitated the adsorption and diffusion of NH3 molecules.
Composite of SnO2 /GN could be facilely fabricated via a hydrothermal procedure with SnCl2 and GO as the precursor. The resulting composite exhibited a 3D network nanostructure, in which GN lamellas were separated by radial flower-like SnO2 microspheres. Moreover, the GN nanosheets were decorated with plenty of tiny SnO2 nanoparticles. The existence of GN significantly increased the conductivity of the composite, and led to very high specific surface area of ∼100 m2 /g. Meantime, it promoted the preferential growth of SnO2 nanocrystals along (1 0 1) plane. Gas sensors based on the composite exhibited good gas sensing properties to NH3 of 10 to 50 ppm at room temperature, such as high response magnitude, fast response and recovery, good reversibility and repeatability. The much enhanced gas sensing properties of the composite at room temperature is proposed to relate to the high surface area, 3D nanostructure and special interactions between GN and SnO2 . Acknowledgements This work is financially supported by the National Natural Science Foundation of China (Contract no. 51073134), the Fundamental Research Funds for the Central Universities (Contract no. 2011QNA4024), National “863” program of China (Contract no. 2006AA10Z215) and Zhejiang Provincial Natural Science Foundation of China (Grant no. Y4080128). References [1] A. Lassesson, M. Schulze, J. van Lith, S.A. Brown, Tin oxide nanocluster hydrogen and ammonia sensors, Nanotechnology 19 (2008) 015502. [2] F. Gyger, M. Hubner, C. Feldmann, N. Barsan, U. Weimar, Nanoscale SnO2 hollow spheres and their application as a gas-sensing material, Chemistry of Materials 22 (2010) 4821–4827. [3] X.M. Yin, C.C. Li, M. Zhang, Q.Y. Hao, S. Liu, Q.H. Li, L.B. Chen, T.H. Wang, SnO2 monolayer porous hollow spheres as a gas sensor, Nanotechnology 20 (2009) 455503. [4] Z.W. Chen, D.Y. Pan, B. Zhao, G.J. Ding, Z. Jiao, M.H. Wu, C.H. Shek, L.C.M. Wu, J.K.L. Lai, Insight on fractal assessment strategies for tin dioxide thin films, ACS Nano 4 (2010) 1202–1208. [5] G.K. Fan, Y. Wang, M. Hu, Z.Y. Luo, G. Li, Synthesis of flowerlike nano-SnO2 and a study of its gas sensing response, Measurement Science and Technology 22 (2011) 045203. [6] N. Van Hieu, L.T.B. Thuy, N.D. Chien, Highly sensitive thin film NH3 gas sensor operating at room temperature based on SnO2 /MWCNTs composite, Sensors and Actuators B 129 (2008) 888–895. [7] N.V. Quy, V.A. Minh, N.V. Luan, V.N. Hung, N.V. Hieu, Gas sensing properties at room temperature of a quartz crystal microbalance coated with ZnO nanorods, Sensors and Actuators B 153 (2011) 188–193. [8] G. Rao, D.T. Rao, Gas sensitivity of ZnO based thick film sensor to NH3 at room temperature, Sensors and Actuators B 55 (1999) 166–169. [9] K.K. Khun, A. Mahajan, R.K. Bedi, SnO2 thick films for room temperature gas sensing applications, Journal of Applied Physics 106 (2009) 124509. [10] I. Singh, R.K. Bedi, Surfactant-assisted synthesis, characterizations, and room temperature ammonia sensing mechanism of nanocrystalline CuO, Solid State Science 13 (2011) 2011–2018. [11] M.A. Chougule, S.G. Pawar, S.L. Patil, B.T. Raut, P.R. Godse, S. Sen, V.B. Patil, Polypyrrole thin film: room temperature ammonia gas sensor, IEEE Sensors Journal 11 (2011) 2137–2141. [12] G. De, R. Kohn, G. Xomeritakis, C.J. Brinker, Nanocrystalline mesoporous palladium activated tin oxide thin films as room-temperature hydrogen gas sensors, Chemical Communications (2007) 1840–1842.
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Biographies Qianqian Lin is a postgraduate student in the Department of Polymer Science and Engineering, Zhejiang University. His research interests include polymeric chemical sensors. Yang Li received his Ph.D. degree in polymer chemistry and physics from Zhejiang University in 2000. He has been working in Department of Polymer Science and Engineering, Zhejiang University since 2000 and was appointed associate professor in polymer science in 2002. His research interests include polymer materials and organic/inorganic composites for chemical sensors. Mujie Yang graduated from Zhongshan University, China in 1963. She has been working in Zhejiang University since 1963. She was promoted to full professor in polymer science in 1992. Her research interests include functional polymers with optical and electrical characteristics.