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Controlling synthesis and gas-sensing properties of ordered mesoporous In2O3-reduced graphene oxide (rGO) nanocomposite
Sci. Bull. DOI 10.1007/s11434-015-0852-6
www.scibull.com www.springer.com/scp
Article
Materials Science
Controlling synthesis and gas-sensing properties of ordered mesoporous In2O3-reduced graphene oxide (rGO) nanocomposite Ping Xue • Xiaomei Yang • Xiaoyong Lai Weitao Xia • Peng Li • Junzhuo Fang
•
Received: 21 May 2015 / Accepted: 25 June 2015 Ó Science China Press and Springer-Verlag Berlin Heidelberg 2015
Abstract Herein, we describe a strategy for fabricating ordered mesoporous In2O3-reduced graphene oxide (rGO) nanocomposite through ultrasonic mixing, where ordered mesoporous In2O3 nanoparticles are synthesized via the nanocasting route by using mesoporous silica as a hard template, which possess ordered mesostructure with a large surface area of 81 m2 g-1, and rGO nanosheets are synthesized from graphite via graphene oxide (GO) as intermediate. After coupled with rGO, mesoporous In2O3 could maintain its ordered mesostructure. We subsequently investigate the gas-sensing properties of all the In2O3 specimens with or without rGO for different gases. The results exhibit the ordered mesoporous In2O3-rGO nanocomposite possesses significantly enhanced response to ethanol even at low concentration levels, superior over pure mesoporous In2O3 nanoparticles. Similar strategy could be extended to other ordered mesoporous metal oxide–rGO nanocomposite for improving the gas-sensing property. Keywords Mesoporous material Indium oxide Graphene Nanocomposite Nanocasting Gas sensor
P. Xue X. Yang X. Lai (&) W. Xia P. Li J. Fang Key Laboratory of Energy Resource and Chemical Engineering, State Key Laboratory Cultivation Base of Natural Gas Conversion, School of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, China e-mail: [email protected]
1 Introduction There is an increasing concern on semiconducting metal oxide gas sensors in past decades regarding the awareness of environmental protection and human health. Various nanostructured metal oxides with high surface area have widely investigated as sensing materials [1–5]. Among them, ordered mesoporous metal oxides have attracted considerable attention since their accessible pores benefit not only the diffusion of gas molecules for increasing response rate, but also the reduction in aggregation and sintering for enhancing their thermal stability under high temperature during the fabrication and work process of gas sensor [6–13]. For example, Tiemann and co-workers [13] reported the improved sensitivity of mesoporous In2O3 to CH4. Mao et al. [14] also reported the enhanced sensitivity of hierarchically mesoporous hematite microsphere toward formaldehyde (HCHO). Lai et al. [15] presented a low-cost synthesis of mesoporous In2O3 with tunable pore wall thickness by directly using solvent-extracted mesoporous silica with different pore sizes as a template. The gas testing results showed that the sensitivity of mesoporous In2O3 to HCHO sharply increases with reducing the pore wall thickness. The gas-sensing properties of those mesoporous metal oxide sensors could be further improved by doping noble metals. Tu et al. [16] reported that Pt-doped mesoporous In2O3 possess a significantly higher response than those without doping Pt. Lai et al. [17] reported the enhanced gas-sensing properties of Ag-doped mesoporous In2O3 toward HCHO. Nevertheless, the rising cost resulted from noble metals may limit their practical application. Graphene is a kind of interesting material with some extraordinary properties including ultra-large specific surface area, unusual mechanical strength and high electrical conductivity, which has attracted enormous attention
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[18, 19]. Recently, several groups have reported that the gas-sensing properties of metal oxide sensors could be significantly improved after coupled with graphene. For example, Deng et al. [20] have synthesized an reduced graphene oxide (rGO)-conjugated Cu2O nanowire mesocrystal via a one-pot hydrothermal treatment of copper (II) acetate in the presence of o-anisidine and graphene oxide (GO), which exhibit a higher response to NO2 than individual Cu2O nanowire or rGO. Choi et al. [21] have also reported the enhanced response of SnO2 nanofibers functionalized with rGO to acetone and hydrogen sulfide. To the best of our knowledge, however, there is no report on ordered mesoporous metal oxide–rGO nanocomposite for gas sensors. In this work, we have successfully synthesized ordered mesoporous In2O3 nanoparticles via the nanocasting route directly using mesoporous silica as a hard template and then mixed them with rGO to form an ordered mesoporous In2O3-rGO nanocomposite under the assistant of ultrasonication (Scheme 1). The gas-sensing testing results exhibit that ordered mesoporous In2O3-rGO nanocomposite possesses significantly enhanced response and relatively high selective toward ethanol, which suggests the potential application of the ordered mesoporous In2O3-rGO nanocomposite for detecting ethanol.
2 Experimental 2.1 Synthesis of ordered mesoporous In2O3 nanoparticles Ordered mesoporous silica KIT-6 was synthesized at hydrothermal temperature of 130 °C according to the established procedures [22]; 0.6 g of KIT-6 was dispersed in 10 mL of ethanol, followed by addition of 1.2 g of hydrated indium nitrate under stirring in a Teflon beaker. After all the solvent had evaporated, the resulting powder was heated in a ceramic crucible in an oven at 250 °C for 3 h, in order to decompose indium nitrate. Finally, the
silica template was removed at room temperature using 2 mol L-1 NaOH aqueous solution. The solid product was recovered by centrifugation, followed by washing with water several times and then drying at 70 °C overnight. Other mesoporous metal oxides could be also synthesized by similar procedure. 2.2 Synthesis of GO and its reduction First, GO was synthesized by the modified Hummers’ method [23, 24]. A 2 g of natural graphite powder and 1.5 g of NaNO3 were placed in a flask. Then 46 mL of concentrated H2SO4 was added slowly with stirred in an ice-water bath. A 7 g of KMnO4 powder was added gradually under stirring, and the mixture was stirred for 2 h and then heated at 35 °C for 2 h, followed by adding gradually 100 mL of deionized water. The mixture was heated at 90 °C for 30 min and then added gradually 200 mL of deionized water. After the temperature reduced to 60 °C, 30 mL of H2O2 (5 wt%) was added. The mixture was centrifuged and washed with 100 mL of HCl solution (5 wt%) and 900 mL of deionized water. A 1 g of graphite oxide was dispersed in 1,000 mL of deionized water under ultrasonication for 30 min. The mixture was centrifuged at a speed of 1,000 r min-1 for 10 min, followed by discarding the solid and repeating ultrasonic step for three times. GO was collected from the mixture by centrifugation at a speed of 15,000 r min-1 for 30 min and dried at room temperature for 72 h; 0.2 g of GO was dispersed in 200 mL of deionized water, followed by adding 0.25 g of hydrazine solution (80 wt%). The pH of suspension was adjusted to 10 by adding 2.4 mL of concentrated ammonia solution. The mixture was heated under stirring at 90 °C for 3 h and cooled to room temperature. rGO was collected from the mixture by centrifugation at a speed of 15,000 r min-1 for 30 min and dried at room temperature for 72 h. 2.3 Synthesis of ordered mesoporous In2O3 nanoparticle-rGO nanocomposite A 0.2 g of ordered mesoporous In2O3 nanoparticle was dispersed in 5 mL of deionized water, followed by adding 0.05 wt% of rGO water suspension relative to the amount of ordered mesoporous In2O3 nanoparticles dispersed in the solution. The mixed solution was ultrasonicated for 15 min, and the solid composite was collected by filter and dried at 70 °C overnight. 2.4 Characterization
Scheme 1 Schematic illustration of the fabrication process of ordered mesoporous In2O3-rGO nanocomposite
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The powder X-ray diffraction (XRD) patterns were recorded with a Bruker AXS D8 advanced diffractometer (Bruker,
Sci. Bull.
˚ ). The Germany) employing Cu Ka radiation (k = 1.5405 A nitrogen physisorption isotherms at the temperature of liquid nitrogen (77 K) were measured with an ASAP 2020 HD adsorption analyzer (Micromeritics, USA) with prior degassing under vacuum at 200 °C overnight. Total pore volumes were determined using the adsorbed volume at a relative pressure of 0.99. Multipoint Brunauer–Emmet– Teller (BET)-specific surface area was estimated from the relative pressure ranging from 0.05 to 0.2. The pore size distribution of all mesostructured materials was analyzed using the Barrett–Joyner–Halenda (BJH) algorithm. Scanning electron microscopy (SEM) images were obtained with a JSM-7500 (JEOL, Japan). Transmission electron microscopy (TEM) images were recorded with a JEM-2100F (JEOL), operating at an acceleration voltage of 200 kV. Sensors were fabricated by dip-coating the samples on a ceramic tube with two Au electrodes and four Pt wire, according to the procedure in previous literatures [3, 5]. The powder samples were first dispersed in water and slightly ground, forming into a slurry. The slurry was coated onto the external surface of the ceramic tube to connect the two Au electrodes, and the sensor was dried at room temperature to densify the sensing film. The electrical response of the gas sensor was measured with a WS-60A automatic test system (Weisheng Instruments Co., China). The sensor response (sensitivity) was defined as the ratio of resistance in air (Rair) and that in air containing the test gas (Rgas). Response and recovery time are defined as the time taken to reach 90 % of the variation in electrical conductivity.
3 Results and discussion Low-angle XRD patterns of mesoporous silica template KIT-6, mesoporous In2O3 and mesoporous In2O3-rGO nanocomposite are shown in Fig. 1a. KIT-6 displays three resolved diffraction peaks, which could be indexed as the (211), (220) and (332) reflections of the Ia3d symmetry. Mesoporous In2O3 also exhibits a diffraction peak, suggesting that the ordered mesostructure was to some degree transformed into In2O3 replica via the negative structural replication, although this peak is relatively broader and less resolved than those for KIT-6, possibly due to the smaller periodic order domain in mesoporous In2O3 replica. Wideangle XRD pattern of mesoporous In2O3 shows that assynthesized products were composed of crystalline In2O3 (JCPDS No. 06-0416), without any other phases present (Fig. 1b). SEM image (Fig. 1c) exhibits that mesoporous In2O3 replica are composed of particles with a typical diameter of 100–200 nm, significantly less than those for KIT-6. TEM image further confirms that the resultant In2O3 products possess ordered mesostructure (Fig. 1d). Figure 1e shows the N2 physisorption isotherms of
mesoporous In2O3, which gives typical IV-type isotherms with a characteristic for mesoporous materials. Correspondingly, a narrow peak centered at 4.9 nm is exhibited in the pore size distribution curve (the inset in Fig. 1e). The specific BET surface area and the total pore volume of mesoporous In2O3 were calculated from the physisorption results to be 81 m2 g-1 and 0.31 cm3 g-1. rGO nanosheets have been synthesized according to the literature previously reported. Typical SEM and TEM images, XRD pattern and Raman spectra of rGO are shown in Fig. 1f–h, respectively, which are relatively similar to those of rGO reported in the literatures [24]. A small amount of the resultant rGO nanosheets (about 0.05 wt% relative to the weight of mesoporous In2O3 nanoparticles) were mixed with the mesoporous In2O3 to form nanocomposite. SEM image (Fig. 1i) of mesoporous In2O3-rGO nanocomposite clearly exhibits that rGO nanosheets were well dispersed into mesoporous In2O3 nanoparticles, forming three-dimensional hierarchical structure, and its low-angle XRD pattern (Fig. 1a) also shows a similar diffraction peak with that for mesoporous In2O3, suggesting that the introduction of rGO nanosheets did not significantly affect the ordered mesostructure of mesoporous In2O3. Mesoporous In2O3-rGO nanocomposite is used as gassensing materials and fabricated into gas sensor for investigating its gas-sensing property. The pure mesoporous In2O3 nanoparticles are referred to comparison. The response of the gas sensor fabricated from mesoporous In2O3-rGO nanocomposite to 100 ppm (parts-per-million, 10-6) ethanol in air was tested as a function of operating temperature (Fig. 2), which is relatively low at below 230 °C. It reaches a highest value at about 300 °C and then gradually decreases if further increasing the operating temperature. Here, the gas-sensing mechanism of the gas sensor is mainly based on a conductivity change deriving from the adsorption of oxygen on the surface of sensing materials and the reaction between pre-adsorbed oxygen species and ethanol. When the sensor is placed in air, the pre-absorbed oxygen molecules could capture electrons from the conduction band of In2O3 and transform into chemisorbed oxygen species (O2-, O- and O2-), resulting a decrease in electron concentration and conductivity. When ethanol is introduced, it will react with the oxygen species and the captured electron will be injected back to the conduction band, leading to an increase in electron concentration and conductivity. When the operating temperature increases, the pre-absorbed oxygen molecules could capture more electrons [5, 25, 26], thus resulting a higher response. However, chemisorbed oxygen species would tend to drop from the surface of sensing materials as further increasing temperature, thus leading a lower response. The optimum operating temperature should be determined by the balance of these two procedures [27].
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Fig. 1 a Low-angle XRD pattern of mesoporous silica template KIT-6, mesoporous In2O3 replica and mesoporous In2O3-rGO nanocomposite; wide-angle XRD pattern (b), TEM (c), SEM (d) images, N2 physisorption isotherms (e) and the corresponding pore size distribution curve (inset) of mesoporous In2O3; SEM (f) and TEM (inset) images of rGO; g XRD patterns of graphite, GO and rGO; h Raman pattern of rGO; i SEM image of mesoporous In2O3-rGO nanocomposite
Therefore, the optimum operating temperature for the mesoporous In2O3-rGO nanocomposite-based sensor was 300 °C, and all further tests would be performed at this temperature. Figure 3a shows dynamic response curves of the gas sensors based on mesoporous In2O3 and mesoporous In2O3-rGO nanocomposite to ethanol in the range of 1–1,000 ppm at 300 °C. The response to ethanol increases rapidly with the increasing of gas concentration. The correlation between the response and ethanol concentration (shown in Fig. 3b) is approximately linear in the range of 100–1,000 ppm, which indicates that these sensors are very suitable for the detection of ethanol in a wide range. This unsaturation phenomenon to relatively high ethanol
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concentration may be resulted from the large specific surface area and large pore volume of mesoporous In2O3 providing more surface active sites and accommodating large number of ethanol gas molecules. We could observe that the sensor fabricated from ordered mesoporous In2O3rGO nanocomposite exhibited a response to ethanol over the entire testing concentration range stronger than that of mesoporous In2O3 and the response of the former to 1,000 ppm ethanol gas is almost 23 times higher than that of the latter (2,778 and 119 respectively), which still possesses considerable advantages of the response toward ethanol even compared with other In2O3-based sensing materials reported in previous literatures (Table 1) [28– 34]. The response improvement may be explained by the
Sci. Bull. Table 1 Comparison of the response of In2O3-based sensing materials in this work and those reported in previous literatures toward ethanol
Fig. 2 Response versus operating temperature of the gas sensor fabricated from mesoporous In2O3-rGO nanocomposite toward 100 ppm ethanol
Ethanol concentration (ppm)
Product
Response (Rair/Rgas)
Mesoporous In2O3-rGO nanocomposite
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50
52
100
Ref.
This work
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Mesoporous In2O3
3.4
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In2O3 hollow microsphere
13
50
[28]
Er-doped In2O3 hollow microsphere
40
100
[29] [30]
In2O3 porous nanorod
7.4
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In2O3 nanorod
7.6
50
[31]
In2O3 nanoparticle
5.6
50
[32]
Hierarchical In2O3 nanocube
18
200
[33]
Co-doped In2O3 nanowires
17
100
[34]
Fig. 4 Response of the gas sensor fabricated from mesoporous In2O3 and mesoporous In2O3-rGO nanocomposite toward 100 ppm different gases
Fig. 3 a Dynamic response curve of the gas sensors fabricated from mesoporous In2O3 and mesoporous In2O3-rGO nanocomposite, during cycling between increasing concentration of ethanol and ambient air at 300 °C; b response versus ethanol concentration of the gas sensors fabricated from mesoporous In2O3 and mesoporous In2O3-rGO nanocomposite
fact that the introduction of rGO could result in the electron transfer from mesoporous In2O3 to rGO and reduce the free carrier concentration in mesoporous In2O3, which would lead a larger conductivity change under the same electron injection resulting from the reaction between pre-adsorbed oxygen species and ethanol, i.e., a higher response. Similar result was previously reported in the cases of nanofibrous SnO2-rGO nanocomposite [21]. Figure 4 shows the cross-sensitive response of mesoporous In2O3 and mesoporous In2O3-rGO nanocomposite sensor to various gases, including ethanol, benzene, toluene, acetone, methanol, dichloromethane and benzaldehyde. It is obvious that the mesoporous In2O3-rGO nanocomposite
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exhibits the largest response to ethanol among all the test gases. Moreover, the introduction of rGO did not significantly increase the response time (97 and 98 s for the sensors based on mesoporous In2O3 and mesoporous In2O3rGO nanocomposite toward 1,000 ppm ethanol) and inversely reduce the recovery time (60 and 31 s for those two sensors). All the above-mentioned results indicate that the sensor based on mesoporous In2O3-rGO nanocomposite is potentially useful for detecting ethanol. Furthermore, we have also synthesized other mesoporous metal oxide (such as SnO2, a-Fe2O3)–rGO nanocomposite by combining hard-template and ultrasonic mixing method, which also exhibit the significantly improved gas-sensing properties, suggesting the effectiveness and generality of this present method. Nevertheless, the present research on mesoporous metal oxide–rGO nanocomposite for gas sensors is still in its primary stage and further improvement may be realized by independently and finely manipulating the textual properties (such as pore size, pore volume, pore wall thickness and particle size) of mesoporous metal oxides and rGO as well as adjusting the ratio of them, which is still ongoing. 4 Conclusions In summary, we have successfully synthesized ordered mesoporous In2O3 nanoparticle-rGO nanocomposite via a combining hard-template and ultrasonic mixing method. The resultant mesoporous In2O3 nanoparticle-rGO nanocomposite exhibited much high response to ethanol compared to those pure mesoporous In2O3 without rGO, which suggests the potential application of such novel nanostructured material for detecting ethanol gas. Similar strategy could be extended to other mesoporous metal oxide–rGO nanocomposite gas materials with different compositions, pore structure and size. The research may open up new opportunities for preparing advanced materials based on various ordered mesoporous metal oxide–rGO nanocomposite for multipurpose applications. Acknowledgments This work was supported by the National Natural Science Foundation of China (21006116, 51362024), the Natural Science Foundation of Ningxia (NZ12111, NZ14010) and, the Prophase Research Special Project of the National Basic Research Program of China (2012CB723106). Xiaoyong Lai thanks the West Light Foundation of The Chinese Academy of Sciences. Conflict of interest of interest.
The authors declare that they have no conflict
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www.scibull.com www.springer.com/scp
Article
Materials Science
Controlling synthesis and gas-sensing properties of ordered mesoporous In2O3-reduced graphene oxide (rGO) nanocomposite Ping Xue • Xiaomei Yang • Xiaoyong Lai Weitao Xia • Peng Li • Junzhuo Fang
•
Received: 21 May 2015 / Accepted: 25 June 2015 Ó Science China Press and Springer-Verlag Berlin Heidelberg 2015
Abstract Herein, we describe a strategy for fabricating ordered mesoporous In2O3-reduced graphene oxide (rGO) nanocomposite through ultrasonic mixing, where ordered mesoporous In2O3 nanoparticles are synthesized via the nanocasting route by using mesoporous silica as a hard template, which possess ordered mesostructure with a large surface area of 81 m2 g-1, and rGO nanosheets are synthesized from graphite via graphene oxide (GO) as intermediate. After coupled with rGO, mesoporous In2O3 could maintain its ordered mesostructure. We subsequently investigate the gas-sensing properties of all the In2O3 specimens with or without rGO for different gases. The results exhibit the ordered mesoporous In2O3-rGO nanocomposite possesses significantly enhanced response to ethanol even at low concentration levels, superior over pure mesoporous In2O3 nanoparticles. Similar strategy could be extended to other ordered mesoporous metal oxide–rGO nanocomposite for improving the gas-sensing property. Keywords Mesoporous material Indium oxide Graphene Nanocomposite Nanocasting Gas sensor
P. Xue X. Yang X. Lai (&) W. Xia P. Li J. Fang Key Laboratory of Energy Resource and Chemical Engineering, State Key Laboratory Cultivation Base of Natural Gas Conversion, School of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, China e-mail: [email protected]
1 Introduction There is an increasing concern on semiconducting metal oxide gas sensors in past decades regarding the awareness of environmental protection and human health. Various nanostructured metal oxides with high surface area have widely investigated as sensing materials [1–5]. Among them, ordered mesoporous metal oxides have attracted considerable attention since their accessible pores benefit not only the diffusion of gas molecules for increasing response rate, but also the reduction in aggregation and sintering for enhancing their thermal stability under high temperature during the fabrication and work process of gas sensor [6–13]. For example, Tiemann and co-workers [13] reported the improved sensitivity of mesoporous In2O3 to CH4. Mao et al. [14] also reported the enhanced sensitivity of hierarchically mesoporous hematite microsphere toward formaldehyde (HCHO). Lai et al. [15] presented a low-cost synthesis of mesoporous In2O3 with tunable pore wall thickness by directly using solvent-extracted mesoporous silica with different pore sizes as a template. The gas testing results showed that the sensitivity of mesoporous In2O3 to HCHO sharply increases with reducing the pore wall thickness. The gas-sensing properties of those mesoporous metal oxide sensors could be further improved by doping noble metals. Tu et al. [16] reported that Pt-doped mesoporous In2O3 possess a significantly higher response than those without doping Pt. Lai et al. [17] reported the enhanced gas-sensing properties of Ag-doped mesoporous In2O3 toward HCHO. Nevertheless, the rising cost resulted from noble metals may limit their practical application. Graphene is a kind of interesting material with some extraordinary properties including ultra-large specific surface area, unusual mechanical strength and high electrical conductivity, which has attracted enormous attention
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[18, 19]. Recently, several groups have reported that the gas-sensing properties of metal oxide sensors could be significantly improved after coupled with graphene. For example, Deng et al. [20] have synthesized an reduced graphene oxide (rGO)-conjugated Cu2O nanowire mesocrystal via a one-pot hydrothermal treatment of copper (II) acetate in the presence of o-anisidine and graphene oxide (GO), which exhibit a higher response to NO2 than individual Cu2O nanowire or rGO. Choi et al. [21] have also reported the enhanced response of SnO2 nanofibers functionalized with rGO to acetone and hydrogen sulfide. To the best of our knowledge, however, there is no report on ordered mesoporous metal oxide–rGO nanocomposite for gas sensors. In this work, we have successfully synthesized ordered mesoporous In2O3 nanoparticles via the nanocasting route directly using mesoporous silica as a hard template and then mixed them with rGO to form an ordered mesoporous In2O3-rGO nanocomposite under the assistant of ultrasonication (Scheme 1). The gas-sensing testing results exhibit that ordered mesoporous In2O3-rGO nanocomposite possesses significantly enhanced response and relatively high selective toward ethanol, which suggests the potential application of the ordered mesoporous In2O3-rGO nanocomposite for detecting ethanol.
2 Experimental 2.1 Synthesis of ordered mesoporous In2O3 nanoparticles Ordered mesoporous silica KIT-6 was synthesized at hydrothermal temperature of 130 °C according to the established procedures [22]; 0.6 g of KIT-6 was dispersed in 10 mL of ethanol, followed by addition of 1.2 g of hydrated indium nitrate under stirring in a Teflon beaker. After all the solvent had evaporated, the resulting powder was heated in a ceramic crucible in an oven at 250 °C for 3 h, in order to decompose indium nitrate. Finally, the
silica template was removed at room temperature using 2 mol L-1 NaOH aqueous solution. The solid product was recovered by centrifugation, followed by washing with water several times and then drying at 70 °C overnight. Other mesoporous metal oxides could be also synthesized by similar procedure. 2.2 Synthesis of GO and its reduction First, GO was synthesized by the modified Hummers’ method [23, 24]. A 2 g of natural graphite powder and 1.5 g of NaNO3 were placed in a flask. Then 46 mL of concentrated H2SO4 was added slowly with stirred in an ice-water bath. A 7 g of KMnO4 powder was added gradually under stirring, and the mixture was stirred for 2 h and then heated at 35 °C for 2 h, followed by adding gradually 100 mL of deionized water. The mixture was heated at 90 °C for 30 min and then added gradually 200 mL of deionized water. After the temperature reduced to 60 °C, 30 mL of H2O2 (5 wt%) was added. The mixture was centrifuged and washed with 100 mL of HCl solution (5 wt%) and 900 mL of deionized water. A 1 g of graphite oxide was dispersed in 1,000 mL of deionized water under ultrasonication for 30 min. The mixture was centrifuged at a speed of 1,000 r min-1 for 10 min, followed by discarding the solid and repeating ultrasonic step for three times. GO was collected from the mixture by centrifugation at a speed of 15,000 r min-1 for 30 min and dried at room temperature for 72 h; 0.2 g of GO was dispersed in 200 mL of deionized water, followed by adding 0.25 g of hydrazine solution (80 wt%). The pH of suspension was adjusted to 10 by adding 2.4 mL of concentrated ammonia solution. The mixture was heated under stirring at 90 °C for 3 h and cooled to room temperature. rGO was collected from the mixture by centrifugation at a speed of 15,000 r min-1 for 30 min and dried at room temperature for 72 h. 2.3 Synthesis of ordered mesoporous In2O3 nanoparticle-rGO nanocomposite A 0.2 g of ordered mesoporous In2O3 nanoparticle was dispersed in 5 mL of deionized water, followed by adding 0.05 wt% of rGO water suspension relative to the amount of ordered mesoporous In2O3 nanoparticles dispersed in the solution. The mixed solution was ultrasonicated for 15 min, and the solid composite was collected by filter and dried at 70 °C overnight. 2.4 Characterization
Scheme 1 Schematic illustration of the fabrication process of ordered mesoporous In2O3-rGO nanocomposite
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The powder X-ray diffraction (XRD) patterns were recorded with a Bruker AXS D8 advanced diffractometer (Bruker,
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˚ ). The Germany) employing Cu Ka radiation (k = 1.5405 A nitrogen physisorption isotherms at the temperature of liquid nitrogen (77 K) were measured with an ASAP 2020 HD adsorption analyzer (Micromeritics, USA) with prior degassing under vacuum at 200 °C overnight. Total pore volumes were determined using the adsorbed volume at a relative pressure of 0.99. Multipoint Brunauer–Emmet– Teller (BET)-specific surface area was estimated from the relative pressure ranging from 0.05 to 0.2. The pore size distribution of all mesostructured materials was analyzed using the Barrett–Joyner–Halenda (BJH) algorithm. Scanning electron microscopy (SEM) images were obtained with a JSM-7500 (JEOL, Japan). Transmission electron microscopy (TEM) images were recorded with a JEM-2100F (JEOL), operating at an acceleration voltage of 200 kV. Sensors were fabricated by dip-coating the samples on a ceramic tube with two Au electrodes and four Pt wire, according to the procedure in previous literatures [3, 5]. The powder samples were first dispersed in water and slightly ground, forming into a slurry. The slurry was coated onto the external surface of the ceramic tube to connect the two Au electrodes, and the sensor was dried at room temperature to densify the sensing film. The electrical response of the gas sensor was measured with a WS-60A automatic test system (Weisheng Instruments Co., China). The sensor response (sensitivity) was defined as the ratio of resistance in air (Rair) and that in air containing the test gas (Rgas). Response and recovery time are defined as the time taken to reach 90 % of the variation in electrical conductivity.
3 Results and discussion Low-angle XRD patterns of mesoporous silica template KIT-6, mesoporous In2O3 and mesoporous In2O3-rGO nanocomposite are shown in Fig. 1a. KIT-6 displays three resolved diffraction peaks, which could be indexed as the (211), (220) and (332) reflections of the Ia3d symmetry. Mesoporous In2O3 also exhibits a diffraction peak, suggesting that the ordered mesostructure was to some degree transformed into In2O3 replica via the negative structural replication, although this peak is relatively broader and less resolved than those for KIT-6, possibly due to the smaller periodic order domain in mesoporous In2O3 replica. Wideangle XRD pattern of mesoporous In2O3 shows that assynthesized products were composed of crystalline In2O3 (JCPDS No. 06-0416), without any other phases present (Fig. 1b). SEM image (Fig. 1c) exhibits that mesoporous In2O3 replica are composed of particles with a typical diameter of 100–200 nm, significantly less than those for KIT-6. TEM image further confirms that the resultant In2O3 products possess ordered mesostructure (Fig. 1d). Figure 1e shows the N2 physisorption isotherms of
mesoporous In2O3, which gives typical IV-type isotherms with a characteristic for mesoporous materials. Correspondingly, a narrow peak centered at 4.9 nm is exhibited in the pore size distribution curve (the inset in Fig. 1e). The specific BET surface area and the total pore volume of mesoporous In2O3 were calculated from the physisorption results to be 81 m2 g-1 and 0.31 cm3 g-1. rGO nanosheets have been synthesized according to the literature previously reported. Typical SEM and TEM images, XRD pattern and Raman spectra of rGO are shown in Fig. 1f–h, respectively, which are relatively similar to those of rGO reported in the literatures [24]. A small amount of the resultant rGO nanosheets (about 0.05 wt% relative to the weight of mesoporous In2O3 nanoparticles) were mixed with the mesoporous In2O3 to form nanocomposite. SEM image (Fig. 1i) of mesoporous In2O3-rGO nanocomposite clearly exhibits that rGO nanosheets were well dispersed into mesoporous In2O3 nanoparticles, forming three-dimensional hierarchical structure, and its low-angle XRD pattern (Fig. 1a) also shows a similar diffraction peak with that for mesoporous In2O3, suggesting that the introduction of rGO nanosheets did not significantly affect the ordered mesostructure of mesoporous In2O3. Mesoporous In2O3-rGO nanocomposite is used as gassensing materials and fabricated into gas sensor for investigating its gas-sensing property. The pure mesoporous In2O3 nanoparticles are referred to comparison. The response of the gas sensor fabricated from mesoporous In2O3-rGO nanocomposite to 100 ppm (parts-per-million, 10-6) ethanol in air was tested as a function of operating temperature (Fig. 2), which is relatively low at below 230 °C. It reaches a highest value at about 300 °C and then gradually decreases if further increasing the operating temperature. Here, the gas-sensing mechanism of the gas sensor is mainly based on a conductivity change deriving from the adsorption of oxygen on the surface of sensing materials and the reaction between pre-adsorbed oxygen species and ethanol. When the sensor is placed in air, the pre-absorbed oxygen molecules could capture electrons from the conduction band of In2O3 and transform into chemisorbed oxygen species (O2-, O- and O2-), resulting a decrease in electron concentration and conductivity. When ethanol is introduced, it will react with the oxygen species and the captured electron will be injected back to the conduction band, leading to an increase in electron concentration and conductivity. When the operating temperature increases, the pre-absorbed oxygen molecules could capture more electrons [5, 25, 26], thus resulting a higher response. However, chemisorbed oxygen species would tend to drop from the surface of sensing materials as further increasing temperature, thus leading a lower response. The optimum operating temperature should be determined by the balance of these two procedures [27].
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Fig. 1 a Low-angle XRD pattern of mesoporous silica template KIT-6, mesoporous In2O3 replica and mesoporous In2O3-rGO nanocomposite; wide-angle XRD pattern (b), TEM (c), SEM (d) images, N2 physisorption isotherms (e) and the corresponding pore size distribution curve (inset) of mesoporous In2O3; SEM (f) and TEM (inset) images of rGO; g XRD patterns of graphite, GO and rGO; h Raman pattern of rGO; i SEM image of mesoporous In2O3-rGO nanocomposite
Therefore, the optimum operating temperature for the mesoporous In2O3-rGO nanocomposite-based sensor was 300 °C, and all further tests would be performed at this temperature. Figure 3a shows dynamic response curves of the gas sensors based on mesoporous In2O3 and mesoporous In2O3-rGO nanocomposite to ethanol in the range of 1–1,000 ppm at 300 °C. The response to ethanol increases rapidly with the increasing of gas concentration. The correlation between the response and ethanol concentration (shown in Fig. 3b) is approximately linear in the range of 100–1,000 ppm, which indicates that these sensors are very suitable for the detection of ethanol in a wide range. This unsaturation phenomenon to relatively high ethanol
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concentration may be resulted from the large specific surface area and large pore volume of mesoporous In2O3 providing more surface active sites and accommodating large number of ethanol gas molecules. We could observe that the sensor fabricated from ordered mesoporous In2O3rGO nanocomposite exhibited a response to ethanol over the entire testing concentration range stronger than that of mesoporous In2O3 and the response of the former to 1,000 ppm ethanol gas is almost 23 times higher than that of the latter (2,778 and 119 respectively), which still possesses considerable advantages of the response toward ethanol even compared with other In2O3-based sensing materials reported in previous literatures (Table 1) [28– 34]. The response improvement may be explained by the
Sci. Bull. Table 1 Comparison of the response of In2O3-based sensing materials in this work and those reported in previous literatures toward ethanol
Fig. 2 Response versus operating temperature of the gas sensor fabricated from mesoporous In2O3-rGO nanocomposite toward 100 ppm ethanol
Ethanol concentration (ppm)
Product
Response (Rair/Rgas)
Mesoporous In2O3-rGO nanocomposite
18
50
52
100
Ref.
This work
307
200
Mesoporous In2O3
3.4
50
In2O3 hollow microsphere
13
50
[28]
Er-doped In2O3 hollow microsphere
40
100
[29] [30]
In2O3 porous nanorod
7.4
50
In2O3 nanorod
7.6
50
[31]
In2O3 nanoparticle
5.6
50
[32]
Hierarchical In2O3 nanocube
18
200
[33]
Co-doped In2O3 nanowires
17
100
[34]
Fig. 4 Response of the gas sensor fabricated from mesoporous In2O3 and mesoporous In2O3-rGO nanocomposite toward 100 ppm different gases
Fig. 3 a Dynamic response curve of the gas sensors fabricated from mesoporous In2O3 and mesoporous In2O3-rGO nanocomposite, during cycling between increasing concentration of ethanol and ambient air at 300 °C; b response versus ethanol concentration of the gas sensors fabricated from mesoporous In2O3 and mesoporous In2O3-rGO nanocomposite
fact that the introduction of rGO could result in the electron transfer from mesoporous In2O3 to rGO and reduce the free carrier concentration in mesoporous In2O3, which would lead a larger conductivity change under the same electron injection resulting from the reaction between pre-adsorbed oxygen species and ethanol, i.e., a higher response. Similar result was previously reported in the cases of nanofibrous SnO2-rGO nanocomposite [21]. Figure 4 shows the cross-sensitive response of mesoporous In2O3 and mesoporous In2O3-rGO nanocomposite sensor to various gases, including ethanol, benzene, toluene, acetone, methanol, dichloromethane and benzaldehyde. It is obvious that the mesoporous In2O3-rGO nanocomposite
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exhibits the largest response to ethanol among all the test gases. Moreover, the introduction of rGO did not significantly increase the response time (97 and 98 s for the sensors based on mesoporous In2O3 and mesoporous In2O3rGO nanocomposite toward 1,000 ppm ethanol) and inversely reduce the recovery time (60 and 31 s for those two sensors). All the above-mentioned results indicate that the sensor based on mesoporous In2O3-rGO nanocomposite is potentially useful for detecting ethanol. Furthermore, we have also synthesized other mesoporous metal oxide (such as SnO2, a-Fe2O3)–rGO nanocomposite by combining hard-template and ultrasonic mixing method, which also exhibit the significantly improved gas-sensing properties, suggesting the effectiveness and generality of this present method. Nevertheless, the present research on mesoporous metal oxide–rGO nanocomposite for gas sensors is still in its primary stage and further improvement may be realized by independently and finely manipulating the textual properties (such as pore size, pore volume, pore wall thickness and particle size) of mesoporous metal oxides and rGO as well as adjusting the ratio of them, which is still ongoing. 4 Conclusions In summary, we have successfully synthesized ordered mesoporous In2O3 nanoparticle-rGO nanocomposite via a combining hard-template and ultrasonic mixing method. The resultant mesoporous In2O3 nanoparticle-rGO nanocomposite exhibited much high response to ethanol compared to those pure mesoporous In2O3 without rGO, which suggests the potential application of such novel nanostructured material for detecting ethanol gas. Similar strategy could be extended to other mesoporous metal oxide–rGO nanocomposite gas materials with different compositions, pore structure and size. The research may open up new opportunities for preparing advanced materials based on various ordered mesoporous metal oxide–rGO nanocomposite for multipurpose applications. Acknowledgments This work was supported by the National Natural Science Foundation of China (21006116, 51362024), the Natural Science Foundation of Ningxia (NZ12111, NZ14010) and, the Prophase Research Special Project of the National Basic Research Program of China (2012CB723106). Xiaoyong Lai thanks the West Light Foundation of The Chinese Academy of Sciences. Conflict of interest of interest.
The authors declare that they have no conflict
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