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Reduced graphene oxide modified with hierarchical flower-like In(OH)3 for NO2 room-temperature sensing
Sensors and Actuators B 214 (2015) 36–42
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Reduced graphene oxide modified with hierarchical flower-like In(OH)3 for NO2 room-temperature sensing Peng Wan a,∗ , Wei Yang b , Xinnan Wang a , Jiming Hu b , Hua Zhang a a
Faculty of Chemical, Environmental and Biological Science and Technology, Dalian University of Technology, Dalian 116024, PR China Key Lab of Analytical Chemistry for Biology and Medicine (Ministry of Education), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, PR China b
a r t i c l e
i n f o
Article history: Received 25 December 2014 Received in revised form 6 February 2015 Accepted 22 February 2015 Available online 14 March 2015 Keywords: In(OH)3 Graphene Sensor NO2
a b s t r a c t For gas sensors, graphene modified with functional micro- and nanomaterials has been considered as promising materials to improve the sensing performances. In this report, graphene modified by hierarchical flower-like In(OH)3 , for the first time, was prepared by a one-step microwave-assisted hydrothermal method. The as-obtained In(OH)3 /rGO composites exhibited loosely hierarchical flower morphology, thus was favourable for sensing performance enhancement. A room-temperature NO2 sensor based on as-synthesized In(OH)3 /rGO composites was fabricated, and displayed an excellent selectivity and a significant response to NO2 with a concentration lower to 1 ppm. The results also revealed that the sensor exhibited a rapid recovery in exposure of water vapour, compared to natural recovery in air. The enhanced NO2 sensing performances were attributed to the synergistic effect of functional In(OH)3 and graphene in the unique composites. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Graphene, a unique 2D carbon structure, has been considered as a promising candidate in the fabrication of gas sensors due to its ultra-high electron conductance and large surface area [1–3]. In 2007, the graphene-based sensing device was reported by Novoselov for detecting single molecules of gases [4]. Since then, many efforts have been devoted to develop graphene-based sensors [5–7]. Unfortunately, the bare graphene-based materials usually show poor sensing performances towards gas species, such as low sensitivity and long recovery time [2,3,8]. Several methods have been developed to improve the sensing properties of graphene-based materials, including thermal treatment [9], chemically modified method [10,11], microwave [12], UV irradiation treatment [13], and voltage activation [14]. Since the chemical modification could be performed on a large scale at a relatively low cost [15,16], it plays more important roles among all these methods. Moreover, during the process of chemical modifications, many functional materials, e.g., conductive polymers [17], metal oxides [18–22], metal sulphides [23], noble metal [24,25], carbon nanotubes [26], etc., can be incorporated into the graphene network to form hybrid architectures, which are favourable for sensing
∗ Corresponding author. Tel.: +86 041184986350. E-mail address: [email protected] (P. Wan). http://dx.doi.org/10.1016/j.snb.2015.02.100 0925-4005/© 2015 Elsevier B.V. All rights reserved.
performance enhancement due to the diverse functionalities and synergistic effect in the composites [18,19]. Up to now, several kinds of modified graphene-based materials, including PDDA/rGO [27], Cu2 O/rGO [18], WO3 /rGO [19], ZnO/rGO [20,21], SnO2 /rGO [22], MoS2 /rGO [23], Au-decorated rGO [24], Pd-decorated rGO [25], CNTs-rGO hybrid composites [26], rGO/polymer [27] etc., have been reported, and largely improved the sensing behaviour of graphene materials. In(OH)3 and In2 O3 semiconductor nanostructures have drawn considerable attention due to their physical and chemical properties with potential applications in various fields [28,29]. In most of solution-synthesis method, the In2 O3 nanostructures are prepared from In(OH)3 precursor by annealing treatment at high temperature [23,31]. In general, the properties and applications of In2 O3 nano-materials are strongly depended on their specific morphologies [32]. During the annealing process, however, the specific morphology of In2 O3 could be damaged and agglomerated, resulting a degradation of sensing performances [33]. In this work, we innovatively report graphene materials modified with the hierarchical flower-like In(OH)3 . The composites were directly obtained by a facile one-step microwave-assisted hydrothermal method without further annealing treatment. Compared with the In2 O3 /rGO composites prepared from same batch of flower-like In(OH)3 material, the In(OH)3 /rGO composites displayed a loosely hierarchical flower morphology, thus was more favourable for sensing performance enhancement. The as-obtained
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Fig. 1. (a) Images of the ceramic plate previously coated with a pair of gold electrodes on the frontal sides. (b) Photograph of the sensor. (c–e) Photograph of the homemade sensor testing system.
In(OH)3 /rGO composites were used to fabricate a sensor for NO2 sensing at room temperature. The sensor exhibited an excellent selectivity and a significant response to NO2 with concentration lower to 1 ppm. Moreover, the recovery behaviour of the flower-like In(OH)3 modified graphene-based sensor can be greatly improved in exposure to the water vapour.
450, FEI) with an accelerating voltage of 3 kV. Fourier-transform infrared (FT-IR) spectra of KBr powder pressed pellets was recorded on a Spectrum 100 FT-IR (PerkinElmer Instrument Co.) with a scan range of 4000–400 cm−1 .
2. Experimental
For fabricating the sensing devices, the as-synthesized products were dispersed in ethanol by ultrasonication, and then dropped onto a ceramic plate (1.0 mm × 1.5 mm), which was previously coated with a pair of gold electrodes on frontal sides by screen printing technique, followed by drying at room temperature. The gas sensing experiments were carried out in a home-made testing system (a cylindrical plastic chamber with a volume of 100 mL) (Fig. 1). The tested gas concentration was controlled by injecting volume of the tested gas [35]. For recovery of the sensor resistance, the testing chamber was open in air or the sensor was exposed to water vapour in a 20 mL vial with 10 mL water in it. The changes of the sensor resistance in air or tested gas were monitored by a highresistance metre (VICTOR, 66E, China). Sensor response was defined as S = Ra /Rg , where Ra and Rg are denoted as the sensor resistance exposed in air and tested gas, respectively. All the sensing tests were performed under the room temperature (25 ◦ C, 50% relative humidity).
2.1. Synthesis of materials Graphene oxide (GO) was prepared from natural graphite flakes according to the modified Hummers method [34]. The purified GO was then dispersed in deionized water to form a 2 mg/mL suspension. Exfoliation of GO was achieved by using an ultrasonic bath (Kudos, SK5200H, 200 W, China). The In(OH)3 /rGO composites were synthesized by microwave-assisted hydrothermal method. In a typical synthesis, 1 mL of GO solution (2 mg/mL) was added into 9 mL of deionized water, then 50 mg of InCl3 ·4H2 O was dissolved in above GO solution by sonication for 30 min. Subsequently, 250 mg of urea and 62.5 mg of SDS were added into above solution. After another 10 min sonication, the mixed solution was sealed in a 100 mL Teflon container and transferred into a microwave digestion system (SINEO, MDS-6G, China). The reaction was then performed at 150 ◦ C for 30 min. After cooling to room temperature naturally, the products were collected by centrifugation and washed by water for several times. The solid was transferred into a muffle furnace after vacuum-drying at 70 ◦ C for 12 h and then heated to 400 ◦ C for 2 h. The temperature was held at this temperature for another 3 h under 300 sccm N2 atmosphere. For comparison, bare GO solution was conducted by the same microwave condition without other additives. 2.2. Characterization of materials To determine the chemical composition of the samples, X-ray powder diffraction (XRD) analysis was performed on a Rigaku D/Max 2500 diffractometer with Cu K␣1 radiation ( = 0.15406 nm). The samples were scanned at a rate of 5◦ /min over the 2 range of 5–65◦ . The morphologies of the as-obtained samples were characterized by field-emission SEM (Nova NanoSEM
2.3. Fabrication and gas-sensing measurements
3. Results and discussion 3.1. Characterization of In(OH)3 /rGO composites The chemical composition of the resulting products obtained after microwave-assisted hydrothermal reaction was analyzed by XRD technique. Fig. 2 shows the typical XRD pattern of the as-obtained In(OH)3 /rGO composites. Most of the detectable peaks are indexed as In(OH)3 (JCPDS Card, No. 76-1464), except for two diffraction peaks marked with “”, which are coincident with some chloride derivatives [36]. None of obvious typical peaks belonging to rGO are observed in the XRD pattern of as-obtained product. This could be ascribed to the low diffraction intensity of rGO in the In(OH)3 /rGO composites [37].
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of nanometers in thickness. Such hierarchical flower-like microspheres with loose structures are favourable for low apparent density. These unique structure properties are crucial for sensing performance enhancement of such materials-based gas sensors. 3.2. Mechanism of In(OH)3 growth on graphene sheets
Fig. 2. XRD pattern of the as-prepared product.
The morphologies of the as-obtained In(OH)3 /rGO composites are observed using field emission scanning electron microscopy (FESEM), as shown in Fig. 3. The low magnification SEM image (Fig. 3a) clearly displays that the flower-like In(OH)3 microspheres with size around 1–5 m are anchored on graphene sheets. Furthermore, the graphene sheets exhibit some holes in the composites, which could be attributed to the destruction of the microwave-assisted hydrothermal reaction. The detailed morphology information of In(OH)3 /rGO composites is further investigated. As presented in the high-magnification FESEM image in Fig. 3b and c, the graphene sheets show transparent films, partially overlapped and crumpled at many regions. Moreover, except for flowerlike In(OH)3 , there are some residual flower flakes loading on the graphene sheets. Furthermore, the flower-like microspheres are constructed from numerous 2D nanoflakes with several tens
The growth process of hierarchical flower-like In(OH)3 is schematically illustrated in Fig. 4. As we all know, there are numerous oxygen-containing functional groups, such as OH, and COOH, on the surface of GO [38]. Indium ions (In3+ ) are selectively bonded with these functional groups driven by electrostatic interactions. After the addition of SDS, the counter ions of SDS are adsorbed by In3+ through the electrostatic force [39]. At the same time, the additive urea is coordinated with In3+ , and the complex precursor forms at this stage [40]. During the microwave-assisted hydrothermal process, the urea molecules start to hydrolyse and release ammonia, thus leading a uniform rise of pH value in the solution. Meanwhile, the tiny crystal nucleus derived from the complex precursor tends to nucleate to reduce their surface energies [30]. In addition, the typically insulating GO is reduced into conductive rGO, and acts as the nucleation sites for further crystal growth at the same time [41]. As the reaction continues, the formation of flakes occurs by the aggregation and growth of these crystals. Furthermore, the hierarchical flower-like In(OH)3 materials are self-assembled by these flakes via different driving forces, including electrostatic interactions, crystal face aggregation, and hydrophobic interactions [31,40]. The reduction of GO and the formation of In(OH)3 after the microwave-assisted hydrothermal reaction are also confirmed
Fig. 3. SEM images of the as-obtained In(OH)3 /rGO composites at different magnifications.
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Fig. 4. Schematic illustration of the growth process for hierarchical flower-like In(OH)3 on graphene sheets.
using Fourier transform infrared (FT-IR) spectra. The FT-IR spectra for GO, rGO and In(OH)3 /rGO composites are shown in Fig. 5a. In the case of GO, a broad peak at around 3425 cm−1 is ascribed to the hydroxyl group (O H). The absorption bands at 1722, 1383, and 1077 cm−1 can be assigned to the stretching vibrations of C O, O H, and C O, respectively. The absorption bands at 1630 cm−1 are attributed to the skeletal vibration of C C from graphitic domains
Transmittance (a.u.)
(a) In2O3/RGO
RGO
4000
3400
2800
2200
1000
500
1077
1630
1600
1383 1170
3425
1722
GO
400
-1
Wave number (cm )
(b)
G band
Intensity (a. u.)
D band
39
[42]. Based on such observations of GO, it is suggested that there are affluent functional groups existing in the GO. However, in the case of rGO, only C O and O H bands can be seen in the FT-IR spectrum, and the bands for C C and C O cannot be obviously observed. Furthermore, compared with GO, a dramatic decrease of the intensities of C O and O H peaks in rGO is clearly observed, suggesting that GO is reduced in the microwave-assisted hydrothermal process [43]. For the FT-IR spectra of In(OH)3 /rGO composites, all the peaks of functional groups correspond to rGO, indicating that the GO in the composite has been reduced to rGO. Furthermore, there is a typical absorption peak at 500 cm−1 , which is ascribed to vibration of In-OH in composites [31]. Such results indicate that the as-prepared flower-like In(OH)3 is successfully incorporated into the hybrid architectures. Raman spectra are utilized to further investigate the reduction of GO and the formation of In(OH)3 . As shown in Fig. 5b, Both GO and In(OH)3 exhibit two major peaks, containing the D band at 1350 cm−1 and the G band at 1594 cm−1 . The G-band is ascribed to the first-order scattering of the E2 g mode [41]. The D-band is associated with the structural defects [44]. Furthermore, the intensity ratios of the D to the G band (ID /IG ) for GO and In(OH)3 /rGO are 0.9223 and 1.0172, respectively. These results demonstrate the formation of new graphitic domains for both rGO and In(OH)3 /rGO composites after the microwave-assisted hydrothermal process. Additionally, for Raman spectra of the In(OH)3 /rGO composites, there are two peaks at 299 cm−1 and 490 cm−1 , which could be assigned to the characteristic modes of In(OH)3 [45]. 3.3. Comparison of In(OH)3 /rGO composites and In2 O3 /rGO composites For comparison, the In2 O3 /rGO composites were synthesized by thermal treatment of our flower-like In(OH)3 /rGO composites in N2 atmosphere at 400 ◦ C. The XRD pattern of the product obtained after the heat treatment is investigated (see the SI, Fig. S1). As shown, for the XRD pattern of the product from In(OH)3 /rGO composites annealing in N2 flow, most of the diffraction peaks are attributed to In2 O3 (JCPDS Card, No. 22-0336). Similarly, none of obvious typical peaks assigning to rGO are observed in XRD pattern of the product, which could be ascribed to the low diffraction intensity of rGO in the In2 O3 /rGO composites [19]. The morphologies of the resulting product are characterized by FESEM (see the SI, Fig. S2a). Observed from SEM images, the hierarchical flower-like In(OH)3 is destructed during the annealing process, leading amorphous particles deposited on the surfaces of the broken flower petals. Furthermore, the graphene sheets are not clearly seen, and this may be also due to the damage effect of the heat treatment. The detailed morphology information is exhibited in high-magnification SEM image (see the SI, Fig. S2b). The hierarchical flower-like In2 O3 is composed of spiralling flower petals along the centre point, and contains rather wide space between the each petal. In addition, there are many particles filled in these spaces, hindering the gas penetration, thus may lead a degradation of sensing performances [32].
299
3.4. Sensing performances of as-synthesized In(OH)3 /rGO composites
490 GO (ID/IG=0.9223)
In(OH)3/rGO (ID/IG=1.0172) 200
500
800
1100
1400
1700
2000
Raman shift (cm-1) Fig. 5. (a) FT-IR spectra of GO, rGO and In(OH)3 /rGO composites; (b) Raman spectra of GO and In(OH)3 /rGO.
In this paper, we focused on the sensor fabricated by In(OH)3 /rGO composites. Fig. 6a shows dynamic resistance response of the sensor based on In(OH)3 /rGO composites to various concentrations of NO2 . Dramatic decrease is easily observed with the injection of NO2 , indicating the p-type response for the sensor [46]. The sensing time are controlled at ∼180 s for all sensing tests and comparison. Clearly, as the NO2 concentration goes up, the resistance response of the sensor keeps decreasing
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Table 1 Detection of NO2 using graphene-based sensors. Sensing materials
NO2 (ppm)
Working temp (◦ C)
Response
Reference
In(OH)3 /rGO Al-decorated graphene Flexible graphene based sensors Ag-S-RGO Ethanol-based CVD graphene nanomesh Flexible RGO Ozone-treated graphene
1 1.2 5 5 10 5 5
RT 150 RT RT RT RT RT
Ra /Rg − 1 = 19.79% (cal: (Ra − Rg )/Ra = 16.52%) (Ra − Rg )/Ra = 2.89% (Ra − Rg )/Ra = 3% (Ra − Rg )/Ra = 16.7% (Ra − Rg )/Ra = 11% (Ra − Rg )/Ra = 12% (Ra − Rg )/Ra = 3.7%
Present work [8] [48] [45] [49] [50] [51]
Fig. 6. (a) Dynamic resistance response of the sensor based on In(OH)3 /rGO composites to different NO2 concentrations at room temperature; (b) the linear fitting curve of the sensor response versus the NO2 concentration.
obviously. A linear fitting curve of the sensor response versus the NO2 concentration in the range of 1–5 ppm is exhibited in Fig. 6b, and the correlation coefficient R2 of the fitting curve is 0.9822. Fig. 7 shows the response and recovery curves of as-fabricated sensor for 5 ppm NO2 at room-temperature. As indicated, the sensor exhibited good repeatability for three trials. Meanwhile, the recovery behaviour of the In(OH)3 /rGO composites-based sensor is
further investigated. In our case, like most of the semiconductorbased gas sensor operating at room temperature, the recovery time of our sensor is relatively long. However, the NO2 molecules adsorbed on the composites can be displaced rapidly at room temperature by using water vapour (see the SI, Fig. S3). During the recovery stage, the sensor is exposed upon water vapour for only one minute, and its resistance rapidly increases, even reaches a higher value above baseline. Subsequently, the sensor is transferred to air, and its resistance comes back to the initial baseline within ∼100 s. Compared with the refreshing performances in air (Fig. 6), the recovery time is largely shorten. The similar phenomenon has been also reported by Randeniya [44], and considered as a nondestructive method to improve recovery behaviour of sensors at room temperature. In our experiment, this method is also proven as an effective strategy to remove NO2 molecules from the flower-like In(OH)3 /rGO composites at room temperature. The phenomenon could be explained that the desorption of strongly bound NO2 molecules on the sensor is achieved by displacement of polar water molecules [47]. Selectivity is also evaluated for In(OH)3 /rGO composites-based gas sensor related with practical applications. Fig. 8 reveals the response of the sensor to several possible interferential gases, including NH3 , H2 , ethanol, acetone, and toluene. The sensor exhibits the largest response to NO2 , even though the concentration of other detected gases is 1000 times that of NO2 . Such results imply that the In(OH)3 /rGO composites-based gas sensor has an excellent selectivity towards NO2 gas. A comparison of the sensor responses to NO2 in the present work and literature reports is summarized in Table 1. Apparently, the asfabricated sensor in our work exhibits better sensing performances compared with those reported previously in the literature. As we all know, vacancies and defects are the major charge carriers for typical p-type rGO semiconductor [46], while In(OH)3 is a n-type semiconductor with free electrons as major charge carriers [28]. The enhanced sensing performance of the rGO sheets may be attributed to the modification of flower-like In(OH)3 in the unique hybrid architecture, especially the p-n heterojunction, which is
Resistance (kohm)
350
300
on
on
on
250
200 off 150
0
360
off 720
1080
off 1440
1800
Time (s) Fig. 7. The response and recovery curves of the In(OH)3 /rGO composites-based sensor to 5 ppm NO2 at room-temperature.
Fig. 8. Response of the In(OH)3 /rGO composites-based sensor to different gases at room-temperature.
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formed at the interface between the flower-like In(OH)3 and rGO sheets. The effective electronic interaction between In(OH)3 and rGO facilitates the gas molecule detection via the resistance change of the hybrid composites [19]. Theoretically, oxygen molecules are directly adsorbed onto the sensor surface and capture free electrons from the conductance band of the composites, and thus generate numerous chemisorbed oxygen species [32]. After exposing sensors to NO2 gas, the high electrophilic NO2 molecules can not only further capture the electrons from the conductance band, but also react with the absorbed oxygen species, leading to the formation of adsorbed NO2 species [46]. These adsorption and reaction further capture the electrons from the p-type semiconductor, resulting in an increase of the charge carrier concentration, which eventually decrease the sensor resistance [32]. During the refreshing process, the absorbed NO2 species are released, thus leading a recovery of the initial condition. 4. Conclusions In summary, we have synthesized hierarchical flower-like In(OH)3 modified graphene by a facile one-step microwaveassisted hydrothermal method. The sensor based on as-synthesized In(OH)3 /rGO composites exhibited a significant response to NO2 with a concentration lower to 1 ppm. An excellent selectivity towards NO2 gas was also observed, even though the other tested gas concentrations were 1000 times that of NO2 . Moreover, the recovery behaviour of the flower-like In(OH)3 modified graphenebased sensor can be greatly improved in exposure to the water vapour. The sensor also showed good shelf-life (see the SI, Fig. S4). The enhanced NO2 sensing performances may be attributed to the synergistic effect of functional In(OH)3 and graphene sheets in the unique hybrid architectures. Acknowledgement The work was supported through the foundation (Grant no. DUT12RC (3) 84), which is funded by Dalian University of Technology. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2015.02.100. References [1] X. Huang, X.Y. Qi, F. Boey, H. Zhang, Graphene-based composites, Chem. Soc. Rev. 41 (2012) 666–686. [2] W.J. Yuan, A.R. Liu, L. Huang, C. Li, G.Q. Shi, High-performance NO2 sensors based on chemically modified graphene, Adv. Mater. 25 (2013) 766–771. [3] G.H. Lu, S. Park, K.H. Yu, R.S. Ruoff, L.E. Ocola, D. Rosenmann, J.H. Chen, Toward practical gas sensing with highly reduced graphene oxide: a new signal processing method to circumvent run-to-run and device-to-device variations, ACS Nano 5 (2011) 1154–1164. [4] F. Schedin, A.K. Geim, S.V. Morozov, E.W. Hill, P. Blake, M.I. Katsnelson, K.S. Novoselov, Detection of individual gas molecules adsorbed on graphene, Nat. Mater. 6 (2007) 652–655. [5] V. Dua, S.P. Surwade, S. Ammu, S.R. Agnihotra, S. Jain, K.E. Roberts, S. Park, R.S. Ruoff, S.K. Manohar, All-organic vapor sensor using inkjet-printed reduced graphene oxide, Angew. Chem. Int. Ed. 49 (2010) 2154–2157. [6] J.D. Fowler, M.J. Allen, V.C. Tung, Y. Yang, R.B. Kaner, B.H. Weiller, Practical chemical sensors from chemically derived graphene, ACS Nano 3 (2009) 301–306. [7] J.T. Robinson, F.K. Perkins, E.S. Snow, Z.Q. Wei, P.E. Sheehan, Reduced graphene oxide molecular sensors, Nano Lett. 8 (2008) 3137–3140. [8] B. Cho, J. Yoon, M.G. Hahm, D.H. Kim, A.R. Kim, Y.H. Kahng, S.W. Park, Y.J. Lee, S.G. Park, J.D. Kwon, C.S. Kim, M. Song, Y. Jeong, K.S. Nam, H.C. Ko, Graphenebased gas sensor: metal decoration effect and application to a flexible device, J. Mater. Chem. C 2 (2014) 5280–5285.
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Biographies Peng Wan received her PhD degree in 2009 from Wuhan University. Currently she is a staff member in Faculty of Chemical, Environmental and Biological Science and Technology in Dalian University of Technology. Wei Yang is currently a PhD student in Wuhan University. His research focuses on the development of semiconductor sensors. Xinnan Wang received her PhD degree in 2013 from Jilin University. She is a staff member in faculty of Chemical, Environmental and Biological Science and Technology in Dalian University of Technology. Jiming Hu received his PhD degree in 1988 from Wuhan University. Currently he is a full professor in Wuhan University. His main research focuses on the Raman spectroscopy. Hua Zhang received his PhD degree from Dalian University of Technology. He currently is a full professor in faculty of Chemical, Environmental and Biological Science and Technology in Dalian University of Technology.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Reduced graphene oxide modified with hierarchical flower-like In(OH)3 for NO2 room-temperature sensing Peng Wan a,∗ , Wei Yang b , Xinnan Wang a , Jiming Hu b , Hua Zhang a a
Faculty of Chemical, Environmental and Biological Science and Technology, Dalian University of Technology, Dalian 116024, PR China Key Lab of Analytical Chemistry for Biology and Medicine (Ministry of Education), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, PR China b
a r t i c l e
i n f o
Article history: Received 25 December 2014 Received in revised form 6 February 2015 Accepted 22 February 2015 Available online 14 March 2015 Keywords: In(OH)3 Graphene Sensor NO2
a b s t r a c t For gas sensors, graphene modified with functional micro- and nanomaterials has been considered as promising materials to improve the sensing performances. In this report, graphene modified by hierarchical flower-like In(OH)3 , for the first time, was prepared by a one-step microwave-assisted hydrothermal method. The as-obtained In(OH)3 /rGO composites exhibited loosely hierarchical flower morphology, thus was favourable for sensing performance enhancement. A room-temperature NO2 sensor based on as-synthesized In(OH)3 /rGO composites was fabricated, and displayed an excellent selectivity and a significant response to NO2 with a concentration lower to 1 ppm. The results also revealed that the sensor exhibited a rapid recovery in exposure of water vapour, compared to natural recovery in air. The enhanced NO2 sensing performances were attributed to the synergistic effect of functional In(OH)3 and graphene in the unique composites. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Graphene, a unique 2D carbon structure, has been considered as a promising candidate in the fabrication of gas sensors due to its ultra-high electron conductance and large surface area [1–3]. In 2007, the graphene-based sensing device was reported by Novoselov for detecting single molecules of gases [4]. Since then, many efforts have been devoted to develop graphene-based sensors [5–7]. Unfortunately, the bare graphene-based materials usually show poor sensing performances towards gas species, such as low sensitivity and long recovery time [2,3,8]. Several methods have been developed to improve the sensing properties of graphene-based materials, including thermal treatment [9], chemically modified method [10,11], microwave [12], UV irradiation treatment [13], and voltage activation [14]. Since the chemical modification could be performed on a large scale at a relatively low cost [15,16], it plays more important roles among all these methods. Moreover, during the process of chemical modifications, many functional materials, e.g., conductive polymers [17], metal oxides [18–22], metal sulphides [23], noble metal [24,25], carbon nanotubes [26], etc., can be incorporated into the graphene network to form hybrid architectures, which are favourable for sensing
∗ Corresponding author. Tel.: +86 041184986350. E-mail address: [email protected] (P. Wan). http://dx.doi.org/10.1016/j.snb.2015.02.100 0925-4005/© 2015 Elsevier B.V. All rights reserved.
performance enhancement due to the diverse functionalities and synergistic effect in the composites [18,19]. Up to now, several kinds of modified graphene-based materials, including PDDA/rGO [27], Cu2 O/rGO [18], WO3 /rGO [19], ZnO/rGO [20,21], SnO2 /rGO [22], MoS2 /rGO [23], Au-decorated rGO [24], Pd-decorated rGO [25], CNTs-rGO hybrid composites [26], rGO/polymer [27] etc., have been reported, and largely improved the sensing behaviour of graphene materials. In(OH)3 and In2 O3 semiconductor nanostructures have drawn considerable attention due to their physical and chemical properties with potential applications in various fields [28,29]. In most of solution-synthesis method, the In2 O3 nanostructures are prepared from In(OH)3 precursor by annealing treatment at high temperature [23,31]. In general, the properties and applications of In2 O3 nano-materials are strongly depended on their specific morphologies [32]. During the annealing process, however, the specific morphology of In2 O3 could be damaged and agglomerated, resulting a degradation of sensing performances [33]. In this work, we innovatively report graphene materials modified with the hierarchical flower-like In(OH)3 . The composites were directly obtained by a facile one-step microwave-assisted hydrothermal method without further annealing treatment. Compared with the In2 O3 /rGO composites prepared from same batch of flower-like In(OH)3 material, the In(OH)3 /rGO composites displayed a loosely hierarchical flower morphology, thus was more favourable for sensing performance enhancement. The as-obtained
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Fig. 1. (a) Images of the ceramic plate previously coated with a pair of gold electrodes on the frontal sides. (b) Photograph of the sensor. (c–e) Photograph of the homemade sensor testing system.
In(OH)3 /rGO composites were used to fabricate a sensor for NO2 sensing at room temperature. The sensor exhibited an excellent selectivity and a significant response to NO2 with concentration lower to 1 ppm. Moreover, the recovery behaviour of the flower-like In(OH)3 modified graphene-based sensor can be greatly improved in exposure to the water vapour.
450, FEI) with an accelerating voltage of 3 kV. Fourier-transform infrared (FT-IR) spectra of KBr powder pressed pellets was recorded on a Spectrum 100 FT-IR (PerkinElmer Instrument Co.) with a scan range of 4000–400 cm−1 .
2. Experimental
For fabricating the sensing devices, the as-synthesized products were dispersed in ethanol by ultrasonication, and then dropped onto a ceramic plate (1.0 mm × 1.5 mm), which was previously coated with a pair of gold electrodes on frontal sides by screen printing technique, followed by drying at room temperature. The gas sensing experiments were carried out in a home-made testing system (a cylindrical plastic chamber with a volume of 100 mL) (Fig. 1). The tested gas concentration was controlled by injecting volume of the tested gas [35]. For recovery of the sensor resistance, the testing chamber was open in air or the sensor was exposed to water vapour in a 20 mL vial with 10 mL water in it. The changes of the sensor resistance in air or tested gas were monitored by a highresistance metre (VICTOR, 66E, China). Sensor response was defined as S = Ra /Rg , where Ra and Rg are denoted as the sensor resistance exposed in air and tested gas, respectively. All the sensing tests were performed under the room temperature (25 ◦ C, 50% relative humidity).
2.1. Synthesis of materials Graphene oxide (GO) was prepared from natural graphite flakes according to the modified Hummers method [34]. The purified GO was then dispersed in deionized water to form a 2 mg/mL suspension. Exfoliation of GO was achieved by using an ultrasonic bath (Kudos, SK5200H, 200 W, China). The In(OH)3 /rGO composites were synthesized by microwave-assisted hydrothermal method. In a typical synthesis, 1 mL of GO solution (2 mg/mL) was added into 9 mL of deionized water, then 50 mg of InCl3 ·4H2 O was dissolved in above GO solution by sonication for 30 min. Subsequently, 250 mg of urea and 62.5 mg of SDS were added into above solution. After another 10 min sonication, the mixed solution was sealed in a 100 mL Teflon container and transferred into a microwave digestion system (SINEO, MDS-6G, China). The reaction was then performed at 150 ◦ C for 30 min. After cooling to room temperature naturally, the products were collected by centrifugation and washed by water for several times. The solid was transferred into a muffle furnace after vacuum-drying at 70 ◦ C for 12 h and then heated to 400 ◦ C for 2 h. The temperature was held at this temperature for another 3 h under 300 sccm N2 atmosphere. For comparison, bare GO solution was conducted by the same microwave condition without other additives. 2.2. Characterization of materials To determine the chemical composition of the samples, X-ray powder diffraction (XRD) analysis was performed on a Rigaku D/Max 2500 diffractometer with Cu K␣1 radiation ( = 0.15406 nm). The samples were scanned at a rate of 5◦ /min over the 2 range of 5–65◦ . The morphologies of the as-obtained samples were characterized by field-emission SEM (Nova NanoSEM
2.3. Fabrication and gas-sensing measurements
3. Results and discussion 3.1. Characterization of In(OH)3 /rGO composites The chemical composition of the resulting products obtained after microwave-assisted hydrothermal reaction was analyzed by XRD technique. Fig. 2 shows the typical XRD pattern of the as-obtained In(OH)3 /rGO composites. Most of the detectable peaks are indexed as In(OH)3 (JCPDS Card, No. 76-1464), except for two diffraction peaks marked with “”, which are coincident with some chloride derivatives [36]. None of obvious typical peaks belonging to rGO are observed in the XRD pattern of as-obtained product. This could be ascribed to the low diffraction intensity of rGO in the In(OH)3 /rGO composites [37].
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of nanometers in thickness. Such hierarchical flower-like microspheres with loose structures are favourable for low apparent density. These unique structure properties are crucial for sensing performance enhancement of such materials-based gas sensors. 3.2. Mechanism of In(OH)3 growth on graphene sheets
Fig. 2. XRD pattern of the as-prepared product.
The morphologies of the as-obtained In(OH)3 /rGO composites are observed using field emission scanning electron microscopy (FESEM), as shown in Fig. 3. The low magnification SEM image (Fig. 3a) clearly displays that the flower-like In(OH)3 microspheres with size around 1–5 m are anchored on graphene sheets. Furthermore, the graphene sheets exhibit some holes in the composites, which could be attributed to the destruction of the microwave-assisted hydrothermal reaction. The detailed morphology information of In(OH)3 /rGO composites is further investigated. As presented in the high-magnification FESEM image in Fig. 3b and c, the graphene sheets show transparent films, partially overlapped and crumpled at many regions. Moreover, except for flowerlike In(OH)3 , there are some residual flower flakes loading on the graphene sheets. Furthermore, the flower-like microspheres are constructed from numerous 2D nanoflakes with several tens
The growth process of hierarchical flower-like In(OH)3 is schematically illustrated in Fig. 4. As we all know, there are numerous oxygen-containing functional groups, such as OH, and COOH, on the surface of GO [38]. Indium ions (In3+ ) are selectively bonded with these functional groups driven by electrostatic interactions. After the addition of SDS, the counter ions of SDS are adsorbed by In3+ through the electrostatic force [39]. At the same time, the additive urea is coordinated with In3+ , and the complex precursor forms at this stage [40]. During the microwave-assisted hydrothermal process, the urea molecules start to hydrolyse and release ammonia, thus leading a uniform rise of pH value in the solution. Meanwhile, the tiny crystal nucleus derived from the complex precursor tends to nucleate to reduce their surface energies [30]. In addition, the typically insulating GO is reduced into conductive rGO, and acts as the nucleation sites for further crystal growth at the same time [41]. As the reaction continues, the formation of flakes occurs by the aggregation and growth of these crystals. Furthermore, the hierarchical flower-like In(OH)3 materials are self-assembled by these flakes via different driving forces, including electrostatic interactions, crystal face aggregation, and hydrophobic interactions [31,40]. The reduction of GO and the formation of In(OH)3 after the microwave-assisted hydrothermal reaction are also confirmed
Fig. 3. SEM images of the as-obtained In(OH)3 /rGO composites at different magnifications.
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Fig. 4. Schematic illustration of the growth process for hierarchical flower-like In(OH)3 on graphene sheets.
using Fourier transform infrared (FT-IR) spectra. The FT-IR spectra for GO, rGO and In(OH)3 /rGO composites are shown in Fig. 5a. In the case of GO, a broad peak at around 3425 cm−1 is ascribed to the hydroxyl group (O H). The absorption bands at 1722, 1383, and 1077 cm−1 can be assigned to the stretching vibrations of C O, O H, and C O, respectively. The absorption bands at 1630 cm−1 are attributed to the skeletal vibration of C C from graphitic domains
Transmittance (a.u.)
(a) In2O3/RGO
RGO
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3400
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2200
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1383 1170
3425
1722
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39
[42]. Based on such observations of GO, it is suggested that there are affluent functional groups existing in the GO. However, in the case of rGO, only C O and O H bands can be seen in the FT-IR spectrum, and the bands for C C and C O cannot be obviously observed. Furthermore, compared with GO, a dramatic decrease of the intensities of C O and O H peaks in rGO is clearly observed, suggesting that GO is reduced in the microwave-assisted hydrothermal process [43]. For the FT-IR spectra of In(OH)3 /rGO composites, all the peaks of functional groups correspond to rGO, indicating that the GO in the composite has been reduced to rGO. Furthermore, there is a typical absorption peak at 500 cm−1 , which is ascribed to vibration of In-OH in composites [31]. Such results indicate that the as-prepared flower-like In(OH)3 is successfully incorporated into the hybrid architectures. Raman spectra are utilized to further investigate the reduction of GO and the formation of In(OH)3 . As shown in Fig. 5b, Both GO and In(OH)3 exhibit two major peaks, containing the D band at 1350 cm−1 and the G band at 1594 cm−1 . The G-band is ascribed to the first-order scattering of the E2 g mode [41]. The D-band is associated with the structural defects [44]. Furthermore, the intensity ratios of the D to the G band (ID /IG ) for GO and In(OH)3 /rGO are 0.9223 and 1.0172, respectively. These results demonstrate the formation of new graphitic domains for both rGO and In(OH)3 /rGO composites after the microwave-assisted hydrothermal process. Additionally, for Raman spectra of the In(OH)3 /rGO composites, there are two peaks at 299 cm−1 and 490 cm−1 , which could be assigned to the characteristic modes of In(OH)3 [45]. 3.3. Comparison of In(OH)3 /rGO composites and In2 O3 /rGO composites For comparison, the In2 O3 /rGO composites were synthesized by thermal treatment of our flower-like In(OH)3 /rGO composites in N2 atmosphere at 400 ◦ C. The XRD pattern of the product obtained after the heat treatment is investigated (see the SI, Fig. S1). As shown, for the XRD pattern of the product from In(OH)3 /rGO composites annealing in N2 flow, most of the diffraction peaks are attributed to In2 O3 (JCPDS Card, No. 22-0336). Similarly, none of obvious typical peaks assigning to rGO are observed in XRD pattern of the product, which could be ascribed to the low diffraction intensity of rGO in the In2 O3 /rGO composites [19]. The morphologies of the resulting product are characterized by FESEM (see the SI, Fig. S2a). Observed from SEM images, the hierarchical flower-like In(OH)3 is destructed during the annealing process, leading amorphous particles deposited on the surfaces of the broken flower petals. Furthermore, the graphene sheets are not clearly seen, and this may be also due to the damage effect of the heat treatment. The detailed morphology information is exhibited in high-magnification SEM image (see the SI, Fig. S2b). The hierarchical flower-like In2 O3 is composed of spiralling flower petals along the centre point, and contains rather wide space between the each petal. In addition, there are many particles filled in these spaces, hindering the gas penetration, thus may lead a degradation of sensing performances [32].
299
3.4. Sensing performances of as-synthesized In(OH)3 /rGO composites
490 GO (ID/IG=0.9223)
In(OH)3/rGO (ID/IG=1.0172) 200
500
800
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Raman shift (cm-1) Fig. 5. (a) FT-IR spectra of GO, rGO and In(OH)3 /rGO composites; (b) Raman spectra of GO and In(OH)3 /rGO.
In this paper, we focused on the sensor fabricated by In(OH)3 /rGO composites. Fig. 6a shows dynamic resistance response of the sensor based on In(OH)3 /rGO composites to various concentrations of NO2 . Dramatic decrease is easily observed with the injection of NO2 , indicating the p-type response for the sensor [46]. The sensing time are controlled at ∼180 s for all sensing tests and comparison. Clearly, as the NO2 concentration goes up, the resistance response of the sensor keeps decreasing
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Table 1 Detection of NO2 using graphene-based sensors. Sensing materials
NO2 (ppm)
Working temp (◦ C)
Response
Reference
In(OH)3 /rGO Al-decorated graphene Flexible graphene based sensors Ag-S-RGO Ethanol-based CVD graphene nanomesh Flexible RGO Ozone-treated graphene
1 1.2 5 5 10 5 5
RT 150 RT RT RT RT RT
Ra /Rg − 1 = 19.79% (cal: (Ra − Rg )/Ra = 16.52%) (Ra − Rg )/Ra = 2.89% (Ra − Rg )/Ra = 3% (Ra − Rg )/Ra = 16.7% (Ra − Rg )/Ra = 11% (Ra − Rg )/Ra = 12% (Ra − Rg )/Ra = 3.7%
Present work [8] [48] [45] [49] [50] [51]
Fig. 6. (a) Dynamic resistance response of the sensor based on In(OH)3 /rGO composites to different NO2 concentrations at room temperature; (b) the linear fitting curve of the sensor response versus the NO2 concentration.
obviously. A linear fitting curve of the sensor response versus the NO2 concentration in the range of 1–5 ppm is exhibited in Fig. 6b, and the correlation coefficient R2 of the fitting curve is 0.9822. Fig. 7 shows the response and recovery curves of as-fabricated sensor for 5 ppm NO2 at room-temperature. As indicated, the sensor exhibited good repeatability for three trials. Meanwhile, the recovery behaviour of the In(OH)3 /rGO composites-based sensor is
further investigated. In our case, like most of the semiconductorbased gas sensor operating at room temperature, the recovery time of our sensor is relatively long. However, the NO2 molecules adsorbed on the composites can be displaced rapidly at room temperature by using water vapour (see the SI, Fig. S3). During the recovery stage, the sensor is exposed upon water vapour for only one minute, and its resistance rapidly increases, even reaches a higher value above baseline. Subsequently, the sensor is transferred to air, and its resistance comes back to the initial baseline within ∼100 s. Compared with the refreshing performances in air (Fig. 6), the recovery time is largely shorten. The similar phenomenon has been also reported by Randeniya [44], and considered as a nondestructive method to improve recovery behaviour of sensors at room temperature. In our experiment, this method is also proven as an effective strategy to remove NO2 molecules from the flower-like In(OH)3 /rGO composites at room temperature. The phenomenon could be explained that the desorption of strongly bound NO2 molecules on the sensor is achieved by displacement of polar water molecules [47]. Selectivity is also evaluated for In(OH)3 /rGO composites-based gas sensor related with practical applications. Fig. 8 reveals the response of the sensor to several possible interferential gases, including NH3 , H2 , ethanol, acetone, and toluene. The sensor exhibits the largest response to NO2 , even though the concentration of other detected gases is 1000 times that of NO2 . Such results imply that the In(OH)3 /rGO composites-based gas sensor has an excellent selectivity towards NO2 gas. A comparison of the sensor responses to NO2 in the present work and literature reports is summarized in Table 1. Apparently, the asfabricated sensor in our work exhibits better sensing performances compared with those reported previously in the literature. As we all know, vacancies and defects are the major charge carriers for typical p-type rGO semiconductor [46], while In(OH)3 is a n-type semiconductor with free electrons as major charge carriers [28]. The enhanced sensing performance of the rGO sheets may be attributed to the modification of flower-like In(OH)3 in the unique hybrid architecture, especially the p-n heterojunction, which is
Resistance (kohm)
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300
on
on
on
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200 off 150
0
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off 720
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off 1440
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Time (s) Fig. 7. The response and recovery curves of the In(OH)3 /rGO composites-based sensor to 5 ppm NO2 at room-temperature.
Fig. 8. Response of the In(OH)3 /rGO composites-based sensor to different gases at room-temperature.
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formed at the interface between the flower-like In(OH)3 and rGO sheets. The effective electronic interaction between In(OH)3 and rGO facilitates the gas molecule detection via the resistance change of the hybrid composites [19]. Theoretically, oxygen molecules are directly adsorbed onto the sensor surface and capture free electrons from the conductance band of the composites, and thus generate numerous chemisorbed oxygen species [32]. After exposing sensors to NO2 gas, the high electrophilic NO2 molecules can not only further capture the electrons from the conductance band, but also react with the absorbed oxygen species, leading to the formation of adsorbed NO2 species [46]. These adsorption and reaction further capture the electrons from the p-type semiconductor, resulting in an increase of the charge carrier concentration, which eventually decrease the sensor resistance [32]. During the refreshing process, the absorbed NO2 species are released, thus leading a recovery of the initial condition. 4. Conclusions In summary, we have synthesized hierarchical flower-like In(OH)3 modified graphene by a facile one-step microwaveassisted hydrothermal method. The sensor based on as-synthesized In(OH)3 /rGO composites exhibited a significant response to NO2 with a concentration lower to 1 ppm. An excellent selectivity towards NO2 gas was also observed, even though the other tested gas concentrations were 1000 times that of NO2 . Moreover, the recovery behaviour of the flower-like In(OH)3 modified graphenebased sensor can be greatly improved in exposure to the water vapour. The sensor also showed good shelf-life (see the SI, Fig. S4). The enhanced NO2 sensing performances may be attributed to the synergistic effect of functional In(OH)3 and graphene sheets in the unique hybrid architectures. Acknowledgement The work was supported through the foundation (Grant no. DUT12RC (3) 84), which is funded by Dalian University of Technology. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2015.02.100. References [1] X. Huang, X.Y. Qi, F. Boey, H. Zhang, Graphene-based composites, Chem. Soc. Rev. 41 (2012) 666–686. [2] W.J. Yuan, A.R. Liu, L. Huang, C. Li, G.Q. Shi, High-performance NO2 sensors based on chemically modified graphene, Adv. Mater. 25 (2013) 766–771. [3] G.H. Lu, S. Park, K.H. Yu, R.S. Ruoff, L.E. Ocola, D. Rosenmann, J.H. Chen, Toward practical gas sensing with highly reduced graphene oxide: a new signal processing method to circumvent run-to-run and device-to-device variations, ACS Nano 5 (2011) 1154–1164. [4] F. Schedin, A.K. Geim, S.V. Morozov, E.W. Hill, P. Blake, M.I. Katsnelson, K.S. Novoselov, Detection of individual gas molecules adsorbed on graphene, Nat. Mater. 6 (2007) 652–655. [5] V. Dua, S.P. Surwade, S. Ammu, S.R. Agnihotra, S. Jain, K.E. Roberts, S. Park, R.S. Ruoff, S.K. Manohar, All-organic vapor sensor using inkjet-printed reduced graphene oxide, Angew. Chem. Int. Ed. 49 (2010) 2154–2157. [6] J.D. Fowler, M.J. Allen, V.C. Tung, Y. Yang, R.B. Kaner, B.H. Weiller, Practical chemical sensors from chemically derived graphene, ACS Nano 3 (2009) 301–306. [7] J.T. Robinson, F.K. Perkins, E.S. Snow, Z.Q. Wei, P.E. Sheehan, Reduced graphene oxide molecular sensors, Nano Lett. 8 (2008) 3137–3140. [8] B. Cho, J. Yoon, M.G. Hahm, D.H. Kim, A.R. Kim, Y.H. Kahng, S.W. Park, Y.J. Lee, S.G. Park, J.D. Kwon, C.S. Kim, M. Song, Y. Jeong, K.S. Nam, H.C. Ko, Graphenebased gas sensor: metal decoration effect and application to a flexible device, J. Mater. Chem. C 2 (2014) 5280–5285.
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Biographies Peng Wan received her PhD degree in 2009 from Wuhan University. Currently she is a staff member in Faculty of Chemical, Environmental and Biological Science and Technology in Dalian University of Technology. Wei Yang is currently a PhD student in Wuhan University. His research focuses on the development of semiconductor sensors. Xinnan Wang received her PhD degree in 2013 from Jilin University. She is a staff member in faculty of Chemical, Environmental and Biological Science and Technology in Dalian University of Technology. Jiming Hu received his PhD degree in 1988 from Wuhan University. Currently he is a full professor in Wuhan University. His main research focuses on the Raman spectroscopy. Hua Zhang received his PhD degree from Dalian University of Technology. He currently is a full professor in faculty of Chemical, Environmental and Biological Science and Technology in Dalian University of Technology.