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α-Fe2O3 hybrid nanocomposites for room temperature NO2 sensing
Accepted Manuscript Title: Reduced graphene oxide/␣-Fe2 O3 hybrid nanocomposites for room temperature NO2 sensing Author: Hao Zhang Li Yu Qun Li Yu Du Shuangchen Ruan PII: DOI: Reference:
S0925-4005(16)31687-2 http://dx.doi.org/doi:10.1016/j.snb.2016.10.059 SNB 21117
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
7-7-2016 8-10-2016 14-10-2016
Please cite this article as: Hao Zhang, Li Yu, Qun Li, Yu Du, Shuangchen Ruan, Reduced graphene oxide/␣-Fe2O3 hybrid nanocomposites for room temperature NO2 sensing, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2016.10.059 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Reduced graphene oxide/α-Fe2O3 hybrid nanocomposites for room temperature NO2 sensing
Hao Zhanga,b, Li Yua,b, Qun Lic, Yu Duc*, Shuangchen Ruana,b*
a. Shenzhen Key Laboratory of Laser Engineering, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China. b. Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China c. Shenzhen Key Laboratory of Sensor Technology, College of Physics Science and Technology, Shenzhen University, Shenzhen 518060, China.
*Corresponding authors: E-mail address: [email protected] (Y.Du); [email protected](S.C. Ruan) Tel.: +86 755-26538886;
1
Highlights
We fabricated a gas sensor based on reduced graphite oxide (rGO)/α-Fe2O3 nanocomposite.
The gas sensor exhibited excellent NO2 sensing properties at room temperature.
The
gas
sensing
properties
of
nanocomposite
rely
to
the
α-Fe2O3-functionalization on pure graphene.
Abstract
The reduced graphene oxide/α-Fe2O3 (rGO/α-Fe2O3) hybrid nanocomposites have been prepared by a facile hydrothermal approach. The components and morphologies of the as-synthesized products were analyzed by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). NO2 sensing properties of rGO/α-Fe2O3 nanocomposites are tested at room temperature (25oC) with NO2 concentration ranging from 0.1 to 5.0 ppm. It is demonstrated that the rGO/α-Fe2O3 nanocomposites based sensor exhibited enhanced response (3.86) for 5 ppm NO2 compared to pure rGO (1.38). The improved sensing features can be attributed to this hybrid sensor of synergistic effect between the highly conductive graphene and the α-Fe2O3 nanoparticles.
Keywords: Graphene, α-Fe2O3, NO2 gas sensing, room temperature. 2
1. Introduction
Since air pollution has become an urgent global problem with the development of industry and technology, gas sensors used for process control in chemical industries, detection of toxic environmental pollutants, and the prevention of hazardous gas leaks has become increasingly significant. NO2 as a typical air pollutant mainly comes from the exhaust gases of combustion processes, which affect respiratory system of human beings and animals [1,2], and is a major cause of acid rain, photo-chemical smog and pollution haze. Recently efforts are being made to detect trace levels of NO2 such as chemiluminescence [3], ion chromatography [4], spectrophotometry [5], etc. Among different detection research, Graphene hybrid nanomaterials have been widely studied for gas sensor applications because of their high sensitivity to electrical perturbations from gas molecule adsorption as a result of their ultra-small thickness [6]. Graphene, a one-atom-thick and two-dimensional honeycomb lattice structure, has remarkable properties such as Young’s modulus (∼1100 GPa), fracture strength (125 GPa), thermal conductivity (∼5000 Wm−1 K−1), mobility of charge carriers (200,000 cm2V−1s−1), specific surface area (theoretical value of 2630 m2g−1), magnetism and fascinating transport phenomena[7]. Among the different methods to prepare graphene, the chemical and thermal reduced graphene oxide (rGO) derived from graphene oxide (GO) process based on Hummers’ method was mostly used [8]. Graphene oxide (GO) can form well-dispersed aqueous colloids without stabilizers [9-11], due to electrostatic repulsion of the versatile oxygen-containing groups (OCGs). Also it is of particular interest that the aqueous colloidal dispersion of GO 3
can be used as the starting support material for fabricating advanced graphene-based nanomaterials [12].
Reduced graphene oxide is expected to be promising sensing
materials due to its semiconductor properties. Recently, the synthesis of hybrid nanomaterials based on graphene has flashed enormous research interest [13-17]. To promote the gas sensing characteristic of the rGO based sensors, rGO decorated with metal oxides have been proposed. For instance, Yang and co-workers has fabricated gas sensors based on Pd/SnO2/RGO ternary composites for detection for NH3 at room temperature [18]; Jiang et al. have constructed sensor using rGO/TiO2 layered film as sensing materials for detection of fomaldehyde [19]; Chung and co-authors have fabricated gas sensors based on Pd nanocube–graphene hybrids for detection of H2 at room temperature [20]. Furthermore, In2O3–graphene [21] nanocomposites, Graphene/WO3 [22], CuxO nanoflower/graphene composites [23], SnO2/S-rGO hybrids [24] have also been used for detection of NO2 at room temperature. It’s obvious the introduction of metal oxides has improved the sensing properties of pure graphene. For instance, the response and recovery process can be accelerated from almost an hour to several minutes without heating or ultraviolate light. However, the sensing performances should be further enhanced for their potential applications, especially the room temperature sensitivity. As an important gas sensing material, α-Fe2O3 is an n-type semiconductor (Eg = 2.1 eV). Recently, Wang et al. reported on the synthesis of nanostructures via a simple solvothermal method, and the product presents excellent ethanol sensitivity, quick response and recovery times because of the interaction between graphene and α-Fe2O3 4
[25]. Sun et al. prepared mesoporous α-Fe2O3 nanostructures by the soft template synthesis method, presenting better gas sensing performance towards acetic acid and ethanol gas than its nanosphere and nanowire counterparts [26]. Inspired by the aforementioned concepts, the formation of a heterojunction by introducing metal oxides into graphene composites will enhance the sensing performance. Numerous studies have proved that the heterojunction formed by n-type and p-type materials can play a positive role in the sensing process [27–31]. In our work, we develop a one-step facile hydrothermal route to prepare rGO/α-Fe2O3 nanocomposites. A gas sensor based on rGO/α-Fe2O3 nanocomposites is fabricated and applied for detection of NO2 at room temperature. Compared with pure rGO sensor, the rGO/α-Fe2O3 sensor showed much better gas properties for NO2. In addition, a possible sensing mechanism of the high-performance fabricated sensor for the detection of NO2 is also discussed.
2. Experimental
2.1. Chemicals Graphite powder, H2O2 (30%), NaNO3, H2SO4 (98%), Fe(NO3)3·9H2O, KMnO4, CH3COONa·3H2O were purchased from Beijing Chemical Corp. All chemicals were used as received without further purification. 2.2. Preparation of GO GO was prepared from natural graphite powder through a modified Hummers’ method [32]. In a typical synthesis, 1 g of graphite was added into 23 mL of H2SO4, 5
followed by stirring at room temperature for 24 h. After that, 100 mg of NaNO 3 was introduced into the mixture and stirred for 30 min. Subsequently, the mixture was kept below 5 °C by ice bath, and 3 g of KMnO4 was slowly added into the mixture. After being heated to 35-40 °C, the mixture was stirred for another 30 min. Afterwards, 46 mL of water was added into above mixture during a period of 25min and the mixture was heated to 95 °C under stirring for 15 min. Finally, 140 mL of water and 10 mL of H2O2 were added into the mixture to stop the reaction. After the unexploited graphite in the resulting mixture was removed by centrifugation, as synthesized GO was dispersed into individual sheets in distilled water at a concentration of 1 mg/mL with the aid of ultrasound for further use. 2.3. Preparation of rGO/α-Fe2O3 nanocomposites In a typical synthesis, 0.5 ml of GO (1 mg mL -1) suspension were added into 20 mL of deionized water, followed by stirring for 10 min to obtain a homogeneous yellow-brown colloidal dispersion. Afterwards, 0.024 g of Fe(NO3)3·9H2O was added into the GO solution under stirring, which was sonicated for 40 min at room temperature, followed by adding of CH3COONa·3H2O. After sonication for another 30 min, the aqueous dispersion was transferred into a 40 mL Teflon-lined, stainless-steel autoclave and heated at 180 °C for 12 h. After being cooled to room temperature, the products were filtered, washed with distilled water and ethanol, and dried in a vacuum oven at 60◦C for 24 h. As a comparison, rGO was prepared in the same way without adding of CH3COONa·3H2O and Fe(NO3)3·9H2O. 2.4. Characterizations 6
The composition and phase of the as-obtained products were characterized by X-ray
powder
diffraction
(XRD,
Rigaku
D/max-2500)
with
graphite
monochromatized and Cu Kα λ= 0.15418 nm. X-ray photoelectron spectroscopy (XPS) analysis was measured on an ESCALAB MK II X-ray photoelectron spectrometer using Mg as the exciting source. Raman spectra were obtained on J-YT64000 Raman spectrometer with 514.5 nm wavelength incident laser light. The structure and morphology of the samples were obtained by the transmission electron microscopy (TEM, JEOL JEM-3010). Field emission scanning electron microscope (FESEM: HITACHI, Japan, SU8010) images of rGO/α-Fe2O3 were also taken. 2.5. Fabrication and gas sensing measurements The prepared rGO/α-Fe2O3 nanocomposites were mixed with deionized water to obtain solution in a mortar. Fig. 1 shows a schematic image and a photograph of this sensor substrate. The droplet has covered a ceramic plate (6mm×3mm, 0.5mm in thickness) in which a pair of interdigitated gold electrodes (electrodes width and distance: 0.15mm) was printed by photolithographing. The sensor was heated at 70 °C for 5 h before testing. The measurement was carried out by placing the sensor in a glass vessel with a given concentration of target gas. The response of the gas sensor is defined as the ratio of the resistance of the sensor tested in air (Ra) to that tested in the detection gases (Rg). For oxidizing tested gases, Response = Ra/Rg, while for the reducing tested gases, Response = Rg/Ra. The response (adsorption) or recovery (desorption) time was estimated as the time taken by the sensor resistance output to reach 90% of its total resistance change after each process of applying or clearing the 7
testing gas. The gas-sensing properties of sensors were all measured using a CGS-8 gas-sensing characterization system at room temperature (25oC).
3. Results and discussion
3.1. Structural and morphological characteristics The phase composition of the hybrid nanocomposites is analyzed by XRD. Fig. 2 shows the XRD patterns of the GO and rGO/α-Fe2O3 samples. It demonstrates that GO exhibits a strong diffraction peak at 2θ of 10.92° attributed to (002) diffraction peak of GO (Fig. 2a), indicating the successful preparation of GO by oxidation of graphite [33]. However, no typical diffraction peaks of GO or graphene are observed in the XRD pattern of the rGO/α-Fe2O3 hybrids. The faded peak of GO is attributed to the fact that the regular layers tacking of GO can be destroyed by the growth of α-Fe2O3 nanoparticles during the hydrothermal reaction, leading to the exfoliation of GO sheets [34, 35]. Furthermore, compared to XRD pattern of rGO, seven diffraction peaks at 2θ of 24.48, 33.54, 35.82, 41.24, 49.70, 54.34, 62.84, and 64.44° are observed for the rGO/α-Fe2O3 nanocomposites (Fig. 2b), which are indexed as (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4), (1 1 6), (2 1 4) and (3 3 0) diffractions of α-Fe2O3 (JCPDS No: 79-1741) and no other phase is observed in the patterns [36]. Fig. 3 shows the Raman spectroscopy confirming the reduction of GO. The two strong peaks of the D-band (～1351 cm−1) and G-band (～1595 cm−1) were observed, corresponding to the diamondoid and graphitic graphene structures, respectively. It is noticeable that the intensity ratio of D-band to G-band of the rGO and rGO/α-Fe2O3 8
nanocomposites is larger than that of GO. This change suggests a decrease in the average size of the sp2 domains upon reduction of the exfoliated GO, which can be explained if new graphitic domains were created that are smaller in size to the ones present in GO before reduction, but more numerous in number [37]. To further investigate the compositions of the products, XPS technique was applied to analyze the chemical states of GO and rGO/α-Fe2O3 nanocomposites. Fig. 4a shows the XPS survey spectrum of the rGO/α-Fe2O3 sample. It is well known that the binding energy at 284.6 and 530.2 eV are assigned to the C 1s and O 1s species, respectively [38]. Two distinct peaks at binding energies of ca. 710.9 and ca. 724.8 eV with a shake-up satellite at ca.720.6 eV showed in Fig. 4b demonstrate that the Fe species in the nanocomposites are in the form of hematite (Fe2O3) phase [39], without any impurities of Fe0 [40], verified by the absence of its characteristic peaks. Fig. 4c and Fig. 4d reveal the deconvoluted C 1s spectra of the GO and rGO/α-Fe2O3 samples, respectively. Three peaks at 284.6, 286.6 and 288.4 eV are respectively attributed to the C-C, C-O and C=O species in graphene-based materials [41]. After hydrothermal treatment, the peak intensity of C-O and C=O of the rGO/α-Fe2O3 nanocomposites (Fig. 4d) tremendously decreases compared to that of GO (Fig. 4c), suggesting that the most of oxygen-containing groups are successfully removed after hydrothermal treatment. Taken together, the rGO/α-Fe2O3 hybrid nanocomposites have been successfully prepared after a facile hydrothermal treatment of the precursors including GO, CH3COONa·3H2O and Fe(NO3)3·9H2O. Fig. 5 shows the typical TEM and SEM images of the rGO/α-Fe2O3 9
nanocomposites. It exhibits that the rGO sheet with the size of several micrometers served as a platform is decorated with a large number of uniform α-Fe2O3 nanoparticles with the average size of 40 nm as active sites for gas adsorption (Fig. 5b and d). The absence of isolated Fe2O3 nanoparticles reveals that the hydrothermal treatment of GO and Fe(NO3)3·9H2O solution is an effective method to synthesize rGO/α-Fe2O3 nanocomposites. 3.2. NO2 sensing properties The sensing performance of rGO and rGO/α-Fe2O3 nanocomposites towards NO2 at room temperature is investigated in Fig. 6. The control experiments reveal that the pure rGO used as sensing material exhibits a low response (1.38), ultralong response time (2359 s) and recovery time (40130 s) (Fig 6a) upon exposure to the NO2 gas and air. Thus, the adsorption and desorption of the NO2 gas on rGO-based sensors are considered to be a thorn for gas sensing. However, by using α-Fe2O3 nanoparticles loaded on rGO (i.e., rGO/α-Fe2O3 nanocomposites) as sensing material, it demonstrates that the response/recover speed is dramatically accelerated, suggested by the significantly decreased response time (32 s) and recovery time (1435 s) (Fig. 6b). Moreover, the response of the rGO/α-Fe2O3 nanocomposites (3.86) is 2.8 times higher than that of rGO (1.38). These results indicate that the α-Fe2O3 nanoparticles in the hybrid composites played key roles in determination of sensor performances. Moreover, we can’t evaluate the gas sensing performance of α-Fe2O3, because it couldn’t response to NO2 at room temperature. Fig. 7 shows the response curve of rGO/α-Fe2 O3 sensor orderly exposed to 10
0.1-5.0 ppm NO2 at room temperature. It is clear that the response and recovery performances are consistent with the results exhibited in Fig. 6b. The response gradually decreases as the concentration of NO2 decreases, e.g., the response of the sensor decreased from 3.86 to 2.19, 1.42, 1.26, and 1.17 with the concentration of NO2 decreased from 5.0 to 1.0, 0.5, 0.2 and 0.1 ppm. Fig. 8 illustrates the reproducibility of the temporal curve of the rGO/α-Fe2O3 based sensor. The sensor maintains its initial response amplitude without a significant decrease upon three consecutive sensing tests to 0.1 ppm of NO2. This indicates that the rGO/α-Fe2O3 based sensor possess good stability upon cyclic tests. The selectivity of rGO/α-Fe2O3 nanocomposites based sensor was tested by exposing the sensor to the potential interference gases shown in Fig. 9a. Take 5.0 ppm of the target gases for example, the responses of the sensor to oxidizing gas (NO2, Cl2) and reducing gas (NO, CO, CH4, H2, NH3) are 3.86, 1.15, 1.76, 1.31, 1.18, 1.23, and 1.27, respectively, indicating the rGO/α-Fe2O3 nanocomposites based sensor shows negligible response to these interference gases compared to NO2 gas, and the humid air with relative humidity of 33% (33% RH) doesn’t have effect on gas sensing for rGO/α-Fe2O3 sensor (The relative humidity value in the measurement environment is 25%) either. Additionally, we have measured the NO2 response at different relative humidity, the response doesn’t fluctuate obviously (Fig. 9b). Therefore, it can be concluded that the rGO/α-Fe2O3 nanocomposites exhibit high selectivity for NO2 sensing at room temperature. A Comparison of NO2 sensing performances using rGO/α-Fe2O3 nanocomposites as sensing material and other reported rGO-based 11
sensing materials is presented in Table 1 [42-43, 27-29]. The rGO/α-Fe2O3 nanocomposites displays the highest sensitivity to NO2 at room temperature compared to other rGO-based materials reported elsewhere. 3.3. Gas sensing mechanism of α-Fe2O3 It is found that the pure rGO exhibits relatively weak response and long response and recovery times for detection of NO2 at room temperature [22, 44]. The dissatisfactory sensing properties of rGO result from its constituting carbon atoms. There are two types of carbon atoms in graphene, sp 2 hybridized carbon atoms constituting graphite structure, sp3 hybridized carbon atoms constituting structural defects and forming a chemical bond with oxygen-containing groups, the adsorption energy of the latter is larger (5.7 kcal/mol), resulting in slower adsorption and desorption [45, 46]. The sensing mechanism of the rGO/α-Fe2O3 sensor should follow the surface controlled type, which may be explained by the changing of resistance upon exposing to different gases. In open air, formation of oxygen adsorbates (O2-, O2- and O-) on the surface of α-Fe2O3 and rGO contributes to an electron-depleted surface layer owing to electron transfer from the α-Fe2O3 to oxygen in a similar fashion as for the reported literature [47]. When exposed to an oxidizing gas (NO2), the NO2 molecules react with the chemisorbed oxygen, and could attract electrons from the rGO/α-Fe2O3 because of the high electron affinity of NO2 molecule (Fig 10). The improvement of NO2 sensing performance at room temperature is attributed to the following factors. Firstly, the principle of gas sensing for the resistance-type 12
sensors is based on the conductance variations for target gas, thus the introduction of rGO significantly result in improving the conductivity, leading to gas sensing performance at room temperature. Secondly, benefited from the existence of α-Fe2O3, the gas adsorption and diffusion rate on the active surface is facilitated due to the increasing number of active sites (such as vacancies, defects, oxygen functional groups as well as the sp2-bonded carbon) for the adsorption of NO2 molecules, Additionally, the introduction of α-Fe2O3 prevents the rGO sheets from restacking, thus leading to a good surface accessibility and effective gas diffusion between parallel layers of graphene. Thirdly, the enhancement of gas sensing performance could be attributed to interaction between rGO and α-Fe2O3, when the rGO were decorated with the α-Fe2O3 nanoparticles, a depletion zone may be formed at the interface, the depletion zone around the interface of rGO and α-Fe2O3 nanorods results in the specific capture and migration of gas molecules. Finally, NO2 is a strong oxidizing gas molecule and it tends to interact with the active sites on rGO through its N atom, which enhances the gas sensing ability of the oxygen functional groups [48]. 4. Conclusion RGO/α-Fe2O3 nanocomposites have been synthesized by one-step hydrothermal method. The gas sensing performances indicate that the introducing of the α-Fe2O3 nanoparticles onto rGO matrix greatly enhances the response and adsorption/diffusion rate for NO2 sensing at room temperature. Moreover, it displays excellent selectivity and low detection limit down to 0.1 ppm.
These results indicate that α-Fe2O3
nanoparticles loaded on reduced graphene oxide could be a candidate material to 13
fabricate high performance NO2 sensor at room temperature.
Acknowledgements
This research work was financially supported by National Natural Science Foundation of China (Grant No. 61575129, 61275144), Guangdong Natural Science Foundation (2016A030313059), State Key Laboratory of Inorganic Synthesis and Preparative Chemistry Open Project (2015-10), and Natural Science Foundation of SZU (82700002601).
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[39] Z.G. An, J.J. Zhang, S.L. Pan, F. Yu, Facile Template-free synthesis and characterization of elliptic α-Fe2O3 superstructures, J. Phys. Chem. C 113 (2009) 8092-8096. [40] F. Tihay, G. Pourroy, M. Richard-Plouet, A.C. Roger, A. Kiennemann, Effect of Fischer–Tropsch synthesis on the microstructure of Fe–Co-based metal/spinel composite materials, Appl Catal. A 206 (2001) 29-42. [41] S. Pei, J. Zhao, J. Du, W. Ren, H.M. Cheng, Direct reduction of graphene oxide films into highly conductive and flexible graphene films by hydrohalic acids, Carbon 48 (2010) 4466-4474. [42] G. Lu, L.E. Ocola, J. Chen, Reduced graphene oxide for room-temperature gassensors, Nanotechnology 20 (2009) 445502. [43] H. Zhang, J. Feng, T. Fei, S. Liu, T. Zhang, SnO 2 nanoparticles-reduced grapheneoxide nanocomposites for NO2 sensing at low operating temperature, Sens.Actuators B 190 (2014) 472–478. [44] G. Lu, L.E. Ocola, J. Chen, Gas detection using low-temperature reduced graphene oxide sheets, Appl. Phys. Lett. 94 (2009) 083111. [45] 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. [46] D. Li, M. B. Müller, S. Gilje, R. B. Kaner, G. G. Wallace, Processable aqueous dispersions of graphene nanosheets. Nat. Nanotechnol. 3 (2008) 101-105.
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21
Biographies Hao Zhang received his PhD from the College of Electronics Science and Engineering, Jilin University, China in 2015, majoring in microelectronics and solid-state electronics. Currently he is a post doctor at the college of optoelectronic engineering of Shenzhen University. He is studying the synthesis and characterization of functional materials and gas sensors.
Li Yu received his PhD from Future Industries Institute, University of South Australia in 2016, majoring in Materials and Surface Chemistry. Currently he is a Postdoctoral Fellow at College of Optoelectronic Engineering of Shenzhen University. His research focus is Layer-by-Layer assembly of polyelectrolytes, ultrathin nanosheets and their functionalities.
Qun Li is currently working towards the MS degree in the materials and chemical institute, Hainan University. Her research include the synthesis of metal oxides semiconductor materials and their applications in gas sensor
Yu Du received her PhD from chemistry of Jilin University, China in 2007. After that , she had been worked as the Postdoctoral at Nanyang Technological University, Sing apore for about two years. She is currently an Associate Professor at the college of Ph ysics and Energy Shenzhen University, China. Her current research interests are nano science and gas sensors. 22
Shuangchen Ruan was born on Oct.8 1963. He received the B.S. and M.S. degrees from northwest university in China in 1986 and 1989, and Ph.D. degree in Tianjin University in 2004. He has been the Professor since 1994. His research interests focus on solid state laser, fiber laser, and ultrafast pulse generation.
23
Figure captions: Fig. 1 (a) Schematic illustration and (b) photographic image of the rGO/α-Fe2O3 hybrid nanocomposite-based sensor.
24
Fig. 2 The XRD patterns of the products: (a) GO (black line), (b) rGO/α-Fe2O3 samples.
25
Fig. 3 Raman spectroscopy of the GO (black line), rGO (red line) and rGO/α-Fe2O3 (blue line) samples.
26
Fig. 4 (a) XPS spectrum of rGO/α-Fe2O3, (b) Fe2p spectrum of rGO/α-Fe2O3, (c) C1s spectrum of GO, (d) C1s spectrum of rGO/α-Fe2O3.
27
Fig. 5 (a, b) TEM and (c, d) SEM images of the rGO/α-Fe2O3 nanocomposites. High magnification (b) TEM and (d) SEM images of the selected area marked in (a) and (c), respectively.
28
Fig. 6 The dynamic response curve of (a) the rGO and (b) rGO/α-Fe2O3 nanocomposites based sensors to 5 ppm NO2 at room temperature with 25% RH.
29
Fig. 7 Dynamic NO2 sensing transients curve of the rGO/α-Fe2O3 nanocomposites based sensor to NO2 gases with concentration ranging from 0.1 to 5.0 ppm at room temperature with 25% RH.
30
Fig. 8 The reproducibility of the rGO/α-Fe2O3 based sensor upon 3 cyclic consecutive exposure to 0.1 ppm NO2 at room temperature with 25% RH.
31
Fig. 9 The responses of the rGO/α-Fe2O3 nanocomposites based sensor (a) to 5 ppm of different gases (showed in Figure legends) at room temperature and (b) to 5 ppm NO2 at different relative humidity at room temperature.
32
Fig. 10 The scheme of the proposed gas sensing mechanism: the adsorption behavior of NO2 molecules on the rGO/α-Fe2O3 nanocomposite.
33
Table 1 Performance of graphene-based gas sensors for NO2 detection Materials
NO2 gas concentration
Working temperature
Response
Response/recovery time
References
rGO
100 ppm
RT
1.56
30min/-
[41]
Graphene–WO3
20 ppm
RT
2.80
20 s/-
[42]
CuxO/Graphene
97 ppm
RT
95.1%
-
[26]
SnO2/S-rGO
5 ppm
RT
1.20
40 s/357 s
[27]
SnO2–rGO
5 ppm
50 C
3.31
135 s/200 s
[28]
rGO/α-Fe2O3
5 ppm
RT
3.86
76 s/946 s
This work
o
34
S0925-4005(16)31687-2 http://dx.doi.org/doi:10.1016/j.snb.2016.10.059 SNB 21117
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
7-7-2016 8-10-2016 14-10-2016
Please cite this article as: Hao Zhang, Li Yu, Qun Li, Yu Du, Shuangchen Ruan, Reduced graphene oxide/␣-Fe2O3 hybrid nanocomposites for room temperature NO2 sensing, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2016.10.059 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Reduced graphene oxide/α-Fe2O3 hybrid nanocomposites for room temperature NO2 sensing
Hao Zhanga,b, Li Yua,b, Qun Lic, Yu Duc*, Shuangchen Ruana,b*
a. Shenzhen Key Laboratory of Laser Engineering, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China. b. Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China c. Shenzhen Key Laboratory of Sensor Technology, College of Physics Science and Technology, Shenzhen University, Shenzhen 518060, China.
*Corresponding authors: E-mail address: [email protected] (Y.Du); [email protected](S.C. Ruan) Tel.: +86 755-26538886;
1
Highlights
We fabricated a gas sensor based on reduced graphite oxide (rGO)/α-Fe2O3 nanocomposite.
The gas sensor exhibited excellent NO2 sensing properties at room temperature.
The
gas
sensing
properties
of
nanocomposite
rely
to
the
α-Fe2O3-functionalization on pure graphene.
Abstract
The reduced graphene oxide/α-Fe2O3 (rGO/α-Fe2O3) hybrid nanocomposites have been prepared by a facile hydrothermal approach. The components and morphologies of the as-synthesized products were analyzed by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). NO2 sensing properties of rGO/α-Fe2O3 nanocomposites are tested at room temperature (25oC) with NO2 concentration ranging from 0.1 to 5.0 ppm. It is demonstrated that the rGO/α-Fe2O3 nanocomposites based sensor exhibited enhanced response (3.86) for 5 ppm NO2 compared to pure rGO (1.38). The improved sensing features can be attributed to this hybrid sensor of synergistic effect between the highly conductive graphene and the α-Fe2O3 nanoparticles.
Keywords: Graphene, α-Fe2O3, NO2 gas sensing, room temperature. 2
1. Introduction
Since air pollution has become an urgent global problem with the development of industry and technology, gas sensors used for process control in chemical industries, detection of toxic environmental pollutants, and the prevention of hazardous gas leaks has become increasingly significant. NO2 as a typical air pollutant mainly comes from the exhaust gases of combustion processes, which affect respiratory system of human beings and animals [1,2], and is a major cause of acid rain, photo-chemical smog and pollution haze. Recently efforts are being made to detect trace levels of NO2 such as chemiluminescence [3], ion chromatography [4], spectrophotometry [5], etc. Among different detection research, Graphene hybrid nanomaterials have been widely studied for gas sensor applications because of their high sensitivity to electrical perturbations from gas molecule adsorption as a result of their ultra-small thickness [6]. Graphene, a one-atom-thick and two-dimensional honeycomb lattice structure, has remarkable properties such as Young’s modulus (∼1100 GPa), fracture strength (125 GPa), thermal conductivity (∼5000 Wm−1 K−1), mobility of charge carriers (200,000 cm2V−1s−1), specific surface area (theoretical value of 2630 m2g−1), magnetism and fascinating transport phenomena[7]. Among the different methods to prepare graphene, the chemical and thermal reduced graphene oxide (rGO) derived from graphene oxide (GO) process based on Hummers’ method was mostly used [8]. Graphene oxide (GO) can form well-dispersed aqueous colloids without stabilizers [9-11], due to electrostatic repulsion of the versatile oxygen-containing groups (OCGs). Also it is of particular interest that the aqueous colloidal dispersion of GO 3
can be used as the starting support material for fabricating advanced graphene-based nanomaterials [12].
Reduced graphene oxide is expected to be promising sensing
materials due to its semiconductor properties. Recently, the synthesis of hybrid nanomaterials based on graphene has flashed enormous research interest [13-17]. To promote the gas sensing characteristic of the rGO based sensors, rGO decorated with metal oxides have been proposed. For instance, Yang and co-workers has fabricated gas sensors based on Pd/SnO2/RGO ternary composites for detection for NH3 at room temperature [18]; Jiang et al. have constructed sensor using rGO/TiO2 layered film as sensing materials for detection of fomaldehyde [19]; Chung and co-authors have fabricated gas sensors based on Pd nanocube–graphene hybrids for detection of H2 at room temperature [20]. Furthermore, In2O3–graphene [21] nanocomposites, Graphene/WO3 [22], CuxO nanoflower/graphene composites [23], SnO2/S-rGO hybrids [24] have also been used for detection of NO2 at room temperature. It’s obvious the introduction of metal oxides has improved the sensing properties of pure graphene. For instance, the response and recovery process can be accelerated from almost an hour to several minutes without heating or ultraviolate light. However, the sensing performances should be further enhanced for their potential applications, especially the room temperature sensitivity. As an important gas sensing material, α-Fe2O3 is an n-type semiconductor (Eg = 2.1 eV). Recently, Wang et al. reported on the synthesis of nanostructures via a simple solvothermal method, and the product presents excellent ethanol sensitivity, quick response and recovery times because of the interaction between graphene and α-Fe2O3 4
[25]. Sun et al. prepared mesoporous α-Fe2O3 nanostructures by the soft template synthesis method, presenting better gas sensing performance towards acetic acid and ethanol gas than its nanosphere and nanowire counterparts [26]. Inspired by the aforementioned concepts, the formation of a heterojunction by introducing metal oxides into graphene composites will enhance the sensing performance. Numerous studies have proved that the heterojunction formed by n-type and p-type materials can play a positive role in the sensing process [27–31]. In our work, we develop a one-step facile hydrothermal route to prepare rGO/α-Fe2O3 nanocomposites. A gas sensor based on rGO/α-Fe2O3 nanocomposites is fabricated and applied for detection of NO2 at room temperature. Compared with pure rGO sensor, the rGO/α-Fe2O3 sensor showed much better gas properties for NO2. In addition, a possible sensing mechanism of the high-performance fabricated sensor for the detection of NO2 is also discussed.
2. Experimental
2.1. Chemicals Graphite powder, H2O2 (30%), NaNO3, H2SO4 (98%), Fe(NO3)3·9H2O, KMnO4, CH3COONa·3H2O were purchased from Beijing Chemical Corp. All chemicals were used as received without further purification. 2.2. Preparation of GO GO was prepared from natural graphite powder through a modified Hummers’ method [32]. In a typical synthesis, 1 g of graphite was added into 23 mL of H2SO4, 5
followed by stirring at room temperature for 24 h. After that, 100 mg of NaNO 3 was introduced into the mixture and stirred for 30 min. Subsequently, the mixture was kept below 5 °C by ice bath, and 3 g of KMnO4 was slowly added into the mixture. After being heated to 35-40 °C, the mixture was stirred for another 30 min. Afterwards, 46 mL of water was added into above mixture during a period of 25min and the mixture was heated to 95 °C under stirring for 15 min. Finally, 140 mL of water and 10 mL of H2O2 were added into the mixture to stop the reaction. After the unexploited graphite in the resulting mixture was removed by centrifugation, as synthesized GO was dispersed into individual sheets in distilled water at a concentration of 1 mg/mL with the aid of ultrasound for further use. 2.3. Preparation of rGO/α-Fe2O3 nanocomposites In a typical synthesis, 0.5 ml of GO (1 mg mL -1) suspension were added into 20 mL of deionized water, followed by stirring for 10 min to obtain a homogeneous yellow-brown colloidal dispersion. Afterwards, 0.024 g of Fe(NO3)3·9H2O was added into the GO solution under stirring, which was sonicated for 40 min at room temperature, followed by adding of CH3COONa·3H2O. After sonication for another 30 min, the aqueous dispersion was transferred into a 40 mL Teflon-lined, stainless-steel autoclave and heated at 180 °C for 12 h. After being cooled to room temperature, the products were filtered, washed with distilled water and ethanol, and dried in a vacuum oven at 60◦C for 24 h. As a comparison, rGO was prepared in the same way without adding of CH3COONa·3H2O and Fe(NO3)3·9H2O. 2.4. Characterizations 6
The composition and phase of the as-obtained products were characterized by X-ray
powder
diffraction
(XRD,
Rigaku
D/max-2500)
with
graphite
monochromatized and Cu Kα λ= 0.15418 nm. X-ray photoelectron spectroscopy (XPS) analysis was measured on an ESCALAB MK II X-ray photoelectron spectrometer using Mg as the exciting source. Raman spectra were obtained on J-YT64000 Raman spectrometer with 514.5 nm wavelength incident laser light. The structure and morphology of the samples were obtained by the transmission electron microscopy (TEM, JEOL JEM-3010). Field emission scanning electron microscope (FESEM: HITACHI, Japan, SU8010) images of rGO/α-Fe2O3 were also taken. 2.5. Fabrication and gas sensing measurements The prepared rGO/α-Fe2O3 nanocomposites were mixed with deionized water to obtain solution in a mortar. Fig. 1 shows a schematic image and a photograph of this sensor substrate. The droplet has covered a ceramic plate (6mm×3mm, 0.5mm in thickness) in which a pair of interdigitated gold electrodes (electrodes width and distance: 0.15mm) was printed by photolithographing. The sensor was heated at 70 °C for 5 h before testing. The measurement was carried out by placing the sensor in a glass vessel with a given concentration of target gas. The response of the gas sensor is defined as the ratio of the resistance of the sensor tested in air (Ra) to that tested in the detection gases (Rg). For oxidizing tested gases, Response = Ra/Rg, while for the reducing tested gases, Response = Rg/Ra. The response (adsorption) or recovery (desorption) time was estimated as the time taken by the sensor resistance output to reach 90% of its total resistance change after each process of applying or clearing the 7
testing gas. The gas-sensing properties of sensors were all measured using a CGS-8 gas-sensing characterization system at room temperature (25oC).
3. Results and discussion
3.1. Structural and morphological characteristics The phase composition of the hybrid nanocomposites is analyzed by XRD. Fig. 2 shows the XRD patterns of the GO and rGO/α-Fe2O3 samples. It demonstrates that GO exhibits a strong diffraction peak at 2θ of 10.92° attributed to (002) diffraction peak of GO (Fig. 2a), indicating the successful preparation of GO by oxidation of graphite [33]. However, no typical diffraction peaks of GO or graphene are observed in the XRD pattern of the rGO/α-Fe2O3 hybrids. The faded peak of GO is attributed to the fact that the regular layers tacking of GO can be destroyed by the growth of α-Fe2O3 nanoparticles during the hydrothermal reaction, leading to the exfoliation of GO sheets [34, 35]. Furthermore, compared to XRD pattern of rGO, seven diffraction peaks at 2θ of 24.48, 33.54, 35.82, 41.24, 49.70, 54.34, 62.84, and 64.44° are observed for the rGO/α-Fe2O3 nanocomposites (Fig. 2b), which are indexed as (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4), (1 1 6), (2 1 4) and (3 3 0) diffractions of α-Fe2O3 (JCPDS No: 79-1741) and no other phase is observed in the patterns [36]. Fig. 3 shows the Raman spectroscopy confirming the reduction of GO. The two strong peaks of the D-band (～1351 cm−1) and G-band (～1595 cm−1) were observed, corresponding to the diamondoid and graphitic graphene structures, respectively. It is noticeable that the intensity ratio of D-band to G-band of the rGO and rGO/α-Fe2O3 8
nanocomposites is larger than that of GO. This change suggests a decrease in the average size of the sp2 domains upon reduction of the exfoliated GO, which can be explained if new graphitic domains were created that are smaller in size to the ones present in GO before reduction, but more numerous in number [37]. To further investigate the compositions of the products, XPS technique was applied to analyze the chemical states of GO and rGO/α-Fe2O3 nanocomposites. Fig. 4a shows the XPS survey spectrum of the rGO/α-Fe2O3 sample. It is well known that the binding energy at 284.6 and 530.2 eV are assigned to the C 1s and O 1s species, respectively [38]. Two distinct peaks at binding energies of ca. 710.9 and ca. 724.8 eV with a shake-up satellite at ca.720.6 eV showed in Fig. 4b demonstrate that the Fe species in the nanocomposites are in the form of hematite (Fe2O3) phase [39], without any impurities of Fe0 [40], verified by the absence of its characteristic peaks. Fig. 4c and Fig. 4d reveal the deconvoluted C 1s spectra of the GO and rGO/α-Fe2O3 samples, respectively. Three peaks at 284.6, 286.6 and 288.4 eV are respectively attributed to the C-C, C-O and C=O species in graphene-based materials [41]. After hydrothermal treatment, the peak intensity of C-O and C=O of the rGO/α-Fe2O3 nanocomposites (Fig. 4d) tremendously decreases compared to that of GO (Fig. 4c), suggesting that the most of oxygen-containing groups are successfully removed after hydrothermal treatment. Taken together, the rGO/α-Fe2O3 hybrid nanocomposites have been successfully prepared after a facile hydrothermal treatment of the precursors including GO, CH3COONa·3H2O and Fe(NO3)3·9H2O. Fig. 5 shows the typical TEM and SEM images of the rGO/α-Fe2O3 9
nanocomposites. It exhibits that the rGO sheet with the size of several micrometers served as a platform is decorated with a large number of uniform α-Fe2O3 nanoparticles with the average size of 40 nm as active sites for gas adsorption (Fig. 5b and d). The absence of isolated Fe2O3 nanoparticles reveals that the hydrothermal treatment of GO and Fe(NO3)3·9H2O solution is an effective method to synthesize rGO/α-Fe2O3 nanocomposites. 3.2. NO2 sensing properties The sensing performance of rGO and rGO/α-Fe2O3 nanocomposites towards NO2 at room temperature is investigated in Fig. 6. The control experiments reveal that the pure rGO used as sensing material exhibits a low response (1.38), ultralong response time (2359 s) and recovery time (40130 s) (Fig 6a) upon exposure to the NO2 gas and air. Thus, the adsorption and desorption of the NO2 gas on rGO-based sensors are considered to be a thorn for gas sensing. However, by using α-Fe2O3 nanoparticles loaded on rGO (i.e., rGO/α-Fe2O3 nanocomposites) as sensing material, it demonstrates that the response/recover speed is dramatically accelerated, suggested by the significantly decreased response time (32 s) and recovery time (1435 s) (Fig. 6b). Moreover, the response of the rGO/α-Fe2O3 nanocomposites (3.86) is 2.8 times higher than that of rGO (1.38). These results indicate that the α-Fe2O3 nanoparticles in the hybrid composites played key roles in determination of sensor performances. Moreover, we can’t evaluate the gas sensing performance of α-Fe2O3, because it couldn’t response to NO2 at room temperature. Fig. 7 shows the response curve of rGO/α-Fe2 O3 sensor orderly exposed to 10
0.1-5.0 ppm NO2 at room temperature. It is clear that the response and recovery performances are consistent with the results exhibited in Fig. 6b. The response gradually decreases as the concentration of NO2 decreases, e.g., the response of the sensor decreased from 3.86 to 2.19, 1.42, 1.26, and 1.17 with the concentration of NO2 decreased from 5.0 to 1.0, 0.5, 0.2 and 0.1 ppm. Fig. 8 illustrates the reproducibility of the temporal curve of the rGO/α-Fe2O3 based sensor. The sensor maintains its initial response amplitude without a significant decrease upon three consecutive sensing tests to 0.1 ppm of NO2. This indicates that the rGO/α-Fe2O3 based sensor possess good stability upon cyclic tests. The selectivity of rGO/α-Fe2O3 nanocomposites based sensor was tested by exposing the sensor to the potential interference gases shown in Fig. 9a. Take 5.0 ppm of the target gases for example, the responses of the sensor to oxidizing gas (NO2, Cl2) and reducing gas (NO, CO, CH4, H2, NH3) are 3.86, 1.15, 1.76, 1.31, 1.18, 1.23, and 1.27, respectively, indicating the rGO/α-Fe2O3 nanocomposites based sensor shows negligible response to these interference gases compared to NO2 gas, and the humid air with relative humidity of 33% (33% RH) doesn’t have effect on gas sensing for rGO/α-Fe2O3 sensor (The relative humidity value in the measurement environment is 25%) either. Additionally, we have measured the NO2 response at different relative humidity, the response doesn’t fluctuate obviously (Fig. 9b). Therefore, it can be concluded that the rGO/α-Fe2O3 nanocomposites exhibit high selectivity for NO2 sensing at room temperature. A Comparison of NO2 sensing performances using rGO/α-Fe2O3 nanocomposites as sensing material and other reported rGO-based 11
sensing materials is presented in Table 1 [42-43, 27-29]. The rGO/α-Fe2O3 nanocomposites displays the highest sensitivity to NO2 at room temperature compared to other rGO-based materials reported elsewhere. 3.3. Gas sensing mechanism of α-Fe2O3 It is found that the pure rGO exhibits relatively weak response and long response and recovery times for detection of NO2 at room temperature [22, 44]. The dissatisfactory sensing properties of rGO result from its constituting carbon atoms. There are two types of carbon atoms in graphene, sp 2 hybridized carbon atoms constituting graphite structure, sp3 hybridized carbon atoms constituting structural defects and forming a chemical bond with oxygen-containing groups, the adsorption energy of the latter is larger (5.7 kcal/mol), resulting in slower adsorption and desorption [45, 46]. The sensing mechanism of the rGO/α-Fe2O3 sensor should follow the surface controlled type, which may be explained by the changing of resistance upon exposing to different gases. In open air, formation of oxygen adsorbates (O2-, O2- and O-) on the surface of α-Fe2O3 and rGO contributes to an electron-depleted surface layer owing to electron transfer from the α-Fe2O3 to oxygen in a similar fashion as for the reported literature [47]. When exposed to an oxidizing gas (NO2), the NO2 molecules react with the chemisorbed oxygen, and could attract electrons from the rGO/α-Fe2O3 because of the high electron affinity of NO2 molecule (Fig 10). The improvement of NO2 sensing performance at room temperature is attributed to the following factors. Firstly, the principle of gas sensing for the resistance-type 12
sensors is based on the conductance variations for target gas, thus the introduction of rGO significantly result in improving the conductivity, leading to gas sensing performance at room temperature. Secondly, benefited from the existence of α-Fe2O3, the gas adsorption and diffusion rate on the active surface is facilitated due to the increasing number of active sites (such as vacancies, defects, oxygen functional groups as well as the sp2-bonded carbon) for the adsorption of NO2 molecules, Additionally, the introduction of α-Fe2O3 prevents the rGO sheets from restacking, thus leading to a good surface accessibility and effective gas diffusion between parallel layers of graphene. Thirdly, the enhancement of gas sensing performance could be attributed to interaction between rGO and α-Fe2O3, when the rGO were decorated with the α-Fe2O3 nanoparticles, a depletion zone may be formed at the interface, the depletion zone around the interface of rGO and α-Fe2O3 nanorods results in the specific capture and migration of gas molecules. Finally, NO2 is a strong oxidizing gas molecule and it tends to interact with the active sites on rGO through its N atom, which enhances the gas sensing ability of the oxygen functional groups [48]. 4. Conclusion RGO/α-Fe2O3 nanocomposites have been synthesized by one-step hydrothermal method. The gas sensing performances indicate that the introducing of the α-Fe2O3 nanoparticles onto rGO matrix greatly enhances the response and adsorption/diffusion rate for NO2 sensing at room temperature. Moreover, it displays excellent selectivity and low detection limit down to 0.1 ppm.
These results indicate that α-Fe2O3
nanoparticles loaded on reduced graphene oxide could be a candidate material to 13
fabricate high performance NO2 sensor at room temperature.
Acknowledgements
This research work was financially supported by National Natural Science Foundation of China (Grant No. 61575129, 61275144), Guangdong Natural Science Foundation (2016A030313059), State Key Laboratory of Inorganic Synthesis and Preparative Chemistry Open Project (2015-10), and Natural Science Foundation of SZU (82700002601).
14
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Biographies Hao Zhang received his PhD from the College of Electronics Science and Engineering, Jilin University, China in 2015, majoring in microelectronics and solid-state electronics. Currently he is a post doctor at the college of optoelectronic engineering of Shenzhen University. He is studying the synthesis and characterization of functional materials and gas sensors.
Li Yu received his PhD from Future Industries Institute, University of South Australia in 2016, majoring in Materials and Surface Chemistry. Currently he is a Postdoctoral Fellow at College of Optoelectronic Engineering of Shenzhen University. His research focus is Layer-by-Layer assembly of polyelectrolytes, ultrathin nanosheets and their functionalities.
Qun Li is currently working towards the MS degree in the materials and chemical institute, Hainan University. Her research include the synthesis of metal oxides semiconductor materials and their applications in gas sensor
Yu Du received her PhD from chemistry of Jilin University, China in 2007. After that , she had been worked as the Postdoctoral at Nanyang Technological University, Sing apore for about two years. She is currently an Associate Professor at the college of Ph ysics and Energy Shenzhen University, China. Her current research interests are nano science and gas sensors. 22
Shuangchen Ruan was born on Oct.8 1963. He received the B.S. and M.S. degrees from northwest university in China in 1986 and 1989, and Ph.D. degree in Tianjin University in 2004. He has been the Professor since 1994. His research interests focus on solid state laser, fiber laser, and ultrafast pulse generation.
23
Figure captions: Fig. 1 (a) Schematic illustration and (b) photographic image of the rGO/α-Fe2O3 hybrid nanocomposite-based sensor.
24
Fig. 2 The XRD patterns of the products: (a) GO (black line), (b) rGO/α-Fe2O3 samples.
25
Fig. 3 Raman spectroscopy of the GO (black line), rGO (red line) and rGO/α-Fe2O3 (blue line) samples.
26
Fig. 4 (a) XPS spectrum of rGO/α-Fe2O3, (b) Fe2p spectrum of rGO/α-Fe2O3, (c) C1s spectrum of GO, (d) C1s spectrum of rGO/α-Fe2O3.
27
Fig. 5 (a, b) TEM and (c, d) SEM images of the rGO/α-Fe2O3 nanocomposites. High magnification (b) TEM and (d) SEM images of the selected area marked in (a) and (c), respectively.
28
Fig. 6 The dynamic response curve of (a) the rGO and (b) rGO/α-Fe2O3 nanocomposites based sensors to 5 ppm NO2 at room temperature with 25% RH.
29
Fig. 7 Dynamic NO2 sensing transients curve of the rGO/α-Fe2O3 nanocomposites based sensor to NO2 gases with concentration ranging from 0.1 to 5.0 ppm at room temperature with 25% RH.
30
Fig. 8 The reproducibility of the rGO/α-Fe2O3 based sensor upon 3 cyclic consecutive exposure to 0.1 ppm NO2 at room temperature with 25% RH.
31
Fig. 9 The responses of the rGO/α-Fe2O3 nanocomposites based sensor (a) to 5 ppm of different gases (showed in Figure legends) at room temperature and (b) to 5 ppm NO2 at different relative humidity at room temperature.
32
Fig. 10 The scheme of the proposed gas sensing mechanism: the adsorption behavior of NO2 molecules on the rGO/α-Fe2O3 nanocomposite.
33
Table 1 Performance of graphene-based gas sensors for NO2 detection Materials
NO2 gas concentration
Working temperature
Response
Response/recovery time
References
rGO
100 ppm
RT
1.56
30min/-
[41]
Graphene–WO3
20 ppm
RT
2.80
20 s/-
[42]
CuxO/Graphene
97 ppm
RT
95.1%
-
[26]
SnO2/S-rGO
5 ppm
RT
1.20
40 s/357 s
[27]
SnO2–rGO
5 ppm
50 C
3.31
135 s/200 s
[28]
rGO/α-Fe2O3
5 ppm
RT
3.86
76 s/946 s
This work
o
34