α-Fe2O3 composites

Author’s Accepted Manuscript Trimethylamine sensing properties of graphene quantum Dots/α-Fe2O3 composites Tao Hu, Xiangfeng Chu, Feng Gao, Yongping Dong, Wenqi Sun, Linshan Bai www.elsevier.com/locate/yjssc

PII: DOI: Reference:

S0022-4596(16)30062-7 http://dx.doi.org/10.1016/j.jssc.2016.02.037 YJSSC19284

To appear in: Journal of Solid State Chemistry Received date: 18 October 2015 Revised date: 10 February 2016 Accepted date: 23 February 2016 Cite this article as: Tao Hu, Xiangfeng Chu, Feng Gao, Yongping Dong, Wenqi Sun and Linshan Bai, Trimethylamine sensing properties of graphene quantum Dots/α-Fe2O3 composites, Journal of Solid State Chemistry, http://dx.doi.org/10.1016/j.jssc.2016.02.037 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 galley proof before it is published in its final citable 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.

Trimethylamine Sensing Properties of Graphene Quantum Dots/α-Fe2O3 Composites Tao Hua, Xiangfeng Chua, *, Feng Gaob, Yongping Donga, Wenqi Suna, Linshan Baia a

School of Chemistry and Chemical Engineering, Anhui University of Technology,

Maanshan 243002, P. R. China b

Department of Materials Science and Engineering, Nanjing University, Nanjing

210093, P. R. China * corresponding author. Email: [email protected]; tel: 86-555-2311822; fax: 86-551-2311822.

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Abstract Graphene quantum dots (GQDs) were prepared by pyrolysis of citric acid. The sizes of the as-prepared GQDs were in the range of 2-4 nm. The GQDs/α-Fe2O3 composites were prepared by loading GQDs with α-Fe2O3 via a one-step facile hydrothermal method. The GQDs/α-Fe2O3 composites were characterized by XRD, TGA, FTIR, Raman, SEM and TEM, respectively. The sensor devices were fabricated using the GQDs/α-Fe2O3 composites as sensing materials. The effect of the amount of GQDs in the composites on the gas-sensing responses of the materials and the gas-sensing selectivity was investigated. The experimental results revealed that the sensor based on GQDs/α-Fe2O3 (S-15) composite exhibited high sensitivity and good selectivity to TMA vapor. The responses of the sensor based on GQDs/α-Fe2O3 (S-15) composite to 1000 ppm and 0.01 ppm TMA vapor attained 1033.0 and 1.9 at 270oC, respectively. The response time and recovery time for 0.01 ppm TMA vapor were only 6 seconds and 4 seconds, respectively. Keywords: Graphene quantum dots; α-Fe2O3; Gas sensor; Trimethylamine vapor

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1. Introduction Trimethylamine (TMA) is a pungent gas secreted from dead fish, the concentration of TMA can be used as a criterion to determine the freshness of fish because it increases as the fish decays [1, 2]. Exposure to TMA vapor can lead to cough, irritation of upper respiratory system, difficulty in breathing and lung oedema [3], so the National Institute for Occupational Safety and Health of USA set down permissible exposure limits of 10 ppm for 10 h for long-term exposure and 15 ppm for 15 min for short-term exposure [4]. TMA is also found in exhaled gas of human due to a disorder in the renal system [3], so TMA can be used as a biomarker of renal diseases. Therefore, it is very important to develop TMA gas sensor with high sensitivity and good selectivity for food safety, environmental monitoring and painless diagnosis of diseases. Many traditional analytical methods have been applied to detect TMA, such as gas chromatography (GC), high performance liquid chromatography (HPLC), mass spectrometry (MS), ion mobility spectrometry (IMS) and quartz microbalance [5-10]. But these methods are complicated, expensive and time-consuming, and they need experienced personnel. On the contrary, gas sensors can detect TMA in a rapid, continuous, convenient and non-destructive manner. Much effort has been made to develop various TMA gas sensors based on metal oxide, for example, MoO3 [3, 4], WO3-TiO2 [11], SnO2-ZnO [12], WO3 [13], SnO2 [14] and Cr2O3-SnO2 [15]. Zhang et al. [16] synthesized hollow sea urchin-like α-Fe2O3 nanostructures by hydrothermal method using FeCl3 and Na2SO4 as raw materials and found that the response (Ra/Rg)

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of hollow sea urchin-like α-Fe2O3 nanostructures to 18 ppm TMA reached 5.9. Sun et al. [17] reported that the sensor based on bundle-like α-Fe2O3 nanostructure exhibited high response, quick response-recovery, and good repeatability to acetone. Other researchers also found that the sensors based on α-Fe2O3 showed high response to acetone [18], NO2 [19], LPG [20]. Graphene (G), a single layer of carbon atoms forming a two dimensional (2D) nano-material, has drawn much attention since it was discovered in 2004. Graphene has great potential application in the field of gas sensor due to its superb electronic mobility and high specific surface area. It has been reported that graphene exhibits gas-sensing responses to NO2 [21, 22], CO [22], NH3 [21, 23], TMA [23]. Although graphene had potential application for detecting gases, the experimental results indicated that the adsorption interaction between gas molecules and graphene was very weak [24, 25]. Many researchers attempted to improve the gas-sensing properties of materials by loading graphene with metals or metal oxides. It has been reported that there are many kinds of metals (Pd [26], Pt [27]) or metal oxides (ZnO [28], SnO2 [29], WO3 [30], MnO2 [31] and α-Fe2O3 [32]) that could form composite materials with graphene, and their gas-sensing performances could be improved. Graphene quantum dots (GQDs) consist of single- or few-layer graphene with lateral dimensions smaller than 100 nm and their diameters span in the range of 3-20 nm [33]. As the derivative of graphene, GQDs exhibit some excellent characteristics which are similar to those of graphene. By comparison with conventional graphene based materials, GQDs possess the superior properties of quantum dots (QDs), such

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as strong quantum confinement and edge effects. Combining the advantages of the graphene nature and quantum dots, GQDs have been recognized as an emerging nano-material and applied in numerous areas, such as bio-imaging [34], sensors [35] and optoelectronic devices [36]. GQDs have rich functional groups (epoxy and hydroxyl groups on the basal plane or carboxyl groups at the edges), thus endowing them excellent water solubility and functionalized possibilities [37]. Moreover, the higher specific surface area of GQDs provides a better driving force for gas diffusion, which is important in gas-sensing applications. It is well known that the sensing mechanism of metal oxide gas-sensing materials is based on the reaction between the adsorbed oxygen on the surface of materials and the gas molecules to be detected. The state and the amount of oxygen on the surface of materials are strongly dependent on the microstructure, particle size, as well as specific surface area of materials. The larger specific surface area is beneficial to improving the gas-sensing properties. However, to our knowledge, there is rarely literature that reports the gas-sensing properties of the composites of GQDs and metals or metal oxides. In this work, the GQDs were obained by the pyrolysis reaction of citric acid. The GQDs/α-Fe2O3 composites were prepared via hydrothermal method. The effect of the amount of GQDs in the composites on the gas-sensing responses of the sensors was investigated. It was found that the sensor based on GQDs/α-Fe2O3 (S-15) composite exhibited high response and good selectivity to TMA vapor when operating at 270oC.

2. Experimental

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2.1 Preparation of graphene quantum dots (GQDs) and GQDs/α-Fe2O3 composites The preparation method of GQDs was similar to that reported by Dong et al. [38]. All chemicals used in this work were analytical-grade reagents. 2 g of citric acid was put into a 25 mL beaker and kept at 200oC. As the reaction proceeded, citric acid changed from a white solid into an orange liquid in 30 min. The obtained orange liquid was added drop by drop into 100 mL of 0.25 mol·L-1 NaOH solution under vigorous stirring. The GQDs aqueous solution was obtained by neutralizing this mixed solution to pH 7.0 with NaOH solution. GQDs/α-Fe2O3 composites were prepared by hydrothermal method. In a typical process, 1.24 g of Fe(NO3)3·9H2O was dissolved in 20 mL deionized water. The GQDs aqueous solution was slowly added to the above solution under vigorous stirring for 30 min, the mixture was sonicated to form a suspension. 15 mL of ammonia water (25 wt%) was added into the above suspension under vigorous stirring for 30 min, then the red brown precipitate formed immediately. Finally, the above mixture was transferred into 50 mL of Teflon-lined stainless-steel autoclave and kept at 180oC for 12 h, cooled down to the room temperature naturally. The obtained products were filtered, washed with deionized water and anhydrous ethanol for several times, and dried at 60oC for 24 h. The amount of GQDs in the composite was controlled by altering the volume of the added GQDs aqueous solution (5, 10, 15, 20 and 25 mL). The samples were correspondingly labeled as S-5, S-10, S-15, S-20 and S-25, respectively. Pure α-Fe2O3 which was labeled as S-0 was prepared via a similar route without adding the GQDs aqueous solution.

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2.2 Characterization of GQDs and GQDs/α-Fe2O3 composites The X-ray diffraction (XRD) characterization were performed on a Bruker D8 Advance (40 kV, 40 mA) X-ray diffractometer with Cu-Kα radiation of λ=1.54056 Å and a 2θ scanning range of 10-80º. Fourier transform infrared spectroscopy (FT-IR) spectra were recorded with a Nicolet 6700 Spectrometer using the KBr pellet technique. Thermogravimetric analysis (TGA) measurements were performed using Shimazu DTG-60 thermogravity analyzer at a heating rate of 5oC∙min-1 in air atmosphere. Transmission electron microscopy (TEM) observations were made on a JEM 1200EX with an acceleration voltage of 120 kV. High-resolution transmission electron microscopy (HRTEM) images were obtained on a Tecnai G2 F20 microscope (FEI, American), the instrument had a point-to-point resolution of 0.24 nm and was operated at an acceleration voltage of 200 kV. Scanning electron microscopy (SEM) was carried out with a Hitachi S-4800 electron microscope at an acceleration voltage of 10 kV. Raman spectra were performed with a Renishaw Invia spectrometer with a 532 nm wavelength laser in the range of 3200-100 cm-1. Naoparticle size analyzer (Zetasizer Nano ZS90*, Malvern, England) was used to determine the particle size distribution of GQDs. 2.3 Fabrication and gas-sensing property measurement of gas sensors The sensor device was fabricated by coating the slurry of terpineol with the powder of GQDs/α-Fe2O3 composite or pure α-Fe2O3 onto the surface of Al2O3 tube (as shown in Fig. 1, 8 mm in length, 2 mm in external diameter and 1.6 mm in internal diameter) on which two gold leads had been installed at each end. The Al2O3 tube was

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dried at 80oC for 10 h after coating. A Ni-Cr alloy wire that inserted into the Al2O3 tube was used as a heater to maintain the required operating temperature, the operating temperature could be controlled in the range of 80-450oC by changing the voltage across the Ni-Cr alloy wire. The response (S=Ra/Rg) of the sensor was defined as the ratio of the electrical resistance of the sensor in air (Ra) to that in the gas mixture of a targeted gas and air when the resistance of the sensor reached stable value. The preparing method for the gas mixture of the targeted gas and air was same as the method reported in our previous work [39]. During measurement, a given amount of trimethylamine solution was taken by a microinjector, the solution was injected into 5L of bottle filled with pure air, the bottle was closed and dried at 80oC for 10 min, the bottle was filled with the gas mixture of trimethylamine vapor and air. The electrical resistance of a sensor in air was measured as follow: the sensor was placed in a closed glass bottle filled with pure air, the sensor was placed in the air bottle at least 5 min after the electrical resistance of the sensor was stable; then the sensor was taken out from the air bottle and placed rapidly in a closed bottle filled with the gas mixture of the targeted gas and air; the electrical resistance (Rg) of the sensor was obtained when the electrical resistance reached stable value in the atmosphere of the targeted gas and air. If the resistance of the sensor could not recover from the previous exposure, the operating temperature was adjusted to 300oC and kept for about 10 min to let the targeted gas desorb outside the air bottle. The resistance variation of the sensor was recorded by a computer. The electronic circuit is shown in Fig. 2. In the circuit, Vh is the heating

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voltage, RL is a constant resistance (RL =2.1 MΩ), Vout is the voltage on RL and VC is the constant voltage (1.5 V) which is applied on the RL and gas sensor. 3ˊResults and discussion 3.1 Microstructure of GQDs/α-Fe2O3 composites The as-prepared GQDs were characterized by TEM, HRTEM and nanoparticle size analyzer, respectively. Fig. 3a shows that the obtained GQDs are relatively uniform in size without agglomeration and the sizes of the GQDs are in the range of 2-5 nm. As shown in the inset in Fig. 3a, the average diameter of GQDs is 2.8 nm, which is in accordance with the phenomenon in TEM image (Fig. 3a). The HRTEM image (inset of Fig. 3b) indicates that the GQDs have a high crystalline structure, with a lattice spacing of 0.262 nm, which is corresponding to the (1120) lattice fringes of graphene [40]. Fig. 4 shows the XRD patterns of the as-prepared α-Fe2O3 and GQDs/α-Fe2O3 composites with different amounts of GQDs. All diffraction peaks of the samples can be indexed to the phase of hematite (α-Fe2O3, JCPDS card No. 33-0664), indicating the formation of α-Fe2O3 without impurities. The main characteristic peaks at 2θ = 24.1o, 33.1o, 35.6o, 40.9o, 43.5o, 49.5o, 54.1o, 57.4o, 62.4o and 64.0o correspond to the (012), (104), (110), (113), (202), (024), (116), (122), (214) and (300) crystal planes of α-Fe2O3, respectively. Comparing with the XRD pattern of pure α-Fe2O3 (Fig. 4a), as shown in Fig. 4e and Fig. 4f, the diffraction peaks of (012), (104), (113), (024), (116) and (214) planes of α-Fe2O3 in the XRD patterns of GQDs/α-Fe2O3 composites disappear and the diffraction peaks of (110), (202), (122) and (300) planes become

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broad. According to the literature [41], the reason may be that some interactions between the functional groups on the surface of GQDs and Fe(NO3)3 solution occur when treating the mix solution of GQDs and Fe(NO3)3 with ultrasound, these interactions promote the preferential growth of α-Fe2O3 on the surface of GQDs in the process of hydrothermal reactions, but the crystallization of α-Fe2O3 weakens in the presence of excessive amount of GQDs. However, no diffraction peak can be attributed to GQDs in the XRD patterns of GQDs/α-Fe2O3 composites, the reason may be ascribed to the low amount of GQDs or the lower diffraction intensity of GQDs in comparison with those of the characteristic peaks of α-Fe2O3. Y. Yan et al. [42] also found similar phenomenon and gave similar explanation, they prepared composite of GQDs and TiO2 nanoparticles, the content of GQDs in the composite was about 41%wt and no diffraction peak of GQDs could be found in the XRD pattern of the composite. In order to estimate the mass ratio of α-Fe2O3 to GQDs in the composite, TGA experiment was carried out from room temperature to 800oC at a heating rate of 5oC∙min-1 in air atmosphere. As shown in Fig. 5, there is a weight loss for all samples from 20oC to 200oC owing to the loss of physically absorbed water and the loss of water formed from condensation of hydroxyl groups on the surface of particles [43]. The major weight loss between 200oC and 400oC is due to decomposition of GQDs [44]. Thus, the amounts of GQDs in samples of S-5, S-15 and S-25 are about 4 wt%, 10 wt% and 15 wt%, respectively. Hence, the operating temperature should not exceed 380oC when measuring the gas-sensing properties of the sensors based on

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GQDs/α-Fe2O3 composites. The FT-IR spectra of citric acid, GQDs/α-Fe2O3 (S-15) composite and α-Fe2O3 are presented in Fig. 6. The broad absorption band at 3400-3500 cm-1 in the FT-IR spectra of all samples is associated with the stretching vibration of the -OH group. The absorption band at 1620 cm-1 can be attributed to the bending vibration absorption peak of the adsorbed water molecules. The absorption peaks at 1740 cm-1 in the spectrum of citric acid (Fig. 6a) is associated with the vibration of C=O [38]. The stretching vibration absorption peaks of COOH at 1640 cm-1 and C-OH at 1380 cm-1 appear in both FT-IR spectra of citric acid (Fig. 6a) and GQDs/α-Fe2O3 (S-15) composite, indicating that GQDs samples contain some incomplete carbonized citric acid [38]. Comparing with the results shown in Fig. 6a, the two absorption bonds in the range of 400-600 cm-1 in the spectra of GQDs/α-Fe2O3 (S-15) composite (Fig. 6b) and α-Fe2O3 (Fig. 6c) can be seen, this two absorption peaks at 572 cm-1 and 480 cm-1 can be assigned to the stretching vibration of Fe-O [45]. Raman spectroscopy is an important tool to characterize carbon materials with usual characteristics of G and D bands. The D band is associated with the destruction of the sp2 network by the sp3-bonded C atoms, while the G band was related to the vibration of the sp2-bonded C atoms in the graphene material [46]. The intensity ratio of the D band to the G band (r=ID/IG) is usually used to estimate the defect and disorder of the graphitized structure and the ratio of sp3/sp2 bonded carbon [47]. As shown in Fig. 7, the peaks centered at around 1340 cm-1 and 1570 cm-1 correspond to the D and G peak of GQDs, respectively [40]. When the amount of GQDs in the

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composites increases, the position of D peak slightly moves to higher value while the position of the G peak moves to lower value, and the intensity ratio of the D band to the G band (ID/IG) of composites decreases, indicating that the average size of the sp2 domains decreases with the increasing amount of GQDs. The Raman peaks of α-Fe2O3 in range of 100-700 cm-1 can be seen in Fig. 7, the one centering at 215 cm-1 is attributed to the A1g mode of α-Fe2O3, the others centering at 278, 392 and 602 cm-1 can be assigned to Eg mode of typical hematite [48]. The morphologies of as-prepared α-Fe2O3 and GQDs/α-Fe2O3 (S-15) composite were observed using SEM. As shown in Fig. 8a, pure α-Fe2O3 is composed of nanoparticles which stack together. A large number of nanoparticles in GQDs/α-Fe2O3 (S-15) composite are shown in Fig. 8b, the sizes of these nanoparticles is in the range of 50-80 nm. The microstructure of pure α-Fe2O3 and GQDs/α-Fe2O3 (S-15) composite was characterized by TEM. As can be seen from Fig. 9a, pure α-Fe2O3 nanoparticles with diameters of 100-150 nm distribute uniformly. Fig. 9b shows that the spatial arrangement of small nanoparticles is very tight, that indicating that the nanoparticles in the composite tend to agglomerate due to the existence of GQDs. Fig. 10 illustrates the preparation route and the formation mechanism of GQDs/α-Fe2O3 composites. Firstly, the citric acid molecules are directly pyrolyzed and are converted into GQDs in condition of incomplete carbonization. The oxygen-containing functional groups (hydroxyl, carbonyl and carboxyl groups) on the edge of GQDs can act as anchor sites that interact strongly with the covered species

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and favor in situ formation of particles. Secondly, when treating the mix solution of GQDs and Fe(NO3)3 with ultrasound, Fe3+ ions may be absorbed on the surface of GQDs due to electrostatic interactions, which may promote the in situ growth of particles. Finally, GQDs/α-Fe2O3 composites form with the α-Fe2O3 nanoparticles covering on the surface of GQDs in the process of hydrothermal reactions. GQDs/α-Fe2O3 nanoparticles with small sizes can keep its large specific surface area effectively and offer more active absorption sites, which are beneficial to enhancing the gas-sensing properties of the composites. 3.2 Gas-sensing properties of GQDs/α-Fe2O3 composites The amount of GQDs in the composites has a significant influence on the responses of the sensors based on GQDs/α-Fe2O3 composites. The responses of GQDs/α-Fe2O3 composites with different amounts of GQDs to 1000 ppm TMA vapor at different operating temperature are presented in Fig. 11. The responses of the sensor based on pure α-Fe2O3 (S-0) are very low at different operating temperatures, and the maximum response to 1000 ppm TMA vapor is only 5.2 when operating at 270oC. The sensor based on GQDs/α-Fe2O3 composites with different amounts of GQDs exhibit higher response to TMA vapor in comparison with those of pure α-Fe2O3 when operating at different temperatures. It should be noted that the response of the sensor (S-15) reaches 1033.0 at 270oC. However, the maximum response of the sensor (S-25) is only 9.8 at 270oC. These experimental results suggest that the amount of GQDs in the composites should be optimized to maximize the response of the sensor to TMA vapor. Therefore, the sensitivity of GQDs/α-Fe2O3 composites was

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related to the microstructure and particle size of materials. α-Fe2O3 is a typical n-type metal oxide semiconductor, and the sensor based on α-Fe2O3 belongs to the surface-controlled type sensor. The mechanism can be explained by the space-charge layer model, which involves the formation of a charge depletion layer in the near-surface region of each grain due to electron trapping on absorbed oxygen species [49]. When the sensor based on α-Fe2O3 is exposed to air, the high specific surface area of α-Fe2O3 nanoparticles can offer numerous absorption sites to absorb more oxygen molecules and form negative oxygen ions (O2-, O- or O2-) by capturing electrons from the conductance band of α-Fe2O3. It leads to a decrease of the carrier concentration in the conductance band of α-Fe2O3 and an increase of the resistance of sensor. When the negative oxygen ions react with TMA vapor at a moderate temperature and form N2, H2O and CO2, as depicted in Eq. (4). O2(gas)↔ O2(ads) (<100oC)

(1)

O2(ads)+ e- ↔ O2(ads)- (100~300oC)

(2)

O2(ads)-+ e- ↔ 2O-(ads) (>300oC)

(3)

4N(CH3)3 + 21O2- = 2N2 +18H2O + 12CO2 + 21e-

(4)

The captured electrons are released back to the conductance band, leading to reduction of the number of the absorbed oxygen species on the surface of α-Fe2O3. The depletion layer on the surface of α-Fe2O3 becomes thin and the electrical resistance of sensor decreases. Thus, according to the formula of the response (S=Ra/Rg), the sensor based on α-Fe2O3 is sensitive to TMA vapor. In addition, the sensor based on GQDs/α-Fe2O3 (S-15) composite exhibit higher response to TMA

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vapor than that of pure α-Fe2O3, the reason may be contributed to the following three factors. Firstly, the high specific surface area distributed in the two-dimensional space can facilitate the diffusion of TMA vapor and can improve the reaction of TMA vapor with oxygen adsorbed on the surface of material. Based on the phenomena in SEM images and TEM images, GQDs/α-Fe2O3 (S-15) composite composed of smaller nanoparticles possesses lager specific surface area in comparison to pure α-Fe2O3. The lager specific surface area is of great benefit to adsorption of more oxygen molecules onto the surface of GQDs/α-Fe2O3 composites. Secondly, GQDs possess outstanding electrical conductivity, which can improve conductivity of composites and result in electrons quickly spreading to surface of the materials, leading to quick response time and recovery time [32]. Finally, GQDs may create a Schottky contact at the interface between GQDs and α-Fe2O3, the synergistic effect of α-Fe2O3 and GQDs occurs, the interface of GQDs/α-Fe2O3 composites belongs to a forward-biased Schottky barrier, which can contribute to more easy capture or migration of the electrons from α-Fe2O3 nanoparticles to GQDs [32]. Therefore, the gas-sensing properties have been greatly improved by loading GQDs with α-Fe2O3. According to the literature [50], the operating temperature has influence over the oxygen state on the surface of α-Fe2O3, the order of the ability to give electron back to α-Fe2O3 conduction band in the process of gas-sensing is O2
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GQDs/α-Fe2O3 (S-15) composite material, the optimum operating temperature is 270oC. The responses of the sensors based on α-Fe2O3 (S-0) and GQDs/α-Fe2O3 (S-15) composite to seven kinds of vapors (1000 ppm) at 270oC are shown in Fig. 12. The responses of the sensor based on pure α-Fe2O3 (S-0) to acetone, ethanol, TMA, ammonia, formaldehyde, acetaldehyde and benzene are only 8.5, 12.7, 5.5, 2.3, 4.3, 1.6 and 1.4, respectively. While the responses of the sensor based on GQDs/α-Fe2O3 (S-15) composite to these vapors reach 81.7, 22.1, 1033.0, 2.9, 4.5, 2.8 and 2.3, respectively. The response of the sensor based on GQDs/α-Fe2O3 (S-15) composite to TMA vapor was 187.8 times as high as that of pure α-Fe2O3, while the responses of the sensor to the other six kinds of vapors at 270oC do not increase dramatically. If this sensor is utilized to measure TMA, the above gases do not disturb the measurement obviously. The results reveal that the sensor based on GQDs/α-Fe2O3 (S-15) composite exhibits good selectivity to TMA vapor at 270 oC. According to the literature [51], the selectivity is also related to the reducing ability and the absorbing ability of detected gas on the surface of the sensing materials. CH3- is electron-donating group, the electron cloud density around N atom in trimethylamine is higher than those of O atom in ethanol, acetone and formaldehyde, the attractive force between N atom in TMA and Fe3+ ion on the surface of the composite can facilitate the adsorption of TMA vapor on the surface of the sensing material. The bond energy has a great influence on the stability of compound, the lower the bond energy, the easier the bond breaks. The bond strengths of C-H, C-C, C-N, C=O and

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O-H are 411, 345, 307, 748.2 and 462 KJ/mol, the bond energy of C-N in trimethylamine is low, and trimethylamine is unstable in term of bond energy. The reducing ability of TMA is higher than those of acetone, ethanol, ammonia, formaldehyde, acetaldehyde and benzene, resulting in the highest sensitivity to TMA in all vapors detected. Of course, the gas-sensing selectivity is very complicated, it is also related to the spatial structure of the gas molecule. The real mechanism of the gas-sensing selectivity is under investigation by our group. Fig. 13 depicts the response transients of the sensor based on GQDs/α-Fe2O3 (S-15) composite to TMA vapor (0.01, 0.1, 1, 10, 100 and 1000 ppm) at 270oC. The responses of the sensor based on GQDs/α-Fe2O3 (S-15) composite to 0.01, 0.1, 1, 10, 100 and 1000 ppm TMA vapor at 270oC are 1.9, 2.9, 5.5, 15.4, 293.0 and 1033.0, respectively. The detection limit can reach 0.01 ppm, it is of great significance to detect dilute TMA vapor. Response time and recovery time are very important parameters for a gas sensor. In general, the response time and recovery time are defined as the time for a sensor to reach 90% of the final signal. The response time and recovery time for TMA vapor (0.01, 0.1, 1, 10, 100 and 1000 ppm) are listed in Table 1 when operating at 270oC. The response time and recovery time for 1000 ppm TMA vapor are 11 seconds and 24 seconds, respectively. When the concentration of TMA vapor is 0.01 ppm, the response time and recovery time are only 6 seconds and 4 seconds, respectively. The short response time and recovery time indicate that reducing gas and oxygen can react easily on the surface of the composite materials.

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4. Conclusions In summary, GQDs with sizes of 2-4 nm were prepared by pyrolysis of citric acid. GQDs/α-Fe2O3 composites were prepared via hydrothermal method. The experimental results indicated that the amount of GQDs in the composites had a significant influence on the morphology and the gas-sensing responses of GQDs/α-Fe2O3 composites. The responses of the sensor based on GQDs/α-Fe2O3 (S-15) composite to 1000 ppm and 0.01 ppm TMA vapor attained 1033.0 and 1.9 at 270oC, respectively. The response time and recovery time for 0.01 ppm TMA vapor were only 6 seconds and 4 seconds when operating at 270 oC, respectively. Loading GQDs with α-Fe2O3 enhanced the gas-sensing response and the gas-sensing selectivity to TMA. Therefore, the sensor based on GQDs/α-Fe2O3 (S-15) composite is a potential candidate to measure the concentration of TMA for human health and fish-processing industry.

Acknowledgements This work was supported by NSFC (No. 61271156), Innovation Team Project of AHUT (No. TD201204) and the research project for university personnel returning from overseas sponsored by the Ministry of Education of China.

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Captions Fig. 1 Schematic structure of the gas sensor. Fig. 2 Electronic circuit of gas sensor measurement system. Fig. 3 (a) TEM image, (b) HRTEM image of GQDs, inset of Fig 3a shows the size distribution of GQDs. Fig. 4 The XRD patterns of (a) α-Fe2O3 (S-0) with GQDs/α-Fe2O3 composites with different amounts of GQDs: (b) S-5; (c) S-10; (d) S-15; (e) S-20; (f) S-25. Fig. 5 The TGA curves of GQDs/α-Fe2O3 composites with different amounts of GQDs: (a) S-5; (b) S-15; (c) S-25. Fig. 6 The FT-IR spectra of (a) citric acid; (b) GQDs/α-Fe2O3 (S-15) composite ; (c) α-Fe2O3 (S-0) . Fig. 7 The Raman spectra of GQDs/α-Fe2O3 composites with different amounts of GQDs: (a) S-5; (b) S-15; (c) S-25. Fig. 8 The SEM image of (a) α-Fe2O3 (S-0), (b) GQDs/α-Fe2O3 (S-15) composite. Fig. 9 The TEM image of (a) α-Fe2O3 (S-0), (b) GQDs/α-Fe2O3 (S-15) composite. Fig. 10 A schematic representation of the formation process of GQDs/Fe2O3 composites. Fig. 11 The responses of the sensors based on GQDs/α-Fe2O3 composites with different amounts of GQDs to TMA vapor (1000 ppm) at different operating temperature. Fig. 12 The selectivity of the sensor based on α-Fe2O3 (S-0) and GQDs/α-Fe2O3 (S-15) composite to seven kinds of vapors (1000 ppm) at 270oC. Fig. 13 The response transients of the sensor based on GQDs/α-Fe2O3 (S-15)

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composite to 0.01, 0.1, 1, 10, 100 and 1000 ppm TMA vapor at 270oC. Table 1 The response time and recovery time for TMA vapor of different concentrations. Concentration/ppm

Response time/s

Recovery time/s

1000 100 10 1 0.1 0.01

11 15 14 2 5 6

24 28 34 2 2 4

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Figure(s)

Fig. 1

Figure(s)

Fig. 2

Figure(s)

Fig. 3

Figure(s)

Fig. 4

Figure(s)

Fig. 5

Figure(s)

Fig. 6

Figure(s)

Fig. 7

Figure(s)

Fig. 8

Figure(s)

Fig. 9

Figure(s)

O O O

O

H

HO

O

COOH H2C COOH COOH

Fe2O3

Fe2O3

HOOC

Fe2O3

Fe2O3

OH

Hydrothermal

COOH

Fe3+

Fe3+

O

HO

Fe OH

Fe2O3

3+

Fe3+ 3+ Fe

COOH

CH2 HOOC

Fe2O3

OH O

OH COOH

HOOC

Fe(NO3)3

CH2

C H2

COOH

Adsorption

H2 C OH

HOOC HOOC

H2C

HO

HOOC

COOH

H 2C

COOH

Fe3+

HOOC

Fe2O3

HO

OH

COOH

CH2

CH2

Fe2O3

Fe2O3

OH

HO

O

O

COOH

OH O

Fe2O3

Incompletely carbonization

O

O

H O

HO

HO

O

H

Fe2O3

O

HO

HO H

HO

HO H

H H

Citric acid

H H

H

OH

Fe2O3

HO

HO H

OH

HO

H H

Pyrolysis

O

H H

HOOC

O

H

HOOC

HOOC

HO

OH

OH

HOOC

HO

HO

O

HO

CH2 HO HOOC

HOOC

GQDs

GQDs/α-Fe2O3

Fig. 10

Figure(s)

Fig. 11

Figure(s)

Fig. 12

Figure(s)

Fig. 13

*Graphical Abstract