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Journal of Materials Chemistry AMaterials for energy and sustainabilitywww.rsc.org/MaterialsA

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Volume 4 Number 1 7 January 2016 Pages 1–330

PAPERKun Chang, Zhaorong Chang et al. Bubble-template-assisted synthesis of hollow fullerene-like MoS

2 nanocages as a lithium ion battery anode material

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Zheng, X. Cheng, L. Sui, S. Gao, X. Zhang, Y. Xu, H. Zhao and L. Huo, J. Mater. Chem. A, 2017, DOI:

10.1039/C7TA06392J.

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This journal is © The Royal Society of Chemistry [2015] J. Mater. Chem. A, [2017, [6], 00–00 | 1

Ionic liquid-assisted synthesis of α-Fe2O3 mesoporous nanorod arrays

and their excellent trimethylamine gas-sensing properties for

monitoring fish freshness †

Ping Wang,a,b

Zhikun Zheng,c Xiaoli Cheng,

a Lili Sui,

b Shan Gao,

a Xianfa Zhang,

a Yingming Xu,

a,*

Hui Zhaoa and Lihua Huo

a,* 5

Received (in XXX, XXX) Xth XXXXXXXXX 201X, Accepted Xth XXXXXXXXX 201X

DOI: 10.1039/b000000x

Large-scale and well-aligned α-Fe2O3 mesoporous nanorod arrays in-situ deposited on the ceramic tubes

has been successfully synthesized by ionic liquid (ILs)-assisted hydrothermal reaction followed by the

calcination. The mesoporous sizes in the nanorods can be adjusted by changing the calcined temperature. 10

Such in-situ assembled α-Fe2O3 mesoporous nanorod arrays sensor exhibited not only high sensitivity,

short recovery time and good reproducibility to trimethylamine (TMA), but also good linear relationship

under concentration range of ppm level (0.1-100 ppm). Furthermore, the sensor was evaluated for a fast

analysis of the primary volatiles of carassius auratus (0~11 h) which have been detected using headspace

solid phase microextraction (HS-SPME) and gas chromatography-mass spectrometry (GC-MS). Such α-15

Fe2O3 mesoporous nanorod arrays showed considerable potential in identification of fish freshness. The

superior gas-sensing properties of the obtained nanostructures should be attributed to the uniformly

mesoporous structure in the ordered nanorod arrays with a large specific surface area, as well as

appropriate amount of residual functionalized ILs, which benefit TMA molecules to diffuse and adsorb on

the array surface and the electron transfer. The formation mechanism of the mesoporous nanorod arrays 20

was also discussed in detail.

Introduction

Trimethylamine (TMA) is a gaseous organic amine, mainly generated during the deterioration of the dead fish and seafood. TMA is considered as an effective indicator for evaluation 25

seafood quality because its concentration increases with the degree of corruption of fish and marine product. Usually, the freshness degree based on the content of TMA is defined as no more than 10 ppm.1-3 Besides, it is also a symptom of problems in renal organ system when the TMA concentration is 0.1~0.2 ppm 30

in a person exhaled breath.4,5 Accordingly, it is indispensable to detect ppm levels of TMA in fish processing industry and medical diagnosis. The chemical gas sensors based on metal oxide semiconductors (MOS) can monitor volatile organic compounds (VOC), 35

including TMA, in a fast, cost-effective, facile and real-time way compared with the traditional techniques such as gas chromatography (GC), ion mobility spectrometry (IMS) and liquid chromatography (LC).4,5 To date, many TMA sensors based on semiconducting metal oxides like SnO2,

6 WO3,7 ZnO,8 40

TiO29,10 and In2O3

11 etc. and their composites have been widely investigated. However, only ZnO-Al2O3/TiO2/V2O5

12 and ZnO-

SnO213 composite materials were used as gas sensors for the fish

freshness detection. They exhibited long responses time and high detection limit to TMA at high working temperatures (300~330 45

°C). Until now, the α-Fe2O3 based TMA sensors are reported relatively less, in which the pure α-Fe2O3 sensors showed low response, low selectivity and high operating temperature to TMA. 14-16 In order to improve the TMA sensing properties, TiO2 nanofibers,14 ZnO nanosheets15 and graphene quantum dots16 50

were composited with α-Fe2O3, which exhibited high sensitivity, low detection limit, good selectivity, and rapid response. So it is attractive to develop a fast, simple, reliable and effective gas sensor for fish freshness detection at relatively low working temperature, based on pure α-Fe2O3 nanostructured materials. 55

Hematite (α-Fe2O3) is an environmentally friendly n-type semiconductor (Eg = 2.1 eV), and has been widely applied in gas sensors owing to its low cost, non-toxic and high sensitivity to various oxidizing and reducing gases. The adsorption of tested gases on sensing layer is indicated a key factor that influences the 60

gas sensing performance. So the gas sensing properties are greatly dependent on the surface morphology and architecture of the sensing materials. For years, 1D metal oxide nanostructured arrays capture tremendous attention because their 1 D nanostructures with large specific surface areas can facilitate gas 65

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molecules to adsorb and electrons transfer on sensing layer. Also, the nanomaterials with porous structures are a kind of ideal nanostructures to promote gas molecules transportation and adsorption on the surface due to the particular structure.17 Thus, the 1D nanostructured arrays with pores are remarkable candidate 5

for gas detection. Till now, α-Fe2O3 arrays constructed by 1D nanomaterials (nanorod, nanowire and nanobelt) have been prepared successfully,18-20 but there is only one report about 1D α-Fe2O3 nanorod arrays with pores, which were synthesized in the presence of CH3COOH and urea in the hydrothermal reaction, 10

followed by calcination.20 Such α-Fe2O3 porous nanorod arrays exhibited a high adsorption capacity for Methyl orange. Template-directed approaches including soft or hard template are usually employed in the synthesis of shape-controlled porous nanostructures. As ideal green medium and template, ionic 15

liquids (ILs) have been widely investigated because of their unique functional properties including wide liquid phase range, excellent thermal and chemical stability, high ionic conductivity, low interface tension and interface energy.21-23 Although some types of 1D α-Fe2O3 nanomaterials were studied by using ILs,24-26

20

there is no report about the preparation of porous α-Fe2O3 nanorod arrays assisted with ILs. Besides, in our previous research,27 we found the appropriate amount of ILs retention in α-Fe2O3 nanospheres can selectively adsorb certain gas molecules and is helpful to improve the gas sensing performance. Therefore, 25

it is meaningful to study the intrinsic relationship between ILs addition and microstructure controlling of α-Fe2O3 nanostructure, especially the adjustment of the uniformly mesoporous nanostructure in the nanorods arrays, as well as the influence of residual ILs in α-Fe2O3 arrays on TMA detection. 30

Herein, large-scale and well-aligned FeOOH nanorod arrays were assembled in-situ onto a ceramic tube by one-pot hydrothermal method, where the long-chained IL of 1-dodecyl-3-methylimidazolium bromide ([C12mim][Br]) was used as morphology inducer. Then, the ordered α-Fe2O3 mesoporous 35

nanorod arrays with proper amount of ionic liquid could be easily obtained by controlling the calcination temperature. Such α-Fe2O3 nanorod arrays with suitable size of mesopores and appropriate residual [C12mim][Br] were proved to be highly sensitive to TMA either in gas samples or in the actual gas given off from a dead 40

carassius auratus at early deterioration stage. The fish metabolites were analyzed by headspace solid phase microextraction (HS-SPME) and gas chromatography-mass spectrometry (GC-MS), which could provide potential volatile information and detect the low concentration of volatiles during storage. The detection limit 45

was as low as 100 ppb, which indicated their practical application in chemical detector and biosensor.

Experimental

Synthesis of α-Fe2O3 porous arrays

All the chemicals (A.R. grade) were obtained from Beijing 50

Chemical Reagents Company and used without further purification. The ionic liquid [C12mim][Br] (>99% mass fraction) was supplied by Lanzhou Greenchem ILs, LICP, CAS (Lanzhou, China). The ceramic tubes were carefully cleaned by deionized water (18.2 MΩ), ethanol and acetone, respectively. In-situ 55

deposited α-Fe2O3 mesoporous nanorod arrays were obtained via

appropriate annealing treatment of FeOOH precursor which was synthesized by [C12mim][Br]-assisted hydrothermal reaction described as follows: ferric chloride hexahydrate (FeCl3·6H2O, 0.2 mmol), urea (CO(NH2)2, 0.4 mmol) and [C12mim][Br] (0.02 60

mmol) were dissolved into 30 mL distilled water, and kept under mild magnetic stirring for 30 min. The above solution and the ceramic tubes were transferred into a 50 mL Teflon-lined autoclave, in which the pretreated ceramic tubes were fixed vertically, kept at 120 °C for 6 h, and then cooled to room 65

temperature naturally. FeOOH precursors on the ceramic tubes were swilled with deionized water several times and dried at 80 °C for 12 h. Finally, α-Fe2O3 mesoporous nanorod arrays were obtained by calcination of the precursors at 250 °C, 400 °C and 600 °C for 2 h in air, and the final products annealed at the three 70

temperatures are defined as sample 1-250, sample 1-400 and sample 1-600, respectively. Moreover, a series of control experiments, which were used to investigate the formation process of the α-Fe2O3 mesoporous nanorod arrays, were performed by controlling different hydrothermal reaction times. 75

To understand the effect of [C12mim][Br] addition on the morphology of the α-Fe2O3 architectures, the experiments were also performed without [C12mim][Br] addition. The synthetic procedure of the sample 2 was kept the same with that of sample 1 except that no [C12mim][Br] was added into the reaction 80

mixture. The as-prepared sample 2 was annealed at 250 °C and 600 °C for 2 h to obtain the final product and defined as sample 2-250 and sample 2-600, respectively. In addition, the sample 3 (with some ILs) was also prepared as follows in order to investigate the role of ILs on the improvement 85

of α-Fe2O3 to TMA sensing: the sample 2-600 (~0.04 g) was dispersed in 30 mL mixture of deionized water and [C12mim][Br] (0.02 mmol) for 12 h to adsorb [C12mim][Br], then the powder was washed several times and dried like before. The sample 3-600 was finally obtained after annealed in air at 250 °C for 2 h. 90

Characterization

The phase structure and phase purity of the as-synthesized samples were characterized by X-ray powder diffraction meter (XRD, D/MAX-Ⅲ-B-40KV, Japan) with monochromatized Cu Kα radiation (λ=1.5406 Å). The morphologies and structural 95

observations were characterized using scanning electron microscope (SEM, S-4800, HITACHI, Japan), transmission electron microscope (TEM, JEOL-JEM-2100, Japan), and selected area electron diffraction (SAED) analyses. Thermogravimetric (TG) analysis was carried out using a 100

thermogravimetric analyzer (PERKIN ELMER, TG/DTA 6300, USA) from 25 to 900 °C at a heating rate of 20 °C min-1 in air flow. The specific surface area and pore size distribution were examined from a nitrogen adsorption-desorption analysis conducted at 77 K using the Brunauer-Emmett-Teller method 105

(BET, Tristar 3020, Micromeritics, USA). The as-prepared products were also analyzed by X-ray photoelectron spectroscopy (XPS) using a VG ESCALAB-MK photoelectron spectrometer with Al Kα as the excitation source. All the binding energies were referenced to the C 1s line to 284.6 eV. SPME-GC-MS were 110

performed using gas chromatograph spectrometer (GC, 112A, China) directly coupled to an inert mass spectrometer (MS, 5975C, China) system. The molecular sieve column was a TDX01 MS model at 100 °C in H2 flow with the velocity of 40

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mL min-1.

Fabrication and measurement of gas sensors

In-situ deposited α-Fe2O3 mesoporous nanorod arrays film sensors were fabricated on ceramic tubes on which Au electrodes were pre-deposited onto the surface. Thick film sensors of sample 5

2-250, sample 2-600 and sample 3-600 were also fabricated following to ref. 28 for comparison with the gas sensing properties of arrays film sensors. A Ni-Cr alloy coil through the tube was employed as a heater to control the operating temperature of the sensors. The ceramic tube and heating wire 10

were connected to the base through the solder. Detailed fabrication procedure refers to ref. 29. The gas-sensing properties of the sensors were measured using a gas-sensing characterization system of JF02E intelligent test meter (Kunming, China). In the gas response measurement, a 15

given amount of sample gases was injected into a closed chamber by a microsyringe as described in ref. 29. The sensor was put into the test chamber for the measurement of the sensing properties. After each measurement, the sensor was exposed to the atmospheric air by opening the chamber. The gas response was 20

defined as the ratio Ra/Rg, where Ra and Rg are the sensor resistances measured in air and in test gas, respectively. The response time is defined as the time needed to reach the resistance Ra-(Ra-Rg)×90%, while the recovery time is Rg+(Ra-Rg)×90%. The common organic vapors (e.g., trimethylamine, 25

dimethylamine, phenylamine, ammonia, ethanol and acetone) were chosen as the target gases to evaluate the gas-sensing performances of the α-Fe2O3 sensors. The detection temperature was operated at 90-300 °C. The humidity sensing properties of α-Fe2O3 nanorod arrays film 30

sensors were measured following to ref. 28. Humidity response measurements of the sensors were carried out at 217 °C. The humidity response (SH) is defined as the ratio of the sensor resistance in the test humidity (t %) to that in air. Fresh carassius auratus (about 100 g), purchased from a local 35

market, was placed in a closed chamber at room temperature and analyzed within 11 h after its death. The measurement method of fish volatiles, released at different times (0, 1, 3, 5, 7, 9 and 11 h), is the same as that of the TMA gas. Simultaneously, SPME-GC-MS was used to analyze the primary volatiles of carassius 40

auratus, which were collected at different times (0, 1, 3, 5, 7, 9 and 11 h).

Results and discussion

Morphology and composition

Fig. 1 shows the XRD patterns of the as-prepared precursor and 45

the calcined products of sample 1. The precursor, obtained from a [C12mim][Br]-assisted hydrothermal route, exhibits a similar XRD pattern (Fig. 1a) with that of the tetragonal FeOOH phase (JCPDS No: 34-1226, 14/m(87), a = 0.1053 nm, b = 0.1053 nm, and c = 0.3030 nm). After the precursor is calcined at 250 °C 50

(Fig. 1b), 400 °C (Fig. 1c) and 600 °C (Fig. 1d) for 2 h, all the diffraction peaks of the calcined products can be attributed to those of the α-Fe2O3 phase (JCPDS No: 30-0664 , R-3c(167), a = 0.5036 nm, b = 0.5036 nm, c = 0.1375 nm).30 No other peaks belonging to the impurities are observed, demonstrating high-55

purity of the products. It is also found that the intensity of the

Fig. 1 XRD patterns of the as-prepared precursor (a) and the calcined products of sample 1 after heat treated at 250 °C (b), 400 °C (c) and 600 °C (d) for 2 h in air. 60

diffraction peaks in XRD patterns increases from sample 1-250 to sample 1-600. It indicates that the as-synthesized products crystallize better with increasing the heating temperature. SEM images of the products grown on ceramic tubes are shown in Fig. 2. It can be seen that the FeOOH precursor arrays are 65

highly ordered and vertically aligned and almost completely occupy the surface of ceramic tube. A high magnification image (Fig. 2b) further shows that the arrays are composed of interconnected uniform α-Fe2O3 nanorods. The average diameter and the length of the nanorods are estimated to be about 30 nm 70

and 200 nm, respectively (Fig. 2a). In order to confirm the morphology of the arrays in detail, the SEM images of the powder products precipitated at the bottom of the autoclave under the same hydrothermal conditions and their calcined product are shown in Fig. S1. It shows that the sample 1, either the precursor 75

or the calcined products, are all assembled by the nanoparticles. In order to obtain the information about the fine microstructure and its change when the nanorod arrays were calcined at different temperatures, detailed structural analysis of the products (sample 1) after calcined at 250, 400 and 600 °C was carried out by TEM 80

and SAED observation. It shows that the ordered arrays of the 250 °C-annealed product (Fig. 3a1 and 3b1) are just constructed of the single and long nanorods with diameters of about 30 nm, which has been also observed in SEM images. Besides, the uniformly porous structures with a pore size of 3-5 nm are well-85

distributed in the nanorods (Fig. 3b1 and 3c1). When the calcination temperature is increased to 400 and 600 °C, α-Fe2O3 nanorods with relatively larger porous structure can be observed in Fig. 3a2, 3b2, 3a3 and 3b3, and the higher the temperature is, the larger the pore size is. A large number of pores, the size of 5-90

12 nm for sample1-400 and 5-15 nm for sample 1-600, are distributed clearly in the whole nanorods. This means that the pore sizes of such α-Fe2O3 nanorods arrays can be adjusted by changing the calcined temperature. The corresponding HRTEM images of the three samples (Fig.3c1-3c3) exhibit the high 95

crystallinity and single-crystalline nature. The fringe spacing is about 0.18 nm for sample 1-250 and 0.27 nm for sample1-400 and sample 1-600, corresponding to the (024) and (104) crystal planes of the orthorhombic α-Fe2O3, respectively. The

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Fig. 2 Typical SEM images of precursor sample 1 (a, b), sample 1-250 (c), sample 1-400 (d) and sample 1-600 (e). Photos of device (inset of (b-e))

corresponding Fast Fourier Transform (FFT) images, as shown in insets of Fig. 3c1-3c3, further confirm the single-crystalline nature of the three calcined products. 5

To further investigate the porous structure of as-synthesized α-Fe2O3 nanorods, N2 adsorption-desorption isotherms and corresponding BJH pore size distributions of α-Fe2O3 nanorods are shown in Fig. 4. It can be noted that the N2 adsorption- desorption isotherms of the three samples all display a IV-type 10

curve, indicating the presence of mesopores.31 The pore size distributions of the α-Fe2O3 nanorods calcined at 250, 400 and 600 °C show that peak values of pore sizes are 3.2, 21.7 and 23.9 nm, respectively. The BET specific surface areas of the three samples are 88.41 m2 g-1 for sample 1-250, 63.01 m2 g-1 for 15

sample 1-400 and 54.05 m2 g-1 for sample 1-600, respectively. Obviously, sample 1-250 nanorods have the much higher specific surface area, and the narrower pore size distribution. The presence of mesopores in the α-Fe2O3 nanorods is resulted from the removal of water and the decomposition of [C12mim][Br] in 20

the precursor during thermal treatment. Undoubtedly, the loosely mesoporous structure might provide a higher surface-to-volume ratio than the other structure. This could enhance the gas molecules diffusion on the surface of α-Fe2O3 nanorod arrays, and the interaction between the α-Fe2O3 nanorod surface and gas 25

molecules, hence possibly improving the gas sensing performance of α-Fe2O3 mesoporous nanorods to the test gas.

3.2 Formation mechanism of α-Fe2O3 nanorod arrays

In order to investigate the growing process investigate the formation mechanism of the FeOOH nanorod arrays grown on 30

ceramic tubes, a detailed time course study was carried out. The synthetic times were 1, 2, 4 and 6 h and the images of the relating products are shown in Fig. 5b~5e. Fig. 5a shows the picture of bare ceramic tube substrate. After reacting for 1 h, clear yellow solution became turbidity, and then some precipitates were just 35

obtained as shown in Fig. 5b. When the reaction time was increased to 2 h, the nanoparticles nearly disappeared and the

products transformed into rod-like shapes, which packed together closely. Small nanorods with diameter around 15 nm can still be observed on ceramic tube surface (Fig. 5c). Increasing the 40

reaction time to 4 h, we found that oriented aggregation happened, many nanorods aggregated into a neatly parallel arrangement and the diameters and lengths of the well-constructed nanorods increased as shown in Fig. 5d. When the reaction time was over 6 h, the nanorods did not change 45

obviously and the diameters fixed to 30 nm, and the lengths to 200 nm (Fig. 2a and Fig. 5e). This implies that the optimal reaction time for synthesizing FeOOH nanorod arrays should be around 6 h at 120 °C. The XRD patterns (Fig. 5f) of the time-dependent products are in good agreement with the JCPDS (34-50

1226) data of FeOOH. It means that the products after hydrothermal reaction of 1 h are all FeOOH phase, while those lower than 1 h are solution. Clearly, the FeOOH nanorod arrays with a rough surface formed via the self-assembly of the primary nanoparticles as well as the nanorods arranged more orderedly 55

and tightly. On the other hand, when the reaction was conducted under the same conditions in the absence of the ionic liquid, the disordered FeOOH nanorods with lengths of 50-250 nm and diameters of 40-80 nm were obtained (JCPDS No: 34-1226). It implies the key role of ionic liquid [C12mim][Br] in synthesis of 60

FeOOH nanorod arrays. Based on an examination of the above time-dependent formation process, the growth mechanism of the mesoporous α-Fe2O3

nanorod arrays grown on the ceramic tube is proposed as follows. The reactions involved in the formation of α-Fe2O3 nanorod 65

arrays can be summarized as following three reaction procedures:32

CO(NH2)2 + 3H2O → 2NH4+ + 2OH− + CO2 (1)

Fe3+ + 3OH− → FeOOH → α-Fe2O3 (2) In the first stage, OH− ions produce by the hydrolysis of 70

CO(NH2)2 under the hydrothermal condition, as shown in Eqn (1). Then, the homogeneous precipitation of FeOOH nanocrystals

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Fig. 3 Typical TEM (a, b), HRTEM (c) images and SAED patterns (insets of (c)) of α-Fe2O3 nanorod arrays after calcined at 250 °C (a1~c1), 400 (a2~c2) and 600 °C (a3~c3).

rapidly form by the reaction of Fe3+ with OH− (Eqn (2)). Many neighboring primary FeOOH nuclei further grow into the rod-like 5

nanocrystals with a monoclinic structure through oriented aggregation due to the thermodynamical unstablility resulted from the high surface energy.17 When [C12mim][Br] is not added into the solution, there are only disordered FeOOH nanorods. On the contrary, the addition of [C12mim][Br] promotes the FeOOH 10

nanoparticles continuous assembly into the highly ordered and vertically aligned nanostructures on the surface of ceramic tube under the same hydrothermal conditions. It is reported that [C12mim][Br] is a typical amphiphilic surfactant consisting of a long alkyl chain and a cationic head group.33,34 In the IL-15

assistanted synthesis system, the cationic head group of [C12mim]+ is supposed to be weakly electrostatically attracted to

the side surface of the growing FeOOH while the hydrophobic hydrocarbon chains protrude into aqueous phase, thus forming a so-called protective electrosteric layer on the FeOOH surface, 20

avoiding aggregation.35 Meanwhile, the [C12mim]+ ions may be easily adsorbed on FeOOH by hydrogen bonding due to the existence of (-Fe-O-H) groups on the surface of crystallites and the special structure of [C12mim][Br] can induce the preferential growth of FeOOH crystal nuclei in a certain direction.29 Finally, 25

well-aligned α-Fe2O3 nanorod arrays with different sizes of homogenous mesopores were obtained by calcination the arrays at different temperatures, which have been shown in Fig. 3. However, irregular α-Fe2O3 nanorods obtained without using [C12mim][Br] did not form uniformly porous structure by 30

calcination of FeOOH precursor, which has been demonstrated

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Fig. 4 Nitrogen adsorption-desorption isotherms and pore size distribution curves (inset) of the α-Fe2O3 nanorod arrays calcined at 250 °C (a), 400 °C (b) and 600 °C (c).

5

Fig. 5 Picture of bare ceramic tube substrate (a), SEM images (a~e) and XRD patterns (f) of sample 1 obtained at 120 °C for different reaction times (a: 1 h, b: 2 h, c: 4 h, d: 6 h) on the substrate.

by SEM, TEM (Fig. S2) and TG (Fig. S3) analyses. The results indicate that the presence of [C12mim][Br] has significant 10

influence on the formation of the homogeneously mesoporous nanorods and self-assembled arrays structure as a capping agent or a soft template based on electrostatic attraction, hydrogen

bonds, self assembled mechanism and so on, which is similar to the formation process of Co3O4, SnO2, ZnO and TiO2.

22,36-38 15

3.3 Gas sensing properties

Quantitative measurement of trimethylamine is most widely used method to test the fish freshness. As the fishes lose freshness, they begin to generate volatile basic nitrogen gases such as trimethylamine, dimethylamine, and ammonia. Trimethylamine 20

gas in particular is known to increase rapidly as the freshness of fishes starts to deteriorate.39,40 In order to study the effect of microstructure of sensing material on the sensing property, five gas sensors were fabricated based on the α-Fe2O3 nanorod arrays (sample 1-250, sample 1-400 and sample 1-600) and α-Fe2O3 25

nanorods (sample 2-250 and sample 2-600). The responses of these five sensors to 100 ppm TMA were measured at an

operating temperature range of 130~290 °C, as shown in Fig. 6a. It is obvious that the sensor based on porous α-Fe2O3 arrays annealed at 250 °C exhibits enhanced responses for TMA as 30

compared to those based on α-Fe2O3 arrays annealed at 400 and 600 °C and α-Fe2O3 nanorods annealed at 250 and 600 °C at all the working temperature range. This may be caused by the suitable mesoporous and hierarchical nanostructures which possess the larger specific surface area and more active sites for 35

the TMA gas to adsorb and react. The BET specific surface area of the sample 1-250 is 88.41 m2 g-1, which is much larger than those of sample 1-400 (63.01 m2 g-1), sample 1-600 (54.05 m2 g-1), sample 2-250 (51.42 m2 g-1) and sample 2-600 (41.24 m2 g-1) (Fig. S4). The larger specific surface area of the sensing material is, the 40

higher response of the sensor is. Besides, we think the appropriate amount of [C12mim][Br] retention in α-Fe2O3 nanostructures can selectively adsorb TMA molecules and is also helpful to improve the gas sensing performance. In order to study the effect of [C12mim][Br] on the 45

TMA sensing property of α-Fe2O3 nanorods, two gas sensors were fabricated based on the α-Fe2O3 nanorods: pure α-Fe2O3

sample 2-600 synthesized without ILs addition, and ILs-midified

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α-Fe2O3 sample 3-600 prepared after sample 2-600 adsorbed ILs ([C12mim][Br]). The responses of the two sensors to 100 ppm TMA were measured at the same operating temperature range of 130~290 °C, as shown in Fig. S5. It can be seen that the sample 3-600 sensor exhibits good responses for TMA as compared to 5

sample 2-600 sensor at all the working temperature range. This confirms the important role of appropriate amount of [C12mim][Br] retention in α-Fe2O3 nanorod arrays for the improvement of their TMA gas sensing performance. Thus, the following investigations are all based on the mesoporous 10

nanorods calcined at 250 °C (sample 1-250) to retain appropriate amount of [C12mim][Br]. Fig. 6b shows the responses of the sensor based on sample 1-250 to 100 ppm trimethylamine, dimethylamine, aniline, ammonia, acetone and ethanol, which were tested at different operating 15

temperatures (90~300 °C). It is obvious that the responses to amines rapidly increase and reach its maximum at 217 °C, and then decrease with a further increase of the operating temperature. The gas responses of the sensor to 100 ppm of six gases of trimethylamine, dimethylamine, aniline, ammonia, 20

ethanol and acetone, are 22.3, 18.1, 15.9, 6.7, 10.1 and 1.5, respectively, measured at 217 °C. The excellent response for stronger interaction between the amines and the surface-adsorbed oxygen species (O−), and faster release of trapped electrons in the α-Fe2O3 array nanorods when compared to the same amounts of 25

other gases interacting with the sensor. In addition, good response for amines might be also attributed to the formation of hydrogen bond (C-H…N) between the hydrogen of imidazole ring (residual [C12mim][Br]) and the nitrogen of amines. Because the electron cloud density around N atom in amines is higher than that around 30

O atom in acetone and ethanol,1,41 the interaction between amines and [C12mim]+ is stronger than that between acetone/ethanol and [C12mim]+, thus it is much easier for amines to adsorb on α-Fe2O3 nanorod arrays. The responses of the sesor to acetone and ethanol are low. Moreover, it is worth noting that the responses to 35

(CH3)3N (22.3), (CH3)2NH (18.1), C6H6NH2 (15.9) and NH3 (6.7) fleetly decrease, which might be due to the structural characteristics of amines. The methyl (CH3

−) is electron-donating group, the electrongativity of N atom in (CH3)3N is higher than that in (CH3)2NH, C6H6NH2 and NH3, the interaction between N 40

atom in (CH3)3N and [C12mim]+ on the surface of α-Fe2O3 can facilitate the adsorption of (CH3)3N on the surface of sensing material. In comparison with (CH3)3N and (CH3)2NH molecules, there might be the large steric hindrance of C6H6NH2, but the mutual π-π stacking interaction between aromatic ring of 45

C6H6NH2 and cation of the imidazole ring ([C12mim][Br]) would

Fig. 6 The responses of the α-Fe2O3 nanorod arrays of sample 1-250, sample 1-400, sample 1-600, sample 2-250 and sample 2-600 sensors to 100 ppm TMA measured at different operating temperatures; (b) The 50

responses of the sample 1-250 sensor to 100 ppm various gases at different operating temperatures.

Fig. 7 The responses (a) and the transient sensing characteristics (b) of α-Fe2O3 nanorod arrays sensor to different concentrations of TMA. 55

Fig. 8 The responses of α-Fe2O3 nanorod arrays sensor to different concentrations of relative humidity; (b) stability measurement of α-Fe2O3 nanorod arrays sensor to 100 ppm TMA in duration of 100 days.

hinder the adsorption of C6H6NH2. So, the response to C6H6NH2 60

is a little lower than that to (CH3)3N and (CH3)2NH. Thus, the residual [C12mim][Br] could help the α-Fe2O3 nanorods to adsorb more amines, especially (CH3)3N molecules. As a consequence, we choose 217 °C as the optimal operating temperature to proceed with subsequent investigations, which is lower than other 65

α-Fe2O3 composites. 14-16,42 Monitoring trimethylamine can serve as an indicator for fish spoilage. Trimethylamine in the evaluation of fish freshness is found that 0~10 ppm is regarded as fresh, and the fish decay begins over 10 ppm of TMA.2 With the increasing concentration 70

of trimethylamine, the gas responses of the sensor increase in a good linear relationship (R2=0.9881) for a concentration range from 0.1 to 100 ppm (Fig. 7a). Fig. 7b shows the real-time gas responses of α-Fe2O3 nanorod arrays to different concentrations of trimethylamine gas at the optimal working temperature of 217 75

°C. The sensitivity to 0.1 ppm trimethylamine is 1.5, while the sensitivities to 1, 20, 50 and 100 ppm are 2.0, 5.4, 14.9 and 22.3, respectively. It can be seen that in-situ deposited α-Fe2O3 nanorod arrays film sensors show higher sensitivity to dilute trimethylamine, and the detection limit is 0.1 ppm. Fig. 7b shows 80

the transient sensing characteristics of the sensor to TMA. The response times of the sensor to different concentrations of TMA gas range from 1 s to 5 s. The recovery time of the sensor to different concentrations of TMA gas are a little slow, within 30~165 s. The short response time of such sensor might be 85

resulted from the strong interaction between TMA molecules and residual [C12mim][Br], except the reaction of the TMA and surface adsorbed oxygen on the α-Fe2O3 sensing material. In addition, the large quantities of interconnected mesopores in α-Fe2O3 nanorods are favorable for TMA molecules to diffuse and 90

adsorb easily on the surface of sensing material, thus decreasing the response time greatly. Relative humidity (RH) plays an important role in not only determination the gas response of metal oxide-based gas sensors but also influence TMA detection possessed high humidity in the 95

fish storage environment. Because of the high humidity in the

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fish storage process. Because of the high humidity in the fish storage process, it is very necessary to study the effect of humidity on TMA detection. Fig. 8a shows the responses of α-Fe2O3 nanorod arrays sensor to relative humidity. It can be seen that there is a very small change of gas response when the sensor 5

is put in different humidity atmospheres, suggesting that the relative humidity has almost no effect on the TMA detection of such sensor. Stability is a key quality indicator in the development of gas sensors for real markets, which can be influenced by many 10

factors.2 Fig. 8b shows the stability measurements of α-Fe2O3

nanorod arrays sensor to 0.1 ppm and 100 ppm TMA performed within 100 days. It shows that the sensitivity values are estimated to be 1.5±0.2 and 21.9±0.5 at 0.1 and 100 ppm of TMA, respectively, which indicate good stability of the sensor. All the 15

above results imply that the α-Fe2O3 nanorod arrays film sensor is a good candidate for detection of low concentration of TMA at 217 °C. The superior TMA sensing performance of this sensor could be mainly attributed to the homogenously and hierarchically mesoporous structure, large specific surface area of 20

such nanorod arrays deposited in-situ on the sensor, and residual ionic liquid [C12mim][Br]. In-situ deposited α-Fe2O3 mesoporous nanorod arrays sensor based on sample 1-250 was exposed repeatedly to the odors from a carassius auratus (about 100 g), which was stored in a closed 25

chamber at 0 and 25 °C, respectively. The responses of α-Fe2O3 nanorod arrays sensor to the volatiles released at different storage times and temperatures of the fish, measured at different working temperatures (90~300 °C), are shown in Fig. 9. It can be seen that the responses to carassius auratus volatiles first gradually increase 30

with operating temperature, up to 217 °C, and then decrease, the optimal working temperature of the sensor for fish detection is also 217 °C. The correlation between different times of volatiles from fish and responses of the sensor operated at the optimal temperature is shown in Figure 9c. The responses show a 35

continuous and accelerating increase with the storage time of fish increasing, corresponding to the corruption process of the dead fish, which means that the responses of the sensor increase in a good linear relationship (R2=0.9860, R2=0.9878 ) with the storage time at 0 and 25 °C, respectively. Furthermore, in-situ deposited 40

α-Fe2O3 mesoporous nanorod arrays sensor also displays fast transient response characteristics and fine reversibility to the volatiles of fish. As the sensor is exposed to fresh and 11 h of volatiles from fish at 25 °C, the responses are 4.4 and 12.7, and

response/recovery times are calculated to be 5/1 s and 50/159 s at 45

optimal working temperature, respectively (Fig. S6). This trend also agrees with the responses of the sensor to different concentrations of TMA which was generated from trimethylamine oxide by bacterial enzymes during spoilage in carassius auratus.1-5 So, it is necessary to explore the gaseous 50

components from dead fish stored at different times so as to confirm the relation between TMA and the fish volatiles using SPME GC-MS technique. Fig. S7 shows the gas chromatogram and mass spectrum of the fish volatiles after the fish is exposed from 0 to 11 hours at 25 oC. The results indicate that the primary 55

volatile is trimethylamine. The chromatographic peak at 0.7992 corresponds to the retention time (Rt) of TMA (Fig. S7a) and the corresponding mass spectrum (Fig. S7c) shows the intense molecular ion peak at m/z=59, which could be speculated to TMA gas molecule. In addition, the intensity of these peaks 60

gradually increases, indicating clearly the increase of the amount of TMA with the freshness of fish deterioration. In conclusion, such in-situ deposited α-Fe2O3 mesoporous nanorod arrays film sensor can real-time monitor the volatiles of carassius auratus. Compared with the earlier reports on the fish freshness detection 65

by gas sensors (see Table 1), this sensor exhibits fast response and low detection limit to target gas at lower working temperature. It demonstrates that such sensor would provide a quite promising approach for instant determination of the fish freshness. 70

3.4 Gas sensing mechanism

Based on the above experimental results and reported investigation, α-Fe2O3 nanorod arrays film sensors, exhibiting good response to TMA, might be explained by the following sensing mechanism. It is known that α-Fe2O3 is resistive-type 75

gas-sensing material, the molecular interaction between ILs and organic gas,43 and the surface interaction between adsorbed oxygen and the target gas is a key factor influencing their gas- sensing performance. In air, oxygen molecules are adsorbed onto the surface of the α-Fe2O3 nanorod arrays, and then transferred to 80

O− ions by gaining electrons from the conductive bands of α- Fe2O3 at temperature range of 90~300 °C, thus electron depletion layers with a high-resistance state form (process 1 in Fig. 10). When the α-Fe2O3 nanorod array sensor is exposed to TMA gas, TMA reacts with the oxygen adsorbates (O−), resulting in the 85

oxygen concentration decreasing (Eqn (1) - (3)). These reaction

Fig. 9 The responses of the α-Fe2O3 nanorod arrays sensor to the volatiles from a 100 g carassius auratus storage for different times (0, 1, 3, 5, 7, 9 and 11 h) measured at different working temperatures (storage temperature: a-0 °C, b-25 °C); The responses of α-Fe2O3 nanorod arrays sensor as a function of storage time measured at 217 °C under exposure to the odors from a carassius auratus (c). 90

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Table. 1TMA sensing property of the reported various oxide semiconductor sensors.

Materials TMA conc., (ppm)

S (Ra/Rg)

Topt.

(°C) Tr1/Tr2

(s) Detection

limit (ppm) Fish freshness

Ref.

ZnO-Al2O3/TiO2/V2O5 160 350 300 - 2 yes 12

ZnO-SnO2 50 125 330 2/5 1 yes 13

TiO2-α-Fe2O3 100 33.1 250 0.5/1.5 10 no 14

ZnO-α-Fe2O3 50 5.9 260 0.7/7.1 10 no 15

Graphene quantum dors-α-Fe2O3 100 293 270 15/28 0.01 no 16

CdO-Fe2O3 1000 1527 230 70/170 0.01 no 42

α-Fe2O3 50 14.9 217 1/70 0.1 yes this work

TMA conc. = TMA gas concentration. S = response. Topt. = optimal operating temperature. Ref. = Reference. Tr1/Tr2 =response/recovery time.

Fig. 10 Proposed sensing mechanism of α-Fe2O3 nanorod array to TMA. 5

release electrons that decrease the resistance of the α-Fe2O3 nanorod arrays sensor, shown as processes 2 and 3 in Fig. 10, forming a low-resistance state. The possible reactions taken place on the surface of α-Fe2O3 nanorods are as follows: 2, 42

O2 (ads) ＋ e－→ O2 (ads) (＜100°C) (1) 10

O2 (ads) ＋ 2e－→ 2O− (ads) (100～300°C) (2)

4N(CH3)3(ads)＋21O−(ads)→2N2＋9H2O＋6CO2＋42e－ (3)

Fig. S8 shows the XPS spectra before and after the sample 1-250 sensor exposure to 100 ppm TMA. It can be seen that the surface adsorbed oxygen of sample 1-250 is about 53.7 %, and then 15

decreases to 24.2 % after the sensor exposure to 100 ppm TMA. Thus, it is easily concluded that main reactions can be attributed to more surface adsorbed oxygen ions reacted with the TMA gas molecules. In addition, the enhanced and excellent TMA-sensing performance of the α-Fe2O3 sensor might be resulted from the one 20

dimensional and homogeneously mesoporous nanostructure, which exhibit an ordered and hierarchically organized assembly with uniform pore and distribution, and possess large specific surface areas and pore volumes, as well as appropriate amount of residual functionalized ILs, which enabling them to fast and 25

selectively adsorb TMA molecules.

Conclusions

In summary, we have developed a facile [C12mim][Br]-assisted hydrothermal approach to fabricate large-scale and well-aligned

α-Fe2O3 nanorod arrays precursor on the ceramic tubes. 30

Combined with the calcinations at different temperatures, α-Fe2O3 nanorod arrays with different sizes of homogenous mesopores were obtained. The formation of such uniformly mesoporous nanorod arrays can be ascribed to the self-assembly mechanism, based on the interaction between [C12mim][Br] and 35

α-Fe2O3 crystal planes in the hydrothermal reaction, and the following removal of partial ILs upon calcination at certain temperature. In addition, the appropriate amount of ILs retention in α-Fe2O3 mesoporous nanorods, calcined at 250 oC can selectively adsorb TMA molecules and improve the TMA sensing 40

performance greatly. Such in-situ assembled α-Fe2O3 nanorod arrays sensor exhibits a high response to TMA at 217 °C with good anti-humidity and low detection limit of 100 ppb. The excellent sensing performance can be attributed to the homogeneously and hierarchically mesoporous nanostructure, 45

high surface area and residual functionalized ILs. Meanwhile, the most important thing is such α-Fe2O3 nanorod arrays sensor enables the “real-time” monitoring of fish spoilage, which possesses a fast and high-sensitivity analysis of the volatiles of carassius auratus in this study. The results suggested that such α-50

Fe2O3 sensors can be used as a rapid, low cost and non-destructive measurement in monitoring fish freshness and quality.

Acknowledgments

This work was supported by the Interational Science & Technology Cooperation Program of China (2016YFE0115100), 55

National Natural Science Foundation of China (21771060, 61271126), Program for Science and Technology Project of Heilongjiang province (B2015008), Heilongjiang Educational Department (2013TD002, 135109206).

Notes and references 60

aKey Laboratory of Functional Inorganic Material Chemistry, Ministry of

Education, School of Chemistry and Materials Science, Heilongjiang

University, Harbin 150080, People's Republic of China.

E-mail address: lhhuo68@yahoo.com; xuyingming@hlju.edu.cn; Tel.:

(+86) 0451-86608426; Fax: (+86) 0451-86608040 65 bCollege of Chemistry and Chemical Engineering, Qiqihar University,

Qiqihar 161006, People's Republic of China.

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cKey Laboratory for Polymeric Composite and Functional Materials of M

inistry of Education & Lehn Institute of Functional Materials,

School of Chemistry Sun Yat-Sen University, Guangzhou, 510275, China

† Electronic Supplementary Information (ESI) available. See DOI: 10.1039/b000000x/ 5

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View Article OnlineDOI: 10.1039/C7TA06392J

Ionic liquid-assisted synthesis of mesoporous α-Fe2O3 nanorods

arrays and their excellent trimethylamine gas-sensing properties for

monitoring fish freshness

Ping Wanga,b, Zhikun Zheng

c, Xiaoli Cheng

a, Lili Sui

b, Shan Gao

a, Xianfa

Zhanga,Yingming Xu

a,*, Hui Zhao

a and Lihua Huo

a,*

a Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education

of the People's Republic of China, Heilongjiang University, Harbin 150080 P. R.

China; Tel: (+86) 451 8660 8426;

b College of Chemistry and Chemical Engineering, Qiqihar University, Qiqihar

161006, P. R. China.

c Key Laboratory for Polymeric Composite and Functional Materials of Ministry of

Education & Lehn Institute of Functional Materials, School of Chemistry

Sun Yat-Sen University, Guangzhou, 510275, China

E-mail: lhhuo68@yahoo.com (L. H. Huo), xuyingming@hlju.edu.cn (Y. M. Xu)

Mesoporous α-Fe2O3 nanorod arrays were grown in-situ on ceramic tubes by a

[C12mim][Br]-assisted hydrothermal reaction with a subsequent calcination process.

Such mesoporous nanorod arrays, with appropriate amount of [C12mim][Br] retention,

exhibit excellent sensitivity to trimethylamine and good anti-humidity ability at

217 °C, which can be used in identification of fish freshness.

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View Article OnlineDOI: 10.1039/C7TA06392J