microstructure (0D to 3D)

Accepted Manuscript Size and morphology dependent gas-sensing selectivity towards acetone vapor based on controlled hematite nano/microstructure (0D to 3D) Hao Ding, Jinliang Ma, Fang Yue, Pingyi Gao, Xiao Jia PII:

S0022-4596(19)30202-6

DOI:

https://doi.org/10.1016/j.jssc.2019.04.025

Reference:

YJSSC 20726

To appear in:

Journal of Solid State Chemistry

Received Date: 23 January 2019 Revised Date:

2 April 2019

Accepted Date: 21 April 2019

Please cite this article as: H. Ding, J. Ma, F. Yue, P. Gao, X. Jia, Size and morphology dependent gassensing selectivity towards acetone vapor based on controlled hematite nano/microstructure (0D to 3D), Journal of Solid State Chemistry (2019), doi: https://doi.org/10.1016/j.jssc.2019.04.025. 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.

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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT Size and morphology dependent gas-sensing selectivity towards acetone vapor based on controlled hematite nano/microstructure (0D to 3D)

College of Chemistry, Qishan Campus, Fuzhou University, Fuzhou, Fujian 350116, P. R. China

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Hao Ding 1, Jinliang Ma 1, Fang Yue 1, Pingyi Gao 1, Xiao Jia 1,*

*

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Corresponding authors: Tel.: +86-591-22867963, E-mail: [email protected] (X. Jia)

ACCEPTED MANUSCRIPT ABSTRACT Seven 0D-3D α-Fe2O3 nanostructures with different shapes and sizes have been prepared via a solvothermal route and subsequent calcination. It was found that the dosage of

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hexamethylenetetramine (HMTA) and the kind of solvent played determinant roles in the controlled synthesis of α-Fe2O3 nanostructures. Increasing the amount of HMTA from 0.070 g to 0.420 g, the morphology of α-Fe2O3 product evolves from 1D nanobelt, 2D nanosheet to 3D

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nanoflower with sizes ranging from 4 µm to 650 nm. Changing the kind of solvent with various

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chain lengths or polarity, 0D α-Fe2O3 porous nanospheres, rhombohedra and cylinder microstructure can be obtained. Based on the experimental results, the possible formation mechanism of various dimensional products was speculated. The different gas-sensing behaviors that related with the microstructures of the as-prepared α-Fe2O3 samples were also investigated.

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The fabricated gas sensors all present good stability and high selectivity towards acetone gas, and the comparable responses to acetone should be due to the variation in their size, morphology and specific surface area (SSA). In addition, the gas-sensing conductive mechanism based on the

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α-Fe2O3 samples was also proposed.

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Keywords: α-Fe2O3 nanostructures; solvothermal method; controlled synthesis; gas-sensing performance

1. Introduction

In recent years, a great deal of volatile organic compounds (VOCs) have been widely used in

industry and laboratories, and some of the VOCs even face the risk of explosion [1, 2]. Therefore, proper monitoring techniques of VOCs concentration in the air should become of great importance. Compared with the other traditional gas detection methods (such as electrochemical method,

ACCEPTED MANUSCRIPT solid-state electrolyte method, chemiluminescence method etc.), semiconductor oxide gas sensor has the merit of relatively easy operating, high sensitivity, fast response and recovery time etc [3]. As is well-known, metal oxide semiconductor serves as a signal transducer by utilizing its

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resistance variation, which arises from the charge transfer reactions between the adsorbed oxygen on its surface and target gases [4, 5]. As a result, the sensing performance of gas sensor is closely related to the morphology, size and surface nature of the fabricated semiconductor material [6].

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Even though various metal oxide semiconductors [7-12] have been obtained for the application of

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chemical sensors, the controlled synthesis of peculiar semiconductor nanostructures and the vital roles of their microstructures on the differences in gas-sensing behaviour are still rising research tasks.

Hematite (α-Fe2O3), a low cost and environmentally friendly n-type transition metal oxide

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(Eg=2.1 eV), has been drawn considerable attention in the fields of gas sensor, drug delivery, catalysts and lithium-ion batteries etc [13, 14]. α-Fe2O3 nanostructures with miscellaneous sizes and morphologies have been synthesized through different technologies, including the

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hydrothermal or solvothermal process, sol-gel synthesis and thermal decomposition of organic

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complexes etc [15-21]. Among the diverse morphologies of α-Fe2O3 nanomaterials, the porous 1D nanostructure has been extensively investigated for gas sensor due to its high surface-to-volume ratio and large interfacial contact area for gas transmission. Nevertheless, for preparing porous 1D α-Fe2O3 nanobelt structure, the main synthesis method at present is thermal oxidation deposition technology that using iron substrate at high temperature, which requires careful control of the growth conditions [22, 23]. The synthesis of α-Fe2O3 nanobelt has so far been rarely achieved using the solution-based wet-chemical approach, which may be due to its particular crystal

ACCEPTED MANUSCRIPT structure and the requirement of special templates [24]. Thus, it still remains a challenge to explore a new solution-based reaction system to synthesize the morphology and size controllable α-Fe2O3 nanostructures (especially 1D nanobelt structures) for their applications in the

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microstructure-related gas-sensing field. In this work, the controllable synthesis of α-Fe2O3 nanobelt has been implemented by a facile solution process. Beyond that, α-Fe2O3 nanostructures with different morphologies (nanoflower,

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nanosphere, rhombohedra and cylinder etc.) from 0D to 3D can also be effectively controlled by

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simply adjusting the experiment parameters such as the amount of HMTA, the kind of solvents etc. And the growth mechanism of α-Fe2O3 nanostructures in different dimensionalities was discussed in detail. Correspondingly, the gas sensing performances of the obtained seven products were investigated in detail. So far as we know, the controllable synthesis of α-Fe2O3 nanobelt using a

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solution approach and their related gas sensing performance have seldom been reported. Meanwhile, the systematic investigation of the vital roles of HMTA in the growth process of α-Fe2O3 nanostructures from 0D to 3D architectures is also rarely covered, which would provide

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an alternative method to explore high-performance α-Fe2O3 sensors. Furthermore, in order to

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better explain the differences in the gas sensing response of each sample, the related affecting factors such as crystal microstructure and SSA were studied in detail, and the gas-sensing mechanism was also proposed based on the experimental results. 2. Experimental section

2.1 Preparation of 1D α-Fe2O3 nanobelt structure All chemicals in analytical grade were obtained from Sinopharm Chemical Reagent Co Ltd and used without further purification. In a typical experiment, 0.152 g of FeCl3 and 0.140 g of

ACCEPTED MANUSCRIPT HMTA were dissolved in 40 mL of ethylene glycol (EG). After being stirred at 50℃ for 1 h, the mixed solution was sealed in a 50 mL Teflon-lined autoclave and heated at 220℃ for 6 h. Subsequently, the autoclave was cooled to room temperature. The α-Fe2O3 precursor was

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centrifuged and rinsed with deionized water and ethanol several times. The α-Fe2O3 product (sample S2) was obtained by calcination of the precursor at 500℃ for 3 h, with a temperate heating rate of 1 ℃ min−1.

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A series of α-Fe2O3 nanostructures were also obtained via changing the amount of HMTA or

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the kind of solvents (named as S1, S3, S4, S5, S6 and S7, respectively), with the other experimental conditions remain unchanged, as shown in Table 1. For sample S5, the α-Fe2O3 product was prepared without calcination. 2.2 Characterization

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The structures of products were performed using X-ray powder diffraction (XRD, Rigaku D/Max 2200PC, Cu Kα radiation

λ= 1.5418 Å). The microstructures and morphologies were

observed via field emission scanning electron microscopy (FESEM, Nova NanoSEM 230) and

were

measured

by

a

BET

(Brunauer-Emmett-Teller)

method

with

N2

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product

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high-resolution TEM (HR-TEM, Tecnai G2 F20 S-TWIN, 200 kV). The SSA and porosity of the

adsorption-desorption on a gas adsorption analyzer (Autosorb-1, Quantachrome Corp., USA). 2.3 Gas sensing measurement The Gas Sensing Measurement System (JF02E, Jinfeng Tech. Co. Ltd, Kunming, China) was

used to detect the gas-sensing properties. The as-prepared products were mixed with terpineol and the suspensions were coated on the alumina substrate. After drying in air for 2 days at room temperature, the film sensors were annealed at 300℃ for 2 h, and then aged at 300℃ for a week

ACCEPTED MANUSCRIPT before measurement. In this work, the sensing response is defined as S=Ra/Rg, where Ra and Rg are the resistances of gas sensor in air and the tested gas, respectively [25, 26]. 3. Results and discussions

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3.1 Structural and morphological characteristics XRD patterns of the as-prepared precursor and the calcination product in the typical experiment are shown in Fig. S1 and Fig. 1a. The diffraction peaks of the precursor can match

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well with metal alkoxide, which has a characteristic peak at ca. 11.1o [27-29]. Therefore, we

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conclude that the precursor was a Fe-alkoxide compound. All the diffraction peaks of calcination product in Fig. 1a can be well indexed to the pure phase hematite (JCPDS 33-0664), and the characteristic peaks of the Fe-alkoxide disappeared. The results indicated that the precursor has been completely transformed to α-Fe2O3.

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The low and high-magnification SEM images of the α-Fe2O3 product are shown in Fig. 1b and c. It can be clearly seen that the α-Fe2O3 product exhibits 1D nanobelt structures with lengths of ca. 2 µm, widths of ca. 500 nm and thicknesses of ca. 100 nm. The TEM images in Fig. 1d

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reveal that the as-prepared α-Fe2O3 nanobelts are constructed by several small irregular

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nanoparticles. The magnified TEM image of an individual nanobelt in Fig. 1e clearly shows small holes in its interior, indicating the porous structures. The lattice spacing calculated from the red area in Fig. 1e is 0.25 nm (Fig. 1f), which agrees well with the (110) plane of rhombohedral α-Fe2O3.

3.2 Formation mechanism of α-Fe2O3 nanostructures with different morphologies To further explore how the 1D α-Fe2O3 nanobelt was formed in the typical synthesis process, the time-dependent experiment was carried out (Fig. S2). At an initial stage of 0.5 h, the red

ACCEPTED MANUSCRIPT products were amorphous (Fig. S2f), with small and bulk particles coexisted (Fig. S2a). After reacting for 1 h, the bulk particles were completely disappeared, and plenty of 2D thin sheets combined with a few nanobelt structures could be observed (Fig. S2b). Meanwhile, the color of

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sample started to change to green at 1 h, indicating that the reduction of Fe3+ to Fe2+ occurred with the assistance of EG [30, 31]. The XRD pattern further confirmed that the iron alkoxide structure began to form at 1 h. With increasing the reaction time from 1 to 3 h, the proportion of nanobelt

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structure increased significantly with the thin sheet disappeared gradually. When the reaction time

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was prolonged to 4 h, the thin sheets were almost entirely converted to nanobelt structures, and the product preserved the 1D structure with a relative uniform size at an extended reaction time of 6 h. On the above results, we infer that nanobelt structure are formed by a fast nucleation of amorphous particles with a subsequent preferred orientation growth process. Amorphous particles

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were first obtained, and oriented aggregation to form plenty of thin sheets. Then, the thin sheets gradually grew into longer nanobelts, as their ends had the trend to attract other Fe2+ nuclei, and HMTA that acted as a the capping agent can also attach on the specific facets of the iron ( )

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alkoxide and lead to the anisotropic growth of nanobelt structures [24, 32]. Note that EG

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molecules that acted as chelated ligand and reductant can react with FeCl3 and form iron ( ) alkoxide and HCl [28]. If HCl is failure to remote promptly and accumulated, the further reaction should be hampered. Fortunately, OH- decomposed by HMTA has the role of assisting the removal of H+, and moreover the superfluous OH- can again accelerate the oxidation of EG during the present polyol system [33, 34]. Meanwhile, the release rate of OH- can be effectively adjusted via adding different concentration of HMTA, which might affect the coordination, reduction, and polymerization

ACCEPTED MANUSCRIPT process and ultimately change the nanostructure of the product. Increasing the amount of HMTA from 0.070 g to 0.280 g, the morphology of the α-Fe2O3 product changed from 1D nanobelt (Fig. 2a and 1b) to 3D nanoflower structures (Fig. 2c and S3). To be specific, when 0.070 g of HMTA

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was added, the as-prepared product had nanobelt structures with lengths of ca. 4 µm, widths of ca.700 nm and thicknesses of ca. 50 nm (sample S1, Fig. 2a, and b), which were thinner and longer compared with sample S2 (Fig. 1a and b, 0.140 g of HMTA). When the dosage of HMTA

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was increased to 0.280 g, the nanoflower structures with diameters of ca. 650 nm were prepared

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(Fig. 2c and d). The 3D flower-like nanostructures were composed of interconnected nanosheets with widths of ca. 300 nm and thicknesses of ca. 40 nm. If we increased the dosage of HMTA further to 0.42 g, similar nanoflower structures consisted of thickened nanosheets could be obtained (Fig. S4). Compared with sample S1, S2 possessed smaller size may be attributed to the

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increased pH value of the solution and the promoted nucleation rate of the crystal by adjusting the amount of HMTA from 0.070 g to 0.140 g. Increasing the amount of HMTA to 0.280 g and even to 0.420 g, the hydrolysis rate would be further increased in the high-alkaline pH solution. In order to

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reduce the surface energy, the formed small particles tended to aggregate along a particular

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direction, and the 3D nanoflower structures that assembled by laminated nanosheets were formed eventually. In general, HMTA should act not only as a capping agent but also as a pH regulation agent during the process for forming nanobelt and nanoflower structures [35]. Meanwhile, HMTA played a vital role to provide a suitable alkaline environment for iron alkoxide, enabling their preferred orientation growth. The effect of different solvents with varying polarities on the morphology of the product was also investigated. As shown in Fig. 3 and S5, pure phase α-Fe2O3 nanostructures with adjustable

ACCEPTED MANUSCRIPT size and morphology can be obtained. When methanol was used as the reaction solvent, 0D α-Fe2O3 porous hollow nanostructures with a uniform particle size of ca. 100 nm were prepared (Fig. 3a). When using ethanol instead of methanol, rhombic-shaped α-Fe2O3 nanostructures (ca.

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300nm, Fig. 3b) that consisted of small nanoparticles were synthesized. In case of using EG and cyclohexane as mixed solvents, cylinder-like mircostructures (ca. 1µm) with a deep crack in the middle were obtained (Fig. 3c). 0D diamond α-Fe2O3 nanostructures with smooth surfaces and

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uniform sizes of 100 nm were formed in water-cyclohexane biphasic system (Fig.3d) [36, 37]. The

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precursor has a relatively good dissolution and fast hydrolysis and nucleation rate in a more polar solvent, which is beneficial for the formation of plenty of small nanocrystals [38]. The polarity of series of single solvents used in the experiment is as follows: methanol > ethanol > EG, and the corresponding sizes of as-prepared α-Fe2O3 nanostructures increased from 100 nm to 4 µm

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successively. For samples S6 and S7 synthesized in mixed solvents, the degree of complex formation of HMTA with Fe3+ became more complicated. Compared with EG, water has a higher dielectric constant and polarity index, resulting in a smaller size of sample S7 compared with S6.

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In addition, different alcohols or mixed solvents may have different effects on the agglomeration

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tendency of nanoparticles, leading to the formation of nanostructures with different shapes. 3.3 BET analysis

Fig. S6 presents the nitrogen adsorption-desorption isotherm and the corresponding pore size

distribution curve of the as-prepared samples S1-S7. The SSA of samples S1-S7 calculated by the BET measure was 9.50, 38.15, 29.17, 31.16, 110.80, 27.16 and 41.15 m2 g-1, respectively. The average pore size distribution was calculated from the desorption branch by the BJH method, which shows a narrow distribution centered at 3-5 nm for S2-S7 and an additional wide

ACCEPTED MANUSCRIPT distribution centered at ca. 30-35 nm for samples S1-S3 and S7. As a result, the relatively high SSA and the mesoporous nanostructures of the product could effectively increase the surface-active sites, which are conducive to absorbing molecular oxygen and enhancing

3.4 Gas sensing performance and mechanism

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gas-sensing performance.

The gas-sensing performance of the as-prepared α-Fe2O3 samples S1-S7 with different sizes

the sensitivity curves of the gas sensors under 100 ppm ethanol were measured (Fig. 4a).

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voltage

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and morphologies were investigated, as shown in Fig. 4. To determine the optimum working

With the increase of operating voltage, the responses of all sensors increase evidently at first and then reduce, with a maximum value appear at 3.75 V. Thus, 3.75 V was served as the optimum working voltage in the next gas sensing experiment.

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To investigate the selectivity of the as-prepared gas sensing materials, the responses of the sensors to various flammable or toxic gases (acetone, benzene, carbon monoxide etc., 100 ppm, 3.75 V) were detected (Fig. 4b). Among the eight tested gases, all samples exhibited higher

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responses to acetone. This difference in response degree may be explained by the larger dipole

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moment and the easily catalyzed properties on the α-Fe2O3 surface of acetone molecule [39, 40]. The dynamic response curves of the series sensors under acetone concentrations ranging from

10 to 1000 ppm were also measured (Fig. 4c), and the corresponding sensitivities are shown in Fig. 4d. The responses of samples S1-S7 increased sharply as the concentrations of acetone increased from 10 to 500 ppm, and then increased slightly with a further increase in the concentration of acetone up to 1000 ppm. Towards 1000 ppm of acetone gas, the responses of S1-S7 is 18.87, 23.64, 22.56, 29.84, 31.73, 10.61 and 31.58, respectively. As we known, many factors such as the

ACCEPTED MANUSCRIPT morphology, size and SSA of the fabricated materials can affect the sensing performance of sensors [41, 42]. Usually, the smaller the nanoparticle size, the faster the electron transfer rate, the more conductive to the improvement of gas sensing performance. Among these sensors, samples

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S4, S5 and S7 with relatively smaller size have higher gas-sensing responses, behave in exactly this way, and sample S5 with the largest SSA has the highest sensitivity (up to 31.73). For the other nanostructures with different dimensionality, the as-prepared porous 1D α-Fe2O3 nanobelt

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(sample S2) has the highest gas responses, which should be ascribed to its high surface-to-volume

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ratio and large interfacial contact area for gas transmission. Sample S3 with nanoflower structure bahave the following gas-sensing performance, while S6 with cylinder microstructure has the lowest response. As a note, sample S1 with the smallest SSA has a much higher response than S6, which should be benefit from its unique 1D nanobelt structure. To sum up, it may be indicated that

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the particle size is the most important but not the decisive factor affecting the samples’ gas sensing responses. For multi-dimensional samples, the special 1D porous nanobelt structure also play a key role in improving the sensing properties.

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The repeatability performance is another important parameter to test the quality of the

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fabricated gas sensors. The response-recovery curves in Fig. 4e show that S1-S7 basically maintained their initial responses without significant decrease after four cycle experiments, indicating their good stability and reproducibility. The resistance change that induced by the adsorption and desorption of gas molecules on the

surface of sensing materials is the basis of the gas-sensing mechanism of the semiconductor sensor. When the sensor is exposed to air, oxygen molecules can be ionized to oxygen species on the sensor surfaces by capturing the free electrons from the conductance band of α-Fe2O3. And then,

ACCEPTED MANUSCRIPT the resistance increases due to the formation of electron depletion layer. When exposed to acetone gas, the chemisorbed oxygen species will react with acetone gas molecules and release the electron back to the conductance band of α-Fe2O3, leading to the decrease in resistance. Further study of

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the oxygen species adsorbed on the surface of the sensor was also carried out [43, 44]. The sensor response and acetone concentration in log scale can be written as simplified formulas: log(S-1) = b log(C) + log(a), in which b value obtained from the slope of the plot between log(S-1) and log(C)

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representing oxygen ion species [45]. As shown in Fig. S7, there are linear correlations between

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log(S-1) and log(C) with a slope of b value close to 0.5 for all samples, demonstrating the key media of gas sensing process is O2-. Thus, the reaction process may be described as follows: O2 (g)+ 4e−

2O2− (ads)

C3H6O (ads) + 8O2−

3CO2(g) + 3H2O + 16e−

(2)

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4. Conclusions

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In summary, a series of 0D-3D α-Fe2O3 nanostructures have been synthesized via a facile solvothermal route combined with calcination. By adjusting the dosage of HMTA and the kind of

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solvents, the as-prepared nanostructures in the form of nanobelt, nanoflower, nanosphere,

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rhombohedra and cylinder with sizes from ca. 100 nm to ca. 4µm can be effectively controlled. The detailed study indicated that HMTA can play a non-mutually exclusive dual role of pH regulation and capping agent during the process for forming nanobelt and nanoflower structures. Meanwhile, the larger the polarity of solvent, the smaller the particle size of the product in the solvothermal system. When tested as fabricated materials for gas sensor, all the α-Fe2O3 samples exhibit excellent stability, good selectivity and high gas-sensing response towards acetone vapor. The gas sensing behaviors of the sensors are closely related to the sizes, morphologies, and SSA of

ACCEPTED MANUSCRIPT the fabricated materials. Among the obtained samples S1-S7, rhombohedra sample S5 with relatively smaller particle size and higher SSA shows the best gas-sensing performance. Moreover, the unique 1D porous nanobelt structure also acts a pivotal part in increasing the gas sensing

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responses. Acknowledgments

The work was supported by the National Natural Science Foundation of China (No.

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21201035) and the Natural Science Foundation of Fujian Province (No. 2017J05021).

References

[1] Meng D.; Liu D.Y.; Wang G.S.; Shen Y.B.; San X.G.; Li M.; Meng F.L., Low-temperature formaldehyde gas sensors based on NiO-SnO2 heterojunction microflowers assembled by thin porous

TE D

nanosheets. Sens. Actuators, B 2018, 273, 418-428.

[2] Jo Y.M.; Kim T.H.; Lee C.S.; Lim K.; Na C.W.; Abdel-Hady F.; Wazzan A. A.; Lee J.H., Metal–Organic Framework-Derived Hollow Hierarchical Co3O4 Nanocages with Tunable Size and

EP

Morphology: Ultrasensitive and Highly Selective Detection of Methylbenzenes. ACS Appl. Mater.

AC C

Interfaces 2018, 10 (10), 8860-8868.

[3] Zhang L.H.; Dong B.; Xu L.; Zhang X.R.; Chen J.J.; Sun X.K.; Xu H.W.; Zhang T.X.; Bai X.; Zhang S.; H.W. Song, Three-dimensional ordered ZnO–Fe3O4 inverse opal gas sensor toward trace concentration acetone detection. Sens. Actuators, B 2017, 252, 367-374. [4] Ma J.H.; Ren Y.; Zhou X.R.; Liu L.L.; Zhu Y.H.; Cheng X.W.; Xu P.C.; Li X.X.; Deng Y.H.; Zhao D.Y., Pt nanoparticles sensitized ordered mesoporous WO3 semiconductor: gas sensing performance and mechanism study. Adv. Funct. Mater. 2018, 28 (6), No.1705268.

ACCEPTED MANUSCRIPT [5] Zhang H.; Yi J.X., Enhanced ethanol gas sensing performance of ZnO nanoflowers decorated with LaMnO3 perovskite nanoparticles. Mater. Lett. 2018, 216, 196-198. [6] Geng W.C.; Ge S.B.; Zhang X.W.; He, S.; Gu J.W.; Lai X.Y.; Wang H.; Zhang Q.Y., Volatile

Fast Response. ACS Appl. Mater. Interfaces, 2018, 10 (16), 13702-13711.

RI PT

Organic Compound Gas-Sensing Properties of Bimodal Porous α-Fe2O3 with Ultrahigh Sensitivity and

[7] Kim H.J.; Yoon J.W.; Choi K.I.; Jang H.W.; Umar A.; J.H. Lee, Ultraselective and sensitive

M AN U

nanostructures. Nanoscale 2013, 5 (15), 7066–7073.

SC

detection of xylene and toluene for monitoring indoor air pollution using Cr-doped NiO hierarchical

[8] Rai P.; Yoon J.W.; Jeong H.M.; Hwang S.J.; Kwak C.H.; Lee J.H., Design of highly sensitive and selective Au@NiO yolk–shell nanoreactors for gas sensor applications. Nanoscale 2014, 6 (14), 8292–8299.

TE D

[9] Hagedorn K.; Li W.Y.; Liang Q.J.; Dilger S.; Noebels M.; Wagner M.R.; Reparaz J.S.; Dollinger A.; Günne J.S.A.D.; Dekorsy T.; Schmidt-Mende L.; Polarz S., Catalytically Doped Semiconductors for Chemical Gas Sensing: Aerogel-Like Aluminum-Containing Zinc Oxide Materials Prepared in the Gas

EP

Phase. Adv. Funct. Mater. 2016, 26 (20), 3424-3437.

AC C

[10] Zhu L.; Li Y.Q.; Zeng W., Hydrothermal synthesis of hierarchical flower-like ZnO nanostructure and its enhanced ethanol gas-sensing properties. Appl. Surf. Sci. 2018, 427, 281-287. [11] Xu J.M.; Cheng J.P., The advances of Co3O4 as gas sensing materials: a review, J. Alloys Compd. 2016, 686, 753-769.

[12] Wang L.L.; Deng J.N.; Lou Z.; Zhang T., Cross-linked p-type Co3O4 octahedral nanoparticles in 1D n-type TiO2 nanofibers for high-performance sensing devices. J. Mater. Chem. A 2014, 2 (26), 10022–10028.

ACCEPTED MANUSCRIPT [13] Jiang Z.F.; Wan W.M.; Li H.M.; Yuan S.Q.,; Zhao H.J.; Wong P.K., A Hierarchical Z℃Scheme α-Fe2O3/g-C3N4 Hybrid for Enhanced Photocatalytic CO2 Reduction. Adv. Mater. 2018, 30 (10), No.1706108.

RI PT

[14] Yang B.X.; Liu J.Y.; Qin H.; Liu Q.; Jing X.Y.; Zhang H.Q.; Li R.M.; Huang G.Q.; Wang J., Co3O4 nanoparticle-decorated hierarchical flower-like α-Fe2O3 microspheres: Synthesis and ethanol sensing properties. J. Alloys Compd. 2017, 727, 52-62.

SC

[15] Chen H.M.; Zhao Y.Q.; Yang M.Q.; He J.H.; Chu P.K.; Zhang J.; Wu S.H., Glycine-assisted

M AN U

hydrothermal synthesis of peculiar porous α-Fe2O3 nanospheres with excellent gas-sensing properties. Anal. Chim. Acta 2010, 659 (1-2), 266-273.

[16] Su C.; Zou Y.C.; Xu X.F.; Liu L.; Liu Z.; Liu L.L., Ultrahigh sensitivity of Nd-doped porous α-Fe2O3 nanotubes to acetone, Colloids Surf., A 2015, 472, 63-68.

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[17] Dai M.J.; Zhao L.P.; Gao H.Y.; Sun P.; Liu F.M.; Zhang S.; Shimanoe K.G.; Yamazoe N.; Lu G.Y., Hierarchical Assembly of α-Fe2O3 Nanorods on Multiwall Carbon Nanotubes as a High-Performance Sensing Material for Gas Sensors. ACS Appl. Mater. Interfaces 2017, 9 (10), 8919-8928.

EP

[18] Wang L.B.; Yang H.L.; Liu X.X.; Zeng R.; Li M.; Huang Y.H.; Hu X.L., Constructing Hierarchical

AC C

Tectorum-like α-Fe2O3/PPy Nanoarrays on Carbon Cloth for Solid-State Asymmetric Supercapacitors. Angew. Chem. Int. Ed. 2017, 56 (4), 1105-1110. [19] Mirzaei A.; Janghorban K.; Hashemi B.; Bonavita A.; Bonyani M.; Leonardi S.G.; Neri G., Synthesis, Characterization and Gas Sensing Properties of Ag@α-Fe2O3 Core–Shell Nanocomposites. Nanomaterials 2015, 5 (2), 737-749. [20] Mirzaei A.; Janghorban K.; Hashemi B.; Bonyani M.; Leonardi S.G.; G. Neri, Highly stable and selective ethanol sensor based on α-Fe2O3 nanoparticles prepared by Pechini sol–gel method. Ceram.

ACCEPTED MANUSCRIPT Int. 2016, 42 (5), 6136-6144. [21] Pan H.; Jin L.; Zhang B.B.; Su H.; Zhang H.T.; Yang W.Q., Self-assembly biomimetic fern leaf-like α-Fe2O3 for sensing inflammable 1-butanol gas. Sens. Actuators, B 2017, 243, 29-35.

RI PT

[22] Yuan L.; Cai R.S.; Jang J.I.; Zhu W.H.; Wang C.; Wang Y.Q.; Zhou G.W., Morphological transformation of hematite nanostructures during oxidation of iron, Nanoscale 2013, 5 (16), 7581-7588. [23] Wen X.G.; Wang S.H.; Ding Y.; Wang Z.L.; Yang S.H., Controlled growth of large-area, uniform,

SC

vertically aligned arrays of α-Fe2O3 nanobelts and nanowires. J. Phys. Chem. B 2005, 109 (1), 215-220.

M AN U

[24] Gao G.; Gao W.J.; Guo X.Q.; Wang H.; Wu H.X.; Zhang C.L.; Wang C.; Cui D.X., Synthesis of single-crystalline α-Fe2O3 nanobelts via a facile PEG-200 assisted solution route. CrystEngComm 2011, 13 (20), 6045-6049.

[25] Zhou X.; Wang B.Q.; Sun H.B.; Wang C.; Sun P.; Li X.W.; Hu X.L.; Lu G.Y., Template-free

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synthesis of hierarchical ZnFe2O4 yolk–shell microspheres for high-sensitivity acetone sensors. Nanoscale 2016, 8 (10), 5446-5453.

[26] Zhang J.; Liu X.H.; Neri G.; Pinna N., Nanostructured Materials for Room-Temperature Gas

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Sensors. Adv. Mater. 2016, 28 (5), 795-831.

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[27] Zhang J.J.; Chen Y.L.; Sun Y.F.; Huang T.; Yu A.S., Hierarchical hollow Fe2O3 micro-flowers composed of porous nanosheets as high performance anodes for lithium-ion batteries. RSC Adv. 2013, 3 (43), 20639-20646.

[28] Jin S.L.; Deng H.G.; Long D.H.; Liu X.J.; Zhan L.; Liang X.Y.; Qian W.M.; Ling L.C., Facile synthesis of hierarchically structured Fe3O4/carbon micro-flowers and their application to lithium-ion battery anodes. J. Power Sources 2011, 196 (8), 3887-3893. [29] Ma X.H.; Feng X.Y.; Song C.; Zou B.K.; Ding C.X.; Yu Y.; Chen C.H., Facile synthesis of

ACCEPTED MANUSCRIPT flower-like and yarn-like α-Fe2O3 spherical clusters as anode materials for lithium-ion batteries. Electrochim. Acta 2013, 93, 131-136. [30] Cao S.W.; Zhu Y.J.; Chang J., Fe3O4 polyhedral nanoparticles with a high magnetization

RI PT

synthesized in mixed solvent ethylene glycol–water system. New J. Chem. 2008, 32 (9), 1526-1530.

nanoparticles. Angew. Chem. Int. Ed. 2004, 116 (5), 607-611.

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[31] Sun X.M.; Li Y.D., Colloidal carbon spheres and their core/shell structures with noble-metal

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[32] Shen Q.Q.; Xue J.B.; Liu J.; Jia H.S.; Liu X.G.; Xu B.S., Shape-controlled synthesis of three-dimensional branched CdS nanostructure arrays: structural characteristics and formation mechanism. CrystEngComm 2013, 15 (5), 1007-1014.

[33] Fan T.; Li Y.G.; Zhang H., Surfactant-Free Solvothermal Synthesis of 3D Flowerlike Iron

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Alkoxide (Fe-EG) Micro/Nanostructures: Structure, Formation Mechanism, and Fenton Oxidation of Azo Dyes. Ind. Eng. Chem. Res. 2017, 56 (41), 11684-11696. [34] Joseyphus R.J.; Shinoda K.; Kodama D.; Jeyadevan B., Size controlled Fe nanoparticles

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487-493.

EP

through polyol process and their magnetic properties. Mater Chem Phys. 2010, 123 (2-3),

[35] Avireddy H.; Kannan H.; Shankar P.; Mani G.K.; Kulandaisamy A.J.; Rayappan J.B.B., Non-mutually exclusive dual role of hexamethylenetetramine on the growth of ZnO nanostructures and their sensing footprints. Mater. Chem. Phys. 2018, 212, 394-402. [36] Rui Y.C.; Xiong H.; Su B.; Wang H.Z.; Zhang Q.H.; Xu J.L.; Müller-Buschbaum P.,

Liquid-liquid interface assisted synthesis of SnO2 nanorods with tunable length for enhanced performance in dye-sensitized solar cells. Electrochim. Acta 2017, 227, 49-60.

ACCEPTED MANUSCRIPT [37] Velleman L.; Sikdar D.; Turek V.A.; Kucernak A.R.; Roser S.J.; Kornyshev A.A.; Edel J.B.,

Tuneable 2D self-assembly of plasmonic nanoparticles at liquid|liquid interfaces. Nanoscale 2016, 8 (46) 19229-19241.

RI PT

[38] Chaudhari N.K.; Kim H.C.; Kim C.S.; Park J.; Yu J.S., Solvent controlled synthesis of new

hematite superstructures with large coercive values. CrystEngComm 2012, 14 (6), 2024-2031.

[39] Jia Q.Q.; Ji H.M.; Zhang Y.; Chen Y.L.; Sun X.H.; Jin Z.G., Rapid and selective detection of

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acetone using hierarchical ZnO gas sensor for hazardous odor markers application. J. Hazard. Mater.

M AN U

2014, 276, 262-270.

[40] Tian S.Q.; Yang F.; Zeng D.W.; Xie C.S., Solution-Processed Gas Sensors Based on ZnO Nanorods Array with an Exposed (0001) Facet for Enhanced Gas-Sensing Properties. J. Phys. Chem. C 2012, 116 (19), 10586-10591.

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[41] Zhang L.; Jing X.Y.; Liu J.Y.; Wang J.; Sun Y.B., Facile synthesis of mesoporous ZnO/Co3O4 microspheres with enhanced gas-sensing for ethanol. Sens. Actuators, B 2015, 221, 1492-1498. [42] Joshi N.; Silva L.F.D.; Jadhav H.S.; Shimizu F.M.; Suman P.H.; M’Peko J.C.; Orladi M.O.; Seo

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J.G.; Mastelaro V.R.; Oliveira Jr O.N., Yolk-shelled ZnCo2O4 microspheres: Surface properties and gas

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sensing application. Sens. Actuators, B 2018, 257, 906-915. [43] Xiong Y.; Zhu Z.Y.; Ding D.G.; Lu W.B.; Xue Q.Z., Multi-shelled ZnCo2O4 yolk-shell spheres for high-performance acetone gas sensor, Appl. Surf. Sci. 2018, 443, 114-121. [44] Liu C.; Zhao L.P.; Wang B.Q.; Sun P.; Wang Q.J.; Gao Y.; Liang X.S.; Zhang T.; Lu G.Y., Acetone gas sensor based on NiO/ZnO hollow spheres: Fast response and recovery, and low (ppb) detection limit, J. Colloid Interface Sci. 2017, 495, 207-215. [45] Gao P.Y.; Liu R.; Huang H.H.; Jia X.; Pan H.B., MOF-templated controllable synthesis of α-Fe2O3

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porous nanorods and their gas sensing properties. RSC Adv. 2016, 6 (97) 94699–94705.

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Size and morphology dependent gas-sensing selectivity towards acetone vapor based on controlled hematite nano/microstructure (0D to 3D)

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Table of contents Section 1: Table 1. Seven kinds of typical morphologies of α-Fe2O3 nanostructures. Table 1. Seven kinds of typical morphologies of α-Fe2O3 nanostructures. Solvent/ mL

Morphologies

S1

0.070

EG:40

nanobelt

4 µm

9.50

S2

0.140

EG:40

nanobelt

1-3 µm

38.15

S3

0.280

EG:40

nanoflower

650 nm

29.17

S4

0.140

methanol :40

porous nanosphere

100 nm

31.16

S5

0.140

ethanol:40

rhombohedra

300 nm

110.80

S6

0.140

EG /cyclohexane 20:20

cylinder

1 µm

27.16

S7

0.140

water/ cyclohexane 20:20

diamond

100 nm

41.15

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HMTA/g

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Size

SSA /m2·g-1

Samples

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Table of contents Content

Fig. 1

XRD pattern (a), SEM images (b, c) and TEM images (d, e, f) of the as-prepared α-Fe2O3 nanobelt (S2) in a typical synthesis.

Fig. 2

Low- and high magnification FESEM images of α-Fe2O3 nanostructures with adding different amounts of HMTA: (a, b) 0.070 g (S1) and (c, d) 0.280 g (S3).

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(a) Responses of the as-prepared samples S1-S7 at different operating voltage, to 100ppm ethanol vapor, respectively. (b) Responses of sensors to various gases at 100ppm. (c) Time-dependent responses of the sensors to acetone vapor. (d) Responses of the sensors to different concentrations of acetone vapor. (e) Responses-recovery curves of gas sensors to 100 ppm ethanol vapor.

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Fig. 4

SEM images of α-Fe2O3 nanostructures obtained by changing the reaction solvents. (a) methanol (S4), (b) ethanol (S5), (c) EG+cyclohexane (1:1) (S6) and (d) water+ cyclohexane (1:1) (S7).

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Fig. 1 XRD pattern (a), SEM images (b, c) and TEM images (d, e, f) of the as-prepared α-Fe2O3 nanobelt (S2) in a typical synthesis.

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Fig. 2. Low- and high magnification FESEM images of α-Fe2O3 nanostructures with adding different amounts of HMTA: (a, b) 0.070 g (S1) and (c, d) 0.280 g (S3).

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Fig. 3. SEM images of α-Fe2O3 nanostructures obtained by changing the reaction solvents. (a) methanol (S4), (b) ethanol (S5), (c) EG+cyclohexane (1:1) (S6) and (d) water+ cyclohexane (1:1) (S7).

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Fig. 4 (a) Responses of the as-prepared samples S1-S7 at different operating voltage, to 100ppm ethanol vapor, respectively. (b) Responses of sensors to various gases at 100ppm. (c) Time-dependent responses of the sensors to acetone vapor. (d) Responses of the sensors to different concentrations of acetone vapor. (e) Responses-recovery curves of gas sensors to 100 ppm ethanol vapor.

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The controlled synthesis of 0D-3D α-Fe2O3 nanostructures using a HMTA-assisted solution approach and their mircostructures-related gas-sensing performance have been reported.