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α-Fe2O3 porous microrods with enhanced TEA-sensing performance
Accepted Manuscript Facile synthesis of ZnFe2O4/α-Fe2O3 porous microrods with enhanced TEA-sensing performance Yanwei Li, Na Luo, Guang Sun, Bo Zhang, Guangzhou Ma, Honghong Jin, Yan Wang, Jianliang Cao, Zhanying Zhang PII:
S0925-8388(17)34251-2
DOI:
10.1016/j.jallcom.2017.12.068
Reference:
JALCOM 44154
To appear in:
Journal of Alloys and Compounds
Received Date: 22 September 2017 Revised Date:
6 December 2017
Accepted Date: 7 December 2017
Please cite this article as: Y. Li, N. Luo, G. Sun, B. Zhang, G. Ma, H. Jin, Y. Wang, J. Cao, Z. Zhang, Facile synthesis of ZnFe2O4/α-Fe2O3 porous microrods with enhanced TEA-sensing performance, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2017.12.068. 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.
ACCEPTED MANUSCRIPT
Facile synthesis of ZnFe2O4/α-Fe2O3 porous microrods with enhanced TEA-sensing performance Yanwei Li, Na Luo, Guang Sun∗, Bo Zhang, Guangzhou Ma, Honghong Jin, Yan Wang*, Jianliang Cao and Zhanying Zhang
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School of Materials Science and Engineering, Cultivating Base for Key Laboratory of Environment-friendly Inorganic Materials in University of Henan Province, Henan Polytechnic
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University, Jiaozuo 454000, PR China
∗
Corresponding author Tel.: +86 03913986952 E-mail address: [email protected](Guang Sun); [email protected] (Yan Wang) 1
ACCEPTED MANUSCRIPT Abstract Developing metal oxide nanocomposite with superior gas-sensing properties is an important subject in the field of gas sensor. In this paper, we reported a facile synthesis route for ZnFe2O4/α-Fe2O3 porous microrods (PMRs) by using dual oxalates as sacrificial template.
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FeC2O4·2H2O solid microrods (SMRs) were first synthesized by a chemical precipitation reaction between FeSO4·7H2O and H2C2O4·2H2O, which were then immersed in Zn2+ aqueous solution to obtain the hybrid ZnC2O4·2H2O/FeC2O4·2H2O SMRs through the substitution reaction between Zn2+ and Fe2+. After a proper annealing process, ZnFe2O4/α-Fe2O3 PMRs were successfully
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prepared through the decomposition of ZnC2O4·2H2O/FeC2O4·2H2O SMRs. The prepared ZnFe2O4/α-Fe2O3 PMRs are about 2–5 µm in length and 0.3–0.5 µm in diameter, and constructed
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by numerous loosely stacked nanoparticles with the size about 15 nm. The dominant pore size in ZnFe2O4/α-Fe2O3 PMRs is around 10 nm. The results of gas-sensing tests indicates that through decorating the α-Fe2O3PMRs with ZnFe2O4, the response of the sensor towards TEA is remarkably improved. For example, the response of the composite sensor to 100 ppm TEA is as high as 42.4, which is about 20 times higher than that of the bare α-Fe2O3 PMRs sensor (2.5). The
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enhanced TEA sensing mechanism of ZnFe2O4/α-Fe2O3 PMRs was discussed in detail. Keywords: ZnFe2O4/α-Fe2O3; Porous structure; Sacrificial template; Heterojunction; Gas sensor
1. Introduction
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As one of the most important organic amines, triethylamine (TEA) has been widely used in chemical and food industries as raw material, catalyst, solvent and preservative [1]. However,
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because TEA is volatile and toxic, the volatilization and spillage of TEA can bring a series of threatens to human health and even endanger our environment [2,3]. Thus, fast and timely detecting the existence and concentration of TEA in surroundings is of great importance for environment protection. Metal oxide semiconductor (MOS) based gas sensor has stood out from various gas detection techniques due to its merits of low cost, fast response and easy fabrication, and been regarded as a significant way to monitor flammable, toxic, and corrosive gases [4,5]. Up to now, several kinds of MOSs have been developed as gas-sensing materials to detect TEA, such as SnO2, α-Fe2O3, V2O5, MoO3, NiFe2O4, NiO/ZnO and so on [6-11]. However, due to the limited gas-sensing properties, some of them are far to satisfy the needs of practical application. The 2
ACCEPTED MANUSCRIPT development of high performance TEA-sensing materials is still a challenging work. It has been widely acknowledged that the gas sensitivity of MOS originates from its resistance change when it is exposed to different atmospheres, which is mainly controlled by the surface reactions participated by adsorbed oxygen and tested gases [12]. Therefore, the gas-sensing
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properties of MOSs are closely related with their chemical composition and microstructure. In order to improve the gas-sensing properties of MOSs, fabricating their novel micro/nanostructures with large surface area to promote the surface reactions (including surface gas adsorption, redox reaction, and desorption) has been proved an effective way [13]. Among various structures, porous
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structure, especially mesoporous structure with large surface area, was considered as one of ideal structures that can achieve high gas-sensing performance, because the existence of a particularly
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large number of nano-sized pores can facilitate the gas diffusion and transportation, and the large surface area can provide more available active sites for gas adsorption and reaction [14]. In addition, combining two or more kinds of MOSs together to fabricate their nanocomposite is also a feasible way to improve the gas-sensing properties [15,16]. With the assistance of synergistic effect between different components, the nanocomposite can always exhibit upgrading gas-sensing
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performance as compared with their single component counterpart [17,18]. Thus, developing metal oxide nanocomposite with superior gas-sensing properties via rational design and controlled synthesis of their chemical composition and micro/nanostructure has become one of the most
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active fields.
Hematite (α-Fe2O3), an n-type semiconductor with a band gap of 2.1 eV, has been widely
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investigated in a broad range of applications, such as gas sensors [19], photocatalysts [20], magnetic materials [21], lithium-ion batteries [22], and solar cells [23], because of the features of non-toxicity, low cost, high stability, and environmental compatibility. In order to improve the gas-sensing properties of α-Fe2O3, different α-Fe2O3 based nanocomposites have been fabricated and investigated. Sun et al. reported the synthesis of the hierarchical assembly of α-Fe2O3 nanosheets on SnO2 hollow nanospheres via a microwave hydrothermal method and found that such a hierarchical composite exhibited superior ethanol-sensing performance to the single component of SnO2 hollow spheres [24]. Zhang et al. reported that the hierarchical α-Fe2O3/ZnO nanobranch frame synthesized via a facile two-step continuous hydrothermal process, has a good 3
ACCEPTED MANUSCRIPT sensitivity and better selectivity to trimethylamine [25]. Li et al. reported the synthesis of V2O5-decorated α-Fe2O3nanorods by electrospinning and soak-calcination method. The composite showed high selectivity and stability to diethylamine [26]. Zhang and co-workers reported the good gas sensing properties of ZnO nanoparticle-decorated round-edged α-Fe2O3 hexahedrons
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fabricated through solvothermal method [27]. All these researches firmly demonstrate that the combination of α-Fe2O3 with other MOS is a promising way to achieve high sensing performance. However, to the best of our knowledge, there are few reports on the synthesis and TEA sensing properties of hybrid ZnFe2O4/α-Fe2O3 porous microrods.
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In this work, ZnFe2O4/α-Fe2O3 porous microrods (PMRs) were successfully synthesized via a facile and reliable sacrificial template method, in which a continuous process including
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precipitation, substitution, and decomposition reactions was involved and dual oxalates were applied as sacrificial template. Significantly, the prepared ZnFe2O4/α-Fe2O3 PMRs exhibited superior TEA-sensing performance to the single component α-Fe2O3 PMRs in terms of much higher response. The variation of the potential barrier height of ZnFe2O4-Fe2O3 heterojunction in different gas ambient was suggested to be the main reason for the improved TEA-sensing
2. Experimental
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performance.
2.1 Synthesis of ZnFe2O4/α-Fe2O3 PMRs
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All the chemical reagents involved in the experiments were of analytical purity and used as received without further purification. Distilled water was used throughout the experiments. In
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order to synthesize ZnFe2O4/α-Fe2O3 PMRs, a continuous process including precipitation, substitution and decomposition reactions was employed, as shown in Fig. 1. Firstly, FeC2O4·2H2O solid microrods (SMRs) were synthesized through a chemical precipitation reaction between H2C2O4·2H2O and FeSO4·7H2O. In brief, a homogeneous solution of Fe2+ aqueous solution was prepared by dissolving 0.69 g of FeSO4·7H2O into 100 mL of distilled water with vigorous stirring. Then, 20 mL of ethanol solution containing 0.63 g of oxalic acid (H2C2O4·2H2O) was dropped into above solution with magnetic stirring. After stirring at room temperature for 60 min, FeC2O4·2H2O SMRs were prepared and collected by centrifugation. Secondly, by using above FeC2O4·2H2O SMRs as precursor, hybrid ZnC2O4·2H2O/FeC2O4·2H2O SMRs were prepared via a 4
ACCEPTED MANUSCRIPT mild substitution reaction. Typically, an appropriate amount of as-prepared FeC2O4·2H2O precursor were sufficiently dispersed in 20 mL of aqueous solution containing a designed amount of Zn(CH3COO)2·2H2O, and the molar ratio of Fe: Zn is set at 1:8. After stirring at room temperature for 48 h, the precipitate was harvested by centrifugation, washed with distilled water and ethanol for several times, and dried in air at 70 oC for 5 h. Finally, the prepared hybrid oxalate
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SMRs, being applied as sacrificial templates, were annealed in air at 430 oC for 2 h to obtain the ZnFe2O4/α-Fe2O3 PMRs. For comparison, α-Fe2O3 PMRs were also prepared by directly annealing the FeC2O4·2H2O SMRs at the same condition.
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2.2 Characterization
X-ray diffraction (XRD) measurements were carried on Bruker/D8-Advance diffractometer
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with Cu Kα radiation. The scanning was performed from 10 to 80o with a speed of 0.02o min-1. The morphology and microstructure were investigated by field emission scanning electron microscopy (FESEM, FEI QUANTA FEG250) and transmission electron microscopy (TEM, JEOL, JEM-2100). Elemental analysis was performed by energy dispersive spectroscopy (EDS, INCA ENERGY 250) integrated into the FESEM system. Thermal analysis was carried out on a
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Setaram Evolution 2400 thermal analyzer. The specific surface area of the prepared samples was measured on a Micromeritics Triatar 3020 apparatuses by using Brunauer-Emmett-Teller (BET) method.
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2.3 Sensor fabrication and measurement
The fabrication process for sensor is similar to our previous method [28]. Simply, a proper
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amount of as-prepared samples was dispersed into ethanol to get uniform emulsion, which was then coated onto a ceramic substrate (13.4 mm × 7 mm) screen-printed with Ag-Pd comb-like electrodes to obtain the resistance-type sensor. The schematic diagram of the fabricated sensor is showed in Fig. 2. The gas-sensing tests were carried on an intelligent gas sensing analysis system of CGS-4TPS (Beijing Elite Tech. Co., Ltd., China). Before testing, the sensor was allowed to be aged at 200 oC for 12 h to improve the repeatability and stability. During the test, target gases such as TEA with calculated concentration were injected into the testing chamber (1.8 L) by a microsyringe. The response (S) of the sensor was defined as the ratio of Ra/Rg, where Ra and Rg were the electrical resistance of sensor in air and in target gas, respectively. The response and 5
ACCEPTED MANUSCRIPT recovery times were defined as the time required for a change in response to reach 90% of the equilibrium value after injecting and removing the target gas, respectively. During the test, the operating temperature range was set at 270–330 oC, and the relative humidity was 40%.
3. Results and discussion
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3.1 Characterization The phase and purity of the products were characterized with XRD, and the results are showed in Fig. 3. In Fig. 3a, all of the diffraction peaks can be attributed to standard FeC2O4·2H2O (JCPDS: 72-1305) [29], indicating the formation of pure FeC2O4·2H2O phase in the chemical
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precipitation step. In comparison with Fig. 3a, two additional peaks with low intensity that appeared at 2θ of 22.6o and 30.2o are observed in Fig. 3b, which can be indexed to the (002) and
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(-402) planes of monoclinic ZnC2O4·2H2O (JCPDS: 25-1029) in turn, demonstrating that a small quantity of ZnC2O4·2H2O were produced by immersing the FeC2O4·2H2O precursor in Zn2+ solution. The formation of ZnC2O4·2H2O can be explained by the substitution reaction between Zn2+ and Fe2+ occurred on the FeC2O4·2H2O due to the lower solubility of ZnC2O4 (2.5×10-5 g/cm3) than that of FeC2O4 (8×10-5 g/cm3). Fig. 3c shows the XRD pattern of the
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ZnFe2O4/α-Fe2O3 composite prepared from ZnC2O4·2H2O/FeC2O4·2H2O. In this figure, two definite crystalline phases are identified. The diffraction peaks located at 2θ of 29.9o, 42.8o and 56.6o are corresponding to the (220), (400), (511) planes of ZnFe2O4 (JCPDS: 22-1012),
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respectively. While, the other peaks, from low to high angle area, can be attributed to the (012), (104), (110), (006), (113), (024), (116), (018), (214), (300), (1,0,10) and (220) planes of α-Fe2O3
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(JCPDS: 33-0664), respectively. Such result undoubtedly proves that after annealing at 430 oC for 2 h, crystalline ZnFe2O4/α-Fe2O3 composite was successfully prepared from the hybrid oxalate precursor. In
order
to
get
insight
into
the
decomposition
process
of
the
prepared
ZnC2O4·2H2O/FeC2O4·2H2O, thermal analysis was performed, and the result is displayed in Fig. 4. In this figure, the steep endothermic peak located at 172 oC (DSC curve) should be attributed to the dehydration of ZnC2O4·2H2O and FeC2O4·2H2O, because the corresponding total weight loss (20 %) observed in the TGA curve is very close to the theoretical weight loss of crystal water for ZnC2O4·2H2O (19 %) and FeC2O4·2H2O (20 %) [30]. The other strong endothermic peak centred 6
ACCEPTED MANUSCRIPT at 400 oC with a weight loss about 39.2% (TGA curve) can be attributed to the phase transformation from ZnC2O4/FeC2O4 to ZnFe2O4/α-Fe2O3. The weak endothermic peak observed at about 120 oC should be caused by the releasing of absorbed H2O of the sample. Thus, according to the results given by thermal analysis, in the decomposition step for synthesizing
product and simultaneously prevent collapsing of porous structure.
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ZnFe2O4/α-Fe2O3, the annealing temperature was chosen at 430 oC with an aim to get crystalline
The morphology of the prepared samples was observed by using FESEM. The low-magnification FESEM image displayed in Fig. 5a indicates that the FeC2O4·2H2O sample is
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composed of a large number of ~3µmmicrorods with a diameter about 300-500 nm. The corresponding high-magnification image showed in the inset of Fig. 5a further reveals that these
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formed FeC2O4·2H2O microrods have smooth surfaces and sharp edges. From the typical FESEM of the ZnC2O4·2H2O/FeC2O4·2H2O showed in Fig. 5b, it can be seen that after the substitution reaction between Zn2+ and Fe2+, the obtained hybrid ZnC2O4·2H2O/FeC2O4·2H2O basically reserves the rod-like structure of the FeC2O4·2H2O precursor, although the surface of the microrods seemed to be rougher (inset in Fig. 5b). Considering the decomposition property of
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metal oxalate at high temperature [31], the obtained hybrid oxalate microrods were used as sacrificial templates to synthesize ZnFe2O4/α-Fe2O3. As shown in Fig. 5c and d, after the annealing process, both the α-Fe2O3 and ZnFe2O4/α-Fe2O3 products reserved the rod-like frame
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from their precursors. EDS measurements were performed on the microrods to investigate their chemical composition. In the EDS mappings showed in Fig. 5e, besides of the elements of Fe and
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O, Zn element is also observed, suggesting the existence of ZnFe2O4 on the observed microrods. Moreover, the good dispersion of Zn among Fe and O further demonstrates that ZnFe2O4 is homogeneously distributed on the whole α-Fe2O3 microrods. The structural characteristic of the prepared ZnFe2O4/α-Fe2O3 microrods was further excavated by TEM analysis. The TEM image showed in Fig. 6a clearly reveals the porous structure of the obtained ZnFe2O4/α-Fe2O3 microrods. From the partially enlarged TEM image showed in Fig. 6b, one can further observe that the ZnFe2O4/α-Fe2O3 PMRs are constructed by numerous loosely stacked nanoparticles with the size about 15 nm. The selected area electron diffraction (SAED) pattern showed in Fig. 6c indicates that these ZnFe2O4/α-Fe2O3 PMRs are of polycrystalline in nature. After careful identification, all 7
ACCEPTED MANUSCRIPT the diffraction rings from inside to outside can be indexed as the (012) and (006) planes for rhombohedral α-Fe2O3 (JCPDS: 33-0664) and (311), (331) and (440) planes for cubic ZnFe2O4 (JCPDS: 22-1012), respectively. Fig. 6d displays the typical high resolution TEM (HRTEM) image of the ZnFe2O4/α-Fe2O3 PMRs. The measured interplanar distances of 0.24 nm and 0.27 nm
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are close to the d values of (222) and (104) planes for ZnFe2O4 and α-Fe2O3, respectively. Moreover, the interlaced lattice fringes between adjacent ZnFe2O4 and α-Fe2O3 particles reveal the formation of ZnFe2O4-Fe2O3 heterojunction.
The specific surface area and porous structure of the prepared α-Fe2O3 and ZnFe2O4/α-Fe2O3
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PMRs were investigated by N2 adsorption-desorption. As shown in Fig. 7a, the samples exhibit a type of Ⅲ isotherm with type H3 hysteresis loops, demonstrating the existence of mesoporous
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structure in the product. The BET surface areas of α-Fe2O3 and ZnFe2O4/α-Fe2O3PMRs were calculated to be 30.3 m2/g and 27.9 m2/g, respectively. Fig. 7b shows the pore size distribution curve calculated from the Barret–Joyner–Halenda (BJH) method, from which it can be seen that the pore size distributions of the α-Fe2O3 and ZnFe2O4/α-Fe2O3 samples are around 5–11 nm and 5–15 nm, respectively.
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3.2 Gas sensing performance
Due to the prominent structure and composition-dependent gas-sensing properties of the MOS composite, the gas sensing properties of the prepared ZnFe2O4/α-Fe2O3 PMRs towards TEA were
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tested to explore their potential application. Considering that the operating temperature can affect the gas sensitivity of MOS sensor by exerting great influence on the surface absorption and
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reaction between the sensing film and gas molecules, the temperature-dependent responses of the ZnFe2O4/α-Fe2O3 PMRs sensor toward 100 ppm TEA were firstly measured, as well as the prepared pure α-Fe2O3 PMRs for comparison. As presented in Fig. 8, with the operating temperature increasing from 290 to 330 oC, the responses of the sensor based on ZnFe2O4/α-Fe2O3 PMRs undergoes an obvious "increase-maximum-decrease" process, and reaches its maximum value at 305 oC. Similar tendency is observed for the sensor based on the pure α-Fe2O3 PMRs (inset in Fig. 8), and its maximum response appears at 280 oC. Thus, the optimal operating temperatures for the sensors based on α-Fe2O3 PMRs and ZnFe2O4/α-Fe2O3 PMRs are determined as 280
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C and 305
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C, respectively. By contrast, the optimal operating temperature of 8
ACCEPTED MANUSCRIPT ZnFe2O4/α-Fe2O3 PMRs sensor is 25 oC higher than that of the α-Fe2O3PMRs sensor. However, the much higher response of ZnFe2O4/α-Fe2O3 PMRs implies its superior ability to detect TEA. For instance, the response of ZnFe2O4/α-Fe2O3 sensor at the optimal operating temperature of 305 o
C is as high as 42.4, which is about 20 times higher than that of the α-Fe2O3 sensor (2.5) at the
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optimal operating temperature of 280 oC. Fig. 9a shows the dynamic response and recovery curves of the two sensors to varied TEA concentrations at 305 oC. In the whole concentration range, the sensor based on α-Fe2O3 PMRs only gives a weak response, and even to the concentration as high as 1500 ppm, its response
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amplitude is almost undetectable. While, the sensor fabricated with ZnFe2O4/α-Fe2O3 PMRs can gives obvious response, and the response amplitude increased gradually with the increase of TEA
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concentration. The sharp contrast in response between two sensors clearly reveals that the TEA sensing-performance was remarkably improved by decorating the α-Fe2O3 PMRs with ZnFe2O4. The relationships between responses and TEA concentrations for the ZnFe2O4/α-Fe2O3 and α-Fe2O3 based sensors are showed in Fig. 9b. It can be found that the responses of both sensors increase gradually with TEA concentration increasing from 30 to 1500 ppm. At each tested
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concentration points, the ZnFe2O4/α-Fe2O3 sensor shows much higher response than the α-Fe2O3 sensor. The response-recover speed is another key parameter for evaluating a gas sensor. Thus, from the dynamic response-recover curve displayed in Fig. 9a, the response and recovery times of
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the ZnFe2O4/α-Fe2O3 PMRs sensor towards different TEA concentrations were measured, and the results are presented in Fig. 9c. It can be seen that with the TEA concentration increasing from 30
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to 1500 ppm, the response and recover times prolong correspondingly. Significantly, even to the TEA concentration as high as 1500 ppm, the sensor still exhibits a fast response (22 s) and recover (43 s). The fast response and recover characteristic of ZnFe2O4/α-Fe2O3 PMRs in such a wide TEA concentration range (30–1500 ppm) is believed to be great valuable for real-time detection of TEA in practical application. The selectivity of the sensor based on ZnFe2O4/α-Fe2O3 PMRs was also measured. Fig. 9d shows the responses of the sensor to several volatile organic compounds (VOCs) at 100 ppm at 305 oC. Clearly, the sensor's response to TEA is much higher than to other reference gases. For example, its response to TEA is as high as 42, which is about 2~4 times higher than that to acetone (25), ethanol (21), formaldehyde (11) and methanol (14), revealing its good selectivity 9
ACCEPTED MANUSCRIPT to TEA. The TEA sensing performance of our sensor is compared with that of the previous sensors reported in literatures. As shown in Table 1, an obvious improvement of response was found in the present ZnFe2O4/α-Fe2O3 sensor, demonstrating its superior ability to detect TEA. The repeatability and long-term stability are two important parameters for a successful sensor.
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Therefore, the repeatability and long-term stability of the sensor based on ZnFe2O4/α-Fe2O3 PMRs were also measured to further evaluate its quality. As shown in Fig. 10, the response amplitude of the sensor only changes slightly during the cycle test (Fig. 10a), and can still keep about 90 % of
stability to detect TEA. 3.3 Gas sensing mechanism of the ZnFe2O4/α-Fe2O3PMRs
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initial response value after 30 days (Fig. 10b), revealing its good repeatability and long-term
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In our experiment, the TEA-sensitivity of the fabricated sensor was remarkably improved through decorating α-Fe2O3 PMRs with ZnFe2O4 nanoparticles. As is well known that both Fe2O3 and ZnFe2O4 are n-type MOSs and the most widely accepted sensing mechanism for n-type MOS is the remarkable change in sensor resistance when it is exposed to different gas ambient [14]. Take α-Fe2O3 as an example, in air ambient, oxygen molecules will be absorbed on the surface of
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α-Fe2O3 and capture free electrons from its conduction band to form surface absorbed oxygen anions (O2−, O−, or O2−) according to the following reactions (Eq. 1-3). As a result, a relatively thick electron depletion layer (EDL) is formed on the surface domains of α-Fe2O3, which will lead
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to a high sensor resistance (Ra) due to the decrease of carrier concentration and the formation of a relatively high potential barrier (as schematically illustrated in Fig. 11a). In contrast, if a certain
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concentration of reducing gas like TEA is introduced at this moment, the formerly absorbed oxygen anions will react with the reducing gas (Eq. 4), after which the trapped electrons by absorbed oxygen anions will be released back to the conduction band of α-Fe2O3. This process will result in a reduction of sensor resistance (Rg) due to the shrink of the EDL width and the decrease of potential barrier height (as schematically illustrated in Fig. 11b). O + e → O T < 100℃
(1)
O + e → 2O 100℃ ≤ T ≤ 300℃ O + e → O T > 300℃
(2) (3)
Et2N–CH2–CH3+ O → Et2N–CH=CH2+ H O + 2e 10
(4)
ACCEPTED MANUSCRIPT When ZnFe2O4 nanoparticles are implanted into the α-Fe2O3 PMRs, three kinds of n-n junctions should be formed in the composite, including the heterojunction of ZnFe2O4-Fe2O3 and the homojunctions of ZnFe2O4-ZnFe2O4 and Fe2O3-Fe2O3. While, for pure α-Fe2O3 only Fe2O3-Fe2O3 homojunction is contained. The additionally formed ZnFe2O4-Fe2O3 heterojunction is believed to
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play an important role in the improved TEA sensitivity. In previous studies, it has been widely demonstrated that the combination of two different metal oxides can bring significant improvement in gas-sensitivity due to the formation of p-n or n-n heterojunction [5,37]. In the case of our experiment, when the composite sensor was exposed to air ambient, the barrier height
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of ZnFe2O4/α-Fe2O3 heterojunction increased significantly due to the emergence of EDL and the bend of conduction band (Fig. 11c). On this occasion, the migration of electron carriers in the
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sensing film will become more difficult because of the higher potential barriers, thus leading to an obvious increase in the sensor resistance (Ra). In contrast, when the composite sensor is exposed to TEA, the potential barrier height will decrease due to the reinjection of electrons into the conduction bands of ZnFe2O4 and α-Fe2O3 as a result of the surface redox reaction between oxygen anions and TEA molecules (Eq. 4). Therefore, the potential barrier height of
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ZnFe2O4/α-Fe2O3 heterojunction is decreased, and sensor resistance (Rg) is decreased correspondingly (Fig. 11d). Since the response is determined by the ratio of Ra/Rg, the great variation of sensor resistance in air (Ra) and TEA (Rg) eventually resulted in the better sensing
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performance of the ZnFe2O4/α-Fe2O3 PMRs based sensor than the bare α-Fe2O3 PMRs. In addition, due to the loose structure, the existence of a large amount of in situ 5–15 nm pores (as shown in
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Fig. 6b and Fig. 7b), and the good dispersion of ZnFe2O4 nanoparticles on the α-Fe2O3 PMRs (Fig. 5e), gas molecules can easily access to the surfaces of α-Fe2O3 and ZnFe2O4 when the composite sensor is exposed to air and TEA. In other words, during the gas-sensing process, both α-Fe2O3 and ZnFe2O4 can contribute to the response, leading to a higher TEA sensitivity of ZnFe2O4/α-Fe2O3 PMRs.
4. Conclusion In summary, a facile and reliable sacrificial template route was successfully developed to synthesize ZnFe2O4/α-Fe2O3 PMRs, in which sacrificial template of ZnC2O4·2H2O/FeC2O4·2H2O was synthesized via a chemical precipitation reaction with subsequent substitution reaction. The 11
ACCEPTED MANUSCRIPT synthesized ZnFe2O4/α-Fe2O3 PMRs were constructed by numerous loosely stacked nanoparticles with the size about 15 nm and the dominant pore size in them is around 5–15 nm. Gas-sensing tests indicated that the sensor based on ZnFe2O4/α-Fe2O3 PMRs showed an enhanced response towards TEA as compared with that based on bare α-Fe2O3 PMRs. Its response to 100 ppm TEA is
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as high as 42.4, which is about 20 times higher than that of α-Fe2O3 counterpart (2.5). Besides of high sensitivity, the composite sensor also exhibited some good performances, such as fast response and recovery, high selectivity, and good stability. It is believed that the improved gas-sensing performance of the ZnFe2O4/α-Fe2O3PMRs mainly originates from the formation of
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ZnFe2O4-Fe2O3heterojunction. The present research not only provides a reliable method for fabricating ZnFe2O4/α-Fe2O3 porous microrods, but also demonstrates that the introduction of
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heterojunction is a feasible way to improve the gas-sensing performance of metal oxides.
Acknowledgements
This work is supported by the National Natural Science Foundation of China (U1404613), Program for Science & Technology Innovation Talents in Universities of Henan Province (18HASTIT010, 17HASTIT029), Young Core Instructor Project of Colleges and Universities in
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Henan Province (2015GGJS-063, 2016GGJS-040), the Education Department Natural Science Foundation of fund Henan province (16A150051), Natural Science Foundation of Henan Province of China (162300410113), the Fundamental Research Funds for the Universities of Henan
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Province (NSFRF1606, NSFRF1614), and Foundation for Distinguished Young Scientists of Henan Polytechnic University (J2016-2, J2017-3).
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ACCEPTED MANUSCRIPT [23] S.S. Shinde, R.A. Bansode, C.H. Bhosale, K.Y. Rajpure, Physical properties of hematite α-Fe2O3 thin films: application to photoelectrochemical solar cells, Journal of Semiconductors, 32 (2011) 013001. [24] P. Sun, C. Wang, J.Y. Liu, X. Zhou, X.W. Li, X.L. Hu, G.Y. Lu, Hierarchical Assembly of α-Fe2O3 Nanosheets on SnO2 Hollow Nanospheres with Enhanced Ethanol Sensing Properties, ACS Appl Mater Inter, 7 (2015) 19119-19125. [25] R. Zhang, L.L. Wang, J.A. Deng, T.T. Zhou, Z. Lou, T. Zhang, Hierarchical structure with
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Z.Y. Zhang, Synthesis and improved gas sensing properties of NiO-decorated SnO2 microflowers assembled with porous nanorods, Sensor Actuat B-Chem, 233 (2016) 180-192. [29] W. Zhou, K. Tang, S. Zeng, Y. Qi, Room temperature synthesis of rod-like FeC2O4·2H2O and its transition to maghemite, magnetite and hematite nanorods through controlled thermal decomposition, Nanotechnology, 19 (2008) 060957-064484.
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flowerlike architectures assembled with SnO2 submicron rods, Sensors and Actuators B: Chemical, 173 (2012) 643-651.
[34] M. Wu, X. Zhang, S. Gao, X. Cheng, Z. Rong, Y. Xu, H. Zhao, L. Huo, Construction of monodisperse vanadium pentoxide hollow spheres via a facile route and triethylamine sensing property, Crystengcomm, 15 (2013) 10123-10131.
[35] T. Guo, Y. Luo, Y. Zhang, Y.-H. Lin, C.-W. Nan, ZnO-NiO hetero-nanostructures as highly sensitive and selective triethylamine sensor, Journal of Applied Physics, 116 (2014) 040021-048979. [36] X.F Chu, D.L. Jiang, Y. Guo, C.M Zheng, Ethanol gas sensor based on CoFe2O4 nano-crystallines prepared by hydrothermal method, Sensors and Actuators B: Chemical, 120 (2006) 177-181. [37] S. Park, S. Kim, H. Kheel, C. Lee, Oxidizing gas sensing properties of the n-ZnO/p-Co3O4 composite nanoparticle network sensor, Sensor Actuat B-Chem, 222 (2016) 1193-1200.
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ACCEPTED MANUSCRIPT Table and Figure captions Table 1 Compared TEA-sensing properties of different sensors. Fig. 1 Schematic illustration for the formation of α-Fe2O3 and ZnFe2O4/α-Fe2O3 PMRs. Fig. 2 Schematic diagram of the fabricated sensor.
·2H2O SMRs, and (c) ZnFe2O4/α-Fe2O3 PMRs.
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Fig. 3 XRD patterns of the as-prepared (a) FeC2O4·2H2O SMRs, (b) hybrid ZnC2O4·2H2O/FeC2O4
Fig. 4 TGA and DSC curves of the prepared ZnC2O4·2H2O/FeC2O4·2H2O.
Fig. 5 FESEM images of (a) FeC2O4·2H2O SMRs, (b)ZnC2O4·2H2O/FeC2O4·2H2O SMRs, (c)
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α-Fe2O3 PMRs, and (d) ZnFe2O4/α-Fe2O3 PMRs; (e) The EDS element mappings corresponding to (d).
ZnFe2O4/α-Fe2O3PMRs.
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Fig.6 (a, b) TEM images, (c) SAED pattern and (d) HRTEM image of the prepared
Fig. 7 (a) N2 adsorption-desorption isotherms and (b) pore size distribution curves of the prepared α-Fe2O3 and ZnFe2O4/α-Fe2O3 samples.
ppm TEA.
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Fig. 8 Temperature-dependent responses of the sensors based on different samples towards 100
Fig. 9 (a) Dynamic response-recover curves and (b) concentration-dependent responses of different sensors toward TEA at 305
o
C, (c) response and recover times of the
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ZnFe2O4/α-Fe2O3 sensor to different concentrations of TEA, and (d) response of the ZnFe2O4/α-Fe2O3 sensor to 100 ppm different gases.
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Fig. 10 (a) Repeatability and (b) stability measurements of the ZnFe2O4/α-Fe2O3 PMRs sensor to 100 ppm TEA at 305 oC.
Fig. 11 Schematic illustration for the varied EDL width and potential barrier height of (a, b) α-Fe2O3 and (c, d) ZnFe2O4/α-Fe2O3 when exposed to different gases.
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TEA concentration (ppm)
Response (Ra/Rg)
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Al-doped ZnO-NiO hetero-nanostructure
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100
21
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The synthesized hybrid PMRs exhibit an enhanced TEA-sensing performance.
The improved gas-sensing mechanism was discussed.
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S0925-8388(17)34251-2
DOI:
10.1016/j.jallcom.2017.12.068
Reference:
JALCOM 44154
To appear in:
Journal of Alloys and Compounds
Received Date: 22 September 2017 Revised Date:
6 December 2017
Accepted Date: 7 December 2017
Please cite this article as: Y. Li, N. Luo, G. Sun, B. Zhang, G. Ma, H. Jin, Y. Wang, J. Cao, Z. Zhang, Facile synthesis of ZnFe2O4/α-Fe2O3 porous microrods with enhanced TEA-sensing performance, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2017.12.068. 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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Facile synthesis of ZnFe2O4/α-Fe2O3 porous microrods with enhanced TEA-sensing performance Yanwei Li, Na Luo, Guang Sun∗, Bo Zhang, Guangzhou Ma, Honghong Jin, Yan Wang*, Jianliang Cao and Zhanying Zhang
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School of Materials Science and Engineering, Cultivating Base for Key Laboratory of Environment-friendly Inorganic Materials in University of Henan Province, Henan Polytechnic
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University, Jiaozuo 454000, PR China
∗
Corresponding author Tel.: +86 03913986952 E-mail address: [email protected](Guang Sun); [email protected] (Yan Wang) 1
ACCEPTED MANUSCRIPT Abstract Developing metal oxide nanocomposite with superior gas-sensing properties is an important subject in the field of gas sensor. In this paper, we reported a facile synthesis route for ZnFe2O4/α-Fe2O3 porous microrods (PMRs) by using dual oxalates as sacrificial template.
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FeC2O4·2H2O solid microrods (SMRs) were first synthesized by a chemical precipitation reaction between FeSO4·7H2O and H2C2O4·2H2O, which were then immersed in Zn2+ aqueous solution to obtain the hybrid ZnC2O4·2H2O/FeC2O4·2H2O SMRs through the substitution reaction between Zn2+ and Fe2+. After a proper annealing process, ZnFe2O4/α-Fe2O3 PMRs were successfully
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prepared through the decomposition of ZnC2O4·2H2O/FeC2O4·2H2O SMRs. The prepared ZnFe2O4/α-Fe2O3 PMRs are about 2–5 µm in length and 0.3–0.5 µm in diameter, and constructed
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by numerous loosely stacked nanoparticles with the size about 15 nm. The dominant pore size in ZnFe2O4/α-Fe2O3 PMRs is around 10 nm. The results of gas-sensing tests indicates that through decorating the α-Fe2O3PMRs with ZnFe2O4, the response of the sensor towards TEA is remarkably improved. For example, the response of the composite sensor to 100 ppm TEA is as high as 42.4, which is about 20 times higher than that of the bare α-Fe2O3 PMRs sensor (2.5). The
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enhanced TEA sensing mechanism of ZnFe2O4/α-Fe2O3 PMRs was discussed in detail. Keywords: ZnFe2O4/α-Fe2O3; Porous structure; Sacrificial template; Heterojunction; Gas sensor
1. Introduction
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As one of the most important organic amines, triethylamine (TEA) has been widely used in chemical and food industries as raw material, catalyst, solvent and preservative [1]. However,
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because TEA is volatile and toxic, the volatilization and spillage of TEA can bring a series of threatens to human health and even endanger our environment [2,3]. Thus, fast and timely detecting the existence and concentration of TEA in surroundings is of great importance for environment protection. Metal oxide semiconductor (MOS) based gas sensor has stood out from various gas detection techniques due to its merits of low cost, fast response and easy fabrication, and been regarded as a significant way to monitor flammable, toxic, and corrosive gases [4,5]. Up to now, several kinds of MOSs have been developed as gas-sensing materials to detect TEA, such as SnO2, α-Fe2O3, V2O5, MoO3, NiFe2O4, NiO/ZnO and so on [6-11]. However, due to the limited gas-sensing properties, some of them are far to satisfy the needs of practical application. The 2
ACCEPTED MANUSCRIPT development of high performance TEA-sensing materials is still a challenging work. It has been widely acknowledged that the gas sensitivity of MOS originates from its resistance change when it is exposed to different atmospheres, which is mainly controlled by the surface reactions participated by adsorbed oxygen and tested gases [12]. Therefore, the gas-sensing
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properties of MOSs are closely related with their chemical composition and microstructure. In order to improve the gas-sensing properties of MOSs, fabricating their novel micro/nanostructures with large surface area to promote the surface reactions (including surface gas adsorption, redox reaction, and desorption) has been proved an effective way [13]. Among various structures, porous
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structure, especially mesoporous structure with large surface area, was considered as one of ideal structures that can achieve high gas-sensing performance, because the existence of a particularly
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large number of nano-sized pores can facilitate the gas diffusion and transportation, and the large surface area can provide more available active sites for gas adsorption and reaction [14]. In addition, combining two or more kinds of MOSs together to fabricate their nanocomposite is also a feasible way to improve the gas-sensing properties [15,16]. With the assistance of synergistic effect between different components, the nanocomposite can always exhibit upgrading gas-sensing
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performance as compared with their single component counterpart [17,18]. Thus, developing metal oxide nanocomposite with superior gas-sensing properties via rational design and controlled synthesis of their chemical composition and micro/nanostructure has become one of the most
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active fields.
Hematite (α-Fe2O3), an n-type semiconductor with a band gap of 2.1 eV, has been widely
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investigated in a broad range of applications, such as gas sensors [19], photocatalysts [20], magnetic materials [21], lithium-ion batteries [22], and solar cells [23], because of the features of non-toxicity, low cost, high stability, and environmental compatibility. In order to improve the gas-sensing properties of α-Fe2O3, different α-Fe2O3 based nanocomposites have been fabricated and investigated. Sun et al. reported the synthesis of the hierarchical assembly of α-Fe2O3 nanosheets on SnO2 hollow nanospheres via a microwave hydrothermal method and found that such a hierarchical composite exhibited superior ethanol-sensing performance to the single component of SnO2 hollow spheres [24]. Zhang et al. reported that the hierarchical α-Fe2O3/ZnO nanobranch frame synthesized via a facile two-step continuous hydrothermal process, has a good 3
ACCEPTED MANUSCRIPT sensitivity and better selectivity to trimethylamine [25]. Li et al. reported the synthesis of V2O5-decorated α-Fe2O3nanorods by electrospinning and soak-calcination method. The composite showed high selectivity and stability to diethylamine [26]. Zhang and co-workers reported the good gas sensing properties of ZnO nanoparticle-decorated round-edged α-Fe2O3 hexahedrons
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fabricated through solvothermal method [27]. All these researches firmly demonstrate that the combination of α-Fe2O3 with other MOS is a promising way to achieve high sensing performance. However, to the best of our knowledge, there are few reports on the synthesis and TEA sensing properties of hybrid ZnFe2O4/α-Fe2O3 porous microrods.
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In this work, ZnFe2O4/α-Fe2O3 porous microrods (PMRs) were successfully synthesized via a facile and reliable sacrificial template method, in which a continuous process including
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precipitation, substitution, and decomposition reactions was involved and dual oxalates were applied as sacrificial template. Significantly, the prepared ZnFe2O4/α-Fe2O3 PMRs exhibited superior TEA-sensing performance to the single component α-Fe2O3 PMRs in terms of much higher response. The variation of the potential barrier height of ZnFe2O4-Fe2O3 heterojunction in different gas ambient was suggested to be the main reason for the improved TEA-sensing
2. Experimental
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2.1 Synthesis of ZnFe2O4/α-Fe2O3 PMRs
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All the chemical reagents involved in the experiments were of analytical purity and used as received without further purification. Distilled water was used throughout the experiments. In
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order to synthesize ZnFe2O4/α-Fe2O3 PMRs, a continuous process including precipitation, substitution and decomposition reactions was employed, as shown in Fig. 1. Firstly, FeC2O4·2H2O solid microrods (SMRs) were synthesized through a chemical precipitation reaction between H2C2O4·2H2O and FeSO4·7H2O. In brief, a homogeneous solution of Fe2+ aqueous solution was prepared by dissolving 0.69 g of FeSO4·7H2O into 100 mL of distilled water with vigorous stirring. Then, 20 mL of ethanol solution containing 0.63 g of oxalic acid (H2C2O4·2H2O) was dropped into above solution with magnetic stirring. After stirring at room temperature for 60 min, FeC2O4·2H2O SMRs were prepared and collected by centrifugation. Secondly, by using above FeC2O4·2H2O SMRs as precursor, hybrid ZnC2O4·2H2O/FeC2O4·2H2O SMRs were prepared via a 4
ACCEPTED MANUSCRIPT mild substitution reaction. Typically, an appropriate amount of as-prepared FeC2O4·2H2O precursor were sufficiently dispersed in 20 mL of aqueous solution containing a designed amount of Zn(CH3COO)2·2H2O, and the molar ratio of Fe: Zn is set at 1:8. After stirring at room temperature for 48 h, the precipitate was harvested by centrifugation, washed with distilled water and ethanol for several times, and dried in air at 70 oC for 5 h. Finally, the prepared hybrid oxalate
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SMRs, being applied as sacrificial templates, were annealed in air at 430 oC for 2 h to obtain the ZnFe2O4/α-Fe2O3 PMRs. For comparison, α-Fe2O3 PMRs were also prepared by directly annealing the FeC2O4·2H2O SMRs at the same condition.
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2.2 Characterization
X-ray diffraction (XRD) measurements were carried on Bruker/D8-Advance diffractometer
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with Cu Kα radiation. The scanning was performed from 10 to 80o with a speed of 0.02o min-1. The morphology and microstructure were investigated by field emission scanning electron microscopy (FESEM, FEI QUANTA FEG250) and transmission electron microscopy (TEM, JEOL, JEM-2100). Elemental analysis was performed by energy dispersive spectroscopy (EDS, INCA ENERGY 250) integrated into the FESEM system. Thermal analysis was carried out on a
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Setaram Evolution 2400 thermal analyzer. The specific surface area of the prepared samples was measured on a Micromeritics Triatar 3020 apparatuses by using Brunauer-Emmett-Teller (BET) method.
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2.3 Sensor fabrication and measurement
The fabrication process for sensor is similar to our previous method [28]. Simply, a proper
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amount of as-prepared samples was dispersed into ethanol to get uniform emulsion, which was then coated onto a ceramic substrate (13.4 mm × 7 mm) screen-printed with Ag-Pd comb-like electrodes to obtain the resistance-type sensor. The schematic diagram of the fabricated sensor is showed in Fig. 2. The gas-sensing tests were carried on an intelligent gas sensing analysis system of CGS-4TPS (Beijing Elite Tech. Co., Ltd., China). Before testing, the sensor was allowed to be aged at 200 oC for 12 h to improve the repeatability and stability. During the test, target gases such as TEA with calculated concentration were injected into the testing chamber (1.8 L) by a microsyringe. The response (S) of the sensor was defined as the ratio of Ra/Rg, where Ra and Rg were the electrical resistance of sensor in air and in target gas, respectively. The response and 5
ACCEPTED MANUSCRIPT recovery times were defined as the time required for a change in response to reach 90% of the equilibrium value after injecting and removing the target gas, respectively. During the test, the operating temperature range was set at 270–330 oC, and the relative humidity was 40%.
3. Results and discussion
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3.1 Characterization The phase and purity of the products were characterized with XRD, and the results are showed in Fig. 3. In Fig. 3a, all of the diffraction peaks can be attributed to standard FeC2O4·2H2O (JCPDS: 72-1305) [29], indicating the formation of pure FeC2O4·2H2O phase in the chemical
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precipitation step. In comparison with Fig. 3a, two additional peaks with low intensity that appeared at 2θ of 22.6o and 30.2o are observed in Fig. 3b, which can be indexed to the (002) and
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(-402) planes of monoclinic ZnC2O4·2H2O (JCPDS: 25-1029) in turn, demonstrating that a small quantity of ZnC2O4·2H2O were produced by immersing the FeC2O4·2H2O precursor in Zn2+ solution. The formation of ZnC2O4·2H2O can be explained by the substitution reaction between Zn2+ and Fe2+ occurred on the FeC2O4·2H2O due to the lower solubility of ZnC2O4 (2.5×10-5 g/cm3) than that of FeC2O4 (8×10-5 g/cm3). Fig. 3c shows the XRD pattern of the
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ZnFe2O4/α-Fe2O3 composite prepared from ZnC2O4·2H2O/FeC2O4·2H2O. In this figure, two definite crystalline phases are identified. The diffraction peaks located at 2θ of 29.9o, 42.8o and 56.6o are corresponding to the (220), (400), (511) planes of ZnFe2O4 (JCPDS: 22-1012),
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respectively. While, the other peaks, from low to high angle area, can be attributed to the (012), (104), (110), (006), (113), (024), (116), (018), (214), (300), (1,0,10) and (220) planes of α-Fe2O3
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(JCPDS: 33-0664), respectively. Such result undoubtedly proves that after annealing at 430 oC for 2 h, crystalline ZnFe2O4/α-Fe2O3 composite was successfully prepared from the hybrid oxalate precursor. In
order
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decomposition
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of
the
prepared
ZnC2O4·2H2O/FeC2O4·2H2O, thermal analysis was performed, and the result is displayed in Fig. 4. In this figure, the steep endothermic peak located at 172 oC (DSC curve) should be attributed to the dehydration of ZnC2O4·2H2O and FeC2O4·2H2O, because the corresponding total weight loss (20 %) observed in the TGA curve is very close to the theoretical weight loss of crystal water for ZnC2O4·2H2O (19 %) and FeC2O4·2H2O (20 %) [30]. The other strong endothermic peak centred 6
ACCEPTED MANUSCRIPT at 400 oC with a weight loss about 39.2% (TGA curve) can be attributed to the phase transformation from ZnC2O4/FeC2O4 to ZnFe2O4/α-Fe2O3. The weak endothermic peak observed at about 120 oC should be caused by the releasing of absorbed H2O of the sample. Thus, according to the results given by thermal analysis, in the decomposition step for synthesizing
product and simultaneously prevent collapsing of porous structure.
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ZnFe2O4/α-Fe2O3, the annealing temperature was chosen at 430 oC with an aim to get crystalline
The morphology of the prepared samples was observed by using FESEM. The low-magnification FESEM image displayed in Fig. 5a indicates that the FeC2O4·2H2O sample is
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composed of a large number of ~3µmmicrorods with a diameter about 300-500 nm. The corresponding high-magnification image showed in the inset of Fig. 5a further reveals that these
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formed FeC2O4·2H2O microrods have smooth surfaces and sharp edges. From the typical FESEM of the ZnC2O4·2H2O/FeC2O4·2H2O showed in Fig. 5b, it can be seen that after the substitution reaction between Zn2+ and Fe2+, the obtained hybrid ZnC2O4·2H2O/FeC2O4·2H2O basically reserves the rod-like structure of the FeC2O4·2H2O precursor, although the surface of the microrods seemed to be rougher (inset in Fig. 5b). Considering the decomposition property of
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metal oxalate at high temperature [31], the obtained hybrid oxalate microrods were used as sacrificial templates to synthesize ZnFe2O4/α-Fe2O3. As shown in Fig. 5c and d, after the annealing process, both the α-Fe2O3 and ZnFe2O4/α-Fe2O3 products reserved the rod-like frame
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from their precursors. EDS measurements were performed on the microrods to investigate their chemical composition. In the EDS mappings showed in Fig. 5e, besides of the elements of Fe and
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O, Zn element is also observed, suggesting the existence of ZnFe2O4 on the observed microrods. Moreover, the good dispersion of Zn among Fe and O further demonstrates that ZnFe2O4 is homogeneously distributed on the whole α-Fe2O3 microrods. The structural characteristic of the prepared ZnFe2O4/α-Fe2O3 microrods was further excavated by TEM analysis. The TEM image showed in Fig. 6a clearly reveals the porous structure of the obtained ZnFe2O4/α-Fe2O3 microrods. From the partially enlarged TEM image showed in Fig. 6b, one can further observe that the ZnFe2O4/α-Fe2O3 PMRs are constructed by numerous loosely stacked nanoparticles with the size about 15 nm. The selected area electron diffraction (SAED) pattern showed in Fig. 6c indicates that these ZnFe2O4/α-Fe2O3 PMRs are of polycrystalline in nature. After careful identification, all 7
ACCEPTED MANUSCRIPT the diffraction rings from inside to outside can be indexed as the (012) and (006) planes for rhombohedral α-Fe2O3 (JCPDS: 33-0664) and (311), (331) and (440) planes for cubic ZnFe2O4 (JCPDS: 22-1012), respectively. Fig. 6d displays the typical high resolution TEM (HRTEM) image of the ZnFe2O4/α-Fe2O3 PMRs. The measured interplanar distances of 0.24 nm and 0.27 nm
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are close to the d values of (222) and (104) planes for ZnFe2O4 and α-Fe2O3, respectively. Moreover, the interlaced lattice fringes between adjacent ZnFe2O4 and α-Fe2O3 particles reveal the formation of ZnFe2O4-Fe2O3 heterojunction.
The specific surface area and porous structure of the prepared α-Fe2O3 and ZnFe2O4/α-Fe2O3
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PMRs were investigated by N2 adsorption-desorption. As shown in Fig. 7a, the samples exhibit a type of Ⅲ isotherm with type H3 hysteresis loops, demonstrating the existence of mesoporous
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structure in the product. The BET surface areas of α-Fe2O3 and ZnFe2O4/α-Fe2O3PMRs were calculated to be 30.3 m2/g and 27.9 m2/g, respectively. Fig. 7b shows the pore size distribution curve calculated from the Barret–Joyner–Halenda (BJH) method, from which it can be seen that the pore size distributions of the α-Fe2O3 and ZnFe2O4/α-Fe2O3 samples are around 5–11 nm and 5–15 nm, respectively.
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3.2 Gas sensing performance
Due to the prominent structure and composition-dependent gas-sensing properties of the MOS composite, the gas sensing properties of the prepared ZnFe2O4/α-Fe2O3 PMRs towards TEA were
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tested to explore their potential application. Considering that the operating temperature can affect the gas sensitivity of MOS sensor by exerting great influence on the surface absorption and
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reaction between the sensing film and gas molecules, the temperature-dependent responses of the ZnFe2O4/α-Fe2O3 PMRs sensor toward 100 ppm TEA were firstly measured, as well as the prepared pure α-Fe2O3 PMRs for comparison. As presented in Fig. 8, with the operating temperature increasing from 290 to 330 oC, the responses of the sensor based on ZnFe2O4/α-Fe2O3 PMRs undergoes an obvious "increase-maximum-decrease" process, and reaches its maximum value at 305 oC. Similar tendency is observed for the sensor based on the pure α-Fe2O3 PMRs (inset in Fig. 8), and its maximum response appears at 280 oC. Thus, the optimal operating temperatures for the sensors based on α-Fe2O3 PMRs and ZnFe2O4/α-Fe2O3 PMRs are determined as 280
o
C and 305
o
C, respectively. By contrast, the optimal operating temperature of 8
ACCEPTED MANUSCRIPT ZnFe2O4/α-Fe2O3 PMRs sensor is 25 oC higher than that of the α-Fe2O3PMRs sensor. However, the much higher response of ZnFe2O4/α-Fe2O3 PMRs implies its superior ability to detect TEA. For instance, the response of ZnFe2O4/α-Fe2O3 sensor at the optimal operating temperature of 305 o
C is as high as 42.4, which is about 20 times higher than that of the α-Fe2O3 sensor (2.5) at the
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optimal operating temperature of 280 oC. Fig. 9a shows the dynamic response and recovery curves of the two sensors to varied TEA concentrations at 305 oC. In the whole concentration range, the sensor based on α-Fe2O3 PMRs only gives a weak response, and even to the concentration as high as 1500 ppm, its response
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amplitude is almost undetectable. While, the sensor fabricated with ZnFe2O4/α-Fe2O3 PMRs can gives obvious response, and the response amplitude increased gradually with the increase of TEA
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concentration. The sharp contrast in response between two sensors clearly reveals that the TEA sensing-performance was remarkably improved by decorating the α-Fe2O3 PMRs with ZnFe2O4. The relationships between responses and TEA concentrations for the ZnFe2O4/α-Fe2O3 and α-Fe2O3 based sensors are showed in Fig. 9b. It can be found that the responses of both sensors increase gradually with TEA concentration increasing from 30 to 1500 ppm. At each tested
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concentration points, the ZnFe2O4/α-Fe2O3 sensor shows much higher response than the α-Fe2O3 sensor. The response-recover speed is another key parameter for evaluating a gas sensor. Thus, from the dynamic response-recover curve displayed in Fig. 9a, the response and recovery times of
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the ZnFe2O4/α-Fe2O3 PMRs sensor towards different TEA concentrations were measured, and the results are presented in Fig. 9c. It can be seen that with the TEA concentration increasing from 30
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to 1500 ppm, the response and recover times prolong correspondingly. Significantly, even to the TEA concentration as high as 1500 ppm, the sensor still exhibits a fast response (22 s) and recover (43 s). The fast response and recover characteristic of ZnFe2O4/α-Fe2O3 PMRs in such a wide TEA concentration range (30–1500 ppm) is believed to be great valuable for real-time detection of TEA in practical application. The selectivity of the sensor based on ZnFe2O4/α-Fe2O3 PMRs was also measured. Fig. 9d shows the responses of the sensor to several volatile organic compounds (VOCs) at 100 ppm at 305 oC. Clearly, the sensor's response to TEA is much higher than to other reference gases. For example, its response to TEA is as high as 42, which is about 2~4 times higher than that to acetone (25), ethanol (21), formaldehyde (11) and methanol (14), revealing its good selectivity 9
ACCEPTED MANUSCRIPT to TEA. The TEA sensing performance of our sensor is compared with that of the previous sensors reported in literatures. As shown in Table 1, an obvious improvement of response was found in the present ZnFe2O4/α-Fe2O3 sensor, demonstrating its superior ability to detect TEA. The repeatability and long-term stability are two important parameters for a successful sensor.
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Therefore, the repeatability and long-term stability of the sensor based on ZnFe2O4/α-Fe2O3 PMRs were also measured to further evaluate its quality. As shown in Fig. 10, the response amplitude of the sensor only changes slightly during the cycle test (Fig. 10a), and can still keep about 90 % of
stability to detect TEA. 3.3 Gas sensing mechanism of the ZnFe2O4/α-Fe2O3PMRs
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initial response value after 30 days (Fig. 10b), revealing its good repeatability and long-term
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In our experiment, the TEA-sensitivity of the fabricated sensor was remarkably improved through decorating α-Fe2O3 PMRs with ZnFe2O4 nanoparticles. As is well known that both Fe2O3 and ZnFe2O4 are n-type MOSs and the most widely accepted sensing mechanism for n-type MOS is the remarkable change in sensor resistance when it is exposed to different gas ambient [14]. Take α-Fe2O3 as an example, in air ambient, oxygen molecules will be absorbed on the surface of
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α-Fe2O3 and capture free electrons from its conduction band to form surface absorbed oxygen anions (O2−, O−, or O2−) according to the following reactions (Eq. 1-3). As a result, a relatively thick electron depletion layer (EDL) is formed on the surface domains of α-Fe2O3, which will lead
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to a high sensor resistance (Ra) due to the decrease of carrier concentration and the formation of a relatively high potential barrier (as schematically illustrated in Fig. 11a). In contrast, if a certain
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concentration of reducing gas like TEA is introduced at this moment, the formerly absorbed oxygen anions will react with the reducing gas (Eq. 4), after which the trapped electrons by absorbed oxygen anions will be released back to the conduction band of α-Fe2O3. This process will result in a reduction of sensor resistance (Rg) due to the shrink of the EDL width and the decrease of potential barrier height (as schematically illustrated in Fig. 11b). O + e → O T < 100℃
(1)
O + e → 2O 100℃ ≤ T ≤ 300℃ O + e → O T > 300℃
(2) (3)
Et2N–CH2–CH3+ O → Et2N–CH=CH2+ H O + 2e 10
(4)
ACCEPTED MANUSCRIPT When ZnFe2O4 nanoparticles are implanted into the α-Fe2O3 PMRs, three kinds of n-n junctions should be formed in the composite, including the heterojunction of ZnFe2O4-Fe2O3 and the homojunctions of ZnFe2O4-ZnFe2O4 and Fe2O3-Fe2O3. While, for pure α-Fe2O3 only Fe2O3-Fe2O3 homojunction is contained. The additionally formed ZnFe2O4-Fe2O3 heterojunction is believed to
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play an important role in the improved TEA sensitivity. In previous studies, it has been widely demonstrated that the combination of two different metal oxides can bring significant improvement in gas-sensitivity due to the formation of p-n or n-n heterojunction [5,37]. In the case of our experiment, when the composite sensor was exposed to air ambient, the barrier height
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of ZnFe2O4/α-Fe2O3 heterojunction increased significantly due to the emergence of EDL and the bend of conduction band (Fig. 11c). On this occasion, the migration of electron carriers in the
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sensing film will become more difficult because of the higher potential barriers, thus leading to an obvious increase in the sensor resistance (Ra). In contrast, when the composite sensor is exposed to TEA, the potential barrier height will decrease due to the reinjection of electrons into the conduction bands of ZnFe2O4 and α-Fe2O3 as a result of the surface redox reaction between oxygen anions and TEA molecules (Eq. 4). Therefore, the potential barrier height of
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ZnFe2O4/α-Fe2O3 heterojunction is decreased, and sensor resistance (Rg) is decreased correspondingly (Fig. 11d). Since the response is determined by the ratio of Ra/Rg, the great variation of sensor resistance in air (Ra) and TEA (Rg) eventually resulted in the better sensing
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performance of the ZnFe2O4/α-Fe2O3 PMRs based sensor than the bare α-Fe2O3 PMRs. In addition, due to the loose structure, the existence of a large amount of in situ 5–15 nm pores (as shown in
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Fig. 6b and Fig. 7b), and the good dispersion of ZnFe2O4 nanoparticles on the α-Fe2O3 PMRs (Fig. 5e), gas molecules can easily access to the surfaces of α-Fe2O3 and ZnFe2O4 when the composite sensor is exposed to air and TEA. In other words, during the gas-sensing process, both α-Fe2O3 and ZnFe2O4 can contribute to the response, leading to a higher TEA sensitivity of ZnFe2O4/α-Fe2O3 PMRs.
4. Conclusion In summary, a facile and reliable sacrificial template route was successfully developed to synthesize ZnFe2O4/α-Fe2O3 PMRs, in which sacrificial template of ZnC2O4·2H2O/FeC2O4·2H2O was synthesized via a chemical precipitation reaction with subsequent substitution reaction. The 11
ACCEPTED MANUSCRIPT synthesized ZnFe2O4/α-Fe2O3 PMRs were constructed by numerous loosely stacked nanoparticles with the size about 15 nm and the dominant pore size in them is around 5–15 nm. Gas-sensing tests indicated that the sensor based on ZnFe2O4/α-Fe2O3 PMRs showed an enhanced response towards TEA as compared with that based on bare α-Fe2O3 PMRs. Its response to 100 ppm TEA is
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as high as 42.4, which is about 20 times higher than that of α-Fe2O3 counterpart (2.5). Besides of high sensitivity, the composite sensor also exhibited some good performances, such as fast response and recovery, high selectivity, and good stability. It is believed that the improved gas-sensing performance of the ZnFe2O4/α-Fe2O3PMRs mainly originates from the formation of
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ZnFe2O4-Fe2O3heterojunction. The present research not only provides a reliable method for fabricating ZnFe2O4/α-Fe2O3 porous microrods, but also demonstrates that the introduction of
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heterojunction is a feasible way to improve the gas-sensing performance of metal oxides.
Acknowledgements
This work is supported by the National Natural Science Foundation of China (U1404613), Program for Science & Technology Innovation Talents in Universities of Henan Province (18HASTIT010, 17HASTIT029), Young Core Instructor Project of Colleges and Universities in
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Henan Province (2015GGJS-063, 2016GGJS-040), the Education Department Natural Science Foundation of fund Henan province (16A150051), Natural Science Foundation of Henan Province of China (162300410113), the Fundamental Research Funds for the Universities of Henan
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Province (NSFRF1606, NSFRF1614), and Foundation for Distinguished Young Scientists of Henan Polytechnic University (J2016-2, J2017-3).
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ACCEPTED MANUSCRIPT Table and Figure captions Table 1 Compared TEA-sensing properties of different sensors. Fig. 1 Schematic illustration for the formation of α-Fe2O3 and ZnFe2O4/α-Fe2O3 PMRs. Fig. 2 Schematic diagram of the fabricated sensor.
·2H2O SMRs, and (c) ZnFe2O4/α-Fe2O3 PMRs.
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Fig. 3 XRD patterns of the as-prepared (a) FeC2O4·2H2O SMRs, (b) hybrid ZnC2O4·2H2O/FeC2O4
Fig. 4 TGA and DSC curves of the prepared ZnC2O4·2H2O/FeC2O4·2H2O.
Fig. 5 FESEM images of (a) FeC2O4·2H2O SMRs, (b)ZnC2O4·2H2O/FeC2O4·2H2O SMRs, (c)
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α-Fe2O3 PMRs, and (d) ZnFe2O4/α-Fe2O3 PMRs; (e) The EDS element mappings corresponding to (d).
ZnFe2O4/α-Fe2O3PMRs.
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Fig.6 (a, b) TEM images, (c) SAED pattern and (d) HRTEM image of the prepared
Fig. 7 (a) N2 adsorption-desorption isotherms and (b) pore size distribution curves of the prepared α-Fe2O3 and ZnFe2O4/α-Fe2O3 samples.
ppm TEA.
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Fig. 8 Temperature-dependent responses of the sensors based on different samples towards 100
Fig. 9 (a) Dynamic response-recover curves and (b) concentration-dependent responses of different sensors toward TEA at 305
o
C, (c) response and recover times of the
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ZnFe2O4/α-Fe2O3 sensor to different concentrations of TEA, and (d) response of the ZnFe2O4/α-Fe2O3 sensor to 100 ppm different gases.
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Fig. 10 (a) Repeatability and (b) stability measurements of the ZnFe2O4/α-Fe2O3 PMRs sensor to 100 ppm TEA at 305 oC.
Fig. 11 Schematic illustration for the varied EDL width and potential barrier height of (a, b) α-Fe2O3 and (c, d) ZnFe2O4/α-Fe2O3 when exposed to different gases.
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TEA concentration (ppm)
Response (Ra/Rg)
Reference
100
42.4
This work
100
>19
[32]
45
2.97
[33]
500
9.7
[34]
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Sensor material
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Table 1
305
Sea urchin-like α-Fe2O3
350
Flower-like SnO2 architectures
350
V2O5 hollow spheres
370
Al-doped ZnO-NiO hetero-nanostructure
250
100
21
[35]
190
100
8
[36]
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Fig. 9
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Fig. 10
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Fig. 11
ACCEPTED MANUSCRIPT ZnFe2O4/α-Fe2O3 PMRs were synthesized via a sacrificial template method.
The synthesized hybrid PMRs exhibit an enhanced TEA-sensing performance.
The improved gas-sensing mechanism was discussed.
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