Enhanced CH4 sensitivity of porous nanosheets-assembled ZnO microflower by decoration with Zn2SnO4

Journal Pre-proof Enhanced CH4 sensitivity of porous nanosheets-assembled ZnO microflower by decoration with Zn2 SnO4 Xiaojie Li, Yanwei Li, Guang Sun, Bo Zhang, Yan Wang, Zhanying Zhang

PII:

S0925-4005(19)31573-4

DOI:

https://doi.org/10.1016/j.snb.2019.127374

Reference:

SNB 127374

To appear in:

Sensors and Actuators: B. Chemical

Received Date:

24 September 2019

Revised Date:

28 October 2019

Accepted Date:

1 November 2019

Please cite this article as: Li X, Li Y, Sun G, Zhang B, Wang Y, Zhang Z, Enhanced CH4 sensitivity of porous nanosheets-assembled ZnO microflower by decoration with Zn2 SnO4 , Sensors and Actuators: B. Chemical (2019), doi: https://doi.org/10.1016/j.snb.2019.127374

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Enhanced CH4 sensitivity of porous nanosheets-assembled ZnO microflower by decoration with Zn2SnO4

Xiaojie Lia, Yanwei Lia,b, Guang Sunb,c*, Bo Zhanga, Yan Wangb,c*, Zhanying Zhanga,c

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School of Materials Science and Engineering, Cultivating Base for Key Laboratory of

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Environment-friendly Inorganic Materials in University of Henan Province, Henan Polytechnic University, Jiaozuo 454000, China. b

The Collaboration Innovation Center of Coal Safety Production of Henan Province, Henan

Polytechnic University, Jiaozuo 454000, China.

School of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo

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c

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454000, China.

Corresponding

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author Tel.: +86 03913986952 E-mail address: [email protected] (Guang Sun); [email protected] (Yan Wang)

Highlights

Heterostructured Zn2SnO4/ZnO microflowers were successfully prepared.

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The Zn2SnO4/ZnO microflowers feature an assembly of porous nanosheets.

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The Zn2SnO4/ZnO microflowers showed an improved CH4 sensing properties.

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Abstract

In recent years, design and synthesis of heterostructured nanomaterials with high gas-sensing

performance for detection of flammable and toxic gases has attracted much research interest. In this paper, pristine and Zn2SnO4-decorated ZnO microflowers with the size about 2˗5 μm and assembled by porous nanosheets of about 15 nm in thickness were successfully synthesized via a simple solvothermal method and subsequent calcination process. The methane (CH4) sensing

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properties of the prepared Zn2SnO4/ZnO were investigated and compared with that of the pure ZnO counterpart. It was found that after decorating with a small amount of Zn2SnO4, the ZnO sensor showed an improved gas sensing properties to CH4. At optimum operating temperature of 250 oC, the sensor based on SZ2 (Zn2SnO4/ZnO composite with the optimal Zn2SnO4 contend) shows a response as high as 27.2 to 1000 ppm CH4, which is about 3.2 times higher than that of SZ0 (pristine ZnO) sensor. Meanwhile, the SZ2 sensor also shows a fast response and recover characteristic, low detection limit (1.48 ppm), good repeatability and long-term stability. The Zn2SnO4/ZnO

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heterostructures related CH4 sensing mechanism was discussed.

Key words: Zn2SnO4; Hierarchical structures; CH4; Heterostructures; Gas sensor

1. Introduction

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Methane (CH4), as a main component of nature gas, has been widely used as an energy source

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for industrial and domestic applications [1-2]. However, due to its flammable and explosive nature, CH4 can bring great threatens to human health and environment by causing fire and explosion

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accidents once its volume concentration in air reaches 4.9-15 % [3]. Furthermore, CH4 is also a well-known greenhouse gas whose effect on global warming is 25 times higher than CO2 [4]. Therefore, realizing fast and efficient detection of CH4 in our surroundings through simple and

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convenient methods is very important from the view point of environment and health protection. As one of promising gas detection techniques, metal oxide semiconductor (MOS) based gas

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sensor has attracted an increasing attention in recent decades due to the merits of low cost, easy fabrication and application, high sensitivity and fast response speed [1-4]. Many MOSs such as ZnO

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[5], SnO2 [6], In2O3 [7], Fe2O3 [8], Zn2SnO4 [9]and ZnCo2O4 [10-11] have been investigated for gas sensor applications. Among them, ZnO, a typical n-type MOS with a band gap of 3.3 eV, has been regarded as one of most promising gas sensing materials. In previous studies, the CH4 sensing properties of ZnO have been also investigated [12-14]. However, the relatively low sensitivity and long response-recovery time restrain their practical application [15-16]. Therefore, developing efficient methods to improve the CH4 sensitivity of ZnO is an urgent work.

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In order to improve the gas sensing properties of MOS, two main strategies have been applied. One is fabricating their micro/nanostructures with novel morphology. In the respect, different morphologies of MOS micro/nanostructures have been designed and synthesized to achieve high gas sensing performance [17-19]. Among diverse micro/nanostructures, three dimensional (3D) hierarchical structure that assembled from low-dimensional nano units has been considered as a promising candidate for developing advanced gas sensing materials. The 3D hierarchical structure can always endow the MOS sensing materials with high specific surface area and high porosity, which are considered to be favorable for gas sensing process [19-21]. The other strategy is building

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heterostructures by introducing the second phase into the host MOS material [22-24]. The heterostructured nanocomposite usually exhibited better gas sensing properties than their single

phase counterparts [16, 25-26]. In order to upgrade the gas sensing properties of ZnO, various ZnO-

based heterostructures have been synthesized and investigated [25, 27-28]. Yang et al. [25]

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synthesized the SnO2/ZnO heterostructures and found their improved ethanol sensing properties as compared with the pure ZnO counterpart. Xu et al. [27] reported that the sensor fabricated with p-n

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CuO/ZnO heterostructures showed a lower optimum working temperature and higher response than

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the pure ZnO during sensing trimethylamine vapor. Song et al. [28] reported the synthesis of ZnO/ZnFe2O4 heterostructures via a co-precipitation method combined with subsequent calcination process. They found that after formation of ZnO-ZnFe2O4 heterojunctions, the fabricated sensor

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exhibited excellent acetone sensing performance. Zn2SnO4, as a ternary oxide semiconductor with a band gap of 3.6 eV, has been also used as a sensitizer to improve the gas sensing properties of

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ZnO. Park et al. [29] prepared Zn2SnO4/ZnO heterostructures by thermal evaporation and chemical deposition method. Gas sensing test showed that the Zn2SnO4/ZnO nanocomposites had a higher

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response to NO2 than the pure Zn2SnO4 and ZnO counterparts. Although, significant advance has achieved in improving the gas sensing properties of ZnO by fabricating novel hierarchical structure and constructing different heterostructures [30-32], there are few reports, as far as we known, on improving the CH4 sensing properties of ZnO by introduction of Zn2SnO4-ZnO heterojunction. In comparison with widely adopted noble metal modification method [33, 34], the strategy of constructing Zn2SnO4-ZnO heterojunction can avoid using expensive source materials (noble metals such as Pd and Pt), which makes it more suitable for low-cost production.

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In this work, we report a simple solvothermal method for the synthesis of pristine ZnO and Zn2SnO4-decorated ZnO microflowers that assembled from porous nanosheets with the thickness about 15 nm. The prepared samples were characterized by using X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), energy dispersive X-ray spectroscopic (EDS), transmission electron microscope (TEM), Nitrogen adsorption-desorption isotherms, X-ray photoelectron spectroscopy (XPS) and UV-visible diffuse reflectance spectroscopy. The CH4 sensing performance of the prepared materials were investigated. It was found that after decorating with different amount of Zn2SnO4, the ZnO microflowers showed a remarkable improvement in

long-term stability.

2. Experimental 2.1 Synthesis of heterostructured Zn2SnO4/ZnO microflowers

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CH4 sensitivity, such as outstanding response, fast response-recover speed, good repeatability and

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All chemicals were analytical grade and used as received without further purification. In a typical

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synthesis procedure for Zn2SnO4/ZnO microflowers, 10 mL of NaOH aqueous solution (13.3 mmol/L, ≥99.0%, Shanxi-Tongjie, China) was slowly dropped into 10 mL ethanol solution of

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SnCl2·2H2O (1 mmol/L, ≥99.0%, Tianjin-Kemiou, China) under vigorous stirring to obtain solution A. Then, solution B was prepared by dissolving a desired amount of Zn(NO3)2·6H2O (≥99.0%, Chengdu-Aikeda, China) in a mixture solution of glycerin (3 mL) and ethanol (10 mL). After mixing

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solution A with solution B, the obtained mixture solution was transferred into a 50 mL Teflon-lined autoclave and maintained at 160 oC for 24 h. After the solvothermal reaction, the white precipitation

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was collected by centrifugation, washed with deionized water and ethanol, and dried at 60 oC for 12 h. The collected precipitation was annealed in air at 450 oC (2 oC/min) for 2 h to obtain the final

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Zn2SnO4/ZnO samples. By controlling the molar ratio between SnCl2·2H2O and Zn(NO3)2·6H2O as 1:3, 1:4 and 1:5, Zn2SnO4/ZnO nanocomposites with different Zn2SnO4 contents were prepared and denoted as SZ1, SZ2 and SZ3, respectively. For comparison, pristine ZnO was also prepared under the same conditions but without adding of SnCl2·2H2O, which was labeled as SZ0. The synthesis of pristine ZnO and Zn2SnO4/ZnO microflowers were illustrated in Fig. 1. 2.2 Characterization

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The crystalline phase and structure of prepared samples were analyzed by the X-ray diffraction (XRD, Bruker-AXS D8, Bruker, Madison, MI, USA) using Cu Kα radiation with a wavelength of 0.154 nm at scanning rate of 15o/min. Elemental mapping analysis was performed via energy dispersive X-ray spectrometry (EDS) (FEI, Eindhoven, Netherlands). The morphologies and microstructures were studied with field emission scanning electron microscope (FESEM, QuantaTM250 FEG) and transmission electron microscope (TEM, Tecnai G2 F20, FEI, USA). The optical absorption properties were investigated by UV-visible diffuse reflectance spectroscopy (TU1901, Beijing, China). Nitrogen adsorption-desorption isotherms was obtained on a Quantachrnme

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Autosorb-iQ sorption analyzer (Quantachrome, Boynton Beach, FL, USA). X-ray photoelectron spectroscopy (XPS) analysis were performed on an ESCALAB 250Xi (Thermofisher Scientific) using an Al Kα monochromated (150 W, 500 μm spot size) and bonding energy was calibrated with

2.3 Fabrication and measurement of gas sensor

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reference to C1 s peak (284.8 eV).

The sensor was fabricated according to our previous method [16]. Typically, a ceramic substrate

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(13.4 mm×7 mm) screen-printed with Ag-Pd interdigitated electrodes was cleaned with deionized

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water and ethanol by sonication. Then, 10 mg of as-prepared powders were mixed with two drops deionized water to form a homogenous paste. The paste was brush-coated on the surface of the ceramic substrate and then dried to obtain a resistance-type sensor. Gas sensing tests of the

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fabricated sensors were performed on the gas sensing analysis system of CGS-4TPS (Beijing Elite Tech. Co., Ltd., Beijing, China). The measuring system consists of four parts including cyclic water

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installation, gas sensor testing device, gas-sensing analytical equipment and computer. In order to obtain the desired concentration of CH4, a calculated volume of CH4 was injected into a closed test

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chamber (volume: 1.8 L) via a syringe. During the test, the operating temperature of sensors was controlled by heating the bottom layer of substrate and the resistance signals were collected by AgPd interdigitated electrodes as shown in Fig. 2. In order to obtain a stable resistance signals, the sensors were aged at 60 oC and 200 oC for 2 h, respectively. The response value of sensors to CH4 was defined as Ra/Rg, where Ra and Rg were the resistance of sensor in air and in CH4, respectively. The response (Tres) and recovery time (Trec) was defined as the time taken for the sensor to reach

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90% of the total resistance change in case of CH4 injection and release, respectively. During the tests, the relative humidity (RH) in the test chamber was 30 %.

3. Results and discussion 3.1 Morphology and structure of the prepared samples Fig. 3 shows the XRD patterns of different samples. In the XRD pattern of SZ0, all diffraction peaks match well with that of hexagonal ZnO (JCPDS no.74-0534), and no diffraction peaks from any other impurities were detected, demonstrating the formation of pure ZnO phase. In SZ2 and SZ3, in addition to the peaks from ZnO, two small peaks locating at 2θ of 34.5o and 60.8o are

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observed, which can be assigned to the (311) and (440) planes of Zn2SnO4 (JCPDS no.73-1725), respectively. While, in SZ1, such two peaks of Zn2SnO4 are absent, which may be attributed to the low content of Zn2SnO4 in this sample. Moreover, the intensity ratio between (002) and (101) planes

(I(002)/I(101)) in SZ1, SZ2 and SZ3 is higher than that in SZ0, which suggests that in the composite

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samples, the ZnO nanocrystals may have a larger fraction of exposed (002) plane than the pure ZnO

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sample (SZ0) [35-37].

The morphology and microstructure of the prepared samples were characterized by FESEM and

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TEM, as shown in Fig. 4. From the FESEM image showed in Fig. 4a and b, one can see that the SZ0 sample is composed of many flower-like hierarchical architectures with the size about 2˗5 µm. The nanosheets contained in the observed microflowers are about 15 nm in thickness. Fig. 4c shows

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a representative FESEM image recorded from SZ2. It is interesting to observe that after introducing a small amount of Zn2SnO4 into the host ZnO, similar flower-like architecture can be also formed

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in the composite product. The chemical composition of the microflowers in SZ2 were investigated by EDS. Fig. 4g shows the EDS element mappings recorded from the denoted area in Fig. 4c. Three

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elements of Sn, Zn and O with good dispersion were clearly observed, suggesting the coexistence of ZnO and Zn2SnO4 in the microflower. The formation of flower-like architecture in SZ2 was also confirmed by TEM observation, as shown in Fig. 4d. In order to get more structure information of the microflower, a representative TEM image recorded from the sheet-like petals is showed in Fig. 4e, in which a large number of randomly dispersed pores with a size about 10˗25 nm were observed on the petals. These embedded nanopores can provide efficient channels for gas transmission and diffusion, which is considered to be helpful for the gas sensing process. Fig. 4f shows a

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representative high resolution TEM (HRTEM) recorded from the porous petals. Two kinds of lattice fringes with the interplanar distance of 0.260 and 0.304 nm were observed, which can be attributed to the (002) plane of hexagonal ZnO and (220) plane of Zn2SnO4, respectively. At the interface of ZnO and Zn2SnO4 nanocrystals, interlaced lattice fringes were observed, suggesting the formation of heterojunctions between Zn2SnO4 and ZnO. The BET specific surface area of SZ0 and SZ2 was investigated by using nitrogen adsorption and desorption. The adsorption-desorption isotherms of two samples are showed in Fig. 5, both of which exhibits a typical type IV isotherm with H2-type hysteresis loops. The BET specific surface areas

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of the two samples were calculated, which are found to be 16.05 and 28.02 m2/g for SZ0 and SZ2, respectively. Obviously, after introduction of Zn2SnO4, the specific surface area of the ZnO microflower was increased.

XPS analysis was used to investigate the chemical composition of SZ0 and SZ2, during which C

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1s peak at 284.8 eV was used as the reference in the obtained binding energies. The full wide-scan

survey spectra of SZ0 and SZ2 were shown in Fig. 6a. The characteristic peaks of Sn, Zn and O

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were found in SZ2, while only Zn and O were obtained in SZ0. Fig. 6b shows the high resolution

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Sn 3d spectra. The peaks arising from Sn 3d5/2 and Sn 3d3/2 at the banding energy of 486.38 eV and 494.83 eV were observed, which indicated a normal oxidation valence sate of Sn4+. Fig. 6c presents the high resolution Zn2p spectra of SZ0 and SZ2, in which two distinct peaks of 2p3/2 and 2p2/1 were

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detected. Their spin-orbit splitting energy was about 23.10 eV, whose result was consistent with previous reports [31, 39]. As compared with SZ0, the Zn2p peaks of SZ2 shift to higher energy

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(about 0.1 eV), which can be explained by the chemical interaction between Zn2SnO4 and ZnO due to the formation of heterostructure [40]. The high resolution O 1s spectra of SZ0 and SZ2 (Fig. 6d)

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can be divided into two peaks. The peak at around 530.00 eV can be ascribed to the O2- in the crystal lattice (Olattice) and the peak at 531.03 eV belongs to the oxygen-related vacancies or oxygen adsorbed (Oads) [41]. It was reported that the Oads can play an important role in improving the gas sensing property of MOS materials [42]. From Fig. 6c, the relative percentages of Olattice and Oads components were approximately 59.63 % and 40.36 % for SZ0 and 47.40 % and 52.60 % for SZ2, respectively.

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The band structures of SZ0 and SZ2 were investigated by measuring their UV-vis absorption spectra. As shown in Fig. 7, the absorption curve of SZ2 had a slight red shift as compared with SZ0. In order obtain the band gap of the two samples, their spectrum of (αhv)2 vs. hv was ploted and showed in inset of Fig. 7. The band gap of SZ0 and SZ2 was found to be 3.24 eV and 3.18 eV, respectively. The narrowed band gap of SZ2, as compared with SZ0, may be attributed to the formed Zn2SnO4/ZnO n-n heterojunction [43-45]. 3.2 Gas sensing properties The CH4 sensing properties of the prepared Zn2SnO4-decorated ZnO microflowers were

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investigated to explore their possible application in CH4 sensor. Considering the fact that the operating temperature can exert severe influences on the gas sensing performance of a MOS sensor,

such as sensitivity, selectivity, and response-recover speed, the temperature-dependent responses of the fabricated sensors to CH4 were first tested. Fig. 8 shows the response values of the fabricated

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sensors to 1000 ppm CH4 at different working temperatures. From this figure, it can be seen that in

the testing temperature range of 200-300oC, all sensors exhibit a similar trend of “increase-

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maximum-decrease” in response, and reach their maximum response values at 250 oC. Such

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temperature-dependent response variation can be explained by the temperature-dependent gas adsorption and desorption behaviors occurring on the surface of sensing materials. In detail, when the temperature was lower than 250 oC, the speed of gas adsorption is considered to be faster than

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that of the gas desorption. Thus, with the temperature increasing from 200 to 250 oC, the response of sensors increased correspondingly due to the enhanced gas adsorption. At the optimum working

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temperature of 250 oC, the speed of gas adsorption and desorption on the surface of sensing materials tends to be equal, resulting in the maximum response of the sensors. While, as the operating

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temperature is over the 250 oC, the equilibrium between gas adsorption and desorption could move to the gas desorption side, leading to a continuously decreased response with further increasing the temperature to 300 oC. From Fig. 8, one can further observe that at different working temperatures, all the composite sensors (SZ1, SZ2 and SZ3) show higher response than the pure ZnO sensor (SZ0). Such result indicates that during sensing CH4, Zn2SnO4 may play a role of sensitizer of ZnO. Moreover, among the three composite sensors, the SZ2 sensor shows the highest response. At the

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optimum working temperature of 250 oC, the responses of the SZ0, SZ1, SZ2 and SZ3 sensors towards 1000 ppm CH4 are 8.2, 18.5, 27.2, and 17.7, respectively. Since the SZ2 sensor shows the highest response among the three composite sensors, its CH4 sensing properties was further investigated and compared with SZ0 by testing their responses to different concentrations of CH4. Fig. 9a displays the recorded dynamic response and recover curves of the SZ0 and SZ2 sensors when they were used to detect different concentrations of CH4 at the optimum working temperature of 250 oC. It can be seen that once exposed to different concentrations of CH4, the SZ2 sensor can give faster and higher responses than the SZ0 sensor, further

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demonstrating the sensitization effect of Zn2SnO4 on ZnO. In addition, with the CH4 concentration increasing from 10 to 5000 ppm, the response amplitude of the SZ2 sensor enlarges gradually,

demonstrating its good sensing ability to different concentrations of CH4. Fig. 9b shows the concentration-dependent responses of the SZ0 and SZ2 sensors. It can be seen that the response of

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SZ2 increases fast and almost linearly with the CH4 concentration increasing from 10 to 500 ppm.

While, as the CH4 concentration is over 500 ppm, the increase of response slows down, and the

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sensors tends to be saturation when the CH4 concentration increases to 5000 ppm. In contrast, the

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SZ0 sensor only gives a slight increase in response when the CH4 concentration is increased from 10 to 5000 ppm. The slop of the fitting line of the SZ2 sensor in the linear range of 10-500 ppm is found to be 0.03416 (inset of Fig. 9b), meaning that in such a concentration range, the sensitivity of

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SZ2 sensor to CH4 can be as high as 0.03416/ppm. Based on the definition of International Union of Pure and Applied Chemistry (IUPAC), the detection limit (DL) of the SZ2 sensor for CH4 was

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estimated according to the equation of DL = 3 Noiserms/slop [16]. The calculated result shows that the present SZ2 sensor can give a DL as low as 1.48 ppm. The transient resistance changes of the

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SZ0 and SZ2 sensors as they were exposed to 5000 ppm CH4 are displayed in Fig. 9c and d, respectively. Both sensors show a rapid decrease of resistance when they are exposed to CH4 atmosphere, exhibiting a typical response of n-type semiconductor. The resistance base line of SZ2 in air (108MΩ) is much lower than that of the SZ0 sensor (520 MΩ). The lower sensor resistance baseline of SZ2 can be attributed to the formation of Zn2SnO4-ZnO heterojunctions, which will be discussed in the following part. Based on Fig. 9c and d, the response/recover time (Tres/Trec) of the SZ0 and SZ2 sensors were measured to be 28/135 s and 7/30 s, respectively. The faster

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response/recover speed as well as the higher response value of SZ2 makes it more suitable for practical detection of CH4. The CH4 sensing properties of the prepared Zn2SnO4-decorared ZnO microflower were compared with previously reported materials. As shown in Table 1, our SZ2 sensor shows much higher response and shorter response-recover speed than these reported CH4 sensing materials. The selectivity of the SZ0 and SZ2 sensors to CH4 was investigated to evaluate their possible application in coal mine environment. As is well known that the coal mine gas usually contains of a great amount of CH4, a small amount of carbon monoxide (CO) and carbon dioxide (CO2), and a

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trace amount of other gases [35]. Thus, the responses of SZ0 and SZ2 sensors to 200 ppm CH4 and 50 ppm other harmful gases including CO, NH3 and H2O were measured at 250 oC. As shown in

Fig. 10a, as compared with the SZ0 sensor, the SZ2 sensor shows better selectivity to CH4, demonstrating its advantage in detection of CH4. The repeatability and long-term stability of the

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SZ2 sensor were also tested. As shown in Fig. 10b, in two successive cycles for detection different concentrations of CH4, the SZ2 sensor can give almost the same response amplitudes to the same

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concentration of CH4, demonstrating its good repeatability. Fig. 10c shows the results of long-term

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stability test of the SZ2 sensor, in which the time-dependent response values of the sensor to 10, 100 and 1000 ppm within 30 days were displayed. One can see that the response values of the SZ2

for sensing CH4.

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sensor to the same concentration of CH4 only fluctuate in a small range, revealing its good stability

3.3 gas sensing mechanism

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In general, the gas sensing properties of MOS originates from the surface resistance change when the sensor is exposed to different gases [50]. Take our experiment as an example, as illustrated in

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Fig 11a, when the sensor based on SZ0 (pure ZnO) is exposed in air, oxygen molecules can adsorb on the surface of ZnO and trap a lots of electrons from the conduction band (Ec) of ZnO to form chemisorbed oxygen species (O2-, O- and O2-) [42], causing an upward bending of the bands and the formation of a thick electron depletion layer (EDL) on the surface of ZnO. As a result, a high sensor resistance (Ra) is obtained because of the low conductivity of EDL. When the sensor is transferred to CH4 atmosphere, CH4 molecules will adsorb on the surface of ZnO and then react with the originally created chemisorbed oxygen species. After the reaction, the electrons trapped by

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chemisorbed oxygen will be released back to ZnO to increase the electron concentration and decrease the EDL thickness, as shown in Fig 11b. In this case, the sensor resistance (Rg) will be decreased. Since the response of the ZnO sensor to CH4 is defined as Ra/Rg, the varied sensor resistance in different gas atmospheres endows ZnO with the ability to detect CH4 [51]. In our experiment, it was found that after decorating the ZnO microflower with Zn2SnO4, the response of the sensor to CH4 was remarkably enhanced. The improved gas sensitivity of Zn2SnO4/ZnO microflowers can be mainly attributed to the Zn2SnO4-ZnO heterojunctions. Considering the fact that ZnO is the dominate phase in Zn2SnO4/ZnO composite, the resistance of

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the sensor should be mainly controlled by ZnO. So, the enhanced response of the Zn2SnO4/ZnO sensor should be mainly attributed to the varied thickness of DEL on the surface of ZnO. From the

results of HRTEM and XPS analysis, it was confirmed that after decorating Zn2SnO4 on the ZnO

microflower, Zn2SnO4-ZnO heterojunctions were formed. Because the work function of Zn2SnO4

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(4.9 eV) is a lower than that of ZnO (5.0 eV), at the interface of Zn2SnO4-ZnO heterojunctions, electrons will migrate from Zn2SnO4 to ZnO until their Femi levels reaches balance (Fig. 11c). In

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this case, the EDL thickness of ZnO will be decreased, resulting in a lower sensor resistance of the

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Zn2SnO4/ZnO sensor. In fact, the resistance baseline of SZ2 (108 MΩ) is found to be lower than that of the SZ0 sensor (520 MΩ) (Fig. 9c and d), whose result is consistent with above speculations. In addition, the higher electrical conductivity of Zn2SnO4 (102~103 s/cm) than ZnO (5~102 s/cm)

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may be another reason for the lower resistance of SZ2. In general, for an n-type MOS sensor, the higher Ra value is disadvantageous for achieving high response, because the response of an n-MOS

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to reducing gas is usually defined as Ra/Rg. In our experiment, it was found that although the SZ2 sensor has a lower resistance baseline, it still exhibits higher response to CH4 than the pure ZnO

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sensor. This phenomenon may be explained by the second effect of Zn2SnO4-ZnO heterojunctions. Due to the lattice mismatch between ZnO and Zn2SnO4, a lot of defects will be formed near the region of Zn2SnO4-ZnO heterojunctions. These defects will become potential active sites for gas adsorption and sensing reaction. Thus, as compared with pure ZnO microflower, more CH4 molecules will adsorb on the surface of Zn2SnO4/ZnO microflower and then take part in the sensing reaction between CH4 and chemisorbed oxygen species. In this case, after the gas sensing reaction, more electrons will be released back to ZnO, resulting in a much lower sensor resistance (Rg) of

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Zn2SnO4/ZnO. In addition, after the gas sensing reaction, the migration of electrons from Zn2SnO4 to ZnO can further decrease the resistance of the SZ2 sensor, which will be also responsible for the higher response of SZ2, as shown in Fig. 11d. Besides of Zn2SnO4-ZnO heterojunction, the larger specific surface area of SZ2 is also considered to be an unnegligible factor for the improved CH4 sensitivity. According to the results of the N2 adsorption-desorption measurements, the specific surface area of SZ2 (28 m2/g) is larger than that of SZ0 (16.05 m2/g). The larger specific surface area of SZ2 means that more active sites can be provided for gas adsorption and redox reaction during the gas sensing process. Moreover, the larger

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exposure fraction of (002) plane of ZnO may be also responsible for the improved CH4 sensitivity of SZ2 [35-37]. 4. Conclusions

In summary, Zn2SnO4-decorated ZnO microflowers were successfully synthesized via simple

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solvothermal route and subsequent calcination. The prepared Zn2SnO4/ZnO microflowers were

assembled by porous nanosheets with the thickness about 15 nm. It was found that after decorating

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the ZnO microflowers with Zn2SnO4, the CH4 sensing properties of the obtained Zn2SnO4/ZnO

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hybrid microflowers were remarkably improved, especially in terms of sensor response and response/recover speed. In the concentration range of 10˗500 ppm, the SZ2 sensor shows good response linearity to CH4 and the detect limit can be as low as 1.48 ppm. Moreover, the SZ2 sensor

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also shows good repeatability and long-term stability. The enhanced CH4 sensing performance of the Zn2SnO4/ZnO microflowers can be mainly attributed to Zn2SnO4-ZnO n-n heterojunctions. Our

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research not only provides a simple route for synthesizing hierarchical Zn2SnO4/ZnO microflowers that assembled with porous nanosheets, but also demonstrates that decorating with Zn2SnO4 is a

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feasible method to improve the CH4 sensing properties of ZnO. Acknowledgments This work is supported by the National Natural Science Foundation of China (U1704255,

U1404613), Program for Science & Technology Innovation Talents in Universities of Henan Province (18HASTIT010), Foundation of Henan Scientific and Technology key project (182102310892), the Education Department Natural Science Foundation of fund Henan province

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(16A150051), and the Program for Innovative Research Team of Henan Polytechnic University (T2019-1).

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Author biographies Xiaojie Li received her BS degree in 2017. She is currently a master course student at Henan Polytechnic University. Her major is materials physics and chemistry. Yanwei Li received her master's degree in 2002 from Hebei University, China. She is currently a lecture in Henan Polytechnic University, China. Her research focus is on the design and synthesis of metal oxide nanostructures and their application in gas sensor. Guang Sun received his PhD degree in materials science in 2007 from Yanshan University, China. He is currently an associate professor in the School of Materials Science and

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Engineering of Henan Polytechnic University, China. His research interests include the design and synthesis of nanostructured metal oxide semiconducting materials and their applications in catalyst, gas sensor and lithium ion rechargeable battery.

Bo Zhang received his master’s degree in 2007 from Capital Normal University, China. He is

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currently a doctoral candidate at Henan Polytechnic University, China. His research interests include the theory calculation and synthesis of metal oxide semiconducting materials and

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their applications in gas sensor.

Yan Wang received her PhD degree of Chemistry in 2009 from Nankai University, China. She

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is currently an associate professor in the School of Safety Science and Engineering of Henan Polytechnic University, China. Her research topic is the design and synthesis of metal oxide

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materials and their applications in gas sensor.

Zhanying Zhang received his PhD degree of materials science and engineering in 1982. Now,

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he is a professor and the vice-president of Henan Polytechnic University, China. His research interests are focused on the design, synthesis and application of nanostructured metal oxide

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semiconducting materials.

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Table and Figure captions Fig. 1. Schematic illustration for the synthesis of Zn2SnO4/ZnO microflowers.

Zn( NO3

N a 160 o C,

450 o C, Zn2SnO 4/ZnO

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Sn Cl

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Fig. 2. Schematic diagram the test platform and the sensor element.

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Fig. 3. XRD patterns of the prepared samples.

Fig. 4. FESEM images of (a, b) SZ0 and (c) SZ2; (d, e) TEM and (f) HRTEM images of SZ2; (g) EDS element mappings corresponding to the denoted area in (c).

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Fig. 5. Nitrogen adsorption-desorption isotherms of SZ0 and SZ2.

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and SZ2; (d) O 1s spectra of SZ0 and SZ2.

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Fig. 6. (a) XPS survey spectra of SZ0 and SZ2; (b) Sn 3d spectrum of SZ2; (c) Zn 2p spectra of SZ0

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Fig. 7. UV–vis absorption spectra of SZ0 and SZ2. Inset is the (αhv)2 vs. hv plots of SZ0 and SZ2.

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Fig. 8. Temperature-dependent responses of the sensors to 1000 ppm CH4.

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Fig. 9. (a) The dynamic response-recovery curves of the SZ0 and SZ2 sensors to different concentrations of CH4 at 250 oC; (b) the responses of the SZ0 and SZ2 sensors to 10˗5000 ppm CH4

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(inset is the fitted line of the SZ2 sensor in the CH4 concentration range of 10˗500 ppm); the transient responses of SZ0 (c) and SZ2 (d) sensors to 5000 ppm CH4 at 250 oC.

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Fig. 10. (a) Selectivity of the SZ0 and SZ2 sensors expose to CH4 at 250 oC; (b) repeatbiltiy and (c)

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Long-term stability tests of the ZS2 sensor to different concetrations of CH4 at 250 oC.

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Fig. 11. The energy-band structure diagram of (a, b) SZ0 and (c, d) SZ2 during exposure in air and

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CH4.

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Table 1 The CH4 sensing properties of different materials.

Temperature (oC)

Materials SnO2/NiO Pd-SnO2-rGO ZnO2-rGO SnO2 NiO/rGO

330 RT 190 150 260

ZnO/Zn2SnO4

250

400

10/30

Response (RaRg)/Ra*100 % 15.2 % 0.2 % 9.5 % 24.9 % 15 % 81.38 % (Ra/Rg = 15.36)

References [46] [47] [16] [48] [49] This work

according to the corresponding reference.

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a Estimated

Concentration (ppm) Tres/ Trec (s) 500 28/44 800 300/420 500 30/200 1000 369/350a 1000 16/20

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