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Enhanced methane sensing properties of porous NiO nanaosheets by decorating with SnO2
Accepted Manuscript Title: Enhanced methane sensing properties of porous NiO nanaosheets by decorating with SnO2 Authors: Saisai Zhang, Yanwei Li, Guang Sun, Bo Zhang, Yan Wang, Jianliang Cao, Zhanying Zhang PII: DOI: Reference:
S0925-4005(19)30371-5 https://doi.org/10.1016/j.snb.2019.03.024 SNB 26243
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
30 December 2018 5 March 2019 6 March 2019
Please cite this article as: Zhang S, Li Y, Sun G, Zhang B, Wang Y, Cao J, Zhang Z, Enhanced methane sensing properties of porous NiO nanaosheets by decorating with SnO2 , Sensors and amp; Actuators: B. Chemical (2019), https://doi.org/10.1016/j.snb.2019.03.024 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.
Enhanced methane sensing properties of porous NiO nanaosheets by decorating with SnO2
Saisai Zhang a, Yanwei Li a, Guang Sun a,b,c*, Bo Zhang a, Yan Wang b,c,
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Jianliang Cao b,c, Zhanying Zhang a,b,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
State Key Laboratory Cultivation Bases for Gas Geology and Gas Control (Henan Polytechnic
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c
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Polytechnic University, Jiaozuo 454000, China.
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University), Jiaozuo 454000, China.
Corresponding
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author Tel.: +86 03913986952 E-mail address: [email protected] (Guang Sun), [email protected] (Zhanying Zhang)
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Graphical Abstract
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Pure and SnO2-decorated NiO porous nanosheets were synthesized via a sacrificial template method. The CH4 sensing properties of NiO was improved by composition evolution (decorating
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with SnO2). The improved gas-sensing mechanism was discussed.
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Abstract
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Two dimensional (2D) nanomaterials with porous structure have stimulated much research
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interest owing to their unique structure and fascinating physical and chemical properties. Here,
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ultrathin 2D porous nanosheets (PNSs) of pure and SnO2-decorated NiO with uniform hexagonal shape were synthesized by using the pre-synthesized Ni(OH)2 nanosheets precursor. The obtained
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SnO2/NiO PNSs were about 100~150 nm in size and their mean thickness was about 7.5 nm. The gas sensing properties of the prepared pure and SnO2-decorated NiO PNSs were investigated
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intensively. It was found that after decorating with different amount of SnO2, the sensor based on NiO PNSs showed an improved sensing properties to methane (CH4), and the optimal SnO2 content
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in the composite was determined to be 2 mol%. At the optimum working temperature of 330 oC, the SnO2/NiO-2 sensor showed higher response and faster response/recover speed towards 500 ppm CH4 than the pure NiO sensor. In addition, the SnO2/NiO-2 sensor also exhibited a good long-term
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stability within 28 days, demonstrating its potential application for CH4 detection. The porous structure and p-n junction related sensing mechanism of SnO2/NiO PNSs was also discussed. Keywords: 2D nanostructure; porous nanosheets; SnO2/NiO; p-n junction; CH4; gas sensor
1. Introduction
Methane (CH4), as a simple colorless and odorless organics, is the main component of natural gas and coal mine gas. As a kind of fuel, it is widely used in civil life and industrial production [1, 2]. It can be used as a raw material for the preparation of hydrogen, carbon black, carbon monoxide, acetylene and formaldehyde [3]. Besides, methane can be employed as the carbon source for producing solar cell and amorphous silicon film. However, once the concentration of CH4 is too high, the oxygen content will remarkably reduce which may result in headache, dizziness,
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inattention and suffocation [4]. In addition, as one of the gases resulting in climate changes, the
greenhouse effect of methane is 25 times greater than CO2 [5]. More seriously, when methane is
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mixed with air in coal mine production, it will be easy to explode with the volume concentration ranges from 4.9% to 15.4% [2, 3, 6]. Therefore, the rapid and real-time detection of CH4 is becoming more and more important in our daily life.
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In the past few decades, various kinds of gas sensors such as catalytic combustion sensor [7, 8],
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infrared sensor [9, 10] and semiconductor sensor [3, 11, 12] have been employed to detect CH4.
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Among them, metal oxide semiconductor (MOS) based sensor has been widely used in practical
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application with the advantages of low-cost, easy fabrication and integration, good safety and long service life in the comparison with the other kind of sensors [13-15]. Until now, there are many
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kinds of MOS have been reported to detect methane, including TiO2 [11], SnO2 [16], NiO [5] and so on. In contrast, p-type MOSs has received less attention because of their relative low gas response.
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Hübner suggested that when the morphological structure of the sensing material is identical, the response of a p-type MOSs to a given gas is usually larger than that of n-type MOSs [17]. Hence,
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p-type MOSs may be the promising potentials for practical applications for showing low humidity dependence and rapid recovery kinetics[18]. Among numerous MOS materials, NiO, as a typical p-type MOS, has received widespread
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attention and exhibits various application fields in gas sensors [19-22], lithium-ion batteries [23], catalysts [24], magnetic materials [25, 26] and so on. When applied as gas sensors, NiO exhibit considerable ability to detect some organic and inorganic gases. In order to improve the gas sensitivity of NiO, many attempts have been made. On the one hand, since the sensing properties of MOS are mostly dependent on its morphology and size, structure and composition, the preparation of MOS materials with unique structure has become a powerful strategy for improving the gas
sensitivity of MOS sensors. It’s reported that porous structure can provide more active sites and usually used to improve the sensitivity [27, 28]. On the other hand, there are some limitations to restrict the development of single MOS based sensor, such as low response and poor selectivity. Therefore, more and more researchers are working on the construction of composite sensitive materials, such as ZnO/ZnCo2O4 [29-31], MoS2/CuO [32], α-Fe2O3/NiO [33] and SnO2/Co3O4 [34], and the results indicated that the heterojunctions are beneficial for enhancing the gas sensitivity of
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p-type MOSs. Thus, we can also decorate NiO with an n-type MOS to construct composite sensitive
material. For instance, Zeng et al [35] reported the preparation of NiO/SnO2 nanocomposites
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through annealing the mixtures of Ni-Sn precursor and found that the composites showed an
enhanced NO2 sensitivity than bare NiO. Chen et al [36]successfully synthesized NiO/ZnO nanoplates and the gas sensing results showed that the ethanol sensitivity of NiO/ZnO composites
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is improved after decorating with ZnO. In recent years, some researchers reported that NiO or SnO2
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based sensors can be employed to fabricate sensors for detecting CH4 [5, 16]. For instance, Zhou et
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al. [5] prepared hierarchical NiO nanoflakes via hydrothermal method, the sensing performance
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results indicated that the hierarchical NiO nanoflakes sensor exhibited excellent selectivity and stability to CH4. However, besides the encouraging results that have been obtained, there are few
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reports about the preparation of porous SnO2/NiO nanosheets composites and their application as sensing materials to detect CH4.
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Here, we report the successful synthesis of NiO PNSs and SnO2/NiO PNSs (with different amounts of SnO2) through a facile hydrothermal route combined post-heat treatment. The SEM and
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TEM results indicate that it is easier to obtain NiO PNSs and SnO2/NiO PNSs by employing Ni(OH)2 precursors as template. The gas sensing tests demonstrate that SnO2/NiO PNSs present higher sensitivity to CH4 than pure NiO PNSs.
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2. Experimental section The chemicals of Nickel nitrate, polyvinylpyrrolidone (PVP) and ammonia were of analytical
grade and used without further purification. 2.1 Preparation of hexagonal Ni(OH)2 nanosheets The Ni(OH)2 nanosheets were prepared via a hydrothermal route. Typically, 0.2 g of PVP were dispersed into 80 mL of Ni(NO3)2 solution (0.04 mol/L) with vigorous stirring for 20 min. Then, a
desired amount of ammonia water was dropped into the above solution under vigorously stirring. After 1 h, the mixed solution was sealed into a 100 mL Teflon-lined autoclave and heated at 180 oC for 9 h. After cooling down to room temperature naturally, the precipitate was collected by centrifugation, washed with deionized water and ethanol for several times, and finally dried at 80 o
C to obtain the final Ni(OH)2 product. 2.2 Preparation of SnO2/NiO porous nanosheets (PNSs)
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The SnO2/NiO PNSs were synthesized through an immersion-calcination method by using as-
prepared Ni(OH)2 nanosheets as precursor. As shown in Fig.1, 0.092 g of as-prepared Ni(OH)2 (1
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mmol) was first dispersed in 10 ml of ethanol under ultrasonic condition, then, different amounts of
SnCl4·5H2O and urea were dissolved into above solution in sequence. After ultrasonic treatment for 1 h, the green precipitates were collected, washed with ethanol for three times, dried in an oven, and
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finally calcinated at 400 oC (heating rate: 2 oC/min) for 3 h to obtain the SnO2/NiO samples. By
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adjusting the using amount of SnCl4·5H2O, the with SnO2/NiO samples with the SnO2 contents of
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1, 2 and 3 mol % were prepared and labeled as SnO2/NiO-1, SnO2/NiO-2 and SnO2/NiO-3,
under the same condition.
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respectively. Pure NiO PNSs were also prepared by directly annealing the Ni(OH)2 nanosheets
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2.3 Fabrication and measurement of gas sensor
Gas-sensing tests were carried out on an intelligent gas-sensing analysis system of CGS-4TPS
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(Beijing Elite Tech Co., Ltd., China), which the sensor temperature can be adjusted by an external temperature control system. The sensor fabrication is similar to our previous works [37]. Simply,
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the prepared sample was mixed with deionized water to form a paste. Then, the paste was coated onto the ceramic substrate to form a thick uniform film. In order to stabilize the sensor, it would be aged at 260 oC for several days before sensing test. The response (S) of the sensor was defined as
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|Ra-Rg|/Ra*100, where Ra and Rg were the sensor resistance in air and target gas, respectively. And the conditions of the test chamber during the test were about 16% RH and 35 oC. 2.4 Characterization The phase of the obtained samples were analyzed by X-ray power diffraction (XRD) on a Bruker/D8Advance diffract meter with Cu Kα radiation, and thermogravimetry and differential scanning calorimetry (TG-DSC, Setaram Evolution 2400). The morphologies and microstructures
were examined by field-emission scanning electron microscopy (FSEM, Quanta 250 FEG) and transmission electron microscopy (TEM, JEOL JEM-2100 microscope). Atomic force microscopy (AFM) analysis was carried out by using a FM-Nanoview 6800 equipment (Suzhou, China). The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method on a Micromeritics Triatar 3020 apparatuses. The element analysis was carried out by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI electron spectrometer using Al Kα
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radiation). 3. Results and discussion
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3.1 Characteristics of the prepared samples
The formation of Ni(OH)2 in the hydrothermal step was confirmed by XRD. As shown in Fig. 2a, all the diffraction peaks can be well assigned to the standard data of Ni(OH)2 (JCPDS: 74-2075),
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and no other peaks from impurities were detected, indicating the high purity of the Ni(OH)2 product.
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Ni(OH)2 is thermolabile and can decompose to NiO at elevated temperature. So, thermal analysis
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was performed on the prepared Ni(OH)2 to make clear its thermal decomposition process. As shown
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in Fig. 2b, with the temperature increasing from 25 oC to 600 oC, a strong endothermic peak at 300~340 oC was observed (DTA curve), which corresponds to a weight loss of 17 % (TG curve).
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Such value is close to the theoretical weight loss (16.5%) of the phase transformation from Ni(OH)2 to NiO. Thus, based on above results, the calcination temperate was set at 400 oC to ensure a
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complete decommission of the Ni(OH)2 precursor, as well as a good crystallinity of the NiO product. Fig. 3 shows the XRD patterns of the prepared pure NiO and SnO2/NiO composites. In Fig. 3a, the
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strong diffraction peaks arising from bunsenite NiO (JCPDS: 71-1179, lattice constants of a=b=c=4.178) were clearly observed, demonstrating the formation of pure NiO with good crystallinity. In Fig. 3b-d, besides of the peaks from NiO, some broaden peaks arising from SnO2
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were also observed. Therein, the peaks located at 26.58◦, 33.87◦ and 51.77◦ can be indexed as the (110), (101) and (211) planes of SnO2 (JCPDS: 71-0652), respectively, demonstrating the coexistence of NiO and SnO2 phase. The FESEM and TEM analysis were carried out to investigate the structure and morphology of the prepared samples. The FESEM image showed in Fig. 4a reveals that the obtained Ni(OH)2 precursor is composted of a large number of hexagonal nanosheets with smooth surface. The TEM
image displayed in Fig. 4b further reveals that these observed nanosheets have solid structure. The size of Ni(OH)2 nanosheets measured from Fig. 4a and b were about 100~150 nm in diameter and 7~8 nm in thickness. In order to further confirm the thickness of the Ni(OH)2 nanosheets, atomic force microscope (AFM) analysis was performed and the results are shown in Fig. 4e and f. The height profile diagram displayed in Fig. 4f indicates the thickness of an individual nanosheet is about 7.5 nm, being consistence with the result given by FESEM analysis. Fig. 4c and d present the
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typical FESEM and TEM images of the obtained NiO product, respectively. As can be seen from Fig. 4c, the obtained NiO product has the similar morphology and size with the Ni(OH)2 precursor.
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While, from the TEM image displayed in Fig. 4 d, we can observe that many randomly dispersed
nanopores with the size about 2~18 nm were embedded on the NiO hexagonal nanosheets. The formation of these randomly dispersed nanopores can be put down to the thermal decomposition
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process of the Ni(OH)2 precursor. The selected area electron diffraction (SAED) pattern taken from
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the porous nanosheets is displayed in the inset of Fig. 4 d. The sharp and regular diffraction dots
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reveal the single-crystalline nature of the NiO porous nanosheets. Fig. 5 presents the representative
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TEM image taken from the SnO2/NiO-2 sample, in which the porous nanosheets structure was also observed, indicating the introduction a small amount of SnO2 has almost no influence on the
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morphology of NiO product. Fig. 5 b shows a typical HRTEM image taken from the SnO2/NiO PNS. The measured interplanar distances were 0.241 nm, 0.335 nm and 0.264 nm, corresponding to the
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(111) of NiO, (110) and (101) planes of SnO2, respectively. Moreover, at the interfaces of NiO and SnO2 nanoparticles, an obvious lattice connection was observed, demonstrating the formation of
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NiO-SnO2 p-n junctions in the porous nanosheets. The element composition of the SnO2/NiO PNSs were investigated by EDS. Fig. 5 d shows the EDS mappings taken from the area marked in Fig. 5c, in which Ni, O, and Sn elements with good dispersion were detected, further confirming that the
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porous nanosheets are composed of SnO2 and NiO and SnO2 was homogeneously distributed on the NiO PNSs. In addition, the observed C elements should attribute to the carbon conductive adhesive. The specific surface area and the porosity of the prepared samples were investigated with N2 adsorption-desorption analysis. The N2 adsorption-desorption isotherms (shown in Fig. 6a) of all samples display type IV cures with H2-type hysteresis loops according to the IUPAC. The cures of pore size distribution shown in Fig. 6b reveal that both samples have relatively pores with a size of
2~20 nm, which is consistent with the results of TEM. The BET calculated result shows that the specific surface areas of pure NiO and SnO2/NiO-2 are 51.977 and 68.182m2g-1, respectively. These results indicate that a higher surface area can be obtained when SnO2 nanoparticles decorated on the surface of NiO PNSs, which can provide more surface active sites and available space for gas molecules adsorption and surface reaction. XPS measurements were performed on pure NiO and SnO2/NiO-2 samples to investigate their
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surface composition and chemical state. Fig. 7a presents the survey spectra, in which the characteristic peaks of Ni and O were observed in the pure NiO, as well as the peaks of Ni, O and
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Sn in the SnO2/NiO-2. The unexpectedly existence of C is ascribed to the intrinsically introduced carbon source in XPS tests. Fig. 7b shows the O 1s spectra of the two samples. Both the O 1s spectra
are fitted into three peaks, and the binding energies at ~529.28 eV, ~530.88 eV and ~532.05 eV of
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pure NiO correspond to the lattice oxygen (OL), oxygen vacancies (OV), and chemisorbed oxygen
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species (OC) or OH species, respectively. In contrast, the binding energies at 531.03 eV of SnO2/NiO
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exhibit a little shift compared with pure NiO. The phenomenon may be put down to the different
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chemical environments of O in the composite oxides can lead to the characteristic peak shift [35]. In addition, the calculated content of OC in SnO2/NiO is higher than that in pure NiO, indicating
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that the introduction of SnO2 could create more active sites for gas adsorption and surface reaction. Subsequently, the Sn 3d high-resolution XPS spectra of SnO2/NiO are carried out as shown in Fig.
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7c. The sample exhibits two peaks with binding energies around 495.58 eV and 487.18 eV originated from Sn 3d3/2 and Sn 3d5/2, respectively. The detail Ni 2p XPS spectra of the both samples
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was present in Fig. 7d. It can be seen that Ni 2p spectra of both samples can be fitted into six peaks. The binding energies at 860.53 eV and 873.23 eV of SnO2/NiO are attributed to the Ni 2p3/2 and Ni 2p1/2 peaks, respectively, exhibiting a little shift (0.15 eV and 0.1 eV) compared with pure NiO. The
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little binding energy shift in the SnO2/NiO composites may be ascribed to the strong interaction between SnO2 and NiO due to the formation of p-n junctions, where the electrons will transfer from SnO2 to NiO [38, 39]. 3.2 Gas sensing characteristics The unique 2D porous nanosheets structure as well as the formed p-n junctions in the SnO2/NiO PNSs were expected to endow the composite materials with good sensing properties, so, the gas
sensitivity of the prepared SnO2/NiO PNSs towards CH4 were tested to explore their possible application, during which the pure NiO PNSs was used as a reference. Considering that the operating temperature can exert severe influences on the gas sensing properties of a MOS sensor, the responses of the sensors to 500 ppm CH4 were first measured to find out the optimum working temperature (OWT). As shown in Fig. 8, with the temperature increasing from 220 to 360 oC, all sensors exhibited a gradual increase in response and reached their maximum response vales at 330 oC. While,
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with further increasing the working temperature to 360 oC, the responses of the sensors decayed obviously. In the whole temperature range, the sensors based on SnO2/NiO PNSs always show
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higher response than that based on pure NiO PNSs, demonstrating the sensitization effect of n-type
SnO2 on p-type NiO. Moreover, at the OWT of 330 oC, the response of the sensor based on SnO2/NiO-2 is 15.2%, which is higher than that based on SnO2/NiO-1(12.3%) and SnO2/NiO-3
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(13.5%). Such result suggested that the optimal SnO2 content in the present SnO2/NiO composite
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was 2 mol %.
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The dynamic responses of the sensors to varied concentrations of CH4 were also tested. As
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depicted in Fig. 9a, once exposing to 500 ppm CH4, all sensors exhibit an obvious response, and the response amplitudes increased correspondingly with the concentration gradually increasing from
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500 to 7000 ppm, and then returned back to their initial values as the sensors were exposed to air atmosphere again, demonstrating the good response ability of the sensors to different concentrations
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of CH4. It is worth mentioning that the SnO2/NiO sensors always showed higher response amplitudes than the pure NiO sensor when they were exposed to different concentrations of CH4,
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further demonstrating the sensitization effect of n-type SnO2 on p-type NiO. The concentrationdependent responses of the sensors were plotted to exhibit the relationship between the response and gas concentration (presented in Fig. 9b). With increasing the concentration of CH4 from 500 to
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7000 ppm, the responses of the SnO2/NiO sensors exhibit higher response values than pure NiO sensor, and the highest CH4 sensing performance is obtained at an optimized SnO2 content of 2 mol%. For instance, the response of SnO2/NiO-2 is 48.9% to 7000 ppm CH4, which is about 3 times of pure NiO (15.4%). Such results indicating that through the component optimization, the improvement in the response of NiO PNSs can be achieved by the decoration of SnO2 on the NiO porous nanosheets. In addition, it can be also observed that the response of the sensors undergone a
rapid increase (ranging from500 to 2000 ppm) and then the increase speed slowed down with further increasing the CH4 concentration, which could be explained by the dynamic equilibrium between the re-adsorbed oxygen species and consumed adsorbed oxygen species by reacting with CH4 species [39]. Fig. 9c displays the dynamic response-recovery transient of SnO2/NiO-2 and pure NiO sensors at 330 oC. As we can see that once exposing to 500 ppm CH4, the resistances of both sensors increased
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immediately, then reach fast to a steady state, and finally returned back to their original values as
the CH4 gas was released, exhibiting the typical response of a p-type semiconductor. The response
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and recovery times were defined as the time required for a change in sensor resistance to reach 90 % of the equilibrium state value after injecting and removing the tested gas, respectively. Thus, according to the results given by Fig. 9c, the response/recover times of SnO2/NiO-2 were measured
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to be 28/44 s, which are shorter than that of the pure NiO sensor (30/68 s). The faster
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response/recover speed of SnO2/NiO-2 makes it easier for real time detection in practical application.
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In addition, the sensor resistance in air (Ra) of SnO2/NiO-2 (2.2 KΩ) is higher than that of pure NiO
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(0.93 KΩ), which may be attributed to the NiO-SnO2 p-n junctions formed on the porous SnO2/NiO nanosheets. The CH4 sensing performance of SnO2/NiO-2 was compared with other reported
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materials. As shown in Table 1, the present sensor can give a comparable response and faster response-recovery speed to a lower CH4 concentration.
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The good long-term stability is also an important parameter for a successful sensor. Thus, the long-term stability test of the SnO2/NiO-2 sensor was carried out to further evaluate its quality, as
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shown in Fig. 9d. The response vales only changed slightly within 28 days, demonstrating its good long-term stability. On the basis of its good stability, the present SnO2/NiO-2 sensor may be the potential candidate for practical application.
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3.2 Sensing mechanism of pure NiO and SnO2/NiO PNSs NiO is a typical p-type semiconductor and its sensing mechanism can be explained by the surface-
resistance controlled model [42]. Take our experiment as an example, in air atmosphere, oxygen molecules will be absorbed on the surface of NiO PNSs and form chemisorbed oxygen ions (O2-, Oand O2-) by trapping electrons from the conduction band of NiO, which will lead to the increase of the hole concentration and create a hole accumulation layer (HAL) on the surface of NiO PNSs.
Since positive holes are the dominant charge in NiO, the formation of HAL can result in a lower sensor resistance (Ra). While, as the sensor is exposed to reducing gases, such as CH4 in our case, the chemisorbed oxygen ions will react with CH4 to produce oxidation products, after which the trapped electrons by chemisorbed oxygen will be released and return back to NiO to neutralize some of the holes in HAL. As a result, the wideness of HAL is decreased and a relative higher sensor resistance (Rg) is obtained, as shown in Fig. 10 a and b. According to the definition of response, the
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varied resistance NiO PNSs in air and CH4 atmospheres eventually endows the materials with the ability to response CH4.
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When SnO2 nanoparticles were decorated on the NiO PNSs, a remarkably enhanced gas response was observed. The enhanced response of SnO2/NiO PNSs towards CH4 can be mainly attributed to the p-n junction formed on the SnO2/NiO PNSs. From Fig. 9c, one can see that after decorating NiO
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PNSs with a small amount of n-type SnO2, the composite material still exhibited a p-type response,
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which indicated that the response of SnO2/NiO PNSs towards CH4 is mainly controlled by the
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resistance change of dominant NiO. So, the gas response of SnO2/NiO PNSs was mainly determined
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by the varied HAL thickness of NiO. In addition, when SnO2 is introduced into NiO PNSs, p-n junctions are formed between NiO and SnO2, which has been confirmed by HRTEM analysis (Fig.
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5b). The formation of p-n junctions can bring important influences on the gas sensing process of SnO2/NiO PNSs. On one hand, because the Fermi level of SnO2 is higher than that of NiO, at the
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interface of p-n junctions, electrons will flow from SnO2 to NiO and holes will flow along the opposite direction until the Fermi level of the two materials reaches equilibrium, as illustrated in
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Fig. 10 c. In this case, the hole concentration in NiO will decrease and the wideness of HAL will become narrow correspondingly, leading to a higher resistance of SnO2/NiO as compared with pure NiO (Fig. 10a and b). As shown in Fig. 9c, the resistance value of SnO2/NiO-2 is about 2.2 KΩ (330 which is higher than that of NiO (0.93 KΩ). On the other hand, due to the different crystal
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oC),
lattice parameters of NiO and SnO2, a lot of defects should be created at the interface of NiO (p)SnO2 (n) junctions, which will become potential active sites for gas adsorption. Thus, more gas molecules could absorb on the surface of SnO2/NiO PNSs and then take part in the following reactions between chemisorbed oxygen and CH4 molecules, resulting in a much decreased wideness of HAL (shown in Fig. 10d), as well as a much higher resistance of the SnO2/NiO sensor as
compared with pure NiO sensor, because more electrons will be released back to NiO to neutralize holes. In addition, because of the higher Fermi level of SnO2 than NiO, electrons will flow from SnO2 to NiO after the chemical reaction between chemisorbed oxygen and CH4 molecules, which will result in further decrease of the hole concentration in NiO. At this moment, the sensor resistance of SnO2/NiO PNSs can be further increased correspondingly. The unique porous nanosheets structure is also considered to be helpful for the gas sensing
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properties of the SnO2/NiO composite. As shown in Fig. 5a, a lot of nano-sized pores were creased
on the SnO2/NiO nanosheets. These nanopores can not only provide much larger surface area for
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gas adsorption, but also provide enough channels for gas diffusion and transportation and then
facilitate gas adsorption and desorption process in sensitive materials. Additionally, the mean thickness of the porous SnO2/NiO nanosheets is only 7~8 nm. Such thin structure can also ensure
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the thickness of the HAL occupies almost the entire material in some extent, resulting in further
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reduction of carriers concentration. Thus a higher material resistance would be obtained, which is
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contribution to improving the sensor response.
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4. Conclusion
In summary, the SnO2 nanoparticles were successfully decorated on the NiO PNSs via a
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hydrothermal method combined with post-heat treatment. The prepared NiO PNSs were about 100~150 nm in diameter and 7.5 nm in thickness. The successful decoration of SnO2 nanoparticles
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on NiO PNSs were confirmed by XRD, XPS, TEM and SEM. The as-prepared pure NiO PNSs and SnO2/NiO PNSs (with different SnO2 contents) were applied as sensing materials for detecting CH4.
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It was found that at the optimum working temperatures of 330 oC, the sensor based on SnO2/NiO PNSs with the optimal SnO2 content of 2 mol % exhibited higher response and rapid responserecover speed to CH4 in comparison with pure NiO PNSs. The improved sensitivity of SnO2/NiO
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PNSs can be attributed to the porous structure and the p-n heterojunctions. Our research not only provides a reliable route for fabricating SnO2/NiO PNSs, but also demonstrates that decorating with n-type is an effective way to enhance the CH4 sensitivity of p-type NiO. Acknowledgement This work is supported by the National Natural Science Foundation of China (U1704255), Program for Science & Technology Innovation Talents in Universities of Henan Province
(18HASTIT010, 17HASTIT029), Young Core Instructor Project of Colleges and Universities in Henan Province (2016GGJS-040), Foundation of Henan Scientific and Technology key project (182102310892), the Education Department Natural Science Foundation of fund Henan province (16A150051), and the Program for Innovative Research Team of Henan Polytechnic University (T2019-1, T2018-2).
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Reference [1] W. Lu, D. Ding, Q. Xue, Y. Du, Y. Xiong, J. Zhang, X. Pan, W. Xing, Great enhancement of
CH4 sensitivity of SnO2 based nanofibers by heterogeneous sensitization and catalytic effect,
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Sensor Actuat B-Chem., 254 (2018), pp. 393-401.
[2] S. Nasresfahani, M.H. Sheikhi, M. Tohidi, A. Zarifkar, Methane gas sensing properties of Pddoped SnO2/reduced graphene oxide synthesized by a facile hydrothermal route, Mater Res Bull.,
U
89 (2017), pp. 161-169.
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[3] N.M. Vuong, N.M. Hieu, H.N. Hieu, H. Yi, D. Kim, Y.-S. Han, M. Kim, Ni2O3-decorated SnO2
A
particulate films for methane gas sensors, Sensor Actuat B-Chem., 192 (2014), pp. 327-333.
M
[4] S. Ghosh, C. Roychaudhuri, R. Bhattacharya, H. Saha, N. Mukherjee, Palladium-silver-activated ZnO surface: highly selective methane sensor at reasonably low operating temperature, ACS
ED
Appl Mater Inter., 6 (2014), pp. 3879-3887.
[5] Q. Zhou, Z. Lu, Z. Wei, L. Xu, Y. Gui, W. Chen, Hydrothermal synthesis of hierarchical ultrathin
PT
NiO nanoflakes for high-performance CH4 sensing, Front Chem, 6 (2018). [6] D. Haridas, V. Gupta, Enhanced response characteristics of SnO2 thin film based sensors loaded
CC E
with Pd clusters for methane detection, Sensor Actuat B-Chem., 166-167 (2012), pp. 156-164. [7] D. Nagai, M. Nishibori, T. Itoh, T. Kawabe, K. Sato, W. Shin, Ppm level methane detection using micro-thermoelectric gas sensors with Pd/Al2O3 combustion catalyst films, Sensor Actuat
A
B-Chem., 206 (2015), pp. 488-494.
[8] L. Sun, F. Qiu, B. Quan, Investigation of a new catalytic combustion-type CH4 gas sensor with low power consumption, Sensor Actuat B-Chem., 66 (2000), pp. 289-292.
[9] Z. Zhu, Y. Xu, B. Jiang, A one ppm NDIR methane gas sensor with single frequency filter denoising algorithm, Sensors-basel 12 (2012), pp. 12729-12740. [10] M. Dong, C. Zheng, S. Miao, Y. Zhang, Q. Du, Y. Wang, F.K. Tittel, Development and
measurements of a mid-infrared multi-gas sensor system for CO, CO2 and CH4 detection, in: Sensors-Basel., 17(2017), pp. 2221. [11] Z.P. Tshabalala, K. Shingange, B.P. Dhonge, O.M. Ntwaeaborwa, G.H. Mhlongo, D.E. Motaung, Fabrication of ultra-high sensitive and selective CH4 room temperature gas sensing of TiO2 nanorods: Detailed study on the annealing temperature, Sensor Actuat B-Chem., 238 (2017), pp. 402-419.
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[12] O. Lupan, O. Lupan, V. Postica, J. Gröttrup, A.K. Mishra, N.H. de Leeuw, J.F. Carreira, J.
Rodrigues, S.N. Ben, M.R. Correia, Hybridization of zinc oxide tetrapods for selective gas
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sensing applications, ACS Appl Mater Inter., 9 (2017), pp. 4084-4099.
[13] C. Zhao, H. Gong, W. Lan, R. Ramachandran, H. Xu, S. Liu, F. Wang, Facile synthesis of SnO2 hierarchical porous nanosheets from graphene oxide sacrificial scaffolds for high-
U
performance gas sensors, Sensor Actuat B-Chem., 258 (2018), pp. 492-500.
N
[14] Y. Gong, Y. Wang, G. Sun, T. Jia, L. Jia, F. Zhang, L. Lin, B. Zhang, J. Cao, Z. Zhang, Carbon
A
nitride decorated ball-flower like Co3O4 hybrid composite: Hydrothermal synthesis and ethanol gas sensing application, Nanomaterials-basel., 8 (2018), pp. 132.
M
[15] F. Ren, L. Gao, Y. Yuan, Y. Zhang, A. Alqrni, O.M. Al-Dossary, J. Xu, Enhanced BTEX gas-
ED
sensing performance of CuO/SnO2 composite, Sensor Actuat B-Chem., 223 (2016), pp. 914920.
[16] M. Kooti, S. Keshtkar, M. Askarieh, A. Rashidi, Progress toward a novel methane gas sensor
96-106.
PT
based on SnO2 nanorods-nanoporous graphene hybrid, Sensor Actuat B-Chem., 281 (2019), pp.
CC E
[17] M. Hübner, C.E. Simion, A. Tomescu-Stănoiu, S. Pokhrel, N. Bârsan, U. Weimar, Influence of humidity on CO sensing with p-type CuO thick film gas sensors, Sensor Actuat B-Chem., 153 (2011), pp. 347-353.
A
[18] H.-R. Kim, A. Haensch, I.-D. Kim, N. Barsan, U. Weimar, J.-H. Lee, Gas Sensors: The role of NiO doping in reducing the impact of humidity on the performance of SnO2-Based gas sensors: Synthesis strategies, and phenomenological and spectroscopic studies (Adv. Funct. Mater. 23/2011), Adv Funct Mater., 21 (2011), pp. 4402-4402. [19] R. Miao, W. Zeng, Q. Gao, Hydrothermal synthesis of novel NiO nanoflowers assisted with CTAB and SDS respectively and their gas-sensing properties, Mater Lett., 186 (2017), pp. 175-
177. [20] S.M. Majhi, G.K. Naik, H.-J. Lee, H.-G. Song, C.-R. Lee, I.-H. Lee, Y.-T. Yu, Au@NiO coreshell nanoparticles as a p-type gas sensor: Novel synthesis, characterization, and their gas sensing properties with sensing mechanism, Sensor Actuat B-Chem., 268 (2018), pp. 223-231. [21] N.D. Hoa, C.M. Hung, N. Van Duy, N. Van Hieu, Nanoporous and crystal evolution in nickel oxide nanosheets for enhanced gas-sensing performance, Sensor Actuat B-Chem., 273 (2018),
IP T
pp. 784-793.
[22] F. Qu, W. Shang, D. Wang, S. Du, T. Thomas, S. Ruan, M. Yang, Coordination polymer-
xylene, ACS Appl Mater Inter., 10 (2018), pp. 15314-15321.
SC R
derived multishelled mixed Ni-Co oxide microspheres for robust and selective detection of
[23] V.A. Agubra, L. Zuniga, D. Flores, H. Campos, J. Villarreal, M. Alcoutlabi, A comparative
U
study on the performance of binary SnO2 /NiO/C and Sn/C composite nanofibers as alternative
N
anode materials for lithium ion batteries, Electrochimica Acta, 224 (2017), pp. 608-621.
A
[24] H. Derikvandi, A. Nezamzadeh-Ejhieh, Synergistic effect of p-n heterojunction, supporting
Sci., 490 (2017), pp. 314-327.
M
and zeolite nanoparticles in enhanced photocatalytic activity of NiO and SnO2, J Colloid Inter
ED
[25] J. Fan, M. Guerrero, A. Carretero-Genevrier, M.D. Baró, S. Suriñach, E. Pellicer, J. Sort, Evaporation-induced self-assembly synthesis of Ni-doped mesoporous SnO2 thin films with tunable room temperature magnetic properties, J Mater Chem C., 5 (2017), pp. 5517-5527.
PT
[26] K. Srinivas, S.M. Rao, P.V. Reddy, Structural, electronic and magnetic properties of Sn0.95Ni0.05O2 nanorods, Nanoscale., 3 (2011), pp. 642-653.
CC E
[27] Y. Zou, S. Chen, J. Sun, J. Liu, Y. Che, X. Liu, J. Zhang, D. Yang, Highly efficient gas sensor using a hollow SnO2 microfiber for triethylamine detection, Acs Sensors., 2 (2017), pp. 897902.
A
[28] X. Liu, L. Long, W. Yang, L. Chen, J. Jia, Facilely electrodeposited coral-like copper micro/nano-structure arrays with excellent performance in glucose sensing, Sensor Actuat B-Chem., 266 (2018), pp. 853-860. [29] F. Qu, H. Jiang, M. Yang, Designed formation through a metal organic framework route of ZnO/ZnCo2O4 hollow core-shell nanocages with enhanced gas sensing properties, Nanoscale., 8 (2016), pp. 16349-16356.
[30] K.T. Alali, J. Liu, Q. Liu, R. Li, Z. Li, P. Liu, K. Aljebawi, J. Wang, Tube in tube ZnO/ZnCo2O4 nanostructure synthesized by facile single capillary electrospinning with enhanced ethanol gassensing properties, RSC Adv., 7 (2017), pp. 11428-11438. [31] Y. Li, N. Luo, G. Sun, B. Zhang, H. Jin, L. Lin, H. Bala, J. Cao, Z. Zhang, Y. Wang, Synthesis of porous nanosheets-assembled ZnO/ZnCo2O4 hierarchical structure for TEA detection, Sensor Actuat B-Chem., 287 (2019), pp. 199-208.
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[32] S. Sharma, A. Kumar, N. Singh, D. Kaur, Excellent room temperature ammonia gas sensing
properties of n-MoS2/p-CuO heterojunction nanoworms, Sensor Actuat B-Chem., 275 (2018),
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pp. 499-507.
[33] W. Chen, X. Cheng, Z. Xin, S. Peng, X. Hu, K. Shimanoe, G. Lu, N. Yamazoe, Hierarchical α-Fe2O3/NiO composites with a hollow structure for a gas sensor, ACS Appl Mater Inter., 6
U
(2014), pp. 12031-12037.
N
[34] J.-H. Kim, J.-H. Lee, A. Mirzaei, H.W. Kim, S.S. Kim, Optimization and gas sensing
A
mechanism of n-SnO2-p-Co3O4 composite nanofibers, Sensor Actuat B-Chem., 248 (2017), pp. 500-511.
M
[35] J. Zhang, D. Zeng, Q. Zhu, J. Wu, Q. Huang, W. Zhang, C. Xie, Enhanced room temperature
ED
NO2 response of NiO-SnO2 nanocomposites induced by interface bonds at the p-n heterojunction, Phys Chem Chem Phys., 18 (2016), pp. 5386-5396. [36] X. Deng, L. Zhang, J. Guo, Q. Chen, J. Ma, ZnO enhanced NiO-based gas sensors towards
PT
ethanol, Mater Res Bull., 90 (2017), pp. 170-174. [37] S. Zhang, G. Sun, Y. Li, B. Zhang, L. Lin, Y. Wang, J. Cao, Z. Zhang, Continuously improved
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gas-sensing performance of SnO2/Zn2SnO4 porous cubes by structure evolution and further NiO decoration, Sensor Actuat B-Chem., 255 (2018), pp. 2936-2943.
A
[38] Z. Zhang, C. Shao, X. Li, C. Wang, M. Zhang, Y. Liu, Electrospun nanofibers of p-type NiO/ntype ZnO heterojunctions with enhanced photocatalytic activity, ACS Appl Mater Inter., 2 (2010), pp. 2915-2923.
[39] F. Qu, T. Thomas, B. Zhang, X. Zhou, S. Zhang, S. Ruan, M. Yang, Self-sacrificing templated formation of Co3O4/ZnCo2O4 composite hollow nanostructures for highly sensitive detecting acetone vapor, Sensor Actuat B-Chem., 273 (2018), pp. 1202-1210. [40] D. Zhang, N. Yin, B. Xia, Facile fabrication of ZnO nanocrystalline-modified graphene hybrid
nanocomposite toward methane gas sensing application, J Mater Sci-Mater El., 26 (2015), pp. 5937-5945. [41] D. Zhang, H. Chang, P. Li, R. Liu, Characterization of nickel oxide decorated-reduced graphene oxide nanocomposite and its sensing properties toward methane gas detection, J Mater Sci-Mater El., 27 (2016), pp. 3723-3730.
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PT
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M
A
N
U
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semiconductors: Overview, Sensor Actuat B-Chem., 192 (2014), pp. 607-627.
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[42] H.J. Kim, J.H. Lee, Highly sensitive and selective gas sensors using p-type oxide
Biographies Saisai Zhang received her master's degree in 2016. She is currently a doctoral candidate in the School of Materials Science and Engineering of 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
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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.
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He is currently an associate professor in the School of Materials Science and 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
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and lithium ion rechargeable battery.
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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
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the theory calculation and synthesis of metal oxide semiconducting materials and their applications in gas sensor.
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Yan Wang received her PhD degree of Chemistry in 2009 from Nankai University, China. She is currently an associate professor in the School of Safety Science and Engineering of Henan
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Polytechnic University, China. Her research topic is the design and synthesis of metal oxide materials and their applications in gas sensor.
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Jianliang Cao received his PhD degree of Chemistry in 2009 from Nankai University, China. He is currently an associate professor in the School of Materials Science and Engineering of Henan Polytechnic University, China. His research topic is the synthesis of nanostructured metal oxide
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materials and their applications in catalyst and gas sensor. Zhanying Zhang received his PhD degree of materials science and engineering in 1982. Now, 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 semiconducting materials.
Table and Figure captions
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Fig. 1. Schematic illustration for the formation of pure NiO PNSs and SnO2/NiO PNSs.
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Fig. 2. XRD patterns and TG/DTA curves of Ni(OH)2.
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Fig. 3. XRD patterns of (a) NiO PNSs, (b) SnO2/NiO-1, (c) SnO2/NiO-2 and (d) SnO2/NiO-3.
Fig. 4. SEM and TEM images of (a, b) Ni(OH)2, (c, d) pure NiO PNSs, and (e, f) AFM images of
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Ni(OH)2.
Fig. 5. (a) TEM, (b) HRTEM and (c) SEM images of SnO2/NiO-2, and (d) the EDS element
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mappings recorded from (c).
Fig. 6. (a) N2 adsorption-desorption isotherm and (b) pore-size distribution curves of NiO and
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SnO2/NiO-2.
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Fig. 7. XPS spectra of NiO and SnO2/NiO-2: (a) Survey, (b) O 1s, (c) Sn 3d and (d)Ni 2p spectrums.
Fig. 8. Temperature-dependent responses of the sensors based on different samples towards 500
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ppm CH4.
Fig. 9. (a)The dynamic response curves toward varied CH4 concentration of both sensors and (b) concentration-dependent responses of the sensors based on different samples at 330 oC; (c) Response transient of both sensors to 500 ppm CH4 and (d) stability measurements of the SnO2/NiO-
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2 sensor to 5000 ppm CH4.
Fig. 10. Energy band diagram of (a, b) pure NiO and (c, d) SnO2/NiO as exposure to air and CH4
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atmosphere, respectively.
Table 1 Compared CH4 sensing properties of different materials.
Sensor type
Operating temperature/oC
Concentration (ppm)
ZnO/rGO
190
NiO/ rGO
Tres/Trec
Ref.
4000
18.5%
~50s/60s
[40]
260
1000
15.2%
~16s/20s
[41]
Pd-SnO2 nanofibers
350
1000
4.5
30s/150s
[1]
Pd-SnO2/rGO
RT
12000
~9.3%
~5min/7min
[2]
SnO2 nanorods
150
[16]
330
24.9% ~48.5% 49.5% 15.2%
369s/-
SnO2/NiO porous nanosheets
1000 10000 7000 500
28s/44s
This work
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Response
S0925-4005(19)30371-5 https://doi.org/10.1016/j.snb.2019.03.024 SNB 26243
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
30 December 2018 5 March 2019 6 March 2019
Please cite this article as: Zhang S, Li Y, Sun G, Zhang B, Wang Y, Cao J, Zhang Z, Enhanced methane sensing properties of porous NiO nanaosheets by decorating with SnO2 , Sensors and amp; Actuators: B. Chemical (2019), https://doi.org/10.1016/j.snb.2019.03.024 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.
Enhanced methane sensing properties of porous NiO nanaosheets by decorating with SnO2
Saisai Zhang a, Yanwei Li a, Guang Sun a,b,c*, Bo Zhang a, Yan Wang b,c,
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Jianliang Cao b,c, Zhanying Zhang a,b,c*
a
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
State Key Laboratory Cultivation Bases for Gas Geology and Gas Control (Henan Polytechnic
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c
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Polytechnic University, Jiaozuo 454000, China.
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University), Jiaozuo 454000, China.
Corresponding
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author Tel.: +86 03913986952 E-mail address: [email protected] (Guang Sun), [email protected] (Zhanying Zhang)
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Graphical Abstract
Highlight
Pure and SnO2-decorated NiO porous nanosheets were synthesized via a sacrificial template method. The CH4 sensing properties of NiO was improved by composition evolution (decorating
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with SnO2). The improved gas-sensing mechanism was discussed.
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Abstract
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Two dimensional (2D) nanomaterials with porous structure have stimulated much research
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interest owing to their unique structure and fascinating physical and chemical properties. Here,
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ultrathin 2D porous nanosheets (PNSs) of pure and SnO2-decorated NiO with uniform hexagonal shape were synthesized by using the pre-synthesized Ni(OH)2 nanosheets precursor. The obtained
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SnO2/NiO PNSs were about 100~150 nm in size and their mean thickness was about 7.5 nm. The gas sensing properties of the prepared pure and SnO2-decorated NiO PNSs were investigated
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intensively. It was found that after decorating with different amount of SnO2, the sensor based on NiO PNSs showed an improved sensing properties to methane (CH4), and the optimal SnO2 content
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in the composite was determined to be 2 mol%. At the optimum working temperature of 330 oC, the SnO2/NiO-2 sensor showed higher response and faster response/recover speed towards 500 ppm CH4 than the pure NiO sensor. In addition, the SnO2/NiO-2 sensor also exhibited a good long-term
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stability within 28 days, demonstrating its potential application for CH4 detection. The porous structure and p-n junction related sensing mechanism of SnO2/NiO PNSs was also discussed. Keywords: 2D nanostructure; porous nanosheets; SnO2/NiO; p-n junction; CH4; gas sensor
1. Introduction
Methane (CH4), as a simple colorless and odorless organics, is the main component of natural gas and coal mine gas. As a kind of fuel, it is widely used in civil life and industrial production [1, 2]. It can be used as a raw material for the preparation of hydrogen, carbon black, carbon monoxide, acetylene and formaldehyde [3]. Besides, methane can be employed as the carbon source for producing solar cell and amorphous silicon film. However, once the concentration of CH4 is too high, the oxygen content will remarkably reduce which may result in headache, dizziness,
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inattention and suffocation [4]. In addition, as one of the gases resulting in climate changes, the
greenhouse effect of methane is 25 times greater than CO2 [5]. More seriously, when methane is
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mixed with air in coal mine production, it will be easy to explode with the volume concentration ranges from 4.9% to 15.4% [2, 3, 6]. Therefore, the rapid and real-time detection of CH4 is becoming more and more important in our daily life.
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In the past few decades, various kinds of gas sensors such as catalytic combustion sensor [7, 8],
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infrared sensor [9, 10] and semiconductor sensor [3, 11, 12] have been employed to detect CH4.
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Among them, metal oxide semiconductor (MOS) based sensor has been widely used in practical
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application with the advantages of low-cost, easy fabrication and integration, good safety and long service life in the comparison with the other kind of sensors [13-15]. Until now, there are many
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kinds of MOS have been reported to detect methane, including TiO2 [11], SnO2 [16], NiO [5] and so on. In contrast, p-type MOSs has received less attention because of their relative low gas response.
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Hübner suggested that when the morphological structure of the sensing material is identical, the response of a p-type MOSs to a given gas is usually larger than that of n-type MOSs [17]. Hence,
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p-type MOSs may be the promising potentials for practical applications for showing low humidity dependence and rapid recovery kinetics[18]. Among numerous MOS materials, NiO, as a typical p-type MOS, has received widespread
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attention and exhibits various application fields in gas sensors [19-22], lithium-ion batteries [23], catalysts [24], magnetic materials [25, 26] and so on. When applied as gas sensors, NiO exhibit considerable ability to detect some organic and inorganic gases. In order to improve the gas sensitivity of NiO, many attempts have been made. On the one hand, since the sensing properties of MOS are mostly dependent on its morphology and size, structure and composition, the preparation of MOS materials with unique structure has become a powerful strategy for improving the gas
sensitivity of MOS sensors. It’s reported that porous structure can provide more active sites and usually used to improve the sensitivity [27, 28]. On the other hand, there are some limitations to restrict the development of single MOS based sensor, such as low response and poor selectivity. Therefore, more and more researchers are working on the construction of composite sensitive materials, such as ZnO/ZnCo2O4 [29-31], MoS2/CuO [32], α-Fe2O3/NiO [33] and SnO2/Co3O4 [34], and the results indicated that the heterojunctions are beneficial for enhancing the gas sensitivity of
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p-type MOSs. Thus, we can also decorate NiO with an n-type MOS to construct composite sensitive
material. For instance, Zeng et al [35] reported the preparation of NiO/SnO2 nanocomposites
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through annealing the mixtures of Ni-Sn precursor and found that the composites showed an
enhanced NO2 sensitivity than bare NiO. Chen et al [36]successfully synthesized NiO/ZnO nanoplates and the gas sensing results showed that the ethanol sensitivity of NiO/ZnO composites
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is improved after decorating with ZnO. In recent years, some researchers reported that NiO or SnO2
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based sensors can be employed to fabricate sensors for detecting CH4 [5, 16]. For instance, Zhou et
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al. [5] prepared hierarchical NiO nanoflakes via hydrothermal method, the sensing performance
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results indicated that the hierarchical NiO nanoflakes sensor exhibited excellent selectivity and stability to CH4. However, besides the encouraging results that have been obtained, there are few
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reports about the preparation of porous SnO2/NiO nanosheets composites and their application as sensing materials to detect CH4.
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Here, we report the successful synthesis of NiO PNSs and SnO2/NiO PNSs (with different amounts of SnO2) through a facile hydrothermal route combined post-heat treatment. The SEM and
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TEM results indicate that it is easier to obtain NiO PNSs and SnO2/NiO PNSs by employing Ni(OH)2 precursors as template. The gas sensing tests demonstrate that SnO2/NiO PNSs present higher sensitivity to CH4 than pure NiO PNSs.
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2. Experimental section The chemicals of Nickel nitrate, polyvinylpyrrolidone (PVP) and ammonia were of analytical
grade and used without further purification. 2.1 Preparation of hexagonal Ni(OH)2 nanosheets The Ni(OH)2 nanosheets were prepared via a hydrothermal route. Typically, 0.2 g of PVP were dispersed into 80 mL of Ni(NO3)2 solution (0.04 mol/L) with vigorous stirring for 20 min. Then, a
desired amount of ammonia water was dropped into the above solution under vigorously stirring. After 1 h, the mixed solution was sealed into a 100 mL Teflon-lined autoclave and heated at 180 oC for 9 h. After cooling down to room temperature naturally, the precipitate was collected by centrifugation, washed with deionized water and ethanol for several times, and finally dried at 80 o
C to obtain the final Ni(OH)2 product. 2.2 Preparation of SnO2/NiO porous nanosheets (PNSs)
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The SnO2/NiO PNSs were synthesized through an immersion-calcination method by using as-
prepared Ni(OH)2 nanosheets as precursor. As shown in Fig.1, 0.092 g of as-prepared Ni(OH)2 (1
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mmol) was first dispersed in 10 ml of ethanol under ultrasonic condition, then, different amounts of
SnCl4·5H2O and urea were dissolved into above solution in sequence. After ultrasonic treatment for 1 h, the green precipitates were collected, washed with ethanol for three times, dried in an oven, and
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finally calcinated at 400 oC (heating rate: 2 oC/min) for 3 h to obtain the SnO2/NiO samples. By
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adjusting the using amount of SnCl4·5H2O, the with SnO2/NiO samples with the SnO2 contents of
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1, 2 and 3 mol % were prepared and labeled as SnO2/NiO-1, SnO2/NiO-2 and SnO2/NiO-3,
under the same condition.
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respectively. Pure NiO PNSs were also prepared by directly annealing the Ni(OH)2 nanosheets
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2.3 Fabrication and measurement of gas sensor
Gas-sensing tests were carried out on an intelligent gas-sensing analysis system of CGS-4TPS
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(Beijing Elite Tech Co., Ltd., China), which the sensor temperature can be adjusted by an external temperature control system. The sensor fabrication is similar to our previous works [37]. Simply,
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the prepared sample was mixed with deionized water to form a paste. Then, the paste was coated onto the ceramic substrate to form a thick uniform film. In order to stabilize the sensor, it would be aged at 260 oC for several days before sensing test. The response (S) of the sensor was defined as
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|Ra-Rg|/Ra*100, where Ra and Rg were the sensor resistance in air and target gas, respectively. And the conditions of the test chamber during the test were about 16% RH and 35 oC. 2.4 Characterization The phase of the obtained samples were analyzed by X-ray power diffraction (XRD) on a Bruker/D8Advance diffract meter with Cu Kα radiation, and thermogravimetry and differential scanning calorimetry (TG-DSC, Setaram Evolution 2400). The morphologies and microstructures
were examined by field-emission scanning electron microscopy (FSEM, Quanta 250 FEG) and transmission electron microscopy (TEM, JEOL JEM-2100 microscope). Atomic force microscopy (AFM) analysis was carried out by using a FM-Nanoview 6800 equipment (Suzhou, China). The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method on a Micromeritics Triatar 3020 apparatuses. The element analysis was carried out by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI electron spectrometer using Al Kα
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radiation). 3. Results and discussion
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3.1 Characteristics of the prepared samples
The formation of Ni(OH)2 in the hydrothermal step was confirmed by XRD. As shown in Fig. 2a, all the diffraction peaks can be well assigned to the standard data of Ni(OH)2 (JCPDS: 74-2075),
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and no other peaks from impurities were detected, indicating the high purity of the Ni(OH)2 product.
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Ni(OH)2 is thermolabile and can decompose to NiO at elevated temperature. So, thermal analysis
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was performed on the prepared Ni(OH)2 to make clear its thermal decomposition process. As shown
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in Fig. 2b, with the temperature increasing from 25 oC to 600 oC, a strong endothermic peak at 300~340 oC was observed (DTA curve), which corresponds to a weight loss of 17 % (TG curve).
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Such value is close to the theoretical weight loss (16.5%) of the phase transformation from Ni(OH)2 to NiO. Thus, based on above results, the calcination temperate was set at 400 oC to ensure a
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complete decommission of the Ni(OH)2 precursor, as well as a good crystallinity of the NiO product. Fig. 3 shows the XRD patterns of the prepared pure NiO and SnO2/NiO composites. In Fig. 3a, the
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strong diffraction peaks arising from bunsenite NiO (JCPDS: 71-1179, lattice constants of a=b=c=4.178) were clearly observed, demonstrating the formation of pure NiO with good crystallinity. In Fig. 3b-d, besides of the peaks from NiO, some broaden peaks arising from SnO2
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were also observed. Therein, the peaks located at 26.58◦, 33.87◦ and 51.77◦ can be indexed as the (110), (101) and (211) planes of SnO2 (JCPDS: 71-0652), respectively, demonstrating the coexistence of NiO and SnO2 phase. The FESEM and TEM analysis were carried out to investigate the structure and morphology of the prepared samples. The FESEM image showed in Fig. 4a reveals that the obtained Ni(OH)2 precursor is composted of a large number of hexagonal nanosheets with smooth surface. The TEM
image displayed in Fig. 4b further reveals that these observed nanosheets have solid structure. The size of Ni(OH)2 nanosheets measured from Fig. 4a and b were about 100~150 nm in diameter and 7~8 nm in thickness. In order to further confirm the thickness of the Ni(OH)2 nanosheets, atomic force microscope (AFM) analysis was performed and the results are shown in Fig. 4e and f. The height profile diagram displayed in Fig. 4f indicates the thickness of an individual nanosheet is about 7.5 nm, being consistence with the result given by FESEM analysis. Fig. 4c and d present the
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typical FESEM and TEM images of the obtained NiO product, respectively. As can be seen from Fig. 4c, the obtained NiO product has the similar morphology and size with the Ni(OH)2 precursor.
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While, from the TEM image displayed in Fig. 4 d, we can observe that many randomly dispersed
nanopores with the size about 2~18 nm were embedded on the NiO hexagonal nanosheets. The formation of these randomly dispersed nanopores can be put down to the thermal decomposition
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process of the Ni(OH)2 precursor. The selected area electron diffraction (SAED) pattern taken from
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the porous nanosheets is displayed in the inset of Fig. 4 d. The sharp and regular diffraction dots
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reveal the single-crystalline nature of the NiO porous nanosheets. Fig. 5 presents the representative
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TEM image taken from the SnO2/NiO-2 sample, in which the porous nanosheets structure was also observed, indicating the introduction a small amount of SnO2 has almost no influence on the
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morphology of NiO product. Fig. 5 b shows a typical HRTEM image taken from the SnO2/NiO PNS. The measured interplanar distances were 0.241 nm, 0.335 nm and 0.264 nm, corresponding to the
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(111) of NiO, (110) and (101) planes of SnO2, respectively. Moreover, at the interfaces of NiO and SnO2 nanoparticles, an obvious lattice connection was observed, demonstrating the formation of
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NiO-SnO2 p-n junctions in the porous nanosheets. The element composition of the SnO2/NiO PNSs were investigated by EDS. Fig. 5 d shows the EDS mappings taken from the area marked in Fig. 5c, in which Ni, O, and Sn elements with good dispersion were detected, further confirming that the
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porous nanosheets are composed of SnO2 and NiO and SnO2 was homogeneously distributed on the NiO PNSs. In addition, the observed C elements should attribute to the carbon conductive adhesive. The specific surface area and the porosity of the prepared samples were investigated with N2 adsorption-desorption analysis. The N2 adsorption-desorption isotherms (shown in Fig. 6a) of all samples display type IV cures with H2-type hysteresis loops according to the IUPAC. The cures of pore size distribution shown in Fig. 6b reveal that both samples have relatively pores with a size of
2~20 nm, which is consistent with the results of TEM. The BET calculated result shows that the specific surface areas of pure NiO and SnO2/NiO-2 are 51.977 and 68.182m2g-1, respectively. These results indicate that a higher surface area can be obtained when SnO2 nanoparticles decorated on the surface of NiO PNSs, which can provide more surface active sites and available space for gas molecules adsorption and surface reaction. XPS measurements were performed on pure NiO and SnO2/NiO-2 samples to investigate their
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surface composition and chemical state. Fig. 7a presents the survey spectra, in which the characteristic peaks of Ni and O were observed in the pure NiO, as well as the peaks of Ni, O and
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Sn in the SnO2/NiO-2. The unexpectedly existence of C is ascribed to the intrinsically introduced carbon source in XPS tests. Fig. 7b shows the O 1s spectra of the two samples. Both the O 1s spectra
are fitted into three peaks, and the binding energies at ~529.28 eV, ~530.88 eV and ~532.05 eV of
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pure NiO correspond to the lattice oxygen (OL), oxygen vacancies (OV), and chemisorbed oxygen
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species (OC) or OH species, respectively. In contrast, the binding energies at 531.03 eV of SnO2/NiO
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exhibit a little shift compared with pure NiO. The phenomenon may be put down to the different
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chemical environments of O in the composite oxides can lead to the characteristic peak shift [35]. In addition, the calculated content of OC in SnO2/NiO is higher than that in pure NiO, indicating
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that the introduction of SnO2 could create more active sites for gas adsorption and surface reaction. Subsequently, the Sn 3d high-resolution XPS spectra of SnO2/NiO are carried out as shown in Fig.
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7c. The sample exhibits two peaks with binding energies around 495.58 eV and 487.18 eV originated from Sn 3d3/2 and Sn 3d5/2, respectively. The detail Ni 2p XPS spectra of the both samples
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was present in Fig. 7d. It can be seen that Ni 2p spectra of both samples can be fitted into six peaks. The binding energies at 860.53 eV and 873.23 eV of SnO2/NiO are attributed to the Ni 2p3/2 and Ni 2p1/2 peaks, respectively, exhibiting a little shift (0.15 eV and 0.1 eV) compared with pure NiO. The
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little binding energy shift in the SnO2/NiO composites may be ascribed to the strong interaction between SnO2 and NiO due to the formation of p-n junctions, where the electrons will transfer from SnO2 to NiO [38, 39]. 3.2 Gas sensing characteristics The unique 2D porous nanosheets structure as well as the formed p-n junctions in the SnO2/NiO PNSs were expected to endow the composite materials with good sensing properties, so, the gas
sensitivity of the prepared SnO2/NiO PNSs towards CH4 were tested to explore their possible application, during which the pure NiO PNSs was used as a reference. Considering that the operating temperature can exert severe influences on the gas sensing properties of a MOS sensor, the responses of the sensors to 500 ppm CH4 were first measured to find out the optimum working temperature (OWT). As shown in Fig. 8, with the temperature increasing from 220 to 360 oC, all sensors exhibited a gradual increase in response and reached their maximum response vales at 330 oC. While,
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with further increasing the working temperature to 360 oC, the responses of the sensors decayed obviously. In the whole temperature range, the sensors based on SnO2/NiO PNSs always show
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higher response than that based on pure NiO PNSs, demonstrating the sensitization effect of n-type
SnO2 on p-type NiO. Moreover, at the OWT of 330 oC, the response of the sensor based on SnO2/NiO-2 is 15.2%, which is higher than that based on SnO2/NiO-1(12.3%) and SnO2/NiO-3
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(13.5%). Such result suggested that the optimal SnO2 content in the present SnO2/NiO composite
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was 2 mol %.
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The dynamic responses of the sensors to varied concentrations of CH4 were also tested. As
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depicted in Fig. 9a, once exposing to 500 ppm CH4, all sensors exhibit an obvious response, and the response amplitudes increased correspondingly with the concentration gradually increasing from
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500 to 7000 ppm, and then returned back to their initial values as the sensors were exposed to air atmosphere again, demonstrating the good response ability of the sensors to different concentrations
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of CH4. It is worth mentioning that the SnO2/NiO sensors always showed higher response amplitudes than the pure NiO sensor when they were exposed to different concentrations of CH4,
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further demonstrating the sensitization effect of n-type SnO2 on p-type NiO. The concentrationdependent responses of the sensors were plotted to exhibit the relationship between the response and gas concentration (presented in Fig. 9b). With increasing the concentration of CH4 from 500 to
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7000 ppm, the responses of the SnO2/NiO sensors exhibit higher response values than pure NiO sensor, and the highest CH4 sensing performance is obtained at an optimized SnO2 content of 2 mol%. For instance, the response of SnO2/NiO-2 is 48.9% to 7000 ppm CH4, which is about 3 times of pure NiO (15.4%). Such results indicating that through the component optimization, the improvement in the response of NiO PNSs can be achieved by the decoration of SnO2 on the NiO porous nanosheets. In addition, it can be also observed that the response of the sensors undergone a
rapid increase (ranging from500 to 2000 ppm) and then the increase speed slowed down with further increasing the CH4 concentration, which could be explained by the dynamic equilibrium between the re-adsorbed oxygen species and consumed adsorbed oxygen species by reacting with CH4 species [39]. Fig. 9c displays the dynamic response-recovery transient of SnO2/NiO-2 and pure NiO sensors at 330 oC. As we can see that once exposing to 500 ppm CH4, the resistances of both sensors increased
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immediately, then reach fast to a steady state, and finally returned back to their original values as
the CH4 gas was released, exhibiting the typical response of a p-type semiconductor. The response
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and recovery times were defined as the time required for a change in sensor resistance to reach 90 % of the equilibrium state value after injecting and removing the tested gas, respectively. Thus, according to the results given by Fig. 9c, the response/recover times of SnO2/NiO-2 were measured
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to be 28/44 s, which are shorter than that of the pure NiO sensor (30/68 s). The faster
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response/recover speed of SnO2/NiO-2 makes it easier for real time detection in practical application.
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In addition, the sensor resistance in air (Ra) of SnO2/NiO-2 (2.2 KΩ) is higher than that of pure NiO
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(0.93 KΩ), which may be attributed to the NiO-SnO2 p-n junctions formed on the porous SnO2/NiO nanosheets. The CH4 sensing performance of SnO2/NiO-2 was compared with other reported
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materials. As shown in Table 1, the present sensor can give a comparable response and faster response-recovery speed to a lower CH4 concentration.
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The good long-term stability is also an important parameter for a successful sensor. Thus, the long-term stability test of the SnO2/NiO-2 sensor was carried out to further evaluate its quality, as
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shown in Fig. 9d. The response vales only changed slightly within 28 days, demonstrating its good long-term stability. On the basis of its good stability, the present SnO2/NiO-2 sensor may be the potential candidate for practical application.
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3.2 Sensing mechanism of pure NiO and SnO2/NiO PNSs NiO is a typical p-type semiconductor and its sensing mechanism can be explained by the surface-
resistance controlled model [42]. Take our experiment as an example, in air atmosphere, oxygen molecules will be absorbed on the surface of NiO PNSs and form chemisorbed oxygen ions (O2-, Oand O2-) by trapping electrons from the conduction band of NiO, which will lead to the increase of the hole concentration and create a hole accumulation layer (HAL) on the surface of NiO PNSs.
Since positive holes are the dominant charge in NiO, the formation of HAL can result in a lower sensor resistance (Ra). While, as the sensor is exposed to reducing gases, such as CH4 in our case, the chemisorbed oxygen ions will react with CH4 to produce oxidation products, after which the trapped electrons by chemisorbed oxygen will be released and return back to NiO to neutralize some of the holes in HAL. As a result, the wideness of HAL is decreased and a relative higher sensor resistance (Rg) is obtained, as shown in Fig. 10 a and b. According to the definition of response, the
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varied resistance NiO PNSs in air and CH4 atmospheres eventually endows the materials with the ability to response CH4.
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When SnO2 nanoparticles were decorated on the NiO PNSs, a remarkably enhanced gas response was observed. The enhanced response of SnO2/NiO PNSs towards CH4 can be mainly attributed to the p-n junction formed on the SnO2/NiO PNSs. From Fig. 9c, one can see that after decorating NiO
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PNSs with a small amount of n-type SnO2, the composite material still exhibited a p-type response,
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which indicated that the response of SnO2/NiO PNSs towards CH4 is mainly controlled by the
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resistance change of dominant NiO. So, the gas response of SnO2/NiO PNSs was mainly determined
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by the varied HAL thickness of NiO. In addition, when SnO2 is introduced into NiO PNSs, p-n junctions are formed between NiO and SnO2, which has been confirmed by HRTEM analysis (Fig.
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5b). The formation of p-n junctions can bring important influences on the gas sensing process of SnO2/NiO PNSs. On one hand, because the Fermi level of SnO2 is higher than that of NiO, at the
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interface of p-n junctions, electrons will flow from SnO2 to NiO and holes will flow along the opposite direction until the Fermi level of the two materials reaches equilibrium, as illustrated in
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Fig. 10 c. In this case, the hole concentration in NiO will decrease and the wideness of HAL will become narrow correspondingly, leading to a higher resistance of SnO2/NiO as compared with pure NiO (Fig. 10a and b). As shown in Fig. 9c, the resistance value of SnO2/NiO-2 is about 2.2 KΩ (330 which is higher than that of NiO (0.93 KΩ). On the other hand, due to the different crystal
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oC),
lattice parameters of NiO and SnO2, a lot of defects should be created at the interface of NiO (p)SnO2 (n) junctions, which will become potential active sites for gas adsorption. Thus, more gas molecules could absorb on the surface of SnO2/NiO PNSs and then take part in the following reactions between chemisorbed oxygen and CH4 molecules, resulting in a much decreased wideness of HAL (shown in Fig. 10d), as well as a much higher resistance of the SnO2/NiO sensor as
compared with pure NiO sensor, because more electrons will be released back to NiO to neutralize holes. In addition, because of the higher Fermi level of SnO2 than NiO, electrons will flow from SnO2 to NiO after the chemical reaction between chemisorbed oxygen and CH4 molecules, which will result in further decrease of the hole concentration in NiO. At this moment, the sensor resistance of SnO2/NiO PNSs can be further increased correspondingly. The unique porous nanosheets structure is also considered to be helpful for the gas sensing
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properties of the SnO2/NiO composite. As shown in Fig. 5a, a lot of nano-sized pores were creased
on the SnO2/NiO nanosheets. These nanopores can not only provide much larger surface area for
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gas adsorption, but also provide enough channels for gas diffusion and transportation and then
facilitate gas adsorption and desorption process in sensitive materials. Additionally, the mean thickness of the porous SnO2/NiO nanosheets is only 7~8 nm. Such thin structure can also ensure
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the thickness of the HAL occupies almost the entire material in some extent, resulting in further
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reduction of carriers concentration. Thus a higher material resistance would be obtained, which is
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contribution to improving the sensor response.
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4. Conclusion
In summary, the SnO2 nanoparticles were successfully decorated on the NiO PNSs via a
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hydrothermal method combined with post-heat treatment. The prepared NiO PNSs were about 100~150 nm in diameter and 7.5 nm in thickness. The successful decoration of SnO2 nanoparticles
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on NiO PNSs were confirmed by XRD, XPS, TEM and SEM. The as-prepared pure NiO PNSs and SnO2/NiO PNSs (with different SnO2 contents) were applied as sensing materials for detecting CH4.
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It was found that at the optimum working temperatures of 330 oC, the sensor based on SnO2/NiO PNSs with the optimal SnO2 content of 2 mol % exhibited higher response and rapid responserecover speed to CH4 in comparison with pure NiO PNSs. The improved sensitivity of SnO2/NiO
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PNSs can be attributed to the porous structure and the p-n heterojunctions. Our research not only provides a reliable route for fabricating SnO2/NiO PNSs, but also demonstrates that decorating with n-type is an effective way to enhance the CH4 sensitivity of p-type NiO. Acknowledgement This work is supported by the National Natural Science Foundation of China (U1704255), Program for Science & Technology Innovation Talents in Universities of Henan Province
(18HASTIT010, 17HASTIT029), Young Core Instructor Project of Colleges and Universities in Henan Province (2016GGJS-040), Foundation of Henan Scientific and Technology key project (182102310892), the Education Department Natural Science Foundation of fund Henan province (16A150051), and the Program for Innovative Research Team of Henan Polytechnic University (T2019-1, T2018-2).
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Reference [1] W. Lu, D. Ding, Q. Xue, Y. Du, Y. Xiong, J. Zhang, X. Pan, W. Xing, Great enhancement of
CH4 sensitivity of SnO2 based nanofibers by heterogeneous sensitization and catalytic effect,
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Sensor Actuat B-Chem., 254 (2018), pp. 393-401.
[2] S. Nasresfahani, M.H. Sheikhi, M. Tohidi, A. Zarifkar, Methane gas sensing properties of Pddoped SnO2/reduced graphene oxide synthesized by a facile hydrothermal route, Mater Res Bull.,
U
89 (2017), pp. 161-169.
N
[3] N.M. Vuong, N.M. Hieu, H.N. Hieu, H. Yi, D. Kim, Y.-S. Han, M. Kim, Ni2O3-decorated SnO2
A
particulate films for methane gas sensors, Sensor Actuat B-Chem., 192 (2014), pp. 327-333.
M
[4] S. Ghosh, C. Roychaudhuri, R. Bhattacharya, H. Saha, N. Mukherjee, Palladium-silver-activated ZnO surface: highly selective methane sensor at reasonably low operating temperature, ACS
ED
Appl Mater Inter., 6 (2014), pp. 3879-3887.
[5] Q. Zhou, Z. Lu, Z. Wei, L. Xu, Y. Gui, W. Chen, Hydrothermal synthesis of hierarchical ultrathin
PT
NiO nanoflakes for high-performance CH4 sensing, Front Chem, 6 (2018). [6] D. Haridas, V. Gupta, Enhanced response characteristics of SnO2 thin film based sensors loaded
CC E
with Pd clusters for methane detection, Sensor Actuat B-Chem., 166-167 (2012), pp. 156-164. [7] D. Nagai, M. Nishibori, T. Itoh, T. Kawabe, K. Sato, W. Shin, Ppm level methane detection using micro-thermoelectric gas sensors with Pd/Al2O3 combustion catalyst films, Sensor Actuat
A
B-Chem., 206 (2015), pp. 488-494.
[8] L. Sun, F. Qiu, B. Quan, Investigation of a new catalytic combustion-type CH4 gas sensor with low power consumption, Sensor Actuat B-Chem., 66 (2000), pp. 289-292.
[9] Z. Zhu, Y. Xu, B. Jiang, A one ppm NDIR methane gas sensor with single frequency filter denoising algorithm, Sensors-basel 12 (2012), pp. 12729-12740. [10] M. Dong, C. Zheng, S. Miao, Y. Zhang, Q. Du, Y. Wang, F.K. Tittel, Development and
measurements of a mid-infrared multi-gas sensor system for CO, CO2 and CH4 detection, in: Sensors-Basel., 17(2017), pp. 2221. [11] Z.P. Tshabalala, K. Shingange, B.P. Dhonge, O.M. Ntwaeaborwa, G.H. Mhlongo, D.E. Motaung, Fabrication of ultra-high sensitive and selective CH4 room temperature gas sensing of TiO2 nanorods: Detailed study on the annealing temperature, Sensor Actuat B-Chem., 238 (2017), pp. 402-419.
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[12] O. Lupan, O. Lupan, V. Postica, J. Gröttrup, A.K. Mishra, N.H. de Leeuw, J.F. Carreira, J.
Rodrigues, S.N. Ben, M.R. Correia, Hybridization of zinc oxide tetrapods for selective gas
SC R
sensing applications, ACS Appl Mater Inter., 9 (2017), pp. 4084-4099.
[13] C. Zhao, H. Gong, W. Lan, R. Ramachandran, H. Xu, S. Liu, F. Wang, Facile synthesis of SnO2 hierarchical porous nanosheets from graphene oxide sacrificial scaffolds for high-
U
performance gas sensors, Sensor Actuat B-Chem., 258 (2018), pp. 492-500.
N
[14] Y. Gong, Y. Wang, G. Sun, T. Jia, L. Jia, F. Zhang, L. Lin, B. Zhang, J. Cao, Z. Zhang, Carbon
A
nitride decorated ball-flower like Co3O4 hybrid composite: Hydrothermal synthesis and ethanol gas sensing application, Nanomaterials-basel., 8 (2018), pp. 132.
M
[15] F. Ren, L. Gao, Y. Yuan, Y. Zhang, A. Alqrni, O.M. Al-Dossary, J. Xu, Enhanced BTEX gas-
ED
sensing performance of CuO/SnO2 composite, Sensor Actuat B-Chem., 223 (2016), pp. 914920.
[16] M. Kooti, S. Keshtkar, M. Askarieh, A. Rashidi, Progress toward a novel methane gas sensor
96-106.
PT
based on SnO2 nanorods-nanoporous graphene hybrid, Sensor Actuat B-Chem., 281 (2019), pp.
CC E
[17] M. Hübner, C.E. Simion, A. Tomescu-Stănoiu, S. Pokhrel, N. Bârsan, U. Weimar, Influence of humidity on CO sensing with p-type CuO thick film gas sensors, Sensor Actuat B-Chem., 153 (2011), pp. 347-353.
A
[18] H.-R. Kim, A. Haensch, I.-D. Kim, N. Barsan, U. Weimar, J.-H. Lee, Gas Sensors: The role of NiO doping in reducing the impact of humidity on the performance of SnO2-Based gas sensors: Synthesis strategies, and phenomenological and spectroscopic studies (Adv. Funct. Mater. 23/2011), Adv Funct Mater., 21 (2011), pp. 4402-4402. [19] R. Miao, W. Zeng, Q. Gao, Hydrothermal synthesis of novel NiO nanoflowers assisted with CTAB and SDS respectively and their gas-sensing properties, Mater Lett., 186 (2017), pp. 175-
177. [20] S.M. Majhi, G.K. Naik, H.-J. Lee, H.-G. Song, C.-R. Lee, I.-H. Lee, Y.-T. Yu, Au@NiO coreshell nanoparticles as a p-type gas sensor: Novel synthesis, characterization, and their gas sensing properties with sensing mechanism, Sensor Actuat B-Chem., 268 (2018), pp. 223-231. [21] N.D. Hoa, C.M. Hung, N. Van Duy, N. Van Hieu, Nanoporous and crystal evolution in nickel oxide nanosheets for enhanced gas-sensing performance, Sensor Actuat B-Chem., 273 (2018),
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pp. 784-793.
[22] F. Qu, W. Shang, D. Wang, S. Du, T. Thomas, S. Ruan, M. Yang, Coordination polymer-
xylene, ACS Appl Mater Inter., 10 (2018), pp. 15314-15321.
SC R
derived multishelled mixed Ni-Co oxide microspheres for robust and selective detection of
[23] V.A. Agubra, L. Zuniga, D. Flores, H. Campos, J. Villarreal, M. Alcoutlabi, A comparative
U
study on the performance of binary SnO2 /NiO/C and Sn/C composite nanofibers as alternative
N
anode materials for lithium ion batteries, Electrochimica Acta, 224 (2017), pp. 608-621.
A
[24] H. Derikvandi, A. Nezamzadeh-Ejhieh, Synergistic effect of p-n heterojunction, supporting
Sci., 490 (2017), pp. 314-327.
M
and zeolite nanoparticles in enhanced photocatalytic activity of NiO and SnO2, J Colloid Inter
ED
[25] J. Fan, M. Guerrero, A. Carretero-Genevrier, M.D. Baró, S. Suriñach, E. Pellicer, J. Sort, Evaporation-induced self-assembly synthesis of Ni-doped mesoporous SnO2 thin films with tunable room temperature magnetic properties, J Mater Chem C., 5 (2017), pp. 5517-5527.
PT
[26] K. Srinivas, S.M. Rao, P.V. Reddy, Structural, electronic and magnetic properties of Sn0.95Ni0.05O2 nanorods, Nanoscale., 3 (2011), pp. 642-653.
CC E
[27] Y. Zou, S. Chen, J. Sun, J. Liu, Y. Che, X. Liu, J. Zhang, D. Yang, Highly efficient gas sensor using a hollow SnO2 microfiber for triethylamine detection, Acs Sensors., 2 (2017), pp. 897902.
A
[28] X. Liu, L. Long, W. Yang, L. Chen, J. Jia, Facilely electrodeposited coral-like copper micro/nano-structure arrays with excellent performance in glucose sensing, Sensor Actuat B-Chem., 266 (2018), pp. 853-860. [29] F. Qu, H. Jiang, M. Yang, Designed formation through a metal organic framework route of ZnO/ZnCo2O4 hollow core-shell nanocages with enhanced gas sensing properties, Nanoscale., 8 (2016), pp. 16349-16356.
[30] K.T. Alali, J. Liu, Q. Liu, R. Li, Z. Li, P. Liu, K. Aljebawi, J. Wang, Tube in tube ZnO/ZnCo2O4 nanostructure synthesized by facile single capillary electrospinning with enhanced ethanol gassensing properties, RSC Adv., 7 (2017), pp. 11428-11438. [31] Y. Li, N. Luo, G. Sun, B. Zhang, H. Jin, L. Lin, H. Bala, J. Cao, Z. Zhang, Y. Wang, Synthesis of porous nanosheets-assembled ZnO/ZnCo2O4 hierarchical structure for TEA detection, Sensor Actuat B-Chem., 287 (2019), pp. 199-208.
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[32] S. Sharma, A. Kumar, N. Singh, D. Kaur, Excellent room temperature ammonia gas sensing
properties of n-MoS2/p-CuO heterojunction nanoworms, Sensor Actuat B-Chem., 275 (2018),
SC R
pp. 499-507.
[33] W. Chen, X. Cheng, Z. Xin, S. Peng, X. Hu, K. Shimanoe, G. Lu, N. Yamazoe, Hierarchical α-Fe2O3/NiO composites with a hollow structure for a gas sensor, ACS Appl Mater Inter., 6
U
(2014), pp. 12031-12037.
N
[34] J.-H. Kim, J.-H. Lee, A. Mirzaei, H.W. Kim, S.S. Kim, Optimization and gas sensing
A
mechanism of n-SnO2-p-Co3O4 composite nanofibers, Sensor Actuat B-Chem., 248 (2017), pp. 500-511.
M
[35] J. Zhang, D. Zeng, Q. Zhu, J. Wu, Q. Huang, W. Zhang, C. Xie, Enhanced room temperature
ED
NO2 response of NiO-SnO2 nanocomposites induced by interface bonds at the p-n heterojunction, Phys Chem Chem Phys., 18 (2016), pp. 5386-5396. [36] X. Deng, L. Zhang, J. Guo, Q. Chen, J. Ma, ZnO enhanced NiO-based gas sensors towards
PT
ethanol, Mater Res Bull., 90 (2017), pp. 170-174. [37] S. Zhang, G. Sun, Y. Li, B. Zhang, L. Lin, Y. Wang, J. Cao, Z. Zhang, Continuously improved
CC E
gas-sensing performance of SnO2/Zn2SnO4 porous cubes by structure evolution and further NiO decoration, Sensor Actuat B-Chem., 255 (2018), pp. 2936-2943.
A
[38] Z. Zhang, C. Shao, X. Li, C. Wang, M. Zhang, Y. Liu, Electrospun nanofibers of p-type NiO/ntype ZnO heterojunctions with enhanced photocatalytic activity, ACS Appl Mater Inter., 2 (2010), pp. 2915-2923.
[39] F. Qu, T. Thomas, B. Zhang, X. Zhou, S. Zhang, S. Ruan, M. Yang, Self-sacrificing templated formation of Co3O4/ZnCo2O4 composite hollow nanostructures for highly sensitive detecting acetone vapor, Sensor Actuat B-Chem., 273 (2018), pp. 1202-1210. [40] D. Zhang, N. Yin, B. Xia, Facile fabrication of ZnO nanocrystalline-modified graphene hybrid
nanocomposite toward methane gas sensing application, J Mater Sci-Mater El., 26 (2015), pp. 5937-5945. [41] D. Zhang, H. Chang, P. Li, R. Liu, Characterization of nickel oxide decorated-reduced graphene oxide nanocomposite and its sensing properties toward methane gas detection, J Mater Sci-Mater El., 27 (2016), pp. 3723-3730.
A
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PT
ED
M
A
N
U
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semiconductors: Overview, Sensor Actuat B-Chem., 192 (2014), pp. 607-627.
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[42] H.J. Kim, J.H. Lee, Highly sensitive and selective gas sensors using p-type oxide
Biographies Saisai Zhang received her master's degree in 2016. She is currently a doctoral candidate in the School of Materials Science and Engineering of 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
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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.
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He is currently an associate professor in the School of Materials Science and 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
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and lithium ion rechargeable battery.
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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
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the theory calculation and synthesis of metal oxide semiconducting materials and their applications in gas sensor.
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Yan Wang received her PhD degree of Chemistry in 2009 from Nankai University, China. She is currently an associate professor in the School of Safety Science and Engineering of Henan
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Polytechnic University, China. Her research topic is the design and synthesis of metal oxide materials and their applications in gas sensor.
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Jianliang Cao received his PhD degree of Chemistry in 2009 from Nankai University, China. He is currently an associate professor in the School of Materials Science and Engineering of Henan Polytechnic University, China. His research topic is the synthesis of nanostructured metal oxide
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materials and their applications in catalyst and gas sensor. Zhanying Zhang received his PhD degree of materials science and engineering in 1982. Now, 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 semiconducting materials.
Table and Figure captions
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Fig. 1. Schematic illustration for the formation of pure NiO PNSs and SnO2/NiO PNSs.
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Fig. 2. XRD patterns and TG/DTA curves of Ni(OH)2.
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Fig. 3. XRD patterns of (a) NiO PNSs, (b) SnO2/NiO-1, (c) SnO2/NiO-2 and (d) SnO2/NiO-3.
Fig. 4. SEM and TEM images of (a, b) Ni(OH)2, (c, d) pure NiO PNSs, and (e, f) AFM images of
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Ni(OH)2.
Fig. 5. (a) TEM, (b) HRTEM and (c) SEM images of SnO2/NiO-2, and (d) the EDS element
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mappings recorded from (c).
Fig. 6. (a) N2 adsorption-desorption isotherm and (b) pore-size distribution curves of NiO and
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SnO2/NiO-2.
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Fig. 7. XPS spectra of NiO and SnO2/NiO-2: (a) Survey, (b) O 1s, (c) Sn 3d and (d)Ni 2p spectrums.
Fig. 8. Temperature-dependent responses of the sensors based on different samples towards 500
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ppm CH4.
Fig. 9. (a)The dynamic response curves toward varied CH4 concentration of both sensors and (b) concentration-dependent responses of the sensors based on different samples at 330 oC; (c) Response transient of both sensors to 500 ppm CH4 and (d) stability measurements of the SnO2/NiO-
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2 sensor to 5000 ppm CH4.
Fig. 10. Energy band diagram of (a, b) pure NiO and (c, d) SnO2/NiO as exposure to air and CH4
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atmosphere, respectively.
Table 1 Compared CH4 sensing properties of different materials.
Sensor type
Operating temperature/oC
Concentration (ppm)
ZnO/rGO
190
NiO/ rGO
Tres/Trec
Ref.
4000
18.5%
~50s/60s
[40]
260
1000
15.2%
~16s/20s
[41]
Pd-SnO2 nanofibers
350
1000
4.5
30s/150s
[1]
Pd-SnO2/rGO
RT
12000
~9.3%
~5min/7min
[2]
SnO2 nanorods
150
[16]
330
24.9% ~48.5% 49.5% 15.2%
369s/-
SnO2/NiO porous nanosheets
1000 10000 7000 500
28s/44s
This work
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Response