Effective hydrogen gas sensor based on [email protected] nanocomposite

Accepted Manuscript Title: Effective hydrogen gas sensor based on NiO@rGO nanocomposite Authors: Haibo Ren, Cuiping Gu, Sang Woo Joo, Jingjuan Zhao, Yufeng Sun, Jiarui Huang PII: DOI: Reference:

S0925-4005(18)30629-4 https://doi.org/10.1016/j.snb.2018.03.130 SNB 24415

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

30-9-2017 17-3-2018 22-3-2018

Please cite this article as: Haibo Ren, Cuiping Gu, Sang Woo Joo, Jingjuan Zhao, Yufeng Sun, Jiarui Huang, Effective hydrogen gas sensor based on NiO@rGO nanocomposite, Sensors and Actuators B: Chemical https://doi.org/10.1016/j.snb.2018.03.130 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.

Effective hydrogen gas sensor based on NiO@rGO nanocomposite

Haibo Ren1,2,†, Cuiping Gu1,†, Sang Woo Joo2,*, Jingjuan Zhao1, Yufeng Sun3,* and Jiarui Huang1,*

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Key Laboratory of Functional Molecular Solids, Ministry of Education, Center for Nano Science

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and Technology, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241002, P. R. China.

School of Mechanical Engineering, Yeungnam University, Gyeongsan 38541, R. Korea.

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School of Mechanical and Automotive Engineering, Anhui Polytechnic University Wuhu, 241000,

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Corresponding authors. E-mail: [email protected] (J.R. Huang); [email protected] (S.W. Joo),

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*

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P.R. China

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[email protected] (Y.F. Sun). † Equal contribution as the first author.

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Highlights

NiO@rGO nanocomposite consisting of NiO NPs uniformly anchored on rGO is developed.

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The preparation method involves freeze-drying followed by heat treatment.

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NiO@rGO sensor shows high response and selectivity to H2 at low working temperature.

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High performance is ascribed to the electron transfer between NiO NPs and rGO.

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Abstract: A NiO@rGO nanocomposite is prepared by a freeze-drying method combined with heat treatment. The morphology and structure are analyzed through X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman spectra, and X-ray photoelectron spectroscopy. The results clearly show that NiO nanoparticles are uniformly anchored on the

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surface of the rGO nanosheet. The gas-sensing performance of the nanocomposite is also investigated to detect hydrogen. The NiO@rGO sensor exhibits good gas-sensing performance with high sensitivity, fast response, good reversibility, and selectivity toward hydrogen. The relative response of the sensor to 1% hydrogen is 0.64, the response time is 28 s, and the recovery time is 142 s. The improved sensing performance is due to the effective electron transfer between the NiO

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nanoparticles and rGO nanosheet.

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Keywords: NiO@rGO nanocomposite; Freeze-drying method; Hydrogen; Gas sensor; Low

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working temperature

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1. Introduction

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Hydrogen gas is widely used in various commercial and industrial applications as a fuel or

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reducing agent in epitaxial chemical reactions. The rapid development of hydrogen as a clean energy source for fuel cell vehicles, spacecrafts, automobiles, and aircrafts has been noted in recent

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years [1-3]. However, hydrogen is an odorless, flammable, and explosive gas that cannot be detected by human nose or eyes even at a high explosive level concentration of 4% [4]. Also, the

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molecules of H2 are very small and diffuse very easily into air. Hydrogen gas leakage can result in disastrous consequences and disasters. Therefore, sensors with high sensitivity, good selectivity and

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low working temperature for the detection of H2 are necessary during production, storage, transportation, and use of hydrogen in both stationary and mobile applications.

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Resistive gas sensors based on the change in conductivity of semiconductors upon exposure to gaseous molecules are the most beneficial because of their simple, low cost, real-time monitoring, and easy application [5]. Various semiconducting metal oxides including n-type (TiO2, WO3, ZnO, and SnO2) and p-type (CuO, Co3O4 and NiO) have been developed for resistive H2 sensor applications [6,7]. Among these metal oxides, NiO is a typical p-type semiconductor with a band

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gap of 3.5 eV. It has been studied as a good material due to its remarkable electrical properties [8,9], thermal stability [10], and small electron affinity [11]. NiO nanofilm [12], NiO-ZnO nanowires [13] and Pd modified NiO mesoporous nanosheets [14] have been developed for H2 detection. However, their usual high working temperature requirement over 250°C limits their application. As well as we know, compositing of the materials is a common method which is adopted for the improvement of

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some properties of the sensing materials.

Reduced graphene oxide (rGO) has attracted great interest due to its advantages of large surface

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area [15,16], excellent thermal and electrical properties [17], and mass production [18]. Besides from pure form, rGO is an ideal choice for loading into some other materials to give various

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applications such as energy storage [19,20], photoconductive switching [21], bioimaging [22], and

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gas sensors [23]. As a sensing material, chemical bonds on the surface of rGO weakly interact with

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gas molecules, which greatly reduces the sensitivity and selectivity of rGO when trying to detect a

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gas [24]. To enhance the sensing properties of the sensors based on rGO, an effective method was functionalized with some metal oxide[25], noble metal[26], or polymer[27]. For example,

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Khaleed et al reported a NiO/graphene foam electrode, which displayed good sensing performance towards reductive gas CO at a low working temperature [28]. Yang et al synthesized a hierarchical

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NiO cube/nitrogen-doped reduced graphene oxide composite for H2S detection at a low operating temperature [29]. Similarly, Zhang et al reported that nickel oxide decorated-reduced graphene

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oxide nanocomposite detected methane gas [30]. Noble metals and metal oxide have been employed to fabricate catalytic combustion sensors

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because of their high catalytic activity to flammable gases [5]. As for the metal oxide based gas sensor, SnO2 and WO3 are the best materials for H2 sensing, particularly when decorated with Pd and Pt [31-36]. Recently, many kinds of graphene based nanocomposites including noble metals (Pt and Pd) and metal oxides (TiO2, ZnO, SnO2, MoS2, and NiO) have been developed for H2 detection [5,37-45]. For example, Russo et al synthesized SnO2/rGO for H2 detection at room temperature

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[41]. Kamal reported that NiO/rGO hybrid was used in detecting H2 at lower operating temperatures [43]. The comparison of sensing performance of various graphene based nanocomposites and some metal oxides was shown in Table S1 in the supporting information, from which it can be seen that most of the noble metal/rGO sensors operated at room temperature. However, most metal oxide/rGO hybrid sensors operated at high temperature. Although it is clear from the literature

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survey that mixing of graphene with other noble metal or metal oxides can sense H2 at lower operating temperatures, there is strong need to develop a good and reliable H2 sensor. The

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morphology and the structure of the sensing materials significantly influence their sensing

performance, so it is necessary to design and prepare special morgphologies of graphene-based

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nanocomposites for improving its sensing performance.

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In present study, we successfully synthesized NiO@rGO nanocomposite by combing a

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freeze-drying route with a heat treatment process. The sensing properties of the nanocomposite

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were investigated for the detection of H2 at low working temperature. 2. Experimental details

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2.1 Sample preparation

Preparation of graphene oxide: GO was prepared by the oxidation of natural graphite flakes using

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the modified Hummer’s method [46]. First, concentrated H2SO4 (180 mL) and concentrated H3PO4 (20 mL) were mixed in an ice bath under continuous stirring. Then, 1.0 g of graphite and 6.0 g of

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KMnO4 were added to the mixture under stirring for 2 h. Subsequently, the mixture was kept at 50°C for 10 h. After cooling to room temperature, deionized water (400 mL) was added to the

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mixture until the solution turned brownish yellow. When the solution completely changed color, 30% hydrogen peroxide (3 mL) was added. The product was washed in ethanol and deionized for several times and centrifuged at 10000 rpm for 40 min to remove unexfoliated graphite and residual acid. After drying in a vacuum for 12 h, graphite oxide was obtained. Finally, 5 mg of graphite oxide was added to deionized water (20 mL) and ultrasonicated for 12 h to form a homogeneous

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mixture. Preparation of NiO@rGO nanocomposites: 4.0 mg of NiO nanoparticles (Aladdin Industrial Corporation, Shanghai) with a diameter of 20 nm were mixed with 5 mL of the GO mixture (0.4 mg mL-1). The mixture was ultrasonicated for 15 min to form a uniform solution and then freeze-dried at -50°C for 2 days. Finally, the NiO@rGO nanocomposites were obtained by heat treatment at

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200°C for 2 h. 2.2 Characterization

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The morphology and structure of the obtained products were investigated using scanning electron microscopy (SEM, Hitachi S-4800, operated at 5 kV), X-ray diffraction (Shimadzu

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XRD-6000, high-intensity Cu Kα radiation with characteristic wavelength of 1.54178 Å), and

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transmission electron microscopy (TEM, Hitachi H-800, operated at acceleration voltage of 200

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kV). The composition and oxidation state of the products were explored by X-ray photoelectron

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spectroscopy (XPS, Thermo ESCALAB 250 system) with an Al-K non-monochromatized X-ray source. The detailed structure was investigated using Raman spectra (Brooke-Senterra, wavelength

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of 532 nm).

2.3 Gas-sensor fabrication and response test

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The gas-sensor fabrication was very similar to our previous study [47]. The sample

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nanocomposites were first dispersed in an ethanol solution and then coated on the outer surface of an aluminum tube-like substrate with a pair of Au electrodes attached. They were then placed in an oven for 2 h at 50°C. To provide the working temperature of the gas sensor, a small Ni-Cr alloy coil

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heater was inserted into the tube. The sensors were kept at the working temperature (70°C) for 2 days to improve the long-term stability. A stationary-state gas distribution method was used to test the gas response. The sensor response measurement was performed on an electrochemical work station (CHI-660E, Shanghai Chenhua Instruments Co., Ltd). Hydrogen was injected into the test chamber and mixed with air. The sensor response (%) was defined as ∆R/RAir×100, where

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∆R=RHydrogen−RAir, RAir is the resistance of air, and RHydrogen is the resistance of air mixed with hydrogen. The response or recovery time was expressed as the time required for the sensor output to reach 90% saturation after applying or switching off the gas in a step function. The optimal operating temperature of the rGO nanocomposite sensor was 50°C, and all response measurements were performed at this temperature.

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3. Results and discussion 3.1 Structure and morphology

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The NiO@rGO nanocomposites were prepared via three steps. Scheme 1 shows the fabrication procedure of the NiO@rGO nanocomposites. Firstly, NiO nanoparticles were mixed with the GO

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solution. Secondly, the mixture was ultrasonicated to form a uniform solution and then freeze-dried.

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Finally, the NiO@rGO nanocomposites were obtained by a heat treatment process. XRD used to

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investigate the crystal phase and structure of the obtained products. Fig. 1 shows the XRD pattern of

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the NiO nanoparticles and NiO@rGO nanocomposites after treatment for 2 h at 200°C. The characteristic diffraction peaks at 79.3°, 75.2°, 62.5°, 43°, and 37° can correspond well to the (222),

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(311), (220), (200), and (111) planes of NiO nanoparticles, respectively, and they can be indexed to the structure of NiO (JCPDS NO.47-1049). When the nanocomposites were annealed for 2 h at

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200°C, a typical peak of the NiO@rGO nanocomposites was observed at 23.5°, which corresponds to the (002) plane of pure graphene [48]. In addition, the characteristic diffraction peak of GO at

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10° was not observed, which clearly suggests that the GO was completely transformed to rGO. The NiO nanoparticles were well dispersed on the surface of the rGO based on the fact that the width

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and diffraction peak intensity of NiO are higher than those of rGO. Fig. 2a shows the SEM image of the NiO@rGO nanocomposite. There are a few wrinkles on

the nanocomposite, and a large number of NiO nanoparticles were evenly distributed on the surface of the rGO. The surface of rGO shows small areas of agglomeration. Fig. 2b shows the TEM iamge of the NiO@rGO nanocomposite. The layered rGO nanosheet is transparent, and the numerous NiO

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nanoparticles were uniformly anchored on the surface. But some NiO nanoparticles were aggregated. The lattice fringe with spacing of 0.24 nm is confirmed by the high-resolution TEM (HRTEM) image (Fig. 2c), which is assigned to the plane (111). The selected area electronic diffraction (SAED) pattern (Fig. 2d) shows ring-like profiles that are indexed to crystal planes (111), (200), (220), (311), (222), (331), and (400), respectively, which is accordance with the XRD results

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in Fig. 1.

The Raman spectra of the GO and NiO@rGO nanocomposite are shown in Fig. 3. Two

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characteristic peaks at 1598 cm−1 (G-band) and 1335 cm−1 (D-band) were observed, which

correspond to graphitic and diamond structures, respectively. A D-band was formed because of the defects and disorder of the graphene structure. A G-band was formed due to the plane vibration of

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sp2 hybridized C-C bonds. The G peak in the NiO@rGO nanocomposite and GO is red shifted from

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1598 cm−1 to 1591 cm−1. The ID/IG values of the NiO@rGO and GO are 1.19 and 1.04, respectively, which suggest that the NiO nanoparticles were successfully hybridized with the rGO nanosheet. The

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reduction of GO to rGO [49].

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ID/IG value of the rGO is larger than that of GO because the average size of sp2 decreased after the

The XPS spectra are shown in Fig. 4. Fig. 4a shows three sharp peaks at 284.38, 530.11, and

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854.8 eV in the XPS spectra of the nanocomposite. These suggest that the nanocomposite consists of carbon, oxygen, and nickel. Fig. 4b shows peaks at Ni 2p, Ni 2p1/2 (approximately 872 eV), and

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Ni 2p3/2 (approximately 865 eV), which are attributed to spin orbital coupling [50]. Figs. 4c and d show the XPS spectra for rGO, and there are four sharp peaks at approximately 288.13, 287.01,

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286.69, and 284.48 eV. These correspond to O-C=O (carboxyl group), C=O (carbonyl group), C-O (epoxy and alkoxy), and C=C (sp2 carbon), respectively. The peak intensity of the C=C bond is much higher than those of the other chemical bonds, which further indicated that oxygen-containing functional groups of the NiO@rGO hybrid were successfully reduced in the preparation process. The results are accordance with the Raman spectroscopy results.

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3.2 Gas-sensing properties of NiO@rGO nanocomposite The nanocomposite was used to fabricate gas sensors. The operating temperature significantly affects the performance of semiconductor gas sensors. Therefore, selecting the optimal working temperature is a key step to measure the sensor performance. To determine the optimal operating temperature, the response of the NiO@rGO nanocomposite to a concentration of 3% hydrogen (H2)

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was tested as a function of working temperature, as shown in Fig. 5. It is clear that the response of the gas sensor varies with working temperature. In the range of 30 to 50°C, the sensor responses to

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3% H2 were increased with the increase of working temperature, and up to a maximum value of 3.2 at the working temperature of 50°C, which may be attributed to the thermal energy provided to

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overcome the barrier height of the depletion layer [51]. After that, the sensor responses were

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decreased with the increase of working temperatures. This probably happened because the majority

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of the gas desorption and electron excitation had already occurred [52]. Therefore, the optimal

were performed at this temperature.

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operating temperature of the rGO nanocomposite sensor was 50°C, and all response measurements

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The real-time response and response curve of the NiO@rGO sensor are shown in Fig. 6 for different concentrations of H2 at 50°C. Fig. 6a shows that the sensor has a high response to

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hydrogen. When the H2 concentration is very low (0.5%), the sensor shows an obvious response to H2 with a relative response of about 0.22. The standard deviation of the noise level is ca. 6.0 Ω, and

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the detection limits of hydrogen is approximately 0.3% (signal-to-noise ratio, S/N = 3), which is much lower than the limit concentration for hydrogen explosions (4%). When the H2 concentration

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increased to 0.7%, the relative response was about 0.51. As the H2 concentration increases, the response of the sensor increases remarkably, and the sensor response exhibited a fine linear relationship with the concentration of hydrogen (Fig. 6b). The sensor response increases linearly with concentration in the range of 0.5% to 10%. The linear equation can be expressed as S=-0.0018+0.6059(CHydrogen/v%), and the correlation coefficient (R2) is 0.9984, which suggests that

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the sensor could detect H2 gas at a low working temperature. S is the sensor response, and C is the concentration of the tested gas (H2). The sensitivity of the sensor determined using the slope of the calibration plot between 0.5% and 10% was found to be, on an average, 0.6059 v%−1. When the H2 concentration increases to 10%, the relative response is approximately 6.1. The response to 1% H2 is 0.64, and the response and recovery times of the sensor are 28 s and 142 s, respectively.

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Repeatability is a key basis for judging the performance of a sensor. To test the repeatability of the NiO@rGO sensor, 7 cycles of measurements were performed using the same sensor with a

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hydrogen concentration of 3% at the working temperature of 50°C, as shown in Fig. 7a. The

resistance of the sensor showed a remarkable response, and when the hydrogen was removed, the

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resistance quickly returned to the initial state. Even after many cycles of H2 absorption and

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desorption, the sensor still has the same response with no obvious decline. This indicated that the

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NiO nanoparticles were closely hybridized with the rGO sheet, resulting in good repeatability of the

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sensor.

To understand the effect of humidity on the response of NiO@rGO sensor, three different

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conditions 40, 58 and 70% of humidity conditioning at 18°C were investigated. The H2 gas sensing properties of the as-fabricated the NiO@rGO sensor at different relative humidity (RH) levels are

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shown in Fig. 7(b). It can be seen that the NiO@rGO sensor exhibits good response towards different concentrations of hydrogen as the humidity is varied from 40 to 70%, conditioning at 18°C.

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The response slightly increases in magnitude as the humidity increases. Generally, the humidity effect on the sensing material is negligible in this study. To verify the catalytic oxidation of

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hydrogen in present of the NiO@rGO nanocomposites, 0.01% H2 mixed with air at RH 70% conditioning at 18°C was loaded into a 50 ml flask loaded with 0.1 g of NiO@rGO nanocomposites. Then the flask was kept at 50°C for 12 h in a water bath. For controlled experiment, 0.01% H2 mixed with air at RH 70% conditioning at 18°C was loaded into a 50 ml flask. The setup was shown in Fig. S1. After that, the mixed gases was tested with a commercial gas sensor (MQ-3,

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Shenzhen Yuansheng Electronics Co., Ltd.) at a working temperature of ca. 200°C. The test result was shown in Fig. S2. It can be seen that the sensor response is obviously low after the mixed gases kept at 50°C for 12 h in present of the NiO@rGO nanocomposites. This result demonstrates the reaction of H2 with oxygen adsorbates at 50°C under wet environment. Selectivity is another key factor for determining sensor performance. Therefore, the NiO@rGO

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sensor was exposed to NH3, CO, CH4, HCHO, and EtOH gases at concentrations of 0.5, 0.7, 1.0, 1.5, 3.0, 5.0, 8.0, and 10% at the working temperature of 50°C. It was found that the NiO@rGO

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sensor showed no obvious response to these gases even at the highest concentration of 10%, meaning that the as-prepared NiO@rGO sensor has good selectivity to hydrogen. The good

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selectivity may have resulted from the much stronger competitive absorption ability of H2

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molecules than other molecules at the low working temperature.

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3.3 Gas-sensing mechanism of NiO@rGO nanocomposite sensor

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A gas sensing mechanism is proposed to explain the good performance of the NiO@rGO nanocomposite sensor, as shown in Fig. 8a. The gas-sensing reaction mainly involves the

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adsorption/desorption of gases, as well as electron transfer between oxygen ions (O-, O2- and O2-) absorbed on the face of the material and the detected gas. When the sensor is exposed to air, oxygen

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molecules can quickly spread and adsorb on the surface of the NiO@rGO sensing material. The adsorbed oxygen can extract electrons from the surface of the NiO@rGO material and form

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strongly active oxygen anions, as shown in Eqs. (1) and (2). To further detail the sensing mechanism of the NiO@rGO nanocomposite, the energy band diagram of the nanocomposite and

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the hole transfer between NiO and rGO were given in Fig. 8b. Since the level of the valence band (-4.64 eV) in NiO is lower than that of rGO (-4.40 eV) [48,51,52], the holes of NiO will quickly transfer to the rGO until the Fermi level in the nanocompositione reaches the same level. These increase the number of holes and decrease the resistance of the NiO@rGO sensing material (Ra). O2 (g) + e- ↔ O2-ads

(1)

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O2-ads +e- ↔ O-ads

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H2 (g) ↔ (H2) ads

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(H2) ads+O2-ads → H2O (g) + xe-

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When the strongly reducing H2 gas contacts the surface of the material, the active oxygen anions quickly react with it, and the extracted electrons are immediately released to the conduction band of

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the NiO nanoparticles (Eqs. (3) and (4)). Thus the holes of rGO will quickly transfer to the NiO

nanoparticles. Since the resistance of the p-type semiconductor NiO@rGO will greatly increases

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when H2 molecules are oxidized by the active oxygen anions on the surface of the sensing materials.

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4. Conclusions

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A NiO@rGO nanocomposite was synthesized by a freeze-drying method followed by heat

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treatment. A NiO@rGO sensor was then fabricated and applied to detect hydrogen. The sensor

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exhibited high sensitivity, fast response, good reversibility, and selectivity toward hydrogen. The relative response of the sensor to 1% hydrogen was 0.64, the response time was 28 s, and the

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recovery time was 142 s. The good gas-sensing performance can be attributed to the tiny NiO nanoparticles uniformly anchored on the rGO, their heterojunctions, and the catalytic oxidation of

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the nanoparticles towards hydrogen. The improved gas-sensing performance and low working temperature of the sensor make it a highly promising candidate for the quick and accurate detection

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of hydrogen in practical applications. Acknowledgements

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This study was supported by National Natural Science Foundation of China (Project Nos. 21471005 and 61203212), Anhui Provincial Natural Science (Project No. 1708085MB36) and the grant NRF-2018R1A2B3001246 of the National Research Foundation of Korea.

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References [1] R. Thimmappa, S. Shafi, S. Freunberger, M.O. Thotiyl, Proton conducting hollow graphene oxide cylinder as molecular fuel barrier for tubular H2-air fuel cell, International Journal of Hydrogen Energy 41 (2016) 22305-22315. [2] A.U. Haque, W. Asrar, A.A. Omar, E. Sulaeman, J.M. Ali, Assessment of engine‫ ׳‬s power budget

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for hydrogen powered hybrid buoyant aircraft, Propulsion and Power Research 5 (2016) 34-44.

Compendium of Hydrogen Energy 4 (2016) 87-107.

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[3] N. Frischauf, 5-Hydrogen-fueled spacecraft and other space applications of hydrogen,

[4] J.G. Firth, A. Jones, T.A. Jones, The principles of the detection of flammable atmospheres by

U

catalytic devices, Combustion and Flame 20 (1973) 303-311.

N

[5] F.L. Meng, Z. Guo, X.J. Huang, Graphene-based hybrids for chemiresistive gas sensors, TrAC

A

Trends in Analytical Chemistry 68 (2015) 37-47.

M

[6] Y.F. Luo, C. Zhang, B.B. Zheng, X. Geng, M. Debliquy, Hydrogen sensors based on noble metal doped metal-oxide semiconductor: A review, International Journal of Hydrogen Energy 42 (2017)

ED

20386-20397.

[7] A.I. Ayesh, Metal/metal-oxide nanoclusters for gas sensor applications, Journal of

PT

Nanomaterials 2016 (2016) 2359019.

[8] M. Predanocy, I. Hotový, M. Čaplovičová, Structural, optical and electrical properties of

CC E

sputtered NiO thin films for gas detection, Applied Surface Science 395 (2017) 208-213. [9] J. Keraudy, A. Ferrec, M. Richard-Plouet, J. Hamon, A. Goullet, P.Y. Jouan, Nitrogen doping on

A

NiO by reactive magnetron sputtering: A new pathway to dynamically tune the optical and electrical properties, Applied Surface Science 409 (2017) 77-84. [10] M.J. Wang, X.F. Song, S.G. Dai, W.N. Xu, Q. Yang, J.L. Liu, C.G. Hu，D.P. Wei, NiO nanoparticles supported on graphene 3D network current collector for high-performance electrochemical energy storage, Electrochimica Acta 214 (2016) 68-75.

12

[11] S.B. Narendranath, S.V. Thekkeparambil, L. George, S. Thundiyil, R.N. Devi, Photocatalytic H2 evolution from water–methanol mixtures on InGaO3(ZnO)m with an anisotropic layered structure modified with CuO and NiO cocatalysts, Journal of Molecular Catalysis A: Chemical 415 (2016) 82-88. [12] H. Steinebach, S. Kannan, L. Rieth, F. Solzbacher, H2 gas sensor performance of NiO at high

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temperatures in gas mixtures, Sensors and Actuators B: Chemical 151 (2010) 162-168.

[13] C. Baratto, R. Kumar, E. Comini, M. Ferroni, M. Campanini, Bottle-brush-shaped

SC R

heterostructures of NiO-ZnO nanowires: growth study and sensing properties, Nanotechnology 28 (2017) 465502.

U

[14] P.V. Tong, D.H. Nguyen, V.D. Nguyen, V.Q. Vu, T.L. Nguyen, V.H. Nguyen, In-situ

N

decoration of Pd nanocrystals on crystalline mesoporous NiO nanosheets for effective hydrogen gas

A

sensors, International Journal of Hydrogen Energy 38 (2013) 12090-12100.

M

[15] S. Liu, G. Wang, H.Y. Hou, X.X. Liu, J.X. Duan, Q.S. Liao, Binder-free combination of large area reduced graphene oxide nanosheets with Cu foil for lithium ion battery anode, Diamond and

ED

Related Materials 68 (2016) 102-108.

[16] J. Fang, Z.G. Xie, G. Wallace, X.G. Wang, Co-deposition of carbon dots and reduced graphene

PT

oxide nanosheets on carbon-fiber microelectrode surface for selective detection of dopamine, Applied Surface Science 412 (2017) 131-137.

CC E

[17] S. Colonna, O. Monticelli, J. Gomez, C. Novara, G. Saracco, A. Fina, Effect of morphology and defectiveness of graphene-related materials on the electrical and thermal conductivity of their

A

polymer nanocomposites, Polymer 102 (2016) 292-300. [18] M. Fathy, A. Gomaa, F.A. Taher, M.M. El-Fass, A.E.H.B. Kashyout, Optimizing the preparation parameters of GO and rGO for large-scale production, Journal of Materials Science 51 (2016) 5664-5675. [19] J.R. Huang, W. Wang, X.R Lin, C.P. Gu, J.Y. Liu, Three-dimensional sandwich-structured

13

NiMn2O4@reduced graphene oxide nanocomposites for highly reversible Li-ion battery anodes, Journal of Power Sources 378 (2018) 677-684. [20] W. Wang, X.J. Song, C.P. Gu, D.X. Liu, J.Y. Liu, J.R. Huang, A high-capacity NiCo2O4@reduced graphene oxide nanocomposite Li ion battery anode, Journal of Alloys and Compounds 741 (2018) 223-230.

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[21] J. Wei, Z.G. Zang, Y.B. Zhang, M. Wang, J.H. Du, X.S. Tang, Enhanced performance of

light-controlled conductive switching in hybrid cuprous oxide/reduced graphene oxide (Cu2O/rGO)

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nanocomposites, Optics Letters 42 (2017) 911-914.

[22] Z.G. Zang, X.F. Zeng, M. Wang, W. Hu, C.R. Liu, X.S. Tang, Tunable photoluminescence of

U

water-soluble AgInZnS-graphene oxide (GO) nanocomposites and their application in-vivo

N

bioimaging, Sensors and Actuators B: Chemical 252 (2017) 1179-1186.

A

[23] E. Singh, M. Meyyappan, H.S. Nalwa, Flexible graphene-based wearable gas and chemical

M

sensors, ACS Applied Materials & Interfaces 9 (2017) 34544-34586. [24] S. Nasresfahani, M.H. Sheikhi, M. Tohidi, A. Zarifkar, Methane gas sensing properties of

ED

Pd-doped SnO2/reduced graphene oxide synthesized by a facile hydrothermal route, Materials Research Bulletin 89 (2017) 161-169.

PT

[25] Q.X. Feng, X.G. Li, J. Wang, Percolation effect of reduced graphene oxide (rGO) on ammonia sensing of rGO-SnO2 composite based sensor, Sensors and Actuators B: Chemical 243 (2017)

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1115-1126.

[26] R. Furue, E.P. Koveke, S. Sugimoto, Y. Shudo, S. Hayami, S.I. Ohira, K. Toda, Arsine gas

A

sensor based on gold-modified reduced graphene oxide, Sensors and Actuators B: Chemical 240 (2017) 657-663. [27] Y.J. Yang, X.J. Yang, W.Y. Yang, S.B. Li, J.H. Xu, Y.D. Jiang, Porous conducting polymer and reduced graphene oxide nanocomposites for room temperature gas detection, RSC Advances 4 (2014) 42546-42553.

14

[28] A.A. Khaleed, A. Bello, J.K. Dangbegnon, M.J. Madito, F.U. Ugbo, A.A. Akande, B.P. Dhonge, F. Barzegar, D.Y. Momodu, B.W. Mwakikunga, N. Manyala, Gas sensing study of hydrothermal reflux synthesized NiO/graphene foam electrode for CO sensing, Journal of Materials Science 52 (2017) 2035-2044. [29] M. Yang, X.F. Zhang, X.L. Cheng, Y.M. Xu, S. Gao, H. Zhao, L.H. Huo, Hierarchical NiO

low temperature, ACS Applied Materials & Interfaces 9 (2017) 26293-26303.

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cube/nitrogen-doped reduced graphene oxide composite with enhanced H2S sensing properties at

SC R

[30] D.Z. Zhang, H.Y. Chang, P. Li, R.H. Liu, Characterization of nickel oxide decorated-reduced graphene oxide nanocomposite and its sensing properties toward methane gas detection, Journal of

U

Materials Science: Materials in Electronics 27 (2016) 3723-3730.

N

[31] I.H. Kadhim, H. Abu Hassan, Hydrogen gas sensing based on SnO2 nanostructure prepared by

A

sol-gel spin coating method, Journal of Electronic Materials 46 (2017): 1419-1426.

M

[32] K. Nguyen, C.M. Hung, T.M. Ngoc, D.T.T. Le, D.H. Nguyen, D.N. Van, H.N. Van, Low-temperature prototype hydrogen sensors using Pd-decorated SnO2 nanowires for exhaled

ED

breath applications, Sensors and Actuators B: Chemical 253 (2017) 156-163. [33] Y.X. Li, D.Y. Deng, N. Chen, X.X. Xing, X. Liu, X.C. Xiao, Y.D. Wang, Pd nanoparticles

PT

composited SnO2 microspheres as sensing materials for gas sensors with enhanced hydrogen response performances, Journal of Alloys and Compounds 710 (2017) 216-224.

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[34] M.B. Rahmani, M.H. Yaacob, Y.M. Sabri, Hydrogen sensors based on 2D WO3 nanosheets prepared by anodization, Sensors and Actuators B: Chemical 251 (2017) 57-64.

A

[35] M. Horprathum, T. Srichaiyaperk, B. Samransuksamer, A. Wisitsoraat, P. Eiamchai, S. Limwichean, C. Chananonnawathorn, K. Aiempanakit, N. Nuntawong, V. Patthanasettakul, C. Oros, S. Porntheeraphat, P. Songsiriritthigul, H. Nakajima, A. Tuantranont, P. Chindaudom, Ultrasensitive hydrogen sensor based on Pt-decorated WO3 nanorods prepared by glancing-angle dc magnetron sputtering, ACS Applied Materials & Interfaces 6 (2014) 22051-22060.

15

[36] J.X. Dai, W. Peng, G.P. Wang, F. Xiang, Y.H. Qin, M. Wang, M.H. Yang, H. Deng, P.C. Zhang, Improved performance of fiber optic hydrogen sensor based onWO3-Pd2Pt-Pt composite film and self-referenced demodulation method, Sensors and Actuators B: Chemical 249 (2017) 210-216. [37] A. Kaniyoor, R.I. Jafri, T. Arockiadoss, S. Ramaprabhu, Nanostructured Pt decorated graphene

IP T

and multi walled carbon nanotube based room temperature hydrogen gas sensor, Nanoscale 1 (2009) 382-386.

SC R

[38] H. Vedala, D.C. Sorescu, G.P. Kotchey, A. Star, Chemical sensitivity of graphene edges decorated with metal nanoparticles, Nano Lett 11 (2011) 2342-2347.

U

[39] P.A. Pandey, N.R. Wilson, J.A. Covington, Pd-doped reduced graphene oxide sensing films for

N

H2 detection, Sensors and Actuators B: Chemical 183 (2013) 478-487.

A

[40] Y. Pak, S.M. Kim, H. Jeong, C.G. Kang, J.S. Park, H. Song, R. Lee, N.S. Myoung, B.H. Lee, S.

M

Seo, J.T. Kim, G.-Y. Jung, Palladium-decorated hydrogen-gas sensors using periodically aligned graphene nanoribbons, ACS Applied Materials & Interfaces 6 (2014) 13293-13298.

ED

[41] P.A. Russo, N. Donato, S.G. Leonardi, S. Baek, D.E. Conte, G. Neri, N. Pinna, Room-temperature hydrogen sensing with heteronanostructures based on reduced graphene oxide

PT

and tin oxide, Angewandte Chemie International Edition 51 (2012) 11053-11057. [42] K. Anand, O. Singh, M.P. Singh, J. Kaur, R.C. Singh, Hydrogen sensor based on graphene/ZnO

CC E

nanocomposite, Sensors and Actuators B: Chemical 195 (2014) 409-15. [43] T. Kamal, High performance NiO decorated graphene as a potential H2 gas sensor, Journal of

A

Alloys and Compounds 729 (2017) 1058-1063. [44] D. Dutta, S.K. Hazra, J. Das, C.K. Sarkar, S. Basu, Studies on p-TiO2/n-graphene heterojunction for hydrogen detection, Sensors and Actuators B: Chemical 212 (2015) 84-92. [45] A. Venkatesan, S. Rathi, I.-Y. Lee, J.W. Park, D.S. Lim, M. Kang, H.-I. Joh, G.-H. Kim, E.S. Kannan, Molybdenum disulfide nanoparticles decorated reduced graphene oxide: highly sensitive

16

and selective hydrogen sensor, Nanotechnology 28 (2017) 365501. [46] J.R. Huang, L.W. Wang, C.C. Shi, Y.J. Dai, C.P. Gu, J.H. Liu, Selective detection of picric acid using functionalized reduced graphene oxide sensor device, Sensors and Actuators B: Chemical 196 (2014) 567-573. [47] J.R. Huang, L.Y. Wang, C.C. Shi, Y.J. Dai, C.P. Gu, J.H. Liu, Elective detection of picric acid

IP T

using functionalized reduced graphene oxide sensor device, Sensors and Actuators B: Chemical 196 (2014) 567-573.

SC R

[48] Y.L.T. Ngo, H.S. Hur, Low-temperature NO2, gas sensor fabricated with NiO and reduced graphene oxide hybrid structure, Materials Research Bulletin 84 (2016) 168-176.

U

[49] H. Zhang, L. Yu, Q. Li, Y. Du, S.C. Ruan, Reduced graphene oxide/α-Fe2O3 hybrid

A

N

nanocomposites for room

M

temperature NO2 sensing, Sensors and Actuators B: Chemical 241 (2017) 109-115. [50] L.T. Hoa, H.N. Tien, V.H. Luan, J.S. Chung, S.H. Hur, Fabrication of a novel

ED

2D-graphene/2D-NiO nanosheet-based hybrid nanostructure and its use in highly sensitive NO2 sensors, Sensors and Actuators B: Chemical 185 (2013) 701-705.

PT

[51] N.H. Al-Hardan, M.J. Abdullah, A. Abdul Aziz, Performance of Cr-doped ZnO for acetone sensing, Applied Surface Science 270 (2013) 480-485.

CC E

[52] M.K. Khalaf, R.H. Mutlak, A.I. Khudhair, Q.G. Hial, Influence of discharge voltage on the sensitivity of the resultant sputtered nio thin films toward hydrogen gas, Physica B Condensed

A

Matter 514 (2017) 78-84.

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Author Biographies

Haibo Ren was born in Suzhou Anhui Province. He received the B.S. degree in chemistry from Hefei Normal University, Department of Chemical and Chemical Engineering, in 2010, his M.S. in College of Chemistry and Materials Science of Anhui Normal University. He is currently working

work mainly focuses on the sensing materials and chemical sensors.

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toward the Ph.D. degree at School of Mechanical Engineering of Yeungnam University. Now his

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Cuiping Gu was born in Xuancheng Anhui Province. She received her B.S. in Chemistry from the College of Chemistry and Materials Science of Anhui Normal University in 2000, her M.S. in

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Polymer Chemistry from the School of Chemistry and Chemical Engineering of Anhui University

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in 2005, and her Ph.D. in Inorganic Chemistry from the School of Chemistry and Chemical

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Engineering of Anhui University in 2008. She is an associate professor of inorganic chemistry at

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Anhui Normal University. Her present work mainly focuses on sensing materials and biochemistry sensors.

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Sang Woo Joo received his B.S. in School of Mechanical Engineering from Seoul National University in 1982, his M.S. in School of Mechanical Engineering from Seoul National University

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in 1984, and his Ph.D. in School of Mechanical Engineering from University of Michigan, Ann Arbor in 1989. He is a professor in the School of Chemical Engineering of Yeungnam University.

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His present work mainly focuses on microfluidics, nanobiotechnology, interfacial instabilities, phase change, and sensing materials.

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Jingjuan Zhao was born in Fuyang Anhui Province. She received her B.S. in Chemistry from Wanjiang College of Anhui Normal University in 2017. She is currently working toward attaining a M.S. in Inorganic Chemistry at Anhui Normal University. Her present work mainly focuses on sensing materials and chemical sensors. Yufeng Sun was born in Xuancheng Anhui Province. He received the B.S. degree in material

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chemistry from Hefei University of Technology, in 1985 and the M.S. degree in material chemistry from Hefei University of Technology, in 1995 and the Ph.D. degree in inorganic chemistry from University of Science and Technology of China, Department of Chemistry, in 2005. He is a Professor of School of Mechanical and Automotive Engineering, Anhui University of Technology and Science. Now his work mainly focuses on the sensing materials and gas sensor.

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Jiarui Huang was born in Shouxian Anhui Province. He received his B.S. in Chemistry from the Chemistry Department of Anhui Normal University in 2000, his M.S. in Synthetic Organic

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Chemistry from the Chemistry Department of Nanjing University of Technology in 2003, and his Ph.D. in Inorganic Chemistry from the Chemistry Department of the University of Science and

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Technology of China in 2006. He is a professor of Inorganic Chemistry at Anhui Normal University.

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Figure captions

Scheme 1 Fabrication procedure of NiO@rGO nanocomposites. Fig. 1 XRD pattern of NiO@rGO nanocomposite. Fig. 2 (a) SEM image, (b) TEM image, (c) HRTEM image of the NiO@rGO nanocomposite, and (d)

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the corresponding SAED pattern. Fig. 3 Raman spectra of NiO@rGO nanocomposite.

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Fig. 4 XPS spectra of the NiO@rGO composites: (a) survey spectrum, (b) Ni 2p spectrum, (c) C 1s spectrum, and (d) O 1s spectrum.

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operating temperature at RH 40%, conditioning at 18°C.

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Fig. 5 Resistance change of NiO@rGO nanocomposite sensors exposed to 3% hydrogen at different

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Fig. 6 (a) Resistance change curves and (b) corresponding responses of NiO@rGO sensor exposure

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to various H2 concentrations at the operating temperature of 50°C at RH 40%, conditioning at 18°C. Fig. 7 Resistance change of NiO@rGO nanocomposite sensors exposed to 3% hydrogen under 7

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measurement cycles at the operating temperature of 50°C at RH 40%, conditioning at 18°C. (b) Responses of NiO@rGO sensor for various H2 concentrations at the operating temperature of 50°C

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at different humidities, conditioning at 18°C. Fig. 8 (a) Schematic diagram of a possible sensing mechanism of the NiO@rGO nanocomposite

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exposed to H2 gas and (b) energy band diagram of the NiO@rGO and the hole transfer between

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NiO and rGO.

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