WO3 nanowire nanocomposites for highly sensitive toxic NH3 gas sensors

Journal Pre-proof Facile synthesis of ultrafine rGO/WO3 nanowire nanocomposites for highly sensitive toxic NH3 gas sensors Chu Manh Hung, Do Quang Dat, Nguyen Van Duy, Vu Van Quang, Nguyen Van Toan, Nguyen Van Hieu, Nguyen Duc Hoa

PII:

S0025-5408(19)32353-0

DOI:

https://doi.org/10.1016/j.materresbull.2020.110810

Reference:

MRB 110810

To appear in:

Materials Research Bulletin

Received Date:

13 September 2019

Revised Date:

18 January 2020

Accepted Date:

28 January 2020

Please cite this article as: Hung CM, Dat DQ, Van Duy N, Van Quang V, Van Toan N, Van Hieu N, Hoa ND, Facile synthesis of ultrafine rGO/WO3 nanowire nanocomposites for highly sensitive toxic NH3 gas sensors, Materials Research Bulletin (2020), doi: https://doi.org/10.1016/j.materresbull.2020.110810

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Facile synthesis of ultrafine rGO/WO3 nanowire nanocomposites for highly sensitive toxic NH3 gas sensors Chu Manh Hung1**, Do Quang Dat1,2, Nguyen Van Duy1, Vu Van Quang1, Nguyen Van Toan1, Nguyen Van Hieu3,4, Nguyen Duc Hoa1* 1

International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST), No 1 - Dai Co Viet Str. Hanoi, Vietnam Department of Natural Science, Hoa Lu University, Ninh Nhat commune, Ninh Binh city, Ninh

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2

Binh province, Vietnam

Faculty of Electrical and Electronic Engineering, Phenikaa Institute for Advanced Study

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4

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(PIAS), Phenikaa University, Yen Nghia, Ha-Dong district, Hanoi 10000, Viet Nam; Phenikaa Research and Technology Institute (PRATI), A&A Green Phoenix Group, 167 Hoang

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Ngan, Hanoi 10000, Viet Nam.

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Corresponding authors

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* Nguyen Duc Hoa, PhD., Assoc. Professor ** Chu Manh Hung, PhD. International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST) No. 1, Dai Co Viet Road, Hanoi, Vietnam Phone: 84 24 38680787 84 24 38692963 Fax: [email protected] (N D Hoa) [email protected] (C M Hung) E-mail: Post address:

No. 1, Dai Co Viet Road, Hanoi, Vietnam

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Graphical abstract SMO

(A) Graphite

SMO

Adding Metal salts & Hydrothermal synthesis

(B) Graphene oxide

(C) Reduce Graphene oxide/SMO W eV0=m -n CB

B

EC Fn

FG

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Chemical exfoliation

EV VB

11

10 o

@ 100 ppm & 300 C

8 6 4 1.8

2 NH3

CO

1.1 H2

Tested Gas

1.3

SO2

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0

-p

S =(Ra/Rg)

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(D) Graphene/SMO heterojunction 14

Highlights

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 Nanocomposites of rGO and WO3 nanowires were hydrothermally synthesized  The rGO/WO3 nanocomposites showed excellent sensing performance with theoretical detection limit of 138 ppb

 Gas-sensing mechanism was discussed based on the p-n heterojunction  The rGO/WO3 nanocomposites are capable for monitoring highly toxic NH3 gas in air

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Abstract: We introduce a facile and scalable synthesis of rGO/WO3 nanocomposites by hydrothermal method for gas sensing applications. The characterization of rGO/WO3 nanocomposites by some advanced techniques such as scanning electron microscopy, high resolution transmission electron microscopy and Raman spectroscopy revealed that high quality rGO/WO3 nanocomposites

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included single crystal WO3 nanowires (average diameter of 10 nm) entangled by thin rGO layers.

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Sensing measurements demonstrated that the rGO/WO3 nanocomposite-based sensor can detect highly toxic NH3 gas at low concentrations ranging from 20 ppm to 500 ppm and fulfills practical

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applications. The developed gas sensors based on rGO/WO3 nanocomposites have significant

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application prospects in environment pollution monitoring at detection limit of about 138 ppb. We also discussed the gas sensing mechanism of the rGO/WO3 nanocomposites based on the p-n

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junction.

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Keywords: WO3 nanowires; rGO; Nanocomposites; Hydrothermal; Highly toxic NH3, Gas sensors

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1. Introduction Since its first invention [1], resistive gas sensor has attracted massive attention from researchers worldwide [2–7] because it has large potential application in various fields [8], such as chemical processing, agriculture, environmental monitoring, gas exhaust control, industrial process control, military, security, smart house, food analysis, and lung cancer diagnosis [9,10].

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Attempts have been made to fabricate novel nanomaterials [11] for gas sensing devices with high sensitivity and selectivity for practical applications in modern society [12–14]. Metal oxide-based

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gas sensors [15] are prospective candidates, which have been extensively studied due to their low

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cost, compact size, real-time detection, portability, and low power consumption [16,17]. Among the materials used as gas sensitive layers, pristine WO3 material with different morphologies is

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used as a gas sensor because of its high sensitivity to NO2 [18,19]. Response value of WO3 nanowires to NH3 1500 ppm at 250 oC is only 9.7 [20]. Recently, WO3 synthesized by

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hydrothermal method and decorated with Pd nanoparticles has been found to enhance gas sensing performance [21]. However, the pristine metal oxide-based gas sensors have low sensitivity and

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poor selectivity to some certain gases with low signal to noise due to its low conductivity [22–24]. In addition to metal oxides, 2D materials, such as graphene [25] and reduced oxide graphene (rGO) [26], have recently been used as sensing materials for the detection of hazardous and

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explosive gases such as H2S, NO2, H2, and NH3, due to their large specific surface area and tunable electrical properties [27–29]. However, pristine graphene and nanocarbon-based sensors suffer from the slow response and recovery characteristics [27], thereby limiting their potential application [28]. Recently, the hybrids or composites of metal oxide and carbon allotropes [29] have been extensively studied as gas-sensitive materials and reported to exhibit significantly high sensitivity [30–32]. The composites of metal oxide and carbon allotropes like rGO were used to 4

enhance the gas sensing ability of sensors [33,34] by utilizing the heterojunction between nanocarbon and metal oxides [35,36]. Therefore, many efforts have focused on the synthesis of rGO and metal oxide composites for gas sensor applications [37]. For instance, the rGO/hexagonal WO3 nanosheets composites were synthesized for enhanced H2S sensing performance [38], whereas the WO3 nanorods/graphene nanocomposites were prepared for NO2 gas sensing

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application [39]. The rGO/WO3 nanolamellae nanocomposites were prepared by hydrothermal method for acetone sensing application [40], while the Pd–WO3/rGO hierarchical nanostructures

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were used as efficient H2 gas sensors [41]. Composites of rGO and WO3 nanoparticles were also prepared by ultrasonicating WO3 nanospheres and rGO layers for the NH3 sensor [33]. It is clearly

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that the gas sensing characteristics of rGO based nanocomposites are strongly dependent on the

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morphology and the size of WO3 nanostructures. The smaller diameter of WO3 is expected to show higher sensitivity [42]. However, none of the report dedicating on the synthesis and application of

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WO3 nanowires/rGO for gas sensors despite the nanowires were reported to exhibit better sensing performance. Advantages of WO3 nanowires/rGO include the small diameter of WO3 nanowires

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which is comparable with the Debye length, and the p-n junction between rGO and WO3, thus enhancing the gas sensing performance [40]. Herein, we introduce our study on the synthesis of rGO/WO3 nanowire composites for gas sensor application. The design of gas sensor based on rGO/WO3 nanowire composites is shown in

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Figure 1(A). The nanocomposites of rGO and WO3 nanowires were prepared by a green facile and scalable hydrothermal method for gas sensor applications. The advantages of this method include simple synthesis and good dispersion of rGO and WO3 nanowires. Given that the hydrothermal synthesis of WO3 requires low pH, we used ascorbic acid (C6H8O6) as a reducing agent to reduce GO into rGO, control the pH, and form ultrafine WO3 nanowires. Under these synthesis conditions,

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the surface of rGO was highly functionalized and could be easily dispersed in solution for sensor fabrication. Homogenous WO3 nanowires with an average diameter of 10 nm were obtained, and the rGO layers were entangled over the WO3 nanowires. The gas sensing properties of the synthesized materials were tested using various gases, including of SO2, NH3, CO, and H2. We also discussed the gas sensing mechanism of the synthesized rGO/WO3 nanocomposites based on

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the p-n heterojunctions.

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2. Experimental

The materials used in this study were graphite, NaNO3, H2SO4, KMnO4, H2O2, HCl,

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Na2WO4.2H2O, NaCl, C6H8O6, C2H5OH, and deionized (DI) water. All analytical grade reagents

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were used without further purification. 2.1 rGO/WO3 nanowire nanocomposite synthesis

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Graphene oxide (GO) was synthesized using a modified Hummers method, as reported elsewhere [43]. Detail about the synthesis of GO is summarized in Fig. S1 (Supplementary). The

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nanocomposites of rGO/WO3 nanowires were synthesized by a facile hydrothermal method using as-prepared GO, Na2WO4.2H2O and C6H8O6 as precursors. Schematic for the synthesis of the nanocomposites of rGO/WO3 nanowires is shown in Figure 1(B). In a typical synthesis, 1000 µL of the GO solution synthesized above was dispersed in a mixture of 80 g of DI water, 1.5 g of

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Na2WO4.2H2O, 1.0 g of NaCl, and 2.5 g of C6H8O6 via ultrasonication. Herein, we used C6H8O6 as reducing and pH control agent. The mixture was further stirred for 15 min at room temperature, and then pH of the solution was adjusted to 2. The mixture solution was transferred into a 100 mL Teflon lined autoclave for hydrothermal treatment at 180oC for 12 h. The precipitate product was washed with DI water and ethanol, and then collected by centrifugation at a rate of 5800 rpm for

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15 min. Finally, the product was dried at 60oC for 24 h before characterization. The crystal structures and morphology of the synthesized materials were characterized by X-ray diffraction (XRD; Advance D8, Bruker), field-emission scanning electron microscopy (SEM, JEOL 7600 F), and high-resolution transmission electron microscopy (TEM, JEM2100). The Raman spectra were studied using LabRAM HR (HORIBA Jobin Yvon (exc = 632.8 nm). Fourier transform infrared

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(FTIR) spectra were obtained by Thermo Nicolet Nexus 670 Fourier Transform Infrared Spectroscopy. UV-vis spectra were obtained in the wavelength range 190–900nm by using a

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2.2 rGO/WO3 nanocomposite-based sensor fabrication

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spectrophotometer (PG-T90, UK).

Gas sensors were fabricated via a thick film technology as reported elsewhere [14]. In brief, the

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synthesized rGO/WO3 nanocomposite was dispersed in solution and then coated onto a thermally

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oxidized Si substrate equipped with a pair of interdigitated Pt electrodes. The synthesized sensors were heat treated at 400oC for 2h in air to remove the binder before gas sensing measurements.

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The gas sensing properties were measured at different temperatures under dynamic conditions, where the dry air and analytic gas was continuously flowed through the chamber while the resistance was recorded using a Keithley instrument (Model 2602). The sensor was placed on a heating plate to control the working temperatures of approximately 250, 300, 350, and 400 °C. The

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gas sensing characteristics were tested for NH3, H2, CO and SO2 at various temperatures. The sensor response (S) is defined by the ratio of Ra/Rg (for reducing gases) or Rg/Ra (for oxidizing gases), where Ra and Rg are the resistances of the sensors in dry air and in the tested gas, respectively.

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3. Results and discussion 3.1 Characterization of materials The morphology of the synthesized rGO/WO3 nanocomposites studied by SEM is shown in Figure 2. The low-magnification SEM image (Figure 2(A)) revealed that the sample was composed of rGO flakes and WO3 nanowire bundles. Given that the amount of rGO was low (less than 1% wt.),

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the sample mainly looked like bundles of WO3. The average length of the WO3 bundles is about 1

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µm. The high-resolution SEM image (Figure 2(B)) demonstrated that WO3 bundles were composed of very fine nanowires with an average diameter of less than 10 nm. Given that the

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length of the WO3 nanowires in a bundle differed, the bundle size was small at the top but large at the center. In our previous study, microwheels composed of WO3 nanorods, which were

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approximately 1.5 μm long and 10 nm in diameter, were obtained by hydrothermal method at

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temperature of 200oC [44]. In the study by Xiong et al. [45], nanowire bundle-like WO3-W18O49 was obtained through solvothermal method using WCl6 as precursor and isopropanol as reaction medium. The nanowire WO3 bundle was obtained without using isopropanol as reaction medium.

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Compared with the hexagonal WO3 nanorods prepared by hydrothermal method reported in the literature [21], the diameter of our product was considerably smaller, which was advantageous to enhance gas sensing performance.

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The crystal structures of the calcinated rGO/WO3 nanocomposites studied by XRD is shown in Figure 3(A). The XRD pattern showed the typical diffraction peaks of hexagonal structure, and those peaks were consistent with the standard card of WO3 (JCPDS, No. 33-1387), with lattice constants a = b = 0,7298 nm and c = 0,3899 nm [44]. These findings indicated that the hexagonal

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structure of WO3 was stable with thermal treatment up to temperature of 400oC. This result was consistent with other report, where the authors pointed out that the XRD pattern of hexagonal WO3 remains nearly the same up to 450°C [46]. This phenomenon is crucial for gas sensing application because the high working temperature of the device does not influence to the stability of the sensor. In addition, no impurity peaks were observed in the XRD pattern, suggesting the high purity of the

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hexagonal WO3 phase. The diffraction peaks with large and sharp intensity showed that the hexagonal phase of the WO3 crystal in the synthesized samples exhibited high crystallinity. This

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result was consistent with other reports on the hydrothermal synthesis of hexagonal rGO/WO3 nanosheets [38]. However, Figure 3(A) shows no obvious diffraction peak of rGO in the XRD

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pattern due to small amount of rGO (less than 1 wt.%) in the synthesized nanocomposites. Further

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characterization by Raman spectroscopy, TEM images, and FTIR spectroscopy was performed to confirm the presence of rGO in the obtained nanocomposites.

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Figure 3(B) reveals the Raman spectrum of the rGO/WO3 nanocomposites. In addition to the vibrational modes between W and O (WO and WOW) of the crystalline hexagonal-WO3

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nanowires in the range of 1501050 cm-1 [47,48], the spectrum exhibited two distinct Raman modes centered at approximately 1330 and 1593 cm-1, which correspond to the vibrational modes of the D and Gbands of rGO, respectively. The Dband originated from the vibration of the

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CC bond induced by the disorder [49], whereas the Gband was attributed to the first-order scattering of the E2g vibration mode of sp2 carbon domains [48]. These results confirmed the existence of rGO in the synthesized nanocomposites. Intensity ratio between the D and Gband peaks (ID/IG) is often used as a parameter to estimate the defects and disorders of carbonaceous materials [48,50]. The calculated ID/IG intensity ratio (Figure 2(B)) was approximately 1.09, which was considerably smaller than that of rGO/WO3 nanosheets (1.28) synthesized through the 9

hydrothermal method in a previous report [48]. Thus, less carbonaceous defects and disorders were generated in the synthesized rGO presented in the current work. In our study, we used ascorbic acid as pH adjusting and reducing agent, thus during hydrothermal process the ascorbic further reduced GO to enhance the quality of rGO and decreased the ID/IG intensity ratio.

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The representative TEM images of the rGO/WO3 nanocomposites are displayed in Figure 4. The

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low-magnification TEM image of Figure 4(A) revealed extremely thin WO3 nanowires of approximately 10 nm, which was exfoliated from the bundles of rGO/WO3 nanowires (as shown

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in the SEM image). Curled and corrugated rGO sheets were also observed in the TEM images. The extremely thin rGO sheets appeared to cover the WO3 nanowires, thereby enhancing the gas

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sensing characteristics of the rGO/WO3 nanocomposites [51]. High-resolution TEM image of the

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rGO/WO3 nanocomposites (Figure 4(B)) showed homogeneous lattice structures along the WO3 nanowires. The d-spacing of the lattice planes was calculated to be approximately 0.39 nm, which

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corresponded to the (001) lattice plane of WO3 [52].

The optical absorption properties and band gap energy of the rGO/WO3 nanocomposites were characterized by UV-vis diffuse reflectance spectroscopy (DRS). Figure 5(A) shows the UV-Vis

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spectrum of the rGO/WO3 nanocomposites in the spectral range of 200800 nm, and the Tauc plot (inset graph) shows the effect of ℎ𝜈 on the corresponding (𝛼ℎ𝜈)1/2 . The UV-vis DRS spectrum revealed a characteristics absorption band at 390 nm, indicating that WO3 with high crystalline structure was formed in the rGO/WO3 nanocomposites [53]. The absorption intensity of the synthesized rGO/WO3 nanocomposites in the visible region from 400 nm to 800 nm was higher

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than that of the pristine WO3 nanowires grown via a similar method as reported by Nagy et al. [54]. This result was due to the presence of rGO in the synthesized nanocomposites, which formed additional defects in the WO3 crystals. The absorption edge of the rGO/WO3 nanocomposites showed a blue shift compared with the pristine WO3 nanowires [54]; this result was in good agreement with a previous report [52]. The band gap energy of the rGO/WO3 nanocomposites was

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estimated from the UV-vis spectrum using the following expression [55,56]: (𝛼ℎ𝜈)1/2 = 𝐾(ℎ𝜈 − 𝐸𝑔 ),

(1)

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where  is the absorption coefficient, ℎ is Planck’s constant, 𝜈 is the incident photon’s frequency,

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K is a proportionality constant, and Eg is the band gap energy. The inset in Figure 5(A) displays the linear extrapolation of the Tauc plot in which the X-axis is ℎ𝜈 and the Y-axis is the Tauc term

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((𝛼ℎ𝜈)1/2 ) in the case of indirect band gap rGO/WO3 nanocomposites. The linear extrapolation

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intercepts the X-axis at a certain value, which is generally known as the optical band-gap energy [55,56]. Therefore, the band gap energy of rGO/WO3 nanocomposites was approximately 2.9 eV, which was consistent with that of rGO-WO3 nanorod composites synthesized by the one-pot

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hydrothermal method in a previous work [55].

The functional groups in rGO and rGO/WO3 nanocomposites were investigated by FTIR spectroscopy. Figure 5(B) shows the FTIR spectra of rGO and the synthesized rGO/WO3

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nanocomposites. The FTIR spectrum of rGO (on top) showed characteristic absorption peaks at 3427.7, 1728.9, 1583.5, 1225.4, 1046.7, and 585.6 cm-1, which corresponded to the OH, CO, CC, phenolic COH, alkoxy CO, and epoxy CO stretching vibrations of rGO, respectively [57,58]. However, only the peaks of OH and CC stretching vibrations were not observed in the FTIR spectrum of rGO/WO3 nanocomposites (at the bottom). It indicates a reduction of oxygen-

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containing functional groups of the rGO/WO3 nanocomposites caused by the thermal treatment. The results imply an occurrence of the interaction between rGO and WO3, which may enhance the electron transfer, and therefore improve the gas sensing properties of the rGO/WO3 nanocomposites [59]. In addition, the presence of a broad absorption peak at 820 cm-1 in the spectrum of the rGO/WO3 nanocomposites was ascribed to the WOW stretching mode in the

nanosheets in a previous report [38].

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ultrafine WO3 nanowires, which is consistent with the stretching mode of the crystalline WO3

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3.2 Gas sensing characteristics

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The gas sensing properties of the rGO/WO3 nanowire composites were tested at 20500 ppm NH3 in a temperature range of 300450 °C. Figure 6(A) shows the transient resistances of the rGO/WO3

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compositebased sensor at different temperatures. The data presented a decrease in sensor resistance with increasing working temperature. This occurrence was attributed to the fact that the

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density of carrier electron, and the electron mobility degree in the rGO/WO3 nanocomposites increased with increasing the working temperature, which was in good agreement with the general characteristics of semiconductor materials [60]. Moreover, the rGO/WO3 nanocomposite sensor revealed a typical n-type gas sensing response upon exposure to NH3 reducing gas within the

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temperature range. This observation was consistent with the n-type semiconducting property of pristine WO3 materials [61]. The gas responses of the rGO/WO3 nanocomposite sensor at different temperature as a function of NH3 gas concentration are shown in Figure 6(B). The gas response of the sensor increased with increasing NH3 concentration from 20 ppm to 500 ppm. At high NH3 concentrations, more gas 12

molecules participated in the adsorption, diffusion, and reaction processes, resulting in elevated gas sensor response. The increase in the sensor response was rather linear with the increase in gas concentration; this factor is important to evaluate potential application in practical devices for NH3 monitoring [62]. Figure 6(B) also shows that the gas response of the rGO/WO3 nanocomposite sensor increased as the working temperature decreased. The highest sensing performance was achieved at 300 °C, and the response value to 500 ppm NH3 gas was 35. At a low gas concentration

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of 250 ppm and operating temperature of 300 °C, the NH3 gas response of the sensor also reached

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the value of approximately 16, which is about 5 times higher than that of the sensor based on the Fe2O3/WO3 nanorod composites [63]. As the working temperature of the rGO/WO3

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nanocomposite sensor further decreased, the NH3 gas response decreased and the response and

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recovery times were very long, which hampered the practical applications of devices (Fig. S2,

measurements.

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Supplementary). Thus, temperatures ranging from 300 °C to 450 °C were selected for gas sensing

Figures 6(C) and (D) show the response and recovery times of the sensors as function of NH3 gas

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concentrations at different working temperatures. As shown in Figure 6(C), at all working temperatures, the response time of the sensor decreased as the NH3 gas concentration increased. At high gas concentration, more NH3 molecules were absorbed on the active sites of the sensing

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layers, leading to the rapid saturation of sensor resistance; hence, the response time of the sensor was shortened. However, a longer time is needed for the sensor to recover to its initial state after stopping exposure to high gas concentration, as shown in Figure 6(D). It is clearly attributed to the fact that at high gas concentration, more NH3 molecules can be absorbed on the active sites of the sensing layers, leading to quickly reach to the saturated resistance of the sensor. Thus, the response time of the sensor was shorter. Whereas, longer time 13

was needed to recover to the initial state after stopped exposure of the sensor to high gas concentration for all working temperatures as shown in Figure 6(D). In addition, the response/recovery times increased with decreasing working temperature; at 300 °C, the response and recovery times were approximately 37 and 711 s, respectively.

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Table 1 summarizes the recent studies on the NH3 sensing performance of sensors based on different WO3 materials and composites. The sensors based on pristine WO3 demonstrated low

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sensitivity to NH3 gas. For instance, the WO3 particles prepared by thermal decomposition of

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ammonium wolframate showed the response value of 6 to 1000 ppm NH3 at 300 oC [64], while the WO3 nanowire prepared by sputtering and calcination using carbon nanotubes as templates

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exhibited the response value of 9.7 to 1500 ppm NH3 at 250 oC [20]. Table 1 clearly documents that the NH3 gas sensing performance of the sensors improved with surface decoration, doping,

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and/or formation of composite compounds with other materials. The rGO/WO3 nanoparticles prepared by ultrasonication showed the response value of 15.83 to 100 ppm NH3 at room temperature [33], whereas the rGO/WO3 nanocomposite prepared by hydrothermal method showed the response value of 27.7 to 100 ppm NH3 at 150 oC [65]. The rGO/WO3 nanowires

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prepared in this study showed the response value of approximately 11 to 100 ppm NH3, which is higher than the response of GO/WO3 nanorods [66]. Anyhow, on the basis of our study, the rGO/WO3 nanowire composites demonstrated the enhanced NH3 gas response compared to the pristine WO3 nanorods, nanowires, and nanoparticles.

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Table 1. Comparison of NH3 sensing performance between the present work and other literature reports Materials

NH3

Temp.

(ppm)

(°C)

Response

Detection

Method

Ref.

range

100

350

3.2

20-100

Two-step hydrothermal

[67]

1000

250

25

2-1000

Hydrothermal

[45]

500

300

12.5

25-1000

Reactive sputtering

[68]

Fe doped WO3 film

20

250

0.15

20

Electron beam evaporation

WO3 nanocrystals

74

325

3.2

74

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[69]

Hydrothermal

[70]

WS2/WO3 composites

10

150

4.9

1-10

Oxidation

[71]

Ru loaded WO3 nanosheets

20

300

17.8

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(ppm) Closed pack WO3

Acidification with impregnation

[72]

microspheres WO3-W18O49 nanowire

Bilayer SnO2-WO3

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nanofilms

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bundle

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process Hydrothermal

WO3 nanowires

1500

250

9.7

300-1500

Sputtering & Calcination

[20]

WO3 particles

1000

300

6

10-1000

Thermal decomposition

[64]

rGO/WO3 nanoparticles

100

RT

15.83

10-100

ultrasonication

[33]

rGO/WO3 nanocomposite

100

150

27.7

1-100

hydrothermal

[65]

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3-20

GO/WO3 nanorods

100

200

1.17

10-100

Hydrothermal

[66]

rGO/WO3 nanowire

100

300

11

20-500

Hydrothermal

This

Fe2O3/WO3 nanorod

300

composites

composites

100

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Ag-doped -Fe2O3/SiO2

375

4

25-300

[63]

work

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In addition to the high gas response and fast response/recovery times, good limit of detection (LOD), high gas selectivity, and good stability are also important characteristics of the sensor for practical applications. As mentioned above, the response of the rGO/WO3 nanocomposite sensor increased when the NH3 gas concentration increased. Figure 7 (A) shows the representative response of the sensor to 25500 ppm NH3 at 300 oC, which indicated a linear response to the

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increase in NH3 concentration. Through linear fitting, the rGO/WO3 nanocomposite sensor exhibited a slope (S) of 0.0615 and coefficient of determination (R2 value) of 0.9916. The LOD of

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the rGO/WO3 nanocomposite sensor was calculated using the equation (2), where  and S are the root-mean square deviation calculated by the fifth-order polynomial fit of base response versus

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time, and slope value of the linear fit of the gas response versus gas concentration, respectively

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[73]. Detail about the fifth-order polynomial fit of 15 data points taken on the base of the response curve as a function of time for calculation of detection limit is shown in Figure 7 (B). 3𝜎 𝑆

lP 𝐿𝑂𝐷 =

(2)

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The LOD for the rGO/WO3 nanocomposite sensor was calculated to be 138 ppb, which demonstrated the potential application sensors with low LOD toward NH3 gas.

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To evaluate gas selectivity, the rGO/WO3 nanocomposite sensor was exposed to 100 ppm CO, H2, and SO2 gases at 300 oC. The transient resistances of the sensors upon exposure to these gases are provided in the supporting information (Figure S35, Supplementary), which showed that the resistance of the sensor decreased and increased upon exposure to reducing gases (CO and H2) and oxidizing gas SO2, respectively. This finding again confirmed the typical n-type sensing behavior of rGO/WO3 nanocomposite sensor under different working temperature ranges, as shown in the 16

supporting information (Figure S35, Supplementary). The highest sensor response to 5100 ppm CO and 25500 ppm H2 was as low as 1.6 and 1.42 at 300 °C and 350 °C, which were similar optimal working temperatures to that of the NH3 sensor based on the rGO/WO3 nanocomposite. By contrast, the optimal working temperature of the rGO/WO3 nanocomposite sensor to 110 ppm SO2 gas was room temperature with a sensor response of only 1.31. For comparison, Figure 8(A)

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presents the gas responses of the rGO/WO3 nanocomposite sensor to NH3, CO, H2, and SO2 gases under identical working conditions. At the same concentration and working temperature, the

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response of the sensor toward NH3 gas was considerably higher than that toward the other gases, indicating good selectivity to NH3. Figure 8(B) illustrates the short-term stability of the rGO/WO3

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nanocomposite sensor to NH3 gas at 300 °C with 10 response/recovery cycles. The data showed

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that the sensor nearly maintained its original response and recovery even after 10 cycles, thereby confirming the good short-term stability and reproducibility of the rGO/WO3 nanocomposite

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sensor. In addition, the long-term stability of the sensor was also studied by measuring the NH3 response characteristics of the sensor six months storage in ambient environment and a week

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continuously working at 300oC, as shown in Figure 8(C). The sensor distorted its performance after a week of continuously working at high temperature. Such data confirms that the rGO/WO3 nanocomposite is suitable for disposable sensor for monitoring of toxic NH3 gas.

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Gas sensing mechanism of pristine metal oxide was based on the surface reaction between the preadsorbed oxygen species and analytic gas molecules, which modulated the depletion region and altered sensor resistance [18]. However, herein, rGO/WO3 nanocomposite was used; so the heterojunction formed between rGO and WO3 also determined the gas sensing performance [43]. The gas sensing mechanism of the rGO/WO3 nanocomposites-based sensor is proposed in Figure 17

9 [40]. Hexagonal WO3 is an n-type wide band gap (2.9 eV) semiconductor with a work function of approximately 4.7–6.4 eV [74]. By contrast, rGO is a p-type semiconductor [75] with low work function ranging from 4.2 eV to 4.45 eV [76]. The band gap of rGO can vary from 1.00 eV to 1.69 eV, depending on reduction method [77]. Therefore, when rGO came into contact with WO3 in the form of rGO/WO3 nanocomposites, the p-n junction formed, which was highly sensitive to

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gas adsorption/desorption [40]. Upon exposure to NH3, the gas molecule further reduced rGO until zero bandgap was reached or metallic graphene formed. The p-n junction was changed to Ohmic

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contact, which significantly reduced sensor resistance and improved sensing performance.

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4. Conclusion

We introduced a facile hydrothermal method of synthesizing high quality rGO/WO3

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nanocomposites for gas sensor applications. The nanocomposites composed of single crystal WO3 nanowires (average size of 10 nm) entangled by thin rGO layers. The diameter of the WO3

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nanowires was very small and comparable with the Debye length, which ensured the total depletion of the oxide crystal. Meanwhile, the entanglement of rGO over the nanowires provided additional adsorption sites, thereby enhancing sensing performance. Gas sensing measurements demonstrated that the rGO/WO3 nanocomposite could monitor NH3 at low concentrations with

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detection limit of 138 ppb. The gas sensing mechanism of the of the rGO/WO3 nanocomposites was also discussed based on the heterojunction between highly conductive p-type rGO and n-type semiconducting WO3 and the enhancement of gaseous adsorption sites for maximizing sensing performance.

18

Authors statement: All authors equally contributed to the manuscript. Professors Nguyen Duc Hoa On behalf of all authors

Declaration of interests

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

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Acknowledgment

Research under Vingroup Innovation Foundation (VINIF) annual research support program in project

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Conflicts of Interest

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code VINIF.2019.DA10.

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The authors declare that they have no conflicts of interest.

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Abid, P. Sehrawat, S.S. Islam, P. Mishra, S. Ahmad, Reduced graphene oxide (rGO) based wideband optical sensor and the role of Temperature, Defect States and Quantum

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Efficiency, Sci. Rep. 8 (2018) 3537. doi:10.1038/s41598-018-21686-2.

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Figure caption

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Figure 1. (A) Deign of gas sensor based on rGO/WO3 nanowire composites, and (B) Materials and sensor fabrication processes

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Figure 2. (A) low and (B) high magnification SEM images of the synthesized rGO/WO3

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nanowire nanocomposites

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Figure 3. (A) XRD pattern, and (B) Raman spectrum of the synthesized rGO/WO3 nanowire

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nanocomposites

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Figure 4. (A) low and (B) high magnification HRTEM images of the synthesized rGO/WO3

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nanowire nanocomposites

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Figure 5. (A) UV-Vis spectrum; and (B) FTIR spectra of the synthesized rGO/WO3 nanowire

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nanocomposites

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Figure 6. NH3 gas sensing characteristics of the synthesized rGO/WO3 nanowire nanocomposites measured at different temperatures: (A) transient resistance vs. time upon exposure to different NH3 concentrations; (B) sensor response, (C) response time, (D) recovery

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time as a function of NH3 concentrations

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Figure 7. Sensor response plot of the synthesized rGO/WO3 nanowire nanocomposites on

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exposure to 25-500 ppm NH3 and its linear fitting

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Figure 8. (A) Selectivity and (B) Stability of the as-made rGO/WO3 gas sensor; (C) stability

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after storage of six months in ambient environment and a week working at 300oC

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Figure 9. Proposed gas sensing mechanism of the rGO/WO3 nanocomposites

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