Reduced graphene oxide-CuFe2O4 nanocomposite: A highly sensitive room temperature NH3 gas sensor

Accepted Manuscript Title: Reduced Graphene Oxide-CuFe2 O4 Nanocomposite: A Highly Sensitive Room Temperature NH3 Gas Sensor Authors: L. Satish K. Achary, Aniket Kumar, Bapun Barik, Pratap S. Nayak, N. Tripathy, J.P. Kar, Priyabrat Dash PII: DOI: Reference:

S0925-4005(18)30994-8 https://doi.org/10.1016/j.snb.2018.05.093 SNB 24749

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

31-1-2018 9-5-2018 16-5-2018

Please cite this article as: L.Satish K.Achary, Aniket Kumar, Bapun Barik, Pratap S.Nayak, N.Tripathy, J.P.Kar, Priyabrat Dash, Reduced Graphene Oxide-CuFe2O4 Nanocomposite: A Highly Sensitive Room Temperature NH3 Gas Sensor, Sensors and Actuators B: Chemical https://doi.org/10.1016/j.snb.2018.05.093 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.

Reduced Graphene Oxide-CuFe2O4 Nanocomposite: A Highly Sensitive Room Temperature NH3 Gas Sensor L. Satish K. Acharya, Aniket Kumara, Bapun Barika, Pratap S. Nayaka, N. Tripathyb, J.P. Karb and

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Priyabrat Dash*a Department of Chemistry, National Institute of Technology, Rourkela, Orissa, 769008, Department of Physics, National Institute of Technology, Rourkela, Orissa, 769008,

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Graphical Abstract

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Highly sensitive rGO-CuFe2O4 has been synthesized via simple solution combustion method and

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used as an ultra fast ammonia gas sensor at room temperature.

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Highlights

rGO-CuFe2O4 was prepared via simple and non-expensive solution combustion method.

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This nanocomposite is used as highly sensitive room temperature ammonia gas sensor.

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The sensor has good stability and ultra fast response and recovery time.

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Enhanced charge transfer and high surface area played role in the final activity.

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Abstract Sensitive and selective detection of NH3 at room temperature is required for proper environmental monitoring and also to avoid any health hazards in the industrial areas. Towards this objective, a low-cost, one-step and combustion route mediated reduced graphene oxide (rGO)–CuFe2O4 nanocomposite was exploited as a high–performance NH3 gas sensor by combining the excellent electrical properties of rGO and sensing capabilities of CuFe2O4. This

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nanocomposite was characterized using Fourier transform infrared spectra (FTIR), Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Transmission

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electron microscopy (TEM) and N2 adsorption-desorption analysis. The TEM images

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demonstrate the uniform distribution of the nanoparticles on the rGO surface and high-resolution

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transmission electron microscopy (HRTEM) confirms an average particle size of 15-20 nm. The designed sensor can detect NH3 at low concentrations up to 5 ppm at room temperature. Besides

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high sensing ability, the designed nanocomposite showed good recyclability suggesting its potential applications for the detection of environmentally toxic gases. The enhanced sensing

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behavior can be attributed to the synergistic behavior between individual rGO and CuFe2O4

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component.

Keywords: Reduced Graphene Oxide(rGO), Spinel CuFe2O4, NH3 gas, Gas, sensing,

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Nanocomposite

Introduction

The rapid development of several industrial sectors become the major sources for the production of toxic gases like CO2, NOx, SO2, CO and NH3 which endanger human health in the long term [1, 2]. Among these gases, NH3 is the most frequently found toxic pollutant, mostly generated by industries and agriculture. It leads to several health hazards such as irritation in the respiratory track, eye and skin and causes cell damage even to the exposure of 35 ppm of NH3 for 15min [3].

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Hence, it is essential to detect the presence of NH3 for both environmental as well as health point of view. In this regard, research has been going on to develop highly sensitive, selective and cost-effective NH3 gas sensor. Among them, metal oxides semiconductors are found to be promising materials for the detection of NH3 gas. In these materials, the gas sensing ability is due to the reversible interaction of the analyte gas with the pre-adsorbed ambient oxygen from air, resulting in the significant change in electrical property [4]. Till date, a wide range of metal

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oxide-based NH3 gas sensors have been reported in the literatures. Some examples are ZnO

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nanorod [5], V2O5-WO3-TiO2 electrode [6], TiO2 thin film [7], polycrystalline WO3 nanofiber [8], nanocrystalline ZnO coated fiber [9] and nanostructured vanadium oxide thin film [10]. Gas

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sensing property of these metal oxide based systems greatly influenced by its structural

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composition. Though they are found to be active materials for gas sensing applications, several drawbacks like low sensitivity at room temperature, poor selectivity and high power

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consumption demand the design of suitable material for efficient detection of NH3 gas.

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Spinels, having a general formula of (M2+M23+O4) have shown great promise in lithium-ion battery [11], catalysis, gas sensing [12], energy storage [13] and supercapacitor applications [14]

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owing to its unique electrical , magnetic, catalytic and dielectric properties [15]. Among various spinel structures, spinel ferrites are promising materials having a common chemical composition

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of MFe2O4 (M= Cu, Mn, Co, Zn, Ni, etc.). Most of these spinel ferrite structures show semiconducting behavior in which two different cation sites are occupied by either transition or post-transition cations [16]. Due to this, spinel ferrites brought new possibilities for the design of

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suitable gas sensing material with enhanced sensitivity and selectivity. ZnFe2O4 [17], NiFe2O4 [18], CdFe2O4 [19], MnFe2O4 [20], MgFe2O4 [21] and CoFe2O4 [22] are some of the most widely investigated gas sensors for the detection of toxic gases. One such interesting spinel ferrite is CuFe2O4, which possess unique properties in nano scale range like enhanced magnetic and dielectric properties than bulk. It has been found that the magnitude of conductivity is normally influenced by the dielectric and magnetic properties of the material [23]. Due to the unique

dielectric and magnetic property of CuFe2O4, it has shown promising behaviour towards gas sensing application. However, the low sensitivity, high power consumption and poor selectivity of individual CuFe2O4 nanostructure pose a serious challenge in gas sensing application. Recent literatures reveal that modification of spinels with suitable support materials is found to be an efficient way to further improve its sensing performance. Towards this direction, graphene oxide

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(GO), due to its two-dimensional (2D) carbon monolayer structure, presence of heavy oxygenated functional groups and unique properties like high carrier mobility, good catalytic activity, thermal stability and good mechanical property [24] have shown great promise as a support material. It has shown desirable properties for the detection of many toxic gases due to its large surface area and high charge carrier mobility in its reduced form [25, 26]. Moreover, the availability of oxygen-containing functional groups on the GO sheets allows them to be modified

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with a wide range of metal/metal oxide nanoparticles with covalent or non-covalent approaches

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to convert it as a potential candidate for a wide range of applications. Therefore, we envisioned that modification of GO with copper ferrite would efficiently improve the gas sensing properties,

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owing to the unique properties of copper ferrite and GO for gas sensing application. To the best

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of our knowledge, the use of rGO-CuFe2O4 as an efficient gas sensor has not been thoroughly

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studied.

Keeping this in mind, herein we demonstrate the use of rGO-CuFe2O4 nanocomposite as a novel

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sensor for the detection of NH3. The structural modification of graphene oxide by copper ferrite became an excellent platform for NH3 sensing application. The material shows fast sensitivity,

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fine selectivity and reversibility towards NH3 with excellent response and as well as recovery time at room temperature. Experimental

1.1.

Materials

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1.

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Graphite powder, Fe (NO3)3.9H2O, Cu (NO3)2.3H2O and citric acid were purchased from SigmaAldrich. H2O2 (30%), ethanol, NaNO3, KMnO4, HCl and H2SO4 (98%) were purchased from HiMedia. All the chemicals were used without further purification. 18 mΩ Milli-Q water was used throughout the synthesis. 1.2. Preparation of rGO-CuFe2O4

Graphene oxide was prepared from natural graphite flakes according to the modified Hummers’ method [27]. Later on, rGO-CuFe2O4 nanocomposite was prepared by combustion method as shown in scheme 1. Cu (NO3)2.3H2O and Fe (NO3)3.9H2O were used as cation precursor and citric acid was used as fuel. In 20ml of distilled water, 100 mg of GO was dispersed in a beaker and sonicated for 30 min to get a well dispersion of GO. The cation precursors were dissolved in

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minimum amount of distilled water with a molar ratio as 1:2 of Cu2+ to Fe3+ and stirred for 30 min. Then, the metal solution was added to the GO dispersion drop wise followed by the addition of citric acid. The resulting mixture was stirred for an appropriate time at 100 °C to get a viscous solution. The beaker with the resulting solution was placed in muffle furnace and then heated at around 450 °C. Initially, the dispersion was boiled, dehydrated and then decomposed with evolution of huge amount of heat and gases. The mixture was then frothed, swelled forming

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foam and ruptured with flame and finally gave a foamy powder of rGO-CuFe2O4 nanocomposite.

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1.3. Sensor fabrication and gas sensing measurements

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Gas sensing measurement was done by depositing the nanocomposite on an ITO (Indium Tin

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Oxide) glass substrate according to a previous literature [28]. Typically, the sample was well dispersed in ethanol (1mg in 5ml) by ultrasonication for 30 min and then coated over the

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substrate. Two copper tap strips with dimension of 0.5cm × 2cm was used to create the electrical contact. All measurements were done in an air tight stainless steel sealed chamber with two

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microprobes at room temperature with relative humidity (RH) nearly 40-45 %. The sensing measurement was carried out by injecting different concentration of analyte through a tiny hole

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into the small furnace present inside the gas sensing chamber and then heated to vaporize after the chamber was sealed [29]. The sensor recovery was achieved by exposing the samples to

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ambient air. Keithley 6487 picoammeter/voltage source IV setup was used for all the gas sensing measurements by measuring the change in resistance as a function of time upon exposure to gas

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at a constant voltage. The sensor response (S) was calculated by using the following formula

𝑆(%) =

(𝑅𝑔 − 𝑅𝑎 ) × 100% 𝑅𝑎

Where Rg and Ra are the electrical resistances in gas and air, respectively. Response and recovery times were defined as the times needed for 90% of total change on exposure to gas and fresh air respectively. 2. Characterizations

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Philips PW 1830 X-ray diffractometer with Cu Kα source was used for X-ray diffraction study. FTIR spectra of the product were recorded using a Perkin-Elmer FTIR spectrophotometer with NaCl support. The information regarding composition of the products was performed using EDX (JEOL JSM-6480 LV). The XPS analysis was carried out on a K-Alpha instrument supplied by Thermo-scientific, UK, equipped with an Al Kα (1486.6 eV) monochromatic X-ray source. The measurement was carried out in the range of 0–1350 eV and at extremely high vacuum of 10–9

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mbar. Bruker RFS 27 spectrometer with 1064 nm wavelength incident laser light was used to record Raman spectra. Philips CM 200 equipment using carbon coated Nickel grids was used for

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transmission electron micrographs (TEM) analysis of the sample. The impedance measurement

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was done by using a computer-controlled impedance analyzer (Hioki Impedance Analyzer 1352)

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as a function of frequency (100 Hz to 1 MHz) at room temperature. Results and discussion

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3.1. Structure and morphology characterization

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During the synthesis of rGO-CuFe2O4 via solution combustion method, the oxygen functional groups on GO surface got reduced due the thermal treatment. This leads to high charge carrier mobility and makes it a promising supporting material for NH3 sensing application. To

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investigate the successful reduction and decoration of CuFe2O4 nanoparticle on rGO surface, the resulting nanocomposite was thoroughly characterized by different analytical techniques such as

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PXRD, FT-IR, TEM, XPS and Raman analysis. Initially, the phase purity and presence of various chemical groups on the composite was

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characterized by XRD and FTIR spectroscopy. In the XRD spectra of GO (figure 1 a), the x-ray diffraction peak at 2θ of 10.5° corresponding to the (002) plane, indicating the successful synthesis of GO by oxidation of graphite powder. The sharp peaks in the XRD spectra of rGOCuFe2O4 nanocomposite indicate the high crystalline nature of the nanocomposite which was later confirmed from the SAED pattern. For the rGO-CuFe2O4 nanocomposite, all the diffraction peaks can be indexed as spinel CuFe2O4 while no typical diffraction peak observed for GO (002)

indicating the formation of reduced GO. Besides, it is also possible that the GO in the rGOCuFe2O4 nanocomposite are exfoliated due to the crystal growth of CuFe2O4 nanoparticles between the interlayer of GO sheets [30]. All the indexed diffraction peaks in XRD pattern show the presence of cubic CuFe2O4 lattice with lattice constant a = b = c = 8.37 Å which are with good agreement with JCPDS: 77-0010. These results suggest the purity and high crystalline

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nature of the rGO-CuFe2O4 nanocomposite.

To further investigate the change in chemical environment on graphene oxide during the nanocomposite formation, FT-IR analysis was performed. As expected, the FTIR spectrum of GO is in good agreement with those from previous works [31]. In figure 1(b), the broad and intense band observed at 3437 cm−1 is ascribed to the stretching vibration of O−H. The bands at 1716,

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1380, and 1090 cm−1 correspond to the C−O stretching vibration, the C−O−H deformation vibration, and the C=O stretching vibration, respectively. On examining the FTIR spectra of

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rGO-CuFe2O4, it has been found that the intensity of the peaks at 1716 cm-1 and 1090 cm-1

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decreased considerably indicating the formation of rGO due to thermal reduction via solution combustion synthesis of the nanocomposite. Moreover, two new prominent absorption bands at

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about 529 and 436 cm−1 appeared in the FTIR spectrum of the rGO-CuFe2O4 nanocomposite, which can be assigned to vibrational modes in CuFe2O4 nanoparticles. The band at 436 cm-1 is

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due to the stretching vibration of Fe3+ in tetrahedral site and the band at 529 cm-1 is due to the

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stretching vibration of Cu2+ ion in octahedral site of the CuFe2O4 nanocomposite [32].

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Further, to investigate the possible electronic interactions between CuFe2O4 with GO surface in the rGO-CuFe2O4 nanocomposite, Raman analysis was carried out. Figure 1(c) shows the Raman

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spectra of both GO and rGO-CuFe2O4. Two predominant bands such as D (due to out of plane vibrations attributed to the presence of structural defects) and G (due to in-plane vibrations of sp2 bonded carbon atoms) band appeared in the case of both GO and rGO-CuFe2O4. The Raman

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spectra of pure GO showed the D band at 1306 cm-1 and the G band at 1601 cm-1 [15]. In comparison, the D and G band of rGO-CuFe2O4 nanocomposite appeared at 1286 cm-1 and 1588 cm-1 respectively, showing a slight shifting of bands due to the increase in disorders on GO surface. Further, the ratio of D- to G-band intensity (ID/IG) of GO was found to be 0.89 whereas for the rGO-CuFe2O4 it was found to be 0.96. This suggests possible electronic interaction between the CuFe2O4 structure with the GO sheets, which possibly play an important role in the

enhanced sensing performance of the nanocomposite as described in the later part of the discussion.

Later on, the morphology of the nanocomposite was investigated through transmission electron microscopy (TEM) as depicted in figure 2. The CuFe2O4 nanoparticles can be seen uniformly

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distributed on rGO surface having several nanometers size range (15-20 nm) as shown in figure.2a. It further revealed the typical spherical platelet structural characteristic of CuFe2O4 nanoparticles anchored on graphene oxide sheets in the composites. Figure 2(c) shows the HRTEM image of an individual CuFe2O4 nanoparticle with a lattice spacing of 0.25 nm. Above results confirm the formation of spinel CuFe2O4 nanoparticles on GO surface, which is in good agreement with the XRD results. Moreover, to validate the highly crystalline nature of the

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nanocomposite, SAED pattern was taken which justifies it by showing clear ring structure as seen

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in figure 2(d).

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XPS analysis was further carried out to investigate the chemical composition in our sample.

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Figure 3 (a) and (b) show the C1s spectra of GO and rGO-CuFe2O4, respectively. In C1s spectra of GO, the intense peak located at around 284.4 eV is due to non-oxygenated ring carbon

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molecules. On the other hand, the peaks at 286.8, 287.9 and 289.3 eV are assigned to the oxygencontaining groups such as (C–OH), (C= O), and (O= C–OH), respectively. On comparing the

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peaks of oxygen conaining functional groups on GO and nanocomposite, it was found that the intensities of these peaks decreased drastically in the nanocomposite. This is mainly due to the

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reduction of the GO surface during the solution combustion synthesis of rGO-CuFe2O4. This further suggests the presence of rGO in our composite.

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Surface area of the nanocomposite plays an important role in gas sensing behavior as it depends upon the adsorption and desorption of the analyte gas. To find out the surface area of our

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nanocomposite (rGO-CuFe2O4), N2 adsorption-desorption analysis was carried out and are shown in figure 4. For better comparison, the surface area of GO and rGO (synthesized under similar solvothermal condition to that of nanocomposite) was also measured. The BET surface area of GO, rGO and rGO-CuFe2O4 nanocomposite exhibit type IV isotherm based on the IUPAC classification [33]. The specific surface area of GO (rGO) was found to be 154 m2 g_1. In comparison, the surface area of the composite after solution combustion method was found to be

187 m2/g. To check that GO was reduced during the composite synthesis, GO was treated with the similar solution combustion method as that of the composite. The surface area of chemically modified GO (rGO) was found to be 265 m2/g. This increase in surface area suggest the removal of oxygen functional groups during the combustion method, thereby forming a reduced form of GO (rGO). After the decoration of CuFe2O4 nanoparticles, some of the pores of rGO were

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blocked and hence, the surface area of the composite was decreased to 187 m2/g. This study further confirms the existence of rGO in our composite. Gas Sensing Studies and Discussion

After successful characterization of the nanocomposite, its sensing capacity towards NH3 was then investigated. In order to verify the good electrical properties of the nanocomposite, its

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current-voltage characteristic was first investigated by monitoring the change in current with

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voltage. Figure 5 shows the I-V curve of the rGO-CuFe2O4 nanocomposite sensor measured from -5V to +5V to investigate the electrical contact between the sensing material and the gold

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coated microprobes (electrode) at room temperature. When the voltage increased, the current

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also increased linearly which shows that the electrical contact between probe and rGO-CuFe2O4 is ohmic type. This linear behavior of the sensor is due to the smooth movement of charge

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carriers within the material [34]. The I-V curves of GO, rGO and CuFe2O4 were also measured

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for comparison with the nanocomposite in same conditions. It is clear from the I-V curve that GO has lowest conductivity. However, the conductivity of rGO increased sharply due to the reduction of oxygen functional groups. Also, CuFe2O4 exhibit low conductivity than rGO-

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CuFe2O4 nanocomposite.

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Figure 6 (a) shows the resistance change of the rGO-CuFe2O4 nanocomposite gas sensor towards 50 ppm NH3 at room temperature in 42% RH. The resistance of the nanocomposite increased when the sensor was subjected to interact with the analyte gas and decreased when the gas was

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allowed to release. As NH3 is an electron donating gas, the nanocomposite sample can be regarded as p-type semiconductor. In order to investigate the role of the amount of CuFe2O4 in the nanocomposite towards gas sensing behavior, its wt% was gradually increased to get the optimized hybrid sensor. It was observed that with an increase in the amount of CuFe2O4 in the rGO-CuFe2O4 nanocomposite, the response behavior enhanced till 10:5 ratio with a maximum value of response 9.9% (figure 6(b)). However, when GO and CuFe2O4 were mixed beyond this

ratio such as 10: 8 and 1:1, the resistance response of the samples decreased to 9.7 and 9.5 respectively. Therefore, all the NH3 sensing tests were carried out at room temperature with the optimized hybrid rGO-CuFe2O4 (10:5 ratio) sensor. Further, to check the role of rGO in rGOCuFe2O4 nanocomposite in the enhanced sensing behaviour, the behaviour of bare CuFe2O4 nanoparticle was studied and shown in figure S1. It has been found that without rGO, the

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composite showed a response of 2.9%. However, with the introduction of rGO in the nanocomposite, the sensing behaviour exhibits significantly higher response, suggesting the key role of rGO in the composite. This can be attributed to various factors. Firstly, the superior electrical property generated in the reduced form of GO (rGO) contributes to the enhanced conductivity of the composite. Secondly, the higher surface area of rGO (table 1) facilitates

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improved gas adsorption and diffusion on its surface.

The reproducibility of the above sensor was then tested towards 50 ppm NH3 gas at room

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temperature, demonstrating good reproducibility up to four successive cycles (figure S2). In

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addition, the sensitivity increases dramatically when the nanocomposite sensor was subjected to

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a wide range of NH3 concentrations and recovers to the original values when the NH3 is replaced by air (figure 6 c), which is an essential property of a good sensor. The change in sensitivity

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behavior of the sensor can be ascribed to the increased adsorption and desorption of NH3 molecules on the nanocomposite sensing material which has further been discussed thoroughly in

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the mechanism section. Moreover, good reproducibility was also obtained for all the different concentrations as shown in figure 6 (d).

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A practically applicable gas sensor should have essential properties like good reproducibility with fast response and recovery time. In order to demonstrate these features in our sensor, the

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response-recovery characteristic of rGO-CuFe2O4 nanocomposite was measured by exposing to 50 ppm NH3 at room temperature. When NH3 was allowed to enter the testing chamber, the

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sensor response increased rapidly and when subjected to air, the sensor recovered to the initial state very fast i.e., 3 sec and 6 sec, respectively(figure 7(a)). The response and recovery times are defined as the time period required to reach 90% of the final equilibrium value after the analyte gas was allowed to enter and removed, respectively [35]. The rapid response and recovery of our sensor can be attributed to the high surface area of our nanocomposite which is confirmed from BET data. The increased surface area facilitates the fast mass transfer of NH3 molecules to the

active sites which improves the rate of charge carriers’ motion to cross over the barriers. Therefore, its response and recovery are quicker than that of pure graphene oxide surface. For further checking its suitability, the fitting curve of sensor response versus different NH3 gas concentration in the range of 5-200 ppm was measured. The correlation coefficient R2 of the

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fitting curve in the range 5-20 ppm was found to be 0.997 and for 30-200 ppm it was 0.996, indicating good linearity in both the cases (figure 7(b)). This type of linear behaviour is suitable for estimation of NH3 in practical sensing application. Further, the sensitivity of the sensor was 𝛥𝑆

determined by calculating the slope (𝛥𝐶) from the plot. The slope in the range or 5-20 was found to be 0.28 which rapidly deceased to 0.098 after 30 ppm of NH3 gas. Therefore, from the above data it can be concluded that, the availability of active sites for adsorption on the sensor material

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decreased with the increase in NH3 concentration.

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To validate the usefulness of our sensor, a comparison of our hybrid sensor with the other

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available NH3 sensor has been carried out and is displayed in table 2. Our nanocomposite based sensor shows very good response toward NH3 gas with fast response time and recovery time in

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comparison to other sensors available in the literatures [36-45].

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As the sensing ability of a sensor mostly depends on temperature change, the influence of operating temperature on the sensor response towards 50 ppm of NH3 was then carried out

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(figure 8 (a)). It was found that with the increase in operating temperature of the sensor, the response increased. At higher temperature, the pre-adsorbed water molecule desorb, thereby

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exposing more active surface for NH3 gas. Besides temperature, another important parameter is the effect of humidity on the sensing performance of a sensor. Figure 8 (b) shows the influence of

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humidity on the sensing response to 50 ppm NH3. It was found that the response in air decreased as the humidity increased from 28% to 86%. This can be attributed to the stronger adsorption

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ability of water molecule on rGO surface, resulting in blockage of most of the active sites. This in turn allow less NH3 adsorption leading to lower sensor response. Similar behaviour has also been reported in other literature [46]. In the end, the long-term stability of our hybrid sensor was studied in order to showcase its practical applications. Figure 8(c) shows the stability of the sensor for 45 days. The average response towards 50 ppm of NH3 gas was found to be 9.3 ± 0.5% to that of initial value of 9.8%,

indicating good stability of our sensor. Besides good stability, selectivity is also an important property of a sensor for the effective detection of a particular gas. To showcase this property, the sensitivity of the hybrid sensor was investigated towards various volatile vapours. Figure 8(d) represents the response of rGO-CuFe2O4 nanocomposite towards different gases like NH3, acetone, toluene, methanol, water, CO2 and NO2 at room temperature. It can be seen that our

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rGO-CuFe2O4 nanocomposite sensor has a larger response (9.9 %) to NH3 compared to the other reducing/oxidising gases. The increased selectivity can be attributed to the unique surface and structural property of rGO as reported by various research groups [47]. For example, Mattson et al demonstrated surface functional groups on rGO effectively react with NH3 molecules resulting in good selectivity [48]. Similarly, Cao and co-workers presented the effective adsorption of NH3 molecules on rGO surface owing to its unique surface and structural

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property as a major factor for its enhanced selectivity [49]. Similar to these observations, as

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seen from Raman (figure 1(c)) and XPS (figure 3), rGO in our composite consists of large number of defect sites and some unreduced functional groups. When NH 3 molecules enter the

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sensing system, it diffuses to the surface of rGO and interacts with the defect site and carboxyl,

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hydroxyl, carbonyl and epoxy functional groups via hydrogen bonding [48], ensuring enhanced selectivity. Because of these suitable properties like enhanced response and recovery, good

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stability, reproducibility and selectivity towards NH3 at room temperature, our designed rGO-

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CuFe2O4 composite has shown to be a potential candidate in NH3 gas sensing application. 4. Sensing mechanism

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Based on the above results, a plausible mechanism for the enhanced gas sensing behaviour of rGO-CuFe2O4 nanocomposite towards NH3 has been presented in scheme 2. First, impedance

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measurements were carried out to study the effect of oxygen adsorption which would subsequently suggest the plausible mechanism of our composite (figure S3). As oxygen adsorption play an important role in the performance of a gas sensor, bare rGO and CuFe2O4

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was initially exposed to O2 and the comparative impedance spectra were measured. Both showed decreased resistances indicating a p-type conduction mechanism, cases similar to those reported in the literatures [50]. At first, the oxygen molecules present in air get adsorbed on the surface of nanocomposite via chemisorptions process by trapping electrons from the conduction band of the material. This results in increase in the concentration of vacancies and subsequently decreases its resistance in ambient air. This decrease in resistance can be seen the form of a

small arc in impedance spectra of the rGO-CuFe2O4 composite as shown in figure S3 [51]. When the analyte gas (NH3) interacts with the sensing material surface, the chemisorbed oxygen molecules react with NH3 and electrons are supplied to the conduction band of the sensing material. Thus, the concentration of the charge carrier (hole) decreases and the resistance increases. When the analyte gas sample (NH3) breaks the bond in desorption process, the

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resistance of the sensor restores to its initial state.

In the end, it can be concluded that rGO-CuFe2O4 nanocomposite showed enhanced activity and selectivity in compare to that of GO and CuFe2O4 nanoparticles. This enhanced sensing behaviour can be attributed to three factors as discussed bellow. Firstly, the effective electronic interaction between rGO and CuFe2O4 nanoparticles help in the sensing behaviour via the

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resistance change in the rGO-CuFe2O4 composite. Figure S1 justifies this effective electronic interaction in our composite. Secondly, the introduction of CuFe2O4 nanoparticles avoids the

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restacking problem of GO-sheets, as confirmed from XRD analysis, leading to good surface

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accessibility in the composite. This in fact allows more amount of oxygen to be adsorbed and

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ionised in rGO-CuFe2O4 sensor [52]. In the end, creation of more defect sites and high surface area help in the enhanced diffusion of NH3 gas in the composite, leading to increased sensing

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behaviour. Therefore, the synergistic effect between rGO and CuFe2O4 result in improved conductivity, good surface accessibility and increased NH3 adsorption. All the above properties

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Conclusions

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play an important role in the enhanced sensing behaviour of our nanocomposite sensor.

In conclusion, an inexpensive rGO-CuFe2O4 nanocomposite sensor exhibiting superior gas sensing property was synthesized by a simple combustion method. The sensor was thoroughly

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characterized by a set of analytical techniques such as FTIR, Raman, XRD, TEM and XPS analysis. The formation of spinel ferrite nanoparticles was demonstrated by XRD and FTIR

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analysis. We have for the first time demonstrated the use of rGO-CuFe2O4 as highly sensitive and selective gas sensor for the detection of NH3 gas at room temperature. The combination of CuFe2O4 with rGO results in the enhancement of the sensing ability of CuFe2O4. The sensor showed good sensitivity (25% for 200ppm and 2% for 5ppm) for a wider range of NH3 gas among other gases at room temperature with fast response (3 sec) and recovery (6 sec) time. Our

findings hold great promise for the design of graphene oxide-based nanocomposites towards real world sensing applications.

Acknowledgements

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The authors are thankful to Ministry of Human Resource Development (MHRD), Govt. of India, for funding. Sensing facility at Department of Physics, NIT Rourkela and Department of Science and Technology (DST). IISc Bangalore for providing XPS facility and IIT Madras for providing FT-Raman facility.

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Reference

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BIOGRAPHIES L. Satish K. Achary is a Ph.D student in the Department of Chemistry, National Institute of Technology Rourkela, India. His research interests include Graphene/Reduced graphene oxide based sensors.

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Aniket Kumar has received his Ph.D degree in the Department of Chemistry, National Institute of Technology Rourkela, India. His research interests include Carbon based materials for catalysis and sensing applications. Bapun Barik is a Ph.D student in the Department of Chemistry, National Institute of Technology Rourkela, India. His research interests include Biopolymer based materials for catalytic and environmental applications. Pratap Sagar Nayak is a Ph.D student in the Department of Chemistry, National Institute of Technology Rourkela, India. His research interests include Metal oxide based materials for catalytic applications.

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Nilakantha Tripathy is a Ph.D student in the Department of Physics, National Institute of Technology Rourkela, India. His research interests include metal oxide based materials for high K dielectric.

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Jyoti Prakash Kar received his Ph.D. degree in physics from Indian institute of Technology Delhi, India. His research interests include solid state sensors and devices.

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Priyabrat Dash received his Ph.D. degree in the Department of Chemistry, University of Saskatchewan, Canada, in June, 2010. His research interests include rational design of highly selective nanocatalysts and ultrasensitive solid-state sensors.

Figure Captions 1. (a) XRD (b) FTIR and (c) Raman spectra of GO and rGO-CuFe2O4. 2. (a-c) TEM image of rGO-CuFe2O4 in different magnification, (d) SAED pattern

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of rGO-CuFe2O4. 3. (a) C1s XPS spectra of GO and (b) C1s XPS spectra of rGO-CuFe2O4.

4. N2 adsorption-desorption study of (a) GO, (b) rGO-CuFe2O4 and (c) rGO nanocomposite.

5. I-V curves of GO, rGO, CuFe2O4 and rGO-CuFe2O4 composite based sensor at room temperature.

6. (a) Response and recovery of rGO-CuFe2O4 nanocomposite sensor, (b)

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Comparative responsive of GO,CuFe2O4 and rGO-CuFe2O4 nanocomposite sensor

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prepared by combustion method and by varing the wt. % of GO and CuFe2O4

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spinel nanoparticle towards 50 ppm ammonia gas, and (c, d) Response of the sensor towards various concentration (5-200) ppm of ammonia gas.

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7. (a) Response and recovery time of rGO-CuFe2O4 based gas sensor towards 50 ppm of ammonia gas and (b) linear dependence of response of rGO-CuFe2O4

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sensor on different concentration of ammonia at room temperature.

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8. (a) Effect of temperature and (b) Effect of humidity (c) Long term stability on response of rGO-CuFe2O4 sensor towards 50 ppm of ammonia at room

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temperature and (d) Selectivity of the response of the hybrid rGO-CuFe2O4 based sensor towards 50 ppm of different oganic volatile solvents. (a) Long term

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stability of rGO-CuFe2O4 nanocomposite sensor towards 50 ppm of ammonia and (b) Selectivity of the response of rGO-CuFe2O4 sensor towards 50 ppm of

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different oganic volatile solvents.

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Fig.1. (a) XRD (b) FTIR and (c) Raman spectra of GO and rGO-CuFe2O4.

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Fig.2. (a-c) TEM image of rGO-CuFe2O4 in different magnification, (d) SAED pattern of rGO-

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CuFe2O4.

Fig. 3. (a) C1s XPS spectra of GO, (b) C1s XPS spectra of rGO-CuFe2O4.

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Fig.4. N2 adsorption-desorption study of (a) GO, (b) rGO-CuFe2O4 and (c) rGO nanocomposite.

Fig.5. I-V curves of GO, rGO, CuFe2O4 and rGO-CuFe2O4 composite based sensor at room temperature.

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Fig.6 (a) Response and recovery of rGO-CuFe2O4 nanocomposite sensor, (b) Comparative

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responsive of GO,CuFe2O4 and rGO-CuFe2O4 nanocomposite sensor prepared by combustion method and by varing the wt. % of GO and CuFe2O4 spinel nanoparticle towards 50 ppm

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ammonia gas, and (c, d) Response of the sensor towards various concentration (5-200) ppm of

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ammonia gas.

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Fig.7 (a) Response and recovery time of rGO-CuFe2O4 based gas sensor towards 50 ppm of

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ammonia gas and (b) linear dependence of response of rGO-CuFe2O4 sensor on different

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concentration of ammonia at room temperature.

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Fig.8 (a) Effect of temperature and (b) Effect of humidity (c) Long term stability on response of rGO-CuFe2O4 sensor towards 50 ppm of ammonia at room temperature and (d) Selectivity of the

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response of the hybrid rGO-CuFe2O4 based sensor towards 50 ppm of different oganic volatile solvents. (a) Long term stability of rGO-CuFe2O4 nanocomposite sensor towards 50 ppm of

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ammonia and (b) Selectivity of the response of rGO-CuFe2O4 sensor towards 50 ppm of different oganic volatile solvents.

Scheme Captions

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1. Schematic representation of the synthesis of rGO-CuFe2O4 nanocomposite through solution combustion method. 2. Schematic representation of the sensing mechanism.

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Scheme 1: Schematic representation of the synthesis of rGO-CuFe2O4 nanocomposite through solution combustion method.

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Scheme 2: schematic representation of the sensing mechanism.

Table captions

rGO

265

rGO-CuFe2O4

187

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Table 1. Surface area and pore size distribution of GO, rGO and rGO-CuFe2O4. Sample BET surface area (m2 g-1) GO 154

Table 2: Comparison of the sensor performance of the present work and previous reported literature. Tres(Sec)

PEDOT nanowire

2 (10 ppm), 12 (70 ppm)

10

PEDOT nanotube

2.1 (5 ppm), 24 (100 ppm)

<1

Graphene/PEDPTPSS SWCNTs/ PEDOTPSS Pf-MWCNT/PANI

0.9 (5 ppm), 7 (1000 ppm)

A

M

180

Trec (Sec)

Tm(°C)

references

10

rt

36

30

rt

37

-

rt

38

U

S (%)

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M

12

18

rt

39

0.015 (20 ppm), 0.075 (100 ppm)

100

700

rt

40

PANI/TiO2

1.67 (23 ppm), 5.55 (117 ppm)

18

58

rt

41

palladium/ polypyrrole

0.14 (20 ppm), 0.2 (100 ppm)

14

148

rt

42

53.6% (50 ppm)

4

300

rt

43

25% (50ppm) (calculated)

14

--

rt

44

rGO decorated TiO2

4.8 % (50 ppm) (calculated)

--

--

rt

45

GO-CuFe2O4

2.35 (5ppm), 24.41 (200 ppm)

3

6

rt

Present work

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rGO–Co3O4 nanofibers

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0.1 (2 ppm), 33 (300 ppm)

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0.75% rGO-SnO2

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Materials (M), response (S(%)), response time (Tres), recovery time (Trec), and measurement temperature (Tm (°C)).