rGO nanocomposites for high performance electrodes

Available online at www.sciencedirect.com

ScienceDirect Materials Today: Proceedings 18 (2019) 5012–5018

www.materialstoday.com/proceedings

ICMPC-2019

Mg(OH)2/rGO nanocomposites for high performance electrodes Richa Bhargavaa, Shakeel Khana,*, Naseem Ahmada and Mohd Mohsin Nizam Ansaria a

Department of Applied Physics, Z.H College of Engineering and Technology, Aligarh Muslim University, Aligarh-202002, India

Abstract Mg(OH)2 nanoparticles and Mg(OH)2–rGO nanocomposites were prepared by microwave assisted co-precipitation method. The morphological study of the as-prepared samples was done by Transmission electron microscopy (TEM). The real and imaginary parts of complex impedance were studied as a function of frequency over a range of 50Hz to 5MHz at room temperature. The Nyquist plots were recorded to know the resistance contribution from grains and grain boundaries. DC conductivity (𝜎 ) has been studied as a function of temperature by using two probe method. A minor increase in GO concentration causes increases in the value of 𝜎 in the nanocomposites. The activation energy (𝐸 ) was calculated by using Arrhenius equation and its value decreases with the increases in the GO concentration. concentration. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: TEM; Nyquist plot; Mg(OH)2; activation energy

1. Introduction Highly efficient energy storage devices have generated a lot of interest over the last few decades due to the drastic reduction of fossil fuel assets. At present, the electronic devices require scaling down to nanoscale dimensions due to the extensive demand of small, supple, lightweight and portable electronic devices [1]. These electronic devices require high backup, sustainable storage and transformation system like a capacitor, supercapacitor, battery and fuel cell. These systems need nanomaterials which show excellent conductivity, good stability and high dielectric constants. * Corresponding author. Tel.: +91-9358210751.

E-mail address: [email protected]

2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019

Shakeel Khan et al./ Materials Today: Proceedings 18 (2019) 5012–5018

5013

Graphene and reduced graphene oxide (rGO) have been frequently used as promising materials to modify electrodes over the past decade [2]. Graphene a 2D carbon material is having flat monolayer of sp2 carbon atom which is largely used as an electrode material due to its unique properties viz. high specific surface area, high electrical conductivity, good chemical stability and high specific capacitance [3]. Several graphene based materials have been studied as electrode materials due to their fascinating properties including low cost, eco-friendly and high stability. Many efforts has been put in the synthesis of metal oxide-based graphene composites such as TiO2 [4], RuO2 [5], MnO2 [6], CuO [7], and NiO [8]. Mg(OH)2 is an insulating metal oxide with large band gap and has many interesting features such as nontoxicity, noncorrosivity, thermal stability and high dielectric property [9]. Mg(OH)2 shows poor electrical conductivity and has limited use in various electronic applications. To overcome this limitation the conductive rGO sheets are incorporated into Mg(OH)2 which would enhanced the conductivity, high surface area and mechanical flexibility of Mg(OH)2 electrode. The enhancement in the conductivity leads to enhanced capacitive performance and makes Mg(OH)2–rGO nanocomposite a good electrode material. In this article, we report the microwave assisted coprecipitation route for the synthesis of Mg(OH)2-rGO nanocomposites. The results presented in this paper showed enhanced performance of the nanocomposite in terms of increased electrical conductivity which can have applications as an electrode material in various devices such as supercapacitors, batteries etc. 2. Sample Preparation and Characterization: Graphene Oxide (GO) was synthesized by modified Hummers method [10]. In this process, the oxidation of graphite powder takes place in the presence of strong oxidizing agent (Conc.H2SO4) to form graphene oxide. Pure Mg(OH)2, Mg(OH)2-rGO1 and Mg(OH)2-rGO2 nanocomposites with GO concentration (100, 200 mg) were prepared through microwave assisted co-precipitation route by using AR grade magnesium nitrate (Mg(NO3)2.6H2O) as initial precursor. Details of preparation technique and the results of some characterization techniques such as X-ray diffraction measurement (XRD), Scanning electron microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR). Raman spectroscopy, UV–Visible spectroscopy, and Room temperature dielectric properties etc. have been discussed in an earlier report [9]. The morphological study of the nanocomposites was carried out using transmission electron microscope TEM (JEOL, JEM 2010, JAPAN). Impedance analysis was done by using LCR meter (HIOKI, JAPAN) in the frequency range of 50 Hz to 5 MHz. The DC resistivity of the as prepared samples was studied as a function of temperature in the range 320–400 K by two-probe method using Keithley 6517B electrometer. For these measurements, each sample was pressed into a circular pellet having approximate 1-2 mm thickness and 12.3 mm diameter and was coated with silver paste on the adjacent faces to obtain parallel plate capacitor geometry. Result and Discussion: 3. Results and discussion: The TEM micrographs for Mg(OH)2-rGO1 and Mg(OH)2-rGO2 nanocomposites are shown Fig.1(a-b) respectively. These TEM micrographs show the existence of abundant irregular graphene sheets. The rod-like structures of Mg(OH)2 are randomly anchored and agglomerated onto the surfaces of the rGO sheets. Mg(OH)2rGO1 and Mg(OH)2-rGO2 nanocomposites show similar morphology.

5014

Shakeel Khan et al./ Materials Today: Proceedings 18 (2019) 5012–5018

Fig. 1: TEM micrographs of (a) Mg(OH)2-rGO1 and (b) Mg(OH)2-rGO2 nanocomposites.

The complex impedance analysis is a powerful technique to understand the influence of grains and grain boundaries on the electrical properties of the system. The complex impedance of the system can be stated as the sum of real and imaginary part, as follows

Z * = Z ′ − jZ ′′

(1)

|𝑍|𝑐𝑜𝑠𝜃 and 𝑍 |𝑍|𝑠𝑖𝑛𝜃 are the real and imaginary parts of complex impedance, θ is the phase Here, 𝑍 angle. The Bode plots are the graphical illustration of the frequency dependence of real and imaginary part of complex impedance. Fig. 2(a-b) shows the Bode plots for Mg(OH)2, Mg(OH)2-rGO1 and Mg(OH)2-rGO2 nanocomposites. The real part of the complex impedance gives the resistive contribution in the synthesized samples. The frequency dependence of the real part of impedance 𝑍 is shown in Fig. 2(a). It can be noticed that the value of 𝑍 gradually decreases with the rise in the frequency and becomes constant at the higher frequency. At lower frequency the value of 𝑍 is higher because of the contribution from space charge polarisation, electronic polarization and dipolar polarization. As the frequency is increased the value of 𝑍 becomes lower and attains a constant value because the space charge polarisation and dipolar polarization do not follow the rapid field variations. The Mg(OH)2-rGO nanocomposite shows better resistive component as compared to Mg(OH)2. The reduction in the value of 𝑍 in the nanocomposites is due to the presence of trapping sites created in the form of imperfection which offer space charge polarization and increase the electron exchange interaction of the charge carriers between the rGO sheets [11]. The value of 𝑍 decreases with the increase in the concentration of GO in the composites, which supports a growth in the conductivity. The imaginary part of the complex impedance gives the capacitive contribution in the synthesized samples. The frequency dependence of the imaginary part of impedance 𝑍 is shown in Fig. 2(b). It can be noticed that the value of 𝑍 gradually decreases with the rise in the frequency and becomes constant at the higher frequency. Additionally, a relaxation peak is detected at lower frequency region in the imaginary part of the complex impedance plot in all the samples. This relaxation peak indicates an activation mediated conduction mechanism [12]. The capacitive component rises with the concentration of the GO in the nanocomposite. The Mg(OH)2-rGO nanocomposites show better capacitive component as compared to Mg(OH)2. In the presence of an external field, the movement of free charge at the interface surges the space charge polarization and enhances the capacitive component in the nanocomposites [13]. A decline in 𝑍 and 𝑍 values with the increasing amount of GO in the nanocomposite, supports an increase in conductivity. This increasing conductivity behavior is useful for this composite as an electrode material.

Shakeel Khan et al./ Materials Today: Proceedings 18 (2019) 5012–5018

Fig. 2: (a) Real part and (b) Imaginary part of complex impedance (Bode plots) for Mg(OH)2, Mg(OH)2-rGO1 and Mg(OH)2-rGO2 nanocomposites.

Fig. 3: Nyquist plot for (a) Mg(OH)2, (b) Mg(OH)2-rGO1 and (c)Mg(OH)2-rGO2 nanocomposites.

5015

5016

Shakeel Khan et al./ Materials Today: Proceedings 18 (2019) 5012–5018

Nyquist Plot or Cole-Cole plot is usually a representation of an imaginary component of complex impedance 𝑍 versus real component of complex impedance (𝑍 ). The Nyquist plots can show two semicircles, one at low frequency is ascribed to grain contribution and another at high frequency is ascribed to the grain boundary contribution. The Fig 3(a-c) shows the Nyquist plots of Mg(OH)2, Mg(OH)2-rGO1 and Mg(OH)2-rGO2 nanocomposites which shows a single semicircle indicating the dominance of grain boundaries contribution. From the Fig 3(a-c), it is seen that the addition of graphene considerably reduces the width of the semicircle. This reduction in the width is due to the decrement in the charge transfer resistance on the surface of rGO sheets [14]. In order to determine the values of grain boundary resistance (𝑅 ) the diameter of the semicircles in the Nyquist plots is measured. The resonance frequency (𝜔 ) is determined by considering the maximum peak value of the imaginary part of complex impedance. The values of capacitance (𝐶 ) and relaxation time (𝜏 ) can be calculated by using the relations:

C gb =

1

(2)

Rgbω gb

τ gb = R gb C gb

(3)

Table 1. Impedance and electrical parameters of Mg(OH)2, Mg(OH)2-rGO1 and Mg(OH)2-rGO2 nanocomposites. Samples

Rgb (MΩ)

Cgb (nF)

τgb (μs)

σdcx 10-6(MΩ-cm)-1

Activation energy(Ea) (eV)

Mg(OH)2

1.12

0.65

726

6.98

0.70

Mg(OH)2-rGO1

0.81

1.29

1039

35.39

0.48

Mg(OH)2-rGO2

0.45

4.15

1867

51.25

0.27

The nonlinear least square (NLLS) fit method was used to calculate the impedance parameters (𝑅 , 𝐶 𝑎𝑛𝑑 𝜏 ) by examining the impedance data and are listed in Table-1. As seen from the Table-1 the value of the 𝑅 decreases whereas 𝐶 increases with the increasing rGO content in the composites. This behavior can be due to the introduction of conductive rGO sheets into the nanocomposites, which increase the electron-transfer rate by lowering the electron-transfer distance and offering more paths for the conduction between the rGO sheets [15]. The addition of rGO in the nanocomposites enhances the value of relaxation time in the nanocomposites. This can be due to the presence excited free charge carriers in the space charge region produced by the creation of oxygen or cation vacancies. The DC conductivity (𝜎 ) of Mg(OH)2, Mg(OH)2-rGO1 and Mg(OH)2-rGO2 nanocomposites are studied by using two-probe method as a function of temperature in the range 320–400 K. The Fig. 4(a), illustrates that 𝜎 of all the samples shows a growing trend with increase in temperature, thus approving the semiconducting electrical behavior. The introduction of GO overcomes the limit of poor conductivity in Mg(OH)2. The room temperature values of 𝜎 of Mg(OH)2, Mg(OH)2-rGO1 and Mg(OH)2-rGO2 nanocomposites are listed in Table-1, which shows that the value of 𝜎 increases with the addition of GO in the nanocomposites. These results can be due the increased conductive networks because of the removal of oxygen functional groups in the GO which improves electron transfer in the nanocomposites [16]. The room temperature values of 𝜎 of Mg(OH)2, Mg(OH)2-rGO1 and Mg(OH)2-rGO2 nanocomposites are listed in Table-1, which shows that the value of 𝜎 increases with the addition of GO in the nanocomposites. These results can be due the increased conductive networks because of the removal of oxygen functional groups in the GO which improves electron transfer in the nanocomposites.

Shakeel Khan et al./ Materials Today: Proceedings 18 (2019) 5012–5018

5017

Fig. 4: (a) DC conductivity as a function of temperature (b) Plot of ln σT vs (1000/T) for Mg(OH)2, Mg(OH)2-rGO1 and Mg(OH)2-rGO2 nanocomposites.

The Fig. 4(b) show the plot of ln σT vs (1000/T) for Mg(OH)2, Mg(OH)2-rGO1 and Mg(OH)2-rGO2 nanocomposites. The solid dots shown in figure are the experimental points and the solid line represents the linear fitted curve. The DC conductivity followed the Arrhenius law given by the relation [17]:

 − Ea    KT 

σ dcαT −1 exp

(4)

where K the Boltzmann constant, T is the absolute temperature and Ea is the activation energy for dc conduction which characterizes the position of trap levels below the conduction band. From the slopes of these Arrhenius plots the values for Ea of Mg(OH)2, Mg(OH)2-rGO1 and Mg(OH)2-rGO2 nanocomposites are calculated and are given in Table-1. It can be noted that the activation energy is higher for Mg(OH)2-rGO nanocomposites than pure Mg(OH)2. This may be due to the increase in the amount of sp2 carbon atoms because of the elimination of the functional groups (-OH, -COOH, and –CO) and also due to the increment in the conductivity in the nanocomposites in comparison with Mg(OH)2 [18]. In the nanocomposite, the GO is completely reduced to rGO due to which 𝜎 shows higher values and it leads to lower activation energies. 4. Conclusion We have successfully synthesized Mg(OH)2 NPs and Mg(OH)2-rGO NCs by microwave assisted coprecipitation method. The Bode plots shows decrement in that the value of 𝑍 and 𝑍 which increases the conductivity. The Nyquist plot exhibits only one semicircle for Mg(OH)2 NPs and Mg(OH)2–rGO nanocomposites and suggests that the interfaces in the composites are more active than Mg(OH)2 grains. The morphological studies done by TEM reveals that the Mg(OH)2 nanorods were grown on the rGO sheets. DC conductivity were studied using two probe measurement and it is found that the value 𝜎 increases (i.e., from 6.98x10-6 to 51.25x10-6 Ω-1-cm1 ) in the nanocomposites. The activation energy found to decrease from 0. 70 eV (Mg(OH)2) to 0.27 eV (Mg(OH)2rGO2). These outcomes recommend that produced nanocomposites can be promising materials for practical applications as an electrode materials in various devices..

5018

Shakeel Khan et al./ Materials Today: Proceedings 18 (2019) 5012–5018

Acknowledgements One of the authors (Richa Bhargava) is thankful to the University Grants Commission (UGC), New Delhi for providing financial support in the form of fellowship. References [1] J. Kim, W.-H. Khoh, B.-H. Wee, J.-D. Hong, Fabrication of flexible reduced graphene oxide–TiO 2 freestanding films for supercapacitor application, RSC Adv. 5 (2015) 9904–9911. doi:10.1039/C4RA12980F. [2] R.K. Jammula, V.V.S.S. Srikanth, B.K. Hazra, S. Srinath, ZnO nanoparticles’ decorated reduced-graphene oxide: Easy synthesis, unique polarization behavior, and ionic conductivity, Mater. Des. 110 (2016) 311–316. doi:10.1016/j.matdes.2016.08.001. [3] Z. Xing, Q. Chu, X. Ren, J. Tian, A.M. Asiri, K.A. Alamry, A.O. Al-Youbi, X. Sun, Biomolecule-assisted synthesis of nickel sulfides/reduced graphene oxide nanocomposites as electrode materials for supercapacitors, Electrochem. Commun. 32 (2013) 9–13. doi:10.1016/j.elecom.2013.03.033. [4] A. Ramadoss, G.-S. Kim, S.J. Kim, Fabrication of reduced graphene oxide/TiO2 nanorod/reduced graphene oxide hybrid nanostructures as electrode materials for supercapacitor applications, CrystEngComm. 15 (2013) 10222. doi:10.1039/c3ce41517a. [5] F. Yang, L. Zhang, A. Zuzuarregui, K. Gregorczyk, L. Li, M. Beltrán, C. Tollan, J. Brede, C. Rogero, A. Chuvilin, M. Knez, Functionalization of Defect Sites in Graphene with RuO 2 for High Capacitive Performance, ACS Appl. Mater. Interfaces. 7 (2015) 20513– 20519. doi:10.1021/acsami.5b04704. [6] S. Ghasemi, S.R. Hosseini, O. Boore-talari, Sonochemical assisted synthesis MnO2/RGO nanohybrid as effective electrode material for supercapacitor, Ultrason. Sonochem. 40 (2018) 675–685. doi:10.1016/j.ultsonch.2017.08.013. [7] Y.N. Sudhakar, H. Hemant, S.S. Nitinkumar, P. Poornesh, M. Selvakumar, Green synthesis and electrochemical characterization of rGO– CuO nanocomposites for supercapacitor applications, Ionics (Kiel). 23 (2017) 1267–1276. doi:10.1007/s11581-016-1923-7. [8] X. Sun, H. Lu, P. Liu, T.E. Rufford, R.R. Gaddam, X. Fan, X.S. Zhao, A reduced graphene oxide–NiO composite electrode with a high and stable capacitance, Sustain. Energy Fuels. 2 (2018) 673–678. doi:10.1039/C7SE00420F. [9] R. Bhargava, S. Khan, Effect of reduced graphene oxide (rGO) on structural, optical, and dielectric properties of Mg(OH) 2 /rGO nanocomposites, Adv. Powder Technol. 28 (2017) 2812–2819. doi:10.1016/j.apt.2017.08.008. [10] D.C. Marcano, D. V Kosynkin, J.M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L.B. Alemany, W. Lu, J.M. Tour, Improved Synthesis of Graphene Oxide, ACS Nano. 4 (2010) 4806–4814. doi:10.1021/nn1006368. [11] Y. Feng, W.L. Li, J.P. Wang, J.H. Yin, W.D. Fei, Core–shell structured BaTiO 3 @carbon hybrid particles for polymer composites with enhanced dielectric performance, J. Mater. Chem. A. 3 (2015) 20313–20321. doi:10.1039/C5TA04777C. [12] G. Khurana, N. Kumar, S. Kooriyattil, R.S. Katiyar, Structural, magnetic, and dielectric properties of graphene oxide/Zn x Fe 1− x Fe 2 O 4 composites, J. Appl. Phys. 117 (2015) 17E106. doi:10.1063/1.4908146. [13] J. Varghese, S. Jasimudeen, K.T. Varghese, Study of the dielectric properties of graphene/CuS/ZnO hybrid nanocomposites for high performance supercapacitor applications, RSC Adv. 5 (2015) 107142–107149. doi:10.1039/C5RA20099G. [14] Y. Fu, H. Chen, X. Sun, X. Wang, Combination of cobalt ferrite and graphene: High-performance and recyclable visible-light photocatalysis, Appl. Catal. B Environ. 111–112 (2012) 280–287. doi:10.1016/j.apcatb.2011.10.009. [15] W. Huang, S. Ding, Y. Chen, W. Hao, X. Lai, J. Peng, J. Tu, Y. Cao, X. Li, 3D NiO hollow sphere/reduced graphene oxide composite for high-performance glucose biosensor, Sci. Rep. 7 (2017) 1–11. doi:10.1038/s41598-017-05528-1. [16] K. Anand, O. Singh, M.P. Singh, J. Kaur, R.C. Singh, Hydrogen sensor based on graphene/ZnO nanocomposite, Sensors Actuators B Chem. 195 (2014) 409–415. doi:10.1016/j.snb.2014.01.029. [17] A.K. Jonscher, The ‘universal’ dielectric response, Nature. 267 (1977) 673–679. doi:10.1038/267673a0. [18] S. Pei, H.-M. Cheng, The reduction of graphene oxide, Carbon N. Y. 50 (2012) 3210–3228. doi:10.1016/j.carbon.2011.11.010.