Temperature-Dependent Conductivity of Graphene Oxide and

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Temperature Dependent Conductivity of Graphene Oxide and Graphene Oxide-Polyaniline Nanocomposites Studied by Terahertz Time-Domain Spectroscopy Partha Dutta, Jessica Afalla, Arnab Halder, Sudeshna Datta, and Keisuke Tominaga J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10412 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 7, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Temperature-Dependent Conductivity of Graphene Oxide and Graphene Oxide-Polyaniline

Nanocomposites

Studied

by

Terahertz

Time-Domain

Spectroscopy

Partha Dutta1,*, Jessica Afalla2, Arnab Halder3, Sudeshna Datta4, and Keisuke Tominaga2,*

1. Department of Chemistry, Maharaja Manindra Chandra College, University of Calcutta, Kolkata - 700003, India 2. Molecular Photoscience Research Center, Kobe University, Rokkodaicho 1-1, Nada, Kobe, 657-8501, Japan 3. Department of Chemistry, Presidency University, Kolkata - 700073, India 4. Department of Polymer Science & Technology, University of Calcutta, Kolkata 700009, India

*: corresponding authors ＊

E-mail: [email protected] (P. Dutta), [email protected] (K. Tominaga),

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ABSTRACT In this work, we have studied the temperature-dependent conductivity of graphene oxide (GO) and graphene oxide-polyaniline (GO-PANI) nanocomposites by terahertz time-domain spectroscopy (THz-TDS) from 78 K to 293 K. The refractive index and absorption coefficient are related to the conductivity, and it has been found that in the THz region, the real part of the complex conductivity of GO is less than that of GO-PANI over the entire range of experimental temperatures. Both the systems exhibit an increase in these physical parameters with increasing temperature. The complex conductivity spectra of these systems in the THz region are well fitted by an analytical model that has contributions from the scattering of free electrons (Drude-Smith term) and from bound-electron oscillation (Lorentz term). The fitting analysis suggests a more ordered structure and anharmonicity for both the systems with increasing temperature and reports that GO-PANI has a greater free electron density and damping frequency of oscillation and more ordered structures than GO at all temperatures.

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Introduction Since its discovery in 2004, graphene, a two-dimensional, entirely sp2-hybridized, single-layered form of carbon has been the focus of intense fundamental and applied research due to its unique electronic, optical, and mechanical properties.1, 2 However, because of its lack of solubility in water, applications in aqueous environments have been limited. On the other hand, graphene oxide (GO) is an excellent candidate for dispersion in aqueous environments due to its hydrophilic nature. GO is a possible intermediate for the manufacture of graphene and is a compound of carbon, oxygen, and hydrogen in variable ratios. The maximally oxidized bulk product is a yellow solid that retains the layer structure of graphite but with much larger and more irregular spacing.3 The oxidized moieties of GO can be compatible with various substrates and are able to interact with different inorganic, organic or biomaterials. Thus, GO is not only a precursor material of graphene but also a polymeric material with its own properties. GO has been used in polymer composites,4 dielectric layers in electronic devices,5 solar cells6 and various biological systems7 such as DNA analysis.8 Unlike graphene, GO is an electrically insulating material. Many defect sites can be generated on the surface of GO due to the disruption of the sp2 bonding network of graphene by the oxidized portion of GO. These defects may be reduced by means of thermal reduction or

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chemical processes through the preparation of reduced graphene oxide, and the electrical properties of graphene can be restored.9-11 One such restoration process is by the formation of a GO-polyaniline (GO-PANI) nanocomposite.10-11 Polyaniline (PANI) is a very important polymer with a broad range of environmentally friendly industrial applications due to its desirable electrical, electrochemical, and optical properties.12 PANI is a conjugated polymer containing overlapped π-electrons that enable charges to move along the material. The electrical properties of PANI can be controlled by the protonation of the imine nitrogen atoms and by changing the oxidation state of the main chain using dopant acids.13 The excellent thermal, electrical and mechanical properties of different nanostructures of PANI offer potential applications in various fields, such as sensors, batteries, separation science, electro-optic devices and electro-chromic devices.14 A new type of functional nanocomposite based on GO-PANI has been extensively studied to produce materials with desired electrochemical properties.10-11, 15-20

The polar oxygen groups on the GO surface are able to interact with the polar

portion

of

the

polymer

to

form

intercalated

or

exfoliated

GO-polymer

nanocomposites.16-17, 21 Through the exfoliation of GO, a larger surface area may be generated, resulting in stronger interactions with PANI relative to tubular carbon

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nanotubes, thereby improving the thermal and electrical properties of the nanocomposites.18 The conductivity of PANI arises from intra-chain and inter-chain electron movement.22-23 For the coil-like conformation of PANI, the intra-chain conductivity is dominant, but for highly crystalline PANI, inter-chain conductivity is more significant. In general, the conductivity of GO-PANI nanocomposites depends on the addition of dopant acids15, 19 and the weight ratio of GO to aniline.11, 20 In most reports, in situ protonation of the aniline in GO-PANI nanocomposites has been achieved by the addition of external acids.11,

15, 19

In a recent report, Saha et al.24

followed a synthetic methodology similar to that of Rana et al.10 and reported the formation of a ground state charge transfer complex between GO and PANI. In the GO-PANI nanocomposite, interactions such as π-π stacking, electrostatic interactions, hydrogen bonding and donor–acceptor interactions favor aniline nucleation and polymerization to form nanotubes.10, 11 The reduction of GO through the formation of the GO-PANI nanocomposite results in the enhancement of electrical conductivity.10 In recent years, terahertz time-domain spectroscopy (THz-TDS) has received considerable attention in studies of low-frequency spectra in condensed phases.25-40 The oscillating field applied in this technique measures the AC conductivity of the samples in the THz region, and thus the low-frequency spectra of various materials have been

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studied using THz-TDS. In an earlier study,37 the THz spectra of GO at room temperature were reported; however, the mechanism of charge dynamics and temperature dependence was not discussed. In contrast, although the conductivity mechanism of PANI has been extensively reported,28 no report on the GO-PANI nanocomposite in the THz region has been published to the best of the authors' knowledge. In this work, we have studied the temperature-dependent THz spectra of GO and GO-PANI using the THz-TDS technique. The THz spectral change observed between GO and GO-PANI has been investigated by considering an analytical model composed of both free and bound electron responses.

Experimental Details of the sample preparation are described in the Supporting Information (SI). All the characterization procedures supported the formation of a new nanocomposite of GO-PANI that is quite different from GO or PANI.24, 41 The THz-TDS spectrometer and cryogenic system have been described elsewhere in detail.27, 28 The real ( σ / (ν~ ) ) and the imaginary part ( σ // (ν~ ) ) of the complex conductivity ( σˆ (ν~ ) ) are related to the refractive index (η (ν~ ) ) and absorption coefficient ( α (ν~ ) ) and are calculated using the following relations:42

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σ / (ν~ ) = 4πc ν~ε 0 (η (ν~ )κ (ν~ )) , σ // (ν~ ) = 2πc ν~ε 0 (1 − η 2 (ν~ ) + κ 2 (ν~ )) where ε 0 is the permittivity of free space, and α (ν~ ) is related to the extinction

4πν~ ~ coefficient ( κ (ν~ ) ) by the relation α (ν~ ) = κ (ν ) . In the SI, we briefly mention the ln 10 apparatus. Dried powdered samples of GO and GO-PANI were pressed into pellets at 10 MPa for the THz-TDS experiments. The thickness of the GO and GO-PANI sample pellets were 0.442 mm and 0.351 mm, respectively.

Results Figures 1(a) and (b) display the plots of the real and imaginary parts of the complex conductivity of GO, respectively, whereas Figures 2(a) and (b) represent the plots of the real and imaginary parts of the complex conductivity of GO-PANI, respectively, at different temperatures. In the SI, we show the time-domain THz signals and their power spectra for the two compounds. Spectra of the refractive index (η (ν~ ) ) are also shown in the SI. The imaginary part of the complex conductivity of GO-PANI shows a larger change in magnitude with temperature than that of GO. The real part of the complex conductivity shows significant temperature and frequency dependence for both systems. For all temperatures, GO-PANI has a greater spectral intensity than GO. This difference

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may be directly related to the charges present in the system, which are temperature dependent.

Discussion We aim to understand the difference in temperature dependence between the two samples by analyzing the complex conductivity spectra. The most widely used analytical models, such as the empirical power law,31 Mott-Davis model,28, 43-45 Drude model,46-48 or Drude-Smith model49, were not able to fit our spectra. A detailed discussion regarding the spectral analysis using these models is included in the SI. In these models, conductivity is mainly derived from the motion of free carriers. However, in the literature, it has been reported that both free electrons moving along a nanocomposite and bound electrons contribute to the complex conductivity.36, 50 When bound electrons oscillate at certain frequencies, the Lorentz equation describes their contribution to conductivity. Thus, we propose a model that has a contribution from the scattering of free electrons and a contribution from bound electrons. The equation used in our fitting analysis is expressed by ε ( 2 π c ν~p ) 2 τ σˆ (ν~ ) = σˆ Free (ν~ ) + σˆ Bound (ν~ ) = 0 1 − i ( 2 π c ν~ )τ  + ( − i 2 π c ν~ε 0 )  ~  { (2 π c ν k

 c1 1 +  (1 − i ( 2 π c ν~ )τ 

)

( A s )2 +ε )2 − (2 π c ν~ )2 } − {i Γ k (2 π c ν~ )} ∞ 8


     − 1  

(1)

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where σˆ (ν~ ) = σ / (ν~ )+ iσ // (ν~ ) . In eq. (1), the first term ( σˆ Free (ν~ ) ) is the Drude-Smith model expression,49 and the second term ( σˆ Bound (ν~ ) ) expresses the contribution of the bound electrons. In the Drude-Smith term, ε0 is the permittivity of free space, τ is the mean time interval between collision of free carriers (scattering time), ν~P is the plasma frequency, and c1 is a unitless parameter describing the persistence of velocity after one scattering event.49 In the Lorentz term, As is the Lorentz oscillator strength, ε ∞ is the background permittivity at infinite frequency, ν~k is the kth vibrational mode or phonon frequency, and Γk corresponds to the spectral width of the kth Lorentzian mode. Earlier in the results section, Figures 1 and 2 are described. These plots have been fitted using eq. (1). The spectral fitting curves have been shown in gray in the figures. It is quite clear from the fitting curves that eq. (1) reproduces all of the spectra at different temperatures quite well for both systems. To understand the contribution of the free and bound electrons on the spectral response, we decomposed the spectra according to eq. (1). Figures 3 (a-d) show the decomposed spectra of the real and imaginary parts at 78 K and 293 K for the GO-PANI system. At all temperatures, both the Drude-Smith and Lorentz terms have significant contributions in the experimental frequency region. From global fitting, a resulting error margin of ±10% has been found for all

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fitting parameters, except τ and c1, which have an error margin of ±1%. We now discuss the comparison of the complex conductivities of the GO and GO-PANI systems and their temperature dependence starting with the Lorentz term. Figures 4(a), (b), and (c) show the temperature-dependence spectra of As, Γk and ν~k , respectively, i.e., the parameters related to the bound-electron contribution for both the GO and GO-PANI systems. The Lorentz oscillator strength As changes with temperature for both the systems (Figure 4 (a)). Notably, there is a significant difference in the magnitude of As between the GO and GO-PANI systems (Figure 4(a)). Both the GO and GO-PANI show a 2 to 2.5-fold increase in As with the temperature, and at all the temperatures GO-PANI has a greater magnitude than GO. If the resonant frequency of vibration is related to the graphitic vibrational mode, the parameter As, which is an indicator of the number of the bound electrons, may be considered a measure of the graphitic order of the samples. Thus, with an increase in temperature, both the systems become more ordered. Furthermore, GO is comparatively much more disordered than GO-PANI at any given temperature. According to the XRD patterns and FTIR spectra of the GO-PANI nanocomposites, the GO moiety may be reduced.24 This may result in a more ordered structure or fewer defect sites in GO-PANI. Such a physical change is also supported by our fitting analysis with the parameter As. Subsequently, Figure 4(b) shows that for both

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systems and at all temperatures, there are no changes in the frequency (ν~k ), which we assume to be the graphitic vibrational mode. It should be mentioned that for these types of complex systems, there may be more than one oscillation mode. However, for simplification, the proposed model only considers a single mode, which may be the average mode of oscillations present in these systems. In addition, Figure 4(c) demonstrates how the spectral width Γk is affected by temperature and by changing the systems. At all temperatures, it can be found that GO-PANI has more spectral width than GO and that for both systems, increasing the temperature increases the spectral width (Figure 4(c)). Γk is related to the retardation of the phonon mode of oscillation (damping frequency) and gives an idea of the degree of anharmonicity present in the system. The fitting therefore shows that increasing the temperature induces anharmonicity in both systems, with GO-PANI showing greater damping than GO. On the other hand, the temperature dependence of the parameters of the Drude-Smith term is shown in Figure 5. The plots of ν~P as a function of temperature for both the systems reveal that GO-PANI has an approximately 1.7 to 2 times greater plasma frequency than GO, and there is no temperature dependence for either system (Figure 5(a)). Since the plasma frequency of a material is proportional to the square root of its free electron density,36, 48 GO-PANI therefore has 3-4 times greater free electron

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density than GO. Figure 5(b) shows that the scattering time (τ) remains almost unchanged (~ 9 - 13 fs) with respect to the temperature and system. Although GO-PANI has a higher free electron density than GO, no temperature dependence has been observed for the scattering time of either system. Next, we examine the extent of backscattering, c1, which can be an indication of the disorder of the system.49 Over the entire range of temperatures for both systems, our analysis shows that c1 is close to -1 (Figure 5(c)). Thus, the back scattering or localization of free electrons is present in its full effect, and the systems are too disordered. However, c1 varies less than 1% with an increase in temperature. Apparently, this magnitude of change is so negligible that it does not affect the THz spectra. To investigate such a small change of c1 in the spectra, we constructed THz spectra of the real conductivity (Figure S7) with a magnitude of c1 as -1 and -0.993, keeping all other parameters constant. Figure S7 shows that even a small change in the extent of back scattering can affect the magnitude of the real conductivity, and THz-TDS can detect it. From Figures 5(c) and S7, we conclude that increasing the temperature decreases the backscattering or the disorder of the material, which is reflected in the spectral intensity of the real part of the conductivity. The temperature dependence of the conductivity and the difference of conductivity of the two different samples in the THz region have been analyzed by the

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model proposed in this work. The most widely used analytical models are unable to fit the spectra, whereas our model reproduces the complex conductivity spectra quite well, and the conductivity mechanisms are discussed with the help of different parameters introduced in the complex conductivity equation, i.e., eq. (1). The characteristic differences of the samples and their temperature dependence are shown in Scheme 1. In short, the free electron density, graphitic order of the sample and anharmonicity present in the systems are the main factors differentiating the two samples and explaining the temperature dependence of conductivity.

Conclusion We have performed THz-TDS measurements of GO and GO-PANI from 78 K to 293 K. The spectral characteristics of GO-PANI are quite different than GO. With the increase in temperature, the real part of the conductivity of both systems increases. Widely used free electron-based analytical models are unable to fit the experimental data, and thus an analytical model function with both a free electron scattering term (Drude-Smith) and a bound electron oscillation term (Lorentz) has been introduced. This function fit our data very well in all systems. Our analysis suggests that GO-PANI is more ordered and has greater free electron density and damping frequency of oscillation than GO. The heat

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treatment makes GO and GO-PANI less disordered and induces anharmonicity in both systems. Furthermore, for both systems, no temperature dependence of the free electron densities is observed. The mean time interval between the collision (scattering time) of free electrons and the Lorentz oscillation frequency of bound electrons are found to be independent of temperature and system. The outcome of this work demonstrates the usefulness of the THz-TDS technique to probe the electron landscape, along with the dynamics present in the system, which helps to characterize the materials.

Acknowledgments PD thanks the Governing Body of the Maharaja Manindra Chandra College, Kolkata, India and Hyogo Overseas Research Network (HORN), Japan for their kind support. AH thanks the Head of the Department of Chemistry and Vice-Chancellor of Presidency University, Kolkata for the laboratory facilities.

Supplementary Information (SI) Details of the sample preparation, characterization, and the apparatuses. Discussion of different analytical models. Temperature dependence of the THz time-domain signals, power spectra, absorption spectra and spectra of the refractive index of GO and

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GO-PANI. Spectral analysis of the complex conductivities of GO and GO-PANI.

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12/12/2016

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Free Electron

Lesser Back Scattering

Temp. More Ordered, Fewer Defect Sites

Conductivity

Bound Electron More Damping

Temp. Effect for GO & GO-PANI

Frequency

More Free Electron Density

Free Electron Conductivity

More Ordered, Fewer Defect Sites

Bound Electron

More Damping Frequency

GO

GO-PANI

SCHEME 1: Characteristic Differences of the Samples and Their Temperature Dependence 22


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FIGURE CAPTIONS Figure 1. The spectral fitting curve (gray) according to eq. (1) of the plots of (a) the real part (σ/) and (b) the imaginary part (σ//) of the complex conductivity of GO at 78, 133, 186, 213, 240, 267, and 293 K.

Figure 2. The spectral fitting curve (gray) according to eq. (1) of the plots of (a) the real part (σ/) and (b) the imaginary part (σ//) of the complex conductivity of GO-PANI at 78, 133, 186, 240 and 293 K.

Figure 3. The decomposed spectra according to eq. (1) of the plots of the real part (σ/) of the complex conductivity at (a) 78 K and (b) 293 K, and of the plots of the imaginary part (σ//) at (c) 78 K and (d) 293 K of GO-PANI. The raw experimental data and the fitted curve are shown by green solid lines and gray dashed lines, respectively. The Drude-Smith term and the Lorentz term are shown by blue and violet lines, respectively.

Figure 4. The plots of (a) As, (b) ν~k , and (c) Γk as a function of temperature of the GO (♦) and GO-PANI (♦) systems.

Figure 5. The plots of (a) ν~P , (b) τ and (c) c1 as a function of temperature of the GO (♦) and GO-PANI (♦) systems.

23



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Figure 1. Dutta et al.

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Figure 2. Dutta et al. 25



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“TOC Graphic”

Temp

graphene oxide

Temp

graphene oxide -polyaniline

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Temperature-Dependent Conductivity of Graphene Oxide and

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