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Swift Heavy Ion Irradiation as a Tool for Homogeneous Dispersion of Nanographite Platelets within the Polymer Matrices: Towards Tailoring the Properties of PEDOT:PSS/Nanographite Nanocomposites Prachi Singhal, and Sunita Rattan J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b11240 • Publication Date (Web): 16 Mar 2016 Downloaded from http://pubs.acs.org on March 18, 2016

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The Journal of Physical Chemistry

Swift Heavy Ion Irradiation as a Tool for Homogeneous Dispersion of Nanographite Platelets within the Polymer Matrices: Towards Tailoring the Properties of PEDOT:PSS/Nanographite Nanocomposites Prachi Singhala, Sunita Rattanb* a

Directorate of Innovation and Technology Transfer, Amity University Uttar Pradesh, Sec-125,Noida, India Amity Institute of Applied Sciences, Amity University Uttar Pradesh, Sec-125, Noida, India

b

ABSTRACT: Performance of the polymer nanocomposites is dependent to a great extent on efficient and homogeneous dispersion of nanoparticles in polymeric matrices. The dispersion of Nanographite Platelets (NGPs) in polymer matrix is a great challenge because of the inherent inert nature of the NGPs, poor wettability towards polymer matrices and easy agglomeration due to Van der Waals interactions. In the present study, attempts have been made to use a new approach involving the irradiation of polymer nanocomposites through Swift Heavy Ion [SHI] to homogeneously disperse the NGPs within the polymer matrices. Poly (3, 4- ethylenedioxythiophene) poly (styrene sulphonate) (PEDOT: PSS)/nanographite nanocomposite (NC) films prepared by solution blending method were irradiated with SHI [Ni ion beam, 80 MeV] at a fluence range of 1 × 1010 to 1 × 1012 ions/cm2. XRD studies revealed that, ion irradiation results in delamination and better dispersion of NGPs in the irradiated nanocomposite films compared to unirradiated films, which is also depicted through SEM, AFM, TEM and Raman studies. In the irradiated polymer nanocomposite films, the conformation of PEDOT chains changes from coiled to extended coiled structure, which along with homogeneously dispersed NGPs in irradiated NCs, shows excellent synergistic effect facilitating charge transport. The remarkable improvement in conductivity from 1.9x10-2 in unirradiated NCs to 0.45 S/cm in irradiated NCs is observed with marked improvement in sensing response towards nitroaromatic vapours at room temperature. The temperature induced conductivity studies have been carried out for PEDOT:PSS/nanographite NCs to comprehend the charge transport mechanism in NC films using 3D Mott variable range hopping model also. The study reveals SHI as a novel method, addressing the challenge associated with the dispersion of NGPs within the polymer matrix for their enhanced performance towards various applications.

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1. INTRODUCTION Nanographite based polymer nanocomposites (NCs) have generated great enthusiasm in world of material science1-3 since isolation of graphene by Novoselov et al. in 2004.4 Tremendous amount of work has been carried out on the preparation of single or few layer graphite platelets. Nanographite platelets (NGPs) with unique superlative properties such as flexibility, mechanical strength, chemical stability and its extended double bond conjugation have shown to be the potential fillers in conducting polymeric materials towards the preparation of polymer NCs.5-7 Combination of NGPs with host conducting polymer matrices in NCs provide incredible physical and structural properties which have an impressive potential for various applications such as light-emitting devices, energy and memory storage, electromagnetic shielding, sensors, actuators, super capacitors, antistatic coatings etc.8-9 Polymer nanographite based NCs has drawn enormous attention as active material for sensors.10 The excellent sensor properties of Polymer nanographite NCs is attributed to large specific surface area, tuneable electrical conductivity, chemical manipulability, thermal and chemical stability which facilitates vapour sensing with high sensitivity, fast response and stable performance.11 However, effective dispersion of the NGPs is essential to realize their full potential in advanced polymer NCs.12-13 High homogeneity of graphitic nanoplatelets in polymer matrix is difficult to achieve because of the inherently inert nature of the NGPs, poor wettability towards polymer matrices and easy agglomeration due to Van der Waals interactions. Similar to other layered nanofillers such as clay, these are prone to restacking due to the high aspect ratio and strong interparticle interaction14-16 which limits the application of polymer nanographite NCs. Therefore, improving the dispersion and interfacial interaction of nanographitic platelets in polymeric matrices remains the most significant challenge in the development of high performance polymer nanographite NCs. Extensive research efforts are reported in literature to prepare polymer nanographite NCs using techniques such as in-situ polymerization, solution mixing, and melt blending.17 Various methods were used to avoid agglomeration of nanographite platelets such as high-power ultrasonic mixers, surfactants18, solution mixing19, in situ polymerization20-21, grafting to nanofiller or grafting to polymer22,23, plasma treatment, functionalization of nanoparticles with organic molecules (including polymers) through covalent and non-covalent interactions.2425

In spite of the considerable advances in area of exfoliated graphene-based polymeric NCs, substantial research is still necessary to provide a method for homogeneously dispersed nanographite and enable full exploitation of their nano engineering potential. Previously, we have reported the use of γ-ray radiations and click chemistry in an attempt to achieve the homogeneous dispersion of graphite within the polymer matrix.26-27 SHI irradiation is used as an effective way to tailor the structural and physio-chemical properties of materials such as metals28, semiconductors, polymers29 etc. to enhance their applicability towards high-end applications. SHI have also been used to change the structure and morphology of carbon nanomaterials in a controllable manner.30 The morphological and electrical changes in materials induced by SHI are of

profuse interest for various applications.31 The changes in the macroscopic properties on ion irradiation are due to large amount of energy deposition by SHI beams which may lead to modifications at the electronic level of any material 32. The changes may be attributed to various factors such as crosslinking, creation of defect sites in molecular structure, structural rearrangements, molecular emission etc.33 The modifications induced are influenced by ion parameter such as energy, fluence etc. and material characteristics such as composition and molecular weight. Two phenomenological models i.e. thermal spike model and coulomb explosion model are used for explaining the transfer of electronic energy generated due to ion-matrix interaction34. According to Coulomb explosion model, swift heavy ion while passing through the target material causes ionization around its path, followed by strong electrostatic repulsion and thus explosion among these charges, leading to the modification of the target material. Coulomb explosion is predominant in insulators where there is little conduction of electrons. However, in conducting materials neutralization of charges occurs much before the Coulomb explosion 35 and hence thermal spike model seems to be predominant The ion irradiation and energy deposition process in polymer/NGP nanocomposites can be explained based on thermal spike model. According to this model, SHI transmits its energy to the electrons of the target materials as kinetic energy via inelastic collision. The deposited energy is transmitted to the atomic lattice by electron-phonon coupling that can induce a localized heating along the ion track of nanometric dimension resulting in transient thermal spike lasting for a few picosecond. The deposited energy reorganizes the basic molecular orientation of the target material in a remarkable way36, 37. Few studies in literature report the applicability of SHI irradiation as a potential tool for tailoring the structural and electronic characteristics of polymer composites.38-39 However, no literature, to date, has reported the use of SHI ion irradiation as a tool for dispersion of nanoparticles within the polymer matrix. The present work reports a novel method of utilizing Swift Heavy Ion [SHI] Irradiation on polymer nano composites to bring about uniform dispersion of NGPs within the polymer matrix. Polymer/nanographite NCs using [Poly (3, 4ethylenedioxythiophene) / poly (4-styrene sulphonate)] [PEDOT: PSS] as polymer matrix were prepared and subjected to SHI irradiation to improve the dispersion of nanographite in the polymer matrix. The irradiated polymer NC films were characterized for structural and morphological properties and temperature dependent electrical conductivity at 300500K. The conduction mechanism in the NCs is explained based on Mott’s variable range hopping model. The investigation is helpful in optimizing the correlation among morphology and transport properties of conducting polymer nanographite NCs. The prepared NCs are evaluated for sensing behaviour towards nitro aromatics. 2. EXPERIMENTAL SECTION 2.1. Materials. Commercially available natural graphite flakes [Asbury carbons Inc.] were used to prepare nanographite platelets. Polymer [Poly (3, 4ethylenedioxythiophene) / poly (4-styrene sulphonate)] in aqueous solution was procured from HC Starck


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Gmbh, Germany. Other reagents and solvents used were of analytical grade. 2.2 Synthesis of Nanographite Platelets. The nanographite platelets were prepared through intercalation-exfoliation method as reported in literature40. Natural graphite flakes (few µm) were acid intercalated and then given a thermal shock to form expanded graphite (100-400 nm). The resulted expanded graphite was subjected to ultrasonication to further exfoliate it into the nanographite platelets (20-80 nm). 2.3. Preparation of PEDOT: PSS/nanographite NC Films. PEDOT: PSS/nanographite NCs were prepared by solution blending technique as shown in scheme 1. A definite amount of PEDOT: PSS dispersion was taken and weighed amount of nanographite is added to prepare 1 wt% of PEDOT: PSS/nanographite NC as reported in our earlier work.41 The solution was thoroughly stirred for a few hours and left to stand overnight and then sonicated for 6 hours. The PEDOT: PSS/nanographite nano composite suspension was poured onto the glass substrates, which were cleaned ultrasonically with acetone and deionized water prior to casting. The casted films were dried at room temperature to evaporate the solvent completely. These polymer/nano graphite nano composite films were used for SHI irradiation. 2.4. Swift Heavy Ion Irradiation. The prepared nano composite polymer films were cut into the size of 1 × 1 cm2. These samples were irradiated by Ni ion beam [80 MeV] under high vacuum [~10–6 mbar], at fluence, 1 × 1010 to 3 × 1012 ions/cm2, available from the 15 UD tandem pelletron accelerator at IUAC, New Delhi, India [Table 1]. The energy used for irradiation was determined by utilizing Stopping and Range of Ions in Matter [SRIM] software.42 The energy of the ion beams used was selected such that the projected range of the ions was more than the thickness of the NC films to ensure that the ion beam pass through the materials and were not implanted in the film. 2.5. Characterizations. 2.5.1. Morphological Studies. The mor

Scheme 1: Schematic representation of the preparation of PEDOT:PSS nanographite NC through solution blending technique

Ion

Ni Table 1. dia

Ion energy [MeV]

Projected Range [µm]

Thickness of the nano composite film [µm]

Fluence (ion/cm2)

80

26.11

6-10

3x1010 to 1x 1012

Energy and Fluence used for Ni ion Beam Irration

phology of NGPs before blending (Fig S1 & S2) and the dispersion of NGPs in PEDOT:PSS matrix before and after Ni ion beam irradiation were investigated by Leo 435 V P Scanning electron microscope [SEM], Atomic force microscopy [AFM], X-ray diffraction technique [XRD], Transmission Electron Spectroscopy (TEM) and Raman spectroscopy. AFM images were recorded by Atomic Force Microscope [NanoscopeIIIa] in the tapping mode with an antimonydoped silicon tip. The XRD analysis was performed by using Bruker AXS, X-ray diffractometer instrument with Cu-Kα [λ=0.154nm] radiation at room temperature. The scanning rate was 2o/min and the scanning range was 1040o. Raman spectra was recorded using Ar laser with excitation wavelength of 514.5 nm. 2.5.2. Electrical conductivity and Gas Sensing Experiments The conductivity measurements for the polymer nanographite NC films were performed using a four probe technique with Keithley 6517B electrometer. Ohmic contacts were made on the NC films using silver paste, connected to the probes of the electrometer and electrical measurements were made. The conductivity measurements with increasing temperature (300-500K) for the polymer nanographite NC films were performed by placing the above set-up in the oven, the temperature was allowed to rise gradually and corresponding electrical measurements were made. For gas sensing measurements, the NC film was placed in a closed chamber and exposed to the nitrobenzene vapours. The change in resistance of the films was measured using Keithley 6517B electrometer interfaced with PC. All the measurements were performed at room temperature. When the electrical resistance of the NC film approached its equilibrium value, the sensor was removed from the closed vessel and exposed to air to recover. 3. RESULTS AND DISCUSSION 3.1. Scanning Electron Microscopy. SEM studies provide direct evidence for improved dispersion and exfoliation of NGPs in the polymer NCs after irradiation. Fig 1 (a-c) represents the SEM micrograph for PEDOT: PSS/nanographite NC films before irradiation which elucidates the heterogeneous distribution of aggregated NGPs throughout the PEDOT: PSS matrix before irradiation, reflecting poor dispersion of NGPs. The PEDOT: PSS/nanographite NC films were subjected to Ni (80 MeV) ion irradiation in the fluence range 1×1010 to 3×1012. After irradiation, significant changes in surface morphology were



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Figure 1: SEM Images of PEDOT: PSS/nanographite NC before irradiation (a)1.04 KX (b) 3.07 KX (c) 5 KX; after irradiation at fluence 3x1010 (d) 177 X (e) 1.01 KX (f) 3.12 KX ; after irradiation at 3x1011 (g) 248 X (h) 1.06 KX (i) 5.17 KX the range of 10-100 µm, which decreases to more uniform and observed. Fig. 1(d-i) show the SEM images for PEDOT: narrow distribution profile 43in the range of 10-40 µm in nanoPSS/nanographite NCs after Ni ion beam (80 MeV) irradiation at composites after irradiation. Fig. 3a and 3b compares the SEM images of PEDOT: 3 x1010 & 3 x 1011 fluences at different magnifications. 11 The irradiated PEDOT:PSS/nanographite NC films exhibits sig- PSS/nanographite NCs before and after irradiation [3 x 10 ] respectively at 50 KX magnification. Fig. 2b clearly shows or nificant homogenisation of NGPs within the polymer matrix unveils the wafer like structure of the nanographite platelets after which improves with increasing fluence. However, at higher fluence of 1 x1012, some deterioration in the NGPs is observed as irradiation. shown in Fig 2 (a-b). It is assumed that during the irradiation of polymer NCs, SHI The SEM image analysis (Fig S3) of PEDOT: PSS/NGP nano- creates a nanometric cylindrical molten zone, during which the composites before and after irradiation shows that the lateral size tempera of nanofiller in NCs before irradiation shows broad distribution profile in

Figure 2: SEM images of PEDOT:PSS/nanographite Nano composite after irradiation at fluence 3x1012 (d) 177 X (e) 1.01 KX

Figure 3: SEM images of PEDOT: PSS/nanographite Nano composite at 50 KX magnification (a) before irradiation (b) after irradiation at fluence 3 x 1011


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Scheme 2: Schematic representation of effect of swift heavy ion irradiation on PEDOT: PSS/nanographite NC film ture of the sample is quite high and the polymer chains can easily diffuse into the gallery of nanofiller resulting in its exfoliation and dispersion of the NGPs. The effect of SHI irradiation on polymer nanographite NC is schematically represented in scheme 2. XRD, AFM and Raman results further supports the significant improvement in exfoliation and dispersion of nanographite platelets within the polymer matrix as discussed in next section. 3.2. X-Ray Diffraction Studies. XRD is the other dominant method to characterize the dispersion of nanographitic sheets in the NCs. Fig 4 illustrates the XRD pattern of NGPs, PEDOT: PSS/nanographite NCs before irradiation, PEDOT: PSS/nnaographite NCs after irradiation at fluence 3x1011 and PEDOT: PSS/nanographite NC after irradiation at fluence 1x1012. NGP exhibits a diffraction peak at 2θ= 26.7o corresponding to (002) graphite plane with interlayer spacing of 0.334 nm.44 This shows that these are dispersed in the form of few layer graphene platelets. A small peak at 2θ = 11.6o corresponds to small amount of graphene oxide intercalated with oxide functionalities with interlayer spacing of 0.841nm.45 PEDOT: PSS/NGP NCs retained diffraction peak at 26.56o corresponding to 002 graphite plane of NGPs. After irradiation, the (002) peak intensity in PEDOT:PSS/NGP NC shows significant decrease which is attributed to the delamination of the NGPs towards formation of graphene sheets.46,47 XRD shows the delamination and dispersion of graphite nanoplatelets as also has been reported in the literature by some authors. Benes et al. 48 reports the drastic decrease in the intensity of the 002 peak (2θ= 26.5) diffraction peak of graphite indicating structural disordering and graphite exfoliation. Similarly, Matsumoto et al.49 also discussed the decrease in the intensity of the 002 peak (2θ= 26.5) diffraction peak in the XRD diffractogram due to the exfoliation of graphite into pristine ‘single-layer’ graphene. The decrease in peak intensity is successive with in-

creasing fluence of irradiation indicating that increase in fluence further delaminates the NGP platelets. This is an important observation in which SHI ions are capable of converting NGPs to few layer graphene sheets directly within the polymer matrix. Further, after irradiation the peak at 2θ=11.6o disappears indicating that the high temperature during irradiation leads to removal of oxide functional groups resulting in exfoliation of graphene nanoplatelets. XRD pattern of PEDOT:PSS/NGP NC after irradiation at 1x1012 shows a single peak of diminished intensity at 26.56 with no other dominant peak in the spectra indicating that NGPs has been efficiently exfoliated and well-dispersed within the PEDOT:PSS matrix

Figure 4: X-ray Diffraction Curves of PEDOT:PSS/nanographite NCs Before and After Ni ion beam Irradiationduring irradiation.



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After irradiation, there is slight shift in 2θ (26.56o) of polymer NC to 2θ (26.6o) which may be due partial restoration of π- network showing annealing of the graphene layers which is also confirmed through raman studies.50 Thus, it is observed that ion irradiation results in delamination and dispersion of NGPs in the polymer matrix with annealing effect on NGPs. 3.3. Atomic Force Microscopy. AFM reveals surface structures with spatial resolution to get their size and thickness. Fig. 5 (a-b) shows the AFM images of PEDOT: PSS/nanographite NC in 2D before and after Ni Ion irradiation respectively. Fig 5(c-d) shows the AFM images of PEDOT: PSS/nanographite NC in 3D before and after Ni Ion irradiation respectively. Fig 5 (e-f) shows the PEDOT: PSS/NGP NC films with their height profiles before and after Ni ion irradiation respectively. The decrease in thickness of the nanographite platelets can be correlated to the reduction in number of layers which accounts for their improved dispersion in the polymer matrix. 51-52 The platelet thickness was estimated by taking measurements at 5–6 different spots near the edge of the NGP flakes in the same sample both before and after irradiation. The average thickness of the platelets in the nanocomposites before irradiation is in the range between 20-50 nm (Fig S3 in ESI) and after irradiation is in the range 3- 11 nm which shows the wafer like structure and overall decrease in the thickness of the NGPs (Fig S4 in ESI) 3.4. Transmission Electron Microscopy (TEM). Nanocomposites were further characterized by TEM which were in good agreement with AFM results. Fig 6 (a-d) shows TEM images of PEDOT: PSS/NGP nanocomposites after irradiation indicating few layered NGPs. Fig 6 (d) shows a cross-section of one such stack of NGP in NCs after irradiation. The 2.6nm thick stack of NGPs indicate 7-8 layers of graphene with inter layer distance of 0.335 nm. 3.5. Raman Spectroscopy. Fig 7 (a) shows the Raman spectra of PEDOT: PSS/nanographite NCs before and after ion irradiation in fluence range from 3x1010 to 1x 1012. The spectra have been staggered in y-axis for clarity. The characteristic vibrational modes of PEDOT are located at 1524 cm-1, 1452 cm-1, 1383 cm-1, 1272 cm-1, which are assigned to the Cα = Cβ, asymmetrical, Cα = Cβ symmetrical, Cβ- Cβ stretching, and Cα-Cα’ inter-ring stretching vibrations respectively. The vibrational modes of PSS are located at 1100 cm-1 and 1000 cm-1.53-54 PEDOT chains exhibits resonance with simultaneous existence of both benzoid and quinoid structure as shown in scheme 3. In the

Scheme 3: Schematic representation of resonance between benzoid and quinoid structure of PEDOT

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Figure 5: (a&b) 2D AFM Images of PEDOT:PSS/nannographite Nano composites images Before and After Irradiation respectively; (c&d) 3D AFM Images of PEDOT:PSS/ nanographite nano composites Before and After Irradiation respectively; (e&f) AFM images and Corresponding Height Profiles of PEDOT:PSS/ nanographite Nano composites Before and After Irradiation respectively benzoid form, Cα-Cβ bond between the two thiophene rings in the PEDOT chain bond have lower density of conjugated π electrons. While in the quinoid form, Cα-Cβ bond between the two thiophene rings show more π character and results in the delocalization of pi-electrons throughout the entire chain. The benzoid structure favours the coiled conformation of the PEDOT chains, while the quinoid structure favours a linear or expanded coil conformation. After irradiation the band between 1400-1500 cm-1 undergoes red shift with decrease in peak intensity indicating the change of dominant resonant structure of PEDOT chains from benzoid (coil conformation) to quinoid (expanded-coil) conformation as represented in scheme 2.55 It is assumed that during irradiation of the polymer/nanographite NCs the PEDOT chains easily diffuses into the gallery of the nanographite layers resulting in further exfoliation of NGPs. This enhances the interaction between PEDOT chains and graphite nanoplatelets and thus extending the conjugation length. The characteristic vibrational modes of NGPs are located as D [peak (at 1350 cm-1), G peak (at 1569 cm-1) and 2D peak (at 2,700 cm1 ) but are suppressed in PEDOT: PSS/nanographite NCs.56 G peak corresponds to stretching vibrational mode of carbon atoms both in the rings and in the chain. There is upshift in position of


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G-peak from 1569 cm-1 to 1571 cm-1 after irradiation at fluence 3x1010 which further upshifts with increasing fluence and reaches to 1580 cm-1 after irradiation at fluence of 1x1012 ions/cm2. The upshift in G-peak occurs mainly due to induced strain in graphite nanoplatelets and reduction in layer number of NGPs.57 The result is consistent with that of SEM, AFM and XRD observations. D peak involves double resonance phenomenon which activates due to defects in the perfect lattice. Thus, intensity of D peak is directly related to the disorder present in the nanographite. The defects induced in carbon based material are studied using relative intensity ratio of D and G peak or disorder parameter α=ID/IG, where ID& IG are intensities of D and G vibrational modes. The variation of disorder parameter (α) as a function of ion fluence is plotted in Fig 7 (b). It is evident from the Fig that disorder parameter decreases upto fluence 3x1011ion/cm2. This signifies the purification or annealing of NGPs at low fluence of incident irradiation i.e. partial restoration of π-network.58 The annealing effect of the ion irradiation in the low range of fluence can be explained through thermal spike model. As the energetic ion passes through the film, a large amount of energy is transferred to the electronic subsystem of the target material which is dissipated to the atomic system through electron phonon coupling, generating a localized heating of the atomic system in the ion track of nanometric dimension. This results in the transient thermal spike lasting for a few microseconds in ion track core region. And as the radial distance from the ion track increases, the temperature shows a gradual radial decay around the ion track core resulting in ion track halo region with less intense temperature. Both damage and annealing occurs simultaneously on ion irradiation. High temperature of core region near the ion track induces disorder while the low temperature halo region is responsible for annealing effect. At lower fluence with lesser number of ion tracks, less intense temperature of halo region with much larger radii is responsible for the net annealing effect. With increase in ion fluence, core region starts to overlap, thus diminishing the annealing effect.59-60

Fig 6: TEM images of irradiated PEDOT:PSS/NGP NCs (a-c) indicating few layered NGPs in the irradiated NCs (d) cross section of NC showing 2.6 nm thick NGP stack with characteristic 0.34- nm inter laminar spacing 3.6 Electrical Conductivity Measurements. The conductivity of PEDOT: PSS/nanographite NCs is much higher than the PEDOT: PSS pristine film. This is due to the fact that functional

groups on NGPs effectively separates the positively charged conducting PEDOT chains from the negatively charged PSS. The PEDOT chains interact with NGPs through coulombic attraction resulting in extended conductive network.61 After, Swift heavy ion irradiation, there is significant jump in the conductivity of PEDOT: PSS/nanographite NC from 0.019 to 0.0824 S/cm at 3x1010 ions/cm2. The conductivity improves further with increase in ion fluence and reaches to 0.454 S/cm at 1x1012 ions/cm2 as shown in Table 2. The possible reason is that as the PEDOT: PSS/nanographite NCs are irradiated, a cylindrical molten zone of few nm is created, during which the temperature of the sample is quite high and the polymer can easily diffuse into the gallery of nanofiller to increase the exfoliation. This diffusion of polymers results in exfoliation of NGP layers at high temperature decreasing the number of layers in the NGP stack and further enhances their dispersion into the molten polymer matrix. The well dispersed NGPs promote the conductive pathways due to increase in the charge carriers and modify the electronic properties. Further, as indicated in Raman studies, conformation change in

Figure 7: a) Raman spectra of PEDOT:PSS/nanographite NC before irradiation and after irradiation at different fluencies; b) Variation of ID/IG with Ni Ion fluence


(


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0.0824

To can be obtained from the experimental data. With the help of To value, n(Ef) (Density of states at Fermi level), R (mean hopping distance), E (mean energy necessary for hopping) can be determined using the following equations:

1x1011

0.176

To = λ k Bn(E F )α 3

3x1011

0.278

Ion irradiation (ions/cm2)

Fluence

Without irradiation 3x10

DC Conductivity (S/cm) 0.019

10

[

[

[

(2)

]

R = 9α {8πk BTn(E F )}

1x1012 0.454 Table 2: DC conductivities of PEDOT: PSS/nanographite NCs before irradiation and after irradiation at fluence range from 3x1010 to 1x1012 PEDOT chains from benzoid to quinoid form after irradiation leads to conductivity enhancement. In case of benzoid structure, Cα - Cβ bond between the two thiophene rings in the PEDOT chain have lower density of conjugated pi-electrons. While in expanded-coil conformation, thiophene rings results in the delocalization of pielectrons throughout the entire chain which results in increased conjugation length of the PEDOT chain and leads to increased conductivity. Study of DC conductivity with temperature can provide important information regarding the phenomenon of charge transport in the NCs. Temperature dependent electrical conductivity of PEDOT: PSS/NGP NCs was examined from 300K to 500K. Fig 8(a) shows the log conductivity vs 1000/T for PEDOT: PSS/nanographite NCs before and after ion irradiation. Results show that the conductivity increases with increasing temperature, displaying the semiconducting nature of the NCs. PEDOT: PSS/nanographite NCs behave as disordered semiconductors. In disordered organic semiconductors, conductivity vs temperature is commonly demonstrated by Mott’s variable range hopping (VRH) mechanism.62 According to VRH mechanism of charge transport, the following relation was considered σ = σoexp − (To T )1/4

]

]

[

]

E = 3 4πR 3 n(E F )

1/ 4

(3) (4)

The parameters of hopping conduction mechanism were determined using above equations are summarized in Table 3. A lower To value indicates lower hopping distance and lower hopping energy and consequent rise in conductivity. Both the hopping distance and the hopping energy decreases with ion irradiation which further shows successive decrease with increasing ion fluence as shown in Table 3. As explained in prior sections, on irradiation of the PEDOT: PSS/nanographite NCs, a cylindrical molten zone of few nm is created, during which the temperature of the sample is quite high and the polymer can easily diffuse in to the gallery of nanofiller which which decreases the number of layers in the nanographitic stack and promotes exfoliation and dispersion of nanographitic

(1)

with 3D charge transport for conducting polymer NCs, where, σo and To are the high temperature limit of conductivity. Fig 8(b) shows the variation of log σ vs 1/T-1/4. NCs

To (106 K)

Without irradiation 3x1010

Hopping distance (10-7 cm)

Average Hopping Energy (meV)

23.04

Density of states at Fermi level (1020) (eV-1cm3 ) 0.09

6.24

109

20.1

0.11

5.95

103

1x10

11

15.3

0.14

3.15

97

3x10

11

12.2

0.17

3

92

1x10

12

9

0.23

2.78

85

Table 3: Mott’s variable parameters PEDOT:PSS/nanographite NC before and After irradiation

for

Figure 8: a) Conductivity versus inverse of absolute temperature for PEDOT: PSS/nanogrpahite NCs before and after irradiation at different fluencies; b) Log σ versus T-1/4 for PEDOT:PSS/nanographite NCs before and after irradiation


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platelets within the polymer matrix. This leads to enhancement of the localized states of the charge carriers which is indicated by increase in n(EF) and decreased the hopping distance and hence less energy is needed for the carriers to make transition between two localized states. Further, there is transformation in structure of PEDOT chains from coiled to extended coiled as shown by Raman spectra, which leads to longer localization length of the charge and lowers energy barrier among the PEDOT chains. This further lowers the hopping energy barrier and is consistent with the above calculated Motts variable parameters. 3.7. Sensing Properties. The sensing response or sensitivity of the NC films to the analyte vapours is defined as

[

PEDOT:PSS/ nanographite NC

Response Time [min]

R0 x 102 [ohms]

R1x102 [ohms]

R 0R1/R0

Before irradiation

2.25 (135 sec)

701.9

817.7

16.5%

After tion

0.84 (50 sec)

15.01

19.7

38.3%

irradia-

Table 4: Sensitivity of PEDOT: PSS/nanographite Nano composite for Nitrobenzene Vapours before and after Irradiation

]

S = (R 1 − R o ) R o x100% (5) Where Ro and R1 are the resistance of the films before and after exposure to the analyte vapours respectively. Fig. 9(a) and 9(b) represent the sensing response of the NC films towards nitrobenzene vapours before and after irradiation respectively. Irradiated films show a significant increase in the response to the nitrobenzene vapours compared to unirradiated films as represented in Table 4.

Figure 9: a) Electrical Resistance of PEDOT: PSS/ nanographite Nano composite to Nitrobenzene Vapours Before Irradiation ; b) Electrical Resistance of PEDOT:PSS/nanographite Nano composite to Nitrobenzene Vapours after Irradiation

The sensitivity of polymer nanographite composites is based on two important aspects, the dispersibility of nanographite in the polymer and electronic properties of the composites.63 The combination of nanographite platelets and PEDOT: PSS results into the synergistic contribution towards the sensing of nitro benzene vapours. The exposure of the PEDOT: PSS/NGP NC sensors to nitrobenzene vapour molecules, leads to the swelling of the polymer at the nano junctions of the NCs which increases the NGP/polymer average gap and partially destroys the interconnected conductive networks of the nanocomposites.64 The change in conductivity is observed within few seconds of the exposure of NCs to nitrobenzene. The increase in sensitivity from 16.5% in unirradiated NC to 38% in radiated NCs can be explained on the basis of better dispersion of the NGPs in the NC after irradiation as discussed in AFM, Raman and XRD studies. The delaminated NGPs in irradiated films provide larger surface area for interaction with the nitrobenzene vapours and hence the sensitivity increases by 132%. 4. CONCLUSION The present work reports the use of SHI as a novel technique to exfoliate the NGPS within the polymer matrix along with their homogenous dispersion. The NCs shows a significant enhancement in electrical properties which is mainly attributed to the exfoliation and homogeneous dispersion of NGPs which is not possible by using chemical methods known till date. The nanocomposites of PEDOT: PSS and NGPs synthesized by solution blending technique, were irradiated by SHI [Ni ion beam, 80 MeV] in a fluence range of 1 × 1010 to 1 × 1012 ions/cm2. The SEM, XRD, AFM, Raman and electrical studies provides the understanding of the effect of ion irradiation on the structural framework and electronic transport properties of the NC. Significant improvement in electrical properties of the NCs is observed after irradiation due to the synergistic effect of the exfoliated and homogeneously dispersed NGPs within the polymer matrix and transformation in structure of PEDOT chains. The observations were discussed through Mott’s Variable range hopping model. The DC conductivity of NCs before and after irradiation at different fluences follows hopping type of charge conduction mechanism with change in only hopping parameters. After irradiation sensor based on PEDOT: PSS/nanographite NC film towards nitrobenzene vapours showed increase in sensitivity by 132% over unirradiated NC. Hence, ion irradiation technique is explored as an effective approach for dispersion of nanofillers within the polymer matrix thereby significantly enhancing the electrical and sensor


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properties of these materials which can be used for different applications. ACKNOWLEDGEMENT The authors are thankful to Inter University Accelerator Centre (IUAC), New Delhi, for providing SHI irradiation facility and Dr, D.K Avasthi and Dr. Ambuj Tripathi (IUAC) for providing technical support. The authors are also thankful to Defence Research Organization (DRDO, LASTEC) for providing financial assistance to carry out the research work. SUPPORTING INFORMATION SEM images of NGPs; Cross-sectional SEM image of PEDOT:PSS/NGP nanocomposite; Size Distribution Profiles of PEDOT:PSS/NGP nanocomposites before and after irradiation; AFM images of PEDOT:PSS/NGP nanocomposites before and after irradiation with corresponding height profiles. REFERENCES (1) Alateyah, A.I.; Dhakal, H.N.; Zhang, Z.Y. Processing, Properties, and Applications of Polymer Nanocomposites Based on Layer Silicates: A Review. Adv. Polym. Technol. 2013, 32, 21368. (2) Anderson, J.A.; Sknepnek, R.; Travesset, A. Design of Polymer Nanocomposites in Solution by Polymer Functionalization. Phys. Rev. E . 2010, 82, 021803. (3) Safdari, M.; Al-Haik, M.S. Synergistic Electrical and Thermal Transport Properties of Hybrid Polymeric Nanocomposites Based on Carbon Nanotubes and Graphite Nanoplatelets. Carbon, 2013, 64, 111. (4) Novoselov, K.S.; Geim, A.K.; Morozov, S.V.; Jiang, D.; Zhang, Y.; Dubonos, S.V. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666. (5) Potts, J.; Dreyer, D.R.; Bielawski, C.W.; Ruoff, R.S. Graphene-based Polymer Nanocomposites. Polymer 2011, 52, 5. (6) Du, J.; Cheng, H.M. The Fabrication, Properties and Uses of Graphene/Polymer Composites. Macromol. Chem. Phys., 2012, 213, 1060. (7) Ramanathan, T.; Stankovich, S.; Dikin, D.A.; Liu, H.; Shen, H.; Nguyen, T.; Brinson, C. Graphitic Nanofillers in PMMA Nanocomposites—An Investigation of Particle Size and Dispersion and their Influence on Nanocomposite Properties. J. Polym. Sci. Part B: Polym. Phys. 2007, 45, 2097. (8) Luo, Y.; Zhao, P.; Yang, Qi.; He, D.; Kong, L.; Peng, Z. Fabrication of Conductive Elastic Nanocomposites via Framing Intact Interconnected Graphene Networks Compos. Sci. Technol. 2014, 100, 143. (9) Xiong, J.; Jiang, F.; Shi, H.; Xu, J.; Liu, C.; Zhou, W.; Jiang, Q.; Zhu, Z.; Hu, Y. Liquid Exfoliated Graphene as Dopant for Improving the Thermoelectric Power Factor of Conductive PEDOT:PSS Nanofilm with Hydrazine Treatment. , 2015, 7, 14917–14925 (10) Coiai, S.; Passaglia, E.; Pucci, A.; Ruggeri, G. Nanocomposites Based on Thermoplastic Polymers and Functional Nanofiller for Sensor Applications Materials, 2015, 8, 3377.

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