Electrically conductive and optically transparent polyaniline/montmorillonite nanocomposite thin films

Thin Solid Films 562 (2014) 319–325

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Thin Solid Films

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Electrically conductive and optically transparent polyaniline/montmorillonite nanocomposite thin films

Lenka Kulhánková a, Jonáš Tokarský b,c,⁎, Vlastimil Matějka b, Pavlína Peikertová b, Silvie Vallová a,Kateřina Mamulová Kutláková b, Vítězslav Stýskala d, Pavla Čapková e

a Faculty of Metallurgy and Materials Engineering, VŠB-Technical University of Ostrava, 17, listopadu 15/2172, 70833 Ostrava-Poruba, Czech Republicb Nanotechnology Centre, VŠB-Technical University of Ostrava, 17, listopadu 15/2172, 70833 Ostrava-Poruba, Czech Republicc CE IT4Innovations, VŠB—Technical University of Ostrava, 17, listopadu 15, 70833 Ostrava, Czech Republicd Faculty of Electrical Engineering and Computer Science, VŠB—Technical University of Ostrava, 17, listopadu 15/2172, 70833 Ostrava-Poruba, Czech Republice Faculty of Science, J. E. Purkyně University, České mládeže 8, 40096 Ústi nad Labem, Czech Republic

⁎ Corresponding author at: Nanotechnology CentreOstrava, 17, listopadu 15, 70833 Ostrava, Czech Repubfax: +420 597 321 640.

E-mail address: [email protected] (J. Tokarský).

http://dx.doi.org/10.1016/j.tsf.2014.05.0060040-6090/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

Article history:Received 4 July 2013Received in revised form 8 May 2014Accepted 9 May 2014Available online 15 May 2014

Keywords:PolyanilineMontmorilloniteNanocompositeThin filmConductivityTransparency

Simple technology (i.e. one-step polymerization of anilinium sulfate in the presence of montmorillonite) hasbeen used to prepare polyaniline/montmorillonite (PANI/MMT) nanocomposite thin films. The present work isfocused on the effect of MMT content on the thickness, morphology and electrical conductivity of PANI/MMTnanocomposite films. An increase in MMT content led to an increase in film thickness and surface roughnessand to a decrease in conductivity from a maximum value of 356 S/m to a minimum value of 76 S/m, while forpure PANI films, prepared using the same polymerization procedure (from anilinium sulfate), the conductivitywas 59 S/m. On the other hand, the MMT content exhibited a significant impact on the optical transparency ofPANI/MMT nanocomposite films, which was limited to the range of 450–650 nm in contrast with pure PANIfilms showing non-zero transparency in thewholemeasured range ofwavelength 300–800 nm. Atomic forcemi-croscopy clearly proved the alignment of PANI chains in nanocomposite films in comparison with pure PANIfilms. Scanning electron microscopy revealed in detail the significant difference in surface morphology betweenpure PANI films and PANI nanofilms covering the MMT particles.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Hybrid nanocomposite materials based on conducting organic poly-mers are of interest in a wide range of applications in many fields, suchas electrochromism, electroluminescence, sensors, corrosion protection,and energy storage systems [1–5]. In conducting polymer systems,chain alignment plays a key role because a higher degree of orderingleads to a low value of percolation threshold and enhances the conduc-tivity [6–16]. Ordering of polymer chains can be achieved by variousmethods such as mechanical orientation of polyaniline (PANI) films[12], using blends with insulating polymers [13,14], incorporation ofmetal nanoparticles [15], using electric field [16] or high pressure [17].Hybrid nanocomposites PANI/phyllosilicate [18–29], or PANI/graphite[11] offer another way of PANI chain ordering due to the inclusion ofplaty particles of layered inorganic structures into a polymeric matrix.Moreover, the non-bond interactions between PANI chains and

, VŠB—Technical University oflic. Tel.: +420 597 321 519;

inorganic materials can result in improved properties of a nanocompos-ite, such as thermal and mechanical stability, and optical properties [1].

Variousmethods of PANI/phyllosilicate nanocomposites preparationusing a number of steps and different precursors (anilinium hydrochlo-ride, anilinium sulfate) have been described in literature (for more de-tails see [28]). Previous studies of PANI/phyllosilicate nanocompositeshave dealt with the preparation and characterization of either powderform or pressed pellets. If the PANI/phyllosilicate nanocomposite waspresent in thin film, it was only a small amount dispersed in an essentialcomponent of the coating [4,25,30]. For example, Špírková et al. [30]used 4 wt.% of PANI/montmorillonite (PANI/MMT) nanocompositein the thin film prepared from organosilicon precursors and eitherα,ω-oligo(propylene oxide)diamine or α,ω-oligo(propylene oxide)triamine.

Conductive composite films based on conducting polymers havebeen prepared bymany authors, as blending PANI with other polymersor fillers provides a chance to create materials possessing improvedconductivity and processability [31–33]. Nevertheless, all these previ-ous studies of PANI nanocomposite films dealt with other components(CdS [34], carbon nanotubes [35], carboxymethyl cellulose [36]) andnot with the phyllosilicate.

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Ourwork presents the PANI/phyllosilicate nanocomposite thin filmscontaining no other components like binders or additional polymericcarriers. In addition to the electrical conductivity, an attention waspaid also to the optical properties of nanocomposite films and, there-fore, optically transparent clay mineral MMT [37] was chosen. Anotheradvantage of the MMT is a layered structure with strong tendency tocreate platy particles, which significantly affects the ordering of PANIchains, i.e. the PANI nanostructure [28].

PANI/MMT nanocomposite films exhibit unique electrical andoptical properties in comparison with pure PANI thin films.

2. Experimental details

2.1. Sample preparation

Aniline, sulfuric acid, and ammonium peroxydisulfate werepurchased from Lach-Ner, Czech Republic, and used as received. Micro-scope glass slides (76mm×26mm×1mm)were used as substrates forPANI/MMT films. The glass slides were carefully washed in a soap solu-tion, rinsedwith distilledwater, thenwith ethanol and dried. In order toprevent a double-sided coatingwith the PANI/MMT film, one side of theglass slidewas covered by a scotch tape. PANI/MMT films on such treat-ed glass slides were prepared as follows using anilinium sulfate as aprecursor.

Two solutions (0.2 M aniline prepared in 0.5 M sulfuric acid and0.1 M ammonium peroxydisulfate dissolved in distilled water)were mixed together with appropriate amounts of MMT and withtwo different sizes of MMT particles, as described in Table 1.

Glass slides attached by clips and hanging on strings were intro-duced into the beaker. After several hours of polymerization and forma-tion of thin film, glass slides with PANI/MMT film were removed fromthe beaker, and rinsed with 0.2 M hydrochloric acid in order toexchange the sulfate ions for chloride ions. Then, the samples wererinsed with ethanol and dried.

Powder samples of PANI/MMT nanocomposite were prepared in thesameway but without glass slide. The green solid was collected on a fil-ter, rinsed with 0.2 M hydrochloric acid, then with ethanol and dried. Apure PANI filmwas also prepared in the sameway butwithout additionof MMT.

2.2. Structure, morphology and film thickness

X-raydiffraction (XRD) analysiswas used to compare theMMTbasalspacing in pure MMT and PANI/MMT powder samples. The XRDpatterns were recorded using Bruker D8 Advance diffractometer(Bruker AXS) equipped with fast position-sensitive detector VÅNTEC1. CoKα irradiation (λ = 0.178897 nm) was used. Measurements of allsamples were carried out in reflection mode in symmetrical Bragg–Brentano arrangement. Raman spectra of all samples were measuredusing a Smart Raman Microscopy System XploRATM (HORIBA JobinYvon). Raman spectra were acquired with 532 nm excitation laserlight and using 1200 gr./mm grating. In order to determine the degreeof homogeneity in PANI/MMT films, Raman spectra were measured insix points evenly distributed on the surface of samples.

Table 1MMT content in 200 ml of polymerization mixture (MMT cont.) and the size fraction ofMMT particles are listed for each prepared sample.

MMT cont. Size fraction

Sample [g/200ml] [μm]

PANI 0.00 –

P/M_0.10_5 0.10 b5P/M_0.20_5 0.20 b5P/M_0.25_5 0.25 b5P/M_0.35_5 0.35 b5P/M_0.35_1 0.35 b1

The morphology of the surfaces of PANI/MMT films was studiedusing SolverNEXT (NT-MDT) atomic force microscope (AFM) operatedin contact mode. The contact probe PPP-CONTR (Nanosensors) wasused for imaging. The images were evaluated using IA P9 software(NT-MDT). Surfacemorphologywas also observed by scanning electronmicroscope (SEM) Philips XL30 (PHILIPS). SEM images were obtainedusing a secondary electron detector. Accelerating voltage was 5.0 kV.Elemental composition of samples was determined using energy dis-persive X-ray analysis (EDS) with an accelerating voltage of 15.0 kV.Film thickness was determined by AFM. A streak perpendicular to thesurface has been made by razor blade and subsequently the height ofsuch prepared “step” above the glass surface was measured.

2.3. Thermogravimetric analysis

Measurementswere carriedout using simultaneous thermogravimeter-differential scanning calorimeter (TG-DSC) STA 409 EP (Netzsch)equipped with a high-sensitive analytical balance for measuringthe mass change of the sample as a function of temperature andtime. The sample carrier system contains the type S thermocouples(Pt10%Rh-Pt) to measure the temperature and the temperaturedifference. All measurements were conducted under the identicalconditions: samples (10 mg in weight) were heated up to 800 °C inthe crucibles (α-Al2O3) in a dynamic atmosphere of dry air with aflow rate of 100 cm3 min−1. Heating rate was 10 °C min−1.

2.4. Measurements of electrical conductivity

In order to eliminate the effect of structural inhomogeneity on theconductivity of PANI/MMT films, a special measuring apparatus hasbeen developed (Fig. 1). Conductivity of samples has been measuredby static applied voltage in the DC regime using linear Cu electrodeswith a triangular cross-section. The distance between electrodes was20 mm (see Fig. 1). The DC voltage used for measurements was 9.5 Vand the value of electrical conductivity was determined as an averageof 300 values measured during a 30 sec interval. The design of themea-suring apparatus guarantees the perfect fit of surface and electrodes forall measurements. During and after repeated measurements, no visibledamage of PANI/MMT thin films was observed.

2.5. Optical properties of PANI/MMT thin films

UV–VIS spectra used for the characterization of optical properties ofPANI/MMT thin film were registered using spectrophotometer CINTRA303 (GBC Scientific Equipment). The optical transmittance spectra ofPANI/MMT films were measured at normal incidence at room tempera-ture in the spectral range of 300–800 nm under the followingconditions: speed was 1000 nm min−1 with a step size of 0.427 nm,and slit width was 2.0 nm.

3. Results and discussion

It can be expected that during the preparation process aniliniumions at least partially enter the MMT interlayer space. In order toconfirm or disprove this assumption the XRD analysis of samples wasperformed. Unfortunately, this direct measurement failed (due to therelatively small amount of PANI/MMT composite in comparison withthe glass) and, therefore, we used the XRD analysis of pure MMT andPANI/MMT composite in powder form (see section 2.1. Sample prepara-tion). XRD profiles of 001 basal reflections are compared in Fig. 2.

The d001 values for pure MMT andMMT in PANI/MMT powder com-posite are 1.251 nm, and 1.278 nm, respectively. The difference0.027 nm is too small to prove the presence of anilinium in the MMT,and, therefore, it can be concluded that anilinium does not enter theMMT interlayer space.

Fig. 1. Experimental apparatus for measuring the DC conductivity. 1) sample—thin PANI film deposited on a glass slide (see the dimensions of glass slide in the upper left corner);2) Cu-electrodes (distance 20 mm); 3) flexible insulator; 4) voltage terminals; 5) weights to ensure a constant load; 6) measuring card; 7) PC+ software.

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The higher intensity of PANI/MMT 001 basal reflection reveals thefact that MMT particles are well ordered in the PANI/MMT composite.

TG analysis was also performed on powder samples in order to avoidthe influence of glass. TG/DTG curves can be found in Fig. 3. Weightlosses, determined from TG curves, indicate the following particularprocesses: loss of water layer by layer (30–200 °C), release of organicmatter (200–690 °C), and loss of constitutional water (600–780 °C).From the DTG curves, themaximum temperatures Tmax were identified.The total weight loss of pure MMT was only 14.3% (Fig. 3a) while purePANI was completely decomposed at 690 °C (Fig. 3b). The presence ofPANI in PANI/MMT powder is apparent from Fig. 3c where TG andDTG curves indicate weight loss in the temperature range 200–690 °Ccorresponding to release of organic matter. The total weight loss ofPANI/MMT powder composite increased to 39.4% compared with pureMMT. The decomposition of organic matter occurs in the same temper-ature interval for both pure PANI and PANI/MMT powder (compareFig. 3b and c). Therefore, an immobilization of PANI chains on theMMT surface could not be proved, probably due to the excess of PANIin the PANI/MMT composite.

Results of DC conductivity for all prepared samples are summarizedin Table 2. As one can see, themaximum conductivity 356 S/m has beenobserved for the samplewith the lowestMMT content (0.1 g per 200mlof polymerization mixture). An increase of MMT content is accompa-nied with an increase of film thickness and a decrease of the conductiv-ity to the minimum value of 76 S/m for the sample P/M_0.25_5. Afurther increase in MMT content to 0.35 g per 200ml of polymerizationmixture (i.e. sample P/M_0.35_5) led to cracks and peeling of thecomposite film. For this reason we prepared an additional sampleP/M_0.35_1, where we used the same amount of MMT as in the sampleP/M_0.35_5 but with lower particle size (size fraction b1 μm). This

Fig. 2. XRD profiles of 001 basal reflections of (a) pure MMT and (b) MMT in PANI/MMTpowder.

sample exhibits a similar ordering and conductivity as sampleP/M_0.25_1; however, the thickness was 3.6 times lower (380 nm).

Last year, Huang et al. reported the preparation of sandwiched PANI/ZnO/PANI free-standing nanocomposite film exhibiting the conductivi-ty ~940 S/m [38]. Although this conductivity is 2.6 times higher, the

Fig. 3. TG (solid)/DTG (dashed) curves of powder samples: (a) pure MMT, (b) pure PANI,and (c) PANI/MMT powder.

Table 2Thickness of PANI/MMT nanocomposite thin film and value of conductivity are providedfor each prepared sample.

Film thickness Conductivity

Sample [nm] [S/m]

PANI 150 59P/M_0.10_5 370 356P/M_0.20_5 600 205P/M_0.25_5 1370 76P/M_0.35_5 Not availablea Not availablea

P/M_0.35_1 415 91

a Cracked film peeled off the glass.

Fig. 5. SEMmicrographs and EDS spectra of (a) PANI/MMT nanocomposite film (sampleP/M_0.20_5), and (b) pure PANI film.

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preparation of this material is much more difficult and expensive thanPANI/MMT nanocomposite films.

Other PANI-based nanocomposite thin films reported by various au-thors exhibit significantly lower conductivities. For example, Raut et al.[34] reported PANI/CdS nanocompositefilmswith the thickness of 366–703 nm and conductivity σ = 6.91 × 10−4 S/m − 3.14 × 10−2 S/m.Branzoi et al. [35] prepared conducting composite films PANI/carbonnanotubes for electrode modification showing significantly lower resis-tance than pure PANI films and Basavaraja et al. [36] reported the roomtemperature electrical conductivity for PANI/carboxymethyl cellulosenanocomposite films σ = 1.07 S/m.

The fact that the conductivity of PANI/MMT nanocomposite films ishigher than the conductivity of pure PANI thin film (see Table 2), canbe ascribed to the influence of MMT on the arrangement of PANI chains.The negative layer charge of MMT particles attracts the positivelycharged PANI chains that are immobilized on the MMT surface. It isalso very probable that some amount of sulfuric acid attacks the MMTlayers and causes a further increase in the negative layer charge (see,for example, [39]) which, as a consequence, leads to stronger immobili-zation of PANI chains on the MMT and better alignment.

AFM results confirmed the assumption that thenanocomposite filmsare ordered uniformly. Fig. 4 shows the three examples of surface mor-phology for samples P/M_0.20_5, P/M_0.35_1 and pure PANI film. Onecan see the distinct surface morphology and certain degree of micro-structure ordering for samples P/M_0.20_5 and P/M _0.35_1 in contrastwith surface morphology for the pure PANI film (see Fig. 4c) where thePANI chains form the nanorods perpendicular to the PANI layer plane.This structural feature of PANI layers has been already reported assecondary nucleation on the top of the PANI film by Trchová et al. [40].

SEM micrographs in Fig. 5 reveal the difference in surface morphol-ogy between the PANI/MMT nanocomposite film (P/M_0.20_5; Fig. 5a)and the pure PANI film (Fig. 5b). It is evident that PANI film covering theMMT particle has a flat surface without tendency to secondary nucle-ation forming the prickles shown in the Fig. 4c. EDS spectra prove thepresence and absence of MMT in the nanocomposite film (Fig. 5a) andthe pure PANI film (Fig. 5b), respectively. The Si atoms detected in theEDS spectrum of the pure PANI film belong to the glass substrate.

Homogeneity of PANI/MMT nanocomposite films was determinedby Raman spectra measured in six different points evenly distributed

Fig. 4. AFM images show the surface morphology of PANI/MMT nanocomposite film

on the sample's surface (P/M_0.20_5; Fig. 6). For comparison, the spec-tra of pure PANI thin film and PANI/MMT powder are shown in Fig. 7.Only one spectrum for each sample is provided in Fig. 7 because nearlythe same spectra were obtained from all measured points.

Fig. 7a shows the typical PANI spectrum. Bands of out-of-plane vi-brations of aromatic rings can be found at 408 and 505 cm−1, while abroad band at 800 cm−1 corresponds to the deformation vibrations ofvarious substituted aromatic rings [41]. The most important band at1327 cm−1 is characteristic for the protonation state (C\N+ ) of PANI[42]. Bonds C\N and C_N in PANI chains are confirmed by bands at1220 cm−1 and 1496 cm−1, respectively. The C_N bond proves thepresence of quinonic units [42–44]. Further, the bands of C\C andC_C stretching vibrations (1610 cm−1 and 1582 cm−1), and C\Hbending (1161 cm−1) can be seen [45,46]. Broad band in the area ofwavenumbers higher than 2000 cm−1 is typical for the conductingform of PANI [47].

The spectrum of PANI/MMT powder (Fig. 7b) has very similar shapebut followingdifferences can be found. Allmaxima are slightly shifted to

s P/M_0.20_5 (a), and P/M_0.35_1 (b) in comparison with pure PANI film (c).

Fig. 6. Raman spectra measured in six points (a–f) evenly distributed on the surface of PANI/MMT nanocomposite film (sample P/M_0.20_5).

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higher wavenumbers and two bands, assigned to phenoxazine-typeunits, appear at 584 cm−1 and 1402 cm−1 [48,49]. Moreover, one addi-tional band showing the presence of quinonic units can be found at1643 cm−1 [50].

All six spectra of P/M_0.20_5 sample are much more similar to thespectrum of PANI/MMT powder than to the spectrum of pure PANI(compare Figs. 6 and 7). Moreover, the fact that all six spectra are

Fig. 7. Raman spectra of (a) pure PANI thin film depo

quite similar to each other suggests that PANI/MMT nanocompositefilms are homogeneous. Protonation band at 1337 cm−1 vary in the in-tensity of measured points, but its intensity is generally very high,which corresponds to a good protonation state of PANI chains in PANI/MMT nanocomposite films. The presence of phenoxazine-type units(595 cm−1, shifted from 584 cm−1 as present in Fig. 7b) is confirmedby spectra obtained from five points (Fig. 6b–f) and only in the first

sited on glass slide, and (b) PANI/MMT powder.

Fig. 8.Optical transmittanceof P/M_0.20_5 and P/M_0.35_1films, comparedwith thepurePANI film.

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point the band at 595 cm−1 is missing (Fig. 6a). Other interestingchanges are in the region 1500–1620 cm−1 where one can see lessstructured bands in spectra obtained from the second (Fig. 6b) andthe fourth (Fig. 6d) points. In Fig. 6d, for example, the C\C band at1612 cm−1 is visible only as a small shoulder contrary to Fig. 6awhere this band is visible more clearly. These changes correspond tochanges in the benzene/quinone ratio in PANI chains.

Optical transmittances of PANI/MMT nanocomposite films and purePANI film are shown in Fig. 8. Pure PANI film is transparent in thewholemeasured wavelength range (300–800 nm) but PANI/MMT nanocom-posite films exhibit a transmittance only in the limited wavelengthrange (450–650 nm) that coincides with the area of maximum trans-mittance of the pure PANI film. The fact that nanocomposite films arestill transparent even after addition of MMT particles can be attributedto the ability of MMT itself to form transparent layers [37].

4. Conclusions

Simple one-step methodwas used for the preparation of PANI/MMTnanocomposite thin films deposited on the glass surface. These PANI/MMT thin films exhibit relatively high conductivity in comparisonwith so far published data for PANI nanocomposite thin films. Whilein the present work the maximum conductivity obtained for the PANI/MMT thin film is σ = 356 S/m, published values for nanocompositefilms are σ = 6.91 × 10−4 S/m − 3.14 × 10−2 S/m for PANI/CdS [34]and σ = 1.07 S/m for PANI/carboxymethyl cellulose nanocompositefilms [36]. The present results of AFM measurements fully confirmedthe assumption that the MMT particles organize PANI chains and, con-sequently, lead to an increase in conductivity of PANI/MMT nanocom-posites compared to pure PANI. SEM images revealed a difference inthe mechanism of growth of pure PANI and PANI/MMT compositefilms. The well known mechanism of forming PANI nanotubes and sec-ondary nucleation resulting in prickles does not occur in case of PANIcovering the silicate particles (see Figs. 4 and 5). PANI/MMT films exhib-it optical transparency, which is lower than that for pure PANI films, butselective in the range of 450–650 nm.

Acknowledgments

This research has been funded by the Grant Agency of CzechRepublic (project P108/11/1057), IT4Innovations Centre of Excellenceproject (reg. no. CZ.1.05/1.1.00/02.0070), and theMinistry of Education,Youth and Sports of Czech Republic (project SP2014/37).

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