Electron. Mater. Lett., Vol. 11, No. 4 (2015), pp. 559-564

DC Electrical Conductivity Retention and Electrical Compensation of Polyaniline by TiO2 at Higher Loading Percentages in

Polyaniline@TiO2 Nanocomposites

Mohd Omaish Ansari,1,* Mohammad Mansoob Khan,

1,2,* Sajid Ali Ansari,

1 and Moo Hwan Cho

1,*

1School of Chemical Engineering, Yeungnam University, Gyeongsan-si, Gyeongbuk 712-749, Korea2Chemical Sciences, Faculty of Science, Universiti Brunei Darussalam, Jalan Tungku Link,

BE 1410, Brunei Darussalam

(received date: 13 February 2015 / accepted date: 7 April 2015 / published date: 10 July 2015)

1. INTRODUCTION

Conducting inorganic/organic nanocomposites haveattracted considerable attention due to their wide range ofpotential applications.[1] A composite material can combinethe advantages of both classes of materials to access thecomplementary properties and synergetic effects. Among themany conducting polymers, polyaniline (Pani) has been theprime choice of researchers because of its outstandingintegrated performance, such as low cost, ease of synthesis,easy control of its redox states, etc.[2] Recently, Pani nano-composites with metal oxides have attracted considerable

attention because of their high performance in a range ofapplications, where a large surface contact area between thenanostructure and its environment is required, which mayhave applications in future technologies.[3,4]

Among the different semiconducting metal oxides, TiO2

nanoparticles are appealing because of their excellentphysical and chemical properties as well as their extensiveperceived applications in a wide range of areas, such ascoatings, solar cells and photocatalysis.[5-7] Recently, nano-composites of Pani with TiO2 have attracted attentionbecause it is a potential route towards improving theperformance of materials in various devices. The effects ofTiO2 on the electrical conductivity of Pani have beenreported. Previous studies examined the effects of TiO2 onthe thermal stability, electrical conductivity and gas sensingresponse of Pani.[8,9] On the other hand, there are few reportson the detailed chemical interactions affecting the electrical

Electrically conductive HCl-doped polyaniline (Pani)@titanium dioxide(TiO2) nanocomposites were prepared by the in-situ oxidativepolymerization of aniline in the presence of different amounts of TiO2

nanoparticles. The synthesized Pani@TiO2 nanocomposites werecharacterized by transmission electron microscopy, X-ray diffraction,UV-visible spectroscopy, and thermogravimetric analysis. The stabilityof the Pani@TiO2 nanocomposites in terms of their electricalconductivity retention was examined under isothermal and cyclic agingconditions, and compared with that of pure Pani. The Pani@TiO2

nanocomposites showed higher thermal stability than pure Pani. Theeffect of competitive doping/de-doping by TiO2 was examined atdifferent Pani to TiO2 weight ratios. TiO2 at a higher weight percentagewas found to be involved in the de-doping type of interaction with Pani,leading to its neutralization, which was found to be similar to the de-doping phenomenon, as in the case of neutralization in basic media. Thenovel mechanism for this de-doping type of interaction of TiO2 with Panihas also been proposed.

Keywords: polyaniline, TiO2, nanocomposite, electrical compensation,doping-dedoping

DOI: 10.1007/s13391-015-5073-2

*Corresponding author: omaishchem@gmail.com*Corresponding author: mhcho@ynu.ac.kr*Corresponding author: mmansoobkhan@yahoo.com©KIM and Springer

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properties. TiO2 might also be involved in the doping/de-doping of Pani, thereby affecting its electrical conductivity,which in turn affects its thermal stability and gas sensingresponse.[8] Therefore, in this study, HCl-doped Pani@TiO2

nanocomposites at different ratios were prepared, and theeffects of TiO2 on the electrical properties of Pani wereexamined. The structural and optical properties of thenanocomposites were determined, and a novel mechanismfor doping/de-doping by TiO2 is proposed.

2. EXPERIMENTAL PROCEDURE

2.1 Materials

Aniline, N-methyl 2-pyrrolidone and TiO2 nanoparticles(size < 25 nm) were purchased from Sigma Aldrich.Potassium persulphate (PPs), HCl, acetone and methanolwere obtained from Duksan pure chemicals, South Korea,and used as received. The water used in these experimentswas de-ionized water obtained from a PURE ROUP 30water purification system.

2.2 Methods

UV-visible spectrophotometry (Optizen 2120UV, Korea)was performed to record the absorbance spectra of thematerials. The microstructures were observed by fieldemission transmission electron microscopy (FE-TEM,Tecnai G2 F20, FEI, USA) at an accelerating voltage of200 kV. Thermogravimetric analysis (TGA) was performedusing a Perkin Elmer (Pyris Dimond) instrument heatingfrom ~25 to ~800°C at a rate of 10°C min−1 and a N2 flowrate of 200 mL/min. X-ray diffraction (XRD, PANalytical,X’Pert PRO-MPD) was performed using Cu Kα radiation at1.540 Ǻ in the range, 20° ≤ 2θ ≤ 70°, at 40 kV. The DCelectrical conductivity (σ) was measured using a 4-in-lineprobe electrical conductivity instrument (Scientific Equipment,India) and the calculations were done using the equation, σ =[ln2(2S/W)]/[2πS(V/I)], where I, V, W, and S are the current(A), voltage (V), film thickness (cm), and probe spacing(cm), respectively, and σ is the DC electrical conductivity(S/cm).[6]

2.3 Synthesis of Pani@TiO2 nanocomposites

The Pani@TiO2 nanocomposites were synthesized by thein-situ oxidative polymerization of aniline in the presence ofdifferent amounts of TiO2. Different TiO2 nanoparticle toaniline weight ratios were taken and dispersed in 200 mL of1 M HCl in an ultrasonic bath for 10 min. Aniline was addeddrop wise to the above dispersion of TiO2 to allow adsorptionon the TiO2 nanoparticles, after which a solution of oxidant(PPs prepared in 1 M HCl) was added to induce thepolymerization reaction. The reaction mixture transformedto greenish-black slurry, which was filtered after 2 h, washedsequentially with distilled water, acetone and methanol to

remove the dissolved impurities and Pani oligomers. Sub-sequently, the prepared Pani@TiO2 nanocomposites weredried at 80°C for 6 h in an air oven, converted to finepowders and stored in a desiccator for further experiments.The as-synthesized samples were then named Pani, Pani@TiO2-1, Pani@TiO2-2, Pani@TiO2-3, and Pani@TiO2-4, aslisted in Table 1.

2.4 Electrical conductivity measurements

The electrical conductivity was measured using pelletizedsamples of the Pani@TiO2 nanocomposites (0.1 g) with theassistance of a hydraulic pressure instrument at a pressure of50 kN for 10 min. The stability of the nanocomposite wasexamined in terms of their retention of the DC electricalconductivity using isothermal and cyclic aging techniquesunder ambient conditions. In the isothermal aging experiments,pellets of the nanocomposites were heated to 50, 70, 90, 110,and 130°C in a PID controlled temperature oven. Theelectrical conductivity was measured at 10 min intervals. Inthe cyclic aging experiments, the DC electrical conductivitywas measured five times at 45 min intervals at temperaturesranging from 40 to 150°C.

3. RESULTS AND DISCUSSION

3.1 Morphological, structural and thermal properties of

Pani@TiO2 nanocomposites

The morphology and inner nanostructure of the as-preparedPani@TiO2 nanocomposite was examined by transmissionelectron microscopy (TEM) (Fig. 1a). Short tubules of Paniwith many imbedded TiO2 nanoparticles were observed.HR-TEM (Fig. 1b) revealed single TiO2 crystals with a highdegree of structural uniformity. The SAED pattern alsoshowed a distinct diffraction maximum and the sequentialappearance of dark and bright fringes, which are characteristicof the polycrystalline structures of TiO2.

Figure 1c shows the characteristic X-ray diffraction(XRD) peaks of the Pani@TiO2 nanocomposites at differentPani to TiO2 weight ratios. The XRD patterns have allprominent peaks corresponding to the anatase and rutilephases of TiO2, as mentioned in previous reports.[3,4] Thecrystallite size of the TiO2 nanoparticles was calculatedusing the method reported elsewhere[4] and the meandiameter was ~17 nm, which corresponds to the TEM

Table 1. Preparation details of the Pani@TiO2 nanocomposites.

Sample I.D. Amount of TiO2 (g) Volume of aniline (mL)

Pani 0 1

Pani@TiO2-1 0.5 1

Pani@TiO2-2 0.5 0.50

Pani@TiO2-3 0.5 0.25

Pani@TiO2-4 0.5 0.15

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images. The characteristic peaks of Pani at ~15.1, 20.7 and25.5° 2θ [10] was not observed, suggesting that as the oxidantwas added, polymerization proceeded initially on TiO2

surface, leading to its encapsulation; hence, the crystallinebehavior of the Pani@TiO2 nanocomposite is hampered andthe peaks of Pani cannot be distinguished. Another reasonfor the non-observance of the Pani peak is its small amountcompared to TiO2. A comparison of the XRD patterns ofPani with those of the Pani@TiO2 nanocomposite showedthat TiO2 has retained its structure, even though it wasdispersed/encapsulated by Pani after the polymerizationreaction. On the other hand, Pani deposited on the surface ofTiO2 also had a slight effect on the crystallization performanceof TiO2 and the crystallinity of TiO2 in the nanocompositedecreased with increasing amount of Pani for the above-mentioned reasons.

TGA was used to examine the thermal stability of Paniand the Pani@TiO2 nanocomposite (Fig. 1d). Pani showeddistinct weight loss, as reported earlier.[10] The Pani@TiO2

nanocomposite showed 6% weight loss and there is no rapidweight loss due to exothermic degradation, which might beexplained by the very small amount of Pani on TiO2 as well

as the increased stability of Pani due to the presence of TiO2

nanoparticles.

3.2 Electrical conductivity

The electrical conductivity of the Pani@TiO2 nanocom-posites showed a decrease with increasing TiO2 loading from4.65 to 0.054 S/cm for Pani and the Pani@TiO2-4, respectively(Table 2). According to a previous report, a small amount ofTiO2 nanoparticles acts as an efficient network for chargetransport, resulting in an increase in electrical conductivity.[9]

In contrast, at a higher loading, the percentage of theconducting part, i.e., Pani, decreased; hence, the overallconductivity decreased. The Pani@TiO2-1 nanocomposite,

Fig. 1. TEM and HR-TEM images of Pani@TiO2-4 nanocomposite (a and b), XRD patterns of Pani@TiO2 nanocomposites (c), and TGA data ofthe Pani and Pani@TiO2-4 nanocomposite (d).

Table 2. DC electrical conductivity of the Pani@TiO2 nanocomposites.

Sample I.D. DC Electrical Conductivity (S/cm)

Pani 4.65

Pani@TiO2-1 1.11

Pani@TiO2-2 0.11

Pani@TiO2-3 0.076

Pani@TiO2-4 0.054

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which had the highest conductivity, was used for the DCelectrical conductivity retention studies.

3.3 DC Electrical conductivity retention studies

3.3.1 Cyclic studiesThe stability in terms of the DC conductivity retention of

Pani and the Pani@TiO2-1 nanocomposite was also studiedusing a cyclic technique. Figure 2 shows that both Pani andthe Pani@TiO2-1 nanocomposite exhibited an initial increasefollowed by a decrease in the electrical conductivity in thefirst cycle. This behavior in the first cycle was attributed tothe annealing behavior of the composite as well as theunquantified amount of moisture in the sample. The increasein electrical conductivity was initially due to the thermalactivation behavior as well as to the moisture present in thesamples. The moisture content decreased at higher tem-peratures, resulting in a decrease in conductivity; hence,irregular behavior was observed in the first cycle. Theconductivity decreased in the subsequent cycles, but moreuniform behavior was observed. In the case of Pani, therewas an increase in electrical conductivity until ~130°C, afterwhich a decrease was observed, which was attributed to theloss of dopant HCl as well as to degradation of the polymerbackbone.[6] The Pani@TiO2-1 nanocomposite showed muchhigher stability and no decrease in the electrical conductivityuntil 150°C was observed. The percentage retention of theelectrical conductivity for Pani and the Pani@TiO2-1nanocomposite was calculated to be 12 and 27%, respectively.The higher stability of the Pani@TiO2-1 nanocomposite wasattributed to the thermally more stable TiO2 component,which imparts additional stability due to a synergistic effect.Similar behavior was also reported for the Pani/grapheneand Pani/CNT nanocomposites.[11,12]

3.3.2 Isothermal studiesUnder isothermal conditions, there was gain/loss of

electrical conductivity at different temperatures. The relativeelectrical conductivity of Pani showed some loss at alltemperatures, whereas the Pani@TiO2-1 nanocompositeshowed an initial loss, followed by gain at higher temperatures(Fig. 3). This shows that the Pani@TiO2-1 nanocompositebehaves as a perfect semiconducting material. The higherstability of the Pani@TiO2-1 nanocomposite was attributedto the above mentioned reasons.

3.4 Electrical compensation studies

Digital photographs (Fig. 4a) of the samples showed thatthe emeraldine salt (ES) form of Pani changed to theemeraldine base (EB) form at higher TiO2 loadings, i.e. thegreen color was diluted with an increasing bluish tingecorresponding to Pani-EB. Hence, in addition to the reasonsmentioned above, the decrease in electrical conductivity canbe explained by neutralization of the Pani backbone due tothe de-doping effect of the TiO2 nanoparticles. This is alsosupported by the increased solubility of the Pani@TiO2

Fig. 2. DC electrical conductivity retention of (a) Pani, and (b) Pani@TiO2-1 nanocomposite under cyclic aging conditions.

Fig. 3. DC electrical conductivity retention of Pani and thePani@TiO2-1 nanocomposite under isothermal aging conditions.

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nanocomposites in NMP with increasing amounts of TiO2

nanoparticles (Fig. 4b). Therefore, at higher TiO2 loadings,Pani-ES is converted to Pani-EB, which is soluble in NMP.

For detailed analysis, different ratios of the Pani@TiO2

dispersion in NMP were centrifuged to remove TiO2, afterwhich the Pani solution in NMP was analyzed by UV-visiblespectroscopy (Fig. 4c). HCl-doped Pani showed an absorbancepeak at ~380 nm, corresponding to Pani-ES, whereas Pani-EB showed an absorbance peak at ~327 and 630 nm, whichis in agreement with previous reports.[13] The absorption inthe visible region, i.e. 630 nm, was attributed to excitonformation in the quinoid rings, which gives rise to the bluecolor of the Pani-NMP solutions.[14] A NMP solution ofPani-EB showed an intense blue color with high absorptionin the visible region. This peak was less intense in allPani@TiO2 nanocomposites and the peak intensity increasedwith increasing TiO2 content. Pani@TiO2-4 showed muchhigher absorption in the visible region than the nanocompositeswith much lower weight percentage of TiO2. Therefore, TiO2

nanoparticles are involved in the de-doping type of theinteraction with Pani; hence converting it to the blue form ofPani. Moreover, this phenomenon increased with increasingpercentage of TiO2, leading to much higher absorption in thevisible light region.

3.5 Mechanism of electrical compensation of Pani by

TiO2

The electrical conductivity of the Pani@TiO2 nanocom-

posite is affected by the reaction of the doping acid, i.e., HCl,with the TiO2 nanoparticles, leading to the formation ofTiCl6

2−. It was assumed that in a Pani@TiO2 systemcontaining a high percentage of TiO2, some TiO2 might alsoexist as TiCl6

2−, as reported by Nageh et al.[13].

TiO2 + H+ + Cl− → TiO(OH)Cl (1)TiO(OH)Cl + 3H+ + Cl−

→ TiCl2

2+ + 2H2O (2)

TiCl22+ + 4Cl− → TiCl6

2− (3)

The decrease in electrical conductivity was attributed tomany factors, such as the reaction of TiO2 with HCl, leadingto a decrease in the concentration of doping acid, whichdecreases the electrical conductivity. In addition, some TiO2

is converted to TiCl62−, as mentioned in the above reaction,

in which TiCl62− acts as a de-doping agent.[15] TiCl2

2+ isconverted to TiCl6

2− because of its high stability, as its freeenergy of formation is −5.12 to −10.90 kcal/mol.[16]

Ansari et al.[17] reported a similar increase/decrease in

electrical conductivity in their ZnO@Pani nanocomposite.They suggested that the de-doping effect dominates at ahigher loading of ZnO nanoparticles, which eventually leadsto a decrease in conductivity. In the light of their observations,the decrease in electrical conductivity can be attributed tocompetitive doping/de-doping, and at higher TiO2 concen-trations, the de-doping phenomenon dominates because ofthe high stability of TiCl6

2− compared to TiCl22+. TiCl6

2− leadsto de-doping of the Pani backbone, which leads to electrical

Fig. 4. (a) Digital photographs, (b) dispersion in NMP, (c) UV-visible spectra and (d) DC electrical conductivity of the Pani@TiO2 nano-composites.

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compensation. Fig. 5 presents a schematic diagram of theinteraction of TiCl6

2− with the Pani backbone.

4. CONCLUSIONS

Pani@TiO2 nanocomposites were prepared by incorporatingvarious weight percentages of TiO2 in Pani using theoxidative polymerization technique. The amount of TiO2 hasa significant effect on the crystallinity of Pani and at higherTiO2 loading, the Pani peaks were not distinct. ThePani@TiO2 nanocomposite showed increased DC electricalconductivity retention under isothermal and cyclic agingconditions compared to Pani, which was attributed to thestabilizing effect of TiO2, giving a highly stable semiconductingnanocomposite. The DC electrical conductivity decreasedwith increasing amount of TiO2, leading to the electricalcompensation/neutralization of Pani, which was similar tothe de-doping phenomenon, as in the case of neutralizationby a base. The improved solubility of the Pani@TiO2

nanocomposite in NMP also supports the theory of electricalcompensation of Pani by TiO2. A mechanism was alsoproposed for the interaction of TiCl6

2− with Pani leading toelectrical compensation.

ACKNOWLEDGEMENTS

This study was supported by Priority Research Centers

Program through the National Research Foundation ofKorea (NRF) funded by the Ministry of Education(2014R1A6A1031189).

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Fig. 5. Interaction of TiCl62− with Pani leading to the compensation/

decrease in conductivity.