synthesis and characterization of conducting self-assembled polyaniline nanotubes-zeolite...

Synthesis and Characterization of Conducting Self-AssembledPolyaniline Nanotubes/Zeolite Nanocomposite

Gordana Ciric-Marjanovic,*,† Vera Dondur,† Maja Milojevic,† Milos Mojovic,†

Slavko Mentus,† Aleksandra Radulovic,‡ Zorica Vukovic,§ and Jaroslav Stejskal|

Faculty of Physical Chemistry, UniVersity of Belgrade, and Institute of General and Physical Chemistry,Studentski Trg 12-16, 11158 Belgrade, Serbia, Department of Catalysis and Chemical Engineering, ICTM,NjegoseVa 12, Belgrade, Serbia, and Institute of Macromolecular Chemistry, Academy of Sciences of the

Czech Republic, 162 06 Prague 6, Czech Republic

ReceiVed September 16, 2008. ReVised Manuscript ReceiVed NoVember 4, 2008

Self-assembled conducting, paramagnetic polyaniline nanotubes have been synthesized by the oxidative polymerizationof aniline with ammonium peroxydisulfate in aqueous medium in the presence of zeolite HZSM-5, without addedacid. The influence of initial zeolite/aniline weight ratio on the conductivity, molecular and supramolecular structure,paramagnetic characteristics, thermal stability, and specific surface area of polyaniline/zeolite composites was studied.The conducting (∼10-2 S cm-1), semiconducting (3 × 10-5 S cm-1), and nonconducting (5 × 10-9 S cm-1) compositesare produced using the zeolite/aniline weight ratios 1, 5, and 10, respectively. The coexistence of polyaniline nanotubes,which have a typical outer diameter of 70-170 nm and an inner diameter of 5-50 nm, accompanied by nanorodswith a diameter of 60-100 nm and polyaniline/zeolite mesoporous aggregates, distinct from the morphology ofmicroporous zeolite HZSM-5, was proved in the conducting nanocomposite by scanning and transmission electronmicroscopies. FTIR spectroscopy confirmed the presence of polyaniline in the form of conducting emeraldine saltand suggested significant interaction of polyaniline with zeolite. The evolution of molecular and supramolecularstructure of polyaniline in the presence of zeolite was discussed.

Introduction

Intrinsically conducting polymers have received considerableattention owing to their wide potential applications in erasableinformation storage, shielding of electromagnetic interference,radar-absorbing materials, sensors, indicators, actuators, cata-lysts, electronic and bioelectronic components, rechargeablebatteries, membranes, electrochemical capacitors, electrochromicdevices, nonlinear optical devices, light-emitting diodes, andantistatic and anticorrosion coatings.1 Polyaniline (PANI) is oneof the most important conducting polymers, frequently studieddue to its ease of synthesis by standard chemical or electrochemicaloxidative polymerization, low cost, high conductivity, and goodenvironmental and thermal stability.2 PANI exists in variousacid-base and redox forms with substantially different chemicaland physical properties.3 Only the green emeraldine salt form(Scheme 1) is conducting (∼100 S cm-1). The preparation ofbulk quantities of conducting PANI is usually performed by thechemical oxidative polymerization of aniline in aqueous solutionin the presence of strong acids (initial pH < 2.0), ammonium

peroxydisulfate (APS) being the most frequently used oxidant.4

A granular morphology has invariably been observed for PANIprepared under such conditions.

Nowadays, there is great interest in the research of nano-structured conducting PANI because its dispersibility andprocessability have significantly improved, and its performanceis substantially enhanced in many conventional applications incomparison with granular PANI.5 The oxidative polymerizationof aniline with APS in aqueous solution starting from alkaline,neutral, and slightly acidic reaction conditions at pH > 4.0 andfinishing at pH < 2.0 (falling pH method) has been recognizedas a reliable template-free synthetic route to PANI nanotubes.6-18

* Corresponding author. E-mail: [email protected].† Faculty of Physical Chemistry, University of Belgrade.‡ Institute of General and Physical Chemistry.§ ICTM.| Academy of Sciences of the Czech Republic.(1) ConductiVe ElectroactiVe Polymers: Intelligent Materials Systems; Wallace,

G. G., Spinks, G. M., Kane-Maguire, L. A. P., Teasdale P. R., Eds.; CRC Press:Boca Raton, FL, 2003.

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Scheme 1. Polyaniline-Emeraldine Salt Form

3122 Langmuir 2009, 25, 3122-3131

10.1021/la8030396 CCC: $40.75 2009 American Chemical SocietyPublished on Web 01/26/2009

Conducting PANI nanotubes have been synthesized by thechemical oxidative template-free method in the presence ofvarious inorganic acids,6 sulfonic acids,7,8 carboxylic acids,9-12

polymeric acids,13 sulfonated carbon nanotubes,14 sulfonateddendrons,15 and titanium dioxide.16 The new simplified template-free method of the synthesis of conducting self-assembled PANInanotubes by the oxidation of aniline with APS in water withoutany added acid has recently been created.10,17 This facile andefficient synthetic method not only omits hard-template and post-treatment of template removal, but also simplifies reagents.

Conducting PANI/inorganic porous solid composites have beenthe subject of considerable interest because of their improvedmechanical and chemical performance compared with the purePANI. Among various inorganic porous materials for thedevelopment of the PANI/inorganic composites, zeolites havereceived growing attention during last two decades.19-36 PANI/zeolite composites were prepared by the oxidation of anilinewithin the zeolite (Y, HZ, HS, HY, MCM-41, 13X, ZSM-5, �)

channel system,19,21,22,24,27 by chemical,23,29,31-34 electrochemi-cal,20 emulsion,25 and enzymatic36 oxidative polymerizations ofaniline in the presence of zeolites (MCM-41, 13X, �, FUYB, Y),dry mixing of PANI powder with the zeolites (Y, 13X, MCM41,LTA),28,30 and addition of zeolite (Zenith-N, LTN) to the PANIsolution.26,35 These hybrid materials were proposed to havepotential applications as electronic components,19 as sensors (e.g.,for gases or pH),28,30,32,33 as antiferromagnetic materials,24 ascathodes in a primary cell,26 for the curing of epoxy resin,25 andfor enhancing the thermal conductivity of zeolite.23

Here we report, for the first time, the synthesis of conductingPANI nanotubes/zeolite nanocomposite, which combines uniqueproperties of one-dimensional PANI nanostructures and PANI/zeolite hybrid materials, by a self-assembly process without addedacid. We expect that this simple and versatile synthetic strategycould provide new opportunities for producing a wide range ofother nanocomposites of conducting self-assembled PANInanotubes and various inorganic materials. Prepared PANI/zeolitecomposites were characterized by FTIR, UV-visible, and EPRspectroscopies, scanning (SEM) and transmission (TEM) electronmicroscopies, thermogravimetric analysis, and specific surfacearea and electrical conductivity measurements. Attention hasbeen focused on the effect of initial zeolite/aniline weight ratioon the structure and physicochemical properties of synthesizedPANI/zeolite composites.

Experimental SectionMaterials. Aniline (p.a., Centrohem, Serbia) was distilled under

reduced pressure and stored at room temperature, under argon, priorto use. APS (analytical grade, Centrohem, Serbia) and zeoliteHZSM-5 [H6(H2O)16(Al6Si90O192), Zeolyst International, SiO2/Al2O3

) 30, specific surface area SBET ) 400 m2 g-1, average particle size1-2 µm] were used as received.

Synthesis of PANI Nanotubes/Zeolite HZSM-5 Nanocomposite.A typical procedure for preparing PANI nanotubes/zeolite HZSM-5nanocomposite was as follows: zeolite HZSM-5 (0.93 g) wasdispersed in 20 mL of distilled water, and then aniline (0.93 g; 1.0× 10-2 mol) and water, up to 25 mL total volume of dispersion, wereadded. This suspension was stirred for 10 min, and then 25 mL ofthe aqueous solution containing 2.85 g of APS (1.25 × 10-2 mol)was poured into the zeolite/aniline (w/w)1) suspension with constantstirring. The resulting mixture was allowed to react 2 h at roomtemperature, with stirring. The precipitated PANI-zeolite nano-composite was then collected on a filter, rinsed with 5 × 10-3 MH2SO4, and dried in vacuo at 60 °C for 3 h. Various weight ratiosof zeolite HZSM-5 to aniline were used: 1, 5, and 10, and compositesprepared at these ratios are designated as PANIZ-1, PANIZ-2, andPANIZ-3, respectively. For each experiment, the molar ratio of APSto aniline was 1.25 and the amount of zeolite was 0.93 g. As areference sample, pure PANI was prepared by the same procedure,without zeolite. The pH values of the starting zeolite/anilinesuspensions were 6.3, 6.0, and 5.1 for the experiments with zeolite/aniline weight ratios of 1, 5, and 10, respectively. The polymerizationtimes, tpol, were specified to achieve a final pH value 1.5-1.7, i.e.,tpol ) 2 h for pure PANI and PANIZ-1, tpol ) 24 h for PANIZ-2,and tpol ) 48 h for PANIZ-3. A portion of product (pure PANI orPANI/zeolite composite) was treated with an excess of 5% ammoniumhydroxide for 3 h, to transform it to base (deprotonated) form, andthe resulting precipitate was collected on a filter, rinsed with 5%ammonium hydroxide, and dried in vacuo at 60 °C for 3 h.

Characterization. A scanning electron microscope JEOL JSM6460 LV and a transmission electron microscope Tecnai G2 Spirit(FEI, Brno, Czech Republic) have been used to characterize themorphology of the samples. Powder materials were deposited onadhesive tape fixed to specimen tabs and then ion sputter coatedwith gold using a BAL-TEC SCD 005 Sputter Coater before SEMmeasurements. For conductivity measurements, the samples werepressed into pellets, 10 mm in diameter and 1 mm thick, under a

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Polyaniline Nanotubes/Zeolite Nanocomposite Langmuir, Vol. 25, No. 5, 2009 3123

pressure of 124 MPa using a hydraulic press. The conductivity wasmeasured between stainless steel pistons, at room temperature, bymeans of an ac bridge (Waynne Kerr Universal Bridge B 224), atfixed frequency of 1.0 kHz. During the measurement, pressure wasmaintained at the mentioned value. All samples were dried in vacuumat 60 °C for 3 h before the conductivity measurement. Thethermogravimetric analysis was carried out using a TA Instrumentsmodel SDT 2960 thermoanalytical device in an air stream (35 mLmin-1) at a heating rate of 10 °C min-1. Elemental analysis (C, H,N, and S) was performed using an Elemental Analyzer VARIO ELIII (Elementar). The content of Al and Si in the composites wasdetermined by inductively coupled plasma (ICP), using a iCAP6500Duo ICP spectrometer (Thermo Scientific). The sample was placedin a platinum crucible, and the PANI part of the composite wasburned using a Bunsen burner. Then, the inorganic residue (zeolite)was fused with sodium carbonate (weight ratio Na2CO3/zeolite )16) for 45 min, dissolved in water, and analyzed by ICP. FTIRspectra of the samples were recorded in the range of 400-4000cm-1 using a MIDAC M 2000 Series Research Laboratory FTIRSpectrometer at 4 cm-1 resolution. Powdered samples were dispersedin KBr and compressed into pellets. UV-visible spectra of thedeprotonated samples dissolved in N-methyl-2-pyrrolidone (NMP)were recorded using a UV-vis Spectrometer GBC Cintra 10e. TheEPR spectra of solid-state samples were recorded at room temperatureusing a Varian E104-A EPR spectrometer operating at X-band (9.3GHz) using the following settings: 1 G modulation amplitude, 100kHz modulation frequency, 10 mW microwave power, 200 G scanrange, and 4 min scan time. Spectra were recorded and analyzedusing EW software (Scientific Software). The X-ray powderdiffraction (XRPD) patterns were obtained on a Philips PW-1710automate diffractometer using a Cu tube operated at 40 kV and 35mA. Diffraction data were collected in the 5-65° 2θ region countingat every 0.02°. Nitrogen adsorption-desorption isotherms weredetermined on a Sorptomatic 1990 Thermo Finningen at -196 °C.Samples were degassed at 95 °C for 24 h. The specific surface areaof samples, SBET, was calculated according to the Brunauer, Emmett,Teller method from the linear part of the nitrogen adsorption isotherms(0.05< p/p0< 0.35, where p and p0 are the equilibrium and saturationpressure of N2 at the temperature of adsorption).37,38 Total porevolume was calculated according to the Gurvitch method for p/p0

) 0.98.37,38 The pore size distribution for mesopores was calculatedaccording to the Dollimore-Heal method from the desorption branchof the isotherm.39 Porosimetry measurements were carried out ona Carlo Erba Porosimeter 2000 using the Milestone 100 SoftwareSystem. This high-pressure mercury intrusion porosimeter operatesin the interval 0.1-200 MPa, enabling estimation of pores in theinterval 7.5-15 000 nm. In the porosity measurements, all compositesamples were analyzed as pellets because powder samples havelarge interparticle distances, which would give irrelevant poro-simetry data. When pure PANI is pressed into pellets it loses bothmicro- and mesoporosity; therefore, Hg porosimetry and BETanalysis of PANI in pellet form could not be performed. For thatreason pure PANI was analyzed as a powder by Hg porosimetry;however, because of small PANI particle size, BET analysis wasnot operational due to the vacuum required by this method. Anilineadsorption on zeolite HZSM-5 was analyzed using a UV-visSpectrometer GBC Cintra 10e, at room temperature. HZSM-5zeolite (200 mg) was dispersed in the aqueous solution (20 mL)of aniline, and the resulting suspension was allowed to equilibratefor 1 h. After centrifugation, aniline concentration was determinedby monitoring the decrease in absorbance at the wavelength ofabsorption maximum of 280 nm.

Results and Discussion

The oxidative polymerization of aniline with APS in aqueouszeolite suspensions without added acid is an exothermic process

(Figure 1). Thermochemistry of these oxidations substantiallydepends on the zeolite/aniline weight ratio. The synthesis ofPANIZ-1 proceeds in two exothermic phases which are wellseparated by an athermal period (Figure 1), similarly to thecorresponding oxidation of aniline in water,10,17 but with lowerpolymerization rate. This decrease of the rate of aniline oxidationin the presence of zeolite is due to the decrease of the concentrationof aniline molecules in the bulk of the aqueous zeolite suspension,because of the partial adsorption of aniline molecules on zeoliteHZSM-5. In the first phase at pH > 4, which is accompaniedby rapid heat evolution, the fast oxidative oligomerization ofaniline with peroxydisulfate occurs (phase I, Figure 1). The acidityof the reaction mixture continuously increases because of theformation of sulfuric acid as a byproduct.10,17 The linear low-molecular-weight N-C4 coupled aniline oligomers (4-amino-diphenylamine, etc.) as well as branched oligoanilines are formed(Figure 2).17,40-43 The intramolecular oxidative cyclization oflow-molecular-weight branched aniline oligomers with peroxy-disulfate leads to the formation of substituted phenazines(pseudomauveine, etc., Figure 2).17,40-43 Because the concentra-tion of peroxydisulfate (a strong oxidant) rapidly decreases, thepolymerization mechanism becomes based on the redox reac-tions of nonprotonated nigraniline- [(-C6H4NdC6H4dN-)3n

(C6H4NH)2n-] and pernigraniline-like oligoanilines [(-C6H4NdC6H4dN-)n] (weak oxidants) with aniline and its low-molecular-weight protoemeraldine- [(-C6H4NdC6H4dN-)n(C6H4NH)6n-]and leucoemeraldine-like oligomers [-(C6H4NH)n-] (phase II,Figures 1 and 2). During this athermal phase of anilinepolymerization, the acidity slowly increases to pH 2.5 andanilinium cation (pKa ) 4.6) becomes prevalent over nonpro-tonated aniline molecule.

The protonation of pernigraniline-like oligoanilines, causingthe significant increase of their oxidant power and solubility,coincides with the autoacceleration of aniline polymerization atpH < 2.5 (phase III, Figure 1). In this polymerization phase,sulfate anions (pKa2 of sulfuric acid is∼2) became also protonated,

(37) Gregg, S. H.; Sing, K. S. Adsorption, Surface Area and Porosity; AcademicPress: New York, 1967.

(38) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by Powders and PorousSolids; Academic Press: London, 1999.

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Figure 1. Temperature changes during the oxidation of aniline (0.2 M)with APS (0.25 M) in water: without added zeolite HZSM-5 (0) and forthe synthesis of PANIZ-1 (b), PANIZ-2 (2), and PANIZ-3 (O) composites.Phases I-IV correspond to the course of aniline polymerization in thepresence of zeolite HZSM-5, for the synthesis of PANIZ-1.

3124 Langmuir, Vol. 25, No. 5, 2009 Ciric-MarjanoVic et al.

thus promoting the charge separation process in the emeraldinesalt form of oligoanilines and PANI, i.e., formation of delocalizedpolaronic form.41,43

Protonated pernigraniline-like oligoanilines [(-C6H4NH+dC6H4dNH+-)n] react further with remaining anilinium cationsand reduced segments of partly oxidized oligoanilines, via theredox equilibrating process, leading thus to the formation of

longer PANI chains in the form of emeraldine salt (PANIhydrogen sulfate) with prevalent N-C4 coupling mode betweenaniline units (Figure 2). After the temperature reached itsmaximum, the medium cools down (postpolymerization period,phase IV, Figure 1).

The syntheses of PANIZ-2 and PANIZ-3 proceed with slowincrease in temperature of the reaction mixture (Figure 1),similarly to the oxidative polymerization of aniline in an acidic

Figure 2. Mechanism of the oxidative polymerization of aniline with APS, in aqueous suspension of zeolite HZSM-5, at zeolite/aniline weight ratio) 1 (PANIZ-1).

Figure 3. Adsorption isotherm of aniline adsorbed on HZSM-5 zeolite(ceq denotes an equilibrium concentration of aniline in the solutionobtained after centrifugation of the aqueous suspension containing anilineand zeolite HZSM-5).

Figure 4. Size of aniline molecule in aqueous medium, determined bythe MNDO-PM3/COSMO semiempirical quantum chemical method.The literature value of 0.12 nm for the van der Waals radius of thehydrogen atom was taken into account.


aqueous solution at pH < 2,10,42 where much less oxidizableanilinium cations are prevalent over nonprotonated anilinemolecules.42 This can be explained by much more efficientadsorption of aniline during syntheses of PANIZ-2 (0.04 M f0.026 M; 34.4% adsorbed aniline) and PANIZ-3 (0.02 M f0.008 M; 59.0% adsorbed aniline), compared with the adsorptionof aniline during PANIZ-1 synthesis (0.2 M f 0.184 M; 8.1%adsorbed aniline). Since adsorbed aniline molecules are protonatedby zeolite

HZSM-5+C6H5NH2f (C6H5NH3)+ZSM-5

it follows that ratios ([C6H5NH3+

(adsorbed)] + [C6H5NH3+

(aq)])/[C6H5NH2(aq)] at the beginning of PANIZ-2 and PANIZ-3syntheses are much higher than corresponding initial ratios[C6H5NH3

+(aq)]/[C6H5NH2(aq)] in the bulk of aqueous dispersions,

indicated by the initial pH of reaction mixtures. It is importantto note that, despite the higher local concentration of aniline at

the HZSM-5/water interface, the rate of the polymerization ofadsorbed aniline is slowed down because adsorbed anilinemolecules are transformed to much less oxidizable aniliniumcations. It should also be noted that the zeolite surface changesfrom hydrophilic to hydrophobic because of the aniline adsorptionvia the amino group (the hydrophobic benzene ring of anilineremains directed toward the aqueous solution), being thus muchless available for the attack of hydrophilic peroxydisulfate anion.The adsorption isotherm of aniline/HZSM-5 zeolite, obtainedfrom UV-visible spectrometry measurements, is shown in Figure3. The size of the aniline molecule in the most stable conformationin aqueous solution (Figure 4), determined by the MNDO-PM3/COSMO semiempirical quantum chemical method,8,40-42,44,45

(44) Ciric-Marjanovic, G.; Blinova, N. V.; Trchova, M.; Stejskal, J. J. Phys.Chem. B 2007, 111, 2188.

(45) Ciric-Marjanovic, G.; Trchova, M.; Konyushenko, E. N.; Holler, P.;Stejskal, J. J. Phys. Chem. B 2008, 112, 6976.

Figure 5. SEM images of (A) pure PANI, (B, C) PANIZ-1, (D) PANIZ-2, (E) PANIZ-3, and (F) pure zeolite HZSM-5.


indicates that the sorption/diffusion of aniline in the HZSM-5zeolite channel system is a complex process. It is possible onlywhen the aniline molecule is oriented properly (Figure 4) forcompression to the size of the HZSM-5 zeolite channels46 [near-circular (0.54 × 0.56 nm) zigzag channels and elliptical (0.51× 0.55 nm) straight-chain channels].47Since the compression(conformation change from the most stable to the unstablecompressed/twisted conformation) of aniline molecule is anendothermic process, it can be concluded that aniline moleculesare adsorbed initially on external surfaces of zeolite. Sub-

sequently, adsorption took place in the zeolite HZSM-5 channelsystem.

SEM and TEM images show the crucial influence of the initialzeolite/aniline weight ratio on the morphology of PANI/zeolitecomposites (Figures 5 and 6). PANI nanotubes and nanorods arerevealed in the nanocomposite PANIZ-1 by SEM (Figures 5Band C) and TEM (Figures 6A-C). PANI nanotubes have anouter diameter of 70-170 nm, an inner diameter of 5-50 nm,and a length extending from 0.4 to 1.0 µm. PANI nanorods havea diameter in the range 60-100 nm and a similar length. Itshould be noted that the pure PANI, synthesized under similarreaction conditions without added zeolite, contains nanotubes ofan average diameter 110-190 nm, Figure 5A. The relative amountof PANI nanotubes and nanorods is significantly reduced in

(46) Choudhary, V. R.; Nayak, V. S.; Choudhary, T. V. Ind. Eng. Chem. Res.1997, 36, 1812.

(47) Kokotailo, G. T.; Lowten, S. L.; Olson, D. H.; Meir, W. M. Nature 1978,272, 437.

Figure 6. TEM images of (A, B, and C) PANIZ-1 and (D) PANIZ-2.


the sample PANIZ-2 (Figure 5D) in comparison with PANIZ-1. The sample PANIZ-3, prepared with the highest amount ofzeolite HZSM-5, consists of PANI/zeolite aggregates of irregularshape and size (Figure 5E) without the presence of PANInanotubes and nanorods. PANI/zeolite aggregates, distinct fromthe morphology of unmodified zeolite HZSM-5 (Figure 5F), arealso present in the composites PANIZ-1 and PANIZ-2 (Figures5B-D). The study of prepared composites by electron mi-croscopies indicates that PANIZ-3 composite is homogeneous,while PANIZ-1 and PANIZ-2 are heterogeneous. Heterogeneityis much more pronounced in PANIZ-1, which contains distinctnanodomains (nanophases) of PANI nanotubes.

We proposed that the growth of PANI nanotubes and nanorodsoccurs in the bulk of the aqueous zeolite HZSM-5 suspension.During the early stages of the oxidative polymerization of anilinein water without added acid, at higher pH, nonprotonated low-molecular-weight oligoanilines are precipitated as hydrophobiccrystallites, which do not adhere to the hydrophilic zeolite crystalsurfaces. The nonconducting needlelike nanocrystallites, withhigh content of fully oxidized oligoanilines which show relativelylow redox reactivity (substituted phenazines, which have tendencyto build columnar aggregates by stacking,8 and nonprotonatedpernigraniline-like oligoanilines), become coated with a con-ducting PANI hydrogen sulfate film during the third polymer-ization phase at pH e 2. This leads to the formation of PANIhydrogen sulfate nanorods with nonconducting core and con-ducting walls. PANI nanotubes are formed by the dissolution ofthe cores of nanorods,8 induced by the protonation of fullyoxidized oligoanilines at pH e 2.

The mechanism of polymerization of anilinium cationsadsorbed at the surface of zeolite HZSM-5 is proposed to be thesame as that of aniline in the presence of strong acids.42 Thismeans that some induction period, caused by the much loweroxidizability of anilinium cations in comparison with anilinebase,40 is followed by the rapid exothermic polymerizationstadium, which leads to the formation of the ordinary conductingthin PANI film at the crystal aluminosilicate surfaces of zeolite.One-dimensional fibrillar PANI growth within the zeolite channelsystem is also possible.19

The weight ratio of zeolite/PANI in composites wasdetermined by thermogravimetric analysis (TGA), Figure 7.Taking into account that the combustion of PANI in air streamis completed at 640 °C, and the residual weight refers to thecontent of zeolite in the composite, as well as that the weightloss from 25 to 200 °C corresponds to the release of residualwater, the weight ratio zeolite/PANI

wzeolite/PANI )residual mass at 640 °C (%)

residual mass at 200 °C (%)- residual mass at 640 °C (%)

was determined to amount to 0.98, 4.72, and 8.79 for compositesPANIZ-1, PANIZ-2, and PANIZ-3, respectively. The weightratio zeolite/PANI in PANIZ composites corresponds well to theinitial zeolite/aniline weight ratio. Based on TGA results, contentof zeolite HZSM-5, PANI hydrogen sulfate, and water incomposites is determined, Table 1.

It should be noted that composites PANIZ-2 and PANIZ-3adsorb a lesser amount of water than pure zeolite (∼8%), becausePANI chains in these composites blocked a part of active surfaceof zeolite framework for bonding of water molecules. We havealso found that the extent of the thermal decomposition of PANIin PANI/zeolite composites (31.5, 24.9, and 20.9 wt % at 400°C for PANIZ-1, PANIZ-2, and PANIZ-3 composites, respec-tively) decreases with the increase of zeolite content, and it islower than the extent of the thermal decomposition of pure PANI(32.8 wt % at 400 °C). Increased thermal stability of PANI inPANI/zeolite composites can be explained by the stronginteraction between PANI and zeolite, which restricts thermalmotion of PANI chains.

The elemental composition of PANIZ composites found byelemental analysis and ICP measurements corresponds well tothat calculated on the basis of the content of PANI hydrogensulfate [C12H11N2SO4]n, zeolite H6(H2O)16(Al6Si90O192), and waterin PANIZ composites, determined by TGA (Table 2). It can beseen that the experimentally determined content of sulfur inPANIZ composites is significantly lower than the calculatedcontent of sulfur. This result indicates that the positive chargeon the PANI chains in PANIZ composites is not compensatedexclusively by hydrogen sulfate counterions, but also is

Figure 7. TGA curves for PANIZ-1, PANIZ-2, and PANIZ-3 composites,pure PANI, and pure zeolite HZSM-5, recorded in an air stream.

Table 1. Content of Zeolite HZSM-5, PANI, and Water inPANI/Zeolite HZSM-5 Composites, Determined by TGA

content (%)

sample zeolite HZSM-5 PANI hydrogen sulfate H2O

PANIZ-1 45.50 46.42 8.08PANIZ-2 77.90 16.52 5.58PANIZ-3 86.32 9.80 3.88

Table 2. Elemental Composition of PANI/Zeolite HZSM-5Composites Determined by the Elemental Analysis (C, H, N,and S), ICP Measurements (Al and Si), and Difference (O)a

content (%)

sample C H N S Al SiO

(by difference)

PANIZ-1 calcd 24.06 2.73 4.66 5.32 1.21 19.00 43.02found 28.15 2.77 5.76 3.19 1.10 18.75 40.28



a The elemental composition calculated on the basis of the content ofPANI hydrogen sulfate, zeolite, and water in PANIZ composites (determinedby TGA) is also shown.

Table 3. Conductivity and Color of Pure PANI, Pure ZeoliteHZSM-5, and PANI/Zeolite HZSM-5 Composites

sampleinitial zeolite/aniline

weight ratio color conductivity (S cm-1)

PANI - dark green 3.8 × 10-2

PANIZ-1 1 dark green 2.6 × 10-2

PANIZ-2 5 green 2.6 × 10-5

PANIZ-3 10 light green 4.8 × 10-9

HZSM-5 - white 1.4 × 10-8


compensated with negatively charged zeolite surface to aconsiderable extent.

The use of zeolite/aniline weight ratio 1 leads to the formationof conducting dark green nanocomposite PANIZ-1, with theconductivity of 2.6 × 10-2 S cm-1, Table 3. This conductivityis of the same magnitude as that of pure PANI, prepared underthe same conditions in the absence of zeolite, 3.8 × 10-2 S cm-1.The increase of zeolite content is accompanied by the dramaticdecrease of conductivity. The semiconducting (3 × 10-5 S cm-1)PANIZ-2 and nonconducting (5 × 10-9 S cm-1) PANIZ-3composites were produced using zeolite/aniline weight ratios 5and 10, respectively, Table 3. It is interesting to note that thesesamples were also green, PANIZ-3 having light green color.This fact, indicating the presence of conducting PANI emeraldinesalt in the semiconducting and nonconducting PANI/zeolitecomposite materials, could be explained by the formation of“islands” of conducting PANI on the zeolite crystal surfaces.Large interisland contact resistance could significantly reduceconductivity of the bulk sample.

It is also interesting to note that the conductivity of pure zeoliteHZSM-5 was 1 order of magnitude higher than that of the

composite PANIZ-3, all samples being dried in the same manner(at 60 °C, in vacuo) before the conductivity measurement. Thisfact can be explained by the effect of residual adsorbed waterin pure zeolite HZSM-5, which participates in the conduction,assisting the proton mobility.48 The water sorption in the zeolitechannel system in the composite PANIZ-3 is significantly reducedbecause the PANI chains blocked the active Brønsted centersresponsible for the ionic conduction.

(48) Higazy, A. A.; Kassem, M. E.; Sayed, M. B. J. Phys. Chem. Solids 1992,53, 549.

Figure 8. FTIR spectra of (a) pure PANI, (b) PANIZ-1, (c) PANIZ-2, (d) PANIZ-3, and (e) pure zeolite HZSM-5 in the wavenumber range (A)2000-400 cm-1 and (B) 1700-1200 cm-1 (a detail of Figure 8A).

Figure 9. UV-visible spectra of the deprotonated form of (a) purePANI, (b) PANIZ-1, (c) PANIZ-2, and (d) PANIZ-3 in N-methyl-2-pyrrolidone.

Figure 10. EPR spectra of solid samples of pure PANI (A), PANIZ-1(B), PANIZ-2, (C), and PANIZ-3 (D), measured at room temperature.


FTIR spectra of PANI/zeolite HZSM-5 composites exhibitedthe characteristic bands of conducting PANI emeraldine salt(Figure 8). The relative intensities of PANI bands decrease withdecreasing PANI content. Characteristic bands of PANI emer-aldine salt at 1577 [quinonoid (Q) ring stretching], 1493[benzenoid (B) ring stretching], 1304 (the C-N stretching ofsecondary aromatic amine), and 1146 cm-1 (the BsNH+)Qstretching),8,17,49,50 are observed in the FTIR spectrum ofPANIZ-1 nanocomposite (Figure 8A, B). The bands originatingfrom the zeolite HZSM-5 appeared at 1227 (external tetrahedronlinkages-asymmetric stretching), 1097 (internal tetrahedron-asymmetric stretching), 546 (double ring), and 455 cm-1 (TsObending, internal tetrahedron).51,52 The characteristic band ofthe conducting emeraldine salt form assigned to C-N•+ stretchingvibration, observed at 1246 cm-1 in the spectrum of pure PANI,is overlapped with the stronger zeolite band at 1227 cm-1 in thespectrum of the PANIZ-1 nanocomposite.8,49 The band at 804cm-1 in the spectrum of PANIZ-1 is due to the mixed contributionof the γ(C-H) vibration of 1,4-disubstituted benzene ring in thelinear PANI backbone, and the symmetric stretching vibrationof external linkages in zeolite, which appear at 810 and 796 cm-1

in spectra of pure PANI17 and zeolite,51,52 respectively. It isimportant to note that the band at 879 cm-1 [γ(C-H) vibrationof 1,2,4-trisubstituted benzene ring],50 indicative of branchedPANI chains,17 and/or hydrogen sulfate ions,50 is also present.

The characteristic PANI bands are blue-shifted to 1593, 1498,and 1306 cm-1 in the spectrum of PANIZ-2, in comparison withthe corresponding bands in the spectrum of PANIZ-1 at 1577,1493, and 1304 cm-1 and those in the spectrum of pure PANIat 1577, 1483, and 1304 cm-1. These shifts indicate the increased

interaction of PANI chains with zeolite, with the increase ofzeolite content, through electrostatic attraction of positivelycharged PANI chains with negatively charged aluminosilicatesurfaces, hydrogen bonding, and ion-dipole and dipole-dipoleinteractions. Because PANI nanotubes and nanorods are muchless adherent to zeolite crystal surfaces than PANI film, it canbe proposed that increased PANI-zeolite interactions are mostprobably caused by the increased weight ratio PANI film/PANInanostructures. In the FTIR spectrum of PANIZ-3, the bands ofzeolite HZSM-5 are prevalent over the PANI bands. The PANIband due to the B-ring stretching is observed at 1498 cm-1,Figure 8B, but the band due to the Q-ring stretching is maskedby the band of water, adsorbed by zeolite, at 1626 cm-1.51 The

(49) Sapurina, I.; Osadchev, A. Yu.; Volchek, B. Z.; Trchova, M.; Riede, A.;Stejskal, J. Synth. Met. 2002, 129, 29.

(50) Infrared and Raman Characteristic Group Frequencies; Socrates, G.,Ed.; Wiley: New York, 2001, pp 94-9, 107, 221.

(51) Zeolite Molecular SieVes: Structure, Chemistry, and Use; Breck, D. W.,Ed.; Wiley: New York, 1974, pp 414-425, 460-465.

(52) Narayanan, S.; Sultana, A.; Le, Q. T.; Auroux, A. Appl. Catal., A 1998,168, 373.

Figure 11. XRPD patterns of parent HZSM-5 zeolite, PANI/zeolite composites, and pure PANI.

Table 4. Specific Surface Area (SHg), Specific Pore Volume (Vp),and Pore Diameter (D) of PANI/Zeolite HZSM-5 Composites,

Determined by the Mercury Porosimetry Method

sample SHg/m2 g-1 Vp/cm3 g-1 D/nm

PANIa 34.6 2.2 3616PANIZ-1 11.9 0.31 92PANIZ-2 0.8 0.01 115

a PANI is analyzed as a powder sample.

Figure 12. Cumulative pore-size distribution curves of PANI/zeolitecomposites.


band corresponding to the bending vibration of water adsorbedby zeolite is red-shifted from 1630 cm-1 in the spectrum of neatzeolite to 1606 (shoulder), 1614 (shoulder), and 1626 cm-1 inthe spectra of PANIZ-1, PANIZ-2, and PANIZ-3, respectively,Figure 8A and B.

The UV-visible spectra of the deprotonated PANIZ compositesand deprotonated pure PANI show two absorption maxima in NMP(Figure 9). The UV-visible spectrum of pure nanotubular PANIbase consists of the bands at 339 and 620 nm. The band at 339nm has been assigned to the π f π* electronic transition, andits position corresponds well to that observed for standard PANIemeraldine base at ∼330-340 nm.53-55 This band is sensitiveto the number of aniline units. The band at 620 nm correspondsto the “exciton” band which has been attributed to a chargetransfer from the highest occupied energy level, centered on thebenzenoid ring, to the lowest unoccupied energy level, centeredon the quinonoid ring.53,54 The exciton band can be used as ameasure of the oxidation state of PANI, and it is observed at∼637 nm for the “standard” emeraldine base.53,54 The UV-visible spectra of PANIZ composites exhibit two absorptionmaxima, which show blue-shifting with the increase of the zeolitecontent in composites: from 320 and 616 nm for PANIZ-1 to 311and 591 nm for PANIZ-2, and to 309 and 565 nm for PANIZ-3.It can be concluded that PANI is partly oxidized in all composites,the oxidation state of PANIZ-1 being very close to that of purenanotubular PANI, i.e., the emeraldine state, in agreement withthe blue color of the solutions of PANIZ-1 and pure nanotubularPANI in NMP. Since fully oxidized pernigraniline base hasabsorption maxima at about 530 and 320 nm,54 the UV-visiblespectra of PANIZ-2 and PANIZ-3 indicate that the oxidationstate of PANI in these composites is between emeraldine andpernigraniline state, and/or that their chain length is lower incomparison to PANI and PANIZ-1.

The EPR spectra confirm the presence of radical cations,characteristic for conducting PANI emeraldine salt (Scheme 1),in all PANIZ samples (Figure 10).

The EPR signal of pure PANI and all composites representa singlet with g) 2.003. The EPR signals of pure PANI, PANIZ-1, and PANIZ-2 have similar shape, while PANIZ-3 showssignificantly broader EPR signal. The broadening of the EPRsignal can be explained by the reduction of the electron diffusioncaused by the presence of zeolite. Similar observation was reportedfor the PANI/Na+-montmorillonite clay nanocomposite.56 Nano-composite PANIZ-1 shows a signal of slightly smaller peak areacompared to pure PANI. The EPR peak area relative to purePANI substantially decreases with increasing zeolite content inthe order 0.95, 0.19, and 0.09 for the samples PANIZ-1, PANIZ-2, and PANIZ-3, respectively. Zeolite HZSM-5 has no EPR signal.

X-ray powder diffraction analysis proved that the crystallinityof zeolite HZSM-5 in the composites is the same as that of theoriginal HZSM-5, Figure 11.

Mercury intrusion porosimetry measurements (Table 4, Figure12) indicate that the increase of zeolite content in PANIZcomposites leads to lower specific surface area and lower specificpore volume.

PANIZ-1 is mesoporous, while PANIZ-2 contains onlymicropores. Therefore, composites with higher content of zeolite,PANIZ-2 and PANIZ-3, were analyzed using the BET method(Table 5, Figure 13). The nitrogen adsorption-desorptionisotherms (Figure 13) have a reversible part at low relativepressures and hysteresis loops at higher relative pressures (H3type of hysteresis cycle), characteristic for aggregated planeparticles which form slit-shaped pores.37,38

The decrease of specific surface area and specific pore volumewith the increase of PANI content in PANI/zeolite compositesis caused by the blockade of the zeolite HZSM-5 pores withPANI.

Conclusion

In summary, we have demonstrated an efficient template-free method for the synthesis of conducting PANI nanotubes/zeolite nanocomposite, through the oxidative polymerizationof aniline with ammonium peroxydisulfate in aqueoussuspension of zeolite HZSM-5 without added acid, by usinginitial zeolite/aniline weight ratio ) 1. This simple and versatilesynthetic method could be extended to prepare a wide range ofother nanocomposites of conducting self-assembled PANInanotubes and various inorganic materials. PANI/zeolite HZSM-5nanocomposite contains PANI nanotubes, which have a typicalouter diameter of 70-170 nm, an inner diameter of 5-50 nm,and a length extending from 0.4 to 1.0 µm, accompanied byPANI nanorods with a diameter of 60-100 nm and PANI/zeolitemesoporous aggregates. PANI nanotubes/zeolite nanocompositeshows similar conductivity (∼ 10-2 S cm-1), oxidation state(emeraldine), and paramagnetic properties (g)2.003), as well asimproved thermal stability of the PANI chains in comparisonwith pure self-assembled PANI nanotubes. This novel composite,which combines unique properties of 1D PANI nanostructuresand mesoporous PANI/zeolite hybrid materials, could be appliedas an electronic component, sensor, and catalyst.

Acknowledgment. The authors wish to thank the Ministry ofScience and Technological Development of Serbia (ContractsNos. 142055 and 142047) and the Grant Agency of the Academyof Sciences of the Czech Republic (IAA 400500405) for thefinancial support.

LA8030396

(53) Kang, E. T.; Neoh, K. G.; Tan, K. L. Prog. Polym. Sci. 1998, 23, 277.(54) Quillard, S.; Louarn, G.; Lefrant, S.; MacDiarmid, A. G. Phys. ReV. B

1994, 50, 12496.(55) Laska, J. J. Mol. Struct. 2004, 701, 13.(56) Kim, B. H.; Jung, J. H.; Joo, J.; Kim, J. W.; Choi, H. J. J. Korean Phys.

Soc. 2000, 36, 366.

Table 5. Specific Surface Area (SBET), Specific Pore Volume(Vp 0.98), and Pore Diameter (D) of PANI/Zeolite HZSM-5

Composites, Determined by the BET Method

sample SBET/m2 g-1 Vp 0.98/cm3 g-1 D/nm

HZSM-5 400 - -PANIZ-2 125 0.16 3.7PANIZ-3 193 0.24 3.8

Figure 13. Nitrogen adsorption-desorption isotherms of the PANI/zeolite composites.


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