Controlled Growth of Polyaniline Fractals on HOPG throughPotentiodynamic ElectropolymerizationDhrubajyoti Bhattacharjya‡ and Indrajit Mukhopadhyay*,†

†School of Solar Energy, Pandit Deendayal Petroleum University, Raisan Gandhinagar 382007, Gujarat, India‡Central Salt and Marine Chemicals Research Institute (CSIR), G. B. Marg Bhavnagar 364002, Gujarat, India

*S Supporting Information

ABSTRACT: Polyaniline (PANI) in fractal dimension hasbeen electrodeposited reproducibly on highly oriented pyro-lytic graphite (HOPG) from 0.2 M aniline in 1 M aqueousHCl solution by potentiodynamic sweeping in the range of−0.2 to 0.76 V vs Ag/AgCl at room temperature. Fractalgrowth of PANI dendrimers is affected by diffusion limitedpolymerization (DLP) at a sweep rate of 15 mV s−1 for 43 min.This type of PANI dendrimer is prepared for the first time onsuch large area HOPG substrate by electrochemical techniqueusing rather simple cell setup. The fractal dimension has beendetermined by chronoamperometry (CA) and box countingtechnique and is found to vary from 1.4 to 1.9 with theduration of electropolymerization. The sweep rate, terminal oxidation potential, and the diverse surface anisotropy of the HOPGsurface are found to be crucial factors in controlling the growth of such PANI fractals.

■ INTRODUCTIONStudies on the electrochemical and surface properties of con-ducting polymers has been a matter of intense investigationin last few decades due to their potential in wide variety ofapplications like in sensors,1,2 actuators,3 supercapacitors,4−6

electrochromic display devices,7,8 and microelectronic devi-ces.9,10 Of these conducting polymers, polyaniline (PANI) haselicited the most interest due to its wide range of conductivityfrom insulating to metallic regime, unique redox tunability,good environmental stability, low cost, ease of synthesis, andpromising applications.11−16 Various synthetic routes have beenemployed for controlled growth of PANI microstructure, suchas microemulsion,17 interfacial polymerization,18 template syn-thesis,19 and self-assembly.20 Electrochemical methods are theversatile, cost-effective, and simple methods to synthesize nano-and microstructures of PANI. Cyclic voltammetry, potentiostatic,galvanostatic methods,21,22 and pulse potentiostatic methods23 arethe classical electrochemical techniques employed to synthesizePANI having different morphologies.Dendrimer is a class of architecture having highly branched

structural design. This kind of architecture is a matter of scien-tific attraction due to the possibility of structure allied novelphysical and chemical property. The diffusion-limited aggrega-tion (DLA) model has been widely used to explain and analyzethe formation of dendritic structure. The DLA model considersthe growth of aggregate by random walk of particle on a latticecontaining a seed and then rapid growth of these clusterstoward the exposed end than other perimeter sites.24 Mandelbortpromulgated the description of this complex pattern in termsof fractal geometry.25 Its research object is the rough and

nondifferential body in a nonlinear system which cannot besolved in a Newtonian system. Due to strong interaction be-tween molecules, conjugated conductive polymers have a ten-dency for self-aggregation and it is easy to form fractalaggregates.26 Electrodeposition is known to give rise to fractalstructures under certain experimental conditions. Fractal growthof polypyrrole in an area of 1.5 cm2 by electropolymerization wasfirst reported by Melroy et al. using a custom designedcomplicated electrochemical cell.27 B. Villeret et al. evidencedthe preparation of polyaniline fractals by elctropolymerization byanalyzing only the cyclic voltametric data.28 Although a numberof theoretical works have been carried out on the diffusionlimited polymerization of various polymers, only a fewexperimental reports were found dealing with the same subject.Haberko et al. reported on the synthesis of dendrites and pillarsof PANI through spin-casting.29 Detailed work on the dispersionof symmetric triblock copolymer based on PANI has beenreported by Knaapila et al.30 Small angle X-ray scattering (SAXS)and electrochemical methods have been employed by Neves andFonseka for the determination of fractal dimension of PANI.31

In the present Article, we report on controlled and repro-ducible synthesis of large scale fractal aggregates of polyanilineby conventional electropolymerization using potentiodynamicsweeping method on HOPG substrate at room temperature.The fractal dimension of the aggregates is shown to vary withthe time of polymerization. The crucial role of the terminal

Received: July 8, 2011Revised: March 20, 2012Published: March 21, 2012

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polymerization potential and the substrate surface chemistry onthe aggregate structure is also revealed.

■ EXPERIMENTAL SECTIONAniline (99.5%) was purchased from Sigma Aldrich and purified bydistillation and then stored in a nitrogen glovebox (with moisture andoxygen content ≤2 ppm) prior to use. Analytical reagent grade HClwas used to make a monomer solution of aniline in MilliQ water.HOPG substrate (ZYH grade, 12 × 12 mm2 area and 2 mm thick) waspurchased from Advanced Ceramics Corporation. The surfaces ofsubstrate were cleaved and detached by adhesive tape to get a freshsurface prior to each experiment. The fluctuation in the number ofdefects of the new graphite surface exposed during cleavage is about10−20%.32The electrochemical experiments were performed by using a bi-

potentiostat (Pine Instrument Company). A vertical Teflon electro-chemical cell with a three electrode configuration was employed forelectropolymerization. The working HOPG electrode was sealed tothe Teflon cell by a Teflon coated O-ring with an active surface area of3.6 × 10−5 m2.33 A Pt ring was used as counter electrode and Ag/AgCl(1 N KCl) (CH instruments) served as reference electrode. The celland counter electrode were cleaned with freshly prepared 1:1 (v/v)H2O2/H2SO4 solution followed by ultrasonication in MilliQ water(18 MΩ·cm resistivity) prior to each experiment. 2% aniline inaqueous 1 M HCl (v/v) solution was used as monomer for electro-polymerization. Electropolymerization was performed by potentiody-namic sweeping from −200 to 760 mV at a scan rate of 15 mV s−1 fordifferent times (no. of cycles). After each experiment, the remainingelectrolyte on the electrode surface was soaked with filter paper andthen kept for drying. The morphology of polyaniline microstructureswas investigated by scanning electron microscopy (SEM, LEO series1430 VP). It is important to mention here that the microstructures ofPANI were always produced by terminating the applied potential atthe initial value.The surface coverage of the fractals was calculated by analyzing the

SEM images using standard image analysis software with a maximumdigital resolution of 512 × 512. Harmonic and Fractal Image Analyzersoftware (HarFA 5.4) developed by Zmeskal et al. of Institute ofPhysical and Applied Chemistry, Brno University of Technology,Czech Republic was used for measurement of fractal dimension of theelectropolymerized polyaniline microstructures. The standard boxcounting method was used in this software and all the images pro-cessed were of identical digital maximum resolution of 512 × 512.34

The fractal dimensions were calculated by linear regression analysis.In an electrochemical system with high rate constants, the time

dependence of diffusion limited current is expressed by theconventional Cottrell equation35 which is given by

=π

I tnFD C

t( )

1/2

1/2 1/2

However, for a rough (fractal) electrode, the Cottrell equationtransforms to an extended form:

= σ −αI t t( ) F

where σF is a proportionality factor, known as the Warburg coefficient,and α is a fractal parameter.36 It is thus observed that the fractalparameter can be calculated for a system with constant σF if the currentvaries as the power law of time in a certain time range. The fractalparameter is related to fractal dimension (D) by: α = (D − 1)/2.Hence, fractal dimension can be calculated from the slope of thecurrent−time relationship plotted in a log−log scale.35 All thepotentials are mentioned with respect to standard normal Ag/AgClreference electrode.

■ RESULTS AND DISCUSSIONThe cyclic voltammogram for the electropolymerization ofPh-NH2·HCl at a sweep rate of 15 mV s−1 is shown in Figure 1.

The voltammogram shows prominent redox couple “a1−c1”which may be attributed to the electroformation and reductionof leucoemeraldine form of PANI.37 The current rise at theextreme anodic scan, a3 is generally attributed to the formationof polyemeraldine.37 The weak redox couple a2−c2 indicatesthat there is no significant formation of benzoquinone typedecomposition product due to restriction of terminal oxidationpotential to 0.76 V. The initial cycle shows very low peakcurrent for the polymer oxidation and reduction processes. Themorphology of the HOPG substrate prior to electropolymeri-zation was found to be associated with the basal plane andsteps. The morphology of the deposits after various length ofelectropolymarization is shown in Figure 2. It can be seen fromFigure 2a that the polyaniline forms discrete dendrites and theclusters are arranged in a treelike morphology, typical for thediffusion limited polymerization (DLP) process. In electro-polymerization, nucleation and growth are two importantcompetitive processes. The result of this competition isreflected on the morphology of the deposited polymer. Twoimportant parameters that effectively determine the outcome ofthe competition are surface diffusion (area migrated by anadatom in unit time) and deposition flux (atoms deposited perunit area per unit time).36 An adatom stops diffusing when ithits a stable aggregate as it condenses. The nucleation densityincreases with increasing coverage until it reaches a saturatedvalue. Thereafter, the impinging atoms condense only on theexisting aggregates. At this stage, the aggregates migrate anaverage distance Λa. The morphology (shape) of these aggre-gates is determined by the directional anisotropy of Λa and theaverage diffusion length of atoms adsorbed on the perimeter ofaggregate Λ1.

38 In the case of DLP (or DLA) atoms attachingto an aggregate stick where they hit and as a result ofsubsequent propagation, a fractal structure is formed.38 Hence,generation of the fractal surface by potentiodynamic electro-polymerization is strongly influenced by the employed sweeprate, which directly effect the deposition flux. In the presentinvestigation, we found that the fractals can be obtained at anoptimum sweep rate of 15 mV s−1. At higher sweep rate, atubular bush of bulk PANI is found to form on HOPG, while atlower sweep rate the coverage of fractal dendrimers reduced.In the case of DLP similar to that of DLA, the integrated two

point correlation function, M(r) should be scaled with the

Figure 1. Cyclic voltammogram of 0.2% aniline in 1 M HCl on HOPGsubstrate obtained at a sweep rate of 0.015 V s−1 at room temperature.The highest current peaks belong to the 20th (the last) cycle.

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radius of the aggregates, r, by M(r) = rD in two dimensions,with D being the fractal dimension.27

It can be seen in Figure 2 that the inner core or the center ofthe tree-structure is less dense in comparison to the growingbranches. After 32 min of electropolymerization (about 15cycles), the surface coverage is found to be about 13%(Supporting Information Figure 1). As the time of electro-polymerization is increased to 42.6 min (20 cycles) underidentical conditions, the relatively empty inner core is found tobe covered by the polymer deposits almost to the same degree(Figure 2b) as that of the outer growing branches and thesurface coverage increases to 61% (Supporting InformationFigure 2). At this point, it is interesting to note that the growthof the outer branches is terminated at the extreme edges leavingan uncovered HOPG surface in between. We attribute thistermination process to two probable reasons. Since DLPgenerally occurs under overpotential deposition (OPD) con-ditions, the monomer concentration becomes effectively zeroinside the fractals and at the growing surface.27 Hence, after acertain duration of electropolymerization, a typical emptyregion is expected to appear in between two growing fractalsdue to lack of sufficient number of monomers,. The otherreason, which will be evidenced later, may be the existence ofhigh surface energy zone on HOPG in between two adjacentgrowing fractal faces, which is not favoring nucleation ofnew cluster for further growth. When the time of electro-polymerization is increased to around 53 min (25 cycles),the fractal domains of polyaniline dendrimers become moredefined and the denuded zones between the edges of the

domains are observed clearly in the SEM image (see Figure 2c).Although the coverage is increased to 73% (SupportingInformation Figure 3), the braches of the fractal becomemore dense and compact. The interesting observation fromFigure 2c is the enhanced growth of terminal polymer clusters.The bright feature of the micrograph can be due to increasedheight of the edges, or it may be related to excess surface chargedensity. The overgrowth of PANI takes place when thepolymerization time is increased to 64 min (30 cycles). Thefractal dendrimeric morphology starts to disappear and a coil-like structure of PANI is formed. The surface coverageincreased to 87% (Supporting Information Figure 4), and thegrowth in the vertical direction is also evident from Figure 2d.The fractal dimension of the PANI dendrimer obtained by

electropolymerization for 53 min (25 cycles) was determinedby chronoamperometry (CA) and was found as 1.92 (the corre-lation coefficient for this evaluation based on the best straightline plotted for the experimental data was 0.992, SupportingInformation Figure 5) which was close to the calculated valueof 1.89 by box counting method. It is seen that the CA methodis based on the right choice of the time domain where current isa power law function of time.35 Since the overall current isinfluenced by the double layer charging current at shorter timedomain and perturbation of planar diffusion takes place at longtime domain, the measured fractal dimension by CA is moreprone to error.35 Hence, we have adopted the box countingmethod for the determination of fractal dimension, which isbased on the analysis of the image of fractals that already existon the substrate. The variation of fractal dimension of the

Figure 2. SEM image of the PANI fractals obtained by potentiodymanic diffusion controlled electropolymerization at a sweep rate of 0.015 V s−1 for(a) 15, (b) 20, (c) 25, and (d) 30 cycles.

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PANI films over HOPG substrate for various length of poly-merization is shown in Figure 3. The micrograph in Figure 2a

and its fractal dimension, D, of 1.59 led us to conclude that thePANI formation proceeds via two dimension diffusion limitedaggregation (DLA) or DLP processes.38,39 As the polymer-ization time is increased, the fractal dimension increasessequentially which is clearly supported by the micrographs inFigure 2b−d where enlarged inner cores with successivelydense radial structures are observed.We have next followed the effect of terminal oxidation poten-

tial of monomers on the dendrimer structure and also the DLAprocess as a whole. For this purpose, we polymerize monomerby potentiodynamic method for 42 min in the potentialwindow where the maximum anodic potential was restricted,separately, to 740 and 780 mV, keeping all other depositionparameters identical. As we can see in Figure 4a that therestriction of terminal anodic potential by 20 mV in thecathodic side, that is, at 740 mV compared to 760 mV, wellpatterned PANI fractals are formed (see Figure 2b) with lessoverall coverage of the HOPG substrate. Monomer oxidation atlow applied potential leads to less number density of PANIclusters, and hence, DLP proceeds with the formation oflocalized fractals. The edges of the fractal domains are alsofound to be terminated by the accumulated excess polymer

mass, the reason of which is not very clear at present. The mor-phology changes entirely when the terminal oxidation potentialis made 20 mV anodic as compared to 760 mV. It can be seenfrom Figure 4b that the entire HOPG surface is covered with atubular bush of PANI with localized growth of PANI clusters inthe vertical direction over the tubular structures. The primaryreason may be associated with the usual over potentialdeposition process where three-dimensional PANI clusters arefound to exist over the localized tubular structures. It is furthernoted that the growth rate and surface coverage of PANIfractals at higher sweep rate is enhanced (see SupportingInformation Figure 6). The role of diverse surface anisotropy ofHOPG in facilitating the growth of PANI fractals was disclosedfrom the fact that, under identical experimental conditions, noPANI fractals were formed on Au(111) substrate (seeSupporting Information Figure 7).It is well-known that HOPG having very low surface energy

(35 dyn cm−1) shows extremely anisotropic electrochemicalproperties like defect free terraces that are electrochemicallyinert or nearly so while the step edges behave like linearmicroelectrodes. Hence, it became very crucial to ascertainwhether the observed fractal growth in the present investigationis a genuine case of diffusion limited polymerization oranisotropic HOPG surface assisted nucleation and growth.30

Figure 5a shows the usual feature of the HOPG substrate withvery large terraces and step edges. Further confirmation that theobserved fractals are not formed by usual step edge decorationis provided in the SEM image of Figure 5b. It can be clearlyseen that, under identical electropolymerization conditions, thestep edges are decorated with larger clusters of PANI and thefractals are grown preferably on the basal plane.The nature of nucleation and growth over the HOPG surface

was also followed from the current time (I−t) transients. Figure 6ishows the I−t transients when the potential was stepped to 200and 760 mV vs Ag/AgCl. Both the transients reflect a typicalnature matching with two-dimensional nucleation andgrowth.40−42 In order to know about the specific nucleationprocess of PANI over HOPG, the I−t transient obtained at astep potential of 200 mV was analyzed for the shorter timedomain. Figure 6ii shows the dimensionless plot and revealsthat the instantaneous nucleation of PANI occurs on theHOPG surface.40 When the similar task is carried out for theI−t transient obtained at a step potential of 760 mV, we do find(see Figure 6iii) that the response is not exactly matching with

Figure 3. Variation of fractal dimensions with the time of poly-merization.

Figure 4. SEM images obtained by potentiodynamic polymerization of aniline on HOPG at a sweep rate of 0.015 V s−1 by limiting the anodicpotential to (a) 0.74 and (b) 0.78 V.

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instantaneous nucleation but a major share (about 90%) of thenucleation process belongs to it.36 The two-dimensionalinstantaneous nucleation and diffusion limited polymerizationcan be further supported from a SEM image (see Figure 7) that

was acquired at very early stage of (after third cycle) poly-merization process. The narrow size distributions of the PANIclusters in Figure 7 indicate the instantaneous nucleation, whereasthe growth of the fractals supports the DLP process.

■ CONCLUSIONS

In summary, we have demonstrated that the controlledsynthesis of PANI fractals on HOPG by DLP is possibleusing a simple electrochemical cell by potentiodynamic method.It is shown that the surface coverage and the nature of thefractals change to an observable extent with the duration ofpotential sweeping. The fractal dimension changes from 1.56to 1.94 as the time of electropolymerization increased from15 to 30 cycles. It is established through SEM studies thatthe fractal structure and consequently, the fractal dimensioncan be nicely controlled by tuning the monomer oxidationpotential limit in the present method. The role of HOPG surfacein facilitating the fractal growth by DLP is also revealed. More-over, the DLP process is established by excluding the possibilityof usual step edge assisted growth on HOPG throughindependent experiment. Since the method is highly reprodu-cible and PANI fractals are grown over large surface, it isbelieved that these structures can be used easily for manyinteresting awaited studies.

Figure 5. SEM image of (a) HOPG surface prior to electropolymerization and (b) fractal grown on the basal plane as well as on the step edge ofHOPG indicating diffusion limited polymerization process.

Figure 6. Current−time transients (I−t) for the electropolymerization process: (i) the step potential is at (a) 0.2 V and (b) 0.76 V. Dimensionlessplots of I/Im vs t/tm for the shorter time domain when the step potential was at (ii) 0.2 V and (iii) 0.76 V. Empty (○) and filled (●) symbols indicatethe experimental and simulated data, respectively.

Figure 7. SEM images revealing the nature of the PANI fractals at anearly stage of electropolymerization indicating dominant instantaneousnucleation process.

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■ ASSOCIATED CONTENT

*S Supporting InformationImage analysis for the determination of surface coverage.Determining the fractal dimension by using CA method. SEMimage of the PANI fractals obtained at different sweep rate onHOPG surface. SEM image revealing the effect of substrate onthe morphology of the electrochemically synthesized PANI.SEM showing the absence of PANI in the fractal boundary.This material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: indrajit.m@sse.pdpu.ac.in. Fax: +91-79-23275030.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe financial support for carrying out this work has beenreceived from CSIR net work project (NWP 0010) and DSTsponsored project (SR/S1/PC-01/2010).

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