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Materials Chemistry and Physics 107 (2008) 404–409

Electropolymerization of 3-aminophenol on carbon graphite surface:Electric and morphologic properties

Diego L. Franco a, Andre S. Afonso a, Sabrina N. Vieira a, Lucas F. Ferreira a,Rafael A. Goncalves b, Ana G. Brito-Madurro a, Joao M. Madurro a,∗

a Institute of Chemistry, Federal University of Uberlandia, Av. Joao Naves de Avila 2121, 38400-902 Uberlandia, Brazilb School of Mechanical Engineering, Federal University of Uberlandia, Av. Joao Naves de Avila 2121, 38400-902 Uberlandia, Brazil

Received 21 January 2007; received in revised form 1 August 2007; accepted 10 August 2007

bstract

This paper reports the formation of electropolymerized films derived from 3-aminophenol on graphite electrode by cyclic voltammetry, preparedn different pH conditions. With increase of pH values, a shift of the oxidation potential of 3-aminophenol to more cathodic potentials was observed.-Aminophenol electrooxidation, in acid and basic media, yielded polymeric films onto graphite surface. In ferrocyanide/ferricyanide solution,

he polymer produced in acid medium showed higher electron transfer efficiency. Scanning electron microscopy (SEM), atomic force microscopyAFM), and FT-IR were used to investigate some properties of the graphite electrode modified with poly(3-aminophenol). Scanning electronicroscopy showed that the morphology of the films is strongly dependent on the pH of the electropolymerization medium. FT-IR spectra of

olymer films produced for either acid or basic media suggest that the monomer is polymerized by NH2 group. 2007 Elsevier B.V. All rights reserved.

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eywords: Coatings; Electrochemical techniques; Polymers; Surfaces

. Introduction

Mankind has used polymeric materials since prehistoric timen the form of wood, natural fibers or during the last century asylon, PVC, polyethylene, polyaniline, etc. Synthetic polymersan be prepared by chemical or electrochemical processes [1–4].nodic oxidation is a widely used method for polymeric film

ormation from electroactive monomers. This can be attributedo the facility of potential control allowing the production oflms with good quality [5].

The discovery of polyaniline in 1862 [6] was followed byhe investigation of its oxidation states (leucoemeraldine, pro-oemeraldine, emeraldine, nigraniline and pernigraniline) [7,8].olyaniline is the most successfully used material for gasensors [9] being one of the most important conducting poly-ers [10–13]. Polyaniline was the first conducting polymer

o be commercialized and currently has applications rangingrom batteries [14,15] to biosensors [16]. Monomers, such ashenol, can be electropolymerized by ortho- or para-coupling

∗ Corresponding author. Tel.: +55 34 3239 4143; fax: +55 34 3239 4208.E-mail address: jmadurro@ufu.br (J.M. Madurro).

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254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2007.08.006

f phenolate radicals generated by oxidation of phenolatenion. Subsequent reactions produce oligomers and, finally,oly(phenyleneoxide) films are polymerized on the surface ofhe electrode [17–19]. Non-conducting polymers show highesistivity, their growth is self-limited and the polymeric film isuch thinner than typical conducting polymer films. The perms-

lectivity of the films conferred improved biosensor selectivitynd anti-fouling properties, with fast response time [20]. Non-onducting polymers do not present a strict definition since thelectric conductivity of many conducting polymers depends onheir electron and proton doping level; so the concept of con-ucting or non-conducting polymers is transferable. Typically,veroxidized polypyrrole or that was polymerized under alkalionditions is electroinactive [13]. Non-conducting polymersnclude poly-benzene or poly-phenol derivatives (phenol; 2-minophenol; 3-aminophenol; 4-aminophenol; 3-methylphenol;-nitrophenol; 1,3-dihydroxybenzene; acetaminophen; 1,2-ihydroxybenzene; 1,3-dihydroxybenzene; 1,4-dihydroxyben-ene; 1,3,5-trihydroxybenzene; 1,2,3-trihydroxybenzene), poly-

henylenediamines (1,2-diaminobenzene; 1,3-diaminobenzene;,4-diaminobenzene), etc. [13]. Insulating electropolymerizedlms like polyphenol, poly(o-phenylethylenediamine), overox-

dized polypyrrole [20,21] and aminophenols [22–24] have

D.L. Franco et al. / Materials Chemistry and Physics 107 (2008) 404–409 405

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For pH values above 10 (Fig. 1B), the results denounce asignificant decrease of the oxidation potential (+0.89 to +0.36 V)

ig. 1. First cyclic voltammograms of a graphite electrode in aqueous solution.0 (c). Inset: peak potentials (first scan) as function pH. (B) pH 10.0 (a) and 12

pecific advantages, such as increase of the electrocatalytic prop-rties to development of analytical devices.

Aminophenols are interesting electrochemical materialsince, differently from aniline [25–27] and other substituted ani-ines [28] they present two groups which could be oxidized (NH2nd OH). They could show electrochemical behavior resemblingnilines [26,29] or phenols [30,31]. These groups bonding upith the polymeric molecules can constitute reactive sites for

ncorporation of metals [32,33] or biomolecules [22,34], withpplications in electrocatalysis, and electrochemical biosensors.

Due to the presence of these two groups there is a contro-ersy in the literature about the polymerization mechanism.-Aminophenol was found to polymerize through the NH2 group35,36] through the OH group [37] or both the hydroxyl andmino groups of 3-aminophenol seem to participate in the elec-ropolymerization [21] making this problem the subject of thisnvestigation.

3-Aminophenol electropolymerization was studied on carbonaste [21], platinum [35,37] and SnO2 [36] electrodes, showinghat the films present non-conducting character.

In this work, cyclic voltammetry, infrared and surface anal-sis had been combined to study the formation, behavior andharacterization of the 3-aminophenol electropolymerization onhe surface of graphite electrode under acid and basic conditions.o the best of our knowledge, this is the first comparative studyolymeric films derived from 3-aminophenol electrogeneratedn acid and basic medium.

. Experimental

All of chemicals used were of analytical grade. 3-Aminophenol was providedy Sigma. The solutions were prepared using deionized water from a Milli-ore Milli-Q system (resistivity = 18.2 M� cm). 3-Aminophenol was preparedn aqueous solutions immediately before use. HClO4 solution (0.5 mol L−1)as used for all experiments and pH adjustment was carried out using NaOH

olution.Voltammetric measurements and electropolymerization were performed in

three-electrode cell using a model 273A potentiostat from PAR. The work-ng electrode was a 6 mm diameter graphite disk electrode or a 1 cm × 2 cmuoride-doped tin oxide electrode (FTO) with one side coated on glass (Flex-

Tec Eletronica Organica). A platinum plate was used as auxiliary electrode in allxperiments. All potentials are reported against the saturated calomel electrodeSCE).

The graphite electrode was polished with alumina oxide (0.3 �m slurry)efore each electrochemical assay. After polishing, the electrode was rinsed

Sm

ning 3-aminophenol (2.5 × 10−3 mol L−1), 50 mV s−1: (A) pH 1.0 (a); 2.0 (b);.

ith deionized water. The electrode was sonicated in an ultrasound bath andinsed again with water. The FTO was cleaned with petroleum ether and thenith acetone and dried with nitrogen.

Monomer solutions were deoxygenated with N2 prior to electropolymer-zation. 3-Aminophenol (2.5 × 10−3 mol L−1) was electropolymerized on theurface of the working electrode from 0.1 mol L−1 HClO4 solution by cycling theotential between −0.4 and +1.1 V. After electropolymerization, the modifiedlectrode was rinsed in deionized water to remove unreacted monomer.

The morphology of the graphite electrode modified with poly(3-minophenol) was analyzed by scanning electron microscopy (SEM) using aEO 940 A, ZEISS. Film roughness and thickness were measured by atomic

orce microscopy (AFM) using a Nanoscope IIIa, Digital Instruments.The FT-IR spectrum of KBr disc samples were recorded with a model

pectrum 1000 FTIR spectrometer from Perkin-Elmer.

. Results and discussion

The effect of pH on the electrochemical behavior of 3-minophenol on graphite electrode is shown in Fig. 1.

Fig. 1A shows a shift of the first oxidation potential of 3-minophenol at the graphite electrode with increasing pH. Upo pH 8.0, the results showed an exponential displacement of therst oxidation wave (first scan) to more cathodic potential thatecreases with pH increase (see inset Fig. 1A). This dependences described by Eq. (1).

(V) = 0.8078 + 0.0845 exp

[−pH − 1

2.0840

](1)

Up to pH 4.0 a rapid decrease in the Eox value occurs.n the pH 4.0–8.0 range a much slower reduction of the Eoxalue was observed. These results indicate that the aniliniumation/aromatic amine equilibrium (Scheme 1) with pKa = 4.3738] is an important feature of the oxidation mechanism, which

cheme 1. Acid/base equilibrium of 3-aminophenol (protonated/neutralolecule).

406 D.L. Franco et al. / Materials Chemistry and Physics 107 (2008) 404–409

Fig. 2. Cyclic voltammograms of 3-aminophenol (2.5 × 10−3 mol L−1) recorded conthe direction of scan increase.

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poly(3-aminophenol) prepared in basic pH (see Fig. 4). The fastdecrease of the oxidation current in the electropolymerizationwith continuous potential cycling suggests that the growth ofthese films is self-limiting (see Fig. 2B). These films tend to be

Scheme 2. Acid/base equilibrium of 3-aminophenol/3-aminophenoxide.

hich can be attributed to the equilibrium phenol/phenoxidequilibrium (Scheme 2) with pKa = 9.82 [38].

Cyclic voltammograms showing the electropolymerizationf 3-aminophenol at pH 1.0 and 12.0 are presented in Fig. 2.

In acid medium, as illustrated in Fig. 2A, the first cycleresents a single irreversible wave (Ep,a ∼= 0.8 V), which corre-ponds to the formation of cation-radical intermediate. Duringontinuous potential cycling a gradual decrease in its oxidationurrent is observed. From the second cycle a pair of peaks cov-ring the 0.00 to +0.60 V potential range appears. The currentncreases with increasing number of potential cycles reflectinghe coverage of the electrode surface by the polymeric film.

As shown in Fig. 2B, in alkaline media (pH > 10.0), therst cycle presents a single irreversible wave and a rather fastecrease of the oxidation current with continuous potential

ycling, suggesting that passivation of the electrode takes placencreasing the number of potential cycles.

The electrochemical behavior of the modified electrodes pre-ared under acid conditions is shown in Fig. 3.

ig. 3. Cyclic voltammogram of the bare graphite electrode (a) and the electrodeodified (pH 1.0) with poly(3-aminophenol) in 0.5 mol L−1 HClO4 solution (b).

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tinuously at 50 mV s−1 (100 scans). (A) pH 1.0. (B) pH 12.0. Arrows indicate

Both oxidation (+317 mV) and reduction (+263 mV) peaks ofoly(3-aminophenol) were observed for the modified electroderepared at pH 1.0. Current peaks were observed for modifiedlectrodes prepared up to pH 8.0. For pH higher than 10.0 noolymer electroactivity was observed.

Electropolymerization from acid or basic media produceslms that significantly affect the electron transference propertiest the electrode surface (Fig. 4).

In the presence of the aqueous K3Fe(CN)6/K4Fe(CN)6 cou-le, when contrasted with the signal of graphite electrodeithout film, the current peaks on the modified electrodes dimin-

sh. The inclination angle and the area of the peaks decreaseoo, suggesting a higher resistance of the system. This reflectshe covering of the electrode surface by the insulator polymericlm. This characteristic does not change with the increase of theH of the polymerization medium.

The passivation is more accented in electrodes coated with

ig. 4. Cyclic voltammograms of a bare graphite electrode and a graphitelectrode modified with poly(3-aminophenol) (100 potential scans) inqueous solution containing K3Fe(CN)6 (5 × 10−3 mol L−1), K4Fe(CN)6

5 × 10−3 mol L−1) and KNO3 (0.1 mol L−1), 100 mV s−1. (a) Graphite elec-rode without film and modified with poly(3-aminophenol) at (b) pH 1.0; (c) pH2.0.

D.L. Franco et al. / Materials Chemistry

Fig. 5. Cyclic voltammograms of the bare graphite electrode or electrodes mod-ified with poly(3-aminophenol) in HClO4 solution 0.5 mol L−1, 50 mV−1. (a)Gc(

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raphite electrode; electrode modified prepared in HClO4 solution (pH 1.0)ontaining 3-aminophenol (2.5 × 10−3 mol L−1), formed by (b) 50 cycles andc) 100 cycles, 50 mV s−1.

ery thin, continuous, generally resulting in low permeability ofompounds for its inside [19].

The low current indicates that few ions of3Fe(CN)6/K4Fe(CN)6 penetrate in the film. The minorE to the redox pair K3Fe(CN)6/K4Fe(CN)6, observed in

lectrodes modified with films produced in pH 12, comparedith films prepared in pH 1, suggests that these films act only asvery thin barrier which does not modify the redox potentialsf K3Fe(CN)6/K4Fe(CN)6.

Fig. 5 shows the behavior of poly(3-aminophenol) at pH 1.0fter 50 or 100 potential scans in HClO4 solution (Fig. 5).

Increasing the number of potential scans to electropolymer-zation of 3-aminophenol on the graphite electrode, an increasen the amount of polymer deposited on the electrode surface isbserved (see Fig. 5).

The morphology of the electrode modified with poly(3-minophenol) produced in acid or basic media was analyzed

y SEM (Fig. 6).

These results show good coverage of the surface, presentinglobular structures randomly distributed on the electrode surfaceFig. 6A). Up to pH 10 the coating is not capable of filling the

CoN

Fig. 6. SEM image of (A) graphite electrode with film of p

and Physics 107 (2008) 404–409 407

raphite cavities (Fig. 6B). This result suggests the formation ofnsulator polymer layer that presents self-limited growth. This isonsistent with the CV behavior shown in Fig. 2B, which showsstrong reduction of the current after the first potential cycle.

AFM imaging was used to provide further information onhe polymer morphology. Typical 3-D AFM images of the filmsolymerized in acid medium were investigated (Fig. 7).

The AFM image reveals globular structures on the surfacef the modified electrode. Roughness is a measurement ofhe small-scale variations in the height of a surface. Rough-ess values for bare graphite and poly-3-aminophenol are 1115nd 518 nm, respectively. Poly-3-aminophenol prepared in acidedium is capable of filling the graphite cavities, diminishing

ts roughness.The film presented thickness of ca. 180 nm, following the

rocedure developed by Lobo and co-workers [39].Infrared spectra of 3-aminophenol and poly(3-aminophenol)

re shown in Fig. 8.In the FT-IR spectra of 3-aminophenol, the bands at 3360 and

294 cm−1 are related to the N H asymmetric and symmetrictretching modes. The hydrogen bonded O H band is a peakn 3422 cm−1. The C H stretching modes are observed at 3042nd 3027 cm−1. C C stretching modes are observed at 1505,464 and 1427 cm−1. The NH2 scissor is assigned to the band at598 cm−1. The N H vibrations in aromatic compounds oftenverlap the aromatic C C ring absorptions, which also appear inhis region [40]. N H wag fundamentals are assigned to the bandt 688 cm−1. The bands at 1258 and 1389 cm−1 are related to the

O stretching and to the O H bending vibration, respectively.his analysis is in agreement with literature [40–42].

In FT-IR spectra of poly(3-aminophenol) produced in acidnd basic media, the bands related to primary amine disappearsnd one band localized at ca. 3430 cm−1 (hydrogen bonded

H stretching band) and one band in ca. 3240 cm−1 relatedo aromatic secondary amines is observed [42]. The band in ca.655 cm−1 is attributed to C N stretching vibrations of imines.ll peaks assignments are represented in Table 1.

The presence of a band in ca. 1650 cm−1, attributed toN stretching vibrations of imines, as well as the absence

f NH2 bending (scissoring), N H out-of-plane bending andH asymmetric and symmetric stretching of primary amine

oly(3-aminophenol) in: (A) pH 1.0 and (B) pH 12.0.

408 D.L. Franco et al. / Materials Chemistry and Physics 107 (2008) 404–409

Fig. 7. AFM image of graphite electrode with film of poly(3-aminophenol) in pH 1.0.

Fig. 8. IR spectra of 3-aminophenol and poly(3-aminophenol) prepared in acid(pH 1.0) and basic (pH 12.0) media.

Table 1Peak assignments of FT-IR absorption bands

Group contribution 3-Aminophenol(cm−1)

Poly(3-aminophenol)prepared in acidmedium (cm−1)

Poly(3-aminophenol)prepared in basicmedium (cm−1)

O H stretching 3422 3429 3427N H stretching 3360; 3294 3237 3242C H stretching 3042; 3027 3024 3026C C stretching 1505; 1464;

14271622; 1491;1462; 1424

1625; 1476; 1458;1430

Primary N Hbending

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C N stretching – 1652 1659OCN

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H bending 1389 1380 1384O stretching 1258 1237 1243H wagging 688 – –

uggests that the 3-aminophenol polymerization is occurring byttack of nitrogen on aromatic ring, forming quinonoid iminenits, similar to the aniline polymerization [43].

. Conclusions

The formation of polymeric film derived of 3-aminophenol athe surface of graphite electrodes is viable. With pH increase, thexidation peak of 3-aminophenol shifts to more cathodic poten-ials, but the polymer produced maintains its non-conductingroperties. The increase in the number of potential scansroduces increase in the amount of material formed. Elec-ropolymerization in acid medium resulted in complete coverage

f the graphite surface with a film thickness of ca. 180 nm. IRpectra suggest that the electropolymerization affects the NH2roups by formation of C NH C bonds while the OH groupsre preserved.

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D.L. Franco et al. / Materials Chem

cknowledgments

The authors are grateful for financial support from Conselhoacional de Desenvolvimento Cientıfico e Tecnologico (CNPq),

nd Fundacao de Amparo a Pesquisa do Estado de Minas GeraisFAPEMIG).

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