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Journal of Electroanalytical Chemistry 608 (2007) 22–30

ElectroanalyticalChemistry

A study of the oxidation and polymerisation of meta substitutedphenol and aniline derivatives

Brendan Kennedy a, Andrew Glidle b, Vincent J. Cunnane a,*

a Materials and Surface Science Institute, CES Department, University of Limerick, Irelandb Department of Electronics and Electrical Engineering, Rankine Building, Oakfield Avenue, Glasgow, G12 8LT, United Kingdom

Received 26 January 2006; received in revised form 15 February 2007; accepted 9 May 2007Available online 24 May 2007

Abstract

The objective of this paper is the study of the electrochemical oxidation and subsequent polymerisation of 1,3-dihydroxybenzene, 3-aminophenol and 1,3-phenylenediamine at pH 7. These related monomers were subsequently utilised by the authors in the production ofpinhole free masks for the construction of features on gold surfaces of nanometer dimensions (neutral pH was required due to the use ofbiological templates). It is shown that whilst the electrochemical oxidation of 1,3-phenylenediamine is very similar to that of 3-amino-phenol, it is quite dissimilar to that of 1,3-dihydroxybenzene and this can be attributed to the lack of interaction between the substituentson the ring due to their meta orientation. This behaviour can be correlated with differences in the HOMO values of the molecules ascalculated using Crystal98. X-ray photoelectron spectroscopy was used to characterise the polymer films and their thin layer nature.� 2007 Elsevier B.V. All rights reserved.

Keywords: Electropolymerisation; 1,3-Phenylenediamine; 3-Aminophenol; 1,3-Dihydroxybenzene; X-ray photoelectron spectroscopy

1. Introduction

When compared to the large number of papers that havebeen written on the electropolymerisation of aniline it isclear that relatively little work has been done on the poly-merisation of 1,3-phenylenediamine. Si et al. [1] and Maz-eikiene et al. [2] have both published papers on theelectrochemical copolymerisation of aniline and 1,3-pheny-lenediamine from acidic media, at gold electrodes and irid-ium oxide coated titanium electrodes, respectively. Workhas also been done on the usefulness of poly(1,3-phenylene-diamine) in the construction of biosensors [3–5]. Wanget al. polymerised 1,3-phenylenediamine from acidic solu-tions at a carbon paste electrode and found that it cataly-sed the oxidation of hydrogen peroxide. Marzouk andco-workers also employed films of 1,3-phenylenediamineto detect hydrogen peroxide whilst Rhemrev-Boom et al.

0022-0728/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.jelechem.2007.05.006

* Corresponding author. Tel.: +353 61 202 308; fax: +353 61 202 568.E-mail address: vincent.cunnane@ul.ie (V.J. Cunnane).

immobilised an oxidoreductase enzyme in a poly(1,3-phenylenediamine) film to achieve the same ends.

The electrochemical copolymerisation of 1,3-dihydroxy-benzene and 1,3-phenylenediamine has also received someconsideration. Again this attention is concentrated on thefield of biosensors. Geise et al. [6,7] looked at various elec-tropolymerised films at several electrodes and concludedthat a film of poly (1,3-phenylenediamine-co-1,3-dihydr-oxybenzene) gave the best stability and protection. Bassiet al. [8] immobilised the enzyme fructose dehydrogenasebehind a non-conducting film of poly (1,3-phenylenedi-amine-co-1,3-dihydroxybenzene) so as to create a fructosebiosensor. In the same year Park and co-workers [9] pub-lished a paper on a polymer-coated, carbon fibre nitricoxide sensor. They found that a coating of a compositepolymer generated from nafion, 1,3-phenylenediamineand 1,3-dihydroxybenzene gave the best results. Pontieet al. [10] came to similar conclusions in their 1999 paper.

The published work on 3-aminophenol is also quite lim-ited. Prater [11] has studied the polymerisation of variousmeta substituted anilines, including 3-aminophenol from

B. Kennedy et al. / Journal of Electroanalytical Chemistry 608 (2007) 22–30 23

acetonitrile. He suggested that the meta positioning of thesubstituent activates the ring towards double electrophillicsubstitution. This leads to a highly cross-linked film lackingelectronic conjugation. The result of this is non-conductingfilms of low permeability. Barbero [12] in his study of 2-aminophenol reports that similar results are observed when3-aminophenol is polymerised from aqueous acid media atglassy carbon electrodes. Taj et al. [13] successfully poly-merised 3-aminophenol from both acetonitrile and metha-nol at a platinum electrode to obtain an insulating deposit.Sankarapapavinasam [14,15] produced two papers in 1993on the polymerisation of 3-aminophenol. In one, hedetailed his attempts to elucidate the mechanism and kinet-ics of the polymerisation, at platinum, from both acetoni-trile and methanol. He concluded that whilst the rate ofpolymerisation was a function of current density at lowcurrent densities, it was independent of current density athigher values. His second paper was concerned with theeffect the polymer film had on the oxidation of certainchemical species. Both Taj [13] and Sankarapapavinasam[14,15] propose a mechanism for the polymerisation of 3-aminophenol in acetonitrile and methanol. They suggestthat the hydroxyl group on 3-aminophenol dissociatesyielding a proton and an anion. Taj claims that the pres-ence of the meta amino group reinforces the acidity ofthe hydroxyl group. This anion is then oxidised at the elec-trode to form a radical. The radical then initiates polymer-isation at the electrode surface, the monomer constituentsbeing linked by ether linkages.

Nakabayashi and co-workers [16,17] have also pub-lished two papers on poly(3-aminophenol) in the field ofbiosensors. In 1998, they reported that 3-aminophenolseemed to be the best of a range of phenols for the produc-tion of polymer films to immobilise glucose oxidase at acarbon paste electrode. In 2000, they produced a non-con-ducting film of poly (3-aminophenol) at a carbon pasteelectrode in which they could immobilise horseradish per-oxidase. Fung et al. [18] polymerised 3-aminophenol at agold plated piezoelectric crystal and managed to immobi-lise antibodies in the resulting film. They claimed that thiswas a step towards the production of a biosensor for thedetection of Salmonella Enteritidis. In the present work,we seek to contrast the oxidation of 1,3-phenylenediamine,1,3-dihydroxybenzene and 3-aminophenol.

2. Experimental

2.1. Film preparation

All electrochemical experiments were carried out in athree-electrode glass cell. A Solartron SI-1287 potentiostatwas used throughout the course of the work. 3-aminophe-nol, 1,3-phenylenediamine and 1,3 dihydroxybenzene wereoxidatively electropolymerised onto gold electrodes frompH 7 aqueous solutions. 1,3-phenylenediamine and 3-ami-nophenol were polymerised by sweeping the potential ofthe working electrode between 0 V and 0.85 V vs. Ag/AgCl

at various sweep rates (1–10 mV s�1) for 5 sweeps of poten-tial. 1,3-dihydroxybenzene was polymerised by sweepingthe potential of the working electrode between 0 V and1 V vs. Ag/AgCl at various sweep rates (1–10 mV s�1) for5 sweeps of potential. 3-Aminophenol was procured fromFluka (>98%). Potassium Chloride (>99%), 1,3-phenylene-diamine (>99%), and 1,3-dihydroxybenzene were suppliedby Sigma Aldrich. Ultra pure water (resistivity: 18.2 MXcm) was used to make up all aqueous solutions. Electrodesof evaporated gold on silicon (100) (gold chip electrodes)were used in this work. These gold chip electrodes wereconstructed in-house using an Edwards E306A coatingsystem. It was necessary to lay down a 9 nm adhesion layerof titanium prior to evaporating the gold. A 75 nm layer ofgold was evaporated onto the titanium layer.

2.2. Molecular orbital calculations

The eigenvalues of the HOMO of the various com-pounds under study here have been calculated using theCrystal98 software package. The HOMO values of a num-ber of reference compounds have also been calculated. Thisdata is presented in Table 1. The geometry of the moleculesis based on the data presented in the paper of Jiang and Lin[19]. A full basis set was used to describe the orbitals of thehydrogen atoms. The hydrogen 1s orbital was described bya Pople standard inner valence shell basis set built into theCrystal98 program. Pople standard outer valence shellbasis sets (also built into the Crystal98 program) describedthe hydrogen 2s and 2p orbitals. The carbon, nitrogen andoxygen atomic orbitals were described by pseudopotentialspublished by the Stuttgart–Dresden theoretical chemistrygroup [20].

2.3. X-ray photoelectron spectroscopy

XPS was used to characterize the gold surface and thepolymer films. XPS measurements were performed on aScienta ESCA-300 instrument (Daresbury Laboratory,CCLRC, UK) using monochromated Al Ka (1486.7 eV)radiation (rotating anode source), a takeoff angle (TOA)of 90�, and an analyser entry slit of 0.8 mm. To compensatefor surface charging effects all binding energies werereferenced to the neutral C 1s peak at 285 eV. The fullwidth at half maximum (FWHM) was kept constant forany given spectrum in the deconvolution of the spectraobtained.

3. Results and discussion

The cyclic voltammograms for the polymerisation of1,3-phenylenediamine at various sweep rates are presentedin Fig. 1a. It is clear that there are two distinct oxidationpeaks in each cyclic voltammogram. There is no reductionpeak on the reverse sweep. In all cases, the height of thesecond peak is greater than that of the first peak. In situ,visual inspection of the electrode surface reveals that a

Table 1HOMO values calculated using Crystal98

HOMO energy values

Species HOMO (eV)

Aniline NH2�8.02

Phenol OH �8.51

3-Aminophenol* NH2

OH

�8.08

1,3-Dihydroxybenzene*

OH

OH �8.48

1,3-Phenylenediamine* NH2

NH2

�7.80

4-Aminophenol NH2

OH

�7.71

1,4-Dihydroxybenzene OH

OH

�8.06

1,4-Phenylenediamine NH2

NH2

�7.51

p-Aminodiphenylamine

NH2

NH2�6.41

* Compounds investigated electrochemically.

-100

100

300

500

12345

-100

200

500

800

0 0.2 0.4 0.6 0.8 1.2E (Volts) vs. Ag/AgCl

10mV/s

5mV/s

1mV/s

-100

100

300

500

0 0.5E (Volts) vs. Ag/AgCl

1

234

1

0 0.2 0.4 0.6 0.8E (Volts) vs. Ag/AgCl

1

1

a

b

c

I (μA

cm-2

)I (

μAcm

-2)

I (μA

cm-2

)

Fig. 1. (a) Cyclic voltammogram of the polymerisation of 0.1 M 1,3-phenylenediamine from an aqueous solution (pH neutral) containing0.1 M KCl between 0 V and 0.85 V vs. Ag/AgCl on a gold workingelectrode. The sweep rates employed were 1 mV s�1, 5 mV s�1 and10 mV s�1. (b) Cyclic voltammograms of the polymerisation of 0.1 M1,3-phenylenediamine from an aqueous solution (pH neutral) containing0.1 M KCl between 0 V and 0.85 V vs. Ag/AgCl. The sweep rate employedwas 5 mV s�1. Sweeps 1–5 are shown. (c) Potential sweeps of a poly(1,3-phenylenediamine) modified gold electrode in an aqueous solution(pH neutral) containing 0.1 M KCl between 0 V and 0.85 V vs. Ag/AgCl.The sweep rate employed was 5 mV s�1. Sweeps 1–4 are shown.

24 B. Kennedy et al. / Journal of Electroanalytical Chemistry 608 (2007) 22–30

reddish polymer layer is deposited once the potential of thesystem has swept past 0.5 V vs. Ag/AgCl. Thus the secondpeak is most likely due to electrochemical reactions withinthe polymer or at the surface of a polymer-modified elec-

trode as opposed to electrochemical reactions at a goldelectrode. The increase in peak height seems to indicatethat the polymer produced does not passivate the electrodesurface. This is not confirmed by the nature of successivesweeps of the electrode in the polymerisation solution.Fig. 1b shows five successive potential sweeps of a gold

-20

20

60

100

140

E (Volts) vs. Ag/AgCl

1

234,5

-50

0

50

100

150

200

250

0 0.5 1 1.5E (Volts) vs. Ag/AgCl

10mV/s

5mv/s

1mV/s

I (μA

cm-2

)I (

μAcm

-2)

0 0.5 1 1.5

a

b

Fig. 2. (a) Cyclic voltammogram of the polymerisation of 0.1 M 1,3-dihydroxybenzene from an aqueous solution (pH neutral) containing0.1 M KCl between 0 V and 1 V vs. Ag/AgCl on a gold working electrode.The sweep rates employed were 1 mV s�1, 5 mV s�1 and 10 mV s�1. (b)Cyclic voltammogram of the polymerisation of 0.1 M 1,3-dihydroxyben-zene from an aqueous solution (pH neutral) containing 0.1 M KCl between0 V and 1 V vs. Ag/AgCl. The sweep rate employed was 5 mV s�1. Sweeps1–5 are shown.

B. Kennedy et al. / Journal of Electroanalytical Chemistry 608 (2007) 22–30 25

electrode immersed in the polymerisation solution. Thefirst peak at 0.48 V vs. Ag/AgCl is only seen on the firstsweep of potential. The second peak at 0.77 V vs. Ag/AgClshifts to higher potentials on each successive scan. This iseither due to electrochemical reactions occurring at a poly-mer–electrolyte interface and to the over-oxidation of thepolymer-film produced in the first sweep. To test this a sim-ple experiment was performed. The polymer film wasgrown as per Fig. 1a (5 mV s�1). Then the polymer filmthus grown was scanned at the same sweep rate in a solu-tion of 0.1 M KCl (the polymerisation matrix minus 1,3-phenylenediamine). The results are presented in Fig. 1c.It is clear that most of the current observed on successivepotential sweeps is due to reactions in the polymer.

The cyclic voltammograms for the polymerisation of1,3-dihydroxybenzene at various sweep rates are presentedin Fig. 2a. There is no reduction peak on the reverse sweep.It is clear that oxidation of 1,3-dihydroxybenzene is more

difficult than the oxidation of 1,3-phenylenediamine. Thedifference in the peak potentials for the initial oxidationpeaks is nearly 260 mV (�0.48 V for 1,3-phenylenediamineand �0.74 for 1,3-dihydroxybenzene, both potentials ver-sus Ag/AgCl for a sweep rate of 5 mV s�1). After onlyone sweep of potential the film of poly(1,3-dihydroxyben-zene) is already formed. This is illustrated in Fig. 2b whereit is demonstrated that virtually no current flows when thepotential of the electrode is swept for a second time in thepolymerisation solution.

The cyclic voltammograms for the polymerisation of 3-aminophenol at different sweep rates are very similar tothose of 1,3-phenylenediamine. These results are presentedin Fig. 3a. It is clear that there are two distinct oxidationpeaks in each cyclic voltammogram. There is no reductionpeak on the reverse sweep. In all cases, the height of thesecond peak is approximately the same as that of the firstpeak. In situ, visual inspection of the electrode surface doesnot indicate that any polymer forms, as the working elec-trode does not change colour (although ex situ inspectionreveals that the polymer formed has some colour). It wasnoted that for 1,3-phenylenediamine, polymer had formedon the surface of the electrode once the potential had sweptpast 0.5 V vs. Ag/AgCl. The same is true for 3-aminophe-nol. This is proven in Fig. 3b where the switching potentialis 0.55 V vs. Ag/AgCl. Successive scans clearly indicate thatpolymer has formed at this point. Therefore, the secondpeak is due to polymer growing at a polymer–electrolyteinterface as opposed to a gold–electrolyte interface.Fig. 3c shows the electrochemical behaviour observed uponsuccessive potential sweeps of the working electrode in thepolymerisation solution. Although the first sweep showsthat the oxidation of 3-aminophenol is very similar to theoxidation of 1,3-phenylenediamine, sweeps subsequent tothe first sweep show that poly(3-aminophenol) is more sim-ilar to poly (1,3-dihydroxybenzene). There are no peaksobserved on the successive sweeps and the currents arelow as were observed for 1,3-dihydroxybenzene.

The mechanism by which 1,3-phenylenediamine is oxi-dised is probably very similar to the oxidation of anilinewhich has been extensively studied by many authors[19,21–27]. The amine groups on 1,3-phenylenediamineare orientated so that the only interaction between thegroups is of an inductive nature [28]. Thus 1,3-phenylenedi-amine behaves very much as if there were two aniline mol-ecules present on the same ring acting independently of eachother. This is borne out to some extent by the HOMO cal-culations presented in Table 1. The energy of the HOMOfor 1,3-phenylenediamine is greater than that of 1,4-pheny-lenediamine where resonance interaction of the groups ispermitted. There has been considerable debate [19,21–27]as to the exact nature of the oxidation reaction in anilineand the subsequent polymerisation reactions. The mainbody of opinion seems to favour the formation of a radicalcation followed by radical cation polymerisation [21,23]. AtpH 7 1,3-phenylenediamine exists in an uncharged state [29]in solution. The oxidation of 1,3-phenylenediamine to a

NH2

NH2

NH2+

NH2

C

NH2+

NH2C

NH2+

NH2

NH2

NHNH2

NH2

NH2

NH2

NH2

NH2NH2

NHNH

NH2

e--a

b

Fig. 4. (a) The initial oxidation of 1,3-phenylenediamine. (b) The dimersformed upon coupling of the radicals.

-10

30

70

110

150

0 0.4 0.8 1.2E (Volts) vs. Ag/AgCl

10mV/s

5 mV/s

1 mV/s

-10

30

70

110

150

0 0.2 0.4 0.6

1

2

5

-10

10

30

50

70

90

0 0.5 1

1

2345

E (Volts) vs. Ag/AgCl

E (Volts) vs. Ag/AgCl

I (μA

cm-2

)I (

μAcm

-2)

I (μA

cm-2

)a

b

c

Fig. 3. (a) Cyclic voltammogram of the polymerisation of 0.1 M 3-amino-phenol from an aqueous solution (pH neutral) containing 0.1 M KClbetween 0 V and 0.85 V vs. Ag/AgCl on a gold working electrode. Thesweep rates employed were 1 mV s�1, 5 mV s�1 and 10 mV s�1. (b) Cyclicvoltammogram of the polymerisation of 0.1 M 3-aminophenol from anaqueous solution (pH neutral) containing 0.1 M KCl between 0 V and0.55 V vs. Ag/AgCl. The sweep rate employed was 5 mV s�1. Sweeps 1, 2and 5 are shown. (c) Cyclic voltammogram of the polymerisation of 0.1 M3-aminophenol from an aqueous solution (pH neutral) containing 0.1 MKCl between 0 V and 0.85 V vs. Ag/AgCl. The sweep rate employed was5 mV s�1. Sweeps 1–5 are shown.

26 B. Kennedy et al. / Journal of Electroanalytical Chemistry 608 (2007) 22–30

radical cation is shown in Fig. 4a. These radical cations arehighly reactive [30]. They react with each other to formdimers (similar to the formation of p-aminodiphenylamineupon oxidation of aniline [23]). The types of dimers that

are possible to form are shown in Fig. 4b. The hydrazinedimer is unlikely to be stable as the nitrogen–nitrogen bondis much weaker than that of the carbon–carbon bonds [31].These dimers are themselves more easily oxidised than themonomers [23,32]. This is because the p electrons can bedelocalised over a greater number of atoms, which has theeffect of raising the energy of the HOMO (see p-aminodi-phenylamine in Table 1). Indeed it is to be noted that asthe number of monomer units in the oligomer grows theoxidation of the oligomer becomes easier for much the samereasons. Therefore, the dimers are oxidised and the polymerchain propagates. Polymerisation of aniline tends to resultin the production of linear chains [22,23]. This is not thecase for 1,3-phenylenediamine. Poly(1,3-phenylenediamine)films tend to be highly cross-linked [11]. The argument is asfollows. The amine groups of 1,3-phenylenediamine in thisuncharged state are ortho-para directing [33]. As alreadyhighlighted the only influence one amine group can haveon the other is of an inductive nature [28]. The groups donot interact via resonance effects. This makes the meta iso-mers of benzene, in most cases, the most difficult of the dia-mine isomers to oxidise [34] (given their inability to stabilisethe radical formed upon oxidation via relocation onto thesecond amine group). It is this orientation of the functionalgroups that leads to the polymers formed from the meta iso-mers of benzene being highly cross-linked [11]. This is dueto the fact that there are three positions on the ring doublyactivated for substitution (see Fig. 5).

Gattrell and Kirk [35] proposed in 1992 a mechanismfor the oxidation of phenol. The analogous reaction for1,3-dihydroxybenzene is shown in Fig. 6a. As already high-lighted the oxidation potentials of 1,3-dihydroxybenzeneare higher than those of 1,3-phenylenediamine. A questionarises as to why this is so? With reference to Table 1 andthe literature [36–40], it is clear that the HOMO of 1,3-dihydroxybenzene (and its’ analogue phenol) has a lowerenergy than the HOMO of 1,3-phenylenediamine (and its’analogue aniline) making it more difficult to oxidise. The

NH2

NH2

ortho to both groups : doubly activated

para to one group and ortho tothe other : doubly activated

para to one group and ortho to the other :doubly activated

meta to both groups :unactivated

Fig. 5. Activation of 1,3-phenylenediamine at pH 7.

OH

OH

O

OH

H

C

O

OH

H

C

O

OH

H

OH

OH

O

HO

OH

OH

OH

OH

OO

OH

OH

e--+ + +

a

b

Fig. 6. (a) The initial oxidation of 1,3-dihydroxybenzene. (b) The dimersformed upon coupling of the radicals.

NH2

OH

NH2+

OH

C

NH2+

OH C

NH2+

OH

NH

NH2

OH

OH

NH2

OHOH

NH2

OH

NHNH

OH

e--a

b

Fig. 7. (a) The initial oxidation of 3-aminophenol. (b) The dimers formedupon coupling of the radicals.

B. Kennedy et al. / Journal of Electroanalytical Chemistry 608 (2007) 22–30 27

substantial effect of the absence of resonance interactions isalso clear when the eigenvalues for 1,3-dihydroxybenzeneand 1,4-dihydroxybenzene are compared. The radical soformed then reacts to form dimers (see Fig. 6b). Thesedimers are more easily oxidised than the monomer forthe same reasons as pertained for 1,3-phenylenediamine.Oxidation of theses dimers results in oligomer formation,leading to polymer growth. The dimers that will predomi-nate are those which are ether linked and those that arecarbon linked. The peroxide linked dimers will be highlyunstable and will break up as soon as they are formed[41]. The bond dissociation energies of oxygen–oxygenbonds are less than half that of carbon–carbon bonds.For phenol the predominant form of bonding in the poly-mer would be carbon–carbon, but for more highly substi-tuted phenols a greater proportion of the bonds tend tobe of the ether variety [35]. Hydroxyl groups in a meta ori-entation interact in the same fashion as amine groups metato each other. That is to say that one group exerts influenceon the other via induction effects but not via resonanceeffects. Since induction effects will be minimal over the dis-tance of three carbon atoms the groups can effectively beviewed as being isolated from each other. As before the

meta orientation of the groups leads to a high degreeof cross-linking in the polymer film produced [11]. It hasto be said that phenol itself produces a highly cross-linkedstable polymer film, indicating that the meta orientation isnot as critical for 1,3-dihydroxybenzene as it is for 1,3-phenylenediamine.

It has been noted that the energy of the HOMO of 1,3-dihydroxybenzene was lower than that of 1,3-phenylene-diamine and that this difference could explain why1,3-dihydroxybenzene was more difficult to oxidise. FromTable 1 it is clear that 3-aminophenol has eigenvalues moresimilar to those of aniline than those of phenol. In terms ofchemistry this also makes sense. It has already been notedthat there is no resonance interaction between meta-orien-tated substituents on a benzene ring. Therefore, the aminogroup on 3-aminophenol is free to behave similarly to 1,3-phenylenediamine and indeed that is what is observed.However, the eigenvalues of 3-aminophenol and 1,3-phenylenediamine are somewhat different. The fact thatthe electrochemistry of the compounds is so similar maybe because in reality the true model is that of the moleculesadsorbed onto gold. In the literature [36–40], the energy ofthe HOMO for 3-aminophenol and 1,3-phenylenediamineare quoted as being quite similar. The oxidation of 3-ami-nophenol is shown in Fig. 7a. As was noted for 1,3-pheny-lenediamine these radical cations are highly reactive. Theyreact with each other to form dimers. The types of dimersthat it is possible to form are shown in Fig. 7b. Due to themeta orientation of the hydroxyl and amine groups theradical cannot be stabilised by location on the hydroxylgroup. A consequence of this is that ether bonds cannotlink the dimers. This is a supposition based on the pro-posed reaction mechanism. There is no direct experimentalevidence to verify this. However, we believe that the oxida-tion (and polymerisation) of the dimer itself will lead to theformation of ether bonds. The same can be said for theoligomers. Again these dimers are themselves more easilyoxidised than the monomers [23] and the formation ofpolymer is propagated.

a

b

c

28 B. Kennedy et al. / Journal of Electroanalytical Chemistry 608 (2007) 22–30

The XPS surveys of the polymer films are shown inFig. 8. The changing intensity of the gold peaks (due tothe underlying gold electrode) in the polymer surveys isinteresting. It gives an idea as to the relative thickness ofthe various polymer films. In the survey of poly(1,3-dihydr-oxybenzene) two gold peaks are present. These are the 4fdoublet and the Au 4d 5/2 peak. This indicates that filmsof poly (1,3-dihydroxybenzene) produced in the mannerdescribed above must be very thin (of about three to sixnm in thickness [42]). Were the films any thicker than thisthen the overlying polymer layer would almost certainlyobscure the Au 4d peak. Moving to poly(3-aminophenol),it is clear that there is a dramatic reduction in the intensityof the 4f peaks relative to the intensity of the other peaks inthe survey. Also the Au 4d peak is no longer present in thesurvey. This indicates that films of poly(3-aminophenol)are substantially thicker than those of poly(1,3-dihydroxy-benzene). In the survey of poly (1,3-phenylenediamine)there are no gold peaks present. This demonstrates thatthe films of poly(1,3-phenylenediamine) produced aremuch thicker than those of poly(3-aminophenol) and poly(1,3-dihydroxybenzene).

The carbon 1s peak for poly(1,3-dihydroxybenzene) isshown in Fig. 9a. It is clear, even upon initial inspection,that there are three peaks convoluted together in the largerpeak. With reference to the XPS survey for poly(1,3-dihydroxybenzene) shown in Fig. 9a the only bonds thatcan possibly be represented in the peak are carbon–carbonbonds and carbon–oxygen bonds. This is due to the factthat these are the only elements present in the survey.The presence of the carbon–oxygen bond at 286.5 eV is

280 282 284 286 288 290 292Binding Energy (eV)

Fig. 9. The C1s spectra of (a) poly(1,3-dihydroxybenzene), (b) poly(1,3-phenylenediamine) and (c) poly(3-aminophenol). The films were preparedas per Figs. 1b, 2b and 3c.

0 1 00 200 300 400 500 600Binding Energy (eV)

Au 4f

C1s

Au 4d

O 1s

N 1s

a

b

c

Fig. 8. The XPS surveys of (a) poly(1,3-dihydroxybenzene), (b) poly(3-aminophenol) and (c) poly(1,3-phenylenediamine). The films wereprepared as per Figs. 1b, 2b and 3c.

to be expected as this is due to ether and hydroxyl bonds.The presence of the carbonyl group at 288 eV is evidencefor the over-oxidation of the films or for the atmosphericoxidation of the films [43]. The carbon 1s peak forpoly(1,3-phenylenediamine) is shown in Fig. 9b. In thispeak, it is not clear that there is more than one bondingcontribution to the overall peak. Due to the lower electro-negativity value of nitrogen compared to oxygen (3.04 vs.3.44) the shift caused by nitrogen on carbon is less distinct.The peak for carbon–nitrogen has been deconvoluted to avalue of 286 eV [44]. There is probably some carbonylbonding in the polymer. This is confirmed upon analysingthe oxygen 1s peak (see Fig. 10). This carbonyl group is

529 531 533 535 537 539Binding Energy (eV)

a

b

c

Fig. 10. The O1s spectra of (a) poly(1,3-dihydroxybenzene), (b) poly(1,3-phenylenediamine) and (c) poly(3-aminophenol). The films were preparedas per Figs. 1b, 2b and 3c.

395 397 399 401 403 405Binding energy (eV)

a

b

Fig. 11. The N1s spectra of (a) poly(1,3-phenylenediamine) and (b)poly(3-aminophenol). The films were prepared as per Figs. 1b and 3c.

B. Kennedy et al. / Journal of Electroanalytical Chemistry 608 (2007) 22–30 29

present due to the reaction of the film with oxygen in theair as per the mechanism outlined by Kieffel et al. for poly-aniline [43]. The carbon 1s peak for poly(3-aminophenol) isshown in Fig. 9c. There is both oxygen and nitrogen in 3-aminophenol and deconvolution of the peak reveals bothto be present (carbon–nitrogen bonds appearing at286 eV and carbon–oxygen bonds appearing at 286.5 eV).There is also some carbonyl bonding present. There is, rel-ative to the other contributions, a much lower amount ofcarbonyl bonding than was observed for poly (1,3-dihydr-oxybenzene). These carbonyl bonds are probably formedin a similar manner to those formed in poly(1,3-dihydroxybenzene).

The oxygen 1s peak for poly(1,3-dihydroxybenzene) isshown in Fig. 10a. The peak is clearly asymmetric andcan be deconvoluted into two contributory peaks. Theseare due to carbonyl oxygen at 532.4 eV and ether/hydroxyloxygen at 533.8 eV. This is in agreement with the informa-

tion obtained from the carbon peak. It has already beenpostulated that oxygen is incorporated into poly(1,3-phenylenediamine), in the carbonyl form, via oxidationby the atmosphere [43]. This is proven in Fig. 10b. Thepeak is symmetric and is located at 532.5 eV, which iswhere the carbonyl peak occurs. It is noticeable that thescan is quite noisy compared to that of the oxygen 1s peakfor poly (1,3-dihydroxybenzene). This is a good indicationthat there is much less oxygen in the poly(1,3-phenylenedi-amine) than there is in poly (1,3-dihydroxybenzene). This,of course, is to be expected. The oxygen 1s peak forpoly(3-aminophenol) is shown in Fig. 10c. It mirrors clo-sely the structure of the oxygen 1s peak for poly (1,3-dihydroxybenzene) in so far as it is asymmetric. The peakcan be deconvoluted into two constituent peaks: one forthe carbonyl carbon at 532.4 eV and the other for the ethercarbon at 533.8 eV. However, the contribution of the car-bonyl peak is reduced from that for the oxygen 1s peakfor poly(1,3-dihydroxybenzene). This is consistent withthe differing nature of the carbon 1s peaks for poly(1,3-dihydroxybenzene) and poly(3-aminophenol).

In Fig. 11a, the nitrogen 1s peak for poly(1,3-phenylenedi-amine) is shown. The data is perfectly modelled by oneGaussian peak once a Shirley background has been fittedto account for the variation in the baseline. The peak appearsat 400 eV, which is where the peak for the secondary amine

30 B. Kennedy et al. / Journal of Electroanalytical Chemistry 608 (2007) 22–30

is located. Thus the nitrogen bonds in poly (1,3-pheny-lenediamine) most closely mirror those of leucoemeral-dine. Leucoemeraldine is an insulating, reduced form ofpolyaniline and thus poly(1,3-phenylenediamine) grownfrom solution at neutral pH is insulating. The nitrogen 1speak for poly(3-aminophenol) is shown in Fig. 11b. Ittoo demonstrates the same form of nitrogen bonding asleucoemeraldine.

4. Conclusions

It has been shown, that the cyclic voltammograms forthe oxidation and polymerisation of 1,3-phenylenediamineand 3-aminophenol at neutral pH, are similar. The cyclicvoltammograms for the oxidation and polymerisation of1,3-dihydroxybenzene at neutral pH are different. Thisdifference in behaviour can be attributed to the greater energyneeded to liberate an electron from 1,3-dihydroxybenzeneover 1,3-phenylenediamine and 3-aminophenol. This isborne out by molecular orbital calculations, which show thatthe HOMO of 1,3-dihydroxybenzene is lower in energy thanthe HOMO of 1,3-phenylenediamine and 3-aminophenol.

XPS measurements on the films produced show that thethickness of the films increases in the following order:poly(1,3-dihydroxybenzene), poly(3-aminophenol) andpoly(1,3-phenylenediamine). XPS also shows that whilsttwo differently bonded forms of oxygen may be presentin the polymers only one bonded form of nitrogen is pres-ent. Polymerisation results in the formation of ether bondswhilst carbonyl bonds are formed through atmosphericoxidation. The current densities observed for the oxidationof 1,3-phenylenediamine are higher than those observed forthe oxidation of 3-aminophenol. It is believed that the dif-fering ability of the various polymer films to passivate thegold surface is the primary factor influencing current den-sity. Comparing the XPS surveys for 1,3-phenylenediamineand 3-aminophenol it is clear that a thicker layer of theformer is grown. This ties in with observation that greatercurrent densities are observed for the oxidation of 1,3-phenylenediamine.

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