Electrochimica Acta 50 (2005) 1917–1924

Electropolymerization of 4-nitro-1,2-phenylenediamine andelectrochemical studies of poly(4-nitro-1,2-phenylenediamine) films

Bin Yu, Soo Beng Khoo∗

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore

Received 29 May 2004; received in revised form 19 August 2004; accepted 21 August 2004Available online 5 October 2004

Abstract

In this work, we report the anodic electropolymerization of 4-nitro-1,2-phenylenediamine (4NoPD) in different supporting electrolytes atdifferent pH. The feasibilities of forming the polymer poly(4-nitro-1,2-phenylenediamine) (P4NoPD) on gold and glassy carbon electrodes(GCE) were shown. The P4NoPD films were generated by continuous potential cycling between−0.15 V and +1.10 V (versus Ag|AgCl,saturated KCl). The pH of the electropolymerization medium, the nature of the background electrolyte and the number of cycles used weref ed©

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ound to influence strongly the amount of polymer deposited. The reduction of nitro-groups of the P4NoPD films was also found to bependent on solution conditions, especially pH.2004 Elsevier Ltd. All rights reserved.

eywords:Polymer-modified electrodes; Anodic electropolymerization; 4-Nitro-1,2-phenylenediamine; Poly(4-nitro-1,2-phenylenediamine) film; Nigroupeduction

. Introduction

Conducting polymers, in particular electrodes modifiedith conducting polymer films, have attracted considerable

nterest in the last two decades. A multitude of reviews[1–5]nd monographs[6–10] have been written on the subject.hese materials possess a wide range of applications in elec-

roanalysis[11], energy storage[12], electrocatalysis[13],iosensing[14], corrosion protection[15], sensors and elec-

ronic devices[1,2], electrochromic displays[16], etc. Whileonducting polymer films on electrodes can be formed ineveral ways, electropolymerization is one of the most con-enient and advantageous[17].

Poly(thiophene), poly(pyrrole), and poly(aniline) (PANI)re amongst the three most widely studied conducting poly-ers. This is probably due to the high conductivities of theirxidized forms and their ability to reversibly switch betweenonducting and insulating states by doping and undoping

∗ Corresponding author. Tel.: +65 6874 2919; fax: +65 6779 1691.

[18]. These properties enable possible applications in sof the areas mentioned above. The structure of PANI is slar to that of emeraldine, with benzene ring para-couple

NH groups (in the reduced form)[19]. In this aspect, it is ointerest to investigate ladder analogues of the PANI strucSuch analogues are expected to extend the�-conjugated system as well as to increase the thermal stability and mechastrength[20]. One such candidate which has received ation is poly(ortho-phenylenediamine) (PoPD) [20–22]. Thispolymer has a phenazine-like chain structure and is reobtained by electropolymerization from aqueous acidiclution.

The nitro-group substituent of a benzene ring is an ereduced group, and is reported to undergo an irreversibleor six electron (overall) reduction in various media[23–26].However, there have been few reports of the electropmerization of nitro-substituted monomers. This scarcitybe attributed to the strong electron-withdrawing naturthe nitro-group that destabilizes the initially formed catiradicals, thereby limiting the following coupling reactionthese that leads to polymerization[27,28].

E-mail address:chmksb@nus.edu.sg (S.B. Khoo).

013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2004.08.048

1918 B. Yu, S.B. Khoo / Electrochimica Acta 50 (2005) 1917–1924

In this work, we report the electropolymerization of 4-nitro-1,2-phenylenediamine (4NoPD). Our interest stemsfrom the reported difficulty of electropolymerization of nitro-substituted benzenes. Further, the electropolymerization, iffeasible, could be expected to give rise to a polymer struc-turally different from PoPD because of site-blockage at thepara-position. Of even more interest is the reduction of thenitro-group on the benzene ring. Chemical and/or electro-chemical conversion of the nitro-group may open up possibleapplications in immobilization, sensors, electrocatalysis, etc.Therefore, the main objectives of the present work are to in-vestigate the feasibility of electropolymerization of 4NoPDand also the electrochemical behaviors of the polymer filmformed and the nitro-group substituent.

2. Experimental

2.1. Reagents

All chemicals were of analytical reagent grade unlessotherwise specified. 4NoPD was purchased from Merck(Germany) and was recrystallized from a mixture ofacetone–acetonitrile (1:1, vol/vol). The purified 4NoPD, af-ter drying in a vacuum oven at 80◦C for 2 days, gave a melt-i ◦ ◦s liter-a terp

2

weres -i TAT3 com-p roxi-m per-i )w elec-t om-p desw untere

2

era-t org nt byp gc illi-p in.F d in0 att

Solutions were purged with high purity nitrogen gas for atleast 10 min before electrochemical measurements. P4NoPDfilms were formed on the electrode surfaces by continuouspotential cycling between−0.15 V and +1.10 V at 50 mV s−1

in 0.50 M H2SO4 containing 4.50 mM 4NoPD. Typically, 25cycles were employed (although other cycle numbers werealso studied). When not in use, the modified electrode wasstored in Millipore water.

3. Results and discussion

3.1. Electropolymerization of 4NoPD

We initially investigated several aqueous and nonaque-ous media (e.g. HNO3, HClO4, H2SO4, CH3CN, MeOH,dimethyl sulfoxide (DMSO), etc.) for the electropolymer-ization of 4NoPD at the gold electrode. However, the ma-jority of these media gave difficulties relating to the lowsolubility of 4NoPD. In the case of HClO4, although sol-ubility of 4NoPD was reasonable (up to about 15 mM), littlepolymer film was observed due to the increased solubility ofoligomers of 4NoPD in this medium. Similarly, DMSO wasfound to dissolve P4NoPD, giving rise to a reddish-coloredsolution. The final choice of medium was 0.50 M H2SO4( to1 sol-u osi-t t4

Vs)fH t cy-c peak(f -t omert ow-e i-c tione

F n0+ e lastc

ng point 201–202C (literature 199–201C). IR and NMRpectra were also compared with those available in theture. Water was obtained from a Millipore Alpha-Q waurification system (Millipore Corporation, USA).

.2. Apparatus

Containers (glassware, polyethylene bottles, etc.)oaked overnight in 10% HNO3 prior to use. Electrochemcal experiments were performed with an Autolab PGS0 (Eco Chemie, Netherlands) controlled by a personaluter. A home-made three electrode glass cell of appately 5 ml capacity was used for all electrochemical ex

ments. The reference electrode (Ag|AgCl, saturated KClas placed in a compartment containing the supporting

rolyte solution separated from the -working electrode cartment by a 4 mm diameter Vycor frit. Working electroere gold and glassy carbon disks (3-mm diameter). Colectrodes were 3-mm diameter platinum disks.

.3. Procedure

All experiments were performed at an ambient tempure of 25± 2◦C. The substrate electrode surface (goldlassy carbon) was pretreated prior to each experimeolishing with alumina (0.3�m)/water slurry on polishinloth. The electrode was then rinsed copiously with More water and sonicated in a Millipore water bath for 5 minally, the electrode was electrochemically conditione.50 M H2SO4 by potential cycling from 0.00 V to 1.40 V

he scan rate of 10 mV s−1 for 40 min.

concentrations of H2SO4 were examined from 0.10 M.00 M, but 0.50 M was used as optimum to give goodbility of 4NoPD and not too early a cathodic decomp

ion potential) that gave a solubility of 4NoPD up to abou.50 mM.

Fig. 1 shows a series of cyclic voltammograms (Cor oxidative polymerization of 4.50 mM 4NoPD in 0.50 M

2SO4 at the gold electrode. The forward scan of the firsle revealed the presence of one irreversible oxidationO1) with peak potential (Ep,O1) at +0.81 V. O1 clearly aroserom the oxidation of the monomer. O1 decreased with coninuous cycling attesting to restricted access of the mono the electrode surface as the polymer film was formed. Hver, the current for O1 did not go completely to zero, indating that there was a small amount of monomer oxidaven after many cycles.

ig. 1. Cyclic voltammograms (CVs) (50 mV s−1) at the Au electrode i.50 M H2SO4 solution, containing 4.50 mM 4NoPD, between−0.15 V and1.10 V. The cycles numbers shown are 1, 10, 15, 20, 25 (being thycle).

B. Yu, S.B. Khoo / Electrochimica Acta 50 (2005) 1917–1924 1919

Two reduction peaks, R1 (Ep,R1 = 0.07 V) and R2(Ep,R2 = 0.01 V) (Fig. 1), were observed during the reversedscans, growing in height with repeated cycling. Peak R1 was asharp peak, which broadened and merged with R2 on repeatedcycling and aging of the polymer film after formation. Thereason for this may be that R1 was due to occluded oligomersand shorter polymer chains, which existed during the forma-tion process and in freshly, prepared films. A similar peak wasseen for the electropolymerization ofo-phenylenediamine(oPD) [21]. It should be noted that at the end of the 25-cycle electropolymerization, the solution developed a lightpinkish coloration, indicating the presence of some solubleoligomers. The main reduction peak for P4NoPD is R2, andassociated with this an oxidation peak O2 (Ep,O2 = 0.05 V),both of which increased in height with increasing film thick-ness. It is interesting to observe that polymer films resultingfrom different number of potential cycles gave rise to dif-ferences in colors; for example, a 25-cycle film was lightreddish, a 40-cycle film was dark red and 70-cycle was darkgreen. The last may be due to over-oxidation of the film giv-ing rise to a color change and/or interference (combination)of colors. The actual cause of this needs to be further inves-tigated. Previous studies with PoPD [29] reported that thispolymer film was red but a dark green color has not beenmentioned.

ep ouldb l-l then eb ow-e en”s -j is tob sibler -d ee

the4 ac lledr )a ionsf en-d

i

C to etF ove0 ma-t be3 t al.

Fig. 2. Plot ofi vs. t−1/2 for the chronoamperometric response at the Auelectrode in 0.50 M H2SO4 containing 0.05 mM 4NoPD. The potential wasstepped from 0.4 V to 1.0 V.

[21] have estimated ann-value of 2.7 per monomer unit ofoPD for its electropolymerization.

3.2. Electrochemical characterization of P4NoPD films

3.2.1. Films formed using different number of cycles anddifferent supporting electrolytes

The P4NoPD film was insoluble in aqueous solutions aswell as in many organic solvents such as acetonitrile andethanol. It was soluble to some extent in DMSO to give a red-dish solution.Fig. 3a shows typical CVs of P4NoPD filmsformed from different number of cycles. These CVs weretaken in 0.50 M H2SO4 immediately after electropolymer-ization. It can be observed that, in general, the anodic andcathodic peaks are not mirror images of each other expectedfor reversible electron transfer process of adsorbed thin films.Rather, the cathodic film was sharper than the anodic peak.As expected, films formed with more cycles involved highercharges (i.e. the thicker films as evidenced by the higher peaksin Fig. 3a). Further, in all three cases, it is noteworthy that theratios of cathodic to anodic peak height were larger than unity.A possible explanation for this is that the reduced state of theP4NoPD film is more conducting that the oxidized state, asshown by the impedance data ofFig. 3b, which demonstratesthat for a given film, the impedances were generally higher atmfi tialscF nse,w film.T

onfifim ol-u efis so-l vorst ts t

The view in the literature is that PoPD has a ladder-liklanar structure with phenazine rings, some of which ce open. In the case of P4NoPD, a ladder-like structure fo

owing that of PoPD is unlikely because of the presence ofitro-groups in thepara-position. The structure is likely to branched, with more partially-opened phenazine rings. Hver, even with the likelihood of more branching and “optructure for the P4NoPD compared to PoPD, extensive con

ugation of the benzene rings via the amine functionalitye expected. Indeed, this is what gives rise to the reveredox processes of the peaks O2/R2 and the electronic conuctivity of the P4NoPD film (features related to polyanilinlectrochemistry).

We investigated the solution-based oxidation ofNoPD just prior to film formation. This was done byhronoamperometric experiment to the diffusion-controegion of peak O1 at low 4NoPD concentration (0.05 mMnd at short times (below 0.34 s) to minimize complicat

rom film formation. For this experiment, the current depence on time is given by the Cottrell equation:

= nFADCb

π1/2t1/2

b is bulk monomer concentration;D is diffusion coefficienf 4NoPD, 6.5× 10−6 cm2 s−1 [30]; the other terms hav

heir usual meanings. A plot ofi versust−1/2 is shown inig. 2, where deviation from linearity was observed ab.75 s, possibly signalling the commencement of film for

ion. For the linear region, then-value was determined to.1 (average of five determinations, R.S.D. 7%). Dai e

ore positive bias potentials. This is different from the PoPDlm whose impedances were more or less similar at potenorresponding to the fully reduced and oxidized state[20].ilm conductivity discussed here is in the electronic sehich has as its source the extensive conjugation in thehis is similar to conductivity in polyaniline films.

The effect of pH of the electropolymerization mediumlm formation was examined. The CVs in 0.50 M H2SO4 forlms freshly formed in 0.50 M H2SO4 and H2SO4–Na2SO4ixtures (0.50 M Na2SO4 was not studied due to poor sbility of 4NoPD) are shown inFig. 3c. Since the threlms were formed with 25 cycles, the data ofFig. 3c clearlyhows that thicker films were obtained from more acidicutions. Therefore, it can be deduced that protonation fahe electropolymerization process[8,9]. Further, data (nohown here) for the film formed in 0.50 M HClO4 shows tha

1920 B. Yu, S.B. Khoo / Electrochimica Acta 50 (2005) 1917–1924

Fig. 3. (a) Cyclic voltammograms (CVs) (50 mV s−1) of three P4NPoD filmson Au electrodes (electropolymerizations were performed in 0.50 M H2SO4

containing 4.50 mM 4NoPD) in 0.50 M H2SO4 solution: 25-cycle film (—);40-cycle film (- - -); 70-cycle film (. . .). (b) Plot of resistance,Z vs. biaspotential for three P4NPoD films on Au electrodes (electropolymerizationswere performed in 0.50 M H2SO4 containing 4.50 mM 4NoPD) in 0.50 MH2SO4 solution: 25-cycle film (�); 40-cycle film (�); 70-cycle film (�). Forresistance measurements, a 5 mV amplitude sine wave at the frequency of20 kHz was used. (c) CVs (50 mV s−1) in 0.50 M H2SO4 for three P4NPoDfilms, electropolymerized in different H2SO4–Na2SO4 solutions containing4.50 mM 4NoPD on Au electrodes: 0.10 M H2SO4–0.40 M Na2SO4 (—);0.20 M H2SO4–0.30 M Na2SO4 (- - -); 0.50 M H2SO4 (. . .).

in this medium, the film formed was much thinner than in0.50 M H2SO4. This is attributed to the higher solubilities ofthe oligomers of 4NoPD in this medium resulting in lowerdeposits of the polymer on the gold electrode surface.

3.3. Scan rate studies

The influence of potential scan rates on CVs at filmsformed in 0.50 M H2SO4 for 25 and 70 cycles were stud-ied. For this purpose, aged films that had been stabilized byrepeated cycling and stored in water for a few days were used.This precaution was to ensure that variations in peak currentsresulted entirely from scan rate variations and were not com-plicated by polymer strands rearrangements and/or aging forfreshly prepared polymer films. The plot of peak current ver-sus scan rate was linear through the origin up to 50 mV s−1

for the thinner film (25 cycles), but only up to 10 mV s−1

for the thicker film (70 cycles). Thus, surface behavior, asmanifested in linear peak current versus scan rate plot, wasobserved for a larger range of scan rates in the thinner film.This is expected, as diffusional charge propagation tends toset in for the thicker film[31].

3.4. Stabilities of P4NoPD films

The stability of an electropolymerized film on gold was in-vestigated by monitoring the anodic peak current (ip,a) fromfilm oxidation (Fig. 4a). A freshly formed film (25 cyclesin 0.50 M H2SO4 containing 4.50 mM 4NoPD) was contin-uously cycled in 0.50 M H2SO4. It can be seen that aftersome initial decrease in peak current, the current stabilizedand was essentially constant from about 4 h onwards up tothe maximum cycling time studied of over 14 h. The initialdecrease in anodic peak current (from 45 to 37�A after 4-h cycling) was attributed to loss of the occluded oligomers

Fig. 4. Plots of anodic peak currentsip,a of P4NoPD films on Au electrodes(25-cycle, fabricated in 4.50 mM 4NoPD in 0.50 M H2SO4) under differenttime and storage regimes: (a) continuous cycling in 0.50 M H2SO4; (b) inbetween CVs in 0.50 M H2SO4, electrode was stored in 0.50 M H2SO4; (c)in between CVs in 0.50 M H2SO4, electrode was stored in Millipore water.In all cases, the scan rate was 50 mV s−1.

B. Yu, S.B. Khoo / Electrochimica Acta 50 (2005) 1917–1924 1921

and/or rearrangement of polymer strands (aging effect) inthe freshly formed film. On a longer term basis,ip,a for afreshly prepared film was monitored at intervals over a 2-week period with storage in 0.50 M H2SO4 between CVs.The results are shown inFig. 4b. It can been seen by theslow decay ofip,a that storage in 0.50 M H2SO4 apparentlycaused deterioration of the polymer film. After 11 days ofstorage, rapid degeneration was observed andip,a droppedsharply. Film deterioration in acidic solution was probablydue to loosening of the attachments of the film to the elec-trode surface leading to partial film detachment. In a similarstudy, CVs of the film were obtained at various time intervalsin 0.50 M H2SO4, but in between measurements the electrodewas stored in Millipore water. As can be seen inFig. 4c, inthe absence of the destabilizing effect of storage in 0.50 MH2SO4, the polymer film was stable for at least 14 weeks.This long-term stability of the film is advantageous for anyapplication.

3.5. pH effects

As shown above, over time, pH affects the stability ofP4NoPD film. pH is also expected to have a strong effecton the redox reaction of P4NoPD films. We examined thiseffect in two media, HSO and H PO . We observed thati po-t shiftw lfatem d lin-e k po-t(p an-o n inp r therfia tyf ept argen argeo as-i f thefi dia,p untilt ve pH4 hatem ris-t ates andd duet atao pHo

Table 1Peak potentials and currents for CVs of P4NoPD filma on Au electrode insulfate solutions of different pH

pH ip,a (�A) ip,c (�A mV) Ep,a (mV) Ep,c (mV) Ep,a−Ep,c (mV)

0.73 76.87 95.63 85 55 301.45 41.25 75.01 −8 −9 12.47 35.62 61.75 −75 −116 414.12 10.05 13.12 −175 −254 796.00 – – – – –7.90 – – – – –

11.17 – – – – –a Electropolymerized in 0.50 M H2SO4 containing 4.50 mM 4NoPD with

25 cycles.

Table 2Resistance dataa of P4NoPD filmb on Au electrode at the peak potentials oftheir respective CVs in sulfate solutions of different pH

pH 0.70 1.56 2.47 4.12 6.00 7.90 11.00Resistance (�) 8 20 28 40 48 55 72

a Resistance obtained using a 5 mV amplitude sine wave at the frequencyof 20 kHz.

b Electropolymerized in 0.50 M H2SO4 containing 4.50 mM 4NoPD with25-cycles.

3.6. Reduction of the nitro-groups of 4NoPD monomer

Numerous studies have been reported on the reduction ofaromatic nitro-groups[25,33–35]. Generally, the reductionscheme is given as:

RNO22e−, 2H+

−→ RN(OH)2−H2O−→ RNO

2e−, 2H+−→ RNHOH

2H+, 2e−, −H2O−→ RNH2

The overall process involves a total of six electrons withan amine as the final product. Prior to investigating the re-duction of nitro-groups in the P4NoPD film, we thought itwould be instructive to first examine the reduction of 4NoPDmonomer, as there is no data in the literature for the reduc-tion of 4NoPD. The glassy carbon electrode (GCE) was usedfor this study as it allowed a larger potential window in theacidic sulfate medium used.Fig. 5 shows three consecutiveCVs for the reduction of 4NoPD in 0.10 M H2SO4. A sin-gle irreversible wave (Ep,c=−0.60 V) was observed, and the

FG thirdc

2 4 3 4ncreasing pH shifted the anodic peak to less positiveentials in both media. Further, the magnitude of theas much larger in the phosphate medium than the suedium. In both media, the peak potentials decreasearly with increasing pH. These linear decreases in pea

entials have the equation ofEp,a (mV) =−73.5, pH +118r2 = 0.971) for the sulfate medium andEp,a (mV) =−76.9,H +82.3 (r2 = 0.985) for the phosphate medium. Thedic peak potentials in sulfate were more positive thahosphate medium, for the same pH (this is also true foeduction peaks). It has previously been reported that PoPDlm has high selectivity for SO42−/HSO4

− ions over ClO4−

nd Cl− anions[32]. If we extrapolate this to higher affinior SO4

2− over PO43− for the P4NoPD, this can explain th

ositive potential shifts as the greater availability of SO42− in

he film served to promote increased protonation via cheutralization, and therefore enhanced the positive chn the film. Increasing protonation level, with its incre

ng positive charge environment, made the oxidation olm more difficult. For both the sulfate and phosphate meeak currents also became smaller with increasing pH

he peaks essentially disappeared. This occurred at abo.12 for the sulfate medium and pH 7.00 for the phospedium.Table 1summarizes the redox peaks characte

ics of P4NoPD in sulfate solution (the results for phospholutions, not shown here, were similar). The decreaseisappearance of the redox peaks of higher pH may be

o decreasing film conductivity. This is confirmed by the df Table 2, which shows that film resistance increased asf the bathing solution increased.

ig. 5. Reduction of 4.50 mM 4NoPD monomer in 0.10 M H2SO4 at theCE for three successive cycles: first cycle (—); second cycle (- - -);

ycle (. . .). The scan rate was 50 mV s−1.

1922 B. Yu, S.B. Khoo / Electrochimica Acta 50 (2005) 1917–1924

Fig. 6. Reduction of nitro-groups of P4NoPD on GCE (formed with 25 cyclein 4.50 mM 4NoPD in 0.50 M H2SO4) in 0.10 M H2SO4 for three successivecycles: first cycle (—); second cycle (- - -); third cycle (. . .). The scan ratewas 50 mV s−1.

peak height was found to vary linearly with square root ofscan rate up to 200 mV s−1 indicating diffusion-controlledbehavior. Subsequent cycles after the first gave no additionalpeaks. This is in contrast to nitrobenzene[33–35], which ex-hibited an additional pair of redox peaks following the firstscan.

3.7. Electrochemical behaviors of the nitro-groups ofP4NoPD

While the reduction of solution phase nitro-compoundshave been studied as mentioned above, the voltametric reduc-tion of the nitro-group in a polymer chain have rarely beenreported.Fig. 6depicts the reduction of the P4NoPD film onGCE for three continuous cycles in 0.10 M H2SO4. A broadand irreversible reduction peak for the nitro-group (the reduc-tion of an PoPD film under identical conditions did not showthis peak) can be seen withEp,c at−0.34 V, less negative thanthe reduction of the solution-based 4NoPD monomer in thesame solution withEp,cat−0.60 V. This is probably due to theenhanced conjugative effect of the polymer film. In contrastto the solution-phase monomer reduction, the reduction peakof the P4NoPD film disappeared after the first cycle (Fig. 6).This can be understood as the nitro-groups in P4NoPD filmwere irreversibly reduced and therefore after the first cycle,t h as caledl

chedt ver,t s thata ionalg tionp merfi ert -d ighta actorc ncesf asd

Fig. 7. (a) Redox peaks for the P4NoPD film (25 cycle in 4.50 mM 4NoPD in0.50 M H2SO4) in 0.10 M H2SO4; comparison of fresh film (—) with a filmin which nitro-groups had been reduced in 0.10 M H2SO4 (see Fig. 6) (- - -),CVs were obtained at 50 mV s−1. (b) Plot of resistance Z vs. bias potentialfor the P4NoPD film (25 cycle in 4.50 mM 4NoPD in 0.50 M H2SO4) in0.10 M H2SO4; comparison of fresh film (�) with a film in which nitro-groups had been reduced in 0.10 M H2SO4 (see Fig. 6) (�). For resistancemeasurements, a 5 mV amplitude sine wave at the frequency of 20 K wasused.

The reduction of nitro–aromatic compounds in alkalinesolutions may exhibit completely different behavior com-pared to acidic solutions. For example, at a gold ring-diskelectrode in 0.10 M NaOH, an oxidation wave was observedfor the radical anion of nitrobenzene produced by a one-electron electrode reduction[37]. Nitrobenzene was alsofound to give a one-electron reduction wave followed by afour-electron reduction wave in 0.10 M NaOH[38]. Fig. 8ashows the reduction of freshly formed P4NoPD film in 0.10 Mand 1.00 M NaOH. In both cases, a reduction peak (Ep,c at−0.78 V in 0.10 M NaOH,Ep,c at−0.86 V in 1.00 M NaOH)with a corresponding oxidation peak (Ep,a at −0.61 V in0.10 M NaOH,Ep,a at −0.79 V in 1.00 M NaOH) are ob-served. The oxidative peak is broader and less negative in0.10 M NaOH compared to 1.00 M NaOH. Additionally, thepeak separation is larger in 0.10 M NaOH (�Ep = 0.17 V)than in 1.00 M NaOH (�Ep = 0.07 V). These data suggestthat the reduction of the nitro-groups in P4NoPD film un-der these conditions produced a chemically stable product(in the time frame of the experiment) that was re-oxidizedon scan reversal. In such alkaline media, the anionic radicalinitially produced by reduction of the nitro-groups may bemore stable, leading to a corresponding oxidation peak onscan reversal, as shown by previous investigations for solu-tion phase nitro-aromatic compounds[37,38]. The CVs of

here were no more available for reduction. In line witurface-controlled redox reaction, the peak currents sinearly with scan rate up to 50 mV s−1.

The presence of redox active functional groups attao conducting polymer skeleton is often desirable. Howehese functionalized polymers may possess propertiere not simply superpositions of the skeleton and functroups[36]. In the present case, the nitro-group reducroduced an attenuation of the redox peak for the polylm, as can been seen inFig. 7a. The figure shows that afthe reduction of the nitro-groups in the P4NoPD film, the reox waves for the polymer film were much reduced in hend the potential was pushed more negative. The major fontributing to this behavior is the increased film resistaollowing the irreversible reduction of the nitro-groups,emonstrated by the data ofFig. 7b.

B. Yu, S.B. Khoo / Electrochimica Acta 50 (2005) 1917–1924 1923

Fig. 8. (a) CVs (50 mV s−1) of the P4NoPD film (25 cycles in 4.50 mM4NoPD in 0.50 M H2SO4) on GCE in: 0.10 M NaOH (—); 1.00 M NaOH(- - -). (b) CVs (50 mV s−1) of P4NoPD film (as above) on GCE in 0.10 MH2SO4, comparison of fresh film (—) and film after reduction of nitro-groupsin 1.00 M NaOH (- - -).

Fig. 8a are reminiscent of stripping voltammograms. If wetake the reduction product (possibly the anionic radical ofthe nitro-groups) to be stable, then these CVs suggest thatthe reduction of the nitro-groups throughout the polymerchains are not instantaneous but rather has a time effect,resulting in a “preconcentration” effect. This explains thesignificantly larger area of the oxidation peaks during thereverse scan. An alternative explanation for the larger areaof the oxidation peaks is that these arise from a redox pro-cess involving more electrons than the forward process. Ifthis is true, then the argument above for the proposed stabil-ity of the anionic radical and its re-oxidation fails.Fig. 8bshows that the redox behavior of the film in 0.10 M H2SO4did not change much, when compared with a fresh film, af-ter nitro-groups reduction in 1.00 M NaOH. This is in con-trast to the reduction of the nitro-groups in acidic solutionthat resulted in attenuation of the redox peaks for the film(Fig. 7). This observation further supports the chemical re-versibility of the redox reaction of the nitro-groups in al-kaline media, as suggested above, because an irreversiblechange in the nitro-groups would significantly affect the re-dox behavior of the film as noticed for acidic solution ear-lier.

The above studies demonstrated that pH greatly influencedthe electrochemical behavior of the attached nitro-groups.T 0.08f ob f then ose,0 ith

Fig. 9. (a) CV (50 mV s−1) of P4NoPD film (25 cycles in 4.50 mM 4NoPDin 0.50 M H2SO4) on GCE in 0.10 M Na2SO4 (pH = 5.41): first cycle (—);second cycle (- - -); (3) third cycle (. . .). (b) CVs (50 mV s−1) of P4NoPDfilms (as above) on GCE in the sulfate media of different pH for first cycle:pH = 0.30 (—); pH = 5.41 (-·-·-); pH = 7.90 (- - -); pH = 11.00 (. . .).

pH adjustments to the pH 0.30–11.00 region by additions ofconcentrated H2SO4 or NaOH.

The CVs for the continuous cycles for reduction in 0.50 MNa2SO4 (pH 5.41) are shown inFig. 9a starting from +0.30 Vto a final potential at−1.30 V. It was earlier established(also, seeTable 1) that at pH higher than about 4.12, withinthe potential region (from 400 mV to−400 mV) studied forP4NoPD film redox activities in the sulfate medium, P4NoPDfilm did not exhibit any redox peaks. Therefore, these redoxpeaks inFig. 9a are most likely to be due to the reductionof nitro-groups. In the first forward scan, no redox reactionwas found until about−0.30 V. Further negative, a small pre-peak was seen withEp at−0.42 V, quickly followed by a mainreduction peak, P1, with Ep at −0.70 V. On scan reversal, asingle oxidation peak P2 was observed withEp at−0.02 V. Inthe second and third cycles (Fig. 9a), a small reduction peakP3 (Ep =−0.10 V) was evident in the forward, cathodic scanwhich was not present during the first cycle. P1 decreasedsignificantly in height from the first to the second cycle butminimally on the third cycle. P2 decreased slight from thefirst to the third cycle. The couple P2/P3 may be due to the

NHOH/ NO couple observed in the solution phase studiesof nitro aromatic compounds[37,38]. This result is in con-trast to that in 0.10 M H2SO4 (seeFig. 7), where there is noadditional reduction peaks after the first cycle.

-t edi res om

hese studies were carried out at very low and high pH (or 0.50 M H2SO4, 13.75 for 1.00 M NaOH). It would alse meaningful to study the electrochemical reduction oitro-groups at intermediate pH values. For this purp.50 M Na2SO4 was used as background electrolyte, w

The CVs between +0.40 V and−1.30 V for the reducion of the nitro-groups (first cycle, fresh films producn 0.50 M H2SO4) for various pH from 0.30 to 11.00 ahown inFig. 9b. For pH 0.30, the scan range was fr

1924 B. Yu, S.B. Khoo / Electrochimica Acta 50 (2005) 1917–1924

−0.15 V to−1.30 V because of film oxidation at less neg-ative potentials at this pH. The nitro-group reduction peakpotentials shifted negatively, approximately linearly,Ep,c(mV) =−64.6pH− 370 (r2 = 0.985) up to about pH 4.1, afterwhich it was essentially constant. This is depicted inFig. 9b.Oxidation peak potentials, on scan reversal, shifted contin-ually positive with increasing pH i.e. pH 5.41 to 11.00 (thispeak was non-existent or not observable at more acidic pH).Further, peak heights (P1 and P2) also increased with increas-ing pH. This is rather contradictory as film resistance has beenshown to increase with increasing pH. The reason for this isnot clear at the moment and more detailed investigations arecontinuing.

4. Conclusions

As mentioned in the introduction, aromatic compoundscontaining the nitro-group as substituent are known to be dif-ficult to electropolymerize to give film formation. We haveshown here that the anodic electropolymerization of 4NoPDon Au and GC electrodes is feasible even though film thick-ness is thought to be low. The electropolymerization wasfound to be dependent on the acidity as well as the nature ofanions in the supporting electrolyte. The low solubility of them , asdt reas-i tiona

itye mea-s s re-d e oft hasb po-t t then re-gfi ash sors,e

A

12)f

R

in:eric

Systems, vol. 1, Marcel Dekker, New York, 1988, p. 97 (Chapter3).

[4] K. Doblhofer, Thin Polymer Films on Electrodes: A PhysicochemicalApproach, VCH Publishers, NY, 1994.

[5] G. Inzelt, M. Pineri, J.W. Schultze, M.A. Vorotyntsev, Electrochim.Acta 45 (2000) 2403.

[6] R.G. Linford (Ed.), Electrochemical Science and Technology ofPolymers, vols. 1 and 2, Elsevier, London, 1987.

[7] R.W. Murray (Ed.), Molecular Design of Electrode Surfaces, Tech-niques of Chemistry Series, vol. 22, Wiley Interscience, New York,1992.

[8] M.E.G. Lyons (Ed.), Electroactive Polymer Electrochemistry. Part 1.Fundamentals, Plenum Publishing Corporation, New York, 1994.

[9] M.E.G. Lyons (Ed.), Electroactive Polymer Electrochemistry. Part2. Methods and Applications, Plenum Publishing Corporation, NewYork, 1994.

[10] M.D. Levi, E.Yu. Pisarevskaya, Electochim. Acta 37 (1992) 635.[11] A. Ivaska, Electroanalysis 3 (1989) 247.[12] H.N. Dinh, S.M. Ren, F.H. Garzon, J. Electroanal. Chem. 491 (2000)

48.[13] K.M. Kost, D.E. Bartak, B. Kazee, T. Kuwana, Anal. Chem. 60

(1988) 2379.[14] S.J. Dong, J.H. Li, Bioelectrochem. Bioenerg. 42 (1999) 7.[15] R. Nouti, A.J. Nozik, J. White, L. Warren, J. Electrochem. Soc. 129

(1982) 1625.[16] T. Kobayashi, H. Yoneyama, H. Tamura, J. Electroanal. Chem. 161

(1984) 419.[17] R.A. Durst, A.J. Baumner, R.W. Murray, R.P. Buck, C.P. Andrieux,

Pure Appl. Chem. 69 (1997) 1317.[18] T. Komura, Y. Ito, T. Yamaguti, K. Takahasi, Electochim. Acta 43

[ 42.[ anal.

[ 456

[ 997)

[ ub,

[ 987)

[ ker,

[ ard,

[ llypo-

[ 96)

[[ 7)

[ ntals

[[[ , J.

[ anal.

[ ld-

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onomer 4NoPD (as well as the presence of nitro-groupsiscussed earlier) limits the ability to increase P4NoPD film

hickness by increasing the concentration. However, incng number of potential cycles during electropolymerizalleviates this solubility problem somewhat.

The P4NoPD film exhibits good electronic conductivspecially in acidic solutions as indicated by resistanceurements. Also, the nitro-group is electroactive, and itox property is highly pH dependent. The dependenc

he stability of the product(s) of its reduction on pHeen demonstrated. This has important implications in

ential applications, whether there is a need to converitro-group to another form (e.g. amine) or to be able toain it after reduction. Possible applications of the P4NoPDlm may include surface protection, electronic relay,ost matrix through encapsulation or bonding, in sentc.

cknowledgement

This work was supported by a grant (R-143-000-113-1rom the National University of Singapore.

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