Journal of The Electrochemical Society, 161 (12) G103-G113 (2014) G1030013-4651/2014/161(12)/G103/11/$31.00 © The Electrochemical Society

Post-Polymerization Electrochemical Functionalizationof a Conducting Polymer: Diazonium Salt Electroreductionat Polypyrrole ElectrodesMatei Raicopol,a Corina Andronescu,b Ruxandra Atasiei,c Anamaria Hanganu,dand Luisa Pilane,z

aUniversity Politehnica of Bucharest, “Costin Nenitzescu” Department of Organic Chemistry, Bucharest, RomaniabUniversity Politehnica of Bucharest, Department of Bioresources and Polymer Science, Bucharest, RomaniacUniversity Politehnica of Bucharest, Department of Physics, Bucharest, Romaniad“C.D. Nenitzescu” Institute of Organic Chemistry of the Romanian Academy, Bucharest, RomaniaeUniversity Politehnica of Bucharest, Department of Inorganic Chemistry, Physical Chemistry and Electrochemistry,Bucharest, Romania

In this paper we report the functionalization of conductive polypyrrole (PPY) films via electrochemical reduction of the aryl di-azonium salts in a manner that is similar to the one employed for other conductive surfaces. To understand the general trendsof the grafting behavior of diazonium salts and to establish the optimal conditions for the covalent functionalization of the PPYfilms, we have compared the grafting behavior of four p-substituted phenyldiazonium salts: p-nitrophenyl diazonium tetrafluorob-orate (PNBDBF4

−), p-tolyl diazonium tetrafluoroborate (TDBF4−), p-fluorophenyl diazonium tetrafluoroborate (FPDBF4

−) and4-diazo-N,N-dimethylaniline tetrafluoroborate (DDMABF4

−). The selection of the molecules to be grafted was done both for theirelectroactivity after grafting and the contrasted electronegativityof the substituents at the benzene ring. For all investigated diazoniumsalts, a linear relationship between their reduction potential at the PPY electrodes and Hammett substituent constants was obtained,suggesting a similar electrochemical reaction mechanism. The functionalization of the polypyrrole films has been evaluated usingelectrochemical methods like EQCM, CV and EIS. The presence at the polymeric films surface of the functional groups introducedby the electrochemical reduction of diazonium salts was evidenced also by XPS. This approach enables new functionalities on PPYthat could otherwise not withstand the polymerization conditions.© 2014 The Electrochemical Society. [DOI: 10.1149/2.0871412jes] All rights reserved.

Manuscript submitted June 26, 2014; revised manuscript received August 21, 2014. Published September 9, 2014.

Conducting polymeric systems are still at the foreground of re-search activity in electrochemistry due to the wide range of promisingapplications of these compounds in the fields of energy storage,1,2 so-lar cells,3 electrochromic displays,4 corrosion protection,5,6 chemicalsensors or biosensors.7–9 Among conducting polymers, polypyrrole(PPY) is by far the most extensively studied, since it can be easilydeposited from aqueous and non-aqueous media on many types ofsubstrates, and is well-conducting and stable. Conducting polymerscan be synthesized either chemically or electrochemically. The elec-trochemical approach for synthesis of electroactive/conductive filmsis very versatile and provides a facile way to vary the film proper-ties by simply modifying the electrolysis conditions in a controlledmanner.

For the development of new functional conducting polymers manychallenges have to be met, including the synthesis of monomersposessing the required functionality and establishment of polymer-ization conditions compatible with the targeted function. The usualway to produce functionalized conductive polymers implies the syn-thesis of a monomer substituted with functional groups of interestfollowed by its homopolymerization or co-polymerization. However,inductive and steric effects could make the monomer difficult or im-possible to polymerize.10 The functionalized conductive polymers canbe obtained, also, as films at an electrode surface by carrying out theelectropolymerization of the monomer in a medium containing an an-ionic species as the supporting electrolyte.11 Such electrode materialsare less stable than those in which the substituents are covalently at-tached to the polymeric skeleton, owing to some exchange of dopantanions with counter-anions of the electrolyte. A way to overcome suchlimitations and, in the same time, a less explored route to produce thesematerials involves post modification of already synthesized and wellcharacterized, conducting polymers by means of covalent bonding ofthe functional group to the polymer backbone.12

The methods for attachment of thin organic layers to conductingsubstrates via covalent bonds directly between the modifier and sur-face atoms of the bulk material are receiving increasing attention. Themost studied reaction is the reduction of an aryldiazonium cation to

zE-mail: luisa_pilan@yahoo.com

yield an aryl radical which binds to the surface.13–16 The stability ofthe covalent attachment and the simplicity of the procedure are at-tractive features of this method. Most commonly, the reactions areinitiated electrochemically, a radical being generated in solution atthe electrode (substrate) surface by reduction of the modifier. Theradical appears to couple to the surface with formation of a covalentbond. The diazonium chemistry gives us the capability of tailoringthe functionalized materials by changing the addends. Previous workin our group has shown that aryl diazonium treatment of single-wallcarbon nanotubes (SWCNTs)17,18 produces functionalized SWCNTs.Likewise, a large variety of materials have been also modified by thismethod: carbon (GC, HOPG, pyrolized photoresists, pyrolized Teflon,carbon fibers, carbon blacks, diamond), semiconductors (Si,GaAs),and metals (including noble metals such as Au and Pt).13

In this context, the electrochemical reduction of aryl diazoniumsalts represents a promising route for obtaining functionalized con-ducting polymers. Based on our knowledge, the electrochemical re-duction of aryl diazonium salts modification technique is employedfor the first time as a method for functionalization of PPY to ob-tain conductive polymer surfaces with various functionalities. A con-ductive PPY film is first deposited at the electrode surface. Theconductivity of the film enables post polymerization functionaliza-tion of the conducting polymer using diazonium salts electroreduc-tion. Four p-substituted phenyldiazonium salts: p-nitrophenyl dia-zonium tetrafluoroborate (NBDBF4

−), p-tolyl diazonium tetrafluo-roborate (TDBF4

−), and, p-fluorophenyl diazonium tetrafluorobo-rate (FPDBF4

−) and N,N-dimethyl-p-phenilenediamine diazoniumtetrafluoroborate (DMPDBF4

−) have been chosen to be attached andtested on the films. The selection of the molecules to be grafted wasdone both for their electroactivity after grafting and the contrastedelectronegativity (i.e, electron-withdrawing nitro group, electron-donating aminophenyl group) of the substituents at the benzene ring.The influence of the substituent could be deduced in our previous stud-ies at GC and SWCNTs electrodes from the electrochemical behaviorof diazonium salts and the electron withdrawing substituents werefound to graft faster and more easily than the more electron donatingsubstituent.16–18 The functionalization of the PPY film was evalu-ated using electrochemical methods (Electrochemical Quartz CrystalMicrobalance (EQCM) measurements, cyclic voltammetry (CV) and

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N NBF4

NO2

N NBF4

CH3

N NBF4

F

N NBF4

N(CH3)2

Scheme 1. Diazonium salts employed for electrochemical graftingonto polypyrrole electrodes.

electrochemical impedance spectroscopy (EIS)), as well as X-ray pho-toelectron spectroscopy (XPS).

This method of covalent immobilization of different functionalgroups on PPY film surface offers the advantages of preserving theelectrical conductivity and mechanical strength of the film. This ap-proach enables new functionalities on PPY that could otherwise notwithstand the polymerization conditions. Electrochemical reductionof aryl diazonium salts on PPY films shows also the potential for beingan important platform for biological devices and sensors.

Experimental

Reagents, solutions, and instrumentation.— Pyrrole was pur-chased from Aldrich. Anhydrous acetonitrile (99.8%, noted ACN),tetra-n-butylammonium tetrafluoroborate (99%, noted nBu4N+BF4

−)were obtained from Aldrich and were used as received.

Scheme 1 shows the four studied compounds. These diazoniumsalts, p-nitrophenyl diazonium tetrafluoroborate (PNBDBF4

−), p-tolyl diazonium tetrafluoroborate (TDBF4

−), p-fluorophenyl diazo-nium tetrafluoroborate (FPDBF4

−) and 4-diazo-N,N-dimethylanilinetetrafluoroborate (DDMABF4

−) were prepared by standard diazona-tion of the corresponding amines with NaNO2 in acidic medium asdescribed in our previous studies.17,18

All of the electrochemical experiments were performed in an ar-gon atmosphere at room temperature and controlled by a potentio-stat/galvanostat (Eco-Chemie, Autolab, 128N). A three-electrode con-figuration consisting of bare or modified Pt, glassy carbon (GC) or Au(Metrohm, disks, diameter 2 mm) as working electrodes, Ag/10 mMAgNO3, 0.1 M nBu4N+BF4

− as reference electrodes and a Pt wireas counter electrode was used. EIS measurements were carried outin 0.1M Bu4N+BF4

−/ACN, in the frequency range of 10−1 - 105 Hzat the constant potential of 0.0 V for an amplitude of the sinusoidalsignal of 5 mV.

EQCM measurements.— The EQCM measurements were per-formed with the Autolab 128N EQCM module fitted with a 6 MHzcrystal oscillator, in a three-electrode electrochemical cell, in whicha gold wire served as a counter-electrode, a home-made Ag/10 mMAgNO3, 0.1 M nBu4N+BF4

− as reference electrode and a gold-coatedquartz crystal electrode modified with PPY films as the working elec-trode. The working electrode had an area of 0.196 cm2.The resonantfrequency depends on the thickness of the crystal. As mass is depositedon the surface of the crystal, the thickness increases; consequently thefrequency of oscillation decreases from the initial value. With somesimplifying assumptions, this frequency change can be quantified andcorrelated precisely to the mass change using Sauerbrey’s equation:

� f = − 2 f 20

A√

ρqμq· �m

where �f is the change in frequency, in Hz, f0 is the nominal resonantfrequency of the crystal (6 MHz), �m is the change in mass, in g cm−2,A is the area of the crystal in cm2, ρq is the density of quartz, in g cm−3

and μq is the is the shear modulus of quartz, in g cm−1 · s−2. For a 6MHz crystal, the same equation can be reduced to: −� f = �m · C f ,where Cf is 0.0815 Hz ng−1 cm2.

Preparation of the modified electrodes.— Prior to electrode modi-fication, the electrode surface was polished in 0.05 μm alumina slurryon a microcloth pad. After polishing the electrode was thoroughlyrinsed with water and sonicated for 5 min in water and, then, in ACN.

The PPY electrodes used for functionalization were different thick-ness (30 nm, 60 nm, 90 nm and 120 nm) PPY films doped with BF4

−

anion, obtained galvanostatically (j = 0.4 mA cm−2) at a Pt elec-trode surface. For every measurement a freshly obtained PPY filmthat was first cycled in 0.1 M nBu4N+BF4

− supporting electrolyteACN solutions was used.

The functionalization of the PPY electrodes was carried out inACN solutions containing different concentrations of diazonium saltsand 0.1 M nBu4N+BF4

− by CV in the cathodic range at a scanrate of 0.1 Vs−1 or under controlled-potential electrolysis at im-posed potentials more cathodic than diazonium salt electroreductionpeak. For comparison we performed also the same electrochemicalgrafting at GC, Pt and Au electrodes. The grafted electrode wasthen removed from the grafting solution and rinsed with large vol-umes of ACN. Functionalized PPY were denoted PNBPPY/BF4

−,TPPY/BF4

−, FPPY/BF4− and DMAPPY/BF4

−, corresponding to thereactions with NBDBF4

−, TDBF4−, FPDBF4

− and DDMABF4− dia-

zonium salts, respectively.

Results and Discussion

The electrochemical behavior of the studied diazonium salts wascomparatively investigated by CV at the PPY/BF4

− (60 nm), GC, Ptand Au electrodes. The cyclic voltammograms (CVs) obtained at GCand PPY electrodes are comparatively presented in Figure 1, whilethe electrochemical parameters obtained from these experiments forall studied electrodes can be compared from Table I.

The CVs recorded in 1 mM solutions of the PNBDBF4− salt (in

0.1 M nBu4N+BF4−, ACN) at different substrate electrodes showed

an irreversible cathodic process assigned to the reduction of the dia-zonium species at a potential in the range (−0.3; +0.2)V, dependingon the substrate electrode nature (Table I). It can be seen that afterthe first scan, the modified GC, Pt and Au electrodes present a block-ing effect toward the reduction of diazonium in solution because, inthis potential range, no electroactive relay continues to exist for thegrafted molecules.16 Performing several scans leads to a more diffi-cult electron transfer induced by thicker layers formed on the electrodesurface. In the case of the PPY film, in the first voltammetric cycle,a less intense reduction peak (at +0.16 V) occurs due to reductionof the diazonium salt, but also the large peaks corresponding to re-duction and oxidation of the polymer can be observed. In the nextcycle, only the reduction and oxidation peaks of the polymer can beobserved, the latter presenting an anodic shift of approximately 0.1 Vafter each cycle. The PPY cycling behavior in PNBDBF4

− solutionis better shown in Figure 2, where the corresponding voltammogramsof one cycle (a) and that of 10 cycles (b) are compared. Thus, in thecase of the PPY electrode, the phenomenon of the surface “blocking”did not appear even after 10 cycles. It was obvious, instead, the shiftof the oxidation peak of the polymer toward anodic potentials, whileits reduction peak in the cathodic direction, indicating that the func-tionalized polymer retains its electrochemical activity, even thoughoxidation and reduction occur more difficult in this case.

The studies performed for the TDBF4−, FPDBF4

− andDMPDBF4

− salts showed also, for the first cycle, the characteristicpeaks of the diazonium salt electroreduction. The difference consistedin the fact that the phenomenon of surface “blocking” did not appearat GC and the metallic electrodes. In fact, because of the electronictransfer, diazonium reduction is still observed after the first scan,but the signal is displaced toward more negative values. This maydenote a lower reactivity of the radical with the electrode surface

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Figure 1. The electrochemical reduction of (a) PNBDBF4−, (b) TDBF4

−, (c) DMPBF4− and (d) FDBF4

− (1 mM in 0.1 M nBu4N+BF4−/ACN) at PPY/BF4

−(60 nm) and GC electrodes, by CV in the potential range of (+ 0.4; −1.0) V, at a scan rate of 0.1 Vs−1, for 3 cycles.

than for PNBDBF4−, that can be attributed to a radical stabilization

and solution diffusion with further coupling side reactions. Perform-ing several scans leads to a more difficult electron transfer inducedby thicker layers formed on the electrode surface. Again, for thePPY electrodes, the reduction peak of diazonium salts, as well as

the reduction and oxidation peaks of the polymer, appear in the firstcycle.

The potential value of the diazonium reduction peak suggests thatthe reduction occurs most easily on conductive polymer electrodes,the peak potentials being shifted in the cathodic direction in the order

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Table I. The electrochemical parameters obtained for theelectroreduction of diazonium salts (1 mM in 0.1 MnBu4N+BF4

−/ACN) by CV (1st cycle, potential range of (+0.4 ÷−1.0) V, scan rate of 0.1 V s−1) at the PPY/BF4

− (60 nm), GC, Ptand Au electrodes.

Diazonium salt Electrode Ep,c mV jp,c μA/cm2 Qp,c μC/cm2

PNBDBF4− PPY +160 - -

GC −290 107.8 379Pt −140 138.8 465Au −110 188.0 548

TDBF4− PPY −90 - -

GC −590 121.3 419Pt −450 151.9 532Au −330 175.9 592

FPDBF4− PPY −50 - -

GC −340 58.5 246Pt −390 46.4 142Au −180 79.1 212

DDMADBF4− PPY −430 235.8 615

GC −710 102.5 328Pt −820 128.0 246Au −700 162.1 446

Ep,c(PPY) > Ep,c(Au) > Ep,c(Pt) > Ep,c(GC). For the 2nd and the 3rd

cycles, the redox potentials of the PPY films stabilizes at about −0.4 Vfor reduction and at about 0.3 V for oxidation. In the case of TDBF4

−

and FPDBF4− salts, the anodic peak current densities present a small

increase with the number of cycles (Figure 1b and 1d), while in thecase of DMPDBF4

− salt (Figure 1c), starting with the 2nd cycle thevoltammograms become reproducible for the next several cycles.

We can conclude that the electroreduction potential of the aryldiazonium salts is dependent on the nature of existing substituentson the benzene ring. In the case of electron-withdrawing substituentssuch as -NO2 group, as the electron density on the benzene ringis reduced, the reduction will take place more easily than for thesubstituents having a significant electron-donating effect (-N(CH3)2

and -F). The dependency of the diazonium salts reduction potentialat PPY electrodes on the Hammett constants can be expressed byHammett-Zuman equation:19

E Xp = ρ ·σ+ E H

p , where E Xp is the reduction peak potential for the

diazonium salt containing the X substituent, E Hp is the reduction peak

Figure 3. The relationship between the reduction peak potential and the Ham-mett substituent constant for the diazonium salts electroreduction at PPYelectrodes.

potential for the unsubstituted diazonium salt, while ρ and σ are theHammett substituent constant and the reaction constant, respectively.

Figure 3 shows, for all investigated diazonium salts, a linear de-pendency between the reduction peak potential and the Hammettsubstituent constant, suggesting a similar electrochemical reactionmechanism.

EQCM study.— The current density and the synchronous resonantfrequency variation of the PPY/BF4

− film during the first 5 potentio-dynamic cycles obtained in the pure electrolyte solution and in thepresence of 1 mM PNBDBF4

− salt are presented in the Figures 4aand 4b, respectively. The same recordings for the PNBPPY/BF4

− filmin the pure electrolyte solution can be seen in the Figure 4c.

Figure 2. The electrochemical reduction of PNBDBF4− salt (1 mM, in 0.1 M nBu4N+BF4

−/ACN) at PPY/BF4− electrodes (60 nm) in the potential domain of

(+ 0.4; −1.0) V, for a scan rate of 0.1 Vs−1, (a) one cycle; (b) 1 to 10 cycles.

Journal of The Electrochemical Society, 161 (12) G103-G113 (2014) G107

Figure 4. In situ CV–EQCM results (5 cycles, (+ 0.4 ÷ 1.0)V, 0.1 Vs−1)for (a) PPY/BF4

− film (60 nm) in 0.1M Bu4N+BF4−/ACN; (b) PPY/BF4

−film (60 nm) in 1mM PNBDBF4

− + 0.1 M Bu4N+BF4−/ACN and (c)

PNBPPY/BF4− film in 0.1 M Bu4N+BF4

−/ACN. Dotted (red) line corre-sponds to frequency (mass) change and the solid (blue) line to the currentdensity.

All the measurements obtained in a pure electrolyte solutionshowed reasonably good reversibility, since the trajectories of �fvs. E curves were almost the same. In the case of PPY / BF4

− filmcycling in 1 mM PNBDBF4

− in 0.1M Bu4N+BF4−/ACN, obvious

mass alteration was observed.With the Sauerbrey equation20 shown in Eq. (1), the resonant

frequency variation obtained from EQCM measurements and pre-sented in Figure 4a could be transferred into the mass changeof this film electrode. Thus, Figure 5 shows the mass change ofPPY/BF4

− during the first potentiodynamic cycle in a pure electrolytesolution.

The mass change was divided into four stages (1÷4) and, there-fore, a better understanding of the insertion and de-insertion condi-tions of the ions could be obtained for different potential windows.In the first potential window (potential ranging from 0.5 to −0.5 V),the mass change of the PPY/ BF4

− film decreased significantly (�m= −340 ng cm−2) due to dopant anions moving out the film. At

further decrease of the potential from −0.5 to –1 V (2nd potential win-dow), the increase in mass (+460 ng cm−2) can be explained by theBu4N+cations insertion from the electrolyte. Simultaneously anionscan be extracted out of the reduced film to neutralize the charge, butthe cations play the predominant role for the mass change of the film,Bu4N+ having a higher molar mass than that of BF4

−. The partici-pation of bulky cations such as Bu4N+ at redox transformations inpolypyrrole was evidenced experimentally only recently.20 Most ofthe previous studies have shown that the insertion can occur only forsmall cations (eg, Na +) and, generally, in aqueous electrolyte, con-sidering that the ion transport number of Bu4N+ in PPY is practicallyzero (at least for polymer films having a thickness greater than 1 μm).After reaching the switching potential, by sweeping the potential inthe anodic direction, the polymeric film is oxidized. In the 3rd poten-tial range (from −1 V to −0.15 V) oxidation is accompanied by amass decrease (−415 ng cm−2) due to the cations expulsion, whilein the 4th potential range (from −0.15 V to +0.4 V) the insertionof the anions as dopants occurs, so that a mass increase takes place(285 ng cm−2).

When cycling the PPY/BF4− film in the electrolyte solution

containing 1mM PNBDBF4−, the mass change can provide in-

formation about the mechanism of the polypyrrole functional-ization by reduction of the diazonium salts. As in the previ-ous case, the mass change was divided into four stages (1÷4)(Figure 6).

The potential is scanned first in the cathodic direction, so thediazonium salt electroreduction (at + 0.16 V) and the reduction ofpolymer take place at the same time. It is interesting that in the 1st

potential window, from +0.4 V to −0.35 V, the mass change ofthe PPY/ BF4

− film is decreasing (�m = −185 ng cm−2), and isfollowed by an increase (215 ng cm−2) in the 2nd potential range (from−0.35 V to −1 V), most likely due to expulsion of the dopant anionsand the cations insertion. After reaching the switching potential, amass decrease (−70 ng/cm2) is observed in the 3rd potential rangeand, again, from −0.25 V to + 0.4 V a significant mass increase(440 ng cm−2) takes place. The difference between the initial andthe final mass of the PPY film is of 440 ng cm−2. In contrast to thereduction of PNBDBF4

− at GC, Au or Pt electrodes, where the EQCMexperiments have shown that, in the potential window defined by thepeak reduction, a monolayer grafting at electrode surface occurs,21 inthe case of PPY more than 50% of the mass increase due to the graftingprocess takes place during the oxidation cycling of the polymer, in the4th potential range (−0.25 ÷+0.4) V.

Considering these results, it can be assumed that the reduction ofdiazonium salts at the PPY electrode occurs in the diffusion layer andthe formed aryl radicals react with the polymer surface mainly whenit is in its oxidized form.

These results can be confirmed by previous studies on the potentialof zero charge (PZC) of PPY/ClO4

− electrodes.22 It was demonstratedby dynamic contact angle measurements that the PZC for PPY hasa value close to the redox potential (Ep) of PPY (more exactly, Ep

+ 0.1 V), i.e. −0.28 V vs. Ag / Ag+ 0.01M in our case. It is wellknown that PZC affects the adsorption on the electrode of ionic speciessuch as the diazonium salts considered in our study. At more posi-tive potentials, the surface has an excess of positive charge, so theadsorption of cationic species is hindered. It should be taken into ac-count, also, the conformational changes that occur concurrently withthe redox changes in the PPY. For the reduced form of the poly-mer, the spaces between the polymer chains are smaller, so that amore compact structure of the polymer does not allow an efficientgrafting.

For the next voltammetric cycles, the difference between the initialand the final mass of the PPY film decreases, reaching 90 ng cm−2

at the 5th cycle (Figure 7a). However, between +0.4 and −0.5 V amore pronounced mass decrease (−435 ng cm−2) is observed, beingcompensated at the potential sweep in the anodic direction by anincrease of 470 ng cm−2. This may indicate a loss of a part of thegrafted layer which is not covalently bonded to the polymer duringthe potential cycling.

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Figure 5. The in situ mass change of PPY/BF4−

film during the cycling in 0.1 M Bu4N+BF4−/ACN

(1st cycle, potential range of (+ 0.4 ÷ 1.0)V, 0.1Vs−1).

Figure 6. In situ mass change of PPY/BF4− film during the cycling in 1mM PNBDBF4 + 0.1 M Bu4N+BF4

−/ACN (1st cycle, potential range of (+ 0.4 ÷ 1.0)V,0.1 Vs−1).

Figure 7. In situ mass change of (a) PPY/BF4− film in 1mM PNBDBF4 + 0.1 M Bu4N+BF4

−/ACN and (b) PNBPPY/BF4− film in 0.1 M Bu4N+BF4

−/ACNduring CV (5th cycle, potential range of (+ 0.4 ÷ 1.0)V, 0.1 Vs−1).

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The behavior of the PNBPPY/BF4− in a pure electrolyte solution

is apparently similar to PPY/BF4− (Figure 7b), the difference being

smaller values for the mass decrease between +0.4 and −0.25 V(−210 ng cm−2) and the mass increase between −0.25 and –1 V (+205ng cm−2). After the potential switching a mass decrease between −1and −0.1 V (−175 ng cm−2) is initially recorded, followed by a massincrease between −0.1 and +0.4 V (200 ng cm−2). These values seemto indicate a lower doping degree of the functionalized PPY.

Cyclic voltammetry study.— The CV characterization in a pureelectrolyte solution of the polymeric films before and after the func-tionalization in 1mM PNBDBF4

− is presented in Figure 8. One canobserve a similar behavior of the polymeric films with different de-grees of functionalization, being evidenced a shift toward anodic po-tentials of the oxidation and reduction peaks for PNBPPY/BF4

− incomparison with PPY/BF4

− electrodes. In all cases, during the firstten voltammograms, the peaks current densities decrease and, simul-taneously, they shift to lower potential values until a reproduciblevoltammogram is obtained. This behavior is more evident as the func-tionalization is performed at a higher concentration of the diazoniumsalt.

The occurrence of the oxidation peak of the functionalized polypyr-role at a more positive potential and its shift in the cathodic directionafter the cycling in the electrolyte solution shows that a part of thegrafted layer, that makes difficult its oxidation, is removed duringcycling.

The CVs of the polymeric films before and after functionalization,having different thicknesses (30 ÷ 120 nm) are similar, showing onlyan increase in the difference between the peak current densities be-fore and after functionalization with the increasing thickness of thepolymer film (not shown).

Electrochemistry is a useful characterization choice when a re-dox active functional group is attached to the surface, the presenceof the modifier can be easily evidenced using cyclic voltammetry. Inthe case of a p-nitrophenyl grafted GC electrode by diazonium saltelectroreduction, the CV measurements, in the electrolyte thoroughlydegassed, show the appearance of a reduction peak at −1.24 V andsubsequent oxidation peak at −1.08 V (Figure 9a). These can be at-tributed to a quasi-reversible couple (�Ep = 0.16 V) correspondingto NO2 group reduction to its radical anion.23 The CV of the function-alized PPY electrode obtained in similar conditions (functionalizationby CV in the potential range (+0.4 ÷ −1.0) V, 0.1 Vs−1, 3 cycles,1 mM PNBDBF4

− solution) shows, besides the reduction and theoxidation characteristic peaks of the polymer, the NO2 group reduc-tion peak at −1.26 V (Figure 9b). The oxidation peak of the NO2

radical anion can be observed as a “shoulder”, being masked by thepolypyrrole capacitive current.

EIS study.— EIS has been used as a method of investigation toprovide additional information about the nature of the changes re-sulting from the reduction of diazonium salts on PPY electrodes. Theimpedance spectra of the PPY/BF4

− films in 0.1M nBu4N+BF4−/ACN

have been recorded before and after functionalization, as well as afterthe cycling in the electrolyte solution (10 cycles in the potential range(+0.4 ÷ −1.0) V, 0.1 Vs−1). Figure 10 presents the Nyquist diagramsthroughout the investigated frequency range, on the right side beinghighlighted the corresponding high frequency region.

One can see that the spectra have an appearance typical for theconducting polymers, with a semicircle in the high frequency rangecharacteristic to the interfacial charge transfer mechanisms and be-come purely capacitive at low frequencies due to the phenomenonof saturation of the electric charge. For the intermediate frequenciesrange, the straight line characteristic to diffusion processes spans alimited range due to the reduced thickness of the investigated poly-mer films. From the charge transfer resistance values (RCT) obtainedfrom the impedance spectra of the PPY/BF4

− films with different de-grees of functionalization (Table II), one can see an increase of theRCT upon functionalization. This can be explained by the fact thatfor the functionalized PPY the redox processes are prevented to a

Figure 8. CVs (0.1 Vs−1) of the PPY/BF4− films (60 nm) in 0.1M

nBu4N+BF4−/ACN before (dotted/blue line) and after (solid/red line) func-

tionalization by electroreduction of PNBDBF4− salt (1 mM) by CV for (a) 1

cycle; (b) 5 cycles; (c) 10 cycles in the potential range (+0.4 ÷ −1.0) V, 0.1Vs−1.

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Figure 9. The NO2 group signature evidencedin the CV experiments (0.1 Vs−1) in 0.1 MnBu4N+BF4

−/ACN for (a) GC electrode and (b)PNBPPY/BF4

− films (60 nm) after functionaliza-tion by CV (1mM PNBDBF4

−, potential range(+0.4 ÷ −1.0) V, 0.1 Vs−1, 3 cycles).

Figure 10. The EIS spectra of the PPY/BF4−

films (60 nm) in 0.1 M nBu4N+BF4−/ACN be-

fore (●), after the functionalization (◦) and af-ter 10 cycles in 0.1M nBu4N+BF4

−/ACN(�).The PPY/BF4

− films functionalization wasperformed by PNBDBF4

− (1 mM) electrore-duction by CV for (a) 1 cycle; (b) 5 cycles;(c) 10 cycles, in the potential range (+0.4 ÷−1.0) V, 0.1 Vs−1.

Journal of The Electrochemical Society, 161 (12) G103-G113 (2014) G111

Table II. The charge transfer resistance values (RCT) obtained from the impedance spectra of the PPY/BF4− films with different degrees of

functionalization.

RCT (�) 1 cycle RCT (�) 5 cycles RCT (�) 10 cyclesof functionalization of functionalization of functionalization

PPY/BF4− films 41 50 48

PNBPPY/BF4− films 690 13800 11800

PNBPPY/BF4− films (after 10 cycles in the

supporting electrolyte)180 280 460

certain extent, probably due to a surface “blocking effect”, similar towhat was observed for the metallic or GC electrodes functionalizedby the diazonium salts electroreduction.24 It should be noted that RCT

value increases after each cycle of functionalization until 5 cyclesand remains at approximately same value if the number of cyclesis further increased, suggesting that a saturation limit is attained. Adecrease of the RCT value of the PNBPPY/BF4

− electrodes after 10cycles in the supporting electrolyte solution was also observed. Thismay indicate the removal of a part of the grafted layer depositedon the polymer surface, and it is in agreement with the CV results.However, the RCT values of the PNBPPY/BF4

− electrodes after cy-cling are one order of magnitude higher than for the PPY/BF4

− films,suggesting that the grafted groups are still present on the polymersurface.

XPS study.— X-ray photoelectron spectroscopy (XPS) is a quali-tative and (semi)quantitative spectroscopic technique used for deter-mining the elemental composition of materials at their surface. In thecase of PPY film, due to its insolubility, XPS is one of the few methodsthat provide structural information.

In the N 1s high resolution spectra of the functionalized PPY byPNBDBF4

− electroreduction (Figure 11a) two peaks can be observed,one at 406.5 eV that can be attributed uniquely to N atoms of the -NO2

groups.23 From the second peak, four secondary peaks were obtainedby deconvolution, corresponding to the four types of N atom of thedoped polymer structure. Based on previous studies reported in theliterature,25 they were attributed as follows: the peak at 400.2 eV tothe amine nitrogen of the -NH- group, the peak at the 399.5 eV tothe imine nitrogen (-N = ), and the two peaks at 401.1 and 402.3to the positively charged nitrogen polaron (-NH+-) and bipolaron(-NH+ =), respectively.

Upon functionalization, a clear increase in the intensity of the peakcorresponding to the positively charged nitrogen appears, comparedwith unfunctionalized polypyrrole.26,27 From the ratio of peak areaat 406.5 eV (nitrophenyl nitrogen) and the areas of the other peaks(nitrogen in polypyrrole), we can conclude that after PPY functional-ization by CV, a 4-nitrophenyl moiety is inserted to 1.25 pyrrole rings(after 1 voltammetric cycle) or to 1.1 pyrrole rings (after 5 CVs). It isnot ruled out any possibility of the formation of multilayers of substi-tuted poly(phenylene) on the surface of the PPY,28 in which case theactual degree of functionalization of the pyrrole rings is lower. Also,the spectrum of the functionalized PPY that was subjected to voltam-metric cycling in the electrolyte solution (Figure 11b) shows that afterthe cycling a major part of the nitrophenyl groups remain attached tothe surface of the polymer (a 4-nitrophenyl moiety is attached to 1.2pyrrole rings after 10 cycles).

In the case of PPY/ BF4− films functionalized by FPDBF4

− elec-troreduction, the high-resolution F 1s spectrum of the functionalizedPPY (Figure 12a) shows one peak at the binding energy of 695.6eV. This can be attributed to the fluorine atoms linked to the sp2 hy-bridized C of the 4-fluorophenyl groups. From the F/N ratio of thepeaks areas of the survey spectrum and considering the correctionfactors, it can be concluded that the functionalized polymer containsa 4-fluorophenyl moiety attached to 1.04 pyrrole rings. The absenceof the peaks corresponding to boron and the very small amount offluorine in the XPS spectra can be explained by the PPY/ BF4

− film

dedoping, upon storage, as a result of the slow decomposition at roomtemperature of the BF4

− ion in BF3 and F−.29

In the case of the functionalized PPY by TDBF4−electroreduction,

the relevant high-resolution XPS spectrum is C1s (Figure 12b). Fivesecondary peaks were resulted after deconvolution, but their allocationis complicated by the fact that the binding energies of different typesof carbon atoms have very close values.

The N1s high resolution spectra of the functionalized PPY/ BF4−

films by DDMABF4− electrochemical reduction (Figure 12c) shows

one peak. After deconvolution, three secondary peaks were obtained,but the degree of functionalization cannot be calculated based on thepeaks area ratio, the peak at 400.7 eV corresponding to both N atoms

Figure 11. High-resolution XPS N1s spectra of PNBPPY/BF4− films (60

nm, deposited on ITO substrates) functionalized by CV (1 mM PNBDBF4−,

5 cycles, potential range (+0.4 ÷ −1.0) V, 0.1 Vs−1). The functionalizedPPY films were characterized (a) before and (b) after 10 cycles in 0.1MnBu4N+BF4

−/ACN.

G112 Journal of The Electrochemical Society, 161 (12) G103-G113 (2014)

Figure 12. High-resolution XPS F1s spectrum of FPPY/BF4− (a), XPS C1s

spectrum of TPPY/BF4− (b) and XPS N1s spectrum of DMAPPY/BF4

− (c).The functionalized PPY/ BF4

− films (60 nm, deposited on ITO substrates)were obtained by electroreduction by CV (5 cycles, potential range (+0.4 ÷−1.0) V, 0.1 Vs−1) of 1mM diazonium salts.

of the N,N-dimethylaminophenyl and positively charged N atoms ofpolypyrrole.

Conclusions

The functionalization of PPY/BF4− films of different thicknesses

via electrochemical reduction of the aryl diazonium salts in a man-ner that is similar to the one employed for functionalization of other

conductive surfaces is reported. The aromatic diazonium salts elec-troreduction at PPY, GC, Au and Pt electrodes was comparativelystudied and a similar electrochemical behavior was evidenced in allcases. In the case of the PPY electrodes, the reduction peak of thediazonium salt, as well as the reduction and oxidation peaks of thepolymer are observed in the first voltammetric cycle. The compari-son of the diazonium reduction peak potentials at different electrodesled to the conclusion that the functionalization occurs most easily onconductive polymer electrodes, the reduction peak potentials beingshifted in the cathodic direction in the order Ep,c(PPY) > Ep,c(Au) >Ep,c(Pt) > Ep,c(GC).

In order to establish the optimal conditions for the covalent func-tionalization of the PPY films, four p-substituted phenyldiazoniumsalts obtained as tetrafluoroborates from p-nitroaniline, p- toluidine, p-fluoroaniline and N,N-dimethyl-p-phenilenediamine have been used.For all investigated diazonium salts, a linear relationship betweentheir reduction potential at the PPY electrodes and Hammett sub-stituent constants was obtained, suggesting a similar electrochemicalreaction mechanism.

The functionalization of the PPY film was evaluated using electro-chemical methods like EQCM, CV and EIS, as well as XPS technique.

EQCM studies performed for PNBDBF4− electroreduction at the

PPY electrodes showed that, unlike metallic or GC electrodes func-tionalization, where the grafting occurs in the potential range limitedby the diazonium reduction peak, in the PPY case over 50% of themass increase due to grafting is recorded during the oxidation of thepolymer, in the potential range (−0.25 ÷ +0.4) V. It can be con-cluded that the reduction of diazonium salts at the PPY electrodesoccurs in the diffusion layer and the resulting aryl radicals react withthe polymer surface mainly when it is the oxidized form.

The CV characterization of the functionalized PPY films in a pureelectrolyte solution showed a similar behavior for different degreesof functionalization. The appearance of oxidation peak of PPY atmore positive potentials after functionalization and its movement inthe cathodic direction after the electrolyte cycling indicate that a partof the grafted layer at the polymer surface, that impede its oxidation,is removed by repetitive cycling.

The charge transfer resistance values determined from EIS ofpolypyrrole films with different degrees of functionalization led tothe conclusion that, for the functionalized films, the redox processesof PPY are hindered to some extent, probably due to a “blocking ef-fect” of the surface similar to that observed in the case of the metallicor GC functionalized electrodes by reduction of diazonium salts.

The functionalized PPY films have been also characterized by XPSand the presence on their surface of the functional groups introducedby the electrochemical reduction of diazonium salts was evidenced.

The approach of post-polymerization functionalization of PPY bydiazonium salt electroreduction enables new functionalities on thepolymeric films that otherwise could not withstand the polymeriza-tion conditions. Electrochemical reduction of aryl diazonium salts onconducting polymer films shows the potential for being an importantplatform for biological devices and sensors.

Acknowledgments

The authors gratefully acknowledge the financial support of theRomanian National Authority for Scientific Research, CNCSUEFIS-CDI, under grant PN-II-ID-PCE-2011-3-0535.

The work has been funded by the Sectoral Operational Pro-gramme Human Resources Development 2007-2013 of the Min-istry of European Funds through the Financial Agreement POS-DRU/159/1.5/S/134398.

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