Study by EQCM on the voltammetric electrogeneration of poly(neutral red). The effect of the pH and...

Electrochimica Acta 48 (2003) 4039�/4048

www.elsevier.com/locate/electacta

Study by EQCM on the voltammetric electrogeneration ofpoly(neutral red). The effect of the pH and the nature of cations and

anions on the electrochemistry of the films

D. Benito a,b, C. Gabrielli b,1, J.J. Garcıa-Jareno a,*, M. Keddam b, H. Perrot b,F. Vicente a,1

a Department de Quımica-Fısica, Universitat de Valencia, Dr Moliner 50, 46100 Burjassot, Spainb Laboratoire de Physique des liquides et Electrochimie, UPR 15 du CNRS, Universite Pierre et Marie Curie, 4 Place Jussieu, tour 22, 75252 Paris,

France

Received 20 February 2003; received in revised form 5 May 2003; accepted 27 June 2003

Abstract

Generation of poly(neutral red) films has been studied by means of the simultaneous measurements of current�/potential and

mass�/potential curves during cyclic voltammetry (CV) experiments. It has been proved that the presence of molecular oxygen in the

solution increases the amount of polymer deposited on the electrode. Otherwise, using the mass/charge ratio it is possible to obtain

quantitative information about the electrodeposition by different procedures. It is observed that this ratio decreases when the

amount of polymer electrogenerated increases, except when the polymer is not reduced and oxidised after its electrogeneration. The

study of poly(neutral red) by CV and quartz crystal microbalance in solutions without monomer allows to discern between the role

of different charged species which are present in the solution: salt cations (Cs�, Na� and K�), salt anions (NO3�, Cl�, I� and

Br�) and hydrated protons that can compensate electrical charge within the film during electrochemical processes.

# 2003 Elsevier Ltd. All rights reserved.

Keywords: Poly(neutral red); EQCM; pH dependence; Charge transport; Counterions

1. Introduction

In the past few years, a new kind of electroactive

polymers derived from phenazine and phenothiazine

dyes has been synthesised [1�/7]. The monomer dyes

have proved to be good electronic mediators that can

undergo fast redox reactions with NADH [8�/11], but

the modified electrodes made up of adsorbed dyes have

a low long-term stability. The polymerisation of these

dyes increases the stability of the modified electrodes,

and the obtained polymer retains the electrochemical

* Corresponding author.

E-mail address: [email protected] (J.J. Garcıa-Jareno).1 ISE Member.

0013-4686/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0013-4686(03)00561-9

properties of the monomer. For this reason, several

electroanalytical sensors based on polyphenazines [12]

and polyphenothiazines [13�/17] with good sensitivities,

have been built. However, the properties and technolo-

gical usefulness of those films are not well known yet.

According to the proposed mechanism for the poly-

merisation of the analogous methylene blue dye [1], the

polymerisation of the neutral red (N8,N8,3-trimethyl-

phenazine-2,8-diamine) requires the formation of a dye

radical cation, which is the precursor of the polymer

[1,3,18].

IR spectra for the neutral red monomer and poly(neu-

tral red) (PNR) show that the phenazine ring remains in

the polymer structure and, therefore, the electroactivity

of polymer has also been attributed to these groups

[2,4,7]:

mailto:[email protected]

D. Benito et al. / Electrochimica Acta 48 (2003) 4039�/40484040

or

where A� represents an anion doping the film. This

reaction schema can be modified when pH is high

enough. Then, anions can participate as counterions

during the redox processes [19]:

This reaction schema leads one to suppose that

hydrogen ions but also anions play a very important

role in the electrochemical processes and during the

electron conduction through these films, since the

reduced phenazine group losses in part their p-electronic

character.

Voltammograms of PNR films generated on different

kind of electrodes showed a well-defined redox couple

near the hydrogen evolution potentials. These peak

potentials move to more negative values as the pH

increases while peak current strongly decreases [7,11,18].

Besides, none of these peaks are recorded when the pH

reaches values close to the pKa of the monomer [4,7,18].

In spite of the fact that voltammetric experiments give

important information, in most cases it is not enough to

discern between the role of anions or cations during the

charge compensation process. In recent years, several

techniques have been used to obtain this information

such as measuring the electrochemical impedance of the

coated electrode [20�/23], by probe beam deflection

(mirage effect) [24�/31] or the combined quartz crystal

microbalance and radiotracer methods [32�/36]. Also,

the use of the quartz crystal microbalance coupled with

other electrochemical techniques such as CV or EIS has

proved very adequate to obtain information about the

nature of the species that enter or leave the film during

the reduction or oxidation processes in polyaniline

[37,38], polypyrrole [39,40] or Prussian Blue films

[41,42] among others.

In PNR films, EQCM has been used to follow the

films growth during the electrogeneration reaction

[7,11,43]. The use of ac-electrogravimetry has allowed

to obtain information on the different species that can

participate during electrochemical processes at different

D. Benito et al. / Electrochimica Acta 48 (2003) 4039�/4048 4041

pH [19] In spite of the fact that this technique provides

more information it has some problems when changes of

mass are not very important. Thus it is only possible to

obtain valuable information at potentials near the redoxpotential of this system.

In this work, EQCM has been used for the study the

growth of the polymer and some particular aspects such

as the role of molecular oxygen present in the solution.

Then, EQCM and CV have been used for elucidating the

role of hydrogen ions and other ions during the

electrochemical processes in different aqueous media

and at different potentials and different pH.So as to obtain more information from electrochemi-

cal quartz crystal microbalance (EQCM) data, it is

useful to relate changes in mass with the electrical charge

passed. If during an interval of time the mass changes in

an amount Dm and during the same interval the

electrical charge passed is DQ, it is possible to obtain

an estimation of the mass/electrical charge ratio:

mass=electrical charge (g mol�1)�Dm

DQF;

where F is the Faraday constant.

This same analysis can be done punctually at any

potential by:

mass=electrical charge (g mol�1)�F

dm

dt

I�F

dm

dt

dQ

dt

�Fdm

dQ

These functions are negative for mass increase during

cathodic processes (dm�/0; dQB/0) or mass decreaseduring anodic processes (dmB/0; dQ�/0) and positive

for mass increase during anodic processes (dm�/0;

dQ�/0) and mass decrease during cathodic processes

(dmB/0; dQB/0).

That way, the function Fdm

dQgives information about

the species that participate in the charge compensation

reaction at different potentials. If only one species

participates, this function is equal toM

z(molecular

mass/charge of ion). When many species participate,

values for this function are a ponderated mean ofM

z:/

This paper aims at studying some factors that affect

the reaction of generation of polymer on the electrode

surface, such as the presence of oxygen in the solution orthe previous treatment. It is also studied the dependence

of voltammograms on the nature of cation or anion

present in the aqueous solution, the dependence on the

concentration of the salt present in the solution and the

dependence on the pH. All these studies were carried out

by measuring the current and mass changes simulta-

neously during voltammetric experiments.

2. Experimental

Cyclic voltammetry (CV) and electrochemical impe-

dance spectroscopy (EIS) studies have been carried out

by means of a typical three-electrode cell thermostatised

and the cell temperature was measured (2989/0.1 K).

Prior to any experiment, the solution was deareated bybubbling an inert gas for 5 min: N2 (from Air�/Liquide)

except in specified cases. A platinum mesh was used as

counter electrode and saturated calomel electrode (SCE)

was the reference electrode to which all potentials refer.

Potentiostat SOTELEM was used to perform electro-

chemical experiments.

Neutral red dye was supplied by Panreac (for micro-

scopy), and it was used as received. All other chemicalsas alkaline salts were of analytical reagent quality from

Panreac, except HCl and KOH (chemically pure). All

the solutions were prepared from previously bi-distilled

water.

The polymerisation of the neutral red was carried out

from a 5�/10�4 M neutral red, 0.1 M KNO3 and pH

buffered to 6.6 by means of a phosphate 0.05 M buffer

solution. PNR electrodes used for the pH and saltcations and anions studies were obtained by means of

CV between �/0.8 and �/0.8 V (eight cycles) in

deareated solutions (deposited mass :/15 mg cm�2).

The EQCM takes advantage of the resonance fre-

quency of a 6 MHz ‘‘AT-cut’’ quartz crystal (CQE,

Troyes, France) due to a minute mass change of one of

its electrodes exposed to the solution. This technique has

been used to follow and to study the mass change of thegold electrode during the PNR electrogeneration. The

experimental details of this technique were previously

described [44�/47]. An experimental value for the mass/

frequency coefficient sensitivity �/7.5 10�7 Hz g�1 cm2

was used for the experimental treatment of all the

gravimetric data; this coefficient was previously esti-

mated through copper electrodeposition. Mass changes

obtained from frequency changes can be affected byother ‘non-ideal’ effects such as shifts of frequency due

to the non-rigid character of the film or the swelling

effect during electrochemical reactions [48,49]. For

rubbery materials, the application of the Sauerbrey’s

equation [50] causes an amplification of the mass

estimated, while for glassy materials this effect proves

smaller. However, this error depends on the thickness of

the film, being negligible for thin films. In this work, it isdifficult to obtain a good estimation of the thickness of

the film. In studies of polypyrrole films by EQCM it is

considered that viscoelastic effects will be minimal for

Fig. 1. Voltammograms of deposition of poly(neutral red). The

solution was 5�/10�4 M neutral red, 0.1 M KNO3 and pH buffered

to 6.6 by means of a phosphate 0.05 M buffer. The solution was

previously deareated by bubbling N2 for 5 min before the deposition.

Scan rate was 5 mV s�1. Continuous line represents the current�/

potential curve and the dot curve represents the mass�/potential curve.


films of about 25 mg cm�2 [51]. In our work, PNR films

are about 15 mg cm�2. In spite of the fact that physical

properties of PNR films does not necessary match with

those of polypyrrole, it could be said that films are thin

enough to consider that frequency variations are mainly

due to mass effects. However, we prefer that mass

changes in this work are interpreted as apparent mass

changes and as a first approach to the true value of themass.

For the characterisation studies in different salt

solutions, the pH was buffered to 4.5 by means of

CH3COO�/CH3COOH 0.1 M buffer solutions. For the

pH dependence study, the solution was NaNO3 0.38 M,

and the pH was regulated by means of CH3COONa/

CH3COOH 0.1 M buffer solutions.

To obtain a good estimation for dm/dt from themass�/t curve, raw data is previously smoothed by

means of a numerical fast Fourier transform (FFT)

procedure.

3. Results

3.1. Study of generation of poly(neutral red) films

3.1.1. Cyclic voltammetry study

The generation of the poly(neutral red) requires the

formation of a neutral red radical cation, which is the

precursor of the polymer [1,3,18]. This polymer isusually obtained by immersing the working electrode

in a solution which contains the monomer and arriving

by CV at anodic potentials where the radical cation is

formed. In this section it is studied how some factors,

such as the redox reaction of the generated polymer, the

presence of oxygen in the solution and the previous

adsorption of monomer can affect the electrogenerationreaction. That way, deposits of PNR were obtained

from 5�/10�4 M monomer solutions by following four

different procedures.

1) By CV (eight cycles): starting at E�/0.0 V and going

to �/0.8 V (first reversing potential) and then from

E�/�/0.8 to �/0.8 V (second reversing potential).(E

(V)�/0.00/�/0.80/�/0.80/0.0). Scan rate was 5

mV s�1 and N2 was bubbled before the generation

(deareated solution). This experiment corresponds

to Fig. 1 where three peaks can be observed. Theoxidation peak that appears at positive potentials

(about �/0.7 V, peak I) corresponds to the forma-

tion of the radical cation dye [1]. The other anodic

peak (peak II) and the cathodic peak (peak III) can

be attributed to the oxidation and reduction pro-

cesses of the polymer [12]. That way, peak II and

peak III currents increase with the number of cycles,

at least for the first eight cycles, indicating a regularincrease in the amount of electroactive film depos-

ited on the electrode and peak I increases for the

three first cycles and after decreases.

2) By CV between �/0.2 V (start potential) and �/0.8 V

(reversing potential) at 5 mV s�1 (partial potential

scan) for eight cycles and bubbling N2 before the

generation (deareated solution). The shape of this

voltammogram is similar to peak I in Fig. 1 but thepeak current decreases from the first to the last cycle

and peak potential displaces to more positive

potentials.

3) By CV (0.00/�/0.80/�/0.80/0.0 V) at 5 mV s�1

(complete potential scan) for eight cycles, but with-

out bubbling N2 before the generation (non-

deareated solution). The shape of voltammetric

curves is similar to Fig. 1, but peaks are significantlygreater, i.e. the peak I current is more than three

times the peak I current corresponding to Fig. 1. It

is also found out that the current of peak I decreases

form the first to the last cycle. Besides, peaks II and

III increase faster when O2 is present in the solution

than they do when solutions are previously

deareated.

4) By CV (�/0.30/�/0.80/�/0.80/�/0.3 V) at 5 mV
s�1 (complete potential scan) for eight cycles, and
bubbling N2 before the generation (deareated solu-

tion). The shape of voltammetric curves is similar to

Fig. 1.

3.1.2. Quartz crystal microbalance analysis

These results are better analysed by means of EQCM

data. In Fig. 1 are plotted changes of mass that

accompany a cycle of voltammetry during the deposi-

Tab

le1

Mass

chan

ges

du

rin

gth

eel

ectr

od

epo

siti

on

of

PN

Rfi

lms

Pro

ced

ure

Cy

cle

1C

ycl

e2

Cy

cle

5

Dm

(mg

cm�

2)

DQ (m

Ccm

�2)

/FD

m DQ

(gm

ol�

1)

/Fd

m

dQ

(gm

ol�

1)

Dm

(mg

cm�

2)

DQ

(mC

cm�

2)

/FD

m DQ

(gm

ol�

1)

/Fd

m

dQ

(gm

ol�

1)

Dm

(mg

cm�

2)

DQ

(mC

cm�

2)

/FD

m DQ

(gm

ol�

1)

/Fd

m

dQ

(gm

ol�

1)

12

.03

09

06

28

02

.13

93

05

25

01

.53

86

03

82

3

21

.82

75

06

38

81

.21

76

06

68

20

.68

20

71

70

36

.11

36

50

43

90

3.9

13

35

02

85

02

.21

07

30

20

25

42

.84

63

05

88

12

.55

33

04

54

31

.64

65

03

32

1

Dm

an

dD

Qa

rem

easu

red

fro

mE�

/�/0

.4V

,d

uri

ng

the

an

od

icsc

an

,b

efo

rep

eak

I,a

nd

E�

/�/0

.4V

du

rin

gth

eca

tho

dic

sca

n,

aft

erp

eak

I.F

dm

dQ

isth

em

axim

um

va

lue

ob

tain

edfo

rth

isfu

nct

ion

in

thes

era

ng

eo

fp

ote

nti

als

(bet

wee

n0

.6a

nd

0.7

Va

ga

inst

SC

E).

Sca

nra

tew

as

alw

ay

s5

mV

s�1.


tion reaction for a film generated according to proce-

dure 1. The shape of this curve is similar when films are

generated by procedures 3 and 4. Table 1 collects mass

changes during the first cycles for the different kind of

deposits.

Changes of mass during the first cycle in presence of

oxygen (procedure 3, Dm�/6.1 mg cm�2) are three times

the changes of mass when deposits are generated in

absence of oxygen (procedure 1, Dm�/2.0 mg cm�2).

This result corroborates that deposits are fastest gener-

ated in presence of O2, which can favour the formation

of initiator species for the polymerisation reaction such

as it could be some radical species. However, from these

results it is not possible to identify, which species are

formed, but it is possible to say that these species favour

the electrogeneration of polymer.

The mass/charge ratio gives information on the nature

of species or on the mechanism of a chemical reaction

involving changes of mass on the electrode surface. In

spite of the fact that mass changes are different

depending on the procedure used to generate the

polymer, this maximum mass/charge ratio (/Fdm

dQ) is

very similar in all cases, about 80�/90 g mol�1 during

the first cycle, at potentials about 50 mV before the peak

potential (see Table 1). Taking into account that the

molecular mass of the monomer is 252 g mol�1 and

considering that all the change of mass is due to the

polymerisation reaction, three electrons per each mono-

mer that polymerises are needed. However, this calculus

is only a first approach since other factors such as the

polymer doping are not considered.

During next cycles, this ratio decreases significantly,

except for PNR films generated by procedure 2. A

possible explanation can be the fact that by procedure 1,

3 and 4, after generation, the polymer is reduced at

negative potentials and after oxidised (peaks III and II

in Fig. 1). However, this re-oxidation is only partial,

since electrical charge in peak II is smaller than electrical

charge in peak III. Then, it should be expected that the

oxidation of the polymer continue at these potentials

making that current recorded was not only due to the

monomer oxidation and deposition. The same analysis

can be made from the global FDm

DQfunction. The mass/

charge ratio reaches maximum values for the first cycle,

and then decreases, except for PNR generated following

procedure 2. Nevertheless, minimum values for this

function are always for PNR films generated by

procedure 3 since the molecular oxygen present in the

solution favours other oxidation reactions at these

potentials.

However, there are other factors that can affect these

values such as a possible adsorption of the monomer at

negative potentials [7,52]. In PNR films generated by


procedure 4, from E�/�/0.3 V (before peak III) and

E�/�/0.3 V (after peak II) mass significantly increases

during the first cycle (about 0.90 mg cm�2). During the

next cycles, and also in PNR films generated byprocedures 1 or 3, mass only increases slightly. This

result indicates that a non-reversible adsorption of

monomer on the electrode surface can take place, but

preferably, if no polymer has been generated before on

the electrode surface.

3.2. Dependence of voltammograms and EQCM data on

the pH

PNR films were studied in KNO3 solutions at

different pH as shown in Fig. 2a and b. It is obtained

that the peak potential displaces to more negative values

and peak current decreases when pH increases. This

trend has already been analysed and it has been foundthat peak potential follows a quasi-nernstian behaviour

on the pH [7,11].

During the voltammogram at pH 2.5 it is observed

that mass does not vary initially and then, at potentials

near the peak potential, mass increases (Fdm/dQ�/�/10

Fig. 2. Voltammogram (a) and mass changes (b) of a PNR film in

KNO3 0.38 M at pH 2.49, 3.46 and 4.70 solutions. The pH was

buffered by means of CH3COO�/CH3COOH 0.1 M buffer solutions.

Scan rate was 5 mV s�1.

g mol�1) that means that the main contribution to the

process of charge compensation is a cation that enters

the film. During the anodic peak, mass decreases (Fdm/

dQ�/�/9 g mol�1) indicating that a cation leaves thefilm.

At pH 3.5 the same behaviour is found out during the

cathodic peak, but in this case, changes of mass are less

pronounced. Initially, mass slightly decreases or keeps

constant and at the cathodic peak potential mass slightly

increases (Fdm/dQ�/�/2 g mol�1) and at the anodic

peak potential mass slightly decreases (Fdm/dQ�/�/3 g

mol�1).At pH 4.70 mass decreases during the cathodic peak

(Fdm/dQ�/�/20 g mol�1) and increases during the

anodic one (Fdm/dQ�/�/48 g mol�1).

These results indicate that the role of different species

during electrochemical processes depends on the pH. It

seems that at lower pH anions participate less than at

higher pH. Therefore, the influence of the salt electrolyte

will be studied at two pH 2.5�/2.8 were cations play themost important role and 4.5�/4.7 were anions play the

most important role.

3.3. Dependence on the electrolyte at acid pH 2.5�/2.6

PNR films were studied by CV around the system of

peaks corresponding to the oxidation and reduction ofthe polymer (peaks II and III in Fig. 1) in NaNO3,

NaCl, KCl and KI solutions. The shape of voltammetric

peaks and changes of mass�/potential curves are very

similar to that presented in Fig. 2a and b at pH 2.5.

Table 2 collects values for parameters obtained from

voltammograms and mass�/potential curves. In all cases,

mass increases during cathodic scan and decreases

during the anodic scan. However, it proves interestinganalysing differences obtained in the Fdm/dQ function

at peak potential or the global F/

Dm

DQratio in the different

aqueous salt electrolyte solutions studied. Firstly, mass

changes indicate that cations participate preferably

during the reduction reaction entering the film and

during the oxidation reaction leaving the film. However,

the mass/charge ratio is very similar in NaCl and KCl

solution during cathodic and anodic scans. The most

important differences are found out between KCl and

KNO3 or KI solutions and between NaCl and NaNO3

solutions.

3.4. Dependence on the electrolyte at less acid pH 4.5�/

4.7

3.4.1. Effect of cation

Voltammetry of PNR films and mass�/potential

curves were studied at this pH in different salt electrolyte

aqueous solutions (NaNO3, NaBr, NaCl, CsCl, KNO3,

Tab

le2

Pa

ram

eter

so

bta

ined

fro

mvo

lta

mm

og

ram

sa

nd

ma

ss� /p

ote

nti

al

curv

esa

tp

H2

.5�/

2.6

So

luti

on

pH

Ec

(V)

I c(m

Acm

�2)

/Fd

m

dQj c

(gm

ol�

1)

Dm

c(m

gcm

�2)

/Dm DQj c/(g

mo

l�1)

Ea

(V)

I a(m

Acm

�2)

/Fd

m

dQj a

(gm

ol�

1)

Dm

a(m

gcm

�2)

/Dm DQj a

(gm

ol�

1)

KN

O3

2.5

�/0

.31

�/1

03

�/1

00

.46

�/8

�/0

.24

80

�/9

�/0

.36

�/7

KC

l2

.6�

/0.3

5�

/84

�/4

0.0

9�

/2�

/0.2

74

5�

/3�

/0.0

6�

/2

KI

2.6

�/0

.37

�/8

1�

/80

.16

�/9

�/0

.23

43

�/8

�/0

.12

�/9

Na

NO

32

.6�

/0.3

3�

/10

6�

/70

.27

�/6

�/0

.26

38

�/5

�/0

.20

�/1

0

Na

Cl

2.6

�/0

.28

�/9

5�

/40

.08

�/2

�/0

.21

41

�/3

�/0

.09

�/4

Su

bsc

rip

tc

an

da

corr

esp

on

dto

cath

od

icp

roce

ssa

nd

an

od

icp

roce

ss,

resp

ecti

vel

y.

Ea

reth

ep

eak

po

ten

tia

l,I

the

pea

kcu

rren

t,D

mre

pre

sen

tsch

an

ges

of

ma

ssd

uri

ng

the

vo

lta

mm

etri

cp

eak

an

dD

Q

the

elec

tric

al

charg

een

clo

sed

invo

ltam

met

ric

pea

ks.

So

luti

on

sw

ere

alw

ays

aq

ueo

us

solu

tio

ns

of

salt

con

cen

trati

on

0.3

8M

.T

he

pH

wa

sb

uff

ered

by

mea

ns

of

CH

3C

OO

H/C

H3C

OO

�.


KI, and KCl) 0.38 M. Some representative voltammo-

grams and mass�/potential curves are plotted in Fig. 3a

and b.

In solutions containing CsCl or NaCl, it is recorded

that mass increases at the beginning of the first cycle,

0.032 mg in NaCl solutions and 0.068 mg in CsCl

solutions. These changes of mass are not accompanied

by an important flux of current and, therefore, they

could be attributed to an ion exchange between the inner

cation, K�, and the cation present in the solution Na�

or Cs�. It should be noted that PNR films were

generated in a medium where KNO3 was the support

electrolyte. The increase of mass for an exchange

between potassium and sodium ions is explained if it is

considered that the mass of the hydrated sodium cations

is larger than the mass of the hydrated potassium

cations. That way, it is possible to explain values for

the Fdm/dQ function in these range of potentials: �/250

g mol�1 in CsCl solutions and �/400 g mol�1 in NaCl

solutions, by the contribution to this function of

processes that imply a change of mass but not a net

electrical charge. During the next cycles this change of

mass at the start of the cathodic peak is not observed.

There are other possibilities such as a partial hydration

Fig. 3. Voltammogram (a) and mass changes (b) of a PNR film in

CsCl 0.38 M, NaCl 0.38 M and NaBr 0.38 M and NaNO3 0.38 M

solutions. The pH was buffered to 4.5 by means of CH3COO�/

CH3COOH 0.1 M buffer solutions. Scan rate was 5 mV s�1.

Tab

le3

Pa

ram

eter

so

bta

ined

fro

mvo

lta

mm

og

ram

sa

nd

ma

ss� /p

ote

nti

al

curv

esa

tp

H4

.5�/

4.7

Sa

ltp

HE

cI c

/Fd

m

dQj c

/Fd

m

dQj cb

/Fd

m

dQj ca

Dm

c/FD

m DQj c

Ea

I a/FD

m DQj a

Dm

a/FD

m DQj a

(V)

(mA

cm�

2)

(gm

ol�

1)

(gm

ol�

1)

(gm

ol�

1)

(mg

cm�

2)

(gm

ol�

1)

(V)

(mA

cm�

2)

(gm

ol�

1)

(mg

cm�

2)

(gm

ol�

1)

KN

O3

4.7

�/0

.52

�/6

22

01

92

0�

/0.4

91

3�

/0.3

44

44

80

.46

31

KC

l4

.5�

/0.5

2�

/11

83

27

�/0

.25

4�

/0.3

82

89

0.1

09

KI

4.5

�/0

.48

�/7

82

01

82

3�

/0.3

91

9�

/0.3

64

85

20

.43

52

Na

NO

34

.5�

/0.4

6�

/63

55

6�

/0.0

83

�/0

.36

36

70

.09

5

Na

Cl

4.5

�/0

.49

�/8

61

06

�/0

.06

2�

/0.3

94

75

0.0

63

Na

Br

4.5

�/0

.49

�/1

10

21

3�

/0.1

53

�/0

.37

37

80

.18

9

CsC

l4

.5�

/0.5

1�

/95

21

5�

/0.1

74

�/0

.38

33

50

.10

6

Su

bsc

rip

tc

an

da

corr

esp

on

dto

cath

od

icp

roce

ssa

nd

an

od

icp

roce

ss,

resp

ecti

vel

y.

Ea

reth

ep

eak

po

ten

tia

l,I

the

pea

kcu

rren

t,F

dm

dQj cb

an

dF

dm

dQj ca

are

the

va

lues

for

the

Fd

md

Qjf

un

ctio

na

t0

.07

5V

bef

ore

the

cath

od

icp

eak

po

ten

tia

la

nd

at

0.0

75

aft

erth

eca

tho

dic

pea

kp

ote

nti

al,D

mre

pre

sen

tsch

an

ges

of

ma

ssd

uri

ng

the

vo

lta

mm

etri

cp

eak

an

dD

Qth

eel

ectr

ical

charg

een

clo

sed

invo

lta

mm

etri

c

pea

ks.

So

luti

on

sw

ere

alw

ays

aq

ueo

us

solu

tio

ns

of

salt

con

cen

trati

on

0.3

8M

.T

he

pH

was

bu

ffer

edb

ym

ean

so

fC

H3C

OO

H/C

H3C

OO

�.


of the film that could be excluded since these changes of

mass are not observed in KCl, KNO3 and KI solutions

and only slightly in some cases in NaNO3 and NaBr

solutions at this pH.After this important mass increase, mass decreases

quickly and values for the Fdm/dQ ratio of 64 g mol�1

in CsCl solutions and 12 g mol�1 in NaCl solutions

were found indicating that these changes of mass have a

greater influence of the electrochemical processes that

take place. In the case of CsCl solutions, the value of 64

g mol�1 for the Fdm/dQ function coincides with the

expected for a pure participation of inner NO3� anions

(62 g mol�1) as counteranions during the electrochemi-

cal processes. However, this coincidence is perhaps only

casual.

It proves very interesting to compare values for the

Fdm/dQ at the peak potential or the global F/

Dm

DQratio

during voltammetric peaks in the different aqueous

media studied (Table 3). That way it is found that

values for these mass/charge ratio are smaller in solu-

tions containing NaCl than in solutions containing KCl,

and also smaller in solutions containing NaNO3 than insolutions containing KNO3. Taking into account that at

these potentials, the main contribution to mass changes

is due to the anions participation, it can be said that the

insertion or expulsion of sodium cations is easier than

that of potassium cations. Values for this ratio in CsCl

solutions are slightly larger than in NaCl solutions,

indicating that cesium cations participation is smaller

than sodium participation.

3.4.2. Effect of anion

At this pH, it is found out that changes of mass

during voltammetric experiments correspond mainly to

insertion�/expulsion of anions. From the analysis ofvoltammograms and mass�/potential curves some infor-

mation can be extracted. Table 3 shows that the mass/

charge ratio is larger in solutions containing KNO3 or

KI than KCl. A similar behaviour is found out when

analysing values obtained in NaNO3 and NaBr solu-

tions larger than in NaCl solutions, but in this case,

values for this function are significantly smaller.

Looking at mass�/potential curves of Fig. 3.b thereare some differences that should be discussed. In

solutions containing Cl� anions, changes of mass

during the cathodic peak are especially important after

the peak potential, while in KNO3, NaNO3, NaBr or KI

solutions this mass decrease is more regular during the

entire peak. During the anodic peak, the increase of

mass is more regular. These differences can be quanti-

fied by means of the Fdm/dQ function at severalpotentials during the cathodic scan(see Table 3). That

way in KCl, NaCl and CsCl solutions it is clearly

observed that the mass/charge ratio is larger at more


negative potentials, while in other solutions containing

NO3� or I� or Br� these differences are less important.

3.4.3. Effect of NaNO3 concentration

Current�/potential curves and mass�/potential curveshave been obtained for a PNR film in NaNO3 solutions

at pH 4.5 and different electrolyte concentration (0.38,

0.76 and 1.14 M). It is obtained that peak potentials,

peak currents and mass changes in these three experi-

ments are very similar and that no peak potential

displacement is obtained. The quantitative analysis by

means of the mass/electrical charge ratio gives also very

similar values for the different concentration.

4. Discussion and conclusion

The deposition study has proved that the presence of

molecular oxygen in the solution favours the electro-

generation reaction since more polymer is generated

when solutions are not previously deareated. However,the Fdm/dQ function reaches values slightly smaller in

these conditions. This is due to the fact that molecular

oxygen in the solution favours also other oxidation

reactions at anodic potentials which does not involve

changes of mass on the electrode surface.

By analysing differences in the Fdm/dQ function, it

can be concluded that the polymer oxidation continues

at potentials where the electrogeneration reaction takesplace since values for this function are largest when the

polymer has not been previously reduced. This hypoth-

esis is corroborated by the fact that peaks II and III

proves more stable against successive cycling when

potentials, during the anodic scan, arrive at more

positive values in solutions free of monomer.

On the other hand, a possible monomer adsorption

has been detected at cathodic potentials, but this processtakes place preferably when no polymer has been

deposited on the electrode surface. Therefore, it is

possible that PNR films generated by different techni-

ques present a different behaviour in some conditions.

Focussing on the electrochemical behaviour of the

electrogenerated polymer, it proves clear that the

relative participation of cation/anions during the charge

compensation process mainly depends on the pH of thesolution, but also on the nature of cations and anions.

That way, at pH 2.5�/2.6 values of the mass/charge

ratio does not depend clearly on the nature of the salt

cation present in the solution (sodium or potassium) but

it depends on the anion. However, changes of mass

show clearly that there are cations that enter the film

during the reduction processes and leave the film during

the oxidation reaction. Therefore, it can be concludedthat the cation that participates at this pH is mainly the

hydrated proton, and that a partial participation of

anions, especially Cl� makes the Fdm/dQ function to be

smaller than the 19 g mol�1 expected for the pure

hydrated proton participation. According to values of

the mass/charge ratio, at this pH smaller Cl� anions are

preferred instead of largest NO3� or Br� anions.

At higher pH 4.5�/4.7 mass changes are very different

as it is described in the text. It is possible to analyse the

relative participation of anions and cations by means of

the mass/charge ratio values. Values of Fdm/dQ at peak

potentials in NaNO3 (5�/7 g mol�1) and KNO3 (20�/48 g

mol�1) solutions at pH 4.5�/4.7 can be explained by

considering that sodium cations have a larger participa-

tion than potassium. Less pronounced differences areobserved between values obtained in solutions contain-

ing NaCl and in solutions containing KCl or CsCl. A

possible explanation can be found in the fact that the

radius of hydrated sodium cation (5.6�/7.9 A) is larger

than the radius of hydrated cesium (3.6�/5.0 A) and the

radius of hydrated potassium (3.8�/5.3 A). Therefore,

electrical charge is more distributed in sodium cations

and this fact allows a small electrical repulsion withinthe polymer that at these potentials prefers insertion�/

expulsion of anions to balance electrical charge. Small

differences between Fdm/dQ function in CsCl and KCl

solutions can be explained by the fact that the molecular

mass of cesium cations is larger than that of potassium

cations, and therefore, with the same molar participa-

tion of both cations, cesium insertion�/expulsion will

affect more to the mass/charge ratio.On the other hand, values of this mass/charge ratio at

peak potential depends also on the nature of the salt

anion, being largest for NO3� and I� than Cl�. In this

case, it could be said that the PNR films is more selective

to anions when largest anions are present in the

solution, perhaps due to a shielding effect that makes

difficult the participation of cations, especially potas-

sium cations. The other possible explanation is that inlargest anions the electrical charge is more distributed

and this fact makes smaller repulsion between inner

chains in the polymer and the anion that moves through

it. This high selectivity for the anions can explain why

mass increases, due to a possible cation exchange, at the

start of the first cycle during the cathodic scan in CsCl

and NaCl solutions, while this behaviour is not observed

in NaNO3 or NaBr since these anions make difficult theinsertion or expulsion of cations.

The different behaviour at these two pH can be

explained by the fact that changing the pH from 2.5�/

2.6 to 4.5�/4.7 means also a change in the polymer

structure since there is a pKa of the polymer between

these pHs.

Another result that should be analysed is the apparent

no dependence of voltammograms on the electrolyteconcentration (NaNO3) at pH 4.5. A possible explana-

tion can be found in the fact that at this pH and in

NaNO3 solutions, values of Fdm/dQ are very small at

peak potentials. That means, that both the salt cations


and the salt anions participate during the charge

compensation and it can be considered this overall

process if nitrate and sodium participate 1:1:

�PNR; NO�3 ��2e��Na�?�PNR; Na��NO�

3

Therefore, possible changes of peak potentials are

compensated and it is found out an apparent no

dependence. Another effects due to low concentration

of electrolyte in the solution are not observed since this

concentration is always enough to not control electro-

chemical processes of PNR films.From these results, it proves clear that peak potential

have no large dependence on the nature of the salt

cation present in the solution, nor on the nature of the

salt anion nor on the electrolyte concentration. How-

ever, peak potentials have a clear dependence on the pH.

Therefore, it can be concluded that redox processes

takes part by the main participation of H� species and

that the role of salt cations or anions is only to keepelectroneutrality, especially when the pH of the solution

is not enough acid.

Acknowledgements

Part of this work was supported by Project CICYT-

MAT 2000-100-P4-03. J.J. Garcıa-Jareno acknowledgesthe financial support from Ministerio de Ciencia y

Tecnologıa and FEDER (Programa Ramon y Cajal).

References

[1] J.M. Bauldreay, M.D. Archer, Electrochim. Acta 28 (1983) 1515.

[2] A.A. Karyakin, A.K. Strakhova, E.E. Karyakina, S.D. Varfolo-

meyev, A.K. Yatsimirsky, Bioelectrochem. Bioenerg. 32 (1993)

35.

[3] D.D. Schlereth, A.A. Karyakin, J. Electroanal. Chem. 395 (1995)

221.

[4] F. Vicente, A.R.oig Garcıa-Jareno, J.J. Trijueque, J. Navarro-

Laboulais, J. Scholl, H. Port, Electrochim. Acta 28 (1995) 235.

[5] A. Roig, Thesis, Universitat de Valencia, 1994.

[6] A.A. Karyakin, E.E. Karyakina, H.L. Schmidt, Electroanalysis

11 (1999) 149.

[7] G. Inzelt, E. Csahok, Electroanalysis 11 (1999) 744.

[8] L. Gordon, G. Johansson, A. Torstensson, J. Electroanal. Chem.

196 (1985) 81.

[9] Z. Lu, S. Dong, J. Chem. Soc., Faraday Trans. 1 (84) (1988) 2979.

[10] Q. Chi, S. Dong, Electroanalysis 7 (1997) 147.

[11] S.M. Chen, K.C. Lin, J. Electroanal. Chem. 511 (2001) 101.

[12] A.A. Karyakin, O.A. Bobrova, E.E. Karyakina, J. Electroanal.

Chem. 399 (1995) 179.

[13] A.A. Karyakin, E.E. Karyakina, W. Schuhmann, H.L. Schmidt,

S.D. Varfolomeyev, Electroanalysis 6 (1994) 821.

[14] A. Silber, N. Hampp, W. Schuhmann, Biosens. Bioelectron. 11

(1996) 215.

[15] S. Cosnier, K. Le Lous, J. Electroanal. Chem. 406 (1996) 243.

[16] C.X. Cai, K.H. Xue, J. Electroanal. Chem. 427 (1997) 147.

[17] K. Tanaka, S. Ikeda, N. Oyama, K. Tokuda, T. Ohsaka, Anal.

Sci. 9 (1993) 783.

[18] D. Benito, J.J. Garcıa-Jareno, J. Navarro-Laboulais, F. Vicente,

J. Electroanal. Chem. 446 (1998) 47.

[19] D. Benito, C. Gabrielli, J.J. Garcıa-Jareno, M. Keddam, H.

Perrot, F. Vicente, Electrochem. Commun. 4 (2002) 613.

[20] G. Maiai, R.M. Torresi, E.A. Ticianelli, F.C. Nart, J. Phys.

Chem. 100 (1996) 15910.

[21] C. Deslouis, M. Musiani, B. Tribollet, Phys. Chem. 100 (1996)

8994.

[22] C. Deslouis, M. Musiani, B. Tribollet, M.A. Vorotyntsev, J.

Electrochem. Soc. 142 (1995) 1902.

[23] S.H. Glarum, J.H. Marshall, J. Electrochem. Soc. 134 (1987) 142.

[24] M.F. Mathias, O. Haas, J. Phys. Chem. 96 (1992) 3174.

[25] T. Matencio, E. Vieil, Synth. Met. 44 (1991) 349.

[26] M. El Rhazi, C. Lopez, C. Deslouis, M.M. Musiani, B. Tribollet,

E. Vieil, Synth. Met. 78 (1996) 59.

[27] C. Barbero, M.C. Miras, R. Kotz, O. Haas, J. Electroanal. Chem.

437 (1997) 191.

[28] V. Plichon, S. Besbes, J. Electroanal. Chem. 284 (1990) 141.

[29] O. Haas, J. Rudnicki, F.R. Mc Larnon, E.J. Cairns, J. Chem. Soc.

Faraday Trans. 87 (1991) 939.

[30] V.M. Schmidt, C. Barbero, R. Kotz, J. Electroanal. Chem. 352

(1993) 301.

[31] V. Kertesz, N.M. Dunn, G.J. Van Berkel, Electrochim. Acta 47

(2002) 1035.

[32] R.M. Torresi, S.L.D. Maranhao, J. Electrochem. Soc. 146 (1999)

4179.

[33] G. Inzelt, G. Horanyi, J. Electroanal. Chem. 471 (1999) 73.

[34] E. Csahok, E. Vieil, G. Inzelt, J. Electroanal. Chem. 457 (1998)

251.

[35] S. Pruneanu, E. Csahok, V. Kertesz, G. Inzelt, Electrochim. Acta

43 (1998) 2305.

[36] G. Inzelt, Electroanalysis 7 (1995) 895.

[37] C. Gabrielli, M. Keddam, N. Nadi, H. Perrot, J. Electroanal.

Chem. 485 (2000) 101.

[38] C. Gabrielli, M. Keddam, N. Nadi, H. Perrot, Electrochim. Acta

44 (1999) 2095.

[39] C. Gabrielli, M. Keddam, F. MinoufletLaurent, N. Nadi, H.

Perrot, Polish J. Chem. 71 (1997) 1171.

[40] C. Gabrielli, J.J. Garcıa-Jareno, H. Perrot, Electrochim. Acta 46

(2001) 4095.

[41] J.J. Garcıa Jareno, A. Sanmatıas, F. Vicente, C. Gabrielli, M.

Keddam, H. Perrot, Electrochim. Acta 45 (2000) 3765.

[42] J.J. Garcıa Jareno, C. Gabrielli, H. Perrot, Electrochem. Com-

mun. 2 (2000) 195.

[43] F. Vicente, J. Navarro, J.J. Garcıa, D. Benito, H. Perrot, D.

Gimenez, in: Aplicabilidad de la Microbalanza de Cuarzo:

Ensayos con el Poli(rojo neutro), Ed. Moliner-40, Burjassot,

Spain, 2001.

[44] S. Bruckenstein, A.R. Hillman, J. Phys. Chem. 92 (1988) 4837.

[45] D. Orata, D.A. Buttry, J. Am. Chem. Soc. 109 (1987) 3574.

[46] R.M. Torresi, S.I. Cordoba-Torresi, C. Gabrielli, M. Keddam, H.

Takenouti, Synth. Met. 61 (1993) 291.

[47] K. Naoi, M. Lien, W.H. Smyrl, J. Electrochem. Soc. 138 (1991)

440.

[48] V.E. Granstaff, S.J. Martin, J. Appl. Phys. 75 (1994) 1319.

[49] R. Lucklum, P. Hauptmann, Sens. Actuat. B 70 (2000) 30.

[50] G. Sauerbrey, Z. Phys. 155 (1959) 206.

[51] S. Bruckenstein, K. Brzezinska, A.R. Hillman, Electrochim. Acta

45 (2000) 3801.

[52] V. Kertesz, J. Bacskai, G. Inzelt, Electrochim. Acta 41 (1996)

2877.

Study by EQCM on the voltammetric electrogeneration of poly(neutral red). The effect of the pH and...

Documents

Transcript of Study by EQCM on the voltammetric electrogeneration of poly(neutral red). The effect of the pH and...