Electrochemistry of platinum phthalocyanine microcrystals: I. Electrochemical behaviour in acetonitrile electrolytes

Article abstract of DOI:10.1016/S0013-4686(00)00318-2

Platinum phthalocyanine (PtPc) microcrystals deposited upon platinum, glassy carbon and gold by a process of dry abrasion have been characterized by electrochemical techniques. This mechanical abrasion can produce good electronic contact and adhesion between the microcrystals and the electrode. The redox process of PtPc microcrystals is accompanied by two reversible electrochemical phase transformations, evidenced by a sharp peak and unusually large peak potential differences. Similar to conductive polymers, an obvious first-scan discrepancy and large capacitance are observed during electrochemical oxidation. Some of the intercalated anions remain in the re-reduced microcrystals and lead to a conductivity enhancement of the microcrystals, supported by AC impedance and X-ray photoelectron spectroscopy (XPS) results. Coulometry shows that the size of the anion determines the rate and degree of oxidation, and influences the reversible phase transformations. Chronoamperometry shows diffusion-controlled nucleation and growth kinetics, controlled by the diffusion of anions into the solid films. The conformational relaxation model was used to describe this process.

Full text of DOI:10.1016/S0013-4686(00)00318-2

Electrochimica Acta 45 (2000) 2227–2239
www.elsevier.nl/locate/electacta
The electrochemistry of platinum phthalocyanine
microcrystals
I. Electrochemical behaviour in acetonitrile electrolytes
Junhua Jiang, Anthony Kucernak *
Department of Chemistry, Imperial College, London SW7 2AZ, UK
Received 27 August 1999; received in revised form 13 December 1999
Abstract
Platinum phthalocyanine (PtPc) microcrystals deposited upon platinum, glassy carbon and gold by a process of dry
abrasion have been characterised by electrochemical techniques. This mechanical abrasion can produce good
electronic contact and adhesion between the microcrystals and the electrode. The redox process of PtPc microcrystals
is accompanied by two reversible electrochemical phase transformations, evidenced by a sharp peak and unusually
large peak potential differences. Similar to conductive polymers, an obvious first-scan discrepancy and large
capacitance are observed during electrochemical oxidation. Some of the intercalated anions remain in the re-reduced
microcrystals and lead to a conductivity enhancement of the microcrystals, supported by AC impedance and X-ray
photoelectron spectroscopy (XPS) results. Coulometry shows that the size of the anion determines the rate and degree
of oxidation, and influences the reversible phase transformations. Chronoamperometry shows diffusion-controlled
nucleation and growth kinetics, controlled by the diffusion of anions into the solid films. The conformational
relaxation model was used to describe this process. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Platinum phthalocyanine; Electrochemical behaviour; Phase tranformation; Microcrystals; Abrasion
1
. Introduction
Compared with the first-row transition metal ph-
thalocyanines, precious metal phthalocyanines have
higher thermal stability and more effectively resist elec-
trochemical and chemical oxidation [11]. Platinum ph-
thalocyanine (PtPc) has been investigated as a charge
transfer salt [12–14] and electro-catalyst for oxygen
reduction [11,15–17]. Electrochemically doped PtPc
shows highly metal-like conductivity [12]. Deposition of
PtPc onto an inert substrate has been achieved through
both its evaporation in vacuum and through encapsu-
lating PtPc microcrystals in a polymer matrix [11–13],
although it is difficult to obtain good electrode repro-
ducibility using the above two methods. The electro-
chemical characterisation of PtPc has been limited
The electrochemical and photochemical properties of
metallophthalocyanine (MPc) have been extensively
studied because of their characteristic molecular struc-
ture. Both applied and fundamental studies have been
reported [1,2], and these materials have been applied
successfully to electrochromic devices [3,4], organic so-
lar cells [5], switching devices [6] and power sources
[
7–9]. An extensive review of phthalocyanine electro-
chemistry has been presented [10].
*
Corresponding author.
0
013-4686/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0013-4686(00)00318-2

2
228
J. Jiang, A. Kucernak / Electrochimica Acta 45 (2000) 2227–2239
because of its low solubility even in organic solvent at
high temperature. PtPc dissolved in 1-chloronaph-
thalene at 150°C produces two reduction waves at
2. Experimental
.1. Chemicals and instrumental
2
−
1.23 and −1.61 V against a Fe(C H5)2 reference
5
Tetrabutylammonium
Fluka), tetrabutylammonium tetrafluoroborate (TBA-
perchlorate
(TBAClO4,
electrode [18]. A new composite electrode PtPc-poly-
bisphenol-A-carbonate reticulate structure is character-
ised by two anodic and four cathodic redox transitions
in propylene carbonate electrolyte [12].
BF , Aldrich) and tetrabutylammonium hexafluoro-
4
phosphate (TBAPF , Fluka) were of electrochemical
6
grade and used as received. Acetonitrile (AN, Analar)
was distilled under vacuum into a flask filled with the
molecular sieve. PtPc was synthesized by heating finely
crushed phthalonitrile and PtCl2 at 200°C in the ab-
sence of a solvent for one hour [10]. Two separate
sublimation steps purified the product, each repeated
three times. In the first, unreacted phthalonitrile was
sublimed from the crude product at 140°C and 0.1
mbar pressure, and in the latter, PtPc was sublimed
from the crude product at 550°C and 0.1 mbar
pressure.
In this work, a systematic study of the electrochemi-
cal behaviour in acetonitrile of PtPc microcrystals at-
tached by a process of dry abrasion to an electrode
surface without the need of a polymeric binder is
described. A recent review of voltammetry of solid
microparticles immobilized on electrode surfaces pro-
vides useful background to the techniques for immobi-
lizing powders on electrodes [19].
Voltammetric, chronoamperometric and coulometric
measurements were performed at room temperature
(
ꢀ20°C) using an Autolab General Purpose Electro-
chemical Systems (GPES, Ecochemie, Netherlands).
The electronic conductivity of the PtPc powder micro-
crystals was measured using an Autolab Frequency
Response Analyser (FRA). A following paper will de-
scribe AC impedance experiments. X-ray photoelectron
spectroscopy (XPS) was measured using a Kratos
XSAM 800 ultrahigh vacuum photoelectron spectrome-
ter using the Mg Ka line as the excitation source.
Electrochemical measurements were performed in an
oxygen-free three-compartment electrochemical cell.
−
3
−3
The reference half-cell was Ag/10
mol dm
−
3
AgNO +0.1 mol dm
TBAClO +AN, and the
3
4
counter electrode was a platinum foil. Solutions were
deoxygenated with high purity argon.
2.2. The preparation of PtPc microcrystals electrode
A gold disk (0.5 mm diameter, 99.99%, Goodfellows,
UK), platinum disk (0.5 mm diameter, 99.95%, Good-
fellows, UK) or glassy carbon (5 mm diameter, Sigadur
G, Hochtemperatur Werkstoffe GmBH, Germany)
served as the working electrode.
SEM images (Fig. 1(a)) of PtPc fixed onto a glassy
carbon substrate using polybenzimidazole (PBI) showed
needle-like microcrystals with an average diameter of
10–20 mm.
Microcrystals of PtPc were attached to the electrode
surfaces without polymer binder as follows. First, a
small amount of the microcrystal powder was placed on
a smooth glass plate. The electrode was then rubbed
heavily on the powder covered glass until the electrode
surface was thoroughly coated by a thin-layer of PtPc.
Successful coating was evidenced by a shiny purple
colour. Finally, the electrode was transferred to the
electrochemical cell containing non-aqueous electrolyte.
Fig. 1. SEM images of a thin composite platinum phthalocya-
nine (PtPc)-polybenzimidazole (PBI) film (a) and of an abra-
sively obvious PtPc layer on a glassy carbon electrode (b). In
the latter, loosely adherent PtPc particles (white) rest on a
dense PtPc layer covering the entire glassy carbon electrode.

J. Jiang, A. Kucernak / Electrochimica Acta 45 (2000) 2227–2239
2229
−
3
g cm
[21,22]. An SEM image of the mechanically
deposited PtPc microcrystals on a glassy carbon sub-
strate is shown in Fig. 1(b). The mechanically deposited
PtPc is found to uniformly coat the entire surface, with
small excess particles loosely adherent to the top layer.
SEM images of cross-sections through the film do not
show excessive porosity. The mechanical abrasion has
severely broken the microcrystals and damaged their
external faces. There was no evidence of recrystallisa-
tion of the material after potentiodynamic cycling.
Electrodes prepared in this way exhibit the purple
metallic sheen indicative of PtPc. All of the results
presented in the following paper will be from electrodes
prepared in this manner.
Application of Nafion to selected electrode surfaces
was readily achieved by dropping 5 ml 5% (wt.%)
Fig. 2. Cyclic voltammograms of platinum phthalocyanine
(
PtPc) microcrystals on a glassy carbon electrode in 0.1 mol
®
Nafion solution (Aldrich) onto the PtPc covered
−
3
dm
tetrabutylammonium perchlorate (TBAClO )+acetoni-
4
−
1
glassy carbon surface. The recast film was dried at
room temperature.
trile (AN) (w=50 mV s ). —, the first scan; -·-·-·, the
second;-- -, the fifth.
After each series of experiments, the electrode sur-
faces were renewed by polishing and cleaning with
acetone and Millipore water. The cleaned electrodes
were dried in an oven at 80°C.
The microcrystals were found to have good electronic
contact and adhesion with the substrate, and electrodes
produced in this manner show good reproducibility.
The amount of PtPc film on the electrode surface was
measured by coulometry after exhaustive electrolysis.
The coulometric measurements were calibrated by dis-
solving the deposited microcrystals in 1-chloronaph-
thalene and measuring the absorbance at umax (650 nm)
on the basis of the measured extinction coefficient
3. Results
3.1. General electrochemical beha6iour
Cyclic voltammograms of PtPc microcrystals on
(
lg m=5.18), finding a good linear relationship between
−
3
glassy carbon in 0.1 mol dm
TBAClO +AN are
4
the two. This extinction coefficient is consistent with
those reported for other MPcs in the same medium [20].
The thickness of PtPc film was calculated to be in the
range of 5–10 mm on the basis of a PtPc density of 1.96
shown in Fig. 2. There is a large difference between the
first and successive scans, which we will comment upon
below. The following discussion considers the voltam-
metry of the film after the first scan.
On increasing the potential the oxidation current
rises at about 0.20V and grows gradually with potential
until about 0.60 V. There is a broad difficult to measure
peak (1) occurring in the range (0.2–0.6 V). The current
then grows rapidly and reaches a maximum (peak 2) at
about 0.75 V. At higher potentials, peak 3 is seen at
1.07 V accompanied by peak 3’ at about 1.14 V. There
is a long flat-current region between peak 2 and peak 3.
On reversing the scan four peaks are seen. A very small
peak (4) at 0.82 V is followed by a sharp peak (5) at
0.67 V; a rather broad peak (6) at about 0.40 V is
followed by a small peak (7) at 0.06 V.
Fig. 3 displays the cyclic voltammograms with differ-
ent upper potential limits for PtPc micro-crystals on the
−
3
glassy carbon electrode in 0.1 mol dm
TBAClO +
4
AN. Peaks 1 and 6 correspond to a chemically re-
versible couple. The potential difference between the
two peaks is estimated to be 0.15 V. Peak 2 is responsi-
ble for peak 7. The potential difference between these
two peaks is unusually large at 0.71 V. Peak 2 shows a
larger peak current and peak area than peak 7, and so
Fig. 3. Cyclic voltammograms with different potential ranges
for platinum phthalocyanine (PtPc) microcrystals on a glassy
−
3
carbon electrode in 0.1 mol dm
tetrabutylammonium
−
1
perchlorate (TBAClO )+acetonitrile (AN) (w=50 mV s ).
4

2
230
J. Jiang, A. Kucernak / Electrochimica Acta 45 (2000) 2227–2239
0.7–0.8 for scan rates in the range of 5–100 mV s
−
1
ꢀ
,
rather than being proportional to the scan rate or its
square root. The charge involved both in oxidation and
reduction is independent of the scan rate.
3.2. First scan discrepancy in the 6oltammetry
The voltammetry of the PtPc microcrystals changes
considerably after the first scan. Compared to following
scans, little oxidation occurs at peak 1 in the potential
range 0.2–0.6 V during the first scan. However, peak 6,
which is the corresponding reduction process, does not
alter in magnitude between the first and successive
scans, suggesting that the process responsible for peak 1
still occurs, although presumably at higher potentials.
In support of this, peak 2 shows a larger magnitude
while also occurring at a higher potential (0.8 V) than
successive scans. Furthermore, the flat-current region
between peaks 2 and 3 is not evident during the first
scan. Peak 3 occurs at the same potential but is slightly
decreased in size whereas peak 3% is significantly de-
creased in size and slightly shifted in potential. The
peaks due to reduction of the film are essentially the
same as seen after further scans, except peak 5, which
shows a smaller current during the first scan.
−
1
Fig. 4. Cyclic voltammograms at 5 mV s
for platinum
phthalocyanine (PtPc) microcrystals deposited on to a glassy
−
3
carbon electrode in 0.1 mol dm
tetrabutylammonium
perchlorate (TBAClO )+acetonitrile (AN).
4
it must be expected that some of the charge associated
with peak 2 is also recovered in peak 6. Peak 3 leads to
the formation of peak 5 with a small shoulder at peak
4
. The potential difference between peak 3 and peak 5
is about 0.42 V. Furthermore, an unusually large dou-
ble-layer current is observed in the flat-current region
between peak 2 and peak 3, compared to the double
layer current at the re-reduced electrode (EB0 V).
Lastly, besides an obvious influence on the height and
potential of peak 4 and peak 5 the oxidation potentials
around peak 3 have an influence on the peak potential
of peak 7. A more positive potential leads to a shift of
peak 7 to more negative potentials.
The ratios of the anodic to cathodic charge are
independent of the upper potential limit and remain
close to 1. This ratio implies that every electrochemical
step is chemically reversible.
The anodic charge (Q ), cathodic charge (Q ) and
a
c
their ratios (Q /Q ) for five successive scans over the
c
a
full potential range are listed in Table 1. The anodic
charge is larger than the cathodic charge during the first
scan with an anodic charge excess of 0.35 mC. There
are two possibilities for this difference. One is that the
original electrochemical reaction is not chemically re-
versible. The other is that there is some physical loss of
material during the oxidation. The anodic charge is
almost equivalent to the corresponding cathodic charge
during subsequent successive scans, and charge re-
versibility is maintained.
A cyclic voltammogram at low scan rates for PtPc
micro-crystals on the glassy carbon electrode in 0.1 mol
In order to rule out the possibility of physical loss of
PtPc microcrystals during scanning, a thin Nafion film
was applied to cover the PtPc coating on the electrode.
A similar first-scan effect can still be seen from the
cyclic voltammograms of Nafion coated PtPc micro-
−
3
dm
TBAClO +AN is shown in Fig. 4. It is found
4
that the scan rate has an influence on the peak shape.
Peak 2 and peak 3 have a smaller half-height width at
lower scan rates than at high scan rates. The peak
current is proportional to the scan rate to the power of
−
3
crystals on glassy carbon in 0.1 mol dm
TBAClO +
4
AN, shown in Fig. 5. The recast Nafion film is sparsely
cross-linked and has poor ion-selectivity because of a
large number of pinholes [23], and as a result the anions
can easily penetrate the film. The first-scan effect is also
observed on a PBI-PtPc composite film electrode [24].
These similar first-scan effects indicate that the anodic
charge loss was caused by chemical irreversibility of the
first scan rather than by the physical loss of PtPc
micro-crystals from the electrode surface.
The significant difference between the first and suc-
cessive scans has also been reported in the voltam-
mograms of polyaniline [25,26]. Aoki et al. explained
this effect on the basis of a percolation threshold poten-
Table 1
The anodic charge (Q ), cathodic charge (Q ) and their ratios
a
c
(
Q /Q ) for successive five scans
c
a
Scan no.
103 Q /C
103 Q /C
Q /Q
a
c
c
a
1
2
3
4
5
2.85
2.50
2.43
2.42
2.42
2.35
2.42
2.40
2.40
2.41
0.82
0.97
0.99
0.99
1.0

J. Jiang, A. Kucernak / Electrochimica Acta 45 (2000) 2227–2239
2231
rapidly giving sharp voltammetric peaks. The conduc-
tivity of PtPc microcrystals at the open circuit potential
−
3
−1
(
1
−0.22 V) increases from 2.9×10
S cm
to 2.1×
−
2
−1
0
S cm
after one scan, as measured by electro-
chemical impedance in the following paper. This higher
conductivity facilitates electrochemical oxidation at the
electrochemically cycled PtPc microcrystals. The con-
ductivity enhancement of normal semiconductors can
be achieved through doping, and it is possible that the
PtPc semiconductor studied here undergoes a doping
process through an electrochemical cycle and that some
anions remain in the cycled film to permanently dope
the PtPc semiconductor.
The incomplete removal of anions from a MPc film
after potential cycling has been previously reported
[
29]. The presence of anions in re-reduced PtPc films
Fig. 5. Cyclic voltammograms for Nafio-coated platinum ph-
thalocyanine (PtPc) microcrystals deposited on to a glassy
have been confirmed by XPS for systems in which
perchlorate is the anion (the detection limits for phos-
phorus and boron are too low to allow comparison
−
3
carbon electrode in 0.1 mol dm
perchlorate (TBAClO )+acetonitrile (AN) (w=50 mV s ).
tetrabutylammonium
−
1
4
−
−
—
, first scan; -·-·-, second scan.
with the PF and BF anions). The re-reduced sample
6 4
was taken out from the electrolyte and rinsed multiple
times with fresh acetonitrile. Typical surface spectra for
the original film and the re-reduced sample are shown
in Fig. 7. The presence of chlorine (Cl2p at 204 eV) is
evident in the re-reduced film.
tial [27]. Fig. 6 shows the dependence of the conductiv-
ity of PtPc microcrystals on potential measured using
a.c. impedance. The original PtPc is a semi-conductor
and has low conductivity. The conductivity only
slightly increases with potential up to 0.6 V. An abrupt
growth in conductivity occurs at about 0.7 V, at the
same point at which the oxidation current begins to
grow rapidly during the first scan in Fig. 2. It is well
accepted that it is difficult to initiate electrochemical
reactions at materials with insulating or semiconducting
films at which electron transfer through that film is
hindered. In such situations the electron has to be
transferred at the electrode/film interface, while at the
same time counterions have to cross the electrolyte/film
interface in order to maintain charge neutrality within
the film. In order to incorporate anions into the PtPc
films, conducting pathways through the polymer layer
have to be nucleated [28]. Once there is sufficient over-
voltage for this process, the oxidation can proceed
3.3. The effect of electrode substrate
To test if the electrode material has any effect on the
voltammetry of PtPc microcrystals, three different elec-
trodes were used as substrates (5 mm diameter glassy
carbon, 0.5 mm platinum, 0.5 mm gold). In all cases,
the responses were almost identical, with voltammetric
Fig. 7. X-ray photoelectron spectroscopy (XPS) spectra for
untreated platinum phthalocyanine (PtPc) microcrystals (a),
and for electrochemically oxidised and then reduced micro-
crystals (b).
Fig. 6. Dependence of AC conductivity of deposited platinum
phthalocyanine (PtPc) microcrystals on the applied potential.

2
232
J. Jiang, A. Kucernak / Electrochimica Acta 45 (2000) 2227–2239
Table 2
The peak parameters for platinum phthalocyanine (PtPc) microcrystals on different electrode substrates in 0.1 mol dm
−
3
−
1
tetrabutylammonium perchlorate (TBAClO )+acetonitrile (AN) at 50 mV s
after five cycles
4
Electrode
Peak potentials (E /V)
p
Peak potential difference (DE /V)
p
Ep,1
Ep,2
Ep,3
Ep,5
Ep,6
Ep,7
DEp1,6
DEp2,7
DEp3,5
Glassy carbon
Platinum
Gold
0.535
0.514
0.517
0.751
0.688
0.676
1.084
1.035
1.026
0.663
0.652
0.653
0.385
0.395
0.380
0.040
0.038
0.052
0.150
0.119
0.131
0.711
0.650
0.624
0.421
0.383
0.373
−
steady states being achieved within five cycles. For ease
of comparison, typical peak parameters are summarised
in Table 2. The different peak currents are dependent
upon the electrode area and the loading of PtPc micro-
crystals on the electrode. Within experimental error and
the effect of uncompensated Ohmic drop, the peak
parameters for redox reactions are independent of the
underlying electrode materials. This strongly suggests
that the phenomena we have reported above are associ-
ated with the bulk of the PtPc microcrystals rather than
at the interface between the electrode and microcrystals.
with the curves in Fig. 3, the BF4 curves have similar
characteristics except a smaller shoulder at peak 4,
−
while the PF6 curves show more significant differences.
The PF6 voltammogram shows an additional oxidation
peak at about 1.18 V. This peak is responsible for two
reductive peaks during the reverse scan. These peaks
indicate that the doping and undoping of PF6 pro-
duces a more complicated phase transformation.
Comparing the voltammetric response for the three
different anions it was seen that the peak potentials are
anion dependent. The potentials of peak 2 and peak 3
shift positive with the anion size. This indicates that it
is difficult to drive large anions into the PtPc crystals.
The potentials of peak 7 also shift positively with the
anion size, indicating that it is relatively easy to expel
large anions during the reduction of the film.
−
−
3
.4. The effect of charge compensating anion
To maintain the charge balance inside PtPc micro-
crystals, anions have to be incorporated during the
oxidation process and must be expelled from the solid
during the reduction process. The influence of anions
on the redox behaviour of transition-metal phthalocya-
nine has been reported by some authors. Faulkner et al.
reported that the size of anions affected their ability to
enter the molecule, which determines the rate and de-
gree of the oxidation of MgPc and ZnPc [29,30].
Toshima et al. found that the electrolyte anions play an
important factor in the reversibility of electrochromism
of CuPc [31]. In order to accomplish reversible elec-
trochromism of CuPc film, the anion should be stable
at the most positive voltage applied, and its Stokes radii
should not be too large. Moreover, an anion that is too
small is not suitable for reversible electrochromism,
because a small anion easily remains within the film,
even after reduction.
The effect of anions with different size on the extent
of oxidation of the PtPc film was determined by using
chronocoulometry. The number of electrons transferred
per PtPc molecule obtained at different potentials as a
function of anion are collected in Table 3. The oxida-
tion extent increases as the anion size decreases at the
same potential and increases with the oxidation poten-
−
tial. The BF4 anion is small enough to occupy more
−
interlayer positions than the larger PF6 anion which is
only able to occupy some larger sites within the crystal
structure.
3.5. The anion concentration dependence
Bond et al. examined redox cycling of 7,7,8,8, tetra-
cyanoquinodimethane (TCNQ) nanocrystals immobi-
lized on a variety of electrodes in aqueous solutions
[23]. They observed an ‘inert zone’ between the chemi-
cally reversible oxidation and reduction peaks of one of
the redox couples of TCNQ. This significant separation
between peaks is strikingly similar to the results that we
have presented for PtPc microcrystals. By employing a
random array of microelectrodes (RAM) electrode,
they were able to examine the electrochemistry of the
immobilized TCNQ crystals in solutions that varied by
over four orders of magnitude in potassium chloride
concentration without the effects of excessive Ohmic
drop in solution. From these results they were able to
−
−
−
PF , ClO and BF4 are spherical ions and are
6
4
poorly solvated in acetonitrile [32,33]. Their size de-
−
−
−
creases from PF6 to BF
(r (PF )=0.301 nm,
4
i 6
−
−
r (ClO )=0.290 nm, r (BF )=0.284 nm) [34]. Their
i
4
i
4
Stokes Radii also decrease in this sequence [31]. The
voltammetric responses with different potential ranges
after 5 cycles were recorded for each of these different
anions using the tetrabutyl ammonium cation. Fig. 8
shows cyclic voltammograms with different potential
upper limits for PtPc microcrystals on the glassy carbon
−
3
electrode in 0.1 mol dm
TBABF +AN (a), and 0.1
4
−
3
mol dm
TBAPF +AN (b), respectively. Compared
6

J. Jiang, A. Kucernak / Electrochimica Acta 45 (2000) 2227–2239
2233
−
1
−
measure a shift in potential of close to 60 mV decade
for both the anodic and cathodic peaks.
concentration effect indicates that ClO4 is involved in
the nucleation and growth kinetics occurring at these
potentials. Peak 2 and peak 3 have a negative potential
Cyclic voltammograms with Ohmic drop correction
for PtPc films on glassy carbon in TBAClO +AN
solution with different ClO4 concentration are shown
in Fig. 9. The oxidation charge increases with the
electrolyte concentration, but the peak current is not
proportional to the concentration. Because both peak 2
and peak 3 are caused by the nucleation and growth of
a new phase within the PtPc film (see below), the
−
1
−1
shift of about 27 mV decade
and 18 mV decade
of
4
−
concentration, respectively. Some authors emphasized
the role of junction potential and ionic activities in the
comparative study of voltammetric response at various
+
concentrations [35]. The potential of Ag/Ag is mainly
+
determined by the concentration of Ag in the refer-
ence half-cell and is not affected by the concentration
Fig. 8. Cyclic voltammograms with different potential ranges for platinum phthalocyanine (PtPc) microcrystals on a glassy carbon
−
3
−3
in 0.1 mol dm
tetrabutylammonium tetrafluoroborate (TBABF )+acetonitrile (AN) (a) and in 0.1 mol dm
tetrabutylammo-
4
−
1
nium hexafluorophosphate (TBAPF )+AN (b) (w=50 mV s ).
6

2
234
J. Jiang, A. Kucernak / Electrochimica Acta 45 (2000) 2227–2239
Table 3
Evidence for nucleation/growth kinetics during the
oxidation is provided by chronoamperometric (single
potential step) experiments. The current–time tran-
sients at different potential steps are shown in Fig. 10.
All experiments are stepped from the reduced state of
the film at 0 V to a different oxidising potential. The
curve at 0.65 V appears to be diffusion-controlled. In
comparison, the step to 0.7 V shows significant differ-
ences at short time, while at longer time intervals the
response is similar to the 0.65 V transient. The response
can be considered as being due to the overlap of a peak
at short time intervals and a curve similar to that
occurring at 0.65 V. Because the electrochemical oxida-
tion of the PtPc film is not adsorption-controlled, the
overlapping peak provides evidence of the nucleation
and growth of a second phase within the film. Increas-
ing the step potential to 0.75 V results in a shortening
of the duration of this phase transformation. When the
potential reaches 1.0 V, the first transformation occurs
so quickly that it is lost in the double layer charging
transient, and the response again looks similar to that
obtained at 0.65 V. Performing the experiment with a
step potential of 1.1 V provides evidence of another
nucleation and growth transient at short times when
compared to the 1.0 V transient. Therefore, the doping
of the PtPc film undergoes two independent solid–solid
phase transformations occurring via nucleation/growth
mechanisms. These electrochemical transformations are
potential dependent and high overpotentials shorten the
relaxation time. The current at longer time has a linear
Electrons transferred per phthalocyanine molecule (n) for the
oxidation of PtPc microcrystals at different potential in 0.1
mol dm tetrabutylammonium salt of the respective anion+
acetonitrile (AN)
−
3
Anions
n1 (0.6 V)
n2 (0.9 V)
n3 (1.2 V)
−
BF4
0.25
0.17
0.09
0.43
0.27
0.22
0.68
0.45
0.30
−
4
ClO
−
PF6
of anions. The influence of junction potential on the
peak potential can be ruled out for the system. Simi-
larly, the ionic activity coefficient of the electrolyte
should have no great contribution to the change of the
peak potential, as it varies by no more than a factor of
three over this concentration range [36]. Therefore the
−
above peak shifts imply that the ClO4 in the solution
indirectly takes part in the phase transformation.
3
.6. Chronoamperometric e6idence for
nucleation/growth kinetics
The chronoamperograms obtained when potential
steps are applied to conducting polymers in different
electrolytes and solvents have been modelled theoreti-
cally from several different points of view, including
nonmetal–metal transitions in the amorphous phase
[
37], solid state electrochemical reactions [38], the intro-
−
1/2
duction of a percolation threshold potential [24], and
conformational relaxation control [39].
relation with t
. This indicates that the electrochem-
ical processes are completed under diffusion control.
Fig. 9. Cyclic voltammograms for platinum phthalocyanine (PtPc) microcrystals deposited on a glassy carbon electrode tetrabuty-
−
1
lammonium perchlorate (TBAClO )+acetonitrile (AN) as a function of electrolyte concentration (w=5 mV s ). —, 0.001 mol
4
−
3
−3
−3
−3
dm ; -·-·-, 0.01 mol dm ; · · · ·, 0.1 mol dm ; -··-··-, 1 mol dm
.

J. Jiang, A. Kucernak / Electrochimica Acta 45 (2000) 2227–2239
2235
Fig. 10. Current–time transients at different potentials for platinum phthalocyanine (PtPc) microcrystals deposited on a glassy
−
3
carbon electrode in 0.1 mol dm
tetrabutylammonium perchlorate (TBAClO )+acetonitrile (AN).
4
Usually systems which undergo nucleation/growth
kinetics display inductive loops in their voltammetry
immediately after scan reversal [40]. One might expect
to see such inductive loops in the scans in which the
potential reverses part way up peaks 2 and 3 in Fig. 3.
However, no such phenomenon is evident. Such an
absence is possible when a large number of nucleation
sites are formed and the growing sites collide at an
early stage of growth. In such a case, the diffusion
zones surrounding the growing zones quickly overlap,
and there is no maxima on reversing the scan direction
4.1. Molecular changes in the PtPc films during
6oltammetry
Many kinds of MPc have rod-like crystallites with a
width of about 20 nm, evidenced by SEM images [31],
and XRD patterns of the MPc films exhibiting a single
sharp peak, indicating that MPc molecules are aligned
in crystal grains. Great advances have been made in
modifying the structure of phthalocyanine molecular
conductive materials by using counterions other than
halides [1]. Anions can take part in self-assembly and
distribute evenly along segregated closely arrayed
stacks of the cationic MPc. The resulting structure
depends upon many factors — for instance the level of
partial oxidation of the phthalocyanine. A partially
oxidised PtPc(ClO4)0.5 crystal was reported to be com-
posed of molecular columns piled up along the c-axis
with anions located at the interlayer positions [14].
[
40].
4. Discussion
For many MPc complexes, it seems likely that the
oxidation and reduction take place reversibly on the
phthalocyanine ring [41,42]. It would seem plausible
that the electrochemical oxidation occurring on the
PtPc involves reversible insertion of anions from the
electrolyte to maintain overall charge neutrality, imply-
ing an overall reaction of the form
−
Since the diameter of ClO4 is larger than half the
c-axis, the anions and voids should be located alter-
nately along the c-axis.
The first voltammetric cycle is special, as during this
process anions are irreversibly inserted into the PtPc
film structure leading to a discrepancy in the cathodic/
anodic charge balance, Table 1. During the first scan,
the process responsible for peak 1 does not occur
appreciably until after the start of peak 2. As will be
described below, peak 2 corresponds to the nucleation
and growth of a new phase, and concomitant with this
is an increase in the conductivity of the film. It thus
seems likely that the delayed onset of peak 1 is due to
−
n+
−
4 n
−
PtPc+nClO ?PtPc (ClO ) +ne
(1)
4
(solution)
(
solid)
(solid)
To incorporate the anions within the PtPc film,
electronically conducting pathways within the micro-
crystals have to be nucleated. The formation of these
pathways follows a nucleation/growth type mechanism
in which the PtPc film undergoes a phase transforma-
tion.

2
236
J. Jiang, A. Kucernak / Electrochimica Acta 45 (2000) 2227–2239
the initial low conductivity of the freshly prepared film.
Subsequent voltammetric scans are of a film which
retains considerable amounts of anions and which
shows much higher conductivity, resulting in a shift of
peak 1 to lower potentials. Subsequent discussion will
be confined to such pre-conditioned films.
Firstly, the current–voltage curve has steeper gradi-
ents at low scan rates than at high scan rates. This is a
tell-tale sign of nucleation and growth [48].
Secondly, the most important feature of the CV is the
very large potential difference between the anodic peak
and its corresponding cathodic peak. Such large differ-
ence can not be explained as electrochemical irre-
versibility, as the scan rate has no obvious effect on the
peak potential. Kogan et al. reported this large peak
potential difference for single crystal PtPc electrode in
A PtPc crystal grain seems to have a number of
interlayer positions of different sizes. Some of them are
large enough to directly accommodate anions. The
anions can enter and leave these positions easily. Some
sites are not sufficiently large to accept the anions, but
they can become large enough to accommodate them
after a phase transformation. Others are too small and
it is difficult for them to grow larger through a phase
transformation of the material. Dimensional changes in
the c-axis direction are possible for the rod-like MPc,
resulting in a change of interlayer distance. Chain-di-
rection dimensional changes are observed in the dop-
ing/undoping process of various polymers [43–45]. This
might be used to explain the reversible phase transfor-
mations and the size effect of the anions. When the
PtPc microcrystals are oxidised in the region of peak 1,
anions enter the microcrystals and occupy some of the
large interlayer positions. During this process there is
no structural change in the crystals. When an overpo-
tential is large enough to initiate the phase transforma-
tion, the microcrystals swell to produce more large
interlayer positions that are occupied by a certain
amount of the anions at the potential of peak 2. At
even higher potentials, the repulsive interactions be-
tween the charged columns increase and the microcrys-
tals will undergo another phase transformation
resulting in more anions being taken up. During the
reverse process, the repulsive interactions between the
columns decrease. The microcrystals will gradually re-
cover their original structure, during which process the
intercalated anions are expelled.
−
3
0.1 mol dm
TBAPF -propylene carbonate solution
6
[12]. They introduced two models. One is based on an
N-shaped free energy dependence of the doped system
giving rise to a hysteresis-like behaviour [49]. In this
case the chemical composition of PtPc(ClO ) can not
4
x
be continuously controlled by electrochemical doping
as the intermediate compositions are not stable. An-
other proposal to explain the large potential difference
involves the introduction of a percolation threshold
potential [27], at which the electronic connection be-
tween the conducting crystals generates a well-coupled
conductive network. An alternative proposal, put for-
ward by Bond et al. in their study of TCNQ is that the
high specific interfacial free energy between the two
−
2
interconverting solid phases (hundreds of mJ cm
)
plays a critical part in determining the nucleation rate
as a function of overpotential [23]. There analysis
shows that for reasonable values of the surface free
energy, there exists a critical overpotential, below which
the rate of nucleation is vanishingly small, and the
system is inert.
Finally, the relation between the peak current and
the scan rate is diagnostic of nucleation and growth
kinetics. The experimental dependence of Ip on w for
several known cases of nucleation/growth is of the form
[48]
x
Ip8w
(2)
Some authors reported that polarization of
polypyrrole films at high cathodic potentials leads not
only to the reduction of the polymer but also to confor-
mational changes in the structure which results in a
highly compact film [46,47]. Reoxidation of the film is
thus controlled by the conformational relaxation pro-
cess (i.e. swelling of the polymer by solvent), and a
dependence of the potential of peak oxidation current is
seen with cathodic pre-treatment potential. This phe-
nomenon has not been observed in the PtPc system
where x is dependent upon the precise nature of the
nucleation/growth kinetics and may even depend upon
0.7–0.8
the scan rate [50,51]. In this case, an I 8w
linear
p
relationship rules out the possibility of diffusion-con-
trolled or adsorption-controlled kinetics, as usually seen
in electrochemical systems.
Based upon the above analysis we ascribe both the
peaks 2 and 3 to electrochemical phase transformations
which are governed by nucleation/growth kinetics. As
peak 2 has a lower overpotential and a smaller half-
height width, the PtPc film at this point shows a smaller
hindrance to the phase transformations and a higher
rate. Kogan et al. explained the large peak potential
difference on the basis of a steric barrier [13]. Crystals
within the film consist of two regions. One is the core
region in which MPc molecules align compactly with
each other; the other is the surrounding region where
the molecules align rather poorly [31]. The reversible
[
24], and so the re-reduced PtPc has a stable structure
independent of the cathodic potential.
4
.2. Phase transformation of the PtPc film under
nucleation/growth kinetics
There are three pieces of evidence, which point to a
nucleation and growth mechanism during the electro-
chemical oxidations occurring at peaks 2 and 3.

J. Jiang, A. Kucernak / Electrochimica Acta 45 (2000) 2227–2239
2237
process is the result of the unimpeded movement of
anions into and out of the non-aligned regions at the
boundaries of MPc crystals. On the other hand, the
irreversible process results from the anions entering the
core regions of a crystal grain driven by a rather large
overpotential, where they remain even after re-reduc-
tion. The two oxidation peaks of CuPc are explained
based on this model. But the reversible phase transfor-
mations of PtPc microcrystals are difficult to explain
using this model especially the voltammetric response at
peaks 3 and 5 and the anion size effects.
where m is the number of moles in a monolayer of area,
A. In this case the diffusion coefficient, D, and the
concentration, C, correspond to those of the anion
within the film, and not in solution. Although one could
attempt to use Eq. (5) to fit the transients, the large
number of unknown parameters means that one would
have to assume values for some of them. This equation
is simplified to the following when the whole of the
surface appears to be active to anions approaching the
surface:
1/2
nFAD
C
ꢂ ꢀ−nFpꢁn
In order to describe the nucleation and growth pro-
cess, it is necessary to determine the rate-controlling
step. If the formation and growth of nuclei are the
rate-controlling step, the current–time transient should
be independent of the anion concentration. However,
I=
1−exp
(6)
y1
/2 1/2
t
RT
This equation reduces to the Cottrell equation for
large p. Thus the appearance of a dependence of cur-
−
1/2
rent with t
at long time supports the conclusion
−
that diffusion within the film is the limiting factor
during oxidation of the film. Under such conditions,
inductive loops are not expected to occur during
voltammetric cycling when the scan direction is re-
versed within the potential range at which PtPc oxida-
tion occurs.
the experiments show that the concentration of ClO4
in the electrolyte has a large influence on the current–
time transients. Low concentrations of anion lead to
lower transient currents and a longer relaxation time
[
24]. This dependence indicates that the diffusion of
−
ClO4 may be the rate-controlling step, supported by
the t
−
1/2
dependence of current at long time. According
4.3. Pseudo-capacitance of the partially oxidised PtPc
to the Cottrell equation
film
1
/2
nFACD
I=
(3)
y1
/2 1/2
The large capacitance effect revealed clearly in Fig. 3
t
between 0.8 and 1.0 V is an established effect in the
doping process of polypyrrole and etherocyclic poly-
mers [53,54]. The related capacitance CL can be ob-
tained from the voltammetric response by simple
calculations based on the expression
−
1/2
The slope (K) of Iꢀt
following form
line should have the
1/2
nFACD
K=
(4)
y1/2
C =I/w
(7)
L
K should be proportional to C according to this
equation. However, the slope has no such relation with
C in the experiments. This indicates that the diffusion
where w is the scan rate and I the double layer current,
−
2
CL is found to reach 8 mF cm
within this potential
range. This capacitance is referred to as the redox
capacity in conductive polymers and redox polymers
−
of ClO4 in the solution as the rate-controlling step can
−
be ruled out, and that the diffusion of ClO4 in the solid
[55], and indicates that the oxidised PtPc microcrystals
film must be the rate-controlling step.
have the ability to store charge. The origin of such large
capacitances in polymer electrodes has been discussed
in detail by Mermillod and Tanguy and is explained on
the basis of charge saturation effects which take place
at the end of the doping processes [56,57]. Both
faradaic and capacitative currents can be associated
with these processes, and in effect the oxidation reac-
tion is completed within this potential range. The large
value of the related capacitance can be considered to
arise from a double-layer model in which weakly
trapped ions are held near surface of piled molecular
columns.
Because the current–time transient is similar to that
caused by the adsorption of organics, it can be assumed
that the individual diffusion zones soon disappear after
a large number of very small nuclei are formed. Under
such conditions the rate of growth is controlled by
planar diffusion perpendicular to the surface. The ex-
pression assuming a linear isotherm is [52]
1
/2
nFACD
I=
y1
/2 1/2
t
2
2
!
nFA C D
ꢀ
−nFp
ꢁ
−
exp
m
RT
2
2
ꢂ
A C Dt
ꢀ
−2nFpꢁn
exp
exp
5. Conclusions
2
m
RT
1
/2 1/2
ꢂ
ACD
t
ꢀ
−nFpꢁn"
The following conclusions can be drawn on the basis
of the results discussed.
erfc
exp
(5)
m
RT

2
238
J. Jiang, A. Kucernak / Electrochimica Acta 45 (2000) 2227–2239
[11] C. Paliteiro, A. Hamnett, J.B. Goodenough, J. Elec-
(
1) The mechanical abrasion of PtPc microcrystals
troanal. Chem. 239 (1988) 273.
can be used to deposit material onto inert electrically
conductive substrates, and that this technique is espe-
cially useful for studying these materials which are
insoluble in aqueous and non-aqueous media. The
transfer of microcrystals is simple, it can avoid contam-
ination, and avoids the use of a binder. The abrasion
only affects minor physical properties of the
microcrystals.
[
[
[
12] I.L. Kogan, K. Yakushi, Electrochim. Acta 43 (1998)
2
053.
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2
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(
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mations, independent of the electrode substrate. Anions
have some influence on the process of phase transfor-
mation. The reversible transformations imply that PtPc
microcrystals may be applied to molecular metal artifi-
cial muscles.
[
[
[
17] C. Paliteiro, A. Hamnett, J.B. Goodenough, J. Elec-
troanal. Chem. 160 (1984) 359.
18] R.H. Campbell, G.A. Heath, E.T. Hefter, R.C.S. Mc-
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(
3) Like other conductive polymers, an obvious first-
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and the dependence of conductivity on the potential are
observed. Partially oxidized PtPc microcrystals have the
ability to store charge and have high conductivity.
[20] M. Whalley, J. Chem. Soc. (1961) 866.
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[
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[
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1
[
[
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Acknowledgements
1
We thank Dr Long and Mr Mongay-Batalla for
providing the PtPc samples. We thank Dr Karol
Senkiw for performing the XPS work. This project has
been funded by the United Kingdom Engineering and
Physical Sciences Research Council, Grant GR/L
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