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A comparative potentiostatic study of the electrocrystallization of PbO2 on platinum and glassy carbon electrodes
1. EtectmannL Chcm. i38 (1982) 425-?33 Hsevier Seqioia’SA. L2 uanne.-. Printed in The Netherlands _:;
425
A C&PkA.IXVE POTENTIOSTATIC STUDY OF THE ELECliR6CRYSTALLIZATON bF PbO, ON PLATINUM CARBON ELE&RODES
RG- EIARRADAS and AQ. CONTRACTOR Department of Chemistry, Corleron Uniwsify,
AND GLASSY
* 011auq
Onrario KIS
5B6 (Canado)
(Received 29th January 1982)
ABSTRACX The deposition of Pb& on plakmm and glassy carbon eleckodes wx invesrigted experimentally. The comparison of the different behaviour on the two types of electrodes is presented. The effects of variation of [emperkre, potential. nucleation order and induction time-lags on the elccrrocrystallisation proce.G are discu&ec.
INTRODUCTION
In an earlier publication we have discussed the essential characteristics of the potentiostatic transients due to electrochemical nucleation and growth of PbO, on ghsy carbon [l]_ In the present communication we examine this process in greater detail and compare it with the deposition of PbO, on platinum. The concept of induction times for nucleation has been investigated and its significance to electrocrystallization data will be assessed. Data on steady state rate of the process will also be presented. In addition we discuss some results which indicate that platinum is not a fully in&t substrate for the electroc~&llisation of PbO,_ EXPERIMENTAL
~.
.The experimental conditions used in these studies have been described previotisly [l]. The experiments with platinum were carried out in one case with a platinum rotating disc electrode and iri other c&&s with platinum foil electrodes (area -0.5 cm’) in unstirred solutions_ The results from both types of platinum electrodes were rather ~Similar; the quantitative data reported here pertain to platinum foil ekctrades. Whenever ‘rotating platinum or rotating glassy carbon electrodes were used,
f-tint
addrcsx J&rtment
of-Chemislrv;Texz-A
and M University. CoIlwe Station. Texas 77843.
U.S.A. 0022kW28/82/KIOO-OOOO/SO2.75
0 1982 Ekvin
Sequoia S.A.
426
the experiments were carried at a rotation speed of 10 Hz. A Stonehart B.C. 1200 Potentiostat was used in all electrochemical experiments. RESULTS
AND
DISCUSSION
Initial stage of deposition on glassy carbon electrodes
The electrocrystallisation of PbO, on glassy carbon was investigated at temperatures between 20” to 60°C and at overpotentials from 200-235 mV_ The form of the growth transient is similar to that reported earlier [I]. In all cases the electrocrystallisation transients correspond to a nucleation order of three, indicating that the deposition occurs by progressive nucleation and growth of three-dimensional centres_ The data fit the geometrical model reasonably well without the need to invoke induction times. This aspect must be stressed because of the frequent incorporation of induction times in most previously published electrocrystallisation studies. The concept of time-lag in nucleaiion has been discussed in an excellent review by Toschev [2]. In electrocrystallisation, induction times may arise in three ways. The first and most generally considered cause of time-lag in electrochemical nucleation is related to the non-steady state of nuclea:ion [3-91. Induction times could also be attributed to the stochastic nature of nucleation_ This type of induction time is given by an exponential law_ Therefore, with a macro-electrode having a large number of possible nucleation sites, this time-lag would be vanishingly small_ Finally, a time-lag could also appear if some kind of electrochemical transformation of the electrode surface is required before nucleation and growth can occur. Apparently for the deposition of PbO, on glassy carbon, time-lags from the above mentioned sources are not significant compared to the time scale of nucleation and growth. Initial stage of deposition on platinum
Electrocrystallisation of PbO, on platinum RDE and platinum foil electrodes was investigated under the same conditions as those used for glassy carbon. A specimen nucleation and growth transient at a stationary platinum electrode is shown in Fig. 1. In contrast to glassy carbon, the nucleation orders obtained from the transients using platinum electrodes vary considerably over the potential range studied. The nucleation orders are also sensitive to the time elapsed at the reversible potential (i.e.. at q = 0) prior to stepping to the deposition potential. The variation of nucleation order with deposition potential is shown in Pig. 2, where the elecirode was held at the reversible potential until the residual current was less than 1pA before stepping to the deposition potential. These results do not conform to any of the geometric models for nucleation and growth, and are at variance with those reported earlier [lo]. For this reason a large number of experiments were performed with both the rotating disc and platinum foil electrodes before presenting our findings_ Though the nucleation orders do not seem to agree with a three-dimensional nucleation and growth model, the micrographs of the deposit reveal hemi-
427
B 0
=_ .u-l
‘0
” -a
t!?
0
0 a
0
0
a 123LS6
125
175
225
0
275
.t/s
t&V
Fig. 1. Initial portion of the current transient in reqonse in 0.1 M Pb(OAc),
+ I M HOAc;
to a potential step from 0 to t275 mV at 23’C
stationary platinum electrode.
Fig. 2. Variation of nucleation order with deposition poLential at (0) 23T Pb(OAc), + 1 M HOAc; stationary platinum electrode.
spherical
325
crystals (Fig. 3)
Electrocrystallisation
transients
60°C;
on platinum
seem to be
by contributions
the time required
for decay of current after the initial sharp rise is quite significant
carbon,
to the duration
of
the transient_
Under
processes.
0.1 M
severely distorted compared
fro-m background
and (a)
similar
As shown in Fig. 1,
conditions
initial portion of the transient does not affect the results significantly. the large contribution the substrate,
to the background
as shown
of transients obtained
H,SO,
prior to being introduced
was held at the reversible .potential potential_ Nucleation orders derived listed in Table
1. Pre-oxidised
transients contain
followed
of
anodically
polarized
in
electrolyte_ The electrode
for 10 s before stepping up to the deposition from the transients under these conditions are
model.
a large contribution
oxidation
Clearly,
the normal
orders closer to
electrocrystallisation
from substrate oxidation
and therefore were
suitable for further analysis.
Some interesting was examined
were
of the
with pre-oxidised
electrodes consistently gave nucleation
that predicted ~by the geometric not considered
which
into the working
glassy
With platinum
current is from simultaneous
by 2 comparison
electrodes and initially oxide-free electrodes. Pre-oxidised electrodes refer to electrodes
0.05 M
on
limiting current is not attained even after 25 rnin and hence distortion
features were revealed
using a cathodic
when
linear potential
and the resulting voltammograrns
the deposit sweep.
are recorded
The
formed
at short times
potential
in Fig. 4. When
programme a cathodic
linear potential sweep was applied after deposition at 150 mV for 2Os, a single reduction.peak was observed_ If deposition was allowed to continue for 40s two reduction increasing
peaks were observed on the subsequent linear potential sweep. With deposition-times, charge under the second peak increases whereas the first
Fig. 3. Scanning sktron micrograph of deposit formed a~ +275 Pb(OAc), + 1 M HOAc; magnification 10000X. This deposit st;lrionary Pr wire for convenience of SEM observation.
mV. 23°C on platinum electrode; 0.1 M was formed on the cross seetion of a
429 TABLE
I
Deposition
Nucleation
order. n-d log i/d
log r
potential 7/mV
Pre-oxidized
300
275 225
2.27 2.70 2.80
1.25 I.!90 2.40
175
2.70
2.20
peak disappears initial and
completely.
monolayer growth
of
of
PbO,
PbO,
deposition
of PbO,
nucleation
of
ground
electrode
processes,
The first reduction which
can but
no
due
must
occur_
on tin oxide
PbO,
Oxide-free
A
electrode
peak may therefore
be deposited similar
and
correction
was
distortion
attempted_
A
was
introduce of
correspond
top of which
phenomenon
[ 111. This can effectively
to the severe
on
observed
notable
for
a time-lag
the transients feature
reduction peak is that it occurs a few mtlhvolts anodic to the reversible There was no evidence of similar monolayer formation on glassy carbon.
to an
nucleation
for the
from of
the
the
backfirst
potential.
(a) +lM
0
-150
-300
-LSO
0
-150
-300
-L50
Q -10 3._ -20 P It.1
l150
Fig. 4. Reduction (b) 40 s.
of PbO,
during cathcdic
sweep following
elrctrodrposition
at 150 mV for (a) 20 s and
430
Potential and temperature dependence of the eiecrrocgxtallisation rate constants on ghssy carbon disc-electrodes At
short times, the rate of a process which is limited by progressive
and growth of three-dimensional
nucleation
centres is given by
i = rFzM2Ak7r’/3p2
Owing to the observed spherical symmetry constants
parallel
to the substrate
of PbO,
crystallites
and perpendicular
[l],
the growth rate
to the substrate
have been
replaced by a single rate constant k.- Thus the transient rate gives a measure of the combined
nucleation
and growth rate constants.
Usually
a double pulse technique is
used to obtain an estimate of the growth rate constant independently,
but because of
the limitations mentioned previously potential dependence of the combined
adopted. The is depicted in
Fig. 5. Using
this data, Arrhenius
activation energy corresponding
[I], this technique was not rate at various temperatures
plots were constructed
to the combined
to obtain estimates of the
rate constant,
EpLk3_The values are
listed in Table2 The steady state deposition rate was also measured over a wide range of potential at various temperatures (Fig.@. The potential dependence does not follow the
Fig. 5. Variation gh~~y-carbon
RDE
OK elccrronys~allisation
“cohbincd-rate-constant”
with
potential
and
tcmpcrature;
431
TABLE
2
Energies of activation corresp4mding to the various rate constants PotentiJ
EA=
EC/
Ee.--3Ei/
v/m+
k.kJ/ kJ mol-’
250
291.6
36.2
183.0
275 ho
280.5 266.8
33.3 31.7
180.6 171.7
kJ mol-’
kI mol-’
expected Tafel behaviour and it is in disagreement with earlier results [IO]. For these reasons the measurements were carefully repeated in order to confirm the trend observed.
Similar
dependence
is observed
(Fig. 7), thereby
ruling out the presence
dence is similar
to that observed
steep
absence
[12]. The
deposition
state deposition, growth,
If
of elimination
lattice incorporation
this argument
.deposition
is correct
brings
remains
then i,
indicate-s that the steady-state Under
the conditions
of steady
us to the possibility
that even at
to E and
the steady
rate constant_ From an Arrhenius
the activation energies at different potential
values are included
in Table 2. From-values
of EA_k~-
EL are obtained
state
plot of
and these
EC = EA, values of activation
C
.-. 50.
loo
150
for
the slow step, as it is at low coverages.
is proportional
rate is a measure of the growth
log i,, vs l/T
depen-
but it is not as
cannot be rate limiting ualess death and re-birth processes are
[13j. This process
high coverages
effect_ The potential
rate constant”
behaviour controlled_
on platinum
there is a large and virtually constant number of nuclei available
so nucleation
involved
transfer
state deposition
of a substrate
For the “combined
of a Tafel-like
rate is not charge
for steady
200
250
300 d
mv
.Fig 6; Dependence of steady-state deposition rate on potential and temperature; glassy-carbon RDE
100
150
200
250
300
350
LOO
Fig. 7. Dependence of steady-stale deposirion raw on potential; stationary plahum
energies compared involves one Some
atom
fx
nucleation 40
that
for
the interaction or molecule
evidence
foil elecrrod~ 23°C.
are deduced. The activation energy for nucleation is high growth. This seems reasonable because a nucleation event of several
atoms
or molecules,
whereas
growth
involves
only
per event_
of diffuiorr
of PbO,
into t.ie Pr disc-substrate
Whife making a microscopic study of the PbO, deposit on a platinum disc, it was observed that the PbO, is unstable. A bright Pt disc electrode covered with PbO, has the appearance of rusted steel. On leaving the electrode in distilled water for a few minutes immediately after PbO, deposition, there is a distinct lightening_of the rust colocr. This deposit was dissolved in hydrochloric acid and the bright platinum surface was regained. When the cleansed platinum electrode was left standing iu water for a few hours, the surface was re-tarnished with a faint rust-coloured deposit. This deposit was scraped with a blade and analysed using atomic absorption spectroscopy, which showed a strong absorption peak corresponding to lead-~What had probably occurred was a surface reconstruction involving the deposited PbO, and the underlying platinum oxide_ PbO, diffused into the underlying platinum
.~ oxide and after the surface deposit had been dissolved, re-tarnish
the platinum
the lead oxide diffused
out to
surface.
SUMMARY
The
present
investigations
occurs through 3;D nucleation on platinum.
have
confirmed
that electrocrystallisation
and growth on glassy carbon
The data on platinum
contains
considerable
of
and most probably
PbO, also
noise from simultaneous
processes and is not amenable for analysis in a simple way. The steady state behaviour is unusual because it has generahy been assumed that charge transfer is rate dete i-mining under most conditions. However,’ as shown here, if lattice incorporation is slow even at the steady state, it provides a method for separating the activation energies associated with the nucleation and growth rate constants. Finally, the dissolution of PbO, into the platinum substrate and its implication on transient rates merits more attention_ In this context the Pt-PbO, system is probably not a good choice because of the large contribution of substrate oxidation to the measured chemical
transient
rates.
ACKNOWLEDGMENT
This work has been supported by a strategic grant from the Natural Sciences and Engineering
Council
of Canada.
REFERENCES 1 R-G. Barradz and A-Q. Contractor, J. Ekcrroanal. Chem., 129 (1981) 327. 2 S. Toszhev in P. Human (Ed.), Crystal Growth: An Introduclion, North-Holland Amsterdam. 1973. 3 J. Zeldovich. J. Ekpd. Theorel. Phys. (U.S.S.R), 12 (1942) 525. 4 J-C. Fisher, J.H. Holloman and D. Turnbull, J. Appl. Phys.. 19 (1948) 775. 5 A. Kanuowie J. Chem Phys.. 19 (1951) 1097. 6 F.C. Collks, Z. Elektrochem.. 59 (1955) 404. 7 H-L. Frish, J. Chcm_ Phys., 27 (1957) 90. 8 W.G. Courmey, J. Chem: Phys.. 36 (1962) 2009. 9 D. Kaishchiev, Surface sci., 14 (1969) 209. 10 M. Fkkchmann and M. L&r, Trans. Farad Sot.. 51 (1953) 1370. II H.H. Laitinen and N.H. Watkins, J. Electrochem. Sot., 123 (1976) 804. 12 RG. Ilan-adas and J.D. Porter, Electrochim. Acta, 27 (1982) 541. 13 M.Y. Abyaneh and M. Fkkchmann, J. Eleccroanal. Chcm.. 119 (1981) 187.
Publ. Co.,
425
A C&PkA.IXVE POTENTIOSTATIC STUDY OF THE ELECliR6CRYSTALLIZATON bF PbO, ON PLATINUM CARBON ELE&RODES
RG- EIARRADAS and AQ. CONTRACTOR Department of Chemistry, Corleron Uniwsify,
AND GLASSY
* 011auq
Onrario KIS
5B6 (Canado)
(Received 29th January 1982)
ABSTRACX The deposition of Pb& on plakmm and glassy carbon eleckodes wx invesrigted experimentally. The comparison of the different behaviour on the two types of electrodes is presented. The effects of variation of [emperkre, potential. nucleation order and induction time-lags on the elccrrocrystallisation proce.G are discu&ec.
INTRODUCTION
In an earlier publication we have discussed the essential characteristics of the potentiostatic transients due to electrochemical nucleation and growth of PbO, on ghsy carbon [l]_ In the present communication we examine this process in greater detail and compare it with the deposition of PbO, on platinum. The concept of induction times for nucleation has been investigated and its significance to electrocrystallization data will be assessed. Data on steady state rate of the process will also be presented. In addition we discuss some results which indicate that platinum is not a fully in&t substrate for the electroc~&llisation of PbO,_ EXPERIMENTAL
~.
.The experimental conditions used in these studies have been described previotisly [l]. The experiments with platinum were carried out in one case with a platinum rotating disc electrode and iri other c&&s with platinum foil electrodes (area -0.5 cm’) in unstirred solutions_ The results from both types of platinum electrodes were rather ~Similar; the quantitative data reported here pertain to platinum foil ekctrades. Whenever ‘rotating platinum or rotating glassy carbon electrodes were used,
f-tint
addrcsx J&rtment
of-Chemislrv;Texz-A
and M University. CoIlwe Station. Texas 77843.
U.S.A. 0022kW28/82/KIOO-OOOO/SO2.75
0 1982 Ekvin
Sequoia S.A.
426
the experiments were carried at a rotation speed of 10 Hz. A Stonehart B.C. 1200 Potentiostat was used in all electrochemical experiments. RESULTS
AND
DISCUSSION
Initial stage of deposition on glassy carbon electrodes
The electrocrystallisation of PbO, on glassy carbon was investigated at temperatures between 20” to 60°C and at overpotentials from 200-235 mV_ The form of the growth transient is similar to that reported earlier [I]. In all cases the electrocrystallisation transients correspond to a nucleation order of three, indicating that the deposition occurs by progressive nucleation and growth of three-dimensional centres_ The data fit the geometrical model reasonably well without the need to invoke induction times. This aspect must be stressed because of the frequent incorporation of induction times in most previously published electrocrystallisation studies. The concept of time-lag in nucleaiion has been discussed in an excellent review by Toschev [2]. In electrocrystallisation, induction times may arise in three ways. The first and most generally considered cause of time-lag in electrochemical nucleation is related to the non-steady state of nuclea:ion [3-91. Induction times could also be attributed to the stochastic nature of nucleation_ This type of induction time is given by an exponential law_ Therefore, with a macro-electrode having a large number of possible nucleation sites, this time-lag would be vanishingly small_ Finally, a time-lag could also appear if some kind of electrochemical transformation of the electrode surface is required before nucleation and growth can occur. Apparently for the deposition of PbO, on glassy carbon, time-lags from the above mentioned sources are not significant compared to the time scale of nucleation and growth. Initial stage of deposition on platinum
Electrocrystallisation of PbO, on platinum RDE and platinum foil electrodes was investigated under the same conditions as those used for glassy carbon. A specimen nucleation and growth transient at a stationary platinum electrode is shown in Fig. 1. In contrast to glassy carbon, the nucleation orders obtained from the transients using platinum electrodes vary considerably over the potential range studied. The nucleation orders are also sensitive to the time elapsed at the reversible potential (i.e.. at q = 0) prior to stepping to the deposition potential. The variation of nucleation order with deposition potential is shown in Pig. 2, where the elecirode was held at the reversible potential until the residual current was less than 1pA before stepping to the deposition potential. These results do not conform to any of the geometric models for nucleation and growth, and are at variance with those reported earlier [lo]. For this reason a large number of experiments were performed with both the rotating disc and platinum foil electrodes before presenting our findings_ Though the nucleation orders do not seem to agree with a three-dimensional nucleation and growth model, the micrographs of the deposit reveal hemi-
427
B 0
=_ .u-l
‘0
” -a
t!?
0
0 a
0
0
a 123LS6
125
175
225
0
275
.t/s
t&V
Fig. 1. Initial portion of the current transient in reqonse in 0.1 M Pb(OAc),
+ I M HOAc;
to a potential step from 0 to t275 mV at 23’C
stationary platinum electrode.
Fig. 2. Variation of nucleation order with deposition poLential at (0) 23T Pb(OAc), + 1 M HOAc; stationary platinum electrode.
spherical
325
crystals (Fig. 3)
Electrocrystallisation
transients
60°C;
on platinum
seem to be
by contributions
the time required
for decay of current after the initial sharp rise is quite significant
carbon,
to the duration
of
the transient_
Under
processes.
0.1 M
severely distorted compared
fro-m background
and (a)
similar
As shown in Fig. 1,
conditions
initial portion of the transient does not affect the results significantly. the large contribution the substrate,
to the background
as shown
of transients obtained
H,SO,
prior to being introduced
was held at the reversible .potential potential_ Nucleation orders derived listed in Table
1. Pre-oxidised
transients contain
followed
of
anodically
polarized
in
electrolyte_ The electrode
for 10 s before stepping up to the deposition from the transients under these conditions are
model.
a large contribution
oxidation
Clearly,
the normal
orders closer to
electrocrystallisation
from substrate oxidation
and therefore were
suitable for further analysis.
Some interesting was examined
were
of the
with pre-oxidised
electrodes consistently gave nucleation
that predicted ~by the geometric not considered
which
into the working
glassy
With platinum
current is from simultaneous
by 2 comparison
electrodes and initially oxide-free electrodes. Pre-oxidised electrodes refer to electrodes
0.05 M
on
limiting current is not attained even after 25 rnin and hence distortion
features were revealed
using a cathodic
when
linear potential
and the resulting voltammograrns
the deposit sweep.
are recorded
The
formed
at short times
potential
in Fig. 4. When
programme a cathodic
linear potential sweep was applied after deposition at 150 mV for 2Os, a single reduction.peak was observed_ If deposition was allowed to continue for 40s two reduction increasing
peaks were observed on the subsequent linear potential sweep. With deposition-times, charge under the second peak increases whereas the first
Fig. 3. Scanning sktron micrograph of deposit formed a~ +275 Pb(OAc), + 1 M HOAc; magnification 10000X. This deposit st;lrionary Pr wire for convenience of SEM observation.
mV. 23°C on platinum electrode; 0.1 M was formed on the cross seetion of a
429 TABLE
I
Deposition
Nucleation
order. n-d log i/d
log r
potential 7/mV
Pre-oxidized
300
275 225
2.27 2.70 2.80
1.25 I.!90 2.40
175
2.70
2.20
peak disappears initial and
completely.
monolayer growth
of
of
PbO,
PbO,
deposition
of PbO,
nucleation
of
ground
electrode
processes,
The first reduction which
can but
no
due
must
occur_
on tin oxide
PbO,
Oxide-free
A
electrode
peak may therefore
be deposited similar
and
correction
was
distortion
attempted_
A
was
introduce of
correspond
top of which
phenomenon
[ 111. This can effectively
to the severe
on
observed
notable
for
a time-lag
the transients feature
reduction peak is that it occurs a few mtlhvolts anodic to the reversible There was no evidence of similar monolayer formation on glassy carbon.
to an
nucleation
for the
from of
the
the
backfirst
potential.
(a) +lM
0
-150
-300
-LSO
0
-150
-300
-L50
Q -10 3._ -20 P It.1
l150
Fig. 4. Reduction (b) 40 s.
of PbO,
during cathcdic
sweep following
elrctrodrposition
at 150 mV for (a) 20 s and
430
Potential and temperature dependence of the eiecrrocgxtallisation rate constants on ghssy carbon disc-electrodes At
short times, the rate of a process which is limited by progressive
and growth of three-dimensional
nucleation
centres is given by
i = rFzM2Ak7r’/3p2
Owing to the observed spherical symmetry constants
parallel
to the substrate
of PbO,
crystallites
and perpendicular
[l],
the growth rate
to the substrate
have been
replaced by a single rate constant k.- Thus the transient rate gives a measure of the combined
nucleation
and growth rate constants.
Usually
a double pulse technique is
used to obtain an estimate of the growth rate constant independently,
but because of
the limitations mentioned previously potential dependence of the combined
adopted. The is depicted in
Fig. 5. Using
this data, Arrhenius
activation energy corresponding
[I], this technique was not rate at various temperatures
plots were constructed
to the combined
to obtain estimates of the
rate constant,
EpLk3_The values are
listed in Table2 The steady state deposition rate was also measured over a wide range of potential at various temperatures (Fig.@. The potential dependence does not follow the
Fig. 5. Variation gh~~y-carbon
RDE
OK elccrronys~allisation
“cohbincd-rate-constant”
with
potential
and
tcmpcrature;
431
TABLE
2
Energies of activation corresp4mding to the various rate constants PotentiJ
EA=
EC/
Ee.--3Ei/
v/m+
k.kJ/ kJ mol-’
250
291.6
36.2
183.0
275 ho
280.5 266.8
33.3 31.7
180.6 171.7
kJ mol-’
kI mol-’
expected Tafel behaviour and it is in disagreement with earlier results [IO]. For these reasons the measurements were carefully repeated in order to confirm the trend observed.
Similar
dependence
is observed
(Fig. 7), thereby
ruling out the presence
dence is similar
to that observed
steep
absence
[12]. The
deposition
state deposition, growth,
If
of elimination
lattice incorporation
this argument
.deposition
is correct
brings
remains
then i,
indicate-s that the steady-state Under
the conditions
of steady
us to the possibility
that even at
to E and
the steady
rate constant_ From an Arrhenius
the activation energies at different potential
values are included
in Table 2. From-values
of EA_k~-
EL are obtained
state
plot of
and these
EC = EA, values of activation
C
.-. 50.
loo
150
for
the slow step, as it is at low coverages.
is proportional
rate is a measure of the growth
log i,, vs l/T
depen-
but it is not as
cannot be rate limiting ualess death and re-birth processes are
[13j. This process
high coverages
effect_ The potential
rate constant”
behaviour controlled_
on platinum
there is a large and virtually constant number of nuclei available
so nucleation
involved
transfer
state deposition
of a substrate
For the “combined
of a Tafel-like
rate is not charge
for steady
200
250
300 d
mv
.Fig 6; Dependence of steady-state deposition rate on potential and temperature; glassy-carbon RDE
100
150
200
250
300
350
LOO
Fig. 7. Dependence of steady-stale deposirion raw on potential; stationary plahum
energies compared involves one Some
atom
fx
nucleation 40
that
for
the interaction or molecule
evidence
foil elecrrod~ 23°C.
are deduced. The activation energy for nucleation is high growth. This seems reasonable because a nucleation event of several
atoms
or molecules,
whereas
growth
involves
only
per event_
of diffuiorr
of PbO,
into t.ie Pr disc-substrate
Whife making a microscopic study of the PbO, deposit on a platinum disc, it was observed that the PbO, is unstable. A bright Pt disc electrode covered with PbO, has the appearance of rusted steel. On leaving the electrode in distilled water for a few minutes immediately after PbO, deposition, there is a distinct lightening_of the rust colocr. This deposit was dissolved in hydrochloric acid and the bright platinum surface was regained. When the cleansed platinum electrode was left standing iu water for a few hours, the surface was re-tarnished with a faint rust-coloured deposit. This deposit was scraped with a blade and analysed using atomic absorption spectroscopy, which showed a strong absorption peak corresponding to lead-~What had probably occurred was a surface reconstruction involving the deposited PbO, and the underlying platinum oxide_ PbO, diffused into the underlying platinum
.~ oxide and after the surface deposit had been dissolved, re-tarnish
the platinum
the lead oxide diffused
out to
surface.
SUMMARY
The
present
investigations
occurs through 3;D nucleation on platinum.
have
confirmed
that electrocrystallisation
and growth on glassy carbon
The data on platinum
contains
considerable
of
and most probably
PbO, also
noise from simultaneous
processes and is not amenable for analysis in a simple way. The steady state behaviour is unusual because it has generahy been assumed that charge transfer is rate dete i-mining under most conditions. However,’ as shown here, if lattice incorporation is slow even at the steady state, it provides a method for separating the activation energies associated with the nucleation and growth rate constants. Finally, the dissolution of PbO, into the platinum substrate and its implication on transient rates merits more attention_ In this context the Pt-PbO, system is probably not a good choice because of the large contribution of substrate oxidation to the measured chemical
transient
rates.
ACKNOWLEDGMENT
This work has been supported by a strategic grant from the Natural Sciences and Engineering
Council
of Canada.
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