extracting nucleation rates from current–time transients. concluding remarks
TRANSCRIPT
Discussion
Extracting nucleation rates from current�/time transients. Concludingremarks
Stephen Fletcher *
Department of Chemistry, Loughborough University, Ashby Road, Loughborough, Leicestershire LE11 TU, UK
Received 2 December 2001; accepted 24 May 2002
Abstract
Some time ago, Abyaneh and Fleischmann submitted several papers to this journal in which they modelled the heterogeneous
nucleation of crystals as a first order kinetic process. In an invited response, I argued that such an approach was seriously flawed,
because it ignored nucleation rate dispersion. More recently, instead of responding directly to this criticism, Abyaneh and
Fleischmann have challenged the validity of the Deutscher�/Fletcher experiments that first established the physical reality of
nucleation rate dispersion. This note, therefore, serves two purposes. Firstly, it confirms the validity and high rigour of the original
Deutscher�/Fletcher experiments. Secondly, it identifies the errors in the Abyaneh�/Fleischmann papers. In conclusion, it is
emphasised that nucleation rate dispersion is real, universal, and must always be taken into account in experiments involving
heterogeneous nucleation. # 2002 Elsevier Science B.V. All rights reserved.
Keywords: Heterogeneous nucleation; Nucleation rate dispersion; Active sites; Current�/time transients; Alamethicin
I am grateful to Abyaneh and Fleischmann [1] for
their comments.On one matter, at least, we are in full agreement. . .
they surmise, and I can confirm, that I am indeed
dismayed by their lack of appreciation for the works of
Deutscher and Fletcher! I had hoped that Professor
Fleischmann in particular would have appreciated the
subtlety and beauty of the modern theory, given his
early efforts to establish nucleation as an electrochemi-
cal phenomenon. He was instrumental in proving the
widespread existence of nucleation phenomena in elec-
trochemical systems, an achievement still widely under-
rated in the electrochemical literature. For example, you
will search in vain for the word ‘nucleation’ in the index
to Bard and Faulkner’s textbook [2]. Yet many systems
of interest in electrochemistry, ranging from metal
deposition to corrosion, exhibit a phase change con-
trolled by nucleation kinetics. It is an important
problem.
Given the importance of nucleation, it is desirable
that researchers in the field should reach a common
understanding of the underlying physics, and a common
understanding of the mathematical tools for describing
them. Inevitably, the only way to attain this common
understanding is to embark on a focussed dialogue
concerning the existing theory.
Unfortunately, what Abyaneh and Fleischmann have
done in their latest comments is to expand the discussion
beyond purely theoretical questions, and criticised the
experimental work of Deutscher and Fletcher [3�/6].
This is unfortunate, because the experimental work of
Deutscher and Fletcher contains some of the best data
that we have. Widening the discussion in this way also
makes it difficult to provide a brief response, though I
shall try. Essentially, I shall assert that the works of
Deutscher and Fletcher stand unblemished, whereas the
experimental and theoretical works of Abyaneh and
Fleischmann stand in need of correction. Before em-
barking on this course, however, let me first present to
the general reader, who is perhaps unfamiliar with some
of the arcana of nucleation theory, a simple thought
experiment that distinguishes the approaches of the two
groups.Consider a ‘perfect’ electrode, i.e. one without defects,
impurities, grain boundaries, inclusions, etc. which
terminates in a ‘perfect’ surface without reconstruction.* Tel.: �/44-1509-222-561; fax: �/44-1509-223-925
E-mail address: [email protected] (S. Fletcher).
Journal of Electroanalytical Chemistry 530 (2002) 119�/122
www.elsevier.com/locate/jelechem
0022-0728/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 0 7 2 8 ( 0 2 ) 0 0 9 7 7 - 4
Every location on the surface of such an electrode is
defined to be identical to its neighbours; ipso facto
nucleation takes place at the same rate everywhere, and
nucleation rate dispersion is absent. Now consider whathappens if I pick up a piece of emery paper, or
sandpaper, or indeed any rough object, and scratch
the surface of the electrode. Does the new surface now
present only one type of active site towards nucleation
of a second phase, or more than one type?
If you are absolutely certain that only one type of site
exists on the scratched surface, you need never consult
the papers of Deutscher and Fletcher. They would beirrelevant. If, however, you even suspect that there may
be more than one kind of nucleation site, with each site
having its own activation energy towards nucleation,
then you will need a different formulation of nucleation
theory than that provided by Fleischmann. This differ-
ent formulation is the one provided by Deutscher and
Fletcher, and it has been justified both experimentally
[3] and theoretically [4]. Unfortunately, in the matter ofreal-world data, no compromise is possible between
these formulations. You have to choose either the
general model of Deutscher and Fletcher, or the
particular model of Fleischmann.
It is natural to wonder if a ‘reasonably uniform’
electrode surface might perhaps be prepared, upon
which the number of scratches, steps, or point defects
was decreased to an insignificant level. Alas, no. Thepoint is, with �/1013 atoms cm�2, even a few hundred
misplaced atoms would be sufficient to destroy the
energetic uniformity. This is because, by definition,
nucleation always occurs most rapidly on the active
sites that constitute the upper outliers of the rate
distribution, and this distribution is phenomenally
wide due to the high sensitivity of nucleation rates to
the interfacial free energies of the active sites. If therewere 100 highly energetic sites, for example, then that is
where nucleation would occur first. In short, nucleation
dispersion is unavoidable. This is the reason why
nucleation is so irreproducible in practice, and why
crystals decorate scratches. In the final analysis, any
satisfactory theory of heterogeneous nucleation MUST
take into account the fact that the interface is energe-
tically non-uniform.One can also reach the same conclusion from a
different angle. Imagine I have a collection of fine wires
made of different metals (such as Ag, Pt, Cu, etc.), and
suppose that I collect these together in a tightly packed
bundle and use their cross-sections as an electrode
surface. Thus a patchwork of different metals is exposed
to the solution, creating a micro-heterogeneous surface.
Suppose further that I deposit nuclei of a new phase onthis micro-heterogeneous surface. Do all regions of the
surface exhibit the same nucleation rate? Of course not.
The rate will vary from region to region, depending on
the local composition and structure. In this extreme case
it would be absurd to dispute the very existence of
nucleation rate dispersion. It follows, therefore, that we
are not arguing about the existence of nucleation rate
dispersion per se*/it clearly exists*/but rather, whetherit exists to a significant extent on an ‘ordinary’ electrode
surface prepared in an ‘ordinary’ way. To decide this
question, we merely need to know how uniform the
interfacial free energy must be before nucleation rate
dispersion can be ignored. The answer, as shown in ref.
[4], is that the interfacial free energy must be very, very
uniform indeed. Indeed, it needs to be uniform beyond
what has been achieved by anyone anywhere in theworld today, and possibly uniform beyond what is
thermodynamically possible! So once again we are
forced to conclude that any satisfactory theory of
heterogeneous nucleation MUST take into account the
fact that the interface is energetically non-uniform.
This is as clear as I can explain the existence of
nucleation rate dispersion without using maths. The rest
of this reply to the Abyaneh�/Fleischmann commentsmust necessarily be mathematical and historical.
Turning now to the specific comments of Abyaneh
and Fleischmann [1], they basically contain three types
of error. First, misrepresentations of the original papers
of Deutscher and Fletcher. Second, a hubristic insistence
that the assumptions of Fleischmann et al. are ‘well
established principles’. Third, terminological inexacti-
tudes. Let us consider each of these in turn.First, concerning the misrepresentations of the origi-
nal papers of Deutscher and Fletcher, we make the
following corrective remarks. (1) No errors of experi-
mental design were made. (2) Deconvolution does not
depend on the probabilistic character of nucleation. It
merely requires that nucleation events be independent.
Independence can readily be established by a series of
simple tests as discussed in the original papers. (3)Deconvolution does not depend on the reversibility of
the growth process, because the shapes of the growth
curves simply cancel. (4) Nucleation rate dispersion is
predicted by both the classical and atomistic models of
nucleation. We chose the classical model only as an
example; we could just as easily have chosen the
atomistic model. In any case, the existence of nucleation
rate dispersion is an empirical fact that is completelyindependent of any assumed model. (5) Deutscher and
Fletcher have nowhere claimed to have observed a
‘marked time dispersion’ as erroneously stated several
times by Abyaneh and Fleischmann in their reply. In
fact, what we have claimed is a marked rate dispersion.
(6) Deutscher and Fletcher have never described the
work of Abyaneh and Fleischmann as ‘heretical’.
Metaphors drawn from religion seem to us whollyinappropriate in a scientific discussion.
Second, regarding the theories of Fleischmann et al.,
we emphasise the following points. (1) The number of
crystals is not, in general, a Poisson random variable,
S. Fletcher / Journal of Electroanalytical Chemistry 530 (2002) 119�/122120
though it may appear so under some special conditions
(infinite N0). (2) The first order nucleation law is
physically unrealistic and there is no credible evidence
for it anywhere in the electrochemical literature. (3)‘Instantaneous nucleation’ is a contradiction in terms,
and simply does not exist. (4) The double potential step
method does not work as advertised. You cannot switch
nucleation on and off instantaneously. If you could, it
would violate the whole spirit of nucleation theory,
because in the real world the population of critical nuclei
responds slowly to rapid changes in driving force, due to
the fact that the nuclei must assemble molecule bymolecule via a series of step-wise size fluctuations.
Third, concerning terminological inexactitudes, there
are a number of quite bizarre examples in the Abyaneh�/
Fleischmann comments that, at times, make them
completely incomprehensible. Take, for example, the
word ergodic. As is well known, a random process is
stationary if all of its statistical moments are time
invariant, and in addition it is ergodic if its expectationvalues correspond to its time average values. By defini-
tion, therefore, all ergodic processes are stationary
processes. Now, since nucleation is not even stationary,
having both an acceleratory period (induction time) and
a deceleratory period (due to the exhaustion of active
sites) it is trivially obvious that nucleation is not ergodic.
Indeed, no transient process is ergodic. So why do the
authors insist that ‘We believed (and we still believe)that such a discussion (of the work of Deutscher and
Fletcher) should have been delayed until proper tests of
the ergodicity of the systems have been carried out’?
With respect, this is nonsense. The non-ergodicity of
nucleation has no bearing whatever on the reality of
nucleation rate dispersion.
The strange references to ‘time dispersion’ have
already been commented upon: that phrase too ismeaningless. In addition, I have no idea what is meant
by their phrase ‘compound stochastic system.’ Conven-
tional compound stochastic processes are well defined;
they are processes in which one time-dependent random
process triggers another; but Abyaneh and Fleischmann
seem to mean something quite different. My guess is
that they are simply distinguishing multi-nuclear ob-
servations from single nucleus observations, but if so Icannot see what point is being made. If my interpreta-
tion is correct, ‘compound nucleation’ is just the same
nucleation that we have all been dealing with for the
past several decades. There are also scattered references
to ‘nucleation rate constants’ when what are meant are
nucleation rates, an error which I have pointed out on
many occasions and which Professor Fleischmann
unaccountably refuses to correct. My guess is that‘ensemble average’ refers to the sample mean. There is
also a reference to ‘death processes’, without any further
explication. How are we to understand this? The only
death process normally considered in the theory of
stochastic processes arises in the description of reversible
processes (Kolmogoroff’s equations) rather than in
irreversible processes such as the nucleation of metals.
Are the authors seriously suggesting that macroscopi-cally growing metal crystals disappear again at over-
potential? If not, what is the point of this remark?
Examples of woolly thinking of this type abound, and
could be discussed almost indefinitely. However, this
small sample of errors concerning stochastic processes
should be enough to convince even the most sceptical
reader that something is seriously wrong with the
Abyaneh�/Fleischmann approach. Indeed, the wholetenor of the reply of Abyaneh and Fleischmann suggests
that they have completely missed the point of the
Deutscher�/Fletcher papers. Could it be that they have
confused nucleation rate dispersion with some sort of
induction time dispersion? I am beginning to think so.
Given the terminological confusions that evidently
exist regarding the probabilistic formulation of nuclea-
tion, let me briefly try to summarise present knowledgeusing standard notation. The observation of one active
site chosen at random constitutes a Bernoulli trial (i.e. a
simple yes/no decision depending on whether there is a
growing crystal or not), and so the number of crystals
N (t ) observed during a series of experiments on a fixed
number of N0 active sites of equal activity (Fleisch-
mann’s own model) is in fact a binomial random
variable not a Poisson random variable [7�/9]. Thevariance is zero when all the sites are empty and zero
when all the sites are full, as we would expect. The paper
of Bindra et al. [10], mentioned in the comments [1],
completely misses this latter point. The defects of the
Bindra approach were pointed out 17 years ago [8].
Nowadays, the field has moved on and contemporary
interest centres on N0 active sites of different activity, i.e.
nucleation rate dispersion. Unfortunately this area iscomparatively unexplored from a stochastic point of
view and only one analytical solution is known. This is
for the special case where the distribution of nucleation
rates can be approximated as a gamma random process
and N0 is asymptotically large. The probability density
function of the number of observed crystals N (t) turns
out to be that of the negative binomial distribution, and
the corresponding mean and variance have been pub-lished [7]. A unique and interesting feature of nucleation
rate dispersion is that the standard deviation of N (t) can
exceed its mean. (Unfortunately, this signature effect
will probably be hard to see for the case of finite N0.)
This is the current state of the art: further studies are
desirable.
Concerning the experimental systems mentioned by
Abyaneh and Fleischmann, there is little to add, exceptto comment on their unrepresentative character. The
nucleation of PbO2 on platinum, over most of the
overpotential range, is a rare example of nucleation at
super-high nuclear density (I know of only nickel that
S. Fletcher / Journal of Electroanalytical Chemistry 530 (2002) 119�/122 121
behaves similarly), and this would exclude it from
analysis by the Deutscher�/Fletcher experimental
method (though not the Deutscher�/Fletcher theory)
because the nuclei are too close together to remainindependent for long enough for decent measurements
to be made. From the point of view of stochastic
measurements there are other serious problems with
this system. Once crystals of PbO2 have been deposited,
it is almost impossible to remove vestigial fragments of
material left behind by the re-reduction process. These
are readily seen by microscopy and tend to act as new
nucleation sites in future measurements. As a conse-quence, the results become time-dependent.
The formation of ion channels in lipid bilayers by
alamethicin is an interesting example of a reversible
stochastic process, but the ‘quantized’ currents observed
should have raised serious doubts in the authors’ minds
about whether the process really is one of nucleation. I
strongly doubt it. The fluctuating sizes of nuclei inherent
in all nucleation models should manifest as fluctuatingcurrent responses, whereas constant current responses
are seen. In other words, the ion channels behave more
like stochastically triggered trapdoors. This suggests a
rate-determining step connected with a conformational
change rather than a nucleation process. In any case
such a system is hardly relevant to metal deposition
processes, which, to the best of my knowledge, have
never generated even one example of reversible nuclea-tion. So the death of nuclei is irrelevant in the present
context. Finally, the extraordinary claim at item (ix) of
ref. [1] cannot be allowed to pass without comment. This
states that ‘. . . in the development of work on the
compound stochastic systems (sic), it will not be
necessary to use arrays of microelectrodes in order to
ensure adequate separation of the diffusion fields; the
same objective can be achieved using vitreous carbonsubstrates.’ Well, if Abyaneh and Fleischmann were
actually to try this experiment, they would immediately
find that the large and slow capacitive charging currents
of vitreous carbon obscured the small currents needed to
ensure the diffusional independence of the nuclei.
In summary, all the criticisms of Abyaneh and
Fleischmann regarding nucleation rate dispersion are
ill founded. In a short note such as this, it is not possible
to track every error in their comments to its source, but
the principal defects are clear enough. The reader who
wishes to follow the correct mathematical arguments in
their full rigour is strongly recommended to consult refs.
[3�/9].
The first-order nucleation law for N (t) was a reason-
able hypothesis 40 years ago, before microelectrode
arrays were developed, but today it is unsustainable. It is
not a ‘well established’ principle. It is a discredited
principle.
References
[1] M.Y. Abyaneh, M. Fleischmann, J. Electroanal. Chem. 530
(2002) 108.
[2] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Fundamen-
tals and Applications, 2nd ed., Wiley, New York, 2001.
[3] R.L. Deutscher, S. Fletcher, J. Electroanal. Chem. 239 (1988) 17.
[4] R.L. Deutscher, S. Fletcher, J. Electroanal. Chem. 277 (1990) 1.
[5] S. Fletcher, in: M.I. Montenegro, M.A. Queiros, J.L. Daschbach
(Eds.), Microelectrodes: Theory and Applications NATO-ASI,
vol. 197, Kluwer Academic Publishers, Dordrecht, 1991, pp. 341�/
355.
[6] R.L. Deutscher, S. Fletcher, J. Chem. Soc. Faraday Trans. 94
(1998) 3527.
[7] R.L. Deutscher, S. Fletcher, J. Electroanal. Chem. 164 (1984) 1.
[8] S. Fletcher, J. Electroanal. Chem. 164 (1984) 11.
[9] S. Fletcher, J. Electroanal. Chem. 215 (1986) 1.
[10] P. Bindra, M. Fleischmann, J.W. Oldfield, D. Singleton, Faraday
Disc. Chem. Soc. 56 (1973) 180.
S. Fletcher / Journal of Electroanalytical Chemistry 530 (2002) 119�/122122