The role of defects, or active states, in surface electrochemistry with particular
reference to gold in neutral solution
L.D. BURKE, A.M. O’CONNELL and A.P. O’MULLANE
Chemistry Department, University College Cork, Cork, Ireland
Received 9 September 2002; accepted in revised form 13 June 2003
Key words: active states, electrocatalysis, gold, hydrous oxides, neutral solution
Abstract
Metastable, active, or nonequilibrium states due to the presence of abnormal structures and various types of defects
are well known in metallurgy. The role of such states at gold surfaces in neutral aqueous media (an important
electrode system in the microsensor area) was explored using cyclic voltammetry. It was demonstrated that, as
postulated in earlier work from this laboratory, there is a close relationship between premonolayer oxidation,
multilayer hydrous oxide reduction and electrocatalytic behaviour in the case of this and other metal electrode
systems. Some of the most active, and therefore most important, entities at surfaces (e.g., metal adatoms) are not
readily imageable or detectable by high resolution surface microscopy techniques. Cyclic voltammetry, however,
provides significant, though not highly specific, information about such species. The main conclusion is that further
practical and theoretical work on active states of metal surfaces is highly desirable as their behaviour is not simple
and is of major importance in many electrocatalytic processes.
1. Introduction
Little attention has yet been given in surface electro-
chemistry to the detail that solid metals can trap and
store energy and exist in metastable or nonequilibrium
states. Metal atoms at defect sites are often protruding
species which are thermodynamically active as they have
a low lattice stabilization energy; also, atoms present in
microcluster states are unusually active due to quantum
confinement effects. Such states are well known for bulk
metals in metallurgy [1, 2] and for thin metal films in the
microelectronics fabrication industry [3, 4]. Generally,
the equilibrium state of a metal is highly ordered, with
virtually all lattice points occupied. Metastable states
and energy storage are associated with lattice defects,
for example, the metal may be amorphous or poorly
crystallized or contain an unusually high density of
extended defects (dislocations) [2]. The metastable state
is difficult to investigate as (i) it is almost infinitely
variable, the type and density of defects, or the degree of
noncrystallinity, being not easily controlled, and (ii)
since such states are intrinsically unstable they are prone
to alter easily with time or treatment. However, the
behaviour of metals in such states is of increasing
importance as may be judged by the market value of
metastable metals which was quoted in 1991 at about
US $0.3 billion per annum [1].
It is widely accepted in surface and interfacial science
that defects, which are the essence of energy storage in
metals, are common at real surfaces. According to
Adamson [5] ‘not all atoms on the surface are equivalent
in nature; those present at ragged asperities are much
more energy-rich than those with a normal number of
nearest neighbours and possess a higher than average
surface energy and surface mobility’. Attempts to
circumvent the surface defect problem by the use of
single crystal plane surfaces seem to be frustrated by the
fact that these model systems usually contain plenty of
imperfections [6, 7]. More importantly, from a surface
catalysis viewpoint, it has been pointed out that rough-
ness, or disorder, is a prime requirement for high
catalytic activity [8]; this may be regarded as a restate-
ment of Taylor’s active site theory of heterogeneous
catalysis [9].
Metal activation and its effect on surface electro-
chemistry has been a topic of research in this laboratory
for several years. The type of activation in question here
is energetic (or thermodynamic, l
Me
) l8
Me
) rather
than kinetic, and in most cases the high energy state was
achieved by doing work on the metal sample under
investigation. Activation techniques used earlier include
potential cycling + abrasion (for Pt [10]), thermal
pretreatment (for Pt and Au [11–13]), cathodization or
hydrogen embrittlement (for Au, Pt and Cu [14, 15]) and
repeated multilayer oxide growth and reduction (for Pd
[16]). The mechanism of energy storage involves exten-
sive defect generation; this may entail an increase in
surface area but, much more importantly, it involves a
severe reduction in the lattice stabilization energy of
surface metal atoms. These more active atoms tend to
Journal of Applied Electrochemistry 33: 1125–1135, 2003.
1125
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2003 Kluwer Academic Publishers. Printed in the Netherlands.