KINETICS AND MECHANISM OF THE SILVER (I) OXIDE TO SILVER (II) OXIDE LAYER ELECTROOXIDATION REACTION

Ah&act-The electroformation of Ag(II) oxide layer during the anodization of silver in 0.1 M NaOH is investigated under potentiostatic and potentiodynamic conditions. Results are discussed in terms of nucleation and growth models and statistical analysis ofinduction times related to the nucleation kinetics of Ag(II) oxide crystals. The best fitting of results comes out from the application of a progressive nucleation and 3-D growth model under mass transfer control where diffusion of species from the electrode to growing sites is essential for further expansion.


INTRODUCTION
The anodization of silver electrodes in alkaline solutions has been extensively investigated.Reviews on the subject are given in references[l-51.
The anodic reaction involves two oxidation levels corresponding to Ag,O and AgO formation.The corresponding reactions were principally investigated at 298 K, although data down to 258 KC33 and up to 478K has been reportedC6, 71.
The first oxidation level comprises as a first step the formation of a AgOH monolayer[S-111 followed by thickening to produce a homogeneous Ag(1) oxide primary layer.A second An(I) oxide layer is formed through nucleation and 3-D growth under diffusion control of isolated Aa oxide centersrl2-151.The second oxidation level of silver anodization corresponds to the formation of a Ag(I1) oxide layer on top of the complex A&I) oxide layer [5].Earlier kinetic studies of Ag(I1) oxide formation[ll,  were interpreted through different mechanisms, that is either a conventional heterogeneous electrochemical reaction mechanism, such as the autocatalytic mechanism [17] and the transfer of O* -ions at the Ag,O-AgO interface [16], or nucleation and growth rate controlling processes [5,10,11,18,193.These models, however could not explain the marked decrease in the Ag(I1) oxide charge with potential result-.ing from the crystal formation under a constant applied potential set in the 0.51-0.56V range (vs see).Nevertheless, despite these discrepancies it should be noticed that the kinetic interpretation based upon a progressive nucleation of AgO centers coupled with 3-D growth appears as the most satisfactory one[l8].It implies a mechanism of lattice formation based on the concentration and potential dependence of crystal growth.
The present paper refers to the electroformation of the Ag(I1) oxide layer under potentiodynamic and potentiostatic conditions.Data are discussed in terms of both a nucleation and growth model which accounts for the entire current transient bchaviour complemented with a statistical analysis of the induction times related to the nucleation kinetics of the Ag(II) oxide crystals.

Working
electrodes were made of polycrystalline (PC) Ag (99.99% purity) rods axially embedded in Araldite cylindrical holders to obtain circular silver exposed areas of 0.05 cm' apparent area surrounded by an insulating ring of 0.6 cm outer dia.The electrodes were mechanically polished starting with fine grained emery paper and followed with alumina paste of 1 pm dia. to obtain mirror polished silver electrode surfaces.Before the electrochemical measurements the electrodes were degreased with alcohol and rinsed with triply distilled water.Precautions were taken to avoid crevices between Ag rods and Araldite holders which could lead to artifacts in the electrochemical measurements.The counter electrode was a large Pt sheet located in a separate cell compartment.The potential of the working electrode was measured against a saturated calomel electrode (see) connected to the working electrode cell compartment through a Luggin-Haber capillary tip.Potentials in the text are referred to the see.The electrolyte solution was 0.1 M NaOH prepared from triply distilled water and AR chemicals.Solutions were bubbled with purified nitrogen for 3 h prior to the electrochemical runs.The working electrode was subjected only to a single triangular potential scan between the cathodic (E,,.) and the anodic (E,,+) switching potentials.Repetitive potential scans were specifically avoided as the electroreduction of the silver oxide layer results in reformed [5] Ag surfaces made of a large number of overlapping nuclei with complex diffusional paths.For this reason a new fresh polished Ag electrode was required for each measurement.Current transients at constant potential were obtained in the conventional way by using the perturbing potential programs described in the text.In all these cases the Ag electrodes were held at Es., = -1.20 V, for 60 s, to start every electrochemical run with a reproducible electroreduted silver electrode surface.All measurements were made at T=25 "C.Experiments were also made by using a working electrode consisting of a Ag(1) oxide layer chemically precipitated on the base of a spectroscopy grade graphite rod (0.28 cm').The as indicated elsewhere [20].

Voltammetric data
The j-E profile of a polycrystalline silver electrode immersed in 0.1 MNaOH run at u=O.Sx 10m3 Vs 1 between E,,, = -0.2V and E,,.=0.65 V shows up in the positive potential going scan peaks A', and A;' at 0.23 V and 0.28 V, respectively, followed by a current decay until the potential reaches 0.53 V (Fig. 1).By further increasing the potential just a few mV a sudden increase in current defining a very sharp peak (AZ) located at 0.55 V can be observed.The returning scan from 0.6 V downwards shows a broad peak (C,) at 0.37 V and a sharp peak (C,) at 0. The current transients exhibit the following features.Initially the current remains nearly constant (background current) for a certain time.This time can be considered as an induction time (ti) related to the oxide conversion process (insert in Fig. 4).Later for t, z ti, the current increases to attain a maximum (Id at the time t,, and finally for t>t, the current decreases markedly to approach the initial background current (Fig. 4).As E, shifts in the positive direction both ti and t, decrease and I, increases, but 4. the charge density involved in the transients decreases sharply as E, increases (Fig. 5) in contrast to the expectations for a single layer nucleation and growth process [lS].
The behaviour of these current transients is similar to those described previously.This charge decrease can be related to the change in the multilayer structure of the anodic layer.As the E, is positively increased it appears as if a more compact outer AgO layer is produced, ie the passive character of this layer increases, so that only a relatively smaller fraction of Ag,O transforms into AgO.This decrease in the yield of Ag,O to AgO reaction has been observed at low temperatures due to slow nucleation [3].
Relevant information about the growth mode of the Ag(I1) oxide nuclei can also be derived from the analysis of the initial rising part of the current transients which in all cases fits j us t3 linear relationships (Fig. 6) which go through the origin of coordinates because the time scale was set for t = ti = 0. Furthermore, linear log jM us E, and log rM us E, plots are also obtained, the slope of the former being close to 0.06 V dec-' (Figs 7 and 8).
The time dependence of P,> i, the probability for the formation of at least one nucleus, can be used to probe new aspects of the nucleation kinetics.For a Poisson distribution and a stationary nucleation rate, <, the mean value of the induction time, is given by [22]: (1) 0 where aN, is the nucleation rate, a is the nucleation rate constant and N, denotes the number of sites available for nucleation.To test Equation (1) a large number of ti were measured for each E. value so that P n L 1 was obtained as a function of time.The P, z 1 us t plots (Fig. 9) indicate that as Es moves in the positive direction the survival time, i.e. the time for which the sample remains free of AgO nuclei, is drastically reduced.

DISCXBSION
The present results can be directly discussed within the framework of certain controversial theories about the growth mode of the Ag(I1) oxide centers during silver anodization at relatively high anodic potential in basic solutions, in particular the participation of a  ence of mass transport limitations (Fig. 2).Otherwise, it should be noticed that 3-D growth under diffusion control requires either Z us t"' or Z us t312 linear relationships for the initial rising part of the current transients and linear Z us t iI2 relationship for t + co.
However, the initial rising part of the transients actually fit linear Z OS t3 instead of Z vs t312 linear relationships (Fig. 6) and the portion of the transients after the maximum decays faster than that expected for a Z us tell2 plot.Nevertheless, despite these drawbacks for the present system, the progressive nucleation and 3-D growth under charge transfer control where diffusion of species from the electrode to the growing sites is essential for further expansion seems to be applicable [23, 241.For such a model the current-time transients for the case where pyramids are right circular cones is given by the following equations [23]: where j(t) is the instantaneous current density, P, =zFk,, P, = aM'kfuN,f 3p2, M is the molecular weight and p is the density of the electroformed deposit, N, is the number of sites available for nucleation, a is the nucleation rate constant, so that aN, is the nucleation rate, and k, and k, are the specific rate constants for crystal growth perpendicular and parallel to the electrode surface, respectively.The final factor in Equations (2) and (3) accounts for the reduction of growth caused by the diffusion of species to the growth sites.It can be noticed that the decrease in q as E, increases (Fig. 5) indicates that certain "passivation" of the electrode probably occurs through the formation of a more compact Ag(II) oxide layer at positive potentials which prevents the access of the .,r\ ._reactants to the Ag(1) oxide layer.From the q us E, dependence (Fig. S), the change in the average thickness of the AgO layer with E, can be estimated from: h= $ q.

BY taking
MAgZoI = 246 g mol-*, z=2, P' 7.143gcmm3; h becomes 2 and 0.4 pm for ES= 0.51 and 0.55 V, respectively.Equation (3) predicts a linear I IX t3 plot for t-0, and linear log I, vs E, and log t, vs E, relationships which have been proved experimentally (Figs 7 and 8).Furthermore, a reduced variable test for different E, after ti and background current subtraction, shows an excellent agreement with the theoretical curve (Fig. 10).Unfortunately, information about the nucleation stage can not be obtained from the transients as P, contains the composite kfaN, term, but it can be derived through the statistical analysis of ti and by using the [1 -P,> 1] us t and In /? us q plots, where 1 corresponds to an average nucleation rate derived from the plots shown in Figs 11 and 12. Therefore, on the basis of an irreversible electrochemical nucleation kinetic formalism [25], the value of the rate aNO as derived from n/u ratio, a being the electrode area, allows now the calculation of the rate constant k, from the value of P,, and the rate constant k, directly from P,.Both rate constants fit log k, and log k, vs ,I plots with slopes close to 0.06 V decade (Fig. 13) but at a fixed potential the value of k1 is practically three orders of magnitude greater than that of k,.This difference would explain the fact that under certain conditions the overall process would approach a 2-D growth model.Furthermore, it should be noticed that for a progressive nucleation and 2-D growth under charge transfer control j(t) is given by [26]:

Fig. 1 .Fig. 2 .Fig. 3 .
Fig. 1 with nearly the same qc2/qr, ratio.This fact is a clear indication that the process occurring in the