Charge transfer reactions across oxide covered Titanium Electrodes

Abstract

The kinetics of the charge transfer process of the ferro-/ferricyanide Redox system on anodically formed titanium oxide layers is studied. The oxides were formed potentiodynamically in borate buffer solutions of pH = 8. Anodic and cathodic Tafel lines were measured at a scan rate of 1.0 mV . s-l. Oxide thickness was measured using capacitance technique at a frequency of 1.0 KHz.The log of the exchange cd, 6 for the Redox system was found to decrease linearly with the oxide thickness in agreement with the lntroduct ionSince the pioneering work of Makrides [l], the study of the kinetics of Redox reactions on anodically formed oxide films on different metals found increasing interest. He measured anodic and cathodic Tafel lines on oxide films formed on Ni, Fe and Ti electrodes. It was early established [2-41, that the exchange cds of a Redox reaction is much smaller on oxide covered electrodes (8 or 9 orders of magnitude) compared to those obtained on "bare" metals. Generally, polarization curves are reliable when changes of film thickness and film composition are avoided [5]. The process was studied later in details on several oxide covered electrodes using different Redox systems [6-121. The development of the theory of electron transfer [13-151 contributed to further understanding of the relation between exchange cd and film thickness. Tunneling of electrons was proposed to explain the results obtained for thin films. The transfer coefficient of the anodic process was smaller than the usual value of 0.5.In an earlier publication, we studied the formation of anodic oxide films onTi electrodes in different media [16]. Mainly Ti02 was found to be formed on the electrodes within a large potential range (-0.6-5.0 V NHE). The stability of the anodic oxide film on Ti, made it possible to study the kinetics of the ferdferrocyanide Redox couple on oxide-covered electrodes, which we report here.Heusler et al. [17] reviewed the theoretical consideration of the theory of electron transfer across semiconductor oxide films. The work presented here is influenced by Heusler's theoretical treatment. it possible to avoid the use of supporting electrolyte. The oxide was formed potentiodynamically until a certain denoted potential in pure borate buffer at pH 8. The use of galvanostatic or potentiostatic technique to form the oxide led to irreproducible Tafel lines for the Redox system. The electrode was transferred from the solution in which it was formed (pH = 8) to the solution which contains equal concentrations of the Redox couple. The solution was Nzdeaerated for 30 min during which the potential was followed as a function of time. Generally steady-state open circuit potentials were established within this period of time.The potential was then scanned in the anodic and then in the cathodic direction using a scan rate of 1 mV . s-l. ExperimentalA simple and approximate method was used to estimate the oxide thickness using the capacitance measurements. This was performed at a frequency of 1.0 KHz and oxide thickness was calculate...

Charge transfer reactions on oxide‐covered Ti Electrodes: Effect of dielectric constant variation and oxide stabilization

“…From this relation, one can determine the order of reaction with respect to Fe'+/Fe*+ concentration form the slope of the straight line according to the equation [8]:i, = K Cn (4)where K is a constant, C is the redox concentration and n is the reaction order. A value of 0.9 was obtained for the reaction order which is in good agreement with the values obtained for other redox systems, like vanadium (V/IV) [l] and Fe(CN)63-/Fe(CN)64-[4] on passive titanium. Log Credox I M Relation between log io calculated for oxide formed at 0.6 V (Hg/HgzSO4) in aerated Na2S04 + HzS04 solution (pH = 3) and the logarithm of redox concentration…”

“…In a previous work [4], the kinetics of the charge transfer process of the ferri-ferrocyanide redox couple on oxide covered Ti electrodes formed in aerated and deaerated borate buffer (pH = 8) were discussed. The results indicated an independency of the transfer process on the conditions of oxide film formation, and tunnelling of the electrons represents the main process for the charge transfer through thin oxides.…”

“…These flaws are then repaired as the potential increases leading to a decrease of log io due to the increase of oxide thickness [4,8,161.…”