Computer Simulations of the Semiconductor‐Metal Transition in Ru/Ti and Ir/Ti Mixed Oxides

Abstract

The mixed oxide films, MxTi~ =O2 (M = Ru, Ir), formed by anodic oxidation of M-implanted titanium alloys exhibit a transition from semiconducting to metallic behavior for a M/Ti ratio on the order of 0.1. Two-dimensional computer simulations are used to describe the process by a percolation model. Analysis of results of the calculations in terms of the composition dependence of the number of "metallic" clusters and of the size of the largest one suggests a mechanism involving the creation and the spreading of impurity bands in the forbidden bandgap. Metallic behavior would be obtained by the overlap of impurity bands with the conduction band.Mixed oxide layers with electrocatalytic activity for the chlorine reaction have been formed (1) by anodic oxidation of titanium alloys produced by ion implantation of Ru (or Ir) into titanium. Ion implantation produces a Gaussian composition profile in the titanium. By a continuous (slow) oxidation/corrosion process, with the electrode held at a constant potential, the implant profile is exposed at the electrode/solution interface. In situ photoacoustic studies of optical absorption (2, 3) of these oxide layers indicated that the valence to conduction band transition observed in titanium dioxide (~3 eV) disappears for a critical Ru (or Ir)/Ti ratio of the order of 0.07-0.10. Figure 1 a shows the variation of the absorption bandedge with the surface concentration of Ru (or Ir) in the oxide. Figure lb shows the correlation between the collapse of the absorption bandedge, and the establishment of a 40 mV Tafel slope in potentiodynamic scans in the chlorine reaction region. The x axis is the reduced time (t~ = 1 when a 40 mV Tafel slope is first observed) of corrosion through the implant profiles. The electrochemical profiling progresses with no marked preferential dissolution (1), although the corrosion rates for ruthenium-and iridium-implanted titanium differ markedly. The collapse of the absorption bandedge was linked (1-3) with the acquisition of metallic properties by the mixed oxides. An abrupt change with composition of the electrical conductivity of powder mixtures of TiO2 and RuO2 was reported (4) at a 30% RuQ-70% TiO2 composition. Such changes in conductivity of composite materials can be treated with a percolation model (5). This critical composition corresponds to a two-dimensional problem, probably because the samples have a depth several orders of magnitude smaller than the lateral dimensions and the electronic repartition at the electrode/solution interface controls the electrode response to changes in potential. Metallic conductivity is achieved in the sample when the metallic phase (e.g., RuO2) forms the infinite cluster and therefore provides a continuous metallic path through the sample. The observation of a semiconductor/metal transition at a smaller ratio of Ru/Ti in implanted samples points to the quantum mechanical aspect of the problem when ions of different electronic configurations are mixed on an atomic scale. This paper is an attempt to correlate the ...

Deactivation of Thermally Formed Ru/Ti Oxide Electrodes: An AC Impedance Characterization Study

“…9,12,20,23,25 This may take place by erosion and/or dissolution, or via the loss of Ru-based adsorbed intermediates participating in the chlorine and oxygen evolution reactions. In support of the suggested loss of Ru, it is known that a notable increase in the resistivity of mixed Ru/Ti oxides occurs [26][27][28][29][30] when the RuO 2 content is Ͻ20 atom %. Focusing on the latter explanation, it is unequivocally agreed that during electrolysis some RuO 2 does dissolve electrochemically ͑al-though the magnitude of the rate of dissolution is not known with certainty͒ and that some of it is also lost by erosion.…”

“…9,12,20,23,25 This may take place by erosion and/or dissolution, or via the loss of Ru-based adsorbed intermediates participating in the chlorine and oxygen evolution reactions. In support of the suggested loss of Ru, it is known that a notable increase in the resistivity of mixed Ru/Ti oxides occurs [26][27][28][29][30] when the RuO 2 content is Ͻ20 atom %.…”

Electrochemical Impedance Spectroscopy (EIS) Study of Ru-La Oxide Coatings in NaCl Solution

“…Complex plane (a) and bode plots (b) for Ti/Ru 0.3 La 0.7 O y coating in 3.5% NaCl solution at 1.1 V (vs. SCE). Scatterexperimental data, real linefitting result from [29] , 表面不均匀性 [30] 及特性吸附 [26]…”