R.T. Atanasoski

Surface properties of RuO2 + IrO2 mixed oxide electrodes

The surface properties of RuO2 + IrO2 mixed oxide layers deposited on Ti by thermal decomposition of the corresponding chlorides in aqueous solutions have been investigated by means of cyclic voltammetry and ex situ UHV (AES and XPS) techniques. The data obtained have shown that the surface is enriched with Ir and also that the bulk composition is likely to be inhomogenous. The surface composition estimated on the basis of the voltammetric curves has been found to agree excellently with the results of ex situ surface analysis. Finally, the samples have been found to “absorb” protons from the solution through a mechanism which does not interfere with the estimation of the electrochemically active surface area on the basis of the voltammetric charge.

Effect of preparation on the surface and electrocatalytic properties of RuO2 + IrO2 mixed oxide electrodes

Abstract RuO2 + IrO2 electrodes of various compositions from 0 to 100% RuO2 have been prepared by thermal decomposition of RuCl3 + IrCl3 in isopropanol solutions. The surface and electrocatalytic properties have been investigated by voltammetric curves, ex situ XPS analysis, and O2 evolution. Dramatic differences have been observed with respect to mixed oxides prepared from an aqueous solution of the precursors in a previous work. In situ surface analysis based on some features of the voltammetric curves has proved to be as sensitive and accurate as XPS. The data have shown that the surface is more enriched with Ir than in the previous work. Electrocatalytic results have been explained in terms of additive behaviour of the two components. It is concluded that surface enrichment and bulk inhomogeneity go parallel, the samples prepared from aqueous solutions showing some synergetic effects which are absent in the case where the precursors are dissolved in isopropanol.

Photoelectrochemical microscopy of oxide films on metals: Ti/TiO2 interface

Abstract Thin electrochemically grown oxide films on polycrystalline titanium substrates were examined by photoelectrochemical and electron diffraction techniques. Photoelectrochemical microscopy, based on illumination by a focused laser beam, was used to measure the local photoresponse of the oxide films. For oxide films grown slowly (1 nm/h)if, by ramping the electrode potential at 0.1 mV/s, the oxide photoresponse was found to map the grain structure of the underlying substrate. As the oxide growth rate was increased the photoresponse became more homogeneous and the average photocurrent decreased. The photoresponse heterogeneities were associated with variations in the structure of the oxide film. Electron diffraction studies showed that for slowly grown films, an ordered oxide structure was present with a preferential growth direction of [110] occurring on some substrate grains. As the oxide growth rate was increased the film structure became more homogeneous and the average oxide crystalline size decreased. The decrease in oxide crystallinity with increase in oxide growth rate was also observed in photospectroscopy measurements as a loss of the 3.7 eV direct bandgap. For all oxide films studied here, rutile was the only identifiable phase.

Local film thickness and photoresponse of thin anodic TiO2 films on polycrystalline titanium

Abstract Auger depth profiling was used to determine the local film thickness of a thin anodic oxide grown on a polycrystalline Ti substrate. The oxide thickness was studied as a function of substrate crystallography and final growth voltage. These results were related to local photocurrent measurements obtained using photoelectrochemical microscopy. Variations in the film thickness are too small to account for the non-uniform photocurrent response. The non-uniform photoresponse is instead attributed to variations in the defect density of the oxide.

The influence of the aging time of RuO2 and TiO2 sols on the electrochemical properties and behavior for the chlorine evolution reaction of activated titanium anodes obtained by the sol-gel procedure

Abstract The influence of the aging time of RuO 2 and TiO 2 sols used for the preparation of (40% RuO 2 +60% TiO 2 )/Ti anodes by the sol-gel procedure on the electrochemical properties and behavior for the chlorine evolution reaction of obtained anodes was studied. The electrochemical (active) surface area of the anode coatings was examined by cyclic voltammetry. The electrocatalytic activity and stability of obtained anodes for the chlorine evolution reaction were investigated by polarization measurements and accelerated stability test. The dependence of electrochemical properties of obtained activated titanium anodes on RuO 2 and TiO 2 sol particle size was established.

The roles of the ruthenium concentration profile, the stabilizing component and the substrate on the stability of oxide coatings

Abstract Electrocatalytic oxide coatings with variable concentration profiles of RuO2 as the active component were obtained through a combination of separately applied layers of RuO2, TiO2, IrO2, RuO2 + TiO2 and RuO2 + IrO2 on titanium and glassy carbon substrates. The stability of the samples was examined by accelerated tests performed at high anodic current densities. Electrochemical techniques, cyclic voltammetry for assessing the charge associated with the coating, polarization measurements for assessing the electrocatalytic activity, and Auger electron spectroscopy to register surface composition of the coatings, were applied to follow changes due to the stability experiments. The stability and the charge depended strongly on the sequence of layers in the RuO2−TiO2 coating, with the samples having the RuO2 + TiO2 layer facing the electrolyte exhibiting the highest values for both properties. In contrast to this, the stability of the RuO2−IrO2 coatings, besides being lower than the stability of RuO2−TiO2 coatings, showed no dependence on the sequence of the applied layers. Much lower stability was exhibited by the coatings applied on glassy carbon rather than on titanium. A mechanism of the stability of the coatings based on the interaction of lower than four valency state titanium with higher than four valency ruthenium, proposed for single-crystal surfaces, is corroborated. Finally, during the thermal treatment a diffusion of titanium originating in the titanium substrate through the coating was established.

XPS and AES study of mixed layers of RuO2 and IrO2

The surface composition of RuO2+IrO2 mixed oxide layers (1–2 μm thick) deposited on titanium by thermal degradation of the corresponding chlorides in aqueous or isopropanol solutions has been investigated. A surface enrichment with Ir, more pronounced for the layers obtained from isopropanol, has always been found. Electrochemical data reflecting the surface properties of RuO2+IrO2 at the surface.

Anodic stability and electrochromism of electrodeposited ruthenium-iridium coatings on titanium

Abstract The anodic behaviour of galvanically plated ruthenium-iridium coatings on titanium was investigated. The dependence of the Tafel slopes and the stability of the coatings for the oxygen evolution reaction in sulphuric acid on the electrode composition was established. A change in the Tafel slope from 33 to 90 mV per decade on going from pure ruthenium to pure iridium was found. The presence of 40% Ir in the coating yielded 300 times higher stability in comparison with the coating of pure ruthenium. In addition, a square-wave pulsing between −0.25 and 1.25 V vs. the saturated calomel electrode produced an oxide film with electrochromic properties which were directly related to the charge enhancement factor of the grown oxide.

Single crystal RuO2/Ti and RuO2/TiO2 interface: LEED, Auger and XPS study

The interactions at the evolving RuO2/titanium interface have been studied by LEED, AES and XPS. Titanium films of up to 5 monolayers were evaporated onto well ordered and ion sputtered ruthenium dioxide crystal surfaces of (110) and (100) orientation. Stabilization of the surface oxygen content under thermal treatment in UHV (up to 600°C) with increasing titanium coverage was established. After extended (up to 4 h) annealing in O2 at 600°C an epitaxial ordering of TiO2 on RuO2(110) was observed. The (1 × 1) LEED patterns from the epitaxial layer exhibit a reduced background level when compared to the RuO2 substrate itself. These findings are correlated with the XPS data and are interpreted in connection with the disappearance of the defect RuO2 phase in the surface layer of the RuO2. The appearance of the (1 × 2) surface reconstruction at the RuO2(100)/Ti interface is discussed in the context of maximum cation coordination by oxygen atoms.

Anodic oxidation of small organic molecules on silver modified glassy carbon electrodes

Small organic molecules (HCOOH, C,H,(OH),, CH,OH, CH,O, etc.) have become more interesting with the possibility of their application in fuel cells. Therefore, the anodic oxidation of such molecules has been studied on a number of noble metals [1,2]. The present paper is intended to expand these investigations by introducing glassy carbon as an electrode material and/or a support to enhance the catalytic activity of silver deposited on its surface. Glassy carbon (GC) has been widely used in electrochemistry due to its chemical inertness, good conductivity, rather small porosity and large electrochemical window [31 that allow examination and assessment of a number of reactions over a wide range of potentials. If appropriately treated, glassy carbon can be used either as an electrocatalyst [4,5] or as a substrate for other catalysts [6]. According to Jenkins and Kawamura [7], glassy carbon consists of randomly oriented ribbon molecules tangled in an intricate way. Several investigations [&lo] have demonstrated the presence of different oxygen-containing functional groups on the surface of this material. A glassy carbon surface can be easily modified by metals simply by exposing it to solutions with the corresponding metal ions 1111. In prolonged contact with AgNO, solution, GC acquires a large percentage of zero valency silver on the surface and in the near surface region [11,12]. In general, the aim of this work has been to attain a simple and coherent procedure for achieving an active and stable, silver-modified glassy carbon electrodes for the anodic oxidation of small organic molecules. In this note we report the effects of silver modified GC on the oxidation of HCOOH, C,H,(OH),, CH,OH, and CH,O in alkaline solution. As observed by cyclic voltammetry,