Electrochemical versus Gas-Phase Oxidation of Ru Single-Crystal Surfaces

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Abstract

The electrochemical uptake of oxygen on a Ru(0001) electrode was investigated by electron diffraction, Auger spectroscopy, and cyclic voltammetry. An ordered (2 × 2)-O overlayer forms at a potential close to the hydrogen region. At +0.42 and +1.12 V vs Ag/AgCl, a (3 × 1) phase and a (1 × 1)-O phase, respectively, emerge. When the Ru electrode potential is maintained at +1.12 V for 2 min, RuO2 grows epitaxially with its (100) plane parallel to the Ru(0001) surface. In contrast to the RuO2 domains, the non-oxidized regions of the Ru electrode surface are flat. If, however, the electrode potential is increased to +1.98 V for 2 min, the remaining non-oxidized Ru area also becomes rough. These findings are compared with O overlayers and oxides on the Ru(0001) and Ru(101̄0) surfaces created by exposure to gaseous O2 under UHV conditions. On the other hand, gas-phase oxidation of the Ru(101̄0) surface leads to the formation of RuO2 with a (100) orientation. It is concluded that the difference in surface energy between RuO2(110) and RuO2(100) is quite small. RuO2 again grows epitaxially on Ru(0001), but with the (110) face oriented parallel to the Ru(0001) surface. The electrochemical oxidation of the Ru(0001) electrode surface proceeds via a 3-dimensional growth mechanism with a mean cluster size of 1.6 nm, whereas under UHV conditions, a 2-dimensional oxide film (1−2 nm thick) is epitaxially formed with an average domain size of 20 μm.

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