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

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, RuO 2 grows epitaxially with its (100) plane parallel to the Ru(0001) surface. In contrast to the RuO 2 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 O 2 under UHV conditions. On the other hand, gas-phase oxidation of the Ru(101̄0) surface leads to the formation of RuO 2 with a (100) orientation. It is concluded that the difference in surface energy between RuO 2 (110) and RuO 2 (100) is quite small. RuO 2 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.