Growth of RuO2 by electrochemical and gas-phase oxidation of an Ru(0001) surface

Rectangular monolayer stripes of RuO2(110) obtained in a simple gas-phase oxidation of an Ru(0001) surface in argon (Ar) containing 115 ppm of O-2 were compared with the electrochemical oxidation of an Ru(0001) surface in sulfuric acid at high positive potentials. In situ scanning tunneling microscopy (STM) showed that the electrochemical oxidation of the Ru(0001) surface starts at step edges, and proceeds through the formation of randomly distributed RuO2 islands. The nucleation process is instantaneous, as verified by potential step experiments. The oxidation of Ru(0001) in a mixture of O-2 (115 ppm) and Ar produces a layer of ordered RuO2(110) oxide in the form of rectangular stripes of monolayer height. The growth of these stripes, globally initiated at the step's edges, is unidirectional, forming a 60degrees angle to the steps' direction. The entirely different morphologies of RuO2 grown in electrochemical- and gas-phase oxidation environments probably reflect the difference in the reaction temperature of two processes. In the latter, the elevated temperatures (600-800 K) appear to confer mobility on the reaction species that facilitates ordering of the oxide layer. (C) 2004 Elsevier B.V. All rights reserved.