Electrochemical faceting is a term recently coined to denote the preferred crystallographic orientation of grains in polycrystalline metals developed when they are subjected to periodic potential perturbations of determined characteristics. Electrochemical faceting can also be applied to small single crystal beads resulting from melting polycrystalline metal wires. By properly adjusting the conditions defining the periodic perturbation, the resulting electrode surface acquires different preferred orientations which, depending on the nature of the electrode metal, can be followed either electrochemically through conventional voltammetry in the H and 0 electroadsorption/electrodesorption potential range, and in upd of different metals, or by scanning electron microscopy. The procedure has already been successfully applied to platinum, gold, rhodium and palladium. Electrochemical faceting involves at least two stages, namely, the initiation stage related to an electroadsorption process and a propagation stage associated with the electrodissolution and electrodeposition of the base metal in the acid electrolyte. Stabilization procedures for the freshly oriented surfaces and roughness development are also considered.

Electrochemical faceting of polycrystalline (pc) gold electrodes in 1 M HaSO., was investigated by employing wire and bead-shaped electrodes. Electrochemical faceting was produced by applying a repetitive symmetrical square wave potential signal in the 2-4 kHz range and upper and lower potential limits ranging between 1.44 and 1.60 V (vs. RHE) and 0.10 and 1.10 V, respectively. The degree of faceting was followed voltammetrically principally through the 0-electroadsorption/electrodesorption process. Electrochemical information was complemented with scanning electron microscopic observations. The results are discussed in terms of the equilibrium potential of reactions involving different gold species, and the potential of zero charge, hydrophobicity and anion adsorbability of the (llO), (100) and (111) crystallographic faces of gold. The kinetics of electrochemical faceting of gold can be explained through the two-stage mechanism proposed earlier for polycrystalline platinum in acid electrolyte.

The electrochemical treatment of Pd electrodes in acid solutions by means of periodic potentials applied within certain potential windows in the range O-l.5 V (vs. RHE) and frequencies in the range 0.5-6.25 kHz, produces surface modifications involving different types of roughening and faceting. These modifications are the consequences of various electrochemical reactions occurring during the oxidation-reduction cycles, mainly associated with the electrodissolution-electrodeposition of Pd, and oxide layer formation-ehxtroreduction processes. Quantitative data are shown about the former conjugated processes which indicate that, depending on the frequency range, at least three different mechanisms operate for the entire reaction. Surface modifications are followed through voltammetry, scanning electron microscopy and scanning tunneling microscopy.

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