Stability of amorphous alumina layers deposited on platinum films by r.f. magnetron sputtering in water solutions. A microscopic approach based on scanning electrochemical microscopy

A series of bilayers, formed by amorphous Al₂O₃ (thickness in the range 60–1000 nm) and Pt (thickness 300 nm) films, deposited by r.f. magnetron sputtering on a silicon substrate, was investigated by scanning electrochemical microscopy (SECM) to establish the stability of Al₂O₃ in contact with aqueous solutions, containing various electrolytes at different pH. Under mild acidic conditions (pH 6.5–4.6), SECM responses indicated that the Al₂O₃ layers were stable. To achieve more acidic (pH < 3) and basic (pH > 9) conditions, avoiding the use of harmful strong acids or bases, local H⁺ and OH^- challenges were electrogenerated at the SECM tip through water electrolysis at constant currents. pH 2.3 and 11.6 were attained on the Al₂O₃ surface, as predicted by finite element simulation. At low pH, a slow dissolution kinetic was involved. At high pH, the formation of the soluble AlO₂^- species triggered a faster alumina dissolution, which led to the formation of etch pits. The geometric features of the etch pits, evaluated by SECM as a function of the electrolysis time, afforded to a mass dissolution rate of 6.7 (± 0.6) × 10⁻¹² moles·min⁻¹ and to a vertical dissolution rate of 308 (± 15) nm·h⁻¹.