In-Situ Imaging of Ionic Crystal Dissolution Using an Integrated Electrochemical/AFM Probe

The kinetics and mechanism controlling dissolution from the (100) cleavage face of potassium bromide single crystals in acetonitrile solutions have been identified using a novel integrated electrochemical/AFM probe and a scanning electrochemical microscope (SECM). With both techniques, dissolution is induced by perturbing the dynamic dissolution/growth equilibrium at the crystal/solution interface through the electrochemical oxidation of bromide ions. SECM measurements demonstrate that the dissolution reaction is diffusion-limited under the experimental conditions, suggesting that the surface reaction is characterized by a rate constant in excess of 5 cm s-1 (assuming a first-order dissolution process). The topography of the dissolving surface has been imaged in situ, under conditions which closely mimic those of the SECM measurements, using an electrochemically active AFM probe. These studies provide the first direct experimental evidence of the operation of the spiral mechanism in the dissolution of an ionic single crystal, in which steps of unit cell height unwind from screw dislocations emerging on the crystal surface.