In situ observation of the surface processes involved in dissolution from the cleavage surface of calcite in aqueous solution using combined scanning electrochemical-atomic force microscopy (SECM-AFM)

The surface processes involved in the initial stages of the proton-assisted dissolution of the calcite single crystal cleavage plane (10 (1) over bar4) have been identified using a combined scanning electrochemical-atomic force microscope (SECM-AFM). This instrument employs a platinum-coated AFM probe, which functions as an electrode as well as a high-resolution topographical sensor. Dissolution in this arrangement is effected by the local electro-generation of protons, produced by oxidation of water at the probe electrode. By careful control of the applied potential, it is possible to vary the magnitude of the electrogenerated flux of protons from the probe towards the calcite surface. Crucially, by generating a small proton flux for short time periods (0.5 s) it is possible to observe and monitor the initial sites in the dissolution process. Topographical images were recorded in the same area of the surface both prior to and after inducing dissolution, as a function of the proton flux. At low proton fluxes, of the order of 1 nmol cm(-2) s(-1) or less, the surface was observed to dissolve by the nucleation of monolayer deep pits, with densities of about 10(8) cm(-2). These pits are likely to be formed at point vacancies or atomic (impurity for example) defects in the crystal lattice. As the proton flux was increased (over two orders of magnitude), these same etch pits were found to open into wider macro-pits, with an outline morphology that reflected the crystallographic orientation of the surface. At the highest proton fluxes, dissolution from macroscopic step edges become significant.