In situ AFM observations of the interaction between calcite surfaces and Cd-bearing aqueous solutions

In situ atomic force microscopy (AFM) observations of the interaction between calcite ( 1 0 1 ¯ 4 ) surfaces and Cd-bearing aqueous solutions have been carried out, by maintaining the solutions static in the AFM fluid cell. The interaction involves the dissolution of the original surface and the simultaneous epitaxial growth of multilayer three-dimensional islands ∼2.75 nm in height of Cd-rich members of the CdxCa1−xCO3 solid solution. Dissolution occurs by the retreating of steps and the formation and growth of etch pits. Both the etch pits and the multilayer islands are elongated along the [ 4 2 1 ¯ ] direction. Such an unusual elongation direction is interpreted as a kinetic effect controlled by both the structural characteristics of the calcite ( 1 0 1 ¯ 4 ) surface and the structure and elastic properties of the overgrowth. Using aqueous solutions highly concentrated in cadmium, the formation of ∼2.75 nm thick islands occurs after the initial growth of a solid solution monolayer ∼0.3 nm thick, which finally dissolves as the islands grow. This effect seems to be a result of the coherent strain energy accumulated in the substrate–monolayer interface. Significantly, the dissolution rate of the calcite surface decreases when the concentration of Cd2+ in the aqueous solution increases, suggesting that, together with co-precipitation, Cd adsorption plays an important role in the interaction process. During co-precipitation, the progressive coalescence of the Cd-rich islands results in formation of a nanometric epitaxial layer on the calcite ( 1 0 1 ¯ 4 ) surface. This layer armours the substrate from further dissolution and determines the end of the process at a “partial” pseudo-equilibrium endpoint.