Heteroepitaxial growth of cadmium carbonate at dolomite and calcite surfaces: Mechanisms and rates

Abstract

The systematic variation of rates and the mechanism of cadmium uptake on the (104) surface of dolomite (CaMg(CO3)2) were investigated using in situ and ex situ atomic force microscopy (AFM), ex situ specular X-ray reflectivity (XR), and ex situ X-ray fluorescence (XRF). Selected experiments were performed on the calcite (CaCO3) (104) surface for comparison. Aqueous solutions of CdCl2, CaCl2, and NaHCO3, undersaturated with respect to calcite and supersaturated with respect to otavite (CdCO3) and the (CdxCa1−x)CO3 solid solution, were reacted with dolomite surfaces for minutes to days. Calcite substrates were reacted with solutions containing 1–50μM CdCl2, and with no added Ca or CO3. Thin carbonate films following the Stranski–Krastanov growth mode were observed on both substrates. Specular XR and XRF revealed the formation of nm-thick Cd-rich carbonate films that were structurally ordered with respect to the dolomite (104) plane. Epitaxial films adopted the calcite crystal structure with a d104-spacing (3.00Å) larger than those of pure dolomite (2.88Å) and otavite (2.95Å) indicating either a solid solution with x≈0.5, or a strained Cd-rich carbonate with a composition near that of otavite. The growth rate r of this phase increases with the initial supersaturation of the solution with respect to the solid solution, βmax, and follows the empirical relationship, as determined from XRF measurements, given by: r=10-4.88±0.42(βmax2.29±0.24-1),(in units of atoms of Cd/Å2/h).The morphology of the overgrowth also varied with βmax, as exemplified by AFM observations. Growth at step edges occurred over the entire βmax range considered, and additional growth features including 3Å high monolayer islands and ∼25Å high tall islands were observed when logβmax>1. On calcite, in situ XR indicated that this phase is similar to the Cd-rich overgrowth formed on dolomite and images obtained from X-ray reflection interface microscopy (XRIM) reveal the existence of laterally variable Cd-rich domains.