Heterogeneous growth of cadmium and cobalt carbonate phases at the 101¯4 calcite surface

- Cd- and Co-reacted CaCO3 single crystals were imaged in situ with AFM and analyzed ex situ with XPS.
- Cd-reacted crystals showed evidence of the formation of Cd-rich (Ca,Cd)CO3 heteroepitaxial solid solutions.
- A hydroxy-carbonate Co phase that did not grow epitaxially formed on Co-reacted crystals.
- Observed differences are rationalized based on differences in lattice misfits between CdCO3 or CoCO3 and CaCO3.
- The satellite structure of the Co 2p photoelectron line is a reliable tool to identify Co(II) coordination environments.

The ability of surface precipitates to form heteroepitaxially is an important factor that controls the extent of heterogeneous growth. In this work, the growth of cadmium and cobalt carbonate phases on 101¯4 calcite surfaces is compared for a range of initial saturation states with respect to otavite (CdCO3) and sphaerocobaltite (CoCO3), two isostructural metal carbonates that exhibit different lattice misfits with respect to calcite (− 4% and − 15%, respectively, based on 101¯4 surface areas). Calcite single crystals were reacted in static conditions for 16 h with CdCl2 and CoCl2 aqueous solutions with initial concentrations 0.3 ≤ [Cd2 +]0 ≤ 100 μM and 25 ≤ [Co2 +]0 ≤ 200 μM. The reacted crystals were imaged in situ with atomic force microscopy (AFM) and analyzed ex situ with X-ray photoelectron spectroscopy (XPS). AFM images of Cd-reacted crystals showed the formation of large islands elongated along the 421¯ direction, clear evidence of heteroepitaxial growth, whereas surface precipitates on Co-reacted crystals were small round islands. Deformation of calcite etch pits in both cases indicated the incorporation of Cd and Co at step edges. XPS analysis pointed to the formation of a Cd-rich (Ca,Cd)CO3 solid solution coating atop the calcite substrate. In contrast, XPS measurements of the Co-reacted crystals provided evidence for the formation of a mixed hydroxy-carbonate cobalt phase despite supersaturation with respect to CoCO3. The combined AFM and XPS results suggest that the lattice misfit between CoCO3 and CaCO3 is too large to allow for heteroepitaxial growth of a pure cobalt carbonate phase on calcite surfaces in aqueous solutions and at ambient conditions. The use of the satellite structure of the Co 2p3/2 photoelectron line as a tool for determining the nature of cobalt surface precipitates is also discussed.