Kinetics and mechanisms of cadmium carbonate heteroepitaxial growth at the calcite (101¯4) surface

Abstract

Elucidating the kinetics and mechanisms of heteroepitaxial nucleation and growth at mineral-water interfaces is essential to understanding surface reactivity in geochemical systems. In the present work, the formation of heteroepitaxial cadmium carbonate coatings at calcite-water interfaces was investigated by exposing calcite ð10 1 4Þ surfaces to Cd-bearing aqueous solutions. In situ atomic force microscopy (AFM) was employed as the primary technique. The AFM results indicate that the heteroepitaxial growth of cadmium carbonate proceeds via three different mechanisms depending on the initial supersaturation of the aqueous solution: advancement of existing steps, nucleation and growth of three-dimensional (3D) islands, and nucleation and spread of two-dimensional (2D) nuclei. The 3D islands and 2D nuclei exhibit different morphologies and growth kinetics. The effects of supersaturation on heteroepitaxial growth mechanisms can be interpreted in terms of the free energy barrier for nucleation. At low initial supersaturation, where 3D nucleation dominates, it is hypothesized, from the growth rate and morphology of the 3D islands observed with AFM, that the crystallization of the overgrowth follows a non-classical pathway involving the formation of a surface precursor that is not fully crystalline, whereas high supersaturation favors the formation of crystalline 2D nuclei whose morphology is based on the atomic structure of the calcite substrate. Cross-sectional transmission electron microscopy (TEM) images reveal that the atomic structure of the interface between the cadmium carbonate coating and calcite shows perfect, dislocation-free epitaxy.

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

“…For [Cd 2+ ] 0 ≥ 1 μM ( Fig. 2a and b), the islands displayed an elongated shape, with the long axis following the 421 h i direction and the short axis parallel to [010], which is consistent with time-sequenced AFM images of CdCO 3 growth on calcite surfaces observed previously for up to 6 h (Xu et al, 2014). This previous study demonstrated that these morphologies are clear evidence of heteroepitaxial growth and also included high-resolution transmission electron microscopy images of cross sections of the interface between a calcite single crystal and CdCO 3 overgrowth showing dislocation-free epitaxy.…”

“…At the point of introduction of CdCl 2 or CoCl 2 , the aqueous solutions are moderately undersaturated with respect to calcite (Table 1). Adsorption of Cd or Co at the calcite surfaces can inhibit the dissolution of the calcite substrate, as demonstrated for Cd in our previous work (Xu et al, 2014). However, this previous work also showed that, while this phenomenon has a mild effect on the growth rate of the surface precipitates, it does not affect the nature and morphology of the overgrowths, which are the focus of this work.…”

“…Cd 2+ -or Co 2+ -bearing solutions were prepared from high-purity (99.999%) CdCl 2 or CoCl 2 salts dissolved in deionized water (resistivity~18 MΩ). Similar to the in situ AFM experiments of CdCO 3 growth performed previously (Xu et al, 2014), the calcite specimens were first contacted with 2.3 mL of deionized water for 30 min, followed by the introduction of concentrated Cd 2+ -or Co 2+ -bearing solutions to obtain a specific initial concentration of Cd 2+ or Co 2+ , denoted [Cd 2+ ] 0 and [Co 2+ ] 0 , respectively. Initial Cd 2+ concentrations were in the range 0.3 to 100 μM and initial Co 2+ concentrations were in the range 25 to 200 μM.…”

“…Models of divalent metal interaction with calcite surfaces attributed the second stage to the growth of surface precipitates, specifically, to the formation of carbonate solid solutions (Comans and Middelburg, 1987;Davis et al, 1987). Indeed, the ability of CdCO 3 and MnCO 3 , which present misfit values of −4% and −10%, respectively, based on their 1014 surface areas, to form heteroepitaxially on calcite was later demonstrated by a combination of surface sensitive spectroscopic, microscopic, and scattering techniques (Stipp et al, 1992;Chiarello and Sturchio, 1994;Chiarello et al, 1997;Hay et al, 2003;Lea et al, 2003;Pérez-Garrido et al, 2007, 2009Xu et al, 2014). In contrast, the propensity of sphaerocobaltite (CoCO 3 ) to grow as a heteroepitaxial phase on calcite has neither been confirmed nor refuted, despite a demonstrated affinity of Co for calcite surfaces (Kornicker et al, 1985;Zachara et al, 1991;Reeder, 1996;Xu et al, 1996;Cheng et al, 2000).…”

“…Moreover, if the atomic arrangement of the overgrowing phase exhibits structural similarities with the calcite substrate, heteroepitaxial growth can take place. For example, otavite (CdCO 3 ), which yields the lowest lattice misfit with respect to calcite of all the metal carbonates, has been shown to form readily as a heteroepitaxial coating on calcite surfaces (Stipp et al, 1992;Chiarello and Sturchio, 1994;Chiarello et al, 1997;Hay et al, 2003;Pérez-Garrido et al, 2007;Cubillas and Higgins, 2009;Xu et al, 2014).…”