Article pubs.acs.org/crystal
Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX
Anisotropic Growth of Otavite on Calcite: Implications for Heteroepitaxial Growth Mechanisms Shawn L. Riechers* and Sebastien N. Kerisit Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352, United States S Supporting Information *
ABSTRACT: Elucidating the molecular-scale mechanisms driving complex growth behaviors during heteroepitaxial growth in aqueous media has remained a challenging endeavor. Toward this goal, in situ atomic force microscopy was employed to image the heteroepitaxial growth of otavite (CdCO3) at the (101̅4) surface of calcite (CaCO3) single crystals in static aqueous conditions. Heteroepitaxial growth proceeded via spreading of three-dimensional (3D) islands and two-dimensional (2D) atomic layers at low and high initial saturation levels, respectively. Experiments were carried out as a function of applied force and imaging mode thus enabling determination of growth mechanisms unaltered by imaging artifacts. This approach revealed the significant anisotropic nature of heteroepitaxial growth on calcite in both growth modes and its dependence on supersaturation, film thickness, and substrate topography. The 3D islands not only grew preferentially along the [421̅] direction relative to the [010] direction, resulting in rod-like surface precipitates, but also showed clear preference for growth from the island end rich in obtuse/obtuse kink sites. Pinning to step edges was observed to often reverse this tendency. In the 2D growth mode, the relative velocities of acute and obtuse steps were observed to switch between the first and second atomic layers. This phenomenon likely stems from significant Cd−Ca intermixing in the first layer, despite bulk thermodynamics predicting the formation of almost pure otavite. Composition effects may also be responsible for the inability of 3D islands to grow on 2D layers in cases where both modes were observed to occur simultaneously. Overall, the complex heteroepitaxial growth in general and thickness-dependent growth mechanisms in particular provide insights into the interplay of several growth factors and suggest that intermixing may play an important role.
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INTRODUCTION Nucleation and growth at the solid/water interface are key processes controlling biomineralization,1−5 material synthesis,6,7 and the transport and fate of contaminants.8 In particular, mineral surfaces are known to catalyze nucleation leading to heterogeneous growth of pure precipitates or, as expected in complex systems, solid solutions.9−12 When the crystal structure of the substrate is similar to that of the growing phase, heteroepitaxial coatings may form.13,14 This can be critical in predicting the fate of toxic metals such as Cd2+ in aqueous environments. Indeed, in the presence of carbonate minerals such as calcite (CaCO3), sequestration of Cd2+ may take place through heteroepitaxial growth of otavite (CdCO3).8,15 In contrast to the homoepitaxial growth of calcite, which has served as a model system for aqueous crystal growth and for which the effects of numerous variables, such as pH,16 supersaturation,17 cation to anion ratio,17,18 step edge orientation,19 ionic radius of impurities,20,21 etc., have been quantified, the molecular-scale controls that determine the growth rate, structure, and composition of heteroepitaxial carbonate coatings are not as well understood. Heteroepitaxial growth is more complex as the lattice mismatch between the coating and the mineral substrate induces stress in the growing phase, which can affect its structure and growth rate. Additionally, intermixing can occur thereby relieving stress in © XXXX American Chemical Society
the growing phase but also introducing further complexity by altering its composition. For example, seminal work by Chiarello et al.22,23 demonstrated from synchrotron X-ray scattering the formation of (Cd, Ca)CO3 solid solutions on the calcite (101̅4) cleavage surface. Also, the atomic force microscopy (AFM) images, Cubillas and Higgins24 suggested that changes in composition could occur from one atomic layer to the next. More recently, the heteroepitaxial growth of rhodochrosite (MnCO3) on the (101̅4) calcite surface was shown to be possible despite a −10% lattice mismatch because of the strain relaxation afforded by the incorporation of Ca within the first 2−3 nm of the growing film.25 Although the underlying principles of crystal growth revealed by the many studies of calcite growth are expected to hold in the case of heteroepitaxial carbonate growth, the extent to which growth mechanisms are affected by the direct (stress) and indirect (composition) effects of the lattice mismatch remains to be fully elucidated. Determining the direction dependence of growth rates and mechanisms may be helpful toward this goal because of the anisotropic elastic properties of carbonates and the known element-specific affinities of nonequivalent growth steps on the (101̅4) calcite surface.17,18,20,21,26 Received: July 30, 2017 Revised: December 8, 2017 Published: December 8, 2017 A
DOI: 10.1021/acs.cgd.7b01055 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
solution conditions). AFM imaging was then resumed for 2−6 h. All experiments were conducted at ∼22 °C. AFM Imaging. A Bruker Dimension Icon AFM with Nanoscope 8.10 software was used for imaging. Contact mode was used to obtain topography, deflection, and lateral force images using NP-10 (Bruker, nominal k = 0.58 or 0.06 N m−1) and MLCT (Bruker, k = 0.02 N m−1) tips. Contact mode imaging forces, FCM, between 0.5 and 20 nN were used after calibration, as outlined in previous work.33 Images typically took 5−10 min each, resulting in ∼12 images per hour. SCANASYST-FLUID+ tips (Bruker, k = 0.07 N m−1) were used for PeakForce tapping (PFT) in ScanAsyst mode. PFT, which does not apply shear force, was used to ensure passive observation with the least amount of perturbation from tip scanning.35,36 PFT imaging force, FPFT, was estimated from the imaging set point and typical cantilever sensitivity and spring constants and ranged from 0.5 to 2 nN. Each image collection required 10−30 min resulting in ∼3.5 images per hour. Scanning frequencies for contact and PFT imaging ranged from 0.5 to 4 Hz, with 512 or 1024 sampling points per scan line and scanning areas ranging from 2 × 2 to 10 × 10 μm2. Image Analysis. Image processing and analysis were carried out using Gwyddion (version 2.38) and ImageJ (version 1.47v). The Gwyddion functions for plane subtraction, median line correction, and polynomial background removal were used to flatten the background along a single atomic step, using masking where appropriate. ImageJ was used to create time-lapse videos. An image stabilization macro (NMS_fixTranslation_ver1.ijm, 2014, N. M. Schneider) was used to compensate for translation between images by selecting a small unchanging feature present in each image in a series. Mineral growth was then measured by cursor from an initial fixed point along each step direction as each frame was advanced.
Further, for the case of otavite growth on calcite in particular21,24,27−31 but also for other systems,13,26,32 growth has been shown to proceed by the formation and coalescence of three-dimensional (3D) islands or two-dimensional (2D) nuclei, depending on growth conditions. The presence of two growth modes is thought to stem from two different nucleation pathways: a nonclassical nucleation pathway through an amorphous precursor for 3D growth33 and a classical heterogeneous nucleation mechanism for 2D growth.28−30 Therefore, a complete picture of heteroepitaxial growth mechanisms needs to include both growth modes. In situ interface-sensitive techniques such as AFM are undoubtedly needed to observe the nanoscale processes involved in the formation of heteroepitaxial coatings and determine growth mechanisms. However, as recently shown in the case of otavite nucleation on calcite, care must be taken to ensure the observational methods employed do not impact the mechanisms of growth being observed.33 Indeed, that work showed that, under typical imaging forces (≥15 nN),24,27,30,34 nuclei were removed from the surface, whereas under low force imaging (
