Nucleation and Epitaxy-Mediated Phase Transformation of a

Article pubs.acs.org/JPCC

Nucleation and Epitaxy-Mediated Phase Transformation of a Precursor Cadmium Carbonate Phase at the Calcite/Water Interface Shawn L. Riechers,* Kevin M. Rosso, and Sebastien N. Kerisit* Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352, United States S Supporting Information *

ABSTRACT: Mineral nucleation can be catalyzed by the presence of mineral substrates; however, the mechanisms of heterogeneous nucleation remain poorly understood. A combination of in situ time-sequenced measurements and nanomanipulation experiments were performed using atomic force microscopy (AFM) to probe the mechanisms of heteroepitaxial nucleation of otavite (CdCO3) on calcite (CaCO3) single crystals that exposed the (101̅4) surface. Otavite and calcite are isostructural carbonates that display a 4% lattice mismatch, based on their (101̅4) surface areas. AFM observations revealed a two-stage process in the nucleation of cadmium carbonate surface precipitates. As evidenced by changes in the height, shape, growth behavior, and friction signal of the precipitates, a precursor phase was observed to initially form on the surface and subsequently undergo an epitaxy-mediated phase transformation to otavite, which then grew epitaxially. Nanomanipulation experiments, in which the applied force was increased progressively until precipitates were removed from the surface, showed that adhesion of the precursor phase to the substrate was distinctively weaker than that of the epitaxial phase, consistent with that of an amorphous phase. These findings demonstrate that heterogeneous mineral nucleation can follow a nonclassical pathway like that found in homogeneous aqueous conditions.

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INTRODUCTION Nucleation is the obligatory first step in the formation and growth of crystalline solids, and as such, it underlies a wide range of phenomena in geochemistry, biomineralization, and materials synthesis. Nucleation is also an extremely challenging phenomenon to study due to its transient and nanoscale nature. As a result, the picture offered by classical nucleation theory1,2 has remained mostly unchallenged, for well over a century, until recently. In classical nucleation theory, stochastic density fluctuations of the solute cause the formation of crystalline nuclei that behave as the bulk phase. Nuclei that reach a critical size determined by the balance between bulk and interfacial energies lead to growth, whereas those below the critical size dissolve. Contrary to this classical picture, recent work has established the importance of prenucleation clusters and amorphous phases serving as precursors to thermodynamically stable crystalline products. Much of this work has been focused on carbonate minerals, because of their widespread role in environmental, geological, and biological systems. For example, homogeneous nucleation of calcium carbonate3 showed the formation of a transient amorphous phase (amorphous calcium carbonate or ACC) and stable prenucleation clusters4 as precursors to calcite and vaterite crystallites. In addition, at very high supersaturation levels, calcite growth was shown to be facilitated by the attachment of ACC from solution to the calcite surface, followed by transition to calcite.5 These key findings have © 2017 American Chemical Society

brought to light nucleation pathways that involve cluster and particle-based intermediates, the roles of which are prominent but not well understood. The situation is equally complex in the case of heterogeneous nucleation and growth. In keeping with the example of calcium carbonate, Pouget et al.6 showed that ACC nanoparticles formed in aqueous solution by aggregation of prenucleation clusters could deposit on a stearic acid monolayer template and later transform to crystalline calcium carbonate, thereby demonstrating the relevance of nonclassical nucleation pathways to heterogeneous nucleation and growth. Whether a similar pathway is available in a fully inorganic system in aqueous solutions of low supersaturation has never been established. In situ measurements of heterogeneous nucleation at mineral/water interfaces, and in particular those of Jun and co-workers, have demonstrated the effects of the nature of the substrate,7 ionic strength,8 and pH9 and have allowed for quantification of interfacial energies,7−10 but comparatively little is known about the mechanisms of heterogeneous nucleation for cases where sufficient structural and chemical similarities between the nucleating phase and the substrate exist, allowing, in principle, growth to occur catalytically. Received: November 21, 2016 Revised: January 16, 2017 Published: February 10, 2017 5012

DOI: 10.1021/acs.jpcc.6b11727 J. Phys. Chem. C 2017, 121, 5012−5019

Article

The Journal of Physical Chemistry C

solution conditions). No additional electrolytes (e.g., for ionic strength control) were added. AFM imaging was then resumed for 2−6 h. Control experiments without the addition of Cd2+ were also performed for comparison. 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. The particle diameter for circular particles or length and width for elongated particles were measured laterally using the full width at half-maximum to compensate for the tip broadening effect. These values were then used to calculate the surface contact area of either a circle or an ellipse. ImageJ was used to generate time-lapse videos with image stabilization. AFM Setup. 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) tips for high-force imaging (10−300 nN), while MLCT tips (Bruker, k = 0.02 N m−1) were used for low-force imaging (0.5−30 nN). SCANASYST-FLUID+ tips (Bruker, k = 0.07 N m−1) were used for PeakForce tapping in ScanAsyst mode. In order to precisely control low forces during imaging, each cantilever was calibrated after laser alignment in solution by first determining the cantilever sensitivity (nm V−1) by performing a force curve. Typical values ranged from 50 to 150 nm V−1. Thermal tuning was then used to determine the spring constant of the cantilever. Cantilevers were found to vary by up to 85% from their listed nominal values. After engaging the tip, the imaging set point was then decreased incrementally to determine the point of contact. This step was repeated after every image to mitigate the effect of z-drift. In this way, the exact set point required to reach a given force was known and imaging could routinely be carried out at 0.5 nN. The scanning frequencies used ranged from 0.5 to 4 Hz, with 512 or 1024 sampling points per scan line and scanning areas ranging from 4 to 100 μm2. Three types of time-sequential in situ AFM experiments were utilized: PeakForce tapping mode, low-force contact mode, and high-force contact mode. PeakForce tapping mode was used to ensure passive observation with the least amount of perturbations from tip scanning,18,19 with each image taking 10−20 min resulting in ∼3.5 images per hour. Low-force contact mode was used with an imaging force kept between 0.5 and 2 nN to minimize perturbation while still allowing for lateral force measurements. Images typically took 5−10 min each, resulting in ∼12 images per hour. High-force contact mode utilized the tip to probe the adhesion of surface bound particles. During these experiments, the lowest possible force (5−10 nN) was used to find a suitable region with particles present. Once a region was selected, the imaging force was increased after each image (4−7 min each) by 1 nN until 10 nN was achieved, by 5 nN until 50 nN was achieved, and by 50 nN until the highest force possible was achieved, typically 250− 300 nN. In this way, the largest range of forces could be carefully probed while having as little time elapsed as possible resulting in ∼25 images per hour. All experiments were conducted at ∼22 °C.

Following previous work on heteroepitaxial growth of metal carbonates,11−15 we recently examined this prospect16 for the growth of otavite (CdCO3) (or a Cd-rich (Cd,Ca)CO3 solid solution17) on calcite (CaCO3) single crystals, which hinted at the formation of a precursor phase during heteroepitaxial nucleation. Atomic force microscopy (AFM) studies suggested the nucleation of a precursor phase as the first step of a twostage process in the growth of three-dimensional islands. In the first stage, growth was observed to be mostly vertical with no direction dependence in the plane of the surface, followed by a second stage with no vertical but extensive lateral growth along a preferred crystallographic direction. Although this observation of two stages offered a tantalizing clue on the mechanisms of heterogeneous nucleation, that study did not provide evidence for the phase transformation between two distinct phases, and it did not investigate to what extent artifacts could have resulted from the imaging mode. In the present study, we specifically pursued this objective, hypothesizing that heteroepitaxial nucleation of metal carbonates occurs via the formation of a metastable phase that subsequently transforms to a stable crystalline phase under the influence of the mineral substrate. We report AFM in situ timesequenced measurements and nanomanipulation experiments that probe the nucleation and growth of otavite on calcite single crystals, which definitively establish that the phase transformation of a precursor cadmium carbonate phase to otavite is heteroepitaxy-mediated.

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EXPERIMENTAL METHODS In Situ Sample Preparation. Optically clear calcite crystals (Ward’s Natural Science Establishment, Inc.) sourced from Minas Gerais, Brazil, were used to prepare fresh specimens of approximately 10 mm × 12 mm × 1.5 mm in size by cleaving with a dull knife along the (101̅4) cleavage plane minutes before performing the experiments. Aqueous solutions were prepared from high-purity CdCl2 (99.999%, Sigma-Aldrich) dissolved in deionized water (resistivity ∼18 MΩ, Barnstead). All the growth experiments were carried out in the AFM fluid cell. The fluid cell was a custom-built Kel-F cell with a diameter of 2.7 cm and a volume of approximately 2.5 mL. Experimental procedures were slightly optimized from previous protocols.16,17 The calcite specimens were fixed in the fluid cell with Crystalbond (Crystalbond 509, SPI Supplies) by heating on a hot plate. The samples were allowed to cool on an aluminum block for 3 min before rinsing and then filling the cell with 2.3 mL of water. The fluid cell remained open to the air throughout the experiments. In each experiment, the calcite specimen was allowed to dissolve for exactly 30 min in order to provide a carbonate source. During this time the AFM laser was aligned and initial images were obtained to locate suitably flat and clean regions. The AFM tip was then pulled out of solution while a small volume (50−200 μL) of concentrated CdCl2 solution was slowly injected with a pipet in small aliquots around the rim of the fluid cell to obtain an initial Cd2+ concentration, [Cd2+]0, of 1−10 μM. The solution was then aspirated several times to produce a more homogeneous solution. As determined in previous work,16,17 these solution conditions resulted in initial saturation states (Ω = Q/Ksp, where Q is the ionic activity product and Ksp is the solubility product) of Ωcalcite = 0.10−0.12, and Ωotavite = 6−56, an initial pH of 8.9, and a final pH ranging from approximately 7.6 to 8.1 (see Table S1 of the Supporting Information for information on the ionic activities and saturation states for a range of initial 5013


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Figure 1. AFM deflection images of an epitaxial island and small particles on calcite (3 × 3 μm2) obtained with 2 (a) and 15 nN (b) imaging forces, 50 (a) and 121 min (b) after CdCl2 injection to produce a 1 μM solution. (c) Threshold force required to remove the particles as a function of their surface contact area taken from six experiments.

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RESULTS Low-Force Contact AFM. In previous studies conducted under the conditions of the present experiments, threedimensional epitaxial islands were observed to form for aqueous solutions with initial Cd2+ concentrations up to 50 μM.14,16 X-ray photoelectron spectroscopy (XPS) measurements17 confirmed that the epitaxial islands were composed of a Cd-rich (Cd,Ca)CO3 solid solution, which was consistent with thermodynamics analyses that predicted CdCO3 molar fractions of 0.98 and higher for the conditions of the present experiments.16 For the sake of simplicity, due to the predicted very high Cd content, the surface precipitates will be referred to as otavite hereafter, with the understanding that some amount of Ca might also be present. AFM images14,16 showed that the islands were ∼2.5 nm in height and grew preferentially along the [421̅] direction, leading to elongated or rodlike shapes. Possible explanations for the shape of the epitaxial islands include surface elastic modulus considerations, as debated in previous studies,14,16,20,21 but this aspect remains unresolved. A deflection AFM image (Figure 1a) of features observed on a (101̅4) calcite surface during in situ imaging in a 1 μM CdCl2 solution with an imaging force of 2.0 nN illustrates the presence of small particles 16−59 nm in diameter (201−2734 nm2 in surface contact area) in addition to a typical epitaxial island in the upper-right-hand corner. The smaller particles appear to represent initial stages of nucleation. When the imaging force reaches 15 nN (Figure 1b), most of these small particles are removed. Using the results of six typical experiments, in which the imaging force was increased after each scan following the highforce imaging protocol outlined in Experimental Methods, the threshold force at which each particle was removed was plotted as a function of its surface area (Figure 1c). Once a typical imaging force of 15 nN was reached, 41% of the 334 particles (27% of which had a diameter of 50 nm or less) were removed, including 89% of the particles under 50 nm in diameter (which made up 60% of all the particles removed with a force of up to 15 nN). This result indicates that the smallest particles, which are critical to understanding the mechanisms of nucleation, would not be observed under typical imaging forces. Imaging forces must be low enough such that weakly bound particles are not removed by the tip during imaging. In many AFM studies of mineral surfaces, the applied force is not listed and what is considered “low force” can range from 1 to 40 nN13,15,17,22 and often may be an estimate rather than a carefully calibrated and controlled value. In order to accurately obtain the required low

imaging forces of 0.5−2 nN, the procedure discussed in Experimental Methods was followed, including the use of low spring constant cantilevers that were calibrated before use and the determination and compensation for z-drift prior to each image. Particle Characterization. The particles observed with imaging forces below 2 nN fell into three types denoted here as epitaxial, precursor, and dynamic, based on their shape, growth behavior, mobility, and lateral signal (Table 1). Table 1. Characteristic Properties of the Three Particle Types Observed in in Situ AFM Measurements type

growth

growth direction

shape

lateral signal

mobility

epitaxial precursor dynamic

yes no no

[421̅] none none

elongated round irregular

high low high

none none high

Epitaxial islands have been well-characterized previously and can serve as an important internal reference. Figure 2a,b highlights a typical epitaxial island (circled in orange), which elongated along the [421]̅ direction from 408 to 714 nm over 38 min. In this study, the epitaxial islands grew on average from 1.3 to 5.0 nm min−1 along the [421̅] direction. The height of the epitaxial islands varied little over time and showed a narrow distribution with an average height of 2.6 ± 0.4 nm (Figure 3). Previous studies reported growth rates ranging from 1.2 to 7.8 nm min−1 and heights of 2.2−2.7 nm16 and 2.75 ± 0.25 nm.14 Dynamic particles were irregular in shape and size and had diameters ranging from 28 to 68 nm. These particles were unique in their translation across the substrate (position of the red circle relative to the white line in Figure 2a,b and Movie M1 of the Supporting Information). The translation of dynamic particles took place primarily along the fast-scan direction, which suggests the movement was due in part to interactions with the AFM tip. Unlike other particles and despite their movement, the size of dynamic particles did not change significantly over time. Under higher imaging forces, the dynamic particles were generally not observed as they were readily removed. While the dynamic particles were more prevalent in the presence of Cd2+, similar features were observed in control experiments of the calcite substrate without the addition of Cd2+. Furthermore, no evidence was found to suggest that they served any role in the formation of either precursor particles or epitaxial islands. 5014


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Figure 2. AFM deflection (3 × 3 μm2) (a, b) and lateral force (2 × 2 μm2) (c) images of the three types of particles observed on calcite 153 (a) and 199 min (b, c) after CdCl2 injection to produce a 7 μM solution. An epitaxial island is highlighted in orange, two precursor particles are highlighted in blue, and a dynamic particle is highlighted in red, which is shown to translate relative to the white line added for reference. The corresponding sequence of in situ AFM topography images is shown in Movie M1 of the Supporting Information.

(Figure 4a−c). During this time, most of the particles behaved as described abovethe epitaxial particles grew along the [421̅] direction, the dynamic particles translated across the surface, and the precursor particles were removed from the surfacebut three precursor particles (labeled 1−3) transitioned to epitaxial particles (Figure 4a−f). Figure 4a,b illustrates the moment of transition, evidenced by changes in shape, height, growth behavior, and lateral signal. Particles 1, 2, and 3 began to transition 192, 187, and 189 min after the introduction of Cd2+, respectively (dashed lines in Figure 4g−i). The change in shape can be described in terms of eccentricity (ε), which is a measure of the deviation from circularity and is given by ε = ((a2 − b2)/a2)1/2, where a and b are half of the major and minor axes of an ellipse, respectively. A transition from a stable lateral geometry to an increasingly elliptical geometry is clearly evident for each of the particles (Figure 4g). In the same way the height of particles 1−3 changed from stable values of 5.6 to 2.6 nm, 4.1 to 2.6 nm, and 3.4 to 2.6 nm, respectively, over 40 min, matching the height of the adjacent epitaxial particles (2.7 ± 0.1 nm). After the transition, the particles began to grow laterally as measured by an increase in surface contact area (Figure 4h). In addition, the lateral force signal of the central portion of the island shifted from a negative to a positive value relative to the substrate, matching that of the epitaxial islands (Figure 4d−f). Figure 4i further highlights this shift. In general, as the imaging force is increased, the lateral force signal increases in magnitude in a linear fashion.23 The particles labeled 1−3 began with a lateral signal of ∼ −8 mV, matching the value obtained for four other precursor particles (shown in gray in Figure 4i). Once the particles transitioned, their lateral signal increased linearly in the positive direction, matching that of four neighboring epitaxial islands (shown in black in Figure 4i). These measurements provide strong and direct evidence for the formation and transformation of a precursor phase that is structurally distinct from the eventual epitaxial phase. Precursor particles were sensitive to repeated imaging in contact mode, and even epitaxial islands showed a slight decrease in height with imaging forces below 15 nN and a significant decrease in height with imaging forces above 20 nN (Figure S1 of the Supporting Information). In order to confirm that the observed decrease in height of precursor particles was due to a phase transition rather than a tip induced effect, PeakForce tapping mode was utilized. PeakForce tapping mode is known to be a suitably controlled low interaction force

Figure 3. Height distributions of 94 precursor particles and 56 epitaxial islands measured from nine experiments.

Precursor particles (highlighted in blue in Figure 2) averaged 63 ± 13 nm in diameter with a greater range of measured heights than the epitaxial islands, from 0.8 to 7.4 nm (Figure 3). Precursor particles were easily distinguishable from epitaxial and dynamic particles in lateral force images as they showed a negative contrast relative to the substrate in the center of the particles compared to a positive contrast for the other two particle types (Figure 2c). Lateral force is a measure of the tilt of the cantilever due to the local tip/substrate interaction as the tip glides across the surface. The positive contrast of the epitaxial islands and dynamic particles and the negative contrast of the precursor particles relative to the calcite substrate indicate an increase and decrease, respectively, in adhesion between the tip and particle, relative to the adhesion between the tip and substrate, due to differences in the physical and chemical properties of their surfaces. In addition, unlike the other particles, which increased or remained the same size over time, a majority of the precursor particles decreased in size or were removed after multiple scans, even at forces below 1 nN, suggesting lower adhesion and/or structural integrity. Phase Transition. Precursor particles were observed to transition to epitaxial islands over time. In experiments where the moment of transition was observed, 13−72% of the precursor particles transitioned. An example observed during an incremental force experiment is depicted in Figure 4 and in Movies M2 and M3 of the Supporting Information. Early on, both epitaxial and precursor particles were present on the calcite surface (Figure 4a). Over time, the substrate continued to dissolve as can be seen by the advance of two step edges 5015


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Figure 4. Moment of transition from precursor particles to epitaxial islands. Topographical (a−c) and lateral force (d−f) AFM images taken 164 (a, d), 197 (b, e), and 216 min (c, f) after CdCl2 injection to produce a 1 μM solution under imaging forces of 1.5 (a), 5 (b), and 20 nN (c). Images are 3 × 3 μm2; insets are 500 × 500 nm2. The points of transition for particles 1 (blue), 2 (red), and 3 (green) are marked by vertical dashed lines of the same color in (g), (h), and (i). (g) Eccentricity of the particles from the time of injection. (h) Surface contact area assuming an elliptical geometry (dots) as well as the height (lines) of particles 1−3 over time from the point of injection. (i) Lateral force signal of particles 1−3, four epitaxial islands (dashed black), and four additional precursor particles (dashed gray) as a function of increasing imaging force. The corresponding sequences of in situ AFM topography and lateral force images are shown in Movies M2 and M3 of the Supporting Information.

imaging method.18,19 PeakForce tapping mode uses minimal force curves as the imaging feedback mechanism rather than maintaining a constant tip deflection, as in contact mode, or by oscillating the tip and applying an unknown force, as in traditional tapping mode. The progression of 15 precursor islands from initial formation to growing epitaxial islands is shown in Figure 5a,b. The change in height of these particles was measured from 12 to 217 min after Cd2+ injection (Figure 5c). The precursor particles initially ranged in height from 2.08 to 7.40 nm. As the precursors transitioned to epitaxial islands, their heights converged to 2.98 ± 0.13 nm, confirming this process is a characteristic of epitaxial island formation and is not a tip induced phenomenon. It is important to note that precursor islands as short as 1.6 nm and as tall as 7.4 nm were observed to transition into epitaxial islands. This finding has important implications when considering the thermodynamic and kinetic driving forces that play a role in the formation of epitaxial islands.

Particle Adhesion. To further probe the phase of the precursor particles, the adhesion of the particles was compared to that of the epitaxial islands. If the precursor particles represent a metastable nonepitaxial phase, one would expect these particles to have a lower adhesion force relative to the epitaxial islands. To determine adhesion force, the AFM tip was used as a probe. Experiments were conducted in lateral force contact mode where the imaging force was increased incrementally after each scan until a maximum force was achieved. As the normal force applied increased, the particles were removed and the degree to which the AFM tip tilted was recorded as the lateral signal. The highest lateral signal attained for each particle before it was removed was then used as a measure of its relative adhesion. This method is similar to nanoshaving,24 and has previously been demonstrated as a method suitable for the study of local adhesion in general25,26 and particle adhesion during mineral growth in particular.27 Due to the difficult and unreliable nature of lateral force 5016


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Figure 5. PeakForce tapping mode images, 10 × 10 μm2, of particle nucleation and growth 12 (a) and 217 min (b) after the introduction of CdCl2 to produce a 5 μM solution. (c) Time evolution of the height of 15 precursor particles shown in (a) as they transitioned to epitaxial islands shown in (b).

Figure 6. Highest lateral voltage signal achieved for individual particles before their removal, normalized relative to epitaxial islands at 10 nN, as a function of surface contact area (a) and height (b), representing the relative adhesion of precursor particles (blue symbols) and epitaxial islands (red symbols).

calibration, relative values were used rather than absolute lateral force values.27−30 For each experiment the average lateral signal value obtained for the epitaxial islands at 10 nN was used as an internal standard due to their constant height and known phase. This was then used to normalize each threshold value. This approach ensured that changes in lateral signal due to tip radius, shape, spring constant, alignment, etc. could be mitigated and data from separate experiments could reliably be compared. Data from five representative experiments with imaging forces ranging from 1 to 260 nN and [Cd2+]0 ranging from 1 to 10 μM were compiled (Figure 6). Each particle was identified as an epitaxial island or precursor particle prior to removal at low force by lateral force contrast, particle height, shape, and growth behavior. The relative adhesion as a function of surface contact area (Figure 6a) and particle height (Figure 6b) showed a clear difference in adhesion. Overall, the average relative lateral force signal for precursor particles was 0.69 ± 0.55 compared to 3.20 ± 0.80 for epitaxial islands. Assuming a linear correlation between lateral force and adhesion,31,32 these measurements would suggest the adhesion of precursor particles is ∼5 times weaker than that of the epitaxial islands. The increased adhesion of the epitaxial islands is not due to the increased surface contact area which would result in a linear increase in relative adhesion.27,33 This finding is consistent with the hypothesis that precursor particles are metastable particles that undergo a phase change to three-dimensional (3-D) islands, which are epitaxially oriented crystals with respect to the calcite substrate and thus exhibit a higher adhesion force.

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heterogeneous nucleation and growth is schematically illustrated in Figure 7. The collective evidence suggests that the precursor particles formed in solution and were deposited on the surface (Figure 7a), rather than nucleating on the surface. Indeed, they were present on the calcite surface within minutes of the introduction of Cd2+ and exhibited a fairly wide distribution of heights from the very first image (Figures 3 and 5c), whereby the larger particles were never observed to grow from smaller particles. Instead, changes in height were associated with phase transformation and tended toward the typical epitaxial island height of approximately 3 nm (Figure 5). The formation of amorphous particles in solution has been reported for several metal carbonates at much higher supersaturations than used here.34−36 However, our conclusion that amorphous particles first form in solution at lower supersaturation is consistent with the previous work on homogeneous calcium carbonate nucleation discussed in the Introduction, as well as the Ostwald−Lussac law of phases, which suggests that nucleation of stable phases will first proceed through less stable states in order of increasing stability.37 In our system we can expect the lower stability phases will be an amorphous phase and spherical in shape as this provides the lowest free energy of any shape.1,2 Future work will focus on isolating the precursor particles from solution. Within 20 min to 3 h, precursor particles began to transform to a fully crystalline and epitaxial phase. This transformation was demonstrated by changes in particle size, shape, friction, and adhesion to the surface (Figures 4−6). Because calcite and otavite adopt the same crystal structure, the interfacial energy between the calcite substrate and the otavite nucleating crystal, αsc, should be lower than that between calcite and the precursor particle, αsp. As a result, nucleation of otavite within the

DISCUSSION

This work provides direct evidence for the formation of a metastable precursor phase during heteroepitaxial nucleation and growth of cadmium carbonate islands at low supersaturation on the surface of calcite. The apparent mechanism of 5017


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Figure 7. Proposed nucleation and growth pathway of CdCO3 on the calcite substrate. Amorphous precursors are formed (a), which first attach to the calcite substrate (b), then crystallize mediated by the surface, adopting an optimal height (c), and finally undergoing lateral heteroepitaxial growth (d).

grew laterally and preferentially along the [421̅] direction (Figure 7d). Possible explanations for this preference, which include site accessibility, kink site density, direction-dependent stiffness, or some combination thereof, were discussed in previous studies.16,20,21 This example of the nucleation of a low-solubility inorganic phase at the calcite/water interface and at low supersaturations significantly extends the range of systems and conditions for which nonclassical nucleation is operative. Overall the observed pathway of heteroepitaxial nucleation and growth by way of an amorphous precursor provides important new insights into the complex nature of metal carbonate nucleation and growth. The findings will help in the development of improved conceptual models of mineral growth and transformations across a wide range of environmental, geological, and biological systems.

precursor phase should be facilitated, and is thus expected to occur, at the interface with the calcite substrate (Figure 7b). Precursor heights increased or decreased from a range of 1.6 to 7.4 nm to conform to the typical 2.5−3 nm height of epitaxial islands (e.g., Figure 5). This observation strongly suggests that the island height is dictated by the thermodynamics of the system. Following the conceptual models put forward by Jun et al.,38 the free energy of the epitaxial phase can be divided into three contributions: the bulk energy, g; the interfacial energy with the substrate, αsc, and with the precursor particle, αcp; and the strain energy, U, that arises from the lattice mismatch between calcite and otavite. The change in bulk energy, Δg, is negative as otavite is thermodynamically more stable than the precursor phase, whereas the change in strain energy, ΔU, is positive as the less-structured precursor phase is more loosely attached to the substrate (Figure 6). If the effects of the island’s side surfaces are neglected due to their small contribution to the overall surface area, both the bulk and strain energies will vary linearly with the island height, h, as the island grows vertically. The net change in interfacial energy, Δα, will be negative if the energy gained from replacing the substrate− precursor interface by an otavite−precursor interface is greater than the calcite−otavite interfacial energy, a likely proposition as otavite and calcite adopt the same structure and otavite and the precursor phase are similar chemically. In such a case, a limiting height will be observed if |ΔU| > |Δg|, whereby the sum of h|Δg| and |Δα| initially exceeds h|ΔU|, and growth is favorable until the increase in strain energy prevents further growth (Figure 7c).38 Molecular simulation will be employed in future work to quantify all contributions and confirm this model. Because the precursor phase is the growth medium in this picture, precursor particles with heights lower than the equilibrium height would have had to draw materials from the sides of the particle for vertical growth to extend to the equilibrium height. If such transport is possible, why was the growth primarily vertical at that stage instead of lateral as observed in the last stage (Figure 7d)? One possible explanation is the fact that the precursor particle has to dehydrate. AFM images can be misleading in that the lateral scale is typically much larger than the vertical scale, so that the precursor particles and epitaxial islands were actually significantly wetting the surface. As a result, vertical growth might have been kinetically favored during crystallization of the precursor phase as it provided the shortest route for water to be expelled from the growing phase. Once the phase transformation was complete, the particles were crystalline and epitaxially oriented with respect to the calcite substrate. The growth medium was then the aqueous solution, resulting in lower growth rates than the rapid precursor-to-otavite phase transformation. The epitaxial islands

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b11727. Sequence of in situ AFM topography images showing growth of CdCO3 on calcite substrate over 410 min (AVI) Sequence of in situ AFM topography images showing growth of CdCO3 on calcite substrate over 61 min (AVI) Sequence of in situ AFM lateral force images showing growth of CdCO3 on calcite substrate over 61 min (AVI) Range of initial solution conditions; height of epitaxial 3D islands as a function of imaging force (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Shawn L. Riechers: 0000-0002-5713-5534 Kevin M. Rosso: 0000-0002-8474-7720 Sebastien N. Kerisit: 0000-0002-7470-9181 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division through its Geosciences Program at Pacific Northwest National Laboratory (PNNL). The research was performed using the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research 5018


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and located at PNNL. PNNL is operated for DOE by Battelle Memorial Institute under Contract No. DE-AC06-76RLO1830.

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Nucleation and Epitaxy-Mediated Phase Transformation of a

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