Article Cite This: Chem. Mater. 2018, 30, 700−707
pubs.acs.org/cm
Templating Growth of a Pseudomorphic Lepidocrocite Microshell at the Calcite−Water Interface Ke Yuan,*,† Sang Soo Lee,† Jun Wang,‡ Neil C. Sturchio,§ and Paul Fenter† †
Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States ‡ National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, New York 11973, United States § Department of Geological Sciences, University of Delaware, Newark, Delaware 19716, United States S Supporting Information *
ABSTRACT: The growth of lepidocrocite (γ-FeOOH) has been observed through oxidation of Fe(II) on calcite (CaCO3). Here, we seek to understand the structural relation between lepidocrocite and the calcite substrate and its growth mechanism. The formation of iron oxyhydroxide layers having distinct morphologies was observed during the dissolution of calcite in acidic Fe(II)-rich solutions. A pseudomorphic lepidocrocite shell together with multiple iron oxyhydroxide layers encapsulated within the shell was imaged by optical and transmission X-ray microscopies. The presence of a severalnanometer-thick ordered lepidocrocite film was observed by Xray reflectivity, with the lepidocrocite (100) plane oriented parallel to the calcite (104) surface. Lath-shaped lepidocrocite aggregates formed during the initial precipitation, which eventually grew into clusters of parallel platy crystals. The formation of a nanometer-thick well-ordered lepidocrocite film on a pristine calcite surface appears critical for the subsequent pseudomorphic overgrowth. Detachment of the lepidocrocite film from the dissolving calcite surface yielded a free-standing pseudomorphic iron oxyhydroxide shell, suggesting weak interactions between the shell and the calcite substrate. This growth mechanism yields the potential of using carbonate minerals as templates for pseudomorphic synthesis of iron oxyhydroxides having well-defined size and morphology.
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demonstrated.27 Similar processes may occur in natural environments, where pseudomorphic growth/replacement reactions commonly occur during the diagenesis and weathering of rocks. A secondary phase grows on top of a parent mineral while preserving the dimension of the original parent crystal.28,29 Formation of lepidocrocite on calcite, a ubiquitous mineral in Earth’s crust, has been investigated in the context of remediation of Fe in groundwater, which leads to the enrichment of Fe in calcareous soils.30 Precipitation of platy lepidocrocite crystals on calcite surfaces has been observed by scanning electron microscopy (SEM).31 However, the specific structural relation between calcite substrate and lepidocrocite remains unresolved. Here, we provide new insights into the growth of lepidocrocite at the calcite (104)−water interface, where we observed a pseudomorphic growth process. Multiple imaging techniques (optical microscopy, transmission X-ray microscopy (TXM), SEM, and atomic force microscopy (AFM)) were used to investigate the morphology and structure of the iron oxyhydroxide pseudomorphic layers. Specular X-ray reflectivity
INTRODUCTION Lepidocrocite (γ-FeOOH) is a common iron oxyhydroxide that forms in nature by both abiotic (e.g., corrosion of metallic Fe) and biomineralization processes.1−5 As a polymorph of common goethite (α-FeOOH) and rare akaganéite (βFeOOH), it has multiple uses in environmental and materials science. It is known to sequester heavy metal elements (As(III,V),6−9 Pb(II),10 Se(IV,VI)11,12) and radionuclides (U(VI),13,14 Np(V)15) by adsorption and incorporation. It is active in charge transfer reactions due to the redox sensitive Fe(III) ion.16,17 Lepidocrocite as a semiconducting material demonstrates photochemical properties enabling the decomposition of organic molecules18 and has been investigated as a battery anode through reversible insertion of Na+ and Li+ between the [FeO6] octahedral sheets.19,20 The growth mechanism of lepidocrocite is not wellunderstood and is critical toward its utilization. Previous studies revealed the effects of different substrates and organic modifiers on the homogeneous crystallization and morphological control of lepidocrocite in solution.21−23 Solid state transformation of ferrihydrite, a poorly ordered hydrous ferric oxyhydroxide, to lepidocrocite has also been investigated.24−26 Recently, growth of iron oxyhydroxide pseudomorphic “microboxes” from cube-shaped organic precursors has been © 2018 American Chemical Society
Received: September 15, 2017 Revised: January 4, 2018 Published: January 5, 2018 700
DOI: 10.1021/acs.chemmater.7b03921 Chem. Mater. 2018, 30, 700−707
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Chemistry of Materials
Transmission X-ray Microscopy. Ex situ TXM images resolved three distinct precipitate layers around the remaining calcite core (Figure 2). Each layer was less than 1 μm thick, and
measurements were used to provide detailed structural and morphological data for lepidocrocite grown in situ on the calcite (104) surface. This set of techniques was used to resolve the pseudomorphic growth mechanism of lepidocrocite on calcite at the molecular scale.
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RESULTS Optical Microscopy. The optical images of an isolated calcite crystal with its rhombic (104) surface lying flat on the Kapton support were recorded in situ in a 1 mM Fe(II)Cl2 at initial pH = 3.0 as a function of time. This reaction solution was undersaturated with respect to calcite. Therefore, dissolution of calcite was expected and was confirmed by the edge retreating and corner rounding of the crystal over time (Figure 1a).
Figure 2. (a) 3D reconstructed image of a reacted calcite crystal from TXM data. The crystal was reacted in a solution with 1 mM FeCl2 at initial pH = 3.0 for 3 h. (b) Cross-sectional view, cut across the yellow line in a, shows three layers of precipitates around the dissolved calcite core. (c) Averaged intensity within the dashed line box projected along the horizontal direction of the cross-sectional image (b).
density variation was observed both within each layer and between different layers. For example, a part of layer 2 on the right side of Figure 2b had higher density than that on the left side (Figure 2b and c). Between layers 1 and 2, a low density zone was found, which was presumably filled with solution during reaction in contact with the fluid phase. Layers 1 and 2 preserved the approximate dimensions of the original calcite crystal, indicating formation of a pseudomorphic shell that corresponds to the observed transparent shell on the optical images (Figure 1). Layer 3 was found adjacent to the remaining calcite core, which likely is associated with the dark red surface coating observed in Figure 1b. In addition, flaky precipitates were found on the inner wall of layer 2 (Figure S2). This observation indicates that the secondary precipitation reactions occurred not only on the calcite surface but also on the pseudomorphic shells. Scanning Electron Microscopy. SEM images revealed that the pseudomorphic shell is composed of nanosized precipitates that are typically bladed or platy and up to a few hundred nanometers long (Figures 3a and b). Apparently, pore space exists between the grains. From the broken edges of the outer shell, we found that it consists of either one (Figure 3c) or two layers (Figure 3d). The thickness of each layer ranges from 200 to 400 nm. Although the calcite crystals for SEM were prepared from the same experiment, the observed differences in surface morphology and layered structure indicate a complex growth mechanism during the shell formation. Atomic Force Microscopy. Initial growth of iron oxyhydroxide coatings on the calcite (104) surface was investigated by ex situ AFM. The nucleation of precipitates on the calcite surface was observed after 1 min of reaction. Individual particles, less than 100 nm long and 30 nm high (Figure 4a, inset), aggregated to form larger islands (Figure 4a). After 10 min of reaction, particles exhibited similar orientation with welldefined edges, indicating increased crystallinity (Figure 4b). The height of the particles increased up to ∼160 nm after further reaction of 20 min, and the particles showed well-
Figure 1. (a) Optical images of calcite in a 1 mM FeCl2 solution of initial pH = 3.0 recorded in situ up to 15 h. (b) Two calcite crystals of different orientations reacted for 15 h from the same experiment as the crystal shown in a. The shells and a red iron oxyhydroxide surface coating are indicated by the arrows.
Formation of red precipitates on the calcite (104) surface was observed (Figure 1a) and likely associated with the formation of iron oxyhydroxides (confirmed by X-ray diffraction results in the X-ray Reflectivity section) similar to those observed previously.31 Full coverage of the calcite surface took about 7 h (Supporting Information, Figure S1), after which the red color deepened, suggesting thickening of the film. This red precipitate-coated calcite crystal appeared to be encapsulated by a transparent “shell,” which maintained the shape of the original rhombic calcite crystal (Figure 1a, indicated by the black arrow). This shell likely formed at the early stage of the reaction before significant dissolution of calcite occurred (Figure 1a,h). Similar shells were also observed on other crystals, where different shell shapes reflect different orientations of the original calcite crystals (Figure 1b). The presence of the shell did not stop either the subsequent precipitation of the surface coating on the dissolving calcite crystal or even the dissolution of the calcite core, indicating that the shell was porous. 701
DOI: 10.1021/acs.chemmater.7b03921 Chem. Mater. 2018, 30, 700−707
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Chemistry of Materials
ular to the calcite surface, forming a film of relatively high roughness. X-ray Reflectivity. Further insights into the molecular-scale structure of the iron oxyhydroxide films were obtained from Xray reflectivity measurements at the calcite (104)−1 mM Fe(II) solution interface as a function of time. The crystal truncation rod (CTR) data measured after 1 min of reaction (Figure 5a, 1
Figure 3. Four calcite crystals reacted in 1 mM FeCl2 solution of initial pH = 3.0 for 3 h. a and b show the surface morphologies of two different pseudomorphic shells. Dashed line boxes in the insets indicate the locations of the high-magnification images. Arrows point to individual layers of the exposed edges of the pseudomorphic shells: One or two layers are identified in c and d.
Figure 5. (a) X-ray reflectivity of the calcite (104) surface reacted with a 1 mM FeCl2 solution of initial pH = 3.0 for 1, 5, 10, and 20 min. (b) Normalized reflectivity obtained by dividing out the generic CTR form factor associated with the strong variation of the reflectivity due to calcite Bragg peaks at Q = 2.07, 4.14, and 6.21 Å−1. The red lines represent the reflectivity signal calculated using optimized structural models (Figure S4). The green lines are the reported X-ray reflectivity of the pristine calcite (104)−water interface.34 Positions of the lepidocrocite (2 0 0), (6 0 0), (8 0 0), and (10 0 0) Bragg peaks are indicated by vertical dashed lines.
min) were compared with those for the pristine calcite (104)− water interface.32 Intensities between calcite Bragg peaks for the 1 min reacted sample are generally lower than those for the unreacted interface, indicating roughening of the surface by dissolution in the acidic solution. No additional Bragg peaks or oscillations are observed after 1 min, precluding the formation of a well-ordered film or adsorbed Fe layer on the calcite surface. In the subsequent reaction steps (Figure 5a, 5, 10, and 20 min), thin-film Bragg peaks appeared at Q = 0.99 Å−1, 2.98 Å−1, and 3.89 Å−1, which are very similar to the lepidocrocite (200) (Q = 1.01 Å−1), (600) (Q = 3.04 Å−1), and (800) (Q = 4.05 Å−1) Bragg peak positions, respectively (lepidocrocite, a = 12.40 Å, b = 3.87 Å, c = 3.06 Å, α = β = γ = 90°).33 This agreement suggests that the thin film was composed of lepidocrocite. The measured Q values of these lepidocrocite Bragg peaks were slightly smaller than the reported powder diffraction data, indicating a larger a cell parameter for the film. Additional weak lepidocrocite Bragg peaks, i.e., the (10 0 0) reflection, were also visible, most notably through the changes in the normalized CTRs (Figure 5b). Because the reflectivity was measured at the specular condition with respect to the calcite (104) surface, the prevalence of this series of peaks indicates that the lepidocrocite (100) plane was oriented parallel to the calcite (104) surface. However, no Kiessig fringes were observed around the lepidocrocite Bragg peaks, indicating a highly rough film surface morphology. The lepidocrocite Bragg peaks became more intense with time, indicating an increase in film coverage. In contrast, the peak widths remain largely unchanged, indicating a constant thickness of the film.
Figure 4. Ex situ phase and corresponding height images (rendered as 3D images) of calcite (104) surfaces after reaction in a thin film cell filled with 1 mM Fe2+ solution of initial pH = 3.0 for (a) 1 min, (b) 10 min, and (c) 20 min, respectively.
defined shape and orientations (Figure 4c). From these images, we conclude that these particles grew preferentially perpendic702
DOI: 10.1021/acs.chemmater.7b03921 Chem. Mater. 2018, 30, 700−707
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Chemistry of Materials
1.57, 2.65, and 3.00 Å−1 were found (Figure S3). These peaks are close to the Bragg peak positions of goethite (Table S1), indicating the presence of randomly oriented goethite crystals on the calcite (1 0 4) surface. Structural Model of the Reacted Calcite Surface. The best-fit electron density profile of the 1 min reacted sample had a peak at 2.59 Å with an occupancy of 2.03 ± 0.34 H2O/Auc, where Auc is the area of the unit cell on the calcite (104) plane (Figure S4a and Table S2). The height and occupancy are similar to those attributed previously to the adsorbed water layer at the calcite (104)−water interface.32 Since there were no visible Bragg peaks of lepidocrocite in the CTR of the 1 min sample (Figure 5), the film component was excluded in the analysis. For the analysis of the other CTR data where the Bragg peaks of lepidocrocite were evident (Figure 5), the lepidocrocite was modeled as a film having a uniform layer spacing with a varying occupation factor for each successive layer above the calcite surface. This model also included interface layers to simulate detailed variations in electron density near the calcite surface. Specifically, inclusion of two layers (modeled as two separate Gaussian peaks) between the calcite surface and the lepidocrocite film (Figure 7 and Figure
These observations reveal that the film evolved primarily by increasing its lateral coverage on the calcite surface. On the basis of the peak full width at half-maximum (ΔQ) of 0.12 Å−1 of the (200) Bragg peaks, the average film thickness was estimated to be ∼50 Å (= 2π/ΔQ). This estimated thickness is thinner than those observed in the AFM images (Figure 4). This discrepancy results from the different sensitivity between CTR and AFM to the interfacial structure. While AFM images can show all topographical features formed on the calcite surface, specular CTR is sensitive only to the portion of a film that is structurally ordered with respect to the calcite surface. Therefore, the smaller film thickness estimated by CTR suggests that only part of lepidocrocite films were structurally connected to the calcite substrate. Further insights into the phases of the precipitates and their orientation with respect to the calcite (104) surface were obtained from three-dimensional X-ray diffraction around the specular CTR of the surface reacted with the acidic FeCl2 solution for 15 h (Figure 6). Three intense peaks at Q = 2.07,
Figure 7. Structure model of the lepidocrocite-coated calcite (104)− water interface. The electron density profile was obtained by fitting the X-ray reflectivity measured on the calcite (104) surface reacted in a 1 mM Fe(II) solution for 20 min. The schematic represents the components of the model, including distorted calcite (104) surface, the interface layers (which likely consist of a mixture of adsorbed ionic Fe species and H2O molecules), lepidocrocite film, and bulk water.
S4) significantly improved the quality of fit compared to the model with only one layer. In contrast, the improvement was relatively marginal for the models with more than two peaks, which also showed large covariance among the parameters (data not shown). The best-fit models reveal that the coverages of each ordered lepidocrocite layer seen by XR increased with reaction time while the thickness of the film changed slightly (Figure S5). This indicates the lateral growth of initial ordered nuclei and/or increase in the number of ordered lepidocrocite nuclei. These results complement the AFM images, which revealed the vertical growth of both aligned and nonaligned lepidocrocite layers. The first four to five half-unit-cell layers had nonzero occupancies, which increased as a function of time (Figures 7 and S4). For example, the fractional coverage of the bottom lepidocrocite layer in contact with the calcite surface increased from 6 to 17 to 21% at 5, 10, and 20 min, respectively (Table
Figure 6. Cross-sectional view of a reciprocal space map near the specular crystal truncation rod of the calcite (104) surface reacted with 1 mM FeCl2 solution of initial pH = 3.0 for 15 h as a function of vertical and lateral momentum transfers Qz andQ∥). In these images, the lateral momentum transfer, Q∥, is oriented transverse to the scattering plane. Powder rings are identified at various Q values. The background-subtracted data are shown in logarithmic scale in order to highlight the weak intensity of the crystal truncation rod.
4.14, and 6.21 Å−1 are the first, second, and third order reflections of the calcite (104) Bragg peaks. The lepidocrocite (2 0 0), (6 0 0), and (10 0 0) Bragg peaks are between the calcite Bragg peaks and are broad along Q∥. Powder rings going through the (2 0 0) and (6 0 0) peaks were also observed, indicating that some lepidocrocite crystals grew in random orientations. In addition, four other powder rings at Q = 1.30, 703
DOI: 10.1021/acs.chemmater.7b03921 Chem. Mater. 2018, 30, 700−707
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Chemistry of Materials S2). The derived lattice parameter for the lepidocrocite film was slightly larger than that reported by powder X-ray diffraction (Table S2), consistent with the observed shifts of the Bragg peaks toward smaller Q values in the CTR data. We also accounted for structural changes in the calcite surface by allowing for vertical displacements (Δz) of Ca atoms and CO3 groups and angular displacements (tilt, θ and rotation, Φ) of the CO3 groups as shown in Figure S6. In general, the surface relaxation of calcite primarily occurred within the top unit-cell layer, and the degree of displacement of Ca2+ and CO32− from their ideal positions became more pronounced with increasing reaction time (Figure S7). Given the complexity of this interfacial structure and the partial coverage of the surface by the lepidocrocite films, these structure changes of calcite are likely more indicative rather than uniquely defined.
Figure 8. Schematic model of the layered structure of FeOOH formed on top of calcite.
transport of ions and water), allows the complete detachment of the films from the calcite surface, leaving the freestanding lepidocrocite “pseudomorphic” shell observed by optical microscopy and TXM. Fe−Calcite Interaction and Iron Oxyhydroxide Formation. It is known that Fe(III) and Fe(II) interact with calcite differently.30 Aqueous Fe(III) ions rapidly hydrolyze and polymerize into amorphous colloidal solids.36 These polymerization reactions produce H+, which can bind to CO32− released from calcite to form HCO3−. The formation of the final iron oxyhydroxide products dominated by lepidocrocite and goethite were interpreted mainly as an independent process that does not require specific interactions of Fe(III) with the calcite surface.30 In contrast, Fe(II) ions are more reactive with calcite. Addition of Fe(II) to the growth solution of calcite can dramatically reduce the growth rate of calcite compared to Fe(III).37 This observation was interpreted as a consequence of Fe(II) competing against Ca(II) for sorption sites during calcite growth. Fe(II) can form the carbonate mineral siderite (FeCO3), which is isostructural to calcite. Similar coordination environments of Fe(II) and Ca(II) in these carbonate phases may facilitate strong Fe−calcite surface interactions. We speculate that surface sorption likely leads to the subsequent oxidation from Fe(II) to Fe(III) as surface complexed Fe(II) is known to oxidize substantially faster than free aqueous Fe(II) ions.38 Hydrolysis of Fe(III) on the calcite surface resulted in the precipitation of lepidocrocite film as characterized by X-ray reflectivity. Lepidocrocite is commonly found during the oxidation of Fe(II) bearing solutions.1 A lepidocrocite crystal has a basic lath-shaped morphology and is elongated along the [100] direction. The largest faces commonly found on lepidocrocite are the {010} faces.1 However, the present X-ray reflectivity data reveal that the lepidocrocite crystals are oriented with the (010) plane aligned parallel to the calcite surface normal direction. This result is consistent with reported observations that platy lepidocrocite crystals grow in a perpendicular (“standing”) geometry on calcite (Figure 8).30,31 Additional experiments performed at higher Fe(II) solution concentration (5 mM) reproduced the micrometer-sized lepidocrocite flakes that were grown perpendicular to the calcite surface (Figure S8). This growth pattern is also suggested by the AFM images (Figure 4b and c). The aligned platy particles observed on the 10 min sample were likely due to the initial growth of lepidocrocite, which grew along the [100] direction into taller crystals in the 20 min sample. We infer that the thin ordered lepidocrocite film formed on the calcite surface served as the basis for the growth of the thicker pseudomorphic shell observed by optical microcopy and
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DISCUSSION Templating Growth at the Calcite (104)−Water Interface. The first peaks of the interface layer models for the 5, 10, and 20 min samples are located close to the surface, i.e., ≤ 1 Å from the top oxygen layer (outermost O in the top CO32− group; Figure S4b, c, and d). These short distances indicate that these peaks likely include species that are either adsorbed on or incorporated in the calcite surface. The occupancies of these first peaks were 2.46 ± 0.14 and 4.3 ± 0.16 H2O/Auc for the 10 and 20 min samples, respectively, when they were modeled as H2O. These values are higher than that of water observed at the calcite−water interface (∼2 H2O/ Auc34), indicating the presence of heavy elements in the layer. The most likely candidate is Fe, which has the highest concentration (1 mM) among all cations in the solution. The measured occupancies correspond to 0.97 ± 0.05 and 1.66 ± 0.04 Fe/Auc for the 10 and 20 min samples, respectively (Table S2). It is more likely that the actual structure contained a mixture of Fe cations and water. While the results imply the sorption of Fe on the calcite surface, the chemical forms of the Fe species cannot be identified solely with the XR data. Fe(III)CO3(OH)0, Fe(III)CO3+, and Fe(II)CO30 have been proposed as possible surface species when Fe(II)/Fe(III) adsorbs on calcite,35 but direct experimental data (e.g., EXAFS) on the structures of these surface complexes remain limited. We postulate that this Fe/H2O layer may act as a bridge between the calcite surface and the film. Unlike the prediction from traditional nucleation theories that film growth relies on lattice matching and direct bond formation between the substrate and film, the XR analyses imply that the lepidocrocite film might not form direct bonds with the calcite substrate. For example, the height of the bottom O(H) plane of the film was about 3.4 Å from the top oxygen plane of the substrate. In comparison, the theoretical height calculated from hydrogen bonds between the film and the surface is ca. 1.9 Å, substantially shorter than the measured value. This presumed weak connection between the growing lepidocrocite film and the calcite substrate provides a possible explanation for observations of the detachment of the lepidocrocite film in the later reaction stage leading to a free-standing shell (Figures 1 and 2). After the initial nucleation and growth of ordered lepidocrocite films at the pristine calcite−water interface (Figure 8), we postulate that the “softness” of the hydrated interface layer allows sufficient diffusive flow of the acidic solution to the interface between the film and the surface to enable further dissolution of calcite. This, coupled with the apparent porosity of the lepidocrocite shell (that allows 704
DOI: 10.1021/acs.chemmater.7b03921 Chem. Mater. 2018, 30, 700−707
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Chemistry of Materials TXM. Even though the highest film coverage measured by Xray reflectivity was 21%, with increased solution volume (as shown in samples prepared for optical microcopy and TXM measurements), ordered lepidocrocite films are expected to cover more calcite surface areas. Calcite-surface-mediated formation of lepidocrocite layers likely dominated the early stage of the growth. This explains why the outermost pseudomorphic layer observed on calcite crystals was in the shape of the original calcite crystal. This thin layer provided the seeds for further growth of platy lepidocrocite crystals on top of it (Figure 8). As the calcite crystal dissolved, additional layers of lepidocrocite films might form. If the rate of film formation is faster than that of calcite dissolution, multiple iron oxyhydroxide layers might form, as observed by TXM and SEM (Figures 2b and 3d). The inner layers observed close to the rounded calcite core (Figure 1b, surface coating) likely contained different iron (oxy)hydroxide phases formed in the later stage of the reaction. The primary phase of these precipitates is likely to be goethite as indicated by the X-ray diffraction of the calcite sample (Figure 6 and Table S1). Well-crystalline lepidocrocite and lesscrystalline goethite were reported as the two main products from the reaction of calcite in a 10 mM Fe(ClO4)2 solution.31 For lepidocrocite formation, however, it is suggested that the main reaction includes the production of CO2 as
made by sealing two glass windows onto a PEEK cell body having a fixed volume of 4.2 mL. Calcite crystals from 10 to 200 μm in size were grown on either borosilicate glass or Kapton film using the ammonium diffusion method39 before transferring them to the fluid cell. A 1 mM Fe(II) solution was prepared by dissolving FeCl2·4H2O in deionized (DI) water that was bubbled with N2 for >12 h to reduce dissolved oxygen. The solution pH was adjusted to 3.0 using 0.1 M HCl. Acidic pH was used in order to minimize the oxidation of Fe(II) to Fe(III) (Supporting Information, Figure S9). Freshly prepared Fe(II) solution was injected into the cell loaded with calcite crystals, and then temporal evolution of the shape and color of the crystals was observed optically for 15 h. The reacted samples were gently washed by DI water, dried in N2, coated with a 10-nm-thick Au film, and imaged using a Hitachi S4700 scanning electron microscope (SEM). Transmission X-ray Microscopy. Ex situ transmission X-ray microscopy (TXM) measurements were performed at beamline 8-BMB of the Advanced Photon Source (APS) at the Argonne National Laboratory (ANL) using the TXM relocated from National Synchrotron Light Source II (NSLS-II) at Brookhaven National Laboratory operated by the NSLS-II through the NSLS-II TXM transition program. The X-ray photon energy was 8 keV with a field of view of 40 μm × 40 μm and a pixel size of 40 nm after 2 × 2 pixel binning. Calcite crystals were grown on a Kapton film using the ammonium diffusion method, and reacted with a 1 mM FeCl2 solution at pH 3 for 3 h. The reacted crystals were gently washed by DI water and dried in N2 for ex situ TXM imaging. Each crystal was imaged in 721 projection directions spanning over 180° of rotation angle with an exposure time of 20 s per image. This series of X-ray data was transformed numerically into the 3D object, and this reconstructed image was visualized using a TXM reconstructer (Xradia). Brightness on the reconstructed image is proportional to the density of the material. X-ray Reflectivity and Model Fitting. X-ray reflectivity (XR) measurements were performed using a single-crystal calcite (Chihuahua, Mexico) with dimensions of 8 mm × 8 mm × 2 mm that was cleaved along its (104) plane and washed gently with DI water and methanol. The crystal was dried under a flush of N2 and placed in an X-ray thin-film cell composed of a PEEK cell body and sealed with a Kapton window, as described previously.40,41 The growth of lepidocrocite films was achieved by reacting the calcite sample with ∼0.6 mL of the 1 mM Fe(II) solution at initial pH = 3.0 for 1, 10, and 20 min. After each reaction, excess solution was drained to leave a thin solution layer (∼10 μm) between the Kapton window and the calcite substrate that effectively froze the reaction progress. Specular XR data were measured at beamline 33-ID-D of the APS at ANL using a photon energy of 18.0 keV. The specular reflectivity signal and the associated background were measured using an X-ray area detector (Pilatus) as a function of momentum transfer Q (Å−1). The d104 spacing of calcite is 3.035 Å, and the unit cell area on the (104) plane, Auc, is 20.198 Å2. The 3D crystal truncation rod was reconstructed using rsMap3D. Experimental details can be found elsewhere.32,34,42 The measured reflectivity data were fit to a model in order to obtain insight into the film structure at the interface. The best-fit model to the XR data includes four components: the bulk calcite crystal, the interface layer(s), the lepidocrocite film, and the bulk water. A series of models were used to evaluate the significance of specific structural features at the lepidocrocite-coated calcite (104)−water interface using χ2 (χ2 = 1/Np ∑i[(Ri − Rc,i)/σi]2, where Ri, Rc, σi, and Np are the measured reflectivties, calculated reflectivities, uncertainties of the measurements, and number of data points, respectively).43 Details on the fitting can be found in the Supporting Information. Atomic Force Microscopy. The morphology of lepidocrocite films on the calcite (104) surface was imaged ex situ using the Asylum Research MFP-3D atomic force microscope (AFM) in tapping mode. The samples were prepared in the same procedure used for the XR measurements. The calcite crystal was removed from the cell and quickly washed by N2 bubbled DI water and dried in N2 before imaging. Images in the area of 2 μm × 2 μm were obtained using a silicon tip having a ∼10 nm radius.
4Fe2 + + O2 + 4CaCO3(s) + 2H 2O → 4γ‐FeOOH + 4Ca 2 + + 4CO2
(1)
For the optical microscopy work performed using the liquid cell, growth of lepidocrocite sheets likely led to an increase in partial pressure of CO2, which has been found to favor the growth of goethite rather than lepidocrocite.30 In general, the surface coating found inside the pseudomorphic shell has distinct crystal morphology and lower density (Figure 2b) compared to that of the shell and is likely to be dominated by goethite and poorly crystalline iron (oxy)hydroxide phases (Figure 8).
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CONCLUSIONS Surface-mediated formation of ordered lepidocrocite thin films served as a template for the subsequent growth of an iron oxyhydroxide pseudomorphic shell. Precipitation of lepidocrocite dominated the early stage of Fe(II)−calcite interactions in acidic solutions. Goethite precipitated in the subsequent reaction, but without preferred crystallographic orientation with respect to calcite. The X-ray reflectivity data suggest the presence of adsorbed Fe and water species on the calcite surface, which likely acted as a bridge for the film growth. This nanometer-thick ordered lepidocrocite film facilitated the subsequent overgrowth of thicker iron oxyhydroxide layers, which eventually remained as a pseudomorphic shell after the further dissolution of calcite. This growth mechanism indicates the potential of using ordered solid−water interface to template the film growth and of using carbonate minerals as templates for pseudomorphic synthesis of iron oxyhydroxides in controlled morphologies.
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EXPERIMENTAL SECTION
Optical Microscopy and Scanning Electron Microscopy. The in situ observations of the growth of lepidocrocite on calcite were conducted by using an optical microscope (Nikon Optiphot) equipped with a digital camera and an image recording system. The fluid cell was 705
DOI: 10.1021/acs.chemmater.7b03921 Chem. Mater. 2018, 30, 700−707
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Chemistry of Materials
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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.chemmater.7b03921. Temporal variations of the fractional coverage of iron oxide and oxyhydroxide precipitates on the calcite (104) surface; TXM images of flaky precipitates formed on the inner wall of a lepidocrocite shell; background of crystal truncation rod and peak identification; electron density profiles of reacted calcite (104) surfaces at 1, 5, 10, and 20 min with associated structural parameters; occupancies of the lepidocrocite layers as a function of time; structural model and relaxation of calcite surface; iron (oxy)hydroxide coated calcite crystal; pH-Eh diagram of Fe(II) (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Ke Yuan: 0000-0003-0565-0929 Sang Soo Lee: 0000-0001-8585-474X Paul Fenter: 0000-0002-6672-9748 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the beamline scientist Dr. Jiajun Wang (Brookhaven National Laboratory) at 8-BM-B and Dr. Liguang Wang (Harbin Institute of Technology) for help with the TXM measurements and data reconstructions. This work is supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division (Geosciences Research Program) under Contract No. DE-AC02-06CH11357. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Use of APS beamline 8BM is partially supported by the National Synchrotron Light Source II, Brookhaven National Laboratory under DOE contract No. DE-SC0012704. Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH11357.
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