Article pubs.acs.org/Langmuir
Structural Characterization of Aluminum (Oxy)hydroxide Films at the Muscovite (001)−Water Interface Sang Soo Lee,*,† Moritz Schmidt,§,† Timothy T. Fister,† Kathryn L. Nagy,‡ Neil C. Sturchio,∥,‡ and Paul Fenter† †
Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States ‡ Department of Earth and Environmental Sciences, University of Illinois at Chicago, 845 West Taylor Street, MC-186, Chicago, Illinois 60607, United States S Supporting Information *
ABSTRACT: The formation of Al (oxy)hydroxide on the basal surface of muscovite mica was investigated to understand how the structure of the substrate controls the nucleation and growth of secondary phases. Atomic force microscopy images showed that solid phases nucleated on the surface initially as two-dimensional islands that were ≤10 Å in height and ≤200 Å in diameter after 16−50 h of reaction in a 100 μM AlCl3 solution at pH 4.2 at room temperature. High-resolution X-ray reflectivity data indicated that these islands were gibbsite layers whose basic unit is composed of a plane of Al ions octahedrally coordinated to oxygen or hydroxyl groups. The formation of gibbsite layers is likely favored because of the structural similarity between its basal plane and the underlying mica surface. After 700−2000 h of reaction, a thicker and continuous film had formed on top of the initial gibbsite layers. X-ray diffraction data showed that this film was composed of diaspore that grew predominantly with its [040] and [140] crystallographic directions oriented along the muscovite [001] direction. These results show the structural characteristics of the muscovite (001) and Al (oxy)hydroxide film interface where presumed epitaxy had facilitated nucleation of metastable gibbsite layers which acted as a structural anchor for the subsequent growth of thermodynamically stable diaspore grown from a mildly acidic and Al-rich solution.
1. INTRODUCTION Nucleation and growth of solid phases from aqueous solutions occur by multiple pathways,1 but the mechanisms have not been fully understood at the molecular level. Classical theory for homogeneous nucleation simply describes that the processes occur when a solution is supersaturated with respect to a solid phase.2,3 However, recent work, mostly focused on biomineralization, has shown a new pathway in which metastable clusters3−6 or dense liquid phases5,7,8 are precursors for crystallization. The process can be different for heterogeneous nucleation. It is often observed that formation of nuclei is promoted in the presence of solid surfaces.2,3,9,10 This catalytic effect is understood as a result of reduction in the surface free energy of the nuclei,11−13 e.g., when the interfacial energy between the solid surface and nuclei is lower than that between the nuclei and solution. It can be substantial in the case of good epitaxy (i.e., minimal lattice mismatch between the substrate and growing phase).13−18 Epitaxial control is important in the formation of secondary minerals in many natural and synthetic systems.11,19−22 However, the interface between the nucleated phase and the underlying surface is generally not well characterized. To understand epitaxial nucleation of secondary phases at the atomic scale, we studied the growth of aluminum © 2015 American Chemical Society
(oxy)hydroxide on the basal surface of muscovite mica. The experimental system was chosen because of the ubiquity and environmental significance of Al (oxy)hydroxide, in both large crystalline phases23−30 and small molecular clusters,31,32 and muscovite mica, K2Al4(Al2Si6)O20(OH)4, in many soils and sediments. Heterogeneous nucleation of Al (oxy)hydroxide on surfaces of mica can affect the mobility of Al and other elements that may be sorbed or otherwise incorporated. Furthermore, the basal surface of muscovite is structurally similar to the dominant surfaces of many aluminum (oxy)hydroxide minerals33 and therefore can facilitate their epitaxial growth. A previous atomic force microscopy (AFM) study19 identified three morphologies of Al-containing phases precipitated on a fresh muscovite surface reacted in 1 mM AlCl3 at pH ∼3 and 80 °C for 47 days: films with edges having hexagonal outlines, 30 to 40 Å thick crystals elongated along two distinct directions, and micrometer-scale clumps of intergrown crystals. The experiments were conducted at an elevated temperature to accelerate the reaction kinetics. However, no structural information was obtained about the Received: September 4, 2015 Revised: December 11, 2015 Published: December 17, 2015 477
DOI: 10.1021/acs.langmuir.5b03346 Langmuir 2016, 32, 477−486
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Langmuir incipient states of these precipitates or their crystallographic relationship to the underlying substrate. Here, we focused on investigating the structure of Al (oxy)hydroxide formed at the initial stage of the heterogeneous nucleation process. Experiments were conducted at ambient temperature to resolve time-dependent changes at the interface during the reaction. Our approach is composed of three steps. First, we used AFM to monitor the evolution of the morphology of the precipitated material from a mildly acidic AlCl3 solution. We observed two characteristic time scales for the nucleation of precipitates as two-dimensional (2D) islands and subsequent growth to thick films. Second, we determined the structure of the nucleated phase using in situ crystal truncation rod (CTR) measurements in a specular geometry. The results provide information on the internal arrangements of atomic planes within the nucleated phases with respect to the surface of the underlying mica. Additionally, we explored the location of Al ions in the initial precipitates by probing the distribution of Ga ions substituting for Al from mixed GaCl3/ AlCl3 solutions using element-specific resonant anomalous Xray reflectivity (RAXR). Gallium behaves chemically similar to Al,34 so it is expected to follow generally the reactivity of Al3+ in our system, while its X-ray absorption edge is favorable (i.e., ∼10.37 keV instead of ∼1.56 keV for the K edges) for RAXR experiments (see section 2.3.2 for experimental details). The structural and chemical information was used to identify the growth phases. Last, we examined the stability of the nucleated phase after an extended reaction time over which we observed the formation of thicker films. We utilized X-ray diffraction (XRD) to identify the film phase and its crystallographic orientations. Based on these results, we propose a mechanism by which the muscovite surface controls the nucleation and growth of Al (oxy)hydroxide at the interface in mildly acidic and Al-rich solutions.
Table 1. Dissolved Al and Ga Speciation and Saturation Indices of Solid (Oxy)hydroxide Phases in Two Experimental Solutionsa solutions
100 μM AlCl3
pH Al species (μM) Al3+ AlOH2+ Al(OH)2+ Al2(OH)24+ AlCl2+ Altot mineral saturation indicesb diaspore, α-AlO(OH) gibbsite, α-Al(OH)3 Al(OH)3 (am)c
90 μM AlCl3 + 10 μM GaCl3
4.2
4.2
87 12 0.9 0.04 0.01 100
78 11 0.8 0.03 0.01 90
1.6 0.7 −2.4
1.5 0.7 −2.4
Ga species (μM) Ga(OH)2+ GaOH2+ Ga(OH)4− Ga3+
6.7 2.9 0.22 0.17
Gatot
10
GaOOH Ga(OH)3 (cr)c Ga(OH)3 (am)c
2.8 n/ac 0.7
a
Calculated using The Geochemist’s Workbench and the Minteq database35 with pCO2(g) = 10−3.4 (atm). Only aqueous species with concentrations greater than 0.01 μM are shown. bcalculated as log10(Q/Keq) for the reaction Al3+ (or Ga3+) + 2H2O = Al(or Ga)O(OH) + 3H+ for oxyhydroxide, and Al3+ (or Ga3+) + 3H2O = Al(Ga) (OH)3 + 3H+ for hydroxide phases, where Q is the ion activity product in solution and Keq is the equilibrium constant. cn/a: not available, cr: crystalline, am: amorphous layers using adhesive tape to expose a fresh (001) surface. The cleaved surface was rinsed thoroughly with deionized water (DIW) and blow dried in a stream of purified nitrogen gas. Images were taken in air at three different locations to confirm the quality of the surface. Then, the crystal was immersed vertically in 15 mL of a 100 μM AlCl3 solution at pH 4.2 at room temperature. After 16, 50, and 700 h of reaction, the sample was rinsed with DIW and dried in a stream of purified nitrogen gas. The surface of the reacted sample was imaged in air at several different locations to check for consistency in the surface morphology. After completion of imaging at each time step, the sample was immersed again in a newly prepared 100 μM AlCl3 solution for further reactions. 2.3. X-ray Reflectivity Measurements. X-ray data were collected at beamlines 6-ID-B, 11-ID-D, and 33-ID-D, Advanced Photon Source (APS) (see SI for details). The experiments were conducted using three muscovite crystals (25 mm × 25 mm × 0.15 mm) each of which was cleaved, rinsed with DIW, and transferred to a centrifuge tube containing one of the experimental solutions (50 mL): One crystal was reacted in a 100 μM AlCl3 solution for 18 h; another crystal was reacted in three sets of newly prepared 100 μM AlCl3 solutions, which were exchanged approximately every 700 h, for a total of 2000 h; and the third crystal was reacted in a 90 μM AlCl3 and 10 μM GaCl3 solution for 12 h. After the reactions, the wet crystals were transferred to an X-ray thin-film cell36 for reflectivity measurements. 2.3.1. Crystal Truncation Rod (CTR). In-situ CTR data were measured in a specular geometry as a function of momentum transfer q [= 4πsin(2θ/2)/λ = 2πL/d, where 2θ is the angle between incident and reflected X-rays, λ is the X-ray wavelength, L is the Bragg index of the muscovite (001) reflection, and d = ∼19.96 Å is the (001) layer spacing]37 at fixed photon energy (E). Before the actual measurements we conducted a set of preliminary measurements to check the sensitivity of the interface to X-rays (see section SI-4 of the Supporting Information (SI) for details). To minimize beam-induced effects on data sets, we periodically translated the sample within the scattering plane during the data collection. The translation distance was short (±0.4 mm) so that the whole reflectivity data could be collected within the area illuminated by X-rays at the lowest angle (5 mm along the scattering plane), but long enough to be sure the X-rays illuminated a new spot when the sample was translated at a higher angle. The CTR data were fit to a structural model consisting of rigid bulk muscovite; the interfacial region including muscovite surface atoms (allowed to relax vertically with respect to their crystallographic positions),
2. MATERIALS AND METHODS 2.1. Experimental Solutions. Two experimental solutions, 100 μM AlCl3 at pH 4.2 (“AlCl3”) and 90 μM AlCl3 and 10 μM GaCl3 at pH 4.2 (“(Al0.9Ga0.1)Cl3”), were used for the experiments (see SI-2 in the Supporting Information (SI) for details). The solution pH was selected to make the solutions supersaturated with respect to Al or Ga (oxy)hydroxide minerals at given ion concentrations to facilitate nucleation. For the (Al0.9Ga0.1)Cl3 experiment, the 9:1 ratio of Al:Ga was chosen to ensure sufficient incorporation of a Ga impurity into nucleated Al (oxy)hydroxide while minimizing any changes in the nucleated phases (SI). Thermodynamic calculations (Table 1) indicate that the former solution was supersaturated with respect to diaspore, α-AlOOH, and gibbsite, α-Al(OH)3. However, chemical analyses (SI) showed that the solutions were stable over ∼270 h, indicating that rates of homogeneous nucleation were negligible over the course of the measurements. For the latter solution, calculations indicate that the solution was supersaturated with respect to diaspore and gibbsite, similar to the AlCl3 solution, as well as GaOOH and Ga(OH)3(am). Most dissolved Al ions existed as Al3+ and AlOH2+ whereas the proportions of the other species, if present, were negligible (≤1%). For the (Al0.9Ga0.1)Cl3 solution, the most dominant aqueous Ga species were expected to be Ga(OH)2+ and GaOH2+. 2.2. Atomic Force Microscopy. Morphological changes at the muscovite (001) surface before and after the reactions with the solutions were investigated using atomic force microscopy (Asylum Research MFP-3D AFM) operated in tapping mode. We used a silicon nitride (Si3N4) tip (with nominal radius of 9 ± 2 nm and a resonant frequency of ∼360 kHz) at a scanning rate of 0.5 to 0.75 Hz. A gemquality muscovite crystal (10 mm × 10 mm × 0.15 mm from Asheville Schoonmaker Mica Company) was cleaved by peeling off the top 478
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Figure 1. Atomic force microscopy images of the muscovite (001) surface before (a) and after (b-e) reaction with a 100 μM AlCl3 solution at pH 4.2. Each image is 1 μm × 1 μm, and labeled with the reaction time. The scale bar indicates 0.1 μm. For comparison among the images, the −1σ values of the height-distribution function of the images were chosen as the reference height (z = 0 Å). Note the color scale for images d−e is larger than that for images a−c. Cross sections (A−A′ and B−B′) are shown in (f) and (g). precipitates, and sorbed species; and the bulk solution above the interfacial region (SI). Different models were tested for each data set, and the best-fit model (Table S2) was chosen on the basis of the smallest χ2 (section. SI-2.2) and covariance among fitting parameters.38 2.3.2. Resonant Anomalous X-ray Reflectivity (RAXR). A series of RAXR spectra was measured by scanning the incident photon energy, E, around the X-ray K-absorption edge energy of Ga (EGa = ∼ 10.37 keV) at a fixed momentum transfer q for the (Al0.9Ga0.1)Cl3 experiment. During this experiment, the stability of the interfacial system was monitored by periodically measuring RAXR at q value of 0.38 Å−1 [i.e., at q low enough for the data to be extremely sensitive to the coverage and average height of sorbed Ga(III)]39 (see section SI2.3 for details). A complete set of RAXR data was obtained in about 6 h over 8 different spots on one sample. The RAXR data were fit using 4 different models (Table S3), and the best-fit model was chosen based on the quality of fit (i.e., χ2). The validity of the model was tested by comparing these fitting results with that from modelindependent analysis39 (Figure S6). 2.3.3. Electron-Density Profile. The interfacial electron-density profile derived from the best-fit model is plotted as a function of height (z) from the surface (the origin, z = 0, is chosen at the average height of the oxygen atoms in the basal surface plane of muscovite). All plotted electron-density profiles are broadened by the experimental resolution (π/qmax),36 and normalized to that of bulk water (∼0.33 e−/ Å3).
Ex-situ images were taken after various reaction times (t = 16, 50, and 700 h) with a 100 μM AlCl3 solution at pH 4.2 to determine time-dependent changes in the morphology of the precipitates. After 16 h (Figure 1b), the rms roughness increased significantly from 0.3 to 2.1 Å. The change in surface topography is most likely a result of the growth of Al (oxy)hydroxide whereas any increase in roughness caused by dissolution of the surface should be negligible in the solutions40,41 (for example, no change in rms roughness was observed when a muscovite (001) surface was reacted in a pH 4.2 HCl solution (i.e., without AlCl3) for the same period (16 h), Figure S8). The majority (≥99%) of the nucleated phases appeared as 2D islands that were less than 10 Å in height (among which ∼70% were 4−6 Å high) and less than 200 Å in width. The AFM images taken after 50 h of reaction (Figure 1c) showed similar features, with the same 2 Å rms roughness as observed at 16 h, but with slight increases in the width of the islands (by approximately 50% in average). More considerable changes in surface topography were observed after 700 h (Figure 1d,e). The surface was almost fully covered with a continuous film having rms roughness of 6 Å. We also observed that some part of the film was removed presumably during the drying procedure. The depth of these features from the top of the film was ∼20 Å (Figure 1e−g), which can be interpreted as the minimum thickness of the film (assuming that the underlying muscovite was intact). Larger particles, up to ∼100 Å tall, were also observed but their coverage was small (≤2%). Overall, the AFM results show that the Al (oxy)hydroxide film initially nucleated at the muscovite (001)−AlCl3 solution interface as 2D islands that were less than 10 Å high. These islands maintained a constant thickness for the first 50 h of
3. RESULTS 3.1. Film Formation. The morphology of the aluminum (oxy)hydroxide phase formed on the muscovite surface was investigated using tapping-mode AFM in air. The initial surface before reaction with an AlCl3 solution (Figure 1a) was atomically flat with rms roughness of 0.3 Å. The roughness is similar to the crystallographic corrugation of the top ideal muscovite surface (0.2 Å),37 and, presumably, was also affected by instrumental noise. 479
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Figure 2. High-resolution X-ray reflectivity data at the muscovite−solution interface. (a) In situ and ex situ CTR data from the muscovite (001) surface after 18 and 2000 h, respectively, of reaction in a 100 μM AlCl3 solution at pH 4.2 compared with that in deionized water (DIW).42 The locations of some muscovite Bragg peaks are indicated with their hkl indices. The data for the 2000 h reaction also show the film Bragg peaks at q = 2.68 and 3.09 Å−1 (green arrows). The solid lines through the data points are calculated from the best-fit models (Table S2). The data for the 2000 h reaction were fit without the points near the film Bragg peaks. The data are scaled vertically as indicated below each sample label. (b) The data for the 2000 h reaction are plotted after normalization to the generic CTR shape (1/[qsin(qd/4)2])43 to enhance the visibility of film Bragg peaks. The locations of the major Bragg peaks of diaspore (Dias) and gibbsite (Gibb), two stable solid phases in the solution (Table 1), are shown for comparison. (c) Variation of the film diffraction intensities with respect to the surface normal (specular) transverse to the scattering plane (Δχ) for the two film Bragg peaks. An image of the diaspore (040) Bragg peak (q = 2.68 Å−1) (inset) shows the shape of the Debye ring (yellow short-dashed curve). The small angular deviation (∼1°) of the Bragg peak from the specular direction is noted (two long-dashed white lines).
which predicted that it is the most stable solid phase in the solution (Table 1).45 Only these two Bragg peaks were visible, indicating the diaspore grew with specific crystallographic orientations with respect to the muscovite surface. For example, the (110) reflection, which is reported to be the strongest for diaspore occurring in natural soils,44 is absent in our data. The diffraction images (Figure 2c) have a nonuniform Debye ring structure, where the peak intensities are located near the specular direction (with only small angular deviations), indicating that the film was an aggregate of preferentially oriented particles. The presence of the sharp Bragg peaks indicates that the diaspore films formed at the interface were relatively thick. The full width at half-maximum of the (040) reflection is ∼0.01 Å−1, indicating that some part of the film was ∼600 Å thick, about 30 times the thickness measured by AFM after 700 h of reaction, in contrast with about a factor of 3 increase in reaction time. The results indicate that the growth of the diaspore film was accelerated along the direction perpendicular to the mica surface. The film thickness also implies that a fraction (less than 20%) of the Al in the reacting solution could be removed (SI). The total electron-density profile derived from the best-fit model (χ2 = 1.18 and R-factor = 0.036) for the data collected after 18 h of reaction in the AlCl3 solution has a distinct structure within ∼11 Å of the muscovite surface (Figure 3). The most striking feature is the presence of three sharp peaks at heights of 2.64 ± 0.02, 3.79 ± 0.01, and 4.88 ± 0.02 Å above the average location of surface oxygen atoms in the muscovite
reaction but eventually either evolved into or were covered by a thicker and continuous film after longer reaction time. 3.2. Internal Film Structure. X-ray reflectivity was applied to identify the phases of both the initially nucleated islands and the thicker film and to determine their structural relationship with the underlying muscovite (001) surface. Specular crystal truncation rod (CTR) data were measured for the muscovite (001) surface reacted in a 100 μM AlCl3 solution at pH 4.2 for 18 h (in situ) and 2000 h (ex situ) using two different crystals (Figure 2). Based on the AFM observations after 16, 50, and 700 h (Figure 1), we expected that the data after 18 h (i.e., before the formation of the thick film) would be sensitive exclusively to the nucleated 2D islands while those after 2000 h would be sensitive to both the islands (if still present) and the thicker film. The data collected after 18 h of reaction show substantial differences from those measured in deionized water (DIW)42 (Figure 2a). These differences likely result from the presence of the 2D islands and associated changes in the hydrated structure of the muscovite surface. Significant differences at high momentum transfer (q near 4 Å−1) indicate that the atoms in the islands were well ordered with respect to the surface.36 A similar pattern remained in the ex-situ data collected after 2000 h, indicating that the islands were still present underneath the thicker film. These data also show two Bragg peaks at q = 2.68 and 3.09 Å−1 (Figure 2a,b), which match those of the (040) and (140) reflections of diaspore, α-AlOOH.44 This observation is consistent with the thermodynamic calculations 480
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the layer are occupied.33 The gibbsite layer is a fundamental unit for Al hydroxide minerals, e.g., gibbsite and bayerite, and also for many phyllosilicate minerals, such as kaolinite and dioctahedral micas including muscovite. Therefore, we interpret the three-peak structure as the signature of a gibbsite layer that was nucleated on the muscovite surface with its (001) plane oriented parallel to the (001) plane of the muscovite. The integrated electron density of this layer is less than that expected for an ideal gibbsite layer, indicating incomplete surface coverage. This partial coverage is consistent with the AFM observations for 15−50 h reaction (Figure 1). The gibbsite layer is followed by additional molecular-scale ordering which is best described by two peaks in the electron density located at z = 7.46 ± 0.05 and 9.03 ± 0.21 Å (Figure 3). The distribution width of this layer (gray box, Figure 3) is similar to that of the first gibbsite layer (orange box, Figure 3), implying that it may include a second gibbsite layer. We attempted to fit the CTR data using a model that included two gibbsite layers (each consisting of three peaks). However, the fit resulted in significant covariance among the parameters with only a marginal improvement in the quality of fit (i.e., χ2 from 1.18 to 1.15; data not shown). From this observation, it is concluded that the structure of this second layer is not uniquely defined. For example, it is possible that there are additional adsorbed species on top of the first gibbsite layer (as shown in the schematic in Figure 3), which could be indistinguishable by the X-ray data from a second gibbsite layer. The peak closest to the surface is located at z = 1.49 ± 0.06 Å (Figure 3). This height is similar to one observed in DIW, which was interpreted as water molecules (or hydronium ions) adsorbed in ditrigonal cavities at the muscovite surface.42 The integrated electron density of the peak is 1.35 ± 0.12 water molecules per unit cell area of the muscovite (001) plane (H2O/AUC) in the AlCl3 solution consistent with that measured in DIW (1.3 ± 0.2 H2O/AUC). Note that the first water peak in DIW (Figure 3) appears to be more electron-dense because it overlaps with a tail of the next water peak. The data collected after 2000 h of reaction were more complicated to interpret but the derived electron-density profile was consistent with that observed for the data collected after 18
Figure 3. Total electron-density profile derived from the best-fit models of the in situ CTR data measured at the muscovite (001)−100 μM AlCl3 solution interface at pH 4.2 after 18 h of reaction compared with that at the muscovite (001)−DIW interface.42 The electrondensity profile (red) is plotted with a band (pink) indicating the ±1σ uncertainties of the derived density.38 The profile is compared with a schematic model (top) including the muscovite surface, the first Al(OH)3 layer that partially covers the surface (with the remaining surface covered by water), and the second layer (integrated with any possible adsorbed species) that can form on top of the first layer. The yellow region highlights the gibbsite layer within the muscovite substrate (both in the electron density profile and the structural schematic).
(defined as z = 0 Å). These three peaks have similar electron densities (Table S2), and the overall structure resembles the octahedral sheet, AlO2(OH)2−, found in the muscovite substrate (shown in a yellow-shaded box in Figure 3). This octahedral sheet is generally referred to as a gibbsite layer, a plane of trivalent ions octahedrally coordinated to hydroxyls (or oxygens).33 Only two of three octahedral sites in a unit cell of
Figure 4. X-ray reflectivity data and the derived electron-density profile for the muscovite (001)−mixed AlCl3/GaCl3 solution system. (a) CTR data of muscovite (001) in a 90 μM AlCl3 + 10 μM GaCl3 solution at pH 4.2 in comparison with those in a 100 μM AlCl3 solution at pH 4.2 and in DIW.42 (b) Comparison of the electron-density profiles derived from the best-fit models. The total electron density profile (dark blue with a cyan band) and the element-specific distribution of Ga (cyan shaded area) were obtained from the best-fit models of the CTR and RAXR data, respectively. Also shown is the total electron-density profile of the muscovite (001)−AlCl3 data (red short-dashed line with a pink band). The Ga profile is scaled vertically by a factor of 2 for clarity and compared with that of trivalent Y3+ (black long-dashed line) at the muscovite (001)−100 μM YCl3 interface.46 The profiles are plotted with a color band indicating ±1σ uncertainty.38 481
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A series of RAXR spectra was measured from the same sample at 21 q values ranging from 0.14 to 4.33 Å−1 (Figure S4). The best-fit model of the RAXR data (χ2 = 1.59 and Rfactor = 0.010; see Table S3 for details) identifies four distinct types of Ga species at the muscovite−(Al0.9Ga0.1)Cl3 solution interface (Figure 4b). Two Ga species with the most narrow distribution widths are observed at 3.72 ± 0.02 and 8.56 ± 0.02 Å. Another Ga species with a slightly broader distribution is located in between these at 5.76 ± 0.03 Å. The fourth species is distributed more broadly and extends up to ∼18 Å from the surface. At this stage, it is difficult to uniquely determine the states of these Ga species from this profile. So, we first compare the Ga distribution with the distribution of yttrium, the only trivalent cation whose adsorbed speciation was determined experimentally at the muscovite−water interface with the atomic-scale resolution,46 as a reference. Similarities between Ga and Y distributions can be used to identify the chemical states of Ga species adsorbing on uncoated areas of the surface, whereas differences can be interpreted as resulting from the effect of the film. From a 100 μM YCl3 solution, Y adsorbed to the muscovite surface as three distinct species: inner-sphere (IS at 1.8 Å), adsorbed outer-sphere (OSads at 4.2 Å), and extended outersphere (OSext at 8.6 Å) complexes (Figure 4b).46 The distinction in adsorption height among the species depends on the number of hydration shells (nhyd = 0, 1, and ≥2, respectively) between the ion and the surface.47 Compared to the locations of these three Y species, those of Ga species are somewhat different. The height of the Ga species at 3.7 Å is similar to but slightly lower than that of OSads Y; the Ga peak at 5.8 Å does not match the locations of any of Y3+ species; the Ga peak at 8.6 Å matches the location of OSext Y species, but it is significantly more narrowly distributed; and the broadest Ga species extends farther from the surface than the OSext Y species in a 100 μM YCl3 solution. However, an extended distribution (up to ∼15 Å) of OSext Y species was observed in a 1 mM YCl3 solution46 and is comparable to that seen for Ga. Differences in the Ga distribution from that of Y suggests that sorption of Ga on the muscovite surface cannot be explained as simple ion adsorption. Therefore, we conclude that some of the Ga is likely associated with the nucleated islands whose internal structure is shown in the total electron-density profile (Figure 4b). Next, we compare the Ga distribution with the total electron-density profile to determine possible correlations between these two profiles. The height of the Ga species closest to the surface (∼3.7 Å) is in excellent agreement with the height of the central peak (∼3.6 Å) of the first gibbsite layer, indicating that the electrondensity peak is partially composed of Ga. Considering the expected isomorphic substitution of Ga for Al34 and the lower electron density of the Ga peak with respect to the total electron density, we conclude that the central plane in the layer also includes Al. In contrast, no Ga peak locations match the other two total electron-density peaks in the first layer, supporting the interpretation that these peaks are likely to be hydroxyl groups (or oxygen atoms or water molecules). Together, these results indicate that the first layer observed in the (Al0.9Ga0.1)Cl3 solution is both structurally and compositionally equivalent to a Ga-doped gibbsite layer. The other narrow Ga peak (at z = ∼8.6 Å) is 4.84 ± 0.03 Å above the first Ga peak. This spacing is the same as the Al−Al distance in gibbsite (4.85 Å).33 Furthermore, the peak location approximately matches the center of the two total electron-
h reaction. To make the analyses easier we excluded the data points of the diaspore Bragg peaks, which effectively made the thick film invisible, and therefore, the analyses were only sensitive to the interfacial species that were structurally commensurate with the mica surface.36 Briefly, the derived profile shows a similar three-sharp-peak layer near the surface (Figure S3), indicating that the internal structure of the nucleated 2D islands was largely unchanged. However, the peaks within the layer appear to be less electron dense and slightly broader, implying the nucleated layer could be slightly disordered (e.g., corrugated) with respect to the mica surface. These changes may be caused by the aging of the sample and/ or the drying process. This profile may include a part of the diaspore film, which grew on top of the gibbsite layer. However, it is not possible to determine these features separately mainly because of the complexity of the derived interfacial structure. 3.3. Elemental Composition of the Film. Although the derived electron-density profiles for both CTR data are fully consistent with the formation of a gibbsite layer at the muscovite surface, it is still valuable to investigate the chemical identity of the modeled peaks within the profile. To obtain information on the elemental composition of the individual planes within the film, we included Ga(III) in the AlCl3 solution and probed the distribution of Ga within the profile by using element-specific resonant anomalous X-ray reflectivity (RAXR). Here, it is assumed that isomorphic substitution of Ga for Al34 in the layer would occur, and therefore the Ga distribution could be used to gain insight into the Al distribution at the interface. The data also probe the distribution of other Ga species, such as those adsorbed on the mica surface independently of the nucleated gibbsite-layer, which could not be determined solely by the CTR data. Both CTR and RAXR data were collected after reaction of a freshly cleaved muscovite crystal with a 90 μM AlCl3 solution containing 10 μM GaCl3, (Al0.9Ga0.1)Cl3, at pH 4.2 for 12 h. The CTR data are similar to those obtained in the AlCl3 solution, indicating that similar films were present at the interface (Figure 4a). The data at q ≥ 3.3 Å−1 are almost identical to those from the Ga-free experiment, which shows that many aspects of the interfacial structure (e.g., location, occupation, and sharpness of the atomic planes in the film) remain largely unchanged. However, the reflectivity signal at q between ∼0.6 and 1.9 Å−1 is intermediate between those measured in the AlCl3 and in DIW, suggesting the film had a lower coverage. Given these considerations, it is not surprising that the total electron-density profile derived from the best-fit model of the CTR data (χ2 = 1.04 and R-factor = 0.043) is generally similar to that observed in the AlCl3 experiment (Figure 4b). The total electron-density profile in the (Al0.9Ga0.1)Cl3 solution also has a film structure with three sharp peaks at similar locations (Figure 4b). However, the integrated electron density of the three-peak structure is lower, indicating less film coverage. In addition, the third peak is significantly broader than that in the AlCl3 solution, indicating that the top surface of the first layer is corrugated in the presence of Ga. Besides the first layer, the profile has an electron-density peak at ∼1.42 ± 0.04 Å that matches the water peak in the AlCl3 solution (Figure 4b). However, the electron-density profile above the first film layer is different; the broad peak at 8.77 ± 0.05 Å is ∼50% more electron dense for the muscovite (001)−(Al0.9Ga0.1)Cl3 system compared to the muscovite (001)−AlCl3 system. 482
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Figure 5. Proposed mechanism for the formation of Al (oxy)hydroxide films on the muscovite (001) surface. (a) Hydrated Al(III) adsorbs on the muscovite surface by electrostatic attraction. The adsorbed species can undergo deprotonation reactions depending on pH at the interface. (b) Formation of oligomers via olation (e.g., nAl(H2O)63+ ↔ Aln(OH)2(n−1)(H2O)2(n+2)n+2 + 2(n−1)H3O+) followed by their two-dimensional polymerization along the (001) plane of muscovite partly by the structural epitaxy, resulting in (c) the formation of single-layer gibbsite films and (d) subsequent growth of more stable diaspore on top of the gibbsite layer.
adsorbed species are expected to be mostly hydrated Al3+, the most dominant Al species expected based on the thermodynamic calculation (Table 1), with possibly Al(OH)2+, depending on its relative affinity for the surface or formed by hydrolysis of Al3+ at the interface (Figure 5). Because of its large free energy of hydration, Al ions are expected to adsorb mostly as mobile outer-sphere complexes rather than less mobile inner-sphere complexes, with the majority of ions adsorbed within ∼20 Å of the surface.10,46 The average concentration of Al(III) within this range is estimated to be ∼0.5 M, more than 3 orders of magnitude higher than the bulk Al concentration in our study. The expected high concentration and mobility of cationic Al species can lead to the spontaneous formation of oligomers10,53 (Figure 5). It is expected that the oligomers would orient in preferred directions because of the structural similarity between Al hydroxides and the muscovite surface (i.e., epitaxy). Condensation of these oligomers into a two-dimensional sheet, i.e., formation of Al(OH)3‑δ(H2O)2δδ+ (where δ ≪ 1) and Al(OH)3 when δ = 0, along the presumed basal plane of gibbsite is known to be faster than the growth along the vertical direction.54 Our AFM images, however, showed that the nuclei grew as 2D islands rather than a continuous film, which may result from slight mismatch between the unit cell areas of muscovite and gibbsite basal planes (5.19 × 9.01 and 5.07 × 8.64 Å2, respectively).37,55 Our X-ray reflectivity data indicate that the growth of the gibbsite layers was largely restricted to a single layer. This observation shows that the epitaxy appears to facilitate the growth parallel to the surface but not along the vertical direction where the layer-by-layer growth is controlled mostly by hydrogen bonding33,54 (Figure S1a). Eventually, we observed that diaspore, predicted as the most stable solid phase in the solution, grew to form a thick and continuous film at the interface. It is unknown if this diaspore film grew epitaxially on the underlying gibbsite layer: neither our specular X-ray data nor AFM images were sensitive to the relationship between gibbsite and diaspore. However, the overgrown diaspore had two preferred crystallographic orientations with respect to the interface, indicating that the underlying gibbsite layer exerted some structural control on the overgrowth phase. 4.2. Significance of Al (Oxy)hydroxide Films in the Environment. Our experimental conditions are relevant to many natural environments, including Al-rich argillaceous rocks or soils in contact with acidic waters, in which the observed (oxy)hydroxide films can play an important role in controlling
density peaks located at z from 6 to 11 Å (Figure 4b). Together these results indicate that the Ga peak is presumably associated with a second gibbsite layer that formed on top of the first gibbsite layer. The height of the Ga species at ∼5.7 Å is approximately centered between the first and second gibbsite layers, and therefore may be structurally related to them. For example, Ga may be positioned close to unoccupied octahedral sites in both or one of the two layers. A dehydrated Ga ion assigned to the centerline above this vacancy site at the observed distance from the upper and lower basal planes (∼1.2 Å in average, Tables S2 and S3) would have an average Ga−O distance of ∼2 Å. This distance is consistent with the Ga−O distances (1.964 ± 0.008 and 1.954 ± 0.003 Å) of hydrated Ga, Ga(H2O)63+, in aqueous perchlorate and nitrate solutions measured by large-angle X-ray scattering and extended X-ray absorption fine structure techniques, respectively.48 A similar geometry was proposed for Ca2+ sorbed on the basal plane of a gibbsite mineral on the basis of frequency-modulated AFM measurements accompanied by density functional theory calculations.49
4. DISCUSSION Our results show that Al (oxy)hydroxide initially nucleated as metastable gibbsite rather than stable diaspore at the muscovite (001) - AlCl3 solution interface. In general, it is not uncommon to have a metastable phase nucleate before a stable phase in solutions. For example, it has been widely accepted that metastable phases can form as precursors, independent of homogeneous nucleation of stable phases, and eventually transform to a more stable phase either via recrystallization or through dissolution and reprecipitation processes.1,4,5,50,51 However, our X-ray reflectivity data showed that the metastable gibbsite layer, instead of spontaneously transforming into a more stable phase, maintained its structure and chemistry over the extended time period, and acted as a structural anchor to the growth of the more stable phase. Furthermore, this gibbsite layer is structurally distinct having morphology of 2D islands whose vertical growth was largely restricted to a single layer. The observed chemical stability and structure of the nucleated phase are likely attributed to the unique interfacial relationship between the gibbsite layer and the muscovite surface. 4.1. Interfacial Control on the Nucleation of Al (Oxy)hydroxide on Mica. The muscovite (001) surface has a structural negative charge (1e−/AUC = −0.34 C/m2)52 that attracts cations to the interface, increasing their local concentrations. From the 100 μM AlCl3 solution at pH 4.2, 483
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the reactivity of the mineral−water interfaces. For example, the basal surface of muscovite mica is known to be reactive to cationic aqua ions because of its structural negative charge.47 The sorptive capacity of the mica surface can change in the presence of a gibbsite layer whose basal surfaces are reported to possess only small charges at acidic to near neutral pHs.23,24,56 A similar phenomenon can occur for expandable clay minerals whose structures are essentially identical to micas. For these cases, the formation of a gibbsite layer may occur in the interlayers, similar to intercalation of hydroxy-Al species observed in many acidified soils,57,58 leading to a decrease in the cation exchange capacity as well as swelling property of the minerals. Our results also showed that nucleated gibbsite layers control crystallographic orientations of overgrown diaspore. This observation indicates that diaspore films formed on the basal surface of micas (and also likely clay minerals) dominantly expose specific crystallographic planes for reactions with natural waters. The result also indicates that structural controls in a simple aqueous system can be used for producing oxyhydroxide films with well-defined surface functional groups, an approach that is fully complementary to the fabrication of nanoparticles.31,32 Quantifying the type and population of reactive surface sites is critical for interpreting mineral reactivity data, e.g., from ion uptake or titration experiments. Consequently, both synthesis approaches will provide a fundamental basis on which more accurate and robust predictive models can be built and provide an insight into understanding the molecular-scale reactivity of (oxy)hydroxide minerals in nature.
AUTHOR INFORMATION
Corresponding Author
*Phone: (630)252-6679. Fax: (630)252-9570. E-mail: sslee@ anl.gov. Present Addresses §
Helmholtz-Zentrum Dresden - Rossendorf, Institute of Resource Ecology, Dresden, Germany. ∥ Department of Geological Sciences, University of Delaware, Newark, DE 19716, United States. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Geosciences Research Program, Office of Basic Energy Sciences, United States Department of Energy under Contracts DE-AC0206CH11357 to UChicago Argonne, LLC as operator of Argonne National Laboratory and DE-FG02-03ER15381 to the University of Illinois at Chicago, and was cofinanced (M.S.) by the Helmholtz Gemeinschaft Deutscher Forschungszentren by supporting the Helmholtz-Nachwuchsgruppe “Structures and Reactivity at the Water/Mineral Interface” (VH-NG-942). The manuscript was created at UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. Thoughtful comments from four anonymous reviewers were used in revision.
5. CONCLUSIONS
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We investigated the structure of aluminum (oxy)hydroxide films nucleated at the muscovite (001)−water interface as an exemplary system to understand how the structure of the substrate controls the growth of secondary phases. The results showed that gibbsite layers, Al(OH)3, initially nucleated in a morphology of 2D islands with a vertical extension that was limited to one or two layers. These nucleated gibbsite layers maintained the structure for an extended time period (up to 2000 h) over which a thicker film of thermodynamically more stable diaspore, AlOOH, grew. This diaspore film is composed of aggregates which were aligned in specific crystallographic directions with respect to the interface, indicating that the gibbsite layers preformed at the muscovite surface affects the orientation of the overgrown phase. Overall, these results show the presence of a strong structural linkage among three solid phases at the Al (oxy)hydroxide-coated mica surface, which will affect the stability, and, ultimately, the reactivity of the interface in natural environments.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03346. Structure of Al (oxy)hydroxide and oxide phases; experimental details; beam-induced changes at the interface; additional AFM images (PDF) 484
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