Direct Observation of the Orientational Anisotropy of Buried Hydroxyl

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Direct Observation of the Orientational Anisotropy of Buried Hydroxyl Groups Inside Muscovite Mica Aashish Tuladhar, Zizwe A. Chase, Marcel D. Baer, Benjamin A. Legg, Jinhui Tao, Shuai zhang, Austin D. Winkelman, Zheming Wang, Christopher Mundy, James J. De Yoreo, and Hong-fei Wang J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 7, 2019

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Journal of the American Chemical Society

Direct Observation of the Orientational Anisotropy of Buried Hydroxyl Groups Inside Muscovite Mica Aashish Tuladhar[a], Zizwe A. Chase[a,b], Marcel D. Baer[a,*], Benjamin A. Legg[a,d], Jinhui Tao[a], Shuai Zhang[a], Austin D. Winkelman[a,b],Zheming Wang[a], Christopher Mundy[a,e], James J. De Yoreo[a,d,*], and Hong-fei Wang[a,c,*] [a] Physical & Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 99352 [b] School of Chemical and Biological Engineering, Washington State University, Pullman, WA 99364 [c] Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, China [d] Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195, USA [e] Department of Chemical Engineering, University of Washington, Seattle, WA 98195, USA.

ABSTRACT Muscovite mica (001) is a widely-used model surface for controlling molecular assembly and a common substrate for environmental adsorption processes. The mica (001) surface displays neartrigonal symmetry, but many molecular adsorbates — including water — exhibit unequal probabilities of alignment along its three nominally equivalent lattice directions. Buried hydroxyl groups within the muscovite structure are speculated to be responsible, but direct evidence is lacking. Here we utilize vibrational sum frequency generation spectroscopy (vSFG) to characterize the orientation and hydrogen-bonding environment of near-surface hydroxyls inside mica. Multiple distinct peaks are detected in the O-H stretch region, which we attribute to Si/Al substitution in the SiO4 tetrahedron and K+ ion adsorption above the hydroxyls based on density functional theory simulations. Our findings demonstrate that vSFG can identify the absolute orientation of –OH groups and hence the surface termination at a mica surface, providing a means to investigate how –OH groups influence molecular adsorption and better understand mica stacking-sequences and physical behavior. 1 ACS Paragon Plus Environment

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INTRODUCTION The atomic structure near solid-liquid interfaces underlies diverse phenomena including fluid transport in confinement1, sequestration of heavy metals and organic matter in soils2, formation of colloidal superlattices3, heterogeneous nucleation of minerals4, and molecular self-assembly on surfaces5. Mica has served as a model system to investigate these processes due to its ubiquitous presence in geochemically relevant environments, the ease with which atomically flat mm-scale surfaces can be prepared as substrates for thin film growth and study of molecular adsorption kinetics, and the extent to which the surface charge can be manipulated via cation exchange and solvent pH, making it the canonical model system for solid-liquid interfaces. More specifically, it is unclear why some molecules adsorb on the muscovite surface with the three-fold symmetry of the cation sublattice at the mica surface6-7, whereas other molecules clearly show preference to align along one8-11 or two12 of the three nearest neighbor lattice directions. Additionally, many questions remain about the influence of mica type, solution pH and electrolyte strength and type on the process of molecular self-assembly. This lack of understanding has been attributed to the complexities of water structure at the mica surface13-21, the distribution and ordering of Al atoms in the alumino-silicate network22, and the orientation of the subsurface –OH groups23-25. Consequently, experimental techniques for definitively determining mica surface terminations, water structure and the absolute orientation of subsurface –OH groups are critical for building a comprehensive description of mica-water interfacial structure and assembly processes. Vibrational sum frequency generation (VSFG) spectroscopy is a second order non-linear technique that provides interface-specific vibrational spectra with sub-monolayer sensitivity26-27.

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The interface specificity of vSFG spectroscopy stems from the dipole approximation within which a sum frequency signal is not generated by centrosymmetric environments, but is generated by environments where the centrosymmetry is broken — as at an interface. The vSFG signal is also dependent on the polarization of the beam relative to the vSFG-active surface groups, making vSFG spectroscopy an excellent probe of molecular orientation and structural anisotropy28-30. Muscovite mica has been previously16, 31-32 studied using vSFG spectroscopy but the focus was on probing the structure of interfacial water. Second harmonic generation (SHG) spectroscopy, another surface sensitive technique, has also been used to investigate ion adsorption at the muscovite surface33-34, in order to understand the mechanism behind heterogeneous ice nucleation35-36 and to investigate the influence of birefringence on the SHG signal37. However, none of the above studies have probed the –OH groups present inside the muscovite lattice. Here we use vSFG spectroscopy to exclusively probe the array of –OH groups from the topmost layer of muscovite. Our results show that the O-H stretching regions of the vSFG spectra are comprised of multiple peaks. Density Functional Theory (DFT) simulations provide a rationale for this; the O-H stretching vibrations are shown to shift in frequency as a result of different environments arising from isomorphous Si/Al substitutions in the SiO4 tetrahedron, as well as the influence of the nearby K+ ions. The data also demonstrate that the anisotropically oriented –OH populations give rise to peak intensities exhibiting a strong dependence on azimuthal rotation, which enables a direct determination of the angle of the hydroxyl orientation, as well as the mica stacking sequence and polytypism. By combining these data with high-resolution AFM, we show that the near 3-fold symmetry of the basal plane of muscovite is broken as a consequence of the O-H vector aligning along one of the mica lattice direction and so we are able to determine the absolute orientation of the topmost –OH groups in the mica lattice. To the best of our knowledge, 3 ACS Paragon Plus Environment


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no other technique can directly or easily identify the lattice direction along which the O-H dipole vector projects. This capability will enable future studies to investigate how subsurface –OH groups influence water structure, adsorption dynamics and molecular assembly.

RESULTS AND DISCUSSION Structure of bulk muscovite mica: Muscovite mica (nominal composition KAl2(Si3Al)O10(OH)2 ) is a commonly studied form of mica38-40 and the one employed here. Each layer of muscovite (~1 nm thick) is comprised of an aluminum hydroxyl-octahedral sheet sandwiched between two tetrahedral SiO4 sheets, giving rise to a tetrahedral-octahedral-tetrahedral (T-O-T) layer structure (Figure 1). A partial negative charge is induced in the tetrahedral sheet by isomorphous substitution of Si4+ cations by Al3+ cations in a ratio of 3:1, which is neutralized by K+ ions intercalated between the adjacent T-O-T layers. This allows muscovite to be easily cleaved along its basal plane (001) creating two large atomically flat surfaces, with approximately equal but randomly distributed K+ ions on both surfaces. The common C2/c 2M1 mica polytype consists of T-O-T sheets in an alternating 1-2 stacking sequence. Each octahedral sheet has two oppositely pointing –OH groups: 1OH1 and 1OH2 in the first T-O-T layer and 2OH1 and 2OH2 in the second T-O-T layer, and so on (Figure 1). The OH1 groups are angled approximately 15° above the (001) plane and the OH2 groups approximately 15° below. When projected onto the (001) surface, the O-H dipole vectors are known to point within 2° of the [110] and [110] directions and the projections of 2OH1 and 2OH2 are rotated by approximately 120° relative to that of 1OH1 and 1OH2, respectively (Figure 1b)41-42. (This description is true for bulk muscovite, but close to the surface the rotational relationship of the –OH groups might be slightly different due to surface relaxation.)


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Figure 1. (a) Side and (b) top view of structure of two layers of muscovite color coding the internal OH, depending on their position relative to the surface. Blue (1OH1) and green (2OH2) are surface OH and red (1OH2) and magenta (2OH1) are subsurface OH moieties. The definition of the angle between the surface OH and the surface normal () is shown in (a) and the definition of the rotation angle between adjacent layers () is shown in (b). The black dashed lines represent the lattice-periodicities of 0.52 nm in three nearest neighbor directions of the cation sublattice. The [110] and [110] direction are interchangeable. The blue and the pink dashed line runs across the dipole vector of the –OH groups from the 1st layer and the 2nd layer, respectively.

Azimuthally resolved vSFG spectra of air/muscovite interface: We performed azimuthally resolved vSFG measurements on freshly cleaved mica substrates in air using a scanning vSFG spectrometer. (See SI for details on mica sample preparation and the scanning vSFG spectrometer.) Consistent with the anisotropic orientation of the –OH groups inside mica (Figure 1), the vSFG 5 ACS Paragon Plus Environment


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spectrum of the air/muscovite interface is dependent on azimuthal orientation (Figure 2a). (In what follows, the SSP (S-polarized SFG, S-polarized visible, and P-polarized IR beams) polarization, which gives the largest vSFG signal (Figure 3 and S3), is used unless stated otherwise.) Multiple types of –OH species, with unique azimuthal dependence are detected, but we will focus on the OH stretching region around 3620-3680 cm-1. Previous IR and RAMAN studies have assigned this peak to O-H stretching vibrations of hydroxyl groups in the octahedral aluminum sheets in muscovite43-46. We have also acquired a bulk IR spectrum (Figure S4) of thin mica sample (~10 m) and it features a broad peak centered at ~3630 cm-1, consistent with earlier studies43-44. Given that the hydroxyl groups are both IR and RAMAN active, we expect it to be vSFG active as well (SFG process is a combination of IR and anti-stokes RAMAN transitions). Additional control experiments with hydroxyl-free mica (fluro-phlogopite) and muscovite mica placed in an N2 purged sample cell are shown to conclusively assign these peaks to the hydroxyl groups inside the mica and not from the adsorbed water or carbonates from the surrounding environment (Figure S1 & S2). Since vSFG is an interface specific technique and bulk muscovite is centrosymmetric (vSFG inactive), we expect only the –OH groups close to the interface to produce a vSFG signal. Thus, the azimuthally resolved vSFG spectra (Figure 2a) shows a dominant peak at only one azimuthal angle, which we define as the zero angle (0°, red dotted line). If vSFG was equally sensitive to all the –OH groups in the bulk muscovite, we would expect to see equally intense peaks at both 0° and ~120°. However, only minor peaks around ~105° and ~180° are observed and their origins are discussed later.


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Figure 2. (a) Azimuthally resolved vSFG spectra of air/muscovite interface acquired using the scanning vSFG setup with SSP polarization. Red dashed line represents the 7 ACS Paragon Plus Environment


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center of the azimuthal distribution of the 3620-3680 cm-1 peak, which is defined as the 0° azimuthal angle. (b) vSFG spectra of air/muscovite interface at 0° azimuth angle using scanning instrument (red line with open circle) and high-resolution instrument (green closed circles) using SSP polarization. Black solid line is a fit to the high resolution vSFG spectrum using a Lorentzian line shape function (Eq. S1). (See SI for details.) (C) Vibrational density of states from DFT simulations (see SI for details) for bulk mica, and either surface (1OH1, 2OH2) or subsurface (1OH2, 2OH1), see Figure 1a, for the slab calculation. The oscillators are grouped into black, if the O-H is interacting with a Si-O-Al moiety and red if the O-H is interacting with Si-O- Si. An additional splitting is observed for the surface layer, depending whether a K+ is coordinated above the OH or not.

The vSFG spectra of the air/muscovite interface at 0° acquired using scanning vSFG instrument and high-resolution vSFG instrument are compared in Figure 2b. The details of the vSFG spectrometers can be found in the SI. The two vSFG spectra are quantitatively similar, with multiple peaks clearly visible and lying within the bulk O-H stretching vibration. In the past, there have been many inconsistencies and reproducibility issues regarding vSFG spectra from various interfaces. Usually, this has been explained in terms of differences in vSFG instrument geometry and sensitivity between different labs (for example, different angle of incidence, broadband vs. scanning instruments, and differences in spectral and temporal resolution). The fact that our spectra from two very contrasting vSFG spectrometers are quantitatively similar provides evidence that


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consistent vSFG spectra can be obtained for muscovite surface provided the absolute azimuthal rotation is controlled. The high-resolution spectra, which was fit using a Lorentzian line shape function (Equation S1), reveals the presence of three peaks centered at 3622, 3651, and 3672 cm-1 (Table S1). The fit implies that the highest frequency peak has opposite phase compared to the other two, but additional confirmation with heterodyne detected vSFG measurement are warranted. For more details on fitting analysis, refer to the SI (Figure S5). The vSFG spectra also show slight variations from sample to sample (Figure S4). Mostly, the peak for the 3622 cm-1 species remains dominant, while the relative amplitude of the other two peaks (3651 and 3672 cm-1) vary. We speculate this to be due to uniqueness of each mica sample as a result of the random distribution of K+ ions on the surface and the poorly defined distribution and ordering of Al atoms in the alumino-silicate network. Previous bulk mica studies44-45, 47 have shown that the O-H stretching frequencies red-shift when silicon atoms near the –OH group are substituted by aluminum atoms. In order to determine the origin of the different O-H stretching peaks in the vSFG spectra, DFT molecular dynamics simulations were performed. Vibrational frequencies were calculated using the O-H bond length vibrational density of states through the Fourier Transform of the bond length autocorrelation function48. Two systems are considered, a bulk mica and a two-layer mica slab, see Figure 1b and SI for details. The OH oscillators are categorized as bulk from bulk simulations and either surface (1OH1, 2OH2) or subsurface (1OH2, 2OH1) from slab simulations, see Figure 1a. The resulting frequencies were scaled by 0.98 to match the average of the bulk mica as a reference for easier comparison, as is typical for generalized gradient approximations (GGA) functionals in the hydrogen bond stretch region. Two frequencies are observed for DFT bulk simulations depending 9 ACS Paragon Plus Environment


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on the substitution pattern: OH bonds interacting with either an oxygen bridging a silicon and aluminum (Si-O-Al), at 3616 cm-1, or an oxygen bridging two silicon atoms (Si-O-Si), at 3650 cm-1 (Figure 2c). Upon formation of a surface, the 3650 cm-1 oscillator experiences a blue shift of 10 cm-1 in the subsurface layer. In the surface layer, four distinct frequencies are found, two of which are at the same frequency as those of the bulk. For Si-O-Si sites, compared to the subsurface layer, a K+ above the OH red shifts the oscillator to be equal to the bulk frequency, whereas the absence of any ion blue shifts it by ~30 cm-1. For Si-O-Al sites, the ion introduces a red shift of ~20 cm-1. Therefore, the multiple peaks in the vSFG spectra are assigned to O-H stretching vibrations experiencing heterogeneous environments as a result of isomorphous Si/Al substitutions and the proximity of K+ ions to the –OH groups.

Orientation of the –OH group: Polarization analysis of vSFG spectra is accurate in determining the molecular orientation at interfaces30, 49. The orientation of the –OH groups in bulk muscovite is 72-74° from the surface normal24, 41-42, 50. While it is well known that for a rotationally isotropic interface, any vibrational modes along the surface plane do not generate a vSFG signal, the –OH groups inside the muscovite lattice are rotationally anisotropic (as discussed previously) and hence, these nearly in-plane vibrational modes of the O-H stretch are vSFG active. Additionally, these – OH groups have an out-of-plane component, because they do not lie perfectly flat on the mica (001) plane and therefore contribute to the vSFG signal. We collected the vSFG spectra of the air/muscovite interface for all eight polarization combinations (SSP, PPP, SPP, PSP, SSS, SPS, PPS, and PSS) (Figure S3). In agreement with the simulated vSFG signal (Figure S6) by assuming rotational isotropy for the hydroxyl groups, the SSP intensity is greater than the SPS and the PPP intensity (Figure 3a) for the O-H stretching modes oriented 72-74° from the surface normal. (We 10 ACS Paragon Plus Environment

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note that the orientation tilt angle obtained from the rotationally isotopic surface can only serve as an estimate for the rotationally anisotropic surface. Further analysis will be reported in future publications, as the analysis itself is complex and extensive and constitutes a stand-alone study in its own right.) The orientation extracted from the polarization analysis is in good agreement with the DFT simulations (for details, see SI), which predict an average orientation of outermost –OH groups of ~78°, which is slightly greater than the bulk value (Figure 3b). We also found experimentally that even though –OH groups are achiral, significant vSFG intensity is observed for chiral polarizations (SPP, PSP, and PSS), consistent with previous studies showing achiral molecules at an interface or in an asymmetrical environment (like the rotational anisotropic –OH groups inside mica) exhibit chiral signatures51-53.

Figure 3. (a) vSFG spectra of air/muscovite interface with SSP. SPS, and PPP polarization at 0° azimuthal angle. (b) Angle distribution of the surface O-H bond vector with respect to the



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surface normal (), see Figure 1 for definition. The average angle of 78 is highlighted by a dashed line. See SI for details.

Azimuthal dependence of 3622 cm-1 O-H stretch: Figure 4 shows the azimuthal dependence of the 3622 cm-1 O-H stretch using SSP polarization. There is clearly one dominant peak at 0° and smaller peaks at ~105°, 180°, and ~285°. In order to quantify the exact position of the peaks, two lines (green dotted lines, Figure 4) are drawn across the center of the azimuthal distribution such that it bisects the 0° and 180° peaks and the ~105° and ~285° peaks. We define these lines as the direction of the O-H dipole vectors. For some of the mica samples, we observe an asymmetry in the azimuthal distribution, but its origin is unclear (Figure 4b). The azimuthal dependence of the three O-H stretching frequencies (3622, 3651, and 3672 cm-1) are similar, which means that their O-H dipole vectors are pointing in the same direction (Figure S7). For any birefringent material (like muscovite), anisotropic optical signals due to phase matching in the bulk are common. This can easily obscure the interpretation of the optical signal coming from the interface. However, birefringence effects are negligible in our experiments (as discussed in the SI). We assign the main peak at 0° to be due to –OH groups (1OH1 and 1OH2, Figure 1) in the first T-O-T layer. 1OH1 should be the dominant contributor since it is closest to the surface and hence in a “truly” non-centrosymmetric environment, making it more vSFG active. In contrast, 1OH2 is closer to the bulk compared to 1OH1 and hence its dipole can be cancelled out by the –OH groups above and below it. The peak at 180° is also due to 1OH1 (dominant contribution) and 1OH2 groups, but the large difference in vSFG intensity at 0° and 180° is due to variation in the coupling 12 ACS Paragon Plus Environment

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between the excitation source (P-polarized IR light) and the O-H dipole at the two azimuthal angles (See SI for details; Figure S8). Next, the small peak at ~105° is assigned to 2OH1 (dominant contribution) and 2OH2 from the second T-O-T layer (same argument as 0°). The peak appearing at 105° is consistent with the 2M1 stacking pattern of muscovite which rotates the 2OH1 species with respect to the 1OH1 species (Figure 1b). Even though the second layer –OH groups (2OH1 and 2OH2) exists in a more bulklike environment compared to first layer, it is still SFG active. This can be rationalized in two ways. First, the surface has a single termination but the second layer –OH groups are still vSFG active since their O-H vector is rotated by ~120° with respect to the first layer and therefore its dipole is not completely cancelled out and so remain marginally vSFG active. Second, the mica surface (area under the laser beam) might have small defects giving rise to different surface terminations. The amplitudes at 0° vs. 105° could be indicative of surface coverage of such differences in termination. Additionally, the vSFG intensity of 105° (and 285°) peaks vary from sample to sample (compare Figures 4, 5, and S7), probably indicating that different mica samples have differing amounts of defects. The change in orientation of surface molecules due to the presence of defects and etch pits has been observed previously54. The peak appearing at ~105° instead of 120° could suggest that the stacking pattern of muscovite is slightly different at the interface compared to the bulk. The DFT simulations (details in SI) show a surface relaxation with an accompanied deviation from the perfect 120° having a skewed peak in the distribution at about 112° and an average of 106° in good agreement with experimental findings (Figure S9). The vSFG peak at 285° is also due to 2OH1 and 2OH2 species



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and the difference in amplitude at 105° vs. the 285° is due to the difference in coupling between P-polarized IR light and the O-H dipole at the two azimuthal angles.

Figure 4. Polar plot showing the azimuthal dependence of 3622 cm-1 species with SSP polarization for two different mica samples. (a) sample 1: mica sample was only cleaved once (black) and (b) sample 2: mica sample was cleaved two times (red and blue dots represent the azimuthal dependence after the first and second cleave.). The green dotted line drawn across the center of the profile of the azimuthal distribution at 105° represents the average orientation of the –OH groups. Effect of multiple cleaving cycles on azimuthal rotation is shown in Figure S10.

Effect of cleaving on azimuthal dependence of 3622 cm-1 species: We repeatedly cleaved mica by peeling off layers with tape and measured the azimuthal dependence of 3622 cm-1 species after each peeling (Figure 4 and Figure S10). Our results reveal that the azimuthal angle of the dominant 14 ACS Paragon Plus Environment

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peak is generally not altered — i.e., it remains at zero — after cleaving, which implies that mica cleaves in even numbers of layers. If the cleaving was by an odd number of layers, the azimuthal pattern would have rotated by ~120°. We further tested this hypothesis by razor cleaving the muscovite sample in half and recording the azimuthal dependence of all the four faces (Figure S11). The results similarly show cleaving occurred at an even number of layers. (Occasionally we did observe cleaving in odd numbers of layers when using the tape peeling method, causing the maximum vSFG signal to occur at other angle than 0° (Figure S10b), but this outcome was extremely rare.) We speculate that the preference for cleaving in even layers is due to an inherent feature of the mica, either related to the 2M1 stacking pattern of muscovite whereby only the alternate T-O-T layers are identical, or the pattern of Si/Al substitution in the tetrahedral sheet, which could result in inequivalent forces between the muscovite layers. DFT also provides a rationale for this phenomenon. We calculated the adhesion energy for one versus two layers for a slab containing four layers. (See SI for details.) The overall magnitude of about 520 mJ/m2 is in good agreement with other theoretical studies55, but off by a factor of 2 compared to the experimental estimate56. The adhesion energy for one layer is 40 mJ/m2 larger than for two layers which gives an energetic rationale for why cleaving in even layers is much more common than in odd layers (Figure S12). Alternatively, the preference for even layer cleaving could be related to some artifact inherent to the manufacturing process.

Azimuthal dependence of 3622 cm-1 peak for “top” vs. “bottom” face of muscovite: For any piece of muscovite, two (001) faces are indistinguishable by most, if not all, spectroscopic or 15 ACS Paragon Plus Environment


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imaging techniques. However, the azimuthal dependence of the vSFG signal at 3622 cm-1 for the top and bottom faces are always rotated by 180° (Figure 5) — all mica samples tested (>50) behave similarly. This can only be explained if the first and the last T-O-T layers of the muscovite sample are identical so that the vSFG signal at 0° for the “top” face is due to the blue (or pink) –OH groups and the vSFG signal at 180° for the “bottom” face is due to the red (or green) -OH groups (Figure 5c and d). This scheme is possible in two ways: (1) The sample prepared by the manufacturer always has an odd number of layers (assuming there is no stacking fault between layers). This is surprising given that our cleaving experiments revealed that mica prefers to cleave in even number of layers. (2) A stacking fault that results in the first and the last T-O-T layers being identical is always present in each sample. Our results clearly show that the two faces (“top” and “bottom”) of the muscovite samples differ in the orientations of the hydroxyl groups. We expect this to influence the orientation of molecules adsorbed on the two faces. If possible, it would be interesting to see if the preferred orientation of molecules on the two faces are rotated by 180° as well. However, this experiment could prove challenging since one side of the sample has to be glued to a metal puck during AFM experiments and even if the experiment was conducted, it might prove difficult to differentiate molecules oriented at 0° vs 180°.


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Figure 5.

Polar plot showing the azimuthal dependence of the 3622 cm-1 species with SSP

polarization for the top (a) and bottom (b) face of muscovite. Cartoon representation of the dipoles of the –OH groups in the T-O-T layers of muscovite for (d) “top” and (d)“bottom” faces.



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The green and the pink –OH groups are rotated by -60° and +120° with respect to blue –OH groups, respectively (see Figure 1b).

Relationship between the -OH dipole and the muscovite lattice:

Figure 6 shows the

relationship between the –OH orientation and the muscovite lattice. The AFM image shows latticeperiodicities of 0.52 nm in three directions, which correspond to [100], [110] and [110]. These directions are indistinguishable by AFM, which has historically led researchers to approximate the surface as trigonal57-59. The center of the azimuthal distribution of 3622 cm-1 species, which we assign to be the direction of the O-H dipole vector from the first T-O-T layer, aligns with one of the three directions of 0.52 nm periodicity. This agrees with past work23, which determined that bulk muscovite –OH groups are aligned within 2° of the [110] and [110] directions when projected onto the (001) plane. The dipole vector of the –OH groups from the second layer is not parallel to the [110] direction probably due to surface relaxation (as discussed earlier). The novelty of our measurement is that, for any given mica sample, we can obtain an absolute determination of the lattice direction along which the topmost –OH groups lies. The ability to establish the absolute –OH orientation in the topmost layer of muscovite is an important finding, because the –OH groups have been speculated to underlie the preference that molecules commonly exhibit for adsorbing on muscovite along specific orientations, breaking the surface’s pseudo three-fold symmetry7, 10-11, 60. In at least one instance, corrugations parallel to either the [110] or the [110] direction as a result of tilting of the Si/Al oxide tetrahedra were shown to be correlated with the preferred orientation7,

11.

Our results now establish a method for

determining whether a correlation exists between the O-H vector direction and the preferred 18 ACS Paragon Plus Environment

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orientations of adsorbates and/or the groove direction. Moreover, extending such work to ordering of water, adsorption of soil organic matter, and heterogeneous formation of minerals on mica, would open a new window into key environmental processes. Thus, our vSFG results open a unique opportunity to directly study the structural controls on soft matter assembly and mineral nucleation at muscovite surfaces.

Figure 6. Azimuthal dependence of the 3626 cm-1 species (red dots) overlaid onto an AFM image of the [001] plane of muscovite mica. The solid blue and pink line drawn across the center of the profile of the azimuthal distribution represents the average orientation of the – OH groups from 1st and 2nd layer respectively. The green dashed lines represent the three



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directions of the mica lattice with periodicities of 0.52 nm (i.e., the nearest neighbor directions of the cation sublattice). The [110] and [110] directions are interchangeable. The blue solid line is parallel to [110], but the pink solid line is not parallel to [110] due to surface relaxation (see text for details). The –OH group never lies along the [100] direction.

CONCLUSIONS Our results show that vSFG spectroscopy is unique in its ability to exclusively probe the –OH groups from the topmost layer of muscovite. Because the –OH groups are highly oriented, the azimuthally-resolved vSFG spectra is found to be rotationally anisotropic. The O-H stretching modes of these –OH groups are observed to be sensitive to the heterogeneous environment created by the isomorphous substitution of silicon atoms by aluminum atoms in the SiO4 tetrahedra and the K+ ion adsorption above the hydroxyls. The “top” and “bottom” faces of a muscovite sample are indistinguishable to most spectroscopy and imaging techniques, but our vSFG results clearly show that the –OH groups are oriented in opposite directions for the two faces. The effect of cleaving on –OH orientation suggests that muscovite prefers to cleave in even layers, which could point towards anisotropic forces between mica layers or a presence of defects on the muscovite sample inherent to the synthesis process. For the first time, we are able to accurately determine the vector direction of –OH groups in the first T-O-T layer and correlate it with the [100], [110] and [1 10] directions obtained from the lattice-resolution imaging. To the best of our knowledge, this is the first direct experimental observation and we envision this capability being critical for future investigations of molecular adsorption, self-assembly dynamics and the forces controlling alignment on mica surfaces. 20 ACS Paragon Plus Environment

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ASSOCIATED CONTENT Supporting Information The details of sample preparation, vSFG measurements, AFM measurements, and DFT simulations are available in the Supporting Information, which is available free of charge on the ACS Publications website at DOI:

AUTHOR INFORMATION Corresponding Author * [email protected]; [email protected]; [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This work was supported by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Materials Science and Engineering. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle under Contract No. DE-AC05–76RL01830. Experiments were performed at the Environmental Molecular Sciences Laboratory (EMSL), a DOE office of Science User Facility sponsored by the Office of Biological and Environmental Research that is located at PNNL. We would like to acknowledge Dr. Alan Joly, Dr. Patrick Z. El-Khoury, Dr. Li Fu, and Dr. Luis Velarde for helping with laser troubleshooting. 21 ACS Paragon Plus Environment


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Direct Observation of the Orientational Anisotropy of Buried Hydroxyl

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