Anisotropic Growth of Water-Puckered Pentamers on a Mica Terrace

Article pubs.acs.org/Langmuir

Anisotropic Growth of Water-Puckered Pentamers on a Mica Terrace Omar Teschke,*,† Luiz Orivaldo Bonugli,† and Elizabeth Fátima de Souza‡ †

Laboratório de Nanoestruturas e Interfaces, Instituto de Física, UNICAMP, 13083-859, Campinas, SP, Brazil Faculdade de Química, PUC-Campinas, 13012-970, Campinas, SP, Brazil

‡

ABSTRACT: Ice nucleation at mica terrace edges in air forms mounds of water molecules that grow larger as the step-edge height increases from a few Angstroms to hundreds of nanometers. The structures of the ice deposits at mica terrace edges were characterized by atomic force microscopy (AFM), and the edges were shown to act as nucleators for water pentamers, thereby forming a zigzag structure with lattice parameters of 0.72 ± 0.07 and 0.60 ± 0.06 nm. A threedimensional arrangement of three pentamers of water molecules, which formed a parasol-like structure, was assembled to match the AFM images. Seven three-fused pentamers were clustered to form large hexamers that cover the entire surface. The nucleation at the edges reveals a substantially larger growth rate than that on the mica terraces; consequently, highly terraced mica slabs could be used as new and more efficient structures for seeding clouds and causing rain. On the basis of this finding, a new ice-condensation structure was designed with pyramidal features and steps of 100 nm in height and width

1. INTRODUCTION The relevance of the interaction between water and a material substrate is associated with water’s role in natural and technological processes permeating virtually all areas of biochemical and physicochemical importance. There has been considerable progress made in understanding the fundamental interactions of water with solid surfaces.1 For hydrophilic substrates, the first water layer deposited on a solid surface acts as a structural template for the layers that follow. This template determines both the boundary conditions for water transport and the aqueous surface chemistry. Water molecules bridging proteins/cells and a surface play an important role in the adsorption of protein molecules onto surfaces.2−5 The basic question regarding the interactions between water and solid surfaces is investigated here by observing the interfacial water structure resulting from strong bonding between water and mica substrates. Heterogeneous ice nucleation plays a key role in such fields as atmospheric chemistry in which the kinetics of droplet growth determine the properties of clouds.6,7 Interfacial water properties also determine the mechanism of surface catalysis for chemical reactions. Although the exchange of matter at an interfacial surface between the condensed and the gaseous phases plays a central role in many processes, little work has been performed to date to understand systems involving these small domains.8 Therefore, our understanding of the molecular processes involved in ice nucleation lacks a clear model for heterogeneous nucleation at the nanoscale level. The molecular details of the initial stages of ice nucleation have been discovered using experimental and theoretical surface science techniques.9−15 Kimura investigated the cross-sectional hydration layer on a solid surface (muscovite mica/water) with atomic force microscopy (AFM) by measuring force profiles.16 © 2011 American Chemical Society

The water distribution at a solid/liquid interface (mica/water interface) was also recently visualized above the center of hexagonal cavities.17 Through such studies, an extraordinary variety of water/ice structures have been reported, which range from individual water hexamers to extended, hexagonal, onedimensional (1D), 2D, and 3D overlayers.18 A common feature of such structural models is that they are built from hexagonal arrangements of the water molecules. The first structural study of a water layer on a metal using STM was conducted by Morgenstern et al.,19 who investigated the growth of water on Pt(111) between 120 and 145 K. They found that water adsorbs preferentially at the top and bottom of the step edges. Understanding the growth of larger water clusters and how these clusters link themselves to form a larger network of water molecules remains a challenging scientific endeavor.20−22 Two-dimensional water/ice layers containing large 10-, 12-, 18-, and 45-membered water rings have been observed in the solid state.23−27 These polymeric water morphologies with sizes that range between water clusters and bulk water have physical properties closely associated with those of bulk water. In this work, we obtained high-resolution AFM topographic images of the structured water layer at mica step edges. We also investigated the efficiency of the step edges as ice nucleators at room temperature and ambient humidity. Recently,28 using a molecular dynamics simulation, the twodimensional hydrogen-bond network formed at the surface of muscovite mica was determined. The calculated monolayer structure is formed by a network of highly distorted hexagons. In a previous work,29 we observed that this ice-like structure Received: October 19, 2011 Revised: December 1, 2011 Published: December 8, 2011 1552

dx.doi.org/10.1021/la204096p | Langmuir 2012, 28, 1552−1561

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time of each line is (10/300) × 2 = 2 × 10/300, which results in an image at every 20 s. The drift was estimated by comparing the measured periodicity to the one expected for a surface such as mica. The thermal drift effect is also determined by comparing images taken at different times and comparing the periodicities in at least two sequential images of same scan direction. A period of least 2 h of operation previous to data acquisition was observed.

corresponds to a structure of highly distorted hexagons matching the mica structure and exhibiting monolayer coverage. In the present work, we show that thicker layers may be formed on a mica substrate close to the step edges in which the new structure, with an O−O bond length of ∼0.28 nm, matches the substrate edges. The structure stiffness was compared to the stiffness of other structures,28 and the growth rate was measured and compared with the growth rate on flat terraces. The molecular structure of the layers was investigated using high-scanning-speed (≥100 nm/s) AFM.

3. RESULTS Freshly cleaved mica surfaces exhibit micronized flat terraces separated by steps that have a height distribution beginning at a few Angstroms (∼10 Å). Imaging these sharp mica step edges requires special attention to overcome effects that can appear in the AFM image, such as an overshoot as the tip travels up the slope and an undershoot as the tip travels down the slope. This feedback artifact commonly appears on steep features, represented in AFM images as bright ridges on the uphill side and dark ridges on the downhill side. In this work, before imaging the ice-like structures, a profile of bare ridges was measured and the feedback mechanism adjusted by optimizing the parameters of the proportional integral derivative (PID) controller in the feedback circuit to eliminate these effects. Figure 1 shows the resulting profile for two step edges on a

2. EXPERIMENTAL SECTION We used an AFM (model TMX2000, TopoMetrix, Veeco) with a silicon nitrite (Si3N4) tip (Microlevers, Veeco, model MSCT-AUHW) with a spring constant of approximately 0.03 N/m to scan the mica surfaces. The radius of curvature of the AFM tip is approximately 5 nm. Each map of a sample surface consisted of 300 × 300 grid points. The scan velocity was optimized to obtain the best signal-to-noise ratio, thereby resulting in a value of ≥100 nm/s for mica in air. The constant normal load was also optimized, and a value of ∼4.0 nN was used throughout the experiments. The patterns shown correspond to the best signal-to-noise ratio images of the mica samples scanned over varying regions and after a large number of cleavages. The images displayed were selected from a group of more than 100 images. The AFM was operated in a mode in which the position of the sample is kept constant while scanning. In this mode all feedback loops were opened and there is no scanning velocity limitation by the time constant (∼50 ms) of the piezoelectric translator and feedback circuits.30 The image was obtained by plotting the deflection of the cantilever as a function of its lateral and vertical position. The detector voltage was registered as the output signal. The tangential force along the scanning direction and the force normal to the scanning direction were recorded simultaneously. A polycarbonate controlled-atmosphere chamber enclosed the AFM head in which the relative humidity (RH) was controlled by evaporating deionized Milli-Q water from a container via insertion of a desiccant material or by the flow of dry nitrogen gas. The variation in RH was controlled to less than ±2.5% during the imaging experiments. Mica was cleaved with adhesive tape in air, and the sample was immediately placed into the chamber containing the AFM head. Samples with dimensions of 1 × 1 cm2 and typical thicknesses of several tenths of a millimeter were used without any previous treatment. Under a flow of dry nitrogen gas, the contrast in the lateral force images showed no variations, indicating that the freshly cleaved mica surface was free of contamination. The incubation period was ∼4 h and used for all ice-like deposits. For larger periods of incubation (∼24 h) the same pattern of larger growth rates at the edges compared to terraces was observed. The thickness of the ice structure close to the edge corresponds to the value of the maximum height of the structure at the edge, and the measured width of the structure is the full width at half-maximum (fwhm) value. All images that display a periodic atomic structure were processed as follows: Initially they were horizontally autoleveled in first order, i.e., each line of the image in the scanning direction (x) was fitted to z = ax + b and used to level the line out of the image. Then the unfiltered images acquired directly from the electric signal associated with the tip movement were fast Fourier transformed and higher spatial frequencies were removed, resulting in an image with a regular uniform structure. The calibration procedure used in this study has been described in our previous work.31 Thermal drift effects on measurement of the lattice parameter at room temperature were previously discussed by Rahe et al.32 The thermal drift was minimized by scanning fast (>100 nm/s) small areas (10 nm ×10 nm), resulting in a typical scan speed of less than one image per 25 s, i.e., for vscan = 300 nm/s, 300 lines image the scanning

Figure 1. Contact-mode AFM image of a mica edge at 25 °C and 35% RH after 1 h of exposure. The two-step profile is shown in the inset. The overshoots or undershooting were eliminated by adjusting the feedback signal parameters.

mica substrate scanned in air with a relative humidity below 35%, under these conditions, there are no ice deposits on the substrate.33 The profiles reflect that at the lower part of the curve the tip has a pyramidal shape (apex angle of ∼35°), and close to the terrace edge the slope of the supertip cone is an angle of ∼18°. In this image, there are no dark and/or bright ridges close to the step region; therefore, overshoots or undershoots were minimized and are not observable. Measurement of the ice-like deposit profile grown at the edges was subsequently performed. Figure 2 shows the image of a mica edge covered with an ice-like deposit observed at 25 °C and 65% RH. The measured step height is ∼7 nm. It is possible 1553


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Figure 2. Three-dimensional view of a contact-mode AFM image of an ice-like covered step edge at 25 °C and 65% RH after 6 h of exposure. (Insets) Vertical profile of the step edge at different magnifications.

to verify that the ice deposit at the top edge has grown thicker than deposits in the flat mica region distant from this edge. To confirm the observation that there is preferential growth at the step edges, the vertical profiles of the ice-like deposit at the mica step edges, which range from 2 to 200 nm in height, are plotted in Figure 3. All profiles show a protrusion, indicating

Figure 4. Thickness (a) and width (b) of the ice-like deposits formed at the edges (compared to the terrace deposits level) as a function of the step-edge height.

of 4.4% for the vertical dimension (0.52 ± 0.011 nm) and 4.6% for the horizontal dimension (0.9 ± 0.021 nm). After completion of the calibration procedures, the ice-like layer surfaces were scanned. A contact AFM image of mica scanned in air at 25 °C and 35% RH over a distance of 4 nm at a velocity of 250 nm/s is shown in Figure 5a. Observe the regular pattern of the image; using the vertical profile image (VPI) the lattice parameters of the structure were measured, 0.54 and 0.94 nm, showing errors of 4% for both the vertical and the horizontal dimension when compared to the lattice parameter of mica, which are 0.52 and 0.9 nm. This pattern then corresponds to the substrate before any ice nucleation. In Figure 5b, a mica terrace covered with an ice-like deposit was observed at 25 °C and 65% RH in an atomically flat region away (>1 μm) from any step edge. The ice-like water structure exhibits a zigzag pattern that coincides with the observed molecular arrangement. The width of the zigzag structure observed on mica terraces is 0.40 ± 0.04 nm. The zigzagged corners in the topographic image coincide with the bright features indicated in the figure by red circles connected with red bars. Judging by the distance between the adjacent protrusions, the chain length is estimated to be 0.45 ± 0.05 nm. It is clear that the bright features responsible for the main contrast in the images are probably individual water molecules,

Figure 3. Ice deposit profiles for various steps starting from ∼2 to 200 nm in heights. (Inset) Large amplification.

that ice nucleation at the edge takes place at a faster rate than on the terraces. Also, the profile curves show an accentuated increase with an edge height of 2 nm up to ∼700 nm. The results are summarized in Figure 4, in which the height (Figure 4a) and width (Figure 4b) of the ice deposits are plotted as a function of the step-edge height. To investigate the mechanism of this preferential ice nucleation and growth at the mica step edges, the crystalline structures of the ice-like films grown at the edges and on the flat regions were imaged and compared. Because the ice-like periodicity is on the order of a few tenths of a nanometer, the imaging technique requires a resolution on the order of ∼0.01 nm to differentiate the ice structures. The calibration procedure of the vertical and horizontal piezoelectric was performed using a standard silicon calibration grid with a known pitch size. The method was followed by a finer-scale calibration procedure using a freshly cleaved mica sample scanned in air at 35% RH. The resulting image displays an atomically resolved pattern. Using the vertical profile image (VPI), the lattice parameters of the mica were determined to be 0.52 and 0.94 nm with errors 1554


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Figure 5. (a) Contact AFM image of a cleaved mica surface in air at 25 °C and 35% RH scanning velocity 250 nm/s measured lattice parameters 0.54 and 0.94 nm corresponding to a mica substrate before any ice nucleation. (b) Atomically resolved topography AFM image of an ice-like structure formed on mica terrace surfaces observed at 25 °C and 65% RH (>1 μm away from any edges). Lattice exhibits a zigzag structure indicated by red circles connected by red bars coincident with the light spots pattern formed by scanning action. Period along the zigzag structure is 0.45 and 0.40 nm. (Inset) 2 D FFT spectrum.

Figure 6. Atomically resolved topography AFM image of an ice deposit formed at 25 °C and 65% RH close to a mica edge region. Large magnification of the top terrace forming the edge where light spots with a 0.72 and 0.60 nm periodicity are observed. Lattice exhibits a zigzag structure indicated by red circles connected by red bars coincident with the light spots.

Another image of a region close to a mica edge is shown in Figure 7. To analyze the structural details of the scanned image, a FFT is presented in the inset. The 2D FFT corresponds to the untreated image depicted in Figure 7. The right inset shows spots surrounded by squares, which correspond to the following periodicities: 0.18, 0.28, 0.45, 0.58, and 0.62 nm. The displayed power spectrum is derived by performing a FFT on each scanned line. Each spot subsequently corresponds to a single frequency in one direction. Fourier analysis yields distinct spots that reflect the high degree of order of the imaged surface structure. One-dimensional FFT diagrams, not shown in this work, indicate the following periodicity in the FFT power spectrum: 0.72 nm. The periodicity of the zigzag structure was also measured for a large number of AFM images, and the lattice constant value (0.72 nm) distribution is shown in the right inset.

which dangle away from the surface. The fast Fourier transform (FFT) of the image in Figure 5 is presented in the figure’s lower inset, and the spots marked with circles reflect a periodicity of 0.45 nm. In Figure 6, a mica edge covered with an ice-like deposit was observed at 25 °C and 65% RH in a region close to a step edge with a height of ∼1 nm. A zigzag pattern is visible in the structure shown in Figure 6b that coincides with the observed molecular arrangement. The width of the zigzag structure at the mica step-edge regions is 0.60 ± 0.06 nm, and the chain length is estimated to be 0.72 ± 0.07 nm. This structure is observed at the top of the step-edge region. This high-resolution image shows periodicities that differ from those in Figure 5. 1555


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To demonstrate further that the ice-like structure is epitaxial in nature with respect to the mica step edge, an image of a mica edge with a small amplification is shown in Figure 9a. The

Figure 7. Atomically resolved topography AFM image of an ice deposit close to a mica edge region. The image was acquired with 0.4 nF of applied force and a scanning speed of 100 nm/s. (Right inset) 2D FFT spectrum showing spots at 0.18, 0.28, 0.45, 0.58, and 0.62 nm; (left inset) lattice constant distribution with an average value of 0.72 nm.

An image corresponding to a taller step edge than the one shown in Figure 6 is depicted in Figure 8. An atomic structure

Figure 9. (a) Three-dimensional AFM image of an ice deposit on the mica surface observed at a mica edge region. (b) Large magnification where an atomic resolved image matched by a zigzag structure is indicated by red circles connected by red bars coincident with light spots. Period along the zigzag structure is 0.72 nm, and the width is 0.60 nm. (c) Orientation of the zigzag structure with respect to the edge direction.

image at high magnification is shown in the left top image (Figure 9b). Figure 9c shows the orientation of the zigzag structure axis, which forms a ∼72° angle with respect to the edge. We also investigated the mechanical properties of ice-like layers using force vs distance curves. Force profiles were measured for the mica-covered region close to the edge regions (∼100 nm). The results show that at ∼10 nm from the surface the tip is attracted to the interface, which corresponds to the van der Waals force range,34 and finally stops at ∼10 nm from the interface. There is no meniscus formed as the tip approaches the surface, but an ice-like layer deposits as previously shown.35 This layer corresponds to the thickness of the ice-like structure, which is compressed and removed from the tip interface region when the force on the tip is 0.8 nN.

Figure 8. AFM images of an ice deposit on the mica surface close to an edge. Step edge is indicated by the light region. White zigzag structure with periodicities of 0.72 and 0.60 nm and forming an angle of ∼72° with the edge direction is coincident with the observed molecular arrangement.

is shown only for the bottom portion of the step because the portion of the terrace close to the top-edge image does not show atomic resolution, probably due to curvature in the profile of the surface. The important observation made from this image is that the ice-like structure is aligned and forms an angle of ∼72° with respect to the edge. 1556


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4.1. Structure Builds by Aggregation of Pentamers. For the ice-like structure shown in Figures 6−9 with periodicities of 0.72 and 0.6 nm, we propose a structural model that is based on the water−substrate interaction and should reflect the 2-fold symmetry of the surface. Structural parameters measured using 1D and 2D FFT and the parameters in the modeled structure are listed in Table 1.

The Young’s modulus of 200 MPa, measured for this thin ice-like layer from the force vs distance curves, is much lower than the value for solid-phase ice, which is ∼16 GPa in the c direction,36,37 thereby indicating that properties of these films are strongly influenced by the substrate.

4. DISCUSSION The profiles of ice-like water structures grown on mica edges for various step heights were measured and compared with those in previous reports.29,31 Fletcher38 describes the nucleation processes involved in the freezing of water in the presence of aerosol particles and concludes that size becomes important for particle radii in the range of 10−100 nm. For larger particles, nucleation efficiency is independent of size, whereas for smaller particles, this efficiency is greatly reduced. A similar behavior is shown in Figure 4a and 4b. It is possible to observe that for step-edge heights smaller than 10 nm there is no difference in the deposit thicknesses between the edges and the flat regions. For step heights larger than ∼100 nm, the deposit height is apparently independent of the step height. The results presented here are in agreement with a recent report,18 demonstrating the preferential growth of water pentamers at both the bottom and the top portions of the edges of metal substrates. The preferential growth at the edges is attributed to Smolukosky’s effect, which is associated with the anisotropy of the electronic work function. Because mica is an insulator, a different assumption is necessary. When investigating the ability of mica edges to nucleate ice faster than mica terraces, we examined the mica structure to find possible lattice planes with atomic arrangements similar to those observed in ice. Associated with the surface-deposit periodicity measurements, the stiffness of these layers was measured, yielding a value much lower than that corresponding to bulk ice. This fact suggests that structures other than ice-Ih may have grown at the mica edges. These results are in agreement with our previous work showing that the structure growing outward from the terraces on the mica has a lattice constant influenced by that of the nucleating mica substrate for a monolayer or a few layers of coverage.31 Therefore, water molecules in direct contact with the mica to which they are bound in adjoining layers are organized and embedded in new crystalline structures of ice. Our AFM measurements have enough lateral resolution to differentiate the microscopic structure of the ice-like film formed on the mica edges from the hexagonal structure of iceIh observed on the mica terraces (shown in Figure 5). The presence of two distinct structures on the mica substrates is based on the following observations: (a) The AFM images show two lattice parameter sets: 0.72 and 0.60 nm at the terrace edges and 0.45 and 0.40 nm at the terraces. (b) The difference between 0.72 and 0.45 nm is easily observed in images with a range of 10 nm. (c) The FFT spectrum distributions show two arrangements: one associated with the hexameric structure, which correspond to a hexagon in the K-space, and the second showing a triangular-shaped 2D FFT spectrum. (d) A 0.72 × 0.6 nm periodicity arrangement is only observed close to the mica terrace-edge regions. (e) At the edges, the ice-like (0.72 × 0.6 nm) structure grows at an angle of ∼72° with respect to the mica edge direction. (f) The range of the growth of this structure is coincident with the size of the protrusion observed at the mica edges. (g) There is a substantially larger growth rate for the ice-like deposits at the edge compared with those in the flat mica regions.

Table 1. Measured and Modeled Structural Data on Water Hexamers Forming a Zigzag Pattern measured parameters (nm) 2D FFT 0.18 0.28 0.45 0.58 0.62 1D FFT 0.72

model parameters (nm) 0.18 0.28 0.44 0.58 0.62 0.72

The FFT data shown in the inset of Figure 7 can be compared with the values measured for the oxygen atom pair distribution for liquid water at 25 °C that is derived from neutron diffraction data, which gives the position correlation between the centers of the molecules in liquid water.28 The neutron diffraction data indicates that nearest neighbors are centered at 0.28 nm and second neighbors at 0.45 nm, close to the values shown in the FFT power spectrum (0.28 ± 0.03 and 0.45 ± 0.04 nm, respectively), which indicates a structural resemblance to liquid water. Extended ice structures are typically assumed to be built from cyclic hexagons. Thus, we first considered models based on hexagonal building blocks. However, when modeling the AFM images using the FFT values, it became clear that the ice-Ih structure could not match the protrusions with a period of 0.72 ± 0.07 nm and a 72° angle between the growing layer and the mica step edges. The 0.72 nm ×0.60 nm zigzag structure inferred by mapping the bright spots in the AFM image could be modeled by formation of an alternative arrangement of the H-bonded networks of water molecules formed by water pentamers. Localization of zigzag structure at the mica edge region is shown in Figure 8. The molecular periodicity of the ice-like deposit is coincident with the zigzag structure with a periodicity of 0.72 and 0.60 nm indicated by a white structure. The top and frontal views of the basic building block are shown in Figure 10a and 10b, respectively. For a group of three interconnected pentamers, only one molecule lies in the scanning plane. The assembly of seven three-fused pentamers results in a structure of large hexamers that can cover extended substrate areas close to the mica edges, as shown in Figure 10c. Figure 10d shows two layers of water pentamers superimposed and perfectly overlapping. The O−O distance measured using FFT was approximately 0.28 nm, which corresponds to liquid water; however, because the pentamer structure is puckered, a part of the water molecule bond appears to decrease from 0.28 to 0.24 nm along the horizontal plane. This apparent decrease in bond length, which corresponds to an upward bend of approximately 30° from the horizontal plane, is represented by the bonds connecting the greenish-blue and blue circles in Figure 10b. The two other water molecule bonds between the gray and the 1557


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Figure 10. continued structure of the cyclic water hexamer forming the zigzag structure observed in AFM images. Measured periodicities in the scanned plane are indicated in the diagram and in agreement with the 2D FFT spectra. Coordinate used here for the nearest (0.285 nm) neighbors is that for liquid water at 25 °C. In Figures 6−9 experimentally determined light dots are indicated in this model by greenish-blue circles. Blue circles indicate water molecules in the middle horizontal plane, and gray circles indicate water molecules at a lower plane. (b) Lateral view of the hexameric type structure builds from a puckered arrangement of pentamers. Formed water hydrogen-bond directions are indicated (∼30°). (c) Top view of the arrangement of seven threefused pentamers (shown in a and b) that form a hexameric structure indicated by a black line. (d) Three-dimensional view of the water structure where two layers of water molecules are superposed, indicating that this structure in compatible with water multilayer adsorption.

blue circles of the pentamers were also bent by approximately 30°, but they were bent downward. The upward and downward bending caused a difference in height of approximately 0.28 nm between the top (greenish-blue circles) and the bottom (gray circles) water molecules. This upward and downward bending results in a parasol-like structure with three molecular levels (Figure 10b). Therefore, three-level hexamers are the building blocks of water clustering close to the edges of mica steps. Periodicities of 0.72 and 0.63 nm were observed in our previous work29 in two distinct structures spatially separated by a transition region: the one with a lattice parameter of 0.63 nm corresponds to ice-lc (cubic) and another showing a lattice parameter of 0.76 nm, similar to ice-II (rhombohedral). Ice-Ic was previously shown to be formed in small clusters as reported by Torchet et al,39 Huang and Bartel,40 and Zhang et al.41 The difference between the structures is shown by a comparison of the FFT power spectra of the image in Figure 7 and the one shown in our previous work29 where there is an arrangement with periodicities of 0.63, 0.76, and 0.52 nm, the last one corresponding to mica (0.52 nm), and in this work with periodicity of 0.18, 0.28, 0.45, 0.58, and 0.62 nm. That work29 then shows a spatially fluctuational network with a periodicity corresponding to ice-Ic-type bonding interlaced with ice-II type, here an arrangement formed by water pentamers. 4.2. Pentameric Structure Interaction with Mica Substrate. Recently,28 using a molecular dynamics simulation, the two-dimensional hydrogen-bond network formed at the surface of muscovite mica was determined. This structure was recently observed on the hydrated surface of muscovite mica.29 The evidence is derived from SFM measurements at finite humidity, which have shown that water condenses into a solid in an angular epitaxial relationship with the underlying mica surface.33 The interaction energy of the substrate with the water molecules at hydrophilic interfaces, such as mica, is stronger than that in the bulk, which results in a hydrogen-bonded structure of water molecules next to the interface. The structure reported here for thick deposits is distinct from the conventional picture of ice-like deposits which consists of water molecules that form a ‘‘puckered hexagonal bilayer’’ residing at two heights above the substrate.42 For deposits closed to mica edges we show an H-bonded network forming a pentagon at three heights above the mica substrate. Figure 11a illustrates the pentamer structures built at three levels in which the intermediate-height water molecules are the bridging sites

Figure 10. (a) Top-view illustration of pentameric superstructure on muscovite built by assembling three pentamers that result in one water molecule in the scanned plane. Top-view predicted equilibrium 1558


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dominant topology in liquid water.18 Therefore, in this sense, interfacial water on hydrophilic substrates may mimic the behavior of bulk water. The most important argument against formation of an extended two-dimensional coverage of the surface by pentagons is the geometric impossibility of covering a surface with pentagons. The model presented in this work overcomes this difficulty by connecting seven pentagons into the hexagonal structure indicated by the full black-line hexagons in Figure 10c which depict two-dimensional images of the water structure. Figure 10d shows a three-dimensional view of two superimposed layers of water pentamers which indicates that the two layers overlap perfectly, and the vertical bond length of the water molecules, as represented by greenish-blue circles, is 0.28 nm. Then the models shown in Figures 10d and 11b, which correspond to the water−structure overlayers and the layer attached to mica substrate, respectively, support the inference that the pentagonal structure can be created both by formation of a water multilayer and by direct adsorption. We find that the optimal hydrogen-bonding arrangement for mica edge coverage is not necessarily the traditional hexagonal 2D bilayer, but rather we observed that a structure forms in which the interaction with the substrate takes place along one line, as determined by the step-edge direction at every four Kion rows as shown in Figure 11b. Indeed, fitting commensurate arrangements of hexagons requires a compression of each hexagon by approximately 1 Å compared with the ideal hexagonal geometry of ice. This finding introduces a destabilizing strain in the overlayers of the hexagon-based structure in Figure 5. In contrast, the arrangement of smaller pentagons shown in this work provides a better fit to the substrate. We provided evidence that the two-dimensional extended area comprises lines of edge-sharing water-molecule pentagons. The preference for the pentagonal ring as an alternative bonding geometry to ice-Ih hydrogen bonding is presented. 4.3. Mica Edges As Efficient Water Condensators. One strategy for producing rain by artificially stimulating water condensation in clouds44,45 consists of using agents that would induce the nucleation of ice and, therefore, water condensation. Among these agents, Vonnegut46 proposed inorganic materials as potential candidates, from a purely geometrical basis, with exposed surfaces exhibiting a hexagonal structure with surface lattice constants close to those of the basal plane for hexagonal ice-lh (4.51 Å at 250 K).47 The existence and properties of the active centers are as important to the nucleation process as the similarity of the structures.48 By imaging water interactions on mica edges, we demonstrate that these constructions are efficient condensation structures. Indeed, the edge adsorbs water strongly, and its nucleating ability is greatly improved as compared with flat mica terraces. Analysis of the atomic structures of the various overlayers reveals that edges are favored over flat mica terraces because the edges allow growth of a pentameric configuration. The substances currently used to seed clouds are chosen for their ability to bind hexagonal ice at the surface; however, in this study, we show that edges are more efficient than flat surfaces for growing ice with a pentameric structure. Therefore, highly edged mica particles could be used as a new material for seeding clouds and causing rain where water pentamers serve as seeds for the ice-like deposits. In Figure 12, we show a construction idealized for growing ice in the atmosphere at 25 °C at 65% RH. This configuration may be fabricated on mica plates by initially

Figure 11. (a) Illustration of pentameric superstructure on muscovite built by assembling three pentamers forming a parasol-like structure that covers K ions (indicated by a green circle). White circles indicate other possible positions of the K ion. (b) Top view of the ice structure at mica edges.

and form the parasol structure shown as green circles. The position of the K ion is shown inside the parasol structure. The two white circles indicate the other possible positions of the K ions. The dipole moments of the water molecules point inward at the K-ion site, and P1−P2−P3 positions, associated with the partial solvation of the potassium ion, are equally favored. A fully connected network of hydrogen-bonded water molecules was generated, except for the molecules forming the cage around the K ions, in which a bond is made with only one of the three water molecules. The network of water molecules consists of undistorted pentagons forming a structure of puckered units that solvate K ion, with three water molecules anchoring each K-ion site. The solvation of the K ion pulls the ion out from the surface equilibrium position.28 The K ions, indicated by large green circles, overlap the parasol-like water structure in one out of every four rows as shown in Figure 11b by the parallel black straight lines and indicating the step-edge direction. In this diagram, small gray circles indicate water molecules in the lower plane that are not in contact with the scanning tip and greenish-blue circles indicate arrangements of water molecules that are in contact with the tip as it scans the substrate. The water pentamer topology described in this work agrees with the puckered-ring configuration that was identified in both experimental and theoretical studies by Liu et al.43 This agreement provides strong evidence that the same cyclic geometry of the water pentamer that was observed for isolated clusters also occurs in condensed-phase environments to form extended structures close to mica edges. In addition, molecular dynamics simulations have suggested that water pentagons are a 1559


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ACKNOWLEDGMENTS

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The authors are grateful to J. R. Castro for technical assistance and to the financial support of CNPq 301.282/2009-9.

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Figure 12. Construction with a pyramidal feature for growing ice in the atmosphere at 25 °C and 65% RH in accordance with parameters measured in this work. Size of the steps was experimentally determined to be ∼100 nm (see Figure 4).

cutting cubes with ∼5 μm sides and subsequently forming steps with dimensions (height and width) that were experimentally determined to be ∼100 nm (see Figure 4). Because the edge effects become negligible for dimensions larger than 100 nm, cubic particles of mica with nanosized flat terraces and pyramidal structures with step heights and widths of 100 nm are desirable.

5. CONCLUSION A combination of scanning probe microscopy at high scanning speeds and structural modeling led to characterization of the ice-like structure built on mica step edges. We propose a zigzag 2D chain overlapping higher spots in the observed images of mica substrates. The chain grows exclusively in the direction forming an ∼72° angle with the mica step edges and shows a structure with a periodicity of 0.72 ± 0.07 nm. These novel structures of water adsorbed on mica are assembled in three distinct levels, which are not based on the puckered hexagonal structure of ice-Ih. Instead, we find that the only possible arrangement is a symmetrical distribution of pentamer building blocks forming a pyramidal parasol structure surrounding the K ions. Pentagon-based ice structures have been observed in other environments,49,18 but this study reports the first time that such a structural unit has been seen on a mica surface. The nucleation of the pentagons reveals an unanticipated adaptability of water−ice films to optimize bonding both within the overlayer and to the mica substrate. The detailed arrangement of these water deposits indicates the importance of the pentamer-like patterns of water on hydrophilic substrates. This discovery led to the proposal of a new mica particles configuration for seeding clouds and causing rain. The substances currently used to seed clouds are chosen to bind hexagonal ice, but this work suggests that mica edges bind to pentagons at a higher rate than mica terraces bind to hexagons. Therefore, mica particles with a pyramidal shape formed by ∼100 nm steps could provide a new material for seeding clouds and causing rain.

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Anisotropic Growth of Water-Puckered Pentamers on a Mica Terrace

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