J. Phys. Chem. C 2008, 112, 7605–7610
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Atomic Force Microscopy Study of Self-Assembled Sodium Chloride Nanocrystallites and Their Morphology Transitions Ji Hong Wu, Siau Gek Ang, and Guo Qin Xu* Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Singapore ReceiVed: January 15, 2008; ReVised Manuscript ReceiVed: March 7, 2008
Identically oriented NaCl nanocrystallites have been grown on mica surfaces based on a facile solutionevaporated method. With an increase in humidity, the morphology of the NaCl nanocrystallites evolves in succession from triangular pyramids to cubic islands and then to long lines with lengths of up to millimeters. These nanocrystallites can self-assemble into highly ordered arrays with large spatial extents of ∼10 mm2 under high humidity conditions when the value of relative humidity is higher than 40%. Their morphology transitions and self-assemblies can be understood in terms of controlled epitaxial crystallization together with water adsorption at the surface of the growing nanocrystallites. Introduction Bottom-up self-assembly, being cost-effective and easy-tohandle, is predominantly used in the construction of functional nanostructures with emergent or amplified properties which otherwise are absent in the original building blocks.1–3 The elementary building blocks of self-assembly can be small molecules4,5 biomacromolecules,6,7 polymers,8,9 or even presynthesized materials such as nanoparticles, nanorods, and nanowires.10–13 Reported driving factors for self-assembly include both intermolecular interaction14–17 and external forces, for instance, fluidic, electric, and magnetic fields,18–23 etc. Here we report a novel self-assembly behavior of identically oriented NaCl nanocrystallites into large-scale and highly ordered arrays. The crystal growth in our study takes place within a two-dimensional solution layer under widely varying conditions. When the humidity is increased, we observed the development of the NaCl nanocrystallites from triangular pyramids to cubic islands and subsequently to long lines with lengths of up to millimeters. Interestingly, these NaCl nanocrystallites can spontaneously organize into highly ordered arrays with various spatial arrangements and exceptionally large spatial extents (up to ∼10 mm2). This allows us to simultaneously examine the epitaxial orientation, the morphology transition, as well as the self-assembly behaviors of the NaCl nanocrystallites under a wide span of conditions. Epitaxial crystallizations are usually confined to systems with definite limits of lattice mismatches between the deposits and the substrates. NaCl is a member of alkali halides, and most alkali halides have the NaCl-type crystal structure at room temperature.24 The epitaxial growth of alkali halides on mica is appealing since it presents robust lattice mismatches through a choice of different alkali halides. A common feature of all these epitaxial alkali halides is that they primarily form oriented triangle-pyramidal islands with the (111) basal faces in contact with the (001) face of the mica substrate. Lamelas et al.25 have reported that a larger lattice mismatch between alkali halide and mica leads to a lower nucleation density as well as a smaller span of conditions for the epitaxial growth. Therefore, it is not strange that previous studies largely focused on the alkali halides * To whom correspondence should be addressed. E-mail: chmxugq@ nus.edu.sg. Fax: (65) 6779 1691.
with small lattice mismatches to mica substrates,25–38 such as RbI (0%), KI (-4%), RbBr (-7%), and KBr (-10%). Though the lattice mismatch between NaCl and mica is as large as -23%, it has been found in our study that several kinds of oriented NaCl nanocrystallites, with external shapes other than triangular pyramid, have been grown on mica surfaces, and the condition span for the oriented growth was rather wide. In comparison with previously reported deposition of NaCl on mica,38–40 our study provides a profound insight into the crystallization behavior of NaCl on mica under controlled condition. Experimental Section Crystal Growth. The growth of NaCl nanocrystallites on mica was based on the evaporation of NaCl aqueous solution. The NaCl solution was prepared by dissolving NaCl powder (Merck, g99.5%) in Milli-Q water (Millipore, g18 mΩ cm-1). To ensure that the crystallization of NaCl takes place only on the mica surface and not in solution, low concentrations varying from 0.25 to 2 wt % (far below the saturation concentration) were used in our experiments. For crystal growth, a solution droplet was first pipetted onto a fresh cleaved mica surface and then quickly removed by nitrogen flow. Such a process results in the formation of a thin solution layer on the mica surface, which was subject to rapid evaporation in ambient air. All experiments were carried out at room temperature. The humidity was controlled with the dehumidifier. The value of the relative humidity (RH) was recorded with a portable digital hygrothermometer with (5% accuracy. Atomic Force Microscopy (AFM) Characterization. The morphologies of the NaCl nanocrystallites were examined using AFM (Nanoscope IIIa, Veeco, USA) under ambient condition and at low humidity level (RH < 35%). The images with large scan areas were collected in tapping mode with a “J” scanner and a silicon cantilever (force constant: 0.42 N/m, Nanoworld). On the other hand, images with atomic resolution were achieved in contact mode with a “A” scanner and an oxide-sharpened silicon nitride cantilever (force constant: 0.12 N/m, Veeco). Results Figure 1a shows the NaCl islands grown from a 0.25 wt % solution under a RH of 36%. All the islands are in triangle-
10.1021/jp800358s CCC: $40.75 2008 American Chemical Society Published on Web 04/23/2008
7606 J. Phys. Chem. C, Vol. 112, No. 20, 2008
Wu et al.
Figure 1. (a) AFM height image of NaCl triangle-pyramidal islands grown from a 0.25 wt % solution under a RH of 36%. The inset is the corresponding phase image. (b) AFM height image of NaCl wedged islands from a 0.25 wt % solution under a RH of 56%. The inset is the corresponding phase image. The scale bars represent 1 µm.
pyramidal shape with uniform orientation on the mica surface. Their apexes and side faces are clearly revealed in the phase image (the inset of Figure 1a). Our observation is consistent with the previous studies under vacuum conditions.38,39 When the RH is increased to 56%, NaCl islands with a quasicubic appearance and uniform orientation are formed on the mica surface from the same solution, as shown in Figure 1b. Their uniform orientation distinguishes them from the randomly oriented nonepitaxial ones, suggesting that they are also originated from epitaxy. However, their appearance indicates that the high humidity conditions have resulted in morphology transition during the growth of these NaCl nanocrystallites. Figure 2 gives more details about the morphology transition. At the initial stage of the transformation, the triangle-pyramidal island is modestly elongated (indicated by the arrow in Figure 2a). Subsequently, a new oblique, rectangular face appears, as indicated by the dotted rhombic frame in Figure 2b. One edge slightly expands outward at its center, which eventually develops into a new corner (R). At the same time, the three 60° corners of the original triangular pyramid are also growing slowly. Finally, all four corners become orthogonal with the newly appeared face parallel to the substrate surface, i.e., the original triangular pyramid has converted into a cube (Figure 2d). Of particular interest is that under high humidity conditions (RH > 40%), these NaCl islands can self-assemble into largescale and highly ordered discrete line arrays with the lines either being parallel to each other or making an angle between one another. The acute angle formed by two discrete lines will hereafter be referred to as the “orientation angle”. The value of the orientation angle largely depends on the humidity under
Figure 2. AFM phase images of NaCl islands prepared under RHs of (a) 42%, (b) 52%, (c) 56%, and (d) 58%. The inset images are the corresponding AFM height images. The concentration in all cases is 0.25 wt %. The scale bars represent 200 nm.
which the NaCl islands were prepared. As shown in parts a-c of Figure 3, the angles are 60, 82, and 90° under RHs of 42, 52, and 56%, respectively. On the other hand, no long-rangeordered structures were observed under low humidity conditions (RH < 40%) (data not shown).
AFM Study of Self-Assembled NaCl Nanocrystallites
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Figure 3. AFM height images of (a) NaCl discrete line arrays with an orientation angle of 60° prepared under a RH of 42%, (b) NaCl discrete line arrays with an orientation angle of 82° prepared under a RH of 52%, and (c) NaCl discrete line arrays with an orientation angle of 90° prepared under a RH of 56%. The concentration in all cases is 0.25 wt %. The scale bars represent 5 µm.
A further increase in humidity results in the formation of long NaCl lines with lengths of up to millimeters. However, the NaCl lines grown under this low concentration (0.25 wt %) condition are usually denticulate (data not shown). Instead, very straight and smooth lines with width varying from ∼100 to ∼1600 nm can be obtained from solutions with higher concentrations (Figure 4). The long NaCl lines can also self-assemble into large-scale and highly ordered arrays, in which they are exclusively oriented along two perpendicular directions. The spatial extents of these continuous line arrays are much larger than those of the discrete line arrays, implying a higher stability of the former structure. In many cases, the mica slices with 5 × 5 mm2 size are fully covered with the highly ordered continuous line arrays. It is also interesting to note that the length of NaCl lines along the two perpendicular directions is adjustable upon changing the experimental conditions. As shown in Figure 4a, the NaCl lines grown from a 0.5 wt % solution under a RH of 72% have equivalent lengths along both directions, indicating homoge-
Figure 4. AFM height images of NaCl continuous line arrays. Growth conditions: (a) concentration ) 0.5 wt %, RH ) 72%; (b) concentration ) 0.5 wt %, RH ) 67%; (c) concentration ) 1 wt %, RH ) 72%; (d) concentration ) 2 wt %, RH ) 72%. The scale bars represent 20 µm.
neous growth along both directions. However, after dropping the RH to 67%, the growth becomes more prominent along one direction (Figure 4b). Similar results can also be obtained by increasing the NaCl concentration to 1 wt % under the same RH (Figure 4c). When the concentration is further increased to 2 wt % under a RH of 72%, large areas covered with the NaCl
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Figure 5. (a) Orthogonal ends of NaCl long lines; (b) Fourier filtered atomic resolution AFM deflection image of NaCl long lines; (c) cross section profile of part b. Growth condition: concentration ) 0.5 wt %, RH ) 72%. The scale bar represents 1 µm.
lines oriented along one same direction can be obtained (Figure 4d), implying that the growth along the other direction has been completely inhibited. Close-up views reveal that the NaCl lines have orthogonal ends (Figure 5a), suggesting that they might be (100) oriented. The atomic resolution achievable from the AFM deflection image of the NaCl lines (Figure 5b) exhibits a regular array of protrusions with a 4-fold rotation symmetry and a periodicity of 0.41 nm, further confirming the (100) orientation. Discussion Morphology Transition: From Separated Islands to Continuous Lines. The epitaxial crystallization of NaCl on mica discussed in this paper involves two major steps: the epitaxial nucleation at the solution/mica interface and the subsequent crystal growth within the thin solution layer. At the very beginning, triangle-pyramidal islands are formed upon the epitaxial nucleation of NaCl at the solution/mica interface. The NaCl concentration in the solution is not homogeneous over the surface of the epitaxial islands but highest at their corners and lowest at the centers of their facets.41 The islands are likely to be deformed as the growth evolves into diffusion-controlled growth with crystal size getting larger.42 Under low humidity conditions, the nucleation density is high
Wu et al. and the average crystal size is small since the dimension of the thin solution layer is finite and the supply of NaCl solute is therefore limited. These small islands can keep their trianglepyramidal shape during the growth as the inhomogeneity in the concentration can be compensated by the variation in the kinetic coefficient,43 i.e., the lower step density near the corners of these small islands and the higher step density at their facet centers ensure the uniform growth rate over the entire surface of the growing islands. Under higher humidity conditions, however, the nucleation density is smaller and the crystal size on average is larger. It is not possible to keep the growth rate uniform all over the surface of a large, growing island. Consequently, as the growth continues, the growth at the corners that are exposed to higher concentration becomes faster than the growth at the centers of the facets. This explains the modest elongation of the NaCl islands under the RH of 42% (Figure 2a). Further growth results in the apexes of the NaCl islands protruding out of the solution layer as the solution evaporates and becomes thinner than the growing islands. Consequently, the water vapor from the ambient air adsorbs at the apexes.44–48 The study carried out by Salmeron et al.49 found that water adsorption can cause solvation at the surface of NaCl crystals. At low humidities (RH < 40%), the adsorbed water only induces the solvation of the cations (Na+).49 The solvated cations are preferentially accumulated at the steps. The changes caused by water adsorption are reversible upon drying under this low humidity condition. At high humidities (RH > 40%), however, the adsorbed water induces the solvation of both the ionic species (Na+ and Cl-). The highly mobile solvated ions run down along the oblique faces and re-enter the evaporating solution layer, which in turn influences significantly the morphology evolution. First, it increases the ionic concentration in the solution contacting the edge centers of the NaCl islands, therefore facilitating faster growth and thus developing new corners there. Second, the loss of ions also results in the truncation of their apexes. Higher humidity causes larger amount of ions to re-enter the solution and in turn more dramatic changes in the morphology of the NaCl nanocrystallites (Figure 2). The NaCl islands finally evolve into cubes under high humidity condition because the cubic NaCl terminated with large (100) faces has the lowest surface energy.50 On the other hand, along with the shrinkage in their heights, further growth at their basements also shortens the distance between the separated islands. Under a much higher humidity, it can even lead to the coalescence of the originally separated islands to form long continuous lines. Our off-line data (Figure 6) reveals that the average height of the continuous lines is only about one-fourth of that of the separated islands, strongly supporting the validity of the coalescence mechanism. In summary, the final morphology of NaCl nanocrystallites on the mica surface is controlled by two simultaneous processes, the epitaxy of NaCl on mica and the morphology transition during the crystal growth. At low humidities, the dominant process is epitaxy which favors the triangle-pyramidal form. On the other hand, morphology transition favors the stable cubic form at high humidities. Self-Assembly of NaCl Nanocrystallites. The network shown in Scheme 1 represents the pseudohexagonal surface lattice of the mica substrate. The triangles represent the NaCl epitaxial triangle-pyramidal islands. Berg reported in the study of crystal growth from solution that the solute concentration in contact with the crystal varies from the highest at the corners to the lowest at the centers of the facets.41 The oriented nuclei formed during the primary nucleation therefore induce the
AFM Study of Self-Assembled NaCl Nanocrystallites
J. Phys. Chem. C, Vol. 112, No. 20, 2008 7609 SCHEME 1: Schematic Models Showing (a) the Directed Concentration Gradients and the Discrete Line Array with an Orientation Angle of 60° under the Low Humidity Conditions and (b) the Directed Concentration Gradients and the Discrete Line Array with an Orientation Angle of 90° under the High Humidity Conditionsa
Figure 6. (a) AFM image of NaCl separated islands and continuous lines on mica. Growth conditions: concentration ) 0.25 wt %, RH ) 58%. The scale bar represents 5 µm. (b) Cross section profile of part a: the mean height of the continuous lines (as shown in the below curve) is only about one-fourth of that of the separated islands (as shown in the above curve). The scale bar represents 10 nm.
concentration gradient in the thin solution layer, which in turn dominate the secondary nucleation. At low humidities, the concentration gradient is addressed along the oriented corners of the triangle-pyramidal islands (indicated by the arrows in Scheme 1a). The secondary nucleation is mainly restricted in these solution regions with high concentration, resulting in the formation of discrete line arrays with an orientation angle of 60°. Along with the morphology transition, the distribution of concentration gradients in the thin solution layer is also changing. At high humidities, the concentration gradient is directed along the diagonal directions of the cubic islands (indicated by the arrows in Scheme 1b). The secondary nucleation is mainly restricted in these solution regions with high concentration, resulting in the formation of discrete line arrays with an orientation angle of 90°. Likewise, a similar consideration can also be employed for the cases when the NaCl islands take the intermediate forms. The as-assembled arrays have orientation angles between 60° and 90°. On the basis of the above discussion, it is clear now that the self-assembly of the oriented NaCl nanocrystallites is due to the directed concentration gradient in the thin solution layer. A slow evaporation favors the growth of large-scale and highly ordered crystalline arrays, since it allows more time for the formation of concentration gradient in the thin solution layer. This is in agreement with our observation that the NaCl crystalline arrays formed under higher humidity conditions have larger spatial extents. The fact that ordered crystalline arrays were not observed under the RH of 36% might be due to the extremely fast evaporation at this low humidity level which leaves no time for the building of concentration gradients in the solution. Conclusions In conclusion, large-scale and highly ordered NaCl crystalline arrays have been grown on mica surfaces via a facile solution-
a The network represents the pseudohexagonally arranged potassium ion framework in the mica surface.
based strategy. The epitaxial orientation, the morphology transition of the NaCl nanocrystallites, as well as their selfassembly behaviors were investigated in detail under a variety of experimental conditions. Our study provides a profound understanding of the crystallization behavior of NaCl on mica under controlled conditions. The results of our study present the possible mechanisms for the morphology transition induced by water adsorption at the surface of the growing NaCl nanocrystallites and for the formation of the NaCl crystalline arrays promoted by the directed concentration gradient in the thin solution layer. In addition, the NaCl arrays can further be transferred into other solid thin films which we are going to discuss in our next paper. We believe our method may offer a simple and feasible approach for the development of a new soft lithography approach. Acknowledgment. We acknowledge the financial support from the Ministry of Education, Singapore, Grant No. R-143000-250-112. Ji Hong Wu also acknowledges the award of graduate scholarship from National University of Singapore.
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