Article pubs.acs.org/crystal
Topographical Control of Crystal Nucleation J. L. Holbrough,† J. M. Campbell,† F. C. Meldrum,‡ and H. K. Christenson*,† †
School of Physics and Astronomy and ‡School of Chemistry, University of Leeds, Leeds LS2 9JT, United Kingdom ABSTRACT: Surface topography is here investigated as a route to controlling crystal nucleation. The nucleation from vapor of crystals of neo-pentanol and tetrabromomethane was studied on flat surfaces of glass and mica, and on identical substrates scratched with diamond powders of varying particle size. The result is a study which presents a systematic comparison of the nucleating ability of surfaces with the same chemistry and wettability, but varying surface topography. An increase in nucleation density of up to an order of magnitude was observed on scratched mica surfaces compared to unscratched ones, and there was a decrease in the induction time by up to 60%. Larger diamond particles led to enhanced effects, particularly on the nucleation density. Although the nucleation density and induction time on unscratched glass were similar to those on mica, the surface scratches on glass had no significant effect on nucleation density or induction time. The results suggest that a high density of nanoscale features of the surface topography, such as those produced as the diamond particles fracture the mica, is necessary for an enhancement of nucleation. The apparent length scale of the topographical features on mica is discussed with reference to classical nucleation theory and other models. These results show that both a quantitative reduction in induction time and an increase in nucleation density can be achieved as a result of mechanically produced topographical surface defects, which suggests that the engineering of nanoscale topographical features has real potential for control of heterogeneous nucleation.
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INTRODUCTION Crystallization lies at the heart of a vast array of natural phenomena and technological processes, including the production of pharmaceuticals, nanoparticles, and biomaterials, and processes such as weathering, biomineralization, and formation of ice in the atmosphere. While methods exist to modify the form of a crystal during growth, it is control at the earliest stages at nucleation which is essential for definition of features such as the polymorph, orientation, particle size, and size distribution. Yet, our understanding of nucleation events is poor, and although it is known that growth of a crystal almost always occurs via heterogeneous nucleation, the solid substrate is usually considered to be a perfectly flat surface, which is likely to provide a good model for real surfaces only in the most select situations. In seeking methods for controlling nucleation, the focus has been placed almost entirely on the interaction of molecules/ ions with a developing nucleus. Thus, soluble additives are used to control nucleation in bulk solution,1 while substrates patterned with different functional groups have provided templates for the formation of two-dimensional (2D) arrays of crystals.2,3 Although effective in many systems, this approach suffers from the drawback that additives can be highly specific to a particular crystal system, and that nucleation of some substances such as protein crystals is not readily controlled using additives. There is an alternative approach, which although highly effective, has received relatively little recognition the control of crystal nucleation using surface topography. The effect of surface topography on nucleation has been more closely considered chiefly in three principal areas: the nucleation of water and ice on aerosol particles in the atmosphere,4,5 the © 2011 American Chemical Society
growth of diamond films by chemical vapor deposition (CVD),6−9 and in the crystallization of proteins.10−12 The atmospheric science literature has considered the possibility that undercooled liquid could capillary condense in surface pits and cracks of aerosol particles, and then freeze.5 Deposited material would remain in the cavities as the vapor pressure falls below saturation at elevated temperatures and provide “active sites” for nucleation when the system again becomes supersaturated as it is cooled. Substrates used for CVD growth of diamond films are routinely scratched with abrasive powders of diamond, or other very hard materials such as BN or SiC.7 However, the nucleation of diamond on silicon is believed to involve rather specific mechanisms on an atomic scale, and the wider relevance of these results is uncertain.8,9 Finally, proteins are notoriously difficult to crystallize, and disordered porous media have been found to be very effective in many cases.10−12 Finding effective protein nucleants is an important quest which can enable determination of their structure by X-ray diffraction. The potential generality of using topographically defined materials as nucleating agents is also supported by scattered evidence in the literature. For example, a less preferred polymorph of an organic salt has been grown by exploiting the orienting potential of a ledge on the substrate.13 It has been shown that nanoscale surface pits in BaF2 crystals can lead to enhanced nucleation rates of ice from water,14 while nucleation of Ge quantum dots occurs preferentially in nanoscale pits formed by focused ion-beam (FIB) milling on Si(001) surfaces.15 A positive correlation between nucleation density Received: August 18, 2011 Revised: December 15, 2011 Published: December 15, 2011 750
dx.doi.org/10.1021/cg201084j | Cryst. Growth Des. 2012, 12, 750−755
Crystal Growth & Design
Article
1530 FEG-SEM; the coarsest diamond powder was imaged with an optical microscope (Olympus BX41). The crystal growth was observed at 50× and 100× magnification with the optical microscope, and images were taken using a 2/3 in. CCD camera (Olympus XC-ST70CE, 752 × 582 pixels, 256 gray scale levels) mounted on the microscope. The induction time was measured from first contact between the preheated resistor and the copper plug to the first appearance of a crystal visible in the microscope. The crystals were left to grow for a further 5 min before four images of random areas of the substrate were recorded for nucleation density analysis. Each experiment was repeated three times with different samples, and four areas of each were sampled to obtain the nucleation density. Two separate mica samples, one unscratched and one scratched with the 40−60 μm diamond powder, were used to study in more detail the variation in induction times. Eight separate induction time measurements were carried out with each sample, looking at a different area each time, and the observed induction time distributions of 210 ± 25 s (unscratched) and 70 ± 20 s (scratched) were used to estimate minimum errors in the induction times in other experiments. The temperature of the copper plug TCu and the temperature of the glass substrate Tgl were measured as a function of time after the preheated resistor was placed in contact with the copper plug, using two different voltages. From the temperature difference a value of the relative saturation S was estimated from the Clausius−Clapeyron equation:
and fractal dimension (used as a measure of surface roughness) has been found for nucleation of water droplets from vapor on Mg surfaces.16 There are also results showing that media containing sub-micrometer pores which are heterogeneous in size and shape act as effective nucleating agents for a range of materials including calcium phosphates,17 proteins,10 and semiconductors.18 The effect of confinement in pores on polymorph selectivity of pharmaceuticals has been studied.19,20 Computer simulations have shown enhanced nucleation in pores,21,22 and very recently, investigations of the effects of both surface chemistry and nanopore shape on the nucleation of simple organic molecules from solution have been carried out.23−25 However, despite the above examples, there is a lack of systematic and quantitative studies of the effect of surface topography on crystal nucleation. In this paper we take a step in this direction by carrying out a simple study of nucleation on surfaces scratched with diamond particles and demonstrate conclusively that even a crude method such as scratching with abrasives can greatly enhance nucleation. Our method employs simple organic substances with relatively high vapor pressures at room temperature, namely, neo-pentanol (2,2-dimethylpropanol) and tetrabromomethane. Crystals nucleating from vapor provide a simple system as there is no solvent to complicate the kinetics of crystallization, there can be no confusion between nucleation on the surface and on impurity particles, and observation in the presence of a rarefied background phase (vapor and air) is easy. The substrates selected are freshly cleaved muscovite mica and glass (microscope slides), the smoothness of which means that the only significant surface defects are the scratches made by the diamond particles. The choice of surfaces and nucleating substances also ensures that no specific interactions are involved and geometry and topography should be the only controlling factors.
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⎡p ⎤ ⎞ ΔHsub ⎛⎜ 1 1 ⎟ ln S = ln⎢ Cu ⎥ = − − ⎢ p ⎥ R ⎜⎝ TCu Tgl ⎟⎠ ⎣ gl ⎦
(1)
where ΔHsub is the enthalpy of sublimation, and pCu, pgl are the vapor pressure at the source crystal and substrate, respectively. As TCu increased slightly over the time scale of the experiment the error in S is about ±0.1. Tgl did not change significantly from its starting temperature, 22 °C. The values of ΔHsub used were 54 kJ mol−1 for CBr426 and 52 kJ mol−1 for neo-C5OH.27 Values of S cited subsequently are those estimated at the observed induction time.
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RESULTS Figure 1 shows a representative example of neo-C5OH crystals deposited on unscratched mica (Figure 1a) and mica scratched with the 10 nm diamond powder (Figure 1b) at S = 1.5 ± 0.1. On the smooth surface there are only a few large crystals, while the scratched surface has nucleated numerous crystals. The dendritic growth forms are characteristic of the moderately high supersaturation used. There was no obvious difference in the morphology between the crystals growing on mica and on glass, and no sign of epitaxy on the mica. The nucleation densities and induction times were measured for both glass and mica surfaces scratched with the three diamond grit sizes at two relative saturation levels, S = 1.3 ± 0.1 and S = 1.5 ± 0.1 for neo-C5OH, and at S = 1.5 ± 0.1 only for CBr4 on mica. The results are plotted in Figures 2 and 3, respectively, where each point is the mean value from each of three separate experiments. Newly scratched substrates were employed for each experiment. For the nucleation densities, four areas of each substrate were analyzed, such that each point in the figures is derived from 12 images (where the number of crystals on the images varied from 0 to 38). The error bars are standard deviations. The nucleation density shows a very significant increase with scratched mica surfaces compared to unscratched surfaces, both for neo-C5OH and for CBr4. neo-C5OH shows a larger increase for the higher saturation, and a much larger nucleation density than for CBr4. In all three cases, the increase is monotonic with the diamond particle size, and the difference compared to the
MATERIALS AND METHODS
A small Teflon cell (inner dimensions 4 × 2 × 1 cm) was constructed for in situ observations of crystal growth from vapor. The cell was sealed with a glass microscope slide in a Teflon frame and made airtight with a neoprene O-ring. A small (1 cm) copper plug at one end of the bottom of the cell served to conduct heat from an external resistor (RS-HS10) connected to a variable transformer. A small crystal (50−100 mg) of neo-pentanol (neo-C5OH, Aldrich 99%) or tetrabromomethane (CBr4, Aldrich, 99%) was placed on the copper plug and heated without allowing it to melt. As the vapor pressure in the cell rose, crystals would form on the underside of the glass slide (or on a mica strip under the glass), and these could be observed or imaged from above through a microscope. The temperature of the copper plug and the glass slide were measured using thermocouples and a data controller (Pico TC-8). The microscope slides (76 × 25 mm) were soaked in 1% DECON in Millipore water and ultrasonicated for 30 min before rinsing in Millipore water, ethanol, and drying with nitrogen. The slides were polished with diamond powder using a fingertip in a clean nitrile glove for 5 min to ensure an evenly scratched surface, and the slides were again rinsed with Millipore water, ethanol, and dried with nitrogen. The mica (Paramount Corp, N. Y.) was cut to the same size as a microscope slide, cleaved as thinly as possible, scratched and rinsed in the same way as the glass slides and then mounted directly underneath the glass slide lid. Three kinds of synthetic diamond powder were used, with the following particle size designations:
