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(CH3)3SiCl/SiCl4 Azeotrope Grows Superhydrophobic Nanofilaments Lichao Gao and Thomas J. McCarthy* Polymer Science and Engineering Department, UniVersity of Massachusetts, Amherst, Massachusetts 01003 ReceiVed August 15, 2007. In Final Form: NoVember 14, 2007 We describe the vapor-phase reaction (at room temperature and 40-45% relative humidity) of silicon wafers with the azeotropic mixture of trimethylchlorosilane and tetrachlorosilane. Water contact angle analysis indicates that surfaces become nearly perfectly hydrophobic (θA/θR g176°/g176°) after 2 min of reaction. SEM analysis at various reaction times shows the growth of nanofilaments with diameters of ∼30 nm. X-ray photoelectron spectroscopy of oxidized titanium surfaces that were exposed to the azeotrope vapor indicates that the product is derived from a ∼10:1 ratio of SiCl4 and (CH3)3SiCl. A mechanism for filament growth is proposed.
The hydrophobic nature of certain natural solids and the hydrophobization of solids that are not naturally hydrophobic were topics of chemical research interest ∼60 years ago. In 1944, Fogg reported1 the unusual contact angle behavior of mustard and wheat leaves. Cassie and Baxter commented2 on this report in 1945 and described the mirrorlike reflection of the broccoli leaf-water interface and that the advancing and receding contact angles of duck feathers are “both around 150°.” Imparting water repellency to textiles in ways to retain the fabric’s permeability to air and water vapor was an active research field (hundreds of reports) in the 1940s.3-5 Early in that decade, researchers at the General Electric Company (GE) discovered that mixtures of methylchlorosilanes (MenSiCl4 - n) could be prepared from elemental silicon and chloromethane and that these mixtures could be used to hydrophobize surfaces.6,7 Similar work8 on hydrophobization was carried out at Corning Glass Works in directions that helped spawn Dow Corning. Interest in this topic has been renewed9 in the current decade, and there is now an extensive recent body of literature10-13 of various methods of imparting superhydrophobicity to surfaces.14 We recently reported15 that methyltrichlorosilane reacts with silica surfaces, under appropriate reaction and workup conditions, to render surfaces perfectly hydrophobic. Water contact angles of this surface were measured to be θA/θR ) 180°/180°. Seeger et al. reported16 that MeSiCl3 reacts in the vapor phase with a number of surfaces to impart superhydrophobicity. Static water contact angles of 150-170° were reported. Electron microscopy * Corresponding author. E-mail:
[email protected]. (1) Fogg, G. E. Nature 1944, 154, 515-515. (2) Cassie, A. B. D.; Baxter, S. Nature 1945, 155, 21-22. (3) Schuyten, H. A.; Reid, D. J.; Weaver, J. W.; Frick, J. G. Text. Res. J. 1948, 18, 396-415. (4) Schuyten, H. A.; Reid, D. J.; Weaver, J. W.; Frick, J. G. Text. Res. J. 1948, 18, 490-503. (5) Bartell, F. E.; Purcell, W. R.; Dodd, C. G. Discuss. Faraday Soc. 1948, 3, 257-264. (6) Rochow, E. G. J. Am. Chem. Soc. 1945, 67, 963-965. (7) Patnode, W. I. U.S. Patent 2,306,222, December 22, 1942. (8) Hyde, J. F. U.S. Patent 2,439,689, April 13, 1948. (9) Gao, L.; McCarthy, T. J. Langmuir 2007, 23, 3762-3765. (10) Gao, L.; McCarthy, T. J. Langmuir 2006, 22, 5998-6000. (11) Sun, T.; Feng, L.; Gao, X.; Jiang, L. Acc. Chem. Res. 2005, 38, 644-652. (12) Gao, L.; McCarthy, T. J. Langmuir 2006, 22, 2966-2967. (13) Que´re´, D. Rep. Prog. Phys. 2005, 68, 2495-2532. (14) The scant citations to earlier work in the literature demonstrate that there is a disconnect between what was of interest in the 1940s and the recent research on superhydrophobicity. Publication and patenting of much of the GE and Corning research were delayed or never occurred because of World War II, and the reports that eventually appeared did not report contact angle data. (15) Gao, L.; McCarthy, T. J. J. Am. Chem. Soc. 2006, 128, 9052-9053. (16) Artus, G. R. J.; Jung, S.; Zimmermann, J.; Gautschi, H.-P.; Marquardt, K.; Seeger, S. AdV. Mater. 2006, 18, 2758-2762.
revealed that nanoscopic filaments are formed, and it was suggested that the filaments are composed of polymethylsilsesquioxane. Veinot et al.17 reported that vinyltrichlorosilane deposited on silicon wafers also forms nanofibers and reported an advancing water contact angle of 137°. Patnode’s 1942 GE patent7 did not claim mixtures of SiCl4 and methylchlorosilanes as hydrophobizing agents, and it was likely later when Norton18 (also at GE) recognized that a particular one of these is special and claimed that (CH3)3SiCl and SiCl4, which form a minimum-boiling azeotrope that boils (54.5 °C) ∼3 °C below the two pure components, react with a variety of solids as a vapor mixture to impart water repellency. This was a surprising result and, in retrospect, still is. Norton comments18 that “although pure silicon tetrachloride does not itself confer water-repellent properties to surfaces treated therewith and, of all the known organosilicon halides, pure trimethylsilicon chloride is probably the least effective insofar as this property is concerned, compositions containing both of these chlorosilanes do confer excellent water repellency to surfaces.” We report here reproductions of the experiments using this azeotropic mixture and characterization of the resulting surfaces by contact angle, ellipsometry, scanning electron microscopy, and X-ray photoelectron spectroscopy. The vapor-phase reactions of (CH3)3SiCl and SiCl4 as individual reagents have been studied in some detail. SiCl4 reacts with silicon wafers to form hydrophilic and reactive silica.19 Silica can be “grown” in a controlled step-growth manner by cyclic exposures to air and SiCl4. (CH3)3SiCl vapor reacts with silicon wafers by random covalent attachment that, unknown to Norton, slows significantly in the later stages to yield a monolayer with water contact angles of θA/θR ) 108°/96°.20 We carried out vapor-phase reactions with a mixture of these two compounds (50:50 vol/vol, 52.6:47.4 mol/mol) under controlled humidity conditions at room temperature. Humidity was controlled in a glove bag using an open container of aqueous sodium chloride. It was found that maximum contact angles were obtained at intermediate values of the humidity range studied (10-75% relative humidity). We examined the kinetics of the deposition process at 40-45% relative humidity at room temperature (∼23 °C). The exposure time was controlled by introducing and (17) Rollings, D. A. E.; Tsoi, S.; Sit, J. C.; Veinot, G. C. Langmuir 2007, 23, 5275-5278. (18) Norton, F. J. U.S. Patent 2,412,470, December 10, 1946 (application February 22, 1943). (19) Jia, X.; McCarthy, T. J Langmuir 2003, 19, 2449-2457. (20) Fadeev, A. Y.; McCarthy, T. J Langmuir 1999, 15, 3759-3766.
10.1021/la7025297 CCC: $40.75 © 2008 American Chemical Society Published on Web 12/19/2007
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Figure 1. Water contact angles (2, θA; 1, θR) and ellipsometric thickness (9) of (CH3)3SiCl/SiCl4 azeotrope-derived surface layers.
removing silicon wafer samples to and from a covered crystallizing dish containing an open vial of the mixture of (CH3)3SiCl and SiCl4. Samples were either rinsed with copious toluene, ethanol, ethanol/water (1:1), and water and then dried at 120 °C for 10 min or simply rinsed with copious amounts of water and dried in air. No differences between surfaces rinsed by these two procedures were noted. Figure 1 shows both thickness (determined by ellipsometry21) and water contact angle data for the samples versus time of exposure to the azeotrope vapor. After 2 min of exposure, samples exhibit contact angles of θA/θR g176°/ g176°.22 The thickness of the deposited layer increases with reaction time and is ∼20 nm after 10 min of exposure to the azeotrope vapor. Water droplets do not come to rest on these surfaces (sliding angle is 0°) and move spontaneously in every direction off of horizontal samples. Contact angles of this magnitude are rare12,15,23-25 and require special topography that would cause a three-phase contact line, were it to form and be stable, to be extremely contorted and discontinuous. Water contacts surfaces such as these without a contact line being formed (θA ) θR ) 180°), or if one forms, it is dynamic and hysteresis disappears.26 Figure 2 shows SEM images of modified silicon wafers that were exposed to the (CH3)3SiCl/SiCl4 azeotrope vapor for different times. Contorted filaments with diameters of ∼30 nm grow in an apparent 1D chain-growth fashion from nucleation sites that are visible after 30 s of exposure to the azeotrope vapor. The filaments observed after 10 min of exposure/reaction are very similar in structure to the methyltrichlorosilane/water-derived polymethylsilsesquioxane nanofilaments prepared by Seeger16 and the vinylsiloxane nanofilaments prepared by Veinot.17 Seeger reports that “the mechanism for the formation of filaments is presently unknown” and does not propose one. No evidence of a silsesquioxane structure is reported. The fact that (CH3)3SiCl and SiCl4 react with water to form filaments suggests that a silsesquioxane structure is not important for filament formation and questions whether the methyltrichlorosilane/water-derived surfaces have a silsesquioxane structure. Veinot proposes “a reasonable mechanism of fiber formation” (21) Calculations from ellipsometry data were made as described before.29 These “thickness values” assume smooth surfaces and so are meaningful only in terms of relative amounts of the product formed. (22) Contact angle measurements were made with a Rame`-Hart telescopic goniometer equipped with a Gilmont syringe and a 24-gauge flat-tipped needle. (23) Gao, L.; McCarthy, T. J. Langmuir 2007, 23, 9125-9127. (24) See Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; O ¨ ner, D.; McCarthy, T. J. Langmuir 1999, 15, 3395-3399 for a discussion of and references to earlier reports of extremely hydrophobic surfaces. (25) Gao, L.; McCarthy, T. J. J. Am. Chem. Soc. 2007, 129, 3804-3805. (26) Gao, L.; McCarthy, T. J. Langmuir 2006, 22, 6234-6237.
Figure 2. SEM images of silicon exposed to the (CH3)3SiCl/SiCl4 azeotrope for 30 s (a), 1 min (b), 2 min (c), 4 min (d), 6 min (e), and 10 min (f).
that does not explain uniaxial or even asymmetric growth. We propose a mechanism for filament growth below. Titanium surfaces (containing a native oxide)27 were exposed to the (CH3)3SiCl/SiCl4 azeotrope vapor in an effort to determine the stoichiometry of the reaction and gain insight into the mechanism of filament growth. Surfaces with the same wettability characteristics were obtained. X-ray photoelectron spectroscopy (XPS) of a sample that was reacted for 10 min revealed a Si/C ratio of 3.7. This indicates that SiCl4 and (CH3)3SiCl react in a ratio of ∼10:1. This ratio is likely biased low because any contamination would contain carbon and not silicon and because of the exponential decay of XPS sensitivity with depth: the contact angle data indicate that trimethylsilyl groups must be present at the outermost surface of the filaments and that few silanols are exposed. We propose a qualitative mechanism with a plausible explanation for the 1D growth of the filaments in the following text. SiCl4 and (CH3)3SiCl react with water and surface silanols to form covalently attached solid particles (Figure 2a); XPS indicates that they react in a ratio of ∼10:1. SiCl4 polymerizes with water and grows in three dimensions, but (CH3)3SiCl terminates the polymerization and the trimethylsilyl groups inhibit further reaction near them on the surface of the particle. The resulting trimethylsilylated silica particles must have surfaces that are mostly unreactive toward SiCl4 and its hydrolyzed derivatives (HOSiCl3, (HO)2SiCl2, (HO)3SiCl, Si(OH)4), but reactive defects must be present in small numbers. Growth from one of these defects causes a loss of symmetry, and particles (27) Silicon wafers were coated (magnetron sputtering) with ∼100 nm of titanium and were cleaned with oxygen plasma prior to reaction. No silicon was observed in these samples by XPS prior to reaction. See Fadeev, A. Y.; McCarthy, T. J. J. Am. Chem. Soc. 1999, 121, 12184-12185.
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Figure 3. Selected frames of a video of a (CH3)3SiCl/SiCl4 azeotropederived surface (top) contacting, compressing, and being released from a sessile water droplet. The reflection of the sessile droplet defines the surface of the silicon wafer.
become elongated and form short filaments (Figure 2b,c). (CH3)3SiCl more effectively terminates growth at the sides of the filaments than it does at the ends because of the relative curvature. Surface silanols are more exposed at the ends of filaments because the trimethylsilyl groups are splayed as a result of the curvature. The relative rates of reaction of hydrolyzed derivatives of SiCl4 and (CH3)3SiCl, their relative concentrations (controlled by the azeotrope composition), and the relative reactivity of silanols between trimethylsilyl groups as a function of curvature control the resulting morphology. Monofunctional silanes (e.g., (CH3)3-
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SiCl) form complete, but not close-packed, monolayers that contain defects of various cross-sectional areas smaller than the silane. Smaller silanes can react at these defects.28 It is reasonable to expect that curvature should affect reactivity. We describe the water contact angle behavior of the surfaces prepared from this azeotrope vapor as θA/θR g176°/g176°. Generally, the measured receding angle is higher than the measured advancing angle. We have discussed this behavior in other publications.12,15,23 Contact/compression/release analysis,15,23 which distinguishes perfectly hydrophobic surfaces from surfaces with 179° contact angles, was performed many times on these azeotrope-derived surfaces. Some samples in some locations exhibited perfect hydrophobicity and were indistinguishable (by this technique) from perfectly hydrophobic surfaces. Most of the analyses, however, exhibited a very small affinity for water during release. Figure 3 shows selected frames of a video of a contact/compression/release analysis of a surface exposed for 10 min. An azeotrope-derived surface is lowered onto and raised from a sessile droplet. Defects cause a small amount of receding contact line pinning (Figure 3g). We do not label these surfaces perfectly hydrophobic, but they are very close. This vapor-phase reaction is much more convenient and much less condition-dependent than the methyltrichlorosilane solution-phase reaction.15 Acknowledgment. We thank 3M and the NSF-sponsored MRSEC and RSEC Centers at the University of Massachusetts for support. LA7025297 (28) Fadeev, A. Y.; McCarthy, T. J Langmuir 1999, 15, 7238-7243. (29) Fadeev, A. Y.; McCarthy, T. J Langmuir 2000, 16, 7268-7274.
