Preparation of Silica-on-Titania Patterns with a Wettability Contrast A. Kanta, R. Sedev, and J. Ralston* Ian Wark Research Institute, University of South Australia, Mawson Lakes Campus, Adelaide, SA 5095, Australia Received December 21, 2004. In Final Form: April 11, 2005 The preparation of patterned inorganic surfaces consisting of silica (SiO2) and titania (TiO2) is described. The approach is based on a combination of standard photolithography and plasma-enhanced chemical vapor deposition. Silicon wafers coated with a titania layer (40 nm) were patterned by use of a positive photoresist and then a thin silica layer (10-40 nm) was plasma-deposited. The photoresist was removed by decomposition at 800 °C. The inorganic patterned surfaces possessed excellent high-temperature resistance. Since the silica patches were effectively dehydroxylated during the thermal treatment, the patterns consisted of moderately hydrophobic (silica) and hydrophilic (titania) domains with a significant wettability contrast (40° for water). The surface was further hydrophobized with a self-assembled monolayer of fluoroalkylsilane (FAS) and exposed to UV light. The FAS layer was locally oxidized on the TiO 2 patches and the wettability contrast was maximized to 120° (the highest possible value on smooth surfaces). Introduction If the volume of liquid material that is transported or transformed is reduced, then efficiency generally increases and consumption decreases. As a consequence, minia- turization has always been a leading tendency in technol- ogy but it has gathered significant pace in more recent times. The science of miniaturization has become a topic in its own right. 1 From a colloid science point of view, miniaturization is related to a significant increase in the surface-to-bulk ratio, which in turn amplifies the impor- tance of interfaces, including capillary phenomena and wetting, surface forces, interfacial properties, etc. 2 Sur- faces with a pattern having the desired wettability contrast, for example, a relatively hydrophilic channel crossing a more hydrophobic matrix, are the building blocks of various microfluidic devices. It therefore seems timely to develop protocols for their fabrication and assess their properties in detail. Most liquids will not spread completely on many surfaces. Their spreading will proceed until a finite contact angle is reached. From a macroscopic point of view, the contact angle reflects the balance between the forces of cohesion and adhesion. 3 On a more hydrophobic solid, adhesion is lower and the contact angle is higher. There is a strong correlation between the chemical constitution of the solid surface (the type and packing of the terminal groups in the outermost layer of the solid) and the contact angle for a given liquid. 4 The most hydrophobic surfaces are terminated by densely packed perfluoromethyl groups, and the water contact angle on such surfaces (when homogeneous and smooth) is 115-125°. 4,5 Self-assembled monolayers (SAM) are a popular system due to their availability, versatile termination, and ease of preparation. 6-8 Alkanethiols on gold and organosilane on hydroxylated surfaces adsorb quickly, bond strongly, and form well-arranged monolayers. The surface free energy, and therefore the wettability of the modified surface, can be controlled by altering the terminal group of the SAM: from completely hydrophilic (e.g., -OH or -COOH) to very hydrophobic (e.g., -CH 3 or -CF 3 ). 6 Thiol- based SAM can be easily removed after UV photooxidation and thus, by use of lithography, patterned surfaces can be prepared. This technique is widely used to fabricate model surfaces for wettability studies. 7,9 Our goal in this investigation is to prepare an oxide- on-oxide patterned surface. Oxide surfaces are very common as oxides are ubiquitous and extensively used in industry. Oxides have the advantage of being resistant to heat and aggressive solutions. Oxide surfaces almost always contain surface -OH groups, 10 which can generate a surface charge in aqueous solutions or can be further derivatized under dry conditions. There has been a recent surge of interest in silica- titania patterned surfaces 11-13 as the different chemical affinities of the two oxides can be exploited to selectively adsorb (or in fact prevent adsorption of) proteins, which is therefore of enormous interest in biological applications. We have recently investigated the different wettability of silica and titania after exposure to UV irradiation and heat treatment at elevated temperatures. 14 Both silica and titania recontaminate after cleaning if left in the ambient atmosphere (the process on titania surfaces is * Corresponding author: e-mail john.ralston@unisa.edu.au. (1) Madou, M. J. Fundamentals of Microfabrication: The Science of Miniaturization, 2nd ed.; CRC Press: Boca Raton, FL, 2002. (2) Hunter, R. J. Foundations of Colloid Science, 2nd ed.; Oxford University Press: Oxford, U.K., 2001. (3) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; Wiley: New York, 1997. (4) Zisman, W. A. Adv. Chem. Ser. 1964, 43, 1. (5) Nishino, T.; Meguro, M.; Nakamae, K.; Matsushita, M.; Ueda, Y. Langmuir 1999, 15, 4321. (6) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (7) Ulman, A. Thin Solid Films 1996, 273, 48. (8) Wilbur, J. L.; Whitesides, G. M. In Nanotechnology; Timp, G. L., Ed.; Springer: New York, 1999; p 331. (9) Priest, C. I. Ph.D. Thesis, University of South Australia, Adelaide, Australia, 2004. (10) Hair, M. L. Infrared Spectroscopy in Surface Chemistry; Marcel Dekker: New York, 1967. (11) Michel, R.; Reviakine, I.; Sutherland, D.; Fokas, C.; Csucs, G.; Danuser, G.; Spencer, N. D.; Textor, M. Langmuir 2002, 18, 8580. (12) Michel, R.; Lussi, J. W.; Csucs, G.; Reviakine, I.; Danuser, G.; Ketterer, B.; Hubbell, J. A.; Textor, M.; Spencer, N. D. Langmuir 2002, 18, 3281. (13) Lussi, J. W.; Michel, R.; Reviakine, I.; Falconnet, D.; Goessl, A.; Csucs, G.; Hubbell, J. A.; Textor, M. Prog. Surf. Sci. 2004, 76, 55. (14) Kanta, A.; Sedev, R.; Ralston, J. Langmuir 2005, 21, 2400. 5790 Langmuir 2005, 21, 5790-5794 10.1021/la046837f CCC: $30.25 © 2005 American Chemical Society Published on Web 05/18/2005