A New Route to Covalently Attached Monolayers: Reaction of Hydridosilanes with Titanium and Other Metal Surfaces

O-Si bonds with surface densities as high as those of selfassembled monolayers (SAMs). 2 Thiols can be used on gold substrates and functional silanes with readily hydrolyzable groups (most often chloro or alkoxy) can be used on different forms of SiO2, but if there is a need to modify other materials, however, there are simply not available techniques. The chemistry that we describe is more general in several respects than the results reported here. First, the reaction also works well with a range of other metal oxide surfaces. Second, we have studied a variety of titania surfaces and all react unexceptionally. We discuss here primarily only data obtained using Si-supported titanium ( ∼100 Å of Ti evaporated and deposited on polished Si wafers) and fumed titania (BET nitrogen surface area is 50 m 2/g). Table 1 shows carbon analysis data for several samples of fumed titania (50 m2/g) that were allowed to react with various hydridosilanes in heptane solution ( ∼5 wt % hydridosilane) at 65-70°C (unless indicated otherwise) for various extents of time. Several aspects of these data warrant comment. (1) These reactions are thermally activated and require long reaction times to achieve maximum bonding density. The reaction of n-octylsilane proceeds to ∼60% conversion at room temperature after 30 min, but requires hours at elevated temperature to achieve maximum bonding density. (2) The maximum bonding density for nalkylsilane-derived monolayers is ∼5 chains/nm2 (∼20 Å2/chain); this is similar to the density of SAMs of alkane thiols on gold 3, SAMs of alkyltrichlorosilanes on silica, 4 and Langmuir -Blodgett monolayers of fatty molecules. 5 (3) Dialkyland trialkylsilanes show dramatically decreased coverage relative to the monoalkylsilanes due to sterics. The n-octadecyldimethylsilane monolayer has∼1/3 the bonding density of the n-octadecylsilane monolayer. This trend is also observed in the series of phenylsilane, diphenylsilane, and triphenylsilane monolayers. Figure 1 shows transmission infrared spectra of nctylsilane and a sample of titania that was reacted with octylsilane. The obvious peaks at 2150 ( νSi-H) and 925 cm-1 (δSi-H) present in the starting material are absent in the product, indicating the quantitative conversion of Si -H bonds to Si -O bonds. The broad absorbance from 1100 to 1000 cm -1 indicates the formation of Si-O-Ti bonds (and perhaps Si -O-Si bonds). The hydrocarbon stretching (2950 -2850 cm-1) and bending (1475 -1375 cm-1) vibrations indicate the presence of octyl groups on the titania surface. These changes are consistent with covalent attachment of n-octylsilane moieties to the TiO 2 surface. Spectra of other hydridosilane-treated titania surfaces are similar: no residual Si-H bonds, Ti -O-Si bonds, and characteristic alkyl and aryl group absorbances. We note that n-octadecylsilane-modified surfaces show no Si -H bonds at any stage of reaction (even at low bonding density) and this argues for 3 bonds between silicon and surface titanols being the principal mode of bonding. The volume of hydrogen gas produced as a byproduct was determined for several reactions. Table 1 shows the number of Si-H groups that react (determined from the volume of hydrogen liberated) per silane molecule that is incorporated in the monolayers (determined from carbon analysis). The data for the n-octadecyl monolayers indicate that ∼3 molecules of hydrogen are released at early reaction times that produce incomplete monolayers and that this value decreases to ∼2.7 at later reaction times that produce more dense monolayers. Dehydration of the titania by heating in a vacuum causes a reproducible decrease in the bonding density of attached silanes. XPS data of single surfaces also reflect monolayer formation, with bonding density and thickness values that depend on the sterics of the hydridosilane. After silanization, the titania surfaces (1) On leave from Chemistry Department, M. V. Lomonosov Moscow State University, 119899 Moscow, Vorob. Gory, Russia. (2) Ulman, A.Chem. Re V. 1996, 96, 1533. (3) Bain, C. D. et al.J. Am. Chem. Soc. 1989, 111, 321. (4) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir1989, 5, 1074. (5) Baker, H. R.; Shafrin, E. G.; Zisman, W. A. J Phys. Chem. 1952, 56, 405. Levine, O.; Zisman, W. A. J. Phys. Chem. 1957, 61, 1069. TiO2-OH + R3Si-H f TiO2-OSiR3 + H2 (1) Table 1. Bonding Density Data for Hydridosilane-Modified Titaniaa