AVS 58th Annual International Symposium and Exhibition
    Surface Science Division Thursday Sessions
       Session SS-ThM

Invited Paper SS-ThM3
Organometallics as Probes of Functional Group Distribution on Oxide Surfaces

Thursday, November 3, 2011, 8:40 am, Room 107

Session: Oxide Surface Structure & Reactivity
Presenter: Susannah Scott, University of California, Santa Barbara
Correspondent: Click to Email
The reactivity of the hydroxyl-terminated surface of silica is important for constructing tailored interfaces in many applications, including catalysis, separations, and microelectronics. Amorphous silicas are particularly interesting because of their high surface areas. The hydroxyl content of their surfaces can be reduced thermally, but it decreases slowly because the hydroxyls have very low mobility. Their mutual condensation requires proximity, and, frequently, the formation of strained siloxane rings. Since these silicas have no long-range order, it is often postulated that the hydroxyl groups are distributed randomly, and that when the nominal hydroxyl density is low (e.g., after thermal treatment at temperatures exceeding 500 °C), these hydroxyls can be considered essentially isolated. Curiously, the Ga K-edge EXAFS shows that the reaction of Ga(CH3)3 with such a silica pretreated at 800 °C generates only digallium sites, requiring that the hydroxyls be organized pair-wise, likely as vicinal silanols linked by a single siloxane bond. The origin of the preferred vicinal disposition may be hydrolysis of a strained siloxane bond. Where isolated hydroxyls do exist, they appear to be located on reactive 2-rings whose reactivity resembles that of a masked silanol. Consequently, it is unwise to assume that site isolation results from grafting onto these silicas, although the vicinal disposition of silanols provides an opportunity to create tailored bifunctional surfaces.