Self-Assembled Monolayers of Organosilicon Hydrides Supported on Titanium, Zirconium, and Hafnium Dioxides

This work investigates the preparation of the self-assembled monolayers (SAMs) of organosilicon hydrides (RSiH3) supported on transition metal oxides. The reactions of alkyl-, fluoroalkyl-, and ω-alkenyl-silanes and α,ω-bis-hydridosilanes with nonporous high surface area TiO2 (rutile and anatase), ZrO2 (monoclinic), and HfO2 (monoclinic) powders were studied, and supported SAMs were characterized by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and chemical analysis. SAMs of organosilicon hydrides were closely packed and well-ordered (assessed from CH2 stretchings). Grafting densities of the monolayers were ∼4.6−4.8 group/nm2 for C18H37 SAMs and ∼3.6−3.7 group/nm2 for C6F13 SAMs, which are close to the ultimate values reported in the literature for the best organosilicon SAMs. The reactions with organosilicon hydrides proceed in a noncorrosive environment (no HCl), which distinguishes RSiH3 from RSiCl3 coupling agents. Synthesis of the monolayers was highly reproducible, and SAMs of high quality were prepared from the metal oxides of different surface area and particle size, of different vendors, and of different crystalline forms. Titania, zirconia, and hafnia reacted with RSiH3 unexceptionally, and no influence of the metal oxide on the SAMs' quality was found. According to FTIR, the reactions of RSiH3 with metals oxides yielded cross-linked monolayers (with Si−O−Si bonds) grafted to the metal oxides (via Si−O−MS bonds). The mechanism of the self-assembly of organosilicon hydrides on the surfaces is discussed. Kinetics of the monolayer formation in the solution phase was investigated. The reactions were found to follow first-order kinetics with two rate constants. According to the rate constants, the following range of reactivity was established: H3Si(CH2)8SiH3 > C6F13(CH2)2SiH3 > CH2CH(CH2)6SiH3 ≈ C8H17SiH3 > C18H37SiH3. According to TGA, SAMs supported on titania, zirconia, and hafnia showed good thermal and oxidative stability and no mass loss was observed below 180−200 °C in air. Temperatures of the maximum mass loss rate were independent of the metal oxide and vary from ∼250−300 °C for alkyl- to ∼390 °C for fluoroalkyl-modified surfaces. It is suggested that the degradation of the surfaces proceeds by oxidative destruction of the organic groups and yields silica-like surfaces supported on the metal oxide.