Self-assembled monolayers supported on TiO2:: Comparison of C18H37SiX3 (X = H, Cl, OCH3C18H37Si(CH3)2C1, and C18H37PO(OH)2

The solution-phase reactions of octadecylsilanes with different headgroups (C18H37SiH3, C18H37Si(OCH3)(3), C18H37SiCl3, and C18H37Si(CH3)(2)Cl) and of octadecylphosphonic acid (Cs18H37PO3H2) with titanium dioxide (anatase) were investigated. Chemical analysis and FTIR suggested that all the reactions, with the exception of that of C18H37Si(CH3)(2)Cl, yielded closely packed self-assembled monolayers (SAMs). SAMs were characterized with high grafting density (similar to4.3-4.8 octadecyl groups/nm(2)) and with high degree of ordering of alkyl chains. Reaction of C18H37Si(CH3)(2)Cl yielded less ordered surfaces with grafting density similar to1.5 group/nm(2). The kinetics plots were similar for all the reactions and illustrated two distinct regions, a rapid attachment followed by a slow growth of the grafting density. The uptake curves were adequately described by the first-order kinetics with two rate constants that differed from each other by 1-2 orders of magnitude. According to the rate constants, the following range of reactivity was established: C18H37SiCl3 >> C18H37PO(OH)(2) > C18H37Si(CH3)(2)Cl > C18H37Si(OCH3)(3) > C18H37SiH3. Two distinct types of the SAM growth, uniform and islandlike ones, were proposed on the basis of the FTIR study of the SAMs at submonolayer coverage. Silanes capable of cross-linking (C18H37SiX3, X = H, Cl, OCH3) gave SAMs with a high degree of ordering at relatively low surface coverage, suggesting nonuniform (islandlike) film growth. For SAMs of C18H37Si(CH3)(2)Cl and C18H37PO(OH)(2), the order gradually improved with coverage and highly ordered SAMs were obtained only for high surface coverage, arguing for the uniform mechanism of the film growth. The thermal stability of the supported monolayers was characterized by TGA. All the SAMs showed good thermal and oxidative stability, and no mass loss was observed below similar to200 degreesC in air. The temperatures of the maximum mass loss rate were close for all, SAMs (similar to300 degreesC).