The
surface functionalization of TiO2-based materials
with alkylsilanes is attractive in several cutting-edge applications,
such as photovoltaics, sensors, and nanocarriers for the controlled
release of bioactive molecules. (3-Aminopropyl)triethoxysilane (APTES)
is able to self-assemble to form monolayers on TiO2 surfaces,
but its adsorption geometry and solar-induced photodegradation pathways
are not well understood. We here employ advanced experimental (XPS,
NEXAFS, AFM, HR-TEM, and FT-IR) and theoretical (plane-wave DFT) tools
to investigate the preferential interaction mode of APTES on anatase
TiO2. We demonstrate that monomeric APTES chemisorption
should proceed through covalent Si–O–Ti bonds. Although
dimerization of the silane through Si–O–Si bonds is
possible, further polymerization on the surface is scarcely probable.
Terminal amino groups are expected to be partially involved in strong
charge-assisted hydrogen bonds with surface hydroxyl groups of TiO2, resulting in a reduced propensity to react with other species.
Solar-induced mineralization proceeds through preferential cleavage
of the alkyl groups, leading to the rapid loss of the terminal NH2 moieties, whereas the Si-bearing head of APTES undergoes
slower oxidation and remains bound to the surface. The suitability
of employing the silane as a linker with other chemical species is
discussed in the context of controlled degradation of APTES monolayers
for drug release and surface patterning.