Ultraviolet-Induced Grafting of Alkenes to TiO2 Surfaces: Controlling Multi layer Formation

Photochemical grafting of alkenes has emerged as a versatile way to form functionalized surfaces and enables multistep surface chemistry on a range of substrates. However, the self-terminating nature of the reaction remains poorly understood. Here, we use X-ray photoelectron spectroscopy (XPS) to explore multilayer formation during photochemical grafting of functional alkenes on single-crystal rutile (110) TiO2 surfaces. We demonstrate that simple alkenes such as 1-hexene stop at 1 monolayer, but alkenes with an additional alcohol group easily form multilayers. Atomic force microscopy (AFM) and XPS studies show that the multilayers are highly conformal and that the alcohol groups remain chemically accessible even within rnultilayer films. Small amounts of water enhance multilayer formation by facilitating formation of center dot OH or other radical species that initiate oligomerization processes, while oxygen has little effect. We show that the multilayer formation can be reduced or eliminated by using small amounts of a radical scavenger or by modifying the molecular structure of the alkene. Overall these studies show that multilayer formation is controlled by the ability to form alkyl radicals under ultraviolet (UV) illumination.