Tuning the Hydrophilic, Hydrophobic, and Ion Exchange Properties of Mesoporous TiO2

Abstract

Alkyl phosphonic acids (Pho-C-n-R) of different chain length (6, 10, and 14 carbons) bearing neutral, positive, and negatively charged head groups (R = -H, R- = sulfonate, R+ = pyridinium) were prepared and anchored to the inner walls of randomly sintered mesoporous TiO2 thin films. Quartz crystal microbalance (QCM) and Fourier transform infrared (FT-IR) measurements show that a monolayer coverage was achieved. The monolayer crystallinity is lower as compared to alkyl thiols on gold, but it increases with the length of the carbon chain. The neutral phosphonic acid modifier makes the TiO2 highly hydrophobic and suppresses electrochemistry in aqueous media, and the alkyl phosphonic acids with charged head groups render the TiO2 film as an ion exchanger with a phase separated hydrophilic and hydrophobic portion. Different charged guest molecules were incorporated on top or into the supported membranes. The host-guest interactions were found to be electrostatic, hydrophobic, or both. Highly charged electroactive metal complexes ([Fe(CN)(6)](4-), [IrCl6](2-)) and purpose-synthesized organic electrochromophores (dialkylated viologens with variable chain length, C-1-V+2-C-n, C-n-V+2-C-n, n = 6, 10, and 14) were used as molecular guests, and the assemblies were characterized by cyclic voltammetry and FT-IR. Using the preconcentration phenomenon, [Fe(CN)(6)](4-) concentration as low as 200 nM can be detected on a Pho-C-14-R+ modified TiO2 electrode by conventional cyclic voltammetry. The new surface modification technique simplifies the molecular requirements for functional surface modifiers considerably. Using a limited set of organic anchors with orthogonal coordination properties and adjustable hydrophobicity, a broad range of electrochromophores, redox active wiring compounds, or sensitizers can be adsorbed onto TiO2.