Solid-State NMR Study of Titanium Dioxide Nanoparticles Surface-Modified by Alkylphosphonic Acids

We have modified the surface of TiO2 nanoparticles by propylphosphonic acid (PPA) and decylphosphonic acid (DPA), and have characterized the surface layer of titania (=titanium dioxide) nanoparticles by means of high-resolution solid-state NMR of 31P, 1H, and 13C, X-ray powder diffraction, and themogravimetric analysis. The 31P NMR spectra showed four types of bonding states corresponding to the monodentate and bidentate bonding states. The 1H NMR spectra demonstrated decrease in P–OH groups due to the dehydration condensation reaction. Layered titanium alkylphosphonate was formed by prolonged heating of the mixed solution of the reactants. The layered structure was confirmed by X-ray powder diffraction, and the interlayer distance suggested bilayer arrangement of the n-alkyl chains in the interlayer space. The 31P NMR spectra showed that a tridentate bonding state was formed in the layered structure. The 13C NMR results suggested all-trans conformation of n-alkyl chains, and the methyl group was confined in a small space. The molecular axis of the n-alkyl chains was tilted from the perpendicular of the layer plane. The surface coverage of the organic modifier in the modified TiO2 nanoparticles was estimated from the mass loss of thermogravimetric analysis. The surface coverages are about 50%, except for the samples containing considerable amounts of the layered structure.