Short range order at the amorphous TiO2–water interface probed by silicic acid adsorption and interfacial oligomerization: An ATR-IR and 29Si MAS-NMR study

Abstract

Adsorption and oligomerization of H4SiO4 at the amorphous TiO2–aqueous interface were studied using in situ Attenuated Total Reflectance Infrared (ATR-IR) and ex situ solid state 29Si nuclear magnetic resonance (NMR). The ATR-IR spectra indicate that a monomeric silicate species is present at low silicate surface concentration (ΓSi). Above a threshold ΓSi linear silicate oligomers are formed and these oligomers dominate the surface at high ΓSi. Interestingly the ATR-IR spectra of H4SiO4 on the TiO2 surface are very similar to those previously observed on the poorly ordered iron oxide phase ferrihydrite. The 29Si NMR spectrum of silicate on the TiO2 surface shows the presence of Si in three states with chemical shifts corresponding to isolated monomers (Q0), the ends of linear oligomers (Q1) and the middle of linear oligomers (Q2). The ratio of the area of the Q1 and Q2 peaks was ≈2:1 which is consistent with the proposed formation of linear silicate trimers by insertion of a solution H4SiO4 between adjacent suitably orientated adsorbed silicate monomers. A structural interpretation indicates that the observed interfacial silicate oligomerization behavior is a general phenomenon whereby bidentate silicate monomers on oxide surfaces are disposed towards forming linear oligomers by condensation reactions involving their two terminal Si–OH groups. The high surface curvature of nanometer sized spheres inhibits the formation of interfacial silicates with a higher degree of polymerization.