Silicic acid (H4SiO4) is ubiquitous in natural aquatic systems. Applications of TiO2 in these systems will be influenced by H4SiO4 sorption and oligomerization reactions on the TiO2 surface, and this can affect many aspects of TiO2 reactivity. The spatial arrangement of sorption sites on a metal oxide surface can promote specific lateral interactions, such as oligomerization, between sorbed species. In this work we explore the relationship between surface structure and interfacial H4SiO4 oligomerization by quantifying the extent of H4SiO4 sorption and oligomerization on three TiO2 phases; a rutile phase having well-developed (110) faces (R180), a rutile phase with poorly developed (110) faces (R60), and an amorphous TiO2 (TiO2(am)). The in situ ATR-IR spectra measured over time as 0.2 mM H4SiO4 reacted with TiO2 were quite different on the three TiO2 phases. The percentage of the surface H4SiO4 that was present as oligomers increased over time on all phases, but after 20 h almost all H4SiO4 on the R180 surface was oligomeric, while the H4SiO4 on TiO2(am) was predominantly monomeric. The extent of H4SiO4 oligomerization on R60 was intermediate. When the TiO2 phases reacted with 1.5 mM H4SiO4 the ATR-IR spectra showed oligomeric silicates dominating the surface of all three TiO2 phases; however, after 20 h the percentage of the surface H4SiO4 present as three-dimensional polymers was similar to 30, 10, and 0% on R180, R60, and TiO2(am) respectively. The Si 2s photoelectron peak binding energy (BE) and the H4SiO4 surface coverage (Gamma(Si)) were measured by XPS over a range of Gamma(Si). For any given Gamma(Si) the Si 2s BE's were in the order R180 > R60 > TiO2(am). A higher Si 2s BE indicates a greater degree of silicate polymerization. The ATR-IR and XPS results support the existing model for interfacial H4SiO4 oligomerization where linear trimeric silicates are formed by insertion of a solution H4SiO4 between suitably orientated adjacent bidentate sorbed monomers. The TiO2(am) has previously been shown to consist of similar to 2 nm diameter particles with a highly disordered surface. When compared to the TiO2(am) surface, the regular arrangement of TiO6 octahedra on the rutile (110) face means that sorbed H4SiO4 monomers on adjacent rows of singly coordinated oxygen atoms are oriented so as to favor linear trimer formation. Higher silicate polymers can form between adjacent trimers, and this is favored on the rutile (110) surfaces compared to the TiO2(am). This is also expected on the basis of the arrangement of surface sites on the rutile (110) surface and because the high surface curvature inherent in a similar to 2 nm spherical TiO2(am) particle would increase the spatial separation of adjacent trimers.
