The Influence of Surface Structure on H4SiO4 Oligomerization on Rutile and Amorphous TiO2 Surfaces: An ATR-IR and Synchrotron XPS Study

Abstract

Silicic acid (H(4)SiO(4)) is ubiquitous in natural aquatic systems. Applications of TiO(2) in these systems will be influenced by H(4)SiO(4) sorption and oligomerization reactions on the TiO(2) surface, and this can affect many aspects of TiO(2) 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 H(4)SiO(4) oligomerization by quantifying the extent of H(4)SiO(4) sorption and oligomerization on three TiO(2) phases; a rutile phase having well-developed (110) faces (R180), a rutile phase with poorly developed (110) faces (R60), and an amorphous TiO(2) (TiO(2(am))). The in situ ATR-IR spectra measured over time as 0.2 mM H(4)SiO(4) reacted with TiO(2) were quite different on the three TiO(2) phases. The percentage of the surface H(4)SiO(4) that was present as oligomers increased over time on all phases, but after 20 h almost all H(4)SiO(4) on the R180 surface was oligomeric, while the H(4)SiO(4) on TiO(2(am)) was predominantly monomeric. The extent of H(4)SiO(4) oligomerization on R60 was intermediate. When the TiO(2) phases reacted with 1.5 mM H(4)SiO(4) the ATR-IR spectra showed oligomeric silicates dominating the surface of all three TiO(2) phases; however, after 20 h the percentage of the surface H(4)SiO(4) present as three-dimensional polymers was ∼30, 10, and 0% on R180, R60, and TiO(2(am)) respectively. The Si 2s photoelectron peak binding energy (BE) and the H(4)SiO(4) surface coverage (Γ(Si)) were measured by XPS over a range of Γ(Si). For any given Γ(Si) the Si 2s BE's were in the order R180 > R60 > TiO(2(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 H(4)SiO(4) oligomerization where linear trimeric silicates are formed by insertion of a solution H(4)SiO(4) between suitably orientated adjacent bidentate sorbed monomers. The TiO(2(am)) has previously been shown to consist of ∼2 nm diameter particles with a highly disordered surface. When compared to the TiO(2(am)) surface, the regular arrangement of TiO(6) octahedra on the rutile (110) face means that sorbed H(4)SiO(4) 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 TiO(2(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 ∼2 nm spherical TiO(2(am)) particle would increase the spatial separation of adjacent trimers.