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

Abstract

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 (TiO_2(am)). The <i>in situ</i> 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 TiO_2(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 ∼30, 10, and 0% on R180, R60, and TiO_2(am) respectively. The Si 2s photoelectron peak binding energy (BE) and the H₄SiO₄ 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 H₄SiO₄ oligomerization where linear trimeric silicates are formed by insertion of a solution H₄SiO₄ 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 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 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