Chemistry on the edge: A microscopic analysis of the intercalation, exfoliation, edge functionalization, and monolayer surface tiling reactions of α-Zirconium phosphate

The intercalation and exfoliation reactions of α-zirconium phosphate, Zr(HPO₄)₂·H₂O (α-ZrP), were studied microscopically by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The reaction of α-ZrP with tetra(n-butylammonium) hydroxide (TBA⁺OH^-) initially produces intercalation compounds, which then transform to unilamellar colloids. The rate-determining step in intercalation is the opening of the interlamellar galleries. Subsequent diffusion of TBA⁺ ions into the opened galleries is rapid. The hydrolysis reaction of α-ZrP colloids proceeds from the edges inward, forming ~4-nm hydrated zirconia particles that decorate the edges of the sheets. The reaction does not go to completion, as it is limited by equilibrium associated with the release of phosphate into the solution. The hydrolysis reaction is negligible at 0°C, which permits the synthesis of hydrolysis-free unilamellar colloids. Remarkably, these colloids form monolayer films on amine-derivatized silicon surfaces with a high density that suggests significant surface mobility during the adsorption process. Addition of appropriate phosphonic acids to colloidal α-ZrP suspensions enables modification of the sheet edges, illustrated here by the anchoring of osmium oxide particles to the sheet edges by a vinylphosphonate linker.