Covalent grafting of phenylphosphonate on calcium niobate platelets

This work explores covalent grafting of phenylphosphonate (PPA) onto exfoliated, protonated calcium niobate (HCN), a Dion-Jacobson layered perovskite. The specific hypothesis is that PPA can be readily grafted onto the face surfaces of exfoliated HCN, which has reactive apical oxygen atoms.

Previous research has established the conditions required for full exfoliation of HCN in aqueous solutions of tetrabutylammonium hydroxide (TBAOH), denoted as TBACN. This work first explores the effect of reflux conditions on the dispersion state of TBACN suspensions, and then investigates PPA grafting onto both non-exfoliated HCN and exfoliated TBACN dispersed in deionized (DI) water, TBA solution, and various alcohols. The products are characterized by a variety of techniques including light scattering to assess the TBACN dispersion state, (31)P MAS NMR to confirm PPA grafting, and XPS to estimate PPA grafted amounts.

The results confirm the grafting of PPA on HCN and TBACN, quantify the extent of PPA grafting, and identify various grafting modes (mono-, bi-, and tridentate). All of these aspects are found to be dependent on the layered materials' exfoliation state, suspension processing conditions, and solvent composition. The results are rationalized in terms of a plausible mechanism of the grafting process.