From disordered precursors to hierarchical titanium phosphate architectures: the role of solvent-mediated aging and phosphate speciation

Hierarchical titanium phosphate architectures were synthesized through a combined mechanochemical– sol–gel approach to elucidate the role of precursor activation and solvent-mediated aging in structure formation. While conventional sol–gel routes yield flower-like alfa-Ti(HPO4)2.H2O structures, the chemical factors governing their emergence remain poorly understood. Here, wet ball milling is shown to generate chemically activated but structurally disordered Ti–phosphate precursors, whose evolution is subsequently controlled by aging in alcoholic media. Time-resolved characterization by SEM, XRD, and 31P MAS NMR reveals that hierarchical assembly is not directly induced by milling, but develops progressively during aging through a solvent-mediated reorganization process. In particular, a continuous increase in the [HPO4]/[H2PO4] ratio correlates with the formation of lamellar subunits and the emergence of flower-like architectures. These results indicate that phosphate speciation is closely associated with structural organization and provides information on the progressive condensation and ordering of the phosphate framework beyond that captured by the overall P/Ti stoichiometry. The nature and amount of solvent critically influence both the initial chemical state and the kinetics of structural evolution, determining distinct pathways toward ordered or disordered products. Nitrogen adsorption measurements highlight the mesoporous nature of the hierarchical structure that exhibits a proton conductivity of 6.4 x 10-4 S cm-1 at 90 1C and 90% relative humidity.