The hydrothermal synthesis of SrTiO3 in a Sr­(OH)2/NaOH solution by reaction of four different single crystalline titanium precursors (anatase, rutile, sodium titanate, and hydrogen titanate) having the same nanowire morphology was investigated under stagnant fluid conditions. Owing to the low solubility and dissolution rate of the parent phases, the reaction mainly occurs in a thin interfacial fluid layer. The new phase only grows on the substrate surface, and the morphology evolution is largely controlled by the interface through the coupling of substrate dissolution and SrTiO3 crystallization. The pseudomorphic replacement of the precursor by the product occurs if complete surface coverage is attained. Depending on the crystallographic matching, the parent crystal can either transform in a mesocrystal as happens with anatase via a topochemical transformation or in a polycrystalline product as observed with sodium titanate. In contrast, if the product grows in the form of isolated particles or with dendritic morphology, as in the case of hydrogen titanate and, to a lesser extent rutile, the new compound will not inherit the precursor morphology. When well-defined interfaces are missing, as happens when amorphous titanium hydroxide gel suspensions are used as precursors, the crystallization of SrTiO3 occurs by a completely different pathway, i.e., oriented self-assembly of nanocrystals in mesocrystals.

The mechanisms driving the mutual crystallographic alignment of nanocrystals in mesocrystals are various and not yet fully understood. As discussed in this paper, formation of mesocrystals can result from a topochemical reaction between single crystal particles or templates suspended in a liquid phase and ionic/molecular species in solution. In such a case, the initial particle morphology is retained in the final mesocrystal. If the transformation is not topotactic, the final product will maintain no memory of the precursor morphology. The magnitude of the lattice mismatch as well as the defective state of the precursor surface probably determine the degree of mutual crystallographic alignment of the nanocrystals which nucleate and grow on the substrate. Although identified by studying the hydrothermal crystallization of SrTiO3 from different single crystal titania precursors, this mechanism is very general and applicable to a variety of compounds and experimental situations, including solvothermal and molten salt syntheses.