Hydrothermal Synthesis of SrTiO3: Role of Interfaces

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.