Hydrothermal synthesis of strontium titanate: thermodynamic considerations, morphology control and crystallisation mechanisms

This article highlights the recent developments on the hydrothermal and solvothermal synthesis of strontium titanate (SrTiO3), considered as a model system, by reviewing the literature of the last 10-15 years. The most significant advantage of these solution-mediated crystallisation methods is the effective control of particle composition, size and morphology by varying some physical and chemical parameters such as temperature, concentration, pH and solvent composition, as well as using different precursors and mineralisers, and moreover, adding growth modifiers like polar organic molecules and hydrophilic polymers. Thus, the synthesis process can be designed to obtain pure and doped materials with superior functional and photocatalytic properties, including monodispersed nanoparticles, platelets, wires, porous particles, mesocrystals and heterostructures. The hydrothermal crystallisation mechanisms are critically discussed, considering both the thermodynamic and kinetic aspects. In particular, the nature and morphology of the solid titanium precursor has a significant impact on the hydrothermal crystallisation as in many cases the formation of SrTiO3 occurs on the precursor surface. The dissolution of the precursor and the nucleation and growth of the perovskite are coupled together over rather short distances, and the coupling is mediated by the solid/liquid interfaces. The morphology of strontium titanate is thus determined by the precursor/perovskite crystallographic matching, the surface density of nuclei and the rate-controlling process. Differently, crystallisation from Sr-Ti amorphous gel-like precursors occurs in the absence of crystalline surfaces and often produces mesocrystals by oriented aggregation of the primary nanocrystals. These considerations have a general validity and can be extended to many ternary and even more complex oxides.