Keywords: interfacial tension, nucleation kinetics, growth, aqueous precipitation, thermodynamic stability, Ostwald ripening, crystal phase transformation, nanoparticles, nanorods, akaganeite, hematite, boehmite, bayerite, oxyhydroxide, aqueous stabilisation, crystalline metastable nanostructures, nanotechnology, heteronucleation, aluminium, iron
On the aqueous stabilisation of crystalline metastable nanostructures
Crystal phase transformations in aqueous solutions generally operate through a dissolution-recrystallisation process to fulfil surface energy minimisation requirements. Indeed, when a solid offers several allotropic crystal phases and polymorphs, it is usually the one with the highest solubility and consequently the lowest stability (i.e., the crystallographic metastable phase), which precipitates first. This is understood by considering the nucleation kinetics of the solids. At a given supersaturation ratio, the germ size is as small (and the nucleation rate as high) as the interfacial tension of the system is low. Thus, given that the solubility is inversely proportional to the interfacial tension, the precipitation of the most soluble phase and consequently the less stable thermodynamically, is therefore kinetically promoted. Due to its solubility and metastability, this particular phase is more sensitive to secondary growth and ageing, which leads to crystallographically more stable phases, essentially by heteronucleation. This is typically the case of metal oxides such as aluminium hydroxide (boehmite to bayerite or gibbsite), titanium dioxide (anatase or brookite to rutile), or iron oxyhydroxides (goethite, akaganeite, or lepidocrocite to hematite). Secondary growth and ageing processes are delicate to control and the phase transformations appear within a few hours to a few days in solution, whence the resulting undesired mixing of allotropic phases and polymorphs. However, by careful consideration of the precipitation conditions such phenomena may be avoided when nanosystems are precipitated at low interfacial tension. Thus, the stabilisation of oxide, hydroxide and oxyhydroxide metastable crystal structures can be achieved at thermodynamically stable conditions. As an attempt to assess such behaviour, experimental studies involving iron (akaganeite, β-FeOOH) and aluminium (boehmite, γ-AlOOH) oxyhydroxides were conducted. The results show that it is possible to significantly slow down and even entirely suppress crystal phase transformation and thus stabilise crystalline metastable phases in aqueous solutions.