Water-soluble titanium-oxides: Complexes, clusters and nanocrystals

The water-soluble titanium-oxide structures covered in this review span a continuum from molecular clusters to colloidal nanocrystals. Here, a deliberate distinction is made between titanium clusters and nanoparticles with organic ligands, soluble in organic solvents, and related structures prepared and soluble in water. This distinction is made in response to rapid developments in three areas covered in this article: small titanium-oxide clusters prepared at low pH values in water, heterpolytungstate macroanions with high-nuclearity titanium-oxide cores, and polyoxometalate complexes of anatase-phase TiO₂. The latter nanostructures occupy a unique position between polyoxometalate macroanions and traditional colloidal TiO₂. As such, the progressively larger water-soluble titanium-oxide structures covered in the three main sections of this review establish a continuum that begins to blur the longstanding divide between molecular and colloid science.