Multiform Oxide Optical Materials via the Versatile Pechini-Type Sol−Gel Process:  Synthesis and Characteristics

This feature article highlights work from the authors' laboratories on the various kinds of oxide optical materials, mainly luminescence and pigment materials with different forms (powder, core−shell structures, thin film and patterning) prepared by the Pechini-type sol−gel (PSG) process. The PSG process, which uses the common metal salts (nitrates, acetates, chlorides, etc.) as precursors and citric acid (CA) as chelating ligands of metal ions and polyhydroxy alcohol (such as ethylene glycol or poly ethylene glycol) as a cross-linking agent to form a polymeric resin on molecular level, reduces segregation of particular metal ions and ensures compositional homogeneity. This process can overcome most of the difficulties and disadvantages that frequently occur in the alkoxides based sol−gel process. Using the PSG process, we are able to prepare luminescent powder materials that cannot be well synthesized by the solid-state reaction method, environmentally friendly and highly efficient phosphors that lack metal activator ions, core−shell structured monodisperse and spherical optical materials with tunable physical chemical properties, and thin film phosphors and their patterning combined with soft lithography techniques. The extensive applicability of this process and potential material applications are demonstrated.