Polymorphism in nanocrystalline binary metal oxides

Structural changes occur due to polymorphic transitions in binary metal oxides, and these lead to materials with distinct physical and chemical properties. For the MoO₃ system, for example, its metastable hexagonal phase is more efficient than the stable orthorhombic phase with respect to battery storage capability; furthermore, the orthorhombic phase shows detection specificity to ammonia vapors, whereas the monoclinic phase of the same oxide is a good nitric oxide sensor. It has been observed that high temperature or else metastable or unstable polymorphs are present at room temperature when the oxide is in the form of nanocrystals. In this review, polymorphic forms of key functional binary metal oxides, such as CrO₂, Cr₂O₃, Fe₂O₃, Al₂O₃, Bi₂O₃, TiO₂, SnO₂, ZrO₂, MoO₃ and In₂O₃ are discussed in terms of their observed polymorphism as a function of the synthesis techniques used and the conditions of temperature and particle size, as reported in the literature. The tabulation of literature data on these functional systems is believed to be significant for developing nanomaterial database and structural property maps, ultimately guiding the appropriate nanomaterial selection for specific engineering applications. Supplementary information will be available at http://www.icevirtuallibrary.com/upload/10.1680nme.12.00037_SupplementaryInformation.pdf