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
Tungsten trioxide nanowires of a high aspect ratio have been synthesized in situ in a TEM under an electron beam of current density 14 A/cm 2 due to a massive polymorphic reaction. Sol-gel-processed cubic phase nanocrystals of tungsten trioxide were seen to rapidly transform to one-dimensional monoclinic phase configurations, and this reaction was independent of the substrate on which the material was deposited. The mechanism of the self-catalyzed polymorphic transition and accompanying radical shape change is a typical characteristic of metastable to stable phase transformations in nanostructured polymorphic metal oxides. The findings are important to the controlled electron beam deposition of nanowires for functional applications starting from colloidal precursors.
Binary metal oxides occur in different polymorphic states under applied pressure and temperature. Structural changes occur due to polymorphic transitions in binary metal oxides. It is essential to theoretically predict the conditions of polymorphic transitions so that materials can be effectively used in engineering applications. Temperature and pressure are the two main factors affecting the bulk state phase transformation of materials. For nanomaterials, it has been observed that particle size and temperature are the main factors affecting the phase transformation, e.g., γ‐Fe2O3 to α‐Fe2O3, monoclinic to orthorhombic transformation in MoO3, anatase to rutile transformation in Titania, γ to α Alumina transformation. We compile from literature the main factors which affect the phase stability of a nanocrystalline binary metal oxide. A heuristic approach to formulate particle size is put forth. Factors like surface energy, surface tension, and particle shape are considered, and a value for critical particle size is formulated. The model fits well with the experimental results for nanocrystalline alumina, titania, zirconia, and Fe2O3. Such an approach can be applied to predict the particle size‐dependent stability of a phase at known temperature range.
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