Polymorphism in nanocrystalline binary metal oxides

Abstract

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

Selective detection of naphthalene with nanostructured WO<sub>3</sub> gas sensors prepared by pulsed laser deposition

“…The reason for this is believed to be the small particle size of the samples, similarly as in the samples presented in this study. Also, the existence of the ε phase in the WO 3 composition has been proven to enhance WO 3 structures sensitivity to acetone Righettoni et al, 2010;Sood and Gouma, 2013). The reason was suggested to be the ferroelectricity of the ε phase, namely the spontaneous electric dipole moments it possesses, which are then highly contributing to the chemical reaction between the WO 3 surface and the target gas.…”

Brookite: Nothing New under the Sun?

“…Polymorphism-the ability of a solid material to exist in more than one crystal structure-is a common property of metal oxides [1]. Different polymorphs have distinct chemico-physical properties, such as electronic and optical properties, magnetism [2], ion conductivity [3], photo/electro-chromism [4], and surface energy and atom arrangement, which in turn affect the material performance in various applications, such as sensing [5] and photocatalysis [6,7].…”

Effect of TiO2 polymorph and alcohol sacrificial agent on the activity of Au/TiO2 photocatalysts for H2 production in alcohol–water mixtures

“…The three polymorphs are constructed from TiO 6 octahedra, differing primarily in the connectivity of the octahedra. Rutile has a tetragonal unit cell (P4 2 /mnm, a = 4.5937 Å, c = 2.9587 Å; Z = 2), with a structure that can be described as chains of edge sharing TiO 6 octahedra running parallel to the c-axis [25]. Rutile is the most stable TiO 2 polymorph, though generally has poor photocatalytic activity owing to the fact it is a direct band gap semiconductor which allows fast electronhole pair recombination following photoexcitation [26,27].…”

“…Rutile is the most stable TiO 2 polymorph, though generally has poor photocatalytic activity owing to the fact it is a direct band gap semiconductor which allows fast electronhole pair recombination following photoexcitation [26,27]. Anatase also crystallises with a tetragonal unit cell (I4 1 /amd, a = 3.7845 Å, c = 9.5143 Å; Z = 4), with each TiO 6 octahedra edge sharing with four adjacent octahedra [25]. Anatase is an indirect band gap semiconductor, typically displaying photocatalytic activities 1-2 orders of magnitude higher than rutile due to longer charge carrier lifetimes.…”

“…Anatase is an indirect band gap semiconductor, typically displaying photocatalytic activities 1-2 orders of magnitude higher than rutile due to longer charge carrier lifetimes. In brookite, each octahedron shares three edges with adjoining octahedra, to form an orthorhombic structure (Pbca, a = 5.4558 Å, b = 9.1819 Å, c = 5.1429 Å, Z = 8) [25]. To date, only a limited amount of data is available about the photocatalytic activity of brookite, owing to historical difficulties in synthesising this polymorph in pure form [28,29].…”