Binary transition metal oxides
Binary transition metal oxides BTMOs possess higher reversible capacity, better structural stability and electronic conductivity, and have been widely studied to be novel electrode materials for supercapacitors. Binary transition metal oxides this review, we present an extensive description of BTMO materials and the most commonly used synthetic methods. Furthermore, we review several notable BTMOs and their composites in application of supercapacitors. With the increasing attention for energy storage, more and more exciting results about BTMO materials will be reported in the future.
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The only stable pentahalide is VF 5which is readily hydrolysed and a strong Lewis acid. In the solid state it exists as an infinite chain polymer with cis -bridging fluorides but in the vapour phase it has a binary transition metal oxides monomeric structure. Tetrahalides are formed by Ti and V. The Ti tetrahalides are fairly unreactive in redox and halogenation chemistry, unlike the V compounds. They also tend to halogenate organic material.
All trihalides of the elements from Ti to Cr are known. Chromium III is binary transition metal oxides stable toward both reduction or oxidation. There is a marked tendency toward decreasing ionic character on passing from left to right across the period and from the fluorides to the heavier halides. Ferric chloride and bromide show essentially covalent behaviour such as low MPs and solubility in donor organic solvents.
Many trihalides can be prepared by direct combination of binary transition metal oxides elements. In those cases where direct combination gives a higher oxidation state, trihalides can be produced by either thermal dissociation, disproportionation of the higher halide or by reduction- for example TiCl 3 can be prepared by binary transition metal oxides of TiCl 4 with H 2 at high temperatures.
All these trihalides adopt structures in which the metal is six-coordinate, either octahedral or distorted octahedral. Many of the lattices are complicated, but can be represented as CrCl 3 or BiI 3 types. In some cases it becomes even more complicated since some salts exist in more than one binary transition metal oxides. For example, at low temperature below K CrCl 3 exists in the rhombohedral form mentioned above, but at room temperature it is monoclinic. The difluorides commonly have rutile structuresfor example MnF 2the dichlorides CdCl 2 structures and the diiodides CdI 2 structures.
Dibromides have either CdCl 2 or CdI 2 structures or both. Dihalides are all ionic and typically dissolve in water to give aquo complexes or mixed aquo-halo-complexes. They react extremely rapidly with O 2and Ti II even rapidly reduces water to liberate hydrogen. The Normal Spinel structure, named after a mineral form of MgAl 2 O 4 and of generic formula AB 2 O 4 may be approximated as a cubic close packed lattice of oxide ions with one-eighth of the tetrahedral holes binary transition metal oxides by the A II ions and one-half of the octahedral holes occupied by the B III ions.
Given the fact that this occurs, it is evident that the energy factors directing the two different ions to the different sites are not overwhelmingly large, and it is not surprising that such structures are highly susceptible to defects in actual crystals. One factor that may influence this site selectivity is the crystal field stabilisation energy of transition metal ions.
All the elements from Ti to Fe give stable M 2 O 3 oxides with corundum-type structures. These oxides are all ionic and predominantly basic. Note that CrO 2 is ferromagnetic and used in the production of magnetic tapes. The monoxides show a variety of physical properties.
Thus Ti and V are quasi-metallic, CrO is marginal but Mn to Cu are typical ionic insulators or more precisely, semiconductors.
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