Abstract
Transition-metal oxide nanostructured materials are potentially attractive alternatives as anodes for Li ion batteries and as photocatalysts. Combining the structural and thermal stability of titanium oxides with the relatively high oxidation potential and charge capacity of molybdenum(VI) oxides was the motivation for a search for approaches to mixed oxides of these two metals. Challenges in traditional synthetic methods for such materials made development of a soft chemistry single-source precursor pathway our priority. A series of bimetallic Ti-Mo alkoxides were produced by reactions of homometallic species in a 1:1 ratio. Thermal solution reduction with subsequent reoxidation by dry air offered in minor yields Ti2Mo2O4(OMe)6(OiPr)6 (1) by the interaction of Ti(OiPr)4 with MoO(OMe)4 and Ti6Mo6O22(OiPr)16(iPrOH)2 (2) by the reaction of Ti(OiPr)4 with MoO(OiPr)4. An attempt to improve the yield of 2 by microhydrolysis, using the addition of stoichiometric amounts of water, resulted in the formation with high yield of a different complex, Mo7Ti7+xO31+x(OiPr)8+2x (3). Controlled thermal decomposition of 1–3 in air resulted in their transformation into the phase TiMoO5 (4) with an orthorhombic structure in space group Pnma, as determined by a Rietveld refinement. The electrochemical characteristics of 4 and its chemical transformation on Li insertion were investigated, showing its potential as a promising anode material for Li ion batteries for the first time. A lower charge capacity and lower stability were observed for its application as an anode for a Na ion battery.
Highlights
The interest in early-transition-metal oxide materials has in recent years been fueled by perspectives of their application in photovoltaics,[1] as photocatalysts in the production of solar fuels,[2] and as electrodes for alkali-metal batteries.[3]
Titania was shown to be prospective as a robust anode material for Li ion batteries, competing with graphite due to its reasonable charge capacity and ability to prevent the formation of lithium dendrites.[5,6]
Established approaches to increase the reactivity of metal alkoxides toward complex formation are based on partially replacing the alkoxide groups with sterically available oxide ligands prone to form bridging bonds between metal centers. These approaches are based on either hydrolytic addition of controlled minor amounts of water, which was actively used in the works of Kickelbick et al.[21] or solvolytic approaches, where the mixtures are subjected to heating with decomposition of alkoxide ligands and form oxo ligands instead, actively pursued by Eslava, Wright, et al.[16,22]
Summary
The interest in early-transition-metal oxide materials has in recent years been fueled by perspectives of their application in photovoltaics,[1] as photocatalysts in the production of solar fuels,[2] and as electrodes for alkali-metal batteries.[3]. Compound 1 crystallizes in a centrosymmetric monoclinic structure built up of the centrosymmetric molecules Ti2Mo2O4(OMe)6(OiPr)[6] (1) and is a clathrate with two molecules of interstitial MeOH per molecule of the alkoxide (see Figure 1) It belongs to space group No 14 in P21/c, very characteristic of the layered packing of low-symmetry metal−. In order to approach the heterometallic oxide phase, a sample of single crystals of compound 3 was investigated by TGA, revealing that its decomposition occurs in the temperature interval 140−190 °C with subsequent combustion of the residual carbon at 480−500 °C (see Figure S2). These observations were used to choose the conditions for the preparation of the mixed oxide phase. This is consistent with the behavior of other conversion type anodes known in the literature
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