Novel Materials Based on Organic-Tungsten Oxide Hybrid Systems
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P6.31.1
Novel Materials Based on Organic-Tungsten Oxide Hybrid Systems Bridget Ingham1, Shen V. Chong2 and Jeff L. Tallon1,2 1 Victoria University of Wellington, P O Box 600, Wellington, New Zealand. 2 Industrial Research Limited, P O Box 31 310, Lower Hutt, New Zealand.
ABSTRACT
The physical and electronic properties of tungsten oxide and some related hybrid materials have been examined via various spectroscopic techniques and the results complemented by ab initio computation. Hybrid materials based on intercalated straight-chain α,ω-diaminoalkanes in between sheets of corner-shared tungsten-oxide octahedra exhibit from XRD a linear expansion in the interlayer spacing with increasing number of carbon atoms in the amines. UV-visible diffuse reflectance spectra of several hybrid powders indicate a dominant absorption edge centred at 4.0 eV, with no apparent trend among the different samples as the alkyl chain length changes. This is higher than that of tungsten trioxide powder, which has an absorption edge centred at 2.8 eV. Ab initio calculations show that these experimental values may relate to the indirect band-gap energies of the respective compounds rather than the optical band gap. This was confirmed from the absorption coefficient results of WO3 thin films which yield a strong edge at 4.0 eV. Similar measurements of hybrid films with 1,12-diaminododecane as the organic spacer showed absorption throughout the visible range, with weak features at 4.2 and 4.9 eV.
INTRODUCTION
The electronic structure of tungsten oxide has received a huge amount of attention stemming from the study of its interesting chromogenic properties. Thin tungsten oxide films are well known for their colouration behaviour when they are subjected to different chemical and electrical treatment, which make them a good candidate for electrochromic devices [1-4]. There exist series of tungsten bronzes MxWO3, where M is a foreign atom which occupies interstitial sites in the tungsten oxide framework [5]. The ability of tungsten oxide to accommodate a wide variety of such atoms into the three dimensional framework is remarkable. The “tungsten oxides” mentioned thus far are those of tungsten trioxide which in its crystalline form consists of a three dimensional array of WO6 octahedra sharing all their corners as in ReO3 [6] (although tungsten trioxide also exists in different structural phases – hexagonal, pyrochlore, etc. – depending on synthesis conditions [7,8]). The hydrated forms of tungsten trioxide have two-dimensional layered structures. Tungsten trioxide monohydrate, WO3·H2O, consists of layers of corner shared WO6 octahedra, with W=O and W–OH2 occupying the apical positions and providing the chemical structure for the H-bonding which holds the layers together [9]. Different molecules can also be intercalated in between these layers, such as H2O (forming H2WO4·H2O [10]), monovalent cations [11], and organic species (forming organic-inorganic hybrid materials) [12-14]. The interesting electronic properties that tungsten bronzes possess
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