Growth of nanotubes in sol-gel-derived V 2 O 5 powders and films prepared under acidic conditions
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T. Barone, G. De Santo, and E. Cazzanelli Liquid Crystal Laboratory–Centro Nazionale della Ricerche–Istituto Nazionale Fisica della Materia (LICRYL CNR-INFM) and Centro di Eccellenza dell’ Universita` della Calabria (CEMIF.CAL), Dipartimento di Fisica, Universita` della Calabria, 87036 Arcavacata di Rende (Cosenza), Italy (Received 1 July 2008; accepted 29 October 2008)
The structural evolution with temperature of some V2O5 gels and thin films is presented, and the role of the hydrolysis conditions is investigated. Several techniques, i.e., x-ray diffraction, differential thermal analysis, infrared, and temperature-dependent Raman spectroscopy, have been used to follow the thermal behavior of the samples. When the bulk xerogels begin to change from a nanocrystalline phase to the orthorhombic a-V2O5, in the temperature range 280 to 300 C, a growth of vanadium oxide nanotubes also occurs, while at higher temperatures the crystallization into the a phase prevails. A slightly different evolution is observed for heat treated thin films, which show a structure containing polyvanadate chains near room temperature. They also present a growth of nanotubes for intermediate temperatures and a complete crystallization into the a phase when the temperature is further increased.
I. INTRODUCTION
Vanadium pentoxide has been widely investigated because of its interesting electrochemical performances. Pure V2O5, among several applications, can be used as a catalyst1 and as a cathode in lithium batteries.2 It is already known that binary mixtures of vanadium pentoxide and “bleaching” inorganic oxides can be used as passive counterelectrodes in electrochromic devices3 or as rectifying electrodes in modified nematic liquid crystal (NLC) cells.4,5 These materials, both as thin films and powders, have been produced by several methods, such as electrochemical deposition,6 vacuum deposition techniques including chemical vapor deposition7 and sputtering,8 as well as sol-gel syntheses; with this latter method, vanadate sols have been obtained mainly using protonation of sodium metavanadate solutions,9,10 metal oxidation with hydrogen peroxide,11 or from vanadium alkoxides.12 It has been demonstrated that the conduction properties of the obtained samples depend on their structural order. A particular interest has been developed in the last years for the occurrence of nanostructured forms of vanadium pentoxide, as nanotubes, nanorods, a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0061 J. Mater. Res., Vol. 24, No. 2, Feb 2009
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or nanobelts.13–18 Several methods have been tested to obtain such nanostructured forms of vanadium pentoxide, some of them involving the use of carbon nanotubes or of organic templates.15,18–21 However, vanadium oxide nanotubes (VONT) appear to grow, in particular conditions, also when more usual film-deposition methods are used, such as hydrothermal synthesis22 or direct current (dc) magnetron sputte
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