The direct observation of structural development during vanadium pentoxide gelation
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The sequence of structural evolution in the gelation of vanadic acid to form vanadium pentoxide gels was studied using cryogenic transmission electron microscopy (cryo-TEM) and scanning tunneling microscopy (STM). Small whiskers form from initially homogeneous solutions, and then grow into crystalline ribbon-like colloidal particles. It is proposed that the whiskers form from polymerization of dioxovanadium cations. The ribbons then grow by continued addition of dioxovanadium cations which are supplied by the decomposition of decavanadate ions. In solution, the ribbon-like particles have dimensions of approximately 25 nm X 3 nm X over 1 ^,m. These ribbons are flexible perpendicular to the plane of the ribbon. Upon drying, a flat rigid mass of ribbon-like particles is formed. The ribbons examined by STM showed striations 3 nm wide, a value that corresponds with the width of the unit cell proposed by J. Legendre and J. Livage [J. Colloid, and Interf. Sci. 94, 75 (1983)].
I. INTRODUCTION Vanadium pentoxide gels have received considerable interest recently due to their novel ribbon-like morphology and their humidity-sensitive, semiconducting electronic properties, which make them attractive as antistatic coatings and switching devices.1"3 Much work has focused on characterizing the xerogel (dried gel), especially the electrical properties. 17 It has been found that the properties of these gels are closely linked to their microstructure. Although effort has been made to characterize the final microstructure of the gels,8"11 the development of the structure has not been as well studied. To understand the structural evolution of these gels, it is important to characterize both the chemistry and the microstructure of the gels. A comprehensive review of the chemistry of vanadium pentoxide gels is given in Ref. 1. Here we review the information relevant to this work. Analyses of the chemistry during the gelation process have focused primarily on the soluble species, which are predominantly dioxovanadium cations, VO^~, and decavanadate (V^Of^) • JCH + (where x depends on pH, concentration, and the presence of stabilizing ions). The polycondensed vanadium oxyhydroxide material that precipitates to form the gel has too high a molecular weight to be characterized by a
'Current address: Sandia National Laboratories, Inorganic Materials Chemistry, Division 1846, P.O. Box 5800, Albuquerque, New Mexico 87185. b 'Current address: Department of Mechanical Engineering, Materials Division, University of California-Irvine, Irvine, California 92717. Author to whom correspondence should be addressed. 2530
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J. Mater. Res., Vol. 7, No. 9, Sep 1992
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standard solution techniques. Lemerle and coworkers12 have followed the condensation processes via solution sedimentation and have shown that decavanadate ions are present in these solutions, and that their concentration decreases as a function of time. They postulated that the higher molecular weight species, the fibrillar colloidal partic
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