The direct observation of structural development during vanadium pentoxide gelation

Abstract

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 × 3 nm × over 1 μm. 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)].

Effects of aging time on V2O5 sol-gel coatings

“…Using the thicknesses measured, the approximate range of conductivity for the dried gel is 2.0 to 3.0 x 10-4 (ohm*cm)-I for all coatings measured. This is slightly above the conductivity observed for xerogels obtained from ion-exchanged sols, which has been reported to be 10- 5 (ohm*cm)-l [8].…”

“…The increase in viscosity occurs after the induction time previously observed [5,6]. This increase in viscosity at days 5 to 7 leads to a better overall coverage on the coated substrates and increased coating thickness as the ability of the reacting solution to wet the substrate improves.…”

“…Krom cryo-Transmission Electron Microscopy (TEM) and Nuclear Magnetic Resonance (NMR) spectroscopic studies of the gelation, there are several intermediate structures and reactions in this process (4,5,6), but no information is yet available on the effects of aging on the dependent properties of viscosity, conductance, and film morphology in the coating process. This study observed the changes in these properties over a 30 day period by means of rheological measurements and microscopic observation of the dried coatings obtained.…”

“…This attempt to re-establish equilibrium in the solution speeds the polymerization process which leads to a burst of amorphous particulate in solution, observed in several microscopic studies. It is this amorphous particulate that grows and forms the crystalline ribbon structure in solution [5]. The ribbon itself is a two-dimensional aggregation of linear entities as shown by the striations on the ribbon and observed fraying of the ribbon structure.…”

“…Vanadium pentoxide gels exhibit a ribbon morphology in both solution and solid states [4,5,7,8]. The ribbon-like polymers have been measured to have dimensions of approximately 25nm x 3nm and over I micron in length in solution, and are transverse aggregations of linear polymers as shown by cryo-TEM microscopy[4,51.…”