Desorption influence of water on structural, electrical properties and molecular order of vanadium pentoxide xerogel films

Abstract

Vanadium pentoxide xerogel films were grown by sol-gel method on pre-treated glass substrates, the gelation time was 14 days. The crystallinity of the films was analyzed with X-ray diffraction (XRD), identifying the composite V$_2$O$_5\cdot$nH$_2$O before, and both vanadium pentoxide xerogel and $\alpha$-V$_2$O$_5$ phases after, being subjected to thermal treatment (47, 97, 147, 204, 237, 272, 297 and 330$^{\circ}$C during 15~minutes in each isotherm). The termal treatment reduces the degree of hydrations gel ($n$) from 2.1 to 1.4, besides the secondary phase ($\alpha$-V$_2$O$_5$) has lattice parameters very similar to the precursor powder (which deviate about 0.3$ % $). The electrical conductivity presents a semiconductor behavior in agreement with small polaron model, thermally activated and irreversible. The activation energies for three consecutive cycles were studied and analyzed: a strong dependency between the degree of hidratation's gel $n$ with activation energy for high and low temperature regions was found. $\mu$-Raman Spectroscopy showed the influence of temperature in the vanadium pentoxide gel film, presenting a phase transition from crystalline-amorphous for temperatures above 272$^{\circ}$C and inferring that the water presence in the sample is responsible in some way for the crystallinity of the material.