Nanostructured α- and β-cobalt hydroxide thin films

A new perspective in the use of electrochemical methods to deposit cobalt hydroxide thin films is presented. Ordered arrays of alpha-Co(OH)(2) (hydrotalcite-like (Co-HT)) and beta-Co(OH)(2) nanoparticles were synthesized on transparent conductive oxide (TCO) substrates by localized cathodic electrogeneration of hydroxyl via the reduction of NO2- (or NO3-) ion precursors in solution containing CO2+ in very low concentration. The thin films, analyzed by X-ray diffraction and scanning electron microscopy were found to be composed of vertically oriented platelets with the crystallographic c-axis parallel to the substrate surface. Turbostratic disorder was not observed in the films. UV/Vis spectra and thermal gravimetric analyses (TGA) indicated distinct variation between the Co-HT structures. Films deposited at 60 degrees C using a nitrite precursor generated uniform, vibrant-green mixed-valence Co-HT(Co2+/Co3+). Nitrate precursors yielded a "hydroxyl-deficient" Co-HT (Co2+ only). Films deposited at 95 degrees C in nitrate solution yielded beta-Co(OH)(2). The films obtained in presence of nitrite were thicker than those obtained in nitrate. They were formed of beta-Co(OH)(2) and contained traces of Co-HT. (C) 2009 Elsevier Ltd. All rights reserved.