Studies on the electrochemical deposition of niobium oxide

Various strategies for the electrodeposition of niobium oxide from both non-aqueous and aqueous solution are explored. The methods used were based on the electrochemical manipulation of the pH which leads to hydrolysis of soluble, well defined niobium precursors, leading to oxide deposition at the electrode. The reactions which proceed are believed to be closely related to those involved in conventional sol-gel processing. In non-aqueous systems containing niobium alkoxides, hydroxide ions are generated by the two-electron reduction of tertiary alcohols (at < -2.5 V) and react with the niobium precursor to give a film of niobium oxide on the dropping mercury electrode. Hydroxide generation via the reduction of oxygen to superoxide in O-2-saturated, non-aqueous solvents containing NbOCl3 did not, however, lead to niobium oxide films. In acidic aqueous systems containing [NbOCl5](2-) species, raising the local pH around the electrode by hydrogen evolution does give rise to niobium oxide deposition, but not at the electrode; In aqueous alkaline solutions of the niobates K7H [Nb6O19] or (TMA)(6)[Nb10O28] (TMA = tetramethylammonium), protons generated by the electrochemical oxidation of water at > + 1.5 V cause the formation of mixed niobate and niobium oxide films on the electrode after > 30 min.