Hydrothermal synthesis of bismuth niobates and their application in azo-dyes photo-discoloration

Abstract

Bismuth niobates were obtained by hydrothermal synthesis using bismuth (III) nitrate pentahydrate [Bi(NO3)3·5H2O] and a Lindqvist potassium hexaniobate [K8-xHxNb6O19.nH2O] as soluble precursors. Pure crystalline Bi5Nb3O15, β-BiNbO4 and α- BiNbO4 were obtained as pure phases through co-precipitation at pH = 8, followed by hydrothermal treatment and calcination at 600, 700 or 800 °C, respectively, which are milder temperatures than those used in solid state, citrate sol-gel and ball milling methods. Band gap energies of α-BiNbO4 and β-BiNbO4 were estimated by UV–vis diffuse reflectance spectroscopy (DRS) as 3.2–3.4 eV, which are higher than experimental values reported in the literature. The materials were evaluated in the methyl orange azo-dye photo-discoloration under UV irradiation and complete bleach was attained. The triclinic phase presented a better efficiency as a photocatalyst than the orthorhombic phase. Unexpectedly, the mixture of both phases resulted in the highest photocatalytic efficiency, similar to that found for Degussa P25 commercial catalyst.