Understanding the role of the phase type, and microstructure during pH-dependent synthesis of BiVO4 in the photoreduction of Cr(VI)

Bismuth vanadate nanopowders were obtained by co-precipitation at room temperature using Bi(NO₃)₃ and NH₄VO₃, as precursor salts. The effect of pH was comprehensively evaluated on the crystallization of the monoclinic scheelite-type (ms), and tetragonal zircon-type (tz) phases of BiVO₄. Preferential crystallizations of the tz-phase and ms-phase were observed at acid and basic pH-values, respectively. The mixture of two polymorphs influenced the morphological and optical properties of the materials compared with those of pristine powders. The photoactivity of the as-synthesized powders was evaluated during the photoreduction of Cr(VI) to Cr(III) under visible-light irradiation using formic acid (65 mM) as a hole scavenger in synthetically contaminated water (30 mg L⁻¹). The samples obtained at pH values between 4 and 8 succeeded in achieving complete reduction of Cr (VI) in less than an hour. Thus, the BiVO₄ phase-type in a monoclinic scheelite predominating mixture (pH 4 and 6) seems to be the most important structural factor accounting for the photoactivity towards Cr(VI) reduction, rather than the oxygen vacancy content or crystallite size.