Influence of the Bismuth Precursor on Phase Purity and Photocatalytic Activity of Hydrothermally Synthesised BiVO4 for Dye and Antibiotic Degradation

Bismuth vanadate (BiVO4) is widely studied as a visible-light-responsive photocatalyst, but the influence of the bismuth precursor on phase composition and the resulting photocatalytic activity has rarely been quantified. In this work, two BiVO4 samples were synthesised hydrothermally under identical conditions and calcined at 600 °C, differing only in the bismuth source: basic bismuth nitrate Bi5O(OH)9(NO3)4 (BVO–1) and bismuth nitrate pentahydrate Bi(NO3)3·5H2O (BVO–2), both with NH4VO3 as the vanadium source. Rietveld refinement of X-ray diffraction data shows that BVO–2 is phase-pure monoclinic-scheelite BiVO4 (clinobisvanite), whereas BVO–1 is a three-phase mixture of approximately 25% BiVO4, 59% Bi5O7NO3, and 16–19% β-Bi2O3. Field-emission scanning electron microscopy is consistent with this finding: BVO–2 forms uniform faceted polyhedra (1–2 μm), while BVO–1 is a heterogeneous aggregate of sub-400 nm particles with at least three coexisting morphologies. X-ray photoelectron spectroscopy of BVO–2 shows single Bi(III) and V(V) states, and diffuse reflectance spectroscopy gives optical band gaps of 2.55 eV for BVO–1 and 2.50 eV for BVO–2. Photocatalytic degradation of methylene blue (MB), methyl orange, tetracycline (TC), and ciprofloxacin under germicidal UV-C irradiation at 253.7 nm reveals a pollutant-dependent activity inversion: BVO–1 outperforms BVO–2 for cationic MB (k=7.6 × 10−3 min−1 against 3.9 × 10−3 min−1) in line with its larger dark adsorption of the dye, whereas BVO–2 is the more active for both antibiotics, with the larger margin for TC (kTC=15.2 × 10−3 min−1, 1.8× that of BVO–1). These results identify the bismuth precursor as a determinant of phase purity in the hydrothermal synthesis of BiVO4: Bi(NO3)3·5H2O yields phase-pure BiVO4, whereas Bi5O(OH)9(NO3)4 produces a multi-phase composite whose advantage was observed only for MB, where adsorption contributes strongly.