Visible-light-induced photocatalytic behaviour of bismuth vanadate nanoparticles via mechanochemical milling

Monoclinic BiVO₄ nanoparticles were synthesized via a mechanochemical route using high-energy ball milling to investigate the influence of milling time on their structural, optical and photocatalytic properties. The synthesized materials were characterized by XRD, UV-DRS, SEM, Raman, XPS and TEM. X-ray diffraction analysis confirmed the formation of phase-pure monoclinic BiVO₄, with crystallite size and lattice strain showing a systematic dependence on milling duration. A monotonic blue shift in the optical band-gap energies of BiVO₄ nanoparticles (2.14–2.30 eV) was observed with increasing milling time, reflecting progressive phase purification and structural ordering. Raman spectra showed characteristic vibrational modes at 210, 325, 367, 703 and 825 cm⁻¹, confirming structural uniformity. SEM revealed spherical nanoparticles and TEM for the optimized 10 h sample showed a nanocrystalline nature. XPS confirmed the formation of chemically pure BiVO₄ after 10 h of milling. Photocatalytic activity was evaluated via methylene blue (MB) degradation under visible-light LED irradiation. The BiVO₄ milled for 10 h exhibited the highest degradation efficiency (91% in 120 min) and a pseudo-first-order rate constant of 0.01217 min⁻¹, attributed to optimal crystallinity, reduced strain and a suitable bandgap. Scavenger studies identified superoxide radicals (•O₂⁻) as the dominant reactive species. Alkaline conditions significantly enhanced degradation kinetics and recyclability tests confirmed good catalyst stability over three cycles.