Effect of Calcination Temperature on Lattice Contraction in Hydrothermally Grown Monoclinic BiVO4 for Photocatalytic Dye Degradation

Calcination is a routine step in the preparation of BiVO4 photocatalysts, yet its structural effect is often inferred from peak positions alone rather than from full-profile refinement. Here, hydrothermally synthesized BiVO4 was divided into an as-dried portion and portions calcined between 300 and 700 °C, so that calcination temperature was the only variable. The samples were examined by X-ray diffraction with Rietveld refinement, scanning electron microscopy, diffuse reflectance spectroscopy, and photocatalytic degradation of methylene blue (MB) and methyl orange (MO) under 254 nm irradiation. The as-dried powder is already predominantly monoclinic, with a minor zircon-type fraction (below 2 wt%) that disappears between 400 and 500 °C, so the monoclinic structure forms mainly during the hydrothermal step. The unit-cell volume contracts monotonically from 310.179 to 309.750 Å3, mainly through b (0.081%, against 0.035 and 0.023% for a and c), with no systematic change in the monoclinic angle. The crystallite size stays near 100 nm up to 500 °C and rises to 129 to 158 nm as the particles sinter. The band gap from direct Tauc plots remains between 2.49 and 2.54 eV. Every sample leaves less MB than photolysis alone after 240 min, and the sample calcined at 500 °C gives the highest apparent rate constant, about 2.6 times that of photolysis, whereas MO is not degraded. The MB activity follows neither the lattice contraction nor the band gap, so within the resolution of single runs it is not controlled by the refined structural or optical parameters.