Structure–Property Relationships in Hydrothermally Synthesized Monoclinic Scheelite BiVO4: Role of Preferential Crystallographic Orientation in Photocatalytic Activity

Monoclinic scheelite BiVO4 is a visible-light photocatalyst whose performance is limited by charge–carrier recombination and inefficient charge transport. Here, a surfactant-free 32 full factorial design was used to study the effects of initial pH (0.25, 0.50, and 0.75) and hydrothermal temperature (160, 180, and 200 °C) on the structural, morphological, optical, photoelectrochemical, and photocatalytic properties of BiVO4. All nine samples exhibited the monoclinic scheelite phase, with no detectable secondary crystalline phases within the XRD detection limits, and showed nearly invariant unit-cell parameters, V−O bond length, crystallite size, and optical band gap (2.44 ± 0.06 eV). In contrast, the I(040)/I(110) intensity ratio, used as a descriptor of preferential crystallographic orientation, varied from 2.32 to 3.36 and showed a strong correlation with the apparent rate constant for methylene blue (MB) discoloration (r = 0.931; Bonferroni-corrected p = 0.002), with the correlation remaining robust in leave-one-out sensitivity analysis. Higher ratios were associated with plate-like morphologies, lower charge-transfer resistance, and higher photocurrent. Response-surface analysis identified an optimum within the experimental domain, and the sample prepared at pH 0.50 and 180 °C reached 86.8% MB discoloration in 90 min (kapp = 0.0224 min−1), with an apparent rate constant approximately six times than of the least active sample. Scavenger tests indicated that O2 reduction pathways and photogenerated holes are the main contributors. These results identify preferential crystallographic orientation as a useful, though not exclusive, structural descriptor of photocatalytic activity in monoclinic BiVO4.