Tetragonal to Monoclinic Crystalline Phases Change of BiVO₄ via Microwave-Hydrothermal Reaction: In Correlation with Visible-Light-Driven Photocatalytic Performance

Explicit control of the crystalline phases and morphology of semiconducting BiVO₄ crystals has been successfully synthesized via microwave-hydrothermal condition (MW-HT) without requiring any template/surfactant, doping of metal ions, and altering pH of reaction solution. Unambiguously, the crystalline phase of BiVO₄ crystal has transformed from tetragonal zircon type (tz) to monoclinic scheelite (m) via mixed-phase (m-tz) by altering microwave-irradiation time at fixed microwave-irradiation power (800 W) without changing any precursor concentrations throughout the reaction. X-ray diffraction and Rietveld refinement studies confirmed the phase transformation of BiVO₄ crystals that occurs by controlling the irradiation time (10–22 min) and temperature (116–195 °C). The changes in VO₄^3– tetrahedron bond strength and bond length attributed to phase transitions in BiVO₄ crystals were corroborated by Raman spectra. Field emission scanning electron microscope revealed the sequential growth and rational morphological evolution of spherical-shaped zircon type tz-BiVO₄ particles to preferentially oriented (010) and (110)-faceted decahedron-shaped scheelite m-BiVO₄ crystals. The UV-reflectance and photoluminescence analyses revealed reduction in the optical bandgap and efficient charge separation with tunneling of excitons through interfaces, owing to phase transitions from tetragonal to monoclinic in BiVO₄ crystals. High-resolution transmission electron microscopy images revealed the formation of heterojunctions between both the phases of BiVO₄ crystals. The photocatalytic degradation of Rhodamine-B dye under natural sunlight showed maximum efficiency of 95% with highest rate kinetics (κ_avg = 0.0718/min) using mixed-phase BiVO₄ (m:tz-60:40) crystals, whereas under simulated sunlight, monoclinic phase BiVO₄ crystals showed high degradation efficiency of 87% with low rate kinetics (κ_avg= 0.0436/min) for 200 min. The free-radical trapping tests revealed that superoxide radical (•O₂) and hydroxyl radical (•OH) are active radicals during photocatalysis. Significantly, the MW-HT synthesized mixed-phase BiVO₄ retained photocatalytic activity and structural stability even after three cycles. These findings open possibilities for innovative design of highly efficient semiconductor photocatalyst for environmental remediation applications.