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

Abstract

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 (κavg = 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 (κavg= 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.

Facile and rapid synthesis of a novel spindle-like heterojunction BiVO4 showing enhanced visible-light-driven photoactivity

“…The conduction and valence band potentials of a semiconductor can be determined by following eqn (2) and (3): 21,24 E VB ¼ X À E e + 0.5E g (2)…”

“…Recently, it has been reported that BiVO 4 can be successfully synthesized by a high-pressure microwave method. 24,25 The microwave-assisted synthesis of BiVO 4 under a high pressure can save reaction time and control morphology.…”

Effect of pH on visible-light-driven photocatalytic degradation of facile synthesized bismuth vanadate nanoparticles

“…The factor pH is one of the crucial parameters that play a pivotal role in the synthesis of nanoparticles due to the surface charge properties of the photocatalyst. From recent reports [26][27][28], it was observed that the synthesis of BiVO 4 in acidic medium results in the formation of mixed phases (tetragonal and monoclinic). The pH from acidic to basic favors crystallization of BiVO 4 in monoclinic structure and hence in this work it was aimed to study the structural, optical and photocatalytic behavior of BiVO 4 synthesized in basic medium.…”

Visible light sensitization of TiO2/Ag/N nanostructures synthesized by microwave irradiation for oxidative degradation of organic dyes

“…The main merits of microwave-assisted chemistry (MWAC) over conventional heating methods include homogeneous heating, fast nucleation processes, good reproducibility, simple control of the reaction parameters, possibility to use non toxic organic solvents and last but not least, it provides a scale-up platform for applications in the industry [15][16][17]. TiO 2 nanostructures synthesized by MWAC have been reported in the past [18][19][20] and recently the work was intensified [16,[21][22][23][24][25][26][27][28][29]. Preparation of P-doped nanostructures of Fe 2 O 3 -TiO 2 mixed oxides [21], selective oxidation of TiO 2 nanoestructures [22], noble-metal free co-catalysts decorating inorganic-organic hybrid materials [16], controlled morphologies and phases of BiVO 4 [27], CdS NRs-carbon NTs for the H 2 production [28] and synthesis of Au/SnS 2 nanoflowers [29] are current examples of the efficient use of MWAC for preparation of several new nanostructures.…”