Energetics of bismuth vanadate

• Bismuth vanadate crystallizes in three different polymorphs.
• High temperature calorimetric measurements were made to determine their formation enthalpies.
• Enthalpy of formation decreases in the order BV-ms→BV-ts→BV-tz.
• Photocatalytically active monoclinic-BiVO4 was found to be the most stable polymorph.

Bismuth vanadate has gained considerable interest as a photoanode for water splitting reactions under visible light. It exists in four different polymorphs, out of which three of them have been synthesized. Thermodynamic properties of these three polymorphs are investigated using high temperature oxide melt solution calorimetry. The monoclinic scheelite phase which exhibits photocatalytic activity under visible light is found to be the most stable polymorph, followed by tetragonal scheelite which exhibits activity under UV light. The photocatalytically inactive tetragonal zircon form is found to be the least stable polymorph. The small difference in enthalpy of formation between the two scheelite structures (−8 kJ/mol) is in accord with the reversibility of the transformation between them and the larger difference between the most stable monoclinic phase and the least stable tetragonal zircon phase (−23 kJ/mol) is in accord with the irreversible (monoclinic→tetragonal zircon) phase transformation.

Schematic representation of polymorphic transitions in BiVO4 along with their formation enthalpies.Figure optionsDownload as PowerPoint slide