Intercalated Water Drives Anomalous Thermal Expansion in the Tetragonal Zircon Structured Bismuth Vanadate BiVO 4 Photocatalyst.

The thermal transformation of the tetragonal-zircon (tz-) to tetragonal-scheelite (ts-)BiVO 4 was studied by in situ synchrotron X-ray diffraction, thermogravimetric analysis, and Fourier-transformed infrared spectroscopy. Upon heating, the tetragonal zircon polymorph of BiVO 4 (tz-BiVO 4 ) transitioned to the ts-polymorph between 693-773 K. Above 773 K, single phase ts-BiVO 4 was observed before transitioning to the monoclinic fergusonite (mf-) polymorph upon cooling. An anomaly in thermal expansion was observed between 400-500 K, associated with the loss of intercalated H 2 O/NH 4 + from the coprecipitation procedure. Heating tz-BiVO 4 resulted in contraction of the V-O bond distance and VO 4 polyhedra volume, ascribed to rotation of the tetrahedra groups. Attempts to study this by neutron diffraction failed due to the large incoherent scatter from the hydrogenous species. Efforts to remove these species while maintaining the tz-BiVO 4 structure were unsuccessful, suggesting they play a role in stabilizing the tz-polymorph. The local structure of both mf-BiVO4 and tz-BiVO4 were investigated by X-ray pair distribution function analysis, revealing local distortions.