BiVO is a promising photocatalyst for water splitting using solar energy. Bulk BiVO exists in several crystalline forms; the monoclinic polymorph is believed to have the best photocatalytic properties. Raman spectroscopy is capable of identifying many properties of materials including distinguishing crystalline polymorphs in bulk and thin film solids.
Here it was applied to study thin films of BiVO grown by molecular beam epitaxy on (001)-oriented yittria-stabilized cubic zirconia substrate compared to single BiVO crystals. Raman spectra were measured at several temperatures from 10 to 500 K with 9 different excitation laser lines in the range from 325 to 514.5 nm. Temperature evolution of Raman spectra in the range 10-500 K was observed and attributed to the monoclinic phase; soft mode corresponding to ferroelastic transition observed in the crystal. Raman spectra indicate that the BiVO films and crystals have monoclinic structure, as expected for the growth conditions used and consistent with XRD and STEM. Excitation energy dependences of Raman intensities provide the bandgap estimates of 2.4 - 2.5 eV for the crystal and 2.6 - 2.7 eV for the film, the latter being consistent with spectroscopic ellipsometry and EELS data. The larger bandgap for the films is likely due to the strain.
Supported in part by the NSF Grant DMR-1006136 and the M. J. Murdock Charitable Trust