CdWO4 polymorphs: Selective preparation, electronic structures, and photocatalytic activities

This work explored the selective synthesis of polymorphs of CdWO₄ in either tetragonal or monoclinic phase by optimizing the experimental parameters. Systematic characterization indicated that both polymorphs possessed similar spherical morphologies but different structural building blocks. Electronic structures calculations for both polymorphs demonstrated the same constructions of conduction band or valence band, while the conduction band widths of both polymorphs were quite different. Both CdWO₄ polymorphs exhibited good photocatalytic activity for degradation of methyl orange under UV light irradiation. When comparing to some other well-known tungstate oxide materials, the photocatalytic activity was found to follow such a consequence, monoclinic CdWO₄≈monoclinic ZnWO₄>tetragonal CdWO₄>tetragonal CaWO₄. The specific photocatalytic activity of monoclinic CdWO₄ was even higher than that of commercial TiO₂ photocatalyst (Degussa P25). The increased activity from the tetragonal CdWO₄ to the monoclinic was consistent with the trend of the decreased symmetry, and this could be explained in terms of the geometric structures and electronic structures for both polymorphs.