Topotactic Transformation of Bismuth Oxybromide into Bismuth Tungstate: Bandgap Modulation of Single-Crystalline {001}-Faceted Nanosheets for Enhanced Photocatalytic CO Reduction.

The photocatalytic conversion of CO to energy-rich CH solar fuel is an ideal strategy for future energy generation as it can resolve global warming and the imminent energy crisis concurrently. However, the efficiency of this technology is unavoidably hampered by the ineffective generation and utilization of photoinduced charge carriers. In this contribution, we report a facile topotactic transformation approach where {001}-faceted BiOBr nanosheets (BOB-NS) were employed as the starting material for the formation of single-crystalline ultrathin BiWO nanosheets (BWO-NS). The as-obtained BWO-NS not only preserved the advantageous properties of the 2D nanostructure and predominantly exposed {001} facets but also possessed enlarged specific surface areas as a result of sample thickness reduction. As opposed to the commonly observed bandgap broadening when the particle sizes decrease to an ultrathin nanoscale owing to the quantum size effect, the developed BWO-NS exhibited a fascinating bandgap narrowing compared to those of pristine BiWO nanoplates (BWO-P) synthesized from a conventional one-step hydrothermal approach. Moreover, the electronic band positions of BWO-NS were modulated as a result of ion exchange for the reconstruction of the energy bands, where BWO-NS demonstrated significant upshifting of CB and VB levels; these are beneficial for photocatalytic reduction applications. This propitious design of BWO-NS through integrating the merits of BOB-NS caused BWO-NS to exhibit substantial 2.6 and 9.3-fold enhancements of CH production over BOB-NS and BWO-P, respectively.