Topotactic Transformation of Bismuth Oxybromide into Bismuth Tungstate: Bandgap Modulation of Single-Crystalline {001}-Faceted Nanosheets for Enhanced Photocatalytic CO2 Reduction.

: The photocatalytic conversion of CO2 into energy-rich CH4 solar fuel is an ideal strategy for future energy generation as it can resolve global warming and 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 in situ topotactic transformation approach, where {001}-faceted BiOBr nanosheets (BOB-NS) were employed as the starting material for the formation of single-crystalline ultrathin Bi2WO6 nanosheets (BWO-NS). The as-obtained BWO-NS not only preserved the advantageous properties of 2D nanostructure and predominantly exposed {001} facets, but also possessed enlarged specific surface area as a result of sample thickness reduction. In opposed to the commonly observed bandgap broadening when the particle sizes decrease to ultrathin nanoscale owing to the quantum size effect, the developed BWO-NS exhibited a fascinating bandgap narrowing compared to pristine Bi2WO6 nanoplates (BWO-P) synthesized from conventional one-step hydrothermal approach. Moreover, the electronic band positions of BWO-NS were modulated resulting from the ion exchange for reconstruction of energy bands, where BWO-NS demonstrated significant up-shifting of CB and VB levels, which are beneficial for photocatalytic reduction applications. This propitious design of BWO-NS through integrating the merits of BOB-NS rendered BWO-NS to exhibit a substantial 2.6-fold and 9.3-fold enhancement of CH4 production over BOB-NS and BWO-P, respectively.