Regulating the phase transition of monoclinic Bi4O5Br2 through the synergistic effect of “drag force” and facet recognition by branched polyethyleneimine

In this study, the phase transition from tetragonal BiOBr to monoclinic Bi4O5Br2 was regulated in the presence of branched polyethyleneimine (BPEI) containing massive amino groups via a hydrothermal route. Based on the crystal phase of the products obtained at different concentrations of the BPEI and Br− sources, the relative concentration of BPEI (the concentration ratio of the BPEI and Br− sources, BPEI/Br−) played the decisive role in this phase transition. In particular, a BPEI/Br− value over 0.2 g mmol−1 with excessive BPEI was critical for the phase transition. Further, a reasonable theory referring the synergistic effect of the “drag force” and facet recognition of excessive BPEI underlying this phase transition was confirmed through a series of systematically designed experiments. The protonated amino groups from excessive BPEI could not only act as a “drag force” to extract the bromine ions from the solid-state BiOBr nanosheets but also recognize the (001) facet of BiOBr, thereby distorting the tetragonal structure of the BiOBr nanosheets and forming monoclinic Bi4O5Br2 nanosheets. This work provides a new insight and proof of the regulation of the phase transition of BiOBr to Bi4O5Br2.