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

The phase transition from tetragonal BiOBr to monoclinic Bi4O5Br2 was regulated in the presence of branched polyethyleneimine (BPEI) containing massive amino groups through a hydrothermal route. Based on the crystal phase of products obtained at different concentration of BPEI and Br- sources, the relative concentration of BPEI (the concentration ratio of the BPEI and Br- sources, BPEI/Br) played the decisive role for this phase transition. Especially, for BPEI/Br values over 0.2 g/mmol with excessive BPEI was the critical point for the phase transition. And, a reasonable theory referring the synergistic effect of “drag force” and facet recognition of excessive BPEI was proposed for this phase transition and confirmed through a series of systematical designed experiments. That was, the numerous protonated amino groups from the excessive BPEI could not only act as a “drag force” to extract bromine ions from the solid-state BiOBr nanosheets but could also recognize the (001) facet of BiOBr, distorting the tetragonal structure of BiOBr nanosheets and forming monoclinic Bi4O5Br2 nanosheets. This work provides new insight and evidential proofs into the regulation of the phase transition of BiOBr to Bi4O5Br2.