Synthesis of Nanosheets from Layered Bismuth Oxide Bi_2.2Sr_1.8CaFe₂O₉ without Using Organic Exfoliation Agents

Bismuth oxide nanosheets were synthesized through the exfoliation of layered compounds without any organic exfoliation agents. The layered compound Bi_2.2Sr_1.8CaFe₂O₉, comprising Bi-O layers and Sr-Ca-Fe-O layers, was synthesized as the starting material. The Sr-Ca-Fe-O layers were selectively dissolved by shaking the compound in 0.3 M HNO₃ solution, yielding aggregates consisting solely of bismuth oxide layers. Subsequent ultrasonic treatment of these aggregates in pure water led to the formation of bismuth oxide nanosheets. The bismuth oxide nanosheet exhibited a higher photocatalytic activity for H₂ production compared to α-Bi₂O₃. Characterization using FE-SEM, AFM, and HAADF-STEM revealed that these nanosheets exhibit a complex structure. They consist of a crystalline nanosheet framework of ε-bismuth oxide, interspersed with island-like regions of amorphous or crystalline bismuth oxide. Additionally, self-standing membranes of bismuth oxide nanosheets, aligned in the [001] direction, were successfully synthesized. This method, which involves the partial dissolution of the layered structure through acid treatment followed by ultrasonic processing in water, is anticipated to be applicable to other layered compounds as well.