Single Unit-Cell Layered Bi₂ Fe₄ O₉ Nanosheets: Synthesis, Formation Mechanism, and Anisotropic Thermal Expansion

As an important multiferroic material, pure and low-dimensional phase-stable bismuth ferrite has wide applications. Herein, one-pot hydrothermal method was used to synthesize bismuth ferrite. Almost pure Bi₂ Fe₄ O₉ , BiFeO₃ , and their mixture were successfully obtained by controlling the KOH concentration in the hydrothermal solutions. The as-prepared Bi₂ Fe₄ O₉ products were crystalline with Pbam space group, had nanosheet morphology, and tended to aggregate into nanofloret or random stacking. Each Bi₂ Fe₄ O₉ nanosheet was a single crystal with (001) plane as its exposed surface. Single unit-cell layered Bi₂ Fe₄ O₉ nanosheets had a uniform thickness of 1 nm. The surface energies of various (100), (010), and (001) planes were 3.6-4.0, 5.6-15.1, and 1.7-3.0 J m^-2 , respectively, in the Bi₂ Fe₄ O₉ crystal. The formation mechanism and structural model of the as-prepared single unit-cell layered Bi₂ Fe₄ O₉ nanosheets have been given. The growth of Bi₂ Fe₄ O₉ nanosheets was discussed. Thermal analysis showed that the Bi₂ Fe₄ O₉ phase was stable up to 1260 K. The thermal expansion behavior of the Bi₂ Fe₄ O₉ nanosheet was nonlinear. The thermal expansion coefficients of the ultrathin Bi₂ Fe₄ O₉ nanosheets on the a-, b-, c-axes, and on the unit-cell volume V were determined, showing an anisotropic thermal expansion behavior. This study is helpful for the controllable synthesis of ultrathin Bi₂ Fe₄ O₉ nanosheets.