Single Unit‐Cell Layered Bi2Fe4O9 Nanosheets: Synthesis, Formation Mechanism, and Anisotropic Thermal Expansion

Abstract 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 Bi2Fe4O9, BiFeO3, and their mixture were successfully obtained by controlling the KOH concentration in the hydrothermal solutions. The as‐prepared Bi2Fe4O9 products were crystalline with Pbam space group, had nanosheet morphology, and tended to aggregate into nanofloret or random stacking. Each Bi2Fe4O9 nanosheet was a single crystal with (001) plane as its exposed surface. Single unit‐cell layered Bi2Fe4O9 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 Bi2Fe4O9 crystal. The formation mechanism and structural model of the as‐prepared single unit‐cell layered Bi2Fe4O9 nanosheets have been given. The growth of Bi2Fe4O9 nanosheets was discussed. Thermal analysis showed that the Bi2Fe4O9 phase was stable up to 1260 K. The thermal expansion behavior of the Bi2Fe4O9 nanosheet was nonlinear. The thermal expansion coefficients of the ultrathin Bi2Fe4O9 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 Bi2Fe4O9 nanosheets.