The optical properties of nano-structural α-Fe₂O₃ dependence on the shape

Three different crystal morphologies of α-Fe₂O₃, including uniform hexagonal, square, and rhombic shapes, were prepared according to the aqueous-thermal reaction. The hexagonal-shaped α-Fe₂O₃ was enclosed by the 104 plane, while the square and rhombic structures were enclosed by the 110 plane. Two absorption peaks at 455 and 532 cm^-1 were found for the perpendicular (⊥) modes, and one absorption peak at 650 cm^-1 appeared for the parallel (||) mode for hexagon-shaped α-Fe₂O₃ during analysis by Fourier-transform infrared spectroscopy. However, the peaks of square- and rhombic-shaped α-Fe₂O₃ for perpendicular (⊥) mode blueshifted, and the former two peaks merged together forming a broad band at approximately 480 cm^-1. For Raman spectra determination, the peaks arose from the Brillouin zone center, and two additional peaks were observed at 660 and 1320 cm^-1, belonging to 1 longitudinal optical (1LO) and 2 longitudinal optical (2LO) modes. All three materials exhibited higher intensities when excited at a wavelength of 633 cm^-1. Furthermore, in the polarization state, the centers of all peak positions slightly shifted for hexagon-shaped α-Fe₂O₃, but all peak positions for square-shaped and rhombic-shaped α-Fe₂O₃ exhibited a significant blueshift. The structure of hexagon-shaped α-Fe₂O₃ was relatively tolerant regarding the polarization properties of vibration modes; however, the symmetry of crystal square-shaped and rhombic-shaped α-Fe₂O₃ changed, subsequently revealing different optical properties. RESEARCH HIGHLIGHTS: The hexagon-shaped, square-shaped, and rhombic-shaped α-Fe₂O₃ enclosed by different planes were synthesized. The Fourier Transform Infrared spectrometer peaks of α-Fe₂O₃ depended on their hexagon, square and rhombic shapes. Compared with hexagon-shaped α-Fe₂O₃, the Raman peaks for square and rhombi ones significantly shifted. The hexagon-shaped α-Fe₂O₃ is relatively tolerant regarding the polarization properties.