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

Three different crystal morphologies of alpha-Fe2O3, including uniform hexagonal, square, and rhombic shapes, were prepared according to the aqueous-thermal reaction. The hexagonal-shaped alpha-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 (perpendicular to) modes, and one absorption peak at 650 cm(-1) appeared for the parallel (||) mode for hexagon-shaped alpha-Fe2O3 during analysis by Fourier-transform infrared spectroscopy. However, the peaks of square- and rhombic-shaped alpha-Fe2O3 for perpendicular (perpendicular to) 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 alpha-Fe2O3, but all peak positions for square-shaped and rhombic-shaped alpha-Fe2O3 exhibited a significant blueshift. The structure of hexagon-shaped alpha-Fe2O3 was relatively tolerant regarding the polarization properties of vibration modes; however, the symmetry of crystal square-shaped and rhombic-shaped alpha-Fe2O3 changed, subsequently revealing different optical properties.