Unconventional Disorder by Femtosecond Laser Irradiation in Fe2O3

Abstract

This paper demonstrates that femtosecond laser-irradiated Fe₂O₃ materials containing a mixture of α-Fe₂O₃ and ε-Fe₂O₃ phases showed significant improvement in their photoelectrochemical performance and magnetic and optical properties. The absence of Raman-active vibrational modes in the irradiated samples and the changes in charge carrier emission observed in the photocurrent density results indicate an increase in the density of defects and distortions in the crystalline lattice when compared to the nonirradiated ones. The magnetization measurements at room temperature for the nonirradiated samples revealed a weak ferromagnetic behavior, whereas the irradiated samples exhibited a strong one. The optical properties showed a reduction in the band gap energy and a higher conductivity for the irradiated materials, causing a higher current density. Due to the high performance observed, it can be applied in dye-sensitized solar cells and water splitting processes. Quantum mechanical calculations based on density functional theory are in accordance with the experimental results, contributing to the elucidation of the changes caused by femtosecond laser irradiation at the molecular level, evaluating structural, energetic, and vibrational frequency parameters. The surface simulations enable the construction of a diagram that elucidates the changes in nanoparticle morphologies.

Figure 1

TGA (red line) and DTA (blue line) in the curves of Fe₂O₃ samples.

Figure 2

XRD patterns for the α-Fe₂O₃ and ε-Fe₂O₃ materials heat-treated at 860 °C for 30 min (860-30), at 900 °C for 10 min (900-10), and at 900 °C for 30 min (900-30). In (a) nonirradiated materials and in (b) femtosecond laser-irradiated materials.

Figure 3

Conventional unit cell representation for both (a) α-Fe₂O₃ and (b) ε-Fe₂O₃ phases, highlighting the cluster units. The brown and red balls correspond to iron and oxygen atoms, respectively.

Figure 4

Raman spectra for (a) nonirradiated samples and theoretical spectra simulated for both α-Fe₂O₃ and ε-Fe₂O₃ phases. In (b) irradiated samples.

Figure 5

FESEM micrographs of the nonirradiated and irradiated samples.

Figure 6

HRTEM images of the nonirradiated and irradiated samples.

Figure 7

Representation of different morphologies obtained for α-Fe₂O₃. The E_surf values are reported in J m^–2. The M values correspond to the magnetization index calculated by eq 3.

Figure 8

Representation of different morphologies obtained for ε-Fe₂O₃. The E_surf values are reported in J m^–2. The M values correspond to the magnetization index calculated by eq 3.

Figure 9

Magnetization curves. In (a) nonirradiated samples and in (b) femtosecond laser-irradiated samples.

Figure 10

Magnetization as a function of the applied magnetic field performed at room temperature of irradiated samples, with the nonirradiated sample contribution subtracted.