Radiative properties of hematite particles in the UV-visible spectrum

Abstract

The radiative properties of hematite (α-Fe₂O₃) samples of different particle size distributions obtained by the pressure welding procedure were investigated. Measurements were made in the ultraviolet and visible spectra, and the radiative properties of samples were assessed using the discrete dipole approximation (DDA) in the visible spectrum. The accuracy of DDA was evaluated against the Mie theory results in terms of the optical efficiency in the short and long wavelength range of 0.20–50 μm. The coating on particles due to the aging effect was examined using scanning electron microscopy (SEM) images. The measured absorbance values of the samples displayed significant differences in the wavelength range of 0.20–0.50 μm. Among the measured absorbance of three samples with sizes of 37 μm, 63 μm, and 125 μm, the most prominent change in absorbance was observed in the sample consisting of the smallest diameter, while the sample containing the largest size had the highest absorbance values in the spectral range. The change with oscillation in wavelength of the absorbance was also observed in the DDA results. Both the experimental and calculated results presented in this study indicate that the absorption behaviour of hematite particles depends on the particle size, wavelength and production process.

Introduction

Among many metal oxides that have been utilised in industrial processes, hematite (α-Fe₂O₃) has received considerable attention as an n-type semiconductor due to its abundance, low cost, and environmentally friendly characteristics [1,2]. Iron oxides such as hematite have been shown to strongly absorb solar radiation [3]. In recent studies, it has been shown that iron oxides contribute to heterogeneous reactions and affect the radiation balance [4,5]. Due to the versatile properties of hematite, it has been used in different applications, such as catalysts, magnetic devices, pigments, anticorrosive agents and gas sensors.

The radiative properties of spherical hematite particles in the size range of 0.10–0.51 μm were experimentally investigated in the wavelength range of 380–700 nm and compared with Mie theory by Hsu and Matijević [6]. The effects of the structural properties of hematite (γ- and α-) on absorption were investigated [7,8]. These studies indicated that small-sized particles of hematite (below the micron level) should be investigated to determine the effects of the metal and oxygen interactions. A study into the scattering and absorbing efficiencies of hematite particles with diameters from 0.01 to 0.5 μm in air and water in the 400–700 nm wavelength range was conducted by Bedidi and Cervelle [9]. The common result of these studies is that the radiative properties of the particles are affected by both the particle size and wavelength, as expected. A detailed review explaining both the morphology and the radiative properties of iron oxides particles was provided by Zboril et al. [10]. The optical properties of particles of hematite and rutile were also experimentally and numerically analysed by Muñoz et al. [11]. In the Muñoz et al. study, the effects of the real part n and imaginary part k of the refractive index on the scattering properties at a wavelength of 0.6328 μm were studied in detail. They concluded that the refractive index is more influential than the particle shape on scattering properties. A similar finding was reported by Mishchenko et al. [12], who found by using the T-matrix method that the scattering behaviour of particles is less affected by particle shape when the imaginary part of the refractive index increases. The optical properties of iron oxides, such as hematite or goethite, at 0.47, 0.55 and 0.66 μm, were investigated by Meland et al. [13]. They computed the scattering effects using the Mie and T-matrix methods and found that, while the particle shape does not affect the scattering properties of hematite, it can have a significant effect for goethite. Chakrabarty and Chatterjee [14] studied the effect of different solvents (i.e., ethanol amine, ethylene diamine, ethylene glycol, acetic acid, ethanol and acetaldehyde) on the morphology of hematite nanoparticles and showed that the optical absorption peaks arise in different wavelength ranges due to some morphological (ligand field, spin-flip) transitions. The effect of the shape of hematite nanoparticles and time-dependent growth were investigated by Chen et al. [15]. The authors found that nanostructures of hematite particles change based on crystallography and surface chemistry in both the growth and aging stages. The optical properties of hematite particles with different morphologies and sizes in the 50–300 nm range were comprehensively studied by Distaso et al. [16] using the superposition T-matrix approach and compared to Mie theory and the finite element method. They focused on changes to the internal structure of hematite particles. Because hematite is a birefringent material [17], the authors defined the wavelength-dependent refractive index, i.e., the n- and k-values, of hematite in the range of 400–700 nm. They found that the applied approaches produced results that were in good agreement with experimental studies; moreover, the organics inside the particles or the bonds of the hematite phase influenced the optical properties. Iron oxide particles such as goethite, hematite and maghemite (size ˂ 0.1 μm) have been analysed for use in an industrial process [18]. The authors found that, when the iron oxide samples were studied at different temperatures or times, the samples had different XRD characteristic phases. Absorbance measurements of hematite thin films prepared using different deposition procedures were performed by Chen and Tu [19]. The authors found that the absorbance values of the samples were high due to the occurrence of photocatalytic properties in the visible wavelength. The change in absorption due to the change in the nanostructure of hematite films has been studied in detail by Wheeler et al. [20].

In this study, the radiative properties of “debris” hematite particles produced as the result of a pressure bud welding process were evaluated experimentally using absorbance measurements and numerically using the discrete dipole approximation (DDA). Additionally, the samples were examined in terms of their physical properties by assessing 100 SEM images in total. The distinct feature of our study is that the absorbance measurements of aged hematite particles in the size range of 37–125 μm have not been performed in previous studies. The characteristics of the production and aging process affect the radiative properties of hematite particles. Since iron oxides (hematite, goethite and magnetite) are used as gas sensor material, as photoanod in photoelectrochemical cells, as polishing and corrosion prevention material and in industrial applications as high density coating material, the micrometer-sized particles were analysed in terms of compounds, particle size and radiative properties. Without considering the details of morphological transitions, the capturing of these effects is essential for the in situ characterisation of hematite particles in their reuse in improved engineering applications.