On the electronic transitions of α-Fe₂O₃ hematite nanoparticles with different size and morphology: Analysis by simultaneous deconvolution of UV–vis absorption and MCD spectra

Highlights

• Various α-Fe₂O₃ nanostructures with different morphologies are synthesized.

• We put the main focus on magnetic circular dichroism (MCD) of the nanostructures.

• Size-, shape-, and aggregation-dependent absorption and MCD are investigated.

• Simultaneous deconvolution of both absorption and MCD spectra is successful.

• We discuss the detailed nature of electronic transitions of α-Fe₂O₃ nanoarchitectures.

Abstract

Nanostructures of the naturally-existing most stable iron oxide, hematite (α-Fe₂O₃), gain significant research interest in various recent applications. In this study, we put the main focus on magneto-optical (MO) properties of hematite nanoparticles synthesized by a solvothermal approach, and the effects of size, morphology, and particle aggregation on the magnetic circular dichroism (MCD) and conventional absorption spectra are investigated. Simultaneous deconvolution of UV–vis absorption and MCD spectra allows us to discuss the nature of electronic transitions of various hematite nanoarchitectures on the basis of possible transitions in the 3d⁵ system, giving enhanced spectral resolution and detailed assignments. Consequently, we find (i) characteristic size- and shape-dependent MCD responses at 450–470 nm; and (ii) the presence of both spin-allowed pair excitation and Laporte-forbidden d–d transition with opposite MCD signs in close energy at around 550 nm.

Introduction

Hematite (α-Fe₂O₃), an n-type semiconductor with a band gap of ~2.1 eV, is the most common and thermodynamically stable iron oxide with canting ferromagnetic responses in nature [1], [2], and their detailed understanding of electronic and magnetic properties are still of considerable interest [1], [2]. It has then been extensively investigated in, for example, catalysts [3], [4], [5] pigments [6], and sensors [7], [8] owing to its non-toxicity, low processing cost, and high resistance to corrosion [2]. Many recent efforts have been directed toward the fabrication of hematite nanostructures to enhance their performance [2], [7], [8], [9]. Interestingly, the size and shape variations of hematite nanostructures are known to bring about considerable change in their physical and/or chemical properties (=geometry effect) [1], [10], [11], so tailoring their size and shape represents the renovation of dimension with different properties. In particular, the optical (or spectroscopic) property is one of the most fascinating aspects of hematite nanoparticles since it shows strong dependence on the parameters including size and morphology [1], [12]. For example, the apparent color of hematite nanoparticles can be altered from bloody brown to dark brown, due to the reason that some specific surface planes possess a higher reflectance ability than other surface planes [13]. In addition, non-monotonous increase in the energy of pair excitation (or double exciton transition) process, resulting from the simultaneous excitations of two neighboring Fe³⁺ cations that are magnetically coupled, is reported with a decrease in the nanoparticles size (d), especially for d < ~20 nm [14]. The commonly applied methods for synthesizing α-Fe₂O₃ to obtain desired nanostructures are liquid-based processes, and typically, hematite nanoparticles with various shapes such as nanoplates, nanocubes, nanorods, and nanotubes have often been prepared by hydrothermal and solvothermal routes [15], [16], [17], [18], [19].

Meanwhile, magnetic materials generally possess magneto-optical (MO) activity, arising typically from spin-orbit coupling of electrons, resulting in a magnetic-field induced modulation of intensity and/or polarization of the reflected and transmitted light [20], [21], [22]. In contrast to many studies on traditional optical properties for hematite nanoparticles, those of the MO characteristics are still currently limited, though there are a few reports on the magneto-optical Kerr effects (MOKE) based on the reflected light polarization [20]. On the other hand, magnetic circular dichroism (MCD) originated from Faraday effects based on the transmitted light, which measures CD spectra of the specimen placed in magnetic field, has an advantage that it enables us to directly identify the electronic transitions due to the complexity of the MCD peaks discriminating between different types of electronic transitions [21]; therefore, very recently, MCD spectra for bulk and nano-spherical hematite have been reported [22]. According to this study, two types of MCD features were revealed, that is, Gaussian bands associated with d-d transitions and S-shaped components ascribed to the pair exciton-magnon transitions [22]. However, considering that UV–vis absorption spectra of hematite nanoparticles are strongly dependent on their size and/or shape [12], [13], [14], MCD should also has a strong dependence on their size/shape, and thus, it is important to examine the optical transitions on the basis of both UV–vis absorption and MCD spectra as a function of size and shape for better understanding of the nature of the electronic processes, since the MCD signal arises from the same transitions as those seen in the absorption spectrum [23], [24].

In the present study, we combine UV–vis absorption and MCD spectroscopy for hematite nanoparticles with various size and morphology, which can be synthesized by a solvothermal process, to gain a better insight into their electronic structures. The MCD gives valuable information on the exact energy levels, so it can assist the interpretation of electronic transitions. A companion use of absorption spectra obtained simultaneously is carried out, and consequently, electronic states that are not resolved in the optical spectrum are discussed. On the basis of the above strategy, we find a characteristic size- and shape-dependent MCD response at 450–470 nm (2.75–2.64 eV) as well as a hidden transition at around the pair excitation region (~550 nm: 2.25 eV) in the absorption spectrum.