Original Research PaperAccurate band gap determination of chemically synthesized cobalt ferrite nanoparticles using diffuse reflectance spectroscopy
Graphical abstract
Introduction
Cubic ferrites form an important class of materials, studied extensively owing to the innumerable applications offered through almost all areas of scientific research and technology [1], [2], [3]. One of the most potential candidates belonging to this class is cobalt ferrite CoFe2O4 (CoF), which has a wide spectrum of applications related to magnetic [4], [5], [6], [7], magneto-optic [8], [9], electrical [10], chemical [11], electrochemical [12], thermal [13], photoelectrochemical [14], [15], thermo-acoustic [16] and adsorption [17], [18] properties, both in the bulk form and in the nanoscale. Electronic band structure and optical transitions of this spinel structured ferrite have always been intriguing since it showcases varieties of inter-band and intra-band transitions, spanning wide range of energies [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. Optical and magneto-optical transitions in CoF bulk, as well as nanostructured systems, have been analysed experimentally using techniques such as Magneto-optic Kerr effect spectroscopy [20], [21], optical absorption spectroscopy [22], [23], [24], [25], [26], spectroscopic ellipsometry [27], electron energy loss spectroscopy [28] etc., while the band structure has also been studied theoretically using first principle studies [29], [30]. The possible electronic transitions and the corresponding energy values are mostly calculated based on the energy-dependent variations of the real and imaginary parts of the diagonal and off-diagonal components of the dielectric tensor. While there has been agreement in reports regarding the electronic transitions [20], [21] and optical band gaps of CoF calculated using theoretical approaches [28], [29], [30], the band gap values obtained through experimental procedures differ enormously from one report to another [22], [23], [24], [25], [26], [27]. Through a thorough survey of literature it can be understood that the most common methodology adopted for the calculation of band gap of CoF nanostructures, similar to many other semiconductors, is the determination of absorption coefficients, , as a function of photon energy , from transmission and/or absorption spectroscopy, followed by the band gap estimation from and versus curves [31], [32], [33], [34]. This is usually done by identifying the linear segment corresponding to the absorption onset, from the and versus curves for direct and indirect band gap estimation respectively. This forms a very crucial step, since, an improper selection of the absorption onset region from the graphs may lead to erroneous estimation of band gaps [35], [36]. This becomes specifically important in the case of materials such as CoF that show electronic transitions starting from energy values as low as 0.8 eV to about 4.5 eV. Here, we make use of Diffuse Reflectance Spectroscopy (DRS) for the band gap determination of CoF Nanoparticles (NP). DRS is invariably, the best suited technique for the band gap analysis of nanopowders [37]. The highlight is that no sample treatment is necessary for DRS measurement and the synthesized nanopowders can be analysed as such. The reflectance data collected through DRS measurement is converted into the remission function, , using the most widely accepted Schuster-Kubelka-Munk (SKM) formalism after examining the various requirements to be satisfied for its application [38], [39], [40], [41]. To avoid creeping in of any error while estimating the band gap, we have used a detailed band gap estimation algorithm, that routinely examines all the potential linear segments in the and versus curves and finally chooses the most appropriate ones representing the absorption onset and the corresponding base line, depending on a set of well-defined conditions [42], [43], [44]. To the best of our knowledge, the band gap determination of CoF NP through DRS followed by SKM formalism and a detailed band gap estimation procedure, has not been reported before. We have also tried to explain the phenomena of quantum confinement in CoF NP that have sizes comparable to the bulk exciton-Bohr radius and calculate the band gap enhancement that could occur in them [45]. We have first, theoretically calculated the band gap shift possible in the synthesized nanocrystallites, by applying Brus effective mass model and have shown that the shift accurately corresponds to the experimentally obtained band gap.
Section snippets
Synthesis of CoF NP
CoF NP powder was synthesized according to a wet chemical synthesis technique using CoCl2·6H2O, FeCl3·6H2O and NaOH [Sigma Aldrich] as the precursors [46]. Typically, 0.4 M FeCl3·6H2O and 0.2 M CoCl2·6H2O were prepared separately in 25 ml each of Demineralised (DM) water . The two were then mixed and stirred well for about 30 min, until a homogeneous mixture was obtained. 3 M NaOH solution prepared in 25 ml of DM water was then added drop wise to the mixture with vigorous stirring, while the pH
Structural properties
XRD and SAED patterns of CoF NP are shown in Fig. 1 (a) and (b) respectively. The diffraction angles corresponding to the peaks and the relative intensities of the peaks obtained from XRD, as well as the relative inter-planar distances calculated from the bright spotted rings in the SAED pattern clearly indicate that the sample is highly crystalline with a cubic spinel structure, consistent with ICDD Powder diffraction data 00–022-1086. No impurity phases were detected. Inter-planar distances
Conclusions
CoF NP powder was synthesized using a wet chemical synthesis technique followed by a high temperature annealing. The NP crystallised in a partially inverse spinel structure and were seen to be having cuboidal shape. The optical properties of the nanopowder were analysed using DRS followed by SKM formalism. The indirect and direct band gap values were analysed using GapExtractor© software and were found to be 0.79 ± 0.01 eV and 1.5 ± 0.01 eV respectively. The values were found to be highly
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Authors acknowledge SAIF, STIC, CUSAT for XRD, SEM and HR-TEM characterizations and SAIF, IIT Madras for VSM measurement. Authors also acknowledge DST FIST for providing funding for various instruments used in this work.
Funding
Authors acknowledge the Department of Science and Technology, Government of India for financial support vide reference No. SR/WOS-A/PM-39/2018 under Woman Scientist Scheme-A to carry out this work.
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