Composition assisted bandgap tuning of honey green synthesized Mg-Zn spinel nanoferrite

Abstract

Spinel nanoferrites are represented by AB₂O₄, A - divalent metal ions, B–, trivalent metal ions, display face-cantered cubic (fcc) structure with tetrahedrally coordinated A site, octahedrally coordinated B site. They exhibit very interesting magnetic properties as compared to their bulk counterparts, and hence are useful in wide range of applications such as data storage, magnetically guided drug delivery, ferrofluids etc [1]. Green synthesis of ferrites is shown to be an eco-friendly as compared to other popular chemical, and physical methods [2], expected to exhibit lesser toxicity, has potential application in hyperthermia for cancer treatment [3]. Sol gel auto-combustion method, utilizing non-toxic and/or less toxic natural, economical precursors e. g. - honey, aloe vera plant extract, etc., can be efficiently employed to synthesize ferrite nano-particles at relatively low temperatures (~ 200 ^oC), consumes less energy, and thus is more cost effective, and ensures spinel phase formation in dry gel form, where no heat treatment is needed. Spinel nanoferrites with bandgap ~2 eV, have strong absorption in visible-region (~50% of solar-radiation) is advantageous, and would show improved efficiency in photocatalytic applications [4], can be magnetically recovered for re-use, and has shown significant efficiency towards the degradation of organic, inorganic pollutants [5]. Literature [6] reports composition assisted tuning of bandgap, and thus a spinel ferrite having appropriate bandgap can be more effective in photo-catalytic, solar energy related applications. Thus, in this work we report honey green synthesis of Mg_xZn_1-xFe₂O₄ (x = 0.0  - 1.0) by sol gel auto-combustion protocol, and impact of successive Mg addition on tuning energy bandgap, probed by x-ray diffraction (XRD), ultraviolet–visible (Uv–Vis) spectroscopy). Samples were prepared by utilizing Nitrate-Acetate pre-cursors, as described in [6], and honey as chetaling agent, fuel, keeping pH 7. Formed dry gel samples were used for characterization without any post-preparation thermal treatment. XRD, Uv-Vis data analysis yields structural parameters, cationic distribution, Néel magnetic moment n_N, energy bndgap. XRD (fig. 1a) validates the formation of spinel nano ferrite (grain diameter: 28.1 - 38.5 nm). Presence of a-Fe₂O₃, is ascribed to absence of thermal annealing. Linear decrease of lattice parameter (fig. 1a inset) is attributed to the addition of Mg²⁺ ion with slightly lower ionic radius (0.72 nm) as compared to Zn²⁺ ion (0.074 nm). Linear decrease of a_exp., described by equation: a_exp. = 0.84 – 0.005  [Mg-content], shows strong correlation between Mg-content and a_exp. Addition of Mg²⁺ ion shows transition from  normal to inverse ferrite (fig. 1b), and leads to modification of oxygen parameter ‘u’ (fig. 1b) revealing increase of disorder, and also shows linear decrease of n_N. Fig. 1(c) reveal weakening of B–O–B super-exchange interaction with simultaneous strengthening of A–O–A, A–O–B interaction, affecting n_N, (see fig. 1 c). With increasing Mg-content, Fe³⁺ ion occupation on B-site decreases (affects n_N  as seen in inset of fig. 1d ) , with corresponding increase on A-site. Mg²⁺, Zn²⁺ ions remains more populated respectively on B-, A-site. Presence   of Mg²⁺ ions on A, B site   shows   non- equilibrium   cationic distribution as also reported in [7]. Fig 1e shows Mg-content dependent fine–tuning of band gap (between 1.63 V– 1.97 eV), is a collective result of variation of lattice parameter, grain diameter, and cationic distribution as is also seen in [6]. Fig. 1e(inset) shows dependence of bandgap on lattice parameter, showing strong correlation between structural properties, and bandgap. It is of value to note that, compositional dependent tunability of bandgap is valuable in enhancing photocatalytic efficiency. Current study noticeably shows strong relationship between structural properties, cationic distribution, and bandgap.