Impact of Gd3+ doping on structural, electronic, magnetic, and photocatalytic properties of MnFe2O4 nanoferrites and application in dye-polluted wastewater remediation

Abstract

The present work focuses on developing Gd-doped Mn spinel nanoferrites and their potential application in the photodegradation of water pollutants. The impact of Gd³⁺ ion substitution on structural, electronic, and magnetic characteristics of manganese ferrites has been studied. Nanocrystalline samples of MnGd_xFe_2-xO₄ (x = 0.0 to 0.10, in step size of 0.02) ferrites were prepared via sol-gel self-ignition route. The Rietveld, XPS, HRTEM, and SAED characterization methods confirmed the formation of phase pure ferrite nanoparticles (~ 8-22 nm) in the cubic spinel structure. The Gd³⁺ content in these nanoferrites responded to a systematic reduction in the size of nanocrystallites and an upsurge in the density of nanoferrites. The XPS study revealed fine assimilation of constituent elements in the fcc lattice and ruled out impurities in the nanoferrites. The Fe and the Gd ions were found to be in Fe³⁺ and Gd³⁺ states, respectively. While a major fraction of the Mn ions were found to be in the Mn²⁺ state, a small fraction of Mn⁴⁺ ions was observed on the surface of nanoparticles. The nanoferrites were found to exhibit a soft ferromagnetic state from 300 to 20 K limits. The highest saturation magnetization was observed for x = 0.02 (M_S = 66.6 emu/g at 20 K). The observed magnetic properties can be understood with the competing (Fe³⁺ and Mn²⁺)_A-O^2--[Fe³⁺, Mn²⁺, and Gd³⁺]_B superexchange interactions and magnetocrystalline anisotropy. Due to the small band gap energy of Gd-doped Mn ferrites than that of the pure Mn ferrite, they have demonstrated excellent photocatalytic activity for the degradation of methylene blue (MB) dye under visible light illumination. As much as 96.35% of the MB dye was found to get degraded in 70 min of light illumination over synthesized nanoparticles and the photodegradation reaction followed pseudo-first-order kinetics. The increased optical absorbance due to lower band gap, suppressed recombination rate of charge carriers, and enhanced charge mobility make them effective visible light active photocatalysts. This study revealed that the electronic, optical, and magnetic properties of MnFe₂O₄ nanoferrites could be easily tuned by varying the Gd³⁺ content and the prepared Gd-doped MnFe₂O₄ nanomaterials have boundless potential to be utilized in the future making promising active photocatalysts and degradation of harmful industrial dyes for enhanced protection in the fields of environment and health care.

Keywords: HRTEM; MB dye; Magnetization; Nanoferrites; Photocatalysts; Water pollutant; XRD.