Structure engineering is a powerful tool for tuning various properties and making CoFe2O4 (CFO) a prime candidate for electrocatalyst applications, especially in improving the oxygen evolution reaction (OER) performance. In this work, we explored rhenium substituents to replace cobalt and iron atoms of the CFO and obtained significant differences with the same doping level. The sol–gel process was employed to prepare the nanoparticles of Co1−zRezFe2O4 and CoFe2−zRezO4 (z = 0, 0.05). A detailed structural evaluation using Mössbauer spectroscopy verified the pure phase cubic spinel-type structure with a noticeable change in morphology by substituting Re. The OER and electrochemical impedance spectroscopy (EIS) were performed and showed better performance in the Fe-site doped structure. To explain the differences in the OER performance of the samples, X-ray photoelectron spectroscopy (XPS) on all elements, low-temperature Raman, and the density of states simulated by density-functional theory (DFT) were performed. We found that Re actively changed the properties of the structures and the doping on Fe-site enabled more oxygen vacancy and a higher ratio of cations in octahedral sites, which led to enhanced catalytic performance. This work provides a deep insight into the mechanism that influences the OER properties with Re doped in different sites, indicating a promising application of CFO as a catalyst material.
Yuruo Zheng,Ghulam Hussain,Shuyi Li,Shanta Batool,Xiawa Wang
In this work, nanoparticles of Co1−xRexFe2O4and CoFe2−xRexO4(0 ≤ x ≤ 0.05) were synthesized by the sol-gel method. The Rietveld refinement analysis of XRD and Raman data revealed that all of the prepared samples were single phase with a cubic spinel-type structure. With the substitution of Re, the lattice parameters were slightly increased, and Raman spectra peak positions corresponding to the movement of the tetrahedral sublattice shifted to a higher energy position. Furthermore, Raman spectra showed the splitting of T2gmode into branches, indicating the presence of different cations at crystallographic A- and B-sites. The SEM micrograph confirms that surface Re exchange changes the coordination environment of metals and induces Fe-site structure distortion, thereby revealing more active sites for reactions and indicating the bulk sample’s porous and agglomerated morphology. The vibrating sample magnetometer (VSM) results demonstrated that the synthesized nanoparticles of all samples were ferromagnetic across the entire temperature range of 300–4 K. The estimated magnetic parameters, such as the saturation magnetization, remanent magnetization, coercivity, blocking temperature (TB), and magnetic anisotropy, were found to reduce for the Co-site doping with the increasing doping ratio of Re, while in the Fe site, they enhanced with the increasing doping ratio. The ZFC-FC magnetization curve revealed the presence of spin-glass-like behavior due to the strong dipole–dipole interactions in these ferrite nanoparticles over the whole temperature range. Finally, the dielectric constant (εr′) and dielectric loss (tanδ) were sharply enhanced at low frequencies, while the AC conductivity increased at high frequencies. The sharp increases at high temperatures are explained by enhancing the barrier for charge mobility at grain boundaries, suggesting that samples were highly resistive. Interestingly, these parameters (εr′,tanδ) were found to be higher for the Fe-site doping with the increasing Re doping ratio compared with the Co site.
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