Ferrite-Dichalcogenide Nanocomposites for Energy Storage Applications

Hybrid nanocomposites have emerged as promising materials to address the increasing demand for efficient, high-performance energy storage systems.Ferrite-dichalcogenide nanocomposites have garnered significant attention due to their unique physicochemical properties.This paper focuses on synthesizing, characterizing, and applying nano zinc ferrites (ZnFe 2 O 4 ) and a fewlayered molybdenum disulfide (MoS 2 ) nanocomposite for energy storage applications.The ZnFe 2 O 4 NS were prepared via a hydrothermal approach, which involved dissolving Zn(CH 3 COO) 2 and FeCl 3 in ethylene glycol and varying molar concentrations of CH 3 COONa as a surfactant.The resulting homogeneous mixture was transferred into a stainless-steel autoclave and heated at 180 C for 24 hours.Afterward, the product was collected via centrifugation, thoroughly washed with distilled water, and vacuum-dried at 65 C for 24 hours.The synthesis of few-layered MoS 2 samples was accomplished by dispersing a nominal amount of commercial MoS 2 material in acetone and deionized water.Subsequently, the dispersion underwent ultrasonic treatment in iced water for 1 hour with ultrasonic power set at 550 watts.Following the ultrasonic treatment, the resulting black dispersion was centrifugated, and the supernatant liquid was subsequently collected by drying the solution in a vacuum at 65 C for 24 hours.The MoS 2 /ZnFe 2 O 4 nanocomposites (NCs) were synthesized using the hydrothermal method.Different molar amounts of a few-layered MoS 2 were mixed with Zn (CH 3 COO) 2 , FeCl 3 , and CH 3 COONa and subsequently dissolved in C 2 H 6 O 2 to have NCs with various molar ratios.A field emission scanning electron microscope (FESEM) system of model Quanta 3D from ThermoFisher Scientific was also used to investigate the surface morphology of synthesized materials.The synthesized samples were also analyzed with a transmission electron microscope (TEM) to investigate the morphology, structure, and composition at higher resolution.The TEM analysis was done with a double aberration-corrected microscope of model Titan Themis Z 60 -300 from ThermoFisher Scientific.The accelerating voltage of 300 keV was set during the TEM analysis.Elemental mapping was achieved via STEM-EDS, while oxidation states of iron at the nanoscale were determined by carrying out core-loss STEM-EELS analysis of samples.Meanwhile, bulk chemical and compositional analyses were conducted using X-ray photoelectron spectroscopy (XPS) to correlate bulk material characteristics with nanoscale properties.The electrochemical performance of the MoS2/ZnFe2O4 NCs and individual materials was investigated more deeply by density functional theory (DFT) calculations with the Vienna ab initio Simulation Package (VASP).The magnetic properties of ZnFe2O4 were taken into consideration as well.The DFT calculations included Projected Density of States (PDOS), Band Structure, Bader Charge Analysis, and Charge Density Difference (CDD).These calculations were done on the MoS2/ZnFe2O4 NCs before and after K-insertion.The synthesized samples' morphological, structural, and compositional characteristics were assessed using the FESEM and TEM, as shown in Figure 1.The images reveal that most zinc ferrite particles exhibit spherical and MoS 2 sheet-like morphologies.The nanospheres of zinc ferrite particles are formed via the self-assembly of smaller crystallites, whose dimensions are determined by HRTEM analysis.The STEM-EELS characterization was conducted to study changes in Fe oxidation states across all synthesized samples.The results are presented in Figure 2. In EELS, iron's L 23 ionization edges typically display sharp peaks known as white lines, arising from the electron transition from the 2p to unoccupied 3d states.This excitation causes a transition in the atomic state from 2p 6 3d n to 2p 5 3d n+1 , where "n" is the number of occupied 3d states.The L 3 and L 2 lines represent transitions from 2p 3/2 to 3d 3/2 , 3d 5/2 , and 2p 1/2 to 3d 3/2 , respectively.The intensity of these lines reflects the number of unoccupied 3d states.The intensity ratio L 3 /L 2 values of the synthesized zinc ferrite particles are less than the previously reported values for ferrite/graphene and MnFe 2 O 4 /graphene.The increase in Fe oxidation states following the formation of NS is a beneficial characteristic, making these materials well-suited for use as supercapacitor electrodes [1]. Figure 1 presents the electron microscopy (SEM & TEM) analysis of a zinc ferrite sample whose energy performance was evaluated [2].The presented study provides information on morphology, crystal structure, and the mappings of constituent elements in zinc ferrite samples.The HRTEM images in Figure 1 show interference fringes with spacings in the range of 0.260 nm, 0.252 nm, and 0.253 nm, confirming the cubic structure of synthesized zinc ferrite samples.Figure 2 shows the acquired core-loss EELS spectra of the same zinc ferrite samples containing Fe-L 23 edges for iron.The ratio of Fe-L 2 and Fe-L 3 peaks was calculated from the acquired EELS spectra to determine the oxidation state of iron in the samples.Similarly, the synthesized composite samples' morphological and microstructural characteristics were assessed, and the obtained results are presented in Figure 3.The SEM analysis shows that the exfoliated MoS 2 nanosheets have flat shapes with small dimensions and thickness.At the same time, the TEM analysis confirms that the zinc ferrite phase of nanocomposite has predominantly nanoparticles that have uniform spherical shapes and are agglomerated, which could be attributed to the interactions between magnetic dipole moments.The homogeneous dispersion of the zinc ferrite nanoparticles facilitated by MoS 2 nanosheets can yield a better surface-to-volume ratio, essential for improved charge transfer and increased specific capacitance.The TEM analysis of MoS 2 nanosheets reveals that their layers are significantly transparent, indicating a reduced thickness.The interlayer spacing between adjacent layers, corresponding to the (002) plane, was approximately 0.66 nm for MoS 2 nanosheets, which could