Microwave spin resonance investigation on the effect of the post-processing annealing of CoFe2O4 nanoparticles

Synthesis and Characterization of Highly Crystalline Bi-Functional Mn-Doped Zn2SiO4 Nanostructures by Low-Cost Sol-Gel Process

Herein, we demonstrate a process for the synthesis of a highly crystalline bi-functional manganese (Mn)-doped zinc silicate (Zn₂SiO₄) nanostructures using a low-cost sol-gel route followed by solid state reaction method. Structural and morphological characterizations of Mn-doped Zn₂SiO₄ with variable doping concentration of 0.03, 0.05, 0.1, 0.2, 0.5, 1.0, and 2.0 wt% were investigated by using X-ray diffraction and high-resolution transmission electron microscopy (HR-TEM) techniques. HR-TEM-assisted elemental mapping of the as-grown sample was conducted to confirm the presence of Mn in Zn₂SiO_4. Photoluminescence (PL) spectra indicated that the Mn-doped Zn₂SiO₄ nanostructures exhibited strong green emission at 521 nm under 259 nm excitation wavelengths. It was observed that PL intensity increased with the increase of Mn-doping concentration in Zn₂SiO₄ nanostructures, with no change in emission peak position. Furthermore, magnetism in doped Zn₂SiO₄ nanostructures was probed by static DC magnetization measurement. The observed photoluminescence and magnetic properties in Mn-doped Zn₂SiO₄ nanostructures are discussed in terms of structural defect/lattice strain caused by Mn doping and the Jahn-Teller effect. These bi-functional properties of as-synthesized Zn₂SiO₄ nanostructures provide a new platform for their potential applications towards magneto-optical and spintronic and devices areas.

Operando X-ray Absorption Spectroscopy Study of SnO2 Nanoparticles for Electrochemical Reduction of CO2 to Formate

Tin-based electrocatalysts exhibit a remarkable ability to catalyze CO₂ to formate selectively. Understanding the size-property relationships and exploring the evolution of the active size still lack complete understanding. Herein, we prepared SnO₂ nanoparticles (NPs) with a controllable size supported on commercial carbon spheres (SnO₂/C-n, n = 1, 2, and 3) by a simple low-temperature annealing method. The transmission electron microscopy/scanning transmission electron microscopy images and fitting results of the small-angle X-ray scattering profile confirm the increased size of SnO₂ NPs due to the increase of SnO₂ loading. The catalytic performance of SnO₂ has proved the size-dependent effect during the CO₂ reduction reaction process. The as-prepared SnO₂/C-1 displayed the maximum Faradic efficiency of formate (FE_HCOO-) of 82.7% at -1.0 V versus reversible hydrogen electrode (RHE). In contrast, SnO₂/C-2 and SnO₂/C-3 with larger particle sizes achieved lower maximum FE_HCOO- and larger overpotential. Moreover, we employed operando X-ray absorption spectroscopy to study the evolution of the oxidation state and local coordination environment of SnO₂ under working conditions. In addition to the observed shifts of the rising edge of Sn K-edge X-ray absorption near-edge structure spectra to a lower energy side as the applied voltage decreases, the decreased coordination number of Sn in the Sn-O scattering path and the presence of Sn metal contribution in the extended X-ray absorption fine structure spectra verify the reduction of SnO₂ to SnO_x and metallic Sn.

Facile synthesized zinc oxide nanorod film humidity sensor based on variation in optical transmissivity

Variation in the transmitted light intensity from metal oxide thin films with moisture content provides a great opportunity to use them for humidity sensing. Herein, we have developed a novel and simple humidity sensor based on ZnO nanorod (ZNR) thin films which work as transmission-based sensing elements in an in-house fabricated sensing setup. The ZNR sensing element shows excellent linear sensing performance in the relative humidity (RH) range 10-90% and does not show any hysteresis. A maximum change in optical power of ∼95 μW is observed with the change in RH in the range 10-90%, for the sample with the smallest crystallite size (ZNR1) and highest pore diameter of the ZNR film. Also, a maximum sensitivity of 1.104 μW/% RH is observed for the ZNR1 sample which drops to 0.604 μW/% RH for the highest crystallite size sample (ZNR4). The presence of oxygen vacancies and the micro-porous nature of the film allow the absorption of water vapour on the film which deflects light at different angles that vary with the moisture content. The experimental results suggest that the ZNR film with a smaller crystallite size and larger pore diameter is more sensitive for humidity measurements. Further, an improved sensing performance is perceived in ZNRs because of the larger surface area of the nanorods. The ZNR based sensing elements do not suffer from ageing effects and exhibit high repeatability (88.74%). Further, the humidity sensor has a response time of 62 seconds and recovery time of 100 seconds which can be considered as a fairly quick response.

Deconvolution of ferromagnetic resonance spectrum of magnetic nanoparticle assembly using genetic algorithm

The ferromagnetic resonance (FMR) spectra of dilute random assemblies of magnetite nanoparticles with cubic magnetic anisotropy and various aspect ratios are calculated using the stochastic Landau-Lifshitz equation at a finite temperature, T = 300 K, taking into account the thermal fluctuations of the particle magnetic moments. Particles of non-spherical shape in the first approximation are described as elongated spheroids with a given semiaxes ratio a/b, where a and b are the long and transverse semiaxes of a spheroid, respectively. A representative database of FMR spectra is created for assemblies of randomly oriented spheroidal magnetite nanoparticles with various transverse diameters D = 5-25 nm, moderate aspect ratios a/b = 1.0-1.8, and magnetic damping constants κ = 0.1, 0.2. The basic FMR spectra of assemblies with D = 25 nm at different aspect ratios can be considered as representatives of assemblies of single-domain magnetite nanoparticles with transverse diameters D > 25 nm. The database is calculated at exciting frequency f = 4.9 GHz (S-band) to clarify the details of the FMR spectrum that depend on the particle magnetic anisotropy nature. The data obtained make it possible to analyze arbitrary combined FMR spectra constructed as weighted linear combinations of FMR spectra of the base assemblies. In addition, using a genetic algorithm, the corresponding inverse problem is solved. The latter consists in determining the volume fractions of the base assemblies in some arbitrary nanoparticle assembly, which is represented by its FMR spectrum.

Facile Synthesis, Static, and Dynamic Magnetic Characteristics of Varying Size Double-Surfactant-Coated Mesoscopic Magnetic Nanoparticles Dispersed Stable Aqueous Magnetic Fluids

The present work reports the synthesis of a stable aqueous magnetic fluid (AMF) by dispersing double-surfactant-coated Fe3O4 magnetic nanoparticles (MNPs) in water using a facile ambient scalable wet chemical route. MNPs do not disperse well in water, resulting in low stability. This was improved by dispersing double-surfactant (oleic acid and sodium oleate)-coated MNPs in water, where cross-linking between the surfactants improves the stability of the AMFs. The stability was probed by rheological measurements and all the AMF samples showed a good long-term stability and stability against a gradient magnetic field. Further, the microwave spin resonance behavior of AMFs was studied in detail by corroborating the experimental results obtained from the ferromagnetic resonance (FMR) technique to theoretical predictions by appropriate fittings. A broad spectrum was perceived for AMFs which indicates strong ferromagnetic characteristics. The resonance field shifted to higher magnetic field values with the decrease in particle size as larger-size MNPs magnetize and demagnetize more easily since their magnetic spins can align in the field direction more definitely. The FMR spectra was fitted to obtain various spin resonance parameters. The asymmetric shapes of the FMR spectra were observed with a decrease in particle sizes, which indicates an increase in relaxation time. The relaxation time increased with a decrease in particle sizes (sample A to D) from 37.2779 ps to 42.8301 ps. Further, a detailed investigation of the structural, morphological, and dc magnetic properties of the AMF samples was performed. Room temperature dc magnetic measurements confirmed the superparamagnetic (SPM) characteristics of the AMF and the M-H plot for each sample was fitted with a Langevin function to obtain the domain magnetization, permeability, and hydrodynamic diameter of the MNPs. The saturation magnetization and coercivity of the AMF samples increased with the increase in dispersed MNPs' size of the samples. The improvement in the stability and magnetic characteristics makes AMFs suitable candidates for various biomedical applications such as drug delivery, magnetic fluid hyperthermia, and biomedicines.

High-Performance, Lightweight, and Flexible Thermoplastic Polyurethane Nanocomposites with Zn2+-Substituted CoFe2O4 Nanoparticles and Reduced Graphene Oxide as Shielding Materials against Electromagnetic Pollution

The development of flexible, lightweight, and thin high-performance electromagnetic interference shielding materials is urgently needed for the protection of humans, the environment, and electronic devices against electromagnetic radiation. To achieve this, the spinel ferrite nanoparticles CoFe2O4 (CZ1), Co0.67Zn0.33Fe2O4 (CZ2), and Co0.33Zn0.67Fe2O4 (CZ3) were prepared by the sonochemical synthesis method. Further, these prepared spinel ferrite nanoparticles and reduced graphene oxide (rGO) were embedded in a thermoplastic polyurethane (TPU) matrix. The maximum electromagnetic interference (EMI) total shielding effectiveness (SET) values in the frequency range 8.2-12.4 GHz of these nanocomposites with a thickness of only 0.8 mm were 48.3, 61.8, and 67.8 dB for CZ1-rGO-TPU, CZ2-rGO-TPU, and CZ3-rGO-TPU, respectively. The high-performance electromagnetic interference shielding characteristics of the CZ3-rGO-TPU nanocomposite stem from dipole and interfacial polarization, conduction loss, multiple scattering, eddy current effect, natural resonance, high attenuation constant, and impedance matching. The optimized CZ3-rGO-TPU nanocomposite can be a potential candidate as a lightweight, flexible, thin, and high-performance electromagnetic interference shielding material.

Spin dynamics investigations of multifunctional ambient scalable Fe3O4 surface decorated ZnO magnetic nanocomposite using FMR

Microwave spin resonance behavior of the Fe3O4 surface decorated ZnO nanocomposites (FZNC) has been investigated by ferromagnetic resonance (FMR). Modified hydrothermal method has been adopted to fabricate FZNC samples with Fe3O4 nanoparticles chains were used as seeds in the uniform magnetic field to decorate them on the surface of the ZnO nanoparticles in a unique configuration. Spin dynamics investigation confirms the transition of ZnO from diamagnetic to ferromagnetic as the sharp FMR spectra converts to the broad spectra with Fe3O4 nanoparticles incorporation. A single broad FMR spectra confirms that no isolated Fe3+ or Zn2+ ions exist which is also in agreement with XRD confirming suitable composite formation. Further, the increase in Fe3O4 concentration leads to decrease in g-value which is resulting from the internal field enhancement due to magnetic ordering. Also, various spin resonance parameters were calculated for the FZNC which provides a detail information about the magnetic ordering, exchange coupling and anisotropy. Elemental analysis confirms the presence of Fe and Zn simultaneously and transmission electron microscopy (TEM) image show the presence of Fe3O4 on the grain boundaries of ZnO which has been confirmed by taking high-resolution TEM and electron diffraction patterns on both sides of the interface. These unique structural configuration of the FZNC has tremendous potential in various magneto-optoelectronic, spintronics and electro-chemical applications.

Temperature selectivity for single phase hydrothermal synthesis of PEG-400 coated magnetite nanoparticles

Temperature plays a vital role in the hydrothermal synthesis of the nanoparticles. Herein, we have provided a very detailed spin dynamic investigation on the varying size Fe₃O₄ nanoparticles using FMR technique.