Iron oxides are chemical compounds which have different polymorphic forms, including γ-Fe2O3 (maghemite), Fe3O4 (magnetite), and FeO (wustite). Among them, the most studied are γ-Fe2O3 and Fe3O4, as they possess extraordinary properties at the nanoscale (such as super paramagnetism, high specific surface area, biocompatible etc.), because at this size scale, the quantum effects affect matter behavior and optical, electrical and magnetic properties. Therefore, in the nanoscale, these materials become ideal for surface functionalization and modification in various applications such as separation techniques, magnetic sorting (cells and other biomolecules etc.), drug delivery, cancer hyperthermia, sensing etc., and also for increased surface area-to-volume ratio, which allows for excellent dispersibility in the solution form. The current methods used are partially and passively mixed reactants, and, thus, every reaction has a different proportion of all factors which causes further difficulties in reproducibility. Direct active and complete mixing and automated approaches could be solutions to this size- and shape-controlled synthesis, playing a key role in its exploitation for scientific or technological purposes. An ideal synthesis method should be able to allow reliable adjustment of parameters and control over the following: fluctuation in temperature; pH, stirring rate; particle distribution; size control; concentration; and control over nanoparticle shape and composition i.e., crystallinity, purity, and rapid screening. Iron oxide nanoparticle (IONP)-based available clinical applications are RNA/DNA extraction and detection of infectious bacteria and viruses. Such technologies are important at POC (point of care) diagnosis. IONPs can play a key role in these perspectives. Although there are various methods for synthesis of IONPs, one of the most crucial goals is to control size and properties with high reproducibility to accomplish successful applications. Using multiple characterization techniques to identify and confirm the oxide phase of iron can provide better characterization capability. It is very important to understand the in-depth IONP formation mechanism, enabling better control over parameters and overall reaction and, by extension, properties of IONPs. This work provides an in-depth overview of different properties, synthesis methods, and mechanisms of iron oxide nanoparticles (IONPs) formation, and the diverse range of their applications. Different characterization factors and strategies to confirm phase purity in the IONP synthesis field are reviewed. First, properties of IONPs and various synthesis routes with their merits and demerits are described. We also describe different synthesis strategies and formation mechanisms for IONPs such as for: wustite (FeO), hematite (α-Fe2O3), maghemite (ɤ-Fe2O3) and magnetite (Fe3O4). We also describe characterization of these nanoparticles and various applications in detail. In conclusion, we present a detailed overview on the properties, size-controlled synthesis, formation mechanisms and applications of IONPs.

https://www.mdpi.com/1996-1944/13/20/4644/pdf

Magnetic Iron Oxide Nanoparticle (IONP) Synthesis to Applications: Present and Future.

The goal of this paper is to simplify and reduce the cost of manufacturing tungsten carbide. The work presents the results of experimental studies that prove the possibility of synthesizing fine and ultrafine crystalline phases of tungsten carbide in the DC arc discharge plasma ignited in open air by a developed vacuumless technique. The synthesis is implemented in the process of arc discharge burning between the graphite rod (anode) placed inside the cavity of the graphite cathode in the presence of graphite and tungsten powders used as initial precursors. The cathode cavity (reaction zone) is shielded from oxygen by a stream of carbon monoxide and carbon dioxide gases, which are generated during the arc discharge burning in an open air. After studying the influence of main process parameters (parameters of the discharge circuit and graphite electrode's dimensions) and optimizing the system, the authors obtained tungsten carbide phases without impurities of oxide phases. This allows simplifying significantly the technology of tungsten carbides synthesis by excluding the stage of vacuum creation.

Vacuumless synthesis of tungsten carbide in a self-shielding atmospheric plasma of DC arc discharge

Controlled fabrication of core-shell γ-Fe2O3@C-Reduced graphene oxide composites with tunable interfacial structure for highly efficient microwave absorption.

The major challenges related to the activity, stability, and cost of electrocatalysts are being increasingly raised to achieve highly efficient and cost-effective hydrogen generation. Herein, multiphase nanocomposites of W2C@WC1-x encapsulated within graphitic carbon layers were prepared via a facile and effective process of electrical explosion of wires and subsequent heat treatment to serve as a highly active and stable electrocatalyst without any noble metal for hydrogen generation. The single-phase comprising less than 15 nm WC1-x nanoparticles embedded in a lump of amorphous carbon were successfully synthesized via the EEW process in oleic acid used as a carbon source at room temperature. Subsequent heat treatment facilitates the desired phase transition of WC1-x to W2C without the formation of any secondary phases, maintaining the initial particle size and simultaneously eliminating excess amorphous carbon adhered to the nanoparticles. The few graphitic carbon layer-encapsulated nanoparticles with the main W2C phase prepared by this simple method exhibit high efficiency for hydrogen generation with a low overpotential of 240 mV at a current density of 10 mA cm-2 and a low Tafel slope of 86 mV dec-1. Moreover, the overpotential is well maintained at a constantly injected current density of 10 mA cm-2 for 100 h with a low η100/ηi value of 1.03 (ηi: initial overpotential, η100: overpotential after 100 h), demonstrating superior catalytic stability in acidic media. This work proposes and evaluates a facile strategy for the synthesis of highly efficient electrocatalysts based on metal carbides without noble metals.

Carbon-encapsulated multi-phase nanocomposite of W2C@WC1-x as a highly active and stable electrocatalyst for hydrogen generation.

We report on the synthesis and thermal conductivity of gold nanoparticles (AuNPs) decorated graphene nanosheets (GNs) based nanofluids. The GNs-AuNPs nanocomposites were synthesised using a nanosecond pulsed Nd:YAG laser (wavelength = 1,064 nm) to ablate graphite target followed by Au in ethylene glycol (EG) base fluid to obtain GNs-AuNPs/EG hybrid nanofluid. The characterization of the as-synthesised GNs-AuNPs/EG hybrid nanofluid confirmed a sheet-like structure of GNs decorated with crystalline AuNPs with an average particle diameter of 6.3 nm. Moreover, the AuNPs appear smaller in the presence of GNs which shows the advantage of ablating AuNPs in GNs/EG. The thermal conductivity analysis in the temperature range 25-45 °C showed that GNs-AuNPs/EG hybrid nanofluid exhibits an enhanced thermal conductivity of 0.41 W/mK compared to GNs/EG (0.35 W/mK) and AuNPs/EG (0.39 W/mK) nanofluids, and EG base fluid (0.33 W/mK). GNs-AuNPs/EG hybrid nanofluid displays superior enhancement in thermal conductivity of up to 26% and this is due to the synergistic effect between AuNPs and graphene sheets which have inherent high thermal conductivities. GNs-AgNPs/EG hybrid nanofluid has the potential to impact on enhanced heat transfer technological applications. Also, this work presents a green synthesis method to produce graphene-metal nanocomposites for various applications.

/pdf/thermal-conductivity-enhancement-in-gold-decorated-graphene-4lxj57rpom.pdf

Thermal conductivity enhancement in gold decorated graphene nanosheets in ethylene glycol based nanofluid.

Synthesis of metastable cubic tungsten carbides by electrical explosion of tungsten wire in liquid paraffin

The Fe–FeO–graphene nanocomposite material was produced successfully by pulsed wire discharge in graphene oxide (GO) suspension. Pure iron wires with a diameter of 0.25 mm and a length of 100 mm were used in the experiments. The discharge current and voltage were recorded to analyze the process of the pulsed wire discharge. The as-prepared samples—under different charging voltages—were recovered and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, and transmission electron microscopy (TEM). Curved and loose graphene films that were anchored with spherical Fe and FeO nanoparticles were obtained at the charging voltage of 8–10 kV. The present study discusses the mechanism by which the Fe–FeO–graphene nanocomposite material was formed during the pulsed wire discharge process.

One Step Preparation of Fe–FeO–Graphene Nanocomposite through Pulsed Wire Discharge

Few-layer graphene nanosheets were produced by pulsed discharge in graphite micro-flake suspension at room temperature. In this study, the discharging current and voltage data were recorded for the analysis of the pulsed discharge processes. The as-prepared samples were recovered and characterized by various techniques, such as TEM, SEM, Raman, XRD, XPS, FT-IR, etc. The presence of few-layer graphene (3–9 L) in micrometer scale was confirmed. In addition, it is investigated that the size of recovered graphene nanosheets are influenced by the initial size of utilized graphite micro-flake powder. Based on the process of pulsed discharge and our experimental results, the formation mechanism of few-layer graphene was discussed. The influence of charging voltage on as-prepared samples is also investigated.

https://www.mdpi.com/2073-4352/9/3/150/pdf

Hayato Oda

Papers

Synthesis of metastable cubic tungsten carbides by electrical explosion of tungsten wire in liquid paraffin

One Step Preparation of Fe–FeO–Graphene Nanocomposite through Pulsed Wire Discharge

Preparation of Few-Layer Graphene by Pulsed Discharge in Graphite Micro-Flake Suspension