Experimental investigation of the mechanical grinding effect on graphene structure

Graphene has been proven to be a promising material for various applications due to its outstanding chemical, physical, optical as well as mechanical properties. To further improve these properties of graphene, here we apply a grinding method with various speeds (100–600 rpm) of a planetary ball mill under wet conditions in graphene based aqueous solution. Therefore, the improvements in dispersion and thermal characteristics of the graphene–water solution were investigated based on the morphological and structural changes. The best dispersibility and highest thermal conductivity of graphene–water solution were observed for a grinding speed of 500 rpm. As a result, the grinding speed of 500 rpm is found as the optimum condition of planetary ball milling in the case study. The reason for the grinding speed of 500 rpm revealing the best condition is attributed to the reduced ratio (/D//G=0.221)of the D band and the G band in Raman spectroscopy. We believe that structurally upgraded graphene in this study would greatly improve the performance of the graphene based devices.Munkhshur Myekhlai, B. Munkhbayar, Taejin Lee, Md. Riyad Tanshen, Hanshik Chung and Hyomin Jeon

Thermal conductivity of TiO(2) nanoparticles based aqueous nanofluids with an addition of a modified silver particle

Nanofluid is a colloidal suspension which has received great attention over the past two decades, but its limited heat transfer enhancement is a matter of concern for industrial applications. We demonstrate an improvement in the thermal conductivity of TiO2 nanofluids with an addition of negligible amounts of modified silver “Ag” nanoparticles. In this work, the surface/shape of newly synthesized “Ag” nanoparticles is modified by planetary ball milling. Then, to enhance the thermal conductivity of TiO2 nanofluids, the flattened “Ag” particles are incorporated with the combination of small (15 nm) and large (300 nm) TiO2 nanoparticles in an aqueous solution. The thermal conductivities of Ag/TiO2−water nanofluids with various weight concentrations are measured at temperatures ranging from 15 to 40 °C. As a result, the present study confirms that the thermal conductivity of TiO2 based solution can be improved by introducing the flattened “Ag” particles.Munkhbayar Batmunkh, Md. R. Tanshen, Md. J. Nine, Munkhshur Myekhlai, Heekyu Choi, Hanshik Chung, and Hyomin Jeon

Increasing the Formation of Active Sites on Highly Crystalline Co Branched Nanoparticles for Improved Oxygen Evolution Reaction Electrocatalysis

The electrocatalysis of the oxygen evolution reaction (OER) at the surface of oxidized metal electrocatalysts is highly dependent on the structure and composition of the surface oxide. Here, Au core- Co branched nanoparticles were synthesized using a cubic-core hexagonal-branch growth approach in a slow reductive solution synthesis, resulting in highly crystalline metallic hcp Co branches. Electrochemical surface oxidation of the Co branched nanoparticles resulted in formation of Co(OH)2 that enable the formation of a higher number of active sites under OER conditions compared to Co3O4. Differently from polycrystalline spherical Au−Co core-shell nanoparticles, the oxidized structure on the Co branched nanoparticle surface is retained with electrochemical cycling, resulting in improved OER activity and stability

Highly efficient and stable Ru nanoparticle electrocatalyst for the hydrogen evolution reaction in alkaline conditions

Developing alternatives to platinum-based electrocatalysts for the hydrogen evolution reaction (HER) is an important challenge for realizing the green transition. This is especially the case for alkaline conditions where Pt-based catalysts have very poor stability. Here, we demonstrate new solvothermal synthesis methods with facile allotropism control for selectively obtaining hexagonal-close-packed (hcp) and face-centered cubic (fcc) ruthenium nanoparticles. Both samples are highly active HER catalysts in alkaline conditions outperforming commercial Pt/C. However, the samples show markedly different stabilities. The hcp sample shows exceptional stability for 12 hours constant operation at 10 mA/cm2 with an overpotential that only increases 6 mV whereas the fcc sample increases 50 mV and the commercial Pt/C more than 350 mV. Thus, this study underlines the importance of controlling the crystal structure of nanoparticle electrocatalysts and shows the potential of using Ru as an alternative to Pt in alkaline conditions

Microwave-assisted synthesis of black phosphorus quantum dots: efficient electrocatalyst for oxygen evolution reaction

A simple and efficient approach to produce high quality black phosphorus quantum dots (BPQDs) in a common organic solvent using a microwave technique is developed in this work. This novel approach produces a stable dispersion of crystalline BPQDs with an average lateral size of 2.95 nm and thickness of 3.59 nm. We demonstrated that the as-prepared BPQDs can be an efficient electrocatalyst for the oxygen evolution reaction (OER). Our BPQDs without any supporting catalyst exhibited an impressive electrocatalytic activity for OER with an overpotential of 450 mV at 10 mA cm⁻², whilst the traditional CoOₓ electrocatalyst showed an overpotential of 480 mV. By integrating our BPQDs with CoOₓ, we achieved an outstanding electrocatalytic OER performance with an overpotential of 360 mV at 10 mA cm⁻², a low Tafel slope of 58.5 mV dec⁻¹ and excellent stability, which was even comparable to the commercial IrO₂ and RuO₂ systems. This work introduces a promising protocol to prepare scalable BPQDs for real-world applications including electrocatalysis.Munkhbayar Batmunkh, Munkhshur Myekhlai, Abdulaziz S.R. Bati, Susanne Sahlos, Ashley D. Slattery, Tania M. Benedetti, Vinicius R. Gonçales, Christopher T. Gibson, J. Justin Gooding, Richard D. Tilley and Joseph G. Shapte

Magnetic assembly of transparent and conducting graphene-based functional composites

Innovative methods producing transparent and flexible electrodes are highly sought in modern optoelectronic applications to replace metal oxides, but available solutions suffer from drawbacks such as brittleness, unaffordability and inadequate processability. Here we propose a general, simple strategy to produce hierarchical composites of functionalized graphene in polymeric matrices, exhibiting transparency and electron conductivity. These are obtained through protein-assisted functionalization of graphene with magnetic nanoparticles, followed by magnetic-directed assembly of the graphene within polymeric matrices undergoing sol–gel transitions. By applying rotating magnetic fields or magnetic moulds, both graphene orientation and distribution can be controlled within the composite. Importantly, by using magnetic virtual moulds of predefined meshes, graphene assembly is directed into double-percolating networks, reducing the percolation threshold and enabling combined optical transparency and electrical conductivity not accessible in single-network materials. The resulting composites open new possibilities on the quest of transparent electrodes for photovoltaics, organic light-emitting diodes and stretchable optoelectronic devices