Synthesis of High-Purity Silica Nanoparticles by Sol-Gel Method

Colloidal silica (silica sol) nanoparticles were synthesized by ammonia- and hydrochloric acid-catalyzed hydrolysis of tetraethoxysilane with subsequent condensation and polymerization. Silica particles with the size of 12‒160 nm were obtained at different temperatures and ratios of the initial reactants and studied by means of TEM, AFM, IR spectroscopy and zeta-potential measurements. The reaction conditions providing the minimum particle size in the final product of the most complete hydrolysis were determined. At pH above 8.5, an increase in the SiO2 content of the sol to 23 wt.% did not change the particle size. At a low (~ 1.8 wt.%) SiO2 content of the sol, a wide variation in pH also did not exert a significant effect on the particle size. Stability of the silica sols synthesized in an alkaline medium was enhanced by the replacement of alcohol with water during evaporation at pH 8.5‒9.5. The possibility to produce silica sols with the required characteristics (particle size, pH, stability, purity, and SiO2 content in an aqueous or alcohol medium) makes them applicable in various industries

Low temperature electrical transport in microwave plasma fabricated free-standing graphene and N-graphene sheets

Funding Information: This work was performed under the framework of the PEGASUS (Plasma Enabled and Graphene Allowed Synthesis of Unique nano-Structures) project, funded by the European Union’s Horizon research and innovation program under grant agreement No 766894. Work partially funded by Portuguese FCT - Fundação para a Ciência e a Tecnologia, through EAGER project (PTDC/NAN-MAT/30565/2017) and under projects UIDB/50010/2020 and UIDP/50010/2020. The authors would like to thank S. Russev for the SEM images. M A, E V, K K and Zh K thank the European Regional Development Fund within the Operational Programme ‘Science and Education for Smart Growth 2014–2020’ under the Project CoE ‘National center of mechatronics and clean technologies ‘BG05M2OP001-1.001-0008’. Publisher Copyright: © 2023 The Author(s). Published by IOP Publishing Ltd.In this paper, the electrical transport in free-standing graphene and N-graphene sheets fabricated by a microwave plasma-based method is addressed. Temperature-dependent resistivity/conductivity measurements are performed on the graphene/N-graphene sheets compressed in pellets. Different measurement configurations reveal directional dependence of current flow—the room-temperature conductivity longitudinal to the pellet’s plane is an order of magnitude higher than the transversal one, due to the preferential orientation of graphene sheets in the pellets. SEM imaging confirms that the graphene sheets are mostly oriented parallel to the pellet’s plane and stacked in agglomerates. The high longitudinal electrical conductivity with values on the order of 103 S/m should be noted. Further, the current flow mechanism revealed from resistivity-temperature dependences from 300K down to 10K shows non-metallic behavior manifested with an increasing resistivity with decreasing the temperature d ρ / d T < 0 usually observed for insulating or localized systems. The observed charge transport shows variable range hopping at lower temperatures and thermally activated behaviour at higher temperatures. This allows us to attribute the charge transport mechanism to a partially disordered system in which single graphene sheets are placed predominantly parallel to each other and stacked together.publishersversionpublishe

Prospects for microwave plasma synthesized N-graphene in secondary electron emission mitigation applications

PTDC/NAN-MAT/30565/2017 D01-284/2019 (INFRAMAT) IBB BASE 2020-2023 UID/FIS/00068/2019.The ability to change the secondary electron emission properties of nitrogen-doped graphene (N-graphene) has been demonstrated. To this end, a novel microwave plasma-enabled scalable route for continuous and controllable fabrication of free-standing N-graphene sheets was developed. High-quality N-graphene with prescribed structural qualities was produced at a rate of 0.5 mg/min by tailoring the high energy density plasma environment. Up to 8% of nitrogen doping levels were achieved while keeping the oxygen content at residual amounts ( 1%). The synthesis is accomplished via a single step, at atmospheric conditions, using ethanol/methane and ammonia/methylamine as carbon and nitrogen precursors. The type and level of doping is affected by the position where the N-precursor is injected in the plasma environment and by the type of precursors used. Importantly, N atoms incorporated predominantly in pyridinic/pyrrolic functional groups alter the performance of the collective electronic oscillations, i.e. plasmons, of graphene. For the first time it has been demonstrated that the synergistic effect between the electronic structure changes and the reduction of graphene $-plasmons caused by N doping, along with the peculiar “crumpled” morphology, leads to sub-unitary (textless 1) secondary electron yields. N-graphene can be considered as a prospective low secondary electron emission and plasmonic material.publishersversionpublishe

N2-H2 capacitively coupled radio-frequency discharges at low pressure. Part I. Experimental results: Effect of the H2 amount on electrons, positive ions and ammonia formation

The mixing of N2 with H2 leads to very different plasmas from pure N2 and H2 plasma discharges. Numerous issues are therefore raised involving the processes leading to ammonia (NH3) formation. The aim of this work is to better characterize capacitively-coupled radiofrequency plasma discharges in N2 with few percents of H2 (up to 5%), at low pressure (0.3-1 mbar) and low coupled power (3-13 W). Both experimental measurements and numerical simulations are performed. For clarity, we separated the results in two complementary parts. The actual one (first part), presents the details on the experimental measurements, while the second focuses on the simulation, a hybrid model combining a 2D fluid module and a 0D kinetic module. Electron density is measured by a resonant cavity method. It varies from 0.4 to 5 109 cm-3, corresponding to ionization degrees from 2 10-8 to 4 10-7. Ammonia density is quantified by combining IR absorption and mass spectrometry. It increases linearly with the amount of H2 (up to 3 1013 cm-3 at 5% H2). On the contrary, it is constant with pressure, which suggests the dominance of surface processes on the formation of ammonia. Positive ions are measured by mass spectrometry. Nitrogen-bearing ions are hydrogenated by the injection of H2, N2H+ being the major ion as soon as the amount of H2 is >1%. The increase of pressure leads to an increase of secondary ions formed by ion/radical-neutral collisions (ex: N2H+, NH4 +, H3 +), while an increase of the coupled power favours ions formed by direct ionization (ex: N2 +, NH3 +, H2 +).N. Carrasco acknowledges the financial support of the European Research Council (ERC Starting Grant PRIMCHEM, Grant agreement no. 636829). A. Chatain acknowledges ENS Paris-Saclay Doctoral Program. A. Chatain is grateful to Gilles Cartry and Thomas Gautier for fruitful discussions on the MS calibration. L.L. Alves acknowledges the financial support of the Portuguese Foundation for Science and Technology (FCT) through the project UID/FIS/50010/2019. L. Marques and M. J. Redondo acknowledge the financial support of the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UIDB/04650/2019

Noise reduction in EEDF numerical differentiation technique

In this work we present a review of numerical techniques providing large increase in signal-to-noise ratios in a wide variety of applications, namely, in Langmuir probe measurements intended to determine the electron energy distribution function (EEDF).Dans ce travail on présente une vision générale des techniques numériques permettant une augmentation importante du rapport signal-bruit dans une grande variété d'applications comme, par exemple, dans les mesures par sonde de Langmuir destinées à la détermination des fonctions de distribution de l'énergie des électrons (EEDF)

Emission spectroscopy of a surface wave sustained N-2-H-2 discharge

An experimental investigation on the degree of molecular dissociation in a surface wave-driven discharge (¿/2p=2.45 GHz) operating in N2–H2 mixture at pressure p=(66.5–266 Pa) is performed by means of optical emission spectroscopy. The dissociation degree of nitrogen molecules [N(4S)]/[N2] increases with the amount of hydrogen (up to 50%) in the mixture in correlation with the increase in electron temperature, when the electron density and the pressure are kept nearly constant. The relative number of hydrogen atoms keeps approximately constant with increasing H2 percentage (between 10% and 50%) in the mixture. The degree of molecular dissociation decreases along the plasma column length following the decrease in the electron density

Microwave plasmas applied for the synthesis of free standing graphene sheets

The article of record as published may be located at http://dx.doi.org/10.1088/0022-3727/47/38/385501Self-standing graphene sheets were synthesized using microwave plasmas driven by surface waves at 2.45 GHz stimulating frequency and atmospheric pressure. The method is based on injecting ethanol molecules through a microwave argon plasma environment, where decomposition of ethanol molecules takes place. The evolution of the ethanol decomposition was studied in situ by plasma emission spectroscopy. Free gas-phase carbon atoms created in the plasma diffuse into colder zones, both in radial and axial directions, and aggregate into solid carbon nuclei. The main part of the solid carbon is gradually withdrawn from the hot region of the plasma in the outlet plasma stream where nanostructures assemble and grow. Externally forced heating in the assembly zone of the plasma reactor has been applied to engineer the structural qualities of the assembled nanostructures. The synthesized graphene sheets have been analysed by Raman spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy and x-ray photoelectron spectroscopy. The presence of sp3 carbons is reduced by increasing the gas temperature in the assembly zone of the plasma reactor. As a general trend, the number of mono-layers decreases when the wall temperature increases from 60 to 100 ◦C. The synthesized graphene sheets are stable and highly ordered