Plasma-enhanced Surface Research Articles

Kinetic investigation of plasma catalytic synthesis of ammonia: insights into the role of excited states and plasma-enhanced surface chemistry

The present work investigates the kinetics of catalytic ammonia synthesis in a N2/H2 mixture activated by a nanosecond pulsed discharge plasma experimentally and numerically. X-ray diffraction, high-resolution transmission electron microscopy and x-ray photoelectron spectroscopy are combined to characterize the morphology and surface electronic properties of the catalyst. Special attention is placed on the role of excited species in promoting the formation of important intermediates and the plasma-enhanced surface chemistry. A detailed kinetic mechanism consisting of atoms, radicals, excited species, molecules, ions, and surface species is developed and studied by incorporating a set of the electron impact reactions, reactions involving excited species, ionic reactions, direct and dissociative adsorption reactions, and surface reactions. A zero-dimensional model incorporating the plasma kinetics solver is used to calculate the temporal evolution of species densities in a N2/H2 plasma catalysis system. The results show that the coupling of Fe/γ–Al2O3 catalyst with plasma is much more effective in ammonia synthesis than the Fe/γ–Al2O3 catalyst alone and plasma alone. The numerical model has a good agreement with experiments in ammonia formation. The path flux analysis shows the significant roles of excited species N(2D), H2(v1), N2(v) in stimulating the formation of precursors NH, NH2, and adsorbed N(s) through the pathways N(2D) + H2 → NH + H, H2(v1) + NH → NH2 + H and N2(v) + 2Fe(s) → N(s) + N(s), respectively. Furthermore, the results show that the adsorption reaction N + Fe(s) → N(s) and Eley–Ridel interactions N(s) + H → NH(s), N + H(s) → NH(s), NH + H(s) → NH2(s) and NH2 + H(s) → NH3(s) can kinetically enhance the formation of ammonia, which further highlights the plasma-enhanced surface chemistry. This work provides new insights into the roles of excited species and plasma-enhanced surface chemistry in the plasma catalytic ammonia synthesis.

Plasma-Engineered N-CoOx Nanowire Array as a Bifunctional Electrode for Supercapacitor and Electrocatalysis.

Surface engineering has achieved great success in enhancing the electrochemical activity of Co3O4. However, the previously reported methods always involve high-temperature calcination processes which are prone to induce agglomeration of the nanostructure, leading to the attenuation of performance. In this work, Co3O4 nanowires were successfully modified by a low-temperature NH3/Ar plasma treatment, which simultaneously generated a porous structure and efficient nitrogen doping with no agglomeration. The modified N-CoOx electrode exhibited remarkable performance due to the synergistic effect of the porous structure and nitrogen doping, which provided additional active sites for faradic transitions and improved charge transfer characteristics. The electrode achieved excellent supercapacitive performance with a maximum specific capacitance of 2862 mF/cm2 and superior cycling retention. Furthermore, the assembled asymmetric supercapacitor (N-CoOx//AC) device exhibited an extended potential window of 1.5 V, a maximum specific energy of 80.5 Wh/kg, and a maximum specific power of 25.4 kW/kg with 91% capacity retention after 5000 charge–discharge cycles. Moreover, boosted hydrogen evolution reaction performance was also confirmed by the low overpotential (126 mV) and long-term stability. This work enlightens prospective research on the plasma-enhanced surface engineering strategies.

Improving sensing properties of entangled carbon nanotube-based gas sensors by atmospheric plasma surface treatment

Entangled multi-walled carbon nanotubes (MWCNTs) on polyurethane (PUR) after Ar plasma-treatment and He plasma-treatment have been tested as gas sensors for ethanol sensing. It was found that plasma-treated sensors exhibit higher sensitivity compared to the non-treated samples along with different ethanol concentration. Non-treated sensors exhibit similar sensor response with the increase in ethanol concentration, while Ar plasma-treated sensors displays ~5 times improvement and He plasma-treated sensors show ~3 times improvement with an increase in ethanol concentration. The sensitivity of the plasma-treated sensors is also stable for following two-weeks after the preparation compared to the non-treated sensor. Entangled nanotube network exhibits a significant shift in the baseline resistance after both plasma-treatments. The response time of the sensor was increased after the plasma-treatment, while the recovery was rather quick. Surface analyses revealed that plasma-treatment did not make any significant morphological changes. Thus, the improvements in stability and sensitivity after plasma-treatment are attributed to the plasma-enhanced surface modification and formation of functional bonds on the surface of nanotubes, which are sensitive to the ethanol vapour.

Novel g-C3N4/TiO2/PAA/PTFE ultrafiltration membrane enabling enhanced antifouling and exceptional visible-light photocatalytic self-cleaning

Membrane fouling due to superhydropobicity of polytetrafluoroethylene ultrafiltration membranes (PTFE UFMs) represents a grand challenge for their practical applications in diverse water treatment industries. Surface immobilisation of hydrophilic and chemically stable inorganic metal oxides (TiO2, ZrO2, etc) has been developed to improve hydrophilicity of the PTFE UFMs, though they still suffer from expensive and repeating regenerations once fouled. To address such issues, we strive to firmly immobilize g-C3N4 modified TiO2 (g-C3N4/TiO2, hereafter CNTO) onto PTFE UFM via a facile plasma-enhanced surface graft technique using polyacrylic acid (PAA) as a bridging agent. As reported here, the obtained CNTO/PAA/PTFE UFM shows much smaller surface water contact angle (WCA) of 62.3° than that of bare PTFE UFM(115.8°), leading to enhanced water flux of 830 L m−2 h-1 in the initial ultrafiltration of modelled waste-water containing methylene blue (MB). The CNTO/PAA/PTFE UFM is highly resistant to fouling in the prolonged filtration of 1000 mg/L bovine serum albumin (BSA) solution, while the fouled CNTO/PAA/PTFE UFM is able to regenerate rapidly under either UV or visible-light irradiation. The enhanced performance of the novel CNTO/PAA/PTFE UFM is reasonably attributed to its high wettability and robust photocatalytic activity of the g-C3N4/TiO2 coating that follows different self-cleaning mechanisms under UV and visible light irradiations.

(Invited) Magnetron Sputtering for Deposition of Photocatalyst Nanostructures on Transparent Conductive Oxides for Solar Applications

Photocatalysis has attracted an enormous amount of research interest, in especially with regard to applications such as solar hydrogen generation as well as pollutant degradation. Main research target is the development of corrosion-stable core candidate materials that are able to perform under the abundant visible light in the solar spectrum. Two main approaches are being employed, the development of visible light-sensitive semiconductors and sensitisation of wide band-gap semiconductors. In materials synthesis, plasma-enhanced surface modification and layer deposition methods are based on the presence of non-equilibrium states of reactive species in a plasma environment. They are therefore able to overcome limitations of traditional catalyst synthesis methods, giving rise to new reaction pathways and resulting in unique properties of nanomaterials. This presentation reports on recent work on development of DC magnetron sputtering processes for deposition of TiO2 and WO3 semiconductor layers as well as RF magnetron deposition for synthesis of N-doped graphite electrodes on TCO. Electrode properties are characterised by means of XRD, TEM, UV/Vis, XPS, Raman spectroscopy and photoelectrochemical characterisation including IMVS/IMPS and IPCE measurements. Furthermore, plasma-enhanced deposition methods for surface enhancement of photocatalysts layers by nanoparticles and polymer-encapsulation are also discussed.

Self-organized fabrication of periodic arrays of vertical, ultra-thin nanopillars on GaAs surfaces

This paper presents a low-cost procedure that allows for self-organized fabrication of periodically arranged, sub-50 nm diameter, vertical-sidewall GaAs nanopillars on GaAs surfaces based on nanosphere lithography, and reactive ion etching (RIE). Monodispersed polystyrene (PS) sphere double layers are deposited from a colloidal suspension on pre-treated, hydrophilized GaAs substrates. Ni is thermally evaporated to act as a hard mask for subsequent RIE. Scanning electron microscopy reveals that the Ni nanoparticles left on the substrate after PS sphere removal have polygon-shaped in-plane cross-sections corresponding to the shape of the double layer mask openings. RIE using SiCl4 at low pressure and high power density leads to the formation of vertical nanopillars with circular to oval cross-sections, whose diameters are reduced by ∼1/3 compared to those of the Ni nanoparticles. These findings can be explained by plasma-enhanced surface diffusion and sputtering processes during RIE. The obtained GaAs nanopillars have an average diameter of only ∼23 nm, exhibit an excellent verticality with a sidewall angle of 88.9 ± 0.4° and planar top faces, as visible in transmission electron microscopy images.

Plasma-Enhanced Surface Modification of Sprayed Carbon Nanotube Electrodes for Lithographically Integrated Biosensing System

Plasma-Enhanced Surface Modification of Sprayed Carbon Nanotube Electrodes for Lithographically Integrated Biosensing System

Plasma-Enhanced Surface Modification of Sprayed Carbon Nanotube Electrodes for Lithographically Integrated Biosensing System

A sprayed carbon nanotube (CNT)-modified working electrode was successfully integrated into an electrochemical three-electrode system based on a glass substrate. The integrated biosensing system was fabricated through a series of photolithographic patterning and plasma etching processes. A CNT-dispersed solution was sprayed on the three-electrode system, and the CNT-modified surface was treated with O2 plasma to pattern, clean, and activate the CNT layer. The optimized plasma treatment conditions were verified in terms of plasma power and treatment time by scanning electron microscopy (SEM), cyclic voltammetry (CV), and X-ray photoelectron spectroscopy (XPS).

PVD hard coatings on prenitrided low alloy steel

The combination of traditional surface treatments such as nitriding with modern plasma-enhanced surface technologies reveals the possibility, particularly in the application to low alloy steels, of obtaining mechanical properties comparable with those of high alloy steels. Gas-nitrided samples of the hardened and tempered low alloy steels 30CrMoV9 and 17CrMoV10 were TiN coated by r.f. magnetron sputtering and ion plating. The requirements to obtain a nitrided substrate that can be coated were given special consideration. For this, various surface modifications of the nitrided substrates were realized by bright nitriding, nitriding with a compound layer and additional steps before coating, such as polishing, grinding and sputter cleaning. The properties of prenitrided coated steels essentially depend on the structure and properties of the outer part of the nitrided case. TiN on bright nitrided and nitrided substrates with the compound layer removed has a better adherence than on compound layers. The decomposition of the iron nitride during the plasma sputter cleaning of compound layers results in a lower surface hardness and lower adherence of TiN. The highest wear resistances in the Timken test were registered on samples where the compound layer had been removed before TiN coating.