Combined Spatially Resolved Optical Emission Imaging and Modeling Studies of Microwave-Activated H2/Ar and H2/Kr Plasmas Operating at Powers and Pressures Relevant for Diamond Chemical Vapor Deposition

Plasma-Engineering of Oxygen Vacancies on NiCo2 O4 Nanowires with Enhanced Bifunctional Electrocatalytic Performance for Rechargeable Zinc-air Battery

Designing an efficient, durable, and inexpensive bifunctional electrocatalyst toward oxygen evolution reactions (OER) and oxygen reduction reactions (ORR) remains a significant challenge for the development of rechargeable zinc-air batteries (ZABs). The generation of oxygen vacancies plays a vital role in modifying the surface properties of transition-metal-oxides (TMOs) and thus optimizing their electrocatalytic performances. Herein, a H2 /Ar plasma is employed to generate abundant oxygen vacancies at the surfaces of NiCo2 O4 nanowires. Compared with the Ar plasma, the H2 /Ar plasma generated more oxygen vacancies at the catalyst surface owing to the synergic effect of the Ar-related ions and H-radicals in the plasma. As a result, the NiCo2 O4 catalyst treated for 7.5 min in H2 /Ar plasma exhibited the best bifunctional electrocatalytic activities and its gap potential between Ej = 10 for OER and E1/2 for ORR is even smaller than that of the noble-metal-based catalyst. In situ electrochemical experiments are also conducted to reveal the proposed mechanisms for the enhanced electrocatalytic performance. The rechargeable ZABs, when equipped with cathodes utilizing the aforementioned catalyst, achieved an outstanding charge-discharge gap, as well as superior cycling stability, outperforming batteries employing noble-metal catalyst counterparts.

Imaging and Modeling C2 Radical Emissions from Microwave Plasma-Activated Methane/Hydrogen Gas Mixtures: Contributions from Chemiluminescent Reactions and Investigations of Higher-Pressure Effects and Plasma Constriction

Wavelength and spatially resolved imaging and 2D plasma chemical modeling methods have been used to study the emission from electronically excited C₂ radicals in microwave-activated dilute methane/hydrogen gas mixtures under processing conditions relevant to the chemical vapor deposition (CVD) of diamond. Obvious differences in the spatial distributions of the much-studied C₂(d³Π_g-a³Π_u) Swan band emission and the little-studied, higher-energy C₂(C¹Π_g-A¹Π_u) emission are rationalized by invoking a chemiluminescent (CL) reactive source, most probably involving collisions between H atoms and C₂H radicals, that acts in tandem with the widely recognized electron impact excitation source term. The CL source is relatively much more important for forming C₂(d) state radicals and is deduced to account for >40% of C₂(d) production in the hot plasma core under base operating conditions, which should encourage caution when estimating electron or gas temperatures from C₂ Swan band emission measurements. Studies at higher pressures (p ≈ 400 Torr) offer new insights into the plasma constriction that hampers efforts to achieve higher diamond CVD rates by using higher processing pressures. Plasma constriction is proposed as being inevitable in regions where the local electron density (n_e) exceeds some critical value (n_ec) and electron-electron collisions enhance the rates of H₂ dissociation, H-atom excitation, and related associative ionization processes relative to those prevailing in the neighboring nonconstricted plasma region. The 2D modeling identifies a further challenge to high-p operation. The radial uniformities of the CH₃ radical and H-atom concentrations above the growing diamond surface both decline with increasing p, which are likely to manifest as less spatially uniform diamond growth (in terms of both rate and quality).

Optical and Mass Spectrometric Measurements of the CH4-CO2 Dry Reforming Process in a Low Pressure, Very High Density, and Purely Inductive Plasma

This paper presents a study of a CH₄-CO₂ plasma-reforming process carried out in a high power density (5-50 W/cm³), using toroidal transformer-coupled plasma, and operated at low pressure (0.2-0.7 Torr). Using the intermediate between a thermal and nonthermal plasma (electron density, n_e ≈ 3 × 10¹² cm^-3 and a maximum gas temperature of ∼4000-6000 K along the center line), the low-pressure study provides a unique set of conditions to investigate reaction mechanisms, where three-body reactions can be ignored. Reactive species in the plasma were identified by optical emission spectroscopy. End products of the reforming process were measured by mass spectrometry. Quite high conversions of CO₂ and CH₄ were found (90%), with selectivities for CO and H₂ of 80% at 300 sccm feed gas flow rate in a 0.5 Torr plasma, with a mole ratio CO₂-CH₄ of 1:1. A detailed reaction mechanism is presented, taking into account the combined detection of reactive intermediates in the plasma (H, O, CH, and C₂) and stable product downstream.

Imaging and Modeling the Optical Emission from CH Radicals in Microwave Activated C/H Plasmas

We report a combined experimental/modeling study of optical emission from the A²Δ, B²Σ^-, and C²Σ⁺ states of the CH radical in microwave (MW) activated CH₄/H₂ gas mixtures operating under a range of conditions relevant to the chemical vapor deposition of diamond. The experiment involves spatially and wavelength resolved imaging of the CH(C → X), CH(B → X), and CH(A → X) emissions at different total pressures, MW powers, C/H ratios in the source gas, and substrate diameters. The results are interpreted by extending an existing 2D (r, z) plasma model to include not just electron impact excitation but also chemiluminescent (CL) bimolecular reactions as sources of the observed CH emissions. Three possible CL reactions (of H atoms with CH₂(a¹A₁) and CH₂(X³B₁) radicals and of C(¹D) atoms with H₂) are identified as plausible sources of electronically excited CH radicals (particularly of the lowest energy CH(A) state radicals). Each or all of these could contribute to the observed emissions and, collectively, are deduced to be the major source of the CH(A) emissions observed at the high temperatures (T_gas ∼ 3000 K) and pressures (75 ≤ p ≤ 275 Torr) explored in the present study. We suggest that such CL contributions are likely to be commonplace in such high pressure, high temperature plasma environments and highlight some of the risks associated with using relative emission intensities as an indicator of the electron characteristics in such plasmas.