Plasma-generated Species Research Articles

Plasma-catalytic assisted ammonia synthesis: Reactive molecular dynamics study

Plasma catalytic assisted ammonia synthesis is currently considered a promising approach that enables ammonia synthesis under low temperature and low pressure. In this paper, ReaxFF molecular dynamics (MD) method was first used to study the impact of varying electric fields and different plasma-generated active species on ammonia synthesis, aiming to uncover the formation mechanisms and synthetic pathways of NH3 from the molecular-level under plasma assistance. The results indicate that the electric field has the optimal range. With the increase of electric field strength, the collision frequency between N2 and H2 molecules does not increase linearly due to the polarization phenomenon, but increases first and then decreases, which affects the production of ammonia. As the electric field is greater than −0.01 V/Å, the ammonia production begins to decline due to the decreased molecular collision frequency as well as the electric-field induced ammonia dissociation. The results also show that the plasma-generated active species significantly promote NH3 formation. Compared to the plasma-generated excited state and the ion, plasma-generated radicals such as N, H, NH and NH2 have a more significant promoting effect on ammonia synthesis due to the acceleration of ammonia synthesis elementary reactions and the reduction of starting time significantly. In the aspect of molecular level, a new ammonia synthesis reaction pathway is discovered: N2→N2H→N2H2→N2H3→NH3, which was never reported in previous studies. In addition, by decoupling the gas-phase reactions and dissociative adsorption reactions on the catalyst, it verified a new adsorption reaction path: N(s)→NH(s)→N2H(s)→N2H2(s) at molecular level. This study provides valuable insights into the complete dynamic mechanism of plasma catalyst assisted NH3 synthesis in the molecular level.

Plasma-catalytic dry reforming of CH4: Effects of plasma-generated species on the surface chemistry

By means of steady-state experiments and a global model, we studied the effects of plasma-generated reactive species on the surface chemistry and coking in plasma-catalytic CH4/CO2 reforming at reduced pressure (8–40 kPa). We used a hybrid ZDPlasKin-CHEMKIN model to predict the species densities over time. The detailed plasma-catalytic mechanism consists of the plasma discharge scheme, a gas-phase chemistry set and a surface mechanism. Our experimental results show that the coupling of Ni/SiO2 catalyst with plasma is more effective in CH4/CO2 activation and conversion than unpacked DBD plasma, with syngas being the main products. The highest total conversion of 16 % was achieved at 8000 V and 473 K, with corresponding CO and H2 yields of 15 % and 12 %, respectively. The reactants conversion and product selectivity are well captured by the kinetic model. Our simulation results suggest that vibrational species and radicals can accelerate the dissociative adsorption and Eley-Rideal (E-R) reactions. Path flux analysis shows that E-R reactions dominate the surface reaction pathways, which differs from thermal catalysis, indicating that the coupling of non-equilibrium plasma and catalysis can effectively shift the formation and consumption pathways of important adsorbates. For instance, our model suggests that HCOO(s) is primarily generated through the E-R reaction CO2(v) + H(s) → HCOO(s), while the hydrogenation reaction HCOO(s) + H → HCOOH(s) is the main source of HCOOH(s). Carbon deposition on the catalyst surface is primarily formed through the stepwise dehydrogenation of CH4, while the E-R reactions enhanced by plasma-generated H and O atoms dominate the consumption of carbon deposition. This work provides new insights into the effects of reactive species on the surface chemistry in plasma-catalytic CH4/CO2 reforming.

Advancing DBD Plasma Chemistry: Insights into Reactive Nitrogen Species such as NO2, N2O5, and N2O Optimization and Species Reactivity through Experiments and MD Simulations.

This study aims to fine-tune the plasma composition with a particular emphasis on reactive nitrogen species (RNS) including nitrogen dioxide (NO2), dinitrogen pentoxide (N2O5), and nitrous oxide (N2O), produced by a self-constructed cylindrical dielectric barrier discharge (CDBD). We demonstrated the effective manipulation of the plasma chemical profile by optimizing electrical properties, including the applied voltage and frequency, and by adjusting the nitrogen and oxygen ratios in the gas mixture. Additionally, quantification of these active species was achieved using Fourier transform infrared spectroscopy. The study further extends to exploring the aerosol polymerization of acrylamide (AM) into polyacrylamide (PAM), serving as a model reaction to evaluate the reactivity of different plasma-generated species, highlighting the significant role of NO2 in achieving high polymerization yields. Complementing our experimental data, molecular dynamics (MD) simulations, based on the ReaxFF reactive force field potential, explored the interactions between reactive oxygen species, specifically hydroxyl radicals (OH) and hydrogen peroxide (H2O2), with water molecules. Understanding these interactions, combined with the optimization of plasma chemistry, is crucial for enhancing the effectiveness of DBD plasma in environmental applications like air purification and water treatment.

Precision spectroscopy of non-thermal molecular plasmas using mid-infrared optical frequency comb Fourier transform spectroscopy

Non-thermal molecular plasmas play a crucial role in numerous industrial processes and hold significant potential for driving essential chemical transformations. Accurate information about the molecular composition of the plasmas and the distribution of populations among quantum states is essential for understanding and optimizing plasma processes. Here, we apply a mid-infrared frequency comb-based Fourier transform spectrometer to measure high-resolution spectra of plasmas containing hydrogen, nitrogen, and a carbon source in the 2800–3400 cm–1 range. The spectrally broadband and high-resolution capabilities of this technique enable quantum-state-resolved spectroscopy of multiple plasma-generated species simultaneously, including CH4, C2H2, C2H6, NH3, and HCN, providing detailed information beyond the limitations of current methods. Using a line-by-line fitting approach, we analyzed 548 resolved transitions across five vibrational bands of plasma-generated HCN. The results indicate a significant non-thermal distribution of the populations among the quantum states, with distinct temperatures observed for lower and higher rotational quantum numbers, with a temperature difference of about 62 K. Broadband state-resolved-spectroscopy via comb-based methods provides unprecedented fundamental insights into the non-thermal nature of molecular plasmas—a detailed picture that has never been accomplished before for such complex non-thermal environment.

Design and Development of a Novel Tesla Coil-Based Cold Plasma Device for Plasma Medicine: Decoupling the Effect of Plasma-Generated Species

Design and Development of a Novel Tesla Coil-Based Cold Plasma Device for Plasma Medicine: Decoupling the Effect of Plasma-Generated Species

Experimental and numerical study of stabilized flame in inverse coflow turbulent jet using nanosecond repetitively pulsed discharges

Methane has become increasingly popular in rocket propulsion, but low stability and limited flammability range have always been a concern about methane-powered systems. Many stabilization methods have been developed to change the geometrical or flow characteristics of the burner. However, most of these efforts have yet to be practically successful due to cost and compatibility issues. Alternatively, other methods such as microwave, dielectric barrier, and nanosecond repetitive pulse (NRP) discharges have been proven to be efficient by modifying the kinetic and transport pathways. NRP discharges have shown promising results as one of the most effective low-temperature plasma (LTP) methods. In this paper, chemiluminescence imaging is used to study the effect of NRP discharge on liftoff and blowout, as the important stabilization parameters, by recording the liftoff height and liftoff/blowout velocities under a wide range of discharge (f=0–10 kHz and V=11–19 kV) and jet velocity (ν=2–60 m/s). Depending on these parameters, four different discharge regimes of corona, diffuse, filamentary, and arc were observed. The results have shown that high-intensity plasma in a filamentary discharge regime can provide a significant advantage in delaying the liftoff conditions, but no improvements in the blowout were observed. It was also found that NRP discharge can reduce the liftoff height. To explore the cause of the increased stability, a parametric numerical study is conducted using detailed plasma-assisted methane kinetic modeling coupled to a 1D opposed diffusion flame simulation. Results show that the extinction limits of diffusion flames can be dramatically enhanced by LTP due to the local formation of high radical and excited species concentrations, with subsequent recombination leading to increased temperature and higher reactivity in the flame zone. In addition, a 1D laminar flame speed evaluation shows that the plasma-generated active species can dramatically increase the flame speed, which, in turn, reduces the lifted flame height above the burner surface.

Plasma-liquid interactions in the presence of organic matter-A perspective.

As investigations in the biomedical applications of plasma advance, a demand for describing safe and efficacious delivery of plasma is emerging. It is quite clear that not all plasmas are "equal" for all applications. This Perspective discusses limitations of the existing parameters used to define plasma in context of the need for the "right plasma" at the "right dose" for each "disease system." The validity of results extrapolated from in vitro studies to preclinical and clinical applications is discussed. We make a case for studying the whole system as a single unit, in situ. Furthermore, we argue that while plasma-generated chemical species are the proposed key effectors in biological systems, the contribution of physical effectors (electric fields, surface charging, dielectric properties of target, changes in gap electric fields, etc.) must not be ignored.

Remote inductively coupled plasmas in Ar/N2 mixtures and implications for plasma enhanced ALD

Plasma enhanced atomic layer deposition (PEALD) is a cyclic atomic layer deposition (ALD) process that incorporates plasma-generated species into one of the cycle substeps. The addition of plasma is advantageous as it generally provides unique reactants and a substantially reduced growth temperature compared to thermal approaches. However, the inclusion of plasma, coupled with the increasing variety of plasma sources used in PEALD, can make these systems challenging to understand and control. This work focuses on the use of plasma diagnostics to examine the plasma characteristics of a remote inductively coupled plasma (ICP) source, a type of plasma source that is commonly used for PEALD. Ultraviolet to near-infrared spectroscopy and spatially resolved Langmuir probe measurements are employed to characterize a remote ICP system using nitrogen-based gas chemistries typical for III-nitride growth processes. Spectroscopy is used to characterize the relative concentrations of important reactive and energetic neutral species generated in the remote ICP as a function of gas flow rate, Ar/N2 flow fraction, and gas pressure. In addition, the plasma potential and plasma density for the same process parameters are examined using an RF compensated Langmuir probe downstream from the ICP source. The results are also discussed in terms of their impact on materials growth.

In situ separation of plasma-generated species using metal–organic frameworks

Selectively extracting the desired reactive species from plasma could unlock various applications for plasma. This study uncovers the potential of metal–organic frameworks (MOFs) in selectively extracting plasma-generated species from the plasma environment. To demonstrate this, we utilized a zeolitic imidazolate frameworks-8 (ZIF-8) as the MOF separation membrane and Ar/C3H8 plasma. The ZIF-8 membrane proved effective in selectively extracting H2 molecules produced from C3H8, as compared with the case without the ZIF-8.

Non-thermal atmospheric-pressure positive pulsating corona discharge in degradation of textile dye Reactive Blue 19 enhanced by Bi2O3 catalyst

In this work, monoclinic Bi2O3 was applied for the first time, to the best of our knowledge, as a catalyst in the process of dye degradation by a non-thermal atmospheric-pressure positive pulsating corona discharge. The research focused on the interaction of the plasma-generated species and the catalyst, as well as the role of the catalyst in the degradation process. Plasma decomposition of the anthraquinone reactive dye Reactive Blue 19 (RB 19) was performed in a self-made reactor system. Bi2O3 was prepared by electrodeposition followed by thermal treatment, and characterized by x-ray diffraction, scanning electron microscopy and energy-dispersive x-ray techniques. It was observed that the catalyst promoted decomposition of plasma-generated H2O2 into •OH radicals, the principal dye-degrading reagent, which further attacked the dye molecules. The catalyst improved the decolorization rate by 2.5 times, the energy yield by 93.4% and total organic carbon removal by 7.1%. Excitation of the catalyst mostly occurred through strikes by plasma-generated reactive ions and radical species from the air, accelerated by the electric field, as well as by fast electrons with an energy of up to 15 eV generated by the streamers reaching the liquid surface. These strikes transferred the energy to the catalyst and created the electrons and holes, which further reacted with H2O2 and water, producing •OH radicals. This was indentified as the primary role of the catalyst in this process. Decolorization reactions followed pseudo first-order kinetics. Production of H2O2 and the dye degradation rate increased with increase in the input voltage. The optimal catalyst dose was 500 mg∙dm−3. The decolorization rate was a little lower in river water compared with that in deionized water due to the side reactions of •OH radicals with organic matter and inorganic ions dissolved in the river water.

Harnessing plasma-generated reactive species for the synthesis of different phases of molybdenum oxide to study adsorption and photocatalytic activity.

This study employs plasma-liquid interaction technique to synthesize different phases of molybdenum oxide using air and argon as plasma-forming gases. In situ plasma-generated nitrogen species primarily NO3-/NO2- and hydrogen species (H+) facilitate the reduction of the molybdenum precursor anion (Mo7O24-). The reduced Mo species subsequently reacts with reactive oxygen species, forming MoO6 octahedra, which is the building block of a molybdenum oxide crystal. Varied concentrations of NO3-/NO2- and H+ species in air and argon plasma treatment significantly influence the growth process. Air plasma synthesis yields hexagonal molybdenum oxide microrods, which upon calcination changes its phase to orthorhombic 2D layered structure. Moreover, the argon plasma synthesized sample exhibits a mixed phase of hexagonal and orthorhombic molybdenum oxide due to the heavy argon ion bombardment, inducing material porosity and surface oxygen vacancies. The mixed-phase material exhibits superior adsorption and photo-degradation towards cationic dye compared to the other two phases. The higher photocatalytic performance may be responsible for the extended lifetime of the photo-generated charge carriers possessed by the mixed-phase material. Radical scavenging tests have identified holes and hydroxyl radicals as the key reactive species that take part in the photo-degradation process.

Synergistic role of oxygen vacancy defect and plasmonics on modulating the photocatalytic activity of Ag/CuOx nanocomposites synthesized via solution plasma

Herein, an environmentally friendly green approach for the single-step synthesis of Ag/CuOx nanocomposite by generating plasma between two copper electrodes in an AgNO3 solution is reported. Morphological tunability of the CuOx nanoparticles from spindle to flower-like and then to wire-like is achieved by varying the concentration of the AgNO3 solution, which regulates toxic dye adsorption. Compositional analysis reveals the formation of oxygen vacancies, indicating the presence of sub-stoichiometric CuO due to the bombardment of plasma-generated species on the as-formed nanoparticle. The percentage of oxygen vacancies decreases with increasing the concentration of AgNO3 to fabricate the nanocomposites. At low Ag content in the Ag/CuOx nanocomposites, higher photocatalytic efficiency is observed than the bare CuOx nanoparticles, whereas for high Ag content, photocatalytic efficiency quenches. Electrochemical spectroscopy reveals an increase in the charge-transfer resistance with Ag content. A possible photo-degradation mechanism based on the trapping and transfer of photo-generated charge carriers between the defect-enriched CuOx and plasmonic Ag nanoparticles is presented. Charge carrier lifetime is investigated to support the photo-degradation mechanism. The effect of H2O2 on the photo-degradation by considering the band edge potential of CuOx nanoparticles is also discussed.

Sustainable ammonia production via nanosecond-pulsed plasma oxidation and electrocatalytic reduction

The production of ammonia, powered by renewable energy, in a decentralized manner is of key importance in the transition to a more sustainable future. Recent research has explored the integration of non-thermal plasma and electrochemical processes to achieve this goal. However, the success of this hybrid process is contingent on the energy efficiency of the plasma-generated species. Herein, we developed a plasma bubble reactor, driven by nanosecond pulses interfacing plasma directly with water. This reactor can comprehensively probe gas ionization processes, different energy channels, corresponding plasma catalytic reaction mechanisms, and reactive species in gas and liquid phases. By using on-and-off plasma ignition with rapid pulses, we could regulate energy consumption in cycles and achieved the lowest reported energy consumption of 2.7 ± 0.1 kWh mol-1 NO3− and 3.2 ± 0.1 kWh mol-1 NH4+ after electrocatalytic nitrate reduction. This provides a promising pathway to producing green, renewable ammonia from air and water.

The cold atmospheric pressure plasma-generated species superoxide, singlet oxygen and atomic oxygen activate the molecular chaperone Hsp33.

Cold atmospheric pressure plasmas are used for surface decontamination or disinfection, e.g. in clinical settings. Protein aggregation has been shown to significantly contribute to the antibacterial mechanisms of plasma. To investigate the potential role of the redox-activated zinc-binding chaperone Hsp33 in preventing protein aggregation and thus mediating plasma resistance, we compared the plasma sensitivity of wild-type E. coli to that of an hslO deletion mutant lacking Hsp33 as well as an over-producing strain. Over-production of Hsp33 increased plasma survival rates above wild-type levels. Hsp33 was previously shown to be activated by plasma in vitro. For the PlasmaDerm source applied in dermatology, reversible activation of Hsp33 was confirmed. Thiol oxidation and Hsp33 unfolding, both crucial for Hsp33 activation, occurred during plasma treatment. After prolonged plasma exposure, however, unspecific protein oxidation was detected, the ability of Hsp33 to bind zinc ions was decreased without direct modifications of the zinc-binding motif, and the protein was inactivated. To identify chemical species of potential relevance for plasma-induced Hsp33 activation, reactive oxygen species were tested for their ability to activate Hsp33 in vitro. Superoxide, singlet oxygen and potentially atomic oxygen activate Hsp33, while no evidence was found for activation by ozone, peroxynitrite or hydroxyl radicals.

Immobilization protects enzymes from plasma-mediated inactivation.

Non-thermal plasmas are used in various applications to inactivate biological agents or biomolecules. A complex cocktail of reactive species, (vacuum) UV radiation and in some cases exposure to an electric field together cause the detrimental effects. In contrast to this disruptive property of technical plasmas, we have shown previously that it is possible to use non-thermal plasma-generated species such as H2O2 as cosubstrates in biocatalytic reactions. One of the main limitations in plasma-driven biocatalysis is the relatively short enzyme lifetime under plasma-operating conditions. This challenge could be overcome by immobilizing the enzymes on inert carrier materials. Here, we tested whether immobilization is suited to protect proteins from inactivation by plasma. To this end, using a dielectric barrier discharge device (PlasmaDerm), plasma stability was tested for five enzymes immobilized on ten different carrier materials. A comparative analysis of the treatment times needed to reduce enzyme activity of immobilized and free enzyme by 30% showed a maximum increase by a factor of 44. Covalent immobilization on a partly hydrophobic carrier surface proved most effective. We conclude from the study, that immobilization universally protects enzymes under plasma-operating conditions, paving the way for new emerging applications.

Plasma–liquid interactions: an experiment and simulation study on plasma dynamic behaviors near the gas–liquid interfacial layer

Plasma–liquid interaction processes are essential to various plasma applications such as sewage disposal, biomedicine, and synthesis of nanomaterials. However, the near gas–liquid interfacial behavior of plasma property remains inadequately understood, hindering the controllability of the application process. This study combines experimental diagnostics and simulations to investigate the production and transport of plasma-generated reactive species near (∼35 μm) the gas–liquid interfacial layer. The experimental results are used to benchmark densities obtained from a zero-dimensional plasma chemical kinetics simulation, which reveals the time evolutions of plasma-generated active species. A large number of neutral particles (like OH, H2O2) and water cluster ions (like H9O4 +) were produced as a result of the evaporation effect of the cathode solution surface. The estimation of energy flux from the gaseous plasma to liquid surface showed that the hydration process of positive ions plays the key role in the resulted water evaporation at the gas–liquid interfacial layer.

Rapid hydrophobicity recovery of contaminated silicone rubber using low-power microwave plasma in ambient air

The loss of hydrophobicity of silicone rubber (SR) insulators due to contamination accumulation will increase the risk of flashover and threaten the stability of electrical power system. To solve this problem, a highly efficient and environmentally friendly microwave plasma surface treatment method is developed, which only consumes low-power electricity and ambient air without any chemical agent or rare gas. The hydrophobicity of seriously contaminated SR can be recovered on the order of 10 s, with a nice long-term aging performance under natural transfer. The underlying mechanism is revealed via carefully designed control experiments and systematic surface analysis. It is illustrated that the plasma-generated active species break the long chain of SR into non-crosslinked low-molecule-weight siloxanes (LMWs) and accelerate their transfer to contamination layer, while the bombing and etching effects of plasma manufacture a porous structure and increase the surface roughness. All of these factors will contribute to the increase of hydrophobicity. On the contrary, the oxidation effect of plasma will turn LMWs into polar groups and destroy the hydrophobicity, which should be restricted in practice. The microwave plasma treatment method developed in this work has the on-line application potential in power system for hydrophobicity recovery under severe contamination conditions.

DNA-based surrogates for the validation of microbial inactivation using cold atmospheric pressure plasma and plasma-activated water processing

Cold atmospheric pressure plasma (CAPP) and plasma-activated water (PAW) are emerging nonthermal technologies. Due to the complexity of plasma-generated species as a function of plasma generation conditions, there is a need to develop process validation. This study aims to evaluate plasma interactions with DNA-based surrogates using infrared spectroscopy and gradient boosting decision tree, a machine learning algorithm, for data analysis to validate the decontamination effectiveness of CAPP and PAW. Chitosan-DNA films were developed and treated with CAPP or PAW. Changes in the spectral properties of DNA were characterized with Fourier-transform infrared spectroscopy (FTIR) and correlated to the dosage levels of CAPP and PAW and decontamination efficacy. Using the LightGBM algorithm, both plasma dosage and the inactivation efficacy against bacteria and biofilms were predicted with high accuracy (>89%) based on the spectral features of DNA. In summary, this study illustrates a novel approach for validating the decontamination efficacy of plasma processing.

High degree of N-functionalization in macroscopically assembled carbon nanotubes

Nitrogen doping of carbon nanomaterials has emerged as a method to develop novel material properties, though limitations in the form of extended treatment times, harsh chemical usage and limited total nitrogen content exist. Here, macroscopic ribbon-like assemblies of carbon nanotubes are functionalized with nitrogen using a simple direct current-based plasma–liquid system. This system utilizes the plasma-generated species in an ethanol:water solution with ethylenediamine as a nitrogen precursor for the nitrogen functionalization of the carbon nanotube assembly. These unique, plasma-generated species and pathways enable rapid and high levels of functionalization with the atomic concentration of nitrogen reaching 22.5%, with amine groups, pyrrolic groups and graphitic nitrogen observed in the X-ray photoelectron spectra, the highest ever achieved. This nitrogen content is demonstrated to be significantly higher than a comparative electrolysis process. This demonstrates that this plasma process enhances the availability of nitrogen from the ethylenediamine precursor, facilitating greater functionalization.Graphical abstract

Hollow cathode enhanced capacitively coupled plasmas in Ar/N2/H2 mixtures and implications for plasma enhanced ALD

Plasma enhanced atomic layer deposition (PEALD) is a cyclic atomic layer deposition process that incorporates plasma-generated species into one of the cycle substeps. The addition of plasma is advantageous as it generally provides unique gas-phase chemistries and a substantially reduced growth temperature compared to thermal approaches. However, the inclusion of plasma, coupled with the increasing variety of plasma sources used in PEALD, can make these systems challenging to understand and control. This work focuses on the use of plasma diagnostics to examine the plasma characteristics of a hollow cathode enhanced capacitively coupled plasma (HC-CCP) source, a type of plasma source that has seen increasing attention in recent years for PEALD. Ultraviolet to near-infrared spectroscopy as well as spatially resolved Langmuir probe and emissive probe measurements are employed to characterize an HC-CCP plasma source using nitrogen based gas chemistries typical of nitride PEALD processes. Spectroscopy is used to characterize the relative concentrations of important reactive and energetic neutral species generated in HC-CCP systems as a function of applied RF power, gas chemistry, and pressure. In addition, the electron energy distribution function, electron temperature, plasma potential, and plasma density for the same process parameters are examined using an RF compensated Langmuir probe and emissive probe. These measurements indicated that electron temperature (Te), electron density (ne), and plasma potential (Vp) varied significantly over the operating conditions examined with Te varying from 1.5 to 8 eV, Vp varying from 30 to 90 V, and ne varying between 1015 and low 1016 m−3. This wide range of plasma conditions is mediated by a mode transition from a low Te, high ne mode of operation at low pressure (<100 mTorr) to a high Te, low ne mode at higher pressures (>100 mTorr). These operational modes appear analogous to the classical γ and α modes of traditional capacitively coupled plasmas. Atomic N and H densities also vary significantly over the operating conditions examined.