Plasma Chemistry and Plasma Processing (v.36, #4)

An innovative plasma discharge reactor was developed to fit an infrared cell and to allow the in situ characterization of isopropanol (IPA) and toluene decomposition at the surface of three metal oxides (γ-Al2O3, TiO2 and CeO2). The impact of the plasma discharge on the conversion of these pollutants, with the material placed in the discharge area, was studied under real time conditions at atmospheric pressure via infrared analysis. The plasma treatment of IPA molecules led to the formation of acetone, propene, acetic acid and/or formic acid. By contrast, the toluene oxidation led to the rapid opening of the aromatic ring, followed by the total oxidation through carboxylic formation of the species arising from the toluene molecules fragmentation.
Keywords: In-situ characterization; Infrared spectroscopy; Non-thermal plasma; VOC

Plasma Catalytic Synthesis of Ammonia Using Functionalized-Carbon Coatings in an Atmospheric-Pressure Non-equilibrium Discharge by Jungmi Hong; Morteza Aramesh; Olga Shimoni; Dong Han Seo; Samuel Yick; Amelia Greig; Christine Charles; Steven Prawer; Anthony B. Murphy (917-940).
We investigate the synthesis of ammonia in a non-equilibrium atmospheric-pressure plasma using functionalized-nanodiamond and diamond-like-carbon coatings on α-Al2O3 spheres as catalysts. Oxygenated nanodiamonds were found to increase the production yield of ammonia, while hydrogenated nanodiamonds decreased the yield. Neither type of nanodiamond affected the plasma properties significantly. Using diffuse-reflectance FT-IR and XPS, the role of different functional groups on the catalyst surface was investigated. Evidence is presented that the carbonyl group is associated with an efficient surface adsorption and desorption of hydrogen in ammonia synthesis on the surface of the nanodiamonds, and an increased production of ammonia. Conformal diamond-like-carbon coatings, deposited by plasma-enhanced chemical vapour deposition, led to a plasma with a higher electron density, and increased the production of ammonia.
Keywords: Plasma catalysis; Functionalized-nanodiamond coating; Ammonia production; Diamond-like carbon; Non-equilibrium atmospheric-pressure plasma

Synthesis of Silicon Nanoparticles in Nonthermal Capacitively-Coupled Flowing Plasmas: Processes and Transport by Romain Le Picard; Aram H. Markosyan; David H. Porter; Steven L. Girshick; Mark J. Kushner (941-972).
Control of the size and material properties of silicon nanoparticles plays a critical role in optimizing applications using those nanoparticles, such as photovoltaics and biomedical devices. While synthesis of silicon nanoparticles in low temperature plasmas has many attractive features, the basic mechanisms leading to formation of nanoparticles in these plasmas are poorly understood. A two-dimensional numerical model for synthesis of silicon nanoparticles (<5 nm in diameter) in radio frequency (RF) discharges was developed and used to investigate mechanisms for particle growth for Ar/He/SiH4 gas mixtures. Algorithms for the kinetics of nanoparticle formation were self-consistently embedded into a plasma hydrodynamics simulation to account for nucleation, growth, charging, and transport of nanoparticles. We found that with RF excitation in narrow tubes at pressures of a few Torr, the electric field does not fully confine charged nanoparticles in the axial direction, which then results in a finite residence time of particles in the plasma. We found that because of the high neutral nanoparticle density, coagulation plays a significant role in growth. The model predicts the possibility of synthesizing crystalline silicon nanoparticles under these conditions. Trends in the growth of nanoparticles as a function of power are discussed.
Keywords: Silicon nanoparticle synthesis; Plasma modeling; Nanoparticle charging

A model of an atmospheric pressure nitrogen glow discharge in high-gas temperature regimes is developed. The model considers a fairly complete set of chemical reactions, including several processes with the participation of electronically exited nitrogen atoms describing the energy balance and charged particles kinetic processes in the discharge. It is shown that the thermal dissociation of vibrationally excited molecules plays an essential role in the production of N(4 S) atoms. The dominant ion within the investigated current range (52–187 mA) is the molecular N2 + with an increasing proportion of atomic N+ towards high-current values. The process of production of electrons within the almost whole current range is controlled predominantly by associative ionization in atomic collisions N(2 P) + N(2 P) → N2 + + e; being the N(2 P) atoms mainly produced via quenching of N2(A 3u + ) electronically excited molecules by N(4 S) atoms. The results of calculations are compared with the available experimental data and a good agreement is found.
Keywords: Glow discharge; Nitrogen gas; Electronic metastable atoms; Atmospheric pressure

Defluorination and Mineralization of Difluorophenols in Water by Anodic Contact Glow Discharge Electrolysis by Haiming Yang; Xiaotong Zhao; Giya Mengen; Meguru Tezuka; Baigang An; Lixiang Li; Shaoyan Wang; Maowei Ju (993-1009).
Anodic contact glow discharge electrolysis (CGDE) is a DC-excited atmospheric pressure discharge, in which a steady non-thermal plasma is generated locally between the surface of an electrolytic solution and an anode in contact with it. The I–U characteristics of CGDE were investigated. The plasma temperatures were estimated to be in the range, 1373–2045 K. Hydroxyl radicals and hydrogen peroxide were the main oxidants generated by CGDE. The hydrogen peroxide concentration reached 31.2 mmol/L (mM) in a phosphate buffer solution without organic substrates. During CGDE, the DFPs and the corresponding total organic carbon (TOC) in water were consumed. Most of the fluorine atoms in the DFPs were converted to fluoride ions, and the fluoride concentration increased steadily. An analysis of the hydroxylation of DFPs suggested that the hydroxyl radicals generated by CGDE were the key species responsible for the degradation of DFPs, and the possible mechanistic routes of the mineralization of DFPs are proposed. The disappearance of DFPs and the TOC as well as the defluorination of the DFPs followed first-order kinetics. The rate of TOC disappearance was relatively constant: 1.00 ± 0.05 × 10−2 min−1. The order of disappearance of the DFPs was 2,6-DFP > 2,3-DFP > 2,5-DFP > 2,4-DFP > 3,4-DFP > 3,5-DFP. In contrast, the order of defluorination of the DFPs was 2,5-DFP > 2,3-DFP > 2,6-DFP > 2,4-DFP > 3,4-DFP > 3,5-DFP. Overall, the order of the reaction rates for each DFP was kDFP > kdF > kTOC.
Keywords: Difluorophenol; Decomposition; Defluorination; Anodic contact glow discharge electrolysis; First-order rate law; pKa

Degradation of Organic Pollutants Using Atmospheric Pressure Glow Discharge Plasma by Wenzheng Liu; Qiang zhao; Tahan Wang; Xiaoxia Duan; Chuanhui Li; Xiao Lei (1011-1020).
A method of plasma treatment in which a glow discharge was generated in the small gas gap between an electrode and a water surface was designed and employed in this study. By using this method, many active species were generated on the wastewater surface to degrade organic pollutants. The electric field distribution of the designed electrode model was simulated using the MAXWELL 3D® simulation software, and the discharge parameters were measured to investigate the impact of design optimization. In addition, we designed an equipotential multi-electrode configuration to treat a methyl orange solution and an azobenzene solution. The experimental and simulation results indicate that the designed electrodes can realize glow discharge with a relatively low voltage and that the generated plasma covers a large area and is in a stable state. Accordingly, the method helped reduce the cost of the reactor and improved the effectiveness of wastewater treatment.
Keywords: Wastewater treatment; Water surface; Plasma; Glow discharge; Organic pollutants

Interactions Between Helium Plasma Jets and Electrolytes at Different Driving Voltages by Woo Seok Kang; Min Hur; Jin Young Lee; Jae-Ok Lee; Young-Hoon Song (1021-1029).
This study reports the effects of the driving voltage on interactions between plasma jets and electrolytes. Plasma jets over electrolytes, which were generated using five kinds of driving voltages, were characterized by distinct optical emissions from the gas region and the gas–liquid interface with unique electrical characteristics. The generated plasma jets over electrolytes resemble the characteristics of dielectric barrier discharges; the plasma–liquid interactions are affected by external driving voltages that vary the surface charge at the gas–liquid interface because of the conductive and dielectric nature of the electrolyte. The findings suggest that selecting an adequate driving voltage waveform is critical for specific applications utilizing plasma–liquid interactions.
Keywords: Plasma–liquid interactions; Plasma jet; Electrolyte; DC and bipolar pulse

Interaction of (3-Aminopropyl)triethoxysilane with Pulsed Ar–O2 Afterglow: Application to Nanoparticles Synthesis by M. Gueye; T. Gries; C. Noël; S. Migot-Choux; S. Bulou; E. Lecoq; P. Choquet; K. Kutasi; T. Belmonte (1031-1050).
The interaction of (3-Aminopropyl)triethoxysilane (APTES) with pulsed late Ar–O2 afterglow is characterized by the synthesis of OH, CO and CO2 in the gas phase as main by-products. Other minor species like CH, CN and C2H are also produced. We suggest that OH radicals are produced in a first step by dehydrogenation of APTES after interaction with oxygen atoms. In a second step, the molecule is oxidized by any O2 state, to form peroxides that transform into by-products, break thus the precursor C–C bonds. If oxidation is limited, i.e. a low duty cycle, fragmentation of the precursor is limited and produced nanoparticles keep the backbone structure of the precursor, but contain amide groups produced from the amine groups initially available in APTES. At high duty cycle, silicon-containing fragments contain some carbon and react together and produce nanoparticles with a non-silica-like structure.
Keywords: (3-Aminopropyl)triethoxysilane; Oxygen afterglow; Plasma polymer; Nanoparticles

Highly Sensitive Electrochemical Bioassay for Hg(II) Detection Based on Plasma-Polymerized Propargylamine and Three-Dimensional Reduced Graphene Oxide Nanocomposite by D. L. Peng; H. F. Ji; X. D. Dong; J. F. Tian; M. H. Wang; L. H. He; Z. Z. Zhang; S. M. Fang (1051-1065).
In this study, a nanocomposite consisting of three-dimensional reduced graphene oxide (3D-rGO) and plasma-polymerized propargylamine (3D-rGO@PpPG) was prepared and used as a highly sensitive and selective DNA sensor for detecting Hg2+. Given the high density of amino groups in the resultant 3D-rGO@PpPG nanocomposite, thymine-rich and Hg2+-targeted DNA was preferentially immobilized on the fabricated sensor surface via the strong electrostatic interaction between DNA strands and the amino-functionalized nanocomposites, followed by detecting Hg2+ through T–Hg2+–T coordination chemistry between DNA and Hg2+. The results of electrochemical measurements revealed that the anchored amount of DNA strands anchored on the 3D-rGO@PpPG nanofilm surface affects the determination of Hg2+ in aqueous solution. It showed high sensitivity and selectivity toward Hg2+ within concentrations ranging from 0.1 to 200 nM and displayed a low detection limit of 0.02 nM. The new strategy proposed also provides high selectivity of Hg2+ against other interfering metal ions, good stability, and repeatability. The excellent applicability of the developed sensor confirms the potential use of plasma-modified nanofilms for the detection of heavy metal ions in real environmental samples and water.
Keywords: Plasma-polymerized propargylamine; Three-dimensional reduced graphene oxide; Detection of Hg2+ ; Electrochemical biosensor

PMMA Surface Functionalization Using Atmospheric Pressure Plasma for Development of Plasmonically Active Polymer Optical Fiber Probes by Priyanka Vasanthakumari; Zohreh Khosravi; V. V. R. Sai; Claus-Peter Klages (1067-1083).
In this paper, we demonstrate the development of plasmonically active PMMA optical fiber probes by the attachment of gold nanoparticles to the probe surface functionalized by means of flowing post-discharges from dielectric barrier discharge (DBD) plasmas for the first time. Polymer optical fiber (POF) probes (U shape to improve absorbance sensitivity) were subjected to reactive gas atmospheres in the post-discharge region of a coaxial DBD plasma reactor run at atmospheric pressure in different gases (Ar, Ar + 10 % O2, O2, N2, N2 + 0.5 % H2). Plasma treatments in Ar or N2 gave rise to water-stable electrophilic functional groups on PMMA surface, whereas the amine groups generated by N2-containing plasmas were not stable. Subsequently, PMMA surfaces were treated with hexamethylene diamine (HMDA) to obtain stable amine groups through the reaction of electrophilic groups. Gold nanoflowers (AuNF, 37 nm, peak 570 nm) binding to the amine functionalized fiber probes was monitored in real-time by recording the optical absorbance changes at 570 nm with the help of a UV–vis spectrometer. Absorbance response from Ar or N2 plasma treated probes are 100 and 60 times, respectively, that of untreated control probes. A 25 fold improvement in absorbance response was obtained for Ar plasma treated POF in comparison with only HMDA treated POF. The shelf life of the hence fabricated plasmonically active probes was found to be at least 3 months. In addition, plasmonic activity of U-bent fiber probes treated in Ar plasma is better than the conventional wet-chemical activation by environmentally hazardous acid pre-treatment approaches.
Keywords: Poly(methyl methacrylate); Dielectric barrier discharge (DBD); Polymer optical fiber (POF); U-bent fiber optic probe; Gold nanoflowers

Trichloroethylene Combustion in a Submerged Thermal Plasma: Results and Chemical Kinetics Model by Majdi Mabrouk; Mickael Marchand; Jean-Marie Baronnet; Florent Lemont (1085-1110).
This work deals with incineration of organic liquid wastes using an oxygen thermal plasma jet, submerged in water. The results presented here concern incineration of trichloroethylene (TCE). During a trial run, the CO2 and CO content in the exhaust gas is continuously measured; samples taken periodically from the solution are analyzed by appropriate methods: total organic carbon and chlorine content are measured. Process efficiency during tests with a few L/h of TCE is given by the mineralization rate. The trapping rate of chlorine as HCl is near 100 %. The TCE destruction and removal efficiency, measured by MS/GC, is better than 99.9999 %. A simplified kinetic model of gas quenching was constructed from a single-phase plug-flow reactor model taking into account 14 species and 34 reactions. It satisfies the requirements of heat balance and major components analysis, and reveals the major role of the OH radical on the concentrations of CO as well as HCl and/or Cl2 in the off-gas stream.
Keywords: Submerged thermal plasma; Trichloroethylene incineration; Oxygen plasma torch; 1D chemical kinetics model of off-gas quenching

Reduction of Copper Oxide by Low-Temperature Hydrogen Plasma by K. C. Sabat; R. K. Paramguru; B. K. Mishra (1111-1124).
The paper presents experimental results of reduction of cupric oxide (CuO) by low-temperature hydrogen plasma in a microwave-assisted plasma set-up. The experiments were carried out at low microwave powers in the range of 600–750 W and low hydrogen flow rates in the range of 0.833 × 10−6 to 2.5 × 10−6 m3 s−1. In all the experiments for reduction of CuO with hydrogen plasma, an initial induction period was observed in the kinetic plots. The induction period decreases with increase in pressure or temperature. The induction period leads to the formation of active sites for adsorption of H2. After the induction period, fast autocatalytic reduction takes place followed by a sluggish period towards the end. The reduction process proceeds in sequential steps through the formation of sub-oxides. The kinetic data fits the Avrami-Erofeev equation with ‘n’ value close to 3. The resultant activation energy measured during hydrogen plasma processing is around 75.64 kJ mol−1. This is lower compared to activation energies measured by other methods of reduction indicating a clear advantage.
Keywords: Reduction of cupric oxide; Low-temperature hydrogen plasma; Induction period; Avrami-Erofeev equation

Monitoring Hydrogen Plasma Reduction of Oxides by Na D Lines by Sarita Das; Debi Prasad Das; Priyanka Rajput; Joydeep Ghosh; Bhagyadhar Bhoi; Barada Kanta Mishra (1125-1139).
Out of many applications of hydrogen plasma, reduction of metal oxides is an important one. The reduction can be carried out using carbon or hydrogen. While carrying out the reduction of hematite (Fe2O3) in hydrogen plasma, an attempt was made to characterize the hydrogen plasma by optical emission spectroscopy. The spectroscopic results provide some new and useful information. In addition to the hydrogen emission lines, two prominent lines at 589 and 589.6 nm were observed. These two lines are confirmed to be sodium D1 and D2 (Na D lines) by comparing with a low pressure sodium vapour lamp (LPSVL). The source of the trace amount of sodium is also confirmed to be from the metal oxide sample as an impurity. These lines are found to be very sensitive to various process parameters such as gas flow rate, microwave power, and reduction chamber pressure. The temporal variation of these two Na D lines also shows a characteristic trend during metal oxide reduction in hydrogen plasma. The weight loss and the X-ray diffraction analyses of reduced Fe2O3 sample for different time duration provides the evidence of correlation with Na D lines’ intensity trend. This trend can be used to monitor the state and completion of hydrogen plasma based reduction reaction. In processes where Na is not associated with metal oxide, trace amount of Na in its molecular form such as NaOH can be introduced for monitoring the plasma process parameters as well as the plasma based reduction process.
Keywords: Metal oxide reduction; Microwave hydrogen plasma; Sodium D lines; Reduction of metal oxides

Thermodynamic Properties and Transport Coefficients of CO2–Cu Thermal Plasmas by Aijun Yang; Yang Liu; Linlin Zhong; Xiaohua Wang; Chunping Niu; Mingzhe Rong; Guohui Han; Youpeng Zhang; Yanhui Lu; Yi Wu (1141-1160).
This paper presents the calculated values of equilibrium compositions, thermodynamic properties and transport coefficients (viscosity, electrical conductivity and thermal conductivity) for CO2–Cu thermal plasmas. With several copper mass proportions, the calculation is performed at temperatures 2000–30,000 K and various pressures 0.1–16 bar. Gibbs free energy minimization is used to determine species compositions and thermodynamic properties and the well-known Chapman–Enskog method is applied to calculating transport properties. Furthermore, great attention is paid to cope with the interactions between all the particles in the determination of collision integrals. The results are illustrated indicating the effect of the copper proportions and pressure on the fundamental properties of CO2–Cu thermal plasmas. It can be found that a small quantity of copper (less than 10 %) can significantly modify the charged species densities and electrical conductivity especially at low temperature. While for other properties, the influences can be noticeable only when the copper proportion is above 10 %.
Keywords: Thermal plasma; Thermodynamics properties; Chapman–Enskog method; CO2–Cu; Transport coefficient

With the Gibbs free energy method, we determine the molar fraction in a plasma at and out of thermal equilibrium consisting of air and aluminum for several percentages in the temperature range of 500–6000 K. We take three temperatures into account (T rot  = T h ; T vib ; T ex  = T e ). We indicate the formulae and the numerical method used to perform the calculation taking three condensed phases AlN, Al, Al2O3 into account. We show that the air percentage plays a major role to create these phases. We clarify the role plays on the vaporization temperatures and on the sublimation temperature by the non-thermal equilibrium of the plasma. This kind of plasma is found in arc roots, near a wall, in plasmas with a high value of electrical field,… The influence of the pressures until 30 × 105 Pa. is shown on molar fraction of the chemical species, on the vaporization temperatures and on the sublimation temperature. The vaporization temperatures are given versus the thermal non equilibrium versus various mixtures (air, aluminum) and versus the pressures (105 Pa–30 × 105 Pa).
Keywords: Aluminum nitride; Aluminum oxide; Aluminum; Plasma; Vaporization temperature; Sublimation temperature; Chemical equilibrium; Multitemperature plasma; 52.27.Cm; 52.77.-j; 64.70.fm; 64.75.-g

Silicon carbide powder was successfully transformed from hexagonal SiC into cubic SiC using the electric discharge assisted mechanical milling (EDAMM) method. The milling process was conducted in nitrogen plasma at atmospheric pressure. The effects of pulsed alternating current (AC) and direct current (DC) discharge on product formation were investigated. Products were characterized by X-ray diffractometry, scanning electron microscopy, transmission electron microscopy and nanohardness. It was found that hexagonal SiC can be transformed into cubic SiC under EDAMM processing, near complete transformation occurring within 5 min when applying AC mode electrical pulses, and within 10 min when applying DC mode discharges.
Keywords: Silicon carbide; Electric discharge assisted mechanical milling; Spark plasma