Plasma Chemistry and Plasma Processing (v.36, #6)

The Role of Interfacial Reactions in Determining Plasma–Liquid Chemistry by Carly E. Anderson; Nico R. Cha; Alexander D. Lindsay; Douglas S. Clark; David B. Graves (1393-1415).
In this work, we investigate the production of highly oxidative species in solutions exposed to a self-pulsed corona discharge in air. We examine how the properties of the target solution (pH, conductivity) and the discharge power affect the discharge stability and the production of H2O2. Indigo carmine, a common organic dye, is used as an indicator of oxidative strength and in particular, hydroxyl radical (OH·) production. The observed rate of indigo oxidation in contact with the discharge far exceeds that predicted from reactions based on concentrations of species measured in the bulk solution. The generation of H2O2 and the oxidation of indigo carmine indicate a high concentration of highly oxidizing species such as OH· at the plasma–liquid interface. These results indicate that reactions at the air plasma–liquid interface play a dominant role in species oxidation during direct non-equilibrium atmospheric pressure plasma treatment.
Keywords: Plasma activated water (PAW); Reactive oxygen species; Corona discharge; Indigo carmine; Non-equilibrium atmospheric pressure plasma (NEAPP)

Plasma Treated Sepiolite: A New Adsorbent for Removal of Malachite Green from Contaminated Water by Mustafa Kaya; Mehmet Fatih Dilekoğlu; Ömer Şahin; Cafer Saka (1417-1430).
Surface modification of clay materials has become an important issue to improve the efficiency of the adsorbent. The adsorption capacity of the clay material can be increased by thermal or chemical modifications. In this study, plasma technology was applied for the surface modification of sepiolite to improve the removal of malachite green from contaminated water. This study is novel in preparing and examining the effectiveness of sepiolite in adsorption of malachite green from contaminated water. To achieve the aim, plasma application time, CO2, N2, or Ar plasma gases effect and pH were investigated with respect to the adsorption capacity of MG. The surface properties of raw and plasma treated sepiolite were investigated with SEM, FTIR, BET surface area and XRD measurements. The monolayer adsorption capacity was found to be 143 mg/g.
Keywords: Modification; Plasma; Sepiolite; Malachite green

Molecular-Level Reinforced Adhesion Between Rubber and PTFE Film Treated by Atmospheric Plasma Polymerization by Masaaki Okubo; Takeshi Onji; Tomoyuki Kuroki; Hiroaki Nakano; Eiji Yao; Mitsuru Tahara (1431-1448).
Extremely strong reinforced adhesion between a polytetrafluoroethylene (PTFE) film and butyl rubber is achieved using an atmospheric pressure plasma graft polymerization, involving argon and acrylic acid vapor. The treated PTFE film is then placed over a raw butyl rubber plate and hot-pressed under 157 N/cm2 for 40 min at 150 °C or for 10 min at 180 °C. This procedure results in molecular-level or chemical adhesion between the butyl rubber and the PTFE film. The 180° peeling test results show that a high peeling strength of 3.9 N, per 1 mm sample width, is achieved. Adherend failure of the rubber sheet occurs when the peeling is enforced. From X-ray photoelectron spectroscopy analysis of the treated films, chemical bonds with fluorine atoms are absent from the surface. From scanning electron microscopy analysis, a transparent hydrophilic poly(acrylic acid) layer composed of nanoscale spherical particles is formed. This PTFE-rubber composite material is suitable for high-quality, prefilled medical syringe gaskets.
Keywords: Nonthermal plasma; PTFE; Rubber; Adhesion; Atmospheric plasma; Graft polymerization

Plasma Chemical Functionalisation of a Cameroonian Kaolinite Clay for a Greater Hydrophilicity by B. Sop Tamo; G. Kamgang-Youbi; E. Acayanka; L. Medjo Simo; A. Tiya-Djowe; D. Kuete-Saa; S. Laminsi; L. Tchadjie (1449-1469).
A Cameroonian kaolinite powder was treated with gliding arc plasma in order to increase the amount of hydroxyl functional groups present on its external surfaces. The functional changes that occurred were monitored by Fourier transform infrared spectroscopy. The crystalline changes were followed by the X-ray diffraction. The ionisation effect, acid effect, and water solubility of the treated samples were also evaluated. Results showed that there is breaking of the bonds in the Si–O–Si and Si–O–Al groups, followed by the formation of new aluminol (Al–OH) and silanol (Si–OH) groups at the external surface of kaolinite after exposing the clay to the gliding arc plasma. The increase in hydroxyl groups on the surface of kaolinite leads to the increase of its hydrophilicity. Moreover, new charges appear on its surfaces and no significant change in crystallinity has occurred. This study shows that clays in powder form being can effectively be functionalised by gliding arc plasma in spatial post discharge processing mode. Knowing that the treatment in spatial post discharge offers the possibility to process large amounts of clay, this work is of great interest to the industry.
Keywords: Kaolinite clay; Gliding arc plasma; Hydroxyl groups; Spatial post-discharge

Synthesis of Mixed-Phase TiO2 Nanopowders Using Atmospheric Pressure Plasma Jet Driven by Dual-Frequency Power Sources by Yong Wang; Qianghua Yuan; Guiqin Yin; Yu Zhang; Yadong Zhang; Yang Li; Jiaojiao Li; Tao Wang; Shuyi Ma (1471-1484).
Mixed-phase TiO2 nanopowders with different ratios of anatase and rutile have been successfully synthesized using atmospheric pressure plasma jet driven by dual-frequency power sources. The crystal structures of the TiO2 nanopowders were characterized by X-ray diffraction, SAED, HRTEM, and Raman shift spectroscopy. These results indicated that samples possessed anatase and rutile structure, in addition, the crystallinity of the TiO2 nanopowders increased and the chlorine contamination decreased with discharge RF power increasing. The photocatalytic activity of the TiO2 nanopowders was evaluated by decomposition methylene blue solution. The TiO2 nanopowders which were produced at the discharge RF power of 110 W had the highest photocatalytic activity. Optical emission spectroscopy (OES) was used to detect various excited species in the plasma jet. The results indicate that the various RF power significantly changes the intensities of emission lines (Ar, Ar+, Ti, Ti+, Ti2+, Ti3+ and O), which results in the TiO2 nanopowders a mixture of anatase and rutile phases. The nonequilibrium chemical composition could be formed in one step without anneal. It may have potential applications for synthesizing nanosized particles of high crystallinity by reactive nonthermal plasma processing.
Keywords: Atmospheric pressure dual-frequency (DF) plasma jet; Mixed-phase TiO2 nanopowders; Photocatalytic activity; Discharge RF power

MnOx/TiO2 Catalysts for VOCs Abatement by Coupling Non-thermal Plasma and Photocatalysis by I. Aouadi; J.-M. Tatibouët; L. Bergaoui (1485-1499).
Mn3O4 with different particle sizes was prepared and mixed with titanium oxide to prepare catalytic systems. Those systems were characterized and then used for ethanol removal from air. Commercial β-MnO2 was also used for comparison. Prepared solids were characterized by X-ray Diffraction, N2-physisorption, Raman, Scanning Electron Microscopy and Photoluminescence. MnOx/TiO2 catalysts were tested in the ethanol oxidation reaction (at low concentration: 30 ppm) under ultraviolet light and under non-thermal plasma (Espe = 14 J L−1). The combination of those both technologies was also tested. This study shows that the combination of photocatalysis and non-thermal plasma enhances significantly the oxidation of ethanol. Indeed, very high ethanol conversion rate was obtained with an important carbon dioxide selectivity and low residual ozone concentration. Besides, well dispersed nanoparticles of Mn3O4 are more efficient than β-MnO2 microparticles in the minimization of undesirable byproduct. Actually, under combined ultraviolet light and non-thermal plasma the O3 activation seems to be more efficient on Mn3O4, which additionally enhances the ethanol decomposition and the CO2 selectivity.
Keywords: Non-thermal Plasma; Photocatalysis; TiO2 ; Manganese Oxides; Nanoparticles

Ultraviolet (UV) light with a wavelength of 254 nm was applied to a double dielectric barrier discharge (DDBD) system to decompose of gaseous xylene. The results show that a significantly synergistic effect can be achieved with the introduction of UV light into the DDBD system. When UV light is applied, the system show a 21.8 % increase in its removal efficiency for xylene at 35 kV with an ozone concentration close to 971 ppmv. The CO x (x = CO2 and CO) selectivity of outlet gas rises from 6.54 to 76.2 %. The optimal synergetic effect between UV light and DDBD can be obtained at a peak voltage of 30 kV. The system is robust for humidity, which only slightly reduces the xylene removal efficiency at a high peak voltage (30–35 kV). With the increase of gas flow rate, the removal efficiency for xylene decreases due to a reduced residence time. In addition, the products of xylene degradation were also analyzed. The major products of the degradation were found to be CO2 and H2O while byproducts such as O3 and HCOOH were observed as well.
Keywords: Double dielectric barrier discharges (DDBD); Ultraviolet (UV); Volatile organic compounds (VOCs); Byproducts identification

Application of Flying Jet Plasma for Production of Biodiesel Fuel from Wasted Vegetable Oil by Wameath S. Abdul-Majeed; Ghanim S. AAl-Thani; Jamal N. Al-Sabahi (1517-1531).
Biodiesel, a good partial or total substitute for petrodiesel, is a renewable clean burning fuel which can be produced from transesterification of vegetable oils and animal fats with an alcohol in presence of a catalyst. Since the feedstock costs in this process constitutes more than 70 % of the overall cost, use of wasted vegetable oil (i.e. consumed cooking oil) for biodiesel production is a big challenge in terms of cost reduction and environmental impacts. Nonetheless, the content of residues in the wasted vegetable oil, formed during frying, is a major drawback could be faced in this direction. In this research, we applied an unconventional design of flying jet dielectric barrier discharge plasma torch to treat several specimens of wasted cooking oil collected from different resources before transesterification. In other experiments, the jet plasma itself was used to catalyze the reaction process. The examined plasma torch was found more feasible than conventional DBD reactor design in terms of gas and power consumptions. Upon inducting plasma treatment, the transesterification process resulted in higher biodiesel yield, lower reaction time and easier product separation than the conventional path. Upon catalyzing the reaction by the sole jet plasma effect, the biodiesel content of saturated methyl esters was higher than conventional tranesterification. Also, the yield and properties were found within commercial standards.
Keywords: Wasted cooking oil; Biodiesel; Non-thermal plasma; Fuel characterization

Decomposition of Benzene Using a Pulse-Modulated DBD Plasma by Tianpeng Ma; Huadong Jiang; Jianqi Liu; Fangchuan Zhong (1533-1543).
To improve the energy yield (EY) of plasma volatile organic compound decomposition, a dielectric barrier discharge plasma driven by pulse-modulated AC power was used to experimentally study the abatement of benzene in atmospheric pressure air and at room temperature. The effects of the duty cycle on decomposition efficiency, EY, CO2 selectivity and the formation of ozone and NO2 were investigated. The results show that applying pulse modulation improves the EY and the CO2 selectivity and greatly reduces the wall temperature of the reaction chamber.
Keywords: Nonthermal plasma; Pulse modulation; Energy yield; Benzene decomposition

Simultaneous Removal of H2S and Dust in the Tail Gas by DC Corona Plasma by Wang Xueqian; Xu Ke; Ma Yixing; Ning Ping; Cheng Chen; Wang Langlang; Cheng Jinhuan (1545-1558).
The removal of hydrogen sulfide and dust simultaneously by the DC corona discharge plasma with a wire-cylinder reactor was studied at atmospheric pressure and room temperature. The outlet gases were analyzed by Fourier Transform Infrared. Chemical compositions of the dust collected from ground electrode were analyzed by X-ray fluorescence. The results showed that the DC corona discharge is effective in removing H2S and dust simultaneously. The best H2S conversion was gained with the 2 cm discharge gap. The lower inlet H2S concentration, the higher conversion efficiency was gained at any specific input energy (SIE), while the energy yield was on the contrary. The removal efficiency of H2S decreased gradually as oxygen concentration increased, which means that the H2S decomposition mainly depends on direct electron collisions or short-living species, such as·O, ·OH radicals in the non-thermal plasma. At the initial stage, the conversion efficiency of H2S increased with the increasing of relative humidity, but later decreased while the relative humidity keep increasing with the same SIE. Existing of dust can not only reduce the energy consumption of H2S conversion and improve the removal efficiency, but also inhibit the yield of SO2 for it can further react with some compounds in the dust. With the discharge gap of 2 cm, inlet H2S concentration of 2400 ppm, O2 Of 0.5 %, relative humidity of 41 %, dust content of 4000 ± 5 % mg/m3 and SIE of 600 J/L, the H2S conversion reached 98.8 %, and the dust removal efficiency was close to 100 %.
Keywords: Hydrogen sulfide; Corona plasma; Dust; Simultaneous removal

Comparison of the Active Species in the RF and Microwave Flowing Discharges of N2 and Ar–20 %N2 by André Ricard; Jean-Philippe Sarrette; Soo-Ghee Oh; Yu Kwon Kim (1559-1570).
We report a detailed comparison between RF and microwave (HF) plasmas of N2 and Ar–20 %N2 as well as in the corresponding afterglows by comparing densities of active species at nearly the same discharge conditions of tube diameter (5–6 mm), gas pressure (6–8 Torr), flow rate (0.6–1.0 slm) and applied power (50–150 W). The analysis reveals an interesting difference between the two cases; the length of the RF plasma (~25 cm) is measured to be much longer than that of HF (6 cm). This ensures a much longer residence time (10−2 s) of the active species in the N2 RF plasma [compared to that (10−3 s) of HF], providing a condition for an efficient vibrational excitation of N2(X, v) by (V–V) climbing-up processes, making the RF plasma more vibrationally excited than the HF one. As a result of high V–V plasma excitation in RF, the densities of the vibrationally excited N2(X, v > 13) molecules are higher in the RF afterglow than in the HF afterglow. Destruction of N2(X, v) due to the tube wall is estimated to be very similar between the two system as can be inferred from the γv destruction probability of N2(X, v > 3–13) on the tube wall (2–3 × 10−3 for both cases) obtained from a comparison between the density of N2(X, v > 3–9) in the plasmas to that of the N2(X, v > 13) in the long afterglows. Interestingly enough, densities of N-atoms and N2(A) metastable molecules in the afterglow regions, however, are measured to be very similar with each other. The measured lower density of N2 + ions than expected in the HF afterglow is rationalized from a high oxygen impurity in our HF setup since N2 + ions are very sensitive to oxygen impurity .
Keywords: N2 RF and HF plasmas; N2 RF and HF afterglows; N density; N2(A) and N2(X, v > 13) density; N2 + density; Wall destruction probability

The comparative study of etching characteristics and mechanisms for TiO2 thin films in CF4 + Ar, Cl2 + Ar and HBr + Ar inductively coupled plasmas was carried out. The etching rates for TiO2, Si and photoresist were measured as functions of gas mixing ratios at fixed gas pressure (10 mTorr), input power (800 W) and bias power (300 W). It was found that the maximum TiO2 etching rate of ~130 nm/min correspond to pure CF4 plasma while an increase in Ar fraction in a feed gas results in the monotonic non-linear decrease in the TiO2 etching rates in all three gas mixtures. Plasma diagnostics by Langmuir probes and 0-dimensional (global) plasma modeling supplied the data on the densities of plasma actives specie as well as on particle and energy fluxes to the etched surface. It was concluded that, under the given set of experimental conditions, the TiO2 etching kinetics in all gas systems correspond to the ion-assisted chemical reaction with a domination of the chemical etching pathway. It was found also that the differences in the absolute TiO2 etching rates correlate with the energy thresholds for TiO2 + F, Cl or Br reaction, and the reaction probabilities for F, Cl and Br atoms exhibit the different changes with the ion energy flux according to the volatility of corresponding etching products.
Keywords: TiO2 ; Plasma etching; Diagnostics; Modeling; Etching mechanism

Effects of Pressure and Electrode Length on the Abatement of N2O and CF4 in a Low-Pressure Plasma Reactor by M. Hur; J. O. Lee; J. Y. Lee; W. S. Kang; Y.-H. Song (1589-1601).
The emission of greenhouse gases, such as N2O and fluorinated gases, has been increasingly regulated in the semiconductor industry. Pressure effects on the abatement of N2O and CF4 were investigated in a low-pressure plasma reactor by using Fourier transform infrared (FTIR) spectroscopy. The destruction and removal efficiency (DRE) of N2O and CF4 was significantly lowered below 0.2 Torr. When the pressure was increased, the DRE of CF4 with H2O as the reactant gas increased continuously, but that with O2 or without any reactant gas first increased and then decreased. A larger electrode length yielded a higher DRE of N2O and CF4, especially at lower pressures. To understand this phenomenon, the electrical waveforms for the discharge in N2O were analyzed in conjunction with its optical emission profiles, and the rotational temperatures for different electrode lengths were compared using the N2 + ion band (λ = 391.4 nm). They provided insights into the mechanism involved in terms of plasma property and gas residence time.
Keywords: CF4 ; Electrode length; N2O; Plasma abatement; Pressure