Radical Density Research Articles

Surface layer of Pt‐O‐Ce bonds on CeOx nanowire with high ORR activity converted by proton beam irradiation

AbstractTo develop the state‐of‐the‐art polymer membrane fuel cells. Both maximization of oxygen reduction reaction (ORR) activity on Pt cathode and minimization of Pt content in the cathode are required. For this challenge, the defect interface on oxide support was modified by proton beam irradiation method. Pt‐CeOx nanowire/C (Pt/C = 0.02) was fabricated using the proton beam irradiation method. Since the radical density generated by proton beam irradiation is two orders of magnitude greater than that of electron beam irradiation, the CeOx nanowire surface was fully converted to a thin layer of Pt‐O‐Ce bonds under proton beam irradiation. The ORR activity observed for fabricated sample with above active surface layer was higher than that of conventional Pt/C (Pt/C = 0.2) and comparable to that of Pt‐CeOx nanowire/C (Pt/C = 0.2) fabricated by conventional methods. From the combination of microanalysis characterization and surface atomistic simulation, we concluded that the Pt‐O‐Ce bond was formed on defect‐rich regions of the CeOx nanowire and this leads to a maximized ORR activity on the fabricated sample. Based on all experimental data, it is concluded that the surface modification of CeOx nanowire support using proton beam irradiation is useful for a lowering the Pt content of the cathode with high ORR activity.

Spatially and temporally resolved H and OH densities in a nanosecond pulsed plasma jet: an analysis of the radical generation, transport, recombination and memory effects

Cold atmospheric pressure plasma jets (APPJs) enable the generation of a large flux of highly reactive species to heat sensitive substrates at near ambient temperatures. Nonetheless, the experimental determination of reactive species inside APPJs, while exceedingly important, has not been reported in significant detail. In this work, we report the time-resolved axial density of hydroxyl (OH) and hydrogen radicals (H) in an APPJ device and its effluent operating in a He–H2O mixture using 1D laser induced fluorescence (LIF) and two-photon absorption LIF (TALIF). The results show that H and OH are mainly generated between the electrodes in the APPJ rather than by the guided streamer. The produced H and OH inside the jet are convectively transported to the jet effluent and determine the H and OH densities in jet effluent. The dominant production and destruction mechanisms of H and OH were obtained from a 0D model. The different production mechanisms of H and OH can explain the change in memory effect observed for OH (and not for H) for varying pulse repetition rates of the plasma generation.

Low-frequency plasma activation of nylon 6

In the study reported in this paper, a series of reproducible conditions were employed to uniformly functionalize nylon 6 surfaces using a commercially available, low-frequency (40 kHz), low-pressure plasma system, utilizing oxygen plasma as a reactive gas. Initially, the plasma-treated samples were investigated using static contact angle measurements, showing a progressive increase in wettability with increasing plasma activation time between 10 and 40 s. Such an increase in wettability (and therefore increase in adhesive capabilities of the surfaces) was attributed to the creation of surface C-OH, C=O, and COOH groups. These surface-chemical modifications were characterized using x-ray photoelectron spectroscopy (XPS) and static secondary ion mass spectrometry (SSIMS). Surface radical densities were also shown to increase following plasma activation, having been quantified using a radical scavenging method based on the molecule 2,2-diphenyl-1-picrylhydrazyl (DPPH). The samples were imaged and analyzed using scanning electron microscopy (SEM) and atomic force microscopy (AFM), to confirm that there had been no detectable alteration to the surface roughness or morphology. Additionally, the “hydrophobic recovery” or “ageing” of the activated polymer samples, post-plasma treatment, was also investigated in terms of wettability and surface-chemistry, with the wettability of the sample surfaces decreasing over time due to a reduction in surface-oxygen concentration.

Zero-dimensional numerical simulation of dry reforming of methane in atmospheric pressure non-equilibrium plasma

Recently, atmospheric non-equilibrium plasma has been proposed as a potential and novel type of “reaction carrier” for the activation and conversion of greenhouse gases (methane and carbon dioxide) into value-added chemicals, due to its unique non-equilibrium characteristics. In this paper, a zero-dimensional plasma chemical reaction kinetic model in CH<sub>4</sub>/CO<sub>2</sub> gas mixture is constructed, with an emphasis on reaction mechanism for plasma dry reforming of methane to syngas and oxygenates. Especially, the effect of the CH<sub>4</sub> molar fraction (5%–95%) on plasma dry reforming of methane is investigated. First, the time evolution of electron temperature and density with initial methane content is presented, and the results show that both the electron temperature and electron density vary periodically with the applied triangular power density pulse, and the higher initial methane content in gas mixture is favored for a larger electron temperature and density. Subsequently, the time evolution of number densities of free radicals, ions and molecules at different CH<sub>4</sub>/CO<sub>2</sub> molar fraction are given. The higher the initial methane content, the greater the number densities of H, H<sup>–</sup>, H<sub>2</sub>, and CH<sub>3</sub>, leading to insufficient oxygen atoms to participate in the reaction for oxygenates synthesis. The conversions of inlet gases, the selectivities of syngas and important oxygenates are also calculated. The conversion rate of carbon dioxide increases with the increasing methane content, but the conversion rate of methane is insensitive to the variation of methane content. As methane mole fraction is increased from 5% to 95%, the selectivities of important oxygenates (CH<sub>3</sub>OH and CH<sub>2</sub>O) are relatively low (<5%), and the selectivity of H<sub>2</sub> gradually increases from 13.0% to 24.6%, while the selectivity of CO significantly decreases from 58.9% to 9.7%. Moreover, the dominant reaction pathways governing production and destruction of H<sub>2</sub>, CO, CH<sub>2</sub>O and CH<sub>3</sub>OH are determined, and CH<sub>3</sub> and OH radicals are found to be the key intermediate for the production of valuable oxygenates. Finally, a schematic overview of the transformation relationship between dominant plasma species is summarized and shown to clearly reveal intrinsic reaction mechanism of dry reforming of methane in atmospheric non-equilibrium plasma.

The distribution of OH radicals in a pulsed DC He + H2O plasma jet with the presence of a dielectric material

The distribution and density of OH radicals in a pulsed DC He + H2O plasma jet are studied by a two-dimensional (2D) plasma jet model. This article shows the effect of adding H2O to the pure He plasma jet on the OH density distribution and the changes of the OH density under different H2O contents. The process of streamer propagation toward the dielectric surface and development of the streamer on the dielectric surface are discussed in this article. The total OH radicals are generated from two parts in a He + H2O atmospheric pressure plasma jet (APPJ). The first part is produced with the water in ambient air outside the tube, and the other part is generated with the water in the helium flow. At the time when the streamer approaches the dielectric surface, it is found that electron-impact dissociation of H2O and electron neutralization of H2O+ are the main reactions to generate OH species and three-body combination of OH and OH with He is the main reaction to consume OH species. The OH density shows a donut shape in a pure He APPJ, while the density distribution of OH radicals under the nozzle gradually becomes uniform with the increase in the H2O content added to the helium flow, but the maximum value is at the both ends of the streamer outside the tube. The electron density initially increases with the increase in the proportion of H2O added to the He plasma jet and then gradually decreases with the H2O ratio greater than 0.5%. The enhancement of Penning ionization with the increasing H2O content is the main reason for the increase in electron density, while the increasing consumption of electrons by the excitation energy loss causes the decrease in the electron density with the H2O ratio more than 0.5%. The increase in the H2O content will bring both positive and negative effects. On the one hand, the increase in the H2O content added to the He gas flow will promote the generation of OH species. On the other hand, the decrease in the electron density caused by the increasing excitation energy loss slows the rapid growth of the reaction rates of the two main reactions related to the production of OH species. Eventually, OH density reaches saturation with the H2O ratio at 1.5% under the combined effect mentioned above.

Direct Synthesis of Graphene on Silicon at Low Temperature for Schottky Junction Solar Cells

Graphene thin films synthesized directly at low temperature (550˚C) on silicon substrate by microwave (MW) surface wave plasma (SWP) chemical vapor deposition (CVD) using the cover on substrates for avoiding plasma emission ultraviolet ray’s effect during film deposition. Analytical methods such as Raman spectroscopy, Transmission electron microscopy (TEM) and Scanning electron microscopy (SEM), four-point probe method, and JASCO V-570 UV/VIS/NIR spectrophotometer were employed to characterize the properties of the graphene films. Here, we report that it is possible to grow graphene directly on the silicon substrate (without using catalyst) due to the high radical density of MW SWP CVD. Furthermore, we fabricated graphene/silicon Schottky junction solar cells with an efficiency of up to 6.39%. Compared to conventional silicon solar cells, the fabrication process is greatly simplified; just graphene is synthesized directly on n-type crystalline Si substrate at low temperate.

Temporal evolution of electron energy distribution function and its correlation with hydrogen radical generation in atmospheric-pressure methane needle–plane discharge plasmas

Non-thermal plasmas show great potential in low-temperature activation of methane (CH4) owing to the abundant energetic active species. Motivated by the fact that the chemical reactions in plasma-based CH4 conversion are dominated and regulated by the energetic electrons and various radicals, the temporal evolution of the electron energy distribution function (EEDF) and its relation to hydrogen (H) radical generation in an atmospheric-pressure CH4 needle–plane discharge plasma have been investigated numerically. The simulations are carried out using one-dimensional particle-in-cell Monte-Carlo collision and fluid dynamic models. It can be shown that during the formation and development of the streamer, a characteristic time exists, before and after which the evolution characteristic of the EEDF is reversed. This is mainly attributed to the competition between the energies continuously obtained from the electric field and the increasingly strong inelastic collisions and fast-growing low-energy electron population. When the amplitude of the applied voltage is increased, the fraction of electrons with high enough energy to participate in dissociation or ionization reactions of CH4 increases, leading to an increased H density. Besides, the characteristic time decreases exponentially, and the energy efficiency of the activation of CH4 molecules is decreased. An appropriate electron energy distribution and H radical density should be chosen to ensure acceptable product selectivity and conversion rate without excessive energy consumption; this will depend on the required products. The results presented in this work provide a partial theoretical basis for effectively optimizing the content of high-energy electrons and H radicals.

A Computer Modeling Study of Water Radiolysis at High Dose Rates. Relevance to FLASH Radiotherapy.

"FLASH radiotherapy" is a new method of radiation treatment by which large doses of radiation are delivered at high dose rates to tumors almost instantaneously (a few milliseconds), paradoxically sparing healthy tissue while preserving anti-tumor activity. To date, no definitive mechanism has been proposed to explain the different responses of the tumor and normal tissue to radiation. As a first step, and given that living cells and tissues consist mainly of water, we studied the effects of high dose rates on the transient yields (G values) of the radical and molecular species formed in the radiolysis of deaerated/aerated water by irradiating protons, using Monte Carlo simulations. Our simulation model consisted of two steps: 1. The random irradiation of a right circular cylindrical volume of water, embedded in nonirradiated bulk water, with single and instantaneous pulses of N 300-MeV incident protons ("linear energy transfer" or LET ∼ 0.3 keV/µm) traveling along the axis of the cylinder; and 2. The development of these N proton tracks, which were initially contained in the irradiated cylinder, throughout the solution over time. The effect of dose rate was studied by varying N, which was calibrated in terms of dose rate. For this, experimental data on the yield G(Fe3+) of the super-Fricke dosimeter as a function of dose rate up to ∼1010 Gy/s were used. Confirming previous experimental and theoretical studies, significant changes in product yields were found to occur with increasing dose rate, with lower radical and higher molecular yields, which result from an increase in the radical density in the bulk of the solution. Using the kinetics of the decay of hydrated electrons, a critical time (τc), which corresponds to the "onset" of dose-rate effects, was determined for each value of N. For the cylindrical irradiation model, τc was inversely proportional to the dose rate. Moreover, the comparison with experiments with pulsed electrons underlined the importance of the geometry of the irradiation volume for the estimation of τc. Finally, in the case of aerated water radiolysis, we calculated the yield of oxygen consumption and estimated the corresponding concentration of consumed (depleted) oxygen as a function of time and dose rate. It was shown that this concentration increases substantially with increasing dose rate in the time window ∼1 ns-10 µs, with a very pronounced maximum around 0.2 µs. For high-dose-rate irradiations (>109 Gy/s), a large part of the available oxygen (∼0.25 mM for an air-saturated solution) was found to be consumed. This result, which was obtained on a purely water radiation chemistry basis, strongly supports the hypothesis that the normal tissue-sparing effect of FLASH stems from temporary hypoxia due to oxygen depletion induced by high-dose-rate irradiation.

Optical diagnostics of the characteristics of a square unipolar nanosecond pulse-driven atmospheric pressure helium plasma jet

Compared with the traditional sinusoidal voltage source, a short rising nanosecond voltage source can generate a high electron current for a short rising time. This paper investigates how the nanopulse parameters such as the voltage amplitude, pulse duration, and repetition frequency affect the radical generation and the plasma bullet propagation in an atmospheric pressure helium plasma jet. An intensified charge-coupled device was used to observe the bullet propagation in the nanosecond gate mode. The plasma bullet’s propagation speed is mainly affected by the applied voltage and externally biased electrodes rather than the pulse duration or the driving frequency. In contrast, optical emission spectroscopy diagnostics estimate that the radical density inside the atmospheric pressure plasma jet mainly increases with the repetition frequency. At the same time, the population of high-energy electrons can be controlled with the unipolar voltage amplitude. Thus, unipolar nanosecond pulses make it possible to control the emitting charges and the generated radicals independently.

Measurement of nitrogen dioxide and nitric oxide densities by using CEAS (cavity‐enhanced absorption spectroscopy) in nonthermal atmospheric pressure air plasma

AbstractNonthermal atmospheric pressure plasmas are widely used in biomedicine and agriculture. Currently, radical density is used as an important parameter for the development of plasma biomedicine and agriculture. Especially, nitrogen dioxide (NO2) and nitric oxide (NO) radicals are significant species in these applications. These could be measured by using the cavity‐enhanced absorption spectroscopy (CEAS) technique. For the NO2 and NO radical density measurement, we used a visible LED (light‐emitting diode) of 660 nm and a mid‐infrared LD (laser diode) of 5238.6 nm, respectively. Radical densities can be calculated with the transmission ratio using Beer–Lambert law, which is obtained by the absorbed amount of laser intensity passed through the gas from the plasma jet in an optical cavity of CEAS.

Formation of spherical Sn particles by reducing SnO2 film in floating wire-assisted H2/Ar plasma at atmospheric pressure

A green method to synthesize spherical Sn particles by reducing SnO2 film in atmospheric-pressure H2/Ar plasma at low temperatures for various applications is presented. The floating wire-assisted remotely-generated plasma with a mixture of 0.05% H2/Ar gas formed spherical metallic Sn particles by reducing a SnO2 layer on glass substrate. During the reduction process, H radical density was measured by using vacuum ultraviolet absorption spectroscopy, and plasma properties including electron density and gas temperature were diagnosed by optical emission spectroscopy. The inductively coupled generated plasma with a high electron density of 1014 cm−3, a hydrogen atom density of 1014 cm−3, and a gas temperature of 940 K was obtained at a remote region distance of 150 mm where the SnO2/glass substrate was placed for plasma treatment. The process has been modeled on the spherical Sn formation based on the reduction of SnO2 films using H radicals. Depending on the treatment condition, the total reduction area, where spherical Sn particles formed, was enlarged and could reach 300 mm2 after 2 min. The substrate temperature affected the expansion rate of the total reduction area and the growth of the Sn spheres.

CT, MR and EPR imaging of graphene oxide aerogels

The shape and intrinsic pore structure of pristine and partially reduced graphene oxide (prGO) aerogels/hydrogels was imaged using three complementary techniques: Magnetic Resonance Imaging (MRI), Computer Tomography (CT) and Electron Paramagnetic Resonance Imaging (EPRI). Each method is sensitive to different features of the sample: CT images are contrasted with mass density changes across sample, MRI with hydrogen density, nuclear relaxation times, diffusion, and finally EPRI with radical density. The complementarity of the techniques is presented on exemplary chosen prGO aerogels, which are currently researched for variety of applications, such as biomaterials in tissue engineering. All techniques have their strengths and weaknesses which are thoroughly pointed out and discussed. EPRI technique, which has been perfected only during last years, is used here for the first time to image carbon-based foams. Finally scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) is used to characterize surface and elemental content of fabricated prGO aerogels/hydrogels.

Control of higher-silane generation by dilution gases in SiH4 plasmas

Higher-silane related reactive species (HSRS) is considered as a key factor in increasing the degree of photo-induced degradation, which is a serious problem for hydrogenated amorphous silicon (a-Si:H) film. Dilution gases (H2, He, Ar) are used to control the generation of HSRS. A global model for SiH4-containing discharge is developed to investigate the concentration of HSRS. The model calculations are compared with the available measurements, presenting a reasonable agreement. Increasing pressure contributes to more power loss via volumetric processes, which brings about the decline in the electron density and temperature with the same absorbed power. The average electronegativity rises with the silane content or pressure log-linearly, but decreases with the power log-linearly. As the main precursor of the anion formation, is the starting point for the formation of large silyl anions (). The HSRS density rises first and then falls with the increase of the absorbed power, which can be explained by the decomposition of SiH4. The dilution of H2 promotes the generation of H atomics, enhancing the production of SiH2 with abstraction reaction (H + SiH3 ⇒ H2 + SiH2). However, due to the declined content of SiH4, the insertion reaction (SiH2 + SiH4 ⇒ Si2H6) is inhibited. A higher Ar fraction results in more total ionization and a larger ion density. Due to the high threshold energies of He for both excitation and ionization, the dilution of He does not contribute much to the variation of the neutral radical density.

Study of a linear surface wave plasma source for tin removal in an extreme ultraviolet source

Tin deposition mitigation employs hydrogen radicals and ions, formed in a hydrogen plasma, to interact with tin to form tin hydride (SnH4) in the gaseous state, which is then pumped away. Surface wave plasma (SWP) technology developed at Illinois generates hydrogen radicals and ions, resulting in tin etch rates that are high enough to keep extreme ultraviolet (EUV) lithographic tools clean. An advantage of an SWP antenna is the ability to generate a high density of hydrogen radicals and hydrogen ions directly at the desired etching location. In situ etching of tin enables high availability EUV tools by maintaining high reflectivity of the multilayer mirror of the collector. Additionally, the SWP is characterized with low ion energies and low electron temperature, such that the multilayer mirror does not suffer any damage from sputtering or implantation of hydrogen ions during operation. Here, experiments elucidating the fundamental processes of tin removal are conducted by varying pressure, power, surface temperature, and gas flow rate in order to observe the etch rate behavior. Our results have shown that the presence of hydrogen ions increases etch rates because ion bombardment weakens Sn–Sn bonds, which, in turn, allows for a higher rate of chemical etching by the radicals. The ion bombardment reduces the number of radicals needed to etch a single tin atom to the range of 102–103. The linear SWP antenna yields plasma densities on the order of 1017to1018m−3 and radical densities on the order of 1018to1019m−3, allowing for greater utilization of ion etch enhancement. Etch rates of up to 200 nm/min have been achieved. The surface temperature of the samples is an important factor in the etching process such that the decrease in the surface temperature increases the etch rates and decreases the hydrogen desorption rates. In addition, a kinetic etch model is developed to explain the behavior of etch rates as a function of surface temperature. Furthermore, results from experiments performed in an Illinois NXE:3100 chamber will be discussed.

The kinetics of radical formation on mechanically activated kaolinite surfaces

Kaolinite is a versatile material with many applications in various fields such as in cosmetics or wastewater treatment. Here, we report the first evaluation of mechanoradicals formation on activated kaolinite powders, and its kinetics, via wet-grinding in ethanol using a free-radical scavenger (2,2-diphenyl-1-picrylhydrazyl; DPPH). Increasing the duration of ball milling of two reference kaolinite samples resulted in increased DPPH consumption due to an increase in the surface area of the processed powders. The correlation between the increasing rates of the surface areas and DPPH consumption yielded information on the density of radical species that may react with ethanol to form H radicals on the newly created surfaces of the kaolinite powders. The radical density was much lower than that reported previously for quartz, possibly due to the anisotropic nature of the kaolinite crystal structure. The actual surfaces that take part in the radical formation reaction were mainly newly created edge surfaces rather than basal surfaces. The kaolinite sample with fewer defects was more resistant to delamination, and thus its edge-to-basal surface-area ratio was also higher, which may have resulted in higher reactivity during mechanical processing. This knowledge is key for industrial applications of processed kaolinite powders.

Cause of Etch Pits during the High Speed Plasma Etching of Silicon Carbide and an Approach to Reduce their Size

To reduce the on-resistance in vertical power transistors, backside thinning is required after device processing. However, it is difficult to thin silicon carbide (SiC) wafers with a high removal rate by conventional mechanical processing because their hardness and brittleness cause cracks and chips during thinning. Therefore, the authors have attempted to thin SiC wafers using plasma chemical vaporization machining (PCVM), which is plasma etching using high-pressure plasma. PCVM has a high removal rate because of the high radical density in the high-pressure plasma, and it does not form a damaged layer on the processed surface because of the low ion energy. The authors have already achieved a very high removal rate of 15.6 μm/min by PCVM. However, many etch pits were generated on the wafer during PCVM in these high-speed machining conditions. Therefore, this study, using molten potassium hydroxide (KOH) etching, investigated the cause of such etch pits and found that they may stem from threading screw dislocation in the wafers. In addition, this research considered a process for reducing an etch pit size and succeeded in doing so by controlling wafer temperature.

Green synthesis of spongy Nano-ZnO productive of hydroxyl radicals for unconventional solar-driven photocatalytic remediation of antibiotic enriched wastewater

Herein, novel green/facile approach to synthesize spongy defective zinc oxide nanoparticles (ZnONPs) is presented using for the first time pomegranate seeds molasses as a green capping fuel/reducing mediator during an aqueous solution combustion process. The developed ZnONPs is characterized by UV–Vis. Spectrophotometry and fluorimetry, XRD, Raman spectroscopy, SEM, TEM and BET. Interestingly, pomegranate seeds molasses within a viable content of bio-capping molecules reveal a defective nanoporous ZnO NPs of smaller particle size, greater pore size/volume, and higher surface area compared to the bulky non-biogenic ZnONPs. Moreover, the biosynthesized defective ZnONPs showed narrowed band gap and higher absorption of visible photons that breed higher density of hydroxyl radicals (•OH) under Solar-illumination. Even further, the bulk ZnO and the biosynthesized ZnO photocatalysts were examined in photodegrading flumequine (FL) antibiotic. The bulk ZnO gives 41.46% photodegradation efficiency compared to 97.6% for the biosynthesized ZnO. In highly acidic or highly alkaline media, FL photodegradability is greatly retarded. Scavenging experiment infers considerable contribution of holes over electrons in photodegradation reaction. The biosynthesized ZnO shows high durability in FL photodegradation after four reusing cycles. These promising findings highlight new insights for biogenic synthesis of tuned size/controlled morphology semiconductor NPs relevant to environmental remediation applications.

Global modeling on partial oxidation of methane to oxygenates and syngas in non-equilibrium plasma

In this paper, plasma kinetics of CH4/O2 at atmospheric pressure are of interest for applications in plasma chemistry where partial oxidation of methane to oxygenates and syngas are investigated by zero-dimensional global model. Effect of methane mole fraction ranging from 5% to 99% on plasma kinetics is studied in detail. It is found that electron density and electron temperature change periodically with periodic variation of power density pulse. The number densities of free radicals, ions and stable molecules are discussed as a function of simulation time and CH4/O2 molar ratio. Conversions of inlet gases, selectivities and yields of important products, and optimum methane content for methanol and formaldehyde formation are calculated and presented. In addition, CH3 radical is found to be the key intermediate for oxygenates production by analyzing the underlying dominant reaction pathways. Finally, a schematic overview of the transformation relationship between dominant plasma species is also summarized and shown.

Influence of electrode geometry on the transport of OH radicals in an atmospheric pressure surface micro-discharge

Atmospheric pressure surface micro-discharge (SMD) has recently received considerable interest as an efficient source for its potential applications in biomedical, agricultural and environmental fields. The inherent physical separation between the discharge layer and the downstream target causes a reduction in the flux of reactive chemical species, resulting in the limitation of application efficacy. To obtain the development of healthcare devices, this contribution focuses on the spatial-temporal transport behavior of OH radicals generated by SMD devices in helium using laser induced fluorescence and optical emission spectroscopy with hexagon mesh electrode of lattice distance varying between 3 mm and 11 mm. It was observed that the lattice distance has a significant impact on the distribution of OH radicals and the uniformity of the SMD. The increased lattice distance of the hexagon mesh both increased the density, delivery distance and propagation velocity of OH radicals. When the lattice distance varied from 3 to 11 mm, the generated OH was spread over twice the distance. The maximum delivery distance was found to be approximately 10.4 mm under lattice distance of 11 mm condition, and the peak propagation velocity was estimated to be about 3.8 mm ms−1. For the main distribution region of OH, the optimum lattice distance is 9 mm. In addition, the distribution region rapidly expands between 7 and 9 mm. However, the findings indicated that researchers should make a compromise between the delivery of reactive chemical species and uniformity. The decreased lattice distance facilitated a high degree of uniformity for reactive species on a downstream target, which is a key requirement in healthcare related areas.

Formation of environmentally persistent free radicals and reactive oxygen species during the thermal treatment of soils contaminated by polycyclic aromatic hydrocarbons

Environmentally persistent free radicals (EPFRs) are emerging contaminants of increasing concern due to their toxicity for life and ecosystems, yet their formation, behavior and fate are poorly known. In particular, there is actually no knowledge on the formation of those radicals during the thermal treatment of soils containing polycyclic aromatic hydrocarbons. Such knowledge is important because thermal treatment is a remediation method used to decontaminate soils by removing and degrading PAHs. Here, we studied the formation of radicals in three types of cultivated soils, bauxite soil, fluvo-aquic soil and chernozem soil, artificially contaminated by benzo[a]pyrene, during thermal treatment from 100 to 200 °C for 1 h, using electron paramagnetic resonance. Results show spins densities of radicals up to of 2.079 × 1017 spins/g for bauxite soil, 1.481 × 1017 spins/g for fluvo-aquic soil and 8.592 × 1016 spins/g for chernozem soil at 175 °C. The formed radicals exhibited multiple decays during their observable time and the shortest 1/e lifetimes of radicals up to 757.58 h. These findings are strengthened by EPFR-induction of reactive oxygen species (ROS), O2·− and ·OH, which increased in concentrations from 100 to 200 °C. Overall, our results demonstrates for the first time that thermal treatment of PAHs-contaminated soils induces the formation of EPFRs and suggests that thermal treatment might not be a fully clean remediation method for soils as thermal treatment creates new contaminants.