OH Radical Density Research Articles

Quantitative estimation of ·OH-radical density from fluorescence decay behavior near the plasma-liquid interface by incident microscopic LIF spectroscopy

The formation and transportation of short-lived species on/within the plasma-liquid interfacial layer plays a crucial role in various applications because of their high chemical reactivity. However, the experimental detection and quantification of these short-lived species, such as ˙OH radicals, at the gas–liquid interface still pose formidable challenges. This study confronts this challenge by employing incident microscopic laser-induced fluorescence (mLIF) method to capture the OH-LIF signals on the interfacial layer at different time moments of the post-discharge phase under high spatial resolution. The temporal evolution of ˙OH density is subsequently quantified by fitting the OH-LIF decay behavior to a reaction-dissolution model. Results reveal that increasing the pulse width serves better to enhance ˙OH generation on liquid surface, reaching a density of 1.25 × 1016 cm−3. Furthermore, the cathode-solution interface demonstrates significantly enhanced ˙OH production compared to the anode-solution interface. These results underscore the efficacy of incident mLIF in quantitatively probing short-lived ˙OH-radical production at the interfacial layer in pulsed-driven plasma-solution interactions, with potential applicability to other reactive species.

Mechanism behind polypropylene surface modifications by OH radicals: An experimental study

We quantitatively investigated the roles and mechanisms of OH radicals in polypropylene (PP) surface modification utilizing a previously proposed vacuum ultraviolet photodissociation reactive species supply method. Water contact angle (WCA), attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) were employed to characterize the modifications in surface hydrophilicity and components. The relationship between the WCA decline rate and OH radical density at the PP surface was quantified as (2.1 ± 0.1)[OHs] + (0.5 ± 0.1) degrees/s, where [OHs] is in ppm. The ATR-FTIR and XPS results demonstrated that C–OH, CO, C–O–O, and OCO groups were gradually introduced onto the PP surface at different treatment stages. The formation and behavior of low-molecular-weight oxidized materials (LMWOMs) on the PP surface during OH radical treatment were investigated. A modified layer composed of LMWOMs eventually formed on the PP surface. The physicochemical mechanisms underlying these phenomena at the micro-level were also discussed. A possible surface oxidation model was proposed based on WCA results, surface functional groups, and LMWOMs, indicating that the oxidation of the PP chains by OH radicals was initiated at the topmost surface and advanced in the depth direction.

Plasma‐enhanced evaporation and its impact on plasma properties and gaseous chemistry in a pin‐to‐water pulsed discharge

AbstractThe plasma in contact with liquids has led to various novel applications such as plasma biomedicine, material synthesis, and so on. However, the phenomenon of evaporation under plasma treatment and its impact on plasma–liquid interactions has a limited understanding. In this study, the spatially and temporally resolved behavior of water vapor production and its induced influences on plasma properties and gaseous chemistry were studied in detail in an atmospheric pressure pin‐to‐water pulsed He discharge. Diagnostic methods such as laser‐induced fluorescence (LIF) and high‐resolution optical emission spectroscopy (OES) were applied to determine the water vapor and OH radical densities, as well as key plasma parameters such as the gas temperature and electron density. It shows that the physicochemical properties of plasma vary among different discharge regions due to evaporation behavior stimulated during the pulsed discharge‐on phase. In addition, using simulation based on the experimental data, the mechanisms of how water vapor affects the observed spatiotemporal behaviors of OH radicals in different discharge regions are understood. Compared to the pin‐anode and liquid‐cathode sheath regions, proper electron parameters such as density and temperature, as well as water vapor density in the plasma‐positive column, significantly enhance the production of reactive OH radical through the dominant path of electron‐stimulated H2O dissociation. However, higher levels of electron parameters in the intense discharge region near the positive‐pin boundary enhance OH dissociation and finally result in the hollow distribution of OH density. From the global kinetic plasma simulation, the production of reactive hydroxide species playing key roles in plasma medicine treatments, such as O, H, HO2, H2O2, and hydrated ions including H+(H2O)4 and H+(H2O)5, are promoted noticeably as a result of the enhanced water evaporation process.

Insights into generation of OH radicals in plasma jets with constant power: The effects of driving voltage and frequency

In applications of atmospheric-pressure plasma jets, due to the limitations of common power supplies, the power coupled to the plasma usually varies, which affects the production of reactive species. Here we maintain the power input constant to gain insights into the effects of variations of driving voltage and frequency on the generation of OH reactive species, and give practical recommendations for energy efficient plasma processing. The argon plasma jet generated by AC power is studied under different voltage and frequency under the custom-adjusted constant-power conditions, and optical spectroscopic measurements are carried out. The results show that under the same power, the gas temperature decrease with the increase of frequency from 380 K to 310 K, while the electron density stays the same at about 2.13 × 1015 cm3. The electron excitation temperature decreases from 4689 K to 4176 K with the increase of frequency. The density of ozone rose from 8.65 × 1018 m−3 to 3.20 × 1020 m−3. Though the power is constant, the changes of the plasma parameters affect the density of OH radicals. The latter rises when the driving voltage is increased. These findings provide guiding principles for the development of effective and energy efficient plasma processes.

Effect of atmospheric-pressure plasma irradiation on the surface tension of water

We have shown that measuring the surface tension is a useful scheme to examine the plasma–liquid interface in real-time. The surface tension was measured using a method based on the dispersion relation of an acoustic capillary wave excited on the water surface. The surface tension gradually increased with time, when the water surface was irradiated with the outside region of the spatial afterglow of an atmospheric-pressure plasma. The Marangoni effect associated with the localized increase in the surface tension was observed during the plasma irradiation. The surface tension decreased after the termination of the discharge. A correlation was found between the transient decrease in the surface tension and the variation of the OH radical density in the gas phase. No increase in the surface tension was observed in the solution containing a trapping agent for liquid-phase OH radicals. These experimental results suggest that OH radicals act to increase the surface tension. However, the behavior of the surface tension cannot be explained perfectly by considering only the action of OH radicals.

Absolute OH density and gas temperature measurements by laser induced fluorescence in a microsecond pulsed discharge generated in a conductive NaCl solution

Electrical discharges in bubbles in liquid water have been extensively investigated for many applications including water treatment and often the OH radical is considered to be a key enabler of many applications. Nonetheless, to date, no OH density measurements have been reported for discharges in bubbles or in liquids in general. In this paper, we report an investigation of discharges in bubbles by laser-induced fluorescence (LIF). The plasma was generated by applying a microsecond voltage pulse to a needle electrode submerged in an aqueous NaCl solution in a Hele-Shaw cell. To determine the gas temperature and OH radical density, a LIF model resolving the energy transfer at the rotational level was implemented. Gas compositions at different gas temperature were obtained with a 0-D kinetics model. The LIF model also enabled us to explain the non-equilibrium rotational distribution of the OH ground state by LIF. Gas temperatures and OH radical densities up to 4900 ± 800 K and 3.2 ± 0.5 × 1017 cm−3 are consistent with conditions favoring thermally driven reactions in the plasma-filled vapor bubble.

Influence of dimethylphenol isomers on electrochemical degradation: Kinetics, intermediates, and DFT calculation

Dimethylphenol isomers (DMP) pose a great threat to the environment, and the electrooxidation (EO) process proves to be an extraordinarily effective method to degrade DMP. However, the EO performance is affected by the molecular structure of DMP and the adopted experimental parameters. In this study, the effects of 2,4-DMP and 2,6-DMP on the working potential, limiting current density (Jlim), and pH were systematically analysed, with Ti-mesh plates used as the cathode and Ti/PbO2 as the anode. The peak potentials of 2,4-DMP and 2,6-DMP were determined to be 0.83 V and 0.77 V by cyclic voltammetry, with Jlim were 2.5 mA·cm−2 and 2.0 mA·cm−2, respectively. The whole process exhibited pseudo-first-order kinetics, and the kinetic constants (K) for the degradation of 2,4-DMP and 2,6-DMP were determined to be 0.0041 min−1 and 0.0150 min−1, respectively. Additionally, the optimal initial pH value for 2,4-DMP and 2,6-DMP was 5.0, where the highest hydroxyl (OH) radical density, as determined by the electron spin technique (ESR), was achieved at a higher current density. Comparatively, the OH radical density in the 2,6-DMP solution was lower than that in 2,4-DMP. In situ Fourier infrared (FT-IR) spectroscopy, GC–MS, and density functional theory (DFT) were employed to explore three possible degradation pathways. The main intermediates for 2,4-DMP degradation were determined to be quinone and ether, while that for 2,6-DMP degradation was quinone. According to the results of this study, the molecular structure (different methyl group positions on the benzene ring) has a great influence on the EO process.

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.

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.

Correlation between gas-phase OH density and intensity of luminol chemiluminescence in liquid interacting with atmospheric-pressure plasma

The relationship between the density of OH radicals in the gas phase and the intensity of luminol chemiluminescence was investigated using an atmospheric-pressure plasma system in contact with liquid. A thin luminol chemiluminescence was observed just below the plasma–liquid interface when the plasma was generated on the aqueous solution surface with luminol. The radial region with the chemiluminescence was approximately 2.6 times larger than that of the plasma with the optical emission, and was similar to the diameter of the region with OH radicals in the gas phase. The decay time constant of the intensity of the luminol chemiluminescence in the afterglow phase of the pulsed discharge was approximately 100 s, while the optical emission intensity at the second positive system of molecular nitrogen decayed immediately after the termination of the discharge current. On the other hand, the decay time constant of the OH radical density was approximately 100 s in the afterglow phase. These experimental results indicate that the luminol chemiluminescence is induced by the transport of OH radicals to the plasma–liquid interface. It is considered that the chemiluminescence originates from the reaction with O which is produced by .

Spectroscopic analysis of gold nanoparticle synthesis using plasma liquid interaction technique

Surfactant free AuNPs were prepared via Plasma-Liquid Interaction (PLI) technique, using -plasma, as a function of time, discharge current and gas flow rate. Atomic oxygen [O] and nitrogen [N] densities were determined to study their effect on the intensities of UV radiation and OH radical, which in turns controls the synthesis of AuNPs. UV radiation, OH radical and atomic densities of [O] and [N] were determined through optical emission spectroscopy from gas phase plasma. The increase in UV radiation intensities and OH radicals with time was observed and results were verified by observing increasing trend in the [O] and [N] atomic densities. We report the role of versatile discharge species generated in PLI, like [O] and [N] atomic densities, in the control synthesis of surfactant free AuNPs.

Control of hydrogen peroxide production in plasma activated water by utilizing nitrification

In this study, ammonia (NH3) vapor was added to an atmospheric pressure argon plasma jet generated by a sinusoidal power supply (with an applied voltage of 4 kV and frequency of 35 kHz) and it was analyzed by electrical, optical, and chemical probe measurements. A total gas flow rate of 1200 sccm under 2% flow rate of either water or ammonia solution at concentrations up to 0.75% mixed with 98% argon feeding gas. The dissipated power of the plasma jet estimated by voltage-charge plots was found to decrease continuously with increase in ammonia concentrations at a fixed applied voltage. The optical emission spectra inside the quartz tube obtained for the NH3 mixed plasma indicated the presence of nitrogen emission lines which were absent in the water only mixed argon plasma. Colorimetric measurements of hydrogen peroxide (H2O2) and nitrite () in plasma activated water (PAW) were performed at different consumed energy varied by plasma exposure time. The H2O2 concentration decreased to 4 ppm from 14 ppm, while the concentration simultaneously increased to 6 ppm from 1 ppm as the ammonia concentrations increased to 0.75% from 0% throughout 2.07 × 103 joule energy consumption. We also analyzed the corresponding variation of pH values, in absence of ammonia, PAW leads to acidification but addition of small amount of ammonia in plasma is sufficient to make it alkaline. Additionally, we measured the hydroxyl (OH) radical density in gas and liquid phases by utilizing UV absorption spectroscopy and chemical probe method, respectively. OH radical density in both phases also decreased with the increase in the NH3 concentrations. Experimental results obtained from this experiment can be attributed to control H2O2 concentrations in the PAW via plasma-assisted nitrification.

Effect of discharge polarity on OH density and temperature in coaxial-cylinder barrier discharge under atmospheric pressure humid air

Plasma application for environmental pollution control is desirable, and it is important to clarify the chemical reaction processes in nonthermal plasma. In this study, the discharge polarity dependence of the density and rotational temperature of OH radicals is measured by laser-induced fluorescence (LIF) in coaxial-cylinder pulsed dielectric barrier discharge (DBD) in atmospheric pressure humid air. The density of OH radicals generated by a pulsed positive DBD of +28 kV is estimated to be in the range of 6–10 × 1014 cm−3 at 3 µs after discharge near the central electrode. The rotational temperature rises after the discharge and the rate of temperature rise increases with humidity. In negative discharge, the decay rate of OH density, which directly depends on the initial OH density, is lower than that in positive discharge. The OH amounts contained in the observed area are almost the same in positive and negative discharges. These results indicate that the negative streamer diameter is twice the positive streamer diameter and the OH density in negative DBD is a quarter of that in positive discharge. In the chemical treatments using OH radicals, negative streamer DBD is supposed to be superior to positive streamer DBD from the perspective of ineffective loss. The temperature behavior in negative discharge is similar to that in positive discharge.

Characterization of a novel microwave plasma sheet source operated at atmospheric pressure

Typically, microwave plasma sources (MPSs) operated at atmospheric pressure deliver plasmas in the common form of flames or cylindrical columns. In contrast to that, in this paper, a novel MPS (recently patented by us) for generating a new type of plasma in the form of a plasma sheet, which is sustained within a dielectric flat cuboid box, is presented. It is waveguide based, operated at a frequency of 2.45 GHz and in argon at atmospheric pressure. The plasma sheet consists of many filaments, typical of a microwave argon plasma. The MPS is described and analyzed in terms of optical emission spectroscopy, simulation of the electric field distributions in the MPS and numerical modeling of the working gas flow. Depending on the argon flow rate and absorbed microwave power, the determined electron density and rotational temperatures of OH radicals ranged from 3.6 × 1014 to 5.6 × 1014 cm−3 and from 800–1250 K, respectively. Numerical simulations have confirmed that the dimensions of the box are chosen correctly to ensure a sufficiently homogeneous distribution of the gas flow velocity field. The calculated electromagnetic field distributions in the MPS with plasma have shown that the device is well matched to the feeding line. The relative simplicity of the presented plasma source and industrially convenient plasma shape make it attractive for practical applications in the surface treatment of various materials.

Quantification of plasma produced OH radical density for water sterilization

The interactions between plasma‐generated excited particles and water play an integral role in sustainable degradation of pharmaceutical compounds, improving aerobic respiration of activated sludge, and efficient removal of microorganisms from water, and are fundamental to the intentional transfer of reactivity from plasmas to biological solutions for such medical applications as cancer treatment and wound healing. The physical and chemical mechanisms that govern this transfer of reactivity are complex, and include concomitant generation and consumption of species in the gas and liquid phases, and at the interface. As such, it is challenging to predict the quantities of biologically‐active radicals and molecules in liquid phase from gas phase measurements alone. Rapid and accurate quantification of reactive species, such as OH radicals and H2O2 molecules within the liquid phase and their link to specific biological effects is therefore critical for medical applications of plasma‐activated solutions. Using a simple, low‐cost method for trapping and stabilization of OH radicals by means of salicylic acid, this work seeks to provide further insights into the physics and chemistry of generation of OH radicals within the liquid phase, and integrate these findings with decontamination outcomes for four commonly used processing gases.

OH density measured by PLIF in a nanosecond atmospheric pressure diffuse discharge in humid air under steep high voltage pulses

The spatiotemporal distributions of the OH radical density are measured using planar laser induced fluorescence in the afterglow of a nanosecond diffuse discharge at atmospheric pressure in humid air. The diffuse discharge is generated between a pin and a grounded plate electrodes within a gap of 18 mm. The high voltage pulse applied to the pin ranges from 65 to 85 kV with a rise time of 2 ns. The specific electrical energy transferred to the gas ranges from 5 to 40 J l−1. The influence of H2O concentration is studied from 0.5% to 1.5%. An absolute calibration of OH density is performed using a six-level transient rate equation model to simulate the dynamics of OH excitation by the laser, taking into account collisional processes during the optical pumping and the fluorescence. Rayleigh scattering measurements are used to achieve the geometrical part of the calibration. A local maximum of OH density is found in the pin area whatever the operating conditions. For 85 kV and 1% of H2O, this peak reaches a value of 2.0 × 1016 cm−3 corresponding to 8% of H2O dissociation. The temporal decay of the spatially averaged OH density is found to be similar as in the afterglow of a homogeneous photo-triggered discharge for which a self-consistent modeling is done. These tools are then used to bring discussion elements on OH kinetics.

Synthesis and spectroscopic characterization of gold nanoparticles via plasma-liquid interaction technique

Fabrication of non-functionalized gold nanoparticles is interesting owing to their potential applications in sensing and biomedicine. We report on the synthesis of surfactant-free gold nanoparticles (AuNPs) by Plasma-Liquid Interaction (PLI) technique, using micro-atmospheric pressure D.C. plasma. The effects of discharge parameters, such as discharge current, precursor concentration and gas flow rates on the structure and morphology of AuNPs have been investigated. Optical Emission Spectroscopy (OES) was employed to estimate the UV radiation intensity and OH radical density. Scanning electron microscopy (SEM) and ultraviolet-visible (UV-Vis) optical spectroscopy were employed to study the morphology and structure of AuNPs. The normalized intensities of UV radiation and OH radical density found to increase with increase in discharge current. We observed that the particle size can be tuned by controlling any of the following parameters: intensity of the UV radiation, OH radical density, and concentration of the Au precursor. Interestingly, we found that addition of 1% Ar in the feedstock gas results in formation of relatively uniform size distribution of nanoparticles. The surfactant-free AuNPs, due to their bare-surface, exhibit excellent surface-enhanced Raman scattering (SERS) properties. The SERS study of Rhodamine 6G using AuNPs as substrates, shows significant Raman enhancement and fluorescence quenching, which makes our technique a potentially powerful route to detection of trace amounts of dangerous explosives and other materials.

Simulation of spatio-temporal variation of OH radical density in atmospheric-pressure streamer discharge

The spatio-temporal variation of OH radical density in an atmospheric-pressure plasma discharge is investigated via two-dimensional numerical simulation. The behaviours of OH are characterized in four regions in the inter-electrode gap, namely the ‘Hot anode’, ‘Secondary streamer’, ‘Primary streamer’, and ‘Near-cathode’ regions. In these regions, temporal variations of averaged reduced electric field, electron density, and gas temperature differ and they affect the temporal variation of OH density. In all regions, a relatively large amount of OH is produced by the dissociation reactions of H2O with electronically excited nitrogen molecules and oxygen atom rather than the direct electron-impact dissociation reactions. In the Hot anode and Secondary streamer regions, the instantaneous density of OH just after the discharge is high. However, the subsequent OH decay rates differ in these regions owing to the difference in gas temperatures. The reaction mechanism is discussed by dividing the effect of the diffusion-convection term and chemical reaction term in the continuous equation for OH radical density.

LIF diagnostics of hydroxyl radical in a methanol containing atmospheric-pressure plasma jet**Project supported by the National Natural Science Foundation of China (Grant Nos. 11465013 and 11375041), the Natural Science Foundation of Jiangxi Province, China (Grant Nos. 20151BAB212012 and 20161BAB201013), and the International Science and Technology Cooperation Program of China (Grant No. 2015DFA61800).

In this paper, a pulsed-dc CH3OH/Ar plasma jet generated at atmospheric pressure is studied by laser-induced fluorescence (LIF) and optical emission spectroscopy (OES). A gas–liquid bubbler system is proposed to introduce the methanol vapor into the argon gas, and the CH3OH/Ar volume ratio is kept constant at about 0.1%. Discharge occurs in a 6-mm needle-to-ring gap in an atmospheric-pressure CH3OH/Ar mixture. The space-resolved distributions of OH LIF inside and outside the nozzle exhibit distinctly different behaviors. And, different production mechanisms of OH radicals in the needle-to-ring discharge gap and afterglow of plasma jet are discussed. Besides, the optical emission lines of carbonaceous species, such as CH, CN, and C2 radicals, are identified in the CH3OH/Ar plasma jet. Finally, the influences of operating parameters (applied voltage magnitude, pulse frequency, pulsewidth) on the OH radical density are also presented and analyzed.

Monolithic structure of integrated coaxial microhollow dielectric barrier discharges: Characterization for environmental and biomedical applications

The characteristics of microhollow dielectric barrier discharge devices in a thin monolithic planar structure with many holes were analyzed regarding the production of OH radicals, using optical emission and laser-induced fluorescence (LIF) spectroscopy techniques. Spatial distributions of OH radical density depended on the diameter of electrode holes from 0.6 to 1.5 mm and the discharge operating gas species. Apparent emission intensity from OH radicals and the LIF signals were very high in He and Ar gases but quite low in N2. However, taking into account the LIF quenching rate in each gas, the existing densities of OH radicals in all tested gases were not greatly different from each other. The absolute density of OH radicals estimated by a comparison of the LIF intensity with our measured result on a conventional He plasma jet referring to reported densities in similar situations was on the order of 1014 cm−3.