Motohiro Banno

Development of direct gas injection system for atmospheric-pressure in-solution discharge plasma for plasma degradation and material syntheses

By applying a high pulsed voltage to a gap between two electrodes placed in a solution, an atmospheric-pressure in-solution glow (ASG) plasma is generated. The ASG plasma is applied in a new material processing method, called solution plasma processing (SPP). In order to accelerate the reaction and to add functions to the synthesized materials, it is important to alter the gas content in the ASG plasma. We developed a direct gas injection system for an ASG discharge cell. When O2, CO2, N2 and Ar gases were injected into the ASG plasma, emission bands due to the derivatives of the injected gases were observed in the emission spectra from the ASG plasma. The electron number density in the ASG plasma was increased by the O2, CO2, and N2 injections, probably due to the enhancements of the α and γ processes by the larger molecular weights than H2O. In addition, the first dielectric breakdown of the solution and the formation of the gas bubble processes gradually disappeared due to the gas injection. When O2 was injected, the amount of ·OH generation was increased. By the enhancement of the ·OH generation, the degradation speed of rhodamine B in the ASG plasma was increased by a factor of two. When Au nanoparticles were synthesized utilizing the ASG plasma, the zeta potential of the Au nanoparticles was increased by about 30% by the O2 injection. The plasma parameters and the reactivity of the ASG plasma can be altered more widely by changing the kind of injected gas and the flow rate.

Nanosecond time-resolved microscopic spectroscopy for diagnostics of an atmospheric-pressure discharge plasma formed in aqueous solution

Glow discharge plasma formed in solution under atmospheric pressure has been expected to provide reaction fields with characteristic physical and chemical properties owing to the frequent collisions and reactions of reactive particles inside and the rapid quenching of the products by the surrounding cold solutions. In particular, when an aqueous solution is utilized as the surrounding solution, the atmospheric-pressure in-solution glow (ASG) plasma contains hydrogen and hydroxyl radicals showing large activities for reduction and oxidation, respectively. In addition, because the ASG plasma is formed under atmospheric pressure, the collision frequencies between the particles contained in the plasma are higher than those in other plasmas ordinarily formed under low pressure. This feature should result in rapid energy redistribution among particles contained in the plasma. In the present study, time-resolved optical emission spectroscopy with nanosecond time resolution was applied for the diagnostics of the ASG plasma with chemical species selectivity. The time-resolved measurements revealed that the temporal evolutions of the temperatures of blackbody, hydrogen radical, and hydroxyl radical contained in the ASG plasma consist of two stages: initial rise within 0.15 µs (rising stage) and fluctuation around certain values for about 1 µs (fluctuating stage). In the time region corresponding to the rising stage, the electron number density is about ten times larger than the value temporally averaged during the plasma emission. The initial rise should result from frequent collisions between charged particles accelerated by the applied voltage and unexcited particles. In the fluctuating stage, the electron number density strongly correlates with the increase in the radical temperatures. It is concluded that the electron number density, rather than the electron temperature, is a key parameter determining the temperatures of reactive species in the ASG plasma.

Time- and Space-resolved Optical Diagnostics for Discharge Plasmas Separately Formed in Aqueous Solution

When high voltage is applied to a gap between electrodes in aqueous solution, a discharge plasma is generated that connects the edges of the facing electrodes. When the width of the gap between the electrodes is extended to more than several millimeters, the plasma is separated into two and they are localized at the vicinities of the edges of the anode (+plasma) and cathode (-plasma). Although they are expected to supply characteristic reaction fields, the properties of the plasmas, such as electron number density and temperatures of transient species, have not yet been clarified. In the present study, a time- and space-resolved emission spectrometer with a discharge cell was developed for optical diagnostics of the +plasma and -plasma separately. The electron number density for the +plasma was obviously lower than that for the -plasma. The difference in the electron number density should result from the difference in the ionization energy of the cathode materials. From the temporal evolutions of the emissions from the components, the emissions from the -plasma were sustained for about 0.5 μs after the decay of the applied voltage, probably due to the large number of free electrons in the -plasma. It is also clarified that hydroxyl radicals are effectively generated by the collisions between cations deriving from water and low-energy free electrons in the +plasma. The wide-gap in-solution discharge supplies two plasmas simultaneously with different properties. For plasma reactions, one plasma with suitable properties can be selected.

Time-resolved optical diagnostics of solution plasma formed with graphite electrodes

The glow discharge plasma formed by the application of pulsed high voltage in solution (solution plasma, SP) is expected as a novel reaction field for syntheses of carbon nanomaterials, where temperature control crucially affects the structures of the resultant materials. In the present study, the temperatures of reaction intermediates for carbon material syntheses in SP with graphite electrodes were elucidated by time-resolved optical emission spectroscopy. By comparing the emission bandshapes from C2 and CH radicals with the simulated ones, the temporally averaged temperatures of the radicals were successfully estimated as 3,000 to 6,000 K. From the temporal evolution of the radical temperature, the temperature was almost kept in the range between 2,500 and 4,000 K, except for sudden elevations to more than 7,000 K with durations of less than 0.1 µs. The temperatures were observed to be markedly lower than those in arc discharge plasma, which is conventionally applied to carbon nanomaterial syntheses. It is suggested that SP should provide a reaction field in which the sp2 bonds between carbon atoms in the product nanomaterials are less dissociated than the arc discharge plasma.

Improvement of Spatial Resolution for Nonlinear Raman Microscopy by Spatial Light Modulation

The development of a stimulated Raman scattering (SRS) microscope with a wavefront modulation unit is presented. In the apparatus, two beams for introducing the SRS process were focused into the sample with an objective lens. In the pathway of the Stokes beam, which is one of the two incident beams, a spatial light modulator (SLM) was located. Using the SLM, the wavefront of the Stokes beam was modulated to make the shape of the focal point a concentric circular pattern. By this spot shaping technique, the area where the SRS signal generates is restricted. The instrument response function (IRF) of the SRS microscope was examined by measuring the SRS intensity while scanning the sample position. From the result, the width of the IRF was reduced by about 15% by the wavefront modulation. It is suggested that the introduction of SLM is a way to improve the IRF of vibrational spectroscopic microscopes.

Atmospheric discharge plasma in aqueous solution: Importance of the generation of water vapor bubbles for plasma onset and physicochemical evolution

Discharge plasma formed in aqueous solutions has attracted much attention for its applications in environmental purification and material syntheses. The onset and evolution of the discharge plasma in an aqueous solution and transient reactive species formed in it are successfully monitored with micrometer spatial resolution and nanosecond temporal resolution. The combination of a custom-made microscopic discharge system and a high-speed camera provides direct evidence that water vapor bubbles form before the discharge with the thermal phase transition of aqueous solution at the electrode tip. The water vapor bubbles, i.e., locally formed space in the gas phase, connect the gap between the tips of the opposed electrodes. The local gas area formed in aqueous solution plays a crucial role in the ignition and continuance of the discharge plasma. It is also found that the initially formed plasma lasts for under 100 ns and quenches rapidly. However, plasma regenerates in the water vapor bubble and successively ...

Stimulated Raman Scattering Interferometer for Molecular-Selective Tomographic Imaging

Development of a stimulated Raman scattering (SRS) interferometer is presented. In the apparatus, a SRS signal generated in a sample is collinearly overlapped with a reference beam, and the interference pattern between the signal and reference beams is obtained. From the interference pattern, the phase of the SRS signal can be measured. From the phase of the signal, the spatial distribution of the target molecule can be obtained with high positioning precision. In the present article, the SRS interferometer was developed and the first SRS interference signal was obtained from a polyethylene film. The phase change of the SRS interference pattern caused by scanning the sample position along the direction of the incident light propagation was measured. From the result, the sample position difference for a sub-micron was detected with uncertainties of 15%. The tomographic images of a triple-layered polymer film were successfully obtained. The SRS interferometer would become a core optical technique of a novel molecular-selective tomographic imaging method.

Stimulated Raman photoacoustic spectroscopy for chemical-contrast imaging of a sample deeply buried in scattering media.

The development of a stimulated Raman scattering photoacoustic (SRS-PA) spectrometer is presented. In the apparatus, a molecular vibrational mode is excited by the SRS process. The vibrational excitation energy is converted to heat by vibrational relaxation. The volumes around the excited molecules including the surrounding solvent molecules expand by heating, resulting in the generation of an ultrasonic wave. The ultrasonic wave can be used as a molecular-selective signal. Because the ultrasonic wave is scarcely scattered by media, SRS-PA is expected to be applied for obtaining molecular-selective signals from deeply buried samples. In the present study, a SRS-PA spectrometer was developed and applied to obtain molecular-selective signals from test samples. The SRS-PA signals from water and lipid, which are important components in biological systems, were first obtained. The SRS-PA signal from a polystyrene film buried in a highly light-scattering intralipid suspension was also measured. We succeeded in obtaining the signal from the film when it was buried with a depth of up to 1.8 mm. The results indicate that SRS-PA can be effectively applied for the chemical-contrast imaging of deeply buried samples.