Faculty Profiles - SAITO, Nagahiro

Papers - SAITO, Nagahiro

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Electrocatalytic oxygen reduction activity of boron-doped carbon nanoparticles synthesized via solution plasma process

Panomsuwan G., Saito N., Ishizaki T.

Electrochemistry Communications Vol. 59 page： 81 - 85 2015.7

Language：English Publishing type：Research paper (scientific journal) Publisher：Electrochemistry Communications

The synthesis of boron-doped carbon nanoparticles (BCNP) has been achieved through a solution plasma process without the addition of a metal catalyst source using a mixture of benzene and triphenyl borate as precursor. The electrocatalytic activity toward the oxygen reduction reaction (ORR) of BCNP can be improved in terms of onset potential and current density compared to that of undoped carbon nanoparticles in alkaline solution. Moreover, BCNP possesses superior long-term durability and tolerance to methanol oxidation in the ORR.

DOI： 10.1016/j.elecom.2015.07.005

Thermal plasma treatment of stormwater sediments: Comparison between DC non-transferred and partially transferred arc plasma

Li O.L., Guo Y., Chang J.S., Saito N.

Environmental Technology (United Kingdom) Vol. 36 ( 13 ) page： 1672 - 1679 2015.7

Language：English Publishing type：Research paper (scientific journal) Publisher：Environmental Technology (United Kingdom)

The disposal of enormous amount of stormwater sediments becomes an emerging worldwide problem. Stormwater sediments are contaminated by heavy metals, phosphorus, trace organic and hydrocarbons, and cannot be disposed without treatment. Thermal plasma decontamination technology offers a high decomposition rate in a wide range of toxic organic compound and immobilization of heavy metal. In this study, we compared the treatment results between two different modes of thermal plasma: (1) a non-transferred direct current (DC) mode and (2) a partial DC-transferred mode. The reductions of total organic carbon (TOC) were, respectively, 25% and 80% for non-transferred and partially transferred plasma, respectively. Most of the toxic organic compounds were converted majorly to CxHy. In the gaseous emission, the accumulated CxHy, CO, NO and H2S were significantly higher in partially transferred mode than in non-transferred mode. The solid analysis demonstrated that the concentrations of Ca and Fe were enriched by 500% and 40%, respectively. New chemical compositions such as KAlSi3O8, Fe3O4, NaCl and CaSO4 were formed after treatment in partially DC-transferred mode. The power inputs were 1 and 10 kW, respectively, for non-transferred DC mode and a partially DC-transferred mode. With a lower energy input, non-transferred plasma treatment can be used for decontamination of sediments with low TOC and metal concentration. Meanwhile, partially transferred thermal plasma with higher energy input is suitable for treating sediments with high TOC percentage and volatile metal concentration. The organic compounds are converted into valuable gaseous products which can be recycled as an energy source.

DOI： 10.1080/09593330.2014.1003981

Water-plasma-assisted synthesis of black titania spheres with efficient visible-light photocatalytic activity

Panomsuwan G., Watthanaphanit A., Ishizaki T., Saito N.

Physical Chemistry Chemical Physics Vol. 17 ( 21 ) page： 13794 - 13799 2015.6

Language：English Publishing type：Research paper (scientific journal) Publisher：Physical Chemistry Chemical Physics

Black titania spheres (H-TiO<inf>2-x</inf>) were synthesized via a simple green method assisted by water plasma at a low temperature and atmospheric pressure. The in situ production of highly energetic hydroxyl and hydrogen species from water plasma are the prominent factors in the oxidation and hydrogenation reactions during the formation of H-TiO<inf>2-x</inf>, respectively. The visible-light photocatalytic activity toward the dye degradation of H-TiO<inf>2-x</inf> can be attributed to the synergistic effect of large-surface area, visible-light absorption and the existence of oxygen vacancies and Ti<sup>3+</sup> sites.

DOI： 10.1039/c5cp00171d

Simple one-step synthesis of fluorine-doped carbon nanoparticles as potential alternative metal-free electrocatalysts for oxygen reduction reaction

Panomsuwan G., Saito N., Ishizaki T.

Journal of Materials Chemistry A Vol. 3 ( 18 ) page： 9972 - 9981A 2015.5

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Materials Chemistry A

Fluorine-doped carbon nanoparticles (FCNPs) were synthesized via a simple one-step solution plasma process for the first time. This synthesis strategy can be achieved at relatively low temperature and atmospheric pressure without the involvement of a metal catalyst. A mixture of toluene (C<inf>6</inf>H<inf>5</inf>CH<inf>3</inf>) and trifluorotoluene (C<inf>6</inf>H<inf>5</inf>CF<inf>3</inf>) was used as a precursor for the synthesis. The fluorine doping content can be varied from 0.95 to 4.52 at%, depending on the precursor mixing ratio. The structural analyses reveal that FCNPs mainly exhibit a disordered amorphous structure. The incorporation of fluorine atoms results in the creation of more defect sites and disordered structure in the carbon particles. The electrocatalytic activity toward the oxygen reduction reaction (ORR) of FCNPs in an alkaline solution shows a significant improvement with increasing fluorine doping content, as reflected in an increased limiting current density and a positively shifted onset potential. In association with X-ray photoelectron spectroscopy (XPS) analysis, an improved ORR activity is possibly attributed to the intercalation of ionic C-F and semi-ionic C-F bonds in the carbon structure. In addition, FCNPs possess excellent long-term operation durability and strong tolerance to methanol oxidation compared to those of a commercial Pt-based catalyst. Our results from this study not only confirm the applicability of the solution plasma process to the synthesis of FCNPs with a controllable fluorine doping level but also provide detailed information of FCNPs as potential alternative ORR catalysts for the electrocatalysis research.

DOI： 10.1039/c5ta00244c

A new approach of nonpoint source pollution/stormwater sludge treatment by an integrated thermal plasma system

Li O.L., Guo Y., Chang J.S., Urashima K., Saito N.

International Journal of Environmental Science and Technology Vol. 12 ( 5 ) page： 1769 - 1778 2015.5

Language：English Publishing type：Research paper (scientific journal) Publisher：International Journal of Environmental Science and Technology

In this study, we designed and demonstrated the effectiveness of an integrated waste treatment system for contaminated stormwater sludge. The system is comprised of two components: pulsed arc electrohydraulic discharge (PAED) for aqueous-phase treatment and thermal plasma decontamination for solid-phase treatment. PAED provided a reduction of 80 % in total organic carbon (TOC) within a treatment time of 5 min in the aqueous phase. With a prolonged treatment of 30 min, the decontamination of TOC reached over 90 % in the aqueous phase, accompanied with 20 % carbon and 70 % sulfur decreases in weight in the solid phase. A huge weight reduction can be achieved prior to further solid treatment. The solid phase, named as the PAED-treated sludge, was separated by sedimentation and transferred to thermal plasma treatment. The thermal plasma-treated sludge was slightly melted, and the formation of slag was possible. X-ray energy dispersion solid analysis showed that carbon and sulfur were reduced to zero after treatment. Si, Mg, and Al decreased by 25, 30, and 60 %, respectively, while Ca and Fe enriched by 400 and 25 %, respectively. The emission gas analyses demonstrated a possible energy recovery from gaseous compounds with adequate gaseous treatment devices. This research demonstrates an innovative treatment approach of various sludge types, especially those types that contain high water contents and are severely contaminated by toxic organic compounds.

DOI： 10.1007/s13762-014-0570-7

Nitrogen-doped carbon nanoparticles derived from acrylonitrile plasma for electrochemical oxygen reduction

Panomsuwan G., Saito N., Ishizaki T.

Physical Chemistry Chemical Physics Vol. 17 ( 9 ) page： 6227 - 6232 2015.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Physical Chemistry Chemical Physics

Nitrogen-doped carbon nanoparticles were synthesized via a solution plasma process, with acrylonitrile as a single-source precursor, followed by a post-thermal annealing process. The nitrogen-bonding states can be tuned by varying the annealing temperature. The best electrocatalytic activity for oxygen reduction reaction (ORR) in terms of both onset potential and limiting current density can be achieved for the catalyst annealed at an optimal temperature of 800 °C because of the high content of graphitic-N catalytic sites and a large specific surface area.

DOI： 10.1039/c4cp05995f

One-step facile synthesis of carbon-supported PdAu nanoparticles and their electrochemical property and stability

Hu X., Shi J., Zhang J., Tang W., Zhu H., Shen X., Saito N.

Journal of Alloys and Compounds Vol. 619 page： 452 - 457 2015.1

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Alloys and Compounds

Well-crystallized PdAu nanoparticles supported on Ketjen Black (KB) were successfully fabricated when both Pd and Au wires were served as the electrode pair by a solution plasma technique at atmospheric pressure. The synthesis of PdAu nanoparticles was almost simultaneous with their dispersion on KB. As-obtained PdAu nanoparticles were confirmed to be alloy by ultraviolet-visible spectrophotometer, transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDX) of each particle. PdAu nanoparticles with the average diameter of 2-5 nm were equably distributed on KB. PdAu nanoparticles showed good electrocatalytic activity both in acidic and alkaline solution corresponding to their obvious oxidation and reduction features. PdAu nanoparticles have improved electrochemical stability compared with the electrochemical properties of Pd and Au nanoparticles mixture after long time multi-scan cyclic voltammetry. Multi-scan cyclic voltammetry also presented the PdAu nanoparticles in alkaline solution have better stability than that in acidic solution. Thus as-obtained PdAu alloy nanoparticles would become a promising electrocatalysts for fuel cells or Li-air batteries. This novel process showed its potential applications in designing optimization of catalysts for fuel cells or Li-air batteries in future.

DOI： 10.1016/j.jallcom.2014.09.051

Enhancement of ORR catalytic activity by multiple heteroatom-doped carbon materials

Kim D., Li O., Saito N.

Physical Chemistry Chemical Physics Vol. 17 ( 1 ) page： 407 - 413 2015.1

Language：English Publishing type：Research paper (scientific journal) Publisher：Physical Chemistry Chemical Physics

Heteroatom-doped carbon matrices have been attracting significant attention due to their superior electrochemical stability, light weight and low cost. Hence, in this study, various types of heteroatom, including single dopants of N, B and P and multiple dopants of B-N and P-N with a carbon matrix were synthesized by an innovative method named the solution plasma process. The heteroatom was doped into the carbon matrix during the discharge process by continuous dissociation and recombination of precursors. The chemical bonding structure, ORR activity and electrochemical performance were compared in detail for each single dopant and multiple dopants. According to the Raman spectra, the carbon structures were deformed by the doped heteroatoms in the carbon matrix. In comparison with N-doped structures (NCNS), the ORR potential of PN-doped structures (PNCNS) was positively shifted from -0.27 V to -0.24 V. It was observed that doping with N decreased the bonding between P and C in the matrix. The multiple doping induced additional active sites for ORR which further enhanced ORR activity and stability. Therefore, PNCNS is a promising metal-free catalyst for ORR at the cathode in a fuel cell.

DOI： 10.1039/c4cp03868a

Communication-In situ formation of anticorrosive Mg(OH)<inf>2</inf>/Carbon composite film on magnesium alloy by ascorbic acid-assisted hydrothermal process

Ishizaki T., Kamiyama N., Yamamoto E., Kumagai S., Sudare T., Saito N.

Journal of the Electrochemical Society Vol. 162 ( 14 ) page： C741 - C743 2015

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of the Electrochemical Society

An Mg(OH)2/carbon composite film was prepared on magnesium alloy AZ31 by an ascorbic acid (AA)-assisted hydrothermal method. All films were composed of crystalline brucite type Mg(OH)2. Film thickness decreased with an increase in the amount of AA added. X-ray photoelectron spectroscopy studies revealed that a peak component related to sp2-hybridized carbons contributed to improve film conductivity. Film resistivity decreased with an increase in C content. The film obtained from the aqueous solution containing 10 g/L of AA showed the lowest corrosion current density of all samples.

DOI： 10.1149/2.0351514jes

Direct deposition of gold nanoparticles on cellulose fiber by solution plasma process

Nemoto S., Watthanphanit A., Saito N.

Materials Research Society Symposium Proceedings Vol. 1723 page： 1 - 7 2015

From Cyano-aromatic Molecules to Nitrogen-doped Carbons by Solution Plasma for the Oxygen Reduction Reaction in Alkaline Medium

Panomsuwan G., Saito N., Ishizaki T.

Materials Today: Proceedings Vol. 2 ( 8 ) page： 4302 - 4308 2015

Language：English Publishing type：Research paper (scientific journal) Publisher：Materials Today: Proceedings

Nitrogen-doped carbon nanoparticles (NCNPs) were synthesized by a solution plasma process followed by a thermal annealing at 900°C. Cyano-aromatic molecules, including cyanobenzene (C7H5N), 2-cyanopyridine (C6H4N2), and cyanopyrazine (C5H3N3), were used as single source precursors for the synthesis. The morphological features of NCNPs were governed by the aggregates and agglomerates of uniform nanosized particles (20-40nm). The structural analyses confirmed that NCNPs mainly exhibited a disordered amorphous carbon structure. The nitrogen doping contents determined from an X-ray photoelectron spectroscopy were found to be 0.58, 1.36, and 1.29 at% for NCNPs synthesized from cyanobenzene, 2-cyanopyridine, and cyanopyrazine, respectively. The improvements of oxygen reduction reaction activity and selectivity toward a four-electron pathway in an alkaline solution of NCNPs were correlated with the nitrogen doping content and nitrogen bonding state. The NCNPs also possessed excellent long-term durability and strong methanol tolerance in an alkaline solution compared to those of a commercial Pt/carbon catalyst.

DOI： 10.1016/j.matpr.2015.09.017

Highly durable silica coated Pt/Cs with different surfactant types for proton exchange membrane fuel cell applications

Yaowarat W., Li O., Saito N.

RSC Advances Vol. 5 ( 55 ) page： 44258 - 44262 2015

Language：English Publishing type：Research paper (scientific journal) Publisher：RSC Advances

Platinum supported on carbon Vulcan XC-72 (Pt/Cs) for application as a cathode in proton exchange membrane fuel cells (PEMFCs) was coated with silica layers by a sol-gel method with three types of surfactants with different charging properties. The three various types of surfactants (1) cationic surfactant (cetyltrimethylammonium bromide (CTAB)), (2) anionic surfactant (sodium dodecylbezenesulfonate (SDBS)), and (3) non-ionic surfactant (Pluronic 123 (P123)) were applied to prevent agglomeration of the Pt nanoparticles and prevent detachment of the Pt nanoparticles from the carbon supports during operation. The degree of improvement depended on the type of surfactant applied in the sol-gel method. The formation of silica layers by SDBS and P123 significantly improved the durability of the Pt/Cs catalysts under acid conditions. Silica coated Pt/Cs formed using SDBS and P123 showed improved durability after 500 cycles in a cyclic voltammetry test in 0.5 M sulfuric acid (H<inf>2</inf>SO<inf>4</inf>) by 27.3% and 22.7%, respectively.

DOI： 10.1039/c5ra03343h

In situ solution plasma synthesis of mesoporous nanocarbon-supported bimetallic nanoparticles

Kang J., Saito N.

RSC Advances Vol. 5 ( 37 ) page： 29131 - 29134 2015

Language：English Publishing type：Research paper (scientific journal) Publisher：RSC Advances

Herein, is reported a novel in situ one-step synthesis method of bimetallic nanoparticles (NPs) supported on carbon black, which can markedly prevent the aggregation and growth of NPs, resulting in a small particle size (average 5 nm), good dispersion, and high-electrocatalytic-activity. This journal is

DOI： 10.1039/c5ra04220h

Cellulose conversion to sugar alcohol by solution plasma processing

Watthanaphanit A., Lee H., Saito N.

Materials Research Society Symposium Proceedings Vol. 1745 page： 22 - 27 2015

Language：English Publishing type：Research paper (scientific journal) Publisher：Materials Research Society Symposium Proceedings

Cellulose was selectively converted to sugar alcohols (sorbitol and mannitol) over a supported-metal catalyst ruthenium on carbon (RuJC) by the application of plasma in cellulose aqueous suspension. Generally, conversion of cellulose to sugar alcohol should be done under H2 pressure and high temperature. The goal of using solution plasma process (SPP) in this study is to initiate "self-hydrogenation" by reactive hydrogen species generated from the plasma due to dissociation of water medium. The sugar alcohols were produced at room temperature and atmospheric pressure. Electrospray ionization mass spectrometric analysis indicates that the SPP is a potent tool to promote the conversion of cellulose to sugar alcohols.

DOI： 10.1557/opl.2015.787

A study on electron impact dissociative ionization of organosilicon precursors for plasma processing

Yun Y., Lee M., Inoue Y., Saito N., Takai O.

Journal of Nanoscience and Nanotechnology Vol. 14 ( 12 ) page： 9653 - 9656 2014.12

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Nanoscience and Nanotechnology

We discussed the electron impact dissociation behavior of the organosilicon molecules with methyl groups, based on the fragment pattern and molecular-orbital calculation of the bond dissociation energies for the molecules. From the calculation of bond dissociation energy of the organosilicon molecules, methyl groups, which bonded directly to the silicon atom, were found to have the weakest. Regarding the fragment patterns of the reactants investigated by a quadrupole mass spectrometer, the hexamethyldisiloxane was harder to dissociate than the trimethylmethoxysilane due to the strong Si-O bonding force, which also affected the dissociation in the plasma. From the above considerations, dissociation reactions by electron impact could be partly identified.

DOI： 10.1166/jnn.2014.10175

One-step facile synthesis of Pd nanoclusters supported on carbon and their electrochemical property Open Access

Shi J., Hu X., Zhang J., Tang W., Li H., Shen X., Saito N.

Progress in Natural Science: Materials International Vol. 24 ( 6 ) page： 593 - 598 2014.12

Language：English Publishing type：Research paper (scientific journal) Publisher：Progress in Natural Science: Materials International

Well-crystallized Pd nanoclusters supported on Ketjen Black (KB) were successfully fabricated when Pd wires were served as an electrode pair by a solution plasma technique at atmospheric pressure. The synthesis of Pd nanoclusters was almost simultaneous with their dispersion on KB. Pd nanoclusters with the average diameter of about 2. nm were equably distributed on KB, and showed good electrochemical property corresponding to their obvious characteristic peaks. Multi-scan cyclic voltammetry and chronoamperometry clarified that as-prepared Pd nanoclusters have better electrochemical stability in alkaline solution than that of in acidic solution. Thus as-obtained Pd nanoclusters would become a promising electrocatalyst for fuel cells or Li-air batteries.

DOI： 10.1016/j.pnsc.2014.10.011

In situ solution plasma synthesis of nitrogen-doped carbon nanoparticles as metal-free electrocatalysts for the oxygen reduction reaction

Panomsuwan G., Chiba S., Kaneko Y., Saito N., Ishizaki T.

Journal of Materials Chemistry A Vol. 2 ( 43 ) page： 18677 - 18686 2014.11

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Materials Chemistry A

We report the in situ synthesis of nitrogen-doped carbon nanoparticles (NCNPs) by a solution plasma process without the addition of metal catalysts. Organic liquid mixtures of benzene and pyrazine were used as the precursors for the synthesis. The nitrogen-doping content can be easily controlled by changing the amount of pyrazine in the precursor. The NCNPs synthesized from the solution plasma process exhibit a turbostratic structure with highly uniform nanoscale particles. The nitrogen atoms can be homogeneously incorporated into the entire carbon structure due to the in situ doping during the growth and formation of the carbon particles. The electrochemical activity toward the oxygen reduction reaction (ORR) of the NCNPs in an alkaline medium reveals the improvement in terms of both the onset potential and current density as the nitrogen-doping content increases. The enhanced ORR activity of the NCNPS is mainly attributed to the presence of pyridinic-N and graphitic-N bonding configurations. They also possess long-term durability and excellent tolerance to methanol crossover effects. The results obtained in this study have demonstrated that the solution plasma process has great potential for the synthesis of metal-free nitrogen-doped carbon electrocatalysts for the ORR. We expect that this approach can be extended to the synthesis of other heteroatom-doped carbonaceous materials for a broad range of research applications in energy conversion and storage.

DOI： 10.1039/c4ta03010a

Solution plasma synthesis of nitrogen-doped carbon nanoballs as effective metal-free electrocatalysts for oxygen reduction reaction

Panomsuwan G., Chiba S., Saito N., Ishizaki T.

Prehospital and Disaster Medicine Vol. 1641 ( 4 ) 2014.11

Language：English Publishing type：Research paper (scientific journal) Publisher：Prehospital and Disaster Medicine

Nitrogen-doped carbons nanoballs were synthesized from an organic liquid precursor (a mixture of benzene and pyrazine) by solution plasma process. After synthesis, they were further annealed at 700 and 900 °C under N2 atmosphere. The nitrogen-doped carbon nanoballs before and after thermal annealing process exhibit a similar morphological feature, and their diameters are in the range between 20 and 40 nm. With higher annealing temperature, the graphitization of the nitrogen-doped carbon nanoballs increases. For the electrocatalytic activity in an alkaline solution, the limiting current density and onset potential for the ORR activity can be significantly improved for the samples after thermal annealing at 900 °C. We anticipate that solution plasma process will be a viable alternative way for synthesizing heteroatom-doped carbon electrocatalysts for broad application in the field of fuel cells, metal-air batteries, and supercapacitors.

DOI： 10.1557/opl.2014.319

Influence of the discharge time of solution plasma process on the formation of gold nanoparticles in alginate matrix Open Access

Watthanaphanit A., Heo Y.K., Saito N.

Journal of the Taiwan Institute of Chemical Engineers Vol. 45 ( 6 ) page： 3099 - 3103 2014.11

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of the Taiwan Institute of Chemical Engineers

Gold nanoparticles (AuNPs) stabilized in alginate matrix were synthesized by applying the plasma into alginate aqueous solution containing gold(III) chloride trihydrate precursor. Effect of the solution plasma process (SPP) discharge time on the physical properties, including shape and size, and the optical absorption properties of the synthesized AuNPs were investigated by varying the discharge times to be 0, 5, 10, 20 and 30. min. Results reveal that the increase in the discharge time leads to a size reduction of the AuNPs. Also, the particles tend to be more spherical. Appearance of the obtained AuNPs-alginate suspensions along with the change in their UV-vis absorption spectra intensity after samples were left standing for several time intervals suggest that alginate solution itself can act both as a stabilizer and a reducing agent for the AuNPs formation while the application of plasma helps to improve the controllability of shape and size of the synthesized particles. Furthermore, the method is simple, reproducible, and does not need any manipulative skill. Since the reducing agent is not required, the method is suitable for utilizing in biomedical application.

DOI： 10.1016/j.jtice.2014.09.001

Effect of growth temperature on structural and morphological evolution of epitaxial SrTiO<inf>3</inf> thin films grown on LaAlO<inf>3</inf> (001) substrates by ion beam sputter deposition

Panomsuwan G., Takai O., Saito N.

Vacuum Vol. 109 page： 175 - 179 2014.11

Language：English Publishing type：Research paper (scientific journal) Publisher：Vacuum

A series of epitaxial SrTiO3 (STO) thin films were grown on (001)-oriented LaAlO3 (LAO) substrates by ion beam sputter deposition (IBSD) at various growth temperatures in the range of 350-800 °C. Our results have shown that the growth temperature plays an essential role in influencing the structural and morphological evolution of the STO thin films. X-ray structural analysis demonstrates that the STO thin films are grown under in-plane compressive strain due to the effect of lattice mismatch. The out-of-plane lattice constant is found to decrease with increasing growth temperature. Atomic force microscopy observation reveals that the growth mode of STO thin films transforms from a three-dimensional island mode to a two-dimensional layer-by-layer mode at a sufficiently high growth temperature of 750 °C. We believe that high-quality epitaxial STO thin films with atomically flat surface prepared by IBSD in this study would be beneficial for the fabrication of perovskite STO-based superlattices. © 2014 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.vacuum.2014.07.007

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