Tsutomu Hirakawa

Activation of graphitic carbon nitride (g-C3N4) by alkaline hydrothermal treatment for photocatalytic NO oxidation in gas phase

Photocatalytic activity of graphitic carbon nitride (g-C3N4) was significantly improved by an alkaline hydrothermal treatment. The specific surface area of g-C3N4 obtained by heating melamine at 550 °C was only 7.7 m2 g−1, which was too small for it to be utilized as a catalyst for air purification. By the hydrothermal treatment with NaOH solution at 90–150 °C, the surface area was increased up to 65 m2 g−1, and the oxidation rate of nitrogen oxide (NO) under visible light (380 < λ < 480 nm) was increased by 8.6 times. XRD, ESR, elemental analysis and electron microscopy showed that unstable domains of not-well-ordered carbon nitride were removed by hydrolysis to form a mesoporous structure with a higher surface area. Deactivation of g-C3N4 was not observed during the experimental period, although a small part of carbon nitride was decomposed by self-oxidation.

Removal of high concentration dimethyl methylphosphonate in the gas phase by repeated-batch reactions using TiO2

The aim of our study is to develop apparatuses that use TiO(2) for effective decontamination of air contaminated by Sarin gas. We performed photocatalytic decomposition of gaseous dimethyl methylphosphonate (DMMP) by TiO(2) and identified the oxidization products. The high activity of TiO(2) (0.01 g) was observed under UV-light irradiation and high concentration DMMP (33.5 microM) was removed rapidly. On the other hand, DMMP was not decreased under UV-light irradiation without TiO(2). This indicates that photocatalytic treatment is very effective for the removal of DMMP. Methanol, formaldehyde, formic acid, methyl formate, CO, CO(2) and H(2)O were detected as the primary products. In the gas phase, no highly poisonous substances were detected. In order to examine the performance of photocatalytic activity during long-term reactions, we performed photocatalytic decomposition by repeated-batch reactions using TiO(2). High photocatalytic activities decreased gradually. Meanwhile, the strong adsorption of TiO(2) against DMMP was observed as photocatalytic activities decreased. During the repeated-batch reactions with the sample scaled up (TiO(2): 0.1g), the total amount of removed DMMP reached 968.5 microM by both photocatalytic decomposition and the strong adsorption of TiO(2). These results suggest the possibility of removing large amounts of DMMP.

Fabrication of Inert Silver Nanoparticles with a Thin Silica Coating

We present a genuine experimental technique to fabricate silver nanoparticles with an ultrathin silica coating, thus making the nanoparticles chemically inert. The impact of the coating on plasmonic properties is experimentally quantified and compared with theoretical predictions. Furthermore, numerical simulations of the near-field enhancing properties of the nanoparticles are conducted. It is found that the coatings fabricated are sufficiently thin to make the plasmonic resonance wavelength shift negligible and for observing a significant field enhancement on the surface of the silica shell at the resonance wavelength. Application of such inert nanoparticles to sensitize the absorption of near-ultraviolet light is discussed.

Influence of Microbubbles on Free Radical Generation by Ultrasound in Aqueous Solution: Dependence of Ultrasound Frequency

The influence of microbubbles on sonochemical efficiencies has been investigated under 28, 45, and 100 kHz ultrasound irradiation. For the 28 and 100 kHz ultrasound frequencies, microbubbles suppressed the I3(-) formation from KI solution as well as the 7-hydroxycoumarin formation from coumarin solution caused by the ultrasonic irradiation. On the other hand, for the 45 kHz ultrasound frequency, microbubbles enhanced the I3(-) formation from KI solution as well as 7-hydroxycoumarin formation from coumarin solution caused by the ultrasonic irradiation. Detection of H2O2 after the irradiation of ultrasound in the presence or absence of microbubbles was also performed, and it was found that H2O2 formation was enhanced only when microbubbles were introduced under the 45 kHz ultrasonic irradiation, which was in good agreement with the results of KI oxidation dosimetry measurements and of coumarin fluorescent probe measurements. Based on these present results, plausible mechanisms that explain the dependence of the ultrasound frequency on the enhancement and suppression of free radical formation in the presence of MBs were proposed.

Solution conductivity as a key factor for thin silica coating on colloidal silver

In order to utilize silver nanoparticles for various applications such as photocatalysts while maintaining their chemical stability, enhanced electric fields on the nanoparticle surfaces induced by localized surface plasmon excitation must be used effectively. For satisfying these requirements, an ultrathin silica coating with a thickness of only a few nm was formed around silver nanoparticles by a chemical reaction in a solution, while parameters such as the amount of sodium silicate and the number of dialysis procedures performed were changed. As a result, it was found that a key factor for obtaining stable thin-silica-coated silver nanoparticles is the conductivity of the solution. Using a solution with an appropriate conductivity above 2.7 mS/m, silica films can be coated on silver nanoparticles without causing deterioration of the plasmonic activity resulting from the aggregation of silver.