Noritaka Sakakibara

The dynamics of solvated electrons were visualized using absorption imaging with sub-picosecond time resolution based on a pump–probe measurement during the early stages of femtosecond laser-induced plasma generation in water. The solvated electrons were generated by the propagation of a femtosecond laser pump pulse. In the area with a pump laser intensity over 2 × 1013 W/cm2, where a high density of free electrons was produced, solvated electrons exhibited an additional rapid increase in optical density (OD) at 800 nm, 7–9 ps after the pump pulse excitation. In contrast, no two-step increase in OD was observed when probed at 400 nm, suggesting that the absorption coefficient of the solvated electrons rapidly changed around 800 nm after femtosecond laser excitation for a few picoseconds. This observation might indicate the structural and electronic modulation of solvated electrons owing to the high density of free electrons in water, accompanied by femtosecond-laser-induced plasma generation.

Visible-light-driven conversion of CO2 to useful products is a promising process for a more sustainable energy system in a circular economy. Whilst highly efficient, durable, and selective systems have been reported in organic media, the photocatalytic process performing efficiently in aqueous media is highly desirable for the purpose of its practical application. However, efficiency and selectivity for CO2 reduction versus proton reduction in aqueous solution have proven challenging to date. Herein, this study demonstrates the enablement of highly efficient and durable visible-light-driven photocatalytic CO2 reduction to formate in aqueous media by a hybrid photocatalyst consisting of a silver-loaded conjugated polymer and a binuclear Ru(II) complex. The hybrid photocatalyst exhibited high activity and durability for formate production, with an apparent quantum yield of 4.2% at 460 nm and a turnover number of 38,000 (based on the amount of binuclear complexes adsorbed), both of which are the highest values reported amongst hybrid photocatalysts in aqueous media. Even though the conjugated polymer retains residual amounts of palladium from synthesis (which is an active site for H2 production in aqueous media), the loading of Ag nanoparticles onto the conjugated polymer enhanced the activity and selectivity for photocatalytic CO2 reduction by suppressing H2 production.

This highly efficient hybrid photocatalytic system can quantitatively convert CO₂ to formate and produce 0.4 M of formate solution without additional concentration enrichment.

Recently, cryoplasma has been demonstrated to form reddish coloration on methanol- and water-containing ice surfaces below 150 K. This cryogenic-temperature-only reddish coloration could provide a clue for understanding the composition of the red organic materials observed in the outer solar system. In this study, ice with reddish coloration was analyzed by IR absorption spectroscopy (2500–1700 cm⁻¹) during cryoplasma irradiation at 85 K and subsequent increase of the temperature from 85 to 200 K without cryoplasma. The intensities of the IR peaks, differing from those of the raw materials, increased with the irradiation time during reddish coloration. When the reddish coloration disappeared at 120–150 K, the IR peak intensities at 2170 and 1790 cm⁻¹ decreased. These peaks are thought to originate from C- and/or N-containing triple bonds or cumulative double bonds and carbonyl groups. Therefore, the reddish materials are considered to contain these functional groups.

Photocatalytic CO₂ reduction is in high demand for sustainable energy management. Hybrid photocatalysts combining semiconductors with supramolecular photocatalysts represent a powerful strategy for constructing visible-light-driven CO₂ reduction systems with strong oxidation power. Here, we demonstrate the novel effects of plasma surface modification of graphitic carbon nitride (C₃N₄), which is an organic semiconductor, to achieve better affinity and electron transfer at the interface of a hybrid photocatalyst consisting of C₃N₄ and a Ru(II)-Ru(II) binuclear complex (RuRu'). This plasma treatment enabled the "surface-specific" introduction of oxygen functional groups via the formation of a carbon layer, which worked as active sites for adsorbing metal-complex molecules with methyl phosphonic-acid anchoring groups onto the plasma-modified surface of C₃N₄. Upon photocatalytic CO₂ reduction with the hybrid under visible-light irradiation, the plasma-surface-modified C₃N₄ with RuRu' enhanced the durability of HCOOH production by three times compared to that achieved when using a nonmodified system. The high selectivity of HCOOH production against byproduct evolution (H₂ and CO) was improved, and the turnover number of HCOOH production based on the RuRu' used reached 50 000, which is the highest among the metal-complex/semiconductor hybrid systems reported thus far. The improved activity is mainly attributed to the promotion of electron transfer from C₃N₄ to RuRu' under light irradiation via the accumulation of electrons trapped in deep defect sites on the plasma-modified surface of C₃N₄.

Ice-templating is a powerful technique that exploits self-organized ice structures formed during the freezing of colloidal suspensions as templates to fabricate microstructured materials. However, the active introduction of reactivity to the ice-templating process remains limited. In this study, a Au-TiO₂ composite film composed of gold and titanium oxide (TiO₂) was fabricated on a frozen aqueous colloidal suspension, using cryoplasma to introduce reactive species to the frozen suspension. Auric ions and TiO₂ nanoparticles were segregated on the ice body by one-directional freezing of the aqueous colloidal suspension from the lower side. Meanwhile, a cryoplasma, in which the gas temperature was controlled to be lower than the melting point of water, was irradiated onto the frozen aqueous colloidal suspension to facilitate the chemical reduction of gold ions and the formation of gold bridges between TiO₂ nanoparticles. The face of the fabricated Au-TiO₂ composite film that had been adjacent to the ice structure during its formation was shown to possess both greater roughness and higher photocatalytic activity for methylene blue degradation when the aqueous colloidal suspension was frozen at a higher temperature during the fabrication of the composite film. The temperature dependence of the roughness and photocatalytic activity indicates a potential of the combination of cryoplasma irradiation and ice-templating processes for structure-controlled fabrication of composite film materials, suggesting that alterations in the structure of the freezing ice front that occur with changes in temperature can be harnessed.

In this study, soft X-ray emission spectroscopy of an aqueous colloidal dispersion of multi-walled carbon nanotubes modified via the plasma process in an aqueous solution was performed for investigating the electronic state of water molecules on the colloidal particles. In the aqueous dispersion, reconstruction of the hydrogen-bonded network was implied by the O 1s spectral changes in the 1b₁′ and 1b₁′′ peaks. Furthermore, the O 1s spectral intensity around the 3a₁ state was enhanced to an unusually broad energy range in comparison with previous studies. This unusual spectral change might be attributed to the hybridization of the electronic states of oxygen-containing functional groups on the surface of the plasma-modified multi-walled carbon nanotubes and that of the surrounding water molecules. Our observation indicates not only reconstruction of the hydrogen-bonded network in the aqueous dispersion but also a significant interaction of the electronic states between the water molecules and the plasma-modified multi-walled carbon nanotubes.

We studied the dynamics of solvated electrons in the early stage of plasma generation in water induced with an intense femtosecond laser pulse. According to the decay kinetics of solvated electrons, a fast recombination process of solvated electrons (geminate recombination) occurred with a more prolonged lifetime (500 ps to 1 ns) than that observed in previous pulse photolysis studies (10–100 ps). This unusually longer lifetime is attributed to additional production of solvated electrons due to abundant free electrons generated with the laser-induced plasma, implying significant influence of free electrons on the dynamics of solvated electrons.