Wooyul Kim

DNA damage and mitochondria dysfunction in cell apoptosis induced by nonthermal air plasma

Nonthermal plasma is known to induce animal cell death but the mechanism is not yet clear. Here, cellular and biochemical regulation of cell apoptosis is demonstrated for plasma treated cells. Surface type nonthermal air plasma triggered apoptosis of B16F10 mouse melanoma cancer cells causing DNA damage and mitochondria dysfunction. Plasma treatment activated caspase-3, apoptosis executioner. The plasma treated cells also accumulated gamma-H2A.X, marker for DNA double strand breaks, and p53 tumor suppressor gene as a response to DNA damage. Interestingly, cytochrome C was released from mitochondria and its membrane potential was changed significantly.

Selective Oxidative Degradation of Organic Pollutants by Singlet Oxygen-Mediated Photosensitization: Tin Porphyrin versus C60 Aminofullerene Systems

This study evaluates the potential application of tin porphyrin- and C(60) aminofullerene-derivatized silica (SnP/silica and aminoC(60)/silica) as (1)O(2) generating systems for photochemical degradation of organic pollutants. Photosensitized (1)O(2) production with SnP/silica, which was faster than with aminoC(60)/silica, effectively oxidized a variety of pharmaceuticals. Significant degradation of pharmaceuticals in the presence of the 400-nm UV cutoff filter corroborated visible light activation of both photosensitizers. Whereas the efficacy of aminoC(60)/silica for (1)O(2) production drastically decreased under irradiation with λ > 550 nm, Q-band absorption caused negligible loss of the photosensitizing activity of SnP/silica in the long wavelength region. Faster destruction of phenolates by SnP/silica and aminoC(60)/silica under alkaline pH conditions further implicated (1)O(2) involvement in the oxidative degradation. Direct charge transfer mediated by SnP, which was inferred from nanosecond laser flash photolysis, induced significant degradation of neutral phenols under high power light irradiation. Self-sensitized destruction caused gradual activity loss of SnP/silica in reuse tests unlike aminoC(60)/silica. The kinetic comparison of SnP/silica and TiO(2) photocatalyst in real wastewater effluents showed that photosensitized singlet oxygenation of pharmaceuticals was still efficiently achieved in the presence of background organic matters, while significant interference was observed for photocatalyzed oxidation involving non-selective OH radical.

Tin-porphyrin sensitized TiO2 for the production of H2 under visible light

Hydrogen was successfully produced under visible light irradiation in a tin porphyrin (SnP)-sensitized TiO2 system in the wide pH range (pH 3–11) although SnP hardly adsorbs on TiO2. The number of H2 produced in the SnP/TiO2 system after 9 h irradiation corresponds to the turnover number of 410. The apparent photonic efficiency for H2 evolution was estimated to be 35% with the monochromatic radiation of 550 ± 10 nm. The photochemical production of hydrogen is mediated through the formation of the π-radical anion (SnP˙−) that subsequently transfers electron to TiO2. The photogenerated SnP˙− was monitored by transient absorption spectroscopy and its lifetime is long enough to survive the slow diffusion from the solution bulk to the TiO2 surface, which makes the adsorption of SnP on TiO2 not required for hydrogen production. This is clearly contrasted with the common ruthenium complex-sensitized TiO2 system where the adsorption of the sensitizer complex is essentially required and the hydrogen production is limited to the acidic condition where the adsorption of the sensitizers on TiO2 is allowed. The photocatalytic activity of SnP was mainly attributed to the Q-band (500–650 nm), not to the Soret band (420–430 nm) of which absorption intensity is much higher.

Solar production of H2O2 on reduced graphene oxide–TiO2 hybrid photocatalysts consisting of earth-abundant elements only

A superior cocatalytic behavior of reduced graphene oxide (rGO) was observed for the photocatalytic production of H2O2 in the TiO2-based system. The adsorption of phosphate on TiO2 enhanced the production of H2O2 up to a millimolar level. The in situ formation of cobalt phosphate on rGO/TiO2 enabled the photocatalytic production of H2O2 even in the absence of organic electron donors.

N-doped TiO2 nanotubes coated with a thin TaOxNy layer for photoelectrochemical water splitting: Dual bulk and surface modification of photoanodes

TaON is a good photoanode material with a suitable band structure for water splitting as well as coupling with TiO2 for efficient charge separation. However, the synthesis of TaON that requires high temperature nitridation (850 °C) limits the combination with other materials. In this work, we deposited a thin amorphous TaOxNy layer on N-doped TiO2 nanotubes (N-TNTs) through low temperature nitridation (500 °C) and demonstrated its successful performance as an efficient photoanode for water-splitting. Since the preparation temperature is low, TaOxNy on N-TNTs has a unique amorphous structure with a smooth thin layer (5 nm). It is proposed that the thin amorphous TaOxNy layer plays dual roles: (i) surface sensitization and/or charge rectification at the heterojunction between the TaOxNy layer and N-TNTs, and (ii) passivation of N-TNT surface trap states to retard the charge recombination. TaOxNy layer-decorated N-TNTs as dual modified TNTs (N-doping in the bulk and TaOxNy overlayer deposition on the surface) have significantly improved both visible (ca. 3.6 times) and UV (ca. 1.8 times) activities for PEC water-splitting as well as the faradaic efficiency (ca. 1.4 times, η = 98%) for H2 production. Making the amorphous TaOxNy layer crystalline at higher temperatures reduced the PEC activity of the hybrid photoanode, in contrast, which indicates that the amorphous TaOxNy layer deposition on N-TNTs through low temperature nitridation (500 °C) is optimized for the PEC activity. A range of spectroscopic and electrochemical techniques were systematically employed to investigate the properties of the PEC process.

Nafion layer-enhanced photosynthetic conversion of CO2 into hydrocarbons on TiO2 nanoparticles

Introducing a thin Nafion layer on Pd-deposited TiO2 nanoparticles markedly enhances the photosynthetic conversion of CO2 to hydrocarbons (mainly methane and ethane) in an aqueous suspension (without any sacrificial electron donor) under UV and solar irradiation conditions.

Role of Interparticle Charge Transfers in Agglomerated Photocatalyst Nanoparticles: Demonstration in Aqueous Suspension of Dye-Sensitized TiO2

The interparticle charge transfer within the agglomerates of TiO2 nanoparticles in slurries markedly enhanced the dye-sensitized production of H2 under visible light. By purposely decoupling the light absorbing part of Dye/TiO2 from the active catalytic center of Pt/TiO2, the role of bare TiO2 nanoparticles working as a mediator that connects the above two parts in the agglomerates was investigated systematically. The presence of mediator in the agglomerate facilitated the charge separation and the electron transfer from Dye/TiO2 to Pt/TiO2 through multiple grain boundaries and subsequently produced more hydrogen. The dye-sensitized reduction of Cr(VI) to Cr(III) was also enhanced when Dye/TiO2 nanoparticles were agglomerated with bare TiO2 nanoparticles. The charge recombination between the oxidized dye and the injected electron was retarded in the presence of bare TiO2 nanoparticles, and this retarded recombination on Dye/TiO2 was confirmed by using transient laser spectroscopy. This phenomenon can be rationalized in terms of an interparticle Fermi level gradient within the agglomerates, which drives the charge separation.

Promoting water photooxidation on transparent WO3 thin films using an alumina overlayer

Tungsten trioxide (WO3) is being investigated as one of the most promising materials for water oxidation using solar light. Its inherent surface-related drawbacks (e.g., fast charge recombination caused by surface defect sites, the formation of surface peroxo-species, etc.) are nowadays being progressively overcome by different methods, such as surface passivation and the deposition of co-catalysts. Among them, the role of surface passivation is still poorly understood. Herein, transparent WO3 (electrodeposited) and Al2O3/WO3 (prepared by atomic layer deposition, ALD) thin film electrodes were employed to investigate the role of an alumina overlayer by using both photoelectrochemical and laser flash photolysis measurements. Films with a 5 nm-alumina overlayer (30 ALD cycles) showed an optimum photoelectrochemical performance, portraying a 3-fold photocurrent and Faradaic efficiency enhancement under voltage biases. Moreover, IPCE measurements revealed that alumina effect was only significant with an applied potential ca. 1 V (vs. Ag/AgCl), matching the thermodynamic potential for water oxidation at pH 1 (0.97 V vs. Ag/AgCl). According to the investigation of electron accumulation through optical absorption measurements, the alumina overlayer dominantly decreased the number of electron trapping sites on the WO3 surface, eventually facilitating photoelectron transfer to the external circuit in the presence of a positive bias. In addition, the laser flash photolysis measurements of WO3 and Al2O3/WO3 thin films clearly showed that the electron trapping decreased in the presence of the alumina overlayer whereas the hole trapping relatively increased with alumina, facilitating water photooxidation and rendering a more sluggish recombination process. These results provide a physical insight into the passivation process that could be used as a guideline for further development of efficient photoanodes in artificial photosynthesis.

TiO2 nanodisks designed for Li-ion batteries: a novel strategy for obtaining an ultrathin and high surface area anode material at the ice interface

A rapid and relatively large-scale production of ultrathin TiO2 nanodisks was achieved under mild conditions by developing a novel and simple sol–gel process occurring at the interface of an organic solvent and ice. Owing to the ultrathin structure and unusually high surface area (>400 m2 g−1), the TiO2 nanodisks exhibited high reversible capacity (191.4 mA h g−1 at 0.2 C) and excellent rate performance (58% capacity retention at 20 C) as an anode in lithium ion batteries.

Molecular‐Level Understanding of the Photocatalytic Activity Difference between Anatase and Rutile Nanoparticles

The generation of oxidants on illuminated photocatalysts and their participation in subsequent reactions are the main basis of the widely investigated photocatalytic processes for environmental remediation and selective oxidation. Here, the generation and the subsequent diffusion of (·)OH from the illuminated TiO2 surface to the solution bulk were directly observed using a single-molecule detection method and this molecular phenomenon could explain the different macroscopic behavior of anatase and rutile in photocatalysis. The mobile (·)OH is generated on anatase but not on rutile. Therefore, the photocatalytic oxidation on rutile is limited to adsorbed substrates whereas that on anatase is more facile and versatile owing to the presence of mobile (·)OH. The ability of anatase to generate mobile (·)OH is proposed as a previously unrecognized key factor that explains the common observations that anatase has higher activity than rutile for many photooxidative reactions.