Soonhyun Kim

Effects of surface fluorination of TiO2 on the photocatalytic degradation of tetramethylammonium

Photocatalytic degradation (PCD) of tetramethylammonium (TMA, (CH3)4N+) ions in water was studied using both naked-TiO2 and fluorinated-TiO2 (F–TiO2) in order to investigate how the modification in TiO2 surface functional groups affects the PCD reaction. A comparison between the naked-TiO2 and F–TiO2 systems shows that their relative photoreactivities strongly depend on pH. At pH 3, the addition of fluoride decreases the PCD rate whereas higher degradation rates are obtained at pH 5 and 7 with F–TiO2. Little fluoride effect on the TMA degradation rate is observed at pH 9. The addition of fluoride affects not only the PCD rate but also the mechanistic pathways of TMA degradation and subsequently the intermediates and product distribution. The modeling result of TiO2 surface speciation shows that the fluoride addition at pH 3 shifts the dominant surface species from Ti–OH2+ and Ti–OH to Ti–F (to near completion). This reduces the surface positive charge of TiO2 (at pH 3) upon adding fluoride and consequently lowers the electrostatic repulsion between the TMA cations and TiO2 surface. Accordingly, ATR-FTIR spectroscopic measurements show that the TMA concentration at the water/TiO2 interface is higher on F–TiO2 than naked-TiO2 film at pH 3. However, the PCD of TMA on F–TiO2 at pH 3 is reduced on the contrary, which is ascribed to the depletion of surface OH groups that are the site of surface OH radical formation. At pH 5 and 7, the surface OH sites are not completely diminished even in the presence of fluoride and the presence of surface Ti–F species in fact increases the TMA degradation rate. The fluoride-induced enhancement of PCD is yet to be understood although some speculative arguments are presented.

Comparative Study of Homogeneous and Heterogeneous Photocatalytic Redox Reactions: PW12O40 3- vs TiO2

Polyoxometalates (POMs) as a homogeneous photocatalyst and semiconductor oxide as a heterogeneous photocatalyst share many aspects of similarity in their operating mechanisms. This study systematically compares various photocatalytic oxidation and reduction reactions of PW12O403- (a POM) and TiO2 in water to demonstrate that the two photocatalysts are very different in many ways. Both POM and TiO2 can photooxidize various organic compounds with comparable rates, but the POM-mediated mineralization is markedly slower than the mineralization with TiO2 under the experimental conditions employed in this study. Kinetic studies using tert-butyl alcohol as an OH radical scavenger suggest that OH radicals are the sole dominant photooxidant in POM-mediated degradations regardless of the kind of substrates tested, whereas both OH radicals and direct hole transfers take part in TiO2 photocatalysis. POM immobilization on silica support and surface fluorination of TiO2 significantly modified the kinetics and intermediate distribution. POM-mediated photoreductive dechlorination of CCl4 and trichloroacetate was negligible, whereas the dechlorination with TiO2 was markedly faster. The rate of electron transfer from POM- to reducible substrates seems to be significantly slower than the rate of conduction band electron transfer on TiO2 mainly due to the strong electron affinity of POM. The effects of H2O2 addition on photocatalytic reactivity are also very different between POM and TiO2. Detailed kinetic and mechanistic comparisons between PW12O403- and TiO2 photocatalysts are presented and discussed to understand the similarities and differences.

Surface roughness and strain effects on ZnO nanorod growth

Vertically aligned ZnO nanorods were fabricated on Al2O3(001) substrates with various GaN interlayers by a catalyst-free metal-organic chemical vapor deposition. We observed that the shape and quality of ZnO nanorods grown on the GaN interlayers were considerably sensitive to the surface roughness of the interlayers. We also investigated orientation-dependent residual strain in the ZnO nanorods grown on Al2O3 substrates using polarized x-ray absorption fine structure (XAFS) measurements at Zn K edge. The XAFS revealed that the residual strain relaxation of Zn–O pairs in ab plane played a key role in the ZnO nanorod growth.

Titania nanofibers as a photo-antenna for dye-sensitized solar hydrogen

Directionally grown TiO(2) nano-architectures can serve as effective platforms for photogenerated charges to flow vectorially through the architecture framework, promising an unexpectedly high efficiency. This study demonstrates that directionally aligned TiO(2) nanofibers (TNF) obtained via a simple rearrangement of randomly scattered TiO(2) nanoparticles (TNP) exhibit significantly enhanced activity in terms of hydrogen production from water under visible light (λ > 420 nm). It has been found that Eosin Y (EY)-sensitized hydrogen production with TNF is greater than those with TNP and commercial TiO(2) samples (Degussa P25 and Hombikat UV-100) by a factor of 7 and >140, respectively, in the presence of triethanolamine (TEOA) as an electron donor. The annealing of TNF at elevated temperatures reduces the amount of H(2) produced and changes various physicochemical properties. Attempts have been made to find correlation factors between hydrogen production and reaction parameters (e.g., pH-dependent EY adsorption, surface area, pore size, particle size, and anatase-to-rutile ratio), none of which have provided an apparent correlation. It was suggested that the interparticle electron transfer is facilitated when TiO(2) nanoparticles are physically interconnected, and TNF might work as a robust photo-antenna for efficiently collecting the photogenerated electrons. The photocurrent measurements in visible light-irradiated EY/TiO(2) suspensions indicate that the photocurrent of TNF is 50% higher than that of TNP, supporting the photo-antenna mechanism of TNF.

Synthesis and characterization of platinum modified TiO2-embedded carbon nanofibers for solar hydrogen generation

Photocatalytic water splitting is a significant and promising technology to generate hydrogen, an alternative and clean future fuel. The present work describes the preparation and application of a TiO2 based photocatalyst with an effective architecture focused on the key issues of photocatalyst reusability and photocatalytically stable activity. TiO2 nanoparticles were embedded onto carbon nanofibers (CNF) acting as a support. The electrospinning method was adopted for fabrication of TiO2 embedded carbon nanofibers (TiO2/CNF). The photocatalytic activity of the TiO2/CNF under simulated light was improved by Pt co-catalyst photodeposition. The morphological and structural properties of TiO2/CNF and Pt photodeposited TiO2/CNF (Pt–TiO2/CNF) were investigated by field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM) and powder X-ray diffraction (XRD) techniques. The surface characteristics of these materials were investigated by X-ray photoelectron spectroscopy (XPS), photocurrent and photoluminescence (PL) measurements were also used to elaborate the effect of Pt photodeposition. The reusability and photocatalytic stability of Pt–TiO2/CNF was tested by the generation of approximately the same amounts of photocatalytic hydrogen with the same sample under the same conditions of illumination. The highest production rate of photocatalytic hydrogen generation achieved under simulated light was 3.5 μmol (after 3 hours of illumination with 0.02 g of photocatalyst). These tests show that Pt–TiO2/CNF is a reusable photocatalyt with a promising stable photocatalytic performance for solar energy conversion applications.

Photocatalytic reactivity and diffusing OH radicals in the reaction medium containing TiO2 particles

The generation of OH radicals on UV-illuminated TiO2 surface is mainly responsible for the photocatalytic oxidation of pollutants in various contaminated environmental media. Although the reactivity of OH radicals is largely limited within the surface region, the possibility of OH desorption and diffusion into the reaction medium has been often raised. This study provides several examples for the presence of diffusing OH radicals in aqueous solution and polymer matrix containing TiO2 particles. The photocatalytic degradation rates of (CH3)4N+ in TiO2 suspension were comparable between acidic and alkaline conditions, which could not be explained by a simple electrostatic surface charge model. From the present mechanistic study, it is suggested that the photocatalytic oxidation of (CH3)4N+ at acidic pH mainly proceeds through free OH radicals in the solution bulk, not on the surface of TiO2. The diffusing OH radicals also played the role of main oxidants in the solid phase. The photolysis of TiO2-embedded PVC composite films generated cavities around the imbedded TiO2 particles and the development of cavity diameter continued even after the direct contact between the PVC and TiO2 was prohibited. This implied that active oxygen species that were photogenerated on TiO2 surface desorbed and diffused across a few micrometers to react with the polymer matrix.

Metal nanoparticles decorated PET/PET-TiO2 bi-component filaments by photocatalytic deposition

Gold, silver, and platinum nanoparticles were immobilized on the surface of TiO2 in the sheath part of bi-component filaments. The processes involved include the spinning process used to prepare polyethylene terephthalate (PET)/PET-TiO2 bi-component filaments and the photocatalytic deposition process of gold, silver and platinum nanoparticles. The core part and the sheath part consist of virgin PET and 4 wt.% of TiO2 compounded PET, respectively. The sheath: core ratio of the filament was 50:50. For the photo-deposition of metal nanoparticles, adsorption of the metal ions on the surface of the fabrics was performed by immersing them in AgNO3, HAuCl4, and H2PtCl6 aqueous solutions, with simultaneous addition of methanol as a sacrificial agent. Photo-deposition was then carried out under UV light with an irradiation time of 60 seconds. The structural and antibacterial properties of the bi-component filaments were characterized. The nano-sized noble metal particles in a polka dot form were observed around the surface of the TiO2 particles in sheath region of bi-component filaments after photocatalytic deposition. Ag, Au, and Pt metal photo-deposited fabrics showed excellent antimicrobial effect against the two types of bacteria Staphylococcus aureus and Klebsiella pneumoniae under dark conditions.

Optical properties of hierarchical-nanostructured TiO2 and its time-dependent photo-degradation of gaseous acetaldehyde

The TiO2 hierarchical nanostructures (HNs) composed of rutile TiO2 nanowires on anatase TiO2 nanofibers had higher photocatalytic activities of 62% and 48% than the commercial TiO2 nanoparticles (∼21 nm diameter) in the continuous flow mode and closed-circulation mode, respectively, leading to an efficient degradation of gaseous acetaldehyde under UV-light irradiation. This behavior may be attributed to the effective TiO2 HNs with specific surface area of 85.1 m2/g and lower radiative recombination of self-trapped excitons, enabling an effective electron-hole separation.

Photocatalytic enhancement of cesium removal by Prussian blue-deposited TiO2

After the Fukushima nuclear accident, tremendous efforts were made to treat radiocesium, radiostrontium, and other radioactive materials. For the first time, we demonstrate that a TiO2 photocatalyst can significantly enhance Cs adsorption by Prussian blue-deposited TiO2 (PB/TiO2) under UV irradiation. In this study, we synthesized PB/TiO2 using the photodeposition method. After the Cs ions were adsorbed on the PB/TiO2 in darkness, we then exposed the PB/TiO2 to UV light irradiation. This resulted in a further increase in Cs ion adsorption of more than 10 times the amount adsorbed in darkness. This photocatalytic-enhanced adsorption of Cs ions was not observed on PB mixed with SiO2, nor under visible light irradiation. We investigated the effects of PB concentration, PB/TiO2 concentration, and gas purging on both dark and photocatalytic-enhanced adsorption of Cs ions by PB/TiO2. Based on the results, we suggest that the photocatalytic-enhanced adsorption of Cs ions on PB/TiO2 is due to photocatalytic reduction of PB, which leads to additional adsorption of Cs ions. The change in solution color before and after the reaction, and the change in solution pH in the dark and during UV irradiation strongly support this suggestion. The photocatalytic-enhanced adsorption of Cs ions was equivalent during radioactive 137Cs removal, indicating important applications for pollutant removal from contaminated water.

Anti-bacterial activity of tetragonal and cross-pillar-shaped polyester/TiO2 filaments with photo-deposited silver and platinum nanoparticles

In this work, we tried to enhance anti-bacterial activity of fabric fibers by controlling the shape of filaments, such as tetragonal and cross-pillar, which led to an increase of the surface portion of filaments. With this purpose, silver and platinum nanoparticles were immobilized on the surface of TiO2 nanoparticles within tetragonal and cross-pillar-shaped polyester (PET) filaments. The process consists of preparing 4 wt.% of TiO2 compounding PET chips, melt-spinning of them and photo catalytic deposition of nanoparticles in sequence. To obtain tetragonal and cross-pillar-shaped morphologies of filaments, two different nozzles were used in the melt-spinning process. For the photo-deposition of metal nanoparticles, adsorption of the metal ions on the surface of the filaments was performed by immersing them in AgNO3 and H2PtCl6 aqueous solutions, respectively, with simultaneous addition of methanol as a sacrificial agent. Photo-deposition was then carried out under ultraviolet light with an irradiation time of 300 s. The structural and antimicrobial properties of the tetragonal and cross-pillar-shaped PET/TiO2 filaments with noble metal loaded were systematically characterized. Ag and Pt metal photo-deposited filaments showed excellent antimicrobial effect against the two types of bacteria, Staphylococcus aureus and Klebsiella pneumonia, under the dark condition.