Showing papers by "Wonyong Choi published in 2013"

Surface modification of TiO2 photocatalyst for environmental applications

Abstract: This paper reviews recent studies on the semiconductor photocatalysis based on surface-modified TiO2 of which application is mainly focused on environmental remediation. TiO2 photocatalysis that is based on the photoinduced interfacial charge transfer has been extensively studied over the past four decades. A great number of modification methods of semiconductor photocatalysts have been developed and investigated to accelerate the photoconversion, to enable the absorption of visible light, or to alter the reaction mechanism to control the products and intermediates. In this regard, various modification methods of TiO2 are classified according to the kind of surface modifiers (metal-loading, impurity doping, inorganic adsorbates, polymer coating, dye-sensitization, charge transfer complexation) and their effects on photocatalytic reaction mechanism and kinetics are discussed in detail. Modifying TiO2 in various ways not only changes the mechanism and kinetics under UV irradiation but also introduces visible light activity that is absent with pure TiO2. Each modification method influences the photocatalytic activity and mechanism in a way different from others and the observed modification effects are often different depending on the test substrates and conditions even for the same modification method. Better understanding of the modification effects on TiO2 photocatalysis is necessary to obtain reliable results, to assess the photoconversion efficiency more quantitatively, and to further improve the modification methods.

Strategic Modification of BiVO4 for Improving Photoelectrochemical Water Oxidation Performance

Abstract: To improve the photoelectrochemical (PEC) performance of BiVO4, three different modifications (doping, heterojunction, and catalyst deposition) using earth-abundant elements are performed and their effects are compared in a 0.1 M phosphate electrolyte at pH 7 under AM1.5 light (100 mW/cm2). When a hexavalent element (Cr6+, W6+, or Mo6+) is doped at various levels, the Mo6+-doping effect is most significant at 10 atomic % with about two times higher photocurrent generation at the oxygen evolution potential (1.23 VRHE). Such enhancement is attributed to a decrease in charge transfer resistance (Rct) by donor doping, resulting in an approximate 2-fold increase in charge separation efficiency (ηsep) to about 25%. W6+ is less effective than Mo6+, whereas Cr6+ has a detrimental effect. To further improve the charge separation efficiency of Mo6+-doped BiVO4 (Mo-BiVO4), a approximate 600 nm thick WO3 layer is deposited under a similarly thick Mo-BiVO4 layer. This binary heterojunction (WO3/Mo-BiVO4) exhibits ηsep...

Solar water oxidation using nickel-borate coupled BiVO4 photoelectrodes

Abstract: A naturally abundant nickel-borate (Ni–Bi) complex is demonstrated to successfully catalyze the photoelectrochemical (PEC) water oxidation of BiVO4 electrodes at 1.23 VRHE with nearly 100% faradaic efficiency for oxygen evolution. Ni–Bi is electrodeposited (ED) and photodeposited (PD) for varying times on BiVO4 electrodes in the 0.1 M borate electrolyte with 1 mM Ni2+ at pH 9.2. Surprisingly, optimally deposited Ni–Bi films (ED-10 s and PD-30 min) display the same layer thickness of ca. 40 nm. Both Ni–Bi films enhance the photocurrent generation of BiVO4 at 1.23 VRHE by a factor of 3–4 under AM 1.5-light irradiation (100 mW cm−2) along with ca. 250% increase in the incident and absorbed photon-to-current efficiencies. Impedance analysis further reveals that the charge transfer resistance at BiVO4 is markedly decreased by Ni–Bi deposits. The primary role of Ni–Bi has been suggested to be a hole-conductor making photogenerated electrons more mobile and catalyzing a four-hole transfer to water through cyclic changes between the lower and higher Ni oxidation states. However, thick Ni–Bi films (>∼40 nm) significantly reduce the PEC performance of BiVO4 due to the kinetic bottleneck and charge recombination. Under identical PEC conditions (0.1 M, pH 9.2), the borate electrolyte (good proton acceptor) is found to be better than nitrate (poor proton acceptor), indicative of a proton-coupled electron transfer pathway in PEC water oxidation.

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

Abstract: 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

Abstract: 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.

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

Abstract: 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 r...

Photooxidation of arsenite under 254 nm irradiation with a quantum yield higher than unity.

Abstract: Arsenite (As(III)) in water was demonstrated to be efficiently oxidized to arsenate (As(V)) under 254 nm UV irradiation without needing any chemical reagents. Although the molar absorption coefficient of As(III) at 254 nm is very low (2.49 ± 0.1 M–1cm–1), the photooxidation proceeded with a quantum yield over 1.0, which implies a chain of propagating oxidation cycles. The rate of As(III) photooxidation was highly enhanced in the presence of dissolved oxygen, which can be ascribed to its dual role as an electron acceptor of photoexcited As(III) and a precursor of oxidizing radicals. The in situ production of H2O2 was observed during the photooxidation of As(III) and its subsequent photolysis under UV irradiation produced OH radicals. The addition of tert-butyl alcohol as OH radical scavenger significantly reduced (but not completely inhibited) the oxidation rate, which indicates that OH radicals as well as superoxide serve as an oxidant of As(III). Superoxide, H2O2, and OH radicals were all in situ generat...

Catalytic templating approaches for three-dimensional hollow carbon/graphene oxide nano-architectures

Abstract: We report a catalytic templating method to synthesize well-controlled three-dimensional carbon nano-architectures. Depending on graphene oxide content, the morphology can be systematically tuned from layered composites to 3D hollow structures to microporous materials. The composites with high surface area and high porosity induce a significant enhancement to its capacitance at high current density.

Ruthenium(II) complexes incorporating the bidentate ligand containing an imidazolium moiety: synthesis, characterization, and electrochemical properties and their application in a visible-light induced hydrogen-evolving system

Abstract: Two Ru(II) complexes, [(bpy)2Ru(IL-2)]3+ (bpy = 2,2′-bipyridine) (1) and [(phen)2Ru(IL-2)]3+ (phen = 1,10-phenanthroline) (2), have been prepared by using a newly designed bidentate ligand, 1,3-dimethyl-2,3-dihydro-1H-imidazo[4,5-f][1,10]phenanthroline-2-yl (IL-2). The complexes and the bidentate ligand synthesized have been fully characterized by 1H NMR, 13C NMR, high resolution mass spectrometry (HRMS), elemental analysis, and X-ray crystallography in two cases. All the complexes exhibited typical ligand-based π–π* and metal-to-ligand charge transfer (MLCT) bands in the UV and visible regions, respectively. The absorption maxima occurred at 430 and 428 nm with the lower energy shoulder peaks at 463 and 461 nm for 1 and 2, respectively. 3MLCT emissions were observed at 617 and 610 nm, respectively. The electrochemistry exhibited one reversible oxidation (RuIII/II), and two reversible and one irreversible reductions caused by ligands. A two-component photolysis system consisting of Ru(II) and Pd-precursor complexes has been confirmed to be active for the photo-induced hydrogen production in the presence of triethanolamine (TEOA) as a sacrificial electron donor without an electron mediator (MV2+).

Graphitic domain layered titania nanotube arrays for separation and shuttling of solar-driven electrons

Abstract: Herein, we report a facile method for synthesizing graphitic carbon domains of thin island shapes on the surfaces of titania nanotubes, which were prepared by using hydrothermal and pyrolytic treatments with glucose. The faster decay time of the solar-driven electrons and the lower charge transport resistance on carbon domains as compared to those in the case of bare titania nanotubes serve to increase the solar-to-hydrogen conversion rate.

Immobilization method of escherichia coli for microfluidic application

Abstract: Escherichia coli (E. coli) has been used extensively in various industrial and scientific applications but has not been used widely for microfluidic applications because it is difficult to immobilize properly to a device. Here, we describe the development of an E. coli immobilization method for microfluidic devices using single-walled nanotubes (SWNTs) and dielectrophoresis (DEP) force. SWNTs and E. coli were aligned between two cantilever electrodes by a positive DEP force, forming a film of SWNTs with attached E. coli. For a microfluidic device, our one-step immobilization method has many advantages such as site-specific immobilization, easy density and shape control, an electrically connected structure from the electrodes to the E. coli, and rapid processing.