Showing papers by "Wonyong Choi published in 2017"

Eco-Friendly Photochemical Production of H2O2 through O2 Reduction over Carbon Nitride Frameworks Incorporated with Multiple Heteroelements

Abstract: We report that in situ incorporation of both potassium and phosphate species into a polymeric carbon nitride (CN) framework highly enhanced the photoproduction of hydrogen peroxide (H2O2) without the use of any noble-metal cocatalysts. The incorporation of earth-abundant heteroelements (K, P, and O) (i) introduced the negative surface charge over the entire pH range through surface functionalization by phosphate species, (ii) increased the lifetime of the transient species to a picosecond time scale via the formation of charge separation states, (iii) facilitated the interfacial electron transfer to dioxygen, and (iv) inhibited the decomposition of in situ generated H2O2. As a result, the modified CN showed apparent quantum yields (Φ, for H2O2 production) that are enhanced by about 25 and 17 times (Φ420 = 8.0%; Φ320 = 26.2%) from those of bare CN (Φ420 = 0.32%; Φ320 = 1.55%) under monochromatic irradiation of 420 and 320 nm, respectively. This study clearly demonstrated a simple way to design multiple het...

Sequential Process Combination of Photocatalytic Oxidation and Dark Reduction for the Removal of Organic Pollutants and Cr(VI) using Ag/TiO2

Abstract: We investigated a sequential photocatalysis-dark reaction, wherein organic pollutants were degraded on Ag/TiO2 under UV irradiation and the dark reduction of hexavalent chromium (Cr(VI)) was subsequently followed. The photocatalytic oxidation of 4-chlorophenol (4-CP), a test organic substrate, induced the generation of degradation intermediates and the storage of electrons in Ag/TiO2 which were then utilized for reducing Cr(VI) in the postirradiation period. The dark reduction efficiency of Cr(VI) was much higher with Ag/TiO2 (87%), compared with bare TiO2 (27%) and Pt/TiO2 (22%). The Cr(VI) removal by Ag/TiO2 (87%) was contributed by adsorption (31%), chemical reduction by intermediates of 4-CP degradation (26%), and reduction by electrons stored in Ag (30%). When formic acid, humic acid or ethanol was used as an alternative organic substrate, the electron storage effect was also observed. The postirradiation removal of Cr(VI) on Ag/TiO2 continued for hours, which is consistent with the observation that ...

Photoelectrochemical Degradation of Organic Compounds Coupled with Molecular Hydrogen Generation Using Electrochromic TiO2 Nanotube Arrays

Abstract: Vertically aligned TiO2 nanotube arrays (TNTs) were prepared by electrochemical anodization, and then cathodically polarized with dark blue coloration for the dual-functional photoelectrochemical water treatment of organic substrates degradation and accompanying H2 generation. The resulting Blue-TNTs (inner diameter: ∼40 nm; length: ∼9 μm) showed negligible shift in X-ray diffraction pattern compared with the intact TNTs, but the X-ray photoelectron spectra indicated a partial reduction of Ti4+ to Ti3+ on the surface. The electrochemical analyses of Blue-TNTs revealed a marked enhancement in donor density and electrical conductivity by orders of magnitude. Degradations of test organic substrates on Blue-TNTs were compared with the intact TNTs in electrochemical (EC), photocatalytic (PC), and photoelectrochemical (PEC) conditions (potential bias: 1.64 VNHE; λ > 320 nm). The degradation of 4-chlorophenol was greatly enhanced on Blue-TNTs particularly in PEC condition, whereas the PC activities of the Blue- ...

Distorted Carbon Nitride Structure with Substituted Benzene Moieties for Enhanced Visible Light Photocatalytic Activities

Abstract: Carbon nitride (CN) is being intensively investigated as a low-cost visible light active photocatalyst, but its practical applications are limited because of the fast charge pair recombination and low visible light absorption. Here, we introduce a new strategy for enhancing its visible light photocatalytic activity by designing the CN structure in which the nitrogen of tertiary amine is substituted with a benzene molecule connected by three heptazine rings. The intramolecular benzene doping induced the structural changes from planar symmetric structure to distorted geometry, which could be predicted by density functional theory calculation. This structural distortion facilitated the spatial separation of photogenerated charge pairs and retarded charge recombination via exciton dissociation. Such unique properties of the benzene-incorporated CN were confirmed by the photoluminescence (PL) and photoelectrochemical analyses. The optimal loading of benzene doping reduced the PL of the conjugated ring system (...

Enhanced hydrogen production from ammonia borane using controlled plasmonic performance of Au nanoparticles deposited on TiO2

Abstract: Ammonia borane (AB) is a hydrogen storage material which can produce three equivalent moles of H2 gas by a hydrolytic decomposition reaction in the presence of an appropriate catalyst. The photocatalytic H2 generation from AB decomposition was studied by utilizing the Au/TiO2 plasmonic photocatalyst under both visible and UV light conditions. Different amounts of gold (0.5, 1, 2, and 3%) were photo-deposited on TiO2 (anatase) to prepare Au/TiO2 catalysts. Au(1 wt%)/TiO2 showed the highest activity of H2 generation under visible light and a further increase of Au loading reduced the activity. Although LSPR (localized surface plasmon resonance) excitation should increase with gold nanoparticle (Au NP) loading, excessive Au loading also facilitates the recombination of LSPR charge pairs. The time-resolved photoluminescence (TRPL) decay spectra of Au/TiO2 clearly showed that the recombination rate of LSPR charge pairs is accelerated at higher Au loading. As a result, the photocatalytic activity of Au/TiO2 was optimized at 1 wt% Au loading to produce 88 μmol of H2 in 4 h. However, such an optimized level of Au loading was not observed under UV light irradiation conditions and the UV photocatalytic production of H2 increased with the Au loading up to 3% because higher Au loading facilitates the separation of charge pairs and accumulation of electrons in Au NPs through the Schottky barrier. These two different electron transfer behaviors (accumulation and recombination) can be further confirmed by the Cr(VI) reduction experiments since they showed a similar trend of photocatalytic activity with Au loading. This research provides a fundamental understanding for optimizing the photocatalytic activity of various metal–metal oxide heterojunction photocatalysts.

Freestanding doubly open-ended TiO2 nanotubes for efficient photocatalytic degradation of volatile organic compounds

Abstract: We synthesized freestanding doubly open-ended TiO2 nanotubes (DNT) film and compared their photocatalytic activity and durability during the repeated degradation cycles of volatile organic compounds (VOCs) with those of TiO2 nanotubes (TNT) film. DNT exhibited higher activity and durability for the photocatalytic degradation of gaseous acetaldehyde and toluene than TNT. The doubly open-ended structure of DNT allows O2 molecules to be easily supplied to the active sites, which increases not only the intrinsic photocatalytic activity but also the resistance to catalyst deactivation. The freestanding DNT film was additionally loaded with TiO2 nanoparticles (NP@DNT) in the inner wall to further increase the activity for VOC degradation. The photocatalytic activity of NP@DNT was higher than bare DNT and bare TNT by 1.3 and 1.8 times, respectively. Unlike the case of DNT, the TiO2 nanoparticles loaded TNT (NP@TNT) exhibited a lower activity than bare TNT, probably because the TiO2 nanoparticles blocked the TNT channels with hindering the mass transfer of O2 and VOC molecules. DNT with doubly open-ended structure serves as a versatile platform of fabricating nanostructured photocatalysts with maintaining the open channel structure that facilitates the mass transfer of O2 and VOC molecules.

Iron Oxide Photoelectrode with Multidimensional Architecture for Highly Efficient Photoelectrochemical Water Splitting

Abstract: Nanostructured metal oxide semiconductors have shown outstanding performances in photoelectrochemical (PEC) water splitting, but limitations in light harvesting and charge collection have necessitated further advances in photoelectrode design. Herein, we propose anodized Fe foams (AFFs) with multidimensional nano/micro-architectures as a highly efficient photoelectrode for PEC water splitting. Fe foams fabricated by freeze-casting and sintering were electrochemically anodized and directly used as photoanodes. We verified the superiority of our design concept by achieving an unprecedented photocurrent density in PEC water splitting over 5 mA cm−2 before the dark current onset, which originated from the large surface area and low electrical resistance of the AFFs. A photocurrent of over 6.8 mA cm−2 and an accordingly high incident photon-to-current efficiency of over 50 % at 400 nm were achieved with incorporation of Co oxygen evolution catalysts. In addition, research opportunities for further advances by structual and compositional modifications are discussed, which can resolve the low fill factoring behavior and improve the overall performance.

Investigating the Unrevealed Photocatalytic Activity and Stability of Nanostructured Brookite TiO2 Film as an Environmental Photocatalyst

Abstract: Among three polymorphs of TiO2, the brookite is the least known phase in many aspects of its properties and photoactivities (especially comparable to anatase and rutile) because it is the rarest phase to be synthesized in the standard environment among the TiO2 polymorphs. In this study, we address the unrevealed photocatalytic properties of pure brookite TiO2 film as an environmental photocatalyst. Highly crystalline brookite nanostructures were synthesized on titanium foil using a well-designed hydrothermal reaction, without harmful precursors and selective etching of anatase, to afford pure brookite. The photocatalytic degradation of rhodamine B, tetramethylammonium chloride, and 4-chlorophenol on UV-illuminated pure brookite were investigated and compared with those on anatase and rutile TiO2. The present research explores the generation of OH radicals as main oxidants on brookite. In addition, tetramethylammonium, as a mobile OH radical indicator, was degraded over both pure anatase and brookite phas...

Synergistic combination of bandgap-modified carbon nitride and WO3 for visible light-induced oxidation of arsenite accelerated by in-situ Fenton reaction

Abstract: Photocatalytic oxidation of arsenite (As(III)) to arsenate (As(V)) using non-toxic semiconductor materials has been considered as an environmentally-benign pretreatment process of arsenic contaminated waters, but poor visible light activity hinders the practical applications utilizing solar light. In this study, we designed a ternary photocatalytic system consisting of modified carbon nitride (mCN), WO 3 , and Fe 3+ for efficient oxidation of As(III) which was done by using in-situ generated H 2 O 2 as a Fenton reagent under visible light (λ > 420 nm). While superoxide anion and H 2 O 2 were effectively produced via the reduction of dissolved O 2 by mCN, WO 3 regenerated Fe 2+ from Fe 3+ , which activated in-situ generated H 2 O 2 for Fenton process. The overall photocatalytic oxidation activity of As(III) was optimized at a specific mixing ratio of catalysts (mCN:WO 3 = 60:40) where there is an optimal balance between the conduction band electron transfer to dissolved O 2 (to produce H 2 O 2 on mCN) and the competing electron transfer to Fe 3+ (to regenerate Fe 2+ on WO 3 ). The ternary combination enabled the simultaneous participation of superoxide anions, hydroxyl radicals, and holes to complete the oxidation of 500 μM As(III) within 90 min with 1.0 g/L catalyst and 70 μM Fe 3+ and that of 10 μM As(III) within 60 min with a tenth amount of catalyst (0.1 g/L) and Fe 3+ (7 μM) under visible light without requiring noble metal catalysts and chemical additives. The process consists of earth abundant elements only (C, N, O, W, and Fe) and operates with utilizing visible light photons and dissolved O 2 only, which is eco-friendly and cost effective.

Dual Modification of Hematite Photoanode by Sn-doping and Nb2O5 Layer for Water Oxidation

Abstract: Porous hematite (α-Fe2O3) films doped with Sn(IV) and coated with an ultrathin (∼2 nm thick) Nb2O5 passivation layer were synthesized, and the photoelectrochemical (PEC) water oxidation performance and durability of the hematite were examined in detail. As compared to hematite samples modified by either doping or passivation, dual-modified hematite exhibited a promising PEC water oxidation performance under AM 1.5 irradiation. A stable photocurrent was maintained under prolonged irradiation over 24 h, while O2 was produced from water with a Faradaic efficiency of over 80% without showing any sign of deactivation. This performance and durability could be decoupled into separate effects of Sn doping and Nb2O5 layer via in-depth surface characterization and electrochemical analyses. Sn doping increased the donor density (Nd) of bare hematite by a factor of 20 and significantly improved its conductivity, leading to enhanced charge transfer efficiency. The Nb2O5 layer exerted an effect similar to Sn doping because of the diffusion of a fraction of Nb(V) into the hematite lattice during the annealing process at 700 °C. The primary effect of the Nb2O5 layer is to passivate the hematite surface and make the surface more reactive toward the oxygen evolution through water oxidation. These effects are synergistically combined in the dual-modified hematite electrode.

Vertically Aligned Core–Shell PbTiO3@TiO2 Heterojunction Nanotube Array for Photoelectrochemical and Photocatalytic Applications

Abstract: Vertically aligned core–shell PbTiO3@TiO2 heterojunction nanotube arrays are fabricated on F:SnO2 (FTO) glass substrate via a unique three-step process: an electron beam vapor deposition of Ti thin film on FTO, anodic oxidation of the Ti film to TiO2 nanotubes, and finally formation of PbTiO3 perovskite layer at the inner wall of the TiO2 nanotubes. The PbTiO3@TiO2 nanotube array exhibits dramatically improved photoactivity relative to TiO2 nanotubes or PbTiO3/TiO2 composite powders in photoelectrochemical water splitting and photocatalytic isopropyl alcohol decomposition under visible light irradiation (>420 nm). In the core–shell heterojunction electrodes, PbTiO3 serves as a visible light responsive inorganic photosensitizer with its small band gap and forms a heterojunction with TiO2 for effective charge separation.

Visible light sensitization of TiO2 nanoparticles by a dietary pigment, curcumin, for environmental photochemical transformations

Abstract: The use of curcumin, an active ingredient of turmeric powder (a dye component in curry), as a TiO2 photo-sensitizer was investigated in terms of the photochemical and photoelectrochemical (PEC) properties. Owing to its strong visible light absorption and strong surface complexation, the curcumin-sensitized TiO2 composite exhibited notable activities for the photochemical degradation of organic compounds, the reduction of chromate (Cr(VI)), and the generation of OH radicals and H2O2 through the reduction of O2 under visible light (λ > 420 nm). Various spectroscopic methods confirmed the anchoring of curcumin on TiO2 and the photochemical and PEC properties of curcumin/TiO2 were compared with those of TiO2 sensitized by a ruthenium complex (RuL3) that has been frequently employed as a visible light sensitizer. Curcumin/TiO2 exhibited consistently higher photochemical and PEC activities than RuL3/TiO2 over a wide pH range in an aquatic environment. These results confirm the practical viability of using a natural food dye, curcumin, as an efficient, eco-friendly, and cheap photo-sensitizer of TiO2 for solar environmental applications. However, it should be noted that curcumin on TiO2 like other dye sensitizers is degraded as a result of the sensitizing reactions in water and should be considered as a sensitizing reagent, not a photocatalyst.

Accelerated Reduction of Bromate in Frozen Solution.

Abstract: Bromate is a common disinfection byproduct formed during ozonation. Reducing bromate into bromide can remove this toxic pollutant, however, not many studies have been done for its environmental fate. In this work, we demonstrate a new transformation pathway that bromate can be efficiently reduced to bromide in frozen solution in the presence of organic reductants like humic substances (HS). The results showed that bromate in frozen solution could be removed by 30–40% in dark condition and 80–90% in irradiation condition (λ > 300 nm) in 24 h, while around 1% bromate was reduced in aqueous solution. The bromate reduction by HS induced a partial oxidation of HS, which was confirmed by X-ray photoelectron spectroscopic analysis of the HS sample recovered from the frozen solution. Photoluminescence analysis of HS revealed that the fluorescence quenching by bromate was observed only with very high concentration of bromate (0.1–0.2 M) in aqueous solution whereas the quenching effect in frozen solution was seen w...

Highly efficient hydrogen production using p-Si wire arrays and NiMoZn heterojunction photocathodes

Abstract: Highly efficient photoelectrochemical (PEC) hydrogen production is achieved using p-Si wire arrays loaded with NiMoZn particles in aqueous sulfuric acid under simulated sunlight (AM 1.5 G; 100 mW cm −2 ). Vertically-aligned wire arrays are grown on planar Si wafers via a quick electroless etching process within 5 min, leading to short Si wires of ∼4 μm and diameters of ∼0.2 μm. Despite the short length of the wires, the reflectance of the arrays is I ph ) is enhanced by ∼ 30% relative to planar Si. To further improve the PEC performance, ∼ 100 nm NiMoZn particles are photoelectrochemically deposited onto the wires. The wire arrays with evenly distributed NiMoZn particles show a photocurrent onset potential (E on ) of ∼ + 0.27 V vs. RHE and produce an I ph of ∼1.45 mA cm −2 at 0 V vs. RHE with a Faradaic efficiency of ∼ 100% for H 2 evolution. This I ph value is ∼10-fold greater than that with the planar Si/NiMoZn samples. The excellent performance of the wire arrays and NiMoZn heterojunction is attributed to enhanced light absorption (decreased reflectance), facilitated charge transfer (radial-directional electron transfer), and NiMoZn-catalyzed hydrogen production.

Estimation of greenhouse gas emissions from sewer pipeline system

Abstract: The aim of this study was to estimate the total greenhouse gas (GHG) emissions generated from whole life cycle stages of a sewer pipeline system and suggest the strategies to mitigate GHG emissions from the system. The process-based life cycle assessment (LCA) with a city-scale inventory database of a sewer pipeline system was conducted. The GHG emissions (direct, indirect, and embodied) generated from a sewer pipeline system in Daejeon Metropolitan City (DMC), South Korea, were estimated for a case study. The potential improvement actions which can mitigate GHG emissions were evaluated through a scenario analysis based on a sensitivity analysis. The amount of GHG emissions varied with the size (150, 300, 450, 700, and 900 mm) and materials (polyvinyl chloride (PVC), polyethylene (PE), concrete, and cast iron) of the pipeline. Pipes with smaller diameter emitted less GHG, and the concrete pipe generated lower amount of GHG than pipes made from other materials. The case study demonstrated that the operation (OP) stage (3.67 × 104 t CO2eq year−1, 64.9%) is the most significant for total GHG emissions (5.65 × 104 t CO2eq year−1) because a huge amount of CH4 (3.51 × 104 t CO2eq year−1) can be generated at the stage due to biofilm reaction in the inner surface of pipeline. Mitigation of CH4 emissions by reducing hydraulic retention time (HRT), optimizing surface area-to-volume (A/V) ratio of pipes, and lowering biofilm reaction during the OP stage could be effective ways to reduce total GHG emissions from the sewer pipeline system. For the rehabilitation of sewer pipeline system in DMC, the use of small diameter pipe, combination of pipe materials, and periodic maintenance activities are suggested as suitable strategies that could mitigate GHG emissions. This study demonstrated the usability and appropriateness of the process-based LCA providing effective GHG mitigation strategies at a city-scale sewer pipeline system. The results obtained from this study could be applied to the development of comprehensive models which can precisely estimate all GHG emissions generated from sewer pipeline and other urban environmental systems.