Wan-Kuen Jo

Enhanced visible light-driven photocatalytic performance of ZnO–g-C3N4 coupled with graphene oxide as a novel ternary nanocomposite

This article reports a novel ternary nanocomposite consisting of ZnO, g-C3N4, and graphene oxide (GO) that provides enhanced photocatalytic performance and stability. The ZnO nanospheres disperse evenly and embed themselves in the porous g-C3N4. Composites with various g-C3N4 and GO to ZnO weight ratios were synthesized and characterized systematically. The results indicated that the absorption of binary g-C3N4/ZnO nanocomposites shifted to a lower energy compared to pure ZnO in a fashion consistent with the loading content of g-C3N4. Notably, the loading content of GO in the ZnO-g-C3N4 composite resulted in increased absorption in the visible range and improved charge separation efficiency, thereby drastically improving photocatalytic activity. Successful hybridization of ternary nanocomposite was confirmed by drastic quenching of fluorescence and broader visible light absorption. The optimal content of g-C3N4 in the ZnO-g-C3N4 composite was 50%, which exhibited the effective hybridization between ZnO and g-C3N4, and high photocatalytic efficiency. However, the photocatalytic degradation of the ternary nanocomposite showed performance that was two times greater than ZnO-g-C3N4, exhibiting 99.5% degradation efficiency after just 15 min of light irradiation. The combined heterojunction and synergistic effects of this composite account for the improved photocatalytic activity.

Facile Synthesis of Novel Redox-Mediator-free Direct Z-Scheme CaIn2S4 Marigold-Flower-like/TiO2 Photocatalysts with Superior Photocatalytic Efficiency

Novel redox-mediator-free direct Z-scheme CaIn2S4 marigold-flower-like/TiO2 (CIS/TNP) photocatalysts with different CaIn2S4 weight percentages were synthesized using a facile wet-impregnation method. Uniform hierarchical marigold-flower-like CaIn2S4 (CIS) microspheres were synthesized using a hydrothermal method. Field-emission scanning electron microscopy and transmission electron microscopy analyses suggested that the formation and aggregation of nanoparticles, followed by the growth of petals or sheets and their subsequent self-assembly, led to the formation of the uniform hierarchical marigold-flower-like CIS structures. The photocatalytic degradation efficiency of the direct Z-scheme CIS/TNP photocatalysts was evaluated through the degradation of the pharmaceutical compounds isoniazid (ISN) and metronidazole (MTZ). The direct Z-scheme CaIn2S4 marigold-flower-like/TiO2 (1%-CIS/TNP) photocatalyst showed enhanced performance in the ISN (71.9%) and MTZ (86.5%) photocatalytic degradations as compared to composites with different CaIn2S4 contents or the individual TiO2 and CaIn2S4. A possible enhancement mechanism based on the Z-scheme formed between the CIS and TNP for the improved photocatalytic efficiency was also proposed. The recombination rate of the photoinduced charge carriers was significantly suppressed for the direct Z-scheme CIS/TNP photocatalyst, which was confirmed by photoluminescence analysis. Radical-trapping studies revealed that photogenerated holes (h+), •OH, and O2•- are the primary active species, and suggested that the enhanced photocatalytic efficiency of the 1%-CIS/TNP follows the Z-scheme mechanism for transferring the charge carriers. It was further confirmed by hydroxyl (•OH) radical determination via fluorescence techniques revealed that higher concentration of •OH radical were formed over 1%-CIS/TNP than over bare CIS and TNP. The separation of the charge carriers was further confirmed using photocurrent and electron spin resonance measurements. Kinetic and chemical oxygen demand analyses were performed to confirm the ISN and MTZ degradation. The results demonstrated that the direct Z-scheme CIS/TNP photocatalyst shows superior decomposition efficiency for the degradation of these pharmaceuticals under the given reaction conditions.

Recent developments in photocatalytic dye degradation upon irradiation with energy-efficient light emitting diodes

Abstract Light emitting diodes (LEDs) are gaining recognition as a convenient and energy-efficient light source for photocatalytic application. This review focuses on recent progress in the research and development of the degradation of dyes in water under LED light irradiation and provides a brief overview of photocatalysis, details of the LEDs commonly employed, a discussion of the advantages of LEDs over traditional ultraviolet sources and their application to photocatalytic dye degradation. We also discuss the experimental conditions used, the reported mechanisms of dye degradation and the various photocatalytic reactor designs and pay attention to the different types of LEDs used, and their power consumption. Based on a literature survey, the feasibility, benefits, limitations, and future prospects of LEDs for use in photocatalytic dye degradation are discussed in detail.

Synthesis of MoS2 nanosheet supported Z-scheme TiO2/g-C3N4 photocatalysts for the enhanced photocatalytic degradation of organic water pollutants

MoS2 nanosheets loaded with Z-scheme TiO2/g-C3N4 photocatalysts were prepared using a wetness impregnation method. Exfoliated two-dimensional g-C3N4 and MoS2 nanosheets were used for Z-scheme nanocomposite preparation. Structural, morphological, and photo-physical properties of the prepared photocatalysts were investigated using various spectroscopy and microscopy techniques. Few-layer MoS2 loaded Z-scheme TiO2/g-C3N4 photocatalysts exhibited extended absorbance in the visible region compared to other photocatalysts. The photocatalytic degradation of methylene blue, a common dye, and atrazine, a potent herbicide, in aqueous media were investigated using both pure and composite Z-scheme photocatalysts. 10 wt% g-C3N4 loaded-TiO2 showed higher degradation performance than the other g-C3N4-loaded and bare photocatalysts. This degradation enhancement was due to the improved electron/hole pair separation at the TiO2/g-C3N4 interface imparted by the Z-scheme electron transfer. Furthermore, MoS2 nanosheets loaded with Z-scheme TiO2/g-C3N4 photocatalysts exhibited superior degradation performance compared to all other photocatalysts. Few-layer MoS2 co-catalyst, as an electron acceptor/accelerator, in the Z-scheme composite photocatalyst increased the photocatalytic activity. Suitable mechanism was proposed for this enhanced charge separation performance using PL analysis and band potentials of photocatalysts. The pathway of atrazine degradation was investigated using LC/MS/MS, and a possible degradation mechanism was proposed.

Heterojunction-based two-dimensional N-doped TiO2/WO3 composite architectures for photocatalytic treatment of hazardous organic vapor.

Two-dimensional nanosheet structures of N-doped TiO2/WO3 composites (WO3-N-TNSs) with varying WO3 loadings were synthesized by incorporating WO3 and N sources into sonochemically prepared TiO2 nanosheets (TNSs). These nanostructures were employed as photocatalysts, and their efficacy in the decomposition of hazardous hexane vapor was investigated. The photocatalytic efficiencies of the WO3-N-TNS composites were higher than those of N-doped TNS (N-TNS), which in turn were higher than the corresponding values for un-doped TNS. These variations were ascribed to the different light absorbance efficiencies, adsorption abilities, and charge carrier separations between the samples. An optimal WO3 loading for the performance of WO3-N-TNS was determined. Interestingly, the photocatalytic efficiency for hexane mixed with isopropyl alcohol (IPA) was lower than that for pure hexane, whereas the degradation efficiency for IPA did not vary with the feed method. Also investigated were the hexane conversion into CO2 over a representative WO3-N-TNS sample, the durability of the photocatalyst, and potential byproduct formation. Based on measurements of the hydroxyl radical population, a heterojunction-type mechanism was considered more plausible than a direct Z-scheme-type mechanism for the photocatalytic decomposition of hexane over the WO3-N-TNS photocatalysts.

Photocatalysis of sub-ppm limonene over multiwalled carbon nanotubes/titania composite nanofiber under visible-light irradiation

This study was conducted under visible-light exposure to investigate the photocatalytic characteristics of a multiwalled carbon nanotube/titania (TiO2) composite nanofiber (MTCN) using a continuous-flow tubular reactor. The MTCN was prepared by a sol-gel process, followed by an electrospinning technique. The photocatalytic decomposition efficiency for limonene on the MTCN was higher than those obtained from reference TiO2 nanofibers or P25 TiO2, and the experimental results agreed well with the Langmuir-Hinshelwood model. The CO concentrations generated during the photocatalysis did not reach levels toxic to humans. The mineralization efficiency for limonene on the MTCN was also higher than that for P25 TiO2. Moreover, the mineralization efficiency obtained using the MTCN increased steeply from 8.3 to 91.1% as the residence time increased from 7.8 to 78.0s, compared to the increase in the decomposition efficiencies for limonene from 90.1 to 99.9%. Three gas-phase intermediates (methacrolein, acetic acid, and limonene oxide) were quantitatively determined for the photocatalysis for limonene over the MTCN, whereas only two intermediates (acetic acid and limonene oxide) were quantitatively determined over P25 TiO2. Other provisional gas-phase intermediates included cyclopropyl methyl ketone and 2-ethylbutanal.

Feasibility of Light-emitting Diode Uses for Annular Reactor Inner-coated with TiO2 or Nitrogen-doped TiO2 for Control of Dimethyl Sulfide

Limited environmental pollutants have only been investigated for the feasibility of light‐emitting diodes (LED) uses in photocatalytic decomposition (PD). The present study investigated the applicability of LEDs for annular photocatalytic reactors by comparing PD efficiencies of dimethyl sulfide (DMS), which has not been investigated with any LED‐PD system, between photocatalytic systems utilizing conventional and various LED lamps with different wavelengths. A conventional 8 W UV/TiO2 system exhibited a higher DMS PD efficiency as compared with UV‐LED/TiO2 system. Similarly, a conventional 8 W visible‐lamp/N‐enhanced TiO2 (NET) system exhibited a higher PD efficiency as compared with six visible‐LED/NET systems. However, the ratios of PD efficiency to the electric power consumption were rather high for the photocatalytic systems using UV‐ or visible‐LED lamps, except for two LED lamps (yellow‐ and red‐LED lamps), compared to the photocatalytic systems using conventional lamps. For the photocatalytic systems using LEDs, lower flow rates and input concentrations and shorter hydraulic diameters exhibited higher DMS PD efficiencies. An Fourier‐transformation infrared analysis suggested no significant absorption of byproducts on the catalyst surface. Consequently, it was suggested that LEDs can still be energy‐efficiently utilized as alternative light sources for the PD of DMS, under the operational conditions used in this study.

Visible‐light‐activated photocatalysis of malodorous dimethyl disulphide using nitrogen‐enhanced TiO2

This study evaluated the feasibility of applying a visible‐light‐activated photocatalytic technique to cleanse air dimethyl disulphide (DMDS) at low concentration conditions (0.027–5.4 ppm), by using nitrogen‐enhanced TiO2. In addition, the applicability of a backup adsorption unit for the secondary control of DMDS exiting from the photocatalytic oxidation (PCO) unit was investigated. The PCO unit functioned effectively for the control of DMDS at low concentration levels (≤0.027 ppm) for long‐time periods (at least 603 h). However, rapid photocatalyst deactivation levels were observed during photocatalytic processes with a higher DMDS input concentration (IC) (2.7 ppm). The photocatalyst reactivated with humidified or dried air, under visible‐light irradiation, did not regain all its initial activities. The photocatalytic degradation efficiencies (PDEs) for DMDS were close to 100% for the relative humidity (RH) range of 45–55%, whereas they were between 86% and 91% and between 78% and 82% regarding the RH ranges of 10–20% and 80–90%, respectively. The PDEs via the PCO alone were close to 100% during this time period for the lowest IC conditions (0.027 ppm), whereas they decreased gradually for the other ICs. The FTIR spectra of the photocatalysts, as well as a solid‐liquid extraction method, suggested the formation of sulphate groups on the catalyst surface during a photocatalytic process. Methanol was identified as a gaseous by‐product. In addition, the backup adsorption unit could be effectively utilized to remove methanol, under a broad indoor pollution level (0.027–5.4 ppm), as well as DMDS exiting from the PCO units.

Titania Nanotubes Grown on Carbon Fibers for Photocatalytic Decomposition of Gas-Phase Aromatic Pollutants

This study aimed to prepare titania (TiO2) nanotube (TNT) arrays grown on un-activated carbon fibers (UCFs), with the application of different TiO2 loadings based on the coating-hydrothermal process, and to evaluate their photocatalytic activity for the degradation of sub-ppm levels of aromatic pollutants (benzene, toluene, ethyl benzene, and o-xylene (BTEX)) using a plug-flow photocatalytic reactor. The characteristics of the prepared photocatalysts were determined by scanning electron microscopy (SEM),energy-dispersive X-ray (EDX), transmission electron microscopy (TEM), UV-visible absorption spectroscopy (UV-Vis) and X-ray diffraction (XRD) analyses. Spectral analysis showed that the prepared photocatalysts were closely associated with the characteristics of one-dimensional nanostructured TiO2 nanotubes for TNTUCFs and spherical shapes for TiO2-coated UCF (TUCF). The photocatalytic activities of BTEX obtained from TNTUCFs were higher than those obtained from a reference photocatalyst, TUCF). Specifically, the average degradation efficiencies of BTEX observed for TNTUCF-10 were 81%, 97%, 99%, and 99%, respectively, while those observed for TUCF were 14%, 42%, 52%, and 79%, respectively. Moreover, the photocatalytic activities obtained for TNTUCFs suggested that the degradation efficiencies of BTEX varied with changes in TiO2 loadings, allowing for the optimization of TiO2 loading. Another important finding was that input concentrations and air flow rates could be important parameters for the treatment of BTEX, which should be considered for the optimization of TNTUCFs application. Taken together, TNTUCFs can be applied to effectively degrade sub-ppm levels of gas-phase aromatic pollutants through the optimization of operational conditions.

Volatile organic compound concentrations in newly built apartment buildings during pre- and post-occupancy stages

This study provides updated concentrations of 30 selected volatile organic compounds (VOCs) of indoor and outdoor air in new residential buildings before and after inhabitants moved in. During both the pre- and post-occupancy stages, toluene was the most abundant indoor VOC and, unlike other target VOCs, the indoor concentrations of six chlorinated compounds did not differ significantly from the outdoor concentrations, indicating the absence of any significant indoor source(s). The indoor concentrations of certain VOCs were significantly higher for the one-month post-occupancy stage than the pre-occupancy stage, which was likely attributable to emissions from furiture and household products used by inhabitants after moving in, as well as building finishing materials. The indoor concentrations of individual (excluding naphthalene and six chlorinated VOCs) and total VOCs revealed a decreasing tendency over the 2-y follow-up period. Moreover, there was an initial rapid decrease in indoor VOC concentrations followed by a somewhat slower decrease over the 2-y follow-up period, reflecting a multi-exponential decay model for VOCs. The measured indoor VOC concentrations and their matched measurement times were well fit to exponential models. During the pre-occupancy stage, aromatic hydrocarbons exhibited the highest emission rate. In contrast, terpenes showed the highest emission rate during the post-occupancy stages. The levels of VOCs determined in this study are necessary for establishment of effective VOC control strategies in new residential buildings and linking exposure to the health risk posed to inhabitants.