Young Kwang Kim

Light-harvesting multi-walled carbon nanotubes and CdS hybrids: Application to photocatalytic hydrogen production from water

This study reports the synthesis and surface characterization of multi-walled carbon nanotubes (CNT), CdS, and metal catalyst (M) hybrids (CdS/CNT/M), and their novel application to photocatalytic hydrogen production under visible light (λ > 400 nm) in the presence of electron donor (Na2S and Na2SO3). In the binary hybrids between CNT and CdS (CdS/CNT) the CNT annealed at 500 °C (h-CNT) has the larger amount of hydrogen production than crude (c-CNT) or acid-treated CNT (a-CNT) due to highly improved purity and suitable work function. When hybridized with CdS and M, however, a-CNT has the largest amount of hydrogen production (a-CNT > h-CNT > c-CNT) even though all the CNTs have similar functional groups for binding metal catalyst on their surfaces. Photocurrent measurements also indicated that CdS/a-CNT/Pt ternary generates a higher photocurrent than that of CdS/a-CNT binary (ternary > binary > CdS alone). In such ternary hybrids, Pt, Ni, and Ru are found to be effective in catalyzing proton/water but other metals (Pd, Au, Ag, Cu) showed very low activities with the following order: Pt > Ni > Ru > Pd > Au > Ag > Cu. The enhanced hydrogen production in the binary and ternary hybrids is ascribed partially to suitably positioned work functions among the hybrid components and thereby vectorial charge transfer through the work function energy gradient. Detailed surface studies were also described using Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray photoelectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).

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.

ZnO rods rooted on manifold carbon nanofiber paper as a scalable photocatalyst platform: the effects of ZnO morphology

Crystalline ZnO rods rooted on manifold carbon nanofiber (CNF) paper were synthesized via electrodeposition of ZnO onto electrospun CNF paper (∼300 μm thick) followed by oxidative annealing. The morphology of the ZnO deposited on the conductive CNF paper can be tailored to be polycrystalline rods with convex-shaped ends at a high Zn2+ precursor concentration (0.5 mM) upon annealing (denoted CZ-a-0.5, where “a” refers to annealing), whereas the sample electrodeposited at a low Zn2+ precursor concentration (0.25 mM) results in single crystalline rods with concave-shaped ends (denoted CZ-a-0.25). In order to systematically examine the photocatalytic activity, the annealed and non-annealed CNF/ZnO samples (CZ-a and CZ, respectively) were compared for the oxidation of phenol (one-electron transfer reaction), the production of H2 via water splitting and H2O2 production via oxygen reduction (both two-electron transfer reactions). Irrespective of the type of reaction, the CZ-a samples exhibit superior photocatalytic activities than the CZ samples in the following order: CZ-a-0.25 > CZ-a-0.5 > CZ-0.5 > CZ-0.25. The observed activity order is consistent with the trend observed in the XRD intensity ratio between the (100) and (002) planes (i.e., I100/I002 ratio) of the corresponding samples. The time-resolved photoluminescence decay spectra further reveal that the average lifetime of charge carriers is the shortest for CZ-a-0.25, followed by CZ-a-0.5, CZ-0.5 and CZ-0.25, which is consistent with the trends in the I100/I002 ratio and the photocatalytic activity. The growth mechanism of the samples and the key factors determining the photocatalytic activity are discussed. Finally, the detailed surface characterization of the samples is described.

Preparation of hydrophobic fabric using cross-linked fluorinated poly methacryalte copolymer nanobeads

In this study, we synthesized cross-linked poly methyl methacrylate beads and cross-linked fluorinated poly methacryalte copolymer (F-cPMMA) beads by the dispersion polymerization method with methyl methacrylate monomer, ethylene glycol dimethacrylate, and 2,2,2-trifuloroethyl methacrylate. The size of cross-linked PMMA and F-cPMMA beads was about 100 nm. Then we developed a new geometrical high-hydrophobic film by embedding the F-cPMMA nanobeads on polyurethane (PU) film. The PU film containing fluorinated poly methacryalte copolymer beads showed a greater hydrophobic property than the non-fluorinated one in the range of 1–10 wt.%. For the purpose of industrial application of the PU films, we applied 0.1–1 wt.% of beads on the fabric by the laminating method. The PU film containing 1 wt.% of fluorinated poly methacryalte copolymer beads, which was laminated onto nylon fabric, showed the best performance of hydrophobic property (water contact angle, 79°) and moisture vapor transmission rate (8797 g·m–2·24 h).

Colorimetric hydrogen gas sensor based on PdO/metal oxides hybrid nanoparticles

We have synthesized new colorimetric hydrogen-sensing materials, PdO/metal oxide hybrid nanoparticles, in which palladium oxide was loaded upon surface of substrate materials via an acid-base reaction between a H2PdCl4 solution and substrate materials, ZnO, MgO, TiO2, and SiO2 respectively at 25 °C. The colorimetric hydrogen gas sensing properties of all the samples, PdO/ZnO, PdO/MgO, PdO/TiO2 and PdO/SiO2, were characterized and compared in order to investigate how hydrogen gas sensitivity would be affected by surface property of substrate materials. It was confirmed that the amount of the loaded PdO, which was thought to be closely related with the colorimetric hydrogen sensitivity, was quite different according to the substrate materials and was increased with increasing of the basicity of substrate materials (ZnO > MgO > TiO2 > SiO2). Consequently, among the PdO/metal oxide hybrid nanoparticles, the largest amount of PdO was observed to be loaded on ZnO substrate nanoparticles due to its highest basicity. The best colorimetric hydrogen gas sensing properties (color difference, ΔE = 71.57) was observed in PdO/ZnO hybrid nanoparticles, showing the most prominent color change from brown to black, when the sample was exposed to hydrogen gas of 4 vol% balanced with nitrogen for 2 min.