Soon Wook Kim

Fabrication of Au/Graphene-Wrapped ZnO-Nanoparticle-Assembled Hollow Spheres with Effective Photoinduced Charge Transfer for Photocatalysis

Heterostructures of gold-nanoparticle-decorated reduced-graphene-oxide (rGO)-wrapped ZnO hollow spheres (Au/rGO/ZnO) are synthesized using tetra-n-butylammonium bromide as a mediating agent. The structure of amorphous ZnO hollow spheres is found to be transformed from nanosheet- to nanoparticle-assembled hollow spheres (nPAHS) upon annealing at 500 °C. The ZnO nPAHS hybrids with Au/rGO are characterized using various techniques, including photoluminescence, steady-state absorbance, time-resolved photoluminescence, and photocatalysis. The charge-transfer time of ZnO nPAHS is found to be 87 ps, which is much shorter than that of a nanorod (128 ps), nanoparticle (150 ps), and nanowall (990 ps) due to its unique structure. The Au/rGO/ZnO hybrid shows a higher charge-transfer efficiency of 68.0% in comparison with rGO/ZnO (40.3%) and previously reported ZnO hybrids. The photocatalytic activities of the samples are evaluated by photodegrading methylene blue under black-light irradiation. The Au/rGO/ZnO exhibits excellent photocatalytic efficiency due to reduced electron-hole recombination, fast electron-transfer rate, and high charge-transfer efficiency.

Hydrothermally controlled ZnO nanosheet self-assembled hollow spheres/hierarchical aggregates and their photocatalytic activities

We synthesized ZnO nano-micro structures, including microrods, nanosheets, a nanosheet self-assembled hollow sphere (NSHS), and a hollow hierarchical aggregate (HHA) by changing the concentration of trisodium citrate in a hydrothermal process. The NSHS was formed by the self-assembly of 2D nanosheets on the surface of a formaldehyde bubble, while the HHA was obtained by the self-assembly and hierarchical arrangement of NSHS. The photocatalytic activities of the ZnO structures were evaluated from the degradation of methylene blue under visible and black light irradiation. The NSHS showed better photocatalytic performance due to its excellent photon absorbance, low defect density and low emission quantum yield compared with the HHA. Furthermore, the size and number of the 2D nanosheets hydrothermally grown on the NSHS bubble template were controlled by changing the growth temperature.

Fast and effective electron transport in a Au–graphene–ZnO hybrid for enhanced photocurrent and photocatalysis

A novel ZnO nanosheet aggregated hollow sphere (nSAHS) has been hydrothermally synthesized. Wrapping Au/reduced graphene oxide (Au/rGO) onto the ZnO nSAHS facilitates electron transfer at the ZnO interface. The photoinduced charge transfer of the Au/rGO/ZnO hybrid has been characterized using photoluminescence (PL) and a time-resolved PL decay profile. The quenched PL emission of Au/rGO/ZnO indicates decreased electron–hole pair recombination and facilitated electron transport, which results from the large surface area of the ZnO nSAHS structure, good physical adsorption of Au/rGO onto ZnO, and the excellent chemical interaction at the Au/rGO–ZnO interface. The Au/rGO/ZnO hybrid exhibits fast (4.46 ps) and effective (80.49%) charge transfer at the hybrid interface, indicating that Au/rGO/ZnO has better photocurrent responses and photocatalytic activities than rGO/ZnO and ZnO.

Facile fabrication of thermally reduced graphene oxide–platinum nanohybrids and their application in catalytic reduction and dye-sensitized solar cells

We report the fast synthesis of thermally reduced graphene oxide:platinum (TRGO:Pt) nanohybrids by simply spraying a GO:Pt4+ solution on a hot plate. X-ray photoelectron spectroscopy and atomic force microscopy analyses are performed to investigate the thermal reduction of GO:Pt4+ and the morphologies of the TRGO:Pt hybrid monolayer, respectively. The catalytic performance of TRGO:Pt is evaluated for the reduction of o-nitroaniline. A significant increase in the reaction rate constant for TRGO:Pt compared with pure Pt is due to facilitated electron transfer at the TRGO:Pt interface and enhanced catalytic active sites. Effective electron transfer from TRGO to Pt and significantly increased catalytic active sites in hybrids suggest that TRGO:Pt is a highly potential counter electrode material in dye-sensitized solar cells (DSSCs). The hybrid provides numerous electrons to I−/I3− electrolyte to reduce the recombination at the interface. As a result, the performance of DSSCs with the TRGO:Pt hybrid electrode is significantly increased by 34% in comparison with a pure Pt electrode.

Polyol-mediated synthesis of ZnO nanoparticle-assembled hollow spheres/nanorods and their photoanode performances

ZnO nanoparticle-assembled hollow spheres (raspberry-like) and elliptical nanorods (rice-like) were synthesized via a facile polyol process. Employing ethylene glycol as a polyol led to a ZnO nanoparticle-assembled hollow sphere structure, while diethylene glycol resulted in an elliptical nanorod structure. The ZnO hollow spheres had a higher Brunauer-Emmett-Teller (BET) surface area, better dye adsorption, more incident light trapping, and lower defect density than the ZnO elliptical nanorods. The ZnO hollow sphere-based dye-sensitized solar cells (DSSCs) exhibited a three-times higher current density than the ZnO elliptical nanorod-based DSSCs.

Self-cleaning SiOx-embedded TiO2/SiO2 alternating multilayer

A new method of inserting an eco-sustainable hydrophilic system into an anti-reflection (AR) system was developed and its applicability to photovoltaic systems was determined. We created a TiO2 nanoparticle composite with a densely multi-layered AR system top layer to demonstrate its eco-sustainable, hydrophilic performance and to act as a low refractive index material, which is essential for AR. The hydrophilic system displayed the lowest reflectance and a remarkable contact angle reduction to 0 degrees under UV light irradiation. The annealed hydrophilic system (the eco-sustainable system) exhibited the highest photodecomposition activity of 100% within 90 min by effective separation of electron–hole pairs using SiOx nanoparticles inside the TiO2 layer as trap levels. The successful AR, hydrophilic, and eco-sustainable characteristics of the simplified system composed of only two materials, TiO2 and SiOx, are anticipated to contribute to the enhancement of photovoltaic system efficiency.

Wetting and Photocatalytic Properties of TiO2 Nanotube Arrays Prepared via Anodic Oxidation of E-Beam Evaporated Ti Thin Films

TiO2 nanotube arrays (TNAs) are fabricated on quartz substrate by anodizing E-beam evaporated Ti films. E-beam evaporated Ti films are directly anodized at various anodizing voltages ranging from 20 to 45 V and their morphological, wetting, and photocatalytic properties are examined. The photocatalytic activity of the prepared TNAs is evaluated by the photodecomposition of methylene blue under UV illumination. The TNAs prepared at an anodizing voltage of 30 V have a high roughness of 30.1 nm and a low water contact angle of 7.5°, resulting in a high photocatalytic performance. The surface roughness of the TNAs is found to correlate inversely with the water contact angle. High roughness (i.e., high surface area), which leads to high hydrophilicity, is desirable for effective photocatalytic activity.