Jong Min Kum

Improved conversion efficiency of CdS quantum dot-sensitized TiO2 nanotube-arrays using CuInS2 as a co-sensitizer and an energy barrier layer

A thin layer of CuInS2 and CdS quantum dots (QDs) is deposited on TiO2 nanotube arrays (TNTs) to form CdS/CuInS2/TNTs photoelectrodes. The CuInS2 layer is prepared by a successive ionic layer absorption and reaction method, and the CdS QDs are deposited by a chemical bath deposition method. The CuInS2 layer acts as both a co-sensitizer and an energy barrier layer between TNTs and CdS QDs. The deposited CuInS2 layer significantly extends the visible-light response of CdS-sensitized TNTs into 500–700 nm wavelength range. As a consequence, the photoelectrochemical response of the CdS/CuInS2/TNTs electrodes is much improved compared with CdS sensitized TNTs. The CdS/CuInS2/TNTs electrodes exhibit a maximum power conversion efficiency of 7.3%, which is a 120% improvement compared with the highest efficiency of 3.3% for CdS/TNTs electrodes in our study. The improved efficiency is mainly due to the increased absorbance and the reduced recombination between the photoinjected electrons and the redox ions from the electrolyte, resulting from the formation of a CuInS2 layer.

Fabrication of complete titania nanoporous structures via electrochemical anodization of Ti

We present a novel method to fabricate complete and highly oriented anodic titanium oxide (ATO) nano-porous structures with uniform and parallel nanochannels. ATO nano-porous structures are fabricated by anodizing a Ti-foil in two different organic viscous electrolytes at room temperature using a two-step anodizing method. TiO2 nanotubes covered with a few nanometer thin nano-porous layer is produced when the first and the second anodization are carried out in the same electrolyte. However, a complete titania nano-porous (TNP) structures are obtained when the second anodization is conducted in a viscous electrolyte when compared to the first one. TNP structure was attributed to the suppression of F-rich layer dissolution between the cell boundaries in the viscous electrolyte. The structural morphologies were examined by field emission scanning electron microscope. The average pore diameter is approximately 70 nm, while the average inter-pore distance is approximately 130 nm. These TNP structures are useful to fabricate other nanostructure materials and nanodevices.

Tuning the electronic band structure of PCBM by electron irradiation

Tuning the electronic band structures such as band-edge position and bandgap of organic semiconductors is crucial to maximize the performance of organic photovoltaic devices. We present a simple yet effective electron irradiation approach to tune the band structure of [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM) that is the most widely used organic acceptor material. We have found that the lowest unoccupied molecular orbital (LUMO) level of PCBM up-shifts toward the vacuum energy level, while the highest occupied molecular orbital (HOMO) level down-shifts when PCBM is electron-irradiated. The shift of the HOMO and the LUMO levels increases as the irradiated electron fluence increases. Accordingly, the band-edge position and the bandgap of PCBM can be controlled by adjusting the electron fluence. Characterization of electron-irradiated PCBM reveals that the variation of the band structure is attributed to the molecular structural change of PCBM by electron irradiation.

A novel route to large-scale and robust free-standing TiO2 nanotube membranes based on N2 gas blowing combined with methanol wetting

We present a novel and straightforward approach to fabricate large-scale and robust free-standing TiO(2) nanotube (TNT) membranes. Simply by blowing N(2) gas onto as-anodized TNTs that are wetted with methanol, free-standing TNT membranes are produced. The approach also provides homogeneous and honeycomb-like Ti substrates after the detachment of TNT membranes. Through the second anodization of the honeycomb-like Ti substrates following the N(2) blowing, TNT membranes comprising hexagonally close-packed and regularly ordered TNTs with clear open ends can be achieved. Characterization of the free-standing TNT membranes using Raman spectroscopy and a high-resolution transmission electron microscope reveals that anatase TiO(2) and crystalline graphitic carbon are embedded in the bottom surface of the free-standing TNT membranes.

Plasmon-enhanced photocatalytic hydrogen production over visible-light responsive Cu/TiO2

Cheap and visible-light responsive Cu/TiO2 photocatalysts were fabricated by illuminating ultraviolet (UV) to a mixture of TiO2 nanoparticles (NPs) and Cu2O NPs in an evacuated reaction chamber. The Cu2O NPs were reduced by UV in an oxygen-free reaction chamber, and hence, metallic Cu NPs with size less than 5 nm were uniformly loaded on TiO2. Due to the plasmon resonance of the Cu NPs, the Cu/TiO2 exhibited a good performance of water-splitting hydrogen production under visible light in the presence of glycerol as a hole scavenger. The optimum hydrogen production rate of Cu/TiO2 was 0.24 mmol h(-1) g(-1). The Cu/TiO2 also showed high stability of the photocatalytic performance in the evacuated chamber; however, the visible-light responsive photocatalytic properties dramatically and rapidly decreased when exposed to air.

Rapid synthesis of TiO2 nanoparticles by electrochemical anodization of a Ti wire

We present a simple and novel strategy to synthesize TiO2 nanoparticles (NPs) based on electrochemical anodization of a Ti wire in an aqueous KCl electrolyte. The Ti wire is very rapidly and directly converted to TiO2 NPs by the anodization process, allowing mass production of TiO2 NPs. The size of the synthesized NPs can be readily tuned by changing the concentration of the electrolyte. We found that the field-assisted etching related to a strong electric field and the rapid etching rate caused by chloride ions play important roles for the formation of TiO2 NPs. This approach can also be applied to the mass production of other semiconducting metal oxide NPs such as tungsten-oxide NPs. TiO2 NPs showed higher photocatalytic activity compared to Degussa (P 25) under the same conditions. The higher photocatalytic activity of TiO2 NPs is attributed to the polymorphism. We believe that our approach can be used in broad areas including biomedical applications, photovoltaics, optics, and electronics.

Improvement in the photoelectrochemical responses of PCBM/TiO2 electrode by electron irradiation.

The photoelectrochemical (PEC) responses of electron-irradiated [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM)/TiO2 electrodes were evaluated in a PEC cell. By coating PCBM on TiO2 nanoparticle film, the light absorption of PCBM/TiO2 electrode has expanded to the visible light region and improved the PEC responses compared to bare TiO2 electrode. The PEC responses were further improved by irradiating an electron beam on PCBM/TiO2 electrodes. Compared to non-irradiated PCBM/TiO2 electrodes, electron irradiation increased the photocurrent density and the open-circuit potential of PEC cells by approximately 90% and approximately 36%, respectively at an optimum electron irradiation condition. The PEC responses are carefully evaluated correlating with the optical and electronic properties of electron-irradiated PCBM/TiO2 electrodes.