Gui Young Han

Controlled Synthesis of Vertically Aligned Hematite on Conducting Substrate for Photoelectrochemical Cells: Nanorods versus Nanotubes

This paper describes two different processes to synthesize vertically aligned hematite nanorod and nanotube arrays, respectively, on a conductive substrate by the electrochemical deposition method with the help of an anodized aluminum oxide nanotemplate. The two types of nanostructured hematite were used as the photoanode for photoelectrochemical cells. The hematite nanotubes exhibited much higher photoelectrochemical activity than the hematite nanorods, including an improved photocurrent density, more negative onset potential, better photon harvesting, and better charge carrier transfer ability. The observed behavior may offer new information to enhance the photocatalytic ability of hematite, which is considered to be one of the best photoanode materials in the research field of photoelectrochemical cells.

Controlled growth of vertically oriented hematite/Pt composite nanorod arrays: use for photoelectrochemical water splitting.

Highly ordered and vertically grown Pt-doped α-Fe(2)O(3) nanorod arrays on a gold substrate were successfully prepared by the electrochemical co-deposition method using an anodized aluminum oxide template. The effect of the Pt doping in α-Fe(2)O(3) nanorod arrays on their water splitting ability was investigated for the first time. The elemental maps obtained by energy dispersive spectroscopy showed that the Pt was uniformly dispersed in the α-Fe(2)O(3) nanorod arrays. The photoelectrochemical properties of the α-Fe(2)O(3)/Pt composite nanorod arrays as a function of the Pt content were studied by measuring their photocurrent-potential behavior in 1 M NaOH electrolyte under AM 1.5 100 mW cm(-2) illumination. The Pt-doped α-Fe(2)O(3) nanorod arrays show an improvement in solar-to-hydrogen conversion efficiency (∼5%) for photoelectrochemical water splitting compared to undoped samples. To the best of our knowledge, it is the highest value yet obtained from α-Fe(2)O(3).

Synthesis and photoelectrochemical cell properties of vertically grown α-Fe2O3 nanorod arrays on a gold nanorod substrate

Photoelectrochemical cells prepared from highly ordered and vertically grown α-Fe2O3 nanorod arrays on a Au nanorod substrate showed about 8 mA cm−2 photocurrent density under 1 sun condition without any hole scavenger. To the best of our knowledge, it is the highest value obtained from α-Fe2O3. The Au nanorod arrays were firstly grown inside the AAO nanotubes to provide a conductive surface, not only for the electrochemical deposition of the target material, but also to act as the current collector of a photoelectrochemical cell. Then, the Fe nanorods were electrodeposited on the gold nanorods and annealed in an O2 atmosphere to convert them into α-Fe2O3 nanorod arrays. The α-Fe2O3 nanorod arrays stood freely on the gold nanorod arrays after the removal of the AAO template. The photoelectrochemical properties of the α-Fe2O3 nanorod arrays as a photoanode were studied by evaluating their photocurrent-potential behavior in 1 M NaOH electrolyte under AM 1.5 100 mW cm−2 illumination. Also, the dependence of the photocatalytic ability of the α-Fe2O3 nanorod arrays on their length was studied and the optimum rod length was determined.

Hydrogen Production by Catalytic Decomposition of Methane over Activated Carbons: Deactivation Study

The amount of deposited carbon on activated carbons was adjusted by varying the space time and the time on stream. Carbon nuclei formation appeared to occur initially but was terminated soon and then the carbon crystallite growth became dominant. The methane decomposition rate over activated carbons had a nearly linear relationship with the amount of carbon deposited. This suggests that the carbon deposition occurs uniformly and the activity decreases due to pore blocking, resulting in loss of accessibility to the active sites. The rate was also nearly proportional to the surface area for the same kind of activated carbon, which is contrasted with the previous finding that no discernible trend was observed between the initial activity and the surface area among different kinds of the activated carbons.

Removal of paraquat in aqueous suspension of TiO2 in an immersed UV photoreactor

Experiments were performed to investigate the effect of operating parameters on the photodegradation efficiency of paraquat in a TiO2-suspended photoreactor with immersed UV lamps. TiO2 particles were prepared by a hydrothermal method. The removal rate of paraquat in the reactor was 0.54 mg/l/h with only air-sparging. The removal rate in 24 h with both the UV radiation and air-sparging was 50% higher than that with only the UV radiation. Variations of the paraquat concentration at the UV intensities of 4 and 8 W/m2 decreased slowly with time, but that at 12 W/m2 decreased more rapidly. The removal efficiency at the air-sparging flow rate of 1 //min increased as a UV light intensity increased. pH value in the reactor at the UV intensity of 12 W/m2 decreased with time until 12 h and then increased with time over 12 h.

A photocatalytic performance of TiO2 photocatalyst prepared by the hydrothermal method

Anatase phase nanocrystalline TiO2 powders were prepared by hydrothermal method with the TTIP (titanium tetra isopropoxide) at 200 oC in a stirred autoclave system. The effects of synthesis conditions on the physical properties of catalyst were investigated by using XRD, SEM, DLS, DSC and BET. The TiO2 powders obtained from the optimum condition showed uniform spherical shape, crystalline structure, submicron size with a sharp size distribution and few agglomerates. The optimum synthesis conditions were obtained within the covered experimental ranges. The photocatalytic activity of TiO2 powders prepared by the hydrothermal method was tested for photooxidation of methyl orange.

Axial Gas Dispersion in a Fluidized Bed of Polyethylene Particles

Gas mixing behavior was investigated in a residence time distribution experiment in a bubbling fluidized bed of 0.07 m ID and 0.80 m high. Linear low density polyethylene (LLDPE) particles having a mean diameter of 772 Μm and a particle size range of 200-1,500 Μm were employed as the bed material. The stimulus-response technique with CO2 as a tracer gas was performed for the RTD study. The effects of gas velocity, aspect ratio (H0/D) and scale-up on the axial gas dispersion were determined from the unsteady-state dispersion model, and the residence time distributions of gas in the fluidized bed were compared with the ideal reactors. It was found that axial dispersion depends on the gas velocity and aspect ratio of the bed. The dimensionless dispersion coefficient was correlated with Reynolds number and aspect ratio.

An Analysis of Pressure Drop Fluctuation in a Circulating Fluidized Bed

The characteristics of pressure drop fluctuation in a 5.0 cm I.D.×250cm high circulating fluidized bed with fine polymer particles of PE and PVC were investigated. The measurements of time series of the pressure drop were carried out along the three different axial locations. To determine the effects of coarse particles and relative humidity of air on the flow behavior of polymer powders-air suspension in the riser, we employed deterministic chaos analysis of the Hurst exponent, correlation dimension and phase space trajectories as well as classical methods such as standard deviation, probability density function of pressure drop fluctuation. From a statistical and chaos analysis of pressure fluctuations, the upper dilute region was found to be much more homogenous flow compared to that in the bottom dense region at the same operating conditions. It was also found that the addition of coarse particles and higher humidity of air reduced the pressure fluctuations, thus enhancing flow stability in the riser. The analysis of pressure fluctuations by statistical and chaos theory gave qualitative and the quantitative information of flow behavior in the circulating fluidized bed.

Phase Controllable Transfer Printing of Patterned Polyelectrolyte Multilayers

We introduce a simple and robust method for varying the phase of the patterns in transfer printing of polyelectrolyte multilayers. Simply transferred positive patterns, edge-defined intermediate patterns, and negatively engraved patterns were obtained by manipulating the capillarity of the plasticized polymeric layer from weak to strong. A phase diagram of the pattern transfer was suggested to account for the influences of the temporal and spatial factors in the experiments. In addition, the main role of adhesion at the interfaces in the successful realization of multilayer transfer printing was investigated. It is anticipated that the strategy and ability to control the phase of the patterns transferred may offer a variety of opportunities for functional devices in optics, organic electronics, membranes, and microfluidics.

Characterization and photocatalytic performance of nanosize TiO2 powders prepared by the solvothermal method

The solvothermal reaction of titanium tetra-isopropoxide (TTIP) in different alcohol solvents was investigated in the pressure range 40±2 bar to prepare Titanium (IV) oxide. The results show that the physical properties of the products, such as crystal size, shape, and structure, are strongly in fluenced by the types of solvents and temperature during the reaction. The effects of reaction conditions on the physical properties and the crystal structure of powder were investigated by using XRD, SEM, DLS, DSC and BET. The obtained TiO2 powder prepared at an organic solvent condition exhibited submicron size and huge surface area with a narrow size distribution but some agglomeration. TiO2 powder prepared at 1,4-butanediol and 623 K shows the highest photoactivity on the photodegradation rate of methyl orange.