Hwichan Jun

Phosphate doping into monoclinic BiVO4 for enhanced photoelectrochemical water oxidation activity.

Phos-phorus is a typical dopant for silicon or germanium to make itan n-type semiconductor. However, it has been rarely used asdopant for semiconductor photocatalysts. This is rathersurprising because other non-metallic elements, such as N,C, and S, have been widely used as anionic dopants forphotocatalysts to reduce their band-gap energies.

Fabrication of CaFe2O4/TaON Heterojunction Photoanode for Photoelectrochemical Water Oxidation

Tantalum oxynitride photoanode is fabricated and modified with calcium ferrite to form a heterojunction anode for a photoelectrochemical water splitting cell. The synthesized powders are loaded sequentially to the transparent conducting glass by electrophoretic deposition, which is advantageous to form a uniform layer and a junction structure. X-ray diffraction, UV-vis diffuse reflectance spectroscopy, scanning electron microscopy, and impedance spectroscopy analysis are conducted to investigate the structural, morphological, and electrochemical characteristics of the anode. The introduction of CaFe2O4 overlayer onto TaON electrode increases the photocurrent density about five times at 1.23 V vs reversible hydrogen electrode without any co-catalyst. Impedance spectroscopy analysis indicates that the junction formation increased photocurrent density by reducing the resistance to the transport of charge carriers and thereby enhancing the electron-hole separation. This photocurrent generation is a result of the overall water splitting as confirmed by evolution of hydrogen and oxygen in a stoichiometric ratio. From the study of different junction configurations, it is established that the intimate contact between TaON and CaFe2O4 is critical for enhanced performance of the heterojunction anode for photoelectrochemical water oxidation under simulated sun light.

Photoelectrochemical water splitting over ordered honeycomb hematite electrodes stabilized by alumina shielding

Highly ordered, honeycomb-like iron oxide (hematite) films were fabricated by double-step anodic oxidation of iron foil. The honeycomb structure obtained by double step anodization was found to be more effective in producing a large area film with homogeneous pore distribution compared to nanotubes fabricated by the conventional single-step anodic oxidation process. To prevent agglomeration of the hematite film during the annealing process, a thin alumina layer was deposited on the hematite film surface by atomic layer deposition. With this alumina shielding and subsequent removal by alkaline treatment, one-dimensional (1-D) hematite nanostructure was preserved perfectly after annealing at 550 °C. This highly ordered 1-D nanostructure film showed much enhanced photoelectrochemical cell performances relative to hematite films with low degrees of ordering.

Phase and photoelectrochemical behavior of solution-processed Fe2O3 nanocrystals for oxidation of water under solar light

Monodispersed iron-oxide nanocrystals of controlled sizes (5–16nm) were prepared by the thermal decomposition of iron-oleate complex to fabricate photoanode films for photoelectrochemical oxidation of water under simulated solar light. The smallest 5nm particles were made mostly of γ-Fe2O3, but other sizes showed mixed phases with Fe3O4, whose fraction increased with size. Size-dependent photoanodic currents were also observed, showing maximum photocurrent density for 12nm particles. The high photocurrents were attributed to the lowered electron-hole recombination rates for the nanocrystals with the sizes comparable to the hole diffusion length.

The fabrication of highly uniform ZnO/CdS core/shell structures using a spin-coating-based successive ion layer adsorption and reaction method.

We developed a successive ion layer adsorption and reaction method based on spin-coating (spin-SILAR) and applied the method to the fabrication of highly uniform ZnO/CdS core/shell nanowire arrays. Because the adsorption, reaction, and rinsing steps occur simultaneously during spin-coating, the spin-SILAR method does not require rinsing steps between the alternating ion adsorption steps, making the growth process simpler and faster than conventional SILAR methods based on dip-coating (dip-SILAR). The ZnO/CdS core/shell nanowire arrays prepared by spin-SILAR had a denser and more uniform structure than those prepared by dip-SILAR, resulting in the higher power efficiency for use in photoelectrochemical cells.

Morphology controlled fabrication and ferroelectric properties of vertically aligned one-dimensional lead titanate nanoarrays

Vertically aligned nanowires and highly uniform nanoporous array thin films of PbTiO(3) are synthesized by varying anodic oxidation conditions of Ti foil followed by hydrothermal reaction in an aqueous Pb(II) acetate trihydrate solution. As-synthesized samples have single crystalline nanowire structure and polycrystalline nanoporous structure, although both are pure PbTiO(3) with a tetragonal phase. The structure of intermediate TiO(2) films obtained from different anodic oxidation conditions determines the structure of the product PbTiO(3). The relationships between these morphological structures and ferroelectric properties are investigated. Piezoresponse force microscopy reveals that both these films show ferroelectricity with clear phase contrast and well-defined hysteresis loops. The saturated longitudinal piezoelectric coefficient field (E(c)) of the nanowire sample is smaller than that of nanoporous thin film. Thus, polarization of nanowire thin film is larger in magnitude and easier to flip than that of nanoporous film.

Fabrication of nanoporous MTiO3 (M = Pb, Ba, Sr) perovskite array films with unprecedented high structural regularity

Nanoporous MTiO3 (M = Pb, Ba, Sr) array films with unprecedented high structural regularity are fabricated on Ti substrate by hydrothermal treatment of anodic TiO2 film prepared by two-step anodization. The films contain a high density of uniform cylindrical pores that are aligned perpendicular to the Ti substrate. One-step anodization, however, produces individually-separated nanotubes.