Dong Won Hwang

Mg-Doped WO3 as a Novel Photocatalyst for Visible Light-Induced Water Splitting

Mg-doped WO3 with band gap energy of about 2.6 eV is a viable photocatalyst for visible light-induced water splitting in the presence of hole scavengers. The conduction band edge position of p-type Mg-doped WO3 was -2.7 V versus SCE at pH 12. By doping Mg on WO3, the conduction and valence band positions were shifted by 2.25 V negatively, leading to a conduction band edge position which was negative enough for H+ ions to be reduced thermodynamically, with little change in band gap energy. The negative shift in band position might be ascribed to lowering of the effective electron affinity of WO3 by doping Mg with a very low electron affinity.

Photocatalytic Water Splitting over La2Ti2O7 Synthesized by the Polymerizable Complex Method

Highly donor-doped (110) layered perovskite materials, La2Ti2O7, with high surface areas were synthesized by the polymerizable complex (PC) method. Relative to La2Ti2O7 prepared by the solid state reaction (SSR) method, PC catalysts showed higher surface areas, crystallization at lower temperatures, higher phase purity, more uniform morphology and better-distributed nickel on the outer surface of La2Ti2O7. All these factors led to higher photocatalytic activity for overall water splitting under UV irradiation. The quantum yield of the reaction over La2Ti2O7 prepared by the PC method was as high as 27%, which was about twofold greater than that over La2Ti2O7 prepared by the SSR method.

Photocatalytic hydrogen production from water-methanol mixtures using N-doped Sr2Nb2O7 under visible light irradiation: effects of catalyst structure.

Nitrogen-doped perovskite type materials, Sr2Nb2O7-xNx (0, 1.5 < x < 2.8), have been studied as visible light-active photocatalysts for hydrogen production from methanol-water mixtures. Nitrogen doping in Sr2Nb2O7 red-shifted the light absorption edge into the visible light range and induced visible light photocatalytic activity. There existed an optimum amount of nitrogen doping that showed the maximum rate of hydrogen production. Among the potential variables that might cause this activity variation, the crystal structure appeared to be the most important. Thus, as the extent of N-doping increased, the original orthorhombic structure of the layered perovskite was transformed into an unlayered cubic oxynitride structure. The most active catalytic phase was an intermediate phase still maintaining the original layered perovskite structure, but with a part of its oxygen replaced by nitrogen and oxygen vacancy to adjust the charge difference between oxygen and doped nitrogen. These experimental observations were explained by density functional theory calculations. Thus, in Sr2Nb2O7-xNx, N2p orbital was the main contributor to the top of the valence band, causing band gap narrowing while the bottom of conduction band due to Nb 4d orbital remained almost unchanged.

Photocatalytic water splitting over ZrO2 prepared by precipitation method

ZrO2 prepared by the precipitation method of zirconium oxychloride with various hydrolyzing agents was studied for photocatalytic water splitting reaction under UV light irradiation. The crystal structure as well surface properties were varied with the hydrolyzing agent of KOH, NH4OH, and NH2CONH2. Especially, the surface area of the prepared ZrO2 calcined at the same temperature of 750 °C for 6 h was dependent highly on the hydrolyzing agent, and thus the highest photocatalytic activity was obtained for ZrO2 with the highest surface area when KOH was used as a hydrolyzing agent, In the presence of Na2CO3, the photocatalytic activity of ZrO2 increased by 2–3 fold, which was ascribed to the physically adsorbed carbonate species on the ZrO2 surface.

Effect of Zr Substitution for Ti in KLaTiO4 for Photocatalytic Water Splitting

The effect of substitution of Zr for Ti in KLaTiO4 with a layered perovskite structure has been studied on the photocatalytic decomposition of water under the UV light irradiation. Both the optical property and the crystallinity of KLaTiO4 were varied by the substitution of Zr for Ti. As Zr content was increased, the crystallinity of KLaZrxTi1-xO4 was increased, which had a positive effect on the photocatalytic activity in the water splitting reaction. However, the direct band gap property was lost gradually with increase of Zr content, which resulted in lowering the photocatalytic activity. As a result, the highest activity was obtained for KLaZr0.3Ti0.7O4. Although the absorption coefficients of photons for KLaZr0.1Ti0.9O4 and KLaTiO4 were higher than those for KLaZr0.3Ti0.7O4, the crystal structure was disordered, which resulted in lowering the activity. The nickel-loaded catalysts showed a higher activity than unloaded perovskites by a factor of greater than 2.

Effect of precursors on the morphology and the photocatalytic water-splitting activity of layered perovskite La2Ti2O7

A [110] layered perovskite, La2Ti2O7, was a good photocatalyst under ultraviolet light in water splitting reaction. The material was synthesized with La2O3 and TiO2 as precursors by solid-state transformation. The morphology and photocatalytic activity of La2Ti2O7 depended on the preparation methods, as well as purity and morphology of the precursors. Wet-grinding of precursors in ethanol gave a product with higher crystallinity and phase purity, and thus higher photocatalytic activity, than dry-grinding without solvent. It was important to reduce the particle size of La2O3, as it usually had larger initial particle sizes than TiO2. Thus, the particle size of La2O3 had a strong effect on the crystallinity and surface area of the product La2Ti2O7. On the other hand, a severe chemical purity control was required for TiO2, while the effect of morphology was relatively small. In all cases, a high degree of crystallinity and purity of the prepared La2Ti2O7 was critical to show a high photocatalytic water-splitting activity.

Nickel-loaded La2Ti2O7 as a bifunctional photocatalyst

Nickel-loaded La2Ti2O7, one of the highly donor-doped (110) layered perovskite materials, has been found to be an efficient photocatalyst for simultaneous H2 production from water splitting and decomposition of tetramethylammonium hydroxide (TMAH) under UV irradiation.