Hyun Gyu Kim

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.

Band gap tuning of lead-substituted BaSnO3 for visible light photocatalysis

The Pb substitution effect was investigated experimentally and theoretically on the crystal structure of BaSnO3 and on the photo-oxidation activity of H2O. The chemically doped Pb in BaSnO3 induced a concentration-dependent redshift of the experimental band gap (BG). The BaPb0.8Sn0.2O3 system produced 32μmol∕h of O2 under λ⩾420nm photons, but no O2 for BaSnO3. The DFT calculations of BaPbxSn1−xO3 (x=0,0.5,1) by using generalized approximation, implying the BG alteration and the photocatalytic activity of BaPbxSn1−xO3, are due to the induced Pb 6s orbital in the BG of BaSnO3. Thus Pb modified the insulating nature of BaSnO3 to semiconducting and semimetallic.

Engineered Nanorod Perovskite Film Photocatalysts to Harvest Visible Light

The recent fl ourishing of nanostructured materials has widened its potential applications in the much-desired effi cient energy materials. Specifi cally, materials for solar hydrogen production by water splitting should possess superior optoelectric properties in addition to suitable band energetics and durability in aqueous solutions. The tunability of the physicochemical properties of nanostructred materials by virtue of their size and shape renders a wider applicability. Since the critical limitation of popular TiO 2 photocatalyst that absorbs only UV light, [ 1 , 2 ] the fi eld of visible light water splitting photocatalyst is currently being nurtured by various kinds of conventional and new single component materials (CdS, WO 3 , Fe 2 O 3 , TaON, etc.), as well as composite materials. [ 3–20 ] However, there is still much potential for nanostructured materials in this area. There have been several reports on the binary [ 3–6 ] and ternary metal oxides, [ 7–13 ]

Photocatalytic Ohmic layered nanocomposite for efficient utilization of visible light photons

The WO3∕W∕PbBi2Nb1.9Ti0.1O9 photocatalyst was fabricated by depositing the tungsten clusters over the p-type perovskite base material with the chemical vapor deposition method, and later partly oxidizing the surfaces of these clusters to obtain n-type WO3 overlayers and W metal layer as an Ohmic junction. This NCPC showed unprecedented high activity for the photocatalytic oxidation of water, photocurrent generation, and acetaldehyde decomposition under visible light irradiation (λ⩾420nm).

Indium induced band gap tailoring in AgGa1−xInxS2 chalcopyrite structure for visible light photocatalysis

Indium was substituted at gallium site in chalcopyrite AgGaS(2) structure by using a simple solid solution method. The spectroscopic analysis using extended x-ray absorption fine structure and x-ray photoelectron spectroscopy confirmed the indium substitution in AgGaS(2) lattice. The band gap energy of AgGa(1-x)In(x)S(2) (x=0-1) estimated from the onset of absorption edge was found to be reduced from 2.67 eV (x=0) to 1.9 eV (x=1) by indium substitution. The theoretical and experimental studies showed that the indium s orbitals in AgGa(1-x)In(x)S(2) tailored the band gap energy, thereby modified the photocatalytic activity of the AgGa(1-x)In(x)S(2).

Nanostructure-Preserved Hematite Thin Film for Efficient Solar Water Splitting.

High-temperature annealing above 700 °C improves the activity of photoelectrochemical water oxidation by hematite photoanodes by increasing its crystallinity. Yet, it brings severe agglomeration of nanostructured hematite thin films and deteriorates electrical conductivity of the transparent conducting oxide (TCO) substrate. We report here that the nanostructure of the hematite and the conductivity of TCO could be preserved, while the high crystallinity is attained, by hybrid microwave annealing (HMA) utilizing a graphite susceptor for efficient microwave absorption. Thus, the hematite thin-film photoanodes treated by HMA record 2 times higher water oxidation photocurrents compared to a conventional thermal-annealed photoanode. The enhanced performance can be attributed to the synergistic effect of a smaller feature size of nanostructure-preserved hematite and a good electrical conductivity of TCO. The method could be generally applied to the fabrication of efficient photoelectrodes with small feature sizes and high crystallinity, which have been mutually conflicting requirements with conventional thermal annealing processes.

Carbon monoxide hydrogenation over molybdenum carbide catalysts

Supported and unsupported molybdenum and molybdenum carbides have been studied as catalysts of CO-H2 reactions at 570 K and atmospheric pressure. The initial turnover rates of these catalysts were comparable to those of the more active group VIII elements. However, all molybdenum-based catalysts showed a hydrocarbon product distribution different from those for typical group VIII metals. Furthermore, production of a large amount of CO2 (instead of water) and a high paraffin/olefin ratio reflected high activities of these catalysts for the water-gas shift reaction and hydrogenation, respectively. The high water-gas shift reaction activity allowed a CO-rich synthesis gas to be used efficiently over Mo-based catalysts. The CO-H2 reactions appear to be structure insensitive on molybdenum carbide catalysts, since the rates were independent of particle size and crystal structure of unsupported catalysts and of metal loading of supported catalysts.

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.

Formation of La2Ti2O7 Crystals from Amorphous La2O3-TiO2 Powders Synthesized by the Polymerized Complex Method

Amorphous La2O-TiO2 powders were synthesized by the polymerized complex (PC) method. The activation energies for crystallization and grain growth of La2Ti2O7 from these precursors were determined from results of XRD and DTA and compared with those for La2Ti2O7 precursors by the conventional solid-state reaction (SSR). Activation energy of grain growth of La2Ti2O7 in PC-sample was determined to be 7.1 kJ/mol while that of SSR sample was 14.8 kJ/mol. The energy required for the phase transformation from amorphous PC sample to layered perovskite was 432 kJ/mol, while the SSR sample did not show this transition below 900‡C. It was clearly demonstrated that the La2Ti2O7 crystals were formed at a lower temperature and they grew in size faster in the sample prepared by the PC method relative to the sample prepared by the SSR method. Mixing of elements in molecular level in PC preparation appeared responsible for these differences.