Tetsuya Kako

Hydrogen production using zinc-doped carbon nitride catalyst irradiated with visible light

Abstract Recently, graphitic carbon nitride (g-C3N4) has been investigated as a photocatalyst for water splitting and organic dye degradation. In this study, we have developed a simple soft-chemical method of doping Zn into g-C3N4 to prepare a metal-containing carbon nitride. The doping was confirmed by x-ray photoelectron spectroscopy, and diffusion reflectance spectra revealed a significant red shift in the absorption edge of Zn/g-C3N4. This hybrid material shows high photocatalytic activity and good stability for hydrogen evolution from an aqueous methanol solution under visible light irradiation (λ≥420 nm). The hydrogen evolution rate was more than 10 times higher for a 10%-Zn/g-C3N4 sample (59.5 μmol h−1) than for pure g-C3N4. The maximum quantum yield was 3.2% at 420 nm.

Photothermal Conversion of CO2into CH4with H2over Group VIII Nanocatalysts: An Alternative Approach for Solar Fuel Production

The photothermal conversion of CO2 provides a straightforward and effective method for the highly efficient production of solar fuels with high solar-light utilization efficiency. This is due to several crucial features of the Group VIII nanocatalysts, including effective energy utilization over the whole range of the solar spectrum, excellent photothermal performance, and unique activation abilities. Photothermal CO2 reaction rates (mol h(-1) g(-1)) that are several orders of magnitude larger than those obtained with photocatalytic methods (μmol h(-1) g(-1)) were thus achieved. It is proposed that the overall water-based CO2 conversion process can be achieved by combining light-driven H2 production from water and photothermal CO2 conversion with H2. More generally, this work suggests that traditional catalysts that are characterized by intense photoabsorption will find new applications in photo-induced green-chemistry processes.

Surface-coordination-induced selective synthesis of cubic and orthorhombic NaNbO3 and their photocatalytic properties

NaNbO3 photocatalysts with cubic and orthorhombic structures were selectively fabricated at a low temperature using different starting reagents through a surface coordination modulation. The samples were characterized by X-ray diffraction, field emission-transmission electron microscopy, ultraviolet-visible absorption spectroscopy and Fourier transform infrared spectroscopy. A surface ligand coordination effect is proposed to understand the crystal growth mechanism of cubic NaNbO3. Induced by the organic ligands, cubic NaNbO3 could be prepared at a temperature as low as 500 °C. Interestingly, the as-synthesized cubic NaNbO3 shows better photocatalytic performances than those prepared at temperatures over 600 °C by the conventional way, both in H2 evolution from methanol aqueous solution and CO2 photoreduction in the gas phase. The enhanced photocatalytic activities could be attributed to the preferred crystal structure induced by the surface ligand coordination and the larger surface area obtained from the low formation temperature.

Nanoarchitectonics of a Au nanoprism array on WO3 film for synergistic optoelectronic response

Abstract A layered photoelectrode consisting of a conductive indium tin oxide substrate, a WO3 nanocrystalline film and an array of Au nanoprisms was fabricated via a multistep process. Scanning electron microscopy and atomic force microscopy showed that the Au nanoprisms had a uniform size and shape and formed periodic hexagonal patterns on the WO3 film. The optical absorption of the photoelectrode combined the intrinsic absorption of WO3 and plasmonic absorption of Au. Using this photoelectrode, we investigated the effect of the Au nanoprism array on the optoelectronic conversion performance of the WO3 film. Photoelectrochemical measurement indicated that the array substantially enhanced the photocurrent in the WO3 film. Electrochemical impedance measurements revealed that the Schottky junctions formed between Au and WO3 can facilitate the separation of photogenerated carriers as well as the interfacial carrier transfer. In this study, we demonstrate that covering a semiconductor with plasmonic noble metal nanoparticles can improve its optoelectronic conversion efficiency.

Ultrafine Zn1−xCuxS (0 ≤ x ≤ 0.066) nanocrystallites for photocatalytic H2 evolution under visible light irradiation

Ultrafine Zn1−xCuxS nanocrystallites were fabricated in a controlled manner by a wet chemical method using thiourea as the complexing reagent. Zn0.951Cu0.049S exhibited high photocatalytic activity for H2 evolution without a cocatalyst under visible light irradiation, which is about 11 times higher than that of CdS prepared by the same procedure.

Photoassisted fabrication of zinc indium oxide/oxysulfide composite for enhanced photocatalytic H2 evolution under visible-light irradiation

Abstract A photoassisted approach has been developed to synthesize a zinc indium oxide (Zn5In2O8)/oxysulfide composite through in situ sulfuration of vacancy-rich Zn5In2O8. It was found that vacancies have a considerable impact on the formation of the composite. The composite exhibited an increased photocatalytic H2 evolution activity under visible-light irradiation, which probably resulted from the enhanced ability to separate photoinduced electrons and holes. The H2 evolution rate over the composite was about 17 times higher when using vacancy-rich rather than conventional Zn5In2O8. This study provides a new method of improving the activity of photocatalysts.