Fuminao Kishimoto

Microwave-enhanced photocatalysis on CdS quantum dots--Evidence of acceleration of photoinduced electron transfer.

The rate of electron transfer is critical in determining the efficiency of photoenergy conversion systems and is controlled by changing the relative energy gap of components, their geometries, or surroundings. However, the rate of electron transfer has not been controlled by the remote input of an external field without changing the geometries or materials of the systems. We demonstrate here that an applied microwave field can enhance the photocatalytic reduction of bipyridinium ion using CdS quantum dots (QDs) by accelerating electron transfer. Analysis of the time-resolved emission decay profiles of CdS quantum dots immersed in aqueous solutions of bipyridinium exhibited the shortening of their emission lifetimes, because of the accelerated electron transfer from QDs to bipyridinium under microwave irradiation. This discovery leads us to a new methodology using microwaves as an external field to enhance photocatalytic reactions.

Microwave assisted synthesis of high-surface area WO3 particles decorated with mosaic patterns via hydrochloric acid treatment of Bi2W2O9

Monoclinic WO3 particles with mosaic structures on the planes of the particles were synthesized from layered bismuth tungstate (Bi2W2O9) with the alternate stacked structure of Bi2O22+ layers and W2O72− layers via a hydrothermal technique using hydrochloric acid at 200 °C under microwave heating. These particles possessed high surface areas, giving a high photocatalytic activity in the degradation of gaseous acetaldehyde. Sequential SEM observations have clarified the dynamic transformations of the structures of Bi2W2O9 under microwave heating in comparison with conventional heating. The WO3 production through the reaction of Bi2W2O9 with HCl consists of two reaction steps, i.e., H2W2O7 generation via replacement of Bi2O22+ with H+ (the first step) and conversion of H2W2O7 to WO3 through dehydration of H2W2O7 (the second step). The first step proceeds even at room temperature, while the second reaction requires temperatures above 180 °C. To investigate the microwave heating effect on the first step (the replacement of Bi2O22+), the reaction of Bi2W2O9 and HCl was carried out at 80 °C under both microwave heating and conventional heating. It has been found that the replacement of Bi2O22+ with H+ is accelerated by the microwave selective heating effect. Interestingly, the WO3 particles with mosaic patterns were produced only under microwave heating. On the other hand, conventional heating of Bi2W2O9 in the presence of HCl resulted in the formation of plate-like WO3 particles without mosaic patterns.

Enhancement of anodic current attributed to oxygen evolution on α-Fe2O3 electrode by microwave oscillating electric field.

Various microwave effects on chemical reactions have been observed, reported and compared to those carried out under conventional heating. These effects are classified into thermal effects, which arise from the temperature rise caused by microwaves, and non-thermal effects, which are attributed to interactions between substances and the oscillating electromagnetic fields of microwaves. However, there have been no direct or intrinsic demonstrations of the non-thermal effects based on physical insights. Here we demonstrate the microwave enhancement of oxidation current of water to generate dioxygen with using an α-Fe2O3 electrode induced by pulsed microwave irradiation under constantly applied potential. The rectangular waves of current density under pulsed microwave irradiation were observed, in other words the oxidation current of water was increased instantaneously at the moment of the introduction of microwaves, and stayed stably at the plateau under continuous microwave irradiation. The microwave enhancement was observed only for the α-Fe2O3 electrode with the specific surface electronic structure evaluated by electrochemical impedance spectroscopy. This discovery provides a firm evidence of the microwave special non-thermal effect on the electron transfer reactions caused by interaction of oscillating microwaves and irradiated samples.

Specific electronic absorptions of alternate layered nanostructures of two metal oxides synthesized via a thiol–ene click reaction

Alternate layered nanostructures are synthesized with thiol-modified niobate nanosheets or tantalate nanosheets and alkene-modified tungstate nanosheets via a thiol–ene click reaction. The stacking distance of the nanosheets is increased linearly with the increase of the carbon number contained in the bridging chain, and controlled within the nanometer order by changing the carbon number generated by the thiol–ene click reaction. Different behaviors observed in the absorption spectra of the two combinations are discussed in terms of the electronic interaction between the neighboring nanosheets. The absorption peak attributed to the bandgap transition of tungstate in the absorption spectrum of the alternate layered nanostructure of niobate and tungstate is blue-shifted with the decrease of the stacking distance. This observation leads us to conclude that the density of states of the tungstate nanosheets is changed by formation of a p–n junction in the alternate layered structure. On the other hand, when the stacking distance of the alternate layered nanostructure of tantalate and tungstate is varied, there are no shifts of the absorption peak attributed to the bandgap transition of tungstate. This result indicates that the electronic structure of the tantalate nanosheets and the tungstate nanosheets in the alternate layered structure are independent. It is discovered that the electronic structures of the alternate stacking structures constructed by thiol–ene click reaction can be modified by changing the stacking distance of the alternate layered nanostructures, controlled by changing the carbon number.

Construction of Highly Hierarchical Layered Structure Consisting of Titanate Nanosheets, Tungstate Nanosheets, Ru(bpy)32+, and Pt(terpy) for Vectorial Photoinduced Z-Scheme Electron Transfer

To imitate the precisely ordered structure of the photoantennas and electron mediators in the natural photosynthesis system, we have constructed the Ru(bpy)32+-intercalated alternate-layered structure of titanate nanosheets and tungstate nanosheets via thiol-ene click reaction. Before nanosheet stacking, Pt(terpy) was immobilized at the edge of the titanate nanosheets. The visible-light-induced vectorial Z-scheme electron transfer reaction from the valence band of tungstate to the conduction band of titanate via the photoexcited Ru(bpy)32+ was demonstrated by the following two evidences: (1) From the results of the fluorescence decay of Ru(bpy)32+, the rate of the forward electron transfer from the photoexcited Ru(bpy)32+ to the conduction band of titanate was estimated as 1.16 × 108 s-1, which was 10 times faster than the backward electron transfer from the photoexcited Ru(bpy)3 to the conduction band of tungstate (1.02 × 107 s-1) due to a localization of Ru(bpy)32+ on the titanate nanosheets. (2) We observed the decrease of the electrons accumulated in the conduction band of the tungstate induced by photoexcitation of Ru(bpy)32+, demonstrating the forward electron transfer from the conduction band of tugnstate to the vacant highest occupied molecular orbital level of the photoexcitation Ru(bpy)32+. Finally, H2 gas was produced from the water dispersion of the alternate-layered structure under visible light irradiation, suggesting that the electrons getting to the conduction band of the titanate were transferred to the Pt(terpy) placed at the edge of the nanosheets, and reduced water to dihydrogen. Herein, n-octylamine species at the interlayer space played a role as hole scavenger; in other words, these molecules were oxidized by the hole in the conduction band of the tungstate nanosheets.