Synthesis of meso-porous α-Ga2O3 from liquid Ga metal having significantly high photocatalytic activity for CO2 reduction with water.

We have succeeded in synthesizing meso-porous α-Ga2O3 which shows significantly high photocatalytic activity for CO2 reduction with water. The sample was synthesized by hydroxidation of liquid Ga metal in water to obtain GaOOH and Ga(OH)3, followed by the calcination of the mixed hydroxides at 773 K for 1 hour which converted them to meso-porous α-Ga2O3. The nano-pores remained as the trace of the evaporation of water produced by the oxidation of the hydroxides during the calcination. The photocatalytic activity of the synthesized meso-porous α-Ga2O3 for CO2 reduction with water was as high as or higher than previous studies using various types of Ga2O3 with and without cocatalysts.

Synthesis of α-Ga2O3 by Water Oxidation of Metallic Gallium as a Photocatalyst for CO2 Reduction with Water.

We have succeeded to synthesize gallium oxide consisting of α-phase (α-Ga2O3) with the calcination of GaOOH obtained by a direct reaction of liquid Ga metal with water for the first time and found that α-Ga2O3 exhibits photocatalytic activity for CO2 reduction with water and water splitting as well. The calcination above 623 K converted GaOOH to α-Ga2O3, and the samples calcined at 723-823 K were well crystallized to α-Ga2O3 and promoted photocatalytic CO2 reduction with water, producing CO, H2, and O2. This is observed for the first time that α-Ga2O3 without a cocatalyst has shown very high photocatalytic activity for the conversion of CO2 to CO.

TiO x N y /TiO2 Photocatalyst for Hydrogen Evolution under Visible-Light Irradiation. I: Characterization of N in TiO x N y /TiO2 Photocatalyst.

TiO x N y /TiO2 was synthesized by nitriding of TiO2 in NH3 gas. TiO x N y /TiO2 generated hydrogen from methanol aqueous solution under visible-light irradiation. It was revealed by N K-edge XANES and N 1s XPS measurements that the N species contributing to visible-light responsiveness was the O-Ti-N species. The structure of TiO x N y /TiO2 showing the photocatalytic activity was a double shell type with thin layers of TiO x N y that covers the TiO2 core. Although N content on the surface decreased during the photocatalytic reaction, N was supplied from the deeper side to keep the TiO x N y phase at the surface and the activity as well.

TiO x N y /TiO2 Photocatalyst for Hydrogen Evolution under Visible Light Irradiation II Degradation of Photocatalytic Activity of TiO x N y /TiO2 with Mild Oxidation.

We have studied degradation of photocatalytic activity of TiO x N y for water splitting under visible light irradiation with heat treatment in O2/N2 mixed gas. The reduction of the N content by oxidation through the formation of O-N-O species (NO x ) was confirmed as the result of the reduction of the catalytic activity. The catalytic activity is not simply related to the amount of N remained but that of N taking the chemical state of O-Ti-N in TiO x N y , which is the active species for visible light responsiveness on hydrogen evolution. N in TiO x N y is first oxidized to NO2 species during the oxidation, which reduces the activity. Then, O-N-O species (NO x ) is removed as NO x gas from the surface. Because the formation of O-N-O in TiO x N y could induce an impurity energy level to enhance charge recombination, the loss of catalytic activity might be influenced by the formation of O-N-O species (NO x ) rather than the loss of the N content from TiO x N y .

Photocatalytic Activity of Ga2O3 Supported on Al2O3 for Water Splitting and CO2 Reduction.

We have examined the photocatalytic activity of Ga2O3 supported on Al2O3 (Ga2O3/Al2O3 catalyst) without a noble metal cocatalyst for water splitting and reduction of CO2 with water under UV light irradiation by changing the loading amount of Ga2O3. All prepared Ga2O3/Al2O3 catalysts show photocatalytic activities for both water splitting and CO2 reduction, and their activities are significantly improved compared to those of nonsupported Ga2O3 and Al2O3. The water splitting is dominated for Ga2O3/Al2O3 with less than 1.0 vol % of Ga2O3 loaded, whereas the CO2 reduction, for higher Ga2O3-loaded samples (2.6, 4.2 vol %). Crystalline structure characterizations of Ga2O3/Al2O3 catalysts indicate that active sites for both reactions are different. The water splitting proceeds on nanometer-sized Ga2O3 rods dispersed on an Al2O3 support consisting of a little distorted α-Ga2O3 phase. On the other hand, the CO2 reduction proceeds on sub-micrometer-sized Ga2O3 particles consisting of mixed phases of α-Ga2O3 and γ-Ga2O3 or with appearance of boundaries between the α and γ phases, which plays a critical role. Al2O3 used as the support of the Ga2O3 particles does not seem to play an important role in the photocatalytic CO2 reduction.

Reverse Monte Carlo analysis of extended energy-loss fine structure for disordered structures of tetrahedrally coordinated materials: its applicability.

Capabilities of the reverse Monte Carlo method for characterizing the short-range structural correlations of solids were examined in the analysis of extended energy-loss fine structure (EXELFS). Since this method gives a three-dimensional atomic configuration consistent with the experimental EXELFS spectra, one can extract the statistical distributions of bond lengths, coordination numbers and bond angles of atom species of interest, which have been hard to achieve by conventional EXELFS analysis. The method was applied for the determination of amorphous Si structures and the damaged structure of an electron-irradiated SiC crystal. Advantages and disadvantages of the present method over the precise diffraction intensity measurement are discussed.

Development of new TEM specimen holder for cathodoluminescence detection.

A simple, new TEM specimen holder to detect cathodoluminescence (CL) in a transmission electron microscope has been developed. This holder accommodates a flat light-collecting reflector below the specimen and CL signals are guided to an optical spectrometer outside of the microscope through a quartz optical fibre. The compact system requires no modification of the microscope itself and allows large-angle specimen tilting and easy combination with other associate analytical tools. Test data are presented to aid the future application of the holder.