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.

Influence of Ag Clusters on the Electronic Structures of ß-Ga2O3 Photocatalyst Surfaces.

In order to understand the photocatalytic carbon dioxide reduction over Ag-loaded ß-Ga2O3 photocatalysts, first principles calculations based on density functional theory were performed on the surface model of a Ag cluster-adsorbed ß-Ga2O3 system. The stable adsorption structures of Ag n (n = 1 to 4) clusters on the ß-Ga2O3 (100) surface were determined. In the electronic structure analysis, the valence states of all Ag clusters mixed with the top of the O 2p valence band of Ga2O3, leading the Fermi level of Ag n /ß-Ga2O3 to shift to the bottom of the conduction band. It was also revealed that the unoccupied states of Ag n clusters overlapped with the Ga unoccupied states, and occupied electronic states of Ag clusters were formed in the band gap. These calculation results corresponded to the experimental ones obtained in our previous study, i.e., small Ag clusters had strong interaction with the Ga2O3 surface, enhancing the electron transfer between the Ag clusters and the Ga2O3 surface. That is, excited electrons toward Ag n clusters or the perimeter of Ag-Ga2O3 should be the important key to promote photocatalytic CO2 reduction.

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.

Comparison of platinum photodeposition processes on two types of titanium dioxide photocatalysts.

The photodeposition method is useful for the preparation of metal-loaded photocatalysts, by which the metal precursors are adsorbed on the photocatalyst surface and reduced by photoexcited electrons to typically form metallic nanoparticles. In the present study, the photodeposition process of Pt nanoparticles was investigated on anatase and rutile TiO2 photocatalysts. It was found that on the anatase surface, only some of the Pt4+ precursors were first adsorbed in an adsorption equilibrium and reduced to form a smaller number of initial metal species; then, they functioned as electron receivers to reduce the remaining precursors on their metallic surfaces and become larger particles. In contrast, the rutile surface can adsorb most of the precursors and quickly reduce them upon photoirradiation to form nanoparticles, giving a larger number of small nanoparticles. As a result, the Pt-loaded rutile photocatalyst exhibited higher activity in hydrogen evolution from an aqueous methanol solution than the Pt-loaded anatase photocatalyst.

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.

Realization of red shift of absorption spectra using optical near-field effect.

In many applications such as CO2 reduction and water splitting, high-energy photons in the ultraviolet region are required to complete the chemical reactions. However, to realize sustainable development, the photon energies utilized must be lower than the absorption edge of the materials including the metal complex for CO2 reduction, the electrodes for water splitting, because of the huge amount of lower energy than the visible region received from the sun. In the previous works, we had demonstrated that optical near-fields (ONFs) could realize chemical reactions, by utilizing photon energies much lower than the absorption edge because of the spatial non-uniformity of the electric field. In this paper, we demonstrate that an ONF can realize the red shift of the absorption spectra of the metal-complex material for photocatalytic reduction. By attaching the metal complex to ZnO nano-crystalline aggregates with nano-scale protrusions, the absorption spectra by using diffuse reflection of the metal complex can be shifted to a longer wavelength by 10.6 nm. The results of computational studies based on a first-principles computational program including the ONF effect provide proof of the increase in the absorption of the metal complex at lower photon energies. Since the near-field assisted field increase improves the carrier excitation in the metal-complex materials, this effect may be universal and it could applicable to CO2 reduction using the other metal-complex materials, as well as to the other photo excitation process including water splitting.

Robust Binding between Carbon Nitride Nanosheets and a Binuclear Ruthenium(II) Complex Enabling Durable, Selective CO2 Reduction under Visible Light in Aqueous Solution.

Carbon nitride nanosheets (NS-C3 N4 ) were found to undergo robust binding with a binuclear ruthenium(II) complex (RuRu') even in basic aqueous solution. A hybrid material consisting of NS-C3 N4 (further modified with nanoparticulate Ag) and RuRu' promoted the photocatalytic reduction of CO2 to formate in aqueous media, in conjunction with high selectivity (approximately 98 %) and a good turnover number (>2000 with respect to the loaded Ru complex). These represent the highest values yet reported for a powder-based photocatalytic system during CO2 reduction under visible light in an aqueous environment. We also assessed the desorption of RuRu' from the Ag/C3 N4 surface, a factor that can contribute to a loss of activity. It was determined that desorption is not induced by salt additives, pH changes, or photoirradiation, which partly explains the high photocatalytic performance of this material.

Effective nitrogen doping into TiO2 (N-TiO2) for visible light response photocatalysis.

The thickness-controlled TiO2 thin films are fabricated by the pulsed laser deposition (PLD) method. These samples function as photocatalysts under UV light irradiation and the reaction rate depends on the TiO2 thickness, i.e., with an increase of thickness, it increases to the maximum, followed by decreasing to be constant. Such variation of the reaction rate is fundamentally explained by the competitive production and annihilation processes of photogenerated electrons and holes in TiO2 films, and the optimum TiO2 thickness is estimated to be ca. 10nm. We also tried to dope nitrogen into the effective depth region (ca. 10nm) of TiO2 by an ion implantation technique. The nitrogen doped TiO2 enhanced photocatalytic activity under visible-light irradiation. XANES and XPS analyses indicated two types of chemical state of nitrogen, one photo-catalytically active N substituting the O sites and the other inactive NOx (1⩽x⩽2) species. In the valence band XPS spectrum of the high active sample, the additional electronic states were observed just above the valence band edge of a TiO2. The electronic state would be originated from the substituting nitrogen and be responsible for the band gap narrowing, i.e., visible light response of TiO2 photocatalysts.