Authorship：Last author   Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1039/d1ra09039a

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Language：English   Publishing type：Research paper (scientific journal)  

The photocatalytic activity of mixed phases of GaOOH/β-Ga2O3 and α-Ga2O3/β-Ga2O3 for CO2 reduction with water is investigated for the first time. GaOOH/β-Ga2O3 is synthesized by high energy ball milling of β-Ga2O3 in water, and α-Ga2O3/β-Ga2O3 are prepared by the calcination of GaOOH/β-Ga2O3 at 773 K. Both show higher activity for both CO2 reduction and the accompanying H2 evolution than the original material of β-Ga2O3. This is the first observation that the mixed phase of GaOOH/β-Ga2O3 has high photocatalytic activity. The H2 production rate is correlated to the specific surface area irrespective of the GaOOH/β-Ga2O3 and α-Ga2O3/β-Ga2O3 samples. The CO production rate for the mixed phase of α-Ga2O3/β-Ga2O3 is high, agreeing with the previous observation that the mixed phases or phase boundaries enhance the photocatalytic activity, and the CO production rate increases with the abundance rate of the α-Ga2O3 phase. Although the selectivity of the CO production remains low, it could be enhanced if an Ag cocatalyst was used. As an alternative mechanism of H2 production (H2 evolution) over the samples of GaOOH/β-Ga2O3 and α-Ga2O3/β-Ga2O3, a redox type reaction mechanism is proposed, in which the H2 evolution proceeds with the reduction of GaOOH to α-Ga2O3 emitting H2 and O2 by UV illumination, while α-Ga2O3 returned to GaOOH in water without illumination. Although some of the H2 thus evolved would cause CO2 reduction, CO2 reduction to CO requires specific active sites on the α-Ga2O3 surface.

DOI： 10.1039/d1nj05245d

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A mixed layer of IrO2 and Ta2O5 deposited on a Ti substrate is an effective electrode for the catalysis of oxygen evolution reactions (OERs) in acidic solutions. In our previous work, IrO2-Ta2O5 catalysts supported on Ti fibers exhibited an amorphous structure, a large electrochemically active surface area (ECSA), and a high stability in acidic media. However, the local structure of amorphous OER electrocatalysts has not been studied extensively. In this study, the properties of an amorphous IrO2-Ta2O5 layer were analyzed through X-ray absorption spectroscopy (XAS) to understand the factors that contribute to its high OER stability. Herein, the addition of polyethylene glycol (PEG) to a precursor composed of H2IrCl6 and TaCl5 resulted in the formation of IrO2 nanoparticles with low chlorine residues even after thermal decomposition at a low temperature of 350 °C. X-ray diffraction and Raman spectroscopy results showed that the synthesized IrO2 nanoparticles were amorphous. However, from the XAS results, the local [IrO6] octahedron structure, which is similar to that of rutile IrO2 crystals, was revealed. During the preparation process, PEG assisted a complete ligand exchange from the [IrCl6] octahedron to the [IrO6] octahedron. This local structure of the IrO2 nanoparticles makes the amorphous IrO2-Ta2O5 layer on the Ti fibers stable during OER in acidic media. Furthermore, sintering is unlikely to occur during the synthesis because of the low thermal decomposition temperature of the precursor. As such, the synthesized amorphous IrO2 nanoparticles exhibited a large ECSA. In addition, the high intrinsic activity of the [IrO6] local structure resulted in a small Tafel slope. Thus, the amorphous IrO2 nanoparticles with an [IrO6] local structure are essential not only to achieve high activity but also high stability during the OER.

DOI： 10.1021/acs.jpcc.1c09775

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Language：English  

Nitrogen is a very common element, comprising approximately 78% of Earth's atmosphere, and is an important component of various electronic devices while also being essential for life. However, it is challenging to directly utilize dinitrogen because of the highly stable triple bond in this molecule. The present review examines the use of non-equilibrium plasmas to generate controlled electron impacts as a means of generating reactive nitrogen species (RNS) with high internal energy values and extremely short lifetimes. These species include ground state nitrogen atoms, excited nitrogen atoms, etc. RNS can subsequently react with oxygen and/or hydrogen to generate new highly reactive compounds and can also be used to control various cell functions and create new functional materials. Herein, plasma-processing methods intended to provide RNS serving as short-lived precursors for a range of applications are examined in detail.

DOI： 10.35848/1347-4065/ac25dc

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

To prepare photocatalysts promoting water splitting under visible light irradiation, GaOOH was nitrided by the calcination at a temperature ranging from 773 to 1273 K under a NH3 flow to produce nitrogen doped Ga2O3. X-ray diffraction measurements suggested that even under NH3 atmosphere dehydration of GaOOH to α-Ga2O3 occurred by calcining below 873 K. In the samples nitrided above 973 K, gallium nitride (GaN) phase started to appear and the crystallinity became better with increasing the nitriding temperature. In diffuse reflectance ultraviolet-visible measurements, absorption bands appeared in the visible light region after the nitrization, and the shift of the absorption edge with nitriding temperature suggested the formation of a new phase like gallium oxy-nitride. X-ray photoelectron spectroscopy and scanning transmission electron microscopy-electron energy-loss spectroscopy analyses revealed that oxygen atoms were distributed homogeneously and mixed with nitrogen atoms in the sample nitrided at 1173 K [referred as S(1173 K)] while oxygen atoms were mainly distributed at the surface of the sample nitrided at 1273 K [S(1273 K)]. The differences in the composition and chemical state near the sample surface closely related to the activity and stability during water splitting reaction under visible light irradiation between S(1173 K) and S(1273 K), i.e., the former sample provided stable H2 and O2 evolution while the latter sample was inactive and unstable.

DOI： 10.1380/EJSSNT.2022-003

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

In order to understand the photocatalytic carbon dioxide reduction over Ag-loaded β-Ga2O3 photocatalysts, first principles calculations based on density functional theory were performed on the surface model of a Ag cluster-adsorbed β-Ga2O3 system. The stable adsorption structures of Agn (n = 1 to 4) clusters on the β-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 Agn/β-Ga2O3 to shift to the bottom of the conduction band. It was also revealed that the unoccupied states of Agn 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 Agn clusters or the perimeter of Ag-Ga2O3 should be the important key to promote photocatalytic CO2 reduction.

DOI： 10.1021/acsomega.1c04730

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

Ag loaded Ga2O3 photocatalysts are well known to be highly active for the CO2 reduction with water to CO. However, Ag changes its chemical state during the reaction, resulting in the decrease of its photocatalytic activity. To examine the chemical state change during the photocatalytic CO2 reduction, we have attempted a real time observation of the change in the chemical and physical states of Ag co-catalyst loaded on Ga2O3 by monitoring UV–vis reflectance spectra under the atmosphere simulating the photocatalytic CO2 reduction. In UV–vis spectra, two characteristic absorptions appeared at around 450 nm and 600 nm corresponding to resonance absorptions of localized surface plasmon (LSPR) of Ag nanoparticles (Ag-NPs) and Ag in metallic state. The LSPR absorption appeared as light irradiation started and grew with the time. It was revealed that Ag on Ga2O3 was initially in the oxidized state and reduced by the light irradiation to be Ag-NPs. Further light irradiation enlarged the size of Ag-NPs to be the metallic state especially under atmosphere with water. Under CO2 atmosphere, Ag-NPs remained small after further light irradiation. These results suggest that Ag initially loaded on Ga2O3 in the oxidized state was reduced to be dissolved as Ag+ in water and precipitated as Ag-NPs. Then, some of Ag-NPs aggregated to be larger Ag particles or in the metallic state. Such reduction process depends on gas species of the system, i.e. water promoted the formation of Ag-NPs while CO2 suppressed the formation Ag-NPs.

DOI： 10.1016/j.cattod.2020.04.063

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1021/acsomega.1c02088

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

Since the activity of semiconductor photocatalysts for CO2 reduction with water is still low, it is necessary to improve their activity. To do this, the optimization of particle size and crystal structure of the photocatalysts is one of the promising ways. In previous studies, we have reported that micro-particulate Ga2O3 deposited on Al2O3 support significantly improves the photocatalytic activity of CO2 reduction. However, it is quite difficult to synthesize nano-particulate Ga2O3 with high crystallinity by calcination at higher temperatures. In this work, we have tried to synthesize sheet-like nano-particulate Ga2O3 using graphene oxide (GO) as a template for the first time. After loading gallium butoxide on GO, it was calcined to oxidize the gallium butoxide into Ga2O3 and also to remove GO. Various spectroscopic analyses revealed that the synthesized materials were planar aggregation of nanometer-sized Ga2O3 particles (nsGa2O3), of which sizes and crystallinity could be controlled with the calcination temperature. The nsGa2O3 calcined at 1123 K was well crystallized and showed β-phase with a quite large specific surface area. Furthermore, Ag loaded nsGa2O3 calcined at 1123 K (Ag/nsGa2O3(1123 K)) showed very high photocatalytic activity for CO production in the photocatalytic CO2 reduction test in water including methanol under UV light irradiation. Thus, it is confirmed that the planar aggregation of nanometer-sized Ga2O3 significantly improves the photocatalytic activity of Ga2O3 on the CO2 reduction.

DOI： 10.1016/j.apsusc.2020.148680

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

To reduce the effects of global warming, visible and near-infrared light must be used more efficiently. Deep ultraviolet light (8 eV) is required for the direct dissociation of CO2 by light; however, the introduction of a metal complex has made it possible to realize CO2 reduction with visible light. We demonstrate that the optical near field (ONF) can increase the CO2 reduction rate. For this, we used gold clusters, because they can be a suitable source for ONFs, as their size and density can be controlled by the number of gold atoms. By attaching a metal complex near gold clusters with diameters of 1.0 to 1.3 nm, we confirm that the reduction rate of CO2 to CO increased by 1.5 to 2.1 times. The gold clusters were sufficiently small; therefore, there was no plasmonic resonant peak or heat generation. Because the near-field effect is based on a photochemical reaction, it can be applied to other metal complexes used in CO2 reduction, and it has other applications such as water splitting and water purification.

DOI： 10.1117/1.JNP.14.046011

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Language：English   Publishing type：Research paper (scientific journal)  

We have succeeded to prepare Au nanopareticle deposited TiO2 photocatalysts (Au/TiO2) with control of Au nanoparticle size to be around 8 nm and variation of number density using a colloid photodepostion method. The prepared Au/TiO2 exhibits activity on decomposition (oxidation) of formic acid by thermally activated and photo-activated catalytic reactions. The thermally activated catalytic decomposition gradually increases with increasing the number density of deposited Au NPs and saturated, suggesting that the decomposition occurs on Au NPs surface and/or near the interface of TiO2 and Au NPs. On the other hand, the photocatalytic decomposition is significantly improved with rather small number density deposition of Au NPs and disappeared with high number density deposition. ESR measurements of Au/TiO2 in the surrounding similar to the photocatalytic decomposition suggests that electrons excited by plasmon resonance absorption in the Au NPs transfer to TiO2 to promote the decomposition. However, high number density deposition enhances electron capture by neighboring Au NPs and reduces the photocatalytic activity. Thus there should be the optimum number density of Au NPs on TiO2 for photocatalytic decomposition of formic acid.

DOI： 10.1016/j.cattod.2019.12.035

Scopus

Language：English   Publishing type：Research paper (international conference proceedings)  

It is well known that Ag loaded Gallium oxide (Ag/Ga2O3) promotes photocatalytic CO2reduction to CO. However, the role of the Ag co-catalyst in the CO2reduction has not been clarified. We have intended to find the relationship between the state of Ag loaded on Ga2O3and their activity for the photocatalytic CO2reduction. To achieve this, we have tried to control and stabilize the loading state of Ag on Ga2O3during their use for the photo-catalytic CO2reduction by adding methanol as a reducing agent. It is confirmed that methanol stabilizes the particle sizes of Ag nanoparticles (Ag-NPs) loaded on Ga2O3under the photocatalytic CO2reduction condition and is hardly decomposed to produce CO. Using Ga2O3loaded with size-controlled Ag-NPs as a photocatalyst for the CO2reduction, it is found that Ag-NPs with their sizes within 10-30 nm are active sites for the photocatalytic CO2reduction and the catalytic activity linearly increases with the increase of the number density of Ag-NPs. However, a higher Ag loading amount over 1.0 wt% promotes aggregation of Ag to be larger metal particles over 70 nm which are not active for the CO2reduction. (Figure Presented).

DOI： 10.1380/EJSSNT.2020.168

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Language：English   Publishing type：Research paper (scientific journal)  

We have succeeded in synthesizing black phosphorus (BP) by a one-pot solvothermal reaction of red phosphorus (RP) and ethylenediamine (ED) used as a solvent. To examine the reaction mechanism, we have investigated the influence of the synthesis conditions on BP, and the valence of dissolved phosphorus in the solvent after the reaction. P0 species and P3+ like species are very likely dissolved in ED as intermediates for BP production. Optimizing the reaction conditions, i.e., temperature, the charging amount of RP in ED and the particle size of RP, BP was synthesized with high yield. The synthesized BP has an extra peak at 10.2° in the X-ray diffraction pattern, which was assigned to stacking faults or periodic distortion in the direction of the c axis by simulation of diffraction. The synthesized BP with a Co-P cocatalyst showed high photocatalytic activity for hydrogen evolution from methanol aqueous solution under visible light irradiation.

DOI： 10.1039/c9ta13441g

Scopus

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

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 TiO2photocatalysts. 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.

DOI： 10.1039/c9cp06988g

Scopus

PubMed

Language：English   Publishing type：Research paper (international conference proceedings)  

We have prepared La2O3supported Ga2O3(Ga2O3/La2O3) photocatalyst to improve the photocatalytic activity of Ga2O3for CO2reduction with water under ultraviolet light irradiation without a noble metal cocatalyst. Significant improvement on both CO2reduction and water splitting was attained compared with those for non-supported Ga2O3and La2O3. The improve-ment is attributed to the transformation of the La2O3surface to NaLa(CO3)2 during the photocatalytic reduction tests in aqueous solution of NaHCO3. NaLa(CO3)2 was preferentially formed on the surface of La2O3without changing Ga2O3resulting in the structure of Ga2O3supported by NaLa(CO3)2. Once NaLa(CO3)2 fully covered the surface of La2O3, the high catalytic activity was kept long. (Figure Presented).

DOI： 10.1380/EJSSNT.2020.110

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

Ga2O3 samples were synthesized by calcination of gallium nitrate powder at a given temperature ranging from 673 K to 1173 K (denoted as T-Ga2O3 with T, calcination temperature). XRD and XAFS structural analyses revealed that samples calcined at temperatures below 673 K were consisted of defective ε phase, whereas those calcined above 773 K of β and γ phases, and β phase dominated above 923 K. All calcined Ga2O3 samples promoted photocatalytic CO2 reduction with water to produce CO, H2 and O2. As for the H2 production by water splitting, the H2 production rates were well correlated with the surface areas of calcined Ga2O3 samples. It was also found that the water splitting and the CO2 reduction proceeded fully independently. 823-Ga2O3 showed the highest activity for the CO2 reduction, suggesting that the coexistence of β and γ phases plays a key role on the photocatalytic CO2 reduction.

DOI： 10.1016/j.apcatb.2019.118247

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1246/BCSJ.20190366

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Language：English   Publishing type：Research paper (scientific journal)  

We have studied degradation of photocatalytic activity of TiOxNy 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 (NOx) 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 TiOxNy, which is the active species for visible light responsiveness on hydrogen evolution. N in TiOxNy is first oxidized to NO2 species during the oxidation, which reduces the activity. Then, O-N-O species (NOx) is removed as NOx gas from the surface. Because the formation of O-N-O in TiOxNy 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 (NOx) rather than the loss of the N content from TiOxNy.

DOI： 10.1021/acsomega.9b02979

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Language：English   Publishing type：Research paper (scientific journal)  

TiOxNy/TiO2 was synthesized by nitriding of TiO2 in NH3 gas. TiOxNy/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 TiOxNy/TiO2 showing the photocatalytic activity was a double shell type with thin layers of TiOxNy 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 TiOxNy phase at the surface and the activity as well.

DOI： 10.1021/acsomega.9b02977

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

We have performed dehydronitrization of GaOOH under NH3 flow to produce nitrogen doped Ga2O3 and examined their photocatalytic activities for H2 evolution from an aqueous methanol solution under visible light irradiation. GaOOH was synthesized by hydrothermal treatment and dehydronitrided at a temperature ranging from 773 K to 1273 K under NH3 flow. At first, GaOOH was dehydrided to Ga2O3 under 873 K and followed nitrization. With increasing dehydronitrization temperature, the products were getting closer to full nitride (GaN). Among all dehydronitrided samples, only one sample sintered at 1173 K showed photocatalytic activity under visible light irradiation and its crystalline structure had not changed before and after the reactions, while other samples did not show the activity and were oxidized to GaOOH. From thermodynamical aspect, if nitrogen dissolved into oxide or making oxynitride, its chemical potential must be lower than that of N in GaN. Therefore, there should be some gallium oxinitride phase like GaNyO3-x stable in water showing photocatalytic activity.

DOI： 10.1016/j.apcatb.2019.03.009

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Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1088/1361-6528/ab2092

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PubMed

Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

We have examined the photocatalytic activity of Ga 2 O 3 supported on Al 2 O 3 (Ga 2 O 3 /Al 2 O 3 catalyst) without a noble metal cocatalyst for water splitting and reduction of CO 2 with water under UV light irradiation by changing the loading amount of Ga 2 O 3 . All prepared Ga 2 O 3 /Al 2 O 3 catalysts show photocatalytic activities for both water splitting and CO 2 reduction, and their activities are significantly improved compared to those of nonsupported Ga 2 O 3 and Al 2 O 3 . The water splitting is dominated for Ga 2 O 3 /Al 2 O 3 with less than 1.0 vol % of Ga 2 O 3 loaded, whereas the CO 2 reduction, for higher Ga 2 O 3 -loaded samples (2.6, 4.2 vol %). Crystalline structure characterizations of Ga 2 O 3 /Al 2 O 3 catalysts indicate that active sites for both reactions are different. The water splitting proceeds on nanometer-sized Ga 2 O 3 rods dispersed on an Al 2 O 3 support consisting of a little distorted α-Ga 2 O 3 phase. On the other hand, the CO 2 reduction proceeds on sub-micrometer-sized Ga 2 O 3 particles consisting of mixed phases of α-Ga 2 O 3 and γ-Ga 2 O 3 or with appearance of boundaries between the α and γ phases, which plays a critical role. Al 2 O 3 used as the support of the Ga 2 O 3 particles does not seem to play an important role in the photocatalytic CO 2 reduction.

DOI： 10.1021/acsomega.9b00048

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Various metal oxides with 0.1 wt% Ag loaded as a cocatalyst were prepared by an impregnation method and examined their photocatalytic activity for CO2 reduction with water. Among all the prepared Ag-loaded metal oxides, Ga2O3, ZrO2, Y2O3, MgO, and La2O3 showed activities for CO and H2 productions under ultraviolet light irradiation. Thus, metal oxides involving metal cations with closed shell electronic structures such as d0, d10, and s0 had the potential for CO2 reduction with water. In situ Fourier transform infrared measurement revealed that the photocatalytic activity and selectivity for CO production are controlled by the amount and chemical states of CO2 adsorbed on the catalyst surface and by the surface basicity, as summarized as follows: Ag/ZrO2 enhanced H2 production rather than CO production due to very little CO2 adsorption. Ag/Ga2O3 exhibited the highest activity for CO production, because adsorbed monodentate bicarbonate was effectively converted to bidentate formate being the reaction intermediates for CO production owing to its weak surface basicity. Ag/La2O3, Ag/Y2O3, and Ag/MgO having both weak and strong basic sites adsorbed larger amount of carbonate species including their ions and suppressed H2 production. However, the adsorbed carbonate species were hardly converted to the bidentate formate.

DOI： 10.1002/sia.6542

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

Ga2O3 samples with different crystalline structures were prepared by calcination of a gallium nitrate powder around 800 K. Ga2O3 samples with mixed phases of γ and β showed high photocatalytic activity for CO production from CO2 reduction with water, and the activity was even higher than that for an Ag-loaded β-Ga2O3. The photocatalytic activity increased with time. The increase was attributed to the appearance of GaOOH resulting from the interaction of Ga2O3 with water during the reaction as revealed by XRD and XPS analyses. In situ FT-IR measurements revealed that bicarbonates and bidentate carbonate species were adsorbed on GaOOH. Therefore, the increase of the photocatalytic activity with time would be derived from the formation of GaOOH phase on the γ-Ga2O3 and β-Ga2O3 sample.

DOI： 10.1002/sia.6552

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With a view to rational designing of a highly functional visible-light TiO2 photocatalyst, nitrogen atoms were doped into TiO2 samples by an ion implantation technique which enables to control the depth and concentration of dopants. Although the absorbance in the visible-light region of the sample increased by the nitrogen doping, photocatalytic activity of the sample was not directly connected with the photo-absorbance. The N K-edge X-ray absorption near edge structure (XANES) spectrum of the photocatalytic active sample (A-cat) showed a characteristic double peak at 398 and 401 eV, and the XANES spectrum of the inactive sample (I-cat) a distinct single peak around 401 eV. These features of the XANES spectra were well reproduced by theoretical simulations based on the model where an O atom in TiO2 was replaced by N ((N)s) for A-cat, and that of quasi NO2 molecule ((NO2)s) for I-cat. Therefore, we have concluded that the nitrogen atom occupying the oxygen site of TiO2 is photocatalytic active species effective for visible light photocatalysis. In addition, the quantitative XANES/ELNES analysis has revealed that the photo-absorbance ratio of I-cat to A-cat corresponds well to the ratio of total doped nitrogen concentration rather than photocatalytic active nitrogen ((N)s) concentration. This result indicates that not only (N)s but also (NO2)s also absorb the visible light. Thus, the absorbance in the visible-light region is not necessarily an indication of the visible-light response of a photocatalyst.

DOI： 10.1002/sia.6543

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Language：English   Publishing type：Research paper (international conference proceedings)  

To promote photocatalytic activity of gallium oxides (Ga2O3) on CO2 reduction with water under visible light irradiation, we have tried nitrogen doping into Ga2O3 with different crystalline structures. In diffuse reflectance UV-vis spectra, absorption bands appeared in the visible light region after the nitrogen doping and the absorption edge shifted to a longer wavelength region with increasing nitrogen doping temperature. N K-edge XANES analysis clearly showed two kinds of nitrogen species doped in the samples; gallium nitride (GaN) species and molecular like nitrogen. In XRD patterns, nitrogen doping at temperatures above 823 K, gallium nitride phases appeared while the original crystal structures of gallium oxide samples maintained when nitrogen doping temperature was less than 823 K. However, photocatalytic CO2 reduction under visible light irradiation was insignificant for all the nitrogen doped samples, because nitrogen doped in Ga2O3 samples was unstable in water under the visible light

DOI： 10.1380/ejssnt.2018.262

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Ga2O3 prepared by calcination of a gallium nitrate powder showed high activity for CO2 reduction with water to produce CO, and the CO production rate was 150 times that of commercially available Ga2O3 (CA-Ga2O3). Although we also prepared Ag loaded Ga2O3 (Ag/Ga2O3) for further improvement of CO production, the CO production activity decreased by Ag loading on the prepared Ga2O3. On the other hand, CO production was enhanced by Ag loading on CA-Ga2O3. The crystalline structures of the prepared Ga2O3 and CA-Ga2O3 were coexistence of γ- and β- phases of Ga2O3 and single β-phase of Ga2O3, respectively. The crystalline structure of CA-Ga2O3 was maintained after Ag loading with various methods, however, that of the prepared Ga2O3 changed by Ag loading. Therefore, the CO production activity increased by Ag loading depended on the crystalline structure of Ga2O3 supports. In addition, we revealed that Ga2O3 with mixed phases of γ- and β- not loaded with Ag exhibited the highest photocatalytic activity for CO production.

DOI： 10.1016/j.jphotochem.2017.11.010

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Silver nanoparticles (Ag NPs) were synthesized by a solution plasma method (SPM) in an aqueous solution of ammonia. Optical emission spectra of the plasma revealed that Ag NPs are fabricated with the sputtering of Ag rods as electrode by the produced energetic plasma particles such as H, OH and O radicals. In-situ optical absorption measurements of the solution during the discharge directly presented the concerted formation and aggregation processes of the Ag NPs, which controlled the size of Ag NPs. The synthesized Ag NPs were loaded on gallium oxide (Ga2O3) photocatalyst, and the photocatalytic activities of the obtained Ag loaded Ga2O3 (Ag/Ga2O3) samples were evaluated. Although the photocatalytic reaction proceeded over all the samples to produce CO, the CO production rates decreased with the reaction time. Measurements of DR UV–vis spectra and TEM images revealed that a part of the Ag NPs migrated and aggregated on the photocatalyst surface to become larger particles during the photocatalytic reaction, which would be related to the decrease of the photocatalytic activity. It was also found that the photoirradiation treatment on the prepared Ag/Ga2O3 sample before the use for the photocatalytic reaction improves the photocatalytic performance.

DOI： 10.1016/j.cattod.2017.08.047

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

Ag species in the Ag loaded Ga2O3 (Ag/Ga2O3) photocatalysts were investigated with regard to changes in their structures and the chemical states before and after the photocatalytic CO2 reduction with water. Ag clusters with the size of around one nanometer in as prepared 0.1 wt% Ag/Ga2O3 sample became larger metallic Ag nanoparticles with the size of several nanometers after the reaction. In the 1.0 wt% Ag/Ga2O3 sample, Ag nanoparticles having the AgGaO2 interface structure decomposed to aggregated Ag metal particles after the reaction. Such transition in the morphology and chemical states of Ag species could explain the variations in photocatalytic activities of these samples during the reaction, and suggested that at least metallic Ag particles would function as effective co-catalysts. In-situ FT-IR measurements of the Ag/Ga2O3 samples before and after the reaction, we could not detect CO2 molecules adsorbing on metallic Ag nanoparticles in dark. Under light irradiation, however, the absorption bands originated from the photo-adsorbed CO2 species on Ag metal nanoparticles appeared. Thus, we detected the interaction of CO2 molecules and metallic Ag nanoparticles as the effective co-catalysts for CO2 reduction.

DOI： 10.1016/j.cattod.2017.09.025

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

Nitrogen/fluorine-codoped rutile TiO2 (R-TiO2:N,F) was newly synthesized, and its photocatalytic activity for water oxidation was evaluated. R-TiO2:N,F could be prepared by nitridation of the rutile TiO2 (R-TiO2) and (NH4)2TiF6 mixture at 773 K. The prepared samples produced O2 from aqueous AgNO3 solution under visible light irradiation, while R-TiO2 nitrided at the same temperature without any fluorine source showed negligible activity. The highest activity was obtained with the sample prepared at the (NH4)2TiF6/R-TiO2 ratio of 15/85, exhibiting water oxidation activity even in the presence of a reversible electron acceptor such as IO3- or Fe3+ with the aid of a RuO2 cocatalyst. Stoichiometric water splitting into H2 and O2 was achieved using a mixture of Ru/SrTiO3:Rh and RuO2/TiO2:N,F in the presence of [Co(bpy)3]3+/2+ (bpy = 2,2′-bipyridine) as a shuttle redox mediator without noticeable degradation of activity under visible light and even under AM1.5G simulated sunlight. Transient absorption spectroscopy revealed that appropriate nitrogen/fluorine codoping reduces the density of mid-gap states working as deep traps of photogenerated electrons, and increases the number of free electrons compared to only nitrogen-doped R-TiO2. Experimental results highlighted that the photocatalytic activity of R-TiO2:N,F could be enhanced by improving visible-light absorption capability through N/F codoping while suppressing the density of deep trap sites.

DOI： 10.1039/c8se00191j

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

A Pt-loaded TiO2 photocatalyst successfully catalyzed the direct cross-coupling between various alkenes and tetrahydrofuran (THF) without any additional oxidizing agent. The reaction between cyclohexene and THF gave three cross-coupling products, namely, 2-cyclohexyltetrahydrofuran (A), 2-(cyclohex-2-en-1-yl)tetrahydrofuran (B) and 2-(cyclohex-1-en-1-yl)tetrahydrofuran (C), along with gaseous hydrogen. The mechanistic study revealed that these products were formed through different individual mechanisms: successive addition of two radical species, a 2-tetrahydrofuranyl radical and a hydrogen radical, to the double bond of cyclohexene for A, coupling of a 3-cyclohexenyl radical and a 2-tetrahydrofuranyl radical for B, and 2-tetrahydrofuranyl radical addition and hydrogen radical elimination at the double bond of cyclohexene for C. Among these three mechanisms, those for B and C are dehydrogenative. In this photocatalytic reaction system, since the cyclohexene molecule has enough reactivity, due to the localized π electron density, the Pt nanoparticles loaded on the TiO2 function not as a metal catalyst but as an electron receiver to enhance the charge separation, although the dehydrogenative cross-coupling of benzene with THF requires Pd metal catalysis.

DOI： 10.1039/c8cy00129d

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

Carbon nitride nanosheets (NS-C3N4) were found to undergo robust binding with a binuclear ruthenium(II) complex (RuRu′) even in basic aqueous solution. A hybrid material consisting of NS-C3N4 (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/C3N4 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.

DOI： 10.1002/anie.201701627

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PubMed

Language：English   Publishing type：Research paper (international conference proceedings)  

A photodeposition process of Pt metal particles on anatase TiO2 in the aqueous solution of H2PtCl6 (precursor) and methanol (reductant) was studied using transmission electron microscopy, UV-vis spectroscopy and X-ray absorption fine structure spectroscopy. These analyses proposed the photodeposition mechanism of Pt on TiO2 by the photogenerated electrons: the photo-assisted adsorption of Pt4+ complexes on TiO2 through the ligand exchange at the initial stage, followed by the successive rapid reduction of Pt4+ to Pt0 to grow the Pt metal particles, which gives small and almost uniform size of Pt metal nanoparticles.

DOI： 10.1088/1742-6596/712/1/012076

Scopus

Language：English   Publishing type：Research paper (international conference proceedings)  

Ga2O3 loaded Al2O3 samples (Ga2O3/Al2O3) were prepared to change coordination structures around Ga atoms. Ga K-edge XANES spectra of the Ga2O3/Al2O3 samples showed two peaks assigned to Ga atoms having tetrahedral coordination structure (Ga(t)) and octahedral one (Ga(o)). Curve-fitting analysis of XANES spectra was carried out with a set of pseudo- Voight and arctangent functions, and the fractions of Ga(t) and Ga(o) were quantitatively estimated from the ratio of the peak areas. EXAFS curve-fitting analysis also evaluated the fractions of Ga(t) and Ga(o) and they were in good agreement with those obtained by XANES analysis. It was revealed that the fraction of Ga(t) increased with the decrease in the loading amount of Ga2O3 due to the interaction of Ga species with Al2O3. The fractions of Ga(t) and Ga(o) might relate to the photocatalytic activity for CO2 reduction with H2O over the Ga2O3/Al2O3 samples.

DOI： 10.1088/1742-6596/712/1/012056

Scopus

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)  

Ag loaded Ga2O3 (Ag/Ga2O3) shows photocatalytic activity for reduction of CO2 with water. Ag L3-edge XANES and K-edge EXAFS spectra were measured for various Ag/Ga2O3 samples, which suggested that structural and chemical states of Ag species varied with the loading amount of Ag and the preparation method. The Ag species were metallic Ag particles with an AgGaO2-like interface structure in the sample with high loading amount of Ag while predominantly Ag metal clusters in the sample with low loading amount of Ag. The XANES feature just above the edge represented the interaction between the Ag species and the Ga2O3 surface, showing that the Ag metal clusters had more electrons in the d-orbitals by interacting with the Ga2O3 surface, which would contribute the high photocatalytic activity.

DOI： 10.1088/1742-6596/712/1/012074

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

We investigated the photocatalysis of dendritic nanostructured WO3/W composite materials fabricated by He plasma irradiation to tungsten plates, followed by the surface oxidation. The samples promoted the decolorization reaction of methylene blue (MB) aqueous solution under near infrared (NIR) light irradiation. To verify the MB molecule is actually decomposed by the photocatalysis of the samples, reaction products were analyzed by S K-edge XANES measurements for the MB solution kept with the samples under the light irradiation or in the dark. By the light irradiation, the σ∗(S-C) peak in the XANES spectra reduced and a new peak originated from SO42- species was clearly observed, suggesting that S-C bonds in a MB molecule are broken by the NIR light irradiation and finally the sulfur species exists in the solution in the state of SO42- ion. After the adsorption reaction in the dark, the XANES spectra of the sample surfaces showed a sharp π∗(S-C) peaks, indicating that MB molecules are adsorbed on the sample surfaces and stacked each other by the π-π interaction. These results demonstrate that the photocatalytic decomposition of MB molecules really proceeds over WO3/W composite materials even under NIR light irradiation.

DOI： 10.1016/j.nimb.2015.07.035

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

A nitrogen doped TiO2 as a visible-light response photocatalyst was prepared by N+ implantation technique. N+-implanted TiO2 samples promoted the photocatalytic activity for degradation of methylene blue 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 activity sample, the additional electronic states were observed just above the valence band edge of a TiO2. The electronic state would be originated from the substitutional nitrogen and be responsible for the band gap narrowing, i.e., visible light response of TiO2 photocatalysts.

DOI： 10.1016/j.nimb.2015.04.010

Scopus

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

Ag loaded Ga2O3 (Ag/Ga2O3) photocatalysts were prepared by an impregnation method and examined for the photocatalytic reduction of CO2 with water where CO, H2 and O2 were formed as products. TEM and X-ray absorption near edge structure (XANES) measurements revealed that around 1 nm sized Ag clusters were formed predominantly in an active Ag/Ga2O3 sample while partially oxidized large Ag particles with the size of several-several tens of nm were observed in a less active Ag/Ga2O3 sample. Both Ag L3-edge and O K-edge XANES analyses suggested that the small Ag clusters accepted more electrons in the d-orbitals as a result of the strong interaction with the Ga2O3 surface. In situ FT-IR measurements of the Ag/Ga2O3 samples showed CO3 stretching vibration bands assignable to monodentate bicarbonate and bidentate carbonate species chemisorbed on the Ga2O3 surface, and to monodentate carbonate species on the large Ag particles. Among these chemisorbed species, the monodentate bicarbonate and/or the bidentate carbonate species changed to bidentate formate species, as the reaction intermediate, under UV light irradiation. The bidentate formate species was formed not by the plasmonic excitation of the Ag nanoparticles but by the photoexcitation of the Ga2O3 semiconductor, and the formation process would be promoted at the perimeter of the Ag clusters on the Ga2O3 surface by the effective separation of electron-hole pairs.

DOI： 10.1039/c5ta04815j

Scopus

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

Ag loaded Ga2O3 (Ag/Ga2O3) photocatalysts for the reduction of CO2 with water have been prepared by impregnation (IMP) and two types of solution plasma methods (SPM1 and SPM2). Using X-ray absorption near edge structure (XANES) and Fourier transform infrared (FT-IR) spectroscopies, we have investigated the local electronic structures of Ag/Ga2O3 photocatalysts as well as the adsorption behaviors of CO2 during the reaction. Both Ag L3-edge and O K-edge XANES analyses reveal the Ag-Ga2O3 interaction, i.e., the charge-transfer from O atoms to Ag atoms, by demonstrating the decrease in the unoccupied Ag 4d-state density and increase in the unoccupied O 2p-state density. The strength of the interaction depends on the preparation method, and increases in the order of Ag/Ga2O3 (SPM2), Ag/Ga2O3 (SPM1) and Ag/Ga2O3 (IMP). In addition, FT-IR measurements have disclosed that Ag/Ga2O3 (IMP) obtains a larger amount of strongly basic sites as a result of the strongest interaction between Ag and Ga2O3. Although the amount of the adsorbed CO2 is different in each Ag/Ga2O3 sample, in the following formation process of bidentate formate species, no remarkable difference is detected among all samples. The bidentate formate species are likely to interact with H2O molecules to produce CO under photoirradiation, and this process would be affected by the strength of the Ag-Ga2O3 interaction, because this reaction hardly proceeds over Ag/Ga2O3 (SPM2) having the weakest Ag-Ga2O3 interaction.

DOI： 10.1016/j.nimb.2015.07.031

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1016/j.jcis.2014.12.097

Scopus

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)  

Ag loaded Ga2O3 (Ag/Ga2O3) has exhibited photocatalytic activity for CO2 reduction with water to produce COas well as for water splitting to H2 and O2. In-situ FT-IR measurements have shown CO3 stretching vibration bandsassignable to carbonate and bicarbonate species when CO2 molecules chemisorbed on the catalyst surface. Thesespecies change to bidentate formate species under photoirradiation. It subsequently converts to CO by interactingwith water molecules, not chemisorbed OH on the catalyst surface. This result suggests that formate species isan intermediate of the photocatalytic CO2 reduction. On the other hand, Au loaded Ga2O3 (Au/Ga2O3) hasproduced H2 predominantly with a very small amount of CO formation, since the less amount of CO2 moleculeschemisorb on Au/Ga2O3. FT-IR measurements of the Ag/Ga2O3 with high Ag loading have shown carbonatespecies due to the adsorption of atmospheric CO2 on hydroxyl group. The carbonate species is too unstable underphotoirradiation to convert into formate species, but is stabilized as CO2-3 species by the reaction with water.XANES analysis has revealed that atomically dispersed Ag metal species may be effective for the adsorption ofCO2 and the subsequent conversion into formate species to promote the photocatalytic CO2 reduction. © 2014 The Surface Science Society of Japan.

DOI： 10.1380/ejssnt.2014.299

Scopus

Authorship：Lead author,　Corresponding author   Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (bulletin of university, research institution)