Ga2O3 Photocatalysts Research Articles

Photocatalytic conversion of CO2 using K2YTa5O15 photocatalyst in the presence of silver-iron dual cocatalyst

Abstract Photocatalytic conversion of CO2 is a promising technique for realising a carbon neutral society based on the use of renewable energy sources. The application of dual cocatalysts consisting of silver-cobalt or silver-iron significantly improved the photocatalytic activity of an Al−SrTiO3 photocatalyst for the conversion of CO2 to CO in water. In this work, the photocatalytic conversion of CO2 using Ga2O3 and K2YTa5O15 photocatalysts was successfully performed in the presence of a silver-iron dual cocatalyst loaded via a photodeposition method, and the formation rate of CO and selectivity toward CO production were simultaneously increased by the addition of Fe species. The promotion of the H2O oxidation reaction through Fe addition ensures superior activity for the CO2 conversion into CO.

Facilitating methane conversion and hydrogen evolution on platinized gallium oxide photocatalyst through liquid-like water nanofilm formation

The dehydrogenative coupling of methane, an approach aimed at converting methane into valuable chemicals like ethane and hydrogen, can be facilitated under ambient thermal conditions using semiconductor photocatalysts. Herein, we present an efficient continuous-flow photocatalytic system for methane conversion by inducing the formation of physically adsorbed water nanofilms on the surface of a Pt-loaded Ga2O3 photocatalyst. The microscopic properties of the adsorbed water were examined by diffuse reflectance infrared (IR) spectroscopy and ab initio molecular dynamics (AIMD) calculations. In comparison to the photocatalytic non-oxidative coupling of methane operated in the absence of water vapor, the presence of the interfacial water resulted in a 90-fold and 500-fold enhancement in the production rate of hydrogen and ethane, respectively. The rate of ethane formation significantly increased with increasing water vapor pressure (PH2O) under a high methane pressure (PCH4) of 200 kPa. The selectivity for ethane reached 65 % based on carbon content at PH2O = 3 kPa and PCH4 = 200 kPa. Through diffuse reflectance IR spectroscopy, volumetric water vapor adsorption analysis, and AIMD calculations conducted under the reaction conditions, we observed the formation of a liquid-like water nanolayer on the Ga2O3 surface at 300 K. Thus, this strategy of inducing water nanofilms affords a highly efficient photocatalytic system for hydrogen and ethane production.

Methane coupling and hydrogen evolution induced by palladium-loaded gallium oxide photocatalysts in the presence of water vapor

Non-oxidative coupling of methane (NOCM, 2CH4 → C2H6 + H2) is a reaction that can directly produce ethane and hydrogen at the same time, and gallium oxide (Ga2O3) powder has been reported as an effective photocatalyst for NOCM at room temperature. In this study, we investigated the reaction conditions for Pd-loaded Ga2O3 photocatalysts to improve the production rate of C2H6 and H2. We found that the 0.1 wt% Pd/Ga2O3 exhibited high selectivity of C2H6 (75.8%, carbon-based) under the conditions of steam reforming of methane. Photocatalytic NOCM seems to proceed in the presence of small amount of water. An increase in water vapor pressure (PH2O) was essential for the steady production of C2H6 and H2. The C2H6 production rate was 0.79 μmol min−1 for 50 mg of Pd/Ga2O3 powder at PH2O = 3.6 kPa. The apparent quantum efficiency (AQE) for C2H6 production was 5.1%, which is much higher than that of conventional photocatalytic NOCM in the absence of water vapor. The importance of water adsorbates on the photocatalyst surface was suggested by water vapor adsorption isotherm and Fourier transform infrared (FT-IR) spectroscopy. It is revealed that multilayered water molecules adsorbed on the photocatalyst surface play a role as a reaction field that promotes the dehydrogenative coupling of CH4.

Time-resolved infrared spectroscopic investigation of Ga2O3 photocatalysts loaded with Cr2O3-Rh cocatalysts for photocatalytic water splitting

Investigation of the charge dynamics and roles of cocatalysts is crucial for understanding the reaction of photocatalytic water splitting on semiconductor photocatalysts. In this work, the dynamics of photogenerated electrons in Ga2O3 loaded with Cr2O3-Rh cocatalysts was studied using time-resolved mid-infrared spectroscopy. The structure of these Cr2O3-Rh cocatalysts was identified with high-resolution transmission electron microscopy and CO adsorption Fourier-transform infrared spectroscopy, as Rh particles partly covered with Cr2O3. The decay dynamics of photogenerated electrons reveals that only the electrons trapped by the Rh particles efficiently participate in the H2 evolution reaction. The loaded Cr2O3 promotes electron transfer from Ga2O3 to Rh, which accelerates the electron-consuming reaction for H2 evolution. Based on these observations, a photocatalytic water-splitting mechanism for Cr2O3-Rh/Ga2O3 photocatalysts has been proposed. The elucidation of the roles of the Cr2O3-Rh cocatalysts aids in further understanding the reaction mechanisms of photocatalytic water splitting and guiding the development of improved photocatalysts.

Preparation of Ag-Loaded Ga2O3 Particles by the Ultrasonic Reduction Method and their Photocatalytic Activities for CO2 Reduction

Photocatalytic reduction of CO2 is attracting attention as one of the CCUS (Carbon dioxide Capture, Utilization and Storage) technologies which are necessary for preventing global warming. Silver (Ag)-loaded Ga2O3 photocatalyst is a promising material used for the reduction of CO2 to CO. In this research, the ultrasonic reduction method was investigated as a method of loading Ag nanoparticles on Ga2O3. With this method, Ag nanoparticles were synthesized only by ultrasonic irradiation of silver oxide (Ag2O) in ethanol solvent. Silver nanoparticles with the particle sizes of 20 ± 2 nm were homogeneously loaded on Ga2O3 particles when the loading amount of Ag was in the range of 0.4 wt% to 3.8 wt%. The particle sizes of the loaded Ag particles were ca. 250 nm when using the conventional impregnation and photoreduction methods. Regarding photocatalytic activities for CO2 reduction, the selectivity toward CO evolution of Ag-loaded Ga2O3 prepared by the ultrasonic reduction method was higher than that by the conventional methods. It was shown that Ag loading by the ultrasonic reduction method was not only advantageous for industrial production, but also superior in the photocatalytic performance.

Photocatalytic Activity of Metal Oxide Supported Gallium Oxide for CO2 Reduction with Water

Abstract We have examined photocatalytic activities of Ga2O3 supported by various metal oxides for water splitting and CO2 reduction with water under UV light irradiation. The Ga2O3 photocatalyst supported by metal oxides was prepared by an impregnation method. We have found that some oxides used as a supporting oxide of Ga2O3 photocatalyst improve activities of water splitting and CO2 reduction. In particular, the CO production rates are well correlated to various properties of the supporting oxide. These oxides satisfy the following three criteria significantly enhancing the CO production rate. (1) Their band gap is wider than that of Ga2O3 and the position of its conduction band is at a negative potential with respect to that of Ga2O3. (2) They are not miscible with Ga2O3 and (3) mixed phases of α and γ or defective γ phase of Ga2O3 are well dispersed on them exhibiting columnar shape. These criteria would be useful for further improvement of the activity for photocatalytic CO2 reduction with water by Ga2O3.

Comparison of Ga2O3 and TiO2 Nanostructures for Photocatalytic Degradation of Volatile Organic Compounds

The photocatalytic degradation of formaldehyde, acetaldehyde, toluene, and styrene are compared using monoclinic Ga2O3 and anatase TiO2 nanostructures under ultraviolet-C irradiation. These Ga2O3 and TiO2 photocatalysts are characterized using a field emission scanning electron microscope, a powder X-ray diffraction system, the Brunauer–Emmett–Teller method, and a Fourier transform infrared spectrometer. The Ga2O3 shows a higher reaction rate constant (k, min−1) than TiO2 by a factor of 7.1 for toluene, 8.1 for styrene, 3.1 for formaldehyde, and 2.0 for acetaldehyde. The results demonstrate that the photocatalytic activity ratio of the Ga2O3 over the TiO2 becomes more prominent toward the aromatic compounds compared with the nonaromatic compounds. Highly energetic photo-generated carriers on the conduction/valence band-edge of the Ga2O3, in comparison with that of the TiO2, result in superior photocatalytic activity, in particular on aromatic volatile organic compounds (VOCs) with a high bond dissociation energy.

Real time measurements of UV-vis diffuse reflectance of silver nanoparticles on gallium oxide photocatalyst

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.

Non-oxidative coupling of methane over Pd-loaded gallium oxide photocatalysts in a flow reactor

Photocatalytic methane conversion is attractive for utilization of renewable biogas and solar energy to directly produce useful compounds. In the present study, gallium oxide (Ga2O3) photocatalyst was examined for non-oxidative coupling of methane (NOCM) around room temperature in a flow reactor. It was found that ethane and hydrogen were continuously yielded at constant rates from methane upon photoirradiation around room temperature, confirming that NOCM can be promoted photocatalytically over Ga2O3. In addition, Pd cocatalyst was found to improve the activity of the Ga2O3 photocatalyst for NOCM to produce ethane and hydrogen at almost the same constant rate and achieve more than three times higher formation rate of ethane such as 0.22 μmol h−1 in a flow of 10% methane at 30 mL min−1 with 0.8 g of photocatalyst. The methane conversion achieved to 0.006% within a short contact time of 0.8 s, which is higher than the thermodynamic equilibrium conversion.

Photoelectrochemical investigation of the role of surface-modified Yb species in the photocatalytic conversion of CO2 by H2O over Ga2O3 photocatalysts

Abstract Modification of Ga2O3 photocatalysts with rare earth elements is known to enhance the activity and selectivity of CO formation during the photocatalytic conversion of CO2 by H2O as an electron donor, but the role of the rare earth species requires clarification. Photocurrent measurements revealed that modification with Yb species via hydrothermal treatment enhanced the anodic photocurrent of the Ga2O3 photoelectrode in CO2-saturated NaHCO3 aqueous solution while maintaining the onset potential of the anodic photocurrent. Impedance measurements revealed that the band structure of Ga2O3 was maintained following Yb modification, suggesting that the enhanced anodic photocurrent was due to the surface properties. The anodic photocurrent values of the Yb-modified Ga2O3 photoelectrode increased with an increase in the surface adsorption ability of CO2. The positive relationship between the anodic photocurrent value and the concentration of HCO3− species, as measured in two-component NaHCO3 and Na3PO4 electrolytes, indicated that the concentration of HCO3− species near the surface strongly influenced the anodic O2-formation activity of the Yb-modified Ga2O3 photoelectrode. We concluded that Yb modification improved the oxidation activity of H2O on the surface of Ga2O3 by enhancing the affinity for intermediate HCO3− species, resulting in a high activity for the photocatalytic conversion of CO2 by H2O.

Preparation of Ga2O3 photocatalyst highly active for CO2 reduction with water without cocatalyst

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.

Effect of Thickness of Chromium Hydroxide Layer on Ag Cocatalyst Surface for Highly Selective Photocatalytic Conversion of CO2 by H2O

In this study, we developed a Ag–Cr core–shell-structured (Ag@Cr) cocatalyst that modified the surface of the Ga2O3 photocatalyst. Compared to results provided by modifications with Ag cocatalysts and M–Cr dual cocatalysts (M = Au, Cu, Pd, and Pt), this change significantly improved the formation rate of CO and selectivity toward CO evolution in the photocatalytic conversion of CO2 by the electron donor H2O. As a result of this modification, the Cr(OH)3·xH2O shell changed to Cr(OH)x(CO3)y during the photocatalytic conversion. Furthermore, the thickness of the Cr(OH)3·xH2O shell was found to influence the photocatalytic performance. More specifically, Cr(OH)3·xH2O shells that were too thick or too thin were not beneficial to the CO evolution and suppression of H2 evolution. Notably, the highest photocatalytic activity (525.3 μmol h–1), selectivity toward CO evolution (85.2%), and turnover number of CO to Ag (167) was achieved over 0.25 mol % (Ag@Cr)/Ga2O3. In addition to Ga2O3, the Ag@Cr cocatalyst modific...

Model building of metal oxide surfaces and vibronic coupling density as a reactivity index: Regioselectivity of CO2 adsorption on Ag-loaded Ga2O3

The step-by-step hydrogen-terminated (SSHT) model is proposed as a model for the surfaces of metal oxides. Using this model, it is found that the vibronic coupling density (VCD) can be employed as a reactivity index for surface reactions. As an example, the regioselectivity of CO2 adsorption on the Ag-loaded Ga2O3 photocatalyst surface is investigated based on VCD analysis. The cluster model constructed by the SSHT approach reasonably reflects the electronic structures of the Ga2O3 surface. The geometry of CO2 adsorbed on the Ag-loaded Ga2O3 cluster has a bent structure, which is favorable for its photocatalytic reduction to CO.

Synergistic interplay of Zn and Rh-Cr promoters on Ga2O3 based photocatalysts for water splitting.

The photocatalytic water splitting activity of a wide-bandgap material, Ga2O3, is greatly boosted with the addition of a Zn and Rh-Cr co-catalyst at optimum loadings. To date, however, the exact roles of the co-catalysts and particularly the origin of their synergistic functions have not been clarified. Herein, we present how the optimum Zn loading on Ga2O3 leads to creation of a ZnGa2O4/Ga2O3 heterojunction favorable for charge separation through information on the occupied and unoccupied electronic states of Zn and Ga elucidated by X-ray absorption and emission spectroscopic methods. The function of Rh-Cr as an electron sink and reduction site was proven by photocatalytic experiments using an electron scavenger (Ag+) and by learning where Ag deposits and its effects on photocatalytic activity. Finally, perturbation of the Zn electronic structure by photoactivation was evidenced by modulation excitation X-ray absorption spectroscopy. Importantly, Rh-Cr markedly enhanced the level of the perturbation, serving as proof of the direct communication and synergy between the electronic states of Zn, present in ZnGa2O4, and Rh-Cr deposited on Ga2O3.

Enhancement of CO Evolution by Modification of Ga2O3 with Rare-Earth Elements for the Photocatalytic Conversion of CO2 by H2O.

Modification of the surface of Ga2O3 with rare-earth elements enhanced the evolution of CO as a reduction product in the photocatalytic conversion of CO2 using H2O as an electron donor under UV irradiation in aqueous NaHCO3 as a pH buffer, with the rare-earth species functioning as a CO2 capture and storage material. Isotope experiments using 13CO2 as a substrate clearly revealed that CO was generated from the introduced gaseous CO2. In the presence of the NaHCO3 additive, the rare-earth (RE) species on the Ga2O3 surface are transformed into carbonate hydrates (RE2(CO3)3·nH2O) and/or hydroxycarbonates (RE2(OH)2(3-x)(CO3)x) which are decomposed upon photoirradiation. Consequently, Ag-loaded Yb-modified Ga2O3 exhibits much higher activity (209 μmol h-1 of CO) than the pristine Ag-loaded Ga2O3. The further modification of the surface of the Yb-modified Ga2O3 with Zn afforded a selectivity toward CO evolution of 80%. Thus, we successfully achieved an efficient Ag-loaded Yb- and Zn-modified Ga2O3 photocatalyst with high activity and controllable selectivity, suitable for use in artificial photosynthesis.

Wide band gap Ga2O3 as efficient UV-C photocatalyst for gas-phase degradation applications.

α, β, γ, and δ polymorphs of 4.6-4.8eV wide band gap Ga2O3 photocatalysts were prepared via a soft chemistry route. Their photocatalytic activity under 254nm UV-C light in the degradation of gaseous toluene was strongly depending on the polymorph phase. α- and β-Ga2O3 photocatalysts enabled achieving high and stable conversions of toluene with selectivities to CO2 within the 50-90% range, by contrast to conventional TiO2 photocatalysts that fully deactivate very rapidly on stream in similar operating conditions with rather no CO2 production, no matter whether UV-A or UV-C light was used. The highest performances were achieved on the high specific surface area β-Ga2O3 photocatalyst synthesized by adding polyethylene glycol (PEG) as porogen before precipitation, with stable toluene conversion and mineralization rate into CO2 strongly overcoming those obtained on commercial β-Ga2O3. They were attributed to favorable physicochemical properties in terms of high specific surface area, small mean crystallite size, good crystallinity, high pore volume with large size mesopore distribution and appropriate surface acidity, and to the possible existence of a double local internal field within Ga3+ units. In the degradation of hydrogen sulfide, PEG-derived β-Ga2O3 takes advantage from its high specific surface area for storing sulfate, and thus for increasing its resistance to deactivation and the duration at total sulfur removal when compared to other β-Ga2O3 photocatalysts. So, we illustrated the interest of using high surface area β-Ga2O3 in environmental photocatalysis for gas-phase depollution applications.

Effect of Ag co-catalyst on CO2 adsorption states over Ga2O3 photocatalyst

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 0.1wt% Ag/Ga2O3 sample became larger metallic Ag nanoparticles with the size of several nanometers after the reaction. In the 1.0wt% 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.

Photocatalytic Degradation of Perfluorooctanoic Acid (PFOA) From Wastewaters by TiO2, In2O3 and Ga2O3 Catalysts

The aim of the work was to prepare nanosized In2O3 and Ga2O3 photocatalysts for degradation of perfluorooctanoic acid (PFOA) in water. Their commercial references along with TiO2 were used as a comparison basis. The characterization of the materials proved that successful preparation of cubic In2O3 and monoclinic β-Ga2O3 were achieved via solvothermal and hydrothermal methods, respectively. The effect of different parameters such as catalyst dosage, UV light source and utilization of inorganic oxidant in PFOA treatment were evaluated. In2O3, photocatalyst was the most efficient in the degradation of 15 mg L−1 PFOA under UVB irradiation and synthetic air reaching 27% of degradation, which was 20 percentage points higher than for commercial In2O3. This is proposed to be partly due to significantly higher specific surface area of the self-made In2O3 and smaller crystallite size and partly due to more efficient absorption of UVB light compared to the other tested materials. Addition of KBrO3 did not improve the activity of self-made In2O3.

Drastic improvement in the photocatalytic activity of Ga2O3 modified with Mg–Al layered double hydroxide for the conversion of CO2 in water

Modification of Ga2O3 photocatalyst with a Mg–Al layered double hydroxide (LDH) considerably enhanced not only the amount of CO evolved but also the selectivity toward CO evolution in the photocatalytic conversion of CO2 in water.

XAFS analysis for quantification of the gallium coordinations in Al2O3-supported Ga2O3 photocatalysts

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.