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

Abstract

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.

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

“…It should be noted that the selectivity of CO production is nearly 40% which is quite high for the photocatalytic CO 2 reduction without the Ag cocatalyst. Furthermore, the CO production rate is much higher than those observed in the previous study 11 in which α-Ga 2 O 3 was synthesized under higher temperature and longer calcination time was employed and hence the nano-pores might have been disappeared. Considering the small mass of the sample used as the photocatalyst (0.05 g) and non-use of a co-catalyst, it can be concluded that the synthesized meso-porous α-Ga 2 O 3 showed quite high activity for photocatalytic reduction of CO 2 .…”

“…In a previous work, Sonoda et al 11 have reported first time that α-Ga 2 O 3 formed by the calcination of GaOOH made of liquid metal Ga hydroxidation with water showed high photocatalytic activity for the CO 2 reduction with water. Following the previous work, in this work, we have succeeded to synthesize meso-porous α-Ga 2 O 3 which shows significantly high photocatalytic activity for CO 2 reduction with water under UV light illumination, using liquid Ga metal as a starting material, through the hydroxidation to GaOOH and Ga(OH) 3 followed by the calcination of these hydroxides.…”

Overall Photocatalytic CO2 Reduction over Heterogeneous Semiconductor Photocatalysts

“…[31] For example, Cu 2 O crystals with different exposed crystal facets exhibit different bandgaps owing to the ultrathin surface layers (~1.5 nm) on the crystal facets, which have different band-bending degrees and structures, resulting in dissimilar band energies. [32] Therefore, the bandgaps and band edges of Cu 2 O with various morphologies, such as spheres, cubes, rhombic dodecahedra, and octahedra, differ because of the different ratios of the (100), (111), and (110) crystal facets. [32][33] In addition, the electronic structure of every crystal facet is distinctive, which directly affects the interactions between the catalyst surface and reactants, intermediates, or products.…”

“…From this perspective, the selective exposure of specific crystal facets yields extremely different results, which can be used to improve reaction efficiency. For example, Cheng et al [31c] observed that the (111) facet of the designed NH 2 -MIL-125(Ti) MOF was active for the PCRR, and that increasing the proportion of exposed (111) facets could improve photocatalytic performance. Wu et al [11] demonstrated that the (110) facet of the Cu 2 O photocatalyst exhibited activity for the overall PCRR to produce CH 3 OH, whereas the (100) facet was inert (Figure 2d).…”