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.


Introduction
Gallium sesquioxide (Ga 2 O 3 ) is well known as one of the promising photocatalysts for CO 2 reduction with water and has been extensively studied. 1 Nevertheless, its photocatalytic activity is not high enough and signicant improvement is required for practical use. Nanoparticulation of photocatalysts is oen employed for their improvement. 2,3 In addition, because Ga 2 O 3 shows polymorphism having six different crystalline phases, a, b, g, d, 3 and k, the effects of articial control of phases and morphology for improvement have been investigated. [4][5][6][7] Li et al. 8 and Das et al. 9 have claimed that meso-porous a-Ga 2 O 3 having a large specic surface area was formed easily by the calcination of a-GaOOH and showed high photocatalytic activity for the degradation of dyes. Although, Rodriguez et al. 10 also have reported that a-Ga 2 O 3 formed by the calcination of GaOOH made from liquid Ga metal oxidation degraded MG dye well, little investigation on the photocatalytic activity of a-Ga 2 O 3 for CO 2 reduction with water has been done.
In a previous work, Sonoda et al. 11 have reported rst time that a-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 a-Ga 2 O 3 which shows signicantly 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.
Although it is well known that utilization of silver (Ag) as a cocatalyst enhances the CO production in the photocatalytic reduction of CO 2 , we are focusing the photocatalytic activity of Ga 2 O 3 and the mechanism of photocatalytic reduction of CO 2 with water without the Ag cocatalyst.

Experimental
Sample preparation 1 g of liquid Ga metal (Kojundo Chemical Laboratory Co. Ltd purity 99.99%) and 100 mL of distilled water were placed in a Teon beaker and agitated for 6 days until Ga was mostly dispersed in distilled water as hydroxides, GaOOH and Ga(OH) 3 , which were the reaction products with water. The temperature was kept constant at around 323 K and the loss of the water by evaporation was compensated with water injection. Aer the hydroxidation, the product was ltered and dried in air, and then calcined in air at 773 K for 1 hour to convert to a-Ga 2 O 3 which is referred as a synthesized sample hereaer.

Characterization
The characterization of the hydroxidized and synthesized sample was performed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), specic surface area (SSA) measurements and UV-Vis diffuse reectance spectroscopy. The XRD analysis was done with MiniFlex600 (Rigaku) in which Cu Ka was used as a radiation source under operating voltage of 40 kV and current of 15 mA. The angle scanning rate of an X-ray detector was 10 min À1 . SEM images were observed by a eld emission scanning electron microscope (FE-SEM, JSM-6500F, JEOL Ltd) under an acceleration voltage of 15 kV. TEM images were obtained by a transmission electron microscope (JEM-2100F, JEOL Ltd) under an acceleration voltage of 200 kV. Specic surface areas (SSA) of the samples were determined by the Brunauer-Emmett-Teller (BET) method with using MonosorbTM (QUANTACHROME). Before the BET measurement, each sample was heated in N 2 gas at 423 K for 3 h, which did not give signicant changes in morphology and crystalline phases. UV-Vis spectra were obtained with an UV-visible near-infrared absorbance photometer (V-670, JASCO) with the diffuse reection method using the BaSO 4 as a reference.

Photocatalytic reduction of CO 2 with water
The photocatalytic CO 2 reduction tests were conducted with using synthesized samples as the catalysts. A sample of 0.05 g was placed in a quartz reaction cell together with aqueous solution of 0.5 M NaHCO 3 . The UV light illumination was done by a Xe lamp through the UV cold mirror. The light intensity was 30 mW cm À2 at 254 AE 10 nm. CO 2 gas was owed in the cell with the ow rate of 3 mL min À1 . The reaction products (H 2 , O 2 and CO) were analysed quantitatively by a gas chromatograph equipped with a thermal conductivity detector every one hour up to 24 hours including 1 hour for a background measurement performed without the UV light illumination.

Results and discussion
Sample characterization  Fig. 1(a) clearly shows the hydroxidation of liquid Ga metal to be orthorhombic GaOOH and cubic Ga(OH) 3 with no trace of the metallic phase of Ga. The morphologies of the samples corresponded well to their XRD pattern, i.e., GaOOH exhibits smaller columnar particles, while Ga(OH) 3 larger rectangular particles.
As seen in the XRD pattern ( Fig. 1(b)), the synthesized sample was fully converted to a-Ga 2 O 3 . Although the morphology of the synthesized samples did not change appreciably from that of the hydroxidized ones (compare Fig. 2(c) and (a)), the enlarged image of the former (Fig. 2(d)) shows nanopores which was introduced by the calcination. In TEM images given in Fig. 2(e) and (f), a few nm sized pores were homogeneously distributed in a-Ga 2 O 3 . Unfortunately, it is difficult to distinguish closed or open pores in the TEM image. In addition, particle sizes of a-Ga 2 O 3 were widely distributed so that it was hard to correlate the SSA with the pore size distribution.
The XRD pattern and morphology of the sample aer use as a photocatalyst for CO 2 reduction with water stayed nearly the same as that before use, except the appearance of small XRD peaks assigned to be GaOOH as shown in Fig. 1(c).
Specic surface areas determined by BET analysis of the samples hydroxidized and synthesized were 2.52 and 19.2 m 2 g À1 , respectively. Considering the morphology changes appeared in SEM images, the signicant increase of specic  surface area is most probably caused by the formation of nanoporous by the calcination as seen in Fig. 2(d) and (f). The pores should be formed by the evaporation of H 2 O released by the oxidation of the hydroxides during the calcination as, Photocatalytic reduction of CO 2 with water With using the synthesized meso-porous a-Ga 2 O 3 as a photocatalyst, CO 2 reduction with water under UV light illumination was conducted. Fig. 3  ] being respective production rates, is also shown. Although all production rates initially increased with the time, they were saturated aer 5 hours and aerwards stayed constant keeping the stoichiometry, indicating that the CO 2 reduction proceeded photocatalytically. The high CO production rates continued over 24 hours. 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 a-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 a-Ga 2 O 3 showed quite high activity for photocatalytic reduction of CO 2 . Although, the direct comparison of photocatalytic activities obtained with different reaction cells appeared in the literature, the present CO production rate (20 mmol h À1 0.05 g À1 ) is much higher than the previously observed highest production rate of CO in photocatalytic CO 2 reduction with water using the mixed phase of band g-Ga 2 O 3 without cocatalyst (10 mmol h À1 0.1 g À1 ). 12 The present CO production rate is even comparable to previous studies using cocatalysts like rare-earth, Ag, Pt, transition metal doping and utilization of support. [13][14][15][16][17] It should be noted that in the early stage of the photocatalytic reduction, the stoichiometry among the product was not kept with higher H 2 production rate (see Fig. 3). Most probably some Ga remained in the sample, which were not appreciable in XRD, was oxidized with water, producing H 2 as, This is conrmed by the change of the UV-Vis spectra of the sample before and aer use as the photocatalyst as shown in Fig. 4. The sample before use showed the absorption in visible wavelength region indicating the existence of Ga with metallic state, while that aer use showed no absorption.
In the XRD of the sample aer use, the peak attributed to GaOOH was clearly observed. Kawaguchi et al. have indicated that the formation of GaOOH during the photocatalytic reaction with water on the mixed phase of b-Ga 2 O 3 /g-Ga 2 O 3 enhanced the CO 2 reduction. 18 Therefore, some GaOOH formation on Ga 2 O 3 surface could have important contribution in the reduction process. As for the reaction mechanism, we have proposed the following redox like chemical reaction. 19 Initially the surface of a-Ga 2 O 3 is hydroxidized with water to be GaOOH. Then UV light illumination dissociated the GaOOH as 2GaOOH / a-Ga 2 O 3 + H 2 + 1/2O 2 (5) During this reduction process, some H 2 involved in the reduction of CO 2 . Without the UV illumination, Ga 2 O 3 on the surface retuned to GaOOH by reacting with water.
Because in water near room temperature, GaOOH is thermodynamically more stable than a-Ga 2 O 3 , the surface of a-Ga 2 O 3 would be converted to GaOOH, i.e. the surface of both phases in water are in thermodynamical equilibrium. Fig. 3 Time sequences of production rates of H 2 , CO and O 2 , and the electron and hole ratio consumed for the gaseous product formation, R (e À /h + ) under CO 2 reduction tests for sample. At present the details of CO 2 reduction mechanism is not clear. Probably nanopores in a-Ga 2 O 3 would enhance CO 2 adsorption, however the further study is needed to conrm this and to know the effect of GaOOH.

Conclusions
We have succeeded to synthesize meso-porous a-Ga 2 O 3 which shows signicantly high photocatalytic activity of CO 2 reduction with water under UV light illumination. The activity is one of the highest among those reported including Ag co-catalyst assisted or other metal doped ones. The synthesis was done in the following way. At rst, liquid Ga metal was hydroxidized in water at 323 K resulting in mixed hydroxides of GaOOH and Ga(OH) 3 . Then the mixed hydroxides were calcined at 773 K for 1 hour, which fully oxidized them into a-Ga 2 O 3 . Thus synthesized a-Ga 2 O 3 included meso-pores caused by the vaporization of water during the calcination as evidenced by high specic surface area. The mesoporous structure is most likely the main cause for the signicantly high photocatalytic activity. Since the present method is rather easy to control the conditions of a-Ga 2 O 3 synthesis, further improvement of the activity would be possible with optimization of morphology and meso-porous structures.

Conflicts of interest
There are no conicts to declare.