Analysis of Optical Properties and Structures of Nitrogen Doped Gallium Oxide

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 irradiation. [DOI: 10.1380/ejssnt.2018.262]


I. INTRODUCTION
Photocatalytic CO 2 reduction with water has attracted much attention because of its potential to realize an artificial photosynthesis [1][2][3]. To use water as a reductant for CO 2 reduction, the activation of water is intrinsically required. Therefore, heterogeneous metal oxide photocatalysts which can activate water have been widely studied targeting CO 2 reduction with water [4][5][6][7][8][9][10][11]. Recently, Akatsuka et al. and Teramura et al. have found that gallium oxide (Ga 2 O 3 ) shows photocatalytic activity for the CO 2 reduction with water producing CO under ultra violet (UV) light irradiation [5,6,[12][13][14]. However, the * This paper was presented at the 11th International Symposium on Atomic Level Characterizations for New Materials and Devices '17, Aqua Kauai Beach Resort, Kauai, Hawaii, USA, December 3-8, 2017. † Corresponding author: m17tc011@eb.osaka-cu.ac.jp reduction was not observed under visible light irradiation because the band gap of Ga 2 O 3 is too wide to absorb the visible light.
It is well known that only 6% or less energy of sunlight is given by UV light, while more than 50% by visible light [15]. For the efficient use of the sunlight energy, it is important to prepare photocatalysts driven by the visible light. Recently, it has been reported that nitrogen doping into metal oxides contributes to narrowing of their band gap, providing a visible-light response because N 2p orbitals are newly formed above O 2p orbitals in their valence band [16,17]

A. Sample preparation
It has been reported that Ga 2 O 3 prepared by calcination of gallium nitrate at around 800 K showed high photocatalytic activity for CO 2 reduction with water under UV light irradiation [18]. In this study, Ga

B. Characterization
The crystalline structures of the nitrogen doped samples were investigated by X-ray diffraction (XRD) analysis. XRD patterns of the samples were recorded on a Mini-Flex600 (Rigaku) using Cu Kα as the radiation source with an operating voltage of 40 kV and current of 15 mA. The XRD patterns were collected at 2θ angles of 20-60 • . The 2θ step size was 0.02 • , and the scanning rate was 10 degree/min. Diffuse reflectance (DR) UV-visible spectra were measured using UV-visible spectrophotometer (JASCO, V-670). The spectra of BaSO 4 powder was used as the reference.
N K-edge and Ga K-edge XAFS measurements were carried out at BL7U and BL5S1 of Aichi Synchrotron Radiation Center, respectively. N K-edge XAFS spectra were measured in a total electron yield mode and Ga K-edge XAFS spectra in a transmission mode.

C. Photocatalytic CO2 reduction
Photocatalytic CO 2 reduction with water was carried out using the nitrogen doped samples. Before the reac- tion, the sample (0.1 g) was exposed to visible-light generated by a 300 W Xe lamp with a cut filter for λ > 420 nm for 1 h under CO 2 gas with a flow rate of 3 mL/min, where the light intensity measured in the range of 415 ± 55 nm was 127 mW/cm 2 . Then a NaHCO 3 aqueous solution (H 2 O 10 mL, NaHCO 3 0.92 g) was added to the reactor cell in dark. After 1 h, the background in dark was measured with an online gas chromatograph equipped with a thermal conductivity detector (GC-TCD, Shimadzu GC-8APT). Subsequently, photocatalytic reduction of CO 2 under the visible-light irradiation was started and CO, H 2 , and O 2 production rates were measured for 5 h. Fig. 1 shows DR UV-vis spectra of CA-Ga 2 O 3 -Y and 873-Ga 2 O 3 -Y samples. Intensity of these spectra was converted to the Kubelka-Munk function. The DR spectra indicate that absorption bands appeared in the visible light region after the nitrogen doping. The absorption edge shifted to a longer wavelength region with increasing the temperature of nitrogen doping, suggesting the enhancement of the nitrogen doping at higher temperatures. It is noticeable that the absorption band in the visible light region was much smaller for CA-Ga 2 O 3 -823 compared with 873-Ga 2 O 3 -823, though both samples were nitrogen-doped at the same temperature. In the spectrum of 873-Ga 2 O 3 -873, the band shifted to 550 nm in a similar way to CA-Ga 2 O 3 -973. These results suggest that the 873-Ga 2 O 3 includes larger amount of doped nitrogen than that for CA-Ga 2 O 3 .

B. Chemical states of doped nitrogen
N K-edge XANES spectra of the nitrogen doped samples shown in Fig. 2 confirm that the all samples contain nitrogen as the appearance of clear absorptions originated from σ and π bonds of nitrogen species [19,20]. The chemical sifts or peak broadening of the XANES spectra   [19,20]. Therefore, the 400 eV peak could be associated with the presence of interstitial molecular like nitrogen or small bubbles containing N 2 . Thus, the XANES analysis indicates the existence of at least two different kinds of nitrogen chemical states in the samples; i.e., nitrogen bonded to Ga atom (Ga-N species) and molecular like nitrogen. In the RSF of CA-Ga 2 O 3 , the peak around 3Å at-tributed to Ga-O-Ga bond was higher than the peak at 1-2Å due to Ga-O bond [ Fig. 4A(a)], which should be characteristic of the local structure of β-phase Ga 2 O 3 . Although the RSF of CA-Ga 2 O 3 was not changed by the nitrogen doping at 823 K [ Fig. 4A(b)], a large peak around 3Å newly appeared with the nitrogen doping at 923 K [ Fig. 4A(c)]. This large peak is attributed to GaN structure (Ga-N-Ga bond), referring the RSF of commercially available GaN.

C. Structural analysis of Ga2O3 samples before and after the nitrogen doping
On the other hand, in RSF of 873-Ga 2 O 3 in Fig. 4B whose crystalline structure was assigned to be mixture of γ and β phases of Ga 2 O 3 as revealed from the XRD analysis, the peak at 1-2Å is much higher than the peak around 3Å. Nitrogen doping into 873-Ga 2 O 3 samples reduced both peaks attributed to Ga-O and Ga-O-Ga bonds with more significant reduction in 873-Ga 2 O 3 -823 compared with that in CA-Ga 2 O 3 -823. This result suggests that the original local structure of 873-Ga 2 O 3 changed since larger amount of Ga-N bonds were formed at higher nitrogen doping temperature.
As discussed above, XRD measurements suggested that both crystal structures of CA-Ga 2 O 3 and 873-Ga 2 O 3 samples fundamentally maintained after the nitrogen doping at 823 K. However, the local structure of 873-Ga 2 O 3 already changed by introducing some amount of nitrogen molecules whereas the local structure of CA-Ga 2 O 3 showed no remarkable change. Generally, it is well known that Ga 2 O 3 is a so-called crystal polymorph, which could take five different crystal structures (α- . Among them, γ-Ga 2 O 3 exhibits the defective amorphous-like phase, while β-Ga 2 O 3 is the most stable phase [21,22]. Therefore, the amount of doped nitrogen was larger in 873-Ga 2 O 3 rather than that in CA-Ga 2 O 3 . In CA-Ga 2 O 3 , nitrogen would be introduced into β-Ga 2 O 3 phase accompanying defects formation. Thus, defective structure such as γ-Ga 2 O 3 rather than stable β-Ga 2 O 3 structure allows larger amount of nitrogen doping.

D. Photocatalytic CO2 reduction with water
Although photocatalytic CO 2 reduction with water under visible light irradiation was conducted using the ni-trogen doped samples (823-Ga 2 O 3 -Y and CA-Ga 2 O 3 -Y), the photocatalytic activity was insignificant. Instead the color of the nitrogen doped samples changed from yellow to white. The color change suggests that the doped nitrogen atoms were oxidized by photogenerated holes and released from the samples (2N 3− + 6h + → N 2 ) [23,24]. In particular, the defective amorphous like γ-Ga 2 O 3 phase might enhance the nitrogen evolution from the samples. Thus, the nitrogen doped Ga 2 O 3 samples were unstable in water under the visible light irradiation, even if they had initially some activity for CO 2 reduction. Now we are trying to suppress the self-oxidation and nitrogen evolution of the samples.

IV. CONCLUSION
Based on our previous work indicating potential photocatalytic activity of Ga 2 O 3 for CO 2 reduction with water under UV light irradiation, we have tried nitrogen doping into gallium oxide samples having different crystalline structures with using NH 3 in order to enhance their photocatalytic activity under visible light. The nitrogen doping was successfully done and optical properties were improved to absorb visible light. However, nitrogen doped into Ga 2 O 3 samples was unstable in water under the visible light irradiation. Hence, photocatalytic CO 2 reduction under visible light irradiation was insignificant. Stabilization of doped nitrogen in Ga 2 O 3 in water remains an important issue to resolve.