Title : Influence of chemical states of doped nitrogen in NaTaO3 on photocatalytic activity for CO2 reduction
Reduction of CO2 in atmosphere is an important environmental issue, and photocatalytic conversion of CO2 to useful compounds like CO has been studied extensively. As Nakanishi et al. reported, NaTaO3 shows high photocatalytic activity for CO2 reduction and attracts much attention. However, owing to its rather wide gap nature (4.1ev), NaTaO3 shows the photocatalytic activity only under UV light irradiation up to 300 nm in wavelength region which corresponds to about 1.7×10-4 % in the total solar energy. If NaTaO3 exhibits the photocatalytic activity under visible light, for example, up to 350nm, the usable solar energy will increase about 5000 times, 0.89% in total solar energy. Therefore the band gap narrowing of NaTaO3 is encouraged. Although nitrogen doping into NaTaO3 could narrow its band gap from 4.1 eV to 2.2 eV, the effects of the nitrogen doping on photocatalytic CO2 reduction of NaTaO3 have not been studied well. In this study, nitrogen doped NaTaO3 was synthesized by a solid state reaction or hydrothermal reaction to investigate the relationship between chemical states of doped nitrogen and the photocatalytic activity of the samples.
In the solid state reaction, nitrogen doping was done under NH3 flow at 1223 K for 7 hours on NaTaO3 synthesized as precursor in advance, resulting red colored powders. Analyses of the powders by XPS, XANES and UV-vis showed no nitrogen doped.
In the hydrothermal reaction, Ta3N5 and NaOH aqueous solution were reacted under hydrothermal condition giving orange colored powders. It was confirmed that nitrogen was doped in the powders. The amount of doped nitrogen could be controlled by changing the mixing ratio of Ta3N5 and NaOH. The nitrogen doped NaTaO3 is referred as NaTaO3-xNx hereafter. Depth distribution of the doped nitrogen was determined by XPS analysis under stepwise Ar sputtering. The concentration of the doped nitrogen was highest at the surface and decreased with depth showing deeper penetration in the particles having higher surface concentration.
The synthesized NaTaO3-xNx samples were modified by Ag nanoparticles as cocatalyst, and their activities on photocatalytic CO2 reduction under UV light or visible light irradiation were examined. Unfortunately, the photocatalytic CO2 reduction under visible light irradiation did not proceed. Under UV light irradiation, NaTaO3-xNx showed higher activity for the CO2 reduction than NaTaO3. In the anodic photocurrent measurements under UV light irradiation, NaTaO3-xNx showed higher anodic photocurrent corresponding with their higher activities on the CO2 reduction. Different from the UV irradiation the anodic photocurrents of NaTaO3-xNx under visible irradiation monotonously increased with increasing nitrogen content (x). This result suggested that the amount of electron-hole pairs generated by visible light irradiation depends on the amount of doped nitrogen in NaTaO3.