(Invited) Development of Nitride Photocatalysts for Artificial Photosynthesis
Authors: Ohkawa Kazuhiro 
Published: February 2020

Nitride semiconductors are attractive materials not only for energy-saving devices such as LEDs and LDs but also for clean-energy production systems. Nitride photocatalysts are possible to generate “clean hydrogen” from water and to produce hydrocarbon from CO2, so-called artificial photosynthesis. Energy conversion from light energy to these clean energies are efficient by using nitride photocatalysts.

Efficient LEDs in regions of UV, blue, green are based on nitride semiconductors since nitrides have direct bandgap. Direct bandgap also causes efficient photo-absorption. InGaN alloys have a potential of photo-absorption devices such as solar cells and photocatalysts because their bandgaps cover 0.65-3.42 eV. The energy range is the same with photon energy of 362-1900 nm in wavelength. InGaN seems to be possible to utilize solar light efficiently. On the other hand, nitrides are, generally, chemically stable. Chemical stability gives us a hint that semiconductors in solution seem to be possible. Based on these situations, I invented nitride photocatalysts [K. Ohkawa, Japanese Patent 3730142 (2001)]. Nitride photocatalysts are possible to produce “clean hydrogen” from water using light energy [K. Fujii, T. Karasawa, and K. Ohkawa, Jpn. J. Appl. Phys.
44, L543 (2005); M. Ono and K. Ohkawa et al., J. Chem. Phys. 126, 054708 (2007); K. Ohkawa et al., Jpn. J. Appl. Phys. 52, 08JH04 (2013)]. Nitride photocatalysts can reduce CO2 which causes the global warming into “renewable energy” such as HCOOH, CH4 and C2H4 and C2H5OH [S. Yotsuhashi, K. Ohkawa et al., Appl. Phys. Express
4, 117101 (2011); M. Deguchi, K. Ohkawa et al., Jpn. J. Appl. Phys.
52, 08JF07 (2013)]. Concerning HCOOH production, our recent energy conversion efficiency reached 0.97% which is greater than average of global biological photosynthetic one [T. Sekimoto, K. Ohkawa et al., Appl. Phys. Lett.
106, 073902 (2015)].

Nitride photoelectrodes consist of uid-InGaN photo-absorption layer/n+-GaN conductive layer/sapphire or GaN substrate. These structures were grown by metalorganic vapor-phase epitaxy. We used a nitride photoelectrode as a working electrode, a Pt wire as a counterelectrode. These electrodes are connected with a metal wire through their Ohmic contacts. These working- and counter-electrodes are immersed into electrolyte such as 1 M NaOH. We irradiated light from a Xe-lamp or a solar simulator on the nitride photoelectrode. Photoabsorption is happen in the InGaN layer mainly, and absorbed photons create electron-hole pairs. The uid-InGaN is slightly n-type and its surface band bends to the upper energy side at the surface. The band bending causes electron-hole separation. Holes are emitted from InGaN and/or NiO co-catalyst surface, and oxidize water (produce O2 gas). Separated electrons go out from the Ohmic contact to the Pt counterelectrode, and reduce water (produce H2 gas). In case of InGaN with Eg=3.16 eV and a Xe lamp, applied bias photon-to-current efficiency (ABPE) was as high as 4.6% at 0.48 V between the nitride photoelectrode and Pt wire. Decreasing InGaN bandgap, photoabsorption is enhanced then we will realize higher ABPE. Since we are possible to grow high-quality high-In-content InGaN layers, we will get more “clean hydrogen” using nitride photocatalysis phenomenon.