(Invited) Artificial Photosynthesis Using Plasmonic Photoanode
Authors: Oshikiri Tomoya , Takakura Ryohei , Shi Xu , Ueno Kosei , Misawa Hiroaki 
Published: February 2020

Previously, we have demonstrated plasmonic photocurrent generation from visible to near-infrared wavelengths without deteriorating photoelectric conversion using photoanodes in which gold nanorods were elaborately arrayed on the surface of a TiO2 single crystal [1-3]. We have also reported the stoichiometric evolution of oxygen via water oxidation by irradiating the plasmon-enhanced photocurrent generation system with near-infrared light [4-6]. Additionally, we developed a plasmon-assisted water splitting system that operated under irradiation by visible light; the system was based on the use of two sides of the same strontium titanate (SrTiO3) single crystal substrate as the anode and cathode [7,8]. According to the similar method of the water splitting system, we have also successfully constructed the artificial-photosynthesis system that produced the ammonia by a photofixation of a nitrogen molecule based on visible light irradiation [9,10]. However, the reaction efficiency in the plasmon-induced artificial photosynthesis is still low.
In this study, we employed two approaches to improve the efficiency. First, we tried to enlarge the specific surface area of the plasmonic photoanode to enhance the light-harvesting ability and reaction area. We fabricated three-dimensional plasmonic photoanode of titanium dioxide nanotubes array (TNTs) loaded with gold nanoparticles (Au-NPs) [11]. Anodized TNTs are promising due to their unique hollow one-dimensional nanostructures that exhibit high electron mobility, large specific surface area, and high mechanical strength. We successfully observed plasmon-induced photocurrent generation and water splitting using TNTs loaded with Au-NPs. Additionally, the reaction rate is approximately tenfold greater than that obtained in a previous study using a single crystal of strontium titanate loaded with Au-NPs [7].
Second, we reconstructed the photosynthetic device to enhance the ion and electron transport path. The photoelectrochemical artificial photosynthesis device using Au-NPs/niobium-doped SrTiO3 (Nb-SrTiO3) plasmonic photoanode was fabricated as follows. Au-NPs were fabricated on a 0.05wt% Nb-SrTiO3 single crystalline substrate using a sputtering and annealing method. The nitrogen reduction device comprised reaction cells with two reaction chambers separated by ion exchange membrane. The Au-NPs/Nb-SrTiO3 photoanode was installed in the one chamber, and a zirconium coil as a co-catalytic cathode was put in another chamber. The cathodic chamber was bubbled with nitrogen gas during the reaction. Two electrodes were connected via the electrochemical analyzer. We performed the plasmon-induced ammonia synthesis on the photoelectrochemical artificial photosynthesis device. As a result, the produced ammonia under visible light irradiation was significantly increased than that of the previous result [10]. Furthermore, we report on a quantitative evaluation of the plasmon-induced ammonia synthesis, such as bias effect, pH effect, stoichiometry, and intermediate. Based on these results, a novel reaction mechanism of plasmon-induced ammonia synthesis is proposed.
These findings blaze new methods for energy-efficient photocatalytic production of ammonia using solar light, water, and nitrogen gas, which are entirely different from conventional methods of ammonia synthesis.

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