We have demonstrated plasmonic photocurrent generation from visible to near-infrared wavelengths without deteriorating photoelectric conversion using electrodes 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]. In the present study, we developed a plasmon-assisted water splitting system that operates under irradiation by visible light; the system is based on the use of two sides of the same strontium titanate (SrTiO₃) single crystal substrate [7,8]. The water splitting system contains two solution chambers to separate hydrogen (H₂) and oxygen (O₂), respectively. To promote water splitting, a chemical bias was applied by pH values regulations of those chambers. The quantity of H₂ evolved from the surface of platinum, which was used as a reduction co-catalyst, was twice of O₂ evolved from an Au nanostructured surface. Thus, the stoichiometric evolution of H₂ and O₂ was clearly demonstrated. The hydrogen evolution action spectrum closely corresponded to the localized surface plasmon resonance spectrum, indicating that the plasmon-assisted charge separation at the Au/SrTiO₃ interface promotes water oxidation and the subsequent reduction of a proton on the backside of the SrTiO₃ substrate.

According to the analogous method of the water splitting system, we have successfully constructed the artificial-photosynthesis system that produces the ammonia by a photofixation of a nitrogen molecule based on visible light irradiation [9]. Unlike the water splitting system, ruthenium was used as a co-catalyst instead of a platinum for the ammonia synthesis, and not a solution system but a gas system was used to reduce nitrogen gas. The action spectrum of the apparent quantum efficiency of ammonia evolution showed good agreement with the plasmon resonance spectrum. In addition, we revealed that ammonia could be obtained with ~100% selectivity by using zirconium/zirconium oxide cocatalyst [10]. 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.

 References

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