Approaches to Improvement of Stabilities and Efficiencies of Photoenergy Conversion Devices Based on Plasmon Induced Charge Separation
Authors: Takahashi Yukina 
Published: December 2020

It is known that metal nanoparticles exhibit localized surface plasmon resonance (LSPR) with the resonant light irradiation. When the resonant light irradiates to a plasmonic metal nanoparticle, light energy can be localized in nanospaces around the surface of the nanoparticle. Hence, LSPR is expected to be an innovative technology in order to utilize photoenergy with high efficiency. Furthermore, a plasmonic metal nanoparticle combined with a semiconductor exhibit plasmon induced charge separation (PICS) [1]. When the resonant light irradiates to a plasmonic metal nanoparticle on an n-type semiconductor, electrons in the metal nanoparticle transfer to the semiconductor. As a result, oxidative and reductive reactions occur on the surfaces of the metal nanoparticle and the semiconductor, respectively. Therefore, PICS can be applied to various photoenergy conversion devices, such as photocatalysts and phorovoltaic cells. However, conventional PICS system has some problems to be solved. For example, the oxidation power generated at the metal nanoparticles promotes oxidative dissolution of the metal nanoparticles. Furthermore, the detail mechanism of PICS has been still unclear. In addition, the efficiencies of charge separation have not achieved high enough.
In order to overcome these problems, the chemical and thermal stabilities of plasmonic metal nanoparticles, such as silver nanospheres, gold nanorods (AuNRs), and silver nanoplates (AgPLs), were improved by introducing an Al2O3 nanomask and/or a TiO2 thin film [2-4]. Accordingly, these plasmonic metal nanoparticles has come to be applied to PICS system even in a severe condition.
We also investigated the positive charge generated by PICS on the surface of gold nanoparticles (AuNPs) deposited on a TiO2 substrate [5,6]. As a result, oxidative photopolymerization based on PICS were observed site-selectively in nanospaces on the surface of AuNP. From the results of the nanoscale photopolymerization of phenol, pyrrole, and 3,4-ethylenedioxythiophene (EDOT), we proposed a possible mechanism of generation of oxidation power on the metal nanoparticles based on PICS (Fig. 1).
Furthermore, we engaged to develop PICS system consisting of plasmonic metal nanoparticles and a p-type semiconductor (p-type PICS). Because the p-type PICS can be expected to exhibit charge separation opposite direction to conventional PICS, which employ an n-type semiconductor. As a result, the system overcome the problem of oxidative dissolution of the metal nanoparticles based on PICS. It is also expected to realize the charge separation with high efficiency.
These technologies should progress the field of photoenergy conversion systems based on PICS with high efficiency.

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Figure 1