(Invited) Plasmonic Photocatalysis and Near-Field Photocatalysis
Authors: Tatsuma Tetsu 
Published: December 2024

It is well known that finding of Honda-Fujishima effect1 opened up the way to semiconductor photocatalysis. The semiconductor photocatalysts, typically TiO2, are often modified with noble metal nanoparticles such as platinum as co-catalysts, for improvement of the photocatalytic activity. We modified TiO2 photocatalysts with plasmonic silver2 or gold3 nanoparticles and found that plasmon-induced charge separation (PICS) occurs at the metal-TiO2 interface.3,4 If a metal nanoparticle such as silver or gold is irradiated with light of the resonant wavelength, energetic electrons and holes are generated in the course of relaxation of the photoexcited plasmons. The electron can be injected into the conduction band of the TiO2 in contact with the resonant metal nanoparticle, and PICS is achieved (Figure 1a).3,4

We have applied PICS to visible-light-driven and NIR-light-driven photovoltaics and optoelectronic devices,3,4 photocatalysts,3,4 plasmonic chemical sensors,4 multicolor photochromic materials,2,4 and photo-morphing gels.4 For PICS, we can employ not only silver and gold nanoparticles, but also other plasmonic metal nanoparticles such as copper and plasmonic and electroconducting compound nanoparticles such as ITO. We have developed photovoltaic devices responsive to NIR light by coupling TiO2 with plasmonic ITO nanoparticles.5

If the plasmonic nanoparticles are morphologically or optically anisotropic, plasmonic electron oscillation is localized at certain resonance sites. In other words, optical near field is confined at the resonance sites. We can take advantage of the localization for nanoscale photo-fabrication beyond the diffraction limit, on the basis of deposition and dissolution reactions.6 We have fabricated variety of nanostructures, for instance anisotropic composite nanostructures and chiral plasmonic nanostructures (Figure 1b).7
,8

Optical near field is generated by various processes other than plasmon resonance, for instance Mie resonance. Mie resonance occurs not only at metal nanoparticles but also at dielectric and semiconducting nanomaterials. We employed hexagonal ZnO nanoplates to achieve near-field photocatalysis based on Mie resonance.9 The ZnO nanoplates were adsorbed onto a glass plate, and irradiated with linearly polarized UV light for reductive deposition of silver. The optical near field was localized at sites corresponding to the polarization direction of the irradiated light. Namely, silver deposition site can be controlled by the polarization angle (Figure 1c). Self-oxidative etching of the ZnO nanoplates themselves and oxidative deposition of cobalt oxide onto the ZnO nanoplates can also be conducted in the site-selective manner.
Thus, near-field photocatalysis has been achieved via plasmon resonance and Mie resonance. The technique enables photonic nanofabrication beyond the diffraction limit and design of nanomaterials and nanodevices including sophisticated photocatalysts and metamaterials.

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Figure 1