Plasmonic Photoelectrochemistry: Functional Materials Based on Photoinduced Reversible Redox Reactions of Metal Nanoparticles

Plasmonic metal nanoparticles absorb and scatter visible light due to localized surface plasmon resonance (LSPR). The optical properties including resonance wavelength and absorption intensity depend on particle size, shape, orientation, spacing, and dielectric environments. If silver nanopatticles are placed on TiO2, plasmonic excitation of the particles by visible or near-infrared light results in their oxidation due to plasmon-induced charge separation. Silver ions thus released are reduced back to silver nanoparticles by UV-excitation of TiO2. These pseudo-reversible processes induced by visible and UV light between silver nanoparticles and silver ions are applied to various photoelectrochemical devices and materials. Multicolor photochromism is a typical example, in which an ensemble of silver nanoparticles on TiO2 is colored by monochromated light to a color corresponding to the excitation wavelength. The use of silver nanorods allows infrared and polarization photochromism. The photoinduced oxidation is also applied to size, shape, and orientation control of silver nanoparticles. A photomorphing gel is an additional application of the reversible processes.