Charge separation induced by localized surface plasmon resonance (LSPR) of gold and silver nanoparticles (AuNPs and AgNPs) are applied to various devices and photoelectrochemical functionalities. Here, we develop all solid state In/TiO2/MNPs/ITO photovoltaic cells (MNP = AuNP or AgNP) by using two-dimensional MNP ensembles. Their quantum efficiencies are higher than those of previously reported solid state cells with hole-transport materials (HTMs) (ITO/TiO2/AuNPs/HTM/Au). The photoresponses from cells without HTMs suggest that the photovoltage generates at the TiO2-MNP interface.

Plasmonic metal nanoparticles are known to work as light-harvesting antennae and to enhance photocurrents of photovoltaic cells and reaction rates of photocatalysts. The effects are expected to increase the energy conversion efficiency and to reduce the thickness of a light-absorbing layer and costs for materials. In this work, we examined the plasmonic enhancement of dye-sensitized photocurrents by Au nanoparticle ensembles with different particle densities to study the effects of interparticle plasmon coupling on the photocurrent enhancement. The coupling effects allow enhancement in a longer wavelength region. The optimum particle size for the enhancement by coupled nanoparticles is 100 nm, whereas that for isolated nanoparticles is 40 nm because the plasmon coupling effect is more significant for larger nanoparticles. Theoretical calculations reproduce those results.

A TiO2-coated indium tin oxide electrode was further coated with Ni(OH)2 by electrodeposition to obtain a TiO2-Ni(OH)2 bilayer film. Upon irradiation of the bilayer film with UV light in a pH 10 buffer, the Ni(OH)2 layer was oxidized, and it turned from colorless to brown; oxidative energy was stored in the layer. The potential of the oxidative energy thus stored was about +0.7 V versus Ag|AgCl. The stored energy could be reversibly taken out of the film by chemical and electrochemical means. The photooxidized Ni(OH)2 was rereduced by alcohols, aldehydes, phenol, I-, H2O2, formate, and acetone.