Yukina Takahashi

Solid state photovoltaic cells based on localized surface plasmon-induced charge separation

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.

Plasmon-resonance-based generation of cathodic photocurrent at electrodeposited gold nanoparticles coated with TiO2 films.

Reversed photoresponse: Indium tin oxide (ITO)/Au nanoparticle (NP)/TiO(2) electrodes (see picture) exhibit cathodic photocurrents and positive photopotentials under visible light, whereas ITO/TiO(2)/Au NP electrodes show an inverted response. This behavior indicates that electron transfer occurs from the plasmon-excited Au NPs to the TiO(2) film. An enhanced O(2) photoreduction activity is found for ITO/Au NP/TiO(2)/Pt electrodes.

Enhancement of dye-sensitized photocurrents by gold nanoparticles: effects of dye–particle spacing

Photocurrents of a ruthenium dye-TiO(2) system are enhanced by gold nanoparticles (100 or 40 nm diameter) embedded in TiO(2). As dye-particle spacing decreases to 10 nm, enhancement factor and intensity of localized electric fields at the TiO(2) surface increase. A further decrease in the spacing suppresses the enhancement.

Electrodeposition of thermally stable gold and silver nanoparticle ensembles through a thin alumina nanomask

Hemispheric gold or silver nanoparticle (Au and AgNP) ensembles were electrodeposited on a smooth ITO electrode through a thin Al(2)O(3) nanomask. The nanomask reduced the deviation in the particle size and interparticle distance. The absorption peak based on localized surface plasmon resonance (LSPR) of the AuNP ensemble redshifted with increasing environmental refractive index, suggesting that the ensemble would be used as a LSPR sensor for chemical analysis and bioanalysis. The Al(2)O(3) nanomask prevented the Au and AgNPs from thermal coalescence even at 500 degrees C, and consequently, it improved thermal stabilities of nanoparticle ensembles. The ensembles exhibit LSPR-based absorption peak in the visible region, even after annealing. The nanomask allowed AgNPs, which are thermally and chemically less stable than AuNPs, to be coated with sintered TiO(2). The ITO/AgNP/TiO(2) electrode thus obtained functions as a photocathode on the basis of photoinduced electron transfer from silver nanoparticles to TiO(2).

Electropolymerized Polythiophene Photoelectrodes with Density-Controlled Gold Nanoparticles

A polythiophene thin film was fabricated on gold nanoparticle (AuNP)-deposited indium-tin-oxide (ITO) electrodes with electropolymerization, whereas AuNPs were predeposited on the ITO surface. A photocurrent via photoexcited polythiophene increased with AuNPs which was attributed to the localized surface plasmon resonance. Investigation of the AuNP-density dependence on the relative enhancement of photocurrent revealed the maximum effect at 14% of AuNP-density, while 68% of AuNP-density exhibited smaller photocurrent than the polythiophene electrode without AuNPs. We have revealed that the effects of AuNPs saturate in the fairly low density region, and that the excess AuNPs even in the range of submonolayer resulted in the decrement of photocurrents.

Visible light driven photocatalysts with oxidative energy storage abilities

Visible light driven photocatalysts, N-doped TiO2, Fe(III)-modified TiO2, WO3, Cu(II)-modified WO3 and Pt nanoparticle-loaded WO3, were combined with Ni(OH)2 as an oxidative energy storage material, to overcome two serious limitations of TiO2 photocatalyst; it works only under light, and the light must contain UV. Among the photocatalysts examined, 1 wt% Pt-loaded WO3/Ni(OH)2 bilayer film exhibits the highest efficiency of oxidative energy storage under visible light (≥420 nm), sunlight and fluorescent light. The energy storage is possible even under 460 nm light. Other Pt-loaded WO3 (0.1–10 wt%), Fe(III)-modified TiO2 and Cu(II)-modified WO3 also exhibit energy storage abilities under visible light when combined with Ni(OH)2.

Effects of Film Thickness on the Photocurrent Generation from Polythiophene–Fullerene Thin Films Containing Silver Nanoparticles

We have investigated the incident-photon-to-current conversion efficiency (IPCE) of thin films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methylester (PCBM) as a function of film thickness, in the presence or absence of silver nanoparticles (AgPs) between the films and the indium–tin-oxide (ITO) electrode. The thickness of the film was evaluated by atomic force microscopy. The absorption and photocurrent action spectra of the films were measured to investigate the effect of the thickness of the P3HT:PCBM film. The results show that IPCE increased steeply in thinner films and the optimal range of thicknesses of the P3HT:PCBM film was 50–120 nm. In this optimal range, the IPCEs were 1.5–1.8 times larger in the presence of AgPs.

Silver-nanoparticle-assisted photocurrent generation in polythiophene-fullerene thin films

We have investigated the effect of silver nanoparticles (AgPs) on the photocurrent generation of a polyphiophene-fullerene photovoltaic film. Poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methylester (PCBM) were used for the electron donor and acceptor, respectively. First, AgPs were electrostatically deposited upon the surface of an indium tin oxide (ITO) electrode via a polycation. Then, a film of P3HT or a mixture of P3HT and PCBM was prepared by spin coating. The thickness of the film was evaluated by atomic force microscopy. Absorption and fluorescence spectral measurements were carried out to investigate the effects of AgPs. Photocurrent spectra were also measured, and the effects of AgPs on photocurrent enhancement were verified.

Anisotropic light absorption by localized surface plasmon resonance in a thin film of gold nanoparticles studied by visible multiple-angle incidence resolution spectrometry.

Anisotropic light absorptions via localized surface plasmon resonance in a gold-evaporated film parallel (in-plane; IP) and perpendicular (out-of-plane; OP) to the film surface are studied using visible multiple-angle incidence resolution spectrometry (Vis-MAIRS). When the thin film was aged for eighteen days, the time-dependent Vis-MAIRS IP spectra exhibited significantly different variation from that of the OP spectra: the IP spectra exhibited a large shift to the shorter wavelength side, whereas the OP spectra were explained by a linear combination of three-constituent spectra. The surface topographical analysis of the film revealed that a continuous film coalesced to form aggregates of metal particles. The intrinsic difference between the IP and OP spectra was readily elucidated by considering the surface-parallel and -perpendicular dipoles interaction depending on the topographical changes, which was confirmed by performing spectral simulation using metal particle array models.

Oxidation of methanol and formaldehyde to CO2 by a photocatalyst with an energy storage ability

A TiO(2)-Ni(OH)(2) bilayer photocatalyst is known as a photocatalyst with energy storage abilities. Oxidative energy from the UV-irradiated TiO(2) underlayer can be stored in the Ni(OH)(2) overlayer. We investigated oxidation and mineralization of methanol and formaldehyde by the stored oxidative energy by mean of gas chromatography. When the methanol concentration in air is as low as 10 ppm, the mass conversion efficiency from methanol to CO(2) is approximately 86%. Formaldehyde can also be oxidized to CO(2) by the stored energy.