Mechanisms and Applications of Plasmon-Induced Charge Separation at TiO₂ Films Loaded with Gold Nanoparticles

Plasmon-induced photoelectrochemistry in the visible region was studied at gold nanoparticle−nanoporous TiO2 composites (Au−TiO2) prepared by photocatalytic deposition of gold in a porous TiO2 film. Photoaction spectra for both the open-circuit potential and short-circuit current were in good agreement with the absorption spectrum of the gold nanoparticles in the TiO2 film. The gold nanoparticles are photoexcited due to plasmon resonance, and charge separation is accomplished by the transfer of photoexcited electrons from the gold particle to the TiO2 conduction band and the simultaneous transfer of compensative electrons from a donor in the solution to the gold particle. Besides its low-cost and facile preparation, a photovoltaic cell with the optimized electron mediator (Fe2+/3+) exhibits an optimum incident photon to current conversion efficiency (IPCE) of 26%. The Au−TiO2 can photocatalytically oxidize ethanol and methanol at the expense of oxygen reduction under visible light; it is potentially applicable to a new class of photocatalysts and photovoltaic fuel cells

Highly Crystalline Wurtzite CdS Prepared by a Flux Method and Application to Photocatalysis

Highly crystalline wurtzite CdS particles with exposed crystal facets and fewer sulfur defects have been successfully prepared by heat treatment of zincblende CdS in a mixed molten salt. This method does not require a vacuum or highly toxic hydrogen sulfide conditions as in conventional annealing methods or high pressure as in conventional flux methods. The obtained wurtzite CdS photocatalyst with a Pt cocatalyst shows hydrogen evolution activity under visible light, in the presence of sulfide sacrificial reagents, and its external quantum yield at 420 nm is 19.2%. The activity is ∼50-fold higher than that of the zincblende CdS precursor and about twice as high as that of wurtzite CdS prepared by vacuum annealing, both with a Pt cocatalyst

Plasmonic Manipulation of Color and Morphology of Single Silver Nanospheres

Optical control of size, shape, or orientation of a metal nanoparticle is important for development of nanoscale optical devices and elements of photonic circuits. Thus far, however, independent control of two or more parameters has not yet been achieved. Here we place a simple spherical Ag nanoparticle on TiO₂ with high refractive index and separate a plasmon mode localized at the Ag-TiO₂ interface from the other mode distributed over the nanoparticle. Selective excitation of each mode gives rise to a corresponding morphological change and selective suppression of the plasmon mode, resulting in multicolor changes of scattering light from orange to red, green, or a dark color

Facet-Selective Photoelectrochemical Reactions on Wurtzite CdS Photocatalysts

Some of the photocatalytic reactions take place selectively at specific crystal planes, and knowledge in this regard has attracted much attention from the viewpoint of designing advanced and sophisticated photocatalysts. Here we report some novel facet-selective photoreduction and photo-oxidation reactions for wurtzite CdS. First we deposited Pt nanoparticles onto CdS by direct photodeposition (PD method) or an indirect process via entrapment of photoexcited electrons by reduction of Cd(II) to metallic Cd, followed by its galvanic replacement with Pt (ETD method). The PD method allowed small Pt nanoparticles to be deposited selectively on the CdS(101) facets, whereas Pt nanoparticles were deposited almost uniformly on all of the facets by the ETD method. Those CdS particles with Pt nanoparticles also showed facet-selective photocorrosion reactions based on self-oxidation. CdS–Pt prepared by the PD method exhibited selective photocorrosion at the CdS(101) facets in pure water. In the case of CdS–Pt prepared by the ETD method, however, the photocorrosion occurred selectively at the CdS(001) facets in the presence of lactic acid. These facet-selective reactions would allow the wurtzite CdS to be designed for sophisticated photocatalytic systems

Control of Asymmetric Scattering Behavior of Plasmonic Nanoparticle Ensembles

A transparent or translucent film shows the same color for both front and back sides in general. However, different colors can be shown by the aid of strong plasmonic scattering of metal nanoparticles. Here we propose a design strategy of films exhibiting asymmetric scattering behavior. Typically two different types of nanoparticles are embedded in a SiO₂ thin film that is coated on a TiO₂ thin film. One of the particle types is selectively excited for the front incidence, and the other type for the back incidence, by tuning the electric field distribution inside the SiO₂ film. We experimentally prepared two different samples that scatter light of different colors from the front and the back sides by using Au nanorods and Ag nanospheres

Chiral Plasmonic Nanostructures Fabricated by Circularly Polarized Light

The chirality of materials results in a wide variety of advanced technologies including image display, data storage, light management including negative refraction, and enantioselective catalysis and sensing. Here, we introduce chirality to plasmonic nanostructures by using circularly polarized light as the sole chiral source for the first time. Gold nanocuboids as precursors on a semiconductor were irradiated with circularly polarized light to localize electric fields at specific corners of the cuboids depending on the handedness of light and deposited dielectric moieties as electron oscillation boosters by the localized electric field. Thus, plasmonic nanostructures with high chirality were developed. The present bottom-up method would allow the large-scale and cost-effective fabrication of chiral materials and further applications to functional materials and devices

Photoelectrochemical Analysis of Allowed and Forbidden Multipole Plasmon Modes of Polydisperse Ag Nanorods

Resonant wavelengths of multipole (i.e., higher-order) plasmon resonance of polydisperse Ag nanorods (NRs) and their dependence on incident angle are analyzed by a photoelectrochemical means coupled with spectroscopy. It also allows us to observe nanostructures of multipole-resonant Ag NRs. Multiple dip formation in extinction spectra is caused by selective oxidation of multipole-resonant NRs on the basis of multipole plasmon-induced charge separation on a TiO₂ substrate. The dip wavelengths are the dipole resonant wavelengths of the NRs that are dipole- or multipole-resonant at the irradiation wavelength. Even order plasmon modes (mode index number <i>m</i> = 2, 4) of the NRs are forbidden at perpendicular incidence (θ = 0°), and in the present system, the third-order plasmon mode (<i>m</i> = 3) is also forbidden at a specific incident angle, θ = 37.5°

Control of Asymmetric Scattering Behavior of Plasmonic Nanoparticle Ensembles

A transparent or translucent film shows the same color for both front and back sides in general. However, different colors can be shown by the aid of strong plasmonic scattering of metal nanoparticles. Here we propose a design strategy of films exhibiting asymmetric scattering behavior. Typically two different types of nanoparticles are embedded in a SiO₂ thin film that is coated on a TiO₂ thin film. One of the particle types is selectively excited for the front incidence, and the other type for the back incidence, by tuning the electric field distribution inside the SiO₂ film. We experimentally prepared two different samples that scatter light of different colors from the front and the back sides by using Au nanorods and Ag nanospheres

Photoregulated Nanopore Formation via Plasmon-Induced Dealloying of Au–Ag Alloy Nanoparticles

We propose and demonstrate the third method to dealloy and porosify plasmonic alloy nanoparticles (NPs). Au–Ag alloy NPs were deposited on a TiO₂ thin film and irradiated with visible light in water. As a result, Ag was dissolved from the NPs and nanopores were formed, on the basis of plasmon-induced charge separation (PICS) at the alloy–TiO₂ interface. The dissolution rate can be regulated by tuning the irradiation wavelength or intensity, resulting in different pore and ligament sizes. Porous NPs smaller than 35 nm can also be prepared by the aid of an auto-termination mechanism, which is unique to the present photoelectrochemical dealloying method