Active control of plasmon coupling via simple electrochemical surface oxidation/reduction of Au nanoparticle agglomerates.

Reversible tuning of plasmon coupling of Au nanoparticle (AuNP) agglomerates containing dimers as the main component was achieved via electrochemical surface oxidation/reduction of the AuNP surface. The system required no reactant except for water and was almost finished within a unit second, which leads to novel active plasmonic devices.

Site-selective introduction of MnO2 co-catalyst onto gold nanocubes via plasmon-induced charge separation and galvanic replacement for enhanced photocatalysis.

For energy harvesting with plasmonic photocatalysis, it is important to optimize geometrical arrangements of plasmonic nanomaterials, electron (or hole) acceptors, and co-catalysts so as to improve the charge separation efficiency and suppress charge recombination. Here, we employ a photocatalytic system with Au nanocubes on TiO2 and introduce MnO2 as an oxidation co-catalyst onto the nanocubes via site-selective oxidation based on plasmon-induced charge separation (PICS). However, it has been known that PbO2 is the only material that can be deposited onto Au nanomaterials through PICS with sufficient site-selectivity. Here we addressed this issue by introducing an indirect approach for MnO2 deposition via site-selective PbO2 deposition and subsequent galvanic replacement of PbO2 with MnO2. The indirect approach gave nanostructures with MnO2 introduced at around the top part, bottom part, or entire surface of the Au nanocubes on a TiO2 electrode. The activity of those plasmonic photocatalysts was strongly dependent on the location of MnO2. The key to improving the activity is to separate MnO2 from TiO2 to prevent recombination of the positive charges in MnO2 with the negative ones in TiO2.

Spatial distribution of single guest molecules along thickness of thin films of poly(2-hydroxyethyl acrylate).

We have investigated three-dimensional distribution and diffusion behaviors of single guest dyes in 1-µm thick films of poly(2-hydroxyethyl acrylate) (PHEA) by using astigmatism imaging method. Perylene diimide derivative (BP-PDI) in the PHEA films localized along the Z-axis at ca. Z = 600-700 nm distant from the interface (Z = 0) between PHEA and glass substrate. This Z-localization was not observed in different polymer films of poly(methyl methacrylate) (PMMA), poly(methyl acrylate) (PMA), and polystyrene (PSt). Because the glass transition temperature of the PHEA is lower than the room temperature, BP-PDI in the PHEA films exhibited Brownian motion, normal diffusion on the XY plane and confined motion along the Z-direction. For elucidating the mechanism of the peculiar localization of the guest dyes along film thickness in the PHEA films, we measured diffusion behaviors of different dyes, R6G and Atto 488, in 1-µm thick PHEA films, obtaining result that the Z-distributions of the dyes were overall similar to that of BP-PDI. The result indicates that the Z-localization of the guest dyes should be ascribed not to the interaction between glass surface and guest dye but mainly to the Z-dependent property of the PHEA film. Indeed, the lateral diffusion coefficients of the guest dyes depended on their Z-positions.

Visualization of nano-localized and delocalized oxidation sites for plasmon-induced charge separation.

Oxidation reaction sites for plasmon-induced charge separation at Au nanocubes on TiO2 were visualized on the basis of deposition and dissolution reactions. For Pb2+ oxidation, PbO2 was deposited selectively at resonance sites of the nanocube, while oxidation polymerization of pyrrole to polypyrrole and oxidative dissolution of Au took place over the entire nanocube surface. The localized and delocalized reaction sites are explained in terms of a relationship between oxidation potentials of the electron donors and potentials of the entire nanocube and localized holes.

Plasmonic hole ejection involved in plasmon-induced charge separation.

Since the finding of plasmon-induced charge separation (PICS) at the interface between a plasmonic metal nanoparticle and a semiconductor, which has been applied to photovoltaics including photodetectors, photocatalysis including water splitting, sensors and data storage in the visible/near-infrared ranges, injection of hot electrons (energetic electrons) into semiconductors has attracted attention almost exclusively. However, it has recently been found that behaviours of holes are also important. In this review, studies on the hot hole ejection from plasmonic nanoparticles are described comprehensively. Hole ejection from plasmonic nanoparticles on electron transport materials including n-type semiconductors allows oxidation reactions to take place at more positive potentials than those involved in a charge accumulation mechanism. Site-selective oxidation is also one of the characteristics of the hole ejection and is applied to photoinduced nanofabrication beyond the diffraction limit. Hole injection into hole transport materials including p-type semiconductors (HTMs) in solid-state cells, hole ejection through a HTM for stabilization of holes, hole ejection to a HTM for efficient hot electron ejection, voltage up-conversion by the use of hot carriers and electrochemically assisted hole ejection are also described.

Accelerated site-selective photooxidation on Au nanoparticles via electrochemically-assisted plasmonic hole ejection.

In order to induce electrochemical reactions by localized surface plasmon resonance (LSPR), semiconductors have been employed as electron or hole acceptors for plasmon-induced charge separation (PICS) in most cases. Here we replaced a semiconductor with a potential-controlled transparent electrode, and achieved accelerated photooxidation reactions at selected local sites on plasmonic metal nanoparticles. We demonstrate site-selective PbO2 deposition at the tips and sides of Au nanorods and PbO2 deposition and Au dissolution at the top and bottom of Au nanocubes, through the selective excitation of different LSPR modes. Energetic electron-hole pairs are generated at a plasmonic resonance site, and oxidation reactions are driven by hole ejection at the site. The complementary electrons are removed via the positively biased electrode, and consumed at a counter electrode by reduction reactions. In the case of the PbO2 deposition, formation of PbO2 nuclei is triggered by the hole ejection, and PbO2 is grown further in an electrochemical manner at an improved rate.

Plasmon-induced charge separation at the interface between ITO nanoparticles and TiO2 under near-infrared irradiation.

Plasmon-induced charge separation (PICS) by continuous electron injection from plasmonic compound nanoparticles to a semiconductor was achieved by using solid-state cells based on tin-doped indium oxide (ITO) nanoparticles with a short capping agent and a TiO2 film. The cells extended the PICS range to longer wavelengths and exhibited photoresponses to 1500-2200 nm near-infrared light.

Local trapping of energetic holes at gold nanoparticles on TiO2.

Oxidation sites for plasmon-induced charge separation at gold nanocubes and nanorods on TiO2 were visualized by PbO2 deposition, and the sites were localized at plasmonic resonance sites. This indicates that energetic holes generated at those sites dominate the reactions, which can be applied to photo-nanofabrication beyond the diffraction limit.

Effects of particle size and annealing on plasmon-induced charge separation at self-assembled gold nanoparticle arrays.

Two-dimensional periodic Au nanoparticle arrays were constructed on TiO2 thin films by a micelle lithography method and seed-mediated photoelectrochemical growth. Their adjustable interparticle distance allows investigation of a particle size effect on plasmon-induced charge separation (PICS) efficiencies without interference from particle aggregation or plasmon coupling. External or internal PICS efficiencies were found to increase and decrease, respectively, with an increase in particle diameter from 25 to 38 nm. Improvement of the contact between Au nanoparticles and TiO2 by annealing enhanced the intensity of a plasmonic interface mode and both external and internal PICS efficiencies.

Plasmonic behaviour and plasmon-induced charge separation of nanostructured MoO3-x under near infrared irradiation.

Plasmon-induced charge separation (PICS) allows direct conversion of localized surface plasmon resonance (LSPR) to electron flows and photoelectrochemical reactions. However, PICS has only been achieved using plasmonic noble metal nanoparticles, not with compound nanoparticles. In order to achieve compound PICS, MoO3-x nanostructures were prepared that exhibit LSPR in the near infrared region by using metal oxides or metal nanoparticles as templates. Solid-state cells based on the MoO3-x nanostructure were developed. Their photoresponse to 700-1400 nm infrared light was investigated and analyzed on the basis of their PICS mechanisms.

Hydrogen evolution from water based on plasmon-induced charge separation at a TiO2/Au/NiO/Pt system.

Metal-semiconductor plasmonic nanostructures are capable of converting light energy through plasmon-induced charge separation (PICS), providing fruitful new strategies to utilize solar energy in various fields, including photocatalysis. Here, we enhance the PICS efficiencies for hydrogen evolution from water at a Pt cathode coupled with a TiO2/Au photoanode by coating the TiO2/Au with a p-type NiO layer on which a Pt co-catalyst is deposited. PICS occurs at the Au-TiO2 interface under visible light. The electrons injected from the Au nanoparticles into TiO2 are transported to the Pt cathode and cause hydrogen evolution from water, the action spectrum of which matches the plasmonic extinction spectrum of the Au nanoparticles. The NiO layer extracts the separated positive charges from the Au nanoparticles, accumulates the charges and drives methanol oxidation at the Pt co-catalyst on NiO with the positive charges. As a result of the introduction of the Pt-modified NiO layer, the rates of methanol oxidation and accompanying hydrogen evolution at zero bias voltage were improved by ∼3.5 times. The NiO layer may also protect the Au nanoparticles from self-oxidation.

Tunable plasmon resonance of molybdenum oxide nanoparticles synthesized in non-aqueous media.

Plasmonic compound nanoparticles (NPs) have attracted great interest because they are prepared at lower cost and show unique optical properties. However, full replacement of the plasmonic noble metal NPs with the compound NPs has been difficult because most of the compound NPs exhibit plasmon resonance in the infrared range owing to low free carrier density and mobility. In order to overcome this limitation, we developed a new synthetic method for plasmonic MoO2 and MoO3-x NPs. Those NPs exhibit plasmon resonance at ∼500 nm and 600-1000 nm, respectively, likely because of high carrier densities. The plasmonic properties of the NPs are tunable by changing the synthetic conditions or oxidizing and reducing the NPs. Their refractive index sensitivities are 115-260 nm RIU-1. Those molybdenum oxide NPs are expected to substitute for plasmonic noble metal NPs in optical, electronic, sensing and light harvesting devices and materials.

Plasmonic Photovoltaic Cells with Dual-Functional Gold, Silver, and Copper Half-Shell Arrays.

Solid-state photovoltaic cells based on plasmon-induced charge separation (PICS) have attracted growing attention during the past decade. However, the power conversion efficiency (PCE) of the previously reported devices, which are generally loaded with dispersed metal nanoparticles as light absorbers, has not been sufficiently high. Here we report simpler plasmonic photovoltaic cells with interconnected Au, Ag, and Cu half-shell arrays deposited on SiO2@TiO2 colloidal crystals, which serve both as a plasmonic light absorber and as a current collector. The well-controlled and easily prepared plasmonic structure allows precise comparison of the PICS efficiency between different plasmonic metal species. The cell with the Ag half-shell array has higher photovoltaic performance than the cells with Au and Cu half-shell arrays because of the high population of photogenerated energetic electrons, which gives a high electron injection efficiency and suppressed charge recombination probability, achieving the highest PCE among the solid-state PICS devices even without a hole transport layer.

Potential-Scanning Localized Plasmon Sensing with Single and Coupled Gold Nanorods.

Single plasmonic nanoparticles can potentially serve as optical sensors for detecting local refractive index changes. However, simultaneous monitoring of the scattering spectra from multiple nanoparticles is not practical. Here we perform potential-scanning localized surface plasmon resonance (LSPR) sensing. Gold nanorods are immobilized on an ITO electrode. Instead of collecting the full spectrum, as is done in conventional LSPR sensing, the electrode potential is scanned while the rod spectra are monitored at a single wavelength. We demonstrate that refractive index changes can be determined from single wavelength experiments and we further find that gold nanorod (NR) dimers exhibit higher refractive index sensitivities than single NRs in both potential-scanning and conventional wavelength-scanning based LSPR sensing.

Plasmon-induced charge separation: chemistry and wide applications.

Recent development of nanoplasmonics has stimulated chemists to utilize plasmonic nanomaterials for efficient and distinctive photochemical applications, and physicists to boldly go inside the "wet" chemistry world. The discovery of plasmon-induced charge separation (PICS) has even accelerated these trends. On the other hand, some confusion is found in discussions about PICS. In this perspective, we focus on differences between PICS and some other phenomena such as co-catalysis effect and plasmonic nanoantenna effect. In addition, materials and nanostructures suitable for PICS are shown, and characteristics and features unique to PICS are documented. Although it is well known that PICS has been applied to photovoltaics and photocatalysis, here light is shed on other applications that take better advantage of PICS, such as chemical sensing and biosensing, various photochromisms, photoswitchable functionalities and nanoscale photofabrication.

Labeling and in vivo visualization of transplanted adipose tissue-derived stem cells with safe cadmium-free aqueous ZnS coating of ZnS-AgInS2 nanoparticles.

The facile synthesis of ZnS-AgInS2 (ZAIS) as cadmium-free QDs and their application, mainly in solar cells, has been reported by our groups. In the present study, we investigated the safety and the usefulness for labeling and in vivo imaging of a newly synthesized aqueous ZnS-coated ZAIS (ZnS-ZAIS) carboxylated nanoparticles (ZZC) to stem cells. ZZC shows the strong fluorescence in aqueous solutions such as PBS and cell culture medium, and a complex of ZZC and octa-arginine (R8) peptides (R8-ZZC) can achieve the highly efficient labeling of adipose tissue-derived stem cells (ASCs). The cytotoxicity of R8-ZZC to ASCs was found to be extremely low in comparison to that of CdSe-based QDs, and R8-ZZC was confirmed to have no influence on the proliferation rate or the differentiation ability of ASCs. Moreover, R8-ZZC was not found to induce the production of major inflammatory cytokines (TNF-α, IFN-γ, IL-12p70, IL-6 and MCP-1) in ASCs. Transplanted R8-ZZC-labeled ASCs could be quantitatively detected in the lungs and liver mainly using an in vivo imaging system. In addition, high-speed multiphoton confocal laser microscopy revealed the presence of aggregates of transplanted ASCs at many sites in the lungs, whereas individual ASCs were found to have accumulated in the liver.

Controlling Shape Anisotropy of ZnS-AgInS2 Solid Solution Nanoparticles for Improving Photocatalytic Activity.

Independently controlling the shape anisotropy and chemical composition of multinary semiconductor particles is important for preparing highly efficient photocatalysts. In this study, we prepared ZnS-AgInS2 solid solution ((AgIn)xZn2(1-x)S2, ZAIS) nanoparticles with well-controlled anisotropic shapes, rod and rice shapes, by reacting corresponding metal acetates with a mixture of sulfur compounds with different reactivities, elemental sulfur, and 1,3-dibutylthiourea, via a two-step heating-up process. The chemical composition predominantly determined the energy gap of ZAIS particles: the fraction of Zn2+ in rod-shaped particles was tuned by the ratio of metal precursors used in the nanocrystal formation, while postpreparative Zn2+ doping was necessary to increase the Zn2+ fraction in the rice-shaped particles. The photocatalytic H2 evolution rate with irradiation to ZAIS particles dispersed in an aqueous solution was significantly dependent on the chemical composition in the case of using photocatalyst particles with a constant morphology. In contrast, photocatalytic activity at the optimum ZAIS composition, x of 0.35-0.45, increased with particle morphology in the order of rice (size: ca. 9 × ca. 16 nm) < sphere (diameter: ca. 5.5 nm) < rod (size: 4.6 × 27 nm). The highest apparent quantum yield for photocatalytic H2 evolution was 5.9% for rod-shaped ZAIS particles, being about two times larger than that obtained with spherical particles.

Oxidation Ability of Plasmon-Induced Charge Separation Evaluated on the Basis of Surface Hydroxylation of Gold Nanoparticles.

The oxidation ability of plasmonic photocatalysts, which has its origins in plasmon-induced charge separation and has not yet been studied quantitatively and systematically, is important for designing practical photocatalytic systems. Oxidation ability was investigated on the basis of surface hydroxylation of Au nanoparticles on TiO2 at various irradiation wavelengths and electrolyte pH values. The reaction proceeds only when the sum of the flat band potential of TiO2 and the irradiated photon energy is close to, or more positive than, the theoretical potential for the reaction.

Electrochemical redox-based tuning of near infrared localized plasmons of CuS nanoplates.

Fast and reversible control of the plasmonic properties of compound nanoparticles (i.e. CuS nanoplates) was achieved through electrochemical redox reactions. Their electrochemical tunability can be applied to fast-switching near infrared electrochromic devices, whose visible appearance is not changed by switching.

Potential-Scanning Localized Surface Plasmon Resonance Sensor.

Localized surface plasmon resonance (LSPR) sensors based on plasmonic nanoparticles attract much attention recently. Here we propose a new class of LSPR sensor, that is, a potential-scanning LSPR sensor, in which electron density of the plasmonic nanoparticles is controlled by potential scanning. The sensor exhibits a resonance peak during the potential scan, which negatively shifts with increasing local refractive index. Therefore, the present sensor can be applied to affinity biosensors and chemical sensors based on potential scan instead of wavelength scan. The potential-scanning LSPR sensors do not require space and a mechanical device for wavelength scanning, so the sensors are advantageous for miniaturization and cost reduction, in comparison with the conventional LSPR sensors. We explain the principle and theoretical sensitivities of the potential-scanning LSPR sensors, and refractometry is demonstrated using a sensor with an ITO electrode loaded with gold nanospheres (13 or 40 nm diameter) or nanorods. The smaller and larger nanospheres are suitable for sensing with a wider dynamic range and with a higher sensitivity, respectively. The use of nanorods further improves the sensitivity and figure of merit.