Plasmon-induced Charge Separation Research Articles

Wavelength- and efficiency-tunable plasmon-induced charge separation by the use of Au-Ag alloy nanoparticles.

TiO2 electrodes loaded with Au-Ag alloy nanoparticles, Ag content of which is about 0.25-0.90, exhibit stable anodic photocurrents due to plasmon-induced charge separation (PICS) in the presence of an electron donor. The PICS wavelength is blueshifted and the PICS efficiency is enhanced by increasing the Ag content in the nanoparticles.

Direct output of electrical signals from LSPR sensors on the basis of plasmon-induced charge separation

Potentiometric and conductometric sensors based on localized surface plasmon resonance were developed. The sensors can be applied to coloured and turbid samples because light need not pass through the sample solution.

プラズモン誘起電荷分離（PICS）の機構について

プラズモン誘起電荷分離（PICS）の機構について

Photoelectrochemical synthesis, optical properties and plasmon-induced charge separation behaviour of gold nanodumbbells on TiO₂.

Chemically synthesized, commercially available Au nanorods were adsorbed on a TiO₂ thin film, and photoelectrochemically transformed to Au nanodumbbells by photoelectrochemical deposition of Au at both ends of the NRs under UV irradiation. The nanodumbbells show about fourfold greater light absorption than the nanorods based on localized surface plasmon resonance (LSPR) in the visible to near infrared region. The absorption intensities and wavelengths of the Au nanodumbbells depend on the size of their spheroidal caps, which can be controlled by UV exposure time. The nanodumbbells can be applied to LSPR sensors, as their absorption peak redshifts with increasing local refractive index near the metal surface. The Au nanodumbbells on TiO₂ are also suitable for photofunctional materials and devices based on plasmon-induced charge separation (PICS) at the Au-TiO₂ interface, because of their higher photoabsorption intensity, better wavelength tunability and greater PICS efficiency than nanorods.

Plasmon-induced photoelectrocatalytic activity of Au nanoparticles enhanced TiO2 nanotube arrays electrodes for environmental remediation

A pulse electrodeposition (PED) technique was adopted to construct highly dispersed Au nanoparticles (Au-NPs) on TiO2 nanotube arrays (TiO2-NTs) electrodes prepared by electrochemical anodic oxidation. Both the particle size and loading amount were facilely controlled via adjusting electrochemical parameters. The morphology, crystallinity, elemental composition and light absorption capability of as-obtained Au/TiO2-NTs were distinguished based on various characterizations. Compared with pure TiO2-NTs, Au/TiO2-NTs electrodes exhibited much higher photocurrent density and greatly enhanced photoelectrocatalytic (PEC) activity towards the degradation of methyl orange (MO) under visible-light irradiation (λ>420nm). The synergy effect between nanotubular structures of TiO2 and uniformly dispersed Au nanoparticles, as well as the small bias potential and strong interaction between Au and TiO2, facilitated the Au plasmon-induced charge separation and transfer, which lead to highly efficient and stable visible-light PEC activity.

Plasmon-induced ammonia synthesis through nitrogen photofixation with visible light irradiation.

We have successfully developed a plasmon-induced technique for ammonia synthesis that responds to visible light through a strontium titanate (SrTiO3) photoelectrode loaded with gold (Au) nanoparticles. The photoelectrochemical reaction cell was divided into two chambers to separate the oxidized (anodic side) and reduced (cathodic side) products. To promote NH3 formation, a chemical bias was applied by regulating the pH value of these compartments, and ethanol was added to the anodic chamber as a sacrificial donor. The quantity of NH3 formed at the ruthenium surface, which was used as a co-catalyst for SrTiO3, increases linearly as a function of time under irradiation with visible light at wavelengths longer than 550 nm. The NH3 formation action spectrum approximately corresponds to the plasmon resonance spectrum. We deduced that plasmon-induced charge separation at the Au/SrTiO3 interface promotes oxidation at the anodic chamber and subsequent nitrogen reduction on the cathodic side.

Plasmon-assisted water splitting using two sides of the same SrTiO₃ single-crystal substrate: conversion of visible light to chemical energy.

A plasmon-induced water splitting system that operates under irradiation by visible light was successfully developed; the system is based on the use of both sides of the same strontium titanate (SrTiO3) single-crystal substrate. The water splitting system contains two solution chambers to separate hydrogen (H2) and oxygen (O2). To promote water splitting, a chemical bias was applied by regulating the pH values of the chambers. The quantity of H2 evolved from the surface of platinum, which was used as a reduction co-catalyst, was twice the quantity of O2 evolved from an Au-nanostructured surface. Thus, the stoichiometric evolution of H2 and O2 was clearly demonstrated. The hydrogen-evolution action spectrum closely corresponds to the plasmon resonance spectrum, indicating that the plasmon-induced charge separation at the Au/SrTiO3 interface promotes water oxidation and the subsequent reduction of a proton on the backside of the SrTiO3 substrate. The chemical bias is significantly reduced by plasmonic effects, which indicates the possibility of constructing an artificial photosynthesis system with low energy consumption.

Plasmon-Induced Charge Separation (PICS) and Plasmonic Nanoantenna Effects: Similarities and Differences

Metal nanoparticles including gold, silver, and copper nanoparticles absorb visible and near infrared light due to localized surface plasmon resonance (LSPR). We found plasmon-induced charge separation (PICS)1,2 at the interface between plasmonic metal nanoparticles and semiconductor such as TiO2 and applied it to photocatalysis,2 photovoltaic cells,2,3 multicolor photochromism,1,4 infrared photochromism,5 single particle multicolor changes,6 and photomorphing gells.7 PICS is also applied by other groups in particular to photocatalysis8,9 and photovoltaic cells.10,11 However, PICS is often confused with plasmonic nanoantenna effects. Au or Ag nanoparticles assist semiconductors and dye molecules in light absorption by trapping photons based on LSPR. As a result, nanoparticles enhance photocurrents, photocatalytic reactions, and other photochemical reactions. The enhancement based on the plasmonic nanoantenna effects occurs at wavelengths at which both of the semiconductor/dye molecule and the metal nanoparticles absorb light. In addition, the maximum enhancement is reached when the distance between the metal nanoparticle and the semiconductor/dye is about 10 nm.1 2-1 4 The enhancement is suppressed in the case of direct contact.On the other hand, PICS occurs at LSPR wavelengths even if those are longer than the absorption edge wavelength of the semiconductor.1- 11 PICS is based on direct electron transfer from the resonant metal nanoparticles to semiconductor2,15due to external photoelectric effect or hot electron injection. Therefore, it occurs when the metal nanoparticle is in direct contact with the semiconductor, if electron tunneling or hopping is not possible between them. In the paper, recent developments related to PICS would also be described. This work was supported in part by “R&D on Innovative PV Power Generation Technology” which The University of Tokyo contracted with New Energy and Industrial Technology Development Organization (NEDO) and a Grant-in-Aid for Scientific Research (No. 25288063, No. 25107511).ReferencesY. Ohko, T. Tatsuma, T. Fujii, K. Naoi, C. Niwa, Y. Kubota, and A. Fujishima, Nature Mater., 2, 29 (2003).Y. Tian and T. Tatsuma, J. Am. Chem. Soc., 127, 7632 (2005).Y. Takahashi and T. Tatsuma, Appl. Phys. Lett., 99, 182110 (2011).K. Naoi, Y. Ohko, and T. Tatsuma, J. Am. Chem. Soc., 126, 3664 (2004).E. Kazuma, and T. Tatsuma, Chem. Commun., 48, 1733 (2012).I. Tanabe and T. Tatsuma, Nano Lett., 12, 5418 (2012).T. Tatsuma, K. Takada, and T. Miyazaki, Adv. Mater., 19, 1249 (2007).E. Kowalska, R. Abe, and B. Ohtani, Chem. Commun., 2009, 241.C. G. Silva, R. Juárez, T. Marino, R. Molinari, and H. Garcìa, J. Am. Chem. Soc., 133 , 595 (2011). M. W. Knight, H. Sobhani, P. Nordlander, and N. J. Halas, Science, 332, 702 (2011).P. Reineck, G. P. Lee, D. Brick, M. Karg, P. Mulvaney, and U. Bach, Adv. Mater., 24, 4750 (2012).T. Kawawaki, Y. Takahashi, and T. Tatsuma, Nanoscale, 3, 2865 (2011).T. Torimoto, H. Horibe, T. Kameyama, K. Okazaki, S. Ikeda, M. Matsumura, A. Ishikawa, and H. Ishihara, J. Phys. Chem. Lett., 2, 2057 (2011).T. Kawawaki and T. Tatsuma, Phys. Chem. Chem. Phys., 15, 20247 (2013).N. Sakai, Y. Fujiwara, Y. Takahashi, and T. Tatsuma, ChemPhysChem, 10, 766 (2009).E. Kazuma, N. Sakai, and T. Tatsuma, Chem. Commun., 47, 5777 (2011).

Photoinduced Multiple Spectral Changes of Single Plasmonic Gold Nanospheres by the Aid of Coordination

Abstract One or both of two localized surface plasmon modes (full-surface mode and interface mode) of a commercially available single Au nanosphere on TiO2 are excited, and the corresponding spectral changes are induced in the presence of thiocyanate. Similar spectral changes can be simulated by assuming site-selective oxidation of the Au nanosphere by plasmon-induced charge separation (PICS).

In Situ Nanoimaging of Photoinduced Charge Separation at the Plasmonic Au Nanoparticle‐TiO2 Interface

Photoinduced charge distributions for single Au nanoparticles deposited on single crystalline TiO2 are visualized in situ by Kelvin probe force microscopy. Under UV light, the electrons in excited TiO2 transfer to the Au nanoparticles. In contrast, when the Au nanoparticles are selectively excited by visible-infrared light, electrons are injected from the nanoparticles to TiO2, resulting in plasmon-induced charge separation.

Localized surface plasmon resonance sensors based on wavelength-tunable spectral dips

Localized surface plasmon resonance (LSPR) sensors serve as sensitive analytical tools based on refractive index changes, which can be applied to affinity-based chemical sensing and biosensing. However, to select the monitoring wavelength, monodisperse Au or Ag nanoparticles must be synthesized. Here we developed LSPR sensors that operate at arbitrary wavelengths after preirradiation at the corresponding wavelength. Polydisperse plasmonic Ag nanospheroids or nanorods are photocatalytically deposited on TiO2. The nanoparticle ensemble shows a broad absorption band over the visible and near infrared regions, and absorption dips can be formed at desired wavelengths simply by photoexciting the ensemble at the wavelengths, on the basis of plasmon-induced charge separation. The dips redshift linearly in response to a positive change of refractive index, and the refractive index sensitivity linearly increases with increasing dip wavelength (e.g., 356 nm RIU(-1) at 1832 nm). The dip-based sensor is applied to monitoring of selective binding between biotin and streptavidin. The present system would allow development of miniaturized and cost-effective sensors that operate at the optimum wavelength at which the sensitivity is highest within the optical window of the sample.

(Invited) Recent Developments Related to Plasmon-Induced Charge Separation

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Plasmon-induced oxidation of gold nanoparticles on TiO2 in the presence of ligands

The mechanism of plasmon-induced charge separation of gold nanoparticles (Au NPs) on TiO2 was investigated with the help of anionic ligands. Au NPs are oxidized in the presence of KX (X = SCN, Br or Cl), chiefly via direct oxidation to [AuX4](-). The reactivity of Au NPs depends on the solution pH, the type of anion ligands and the excitation wavelength, suggesting that the photopotential of Au NPs during plasmon-induced charge separation depends on both the flatband potential of TiO2 and the energy of irradiated photons. The results indicate that the reactivity and efficiency of the plasmon-induced charge separation and accompanying reactions can be tuned and optimized by changing those factors.

Stable spectral dip formation and multicolour changes of plasmonic gold nanoparticles on TiO2

Spectral dip formation at arbitrary wavelengths and the corresponding multicolour changes were achieved by using plasmonic Au nanoparticles on TiO(2), based on plasmon-induced charge separation and a coordination effect of iodide ions. The dips were stable in air under white light, in marked contrast to the case of conventional Ag nanoparticles.

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

Abstract 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 nanoparticles 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.

Plasmon Inducing Effects for Enhanced Photoelectrochemical Water Splitting: X-ray Absorption Approach to Electronic Structures

Artificial photosynthesis using semiconductors has been investigated for more than three decades for the purpose of transferring solar energy into chemical fuels. Numerous studies have revealed that the introduction of plasmonic materials into photochemical reaction can substantially enhance the photo response to the solar splitting of water. Until recently, few systematic studies have provided clear evidence concerning how plasmon excitation and which factor dominates the solar splitting of water in photovoltaic devices. This work demonstrates the effects of plasmons upon an Au nanostructure-ZnO nanorods array as a photoanode. Several strategies have been successfully adopted to reveal the mutually independent contributions of various plasmonic effects under solar irradiation. These have clarified that the coupling of hot electrons that are formed by plasmons and the electromagnetic field can effectively increase the probability of a photochemical reaction in the splitting of water. These findings support a new approach to investigating localized plasmon-induced effects and charge separation in photoelectrochemical processes, and solar water splitting was used herein as platform to explore mechanisms of enhancement of surface plasmon resonance.

A Solid‐State Plasmonic Solar Cell via Metal Nanoparticle Self‐Assembly

Sustainable plasmonic photocurrents are generated by gold and silver nanoparticles, located at a TiO2/hole conductor interface. The spectral photocurrent response closely follows the surface plasmon absorption bands of the metal particles. A simple nanoparticle self-assembly method for the solar cell fabrication is presented. Three mechanisms for plasmon-induced charge separation are proposed. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Nanoimaging and Analysis of Localized Surface Plasmon-Induced Charge Separation and Application to Versatile Photochromism

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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 m = 2, 4) of the NRs are forbidden at perpendicular incidence (θ = 0°), and in the present system, the third-order plasmon mode (m = 3) is also forbidden at a specific incident angle, θ = 37.5°.

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.