Surface plasmon-enhanced photochemical reactions

Journal of Photochemistry and Photobiology C: Photochemistry Reviews 15 (2013) 31–52

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Journal of Photochemistry and Photobiology C: Photochemistry Reviews journal homepage: www.elsevier.com/locate/jphotochemrev

Invited review

Surface plasmon-enhanced photochemical reactions Kosei Ueno a,b,∗ , Hiroaki Misawa a a b

Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0021, Japan PRESTO, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan

a r t i c l e

i n f o

Article history: Received 17 December 2012 Received in revised form 29 March 2013 Accepted 3 April 2013 Available online 23 April 2013 Keywords: Plasmonic chemistry Photochemical reactions Gold nanoparticles Electromagnetic field enhancement Nano-lithography Water splitting

a b s t r a c t The electromagnetic field enhancement effect based on the excitation of localized surface plasmon resonance was developed for various photochemical reaction systems, such as nano-lithography, photovoltaic cells, photocatalysis, and water splitting systems. As with most points characteristic of these surface plasmon-enhanced photochemical reactions, spatially selective photochemical reactions can be induced and photons can be efficiently utilized, a concept that could contribute to the development of green nanotechnology. Electromagnetic field enhancement effects based on plasmon excitation have contributed not only to physical processes, such as excitation efficiency, but also to chemical processes, such as photoinduced electron transfer reactions. This review article describes advanced studies on a wide variety of surface plasmon-enhanced photochemical reactions. © 2013 Elsevier B.V. All rights reserved.

Contents 1.

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3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Research background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. Enhancement of light-matter coupling process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3. Enhancement of photochemical reaction process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4. Excitation of propagating surface plasmon polariton and localized surface plasmon resonance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5. Electromagnetic field enhancement based on LSPR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6. The scope of the review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Spatially selective photochemical reactions on the nanometer scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Plasmonic local photochemical reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Photochemical imaging of plasmonically-enhanced optical near-field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. Non-linear photochemical imaging of a hot site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3. Near-field imaging of multipole plasmon resonances using photopolymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Surface plasmon-assisted nanolithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Plasmonic nanolithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. Surface plasmon interference lithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3. Nanogap-assisted plasmonic nanolithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4. Nanolithography using the scattering component of multipole plasmon resonances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photochemical reaction systems realizing the utilization of photons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Photochemical reaction using a weak light source and quantitative analysis of the plasmon-enhanced chemical reactions . . . . . . . . . . . . . 3.1.1. Nonlinear photochemical reactions by a weak incoherent light source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Quantitative analysis of a surface plasmon-enhanced photochemical reaction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3. Control of strong coupling between molecules and plasmon polariton via photochemical reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.4. In situ measurement of plasmon-enhanced photochemical reactions probed by Raman scattering spectroscopy . . . . . . . . . . . . . . 3.1.5. Structural change in a silver nanoparticle using a plasmon-enhanced photochemical reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Plasmon-enhanced photocatalytic reaction and water splitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1. Promotion of a titanium dioxide photocatalysis using the plasmonic effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author at: Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0021, Japan. Tel.: +81 117069318. E-mail address: [email protected] (K. Ueno). 1389-5567/$20.00 © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jphotochemrev.2013.04.001

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3.2.2. Photocatalytic reaction via plasmon-induced charge separation using metallic nanoparticles loaded onto TiO2 . . . . . . . . . . . . . . . 3.2.3. Promotion of photocatalytic reactions by plasmonic effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.4. Plasmonic effect in water splitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction 1.1. Research background Since the discovery of the surface-enhanced Raman scattering (SERS) phenomenon in the 1970s in which the Raman scattering signal from molecules leaving the surface of metallic nanoparticles is markedly enhanced [1–3], many photochemical reactions that are promoted by the electromagnetic field enhancement effect based on this plasmon excitation have been studied. At the time of its discovery, the electromagnetic field enhancement effect attributed to the excitation of plasmon resonance was only applied to chemical sensors or biosensors; however, this phenomenon is now being applied to a wide variety of photochemical reaction systems, such as nano-lithography [4–6], photovoltaic cells [7–9], photocatalysis [10–12], and a water splitting [13]. Fundamental studies to clarify the mechanisms of SERS were performed from the 1980s to the 1990s. These mechanisms are now clearly understood [14–16]. Thus, the only remaining questions pertain to the reproducibility of the system for developing applied technologies and improving quantitative analysis in fundamental research. The enhancement phenomenon of SERS, various nonlinear optical effects, and the surface plasmon-enhanced photochemical reaction were clearly demonstrated in 1980 [17–22]. Therefore, improvements in the reproducibility of a system and the quantitative analysis in experiments involving well-defined metallic nanostructures were considered to be a mere technical problem that could easily be solved. Studies attempting to elucidate a mechanism for these surface plasmon-enhanced chemical effects and to discover their technological applications have been actively pursued from the second half of the 1990s to the 2000s [23–25]. These researches have progressed owing to the development of nanoscience and nanotechnology because various metallic nanoparticles of wellcontrolled size and shape were prepared using bottom-up chemical syntheses and top-down nanofabrication techniques [26–28]. Nanofabrication technologies have seen remarkable progress in the past 15 years, and structural control at the single-nanometer level has become possible [29–31]. With regards to optical measurement or spectroscopic measurement technologies, both spatial and time resolution have improved dramatically, and the propagation and phase relaxation of surface plasmon polaritons, or the nearfield intensity distribution, can now be observed experimentally at the nanometer scale [32–37]. With the development of these technologies, new scientific research has been proposed to understand surface plasmon-enhanced photochemical reactions. The development of structural control and measurement technologies over the past 15 years has not simply repeated earlier work pursuing only the surface plasmon-enhanced chemical effect but can be considered a new area of research, “plasmonic chemistry,” as described in this review article.

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dye-sensitized solar cell [51–53] and an organic solar cell [54–56], have been developed. Therefore, photochemical studies are critical the development of science and technology. These photochemical reactions are initiated from excitation of a molecule by light. The efficiency of the light excitation is determined by the absorption cross section particular to a given molecule, and the absorption cross section of a standard dye molecule is on the order of 1 × 10−16 cm2 [57]. On the other hand, the focused spot area of visible light often used in photochemical reactions is as small as 1 × 10−9 cm2 due to diffraction limit. Therefore, the spot size of light is 107 times larger than the molecular absorption cross section. This means that at least 1 billion molecules are required in a diameter of approximately 350 nm which is tightly focused spot size of light in order to interact between a photon and a molecule [58]. Traditionally, therefore, the light-matter coupling process was not so large. However, the “utilization of photons” concept, in which the photons irradiating the photochemical reaction field are completely absorbed by a small number of molecules, is important to reducing the energy consumed in optical devices [59]. To construct a system that maximizes the light absorption efficiency with a small number of molecules, a new photochemical reaction field is required based on the concept of the light interacting with the molecules by spatially and temporary confining the electromagnetic waves. These can be attained by localizing the electromagnetic waves in a nanometer-sized area that exceeds the diffraction limit and confines the radiation in the space for a certain period. The electromagnetic field enhancement effects based on the excitation of the localized surface plasmon resonance (LSPR) induced at the surface of metallic nanoparticles is effective at building an efficient photochemical reaction field [17,18,20,22,60]. Because the near-field light from a metallic nanoparticle’s surface induced by LSPR is localized in the nanospace, depending on the shape of the nanoparticles as illustrated in Fig. 1, and exists until the phase relaxation of the LSPR, a large electromagnetic field enhancement is induced [59]. Therefore, a molecule in the vicinity of the metallic nanoparticle has a high probability of excitement

1.2. Enhancement of light-matter coupling process To date, many photofunctional materials and optical devices, such as spectral sensitizers for photograph film [38–40], photoresists for photolithography [41–43], illuminants for organic electroluminescence [44,45], photocatalysts [46–50], a

Fig. 1. A schematic illustration in which photons are strongly interacting with a molecule that exists in a nanogap due to the localization of an electromagnetic field [58]. The red dots shows photons and blue one is molecule. The pink region indicates local space showing strong enhancement of electromagnetic field between gold nanostructures (yellow).

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because the molecule experiences a strong electromagnetic field, as shown in Fig. 1 [58]. 1.3. Enhancement of photochemical reaction process In contrast, the plasmonically enhanced optical near-field now plays an important role in not only the excitation process but also in photochemical reaction processes, such as the photo-induced electron transfer reaction. Over the past decade, many researchers demonstrated plasmon-enhanced photocurrent generation or photocatalytic reactions via photo-induced electron transfer from metallic nanoparticle to n-type semiconductor substrate [61–64]. The mechanism was considered that the plasmon-induced charge separation was induced as a result of the hot electron injection to the conduction band of semiconductor via metal/semiconductor interface. In this case, the formation of hot electron is promoted by plasmonically-enhanced optical near-field. On the basis of the speculated mechanism of the plasmon-induced charge separation, the principle is analogous to dye-sensitized solar cell (DSSC) [51], in which plasmonic metallic nanoparticles is used as a sensitizer instead of dye molecule. In a general DSSC system, a porous titanium dioxide (TiO2 ) film conjugated by [Ru(bpy)3 ]2+ -based dye was used as an anode of the solar cell, and the excited state of the dye injects an electron into the conduction band of TiO2 after light absorption. It is well-known that the energy conversion efficiency of DSSC recently reached as high as ∼12% of solar light (100 mW cm−2 ) according to using multiple dyes [65]. Therefore, the efficiency of DSSC is much higher than that of solar cell using plasmon-induced charge separation which was reported so far. However, plasmonic optical antenna is promising because their optical absorption wavelength can be easily controlled by changing their geometries from visible to near-infrared wavelengths. Actually, plasmon-enhanced photocurrent generation was achieved in near-infrared wavelength [63,64]. Furthermore, the design of the plasmonic optical antenna that minify the loss based on light scattering was proposed [66,67]. In addition, photocatalytic water splitting was recently actualized using photoelectordes in the photocurrent generation system [68,69]. Therefore, the plasmonic optical antenna might be also useful as the sensitizer if the detailed mechanism for the photocurrent generation is understood and the structural design of optical antenna and systems are optimized. 1.4. Excitation of propagating surface plasmon polariton and localized surface plasmon resonance In the elementary excitation of a substance, the energy exhibits a nonlinear response to a wavenumber due to interference between the forward and backward waves. Therefore, an electron wave in metal does not couple with light whose energy is linearly related to the wavenumber. According to Maxswell’s theory, surface plasmons can propagate along a metallic film surface and have a spectrum of Eigen frequencies (ω) related to the wave vector (k) by a dispersion relation as followings:



Ksp =

ε1 ε2 ε1 + ε2

(1)

where ε2 = ε2  + iε2  (real part (ε2  ) and imaginary part (iε2  )) and ε1 are the dielectric constant of the metal and the surrounding medium, respectively. The dispersion curve of surface plasmon is shown in Fig. 2. On the other hand, wave vector kc of light at frequency ω propagating through the medium ε1 is expressed by: Kc =

ω√ ε1 c

(2)

where c is the light velocity in vacuum, and its dispersion relation is a straight line kc = ω/c as also shown in Fig. 2. Fig. 2 indicates that the

ω

k Fig. 2. The dispersion curve of surface plasmon (solid curve) and the dispesion relationship of light; kc (broken line) and ksin  (dashed line).

dispersion relation of surface plasmon does not interact with the dispersion relation of light. However, on the metallic film surface, a phenomenon is observed that is generated by the electric field when the total-internal-reflection conditions of light (an evanescent wave and transverse magnetic (TM) polarization) resonate with the compressional wave of the electric charge and propagate with an electric field on a metallic film surface [70–73]. This phenomenon is due to the conditions in which the in-plane component of the wave vector of the light matches that of a surface plasmon because the electric field of the incident light irradiated at an angle has two components that are both parallel and perpendicular to the metallic film surface [72,73]. This means that the dispersion relation of light turn down to the new line (ksin ) where both the light lines and the dispersion curve of surface plasmon cross each other as shown in Fig. 2. Thus, the state in which the light wave and the wave causing the electronic excitation of a metallic film surface are mixed is called a (propagating) surface plasmon polariton and is a mechanism by which evanescent light is generated with an electromagnetic field intensity of at least 10 times that of the incident light [74–76]. Several studies, including those on the development of plasmonic optical devices, such as a waveguide [77–79], fundamental work on efficient second- or higher-order harmonic generation [80–82], and applications such as the surface plasmon resonance (SPR) sensor, which detects a small quantity of adsorbed material utilizing a change in the degree of the incident angle of the light resonant with the surface plasmon polariton by adsorbing the dielectric onto a metal surface [83–85], have been conducted using this principle. In contrast, the spherical metallic nanoparticles (gold, silver, etc.) are sufficiently small in comparison with the wavelength of the light to be resonant with the light coming from all directions and indicate coloration as well as a local electromagnetic field enhancement on the particle surface even without the condition of total internal reflection. The resonance phenomenon between this metallic nanoparticle and the light is called LSPR whose coupling conditions between light and surface plasmon are same as those of the propagating surface plasmon polariton. However, in the case of propagating surface plasmon polariton, plasmons propagate in the x- and y-directions along the metallic film surface, for distances on the order of tens to hundreds of microns, and decay evanescently in the z-direction on the order of 200 nm. In contrast, in the case of LSPR, light interacts with nanoparticles which leads to a plasmon that oscillates locally around the nanoparticle with a given frequency as described in followings. As for metals such as gold, silver or copper, 10 electrons occupy the d orbitals, and a metallic bond forms when the peripheral sorbital electron creates an energy band (sp-conduction band). The metallic bond is stabilized by the conduction band, which has a

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K. Ueno, H. Misawa / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 15 (2013) 31–52

Fig. 3. (a) A schematic illustration of the LSPR excitation by metallic nanoparticles and the formation of an electric dipole on a metallic nanoparticle. (b) An illustration of the near-field enhancement by metallic structures with a dimer-type nanogap. Schematic drawings of the dipole–dipole interaction (c) and the excitation of the electric quadrupole resonances (d) due to the formation of a nanogap.

broad energy level when many metal atoms combine. LSPR is not excited in a metallic nanoparticle with a diameter of 1 nm or less because the metallic character is lost when the particle is composed of an extremely small number of atoms; the conduction electrons are primarily participating in the excitation of the plasmon resonance. A schematic illustration of LSPR excitation by metallic nanoparticles is shown in Fig. 3(a) [86]. A perturbation of the plasmon excitation is based on the oscillating electric field of light, similar to the optical absorption of the ␲ electronic system in an organic molecule. A component of the electric field of light induces a collective motion in the conduction electrons (the free electrons) with a negative electric charge and causes an electric polarization of the surface of the metallic nanoparticle, which results in the formation of an electric dipole [86]. In addition, when a particle becomes comparatively large, dipole and higher-order resonances, such as quadrupole, will be induced [87,88]. The frequency of an LSPR is based on the distribution of the electric charge related to the electron density, the effective mass of an electron, and the size and shape of the metallic nanoparticles. Therefore, the optical properties differ because various shapes of metallic nanoparticles such as spheres, rods, and triangular prisms were proposed and prepared by bottom-up and top-down nanofabrication technologies so far [89–93]. The optical property of spherical metallic nanoparticles can be theoretically predicted using the Mie scattering formula to calculate how the electromagnetic waves are scattered by a material with a certain dielectric function [94]. Theoretical analyses and experiments have been performed by several research groups to examine the optical characteristics of spherical and rod-like metallic nanoparticles [95–97]. 1.5. Electromagnetic field enhancement based on LSPR LSPR induced by the interaction between metallic nanoparticles and an electromagnetic field induces a near-field enhancement on the metallic nanoparticle surface. The electromagnetic field enhancement by LSPR is thought to be due to the localization of electromagnetic field at both corners of metallic nanoparticles along incident polarization direction and the plasma oscillation (a wave motion of charge density by the collective oscillation of conduction electrons), which continues for ∼10 fs (phase relaxation) after LSPR is excited in the metallic nanoparticles, although the light passes after 0.1 fs to extend tens of nanometers in distance.

Thus, the electromagnetic field generated by the oscillating electric field of light, that is near-field light, affects the surface of metallic nanoparticle for a certain time. Therefore, the probability of an interaction between light and a molecule in the vicinity of the metallic nanoparticles increases so that enhancement of fluorescence, SERS, and various nonlinear optical effects is promoted in the presence of metallic nanoparticles [98–104]. A numerical simulation based on Maxwell’s classic electromagnetism theory (the finite-difference time-domain (FDTD) method) can be used to estimate that the induced electromagnetic field enhancement is approximately 10–100 times larger than the incident electromagnetic field intensity, although the enhancement factor depends on the size and shape of the metallic nanoparticles [105]. This enhancement is induced due to localization of light and continuous plasma oscillation as former explained. On the other hand, the electromagnetic interaction between particles is induced when two or more metallic nanoparticles come together at a distance of several nanometers, as shown in Fig. 3(b); then, a dipole–dipole interaction between the neighboring metallic nanoparticles is induced (dipole coupling), as shown in Fig. 3(c) [106]. The plasma oscillation of the neighboring particles is in the same phase because the metallic nanoparticles are small compared with the wavelength of the light, and an electrostatic interaction between the particles is induced [97,107,108]. Naturally, the electromagnetic field intensity at a nanogap increases as the distance between particles decreases because the electromagnetic field is localized at nanometer-sized gap space, and the electromagnetic field intensity can theoretically be enhanced up to ∼105 -fold over that of the incident light [109,110]. Moreover, multipole plasmon resonances, such as quadrupole plasmon resonances, are excited by the dipole–dipole interaction as shown in Fig. 3(d), and an electromagnetic field enhancement is possibly induced at the nanogap over a wide wavelength range as the result [106]. 1.6. The scope of the review In this review article, important discoveries that contributed to development of surface plasmon-enhanced photochemical reactions are introduced. An electromagnetic field can be localized to an area on the scale of one nanometer, and an electromagnetic field enhancement can occur with LSPR that reaches many times the effect of the electromagnetic field intensity when incident light is introduced. Therefore, many studies have been reported on the plasmon-enhanced photochemical reactions using plasmonic nano-lithography, high-density optical memory, a nonlinear photochemical reaction by a weak light source, and effective utilization of photons in a solar cell, etc. In contrast, new discoveries regarding surface plasmon-enhanced photochemical reactions in which an electron transfer reaction from a metal to a semiconductor have been observed, and applications, such as photocatalytic reactions including water splitting and systems to generate photocurrents, have been realized. This review describes studies based on plasmon-enhanced photochemical reactions of spatially selective photochemical reactions on the nanometer scale and photochemical reaction systems that utilize photons effectively. Finally, the future of research in this field is discussed. 2. Spatially selective photochemical reactions on the nanometer scale Electromagnetic field enhancement induced by LSPR was developed to study nano-lithography and high-density optical discs because the spatially selective photochemical reactions that are induced on the nanometer scale are based on plasmon-enhanced photochemical reactions. First, the near-field light localized the

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optical electric field to a region smaller than its diffraction limit; therefore, it was applied to a near-field microscopy technique using spatially resolved spectrum measurement technology [111–113], surface treatment technology [114–116], and other techniques [117–121]. Research in the field of near-field optics has been actively pursued since the 1990s. LSPR was also utilized for nanolithography and surface modifications (photochemical reaction), which were applied to nano-patterning on a photoresist film [5] and optical recording on an optical disk [122,123]. In addition, on an optical disc, LSPR can be used as a read out [124,125]. The studies regarding spatially selective photochemical reactions began in 2000 and were followed by research on the use of precisely prepared metallic nanoparticles [126–128]. In particular, plasmon-enhanced non-linear photochemical reactions via the two-photon absorption of molecules were observed through irradiation using femtosecond laser pulses as an excitation source because long wavelength (near-infrared) plasmon resonance spectra are realized through the development of gold nanorods [129], core–shell nanoparticles [130], and nanofabrication technologies [131], etc. LSPR yields an electromagnetic field enhancement effect that allows the near-field enhancement effect based on the localization of the electromagnetic field to play an important role. Thus, the promotion of plasmon-enhanced non-linear photochemical reactions is affected by the near-field enhancement because a nonlinear optical effect takes place more efficiently due to the localization. The non-linear photochemical reaction is especially promoted at the nanogap region of the metallic nanostructures (called the “hot site”), which has already been cited for its role in the SERS phenomenon [132–134]. Moreover, a propagating surface plasmon polariton has characteristics of a wave; thus, it can produce an interesting spatial pattern using photochemical reactions that utilize the interference of plasmonic electric waves [135–137]. 2.1. Plasmonic local photochemical reactions 2.1.1. Photochemical imaging of plasmonically-enhanced optical near-field The imaging of a near-field intensity distributions based on the LSPR excitation of metallic nanostructures has been intensively studied using scanning probe methodologies, such as photon scanning tunneling microscopy (PSTM), aperture scanning near-field optical microscopy (SNOM), and apertureless scanning near-field optical microscopy (ASNOM) [138–143]. However, problems arise in obtaining a near-field image due to several factors, including noise in the image from the concavo–convex structure that is dependent on the sample and the electromagnetic interaction between the tip and the sample based on multiple scattering and dipole couplings. To avoid these problems, a methodology has been proposed for visualizing the near-field intensity distribution using a photochemical reaction. Hubert et al. successfully observed a near-field spatial distribution of silver (Ag) nanostructures using the surface plasmon-enhanced photochemical reaction of an azobenzene-dye polymer [144]. To observe the near-field intensity distribution of the Ag nanostructures, this study utilized the mass transport properties of poly-(methyl methacrylate) (PMMA) functionalized with azobenzene-dye chromophores. Because azobenzene derivatives give rise to trans–cis isomerization after photoexcitation, the molecular rotation through thermal diffusion within the PMMA matrix induced a reorientation of the chromophores along the electric field; azobenzene derivatives push or pull their polymeric host due to reorientation and create a one-step photoinscription for surface relief [145–147]. In this experiment, an azobenzenedye derivative was spin-coated onto the Ag-nanostructured substrate, and subsequent irradiation with an argon-ion laser at 532 nm, which overlap the 400–600 nm absorption band of the

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azobenzene-dye derivative. The plasmon resonance band of Ag nanoparticles embedded in the polymer film was determined to be approximately 532 nm. Two holes were successfully observed in the polymer via atomic force microscope (AFM) and were close to the particles along the direction of the incident light polarization as shown in Fig. 4(a) and (b). The depth of each hole was approximately 4 nm, and its diameter was approximately 100 nm. Most importantly, the profiles agree remarkably well with the expected dipolar near-field spatial distribution by numerical calculations as shown in Fig. 4(c) and (d), ((d) is a negative image of (c)). The negative image (Fig. 4(d)) almost corresponds with the experimental result, as shown in Fig. 4(b). The near-field photochemical image was clearly obtained using a photosensitive azobenzene-dye polymer [144]. Azobenzene polymer is powerful means visualizing spatial distributions of near-field intensity on metallic nanostructures [148–150]. Ishitobi et al. demonstrated metal (Ag) tip enhanced near-field illumination to azo polymer film using AFM tip for a photochemical imaging of near-field intensity distribution. They successfully visualized near-field intensity distribution with 47 nm full width at half maximum beyond the diffraction limit using the tip-enhanced exposure system [151]. In contrast, Stamplecoskie et al. exhibited the enhanced field around silver nanoparticles in a thin film containing an azo free radical initiator and a triacrylate selectively cross-links the triacrylate within the plasmonic region of the silver nanoparticles [127]. As the other approach, Boneberg et al. investigated the lateral near-field intensity distribution of gold triangular structures using a laser ablation technique [152]. 2.1.2. Non-linear photochemical imaging of a hot site Non-linear photochemical imaging is also a powerful technique for observing a near-field intensity distribution localized on metallic nanoparticles. In the field of plasmonics, control over the width and configuration of a nanogap becomes important because detection of a single molecule can be attained through Raman scattering with an electromagnetic field enhancement at a hot site, such as the nanogap in SERS [153–155]. As previously described, the electromagnetic field enhancement effect is increased 104 or 105 times over that of the incident light at a nanogap when a nanometer-scale gap is formed between two gold nanoparticles, which induces a remarkable nonlinear optical phenomenon. Using this principle, Sundaramurthy et al. successfully visualized a hot site of bowtie gold structures with a nanogap using the two-photon-induced photopolymerization of a negative-type photoresist [156]. In this study, a commercially available negativetype photoresist (SU-8, Microchem. Co.) was employed for the surface plasmon-enhanced photopolymerization. A two-photoninduced polymerization of SU-8 via the two-photon absorption of a photoinitiator included in the photoresist induced a cationic cross-linking chain reaction, which is an established method for fabricating two- and three-dimensional (3D) microstructures with high resolution [157–159]. A scanning electron microscopy (SEM) image of the fabricated bowtie structures is provided in Fig. 5(a). After spin coating the SU-8 onto the bowtie gold nanostructures, a mode-locked Ti:sapphire laser beam (pulse duration ( p ) = 120 fs, repetition rate (f) = 75 MHz, and wavelength (p ) = 800 nm) was irradiated onto the substrate. Fig. 5(b) provides an SEM image of a bowtie structure after irradiation and development performed with the 120-␮W laser light tightly focused on the gold nanostructure. The SEM image indicated a dark spot near the gap position because the polymerized SU-8 formed at the nanogap. To confirm the results of this experiment, the AFM image and cross section of a bowtie structure were determined after irradiation and development were achieved with a 54-␮W laser that was tightly focused on the gold nanostructure, as shown in Fig. 5(c). The AFM image and its cross section clearly demonstrate that the

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Fig. 4. (a) AFM image of an azobenzene-dye polymer surface after exposure, and its high magnification image (b). The arrows indicate the direction of incident polarization. (c) Numerical calculations of the electric field intensity distribution around the Ag nanoparticle and its negative image (d) [144].

two-photon-induced photopolymerization of SU-8 proceeded in the vicinity of the nanogap. Visualization of the hot site of a bowtie structure was successfully accomplished using plasmon-enhanced two-photon-induced photopolymerization [156]. Nah et al. elucidated the mechanism of multi-photon-induced photopolymerization of SU-8 on a gold nanowire [160]. They found that the photopolymerization was correlated with multiphotonabsorption-induced photoluminescence of gold nanowire. On the other hand, Diebold succeeded in non-linear photochemical reaction of positive photoresist on nanostructured metal surface and uncovering the electromagnetic hot spots according to utilizing a selective expose of the positive photoresist covering hot spots [161]. Dostert et al. also demonstrated that the intensity distribution of the near field controls the site and the extent of the two-photon-induced photochemical reaction at the metallic nanostructure surface [162].

Fig. 5. (a) SEM image of the fabricated bowtie structures (gap width = 36 nm). (b) SEM image of the bowtie structures. The dark spot in the SEM image is the polymerized SU-8 region. (c) AFM images and cross sections along the dimer structures exposed at 54 ␮W. The black line in the cross-section figures indicates the bowtie height prior to exposure [156].

2.1.3. Near-field imaging of multipole plasmon resonances using photopolymerization Two-photon-induced photopolymerization was used as a method of elucidating a near-field intensity profile in addition to visualizing the hot site of nanogap structures in several studies [163–165]. Murazawa et al. succeeded in observing a near-field intensity profile of the multipole plasmon resonances induced at a gold nanorod structure using photopolymerization [166]. Studies by Imura and Okamoto achieved the visualization of a

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multi-wave function induced at the gold nanorod via near-field optical microscopy [167,168]. In the study, a gold nanorod was fabricated by electron beam lithography and lift-off techniques on a glass substrate. The structural size of the gold nanorod was 55 nm × 385 nm × 32 nm, and the plasmon resonance spectrum in the longitudinal plasmon mode was approximately 2000 nm. Therefore, a near-field intensity profile at a wavelength of 800 nm, corresponding to an excitation laser wavelength in the experiment that was predicted by FDTD simulation, reflected multipole plasmon resonance modes, not a dipole plasmon resonance mode [168]. In this study, SU-8 was also used for visualizing neaf-field intensity distribution. Actually, photopolymerization proceeded not only at both ends of a gold nanorod but also inside the rod’s surface along the direction of polarization, which corresponded to the near-field intensity profile predicted by the FDTD simulation [166]. On the other hand, Geldhauser et al. successfully performed a quantitative analysis of the electromagnetic field enhancement factor at the hot site [169]. The mechanism for elucidating the enhancement factor using two-photon-induced photopolymerization of SU-8 is as follows. Because the intensity profile of the incident laser beam exhibited a Gaussian distribution, a spatial distribution of the incident light was induced. The incident light intensity differs in each gold nanostructure because the diameter of the laser beam is larger than each gold nanostructure. Using this gradient of incident light intensity, the threshold of the photopolymerization reaction of SU-8 was estimated at a hot site. The threshold of incident light intensity was compared with that without the gold nanostructure to allow for calculation of the enhancement factor for the photopolymerization of SU-8. The enhancement factor was estimated on the order of several thousand and was in good agreement with that predicted by the FDTD simulation [169]. Therefore, the two-photon-induced photopolymerization was confirmed as a powerful tool for not only observing near-field intensity distribution but also quantitatively analyzing the electromagnetic field enhancement factor. 2.2. Surface plasmon-assisted nanolithography 2.2.1. Plasmonic nanolithography Photolithography played an important role in the development of electronic devices, micro-electro-mechanical systems (MEMS) and photonic devices. To improve the photolithography resolution, reducing the wavelengths of the exposure source and adding a non-linear photochemical reaction of a photoresist via a multiphoton absorption have been proposed and developed. In addition, a near-field lithography technique that can transfer nanopatterns smaller than the diffraction limit of light to a photoresist film has been actively studied to minimize the light itself. Surface plasmonassisted nanolithography has been proposed based on near-field lithography [170–172]. The interaction of light and surface plasmons is expressed by the surface plasmon dispersion relationship (Eq. (1)). According to the surface plasmon dispersion curve as shown in Fig. 2, the wavelength of the excited surface plasmons is shorter than that of the exposure light at the same frequency; therefore, patterning smaller than the diffraction limit is expected in the photolithography [173]. Srituravanich et al. investigated the experimental possibility of plasmonic nanolithography [173]. In this study, a quartz substrate with aluminum two-dimensional hole arrays was used as a photomask to resolve the sub-wavelength features in which the hole array, first proposed by Ebbesen et al. [174], exhibits a high transmission of light at an adequate wavelength. In this photomask, the hole array, with a diameter of 40 nm and periodicity of 170 nm, was fabricated on an 80-nm-thick aluminum film using a focused ion beam (FIB) so that light at a wavelength of 370 nm can be adequately transmitted through the photomask hole

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Fig. 6. (a) A schematic drawing of a setup for surface plasmon-assisted photolithography using a hole array mask. PMMA was used as the spacer layer to match the dielectric constant of the quartz substrate. The AFM image of a pattern after exposure and development (b) and its 3D topography image (c) [173].

array. To demonstrate plasmon-assisted nanolithography, the SU-8 was spin-coated onto the photomask substrate with 50-nm-thick PMMA film as a spacer between the photomask and photoresist as shown in Fig. 6(a) [173]. The substrates were subsequently exposed to a radiation at 365 nm (72 mJ cm−2 ). The AFM images, after exposure and development as shown in Fig. 6(b) and (c), indicate that a dot pattern with 90-nm features and a periodicity of 170 nm could be successfully fabricated by irradiation with near-ultraviolet light at a wavelength of 365 nm [173]. Shao and Chen actually demonstrated surface plasmon-assisted nanolithography with a subwavelength resolution for patterning without any additional equipment or added complexity to mask design. In this study, a polarized laser beam of 355 nm wavelength was used as light source to photoinitiate an 80 nm thick SU-8 photoresist on a silicon substrate coated with titanium of 80 nm thick. Array of gold line apertures of 100 nm in width deposited on a quartz substrate was used as a photomask. The pattern after exposure and development indicated a strong dependence of pattern transfer on the polarization of light as well as the energy dosage of the light [175]. Srituravanich et al. also demonstrated surface plasmon-assisted nanolithography with half-pitch resolution down to 60 nm according to using a two-dimensional hole array silver mask [4]. 2.2.2. Surface plasmon interference lithography Surface plasmon-assisted nanolithography can effectively transfer a nanopattern that is smaller than the diffraction limit of light to a photoresist substrate. Although nanopatterning has been realized using a nanohole array photomask, a unique surface plasmon-assisted nanolithography technique was almost simultaneously proposed using the wave characteristics of a plasmon polariton, and the principle was verified experimentally [135,176]. Luo and Ishihara succeeded in confirming surface plasmon interference lithography on a photoresist film [177]. In this study, an Ag nanostructured line and space pattern (line width = 240 nm, space width = 60 nm, thickness = 60 nm) on a quarts glass was used as a photomask. The Ag nanostructured photomask was placed in contact with the g-line positive photoresist coated silicon substrate

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Fig. 8. (a) A schematic illustration of the exposure conditions for nanogap-assisted plasmonic nanolithography. (b) SEM image of periodically arranged pits on the surface of the developed positive photoresist [179].

Fig. 7. (a) Near-field intensity profile of a cross-section of the exposure system. (b) SEM image of a photoresist surface after exposure and development by surface plasmon interference lithography [177].

and was exposed by a mercury lamp. The cross-sectional near-field intensity profile of these substrates predicted by an FDTD simulation is shown in Fig. 7(a). The interference of the surface plasmon polariton can result in a strongly enhanced nanoscale spatial distribution of an electric field near the metal surface. Therefore, the interference pattern of the plasmon polariton is expected to be transferred to the photoresist film. Three lines in a 300-nm period were formed almost homogeneously in a strict reflection of the FDTD simulation results, as indicated in the SEM image of the photoresist surface after exposure and development (Fig. 7(b)). Thus, surface plasmon interference lithography was successfully demonstrated [177]. In contrast, an additional application of three-dimensional lithography was studied using the interference of surface plasmon polariton. Shao and Chen demonstrated the direct patterning of three-dimensional periodic nanostructures by surface plasmon-assisted interference nanolithography [178]. With optical near-field interference patterns generated by surface plasmons, two- and three-dimensional periodic polymeric nanostructures were fabricated using a typical photolithography setup. The size, layout, and defects of nanostructures fabricated by threedimensional surface plasmon-assisted interference lithography were determined to be easily controlled by the photomask design. 2.2.3. Nanogap-assisted plasmonic nanolithography Nanogap metallic structures provide a promising method for transferring a nanometer-sized hot site to a photoresist surface using ultimate near-field lithography to yield the smallest nanopatterns on a photoresist by photolithography. We demonstrated that 5-nm-sized nanopatterns could be fabricated on a positive photoresist surface by nanogap-assisted surface plasmon nanolithography [179]. In this study, dimer-type gold nanostructures (each 80 nm × 80 nm × 35 nm) with a gap width of 4 nm were fabricated on a glass substrate, and the substrate was used as a photomask. The photomask was placed in close contact with a positive photoresist substrate. These substrates were subsequently exposed to a femtosecond laser beam with a center wavelength of

800 nm as shown in Fig. 8(a). The plasmon resonance band indicated at approximately 800 nm under incident polarization parallel to the dimer structure. The photoresist was not directly excited by a one-photon absorption process because the absorption wavelength of the photoresist was below 500 nm; plasmon resonance can only be excited by 800-nm wavelength laser pulses. Therefore, only a two-photon-induced photochemical reaction was expected to be promoted near the nanogap position of the photomask. The SEM image of the photoresist surface after exposure and development using the photomask as shown in Fig. 8(b) indicates that nanopits with diameters of 5 nm were successfully formed with a periodicity of 400 nm, corresponding to the position of the nanogap in the photomask. The induction of the plasmonically enhanced optical near-field light at the hot site from a spatially selective photochemical reaction at the positive photoresist was confirmed for a two-photon absorption process as described in Section 2.1.2 [156]. The nanogap-assisted plasmonic lithography technique can be considered the ultimate lithography technique for generating small nanopatterns with dimensions less than 5 nm [179]. 2.2.4. Nanolithography using the scattering component of multipole plasmon resonances Sections 2.2.2 and 2.2.3 describe plasmonic lithography using a near-field component as an exposure source. Nano-lithography using transmitted light through a nanohole array mask utilized propagation light as an exposure source is described Section 2.2.1 [173]. Therefore, the photoresist substrate and the photomask need not be in complete contact. This principle can be used not only with a nano-hole array mask but also with a photomask containing metallic nanostructures that exhibit LSPR. We proposed a new plasmon-assisted nanolithography system using a scattering component of the multipole plasmon resonances as an exposure source by taking advantage of the characteristic that the spatial distribution of higher-order plasmon resonances agrees with their structure, while that of dipole resonances completely differs from their structure [180]. As estimated from the FDTD simulation, the multipole plasmon resonances were excited so that a wavelength shorter than the dipole resonance band of the metallic nanostructures was used as an excitation wavelength, and the scattering component propagated inside a photoresist film with a phase shift of half ␲ after the plasmon excitation. Using this principle, various nanopattern shapes can be formed on the photoresist film that reflects the structural pattern of the photomask. The SEM images of photomasks with various shapes of gold nanostructures and photoresist patterns after exposure and development are shown in Fig. 9(a) and (b), respectively. These images also confirm that the pattern was transferred onto the photoresist substrate without distorting the shape of the structure of the photomask. The standard deviations of each structural size and curvature radius of the triangular vertex are on the order of a single nanometer, and the resolution of the plasmon-assisted nanolithography is nearly comparable to that of a high-resolution electron beam lithography system. Therefore, the system provides a promising method for

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Fig. 9. SEM images of photomasks (a) and nano-patterns fabricated on a positive photoresist film (b); nanochains, line and space, and triangles [180].

fabricating nanostructures on the scale of ∼100 nm, such as plasmonic nanostructures and two-dimensional photonic crystals for plasmonic solar cells and photonic crystal laser devices [180]. 3. Photochemical reaction systems realizing the utilization of photons The primary points of LSPR are not only to localize an electromagnetic field at a domain smaller than the diffraction limit of light but also to induce a strong enhancement of the electromagnetic field to several orders that of the incident light to induce a surface plasmon-enhanced photochemical reaction. To achieve this result, photons must be effectively utilized in a photochemical reaction system as described in the introduction, and the utilization of photons is now an important topic in science and technology because of the current environmental and energy problems. Approximately 10 years ago, scientists wondered whether a new photochemical system could be built using a laser; the focus has now changed to building an efficient new photochemical reaction system that does not require a laser. Among the important features of metallic nanostructures, nonlinear optical effects, such as two-photon absorption, can be induced because the electromagnetic field is localized in the nanometer spatial domain. Therefore, various optical phenomena, which in the past required a laser light source, can be now induced by an incoherent light source, such as sunlight. The radiation from the infrared spectral region (low energy light) of sunlight, which has not been exploited in the light–energy

conversion system until now, can be used effectively. Moreover, an electric field gradient of nanometric proportions is generated because the light can be localized at the nanometric space, and the long-wavelength approximation for the light excitation no longer applies [181]. Because the optical electric field approximates the size of a molecule, transitions that are forbidden using traditional means can now be excited [181]. Thus, a plasmonic chemical reaction could effectively utilize photons to generate a novel chemical reaction system. In addition, important subjects, such as the quantitative measurement of a photochemical reaction, the reduction of the loss in a reaction yield by energy transfer, and the chemical reaction mechanism for the process, can be studied. Currently, studies on efficient photocatalysis and water splitting caused by visible light have attracted attention in plasmonic chemical reaction systems. In this section, the latest examples of these studies regarding plasmonic chemical reactions are introduced. 3.1. Photochemical reaction using a weak light source and quantitative analysis of the plasmon-enhanced chemical reactions 3.1.1. Nonlinear photochemical reactions by a weak incoherent light source Although the second law of photochemistry in which one molecule absorbs one photon and subsequently one photochemical reaction proceeds was an important research field in the days when photochemical reactions were studied using sunlight, a mercury lamp, etc.; the area of focus changed completely with the

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invention of the laser in 1960 [182]. Göppert-Mayer predicted the theoretical nonlinear optical effect of two-photon absorption in 1931 [183], but their theory could not be verified until 1961 after the development of the laser. Since then, ultrashort-pulse lasers, such as femtosecond lasers, etc., have been developed, and the nonlinear optical effect has been used in technologies such as two-photon laser scanning microscopy [184,185], second harmonic generation spectroscopy [186], and three-dimensional laser lithography [159,187]. These lasers have also been used in physical chemistry research involving ultrafast measurements [188–190]. The probability that a molecule will be excited via simultaneous two-photon absorption is determined by the cross-sectional area in which the two-photon absorption takes place in a particular molecule. However, this probability is generally small, and twophoton absorption is usually observed with a light source that generates a pulse containing many photons per unit time, such as a femtosecond laser, etc. We first demonstrated that simultaneous two-photon absorption was possible without laser excitation using the specific interactions peculiar to metallic nanoparticles; if light with an intensity on the order of ∼W cm−2 is irradiated onto a metallic nanostructure, the electromagnetic field intensity will theoretically be enhanced at a nanogap by ∼MW cm−2 and will serve as a new nonlinear photochemical reaction field [191]. The cationic photopolymerization reaction of SU-8 using a photo-acid generator as the photoinitiator described in Section 2.1.2 provided proof of a non-linear photochemical reaction using an incoherent light source. In the reaction scheme of SU-8, the crosslinking reaction proceeds as the plasmonically enhanced optical near-field at the nanogap promotes the two-photon absorption of a photoinitiator, and an acid molecule is generated [157–159]. In the experiment, the SU-8 was spin coated onto the nanogap gold nanostructured substrate (gap width = 5.5 nm). Then, a halogen light with a wavelength range from 600 nm to 900 nm was irradiated onto the substrate for 3 h. After exposure and development, the photopolymerization proceeded only at the nanogap positions along the polarization direction as shown in Fig. 10(a). Furthermore, all nanogaps were filled by the polymerized SU-8 molecules as shown in Fig. 10(b). This experiment, which used a surface plasmon-enhanced photochemical reaction field, supplied the first proof that a photochemical reaction can proceed via a simultaneous two-photon absorption without laser excitation, confirming the simultaneous two-photon absorption by an incoherent light source that was predicted in 1931 [183,191]. On the other hand, Yokoyama et al. also reported non-linear photochemical reactions in which SU-8 was promoted onto a gold film with a rough surface that had been previously coated on a micrometer-sized silica particle using a weak incoherent light source (Xe Lamp, 500 nm ∼ near IR light) [192]. Because this photopolymerization proceeded not at a smooth surface but at the rough surface of a metallic film, the nonlinear photopolymerization of SU-8 by a weak light source was promoted by the plasmonically enhanced optical near-field [192]. Haruta et al. also reported the dimerization of amino acids, such as tryptophan or tyrosine, by a weak incoherent light source at a wavelength of 500 nm in the plasmon-enhanced photochemical reaction field [193]. 3.1.2. Quantitative analysis of a surface plasmon-enhanced photochemical reaction Tsuboi et al. successfully performed a quantitative analysis of the plasmon-enhanced photochromic reaction of diarylethene (DE) molecules via two-photon absorption [194]. In this experiment, two types of Au-nanoparticle-integrated substrates (type I and type II) prepared on a glass substrates were employed as photochemical reaction fields. Although the type-I substrate contains highly dispersed Au nanoparticles (20 nm) whose plasmon resonance wavelength was near 520 nm, the type II substrate has

Fig. 10. (a) SEM image of the structure after 3 h exposure to the incoherent source polarized linearly along the direction indicated by the arrow. (b) SEM image of the structure after 3 h exposure to an unpolarized source. The polymerized regions are emphasized by the dashed circles. The yellow bar represents 100 nm [191].

condensed Au nanoparticles whose shoulder band based on dipole coupling formed near a wavelength of 600–900 nm. These substrates were covered with a PMMA film (thickness = 15 nm) doped with DE (concentration = 1.3 mol L−1 ), and a two-photon-induced photochromic ring-opening reaction occurred over time under irradiation from a CW laser beam with a wavelength of 808 nm and an arbitrary intensity (0.1–4.0 W cm−2 ). The ring-opening reaction of the DE was observed by measuring the change in the absorption spectrum as shown in Figs. Fig. 1010(b) and Fig. 1111(a)(a) for the type-I and type-II substrates, respectively. Although most absorption spectra of the closed-form DE were unchanging on the type-I substrate, the absorbance value of the spectrum for the closedform DE clearly decreased as the irradiation time increased. The important point of this method is that the ring-closing reaction of the open-form DE proceeded reversibly, and the substrate could be used repeatedly if it was irradiated with an ultraviolet light after the experiment. Therefore, the dependence between irradiation and the light intensity could be examined using the same substrate as shown in Fig. 11(c). Fig. 11(d) indicates that the irradiation-light intensity dependence of the DE ring-opening reaction by plotting the absorbance value at 600 nm. The change in absorbance of the closed-form DE indicated a non-linear relationship between the irradiation and the light intensity; the reaction efficiency indicated a secondary nonlinear response, and a two-photon-induced photochromic reaction of the DE was promoted on the metallic nanostructure without the involvement of a pulsed laser, which indicates a strong enhancement of the electromagnetic field. On the other hand, Nishi et al. succeeded in constructing a unique experimental system to examine a cycloreversion reaction by preparing the gold nanoparticle surface supporting the DE polymer for quantitative measurement of the photochromic DE reaction influenced by the near-field intensity distribution [195]. They synthesized a gold nanoparticle covered with DE polymers (Aupoly(DE)) so that the DE polymer with a thiol group at the end was

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Fig. 11. Absorption spectrum change for DE obtained using a type-I substrate (a) and a type-II substrate (b). (c) Absorption spectrum change for recovery reaction. (d) Absorption change at 600 nm induced by repetitive on-and-off irradiation with 808 nm and UV light. The absorption was normalized as At /A0 in which At is the absorbance after irradiation and A0 is the initial absorbance [194].

attached to the gold nanoparticle surface by a covalent bond [196]. The experiment examined the photochromic reaction of DE that proceeded on the surface of a single gold nanoparticle by dispersing the Au-poly(DE) in a solution (water:THF = 1:1) [195]. Moreover, the quantitative analysis of the plasmon-enhanced photochemical reaction was possible because the quantity of poly(DE) on the nanoparticle surface could be somewhat precisely controlled. When irradiated with the monochromatic light at a wavelength of 650 nm, the ring-opening reaction of the DE on the gold nanoparticle was promoted, as indicated by the change in the absorption spectrum. In addition, this reaction proceeded through a onephoton absorption process. The plot of the DE absorbance value at a wavelength of 650 nm versus the irradiation time is shown in Fig. 12(a). The photochromic reaction of DE was promoted over the change in expected absorbance without gold nanoparticles due to the plasmonic effect. The broken line in Fig. 12(a) is a fitting curve by simulation for the case in which the near-field intensity distribution on a gold nanoparticle was uniform throughout the simulation, and the solid line is a fitting curve for the case in which two-components of the near-field intensity on a gold nanoparticle were considered, as shown by the model in Fig. 12(b). The experimental results were well reproduced by the simulation in which two components of the near-field intensity distribution were assumed. This research succeeded in constructing an experimental system to quantitatively study a surface plasmon-enhanced photochemical reaction while considering the influence of the near-field intensity distribution on a gold nanoparticle surface. Moreover, they demonstrated that the distance of the chromophore from the gold nanoparticle surface affects the reaction efficiency by changing the chain length of the DE polymer [195]. In contrast, Wu et al. succeeded in quantitatively measuring the two-photon-induced photochromic reaction of DE in a solution system without a macromolecule system but with a molecule capable of mass transfer via molecular diffusion [197]. They fabricated

Fig. 12. (a) Relationship between the irradiation time and the absorbance at 650 nm for the DE chromophore in Au-poly(DE). The dashed and solid lines are simulated fitting curves. The dotted line corresponds to the relationship for the Au-free poly(DE) [195]. (b) The model with two-components of the near-field intensity on a gold nanoparticle.

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Fig. 13. (a) SEM image of the dimer-type nanogap gold blocks (gap = 6 nm). (b) Extinction spectra of the dimer blocks; red line, L mode; blue line, T mode. (c) Absorption spectra of the closed-form DE molecules with different irradiation times (peak intensity = 9.0 MW cm−2 ). (d) Laser irradiation time dependence of the absorbance value for the DE molecules at a wavelength of 570 nm [197].

the dimer-type gold nanostructure shown in the SEM image in Fig. 13(a) with a nanogap (gap width = 6 nm) on a glass substrate and induced the photochromic reaction of DE. The plasmon resonance spectra under the conditions of incident polarization parallel (L mode) and perpendicular (T mode) to the dimer structure are provided in Fig. 13(b). In the L mode, the plasmon resonance wavelength was approximately 800 nm, and the two-photon-induced photochromic reaction of DE was expected to be promoted by irradiation with a femtosecond laser whose center wavelength was 800 nm. To pursue the photochromic reaction in a solution system, a photochemical reaction chamber in PDMS was arranged on the gold nanostructure substrates and was filled with a propylene carbonate solution of closed-form DE. Fig. 13(c) indicates the change in the absorption spectrum for the closed-form DE, and Fig. 13(d) indicates the irradiation time dependence of the absorbance value at a wavelength of 570 nm. Because the ring-opening reaction of DE proceeded with laser irradiation, the absorbance value decreased exponentially. Moreover, by measuring the irradiationlight-intensity dependence of the reaction, they demonstrated that the two-photon-induced ring-opening reaction of DE was promoted by a factor of approximately 100 by the nanogap gold structures. 3.1.3. Control of strong coupling between molecules and plasmon polariton via photochemical reaction Ebbesen discovered the phenomenon in which light is anomalously transmitted through a metal substrate with a subwavelength-sized hole array in 1998 and contributed to the development of “plasmonics”, which applies the propagation of a plasmon to energy transmission [174,198]. Recently, Schwartz et al. have induced a strong coupling between molecules and a microcavity or surface plasmon polariton. These studies are attempting to find new functionalities and optical properties. A large vacuum Rabi splitting energy was observed in 2011 through the strong coupling of a micro-cavity and a photochromic molecule [199]. Fig. 14(a) shows the change in the absorption spectrum at the time of irradiation with ultraviolet light onto the spiropyran-derivative neat film used in the experiment. The absorption spectrum in the visible wavelength range increased because the photochromic reaction

from spiropyran to merocyanine proceeded with UV irradiation. When these molecules were introduced into a micro-cavity, the normal-incidence transmission spectrum of the cavity was split into two bands, right and left, to form merocyanine, as shown in Fig. 14(b). The spiropyran derivative, which is a photochromic molecule, was strongly coupled with the mode of the micro-cavity by transposing to merocyanine, which absorbs in the visible wavelength range upon irradiation with UV light, and induced a Rabi splitting. Moreover, they clearly demonstrated that the existence of a strong coupling state can be reversibly switched by light irradiation because the merocyanine was able to transpose to a spiropyran derivative with visible light irradiation. They confirmed that the spectrum was split when the spiropyran derivative transformed to merocyanine due to irradiation with ultraviolet light, and the process was reversed when the material was again irradiated with visible light as shown in Fig. 14(c). Furthermore, the photochromic reaction may also proceed efficiently if a strong interaction is induced between a micro-cavity and a molecule. They succeeded in obtaining a large Rabi splitting energy of up to 700 meV in this report. In addition, they confirmed the large Rabi splitting energy of up to 650 meV in the strong coupling between the surface plasmon polariton and the spiropyran derivative. Thus, a photochemical reaction does not depend only on molecules and can be controlled by a photochemical reaction field. Therefore, this study is important to the field of plasmonic chemistry for the achievement of a controlled photochemical reaction using a plasmon polariton [199]. 3.1.4. In situ measurement of plasmon-enhanced photochemical reactions probed by Raman scattering spectroscopy Many versions of the plasmon-enhanced photochemical reaction have been reported such as the polymerization reaction, the photoisomerization reaction, the photochromic reaction, etc. The principal methods for measuring these reactions have been observed by electron microscopy, atomic force microscope, or the analysis of absorption spectrum changes. In situ Raman scattering spectroscopy is a methodology to measure a surface plasmonenhanced photochemical reaction that is currently attracting attention. Sun et al. succeeded in probing the plasmon-enhanced in situ chemical reaction of 4-nitrobenzenethiol dimerizing to

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Fig. 14. (a) Absorption spectra for neat films in the spiropyran derivative (dashed line) and merocyanine (solid line) forms. (b) Cavity normal-incidence transmission with the molecules in the spiropyran derivative (dashed line) and merocyanine (solid line) forms. The dotted lines in (a) and (b) were taken at a few intermediate states. (c) The all-optical reversible switching represented by the transmission spectrum during conversion. The timescale for the photoconversion is ∼10 min at ∼1 mW cm−2 [199].

Fig. 15. A schematic illustration of a vacuum tip-enhanced Raman spectroscopy system for probing a plasmon-enhanced photochemical reaction [200].

dimercaptoazobenzene using vacuum tip-enhanced Raman spectroscopy (TERS) as shown in Fig. 15 [200]. This chemical reaction was controlled by the incident laser intensity, tunneling current, and bias voltage although the probe of the metallic STM was used as a field to induce a near-field enhancement. Moreover, the temperature change in a chemical reaction can be simultaneously examined by the ratio of the intensity of the Stokes Raman scattering and the anti-Stokes Raman scattering. This system can also analyze a mere plasmon-enhanced chemical reaction, with control of the chemical reaction anticipated as a potential application [200]. In contrast, a method other than TERS using in-situ Raman scattering to measure a surface plasmon-enhanced photochemical reaction was proposed. Dai et al. succeeded in probing a surface plasmon-enhanced photochemical reaction from 4nitrobenzenethiol to p, p -dimercaptoazobenzene on an Ag nanostructured substrate prepared via nanosphere lithography using in-situ Raman scattering spectroscopy [201]. Based on this experimental result, the surface plasmon-enhanced photochemical reaction was shown to be greatly influenced by the duration of the laser exposure, the Ag nanoparticle size, the laser power, etc. The reaction efficiency clearly increased when the wavelength of the incident light and the plasmon resonance band overlapped. These experimental results indicated that electromagnetic field enhancement could effectively induce the transfer of the “hot” electrons that decay from the plasmon to the reactants [201]. Furthermore, Yokoyama et al. succeeded in probing the promotion of photopolymerization in a diacetylene derivative,

1,6-di(N-carbazolyl)-2,4-hexadiyne (DCHD), via the electromagnetic field enhancement effect based on LSPR excitation using in-situ Raman scattering spectroscopy [202]. Fig. 16(a) shows a change in the Raman scattering spectrum of a DCHD monomer nanocrystal in the absence of an Ag nanoparticle, and Fig. 16(b)

Fig. 16. The changes in the Raman scattering spectrum focusing on the acetylene bond as a function of irradiation time: (a) DCHD monomer nanocrystals; (b) silver nanoparticles and DCHD monomer nano-composites [202].

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indicates a Raman scattering spectral change in the DCHD monomer and Ag nanoparticle composite. Although the peak for acetylene binding gradually shifted from 2110 cm−1 to 2083 cm−1 in each figure, the peak shift was accelerated in the presence of Ag nanoparticles. In addition, the peak decreased and disappeared as the irradiation intensity increased, which was attributed to photodegradation by the plasmonically enhanced optical near-field. From the Raman scattering measurements, the photopolymerization reaction was estimated to be accelerated by a factor of 20–40 in the presence of the Ag nanoparticles. Under these irradiation conditions, the polymerization reaction of a diacetylene derivative was thought to be promoted by an electromagnetic field enhancement effect via a two-photon or multi-photon absorption process [202]. 3.1.5. Structural change in a silver nanoparticle using a plasmon-enhanced photochemical reaction The studies in which a photochemical reaction of a molecule in the vicinity of metallic nanostructures was promoted by a strong enhancement of the electromagnetic field have been described. In this section, a phenomenon is introduced in which reactions of the metallic nanostructure itself are promoted by the plasmonically enhanced optical near-field [90,203]. First, the spatially selective chemical reaction of metallic nanoparticles proceeds in solution under the influence of a plasmonic effect, and a variety of phenomena have been reported in which the nanoparticles change shape. Tasuma and his co-workers fabricated Ag nanoparticles loaded onto a titanium dioxide (TiO2 ) substrate and demonstrated photochromism via a morphological change in the Ag nanoparticles with a plasmon-induced charge separation based on electron transfer reaction from the Ag nanoparticles to the TiO2 substrate. Not only was a photochromic reaction possible but its application to an actuator was also found to be possible using this reaction [204–206]. Recently, Tanabe and Tatsuma considered the detailed reaction mechanism of the shape and color change of single Ag nanoparticle accompanied by analysis using a spectrum and FDTD simulation. TiO2 has polymorphic forms such as anatase and rutile. Anatase TiO2 films (150 nm thick) were prepared via spray pyrolysis on Pyrex or ITO (indium tin oxide)-coated glass plates [207]. Commercially available spherical Ag nanoparticles (diameter = 100 nm) were dispersed onto the TiO2 film. The scattering spectrum of the Ag nanoparticles shows two bands near wavelengths of 460 nm and 620 nm as shown in Fig. 17(a). Moreover, the scattered light shown in orange originating from the longer wavelength band was observed in the inset figure. Two bands were similarly observed in the visible wavelength region of the spectrum in the FDTD simulation as shown in Fig. 17(b). Fig. 17(c) indicates the near-field intensity distribution on the Ag-nanoparticle-loaded TiO2 substrate at a wavelength of 405 nm, and Fig. 17(d) indicates the near-field intensity distribution at a wavelength of 405 nm. These intensity profiles were provided as a cross-sectional view. The near-field intensity distribution at each wavelength attributed these two peaks to the full-surface mode and interface mode as shown in Fig. 17(c). The selective excitation of each mode resulted in a corresponding morphological change and selective suppression of the plasmon mode, and multicolor changes occurred in the scattered light [207]. 3.2. Plasmon-enhanced photocatalytic reaction and water splitting 3.2.1. Promotion of a titanium dioxide photocatalysis using the plasmonic effect Current research is focused on a plasmon-enhanced photocatalytic reaction related to a plasmon-enhanced photochemical reaction. Since the discovery of the Honda–Fujishima effect in 1972 [46], titanium dioxide has attracted attention as a material with

Fig. 17. (a) Scattering spectrum of an Ag nanoparticle. The inset shows an image of the scattered light and a SEM image of the Ag nanoparticle on TiO2 . (b) Scattering spectrum of an Ag nanoparticle simulated using the FDTD method. Near-field intensity distribution of an Ag nanoparticle; (c) full-surface mode and (d) interface mode [207].

potential uses in solar energy conversion and as a photocatalyst. Various methods have been proposed to enhance the photocatalytic ability of TiO2 , such as improving the quantum efficiency by surface treatment and control of its crystal structure [208,209], improving the efficiency by controlling the band gap with doping impurities, such as nitrogen [210], and controlling the recombination of an electron–hole pair according to trapping an electron onto metal by platinum coating [211,212]. Hirakawa and Kamat coated the circumference of an Ag nanoparticle with TiO2 and confirmed the spectral shift of the plasmon resonance band to a shorter wavelength based on electron trapping on the Ag nanoparticle induced by the charge separation based on an electron transfer reaction [213,214]. They have also confirmed that the interfacial charge transfer between TiO2 and the metal could be enhanced by a negative shift in the Fermi level of the metal-TiO2 composite resulting from the accumulation of electrons due to the surface plasmon resonance [213,214]. In 2008, Awazu et al. demonstrated that the photolysis of methylene blue (organic dye) was promoted by the plasmonic effect when visible light was irradiated onto Ag nanoparticles placed inside a TiO2 layer whose plasmon resonance band was near the band edge of TiO2 [215]. Fig. 18(a) provides cross-sectional view of the TEM image showing Ag nanoparticles covered with an SiO2 layer embedded in TiO2 (Ag/SiO2 core–shell structure), and Fig. 18(b) provides the absorption spectra of a TiO2 thin film, Ag nanoparticles embedded in the TiO2 , and the Ag/SiO2 core–shell structure. A clear plasmon resonance band was observed near 410 nm in only

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Fig. 18. (a) TEM image of a cross-sectional view of a Ag/SiO2 core–shell structure, (b) absorption spectra of a TiO2 thin film (black line), Ag nanoparticles embedded in TiO2 (red line), and an Ag/SiO2 core–shell structure (blue line). (c) Decomposition rate of methylene blue under near-UV irradiation; TiO2 film on a SiO2 substrate (black plot); Ag/SiO2 core–shell structure (SiO2 = 20 nm (red plot) and 5 nm (blue plot)) [215].

the Ag/SiO2 core–shell structure. Only the TiO2 film also exhibited a broad band near 530 nm, which was attributed to thin film interference. They reported that Ag can be easily oxidized to silver oxide because the Ag may contact the TiO2 without the SiO2 layer, which causes the plasmon band to disappear. Most importantly, the rate of photocatalytic reaction was accelerated in the presence of an Ag/SiO2 core–shell structure as shown in Fig. 18(c). The red and blue lines in the figure indicate the experimental results when the thickness of the SiO2 layer was 20 nm or 5 nm, respectively. From this analysis, the rate of reaction using Ag/SiO2 core–shell structure (SiO2 = 20 nm) was found to be five times that with the TiO2 film without Ag nanoparticles, while the rate of reaction using the Ag/SiO2 core–shell structure (SiO2 = 5 nm) was increased eight fold. Thus, the photocatalytic reaction was promoted by the plasmonically enhanced optical near-field. In contrast, other reports discuss the plasmon-enhanced photocatalytic reaction using gold nanoparticles [215]. 3.2.2. Photocatalytic reaction via plasmon-induced charge separation using metallic nanoparticles loaded onto TiO2 In the gold nanoparticle-TiO2 composite system, a charge transfer is induced from the TiO2 to the gold when it is irradiated with ultraviolet light, and a charge separation occurs from between the gold and the TiO2 when it is irradiated with visible light. Tian et al. demonstrated a plasmonic solar cell that responds to visible light with a plasmon-induced charge separation based on a charge transfer from the gold nanoparticles to the TiO2 and subsequent electron donor oxidation by the generated hole [61,216]. Furube et al. demonstrated that the electron transfer based on the plasmon-induced charge separation occurred within 240 fs as measured using ultrafast transient absorption spectroscopy [217]. Furthermore, Wang et al. reported that the complete separation and recombination process of the plasmon-induced electrons in the Au–TiO2 system occurred on the order of a millisecond [218]. Kowalska et al. developed photocatalyst that responds to visible light utilizing plasmon-induced charge separation [62]. In this study, rod-like gold nanoparticles loaded onto TiO2 whose plasmon resonances are approximately 520–530 nm and 600 nm were employed, and the photocatalytic activity was analyzed to study the photooxidation reactions of acetic acid and 2-propanol. In

this experiment, the evolved carbon dioxide and acetone were measured and analyzed quantitatively using chromatography. This study provided advances in measuring an action spectrum of a photocatalytic reaction. The action spectrum of the 2-propanol oxidation as shown in Fig. 19 almost reproduced the diffuse reflectance spectrum of the gold nanoparticles loaded onto TiO2 in the presence of gold nanoparticles, while without gold nanoparticles, the reaction efficiency was independent of the wavelength. Moreover, they considered that the optical absorption efficiency obtained at the TiO2 band edge was not simply due to an electromagnetic field enhancement because the maximum wavelength of the action spectrum was close to 600 nm. Thus, the photocatalyst activity in the visible wavelength range was demonstrated to be promoted by a plasmon-induced charge separation. In addition, the plasmoninduced charge separation based on the injection of an electron into the semiconductor electrode has been considered using various mechanisms: a charge separation due to an interband or intraband transition of gold could be promoted by a plasmonically enhanced optical near field and an electron transferred into the conduction band of TiO2 , or a hot electron generated by the near-field enhancement could be injected into the conduction band of TiO2 , etc. [62].

Fig. 19. Action spectrum of 2-propanol oxidation on gold nanoparticles loaded onto TiO2 : in the presence of gold nanoparticles (), in the absence of gold nanoparticles () and the diffuse reflectance spectrum of Au/TiO2 [62].

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Fig. 20. A schematic diagram of the charge separation in an Ag/AgCl/TiO2 system with visible light radiation [219].

On the other hand, a plasmon-induced charge separation was applied to various photocatalytic reaction systems. Yu et al. constructed an efficient photocatalytic system by combining silver chloride (AgCl) with Ag nanoparticles loaded onto TiO2 [219]. As shown in the reaction scheme in Fig. 20, the TiO2 nanoparticle was made to support the Ag nanoparticles and the AgCl particles, and visible light was irradiated onto these particles. The excited electron was transferred to the conduction band of the TiO2 , and a hole was subsequently recovered when the chloride ion and chloride radical were formed. The electron injected into the conduction band of the TiO2 was trapped by the oxygen molecule, and superoxide ions were formed. As a result, they concluded that photooxidation reaction was promoted by the superoxide ions, and the organic molecules near the particles were decomposed by the highly active chloride radical. They succeeded in studying the plasmon-enhanced photocatalytic reaction of a hydroxy radical and terephthalic acid using fluorescence spectroscopy [219]. Kochuveedu et al. determined the optimum size and density of gold nanoparticles on the surface of TiO2 to provide maximum photocatalytic efficiency [220]. As a result, gold nanoparticle arrays with a 15-nm diameter and a density of 700 ␮m−2 were found to have the maximum photocatalytic efficiency due to a synergistic effect of the tight contact between the gold nanoparticles and the TiO2 and the maximized degree of LSPR excitation. Kimura et al. recently compared the photocatalytic activity between the gold nanoparticle/anatase and the gold nanoparticle/rutile and suggested that despite the high UV-light activity of the gold nanoparticle/anatase for the reduction of nitrobenzene, Au/rutile indicated a higher visible-light activity for the oxidation of alcohols [221]. 3.2.3. Promotion of photocatalytic reactions by plasmonic effect Various reaction systems have been examined to improve the efficiency of photocatalytic reactions using the plasmonic effect without promoting either the charge separation of TiO2 or the charge transfer to the conduction band of TiO2 . Wang et al. succeeded in fabricating a plasmonic photocatalyst with chemical stability that responds to visible light using Ag and AgCl nanoparticles [222]. Fig. 21(a) shows an SEM image of the AgCl particles with the Ag nanoparticles; the Ag nanoparticles were found to be supported on the surface of AgCl particles. Fig. 21(b) indicates the irradiation time dependence of the photodecomposition of methyl orange via irradiation with visible light with wavelengths longer than 400 nm. The photocatalytic reaction was accelerated eight fold over that with N-doped TiO2 , a photocatalyst known to respond to visible light. They discussed the reaction scheme as follows: The plasmonically-enhanced optical near-field promoted the charge separation at the AgCl particle, and the hole was recovered by the chloride ion so that a highly reactive chloride radical was formed

Fig. 21. (a) SEM image of AgCl particles with Ag nanoparticles. (b) Photodecomposition of methyl orange in solution (20 mg L−1 ) using the AgCl particles with Ag nanoparticles () and N-doped TiO2 () under visible light irradiation ( ≥ 400 nm). C is the concentration of methyl orange at time t, and C0 is the methyl orange concentration [222].

to induce oxidation of the methyl orange near the particles. Furthermore, they considered that the excited electron was trapped and transferred to the superoxide ions (O2 ) and other reactive oxygen species, and the species promoted the oxidation of the methyl orange [222]. On the other hand, Torimoto et al. constructed a plasmonenhanced photocatalytic system using the charge separation of semiconductor nanoparticles not including TiO2 [223]. The photocatalyst was a CdS quantum dot covered with SiO2 that was adsorbed onto the surface of gold nanoparticles covered with SiO2 . The photocatalyst activity increased, as shown by the plasmonically enhanced optical near-field from the action spectrum of hydrogen evolution. They successfully confirmed that the photocatalytic activity could be optimized experimentally and theoretically by controlling the distance between the CdS and gold nanoparticles according to controlling the thickness of the SiO2 layer [223]. In contrast, Plasmon-enhanced photocatalytic reactions were utilized to induce the reduction of graphene oxide. Wu et al. succeeded in inducing the photoreduction of graphene oxide with visible light radiation when Ag nanoparticles were placed onto the graphene oxide [224]. Mori et al. succeeded in synthesizing nanometer-sized photocatalysts composed of core–shell Ag/SiO2 nanoparticles with an anchored [Ru(bpy)3 ]2+ dye and confirmed that the photocatalytic activity estimated by analyzing the oxidation of oxygen gas was related to the luminescence intensity of the [Ru(bpy)3 ]2+ that corresponded to the near-field intensity [225]. 3.2.4. Plasmonic effect in water splitting Since 2011, many studies have been reported on water splitting by the plasmonic effect. Liu et al. constructed a plasmon-enhanced photocurrent generation system and succeeded in measuring hydrogen evolution at the counter electrode of the system [68]. In this study, gold nano-islands were prepared on an N- and F-doped TiO2 photoelectrode, and the photocurrent generation efficiency and amount of evolved hydrogen were measured using

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Fig. 22. I-t curves obtained using TiO2 photoelectrodes with and without Au nanoparticles irradiated with a monochromatic light at 633 nm for 22 s [68].

photoelectrochemistry and mass spectrometry, respectively. They concluded that the activity of a photocatalyst that responds to visible light was promoted by the plasmonically enhanced optical near-field because N- and F-doped TiO2 is known to absorb visible light. In this experiment, a three-electrode conventional photoelectrochemical measurement system was employed. In addition, gold nano-islands loaded onto a TiO2 substrate were used as the working electrode. Fig. 22 shows the I–t curve at 633 nm with and without the gold nanoparticles, and the photocurrent value was found to be enhanced 66 times over that of the electrode without gold nanoparticles. The action spectrum indicated that the photocurrent value increased at the plasmon resonant wavelength, and LSPR could have contributed to the evolution of hydrogen and oxygen [68]. On the other hand, Ingram et al. also succeeded in splitting water using the plasmonic effect and in simultaneously measuring the oxygen evolved at the working electrode and the hydrogen evolved at the counter electrode [69]. They fabricated N-doped TiO2 that responds to visible light and deposited gold or Ag nanoparticles onto the photoelectrode using a three-electrode photoelectrochemical measurement system. The reference electrode was Hg/HgO with an applied potential of +0.3 V vs. Hg/HgO to promote hydrogen evolution. The electron transfer reaction from the metal to TiO2 was prevented by coating the surface of the metallic nanoparticles with nonconducting organic stabilizer molecules [poly(vinylpyrrolidone) (PVP)]. They aimed to enhance the activity of a photocatalyst that responds to visible light in a study similar to one mentioned previously. The plasmon resonance band of the Ag nanoparticles on the TiO2 existed near 400 nm, and overlapped the absorption of N-doped TiO2 . In contrast, the plasmon resonance band of gold nanoparticles was observed near 600 nm and was not in agreement with the absorption of the N-doped TiO2 . Therefore, the quantity of evolved oxygen and hydrogen increased linearly with the irradiation time in the case of Ag nanoparticles, while little increase was noted in the case of gold nanoparticles as shown in Fig. 23(a). The irradiation time dependence of the photocurrent value that responds to turning the light on and off is shown in Fig. 23(b). The photocurrent value obtained by the Agnanoparticle-loaded N-doped TiO2 photoelectrode was also larger, and therefore, the photocurrent value was related to the evolution efficiency of the oxygen and hydrogen gases. The evolution of oxygen and hydrogen proceeded almost stoichiometrically. Therefore, the Honda–Fujishima effect was induced even with the irradiation of visible light using the near-field enhancement based on LSP excitation [69]. In contrast, a variety of studies on water oxidation or water splitting based on plasmon-induced charge separation after electron transfer reaction from metal to semiconductor have recently been

Fig. 23. (a) H2 () and O2 (䊉) evolution with visible light radiation on N-TiO2 (black symbols) and Ag/N-TiO2 (blue symbols) photocatalysts as measured by mass spectrometry. (b) I–V curve responding to the light on and off upon irradiation with a broadband visible light source (400–900 nm) [69].

reported. Primo et al. succeeded in measuring the oxygen evolution based on water oxidation with visible light irradiation using a gold-nanoparticle-loaded cerium oxide semiconductor [226]. They confirmed the reaction scheme as diagramed in Fig. 24. The plasmon-induced charge separation was induced by electron transfer reaction from gold into the conduction band of cerium dioxide. The transferred electron was quenched by Ag+ , and a hole was formed at the gold nanoparticle to induce the oxidation of water [226]. Furthermore, Thomann et al. realized photocurrent generation at a plasmon resonant wavelength using only an aqueous electrolyte solution by employing gold-nanoparticle-loaded iron oxide to obtain water splitting [227]. Nishijima et al. demonstrated photocurrent generation with a near-infrared wavelength using only an aqueous electrolyte solution and a gold-nanostructured TiO2 photoelectrode in 2010 [63]. Remarkably, an efficient photocurrent generation was realized with irradiation of near-infrared light from 800 nm to 1200 nm. They succeeded in achieving the quantitative analysis of stoichiometric oxygen evolution based on the water oxidation of a water isotope in an aqueous electrolyte solution using a gas chromatography/mass spectrometry system [228]. Thus, the overpotential can be ignored in the system, although water oxidation through a four-electron transition generally requires a large overpotential. They considered the mechanism by which the generated multiple holes simultaneously induced oxidation of water molecules because the holes were generated at a spatially selective hot site due to the LSP excitation and trapped on the surface of the TiO2 at the gold/TiO2 /water interface. Therefore, the oxidation of water and the electron transfer from gold to TiO2 proceeded in the nanometer-sized local space indicating a strong enhancement of the electromagnetic field [228].

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Fig. 24. Reaction schemes for the elementary steps occurring in the photocatalytic oxygen evolution upon irradiation of Au/CeO2 with visible light; (i) light absorption; (ii) electron injection from Au to ceria conduction band; (iii) electron quenching by Ag+ ; (iv) water oxidation by h+ [226].

4. Summary and outlook

Acknowledgments

In this review article, current advanced studies of the surface plasmon-enhanced photochemical reactions were described. Although the concept of surface plasmon-enhanced photochemical reaction has been understood since the discovery of SERS, the publication of related studies increased after the development of nanoscience and nanotechnology in the mid-2000s. Studies related to the surface plasmon-enhanced photochemical reaction have focused on photocatalysis and photocatalytic water splitting initiated by nanolithography. All are naturally important areas of investigation for photochemical research. Another study on “surface plasmon-enhanced photochemical reactions” was undertaken in Japan from 2007 to 2011. Approximately 67 researchers took part in the project, which was funded by the Japanese government (the Ministry of Education, Culture, Sports, Science and Technology, KAKENHI Project; Grant-in-Aid for Scientific Research on Priority Area—focused on the strong photon–molecule coupling fields for photochemical reactions). Misawa, one of the authors of this review, was a representative (leader) of the project. This study noted that spatially selective photochemical reactions can be induced and photons can be efficiently utilized to contribute to the development of green nanotechnologies. Therefore, twophoton-induced photochemical reactions using a weak incoherent light source represent an “effective utilization of photons.” The electromagnetic field gradient is almost comparable to the molecular scale because light can be localized on a nanometer-sized region. Therefore, the long-wavelength approximation is not considered, and a local excitation in the molecule becomes possible. Thus, the forbidden transition of molecules is expected to allow for the development of exciting novel photochemical reaction systems. In contrast, the plasmonically enhanced optical nearfield now plays a critical role in not only the excitation process but also in photochemical reaction processes, such as the photoinduced electron transfer reaction. Although water oxidation was realized by irradiation with visible and near-infrared light based on the plasmon-induced charge separation, the detailed mechanism is not completely understood. Although water oxidation by visible and near-infrared light is difficult because the reaction requires a large overpotential, water oxidation was found to proceed almost without an overpotential with near-infrared irradiation. A detailed mechanism must be elucidated to determine how the electron transfer reaction occurred or how a hole was recovered. In the near future, confirmation of the reduction of carbon dioxide via photocatalytic reaction by near-infrared light is expected. The electromagnetic field enhancement effects based on LSP excitation contributed not only to physical processes, such as the excitation efficiency, but also to chemical processes, such as the photo-induced electron transfer reaction, as described in this review article. Thus, the new research field of “plasmonic chemistry” is currently growing and is expected to continue enhancing the study of photochemistry.

The authors acknowledge a PRESTO-JST program and Grantin-Aid for Scientific Research in the Priority Area “Strong Photon-Molecule Coupling Fields” (No. 470 (No. 19049001)), Grantin-Aid for Scientific Research ((Nos. 23225006 and 23686026) and Nanotechnology Platform (Hokkaido University).

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