Reversible control of localized surface plasmon resonance (LSPR) properties in the NIR region was achieved through surface oxidation and reduction reactions of Au nanoporous structures with different pore sizes, fabricated via dealloying of Au-Ag alloy nanoparticles in the presence of L-cysteine. The structures exhibited LSPR in the NIR region, and their optical properties were modulated through the surface redox reactions. Because the LSPR properties of the nanoporous structures originated from ligament-ligament interactions, structures with smaller pores showed larger spectral changes, which was consistent with theoretical calculations. Although further structural changes occurred during the first few cycles, the spectral changes driven by oxidation-reduction cycles became stable and repeatable for at least ten cycles. The nanoporous structures with fine pores therefore show promise as electrochromic and on-demand plasmonic platforms.

Photoelectrodes based on plasmon-induced charge separation (PICS) often consist of Au nanoparticles (NPs) supported on an n-type semiconductor electrode and work as photoanodes. Meanwhile, there are fewer studies on plasmonic photocathodes fabricated by coating a plasmonic NP-modified electrode with the semiconductor. In the present study, plasmonic photocathodes in which Ag NPs electrodeposited on an indium tin oxide (ITO) electrode are covered with TiO2 (ITO/Ag/TiO2) were fabricated via a spray pyrolysis method. The photocathode exhibited efficient absorption and photocurrent responses derived from quadrupolar localized surface plasmon resonance (LSPR) mode. Since there are few studies on the ITO/Ag/TiO2 photocathode and the quadrupole mode shows a smaller scattering/extinction ratio and longer lifetime than the dipolar mode, the insights obtained in the present study would lead to the design of highly efficient plasmonic photocathodes.

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

Periodic nanostructures, including gratings, which have been widely used in the fields of photonics and plasmonics, are generally fabricated through electron beam lithography, photolithography, or laser-induced periodic surface structure (LIPSS) formation. Although photomasks or photoresists are not necessary for the preparation of LIPSSs, a laser is required as a coherent light source. Here, we propose a photoelectrochemical bottom-up approach to obtain semitransparent periodic nanostructures using an incoherent continuous light source. Lead oxide nanoband arrays are formed simply by irradiating linearly polarized light to a potential-controlled indium-tin oxide electrode in the presence of Pb(2+ )ions. The polarized light causes anisotropic growth of lead oxide parallel to the polarization direction, and interference between incident light and light scattered from the nanostructures gives rise to the periodicity. The pitch of the subwavelength scale can be controlled by the irradiation light wavelength.

Chiral plasmonic nanostructures are of significant interest because of their strong chirality compared to typical chiral molecules and their potential for various applications such as enantioselective sensors and metamaterials. Although chemical or photochemical fabrication methods for chiral nanostructures have attracted attention because of their cost-effectiveness and large-area applicability, most of the chemically synthesized chiral nanostructures are two-dimensional ensembles or arrays of individual chiral nanoparticles. In the present study, more three-dimensional, densely interconnected chiral plasmonic nanoporous structures are fabricated via plasmon-induced dealloying of Au-Ag alloy under circularly polarized light (CPL). CPL is used as a sole chiral source and irradiated to a chemically treated Au-Ag alloy film showing absorption due to localized surface plasmon resonance (LSPR). The resulting nanoporous structures exhibit chiroptical responses depending on the handedness CPL illuminated. The mechanism of chirality introduction is discussed on the basis of an electromagnetic simulation.

Metal and compound nanoparticles or nanostructures have unique properties that are different from bulk materials. For instance, metal nanoparticles exhibit light absorption and scattering due to localized surface plasmon resonance (LSPR) while compound nanoparticles such as CdS emit photoluminescence because of quantum size effects. The optical and photoelectrochemical properties of nanomaterials are significantly dependent on their size, shape, orientation, periodicity, and combination with other materials. Therefore, nanofabrication methods to control these factors are of critical importance for photovoltaic, photocatalytic, and other applications. We have successfully demonstrated photoelectrochemical techniques to fabricate different nanostructures composed of metal and/or metal oxide. Au-PbO2[1,2] and Au-Ag[3] hetero-nanostructures were fabricated via plasmon-induced charge separation (PICS)[4,5] in the presence of Pb2+ and Ag+ ions, respectively. Site-selective oxidative or reductive deposition took place at the resonance sites under excitation of LSPR of Au nanoparticles, that are in contact with a semiconductor such as TiO2 or a potential-controlled electrode. The reaction site can be controlled only by changing the irradiation wavelength. PbO2 deposited on the Au nanoparticles was galvanically replaced with MnO2 as a cocatalyst of photocatalytic reaction based on PICS[6]. Recently, we fabricated PbO2 periodic nanostructures by irradiating incoherent linearly polarized visible light to a potential-controlled bare indium-tin oxide electrode in the presence of Pb2+ ions[7] (see the image below). These photoelectrochemical approaches allow us to fabricate a variety of nanostructures with sizes beyond or comparable to the diffraction limit of light. References [1] H. Nishi, M. Sakamoto, T. Tatsuma, Chem. Commun. 2018, 54, 11741. [2] H. Nishi, T. Tatsuma, Nanoscale 2019, 11, 19455. [3] H. Nishi, Y. Zuo, Y. Kuroiwa, T. Tatsuma, in preparation. [4] Y. Tian, T. Tatsuma, J. Am. Chem. Soc. 2005, 127, 7632. [5] T. Tatsuma, H. Nishi, T. Ishida, Chem. Sci. 2017, 8, 3325. [6] K. Kim, H. Nishi, T. Tatsuma, J. Chem. Phys. 2022, 157, 111101. [7] H. Nishi, H. Tojo, A. Kawai, T. Tatsuma, ACS Appl. Nano Mater. 2024, 7, 5426. Figure 1

Plasmonic heteronanostructures are promising building blocks for photofunctional materials and devices including photocatalysts, optical materials, and optoelectronic devices. In the present work, we fabricated Au-Ag bimetallic heteronanostructures based on site-selective and anisotropic Ag deposition and growth on Au nanocubes. Plasmonic Au nanocubes were adsorbed onto a glass plate, and the distal mode or proximal-distal mode of the nanocubes was selectively excited in the presence of Ag+ and citrate ions. Polycrystalline Ag was deposited around the top of the Au nanocubes by the distal mode excitation, and single crystalline Ag was grown laterally from the Au nanocubes by the proximal-distal mode excitation. The present method would be applied to the fabrication of various plasmonic nanostructures composed of two or more heterodomains.

Plasmon-induced charge separation (PICS) is known to take place at the interface between a nanoparticle exhibiting localized surface plasmon resonance (LSPR) and a semiconductor such as titanium oxide. 1 PICS has been applied to photovoltaics, photocatalysis, and so on. 2 For energy harvesting based on plasmonic photocatalysis, it is important to optimize geometrical arrangements of the plasmonic nanoparticles, semiconductor, and co-catalyst if any, to enhance the charge separation efficiency and minimize charge recombination probability. Since LSPR confines light to a nanoscale region smaller than visible light wavelength, PICS allows one to fabricate plasmonic materials in nanoscale beyond the diffraction limit. 3-7 We have reported fabrication of wide variety of plasmonic nanostructures via site-selective reactions such as oxidative dissolution of silver and oxidative deposition of lead oxide. In the present work, gold nanocubes were adsorbed onto a thin film of titanium oxide to obtain a plasmonic photocatalyst. Then, lead oxide was deposited on the top or bottom part of the gold nanocube via the site-selective oxidation of lead ions based on PICS. Lead ions in the deposited lead oxide were replaced with manganese ions by galvanic replacement and the lead oxide was transformed to manganese oxide. The manganese oxide moieties on the gold nanocubes serve as co-catalyst of the plasmonic photocatalyst. We examined photocatalytic currents for ethanol oxidation by PICS. As a result, the photocurrents were enhanced by introducing manganese oxide to the top part of the gold nanocubes. In marked contrast, the photocurrents were suppressed when the manganese oxide was introduced to the bottom part of the nanocubes. In the latter case, charge recombination occurs between the holes injected to manganese oxide and the electrons injected to titanium oxide. On the other hand, the charge recombination is suppressed in the former case, because manganese oxide is not directly contact with titanium oxide. Thus we found that the position of a co-catalyst greatly affect the activity of plasmonic photocatalysts. 8 1. Y. Tian and T. Tatsuma, J. Am. Chem. Soc. , 127, 7632 (2005). 2. T. Tatsuma, H. Nishi, and T. Ishida, Chem. Sci. , 8, 3325 (2017) [review]. 3. I. Tanabe and T. Tatsuma, Nano Lett. , 12, 5418 (2012). 4. K. Saito, I. Tanabe, and T. Tatsuma, J. Phys. Chem. Lett. , 7, 4363 (2016). 5. H. Nishi, M. Sakamoto, and T. Tatsuma, Chem. Commun. , 54, 11741 (2018). 6. K. Saito and T. Tatsuma, Nano Lett. , 18, 3209 (2018). 7. T. Tatsuma and H. Nishi, Nanoscale Horiz. , 5, 597 (2020) [review]. 8. K. Kim, H. Nishi, and T. Tatsuma, J. Chem. Phys. , 157, 111101 (2022).

Chiral plasmonic nanostructures attracts attention because they are potentially applicable to optical materials such as enantioselective sensors and metamaterials, as well as photoelectrochemical devices. Chiral nanostructures are often prepared by electron beam lithography or synthesis based on DNA templates. We have recently developed a photoelectrochemical method, in which handedness of the chiral nanostructure can be controlled by right- or left- circularly polarized light. The photoelectrochemical method is based on plasmon-induced charge separation (PICS),1,2 in which electrons are injected from a plasmonic metal nanoparticle to a semiconductor such as titania in direct contact. In PICS, anodic reactions often occur at the resonance sites of the plasmonic nanoparticle, at which electron oscillation is localized.3,4 Energetic electron-hole pairs generate at the resonance site, and holes are used for the local anodic reaction, probably via trap sites. On the basis of the mechanisms, we have demonstrated site-selective etching of silver nanoparticles and site-selective deposition of lead oxide on gold nanoparticles. Under right-circularly polarized light (CPL), distribution of the resonance sites could be the mirror image of that under left-CPL.5 Therefore, we performed site-selective deposition of lead oxide on gold nanocuboids on titania under right- or left-CPL.6 As a result, lead oxide was deposited on the gold nanocuboids in a chiral geometry. The nanostructures thus obtained exhibited circular dichroism (CD), and the CD spectrum obtained for the structure prepared under right-CPL was opposite to that obtained for the structure prepared under left-CPL. Reversible switching of the handedness of the chiral plasmonic nanostructures can also be possible.7 This method also allows us to fabricate spiral nanostructures. 1. Y. Tian and T. Tatsuma, J. Am. Chem. Soc., 127, 7632 (2005). 2. T. Tatsuma, H. Nishi, and T. Ishida, Chem. Sci., 8, 3325 (2017) [review]. 3. I. Tanabe and T. Tatsuma, Nano Lett., 12, 5418 (2012). 4. T. Tatsuma and H. Nishi, Nanoscale Horiz., 5, 597 (2020) [review]. 5. S. Hashiyada, T. Narushima, and H. Okamoto, J. Phys. Chem. C, 118, 22229 (2014). 6. K. Saito and T. Tatsuma, Nano Lett., 18, 3209 (2018). 7. K. Morisawa, T. Ishida, and T. Tatsuma, ACS Nano, 14, 3603 (2020).

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

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

Plasmon-induced charge separation (PICS) was achieved with compound nanomaterials containing MoO2, which has a plasmon resonance wavelength close to gold.

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

When we applied colloidal quantum dots (QDs) for quantum dot light emitting diodes, it was well known that shell thickness played an important role in core protection, confinement of electrons and holes, and charge injection efficiency. However, although the shell thickness dependence of electroluminescence properties was reported, carrier injection efficiency has not been discussed in detail. In this paper, we investigated the effect of shell thickness on the carrier injection efficiency that was evaluated by photoelectrochemical measurements. By comparing the product of internal quantum yield of photoluminescence and the evaluated carrier injection efficiency with external quantum efficiency (EQE) for QDs with various shell thicknesses, we found that the optimal shell thickness for increasing EQE is determined by the balance between protection of QD's surface and carrier injection efficiency.

Anisotropic growth and modification of plasmonic nanoparticles have been of significant interest because their optical properties are depending strongly on anisotropy of the particle shape and environment. 1,2 Some researchers previously reported anisotropic reduction reactions including Ag and Au nanoparticle growth 3,4 and Pt deposition onto Au nanoparticles 5 under excitation of localized surface plasmon resonance (LSPR). Recently, our group demonstrated “site-selective” oxidative dissolution of Ag nanoparticles 6-8 and deposition of PbO 2 onto Au nanoparticles 9 on the basis of plasmonic hot hole ejection 10 assisted by a semiconductor such as TiO 2 . Irradiation of light with different wavelengths resulted in oxidation reactions at different sites corresponding to excited LSPR modes. However, only oxidation reactions are possible for nanoparticles on n-type semiconductors (electron transport layers). In this study, we found that both oxidation and reduction reactions, including oxidative PbO 2 deposition, coordinative dissolution of Au in the presence of Cl - , and reductive deposition of Pt, take place site-selectively on Au nanoparticles through electrochemically-assisted plasmonic hole/electron ejection without using a semiconductor. A Au nanocube-depoisted indium tin oxide (ITO) electrode was immersed in an electrolyte solution containing Pb(NO 3 ) 2 , KCl, or H 2 [PtCl 6 ] and potential of +0.95, +0.85, or +0.55 V vs. Ag|AgCl (sat. KCl) was applied to the electrode in a three-electrode system, respectively. Distal or proximal mode 2 was excited by visible light after the current became almost constant in the dark. The figure shows SEM images of the electrode surface after light irradiation. Oxidative PbO 2 deposition, Au dissolution, and reductive Pt deposition are observed preferentially at the top face of the nanocubes after distal mode excitation (panels a, b, and c, respectively) while the reactions proceeded at the interfacial region between the nanocube and ITO in the case of proximal mode (panels d, e, and f, respectively). 11 Similar site-selective reactions were also observed on Au nanorods under excitation of transverse and longitudinal modes. 11 These nanofabrication techniques are potentially applicable not only to control of LSPR properties but also to fabrication of high-performance plasmonic photocatalysts and chiral plasmonic materials. S. Link, M. A. El-Sayed, J. Phys. Chem. B 1999 , 103 , 8410. 2. L. J. Sherry, S.-H. Chang, G. C. Schatz, R. P. Van Duyne, Nano Lett. 2005 , 5 , 2034. 3. R. Jin, Y. C. Cao, E. Hao, G. S. Métraux, G. C. Schatz, Nature 2 003 , 425 , 487. 4. Y. Zhai, J. S. DuChene, Y.-C. Wang, J. Qiu, A. C. Johnston-Peck, B. You, W. Guo, B. DiCiaccio, K. Qian, E. W. Zhao, F. Ooi, D. Hu, D. Su, E. A. Stach, Z. Zhu, W. D. Wei, Nat. Mater. 2016 , 15 , 889. N. H. Kim, C. D. Meinhart, M. Moskovits, J. Phys. Chem. C 2016 , 120 , 6750. E. Kazuma, N. Sakai, T. Tatsuma, Chem. Commun. 2011 , 47 , 5777. I. Tanabe, T. Tatsuma, Nano Lett. 2012 , 12 , 5418. K. Saito, I. Tanabe, T. Tatsuma, J. Phys. Chem. Lett. 2016 , 7 , 4363. H. Nishi, M. Sakamoto, T. Tatsuma, Chem. Commun. 2018 , 54 , 11741. T. Tatsuma, H. Nishi, Nanoscale Horiz. 2020 , 5 , 597. H. Nishi, T. Tatsuma, Nanoscale 2019 , 11 , 19455. Figure 1

Plasmon-induced charge separation (PICS)1,2 involves electron transfer from plasmonic noble metal nanoparticles or compound nanoparticles to an n-type semiconductor such as TiO2. Oxidation reactions involved in PICS are based on a charge accumulation mechanism (Figure a) or a hole ejection mechanism (Figure b).3 The latter enables nanoscale local oxidation at relatively positive potentials.4-7 This can be applied to photoinduced nanofabrication beyond the diffraction limit of light. For instance, chiral plasmonic nanostructures can be prepared by PICS under right or left circularly polarized light (Figure c).8,9 Some other applications of the plasmonic hole ejection will also be mentioned. Enhancement of PICS with hole ejection through plasmon coupling10 and its application to photocatalysis will also be described. References Y. Tian and T. Tatsuma, J. Am. Chem. Soc., 127, 7632 (2005). T. Tatsuma, H. Nishi, and T. Ishida, Chem. Sci., 8, 3325 (2017) [review]. T. Tatsuma and H. Nishi, Nanoscale Horiz., 5, 597-606 (2020) [review]. E. Kazuma, N. Sakai, and T. Tatsuma, Chem. Commun., 47, 5777 (2011). I. Tanabe and T. Tatsuma, Nano Lett., 12, 5418 (2012). K. Saito, I. Tanabe, and T. Tatsuma, J. Phys. Chem. Lett., 7, 4363 (2016). H. Nishi, M. Sakamoto, and T. Tatsuma, Chem. Commun., 54, 11741 (2018). K. Saito and T. Tatsuma, Nano Lett., 18, 3209 (2018). K. Morisawa, T. Ishida, and T. Tatsuma, ACS Nano, 14, 3603 (2020). T. Ishida and T. Tatsuma, J. Phys. Chem. C, 122, 26153 (2018). Figure 1

Plasmonic photocathodes based on plasmoninduced charge separation (PICS) were prepared by depositing an Au nanoparticle (AuNP) ensemble on a transparent electrode and coating it with compact TiO2. Its PICS efficiency was improved by coating it further with porous TiO2 or introducing a NiO blocking layer as an underlayer of the AuNP ensemble. The former photocathode was modified further with Pt co-catalyst nanoparticles, and combined with a PICS photoanode fabricated by depositing a AuNP ensemble on compact TiO2 and coating it with a Ni(OH)2 charge accumulation layer. The dual plasmonic photoelectrode system thus obtained exhibited a higher efficiency than the sum of the efficiencies of the single photoanode and single photocathode. Photocatalysts, in particular artificial photosynthesis systems, which convert solar energy to chemical energy, are expected to be one of the promising solutions of the global energy issues. Among the photocatalysts, TiO2 has been most extensively studied and practically used. Although TiO2 drives various redox reactions on the basis of high photovoltage caused by ultraviolet (UV) light, it cannot use visible light for any reactions. One of the options to address this issue is employing plasmonic photocatalysis based on plasmon-induced charge separation (PICS) at the interface between a semiconductor and a plasmonic metal nanoparticle, which exhibits localized surface plasmon resonance (LSPR). Absorption wavelength of plasmonic photocatalysts can be tuned in a wide range by simply changing the particle size and shape. Plasmonic photocatalysts induce oxidative decomposition of alcohols, aldehydes, benzene, and other substances and achieve oxidation or reduction of water under visible light irradiation. For PICS, Au-TiO2 systems are most commonly used. [3] If the theoretical maximum PICS photovoltage is determined by Schottky barrier height at the Au-TiO2 interface, its value is calculated to be 0.9–1.2 V from the work function of Au (5.1 eV) and the electron affinity of TiO2 (3.9–4.2 eV). [12–14] On the other hand, the voltage necessary for water splitting is 1:23þ mO2 þ mH2 V, where mO2 and mH2 are overvoltages required for oxygen evolution and hydrogen evolution from water, respectively. Taking a voltage used for charge separation into account in addition to the overvoltages, photo-induced water splitting without bias voltage should be extremely difficult. However, there are a couple of reports on PICS-based water splitting without bias voltage application, indicating contribution of another mechanism, such as the hole ejection mechanism (Figure 1a). We demonstrated the hole ejection experimentally on the basis of site-selective oxidation reactions, such as selective etching of the top/bottom faces of Ag nanocubes and PbO2 deposition selectively onto the top/ bottom faces of Au nanocubes and the sides/tips of Au nanorods. An electron-hole pair generates at the plasmon resonance site and the hole is consumed by an oxidation reaction directly or indirectly via a surface trap site at around the resonance site, while the electron is injected to the TiO2 conduction band and drives a reduction reaction at the TiO2 [a] Dr. H. Nishi, K. Miyake, Dr. K.-C. Kao, Prof. T. Tatsuma Institute of Industrial Science University of Tokyo Komaba, Meguro-ku, Tokyo 153-8505 (Japan) E-mail: tatsuma@iis.u-tokyo.ac.jp Supporting information for this article is available on the WWW under https://doi.org/10.1002/cnma.201900751 This manuscript is part of a special collection celebrating the 100 Annual Meeting of the Chemical Society of Japan (CSJ). Click here to see the Table of