Decahedral anatase TiO2 particles with exposed (101) and (001) planes were successfully grown in LiCl-KCl molten salts under HF-free conditions. The particle size was adjusted to similar to 30 or similar to 700 nm by tuning annealing temperature, molten salt composition, and annealing time. These particles exhibited UV light absorption based on electron excitation from the valence band to the conduction band, and scattering in the UV-visible range based on Mie resonance. The latter strongly depended on the TiO2 particle size. The optical near-field distributions under circularly polarized light were also size-dependent.

Plasmonic metal nanoband arrays were fabricated by scanning Au or Ag films on dielectric or semiconducting substrates such as TiO(2 )with a cost-effective near-infrared laser marking system. Surface plasmon polaritons were excited through localized surface plasmons at the metal surface with nanoscale roughness, leading to periodic array formation. The nanoband period was controlled by selecting metals and underlying dielectric materials with appropriate dielectric constants. Because the arrays appeared only in the laser-scanned regions, arbitrary patterns of nanoband arrays were easily produced. The resulting arrays and patterns exhibited brilliant visible light diffraction as plasmonic gratings, indicating potential applications to security printing for anticounterfeiting and certification, as well as other optical devices.

We have proposed a magneto-plasmonic system consisting of the [Pt/CoPt/Pt]/Ag stacked films for hydrogen sensing applications. The [Pt/CoPt/Pt] layer provides both the magneto-optical activities and hydrogen reactions. The sensor elements produced significantly hydrogen responses.

Facial electrophysiological signals are crucial to human-machine interfaces and health care monitoring. Soft and skin-conformable electrodes enabled long-term and comfortable signal monitoring. However, the appearance of the electrodes affects the wearer’s social interactions and self-identity, making daily usage difficult and leaving appearance artifacts. Here, we developed fully invisible and unperceivable on-skin electrodes free from appearance artifacts. Neither the wearer nor observers can detect the visual and tactile presence of the electrodes on the skin. The unperceivable property was confirmed with sensory experiments and physical characterizations of the film on skin. Furthermore, our invisible electrodes did not affect the psychological conditions of the wearers, which confirms the feasibility of artifact-free monitoring in daily lives. Last, we demonstrated the functionality of our electrode with successful monitoring of various facial electrophysiological signals, including electrooculogram (EOG), electromyogram (EMG), and electroencephalogram (EEG). Our fully invisible electrodes provide a promising direction in developing on-skin bioelectronics, seamlessly integrating health monitoring and human-computer interaction technologies into people’s daily lives.

Plasmon‐induced charge separation (PICS), which is based on electron injection from plasmonically excited nanoparticles (NPs) into an electron collector, has predominantly employed wide‐bandgap semiconductors such as TiO 2 as the electron collector. If polyoxometalates (POMs), which can serve as capping agents to assist the dispersion of plasmonic metal NPs in water, could also function as electron collectors, metal–POM nanocomposites would be promising as photocatalysts. In the present study, commercially available α‐H 4 SiW 12 O 40 (SiW 12 ) was used as a POM and assembled as thin films on electrodes by a layer‐by‐layer method. PICS via electron injection from Au NPs and that from Ag NPs into SiW 12 was demonstrated by photoelectrochemical measurements and visible absorption spectroscopy, respectively.

Chiral nanostructures are attracting increasing attention, as they can be applied to metamaterials and metasurfaces as well as enantioselective sensors and other optoelectronic devices. Recently, a photonic nanofabrication technique has been developed that enables straightforward fabrication of right- and left-handed chiral plasmonic metal nanostructures depending on the handedness of the circularly polarized light (CPL) used. In the present work, we present a fabrication method for chiral semiconductor nanostructures based on local photocatalytic reactions coupled with Mie-type resonances via optical near fields. Simulations of the optical near fields around polygonal ZnO nanoplates, used as achiral and anisotropic semiconductor precursors, revealed twisted near-field distributions under CPL. Single and stacked hexagonal ZnO nanoplates were experimentally synthesized, and Co3O4 was deposited onto them via photocatalysis driven by UV-CPL. The stacked ZnO nanoplates were converted to three-dimensional spiral ZnO-Co3O4 nanostructures, which exhibited significant circular dichroism.

Electrochromic windows are promising devices for efficiently controlling transmission of light and heat through windows in order to suppress energy consumption of buildings. In particular, versatile electrochromic windows operating in the bright, cool, and dark modes attract attention. In the present work, we synthesized amorphous oxygen-deficient WO3-x nanoparticles and Cu-doped NiO nanoparticles by simple and convenient methods and assembled an electrochromic cell by using the nanoparticles. In the bright mode at +1.0 V, the cell transmitted similar to 90% visible light at 500 nm and similar to 80% near infrared (NIR) light at 1000 nm, whereas the transmittance was suppressed to 10-20 % in both of the visible and NIR ranges in the dark mode at-3.0 V. In the cool mode at-2.5 V, the cell showed a marked contrast in the optical transmittance: similar to 90% in the visible and similar to 20% in the NIR range. The oxygen deficiency of the WO3-x cathode and/or the Cu doping of the NiO anode led to the high visible-NIR transmittance contrast in the cool mode, in addition to excellent cyclability at least for 12000 cycles and improved coloration efficiencies in the visible and NIR ranges. (c) The Author(s) 2025. Published by ECSJ. This is an open access article distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 License (CC BY-NC-SA,https://creativecommons.org/licenses/by-nc-sa/4.0/), which permits non-commercial reuse, distribution, and reproductionin any medium by share-alike, provided the original work is properly cited. For permission for commercial reuse, please email to the corresponding author.

Photonic nanofabrication based on optical near-field generation around nanostructures is a powerful technique for shaping materials at the nanoscale beyond the diffraction limit of light. Recently, we reported that not only localized surface plasmon resonance of metal nanoparticles but also the Mie resonance of photocatalytic semiconductor nanoparticles enables nanofabrication through site-selective excitation via an optical near field and reductive metal deposition at resonance sites. In the present study, we describe the self-etching of ZnO nanoplates in a site-selective manner using holes generated at resonance sites under linearly polarized UV light. This allowed the nanoplates to be shaped into nanorings and exhibit optical anisotropy corresponding to light polarization.

Compound nanoparticles (NPs) attract attention because of their unique electrical, optical, and catalytic properties. Among them, plasmonic tin-doped indium oxide (ITO) NPs are characterized by transparency in the visible wavelength range and tunable localized surface plasmon resonance (LSPR) in the near-infrared (NIR) range due to high electronic conductivity. To date, they have been usually synthesized by a chemical solution process. However, the chemically synthesized ITO NPs are capped with organic protecting agents, which often block exchange of charge carriers and access of chemical species to the NPs, limiting some applications. In the present study, we propose a method for direct deposition of ITO NPs on a glass or plastic substrate using commercially available ITO-coated glass via NIR laser ablation. The LSPR characteristics of the ITO NPs, thus, prepared were controlled by changing the ablation conditions such as laser power. In addition, the potential applications of the ITO NPs were also investigated through measurements of their refractive index sensitivity and magnetic circular dichroism.

The morphology of the CdS-Pt photocatalyst was optimized in terms of photocatalytic activity and stability through photoetching of CdS. This led to a high external quantum efficiency for hydrogen production in an alkaline solution containing a sacrificial electron donor.

The interaction between circularly polarized (CP) light and matter is governed by two fundamental quantities: spin angular momentum (SAM) and optical chirality (OC). While these quantities are inseparable in free space, they can be selectively enhanced in plasmonic near-field regions through appropriately designed structures. We demonstrate that the excitation of circular plasmonic nanostructures with CP light enables the selective or simultaneous enhancement of the SAM and OC through the excitation of rotating plasmon modes. Electromagnetic field analysis reveals that SAM enhancement originates from transverse SAM induced by unidirectional evanescent waves, whereas OC enhancement is governed by the interference between the plasmonic electric field and the incident magnetic field. The finite-element method simulations confirm that circular differential absorption signals arising from these enhanced near-fields clearly depend on the SAM and OC of the local fields, underscoring the importance of structural design in the detection and enhancement of CP light-matter interactions at the nanoscale.

Chiral plasmonic nanostructures attract attention recently, and one of the promising methods for chiral shaping is photoelectrochemical growth of the nanostructures under circularly polarized light (CPL). The CPL-induced method has been based on asymmetric and twisted electric field distribution generated around anisotropic plasmonic nanoparticles irradiated with CPL. In the present work, isotropic and symmetric Au nanodisks were used as plasmonic precursors, around which uniform electric fields were generated. When the Au nanodisks were irradiated with CPL in the presence of Ag ions and citrate ions, initial Ag deposition at an arbitrary site broke the symmetry, and gave rise to anisotropic and asymmetric electric field distribution, which leads to chiral shaping of Au-Ag nanostructures and chiroptical responses.

Nanofabrication is a technology for control of the morphology, orientation, and configuration of nanomaterials. When ordinary, propagating light is employed as a processing tool, subwavelength fabrication is difficult due to the diffraction limit. However, if optical near field, which is localized in a subwavelength region is used, nanofabrication beyond the diffraction limit is possible. We have demonstrated that subwavelength nanofabrication is possible by taking advantage of the optical near field. Since hot electron-hole pairs are generated at around the plasmonic resonance sites, the energetic carriers can be used to drive oxidative dissolution of Ag, 1,2 oxidative deposition of PbO 2 , 3,4 and reductive deposition of Ag, 5 in a site-selective manner. These techniques have been applied to subwavelength photochromic data storage, 1,2 fabrication of chiral nanoparticles, 4-7 and site-selective modification of a plasmonic photocatalyst with a co-catalyst. 8 Generation of optical near field is possible even at non-plasmonic, semiconducting or dielectric nanostructures. Here we employed In-doped ZnO (In:ZnO) as a semiconductor photocatalyst. Hexagonal In:ZnO nanoplates were synthesized and adsorbed onto a glass substrate, and irradiated with linearly polarized UV light in the presence of Ag ions for reductive deposition of Ag. The optical near field generated under polarized UV light gave rise to site-selective Ag deposition. Also, the sample exhibited linear dichroism (LD) signals, indicating that the nanoparticles have optical anisotropy. On the other hand, unpolarized UV light gave no optical anisotropy. Electromagnetic simulations successfully reproduced the LD spectra. We have also performed site-selective oxidation reactions by using the hexagonal In:ZnO nanoplates. Tanabe, I.; Tatsuma, T. Nano Lett. 2012 , 12 , 5418–5421. Saito, K.; Tanabe, I.; Tatsuma, T. J. Phys. Chem. Lett. 2016 , 7 , 4363–4368. Nishi, H.; Sakamoto, M.; Tatsuma, T. Chem. Commun. 2018 , 54 , 11741–11744. Saito, K.; Tatsuma, T. Nano Lett. 2018 , 18 , 3209–3212. Ishida, T.; Isawa, A.; Kuroki, S.; Kameoka, Y.; Tatsuma, T. Appl. Phys. Lett. 2023 , 123 , 061111. Morisawa, K.; Ishida, T.; Tatsuma, T. ACS Nano 2020 , 14 , 3603–3609. Shimomura, K.; Nakane, Y.; Ishida, T.; Tatsuma, T. Appl. Phys. Lett. 2023 , 122 , 151109. Kim, K.; Nishi, H.; Tatsuma, T. J. Chem. Phys. 2022 , 157 , 111101.

Deposition of noble metal nanoparticles onto semiconductor such as TiO2 is important for improving photocatalytic activities with metallic co-catalysts and for achieving plasmon-induced charge separation. In that context, facet-selective deposition of metal nanoparticles is attracting attention. There are many reports on the facet-selective deposition, and the selectivity has been explained in terms of different work functions depending on facets.1,2 However, it is likely that factors other than work function also contribute to the selectivity. Therefore, in this work, we focus also on the number of electrons involved in the reactions and the surface and interfacial energies of the semiconductor and metal, so as to understand the mechanisms of the selective deposition and to control the deposition sites and morphologies of the metal nanoparticles. Here, we employed rutile TiO2 particles with exposed crystal planes as a semiconductor material and deposited Au, Pt, and Ag nanoparticles onto the TiO2 particles by photocatalytic means. Rutile TiO2 particles with exposed crystal planes were synthesized by a flux method. Anatase TiO2 as a precursor was heated at 1000 °C in a NaCl molten salt in air. The TiO2 particles thus obtained (50 mg) was added to 50 vol% aqueous ethanol (5 mL). The solution was purged with nitrogen and a metal precursor, HAuCl4, H2PtCl6, or AgNO3, was added to the solution, followed by irradiating with a UV LED lamp (365 nm) for 10 min, for deposition of Au, Pt, or Ag, respectively, onto the TiO2 particles. From XRD measurements and SEM observation, it was revealed that the obtained TiO2 particles have rutile structure and (110) and (111) planes. The average exposed area ratio between (110) and (111) planes was estimated to be around 7:3. After photocatalytic deposition of Au, we found 87% of the Au nanoparticles were deposited on (110) planes. Even higher selectivity was observed for Pt nanoparticles; 94% of them were deposited on (110) planes. In contrast, interestingly, 75% of Ag nanoparticles were found at (111) planes. Other Ag nanoparticles were found on the edges in between (110) planes. Namely, facet selectivity was different for Au, Pt, and Ag, and the observed difference was difficult to explain solely in terms of work function. We therefore focused on difference in the number of electrons required for the metal deposition reactions. In the case of Au and Pt, these metals are deposited by multi-electron reduction reactions; [AuCl4]- and [PtCl6]2- require 3 and 4 electrons, respectively, to be deposited as metal. It is known that the conduction band minimum of the (110) plane is lower (i.e., more positive in potential) than that of the (111) plane. Therefore, photoexcited electrons should tend to accumulate at (110) planes, and multi-electron reductive deposition of Au and Pt should be promoted there. In contrast, Ag nanoparticles are deposited through one-electron reduction of Ag+, which should not require accumulation of excited electrons, and the deposition occurs even at (111) planes. Because the (111) plane has higher surface energy than the (110) plane, the former could be readily stabilized by coating with Ag, which has much lower surface energy than the rutile TiO2(111) plane. When such an anisotropic growth of metal into nanoplates occurs on a semiconductor, the interfacial energy between them should be low enough. It is expected that a metal crystal lattice is easily distorted to match that of a semiconductor, if the Young's modulus for the metal is low. The Young's modulus of Ag is actually lower than those of Au and Pt,3 indicating that Ag is advantageous to cover the TiO2 surface, even if there is some lattice mismatch between them, resulting in low interfacial energy. In summary, it was shown that facet-selective deposition and morphology control of metal nanoparticles are possible to some extent for rutile TiO2. Also, it was shown that, not only work function, but also the number of electrons involved in the deposition reactions and the surface and interfacial energies for the metal and semiconductor are also important in the facet-selective deposition. These findings would allow us to improve activity of metal co-catalysts and to control plasmon resonance properties of metal nanoparticles. [1] T. Takata, et al., Nature, 581, 411 (2020). [2] R. Li, et al., Nat. Commun., 4, 1432 (2013). [3] C. J. Price and S. P. Hepplestone, J. Mater. Chem. C, 11, 14278 (2023). Figure 1

Shape-controlled Au nanoparticles are synthesized in general by liquid-phase chemical reactions that require reducing and organic protective agents as well as an Au complex, via seed-mediated growth. In the present study, we report a one-step photocatalytic synthesis of Au nanoplates and their dispersion in aqueous solution without using any reducing or organic protecting agents, simply by irradiating a TiO2 substrate with ultraviolet (UV) light in an aqueous solution containing [AuCl4](-). Chemical species necessary for Au nanoplate formation, such as [AuCl2](-), should be generated through photocatalytic reactions, and Au nanoplates without a thick organic protective layer are grown in the solution phase. X-ray diffraction (XRD) measurements revealed that the obtained Au nanoplates are single crystals with (111) as the basal planes. Additionally, it was demonstrated that the nanoplates deposited on glass show sufficient electronic conductivity and that the nanoplates are metallic, and they can directly exchange electrons with each other.

Chiral and magneto-chiral plasmonic nanostructures attract attention because they have various potential applications including catalysts, chemical sensors, and optical and optoelectronic materials and devices. In many cases, those nanostructures are fabricated by lithographic techniques. However, those top down methods are generally time-consuming and expensive. Therefore, we have developed photoelectrochemical methods in which site-selective deposition or dissolution reactions are driven by optical near field generated around anisotropic metal nanoparticles under right- or left-circularly polarized light (CPL). We have used gold or silver nanocuboids,1 nanorods,2 nanocubes,3 triangular nanoplates,4 and intricate nanoporous films5 on semiconducting or dielectric substrates as anisotropic metal precursors for preparation of chiral plasmonic nanostructures. If an anisotropic metal nanoparticle is irradiated with CPL, chiral electric field is generated around the nanoparticle, and energetic electron-hole pairs generate at the resonance sites, where electric field is localized. As a result, reductive deposition of silver or oxidative deposition of lead oxide proceeds preferentially at the resonance sites, resulting in formation of chiral plasmonic nanostructures, which exhibit circular dichroism (CD). Chiral metal nanoparticles were also prepared from less anisotropic, circular metal nanodisks. In this case, initial nucleation of metal at an arbitrary site of the nanodisk edge breaks its symmetry and gives rise to chiral electric field distributions under CPL. As a result, the nanodisk is grown into chiral plasmonic nanostructure. We also prepared magneto-chiral nanostructures by employing superparamagnetic magnetite nanocubes as precursors. The magnetite nanocubes on a glass substrate exhibit magnetic circular dichroism (MCD) but not CD. Magnetite is also reported to be conducting enough for its nanoparticles to show LSPR. Therefore, chiral electric field generates at around a magnetite nanocube under CPL and plasmonic photoelectrochemical reactions can proceed at those localized resonance sites. In addition, magnetite also has valence band and conduction band, and an electron in the valence band can be excited to the conduction band by a photon with energy higher than the band-gap energy. This photoexcitation can also occur at the localized resonance sites.6 As a result of those localized photoelectrochemical reactions induced by CPL, chiral magnetite-silver nanocomposite structures were obtained, and those nanocomposites not only MCD but also CD based on the chiral morphologies. Thus, the nanocomposites break both inversion and time-reversal symmetries. As a result, they exhibit nonreciprocal transmission of visible light, which is recognized also as magneto-chiral dichroism (MChD). K. Saito and T. Tatsuma, Nano Lett. 18, 3209-3212 (2018). K. Morisawa, T. Ishida, and T. Tatsuma, ACS Nano 14, 3603-3609 (2020). K. Shimomura, Y. Nakane, T. Ishida, and T. Tatsuma, Appl. Phys. Lett. 122, 151109 (2023). T. Ishida, A. Isawa, S. Kuroki, Y. Kameoka, and T. Tatsuma, Appl. Phys. Lett. 123, 061111 (2023). H. Nishi, T. Tojo, and T. Tatsuma, Electrochemistry in press (doi: 10.5796/electrochemistry.24-00027). Y. Oba, S. H. Lee, and T. Tatsuma, J. Phys. Chem. C 128, 827-831 (2024). Figure 1

Plasmonic nanofabrication beyond the diffraction limit of light is based on chemical and electrochemical reactions induced by plasmonic near field, and it has been applied to the construction of functional nanomaterials and nanodevices. Generation of optical near field is possible even at nonplasmonic dielectric nanostructures including semiconductors. In the present work, we employed In:ZnO as a semiconductor photocatalyst. Hexagonal In:ZnO nanoplates were adsorbed onto a glass plate and irradiated with linearly polarized UV light to drive reductive deposition of Ag. The optical near field induced by the polarized light gave site-selective Ag deposition and, thereby, optical anisotropy reflected by linear dichroism (LD) signals. The optical anisotropy was not introduced by unpolarized UV light. The LD spectra were successfully reproduced by electromagnetic simulations.