Hiroki Itasaka

The global demand for sustainable freshwater production has intensified interest in interfacial solar vapor generation, which requires efficient broadband photothermal materials. Although plasmonic MoO3−x is a promising candidate, its localized surface plasmon resonance (LSPR) strongly depends on both the valence state and nanoscale morphology, which are difficult to simultaneously control. In this study, we concurrently modify these properties by regulating the valence state of Mo through precursor design and manipulating the nanoscale morphology using a ligand-assisted solvothermal method. In this approach, molybdenum blue acts as a partially reduced precursor composed of mixed-valence Mo5+/Mo6+ clusters generated through mild ethylene-glycol-assisted reduction. Meanwhile, octylamine serves as an organic ligand that promotes cluster aggregation and polymerization while suppressing crystal growth. The synergistic interactions between the precursor and organic ligand effectively modulate both the nanoscale morphology and valence state, yielding reduced MoO3−x with pronounced LSPR characterized by high absorbance over a broad range from visible to near-infrared light. Among the samples, MoO3−x produced with 0.25 mL of EG exhibits the highest LSPR intensity, spanning 400–2600 nm with a peak at 990 nm. Under one-sun irradiation, its temperature reaches 82.4 °C, and it achieves an outdoor solar vapor generation rate of 2.34 kg·m−2·h−1, which is 1.8 times that of carbon felt. These findings demonstrate that the precursor–ligand synergy provides a synthetic strategy for engineering plasmonic metal oxides for efficient solar desalination.

Raman spectroscopy is a powerful tool to analyze materials. The demand for analyzing materials in a smaller analytical region is increasing as technology advances. Tip-enhanced Raman spectroscopy (TERS) is an option. However, measuring the surface of three-dimensional bulk materials is quite challenging, since simultaneously excited micro-Raman signals hide the enhanced nano-Raman signals. In 2024, another approach of using a porous gold membrane was reported to dedicate solid surface analysis. However, this method, for example, sacrifices the lateral analytical spot size because nanopores distribute throughout the membrane. Here, 70-nm thick gold microplates with nano-through-holes in the center are fabricated. The gold microplates provide a smaller analytical spot because nano-through-holes are fabricated only in a spatially limited region. The microplates and the surrounding structures are clearly visible under optical microscopes. We moved gold microplates to another location on a silicon substrate using a manipulator and successfully demonstrated nano-Raman measurements of silicon surface via nano-through-holes. The finite-difference time-domain calculations confirmed that enhanced electric fields are available by the nano-through-holes and revealed that the nano-Raman signals come from the surface of silicon within a depth of 5.4 nm.

Oxide solid electrolyte for all-solid-state lithium-ion batteries have advantages over sulfide solid electrolytes in terms of thermal and mechanical stability, safety, and ease of handling. However, high sintering temperatures required to fabricate oxide solid electrolyte ceramics are on obstacle to integration with cathodes and anodes. We recently reported synthesis of lithium lanthanum titanate (LLTO)-based solid electrolyte nanoplates by a hydrothermal method[1]. The hydrothermally synthesized nanoplates could be sintered at a lower temperature than particles synthesized by the solid phase method. It was revealed that the lower sintering temperature was due to the face-to-face stacking of the nanoplates formed through uniaxial molding, which indicated that sintering behvior of oxide solid electrolytes could be controlled by the shape and stacking structure of nanocrystals. Here, we have synthesized various shaped LLTO-based nanocrystals by changing a Ti source in the hydrothermal method to investigate dependence of sintering behavior on the shape of the nanocrystals and have investigated Li ion conductivity of the ceramics fabricated from the nanocrystals. For synthesis of LLTO-based nanocrystals, we newly introduced two types of titanium sources: anatase-type and rutile-type TiO₂, in addition to titanium(IV) bis(ammonium lactate) dihydroxide (TALH), which was used for the synthesis of the nanoplate in the previous report. Scanning electron microscopy observation showed that anatase-type TiO₂ yielded a mixture of nanocubes and nanorods and rutile-type TiO₂ yielded nanosheets. Scanning transmission electron microscopy observation and electron energy loss spectroscopy confirmed that the nanocrystals yielded from anatase- and rutile-type TiO₂ had the LLTO-based perovskite crystal structure as the nanoplates yielded from TALH. Comparison of the sintering behavior at 1000°C for these different shaped nanocrystals subjected to uniaxial molding revealed that the nanocrystals with high aspect ratio (nanoplate, nanorod, and nanosheet) sintered more easily than the nanocubes and the ceramics of the nanoplates and nanosheets had {100} and {110} orientation, respectively. These oriented ceramics showed ionic conductivity according to the crystallographic orientations. The present result can provide an insight into controlling ionic conductive direction in oxide solid electrolyte ceramics by using shape-controlled nanocrystals. Acknowledgement This study was supported by a project (No. JPNP20005), commissioned by the New Energy and Industrial Technology Development Organization (NEDO), Japan. Reference [1] K. Mimura, N. Hamao, H. Itasaka, Z. Liu, K. Hamamoto, J. Sol-Gel Sci. Tech. 104, 599 (2022).

Oxide type all-solid-state battery has attracted a great deal of attention its high energy density and safety. Perovskite-type (Li_3x,La_2/3-x)TiO₃ (LLTO) has high ionic conductivity and it is one of the candidate electrolyte for an all-solid-state battery. Various efforts are underway to improve the properties. Facet-controlled nanocrystals can assemble into highly ordered nanostructures, and their orientation control and epitaxial grain growth during sintering are expected to improve their electrical properties. In this study, we focused on the orientation of LLTO ceramics. To control the orientation of electrolyte, we tried to fabricate perovskite LLTO platelet nanocrystals (nanoplates). LLTO nanoplates were synthesized by hydrothermal reaction using aqueous Ti complex as a starting material and NaOH as a mineralizer [1]. The pellet made from LLTO nanoplates is sintered at 1000 ºC which is lower sintering temperature than LLTO microparticles. However, the ionic conductivity is as low as 10^-6 S/cm. STEM-EDX analysis revealed that this is due to the Na ions remaining at the Li site of the perovskite structure. We found that Ruddlesden-Popper (R-P) type layered perovskite Li₂La₂Ti₃O₁₀ nanoplates can be synthesized by selecting LiOH instead of NaOH as the mineralizer to prevent substitution of Li to Na ion. The R-P nanoplate is a single crystal having c-plane of R-P phase. The pellet made from R-P nanoplate has highly orientation of c-axis along to the press direction. This pellet is also able to sinter relatively low temperature at 1200 ºC with keeping orientation, but R-P phase shows extremely low ionic conductivity of 3.2x10^-8 S/cm. As Li site of R-P phase is fully occupied by Li ion, Li ions are difficult to move. However, ionic conductivity of the direction of c-plane is higher than that of the c-axis direction. This result suggests that Li ion is located interlayer and Li ion is easy to move in c-plane of R-P structure. When this pellet is sintered at 1400 ºC, R-P phase has transformed to perovskite LLTO with keeping orientation. Transformation to perovskite phase is caused by vaporization of Li ion from R-P structure and it shows good conductivity of 1.1x10^-4 S/cm. Reference [1] K. Mimura, N. Hamao, H. Itasaka, Z. Liu, K. Hamamoto, J. Sol-Gel Sci. Tech. 104, 599 (2022). Acknowledgement: This study is based on results obtained from a project, JPNP20005, subsidized by the New Energy and Industrial Technology Development Organization (NEDO).

Ultra-thin capacitor structures of barium titanate (BaTiO3, BTO) nanocube monolayers are fabricated using graphene as an electrode without high-temperature sintering. The capacitor consists of a self-assembled BTO nanocube monolayer (approximately 20 nm in thickness) and an eight-layer graphene electrode (≤3 nm in thickness) that are sandwiched metal electrodes. The graphene electrode functions as a barrier layer preventing the occurrence of short circuits caused by the penetration of metal species into the dielectric layer during the deposition process of the metal electrode, dramatically reducing the leakage current of the capacitor in comparison with the case without the graphene electrode. Furthermore, this approach is applicable to the fabrication of the graphene/BTO-nanocube-monolayer multi-stacked structures, demonstrating the potential to reduce the thickness of individual electrode/dielectric layers in multilayer ceramic capacitors down to less than 25 nm. This opens up possibilities for energy-efficient production of high-performance ceramic capacitors.

We investigated the ferroelectric properties of barium titanate (BTO) nanocube self-assembled monolayers with and without heat treatment using piezoresponse force microscopy (PFM). Observations of polarization switching behavior confirmed that BTO nanocube monolayers about 15 nm thick are ferroelectric, even without heat treatment. Vertical PFM phase imaging of the monolayers revealed that heat treatment changed the ferroelectric polarization distribution in the monolayers at 800 °C. Atomic force microscopy and transmission electron microscopy suggested that this change originated from the residual stress caused by mechanical interactions between neighboring BTO nanocubes and between the monolayers and the substrate.

It has been reported that the flexoelectric effect could be dominant in the nanoscale. The discrepancy between theory and experiments on the frequency dependence of the dielectric constant of an ordered assembly of BaTiO₃ nanocubes is nearly resolved by assuming the coexistence of flexo- and ferro-electric effects. Although flexoelectric polarizations perpendicular to the applied alternating electric field contribute to the dielectric constant, those parallel to the electric field do not contribute because the magnitude of the flexoelectric polarization does not change due to the mismatch of strain at the interface of the nanocubes. On the other hand, some dielectric response is possible for the ferroelectric component of the polarization parallel to the electric field.

This paper demonstrates structural analysis for single detonation nanodiamond (DND) particles, with a size below 6 nm, by tip-enhanced Raman spectroscopy (TERS). Single DND particles show Raman spectra different from that of the aggregates obtained by conventional micro-Raman spectroscopy. Our results suggest that single-particle TERS measurement is a promising technique for revealing structural distribution of individual DND particles and its dependence on synthesis conditions.

Heat treatment is a key process to determine the dielectric properties of nanocrystalline barium titanate (BaTiO₃) ceramics, which are a prospective candidate to realize further miniaturization of dielectric components in electric devices. Here, we use Raman spectroscopy and scanning microwave impedance microscopy to investigate the dependence of the internal stress and the relative permittivity of BaTiO₃ nanocube three-dimensional assemblies on heat treatment temperature. We show that heat treatment in the temperature range from 700 °C to 1000 °C causes internal compressive stress in the nanocube assemblies without grain growth. The internal compressive stress is caused by formation of tight attachments between neighboring BaTiO₃ nanocubes and of Ti-rich phases in the nanocube assemblies in the lower and higher temperature ranges, respectively. We also show that the relative permittivity of the nanocube assemblies at 3 GHz shows a positive correlation with the internal compressive stress. The result indicates that the internal compressive stress enhances the relative permittivity of BaTiO₃ nanocubes.

Novel cube-shaped nanocrystals of BaTiO₃ (BT) surrounded by CaTiO₃ (CT) that can be used for piezo-driven energy harvesters were synthesized using a one-step hydrothermal reaction with surfactant and additives at 250°C for 24 h. The size distribution of the nanocubes (NCs) was narrow, with an approximate average size of 50 ~ 60 nm. The nanocubes had a core-shell configuration, such that the Pbnm structured CT shell was grown on a P4mm structured BT core by sharing corresponding [001] directions. STEM-EDX and EELS indicated that Ba ions did not diffuse into the CT shell region. Piezoelectric hysteresis loops of BT/CT NCs were obtained by piezoresponse force microscopy.