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A silver nanocolloidal solution was prepared by a cavitation bubble plasma with a pair of silver rod electrodes in a sodium hydroxide water solution with the initial pH = 11.0. The cavitation bubble plasma was generated by the bipolar high voltage pulses of the repetition rate 200 kHz and the pulse width 0.8 μs in the sodium hydroxide water solution in which many cavitation bubbles produced by a high-speed rotor were feeding. The UV–Vis absorption spectra of the silver nanocolloidal solution exhibited a characteristic peak around 400 nm in wavelength. The silver nanocolloidal solution of 250 mL including silver nanoparticles of median diameter 1.3 nm and concentration 4 ppm was prepared at the processing time 1 min. The particle size in the solution increased with increasing the processing time and grew to 14.6 nm in median diameter at the processing time 20 min. The concentration of particles also increased with increasing the processing time. A TEM image of silver nanoparticles revealed that the particles were spherical crystalline.

Niobium-substituted hollandite-type titanium dioxides with different potassium content in the tunnel space, KxTi1–yNbyO2, were synthesized by potassium extraction method using H2SO4/H2O2 solution. Rietveld refinement from X-ray powder diffraction data of the most potassium-extracted specimen, K0.06Ti0.9Nb0.1O2, showed it to have the hollandite-type structure (tetragonal; space group I4/m; a = 10.26725(11) A and c = 2.98759(5) A; Z = 8). Both KxTi0.9Nb0.1O2 (x = 0.19 and 0.06) specimens worked as rechargeable electrode materials for lithium batteries in the voltage range of 1.0 V to 3.0 V, and the initial lithium insertion capacity significantly increased from 15 mAh g−1 to 130 mAh g−1 with decreasing of potassium content. The small amount of residual K+ ions in the tunnel space affected the initial charge–discharge efficiency and the cyclability of the KxTi1-yNbyO2 electrodes. Cyclic voltammograms of the K0.06Ti0.9Nb0.1O2 electrode showed three pairs of redox peaks, and X-ray absorption spectra of the electrodes after charge-discharge measurements elucidated that both Ti and Nb contributed to the redox reaction.

We have successfully fabricated c -axis oriented films of apatite-type lanthanum silicate (LSO) using sputtering technique. Two kinds of thin films, La 2 SiO 5 and La 2 Si 2 O 7 , were successively deposited on yttria-stabilized zirconia (YSZ) or La-doped SrTiO 3 (STO) single crystal substrates by RF magnetron sputtering in pure Ar atmosphere at room temperature. Annealing the films in air at 1100 °C promoted the crystal formation. A surface layer of the obtained specimens were c -axis oriented apatite-type LSO, which was confirmed by in-plane and out-of-plane X-ray diffraction patterns. An activation energy of the ionic conductivity of the LSO sample using STO substrate was 0.74 eV, which agrees well with that of polycrystalline La 9.33 Si 6 O 26 though the conductivity is as low as 2 × 10 − 5 S cm − 1 at 600 °C. Using the laminated thin film method, we have accomplished to fabricate c -axis oriented apatite-type LSO with shorter time and lower temperature than the bulk diffusion synthesis between La 2 SiO 5 and La 2 Si 2 O 7 .

An oxide ion (O2−) conducting membrane cell, based on lanthanum silicate oxyapatite (La9.33+xSi6O26+1.5x, LSO), exhibiting low ohmic and polarization resistances in the intermediate-temperature region (~873 K) was developed. As a solid electrolyte, highly conductive Mg-doped LSO, La9·8(Si5·7Mg0.3)O26.4 (MDLS), modified with another kind of non-conductive lanthanum silicate, La2SiO5 was employed. Two kinds of silicate layers were successively spin-coated on a Ni-MDLS porous anode support, followed by thermal treatment aimed at improving ionic conductivity as well as densification of MDLS through solid state reactive diffusion. In addition, the Gd-doped CeO2, (Ce0.9Gd0.1)O1.95 (GDC) electrolyte layer, which plays an important role to prevent a reaction between the electrolyte and cathode materials, was spin-coated on the modified MDLS electrolyte. Finally, the cathode layer of porous (La,Sr)(Co,Fe)O3-δ was screen printed on the electrolyte layers. The resulting cell obtained from this study showed good fuel cell performance with a maximum power density of 94 mW cm−2 at 873 K, when operated with argon-diluted hydrogen and pure oxygen gasses.

Single phase specimens of hollandite-type MnO2, TiO2, and KxTi1 − yMnyO2, were synthesized, and their electrochemical properties and crystal structures were investigated. The chemically prelithiated MnO2 and sufficiently K+-extracted TiO2 specimens exhibited good charge–discharge properties. The initial Li insertion capacities were about 160 mAh/g for TiO2 and about 230 mAh/g for MnO2. In contrast, the partially K+-extracted solid solution, K0.055Ti0.75Mn0.25O2, showed a poor charge–discharge property. The small amount of residual K+ ions in the tunnel structure affected significantly the charge–discharge property. Electron density distribution images obtained from X-ray powder diffraction data elucidated that stabilizing O atoms locate at the tunnel space of lithiated MnO2 but the tunnel space of TiO2 was essentially empty.

A porous titania was synthesized by spray-drying of titania nanosheets exfoliated by (C4H9)4NOH. Nitrogen adsorption-desorption isotherms showed that the porous titania has a mesoporous structure composed of slit-shaped pores. The porous titania acted as a rechargeable active material in a liquid organic electrolyte lithium cell. An initial lithium insertion capacity was about 150 mAh/g (cut-off voltage of 1.0 V), which approximately correspond to the composition of Li0.45TiO2.

Three hollandite-type K x TiO 2 specimens with different K content in the tunnel space were prepared by the K + ions extraction using HCl and HNO 3 /HCl solutions. The K x TiO 2 specimens worked as rechargeable electrode materials in the voltage range of 1.0 V to 3.0 V, and the initial Li insertion capacities increased from 63 mAh g − 1 to 158 mAh g − 1 with decreasing of K + ions. Neutron powder diffraction investigation elucidated that the tunnel space of K 0.008 TiO 2 with lowest K content was essentially empty, and no water species such as H 2 O and H 3 O + were detected by the nuclear scattering length density distribution calculated by the maximum entropy method based on the Rietveld analysis using neutron powder diffraction data. The K 0.008 TiO 2 specimen showed good cycling and rate performances. The rechargeable capacity at 0.1 C was about 150 mAh g − 1 with a capacity retention rate of 97% over 2–50 cycles, and even at the high rate of 10 C, the capacity exceeded 100 mAh g − 1 . The empty tunnel in the hollandite-type TiO 2 is promising as the storage space for Li + ions.

Single phase specimen of K0.12TiO2 with hollandite-type structure was successfully synthesized by the solid-state reaction under Ar/H2 atomosphere. K+ ions were partially extracted from the parent K0.12TiO2 specimen by the HCl treatment. The K+-extracted K0.08TiO2 specimen worked as a rechargeable electrode material. An initial capacity of 92 mAh/g (cut-off voltage: 1.0 V) could be achieved, which approximately correspond to the composition Li0.28TiO2. A reversible capacity after 50 cycles was about 50 mAh/g.

Porous titania was synthesized through the spray-drying of titania nanosheets. SEM-FIB analyses showed that the spray-dried nanosheets were spherical particles with no hollow center. Nitrogen adsor...