Catalytic NH3 oxidation affected by the nanometric roughness of the platinum overlayer.

Pulsed cathodic arc-plasma deposition was employed to create a few nanometre-thick Pt overlayer on a 50 µm-thick Fe-Cr-Al metal (SUS) foil, resulting in an effective NH3 oxidation catalyst fabrication. This catalyst exhibited a turnover frequency (TOF) exceeding 100 times that of Pt nanoparticles. In this study, Pt overlayer catalysts with varying degrees of surface roughness were fabricated using different metal foil substrates: mirror-polished (Pt/p-SUS), unpolished (Pt/SUS) and roughened by the formation of a surface oxide layer (Pt/Al2O3/SUS). The nanoscale roughness was comprehensively analysed using electron microscopy, laser scanning confocal microscopy and chemisorption techniques. NH3 oxidation activity, measured at 200 °C, followed an increasing trend in the order of Pt/Al2O3/SUS < Pt/SUS < Pt/p-SUS, despite a decrease in the apparent Pt surface area in the same order. Consequently, the calculated TOF was markedly higher for Pt/p-SUS (267 min-1) compared to Pt/SUS (107 min-1) and Pt/Al2O3/SUS (≤22 min-1). The smooth Pt overlayer surface also favoured N2 yield over N2O at this temperature. This discovery enhances our fundamental understanding of high-TOF NH3 oxidation over Pt overlayer catalysts, which holds significance for the advancement and industrial implementation of selective NH3 oxidation processes.

Bandgap Tunable Oxynitride LaNb2 O7-x Nx Nanosheets.

Bandgap tunable lanthanum niobium oxynitride [LaNb2 O7-x Nx ](1+x)- nanosheet is prepared by the delamination of a Ruddlesden-Popper phase perovskite oxynitride via ion-exchange and two-step intercalation processes. The lanthanum niobium oxynitride nanosheets have a homogeneous thickness of 1.6 nm and exhibit a variety of chromatic colors depending on the nitridation temperature of the parent-layered oxynitride. The bandgap energy of the nanosheets is determined by ultraviolet photoemission spectroscopy, Mott-Schottky, and photoelectrochemical measurements and is found to be tunable in the range of 2.03-2.63 eV. Furthermore, the oxide/oxynitride superlattice structures are fabricated by face-to-face stacking of 2D crystals using oxynitride [LaNb2 O7-x Nx ](1+x)- and oxide [Ca2 Nb3 O10 ]- nanosheets as building blocks. Moreover, the superlattices-like restacked oxynitride/oxide nanosheets hybrid exhibits unique proton conductivity and dielectric properties strongly influenced by the oxynitride nanosheets and enhanced photocatalytic activity under visible light irradiation.

Multicolor luminescent material based on interaction between TiNbO5- nanosheets and lanthanide ions for visualization of pH change in inorganic gel electrolyte.

Layered nanosheet materials showing a drastic luminescence change in response to changes in proton concentration (pH) were prepared by sandwiching Eu3+ and Tb3+ cations with anionic TiNbO5- nanosheets using electrostatic interaction. Each trivalent lanthanide ion showed a different response to pH change: a strong red emission from Eu3+ was observed at low proton concentrations (pH: 13) and a green emission from Tb3+ was dominant at high proton concentrations (pH: 1). The photoluminescence intensity was determined by the balance between the photocatalytic activity of TiNbO5- nanosheets and energy transfer from the host layer to the guest lanthanide ions. Moreover, the trivalent lanthanide/TiNbO5- nanosheet hybrid formed a gel-like solid in aqueous solution, which functioned as an inorganic gel electrolyte when mixed with Na2SO4. The multicolor luminescence (red-yellow-green) of the lanthanide/TiNbO5- nanosheet hybrid enabled direct visualization of the diffusion of protons in an inorganic gel electrolyte during water electrolysis.

Preparation of silicate nanosheets by delaminating RUB-18 for transparent, proton conducting membranes.

The present study demonstrated delamination of the layered silicate RUB-18 with no organo-modification of the silanol group. The obtained nanosheets showed a homogeneous thickness. A sponge-like material and a free-standing transparent, dense membrane were reconstructed using the nanosheets.

Preparation and Properties of Organic-Inorganic Hybrid Antireflection Films Made by a Low-Temperature Process Using Hollow Silica Nanoparticles.

Layers made of hollow silica nanoparticles have potential applications as antireflection films with lower refractive index values compared with existing materials such as silica glass (1.50) and magnesium fluoride (1.38). The advantages of such nanoparticles result from interactions between the solid shell, the cavity phase core, and the voids between particles. To obtain practical antireflection films, it is necessary to control the number of layers of these hollow silica nanoparticles and to fill the gaps between particles with a solid. In the present study, antireflection films were prepared by applying a coating of hollow silica nanoparticles dispersed in a UV-curable monomer solution onto plastic substrates. After film formation and exposure to UV light, the voids between the nanoparticles were completely filled with a polymer matrix. Tuning the particle concentration in the coating solution allowed the formation of antireflection films comprising one to three layers of the hollow silica nanoparticles. The reflectance of the films was dependent on the number of layers, and a 100 nm thick film in which two layers of hollow silica nanoparticles were precisely arranged showed the lowest reflectance of 0.92% at 550 nm wavelength, equivalent to a refractive index of 1.23. Because the voids between particles were filled with the polymer, these films resisted contamination during manual handling and so would be expected to maintain low reflectance during practical applications. This work demonstrates that nanosized inorganic-organic hybrid films composed of hollow silica nanoparticles and a UV-curable resin can exhibit optical properties and structural integrity that cannot be achieved by either substance alone.

A perfectly oriented, free-standing and transparent titania nanosheet film with the band gap of a monolayer.

A perfectly oriented, free-standing and transparent titania nanosheet film was prepared using the spin-coating technique. The free-standing film (thickness: ∼210 nm) showed a wide band gap that corresponded to that of the monolayer nanosheet despite the stacking of hundreds of layers.

Photoelectrochemical properties of a well-structured 1.3 nm-thick pn junction crystal.

A 1.3 nm-thick nickel hydroxide (p-type, 0.5 nm)/titania (n-type, 0.8 nm) pn junction prepared by lamination of nanosheets improved the onset potential for photoelectrochemical oxidation and increased the photooxidation current, indicating that ultrathin pn junctions suppress the recombination of photo-generated carriers.

Production of water-dispersible reduced graphene oxide without stabilizers using liquid-phase photoreduction.

We successfully produced water-dispersible reduced graphene oxide (rGO) by pH tuning liquid-phase photoreduction. In this method, the stabilizers and chemical modification usually used for dispersing rGO are not required. The stable carboxyl groups continue to ionize throughout the photoreduction process under alkaline conditions and continue to provide water-dispersible rGO. Moreover, the decomposition of GO into CO2 is prevented, and the production of defects is largely avoided. This is because the epoxide groups on the GO nanosheets that lead to decomposition are converted into hydroxide groups under alkaline conditions. Thus, this simple aggregation-, defect-, and stabilizer-free method is potentially important for the future application of rGO.

All-Graphene Oxide Flexible Solid-State Supercapacitors with Enhanced Electrochemical Performance.

The rapid development of flexible and wearable electronics has led to an increase in the demand for flexible supercapacitors with enhanced electrochemical performance. Graphene oxide (GO) and reduced GO (rGO) exhibit several key properties required for supercapacitor components. Although solid-state rGO/GO/rGO supercapacitors with unique structures are promising, their moderate capacitance is inadequate for practical applications. Herein, we report a flexible solid-state rGO/GO/rGO supercapacitor comprising H2SO4-intercalated GO electrolyte/separator and pseudocapacitive rGO electrodes, which demonstrate excellent electrochemical performance. The resulting supercapacitor delivered an areal capacitance of 14.5 mF cm-2, which is among the highest values achieved for any rGO/GO/rGO supercapacitor. High ionic concentration and fast ion conduction in the H2SO4-intercalated GO electrolyte/separator and abundant CH defects, which serve as pseudocapacitive sites on the rGO electrode, were responsible for the high capacitance of this device. The rGO electrode, well separated by the H2SO4 molecular spacer, supplied highly efficient ion transport channels, leading to excellent rate capability. The highly packed rGO electrode and high specific capacitance resulted in a high volumetric energy density (1.24 mWh cm-3) observed in this supercapacitor. The structure, without a clear interface between GO and rGO, provides extremely low resistance and flexibility for devices. Our device operated in air (25 °C 40%) without the use of external electrolytes, conductive additives, and binders. Furthermore, we demonstrate a simple and versatile technique for supercapacitor fabrication by combining photoreduction and electrochemical treatment. These advantages are attractive for developing novel carbon-based energy devices with high device performance and low fabrication costs.