Eutectic salt mixture-assisted sodium-vapor-induced synthesis of Pt-Ca nanoparticles, and their microstructural and electrocatalytic properties.

We have fabricated Pt-Ca nanoparticles with oxygen reduction reaction catalytic activity via a sodium vapor-induced synthesis method. Prior addition of NaCl to form a eutectic mixture of CaCl2 and NaCl facilitated the formation of intermetallic Pt2Ca nanoparticles. Pt3Mg and Pt5Sr nanoparticles also were suggested to be producible.

Sodium Vapor-Induced Synthesis of Intermetallic Pt5Ce Compound Nanoparticles.

We have developed a synthetic route that uses sodium for the production of intermetallic Pt5Ce nanoparticles (ca. 6 nm average diameter) supported on carbon powder. Sodium melt was demonstrated to reduce a powder mixture of PtCl2 and CeCl3 to form submicrometer Pt5Ce particles with the simultaneous formation of NaCl. The NaCl-CeCl3 melt mixture and Na melt were formed during heating, which led to a uniform reaction between Pt and Ce, and the melt induced grain growth. The synthetic procedures were then modified to supply sodium vapor to the vicinity of the metal sources supported on carbon powder with an aim to suppress grain growth. Pt5Ce nanoparticles were successfully formed on the carbon support with high loading and dispersity.

Highly active and noble-metal-alternative hydrogenation catalysts prepared by dealloying Ni-Si intermetallic compounds.

Noble-metal-alternative Ni-Si catalysts more active than Pd in the hydrogen storage reaction were developed using a unique procedure, i.e., surface dealloying with hydrofluoric acid treatment. The combination of the structural analysis and the DFT calculation revealed a specific active site structure, a Ni cluster embedded in a SiO2 matrix, and its unprecedented role in the molecular conversion.

Na-Melt Synthesis of Fine Ni3Si Powders as a Hydrogenation Catalyst.

Fine Ni3Si powders with high phase purity were successfully produced using a Na melt. The particle surface was less oxidized than that of metallurgically prepared Ni3Si. The Ni3Si catalyst exhibited much higher activities than Ni for the hydrogenation of various unsaturated hydrocarbons.

Photocatalytic NO removal over calcium-bridged siloxenes under ultraviolet and visible light irradiation.

Ca-Bridged siloxenes (Ca-siloxenes) composed of two-dimensional siloxene planes with Ca bridging were prepared and their photocatalytic properties for nitrogen oxide (NO) removal were investigated. Ca-Siloxenes were synthesized via a solid-state metathesis reaction using TaCl5 to extract Ca from CaSi2 with different Cl2/Ca molar ratios of 0.25, 1.25 and 2.5 (CS0.25, CS1.25 and CS2.5, respectively) in an attempt to control the extent of Ca extraction. Ca-Siloxenes have a wide optical absorption band from the visible to ultraviolet region with absorption edges of 1.5, 2.9, and 3.1 eV for CS0.25, CS1.25, and CS2.5, respectively. Ca-Siloxenes exhibited photocatalytic activity for NO removal under irradiation with visible (λ > 400 nm (<3.10 eV)) and ultraviolet light (λ > 290 nm (<4.28 eV)). The photocatalytic activity was particularly improved by mixing the Ca-siloxene with acetylene black as a conductive material, which might have inhibited the recombination of photogenerated electrons and holes. The mixture of Ca-siloxene and acetylene black exhibited improved photocatalytic activity in the presence of 1O2 as one of the active oxygen species formed under ultraviolet light irradiation.

Photocatalytic activity of silicon-based nanoflakes for the decomposition of nitrogen monoxide.

The photocatalytic decomposition of nitrogen monoxide (NO) was achieved for the first time using Si-based nanomaterials. Nanocomposite powders composed of Si nanoflakes and metallic particles (Ni and Ni3Si) were synthesized using a simple one-pot reaction of layered CaSi2 and NiCl2. The synthesized nanocomposites have a wide optical absorption band from the visible to the ultraviolet. Under the assumption of a direct transition, the photoabsorption behavior is well described and an absorption edge of ca. 1.8 eV is indicated. Conventional Si and SiO powders with indirect absorption edges of 1.1 and 1.4 eV, respectively, exhibit considerably low photocatalytic activities for NO decomposition. In contrast, the synthesized nanocomposites exhibited photocatalytic activities under irradiation with light at wavelengths >290 nm (<4.28 eV). The photocatalytic activities of the nanocomposites were confirmed to be constant and did not degrade with the light irradiation time.

Stabilized lithium-ion battery anode performance by calcium-bridging of two dimensional siloxene layers.

A Ca-bridged siloxene (Ca-siloxene) composed of two-dimensional siloxene planes with Ca bridging was synthesized via a solid state metathesis reaction using TaCl5 to extract Ca from CaSi2. Three different Ca-siloxenes synthesized at Cl2/Ca molar ratios of 0.25, 1.25 and 2.5 (CS0.25, CS1.25 and CS2.5, respectively) were fabricated and investigated as anode active materials for lithium-ion batteries. Both secondary and primary Ca-siloxene particles, which serve to increase the contact interfaces with conductive materials and to generate accessible sites for lithium ions, respectively, were found to become smaller and to have increased pore volumes as the Cl2/Ca molar ratio was increased. These Ca-siloxenes exhibited stable charge/discharge performance as anode materials, with 69-99% capacity retention after 50 charge/discharge cycles (compared with 36% retention for a conventional Kautsky-type siloxene). The charge capacity also increased with increases in the Cl2/Ca molar ratio, such that the CS2.5 showed the highest capacity after 50 charge/discharge cycles. This may reflect the formation of Si6Li6 rather than SiLi4.4 and suggests the maintenance of layered Si planes for large capacity retention after charge/discharge cycling. The increase of contact interfaces between acetylene black (as a conductive material) and Ca-siloxenes was found to effectively increase the lithium-ion capacity of Ca-siloxene during high rate charge/discharge cycling.

Ordered CaSi2 Microwall Arrays on Si Substrates Induced by the Kirkendall Effect.

We have specified the synthetic conditions to obtain one-directionally ordered CaSi2 microwall arrays vertically grown on a Si substrate. Our basic concept is based on the utilization of the Kirkendall effect for reactive deposition epitaxy (RDE). We found for the first time that: 1) a much larger Ca vapor supply on the Si substrate than the conventional RDE, 2) the adoption of a two-step heating process, and 3) the selection of the crystal axis of the Si surface are the keys to control the microstructures of CaSi2 on the Si substrate. The CaSi phase was first formed on Si, then the CaSi2 phase was formed at the CaSi/Si interface. Based on the Kirkendall effect, the interdiffusion of Ca and Si was enhanced in the vertical direction rather than in the parallel direction to the Si surface. CaSi2 tends to grow along four equivalent Si{111} planes, however, the specific orientation of the Si surface resulted in CaSi2 microwalls grown along its Si(111‾ ) plane, the only plane directing nearly vertical to the surface among the Si{111} planes. These results suggest that the Kirkendall effect under asymmetric growth of target materials would be a rational strategy to obtain their ordered microstructures.

Facile and scalable synthesis of silicon-based nanocomposites with slitlike nanopores: a solid-state exfoliation reaction using layered CaSi2.

Silicon-based nanocomposites with slitlike nanopores were prepared by heating a mixture of layered CaSi2 and NiCl2. The formation mechanism is based on a solid-state exfoliation reaction wherein the formation of CaCl2 promotes the extraction of Ca from CaSi2, thereby exfoliating the layered structure. The nanocomposites showed anode capacity for lithium ion batteries up to 804 mA h g(-1).

The formation mechanism of a textured ceramic of thermoelectric [Ca2CoO3](0.62)[CoO2] on beta-Co(OH)2 templates through in situ topotactic conversion.

We investigated the formation mechanism of thermoelectric [Ca(2)CoO(3)](0.62)[CoO(2)] (CCO) on beta-Co(OH)(2) templates with maintained orientations by identifying the intermediate phases and specifying the relationship between their crystallographic orientations. We mixed beta-Co(OH)(2) templates with the complementary reactant CaCO(3) and prepared a compact by tape casting, with the developed (001) plane of the templates aligned along the casting plane. High-temperature XRD of the compact revealed that beta-Co(OH)(2) decomposed into Co(3)O(4) by 873 K, and Co(3)O(4) reacted with CaO to form CCO by 1193 K via the formation of the newly detected intermediate phase beta-Na(x)()CoO(2)-type Ca(x)()CoO(2) at 913-973 K. Pole figure measurements and SEM and TEM observations revealed that the relationship between the crystallographic planes was (001) beta-Co(OH)(2)//{111} Co(3)O(4)//(001) Ca(x)()CoO(2)//(001) CCO. The crystal structures of the four materials possess the common CoO(2) layer (or similar), which is composed of edge-sharing CoO(6) octahedra, parallel to the planes. The cross-sectional HRTEM analysis of an incompletely reacted specimen showed transient lattice images from Ca(x)()CoO(2) into CCO, in which every other CoO(2) layer of Ca(x)()CoO(2) was preserved. Thus, it was demonstrated that a textured CCO ceramic is produced through a series of in situ topotactic conversion reactions with a preserved CoO(2) layer of its template.