Yoshitake Toda

Dicalcium nitride as a two-dimensional electride with an anionic electron layer

Recent studies suggest that electrides—ionic crystals in which electrons serve as anions—are not exceptional materials but rather a generalized form, particularly under high pressure. The topology of the cavities confining anionic electrons determines their physical properties. At present, reported confining sites consist only of zero-dimensional cavities or weakly linked channels. Here we report a layered-structure electride of dicalcium nitride, Ca2N, which possesses two-dimensionally confined anionic electrons whose concentration agrees well with that for the chemical formula of [Ca2N]+·e−. Two-dimensional transport characteristics are demonstrated by a high electron mobility (520 cm2 V−1 s−1) and long mean scattering time (0.6 picoseconds) with a mean free path of 0.12 micrometres. The quadratic temperature dependence of the resistivity up to 120 Kelvin indicates the presence of an electron–electron interaction. A striking anisotropic magnetoresistance behaviour with respect to the direction of magnetic field (negative for the field perpendicular to the conducting plane and positive for the field parallel to it) is observed, confirming diffusive two-dimensional transport in dense electron layers. Additionally, band calculations support confinement of anionic electrons within the interlayer space, and photoemission measurements confirm anisotropic low work functions of 3.5 and 2.6 electronvolts, revealing the loosely bound nature of the anionic electrons. We conclude that Ca2N is a two-dimensional electride in terms of [Ca2N]+·e−.

Electron field emission from TiO2 nanotube arrays synthesized by hydrothermal reaction

Conductive TiO2 nanotube arrays were grown on metal Ti substrates by hydrothermal reaction and subsequent postannealing in vacuum. The nanotubes were vertically grown and adhered well to the substrates. The crystal structure of the postannealed TiO2 nanotubes was identified to be oxygen-defective anatase. The nanotube arrays exhibited efficient electron field emission even at room temperature with rather low turn-on fields ∼280V per electrode distance of 100μm. The emission current density exceeded 0.15mA∕cm2 at an extraction voltage of 800V. The emission current was reproducible and stable in the lower voltage (<800V) region.

Activation and splitting of carbon dioxide on the surface of an inorganic electride material

Activation of carbon dioxide is the most important step in its conversion into valuable chemicals. Surfaces of stable oxide with a low work function may be promising for this purpose. Here we report that the surfaces of the inorganic electride [Ca24Al28O64]4+(e−)4 activate and split carbon dioxide at room temperature. This behaviour is attributed to a high concentration of localized electrons in the near-surface region and a corrugation of the surface that can trap oxygen atoms and strained carbon monoxide and carbon dioxide molecules. The [Ca24Al28O64]4+(e−)4 surface exposed to carbon dioxide is studied using temperature-programmed desorption, and spectroscopic methods. The results of these measurements, corroborated with ab initio simulations, show that both carbon monoxide and carbon dioxide adsorb on the [Ca24Al28O64]4+(e−)4 surface at RT and above and adopt unusual configurations that result in desorption of molecular carbon monoxide and atomic oxygen upon heating.

Ammonia decomposition by ruthenium nanoparticles loaded on inorganic electride C12A7:e−

The use of ammonia as a hydrogen carrier has received much attention due to its high hydrogen content and liquid state under mild conditions, which could lead to fuel cell applications. This study demonstrates facile ammonia decomposition on ruthenium nanoparticles loaded on inorganic electride, C12A7:e−. A high turnover frequency (∼12 s−1 at 400 °C) and low activation energy (64 kJ mol−1) for recombinative N2 desorption were obtained for Ru/C12A7:e−. N2-temperature programmed desorption (N2-TPD) and kinetic analyses indicate that the high catalytic performance is due to the low work function of chemically stable C12A7:e−, which enables electron injection to the antibonding orbital of the Ru–N bond formed transiently through the reaction by raising the Fermi level of Ru metal.

Formation of inorganic electride thin films via site-selective extrusion by energetic inert gas ions

Inert gas ion implantation (acceleration voltage 300kV) into polycrystalline 12CaO·7Al2O3 (C12A7) films was investigated with fluences from 1×1016 to 1×1017cm−2 at elevated temperatures. Upon hot implantation at 600°C with fluences greater than 1×1017cm−2, the obtained films were colored and exhibited high electrical conductivity in the as-implanted state. The extrusion of O2− ions encaged in the crystallographic cages of C12A7 crystal, which leaves electrons in the cages at concentrations up to ∼1.4×1021cm−3, may cause the high electrical conductivity. On the other hand, when the fluence is less than 1×1017cm−2, the as-implanted films are optically transparent and electrically insulating. The conductivity is enhanced and the films become colored by irradiating with ultraviolet light due to the formation of F+-like centers. The electrons forming the F+-like centers are photo released from the encaged H− ions, which are presumably derived from the preexisting OH− groups. The induced electron concentration ...

Intense thermal field electron emission from room-temperature stable electride

Thermal field emission (TFE) from a flat surface of 12CaO∙7Al2O3 (C12A7) electride was examined at temperatures up to 900°C and applied external voltages of 0–6kV in a 10−5Pa vacuum. TFE started to occur at ∼650°C and steeply increased at ∼900°C to reach ∼80μA (∼1.5Acm−2) when an extraction field of 105Vcm−1 (an extraction voltage of 6kV) was applied. The work function estimated from the Richardson–Dushman equation was ∼2.1eV, which is rather smaller than that of LaB6 (∼2.7eV). It is experimentally confirmed that the emission with a current of ∼50μA (from a 80μm diameter area) was stably sustained for more than 90h. The efficient and stable emission at relatively low temperature in a moderate vacuum atmosphere strongly suggests that the C12A7 electride has high potential for TFE applications.

Water Durable Electride Y5Si3: Electronic Structure and Catalytic Activity for Ammonia Synthesis

We report an air and water stable electride Y5Si3 and its catalytic activity for direct ammonia synthesis. It crystallizes in the Mn5Si3-type structure and confines 0.79/f.u. anionic electrons in the quasi-one-dimensional holes. These anionic electrons strongly hybridize with yttrium 4d electrons, giving rise to improved chemical stability. The ammonia synthesis rate using Ru(7.8 wt %)-loaded Y5Si3 was as high as 1.9 mmol/g/h under 0.1 MPa and at 400 °C with activation energy of ∼50 kJ/mol. Its strong electron-donating ability to Ru metal of Y5Si3 is considered to enhance nitrogen dissociation and reduce the activation energy of ammonia synthesis reaction. Catalytic activity was not suppressed even after Y5Si3, once dipped into water, was used as the catalyst promoter. These findings provide novel insights into the design of simple catalysts for ammonia synthesis.

Surface of Room-Temperature-Stable Electride [Ca24Al28O64]4+(e−)4: Preparation and Its Characterization by Atomic-Resolution Scanning Tunneling Microscopy

The nanocage compound crystal [Ca24Al28O64]4+(e-)4 (C12A7:e-) is a room-temperature-stable electride. Although bulk C12A7:e- exhibits metallic conduction, the surface of an as-prepared sample or one prepared by mechanical fracture in ultrahigh vacuum is almost insulating and exhibits distinct non-ohmic contact. We studied whether the intrinsic surface of this electride exhibits metallic conduction or not by examining various conditions for preparing the intrinsic surface. A combination of sputtering with thermal annealing led to the emergence of metallic conductivity in a specific condition. Suitably prepared surfaces revealed ohmic contact even in an ambient atmosphere. Atomic-resolution scanning tunneling microscopy (STM) images of the surfaces were consistent with a structural model in which the cage structure in the bulk C12A7:e- electride is conserved at the surface.

n -type conversion of SnS by isovalent ion substitution: Geometrical doping as a new doping route

Tin monosulfide (SnS) is a naturally p-type semiconductor with a layered crystal structure, but no reliable n-type SnS has been obtained by conventional aliovalent ion substitution. In this work, carrier polarity conversion to n-type was achieved by isovalent ion substitution for polycrystalline SnS thin films on glass substrates. Substituting Pb2+ for Sn2+ converted the majority carrier from hole to electron, and the free electron density ranged from 1012 to 1015 cm−3 with the largest electron mobility of 7.0 cm2/(Vs). The n-type conduction was confirmed further by the position of the Fermi level (EF) based on photoemission spectroscopy and electrical characteristics of pn heterojunctions. Density functional theory calculations reveal that the Pb substitution invokes a geometrical size effect that enlarges the interlayer distance and subsequently reduces the formation energies of Sn and Pb interstitials, which results in the electron doping.

Electronic insulator-conductor conversion in hydride ion-doped 12CaO∙7Al2O3 by electron-beam irradiation

We report formation of persistent carrier electrons in hydride ion (H−)-incorporated 12CaO∙7Al2O3 (C12A7) by electron-beam irradiation. The electrical conductivity of H−-doped C12A7 single crystals increases with the electron-beam irradiation dose, accompanied with a green coloration attributable to a carrier electron formation. A 25 keV electron beam with a dose of ∼500μCcm−2 fully converts the conductivity in surface layers to the depth of ∼4μm. Carrier electron formation is most likely due to electron-hole pairs generated in the electron excitation volume and subsequent energy transfer to the H− ions. The estimated carrier formation yield per an incident electron is ∼30. These findings may enable a fine patterning of the conductive area without photomasks and photoresists.