Masato Sasase

Ammonia synthesis over Co–Mo alloy nanoparticle catalyst prepared via sodium naphthalenide-driven reduction

We report the synthesis of Co-Mo alloy nanoparticles with a uniform distribution of the alloy elements on CeO2via sodium naphthalenide-driven reduction. The resulting sample functions as a highly efficient and stable catalyst for ammonia synthesis. Based on the metal weight, the catalytic activity is ca. 20 times higher than that of Co3Mo3N.

Material Design of p‐Type Transparent Amorphous Semiconductor, Cu–Sn–I

Transparent amorphous semiconductors (TAS) that can be fabricated at low temperature are key materials in the practical application of transparent flexible electronics. Although various n-type TAS materials with excellent performance, such as amorphous In-Ga-Zn-O (a-IGZO), are already known, no complementary p-type TAS has been realized to date. Here, a material design concept for p-type TAS materials is proposed utilizing the pseudo s-orbital nature of spatially spreading iodine 5p orbitals and amorphous Sn-containing CuI (a-CuSnI) thin film is reported as an example. The resulting a-CuSnI thin films fabricated by spin coating at low temperature (140 °C) have a smooth surface. The Hall mobility increases with the hole concentration and the largest mobility of ≈9 cm2 V-1 s-1 is obtained, which is comparable with that of conventional n-type TAS.

Effects of working pressure and annealing on bulk density and nanopore structures in amorphous In–Ga–Zn–O thin-film transistors

Microstructures of amorphous In–Ga–Zn–O (a-IGZO) thin films of different densities were analyzed. Device-quality a-IGZO films were deposited under optimum conditions, e.g., the total pressure P tot = 0.55 Pa produced high film densities of ~6.1 g/cm3, while a very high P tot = 5.0 Pa produced low film densities of 5.5 g/cm3. Both films formed uniform high-density layers in the vicinity of the glass substrate, 10–20 nm in thickness depending on P tot, while their growth mode changed to a sparse columnar structure in thicker regions. X-ray reflectivity and in situ spectroscopic ellipsometry provided different results on densification by post deposition thermal annealing; i.e., the latter has a higher sensitivity. High-Z-contrast images obtained by high-angle annular dark-field scanning transmission electron microscopy were also useful for detecting nanometer-size non uniformity even in device-quality a-IGZO films.

Self-organized Ruthenium-Barium Core-Shell Nanoparticles on a Mesoporous Calcium Amide Matrix for Efficient Low-Temperature Ammonia Synthesis

A low-temperature ammonia synthesis process is required for on-site synthesis. Barium-doped calcium amide (Ba-Ca(NH2 )2 ) enhances the efficacy of ammonia synthesis mediated by Ru and Co by 2 orders of magnitude more than that of a conventional Ru catalyst at temperatures below 300 °C. Furthermore, the presented catalysts are superior to the wüstite-based Fe catalyst, which is known as a highly active industrial catalyst at low temperatures and pressures. Nanosized Ru-Ba core-shell structures are self-organized on the Ba-Ca(NH2 )2 support during H2 pretreatment, and the support material is simultaneously converted into a mesoporous structure with a high surface area (>100 m2  g-1 ). These self-organized nanostructures account for the high catalytic performance in low-temperature ammonia synthesis.

Chlorine-Tolerant Ruthenium Catalyst Derived Using the Unique Anion-Exchange Properties of 12 CaO⋅7 Al2O3 for Ammonia Synthesis

Poisoning is defined as deactivation by the strong adsorption of, usually, impurities on active sites, which is a serious problem in many important catalytic processes. For example, the activity of Ru catalysts is significantly degraded by Cl in ammonia synthesis because of its electron‐withdrawing properties. Here we demonstrate that 12 CaO⋅7 Al2O3 with subnanometer‐sized cages prevents the poisoning of a Ru catalyst by Cl− ions in ammonia synthesis. Conventional supported Ru catalysts exhibit a negligible activity if a tiny amount of Cl− ions remain on the catalyst surface. In contrast, the catalytic activity of Ru/C12A7 is not influenced by Cl− ions even though the amount of Cl in Ru/C12A7 is one order of magnitude higher than that in the conventional Ru catalysts. The Cl resistance of Ru/C12A7 is attributed to the unique anion‐exchange properties of C12A7; that is, the Cl− ions are trapped preferentially in the positively charged subnanometer‐sized cages of C12A7 instead of OH− ions under ammonia synthesis conditions.

Large Oblate Hemispheroidal Ruthenium Particles Supported on Calcium Amide as Efficient Catalysts for Ammonia Decomposition

Ammonia decomposition is an important technology for extracting hydrogen from ammonia toward the realization of a hydrogen economy. Herein, it is reported that large oblate hemispheroidal Ru particles on Ca(NH2 )2 function as efficient catalysts for ammonia decomposition. The turnover frequency of Ru/Ca(NH2 )2 increased by two orders of magnitude when the Ru particle size was increased from 1.5 to 8.4 nm. More than 90 % ammonia decomposition was achieved over Ru/Ca(NH2 )2 with large oblate hemispheroidal Ru particles at 360 °C, which is comparable to that of alkali-promoted Ru catalysts with small Ru particle sizes. XAFS analyses revealed that Ru particles are immobilized on Ca(NH2 )2 by Ru-N bonds formed at the metal/support interface, which lead to oblate hemispheroidal Ru particles. Such a strong metal-support interaction in Ru/Ca(NH2 )2 is also substantiated by DFT calculations. The high activity of Ru/Ca(NH2 )2 with large Ru particles primarily originates from the shape and appropriate size of the Ru particles with a high density of active sites rather than the electron-donating ability of Ca(NH2 )2 .

High-Mobility p-Type and n-Type Copper Nitride Semiconductors by Direct Nitriding Synthesis and In Silico Doping Design

Thin-film photovoltaics (PV) have emerged as a technology that can meet the growing demands for efficient and low-cost large-scale cells. However, the photoabsorbers currently in use contain expensive or toxic elements, and the difficulty in bipolar doping, particularly in a device structure, requires elaborate optimization of the heterostructures for improving the efficiency. This study shows that bipolar doping with high hole and electron mobilities in copper nitride (Cu3 N), composed solely of earth-abundant and environmentally benign elements, is readily available through a novel gaseous direct nitriding reaction applicable to uniform and large-area deposition. A high-quality undoped Cu3 N film is essentially an n-type semiconductor, while p-type conductivity is realized by interstitial fluorine doping, as predicted using density functional theory calculations and directly proven by atomically resolved imaging. The synthetic methodology for high-quality p-type and n-type films paves the way for the application of Cu3 N as an alternative absorber in thin-film PV.

Lead-Free Highly Efficient Blue-Emitting Cs3Cu2I5 with 0D Electronic Structure

Halide perovskites, including CsPbX3 (X = Cl, Br, I), have gained much attention in the field of optoelectronics. However, the toxicity of Pb and the low photoluminescence quantum yield (PLQY) of these perovskites hamper their use. In this work, new halide materials that meet the requirements of: (i) nontoxicity, (ii) high PLQY, and (iii) ease of fabrication of thin films via the solution process are explored. In particular, copper(I) halide compounds with low-dimensional electronic structures are considered. Cs3 Cu2 I5 has a 0D photoactive site and exhibits blue emission (≈445 nm) with very high PLQYs of ≈90 and ≈60% for single crystals and thin films, respectively. The large exciton binding energy of ≈490 meV explains well the 0D electronic nature of Cs3 Cu2 I5 . Blue electroluminescence of Pb-free halides is demonstrated using solution-derived Cs3 Cu2 I5 thin films.

Why high-pressure sputtering must be avoided to deposit a-In-Ga-Zn-O films

Film density of amorphous In-Ga-Zn-O (a-IGZO) was varied in a wide range to investigate the origin of the low film density and its effect on thin-film transistor (TFT) characteristics. Device-quality a-IGZO films have the densities ~ 6.1 g/cm3, which is ~5% smaller than that of single-crystal InGaZnO4 (c-IGZO) (6.4 g/cm3). On the other hand, extremely low density of 5.5 g/cm3 was obtained when the sputtering working pressure (PTot) was increased to 5 Pa. High density desorption of H2O and O2 was detected in the low-density films, which are attributed to an origin of the low density. Although the low-density channel produced poor TFTs, good TFT characteristics were obtained by annealing at Tann = 300°C. The densities of the low-density films obtained by X-ray reflectivity (XRR) analysis were, however, almost unchanged up to Tann = 500°C, while spectroscopic ellipsometry (SE) analysis showed that densification started from 100°C. This contradiction is explained by transmission electron microscopy (TEM). Although conventional high-resolution TEM (HR-TEM) observation could not detect a microstructure in the high-density a-IGZO films, high-angle annular dark field scanning TEM (HAADF-STEM) detected nano-scale low-density regions. The low-density films had larger and more voids. These void structures were not found in very thin regions (5-10 nm from the substrate surface) but increased in thicker regions.