Metal species with different size (single atoms, nanoclusters, and nanoparticles) show different catalytic behavior for various heterogeneous catalytic reactions. It has been shown in the literature that many factors including the particle size, shape, chemical composition, metal–support interaction, and metal–reactant/solvent interaction can have significant influences on the catalytic properties of metal catalysts. The recent developments of well-controlled synthesis methodologies and advanced characterization tools allow one to correlate the relationships at the molecular level. In this Review, the electronic and geometric structures of single atoms, nanoclusters, and nanoparticles will be discussed. Furthermore, we will summarize the catalytic applications of single atoms, nanoclusters, and nanoparticles for different types of reactions, including CO oxidation, selective oxidation, selective hydrogenation, organic reactions, electrocatalytic, and photocatalytic reactions. We will compare the results o...

Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles.

Transparent conductors as solar energy materials: A panoramic review

The response of the worldwide scientific community to the discovery in 2008 of superconductivity at T c = 26 K in the Fe-based compound LaFeAsO1−x F x has been very enthusiastic. In short order, ot...

The puzzle of high temperature superconductivity in layered iron pnictides and chalcogenides

Over the past few decades, the design and development of advanced electrocatalysts for efficient energy conversion technologies have been subjects of extensive study. With the discovery of graphene, two-dimensional (2D) nanomaterials have emerged as some of the most promising candidates for heterogeneous electrocatalysts due to their unique physical, chemical, and electronic properties. Here, we review 2D-nanomaterial-based electrocatalysts for selected electrocatalytic processes. We first discuss the unique advances in 2D electrocatalysts based on different compositions and functions followed by specific design principles. Following this overview, we discuss various 2D electrocatalysts for electrocatalytic processes involved in the water cycle, carbon cycle, and nitrogen cycle from their fundamental conception to their functional application. We place a significant emphasis on different engineering strategies for 2D nanomaterials and the influence these strategies have on intrinsic material performance, ...

Emerging Two-Dimensional Nanomaterials for Electrocatalysis

Kamihara and coworkers' report of superconductivity at ${T}_{c}=26\text{ }\text{ }\mathrm{K}$ in fluorine-doped LaFeAsO inspired a worldwide effort to understand the nature of the superconductivity in this new class of compounds. These iron pnictide and chalcogenide (FePn/Ch) superconductors have Fe electrons at the Fermi surface, plus an unusual Fermiology that can change rapidly with doping, which lead to normal and superconducting state properties very different from those in standard electron-phonon coupled ``conventional'' superconductors. Clearly, superconductivity and magnetism or magnetic fluctuations are intimately related in the FePn/Ch, and even coexist in some. Open questions, including the superconducting nodal structure in a number of compounds, abound and are often dependent on improved sample quality for their solution. With ${T}_{c}$ values up to 56 K, the six distinct Fe-containing superconducting structures exhibit complex but often comparable behaviors. The search for correlations and explanations in this fascinating field of research would benefit from an organization of the large, seemingly disparate data set. This review provides an overview, using numerous references, with a focus on the materials and their superconductivity.

Superconductivity in iron compounds

The efficient reduction of atmospheric nitrogen to ammonia under low pressure and temperature conditions has been a challenge in meeting the rapidly increasing demand for fertilizers and hydrogen storage. Here, we report that Ca2N:e−, a two-dimensional electride, combined with ruthenium nanoparticles (Ru/Ca2N:e−) exhibits efficient and stable catalytic activity down to 200 °C. This catalytic performance is due to [Ca2N]+·e1−x−Hx− formed by a reversible reaction of an anionic electron with hydrogen (Ca2N:e− + xH ↔ [Ca2N]+·e1−x−Hx−) during ammonia synthesis. The simplest hydride, CaH2, with Ru also exhibits catalytic performance comparable to Ru/Ca2N:e−. The resultant electrons in these hydrides have a low work function of 2.3 eV, which facilitates the cleavage of N2 molecules. The smooth reversible exchangeability between anionic electrons and H− ions in hydrides at low temperatures suppresses hydrogen poisoning of the Ru surfaces. The present work demonstrates the high potential of metal hydrides as efficient promoters for low-temperature ammonia synthesis.

https://pubs.rsc.org/en/content/articlepdf/2016/sc/c6sc00767h

Essential role of hydride ion in ruthenium-based ammonia synthesis catalysts

Electrides are ionic crystals in which the anionic electrons are confined to interstitial subnanometer-sized spaces. At present, the reported electrides only consist of main-group elements. Here, w...

Two-Dimensional Transition-Metal Electride Y2C

High-throughput ab initio screening of approximately 34000 materials in the Materials Project was conducted to identify two-dimensional (2D) electride materials, which are composed of cationic layers and anionic electrons confined in a 2D empty space. The screening was based on three indicators: (1) a positive total formal charge per formula unit; (2) layered structures for two-dimensionality; (3) empty spaces between the layer units. Three nitrides, Ca2N, Sr2N, and Ba2N, and the carbide Y2C were identified as 2D electrides, where Ca2N is the only experimentally confirmed 2D electride (Lee, K.; et al. Nature 2013, 494, 336-341). Electron density analysis using ionic radii revealed a smaller number of anionic electrons in Y2C than those in the three nitrides as a result of the partial occupation of the anionic electrons in the d orbitals of Y. In addition, no candidates were identified from the p-block elements, and thus the ab initio screening indicates that the s-block elements (i.e., alkali or alkaline-earth metals) are highly preferable as cation elements. To go beyond the database screening, a tailored modeling was conducted to determine unexplored compounds including the s-block elements that are suitable for 2D electrides. The tailored modeling found that (1) K2Cl, K2Br, Rb2Cl, and Rb2Br dialkali halides are highly plausible candidates, (2) Li2F and Na2Cl dialkali halides are highly challenging candidates, and (3) the Cs2O(1-x)F(x) halogen-doped dialkali oxide is a promising candidate.

High-throughput ab initio screening for two-dimensional electride materials.

Utilizing the high stability of calcium and rare-earth hydrides, CaFeAsF${}_{1\ensuremath{-}x}$H${}_{x}$ ($x$ $=$ 0.0--1.0) and SmFeAsO${}_{1\ensuremath{-}x}$H${}_{x}$ ($x$ $=$ 0.0--0.47) have been synthesized using high pressure to form hydrogen-substituted 1111-type iron-arsenide superconductors. Neutron diffraction and density functional calculations have demonstrated that the hydrogens are incorporated as H${}^{\ensuremath{-}}$ ions occupying F${}^{\ensuremath{-}}$ sites in the blocking layer of CaFeAsF. The resulting CaFeAsF${}_{1\ensuremath{-}x}$H${}_{x}$ is nonsuperconducting, whereas, SmFeAsO${}_{1\ensuremath{-}x}$H${}_{x}$ is a superconductor, with an optimal ${T}_{\mathrm{c}}$ $=$ 55 K at $x$ \ensuremath{\sim} 0.2. It was found that up to 40% of the O${}^{2\ensuremath{-}}$ ions can be replaced by H${}^{\ensuremath{-}}$ ions, with electrons being supplied into the FeAs layer to maintain neutrality (O${}^{2\ensuremath{-}}$ $=$ H${}^{\ensuremath{-}}$ $+$ ${e}^{\ensuremath{-}}$). When $x$ exceeded 0.2, ${T}_{\mathrm{c}}$ was reduced corresponding to an electron overdoped region.

Hydrogen in layered iron arsenides: Indirect electron doping to induce superconductivity

Hydrogen is an impurity species having an important role in the physical properties of semiconductors. Despite numerous studies, the role of hydrogen in oxide semiconductors remains an unsolved puzzle. This situation arises from insufficient information about the chemical state of the impurity hydrogen. Here, we report direct evidence for anionic hydrogens bonding to metal cations in amorphous In–Ga–Zn–O (a-IGZO) thin films for thin-film transistors (TFT) applications and discuss how the hydrogen impurities affect the electronic structure of a-IGZO. Infrared absorption spectra of self-standing a-IGZO thin films prepared by sputtering reveal the presence of hydrogen anions as a main hydrogen species (concentration is ∼1020 cm−3) along with the hydrogens in the form of the hydroxyl groups (∼1020 cm−3). Density functional theory calculations show that bonds between these hydride ions with metal centers give rise to subgap states above the top of the valence band, implying a crucial role of anionic hydrogen in the negative bias illumination stress instability commonly observed in a-IGZO TFTs.

/pdf/hydrogen-anion-and-subgap-states-in-amorphous-in-ga-zn-o-1at5l5jpbw.pdf

Satoru Matsuishi

Papers

Essential role of hydride ion in ruthenium-based ammonia synthesis catalysts

Two-Dimensional Transition-Metal Electride Y2C

High-throughput ab initio screening for two-dimensional electride materials.

Hydrogen in layered iron arsenides: Indirect electron doping to induce superconductivity

Hydrogen anion and subgap states in amorphous In–Ga–Zn–O thin films for TFT applications