The structural and electronic properties of the hydrides of silicene and germanene have been studied using ab initio calculations. The trend for the M-H (M=C, Si, Ge) bond lengths, and corresponding bond energies, is consistent with the atomic size trend, and comparable to those of MH_4 hydrides. Band structures were also obtained for the buckled configuration, which is the stable form for both silicene and germanene. Upon hydrogenation, both silicane (indirect gap) and germanane (direct gap) are semiconducting.
Laterally large (~3 micrometers), atomically-thin two-dimensional (2D) Bi2O2CO3 nanosheets (2D bismuth oxycarbonate, 2D bismutite) are fabricated via sonochemically-assisted template-free synthesis. Key to the synthesis of the freestanding, laterally large 2D Bi2O2CO3 nanosheets from bulk Bi powder is choice of suspension medium, controlled reaction temperatures and several hours processing time. Lateral sizes of 2D Bi2O2CO3 can be controlled between micrometer-sized nanosheets and tens of nm sized nanoflakes solely based on the choice of suspension medium. The here introduced 2D Bi2O2CO3 nano…
We have synthesized MoS$_{2}$ incorporated $α$-Fe$_{2}$O$_{3}$/ZnO nanocomposites by the hydrothermal process. The effect of incorporating ultrasonically exfoliated MoS$_{2}$ on the photocatalytic performance of $α$-Fe$_{2}$O$_{3}$/ZnO nanocomposites has been demonstrated. Structural, morphological and optical characteristics of the nanomaterials are investigated by performing Rietveld refinement of powder X-ray diffraction patterns, field emission scanning electron microscopy and UV-visible spectroscopy. The photoluminescence spectra of the nanocomposites show that the recombination of photog…
The rational design of transition metal dichalcogenide electrocatalysts for efficiently catalyzing hydrogen evolution reaction (HER) is believed to lead to the generation of a renewable energy carrier. To this end our work has made three main contributions. At first, we have demonstrated that exfoliation via ionic liquid assisted grinding combined with gradient centrifugation is an efficient method to exfoliate bulk WS2 to nanosheets with a thickness of a few atomic layers and lateral size dimensions in the range of 100 nm to 2 nm. These WS2 nanosheets decorated with scattered nanodots exhibit…
Hydrogen ordering influence on the proton spin-lattice relaxation time in metal-hydrogen compoun Hydrogen ordering influence on the proton spin-lattice relaxation time in metal-hydrogen compounds
2D semiconducting nanosheets of Transition Metal Dichalcogenides are attractive materials for solar energy conversion because of their unique absorption properties. Here, we propose Mo thio- and oxo-thio-complexes anchored on 2D p-WSe2 nanosheets for efficient water splitting under visible light irradiation with photocurrent density up to 2.0 mA cm-2 at -0.2 V/NHE. Besides developing high electro-catalytic activity, the Mo complexe films were shown to display ability to heal surface defects. We propose that the observed healing of surface defects arises from the strong adsorption on point defe…
Computational Study of Water Adsorption and Dissociative Mechanisms Impacting g-C3N4's Optical and Electronic Properties
Hefei Institutes of Physical Science, Chinese Academy of Sciences（中国科学院合肥物质科学研究院）；University of Science and Technology of China（中国科学技术大学）；NOMATEN Centre of Excellence, National Centre for Nuclear Research（国家核研究中心NOMATEN卓越中心）；Institute of Theoretical Physics, Faculty of Physics, University of Warsaw（华沙大学物理学院理论物理研究所）；Institute of Physics Ministry of Science and Education Republic of Azerbaijan（阿塞拜疆共和国教育与科学部物理研究所）；University of Electronic Science and Technology of China（电子科技大学）
We present an evolutionary construction technique of formation energy convex hull to search for thermodynamically stable compounds. In this technique, candidates with a wide variety of chemical compositions and crystal structures are created by systematically applying evolutionary operators, "mating", "mutation", and "adaptive mutation", to two target compounds, and the convex hull is directly updated through the evolution. We applied the technique to carbon-hydrogen binary system at 10 GPa and obtained 15 hydrocarbons within the convex hull distance less than 0.5 mRy/atom: graphane, polybutad…
The search for suitable materials for solid-state stationary storage of green hydrogen is pushing the implementation of efficient renewable energy systems. This involves rational design and modification of cheap alloys for effective storage in mild conditions of temperature and pressure. Among many intermetallic compounds described in the literature, TiFe-based systems have recently regained vivid interest as materials for practical applications since they are low-cost and they can be tuned to match required pressure and operation conditions. This work aims to provide a comprehensive review of…
We investigate a range of possible materials containing the supermolecular form of hydrogen comprising 13 hydrogen molecules arranged in an icosahedral arrangement. This supermolecule consists of freely rotating 12 hydrogen molecules in an icosahedral arrangement, enclosing another freely rotating molecule. To date, this supermolecule has only been observed in a compound with Iodane (HI). The extremely high hydrogen content suggests possible application in hydrogen storage so we examine the possibility of supermolecule formation at ambient pressures. We show that ab initio molecular dynamics c…
Detrimental 2$p$-3$d$ Hybridisation in Ni Nanosheets Supported on Strontium Dioxide for Catalytic H$_2$ Production, Necessitating Thickness Optimisation
Mono- and few-layer transition-metal dichalcogenides (TMDCs) provide opportunities for ideal two-dimensional semiconductors for electronic and optoelectronic devices. For electronic devices on TMDCs, it is essential to incorporate n- and/or p-type dopants which are stable in positions after patterned doping. Here we investigate hydrogen doping for TMDC (MX2 with M = Mo, W and X = S, Se, Te) nanosheets by first-principles calculations to address diffusion and doping properties. We find that adsorbed hydrogen atoms in TMDCs are energetically most stable at the interstitial site right on the Mo o…
The present study investigates the partial substitutions of Mn and Cu for Fe in the TiFe-system to gain better understanding of the role of elemental substitution on its hydrogen storage properties. The TiFe0.88-xMn0.02Cux (x = 0, 0.02, 0.04) compositions were studied. From X-Ray Diffraction (XRD) and Electron Probe Micro-Analysis (EPMA), it was found that all alloys are multi-phase, with TiFe as a major phase, together with \b{eta}-Ti and Ti4Fe2O-type as secondary precipitates, of all them containing also Mn and Cu. Increasing the Cu content augments the secondary phase amounts. Low quantity …
The hybrid nanostructures of nitrogen doped carbon materials and nonprecious transition metals are among the most promising electrocatalysts to replace noble metal catalysts for renewable energy applications. However, the fundamental principles governing the catalytic activity of such hybrid materials remain elusive. Herein, we systematically explore the electrocatalytic properties of transition metals, transition metal oxides and carbides substrates covered by nitrogen-doped graphitic sheets for hydrogen evolution reaction (HER). Our first-principles calculations show that the graphitic sheet…
HCl, a 'textbook' example of a polar covalent molecule, is a well-known compound of hydrogen and chlorine. Inspired by the discovery of unexpected stable stoichiometries of sodium chlorides, we performed systematic searches for all stable compounds in the H-Cl system from ambient pressure to higher pressures up to 500 GPa using variable-composition ab initio evolutionary algorithm USPEX. We found several compounds that are stable under pressure, i.e. HCl, H$_2$Cl, H$_3$Cl, H$_5$Cl and H$_4$Cl$_7$, which display a rich variety of chemical bonding types. At ambient pressure, H$_2$, Cl$_2$ and HC…
The processing of promising magnetocaloric materials into spheres is one of the important issues on developing high-performance magnetic refrigeration systems. In the present study, we achieved in producing spherical particles of a giant magnetocaloric compound HoB2 by a crucible-free gas atomization process, despite its high melting point of 2350 C. The particle size distribution ranges from 100 to 710 micrometers centered at 212-355 micrometers with the highest yield of 14-20wt% of total melted electrode, which is suitable for magnetic refrigeration systems. The majority of the resulting par…
We predict the existence of new two dimensional silicon carbide nanostructure employing ab initio density-functional theory calculations. These structures are composed of tetragonal and hexagonal rings with C-C and Si-C bonds arranged in a buckling plane. They are proven to be thermodynamically and mechanically stable with relatively low formation energy, implying potential fabrication in lab. They exhibit strong ductility and anisotropicity from their strain-stress relation and directional dependence of mechanical moduli. The materials maintain phonon stability upon the application of mechani…
High-pressure metal-hydride (MH) research evolved into a thriving field within condensed matter physics following the realisation of metallic compounds showing phonon mediated near room-temperature superconductivity. However, severe limitations in determining the chemical formula of the reaction products, especially with regards to their hydrogen content, impedes a deep understanding of the synthesized phases and can lead to significantly erroneous conclusions. Here, we present a way to directly access the hydrogen content of MH solids synthesised at high pressures in (laser-heated) diamond an…
We report on the hydrogen sorption investigation of the mixed alkali cluster intercalated fulleride series NaxLi6-xC60. These compounds are isostructural to Na6C60 and Li6C60 while the cubic lattice parameter is linearly dependent on x. The H2 absorption/desorption was studied by means of charge/discharge kinetic and coupled calorimetric - manometric measurements. By varying the stoichiometry, we found the best compromise between absorption rate, temperature and amount of hydrogen in Na1Li5C60. This system is able to reversibly absorb up to 4.3 wt% H2 at 280 °C, which is 70 °C lower in tempera…
Understanding of recently reported putative close-to-room-temperature superconductivity in C-S-H compounds at 267 GPa demands reproducible synthesis protocol as well as knowledge of its structure and composition. We synthesized C-S-H compounds with various carbon composition at high pressures from elemental C and methane CH4, sulfur S, and molecular hydrogen H2. Here we focus on compounds synthesized using methane as these allow a straightforward determination of their structure and composition by combining single-crystal X-ray diffraction (XRD) and Raman spectroscopy. We applied a two-stage s…
Usually the ions of a particular element in solids are in positive or negative oxidation states, depending upon the chemical environment. It is highly unusual for an atom having both positive as well as negative oxidation states simultaneously within the same structural framework in a particular compound. Our structural and chemical bonding analyses show that the hydrogen ions in the transition metal ammine borohydrides (TMABHs) with the chemical formula M(BH4)2(NH3)2 (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) have both positive and negative oxidation states. Our detailed analyses confirm …
Hydrogen-sulfide (H2S) was studied by x-ray synchrotron diffraction (XRD) and Raman spectroscopy up to 150 GPa at 180-295 K and by quantum-mechanical variable-composition evolutionary simulations. The experiments show that H2S becomes unstable with respect to formation of new compounds with different structure and composition, including Cccm and a body-centered-cubic (bcc) like (R3m or Im-3m) H3S, the latter one predicted previously to show a record-high superconducting transition temperature, Tc of 203 K. These experiments provide experimental ground for understanding of this record high Tc. …
Silica, water and hydrogen are known to be the major components of celestial bodies, and have significant influence on the formation and evolution of giant planets, such as Uranus and Neptune. Thus, it is of fundamental importance to investigate their states and possible reactions under the planetary conditions. Here, using advanced crystal structure searches and first-principles calculations in the Si-O-H system, we find that a silica-water compound (SiO2)2(H2O) and a silica-hydrogen compound SiO2H2 can exist under high pressures above 450 and 650 GPa, respectively. Further simulations reveal…
We demonstrate in this paper that high pressure can promote the reactions between the ionic compounds and H2 molecules and form thermodynamically stable hybrid compounds. Using crystal structure search method based on first principles calculations and particle swarm optimization algorithm, we show that many alkali halides XY (X=Na, K, Rb, Cs; Y=Cl, Br, I) can form stable hybrid compounds with various H2 compositions under different pressures that could be as low as 0.8 GPa (KIH2). Especially, the commonly known table salt, NaCl, can form stable NaClH2 and NaCl(H2)4 hybrid compounds under press…
Using first principles calculations, we show the high hydrogen storage capacity of a new class of compounds, metalloboranes. Metalloboranes are transition metal (TM) and borane compounds that obey a novel-bonding scheme. We have found that the transition metal atoms can bind up to 10 H2 molecules.
The two-dimensional (2D) C3N has emerged as a material with promising applications in high performance device owing to its intrinsic bandgap and tunable electronic properties. Although there are several reports about the bandgap tuning of C3N via stacking or forming nanoribbon, bandgap modulation of bilayer C3N nanoribbons (C3NNRs) with various edge structures is still far from well understood. Here, based on extensive first-principles calculations, we demonstrated the effective bandgap engineering of C3N by cutting it into hydrogen passivated C3NNRs and stacking them into bilayer heterostruct…
The stability of different stoichiometric H$_n$Br ($n$=1-7) compounds under pressure are extensively studied using density functional theory calculations. Five new energetically stable stoichiometries of H$_2$Br, H$_3$Br, H$_4$Br, H$_5$Br, and H$_7$Br were uncovered at high pressure. The results show that HBr is stable below 64 GPa, then decomposes into new compound H$_2$Br and Br$_2$ molecular crystal. For H$_2$Br and H$_3$Br compounds, they were found to become stable above 30 GPa and 8 GPa, respectively. In addition, we accidentally discovered the triangular H$_3^+$ species in H$_5$Br compo…
Hybrid perovskites with mixed organic cations such as methylammonium and formamidinium have attracted interest due to their improved stability and capability to tune their properties varying the composition. In this work we report on the local variation of the structural and electronic properties in mixed A-site cation MA/FA lead iodide perovskites FAXMA1-XPBI3 evaluated from static first-principles calculations in certain structures where the orientations of organic cations result from examining the energy landscape of some compositions. The cation replacement at the A-site to form the solid …
We synthesized two C-S-H compounds from a mixture of carbon and sulfur in hydrogen and from sulfur in mixed methane-hydrogen fluids at 4 GPa. X-ray synchrotron single-crystal diffraction and Raman spectroscopy have been applied to these samples up to 58 and 143 GPa, respectively. Both samples show a similar Al2Cu type I4/mcm basic symmetry, while the hydrogen subsystem evolves with pressure via variously ordered molecular and extended modifications. The methane bearing sample lowers symmetry to an orthorhombic Pnma structure after laser heating to 1400 K at 143 GPa. The results suggest that su…
Attempts of synthesis of ionic (NH4)(BH3NH2BH2NH2BH3) using metathetical approach resulted in a mixture of the target compound and a partly dehydrogenated molecular NH3BH2NH2BH2NH2BH3 product. The mixed specimen was characterized by NMR and vibrational spectroscopies, and the crystal structure of their cocrystal was solved from powder x-ray diffraction data, and supplemented by theoretical density functional theory calculations. Despite their impressive hydrogen content, and similarly to ammonia borane, both title compounds release hydrogen substantially polluted with borazine, and traces of a…
A2B7 compounds (A = rare earth or Mg, B = transition metal) are widely studied as active materials for negative electrode in Ni-MH batteries. By playing on the substitution rate of both A and B elements, it is possible to prepare various compositions. This strategy will help to improve the properties, for example get a higher reversible hydrogen capacity and a well-adapted hydrogen sorption pressure. Indeed, A can be almost all light rare earths (La to Gd), yttrium and alkaline earth metals (Mg or Ca), whereas B can contain various late transition metals (Mn to Ni). To understand the effects o…
Intermetallic compounds are key materials for energy transition as they form reversible hydrides that can be used for solid state hydrogen storage or as anodes in batteries. ABy compounds (A = Rare Earth (RE); B = transition metal; 2 < y < 5) are good candidates to fulfill the required properties for practical applications. They can be described as stacking of AB5 and AB2 sub-units along the c crystallographic axis. The latter sub-unit brings a larger capacity, while the former one provides a better cycling stability. However, ABy binaries do not show good enough properties for applications. U…
In the use of hydrogen as large-scale energy vector, metal hydrides based on intermetallic compounds play a key role, thanks to mild temperatures and pressures required for the storage. It is increasingly necessary to develop hydrogen-based devices, and in this work, the intermetallic compound TiFe$_{0.85}$Mn$_{0.05}$ is evaluated and selected as H$_2$-carrier for a storage system of 50 kg of H$_2$. A batch of 5 kg of alloy was synthesized at industrial level and characterized through scanning electron microscopy and powder X-ray diffraction, to determine the structure and phase abundance. Mor…
Through CaH2 chemical reduction of a parent R3+Ni3+O3 perovskite form, superconductivity was recently achieved in Sr-doped NdNiO2 on SrTiO3 substrate. Using density functional theory (DFT) calculations, we find that stoichiometric NdNiO2 is significantly unstable with respect to decomposition into 1/2[Nd2O3 + NiO + Ni] with exothermic decomposition energy of +176 meV/atom, a considerably higher instability than that for common ternary oxides. This poses the question if the stoichiometric NdNiO2 nickelate compound used extensively to model the electronic band structure of Ni-based oxide analog …
In order to identify potential hydrogen storage materials, worldwide attention has been focused on hydrides with high gravimetric and volumetric capacity. Hydrogen is a unique element that possess positive, negative or neutral oxidation state in solids depending upon the chemical environment. If one can find hydrogen storage materials where hydrogen is present in both negative and positive oxidation state within the same structural framework then one can accommodate hydrogen with high volume density. So, it is fundamentally as well as technologically important to identify compounds in which hy…
