Zhenpeng Hu

Hexagonal Cu2SnS3 with metallic character: Another category of conducting sulfides

A hexagonal Cu2SnS3 with uniform and well-dispersed nanoparticle morphology has been synthesized, representing an example of hexagonal system in the Cu–Sn–E (S, Se) ternary chalcogenides. Both theoretical calculation and experimental results give the unique metallic character of Cu2SnS3, which is significantly different from the traditional opinion that I-IV-VI ternary chalcogenides were regarded previously as small or middle band-gap semiconductors. Also, M(I)2SnS3 (M=Ag, Au, Rb, and Cs) serial compounds are another potential family of conducting sulfides. The conducting Cu2SnS3 product with the interlayer space and tunnels in the crystal structures could be fascinatingly introduced to the lithium battery application.

Supported Rhodium Catalysts for Ammonia–Borane Hydrolysis: Dependence of the Catalytic Activity on the Highest Occupied State of the Single Rhodium Atoms

Supported metal nanocrystals have exhibited remarkable catalytic performance in hydrogen generation reactions, which is influenced and even determined by their supports. Accordingly, it is of fundamental importance to determine the direct relationship between catalytic performance and metal-support interactions. Herein, we provide a quantitative profile for exploring metal-support interactions by considering the highest occupied state in single-atom catalysts. The catalyst studied consisted of isolated Rh atoms dispersed on the surface of VO2 nanorods. It was observed that the activation energy of ammonia-borane hydrolysis changed when the substrate underwent a phase transition. Mechanistic studies indicate that the catalytic performance depended directly on the highest occupied state of the single Rh atoms, which was determined by the band structure of the substrates. Other metal catalysts, even with non-noble metals, that exhibited significant catalytic activity towards NH3 BH3 hydrolysis were rationally designed by adjusting their highest occupied states.

Engineering electrocatalytic activity in nanosized perovskite cobaltite through surface spin-state transition.

The activity of electrocatalysts exhibits a strongly dependence on their electronic structures. Specifically, for perovskite oxides, Shao-Horn and co-workers have reported a correlation between the oxygen evolution reaction activity and the eg orbital occupation of transition-metal ions, which provides guidelines for the design of highly active catalysts. Here we demonstrate a facile method to engineer the eg filling of perovskite cobaltite LaCoO3 for improving the oxygen evolution reaction activity. By reducing the particle size to ∼80 nm, the eg filling of cobalt ions is successfully increased from unity to near the optimal configuration of 1.2 expected by Shao-Horns principle. Consequently, the activity is significantly enhanced, comparable to those of recently reported cobalt oxides with eg∼1.2 configurations. This enhancement is ascribed to the emergence of spin-state transition from low-spin to high-spin states for cobalt ions at the surface of the nanoparticles, leading to more active sites with increased reactivity.

Kondo effect in single cobalt phthalocyanine molecules adsorbed on Au(111) monoatomic steps.

The Kondo effect in single dehydrogenated cobalt phthalocyanine (CoPc) molecules adsorbed on Au(111) monoatomic steps was studied with a low temperature scanning tunneling microscope. The CoPc molecules adsorbed on Au(111) monoatomic steps show two typical configurations, which can be dehydrogenated to reveal Kondo effect. Moreover, the Kondo temperatures (T(K)) measured for different molecules vary in a large range from approximately 150 to approximately 550 K, increasing monotonically with decreasing Co-Au distance. A simple model consisting of a single Co 3d(z) (2) orbital and a Au 6s orbital is considered and gives a qualitative explanation to the dependence. The large variation of T(K) is attributed to the variation of the interaction between the magnetic-active cobalt ion and the Au substrate resulted from different Co-Au distances.

Metallic mesocrystal nanosheets of vanadium nitride for high-performance all-solid-state pseudocapacitors

AbstractTransition metal nitrides (TMNs) are of particular interest by virtue of their synergic advantages of superior electrical conductivity, excellent environmental durability and high reaction selectivity, yet it is difficult to achieve flexible design and operation. Herein, mesocrystal nanosheets (MCNSs) of vanadium nitride (VN) are synthesized via a confined-growth route from thermally stable layered vanadium bronze, representing the first two-dimensional (2D) metallic mesocrystal in inorganic compounds. Benefiting from their single-crystalline-like long-range electronic connectivity, VN MCNSs deliver an electrical conductivity of 1.44 × 105 S/m at room temperature, among the highest values observed for 2D nanosheets. Coupled with their unique pseudocapacitance, VN MCNS-based flexible supercapacitors afford a superior volumetric capacitance of 1,937 mF/cm3. Nitride MCNSs should have wide applications in the energy storage and conversion fields because their intrinsic high conductivity is coupled with the reactivity of inorganic lattices.

Identifying atomic geometry and electronic structure of (2×3)-Sr/Si(100) surface and its initial oxidation

We present a joint experimental and theoretical study on the geometric and electronic states and the initial oxidation of the (2x3)-Sr/Si(100) surface. With scanning tunneling microscopy/scanning tunneling spectroscopy (STM/STS) measurements combined with ab initio calculations, the atomic geometry and the electronic states of the (2x3)-Sr/Si(100) surface are identified. The dimerization of the Si atoms in the single atom row based on a (1x3) Si substrate model plays a critical role in stabilization of the surface structure and in determining the electronic properties. At the very initial oxidation of the surface, four features corresponding to the primary adsorption and oxidation sites are determined. Three of them are corresponding to the most favored oxidation sites with single oxygen molecules, whose local density of states gives semiconducting behavior. One is corresponding to the oxidation site with two oxygen molecules, whose local density of states gives metallic behavior. These features all exhibit dark spots with different shapes in the occupied state images but display either dark spots or bright protrusions depending on the different oxidation sites in the empty state images. Compared with the theoretical calculations, the plausible adsorption and oxidation models are proposed.

The atomic structures of carbon nitride sheets for cathode oxygen reduction catalysis.

Carbon nitride sheets are promising Pt replacement materials for cathode oxygen reduction catalysis. Using first principles calculations with a global optimization method, we search for the most stable structures of the monolayer carbon nitrides at various C:N ratios. The results show that the larger the ratio, the more energetically favorable the obtained structures, and the more preferably for the C, N atoms to assume sp(2) configurations. A volcano shape is revealed for the curve of the representative O2 adsorption energies on the sheets vs. the ratios. In the ratio range of 2.0-3.0, the sheets not only have lower formation energies than the stable graphitic-C3N4, but also can potentially catalyze the oxygen reduction as efficiently as Pt.

Imaging metal-like monoclinic phase stabilized by surface coordination effect in vanadium dioxide nanobeam

In correlated systems, intermediate states usually appear transiently across phase transitions even at the femtosecond scale. It therefore remains an open question how to determine these intermediate states—a critical issue for understanding the origin of their correlated behaviour. Here we report a surface coordination route to successfully stabilize and directly image an intermediate state in the metal-insulator transition of vanadium dioxide. As a prototype metal-insulator transition material, we capture an unusual metal-like monoclinic phase at room temperature that has long been predicted. Coordinate bonding of L-ascorbic acid molecules with vanadium dioxide nanobeams induces charge-carrier density reorganization and stabilizes metallic monoclinic vanadium dioxide, unravelling orbital-selective Mott correlation for gap opening of the vanadium dioxide metal–insulator transition. Our study contributes to completing phase-evolution pathways in the metal-insulator transition process, and we anticipate that coordination chemistry may be a powerful tool for engineering properties of low-dimensional correlated solids.

Writing charge into the n-type LaAlO3/SrTiO3 interface: A theoretical study of the H2O kinetics on the top AlO2 surface

Ab initio calculations reveal that H2O binds strongly with the AlO2-terminated LaAlO3(100) surface and stabilizes it. The H2O dissociates into OH and H. An ionic liquid is formed at certain temperature due to the fast diffusion of the H. The OH can then be selectively removed by a Coulomb force, resulting in the charge transfer to the interface. The desorption and diffusion of the remained H atoms are the key factors to the decay of the interface conductivity. While the desorption depends on the thickness of the LaAlO3 film, the diffusion is controlled by the coverage of H2O in a vacuum environment.

Hydrogen Treatment for Superparamagnetic VO2 Nanowires with Large Room-Temperature Magnetoresistance

One-dimensional (1D) transition metal oxide (TMO) nanostructures are actively pursued in spintronic devices owing to their nontrivial d electron magnetism and confined electron transport pathways. However, for TMOs, the realization of 1D structures with long-range magnetic order to achieve a sensitive magnetoelectric response near room temperature has been a longstanding challenge. Herein, we exploit a chemical hydric effect to regulate the spin structure of 1D V-V atomic chains in monoclinic VO2 nanowires. Hydrogen treatment introduced V(3+) (3d(2) ) ions into the 1D zigzag V-V chains, triggering the formation of ferromagnetically coupled V(3+) -V(4+) dimers to produce 1D superparamagnetic chains and achieve large room-temperature negative magnetoresistance (-23.9 %, 300 K, 0.5 T). This approach offers new opportunities to regulate the spin structure of 1D nanostructures to control the intrinsic magnetoelectric properties of spintronic materials.