Noble metal foams (NMFs) are a new class of functional materials featuring properties of both noble metals and monolithic porous materials, providing impressive prospects in diverse fields. Among reported synthetic methods, the sol-gel approach manifests overwhelming advantages for versatile synthesis of nanostructured NMFs (i.e., noble metal aerogels) under mild conditions. However, limited gelation methods and elusive formation mechanisms retard structure/composition manipulation, hampering on-demand design for practical applications. Here, highly tunable NMFs are fabricated by activating specific ion effects, enabling various single/alloy aerogels with adjustable composition (Au, Ag, Pd, and Pt), ligament sizes (3.1 to 142.0 nm), and special morphologies. Their superior performance in programmable self-propulsion devices and electrocatalytic alcohol oxidation is also demonstrated. This study provides a conceptually new approach to fabricate and manipulate NMFs and an overall framework for understanding the gelation mechanism, paving the way for on-target design of NMFs and investigating structure-performance relationships for versatile applications.

In this paper, we review the foundations of the density‐functional tight‐binding (DFTB) method. The method is based on the density‐functional theory as formulated by Hohenberg and Kohn. It introduces several approximations: First, densities and potentials are written as superpositions of atomic densities and potentials. Second, many‐center terms are summarized together with nuclear repulsion energy terms in a way that they can be written as a sum of pairwise repulsive terms. For small distances, the nuclear repulsion dominates, whereas for large distances, these terms vanish. The Kohn–Sham orbitals are expanded in a set of localized atom‐centered functions. They are represented in a minimal basis of optimized atomic orbitals, which are obtained for spherical symmetric spin‐unpolarized neutral atoms self‐consistently. The whole Hamilton and overlap matrices contain one‐ and two‐center contributions only. Therefore, they can be calculated and tabulated in advance as functions of the distance between atomic pairs. In addition, we discuss a self‐consistent charge extension, the treatment of weak interactions, and a linear response scheme in connection with the DFTB method. Finally, some practical aspects are presented. © 2012 John Wiley & Sons, Ltd. This article is categorized under: Electronic Structure Theory > Semiempirical Electronic Structure Methods

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Similar to carbon, several transition-metal chalcogenides are able to form tubular structures. Here, we present results from systematic theoretical investigations of structural and mechanical properties of MoS2 and TiS2 nanotubes in comparison to each other, to carbon nanotubes, and to corresponding experimental results. We have obtained the nanotube’s Young’s moduli (Y), Poisson ratios (ν), and shear moduli (G) as functions of diameter and chirality, using a density-functional-based tight-binding method. Additionally, we have simulated tensile tests by Born–Oppenheimer molecular dynamics simulations. The influence of structural defects on the investigated mechanical properties has been studied as well. As a result of the simulated stretching experiments, we found that TiS2 nanotubes can be stretched only half as much as MoS2 nanotubes.

Crystalline cadmium sulfide is a semiconductor for which the wurtzite and zinc blende structures are energetically almost degenerate. Due to quantum-confinement effects, it is possible to tune the optical properties of finite cadmium sulfide clusters by varying their size. Here, we report results of a theoretical study devoted to the properties of stoichiometric Cd n S n clusters as a function of their size n. We have optimized the structure, whereby our initial structures are spherical parts of either of the two crystal structures, and we have studied systems with up to almost 200 atoms. The calculations were performed by using a simplified LCAO-DFT-LDA scheme. The results include the structure, electronic energy levels (in particular the frontier orbitals HOMO and LUMO), and stability as a function of size. The results allow for a unique definition of a surface region. The Mulliken populations indicate that the bonds within this region are more ionic than in the bulk. Furthermore, whereas the HOMO is delocalized over major parts of the nanoparticle, the LUMO is a surface state, which confirms recent experimental findings. Finally, the relative stability of the zinc blende and wurtzite structures is strongly dependent on the size of the system, and there is a close connection between the HOMO-LUMO energy gap and stability.

Nanotubes from LnS(Se)–TaS2(Se) (Ln = rare earths) misfit compounds produced by relaxation of the layers' mismatch energy and seaming of their edges.

As an emerging class of porous materials, noble metal aerogels (NMAs) have drawn tremendous attention and displayed unprecedented potential in diverse fields. However, the development of NMAs is impeded by the fabrication methods because of their time-and cost-consuming procedures, limited generality, and elusive understanding of the formation mechanisms. Here, by revealing the self-healing behavior of noble metal gels and applying it in the gelation process at a disturbing environment, an unconventional and conceptually new strategy, i.e., a disturbance-promoted gelation method, is developed by introducing an external force field. It overcomes the diffusion limitation in the gelation process, thus producing monolithic gels within 1-10 min at room temperature, 2-4 orders of magnitude faster than for most reported methods. Moreover, versatile NMAs are acquired by using this method, and their superior (photo-)electrocatalytic properties are demonstrated for the first time in light of combined catalytic and optic properties.

Density functional-based calculations employing linear response theory have been performed on cadmium sulfide nanoparticles with up to 2000 atoms. We have considered different stoichiometries, underlying crystal structures (zincblende, wurtzite, rocksalt), particle shapes (spherical, cuboctahedral, tetrahedral), and saturations (unsaturated, partly saturated, completely saturated). We find strong excitonic onset excitations. For the saturated particles, the quantum confinement effect in the lowest excitation is visible as the excitation energy decreases toward the bulk band gap with increasing particle size. Dangling bonds at unsaturated surface atoms introduce trapped surface states that lie below the lowest excitations of the completely saturated particles. Zincblendeand wurtzite-derived particles show very similar spectra, whereas the spectra of rocksalt-derived particles are rather featureless. Particle shapes that confine the orbital wavefunctions strongly (tetrahedron) give rise to less pronounced spectra with lower oscillator strengths. Finally, we find a very good agreement of our data to experimentally available spectra and excitation energies.