Maya Bar-Sadan

MoS2 Hybrid Nanostructures: From Octahedral to Quasi‐Spherical Shells within Individual Nanoparticles

MoS2, a layered compound with tribological and catalytic applications, is known to form a range of hollow closed nanostructures and nanoparticles, including graphene-like structures. These have been demonstrated experimentally through high-temperature synthesis and pulsed laser ablation (PLA), and theoretically with quantum chemical calculations. The smallest allowed structures are nanooctahedra of 3 to 8 nm size. Nicknamed the “true inorganic fullerene” in analogy to carbon fullerenes, they differ from larger multiwalled MoS2 fullerene-like nanoparticles both in their morphology and predicted electronic properties. The larger fullerene-like particles are quasi-spherical (polyhedral) or nanotubular, typically with diameters of 20 to 150 nm. Above a few hundred nm in size, these nanoparticles transform into 2H-MoS2 platelets. Fullerene-like particles have been recognized as superior solid lubricants with numerous commercial applications, and MoS2 nanooctahedra may have catalytic applications. Understanding the fundamental commonality of these two morphologies might prove essential in the development of new materials. The research on hollow MoS2 nanostructures of minimal size (< 10 nm in diameter) was initiated in 1993 upon the first independent proposal of the formation of nanooctahedra of MoS2 [3,5] (and BN) with six rhombi in their corners. In 1999, it was demonstrated that twoto four-walled MoS2 nanooctahedra, 3–5 nm in size and up to ca. 10 atoms, could be obtained by PLA. Similar results were subsequently reported in Ref. [1, 2] as illustrated in Figure 1a. Recent studies of high energy density methods such as laser ablation and arc– discharge resulted in small structures with only a limited number of atoms: Mo–S clusters or double-walled nanooctahedra.

Correlating electron tomography and plasmon spectroscopy of single noble metal core-shell nanoparticles.

The 3D structure reconstruction of gold core-silver shell nanoparticles by electron tomography is combined with optical dark-field spectroscopy. Electron tomography allows segmentation of the particles into core and shell subvolumes and facilitates avoiding Bragg diffraction artifacts inherent in 2D images. This advantage proves essential for accurate correlation of plasmon spectra and structure. We find that for the nanoparticles of near-spherical shape studied here the plasmon resonances depend on the relative size of the core and shell, rather than on their exact shapes and concentricity. A remarkable dependence of the spectral shape on the permittivity of the surrounding medium is also demonstrated, suggesting that core-shell nanoparticles can be used as ratiometric sensors with a very high dynamic range.

Fullerene-like WS2 nanoparticles and nanotubes by the vapor-phase synthesis of WCln and H2S

Inorganic fullerene-like (IF) nanoparticles and nanotubes of WS(2) were synthesized by a gas phase reaction starting from WCl(n) (n = 4, 5, 6) and H(2)S. The effect of the various metal chloride precursors on the formation of the products was investigated during the course of the study. Various parameters have been studied to understand the growth and formation of the IF-WS(2) nanoparticles and nanotubes. The parameters that have been studied include flow rates of the various carrier gases, heating of the precursor metal chlorides and the temperature at which the reactions were carried out. The best set of conditions wherein maximum yields of the high quality pure-phase IF-WS(2) nanoparticles and nanotubes are obtained have been identified. A detailed growth mechanism has been outlined to understand the course of formation of the various products of WS(2).

Designing Bimetallic Co-Catalysts: A Party of Two

The enhanced catalytic properties of bimetallic particles has made them the focus of extensive research. We compare the photocatalytic activity for hydrogen production of core-shell structures of Au@Pd and Au@(Au/Pd alloy) on seeded rods of CdSe@CdS and show that Au@alloy was superior toward hydrogen production. Our finding reveals that the promotion effects of Au in Pd originate both from the alteration of the electronic structure by the Au core as well as by the atomic rearrangement of the surface. Long-term monitoring of the activity of the photocatalysts offered insights into the dynamic processes during the illumination showing that the tip morphology influenced the stability of the hybrid structures. The Au core served as a physical barrier, protecting the CdS rod against cation exchange reactions with the Pd. The coupling of these factors to achieve synergistic effects is therefore a prime aspect in the rational design of efficient cocatalysts.

Atomic-Scale Evolution of a Growing Core–Shell Nanoparticle

Understanding the atomic-scale growth at solid/solution interfaces is an emerging frontier in molecular and materials chemistry. This is particularly challenging when studying chemistry occurring on the surfaces of nanoparticles in solution. Here, we provide atomic-scale resolution of growth, in a statistical approach, at the surfaces of inorganic nanoparticles by state-of-the-art aberration-corrected transmission electron microscopy (TEM) and focal series reconstruction. Using well-known CdSe nanoparticles, we unfold new information that, for the first time, allows following growth directly, and the subsequent formation of CdS shells. We correlate synthetic procedures with resulting atomic structure by revealing the distribution of lattice disorder (such as stacking faults) within the CdSe core particles. With additional sequential synthetic steps, an ongoing transformation of the entire structure occurs, such that annealing takes place and stacking faults are eliminated from the core. The general strategy introduced here can now be used to provide equally revealing atomic-scale information concerning the structural evolution of inorganic nanostructures.

MoS2 FULLERENE-LIKE NANOPARTICLES AND NANOTUBES USING GAS-PHASE REACTION WITH MoCl5

Inorganic fullerene-like (IF) nanoparticles of MoS2 were synthesized using gas-phase reaction starting from MoCl5 and H2S. The IF-MoS2 nanoparticles are spherical and in some cases faceted with diameters in general ranging between 20 and 80 nm. The IF-MoS2 nanoparticles have large hollow cores, filled in some cases with amorphous material. Various parameters have been investigated to understand the growth and formation of the IF-MoS2 nanoparticles. The parameters that have been studied include flow rates of the various carrier gases, temperature at which the reaction was carried out, time of the reaction and heating of the precursor material. The best set of conditions wherein maximum yields of the IF-MoS2 nanoparticles are obtained have been identified. Additionally, annealing the as-obtained samples or heating them in a mixture of H2 along with H2S improves the crystallinity and reduces the amorphous material filling in the core. Apart from the fullerene-like nanoparticles under certain experimental conditions nanotubes of MoS2 have also been obtained nonetheless in small yields.

Weak links and phase slip centers in superconducting MgB2 wires

MgB2 superconducting wires were produced by the Mg diffusion method. Scanning electron microscopy (SEM), optical microscopy, dispersive X-ray analysis (EDS), and XRD diffraction were used to study the physical structure and content of the wires. Magnetic properties (Tcm, Hc1, Hc2, Jc by the Bean model) were obtained with a SQUID magnetometer, and transport properties (Tcr, Hc2, resistivity and residual resistivity ratio) were measured using a standard four-lead configuration. The V-I characteristics of the wires close to the critical temperature showed a staircase response, which was attributed to the presence of weak links, creating phase slip centers. The origin of those weak links is discussed in relation to their formation and structure.