Gopi Krishnan

Thermal stability of gas phase magnesium nanoparticles

In this work we present a unique transmission electron microscopy study of the thermal stability of gas phase synthesized Mg nanoparticles, which have attracted strong interest as high capacity hydrogen storage materials. Indeed, Mg nanoparticles with a MgO shell ( ? 3?nm thick) annealed at 300?°C show evaporation, void formation, and void growth in the Mg core both in vacuum and under a high pressure gas environment. This is mainly due to the outward diffusion and evaporation of Mg with the simultaneously inward diffusion of vacancies leading to void growth (Kirkendall effect). The rate of Mg evaporation and void formation depends on the annealing conditions. In vacuum, and at T = 300?°C, the complete evaporation of the Mg core takes place (within a few hours) for sizes ? 15–20?nm. Void formation and growth has been observed for particles with sizes ? 20–50?nm, while stable Mg nanoparticles were observed for sizes >50?nm. Furthermore, even at relative low temperature annealing (as low as 60?°C), void formation and growth occurs in 15–20 nm sized Mg nanoparticles, indicating that voiding will be even more dominant for nanoparticles smaller than 10 nm. Our findings confirm that Mg evaporation and void formation in nanoparticles with sizes less than 50 nm present formidable barriers for their applicability in hydrogen storage, but also could inspire future research directions to overcome these obstacles.

Mg/Ti multilayers: Structural and hydrogen absorption properties

Mg-Ti alloys have uncommon optical and hydrogen absorbing properties, originating from a “spinodal-like” microstructure with a small degree of chemical short-range order in the atoms distribution. In the present study we artificially engineer shortrange order by depositing Pd-capped Mg/Ti multilayers with different periodicities and characterize them both structurally and optically. Notwithstanding the large lattice parameter mismatch between Mg and Ti, the as-deposited metallic multilayers show good structural coherence. Upon exposure to H2 gas a two-step hydrogenation process occurs, with the Ti layers forming

Tuning structural motifs and alloying of bulk immiscible Mo-Cu bimetallic nanoparticles by gas-phase synthesis

Nowadays bimetallic nanoparticles (NPs) have emerged as key materials for important modern applications in nanoplasmonics, catalysis, biodiagnostics, and nanomagnetics. Consequently the control of bimetallic structural motifs with specific shapes provides increasing functionality and selectivity for related applications. However, producing bimetallic NPs with well controlled structural motifs still remains a formidable challenge. Hence, we present here a general methodology for gas phase synthesis of bimetallic NPs with distinctively different structural motifs ranging at a single particle level from a fully mixed alloy to core-shell, to onion (multi-shell), and finally to a Janus/dumbbell, with the same overall particle composition. These concepts are illustrated for Mo-Cu NPs, where the precise control of the bimetallic NPs with various degrees of chemical ordering, including different shapes from spherical to cube, is achieved by tailoring the energy and thermal environment that the NPs experience during their production. The initial state of NP growth, either in the liquid or in the solid state phase, has important implications for the different structural motifs and shapes of synthesized NPs. Finally we demonstrate that we are able to tune the alloying regime, for the otherwise bulk immiscible Mo-Cu, by achieving an increase of the critical size, below which alloying occurs, closely up to an order of magnitude. It is discovered that the critical size of the NP alloy is not only affected by controlled tuning of the alloying temperature but also by the particle shape.

Determination of the Electronic Energy Levels of Colloidal Nanocrystals using Field-Effect Transistors and Ab-Initio Calculations

Colloidal nanocrystals electronic energy levels are determined by strong size-dependent quantum confinement. Understanding the configuration of the energy levels of nanocrystal superlattices is vital in order to use them in heterostructures with other materials. A powerful method is reported to determine the energy levels of PbS nanocrystal assemblies by combining the utilization of electric-double-layer-gated transistors and advanced ab-initio theory.

Influence of Ti on the formation and stability of gas-phase Mg nanoparticles

The effect of titanium on magnesium nanoparticle formation is investigated in relation to the importance of Mg in hydrogen storage. Addition of Ti reduces the Mg-nanoparticle sizes in the range of 5–20 nm and leads to less protective Mg-oxide shells. This in return accelerates the evaporation of Mg that leads to hollow Mg cores. The presence of oxygen plays a dual role in forming MgO protected stable Mg nanoparticles above a critical size (∼15 nm) and hollow ones below this size both due to evaporation and oxidation associated Kirkendall effects.

Copper nanoparticle formation in a reducing gas environment

Although copper nanoparticles are used as model nanomaterial because of their small nucleation barrier, their oxidization sensitivity hampers production of fully metallic nanoparticles with controlled size and shape. Nevertheless, we demonstrate here synthesis of copper nanoparticles, via high pressure magnetron sputtering, having highly tunable sizes and shapes over a size range spanning two orders of magnitude. This is achieved by exploiting a reducing gas environment to mediate proper nucleation conditions, allowing size control of nanoparticles with robust motifs for particle sizes ∼5–300 nm. However, due to rapid coalescence oxidation-free nanoparticles cannot be produced monodisperse for sizes larger than ∼30 nm.

Improved thermal stability of gas-phase Mg nanoparticles for hydrogen storage

This work focuses on improving the thermal stability of Mg nanoparticles (NPs) for use in hydrogen storage. Three ways are investigated that can achieve this goal. (i) Addition of Cu prevents void formation during NP production and reduces the fast evaporation/voiding of Mg during annealing. (ii) Alloying can prevent Mg evaporation: e.g., Mg with Ni forms a thermally stable core/shell (MgNi2/Ni) preventing Mg evaporation during annealing. (iii) Covering Mg NPs with a Ti film leads to suppression of Mg evaporation during vacuum annealing. Indeed, hydrogenation of the Ti/Mg NPs shows formation of the γ-MgH2 phase as for pure Mg NPs.

Mg/Ti multilayers

Mg-Ti alloys have uncommon optical and hydrogen absorbing properties, originating from a “spinodal-like” microstructure with a small degree of chemical short-range order in the atoms distribution. In the present study we artificially engineer shortrange order by depositing Pd-capped Mg/Ti multilayers with different periodicities and characterize them both structurally and optically. Notwithstanding the large lattice parameter mismatch between Mg and Ti, the as-deposited metallic multilayers show good structural coherence. Upon exposure to H2 gas a two-step hydrogenation process occurs, with the Ti layers forming