Gopi Krishnan1

1, Amrita Ctr for Nanosciences & Molecular Medicine, KOCHI, Kerala, India

Gas phase synthesis based on DC Magnetron sputtering is a promising method to produce nanoparticle and bimetallic nanoparticles (NPs) due to its single step generation of high purity nanoparticles compared to other physical and chemical methods. Although fine tuning of bimetallic NPs with different structural motifs and sizes is very well established and reported for noble and some transition metals, it still remains a challenge to extend this approach towards alkali and alkaline earth metals. This is because these metals with their low reduction potential readily react with oxygen and water. Moreover, limitation associated with nanoparticle production rate and insufficient information on the mono/ and bimetallic NP nucleation mechanisms is a major drawback in understanding the formation process of bimetallic NPs and improve its production rate for large scale synthesis.
We show the influence of CH4/H2 as a trace gas on the nucleation and formation of bimetallic nanoparticles (NPs) prepared by gas phase synthesis. We show this as a strategy to nucleate bimetallic nanoparticles (NPs) made by gas phase synthesis of elements showing difficulty in homogenous nucleation. We illustrate the above mentioned strategy for the case of Mg based bimetallic NPs, which are interesting as hydrogen storage materials and exhibit both nucleation and oxidation issues even at ultra-high vacuum conditions. The above mentioned strategy is associated with tuning dimer bond energy of the formed species to produce stable nuclei for NP formation. We show that not only issue associated with nucleation of NP can be solved but diverse variety of structural motifs can be obtained from alloy to core\shell structures with good control of the NP morphology, size and chemical distribution. Moreover, by tailoring the composition of Ti, Mg and the type of employed trace gas, the as prepared MgTi NPs can be tuned from hexagonal pyramid to platelet shapes. We elucidate the reason based on (i) defects, and (ii) hydrogen and carbon adsorption on (111) planes that alters the growth rates and surface facet stabilization. The shape of MgTi NPs is identified from selected area electron diffraction (SAED) pattern and tomography that is a 3D reconstruction based on a tilt series of Bright-Field transmission electron microscopy (TEM) micrographs. Finally based on our experimental observations and generic geometrical model analysis, we prove that the formation of the various structural motifs is based on two spatially and temporally separated nucleation and growth processes instead of phase separation. This is shown to be associated with the dimer bond energies of the various formed species and the vapor pressures of the metals, which are key factors for NP nucleation.