Direct Gas‐Phase Synthesis of Heterostructured Nanoparticles through Phase Separation and Surface Segregation

Abstract

Heterostructured nanoparticles (NPs) offer great opportunities for applications in areas such as biomedicine, renewable energy, information processing, and data storage by virtue of their embedded multiple functionalities and enhanced magnetic, [1,2] optical, [3] and catalytic performance [4] arising from the interactions of the individual components within the NPs.To understand the fundamental nature of these interactions, as well as to utilize them more efficiently, it is imperative to develop synthesis methods to prepare heterostructured NPs with high-quality interfaces and surfaces. Several heterostructures have been synthesized by the seed-growth approach using wet chemical methods, [5][6][7][8][9] sometimes followed by a chemical vapor deposition (CVD) process. [10,11] However, solution-based approaches make it difficult to obtain contamination-free interfaces and surfaces. Moreover, the integration of solution-synthesized NPs into vacuum-based micro-and nanoelectronics fabrication processes has been remarkably challenging. On the other hand, gas-phase techniques have the intrinsic merit of producing a clean interface and surface, yet only a few attempts have been made to produce heterostructured NPs [12][13][14] other than the widely reported core/shell NPs consisting of metal cores surrounded by an oxide shell of the same metal.[1] Here, we demonstrate that binary metallic (Au-Co and Ag-Fe) NPs with distinctively different heterostructures at the single-particle level can be prepared directly in the gas phase via phase separation and surface segregation. This synthesis approach is ultra high vacuum (UHV) compatible, and yields high quality interfaces and surfaces; the synthesized NPs can also be made water soluble [15] and functionalized for further chemical processing. This approach can also be extended to various other material systems such as alloys and semiconductors. [13] In this work, we have selected a combination of ferromagnetic elements (Co or Fe) and noble metals (Au or Ag) as the model system. Several characteristic properties make this system a good starting point for the synthesis of heterostructured NPs. For instance, at room temperature Co and Au do not alloy and readily phase separate in the bulk form or clusters of more than several hundred atoms. [16,17] The ferromagnetic elements have a smaller atomic size than the noble metals. Consequently, the mismatch in atomic size causes the strain energy to increase when the atoms are mixed as compared to the lower energy segregated state.[18] The ferromagnetic elements generally have a higher surface energy than the noble metals (Co (111) 3.23 J m -2 , Au (111) 1.61 J m -2 ), [19] which makes it more preferable for the noble metals to be present at the surface and leads the system towards surface segregation. Numerical simulations of binary metal NPs also suggest that noble metals are energetically favored to be present at the surface as compared to transition metals. [18,20] Though the combination of phase separation and large differences in s...

Small Adsorbate‐Assisted Shape Control of Pd and Pt Nanocrystals

“…The different effect of CO on the formation of nanocrystals of pure Pt and Pt-M alloys could be due to the phenomenon of surface segregation. [90][91][92][93] Studies have demonstrated that the surface composition of Pt-M alloy nanocrystals could be varied by changing the synthesis. The adsorption of methylamine on the surface of the as-prepared Pt octapods was indeed detected by FT-IR spectroscopy.…”

Tailoring characteristic thermal stability of Ni-Au binary nanocrystals via structure and composition engineering: theoretical insights into structural evolution and atomic inter-diffusion

“…16 These characteristic properties offer another dimension to design and control M-Au NCs structures by engineering the particle size, composition and atomic configuration. 17 Among different approaches, doping heterometals (e.g., Fe, Co and Ni) into Au NCs not only warrants reducing the consumption of the noble material, but also promises to endow distinctive properties benefitting from the synergistic effects. 18 Aiming at the controllable syntheses and the promising applications of M-Au binary NCs, it is paramount important to investigate their thermal stability dynamics dependence on the composition and structure configuration.…”

“…For example, Co-Au NCs with distinctively different heterostructures were synthesized and their magnetic properties can be tuned by metal thermal diffusion with programming the deposition temperature. 17 Au@Co NCs have been demonstrated with a morphology transformation from initial core-shell into a stable "peanut" structure during the heating, 19 while Ni-Au nanospindles were developed along a wrapping diffusion of Au component after heating treatments. 20 The Au-rich surface in Fe-Au NCs was formed upon high temperature annealing in vacuum ambient.…”

Core-Shell Formation and Juxtaposition in Fe and Si Hybrid Clusters Prepared by Controlling the Collision Stages

“…1) A surface segregation effect also contributes to maintain the core-shell morphology. 23) A similar alloy formation has been observed in a TEM column where small hemispherical particles (with less than 10 nm) on substrates accept post-impinging atoms at ambient temperature. 24,25) Such instantaneous alloying is accelerated for element combinations having negative mixing enthalpies.…”