Researchers at the University of Pittsburg’s Department of Chemical Engineering recently proposed a new theory that could possibly provide a rational explanation to the long-standing question, why ligand-protected metal nanoclusters (NCs) are stabilized at specific sizes. Michael G Taylor and Giannis Mpourmpakis from the University of Pittsburg, introduced a theory called ‘thermodynamic stability’ to answer this question.
The ‘thermodynamic stability’ theory is derived from the first principles density functional theory (DFT) calculations, was able to address the stability of thiolate-protected metal NCs as a function of the number of metal core atoms and thiolates surrounding the NC shell1.
Metal NCs are an exciting class of materials which occur as clusters of a small number of atoms composed of single or multiple elements, and these NCs typically measure less than 2 nm in size1. These NCs exhibit special electronic, optical and chemical properties which are very different from the behavior of their atomic scale and bulk counterparts1.
Colloidal NCs are nanometer sized inorganic particles that are solution grown and stabilized by a layer of surfactants attached to their surface2. Even though the inorganic cores are responsible for the NCs desired properties, the surfactant molecules attached to the cores ensure that these structures are easy to fabricate and process further into more complicated structures2. It is due to these special properties, the colloidal NCs can serve as building blocks for advanced materials and devices2.
Colloidal NCs stabilized by thiolate molecules on the surface of the cores have a wide variety of applications ranging from biolabeling of individual cells to targeted drug delivery for catalytic reactions1. Although there are other methods known for synthesis of gold nanoparticles such as Turkevich method, Perrault method, martin method etc., Brust-Schiffrin method is more commonly used.
In Brust-Schiffrin type synthesis, the NCs are produced when the salts of metals such as gold (Au), are reduced in the presence of thiolate (SR) ligands1. The most important determining factor of the NCs’ physico-chemical properties, the size and shape, depends on the reaction conditions used for the synthesis of the NCs1.
Despite the extensive research conducted on metal nanoparticle synthesis, there is no rational explanation about why the nanoparticles are formed1. Some of the proposed theories on the stabilization of nanoparticles were based on empirical electron counting rules which is the number of electrons that form a closed shell electronic structure1. However, these theories failed to explain why certain metal NCs synthesized do not follow this empirical electron counting rules.
The experiments conducted by these University of Pittsburg Researchers revealed that a fine energy balance between the core cohesive energy (CE), which is the bond strength of the NC’s metal core, and the binding energy (BE) of the shell to metal core, is responsible for the stabilization of the NCs1.
The research suggests that the ‘thermodynamic theory’ can be applied to both neutral and charged NCs. Similarly, the Researchers experimentally demonstrated that their theory could be applied to different metals as well. Furthermore, the research highlighted the importance of thiolate ligands on the overall stability as well as size and shape of the NCs.
Based on the thermodynamic stability theory proposed here, it is possible to relate the structural and compositional characteristic of the NCs such as size, number of ligands and number of metal ions.
This research furthered the understanding of the stability of the NC formation, and could potentially benefit Researchers in this area, by allowing them to customize the nanoparticle morphology to achieve the desired properties of the NCs and thereby creating more efficient and sustainable production processes. Therefore, the current study sheds light on new avenues for accelerating the synthesis of stable, atomically precise, colloidal NCs.
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- “Thermodynamic stability of ligand-protected metal nanoclusters” Michael G. Taylor, Giannis Mpourmpakis. Nature Communications (2017). DOI: 10.1038/ncomms15988
- “Review Article Colloidal nanocrystal synthesis and the organic–inorganic interface” Yadong Yin, A. Paul Alivisatos. Nature (2004). DOI: 10.1038/nature04165