Understanding the Synthesis of Na4Ag44(p-MBA)30 Monolayer-Protected Clusters

Abstract

The growth of nanoparticles is generally governed by the classical nucleation and growth model, in which there are no limits to the growth of nanoparticles other than the exhaustion of the building block material. This generally results in broad continuous size distributions that are often Gaussian in shape. On the other hand, the growth of monolayer-protected noble metal clusters, which are a molecular form of nanoparticles, is punctuated by stable sizes. This ultimately forms a family of clusters as a series of different sizes, resulting in size distributions that are neither continuous nor Gaussian. However, in one case, that being Na4Ag44(p-MBA)30 clusters, it was shown that it was possible to form a single-sized product in high yield, although it remains to be explained as to how this could be so different than other nanoparticle syntheses. Here, we study the mechanism of formation of Na4Ag44(p-MBA)30 clusters and show that it is actually not different than other cluster syntheses. Via kinetics studies of the reaction pathway, we reveal that Na4Ag44(p-MBA)30 clusters form via the sequential growth mechanism wherein smaller species form first and then grow into larger species with the final product in the series being Na4Ag44(p-MBA)30 clusters. We show that Na4Ag44(p-MBA)30 clusters form preferentially, i.e. in high-yield, with other less stable species making up a small amount of the synthetic product. Further, we show that Na4Ag44(p-MBA)30 clusters are the only species in the Ag:p-MBA family of cluster that is stable enough to survive post-processing, which involves precipitation and redissolution. These results show why Na4Ag44(p-MBA)30 clusters can be made in large quantities and without other cluster species in the final product, resulting in a high-yield single-sized cluster product, which is a unique result even though the mechanism is not unique. In this model, we found that sequence reactions occur step-by-step, progressing from one stable cluster to another by surpassing the energy barriers between potential wells. In addition, we found Ag17(p-MBA)123- MPCs form first and then gradually transform into a series of reactions, ultimately resulting in the formation of the largest and most stable cluster, Ag44(p-MBA)304-. Additionally, the kinetic mechanism of Ag:p-MBA MPCs demonstrated that the concentration of monomers can play a significant role in determining the size distribution. A higher monomer concentration would produce reactions with rapid kinetics, making it easier to form a wide range of cluster sizes and ultimately leading to a broader size distribution of Ag:p-MBA MPCs. The utilization of isosbestic points to assign the molar absorption coefficient was promising and enabled better spectral deconvolution. In addition, the PAGE results confirmed the optical absorption spectrum of the intermediate, which is observed to have a smaller size than that of Ag44 MPCs. Furthermore, controlling the reaction mechanism of Ag:p-MBA MPCs resulted in varying yields of Ag44(p-MBA)304- MPCs despite the high stability of this cluster. The presence of oxygen has been found to have a significant influence on the formation of Na4Ag44(p-MBA)30. Our research has revealed a distinct variation in reactions occurring in ambient air versus under an inert gas atmosphere. Synthesis under inert conditions was found to lead to the production of Ag17 and Ag44 with slower kinetics. The opposite observations were made in the formation of gold MPCs under inert conditions, indicating that the presence of oxygen may play a different role in the reaction mechanism that synthesizes silver MPCs. Copper MPCs have been successfully synthesized under Ar gas with two different ligands. The syntheses of copper MPCs show slow reduction kinetics compared to silver and gold MPCs. However, the characterization of copper MPCs showed many challenges due to easy oxidation.