Cluster Coagulation and Growth Limited by Surface Interactions with Polymers

The physical and chemical properties of metal nanoparticles differ significantly from those of free metal atoms as well as from the properties of bulk metals, and therefore they may be viewed as a transition regime between the two physical states. Within this nanosize regime, there is a wide fluctuation of properties, particularly chemical reactivity, as a function of the size, geometry, and electronic state of the metal nanoparticles. In recent years, great advancements have been made in the attempts to control and manipulate the growth of metal particles to prespecified dimensions. One of the main synthetic methods utilized in this endeavor, is the capping of the growing clusters with a variety of molecules, e.g., polymers. In this paper we attempt to model such a process and show the relationship between the concentration of the polymer present in the system and the final metal particle size obtained. The theoretical behavior, which we obtained, is compared with experimental results for the cobalt-polystyrene system. The physical and chemical properties of metal nanoclusters differ significantly from those of individual metal atoms, as well as from the properties of bulk metal, 1-5 and hence the size regime of nanoclusters may be viewed as a transition regime between the two physical states. Within the cluster regime, there is a wide fluctuation of properties, particularly chemical reactivity, depending on the size, geometry, electronic state, and packing of the cluster. 6-13 Uniformity of size and spatial distribution of the metal nanoclusters is essential in the study of their properties and can be achieved primarily by conducting their synthesis in the presence of stabilizers. These materials, such as surfactants or polymers, adsorb onto the surfaces of growing clusters and create a “shielding effect”, a chemical barrier that prevents the effects of van der Waals interactions between particles, thus inhibiting the particle aggregation process. 14-23 Polymers are frequently used as stabilizers for metal clusters because they are transparent, permeable, and nonconductive, and as such, do not interfere with and/or mask the potential optical, electrical, and catalytic properties of these clusters. Synthetic routes for the formation of nanoclusters include ultrahigh vacuum techniques (UHV) and “wet” chemical synthesis. The UHV processes, e.g., chemical vapor deposition, have traditionally dominated in the preparation of nanoclusters due to their inherent controllability, but the they lack the flexibility necessary to manipulate a variety of properties within the same system. The chemical synthetic methods, e.g., reduction of metal halides and decomposition of organometallic complexes, are becoming more popular in recent years due to the ability to introduce stabilizing agents and therefore afford limitation of aggregate size. The motivating physical system for this work is the chemical synthesis of nanoscale metal clusters via the thermal decomposition of metal carbonyl complexes in solutions containing either pure solvent or, in addition, polymers, which will result in a more viscous reaction environment. The main factors governing the nucleation and growth of the metal particles during the decomposition process are the metal cluster size, i.e., the number of reactive sites available for chemical reactions, and the mobility of the reactive metal clusters and their ability to diffuse through the solution and collide with each other. Clearly, as particle size grows, the mobility of the particles decreases and a preferred average pseudo-equilibrium size distribution is reached. If the medium in which the thermal decompositions take place is more viscous than a pure solvent, then it is expected that the upper bound on particle mobility will be reached faster, and hence the final average particle size will be smaller. The particular system studied in this work consists of cobalt carbonyl complexes thermally decomposed in the presence of polystyrene (PS). It is important to note that the solvent chosen for the decomposition reactions was toluene, which is a good solvent for polystyrene, and therefore it promoted the solvation of the polymer over a large composition range. Based on this experimental system, we have developed a mathematical model that aims at elucidating the effect of the initial concentration of the polymer present in the reaction solution on the final cobalt particle size obtained via the thermal decomposition mechanism, and correlates the model to our experimental results.