(245a) Effect of Surfactants on Cu Nanoparticle Surface Chemistry and Aggregation
Agglomeration and aggregation in both dry and dispersed nanoparticles is a significant problem. Currently, nanoparticles can be generated by a variety of methods, including liquid and gas phase precipitation and vacuum synthesis techniques. For nanoparticles prepared via the liquid phase route, preventing agglomeration in suspension and in the dry state is highly desirable. Irregular agglomerates that form during the drying process present difficulties for material characterization, and ultimately introduce flaws into the final product. This problem becomes increasingly difficult as the particle diameter decreases to the nanometre region where electrostatic and Van der Waal's forces contribute significantly to inter-particle attraction, and eventually facilitate the formation of hard agglomerates during particulate drying. The mechanical strength of these agglomerates depends on the nature of the binding forces at coordination points between individual particles, at interfaces between the liquid matrix and the solid particles, and/or through liquid bridges between the individual particles. In order to generate dispersed ultra fine metal powers (< 500 nm) for the ceramic and capacitor industry, the mechanisms of metal particle agglomeration must be better understood. We have recently shown that agglomeration in micron-sized copper and nickel particles is aided by the formation of a thin surface oxide layer. Addition of solutions of various surfactants to suspensions of these metallic particles helps prevent agglomeration upon extraction to the dry state in the case of amines, whereas for other molecules such as benzenethiol the degree of agglomeration increased. We also show in atomic force microscope (AFM) experiments that this can be correlated with changes in the forces of adhesion between particle surfaces. A better understanding of the surface chemistry and the interfacial interaction between surfactant and particle is therefore needed for controlling agglomeration as the diameter of metal particles decreases into the nanometre regime. In order to explore these effects we have carried out density functional theory (DFT) molecular modelling studies for small surfactant molecules interacting with a Cu(111) surface, used as an analogue of a Cu nanoparticle surface. Adsorption geometries, adsorption energies and vibrational properties have been calculated for propylamine and benzenethiol. We compare these to the X-ray photoelectron spectroscopy (XPS), diffuse reflectance infrared spectroscopy (DRIFTS) and thermogravimetric analysis (TGA) results obtained from the adsorption of these molecules onto Cu particles. For example, while the amine molecule can potentially bind to a Cu surface with dissociation of either one or two N-H bonds, calculations suggest that the energy barrier to dissociation of the second N-H bond at room temperature is significant. This agrees with IR data obtained from propyl- and butylamine coated Cu particles, which indicate that amine is indeed adsorbed with cleavage of only one N-H bond.