Vaporization of bulk metals into single-digit nanoparticles by non-thermal plasma filaments in atmospheric pressure dielectric barrier discharges

Abstract

A compact, inexpensive and simple dielectric barrier discharge (DBD) design is presented with related electro-thermal properties for the production of metal nanoparticles.Nanoparticle formation and growth mechanisms are depicted from size distributions and chemical analyses of particles collected just after the 70kHz DBD in nitrogen. At first, it is confirmed that the initial local vapor flux is produced from the spots of interaction between plasma filaments and different metal electrodes (Au, Ag, and Cu). Amorphous and crystalline pure metal primary nanoparticles with diameters below 5nm are then produced by physical nucleation in expanding vapors jets. Finally, some small agglomerates with diameters still below 5nm are also formed by ballistic agglomeration of a fraction of these primary particles. This happens at the end of the vapor jet expansion, as well as after the production during the transit between subsequent filaments in the DBD. The first local agglomeration step can be limited at reduced energy per filament by lowering the initial vapor flux in smaller gaps, while the second growth step depends on the transit time in the DBD. Hence, such “low” energy plasma filaments (up to a few tens of µJ) lower the initial vapor flux to control the agglomeration. DBD were thus successfully tested for the production of tailored nanoparticles with tunable size, controlled morphology of spherical agglomerates and the same composition as the metal electrode.The production per unit energy (molJ−1) is related to both plasma and material properties. Besides, neglecting vapor and nanoparticles losses, the mass production rate (gs−1) depends on the input power related to the product of the energy controlling the production per filament times the number of filaments per second, for any given material.This non-thermal plasma process presents great potentialities for nano-technologies since it is performed at atmospheric pressure and can be used to reach size-dependent properties of nano-materials, without any gaseous precursor or solvent.