SWAXS investigations on diffuse boundary nanostructures of metallic nanoparticles synthesized by electrical discharges

Metallic nanoparticles have attracted a particular interest in scientific research and industrial applications due to their unique size-dependent physical and chemical properties. An eco-friendly and cost-effective synthesis method called electrical discharge enables large scale production of metallic nanoparticles. Systematic investigations of such synthesized metallic nanoparticles help to optimize the synthesis process and improve the product quality. In this work, for the first time we have investigated the diffuse interfacial boundary nanostructures of the metallic nanoparticles, which were synthesized under different conditions by electrical glow and arc discharges in the carrier gas, by means of a small-and wide-angle X-ray scattering (SWAXS) technique using a laboratory X-ray source. Meanwhile, this unique SWAXS technique allows simultaneous study of the primary particle size, morphology, and crystallinity. The metallic nanoparticles (copper and nickel) under investigation cover a size range of 10-80 nm, and the determined thickness of the diffuse boundary nanostructured layer of metallic nanoparticles is in the range of 1-3 nm. The experimental results obtained by SWAXS were compared to the TEM/EDX observation and the XRD reference patterns from RRUFF database, and a good agreement was found. Our SWAXS investigations indicated that the existence of a diffuse nanostructured solid layer on the synthesized metallic nanoparticle surface causes a negative deviation of the scattering intensity (I proportional to q(-alpha), alpha > 4) from Porod's law which corresponds to the case of ideal two-phase particle systems with sharp boundaries (I proportional to q(-alpha), alpha = 4). This implies that the electron density profile is not sharp but changes gradually between two phases, and hence the exponent a is greater than four. Two electron density profile models, sigmoidal electron-density gradient model and linear electron-density gradient model, have been taken into account in determining the thickness of the diffuse boundary nanostructured layer. Combined with the SWAXS experimental results, the possible mechanisms of the formation of the diffuse boundary nanostructured layer of the metallic nanoparticles synthesized by electrical glow and arc discharges in the gas phase have been discussed in detail.