Core/Shell Formation of Gold Nanoparticles Induced on Exposure to N,N-Dimethylformamide: Chemical and Morphological Changes

Abstract

Au nanoparticles, ∼15 nm in diameter, were deposited onto both Au and Si substrates and were exposed to droplets of N,N-dimethyl formamide (DMF), which were permitted to dry (∼24 h). When drying was complete, nanoparticle coalescence was found to have occurred in both cases, giving nanoparticles ranging from tens to hundreds of nm in diameter. DMF drying on the Au substrate surface was seen to have occurred by contact line pinning/depinning, to give concentric drying rings, whose edges contained coalesced nanoparticles. No such drying rings were seen for Au nanoparticles after DMF drying on the Si substrate; instead, coalesced Au nanoparticles were found to be uniformly distributed. All Au surfaces, both substrate and nanoparticle, were found to have been covered with a ∼3 nm C,O-containing shell, which formed through the reaction of DMF with Au in the presence of air; the heat produced by this reaction aided the coalescence process, resulting in the larger nanoparticle sizes. The shell was not wet by DMF.