Tuning size and morphology of nanostructures: The fundamental role of transmission electron microscopy in synthesis control

Abstract

In this work, the paramount importance of synthesis variables on shape, size and polydispersity ofmetal and oxide nanostructures is shown with an especial focus put on the role of electronmicroscopy as a powerful tool of characterization. Two types of systems, ferrogels and goldnanostructures were analyzed.Ferrogels with well-dispersed magnetic nanoparticles (NPs) were obtained by the infusion of ironsalts in cross-linked polyvinyl pyrrolidone (PVP) hydrogels followed by coprecipitation. Twodifferent PVP concentrations (PVP 8% and PVP 16%) in the initial solution were used and thegelation of the polymer was carried out with ammonium persulfate.On the other hand, gold nanostructures were obtained by mixing oleylamine, HAuCl4 andtri(isopropylsilane) in tetradecane, the solution is left without agitation for 12hs at room temperatureand 80 ºC.Magnetic gel microstructure was studied by scanning electron microscopy in a JEOL JSM-6460 at15 kV (Figure 1A-D). In order to observe iron oxides nanoparticles inside, magnetic gel was ground,resuspended in water and sonicated. Dispersions were placed on 200 mesh carbonstabilizedFormvar-film copper and observed by transmission electron microscopy, in a JEOL JEM2100 at 200 Kv (Figure 1B-C-E-F). Final size of NPs was controlled by concentration of the polymerin the initial aqueous formulation.Both gold dispersions, room temperature (Figure 2A1-2) and 80ºC (Figure 2B1-2) were placed on 200mesh carbon-stabilized Formvar-film copper grids and were studied by transmission electronmicroscopy in a JEOL JEM 2100 at 200 kV. Completely different structures were obtained bychanging reaction temperature between room temperature and 80ºC. Difference in morphology wasattributed to a changein the stability of a reaction intermediate complex.