Hierarchical Transformation of Silver Morphologies on Clay Film from Spheres, Cubes, Rods to Lengthy Nano-Wires
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Hierarchical Transformation of Silver Morphologies on Clay Film from Spheres, Cubes, Rods to Lengthy Nano-Wires Kai-Ling Liang, Ya-Chi Wang, and Jiang-Jen Lin* Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan ABSTRACT Silver nanoparticles (AgNPs) in silicate clay matrix films were fabricated from solution casting method. The Ag/clay dispersion was first prepared from in situ reduction of silver nitrate in the presence of silicate clay platelets and ethanol as the reducing agent. The morphologies of AgNPs have changed in a hierarchical manner, from sphere to cube and then to rod and wire morphologies during the annealing at 200 °C. The originally homogeneous AgNPs distribution in the clay matrix underwent the transformation of AgNPs in moving to the film surface and coalescing to larger sizes. The hierarchical change continued to form other morphologies. We observed the self-assembled morphologies including spherical (diameter ~ 50 nm), cubic (length ~100 nm), rod-like (length ~ 1.6 μm and width ~300 nm) and then to lengthy wire Ag (length ~10μm). The kinetic mobility of AgNPs to surface and the characterization of Ag composition were confirmed through energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). INTRODUCTION The size- and shape-controlled Ag nanoparticles (NPs) had been investigated previously. Here we report the occurrence of hierarchical changes in morphologies through self-assembly processes. One-dimensional (1D) metal nanostructures such as nanorods, nanowires and nanotubes are extensively studied for their unique chemical, physical and geometric properties and the potential applications [1-4]. Several methods are employed to fabricate 1D silver nanostructures including the uses of hard template, soft template, seed-mediated growth, polyol process, self-assembly, sonoelectrochemical mean and so forth. Most processes are in aqueous-phase process and with a capping agent [5-9]. We observe the hierarchical transformation of silver morphologies from nanoscale spheres, cubes, rods to lengthy wires. We further demonstrate the kinetics and the mechanismof the transformations. . EXPERIMENTAL DETAILS
Sodium montmorillonite (Na+-MMT), was obtained from Nanocor Co. The commercially available clay has exchangeable Na+ counterions with a cationic exchange capacity of 120 mequiv/100g and an average of 8-10 sheets in a primary stack. The size of individual platelets in the polydisperse MMT primary stack is estimated to be 80-100 nm in width and 1 nm in thickness. The nanoscale silicate platelets (NSP) were obtained by the delamination of the layered mineral, reported previously by our research group. [10-12]. Experimentally, the NSP slurry (0.90 g in 45 g water, 2 wt%) was first dispersed well by agitation and soaking in deionized water at room temperature for several minutes, followed by adding 45g ethanol and 1 wt% solution of AgNO3 in water at the designed weight ratios of Ag/NSP, 1/9, 2/8, 3/7 and 5/5. The mixture was stirred vigorously at 80 °C for
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