Nanoscopic materials

Roduner E. 2006. Nanoscopic Materials Size-dependent Phenomena. Cambridge The Royal... 

At the end of the millennium, chemists have become more adventuresome as they prepare and characterize nanoscopic materials and probe the macroscopic regime. There is little doubt that as confidence continues to grow, coupled with the creation of new instrumentation, the next century will unveil unnatural constructs of perfect, predictable nanostructures with molecular weights of currently unprecedented magnitude. 

Pinna, N., M. Willinger, K. Weiss, J. Urban, and R. Schlogl. 2003. Local structure of nanoscopic materials V2Os nanorods and nanowires. Nano Lett. 3 1131-1134. 

This originates from the changes in Ef-LDOS at the nanocrystalline metal surface and at the adsorbate, induced by electrochemical potential control [142]. A layer model analysis is used to describe the Pt NMR spectrum of nanoscopic materials [144]. It is also possible to correlate the electronegativity of the adsorbates with the Knight shift associated with the Pt nanoparticles [138]. The orientation of adsorbates on metallic substrates under potential control conditions has also been explored [122, 131]. Tong and co-workers have recently demonstrated the use of EC-NMR to investigate the electronic environment of the core of MPCs [145]. 

The Pd/CNF catalyst displays a catalytic activity as high as that obtained on a commercial catalyst supported on activated charcoal despite the large difference between the two supports surface area, i.e. 100 m /g for the CNFs instead of 1000 m /g for the activated charcoal. The high hydrogenation activity observed on the CNF-based catalyst was attributed to the high external surface area of the support and to the peculiar interaction existing between the prismatic planes and the metallic particles. Recently, a significant improvement was introduced via a new synthesis route which allows the possibility for these nanoscopic materials, to be supported on a macroscopic host structure [17]. 

In the last decade much attention has been paid to metal nano-clusters including supported nanoparticles as one of the promising advanced nanoscopic materials. Elements easily forming supported metal nanoclusters are Group VIII and IB transition metals as follows Pt, Ir, Pd, Rh, Ru, Ni, Co, and Au, Ag, Cu. It is interesting to note that the heat of formation of the oxides of these metals is low (usually below -AHf = 40 kcal/mol at 25 °C referred to one oxygen atom ). 

Many nanoscopic materials are constructed from nanocrystals or molecular assemblies on a repeating basis. Gold nanoparticles 15 to 20 nm in diameter are made by reduction of AUCI4 in a citrate solution (8). By high resolution TEM the gold nanoparticles often consist of smaller nanocrystals fused together, as shown in Figure 11.2. 

Small-angle X-ray and neutron-scattering (SANS) studies have been used to obtain detailed information on the stmcture of a variety of micelles. According to these studies, the thickness of the Stem layer is 6-9 A for cationic cetyl trimethy-lammonium bromide (CTAB) micelles and anionic sodium dodecyl sulfate (SDS) micelles. For nonionic micelles, the hydrocarbon core is surrounded by a palisade layer, which consists of the polyoxyethylene groups hydrogen-bonded to water molecules. The palisade layer is about 20 A thick for neutral TritonX-100 (TX-lOO) micelles. The radius of the dry, hydrophobic core of TX-lOO is typically 25-27 A [2]. Thus the overall radius of the TX-100 micelle is about 51 A and that of the SDS micelle is about 30 A. So, these stmctures are much bigger in size than a small molecule. The radius of a water molecule is just about 1.5 A. This is why micelles are called nanoscopic materials. We show the stmcture of micelles in Figure 17.1. 

Due to recent improvements in visualization of nanoscopic material with enhanced resolution, huge advances in the characterization technology have been widespread in all the nanotechnology disciplines, compared to just about 20 years ago. A few examples of nanoscopic materials that are less than Ipm are illustrated in Figure 1.2. Current efforts in the nanoparticle—polymer field go into very small metallic and semiconductor nanoparticles with 1—2nm diameter, and use of micron-sized fillers (e.g., layered silicates) (Polymer-Nanoparticle Composites Part 1 (Nanotechnology), 2010). 

Roduner, E. (2006) Nanoscopic Materials. Size-Dependent Phenomena, RSC Publishing, Cambridge. 

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