With the rapid rise in the importance of nanoscale materials, many standard structural characterization methods based on ordered structures have become much less useful, and need to be supplemented by innovative application of techniques such as NMR. In this work two examples in nanomaterials and interfacial chemistry are described where critical atomic level structural details, as well as information on dynamics, were obtained by the use of solid state NMR spectroscopy in conjunction with computational and related experimental approaches.
The first instance addresses the question of the activation of a grafted organometallic rhenium complex by virtue of its interaction with a particular support, namely amorphous silica-alumina. The objectives of this research are to establish the molecular origin of the catalytic activity of this supported Re complex including the structures of adsorption and reaction sites and their relationship to its activity for olefin metathesis. The chemisorption of CH3ReO3 on the surface of a dehydrated, amorphous silica-alumina generates a highly active catalyst for the metathesis of olefins, including functionalized olefins.
Synthesis of this catalytic system is accomplished by chemical vapor deposition onto the support. The principal characterization methods were 1D and 2D solid state NMR (1H, 13C, 27Al), EXAFS, IR, and computational modeling. This catalyst was found to contain two families of chemically and spectroscopically distinct sites, which react differently with olefinic reagents, depending on the grafting conditions, support modification, and/or Re loading. The active grafting site is one with a two-point interaction of the Re complex with the support, wherein one oxo ligand interacts with an Lewis acidic Al site and a support oxygen is weakly coordinated to the Re center. A second, highly mobile CH3ReO3 species is primarily found at higher catalyst loadings, and was determined to be bound by hydrogen-bonding to surface silanols. Its presence furthermore decreases the catalytic activity in propylene self-metathesis reactions. This work demonstrates that the olefin metathesis activity is correlated with the presence of Lewis acid activated CH3ReO3, so that a material with high (10 wt.%) Re loading with a large number of H-bonded sites is no more active than that with low (1 wt.%) Re loading and only Lewis acidic site bonded CH3ReO3.
In the second part of this work the effect of rather different interfacial interactions are examined, namely those of structural changes following surface treatments on the optical properties of nanoscale Group III-V semiconductors and their ternary analogs. Recent advances in chemical synthesis protocols have made certain coated In0.91Ga0.09P nanoparticles attractive, less- toxic alternatives to Cd-, Se-, and Te-based Group II-VI compounds for electrical, opto-electronic, or biological device applications. 1D and 2D MAS NMR experiments (31P, 71Ga, 115In) were used to establish surface reconstruction and changes in local environments of sites linked to increased photoluminescence upon etching of 4.5 nm InGaP quantum dots. These experiments provide a picture of the atomic and molecular distribution on the surface and the interior of the nanoparticle, and show that etching with HF results in significant reorganization of the InGaP nanoparticle surfaces and removal of surface defects. This leads to the 71Ga species being located in chemically and electronically more ordered molecular environments near the periphery of the nanoparticle, and hence improved photoluminescence.