Natural Nanoparticle Structure, Properties and Reactivity from X-ray Studies

Synthetic analogs of naturally occurring nanoparticles have been studied by a range of x-ray techniques to determine their structure and chemistry, and relate these to their novel chemical properties and physical behavior. ZnS nanoparticles, formed in large concentrations naturally by microbial action, have an interesting core-shell structure with a highly distorted and strained outer layer. The strain propagates through the particles and produces unusual stiffness, but can be relieved by changing the nature of the surface ligand binding. Weaker bound ligands allow high surface distortion, but strongly bound ligands relax this structure and reduce overall strain. Only small amounts of ligand exchange causes transformations from the strained to relaxed state. Most remarkably, minor point contacts between strained nanoparticles also relaxes the strain. In TiO2 nanoparticles a similar core-shell structure is observed with distortions severe enough to produce an almost amorphous structure, perhaps similar to a silica glass. Fe oxyhydroxide nanoparticles appear to go through structural transformations dependent on their size and formation conditions, and display a crystallographically-oriented form of aggregation at the nanoscale that alters growth kinetics. At least one Fe oxyhydroxide mineral may only be stable on the nanoscale, and nonstoichiometry observed on the hematite surface suggests that for this phase and possibly other natural metal oxides chemistry may be size-dependent. Numerous questions exist on nanominerals formed in acid mine drainage sites, and by reactions at interfaces.