Multi-Pole Approach to Structural Science

May 10 - 13, 2015

True nanocrystallography
Zbigniew Kaszkur

A popular definition of nanocrystallography is - a branch of science applying methods of crystallography to nanocrystals (NC). This definition already does not apply to novel methods of electron nanocrystallography or femtosecond nanocrystallography that addresses studies of atomic and molecular arrangements in a scale of nanometers, using methods and tools not used previously. It becomes now apparent that structural studies of small nanocrystals of size below 10 nm rise specific questions and require specific methods. NC structure is dominated by surface and is very sensitive to a gaseous environment. With measurable effects of surface relaxation and reconstruction that can be chemically induced the classic tools like Bragg's law, Rietveld refinement, strain analysis etc. are not well applicable. Developed by us for in situ powder diffraction a method to monitor and interpret changes to NC surface structure allows detection of a chemically induced surface reconstruction as well as observations of surface induced symmetry violations and NC reshaping. As most of crystallographic rules for small NC is no longer strictly obeyed, the proposed method builds up new tools of 'true nanocrystallography' basing on atomistic simulations.

After our successful first observation of dynamics of Pt surface reconstruction [1] on hydrogen desorption we were able to measure a degree of surface relaxation (affecting the overall interplanar spacing) on adsorption and relate it to the adsorption energy and the coverage. The observation of changing on adsorption interplanar spacing much exceeding the change expected from adsorption energy and coverage, is indicative of a lateral surface reconstruction phenomenon [2]. Such a tool allowed us to propose an explanation of the observed quick coalescence of Pt in NO atmosphere at 80 deg.C, in terms of a turbulence caused by a self-canceling cyclic surface reconstruction, the reconstruction being detected by our method [3]. The cyclic phenomenon would be caused by a changing on reconstruction number of the atoms exposed to the adsorbate. The caused surface turbulence forms a likely driving force for a nanocluster transport and merger.

The developed tools allows also e.g. explanation and control of the reversible surface segregation phenomena in PdAg nanoalloy giving insight into elementary diffusion mechanisms [4].

This novel nanocrystallography can be well applied to NCs under pressure overcoming known in catalysis so called pressure gap and material gap.

1. Rzeszotarski P., Kaszkur Z., Phys.Chem.Chem.Phys., 11, 5416 – 5421 (2009).
2. Kaszkur Z., Rzeszotarski P., Juszczyk W., J.Appl.Crystallogr., (2014), 47, 2069-2077.
3. Kaszkur Z., Mierzwa B., Juszczyk W., Rzeszotarski P., Łomot D., RSC Adv., 4 (28), 14758 – 14765 (2014) .
4. Kaszkur Z., Juszczyk W., Łomot D., Phys.Chem.Chem.Phys., (2015), under review.