This page extends data provided by the web site of the Institute of Physical Chemistry PAS: http://ichf.edu.pl/res/res_en/lab_xrd/LabXRD.html
Address to contact the author : email@example.com
The author enjoying a night shift in the Institute. (Picture taken arround year 2000)
MOTIVATION : X-Ray Diffraction is widely considered as a major contributor to the development of the XX century chemistry. This status has been achieved in spite of the fact that the technique was used mostly for bulk phase analysis and crystal structure solution - neglecting principal processes of chemical reactions that have their origin e.g. at the solid-gas interface - at the surface . Up to now observation of surface interactions was possible only for internal surface of crystalline porous materials e.g. zeolites. Nowadays with availability of nanopowdered materials, the in-situ diffraction is able to offer a major breakthrough - when number of atoms at the surface is comparable to that of bulk atoms, we can directly observe elementary chemical processes at the surface. The challenging task is to interpret these observations. To this end a large scale atomistic simulations may offer a significant help. I am convinced that in close future a major contribution to chemistry will come from in-situ diffraction and a direct observation of the solid-gas interface via nanopowder diffraction will provide many surprises in the established picture of elementary chemical reactions. It is unbelievable that so far the problem focused so little attention...
- to make possible structural observation (in-situ) of the state of interface (metal-gas) during slow physico-chemical process.
This general goal comprises of two principal tasks:
- be able to detect the signal originating from the interface
- understand this signal and its evolution in terms of structure.
The structural information can be accessed via powder diffraction of X-rays when applied to nanopowdered metal. The scattered intensity due to the interface region is only noticeable when the number of atoms on the metal surface contributes significantly to the overall number of metal atoms. This may be the case for nanopowders. Such small lumps of matter even when apparently crystalline, scatter X-rays in the way not obeying strictly the Bragg law. The terms coined by me are: "nanopowder diffraction" and "diffraction beyond the Bragg law".
One thing is to detect the powder diffraction evolution on some surface process (chemical reaction, chemisorption, disordering etc.), the other is to interpret the observed changes in terms of structure.
To this end already for some years I attempt to develop numerical modelling tools addressing experimentally observed processes and designed to be easily interfaced to the experiment.
On the other hand it appears that designing a right scenario for the in situ structural process to study (employing such parameters as gas composition, temperature, pressure) the nanopowder diffraction alone may suffice in providing satisfactory structural model.
The principal tool developed to complement the in situ nanopowder diffraction experiment is CLUSTER - the building and simulation program with graphical interface.
Some illustration of the program potential is presented graphically on the following slide show. For the next slide just click on the current slide. The slide resolution is 1024x768.
CLUSTER can be downloaded together with its environment in cluster.tar.gz gzipped tar archive. Following guidelines in deployment.txt and correcting few symlinks it should form application working under Linux OS in graphical Xwindow environment. For the moment there is no manual for it and help is limited to scant hints. Works the best with resolution at least 1024x768.
The scope of the project was broadened to interpret the evolution of the structure of bimetallic nanoparticles in some chemical reaction conditions.
The importance of the project may be well understood bearing in mind that most processes of heterogeneous catalysis occur at the surface of metal nanoparticles finely dispersed on some more chemically inert support. Although many physical techniques has already been employed in catalysis to study such surface states and species, these are mostly spectroscopic and do not allow a direct structural insight.
Some general philosophy of the project can be found in a popular article published in journal of Polish Academy of Science - ACADEMIA.
A list of Zbigniew Kaszkur publications.
Brief review of some interesting observations and modelling:
Small nanocrystals do not obey the Bragg law
A direct XRD observation of surface relaxation in nanocrystalline palladium
An observation of a form of Pd with local 5-fold symmetry (icosahedron-like) that does not form hydride PdH
The experimental XRD measurement of a peak shift caused by changing segregation profile and its interpretation (slide show)
Test of applicability of some powder diffraction tools to nanocrystals (presented at EPDIC IX, 2004, PDF).
Surface reconstruction of Pt nanocrystals interacting with gas atmosphere (presented at EPDIC XI, 2008, PDF), published Rzeszotarski,Kaszkur, Phys.Chem.Chem.Phys., 11, 5416-5421 (2009).
Nanocrystalline powder diffraction as a surface science tool (presented at EPDIC XIV, 2014, PDF).
An example of atomistic simulation used in recent author studies:
Au-Pd alloy 3871 atom (magic number 10) model cluster after configurational and spatial energy minimisation. All the atoms were subjected to configurational (Monte-Carlo, Metropolis) and spatial energy minimisation (relaxation of the coordinates). The configurational Monte-Carlo minimisation was supplemented with spatial relaxation at every unlike atom exchange. The resulting segregation profile is presented below:
- gold atom , - palladium atom
The cluster surface was constrained to contain only Pd atoms that, unlike Au atoms, are chemically bonding with chemisorbed surface oxygen. The configurational Monte-Carlo relaxation (with spatial relaxation) then produces segregation profile shown below:
The segregation profiles are presented as number of atoms surrounding the central atom vs. distance. Chemisorption in this model case causes total inversion of the segregation profile.
Home Page of the Laboratory of X-Ray Powder Diffraction and Spectroscopy