Powder diffraction in studies of nanocrystal surfaces: chemisorption on Pt

Abstract

Atoms at the surface of nanocrystals contribute appreciably to the X‐ray diffraction pattern. Phenomena like chemisorption, affecting the displacement of surface atoms with respect to their positions in the perfect crystallographic structure, cause diffraction peak shifts and intensity changes. These effects are easily measurable for small nanocrystals up to 10 nm size. This article reports diffraction effects of chemisorption of adsorbing gases H2, O2, CO and NO for a series of in situ powder diffraction experiments on nanocrystalline Pt supported on silica. On the basis of previous diffraction observation of Pt surface reconstruction during hydrogen desorption, it was possible to quantify this effect versus crystallite size and rationalize the observed diffraction peak shift for the other adsorbing species. This enabled the surface reconstruction to be distinguished from the surface relaxation effect, the latter depending monotonically on the adsorption energy. Even if no phase transition occurs, monitoring of a peak's position, intensity, width and gas composition (via mass spectrometry) during a carefully designed physicochemical process (including surface chemical reaction) enables insight into and understanding of the surface structure evolution (e.g. amorphization, relaxation, reconstruction or changes in the overall morphology). The proposed technique can be used as a surface science tool, allowing studies of nanocrystals under high pressure.

X-ray scattering characterisation of nanoparticles

“…nm in size [225]. Adsorption of gas molecules on the nanoparticle surface changed the surface energy, resulting in atomic reconstruction which was manifested in changes in peak position Downloaded by [New York University] at 06:05 08 June 2015…”

The real background and peak asymmetry in diffraction on nanocrystalline metals

“…ALP evolution showing lattice expansion due to chemisorption has been measured in situ via powder X-ray diffraction. The reported (Kaszkur et al, 2014) values are higher than the calculated relaxation effects, and the net effect causes the diffraction peaks of nanocrystalline platinum to be always shifted to lower angles (higher ALP value) -the greater the shift, the smaller the crystal size. Thus the peak from a crystal size distribution has to show a left-hand-side asymmetry.…”

“…This is illustrated by Fig. 3, showing both the calculated extent of contraction and the measured extent of expansion (Kaszkur et al, 2014). The figure illustrates the diffraction peak shift in terms of apparent lattice parameter (ALP).…”

“…However, for a given reflection the ALP variation follows the structure evolution. Model cluster calculations accounting for inward relaxation followed the global energy relaxation within the Sutton & Chen (1990) potential scheme and were described previously (Kaszkur et al, 2014). ALP evolution showing lattice expansion due to chemisorption has been measured in situ via powder X-ray diffraction.…”

“…From a practical point of view, however, it may be much more important to know what are the changes of the structure during a controlled process, for example chemisorption at the surface. Monitoring and analysis of such changes by exploiting evolution of diffraction peaks is feasible (Kaszkur et al, 2014). An analysis of the peak shape is possible for samples where Bragg diffraction principles still apply.…”