Expansion of interatomic distances in platinum catalyst nanoparticles
by Kui Du, Frank Ernst, Michael C. Pelsozy, Juri Barthel, and Karsten Tillmann
The atomistic structure of Pt catalyst nanoparticles has been studied using quantitative high-resolution transmission electron microscopy. The particles are found to exhibit a faceted, cubo-octahedral shape, extended planar defects, and mono-atomic surface steps. High-resolution imaging with negative spherical aberration yielded atomic-resolution images with a minimum of artifacts. Combining digital image processing, quantitative image analysis, and HRTEM image simulations to determine local variations of the spacing between neighboring Pt atom columns, an expansion of the lattice parameter in the particle core and even larger, locally varying expansion of Pt-Pt next-neighbour distances at the particle surface was observed. The latter likely originates from an amorphous oxide on the nanoparticle surface and/or dissolution of oxygen on subsurface sites. These structural features may significantly impact the catalytic activity of Pt nanoparticles.
The observation of expanded inter-atomic spacings at the particle surface and surface corrugation caused by stacking faults, twin boundaries, and surface steps suggests that the impact of strain and extended structural defects on catalytic activity warrants further investigation. The complex correlation observed between the spatial variation of inter-atomic spacings, surface scales, particle shape, and contact to other particles demonstrates that techniques that average over a large ensemble of particles may lead to non-realistic conclusions. A clear disadvantage of HRTEM is its limitation to ex situ studies, i.e. studies performed not in the same environment and at the same temperature at which respective catalysts operate. A possible route to a deeper understanding could be to combine TEM studies with in situ techniques, such as XRD and XAS, in order to enable more detailed interpretation of XRD and XAS in situ data in terms of structural details of individual particles.
Kui Du, Frank Ernst, Michael C. Pelsozy, Juri Barthel, and Karsten Tillmann: Expansion of interatomic distances in platinum catalyst nanoparticles, Acta Materialia 58 (2010) 836-845