(Invited) Pt Monolayers and Submonolayers on Nobel Metal Nanoparticles: Substrate- and Coverage-Depdendent Atomic Structures and Electrocataltyic Activities

Abstract

Galvanic replacement of underpotentially deposited Cu monolayers (UPD) on noble metal substrates by Pt has been shown a robust strategy to synthesis Pt monolayer catalysts with ideally a 100% Pt utilization efficiency for energy conversion reactions, such as methanol oxidation reaction (MOR) in direct methanol fuel cells. The true atomic structures of the ‘Pt monolayers’ on different noble metal substrates, particularly on nanoscale metal nanoparticles, however, have still remained elusive. By using high resolution (scanning) transmission electron microscopy combined with highly efficient energy-dispersive X-ray spectra mapping, we report here the atomic structures of Pt (sub)monolayers deposited on different noble metal nanoparticles (e.g. Au, Pd, Pt, and Ir, etc.) and at different coverage using the UPD and subsequent galvanic replacement method. Our results reveal that the Pt-substrate interaction has a pronounced influence on the growth mode and atomic structures of the Pt ‘monolayer’, which follows either Volmer-Webber island growth mode or Franck- van der Merwe layered growth mode, leading to different Pt utilization efficiencies and thus different electrocatalytic MOR activities. The nominal coverage of the Pt (sub)monolayers on the noble metal nanoparticles plays an additional role in fine-tuning the atomic structures, Pt utilization efficiency and electrocatalytic MOR activities. Our results provide important atomistic insights into a rational design of Pt (sub)monolayer catalysts with high Pt utilization efficiency and high catalytic activities.