Using iterative under potential deposition (UPD), monolayers layers of Pt catalyst are grown on top of a monolayer graphene sheet, supported on 50 nm (111) Au substrate. X-ray absorption spectroscopic (XAS) and transmission electron microscopy (TEM) analysis show that Pt monolayers growth is dictated by the graphene-teplated epitaxy. Intimate contact between Pt/graphene induces a localized compressive strain on Pt monolayers ranges 3-10% with an overall compressive strain of 3.5% according to extended x-ray absorption fine structure (EXAFS) analysis. In addition, cyclic voltammetry (CV) analysis shows Pt monolayers fully wetting ~100 mm2area of graphene under-layer with only a ~1 nm ultra-thin layer of Pt, while avoiding ripening as shown through TEM images. Atomic Force Microscopy (AFM) analysis demonstrates that Pt monolayers prefer to follow Frank-van der Merwe growth (i.e. layer-by-layer growth mode) rather than Volmer-Weber growth (i.e. island growth mode), where root mean square (rms) of surface roughness remains quite similar while increasing Pt loading is increased.
Pt/graphene hybrid catalysts show superior catalytic activity for ORR relative to the graphene-free counterparts. A combination of the graphene-imposed compressive strain and electron transfer, push the Pt d-band center up, lowering the overpotential needed for ORR to occur. The graphene/Pt hybrid also show superior activity towards ORR for all shallow Pt mass loadings relative to graphene-free cases, which can be attributed to the interface strain. Due to their intimate epitaxial contact, graphene and Pt, therefore, form a new hybrid catalyst that outperforms Pt. Furthermore, the graphene/Pt cap hybrid shows the graphene protecting Pt MLs from both dissolution and from ripening, with almost no Pt loss after 1000 fuel cell operating cycles. Our demonstration of a graphene-Pt hybrid opens the door for graphene/metal or metal/graphene architectures with potential applications in, and not limited to, energy, thermo-electric and electronics field.