Size-Dependent Energy of Ni Nanoparticles on Graphene Films on Ni(111) and Adhesion Energetics by Adsorption Calorimetry

Abstract

The use of carbon supports for late transition-metal nanoparticle catalysts has grown substantially in recent years due to efforts to develop electrocatalysts for clean energy applications and catalysts for new aqueous-phase biomass-related conversions and due to the evolution of new carbon materials with unique properties (e.g., graphene, carbon nanotubes, and so forth). However, much less is known about the bonding energetics of catalytic metal nanoparticles on carbon supports in comparison with oxide supports, which are more common for thermal catalysis. Here, we report the growth morphology and heats of adsorption of Ni vapor deposited onto graphene/Ni(111) at 300 K and 100 K using metal vapor single-crystal adsorption calorimetry and He + lowenergy ion scattering (LEIS). These results provide the Ni chemical potential versus particle size, and the Ni/graphene adhesion energy. LEIS intensities suggest that Ni grows as flat-topped face-centered cubic islands with a nearly constant thickness of ∼1.5 nm when deposited at 300 K. At 100 K, Ni grows as smaller nanoparticles, well modeled as hemispherical hexagonal close-packed nanoparticles with a density of ∼2 × 10 16 particles/m 2 . The Ni chemical potential as a function of average particle diameter in the 0.5 to 4 nm range at 100 K was determined from the heats of Ni gas adsorption. By fitting the measured chemical potential as a function of diameter, we determined an adhesion energy of 3.6 J/m 2 for large Ni particles on graphene/Ni(111). This adhesion energy is in good agreement with previous scanning tunneling microscopy and density functional theory investigations of Ni/ graphene/Ni(111).

Size-Dependent Energy and Adhesion of Pd Nanoparticles on Graphene on Ni(111) by Pd Vapor Adsorption Calorimetry

“…While we have studied the energetics of late transition-metal nanoparticles on oxide supports using single-crystal adsorption calorimetry (SCAC) for decades, 13 there have only been two previous experimental measurements of the energetics of metal nanoparticles on any carbon-based supports: our previous reports of the sizedependent energetics and adhesion energies of Ag and Ni nanoparticles on single-layer graphene(0001) supported on Ni(111). 14,15 Here, we report a similar study of Pd nanoparticles on the same graphene/Ni(111) surface and discover a correlation between the adhesion energy at the metal nanoparticle/support interface (E adh ) for these three metals and the metal element's carbophilicity. Carbonsupported Pd nanoparticles are widely used in both basic and applied catalysis and electrocatalysis research.…”

“…For example, at 0.5 ML, the Pd particle diameter at 100 K is ∼2 nm, while it is ∼5 nm at 300 K. When a new adatom arrives, fewer Pd–Pd bonds form on the smaller particle surface as smaller particles have a much larger fraction of undercoordinated atoms and, therein, less nearest neighboring atoms to form Pd–Pd bonds. In comparison, the larger, flat-topped (herein, faceted) particles formed at 300 K have more nearest neighboring atoms to form more Pd–Pd bonds when a new adatom reaches the particle surface . This continues until the hemispherical caps at 100 K grow sufficiently large that they are dominated by sites where new Ni atoms can make as many Ni–Ni bonds upon adsorption as at 300 K (∼3 ML of Pd coverage).…”

“…The flat-topped shape of Pd particles was also seen in STM on the HOPG surface at room temperature and also at higher temperatures (400 K on graphene and 700 K on HOPG). When Ni is deposited on this same graphene(0001)/Ni(111) surface at room temperature, it also grows rather flat-topped islands with small aspect ratios and a fixed number per unit area. , The kinetic reasons for such a growth mode have been explained. ,, …”

“…We also show there the reported DFT results for Ni clusters on free-standing graphene. ,, As seen, the Ni heats are always slightly higher than those for Pd at the same coverage, but they are slightly lower than those for Pd when compared at the same cluster size. This is because the clusters for Ni are larger than for Pd at the same coverage (due to the smaller number density of clusters for Ni at 100 K), yet atoms in small Pd clusters bind slightly more strongly when compared to Ni at the same cluster size. Since Pd has a 13% lower heat of sublimation than Ni, Pd–Pd bonds are expected to be ∼13% weaker than Ni–Ni bonds, so this small difference for small clusters is dominated by the stronger bonding of the metal clusters to the graphene/Ni(111) surface for Pd compared to Ni at the same tiny cluster size.…”

“…In comparison, the larger, flattopped (herein, faceted) particles formed at 300 K have more nearest neighboring atoms to form more Pd−Pd bonds when a new adatom reaches the particle surface. 15 This continues until the hemispherical caps at 100 K grow sufficiently large that they are dominated by sites where new Ni atoms can make as many Ni−Ni bonds upon adsorption as at 300 K (∼3 ML of Pd coverage).…”