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

Abstract

Interest in the use of carbon supports for late transition metal nanoparticle catalysts has expanded rapidly due to the increasing importance of electrocatalysts for clean energy and environmental technologies and the use and storage of renewable electricity. Compared to oxide supports, almost nothing is known about the effect of metal nanoparticle size on the energies of the metal atoms within carbon-supported nanoparticles, yet these energies are crucial for understanding their surface reactivity and sintering kinetics. Here, the growth morphology and adsorption energetics of vapor-deposited Ag onto clean graphene/Ni(111) surfaces have been studied using a combination of single-crystal adsorption calorimetry (SCAC) and He+ low-energy ion scattering (LEIS). The differential heat of Ag adsorption is 207 kJ/mol for making ∼30 atom Ag particles on graphene terraces at 100 K and 16 kJ/mol higher for making ∼9 atom Ag clusters at defect sites at the same temperature. The heat of adsorption increases rapidly with Ag coverage as 3D Ag nanoparticles nucleate and grow in size, asymptotically reaching within 5 kJ/mol of the bulk Ag sublimation enthalpy (285 kJ/mol) by 2 ML. The heats of adsorption and Ag nanoparticle densities from LEIS (∼1016/m2) were combined to provide the Ag/graphene adhesion energy (E adh = 1.8 J/m2 in the large-particle limit) and the Ag chemical potential (μ) versus effective particle diameter (D). The Ag chemical potential was well-fitted by μ­(D) = (3γv/M – E adh)­(1 + (1.5 nm)/D)­(2V m/D), where γv/M is the surface energy of bulk Ag and V m is its molar volume. The same equation is known to fit similar data for late transition metals on clean surfaces of metal oxide single crystals. The adhesion energy of Ag measured here on graphene falls within the wide range measured for Ag on those oxide surfaces and is almost as large as on the oxide that binds Ag particles most strongly, namely CeO2(111), which is well-known to be very effective at resisting catalyst deactivation by metal sintering. These results imply that carbon supports will be effective at resisting sintering and that Ag particles smaller than 6 nm on graphene will bind small adsorbed reaction intermediates more weakly than supports with weaker adhesion to Ag, like MgO(100).

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

“…The sticking probabilities reported here for Ni atoms deposited onto graphene/Ni(111) are up to 20% larger than those previously reported for Ag deposition on the same surface . This can be attributed to a stronger binding of Ni monomers onto the graphene substrate compared with Ag monomers.…”

“…Graphene films were grown on a clean 1 μm thick Ni(111) single-crystal sample using a direct growth method from ethene gas previously described in the literature. ,,, This procedure is known to produce a single layer of graphene on Ni(111) and not multilayers nor multilayer islands. ,, Before growth, the Ni(111) sample was cleaned by cycles of flashing the sample at 800 °C followed by 1.0 kV Ar + -ion sputtering until the presence of the C 1s carbon peak could no longer be detected by XPS. The clean Ni(111) sample was then annealed in vacuum at 600–650 °C for 5 min before the sample was exposed to 10 –6 Torr of ethylene for 30 min while the sample was continually held at 600–650 °C.…”

“…Ni was vapor deposited onto the graphene/Ni(111) substrate in discrete amounts and the Ni LEIS signal was measured after each dose. The substrate’s LEIS signal (graphitic carbon) could not be measured due to a near unity ion neutralization probability for He + ions scattered from graphitic carbon, as was the case for Ag deposition on graphene/Ni(111). ,− The integrated Ni LEIS signal was normalized to the signal from a thick Ni layer (>10 nm), which served as the Ni reference signal for complete coverage of the graphene surface by Ni. This normalized Ni LEIS signal then gives the fraction of the surface covered (and shadowed) by Ni particles.…”

“…Recently, our group studied the energetics of Ag atoms and nanoparticles on graphene supported on Ni(111) . That was the only previous measurement of metal atom energetics for any late transition metal on any carbon support material.…”

“…It was also found that Ag nanoparticles have a lower adhesion energy (in the large-size limit) to graphene/Ni(111) than onto reducible metal oxides such as CeO 2 (111) and TiO 2 (100) but a higher adhesion energy than onto irreducible metal oxides such as MgO(100). The bonding of Ag to the graphene support is somewhat unique in combining weak monomer bonding with moderately strong nanoparticle adhesion, which has advantages as a catalyst support . This motivates the investigation of other late transition metals on graphene.…”