Energetics of 2D and 3D Gold Nanoparticles on MgO(100): Influence of Particle Size and Defects on Gold Adsorption and Adhesion Energies

Abstract

The adsorption of gold vapor onto MgO(100) films grown on Mo(100) was studied at 300 and 100 K using single crystal adsorption calorimetry (SCAC). The Au particle morphology was investigated using He + low-energy ion scattering spectroscopy (LEIS) and X-ray photoelectron spectroscopy (XPS). The LEIS data combined with particle shape measurements from the literature (Benedetti, S.; Myrach, P.; di Bona, A.; Valeri, S.; Nilius, N.; Freund, H.-J. Phys. Rev. B 2011, 83 (12), 125423) reveal that, at both 300 and 100 K, Au grows as 2D islands with bilayer thickness (∼0.41 nm) up to a diameter of ∼7 nm. At higher coverage, the islands thicken with little increase in diameter. The island densities are 3.0 × 10 11 and 5.4 × 10 11 per cm 2 at 300 and 100 K, respectively. The initial sticking probability of Au is 0.90 at 300 K and 0.95 at 100 K. The surface residence time of the Au atoms that do not stick is <10 ms, implying that gold monomers bind to MgO(100) weakly (<69 kJ/mol). The adsorption energies indicate that Au particles of the same size bind more strongly to MgO(100) when grown at 300 K than at 100 K, which we attribute to Au binding to step edges or defects at 300 K, but at perfect MgO(100) terraces at 100 K (because Au diffusion is too slow to find defects). The adsorption energy of Au onto ∼30-atom Au clusters is 285 kJ/mol at 300 K, ∼68 kJ/mol higher than at 100 K, attributed to the difference between particles on defects versus terraces. Similarly, the adhesion energy of Au nanoparticles to MgO(100) extracted from the adsorption energies at 300 K is much higher (1.8 J/m 2 for ∼7 nm particles at defects) than at 100 K (0.3 J/m 2 for ∼7 nm particles at terraces). This 100 K adhesion energy is close to that estimated from electron-microscopy shape measurements of Au particles at terraces on MgO(100) (0.45−0.67 J/m 2 ). The heat of Au adsorption and Au chemical potential change by >100 kJ/mol as gold's 2D island size increases from 0.7 to 7 nm diameter, implying dramatic changes in catalytic activity and sintering rates with 2D diameter. This is the first experimental measurement of any metal adsorption energy on any oxide as a function of island diameter when making 2D islands, as well as the first direct comparison of any adhesion energy found from calorimetric adsorption energies to that from particle shape analysis.

Trends in Adhesion Energies of Metal Nanoparticles on Oxide Surfaces: Understanding Support Effects in Catalysis and Nanotechnology

“…In our pulsed molecular beam study of Au on MgO(100), Au monomers that hit the surface but did not permanently stick at 100 K had a surface residence time below 10 ms, implying that gold monomers bind to MgO(100) with an adsorption energy weaker than −22 kJ/mol. 54 This weaker binding of Au monomers to MgO(100) compared to CeO 2 (111) is consistent with DFT calculations, which for MgO(100) gives an adsorption energy of only −0.42 eV, 73 well below the value above for CeO 2 (111).…”

“…At the surface temperature (100 K) during Au vapor deposition used here, the Au nanoparticles sit mainly on MgO(100) terraces. 54 A much larger adhesion energy was found by SCAC at 300 K, where the particles sit mainly at step edges. 54 This SCAC value at 100 K for Au at MgO(100) terraces is within 0.36 J/m 2 of the value from particle-shape measurements by TEM (0.67 J/m 2 ), which was also for terraces.…”

“…54 A much larger adhesion energy was found by SCAC at 300 K, where the particles sit mainly at step edges. 54 This SCAC value at 100 K for Au at MgO(100) terraces is within 0.36 J/m 2 of the value from particle-shape measurements by TEM (0.67 J/m 2 ), which was also for terraces. This is an important point, since it is the only system where E adh has been measured by both SCAC and particle shape measurements on clean surfaces.…”

“…Details of the characterization of the Au particle density, particle shape, and particle size and how they vary with Au coverage and surface temperature are described in detail in our much more extensive report of Au adsorption on MgO(100), where we also report how Au adsorption energies vary with these properties. 54 Table 1 shows a summary of metal/oxide adhesion energies we have measured recently with metal adsorption calorimetry, together with other metal/oxide adhesion energies that have been reported based on experimental measurements of the metal nanoparticle shape, along with citations to the references where the previously published data are found. In all cases, these are for solid metal nanoparticles or films prepared by metal vapor deposition in the clean conditions of ultrahigh vacuum where surface cleanliness was verified either directly before metal deposition or in identical control experiments.…”

Approaches to synthesize MgO nanostructures for diverse applications

“…Among the various non-magnetic oxide system, MgO is a technologically important material because of its simple crystal structure and complete absence of d orbital electrons, which persists way of understanding several physical [ 35 , 36 , 37 ] and chemical behavior [ 38 , 39 , 40 , 41 ].…”