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

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 photo-electron 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 (similar to 0.41 nm) up to a diameter of similar to 7 nm. At higher coverage, the islands thicken with little increase in diameter. The island densities are 3.0 x 10(11) and 5.4 x 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,similar to 30 similar to atom Au clusters is 285 kJ/mol at 300 K, similar to 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) similar to 7 nm particles at defects) than at 100 K (0.3 J/m(2) for similar to 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.