Energetics of Ag Adsorption on and Adhesion to Rutile TiO2(100) Studied by Microcalorimetry

The adsorption and adhesion energies of vapor-deposited Ag on rutile TiO2(100) films have been measured using single-crystal adsorption calorimetry and He+ low-energy ion scattering spectroscopy. Ag grows as three-dimensional nanoparticles at 300 and 100 K. The saturation particle density is 8 × 1016 particles/m2 at 300 K and 2.5 × 1017 particles/m2 at 100 K. The differential heat of adsorption starts low, increases with Ag coverage, and finally approaches the sublimation enthalpy of Ag at both 300 and 100 K. At 300 K, the differential heat of adsorption starts from 208 kJ/mol and rises rapidly to 265 kJ/mol by 1 ML. At 100 K, Ag grows nanoparticles with smaller particle size on the terraces, while at 300 K, the Ag nanoparticles have bigger particle size and grow on step edges with stronger binding strength to the nanoparticles. Thus, the heat of Ag adsorption at 100 K starts at 141 kJ/mol (near the monomer size limit on terrace sites) and remains lower than that at 300 K until 1.5 ML. The adhesion energy of Ag(solid) to rutile TiO2(100) is found to be large (∼2.44 J/m2), supporting a trend of decreasing adhesion energy with the enthalpy of oxide reduction.