Silver Nanoparticles on Fe₃O₄(111): Energetics by Ag Adsorption Calorimetry and Structure by Surface Spectroscopies

The heat of adsorption of Ag atoms and the growth morphology of the resulting silver particles on the Fe 3 O 4 (111) surface at 300 K were studied by adsorption microcalorimetry, Auger electron spectroscopy (AES), and low energy ion scattering spectroscopy (LEIS). Thin films (∼6 nm thick) of Fe 3 O 4 (111) were grown on a Pt(111) single crystal. The changes in AES and LEIS signals versus Ag coverage during Ag vapor deposition onto Fe 3 O 4 (111) were indicative of Ag growing as 3D particles with a fixed density of ∼4 × 10 12 particles/cm 2 . The heat of Ag adsorption increased with Ag coverage from ∼230 kJ/mol initially up to within a few percent of the heat of sublimation of bulk Ag (285 kJ/mol) by ∼2 ML Ag. This corresponds to an increase in Ag atom stability (decrease in chemical potential) by ∼55 kJ/mol as the Ag particle size grows from <1 to ∼4 nm effective diameter on Fe 3 O 4 (111). The Ag(solid)/Fe 3 O 4 (111) adhesion energy for the 4 nm particles was estimated from the integral heat of adsorption to be 2.5 ± 0.3 J/m 2 . The sticking probability of Ag atoms was ∼0.96 initially and increased to ∼0.99 by ∼1.0 ML and above.