Surface-supported Ag islands stabilized by a quantum size effect: their interaction with small molecules relevant to ethylene epoxidation
Surface-supported Ag nanoparticles demonstrate quantum-size-effect (QSE) mediated growth behavior. 2-layer Ag islands are stabilized by QSE for Ag/Si(111)-7x7. O2 exposure on such system changes the Ag island height distribution: promoting 3-layer island growth at the expense of 1-, and 2-layer islands. This is an indication that oxygen adsorption changes QSE of Ag/Si(111)-7x7. Mild annealing following O2 exposure promotes growth of even higher islands (up to 9-layer high). Such trend indicates the instability of 3-layer islands with oxygen adsorption. Ethylene exposure on Ag/Si(111)-7x7 leads to protrusions on Ag islands and protrusive rims on island edges. But ethylene does not affect QSE of Ag/Si(111)-7x7 significantly. O2 exposure on Ag/NiAl(110) disrupts the relative stability of Ag bilayer (BL) islands stabilized by QSE. 1-BL island react with O2 to form indentations and protrusions. However, these features do not occur on 2-BL Ag islands. A possible explanation is that the adsorption energy of oxygen is much higher on 2-BL islands than on 1-BL islands.