Properties of silver clusters adsorbed to silver bromide

Density functional calculations of the structure and electronic properties of silver clusters interacting with a fragment of silver bromide that is embedded within a polarizable representation of the AgBr crystal are presented. Flat (001) surfaces and kinked surfaces are considered. The optimized geometry of the silver clusters is nearly planar for the clusters up to four atoms that are considered. We examined the degree to which different surface defect sites could modulate the properties of the adsorbed silver cluster. The positive kink site is unique in providing a location where the neutral cluster can capture photoelectrons. This effect can be rationalized by electrostatic arguments and is consistent with this site functioning as a location for photolytic silver formation. Other kink sites of neutral or partial negative charge do not permit electron trapping at neutral silver clusters. These sites could act as locations for chemically produced silver clusters. Calculations are presented showing that some geometries of Ag-3(+) adsorbed at the negative kink can trap electrons in accord with a proposal by Tani. The calculations are used to compare different mechanisms for photolytic formation of silver clusters. They can be compared to gas-phase calculations adjusted for the interaction with a polarizable surface to show that both calculations agree that cationic silver clusters are unstable with respect to dissociation except on the negative kink. The calculations show a deep electron trap depth for the silver atom consistent with its participation in the nucleation phase of silver cluster growth during photolysis. A sulfide ion substituted for bromide at a positive kink is shown to provide a means of coupling the electron to the crystal ions.