Ab initio study of Ag∕Al₂O₃ and Au∕Al₂O₃ interfaces

Structural stability, adhesion, and chemical bonding of the Ag(111)∕α-Al₂O₃ (0001) and Au(111)∕α-Al₂O₃ (0001) interfaces are investigated by an ab initio approach based on density functional theory. The interfaces are shown to have different stable structures of Al₂, Al, or O termination depending on the chemical potential of aluminum or oxygen atom. A link to thermodynamic factors, i.e., the partial pressure of oxygen gas or the activity of aluminum, is established based on the ab initio thermodynamics developed recently. For condition applicable to sessile drop experiments, the O-terminated interface could exist for the Ag∕Al₂O₃ system but be hard to observe for the Au∕Al₂O₃ interfaces, consistent with the known experiments. The Al₂ termination is possible for the Au∕Al₂O₃ interface at relatively low O₂ pressure or high Al activity but may be hard to form for the Ag∕Al₂O₃ interface. Works of adhesion W_ad of the stoichiometric interfaces are calculated to be 0.33 J∕m² in generalized gradient approximation (GGA) and 0.59 J∕m² in local density approximation (LDA) for the Ag∕Al₂O₃ interface, 0.29 J∕m² in GGA and 0.58 J∕m² in LDA for the Au∕Al₂O₃ interface, in reasonable agreement with measured data.