Comparison of interfacial chemistry at Cu/α-alumina and Cu/γ-alumina interfaces

Different alumina polymorphs (alpha, gamma and a transient phase) are formed in a copper matrix by oxidation of (Cu, Al) at 900 degreesC, at an oxygen activity of a(O2) = 10(-13). Spatially resolved electron energy loss spectroscopy is used to determine bonding and electronic structure at the different interfaces. Compared to adjacent bulk phases, at the interfaces, the near edge fine structures are modified. Cu L-2,L-3 shows an increased intensity compared to metallic copper and a chemical shift to higher energy. The size of both varies for the different interfaces, smallest chemical shift and highest L-2,L-3 intensity are observed at {111}(gamma-alumina)// {111}(Cu) interfaces, while larger chemical shift and smaller L-2,L-3 intensities are found for alpha-alumina/Cu interfaces in the order of {11 (2) over bar0}(alpha-alumina)//{100}(Cu), {0001}(alpha-alumina)// {001}(Cu), {11 (2) over bar3}(alpha-alumina)//Cu- In the interfacial O K edge, an additional pre-edge intensity is observed. For gamma-alumina//Cu, it appears as a clear pre-peak, indicating hybridization with strong contributions of Cu 3d and O 2p states at the interface, while for alpha-alumina/Cu interfaces modifications in the interfacial O ELNES reveal mixed Cu-O-Al bonding with a strong covalent component. While interfacial Al L-2,L-3 shows no chemical shift or new fine structural features compared to the bulk precipitate, a decrease in the aluminum coordination is revealed for the alpha-alumina/Cu interfaces by changes in the intensity distribution in the first peak, while for gamma interfaces no such observation is made. The observations show that interfacial bonding does not only vary with the thermodynamic parameters temperature and oxygen chemical potential, but also with the crystallographic characteristics of the interface.