Crystallography of Impurity Adsorption on Copper Surfaces

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Abstract
The object of the investigation was to determine the sites at which impurity adsorption occurs on crystalline surfaces. This was done by measuring the variation of surface free energy (γ) with orientation (θ). A relation between the torque [τ=(∂γ/∂θ)μ], chemical potential (μ), surface excess (Γ), and orientation is derived: (∂τ/∂μ)θ = — (∂Γ/∂θ)μ. Methods for determining surface torques by measuring dihedral angles at grooves where twin boundaries and grain boundaries intersect surfaces are discussed. Measurements were made of relative torques τ/γ on copper surfaces for the range of oxygen pressures from 10−22 to 10−13 atm at 1000°C using water—hydrogen mixtures. Increasing po2 was found to increase the magnitude of the torque on the (111) and (100) orientations. This indicates preferential adsorption at a range of orientations on the smooth, low‐index (100) and (111) surfaces rather than on stepped surfaces of orientations different from the low‐index orientations. A calculation based on a simple model of the surface indicates that about a half monolayer of oxygen atoms is adsorbed on the (111) surface. The characteristic pressure for oxygen adsorption on (111) was calculated to be 3×10−17 atm oxygen at 1000°C. Measurements of torques after annealing in an atmosphere containing hydrogen sulfide showed that sulfur adsorption decreases the torque near (100) while not affecting that near (111). This indicates preferential sulfur adsorption at steps near the (100) orientation. Integration of the plots of torque as a function of orientation showed that the surface energies of (100) and (111) relative to higher index surfaces are lowered as the oxygen pressure is increased.