X-ray-reflectivity study of the copper-water interface in a transmission geometry underin situelectrochemical control

Using an electrochemical cell that permits x-ray-scattering studies in a transmission geometry under in situ electrochemical control, we performed an x-ray specular-reflectivity study of the copper-water interface as a function of electrochemical potential. It is found that the reflectivity changes upon cyclic oxidation and reduction, indicating a phase transition between copper and copper oxide at the interface. We also find that the thickness of the pure copper and the roughness of the interfaces exhibit electrochemical irreversibility that is consistent with the potentials for the oxidation waves in the anodic cycle and the reduction waves in the cathodic cycle. A standard Fresnel expression for the x-ray specular reflectivity was applied in the data analysis. A smoothly varying Lorentzian interface profile was used for the individual rough interfaces. An incoherent average was additionally used to include the effect of correlated roughness between a pair of interfaces. The results of our data-fitting procedure are discussed in relation to the electrochemical information obtained in the corresponding cyclic I-V curves.