An AFM Evaluation of Rhenium-Nickel Electrodeposit Nucleation on Copper

The electrodeposition of rhenium and rhenium alloys is well documented both in acidic^1-5 and alkaline^6,7 solutions. The effect of complexing agents as well as the catalytic effect of divalent nickel ions has also been extensively investigated^2-4. It has been shown that Re-Ni electrodeposits consist of alternating layers of Ni-rich and Re-rich phases². However, a detailed explanation of the origin of such layers as well as the electrodeposit nucleation process is still missing.  The objective of the present work is threefold.  First, we seek to elucidate  the nucleation mechanism of Re-Ni alloys. AFM will be used to document the formation and growth of electrodeposit nuclei and delineate the progressive vs. instantaneous nucleation mechanism. Second, the multilayered structure of the Re-Ni deposits will be addressed by documenting the formation of the initial layer and its evolution over time. Finally, nickel deposits will be used as substrates for pure rhenium electrodeposition to test the hypothesis that a strike layer of nickel is necessary to promote the deposition of pure rhenium.

Galvanostatic pulse methods were used to electrodeposit pure nickel and pure rhenium salts separately from citrate solutions on (111) single crystal copper substrates as well as mica-supported copper films. The results are compared with the electrodeposition behavior of Re-Ni bath mixtures.  A very small time scale is needed to deposit a monolayer of nickel. Distinct differences in frictional force of the initial deposit nuclei was captured by AFM. Figure 1 shows the difference in frictional force between a bare copper surface (111) and nickel deposit on a copper (111) surface. Nickel was deposited electrochemically by exposing a copper surface (111) to a 96 mM nickel sulfamate solution under galvanostatic polarization for 1 ms at -53.0 mA/cm². The spots observed in the frictional image are attributed to islands of nickel atoms that are too thin to be detected in the topography mode.

Acknowledgements

The authors gratefully acknowledge the financial support from AFOSR under grant FA9550-10-1-0520.

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