(Invited) In Situ Stress and Nanogravimetric Measurements during Underpotential Deposition

The underpotential deposition (UPD) of metals onto foreign metal substrates involves monolayer or submonolayer formation at potentials that are positive of the reversible electrodeposition of the bulk metal. UPD is particularly important to the understanding of electrodeposition where processes in the UPD region can be expected to influence the growth and subsequent properties of bulk thin films. In electrocatalysis, sub-monolayers of metals such as Bi, Pb, and Tl on noble metal surfaces have shown enhanced catalytic activity for a variety of electroreduction processes. Modified catalytic activity has also been observed for Pd and Pt overlayers consisting of a few monolayers. For this reason UPD processes are being examined by a variety of in situ techniques, including wafer curvature, in order to better understand the relationship between the structure of the UPD adlayers and the electrocatalytic activity. Changes in surface stress can be measured by monitoring the curvature of a wafer or cantilever while in solution and under potential control. Surface stresses on the order of 0.01 N/m can be resolved, sufficient to study the adsorption of molecular monolayers onto the electrode surface.

Equation (1) quantifies the change in surface stress associated with epitaxial overlayer growth¹,

                            ΔF = Δf_o + Δf_pzc + Δh + Σ_coht_f                                      (1)

where Δf_o is the intrinsic surface stress difference between the overlayer and the substrate, Δf_pzc is an electrocapillary term that reflects the change in capacitance and potential of zero charge (pzc) associated with changing the surface from the substrate metal to the overlayer metal, Δh is the interface stress, and ΔΣ_coht_f is the product of the coherency stress and the film thickness. For sub-monolayers, Eqn. (1) may also contain an additional term that captures the contribution of the compressive strain due to island size effects². Although the misfit term in Eqn. (1) is often assumed to dictate the overall change in surface stress, this is not always the case. The upd of Al and Cu onto Au are clear examples where the stress change is compressive even though the lattice misfit is positive. In some cases, pseudomorphic film growth of several monolayers allows the coherency term to be distinguished from the interfacial terms, which are independent of film thickness. Measurements of this type have been made for the electrodeposition of Pd onto (111)-textured Au³, and have resulted in estimates of Δh for the Pd-Au interface to be about -1.2 N/m. This is similar in both sign and magnitude to values reported for Ag-Cu⁴.

This talk will examine the surface stress changes associated with underpotential deposition, paying particular attention to the interfacial contributions, in addition to that associated with lattice misfit.

References

1.  T. Trimble, L. Tang, N. Vasiljevic, N. Dimitrov, M. van Schilfgaarde, C. Friesen, C.V. Thompson, S.C. Seel, J.A. Floro, K. Sieradzki, Phys. Rev. Let. 95, 166106 (2005).

2.   S.-E. Bae, D. Gokcen, P. Liu, P. Mohammadi, S. R. Brankovic, Electrocatal. 3, 203 (2012).

3.   G. R. Stafford, and U. Bertocci, J. Phys. Chem. C, 113 (1), 261 (2009).

4.   A. L. Shull and F. Spaepen, J. Appl. Phys. 80, 6243 (1996).