Enhanced catalytic activity has been demonstrated in elastically strained thin films, pseudomorphically grown on substrates with an appropriate lattice mismatch. The enhancement in catalytic activity is typically attributed to both electronic interactions and the epitaxial mismatch strain in the overlayer. The magnitude of the strain is often estimated from lattice-constant measurements using x-ray diffraction. An additional probe gaining popularity in electrochemistry involves the measurement of surface and growth stress using cantilever curvature. Cantilever curvature offers a nice complement to the direct strain measurementand can provide an in situ measurement while the strained layer is being formed. This talk will focus on the biaxial growth stress that develops during the self-terminated electrodeposition of ultrathin Pt films onto a variety of cantilever substrates that introduce different levels of misfit strain into the Pt. The cantilever was a borosilicate glass strip measuring 60 mm × 3 mm × 0.108 mm and was metallized on one side by electron beam evaporation, resulting in substrates with strong (111) crystallographic orientation. The electrolyte was 0.5 mol/L NaCl containing 3 mmol/L K₂PtCl₄. Deposition consisted of stepping the potential from the open circuit potential (~ -0.1V vs SSE) down to -1.2V for 10s and then back to -0.1V for another 30s, following the procedure established by Liu et al.¹Figure 1 shows the stress response for Au, Pt and Ir substrates. Pt grown on Au exhibits considerable tensile stress, which is consistent with the +3.9% lattice mismatch between Au and Pt. In contrast, Pt deposition onto a Pt cantilever deposits homoepitaxially as an unstrained continuation of the substrate. The slight increase in stress with continued deposition is likely microstructural. Pt grown on Ir, with its -2.1% lattice misfit, initially exhibits compressive stress, before leveling off and turning tensile, similar to the growth of Pt on Pt. Although it is tempting to focus on lattice misfit as the primary source of stress, one must consider other contributions, such as step line tension associated with the growth of small islands, possible intermixing, and surface roughness.

References

1. Y. Liu, D. Gokcen, U. Bertocci and T. P. Moffat, Science 338, 1327 (2012).