Sign-inverted surface stress-charge response in nanoporous gold

The surface stress-charge coefficient, ς, quantifies the response of the surface stress - a fundamental property of solid surfaces - to the addition or removal of superficial electric charge. So far, all experiments have found ς negative at clean metal surfaces. Here we report in situ dilatometry experiments on nanoporous gold samples prepared by dealloying. The results reveal a large reversible elastic contraction during the anodic part of cyclic potential scans. The behavior, which indicates an abnormal positive sign of ς, can be linked to a monolayer of strongly adsorbed oxygen formed during dealloying, and to the extraordinarily small (1-2 nm) structure scale of the material while the oxide is in place. After cathodic reduction, ς reverts to the conventional, negative sign of the clean metal surface. We discuss the sign of ς in relation to the electronic screening at metallic and oxide-covered surfaces.