Nanoporous metal samples with millimetre size were prepared either by compacting nanocrystalline powders or by dealloying, the dissolution of the less noble metal(s) of an alloy. The samples were immersed in an electrolyte, and their length was measured as a function of the applied potential in-situ in a dilatometer. The results obtained for nanocrystalline platinum, nanoporous gold and for gold platinum alloys show that the length varies in dependence of the surface charge. The strain amplitude of nanocrystalline platinum was 0.15%, and even larger strains have been measured using an Au-Pt alloy. This strain is comparable to commercial piezoceramics, but it is achieved using smaller voltages.
The strain measured for nanoporous gold prepared by dealloying was smaller than that mainly due to the larger structure size (20 nm structure size compared to 6 nm Pt crystallite size), but in the case of gold, it was possible to prepare stable composite structures of a metal foil and of the nanoporous gold. If such a bimetallic foil is charged, it is found to bend. Due to the mechanical amplification of the contraction or expansion of the nanoporous part of the foil, it was possible to observe the effect of electric charges on the surface stress of metals directly with the naked eye for the first time.
These results demonstrate that nanoporous metals might be useful for actuator applications and for the study of surface strain effects. Furthermore, they are the first realization of a general concept that suggests that most of the properties of conducting nanomaterials can be tuned by controlling the surface charge.