Abstract
Dealloyed nanoporous metals with a hierarchical structure provide model systems for low-density structural and functional nanomaterials. It has been suggested that these materials are distinguished by particularly stringent design principles, with precisely defined characteristic length scales, and with geometrically similar structures on each hierarchy level, and that the length scales can be independently tuned on each level. Studying nanoporous gold made by two-step dealloying, we here demonstrate the tunability of the microstructure, independently for the upper and the lower hierarchy level. Small-angle (SAXS) and ultrasmall-angle x-ray scattering (USAXS) revealed sharp interference peaks corresponding to each of the two levels, confirming the stringent structural definition. Exploiting USAXS, we resolve and study upper-hierarchy-level ligament spacings of up to 600 nm. The length scales inferred from the peak positions correlate excellently with structure sizes determined by analysis of electron micrographs. This suggests a scaling factor that allows for size conversion between the two approaches. Furthermore, the analysis of the small-angle scattering enables a characterization of the volume-specific surface area, in good agreement with the estimate based on the ligament size and the leveled-wave model as an approximate description of the material's microstructure.
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