Nanoporous (np) metals have generated much interest since they combine several desirable material characteristics, such as high surface area, mechanical size effects, and high conductivity. Most of the research has been focused on np Au due to its relatively straightforward synthesis, chemical stability, and many promising applications in the fields of catalysis and actuation. Other materials, such as np-Cu, Ag, and Pd have also been studied. This review discusses recent advances in the field of np metals, focusing on new research areas that implement and leverage structural hierarchy while using np metals as their base structural constituents. First, we focus on single-element porous metals that are made of np metals at the fundamental level, but synthesized with additional levels of porosity. Second, we discuss the fabrication of composite structures, which use auxiliary materials to enhance the properties of np metals. Important applications of these hierarchical materials, especially in the fields of catalysis and electrochemistry, are also reviewed. Finally, we conclude with a discussion about future opportunities for the advancement and application of np metals.

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Hierarchical materials with tailored features provide promising structures for applications requiring light-weight, yet strong, materials. In this study, a method for synthesizing nanoporous gold tubes as a hierarchical structure is presented. Samples are fabricated via electrodeposition, homogenization, and dealloying to generate a random porous network in the tube wall. Electrodeposition parameters are optimized by using a reverse pulse current setup. As-prepared tubes exhibit a ligament diameter of approximately 45 nm and the hierarchy of the tubes is further altered by controlling the ligament size through heat treatments.

Nano-and micro-architected materials generated by ultra-high-resolution 3D printing techniques, such as two-photon polymerization direct laser writing (TPP-DLW) or projection micro-stereolithography (PμSL), have garnered great interest due to their ability to achieve exceptional combinations of material properties. The scalability of these materials, however, remains a crucial challenge as larger high-resolution samples require stitching smaller blocks of the structure of interest together. Herein, scaling techniques and testing methodologies to investigate the effect of stitching on the integrity and mechanical behavior of TPP-DLW parts under tensile load are explored. Specifically, stitched log-pile I-beam specimens with relative densities of 21.5% and 54.7% are tested herein. The micro-tensile tests reveal that the higher-density log-pile samples exhibit brittle behavior with fracture loads at least four times higher than those of lower-density samples. The location of sample failure depends on the type of stitch introduced in the sample, as well as on the relative sample density. Overall, this study highlights the importance of stitching techniques and relative density for the design of nano-and micro-architected materials.

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