Nanoscaled Fractal Superstructures via Laser Patterning-A Versatile Route to Metallic Hierarchical Porous Materials

A laser-based procedure for the preparation of metallic hierarchical porous materials is introduced and exemplified on tin, copper, silicon, titanium, and tungsten surfaces to demonstrate its general applicability. The impact of suitably tuned nanosecond laser pulses triggers a process in which laser-induced metal ablation and instantaneous recondensation of partially oxidized metals lead to cauliflower-like superstructures comprising a hybrid micro-/nanopatterning. Repeated scanning with the intense focused beam over the surface creates microstructures of hierarchically tunable porosity in a layer-by-layer design. The 3D morphology of these superstructures is analyzed using tomographic data based on focused ion-beam scanning electron microscopy to return a fractal dimension of D-f = 2.79-practically identical to a natural cauliflower (D-f approximate to 2.8), even though the plant is four orders of magnitude larger than the superstructures generated through the laser process. The high D-f value signifies a complex morphology that boasts a huge external surface. The introduced concept enables convenient access to a variety of metallic hierarchical porous materials, which are key to performance in environmentally and technologically relevant areas like energy generation, storage, and conversion, as well as sensing and catalysis.