  Many biological materials exhibit a multiscale porosity with small, mostly
nanoscale pores as well as large, macroscopic capillaries to simultaneously
achieve optimized mass transport capabilities and lightweight structures with
large inner surfaces. Realizing such a hierarchical porosity in artificial
materials necessitates often sophisticated and expensive top-down processing
that limits scalability. Here we present an approach that combines
self-organized porosity based on metal-assisted chemical etching (MACE) with
photolithographically induced macroporosity for the synthesis of
single-crystalline silicon with a bimodal pore-size distribution, i.e.,
hexagonally arranged cylindrical macropores with 1 micrometer diameter
separated by walls that are traversed by mesopores 60 nm across. The MACE
process is mainly guided by a metal-catalyzed reduction-oxidation reaction,
where silver nanoparticles (AgNPs) serve as the catalyst. In this process, the
AgNPs act as self-propelled particles that are constantly removing silicon
along their trajectories. High-resolution X-ray imaging and electron tomography
reveal a resulting large open porosity and inner surface for potential
applications in high-performance energy storage, harvesting and conversion or
for on-chip sensorics and actuorics. Finally, the hierarchically porous silicon
membranes can be transformed structure-conserving by thermal oxidation into
hierarchically porous amorphous silica, a material that could be of particular
interest for opto-fluidic and (bio-)photonic applications due to its multiscale
artificial vascularization.
