Wafer‐Scale Fabrication of Hierarchically Porous Silicon and Silica by Active Nanoparticle‐Assisted Chemical Etching and Pseudomorphic Thermal Oxidation

Abstract 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, 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 is presented, i.e., hexagonally arranged cylindrical macropores with 1 µm diameter separated by walls that are traversed by pores 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.