Role of chemical etching and dissolution valence criterion for surface texturing of porous silicon

Capillary-driven surface texturing (CDST) of porous silicon (PSi) plays a critical role in constructing silicon-based special wetting surfaces, particularly in controlling the surface structure and wettability of PSi films. Electrochemical etching is widely recognized for its role in PSi formation, while the contribution of chemical etching during this process is often overlooked, which is essential for CDST. In this study, superhydrophobic PSi surfaces with biomimetic structures were fabricated through a combination of electrochemical etching of silicon, CDST of resulting PSi film, and surface modification with 1-octadecene. The surface structures, wettabilities, effective dissolution valences (EDVs), effective dissolution factors (EDFs), and porosities of PSi films were systematically analyzed. The results reveal that chemical etching is crucial for CDST, as it influences the resulting surface structures and wettabilities by increasing film porosities and creating gradient variations. Based on the intensity of chemical etching and the characteristics of surface structures, the pore-forming region can be categorized into three distinct zones: A CE (chemical etching)-severe region, B surface-texturing region (moderate chemical etching), and C EE (electrochemical etching)-dominating region. CDST primarily occurs in the surface-texturing region, where chemical etching, EDVs, and EDFs fall into the range of 15–25%, 1.5–1.7, and 75–85%, respectively. These results suggest that both EDVs and EDFs can serve as reliable evaluation criteria for CDST of PSi, offering a more precise and professional alternative to current density measurements. Moreover, EDVs and EDFs may also be utilized as control parameters in future applications of such an intriguing phenomenon.