Water-Driven Porous Silicon as a Platform for Blue Materials

Living organisms control water to transport matter, transfer energy and drive chemical reactions. In this perspective, we argue that porous silicon can give engineering materials comparable control over water. Its pore architecture, surface chemistry and semiconducting skeleton can be shaped across length scales with the precision of semiconductor processing and, like the water it can host, it relies on one of the most abundant chemical elements on Earth. Liquids in porous silicon, most often water, can flow as a continuum through pores a few nanometers wide, strain the skeleton through capillary and electrosorption stresses, shift structural color, switch transparency, and modulate or generate electrical signals. We describe three directions to combine and extend these effects and overcome their limitations. Pore hierarchy resolves the classical trade-off between interface area and transport speed by combining large and small pores and thereby allows functions to be scaled from thin planar films to three-dimensional components. Surface modification becomes a design variable that tunes the interaction between water and the pore wall. It can be set locally and, with switchable coatings, on demand. Finally, pores only a few molecules wide alter the properties of water itself. In the absence of a bulk-like volume, water can become dielectrically anisotropic and highly conductive. Altogether, these steps turn porous silicon from a responsive material into an adaptive, water-driven material.