In-situ fabrication of fluoride-free, breathable superhydrophobic structures for the effective protection of sandstone-based cultural heritage

Superhydrophobic coatings are promising for mitigating water-driven deterioration of sandstone relics, but most of them often reduce breathability, causing internal moisture buildup and further damage. Herein, we developed a high-performance, fluoride-free superhydrophobic (water contact angle = 153.7°) and breathable coating on sandstone surfaces via simple in-situ growth of triethoxy(octyl)silane (OTES)-modified ZIF-8 nanosheets. Remarkably, this coating displayed excellent breathable ability, with water vapor permeability (WVP) comparable to that of pristine sandstone. Even at 98% relative humidity (RH), it effectively blocked external water vapor invasion. This performance arises from the ZIF-8 gradient structure, where particle density and size gradually change from the substrate surface to the interior. Moreover, the coating exhibits excellent stability, resisting sandpaper abrasion, tape peeling, rainwater, and acid-rain erosion, acid and alkali solution immersion tests, as well as soluble salt dissolution-crystallization cycles and freeze-thaw cycles. This work would offer a new strategy for high-performance sandstone relic protective materials.