# Natural Stones with a Self-Cleaning Surface via Self-Assembled Monolayers
## 1 Introduction  
Heritage buildings and monuments are mostly made from natural stones because of their earth-abundance, bio-friendliness, and robustness. However, natural stones undergo weathering and dissolution, especially when they are exposed to rains containing SO2 and NOx, which are common nowadays. Contaminants invade the stone bodies, along with water, which causes serious degradation to the stones. Hence, surface modifications aiming at increased water repellency are an important route toward stone protection [1; 2; 3; 4].  
Coatings of acrylic, paraffin, acryl-siliconic, and epoxy resin have been generally used for stone protection [5; 6; 7; 8; 9; 10; 11]. Although the pollution sources can be effectively isolated by these coatings, the breathability (i.e., the moisture permeability) of coated stones might be seriously weakened, especially when the coating is thick. As a consequence, depending on the variation of the ambient temperature, the moisture trapped inside a coated stone experiences cycles of crystallization-dissolution, and such cycles accelerate the degradation of the coated stone. Especially in heavily salt-laden cases, simple treatment by seal coatings can even increase the deterioration fate of treated stones, leading to irreversible scaling, flaking, and cracking [12]. Moreover, the stability of these coatings is limited due to their relatively weak adhesion to the stone. Hence, for a protective coating to function properly on stones, its water repellence, robust adhesion to the stone, and breathability are required.  
For natural stone protection, self-cleaning surfaces are effective due to their excellent water repellency and breathability [13; 14]. Self-cleaning surfaces require low surface energy and a morphology having roughness at both the micro- and nanoscale [15; 16]. Intrinsic micro-roughness exists on natural stone surfaces, while the addition of nanoparticles in protectants effectively produces nano-roughness. For example, a static water contact angle (CA) of 160\({}^{\circ}\) was shown on marble treated with polyalkylsiloxane mixed with silica nanoparticles [17]. Another protectant was a copolymer of epoxy and acrylate, with an additive of nano CaCO3 nanoparticles [18; 19]. The performances of durabilityand resistance to abrasion, contamination, and weathering of natural stones have been greatly improved via the application of those protectants. Meanwhile, the antibacterial and antifungal properties of the treated stones were enhanced, as well. The main issue found in the nanomaterials-enhanced protectants is their adhesion to the stone substrate. Although there are numerous polymers that can be used as binders, they hardly possess the desired properties of adhesion and water repellency. In addition, dispersions of nanoparticles in these polymers are generally poor.  
Reported in this work is a new approach to achieving self-cleaning stone surfaces that includes a self-assembled layer of polydopamine (PDA) on the stone surface to serve as the adhesive layer and Al\({}_{2}\)O\({}_{3}\) nanoparticles derivatized with self-assembled monolayers (SAMs) of a fluorophosphonic acid (FPA) to be deposited on the PDA layer. Using natural marble, Hedishi, and Qingshi, we demonstrated that the stone substrate and the FPA derivatized Al\({}_{2}\)O\({}_{3}\) particles were strongly glued together, due to the robust adhesiveness of PDA layers, resulting in excellent water repellency, as evidenced by highly water static CAs. The thermal stability of the self-cleaning stone surfaces was also investigated.