**5. Smart nanocomposite coatings: Self-cleaning**

Self-cleaning is a surface property consisting in keeping the surface clean under severe environmental conditions [68]. It is inspired from lotus effect with a specific micro- and nano-hierarchical surface morphology and low surface energy, providing superhydrophobicity. A surface can be considered as superhydrophobic when the water contact angle is higher than 150° (WCA > 150°) and the low sliding angle is lower than 10° (SA < 10°). Another opposite approach for getting self-cleaning surfaces is the incorporation of photocatalytic fillers affording hydrophilic surfaces (WCA < 5–10°) able to keep free of organic contaminants and moisture. The selfcleaning surfaces can be developed with nanodoped polymer coatings and with neat graphitic coating, such as it is shown in **Figure 7**.

One of the most studied self-cleaning materials is based on titanium oxide (TiO2) and zinc oxide (ZnO) due to their superhydrophobicity and photocatalytic decomposition ability of organic pollutants. These coatings are usually manufactured by sol-gel process. Modifying these nanofillers with graphene or its derivatives can enhance their visible light response. Hybrid TiO2/graphene nanofillers can exhibit strong electronic overlap and high interfacial binding energy; thus, photoexcited carriers can transfer from TiO2 to graphene, and its band gap is reduced, improving the visible light photoresponse [69, 70]. Nevertheless, graphene enhances the photocatalytic efficiency of ZnO due to that graphene accepts the electron from ZnO nanoparticles, preventing the recombination of photo-generated electron hole in the semiconductor. These nanocoatings show superhydrophobicity when they are irradiated with visible light. An interesting alternative approach is the impregnation of cotton fibers to manufacture industrial self-cleaning textiles [71]. The treated fabrics exhibit an increase antibacteriological behavior and high biocompatibility.

Another possibility to develop superhydrophobic self-cleaning coatings is the use of hydrophobic polymer matrix, such as polysiloxanes or fluoro-polymers,

#### **Figure 7.**

*Scheme of the main approaches of self-cleaning coatings with carbon nanoparticles.*

doped with graphitic nanofillers, giving high electrical conductivity [72]. These coatings present high water contact angle and low sliding angle. In addition, they exhibit high wear resistance and excellent anticorrosion performance [73]. The presence of polymer matrix enhances the adherence with substrate and increases the chemical resistance. When the matrix is not hydrophobic polymer, the nanocomposite coating can be chemically etched (i.e., fatty acids such as stearic acid, palmitic acid, and oleic acid) in order to decrease the surface energy. This approach allows using very different matrices (i.e., acrylic resin, epoxy, etc.), looking for other advantages, such as superior adhesion, mechanical properties, low cost, etc., making it suitable for many industrial fields. Together with self-cleaning behavior, other related properties are commonly developed in smart coating based on carbon nanoparticles, such as antifouling, antimicrobial, anticorrosion, antisoiling, etc. Graphetic nanostructures hold antimicrobial and antibiofilm activities, although the involved mechanisms are not completely understood and accepted [74]. For these applications, other nanoparticles of gold, silver, copper, TiO2, ZnO, Fe2O3, and CuS, are anchored to the graphene nanosheets to enhance the bactericidal efficiency. Now, current efforts consist on developing membranes and films with polymer matrix, such as chitosan, poly(vinyl alcohol), polyamides, polyethyleneimine, etc., for applications in disinfection, gas separation, and water desalination.
