Transparent superhydrophilic and superhydrophobic nanoparticle textured coatings: comparative study of anti-soiling performance

The anti-soiling (AS) performance of highly reflective, superhydrophilic (SPH, 0° water contact angle) coated mirrors was characterized and compared with that of superhydrophobic (SP, >165° water contact angle) coated mirrors. A simple one-step nanotextured silica nanoparticle coating on a mirror exhibited SPH properties associated with hydrophilic rough surfaces. Another mirror surface post-functionalized with low-surface-energy ligand molecules displayed SP behavior. Both coated mirrors, with no solar reflectance loss, demonstrated excellent AS performance because the engineered surface roughness reduced the adhesive force of dust particles. The daily degradation in solar reflectance induced by dust accumulation under outdoor field testing demonstrated that the SPH- and SP-coated mirrors, compared with an uncoated mirror, maintained higher solar reflectance, which was associated with the designed self-cleaning behavior and natural cleaning. However, over the long term, dust-moisture cementation—evidenced by organic hard water stains on the mirror—initiated unrecoverable reflectance loss on the SP-coated mirror after 3 months, whereas the SPH-coated mirror maintained higher reflectance for 7.5 months. Considering fabrication costs and maintenance, SPH-coated nanotextured mirrors offer potential benefits for application in solar energy harvesting.