An eco-friendly building coating with high self-cleaning capacity: Synergetic effect of super-hydrophobicity and photocatalytic degradation

Photocatalytic super-hydrophobic coating is highly appealing due to its exceptional self-cleaning capacity. In this study, recycled concrete powder (RCP) was used to support TiO2, and RCP@TiO2 composites (RCPT) were incorporated into polydimethylsiloxane (PDMS) to fabricate an exterior wall self-cleaning coating. The super-hydrophobicity and photocatalytic activity of the resulting coating were modulated by adjusting the dosage of RCPT. The surface roughness was determined through fractal analysis. The findings indicated that the incorporation of RCPT generated micro-/nano-scale structures on the coating surface, which significantly enhanced its hydrophobicity. The water contact angle (WCA) of the coating surface increased with the content of RCPT. When the mass ratio of RCPT to PDMS reached 1, the sliding angle (SA) sharply decreased, resulting in a super-hydrophobic surface with WCA greater than 150° and SA less than 10°. According to the surface roughness, critical points of wetting state transition can be accurately identified. Meanwhile, the coating with the same addition of RCPT exhibited the highest photocatalytic activity. The synergistic effect of super-hydrophobicity and photocatalytic degradation contributed to the enhanced self-cleaning capacity. Due to the lotus effect, the coating exhibited high stain resistance and water repellency. Most pollutants adhering to the coating surface can be removed by rainwater rolling off. In case of fouling by organic pollutants, photocatalytic degradation can decompose adsorbed pollutants, thus restoring the lotus effect. The coating also demonstrated exceptional stability against photo-thermal erosion, mechanical abrasion, and water impact. Our research has unveiled the correlation between surface structure and properties of coating, providing valuable insights for the development of building self-cleaning coatings.