Photoinduced cooling effect of water-retentive composite of TiO2 and saponite

TiO 2 coating of a building envelope is expected to contribute to the mitigation of the heat-island effect because of the photoinduced cooling effect. However, the limited water adsorbability and retentivity of the TiO 2 thin film can require successive sprinkling of water onto a building to achieve the sufficient cooling effect. In this study, a thin film consisting of a mixture of TiO 2 and saponite was prepared through a sol–gel method. The high water retentivity of saponite was found to enhance the photoinduced cooling effect of the TiO 2 -based thin film. The photoinduced temporal change in the water adsorbability and retentivity of the highly hydrophilic composite was successfully monitored by using a hydrophilic xanthene dye adsorbed onto the coating as a probe. The effect of environmental humidity on the photoinduced cooling effect was also investigated. We found that higher humidity in the surrounding environment triggered stronger photoinduced cooling effects of the composite film. Especially during summer in Japan, when humidity is high, the TiO 2 –saponite composite film did not require intentional water sprinkling to achieve cooling effects. • TiO 2 –saponite composite exhibits higher photoinduced cooling effects than bare TiO 2 . • Water retentivity of saponite contributes to enhanced photoinduced cooling effect. • Surface hydrophilicity is assessed by adsorption of a hydrophilic dye as a probe. • High surrounding humidity is a key factor to obtain high photoinduced cooling effect.