Montmorillonite-derived Mg-doped NaAlSiO4 for radiative cooling applications

Passive daytime radiative cooling (PDRC) technology is considered an effective path to address the challenges of global warming and energy consumption due to its zero-energy feature. To accelerate the wide application of PDRC technology, realization of low-cost preparation is the key and still challenging. Additionally, it is necessary to endow these materials with high UV- and water-resistant characters for outdoor long-term service. In this work, a low-cost fabrication strategy to prepare a Mg-doped NaAlSiO 4 (Mg:NaAlSiO 4 ) with UV- and water-resistant properties is proposed. The material is prepared by annealing Mg-doped montmorillonite clay (Mg 0.5 Al 1.5 (Si 4 O 10 )(OH) 2 ·4H 2 O, Mg:MTT) with additives of Na 2 CO 3 and Al(OH) 3 . Blended with poly(4-methyl-pentene) (TPX) for testing, the hybrid membrane demonstrates a solar reflectance of 91.8 % in the wavelength of 250–2500 nm and an IR emittance of 95 % in the wavelength of 2.5–25 μm. Theoretical analysis shows that a temperature decrease of 5.5 °C is achieved under directed sunlight radiation at noon with a cooling power of 95.3 W m −2 . It is noteworthy that after 480 h of a simulated UV daylight radiation of 32 mW cm −2 there is only a slight decrease in the coefficient of reflection of solar radiation (3.5 %) with a negligible decrease of IR emittance. Additionally, a small (2.8 %) degradation of solar reflectance and a negligible decrease of IR emittance are observed after 480 h exposure in a 95 %-humidity environment at 50 °C. Theoretical calculations show that a large band gap of 4.9 eV and a small absorption energy of −0.083 eV ( i.e. -8.0 kJ mol −1 ) for the Mg:NaAlSiO 4 are the reasons of the UV- and water-resistant properties, respectively.