Facile synthesis of mesoporous VO2 nanocrystals by a cotton-template method and their enhanced thermochromic properties

As a very promising thermochromic material, VO2 (M/R) (Monoclinic/Rutile) has not been widely applied in smart windows due to its intrinsic low solar modulation (ΔTsol) and low luminous transmission (Tlum). To address this issue, porous structures have been introduced into the VO2 film. Herein, mesoporous VO2 powders with pore size of about 2-10 nm were synthesized using cotton as template by hydrothermal methods. The pore and crystal size of the synthesized VO2 powders can be reliably controlled by the hydrothermal temperature. The mesoporous VO2 powders were mixed with PVP to prepare the VO2-based nanocomposite films by spin coating. The VO2-based films show a better performance between ΔTsol and Tlum than that appeared in previous reports. Especially, a larger pore size could lead to a higher visible transmittance and a larger crystal size would facilitate the enhancement in the solar modulation. In this sense, the VO2-based film obtained at the hydrothermal temperature of 180 °C exhibits an outstanding thermochromic performance with ΔTsol of 12.9% and Tlum up to 56.0% due to a larger crystal size and pore size. Therefore, this synthetic route shows a potential method for the application of mesoporous VO2 powders for solar control coatings.