Nanoporous thermochromic VO₂ (M) thin films: Controlled porosity, largely enhanced luminous transmittance and solar modulating ability

Vanadium dioxide is the most widely researched thermochromic material with a phase transition temperature (τ_c) of around 68 C, and its thermochromic performance can be enhanced by adding nanoporosity. Freeze-drying has been employed to fabricate nanostructures with different porosities from 16 to 45% by varying the prefreezing temperature and precursor concentration. The luminous transmittance (T_lum) and solar modulating ability (ΔT_sol) are greatly enhanced as a result of increasing pore size and pore density. The freeze-dried sample with 7.5 mL of H ₂O₂ precursor dip-coated at 300 mm/min gives the best combination of thermochromic properties (T_lum ≈ 50%, ΔT _sol = 14.7%), which surpasses the best combined thermochromic performance reported to date that we are aware of (T_lum ≈ 41%, ΔT_sol = 14.1%).