Size-Controllable M-Phase VO₂Nanocrystals for Flexible Thermochromic Energy-Saving Windows

Ultrafine M-phase VO₂nanocrystals with good crystallinity are desirable for highly efficient thermochromic flexible films. Herein, we report an oxygen-controlled hydrothermal method to achieve the one-step preparation of stoichiometric and size-controllable VO₂(M) nanoparticles for scaled-up production. The specific ratio of oxygen intake to V ions was first determined to guarantee the acquisition of stoichiometric VO₂(M). Then, the hydrothermal conditions were optimized to perform size-controllable growth of VO₂(M) nanoparticles. VO₂(M) nanoparticles with average particle sizes of 25, 32, 41, and 52 nm were prepared by changing the precursor solution concentration under the determined oxygen intake/V ion ratio. Smaller VO₂(M) nanoparticles exhibit better thermochromic performance because a smaller size favors light transmission and promotes blue-shifting of the local surface plasmon resonance. The flexible film fabricated with 25 nm VO₂(M) nanoparticles shows an excellent thermochromic performance with a solar modulation efficiency (ΔT_sol) of 19.2% and a luminous transmittance (T_lum) up to 66.42%. This work proposes a simple and effective hydrothermal process producing size-controllable VO₂(M) nanoparticles with low cost and scalable production and demonstrates that the size reduction of VO₂(M) nanoparticles is favorable for flexible films to exhibit excellent thermochromic properties.