Hydrothermal One-Step Synthesis of Highly Dispersed M-Phase VO₂ Nanocrystals and Application to Flexible Thermochromic Film

Preparation of ultrafine highly dispersed VO₂(M) nanoparticles that are essential materials to fabricate thermochromic flexible films remains a challenge, preventing effective use of their promising properties. Here, we report an original hydrothermal approach by controlling oxidizing atmosphere of reaction with hydrogen peroxide to prepare ultrafine VO₂(M) nanoparticles free from annealing. Hydrogen peroxide is separated from precursor solution in a reactor, which creates a moderate oxygenation environment, enabling the formation of stoichiometric VO₂(M) nanoparticles. The obtained VO₂(M) nanoparticles are well-dispersed, highly uniform, and single-phase, with an average particle size ∼30 nm. The flexible thermochromic films fabricated with the VO₂(M) nanoparticles exhibit excellent thermochromic performance with a solar modulation efficiency of 12.34% and luminous transmittance of 54.26%. While the films prepared with annealed nanoparticles show reduced transmittance due to light scattering of the large size particles resulting from agglomeration and growth during annealing. This work demonstrates a promising technique to realize moderate oxidizing atmosphere by hydrothermal process for preparing well-dispersed stoichiometric nano-oxides.