Size‐Engineered VO ₂ Nanoparticles in a Fabry–Pérot Cavity for High Thermochromic Performance

This study develops a thermochromic Fabry–Pérot (F–P) cavity smart window enabled by particle‐size engineering to mitigate the intrinsic trade‐off between visible transmittance () and solar modulation ability (). The architecture comprises a VO 2 –PVP composite top layer containing two VO 2 nanoparticle populations (average sizes of ∼50 and ∼100 nm), a PMMA spacer, and an Ag‐based low‐emissivity (low‐E) substrate. Tuning both the mixing ratio and the total loading reveals a synergy between the two characteristic sizes. At a fixed total VO 2 loading, an optimized sample incorporating two VO 2 nanoparticle populations with different characteristic sizes delivers = 56.98% and = 14.44%, corresponding to increases of 2.75% and 90% over the single‐size counterpart. With the ratio fixed, further loading optimization yields = 56.09% and = 16.72%. This stems from stronger NIR extinction in the high‐temperature metallic state via enhanced scattering, boosting , while bimodal packing in the low‐temperature insulating state reduces scattering and smooths the refractive index, improving . In addition, the structure shows positive emissivity modulation ( = 0.165) and good optical cycling stability. Overall, synergistic control of particle size and loading provides a practical route to improved thermochromic regulation in F–P cavity smart windows.