Size-Engineered VO₂ 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 (T_avg) and solar modulation ability (ΔT_sol). The architecture comprises a VO₂–PVP composite top layer containing two VO₂ 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 VO₂ loading, an optimized sample incorporating two VO₂ nanoparticle populations with different characteristic sizes delivers T_avg = 56.98% and ΔT_sol = 14.44%, corresponding to increases of 2.75% and 90% over the single-size counterpart. With the ratio fixed, further loading optimization yields T_avg = 56.09% and ΔT_sol = 16.72%. This stems from stronger NIR extinction in the high-temperature metallic state via enhanced scattering, boosting ΔT_sol, while bimodal packing in the low-temperature insulating state reduces scattering and smooths the refractive index, improving T_avg. In addition, the structure shows positive emissivity modulation (Δε = 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. © 2026 Wiley-VCH GmbH.