Vertically distributed VO₂ nanoplatelets on hollow spheres with enhanced thermochromic properties

Vanadium dioxide (VO₂) is a phase-transition material exhibiting great potential in the field of smart windows for energy saving. Herein, VO₂ nanoplatelets vertically distributed on hollow spheres are synthesized via a template-free hydrothermal method. The nanoplatelets are separated and uniformly arranged on the spheres so that the dispersion and optical properties are greatly improved. Compared with the observations for the aggregated sample, visible transmittance (T_vis) and solar modulation (ΔT_sol) improve from 33% to 50% and from 5.2% to 11.2%, respectively. Meanwhile, a large contrast transmittance of 60% at 2500 nm is obtained. T_vis increases with gradual reduction in ΔT_sol as the film thickness is reduced. Ultrahigh T_vis of 87.2% and satisfactory ΔT_sol of 5.3% are obtained. Furthermore, the as-obtained VO₂ hierarchical structure exhibits higher sensitivity than VO₂ nanoparticles owing to 2D-structures of nanoplatelets. This study demonstrates a promising 2D structure for promoting practical applications in smart windows.