Rational design of HSNs/VO2 bilayer coatings with optimized optical performances and mechanical robustness for smart windows

For vanadium dioxide (VO2)-based smart windows, insufficient optical performances, including luminous transmittance (T-lum) and solar modulation efficiency (Delta T-sol), have always been the main problems hindering their practicality as energy-efficient windows, which have attracted numerous studies in recent years. However, the improved T-lum are always obtained at the expense of Delta T-sol, and vice versa. In this work, we report a controllable interfacial engineering strategy for the fabrication of multifunctional hollow SiO2 nanospheres (HSNs)/VO2 bilayer coatings with tunable optical performance and mechanical robustness on the basis of finite difference time domain (FDTD) calculation. The theoretical calculation predicted that the T-lum of the bilayer coatings can be enhanced from 42.7% to 55.2% without sacrificing the Delta T-sol. Surprisingly, experiments showed that not only the T-lum of the optimized bilayer coating increased to 53.8% from 21.4%, but also the Delta T-sol enhanced from 9.8% up to 17.2%. It is believed that the outstanding optical performance comes from the low refractive index of HSNs as well as good dispersibility of VO2 nanoparticles. Moreover, the pencil hardness test, tap peeling test and sand impact abrasion test showed remarkable mechanical robustness of the bilayer coating, which makes it highly attractive for application in the windows of high-rise buildings.