Thermochromic VO2 - SiO2 nanocomposite smart window coatings with narrow phase transition hysteresis and transition gradient width

Thermochromic vanadium dioxide (VO2) window coatings hold the promise of reducing the energy consumption of the built environment by passively regulating solar heat gain in response to changing conditions. Composite materials with embedded VO2 particles have shown greatly improved optical performances compared with thin films, however they typically exhibit broadened phase transition hysteresis and gradient widths, which negatively impacts the overall performance. Here, we present a scalable one-step solution based synthesis for a thermochromic smart window coating based on a vanadium dioxide sol-gel containing silica (SiO2 nanoparticles. We compare the performance of our nanoparticle composite with thin film VO2 along with composites formed by mixing VO2 and SiO2 sol-gels and find that both composites achieve an acceptable visible transmittance (similar to 50%) along with a comparable and competitive solar modulation (12.5% and 16.8% respectively), roughly double that of the plain VO2 film (6.7%). However, our SiO2 nanoparticle containing composite also benefits from a narrow transition hysteresis and gradient width (9.4 degrees C and 2.9 degrees C respectively). We predict that this method may subsequently be combined with metal ion doping to control both the optical and phase transition characteristics to achieve composite films with high overall energy saving performances.