Phase separation of VO2 and SiO2 on SiO2-Coated float glass yields robust thermochromic coating with unrivalled optical properties.

Cindy Yeung, Roberto Habets, Luc Leufkens, Fallon Colberts, Kathleen Stout, Zeger Vroon, Daniel Mann and Pascal Buskens.
Solar Energy Materials and Solar Cells 230, (2021).

Vanadium dioxide displays thermochromic properties based on its structural phase transition from monoclinic VO2 (M) to rutile VO2 (R) and vice versa, and the accompanying reversible metal-insulator transition. We developed a single layer coating comprising VO2 (M) and SiO2. We applied the coating from an alcoholic solution comprising vanadium(IV) oxalate complex and pre-oligomerized tetra ethoxy silane to SiO2-coated float glass using dip coating, and thermally annealed the dried xerocoat in a two-step process. The addition of SiO2 as coating matrix resulted in non-scattering coatings with low surface roughness and random distribution of VO2 nanodomains (≤200 nm). Furthermore, the formation of the coating, comprising a phase separation yielding SiO2 and VO2 nanodomains during the thermal anneal, was studied in detail. The coating displays unrivalled optical properties, combining high visible light transmission Tvis > 60% and large solar modulation ΔTsol ≥ 10%. When applied in insulating glass units, the coating has a positive impact on energy savings for heating and cooling of buildings in intermediate climates, which we demonstrated through building energy simulations. For a typical house in the Netherlands, energy savings up to 24% were obtained. In addition, we demonstrate a coating stability comparable to current energy-efficient window coatings during processing into and in insulating glass units through (accelerated) life time tests.