Towards the optimization of a simple route for the fabrication of energy-efficient VO2-based smart coatings

The present work reports on the optimization of a simple two-step procedure to achieve thermochromic vanadium dioxide coatings. The effect of the layer thickness, the total surface area exposed, as well as different parameters involved in a fast annealing process leads to the selective oxidation in air atmosphere of vanadium films sputtered at glancing angles. Outcomes resulting from structural and optical characterizations of annealed films with different layer thicknesses and porosities disclose two optimal pathways for the fast heating synthesis of high-performance VO2-based coatings with unique thermochromic features. A first one at 475 degrees C with reaction times ranging from 40 to 60 s, ideal for thicker films (-100 nm), which implies reaching pseudo-equilibrium states with the formation of a VO2 + VO2+x phase mixture. A second one more adapted for thinner samples (-25 nm), which involves obtaining VO2-refined products through instantaneous reactions at 500 degrees C. The unusual balance achieved for luminous transmittance (>60%) and solar modulation ability (-5%) through the second route, coupled with a remarkable decrease in the metal-to-insulator transition temperature without the need of using doping (up to 15 degrees C below that of the pure bulk VO2, i.e., -68 degrees C), make these initially porous systems potentially applicable in energy efficient smart glazing or resistive switching while paving the way for further initiatives towards the simple and cost-effective manufacturing of VO2 thermochromic coatings at a large scale.