A simple two-step approach to the fabrication of VO2-based coatings with unique thermochromic features for energy-efficient smart glazing

Vanadium dioxide (VO2) is considered as one of the most promising materials for the next generation of energy-efficient smart glazing since it experiments a reversible metal-to-insulator transition at near room temperature. Nevertheless, the complexity and high cost associated to the fabrication of VO2-based nanostructures limit the transfer of this technology to the industrial scale. Aware of this opportunity, we present a simple and advantageous method for the fabrication of VO2 coatings on glass substrates which comprises the initial sputtering of vanadium thin films at glancing angles and the subsequent very fast oxidation of such systems in open air atmosphere. Relying on the accurate control of the thermal treatment parameters, as well as the enhanced reactivity of the high surface-to-volume porous deposited structures, thermochromic VO2 coatings were achieved and then characterized by means of scanning electron microscopy, grazing incidence X-ray diffraction, variable temperature UV-vis-NIR spectrophotometry, and resistivity measurements. These investigations allowed us to determine the key role that oxidation temperatures and times not only play in modulating the optical performance of the film, but also in the surprising and advantageous decrease in the transition temperature (up to 12 °C lower than the standard value for pure VO2), which is attained without incorporating doping agents. This fact, together with the remarkable values of luminous transmittance (∼50 %) and solar modulation ability (5–10 %) accomplished for our best samples, opens up an alternative and simpler pathway towards the large-scale manufacturing of VO2 coatings for smart window applications, reaching, on a preliminary basis, similar or even better performances than those obtained so far for single and undoped VO2 films.