Optimized thermochromic properties for smart window application of VO2 films by wet-etching process

Vanadium dioxide (VO2) is a potential candidate in smart window application field owing to its reversible and automatic first-order insulator-to-metal transition (IMT) at 341K which can induce dramatic changes of transmittance in near infrared band through MIT. However, the low Tlum (visible light transmittance) below 40% and delta Tsol (solar modulation efficiency) below 10% of pure VO2 films have severely constrained the practical application in smart windows. In this paper, by etching the VO2 films fabricated by magnetron sputtering with sodium hydroxide solution, the VO2 films with improved optical properties were obtained. Specifically, the Tlum increased to 42.6% and the delta Tsol decreased to 15.5% for 150 nm VO2 films after 9 min etching, the Tlum increased to 56.4% and the delta Tsol decreased to 12.2% for 150 nm VO2 films after 12 min etching, respectively, comparing with the unetched film of 150 nm (Tlum:16.9%, delta Tsol:16.3%). Furthermore, we synthetically investigated the influences of etching on the IMT properties and molecular structure of the VO2 films through characterizations such as Raman spectra, XRD, XPS and SEM. The proposed methods represented a facile and cost-effective approach for enhancing thermochromic properties that could promote the application of VO2 thin films in smart windows.