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Vanadium dioxide (VO2) is widely recognized as a thermochromic material with great potential for application in smart glazing for energy-efficient buildings. The monoclinic (M1) VO2 phase undergoes a first-order reversible phase transition from the semiconductor to the rutile metallic state. In this study, an M1 VO2 porous film was synthesized via a polymer-assisted sol-gel route. Processing parameters, such as drying and reduction temperatures, were varied to evaluate their influence on the thermochromic behavior of VO2 and to determine the necessary trade-off between a significant thermochromic effect and high luminous transmittance. Film-silica glass-film systems with luminous transmittance close to 80% and IR solar modulation ability as large as 20% were prepared. By tracking the optical constants of the films during the thermochromic process, the changes produced at the microscopic level in the material could be correlated with its macroscopic behavior when used as an energy-saving material., Y J. Outon acknowledges the support by the Spanish Ministerio de Educacion y Cultura through grant FPU19-02638. M. Dominguez ac-knowledges the support by the Spanish Ministerio de Ciencia, Innovacion y Universidades under project EQC2018-004704-P. The authors thank the University of Cadiz and IMEYMAT for financing the mutual facilities available at the UCA R&D Central Services (SC-ICYT) and the IMEYMAT Institute project reference PLP2020335-1 respec-tively. The authors also acknowledge J. Gonzalez and F. Delgado for their assistance in the preparation of the STEM specimens.

The microstructure and optical properties of TiO2 thin films, prepared by the sol-gel dip coating technique on glass substrates, were inspected. After deposition, the films were annealed at several temperatures in the 400–850 °C range and the resulting nanostructured films were studied by different techniques showing that their structural and optical characteristics evolved significantly with the increased annealing temperature. The analysis of these results by the assumption of the Tauc Lorenz model and the use of Wemple–DiDomenico equation leads to a correlation between microstructural aspects and optical characteristics of the films. Thus, crystallization processes (nucleation, growth and phase transformation) and the evolution of films texture and thickness with increasing annealing temperatures are related with the variation of the refractive index, average gap and extinction coefficient during annealing. Finally, the free-carrier concentration in the films, estimated from the Spitzer–Fan model, ranged from 1.44 × 1019 cm−3 to 3.07 × 1019 cm−3 with the changing annealing temperature, which is in agreement with those obtained in similar anatase thin films from electrical measurement techniques.