Effect of drying temperature on elemental diffusion and performance of antireflection thermochromic TiO2/VO2 sol-gel smart coatings

Thermochromic smart windows based on vanadium dioxide (VO2) dynamically regulate solar radiation transmission with temperature, offering an energy-efficient way to improve indoor comfort and reduce building energy use. As VO2 films often suffer from limited visible transmittance (Tlum), combining them with oxide materials such as titanium dioxide (TiO2) is a promising route to enhance Tlum. Using the sol-gel method - versatile, low-cost and simple - we investigated how processing parameters, and in particular the often overlooked drying temperature (Td), affect TiO2/VO2 bilayers. Through grazing incidence X-ray diffraction (GIXRD), Raman spectroscopy, scanning and high-resolution transmission electron microscopy (SEM/HRTEM), electron energy loss spectroscopy (EELS) and energy dispersive X-ray (EDX), we found that Td strongly influences elemental diffusion, which in turn impacts the optoelectrical properties of the films. High Td values (250 °C) cause significant Ti-V interdiffusion, loss of thermochromic behavior, and the formation of Kirkendall voids and vacancies. By properly combining drying (Td 150 °C) and annealing temperatures (Ta 550 °C), diffusion phenomena can be mitigated, leading to a + 21% improvement in Tlum without compromising thermochromic performance. This work shows that simple tuning of key steps during fabrication can maximize the functional properties of sol-gel multilayers, expanding their applicability across a wide range of fields.