Multilayer windows that are self-cleaning, energy-saving and anti-fogging may be one step closer, thanks to a team of Chinese researchers.

Windows are an important factor in a building’s energy efficiency, and with tall, glass-clad structures becoming the norm in our cities, teams of researchers are looking at ways to improve their efficiency, while maintaining their appearance. In the UK alone, 40% of the nation’s total energy bill comes from the way buildings are lit, heated and used, so even small changes in window technology could have a significant effect in reducing total energy consumption.

Much of the research on “smart windows” has focused on titanium dioxide (or titania, TiO2) which can be used to produce a self-cleaning surface, thanks to its photocatalytic properties. But Chinese researchers have taken this to a new level, by adding another “smart” ingredient, vanadium oxide (VO2), which can control infrared transmittance while maintaining transparency to visible light. The resulting material offers improved thermal insulating properties, is photocatalytically-active and doesn’t fog up. [DOI:10.1016/j.nanoen.2014.09.023]

This performance is the result of the composite’s unique crystal structure – it is effectively a sandwich of two forms of TiO2 (rutile and anatase) and VO2 in its monoclinic phase. In addition, the sandwich structure can be produced using standard thin-film production techniques. The bottom slice of the sandwich consists of TiO2 (rutile), which serves as an antireflection layer. This is followed by the ‘filling’ – a layer of VO2, which controls the amount of solar heat transmitting through the glass in response to temperature changes. The top layer of TiO2 (anatase) provides the photocatalytic properties that make this glass self-cleaning.

The team, led by Ping Jin from the Chinese Academy of Sciences, carried out a series of tests to characterise the final composite thin-film. Optical measurements showed that the 400 x 400 mm3 sample displayed excellent regulation of infrared light, while remaining transparent at visible wavelengths. UV radiation of the material also resulted in a photo-induced hydrophilicity, which produced in an antifogging surface. By measuring the degradation of stearic acid under UV light, the film was found to be highly photocatalytically-active.

The team are confident that their thin film has real applications in the development of a true “smart window”. Their multilayer film offers three functions at once – it is antifogging, self-cleaning and energy-saving – but until the robustness of this film has been measured, it may remain in the research lab.

Nano Energy, Volume 11, January 2015, Pages 136–145 “TiO2(R)/VO2(M)/TiO2(A) multilayer film as smart window: Combination of energy-saving, antifogging and self-cleaning functions.” DOI:10.1016/j.nanoen.2014.09.023