A step forward for smart windows?

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