FE-SEM figures (top-view and cross-sectional view) of TiO2 films annealed at 400 °C.
FE-SEM figures (top-view and cross-sectional view) of TiO2 films annealed at 400 °C.

Anatase titanium dioxide is a promising material for the next generation of “smart windows”, according to new results from researchers in India.

Titanium dioxide (or titania, TiO2) is used in a wide range of applications – in everything from paint pigment to ceramics. It can be found in five main mineral forms, the most common three being rutile, brookite and anatase. The unique catalytic properties of the anatase form have been studied for decades, and in the last few years, has seen a renewed interest, alongside an ongoing debate. Anatase is generally more photocatalytically-active than the other two forms of the material, but as yet, little consensus has been reached to explain the difference.

A team from VIT University in India now reports that in the midst of this debate, they have found a real-world application for nanocrystalline thin films of TiO2. They have reported, in an issue of Materials Science in Semiconductor Processing 26 (2014) 251–258 [DOI: 10.1016/j.mssp.2014.05.006] , that TiO2 may be used to produce a new generation of “smart windows”. Smart windows can apply to any glass products that do more than just ‘keep the elements out’. Due to their surface chemistry, they can break up dirt and be self-cleaning, or with the application of a small voltage, switch from transparent to opaque. The secret to producing a window that can do both is to find a material that combines beneficial photocatalytic and electrochromatic properties.

It has been known for some years that titania’s photocatalytic effect is further pronounced in nanocrystalline films of anatase – an observation generally attributed to the increase surface-to-volume ratio. But Meher and Balakrishnan have demonstrated that alongside this, specially-designed films of anatase also have reasonable electrochromatic properties. Using low-cost sol-gel processes, the team deposited anatase thin films (200 – 300nm thick) onto glass, silicon and ITO substrates, and annealed them at a range of temperatures. They found that the films annealed at 400 °C showed both high-efficiency photocatalytic degradation of the test dye and could switch from transparent (at 550nm) to opaque in 10 s.

These initial results certainly show some potential for the use of nanocrystalline anatase in smart windows. But unanswered questions remain, including a full analysis of the wetting properties of the films. If they are found to be hydrophilic, anatase may yet find its way onto the windows of our future homes.

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