“Deleting” printed content from paper to save natural resources: Researchers at the University of California, Riverside, have now succeeded in what sounds unrealistic at first glance. The novel coating can be written on with UV light, whereby this writing can be erased again under atmospheric oxygen. In the study, up to 50 write-erase cycles were possible without any significant loss of contrast.
Non-toxic components
The material consists of three non-toxic components and is produced through a synthesis, according to researcher Yadong Yin. The semiconductor properties of titanium dioxide were suitable for producing the surface that could be written on with light. Nanocrystalline titanium dioxide changes when exposed to UV light. When excited by light, the titanium atoms change their electrical charge, and the previously transparent film turns dark blue to black.
The color change does not last, however, because atmospheric oxygen ensures direct re-oxidation. The experts have therefore extended the coloring with additives. To do this, they doped the nanocrystals with nitrogen, which they obtained from urea, and incorporated diethylene glycol, which served as a common and non-toxic solvent and even played a crucial role in the color change.
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Retarded re-oxidation
Diethylene glycol scavenges excess charge and delays re-oxidation to the transparent state. Applied to glass or paper, the nanocrystals formed an even coating that could be written on with UV light.
A 30-second illumination with a light source in the wavelength range below 400 nanometers was sufficient for this. It is said that a high contrast is already achieved in the power range of an LED lamp.
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