University of California Riverside researchers have developed a coating material that can be written on with ultraviolet (UV) light and then erased using oxygen.

With the potential to reduce paper waster, this rewritable material was developed using non-toxic components. First, the team used titania (TiO2) nanocrystals to create the light-sensitive, rewritable system. Nanocrystalline titania — a semiconductor — turns dark when exposed to UV light thanks to charge separation and the reduction of titanium atoms. However, this color change is not permanent as oxygen re-oxidizes the titanium and thus encourages a return to transparency.

Source: University of California RiversideSource: University of California Riverside

To sustain the color change for a greater length of time, researchers employed nitrogen obtained from urea to serve as a dopant and further adorned the crystals with the non-toxic substance diethylene glycol. The diethylene glycol, which was added to the synthesis as a solvent, also encouraged the color change — scavenging excess electron holes, thereby delaying re-oxidation and the subsequent return to the transparent state.

Applied to both glass and paper, the nanocrystals reportedly created a uniform coating that is writeable using UV light. The team employed two light writing methods — a photomask and a laser pen — to create high contrast patterns that were stable for several hours and that could be erased with the application of heat or faded over time via oxidation.

The researchers also determined that the life of the print could be prolonged with the application of a protective polymer film layer for reducing oxygen exposure.

Reusable up to 50 times, the light sensitive, rewritable system is being eyed for daily transport tickets, sensor technology, data storage and information boards, for example.

The study, Rapid High‐Contrast Photoreversible Coloration of Surface‐Functionalized N‐Doped TiO2 Nanocrystals for Rewritable Light‐Printing, appears in the journal Angewandte Chemie International Edition.

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