Researchers have converted natural wood into flexible, strong, water-repellent films that glow in different colors. The luminescent films could lead to more sustainable alternatives to the glass and plastic used in today’s lights and displays. They could also find novel uses in applications such as luminescent building facades, ceiling light panels, and even furniture that glows (ACS Nano 2020, DOI: 10.1021/acsnano.0c06110).
Wood has a well-organized structure composed of aligned cellulose fibers made of smaller nanoscale fibers that are bound together with lignin polymers. This hierarchical structure makes wood strong and thermally insulating, and the films made of the cellulose nanofibers can have unique optical properties because of the way they reflect and scatter light.
Because of these properties, scientists have tried to chemically and mechanically alter wood to make novel, renewable substitutes for glass and plastic. By removing lignin, which gives wood its color, some researchers have made transparent wood that is stronger than glass. Others have extracted cellulose nanofibers from wood and used them to make thin films for potential use in electronics and displays. But these wood-based materials are typically mixed or infused with petroleum-based polymers.
The new wood films don’t need these petrochemical binders. To make them, Qiliang Fu of the research institute Scion, Ingo Burgert of ETH Zurich, and their colleagues start with a 1 mm thick piece of balsa wood and then soak it successively in solutions of sodium chlorite and sodium hydroxide to strip out the lignin and most of the hemicellulose. Fu has used this technique before to make flexible wood-based electronics.
Then, to make the materials glow, he and his colleagues immerse the lignin-free wood in a solution of quantum dots—nanoparticles made of a cadmium selenide core and zinc sulfide shell. Finally, the researchers press and dry the wood and coat it with a protective layer of hydrophobic hexadecyltrimethoxysilane.
When exposed to ultraviolet light, the films glow either red or green, depending on the size of the quantum dots used. The films are not entirely ecofriendly because of their use of toxic cadmium, Fu says. But the researchers are now trying to make biobased quantum dots in their laboratory, which would make the material nontoxic and fully biodegradable and may cut cost.
The material is stronger than previous optical films made out of cellulose nanofibers isolated/extracted from wood, says Yulin Deng, a polymer chemist at Georgia Institute of Technology. “The main advantage of this approach is to keep the original alignment of cellulose fibers in wood, so the thin film is stronger,” he says.
The manufacturing cost might be relatively high for practical applications, says Chaoji Chen, a materials scientist and engineer at the University of Maryland. But Fu says that his group is working on making the films on a large scale using a roll-to-roll process. “We can make it any size you want depending on the size of the raw wood veneer,” he says.