Flexible MXene coatings stay put on any surface

Researchers have come up with a simple method to make coatings of novel 2D materials called MXenes on a range of surfaces such as fabric, plastic, and glass. Compared to MXene films made previously, the robust coatings stick strongly even to curved surfaces and maintain their properties when bent and stretched.

The technique is a step toward “spray-on or paint-on batteries or capacitors on surfaces you never dreamed of, or sensors integrated into unusual places like painted on walls or coated on fabric,” said Jodie Lutkenhaus, a chemical engineer at Texas A&M University who presented the work Thursday in a Division of Polymeric Materials session at the American Chemical Society Fall 2021 meeting.

MXenes are fast-growing family of 2D, layered, transition-metal carbides or nitrides that show exciting prospects for energy storage, chemical sensing, catalysis, and wearable electronics. They allow mixing and matching of elements to tailor their electronic, optical, and chemical properties. Researchers typically make MXene films by spraying or dip-coating surfaces. But the films easily crack, flake, or peel off.

Lutkenhaus and Texas A&M colleagues Micah Green and Miladin Radovic make their coatings by alternately depositing layers of MXenes, which are negatively charged, and a positively charged polymer such as poly(diallyldimethylammonium chloride) on a substrate. They deposit the layers onto the substrate by spraying or dipping.

The oppositely charged materials stick to each other and the substrate through electrostatic forces. The forces are strong enough that the coating stays put when the researchers rub a piece of adhesive tape on it and peel it off.

“It’s like a layer cake of positive and negative charges,” Lutkenhaus says. “But it’s more disorganized with MXene sheets from one layer poking into the next so it’s an interconnected network that maintains conductivity.” Less than 10% of the film’s volume is polymer, so it still has all the properties of the MXene, she adds.

By coating carbon fibers with the most commonly studied MXene Ti₃C₂T_x, the team has made flexible supercapacitors that could be woven into fabrics for energy storage. And they have most recently made films with novel MXenes Ti₂CT_z and Nb₂CT_z that heat up when hit with radio frequency waves. “You could use these films for localized heating, say in situ wound healing, or as [a radio frequency] sensor,” she says.

The strongly adherent coatings should be especially useful for making fiber-shaped devices for energy storage or harvesting or for sensing that can be embedded in textiles and wearable electronics, says Yury Gogotsi, a chemist and materials engineer at Drexel University, who first described MXenes in 2011. The technique also offers the opportunity to combine various 2D materials such as graphene, MXenes, and boron nitride so that “each material adds its useful properties leading to hybrid [structures] with tunable properties.”