Researchers at Rice University, who invented laser-induced graphene (LIG), in collaboration with researchers at Ben-Gurion University in Israel, have designed a way to make the spongy graphene either superhydrophobic or superhydrophilic.
Until recently, the Rice lab made LIG in open air only, using a laser to burn part of the way through a flexible polyimide sheet to get interconnected flakes of graphene. However, putting the polymer in a closed environment with various gases changed the product’s properties. Forming LIG in argon or hydrogen makes it superhydrophobic (extremely water-avoiding), a property highly beneficial for separating water from oil or de-icing surfaces. Forming it in oxygen or air makes it superhydrophilic (extremely water-attracting), making it highly soluble.
“Labs could make graphene either hydrophobic or hydrophilic before, but it involved multiple steps of either wet-chemical or chemical vapor deposition processes,” Prof. Tour, leader of the Rice team, said. “We’re doing this in one step with relatively cheap materials in a homemade atmosphere chamber.”
The labs encountered an unexpected surprise when they discovered that fabricating LIG in oxygen increased the number of defects – 5- and 7-atom rings – in the graphene flakes, improving its capacitance and its performance when used as an electrode material for microsupercapacitors.
Changes in the chemical content of the gas and even changes in the direction of the laser raster pattern altered the material, leading the researchers to believe LIG’s hydrophobic or hydrophilic properties could be tuned. They also discovered when they scraped graphene off of a hydrophilic sheet of polymer and turned it into a film, the result was hydrophobic instead. “That leads us to believe the surface orientation of LIG’s flakes have a lot to do with how it reacts with water,” Tour said. “If the edges are more exposed, it appears to be hydrophilic; if the basal planes are more exposed, their hydrophobic properties take over.”
What makes a material “super” in either direction is the angle at which it encounters water. A material with a contact angle of 0 degrees is considered superhydrophilic. In this case, water would lay on the material in a puddle. If the angle is 150 degrees or more, that’s superhydrophobic; the angle is determined by how much the water beads. (An angle of 180 degrees would be a sphere sitting perfectly on top of LIG.)