This is an image of the custom chamber built by researchers at Rice University that allowed them to refine their process for creating laser-induced graphene. Image: Tour Group/Rice University.Scientists at Rice University who invented laser-induced graphene (LIG) for applications like supercapacitors have now figured out a way to make the spongy graphene either superhydrophobic or superhydrophilic – and it's a gas.
Until recently, the Rice lab of James Tour made LIG only in the open air, by using a laser to burn part of the way through a flexible polyimide sheet and produce interconnected flakes of graphene (see Laser-induced graphene produces micro-supercapacitor). But they have now found that putting the polymer in a closed environment with various gases alters the properties of the resultant graphene.
Forming LIG in argon or hydrogen makes it superhydrophobic, or water-avoiding, a property that is highly valued for separating water from oil or de-icing surfaces. Forming LIG in oxygen or air makes it superhydrophilic, or water-attracting, and that makes it highly soluble. Tour and his team conducted this research in collaboration with researchers at Ben-Gurion University in Israel, and report their findings in a paper in Advanced Materials.
"Labs could make graphene either hydrophobic or hydrophilic before, but it involved multiple steps of either wet-chemical or chemical vapor deposition processes," Tour said. "We're doing this in one step with relatively cheap materials in a homemade atmosphere chamber."
As a bonus, the researchers discovered that fabricating LIG in oxygen increased the number of defects – 5- and 7-atom rings – in the graphene flakes, improving its capacitance and thus its performance when used as an electrode material for micro-supercapacitors. Changes in the chemical content of the gas and even changes in the direction of the laser raster pattern altered the material, suggesting that LIG's hydrophobic or -philic properties could be tuned.
The researchers also discovered that when they scraped graphene off of a hydrophilic sheet of polymer and turned it into a film, the result was hydrophobic. "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 interacts with water. A material with a contact angle of 0° is considered superhydrophilic; in this case, water would lay on the material in a puddle. If the angle is 150° or more, that's superhydrophobic; the angle is determined by how much the water beads. (An angle of 180° would correspond to a sphere sitting perfectly on top of LIG.)
The discovery that surface type and chemistry can affect LIG should also allow some flexibility in adjusting the material's properties, Tour said. In fact, when they used a sulfur/fluorine gas, they were able to raise LIG's superhydrophobicity to 160°.
This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.