Graphene helps to crumple up stiff metal oxides

Researchers at Brown University have found a new method for making ultrathin metal oxide sheets containing intricate wrinkle and crumple patterns. In a paper published in ACS Nano, the researchers show that patterning the metal-oxide films in this way improves their performance as photocatalysts and battery electrodes.

These new findings build on previous work done by the same research group in which they developed a method for introducing finely-tuned wrinkle and crumple textures into sheets of the nanomaterial graphene oxide, and then discovered that these textures enhanced some of graphene's properties. The textures made the graphene better able to repel water and enhanced its ability to conduct electricity (see The more wrinkles the better for graphene).

The researchers thought that similar structures might enhance the properties of other materials – specifically metal oxides – but there's a problem. To introduce wrinkle and crumple structures in graphene, the team compressed the sheets multiple times in multiple orientations. Unfortunately, that process won't work for metal oxides.

"Metal oxides are too stiff," explained Po-Yen Chen, a postdoctoral researcher in Brown's School of Engineering who led the work. "If you try to compress them, they crack."

So Chen, working with the labs of Robert Hurt and Ian Wong, both engineering professors at Brown, developed a method in which the crumpled graphene sheets act as templates for making crumpled metal oxide films. "We showed that we can transfer those surface features from the graphene onto the metal oxides," Chen said.

The team started by making stacks of crumpled graphene sheets using the method they had developed previously. They deposited the graphene on a polymer substrate that shrinks when heated; as the substrate shrinks, it compresses the graphene sitting on top, creating wrinkle or crumple structures. The polymer substrate is then removed to leave free-standing sheets of crumpled graphene behind. The compression process can be performed multiple times, creating ever more complex structures.

This process also allows control of what types of textures are formed. Clamping the film on opposite sides and shrinking it in only one direction creates periodic wrinkles; shrinking in all directions creates crumples. These shrinks can be performed multiple times in multiple configurations to create a wide variety of textures.

To transfer those patterns onto metal oxides, Chen placed stacks of wrinkled graphene sheets in a water-based solution containing positively-charged metal ions. The negatively-charged graphene pulls the metal ions into the spaces between the sheet stacks, where they bond together to create thin sheets of metal that follow the wrinkle patterns of the graphene. Finally, the graphene is oxidized away to leave the wrinkled metal oxide sheets. Chen showed that the process works with a variety of metal oxides, including zinc, aluminum, manganese and copper oxides.

Once the researchers had made the materials, they then tested them to see if, as was the case with graphene, the textured surfaces enhanced the metal oxides' properties. They found that wrinkled manganese oxide, when used as a battery electrode, had charge-carrying capacity that was four times higher than a planar sheet. That's probably because the wrinkle ridges give electrons a defined path to follow, say the researchers, allowing the material to carry more of them at a time.

The team also tested the ability of crumpled zinc oxide to perform a photocatalytic reaction – reducing a dye dissolved in water under ultraviolet light – finding that the crumpled zinc oxide film was four times more reactive than a planar film. According to the researchers, that's probably because the crumpled films have a higher surface area, giving the material more reactive sites.

In addition to improving the properties of the metals, Chen points out that the process also represents a way of making thin films out of materials that don't normally lend themselves to ultrathin configurations.

"Using graphene confinement, we can guide the assembly and synthesis of materials in two dimensions," he said. "Based on what we learned from making the metal oxide films, we can start to think about using this method to make new 2D materials that are otherwise unstable in bulk solution. But with our confinement method, we think it's possible."

This story is adapted from material from Brown 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.