Researchers at Daegu Gyeongbuk Institute of Science and Technology (DGIST) have introduced a simple methodology that enables the fine control over the integration of foreign atoms with graphene. From that control, the researchers developed composite graphene-based heterostructures that exhibited synergistic oxygen evolution reaction electrocatalytic performance, and that can be used to store energy at low cost, and fabricate ultrathin, wearable electronics.

In the work, the researchers developed graphene monolayer/metal-oxide nanostructures (GML/MONSs) by using a low-temperature technique known as electrochemical deposition, in which they grew metal oxide nanostructures exclusively on the native defect sites of graphene.

The high surface area, chemical stability, and high mechanical strength and elasticity have made graphene a popular material since its discovery. The DGIST researchers aimed to overcome some of the material’s existing limitations — namely its single-atom thickness, chemical inertness, and lack of an energy gap — by integrating graphene with other materials. The integration of graphene with metal, insulator, and semiconductor materials has already allowed researchers to form composite structures with desirable properties.

Though the addition of metal oxides to graphene to create GML/MONSs has produced examples of this structure, the deposition of uniform layers of metal oxides over graphene without disturbing the characteristics of the graphene layer is extremely difficult.

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The addition of foreign atoms to monolayer graphene supports the ability to develop flexible skin mountable devices that can be used to monitor health. Courtesy of DGIST.

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The researchers immersed a single-atom-thick graphene layer in a metal oxide precursor solution. They adjusted the deposition time and were able to precisely deposit the metal oxide onto the graphene monolayer. This produced composite structures with unique properties in the process.

“Metal oxide integrated graphene monolayers with lower densities (<30 µg/cm²) possess fewer defects, whereas those with higher densities have synergistic characteristics,” Sungwon Lee, a professor from DGIST and a member of the research team, said.

Control of the thickness and density of the metal oxide enabled the scientists to develop high-energy-density cobalt oxide/GML-based microsupercapacitors that could be used as a power source, with strong mechanical stability and durability. They also developed ultrathin zinc oxide/GML-based photoresistors that possessed excellent flexibility and wearability.

“This new class of heterostructures could be adopted for the fabrication of nontoxic and low-cost energy conversion and storage devices as well as the development of ultrathin, lightweight, and skin-mountable devices that can be integrated with real-time health-monitoring systems,” Lee said.

The work, the team said, will pave the way for the development of biocompatible, durable, eco-friendly, and ultralight graphene-based materials.

The research was published in Nano Energy (www.doi.org/10.1016/j.nanoen.2021.106274).