Schematic illustration of the synthesis process of graphene QDs decorated with graphene homostructures (one of the three typical 2D materials explored in the research) using cryo-mediation liquid phase exfoliation.
Schematic illustration of the synthesis process of graphene QDs decorated with graphene homostructures (one of the three typical 2D materials explored in the research) using cryo-mediation liquid phase exfoliation.

Researchers have come up with a new way of preparing extremely thin flakes of material decorated with tiny particles useful for catalyzing the generation of hydrogen gas in a range of energy-related applications [Zhang et al., Materials Today (2020), https://doi.org/10.1016/j.mattod.2020.02.006].

Two-dimensional nanosheets just a few atoms thick and one-dimensional quantum dots of materials like graphene and transition metal dichalcogenides (TMDCs) have unique properties promising for electronics, sensing, energy storage, and catalysis. The high surface area with a large number of active edge states and different phases of these materials are particularly attractive for catalysis. Interest has focused predominantly on the hydrogen evolution reaction (HER), a simple and environmentally friendly electrocatalytic process, which relies heavily on platinum-based catalysts. A cheaper and more readily available catalyst material, which could be easily produced in large quantities, would be highly desirable.

To address this issue, the team from Hefei University of Technology, University of Cincinnati, Rice University, and Indian Institute of Technology combined the promising catalytic properties of zero- and two-dimensional materials by creating nanosheets of graphene, MoS2, and WS2 decorated with matching quantum dots. Existing synthesis routes for these materials tend to be complex, small-scale, and low yield, so the researchers came up with an alternative.

“[We set out to] find a rapid and efficient process for the preparation of 0D quantum dots and 2D nanosheets directly from pristine raw powders of layered materials,” explains Yan Wang, one of the authors of the study. “We [developed] a relatively simple and convenient method to construct the 0D/2D homostructures with quantum dots and nanosheets.”

The new approach uses a rapid temperature change to overcome interlayer Van der Waals forces and split apart the starting materials into nanosheets. To achieve this, graphene, MoS2, and WS2 powders are soaked in liquid nitrogen for a short time and then transferred into a low boiling point solvent. The thermal expansion caused by the extreme temperature change, which allows more contact between the solvent and the layers, combined with the forces of ultrasonication exfoliates the layered materials into few-atom-thick nanosheets, which readily become decorated with matching quantum dots.

“The novelty of our approach is the cryo-pretreatment process in liquid nitrogen and the self-assembly of homostructures,” says Wang. “Both few- and mono-layer quantum dots and ultrathin mesoporous nanosheets can be simultaneously obtained.”

The unique structure of the materials, which have abundant active edge sites available for reactions to take place, combined with the underlying conductivity of the nanosheets, makes them ideal as an effective HER catalyst.

The team is now trying out their cryo-exfoliation approach on other layered and non-layered materials to construct different homo- and hetero-junction materials for electrocatalysis and energy storage.

Click here to read the article in the journal.