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John O | May 2017

Georgia Tech researchers use high temps to create graphene from ethene


Researchers from Georgia Tech University in Atlanta, along with scientists from Germany and Scotland and support from the U.S. Air Force and U.S. Department of Energy, have developed a new process for creating single-layer graphene from ethene (also known as ethylene), which contains just two atoms of carbon.

 


Measured and theoretically simulated images of stages in the dehydrogenation process
observed in programmed surface heating experiments. (U. Landman and B. Yoon)

 

According to a report from Georgia Tech, the process includes heating the ethene in stages to temperatures of more than 700°C to create pure layers of graphene on a rhodium catalyst substrate.

 

The report explained, “Because of its lower cost and simplicity, the technique could open new potential applications for graphene, which has attractive physical and electronic properties. The work also provides a novel mechanism for the self-evolution of carbon cluster precursors whose diffusional coalescence results in the formation of the graphene layers.”

 

Scientists worked off theories that hypothesized that the transformation of ethene into graphene would include the formation of structures as hydrogen atoms leave ethene and carbon atoms self-assemble into graphene’s honeycomb pattern.

 

The article continued, “To explore the nature of the thermally-induced rhodium surface-catalyzed transformations from ethene to graphene, experimental groups in Germany and Scotland raised the temperature of the material in steps under ultra-high vacuum. They used scanning-tunneling microscopy (STM), thermal programed desorption (TPD) and high-resolution electron energy loss (vibrational) spectroscopy (HREELS) to observe and characterize the structures that form at each step of the process.”

 

The heated ethene adsorbed onto the rhodium catalyst, forming 1-D polyaromatic hydrocarbons (PAH), but continued heating led to a crossover into 2-D structures.

 

It also caused “dynamical restructuring processes at the PAH chain ends with a subsequent activated detachment of size-selective carbon clusters, following a mechanism revealed through first-principles quantum mechanical  simulations.  Finally, rate-limiting diffusional coalescence of these dynamically self-evolved cluster-precursors leads to condensation into graphene with high purity.”

 

The scientists saw clusters of 24 carbon atoms in the final stage and used a dehydrogenation process to free the carbon atoms and support the bond-breaking process that was needed to detach the carbon clusters. The resulting graphene is adsorbed into the catalyst and the removal of the carbon layer is part of the next stage in research.

 

The work was recently published in the Journal of Physical Chemistry C. The abstract explained:

 

“Diverse technologies from catalyst coking to graphene synthesis entail hydrocarbon dehydrogenation and condensation reactions on metals and assembly into carbon overlayers. Imperative to gaining control over these processes, through thermal steering of the formation of polyaryl intermediates and the controlled prevention of coking, is the exploration and elucidation of the detailed reaction scheme that starts with adsorbed hydrocarbons and culminates with the formation of extended graphene.

 

“Here we use scanning tunneling microscopy, high-resolution electron energy loss and thermal desorption spectroscopies, in combination with theoretical simulations to uncover the hierarchy of pathways and intermediates underlying the catalyzed evolution of ethene adsorbed on Rh(111) to form graphene.

 

“These investigations allow formulation of a reaction scheme whereby, upon heating, adsorbed ethene evolves via coupling reactions to form segmented one-dimensional polyaromatic hydrocarbons (1D-PAH). Further heating leads to dimensionality crossover (1D → 2D) and dynamical restructuring processes at the PAH chain ends with subsequent activated detachment of size-selective carbon clusters.

 

“Rate-limiting diffusional coalescence of these dynamically self-evolved precursors culminates (≤1000 K) in condensation into graphene of high structural perfection.”

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