A schematic of the pathway describing the evolution of adsorbed ethene (left) to graphene (right). The sequence of intermediates identified in the study and their respective appearance temperatures are indicated. Image: F. Esch, R. Schaub, U. Landman.An international team of scientists has developed a new way to produce single-layer graphene from a simple precursor: ethene – also known as ethylene – the smallest alkene molecule, which contains just two atoms of carbon.
By heating the ethene in stages to a temperature of slightly more than 700°C – hotter than had been attempted before – the researchers produced pure layers of graphene on a rhodium catalyst substrate. The stepwise heating and higher temperatures overcame problems that hampered earlier efforts to produce graphene directly from hydrocarbon precursors.
Because of its lower cost and simplicity, the technique could open new potential applications for graphene, which has attractive physical and electronic properties. This 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.
The research, reported in a paper in the Journal of Physical Chemistry C, was conducted by scientists at the Georgia Institute of Technology, the Technische Universität München in Germany and the University of St. Andrews in the UK. In the US, the research was supported by the US Air Force Office of Scientific Research and the US Department of Energy's Office of Basic Energy Sciences.
"Since graphene is made from carbon, we decided to start with the simplest type of carbon molecules and see if we could assemble them into graphene," explained Uzi Landman, a professor in the Georgia Tech School of Physics who headed the theoretical component of the research. "From small molecules containing carbon, you end up with macroscopic pieces of graphene."
Graphene is currently produced using a variety of different methods including chemical vapor deposition, evaporation of silicon from silicon carbide and simple exfoliation of graphene sheets from graphite. A number of earlier efforts aimed at producing graphene from simple hydrocarbon precursors had proven largely unsuccessful, creating disordered soot rather than structured graphene.
Guided by a theoretical approach, the researchers reasoned that the path from ethene to graphene would involve formation of a series of structures as hydrogen atoms leave the ethene molecules and the remaining carbon atoms self-assemble into the honeycomb pattern that characterizes graphene. To explore the nature of the thermally-induced rhodium surface-catalyzed transformations from ethene to graphene, experimental groups in Germany and the UK raised the temperature of the material in steps under an ultra-high vacuum. They then 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.
They found that, upon heating, ethene adsorbed on the rhodium catalyst evolves via coupling reactions to form segmented one-dimensional polyaromatic hydrocarbons (1D-PAH). Further heating leads to dimensionality crossover – transforming from one dimensional to two dimensional structures – and dynamical restructuring processes at the PAH chain ends. Next comes the 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 their condensation into graphene with high purity.
At the final stage before the formation of graphene, the researchers observed nearly round, disk-like clusters containing 24 carbon atoms, which spread out to form the graphene lattice. "The temperature must be raised within windows of temperature ranges to allow the requisite structures to form before the next stage of heating," Landman explained. "If you stop at certain temperatures, you are likely to end up with coking."
An important component is the dehydrogenation process that frees the carbon atoms to form intermediate shapes. However, some of the hydrogen atoms reside temporarily on, or near, the metal catalyst surface and assist in the subsequent bond-breaking process that detaches the 24-carbon cluster-precursors. "All along the way, there is a loss of hydrogen from the clusters," said Landman. "Bringing up the temperature essentially 'boils' the hydrogen out of the evolving metal-supported carbon structure, culminating in graphene."
The resulting graphene structure is adsorbed onto the catalyst. Although this may be useful for some applications, a way to remove the graphene will have to be developed. "This is a new route to graphene, and the possible technological application is yet to be explored," said Landman.
This story is adapted from material from the Georgia Institute of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.