13 December 2014

Graphene «curtains» and moire effect – metastable domains in carbon materials

Graphene is an ultra-thin material (1 atom thick), which combines a range of unique properties. Graphite, which is composed of graphene layers stacked on each other, is applied in a variety of fields from pencil production to neutron moderation in nuclear reactors, for production of synthetic diamonds, manufacture of heat-resistant lubricants, and adsorbents.

Graphene and graphite have gained outstanding popularity in the recent decade. Researchers are particularly interested in structuring (3D-organization) of these carbon materials. On molecular level graphite consists of number of domains with regular structure, which grow together to form grain boundaries. It is assumed that the most interesting physical and chemical phenomena and processes occur on the grain boundaries.

Researchers of Ananikov laboratory believe that the grain boundaries of graphene layers do modulate chemical interactions on carbon material surfaces. This fact reveals interesting perspectives of applications of carbon material nano-sized effects in organic chemistry and catalysis.

Figure 1. The appearance of different directed alternating dark and light bands is due to the morphology of the sample and its domain structure (graphite sample with deposited Pd nanoparticles; STEM-analysis).

Processes, which are carried out on the carbon grain boundaries, are difficult to investigate due to the lack of unambiguous way to quickly determine the topology of the domain. Direct observation of carbon grains and their boundaries is challenging. Authors paid specific attention to moire effect for detection of the grain boundaries. Moire effect is easily observed in everyday life, when curtains overlap, forming new patterns. Visually, moire pattern is alternating dark and light bands, and the effect is observed during studies of samples of carbon material by electron microscopy (fig. 1).

Another method of domain structure visualization is the deposition of palladium nanoparticles on the sample surface. Supposedly, nanoparticles must be deposited on the surface not chaotically, but on the boundaries of the domains, because the electron density of the inner part of the domain is significantly different from the electron density on its borders. Images were obtained during the study conducted with the help of scanning electron microscopy (SEM), which proves that palladium particles are arranged along grain boundaries. Scanning transmission electron microscopy (STEM) allows to penetrate inside the material and study sub-surface regions, confirming this observation and showing addition chains of nanoparticles. Comparison of the domain boundaries, detected using moire effect and nanoparticles, showed an interesting pattern (fig. 2).

Figure 2. Deposition of Pd nanoparticles on carbon material (graphite sample with deposited Pd nanoparticles; STEM and SEM studies of the same area of sample surface).

Currently, the principle of nanoparticles arrangement is not thoroughly investigated. However, obtained results indicate that particles, located on surface, have the ability to "feel" the changes in the electron density inside graphite. The authors assume the existence of more complex three-dimensional carbon structures, which modulate these interactions.

Systems with metal nanoparticles, deposited on carbon materials surfaces, are wide known catalysts for different fields of chemical industries such as pharmacy, oil refinement and development of new materials. Understanding the carbon matrix effects is crucial for development of industry and creation of new catalysts.

Article "Modulation of chemical interactions across graphene layers and metastable domains in carbon materials" by Evgeniy O. Pentsak, and Valentine P. Ananikov published in Mendeleev Communications (Elsevier B.V.).

Reference: Mendeleev Commun., 2014, 4, 327 - 328; DOI: 10.1016/j.mencom.2014.11.002.

Online link: http://dx.doi.org/10.1016/j.mencom.2014.11.002