Graphene, Graphene Oxide, and Analogues

Graphite is hexagonal mineral. A single layer of the graphite structure is called graphene and was first intentionally prepared in 2004 by a “Scotch-type method” by Russian physicists A. Geim and K. Novoselov, for which they were awarded by Nobel Prize in 2010. In fact, graphene was first prepared already in 1962 by a chemist Hans-Peter Böhm by graphite oxidation by a method reported first in 1859 and adopted by Hummers in 1957. Graphite is oxidized by an explosive cocktail from concentrated sulphuric acid and potassium permanganate and intermediate graphite oxide is reduced, e.g., by glucose.

Graphene can, however, be also prepared without that explosive mix: by an action of intense cavitation field (high-power ultrasound) in strongly polar solvents (DMSO, NMP, DMF) at a pressure of 5-6 atm. In a batch reactor with 2 kW ultrasound generator it is possible to exfoliate (delaminate) graphite to individual single-C layers. The reaction yield can be in order of 10 g and it can also be quantitatively converted to grapheme oxide under substantially milder and safer conditions than what is required for graphite oxidation. The production of graphene or its oxide with quantitative yields is suitable for synthesis of novel materials based on graphene. As an example we can give photocatalytically active composite consisting of grapheme and titania. Graphene oxide obtained after cavitation-exfoliated graphite is much more reactive than graphene oxide from direct oxidation of graphite oxidation, which can be used for materials which are otherwise hard to obtain. The preparation of Graphene Quantum Dots (GQDs) is an example: conventional way to GQDs employs an autoclave, while our more reactive graphene oxide leads to GQDs under laboratory conditions.

Handling synthesis of graphene (or graphene oxide) (and their inorganic analogues) opens way to novel sophisticated materials, such as photocatalysts, sorption of persistent organic pollutants, stoichiometric degradation of organic pollutants, barrier paints or luminescence materials. The same method can be used for exfoliation of inorganic analogues of graphene, such as MoS₂, WS₂, BN, CaF₂, NbSe₂ etc. Exfoliated precursors allowed us preparing yet unknown quantum dots based on MoS₂ and BN.

Selected Papers

• V. Stengl, Preparation of Graphene by Using an Intense Cavitation Field in a Pressurized Ultrasonic Reactor, Chemistry – A European Journal, 18 (2012) 14047-14054.
• V. Stengl, D. Popelkova, P.Vlacil P, TiO₂-Graphene Nanocomposite as High Performance Photocatalysts, Journal of Physical Chemistry C, 115 (2011) pp 25209–25218.