Nanocarbons by High-Temperature Decomposition of Graphite Oxide at Various Pressures Alexandr V. Talyzin,* ,† Tama ´s Szabo ´, ‡ Imre De ´ka ´ny, ‡ Falko Langenhorst, § Petr S. Sokolov, | and Vladimir L. Solozhenko | Department of Physics, Umeå UniVersity, SE-901 87 Umeå, Sweden, Department of Colloid Chemistry and Nanostructured Materials, UniVersity of Szeged, H-6720 Szeged, Hungary, Bayerisches Geoinstitut, UniVersita ¨t Bayreuth, D-95440 Bayreuth, Germany, and LPMTM-CNRS, UniVersite ´ Paris Nord, F-93430 Villetaneuse, France ReceiVed: February 22, 2009; ReVised Manuscript ReceiVed: May 4, 2009 Thermal decomposition of graphite oxide into nanocarbons at pressures from ambient to 5 GPa has been studied by using X-ray diffraction, Raman spectroscopy and TEM. The temperature of graphite oxide decomposition at 5 GPa is close to the temperature of graphite oxide exfoliation at ambient pressure; however, materials formed are strongly dependent on pressure. High-pressure conditions favor formation of rounded graphitic nanoparticles, while exfoliation at ambient pressure results in preservation of paper-like wrinkled micrometer size sheets (with thickness of a few graphene layers) typical for pristine graphite oxide. 1. Introduction Graphite oxide (GO) is a disordered material obtained by strong oxidation of graphite. 1-3 Structure and properties of this material depend on particular synthesis methods and degree of oxidation. A typical feature of strongly oxidized graphite is the turbostratic planar structure with an interlayer spacing about two times higher (∼7 Å) as compared to pristine graphite. 4,5 Strong disorder and turbostratic packing of GO remains a problem for detailed understanding of the structure of this material even after 150 years of studies. Several structural models have been proposed for GO while attempts to clarify the structure of this material have significantly intensified recently. 6-17 According to NMR, XPS, and IR spectroscopy data, GO contains epoxy, carbonyl, and OH - groups attached to the planar graphene skeleton. 10-14 Most recent structural models also suggest significant deviation of geometry of oxidized graphene sheets from the two-dimensional planar structure of graphite. 11,13 According to the most recent study performed at high pressure, the compressibility of “dry” graphite oxide is very similar to that of turbostratic graphite and turbostratic boron nitride, 18 which is evidence of weak van der Waals bonding between oxidized graphene layers. GO is easily hydrated, resulting in a distinct increase of the interplanar distance (up to 12 Å). 4-6,18 Additional water is also incorporated into the interlayer space due to high-pressure- induced effects. 18 Complete removal of water from the structure of GO seems to be almost impossible, especially taking into account that heat treatment of only 40-50 °C above ambient temperature results in partial decomposition and degradation of the material. GO can be easily dispersed into separate sheets in basic solutions; this unusual property was recently used for the preparation of paper with advanced mechanical properties. 19 Single sheets of GO can be deposited as films from the dispersed state in liquid and then converted into graphene by using heat treatment. 15,16 Graphite oxide easily exfoliates in the process of rapid heating at moderately high temperatures (∼550-570 K) with formation of finely dispersed amorphous carbon material somewhat similar to high surface area activated carbons. Recent interest in GO is largely motivated by the synthesis of graphene related materials and composites 20,21 by using high-temperature exfoliation at ambient pressure. However, thermal decomposition at high- pressure conditions was never studied and could, possibly, result in the formation of unusual carbon nanostructured materials. In this study we performed heat treatment experiments at 5 GPa, which proved that thermal decomposition of graphite oxide occurs even at high-pressure conditions but results in different kinds of nanocarbon material. 2. Experimental Section Graphite oxide (GO) was synthesized by using the Brodie method. 1,2 The sample was characterized by IR spectroscopy and X-ray powder diffraction; its structure is found to be in good agreement with literature data. 4,5 High-pressure experi- ments were performed with a “toroid”-type apparatus. 22 The powder of graphite oxide was pressed into pellets and placed into gold capsules. Due to an insufficient amount of starting material we used NaCl in some samples in order to fill the empty volume of capsules. Capsules were not sealed to allow gases formed as a result of GO decomposition to escape. Sample pressure as a function of hydraulic oil pressure was calibrated by using room temperature phase transitions in Bi (Bi I-II at 2.55 GPa and Bi III-V at 7.7 GPa), PbSe (4.2 GPa), and PbTe (5.2 GPa). Temperature calibration was performed with use of Pt/ Pt-10%Rh and chromel-alumel thermocouples without cor- rection for the pressure effect on the thermocouple emf, as well as by using the melting of In (670 K), Sb (950 K), Zn (985 K), Si (1230 K), Al (1430 K), Cu (1670 K), Ni (1970 K), and Fe (2070 K) at 7.7 GPa as temperature reference points. Samples were gradually compressed up to 5 GPa at ambient temperature, and then the temperature was continuously in- creased with a heating rate of about 100 deg/min. The duration of heating at the required temperature was 10-15 min. Then * To whom correspondence should be addressed. Email: alexandr.talyzin@ physics.umu.se. † Umeå University. ‡ University of Szeged. § Universita ¨t Bayreuth. | Universite ´ Paris Nord. J. Phys. Chem. C 2009, 113, 11279–11284 11279 10.1021/jp9016272 CCC: $40.75  2009 American Chemical Society Published on Web 06/04/2009 Downloaded by UMEA UNIV on July 1, 2009 Published on June 4, 2009 on http://pubs.acs.org | doi: 10.1021/jp9016272