Critical temperatures in the synthesis of graphene-like materials by thermal exfoliation–reduction of graphite oxide

Abstract

We prepared a series of graphene-like materials by thermal exfoliation/reduction of a graphite oxide (GO) at temperatures between 127ºC and 2400ºC. The extent of the exfoliation and reduction of the GO at different temperatures, as well as the impact on the resultant graphene-like materials (TRGs), were studied through their chemical/structural characterization. The main oxygen loss was observed at 127ºC during the blasting of the GO, which produced its exfoliation into monolayer functionalized TRG with hydroxyl groups and minor amounts of epoxy and carboxyl groups. Above 600ºC, the reduction continued smoothly, with oxygen and hydrogen loss and the conversion of hybridized carbon atoms from sp 3 into sp 2 . 1000ºC appears to be a critical temperature for the efficiency of the reduction process, as the resulting TRG contained less than 2% oxygen and 81.5% sp 2 -carbon atoms. The materials obtained at 2000ºC and 2400ºC were almost oxygen-free and the layers exhibited a dramatic restoration of the pristine graphite structure, as confirmed by the increase in the average size of the sp 2 -domains. The typical * Corresponding author: Fax. + 34 985 29 76 62; E-mail: rosmenen@incar.csic.es (Prof. R. Menéndez) 2 disordered stacking of TRGs increases with temperature, although they can be dispersed yielding monolayers at 127 and 300°C and stacks of up to 4-6 layers above 1000°C, as determined by AFM.

S- and N-Doped Graphene Nanomaterials for the Oxygen Reduction Reaction

“…Additionally, doped-rGO and carbon Vulcan reveal a small but visible signal close to 45 • that is associated to the (100) facet [17]. The Scherrer equation was used to determine the crystallite size for all the employed catalysts, and Table 1 summarizes the most representative crystallographic parameters attained [14,16]. Doped-rGO materials revealed a crystallite size lower than for graphite and GO, which was in agreement with the number of graphene layers (nl).…”

“…54 • related to the (004) facet [14,15]. The successful oxidation of graphite in GO is revealed by the disappearance of the peaks at 24.6 • and 54 • , also of the appearance of a broader contribution at lower diffraction angles (10.7 • ), which is associated to the (101) diffraction plane [16]. The latter is caused by the growth of oxygen functional groups (OFGs) between the graphitic layers, which caused an expansion of the C-C interplanar spacing from 0.34 to 0.84 nm, and therefore, a weakening of the respective chemical bonds of graphite [8,13].…”

“…The peaks at ca. 1360 cm −1 (D) and 1580 cm −1 (G) are correlated to sp 3 carbon domains and to sp 2 bonds into the graphitic grid, respectively [16]. Both signals have their corresponding overtones between 2500 and 3000 cm −1 , respectively [16].…”

“…Doped-rGO materials revealed a crystallite size lower than for graphite and GO, which was in agreement with the number of graphene layers (n l ). In this sense, n l was estimated from the ratio of the crystallite size and the interplanar spacing, and as expected, graphite showed the highest n l and particle size [16]. As was described above, the oxidation process (i.e., GO formation) creates OFGs that increase the separation of graphene layers (d 001 = 0.84 nm) and reduce the crystallite size and the n l ; meanwhile, the reduction procedure produces a further diminution of both parameters.…”

Optimization of the Synthesis Procedures of Graphene and Graphite Oxide

“…Elemental analysis showed an increase in the percentage of oxygen from graphite to graphite oxide due to the oxidation process. Comparing both graphite oxides, the percentage of each compound (C, O, S, Cl, and Mn) was quite similar [74]. In addition, scanning electron microscope (SEM) images showed an agglomeration of the product after the oxidation process, being several microns in size.…”