Structural Characterization of Graphite Nanoplatelets Synthesized from Graphite Flakes

Graphite nanoplatelets (GNPs) were produced from flake graphite that had been immersed in isopropyl alcohol solution (70%) and converted to powder form in an ultrasonic bath (5 h, room temperature). Scanning and transmission electron microscopy, X-ray diffractometry, and Raman spectroscopy identified 120-nm-thick GNP crystallites and 0.5–21 μm plates with different areas and shapes. Extensive exfoliation was observed by transmission electron microscopy with abundant multilayer and some monolayer GNPs. X-ray diffractometry confirmed 43 GNP layers along the c-axis. Raman spectroscopy indicated well-defined GNPs with few defects and no oxide content. Rietveld analysis indicated a GNP crystal lattice with stacks of parallel two-dimensional graphene layers and tightly bound hybridized carbon atoms stacked in a translational ...ABAB... sequence in hexagonal rings.


Introduction
As the sixth element in the periodic table, carbon can catenate with elemental carbon or other elements to form organic structures as the basis of life. Zero-, one-, two-, and three-dimensionalstructures, which are termed fullerenes, carbon nanotubes, graphene, and graphite, respectively, are basic elemental carbon-based structures [1,2].
Graphite, as one of the oldest known forms of carbon and termed black lead or plumbago, which in Greek "grapho", means "to write" and is widely used in pencils and lubricants. Graphite occurs in metamorphic rocks (sedimentary carbon-compound reduction during metamorphism), in igneous rocks, and in meteorites [3]. Graphite is used in electronic products, such as batteries, electrodes, and solar panels, because of its high conductivity, and low cost. In addition, its high thermal conductivity, and low mass make it an ideal thermal conductive polymer composite in manufacturing [3][4][5][6][7].
Despite graphite appearing to be a well-defined homogeneous product with a known chemical and physical structure, different properties are displayed by different graphite types (amorphous, flake, vein, and synthetic). The two forms or phases of graphite include rhomobohedral and hexagonal crystal structures with similar physical properties (density = 1.9 g/cm 3 , iron-black to steelgray color, and deep blue in transmitted light). The hexagonal and rhombohedral graphite phases have a P63/mcc and R3m space group, 4 and 6 atoms per unit cell (Z), and a = 0.2461 nm and c = 0.6708 nm and a = 2.456 nm and c = 10.044 nm lattice parameters at room temperature, respectively [8,9]. The single-crystal lattice stacking periodicity structure of the hexagonal (2H) and rhombohedral (3R) graphite phases is ABAB and ABCABC, respectively. Graphite can be cleaved along basal planes and its particle size is readily reduced by mechanical grinding because the interlayer bonding force is weaker than that of the intralayers [10][11][12][13][14][15][16]. Graphene, which is the term for individual graphite layers, can be restacked to prepare graphite [17]. In the stacking, carbon atoms are layered in 0.142nm-spaced honeycomb lattices, and planes are separated by 0.335 nm [5]. Their similar structures prevent characterization techniques from being used to distinguish between graphite and graphene [18].
In this study, scanning and transmission electron microscopy (SEM and TEM, respectively), Xray diffractometry (XRD, with Rietveld refinement), and Raman spectroscopy were used to determine the physicochemical properties of GNPs that were synthesized from flake graphite.

Material synthesis
Graphite flakes (molecular weight 12.01 g/mol, product number 332461, Sigma-Aldrich) were used to prepare the GNPs by immersion in isopropyl alcohol (70%) and ultrasonication with frequency of 40 kHz in a bath for 5 h at room temperature, which resulted in shear forces and cavitation by micron-sized bubble growth and collapse. GNPs were formed by filtering and drying the product dispersion.

SEM
The GNP morphology was studied by SEM (IRMC-INSPECT S50, 20 kV acceleration voltage). The GNPs were not coated to prevent charging because they are a good electrical conductor.

TEM
Ground samples of GNPs (~5 mg) were ultrasonicated in an ethanol suspension for 10 min. TEM imaging (FEI MorgagniTM 268, tungsten tip electron source, 100 kV) was carried out on copper grids with two drops of suspension.

Raman spectroscopy
Raman spectroscopy was undertaken on a Labram 1B dispersive Raman spectrometer (2 mW, 150µ m slit, 632.817 nm excitation, 50× objective, 600 lines/mm diffraction grating, Peltier-cooled chargecoupled device detector at -40 °C, with 60 s collection time and 10 s for silicon). The silicon peak correction was undertaken by shifting the Raman spectra by a 520.7 wavenumber.

XRD
The GNP purity and structure were confirmed by XRD (Rigaku Benchtop Miniflex X-ray diffractometer, Cu-Kα radiation, λ = 0.1541 nm, 40 kV, 20 mA, 20-70°, 0.02/min scanning speed). FullProf software (version 7.20) and Rietveld pattern analysis was used with the goodness-of-fit obtained from the weighted pattern R-factor (Rwp), derived Bragg R-factors (RP), and expected R-factor (Rexp). An optimized sample displacement, pattern background, peak shape, preferred orientation, 2θ0, scale factor, and lattice parameters were used to calculate the GNP crystalline phase abundances. Rietveld refinements were undertaken by using the Crystallography Open Database and graphite crystal structures (COD 9000046).

GNP microstructures
SEM and TEM were used to determine the particle size, morphology, and microstructure of GNPs. Typical secondary electron images of sonicated graphite flakes are presented in Figure 1a and (b). Fragmented flake graphite yielded foliated parallelogram graphite plates. The ImageJ ® software (National Institutes of Health, USA, version 1.48e) was used to determine the area and thickness of 70 GNPs, and they were 0.5-21 µ m and ~120 nm, respectively. These values are comparable with the nanosheets that Chen et al. obtained by ultrasonic of graphite powder [21]. The TEM micrographs in Figure 2a and b show graphite in nanoplatelets at a low resolution and granularity in the higher-resolution images, respectively. Therefore, nanoplatelets with varying shapes and areas formed because ultrasonication prior to TEM broke down graphite nanoplatelets into smaller pieces. The weak interlayer van der Waals attraction allowed the nanoplatelets to slide past each other perpendicular to the c-axis, but the sufficiently strong attraction prevented the complete formation of individual graphene layers [17]. TEM diffraction contrast from thickness variations indicates that graphite nanoplatelets of a few layers, including bi-and tri-layers, formed, but some monolayer graphene was also visible. Polycrystalline graphite with randomly oriented crystallites with an interplanar spacing of ~0.40 nm is shown in Figure 2c. Single crystals in natural flake graphite were oriented in a preferred direction, whereas synthetic graphite was oriented more randomly [22].

Raman spectroscopy
Raman spectra of GNPs are shown in Figure 3, where the G and two-dimensional (2D) band shape, position and intensity can be used to estimate the number of GNP layers. The 2D band changes position, width, and shape as the number of layers increases but the peak position of the G band shifts downwards as the layer number increases. Characteristic GNP peaks exist in the D, 2D, and G bands at 1,331, 2,686, and 1,577 cm -1 , respectively. A high sample quality is indicated by the weak and strong D and G peaks (for sp 2 carbon), respectively, and the broad multi-band 2D peak indicates multi-layer graphite features [17]. A primary graphene band characteristic is illustrated by the sp 2 carbon atom vibration in the G band. The D band provides a disordered GNP vibrational peak and is used to characterize GNP structural defects [23]. Well-defined GNPs with few defects are present. Raman-active defect concentrations in parts per million (ppm) are calculated from the ratio of the defect density (nD) and number of carbon atoms (nC) [24,25]: GNP defect concentrations compared well with the literature values for samples of graphite and graphene [25]. Figure 4 shows the diffraction pattern of crystalline GNP structures. The hexagonal crystal structure of GNP with no impurity or second-phase peaks is shown by characteristic diffraction peaks that were indexed as (002), (020), (111), and (004) planes [11,25]. The reflection profile broadening of XRD pattern is used to calculate the crystallite size and the corresponding peak position is used to determine the interlayer spacing. The average crystallite size (Lc) (c-axis crystalline dimension) of GNP was determined from the Scherrer equation and the d002 (dspacing for 2H (002) from 2θ peak at 26. 619°) was calculated from the Bragg's Law [25,26]:

XRD
where θ, β, λ, and k are the Bragg angle, line broadening at half maximum intensity (full width at half maximum, radians), X-ray wavelength (0.15419 nm), and shape factor (0.91), respectively. The computed crystallite size of 14.42 nm compared well with the graphite sample sizes obtained by Gen et al. [21]. The physical origins of the broad (002)-like reflections are interpreted from the uniform interlayer spacing (d002) and are related directly to layer misalignment with average interlayer spacings like crystalline graphite (0.335 nm), and these results agree with the TEM micrographs. Pure crystalline graphite has an identical basal spacing [25,27]. Seehra et al. described the Nc number of layers along the c-axis as [24]: They calculated the interlayer spacing (d002) and apparent crystallite size (Lc) in the c-direction that provide the numbers of GNP layers. Forty-three GNP layers exist along the c-axis.
The existence of parallel 2D graphene layers with a translational …ABAB… sequence of tightly bonded sp 2 hybridized carbon atoms in hexagonal rings is shown in Figure 6 from the Rietveld crystal lattice of GNP. The soft lubricating nature of the GNPs is provided by the covalently bonded carbon atoms in layers that are bound by weak van der Waals forces, which allows for graphene layer sliding. The distance between adjacent graphene layers in graphite (0.336 nm) is half of the hexagonal graphite crystallographic spacing (0.673 nm), which is similar to the calculated (002) peak d-spacing at 2θ of 26.619°. Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 14 August 2020 doi:10.20944/preprints202008.0325.v1 Figure 6. Hexagonal (2H) GNPs with graphene layers stacked in translational …ABAB… sequence with room-temperature 0.336 nm perpendicular interplanar distance.

Conclusions
Expanded flake graphite was ultrasonicated in isopropyl alcohol to prepare GNPs. XRD, SEM, and TEM were used to study the physicochemical properties of GNP. TEM indicated the formation of a large amount of multilayer and some monolayer GNP by exfoliation. XRD analysis indicated the formation of 43 layers along the c-axis. The GNP crystal structure as determined by XRD and Rietveld refinement indicated nanometer-thick worm-like exfoliated graphite nanoplatelets. The 120-nm-thick graphite units from flaky graphite exfoliation are a promising, low-cost, lightweight alternative to carbon-and metal-based electrically conductive reinforcement applications.