Wrinkled graphene: synthesis and characterization of few layer graphene-like nanocarbons from kerosene

The precursor used in this study was kerosene soot (KS), obtained by thermal decomposition of kerosene in air. 2 g of KS was mixed with 2 g of sodium nitrate and 100 mL of sulphuric acid and stirred continuously for 15 minutes in an ice bath, followed by dropwise addition of 12 g of potassium permanganate. The mixture was then allowed to cool down for 30 minutes. After removing from the ice bath, the solution was stirred continuously for 48 hours by means of a Teflon coated magnetic stirrer. This was followed by the addition of 184 mL of distilled water, 560 mL of warm water along with 40 mL of hydrogen peroxide and the mixture was left undisturbed for 12 hours. The GO particles (KS1) formed were separated from the solution by centrifugation and washed repeatedly with water and acetone followed by sonication for a period of 20 minutes (SKS1). The samples obtained were designated as intercalated samples. The sample (KS) was dispersed in isopropyl alcohol (IPA) and was ultrasonicated in water (SKS) resulting in the exfoliation of graphite flakes into graphene sheets or few layer graphene.

The samples obtained before and after Hummers’ treatment and sonication were analyzed using various structural and morphological characterization techniques such as X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning electron microscopy (SEM), electron dispersive spectroscopy (EDS) and transmission electron microscopy (TEM). X-ray diffractograms of the samples were obtained using a Bruker AXS D8 Advance X-ray diffractometer. Raman measurements were performed at a wavelength of 514.5 nm using Horiba LABAM-HR spectrometer. The FT-IR spectra were obtained using a Shimadzu FT-IR-8400 spectrometer. The compositional analysis of the samples was carried out using X-ray photoelectron spectroscope (XPS-Omicron ESCA probe). The AFM images were obtained using a NanoSurf Easy Scan2 AFM machine. The SEM-EDS and TEM micrographs of the samples were recorded by means of JSM-6360 A (JEOL) and JEM-2100 (JOEL) systems, respectively.

The X-ray diffractogram of KS1 (Fig. 1) shows a characteristic peak of heterogeneous structure, indicating a mixture of graphite oxide and amorphous carbon at 25.09° as a result of oxidation of KS. Broadening of this peak is due to the relatively smaller size of the graphene layers or the short range ordering. The peak at 43.02° indicates the hexagonal lattice structure of the carbon. The (I₂₀/I₂₆) ratio of KS1 is found to be increased to 0.27 compared to that of KS, due to the inclusion of functional groups in the carbon backbone during oxidation. The d₀₀₂-spacing of KS1 has increased to 3.4204 Å indicating the defects induced in the graphene planes. L_a, L_c, N and n of KS1 have been determined to be 3.0240 nm, 2.9028 nm, 9 and 25, respectively. L_a is considered as a measure to quantify the dislocations, vacancies and number of graphitic atoms [2, 3, 6, 7]. A considerable decrease in the value of L_a of KS1 indicates the improved quality of the graphene oxide (GO) layers formed.

Fig. 1

AFM images of KS1 (Fig. 2) show mushroom shaped graphene islands (highlighted in red boundaries), resembling a stratocone. These islands are seen to be stacked one over the other. Height analysis of the KS1 reveals the formation of few-layer wrinkled graphene. These results are very well supported by the XRD analysis.

Fig. 2

Raman spectrum of KS1 (Fig. 3) exhibits broad and strongly overlapping D (1354 cm^–1) and G (1589 cm^–1) bands and a broad 2D band (2700 cm^–1). An increase in the intensity of the defect band indicates the incorporation of the functional groups. A blue shift of ∼6 cm^-1 is observed in the case of G band. This can be either due to the presence of D2 band resulting from the defects or the transition of graphite crystal to single graphene sheet [8, 9]. I_D/I_G of KS1 is found to be high (0.91) due to the incorporation of defects during oxidation, thereby disrupting the aromaticity.

Fig. 3

Deconvoluted spectrum of KS1 presented in Fig. 4 shows the existence of five peaks, namely, G (∼1589 cm^-1), D1 (∼1356 cm^-1), D2 (∼1605 cm^-1), D3 (∼1487 cm^-1) and D4 (∼1210 cm^-1). G and D2 bands arise from the graphitic planes whereas D1 band is due to disturbances such as functional groups in the graphene layer. D3 band suggests the amorphous carbon fraction of the soot. D4 can either be due to the sp²-sp³ mixed structure or an increase in defects as a result of oxidation. From Raman analysis, it can be concluded that the oxidative treatment of KS has resulted in the formation of finite sized less defective few-layer GO.

Fig. 4

The peak positions in the FT-IR spectrum of KS1 (Fig. 5) clearly indicate the incorporation of hydroxyl (3414 cm^-1), epoxy (1044 cm^-1, 1168 cm^-1 and 850 cm^-1), carboxyl and car-bonyl functional groups into the graphitic lattice of the KS, during oxidation. The peak observed at 1622 cm^–1 is due to C=C skeletal vibrations from graphitic domains and the one at 1720 cm^–1 is attributed to C=O stretching vibrations of COOH groups situated at the edges and defects of GO lamellae. Yet another peak at 580 cm^-1 is caused by the C–H out-of-plane bending vibration [10, 11].

Fig. 5

The C1s XPS spectrum of KS1 shown in Fig. 6, clearly indicates four components of carbon atoms in functional groups: the non-oxygenated ring C, i.e. the sp2 carbon (∼284.7 eV), the C in C–O bonds, C bound to O either as epoxy or hydroxyl (∼286.5 eV), the carbonyl C, i.e. C=O of alcohols, phenols or ether (∼287.3 eV) and the carboxylate carbon, O–C=O (∼289.0 eV) [12, 13]. The Π-Π* satellite peak (∼290.0 eV) is also observed, which is supported by the UV spectrum [12]. O1s appears at the binding energy of 532.6 eV as it arises from C=O (531.2 eV), C(=O)–(OH) (531.2 eV) and C–O (533 eV). From the XPS spectrum, the incorporation of functional groups in the KS1 can be deduced, which is also supported by FT-IR results.

Fig. 6

Specific resistivity (ρ), sheet resistance and porosity of the prepared graphene nanocarbon, KS and KS1, were obtained by van der Pauw method and the results are presented in Table 1. KS1 showed low value of resistivity (0.168 Ω·cm) and sheet resistance (56.28 Ω/sq) compared to that of KS. Low value of resistivity and decreased sheet resistance of the specific sample makes it an ideal material for the fabrication of devices such as fuel cells or supercapacitors [14, 15].

XRD profile of the sonicated kerosene soot (SKS) is presented in Fig. 7. The X-ray diffractogram of SKS shows two prominent peaks at 2θ = 24.72° and 42.85°, respectively. The (0 0 2) peak positioned at 24.72° has increased in intensity and shows a lower value of FWHM compared to that of KS, indicating the presence of crystalline graphitic carbon. The broadening of the peak at 42.85° upon sonication suggests the presence of amorphous carbon content. The (I₂₀/I₂₆) ratio for the SKS is found to be very low (0.17), thereby confirming the high quality of the carbon nanomaterials. L_a and L_c of the SKS have been determined to be 4.5787 nm and 1.8959 nm, respectively. The d₀₀₂ spacing of the lamellae is 3.3714 Å, which is in a close agreement with that of graphite reported in earlier studies [15–17]. The number of aromatic lamellae (N), average number of carbon atoms per aromatic lamellae (n) and graphitization degree (G) have been calculated to be 6 %, 11 % and 80 %, respectively, which indicates a high order in the stacking of graphene layers.

Fig. 7

The SEM images of Hummers’ treated and sonicated kerosene soot (SKS1) is shown in Fig. 8a. Hummers’ treatment has opened up the carbon nanospheres in KS, forming bundles of graphene islands (KS1). Sonication of KS1 resulted in delamination and leaving behind GO fragments. EDS analysis of SKS1 (Fig. 8b) shows that the sample is composed mainly of carbon along with oxygen, silicon and sulfur. The increase in the percentage of oxygen in the sample confirms the oxidation of KS and incorporation of defects in the carbon backbone.

Fig. 8

TEM micrographs (Fig. 9) reveal nearly spherical aggregates of graphene fragments, forming interconnected chain-like structures. The magnified TEM image shows that the individual graphene lamellae have rolled up as a result of chemical treatment, to assume a conical shape with a wide apex angle and distorted tubule, resembling nanohorns as reported by Masako et al. in 1999 [5]. Nanohorns are the major subclasses of nanocones, distinguished by an aggregate microstructure. The nanohorn aggregates are robust and cannot be separated into individual components. The distortion in the tubular structure of the shortened, bumpy nanohorns, seen in the TEM image, could be a result of a trade-off between the van der Waals force of interaction between the adjacent nanohorns and the stiffness of the individual horns, which depends on the diameter of the tubule [18].

Fig. 9