Facile Synthesis and Characterization of Few-Layer Multifunctional Graphene from Sustainable Precursors by Controlled Pyrolysis, Understanding of the Graphitization Pathway, and Its Potential Application in Polymer Nanocomposites.

The key feature of the present work is the dexterous utilization of an apparently destructive process, pyrolysis, for the synthesis of the most esteemed nanomaterial, graphene. This work is an attempt to synthesize graphene from nonconventional sources such as tannic acid, alginic acid, and green tea by a controlled pyrolysis technique. The precursors used in this work are not petroleum-derived and hence are green. A set of pyrolysis experiments was carried out at different temperatures, followed by a thorough step-by-step analysis of the product morphology, enabling the optimization of the graphitization conditions. A time-dependent morphological analysis was also carried out along with isothermal thermogravimetric studies to optimize the ideal pyrolysis time for graphitization. The specific capacitance of the graphene obtained from alginic acid was 315 F/g, which makes it fairly suitable for application as green supercapacitors. The same graphene was also used to fabricate a rubber-latex-based flexible supercapacitor film with 137 F/g specific capacitance. The graphene and graphene-based latex film exhibited room-temperature magnetic hysteresis, indicating their ferromagnetic nature, which also supports their spintronic applications.

Introduction

Graphene, the two-dimensional, one-atom-thick honeycomb lattice of carbon, was first isolated in 2004 by a simple micromechanical cleavage process.[1] This apparently simple method of isolation of a single layer of graphite manifested to be a great scientific discovery and started a graphene gold rush. Development and study of the atomically thin carbon became a goal of many scientists.[2] Many other mechanical and micromechanical methods have been applied to break down bulk graphite to the nano level for synthesis of graphene in a top-down manner. Ball-milling has been a very popular method for preparation of graphene from graphite.[3] Dry ball-milling of graphite with melamine was also reported to be an effective method for exfoliation of graphite to few-layer graphene.[4] A remarkable scalable method of shear exfoliation of graphite has also been recently reported.[5] The earliest method of preparation of graphene in the form of reduced graphene oxide from graphite oxide prepared by the Hummers method from graphite was in practice even before the announced discovery of graphene.[6] By considering the methodology of exfoliation of graphite, or starting from a different material to yield graphene in a bottom-up reaction approach, graphene can be categorized into several classes, such as graphene oxide, reduced graphene oxide, single-layer graphene, few-layer graphene (2–10), or multilayer graphene.[7] The degree of exfoliation also leads to another classification, including graphite-intercalation compounds, expanded graphites, and graphenes.[8]

Apart from strategies that involve mechanical or chemical conversion of graphite to graphene, there have been techniques in which several organic molecules have been employed as precursors or sources for solid carbon for the growth of graphene from an atomic level in a bottom-up fashion. The most favored bottom-up technique is chemical vapor deposition (CVD), which yields graphene, usually superior in quality compared to that obtained from other established methods. Hydrocarbons, like methane, ethylene, acetylene, and toluene, have been used as precursors for graphene synthesis by CVD on metal substrates such as Cu, Ni, or Si.[9]

However, focus has recently been shifted toward nonconventional precursors. Poly(methyl methacrylate) was used as a solid carbon precursor in a CVD process for the growth of graphene.[10] Other polymers like polyurethane and styrene butadiene rubber have been used as nonconventional precursors owing to the presence of benzene ring and oxygen linkages in their structures.[11] Notably, the search for sustainability has become intensely popular for materials synthesis, starting from fuels and polymers to various carbonaceous materials. Biofuels are being introduced as experimental alternatives to fossil fuels.[12] Similar efforts to achieve sustainability have also been observed for carbonaceous materials. Carbon nanotubes have been synthesized from biological sources such as α-pinene.[13] Sustainable biological sources have gained even more interest for the purpose of graphene synthesis also. Use of novel biological sources like tea tree plant extract[14] and mango peel[15] has been reported.

Alongside these complicated attempts to reach the nanoscale, scientists have also traversed through the classical chemical routes involving high-temperature molecular rearrangements resulting in the formation of nanomaterials from the molecular level. Pyrolysis, an ancient technique, has been utilized and modified by scientists to match the requirements of the modern scientific era, enabling pyrolysis to be used in molecular dynamic simulations to study the degradation of complex polymeric molecules.[16] Sustainability and pyrolysis have voyaged side by side since the disposal of waste polymers became an alarming issue as pyrolysis opened up possible pathways to reach back to the hydrocarbons from the polymers.[17,18] There have also been many ventures using pyrolysis as a productive method to obtain biochars, biofuels, and carbon derived from biomass for energy storage and other applications.[19,20]

Inert-atmosphere pyrolysis or controlled combustions have been practiced as a skill to establish easier and inexpensive bottom-up synthetic routes for graphene such as pyrolysis or combustion of biomass. Rice husk was combusted in air and treated by KOH prior to high-temperature combustion in air at 1123 K for 2 h in a SiC crucible to yield good-quality graphene.[21] Dried wheat straw was treated with KOH in another experiment, followed by pyrolysis at 800 °C temperature for 3 h under a N2 atmosphere to obtain a carbonaceous material, which was washed by acid and finally annealed at 2600 °C temperature in Ar gas for 5 min to synthesize few-layer graphene.[22] Chitosan films were converted to graphene by pyrolysis in the absence of oxygen at a lower temperature range, typically from 600 to 800 °C.[23] Camphor leaves were also pyrolyzed in the N2 atmosphere at 1200 °C temperature to acquire a few layers of graphene.[24] Nitrogen-doped graphene was synthesized from a mixture of urea and edible sugar by means of a simple inert-atmosphere pyrolysis process carried out in a tube furnace at a maximum temperature of 1000 °C.[25] Sucrose, the main component of edible sugar, is known to graphitize at even lower temperatures, such as at 800 °C under a nitrogen flow, yielding graphene.[26] In this work, a set of novel precursors is presented for the synthesis of graphene in a simple pyrolysis method at a considerably lower temperature. Green tea, tannic acid, and alginic acid were chosen as the sources of carbon susceptible to graphitization at high temperatures owing to the labile linkages present in their chemical structures. Green tea leaves are rich with polyphenols such as epigallocatechin gallate, epicatechin 3-gallate, epigallocatechin, epicatechin, and catechin along with caffeine.[27] Tannic acid is also another polyphenol found in oak tree leaves and barks. Alginic acid is abundantly obtained from various seaweeds and is a water-soluble polysaccharide carrying carboxylic and hydroxy groups. These precursors are hence sustainable as well as cost-effective.

Experimental Section

Materials and Reagents

Lipton green tea leaves were obtained from the local market and used for pyrolysis without any prior treatment. Alginic acid was purchased from Alfa Aesar (Mumbai, India), and tannic acid powder was bought from Loba Chemie (Mumbai, India). Nafion D-521 dispersion (5% w/w water and 1-propanol) was purchased from Alfa Aesar (Massachusetts). N-Methyl-2-pyrrolidone (NMP) was purchased from Merck Life Science Private Limited (Mumbai, India). These materials were used in this research work without any further characterization. Toray carbon paper, PTFE-treated (TGP-H-60), was purchased from Alfa Aesar. SBR rubber latex was procured from Apcotex (Mumbai, India).

Experimental Reactor

The pyrolysis experiments were carried out in a custom-made tube furnace with the capacity of being heated up to a temperature of 1200 °C. SiC was used as the heating element in the furnace, and the temperature-sensing device consisted of a thermocouple. The heating tube was made up of alumina, and the inner tube diameter was 6 cm. The tube was surrounded by insulating fibers, and the entire setup was enclosed inside a metal chamber. One end of the tube was sealed with a pipeline attached for nitrogen inlet. There was a detachable metal lid at the other end of the tube with another pipeline for the outlet of nitrogen gas. The sample was placed inside the tube, and the lid was closed followed by joining the gas inlet with a nitrogen cylinder and dipping the gas outlet into a water-filled beaker to monitor the flow of nitrogen. Continuous flow of nitrogen was maintained throughout the pyrolysis as well as the cooling process. A schematic diagram of the reactor is given in Figure S1.

Synthesis of Graphene by Pyrolysis through Step-by-Step Optimization of Graphitization Time and Temperature

Dried green tea leaves as obtained from the local market were weighed and taken into a silica crucible. The crucible was placed inside a tube furnace, and the ends of the tube were sealed.

Nitrogen gas was passed through the tube to create an inert reaction chamber inside the furnace.

The temperature was set at 900 °C at first, and 3 h of pyrolysis time was given. The gas flow rate was kept constant throughout the reaction and also during cooling of the sample to an ambient temperature to avoid oxidation of the graphene formed. Tannic acid and alginic acid were pyrolyzed in the same manner and conditions. The products from tannic acid, alginic acid, and green tea from pyrolysis at 900 °C were named as T9, A9, and G9, respectively. Based on the findings of the preliminary characterizations of these products by field emission electron microscopy and Raman spectroscopy (discussed later), a subsequent second set of experiments was planned in which the temperature was set at 1100 °C and 3 h pyrolysis time was given at the set temperature. The products obtained from tannic acid, alginic acid, and green tea were thereby named as T11, A11, and G11, respectively. The products formed as soon as the temperature reached 1100 °C were cooled down and taken out from the furnace to study their structures by FESEM and Raman spectroscopy. To ensure that graphitization was taking place at this temperature within the given period of reaction time, two more batches were prepared in which the products at 0 min time and the products obtained at 30 min time were also taken out and thoroughly analyzed for their structures.

Analytical Methods

XRD patterns of the powder samples were recorded by X’Pert PRO, made by PANalytical B.V. (The Netherlands), using a Cu Kα radiation. One milligram of each of the powders was dispersed in 3 mL of DMF, dropped on glass slides, and dried to take Raman spectra of the samples using the Trivista 555 spectrograph (Princeton Instruments) using Ar laser of 514 nm wavelength. X-ray photoelectron spectroscopy studies were carried out in Omicron ESCA instrument, Oxford Instrument (Germany). Very dilute dispersions of the samples were dropped on the Al surface and dried for FESEM and carbon-coated Cu grids (300 mesh) for HRTEM imaging. A field emission scanning electron microscope (FESEM) (MERLIN with a tungsten filament; Carl ZEISS, SMT, Germany) and a JEOL JEM-2100F field emission high-resolution transmission electron microscope (HRTEM) were used for the microscopic images provided. The samples were gold-coated before the observation under FESEM. Very dilute dispersions of the samples in DMF were spin-coated on silicon wafer at 1500 rpm for 120 s and dried in an oven for 12 h at 70 °C temperature to prepare samples for atomic force microscopy (AFM, NX10 AFM instrument by Park Systems, South Korea). AFM scans on the samples were done using a silicon nitride tip of 10 nm diameter in the tapping mode. Specific capacitance of the materials was measured by cyclic voltammetry. Cyclic voltammetry experiments on the powder graphene samples were performed in the Metrohm Autolab instrument. The materials were prepared into electrodes by coating on Toray carbon paper and subjected to cyclic voltammetry tests. Then, 1 M potassium chloride solution was used as the electrolyte. Pt was used as the counter electrode, and Ag/AgCl was used as the reference electrode. The materials were prepared into electrodes by coating on Toray carbon paper and subjected to cyclic voltammetry tests. The electrochemical properties of the same were tested in Metrohm Autolab instrument. Mechanical properties of the graphene–rubber nanocomposite were tested in a Zwick/Roell Z010 UTM. The cryofractured surface of the graphene–rubber nanocomposite was gold-coated, and the morphology of the sample was viewed in a field emission scanning electron microscope (FESEM) (MERLIN with a tungsten filament; Carl ZEISS, SMT, Germany). Lake Shore Cryotronics VSM (model no. 7410) instrument was used for vibrating sample magnetometer (VSM) experiments.

Application of Graphene Synthesized by Pyrolysis

Graphene as a Supercapacitor Material

After elaborate characterization on the three graphene samples prepared from three precursors—tannic acid, alginic acid, and green tea—the one derived from alginic acid at 1100 °C, named A11, was tested for supercapacitor application. A11 gave the desired graphene properties as explained later. First, 7.5 mg of the powder sample was taken in a mortar, and 200 μL of the Nafion dispersion was added. NMP was added dropwise, the mixture was ground with the help of a pestle, and a slurry was prepared. The slurry was coated on a piece of Toray carbon paper on an area of 1 cm2. The coated carbon paper was then dried in an oven at 50 °C for 24 h. The piece of carbon paper was weighed before and after coating to determine the mass of the coated sample. The mass of the coated sample was 0.5 mg. This graphene-coated carbon paper was used as the working electrode for the cyclic voltammetry experiments.

Preparation of Graphene-Latex Thin Films (SLA11)

Styrene butadiene rubber latex of 23% styrene content was used to synthesize the flexible supercapacitor film. Then, 10 phr* of A11 filler was predispersed in 20 mL of Millipore water with the aid of 15 mM SDS by ultrasonication for 30 min. Next, 10 mL of latex (weighing 10 g) was added into the filler dispersion and stirred at 900 rpm for 12 h at an ambient temperature. A piece of Toray carbon paper of 1 cm2 area was coated by this latex dispersion and dried overnight at 50 °C for 24 h. The mass of the coated sample was 2.6 mg. The latex-based nanocomposite was henceforth named SLA11. The latex dispersion was poured on a flat Petri dish of 7 cm diameter and dried at an ambient temperature for 3 weeks to cast a thin film of an average thickness of 0.3 mm. This thin film of SLA11 displayed elongation up to 72% when tested for tensile stress–strain properties in a Zwick Roell Z010 universal testing machine (UTM) (phr* in this case indicates parts per 100 g of SBR latex).

Magnetic Properties

Magnetic properties of A11 and SLA11 were studied at 24 °C temperature. Approximately 20 mg of A11 powder was taken for the VSM experiment. The thin film of SLA11, which was cast on a Petri dish, was cut into small pieces, and 20 mg of these pieces was used for VSM studies. Both samples displayed a ferromagnetic hysteresis loop on the application of a magnetic field up to 6000 Oe.

Results and Discussion

Optimization of Pyrolysis Time and Temperature by the Detailed Analyses of Intermediates and End Products

Dried green tea leaves were used for the experiment without extraction of the green tea polyphenols, whereas tannic acid and alginic acid powders were procured as laboratory-grade chemicals and pyrolyzed. Studies of morphology by FESEM revealed that all three precursors after pyrolysis at 900 °C unveiled lumplike structures and no regular wrinkled sheet or flakelike structure of graphene was found (Figure S2, Supporting Information); moreover, G9 revealed a typical microporous structure. The morphology of these products was not satisfactorily graphene-like as observed under the field emission scanning electron microscope (FESEM). The Raman spectra of these samples presented the typical nature of activated carbon with consolidated D and G peaks at 1350 and 1580 cm–1 (Figure S3, Supporting Information). AFM images of the samples prepared at 900 °C are given in Figure S4 (Supporting Information). The temperature was increased to 1100 °C in the second set of experiments. Intermediates generated by the control experiments, as described in the experimental Section , at 0 min at 1100 °C were noted to be devoid of any graphene sheetlike features, as shown in Figure . In the other control experiment, the precursors were pyrolyzed at 1100 °C for 30 min, and the process was terminated; the products were cooled under a N2 atmosphere inside the furnace and taken out. The structural study of the products at this stage revealed formation of sheetlike features as well as the presence of rocklike structures. The structures of the graphitization intermediates were verified by FESEM as presented in Figure . The Raman spectra of these samples had submerged D and G peaks at 1350 and 1580 cm–1 as given in Figure S5 (Supporting Information).[28] The AFM images of the intermediates after 0 and 30 min of graphitization are exhibited in Figures S6 and S7, respectively (Supporting Information).

Figure 1

Morphology of the graphitization intermediates at 0 min time at 1100 °C: (a) tannic acid, (b) alginic acid, and (c) green tea. Morphology of the graphitization intermediates at 30 min time at 1100 °C: (d) tannic acid tea, (e) alginic acid, and (f) green tea.

Isothermal thermogravimetric analysis (isothermal TGA) experiments were carried out to obtain further insights into the graphitization of the precursors at 1100 °C temperature. Tannic acid retained 12% of its initial weight after 108 min when the temperature reached 1100 °C. In the case of alginic acid, it took 107 min for 1100 °C temperature to reach, and 19% of the initial weight was retained. Green tea retained 29% of its initial weight by the time of 107 min when the temperature reached 1100 °C. Though the rate of weight loss decreased when the isothermal test began, gradual weight loss continued to the end of the test till 136 min for all three samples. Tannic acid retained only 5%, while alginic acid and green tea retained 15 and 25% of their respective initial weights. Hence, less than 5% of the weight taken for tannic acid took part in graphitization. Similarly, for alginic acid and green tea, less than 15 and 25% of the initial weight of the raw materials were taken, actually participated in graphitization. Graphical representation of the isothermal TGA experiments is shown in Figure a–c. However, after pyrolysis at 1100 °C for 3 h, all three of the precursors presented well-formed sheetlike structures. These sheets of T11, A11, and G11 had lateral dimensions of about a few microns (8–10 μm) as shown in Figure d–f. On the basis of structural and morphological findings from field emission scanning electron microscope (FESEM) imaging, the temperature of the synthesis was optimized at 1100 °C.

Figure 2

Isothermal TGA at 1100 °C of the raw materials: (a) tannic acid, (b) alginic acid, and (c) green tea; morphology seen under FESEM: (d) T11, (e) A11, and (f) G11.

Plausible Pyrolysis Reaction Mechanism

The raw materials used in the present study are all biologically derived materials with different molecular structures: alginic acid, a polysaccharide; tannic acid, a polyphenol; and green tea, a complex mixture of polyphenols. Alginic acid, a prime component in algae, is known to undergo a major mass loss at around 250 °C temperature in thermogravimetric analysis.[29] Thermal degradation of tannic acid under a nitrogen atmosphere was reported to display minimal weight loss below 200 °C and major weight loss at 350 °C, a temperature higher compared to that observed in alginic acid, and yielded 27% of residue at 800 °C.[30] Green tea extract was also reported to undergo a significant weight loss at around 250–300 °C temperature.[31] Despite the differences in molecular structures of these three raw materials, they have one common feature in their chemical structures: the presence of labile oxygen linkages to six-membered hydrocarbon rings. These labile oxygen linkages are responsible for cyclization and metamorphism of the molecules at high temperature.[9] Evidence also revealed that pyrolytic carbon, produced from biomass precursors at lower temperatures, had a direct relation between the pyrolysis temperature and dehydrogenation of the hydrocarbons along with decomposition of the carbon–oxygen bonds, resulting in unsaturated sp2 carbon networks.[32]

A plausible reaction mechanism for the formation of graphene from alginic acid is given in Figure on the basis of all of the above experiments. The reaction possibly involves radical generation in the initial stages followed by release of carbon dioxide and water molecules and aromatization and subsequent intermolecular condensation reactions at 1100 °C temperature.[33] Green tea polyphenols and tannic acid are expected to undergo similar steps of reactions owing to the carboxyl groups and labile oxygen linkages present in their molecular structures.[9]

Figure 3

Plausible pyrolysis reaction mechanism for synthesis of graphene from alginic acid.

X-ray Photoelectron Spectroscopy (XPS) of Graphene

XPS of the graphene samples confirms the high carbon content in the samples, 91.28% in T11, 92.41% in A11, and 92.75% in G11, and as an obvious consequence quite a low oxygen content such as 8.26% in T11, 6.79% in A11, and 6.81% in G11. Traces of nitrogen were found in the samples: 0.44% in T11, 0.79% in A11, and 0.42% in G11. The XPS survey data shown in Figure establishes the formation of graphene and possibly not graphene oxide in the respective pyrolysis process.[34,35] sp2 carbon was the highest in content in all of the samples, 68.6% in T11, 72.5% in A11, and 75.2% in G11, as stated in Table S1 (Supporting Information). A small amount of sp3 carbon was possibly present in the samples: 13.3, 11.5, and 11.6% in T11, A11, and G11, respectively. Percentages of oxygen-containing groups were also determined and are tabulated in Table S2 (Supporting Information).

Figure 4

(a) XPS survey of T11, (b) XPS survey of A11, (c) XPS survey of G11, (d) C 1s analysis of T11, (e) C 1s analysis of A11, (f) C 1s analysis of G11, (g) O 1s analysis of T11, (h) O 1s analysis of A11, and (i) O 1s analysis of G11.

Metallic impurities are often detected in graphene samples synthesized by chemical vapor deposition (CVD) processes in which metallic substrates or catalysts are involved, and these metallic contaminations severely affect the electronic and electrochemical functions of such graphene samples.[36] However, in the present study, possibilities of having metallic impurities in the samples were neglected as no metal catalyst or substrate was involved in the process and a ceramic crucible was used for the pyrolysis under an inert atmosphere.

X-ray Diffraction Studies of Graphene

XRD patterns, as in Figure a, of T11, A11, and G11 samples showed a wide peak in the region of 2θ between 23 and 25°, T11 at 23.9°, A11 at 23.3°, and G11 at 24.9°, to be more specific. Typically, a multilayer graphite gives a sharp peak at 2θ equaling to 26° for the diffraction from the (002) plane, and this peak is expected to be widened as the structure breaks down to nanometers and is also expected to be shifted to lower 2θ as an obvious outcome of broadening of the d spacing. T11 and G11 also showed peaks corresponding to diffractions from (100), (101), and (102) planes at 2θ values of 42, 43, and 49°. A11, however, showed a broad signal at 43°, possibly for the (101) diffraction.

Figure 5

(a) XRD plots for T11, A11, and G11; (b) Raman spectra for T11, A11, and G11.

The (002) diffraction peak was used to calculate the d spacing of the samples with the help of Bragg’s law of X-ray diffraction as stated in eq .θ in the above equation is the diffraction angle; n is the degree of diffraction, which is considered unity; λ is the wavelength of the Cu Kα radiation being 0.154 nm; and d is the d spacing. The d spacing values thus obtained were 0.37, 0.38, and 0.36 nm for T11, A11, and G11, respectively. The thickness or size of the layers was calculated by the Scherrer equation (eq ) using the FWHM values of the (002) diffraction peak. τ is the size of the crystal inversely related to β, the FWHM in radians, and cos of the diffraction angle. K is the shape factor, which was considered to be unity for the calculations.The crystallite sizes as calculated were 13.2 Å for T11, 8.5 for A11, and 13.1 Å for G11. The values of crystallite size divided by the corresponding d spacing values gave the average values for the number of layers in the graphene sheets.[40] The numbers of layers estimated this way were 3–4 for T11, 2–3 for A11, and 3–4 for G11.

The crystallinity in ideally single-layered graphene is different from that of bulk graphite.[37] Graphite forms a hexagonal lattice in the ABA′B′ pattern with four atoms per unit cell noted as A, A′, B, and B′ in Figure a. On the contrary, graphene has a two-dimensional hexagonal lattice structure with only two atoms A and B in the unit cell as shown in Figure b. The preferred orientation in graphite is along the (002) plane, but deviations may be observed along [100], [101], and [102] directions in some graphite samples depending on the birth of the material. Highly oriented pyrolytic graphite (HOPG) mostly has a crystal orientation along the [002] direction, whereas randomly oriented graphite samples also have crystallite growth in other preferred orientations such as in (100), (101), and (102) planes. Similar observations may prevail in few-layer or multilayer graphene, which are not grown epitaxially but turbostratically. Also, free-standing single-layer graphene habitually pacifies the π orbital valences by rotation of the adjacent layers, relative to one another, forming a multilayered configuration. If we consider a bilayer graphene as an example, this configuration can either be Bernal, in which the position of the A atom in one layer exactly superposes with the position of the B atom in the second layer, or be turbostratic, in which there is a definite angle θ between A and B as illustrated in Figure c,d.[38] In the current study, no substrate was provided for epitaxial nucleation as occurred in chemical vapor deposition processes; the growth of graphene in this study was rather turbostratic, yielding randomly oriented crystals. An illustration of the process of graphitization in different orientations during pyrolysis is given in Figure e for better understanding of the phenomenon.

Figure 6

(a) Unit cell in the graphite crystal, (b) unit cell in the graphene crystal, (c) Bernal arrangement in bilayer graphene, (d) turbostractic arrangement in bilayer graphene, and (e) schematic diagram for the synthesis of turbostratic graphene from alginic acid.

The relatively higher intensity of (101) diffraction compared to (002) diffraction in G11 suggests that [101] was the preferred orientation of the very sample at the given pyrolysis conditions, whereas A11 and T11 displayed fairly comparable degrees of crystallization in different orientations. The absence of a sharp (002) diffraction peak leads to the assumption that the pyrolysis did not result in a highly oriented, stacked multilayered structure but rather a few-layer graphene-like structure. Microcrystalline broadening and stacking configuration of the layers are other important phenomena that enlighten the broad and weak XRD peaks observed in the XRD pattern of these few-layer graphene samples. There are two ways in which the single graphite layers may be stacked with each other, hexagonal (ABAB pattern) or rhombohedral (ABCA pattern), as illustrated in Figure . Stacking faults or defects are often generated if the stacking regularity is hindered owing to the different synthetic procedures and the raw materials used for synthesis and results in further XRD peak broadening, noted as stacking fault broadening. If these broadening effects are taken into consideration, FWHM of an XRD peak, β, can be divided into two terms: βc, the microcrystalline broadening; and βp, the stacking fault broadening.[39]

Figure 7

(a) Hexagonal packing of graphite. (b) Rhombohedral packing of graphite.

Raman Spectroscopy of Graphene

The Raman spectra displayed in Figure b of the three samples showed characteristic D, G, and 2D peaks at 1350, 1590, and 2600 cm–1, respectively. The slight blue shift of the G peak could probably be caused by the defect-induced D′ peak, generally positioned at 1620 cm–1 being merged with the G peak.[41] The ID/IG value was 1.1 for both T11 and A11 and 0.88 for G11. The values of ID/IG were relatable with the values reported in the literature for graphene.[42] The number of layers of graphene was also calculated by the formula stated in eq , where ωG is the value of the Raman shift in cm–1 and n is the number of layers in the graphene sample.[43]Taking the value of the Raman shift as 1590 cm–1 for all three samples, the value of n that predicts the number of layers on an average was calculated to be 3, and this value was in good accordance with the findings from XRD analysis.

The 2D band of graphene is significant for understanding the degree of exfoliation in terms of the number of layers.[44] Highly oriented pyrolytic graphite shows a 2D band with a sharp and intense peak at the higher Raman shift merged with a smaller peak at the lower Raman shift. With increasing degree of layer separation, the band is expected to flatten and the transition is said to take place from four layers with a situation in which the 2D band is deconvoluted into three peaks with a slightly higher intensity at a higher Raman shift. In a typical trilayer graphene sample, six peaks at least, and for a bilayer graphene, four Gaussian peaks could be fitted under the 2D band. The phenomenon of splitting of the 2D band into several peaks is a repercussion of the double-resonance Raman process featuring phonon–electron scattering in graphene. A single-layer graphene thus is expected to give a single 2D signal at a slightly lower value of Raman shift.

Each of the 2D bands of T11 and G11 was fitted with six Gaussian peaks with the use of OriginPro 8.5 software typically resembling trilayer graphene, and the 2D band of A11 was fitted with four Gaussian peaks having the characteristic nature of a bilayer graphene (Figure a–c). The inferences drawn from the analysis of Raman spectra were in good accordance with the information obtained from XRD.

Figure 8

Deconvolution of the Raman 2D band of (a) T11, (b) A11, and (c) G11. (d) TGA plots of T11, A11, and G11.

Small D+G combination peaks were visible near 2900 cm–1 in all three samples.[45,46]

Thermogravimetric Analysis (TGA) of Graphene

Thermogravimetric analysis (TGA) of T11, A11, and G11 is illustrated in Figure d. T11 retained 96.8% of its initial weight on heating up to 800 °C under a N2 atmosphere, and the weight loss was not rapid throughout the heating period. An initial rapid weight loss was observed for each of A11 and G11. A11 lost 4.9% of its initial weight at 60 °C temperature and retained 89.5% of the initial weight until the temperature reached 800 °C. G11 initially lost 4.4% of its weight quite rapidly and retained 88.1% weight until the end of the experiment when the temperature was 800 °C. Hence, from the TGA, we assumed that 3.2% impurity was present in T11, while the percentage of nongraphitic impurities was 10.5 in the case of A11 and 11.9 for G11. The rapid weight loss below 100 °C in A11 and G11 compared to the negligible weight loss in T11 was probably because of the negligible hydroxyl content in T11 as compared to the much higher hydroxyl content in the other two samples (Table S3, Supporting Information).

High-Resolution Transmission Electron Microscope (HRTEM) Imaging of Graphene

The structural study of the graphene samples prepared in the method was committed using characterization of the samples by HRTEM. Images of the graphene sheets obtained under HRTEM and the corresponding selected area electron diffraction (SAED) patterns are given in Figure . The sixfold diffraction pattern obtained for T11, A11, and G11 confirmed the crystallinity of the graphene sheets. Although the concept of crystallinity in graphene and reduced graphene oxide (rGO) from XRD might look similar, the SAED pattern of the two is typically different. There exist literature that show the SAED pattern of rGO to be comprising multiple dots forming concentric rings, indicating a polycrystalline nature.[34] Graphene, on the other hand, shows six clear and distinct dots, indicating the sixfold symmetry in its crystalline structure. All of our samples show SAED patterns characteristic of graphene.

Figure 9

(a) HRTEM image of T11, (b) SAED pattern of T11, (c) HRTEM image of A11, (d) SAED pattern of A11, (e) HRTEM image of G11, and (f) SAED pattern of G11.

Crystalline domains were identified in the samples by the presence of graphene fringes under HRTEM as illustrated in Figure . The d spacing of each sample was determined by measuring the distance between the fringes seen in the atomic resolution images, and the average values were 0.37, 0.38, and 0.37 nm for T11, A11, and G11, respectively. The values of d spacing obtained from HRTEM were comparable to those obtained from XRD.

Figure 10

Graphene fringes observed in atomic resolution HRTEM: (a) T11, (b) d spacing profile for T11, (c) A11, (d) d spacing profile for A11, (e) G11, (f) d spacing profile for G11.

Atomic Force Microscope (AFM) Imaging of Graphene

Atomic force microscopy of the samples was carried out by spin-coating diluted dispersions of the samples on silicon wafer followed by drying. The thicknesses of the graphene sheets roughly obtained from the AFM images were 1.3 nm for T11, 1.2 nm for A11, and 1.5 nm for G11, as illustrated in Figure . Lateral sizes of the observed sheets were 849 nm for T11, 927 nm for A11, and 403 nm for G11. The number of layers as estimated from the information from AFM analysis was three on an average for each of the specimens.

Figure 11

AFM height images and profiles for (a) T11, (b) G11, and (c) A11.

The lateral sizes and the thickness values of the samples reported in the study were based on the graphene sheets detected under the AFM. The numerical values of lateral size and thickness were directly obtained from the AFM images as illustrated in Figure . However, the thickness values of the graphene sheets spotted under AFM were comparable to the values theoretically calculated from XRD. AFM phase images of the same graphene sheets are also given in Figure . The average thickness of the graphene samples and consequently the average number of layers have been calculated by various techniques, such as XRD, Raman spectroscopy, and AFM, in our study, and the average number of layers was found to be two to three. This number is generally associated with few-layer graphene according to the definition. The number of layers in reduced graphene oxide (rGO) is generally higher than that in few-layer graphene.[6−8]

Figure 12

AFM phase images for (a) T11, (b) G11, and (c) A11.

After thorough characterization of the three samples by X-ray diffraction (XRD) and Raman spectroscopy with subsequent theoretical analyses, A11 was found to be a bilayer graphene on the basis of average number of layers and the other two contained three to four layers on an average. The sp2 carbon content of A11 lied in between that of T11 and G11 according to the data obtained from X-ray photoelectron spectroscopy. These observations guided the selection of A11 as a representative sample with a lower average number of layers and fairly low sp3 defects for the rest of the study, including electrochemical and magnetic properties as compared to the other two.

The morphology of the rubber-latex-based graphene nanocomposite (SLA11) was studied on a thin film prepared over a silicon wafer by spin-coating the latex-graphene dispersion, which was prepared by the method described in Section , at 600 rpm for 120 s followed by drying at 50 °C for 24 h. The morphology of the film studied under AFM is illustrated in Figures a and 11b.

Figure 13

(a) AFM height image of SLA11, and (b) AFM phase image of SLA11.

The nanocomposite thin film deposited on the silicon wafer comprised two components: a matrix of rubber latex and graphene sheets as the dispersed phase. The AFM height image reveals the dispersion of graphene in the matrix; the edges of the sheets were distinctly visible, which stipulated a fair degree of dissemination of the graphene sheets by the rubber molecules. The AFM micrograph also disclosed that the graphene sheets did not arrange themselves in a linear fashion in the matrix; rather, they were randomly placed in the matrix, which indicated that the restacking of the graphene sheets was prevented by the rubber molecules and the graphene–graphene interaction was possibly subjugated by the formation of a stable three-dimensional rubber–graphene network.

The phase image of rubber latex is supposed to show a single phase, whereas the graphene-latex dispersion, after being spin-coated on silicon wafer, is supposed to form a uniformly thin film of latex matrix in which graphene is dispersed as the filler phase. As observed in Figure b, the thin film that was formed over the silicon wafer clearly comprised two phases, the darker one being the softer rubber phase and the brighter portions being the stiffer phase of dispersed graphene.

Nitrogen Sorption Experiment

Nitrogen physisorption based on the classical theory of Brunauer–Emmett–Teller (BET) has been competent for the determination of surface area of activated carbons and various types of graphenes. BET has also been a standard method for acquiring insights into the pore volume of carbonaceous samples. Surface area and pore volume are two features of crucial importance when functional properties of graphene and graphene-based composite materials are studied. The larger surface area of graphene compared to graphite or other activated carbon materials has been known to be the prime reason behind the outstanding electrical properties exhibited by graphene.

Representative sample A11 was subjected to a nitrogen sorption experiment. The total BET surface area of A11 was found to be 377 m2/g, and the pore volume of the sample was 0.050 cm3/g. The pore size ranged from 2 to 4 nm, and the pore size distribution was found to be bimodal in nature. The nitrogen sorption plot and the pore size distribution curve are displayed in Figure .

Figure 14

(a) Nitrogen sorption loop for A11. (b) Pore size distribution in A11.

Cyclic Voltammetry and Cyclic Charge–Discharge Tests for Determination of Specific Capacitance of Graphene (A11) and Graphene-Latex Thin Films (SLA11)

The formula used for the determination of specific capacitance included the area under the cyclic voltammogram, active mass of the material (m), potential window (ΔV), and the value of scan rate (S) as stated in eq .[47]The value of specific capacitance obtained for A11 was the highest as 315 F/g at 100 mV/s scan rate. The cyclic voltammograms exhibited electrical double-layer capacitor (EDLC)-type behavior as exhibited in Figure a. The latex-based graphene nanocomposite film covering an area of 1 cm2 of a piece of Toray carbon paper was tested as the working electrode using (M) hydrochloric acid as the electrolyte and Pt as the counter electrode with reference to the Ag/AgCl electrode. The value of capacitance obtained for the nanocomposite film was 137 F/g at 10 mV/s scan rate as plotted in Figure b. Cyclic charge–discharge experiments of the samples A11 and SLA11 were carried out at a current density of 1 A/g. The values of specific capacitance (Cs) of the samples at the specific current density (I) were calculated to be 311 F/g for A11 and 131 F/g for SLA11 using eq involving the slope (Δt/ΔV) of the corresponding discharge curves and the active mass (m) of the coated electrode materials. The voltage vs time plots of the charge–discharge experiments are plotted in Figure c.All carbon-based capacitive materials show electrical double-layer capacitor (EDLC)-type arrangement in contact with an electrolyte, described by the conventional Helmholtz model.

Figure 15

Cyclic voltammograms for (a) A11 and (b) SLA11 at 10 mV/s scan rate. (c) Cyclic charge–discharge plots for A11 and SLA11. (d) Magnetic hysteresis curves at room temperature.

The defined pore size dependence of capacitance is only expected in an ideal situation of perfectly ordered micropores, but this dependency is debatable if the pores have different curvatures and have multimodal size distributions.[48] Specific capacitance of activated carbons is greatly affected by the pore volume, pore size, and pore geometry, but the antithesis of this fact has also been suggested in studies reporting that the effect of pore wall curvatures might be negligible for micropores being too small for the electrolyte ions.[49] The relationship between capacitance and the ability of ion hosting in uniform arrays of pores in mesoporous carbon electrodes has been established in several studies.[50] However, in graphene, the high capacitance is mainly a direct consequence of the large surface area as the surface area of an electrode is proportional to the capacitance in an EDLC system. The crumpled and corrugated nature of graphene sheets further aids the specific capacitance hindering ion release.[51] It has been a recent trend in graphene research to tailor the pore size and geometry in graphene or graphene aerogels to achieve better energy storage.[52−55] The graphene sample A11 in the current study, being nanoporous in nature, might only be displaying fair values of capacitance owing to its larger surface area and crumpled sheetlike graphene structure. Mixing of pristine graphene into certain polymers, especially rubber, is still a challenge because of the absence of chemical interaction sites as in graphene oxide or carbon black fillers. Carbon black pores are conventionally believed to be acting as the adsorption sites for rubber molecules during solid-state mixing. Incorporation of graphene has been more successful when rubber latex was used instead of solid rubber, and these latex-based compositions have displayed enhanced electrical properties.[56] In the current study, graphene A11 was dispersed into rubber latex with the aid of SDS surfactant. The resultant latex–graphene composite acts as a self-binding material, and there was no need for Nafion dispersion to be used in the process of electrode preparation. The rubber–latex composite film SLA11, coated as an electrode material, displayed a fairly high value of capacitance, 137 F/g, which is rare in the cases of similar rubber-based flexible materials.

It was noteworthy that the cyclic voltammetry plot of SLA11 was not perfectly square, probably referring to a significant amount of diffusional resistance. The reason for the diffusional resistance toward the ions in SLA11 might have resulted from the intrinsic hydrophobicity of the rubber molecules and their large entangled macromolecular structure hindering the ions from approaching. Nanofillers, in general, have been reported to form secondary interactions with polymer chains within a polymeric matrix, weakening the local kinking of the polymer chains, drifting the kinked or twisted regions apart, allowing the polymer chains to be somewhat aligned.[57,58] Similarly, graphene in the current system straightens the entangled rubber chains to some extent, opening up a much larger surface area compared to any other polymeric system. The formation of a continuous network of graphene inside the rubber latex matrix enables charge transfer throughout the composite film.

Magnetic Hysteresis Properties of Graphene (A11) and Graphene-Latex Thin Films (SLA11)

The ferromagnetic behavior of graphene on the application of a small magnetic field is believed to result from the nonbonding electron orbitals at the zigzag edges of graphene.[59,60] The representative graphene sample from our study, A11, and the graphene-latex thin film SLA11 displayed ferromagnetic-like hysteresis loops at room temperature on the application of a small magnetic field (Figure d). Remnant magnetization MR values for A11 and SLA11 were 0.018 and 0.029 emu/g, respectively. The saturation magnetization value decreased when A11 was incorporated into rubber latex. A saturation in magnetization is conventionally defined as a state that is reached when an externally applied magnetic field fails to further increase the level of magnetization in a material. A classically ferromagnetic material consists of microscopic magnetic domains, which align themselves parallel to an externally applied magnetic field, inducing an enhanced magnetic field. This alignment of the magnetic domains is only possible to a certain extent, beyond which no further magnetization can be induced and the state of magnetization is saturated. This is phenomenally a primary property of any material, and the saturation magnetization values are directly related to a material’s molecular, specifically electronic, structure. When a ferromagnetic material, in the present case, graphene, is conjugated with a nonmagnetic material, such as rubber, a loss in saturation magnetization is resulted due to the subsided amount of magnetic domains available to be aligned parallel to the external magnetic field, in a given mass of material. Though the saturation magnetization value is less in the nanocomposite compared to pristine graphene, magnetism induced in the rubber nanocomposite still carries substantial importance. This ferromagnetic behavior observed in the representative samples enables them to be used as soft magnets at a small magnetic field.

Conclusions

Tannic acid, alginic acid, and green tea as the sources of polyphenols were decent enough as precursors for the synthesis of graphene by pyrolysis owing to the labile oxygen linkages present in their structure susceptible to undergoing graphitization at high temperatures. The process of graphitization is dependent on temperature and time. Studies on structure and morphology of the intermediates discovered that 1100 °C was a suitable temperature for graphitization. As sheetlike features were not noticed at 0 min time at 1100 °C, it could be concluded that graphitization did not start below this temperature. After 30 min time, growth of graphene sheets was observed, which ensured the suitability of the reaction temperature. The precursors used in this work were unconventional, and they were not derived from petroleum sources. These materials successfully converted to good-quality graphene with an average of three layers as supported by the XRD, Raman, and AFM analyses. Typical SAED patterns confirmed the sixfold crystalline symmetry of the synthesized products. Graphene derived from alginic acid displayed a fair value of specific capacitance, 315 F/g, particularly satisfactory for supercapacitor applications. In succeeding steps, the graphene synthesized from alginic acid was used as a functional filler in styrene butadiene rubber matrix and imparted 137 F/g specific capacitance into a graphene-latex thin film. Ferromagnetism displayed by the rubber-based graphene composite allows the possibility of such materials to be used in spintronic applications and even medical uses such as footwear for diabetic foot. This easy synthesis of graphene from sustainable sources following the preparation of solvent-free rubber–latex composites may have enormous and wide-ranging applications from flexible electronics to medical tools.