Journal of Materials Sciences and Applications 2015; 1(3): 130-135 Published online June 20, 2015 (http://www.aascit.org/journal/jmsa)
Synthesis and Characterization of Graphene by Raman Spectroscopy Hilal S. Wahab1, *, Salam H. Ali2, Adi M. Abdul Hussein2 1 2
Department of Chemistry, College of Science, Al-Nahrain University, Al-Jadriya, Baghdad, Iraq School of Applied Sciences, University of Technology, Baghdad, Iraq
Email address
[email protected] (H. S. Wahab),
[email protected] (H. S. Wahab)
Citation Hilal S. Wahab, Salam H. Ali, Adi M. Abdul Hussein. Synthesis and Characterization of Graphene by Raman Spectroscopy. Journal of Materials Sciences and Applications. Vol. 1, No. 3, 2015, pp. 130-135.
Keywords Graphene, Raman, Graphene Oxide
Received: May 1, 2015 Revised: May 27, 2015 Accepted: May 28, 2015
Abstract In the present study, we focused on the preparation of graphene on a bulk scale from graphite employing affordable and easy chemical and electrochemical techniques. The electrochemical methods produce high quality graphene while the chemical route produces defective graphene. The quality and the characterization of the produced graphene have been examined exploiting Raman spectroscopy. We have reviewed and critically assessed the origin and the meaning of the D and G peaks in the Raman spectra of graphene containing different type of defects.
1. Introduction Graphite is a layered material which contained a number of two dimensional graphene crystals weakly coupled together. Graphene, the world’s thinnest sheet – only a single atom thick – has a great potential to provide a new way in energy, computing and medical research [1]. It is the flat monolayer of carbon atoms in sp2 hybridization. The novel structure of graphene is the center stage for all the calculations on graphite, carbon nanotubes and fullerenes. The first graphene sheets were obtained by extracting monolayer sheets from the three-dimensional graphite using a technique called micromechanical cleavage in 2004 [2]. Chemical preparation processes of graphene sheets use graphite as starting material and delaminate it by various means, namely by chemical fictionalization, oxidation or by intercalation. In order to exhibit a good representation of today’s scientific landscape concerning chemical methods for graphene production, Gengler et al. [3] have divided the subject into two subcategories, the preponderant graphene oxide (GO) production as a first one and all the other methods (intercalation with N-methylpyrrolidone (NMP), functionalized surfactants and perfluorinated aromatic molecules) as a second one. In this work, graphite oxide was synthesized from natural graphite by the electrochemical exfoliation process in which the deionized water was used as solvent (it will be elucidated in detail in the experimental part). Raman spectroscopy has been extensively used to characterize graphene spectrum due to its importance in the field of carbon research and also has historically played an important role in the structural characterization of graphitic materials. Raman spectroscopy is a spectroscopic technique based on inelastic scattering of monochromatic light, usually from a laser source. Inelastic scattering means that the frequency of photons in monochromatic light changes upon interaction with a sample. Photons of the laser light are absorbed by the sample and then re-emitted. The frequency of a part of the re-emitted photons is shifted up or down with respect to the original monochromatic frequency,
Journal of Materials Sciences and Applications 2015; 1(3): 130-135
which is called the Raman effect. This shift provides information about vibrational, rotational and other low frequency transitions in molecules [4, 5]. The wavelength / intensity information is then converted to frequency / intensity which are analyzed to obtain the vibration, rotation and other low-frequency modes specific for the chemical bonds. The Raman spectra of all graphene display a broad G-band around 1570-1580 cm-1 corresponding to the first-order scattering of the E2g mode [6, 7]. In addition, the Raman spectrum of graphene also exhibits the prominent D-band at 1350-1360 cm-1 resulting from the structural defects produced by the bonding of hydroxyl and epoxide groups on the carbon basal plane and the possible decrease in the average size of the sp2 domains, possibly owing to considerable reduction of the graphene oxide. The Raman spectra of all graphene films also contain both G and D bands; however, an increased D/G peak area ratio can be observed when graphene obtained from electrical or thermal reduction. This intensity ratio of the D and G peaks has been used as a metric of disorder in graphene, such as arising from ripples, edges, charged impurities, defect in the graphene structure, a reduction in the graphitic crystal size, presence of domain boundaries, and others. For edges, the intensity of the D peak depends on the edge structure; it is weak at the zigzag edge and strong at the armchair edge [8, 9]. Presence of D and G bands confirms the formation of graphene with lesser defects and crystalline structure. The two most intense features of the D and G peaks as discussed above can be seen in Fig. 1.
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Where (2) where C stands for the pre-factor at 514.5 nm and equals to 4.4 nm, and La is the crystallite size. The intensity of the D and G peaks were estimated by fitting the spectra. Raman spectra of graphene reduced with various reduction conditions reflecting the significant structural changes occurring for each stage of the electro and thermal processing are shown in Fig. 2 [12].
Fig 2. Raman spectra of samples at different stages of treatment (reprinted from ref. 12 ).
The goal of this work is to study the appropriate reduction conditions to obtain the graphene films having the highest electro catalytic feature.
2. Experimental Part
Fig 1. The 1575 cm –1peak (called the “G” peak, after crystalline graphite) is the only Raman active mode of the infinite lattice. The other peak (the “D” peak from disordered graphite) is caused by breakdown of the solid-state Raman selection rules. (reprinted from ref. 10).
The crystallite size was calculated using the Tuinstra and Koening relation [10,11]: (1)
The experimental part of this research includes the preparation of the raw material, GO, which is used for preparing graphene. In this study we used graphite (as electrodes) and the electrolyte baths (electrochemical cell) composed of sulfuric acid and water for exfoliation process. In this work, we performed the electrochemical exfoliation of graphite in sulfuric acid to obtain high quality and large-area graphene thin sheets. Figure 3a illustrates the experimental setup, where graphite foil was employed as an electrode and source of graphene for electrochemical exfoliation. A copper, Cu, foil was chosen as a grounded electrode with a spacing of 5 cm between the electrodes. The electrochemical exfoliation process was carried out by applying DC bias on graphite electrode (from -10 to 10 V). Note that the 10 V activated the exfoliation and oxidized the graphene sheets. The produced functional groups are reduced when the bias is switched to (-10 V). The low bias of 2.5 V was applied for 1 min, and then high voltages were applied (alternating between 10 and -10 V). The initial low bias helps to wet the sample and likely causes gentle intercalation of SO4-2 ions to the grain boundary
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Hilal S. Wahab et al.: Synthesis and Characterization of Graphene by Raman Spectroscopy
of graphite. Before applying a high bias of 10 V, the graphite reserved its status as a single piece. Once the high bias was applied, the graphite was quickly dissociated into small pieces and spread in the solution surface, as shown in Figure 3b. These exfoliated graphene sheets
and related products were collected through filtration and re-dispersed in solution. Figure 3b and c shows the x dispersed graphene sheets in solution and exfoliation of graphite surface, respectively.
Fig 3. a) Experimental setup; b) Dispersed graphene sheets in solution; c) Exfoliation of graphite surface.
We observed that the electrochemical exfoliation of graphene using H2SO4 solution was very efficient as the whole exfoliation process could be finished in a few minutes. To reduce the oxidation of graphene by H2SO4, H2O2 or KOH was added into the H2SO4 solution to alter the acidity of the electrolytic solution (2.5 g of 98% H2SO4 in 100 ml of deionized water (DI) mixed with 1.5g of 30% H2O2 or KOH solution to turn the pH value of the solution to 4.5-5.0). In this work and for common sonication (low energy sonication) an ultrasonic cleaner has been utilized to achieving proper mixing of the graphite flakes with liquid and afterwards it was allowed to settle down and decant the liquid. In order to prepare the graphene sheet suspension, the
exfoliated graphene sheets were filtered and washed with DI. After drying, the sheets were dispersed in Dimethyl Sulfoxide (DMF) solvent via gentle water-bath sonication for 5 min. To remove the unwanted large graphite particles produced in the exfoliation process, the solution was left for 2-3 hours to settle down and eventually used for the film preparation. All the electrochemical exfoliation experiments were carried out at room temperature (25 Co). Figure 4a and b exhibits the solutions of reduced graphene oxide as a resulting process after 2 hours and one weak of preparation, respectively. Figure 5 shows the electrical and thermal reductions of graphene oxide films.
Fig 4. Different solutions of reduced graphene oxide after a) 2 hours and b) one weak.
Fig 5. a) graphene oxide reference sample deposited on Cu (electrical ); b) reduced graphene oxide reference sample on Cu (thermal).
Journal of Materials Sciences and Applications 2015; 1(3): 130-135
To investigate the electrochemical behavior of graphene films reduced by various conditions, we have carried out large scale production of single graphene sheets, controlling individual features for reproducibility and homogeneous layer deposition which are the challenges of the synthesis process.
3. Results and Discussion In this work, graphene films were prepared by electrical and thermal reductions of graphene oxide obtained from oxidation of graphite. Figures 6-8 and Tables 1-3 show Raman spectra of graphene resulted from the thermal, electrothermal and electrical reduction routes of GO films, respectively. In Raman spectroscopy, D band indicates the extent of defects whereas G band indicates the graphitic nature. If the D peak intensity is higher, the sample is having more defects in
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graphite network. These defects will be imparted to the edges of the sheets. If the G band is prominent, sample is said to be crystalline. D and G bands are normally located at 1350 cm-1 and 1574 cm-1, respectively. Existence of D and G bands confirms the formation of graphene with lesser defects and crystalline structure. In addition, an increased intensity of the D line around 1355 cm-1 indicated the considerable reduction in size of the in-plane sp2 domains owing to oxidation and sonication processes, and the formation of graphene nanosheets having highly oriented crystal structure. In the Raman spectrum of graphene based nanosheets obtained after chemical reduction of expanded GO, the intensity of the D line around 1356 cm-1 decreased considerably as a result of an increase of the graphitic domain sizes and an increase of the thickness of graphitic structure after thermal treatment.
Table 1. Spectroscopic function's values for the reduced GO by following the thermal reduction route as illustrated in Fig. 6. sample
D
G
2D
D+G
cm-1
1335.23
1568.77
2672.74
2926.69
Intensity
261.76
376.22
236.34
244.04
I(G)/I(2D)
0.6957
6.32
1.59
Table 2. Spectroscopic function's values for the reduced GO by following the electrical and thermal reduction routes as illustrated in Fig. 7. sample
D
G
2D
D+G
cm-1
1326.913
1571.036
2673.746
2918.877
Intensity
566.555
480.940
108.101
45.484
I(G)/I(2D)
1.178
3.735
4.440
Table 3. Spectroscopic function's values for the reduced GO by following the electrical reduction route as illustrated in Fig. 8. sample
D
G
2D
D+G
cm-1
1357.145
1576.57
2556.60
2935.50
Intensity
258.51
369.82
206.22
169.730
I(G)/I(2D)
0.69901
6.294
Fig 6. Raman spectrum of graphene (reduced GO by following the thermal reduction route of graphene oxide films.
1.79
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Hilal S. Wahab et al.: Synthesis and Characterization of Graphene by Raman Spectroscopy
Fig 7. Raman spectrum of graphene (reduced GO by following the electric and thermal reduction routes of graphene oxide films) display a disordered D peak at 1326.913 cm-1 and a G peak at 1571.036 cm-1, corresponding to the first-order scattering of the E2g mode. (The monolayered specimen is characterized by a single sharp D peak whereas multilayered sample characterized by the two component D peak).
Fig 8. Raman spectrum of graphene ( reduced GO by following the electrical reduction route of graphene oxide films).
For the comparison of the structural changes after the electrical and thermal treatments, another critical factor is stemmed which is the amount of disorder. Based on the above results, we report here that the G peak is due to the relative motion of sp2 carbon atoms, while the D peak is linked to breathing modes of rings. This mode is forbidden in perfect graphite and only becomes active in the presence of disorder. The D mode is dispersive; it varies with photon excitation energy, even when the G peak is not dispersive. Moreover, as the structure changes from graphite to nanocrystalline graphite, the ratio between the intensities of D and G lines, I(D)/I(G), rationalize reciprocally with the size of the crystalline grains or interdefect distance [11]. I(D)/I(G)
values for reduced GO by thermal, reduced GO by electrical and reduced GO by electric and thermal were calculated as 0.6957, 0.6990, and 1.7802, respectively. The highest I(D)/I(G) ratio of reduced GO sample was evidence for the structure with highest order. An in-plane crystallite size La was calculated using the expression La (nm) = 4.4 (IG/ID). The film had a crystalline size of 6.1 nm; this size reduced afterwards to ~ 4.1 nm. When the number of layers is smaller than five, the 2D peak becomes more intense than G peak [12,13]. The increase in the
Journal of Materials Sciences and Applications 2015; 1(3): 130-135
ratio between the intensity of G and 2D peak, I(G)/I(2D), indicated an increase in the number of graphene layers. I(G)/I(D0) values of reduced GO by thermal, reduced GO by electrical and reduced GO by electric and thermal were calculated as 1.59, 4.44, and 1.79, respectively. The highest I(G)/I(2D) ratio of reduced GO by electrical sample demonstrated the largest number of graphene layers.
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4. Conclusions The electrochemical methods produce high quality graphene while the chemical route produces defective graphene, with the defects that affecting the properties of graphene. The Raman spectra characteristics of the three samples of electrical, thermal and electrothermal paths are similar, consisting of the D and G modes. The D band is induced in the first order scattering process by the presence of finite size effects, vacancies and grain boundaries, all of which lower the symmetry of the quasi infinite lattice. The G peak is due to the doubly degenerate zone center E2g mode and also ascribed to the in-plane bond stretching motion of the pairs of carbon atoms. The averaged measure of all possible edge structures present in a bulk nanographite sample reveals that the D band intensity depends on the atomic structure at the edge, and it is actually absent in graphite edges when the atomic structure exhibits the zigzag arrangement. The results have also presented that the thermal treatment process is very effective method among any other solution routes to produce high quality GO reduced graphene.
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