Prashant Tripathia,
Ch. Ravi Prakash Patela,
Abhishek Dixita,
Avanish Pratap Singhb,
Pawan Kumarb,
M. A. Shaza,
Ritu Srivastavab,
Govind Guptab,
S. K. Dhawanb,
Bipin Kumar Guptab and
O. N. Srivastava*a
aNanoscience Centre, Department of Physics (Centre of Advanced Studies), Banaras Hindu University, Varanasi-221005, India
bCSIR-National Physical Laboratory, New Delhi-110 012, India. E-mail: heponsphy@gmail.com
First published on 9th February 2015
Herein, we demonstrate a facile one pot synthesis of graphene nanosheets by electrochemical exfoliation of graphite. In the present study, we report a significant increase in the yield of graphene by electrolyte heating assisted electrochemical exfoliation method. The obtained results of heating assisted electrochemically exfoliated graphene (utilizing H2SO4 + KOH + DW) synthesis clearly exhibit that the yield increases ∼4.5 times i.e. from ∼17% (room temperature) to ∼77% (at 80 °C). A plausible mechanism for the enhanced yield based on lattice expansion and vibration of intercalated ions has been put forward and discussed in details. The quality of graphene was examined by Raman, XPS, FTIR, AFM, SEM, TEM/HRTEM and TGA techniques. The Raman as well as morphogenesis results confirm the quality of the graphene nanosheets. We have used this graphene as electromagnetic interference shielding material where a comparatively large quantity of graphene is required. This graphene exhibits enhanced shielding effectiveness (46 dB at 1 mm thickness of stacked graphene sheets in frequency region 12.4 to 18 GHz) as compared to conventional electromagnetic interference shielding materials, which is greater than the recommended limit (∼30 dB) for techno-commercial applications. Thus the present work is suggestive for future studies on enhancement of yield of high quality graphene by proposed method and the use of synthesized graphene in electromagnetic interference shielding and other possible applications.
At present, graphene is the main hub for scientific and industrial community due to their potential applications in many areas, including EMI shielding. Even, graphene composites are much superior to the well-established CNTs composites for EMI shielding. This is because graphene based composites have better thermal stability, more stiff, low cost (compared to CNTs) and have capability of forming a thin film, paper or coating for EMI shielding and electrostatic charge dissipation (ESD) properties. The ultra-high thermal conductivity and lower density (four times as compared to copper) make graphene preferable for packing microelectronic device in the form of composite thin film, paper or coating. Our previous efforts based on graphene encourage us to search more and more new kind of composites with graphene.1,14 The present investigation is one of the innovative efforts to synthesize graphene on the large scale for EMI shielding, which is highly desirable for practical applications. Most of the synthesis methods have limited their scope due to lack of high yield. At present, a variety of techniques for the synthesis of graphene have been developed. These can be broadly categorized as mechanical, physical and chemical methods. Out of these, chemical methods are best suited for producing graphene of good quality with high yield and low production cost.16 Among all methods, the production of graphene through electrochemical exfoliation of graphite seems to score over the thermal exfoliation of graphite oxide. The main drawback of thermal exfoliation invariably leads to creation of graphene containing several oxygen functionalities. Attempts to remove oxygen functionalities have been made through chemical treatments e.g. processing through hydrazine, NaBH4 etc.18,19 However, this is a tedious job and it involves toxic process as well as introduces impurities and defects in the resulting graphene.17,18 Production of graphene sheets (GS) directly by electrochemical exfoliation in ionic or acidic electrolytes bypasses the above said difficulties. It has been found that electrochemical exfoliation may lead to optimum alternative for large area, good quality graphene with high yield which is required for present application.
Herein, we report the large scale synthesis of graphene through electrochemical exfoliation method and explore the detailed studies about the method and their effect of electrolyte heating on the yield as well as quality of graphene. Although many reports has been published on the exfoliation of graphene nanosheets such as ball milling,19 chemical exfoliation20 and electrochemical synthesis.21 We had earlier noticed that electrolyte heating significantly affects the electrochemical process.22 Addressing on the above problem, we attempted to investigate the effect of increasing temperature of electrolyte on the yield and quality of graphene. It has been found that the electrolyte heating does indeed affect the yield and quality of graphene. For example as against yield of ∼17% for graphite/H2SO4-diluted with KOH/Pt, with electrolyte at room temperature, the corresponding yield for heating assisted electrolyte at 80 °C is ∼77%. To the best of our knowledge, effect of electrolyte heating on graphene higher yield has not been studied till date. Also graphene yield of ∼77% is possibly the highest yield reported so far for the synthesis through electrochemical exfoliation. Therefore, the present work may be one step ahead towards the synthesis of graphene with high yield which is highly required for such applications where large quantities of graphene required for e.g. EMI shielding application. Furthermore, the quality of as-synthesized graphene in present investigation has been examined by several techniques such as Raman, XPS, FTIR, AFM, SEM, TEM/HRTEM and TGA. Moreover, the shielding effectiveness strongly depends on the thickness of the sheets. The tunability of EMI shielding through thickness variation of the graphene has been evaluated for desired value of shielding effectiveness.
In order to find out the effect of heating the electrolyte on the yield of graphene, we heated the electrolyte on a hot plate. After the electrolyte has achieved steady temperature, the voltage was impressed between the electrodes. When voltage of 10 V was applied to the graphite electrode, the graphite foil began to dissociate into the thin sheets and spread into the electrolyte solution. The sheets obtained after exfoliation of graphite were washed repeatedly with distilled water by vacuum filtration. The sheets were then dried and suspended in water and subjected to sonication for 20 min, finally forming a black suspension. After that, the suspension was centrifuged at 4000 rpm for 30 min.19 The supernatant after centrifugation was consisted of few layers of graphene and the precipitated samples were relatively thick graphite flakes. The yield of graphene sheets was calculated by estimating the fractions of graphene sheets and graphite were based on the mass measurement of the dispersed sheets in supernatant (graphene) and precipitated part of solid at bottom (graphite) after centrifugation of exploited solution which is well established and also reported in literature.23 For the better reproducibility, the yield of graphene was estimated after collection of 20 to 25 electrochemical statistical runs under identical conditions. The electrolyte was heated upto three different temperatures namely 40 °C, 60 °C and 80 °C. Since the electrolyte contains water so temperature beyond 100 °C is not feasible. The yield of graphene was obtained by using different electrolytes, which is shown in Table 1. The yield of graphene nanosheets have increased upto ∼13% at 80 °C when 0.2 M H2SO4 as an electrolyte as compared to the yield of ∼7% at room temperature. The graphene yield for electrolyte with H2SO4 diluted with DW and KOH (composition 2.4 g of H2SO4 and 11 ml of 30% KOH in 100 ml DW) was ∼77% at 80 °C as compared to ∼17% at room temperature. It may be pointed out that this yield is very high. Fig. S1 (see in ESI†) shows the optical images of the resulting thin sheets after exfoliation at RT and 80 °C. The electrolyte heating, therefore, provides a convenient way for increasing the yield of graphene. To examine the generality of the electrolyte heating effect, two other electrolytes 0.25 M H3PO4 (Qualigens, 88%) diluted with 11 ml of KOH (30%) and 0.25 M HClO4 (Qualigens, 60%) diluted with 11 ml NaOH {Fisher Scientific} (30%) were taken. The yield of graphene at room temperature and 40 °C, 60 °C, 80 °C were evaluated for all the three electrolytes. For H3PO4 (KOH) and HClO4 (NaOH) the increase in the yield was from ∼12% (RT) to ∼35% (80 °C) and from ∼13% (RT) to ∼43% (80 °C) respectively.
Electrolyte | Percentage yield of graphene (wt%) at different temperatures | |||
---|---|---|---|---|
RT (300 K) | 40 °C (313 K) | 60 °C (333 K) | 80 °C (353 K) | |
2.0 M H2SO4 | ∼6 | ∼6.30 | ∼7 | ∼8 |
0.2 M H2SO4 | ∼7 | ∼8 | ∼12 | ∼13 |
H2SO4 + KOH + DW | ∼17 | ∼28 | ∼44 | ∼77 |
H2PO4 + KOH + DW | ∼12 | ∼13 | ∼19 | ∼35 |
HClO4 + NaOH + DW | ∼13 | ∼17 | ∼24 | ∼43 |
The different extents of increase in yield of graphene for different acidic electrolytes are presumably due to different sizes of the intercalating ions and also the details of the nature of interaction of intercalating ions with graphitic layers. The schematic representation of electrochemical exfoliation of graphene as well as its proposed application (to be described in next section) is shown in Fig. 1.
Fig. 2 exhibits a representative Raman spectrum (excited by 514 nm) for the exfoliated graphitic thin sheets. For comparison a typical Raman spectra from starting graphite material is superimposed. A strong G-band at 1575 cm−1 can be easily seen in Fig. 2. After exfoliation the graphitic sheets shows a strong G-band at 1584 cm−1. However, a D-band at 1351 cm−1 also appears, the D-band is representative of disorder. This is absent in the spectra of starting graphite. The ID/IG ratio in exfoliated graphitic sheets is 0.33. This is suggestive of the partial oxidation on exfoliation of graphene.24,25 The presence of 2D peak around 2698 cm−1 is clear indication that these thin graphitic sheets are graphene. The I2D/IG ratio in exfoliated graphitic sheets is 0.39. The obtained Raman result clearly demonstrates the quality of graphene nanosheets.
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Fig. 2 Raman spectra of starting material (graphite) and the product after electrochemical exfoliation. |
Futhermore, for elemental composition study of exfoliated graphene, XPS had been performed. The survey scan of exfoliated graphene demonstrated in Fig. 3(a). The XPS core level spectra of C1s and O1s region for exfoliated graphene are shown in Fig. 3(b) and (c). Fig. 3(d) reveals the deconvoluted spectra of C1s where the presence of C–C, CC and C–O which confirms the quality of exfoliated graphene, can be clearly seen.
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Fig. 3 (a) XPS survey scan of exfoliated graphene (b) and (c) core-level spectra of C1s and O1s, respectively and (d) deconvoluted XPS spectrum of C1s core level of exfoliated graphene. |
Fig. 4(a) and (b) exhibits 2D and 3D AFM images of the exfoliated graphene sheets. The sample for AFM was prepared by drop casting solution of graphene dispersed in DMF on SiO2 substrate. The high resolution 2D and 3D AFM images of the exfoliated graphene sheets are shown in Fig. 4(c) and (d). The smallest thickness of the graphene sheets are ∼2.4 nm as shown in Fig. S2 (see in ESI†). Some oxygen groups are expected to be present at the graphene surface. The thickness of the exfoliated graphene layers will be larger than the ideal thickness of 0.65 nm on SiO2 for monolayer,26 here it is ∼0.80 nm. Thus the thickness of ∼2.4 nm can be taken to represent three layers of graphene. The statistics histogram for the thickness demonstrated in Fig. S3 (see ESI†). The electrochemically exfoliated graphene layers in the present case thus can be taken to correspond to three/few layers of graphene.
The surface morphology and microstructure analysis of exfoliated graphene have been examined by using SEM and TEM techniques. Fig. 5(a) represents the cross sectional SEM image of stacked block of graphene sheets which is further used for EMI shielding measurements. In order to ensure the quality and surface morphology of the graphene, we have also performed SEM microscopy of as-synthesized electrochemically exfoliated graphene sheets as shown in Fig. 5(b). The SEM micrograph brings out the morphology of the exfoliated graphene sheets. It can be easily seen that the thin graphene regions together with ruptures/cracks produced due to electrochemical exfoliation process. Fig. 5(c) exhibits a typical TEM micrograph of exfoliated graphene supported on a holey carbon grid. The inset of Fig. 5(c) exhibits SAED pattern of exfoliated graphene. The six fold diffraction pattern bringing out the graphitic nature of the exfoliated thin sheets. To identify the number of graphene layers, efforts were made to take HRTEM images of exfoliated graphene. The Fig. 5(d)–(f) reveals the HRTEM images of exfoliated graphene. Fig. 5(d) demonstrated that the estimated d-spacing of exfoliated graphene is ∼0.36 nm which is comparable to graphene. The HRTEM images {Fig. 5(d)–(f)} reveals that the layer number of these exfoliated graphene ranges from two to four. Thus the electrochemical exfoliation process adopted in the present investigation produces few layers of graphene.
Fig.6(a) exhibits the room temperature I–V characteristics of graphene block (thickness ∼0.25 mm). The calculated surface resistance of graphene block (thickness ∼0.25 mm) is ∼2.14 kΩ m−2. To examine the thermal stability of graphene block, the thermogravimetric analysis (TGA) has been performed. The TGA curve of graphene block is illustrated in Fig. 6(b). The TGA curve demonstrates that graphene block is thermally stable in nitrogen ambient until ∼635 °C, above which a continuous weight loss occurs. The weight loss at high temperature of graphene block is due the oxidation of the carbon atoms into CO2. The FTIR spectrum of graphene block is shown in Fig. S4 (see ESI†). The FTIR spectrum of graphene block exhibits the presence of C–O–C, C–H, CC and C–O–H.
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Fig. 6 (a) I–V characteristic of graphene block and inset shows the optical image of graphene block. (b) The TGA curve of exfoliated graphene. |
As regards the effect of electrolyte heating on the yield of graphene is related, the following two effects are relevant-
(1) The increase of graphitic interlayer spacing as a result of electrolyte heating.
(2) The effect of thermal vibrations of the intercalating ions due to electrolyte heating.
The graphite electrode of the electrochemical cell is immersed in the electrolyte. Thus when the electrolyte is heated (up to 80 °C) and steady state is achieved, the graphite will undergo thermal expansion. The expansion in the direction perpendicular to the graphite basal layers (i.e. along c lattice direction) will be relevant. This is so since the graphene is produced due to intercalation of ions of the electrolyte in the successive graphitic layers. It is known that at room temperature the lattice parameters of graphite are a = 2.456 Å, and c = 6.693 Å. The ‘a’ parameter known to exhibit slight decrease up to about 400 °C, on the other hand thermal expansion along the c-direction is large.27 The thermal expansion coefficient at 50 °C is >0.96 × 10−6. The expression for the expansion of interlayer spacing has been found to be d = 3.3525 + 90.54 × 10−6T + (6.33 × 10−9)T2 = 3.3525 + 90.54 × 10−6 Å.28 Thus for 80 °C, the interlayer spacing will increase from 3.3525 (∼3.35 Å) to 3.3595 (∼3.36 Å), thus the change in interlayer spacing is small. However, it should be noted that the van der Waals potential will vary approx as 1/d6. This will facilitate the intercalation of the ions originating from the electrolyte. Thus, the electrolyte ion SO42− (4.60 Å) from H2SO4 will find it easier to enter in expanded graphite with larger interlayer spacing (3.36 Å). Efficient intercalation of ions resulting from the above effects will lead to efficient electrochemical exfoliation and hence higher yield of graphene.
The second effect as outlined in the above is in regards to thermal vibration of the ions coming from heating the electrolyte. The thermal vibration which will be isotropic will increase as a result of increase of the temperature of the electrolyte. This enhancement of thermal vibration would get somewhat damped once the ion gets intercalated. The vibration of the ion perpendicular to basal graphitic planes will produce a kind of vertical jerk to successive graphitic layers as they move under the applied electric field. This will eventually facilitate exfoliation efficiency and thus increases the yield of graphene. It may be pointed out that the graphitic layers are bonded by weak van der Waals bond. The interlayer bonding energy of graphite is ∼31 ± 2 meV.29 The thermal energy at 80 °C (the maximum temperature up to which the electrolyte has been heated) corresponds to ∼30 meV. These estimates suggest qualitatively that the increase in thermal energy of ions coming from heating the electrolyte is expected to weaken/break the van der Waals bond between graphitic layers. This will apparently facilitate electrochemical exfoliation resulting in increase of graphene yield. Based on the present experiments it is difficult to decide which of the two effects, the thermal expansion of the graphite electrode or increased vibration of intercalating ions, will be more effective in increasing the yield of the graphene. Possibly both effects may collectively lead to increase in the yield of graphene.
The EMI Shielding Effectiveness (SE) of a material is defined as the ratio of transmitted power to the incident power and is given by
![]() | (1) |
In two port network, S-parameters S11 (S22), S12 (S21) represents the reflection and the transmission coefficients given as
![]() | (2) |
![]() | (3) |
And absorption coefficient (A) = 1 − R − T.
Here, it is noted that the absorption coefficient is given with respect to the power of the incident electromagnetic wave. If the effect of multiple reflections between both interfaces of the material is negligible, then the relative intensity of the effectively incident electromagnetic wave inside the material after reflection is based on the quantity (1 − R). Therefore, the effective absorbance (Aeff) can be described as Aeff = (1 − R − T)/(1 − R) with respect to the power of the effective incident electromagnetic wave inside the shielding material. It is convenient to express the reflectance and effective absorbance in the form of −10log(1 − R) and −10
log(1 − Aeff) in decibel (dB), respectively, which give SER and SEA as
SER = −10![]() | (4) |
and
![]() | (5) |
For the material, the skin depth (δ) is the distance up to which the intensity of the electromagnetic wave decreases to 1/e of its original strength. The skin depth is related to angular frequency, relative permeability & total conductivity σT = σdc + σac. According to electromagnetic theory, for electrically thick samples (t > δ), EMI SE for the plane electromagnetic wave is given30 as
SE (dB) = SER (dB) + SEA (dB) | (6) |
and dependence of SE on dielectric properties & magnetic properties can be expressed as
SER (dB) ≈ 10![]() | (7) |
![]() | (8) |
For as-synthesized stacked graphene sheets, there are three main reasons for the enhanced microwave attenuation performances of the graphene sheets. First, as mentioned above, the extremely thin and high polarity of the stacked graphene sheets may offer more opportunities to induce polarization by EM wave, which improves dielectric losses.1 Second, the stacking between graphene sheets increases the conductive paths which leads to enhanced microwave attenuation by converting microwave energy into heat.26 Third, extremely thin, flexible and corrugated natures of the graphene sheets increase the propagation paths of the EM wave. The EM wave scattered by corrugated graphene layers, has sufficiently improved attenuation loss with multiple internal reflection loss. When the microwave travels within stacked graphene sheets, the motion of the charge carriers formed oscillatory current31 which would consume much EM energy. Thus, the increased scattering coupled with higher conductivity is responsible for the enhanced SE in stacked graphene sheets. Conducting nature of the graphene is beneficial for energy attenuation, while the increasing conductivity would increase the energy conversion effectiveness. Therefore, the results shown in this work indicate the fundamental principles for achieving high value of shielding effectiveness for graphene. According to the reported results in the literature, the defects in graphene may construct localized states near to the Fermi level to increase the radiation attenuation.31,32 The microstructure investigations show that the electrochemically synthesized graphene contains clustered defects and residual bonds arising during chemical process. There may oxygenic functional groups (as suggested by FTIR result, ESI Fig. S4†) on the graphene, which might increase the EM wave attenuation performance. Meanwhile, because of the thinnest thickness and high electron mobility; graphene has higher conductivity, which induces strong polarization and loss conductance towards the EM wave. While high gravimetric mass of the graphene makes main contribution to get higher conductivity and higher shielding values simultaneously. Thus, the productions approach as well as results of EMI shielding performance of high quality graphene possesses its legitimate applications in EMI shielding.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra17230b |
This journal is © The Royal Society of Chemistry 2015 |