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graphene nanoplatelets
Manufacturing Letters 21 (2019) 66–69
Contents lists available at ScienceDirect
Manufacturing Letters journal homepage: www.elsevier.com/locate/mfglet
Machining induced transformation of graphite flakes to graphite/graphene nanoplatelets Wazeem Nishad, Sathyan Subbiah ⇑ Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 18 December 2018 Received in revised form 23 June 2019 Accepted 11 August 2019 Available online 12 August 2019 Keywords: Few-layer graphene Tool-chip interface Solid lubricant additives, Aqueous graphite lubricants Graphene exfoliation
a b s t r a c t A mechanical exfoliation technique to produce exfoliated graphite-graphene nanoplatelet dispersion as a byproduct of aqueous graphite-based lubricant-assisted metal cutting is presented here. Low cutting speed oscillatory orthogonal metal cutting process has been carried out completely immersed in the lubricant containing graphite flakes. Reduction in the thickness of the graphite flakes from around 30 mm to graphite-graphene nanoplatelets with a size range of 1–15 nm is observed after machining. Characterization of the particles depicts low structural order and induced defects. Optimizing the parameters can tune this machining-induced exfoliation technique to produce high-quality defect and disorder free few-layer graphene sheets. Ó 2019 Society of Manufacturing Engineers (SME). Published by Elsevier Ltd. All rights reserved.
1. Introduction Graphene, the first two-dimensional material to be isolated, has been found in various promising applications due to its interesting electrical and mechanical properties [1–3]. The applicability of these properties largely depends on the method of its synthesis [4]. These methods can be classified broadly into epitaxial growth, colloidal suspension, unconventional methods, and exfoliation (thermal, chemical and mechanical) [1,5–8]. Exfoliation, a topdown approach of synthesis, involves breaking the weak forces of attraction between the layers using external energy like electrical, chemical, thermal or mechanical [9]. Such exfoliations can create few-layer graphene – FLG (1–5 layers), multilayer graphene – MLG (5–10 layers), graphene nanosheets (less than 100 nm lateral dimension), graphene micro sheets (100 nm to 100 mm lateral dimension) and/or graphite nanoplatelets (up to 100 nm thick) [10]. Single layer graphene does not produce a bandgap, while few-layer graphene can be tuned to open bandgap and enhance its applicability in nano-electronics [11]. Few-layer and multilayer graphene nanosheets and graphene-like amorphous carbon nanosheets have also been used as reinforcements in composites due to its improvements in the mechanical properties [12]. Mechanical exfoliation involves mechanical separation of the layers in bulk graphite by peeling off, cutting and/or shearing using ⇑ Corresponding author. E-mail address: [email protected] (S. Subbiah).
a various method [13–16]. The natural lubricating action of the graphite flakes between two solid mechanically rubbing surfaces may also cause exfoliation. Graphite flakes/nano-platelets have been explored as a lubricant in machining [17,18]. The shearing process of machining [19] involves intense high-stress contact rubbing of the chip with the rake face of the cutting tool. If the graphite flakes can be introduced at this tool-chip interface, then the consequent rubbing can shear the flakes into few-layer graphene (FLG) (Fig. 1). This can be expected since the shear stress generated at the tool-chip interface during ductile-metal cutting, estimated between 150 and 300 MPa [19], is in the range of interlayer shear stress required to shear the layers of graphite that were reported to be about 140 MPa [20]. Hence, layered materials (e.g. graphite, hBN, MoS2) while working effectively as a lubricant in metal cutting result in few-layer graphene dispersion as a byproduct. This phenomenon has been explored in the current paper. Machining conditions are optimized with the goal of enhancing the exfoliations and hence are not related to practical machining conditions. 2. Experimental design and methodology 2.1. Materials and methods One gram of sodium cholate (Alfa Aesar (A17074)) and 100 g of graphite flakes (mesh +70, 20) were mixed with one liter of distilled water to prepare the lubricant solution [16]. Sodium cholate prevents aggregation of finer particles. A special double tubular
https://doi.org/10.1016/j.mfglet.2019.08.002 2213-8463/Ó 2019 Society of Manufacturing Engineers (SME). Published by Elsevier Ltd. All rights reserved.
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Fig. 1. Schematic of (a) machining-induced shearing of graphite flakes, (b) oscillatory machining of double tubular workpiece.
workpiece is fabricated with an inner tube width of 4 mm with both sides of the tube filled with the lubricant solution [21]. A horizontal machining centre (Mitsui Seiki HR3C) was retrofitted to perform completely immersed orthogonal tube turning with a carbide tool (5 mm wide parting tool) in vertical arrangement. Intermittent cutting was performed by oscillating the tool allowing the entry of flakes (Fig. 1b). Stirring was done mechanically at intervals to prevent settling of flakes and thereby increasing the probability of trapping. Machining and non-machining conditions which include the feed (0.18 mm/rev), the width of cut (4 mm), oscillation angle (9 degrees forward and 1 degree backward), graphite concentration (100 g/L) and cutting speed (132 mm/min) were kept constant. Tool oscillates in the forward and backward angular direction along the cutting speed direction. The purpose of oscillation in this experiment is to increase the process yield. Backward rotation during oscillation allows sheared flakes to be removed from the tool chip interface. It also allows pristine and also previously partially-sheared flakes to trap at the tool chip interface enabling repeated shearing to happen and thereby improve yield. Machining was performed for a tube height of 50 mm which took 5 h of machining time for one sample. Samples collected after machining for 5, 10 and 15 h were analyzed.
3. Results and discussion 3.1. X-ray diffraction (XRD) results Fig. 2 depicts the X-ray results. Analysis of the Bragg peak of (0 0 2) plane at 2h angles between 25 and 30 degrees was performed to study the effective thickness reduction along the basal plane due to shearing. Peak positions of the graphite flake before
2.2. Characterization To confirm effective reduction in the thickness along the basal plane XRD measurements were carried out using an X-ray Diffractometer (XRD; Rigaku Smartlab) with Cu-k Alpha Excitation wavelength of 0.154 nm. One milliliter of the supernatant from the centrifuged sample was drop cast onto a glass slide for performing XRD. Samples for Atomic Force Microscopy (AFM – Shimazadu SPM – 9700), Raman Spectroscopy (Horiba LabRam-532 nm) and Scanning Electron Microscopy (SEM) were prepared by drop casting 10 ml of the sample onto a heated Si/SiO2 (300 nm) substrate. Samples were then washed with iso-propanol and distilled water to remove surfactant and other impurities.
Fig. 2. XRD spectra of (a) pristine graphite flakes (b) 5,10,15 h samples.
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and after shear were found at peak maximum 26.55 degrees. The d spacing calculated using Braggs law (Fig. 2) [22] for both pristine and sheared flakes was estimated to be 0.335 nm corresponding to the ideal layered structure of graphite [23]. Peak intensity for the pristine graphite flake was in the order of 107 and much lower for 5, 10, 15 h samples. Full width half maximum (FWHM) of the peaks had increased from pristine graphite to sheared flakes due to the size effect. The reduction in basal plane intensity clearly highlights that this was a mechanical exfoliation process caused by shearing along the basal hexagonal planes [24,25]. 3.2. Thickness and lateral dimension study Initial flakes (prior to machining) were observed to be within the mesh size 200–1000 mm with a thickness in the range of 20– 60 mm (Fig. 3a and d). AFM measurements were made on several drop-cast samples. Five hour sample had flakes with thickness in the range of 5–25 nm, while that of 10 h sample ranged from 3 to 15 nm and 15 h sample had flakes with thickness ranging from 1 to 8 nm in addition to lateral dimensions of 40–80 nm (Fig. 3g–i). Fig. 3g depicts the histogram representation of thickness range of few flakes obtained from 15 h. Flakes with smaller thickness also had smaller lateral dimensions (Fig. 3i). Interlayer spacing of graphene being 0.335 nm inferred that flakes with thickness ranging between 1 and 4 nm are few-layer and multilayer graphene flakes with a lateral dimension less than 100 nm, collectively categorized as graphene nanoplatelets. Some edges of the graphite were thinned down to single layer graphene (orange circled Fig. 3f) with a thickness of 0.33 nm which showed single layer graphene edges. Drop casting above the boiling point of water lifts the flakes at one end (Fig. 3e and f blue circled). So, the effective thickness of the flake was calculated by eliminating this error (Fig. 3e and f). Energy Dispersive X-ray Spectroscopy (EDX) confirmed small concentration of sodium and iron
impurities, where sodium from sodium cholate and iron from steel were identified as impurities, thus resulting in thinner flakes than they have been recorded previously. 3.3. Raman spectroscopy The Raman spectroscopy results are summarized in Fig. 4. The figure shows the Raman spectra of pristine graphite, 5, 10 and 15 h samples. A study of peak position, intensity and intensity ratios (using intensity height method) were carried out to understand the structural defects and disorders produced on the flakes [26,27]. Investigations of the shape and size of the 2D band and changes in its peak positions enables estimating number of layers present/thickness. [28]. Pristine graphite used for experiments displayed a 2D band centering at around 2720 cm 1 with double peaks 2D1 and 2D2 at around 1/4th and ½ the position (Fig. 4b black). It displayed a G peak at around 1580 cm 1 with a full width half maximum (FWHM) value of about 15. These confirms that the pristine graphite used is ordered crystalline bulk graphite [29]. Presence of D peak signifies the presence of a basal defect in the pristine graphite flakes. Loss of double peak in the shear thinned flakes from 5 h sample is noticed with a peak shift of about 20 cm 1 (Fig. 4b – red). No significant variation in the I2D/IG was observed when compared to pristine graphite with a very small variation in the ID/IG, 2D peak shape and FWHM are observed in the Raman spectra. These indicates the formation of graphite nanoplatelets [30] supporting AFM thickness measurements of the flakes in the range of 5–20 nm thickness. Due to the smaller lateral dimensions of the 5 h sheared flakes in comparison to the spot size of Raman Spectroscope laser beam, both the arm chair and zig-zag edges of the particles are activated. This leads to an increase in D peak intensity. Tearing of layers can also increase the D peak intensity (Fig. 4a – blue) due to the laser triggering edge defects of the combined armchair and zig-zag layer
Fig. 3. (a) Laser images of pristine graphite flake showing random line along with thickness profile is plotted, (b) 5-hour sample AFM image, (c) 15-hour image AFM image, (d) thickness profile along the pristine flake shown in ‘a’ (e) thickness profile along the 10 h sheared flake shown in ‘b’ (f) thickness profile along the 15 h sheared flakes shown in ‘c’ (g) Histogram showing number of flakes vs. thickness for 15 h sample (h) Histogram depicts the number of flakes vs. lateral dimension for 15 h sample (i) Lateral dimension vs. Thickness for 15 h sample.
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Madras is acknowledged. The authors also acknowledge the staff members of Techno Plaza and Superconducting Materials Laboratory, Department of Materials Science and Engineering, Shibaura Institute of Technology, Tokyo, Japan for providing characterization facilities. References
Fig. 4. (a) Raman Spectra of graphite flakes before and after shear (b) Spectra showing 2D peak before and after shear.
edges formed [21]. As the machining time is increased to 10 h, flakes get re-trapped and thinned resulting in an increase in the I2D/IG enabling the formation of multilayer graphene as indicated in AFM. The FWHM of G peak and D peak (10 h sample) is increased drastically with an increase in the intensity (Fig. 4 – pink) which indicates the loss of structural stability. AFM and Raman spectroscopy examination of 10 h sample shows clearly that it contains multilayer graphene and low amorphous graphene-like carbon nanoplatelets (with both low and high defect particles). After 15 h of machining, the flakes contain less than 5 layers (AFM results) with a combined D peak and G peak (Fig. 4 violet). Combined D and G peak indicates formation of amorphous carbon [31–33]. AFM and Raman spectroscopy analysis of 15 h sample shows clearly that it comprises of few layer graphene and graphene like amorphous carbon. 4. Conclusion Graphite flakes with initial thickness of 20–60 mm when used as an additive in the lubricant during the metal cutting process were observed to be sheared to form graphite nanoplatelets and multilayer graphene as a result of the machining process induced shear. Experiments indicated that increasing the time of machining induces defect and disorder in the layers, thereby forming low amorphous graphene-like carbon nanoplatelets. Additionally, it has been understood that as the thickness reduces the lateral dimension also declines. Few-layer graphene-like amorphous carbon layers with lateral dimension lesser than 100 nm were produced in the current research. Further research must be focused on controlling the stresses and machining time to produce defect-free few-layer graphene by machining induced mechanical exfoliation techniques. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Experimental assistance provided by Mr. Arulanandan. K, Technical Superintendent, Manufacturing Engineering Section, IIT
[1] Novoselov KS et al. A roadmap for graphene. Nature 2012;490:192–200. [2] Geim AK et al. The rise of graphene. Nat Mater 2007;6:183–91. [3] Novoselov KS et al. Two-dimensional atomic crystals. Proc Natl Acad Sci USA 2005;102:10451–3. [4] Allen MJ, Tung VC, Kaner RB. Honeycomb carbon: a review of graphene. Chem Rev 2009;110(1):132–45. [5] Jayasena B, Melkote SN. An investigation of PDMS stamp assisted mechanical exfoliation of large area graphene. Procedia Manuf 2015;1:840–53. [6] Ahmed Mahmoud MM, Imae Toyoko, et al. Defect-free exfoliation of graphene at ultra-high temperature. Colloids Surf, A 2018;538:127–32. [7] Subrahmanyam KS et al. Simple method of preparing graphene flakes by an arc-discharge method. J Phys Che C 2009;113(11):4257–9. [8] Sun G et al. Preparation and characterization of graphite nanosheets from detonation technique. Mater Lett 2008;62(4–5):703–6. [9] Cai Minzhen, Thorpe Daniel, et al. Methods of graphene exfoliation. J Mater Chem 2012;22:24992–5002. [10] Bianco A, Cheng H-M, Enoki T, Gogotsi Y, et al. All in the graphene family – a recommended nomenclature for two-dimensional carbon materials. Carbon 2013;65:1–6. [11] Quhe Ruge, Ma Jianhua, Zeng Zesheng, Tang Kechao. Tunable band gap in fewlayer graphene by surface adsorption. Sci Rep 2013;3:1794. [12] Throckmorton James, Palmese Giuseppe. Direct preparation of few layer graphene epoxy nanocomposites from untreated flake graphite. ACS Appl Mater Interfaces 2015;7(27):14870–7. [13] Novoselov KS et al. Electric field effect in atomically thin carbon films. Science 2004;306(5696):666–9. [14] Jayasena B et al. A novel mechanical cleavage method for synthesizing fewlayer graphene. Nanoscale Res Lett 2011;6:95. [15] Leon V. Few-layer graphenes from ball-milling of graphite with melamine. Chem Commun 2011. [16] Coleman et al. Scalable production of large quantities of defect-free few-layer graphene by shear exfoliation in liquids. Nat Mater 2014;13. [17] Shaji, Radhakrishnan V. Analysis of process parameters in surface grinding with graphite as lubricant based on the Taguchi method. J Mater Process Technol 2003;141(1):51–9. [18] Marcon A, Melkote S, Kalaitzidou K, DeBra D. An experimental evaluation of graphite nanoplatelet based lubricant in micro-milling. CIRP Ann-Manuf Technol 2010;59:141–4. [19] Trent et al. Metal cutting. Butterworth-Heinemann Publications; 2000. p. 446. [20] Liu Z et al. Interlayer shear strength of single crystalline graphite. Acta Mech Sin 2012;28(4):978–98. [21] Nishad Wazeem, Subbiah Sathyan, Miryala Muralidhar. Structural and morphological transformations in aqueous graphite-based lubricant subjected to extreme tribological interactions of low-speed intermittent machining. Tribol Lett 2019;67:20. [22] Bragg WH, Bragg WL. The reflection of X-rays by crystals. Proc Royal Soc A: Math Phys Eng Sci 1913;88(605):428–38. https://doi.org/10.1098/ rspa.1913.0040. [23] Kamimura H. Graphite intercalation compounds. Phys Today 1987;40:64–71. [24] Vittori M, Montone A, Jovic N, et al. Low energy pure shear milling: a method for the preparation of graphite nano-sheets. Scr Mater 2006;55(11):1047–50. [25] Qi X, Qu J, Zhang H, Yang D, Yu Y, Chi C, et al. FeCl3 intercalated few-layer graphene for high lithium-ion storage performance. J Mater Chem A 2015;3:15498–504. [26] Ferrari Andrea C, Basko Denis M. Raman spectroscopy as a versatile tool for studying the properties of graphene. Nat Nanotechnol 2013;8:235–46. [27] Cancado Ferrari et al. Quantifying defects in graphene via raman spectroscopy at different excitation energies. Nano Lett 2011;11:3190–6. [28] Ferrari AC, Meyer JC, Scardaci V, et al. Raman spectrum of graphene and graphene layers. Phys Rev Lett 2006;97:187401. [29] Vidano RP, Fishbach DB, Willis LJ, Loehr TM. Observation of raman band shifting with excitation wavelength for carbons and graphites. Solid State Commun 1981;39:341. [30] Tuinstra Koenig. Raman spectrum of graphite. J Chem Phys 1970;53:1126. [31] Ferrari AC. Raman spectroscopy of graphene and graphite: disorder, electron– phonon coupling, doping and nonadiabatic effects. Solid State Commun 2007;143:47–57. [32] Ferrari AC, Rodil SE, Robertson J. Interpretation of infrared and Raman spectra of amorphous carbon nitrides. Phys Rev B 2003;67:155306. [33] Ferrari AC, Robertson J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys Rev B 2000;61:14095.
Contents lists available at ScienceDirect
Manufacturing Letters journal homepage: www.elsevier.com/locate/mfglet
Machining induced transformation of graphite flakes to graphite/graphene nanoplatelets Wazeem Nishad, Sathyan Subbiah ⇑ Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 18 December 2018 Received in revised form 23 June 2019 Accepted 11 August 2019 Available online 12 August 2019 Keywords: Few-layer graphene Tool-chip interface Solid lubricant additives, Aqueous graphite lubricants Graphene exfoliation
a b s t r a c t A mechanical exfoliation technique to produce exfoliated graphite-graphene nanoplatelet dispersion as a byproduct of aqueous graphite-based lubricant-assisted metal cutting is presented here. Low cutting speed oscillatory orthogonal metal cutting process has been carried out completely immersed in the lubricant containing graphite flakes. Reduction in the thickness of the graphite flakes from around 30 mm to graphite-graphene nanoplatelets with a size range of 1–15 nm is observed after machining. Characterization of the particles depicts low structural order and induced defects. Optimizing the parameters can tune this machining-induced exfoliation technique to produce high-quality defect and disorder free few-layer graphene sheets. Ó 2019 Society of Manufacturing Engineers (SME). Published by Elsevier Ltd. All rights reserved.
1. Introduction Graphene, the first two-dimensional material to be isolated, has been found in various promising applications due to its interesting electrical and mechanical properties [1–3]. The applicability of these properties largely depends on the method of its synthesis [4]. These methods can be classified broadly into epitaxial growth, colloidal suspension, unconventional methods, and exfoliation (thermal, chemical and mechanical) [1,5–8]. Exfoliation, a topdown approach of synthesis, involves breaking the weak forces of attraction between the layers using external energy like electrical, chemical, thermal or mechanical [9]. Such exfoliations can create few-layer graphene – FLG (1–5 layers), multilayer graphene – MLG (5–10 layers), graphene nanosheets (less than 100 nm lateral dimension), graphene micro sheets (100 nm to 100 mm lateral dimension) and/or graphite nanoplatelets (up to 100 nm thick) [10]. Single layer graphene does not produce a bandgap, while few-layer graphene can be tuned to open bandgap and enhance its applicability in nano-electronics [11]. Few-layer and multilayer graphene nanosheets and graphene-like amorphous carbon nanosheets have also been used as reinforcements in composites due to its improvements in the mechanical properties [12]. Mechanical exfoliation involves mechanical separation of the layers in bulk graphite by peeling off, cutting and/or shearing using ⇑ Corresponding author. E-mail address: [email protected] (S. Subbiah).
a various method [13–16]. The natural lubricating action of the graphite flakes between two solid mechanically rubbing surfaces may also cause exfoliation. Graphite flakes/nano-platelets have been explored as a lubricant in machining [17,18]. The shearing process of machining [19] involves intense high-stress contact rubbing of the chip with the rake face of the cutting tool. If the graphite flakes can be introduced at this tool-chip interface, then the consequent rubbing can shear the flakes into few-layer graphene (FLG) (Fig. 1). This can be expected since the shear stress generated at the tool-chip interface during ductile-metal cutting, estimated between 150 and 300 MPa [19], is in the range of interlayer shear stress required to shear the layers of graphite that were reported to be about 140 MPa [20]. Hence, layered materials (e.g. graphite, hBN, MoS2) while working effectively as a lubricant in metal cutting result in few-layer graphene dispersion as a byproduct. This phenomenon has been explored in the current paper. Machining conditions are optimized with the goal of enhancing the exfoliations and hence are not related to practical machining conditions. 2. Experimental design and methodology 2.1. Materials and methods One gram of sodium cholate (Alfa Aesar (A17074)) and 100 g of graphite flakes (mesh +70, 20) were mixed with one liter of distilled water to prepare the lubricant solution [16]. Sodium cholate prevents aggregation of finer particles. A special double tubular
https://doi.org/10.1016/j.mfglet.2019.08.002 2213-8463/Ó 2019 Society of Manufacturing Engineers (SME). Published by Elsevier Ltd. All rights reserved.
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Fig. 1. Schematic of (a) machining-induced shearing of graphite flakes, (b) oscillatory machining of double tubular workpiece.
workpiece is fabricated with an inner tube width of 4 mm with both sides of the tube filled with the lubricant solution [21]. A horizontal machining centre (Mitsui Seiki HR3C) was retrofitted to perform completely immersed orthogonal tube turning with a carbide tool (5 mm wide parting tool) in vertical arrangement. Intermittent cutting was performed by oscillating the tool allowing the entry of flakes (Fig. 1b). Stirring was done mechanically at intervals to prevent settling of flakes and thereby increasing the probability of trapping. Machining and non-machining conditions which include the feed (0.18 mm/rev), the width of cut (4 mm), oscillation angle (9 degrees forward and 1 degree backward), graphite concentration (100 g/L) and cutting speed (132 mm/min) were kept constant. Tool oscillates in the forward and backward angular direction along the cutting speed direction. The purpose of oscillation in this experiment is to increase the process yield. Backward rotation during oscillation allows sheared flakes to be removed from the tool chip interface. It also allows pristine and also previously partially-sheared flakes to trap at the tool chip interface enabling repeated shearing to happen and thereby improve yield. Machining was performed for a tube height of 50 mm which took 5 h of machining time for one sample. Samples collected after machining for 5, 10 and 15 h were analyzed.
3. Results and discussion 3.1. X-ray diffraction (XRD) results Fig. 2 depicts the X-ray results. Analysis of the Bragg peak of (0 0 2) plane at 2h angles between 25 and 30 degrees was performed to study the effective thickness reduction along the basal plane due to shearing. Peak positions of the graphite flake before
2.2. Characterization To confirm effective reduction in the thickness along the basal plane XRD measurements were carried out using an X-ray Diffractometer (XRD; Rigaku Smartlab) with Cu-k Alpha Excitation wavelength of 0.154 nm. One milliliter of the supernatant from the centrifuged sample was drop cast onto a glass slide for performing XRD. Samples for Atomic Force Microscopy (AFM – Shimazadu SPM – 9700), Raman Spectroscopy (Horiba LabRam-532 nm) and Scanning Electron Microscopy (SEM) were prepared by drop casting 10 ml of the sample onto a heated Si/SiO2 (300 nm) substrate. Samples were then washed with iso-propanol and distilled water to remove surfactant and other impurities.
Fig. 2. XRD spectra of (a) pristine graphite flakes (b) 5,10,15 h samples.
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and after shear were found at peak maximum 26.55 degrees. The d spacing calculated using Braggs law (Fig. 2) [22] for both pristine and sheared flakes was estimated to be 0.335 nm corresponding to the ideal layered structure of graphite [23]. Peak intensity for the pristine graphite flake was in the order of 107 and much lower for 5, 10, 15 h samples. Full width half maximum (FWHM) of the peaks had increased from pristine graphite to sheared flakes due to the size effect. The reduction in basal plane intensity clearly highlights that this was a mechanical exfoliation process caused by shearing along the basal hexagonal planes [24,25]. 3.2. Thickness and lateral dimension study Initial flakes (prior to machining) were observed to be within the mesh size 200–1000 mm with a thickness in the range of 20– 60 mm (Fig. 3a and d). AFM measurements were made on several drop-cast samples. Five hour sample had flakes with thickness in the range of 5–25 nm, while that of 10 h sample ranged from 3 to 15 nm and 15 h sample had flakes with thickness ranging from 1 to 8 nm in addition to lateral dimensions of 40–80 nm (Fig. 3g–i). Fig. 3g depicts the histogram representation of thickness range of few flakes obtained from 15 h. Flakes with smaller thickness also had smaller lateral dimensions (Fig. 3i). Interlayer spacing of graphene being 0.335 nm inferred that flakes with thickness ranging between 1 and 4 nm are few-layer and multilayer graphene flakes with a lateral dimension less than 100 nm, collectively categorized as graphene nanoplatelets. Some edges of the graphite were thinned down to single layer graphene (orange circled Fig. 3f) with a thickness of 0.33 nm which showed single layer graphene edges. Drop casting above the boiling point of water lifts the flakes at one end (Fig. 3e and f blue circled). So, the effective thickness of the flake was calculated by eliminating this error (Fig. 3e and f). Energy Dispersive X-ray Spectroscopy (EDX) confirmed small concentration of sodium and iron
impurities, where sodium from sodium cholate and iron from steel were identified as impurities, thus resulting in thinner flakes than they have been recorded previously. 3.3. Raman spectroscopy The Raman spectroscopy results are summarized in Fig. 4. The figure shows the Raman spectra of pristine graphite, 5, 10 and 15 h samples. A study of peak position, intensity and intensity ratios (using intensity height method) were carried out to understand the structural defects and disorders produced on the flakes [26,27]. Investigations of the shape and size of the 2D band and changes in its peak positions enables estimating number of layers present/thickness. [28]. Pristine graphite used for experiments displayed a 2D band centering at around 2720 cm 1 with double peaks 2D1 and 2D2 at around 1/4th and ½ the position (Fig. 4b black). It displayed a G peak at around 1580 cm 1 with a full width half maximum (FWHM) value of about 15. These confirms that the pristine graphite used is ordered crystalline bulk graphite [29]. Presence of D peak signifies the presence of a basal defect in the pristine graphite flakes. Loss of double peak in the shear thinned flakes from 5 h sample is noticed with a peak shift of about 20 cm 1 (Fig. 4b – red). No significant variation in the I2D/IG was observed when compared to pristine graphite with a very small variation in the ID/IG, 2D peak shape and FWHM are observed in the Raman spectra. These indicates the formation of graphite nanoplatelets [30] supporting AFM thickness measurements of the flakes in the range of 5–20 nm thickness. Due to the smaller lateral dimensions of the 5 h sheared flakes in comparison to the spot size of Raman Spectroscope laser beam, both the arm chair and zig-zag edges of the particles are activated. This leads to an increase in D peak intensity. Tearing of layers can also increase the D peak intensity (Fig. 4a – blue) due to the laser triggering edge defects of the combined armchair and zig-zag layer
Fig. 3. (a) Laser images of pristine graphite flake showing random line along with thickness profile is plotted, (b) 5-hour sample AFM image, (c) 15-hour image AFM image, (d) thickness profile along the pristine flake shown in ‘a’ (e) thickness profile along the 10 h sheared flake shown in ‘b’ (f) thickness profile along the 15 h sheared flakes shown in ‘c’ (g) Histogram showing number of flakes vs. thickness for 15 h sample (h) Histogram depicts the number of flakes vs. lateral dimension for 15 h sample (i) Lateral dimension vs. Thickness for 15 h sample.
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Madras is acknowledged. The authors also acknowledge the staff members of Techno Plaza and Superconducting Materials Laboratory, Department of Materials Science and Engineering, Shibaura Institute of Technology, Tokyo, Japan for providing characterization facilities. References
Fig. 4. (a) Raman Spectra of graphite flakes before and after shear (b) Spectra showing 2D peak before and after shear.
edges formed [21]. As the machining time is increased to 10 h, flakes get re-trapped and thinned resulting in an increase in the I2D/IG enabling the formation of multilayer graphene as indicated in AFM. The FWHM of G peak and D peak (10 h sample) is increased drastically with an increase in the intensity (Fig. 4 – pink) which indicates the loss of structural stability. AFM and Raman spectroscopy examination of 10 h sample shows clearly that it contains multilayer graphene and low amorphous graphene-like carbon nanoplatelets (with both low and high defect particles). After 15 h of machining, the flakes contain less than 5 layers (AFM results) with a combined D peak and G peak (Fig. 4 violet). Combined D and G peak indicates formation of amorphous carbon [31–33]. AFM and Raman spectroscopy analysis of 15 h sample shows clearly that it comprises of few layer graphene and graphene like amorphous carbon. 4. Conclusion Graphite flakes with initial thickness of 20–60 mm when used as an additive in the lubricant during the metal cutting process were observed to be sheared to form graphite nanoplatelets and multilayer graphene as a result of the machining process induced shear. Experiments indicated that increasing the time of machining induces defect and disorder in the layers, thereby forming low amorphous graphene-like carbon nanoplatelets. Additionally, it has been understood that as the thickness reduces the lateral dimension also declines. Few-layer graphene-like amorphous carbon layers with lateral dimension lesser than 100 nm were produced in the current research. Further research must be focused on controlling the stresses and machining time to produce defect-free few-layer graphene by machining induced mechanical exfoliation techniques. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Experimental assistance provided by Mr. Arulanandan. K, Technical Superintendent, Manufacturing Engineering Section, IIT
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