Facile synthesis of twisted graphene solution from graphite-KCl
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Cite this: DOI: 10.1039/c3ra43793k Received 19th July 2013, Accepted 5th August 2013 Facile synthesis of twisted graphene solution from graphite-KCl3 DOI: 10.1039/c3ra43793k www.rsc.org/advances Akshaya Kumar Swain a and Dhirendra Bahadur* b A simple technique is demonstrated to produce twisted bi- and tri-layer graphene by mild sonication of exfoliated graphite on a large scale. No oxidation or reduction of graphite is involved in this process. Hydrothermally synthesized graphite-KCl compound is used to prepare exfoliated graphite. Graphene has many superlative properties but due to its 2-dimesnional nature, it becomes hard to conceive immediate applications in our day-to-day life. However, the extraordinary properties of it enables graphene based composite (GBC) materials to be promising for several applications. 1 Additionally, certain electronic and optical properties which are absent in single layer graphene (SLG) could be realized through tri- (TLG) or bi-layer graphene (BLG). These novel properties can also be tuned by varying certain parameters such as interlayer spacing, type of stacking, twinning and twisting angle in it for potential electronic applications. 2–4 The advantage of having graphene in solution form makes it convenient to transfer to a desired substrate or to produce composites of it at ease with fewer parameters involved to manipulate the process. Reduced graphene oxide or graphene produced chemically by subsequent oxidation and reduction of graphite introduces many defects within the material. 5 Although, these processes give a high yield, the properties owing to SLG are lost to a large extent. Here, we report two important results. First, the formation of stable graphite-KCl compound (GKC) is achieved hydrothermally and to the best of our knowledge, reported for the first time. Second, the resulting GKC is used as a precursor to produce BLG and TLG solutions. GKC can be used to make high quality GBC materials which will be reported in near future and is under investigation. Natural graphite powder, KI and 1,2-dichlorobenzene (DCB) were transferred into a pressurized vessel. The mixture was kept at a temperature of 320 uC for 12 h with rigorous stirring. DCB reacts with KI to form 1-iodo-2 chloro-benzene and Cl 2 while the K atoms get intercalated into graphite layers. The alkali metals and its corresponding compounds have been widely used to form graphite intercalated compounds (GIC). 6,7 Wang et al. have reported intercalation of binary compounds of metal chloride systems to produce GIC from its molten salt. 8 It was found that the reaction temperature and the composition of molten salt play key roles on the final stage formation of GIC. Cl is a strong oxidizing agent which oxidizes I 2 . The unreacted KI would react with Cl 2 to form KCl. The formation of KCl was verified by X-ray diffraction (XRD) and supported by energy-dispersive X-ray spectroscopy (EDS) data given in Fig. S1 of ESI. 3 Hsu et al. has found that KCl crystallizes within the space present in carbon nanotube walls. 9 They have produced KCl by treating K-intercalated graphite with CCl 4 . The experimental details for GKC preparation are given in the experimental section. The final product does not have any signs of intercalation due to absence of any noticeable shift in 2h in XRD. However, it appears that K intercalated graphite is an intermediate product. 10 The formation of the intermediate potassium-intercalated graphite in the reaction weakens the van der Waals interaction between the graphene layers of graphite. In addition, the reaction is highly exothermic producing very high pressures due to evolution of chlorine gas which was recorded by the autoclave used for synthesis. This kind of exothermic reactions in aqueous solvents would facilitate exfoliation of graphite. 11 However, removing the high pressure- high temperature restores the graphitic structure in GKC. We propose the formation of GKC by a 2-step technique. First, an intermediate K intercalated graphite is formed as soon as K + ions are produced at the beginning of the reaction. Second, the K + ions within the graphite lattice migrates under high pressure to form KCl (lattice constant: 0.63 nm) whose size is too large to be retained as an intercalate by the host lattice. From XRD data, d 220 spacing of KCl (0.22 nm) is quite close to the distance between two adjacent carbon sites along the zig zag edge of graphite. 9 Also the cubic structure of KCl is observed in a IITB Monash Research Academy, Department of Metallurgical Engineering and Materials Science, IIT Bombay, Mumbai, India 400076 b Department of Metallurgical Engineering and Materials Science, IIT Bombay, Mumbai, India 400076. E-mail: [email protected] 3 Electronic supplementary information (ESI) available: FEG-SEM and EDS of GKC, XRD of KCl compared to that of standard JCPDS data in tabular form, FEG-TEM images of GKC, SAED and diffracted intensity vs. distance plots from SAED pattern of TLG. See DOI: 10.1039/c3ra43793k RSC Advances COMMUNICATION This journal is ß The Royal Society of Chemistry 2013 RSC Adv. Published on 08 August 2013. Downloaded by University of Saskatchewan on 05/09/2013 02:00:24. View Article Online View Journal
Transcript of Facile synthesis of twisted graphene solution from graphite-KCl
Cite this: DOI: 10.1039/c3ra43793k
Received 19th July 2013,Accepted 5th August 2013
Facile synthesis of twisted graphene solution fromgraphite-KCl3
DOI: 10.1039/c3ra43793k
www.rsc.org/advances
Akshaya Kumar Swaina and Dhirendra Bahadur*b
A simple technique is demonstrated to produce twisted bi- and
tri-layer graphene by mild sonication of exfoliated graphite on a
large scale. No oxidation or reduction of graphite is involved in
this process. Hydrothermally synthesized graphite-KCl compound
is used to prepare exfoliated graphite.
Graphene has many superlative properties but due to its2-dimesnional nature, it becomes hard to conceive immediateapplications in our day-to-day life. However, the extraordinaryproperties of it enables graphene based composite (GBC) materialsto be promising for several applications.1 Additionally, certainelectronic and optical properties which are absent in single layergraphene (SLG) could be realized through tri- (TLG) or bi-layergraphene (BLG). These novel properties can also be tuned byvarying certain parameters such as interlayer spacing, type ofstacking, twinning and twisting angle in it for potential electronicapplications.2–4 The advantage of having graphene in solutionform makes it convenient to transfer to a desired substrate or toproduce composites of it at ease with fewer parameters involved tomanipulate the process. Reduced graphene oxide or grapheneproduced chemically by subsequent oxidation and reduction ofgraphite introduces many defects within the material.5 Although,these processes give a high yield, the properties owing to SLG arelost to a large extent. Here, we report two important results. First,the formation of stable graphite-KCl compound (GKC) is achievedhydrothermally and to the best of our knowledge, reported for thefirst time. Second, the resulting GKC is used as a precursor toproduce BLG and TLG solutions. GKC can be used to make highquality GBC materials which will be reported in near future and isunder investigation.
Natural graphite powder, KI and 1,2-dichlorobenzene (DCB)were transferred into a pressurized vessel. The mixture was kept ata temperature of 320 uC for 12 h with rigorous stirring. DCB reactswith KI to form 1-iodo-2 chloro-benzene and Cl2 while the K atomsget intercalated into graphite layers. The alkali metals and itscorresponding compounds have been widely used to formgraphite intercalated compounds (GIC).6,7 Wang et al. havereported intercalation of binary compounds of metal chloridesystems to produce GIC from its molten salt.8 It was found thatthe reaction temperature and the composition of molten salt playkey roles on the final stage formation of GIC. Cl is a strongoxidizing agent which oxidizes I2. The unreacted KI would reactwith Cl2 to form KCl. The formation of KCl was verified by X-raydiffraction (XRD) and supported by energy-dispersive X-rayspectroscopy (EDS) data given in Fig. S1 of ESI.3 Hsu et al. hasfound that KCl crystallizes within the space present in carbonnanotube walls.9 They have produced KCl by treatingK-intercalated graphite with CCl4. The experimental details forGKC preparation are given in the experimental section. The finalproduct does not have any signs of intercalation due to absence ofany noticeable shift in 2h in XRD. However, it appears that Kintercalated graphite is an intermediate product.10 The formationof the intermediate potassium-intercalated graphite in the reactionweakens the van der Waals interaction between the graphenelayers of graphite. In addition, the reaction is highly exothermicproducing very high pressures due to evolution of chlorine gaswhich was recorded by the autoclave used for synthesis. This kindof exothermic reactions in aqueous solvents would facilitateexfoliation of graphite.11 However, removing the high pressure-high temperature restores the graphitic structure in GKC. Wepropose the formation of GKC by a 2-step technique. First, anintermediate K intercalated graphite is formed as soon as K+ ionsare produced at the beginning of the reaction. Second, the K+ ionswithin the graphite lattice migrates under high pressure to formKCl (lattice constant: 0.63 nm) whose size is too large to beretained as an intercalate by the host lattice.
From XRD data, d220 spacing of KCl (0.22 nm) is quite close tothe distance between two adjacent carbon sites along the zig zagedge of graphite.9 Also the cubic structure of KCl is observed in
aIITB Monash Research Academy, Department of Metallurgical Engineering and
Materials Science, IIT Bombay, Mumbai, India 400076bDepartment of Metallurgical Engineering and Materials Science, IIT Bombay,
Mumbai, India 400076. E-mail: [email protected]
3 Electronic supplementary information (ESI) available: FEG-SEM and EDS of GKC,XRD of KCl compared to that of standard JCPDS data in tabular form, FEG-TEMimages of GKC, SAED and diffracted intensity vs. distance plots from SAED patternof TLG. See DOI: 10.1039/c3ra43793k
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GKC (from XRD data) as the hexagonal lattice in graphite acts as atemplate to form KCl crystals arranged in a closed packedstructure where each K+ is surrounded by 6 Cl2 ions. Hence Cl2
ions are most likely to be seen at the center of hexagonal rings ofgraphite while K+ ions occupy a site directly above the C–C bonds(Scheme 1). When this arrangement extends along the c-axis, alocalized strain is developed. In turn, the graphite layers disorientthemselves to attain a stable configuration.12,13 This disorientationbetween graphene layers was confirmed from selected areaelectron diffraction (SAED) pattern obtained by transmissionelectron microscopy (TEM). We suspect that the disorientation ofgraphene layers in GKC may also arise due to high pressure andhigh temperature during the reaction to maintain the stability ofthe system. Under these conditions, graphite can be exfoliated toform high quality large area graphene sheets.14,15 The BLG andTLG were produced by sonicating EG in a solvent preferably inethanol. The production yield was measured to be 0.56 mg ml21.
Comparison of XRD patterns shown in Fig. 1a clearly indicatesthe presence of KCl in GKC and absence of it in exfoliated graphite(EG). The presence of KCl is verified by comparing its standardXRD pattern (JCPDS file No: 01-073-0380, Table 1 in ESI3) with thatof experimental data obtained. Upon washing KCl from graphite,only (002) and (004) reflections of graphite reappear with reducedintensities and peak broadening while (hk0) reflections were notfound in EG.16 This may be seen from the XRD plot of EG andgraphite (zoomed y10 and 50 times along y-axis for EG andgraphite respectively) in Fig. 1b. The absence of (hk0) peaks in EGindicates that there is no intralayer interference in it and the
presence of (00l) peaks only signifies an interlayer interference inthe material.12 However, appearance of (004) peak in EG is a resultof stacking of several graphene layers. A mild sonication of EGdecouples the layers along the c-axis thereby producing BLG andTLG solution. This is due to the repulsive interlayer interactionbetween the pi orbitals of the graphene layers. In graphene, thebonding orbitals are completely filled with pi electrons. And whensuch filled orbitals are stacked together, they become repulsive.Natural bond orbital analysis studies have been used to estimatethe interlayer coupling in graphene stacking.13 When KCl iswashed out of the GKC matrix, charge transfer occurs betweenvarious atomic sites in graphite so as to attain a stableconfiguration. The crystal domains formed during the removalof KCl from graphite may be crosslinked by the grain boundarieswith stacking faults. The weak van der Waals forces that hold thegraphene layers in EG are much weaker than that in graphite.Thus a mild sonication in a solvent results in a graphenesolution.16,17 The emergence of BLG is verified by Raman studies.The exfoliation of graphite is also verified by the Tyndall effect.17
Fig. 1c gives a top view of laser passing through EG in ethanol. Theside view of laser showing scattering (Fig. 1d) from the stableparticles in the dispersion proves the exfoliation of graphite.Images of GKC, EG and few layers of graphene (,3–4) sheetsobtained by field emission gun-scanning electron microscope(FEG-SEM) is given in Fig. S2 (ESI3).
Fig. 2a shows the Raman spectra of graphite and BLG at 5 mWincident laser (514 nm) power. Graphite powder and a drop castedthin film of BLG was used to record the Raman spectra.Deconvolution of G band of BLG in Fig. 2b suggests the formationof defects due to non-zero phonon density of states. This is due tothe occurrence of D9 band at 1620 cm21 which is absent in
Scheme 1 Reaction mechanism of graphite-KCl formation.
Fig. 1 (a) Comparison of XRD patterns of graphite, graphite-KCl and EG. (b)Zoomed (y50 times along y-axis) XRD plot of graphite and EG shows absence of(hk0) planes in EG. Tyndall effect of EG solution (c) top view, (d) side view.
Fig. 2 (a) Raman spectra of graphite and BLG at 5 mW incident laser (514 nm)power. (b) Deconvolution of G-band of BLG suggests the presence of defects in it. (c)Deconvolution of 2D-band of BLG indicates the presence of three separate bands(w1, w2 and w3) signifying the formation of BLG.
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graphite. The disorder in BLG can also be seen due to emergenceof D band in between 1340–1360 cm21.18 The band obtained at1576 cm21 is due to sp2 bonded carbon atoms in graphene. Theexfoliated graphene flakes exhibit characteristics that correspondsto BLG. Similar nature of Raman spectrum for the 2D band is alsoobserved from the BLG flakes obtained by liquid phase exfoliationof graphite.19 The 2D band was found to contain three Lorentziancomponents which signifies a BLG. It should be noted that 2Dband in BLG usually allows 4 peaks to be fitted, however due to thepresence of two degenerate transition processes, one can havethree peaks fitted.20 The positions of deconvoluted bands of the2D peak in BLG are found at 2719 (w1), 2686 (w2) and 2640 (w3)cm21 (Fig. 2c). By optimizing the sonication time of the EG, onecan produce BLG or TLG in a desired solvent. TLG starts to deformto form BLG by increasing the sonication time which is verified byhigh resolution TEM (HRTEM) images.
TEM images of GKC is presented in Fig. S3 (ESI3). Fig. 3a showsthe TEM image of graphene sheets produced by mild sonication ofGKC in formamide. The inset-a gives the angle between the twoSLG sheets in BLG from an indexed SAED pattern. It can be seenin Fig. 3b that TLG starts to deform to give rise to BLG eventuallydepending upon the sonication time (Fig. 3b-inset). However fromthe SAED pattern in Fig. 3c and d and the intensity ratio plotstherein (inset of Fig. 3c and d) indicates the presence of two SLGsheets twisted at an angle.21–23 The critical rotational angle, hc (hc =3aElaser/4ph– Vf), computed using the Dirac dispersion relation ofmonolayer graphene is found to be y12.3u while a is the graphenelattice parameter (2.46 Å), Elaser is the energy of the incident laser(2.14 eV for 514 nm laser), h– is Planck’s constant and Vf is theFermi velocity in SLG (106 m s21).23 Ideally, there exists a strongcoupling between the two graphene layers if the rotational angle h
is smaller than the critical rotational angle and weak coupling if h
. hc. From the SAED patterns of BLG, h was calculated to be y6–8u (Fig. 3a-inset). Because h is close to hc, the properties of theresultant BLG gets modified due to the overlap of the Dirac conesin the momentum space thereby offering different scatteringpaths emanating out of the scattering centers. Change inscattering length will result in a change in phase shifts. Thiscould lead to a deviation of standard properties as expected. Thetwisting of the graphene layers is seen because it suppresses therepulsion between the graphene layers. The orthogonality of piorbitals in graphene is also lost due to this misorientation.13 Theintensity ratio (spots of inner hexagon to that of outer hexagon),{1100}/{2110} . 1 in the insets of Fig. 3c and d and the twohexagonal diffraction spots signifies the presence of disorientedSLG sheets in BLG.19 SAED of several BLG sheets were performedto confirm that majority of the BLG is composed of twomisoriented SLG sheets. Fig. S4 (ESI3) gives the SAED of a TLGwhich indicates the presence of three SLG sheets in it. Theoccurrence of SLG in TLG is verified from the intensity ratio givenin Fig. S4 (ESI3). The deformation of the TLG into BLG and SLGcould be verified by performing height analysis of the sheetsobtained by atomic force microscopy (AFM). The section analysisof graphene sheets are shown in Fig. 4. The height profile inFig. 4a and b indicates the formation of BLG, SLG and TLG fromGKC/EG upon mild sonication.24
Conclusions
In conclusion, we synthesized graphite-KCl compound which isthe precursor for EG. Tyndall scattering is seen upon mildsonication of EG in ethanol. The GKC becomes a promisingmaterial to produce BLG and TLG solutions on a large scale invarious solvents. The formation of BLG is identified by Ramanspectroscopy and verified by HRTEM and AFM. The BLG obtainedin this method shows the characteristics of a twisted BLG whichare potential materials for electronic applications.
Fig. 3 (a) TEM image of graphene sheets. The inset-a figure shows the indexedSAED pattern with the angle between the two graphene sheets in BLG. (b) HRTEMimage of graphene. The inset-b shows BLG/TLG where TLG deforms to form BLG.SAED pattern of twisted BLG is given in (c) and (d) while the diffracted intensityalong the lines drawn is presented in insets-c and d respectively. An intensity ratio({1100}/{2110} . 1) is a signature of SLG.
Fig. 4 (a) and (b) Section analysis of graphene sheets obtained by AFM.
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Experimental section
Preparation of graphite-KCl compound
Natural graphite powder (1.44 g), KI (24.9 g) and 120 ml of 1,2-dichlorobenzene (DCB) were transferred into a pressurized vessel.This was rigorously stirred at room temperature (RT) to dissolvethe KI.10 A pressure of 2 bar is created inside the vessel by purgingit with N2. Further, the solution was kept at a temperature of 320uC for 12 h. Due to temperatures higher than the boiling point ofDCB and formation of chlorine gas in the reaction, high pressure(133 bar) was developed inside the vessel. The solution was autocooled to RT. The resultant solution was washed alternatively withethanol and de-ionized water to remove any unreacted iodine.This was again washed repeatedly with DCB and mixture ofethanol and water to remove un-reacted materials. Graphite-KClcompound was obtained after centrifuging the solution collectedafter washing. This was further vacuum dried at 80 uC for 15 min.The obtained black powder was exfoliated in ethanol by sonicationthereby removing KCl from graphite to form expanded graphite.The particles settled down were separated out to obtain a solutionwhich is comprised up of mainly bi- and tri-layers of graphene init. The production yield of BLG and TLG was calculated to be 0.56mg ml21.
Acknowledgements
The authors would like to thank DST-Nanomission and IITBMonash Research Academy for their financial support.
Notes and references
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