Graphene Synthesis by Ultrasound Energy-Assisted Exfoliation of Graphite in Various Solvents

Article Graphene Synthesis by Ultrasound Energy-Assisted Exfoliation of Graphite in Various Solvents

Betül Gürünlü 1,* , Çi˘gdemTa¸sdelen-Yüceda˘g 2,* and Mahmut Bayramo˘glu 2

1 Institute of Nanotechnology, Gebze Technical University, 41400 Kocaeli, Turkey 2 Chemical Engineering Department, Gebze Technical University, 41400 Kocaeli, Turkey; [email protected] * Correspondence: [email protected] (B.G.); [email protected] (Ç.T.-Y.); Tel.: +90-544-354-9288 (B.G.); +90-262-605-2120 (Ç.T.-Y.)

Received: 13 October 2020; Accepted: 10 November 2020; Published: 14 November 2020

Abstract: The liquid-phase exfoliation (LPE) method has been gaining increasing interest by academic and industrial researchers due to its simplicity, low cost, and scalability. High-intensity ultrasound energy was exploited to transform graphite to graphene in the solvents of dimethyl sulfoxide (DMSO), N,N-dimethyl formamide (DMF), and perchloric acid (PA) without adding any surfactants or ionic liquids. The crystal structure, number of layers, particle size, and morphology of the synthesized graphene samples were characterized by X-ray diﬀraction (XRD), atomic force microscopy (AFM), ultraviolet visible (UV–vis) spectroscopy, dynamic light scattering (DLS), and transmission electron microscopy (TEM). XRD and AFM analyses indicated that G-DMSO and G-DMF have few layers while G-PA has multilayers. The layer numbers of G-DMSO, G-DMF, and G-PA were determined as 9, 10, and 21, respectively. By DLS analysis, the particle sizes, polydispersity index (PDI), and zeta potential of graphene samples were estimated in a few micrometers. TEM analyses showed that G-DMSO and G-DMF possess sheet-like fewer layers and also, G-PA has wrinkled and unordered multilayers.

Keywords: ultrasound; liquid-phase exfoliation; graphene synthesis

1. Introduction Graphene is a versatile nanomaterial with a wide range of chemical, environmental, medical, industrial, and electronical applications by means of its remarkable thermal conductivity (above 1 3000 W m K− ), superior mechanical properties with a Young’s modulus of 1 TPa, an extraordinary 2 1 large specific surface area (2620 m g− ), intrinsic strength of 130 GPa, and an extremely high electronic conductivity (room-temperature electron mobility of 2.5 105 cm2 V 1 s 1)[1–3]. This combination of × − − spectacular properties enables its utilization in the production of different devices, such as electronics with high-speed and radio-frequency logic, sensors, membranes, composites with high thermal and electrical conductivity, displays with superior transparency and flexibility, solar cells, coatings, ultrathin carbon films, electronic circuits, etc. [4]. Few-layered graphene has been synthesized by numerous methods including mechanical cleavage, liquid-phase exfoliation, gas-phase synthesis, Hummers’ method, unrolling of multi-walled carbon nanotubes (MWCNTs), chemical vapor deposition (CVD), epitaxial growth, and electrochemical reaction [2,5–15]. However, these methods have some drawbacks, such as low yield ratio, high-energy consumption, the use of expensive substrates, as well as the difficulty of obtaining a high-quality product. Scientists have studied the development of more efficient synthesis methods apart from the most popular Hummers’ method employing the oxidative exfoliation of natural graphite by harsh acid treatments. The final products attained by Hummers’ method have many functional groups, such as hydroxyls, carbonyls, and carboxyls, which cannot be completely removed by additional reduction and

Crystals 2020, 10, 1037; doi:10.3390/cryst10111037 www.mdpi.com/journal/crystals Crystals 2020, 10, 1037 2 of 12 annealing treatments [6,16,17]. Additionally, the oxidation-reduction processes of Hummers’ method disrupt the original honeycomb structure of graphene [18]. This deteriorated structure reduces the electronical and mechanical properties of the synthesized final graphene products [19,20]. For this reason, selection of the appropriate solvent and process parameters has a critical importance in order to obtain the graphene with a well-arranged structure. Liquid-phase exfoliation (LPE) is one of the most feasible methods for industrial production of graphene through its scalability and low cost. A highly stable dispersion of mono/few-layered graphene products with a defect-free honeycomb structure has been synthesized by LPE of graphite by applying different techniques such as shear mixing, microwave irradiation, or sonication [21]. The application of the LPE method is especially significant for the electronics industry which requires defect-free graphene products without oxide groups [22]. The LPE technique is based on the exfoliation of graphite intercalated compounds in an appropriate solvent. In this technique, there are three subsequent steps: (1) dispersion of graphite in a solvent, (2) exfoliation, and (3) purification [23]. For the intercalation of the solvent molecules, the Van der Waals forces between the graphite layers are overcome by the application of the external driving force, such as ultrasound energy [24]. Then, individual layers start to slip out from the layered structure of graphite under the effect of shear force. By this fragmentation, thinner graphene flakes with a small lateral size can be achieved. The utilization of ultrasound energy in various types of chemical processes relies on the acoustic cavitation in a liquid. The main elements of the acoustic cavitation are the formation, development, and collapse of cavitation bubbles. Steam bubbles emerge due to the pressure drop in some parts of liquid. On the other hand, the rise in pressure and temperature causes the extinction of the cavities. When the liquid is subjected to the ultrasound energy, localized hot points appear with high temperature and pressure [18]. The efficiency of the ultrasound energy is determined by the ultrasound application conditions and the type of ultrasound device. There are two types of ultrasound generators: horn type and bath type. Epoxy/graphene nanocomposites synthesized by using the horn-type device demonstrate higher modulus and flexural strength properties than that of the samples produced via the bath-type device [18]. The reason for obtaining better properties with the horn-type device is the direct immersion of the probe into the sonication liquid. This direct sonication results in turbulent flow conditions and acoustic streaming in suspensions, which prevents the agglomeration of the nanoparticles. In the case of graphene synthesis by the LPE method with ultrasonication, the traditional bath-type sonicator is inadequate to produce intense cavitation necessary for the efficient exfoliation of graphite without chemicals, such as ionic liquids and surfactants [18]. The properties of the liquid establishing the environment for the chemical processes have a great impact on obtaining a stable graphite-solvent dispersion [25]. Many studies were conducted to investigate the most convenient solvent type and optimum process conditions for the synthesis of graphene by ultrasonication [26–28]. Graphite is easily exfoliated in polar aprotic organic solvents due to its hydrophobic character. In this study, polar aprotic dimethyl sulfoxide (DMSO) and dimethyl formamide (DMF) were used for exfoliation of graphite as the sonication medium without addition of any surfactants or ionic liquids. Additionally, these solvents prevent the inverse segregation and micelle cumulation of graphenes by exposure to high-intensity ultrasound energy [29]. The mechanism of the graphite dispersions in solvents of DMSO and DMF is ascribed to restricted solvent layer near the graphene surface blocking the aggregation of graphene layers because of the existence of sterics in the chemical structure of these solvents [30]. As the third sonication solvent, perchloric acid (PA) was chosen due to the benefit of being a highly effective self-intercalating agent. Although PA was utilized at high concentrations for sonication, it still minimizes the defects in the honeycomb lattice of graphene [31–33]. When PA is compared to the other strong acids, such as H2SO4 and HNO3, it requires no excess heating process and no use of oxidizers [34,35]. All these solvents (Figure1) used in this study are capable of successful exfoliation of the graphite and prevention of the graphene transformation back to a graphite structure. The perchlorate ions between the graphite layers separate Crystals 2020, 10, 1037 3 of 12

themCrystals by 2020 the, 10 crystal, x FOR delamination PEER REVIEW under sonication forming an electrolyte including small graphene3 of 12 Crystalsﬂakes [202036,37, 10]., x FOR PEER REVIEW 3 of 12

Figure 1. Lewis structure of the solvents used as liquid medium in the exfoliation of graphite. Figure 1. Lewis structure of the solvents used as liquid medium in the exfoliation of graphite. Figure 1. Lewis structure of the solvents used as liquid medium in the exfoliation of graphite. InIn the the present present study, study, we introduced we introduced one-pot one-pot synthesis sy ofnthesis graphene of graphene via ultrasonication via ultrasonication of graphite inof digraphiteﬀerentIn the organic in present different solvents study, organic without we solvents introduced using without ionic liquidsone-pot using or ionicsy surfactants.nthesis liquids of Theorgraphene surfactants. characterization via Theultrasonication characterization of synthesized of graphitegrapheneof synthesized in samples different graphene was organic performed sample solventss bywas without XRD, performed AFM, using UV-vis, by ionic XRD, liquids DLS, AFM, and or UV-vis, TEMsurfactants. analyses. DLS, Theand characterizationTEM analyses. of synthesized graphene samples was performed by XRD, AFM, UV-vis, DLS, and TEM analyses. 2.2. Experimental Experimental 2. Experimental 2.1.2.1. Materials Materials 2.1. Materials GraphiteGraphite ﬁne fine powder powder (extra (extra pure) pure) and and commercial commercia graphenel graphene (CG) (CG) were were purchased purchased from, from, Asbury Asbury Inc.,Inc.,Graphite Asbury, New Jersey, NJ, fine USA powderand and XG (extra XGSciences, Sciences, pure) Michigan, and Lansing, commercia US, MI, respectively. USA,l graphene respectively. Dimethyl (CG) were Dimethyl sulfoxide purchased sulfoxide (DMSO) from, (DMSO) Asbury(Merck), Inc.,(Merck),N,N-dimethylformamide New N,N-dimethylformamide Jersey, and XG Sciences, (DMF) (Merck), (DMF)Michigan, (Merck), and US, perchloric re andspectively. perchloric acid 70Dimethyl−72% acid (PA) 70 sulfoxide72% (Merck) (PA) (DMSO) were (Merck) of (Merck),analytical were of − N,N-dimethylformamideanalyticalgrade and grade used andas received. used (DMF) as CG received. (Merck),was used CG and as was the perchloric usedcontrol as acidsample the control 70− in72% order sample (PA) to (Merck) compare in order were the to compareofprecision analyticalthe and gradeprecisionpurity and of and usedthe puritysynthesized as received. of the graphene synthesized CG was used products. graphene as the cont products.rol sample in order to compare the precision and purity of the synthesized graphene products. 2.2.2.2. Method Method 2.2. Method First,First, 0.3 0.3 g graphiteg graphite were were dispersed dispersed in a 50-mLin a 50-mL solvent, solvent, such as such DMSO, as DMF,DMSO, and DMF, PA. The and prepared PA. The ® dispersionspreparedFirst, 0.3dispersions were g graphite sonicated were were for sonicated 3dispersed h by using for in3 BANDELINh a by 50-mL using solvent, BANDELINHD 2200such SONOPULS® asHD DMSO, 2200 SONOPULS DMF, equipped and with PA.equipped aThe vs. 190preparedwith T sonotrode a vs. dispersions 190 madeT sonotrode ofwere titanium sonicated made alloy, of for titanium 200 3 W,h by 50% allousing amplitudey, 200BANDELIN W, (Figure 50% ®amplitude 2HD). Afterwards, 2200 SONOPULS(Figure these 2). dispersionsAfterwards, equipped withwerethese a subjected dispersionsvs. 190 T to sonotrode Elektromag,were subjected made M ofto 4812 Elektromag,titanium P for 1allo h M aty, 4812 3000200 W, P rpm for 50% 1 in hamplitudeorder at 3000 to rpm remove (Figure in order the 2). unexfoliatedtoAfterwards, remove the thesegraphiteunexfoliated dispersions particles. graphite were Then, subjectedparticles. the large toThen, aggregates Elektromag, the large were M aggregates settled4812 P down,for were 1 h theat settled 3000 supernatant rpm down, in orderthe part supernatant ofto theremove samples thepart unexfoliatedwasof the decanted, samples graphite and was then decanted, particles. collected and Then, in then separate the collected large vials. aggregates in separate were vials. settled down, the supernatant part of the samples was decanted, and then collected in separate vials.

Figure 2. Ultrasonic treatment unit. Figure 2. Ultrasonic treatment unit. Figure 2. Ultrasonic treatment unit.

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2.3. Characterization Characterization X-ray didiffractionﬀraction (XRD) (XRD) was was conducted conducted by by depositing depositing the samplesthe samples onto onto glass glass pieces pieces (0.7 (0.70.7 mm× 0.72) × mmand2 their) andXRD theirspectra XRD spectra were determined were determined by a Rigaku by a Ri D-Maxgaku D-Max 2200 Series 2200 deviceSeries device equipped equipped with Cu-K withα Cu-Kradiationα radiation (λ = 1.54 (λ Å) = 1.54 at a scanningÅ) at a scanning rate of 3rate◦ per ofminute. 3° per minute. Its tube Its voltage tube voltage and current and current were 40 were kV and 40 kV40 mA,and respectively.40 mA, respectively. Samples Samples for AFM for were AFM prepared were prepared by dropping by dropping the graphene the graphene dispersions dispersions onto glass ontopieces glass (0.7 pieces0.7 mm (0.72 )× and0.7 mm measurements2) and measurements were carried were out carried in contact out in (tapping) contact (tapping) mode, with mode, 10.00 withµm × 10.00scan size,μm andscan 20.35 size, Hzand scan 20.35 rate Hz by scan using rate a Digital by usin Instrumentsg a Digital Nanoscope. Instruments Ultraviolet-visible Nanoscope. Ultraviolet- (UV-vis) visiblespectroscopy (UV-vis) analyses spectroscopy were done analyses by a were Perkin done Elmer by Preciselya Perkin LambdaElmer Precisely 35 UV-vis Lambda Spectrometer 35 UV-vis in Spectrometerthe region between in the region 200–800 between nm wavelengths. 200–800 nm Inwave orderlengths. to elucidate In order the to elucidate particle size the distribution,particle size distribution,polydispersity polydispersity index (PDI) and index zeta (PDI) potential and zeta analyses potential were analyses carried out were by thecarried dynamic out by light the scatteringdynamic light(DLS) scattering method through (DLS) method a Malvern through Zetasizer a Malvern Nano ZS Zetasizer Laser particle Nano size ZS distribution Laser particle meter. size The distribution structural meter.and morphological The structural properties and morphological were determined properti byes awere Hitachi determined HT7800 modelby a Hitachi transmission HT7800 electron model transmissionmicroscope (TEM) electron operating microscope at 120 (TEM) kV. operating at 120 kV.

3. Results Results and and Discussion Discussion The exfoliation mechanism of graphite by the so sonochemicalnochemical process using a horn-type device is given in Figure 33.. ThisThis typetype ofof sonicatorsonicator producesproduces high-intensityhigh-intensity ultrasoundultrasound energy,energy, whichwhich enablesenables stable micromechanical exfoliation of graphite. Additionally, Additionally, the the horn-type sonicator provides the synthesis of graphene with minor functional groups and flat ﬂat and defect-free morphologies. The stable dispersion of graphene into solvents can be provid provideded by sonication that builds cavitation shear stress in the solvent. The The sonication sonication time time and and power power can adjust the concentration of the dispersion.

Figure 3. TheThe exfoliation exfoliation mechanism mechanism of the graphite to graphene.

The stablestable homogeneoushomogeneous dispersion dispersion of of graphite graphite can can be achievedbe achieved in solvents, in solvents, such such as DMSO, as DMSO, DMF, DMF,and PA, and possessing PA, possessing similar similar surface surface tensions tensio closens to close that ofto graphenethat of graphene (~68 mJ /(~68m2)[ mJ/m38,392].) [38,39]. The surface The surfaceenergies energies of DMSO, of DMSO, DMF, and DMF, PA areand 43.5, PA are 37.1, 43.5, and 37.1, 70.0 and mJ/m 70.02, respectively. mJ/m2, respectively. When the When surface the energies surface energiesof the dispersed of the dispersed phase and phase dispersant and dispersant are close toare each close other, to each the enthalpy other, the of enthalpy mixing is of diminished, mixing is diminished,resulting in aresulting stable dispersion in a stable of dispersion graphite. The of gr crystalaphite. structure The crystal and structure the layer and numbers the layer of the numbers natural ofgraphite the natural and the graphite graphene and products the graphene (G-DMSO, produc G-DMF,ts (G-DMSO, and G-PA) G-DMF, were and determined G-PA) were by XRD determined analysis bygiven XRD in analysis Figure4 .given It is well-knownin Figure 4. It that is well-known the peak at that 2 θ◦ the= 26.5 peak indicates at 2θ° = the26.5 graphene indicates characteristicthe graphene characteristic structure. The graphene peak has a lower intensity, which is decreased compared to graphite, showing that the graphite tends to have an amorphous crystal structure of graphene [40].

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Crystals 2020, 10, x FOR PEER REVIEW 5 of 12 structure. The graphene peak has a lower intensity, which is decreased compared to graphite, showing thatIn the the XRD graphite pattern tends of graphite, to have an the amorphous (002) crystal crystal plane structure of graphite of graphene was evident, [40]. In with the XRDa d-spacing pattern of graphite,3.4 Å, which the is (002) typical crystal for planegraphite. of graphite After intercalation was evident, of withgraphite, a d-spacing the produced of 3.4 Å,samples which showed is typical a ford-spacing graphite. of 3.5, After 3.3, intercalation and 3.3 Å for of G-DMSO, graphite, G-DM the producedF and G-PA, samples respectively. showed aThe d-spacing peak (002) of 3.5,crystal 3.3, andplane 3.3 of Å graphenes for G-DMSO, had G-DMF an average and G-PA, value respectively.of 3.4 Å, proving The peak the (002)formation crystal of plane graphenes of graphenes with few- had anlayered average structures. value of 3.4 Å, proving the formation of graphenes with few-layered structures.

700 GRAPHITE 600 G-PA 500 G-DMSO 400 G-DMF 300 Intensity (a.u.) 200

100

0 0 20406080 2θ°

Figure 4. XRD results of synthesized graphene products.

The thickness of graphene products was estimatedestimated byby applyingapplying Scherrer’sScherrer’s equation, which is stated as DD002002 == KKλλ/Bcos/Bcos θθ..D D002002, ,where where K, K, λλ, ,B, B, and and θθ are the thickness of the graphene, a constant based on the crystal shape (0.89), the wavelength of the X-ray (0.15406 nm),nm), the full width half maximum (FWHM)(FWHM) of of the the characteristic characteristic peak peak of graphene, of graphene, and theand scattering the scattering angle, respectivelyangle, respectively [41,42]. The[41,42]. number The number of layers of waslayers calculated was calculated by the by following the following equation: equation: N = ND =002 D/002d002/d002, where, where d d002002 isis thethe interlayer distance [43,44]. [43,44]. The The calculated calculated layer layer numbers numbers of of G-DMSO, G-DMSO, G-DMF, G-DMF, and and G-PA G-PA are are 9, 9, 10, and 20, respectively. Although the layer numbers of G-DMSO and G-DMF were very close to each other, G-PAG-PA gavegave aa higherhigher layerlayer number.number. This higherhigher layer number value may be explained by the inverse segregation ofof graphenegraphene particlesparticles synthesizedsynthesized inin PA.PA. The thickness and surface roughness values of G-DMSO, G-DMF, and G-PA were determineddetermined via AFM analysis analysis shown shown in in Figure Figure 5.5. Roughness Roughness average average (Ra) (Ra) is isdefined deﬁned as asthe the arithmetic arithmetic mean mean of the of theabsolute absolute values values of the of height the height alterations alterations from from the mean the mean line along line along the profile. the proﬁle. The square The square root of root the ofarithmetic the arithmetic mean surface mean roughness surface roughness (Rq) is also (Rq) described is also described to take into to account take into the account big peaks the and big valleys peaks and[45]. valleysAdditionally, [45]. Additionally, the roughness the roughness mean square mean (RMS) square (RMS)is calculated is calculated by using by using height height values values of ofmicroscopic microscopic peaks peaks and and valleys. valleys. As As seen seen from from Figure Figure 5,5 ,the the Ra Ra values values are are 2.94, 6.34, and 15.4515.45 nm,nm, whereas the Rq values are 3.47, 8.05, and and 17.26 17.26 nm nm for for G-DMSO, G-DMSO, G-DMF, G-DMF, and and G-PA, G-PA, respectively. respectively. Furthermore, the the RMS RMS values values are are determined determined as as5.68, 5.68, 8.84, 8.84, and and 19.29 19.29 nm nmfor G-DMSO, for G-DMSO, G-DMF, G-DMF, and andG-PA, G-PA, respectively. respectively.

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Figure 5. The Atomic Force Microscopy (AFM) images of (a) Graphene product obtained in dimethyl Figure 5. The Atomic Force Microscopy (AFM) images of (a) Graphene product obtained in dimethyl sulfoxide (G-DMSO), (b) Graphene product obtained in N,N-dimethyl formamide (G-DMF), and (c) Graphenesulfoxide (G-DMSO), product obtained (b) Graphene in perchloric product acid obtained (G-PA) in drop N,N- casteddimethyl onto formamide a glass piece (G-DMF), showing and the (c) homogeneousGraphene product structure obtained of the in pristine perchloric graphene acid (G-PA) nanosheets. drop casted onto a glass piece showing the homogeneous structure of the pristine graphene nanosheets. In order to confirm the number of graphene layers, AFM results were exploited as well. In order to confirm the number of graphene layers, AFM results were exploited as well. The The following equation was used for the calculation of layer numbers: N = (tmeasured 0.4)/0.335, following equation was used for the calculation of layer numbers: N = (tmeasured − 0.4)/0.335,− where where tmeasured is the vertical distance. The value of 0.335 nm is the thickness of the single-layer graphene.tmeasured is the Since vertical the micadistance. was The used value as a substrateof 0.335 nm material is the inthickness the AFM of measurements,the single-layer thegraphene. height Since the mica was used as a substrate material in the AFM measurements, the height was accepted was accepted as 0.4 nm [46]. The thickness (tmeasured) values of the graphene samples were measured as 1.64,0.4 nm 2.15, [46]. and The 7.28 thickness nm for G-DMSO,(tmeasured) values G-DMF, of the and graphene G-PA, respectively. samples were The measured number of as layers 1.64, 2.15, was calculatedand 7.28 nm from for theG-DMSO, aforementioned G-DMF, and equation G-PA, as respecti 3.69 4,vely. 5.22 The 5, number and 21.05 of layers 21 for was G-DMSO, calculated G-DMF, from andthe aforementioned G-PA, respectively. equation When allas the3.69 results ≅ 4, 5.22 were ≅ assessed,5, and 21.05 the ≅ roughness 21 for G-DMSO, and the layerG-DMF, number and valuesG-PA, ofrespectively. G-DMSO andWhen G-DMF all the are results smaller were than assessed, that of the G-PA. roughness The layer and numbers the layer of number graphene values products of G- obtainedDMSO and from G-DMF XRD are agree smaller with thosethan that estimated of G-PA. from The the layer AFM. numbers It can beof inferredgraphene from products these obtained findings thatfrom while XRD G-DMSOagree with and those G-DMF estimated include from fewer the layers,AFM. It G-PA can be has inferred a multi-layered from these structure. findings Inthat the while case ofG-DMSO preparation and ofG-DMF graphene include in PA, fewer the RMS layers, and layerG-PA number has a aremulti-layered 19.3 and 21.0 structure. nm, respectively. In the case For the of samplepreparation of G-DMSO, of graphene the in RMS PA, and the layerRMS and number layer are number 5.7 and are 4.0 19.3 nm, and respectively. 21.0 nm, respectively. The multilayered For the specimensample of of G-DMSO, G-PA has the a higher RMS RMSand layer value, number which is are in accordance5.7 and 4.0 withnm, respectively. the literature [The47]. multilayered It is believed thatspecimen the combined of G-PA e ffhasect a of higher high-power RMS ultrasoundvalue, which energy is in andaccordance strong acidity with the of PA literature might trigger [47]. It the is formationbelieved that of somethe combined functionalities effect of on high-power the graphene, ultrasound leading toenergy an increase and strong in the acidity surface of roughness. PA might Additionally,trigger the formation the reason of forsome the functionalities higher thickness on maythe graphene, be due to theleading out-of-plane to an increase rippling in behaviorthe surface of grapheneroughness. [48 Additionally,]. the reason for the higher thickness may be due to the out-of-plane rippling behaviorIn order of gr toaphene clarify [48]. the graphene structure, the UV-vis spectra of the graphene dispersions were recordedIn order as seen to clarify in Figure the6. graphene The synthesized structure, graphene the UV-vis products spectra were of characterizedthe graphene by dispersions comparing were the propertiesrecorded as with seen that in ofFigure CG. The6. The graphene synthesized samples graphene labeled products as G-DMSO, were G-DMF, characterized and G-PA, by comparing showed a peakthe properties at the 265-nm with wavelength that of CG. referring The graphene to sp2 C samples= C bonds labeled [49]. as G-DMSO, G-DMF, and G-PA, showed a peak at the 265-nm wavelength referring to sp2 C = C bonds [49].

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3.5 US-G-DMF 3 US-G-DMSO 2.5 US-G-PA 2 CG 1.5

1 Absorbance (arb. unit)

0.5

0 200 250 300 350 400 450 500 550 600 650 700 750 800 Wavelength (nm)

Figure 6 6.. UV-visUV-vis spectra of CG, G-DMSO, G-DMF, and G-PA products.products.

Next, thethe dynamic dynamic light light scattering scattering (DLS) (DLS) technique, techni whichque, which gives thegives apparent the apparent size of the size graphene of the samplesgraphene in samples the aqueous in the dispersion, aqueous wasdispersion, applied towas investigate applied theto investigate particle size the [50 ].particle The size size distribution [50]. The resultssize distribution are presented results in Figureare presented7. in Figure 7. Table1 gives the particle size diameters, polydispersity indexes, and zeta potentials. As seen in Table1, the Z-average hydrodynamic radius (Rh) values of G-DMSO, G-DMF, and G-PA are 6938 408, 3846 18.5, and 7137 2.5 nm, respectively. These results are parallel to the previous ± ± ± studies, which reported graphene samples with a few micrometers of Rh with a less defected structure. The graphene with these particle sizes is promising and favorable for applications in the electronics industry [28,51]. According to the measurements performed by the DLS technique, the particle domain sizes of G-DMSO and G-PA samples demonstrated bimodal distributions, with PDI values of 0.692 0.308 and 0.629 0.150, respectively. Nevertheless, the smallest particle size and PDI value ± ± were obtained as 3846 18.5 and 0.307 0.056, respectively, for the graphene sample exfoliated in ± ± DMF solvent. For G-DMSO, the major part of the material was detected at 2055 nm and the rest of the material was observed at 4103 nm. The majority of the material in the G-PA sample shows an average diameter of 1990 nm. The smaller concentration of particles presents an average size of about 4074 nm.

Table 1. Dynamic light scattering analysis of the obtained graphene samples.

Sample Name Z-Ave (d.nm) Polydispersity Index Zeta Potential (mV) G-DMSO 6938 408 0.692 0.308 8.76 15.4 ± ± − ± G-DMF 3846 18.5 0.307 0.056 11.7 3.38 ± ± − ± G-PA 7137 2.5 0.629 0.150 3.49 4.09 ± ± − ±

As it is seen from Table1, zeta potentials (mV) are measured as 8.76 15.4, 11.7 3.38, − ± − ± and 3.49 4.09 for G-DMSO, G-DMF and G-PA, respectively. For all the sonication solvents, − ± zeta potential values were negative due to the interfacial Lewis charge transfer between the graphene particles and the solvent molecules [30]. Figure8 illustrates the TEM images of graphene products of G-DMSO, G-DMF, and G-PA.

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Figure 7. Particle size analysis results of synthesized samples, (a) G-DMSO, (b) G-DMF, (c) G-PA.

TEMFigure analysis 7. Particle revealed size analysis that G-DMSOresults of synthesized and G-DMF samples, have fewer (a) G-DMSO, layers while(b) G-DMF, G-PA ( hasc) G-PA. multilayers. The light- and dark-colored parts (Figure8a,b) represent the few-layered structures at the edges of theTable sample 1 gives and the the multilayered particle size agglomeratesdiameters, polydispersity at the surface, indexes, respectively. and zeta Additionally, potentials. As Figure seen8 a,bin Tableexhibit 1, the the Z-average sheet-like ﬂakeshydrodynamic of the graphene radius (Rh) structure values overlapping of G-DMSO, at G-DMF, some parts. and FigureG-PA 8arec presents 6938 ± 408,a wrinkled 3846 ± 18.5, and and unordered 7137 ± structure2.5 nm, respectively. of G-PA, which These was results synthesized are parallel by excessive to the previous cavitation studies, in PA. whichDue toreported the high graphene acidic character samples of with PA, a crumbled few microm structure,eters of and Rh with deﬁciencies, a less defected low-quality structure. properties The graphenewere observed with these on the particle surface sizes morphology is promising of the and sample. favorable Moreover, for applications the dark-colored in the partselectronics on the industryimage of [28,51]. G-PA display According the contamination to the measurements arising from performed the residual by the solvent DLS [52technique,]. the particle domain sizes of G-DMSO and G-PA samples demonstrated bimodal distributions, with PDI values of 0.692 ± 0.308 and 0.629 ± 0.150, respectively. Nevertheless, the smallest particle size and PDI value were obtained as 3846 ± 18.5 and 0.307 ± 0.056, respectively, for the graphene sample exfoliated in

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Table 1. Dynamic light scattering analysis of the obtained graphene samples.

Sample Name Z-Ave (d.nm) Polydispersity Index Zeta Potential (mV) G-DMSO 6938 ± 408 0.692 ± 0.308 −8.76 ± 15.4 G-DMF 3846 ± 18.5 0.307± 0.056 −11.7 ± 3.38 G-PA 7137 ± 2.5 0.629 ± 0.150 −3.49 ± 4.09

As it is seen from Table 1, zeta potentials (mV) are measured as −8.76 ± 15.4, −11.7 ± 3.38, and −3.49 ± 4.09 for G-DMSO, G-DMF and G-PA, respectively. For all the sonication solvents, zeta potential values were negative due to the interfacial Lewis charge transfer between the graphene Crystalsparticles2020 and, 10, 1037the solvent molecules [30]. Figure 8 illustrates the TEM images of graphene products9 of of 12 G-DMSO, G-DMF, and G-PA.

FigureFigure 8. 8.TEM TEM imagesimages ofof graphene layers in sample sample ( (aa)) G-DMSO, G-DMSO, (b (b) )G-DMF, G-DMF, and and (c (c) )G-PA. G-PA. Scale Scale bar: bar: (a(a)) 50 50 nm, nm, ( b(b)) 50 50 nm, nm, and and ( (cc)) 200200 nm.nm.

4. ConclusionsTEM analysis revealed that G-DMSO and G-DMF have fewer layers while G-PA has multilayers. The Inlight- summary, and dark-colored the ultrasound parts assisted (Figure LPE8a, b) method represent was the used few-layered for the synthesis structures of grapheneat the edges in theof solventsthe sample of DMSO, and the DMF, multilayered and PA. agglomerates According to at the th XRDe surface, results, respectively. the layer numbersAdditionally, of the Figure samples 8a labelledand 8b exhibit as G-DMSO, the sheet-like G-DMF, flakes and G-PAof the were graphene estimated structure as 9, overlapping 10, and 20, respectively.at some parts. The Figure UV-vis 8c spectrapresents of a all wrinkled the samples and unordered showed a peakstructure at the of 265-nm G-PA, which wavelength, was synthesized indicating by the excessive sp2 C = Ccavitation bonds of graphene.in PA. Due Additionally, to the high the acidic results character of AFM of showed PA, crumbled that the layerstructure, numbers and weredeficiencies, 4, 5, and low-quality 21. The zeta potentialsproperties (mV) were were observed measured on the assurface8.76 morphology15.4, 11.7 of the3.38, sample. and Moreover3.49 4.09, the for dark-colored G-DMSO, G-DMF, parts on the image of G-PA display the contamination− ± − arising± from the residual− ± solvent [52]. and G-PA, respectively, whereas the Z-average hydrodynamic radius (Rh) was 6930 408, 3846 18.5, ± ± and 7137 2.5 nm for G-DMSO, G-DMF, and G-PA, respectively. XRD, AFM, and TEM revealed that 4. Conclusions± G-DMSO and G-DMF contain few layers while G-PA has a multilayer structure. Finally, it can be concludedIn summary, that DMSO the isultrasound a promising assisted solvent LPE for method the one-pot was used synthesis for the of synthesis few-layered of graphene graphene in by the the LPEsolvents method of DMSO, without DMF, using and any PA. surfactants According or ionicto the liquids. XRD results, the layer numbers of the samples labelled as G-DMSO, G-DMF, and G-PA were estimated as 9, 10, and 20, respectively. The UV-vis Authorspectra Contributions: of all the samplesConceptualization, showed a peak B.G., at Ç.T.-Y. the 265-nm and M.B.; wavelength, methodology, indicating B.G., Ç.T.-Y. the andsp2 M.B.;C = C software, bonds B.G., Ç.T.-Y. and M.B.; validation, B.G., Ç.T.-Y. and M.B.; formal analysis, B.G., Ç.T.-Y. and M.B..; investigation, B.G., Ç.T.-Y. and M.B.; resources, B.G., Ç.T.-Y. and M.B.; data curation, B.G., Ç.T.-Y. and M.B.; writing—original draft preparation, B.G., Ç.T.-Y. and M.B.; writing—review and editing, B.G., Ç.T.-Y. and M.B.; visualization, B.G., Ç.T.-Y. and M.B.; supervision, B.G., Ç.T.-Y. and M.B.; project administration, B.G., Ç.T.-Y. and M.B.; funding acquisition, B.G., Ç.T.-Y. and M.B. All authors have read and agreed to the published version of the manuscript. Funding: This work has been partially supported by Research Fund of the Gebze Technical University (project no. 2018-A105-55) Acknowledgments: This work has been partially supported by Research Fund of the Gebze Technical University (project no. 2018-A105-55). The authors also acknowledge the Materials Science and Engineering Department of Gebze Technical University for providing AFM and DLS measurements. Conﬂicts of Interest: The authors declare no conﬂict of interest. Crystals 2020, 10, 1037 10 of 12

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