**3.3 Preparation of graphene sheets via ultrasound method**

0.3 g graphite was dispersed in 50 ml solvent such as DMSO, DMF and PA. Obtained dispersions were sonicated by the means of BANDELIN ® HD 2200 SONOPULS (which is given in **Figure 5**) equipped with a VS 190 T sonotrode, 200 W, 50% amplitude for 3 hours.

Then, these dispersions were subjected to 60 minutes centrifugation (Elektromag, M 4812 P) at 3000 rpm to remove the unexfoliated part of graphite; after the heavier particles were settled down, supernatant parts were decanted and collected in separate vials.

#### **Figure 4.**

*Novel Nanomaterials*

sonication process [26–29].

**Figure 3.**

*Liquid phase exfoliation.*

**3. Experimental section**

**3.1 Materials used**

stable graphite dispersion [25]. Several studies have been performed in order to find the most appropriate solvent as well as the optimum operation conditions for the

The experimental studies consist of two different methods; microwave (MW)

In microwave energy method; graphite (natural flake graphite, grade 3061; purchased from Asbury Graphite Mills, Inc., New Jersey) was used as starting carbon source. Different solvents were used such as 25% ammonia solution (Merck KGaA), N,N-Dimethyl formamide (Merck KGaA), ethylene glycol (ZAG Chemicals) and ethylene diamine (Merck KGaA). Chemicals used in the second cycle of experiments were of analytical grade; n-Hexadecane (Merck, 99.5%), dimethyl sulfoxide (Merck, 99.9%), sodium hydroxide (J.T. Baker, 99%), 1-octanol (Merck, 99%), perchloric acid (Merck, 70–72%), N,N-Dimethyl formamide (Merck, 99.8%), ethylene

Chemicals used in the ultrasound method are as follows: Graphite fine powder

The procedure of MW treatment was summarized as following: First, natural graphite is added to ammonia, then obtained suspension was sonicated by ultrasound energy device (BANDELIN ® HD 2200 SONOPULS), under conditions 200 W,

(Extra pure, Asbury Inc., New Jersey), graphene nanoplatelets (XG Sciences, Michigan, US) Dimethyl sulfoxide - DMSO (Merck), N,N-Dimethylformamide -

energy-assisted method and ultrasound (US) energy-assisted method.

glycol (ZAG Chemicals, 99.3%), and ethylene diamine (Merck, 99%).

DMF (Merck), Perchloric acid 70–72% - PA (Merck).

**3.2 Preparation of graphene sheets via microwave method**

**30**

*The experimental system with a multimode microwave furnace: Reaction was performed inside a Teflon vent-and-reseal vessel.*

**Figure 5.** *Ultrasound device.*

### **3.4 Characterization**

Different characterization techniques were applied to the obtained final products via microwave energy method in order to determine their properties such as thickness, layer number, electrical conductivity. X-ray Diffraction (XRD) analysis was done via Rigaku D-Max 2200 Series equipped with Cu-Kα radiation (λ = 1.54 Å) at a scanning rate of 3° per minute. The tube voltage was 40 kV and the current were 40 mA. The intensity was determined over a 2θ° angular range of 2–90°. Electrical conductivities of synthesized products were measured by Keithley 2400 Sourcemeter which is seen in **Figure 6**.

Each sample was measured by applying following procedure; first, it was placed in a copper cylindrical container which has a copper cap and it was compressed by a hydraulic press under 50 bar for 30 min. The electrical resistivities of obtained products were determined by 4-point probe method. Synthesized powder sample were compressed in copper mold with the help of a joiner's clamp during the electrical conductivity measurement. The conductivity σ was then estimated according to σ = l/AR. The Fourier Transform Infrared (FTIR) spectra of synthesized products were measured by Perkin Elmer Spectrum Two equipped with a germanium (Ge) crystal (Pike Gladi ATR Ge-ATR) in the range of 650–4000 cm−1. The obtained powder was characterized via ultraviolet–visible (UV–Vis) spectroscopy. For UV–vis analysis, the dried filtrate

#### **Figure 6.**

*Electrical resistivity measurement system: (a) copper cylindrical container and a copper cap. (b) Electrical resistivity measurement set-up (joiner'clamp and copper container). (c) Keithley 2400 Sourcemeter.*

**33**

*Investigation of Alternative Techniques for Graphene Synthesis*

which is dried on drying oven at overnight was dispersed in distilled water by agitating via a magnetic stirrer. After that an amount of dispersion was taken into the 10x10 mm vial then it was analyzed by comparing with the water which is reference sample. The spectrum has an operation range (UV Perkin Elmer, Lambda 35) of 200 to 700 nm.

Also, the synthesized products via ultrasound energy method were analyzed via different characterization techniques such as UV–vis spectroscopy, Atomic Force Microscopy, X-ray Diffraction and dynamic light scattering analysis. UV–vis spectral measurements were acquired using a Perkin Elmer Precisely Lambda 35 UV/ vis Spectrometer. UV–Visible spectra (Perkin Elmer, Lambda 35) were measured from 200 to 800 nm. Samples for AFM were prepared by dropping the graphene

contact (tapping) mode, with 10.00 μm scan size, and 20.35 Hz scan rate by using Digital Instruments Nanoscope. Samples for XRD were prepared by depositing onto

with a Rigaku D-Max 2200 Series equipped with Cu-Kα radiation (λ = 1.54 A°) at a scanning rate of 3° per minute. The tube voltage was 40 kV, and the current was 40 mA. Also, an extensive study of the particle size distribution was carried out by an analytical technique such as dynamic light scattering (DLS) method by using

Microwave energy-assisted method and ultrasound energy-assisted method were studied, and the final products were obtained. Synthesized carbon products were analyzed by applying different characterization techniques such as XRD,

All the results of ammonia tests were summarized in **Table 1**. According to the results; sonication did not create a positive effect on electrical conductivity of final product. Lower temperature conditions give better yield and electrical conductivity

According to these results which were given in **Table 1**, low temperature showed

Another set of experiment were done in order to compare the effect of different solvents on graphene synthesis via microwave energy. The results of microwave tests that were conducted by using N,N-Dimethyl formamide (DMF), ethylene

According to the results which were given in **Table 2**, the reaction yields of DMF, EG, and ED are 60, 88, and 75%, respectively. The electrical conductivity values of DMF, EG, and ED are 22.716, 6.0002, 7.0967 S/m, respectively. It can be concluded

XRD spectra of natural graphite, MW assisted expanded graphite products which were obtained in different solvents such as ethylene glycol, ammonia, and

According to XRD results; all the spectrums show the 002 peak of graphite was predominant in all the four types of graphite, at 2θ° = 26.44° peak, which is characteristic for graphite. Natural graphite shows highest intensity peak at 2θ° = 26.44. The intensity of other two peaks 101, 004 was low at all the spectrums. Layer

better electrical conductivity results. Sonication step built a negative effect on electrical conductivity results. Also, after annealing step, electrical conductivity

glycol (EG) and ethylene diamine (ED) were given in **Table 2**.

that; G-DMF shows better conductivity performance.

DMF were given in **Figure 7**, respectively.

Malvern Zetasizer Nano ZS Laser Particle Size Distribution Meter.

) and measurements were made in

) and X-ray diffraction (XRD) patterns were obtained

*DOI: http://dx.doi.org/10.5772/intechopen.94153*

dispersions onto glass pieces (0.7 x 0.7 mm<sup>2</sup>

**4.1 Microwave (MW) assisted method results**

glass pieces (0.7 x 0.7 mm2

**4. Results & discussion**

results slightly increased.

AFM, TEM.

results.

*Investigation of Alternative Techniques for Graphene Synthesis DOI: http://dx.doi.org/10.5772/intechopen.94153*

*Novel Nanomaterials*

**3.4 Characterization**

2400 Sourcemeter which is seen in **Figure 6**.

Different characterization techniques were applied to the obtained final products via microwave energy method in order to determine their properties such as thickness, layer number, electrical conductivity. X-ray Diffraction (XRD) analysis was done via Rigaku D-Max 2200 Series equipped with Cu-Kα radiation (λ = 1.54 Å) at a scanning rate of 3° per minute. The tube voltage was 40 kV and the current were 40 mA. The intensity was determined over a 2θ° angular range of 2–90°. Electrical conductivities of synthesized products were measured by Keithley

Each sample was measured by applying following procedure; first, it was placed in a copper cylindrical container which has a copper cap and it was compressed by a hydraulic press under 50 bar for 30 min. The electrical resistivities of obtained products were determined by 4-point probe method. Synthesized powder sample were compressed in copper mold with the help of a joiner's clamp during the electrical conductivity measurement. The conductivity σ was then estimated according to σ = l/AR. The Fourier Transform Infrared (FTIR) spectra of synthesized products were measured by Perkin Elmer Spectrum Two equipped with a germanium (Ge) crystal (Pike Gladi ATR Ge-ATR) in the range of 650–4000 cm−1. The obtained powder was characterized via ultraviolet–visible (UV–Vis) spectroscopy. For UV–vis analysis, the dried filtrate

*Electrical resistivity measurement system: (a) copper cylindrical container and a copper cap. (b) Electrical resistivity measurement set-up (joiner'clamp and copper container). (c) Keithley 2400 Sourcemeter.*

**32**

**Figure 6.**

which is dried on drying oven at overnight was dispersed in distilled water by agitating via a magnetic stirrer. After that an amount of dispersion was taken into the 10x10 mm vial then it was analyzed by comparing with the water which is reference sample. The spectrum has an operation range (UV Perkin Elmer, Lambda 35) of 200 to 700 nm.

Also, the synthesized products via ultrasound energy method were analyzed via different characterization techniques such as UV–vis spectroscopy, Atomic Force Microscopy, X-ray Diffraction and dynamic light scattering analysis. UV–vis spectral measurements were acquired using a Perkin Elmer Precisely Lambda 35 UV/ vis Spectrometer. UV–Visible spectra (Perkin Elmer, Lambda 35) were measured from 200 to 800 nm. Samples for AFM were prepared by dropping the graphene dispersions onto glass pieces (0.7 x 0.7 mm<sup>2</sup> ) and measurements were made in contact (tapping) mode, with 10.00 μm scan size, and 20.35 Hz scan rate by using Digital Instruments Nanoscope. Samples for XRD were prepared by depositing onto glass pieces (0.7 x 0.7 mm2 ) and X-ray diffraction (XRD) patterns were obtained with a Rigaku D-Max 2200 Series equipped with Cu-Kα radiation (λ = 1.54 A°) at a scanning rate of 3° per minute. The tube voltage was 40 kV, and the current was 40 mA. Also, an extensive study of the particle size distribution was carried out by an analytical technique such as dynamic light scattering (DLS) method by using Malvern Zetasizer Nano ZS Laser Particle Size Distribution Meter.

#### **4. Results & discussion**

Microwave energy-assisted method and ultrasound energy-assisted method were studied, and the final products were obtained. Synthesized carbon products were analyzed by applying different characterization techniques such as XRD, AFM, TEM.

#### **4.1 Microwave (MW) assisted method results**

All the results of ammonia tests were summarized in **Table 1**. According to the results; sonication did not create a positive effect on electrical conductivity of final product. Lower temperature conditions give better yield and electrical conductivity results.

According to these results which were given in **Table 1**, low temperature showed better electrical conductivity results. Sonication step built a negative effect on electrical conductivity results. Also, after annealing step, electrical conductivity results slightly increased.

Another set of experiment were done in order to compare the effect of different solvents on graphene synthesis via microwave energy. The results of microwave tests that were conducted by using N,N-Dimethyl formamide (DMF), ethylene glycol (EG) and ethylene diamine (ED) were given in **Table 2**.

According to the results which were given in **Table 2**, the reaction yields of DMF, EG, and ED are 60, 88, and 75%, respectively. The electrical conductivity values of DMF, EG, and ED are 22.716, 6.0002, 7.0967 S/m, respectively. It can be concluded that; G-DMF shows better conductivity performance.

XRD spectra of natural graphite, MW assisted expanded graphite products which were obtained in different solvents such as ethylene glycol, ammonia, and DMF were given in **Figure 7**, respectively.

According to XRD results; all the spectrums show the 002 peak of graphite was predominant in all the four types of graphite, at 2θ° = 26.44° peak, which is characteristic for graphite. Natural graphite shows highest intensity peak at 2θ° = 26.44. The intensity of other two peaks 101, 004 was low at all the spectrums. Layer


#### **Table 1.**

*Results of experiments that were done by using ammonia.*


#### **Table 2.**

*Microwave tests that were conducted by using DMF, EG and ED.*

#### **Figure 7.**

*XRD spectra of commercial graphite and the MW-assisted graphene products which were obtained in ethylene glycol, ammonia, and DMF.*

numbers of final products calculating by using XRD data were presented at **Table 3**. Layer numbers of expanded graphite products, which were obtained in EG, ammonia, and DMF by using MW energy, were calculated as 1.5 for all solvents. Layer number of natural graphite was calculated as 1.75 by the help of XRD results.

The results of another experiment plan which covering the usage of wide scale of solvents including n-Hexadecane (n-Hexa), Dimethylsulfoxide (DMSO), Sodium

**35**

**Table 4.**

*products.*

*Investigation of Alternative Techniques for Graphene Synthesis*

Hydroxide (50% aq.) (NaOH), 1-octanol (OCTA), Perchloric acid (PA), N,N-Dimethyl formamide (DMF), Ethylene glycol (EG), and Ethylene diamine (ED)

**Code Layer number** Ethylene glycol (EG) 1.5 Ammonia 1.5 N,N-Dimethyl formamide (DMF) 1.5 Natural graphite 1.75

larger, electrical conductivity values of synthesized products increased.

for the MW assisted graphene synthesis as seen in **Figure 8**.

**moment (Debye)**

**Solvent Dipole** 

Sodium Hydroxide (50% aq.)

N,N-Dimethyl formamide

According to the results, MW-G-DMF showed the highest electrical conductivity. Electrical conductivities of MW assisted graphene products were higher when the used chemicals have 2–4 Debye (D) dipole moments. These results are compatible with the dielectric constants and surface tensions of the used chemicals. Layer numbers were calculated by Scherrer equation and the half-width of the diffraction line β(2θ) (in rad) was taken as the experimental half-width (βexp) and was corrected for experimental broading (βinstr) as described in Saberi et al.'s study [30]. Layer numbers show distribution between 10 and 16. MW-G-EG showed the thinnest layer number with the value of 5.5, which is seen at **Table 4**. Solvents that have surface tension bigger than 40 mN/m show better layer number results. Briefly, as the surface tensions increased, layer numbers decreased. These results are supported with Hernandez et al.'s study [29]. Electrical conductivities of MW assisted graphene products were higher when the used chemicals have 2–4 Debye (D) dipole moments as seen as in **Table 4**. When the dielectric constants (ε) get

MW-G-PA showed the optimum electrical conductivity and layer number values

All XRD spectrums showed peak at 26.5° which can be seen in **Figure 9**. XRD spectra of MW- G-PA also proved that graphite peak at 26.5° shows minimum intensity.

> **Dielectric constant (**ε**)**

n-Hexadecane 0.06 2 15.81 27.47 8.174 Dimethylsulfoxide 3.96 46.7 12.36 43.54 7.581

1-octanol 1.76 3.4 14.02 27.6 1.784 Perchloric acid 2.146 115 10 69.69 20.619

Ethylene glycol 2.746 37 5.5 47.7 6.002 Ethylene diamine 1.83 16 10.61 42 7.097

*Electrical conductivities, dipole moments, layer numbers and dielectric constants of MW supported graphene* 

**Layer number**

6.832 57.5 10.33 74.35 10.664

3.86 36.7 15 37.1 22.716

**Surface Tension @ 20 °C (mN/m)**

**Elect. conductivity (S/m)**

*DOI: http://dx.doi.org/10.5772/intechopen.94153*

*Layer numbers of final products calculating from XRD results.*

were presented in **Table 4**.

**Table 3.**

*Investigation of Alternative Techniques for Graphene Synthesis DOI: http://dx.doi.org/10.5772/intechopen.94153*


**Table 3.**

*Novel Nanomaterials*

1 Natural

2 Natural

3 Natural

**Table 1.**

**Exp. No**

**Carbon source**

graphite (0.5 g)

graphite (0.5 g)

graphite (0.5 g)

> **Carbon source**

graphite (0.1 g)

graphite (0.1 g)

graphite (0.1 g)

4 Natural

5 Natural

6 Natural

**Solvent Sonication** 

25% Ammonia

25% Ammonia

25% Ammonia

> DMF (50 ml)

> EG (50 ml)

> ED (50 ml)

*Microwave tests that were conducted by using DMF, EG and ED.*

*Results of experiments that were done by using ammonia.*

**step**

30 min mode 5 power 50%

**Solvent Sonication step React.** 

10′, 200 W, 20kHz, mode 5, power 50%

10′, 200 W, 20kHz, mode 5, power 50%

10′, 200 W, 20 kHz, mode 5, power 50%

**React. Cond.**

1 bar, 50 watt

1 bar, 50 watt

200 °C, 1 bar, 50 watt

— 120°C,

— 120°C,

**Yield (%)**

**Cond.**

30 min 180°C

30 min 180°C

30 min 180°C

**Elec. cond. (S/m)**

94 52.44 58.114

89 12.8 30.647

53.5 9.06 12.047

**Yield (%)**

**E. cond. (After annealing) (S/m)**

> **Elec. cond. (S/m)**

60 22.7

88 6

75 7.1

**Exp. No**

**34**

**Figure 7.**

**Table 2.**

*glycol, ammonia, and DMF.*

*XRD spectra of commercial graphite and the MW-assisted graphene products which were obtained in ethylene* 

numbers of final products calculating by using XRD data were presented at **Table 3**. Layer numbers of expanded graphite products, which were obtained in EG, ammonia, and DMF by using MW energy, were calculated as 1.5 for all solvents. Layer number of natural graphite was calculated as 1.75 by the help of XRD results.

The results of another experiment plan which covering the usage of wide scale of solvents including n-Hexadecane (n-Hexa), Dimethylsulfoxide (DMSO), Sodium *Layer numbers of final products calculating from XRD results.*

Hydroxide (50% aq.) (NaOH), 1-octanol (OCTA), Perchloric acid (PA), N,N-Dimethyl formamide (DMF), Ethylene glycol (EG), and Ethylene diamine (ED) were presented in **Table 4**.

According to the results, MW-G-DMF showed the highest electrical conductivity. Electrical conductivities of MW assisted graphene products were higher when the used chemicals have 2–4 Debye (D) dipole moments. These results are compatible with the dielectric constants and surface tensions of the used chemicals. Layer numbers were calculated by Scherrer equation and the half-width of the diffraction line β(2θ) (in rad) was taken as the experimental half-width (βexp) and was corrected for experimental broading (βinstr) as described in Saberi et al.'s study [30]. Layer numbers show distribution between 10 and 16. MW-G-EG showed the thinnest layer number with the value of 5.5, which is seen at **Table 4**. Solvents that have surface tension bigger than 40 mN/m show better layer number results. Briefly, as the surface tensions increased, layer numbers decreased. These results are supported with Hernandez et al.'s study [29]. Electrical conductivities of MW assisted graphene products were higher when the used chemicals have 2–4 Debye (D) dipole moments as seen as in **Table 4**. When the dielectric constants (ε) get larger, electrical conductivity values of synthesized products increased.

MW-G-PA showed the optimum electrical conductivity and layer number values for the MW assisted graphene synthesis as seen in **Figure 8**.

All XRD spectrums showed peak at 26.5° which can be seen in **Figure 9**. XRD spectra of MW- G-PA also proved that graphite peak at 26.5° shows minimum intensity.


#### **Table 4.**

*Electrical conductivities, dipole moments, layer numbers and dielectric constants of MW supported graphene products.*

**Figure 8.** *Relation between layer numbers and electrical conductivity.*

**Figure 9.** *XRD spectra of MW-assisted graphene products.*

According to the UV–vis spectrums of MW-assisted graphene samples, which are presented in **Figure 10**, synthesized graphene samples, which were labeled as MW- G-PA, MW-G-NaOH, MW-G-n-Hexa, MW-G-ED, MW-G-DMSO, and MW-G-OCTA showed peak at 265 nm wavelength that referring sp2 C=C bonds. This result is in line with the previous literature [31].

#### **4.2 Ultrasound (US) assisted method results**

The US-assisted synthesized graphene products were characterized by using UV–vis spectroscopy, AFM Spectroscopy, and DLS analysis. UV–vis spectrums of US-assisted graphene products are presented in **Figure 11**. Coleman's team calculated the absorption coefficient of graphene dispersion via UV/vis spectroscopy.

**37**

**Figure 12.**

*Investigation of Alternative Techniques for Graphene Synthesis*

*UV–vis spectra of CG, US-G-DMSO, US-G-DMF, and US-G-PA products.*

*The AFM images of (a) US-G-DMSO, (b) US-G-DMF, and (c) US-G-PA drop casted onto glass piece* 

*showing the homogeneous structure of the pristine graphene nanosheets.*

*DOI: http://dx.doi.org/10.5772/intechopen.94153*

**Figure 11.**

**Figure 10.** *UV spectrums of MW based synthesized graphene products.*

*Investigation of Alternative Techniques for Graphene Synthesis DOI: http://dx.doi.org/10.5772/intechopen.94153*

*Novel Nanomaterials*

**Figure 8.**

**Figure 9.**

According to the UV–vis spectrums of MW-assisted graphene samples, which are presented in **Figure 10**, synthesized graphene samples, which were labeled as MW- G-PA, MW-G-NaOH, MW-G-n-Hexa, MW-G-ED, MW-G-DMSO, and

The US-assisted synthesized graphene products were characterized by using UV–vis spectroscopy, AFM Spectroscopy, and DLS analysis. UV–vis spectrums of US-assisted graphene products are presented in **Figure 11**. Coleman's team calculated the absorption coefficient of graphene dispersion via UV/vis spectroscopy.

C=C bonds.

MW-G-OCTA showed peak at 265 nm wavelength that referring sp2

This result is in line with the previous literature [31].

**4.2 Ultrasound (US) assisted method results**

*UV spectrums of MW based synthesized graphene products.*

*XRD spectra of MW-assisted graphene products.*

*Relation between layer numbers and electrical conductivity.*

**36**

**Figure 10.**

**Figure 11.** *UV–vis spectra of CG, US-G-DMSO, US-G-DMF, and US-G-PA products.*

#### **Figure 12.**

*The AFM images of (a) US-G-DMSO, (b) US-G-DMF, and (c) US-G-PA drop casted onto glass piece showing the homogeneous structure of the pristine graphene nanosheets.*

**Figure 13.** *Lateral size results of synthesized samples, (a) US-G-DMSO, (b) US-G-DMF, (c) US-G-PA.*

Concisely, with the help of the Beer–Lambert law, absorption coefficient (A = αcl) of graphene could be found by using dispersion at specific concentrations [29, 32–35]. UV–Vis absorbance spectroscopy was conducted at fixed wavenumbers of 253 nm for graphene. A piercing peak at 210 nm can be noticed and one more peak around 226 nm with a little bit less intensity of absorption peak is also observed due to Π-Π\* bondings of the C-C aromatic rings.

The obtained graphene samples, which are labeled as US-G-DMSO, US-G-DMF and US-G-PA, show peak at 265 nm wavelength that referring sp2 C=C bonds [31].

**39**

*Investigation of Alternative Techniques for Graphene Synthesis*

AFM characterization of final graphene products (US-G-DMF, US-G-DMSO, US-G-PA) were conducted to determine the optimal growth condition by measuring surface roughness and thickness. The AFM images of US-G-DMSO, US-G-DMF, and US-G-PA were presented in **Figure 12**. The Ra values of US-G-DMSO, US-G-DMF, and US-G-PA are 2.937, 6.343, and 10.103 nm, respectively. The Rq values of US-G-DMSO, US-G-DMF, and US-G-PA are 3.471, 8.046, and 11.748 nm, respectively. The RMS values of US-G-DMSO, US-G-DMF, and US-G-PA are 5.675, 8.842, and 11.910 nm, respectively. Vertical distance denotes the thickness of graphene and it is determined for US-G-DMSO, US-G-DMF, and US-G-PA as 1.638, 2.151, and 10.754 nm, respectively. The layer numbers were calculated via following equation:

The layer numbers of US-G-DMSO, US-G-DMF, and US-G-PA are calculated as 4, 5, and 31, respectively. According to AFM results, best result was obtained with DMSO. All these results confirmed that the US-G-DMSO materials had fewer layers

Although these techniques can determine the size of graphene products, dynamic

light scattering (DLS) is also helpful to measure the lateral size. It is an easy and quick method for evaluating the size of graphene samples [36]. The size distribution of the synthesized graphene samples using DLS are shown in **Figure 13**. Z-average hydrodynamic radius (Rh) of US-G-DMF is 3846 nm, Rh of US-G-DMSO is

6930 nm, and Rh of US-G-PA is 7137 nm. According to these results, DMF provides

Microwave (MW)-assisted method was developed. Although many solvents have been studied, carbon product, which was synthesized in DMF, showed the highest electrical conductivity. Electrical conductivities of MW-assisted graphene products were higher when the used solvents have 2–4 Debye (D) dipole moments. These results are compatible with the dielectric constants and surface tensions of the used chemicals. Layer numbers show distribution between 10 and 16. EG has minimum layer number with the value of 5.5. Solvents that have surface tension bigger than 40 mN/m show better layer number results. When the dielectric constants (ε) get larger, electrical conductivity values of synthesized products increased. As the surface tensions increased, layer numbers decreased. PA showed the optimum electrical conductivity and layer number values for the MW-assisted graphene synthesis. According to the UV–vis spectrums of MW assisted graphene samples. The obtained graphene samples, which were labeled as MW-G-PA, MW-G-NaOH, MW-G-n-Hexa, MW-G-ED, MW-G-DMSO, and MW-G-OCTA showed peak at 265 nm wavelength that

Ultrasound (US)-assisted method was studied. Graphene samples were easily synthesized via solution-based process. According to the UV–vis spectrums, all graphene products gave peak at 265 nm wavelengths, which may be caused by the ultrasonication required for proper suspension using the solution-based process. Also, as a result of AFM analyses, US-G-DMSO has four layers, US-G-DMF has five layers and US-G-PA has thirty-one layers. It can be understood that DMSO shows better solvent effect on graphite exfoliation by sonication process. Z-average hydrodynamic radius (Rh) of US-G-DMF is 3846 nm, Rh of US-G-DMSO is 6930 nm, and Rh of US-G-PA is 7137 nm. It can be concluded that, DMF provides graphene

*DOI: http://dx.doi.org/10.5772/intechopen.94153*

graphene products with smallest lateral size.

N = (tmeasured - 0.4)/0.335.

and defects.

**5. Conclusion**

referring sp2

C=C bonds.

products with smallest lateral size.

AFM characterization of final graphene products (US-G-DMF, US-G-DMSO, US-G-PA) were conducted to determine the optimal growth condition by measuring surface roughness and thickness. The AFM images of US-G-DMSO, US-G-DMF, and US-G-PA were presented in **Figure 12**. The Ra values of US-G-DMSO, US-G-DMF, and US-G-PA are 2.937, 6.343, and 10.103 nm, respectively. The Rq values of US-G-DMSO, US-G-DMF, and US-G-PA are 3.471, 8.046, and 11.748 nm, respectively. The RMS values of US-G-DMSO, US-G-DMF, and US-G-PA are 5.675, 8.842, and 11.910 nm, respectively. Vertical distance denotes the thickness of graphene and it is determined for US-G-DMSO, US-G-DMF, and US-G-PA as 1.638, 2.151, and 10.754 nm, respectively. The layer numbers were calculated via following equation: N = (tmeasured - 0.4)/0.335.

The layer numbers of US-G-DMSO, US-G-DMF, and US-G-PA are calculated as 4, 5, and 31, respectively. According to AFM results, best result was obtained with DMSO. All these results confirmed that the US-G-DMSO materials had fewer layers and defects.

Although these techniques can determine the size of graphene products, dynamic light scattering (DLS) is also helpful to measure the lateral size. It is an easy and quick method for evaluating the size of graphene samples [36]. The size distribution of the synthesized graphene samples using DLS are shown in **Figure 13**. Z-average hydrodynamic radius (Rh) of US-G-DMF is 3846 nm, Rh of US-G-DMSO is 6930 nm, and Rh of US-G-PA is 7137 nm. According to these results, DMF provides graphene products with smallest lateral size.
