Synthesis of Graphene Oxide Nanoparticles by Laser Ablation System

In this study, graphene oxide (GO) and reduced graphene oxide were synthesized by pulsed Nd:YAG laser with a fundamental wavelength (1064 nm) focused on the pure graphite target which was immersed in distilled water. Different pulse energies were applied in two cases; with and without magnetic field. The synthesized GO and rGO nanoparticles were characterized by UV-visible spectroscopy, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and atomic force microscopy (AFM) with and without magnetic field. The data show the presence of a magnetic field which illustrated increasing oxygen functional groups of GO. This caused a change in the morphology of the surface of GO, increasing crystallite size from 12.19 nm to 71.2 nm. The interlayer distance (d-space) was reduced from 0.4 nm to 0.25 nm and the absorption peaks that appeared in the spectrum were reduced and shifted toward smallest wavelengths, while the stretching vibration of the O-H group peak was shifted toward largest wavelengths.

fundamental wavelength of 1064 nm and a repetition rate of 6Hz at different pulse laser energy (500, 600 and 700mJ) with and without magnetic field was used for the synthesis of graphene nanoparticles in distilled water with 500 pulse of the graphite target. One permanent magnet which has a value of 4.4mT was used to apply the magnetic field.

Characterization of Synthesized Graphene Oxide Nanoparticles
Several experimental methods are available to characterize the synthesized GO. The absorption spectrum of GO was recorded by using UV-Vis spectrophotometer [Shimadzu model) over a wavelength range of 190-1100nm. The size and morphology of GO nanoparticles were characterized by AFM (Angstrom advanced SPM-A 3000 model) and the presence of elemental GO, in the presence and absence of the magnetic field, was confirmed through X-ray diffraction (SHIMADZ XRD 600C) analysis with Cu-kα x-ray tube (λ=1.5418A 0 ,V=40kV, I= 30mA). The interaction between distilled water and graphene nanoparticles was analyzed by Fourier transforms infrared spectroscopy (FTIR) (SHIMADZU), which provides a record of absorption of electromagnetic radiation by sample in a wavenumber range of 400-4000cm -1 .

Ultraviolet Visible Spectroscopy Analysis (UV-Visible)
The influence of laser energy on the absorption spectrum of GO nanoparticles in distilled water, in two cases of with and without magnetic field, was diagnosed by UV-VIS spectrophotometer. Figure -2 illustrates the influence of laser energy on the measurement of absorbance versus wavelength with and without magnetic field. The absorbance of GO nanoparticles was measured in the UV-Visible region (190-500nm). Figure (2A) shows that when the laser energy is 500 mJ, the absorption peak at 221nm corresponds to the transition of bonds [2]. When the laser energy was increased to 600mJ, the absorption peak was shifted from 221 to 209nm, while when laser energy was increased to 700mJ, the absorption peak remained at 221nm. The intensity of peaks related to oxygen functional groups of the GO were increased dramatically with increasing laser energy from 500 to 600mJ. This result may be attributed to the increase of oxygen functional groups of GO. When the laser energy was increased to 700mJ, the intensity of peaks related to oxygen functional groups of GO was reduced dramatically. These results imply a successful reduction of oxygen functional groups of GO.  demonstrates that when the laser energy is increased, the absorption peak appears at 211nm during the application of the magnetic field. The absorption peak was shifted to 216nm with increasing laser energy from 500 to 600mJ. With further increasing laser energy from 600 to 700mJ, the absorption peak was reduced to become 211nm. This behavior is attributed to the reduction reaction and the formation of oxygen functional groups of GO.
The intensity of peaks was increased with increasing laser energy. This result indicates the increase of oxygen functional groups of GO in the presence of a magnetic field.

Atomic Force Microscope Analysis (AFM)
Figures- (3,4) illustrate the atomic force microscopy images of GO produced by laser with different energy levels and 5 mg of graphite target in 3ml of distilled water, in both cases of with and without magnetic field.
It is clear from Figure-3 that the average thickness values of GO prepared with laser energy levels of 500, 600 and 700 mJ were 10.65, 51.66 and 25.95nm, respectively. On the other hand, the AFM images of GO nanoparticles in the presence of magnetic field show that the average thickness of GO layer was increasing with increasing laser energy; the average thickness values were 12.02, 12.43 and 30.51nm corresponding with laser energy levels of 500, 600 and 700mJ , respectively. The increase of the average thickness may be due to restocking of GO particles [2]. The comparison of Figures- (3) and (4) show that the presence of the magnetic field caused a change in the morphology of the surface. The values of average diameter and roughness of GO in the presence and absence of magnetic field are tabulated in   This table indicates that the average diameters of GO particles were decreased with increasing laser energy in the presence and absence of magnetic field. The presence of magnetic field causes to increase the average diameter of GO particles in all laser energy levels. In addition, the roughness average of GO surface showed a different behavior with different laser energy levels in the presence and absence of the magnetic field. When the magnetic field was not applied and the laser energy was increased from 500 to 600mJ, the roughness average was increased from 2.71 to 13.1nm. The further increase in the laser energy from 600 to 700mJ caused the roughness to be reduced from 13.1 to 5.9nm. With the existence of magnetic field, when the laser energy was increased from 500 to 600mJ, the roughness average was decreased from 3.14 to 2.84nm. The further increase in the laser energy from 600 to 700mJ caused an increase in the roughness average from 2.84 to 5.9 nm. The surface roughness of the prepared films is attributed to the degree of material oxidation, because of the greater oxidation degree will be at spaced more functional layers [5]. The increase and decrease of roughness with changing laser energy may be due to the increased or reduced number of oxygen functional layers, respectively.

4.
X-Ray Diffraction Analysis (XRD) XRD analysis represented a very useful characterization tool that is used to study the physical properties, the crystallographic structure and chemical composition of materials and thin films [3]. Figure-5 shows the structural properties of GO and rGO as characterized using XRD analysis.  , with and without of the magnetic field. The 2θ peak in the range 20-26 0 corresponds to the interlayer distance (d space distance) of ~ 0.4 nm that may be resulting from formation of few graphene oxide (rGO) layers. This interlayer distance indicates the removal of oxygen functional groups [2]. By using Scherrer equation, the crystallite size (C) was calculated as [22]: where θ is the diffraction angle, β is the line width at half height in the radians, and λ is the cu kα wavelength (0.15418nm). According to equation (1), the crystallite size was calculated and shown in Table- (2), the average crystallite size was equal to 12.19nm. While in the presence of magnetic field, the XRD shows the disappearance of the broad peak of the range 20-26 0 and the appearance of new peaks at 2θ in 26.5496, 31.7259 and 64.981 0 . The corresponding internal distance and crystallite size of these peaks are tabulated in Table-3. One can observe from Table-3 that the interlayer distance was decreased when the magnetic field was applied (approximately ~ 0.25nm). While the crystallite size was increased when the magnetic field was applied. This behavior may be attributed to the reduction of oxygen functional groups, which indicates the formation of rGO layers, with and without the magnetic field [22].

Fourier Transform Infrared Spectroscopy (FTIR)
The FTIR spectrum of GO demonstrated in Figure-6, when the magnetic field is not applied, at laser energy of 500mJ shows a broad peak in the high frequency region between 3000 -3700cm -1 together with a sharp peak at 1627.81cm -1 corresponding to the stretching and bending vibration of OH groups of water molecules adsorbed on graphene oxide [22][23][24]. Based on these results, it can be concluded that the sample has strong hydrophilicity. The peaks corresponding to 2921.96cm -1 and 2856.38cm -1 represent the symmetric and anti-symmetric stretching vibrations of CH 2 [2,4,6], while the absorption peak at 2354.92 cm -1 can be attributed to the stretching vibration of C=O of carboxyl group [2]. The stretching vibration of C=C double bond of graphene appears at 1641cm -1 [22,23,25]. The two absorption peaks at 1427.23cm -1 and 1444.58cm -1 correspond to the axial deformation of the C = C bonds [5]. The two absorption peaks at 1110.92cm -1 and 1035.7cm -1 correspond to the stretching vibration of C-O [7,26]. The O-H deformation is exhibited at the peaks range 1300 -1400cm -1 [2].When the laser energy was increased, FTIR spectrum showed that all absorption peaks were reduced and shifted toward the longest wavenumber. The reduction of peak intensities implies a successful reduction of the GO by using the laser beam [3].
Furthermore,, Figure-7 shows the FTIR spectra of GO at different laser energy levels in the presence of the magnetic field. The spectrum reveals absorption peaks between 300-3700cm -1 together with a sharp peak at 1623.95cm -1 which is attributed to the stretching and bending vibrations of OH groups of water molecules adsorbed on GO [22][23][24]. The two peaks at 2802.38cm -1 and 2927.74cm -1 correspond to the symmetric and anti-symmetric stretching vibrations of CH 2 [2,4,6]. The peak at 2366.49cm -1 corresponds to the stretching vibration of C=O of carboxyl group [2]. The stretching vibration of C=C double bond of graphene appears at 16439.38cm -1 [22,23,25]. The two absorption peaks at 1267.14 and 1386.72cm -1 are due to O-H deformation [2]. The axial deformation of the C = C bonds appears at 1429.15cm -1 [5]. The absorption peaks appearing at 1031.85cm -1 and 1105.14cm -1 are due to the stretching vibration of C-O [7,26]. When the laser energy was increased, the FTIR spectrum showed that the peaks that appeared in the spectra were reduced and shifted toward smallest wavelengths, while the stretching vibration of OH group was shifted to the lower wavenumber. Therefore, because not all oxygen functional groups were removed, the material was called reduced graphene oxide (rGO).    Figure 6-FTIR spectra of graphene oxide prepared at different laser energy in the absence of mgnetic field.

Conclusions
By using pure graphite as a laser target in laser ablation system, GO nanoparticles were successfully synthesized with and without magnetic field in this work. The synthesized GO nanoparticles were characterized by using several methods. The results of absorption spectrum of GO showed that the presence of magnetic field caused shifts in the absorption peaks toward longer wavelengths. However, in the absence of magnetic field, the absorption peaks were shifted toward lower wavelengths when the laser energy was increased from 500 to 600mJ, whereas there was no shift in the absorption peak when the laser energy became 700mJ. In addition, the peak intensity was increased when the laser energy was increased from 500 to 600mJ. But the intensity of the peak was decreased dramatically when the laser energy was increased to 700mJ. While in the presence of magnetic field, the absorption peaks were shifted toward longer wavelengths and the intensity of the peaks was increased with increasing laser energy. These results may be attributed to the increase of oxygen functional groups of GO in the presence of the magnetic field. The size and morphology of GO nanoparticles were characterized by AFM and suggested that the presence of magnetic field causes changes in the morphology of the surface and roughness average of GO nanoparticles. While the average diameter of GO nanoparticles was not affected by the applied magnetic field. The elemental GO in the presence and absence of magnetic field was confirmed through XRD which showed that when the magnetic field was applied, the crystallite size was increased while the interlayer distance (d-space) was reduced. This result indicates the successful penetration of oxygen functional groups between the layers. Finally, the interaction between distilled water and graphene nanoparticles was analyzed by FTIR which revealed that when the magnetic field was applied, the absorption peaks that appear in the spectrum were reduced and shifted toward smallest wavelengths, while the stretching vibration of the O-H group peak was shifted toward largest wavelength. This result implies that the presence of magnetic field has a good effect on the structure of graphene oxide.