3. Nitrogen and ammonia plasma

In many works, gases NH3 [38–40] and N2 [32, 41–43] are used to treat GO in the nitrogen containing plasma. Kim and et al. for nitridation of rGO used NH3 inductively coupled plasma with a power of 10 W at a pressure of 100 mTorr [38]. The content of many oxygen groups (such as epoxy, hydroxyl, carbonyl and carboxyl) attached to GO films was significantly reduced after treatment with NH3 plasma, and instead of them, C-N bonds were introduced. The ratios of the nitrogen to carbon concentrations (N/C) and oxygen to carbon concentrations (O/C) in less than 10 min became approximately 10 and 23% and were gradually saturated [38].

Nitrogen forms pyridine (pyridinic-N), pyrrole (pyrrolic-N), and graphite configurations (quaternary, graphitic-N) with carbon atoms in the graphene lattice [36, 44–47]. Pyridinic-N is bonded to two carbon atoms of the hexagonal graphene cell on the edge of vacancy-type defects and introduces 1 p-electron into the π-system; pyrrolic-N introduces 2 p-electrons into the π-system and is connected to two graphene atoms of the pentagonal cell; graphitic-N replaces the carbon atom in the hexagonal ring of graphene [36, 44, 47]. The pyridine and pyrrole configurations of nitrogen form a p-type conductivity, while graphitic N ones increase the electron concentration [44, 48]. Kim et al. [38] showed that when doping in graphene NH3 plasma, pyridine and pyrrole configurations of nitrogen are predominantly introduced, and the ratios N/C and O/C atoms are inversely related to the processing time in the plasma. The same results were received in Refs. [39, 40]. In the work of other authors [32], the concentration of pyridinic-N (48%) was found to be higher than the Pyrrolic-N (29%) and Graphitic-N (15%). At the same time, treatment with N2 plasma can lead to an increase in the amount of oxygen [10, 47].

Kim et al. [38] is shown that an increase in the electrical conductivity occurs with an increase in the exposure time in NH3 plasma with a simultaneous decrease in the optical transmission coefficient. The authors explain the increase in the conductivity from 100 S/m to 1666 S/m rGO films by the combination of the effect of nitrogen doping and the reduction of oxygen. Decreasing of the transmittance is attributed to the restoration of electronic conjugation in rGO film. The surface roughness of the films becomes smoother until the processing time reaches 10 min and further substantially does not change. The main cause of this phenomenon can be explained from the point of view of removing organic impurities in the film, reducing the functional groups of oxygen and extracting the sp<sup>2</sup> -carbon domains [38].

In [39] the plasma treatment with ammonia of GO and RGO in the reaction chamber with parallel plasma-enhanced chemical vapor deposition (RECVD) diodes was carried out. The conditions of plasma treatment were as follows: plasma power of 200 W at 13.56 MHz, a gas flow rate of 400 sccm, the substrate temperature 150<sup>о</sup> C, treatment time 1–5 min. Under these conditions, the level of doping with nitrogen in 6% was reached. Comparison of the intensities of the N1 s peaks associated with the formation of bonds C-N in XPS spectra of GO and RGO after plasma treatments in NH3 showed that the intensity of this peak in RGO than in GO. The authors explain this by the formation of C-N bonds in the interaction of oxygen groups with ammonia. The intensity ratio of Raman peaks ID/IG has a nonmonotonic dependence on the processing time. At initial treatment times (up to 1 min) this ratio increases, then reduces and further gradually enhances with increases exposure time in the plasma. The authors explain the decrease in the ID/IG ratio by the formation of an intermediate chemical species, such as hydrazine radicals. The increase can be attributed to the restoration of the sp<sup>2</sup> bonds in the GO sheets due to the NH3 plasma, which is consistent with the XPS results [39]. Studies of surface morphology, carried out in the same work, are consistent with these results. Studies of surface morphology, carried out in the same work, are consistent with these results. Measurements by atomic force microscopy (AFM) showed that exposure to NH3 plasma for 1 min leads to smoothing of the GO surface. With an increase in the processing time to 5 min, the inhomogeneity of the surface increases due to the destructive effect of the plasma. Four-probe sheet resistance measurements showed that initial treatment with a duration of 1 min leads to a sharp decrease of resistance by 6 orders of magnitude. The resistance reaches a minimum after 3 min of plasma treatment (67.5 4.5 kΩ/sq) and further is a gradual increase of the resistance is observed. The authors attribute the increase in electrical conductivity to the removal of oxygen functional groups and nitrogen doping of graphene oxide.

for the detection of amyloid-beta (Aβ) peptides, the pathological hallmarks of Alzheimer's disease, as the target analytes [9]. Zhao et al. in [6] proposed a chip based gas sensor NH3 with oxygen plasma treated GO surface. Owing to the large surface-to-volume ratio of GO and the rich chemical groups on its surface and edges, the sensitivity of the sensor to gas molecule absorption was improved. The response was further improved by oxygen plasma treatment on

In many works, gases NH3 [38–40] and N2 [32, 41–43] are used to treat GO in the nitrogen containing plasma. Kim and et al. for nitridation of rGO used NH3 inductively coupled plasma with a power of 10 W at a pressure of 100 mTorr [38]. The content of many oxygen groups (such as epoxy, hydroxyl, carbonyl and carboxyl) attached to GO films was significantly reduced after treatment with NH3 plasma, and instead of them, C-N bonds were introduced. The ratios of the nitrogen to carbon concentrations (N/C) and oxygen to carbon concentrations (O/C) in less than

Nitrogen forms pyridine (pyridinic-N), pyrrole (pyrrolic-N), and graphite configurations (quaternary, graphitic-N) with carbon atoms in the graphene lattice [36, 44–47]. Pyridinic-N is bonded to two carbon atoms of the hexagonal graphene cell on the edge of vacancy-type defects and introduces 1 p-electron into the π-system; pyrrolic-N introduces 2 p-electrons into the π-system and is connected to two graphene atoms of the pentagonal cell; graphitic-N replaces the carbon atom in the hexagonal ring of graphene [36, 44, 47]. The pyridine and pyrrole configurations of nitrogen form a p-type conductivity, while graphitic N ones increase the electron concentration [44, 48]. Kim et al. [38] showed that when doping in graphene NH3 plasma, pyridine and pyrrole configurations of nitrogen are predominantly introduced, and the ratios N/C and O/C atoms are inversely related to the processing time in the plasma. The same results were received in Refs. [39, 40]. In the work of other authors [32], the concentration of pyridinic-N (48%) was found to be higher than the Pyrrolic-N (29%) and Graphitic-N (15%). At the same time, treatment with N2

Kim et al. [38] is shown that an increase in the electrical conductivity occurs with an increase in the exposure time in NH3 plasma with a simultaneous decrease in the optical transmission coefficient. The authors explain the increase in the conductivity from 100 S/m to 1666 S/m rGO films by the combination of the effect of nitrogen doping and the reduction of oxygen. Decreasing of the transmittance is attributed to the restoration of electronic conjugation in rGO film. The surface roughness of the films becomes smoother until the processing time reaches 10 min and further substantially does not change. The main cause of this phenomenon can be explained from the point of view of removing organic impurities in the film, reducing

In [39] the plasma treatment with ammonia of GO and RGO in the reaction chamber with parallel plasma-enhanced chemical vapor deposition (RECVD) diodes was carried out. The conditions of plasma treatment were as follows: plasma power of 200 W at 13.56 MHz, a gas


10 min became approximately 10 and 23% and were gradually saturated [38].

plasma can lead to an increase in the amount of oxygen [10, 47].

the functional groups of oxygen and extracting the sp<sup>2</sup>

GO film by introducing numerous site binding defects.

3. Nitrogen and ammonia plasma

10 Graphene Oxide - Applications and Opportunities

Kumar and others investigated the effects of plasma N2 and H2 (50 sccm each) at a power of 500 W for a time of 1 hour on the GO properties [41]. The microwave plasma source was remoted from the GO sample and the temperature was raised only by 10C during plasma treatment. After plasma treatment, a slight increase in the Raman intensity ratio of the ID/IG peaks from 0.97 to 1.05 was observed. From the XPS data it was obtained that the C/O ratio increases from 2.2 to 5.2. It was found that the nitrogen introduced during the plasma treatment was 5.8 at.% of the material. The decrease in oxygen group content was confirmed not only from XPS measurements, but also from Fourier-transform infrared spectroscopy (FT-IR) spectroscopy data, as well as from other works, for example see [38]. After exposure in plasma, the intensities of the FT-IR peaks corresponding to the oxygen functionalities, such as the C〓O stretching vibration peak at 1726 cm<sup>1</sup> , was decreased dramatically.

In [9] GO was processed in an N2 inductively coupled plasma (ICP) (P = 50 W, 50 mTorr, flow rate of 10 sccm). As a result of plasma exposure, the ratio of ID/IG Raman intensities of the samples increased from 0.851, which corresponds to graphene oxide, to 1.08. The authors note that when comparing treatments in nitric and oxygen plasmas, in the latter case, significant surface distortions are observed due to the high-energy particles present in the oxygen plasma. From the results of measurements of the FT-IR spectra, it was found that treatment in plasma N2 leads to a significant decrease of the absorption band corresponding to the O-H group, in contrast to the O2 plasma treatment. Also in the IR spectra new peaks appear at 1331 cm<sup>1</sup> , which confirms the presence of the amide functional group corresponding to N-H in-plane stretching, and the peaks 1608 cm<sup>1</sup> belong to C-N bond stretching.

Lee and et al. used inductively coupled NH3 plasma with a power of 1 <sup>10</sup><sup>3</sup> W/m<sup>2</sup> at a pressure of 500 mTorr [40]. Samples were processed in the two plasma regions: the bulk plasma region (Rbulk) and the sheath region (Rsheath). In both regions, reduction and nitridation processes began immediately once the NH3 plasma was exposed to the GO films. Just like in Kim's work XPS measurements showed that in both cases a gradual increase N-pyrrolic and a decrease N-quoternary with an increase of treatment time of up to 30 min were observed. The authors also observed an increase in the ratio of N/C and a decrease of the O/C, which can be explained by the substitution of nitrogen compounds at the sites of oxygen functional groups on the r-GO films. At the same time, the electrical conductivity of the r-GO films in the bulk plasma region increased significantly after 10 min of treatment. On the other hand, the optical transmittance of the r-GO films in the Rbulk decreases gradually with increasing processing time, while for the sheath plasma region it first decreases, then after 5 min of processing starts to increase gradually. The observed effects are attributed to the fact that the reactions in each region were shown to be different. The authors explain the observed effects by the difference in the reactions in these regions. In the Rsheath, the physical reaction was dominant because of the accelerated ion bombardment by the strong electric field. In general, comparing these sample locations in the reaction chamber, the authors conclude that the reduction in the Rbulk is more effective.

Qin et al. [49] demonstrated that N2 plasma treatment is an efficient technique to prepare rGO with simultaneous introduction of N doping and ferromagnetism. The structural characterizations clearly demonstrate that the pyrrolic N bonding configuration is the main source of ferromagnetic moments. At the same time, the N2 plasma exposure time plays the key role in

Recently Wang et al. have shown that the nitrogen plasma treatment is used to modify graphene oxide to enhance the oxygen reduction reaction (ORR) performance, which implicates a burst

Zhu et al. [51] devoted much attention to the study of the electrical properties of graphene oxide exposed to ammonium and hydrogen plasmas with the addition of argon in the ratio (9:1). Field effect transistors (FET) of rGO were manufactured for the study of electrical characteristics. It was found that the action of ammonium plasma for 8, 5 min leads to electronic conduction. It was found that the action of ammonium plasma for 8, 5 min. Leads

a processing time of up to 5.5 min, the p-type conductivity predominates with a hole mobility

The results of the work performed show that treatment with nitrogen plasma leads to the reduction of graphene oxide due to the removal of oxygen containing groups, as well as to ndoping. From the analysis of the XPS spectrums, the authors believe that nitrogen atoms in the pyridine configuration are responsible for the formation of n-type conductivity. With a short plasma exposure time, oxygen doping dominates, leading to a p-type conductivity of FET

Plasmas of carbon tetrafluoromethane (CF4) and sulfur hexafluoride (SF6) are more often used for the fluorination of GO by plasma [8, 52–54]. In [8], the effect of plasma treatment of SF6 and CF4 ions on GO in the structure of an organic solar cell was investigated by methods of Raman spectroscopy, XPS, ultraviolet (UV) and infrared (IR) spectroscopy, and photoelectric characteristics measurements. For this purpose, reactive ion etching in a plasma with a power of 20 W with a duration of 10 to 60 s at a pressure of 20 mTorr was used. The results of the research showed that the use of GO films functionalized in the SF6 and CF4 plasma makes it possible to increase the conversion efficiency of solar energy from 0.56 to 2.72%. And the best result was achieved by processing in plasma SF6. From measurements of Raman spectra it follows that the ratio of the Raman spectra of ID/IG spectra as a result of plasma fluoride treatment increases slightly (from 1.17 to 1.21). This means an increase in disturbances in the graphene lattice. The XPS method revealed the presence of two peaks of C1s and F1 s in the spectra. C1s peak authors are considered responsible for the formation of C-F and C-F2 bonds with energies at 288.7 and 290.9 eV after SF6 and CF4 plasma treatment. The peak F1 s shows the presence of two components of the C-F bond with energies of 685.4 and 688.1 eV corresponding to the semimetallic and semiconductor bonds. In this case, as the authors

. At intermediate exposure times, ambipolar conductivity is observed.

1

. At the same time, with

Plasma Treatment of Graphene Oxide http://dx.doi.org/10.5772/intechopen.77396 13

open and highenergy electron/ion collision mechanism for doping and exfoliating [50].

to an n-type conductivity with an electron mobility of 5.43 cm<sup>2</sup> V<sup>1</sup> s

tuning the magnetization of nitrogen doped rGO.

of 2.1 cm<sup>2</sup> V<sup>1</sup> s

transistors.

4. Plasma fluorination

1

Mohai et al. in [42] estimated the penetration of nitrogen and argon ions into the GO using the stopping and range of ions in matter (SRIM-2013) program. Calculations have shown that at energies of 20–50 eV, the depth is equal for the two ions. Thus, plasma can only modify narrow near-surface regions.

It was shown in [43] that, as a result of nitrogen plasma treatment, the defect formation causing an increase in the intensity of the Raman peak D of the spectra depends of location of the samples in the reaction chamber. The substrate was placed "face down" into the plasma chamber (Figure 1), which significantly reduced the formation of defects in the rGO. In the investigated rGO samples, the electrical resistance increased, it is possible that this is due to the predominance of defect formation over doping and restoration of the graphene lattice.

Figure 1. Schematic view of plasma system used for nitridation of graphene oxide.

Qin et al. [49] demonstrated that N2 plasma treatment is an efficient technique to prepare rGO with simultaneous introduction of N doping and ferromagnetism. The structural characterizations clearly demonstrate that the pyrrolic N bonding configuration is the main source of ferromagnetic moments. At the same time, the N2 plasma exposure time plays the key role in tuning the magnetization of nitrogen doped rGO.

Recently Wang et al. have shown that the nitrogen plasma treatment is used to modify graphene oxide to enhance the oxygen reduction reaction (ORR) performance, which implicates a burst open and highenergy electron/ion collision mechanism for doping and exfoliating [50].

Zhu et al. [51] devoted much attention to the study of the electrical properties of graphene oxide exposed to ammonium and hydrogen plasmas with the addition of argon in the ratio (9:1). Field effect transistors (FET) of rGO were manufactured for the study of electrical characteristics. It was found that the action of ammonium plasma for 8, 5 min leads to electronic conduction. It was found that the action of ammonium plasma for 8, 5 min. Leads to an n-type conductivity with an electron mobility of 5.43 cm<sup>2</sup> V<sup>1</sup> s 1 . At the same time, with a processing time of up to 5.5 min, the p-type conductivity predominates with a hole mobility of 2.1 cm<sup>2</sup> V<sup>1</sup> s 1 . At intermediate exposure times, ambipolar conductivity is observed. The results of the work performed show that treatment with nitrogen plasma leads to the reduction of graphene oxide due to the removal of oxygen containing groups, as well as to ndoping. From the analysis of the XPS spectrums, the authors believe that nitrogen atoms in the pyridine configuration are responsible for the formation of n-type conductivity. With a short plasma exposure time, oxygen doping dominates, leading to a p-type conductivity of FET transistors.
