1. Introduction

For several decades, plasma technologies are widely used in the fabrication of microelectronic structures. The main directions of the use of plasma treatments are cleaning of the surface of materials, etching, deposition and modification of the surface properties of materials (changes in hydrophilic, adhesion, conductivity and other properties). With the development of nanotechnologies, plasma methods have found application in the growing and functionalization of

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

nanomaterials, providing unique properties in combination with the economy and environmental safety in the production process. Thus, plasma technologies are widely used both for the growth of nanomaterials by the method of plasma enhanced chemical vapor deposition (PECVD) and for the modification of surface properties. A low-temperature radiofrequency (rf) plasma containing chemically active particles provides unique possibilities for this. For the functionalization and reduction of graphene oxide, various thermal, chemical, optical, and other methods are used [1–5]. Plasma treatment is effective, eco-friendly and low cost method of functionalization and reduction of graphene oxide (GO). Plasma treatment of GO is of interest for the creation of various devices such as gas sensors [6, 7], photosensitive devices [8], biological sensors [9–11], flexible electrodes [12, 13] and et al. Various methods are used to perform plasma treatments [14, 15], which can be conditionally divided into actions with "low power" and "remote" [14], also low and atmospheric pressures [15]. This largely determines the effect of plasma on the properties of graphene oxide. The most frequently used for the functionalization of graphene oxide are treatments in plasmas of nitrogen, ammonia, oxygen, methane, hydrogen and fluorine. The treatment in each of these plasmas has features related to the chemical activity of ions, activated by the action of ultraviolet radiation, by electrons and other active spices of plasma. At the same time, it should be noted that most of the work devoted to the effect of plasma on carbon nanomaterials is associated with the study of the properties of graphene and carbon nanotubes. Despite the fact that exposure in plasma leads to an effective modification of the properties of graphene oxide, a limited number of studies have been devoted to the study of its properties. Below we will consider the effects of plasma of various gases on the properties of GO, taking into account the processing conditions.

increasing processing time does not occur [31]. The content of C-O (epoxy, hydroxyl) groups was maximum after a plasma exposure for 1 min, further decreased after treatment for 5 and 10 min. In contrary, the atomic content of C(O)O (carboxyl/lactone) groups increased at the same processing times. The authors explain this by the fusion of carboxyl groups formed at short plasma treatment times and the formation of lactone groups. This process is accompanied by the loss of one oxygen atom, which leads to a decrease in the oxygen content after 5 min of treatment. With the formation of lactone groups, the authors associate the appearance

that in the IR spectra treated in GO plasma, a broad band at 3440 cm<sup>1</sup> bound to hydroxyl

The main peaks of Raman spectra of graphene oxide are G band at ~1580 cm<sup>1</sup> wavelength and D band at 1530 cm<sup>1</sup> [18, 33]. The band G is the result of intraplane oscillations of the bound atoms of sp<sup>2</sup> whereas the D band is due to non-planar oscillations due to the presence of

the structure graphene, are significantly increases the intensity of the D peak [18, 33]. When graphene is exposed in plasma, the intensity of the D band (ID) Raman spectra are significantly increased [26, 27, 34, 35]. Even a short-term O2 plasma exposure for up to 5 s at a power of 2 W leads to a sharp increase ID [34]. The change in Raman intensities associated with the increase in defects occurs also for rGO treated in oxygen plasma [9, 20, 32]. These changes are associated with an increase in the content of oxygen groups and defects formed during plasma oxidation. An increase of the D band intensity is also associated with the formation of sp<sup>3</sup>

An investigation of the electrical conductivity of rGO treated in O2 plasma showed its decrease both with increasing plasma power and processing time [30]. At high powers and long processing times it becomes impossible to measure the conductivity because it was below the

measurements showed that most of the rGO disappears [30]. The authors explain this by the fact that the oxygen plasma destroys sp<sup>2</sup> domains in rGO and introduces various oxygen groups to the sites of rGO defects, which leads to etching of rGO with conversion him to amorphous carbon. At the same time, under 'gentle' processing conditions, it is possible preserve the electrical conductivity of graphene oxidized by plasma [22]. It should be noted that the production of GO from graphene by the plasma treatment method makes it possible to obtain GO with an almost undamaged surface, high hydrophilicity and increased adhesion of

Gokus et al. show that strong photoluminescence (PL) can be induced in single-layer graphene using an oxygen plasma treatment [36]. The samples are then exposed to oxygen/argon (1:2) rf

Surface morphology studies after low-pressure oxygen plasma at 50 W showed that the surface of the GO becomes more porous and corrugated [9, 37]. This leads to an increase in its wettability for both graphene oxide [20] and graphene [22, 27], treated in oxygen plasma. The increase of the wettability allows to improve its surface reactivity with respect to biomolecular interactions [24]. Oxygen-plasma-treated rGO surfaces were employed as reactive interfaces

structural defects. In the presence of defects, vacancies and disorderings in the sp<sup>2</sup>

. It should be noted

Plasma Treatment of Graphene Oxide http://dx.doi.org/10.5772/intechopen.77396 9

). Scanning electron microscopy (SEM)



of new absorption lines in IR spectra at wavelengths at 1730 and 1436 cm<sup>1</sup>

groups is significantly enhanced [20, 32].

measurement range of the instrument (<0.001 Sxcm<sup>1</sup>

plasma (0.04 mbar, 10 W) for increasing time (1–6 s).

hybridized bonds [9, 22].

the surface [21–23].

### 2. Oxygen plasma

Oxygen plasma treatment of graphene oxide can be used to etch surface layers [9, 15–19], to functionalize [6, 9, 20] or to obtain GO from graphene by oxidation [21–25]. On the other hand, the oxygen plasma is very chemically aggressive, usually leading to significant changes in the structural and electronic properties of graphene: a high degree of disorder is induced in the graphene lattice even at low power and after a very short exposure time [26]. At the same time, the intensity of defect formation under the influence of oxygen plasma can be significantly reduced by using mild plasma treatments, which use small power, remote location from the plasma source of samples, the use of protective filters and grids [9, 21, 22, 26–29]. As measurements by the XPS method have shown, the effect of low-pressure O2 plasma on reduced graphene oxide (rGO) leads to the introduction of different oxygen-containing groups [20, 30]. Similar results were obtained when graphene was exposed to oxygen plasma [9, 15, 21, 22]. Oxygen plasma treatment introduces epoxy (C-O-C) and carboxyl (C-OH) groups in the basal plane and edges of graphene, with the epoxy group being the most energetically favorable [1, 8, 17]. This process is accompanied by a decrease in the carbon regions of sp<sup>2</sup> -hybridized bonds [27, 30]. Detailed high resolution X-ray photoelectron spectroscopy (XPS) rGO studies after oxygen plasma treatment with a power of less than 50 W have shown that a monotonous increase in the amount of oxygen and a decrease in the carbon content with increasing processing time does not occur [31]. The content of C-O (epoxy, hydroxyl) groups was maximum after a plasma exposure for 1 min, further decreased after treatment for 5 and 10 min. In contrary, the atomic content of C(O)O (carboxyl/lactone) groups increased at the same processing times. The authors explain this by the fusion of carboxyl groups formed at short plasma treatment times and the formation of lactone groups. This process is accompanied by the loss of one oxygen atom, which leads to a decrease in the oxygen content after 5 min of treatment. With the formation of lactone groups, the authors associate the appearance of new absorption lines in IR spectra at wavelengths at 1730 and 1436 cm<sup>1</sup> . It should be noted that in the IR spectra treated in GO plasma, a broad band at 3440 cm<sup>1</sup> bound to hydroxyl groups is significantly enhanced [20, 32].

nanomaterials, providing unique properties in combination with the economy and environmental safety in the production process. Thus, plasma technologies are widely used both for the growth of nanomaterials by the method of plasma enhanced chemical vapor deposition (PECVD) and for the modification of surface properties. A low-temperature radiofrequency (rf) plasma containing chemically active particles provides unique possibilities for this. For the functionalization and reduction of graphene oxide, various thermal, chemical, optical, and other methods are used [1–5]. Plasma treatment is effective, eco-friendly and low cost method of functionalization and reduction of graphene oxide (GO). Plasma treatment of GO is of interest for the creation of various devices such as gas sensors [6, 7], photosensitive devices [8], biological sensors [9–11], flexible electrodes [12, 13] and et al. Various methods are used to perform plasma treatments [14, 15], which can be conditionally divided into actions with "low power" and "remote" [14], also low and atmospheric pressures [15]. This largely determines the effect of plasma on the properties of graphene oxide. The most frequently used for the functionalization of graphene oxide are treatments in plasmas of nitrogen, ammonia, oxygen, methane, hydrogen and fluorine. The treatment in each of these plasmas has features related to the chemical activity of ions, activated by the action of ultraviolet radiation, by electrons and other active spices of plasma. At the same time, it should be noted that most of the work devoted to the effect of plasma on carbon nanomaterials is associated with the study of the properties of graphene and carbon nanotubes. Despite the fact that exposure in plasma leads to an effective modification of the properties of graphene oxide, a limited number of studies have been devoted to the study of its properties. Below we will consider the effects of plasma of various gases on the properties of GO, taking into account the processing conditions.

Oxygen plasma treatment of graphene oxide can be used to etch surface layers [9, 15–19], to functionalize [6, 9, 20] or to obtain GO from graphene by oxidation [21–25]. On the other hand, the oxygen plasma is very chemically aggressive, usually leading to significant changes in the structural and electronic properties of graphene: a high degree of disorder is induced in the graphene lattice even at low power and after a very short exposure time [26]. At the same time, the intensity of defect formation under the influence of oxygen plasma can be significantly reduced by using mild plasma treatments, which use small power, remote location from the plasma source of samples, the use of protective filters and grids [9, 21, 22, 26–29]. As measurements by the XPS method have shown, the effect of low-pressure O2 plasma on reduced graphene oxide (rGO) leads to the introduction of different oxygen-containing groups [20, 30]. Similar results were obtained when graphene was exposed to oxygen plasma [9, 15, 21, 22]. Oxygen plasma treatment introduces epoxy (C-O-C) and carboxyl (C-OH) groups in the basal plane and edges of graphene, with the epoxy group being the most energetically favor-

able [1, 8, 17]. This process is accompanied by a decrease in the carbon regions of sp<sup>2</sup>

ized bonds [27, 30]. Detailed high resolution X-ray photoelectron spectroscopy (XPS) rGO studies after oxygen plasma treatment with a power of less than 50 W have shown that a monotonous increase in the amount of oxygen and a decrease in the carbon content with


2. Oxygen plasma

8 Graphene Oxide - Applications and Opportunities

The main peaks of Raman spectra of graphene oxide are G band at ~1580 cm<sup>1</sup> wavelength and D band at 1530 cm<sup>1</sup> [18, 33]. The band G is the result of intraplane oscillations of the bound atoms of sp<sup>2</sup> whereas the D band is due to non-planar oscillations due to the presence of structural defects. In the presence of defects, vacancies and disorderings in the sp<sup>2</sup> -domain of the structure graphene, are significantly increases the intensity of the D peak [18, 33]. When graphene is exposed in plasma, the intensity of the D band (ID) Raman spectra are significantly increased [26, 27, 34, 35]. Even a short-term O2 plasma exposure for up to 5 s at a power of 2 W leads to a sharp increase ID [34]. The change in Raman intensities associated with the increase in defects occurs also for rGO treated in oxygen plasma [9, 20, 32]. These changes are associated with an increase in the content of oxygen groups and defects formed during plasma oxidation. An increase of the D band intensity is also associated with the formation of sp<sup>3</sup> hybridized bonds [9, 22].

An investigation of the electrical conductivity of rGO treated in O2 plasma showed its decrease both with increasing plasma power and processing time [30]. At high powers and long processing times it becomes impossible to measure the conductivity because it was below the measurement range of the instrument (<0.001 Sxcm<sup>1</sup> ). Scanning electron microscopy (SEM) measurements showed that most of the rGO disappears [30]. The authors explain this by the fact that the oxygen plasma destroys sp<sup>2</sup> domains in rGO and introduces various oxygen groups to the sites of rGO defects, which leads to etching of rGO with conversion him to amorphous carbon. At the same time, under 'gentle' processing conditions, it is possible preserve the electrical conductivity of graphene oxidized by plasma [22]. It should be noted that the production of GO from graphene by the plasma treatment method makes it possible to obtain GO with an almost undamaged surface, high hydrophilicity and increased adhesion of the surface [21–23].

Gokus et al. show that strong photoluminescence (PL) can be induced in single-layer graphene using an oxygen plasma treatment [36]. The samples are then exposed to oxygen/argon (1:2) rf plasma (0.04 mbar, 10 W) for increasing time (1–6 s).

Surface morphology studies after low-pressure oxygen plasma at 50 W showed that the surface of the GO becomes more porous and corrugated [9, 37]. This leads to an increase in its wettability for both graphene oxide [20] and graphene [22, 27], treated in oxygen plasma. The increase of the wettability allows to improve its surface reactivity with respect to biomolecular interactions [24]. Oxygen-plasma-treated rGO surfaces were employed as reactive interfaces 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 GO film by introducing numerous site binding defects.

flow rate of 400 sccm, the substrate temperature 150<sup>о</sup>

graphene oxide.

stretching vibration peak at 1726 cm<sup>1</sup>

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

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

, 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.

C, treatment time 1–5 min. Under these

Plasma Treatment of Graphene Oxide http://dx.doi.org/10.5772/intechopen.77396 11

,
