The Chemistry and Topography of Stabilized and Functionalized Graphene Oxide Coatings

GO thin films and coatings are regarded as superior in quality to other materials especially for biomedical applications. However, the lack of stability and understanding of their structure and defects hinder their use in value added applications. Here, we stabilized GO by reduction and functionalization through multiple plasma treatments with polymerizing (to deposit a crosslinking and compressing layer of diamond like carbon, DLC) and non-polymerizing precursors (H 2 , O 2 and N 2 ). The hybrid GO and DLC coatings on semi crystalline PEEK (Polyether-ether-ketone) were evaluated using AFM, SEM and XPS. The GO deposited layer showed roughness around 70 nm and, despite care, resulted in several wrinkles and particle aggregations. The hybrid coatings conformed to the roughness and crystalline features of PEEK. XPS showed that the DLC layer cross-linked the GO nano-flakes while not completely masking which enable the partial exposure of GO. The GO-DLC hybrid interface is higher in thickness than the PEEK-GO and is dominating the overall thickness of the hybrid structure ≈ 13± 1 µm. XPS measurements showed that the often unstable C-O functional groups on the surface of the hybrid coating can be reduced by effective plasma treatment. Plasma treatments also generated C=O functional groups that probably originated from the decomposed carboxyl groups. The plasma treatment also contributed to the reduction of GO. Treatment with H 2 was more effective in oxygen reduction than with the N 2 , however, treatment with N 2 increased the reactants on GO as N 2 is heavier tending to deposit more on a surface. Plasma treatment with O 2 increased the surface oxygen content further and hence more defects on the hybrid surface.


Introduction
Graphene oxide (GO) and reduced graphene oxide (rGO), are oxide forms of graphene having the same monatomic structure and hexagonal carbon framework but with different physical and chemical properties 1, 2 ,]. These materials were extensively utilized in current research because of the superior properties and the possibility to produce bulk quantity by the exfoliation of graphite. However, GO contains a considerable amount of residual oxygen which is difficult to be removed leading to a large number of structural defects. Moreover, owing to the high polarity of GO surfaces, water molecules are easily coupled onto GO surfaces and penetrate through leading to detachment of GO flakes, therefore, the stability of GO substrates in aqueous environments is still questionable for bio-applications.
The carbon atoms in graphene are tightly packed together through the sp 2 orbital hybridization that is a combination of orbitals s, px and py constituting of the σ-bond and π-bond. Based on graphene structure, GO is non-stoichiometric, consisting of carbon sheet covalently bonded to oxygen-containing groups including epoxy (R-O-R') and hydroxyl (R-OH) groups at the basal plane and carbonyl (C=O), carboxyl (R-COOH) and lactol (O-C-O) functionalities at the edge [3][4][5][6][7] . The presence of these functional groups offer GO with several advantages, such as tunable electronic property through reduction process changing GO from insulation to conduction 8 , effective solution processability with solvents, hydrophilic characteristics, making GO ready to attract water molecules for stabilization, antirestacking ability of the carbon sheets, and anchoring ability providing reactive sites to immobilize various active species over GO sheets 9 . On the contrary, the oxygen functional groups in GO could degrades the thermal, electrical and mechanical properties relative to graphene 3, 4, 6, 10-12 . The presence of oxygenic moieties causes thermal decomposition of GO at the temperature between 150-200 °C leading to the evolution of CO and CO2 7 ]. During low temperature reduction of GO, the water molecules trapped among the layers are desorbed first in the RT -100 0C temperature range. From 100 0C to 150 0C, hydroxyl and epoxy bonds are gradually decomposed. Concurrently, carbonyl and sp2 bonds are formed 13 . Carbonyl bonds will be then degraded at higher temperatures and hence make it the most thermally stable bond.
The mechanical properties of GO are mostly related to the physical and structural properties that are the thickness and oxidation degree (the ratio of carbon to oxygen) 14,15 , therefore, the Young's modulus and tensile strength have been reported in a wide range of 6-42GPa and 76-293MPa respectively [16][17][18][19][20] .
Structural defects, intrinsic type (sp 2 -type) and extrinsic type (vacancy-type), identified in the graphitic structures of GO and rGO [16][17][18] are the critical determinants for the physical properties. Compton 3 summarized the works done on GO and rGO regarding the production methods with manipulating GO including the synthesis of transparent conductive thin films, their properties, and applications. Banhart 16 reviewed the defects types, generations, and properties of graphene and stated the electronic and optical properties of graphene can be manipulated by managing the intrinsic or extrinsic type defects. It is worth noting that the contained residual oxygen and other heteroatoms in GO and rGO degrades the mechanical strength of defect-free structures as extrinsic type defects 3 . Extrinsic defects also deviate the properties like electrical conductivity of graphite and rGO from defect-free structures, nevertheless, Dreyer and Mao 4, 6 found these defects serve as active sites for reduction-oxidation (redox) reactions.
The restoration strategy could be utilized by supplying additional carbon atoms on GO, which reduces the chemical functional groups attached to the carbon atoms and inhibits the sp 2 -interactions, was found to repair the defective graphitic structure thus improve the stability. In this respect, partial and complete reduction methods of GO including the graphitization at high temperature 21,22 , epitaxial growth 23 , charge-transfer chemical doping 24 , plasma deposition 25 , and chemical vapour deposition (CVD) 26 have been extensively studied in detail 27,28 .
The stability of GO membranes in water is rather in debate still. Yeh 29 showed that GO instability is due to the electrostatic repulsion of negatively charged GO sheets on hydration. Another study showed the stability of GO is not affected by the pH (4-10 value) of the solution, but the salt type and ionic strength have significant influences under the electrostatic layer-to-layer compression 30 . The stability of GO powdered sheets in aqueous solution can be enhanced by converting them into membrane form with few micron thickness through filtering and drying procedures 31 . While the stability of GO membranes is achieved through the crosslinking of individual GO sheets with the implementation of multivalent cationic metal contaminants 29 . Carbonaceous nanomaterials of good conductivity, chemical stability and large surface area enhances the charge storage through generating electric double layer capacitors by adsorption of ions. Another possibility is by forming a pseudo capacitor and producing energy from redox reaction during charge/discharge 32,33 . Accordingly, the intercalation of molecules like carbon nanotube (CNT) and fullerene is useful for charge storage applications by electrostatic adsorption of ions 33 .
Previous studies showed GO membranes exhibit hydration or solvation properties which is not found in their precursor GO powders 34 and GO membranes show a faster water permeation and decreased permeability for alcohols like methanol and ethanol 35,36 . Moreover, GO membranes permeated NaOH faster than Na salts indicating their expansion in alkaline solutions 37 and GO membranes emerged as efficient filters for molecular or ionic separation 35 . Chowdhury also observed the stability of GO in natural aquatic environment and indicated aqueous transportation of GO is possible and useful for effluent treatment. Hence, all the findings mentioned confirm the stability of GO is directly correlated to the carbon-to-oxygen ratio in GO sheets and their cross linkages among the GO sheets.
Polyether-ether-ketone (PEEK), a semi-crystalline thermoplastic with non-toxicity, natural radiolucency, excellent thermal and chemical stability, and good mechanical properties. PEEK have been a substitute for metal alloys in orthopedic applications [38][39][40][41][42] . However being bio-inert and hydrophobic limits its applications. Implementing diamond-like carbon (DLC) coatings on PEEK is a promising strategy for bio-applications owing to their good bio-compatibility, such as their elastic modulus close to the cortical bone in human, excellent tribological and mechanical properties [43][44][45] .
However, the adhesive strength at the interface of DLC coating and PEEK substrate poses a weakness in performing the function when high internal compressive stress and corrosive environment are present in human body. Escudeiro and Dufils 43,44 proposed incorporating with metallic compounds at the interlayers and altering the C to H ratio of the deposited thin film to improve the device performances. Bzaka et al. 46 explained that plasma assisted techniques as suitable option for controlled modification of a variety of biocompatible surfaces by surface oxidation, nitration, hydrolyzation and amination. Plasma treatment were shown to be effective method to decontaminate and sterilization of biomaterials and medically-relevant devices by generating active surfaces of plasma generated radicals.
Moreover, desirable chemistry can be obtained by surface functionalization with specific moiety like amines, hydrogen, oxygen etc. which leads to controlled morphology, desirable substrate adhesion and stability.
The use of plasma treatments with graphene based materials provides an effective means of doping, generation of reactive species and other ionic parameters which modulates the charge transfer characteristics 47 . For instance, NH3 plasma can induce nitrogen radical covalently binding with carbon lattice, oxygen plasma treatment can alter the bandgap of graphene based materials, driving transition from semi-metallic to semiconductor 48 . Conversely, certain optimized plasma treatment at controlled atmosphere (eg. Remote methane plasma) used to reduce GO by lowering its oxygenic content and decreases band gap of the material. Furthermore, plasma-treatments enabled the reduction of GO to graphene that is facilitated by significant defect healing which causes improvement in material quality 25 . Thus, oxidation and reduction reaction carried out by controlled and optimized plasma treatment can regulate defects and functional groups which is useful for enabling life cycle of carbon based precursor.
The present study employs plasma deposition to modify the surface of GO coated PEEK with DLC and implements SEM, AFM and XPS to evaluate the topography and chemical compositions of the DLC/GO solid thin film on PEEK. This engineered material is expected to achieve good stability in an aqueous environment, excellent tribological properties, and electrical characteristics for bioapplications.

Materials
Commercial graphene oxide (GO), dispersion (4mg/ml) in water obtained from Graphenea (Spain), was initially ultrasonicated at 40-50 °C for 2 hours. About 2-3ml of dispersed GO in water was drop-casted over polyether-ether-ketone (PEEK) polymer and dried at room temperature for 24 hours to produce GO/PEEK. PEEK substrates were then introduced in a plasma-enhanced chemical vapour deposition (PECVD) reactor for the deposition of diamond-like carbon (DLC) films to stabilize the GO coating.

PEEK Surface Modification
The plasma treatment apparatus that was used for this study was a custom build instrument. The plasma source is a COPRA GTE200 RF-inductively coupled plasma high density source from CCR-technology GmbH, Germany supplied with a radiofrequency at 13.56 MHz and operated at 200W. In the deposition process, the pressure inside the reactor was maintained constantly at 0.15 mbar by adjusting a butterfly valve. Different plasma precursor mixtures shown in Table 1 were utilized to obtain the DLC films deposited on GO with different chemical compositions and functional groups on their surfaces. Three groups of samples were produced for the study including plasma treated and deposited PEEK substrates (samples 1a-1f), the same plasma treatment adopted on GO-coated PEEK substrates (samples 2a-2f), and oxygen-rich precursors plasma treatment samples employed on GO-coated PEEK substrates (samples 3a and 3b). Pure methane (CH4) and a mixtures of methane and hydrogen (CH4+H2) were utilized to deposit neutral DLC films, the mixture of methane and oxygen (CH4+O2) was utilized to introduce negative carboxyl and hydroxyl functional groups while the mixture of methane and nitrogen (CH4+N2) was utilized to deposit amine-rich coatings.

XPS Characterization
The surfaces of all samples were characterized using X-ray photoelectron spectroscopy (XPS), a non-

Results and Discussion
Topographic analysis from SEM and AFM measurement Figure 1 is showing the topographic images from the SEM and AFM for different coatings namely DLC, GO and the hybrid structure of GO/DLC on the PEEK substrates. The GO has introduced the wrinkled structure (nealy 15 nm altitude) over the semi-crystalline PEEK morphology along with aggregated flakes. The DLC coatings conform the PEEK substrate (Figure 1(b)) revealing comparable values of roughness (rms, root mean square) in Table 2. The GO layers elevated in altitude at the local nucleate site of the PEEK substrate that remarkably increases the roughness up to 3 folds. Unlike DLC, the distribution of GO flakes are not uniform and the interlayer spacing between GO-layers has contributed to the increase in roughness (Figure 1(a, d)). Deposition of DLC over wrinkled and accumulated GO surface further enhances the roughness of the hybrid structure (PEEK+GO+DLC) up to 4 folds. The improper conformation between DLC and GO at the wrinkled region is responsible for such higher roughness.   The thickness and distribution of the GO/DLC over a PEEK substrate is clearly resolved in the crosssection area of the hybrid structure at Figure 2. Schematic view (Figure 2(a)) is showing the arrangement of the heterostructural coatings in which GO is sandwitched between PEEK and DLC.
Cross-section SEM micrograph in Figure 2 PEEK-GO interface, the π-conjugated structure in the GO-graphitic basal plain can form strong π-π stacking interaction with the π-conjugated system (such as benzene ring) in PEEK 49 . The functional groups associated with GO can be used to bind heterostructures for higher mechanical, thermal and chemical stability 50, 51 . Peng et al. 49 found the improvement in thermal stability and mechanical properties at PEEK-GO interface due to π-conjugated structure. It helps to transfer the stress directly from the matrix to GO and stabilize the entire system. The interface between GO-DLC comprises functional groups of -OH, -COOH, -O-bonding to produced hydrogen bonds 52 .

XPS
The elemental abundances of the plasma treated samples are summarized in Table 3 The stability of the GO films depends on the oxygen concentration present on the graphene flakes.
Oxygen defects leads to a change of the carbon hybridization from sp 2 to sp 3 preventing π-π interaction.
It is then expected that the higher is the oxygen content the lower is the coupling between flakes with decreased mechanical properties and stability. Smaller interaction between graphene sheets can also lead to risk of detachment with adverse consequences on its use. A diamond like amorphous hydrogenated films was deposited on the GO film to increase its solidity. The plasma deposition is performed that was generated using mixture of CH4, H2, O2 and N2 in different proportion. A high concentration of ions of the precursor molecules will readily react with the carbon atoms of the graphene flakes leading to the formation of an amorphous network steadily "gluing" the GO flake to flake.
The concentration of the oxygen and nitrogen, listed in Table 3 In Figure 3A the C1s core lines form pristine GO and PEEK together with the same polymer treated in plasma containing CH4, O2 and N2 are shown. As it can be seen, in the C1s from GO the highest component is the one relative to the C-OH bonds. In pure PEEK this component is replicated but with a much less extent. The PEEK sample treated with pure CH4 shows a carbon peak featured essentially by CHx components. Only a mild asymmetry on the high BE side reveals the presence of weak oxidized components. Differently, the O2 plasma treated PEEK shows a distinct C-OH component slightly higher respect to that of PEEK, and those related to carbonyl and carboxyl functional groups. Finally, the N2 containing plasma treatment shows a less structured C1s peak since the -C-N-bonds fall at ~285.7eV rather near to the hydrocarbon CHx main peak. Noticeable also difference among samples is the presence of the graphitic component only in the GO sample while in the other samples this peak is substituted by the hydrocarbon peak at 285eV.

C1s
Binding  Looking at values summarized in Table 4, it appears that the concentration of nitrogen based functional groups is significant only when N2 is present as a plasma precursor. Difference is the case of the presence of graphene on the PEEK substrate. In this case, nitrogen is present in all the plasma treatments.

C1s
Binding  The XPS analyses from Zhao 57 yet well confirms our results which the O2 plasma treatment on GO reduced the graphitic structure C=C-C while increased the other carbon-oxygen structures, C-OH, C=O and O-C=O. This implies O2 plasma introduces numerous defects in GO providing more binding sites for molecule attachment. Finally, the sample treated with a plasma containing N2 the C1s spectrum is dominated by the CHx component. At higher BE Figure 4 shows the presence of a broad tail. This feature can be assigned to multiple bonds of carbon with nitrogen and with the different bonds with oxygen. Another similar study using XPS to analyse the composition and chemical state of GO treated by H2 plasma and a gas mixture of H2 and N2 was done by Chen 58 . Various identified carbon-oxygen bonds, C-O in C-OH, epoxy type C-O-C, carbonyl type C=O, and carboxylic type O-(C=O), are similar to our results. The work also suggests C-O is unstable which can be reduced by effective plasma treatment. C=O is developed most probably from the decomposed carboxyl groups due to the plasma treatments. In terms of reducing GO, H2 shows more effective than with the N2 introduced, but on the other hand, implementing N2 increases the reactants on GO as N2 is heavier tending to deposit on a surface.
The behavior of the C1s associated to the different samples mirrors the concentration of the functional groups listed in Table 4. Peak fitting the core lines pertaining each of the samples it is possible to estimate the abundance of functional groups present on the substrate surface. These abundances will play an important role in the process of the cell adhesion and growth.

Conclusion
The plasma deposition of DLC layer over GO coating created a stable hybrid coating through the actions of crosslinking and compression. XPS showed that plasma treatment enables a much higher