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An Effective Non-Covalent Functionalization of Poly(ethylene-glycol) to Reduced Graphene Oxide Nanosheets through #-Radiolysis for Enhanced Lubrication Bhavana Gupta, Niranjan Kumar, Kalpataru Panda, Ambrose A. Melvin, Shailesh Joshi, Sitaram Dash, and Ashok Kumar Tyagi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08762 • Publication Date (Web): 08 Jan 2016 Downloaded from http://pubs.acs.org on January 12, 2016

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The Journal of Physical Chemistry

An Effective Non-Covalent Functionalization of Poly(ethylene-glycol) to Reduced Graphene Oxide Nanosheets through γ-Radiolysis for Enhanced Lubrication Bhavana Gupta,a Niranjan Kumar,*a Kalpataru Panda,b Ambrose A. Melvin,c Shailesh Joshi,d Sitaram Dash,a Ashok Kumar Tyagia

a

Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, India

b

c

Graduate School of Engineering, Osaka University, Osaka, Japan

Catalysis Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune, India

d

Radiological Safety Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, India

*Corresponding Author: Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, Tamil Nadu, India Tel.: +91 44 27480500 (ext. 22537) Fax: +914427480081 Email: [email protected],

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ABSTRACT: High quality reduced graphene oxide (rGO) nanosheets (NS) were synthesized by oxidation of graphite followed by hydrazine treatment for reduction of oxygen functionality. The γ-radiolysis was used for functionalization of poly(ethylene glycol 200) (PEG200) with rGO-NS. The functionalization exhibits intercalation of PEG200 molecules in rGO through hydrogen bonding between the hydroxyl groups of rGO and oxygen of PEG200 molecules. This resulted increase in d-spacing of graphene sheet while defect density of carbon network is decreased in rGO. Friction coefficient and wear of the sliding steel surfaces was reduced to 38% and 55%, respectively, while using 0.03 mg mL–1 of PEG200 functionalized-rGO dispersed in PEG200. The lubrication properties are described by bipolar interaction between PEG200 and rGO, leading to effective dispersion. Chemical analysis of wear particles showed decomposition of rGO into nanosized graphite domains which was exhibited by mechanical energy produced in tribo-contact. Moreover, these domains formed effective and stable tribofilm on the steel wear track which easily shears under the action of contact stress. This significantly enhanced antifriction and anti-wear properties which results improved oxidation resistance of PEG200 under the tribo-contact. It was investigated that at high rGO concentration, the lubrication efficiency reduces due to graphene-graphene inter-sheet collision, producing mechanical energy and chemical defects at contact interfaces.


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INTRODUCTION Carbon-based materials are widely used for tribology related application in several mechanical components.1 These have various allotropes including diamond, graphite and graphene. Two distinct behaviors, belonging to hard and soft properties, are derived from the carbon allotrope which is based on chemical bonding and crystal structure. The three dimensional sp3 bonding in diamond yields the hardest and the pseudo-two-dimensional sp2 bonding in graphite exhibits one of the softest solid materials. Having quite different properties, both the allotropes of carbon exhibit remarkably low friction and wear.2,3 In diamond, generally low friction and extreme wear resistance is derived from strong, stiff, saturated and directionally coordinated tetrahedral sp3 structure. In graphite, it originates from weak van der Waals interaction, passivated dangling bonds, and incommensurability between rotated graphite layers.3,4 Graphene, a unique allotrope of carbon, is a low dimensional structure of graphite, which consists of a one-atom-thick planar sheet comprised of an sp2 bonded carbon having exceptionally high crystalline structure. It has several applications due to its remarkable physicochemical properties,5-8 due to high specific surface area,5,9 high electron mobility,10,11 excellent thermal-electrical conductivities12 and high mechanical strength.13 One of the most exotic properties of graphene pertains to super low friction behavior which opens up the concept of zero wear. Significant enhancement in tribological properties of bilayer graphene was observed under microscopic loading conditions and this was explained by reduction of electronphonon coupling.14,15 Under macroscopic loading condition, it showed improved lubrication properties due to the formation of stable graphene tribofilm which shears easily at the sliding interface.16-19



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In most of the applications, solid lubricant is not technically practicable due to the extreme contact pressure, high loading condition, specific design of the components and harsh working environment. Therefore, it is necessary to develop environmentally clean, energetically efficient green liquid lubricants. The automobile industry faces challenges due to emission of carbon monoxide and other harmful gases that increases global warming.20 Minimization of the above mentioned harmful substances is possible by improving fuel efficiency by reducing the frictional resistance of components. It is investigated that the graphene-based additives in lube medium effectively reduce friction and improve wear resistance of the contacting surfaces.19,21,22 However, dispersion of graphene in the lube medium is not chemically feasible due to the difference in cohesive energies. Therefore, it is necessary to functionalize graphene and make it hydrophilic, thereby allowing it to interact covalently/non-covalently with the lube medium. It is reported that uniform dispersion of reduced graphene oxide (rGO) in the base oil is effective when polar functional groups such as –OH and –COOH are introduced during rGO synthesis.23,24 The use of covalent or non-covalent approaches to realize functionalized graphene sheets with polymeric chains enhances their dispersion ability.25,26 Covalent functionalization of graphene sheets produces defects within the conjugated sheet causing degradation of physical properties. However, the physical structure is restored when functionalized polymers interact with GO through non-covalent bonding such as hydrogen bonding or other electrostatic forces.25,27 The oxygenated groups in rGO strongly influence its electronic, mechanical, and electrochemical properties. Presence of oxygen functional groups provides potential advantages for using rGO. Such group renders it strongly hydrophilic thereby making it compatible to numerous applications. The polarity increases in functionalized rGO nano-sheet and it can be dispersed through the use of a dispersant, thereby avoiding further chemical reaction. Recent studies show


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that interaction between carbon nanotube and the polymer matrix was enhanced through γirradiation. The γ- rays are high frequency electromagnetic radiations having high energy and short wavelength. The irradiation improved crystallinity, thermal stability and Young’s modulus and induces structural and molecular transition of nanostructured graphene.28 This technique is being studied as a clean and facile method for modifying the nanostructure and properties of carbon materials besides promoting surface chemical reactions. Recently, a γ-irradiation based novel approach was used to tune the physicochemical properties of carbon based materials.29 Results indicate that in graphene, carbon lattice is strongly affected by γ-irradiation and the ensuing materials experience small variations in their oxygen content. This technique has been used in different aqueous media including PEG for reduction of graphene oxide (GO) and its functionalization via intercalation, grafting and exfoliation.30-33 Important advantages of γ-ray synthesis compared to other techniques are summarized as: (a) reduction can be carried out without using excess reducing agent or without producing undesired oxidation products of the reductant (b) reducing agent is uniformly generated in the solution (c) radiation is absorbed regardless of the presence of light-absorbing solutes and products (d) green and facile method (e) simplicity and (f) cost effective. The presence of a small amount of dispersed graphene additives in the lube medium ensures efficient reduction of friction and improvement in anti-wear properties.23,34-36 Such improvement is facilitated by the deposition of graphene/graphite sheets at contact interfaces under the action of shear force.19,37,38 However, the lubrication mechanism of dispersible graphene nanosheets towards friction reduction and protection of contacting interfaces is not fully understood. Recently, investigation showed effective lubrication of dispersed graphite nanosheets while depositing the graphite particles in the contact surfaces.38 However, change in



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chemical characteristics and morphology of graphene/graphite based lubricating particles are yet to be investigated. It was shown that the minute difference in concentration of nanoscale additives in lube medium greatly influences friction and wear behavior.19,22,23,36,38 However, the mechanistic role of graphene concentration on friction and lubrication as lube additives is unclear. The present investigation aims to divulge some of these important above mentioned issues which certainly will improve fundamental understanding of lubrication chemistry of graphene/graphene oxide based system. To gain insight into the problems, γ-irradiated poly(ethylene glycol) (PEG200) functionalized high quality rGO-NS were synthesized. Oxygen functionalization of rGO was carried out by attaching hydroxyl group of the intercalated PEG200 molecules through hydrogen bonding. Concentrations of rGO in PEG200 and γ-irradiation doses are optimized to obtain improved lubrication properties. To disclose the lubrication mechanism, the wear particles and tribofilm at contact interface was investigated by micro-Raman spectroscopy and XPS analysis. Wear track dimension and wear rate were calculated to evaluate the lubrication efficiency.

EXPERIMENTAL METHODS Synthesis of rGO and Functionalization of PEG200 by γ- Radiolysis. GO synthesized by Hummer’s method was reduced to rGO by hydrazine treatment. For GO synthesis, graphite powder (3.0 g) was dispersed in 150 ml of 1 M H2SO4 with constant mild stirring at room temperature for 1 hr under ice cold condition. KMnO4 (8.0 g) was added slowly at room temperature with constant stirring for 2 hrs and 90 ml of water was subsequently added in the reaction mixture followed by heating at 90 °C for 12 hrs. After 12 hrs of heating, 30 ml of 30% H2O2 was slowly mixed in the diluted reaction mixture with constant stirring for another 2 hrs. 6 ACS Paragon Plus Environment

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After completion of reaction, resultant brown color mixture was filtered using Whatsman filter paper fixed in a vacuum filtration unit. Solid precipitates were washed 3 times with 200 ml distilled water followed by 100 ml of 30% HCl and 100 ml of ethanol. Finally, the precipitate was collected in a Petri Dish and dried at 60 °C under vacuum condition. In the next step, the reduction of GO to rGO was carried out by hydrazine treatment of GO. For synthesis, the 100 mg of GO in 30 ml water was treated with 3 ml hydrazine and refluxed at 95 °C for 24 hrs. After completion of reaction, precipitate was filtered, washed and dried by the same procedure as adopted in case of GO. The synthesis of PEG200 incorporated rGO was carried out by γirradiation technique. In this technique, 40 mg of rGO was dispersed in 40 ml of PEG200 and the resultant mixture was sonicated for 1 hr. After sonication, reaction mixture was N2 purged for 5 min to remove extra O2 mixed in the reaction mixture. A homogeneous dispersion of rGO in PEG200 was performed inside a γ- irradiation chamber under sealed condition. The γ-irradiation was carried out for 18, 36 and 54 hrs. Source of γ-irradiation was

60

Co which provided a γ-ray

dose of ~5 KGay/hr. It is noted that after completion of γ-irradiation, the samples was filtered, washed and dried by the same procedure as adopted in case of GO and rGO. For simplicity, the samples with γ-irradiation dose for 0, 18, 36 and 54 hrs are designated as PEG-rGO-NS0, PEGrGO-NS1, PEG-rGO-NS2 and PEG-rGO-NS3, respectively. Such nanocrystalline functionalized entities obtained from radiolysis are termed as non-covalently-PEG-functionalized-rGO (NCPEG-f-rGO). This procedure is a green and facile route to synthesize and functionalize rGO-NS for tribology applications. In this process, PEG200 solvent undergoes high-energy γ-radiation exposure resulting in formation of ions, excited molecules, electrons and free radicals. The electrons can be further solvated with PEG200 solvent molecules. The high reducing activated electrons or solvated electrons are responsible for the reduction of oxygen functionalities,31 and



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therefore, PEG200 radicals react with rGO-NS, leading to functionalization through hydrogen bonding. Selection of PEG200 for rGO functionalization is useful due to its ability to withstand solid–liquid organic phase change, high degree of phase change enthalpy, chemical stability, improved hydrophilicity and modified melting temperature.31,39 These properties of functionalized rGO renders them quite desirable for tribology application. It is well known fact that in several tribology applications, the heat generated from the sliding devices cause lubricant to degrade. Therefore, thermal and phase stabilities are essential to prevent degradation of lubricant. Characterization Techniques. Morphology and microstructure of PEG-rGO-NS samples were analyzed by field emission scanning electron microscope (FESEM, Zeiss Supra 55) and high resolution transmission electron microscope (HR-TEM). Crystallographic structure was analyzed by X-ray diffraction (XRD) using Cu Kα radiation (λ= 0.15418 nm). Chemical composition and bonding aspects was investigated by XPS (ULVAC-Phi ESCA1800) using a monochromatic Mg Kα source (400 W, 1253.6 eV). Chemical structure of samples including wear particles and tribofilm analysis was investigated by micro-Raman spectrometer (Ranishaw) operating in laser wavelength of 514.5 nm. Buried lubricant was processed in solid form to collect the wear debris for chemical analysis using micro-Raman spectroscopy and XPS. An FTIR spectrometer (Bruker Optics), operating in transmission mode with a spectral resolution 4 cm−1 was used for the analysis of functional groups present in PEG-rGO-NS samples. This was also used to analyze the degradation products like solid lubricants and wear debris collected from the contact zone. Thermo-gravimetric analysis (TGA) was carried out using a thermal analyzer (Shimadzu TA-60). This was analyzed in the temperature range of 25−800 °C at a heating rate of 20 °C /minute under a steady flow of nitrogen gas. Wear track dimension and morphology were


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characterized by an optical microscope and FESEM. Contact angle measurement of PEG200 before and after γ–radiolysis and PEG200 functionalized rGO dispersed in PEG200 was carried out by contact angle meter (Holmarc, HO-IAD-CAM-01, and India) equipped with CCD camera. Volume of the droplet used for contact angle measurement was approximately 1 µL. Furthermore, the water contact angle measurement was carried out on solid and smooth surface of rGO and PEG200 functionalized rGO pallet samples. The measurement was performed at ambient conditions and room temperature. Standard deviations in contact angle measurements were typically ± 1.2o. The images obtained were analyzed to measure the contact by using the Image J software. The supernatant rGO nanofluid decanted at specified time intervals to evaluate absorbance using a UV–VIS spectrophotometer. Based on the Lambert–Beer law, the absorbance is described as proportional to concentration. Therefore, the dispersion stability of rGO in PEG200 was evaluated by assessing the absorbance. The PEG200 was used as synthetic lube base oil for tribology evaluation of PEG-rGONS. There are mainly two reasons for selecting the PEG200 as a dispersive medium of rGO nanoadditives. Firstly, the PEG200 is chemically compatible with rGO where both the substances consist of polar groups. Secondly, the PEG is widely used for tribology application in biomedical application for reducing friction and wear of knee joint.40 The PEG is considered as friction and wear reducing additives in the base oil and also it acts as a dispersive lube medium for the dispersion of various lubrication additives.41–43 Before the tribology test, the PEG200 functionalized rGO was thoroughly dispersed in the PEG200 by ultrasonication process and their dispersion stability was monitored for 2 days. The tribo-evaluation of dispersed graphene was carried out by measuring friction and wear using a ball-on-disc standard tribometer (CSM Instrument, Switzerland) operating in linear reciprocating mode. A 100Cr6 spherical steel ball



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with diameter 6 mm was used as a sliding body against the lubricated steel (316 LN) disc. Surface roughness of ball and disc were approximately 32 and 17 nm, respectively. The sliding ball was mounted in a holder which was connected through a stiff lever with the friction force transducer. The friction coefficient was determined by a linear variable differential transformer (LVDT) sensor, measuring the deflection in the elastic arm. All experiments were carried out at a constant load of 1 N, linear sliding speed of 3 cm/s, stroke length of 4 mm and sliding distance of 100 meters. In each tribo-test, two drops of ultrasonicated PEG-rGO-NS was dispersed in PEG200 lubricant that was used to thoroughly lubricate the sliding interfaces. It is noted that two drops of tribo-test sample contains powder form of γ-irradiated PEG-rGO-NS in various concentration (0.02 to 2 mg mL–1) dispersed in PEG200 for tribology evaluation. More detail tribology experimental procedure is given elsewhere.19,23 To calculate the wear rate, the volume of the wear track was estimated by LVDT sensor and optical macrograph. Three repetitive measurements were performed for each wear of the disks, and the averaged values are reported.

RESULTS AND DISCUSSION Morphology and Microstructure of PEG200 Functionalized rGO Samples. The interaction of high-energy γ-irradiation with PEG200 molecules produces excited molecules, ions, electrons and radicals. The irradiation generated electrons are solvated with PEG200 solvent molecules and the solvated electrons further reduce oxygen functional groups present in rGO. In order to investigate tribological properties, two distinct samples such as non-irradiated PEG-rGO-NS0 and γ–radiolysis treated PEG-rGO-NS3 samples were used for morphological, structural and chemical analysis. The PEG-rGO-NS0 shows a diffraction peak of (0 0 2) plane at 24.5o 2θ which is shifted to 22.6o 2θ in PEG-rGO-NS3 sample.44 As inferred from XRD data, the 10 ACS Paragon Plus Environment

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interlayer lattice spacing of (0 0 2) plane in PEG-rGO-NS0 is calculated to be 0.36 nm. This value increased to 0.4 nm in PEG-rGO-NS3 sample (Figure 1). The increase of the interlayer spacing is attributed to the intercalation of PEG200 segments into rGO matrix.33

Interlayer spacing Sheet thickness

(b)

(a)

Figure 1. XRD of (a) PEG-rGO-NS0 and (b) PEG-rGO-NS3 samples. In inset, the typical interlayer spacing and sheet thickness are represented.

FWHM of (0 0 2) plane of PEG-rGO-NS3 is narrow which signifies increase in thickness of the graphene sheet along the c-axis. XRD results show that total thickness of graphene sheet is approximately 2 nm in PEG-rGO-NS0 and this value further increased to approximately 3 nm in PEG-rGO-NS3 sample. The diffraction from (1 0 2) plane are observed at 43o 2θ and 42.8o 2θ for PEG-rGO-NS0 and PEG-rGO-NS3 samples, respectively. The FWHM of this plane is a measure of lateral dimension of the sheet and this dimension is 68 nm in PEG-rGO-NS0 and increased to 82 nm in PEG-rGO-NS3. Well resolved morphology and microstructure of PEG-rGO-NS0 and PEG-rGO-NS3 samples are seen in low resolution (LR) TEM and HR-TEM images (Figure 2). In both these samples, curly morphology of graphene sheets is seen in LR-TEM image, indicating 11 ACS Paragon Plus Environment


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presence of few layers of strained graphene sheets. Moreover, in PEG-rGO-NS0 sample, the transparency of the sheet is enhanced, which clearly indicates presence of reduced number of layers (Figure 2ai). The darker regions indicate presence of thicker graphene sheets entangled with each other. Therefore, it is being pointed out that the sample contains large fraction of thinner as well as thicker graphene flakes. It is seen that single- or few-layer rGO nanosheets possess wrinkles, lateral corrugations and scrolls indicating localized strain that is intrinsic nature of the graphene. The HR-TEM image clearly reveals bunches of rGO-NS, consisting of lattice planes, where lateral dimension of the sheet is significantly larger than the thickness of the sheet (Figure 2bi and 2bii). The lattice fringes shows interlayer distances of 0.4 nm in PEG-rGONS3 sample (Figure 2bii), which belongs to the (0 0 2) plane. Zoomed HR-TEM image shows distorted symmetry of the planer lattice oriented along (0 0 2) plane and this could be due to the formation of oxygen functionalization and PEG200 intercalation (Figure 2bii). The interlayer spacing and total thickness of the graphene sheets are reduced in case of PEG-rGO-NS0 sample (Figure 2bi). Here, the amorphous/highly-distorted region is highlighted in the circle and this phase also exists in PEG-rGO-NS3 sample. Finally, it can be inferred that large volume fraction of thinner and small quantity of thicker crystalline domains of graphene sheets is embedded inside an amorphous carbon matrix. XRD and HR-TEM analysis were obtained randomly from several regions of graphene sample and these data were reproducible. As compared to HR-TEM data on graphene, the XRD data have discrepancy with respect to crystallite size and interlayer spacing.


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(ai)

(bi)

(aii)

(bii)

Figure 2. Low resolution TEM (ai) and (aii) scale bar 100 nm and high resolution TEM (bi) and (bii) scale bar 10 nm of PEG-rGO-NS0 and PEG-rGO-NS3 sample, respectively. The spatial resolution of HR-TEM is significantly high which enables it to locate well resolved regions for analysis and this case is not valid for XRD. Moreover, resolution in electron diffraction (HR-TEM) is significantly high which reveals precise nature of disorder in a crystalline sample. In addition, deducing crystalline parameters from an X-ray amorphous specimen is practically not feasible.



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Vibrational Spectroscopy. The γ-radiolysis of pure PEG200 causes cleavage of water present in PEG200 leads to the formation of hydroxyl radicals45 (Figure S1a). This increases hydroxyl functionality on the polymer backbones while preserving the molecular structure of PEG200. More importantly, FTIR analysis showed enhanced hydrogen bonding coordination in PEG-rGO-NS3 sample (Figure S1b). Change in chemical structures of PEG-rGO-NS0 and PEGrGO-NS3 samples were probed by Raman spectroscopy. In sample PEG-rGO-NS0, the D and G bands are observed at 1343 cm–1 and 1593 cm–1, respectively (Figure 3a). However, the corresponding D and G bands in PEG-rGO-NS3 are slightly up-shifted to1348 cm–1 and 1596 cm–1, respectively (Figure 3b). It is known that the D band originating from the zone boundary phonon and this is attributed either to defects or to breakdown of translational symmetry. On the other hand, the G band corresponds to the first-order scattering of the E2g mode of sp2 domains present in graphite.46-48 In PEG-rGO-NS0 sample, the intensity of D band becomes prominent, which indicates that after oxidation of graphite, some fraction of sp2 bonds are reoriented into sp3 hybridized carbon through the cleavage of C=C double bond present in graphite layers. The relative strength of D band as compared to G band depends strongly on the amount of disorder present in the graphitic materials.49 In this case, the intensity ratio I(D)/I(G) in PEG-rGO-NS0 is 1.05 and this value decreased to 0.89 in PEG-rGO-NS3 sample. Thus, it is inferred that γradiolysis mostly eliminates oxygen functional group existing in graphene and incorporates NCPEG-f-rGO at the limited defect sites. In graphene nanosheets, hydroxyl groups facilitate linkage with intercalated PEG200 molecules. This process increases the dimension of in-plane sp2 domains, thus disorder decreases.48,49 This is well evident from the FTIR analysis which shows interaction of oxygen atoms from the PEG200 with the hydroxyl group of rGO-NS through hydrogen bonding (Figure S1b). Weight loss profile analyzed by thermo-gravimetric analysis


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clearly shows decomposition of PEG200 in PEG-rGO-NS3 sample, indicating functionalization of PEG with rGO (Figure S2).

Figure 3. Raman spectra of (a) PEG-rGO-NS0 and (b) PEG-rGO-NS3 samples A reduction in amount of edge defects is indicated which is further corroborated by Raman results. It is important to note that the interlayer spacing in PEG-rGO-NS3 increases due to PEG200 functionalization and intercalation while the symmetry of planar graphene architecture remains unaltered. This indicates that γ–radiolysis functionalizes PEG200 molecules with rGONS and reduces the oxidized defect domains. In addition, the structural stability of rGO is exhibited by PEG as a stabilizer by γ–radiolysis. The 2D band at 2673–2686 cm–1 arises due to the splitting in π-electron dispersion and it is attributed to interaction between the basal planes of graphite.48,50 Generally, the position and shape of the 2D peak are highly sensitive to the number of graphene layers, and it is useful to distinguish between the single-layer and few-layered graphene. A single sharp 2D peak of monolayer graphene is known to become wider and 15 ACS Paragon Plus Environment


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asymmetric with an upshifted peak position while increasing the number of graphene layers.46,50 The steep decrease in intensity and broadening of the 2D peak in PEG-rGO-NS3 sample is attributed to the increase in number of graphene layers and larger stacking distance across the interlayer spacing through intercalation of PEG200 molecules. X-ray Photoelectron Spectroscopy. XPS was used to investigate the chemical changes that occurred in PEG-rGO-NS0 sample upon exposure to γ-radiolysis (Figure 4). A broad and triplet band in C 1s spectrum was de-convoluted into three chemically shifted components designated as A, B and C at binding energies of 284.4, 285.9 and 288.6 eV, respectively (Figure 4ai). Component A is assigned to sp2 hybridized carbon (C–H/C–C) which represents a fundamental chemical shift of graphene/graphite structure.46-48 The component B is assigned to carbon atoms directly bonded to the oxygen in hydroxyl (C–OH) or epoxide (C–O–C) and a small component at C is attributed to carbonyl (>C=O) and carboxyl groups (COOH or HO– C=O).50-55 The ratio of component A/B+C is 1.2 in PEG-rGO-NS0 sample and this ratio is an important parameter to define the quantity of defects and identify various functional groups present in graphene structure. After the γ-radiolysis, the XPS of PEG-rGO-NS3 sample indicated significant changes in shape of C 1s core level photo emission spectra (Figure 4aii). In this case, the components A in C 1s exhibit a band at 284.5 eV (sp2 hybridized carbon) corresponding to C–H/C–C component. The B (C–O–C/ C–OH) and C (C=O, COOH or HO–C=O) bands are observed at BE of 286.2 and 287.4 eV, respectively.55-57 The ratio of component A/B+C is slightly decreased to a value of 0.92 in PEG-rGO-NS3 sample. The shift in energy band and increase in B/C components are deterministic factors which indicate chemical interaction of rGO with PEG200 molecules. However, a strong band of component A in PEG-rGO-NS3 sample suggests that the symmetry of sp2 framework remains stable after the γ-radiolysis. An important


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issue relates to the presence of carbonyl >C=O group existing only as a part of COOH groups at the edge sites of GO sheets.53 Here, carbonyl >C=O group originates from the PEG200 which is functionalized by the γ-radiolysis into rGO. The sp2 fraction in PEG-rGO-NS0 approximates to 70% and the remaining carbon atoms connected with hydroxyl, epoxy, carbonyl and carboxyl groups are sp3 hybridized. The sp2 fraction decreases to 62.2% in PEG-rGO-NS3 sample (Figure 4aii). This point to intercalation of OH in basal plane of graphene sheet without breaking the symmetry of stacked 2D layers much.55 The retention of symmetry and the structural integrity are supported by the Raman spectral data (Figure 3). In the basal plane, presences of hydroxyl C–OH groups prevail over the sp2 bonded carbon. In accordance with this, the ratio of sp2 fraction to hydroxyl fraction decreases upon γ-radiolysis. This is a signature of PEG200 functionalized graphene where functionalization is mediated through the hydrogen bonding.27 This is also an indication of hydroxylated rGO when structure of sp2 remains stable. Formation of phenolic groups and water molecules in graphene samples are clearly evidenced from the high resolution O1s core level photoelectron spectral analysis (Figure 4bi–bii). In these spectra, three de-convoluted components A, B and C at 530.1, 532.4 and 535.2 eV, respectively, are observed in PEG-rGO-NS0 sample (Figure 4bi). These chemical peaks shift to 530.2, 532.5 and 534.5 in PEG-rGO-NS3 sample (Figure 4bii). Components A and B are assigned to phenolic groups C=O (oxygen doubly bonded to aromatic carbon) and C–O (oxygen singly bonded to aliphatic carbon), respectively.56-58



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(aii)

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PEG-rGO-NS3

A

A

B B C

C

(bi)

B A

PEG-rGO-NS3

(bii)

PEG-rGO-NS0

B A

C

C

Figure 4. XPS of PEG-rGO-NS0 and PEG-rGO-NS3 samples (ai) and (aii) depict C 1s spectra and (bi) and (bii) depict O 1s spectra. Increase in magnitude of component A as inferred from O 1s spectra occurs in PEG-rGONS3 sample. This phenomenon is a deterministic reason which points to non-covalent interaction of PEG200 with rGO occurring vide hydrogen bonding. Component C is assigned to chemisorbed/intercalated H2O originating from PEG200 and marginally exists as impurities. The increase in C=O group (component A) in PEG-rGO-NS3 sample is mostly due to conversion of C–O to C=O and interaction between PEG200 functional groups and functionalized graphene


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sheets. Therefore, ratio of components A/B in O 1s spectra is higher in PEG-rGO-NS3 as compared to PEG-rGO-NS0. This fact is related to γ –irradiation induced conversion of component B to A in PEG-rGO-NS3. This is one of the strong reason for the dominance of C=O in PEG-rGO-NS3 sample. Lubrication in Hybrid PEG-Gr-NS. The average values of friction coefficient and wear rate of PEG200 lubricated system is 0.17 and 5×10–6 mm3/Nm, respectively (Figure 5a, and Figure 5b). The wear rate (k) was calculated from the simple relationship, k=V/F.s, where, V is the wear volume, F is the normal force and s defines the total sliding distance.59

(a)

(b)

Figure 5(a) Friction coefficient of sample (a) pure PEG200 (b) PEG-rGO-NS0 (c) PEG-rGO-NS1 (d) PEG-rGO-NS2 and (e) PEG-rGO-NS3. Inset shows average value of friction coefficient with standard deviation. Figure 5(b) shows wear rate of samples (a) pure PEG200 (b) PEG-rGO-NS0 (c) PEG-rGO-NS1 (d) PEG-rGO-NS2 and (e) PEG-rGO-NS3. Tribo-test conditions: rGO concentration 0.03 mg mL–1 in PEG200, load 1 N, contact pressure 0.82 GPa, sliding speed 3 cm/s, sliding distance 100 m.

It is observed that the value of friction coefficient in PEG200 lubricated condition is unsteady and it increases with sliding distance (Figure 5a, curve (a)). Due to unsteady friction 19 ACS Paragon Plus Environment


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curve, the values exhibit high standard deviation as shown in the inset (Figure 5a). Both friction coefficient and wear rate decreased to average values 0.15 and 2.5×10–6 mm3/Nm, respectively for PEG-rGO-NS0 sample dispersed in PEG200 matrix (Figure 5a, curve (b)). This indicates that rGO additives in the lube medium are capable of lowering friction and wear values. In contrast, GO dispersed in PEG200 shows high friction coefficient with average value of ~0.2 and high wear rate 2.5×10–5 mm3/Nm (Figure S3). In this condition, wider wear track with more scratches adhesive wear, grooves and deformation in the wear track is shown in optical and SEM images (inset of Figure S3). XRD results of GO samples shows (0 0 2) and (1 0 2) diffraction planes centered at 11.32o and 42.4o 2θ, respectively (Figure S4). Considering (0 0 2) plane, the interlayer spacing of 0.78 nm and approximately 44 nm total sheet thickness having approximately 56 graphene sheets are calculated. Here, the interlayer spacing and sheet thickness is much larger than rGO samples (Figure 1) that clearly indicates structure of GO but more close to graphite oxide. High friction and wear in GO is mainly attributed to ineffective shearing of interlayer graphene sheets that contains large volume fraction of defects present in the form of oxygen functional groups. The defect concentration destroy the graphitic structure and enormously increases the hydrophilic behavior.60 In this condition, interlayer shear strength of the sheet increases that leads to increase in friction and wear. In fact, the interaction of GO with PEG200 is effective, however, the lubrication properties is weak due to defects that disrupts the shear mobility of the functionalized graphene plane. Therefore, the optimization of oxygen functional group in rGO necessary for dispersion in PEG200 and in same time preserving the graphene structure is primary requisite for effective lubrication. In case of PEG-rGO-NS3 sample, the friction coefficient and wear rate decreases to 0.12 and 1.5×10–7 mm3/Nm, respectively (Figure 5). Above tribo-test was performed at a minute concentration of 0.03 mg


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mL–1 of rGO powder sample dispersed in PEG200 lube medium. In order to obtain lowest possible values of friction coefficient and wear rate, the concentration of PEG-rGO-NS3 sample in PEG200 medium was optimized (Figure 6). The lowest friction coefficient with a value of 0.12 was obtained at a concentration 0.03 mg mL–1. In this sample, smooth and narrow dimension of wear track is shown in Figure 7. This friction value increased to 0.135 when concentration of PEG-rGO-NS3 was decreased to 0.02 mg mL–1. Earlier reports have also shown a minimization in friction coefficient at certain optimized concentration of γ-irradiated graphite nanosheets dispersed in PEG.38

(b)

(a)

Figure 6(a) Friction coefficient of sample PEG-rGO-NS3 with variation in rGO concentration: (a) 2 (b) 0.2 (c) 0.1 (d) 0.05 (e) 0.03 and (f) 0.02 mg mL–1 dispersed in PEG200. Inset shows average value of friction coefficient with standard deviation. Figure 6(b) shows wear rate at above mentioned concentration of rGO. Tribo-condition: Load 1 N, contact pressure 0.82 GPa, sliding speed 3 cm/s, sliding distance 100 m.

Phenomenologically, at sufficiently small concentration, the graphene sheets do not properly interact with the contact interfaces and lubrication process remains by and large ineffective and 21 ACS Paragon Plus Environment


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only dominated by PEG200. It means that rGO do not effectively participate in lubrication and it is understood that at less concentration the contacting bodies have deficiency of the graphene sheets. This restricts to form low shear strength tribofilm resulting high wear rate. However, at higher concentration, when it exceeds 0.03 mg mL–1, it is presumed that NC-PEG-f-rGO is not properly dispersed in PEG200 and these independent sheets agglomerate and collide randomly during the sliding which generates increased wear and disrupts the symmetric lamellar motion.61

Figure 7. Optical image of wear tracks (a) pristine PEG200 (b) PEG-rGO-NS0 and (c) PEGrGO-NS3 samples, scale bar 100 µm. Magnified SEM images of wear track lubricated with pure (d) PEG200 and (e) PEG-rGO-NS3, scale bar 10 µm. Tribo-conditions: Concentration of rGO 0.03 mg mL–1, load 1 N, contact pressure 0.82 GPa, sliding speed 3 cm/s, sliding distance 100 m. The dispersion stability is high in 0.03 mg.mL–1 rGO concentrations, where absorbance decreases marginally with time and after 5 hours the absorbance value is saturated, indicating good stability (Figure 8a). However, at high concentration 0.2 mg mL–1 time dependent agglomeration


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of the rGO in PEG is observed, indicating weak signature of dispersion stability (Figure 8b). Therefore, dispersion stability of graphene plays an important role for lubrication.

Figure 8. Suspension stability of the lubricating PEG200 oils with rGO concentration (a) 0.03 and (b) 0.2 mg mL–1 as determined by ultraviolet–visible light (UV– VIS) spectrophotometry.

With the use of pristine PEG200 alone for the tribo-test, the friction coefficient and wear rate were found to be highest. Consequently, comparison between lubrication imparted by other two rGO additives samples i.e. PEG-rGO-NS0 and PEG-rGO-NS3 is useful as these two contrastingly exhibit both highest and lowest friction coefficient and wear, respectively. It is clearly shown that wear width is slightly less in PEG-rGO-NS0 as compared to that of PEG200 and this is further significantly reduced in PEG-rGO-NS3 (Figure 7). SEM image of wear track in PEG200 lubricated condition showed deeply inscribed linear scratches, grooves and deformed ripples, suggesting severe plastic deformation along with adhesive wear pattern in the sliding direction. However, wear track is smoother under PEG-rGO-NS3 tribo-test condition. The magnitude of scratches, grooves and adhesive wear patterns are finer which suggests prevalence



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of reduced plastic deformation and mechanical defects. Less deformation is an indication of less frictional energy that enhanced the tribo-thermo-oxidative resistance of PEG-rGO-NS3 sample which is well evident from FTIR analysis (Figure S-5). Optimized concentration of additives is one of the important issues for realizing enhanced lubrication.23,38,62 It was shown that the dispersion of higher concentration of graphene and h-BN in lube medium is not effective and nanosheets easily aggregate in the oil and agglomerates.37,38 This acts as abrasive particles and makes the lubricating oil ineffective thus, the friction and wear become high.35,61 However, the physical and chemical evidence supporting the above concept is not yet proposed. Therefore, in order to understand the mechanism, the detail microscopic and spectroscopic investigation of wear debris is carried out. This is shown in Supplementary Information (Figure S6 to S8). Microscopic analysis showed that morphology of rGO in tribocontact was preserved at high concentration, while graphite-like 3D structure of tribo-chemical product was formed at less (0.03 mg mL–1) concentration (Figure S6). This transformation is chemically analyzed by Raman spectroscopy (Figure S7). C 1s core level spectra in XPS represent oxidation of wear debris and presence of graphene/graphite-like structure (Figure S8). Furthermore, graphite-like chemical species was observed in the wear track of sample PEG-rGONS3 (Figure S8). Based on above results, the lubrication mechanism in PEG-rGO-NS0 and PEGrGO-NS3 samples is proposed in the schematic (Figure 9). The model is based on experimental facts obtained from FTIR, XRD, Raman and XPS analysis. More importantly, Raman analysis of wear particles and detection of chemical substances at wear track reveal in-depth mechanism of lubrication. Sample PEG-rGO-NS0 shows weak interaction of PEG200 with rGO-NS and therefore, deficiency of deposited graphene domains in the steel-steel tribo-contact exhibited (Figure 9a). This is experimentally shown by Raman analysis of wear track (Figure S9a).


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However, in sample PEG-rGO-NS3, the PEG200 molecules are intercalated vide hydrogen bonding with functionalized graphene sheets through γ-radiolysis (Figure 9b). This process makes graphene to have superior dispersion and intercalation in PEG200 which ensures stability of graphene under contact stress. Therefore, PEG200 functionalized graphene existing between the contacting interfaces easily shears and provides facile lubrication. Additionally, under the action of contact stress, the graphene/graphite nanosheets are adsorbed in the wear track forming stable tribofilm (Figure S-9b). The PEG200 functionalized graphene at contact interface acts as a solid-liquid lubricant while deposited graphene/graphite tribofilm behaves as a purely lamellarsolid lubricant. Such effects provide hybrid solid-liquid lubrication that reduce friction and also protect the surfaces against the undesirable wear related damage and deformation.19

(a)

(b)

Figure 9. The lubrication mechanism in (a) PEG-rGO-NS0 and γ-irradiated (b) PEG-rGO-NS3 samples. Steel ball sliding against steel substrate shows graphene deficient contact interface in



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PEG-rGO-NS0 and graphene rich sliding interface in PEG-rGO-NS3 sample. The model is based on chemical analysis of wear debris and wear track.

CONCLUSIONS The γ-radiolysis is proven as a facile, chemically clean and environmentally compatible method for the modification of oxygen functionalization and intercalation of PEG200 with rGO through hydrogen bonding. In this process, interlayer spacing of graphene sheet increased while structural and chemical integrity of carbon network in rGO is preserved. More importantly, intercalation and functionalization provided effective dispersion ability of rGO in hydrophilic PEG200 which is described as a primary criterion for effective lubrication. Using minute concentration 0.03 mg mL–1 rGO, the friction coefficient and wear rate reduced up to 38% and 55%, respectively, in γ-irradiated rGO as compared to un-irradiated rGO sample. Such an improved lubrication characteristic is explained by functionalization of PEG200 with rGO nanosheets which provided additional support for superficial shearing of graphene sheets under the tribo-contact condition. Moreover, under the sliding, the mechnochemical energy transformed the rGO into graphite-like nanosheets which was adsorbed in the wear track of metallic steel surfaces described as a low shear strength tribofilm. This film behaves as a solid lubricant, reduces the friction coefficient and improves the wear resistance. Tribo-thermooxidative resistance of the lubricant is greatly improved in γ-irradiated rGO sample that ensures longevity of lubricant effective performance.


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AUTHOR INFORMATION Corresponding Author *Email: [email protected], [email protected] Tel.: +91 44 27480500 (ext. 22537) Fax: +914427480081 Notes The authors declare that there is no competing financial interest ACKNOWLEDGMENTs B. Gupta would like to acknowledge DST, New Delhi for Inspire Fellowship award. The contribution of IITM, Chennai is acknowledges for TEM characterization. We thank M. P. Janwadker, Director, MSG and Dr. P.R. Vasudev Rao, Director, IGCAR for support.

Supporting Information Available: FTIR analysis of pure PEG200 after and before γ-radiolysis; FTIR of rGO samples; TGA analysis; Friction coefficient of GO; XRD of graphene oxide; FTIR analysis of buried lubricant; SEM of wear debris; Raman spectroscopy of wear debris; XPS analysis of wear debris; Raman spectra of wear tracks. This material is available free of charge via the Internet at http://pubs.acs.org.



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PLGA–PEG–SA–PEG–PLGA

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Copolymer

Nanoparticles:

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to Reduced Graphene Oxide Nanosheets through Î³-Radiolysis for

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