Chemically Functionalized Reduced Graphene Oxide as a Novel

Subscriber access provided by EMORY UNIV

Article

Chemically Functionalized Reduced Graphene Oxide as a Novel Material for Reduction of Friction and Wear Harshal Prakashrao Mungse, and Om Prakash Khatri J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 07 Jun 2014 Downloaded from http://pubs.acs.org on June 10, 2014

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Chemically Functionalized Reduced Graphene Oxide as a Novel Material for Reduction of Friction and Wear

Harshal P. Mungse and Om P. Khatri*

Chemical Science Division, CSIR - Indian Institute of Petroleum, Dehradun - 248005 (India)

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 37

ABSTRACT

Graphene, a lamellar structured material, easily shears at the contact interfaces and exhibits excellent mechanical strength and conductivity, which promises its potential for tribological applications. However, the dispersion of graphene in lube media is a big challenge. Herein, we have developed a chemical approach for selective inclusion of long alkyl chains on the edges and defects sites of reduced graphene oxide sheets through the amide linkage, which facilitates their stable dispersion in the lube oil. Chemical and structural features of site-selective chemically functionalized reduced graphene oxide are monitored by FTIR, XPS, XRD, TG-DTA, FESEM and HRTEM. Tribological test results showed that the chemically functionalized reduced graphene oxide, as an additive to 10W-40 engine oil, significantly reduced both the friction and the wear of steel balls. The stable dispersion of chemically functionalized reduced graphene oxide provides low resistance in a sheared contact owing to weak van der Waals interaction between their lamellas, thus significantly reducing both the friction and the wear.

Keywords: Graphene, Chemical functionalization, Dispersion, Lubricant additive, Friction


2

Page 3 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Ever increasing regulations towards energy conservation and environment protection have propelled an interest to develop energy efficient lubricant, which can improve the fuel-efficiency and environmental value, and lower the emissions. Usually in passenger cars, the direct frictional losses, excluding the braking friction, are estimated to be 28% of the fuel energy.1 One estimation states that reducing friction and wear in engine and its associated components could save the US economy as much as US$ 120 billion per year.2 Graphene, a two-dimensional carbon structure arranged in honeycomb lattice, has attracted wide interest owing to its remarkable electronic, optical, mechanical and thermal properties.3-7 In recent years, graphene has been studied extensively for its potential in electronic, sensors, energy and catalytic applications.8-11 However, the lubrication application of graphene has not been explored significantly, though this material has been identified as an excellent candidate for reducing adhesion, friction and wear.12-14 The number of layers and degree of defects in the graphene, nature of sub-surface on which graphene is bounded, preparation method etc. collectively play an important role to determine its friction, wear and adhesion properties. Friction force microscopic studies on epitaxial graphene, grown on SiC, revealed low friction for bilayer graphene than the single layer graphene.12 Furthermore, nanoscale frictional characteristics of the graphene sheets, exfoliated onto a weakly adherent silicon oxide surface, found to monotonically decrease as the number of layers increased and eventually approached to that of graphite.15 In contrast, recently, Cannara et al. have revealed that friction force increased with increasing number of layers for suspended graphene at low loads owing to competition between local forces that determine the deformations of the surface layer, profile of the membrane as a whole and van der Waals forces between the AFM tip and subsurface layers.16 In


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 37

the load range of µN to mN, epitaxial graphene revealed significantly lower friction than the graphite under the identical experimental conditions.17 The reduced graphene oxide (rGO) sheets assembled on silicon substrate via covalent linkage showed low adhesion and reduced friction force in both micro- and macroscale compared to that of silicon substrate.18 The lubrication properties of rGO sheets ascribed to its intrinsic structure, that is, the covalent bonding to the substrate and low resistance to shear owing to lamellar structure. The most of frictional studies on the graphene have been carried out on the supported thin films, which is important and realistic for MEMS / NEMS applications. Graphene as a freely suspended material in the lube media can also be a good lubricant for reduction of friction and wear between two or more contact surfaces for macro-tribological applications. In general, the nanomaterials as solid lubricants exhibit high specific surface area, low shear strength, excellent Young's modulus, high load-bearing capacity, high thermal stability, conductivity etc., which are favorable from lubrication perspective. A number of research reports revealed that graphene has these desirable properties.6,15,19 Besides that, effectual use of graphene as nano-lubricant for friction and wear reductions relies on its stable dispersion in the lube media. Lin et al. found that the wear resistance, load-carrying capacity and friction coefficient of lubricating oil were improved when graphene sheets modified with stearic/oleic acid, are added to the oil.20 However, friction is found to increase gradually with friction time, which might be because of poor dispersibility of graphene nanosheets in the lubricating oils. The graphene prepared by exfoliation of the GO using focused solar radiation has exhibited significant improvement in both friction and antiwear properties for the lubricant, attributed to the nano-bearing mechanism and high mechanical strength of the graphene.21 The graphene-ionic liquid based hybrid nanomaterials as novel lubricants have shown to improve the


4

Page 5 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


tribological properties, attributed to the formation of tribofilm on the contact interfaces.22 Recently, our group has developed alkylated graphene oxide to improve their dispersion stability in hydrocarbon oil.23 These alkylated GO sheets had very good dispersion stability in nhexadecane and tribo-results revealed 26 % and 9 % reductions in friction and wear, respectively, compared to that of n-hexadecane lube base. Probably, the presence of oxygen functionalities in alkylated GO reduces the Young's modulus of graphene nanosheets6,24, thus, having no significant improvement in antiwear properties of alkylated GO. In tribology, the continuous supply of nano-lubricants on the contact interfaces is very important for optimized performance and this could be achieved by utilizing their stable dispersion in lube media.25 In the present study rGO, which resembles to graphene, is dispersed into a commercial 10W-40 lubricating oil by site selective chemical functionalization. The lubrication properties of chemically functionalized rGO sheets are investigated by measuring the friction coefficient and antiwear properties. It is anticipated that stable dispersion of rGO may find broad potential applications for lubricant development.

EXPERIMENTAL SECTION Preparation of chemically functionalized reduced graphene oxide: The GO was prepared by harsh oxidation of graphite powder using a mixture of NaNO3, H2SO4 and KMnO4 as strong oxidizing reagents, and then exfoliation of oxidized product using the ultrasonic probe. In the subsequent step, reduction of GO was carried out by refluxing the aqueous dispersion of GO using hydrazine monohydrate as a reducing agent, for 24 hours (Scheme 1a-b). The brown color of GO gradually turned into black, owing to elimination of oxygen functionalities and regeneration of -conjugated network during the reduction process. Subsequently rGO as a black


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 37

aggregated material was washed several times with water to remove traces of the hydrazine monohydrate, and the resulting product was obtained using a membrane filter. Prior to siteselective loading of octadecylamine (ODA), the rGO sheets were oxidized under mild condition using the nitric acid as an oxidizing reagent (Scheme 1b-c). The mild oxidation introduces carboxylic groups at the defects sites and edges of rGO.26 This was followed by conversion of carboxylic groups into acyl chlorides using the thionyl chloride. The residual thionyl chloride was removed by distillation and then acylated-rGO product was washed to remove the traces of residual thionyl chloride. Subsequently, acylated-rGO sample was reacted with dried ODA at 120 °C under nitrogen atmosphere. The loading of ODA molecules on acylated-rGO through amide linkage afforded the ODA-rGO sheets (Scheme 1c-d). The synthesized product was washed several times with ethanol to remove the residual content of ODA.

------ Scheme 1 ------

Chemical and structural characterization of ODA-rGO: The site-selective chemical functionalization of rGO was monitored by Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) measurements. FTIR spectra for all samples were recorded using a Thermo-nicolet 8700 research spectrophotometer with a resolution of 4 cm-1. Each sample in known quantity was mixed with potassium bromide and prepared the pellets for their FTIR measurements. XPS (JPS-9010TRX, JEOL Ltd.) measurements were conducted using thin films of the GO and the ODA-rGO samples. All XPS measurements were executed using a Mg K line as the X-ray source. The peak-fitting of the C1s spectra for both samples was carried out using a Gaussian–Lorentzian function after performing a Shirley background correction. X-ray


6

Page 7 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


powder diffraction (XRD) analyses of all samples were carried out using a Bruker D8 Advanced diffractometer at 40 kV and 40 mA using Cu Kα radiation (λ= 0.15418nm). The diffraction data were recorded for 2θ angle between 20° to 60° (step size 0.020o, step time 1s). Thermogravimetric analysis (TGA) of GO and ODA-rGO were carried out using a thermal analyzer (Diamond, Perkin Elmer). Both samples were analyzed in the temperature range of 30550 °C under a flow of nitrogen. Field emission scanning electron microscopy (FESEM) analyses of GO and ODA-rGO samples were carried out using an FEI Quanta 200 F. Both samples were separately sprinkled on a carbon tape to acquire microscopic images of each sample. High resolution transmission electron microscopy (HRTEM) analyses of GO and ODArGO samples were carried out on JEOL 3010 electron microscope at 300kV by drop-casting of their ethanolic dispersion on TEM-grid.

------ Table 1 ------

Tribological Properties of ODA-rGO sheets: The tribo-characteristics in terms of the friction coefficient and the wear scar diameter (WSD) for different samples were examined by using the four-ball machine (Ducom, India). The physico-chemical properties of 10W-40 commercial engine oil and ODA-rGO blended 10W-40 oil are extracted in Table 1. In a typical tribo-experiment, a steel ball ( = 12.7 mm, material: AISI 52100, hardness: 64-66 Rc and surface finish: grade 25 EP, as per ASTM standard requirement) under the applied load was rotated against three stationary steel balls clamped in the holder. The tribological tests were conducted in ambient conditions at 35% humidity. During tribo-tests, the four balls were covered with a lube sample, which was used for the friction and wear evaluation. All tests were carried


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 37

out as per the ASTM D 4172 standard test method under a load of 392 N at a rotating speed of 1200 rpm for 1 h and temperature was maintained 75 °C throughout the experiment. Herein, WSD was measured as per the ASTM D4172 method. The reported values of WSD for each sample is a mean of six balls, which were used in two different tests of each sample. The coefficient of friction values reported here are mean of two different tests of each sample. Furthermore, error bar for each result is provided to know the repeatability of test results. The % reduction of friction and WSD were calculated using mean values of friction coefficient and WSD for 10W-40 oil and ODA-rGO blended oil sample. Morphological features on the worn area of steel balls, lubricated with different samples, were examined using the FESEM.

RESULTS AND DISCUSSION Reduced graphene oxide (rGO) resembles to graphene, is immiscible with most of organic solvents and hydrocarbons (Figure S1, Supporting Information) due to differences in cohesive energies of rGO and dispersible media.27 However, for the efficient lubrication, rGO sheets should be thoroughly dispersed in the lubes. Usually, rGO is stable and chemically inactive to most of the organic moieties. Hence, the first challenge is to generate the active sites in rGO, where the chemical moieties can be introduced to facilitate their dispersion in the lubes. Herein, carboxylic acid groups as active sites were selectively introduced at edges and defects sites of rGO sheets, by mild oxidation using the dilute nitric acid.26 These carboxylic acid groups were then targeted for the chemical functionalization. The octadecylamine, carrying long alkyl chain, was coupled to carboxylic acid groups of rGO sheets through amide linkage using the thionyl chloride as a reaction coupler.28


8

Page 9 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


------ Figure 1 ------

The chemical and structural changes, occurred during the preparation of ODA-rGO using GO as a precursor, were thoroughly monitored by FTIR, XPS, TG-DTA, XRD, FESEM and HRTEM analyses. Figure 1 shows vibration spectra of GO, rGO, orGO and ODA-rGO sheets along with their characteristics vibrational assignments. The GO exhibits strong vibration peaks at 3418 cm1

(O-H stretch attributed to hydroxyl and phenolic groups), 1735 cm-1 (C=O stretch attributed to

carboxyl and carbonyl groups), 1625 cm-1 (bending modes of water molecules and C=C stretch of unoxidized sp2 carbon domain), 1355 cm-1 (O-H bending), 1240 cm-1(C-O stretch ascribed to phenols, ethers and epoxy groups), and 1066 cm-1 (C-O stretch attributed to hydroxyl groups).2930

These vibration signatures reveal the presence of oxygen functional groups such as carboxyl,

hydroxyl, epoxy, carbonyl, phenols, ether etc. Over the years, various structural model of GO were depicted and have shown that most of hydroxyl and epoxy functionalities lie in the basal plane galleries, whereas, carboxyl groups are located along the edges.31,32 The chemical reduction of GO by refluxing with hydrazine hydrate leads to removal of most of these oxygen functionalities including carboxylic group and restoration of -conjugated network as deduced by FTIR spectra (Figure 1). Zettl et al. have revealed that rGO exhibits various defect sites and holes throughput the rGO sheets.33 The carbon atoms located at the edges of rGO and defects sites are susceptible to be oxidized into carboxyl groups in the presence of diluted nitric acid.26 Therefore, the mild oxidation of rGO, using the diluted nitric acid, selectively generates carboxyl groups at the edges and the defects sites of rGO sheets. This was unambiguously confirmed by appearance of C=O stretch at 1727 cm-1. The carboxylic groups in the developed material were then coupled with ODA through amide linkage formation using the thionyl chloride as a reaction


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 37

facilitator. The appearance of strong vibration stretches in the FTIR spectra of ODA-rGO over the range of 2800-3000 cm-1 are attributed to the long alkyl chains, revealing the loading of octadecyl moiety. Furthermore, the characteristics vibrational peaks at 1700 cm-1 (C=O stretch, attributed to the amide group), 1577 cm-1 (assigned to an overlapped signature of the N-H bond and sp2 carbon domain of rGO skeleton), and 1220 cm-1 (attributed to the stretching mode of CN linkage), revealed the loading of ODA through amide linkage.23,28

------ Figure 2 ----------- Figure 3 ------

XPS analyses of GO and ODA-rGO sheets were further carried out to monitor the chemical changes. Figure 2a-b shows the high-resolution C1s spectra of GO and ODA-rGO in the range of 292-280 eV. The C1s spectra of GO exhibits an overlapped double peak structure with broad tail towards higher binding energy, which illustrates that the carbon scaffold is bonded to different types of oxygen functionalities. C1s spectra of GO could be deconvoluted into four chemically shifted components at 284.5, 286.5, 288.2 and 289.2 eV, attributed to carbon skeleton (C=C/CC), hydroxyl/phenolic (C-OH) and epoxide/ether (C-O-C), carbonyl (C=O) and carboxyl (COOH) groups, respectively.34,35 The C1s spectra of ODA-rGO shows single peak with small tail at higher binding energy (Figure 2b). An apparent indication of loading of ODA in rGO sheets was revealed by a very intense peak at 284.5 eV, associated to C=C/C-C. The long alkyl chain of ODA contributes to C1s peak at 284.5 eV, exhibited by significant increase in peak intensity. The deconvoluted C1s spectra of the ODA-rGO shows three more chemically shifted components towards higher binding energy at 285.6, 286.5 and 288.0 eV, attributed to the


10

Page 11 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


amine/hydroxyl (C-N/C-O), ether (C-O-C) and amide / carbonyl (CONH-R / C=O) groups, respectively.34,36 The presence of amide group in the rGO-ODA was confirmed by appearance of N 1s XPS spectra at ~ 400 eV as shown in Figure 2c. The elemental analyses of all samples revealed that rGO and orGO, prepared as intermediate products, are free from nitrogen; hence the nitrogen components in rGO-ODA confirmed the loading of ODA. The thermal stability of the ODA-rGO is an important parameter, for tribological applications. Hence, the thermal decomposition properties of the ODA-rGO were probed using TG-DTA (Figure 3). The ODArGO shows only 4% weight loss up to 250 °C, subsequently, significant weight loss (~50%) was observed in the range of 250 - 530 °C with a peak at the temperature of 410 °C. This could be primarily due to thermal decomposition of chemically anchored long chain amide functionalities and partial contribution from thermal decomposition of carbon skeleton of ODA-rGO along with residual oxygen functionalities. These results suggest that ODA-rGO is thermally stable upto 250 °C, which is fairly good for their use as a lubricant additive. In-contrast, GO shows two significant weight losses (~18 and ~29%) near 100 °C and in the range of 180-240 °C, owing to the evaporation of the trapped water molecules in the material and thermal decomposition of facile oxygen carrying functionalities, respectively.

------ Figure 4 ------

Furthermore, XRD analyses of GO, rGO and ODA-rGO samples were carried out to explore the structural changes, particularly interlayer distance, which is a critical parameter from the lubrication perspective. The XRD pattern of GO shows a diffraction peak at 2 = 10.88° with a corresponding d-spacing of 8.2 Å (Figure 4). The hydrophilic oxygen functionalities located in


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 37

the basal plane of GO sheets absorbs the water molecules, which increases the interlayer distance. The d-spacing of GO varies with the amount of absorbed water molecules in the basal plane galleries and is found to be in the range of 6.1 Å for dry GO to 12 Å for wet GO.37 Probably, hydrogen bonding network between the absorbed water molecules and the oxygen functionalities in the basal plane holds the graphene oxide sheets together with an increased interlayer distance. It has been demonstrated that GO is structurally inhomogeneous with randomly distributed graphitic and oxidized / distorted regions. The distorted regions in GO are likely from basal plane oxygen functionalities.33 Chhowalla et al. have revealed the atomic structure of GO using annular dark field imaging and electron energy loss spectroscopy and found that GO is rough and its structure is predominantly amorphous due to distortions from sp3 C-O bonds.38 The FESEM and HRTEM images of GO (Fig. 5a and c) shows plenty of wrinkle and folded regions, which are highlighted by arrows. These features could be attributed to the sp3 carbons, which are linked to the oxygen functionalities in the basal plane and various structural defects. XRD pattern of rGO shows a broad diffraction peak (200) at 2 = 24.4° with a corresponding interlayer of 3.7 Å. An exfoliation of the graphite oxide by ultrasonic waves breaks down these sheets in lateral direction owing to weak interaction between expanded lamellas. As a result, the low number of lamellas within the rGO sheets and their corrugated structure broaden the diffraction peak. The loading of ODA molecules on the carboxylic groups of orGO through amide linkage shows little shifts in the characteristic diffraction peak, centered at 2 = 25.8°, with a corresponding d-spacing of 3.5 Å. This suggests that no more oxygen functionalities remain in the basal planes of the ODA-rGO sheets, hence the interlayer distance in ODA-rGO is virtually equal to characteristic d-spacing of graphene (3.35 Å). The absence of oxygen functionalities in the basal plane of sp2 carbon skeleton allow the lamellas of ODA-rGO


12

Page 13 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


sheets to be linked by weak van der Waals interactions. As a result, ODA-rGO sheets are found to be highly aggregated with crumpled features as shown in their FESEM and TEM images (Fig. 5b and d). For a comparison, ODA was chemically interacted with the GO and then structural changes in the developed material was probed by XRD analysis. The GO exhibits ample of hydroxyl, epoxy, and carboxyl functionalities. Among these functionalities, hydroxyl and epoxy groups are mainly located in the basal plane galleries, whereas carboxyl groups are decorated along the edges.31,32 XRD pattern (Figure S2, supporting information) of developed material shows a very intense peak at 2 = 3.06°, which is a corresponding to d-spacing of ~29 Å. The polar amino functional group is known to be interacted with hydroxyl and epoxy groups by hydrogen boning and nucleophilic substitution reactions, respectively. Since these functionalities are mostly exists in the basal plane, hence, the alkyl chains of ODA are oriented in the basal plane galleries of GO. As a result, the interlayer distance increased to 29 Å. However, such features were not observed in ODA-rGO sheets. This reveal that ODA molecules are selectively loaded on the edges functionalities of the rGO. ------ Figure 5 ------

------ Figure 6 ------

The surface active additives like friction- and wear-modifiers needs to be on the contact interfaces for their optimized tribo-performance. For the solid nano-lubricants, this can be attained by using their stable dispersion in the lubes during tribo-tests. Herein, the presence of long alkyl chains, at edges of the ODA-rGO, facilitates their stable dispersion in the lube media.


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 37

Figure 6 shows the digital images of dispersion of rGO and ODA-rGO sheets in the 10W-40 commercial engine oil as a function of time. It is noted that rGO nanosheets are immiscible and settle down within couple of hours even after their prolonged sonication as shown in Figure 6b-c, while, the presence of long alkyl chains in the ODA-rGO sheets provided their stable dispersion in the 10W-40 lubricant. The ODA-rGO in the 10W-40 oil shows a zeta potential of 55 mV which falls in the stable dispersion range.39 The stable dispersion of the ODA-rGO is due to van der Waals interaction between the long alkyl chains of the ODA-rGO and alkyl chains of lubricant base oil, which allows these sheets to be thoroughly dispersed. As a result. the viscosity of 10W-40 oil is slightly increased by addition of very little quantity of (0.02 mg.mL-1) ODArGO nanosheets as shown in Table 1.

------ Figure 7 ------

The lubrication properties of the ODA-rGO sheets were evaluated using the 10W-40 commercial engine oil as a lubricant for the steel balls. Figure 7 shows the average friction coefficient and WSD for different doses of ODA-rGO blended with 10W-40 engine oil under a load of 392 N. The average friction coefficient and WSD for 10W-40 oil were found to be about 0.11 and 560 µm, respectively. Both friction coefficient and WSD illustrate a nearly similar trend with increasing concentration of the ODA-rGO in the 10W-40 lube oil. Initially both friction and WSD decrease significantly and then they increase with increasing concentration of the ODArGO nanosheets. The lowest friction coefficient and the smallest WSD were found at the concentration of 0.02 mg.mL-1. This revealed that the ODA-rGO, used as an additive, played a positive role in remarkably lowering the friction and improving the antiwear properties. The


14

Page 15 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


increase of the friction coefficient and the WSD with further increase in dose of the ODA-rGO sheets could be due to the agglomeration of the ODA-rGO sheets under the tribological conditions (high temperature and pressure), which hampers their uninterrupted supply to the contact-interfaces.25

------ Figure 8 ------

Figure 8 illustrates the variation in the friction coefficient of steel balls lubricated with the 10W-40 oil and ODA-rGO sheets blended samples. The 10W-40 oil revealed high friction during initial few minutes of the test and then gradually reduced as a function of run time. The initial high friction could be attributed to the lack of tribo-thin film on the contact interfaces. It is worth to mention that the 10W-40 oil was procured from the market and exhibits all required frictionand wear-modifier additives. The presence of these additives in the 10W-40 oil formed the tribothin film on the tribo-stressed area and such phenomenon increases with increasing contact time.40 Consequently, gradual reduction in friction coefficient (Figure 8). There is a significant reduction in friction coefficient in the presence of 0.02 mg.mL-1 dose of the ODA-rGO sheets in the 10W-40 oil. The chemical and structural characterization of ODA-rGO sheets revealed that graphitic layers are closely bounded by weak van der Waals interaction with an interlayer distance of ~3.5 Å, which is very close to d-spacing of graphite (3.35 Å). This unique layered structure of the ODA-rGO provides easy shearing between their weakly coordinated (van der Waals interaction) lamellas, while they are under the tribo-stressed zone. Furthermore, very good dispersibility of these nanosheets in the lube media ensured their continuous supply on the tribointerfaces. As a whole, this phenomena significantly reduced the friction compared to that of


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 37

10W-40 oil. For a comparison, we have examined the tribo-performance of ODA functionalized GO (ODA-GO), which contains good degree of oxygen functionalities. The ODA-GO is prepared by loading of ODA through amide linkage with carboxylic functionalities of the GO. However, the oxygen functionalities presents in the ODA-GO diminished the easy shearing between the layers of ODA-GO under the contact interfaces, as a result the ODA-GO shows comparatively higher friction than that of the with ODA-rGO. ------ Figure 9 ------

Another important aspect is the influence of ODA-rGO sheets on wear behaviour of the steel balls. Figure 9a-c shows the microscopic images of worn area on steel ball lubricated with 10W40 oil, after one hour test under the load of 392 N. These wear images illustrate the adhesive wear with presence of deep grooves and possibly third-body wear debris generation (Figure 9c) at the contact interfaces. In the presence of ODA-rGO sheets, the wear scar diameter on the steel ball was reduced by ~36 % with smoother features and no evidences of adhesive wear was observed. The 10W-40 commercial engine oil contains all the required additives including friction-modifier and antiwear additive; even though the stable dispersion of the ODA-rGO in 10W-40 oil exhibits significant reduction in the friction and improvement in the antiwear properties, making the lubricant system more energy efficient. The weak van der Waals interaction between the lamellas of ODA-rGO nanosheets provides low resistance to shear under the rolling contact stress, resulting in reduction in friction. Furthermore, continuous supply of these nanosheets on the contact surfaces, due to their stable dispersion in the lube oil, avoids the direct contact between steel balls and improves the antiwear properties. Recently, Sumant et al. have revealed that under the tribo-stress the two-dimensional graphene forms a conformal


16

Page 17 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


protective coating on the contact interfaces, which provided easy shearing and thus resulted in reduced friction and wear.14 It was further supported by build-up of a MoS2 transfer film in the contact region, when two-dimensional MoS2 nanoparticles have been used for lubrication application.41 Herein, the delamination of the ODA-rGO sheets under contact stress could transfers the layers of ODA-rGO to the contact surfaces and generates a protective thin film. A further study on delamination of ODA-rGO sheets and formation of protective thin film on the contact interfaces is in progress and will be in the future communication. This study promises the potential of graphene sheets in reducing energy and materials losses in tribological applications.

CONCLUSIONS The high mechanical strength, good conductivity and lamellar structure of graphene exhibit its potential for tribological applications. Herein, the chemically functionalized rGO, which contains intrinsic properties of graphene along with long alkyl chains attached to the edges and defects sites, was prepared. In this process, the carboxylic functionalities, which were selectively introduced in the rGO by the mild oxidation, were coupled with ODA to prepare the ODA-rGO sheets. The chemical and structural features of the ODA-rGO sheets were examined by FTIR, XPS, TG-DTA, XRD, FESEM, and HRTEM analyses. The presence of long alkyl chains in the ODA-rGO sheets facilitated their stable dispersion in the 10W-40 commercial engine oil, which is very important for their efficient tribological performance. The ODA-rGO as a potential lubricant additive to 10W-40 lube oil was then evaluated using the steel balls as contact surfaces. The tribological test revealed the ability of ODA-rGO sheets to reduce friction and improve the antiwear properties of steel balls under the rolling contact. Excellent performance of the ODArGO as lubricant additive is attributed to (a) the weak van der Waals interaction between the


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 37

lamellas of the ODA-rGO sheets, which provides the low resistance to shear under the rolling contact stress, and (b) the continuous supply of the ODA-rGO sheets on the contact surfaces due to their stable dispersion in the lube oil. Collectively, such occurrence reduced the friction and the wear between the steel balls, which leads to energy conservation and prevents the material loss.


18

Page 19 of 37

FIGURE 1

ODA-rGO

Transmittance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


orGO

C-N

N-H / C=C

C-H

C-H

rGO

GO

C-O

OH

C=O

C-O-C C=C / H2O

4000

3500

3000

2500

2000

1500

1000

-1

Wavenumber (cm )

Figure 1. Vibration spectra of GO, rGO, orGO and ODA-rGO sheets and their characteristics vibrational assignments.


19


FIGURE 2

(a) Measured spectra Fit Fit components

400 cps

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 37

C-OH & C-O-C

C=C/C-C

O=C-OH C=O

292

290

288

286

284

282

280

Binding Energy, eV

(b) C 1s

Measured spectra Fit Fit components

2000 cps

C-N/C-O O=C-N

292

290

C=C/C-C

C-O-C

288

286

284

282

Binding Energy, eV

(c)

N 1s

50 cps

408

406

404

402

400

398

396

394

392

Binding Energy, eV

Figure 2. High-resolution C1s XPS spectra of (a) GO and (b) ODA-rGO sheets along with their deconvoluted peak components. (c) N1s spectra of ODA-rGO sheets.


20

Page 21 of 37

FIGURE 3

(a)

100

0

90

Weight (%)

80 70

-2

60 50 -4

40

Derivative Weight (%/min)

30 100

200

300

400

500



Temperature ( C)

(b)

100

0

90

Weight (%)

80 70

-2

60 50 -4

40

Derivative Weight (%/min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30 100

200

300

400

500



Temperature ( C)

Figure 3. Thermogravimetric and DTA patterns of (a) GO, and (b) ODA-rGO sheets in the range of 25 - 540 °C temperature under the flow of nitrogen.


21


FIGURE 4

ODA-rGO

150

Intensity, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 37

rGO 100

2x 50

GO 0 10

20

30

40

50

2, degree

Figure 4. XRD patterns of GO, rGO and ODA-rGO sheets.


22

Page 23 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FIGURE 5

Figure 5. FESEM images of (a) GO, and (b) ODA-rGO sheets. HRTEM images of (c) GO, and (d) ODA-rGO sheets.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 37

FIGURE 6

Figure 6. Digital images of (a) 10w-40 commercial engine oil, (b-c) dispersion of rGO, and (d-i) ODA-rGO nanosheets in 10W-40 lubricant oil. The rGO nanosheets could not be dispersed thoroughly in the 10W-40 and within couple of hours it settles down, as highlighted in digital image (c). The ODA-rGO sheets are nicely dispersed and exhibit stability for more than one month. Time for each picture is noted on the respective sample bottle. Concentration of nanosheets: 0.04 mg.mL-1.


24

Page 25 of 37

FIGURE 7

(a)

0.14

Friction Coefficient

0.12

0.10

0.08

0.06

0.04 0.00

0.02

0.04

0.06 -1

Concentration of ODA-rGO, mg.mL

700

(b) 600

WSD, m

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


500

400

300

0.00

0.02

0.04

Concetration of ODA-rGO, mg.mL

0.06 -1

Figure 7. Tribological characteristics: (a) average friction coefficient, and (b) wear scar diameter of the ODA-rGO sheets with a function of their concentration blended with 10W-40 oil under a load of 392 N, rotating speed of 1200 rpm, and temperature of 75 °C for one hour.


25


FIGURE 8

0.14

10W-40 ODA-rGO ODA-GO

600

500

WSD, m

0.13 0.12

400

300

0.11

O DA -rG O

O DA -G O

200

10 W -4 0

Friction Coefficient

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 37

0.10 0.09 0.08 0.07 0

10

20

30

40

50

60

Contact Time, Minutes

Figure 8. Tribological characteristics (friction coefficient and WSD) of 10W-40 lube oil and ODA-rGO sheets blended with 10W-40 lube oil at concentration of 20 ppm. Load: 392 N, rotating speed: 1200 rpm, temperature: 75 °C, and test duration: 1 hour.


26

Page 27 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FIGURE 9

Figure 9. FESEM images of worn surfaces of the steel balls lubricated with (a-c) 10W-40 lube oil, and (d-f) ODA-rGO sheets blended 10W-40 lube oil. Load: 392 N, rotating speed: 1200 rpm, temperature: 75 °C, tribo-test duration: one hour, and concentration of the ODA-rGO sheets: 20 ppm.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 37

SCHEME 1

(a)

COOH

OH

COOH

COOH

HO

OH OH HO

HO

OH

OH

O

O

O

O

OH

O

OH

O OH

O

HO

OH HO

O

HO

HOOC OH

OH

HO

HO

OH

COOH

O

COOH

OH

COOH

Reduction N2N4.H20 / 100 oC

(b) O O OH O

HO

HO

Mild Oxidation HNO3

(c)

COOH

COOH O

HOOC O

OH O

HO HOOC

COOH

OH

HO OH O

COOH HOOC COOH

(i) SOCl2 / DMF / 120 oC (ii) CH3-(CH2)17-NH2

(d)

O O

O

OH O

Scheme 1. Schematic model of (a) GO, (b) rGO, illustrating the distribution of various oxygen functionalities in basal plane and at edges of sheets, (c) mild oxidation of rGO selectively introduces carboxylic groups at the edges and defects sites of sheets, and (d) ODA functionalized rGO, prepared by amide linkage on the carboxyl sites of rGO sheets.


28

Page 29 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TABLE 1 Table 1. Physico-chemical properties of lubricants used for tribological evaluation Sample Description

Kinematic viscosity, mm2 s-1

Viscosity

Pour

index

point, °C

Density, g.ml-1

At 40 °C

At 100 °C

10W-40 engine oil

93.3

13.8

151

-36

0.87

ODA-rGO (20 ppm) + 10W-40 engine oil

98.2

13.9

144

-36

0.87


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 37

ASSOCIATED CONTENT Supporting Information Dispersion images of rGO in different solvents. This material is available free of charge via internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected], Fax: +91 135 2660200 Funding Sources Council of Scientific and Industrial Research, Govt. of India, India Notes The authors declare no competing financial interest

ACKNOWLEDGMENT We kindly acknowledge the Director IIP for his kind permission to publish these results. The authors are thankful to CSIR, India for financial support. We are thankful to ASD of IIP, the DST unit of Nanoscience, IIT Chennai, and H. Sugimura, Kyoto University, Japan for providing help in analyses of the samples. H.P.M. thanks the UGC, India for financial support.


30

Page 31 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES (1) Holmberg, K.; Andersson, P.; Erdemir, A. Global Energy Consumption due to Friction in Passenger Cars. Tribo. Inter. 2012, 47, 221-234. (2) Tung, S. C.; McMillan, M. L. Automotive Tribology Overview of Current Advances and Challenges for the Future. Tribo. Inter. 2004, 37, 517-536. (3) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nature Mater. 2007, 6, 183-191. (4) Geim, A. K. Graphene: Status and Prospects. Science 2009, 324, 1530-1534. (5) Zhu, Y.; Murali, S.; Cai, W.; Li, X.; Suk, J. W.; Potts, J. R.; Ruoff, R. S. Graphene and Graphene Oxide: Synthesis, Properties and Applications. Adv. Mater. 2010, 22, 39063924. (6) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene. Science 2008, 321, 385-388. (7) Netro, A. H. C.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K. The Electronic Properties of Graphene. Rev. Mod. Phys. 2009, 81, 109-162. (8) Luo, B.; Lio, A.; Zhi, L. Chemical Approaches toward Graphene-Based Nanomaterials and their Applications in Energy-Related Areas. Small 2012, 8, 630-646. (9) Novoselov, K. S.; Falko, V. I.; Colombo, L.; Gellert, P. R.; Schwab, M. G.; Kim, K. A Roadmap for Graphene. Nature 2012, 490, 192-200.


31


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 37

(10) Wan, X.; Huang, Y.; Chen, Y. Focusing on Energy and Optoelectronic Applications: A Journey for Graphene and Graphene Oxide at Large Scale. Acc. Chem. Res. 2012, 45, 598607. (11) Su, C.; Loh, K. P. Carbocatalysts: Graphene Oxide and its Derivatives. Acc. Chem. Res. 2013, 46, 2275-85. (12) Filleter, T.; McChesney, J. L.; Bostwick, A.; Rotenberg, E.; Emtsev, K. V.; Seyller, T.; Horn, K.; Bennewitz, R. Friction and Dissipation in Epitaxial Graphene Films. Phys. Rev. Lett. 2009, 102, 086102. (13) Kim, K. -S.; Lee, H. -J.; Lee, C.; Lee, S. -K.; Jang, H.; Ahn, J.-H.; Kim, J .-H.; Lee, H. J. Chemical Vapor Deposition-Grown Graphene: The Thinnest Solid Lubricant. ACS Nano 2011, 5, 5107-5114. (14) Berman, D.; Erdemir,; A. Sumant, A. V. Reduced Wear and Friction Enabled by Graphene Layers on Sliding Steel Surfaces in Dry Nitrogen. Carbon 2013, 54, 454-459. (15) Lee, C.; Li, Q.; Kalb, W.; Liu, X. -Z.; Berger, H.; Caprick, R. W.; Hone, J. Frictional Characteristics of Atomically Thin Sheets. Science 2010, 328, 76-80. (16) Deng, Z.; Klimov, N. N.; Solares, S. D.; Li, T.; Xu, H.; Cannara, R. J. Nanoscale Interfacial Friction and Adhesion on Supported versus Suspended Monolayer and Multilayer Graphene. Langmuir 2013, 29, 235-243. (17) Marchetto, D.; Held, C.; Hausen, F.; Wahlisch, F.; Dienwiebel, M., Bennewitz, R. Friction and Wear on Single-Layer Epitaxial Graphene in Multi-Asperity Contacts. Tribol. Lett. 2012, 48, 77-82.


32

Page 33 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(18) Qu, J.; Wang, J.; Liu, S.; Mu, B.; Ren, J.; Wang, H.; Yang, S. Tribology Study of Reduced Graphene Oxide Sheets on Silicon Substrate Synthesized via Covalent Assembly. Langmuir 2010, 26, 15830-15836. (19) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.;, R. D. Piner,; E. A. Stach; Nguyen, S. T.; Rouff, R. S. Graphene-Based Composite Materials. Nature, 2006, 442, 282-286. (20) Lin, J.; Wang, L.; Chen, G. Modification of Graphene Platelets and their Tribological Properties as a Lubricant Additive. Tribol. Lett. 2011, 41, 209-215. (21) Eswaraiah, V.; Sankaranarayanan V.; Ramaprabhu, S. Graphene-Based Engine Oil Nanofluids for Tribological Applications. ACS Appl. Mater. Interfaces 2011, 3, 4221-4227. (22) Khare, V. M.; Pham, -Q.; Kumari, N.; Yoon,; Kim, C. -S.; Park, J. -I.; Ahn, S. -H. Graphene-Ionic Liquid Based Hybrid Nanomaterials as Novel Lubricant for Low Friction and Wear. ACS Appl. Mater. Interfaces 2013, 5, 4063-4075. (23) Choudhary, S.; Mungse, H. P.; Khatri, O. P. Dispersion of Alkylated Graphene in Organic Solvents and its Potential for Lubrication Applications. J. Mater. Chem. 2012, 22, 2103221039. (24) Suk, J. W.; Piner, R. D.; An, J.; Rouff, R. S. Mechanical Properties of Monolayer Graphene Oxide. ACS Nano 2010, 4, 6557-6564. (25) Gusain, R.; Khatri, O. P. Ultrasound Assisted Shape Regulation of CuO Nanorods in Ionic Liquids and their Use as Energy Efficient Lubricant Additives. J. Mater. Chem. A 2013, 1, 5612-5619.


33


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 37

(26) Zhang, J.; Zou, H.; Qing, Q.; Yang, Y.; Li, Q.; Liu, Z.; Guo, X.; Du, Z. Effect of Chemical Oxidation on the Structure of Single-Walled Carbon Nanotubes. J. Phys. Chem. B 2003, 107, 3712-3718. (27) Park, S.; An, J.; Jung, I.; Piner, R. D.; An, S. J.; Li, X.; Velamakanni, A.; Ruoff, R. S. Colloidal Suspensions of Highly Reduced Graphene Oxide in a Wide Variety of Organic Solvents. Nano Lett. 2009, 9, 1593-1597. (28) Gromov, A.; Dittmer, S.; Svensson, J.; Nerushev, O. A.; Perez-Garcia, S. A.; LiceaJimenez, L.; Rychwalski, R.; Campbell, E. E. B. Covalent Amino-Functionalisation of Single-Wall Carbon Nanotubes. J. Mater. Chem. 2005, 15, 3334-3339. (29) Acik, M.; Lee, G.; Mattevi, C.; Pirkle, A.; Wallace, R. M.; Chhowalla, M.; Cho, K.; Chabal, Y. The Role of Oxygen during Thermal Reduction of Graphene Oxide Studied by Infrared Absorption Spectroscopy. J. Phys. Chem. C 2011, 115, 19761-19781. (30) Su, C.; Acik, M.; Takai, K.; Lu, J.; Hao, S.; Zheng, Y.; Wu, P.; Bao, Q.; Enoki, T.; Chabal, Y. J.; Loh, K. P. Probing the Catalytic Activity of Porous Graphene Oxide and the Origin of this Behaviour. Nature Commun. 2012, 3, 1298. (31) Lerf, A.; He, H.; Forster, M.; Klinowski, J. Structure of Graphite Oxide Revisited. J. Phys. Chem. B 1998, 102, 4477-4482. (32) Cai, W.; Piner, R. D.; Stadermann, F. J.; Park, S.; Shaibat, M. A.; Ishii, Y.; Yang, D.; Velamakanni, A.; An, S. J.; Stoller, M.; An, J.; Chen, D.; Ruoff, R. S. Synthesis and SolidState NMR Structural Characterization of 13C-Labeled Graphite Oxide. Science 2008, 321, 1815-1817.


34

Page 35 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(33) Mattevi, C.; Eda, G.; Agnoli, S.; Miller, S.; Mkhoyan, K. A.; Celik, O.; Mastrogiovanni, D.; Granozzi, G.; Grafunkel, E.; Chhowalla, M. Evolution of Electrical, Chemical and Structural Properties of Transparent and Conducting Chemically Derived Graphene Thin Films. Adv. Funct. Mater. 2009, 19, 2577-2583. (34) Choudhary, S.; Mungse, H. P.; Khatri, O. P. Hydrothermal Deoxygenation of Graphene Oxide: Chemical and Structural Evolution. Chem. Asian J. 2013, 8, 2070-2078. (35) Ruck-Braun, K.; Peterson, M. A.; Michalik, F.; Hebert, A.; Przyrembel, D.; Weber, C.; Ahmed, S. A.; Kowarik, S.; Weinelt, M. Formation of Carboxy- and Amide-Terminated Alkyl Monolayers on Silicon(111) Investigated by ATR-FTIR, XPS, and X-ray Scattering: Construction of Photoswitchable Surfaces. Langmuir 2013, 29, 11758-11769. (36) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Synthesis of Graphene-based Nanosheets via Chemical Reduction of Exfoliated Graphite Oxide. Carbon 2007, 45, 1558-1565. (37) Erickson, K.; Erni, R.; Lee, Z.; Alem, N.; Gannett, W.; Zettl, A. Determination of the Local Chemical Structure of Graphene Oxide and Reduced Graphene Oxide. Adv. Mater. 2010, 22, 4467-4472. (38) Mkhoyan, K. A.; Contryman, A. W.; Silcox, J.; Stewart, D. A.; Eda, G.; Mattervi, C.; Miller, S.; Chhowalla. Atomic and Electronic Structure of Graphene-Oxide. Nano Lett. 2009, 9, 1058-1063. (39) Everett, D. H. Basic Principles of Colloidal Science, The Royal Society of Chemistry, London, 1988.


35


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 37

(40) Aktary, M.; McDermott, M. T.; McAlpine, G. A. Morphological and Nanomechanical Properties of ZDDP Antiwear Films As a Function of Tribological Contact Time. Tribo. Lett. 2002, 12, 155-162. (41) Praveena, M.; Bain, C. D.; Jayaram, V.; Biswas, S. K. Total Internal Reflection (TIR) Raman Tribometer: A New Tool for in Situstudy of Friction Induced material transfer. RSC Adv. 2013, 3, 5401-5411.


36

Page 37 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents


37

Chemically Functionalized Reduced Graphene Oxide as a Novel

Recommend Documents