Graphene Oxide Nanocomposite Synthesized in Supercritical CO2

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Au/Graphene Oxide Nanocomposite Synthesized in Supercritical CO2 Fluid as Energy Efficient Lubricant Additive Yuan Meng, Fenghua Su, and Yangzhi Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on October 25, 2017

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ACS Applied Materials & Interfaces

Au/Graphene Oxide Nanocomposite Synthesized in Supercritical CO2 Fluid as Energy Efficient Lubricant Additive Yuan Meng, Fenghua Su* and Yangzhi Chen School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou, China

ABSTRACT Au nanoparticles are successfully decorated onto graphene oxide (GO) sheets with the aid of supercritical carbon dioxide (ScCO2) fluid. The synthesized nanocomposite (Sc-Au/GO) was characterized by X-ray diffraction (XRD), Raman spectroscopy, thermal gravimetric analysis (TGA) and transmission electron microscopy (TEM). The characterization results show that the Au nanoparticles are featured with face-centered cubic crystal structure and disperse well on the GO nanosheet surfaces with average diameters of 4-10 nm. The tribological behaviors of Sc-Au/GO as lubricating additive in PAO6 oil were investigated using a ball-on-disc friction tester, and a control experiment by respectively adding GO, nano-Au particles, and Au/GO produced in the absence of ScCO2 was performed as well. It is found that Sc-Au/GO exhibits the best lubricating performances among all the samples tested. When 0.10 wt. % Sc-Au/GO is dispersed into PAO6 oil, the friction coefficient and wear rate are respectively reduced by 33.6% and 72.8% as compared with that of the pure PAO6 oil, indicating that Sc-Au/GO is an energy efficient lubricant additive. A possible lubricating mechanism of Sc-Au/GO additive in PAO6 oil has been tentatively proposed on the basis of the analyzed results of the worn surface examined by scanning electron microscopy (SEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS).

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KEYWORDS: Graphene oxide; Au nanoparticles; Supercritical CO2; Lubricating additive; Friction and wear

1. INTRODUCTION In modern industry, reducing friction and wear of mechanical elements becomes a critical factor for enhancing durability of mechanical components and improving machine efficiency. Using lubricant additives has been widely regarded as a feasible strategy to improve the lubricating performances of lubricants. Besides traditional organic molecules, numerous metal and metallic oxide nanoparticles have been proven to be effective lubricating additives for reducing friction and wear.1-12 For instance, Chen et al.1 discovered that when nano-nickel particles were added in synthetic PAO6 oil, decreased friction and wear and increased loading-carrying capability were achieved. Gusain et al.2 reported that CuO nanorods as additives in PEG 200 and 10W-40 oil exhibited excellent friction reduction and anti-wear behaviors. Recently, several reports have shown that superior friction-reducing and anti-wear properties can be achieved by the introduction of trace amount of precious metal nanoparticles, such as silver, gold, and palladium as lubricating additives without significant increasing of the costs.13-17 Li et al.13 dispersed 0.5 wt. % dialkyldithiophosphate coated Ag nanoparticles into liquid paraffin and obtained 51 % reduction in wear scar diameter and 60 % enhancement in PB value. Wang et al.14 found that the addition of traces of Au nanoparticles (1.02 × 10-3 wt. %) in ionic liquid could reduce the friction coefficient and wear volume by 13.8 % and 45.4 % respectively. It is extremely challenging to completely explain the lubricating mechanism of nanoparticles as lubricating additive to date, 2


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because of the changeable friction environment and various types of nanoparticles employed. Several hypotheses such as deposited film, cold welding, surface alloying and rolling effect have been proposed to have close relation with the anti-wear mechanism of nanoparticle additives. It has also been realized that the features of chemical composition, grain size and morphology of nanoparticles had important effects on their lubricating performances. Besides metal and metallic oxide nanoparticles, lots of layered materials including graphene, graphite, molybdenum disulfide (MoS2) and hexagonal boron nitride (h-BN) have also been widely considered as effective solid lubricants.18-33 The unique anisotropic crystal structure imparts the layer materials with strong covalent intralayer and weak van der Waals interlayer interactions, which lead to effective lubrication eventually. Among these materials, graphene has been considered as the most promising and attractive material due to its extreme strength and easy shearing capability on the densely packed and atomically smooth sheets.18-27 Tabandeh-Khorshid et al.18 found that 1 wt. % graphene nanoplatelets greatly improved tribological properties of the filled aluminum matrix. Liang et al.19 discovered that in-situ exfoliated graphene in deionized water could offer 81.3% and 61.8% reduction in friction coefficient and wear scar diameter, respectively. Such intriguing anti-wear abilities from graphene was further confirmed by Gupta et al.20, as they found that the friction coefficient and wear rate were remarkably reduced to 70% and 50% through introducing optimized concentration of rGO into neat oil. They thought that the graphene sheets were linked with PEG molecules through hydrogen bonding and thus provided sufficiently lower shear strength for the formed boundary tribofilm. 3



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Since nanoparticles and layered materials are both excellent lubrication materials, it is quite appealing to decorate nanoparticles on layered material substrates as integrated composite as advanced lubricative agent. In fact, several researches have performed such recombination and found that the physicochemical properties including the tribological property can be greatly improved.34-40 Song et al.34 synthesized α-Fe2O3 nanorod/graphene oxide composites in a hydrolysis process and found that the friction and wear properties of this composite was better than that of graphene oxide (GO) and the mechanical mixture of α-Fe2O3 and GO when employed as lubricating additives in paraffin oil. Although the nanocomposites of layered materials dotted with nanoparticles have great potential to obtain superior lubrication performances, it is still a challenge to achieve well-defined nanostructure and optimal combination of those base ingredients at nanoscale dimension, which has direct influences on the play of ingredients’ capability.35,36 Supercritical fluid technique is regarded as an effective and efficient technique for preparing high quality nanocomposites.41-47 Supercritical carbon dioxide (ScCO2) is the most widely employed supercritical medium, thanks to its readily accessible supercritical conditions and various unique properties including gas-like diffusivity, extremely low viscosity, near-zero surface tension and excellent mass-transfer activity. ScCO2 can easily wet substrates, increase loading weight and make metal nanoparticles decorated on substrate surfaces uniformly. Several reports documented that the nanocomposites fabricated by the ScCO2 technique usually presented more regular microstructure and better macro-properties than those prepared without the aid of ScCO2.41,42 In this work, we establish a facile in situ one-step reduction route for synthesizing Au/graphene 4


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oxide nanocomposite (Sc-Au/GO) with the aid of ScCO2 fluids. The tribological performances of this nanocomposite used as lubricating additive in PAO6 oil are investigated by a ball-on-disc friction tester. Other nanomaterials including GO, nano-Au particles, and Au/GO produced in air are also tested as control samples. The lubrication model of the friction pairs lubricated by the Sc-Au/GO dispersed PAO6 oil is discussed and proposed. The findings here provide a viable strategy for preparing nanoparticle/graphene oxide composites with remarkable tribological performances readily for potential industrial applications.

2. EXPERIMENTAL SECTION 2.1. Chemicals All chemicals used in this work were of analytical grade. Gold precursor (HAuCl4·4H2O) was purchased from Shanghai Zhanyun Chemical Co., Ltd. Glucose (C6H12O6) was purchased from Shanghai Rich Joint Chemical Reagent Co., Ltd. Natural graphite with average size around 44 µm was supplied from Tianjin Fuchen Chemical Reagent Factory. Sodium dodecyl sulfate (SDS) was supplied from Sinopharm Chemical Reagent Co., Ltd. Oleyamine (C18H37N) was purchased from Shanghai Hansi Chemical Industry Co., Ltd. Ethanol was produced by Tianjin Fuyu Fine Chemical Co., Ltd. Carbon dioxide of high purity was used in our experiments. 2.2. Synthesis of Sc-Au/GO Graphene oxide (GO) was prepared by following a modified Hummers method48,49 with natural graphite powders as raw materials, and applied as substrate to load Au nanoparticles. Glucose aqueous dispersion (2 mL, 9.1 wt. %) was prepared by dissolving glucose in deionized water. In 5



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a typical experiment to synthesize the nanocomposite of Sc-Au/GO, suitable amounts of GO, AuHCl4·4H2O and SDS were dissolved and ultrasonically dispersed in 48 mL of ethanol, respectively. The suspension was then mixed with 2 mL glucose aqueous dispersion to obtain the final reaction suspension. Subsequently, the reaction suspension was loaded into a stainless autoclave of 100 mL, which was preheated in advance. After flushed with carbon dioxide gas for 2 min, the autoclave was sealed. The autoclave was pressurized by CO2 to 12 Mpa and heated up to 120 oC. The reaction suspension was maintained at this pressure and temperature and vigorously stirred by magnetic agitation at 450 rpm for 2 h, and then cooled to room temperature naturally. After the autoclave was depressurized, the dark precipitate was separated from the suspension by centrifugation, and then washed with copious ethanol and deionized water repeatedly. After vacuum-dried at 65 oC for 8 h, the precipitate was collected and denoted as Sc-Au/GO for further characterizations and friction tests. The gold nanoparticles (nano-Au) were also synthesized using the same method without the addition of GO. In addition, the nanocomposite of Au/GO was also prepared by the same fabrication process but without the introduction of ScCO2. 2.3. Preparation of Sc-Au/GO Dispersed Oil Lubricating performances of nanoparticle additive are seriously influenced by its dispersity and stability in neat oil. To obtain well-dispersed and stable oil dispersions, all the synthesized nanomaterials including GO, nano-Au, Au/GO and Sc-Au/GO are modified by oleylamine. Typically, 100 mg nanomaterials were dispersed in 20 mL ethanol solution of oleylamine (15 mg·mL-1). After sealed in a 100 mL glass flask, the mixture was gently refluxed with slow 6


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magnetic stirring of 60 rpm at 90 oC for 24 h. The products were washed by hot ethanol to remove excess modifiers and then dried naturally in air. The modified nanomaterials with desired mass fraction were ultrasonically dispersed in PAO6 synthetic oil to obtain the dispersed oil samples. 2.4. Characterizations XRD analyses of the nanomaterials were carried out using a Philips X’pert X-ray diffractometer at 40 kV and 40 mA with Cu-κα radiation. The diffraction data were recorded for 2θ angles between 5o to 90o. The morphology and microstructure were observed with a field-emission transmission electron microscope (TEM, JEOL JEM-2010F). Raman spectra were recorded by a multichannel confocal micro-spectrometer (Dilor Labram-1B, excitation laser of 20 mW and 532 nm). Thermal gravimetric analysis (TGA, STA 449C, Germany) was performed in air atmosphere at a heating rate of 10 oC·min-1 from room temperature to 800 oC. Lubrication characteristics of the neat PAO6 oil and the nanomaterial dispersed oils were tested by a ball-on-disc friction tester (MS-T3000, Lanzhou Huahui Instrument Technology Co., Ltd., China). The tests were performed at room temperature and ambient humidity under applied load of 10 N for 30 min. The bottom disc is rotated against the stationary upper ball with a speed of 0.1 m·s-1. The upper ball with diameter of 6.5 mm is composed of GCr15 steel (AISI 52100) and the bottom disc with sliding contact radius of 10 mm consists of stainless steel (AISI 40300). Prior to installment, the steel ball and disc were cleaned ultrasonically in petroleum ether and dried in air. The friction coefficients were recorded automatically by a strain sensor. The wear track on the disc was examined by a Taylor profilometer (Talysurf CLI 1000) to compute the 7



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wear rate. The cross section area (Ss) of the wear track is obtained directly using the analysis software (TalyMap Universal 3.2.0). The wear volume (Vw) is calculated for a half-ellipse track according to Eq. (1) and the load-normalized wear rate (Wr) is calculated according to Eq. (2).: = ∙ 2

(1)

/ = / ∙

(2)

Where Ss = cross section area of wear tracks (mm2), L1 = radius of circular tracks (10 mm). L2 = sliding distance (180 m), F = applied normal load (10 N). Each tribo-test and wear track measurement was carried out at least three times to ensure standard deviations less than 5%. The morphology of the wear scar on the ball and the wear track on the disc was observed using a scanning electron microscope (SEM, JEOL JSM 6700F). X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD; Al-κα radiation) and Raman spectroscopy were employed to analyze the chemical states of typical elements on the wear track surface.

3. RESULTS AND DISCUSSION 3.1. Composition and Morphology X-ray diffraction (XRD) patterns of GO, nano-Au, Au/GO and Sc-Au/GO are presented in Figure 1. The typical diffraction peak at 10.2o is attributed to the graphene (001) lattice plane of GO. However, it completely disappears in the patterns of Au/GO and Sc-Au/GO, which is probably due to effective prevention of graphene oxide restacking and aggregation caused by the deposited Au nanoparticles.46,50 We postulate that the dotted Au nanoparticles can exfoliate the GO layers and prevent them from restacking orderly. In the XRD patterns of nano-Au, Au/GO 8


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and Sc-Au/GO, the peaks at 38.2o, 44.4o, 64.6o, 77.5o, 81.7o are respectively ascribed to gold (111), (200), (220), (311) and (222) crystallographic planes (JCPDS No.04-0784), confirming that the dotted Au nanoparticles on GO sheets adopted with face-centered cubic (FCC) crystal structure. Moreover, these peaks in Sc-Au/GO are broader than those in Au/GO at the same 2θ position, signifying that the smaller grain sized Au crystals in Sc-Au/GO were obtained.

Figure 1. XRD patterns of GO, nano-Au, Au/GO and Sc-Au/GO.

Next, Raman spectroscopy was conducted to further examine the structural features of these samples. As shown in Figure 2, two neighboring sharp peaks can be found for GO in Raman spectra, the G band (κ-point phonon of A1g symmetry) at around 1580 cm-1 and the D band (E2g phonon of C sp2 atoms) at around 1350 cm-1. Interestingly, the intensities of the D and G bands in Sc-Au/GO and Au/GO are distinctly increased due to the adsorption of Au nanoparticles. Such

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phenomenon has been well documented in previous reports 40,51for the nanocomposites of carbonous materials decorated by metal nanoparticles. The so-called surface enhanced Raman scattering (SERS) is mainly caused by chemical enhancement effect.40,51 Note that, the G/D ratio is an important parameter for analyzing the structural properties of graphene oxide. The G/D ratios for GO, Sc-Au/GO and Au/GO are 1.06, 0.99 and 0.84, respectively. It can be noted that, the D band exhibited a higher increase than the G band for the composites especially for Au/GO, indicating that more defects are presented upon the decoration of Au nanoparticles. No other peaks below 1000 cm-1 are observed, which suggests that no impurity of any metallic oxides exist in these nanocomposites. In addition, a relatively weak but broad 2D band of GO is located at ~2701 cm−1, which is ascribed to the graphite-like structures.25 GO restacked during the drying process in vacuum chamber and formed the graphite-like structures which can be broken by the following ultrasound treatment. When Au nanoparticles are decorated on GO sheets, the layer interspaces of GO sheets are expanded by the nanoparticles and the restacking is also impeded, leading to decreased layers of GO sheets in the nanocomposites of Au/GO and Sc-Au/GO. Consequently, the 2D bands of Au/GO (at ~2650 cm−1) and Sc-Au/GO (at ~2688 cm−1) are downshifted and become narrower as compared to that of GO.

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Figure 2. Raman spectra of GO, Au/GO and Sc-Au/GO.

To unveil the components in the composites, thermogravimetric analysis (TGA) was then performed. Figure 3 shows the TGA curves of GO, Au/GO and Sc-Au/GO. The weight loss below 100 oC is ascribed to the evaporation of adsorbed water. GO shows the largest water weight loss, which is probably due to a large number of water molecules absorbed by a multitude of oxygen-containing groups on its surface. The loss in the range of 100~300 oC originates from the thermal decomposition of the oxygen-containing groups, while the loss between 300~600 oC is owing to the pyrolysis of carbon structures that are converted to carbon dioxides in air. All three curves exhibit similar pattern with the increasing of temperature, indicating a very weak reduction degree of GO in Au/GO and Sc-Au/GO nanocomposites. Meanwhile, it can be seen that GO is nearly burned out with few remnants remained (4.81 wt. %), as the temperature 11



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reaches 800 oC. In contrast, the eventual residual weights of Au/GO and Sc-Au/GO are 16.77 wt. % and 38.65 wt. %, respectively. The final residues of Au/GO and Sc-Au/GO should be the leftover Au nanoparticles because of the chemical inertness of gold at high temperature. The higher mass fraction of Au nanoparticles in Sc-Au/GO than that in Au/GO is closely related to the function of ScCO2 during the synthesis process.41,45-47 The unique properties such as gas-like diffusivity, extremely low viscosity and excellent mass-transfer activity of ScCO2 are beneficial for transferring more precursors onto GO surfaces, leading to more metal particles anchored on GO nanosheets.

Figure 3. TGA curves of GO, Au/GO and Sc-Au/GO.

The shape and surface microstructure of the samples were then observed by electron microscopic techniques. The representative TEM images of nano-Au, GO and the nanocomposites are presented in Figure 4. As shown in Figure 4a, GO sheet resembles a transparent crumpled paper 12


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and no micropores or mesopores are observed on the sheet surface. Figure 4b shows that the as-prepared nano-Au particles display spherical shape but heavy aggregation is observed with large chunks easily identified. Apparently, it is easy to recognize the influence of ScCO2 on the morphology and microstructure of the nanocomposites by comparing the TEM images of Au/GO (Figure 4c) and Sc-Au/GO (Figures 4d, e). The Au nanoparticles in Au/GO are dispersed inhomogeneously on GO sheets and show a wide size range between 10 and 90 nm. In sharp contrast, the homogeneous Au grains with average diameter of 4-10 nm are uniformly dispersed on the exfoliated GO sheets in Sc-Au/GO, as shown in Figures 4d, e. According to the previous reports,41,42 ScCO2 is conductive to exfoliate of GO sheets, enhances the adhesion and dispersion of metal precursors on the sheet surfaces, and prevents the growth of metal crystals from out of control during the synthesis process. Accordingly, the Au nanoparticles with smaller grain size and high-quality dispersion are achieved in the nanocomposite of Sc-Au/GO, which agrees well with the XRD results. Additionally, HR-TEM image of typical Au nanoparticles on GO sheet taken from Figure 4e is shown in Figure 4f. The lattice spacing of 0.2355 nm corresponds well to the (111) lattice plane of typical FCC Au crystal.

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Figure 4. Representative TEM images of (a) GO, (b) Nano-Au particles, (c) Au/GO and (d, e) Sc-Au/GO; (f) Typical HR-TEM image of Au nanoparticles on GO sheets from Sc-Au/GO.

3.2. Lubricating Performances As we known, the dispersity and stability of nanoparticle additives in neat oil play a decisive role in the lubricating performances of the dispersed oil. Figure 5 shows the stabilities of the neat PAO6 oil and the oils dispersed with different nanomaterials after 10 days’ standing. It is observed that few precipitates are observed at the bottom of the glass bottles, indicating the qualified dispersity and stability of these oil samples.

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Figure 5. Photographs of the neat PAO6 oil and the nanomaterial dispersed oil samples after standing for 10 days.

Figure 6a shows the variations of friction coefficients (FCs) lubricated with the neat PAO6 oil and the different dispersed oil samples as a function of sliding distance. The FC of the neat PAO6 oil increases from 0.094 in the beginning to 0.120 at around 50 m of sliding and then maintains at this level till the end of sliding, which indicates the lubrication state in neat oil most likely belongs to a mixed lubrication (ML) regime that contains dry contact (DC) and boundary lubrication (BL) during the sliding process. The dispersed oils with these nanomaterials present lower FCs almost during the entire sliding process, when compared to the neat PAO6 oil. The FCs of the nano-Au dispersed oil are lower than that of the GO dispersed oil at the initial 50 m of sliding and become higher with the further increase of sliding distance. The nanocomposite dispersed oils show much lower and steadier FCs than GO or nano-Au dispersed oil, indicating that an improved lubrication state is achieved during the sliding process. 15



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The average FCs and the wear rates (Wrs) of the corresponding bottom discs lubricated with these oils are shown in Figure 6b. It is clear that these nanomaterials are effective for reducing friction coefficient and wear rate as lubricating additive. The nanocomposites of Sc-Au/GO and Au/GO exhibit better lubricating abilities than the individual nanomaterial of Nano-Au or GO, which confirms the synergistic effect of GO sheets and Au nanoparticles.34,36 In addition, Figure 6b also reveals the different friction-reducing and antiwear abilities of Au/GO and Sc-Au/GO, which is due to their different microstructures and morphologies (Figures 4c, d). The smaller grain size and more uniform dispersion of Au nanoparticles anchored on GO surfaces might be favorable for the release of the friction-reducing and antiwear potentials of Au nanoparticles and GO in the nanocomposite. As a result, the Sc-Au/GO dispersed oil presents the better lubricating ability than the Au/GO dispersed one.

Figure 6. (a) Friction coefficient curves for the pure PAO6 oil and the dispersed oils respectively with 0.10 wt. % GO, nano-Au, Au/GO, and Sc-Au/GO with increasing sliding distance; (b) Average friction coefficients (FCs) and wear rates (Wrs) of the corresponding discs lubricated with these oils. 16


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It is well known that, the concentration of additive in lubricating oil plays an important role to determine lubricative characteristics. Effect of the Sc-Au/GO concentration on the FCs and Wrs of the Sc-Au/GO dispersed oil is shown in Figure 7. The FCs and Wrs decrease firstly and then increase slowly with increasing concentration of Sc-Au/GO. And there exist a narrow bottom in the curve where the lowest FC and Wr are presented. At the bottom, the FC and Wr are reduced by 33.6% and 72.8% respectively, in comparison with the neat PAO6 oil. With adding Sc-Au/GO into oil and increasing its concentration, more and more nanocomposites deposit on wear surface and thus greatly reduce the roughness of the surface, and the corresponding lubrication state gradually goes up into a good lubrication regime.52,53 Nevertheless, excessive concentration of Sc-Au/GO in oil will lead to GO piling up between friction pairs, thus blocking the oil film, and oil film will become much more discontinuous, even causing a dry friction.53 As a result, the FC and Wr will increase beyond the bottom point of the curves (about 0.10 wt. %).

Figure 7. Friction coefficients (FCs) and wear rates (Wrs) versus Sc-Au/GO concentration in the 17



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Sc-Au/GO dispersed oil. 3.3. Analyses of Wear Interfaces Figure 8 shows the typical SEM images of the wear scars on the balls and the wear tracks on the corresponding discs lubricated with different oil samples. As shown in Figures 8a, d, the wear scar on the ball lubricated by the neat PAO6 oil is very big and presents many block wear debris and deep furrows, indicating severe scuffing and adhesion wear occurred. It can be concluded that the contact pairs lubricated with the PAO6 oil belongs to the mixed lubrication state. In contrast, the wear scars on the balls lubricated by the Au/GO dispersed oil (Figures 8b, e) and the Sc-Au/GO dispersed oil (Figures 8c, f) become much smaller and shallower and have a small number of wear debris, which indicates the improvement in lubrication state and the reduction in friction and wear. During the sliding process, GO sheets and Au nanoparticles in the nanocomposites easily deposit on contact pair surfaces and form a protective film,13-16,19 which can smooth the surfaces and thus reduce friction and wear. Figures 8g-i shows the wear tracks on the discs lubricated with different oil samples. The width of wear tracks on the discs matches well with the size of wear scars on the corresponding balls. As shown in Figure 8g, the wear track on the disc lubricated by the pure PAO6 oil is fluctuant and very deep and contains lots of wear debris. In contrast, the wear tracks on the discs lubricated by the nanocomposite dispersed oils become clean and small, as shown in Figures 8h, i. In general, the wear surface lubricated by the Sc-Au/GO dispersed oil are the evenest and smoothest and show the smallest wear on the contact pairs, as shown in Figures 8c, f and i. The results are in good accordance with the previous findings in Figure 6, further verifying the best antiwear ability of Sc-Au/GO among all 18


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these nanomaterials as lubricating additives in PAO6 oil.

Figure 8. Typical SEM images of (a-f) wear scars on upper balls and (g-i) wear tracks on bottom discs lubricated with different oil samples. (a, d, g) PAO6 oil, (b, e, h) PAO6 + 0.10 wt. % Au/GO and (c, f, i) PAO6 + 0.10 wt. % Sc-Au/GO.

The 3D simulated images of the wear tracks on the discs lubricated with the PAO6 oil and the nanocomposite dispersed oils are examined by a Taylor profilometer and shown in Figure 9. It is

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clear that the wear track lubricated by the PAO6 oil (Figures 9a, b) is extremely rough and has huge peak and many wide, deep furrows. As 0.10 wt. % Au/GO or Sc-Au/GO are dispersed in the oil, the generated scratches and furrows become much smaller and the surface roughness greatly decreases, as shown in Figures 9c-f. These simulated images give a visual representation of the decreased wear volume of the discs lubricated by the Au/GO and Sc-Au/GO dispersed oils. The Wr of the wear tracks on the discs lubricated with the PAO6 oil, the Au/GO dispersed oil and the Sc-Au/GO dispersed oil are calculated as 3.268×10-6, 1.155×10-6 and 8.889×10-7 mm3(Nm)-1, respectively, which proves the excellent lubricating effects of the nanocomposites in PAO6 oil.

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Figure 9. Taylor profilometer 3D simulated images of the wear tracks on discs lubricated with (a, b) PAO6 oil, (c, d) 0.10 wt. % Au/GO dispersed oil and (e, f) 0.10 wt. % Sc-Au/GO dispersed oil.

Figures 10a-c displays the optical microscopy images of the bare steel surface and the wear track surfaces lubricated with the PAO6 oil and the Sc-Au/GO dispersed oil, and Figure 10d presents the Raman spectra of the typical position (marked with point) on these surfaces to confirm the 21



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deposition action of the Sc-Au/GO additive during rubbing process. The surfaces of the bare steel in Figure 10a and the wear track in Figure 10b are relatively bright and clean, but large dark areas spread on the wear track surface in Figure 10c. As shown in Figure 10d, a dark area (point 5 in Figure 10c) in the wear track surface lubricated with the Sc-Au/GO dispersed oil exhibits strong D band and G band of GO in the Raman spectrum. The two bands with weaker intensities are also observed on the bright area (point 4) of Figure 10c. The Raman spectra of the bare steel surface (point 1) and the bright/dark areas (points 2 and 3) on the wear track surface lubricated by the PAO6 oil (Figure 10b) prove no existence of graphene oxide debris. Therefore, the analysis result verifies the deposition behavior of Sc-Au/GO in dispersed oil during rubbing process, and the whole wear track surface are almost covered by the nanocomposite after tribological test (Figure 10c).

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Figure 10. Optical microscopy images of (a) bare stainless steel surface and wear track surfaces lubricated by (b) PAO6 oil and (c) 0.10 wt. % Sc-Au/GO dispersed oil; (d) Raman spectra of the marked points (1-5) on these surfaces.

To further reveal the friction-reducing and antiwear mechanism of the Sc-Au/GO dispersed oil, the valence states of several typical elements on the wear tracks of the discs are examined by XPS measurements. Figure 11 shows the curve-fitted XPS spectra of C1s, O 1s, Fe2p and Au4f on the bare steel surface (Figures 11a-c) and on the wear track surfaces of discs lubricated with the neat PAO6 oil (Figures 11d-f) and with the Sc-Au/GO dispersed oil (Figures 11g-j). The corresponding mass fractions of these elements are determined by XPS analysis and summarized in Table 1. The peaks from C1s electrons can be de-convoluted into four sub-peaks at 284.6, 285.2, 286.1, 288.5 eV, and they can be attributed to different chemical components. The sub-peaks in Figure 11a mainly correspond to the contaminated carbons and the carbons from bare steel substrate; ones in Figure 11d are mainly ascribed to the contaminated carbons and the oxidation products of oil. The C1s spectrum in Figure 11g is different from the ones in Figures 11a, d. The strong sub-peak at 284.6 eV corresponding to the carbon from GO indicates the existence of Sc-Au/GO on the wear track, which agrees well with the analysis in Figure 10. Furthermore, the enhanced C-O signal at 286.1 eV than that in Figure 11d can also demonstrate the successful deposition of Sc-Au/GO during the rubbing process, because this increased signal arises from the functional groups of GO in Sc-Au/GO. The O1s spectrum in Figure 11b with five peaks around 530, 530.3, 531.3, 532 and 533.7 eV are attributed to the O1s electrons in FeO, 23



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Cr2O3, C=O, FeOOH and absorbed oxygen, respectively. It can be reasonably concluded that the bottom disc surface was oxidized to some extent during the polishing process. As to the O1s spectra in Figure 11e, strong signal from Fe2O3 can be found without the presence of FeO signal, indicating further oxidation occurred to some extent on wear track during the sliding test. Moreover, the enhanced C=O intensity also proves the possible oxidization of neat oil during the rubbing process. In the O1s spectra of the wear track lubricated with the Sc-Au/GO dispersed oil (Figure 11h), the Fe2O3 signal (530 eV) is substituted by Fe3O4 (529.8 eV), which indicates a higher oxidation degree of Fe element than that on bare steel surface and a lower oxidation extent than that on the wear track lubricated with PAO6 oil. In contrast, Figure 11h shows the strongest C=O signal that is from plentiful oxygen-containing groups of GO, which further confirms the deposition of Sc-Au/GO nanocomposites during the rubbing process. The oxidization extent can be clearly recognized by further analyzing the Fe2p spectra in Figures 11c, f and i. In Figure 11c, a part of the iron elements at zero valence state are oxidized to FeO or FeOOH on the bare steel surface. After sliding process lubricated with the PAO6 oil, the iron elements on wear track (Figure 11f) were completely oxidized and transformed to higher valence state of Fe3+. The Fe2p spectrum in Figure 11i shows more Fe2+ than that in Figure 11f, signifying the relatively weak oxidization degree of Fe0 elements. The above analyses of the Fe2p spectra correspond well with the analysis results of the O1s spectra once again. The XPS spectrum of Au4f on the wear track lubricated by the Sc-Au/GO dispersed oil is shown in Figure 11j. The strong signals of Au4f7/2 peak at 83.80 eV and Au4f5/2 peak at 87.45 eV are detected on this wear surface, which suggests that Au particles are released from Sc-Au/GO nanocomposite and deposit on the 24


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sliding steel surfaces during the rubbing process. And the Au nanoparticles show chemical inertness without any chemical reactions as expected.

Figure 11. Curve-fitted XPS spectra of typical elements on (a-c) the bare steel surface and on the wear tracks of steel discs lubricated with (d-f) PAO6 oil and with (g-j) 0.10 wt. % Sc-Au/GO dispersed oil.

Table 1 presents that the wear track lubricated by the Sc-Au/GO dispersed oil has much higher relative mass fraction of C and lower mass fractions of Fe, Cr and Ni than the bare steel surface and the wear track lubricated by the PAO6 oil. Au element only exists on the wear track lubricated by the Sc-Au/GO dispersed oil. It can be reasonably inferred that Sc-Au/GO in dispersed oil are able to form block deposition film with good coverage on rubbing surfaces,

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which finally leads to the significant increase of C element and sharp decreases of other elements. Table 1. Relative mass fraction of typical elements on different disc surfaces Sample

Mass fraction (%) C

O

Fe

Cr

Ni

Au

Bare steel disc

25.23

28.01

27.34

14.53

4.89

/

Disc by PAO6

30.16

30.72

18.96

14.44

5.73

/

Disc by PAO6 + Sc-Au/GO

52.40

27.06

16.68

2.71

0.62

0.52

3.4. Lubricating Mechanism Analysis The lubrication models of the pure PAO6 oil and the Sc-Au/GO dispersed oil are illustrated in Figure 12. In case of lubrication with the neat PAO6 oil, two contact surfaces scratch with each other and many abrasive particles are produced because of the friction force. The stiff abrasive particles slide under high friction stress, leading to the contact surface becoming extremely rough. Once the surface roughness exceeds the oil film thickness, the dry contact will happen, and wide and deep grooves and furrows are formed on the wear surface (Figures 8a, d, g and Figures 9a, b). But as the Sc-Au/GO nanocomposites are added into neat oil, the nanocomposites with oil can penetrate into the interface of contact pairs and gradually deposit and accumulate in original and eventually produced pits and grooves. Quickly, these defects are repaired and the contact surfaces become flat and smooth (Figures 8c, f and Figure 9e), resulting in a decrease in frictional force. The surface roughness of the rubbing surfaces is largely decreased compared with that lubricated with neat PAO6 oil. Finally, the deposited nanocomposites form block physical deposition film and uniformly cover the rubbing surface, as confirmed by Figures 10 26


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and 11, which results in fewer abrasive particles produced. The reduction of abrasive particles is helpful for maintaining the smooth contact surfaces and reducing the wear volume of friction pairs. Moreover, the modified Sc-Au/GO can absorb base oil, which thickens the oil film and prevents the friction pairs from direct contact. As a result, the lubrication state in the Sc-Au/GO dispersed oil has transferred to good boundary lubrication from the mixed lubricating in the pure PAO6 oil, which leads to a significant improvement in friction reduction and antiwear ability. In addition, the components of Sc-Au/GO, i.e., Au nanoparticles and GO sheets, both are good lubricating nanomaterials and show a synthetic lubricating effect. The GO nanosheets are exfoliated by the decorated Au nanoparticles,36,38 so the interlamination sliding becomes easy, which can reduce the friction force effectively. A few GO sheets may be deformed and ruptured by friction force and heat after a period of sliding time,25,27 and then the dotted Au nanoparticles are exposed and released. The released Au nanoparticles can repair the ruptured film and thus continue to improve the durability of deposition film.

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Figure 12. Schematic of the lubricating models of the pure PAO6 oil and the Sc-Au/GO dispersed oil for the ball-on-disc friction tester.

2. CONCLUSIONS In summary, Au nanoparticle decorated-graphene oxide nanocomposite (Sc-Au/GO) has been successfully prepared by a facile chemical reduction in the supercritical carbon dioxide (ScCO2) fluid. The anchored Au nanoparticles show narrow grain size range from 4 to 10 nm and uniform distribution on GO sheet surfaces due to the unique properties of ScCO2. The as-prepared Sc-Au/GO is highly dispersed in the neat PAO6 oil as lubricating additive after modifying with oleylamine. The friction coefficient and wear rate are reduced up to 33.6% and 72.8% respectively, as traces of Sc-Au/GO (0.10 wt. %) were added in the neat oil. The friction-reducing and antiwear abilities of Sc-Au/GO are superior to the contrastive additives of GO, Au nanoparticles, and Au/GO produced by traditional method. During the sliding process, 28


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the nanocomposite of Sc-Au/GO deposits on contact surfaces and forms block physical protective film, which can significantly smooth the contact surface and reduce the roughness. Meanwhile, the Sc-Au/GO can absorb oil molecules, which is beneficial to forming good boundary lubricating state on the sliding surface. In addition, the Au nanoparticles and the GO nanosheets in the Sc-Au/GO present synergistic lubricating effect, leading to the superior lubricating performances as compared to that of the single components (GO and nano-Au) of the nanocomposite. The deposition protective film as well as the synergistic lubricating effect corresponds to the superior lubricating performances of the Sc-Au/GO dispersed oil. The findings here not only provide a new type of excellent lubricating materials, but also can shed light on preparing nanoparticle/graphene oxide nanocomposites with remarkable properties readily for potential industrial applications.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Fenghua Su: 0000-0002-6953-4663 Notes The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the financial support of the National Natural Science Foundation of China (Nos. 21473061 and 51575191), the Guangdong Natural Science Funds for Distinguished Young Scholar (grant: 2015A030306026), and the Science and Technology Planning Project of Guangdong Province (grant: 2016A010102009).

REFERENCES (1) Chen, Y.; Zhang, Y.; Zhang, S.; Yu, L.; Zhang, P.; Zhang, Z. Preparation of Nickel-Based Nanolubricants via a Facile In Situ One-Step Route and Investigation of Their Tribological Properties. Tribol. Lett. 2013, 51, 73-83. (2) 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. (3) Choi, Y.; Lee, C.; Hwang Y.; Park, M.; Lee, J.; Choi, C.; Jung, M. Tribological Behavior of Copper Nanoparticles as Additives in Oil. Curr. Appl.Phys. 2009, 9, 124-127. (4) Zhou, J.; Wu, Z.; Zhang, Z.; Liu, W.; Xue, Q. Tribological Behavior and Lubricating Mechanism of Cu Nanoparticles in Oil. Tribol. Lett. 2000, 8, 213-218 (5) Tarasov, S. Study of Friction Reduction by Nanocopper Additives to Motor Oil. Wear 2002, 252, 63-69. (6) Gulzar, M., Masjuki, H. H., Kalam, M. A.; Varman, M.; Zulkifli, N. W. M.; Mufti, R. A.; Zahid, R. Tribological Performance of Nanoparticles as Lubricating Oil Additives. J. Nanopart. Res. 2016, 18, 223-247. (7) Peng, D. X.; Kang, Y.; Chen, S. K.; Shu, F. C.; Chang, Y. P. Dispersion and Tribological Properties of Liquid Paraffin with Added Aluminum Nanoparticles. Ind. Lubr. Tribol. 2010, 62, 341-348. (8) Akbulut, M.; Belman, N.; Golan, Y.; Israelachvili, J. Frictional Properties of Confined Nanorods. Adv. Mater. 2006, 18, 2589-2592. (9) Padgurskasa, J.; Rukuizaa, R.; Prosyčevasb, I.; Kreivaitisa, R. Tribological Properties of Lubricant Additives of Fe, Cu and Co Nanoparticles. Tribol. Int. 2013, 60, 224-232. (10) Wu, Y.; Tsui, W.; Liu, T. Experimental Analysis of Tribological Properties of Lubricating Oils with Nanoparticle Additives. Wear 2007, 262, 819-825. (11) Battez, A. H.; González, R.; Viesca, J. L.; Fernández, J. E.; Fernández, J. M. D.; Machado, A.; Chou, R.; Riba, J. CuO, ZrO2 and ZnO Nanoparticles as Antiwear Additive in Oil Lubricants. Wear 2008, 265, 422-428. (12) Hernández, B. A. Wear Prevention Behaviour of Nanoparticle Suspension under Extreme Pressure Conditions. Wear 2007, 263, 1568-1574. (13) Li, B.; Wang, X.; Liu, W.; Xue, Q. Tribochemistry and Antiwear Mechanism of Organic–Inorganic 30


Page 30 of 33

Page 31 of 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


(14) (15) (16) (17) (18)

(19) (20)

(21)

(22) (23)

(24) (25)

(26) (27)

(28) (29) (30)

Nanoparticles as Lubricant Additives. Tribol. Lett. 2006, 22(1), 79-84. Wang, B.; Wang, X.; Lou, W.; Hao, J. Gold-Ionic Liquid Nanofluids with Preferably Tribological Properties and Thermal Conductivity. Nanoscale Res. Lett. 2011, 6, 259-268. Wei, Y.; Xun, F. Preparation and Tribological Behavior of Organic Fluid Containing Silver Nanoparticles. Tribology 2004, 24, 425-428. Ma, J.; Mo, Y.; Bai, M. Effect of Ag Nanoparticles Additive on the Tribological Behavior of Multialkylated Cyclopentanes (MACs). Wear 2009, 266, 627-631. Sánchez-López, J. C.; Abad, M. D.; Kolodziejczyk, L.; Guerrero, E.; Fernández, A. Surface-Modified Pd and Au Nanoparticles for Anti-Wear Applications. Tribol. Int. 2011, 44(6), 720-726. Tabandeh-Khorshid, M.; Omrani, E.; Menezes, P. L.; Rohatgi, P. Tribological Performance of Self-Lubricating Aluminum Matrix Nanocomposites: Role of Graphene Nanoplatelets. Eng. Sci. Technol., an Int. J. 2016, 19(1), 463-469. Liang, S.; Shen, Z.; Yi, M.; Liu, L.; Zhang, X.; Ma, S. In-Situ Exfoliated Graphene for High-Performance Water-Based Lubricants. Carbon 2016, 6, 1181-1190. Gupta, B.; Kumar, N.; Panda, K.; Dash, S.; Tyagi, A. K. Energy Efficient Reduced Graphene Oxide Additives: Mechanism of Effective Lubrication and Antiwear Properties. Sci. Rep. 2016, 6, 18372. Doi:10.1038/srep18372 Dorri Moghadam, A.; Omrani, E.; Menezes, P. L.; Rohatgi, P. K. Mechanical and Tribological Properties of Self-Lubricating Metal Matrix Nanocomposites Reinforced by Carbon Nanotubes (CNTs) and Graphene-A Review. Compos. Part B-Eng. 2015, 77, 402-420. Omrani, E.; Dorri Moghadam A.; Menezes, P. L.; Rohatgi, P. K. In Ecotribology; Davim, J., Eds.; Springer: Cham, 2016; Chapter 3, pp 63-103. Ye, X.; Ma, L.; Yang, Z.; Wang, J.; Wang, H.; Yang, S. Covalent Functionalization of Fluorinated Graphene and Subsequent Application as Water-Based Lubricant Additive. ACS Appl. Mater. Interfaces 2016, 8, 7483-7488. Eswaraiah, V.; Sankaranarayanan, V.; Ramaprabhu, S. Graphene-Based Engine Oil Nanofluids for Tribological Applications. ACS Appl. Mater. Interfaces 2011, 3, 4221-4227. Khare, V.; Pham, M. Q.; Kumari, N.; Yoon, H. S.; 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. Belmonte, M.; Ramírez, C.; González-Julián, J.; Schneider, J.; Miranzo, P.; Osendi, M. I. The Beneficial Effect of Graphene Nanofillers on The Tribological Performance of Ceramics. Carbon 2013, 61, 431-435. Zhang, L.; Pu, J.; Wang, L. Xue, Q. Synergistic Effect of Hybrid Carbon Nanotube-Graphene Oxide as Nanoadditive Enhancing The Frictional Properties of Ionic Liquids in High Vacuum. ACS Appl. Mater. Interfaces 2015, 7, 8592-8600. Cho, M.; Ju, J.; Kim, S.; Jang, H. Tribological Properties of Solid Lubricants (Graphite, Sb2S3, MoS2) for Automotive Brake Friction Materials. Wear 2006, 260, 855-860. Chen, B.; Bi, Q.; Yang, J.; Xia, Y.; Hao, J. Tribological Properties of Solid Lubricants (Graphite, h-BN) for Cu-Based P/M Friction Composites. Tribol. Int. 2008, 41, 1145-1152. Cui, G.; Bi, Q.; Zhu, S.; Yang, J.; Liu, W. Tribological Properties of Bronze-Graphite Composites under Sea Water Condition. Tribol. Int. 2012, 53, 76-86. 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 33

(31) Rosentsveig, R.; Gorodnev, A.; Feuerstein, N.; Friedman, H.; Zak, A.; Fleischer, N.; Tannous, J.; Dassenoy,F.; Tenne, R. Fullerene Like MoS2 Nanoparticles and Their Tribological Behavior. Tribol. Lett. 2009, 36, 175-182. (32) Chen, Z.; Liu, X.; Liu, Y.; Gunsel, S.; Luo J. Ultrathin MoS2 Nanosheets with Superior Extreme Pressure Property as Boundary Lubricants. Sci. Rep. 2015, 5, 12869. Doi:10.1038/srep12869 (33) Pawlak, Z.; Kaldonski, T.; Pai, R.; Bayraktar, E.; Oloyede, A. A Comparative Study on The Tribological Behaviour of Hexagonal Boron Nitride (h-BN) as Lubricating Micro-Particles–An Additive in Porous Sliding Bearings for A Car Clutch. Wear 2009, 267, 1198-1202. (34) Song, H.; Jia, X.; Li, N.; Yang, X.; Tang, H. Synthesis of α-Fe2O3 Nanorod/Graphene Oxide Composites and Their Tribological Properties. J. Mater. Chem. 2012, 22, 895-902. (35) Meng, Y.; Su. F.; Chen, Y. Synthesis of Nano-Cu/Graphene Oxide Composites by Supercritical CO2-Assisted Deposition as A Novel Material for Reducing Friction and Wear. Chem. Eng. J. 2015, 281, 11-19. (36) Hong, W.; Bai, H.; Xu, Y.; Yao, Z.; Gu, Z.; Shi, G. Preparation of Gold Nanoparticle/Graphene Composites with Controlled Weight Contents and Their Application in Biosensors. J. Phys. Chem. C 2010, 114, 1822-1826. (37) Altuntas, D. B.; Tepeli, Y.; Anik, U. Graphene-Metallic Nanocomposites as Modifiers in Electrochemical Glucose Biosensor Transducers. 2D Mater. 2016, 3, 034001. Doi:10.1088/2053-1583/3/3/034001 (38) Huang, C. C.; Pu, N. W.; Wang, C. A.; Huang, J. C.; Sung, Y.; Ger, M. D. Hydrogen Storage in Graphene Decorated with Pd and Pt Nano-Particles Using An Electroless Deposition Technique. Sep. Purif. Technol. 2011, 82, 210-215. (39) Sattayasamitsathit, S.; Gu, Y.; Kaufmann, K.; Jia, W.; Xiao, X.; Rodriguez, M.; Minteer, S.; Cha, J.; Burckel, B.; Wang, C.; Polsky, R.; Wang, J. Highly Ordered Multilayered 3D Graphene Decorated with Metal Nanoparticles. J. Mater. Chem. A 2013, 1, 1639-1645. (40) Goncalves, G.; Marques, P. A. A. P.; Granadeiro, C. M.; Nogueira, H. I. S.; Singh, M. K.; Grácio, J. Surface Modification of Graphene Nanosheets with Gold Nanoparticles: The Role of Oxygen Moieties at Graphene Surface on Gold Nucleation and Growth. Chem. Mater. 2009, 21, 4796-4802. (41) Zhang, Y.; Erkey, C. Preparation of Supported Metallic Nanoparticles Using Supercritical Fluids: A Review. J. Supercrit. Fluids 2006, 38, 252-267. (42) Chen, C. Y.; Chang, J. K.; Tsai, W. T.; Hung, C. H. Uniform Dispersion of Pd Nanoparticles on Carbon Nanostructures Using A Supercritical Fluid Deposition Technique and Their Catalytic Performance towards Hydrogen Spillover. J. Mater. Chem. 2011, 21, 19063-19068. (43) An, G.; Ma, W.; Sun, Z.; Liu, Z.; Han, B.; Miao, S. Miao, Z.; Ding, K. Preparation of Titania/Carbon Nanotube Composites Using Supercritical Ethanol and Their Photocatalytic Activity for Phenol Degradation under Visible Light Irradiation. Carbon 2007, 45, 1795-1801. (44) Nguyen, V.; Kim, B.; Jo, Y.; Shim, J. Preparation and Antibacterial Activity of Silver Nanoparticles-Decorated Graphene Composites. J. Supercrit. Fluids 2012, 72, 28-35. (45) Sun, Z.; Liu, Z.; Han, B.; Miao, S.; Miao, Z.; An, G. Decoration Carbon Nanotubes with Pd and Ru Nanocrystals via An Inorganic Reaction Route in Supercritical Carbon Dioxide-Methanol Solution. J. Colloid Interface Sci. 2006, 304, 323-328. (46) Watanabe, M.; Akimoto, T.; Kondoh, E. Synthesis of Platinum-Ruthenium Alloy Nanoparticles on Carbon 32


Page 33 of 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


(47)

(48) (49)

(50) (51)

(52) (53)

Using Supercritical Fluid Deposition. ECS J. Solid State Sci. Technol. 2013, 2(1), 9-12. Zhao, J.; Xue, H.; Zhang, L.; Yu, J.; Hu, H. Decoration of Ultrafine Platinum-Ruthenium Particles on Functionalized Graphene Sheets in Supercritical Fluid and Their Electrocatalytic Property. J. Nanopart. Res. 2012, 14, 935-945. Hummers, J. W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339. Jasim, D. A.; Lozano, N.; Kostarelos, K. Synthesis of Few-Layered, High-Purity Graphene Oxide Sheets from Different Graphite Sources for Biology. 2D Mater. 2016, 3, 014006. Doi:10.1088/2053-1583/3/1/014006 Cai, D. Song, M. Preparation of Fully Exfoliated Graphite Oxide Nanoplatelets in Organic Solvents. J. Mater. Chem. 2007, 17(35), 3678-3680. Mao, A.; Zhang, D.; Jin, X.; Gu, X.; Wei, X.; Yang, G.; Liu, X. Synthesis of Graphene Oxide Sheets Decorated by Silver Nanoparticles in Organic Phase and Their Catalytic Activity. J. Phys. Chem. Solids 2012, 73, 982-986. Erdemir, A. Review of Engineered Tribological Interfaces for Improved Boundary Lubrication. Tribol. Inter. 2005, 38(3), 249-256. Zhang, W.; Zhou, M.; Zhu, H.; Tian, Y.; Wang, K.; Wei, J.; Ji, F.; Li, X.; Li, Z.; Zhang, P.; Wu, D. Tribological Properties of Oleic Acid-Modified Graphene as Lubricant Oil Additives. J. Phys. D: Appl. Phys. 2011, 44, 205303-205306.

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Graphene Oxide Nanocomposite Synthesized in Supercritical CO2

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