Correlation between Molecular Structure and ... - ACS Publications

Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

www.acsanm.org

Correlation between Molecular Structure and Interfacial Properties of Edge or Basal Plane Modified Graphene Oxide Hongmei Yang, Jiu-Sheng Li, and Xiangqiong Zeng* Laboratory for Advanced Lubricating Materials, Shanghai Advanced Research Institute, Chinese Academy of Sciences, University of Chinese Academy of Sciences, Shanghai 201210, China

Downloaded via 180.107.33.206 on June 19, 2018 at 17:09:33 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Although graphene oxide (GO) has been reported to be able to be edge functionalized or basal-plane functionalized separately, no research has been done on comparing both the molecular structure and interfacial properties of them. In this study, an alkyl amine was grafted to the epoxy group on the basal planes of GO (b-GO) and carboxyl group at the edges of GO (e-GO) separately by using different synthetic approach. With the combination of various molecular structure and morphology characterization methodologies, we proved that the reaction site for e-GO was only with the carboxyl group at the edge of GO and that for b-GO was epoxy group on the basal plane of GO, indicating that GO could be controllably functionalized (fGOs), and the structure of fGOs could be tuned. Study of the interfacial behavior of fGOs at liquid−liquid interface showed that the interfacial tension reducing capability of e-GO was broader than that of b-GO, and for alkyl oil phase, b-GO was slightly better than e-GO, and both were better than traditional nonionic surfactant. Study of the interfacial behavior of fGOs at liquid−solid interface demonstrated that, after absorption, b-GO arranged vertically on the metal surface, forming dense, compact, and strong film, while e-GO aligned horizontally to form loosely assembled film, resulting in higher interfacial shear strength than that of b-GO. Our results indicate the possibilities for tuning the interfacial properties of GO at both liquid−liquid and liquid−solid interfaces, which may be promising in the potential applications in controlled drug delivery, surface protection, absorption and separation, lubrication, nanocomposite, and catalyst fields. KEYWORDS: functionalized graphene oxide, basal plane-modification, edge-modification, interfacial properties, mechanism

1. INTRODUCTION Graphene, a two-dimensional (2D) carbon material with large specific surface area, has gained growing research interests currently, due to its outstanding performance, like excellent thermal conductivity, nearly 98% of optical transmittance, high Young’s modulus, high intrinsic mobility and ballistic transport, etc.1 Graphene oxide (GO), as the oxidation product of graphene, contains many oxygen-containing functional groups in its structure, including carbonyl, carboxyl, epoxy, and hydroxyl, which are either covalently or noncovalently bonded (such as hydrogen bonds,2 π−π stacking,3 etc.) to the carbon atoms.4,5 According to the Lerf model,6 the carboxyl and carbonyl groups are mainly at edges, and the hydroxyl and epoxy groups are on basal planes of GO sheets. These functional groups can act as active reaction sites for covalent functionalization of GO (fGOs). Chong Rae Park et al. has provided us guidelines to prepare tailored GO for targeted applications.7 In 2003, Dékány et al. reported modified GOs by grafting primary aliphatic amines and amino acids onto its surface.8 Since then, many researchers have tried different methods to realize the modification of GO, and the fGOs show great potential applications.9−11 The chemical modification © XXXX American Chemical Society

successfully tuned numerous attracting properties of GO, for instance, interfacial properties.12 In terms of epoxy groups on GO basal planes, it can be functionalized via nucleophilic substitution reactions with amine-containing molecules, like primary aliphatic amines, aminosiloxanes, and amino acids.11−19 Khatri et al. reported that octadecylamine basal-functionalized GO revealed remarkable tribological performance.20 Moreover, hydroxyls on GO basal planes can react with silica or carboxyl-containing molecules through corresponding substitution and esterification.21−24 Through these basal plane functionalizations, valuable GO derivatives could be procured, which can be used as adsorbents to remove heavy metal ions from hard water,18 as catalysts for oxidating benzyl alcohol derivatives efficiently and selectively,22 as anticorrosion materials,24 and so on.25,26 When −COOH groups at edges of GO are treated with organic amines or alcohols after activation with thionyl Received: March 20, 2018 Accepted: June 1, 2018

A

DOI: 10.1021/acsanm.8b00405 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Nano Materials chloride, or using dehydrants like N-hydroxysuccinimide, N,N′dicyclohexyl carbodiimide, and N-(3-(dimethylamino)propylN′-ethylcarbodiimide) hydrochloride, it will facilitate the formation of ester or acid amide derivatives.27−30 Khatri found that octadecylamine edge-modified reduced GO shows friction-reducing property due to the low interfacial shear strength under rolling contact stress.31 Chen et al. reported edge-modified GO quantum dots by arylamine-based polyazomethine and used as ternary memory devices to improve the memory capacity of binary memory devices.30 The edgemodified GOs can also be used as DNA biosensors for breast cancer detecting during early diagnosis,27 fluorescent materials for biomedical sensor and low-cost photoelectric devices,28 drug delivery for cancer treatment,29 etc. There are also many reports on GO, which are simultaneously modified with both basal plane and edge oxygen-containing groups. Shahrabi et al. discovered a 3aminopropyltriethoxysilane functionalized GO, which can improve the adhesion strength and anticorrosion performance and cut down the cathodic delamination rate of epoxy coating effectively.32 Application of dopamine-modified nano GO carrier for anticancer drug delivery was reported by Kashanian et al.33 Additionally, the dual-functionalized GOs can be used in dental and orthopedic applications,34 fillers,35,36 and so on.37,38 Most of the GO and fGOs reported could be regarded as an amphiphile with oxygen-containing hydrophilic surfaces and large hydrophobic carbon skeleton. They have distinct length scales, with the thickness similar to approximately a single atomic layer, and lateral dimension up to tens of micrometers. Since GO and fGOs have the characteristics of amphiphile, they could function as a colloidal surfactant or a molecular amphiphile at liquid−liquid and liquid−solid interfaces.39,40 The self-assembly behavior of GO nanosheets at liquid− liquid interfaces has been studied by many researchers due to its promising technology in making advanced functional materials. Thickett and Zetterlund discovered the stabilization energy of GO assembly at nonpolar oil/water interface was greater than that of polar oils. This is because as the polarity of oil increases, the interfacial tension at oil−water interface reduces significantly until it is no longer thermodynamically stable for GO adsorption.41 Additionally, Ren’s group reported that GO can quickly separate oil from the oil-in-water (O/W) emulsion. They disclosed that the GO-driven demulsification was closely associated with the strong n−π and π−π interaction of asphatenes with GO. The findings indicate that GO nanosheets can be used as demulsifiers for separating oil from the crude/heavy O/W emulsions, which is significant for oil industry.42 Because of the presence of many various oxygen-containing groups and the amphilicity, GO shows excellent interfacial properties at liquid−solid interface as well, illustrated by building up low shear strength interfacial film from the GO liquid onto the solid surfaces during sliding, resulting in low friction and wear to protect the solid surfaces under extreme pressure and sliding conditions. Hiroshi et al. discovered that GO can act as water-based lubricants, leading to notable friction-reducing performance due to the adsorption of GO sheets on the sliding surfaces. It is speculated that fresh GOs could be continuously adsorbed onto the surface and desorbed during sliding to reduce the interfacial shear strength.43 Although GO could be edge-functionalized, or basal planefunctionalized separately, or both-functionalized simultaneously, there is no research has been done on comparing both

the molecular structure and interfacial properties of edgefunctionalized GO and basal plane-functionalized GO. In this paper, an alkyl amine was grafted to the epoxy group on basal planes and carboxyl group at edges separately. The two kinds of synthesized GO were carefully characterized to disclose their differences in molecular structure. Their interfacial behavior at both liquid−liquid and liquid−solid interface was studied as well. The interfacial behavior at the liquid−liquid interface was evaluated by measuring the interfacial tension at water−oil interface, in which the oil was with different polarity. To study the interfacial behavior at liquid−solid interface, first, fGO emulsions were constructed with fGO staying at the oil−water interface, in which the oil selected was poly alpha olefins (PAO), one IV class synthetic base oil with neat molecular arrangement and no aromatic hydrocarbons, and with good stability, high viscosity index, and good low-temperature fluidity.44 Using PAO as the oil phase is more in line with the development of high-tier synthetic lubricants to fulfill the increasingly urgent requirements of energy saving, environmental protection, and safety. And then, the interaction between fGO emulsion and two relative sliding steel surfaces were studied under certain pressure and sliding velocity. And the interaction mechanism between the two kinds of fGO emulsion and steel surfaces was discussed.

2. EXPERIMENTAL SECTION 2.1. Materials. Flake graphite (500 mesh, J&K Scientific Ltd., 99%). n-Octylamine (98%), thionyl chloride (SOCl2, 96%), dimethyl formamide (DMF, 98%), dimethyl sulfoxide (DMSO, 98%), hydrochloric acid (HCl, 35%), sulfuric acid (H2SO4, 98%), phosphoric acid (H3PO4, 85%), potassium permanganate (KMnO4, 98%), dichloromethane (CH2Cl2, 98%), sodium hydroxide (NaOH, 98%), and hydrogen peroxide (H2O2, 30% w/w) were obtained from Sinopharm Chemical Reagent Co., Ltd. Tween 20 (TW 20, Haian petrochemical, 99%). TR-1(Guangzhou Shi Ming Chemical Co., Ltd., 98%). All were used as received. The water used was Wow Haha pure water. 2.2. Preparation of GO. Improved Hummer’s method was used to prepare GO from flake graphite for advantages of short reaction time, high oxidation degree, less environmental pollution, and high security. For GO synthesis, a mixture of concentrated H2SO4/H3PO4 (400 mL, v/v = 9:1) was added to a round-bottom flask with graphite powder (3.0 g) under ice-cold condition, followed by slow addition of KMnO4 (18.0 g); the mixture was heated to 50 °C afterward and stirred for 12 h. The reaction solution was cooled and poured into 800 mL of ice water with 30% H2O2 (∼6 mL) and stood overnight. After it was centrifuged, the remaining slurry mixture was washed with pure water (400 mL) followed by 200 mL of 37% HCl. The solution was washed with water and centrifuged, until the supernatant pH value was nearly 7; then, it was dried at 45 °C in a vacuum oven to achieve graphite oxide. After ultrasonication, we got graphene oxide and named GO. 2.3. Functionalization of GO by using Octyl Amine. Asprepared GO is one nonstoichiometric carbon material with tremendous oxygen-containing groups distributed on the basal planes and along the edges.6,45,46 It is reported that the epoxy groups on basal planes of GO could react with −NH2 of alkyl amines via nucleophilic substitution, and the carboxyl groups at edges can also react with alkyl amines to obtain amidated products.4,47 And it is proved that amidation reaction is preferentially occurring at the edges, whereas nucleophilic substitution is expected to predominate on the basal planes. Therefore, by controlling the reaction conditions, octyl amine can selectively react with epoxy groups to achieve basal planefunctionalized GO (b-GO) or selectively react with carboxyl group to obtain edge-functionalized GO (e-GO). For preparing b-GO, a mixture-heating method was used, which could trace back to Imre’s report.8 The synthesis process was as follows: 1.5 g of GO powder was dispersed in a 50:5 mixture of DMSO/DMF (55 mL) by ultrasonic treatment for 1 h, and n-octyl B


Article

ACS Applied Nano Materials

Figure 1. Synthetic route of b-GO and e-GO. A Bruker DRX 500 MHz spectrometer was applied to do the 13C SSNMR measurements. The test was conducted by using ∼100 mg of GO in a magnetic field of 11.7 T with a solid-state probe head (Φ 4 mm), and the ZrO2 rotors were rotated at 5 kHz. The external reference material used was tetramethylsilane with δ to be 0 ppm for 13 C. Before the Fourier transform, a 50 Hz Lorentz window function was applied. GO and fGOs were analyzed by XPS with Al Kα radiation as excitation source. The binding energy of C, N, and O elements was measured at 29.35 eV passing energy, and the binding energy of contaminated carbon (C 1s: 284.8 eV) was used. A D-MAX 2200/PC X-ray diffractometer (Japan Rigaku Corp.) was used to do the XRD analysis of GO and fGOs, with the operation condition to be 2θ = 5−60° at a speed of 4° min−1 with the Cu Kα radiation (λ = 0.154 nm). Thermo behaviors of GO and fGOs were accomplished by Q600 (TA Instruments) under N2 atmosphere at 50.0 mL/min with 10 °C/ min temperature-programmed from room temperature to 800 °C. 2.4.2. Morphology Characterization. TEM of GO, b-GO, and eGO were taken by Tecnai G2 F20 (FEI NanoPorts) after being dispersed in pure water by ultrasonication; the energy used was 200 kV. AFM tests of GO and fGOs were taken by a biological fast atomic force microscope (Bruker); all samples were dispersed in ethanol by ultrasonication to get the dispersion at a low concentration. The dispersions were cast on freshly stripped mica pieces, and tests started after the volatilization of solvent. 2.5. Interfacial Property Study. The interfacial property of fGOs between oil and water was evaluated by using interfacial tension measurement. It was tested through an Automatic Surface Tension meter (K 100, Kruss, Germany) by adding 30.0 mL of water phase dispersed with tested samples to 45 mL of oil phase (referred to Wilhelmy hanging plate method). Since fGOs have interfacial activity at oil−water interface, they can act as emulsifier to make emulsions. A kind of O/W emulsion was made by using fGO and traditional surfactant as emulsifiers together. The formulation of emulsions is displayed in Table S1. To prepare the GO O/W emulsions, GO and fGOs were separately added to a 500

amine (22.0 mL) was dropped into the dispersion. Then the solution was heated under 80 °C for 36 h in the sealing tube. After completion of the reaction, the precipitate was filtrated and washed with 1 M HCl, water, and methanol, then dried at 60 °C in a vacuum oven for 24 h; hereafter, we got the n-octyl amine-modified GO based on basal planes (2.2 g) in powder form, coded as b-GO. The synthetic route was as shown in Figure 1 (Route A). To prepare e-GO, an activation-substitution method was used as reported by Chen et al.48 The synthesis process was as follows: 200.0 mg of GO powder was dispersed in a 20:1 mixture of SOCl2/DMF (64.0 mL) by ultrasonic treatment for 0.5 h and then heated at 70 °C for 24 h in the sealing tube. After completion of the reaction, the precipitate was filtered and washed with dried dichloromethane and methanol, then dried at 45 °C in a vacuum oven for 18 h to achieve acyl chloride-modified GO, coded as GOCl. In the next step, GOCl was dispersed in 20 mL of DMSO after ultrasonic treatment for 0.5 h, and n-octyl amine (7.1 mL) was dropped into the dispersion. Then the solution was heated at 80 °C for 20 h. The precipitate was also filtrated, washed, and dried at 60 °C in the vacuum oven for 24 h; then we got the n-octyl amine-modified GO based on edge (235.5 mg), coded as e-GO. The synthetic route was illustrated in Figure 1 (Route B). 2.4. Characterization. In this study, the molecular structure of GO and fGOs were characterized by Fourier transform infrared (FTIR), solid-state nuclear magnetic resonance (SSNMR), and Raman spectroscopies, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and thermal gravimetric analysis (TGA), and their morphologies were characterized by transmission electron microscope (TEM) and atom force microscope (AFM). 2.4.1. Molecular Structure Characterization. FT-IR was fulfilled by a Paragon 1000 FT-IR Spectrometer (PerkinElmer, Inc). The FT-IR characterization of GO and fGOs were taken using a germanium crystal in the attenuated total reflectance transmission mode by scanning from 4000 to 650 cm−1. Raman analysis of GO, b-GO, and e-GO was performed by a DXRTM xi Raman imaging spectroscope (Thermo Fisher Scientific) in the range from 3500 to 50 cm−1 using a 532 nm laser source. C


Article


Figure 2. (a) FT-IR, (b) SSNMR, (c) Raman, and (d) XRD spectra of GO, b-GO, and e-GO.

vibration shifted to 1090 cm−1 due to the open-ringing substitution of the epoxy groups by n-octyl amine, and there is no obvious C−O−C vibration (peak at 1225 cm−1) any more. All these mean that we achieved the b-GO and e-GO with different molecular structure, and the reaction sites were OC−OH for e-GO and C−O−C for b-GO. SSNMR can provide important insights to structure.52 The results of GO and fGOs (b-GO and e-GO) are shown in Figure 2b. The signal assignments of pristine GO for the three major peaks at 66 ppm (C−O−C and C−OH), 128 ppm (CC), and 189 ppm (CO) are in accordance with reference reported by Lerf et al.6 Because of the poor signal-to-noise ratio for natural abundance 13C, there is no obvious abruption between C−O−C and C−OH.53 The spectra of fGO samples exhibited similar resonance patterns of C−C (∼13 ppm, ∼20 ppm) and C−N (∼27 ppm) peaks in n-octyl amine. The successful grafting of alkylamine onto GO results in a reducing resonance of CC and/or aromatic groups from 128 ppm (GO) to 118 ppm (e-GO) due to the positive electron-donor effect. The signal of CO in e-GO downshifts to 169 ppm because of the formation of amido bond, and the C−O signal reveals a significant downshift from 66 to 40 ppm due to the weakening of intermolecular hydrogen-bond interaction because of the grafting of alkyl chains. The major Raman features of GO are the D peak at ∼1341 cm−1 and G peak at ∼1594 cm−1, respectively54 (Figure 2c), in which, D peak is a disorder-induced mode, and the G peak corresponds to CC stretching vibrations.55 The positions of these two peaks in b-GO and e-GO were slightly downshifted compared to pristine GO. Previous studies demonstrated that, depending on the dopants grafted onto graphene, the G peak could either downshift (electron donor) or upshift (electron acceptor).4 Grafting of n-octyl amine could cause a positive inductive effect on GO surfaces. Therefore, it could be regarded

mL beaker with dispersed TR-1 in water and stirred at 60 °C for 10 min; then PAO 8 and TW 20 was added into the water phase under stirring at 600 rpm for 30 min; subsequently, sodium hydroxide solution was dropped to adjust the pH value to 7, and the the emulsion was cooled to room temperature with stirring. The pictures of the emulsions are shown in Figure S1a−c. The interfacial behavior of the fGO O/W emulsion with solid surface during sliding was studied by using friction tests. A ball-on-disc testing machine (UMT-Tribolab, Bruker), as illustrated in Figure S1d, was used to conduct all the friction tests. The upper stainless steel ball was stationary, and the lower stainless steel plate kept rotation in GO O/W emulsion. The applied nominal pressure was ∼500 MPa with a normal load of 2 N. The speeds were 600 rpm, corresponding to sliding velocities of ∼1200 mm/s. The distance of each sliding test was 100 m. Every test was repeated three times.

3. RESULTS AND DISCUSSION 3.1. Molecular Structure Characterization of the Basal Plane or Edge-Functionalized GO. The FT-IR spectra are presented in Figure 2a. Main IR stretching vibrations of GO are at 1053 cm−1 (C−OH), 1225 cm−1 (C−O−C), 1384 cm−1 (C−O bond of carboxyl), 1738 cm−1 (CO stretching of carboxylic acid), and 3400 cm−1 (O−H). The resonance peak at 1621 cm−1 can be attributed to the absorbed hydroxyl groups and CC stretching in GO.49 The characteristic bands (2853 and 2924 cm−1) of alkyl groups, attributed to symmetric and asymmetric C−H stretching vibrations,50 emerged in both bGO and e-GO spectra due to the modification with n-octyl amine. The peak around 1456 cm−1 can be assigned to C−N vibration.51 The FT-IR spectrum of e-GO revealed a certain amount of epoxy groups and obvious amido bond (CO shifted to 1653 cm−1) but no carboxyl groups (CO at 1738 cm−1), which indicates that the carboxyl groups in e-GO have reacted with n-octyl amine. The carboxyl groups (1738 cm−1 downshifted to 1721 cm−1) still exist in b-GO, but the C−OH D


Article


Figure 3. (a) Full-range XPS spectra and (b) TGA profiles of GO, b-GO, and e-GO.

and 530 eV (O 1s) in survey scan results.61 However, both bGO and e-GO appeared a new peak of N 1s at 400 eV, which suggests that the alkyl amine was successfully grafted onto GO sheets. After analyzing atom contents in the survey scan results, we found that the intensity of C 1s of fGOs increased and that of O 1s peak attenuated, similar to the findings of Chieng et al.51 The difference was verified by the different nitrogen content of b-GO and e-GO, which was 5.05% and 4.59%, respectively. The grafting of n-octyl amine was further proved by a decreasing of O/C ratio from 0.38 (GO), 0.11 (e-GO), to 0.08 (b-GO) and an increasing of N/O ratio from 0.00 (GO), 0.45 (e-GO), to 0.73 (b-GO). The results may also indicate that the content of epoxy on basal planes was higher than that of carboxyl at edges, which is in accordance with the finding from Raman tests. To understand chemical state of elements on GO and fGOs, the deconvolution of C 1s, O 1s, and N 1s was performed, and some representative results are enclosed in Figure S2a−h. After deconvolution, there were three major peaks in the C 1s spectra of GO, which were C−C/CC (284.5 eV; ∼28.55%), C−O (286.5 eV; ∼57.80%), and O−CO (288.5 eV; ∼13.65%). After the grafting of alkyl amines on basal planes of GO (bGO), there was a great rise in C−C/CC (284.5 eV; ∼43.88%), with an appearance of C−N (285.3 eV; ∼40.51%) and C−O (286.5 eV; ∼11.07%) for the formation of C−N after open-ring substitution of epoxy groups, whereas grafting of noctyl amine onto the edges of GO (e-GO) also contributes to an increase in C−C/CC (284.5 eV; ∼41.81%). The analysis of O 1s spectra corroborated the analysis of C 1s spectra. Referring to changes of C 1s peak, we distributed the O 1s peaks of GO at 530.7, 532.3, and 533.5 eV to carbonyl (OC), epoxy (C−O−C), and carboxyl (OC−OH) groups.4 The grafting of alkyl amine to GO edges shows a peak at 531.5 eV (OC-NH), which confirmed the successful functionalization of GO as proposed in Figure 1 (Route B). There are two peaks at 399.9 eV (C−N) and 405.9 eV (OC-NH) in deconvoluted N 1s spectrum of e-GO, which further illustrated this fact. However, there is no new bond emerged in that of bGO except for the C−N peak at 400.3 eV, due to the nucleophilic open-ring substitution of GO basal planes. This proves that e-GO and b-GO are differently modified products of GO after different reactions with the same alkyl amine, which infers that GO can be controllably modified. TGA curves of GO, b-GO, and e-GO are compared in Figure 3b. The characteristics of GO are as follows: releasing of trapped water (∼26%) leads to a mass loss before 130 °C;62 removal of less stable oxygen-containing groups brings about a sharp weight loss from 130 to 220 °C;63 and decomposition of more stable functional groups causes a slow weight loss during

as electron donor, resulting in the G peak. It was reported that the G position of mechanical stripping monolayer graphene varies from 1582 to 1594 cm−1,48,56 suggesting that the prepared GO and modified GOs (b-GO and e-GO) were functionalized single-layer graphene materials. The 2D peak (second-order scattering mode) of GO and fGOs appear at ∼2672 cm−1. It has been demonstrated that the 2D peak would downshift with the enhancement of electron concentration in doped GO.57 Therefore, the n-octyl amine, as donor dopant in b-GO and e-GO, induced a 3 cm−1 downshift of the 2D peak in fGO (2672 cm−1) compared to pristine GO (2675 cm−1). There is another Raman mode at 2926 cm−1 (D+G peak), which appears in high defect density graphite lattice, demonstrating a 12 cm−1 downshift with respect to original GO (2938 cm−1) as well. The gradual high intensity of 2D and D+G peaks in GO, e-GO, and b-GO suggest the increasing disorder in the graphene lattice. The quality of graphene-based materials is widely evaluated by using ID/IG ratio (ID and IG are Raman intensity of D peak and G peak),58 an indicator of disorder of the sample, which may be edges, ripples, or any other defects, like dopants in graphene. We obtained the ID/IG ratio by using Origin 8.0 to fit points in Raman spectra after peak processing for calculating area ratio of D and G peak. The ID/IG ratio of b-GO (ID/IG = 2.13) and e-GO (ID/IG = 1.88) was significantly higher than that of GO (ID/IG = 1.79), which means that the functionalization of graphene skeleton resulted in a large amount of structural disorder in graphene lattice. The disorder degree of b-GO was much higher than that of e-GO, indicating that basal-plane modification can cause more structural disorder than that of edge modification. This may suggest that the amount of epoxy groups on GO basal planes was more than that of carboxyl groups at edges.48 By unscrambling the XRD spectra, we can speculate composition and internal information such as atomic or molecular structure of materials. Figure 2d shows the XRD spectra of GO, b-GO, and e-GO. According to the Bragg Law:59 nλ = 2d sin θ and Scherrer equation:60 D = kλ/β cos θ, we can roughly calculate the distance of interlayer20 in different materials as shown in Table S2. The characteristic peak of GO vanished, which indicated that b-GO and e-GO were successfully prepared. From Table S2, we can see the interlayer distance of b-GO is larger than that of e-GO. The 2θ peak of bGO and e-GO are wider and weaker compared to that of GO; however, the interlayer distance of them are smaller than that of GO. It may be for the reason that after reacting with n-octyl amine, the hydrogen bond between GO layers was weakened because of the alkyl chains on GO sheets. The XPS analyzing results are demonstrated in Figure 3a. GO, b-GO, and e-GO show two strong peaks at 284 eV (C 1s) E


Article


Figure 4. TEM images (9900×) of (a) GO, (b) b-GO, and (c) e-GO. (insets) High-magnification (19 500×) images of 2 μm × 2 μm area.

Figure 5. AFM images and corresponding height profiles of (a) GO, (b) b-GO, and (c) e-GO.

Figure 6. Schematic representation of the structure of pristine GO, b-GO, and e-GO.

220−800 °C. The results show that the trapped water content (step 1) of e-GO (∼19%) is slightly less than that of GO (∼26%) but greatly more than that of b-GO (∼2%) (Table S3), indicating the hydrogen-bond interaction degree can be ranked as GO > e-GO > b-GO. This further proves that the graft of alkyl chain on basal planes can greatly reduce the hydrogen-bond interaction between fGO molecules. Moreover, from differential thermogravimetric (DTG) analysis (Figure S3), it can be found that the great weight loss (step 2) caused by decomposition of less stable groups in GO, e-GO, and b-GO happened at 200, 240, and 250 °C, respectively, indicating the thermal stability is b-GO > e-GO > GO. According to the report of Park, the existence of hydroxyl groups can resist annealing process without pyrolysis from the GO layers,64 there are more hydroxyl and alkyl chains formed in b-GO after epoxy ring-opening during basal plane-functionalization, so its stability is the best. And since the stability of amido is higher than that of the carboxyl, the decomposition temperature of e-GO is higher than that of GO. In addition, because of the different content of epoxy at edges and carboxyl on basal planes, the attached contents of alkyl chains are different as well, resulting in the final weight of b-GO and e-GO to be ∼37% and 23% of

the initial weight, respectively, which may be the residue of carbon skeleton.65 3.2. Morphology Characterization of the Basal Plane or Edge Functionalized GO. Surface morphologies of pristine and alkyl amine-modified GOs (b-GO and e-GO) are shown in Figure 4. TEM results reveal multilayer-fold sheets in GO, which is the characteristic of the graphite oxide prepared by Hummer’s method,66 because the oxidation group mostly exists on surfaces and edges of the pieces, which can give rise to the thin-film structure with curls and folds due to van der Waals force, like hydrogen-bond interaction, π−π stack, etc.67 However, sheets and wrinkles of fGOs are less than that of GO; furthermore, wrinkles in b-GO are mainly distributed on edges, while those of e-GO are mostly distributed on basal planes. It may be due to the different effect of alkyl chains on the weakening of intermolecular hydrogen-bond interaction: the alkyl chains in b-GO mainly decrease the hydrogen-bond interaction of epoxy and hydroxyl groups in basal planes and make its basal plane more flat; the alkyl chains in e-GO reduce the hydrogen-bond interaction of carboxyl groups at edges, so there are still lots of wrinkles on its basal planes, but the edge is more flat. F


Article


Figure 7. Interfacial tension change with different concentrations of (a) GO; TW20, b-GO, and e-GO at (b) toluene/water interface, (c) n-hexane/ water interface, (d) acetic ether/water interface.

The thickness of GO and e-GO is analyzed to be ∼1 nm by AFM (Figure 5), whereas the thickness of b-GO is more than 2 times as much as that of e-GO, which is due to the combination of monolayer graphene skeleton and the grafted alkyl amine groups on GO basal plane.68 The TEM and AFM analysis indicated the lateral sheet sizes of GO and fGOs range from hundreds of nanometers to several microns (Figures 4 and 5). From the above molecular structure and morphology characterization, the structure of pristine GO, b-GO, and eGO could be demonstrated as shown in Figure 6: the alkyl chains of b-GO spread vertically on the GO sheets, while that of e-GO spread horizontally at the edges of the GO sheets. 3.3. Interfacial Properties of fGOs at Oil−Water Interface. As introduced before, GO can act as a 2D amphiphile with a largely hydrophobic center and hydrophilic periphery. Therefore, GO and its derivatives show surfactant and/or polyelectrolyte-like characteristics.69 The interfacial properties of GO and fGOs were studied at three kinds of oil−water interfaces, in which the oil phase is toluene, acetic ether, and n-hexane, respectively, with different molecular structures and polarity. Tween-20 (TW20, poly(oxyethylene) (20) sorbaitan monolaurate), a kind of typical nonionic surfactant, was selected as a comparison. Figure 7 shows the effect of surfactant concentration on the interfacial tension at different oil−water interface. It is found that pristine GO had no obvious effect on oil−water interfacial tension. The effects of TW20 and fGOs on the interfacial tension of acetic ether/water and n-hexane/water are similar. However, the interfacial tension reducing degree for n-hexane/ water interface is greater than that for acetic ether/water interface. The reason may be because, on the one hand, TW20 and fGOs both have alkyl chains and hydroxyl groups and the alkyl chain has the similar structure as n-hexane, so that they could form orderly arranged strong interface. On the other hand, the high polarity of acetic ether makes it have strong

interaction with water by itself, and the interfacial tension is very low originally, so it is difficult to reduce more. It is worth pointing out that different fGOs display different interfacial properties at the toluene/water interface; that is, eGO could reduce the interfacial tension between toluene and water effectively, while b-GO increases the interfacial tension. Because of the edge modification, the graphene lattice of e-GO is not obviously influenced, so it could interact with toluene through π−π stack more efficiently, resulting in effectively reducing interfacial tension between toluene and water. But for b-GO, the basal plane modification causes more disorder in graphene lattice than that of e-GO as proved by Raman analysis discussed above, so the interaction between b-GO with toluene is weak and cannot form compact interface around toluene. For n-hexane/water interface, the interfacial tension reducing capability of fGOs is even better than that of traditional nonionic surfactant TW20, and that of b-GO is slightly better than that of e-GO. Although toluene and n-hexane are both hydrophobic, their molecular structures are different. Toluene with benzene ring in its structure is more similar to that of the skeleton of GO. After functionalization, the alkyl chains grafted on both sides of GO are widely arranged on the basal planes of b-GO. On the one hand, the aromatic rings in its skeleton are difficult to expose; thus, it reveals the increasing interfacial tension of toluene/water. On the other hand, the alkyl chains on the basal planes decrease the interfacial tension of n-hexane/ water due to its similar structure with n-hexane. The results suggest that both b-GO and e-GO have good interfacial tension reducing property, and the interfacial tension reducing capability of e-GO is broader than that of b-GO. It is effective for both alkyl compound oil phase and aromatic oil phase, while b-GO is only effective for alkyl compound oil phase, and its interfacial property is similar to that of traditional nonionic surfactant TW20. G


Article


Figure 8. (a) Friction curves of GO, b-GO, and e-GO emulsions under the same test condition. (b) Micro-Raman spectra after lubrication with GO, b-GO, and e-GO emulsions.

Figure 9. Schematic diagram of emulsion lubrication with GO and fGOs and possible mechanism of adsorption in tribo-film.

surfaces. The order and density of the surface-active film, the formation of the reaction film, and the balance between the buildup and removal of the lubricating film, are all dependent on the molecular structure of the emulsion components. The absorption capability of fGOs depended on the oxygencontaining groups. After n-octyl amine reacted with pristine GO, the b-GO contained carboxyl groups at edges and hydroxyl groups on basal planes, and for e-GO, there are hydroxyl groups and epoxy groups on basal planes. Thus, as demonstrated in Figure 9, we propose that after the rupture of emulsion droplets, GO and e-GO align horizontally on the surface, while b-GO arranges vertically on the surface. For both b-GO and e-GO, their alkyl chains are extended and interacted with PAO stay between the two interacting surfaces. In the meanwhile, TW20 is competitively adsorbed onto the metal surface with GO and fGOs. Since b-GO is vertically arranged on the metal surface and has similar behavior to that of TW20, they could form more densely and compact film on the metal surface, resulting in the lowest friction coefficient. As for e-GO, since it horizontally aligns on the metal surface, it has high steric repulsion and could not form very ordered and compact film, so that COF of e-GO emulsion is high and very unstable. For GO emulsion, GO could both horizontally or vertically adsorb on the metal surface, and TW20 could interact with both the metal surface and GO surface; the film formed could

3.4. Interfacial Behavior of fGO Emulsions at Solid Surface during and after Frictional Sliding. To investigate the interfacial behavior of fGO emulsions at liquid−solid interface, friction tests were conducted with stainless steel surfaces, because the friction property is determined by the film-forming capability of the emulsion components with the interacting surfaces. The results are shown in Figure 8a. Both emulsions with pristine GO and fGOs reveal friction-reducing effect compared to base emulsion. The coefficient of friction (COF) presents the following relationship: b-GO emulsion < GO emulsion < e-GO emulsion < base emulsion. Because the oil phase we used is PAO, which is similar to n-hexane, both the b-GO and e-GO have strong interfacial activity (Figure 7c) and locate at the oil−water interface of the prepared emulsions. However, friction-reducing property of fGOs is quite different; that is, the COF of b-GO emulsion decreases, while that of eGO emulsion increases compared to pristine GO, respectively. The frictional behavior of fGO emulsions can be explained by the plate-out theory, a widely accepted theory for emulsion lubrication.70 At the initial stage of frictional sliding, the emulsion droplets are not ruptured, so they adsorb on the sliding surfaces as droplets. With continuous sliding, the droplets will be ruptured; then the oil phase component (PAO) in the emulsion will come out and, together with emulsifier (TW20 and fGOs), form lubricating films on the sliding H


Article

ACS Applied Nano Materials be dense and compact as well but not as strong as that of b-GO. Therefore, the COF of GO emulsion was between b-GO and eGO emulsion. To better understand the lubricating mechanism of GO and fGOs emulsions, we investigated Raman response of the metal surfaces after friction tests. Micro-Raman results (Figure 8b) demonstrate that, for GO emulsion lubrication, there still shows clear GO signals on the surface after frictional sliding, indicating GO was staying at the interface to reduce shear strength during sliding, while for fGOs emulsion lubrication, after frictional sliding, the GO signals are not obvious, with the D peak disappeared and the intensity of G peak reduced significantly, suggesting that GO sheet part of fGOs interacted with the metal surface, while the top surface was covered by the alkyl groups. And the friction-reducing capability depends on the density and alignment of the alkyl chains. As the result discussed above, we would like to propose the following possible lubricating mechanism, as demonstrated in the schematic diagram of Figure 9: first, the oil-in-water emulsions flowed into the interface between the frictional pairs, and then oil droplets adsorbed onto the interacting surfaces; subsequently, with the continuous sliding, the oil droplets ruptured, and the components in the oil phase (PAO) and at the interfaces (TW20, GO, fGOs) plated out and adsorbed onto the surfaces to form lubricating films. The lubricating film is formed by the competitive adsorption of TW 20 and GO or fGOs. The film formed by b-GO emulsion is dense, compact, and strong. Because based on the structure analysis results illustrated in Figure 6 and the liquid−liquid interfacial property results as shown in Section 3.3, on the one hand, the alkyl chains on b-GO are vertically arranged on the surface of b-GO sheet; on the other hand, the interfacial property of b-GO is the strongest for alkyl compound oil phase. Therefore, the b-GO interacts with the metal surface strongly through its carboxyl groups at the edges, which may result in the similar steric repulsion to that of TW20, so that it could form orderly arranged film together with TW20. Moreover, the Raman and TGA analysis results confirmed that there were more alkyl chains on b-GO than those on e-GO, so that the film formed by b-GO would be with higher density and strength. For GO emulsion, the GO sheets are interacted with the metal surface through their oxygen-containing groups. It may stand horizontally, tilting, or vertically on the metal surface, with the TW20 adsorbed both on the metal surface and GO surface, so that the film formed is dense, stronger than that of TW20, but less stronger than that of b-GO. As for e-GO emulsion, the oxygen groups on basal planes would interact with the metal surface, so that the steric repulsion is very high. Although the TW20 would interspersed interact with the metal surface and the oxygen groups on basal planes of e-GO, while due to the existing of alkyl chains at the edge of its structure and the high steric repulsion, the film formed by e-GO emulsion is even less strong than that of GO emulsion, resulting in the order of the friction value as e-GO emulsion > GO emulsion > b-GO emulsion.

(1) As expected, the molecular structure and morphology characterization of the fGOs confirmed that the reaction site for e-GO was carboxyl groups at edges of GO, and that for b-GO was epoxy groups on basal planes of GO. The modification makes the hydrogen bond between GO layers weakened, resulting in decreased layer distance and reduced wrinkles compared to that of GO itself. In addition, the results also suggest that the amount of epoxy groups on basal planes of GO is more than the amount of carboxyl groups at edges of GO. This indicates that GO could be controllably functionalized and that the structure of fGOs could be tuned. (2) The interfacial behaviors study of the two fGOs at oil− water interface shows that both b-GO and e-GO have good interfacial tension-reducing property, and the interfacial tension-reducing capability of e-GO is broader than that of b-GO. The e-GO is effective for both alkyl compound oil phase and aromatic oil phase, while b-GO is only effective for alkyl compound oil phase, and its interfacial property is similar to that of traditional nonionic surfactant TW20. (3) The interfacial behavior study of fGO emulsions with solid surface during sliding demonstrates that, after absorption, b-GO arranges vertically on the sliding surface, with the film formed by b-GO emulsion dense, compact, and strong, while e-GO aligns horizontally on the sliding surface, with the film formed not dense and compact enough, so that the friction reducing capability of b-GO emulsion is greater than that of e-GO emulsion. The results present interesting possibilities for tuning the interfacial properties of GO at both liquid−liquid and liquid− solid interfaces, which may be promising in the potential applications in controlled drug delivery, surface protection, absorption and separation, lubrication, nanocomposite, and catalyst fields.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b00405. Formulation of emulsions, images of emulsions and friction testing model, theoretical calculation of dspacing, high-resolution analysis of XPS, DTG analysis and summary of the decomposition steps (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-21-20608038. ORCID

Hongmei Yang: 0000-0002-8920-299X Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China (Grant No. 21703279) and the Shanghai Municipal “Science and Technology Innovation Action Plan” International Cooperation Project (No. 15540723600) for financial support.

4. CONCLUSIONS In this study, n-octyl amine was controllably grafted to graphene oxide to achieve basal-planes modified GO (b-GO) and edge-modified GO (e-GO) through different synthetic routes. I


Article


■

cellulose with high metal adsorption. Carbohydr. Polym. 2017, 172, 20−27. (19) Zhang, H.; Li, B.; Pan, J.; Qi, Y.; Shen, J.; Gao, C.; Van der Bruggen, B. Carboxyl-functionalized graphene oxide polyamide nanofiltration membrane for desalination of dye solutions containing monovalent salt. J. Membr. Sci. 2017, 539, 128−137. (20) Mungse, H. P.; Kumar, N.; Khatri, O. P. Synthesis, dispersion and lubrication potential of basal plane functionalized alkylated graphene nanosheets. RSC Adv. 2015, 5, 25565−25571. (21) Zhao, D.; Zhang, Q.; Xuan, H.; Chen, Y.; Zhang, K.; Feng, S.; Alsaedi, A.; Hayat, T.; Chen, C. EDTA functionalized Fe 3 O 4 /graphene oxide for efficient removal of U(VI) from aqueous solutions. J. Colloid Interface Sci. 2017, 506, 300−307. (22) Rana, S.; Jonnalagadda, S. B. Cu doped amine functionalized graphene oxide and its scope as catalyst for selective oxidation. Catal. Commun. 2017, 100, 183−186. (23) Xiao, W.; Yan, B.; Zeng, H.; Liu, Q. Dendrimer functionalized graphene oxide for selenium removal. Carbon 2016, 105, 655−664. (24) Xue, B.; Yu, M.; Liu, J.; Liu, J.; Li, S.; Xiong, L. Corrosion protection of AA2024-T3 by sol-gel film modified with graphene oxide. J. Alloys Compd. 2017, 725, 84−95. (25) Zhang, L.; Li, H.; Lai, X.; Wu, W.; Zeng, X. Hindered phenol functionalized graphene oxide for natural rubber. Mater. Lett. 2018, 210, 239−242. (26) Mu, S.; Li, G.; Liang, Y.; Wu, T.; Ma, D. Hyperbranched polyglycerol-modified graphene oxide as an efficient drug carrier with good biocompatibility. Mater. Sci. Eng., C 2017, 78, 639−646. (27) Saeed, A. A.; Sánchez, J. L. A.; O’Sullivan, C. K.; Abbas, M. N. DNA biosensors based on gold nanoparticles-modified graphene oxide for the detection of breast cancer biomarkers for early diagnosis. Bioelectrochemistry 2017, 118, 91−99. (28) Kumar, S.; Wani, M. Y.; Arranja, C. T.; Castro, R. A. E.; Paixão, J. A.; Sobral, A. J. F. N. Synthesis, physicochemical and optical properties of bis-thiosemicarbazone functionalized graphene oxide. Spectrochim. Acta, Part A 2018, 188, 183−188. (29) Rao, Z.; Ge, H.; Liu, L.; Zhu, C.; Min, L.; Liu, M.; Fan, L.; Li, D. Carboxymethyl cellulose modified graphene oxide as pH-sensitive drug delivery system. Int. J. Biol. Macromol. 2018, 107, 1184−1192. (30) Fan, F.; Zhang, B.; Cao, Y.; Yang, X.; Gu, J.; Chen, Y. Conjugated polymer covalently modified graphene oxide quantum dots for ternary electronic memory devices. Nanoscale 2017, 9, 10610−10618. (31) Mungse, H. P.; Khatri, O. P. Chemically Functionalized Reduced Graphene Oxide as a Novel Material for Reduction of Friction and Wear. J. Phys. Chem. C 2014, 118, 14394−14402. (32) Parhizkar, N.; Shahrabi, T.; Ramezanzadeh, B. A new approach for enhancement of the corrosion protection properties and interfacial adhesion bonds between the epoxy coating and steel substrate through surface treatment by covalently modified amino functionalized graphene oxide film. Corros. Sci. 2017, 123, 55−75. (33) Masoudipour, E.; Kashanian, S.; Maleki, N. A targeted drug delivery system based on dopamine functionalized nano graphene oxide. Chem. Phys. Lett. 2017, 668, 56−63. (34) Zhang, L.; Zhou, Q.; Song, W.; Wu, K.; Zhang, Y.; Zhao, Y. Dual-Functionalized Graphene Oxide Based siRNA Delivery System for Implant Surface Biomodification with Enhanced Osteogenesis. ACS Appl. Mater. Interfaces 2017, 9, 34722−34735. (35) Nguyen, T. T.; Bandyopadhyay, P.; Li, X.; Kim, N. H.; Lee, J. H. Effects of grafting methods for functionalization of graphene oxide by dodecylamine on the physical properties of its polyurethane nanocomposites. J. Membr. Sci. 2017, 540, 108−119. (36) Lei, L.; Shan, J.; Hu, J.; Liu, X.; Zhao, J.; Tong, Z. Co-curing effect of imidazole grafting graphene oxide synthesized by one-pot method to reinforce epoxy nanocomposites. Compos. Sci. Technol. 2016, 128, 161−168. (37) Akhtar, M. A.; Hayat, A.; Iqbal, N.; Marty, J. L.; Nawaz, M. H. Functionalized graphene oxide−polypyrrole−chitosan (fGO−PPy− CS) modified screen-printed electrodes for non-enzymatic hydrogen

REFERENCES

(1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V. Electric field effect in atomically thin carbon films. Science 2004, 306, 666−669. (2) Khaleghi, M.; Didehban, K.; Shabanian, M. Effect of new melamine-terephthaldehyde resin modified graphene oxide on thermal and mechanical properties of PVC. Polym. Test. 2017, 63, 382−391. (3) Omidi, S.; Kakanejadifard, A.; Azarbani, F. Noncovalent functionalization of graphene oxide and reduced graphene oxide with Schiff bases as antibacterial agents. J. Mol. Liq. 2017, 242, 812− 821. (4) Ryu, S. H.; Shanmugharaj, A. M. Influence of long-chain alkylamine-modified graphene oxide on the crystallization, mechanical and electrical properties of isotactic polypropylene nanocomposites. Chem. Eng. J. 2014, 244, 552−560. (5) Wåhlander, M.; Nilsson, F.; Andersson, R. L.; Carlmark, A.; Hillborg, H.; Malmström, E. Reduced and Surface-Modified Graphene Oxide with Nonlinear Resistivity. Macromol. Rapid Commun. 2017, 38, 1700291. (6) Lerf, A.; He, H.; Forster, M.; Klinowski, J. Structure of graphite oxide revisited. J. Phys. Chem. B 1998, 102, 4477−4482. (7) Chang, M. S.; Kim, Y. S.; Kang, J. H.; Park, J.; Sung, S. J.; So, S. H.; Park, K. T.; Yang, S. J.; Kim, T.; Park, C. R. Guidelines for Tailored Chemical Functionalization of Graphene. Chem. Mater. 2017, 29 (1), 307−318. (8) Bourlinos, A. B.; Gournis, D.; Petridis, D.; Szabó, T.; Szeri, A.; Dékány, I. Graphite Oxide Chemical Reduction to Graphite and Surface Modification with Primary Aliphatic Amines and Amino Acids. Langmuir 2003, 19, 6050−6055. (9) Dabhi, S. D.; Jha, P. K. Tuning of electronic properties and dynamical stability of graphene oxide with different functional groups. Phys. E 2017, 93, 332−338. (10) Qiu, C.; Liu, D.; Jin, K.; Fang, L.; Xie, G.; Robertson, J. Electrochemical functionalization of 316 stainless steel with polyaniline-graphene oxide: Corrosion resistance study. Mater. Chem. Phys. 2017, 198, 90−98. (11) Yuan, B.; Bao, C.; Song, L.; Hong, N.; Liew, K. M.; Hu, Y. Preparation of functionalized graphene oxide/polypropylene nanocomposite with significantly improved thermal stability and studies on the crystallization behavior and mechanical properties. Chem. Eng. J. 2014, 237, 411−420. (12) Liu, C.; Du, P.; Zhao, H.; Wang, L. Synthesis of l-HistidineAttached Graphene Nanomaterials and Their Application for Steel Protection. ACS Applied Nano Materials 2018, 1, 1385−1395. (13) Fan, Z.; Wong, K. K.; Chen, S.; Lau, S. P.; Po, K. H. L. Polyethylenimine-Modified Graphene Oxide as a Novel AntibacterialAgent and Its Synergistic Effect with Daptomycin for Methicillin Resistant Staphylococcus aureus. ACS Applied Nano Materials 2018, 1, 1811−1818. (14) Zhang, B.; Yan, Y.; Shen, Q.; Ma, D.; Huang, L.; Cai, X.; Tan, S. A colon targeted drug delivery system based on alginate modificated graphene oxide for colorectal liver metastasis. Mater. Sci. Eng., C 2017, 79, 185−190. (15) Kongkaew, S.; Kanatharana, P.; Thavarungkul, P.; Limbut, W. A preparation of homogeneous distribution of palladium nanoparticle on poly (acrylic acid)-functionalized graphene oxide modified electrode for formalin oxidation. Electrochim. Acta 2017, 247, 229−240. (16) Gupta, R. K.; Malviya, M.; Verma, C.; Quraishi, M. A. Aminoazobenzene and diaminoazobenzene functionalized graphene oxides as novel class of corrosion inhibitors for mild steel: Experimental and DFT studies. Mater. Chem. Phys. 2017, 198, 360− 373. (17) Sun, Y.; Wang, Y.; Li, J.; Ding, C.; Lin, Y.; Sun, W.; Luo, C. An ultrasensitive chemiluminescence aptasensor for thrombin detection based on iron porphyrin catalyzing luminescence desorbed from chitosan modified magnetic oxide graphene composite. Talanta 2017, 174, 809−818. (18) Yakout, A. A.; El-Sokkary, R. H.; Shreadah, M. A.; Abdel Hamid, O. G. Cross-linked graphene oxide sheets via modified extracted J


Article

ACS Applied Nano Materials peroxide detection. J. Nanopart. Res. 2017, 19 (334). DOI: 10.1007/ s11051-017-4029-x (38) Lu, K.-J.; Zuo, J.; Chung, T.-S. Novel PVDF membranes comprising n-butylamine functionalized graphene oxide for direct contact membrane distillation. J. Membr. Sci. 2017, 539, 34−42. (39) Kim, J.; Cote, L. J.; Kim, F.; Yuan, Wa; Shull, K. R.; Huang, J. Graphene Oxide Sheets at Interfaces. J. Am. Chem. Soc. 2010, 132, 8180−8186. (40) Kim, J.-W.; Lee, D.; Shum, H. C.; Weitz, D. A. Colloid Surfactants for Emulsion Stabilization. Adv. Mater. 2008, 20, 3239− 3243. (41) Thickett, S. C.; Zetterlund, P. B. Graphene oxide (GO) nanosheets as oil-in-water emulsion stabilizers: Influence of oil phase polarity. J. Colloid Interface Sci. 2015, 442, 67−74. (42) Liu, J.; Li, X. C.; Jia, W. H.; Li, Z. Y.; Zhao, Y. P.; Ren, S. L. Demulsification of Crude Oil-in-Water Emulsions Driven by Graphene Oxide Nanosheets. Energy Fuels 2015, 29, 4644−4653. (43) Kinoshita, H.; Nishina, Y.; Alias, A. A.; Fujii, M. Tribological properties of monolayer graphene oxide sheets as water-based lubricant additives. Carbon 2014, 66, 720−723. (44) Ma, Y.; Xu, J.; Zeng, X.; Jiang, H.; Li, J. Preparation and performance evaluation of mPAO8 using olefin from coal as raw material. Ind. Lubr. Tribol. 2017, 69, 678−682. (45) Chen, J.; Yao, B.; Li, C.; Shi, G. An improved Hummers method for eco-friendly synthesis of graphene oxide. Carbon 2013, 64, 225− 229. (46) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. The chemistry of graphene oxide. Chem. Soc. Rev. 2010, 39, 228−240. (47) Wang, S.; Wang, J.; Zhang, W.; Ji, J.; Li, Y.; Zhang, G.; Zhang, F.; Fan, X. Ethylenediamine Modified Graphene and Its Chemically Responsive Supramolecular Hydrogels. Ind. Eng. Chem. Res. 2014, 53, 13205−13209. (48) Cao, H.; Wu, X.; Yin, G.; Warner, J. H. Synthesis of adeninemodified reduced graphene oxide nanosheets. Inorg. Chem. 2012, 51, 2954−2960. (49) Lin, Y.; Chen, Y.; Zeng, Z. K.; Zhu, J. R.; Wei, Y.; Liu, L.; Li, F. C.; et al. Effect of ZnO nanoparticles doped graphene on static and dynamic mechanical properties of natural rubber composites. Composites, Part A 2015, 70, 35−44. (50) Samanta, S.; Sahoo, R.; Singh, S. Simultaneous chemical reduction and surface functionalization of graphene oxide for efficient lubrication of steel−steel contact. RSC Adv. 2015, 5, 61888−61899. (51) Ahmad Daud, N.; Chieng, B. W.; Ibrahim, N. A.; Talib, Z. A.; Muhamad, E. N.; Abidin, Z. Z. Functionalizing Graphene Oxide with Alkylamine by Gamma-ray Irradiation Method. Nanomaterials 2017, 7, 135. (52) Cai, W.; et al. Synthesis and Solid-Structural Characterization of 13C-Labeled Graphite Oxide. Science 2008, 321, 1815−1817. (53) He, H.; Riedl, T.; Lerf, A.; Klinowski, J. Solid-state NMR studies of the structure of graphite oxide. J. Phys. Chem. 1996, 100, 19954− 19958. (54) Tuinstra, F.; Koenig, J. Raman spectrum of graphite. J. Chem. Phys. 1970, 53, 1126−1130. (55) Ganguly, A.; Sharma, S.; Papakonstantinou, P.; Hamilton, J. Probing the thermal deoxygenation of graphene oxide using highresolution in situ X-ray-based spectroscopies. J. Phys. Chem. C 2011, 115, 17009−17019. (56) Rao, C. N.; Sood, A. K.; Subrahmanyam, K. S.; Govindaraj, A. Graphene: the new two-dimensional nanomaterial. Angew. Chem., Int. Ed. 2009, 48, 7752−7777. (57) Das, A.; Pisana, S.; Chakraborty, B.; Piscanec, S.; Saha, S. K.; Waghmare, U. V.; Novoselov, K. S.; Krishnamurthy, H. R.; Geim, A. K.; Ferrari, A. C.; Sood, A. K. Monitoring dopants by Raman scattering in an electrochemically top-gated graphene transistor. Nat. Nanotechnol. 2008, 3, 210−215. (58) Bokobza, L.; Bruneel, J.-L.; Couzi, M. Raman Spectra of Carbon-Based Materials (from Graphite to Carbon Black) and of Some Silicone Composites. J. Carbon Res. 2015, 1, 77−94.

(59) Li, J. C.; Zeng, X.; Ren, T. H.; Van Der Heide, E. The Preparation of Graphene Oxide and Its Derivatives and Their Application in Bio-Tribological Systems. Lubricants 2014, 2 (3), 137−161. (60) Abbasi, Z.; Haghighi, M.; Fatehifar, E.; Saedy, S. Synthesis and physicochemical characterizations of nanostructured Pt/Al2O3-CeO2 catalysts for total oxidation of VOCs. J. Hazard. Mater. 2011, 186, 1445−1454. (61) Szabo, T.; et al. Evolution of Surface Functional Groups in a Series of Progressively Oxidized Graphite Oxides. Chem. Mater. 2006, 18, 2740−2749. (62) Eigler, S.; Dotzer, C.; Hirsch, A.; Enzelberger, M.; Müller, P. Formation and Decomposition of CO2 Intercalated Graphene Oxide. Chem. Mater. 2012, 24, 1276−1282. (63) McAllister, M. J.; Li, J.-L.; Adamson, D. H.; Schniepp, H. C.; Abdala, A. A.; Liu, J.; et al. Single sheet functionalized graphene by oxidation and thermal expansion of graphite. Chem. Mater. 2007, 19, 4396−4404. (64) Kang, J. H.; Kim, T.; Choi, J.; Park, J.; Kim, Y. S.; Chang, M. S.; Jung, H.; Park, K. T.; Yang, S. J.; Park, C. R. Hidden Second Oxidation Step of Hummers Method. Chem. Mater. 2016, 28 (3), 756−764. (65) Chen, J.; Li, Y.; Huang, L.; Li, C.; Shi, G. Q. High-yield preparation of graphene oxide from small graphite flakes via an improved Hummers method with a simple purification process. Carbon 2015, 81, 826−834. (66) Botas, C.; Alvarez, P.; et al. Graphene materials with different structures prepared from the same graphite by the Hummers and Brodie methods. Carbon 2013, 65, 156−164. (67) Mattevi, C.; et al. Evolution of Electrical, Chemical, and Structural Properties of Transparent and Conducting Chemically Derived Graphene Thin Films. Adv. Funct. Mater. 2009, 19, 2577− 2583. (68) Botas, C.; Pérez-Mas, A. M.; Á lvarez, P.; Santamaría, R.; Granda, M.; Blanco, C.; Menéndez, R. Optimization of the size and yield of graphene oxide sheets in the exfoliation step. Carbon 2013, 63, 576− 578. (69) Wang, M.; Niu, Y.; Zhou, J.; Wen, H.; Zhang, Z.; Luo, D.; Gao, D.; Yang, J.; Liang, D.; Li, Y. The dispersion and aggregation of graphene oxide in aqueous media. Nanoscale 2016, 8, 14587−14592. (70) Fujita, N.; Kimura, Y. Influence of Plate-out Oil Film on Lubrication Characteristics in Cold Rolling. ISIJ Int. 2012, 52, 850− 857.

K


Correlation between Molecular Structure and ... - ACS Publications

Recommend Documents