Article pubs.acs.org/Langmuir
Intercalating Oleylamines in Graphite Oxide Kaikun Yang,† Si Liang,†,§ Lianfeng Zou,† Liwei Huang,† Cheol Park,∥ Lisheng Zhu,‡ Jiye Fang,‡ Qiang Fu,§ and Howard Wang*,† †
Institute for Materials Research and Department of Mechanical Engineering, and ‡Department of Chemistry, Binghamton University, State University of New York, Binghamton, New York 13902, United States § Department of Polymer Science and Materials, Sichuan University, State Key Laboratory of Polymer Materials Engineering, Chengdu 610065, People’s Republic of China ∥ National Institute of Aerospace, Hampton, Virginia 23666, United States
ABSTRACT: Graphite oxide has been synthesized from raw graphite particles and been treated with various mass amounts of oleylamine as intercalants to form intercalation compounds. X-ray diffraction patterns reveal that the inter-sheet distances strongly depend on the graphite oxide to oleylamine mass ratios. The equilibrium-like behavior implies diffusion-dominated oleylamine adsorption on graphite oxide in solution and excluded volume intercalations among oleylamine-adsorbed graphite oxide during restacking. The intercalation compounds are soluble in organic solvents, and their applications in the fabrication of transparent and conductive coatings have been demonstrated.
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thermal evaporation of the adsorbed surfactant.13 GO films could be further reduced to conductive films through chemical and thermal means if the reduced form of GO is desirable for particular applications. Alkylamines with various lengths of alkyl chains have been studied for intercalating GO; they are shown to be effective intercalants, changing the intersheets distance of GO by various magnitude.4,14,15 In this study, we report GO stacks intercalated by a relative long chain oleylamine (OA) and show near-equilibrium interactions with varying OA/GO ratios, which enable precise control of adsorption states, and therefore the composition and structure of final products.
INTRODUCTION Graphene-based materials have attracted great interest in recent years because of their extraordinary physical properties.1 As pure and individually isolated graphene sheets are difficult to obtain in large quantities for materials applications, much attention has been diverted to its oxidized form, graphite oxide (GO). Similar to graphene, GO is also a two-dimensional nanomaterial, but unlike graphene made of only carbon atoms, GO sheets are synthesized through the oxidation of graphite and have carbon backbones decorated with epoxide and hydroxyl groups on the basal planes and carbonyl and carboxyl groups along the edges.2−4 Those functional groups provide convenient means for synthesizing various GO derivatives containing polymers, hydrophilic molecules, transition metal ions, or surfactants.5−8 Applications of modified GO have been intensively investigated, including GO−polymer composites,9 nanoporous graphitics,10 GO−nanoparticle composites,11 battery electrode materials,12 etc. To render GO processable in nonpolar organic solvents, amphipathic surfactant molecules, such as alkylamine, are often used to intercalate GO. Individually modified GO sheets can be dissolved in various organic solvents upon ultrasonication to form stable suspensions. GO films could be conveniently deposited on various substrates from solution casting followed by © 2012 American Chemical Society
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EXPERIMENTAL SECTION
Following Staudenmaier’s method, GO was synthesized from natural graphite powders through reaction with strong acids (90% HNO3 and 98% H2SO4) and oxidant (KClO3).16 Graphite was purchased from Fisher Scientific and all other chemicals were from Sigma-Aldrich. They were used as received. To prepare OA intercalated GO (OA− GO), fixed amount of GO/H2O (50 mg/8.5 mL) suspensions was mixed with the OA/ethanol solution containing various OA amounts Received: September 26, 2011 Revised: December 28, 2011 Published: January 9, 2012 2904
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Figure 1. SEM micrographs of (a) pristine graphite particles in sizes of a few micrometers and with clean faceted surfaces and sharp edges; (b) graphene oxide (GO) powder without sharp edges; (c) oleylamine (OA) treated graphite oxide (OA−GO), showing layered stacking morphology;
to ensure the OA/GO mass ratios, r, of 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, and 1, respectively. The mixtures were vigorously stirred for 24 hours room temperature followed by vacuum drying of ethanol and water at 80 °C for 12 hours. The resultant solids were collected for the further characterizations. A 0.5 mg/mL OA−GO suspension with r = 1 in toluene was prepared by ultrasonication at the frequency of ∼42 kHz and the power of 155 W. OA−GO films were spun-cast at 2000 round per minute (RPM) on quartz substrates pretreated by 3 wt % aminopropyl triethoxysilane (APTES)/toluene solution for 1 hour. The dried films were annealed at 300 °C in air for 15 min. The morphology of the specimens was examined using a Zeiss Supra 55 field emission scanning electron microscope (FESEM) at a voltage of 5 kV. Atomic force microscope (AFM) images were obtained using a Nanoscope V (Digital Instrument) instrument with tapping-mode. X-ray diffraction (XRD) patterns were recorded using a PANalytical X’Pert Pro diffractometer operated at 30 KV and 45 mA with filtered 0.15 nm Cu Kα radiation. A Cary 50 UV-vis spectrophotometer from Varian Inc. was used to investigate the optical absorption. Thermogravimetric analyses (TGA) were carried out using a TGA Q50 apparatus from TA Instruments Co. from the ambient temperature to 700 °C in a 100 standard cubic centimeter per minute (SCCM) N2 flow at a heating rate of 10 °C/min.
possibly resulting from stress relaxation during the intercalation and drying processes. To gain insights in the intercalation process during OA−GO forming, GO was treated with different amounts of OA, and the resulting composites were examined using XRD. Figure 2a shows the XRD patterns of the pristine graphite, GO, and OA− GOs with r from 0.01 to 1. One common feature of the XRD data is a single dominant peak for all specimens, suggesting layer structures with a well-defined average interlayer distance, although that of the highest OA content (r = 1) at around 5° is somewhat weak and almost out of the range. As compared to the graphite’s layer spacing of 0.33 nm, GO has a much larger value of 0.64 nm, due to the formation of oxygen-containing function groups on carbon basal planes. The (001) peak positions of OA−GOs shift further toward lower angles with increasing OA/GO mass ratio. Note that the lack of a secondary peak at either 0.33 or 0.64 nm in OA−GOs indicates that no portion of graphite or GO stacking remains untouched by OA intercalation, suggesting near-equilibrium structure of OA−GO in each intercalation process. The average intersheet distance from XRD patterns as a function of OA/GO ratio is plotted in Figure 2b in a linearlogarithm scale. It shows that the intersheet distance, lexp, increases with the OA mass, from 0.64 nm, for r = 0.01, to 1.18 nm, for r = 1, respectively. To assess the amount of OA actually intercalating between GO sheets, we assume the density of OA remains constant, the same as that in the bulk. If all OA entered the GO stacking and distributed uniformly, the overall intersheet distance would be L = l0(1 + rρGO/ρOA), where l0 is the intersheet distance of GO stacking, and ρGO and ρOA are the mass densities of GO and OA, respectively. Using l0 = 0.64 nm, ρGO = 1.60 g/cm3, and ρOA = 0.81 g/cm3, L is plotted in Figure 2c as the dashed curve. L appears to be close to the
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RESULTS AND DISCUSSION The morphologies of pristine graphite, GO, and OA−GO (r = 1) particles are shown in Figure 1. The pristine graphite particles are of a few micrometers in size and have clean faceted surfaces and sharp edges (Figure 1a), whereas GO powders in Figure 1b show broad particles size ranges from submicrometer to several micrometers, and morphological features resembling fractured graphite without sharp edges. Figure 1c shows that OA−GO exhibits lamellae-like stacking morphology for many particles viewed from the sides. The high magnification image in Figure 1d clearly illustrates layered structures of OA−GO particles with features such as bending, kinking, and cracking, 2905
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Figure 2. (a) XRD of the OA−GO with mass ratios from 0.01 to 1. The shift of (001) peak to the lower angle indicating larger intersheet distance at higher OA/GO mass ratio. (b) Linear-logarithm plots of intersheet distance as a function of OA/GO mass ratio. The symbols are experimental data, and the dashed curve is the calculation from the assumption of all OA molecules intercalated to GO stacking. The inset shows the percentage of OA intercalation and the fwhm of the (001) peaks. (c) TGA scans of OA, GO, and OA−GO specimens (r = 0.01, 0.05, and 0.5); as OA−GOs with low OA amounts behave very similar to GO, the high OA one shows different mass loss characteristics. (d) Schematics of OA-GO formation at various OA/GO mass ratios.
measured values at r < 0.1, whereas the difference becomes prominent at larger r. The ratio of actual amount of OA entering the GO stacking is (lexp − l0)/(L − l0), which is plotted as the inset of Figure 2b. The actual intercalation ratio drops from the maximum of 70.4% at r = 0.02 to the minimum of 33.8% at r = 0.2. The inset in Figure 2b also shows that the full width at half-maximum (fwhm) of the (001) peaks increases with r, implying larger spacing distribution in OA−GO stacks. The observation points to two possible causes: increasing heterogeneity of OA distribution among the layers, that is, different amount of OA in different layers, and therefore different distances, or increasing thickness fluctuation between the adjacent GO layers, or both. The former is associated with the smaller driving force for intercalation because of more adsorption of OA in GO layers, whereas the latter is due to the smaller attractive van der Waals forces between the adjacent GO layers because of larger interlayer distances. TGA scans of OA, GO, and OA−GOs (r = 0.01, 0.05, and 0.5) are shown in Figure 2c. Those of the GO and OA−GO’s with r = 0.01, 0.05 have similar weight loss behaviors, reflecting their comparable compositions and structures. The weight loss at temperature below 100 °C is typically associated with the evaporation of residual solvent. Loosely bound OA’s evaporate between 100 and 200 °C, whereas those strongly bound could be evaporated only at higher temperatures. For a small amount of OA, it is most likely that the OA fraction leaves the specimen over a wide temperature range. Most of the mass loss at temperatures around 250 °C in GO and OA−GO with small r is attributed to the pyrolysis of oxygen-containing functional groups.4,17 The TGA behavior of OA−GO with r = 0.5 appears to be rather different. The first major loss of ∼35% between 100 and 200 °C is mostly due to the evaporation of loosely
bound OA and possibly also some trapped solvent, whereas the process of the subsequent ca. 40% loss over a wide temperature range is not obvious. It could be due to that the OA−GO complex stabilizes the pyrolysis reduction. Both XRD and TGA data imply that intercalation strongly depends on the OA concentration. The driving force for intercalation can be discussed in the light of free energy differences of OA in solution, in the adsorbed state, and in the OA−GO stacking. Because the solubility of OA in water/ alcohol mixture is limited, there is little free energy penalty for OA adsorption to GO during the process. The enthalpy gain of adsorbing amine groups to GO sheets must exceed entropy loss by confining OA molecules in the stacking. There are several energetic terms involved in the process. The primary driving force favoring intercalation is the attractive interactions between the amine groups and the functional groups on the surface of GO sheets, while interaction against intercalation includes van der Waals forces among the basal planes and the contacts between the hydrophobic alkyl tails and hydrophilic GO. The latter facilitate the excluded volume interactions among the OA−GO sheets during the solvent removal, which play an important role for their more or less uniform restacking. The schematics of OA intercalation are illuminated in Figure 2d, with increasing OA loading. At low OA concentrations, the attractive amine/epoxide interactions dominate the adsorption in solution, whereas van der Waals force and excluded volume interactions among the GO sheets overwhelm the unfavorable alkyl/GO interactions during the solvent drying, causing the stacking of OA−GO sheets. Although OA molecules may prefer to attach on GO at a tilt angle,4 strong van der Waals forces among the basal planes render OA molecules to lay down on the basal planes, resulting in a small but uniform 2906
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Figure 3. (a) AFM image of as-annealed OA−-GO sheets showing 300−600 nm lateral dimensions. The inset is a photograph of the toluene suspension. (b) Cross-section scan of the sheets in (a) showing thickness of ∼1 nm, corresponding to single-layer graphene. (c) SEM image of annealed OA−GO film showing good surface coverage. (d) UV−vis spectrum of the annealed OA−GO film showing high transparency in visible regime and high absorption in UV region. The inset is a photograph of the OA−GO coated quartz plate after annealing at 300 °C for 15 min.
characterized using a UV−vis spectrometer. The optical transmittance spectrum shows a relatively flat with ∼87% transparency in the visible regime, whereas it shows increasing absorption near-UV. Because OA−GOs in nonpolar organic solvents also have good wettability on plastic substrates, and the mild reduction conditions are quite compatible with high temperature engineering plastics, similar results were also obtained using kapton substrates. The electric and optical performances are expectedly lower than ones in previous reports using chemical or thermal reduction methods;13,20 however, here we use an example of mild processing conditions to demonstrate the feasibility of using OA−GOs for producing transparent conductive coating on plastic substrates such as kapton. A comprehensive study on the full applicability and limitations of surfactant-derived GOs will be left to a future publication.
increase in the intersheet distance. As OA loading increases, OA adsorption increases in solution, while in the dry state, the attractive van der Waals force becomes weaker; however, the repulsion is also smaller due to the fewer contacts between alkyl chains and GO, resulting in a net increase of the intersheet distance. At higher OA loading, the GO surface may be saturated with amine functional groups, the alkyl chains extend far from the GO basal plane, van der Waals force becomes rather weak, and consequently the average intersheet distance as well as its variation dramatically increase. It corresponds to the situation of intercalated OA− GO sheets with r > 1, which could be fully exfoliated in organic solvent with brief ultrasonication. The applications of OA−GO as the precursor of transparent conductive films have been examined. The inset in Figure 3a shows a 0.5 mg/mL OA−GO (r = 1) suspension in toluene, which was stable for at least 1 month. Figure 3a shows the AFM morphology of individual OA−GO sheets cast from diluted solution. Their lateral dimensions are about 300−600 nm, and their thickness of ∼1 nm, as shown in the cross-section measurement in Figure 3b, corresponding to single-layer graphene sheets.18,19 Figure 3c shows good coverage of the OA−GO film on quartz after thermal annealing at 300 °C for 15 min. Upon thermal treatment, physisorbed and partially hydrogen-bonded OA as well as labile oxygen-containing functional groups were removed, rendering the remaining film electrically conductive. As compared to using more aggressive schemes such as hydrazine reduction and high temperature thermal annealing, this approach resulted in rather modest reduction. The sheet resistance of the annealed OA−GO film is ∼106 Ω/□ measured using a four-point probe. Figure 3d shows the transparency of annealed OA−GO film on a quartz plate (the inset), which was
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SUMMARY GO was synthesized from graphite powders and treated with various mass amounts of OA to form intercalation compounds, OA−GO. The corresponding XRD patterns reveal that the intersheet distances strongly depend on the OA/GO mass ratios, increasing with OA load toward full exfoliation. The equilibrium-like behavior implies a diffusion-dominated adsorption process, and the subtle balance of weak forces varying with the amount of OA loading. This behavior allows for precise tailoring of GO structures through intercalating organic molecules. Fabrication of transparent and conductive coatings using thermal annealed OA−GO films has also been demonstrated.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 2907
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ACKNOWLEDGMENTS We acknowledge the support by the National Science Foundation CMMI-0928865 and CMMI-0928839 and the Center for Advanced Microelectronics Manufacturing (CAMM) of the State University of New York at Binghamton.
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