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Synthesis, characterization, and surface wettability properties of amine functionalized graphene oxide films with varying amine chain lengths
Journal of Colloid and Interface Science 401 (2013) 148–154
Contents lists available at SciVerse ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Synthesis, characterization, and surface wettability properties of amine functionalized graphene oxide films with varying amine chain lengths A.M. Shanmugharaj, J.H. Yoon, W.J. Yang, Sung Hun Ryu ⇑ Department of Chemical Engineering, Kyung Hee University, Yongin, Kyunggi-Do 336-701, South Korea
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Article history: Received 10 December 2012 Accepted 15 February 2013 Available online 4 April 2013 Keywords: Graphene oxide Superhydrophobicity Contact angle Alkylamines
a b s t r a c t Surface functionalization of graphene oxide (GO) an important graphene precursor using alkylamines of varying chain lengths followed by thermal treatment resulted in the formation of superhydrophobic surfaces. Alkylamines consisting of hydrophobic long chain alkyl groups and hydrophilic amine groups were chemically reacted to the GO surface via two types of reactions viz. (i) amidation reaction between amine groups and carboxylic acid sites of GO and (ii) nucleophilic substitution reactions between amine and epoxy groups on GO surface. Successful grafting of alkylamines was confirmed using Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (1H NMR), and thermogravimetric analysis (TGA). Alkylamine-modified GO surfaces showed enhanced roughness, and this effect was more pronounced with increasing amine chain length. Water contact angle measurements revealed that the hydrophobic nature of graphene depended on the chain length of the grafted alkylamines, and this fact may be corroborated to the decrease in the surface energy values. Our results indicate that superhydrophobic graphene films can be produced by thermal treatment of hexadecylamine- and octadecylaminegrafted GO films. These results will provide valuable guidance for the design and manufacture of graphene-based biomaterials, medical instruments, structural composites, electronics, and renewable energy devices. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction The lotus effect has inspired many studies of self-cleaning superhydrophobic surfaces that have a static water contact angle larger than 150° and a hysteresis of less than 10° [1–3]. Superhydrophobic coating techniques have been widely used as antistiction and anticontamination films in optoelectronics, biochemical sensors, and micro-electromechanical systems (MEMSs) to lower moisture adsorption and improve the reliability of electronic devices [4,5]. Superhydrophobic materials are generally water repellent, a feature that is strongly influenced by both composition and geometric structure (or surface roughness). Several experimental and modeling studies have exploited surface roughness on the micrometer or nanometer scale to engineer superhydrophobic materials [6–10]. A variety of methods and materials have been used to generate superhydrophobic surfaces, including carbon-based materials such as self-cleaning carbon nanotube forests, pillars, films, and nanocomposites [11,12]. However, the fabrication of carbonnanotube-based self-cleaning surfaces has been generally limited by high costs and wafer-sized products. A variety of superhydro⇑ Corresponding author. Fax: +82 31 202 1946. E-mail address: [email protected] (S.H. Ryu). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.02.054
phobic materials have been made, but a facile and cheap production process is still needed. Recently, graphene, an allotrope of carbon, has attracted considerable attention due to its outstanding physical properties such as its high structural strength and excellent electrical properties [13]. In spite of intense research into graphene, very few studies have examined water–graphene interactions [14], which could be an important study, if graphene has to be used in conformal coatings. Recent reports revealed the microscopic structures of adsorbed water on graphene conformal coatings deposited on the hydrophilic [15] and hydrophobic surfaces [16] have been reported. Deposition of graphene material on planar substrates results in the formation of interconnected graphene films, which increases the surface roughness of substrates twofold, leading to improvements in surface properties and the wettability of coated films [17,18]. Studies have revealed that a single layer or a few layers of graphene coating, prepared via epitaxial growth, have a contact angle of 92.5°, which is close to the 91° of highly oriented pyrolytic graphite [19]. Alternatively, graphite oxide (GO), a precursor of graphene synthesized via the chemical exfoliation of graphite, is often used for coatings because it easily forms a fine dispersion in polar solvents [20,21]. Understanding the wettability of GO films is important for the in-depth study of graphene-based functional materials. To date, only a few studies have attempted to tune the
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wettability of GO films by functionalizing GO with phenyl isocynate [22] or octadecylamine [23]. However, no reports are available on tuning the wettability properties of functionalized GO. In this study, we demonstrate a facile method for the synthesis of amine-functionalized GO films using alkylamine of varying chain lengths. The purpose of this work was to understand the effect of alkylamine chain length on the surface wettability properties of functionalized GO films. 2. Experimental section 2.1. Preparation of graphene oxide GO was synthesized from purified natural graphite (Sigma–Aldrich) by a modified Hummer’s method [24,25]. In brief, 1 g of graphite flakes was added to 100 mL of concentrated sulfuric acid and subjected to sonication for 30 min using a Branson digital sonicator (S450D, 500 W, 30% amplitude) followed by the addition of 1 g of sodium nitrate (NaNO3) in an ice bath. To this mixture, 6 g of potassium permanganate was slowly added under ice-cold conditions. The mixture was stirred for 2 h, and the temperature was raised and maintained at 35 °C in a water bath for another 0.5 h. Next, 46 mL of 70 °C water was added dropwise to the solution and the temperature of the system was increased to 98 °C. Finally, 140 mL of 70 °C water was added, followed by 20 mL of 30% (wt) hydrogen peroxide (H2O2) solution to terminate the reaction [26]. The synthesized graphite oxide was suspended in water and purified by dialysis to completely remove residual salts and acids. The resulting GO was dried overnight at 55 °C under vacuum (40 mmHg) to produce GO powder. 2.2. Functionalization of GO and preparation of alkylamine-GO films GO (500 mg) was dispersed in 100 mL of thionyl chloride and heated at 70 °C for 24 h in the presence of 1 mL dimethyl formamide to generate GO-COCl. After purification, 100 mg of GO-COCl was dispersed in ethanol containing 400 mg of hexyl amine (A6), and the suspension was sonicated for 2 h at 60 °C. After cooling to room temperature, excess hexylamine was removed by washing with ethanol several times. The remaining solid was separated by filtration using a 0.2-lm membrane filter. The collected solid was again washed with ethanol several times and dried at 60 °C under vacuum to generate hexylamine-GO powders (GO-A6). Similar synthesis steps were followed in preparing decylamine- (GOA10), hexadecylamine- (GO-A16), and octadecylamine (GO-A18)grafted GO samples. 2.3. Characterizations Fourier transform infrared spectroscopy (FT-IR) characterization of alkylamine-GO samples was performed at ambient temperature using a Perkin Elmer spectrometer (Spectrum one, USA) in transmission mode in the wavenumber range of 4000–400 cm1 at a resolution of 0.4 cm1. Samples for IR characterization were prepared by making KBr pellets using finely dispersed GO (0.1 mg) in 100 mg of KBr powder. Powder X-ray diffraction (XRD) analysis of GO samples in the 2h ranges of 2–40° was carried out using a Mac Science diffractometer (M18XHF-SRA, Japan) equipped with a Cu target (40 kV, 20 mA, Ka1 = 1.54 Å). Two sets of XRD experiment were carried out with the scanning speed of 1°/min (for 2h range of 310°) and 2°/min (for 2h range of 10– 40°). Thermogravimetric characterization (TGA) was carried out using a thermogravimetric analyzer (TGA Q5000 IR, TA instruments, USA) operated at a heating rate of 20 °C/min at nitrogen atmosphere kept at the flow rate of 50 mL/min. A Wyko optical
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profilometer (Veeco Instruments, USA) was used to test film roughness by vertical scanning mode. The surface roughness was measured in two lateral scales (2.0 2.0 mm2). Contact angle measurements were performed under ambient conditions using an FTA 1000 contact angle meter (First Ten Angstroms, Portsmouth, VA). An automated program was employed in which a 3 lL droplet of water was dispensed using a microsyringe, and images were acquired immediately after dispensing using a digital camera. Scanning electron microscopy (SEM) (Leo Supra 55, Genesis 2000, Carl Zeiss, Germany) was used to investigate surface morphology of amine functionalized GO measured under an accelerating voltage of 10 kV.
3. Results and discussion The synthesis of GO from natural graphite using a modified Hummer’s method is described in Section 2. Synthesized GO is a non-stoichiometric material with many oxygen-containing functional groups such as phenolic AOH, epoxy, carbonyl, and carboxylic acid groups dispersed both at the basal planes and along the edges [27]. Lin et al. [23] reported that the wettability of GO can be effectively adjusted by grafting octadecylamine onto the GO surface. In this work, we studied the effect of chain length of alkylamines on the intercalation chemistry and thereby its influence on the structure and wettability properties of the graphene-based nanomaterial films. Scheme 1 represents schematic representation on the synthesis and fabrication steps of alkylamine-grafted GO films. Earlier report revealed that chemically prepared GO consisting of epoxy groups in its basal plane undergo nucleophilic substitution reaction with ANH2 groups of alkylamines [28]. Unlike the previous report [28], in the present study, both nucleophilic substitution and amidation reactions predominated (Scheme 2). As chemically prepared GO consists of epoxy groups in its basal plane and carboxylic sites at the edges, it can be expected that nucleophilic substitution reactions between epoxy and alkylamines predominated in the basal planes of GO sheets and amidation reactions occurred preferentially at the edges of the GO sheets [29]. In addition, hydrogen-bonding and electrostatic attractions occurred due to the presence of hydroxide and carboxylic acid sites on the GO surface. The successful grafting of alkylamines on the GO surface was characterized using FT-IR. Pristine GO showed major IR stretching vibrations at 1710, 1610, 1217, and 1050 cm1 that corresponded to AC@O stretching (ACOOH group), AOH bending (adsorbed water), CAO stretching (ACOOH group), and CAOAC stretching (epoxy group) vibrations (Fig. 1). Upon thionyl chloride treatment, the carboxylic acid sites were converted to acid chlorides. This was indicated by a shift in the stretching vibrations of AC@O (1720 cm1) and CAO (1220 cm1) groups to a higher wavenumber due to the negative inductive effect of the chlorine atom in the ACOCl group (Fig. S1, Supporting Information). Successful formation of COCl groups in GO was further supported by the increase in peak intensity at 615 cm1, which corresponded to CACl stretching vibrations (Fig. S1, Supporting Information). Fig. 1 shows the FT-IR spectra of pristine GO and alkylaminegrafted GO (GO-A6 to GO-A18) that provided informations on the chemical interaction between GO and alkylamines. Compared to pristine GO or GO-COCl, alkylamine-modified GO (GO-A6 and GO-A10) showed a broad peak at around 2900 cm1 corresponding to the ACH asymmetric and symmetric stretching of ACH2 groups. This demonstrated the presence of alkylamine on the GO surface. In contrast to GO-A6 and GO-A10, GO modified with alkylamines of long hydrocarbon chains (GO-A16 and GO-A18) showed distinct intense peaks at 2930 cm1 (ACH asymmetric stretching) and 2854 cm1 (ACH symmetric stretching), demonstrating its
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Scheme 1. Schematic representation of alkylamine-grafted GO films.
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Scheme 2. Possible reaction mechanism of GO and alkylamines.
Fig. 2b. DTG results of amine-grafted GO samples.
Fig. 1. FT-IR spectra of pristine and alkylamine-grafted GO samples.
presence on the GO surface. The nature of the interaction between GO and alkylamines is confirmed by the appearance of peaks at 1575 cm1 (C@O stretching of amide group) and at a broad peak at 1100 cm1 (CAN stretching) along with a broad band at 3500 cm1 (NAH stretching). This indicated the successful formation of amide linkages as well as the occurrence of nucleophilic substitution reactions, as proposed in the reaction mechanism (Scheme 2). 1 H NMR analysis of alkylamine-modified GO was used to elucidate the bonding chemistry between alkylamine and GO. Representative 1H NMR results (GO-A6 and GO-A18) are shown in Fig. S2, Supporting Information. The characteristic signals at d = 3.72 (ANH), 2.17–2.25 (a CH2), 1.25–1.55 (b CH2), and 0.88 (methyl group) clearly confirmed the grafting of alkylamine (hexylamine and octadecylamine) onto the GO surface. Fig. 2a shows the TGA curves of alkylamine-modified GO. Pristine GO showed a major weight loss of about 30% in the temperature range of 110–230 °C corresponding to the decomposition of labile oxygen-containing functionalities and illustrating lower thermal stability. Grafting of alkylamine (A6–A18) increased the
Fig. 2a. TGA results of amine-grafted GO samples.
thermal stability of GO with a major weight loss (30%) in the temperature range of 220–520 °C (Fig. 2a). However, below 100 °C, alkylamine-grafted GO (GO-A6 to GO-A18) showed nearly zero weight loss, indicating an enhanced hydrophobicity that minimized the amount of absorbed water compared to pristine GO, which normally exhibited a weight loss of about 10% below 100 °C. The thermal stability of alkylamine-modified graphene increased substantially below 400 °C (GO-A6 < GO-A10 < GOA16 < GO-A18) with increasing chain length of grafted amines, consistent with its packing density between the graphene layers. Fig. 2b shows the DTG results for amine-grafted graphene samples. Pristine GO showed a major degradation peak at around 200 °C due to the degradation of labile oxygen moieties. Hexylamine-modified graphene (GO-A6) showed a single degradation peak at 312 °C with a small peak at 260 °C. When the chain length of the grafted amines was increased (A10 and A16), two degradation temperatures were observed. The first degradation temperature at 285 °C (GO-A10 and GO-A16) was attributed to the presence of impurities (free amines) within the densely packed grafted amine structures. The second degradation peak at around 450 °C was supported by the degradation of chemically grafted amines intercalated between the GO platelets [30]. Peeters et al., [31] reported similar type of two type desorption mechanism in zirconium phosphate intercalated by n-alkylamine with longer alkyl chain lengths and high amine loadings. Interestingly, octadecylamine-modified graphene (GO-A18) showed a single, broad degradation peak at around 445 °C, which might be due to the degradation of the amines grafted between the graphene layers. The absence of a peak at around 285 °C in GO-A18 samples suggested the absence of free amine impurities in this system.
Fig. 3a. Wide angle X-ray diffraction studies of pristine and alkylamine-grafted GO samples.
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Fig. 3b. X-ray diffractions of pristine and alkylamine-grafted GO (2h range, 2–10°).
Changes in the chemical structure due to grafting of amine into graphene were investigated using X-ray diffraction characterization at two different 2h ranges. Fig. 3a shows the wide angle Xray diffraction results of pristine GO and amine-grafted graphene (GO-A6 to GO-A18) in the 2h range of 10–40°. For the natural graphite sample (Fig. 3a inset), the (0 0 2) peak appeared at 2h = 26°, indicating an interlayer spacing of 0.34 nm between graphene platelets. On oxidation, the diffraction (0 0 2) peak shifted
to 2h = 12.6° (d = 0.7 nm), with no reflections at 26°, indicating that the exfoliation process increased the d-spacing to 0.7 nm due to oxidation and completely eliminated the 0.34 nm graphite interlayer spacing [32,33]. Upon reaction with alkylamines in the presence of ethanol solvent, the peak at 2h = 12.6° of GO disappeared leading to the formation of broad diffusion peaks with peak maxima at approximately at 21–24° (d = 0.39–0.42 nm) and drastic reduction in peak intensity. This fact may be attributed to the formation of certain degree of reaggregated graphene sheets due to the solvothermal reduction [33]. However, it is expected that the grafting of alkylamines should result in increase in d-spacing and thereby affecting the degree of stacking of graphene sheets. To understand the effect of alkylamines on the degree of stacking, X-ray diffraction studies in the 2h range of 2–10° were carried out, and the results are shown in Fig. 3b. Interestingly, hexylamine-grafted GO (GO-A6) showed no peak in this range, confirming that hexylamine does not influence the degree of graphene sheet stacking. However, increasing the chain length of the grafted amines resulted in the appearance of a broad peak in the 2–10° range, and the peak maximum depended on the chain length of the grafted amines. For instance, GO-A10 showed a broad peak at 2h = 5.8° (d = 1.52 nm), whereas GO-A16 and GO-A18 showed broad peaks at 2h = 5.0° (d = 1.77 nm) and at 2h = 3.7° (d = 2.39 nm) (Fig. 3b (inset)). The d-spacing values calculated using Bragg’s equation (nk = 2dsin h) are comparable to the values calculated using Eq. (1) derived by Bourlinos [28]
Fig. 4. SEM images of pristine and alkylamine-grafted GO samples.
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D002 ðAÞ¼ 6:1 þ Ic sin h
ð1Þ
where 6.1 is the thickness of the GO sheets in Å, IC is the length of the hydrocarbon chain of the amine molecule (IC = 1.5 + 1.265 (n 1)); n, number of carbon atoms). Interestingly, the ‘‘h’’ value calculated using Eq. (1) for GO-A10, GO-A16, and GO-A18 was observed to be 54°, which confirmed that the grafted amine molecules oriented relative to the GO sheet were at an 54° angle, rather than parallel. This assumption is valid, since the hydroxyl group in the basal plane caused the basal plane to be hydrophilic, which in turn made the long hydrophobic alkyl chains less likely to contact the basal GO plane. Another sound reason is the self-support effect by the high-density hydrocarbon chains preventing them to attain parallel conformations due to the steric effect [23]. After functionalization, the resulting alkylamine-modified GO samples were dispersed in ethanol and filtered to form a film using a vacuum filtration process. Pristine GO disperses well in polar solvents such as ethanol, leading to a slow vacuum filtration process. The filtration process of amine-grafted GO samples is fast because of their poor dispersibility in ethanol. The fast filtration of the amine-grafted GO samples resulted in the formation of a very rough surface, which was confirmed by morphological characterization and surface roughness measurements. Surface morphologies of the pristine and alkylamine-modified GO film (GO-A6; GO-A10; GO-A16 and GO-A18) were determined using SEM (Fig. 4). Pristine GO showed a relatively smooth surface, while aggregated large domains with lateral dimensions of few micrometer size (0.5–5 lm) and random orientation are formed on the alkylamine-modified GO samples. This morphology resulted in high surface roughness, which was further quantitated using an optical profilometer in vertical scanning mode. The average mean square roughness (Ra) and peak-to-valley value (Rf) of pristine and alkylamine-modified GO samples (GO-A6; GO-A10; GO-A16; and GOA18) measured in the range of 2 2 mm2 are shown in Table S1.
Fig. 5. Contact angle measurements of pristine and alkylamine-modified GO samples.
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Ra and Rf values were in the following order: GO < GO-A6 < GOA10 < GO-A16 < GO-A18, suggesting a rise in surface roughness with increasing amine chain length on the GO surface. Increasing the chain length of amines resulted in the formation of intercalated alkylamines on the GO that substantially alters the dispersibility in polar solvent such as ethanol and thereby formation of aggregated film on vacuum filtration, when compared to pristine GO [23]. The low dispersibility of the amine-grafted GO samples in polar solvents such as ethanol was consistent with the contact angle results obtained through wettability studies. Films prepared through a vacuum filtration process were used to test wettability by measuring static contact angle with water. Fig. 5 shows the water contact angles of pristine and aminegrafted GO samples. Pristine GO had a contact angle of 58° from the hydrophilic oxygen-containing functional groups on the surface. The grafting of alkylamines increased the water contact angle due to the increase in the hydrophobicity of the GO as well as the increase in surface roughness. This effect was more pronounced with increasing alkylamine chain lengths. ODA-grafted GO (GO-ODA) exhibited the highest contact angle value (142°), which was significantly higher than earlier reported values [23]. Higher contact angle value in the present study compared to the earlier reported work may be attributed to the chemical grafting of alkylamine via amide linkage that results in the few alkyl groups (h 90°) perpendicular to the GO surface apart from the tilted angle orientation (h 54°) of alkyl groups. These few perpendicularly oriented alkyl groups were methyl groups (ACH3 groups) on the GO surface rather than the methylene groups of the tilted alkyl groups. These, in turn, enhanced the contact angle of the GO surface [22,34]. To demonstrate the potential applications of alkyl-aminegrafted GO samples in superhydrophobic coatings, a thin film was fabricated by dispersing 1.5 mg/mL of GO samples in ethanol solvent followed by spray coating on silicon substrates using the setup shown in Fig. S3. Spray-coated samples were calcined at 150 °C for 1 h and water contact angles were measured. Thermal annealing of spray-coated samples is expected to cause reduction of the GO films, which in turn changes the hydrophobicity significantly [23]. Fig. 6 shows water contact angle values for the thermally annealed alkylamine-grafted GO samples. Grafting of small chain length alkylamines (thermally annealed GO-A6, 96° and GO-A10, 121°) did not result in superhydrophobic behavior, whereas grafting of long chain alkylamines resulted in contact
Fig. 6. Water contact angle values of thermally reduced pristine and alkylaminemodified GO samples.
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angles values greater than 150° (thermally annealed GO-A16, 152° and GO-A18, 162°). From these results, we concluded that thermally annealed hexadecylamine-grafted GO (GO-A16) and octadecylamine-grafted GO (GO-A18) are practical materials for lowcost and large-scale superhydrophobic coatings. 4. Conclusions In this study, alkylamines of varying chain lengths were used to investigate the effect of chain length on the superhydrophobic wetting behavior of the GO samples; X-ray diffraction studies revealed that the grafted alkylamines on the GO surface alkyl groups oriented at 54°, irrespective of the alkylamine chain length. Water contact angle measurements revealed that the chain length of the grafted alkylamine was crucial for increasing the hydrophobicity of the GO film, which may be corroborated to the decrease in surface energy values and increase in surface roughness. Grafting of long chain alkylamines such as hexadecylamine or octadecylamine on the GO surface followed by thermal reduction resulted in the formation of superhydrophobic surfaces with water contact angle values of 152° and 162° corroborating the role of alkylamine chain length in superhydrophobic wetting control of thermally annealed GO surface. Acknowledgment
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This work was supported by a Grant from the Kyung Hee University in 2011 (KHU-20110248) Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2013.02.054. References [1] R. Blossey, Nat. Mater. 3 (2003) 301–306.
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Contents lists available at SciVerse ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Synthesis, characterization, and surface wettability properties of amine functionalized graphene oxide films with varying amine chain lengths A.M. Shanmugharaj, J.H. Yoon, W.J. Yang, Sung Hun Ryu ⇑ Department of Chemical Engineering, Kyung Hee University, Yongin, Kyunggi-Do 336-701, South Korea
a r t i c l e
i n f o
Article history: Received 10 December 2012 Accepted 15 February 2013 Available online 4 April 2013 Keywords: Graphene oxide Superhydrophobicity Contact angle Alkylamines
a b s t r a c t Surface functionalization of graphene oxide (GO) an important graphene precursor using alkylamines of varying chain lengths followed by thermal treatment resulted in the formation of superhydrophobic surfaces. Alkylamines consisting of hydrophobic long chain alkyl groups and hydrophilic amine groups were chemically reacted to the GO surface via two types of reactions viz. (i) amidation reaction between amine groups and carboxylic acid sites of GO and (ii) nucleophilic substitution reactions between amine and epoxy groups on GO surface. Successful grafting of alkylamines was confirmed using Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (1H NMR), and thermogravimetric analysis (TGA). Alkylamine-modified GO surfaces showed enhanced roughness, and this effect was more pronounced with increasing amine chain length. Water contact angle measurements revealed that the hydrophobic nature of graphene depended on the chain length of the grafted alkylamines, and this fact may be corroborated to the decrease in the surface energy values. Our results indicate that superhydrophobic graphene films can be produced by thermal treatment of hexadecylamine- and octadecylaminegrafted GO films. These results will provide valuable guidance for the design and manufacture of graphene-based biomaterials, medical instruments, structural composites, electronics, and renewable energy devices. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction The lotus effect has inspired many studies of self-cleaning superhydrophobic surfaces that have a static water contact angle larger than 150° and a hysteresis of less than 10° [1–3]. Superhydrophobic coating techniques have been widely used as antistiction and anticontamination films in optoelectronics, biochemical sensors, and micro-electromechanical systems (MEMSs) to lower moisture adsorption and improve the reliability of electronic devices [4,5]. Superhydrophobic materials are generally water repellent, a feature that is strongly influenced by both composition and geometric structure (or surface roughness). Several experimental and modeling studies have exploited surface roughness on the micrometer or nanometer scale to engineer superhydrophobic materials [6–10]. A variety of methods and materials have been used to generate superhydrophobic surfaces, including carbon-based materials such as self-cleaning carbon nanotube forests, pillars, films, and nanocomposites [11,12]. However, the fabrication of carbonnanotube-based self-cleaning surfaces has been generally limited by high costs and wafer-sized products. A variety of superhydro⇑ Corresponding author. Fax: +82 31 202 1946. E-mail address: [email protected] (S.H. Ryu). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.02.054
phobic materials have been made, but a facile and cheap production process is still needed. Recently, graphene, an allotrope of carbon, has attracted considerable attention due to its outstanding physical properties such as its high structural strength and excellent electrical properties [13]. In spite of intense research into graphene, very few studies have examined water–graphene interactions [14], which could be an important study, if graphene has to be used in conformal coatings. Recent reports revealed the microscopic structures of adsorbed water on graphene conformal coatings deposited on the hydrophilic [15] and hydrophobic surfaces [16] have been reported. Deposition of graphene material on planar substrates results in the formation of interconnected graphene films, which increases the surface roughness of substrates twofold, leading to improvements in surface properties and the wettability of coated films [17,18]. Studies have revealed that a single layer or a few layers of graphene coating, prepared via epitaxial growth, have a contact angle of 92.5°, which is close to the 91° of highly oriented pyrolytic graphite [19]. Alternatively, graphite oxide (GO), a precursor of graphene synthesized via the chemical exfoliation of graphite, is often used for coatings because it easily forms a fine dispersion in polar solvents [20,21]. Understanding the wettability of GO films is important for the in-depth study of graphene-based functional materials. To date, only a few studies have attempted to tune the
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wettability of GO films by functionalizing GO with phenyl isocynate [22] or octadecylamine [23]. However, no reports are available on tuning the wettability properties of functionalized GO. In this study, we demonstrate a facile method for the synthesis of amine-functionalized GO films using alkylamine of varying chain lengths. The purpose of this work was to understand the effect of alkylamine chain length on the surface wettability properties of functionalized GO films. 2. Experimental section 2.1. Preparation of graphene oxide GO was synthesized from purified natural graphite (Sigma–Aldrich) by a modified Hummer’s method [24,25]. In brief, 1 g of graphite flakes was added to 100 mL of concentrated sulfuric acid and subjected to sonication for 30 min using a Branson digital sonicator (S450D, 500 W, 30% amplitude) followed by the addition of 1 g of sodium nitrate (NaNO3) in an ice bath. To this mixture, 6 g of potassium permanganate was slowly added under ice-cold conditions. The mixture was stirred for 2 h, and the temperature was raised and maintained at 35 °C in a water bath for another 0.5 h. Next, 46 mL of 70 °C water was added dropwise to the solution and the temperature of the system was increased to 98 °C. Finally, 140 mL of 70 °C water was added, followed by 20 mL of 30% (wt) hydrogen peroxide (H2O2) solution to terminate the reaction [26]. The synthesized graphite oxide was suspended in water and purified by dialysis to completely remove residual salts and acids. The resulting GO was dried overnight at 55 °C under vacuum (40 mmHg) to produce GO powder. 2.2. Functionalization of GO and preparation of alkylamine-GO films GO (500 mg) was dispersed in 100 mL of thionyl chloride and heated at 70 °C for 24 h in the presence of 1 mL dimethyl formamide to generate GO-COCl. After purification, 100 mg of GO-COCl was dispersed in ethanol containing 400 mg of hexyl amine (A6), and the suspension was sonicated for 2 h at 60 °C. After cooling to room temperature, excess hexylamine was removed by washing with ethanol several times. The remaining solid was separated by filtration using a 0.2-lm membrane filter. The collected solid was again washed with ethanol several times and dried at 60 °C under vacuum to generate hexylamine-GO powders (GO-A6). Similar synthesis steps were followed in preparing decylamine- (GOA10), hexadecylamine- (GO-A16), and octadecylamine (GO-A18)grafted GO samples. 2.3. Characterizations Fourier transform infrared spectroscopy (FT-IR) characterization of alkylamine-GO samples was performed at ambient temperature using a Perkin Elmer spectrometer (Spectrum one, USA) in transmission mode in the wavenumber range of 4000–400 cm1 at a resolution of 0.4 cm1. Samples for IR characterization were prepared by making KBr pellets using finely dispersed GO (0.1 mg) in 100 mg of KBr powder. Powder X-ray diffraction (XRD) analysis of GO samples in the 2h ranges of 2–40° was carried out using a Mac Science diffractometer (M18XHF-SRA, Japan) equipped with a Cu target (40 kV, 20 mA, Ka1 = 1.54 Å). Two sets of XRD experiment were carried out with the scanning speed of 1°/min (for 2h range of 310°) and 2°/min (for 2h range of 10– 40°). Thermogravimetric characterization (TGA) was carried out using a thermogravimetric analyzer (TGA Q5000 IR, TA instruments, USA) operated at a heating rate of 20 °C/min at nitrogen atmosphere kept at the flow rate of 50 mL/min. A Wyko optical
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profilometer (Veeco Instruments, USA) was used to test film roughness by vertical scanning mode. The surface roughness was measured in two lateral scales (2.0 2.0 mm2). Contact angle measurements were performed under ambient conditions using an FTA 1000 contact angle meter (First Ten Angstroms, Portsmouth, VA). An automated program was employed in which a 3 lL droplet of water was dispensed using a microsyringe, and images were acquired immediately after dispensing using a digital camera. Scanning electron microscopy (SEM) (Leo Supra 55, Genesis 2000, Carl Zeiss, Germany) was used to investigate surface morphology of amine functionalized GO measured under an accelerating voltage of 10 kV.
3. Results and discussion The synthesis of GO from natural graphite using a modified Hummer’s method is described in Section 2. Synthesized GO is a non-stoichiometric material with many oxygen-containing functional groups such as phenolic AOH, epoxy, carbonyl, and carboxylic acid groups dispersed both at the basal planes and along the edges [27]. Lin et al. [23] reported that the wettability of GO can be effectively adjusted by grafting octadecylamine onto the GO surface. In this work, we studied the effect of chain length of alkylamines on the intercalation chemistry and thereby its influence on the structure and wettability properties of the graphene-based nanomaterial films. Scheme 1 represents schematic representation on the synthesis and fabrication steps of alkylamine-grafted GO films. Earlier report revealed that chemically prepared GO consisting of epoxy groups in its basal plane undergo nucleophilic substitution reaction with ANH2 groups of alkylamines [28]. Unlike the previous report [28], in the present study, both nucleophilic substitution and amidation reactions predominated (Scheme 2). As chemically prepared GO consists of epoxy groups in its basal plane and carboxylic sites at the edges, it can be expected that nucleophilic substitution reactions between epoxy and alkylamines predominated in the basal planes of GO sheets and amidation reactions occurred preferentially at the edges of the GO sheets [29]. In addition, hydrogen-bonding and electrostatic attractions occurred due to the presence of hydroxide and carboxylic acid sites on the GO surface. The successful grafting of alkylamines on the GO surface was characterized using FT-IR. Pristine GO showed major IR stretching vibrations at 1710, 1610, 1217, and 1050 cm1 that corresponded to AC@O stretching (ACOOH group), AOH bending (adsorbed water), CAO stretching (ACOOH group), and CAOAC stretching (epoxy group) vibrations (Fig. 1). Upon thionyl chloride treatment, the carboxylic acid sites were converted to acid chlorides. This was indicated by a shift in the stretching vibrations of AC@O (1720 cm1) and CAO (1220 cm1) groups to a higher wavenumber due to the negative inductive effect of the chlorine atom in the ACOCl group (Fig. S1, Supporting Information). Successful formation of COCl groups in GO was further supported by the increase in peak intensity at 615 cm1, which corresponded to CACl stretching vibrations (Fig. S1, Supporting Information). Fig. 1 shows the FT-IR spectra of pristine GO and alkylaminegrafted GO (GO-A6 to GO-A18) that provided informations on the chemical interaction between GO and alkylamines. Compared to pristine GO or GO-COCl, alkylamine-modified GO (GO-A6 and GO-A10) showed a broad peak at around 2900 cm1 corresponding to the ACH asymmetric and symmetric stretching of ACH2 groups. This demonstrated the presence of alkylamine on the GO surface. In contrast to GO-A6 and GO-A10, GO modified with alkylamines of long hydrocarbon chains (GO-A16 and GO-A18) showed distinct intense peaks at 2930 cm1 (ACH asymmetric stretching) and 2854 cm1 (ACH symmetric stretching), demonstrating its
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Scheme 1. Schematic representation of alkylamine-grafted GO films.
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Scheme 2. Possible reaction mechanism of GO and alkylamines.
Fig. 2b. DTG results of amine-grafted GO samples.
Fig. 1. FT-IR spectra of pristine and alkylamine-grafted GO samples.
presence on the GO surface. The nature of the interaction between GO and alkylamines is confirmed by the appearance of peaks at 1575 cm1 (C@O stretching of amide group) and at a broad peak at 1100 cm1 (CAN stretching) along with a broad band at 3500 cm1 (NAH stretching). This indicated the successful formation of amide linkages as well as the occurrence of nucleophilic substitution reactions, as proposed in the reaction mechanism (Scheme 2). 1 H NMR analysis of alkylamine-modified GO was used to elucidate the bonding chemistry between alkylamine and GO. Representative 1H NMR results (GO-A6 and GO-A18) are shown in Fig. S2, Supporting Information. The characteristic signals at d = 3.72 (ANH), 2.17–2.25 (a CH2), 1.25–1.55 (b CH2), and 0.88 (methyl group) clearly confirmed the grafting of alkylamine (hexylamine and octadecylamine) onto the GO surface. Fig. 2a shows the TGA curves of alkylamine-modified GO. Pristine GO showed a major weight loss of about 30% in the temperature range of 110–230 °C corresponding to the decomposition of labile oxygen-containing functionalities and illustrating lower thermal stability. Grafting of alkylamine (A6–A18) increased the
Fig. 2a. TGA results of amine-grafted GO samples.
thermal stability of GO with a major weight loss (30%) in the temperature range of 220–520 °C (Fig. 2a). However, below 100 °C, alkylamine-grafted GO (GO-A6 to GO-A18) showed nearly zero weight loss, indicating an enhanced hydrophobicity that minimized the amount of absorbed water compared to pristine GO, which normally exhibited a weight loss of about 10% below 100 °C. The thermal stability of alkylamine-modified graphene increased substantially below 400 °C (GO-A6 < GO-A10 < GOA16 < GO-A18) with increasing chain length of grafted amines, consistent with its packing density between the graphene layers. Fig. 2b shows the DTG results for amine-grafted graphene samples. Pristine GO showed a major degradation peak at around 200 °C due to the degradation of labile oxygen moieties. Hexylamine-modified graphene (GO-A6) showed a single degradation peak at 312 °C with a small peak at 260 °C. When the chain length of the grafted amines was increased (A10 and A16), two degradation temperatures were observed. The first degradation temperature at 285 °C (GO-A10 and GO-A16) was attributed to the presence of impurities (free amines) within the densely packed grafted amine structures. The second degradation peak at around 450 °C was supported by the degradation of chemically grafted amines intercalated between the GO platelets [30]. Peeters et al., [31] reported similar type of two type desorption mechanism in zirconium phosphate intercalated by n-alkylamine with longer alkyl chain lengths and high amine loadings. Interestingly, octadecylamine-modified graphene (GO-A18) showed a single, broad degradation peak at around 445 °C, which might be due to the degradation of the amines grafted between the graphene layers. The absence of a peak at around 285 °C in GO-A18 samples suggested the absence of free amine impurities in this system.
Fig. 3a. Wide angle X-ray diffraction studies of pristine and alkylamine-grafted GO samples.
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Fig. 3b. X-ray diffractions of pristine and alkylamine-grafted GO (2h range, 2–10°).
Changes in the chemical structure due to grafting of amine into graphene were investigated using X-ray diffraction characterization at two different 2h ranges. Fig. 3a shows the wide angle Xray diffraction results of pristine GO and amine-grafted graphene (GO-A6 to GO-A18) in the 2h range of 10–40°. For the natural graphite sample (Fig. 3a inset), the (0 0 2) peak appeared at 2h = 26°, indicating an interlayer spacing of 0.34 nm between graphene platelets. On oxidation, the diffraction (0 0 2) peak shifted
to 2h = 12.6° (d = 0.7 nm), with no reflections at 26°, indicating that the exfoliation process increased the d-spacing to 0.7 nm due to oxidation and completely eliminated the 0.34 nm graphite interlayer spacing [32,33]. Upon reaction with alkylamines in the presence of ethanol solvent, the peak at 2h = 12.6° of GO disappeared leading to the formation of broad diffusion peaks with peak maxima at approximately at 21–24° (d = 0.39–0.42 nm) and drastic reduction in peak intensity. This fact may be attributed to the formation of certain degree of reaggregated graphene sheets due to the solvothermal reduction [33]. However, it is expected that the grafting of alkylamines should result in increase in d-spacing and thereby affecting the degree of stacking of graphene sheets. To understand the effect of alkylamines on the degree of stacking, X-ray diffraction studies in the 2h range of 2–10° were carried out, and the results are shown in Fig. 3b. Interestingly, hexylamine-grafted GO (GO-A6) showed no peak in this range, confirming that hexylamine does not influence the degree of graphene sheet stacking. However, increasing the chain length of the grafted amines resulted in the appearance of a broad peak in the 2–10° range, and the peak maximum depended on the chain length of the grafted amines. For instance, GO-A10 showed a broad peak at 2h = 5.8° (d = 1.52 nm), whereas GO-A16 and GO-A18 showed broad peaks at 2h = 5.0° (d = 1.77 nm) and at 2h = 3.7° (d = 2.39 nm) (Fig. 3b (inset)). The d-spacing values calculated using Bragg’s equation (nk = 2dsin h) are comparable to the values calculated using Eq. (1) derived by Bourlinos [28]
Fig. 4. SEM images of pristine and alkylamine-grafted GO samples.
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D002 ðAÞ¼ 6:1 þ Ic sin h
ð1Þ
where 6.1 is the thickness of the GO sheets in Å, IC is the length of the hydrocarbon chain of the amine molecule (IC = 1.5 + 1.265 (n 1)); n, number of carbon atoms). Interestingly, the ‘‘h’’ value calculated using Eq. (1) for GO-A10, GO-A16, and GO-A18 was observed to be 54°, which confirmed that the grafted amine molecules oriented relative to the GO sheet were at an 54° angle, rather than parallel. This assumption is valid, since the hydroxyl group in the basal plane caused the basal plane to be hydrophilic, which in turn made the long hydrophobic alkyl chains less likely to contact the basal GO plane. Another sound reason is the self-support effect by the high-density hydrocarbon chains preventing them to attain parallel conformations due to the steric effect [23]. After functionalization, the resulting alkylamine-modified GO samples were dispersed in ethanol and filtered to form a film using a vacuum filtration process. Pristine GO disperses well in polar solvents such as ethanol, leading to a slow vacuum filtration process. The filtration process of amine-grafted GO samples is fast because of their poor dispersibility in ethanol. The fast filtration of the amine-grafted GO samples resulted in the formation of a very rough surface, which was confirmed by morphological characterization and surface roughness measurements. Surface morphologies of the pristine and alkylamine-modified GO film (GO-A6; GO-A10; GO-A16 and GO-A18) were determined using SEM (Fig. 4). Pristine GO showed a relatively smooth surface, while aggregated large domains with lateral dimensions of few micrometer size (0.5–5 lm) and random orientation are formed on the alkylamine-modified GO samples. This morphology resulted in high surface roughness, which was further quantitated using an optical profilometer in vertical scanning mode. The average mean square roughness (Ra) and peak-to-valley value (Rf) of pristine and alkylamine-modified GO samples (GO-A6; GO-A10; GO-A16; and GOA18) measured in the range of 2 2 mm2 are shown in Table S1.
Fig. 5. Contact angle measurements of pristine and alkylamine-modified GO samples.
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Ra and Rf values were in the following order: GO < GO-A6 < GOA10 < GO-A16 < GO-A18, suggesting a rise in surface roughness with increasing amine chain length on the GO surface. Increasing the chain length of amines resulted in the formation of intercalated alkylamines on the GO that substantially alters the dispersibility in polar solvent such as ethanol and thereby formation of aggregated film on vacuum filtration, when compared to pristine GO [23]. The low dispersibility of the amine-grafted GO samples in polar solvents such as ethanol was consistent with the contact angle results obtained through wettability studies. Films prepared through a vacuum filtration process were used to test wettability by measuring static contact angle with water. Fig. 5 shows the water contact angles of pristine and aminegrafted GO samples. Pristine GO had a contact angle of 58° from the hydrophilic oxygen-containing functional groups on the surface. The grafting of alkylamines increased the water contact angle due to the increase in the hydrophobicity of the GO as well as the increase in surface roughness. This effect was more pronounced with increasing alkylamine chain lengths. ODA-grafted GO (GO-ODA) exhibited the highest contact angle value (142°), which was significantly higher than earlier reported values [23]. Higher contact angle value in the present study compared to the earlier reported work may be attributed to the chemical grafting of alkylamine via amide linkage that results in the few alkyl groups (h 90°) perpendicular to the GO surface apart from the tilted angle orientation (h 54°) of alkyl groups. These few perpendicularly oriented alkyl groups were methyl groups (ACH3 groups) on the GO surface rather than the methylene groups of the tilted alkyl groups. These, in turn, enhanced the contact angle of the GO surface [22,34]. To demonstrate the potential applications of alkyl-aminegrafted GO samples in superhydrophobic coatings, a thin film was fabricated by dispersing 1.5 mg/mL of GO samples in ethanol solvent followed by spray coating on silicon substrates using the setup shown in Fig. S3. Spray-coated samples were calcined at 150 °C for 1 h and water contact angles were measured. Thermal annealing of spray-coated samples is expected to cause reduction of the GO films, which in turn changes the hydrophobicity significantly [23]. Fig. 6 shows water contact angle values for the thermally annealed alkylamine-grafted GO samples. Grafting of small chain length alkylamines (thermally annealed GO-A6, 96° and GO-A10, 121°) did not result in superhydrophobic behavior, whereas grafting of long chain alkylamines resulted in contact
Fig. 6. Water contact angle values of thermally reduced pristine and alkylaminemodified GO samples.
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angles values greater than 150° (thermally annealed GO-A16, 152° and GO-A18, 162°). From these results, we concluded that thermally annealed hexadecylamine-grafted GO (GO-A16) and octadecylamine-grafted GO (GO-A18) are practical materials for lowcost and large-scale superhydrophobic coatings. 4. Conclusions In this study, alkylamines of varying chain lengths were used to investigate the effect of chain length on the superhydrophobic wetting behavior of the GO samples; X-ray diffraction studies revealed that the grafted alkylamines on the GO surface alkyl groups oriented at 54°, irrespective of the alkylamine chain length. Water contact angle measurements revealed that the chain length of the grafted alkylamine was crucial for increasing the hydrophobicity of the GO film, which may be corroborated to the decrease in surface energy values and increase in surface roughness. Grafting of long chain alkylamines such as hexadecylamine or octadecylamine on the GO surface followed by thermal reduction resulted in the formation of superhydrophobic surfaces with water contact angle values of 152° and 162° corroborating the role of alkylamine chain length in superhydrophobic wetting control of thermally annealed GO surface. Acknowledgment
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This work was supported by a Grant from the Kyung Hee University in 2011 (KHU-20110248) Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2013.02.054. References [1] R. Blossey, Nat. Mater. 3 (2003) 301–306.
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