On the Gelation of Graphene

On the Gelation of Graphene

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Graphene oxide (GO) has been recognized as a unique two-dimensional building block for various graphene-based supramolecular architectures. In this article, we systematically studied the three-dimensional self-assembly of GO sheets in aqueous media to form hydrogels. The gelation of GO can be promoted by different supramolecular interactions, including hydrogen bonding, π-stacking, electrostatic interaction, and coordination. Furthermore, the lateral dimensions of GO sheets also have strong influences on GO gelation. The resulting GO hydrogels exhibited low critical gelation concentrations and good reversibility upon chemical stimulations. These findings indicate that GO has rich supramolecular properties, and its hydrogels may have a variety of technological applications.

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10.1021/jp1120299
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ARTICLE pubs.acs.org/JPCC On the Gelation of Graphene Oxide Hua Bai,†,‡ Chun Li,† Xiaolin Wang,‡ and Gaoquan Shi*,† † Key Laboratory of Bio-organic Phosphorous Chemistry and Chemical Biology, Tsinghua University, Beijing, 100084, People's Republic of China ‡ Department of Chemical Engineering, Tsinghua University, Beijing, 100084, People's Republic of China S b Supporting Information ABSTRACT: Graphene oxide (GO) has been recognized as a unique twodimensional building block for various graphene-based supramolecular architectures. In this article, we systematically studied the three-dimensional self-assembly of GO sheets in aqueous media to form hydrogels. The gelation of GO can be promoted by different supramolecular interactions, including hydrogen bonding, π-stacking, electrostatic interaction, and coordination. Furthermore, the lateral dimensions of GO sheets also have strong influences on GO gelation. The resulting GO hydrogels exhibited low critical gelation concentrations and good reversibility upon chemical stimulations. These findings indicate that GO has rich supramolecular properties, and its hydrogels may have a variety of technological applications. ’ INTRODUCTION In the past half decade, graphene oxide (GO), a precursor of graphene,1À4 has attracted a great deal of attention due to its unique structure and outstanding physical and chemical properties.5À8 Particularly, GO behaves like an amphiphilic macromolecule with hydrophilic edges and a more hydrophobic basal plane,9À11 which makes it an attractive building block for the construction of various supramolecular architectures.12,13 Furthermore, the two-dimensional (2D) structure of GO sheets provides them with various new supramolecular behaviors compared with conventional low-dimensional counterparts. Although GO sheets have been assembled into various macrostructures, such as LangmuirÀBlodgett (LB) films and paper-like films,14À17 their 3D assembly behavior has not yet been clearly revealed.18 We have first reported the 3D assembly of GO sheets in water solution by adding poly(vinyl alcohol) (PVA) as a crosslinker, forming a pH-sensitive supramolecular hydrogel.19 Hydrogen bonding between GO sheets and PVA chains is believed to be responsible for the formation of the hydrogel. Recently, single-stranded DNA was also found to be a good cross-linker for preparing a GO/DNA composite hydrogel, in which πÀπ interaction was the dominant driving force.20 Similarly, hydrogels based on chemically concerted graphene (CCG) have also been reported by us and other groups.21À25 These examples reflect that GO and CCG are good gelators. However, the gelation of GO sheets has not yet been studied extensively and the fundamental roles behind gelation phenomena have also not been clearly revealed. Here, we report a systematical study on GO gelation. The GO-based hydrogels were prepared by acidification or adding small organic molecules, polymers, or ions as crosslinkers. The effects of different driving forces (e.g., hydrogen bonding, electrostatic interaction, and coordination) and lateral dimensions of GO sheets on GO gelation are discussed. r 2011 American Chemical Society ’ EXPERIMENTAL SECTION Natural graphite powders were bought from Qingdao Huatai lubricant sealing S&T Co. Ltd. (Qingdao, China). Poly(vinyl pyrrolidone) (PVP), poly(ethylene oxide) (PEO), polydimethyldiallylammonium chloride (PDDA), polyethylenimine (PEI), cetyltrimethyl ammonium bromide (CTAB), tetramethylammonium chloride (TMAC), and melamine were the products of Alfa Aesar. Ethylene diamine tetraacetic acid disodium salt (Na2EDTA) was purchased from Biosharp Co. Ltd. All the inorganic salts, including NaCl, KCl, AgNO3, CaCl2, MgCl2, CuCl2, Pb(NO3)2, CrCl3, and FeCl3, were purchased from Sinopharm Chemical Reagents Co. Ltd. (Beijing, China). All the chemicals were used as received without further purification. GO was prepared from natural graphite powder by a modified Hummers method26,27 and purified by dialysis for 1 week to remove any impurities. Natural graphite with average particle sizes of 325 and 12 000 mesh were used for synthesizing GO sheets with larger and smaller lateral dimensions, respectively. To prepare GO hydrogels, a certain volume GO dispersion was mixed with the solution of acid or other cross-linkers. The blend was then shaken violently for several seconds to form a hydrogel. The formation of the hydrogel was examined by a tube inversion method, in which a polypropylene tube with an inner diameter of 8 mm was used. Atomic force micrographs (AFMs) were recorded on a Nanoscope III MultiMode SPM (Digital Instruments) with an AS-12 (“E”) scanner operated in tapping mode in conjunction with a V-shaped tapping tip (Applied Nanostructures SPM model: ACTA). The images were taken out at a scan rate of Received: December 17, 2010 Revised: January 27, 2011 Published: March 14, 2011 5545 dx.doi.org/10.1021/jp1120299 | J. Phys. Chem. C 2011, 115, 5545–5551 The Journal of Physical Chemistry C ARTICLE Scheme 1. Chemical Structure of GOa Hydrophilic groups are colored in red (carboxyl groups) and blue (hydroxyl and epoxy groups). a 2 Hz. X-ray diffraction (XRD) was carried out using a D8 Advance (Bruker) X-ray diffractometer with Cu KR radiation (λ = 1.5418 Å). All the GO solutions and hydrogels were lyophilized for at least 24 h before XRD measurement. Scanning electron micrographs (SEMs) were taken out by the use of a FEI Quanta 200 scanning electron microscope. Rheological studies were performed on an MCR 300 (Paar Physica) rheometer. All the viscosity data were obtained by using a 25 mm diameter parallel plate with a 1 mm plateÀplate gap. ’ RESULTS AND DISCUSSION A GO sheet can be regarded as a single-layer graphite that brings various hydrophilic oxygenated functional groups (Scheme 1).28À30 Thus, GO sheets can be dispersed in water to form a stable colloidal dispersion. It was also believed that the electrostatic repulsion between GO sheets, resulting from their ionized carboxyl groups, prevented their aggregation in aqueous medium.31 Therefore, acidification of a GO dispersion would weaken the electrostatic repulsion and lead to the formation of a graphite oxide flocculent. This phenomenon was observed by us as well as other researchers. The ζ potential of a GO dispersion increases with the decrease of its pH value, indicating the protonation of carboxyl groups (Figure 1A). Therefore, GO sheets are instable in a strong acidic aqueous medium because of insufficient mutual repulsion.32 However, as the concentration of GO (CGO) was sufficiently high (>4 mg mLÀ1), a hydrogel instead of an amorphous precipitation formed upon acidification. Figure 1A also gives the zero-shear viscosities (η0) of a 5 mg mLÀ1 GO solution at different pH values. Because of the ionization of carboxyl groups, the initial GO solution is acidic (pH = 4.6). Increasing the pH value of the GO solution using NaOH slightly decreased its η0, owing to the increase of the repulsion force between GO sheets. However, when the pH value of the GO solution was lowered from 4.6 to 0.6 by adding hydrochloric acid, its η0 increased dramatically from 290 to 10 850 Pa s. As a result, a hydrogel quickly formed within several seconds, indicating the generation of a 3D infinite network in the solution. The formation of the hydrogel was tested to be independent of the types of acids used for acidification. Thus, the 3D GO network must have resulted from the self-assembly of acidified GO sheets. In this case, the electrostatic repulsion between GO sheets was weakened and their hydrogen-bonding force was enhanced due to the protonation of carboxyl groups. Further lowering the pH value (2 μm) and small ( 3 mg mLÀ1. At 4 mg mLÀ1, a stable hydrogel was formed, as confirmed by the tube inversion method. This is a rather low critical gel concentration (CGC), indicating that GO falls into the concept of “super gelator”.33 An essential difference between GO sheets and conventional amphiphilic molecules is that GO sheets have much larger lateral dimensions. Furthermore, we found that the lateral dimensions of GO sheets have strong effects on their gelation behavior. For example, as the lateral dimensions of most GO sheets are as large as several micrometers (Figure 2A), a stable hydrogel can be easily formed by acidification (Figure 1A). Decreasing the lateral dimension of GO sheets is unfavorable for GO gelation. Actually, if the lateral dimensions of most GO sheets are smaller than 1 μm (Figure 2B), no gelation occurred at any pHs even if the CGO was as high as 9 mg mLÀ1 (Figure 1B). In this case, the acidified solutions of small GO sheets turned cloudy in several minutes, indicating the occurrence of GO precipitation. Therefore, the lateral dimension of GO sheets is a key factor for deciding their gelating ability. The size effect described above can be explained as follows. To some extent, GO can be regarded as an amphiphilic macromolecule. However, the sizes of large GO sheets (several micrometers) are much larger than those of conventional polymers. Therefore, they can contact with each other even in a dilute solution. This assumption was confirmed by the fact that the 5546 dx.doi.org/10.1021/jp1120299 |J. Phys. Chem. C 2011, 115, 5545–5551 The Journal of Physical Chemistry C ARTICLE Figure 2. AFM images of two GO samples with different average lateral dimensions. Scale bar = 5 μm. freeze-dried sample of a 3 mg mLÀ1 GO dispersion can maintain the original volume of the solution. In contrast, lyophilizing a 3 mg mLÀ1 PVA (Scheme 2) produced a shrunken sample (Figure S2, Supporting Information). Furthermore, the lyophilized GO dispersion shows a 3D network composed of GO sheets, as shown by its SEM image (Figure 3A). Therefore, it is reasonable to conclude that a loose dynamic GO network existed in the original dispersion of GO sheets owing to a force balance between electrostatic repulsion and binding interactions (hydrogen bonding, π-stacking, hydrophobic effect, etc.). Gelation occurs as the GO network in solution is reinforced by enhancing the bonding force or weakening the repulsion force. Acidification is an effective way for this purpose, as described above. On the other hand, both gelation and precipitation of GO sheets can be induced by reinforcing their bonding interaction. The difference between a GO hydrogel and graphite oxide precipitation is the stacking states of their GO sheets. GO sheets are randomly orientated in a hydrogel, whereas they adopt a parallel arrangement in their precipitation. The latter staking mode is energetically favorable because of the larger contacting area between GO sheets. However, the mobility of large GO sheets in solution is strongly limited. As a result, it is difficult to adjust their orientation to be parallel with each other. Comparing with the gelation process, precipitation is a kinetically slower process. Moreover, the large conjugated basal planes make the GO sheets stiff and form a stable network. The conversion of the GO network to the energetically more stable state (precipitation) is rather slow. In fact, a very slow precipitation process was observed after 2 weeks of the formation of hydrogels. However, if the sizes of GO sheets were reduced, for example, to be smaller than 1 μm in our experiment, they can change their conformations and positions in solution more easily. Consequently, acidified GO sheets trend to aggregate in an energetically favorable layer-by-layer manner to form a precipitation. The stability of a hydrogel is decided by the strength ratio of repulsion and bonding forces between GO sheets. For example, overacidification of the dispersion of large GO sheets would also lead to the formation of irregular aggregates because of too weak repulsion forces. To reveal the gelation behaviors of GO sheets more extensively, we used the large GO sheets for preparing hydrogels in the following studies. According to the analyses described above, the addition of a cross-linker can increase the bonding force between GO sheets and consequently promote gelation. This deduction Figure 3. SEM images of lyophilized GO solution and three typical GO hydrogels: (A) GO solution, (B) GO/PVP hydrogel with 1 mg mLÀ1 PVP, (C) GO/PDDA hydrogel with 0.1 mg mLÀ1 PDDA, and (D) GO/Ca2þ hydrogel with 9 mM Ca2þ. CGO = 5 mg mLÀ1, and scale bar = 10 μm. was first examined by introducing a polymer component to provide additional hydrogen-bonding interaction. In a previous work, we have reported that a small amount of PVA ( 3 mg LÀ1). The formation of the 3D network is because of the large and flexible 2D structure of GO sheets and the force balance between their static repulsion and bonding interaction. Increasing the bonding force or decreasing the repulsion force between GO sheets in solution will reinforce the 3D GO network and induce GO gelation or participation. These purposes can be realized by adding cross-linkers (polymers, small ammonium salts, and metal ions) or acidizing the GO solution. During the gelation and participation processes, many supramolecular interactions, such as hydrogen bonds, static or hydrophobic interactions, and coordination, are recognized as the possible driving forces. Careful control of the balance between electrostatic repulsion and bonding force is an important factor that governs the formation of GO hydrogels. All the GO hydrogels showed low critical gelation concentrations, and some of them exhibited environmental responsive behavior. This work indicates that GO is a good candidate for preparing “super” and “smart” hydrogels and will enlighten further studies on the supramolecular chemistry of graphene and its derivatives. ’ ASSOCIATED CONTENT S b Supporting Information. Dynamic rheological behavior of GO hydrogels and SEM images, XRD patterns, and photo images of lyophilized GO hydrogels. This material is available free of charge via the Internet at http://pubs.acs.org. ’ AUTHOR INFORMATION Corresponding Author *Phone: þ86-10-6277-3743. 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