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Macroscopic Multifunctional Graphene-Based Hydrogels and Aerogels by a Metal Ion Induced Self-Assembly Process Huai-Ping Cong,† Xiao-Chen Ren,† Ping Wang, and Shu-Hong Yu* Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, People's Republic of China. †These two authors contributed equally to this work.
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elf-assembly has been recognized as one of the most powerful techniques for integrating various nanostructured building blocks into macroscopic materials that can translate properties at the nanoscale into resulting macroscopic devices with hierarchical microstructures and novel functionalities.1 Furthermore, the assembled superstructures are of novel collective physiochemical properties that are different from individual components and the bulk material, which enriches the species in the material fields and improves their capacities for practical applications.2,3 Different interactions, such as dipole interactions, electrostatic attractions or repulsions, hydrophilic/ hydrophobic interactions, and hydrogen bonding, have been involved in the driven self-assembly process.4,5 Graphene, a single layer of carbon atoms patterned in a hexagonal lattice, has attracted great attention all over the world for its potential applications in sensors,6 catalysis,7 energy-storage devices,8 and environmental fields9 due to the excellent mechanical, electronic, and thermal properties. Thus, graphene and its functionalized derivatives with unique two-dimensional (2D) structures can be well used as building blocks for self-assembly of functional carbon-based materials, such as 1D tubein-tube nanostructures, 10 2D graphene films,1113 and 3D graphene/polymer composites.1416 Hydrogels and aerogels, the two typical kinds of 3D macroscopic assemblies, consisting of the microporous and mesoporous networks allowing access and diffusion of ions and molecules, possess attractive potentials in electrode materials, catalysis, and water treatment. Carbon nanotubes1719 CONG ET AL.
ABSTRACT
We report a one-step fabrication of macroscopic multifunctional graphene-based hydrogels with robust interconnected networks under the synergistic effects of the reduction of graphene oxide sheets by ferrous ions and in situ simultaneous deposition of nanoparticles on graphene sheets. The functional components, such as R-FeOOH nanorods and magnetic Fe3O4 nanoparticles, can be easily incorporated with graphene sheets to assemble macroscopic graphene monoliths just by control of pH value under mild conditions. Such functional graphene-based hydrogels exhibit excellent capability for removal of pollutants and, thus, could be used as promising adsorbents for water purification. The method presented here is proved to be versatile to induce macroscopic assembly of reduced graphene sheets with other functional metal oxides and thus to access a variety of graphene-based multifunctional nanocomposites in the form of macroscopic hydrogels or aerogels. KEYWORDS: graphene . self-assembly . hydrogel . aerogel . R-FeOOH . magnetic Fe3O4 . nanocomposite
and cellulose fibers20 have been successfully used as building blocks for the assembly of carbon material gels. However, recent work on the preparation of 3D graphene gels and their functionalities is still limited. Among these reports, chemical or physical cross-linkers are used to prepare monolithic graphene architectures, such as organic binders via the solgel polymerization method,21 DNA molecules,22 ion linkages,23,24 and ion coordination.25 Otherwise, VOL. 6
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* Address correspondence to
[email protected]. Received for review January 7, 2012 and accepted February 3, 2012. Published online February 03, 2012 10.1021/nn300082k C 2012 American Chemical Society
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ARTICLE Figure 1. (a) Photographs of the time-dependent formation process of the hydrogels with 10 mL of GO (2 mg mL1) suspensions at pH 3 in the presence of 0.5 mmol of FeSO4. The hydrogel was prepared in a cylindrical sampler vial with a volume of 25 mL. (b) Photograph of the scale-up synthesis of hydrogels by using 1000 mL of GO and 50 mmol of FeSO4. (c and d) Low- and high-magnified SEM images. (e) TEM image of the microstructures of the freeze-dried graphene/FeOOH hydrogel. (f) XRD pattern of the graphene/FeOOH aerogel.
reducing mediums achieved by hydrothermal processes under high pressure26 or using a large amount of reducing agents (NaHSO3, Na2S, vitamin C, sodium ascorbate, etc.)27,28 have to be provided to satisfy the requirements of the assembly of macroscopic graphene-based hydrogels. An abundance of oxygen-containing functional groups on graphene oxide (GO) sheets make it soluble and easily processable, which leads to the growth of new types of graphene-based materials, that is, decorating the 2D graphene sheets with nanoparticles.2931 However, until now, very few reports were focused on the assembly of 3D graphene-based hydrogels with deposited nanoparticles. Recently, Chen et al. reported the capture of preprepared magnetic nanoparticles into 3D graphene networks under the assistance of reducing agent (NaHSO3, Na2S, vitamin C, HI, and hydroquinone).27,32 Thus, there is still a great challenge to develop facile methods for the preparation of functionalized graphene monolithic hydrogels with integrated functionalities and the advantages of both graphene sheets and nanoparticles. In this article, we demonstrate a facile one-step approach for fabrication of macroscopic graphene/iron oxide hydrogels with 3D interconnected networks under the synergistic effects of the self-assembly of graphene oxide sheets and in situ simultaneous deposition of metal oxide nanoparticles such as R-FeOOH nanorods and magnetic Fe3O4 nanoparticles CONG ET AL.
on graphene sheets induced by ferrous ions as a reducing agent to reduce the graphene oxide sheets under mild conditions. Herein, the compositions of graphene hydrogels can be easily changed via adjusting the pH value of the initial GO suspension with other conditions constant. Interestingly, when burning the oil-saturated graphene/R-FeOOH superhydrophobic aerogel directly, a 3D hematite R-Fe2O3 monolith with porous microstructures can be prepared. Furthermore, the facilely separable and low-cost graphene/metal oxide hydrogels and aerogels exhibit excellent capability for removal of heavy metal ions and oils from water, thus can be potential candidates for efficient adsorbents in water purification or other applications. RESULTS AND DISCUSSION In a typical synthesis, a mixture of 10 mL of GO suspension (2 mg mL1) and FeSO4 (0.5 mmol) was undisturbedly sealed in an oil bath at 90 °C. As shown in the time-dependent photographs of Figure 1a, after 0.5 h, the black, reduced GO sheets were uniformly dispersed in water, without obviously aggregated sheets, but slightly floating from the bottom of vessel. With prolonging the reaction time (e.g., 1 h, 1.5 h), the assembled graphene monolith floated toward the top of the water surface. With prolonging the reaction, the black aggregate further shrunk, as a result of decreasing the diameter of the columnar hydrogel. After 6 h, a VOL. 6
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ARTICLE Figure 2. XPS spectra of (a) the graphene/FeOOH aerogels; (b) core-level Fe2p; (c) core-level C1s of the graphene/FeOOH hydrogel prepared with 10 mL of GO (2 mg mL1) suspensions at pH 3 in the presence of 0.5 mmol of FeSO4; (d) core-level C1s of GO. (e) Raman spectra of the graphene/FeOOH aerogel sample in (a). (f) FT-IR spectra of (1) GO; (2) the graphene/FeOOH aerogel; (3) graphene/Fe3O4 aerogel dried from the hydrogel prepared with 10 mL of GO (2 mg mL1) suspensions at pH 11 in the presence of 0.5 mmol of FeSO4. The hydrogel was prepared in a cylindrical sampler vial with a volume of 25 mL.
well-defined black hydrogel in a columnar shape with an unchanged size of 10 mm in diameter and 7 mm in height formed. Interestingly, except for the integrated cylinder, there were no separated graphene sheets elsewhere, and the transparent solution was left in the vessel. One key advantage of the present synthetic route is that it is easy to scale up, as shown in the photograph in Figure 1b, which presents the successful self-assembly of 1000 mL of GO suspension into the integrated hydrogel by just using a large reaction vessel of 1500 mL without changing other parameters, such as concentrations, temperature, and reaction time (Supporting Information, Figure S1). As demonstrated previously for the synthesis of 3D hydrogels of graphene oxide based on a hydrothermal process using CONG ET AL.
metal ions and glucose as mediators,23,24,26 a metal ion mediated gelation process,25 and adding reducing agents (Na2S, vitamin C, HI, and hydroquinone),27,32 the larger scale-up synthesis of such hydrogels in the present case can be achieved by using the proper reaction vessel with a different volume or shape. The as-prepared graphene hydrogel containing about 95.8 wt % water exhibited a well-defined and interconnected 3D network microstructure with uniformly dispersed pores of several micrometers in size, as shown from the SEM image in Figure 1c. More interestingly, a large number of nanoparticles homogenously enwrapped in the graphene sheets were found by a magnified SEM image (Figure 1d). Furthermore, the TEM image of Figure 1e shows that all of the rod-like nanoparticles with a size of about 60 nm were VOL. 6
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anchored onto the thin graphene sheets; no particles that were disassociated from the graphene sheets were observed. The XRD pattern in Figure 1f illuminated the compositions of the 3D graphene aerogel. All the diffraction peaks can be assigned to a pure orthorhombic phase R-FeOOH (JCPDS No. 29-0713). The presence of the elements C, O, and Fe with high contents in the graphene aerogel was evidenced by the photoelectron lines of the wide-scan XPS spectrum in Figure 2a at 285, 532, and 711 eV, attributed to C1s, O1s, and Fe2p, respectively. In the high-resolution Fe2p XPS spectrum in Figure 2b, the peaks of Fe2p3/2 and Fe2p1/2 at 711.1 and 724.7 eV were the characteristic positions of R-FeOOH,33 indicating the existence of R-FeOOH nanorods in the graphene support. In the formation process of the graphene/R-FeOOH hydrogel, the initial GO was reduced to graphene by ferrous ions, confirmed by significantly improving the intensity of sp2 CC bonds of graphene at 284.6 eV and decreasing the oxygencontaining carbon (epoxy CO at 286.5 eV, carbonyl CdO at 287.9 eV, and carboxyl OdCO at 289.0 eV),34 as analyzed from the deconvoluted C1s spectra of the as-prepared aerogel and GO in Figure 2c and d. In the Raman spectra of GO and graphene/R-FeOOH aerogel, as shown in Figure 2e, the intensity ratio of the welldocumented D band and G band of graphene in the aerogel was enhanced after reduction compared with that for GO, indicating the improvement of the disordered graphene sheets.35 Furthermore, the peak shifts of two bands to 1350 and 1591 cm1 revealed the charge transfer between graphene sheets and R-FeOOH nanorods.36 In comparison with the FT-IR spectra of GO and the graphene/R-FeOOH aerogel in Figure 2f, the absorption bands of carbonyl at 1730 cm1 and epoxy CO at 1225 cm1 of GO were obviously decreased, indicating the effective reduction of graphene sheets. Meanwhile, two bands at 892 and 783 cm1 in the product were the characteristic bending vibrations of Fe-OH.37 Thus, from the above analyses, it was reasonable to conclude that our hydrogels were formed by coassembly of graphene sheets and R-FeOOH nanorods. The rheological measurements in Figure 3a further revealed the structure and property of the graphene/ R-FeOOH hydrogel. As a function of angular frequency (1100 rad 3 s1), the independent storage modulus (G0 ) and slightly sensitive loss modulus (G00 ) to frequency were characteristics of the hydrogel materials. Moreover, G0 was almost 1 order of magnitude larger than G00 over the whole tested range, indicating an elastic rather than viscous response of the composite hydrogel with the permanent cross-linked network. The G0 value of 50 kPa at 10 rad 3 s1 was 1 to 2 orders of magnitude higher than the conventional self-assembled hydrogels.38,39 Moreover, the good thermal stability of such a hydrogel was demonstrated by the
Figure 3. (a) Dynamic rheological behavior of the graphene/FeOOH hydrogel prepared with 10 mL of a GO (2 mg mL1) suspension at pH 3 in the presence of 0.5 mmol of FeSO4. The reaction was performed in a cylindrical sampler vial with a volume of 25 mL. (b) Storage (G0 ) and loss (G00 ) modulus as a function of temperature.
invariable values of G0 and G00 for the entire temperature range (25100 °C), due to the strong interconnected networks, as shown in Figure 3b. The compressionstrain curve of the graphene/FeOOH hydrogel exhibited a linear stage at low strains (
