Letter pubs.acs.org/Langmuir
Controlling the Shell Formation in Hydrothermally Reduced Graphene Hydrogel Kaiwen Hu,†,∥ Xingyi Xie,†,‡,∥ Marta Cerruti,† and Thomas Szkopek*,§ †
Department of Mining and Materials Engineering, McGill University, Montreal, Quebec H3A 2B2, Canada College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials and Engineering, Sichuan University, Chengdu, Sichuan 610065, China § Department of Electrical and Computer Engineering, McGill University, Montreal, Quebec H3A 0E9, Canada ‡
S Supporting Information *
ABSTRACT: Graphene hydrogels/aerogels are emerging three-dimensional graphene macroscopic assemblies of potential use in many applications including energy storage, pollutant adsorption, and gas sensing. In this Letter, we identify, characterize and control the formation of the exterior shell structure of graphene hydrogels prepared via hydrothermal reduction of graphene oxide. Unlike the porous bulk of the hydrogel, the shell is a compact, highly ordered layer with a higher electrical conductivity. Shell formation is dependent upon the surface anchoring of graphene oxide at the liquid−air and liquid-container interfaces. By purposefully weakening surface anchoring of graphene oxide using mild thermal or chemical prereduction method prior to hydrothermal reduction, we have succeeded in completely suppressing shell formation in the graphene hydrogel. The resulting graphene hydrogel shows a lower volume reduction with a porous bulk structure immediately accessible from the surface, in contrast to graphene hydrogels prepared under conventional conditions.
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functionalities.5,7 Two groups recently reported the existence of a densely packed “shell” structure enveloping the porous 3D “bulk” network.4,8 However, the underlying mechanism of shell formation has not been previsouly explained. We report here a detailed study of the structure of the densely packed outer shell; we characterize its significant differences with the porous bulk in terms of morphology, composition and electrical conductivity. We propose a formation scheme of the shell related to the surface-anchoring of GO9,10 at the liquid−container and liquid−air interface. We have successfully removed the shell by prereducing GO before the hydrothermal treatment, thus eliminating its surfaceanchoring ability. The resulting “shell-less” GHG has a porous bulk structure immediately accessible from the surface, and is less mechanically robust than conventional GHGs. Our results provide a facile method to control shell formation and a deepened understanding of the formation of GHGs by the hydrothermal reduction route.
INTRODUCTION Graphene has attracted much attention due to its superior carrier mobility, excellent mechanical properties, thermal conductivity,1,2 and high surface area.1,2 Graphene is thus an excellent candidate for a wide range of applications including energy storage, electronic devices, chemical and biological sensors, adsorption of pollutants and biological engineering.1−4 Structuring two-dimensional (2D) graphene into a threedimensional (3D) macrostructure is an important strategy for applications demanding facile synthesis with potential for largescale production. For example, the use of a graphene hydrogel (GHG) as an electrochemical capacitor electrode brings a significant enhancement in areal capacitance as compared to a 2D thin film structure.5 Among the strategies proposed to prepare 3D graphene structures, the “bottom up” self-assembly of reduced graphene oxide (RGO) achieved by the reduction of graphene oxide (GO) in different solvents stands out as an efficient and cost-effective method to produce multifunctional 3D GHGs, organogels or aerogels.1,2,6 The process usually begins with the dispersion of amphiphilic GO in a solvent that is subjected to a reduction process during which heavily oxidized GO is partially deoxygenated into an sp2 carbon skeleton. Upon reduction, the interactions between graphene sheets are dominated by hydrophobic and π−π interactions, thus driving the formation of the final hydrogel or organogel.1−3 Since the first report of GHG formation via hydrothermal reduction by Shi et al. in 2011, many researchers have used this technique to prepare graphene gels with different © XXXX American Chemical Society
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EXPERIMENTAL SECTION
Synthesis of Graphene Hydrogel with and without Shell. Our starting material was a GO single layer aqueous suspension with a concentration of 6.2 mg/mL (Graphene Supermarket, lateral dimension: 0.5 to 5 μm). 6.45 mL of the suspension was diluted to Received: February 10, 2015 Revised: April 30, 2015
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DOI: 10.1021/acs.langmuir.5b00508 Langmuir XXXX, XXX, XXX−XXX
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Figure 1. Optical, SEM, and TEM images of freeze-dried aerogels. (a) Photograph of an aerogel: the shell exterior has a metallic-gray luster and the interior bulk is black. (b) SEM image of the compact shell formed at the liquid-container interface. (c) High-resolution SEM image of the profile of a shell detached from the bulk. (d) High-resolution SEM image of the macroporous bulk. (e) Compact shell under high resolution TEM; the inset shows the well-ordered RGO stacking. (f) Porous bulk under high resolution TEM.
Figure 2. XPS and conductivity measurements of the shell and the bulk. (a) High resolution XPS O1s depth profile indicating deoxygenation in the shell upon etching. (b) Conductivity versus C/O ratio measured in the bulk and shell, compared with literature results for RGO paper or RGO transparent films deposited on insulating substrates.28−30 10 mL (4 mg/mL) with distilled water in a glass vial that was then sealed in a Teflon-lined autoclave at 180 °C for 6 h. After hydrothermal reduction, the GHG was removed from the vial and freeze-dried prior to characterization. An additional reduction step was performed to prepare shell-less GHGs before hydrothermal reduction. In the thermal prereduction route, GO aqueous suspension was freezedried and then heated to 180 °C for 1 h. The partially reduced GO was dispersed in 10 mL of distilled water (4 mg/mL) by sonication for 15 min. The suspension was subjected to hydrothermal reduction at 180 °C for 6 h, and the shell-less GHG was then freeze-dried. Experimental details for chemical prereduction strategy is included in the Supporting Information. Characterization. The gels were observed using an Inspect-50 field emission scanning electron microscope (FEI, Japan) at 5 kV under secondary electron imaging. The gels were also observed with a CM200 transmission electron microscope (Philips, Netherlands) at 200 kV. X-ray photoelectron spectroscopy (XPS) was performed on a monochromatic Al K-Alpha photoelectron spectrometer (Thermoscientific, USA). At least three survey scans and four high resolutions scans were collected. Depth profiles were obtained by Ar+ ion sputtering at 1 kV with 20 s etch times. The calibrated Ta2O5 etch rate is 0.72 nm/s. The sheet resistance of the bulk and shell were measured in a van der Pauw configuration on an HP4145B semiconductor parameter analyzer (HP, USA). Before measurement, samples were compressed to ∼1 mm thickness. The thickness of the shell was
measured by high-resolution scanning electron microscopy (SEM), while the bulk was measured directly using a thickness gauge (Mitutoyo, Japan).
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RESULTS AND DISCUSSION Figure 1 shows optical, SEM, and transmission electron microscopy (TEM) images of the freeze-dried GHG aerogels. Visually (Figure 1a), the shell on the exterior of the gel has a shiny metallic-gray luster, while the bulk is imbued with a black hue. Note that the shell forms at both the liquid−air (Supporting Information Figure S1) and the liquid−container interfaces (Figure 1b). SEM images in Figure 1b show that the shell is compact and relatively smooth. The cross section of the shell in Figure 1c shows a lamellar stack of RGO with a thickness of more than 0.5 μm. Below the shell, a macroporous bulk is found (Figure 1d); as seen in most literature reports.3,5,7,11,12 The pores found in the bulk range in size from 0.5 to 5 μm, and pore walls are composed of four to seven RGO layers (Figure 1f). The RGO layer spacing in the bulk was measured to be 4.06 Å. In contrast, the shell consists of highly stacked RGO of up to ∼70 layers with a smaller interlayer spacing of 3.68 Å (Figure 1e). The smaller interlayer spacing B
DOI: 10.1021/acs.langmuir.5b00508 Langmuir XXXX, XXX, XXX−XXX
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Scheme 1. GHG Formation under Hydrothermal Conditions: (a) Conventional GHG with Shell; (b) Suppression of Shell Formation in the GHG by Thermal or Chemical Pre-reduction Prior to Hydrothermal Treatment
different from the planar shell observed with a glass container. This difference is likely related to the difference in water wetting behavior at the hydrophobic Teflon surface and at the hydrophilic glass surface. Water droplets with high contact angles may form at the liquid-Teflon interface contributing to the formation of a dimpled shell. The thickness of the shell increases as the concentration of GO solution is increased (Figure S5). This is in line with previous work showing that the thickness of GO membranes assembled at liquid−air interface also exhibited such concentration dependency.20 To fully understand the formation of the shell, we consider the colloidal chemistry of GO. GO exhibits lyotropic liquid crystallinity (LC) in both aqueous and organic media.9,21 The isotropic GO phase transforms into a nematic phase, characterized by orientational order, at a critical concentration.9,21,22 Orientational ordering of lyotropic LCs often occurs at interfaces to minimize free energy by volume exclusion.9 Guo et al. have observed the ordered alignment of GO at the liquid−air interface and at several different liquid−solid interfaces.9 Furthermore, GO was shown by Kim et al. to be an amphiphile that can adsorb to interfaces to reduce surface tensions.10 These two factors together may be used to explain shell formation in GHGs as shown in Scheme 1a. Two domains exist in the GO aqueous dispersion: the face-to-face stacked GO at the liquid− air/container interface and the isotropic GO in the bulk. In the “shell” region, GO aligns with the interface, whereas GO is randomly distributed in the “bulk” region due to the dominant electrostatic repulsion originating from the negatively charged edges of GO. The two domains are simultaneously reduced during hydrothermal treatment. However, since RGOs in the “shell” region are close to each other, there is a greater tendency to restack than in the bulk due to stronger π−π interactions. On the basis of this proposed mechanism, we expect shell formation to be suppressed by weakening the surface anchoring effect by dispersing GO flakes with smaller lateral dimensions, or by enhancing its hydrophobicity.10 We attempted to completely eliminate the shell by prereducing GO before GHG formation by hydrothermal treatment. The mild prereduction of GO removes some of the oxygen functionalities on GO (Figure S6), and thus increases its hydrophobicity. This step is crucial, because the partially reduced GO can still be dispersed in water upon sonication, but we expect that it will not assemble face to face at the liquid−air and liquid−container
and greater number of stacked layers in the shell likely indicate a higher degree of deoxygenation.13 We further analyzed samples by XPS and Raman spectroscopy. The elemental composition of the shell is revealed by XPS. The bulk has a C/O atomic ratio of 6.3 ± 0.3, comparable to other reported values for hydrothermally reduced GO.3 The surface of the shell has a C/O atomic ratio of 6.4 ± 1.5. This ratio increases to 13.0 ± 2.2 as the surface is etched by Ar+ ions, thus indicating that the shell has an oxidized external layer. These results are confirmed by high-resolution C 1s spectra (Figure S3), which show that both the CO (288.4 eV) and C−O (286.4 eV)14,15 components decrease upon the first etching cycle. Overall, the shell has a higher C/O atomic ratio higher than the bulk, again indicating a higher degree of reduction.16 The Raman G band at 1580 cm−1 is a footprint of all sp2 carbon networks, and originates from the in-plane stretching of C−C bonds.17 On the other hand, disorder that breaks the translational symmetry of the sp2 network gives rise to the D band at around 1345 cm−1.17 The Raman spectra of the shell and bulk (Figure S2) reveals a lower D to G peak intensity ratio (ID/IG) in the shell than in the bulk, indicating the presence of a greater density and/or larger lateral size of sp2 carbon domains in the shell of the GHG. A higher degree of deoxygenation implies greater electrical conductivity. To compare the conductivities of the porous bulk and the compact shell material, we compressed the two freezedried GHG samples from a starting thickness of ∼4 mm to a thickness of ∼1 mm. Then, we measured the sheet resistance on the shell side versus the bulk side. The shell was found to have conductivity orders of magnitude greater than the bulk, as shown in Figure 2b. In general, the conductivity of RGO increases as the C/O atomic ratio of the sample increases. The shell has a conductivity close to that of hydrogen iodide (HI)− reduced RGO paper13 and approximately a third of that of a compressed graphite pellet.13,18 We observed the formation of the shell at different interfaces including liquid−glass, liquid−silanized glass, liquid−Teflon (Figure S4), and liquid−air (Figure S1). Consistent with results from our previous study, the shell formed at the liquid−air interface takes the shape of the concave meniscus.19 However, the morphology of the shell formed at the liquid−container interface is dependent on the container surface. For example, when highly hydrophobic Teflon is used as a container, the shell exhibits a dimpled pattern on the surface, which is quite C
DOI: 10.1021/acs.langmuir.5b00508 Langmuir XXXX, XXX, XXX−XXX
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Figure 3. Optical and SEM images of conventional and shell-less GHG. a) Photograph of conventional GHG, b) shell-less GHG prepared via thermal prereduction at 180 °C and c) shell-less GHG prepared via chemical prereduction using vitamin C. SEM images of the exterior (d) and interior (e) structure of shell-less GHG in b) with similar microstructure.
interfaces. Interestingly, the work by Woltornist et al.23 has shown that hydrophobic pristine graphene exhibits a tendency for interfacial trapping at the water−heptane interface, but further work is required to quantify interfacial trapping of graphene oxide versus degree of reduction. GO can be prereduced by either using a simple thermal treatment or a chemical reducing agent such as vitamin C. High-resolution C 1s spectra following prereduction treatment were measured (Figure S7). For a thermal prereduction treatment, we freeze-dried the GO dispersion and reduced it in air at 180 °C for 1 h. The resulting prereduced GO has a C/ O atomic ratio of 3.4 ± 0.1. As shown in Figure 3b,c, a GHG still forms after the hydrothermal reduction but with a larger volume. The shell-less GHG is mechanically weak as compared to a conventional GHG. The integrity of the gel is readily degraded by mechanical shaking (Figure S8). Although the GHG prepared by this method has similar thermogravimetric analysis (TGA; Figure S9) and Fourier transform infrared (FTIR; Figure S10) spectra compared to GHG synthesized without the prereduction step, its structure is different. SEM images (Figure 3b,c) show that the GHG synthesized with the prereduction step has a homogeneous porous structure throughout its entire volume and surface, proving that we have successfully inhibited the formation of the shell during the reduction process. Thermal prereduction at 130 °C can also result in a shell-less GHG with similar structure (Figure S11), thus indicating that the surface anchoring ability is lost even at a lower prereduction degree (C/O ratio: 2.1 ± 0.1). Chemical reduction is yet another means to remove the shell, with a C/O atomic ratio of 4.2 ± 0.1 exceeding that of thermal prereduction at 180 °C. A completely black aerogel is produced after freezedrying with a homogeneous structure throughout (Figure S12).
a Li−S battery cathode.26 The shell may be beneficial when preparing composite materials:19,26,27 in this case, the shell enhances the retention of nanoparticles or polymeric molecules inside the GHG. The GHG shell control discussed in this paper can also be applied to GHG synthesized using other solutionbased reduction methods.
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ASSOCIATED CONTENT
S Supporting Information *
Further experimental details, Raman spectra, summary of GHG shell and bulk, XPS high resolution spectra, TGA, FTIR, more SEM image and electrochemical characterization of GHG. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b00508.
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AUTHOR INFORMATION
Corresponding Author
*Telephone 514-398-3040. E-mail:
[email protected]. Author Contributions ∥
These authors equally contributed to the paper.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS K. Hu thanks the McGill Excellence Doctoral Award and X. Xie thanks Sichuan University for financial support. We thank the Facility for Electron Microscopy Research at McGill and Dr. Xuedong Liu for help with TEM imaging. M. Cerruti and T. Szkopek thank the Fonds de recherche du québéc - nature et technologies, the Center of Self-assembled Chemical Structures, the Natural Sciences and Engineering Research Council of Canada, Canada Research Chairs and Canada Foundation for Innovation for their support.
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OUTLOOK By elucidating the shell formation mechanism, we have identified prereduction routes to enable shell engineering and thus the synthesis of open GHGs with lower densities and potentially improved ionic transport. The compact layered morphology of the shell is expected to govern properties such as ion transport and diffusion into the GHG. Recently, we have shown that the shell can function as a dialysis membrane to remove ionic species inside the GHG.19 In an electrochemical capacitor or battery electrode, the shell may be undesirable as it may act as a barrier for ions to diffuse into the bulk where most of the active material resides;24,25 however, it could also act as a protective layer such as a barrier for dissolution of polysulfide in
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