Surface modification and partial reduction of three-dimensional macroporous graphene oxide scaffolds for greatly improved adsorption capacity

Abstract

A surface modification and partial reduction of graphene oxide (GO) scaffolds by a coating of polymethylsiloxane (PDMS) leads to the production of an efficient pollutant adsorbent consisting of three-dimensional, macroporous, and hydrophobic prGO–PDMS architectures, resulting in great improvement of adsorption capacity by a factor of 3.53 and a reliable recyclability of ∼98% relative to the initial capacity.

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The high demand for environmental remediation of water contamination from oil spills and leakage of toxic organic chemicals has recently motivated the development of novel types of porous materials outperforming traditional pollutant adsorbents.^1,2 These alternative adsorbents are expected to achieve essential properties such as high adsorption capacity, fast adsorption kinetics, and excellent selectivity and recyclability, due to an enlarged specific surface area (SSA) and controllable hydrophobicity.^3,4 As a result of their inherent hydrophobicity and meso/microporosity, conjugated polymers were first employed to selectively sequester and adsorb a wide range of organic solvents and oils. However, complicated synthetic procedures and the high-cost of the conjugated polymers could hamper their commercial applications.^5,6 Other approaches based on simple and cost-effective preparations have included the surface-coating of commercial melamine sponges and the sugar-templating of elastomeric polymethylsiloxane (PDMS), which are readily mouldable into desired shapes and simply reusable.^7,8

More recently, three-dimensional (3D) carbon-based macroporous architectures assembled by carbon nanotubes (CNTs) and graphene, have been intensively explored as a promising adsorbent with a remarkable adsorption capability resulting from their intrinsic surface hydrophobicity and superwetting behavior for organic solvents and oils. Despite the excellent adsorption capacities of CNT-assembled sponges and graphene–CNT hybrid foams, the high-temperature and complicated fabrication process hinders their practical mass-production.^9,10 Alternatively, 3D graphene-based porous scaffolds are regarded as the efficient pollutant sorbents, which are commonly derived from a bulky precursor of graphene oxides (GOs). The self-assembly of GOs into 3D reduced GOs (3D rGOs) in a macroscopic fashion induces the restacking of graphene sheets owing to π–π interactions and van der Waals forces, which diminishes the surface area and porosity of the products. In order to keep the open pores while preventing the irreversible aggregation of graphene nanosheets, hydrazine gaseous species can be evolved or water molecules between the rGO layers can be sublimated through a freeze drying process.^11–14 Therefore, it is postulated that 3D macroporous GO scaffolds framed by thin pore walls can be an advanced adsorbent because their abundance of oxygenated groups allow the surfaces to be easily modified through covalent or non-covalent functionalization. In particular, an ultralight N-doped graphene framework, which was produced by the sequential hydrothermal and high-temperature annealing process using a swelling and N-doping reagent of pyrrole, recorded the most outstanding adsorption capacities for the oil and organic substances based on its highly porous structure with a low density and large volume.¹⁵

Differently from such chemical and thermal annealing approaches, we here employed an ice-templating method to achieve 3D GO macroporous structures and modified its surface with a hydrophobic PDMS layer to efficiently improve the adsorption properties of product. First, highly porous 3D GO scaffolds could be obtainable through a simple and facile ice-templating step because the ice-templating assembly different from the flow-directed assembly¹⁶ allows for the creation of highly macroporous GO structures with the functional groups and a specific 3D cellular network constructed by thin pore walls of a few GO layers.^17,18 Second, the surface-modification of 3D GO scaffolds by a coating of PDMS converted the hydrophilic GO scaffold into the hydrophobic macrostructure while maintaining the open porosity of the original GO assembly. At a given condition for the PDMS-coating, the GO scaffolds can be partially reduced.³ That is, the final product of our work is the surface-modified and partially reduced 3D GO structure that is referred to prGO–PDMS. The hydrophobic nature in the 3D scaffolds is originally different from that of the conventional method based on the reduction of GO sheets into 3D rGO. Furthermore, the PDMS layers coated on the GO surfaces reinforced the cross-linking of prGO nanosheets and thus enabled the 3D prGO–PDMS macrostructure to be reliably recycled, which cannot be accomplished by the brittle GO assembly.

A schematic of the surface modification and partial reduction from the hydrophilic GO sheets into the hydrophobic prGO–PDMS surface is illustrated in Fig. 1a. The GO solution with a concentration of 5 mg mL⁻¹ prepared from a commercial GO dispersion (Angstron Materials, USA) was immersed into a dry ice bath containing liquid nitrogen to freeze the solution for 30 minutes. Without thermal heating, the freeze drying process was carried out to obtain the 3D macroscopic GO scaffolds. The resultant monoliths were modified by the surface-coating of GO sheets through a vapour deposition of PDMS (Sylgard 184A) in a sealed glass container at 235 °C for 1 hour.³ As shown in the scanning electron microscope (SEM) images (Fig. 1b and c), the macrostructure of prGO–PDMS is nearly identical to that of the original GO scaffold, which corresponds to randomly oriented porous structures and pore sizes up to tens of micrometers.^17,19 The adsorption capacities of prGO–PDMS structures were assessed by soaking various oils and organic solvents and weighing the adsorbents. After soaking and weighing, the adsorbates were released by drying in a convection oven for a given period of time and at the specific temperature, and then we repeated the recycling processes.

Fig. 1 (a) A schematic of surface modification and partial reduction through vapour deposition of PDMS on the GO scaffold. SEM images of (b) the GO and (c) prGO–PDMS scaffolds. Insets display optical images of a water droplet on the surface of GO and prGO–PDMS, respectively.

In order to verify the change of surface properties, we investigated the surface wettability of samples by contact angle measurement. As displayed in the insets of Fig. 1b and c, the contact angle of prGO–PDMS macrostructure significantly increased up to 115° from 40.91° of the GO scaffold. Notably, the hydrophobicity of prGO–PDMS surfaces was induced by the surface modification. It is well known that the hydrophobicity of materials relies on various factors such as porous structures, surface roughness, and hydrophobic chemical compositions.^11,20,21 Since the PDMS layer coats with a thickness of ∼3 nm in a given deposition condition,³ the contributions from the porous structures and surface roughness to the high contact angle of prGO–PDMS may be ruled out. Instead, the hydrophobicity of prGO–PDMS structure is beneficially increased by the hydrophobic features of PDMS that is produced by the heterolytic leakage of Si–O bonds during the thermal vapour deposition and coated on the surface of GO sheets.

The surface-coating of PDMS layer on the GO sheets was confirmed by a transmission electron microscopy (TEM) technique. As shown in high-angle annular dark-field scanning TEM (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDS) mapping images in Fig. 2, the Si element was homogeneously distributed and sharply confined within the C atomic mapping region. These findings indicate the coating of PDMS layer on the overall surface of GO scaffolds. In addition, X-ray photoelectron spectroscopy (XPS) analysis ascertains the surface modification of the samples, where the C1s and Si2p XPS spectra displayed the noticeable changes in the corresponding chemical bonding states and binding energies (Fig. S1 in the ESI†).

Fig. 2 (a) HAADF image of the prGO–PDMS structure. (b–e) EDS mapping results of C, O and Si element.

Adsorption properties of the samples were evaluated for a series of commercial oils and organic solvents, defined as k = (W_{saturated adsorption} − W_initial)/W_initial. As shown in Fig. 3a, the adsorption capacities of the hydrophobic prGO–PDMS macrostructures dramatically increased up to ∼151, which was greater by a factor of 3.02 than that of the hydrophilic GO scaffolds. Considering the complex relation between the adsorption capacity and the physicochemical parameters, including density, surface tension, viscosity, polarity, hydrophobicity, and porosity,^4,5,11 the adsorption capability of 3D porous macrostructures is dependent on the physicochemical properties of the adsorbates. Moreover, the correlation is more complicated because of the coupling effects of the physical geometry and the chemical interaction on the adsorption capacity. In this work, the thin PDMS-coating is expected to make a marginable difference in the total porosity between the GO and the prGO–PDMS structures. Accordingly, we reasoned that the chemical identity and properties of the adsorbate play a key role in determining the adsorption capacity of prGO–PDMS macrostructure. The relative adsorption capacities, referred to k_prGO–PDMS/k_GO (Table S1 in the ESI†), for the less-polar solvents were much larger than those for the polar solvents (e.g. k_prGO–PDMS/k_GO = ∼3.53 for chlorobenzene and k_prGO–PDMS/k_GO = ∼1.4 for ethanol). Such a dependence of adsorption behavior on the chemical identity of the adsorbate can be understood by the chemical interaction between the adsorbate and the surface of adsorbent, which strongly influences the wettability and the capillary action for the adsorption. This chemical effect on the adsorption behavior was more prominent for the adsorption of silicone oil, whose backbone of Si–O atoms strongly interacts with the Si molecules of PDMS chains due to their chemical similarity: the adsorption capacity (k_{silicone oil} = ∼117) was much higher than those of other oils (Table S1 in the ESI†). Our statement is further supported by the weak dependence of the adsorption capacities on the densities of adsorbates. Similar to the porosity, form factor, and chemical identity, the adsorption capacity is greater for the higher density of adsorbates due to their smaller volumes at the constant mass.^7,10,14 Despite the fact that toluene and motor oil have nearly identical densities (0.87 and 0.89 g cm⁻³, respectively), the adsorption behavior of prGO–PDMS scaffolds was completely different from that of the GO scaffolds. In other words, GO scaffolds showed very similar capacities for toluene (k = ∼21) and motor oil (k = ∼22), while the GO–PDMS structure revealed a large discrepancy in its capacity depending on the chemical identity of the adsorbates, i.e. toluene (k = ∼65) and motor oil (k = ∼30). Studies on the complex relationship between the adsorption capacities of adsorbents and the properties of adsorbates are in progress.

Fig. 3 (a) Adsorption capacities of the GO scaffold and the prGO–PDMS macrostructure for various oil and organic solvents in a relation with the densities of adsorbates. (b) Recyclability of the prGO–PDMS adsorbent for the solvent of acetone.

Despite the good adsorbing capability of GO scaffolds compared to other 3D macroscopic adsorbents,^3,5,8 it should be noted that the GO scaffold is too brittle to be repeatedly used. The individual GOs have good mechanical property, but the GO scaffold is very brittle due to poor structural integrity arising from weak interactions between individual sheets. After converting to rGO by reduction, the mechanical property of rGO scaffold can be improved as a consequence of strengthened π–π and hydrophobic interactions. Because the GO scaffold is able to be partially reduced to prGO structure at 235 °C as aforementioned, we prepared the thermally reduced samples under the same vapour deposition condition without PDMS. The contact angle and the adsorption capacities of thermally reduced prGO samples with lower values relatively to those of the prGO–PDMS structure (Fig. S2 in the ESI†) indicate that the PDMS-coating of GO scaffolds beneficially contribute to the improved adsorption capacities and the enhanced surface hydrophobicity of in the prGO–PDMS macrostructure. In addition, the extent of partial reduction of GO were characterized by measuring the atomic ratio of carbon to oxygen (C/O ratio) in the GO scaffolds and the prGO–PDMS macrostructures, which were estimated as the C/O ratio of 2.38 and 3.38, respectively (Fig. S1a in the ESI†). This finding indicates that the enhanced contact angle and adsorption capacity was attributed to the coupling effect of PDMS coating and partial reduction. It needs to be pointed out that the thermally reduced prGO samples are also brittle to be regenerated in the recycling processes. On the other hand, the surface-coating of PDMS layer enabled the cross-linking of walls between GO sheets, which reinforced the resultant prGO–PDMS macrostructures to be regenerated in the recycling processes (Fig. 3b). The acetone solvent adsorbed by the prGO–PDMS adsorbent was released into its vapour at 50 °C for 10 m and then the residual acetone was weighed. After five recycling cycles, the adsorption capacity of the sample retained approximately 98% of its initial capacity.

In summary, the hydrophobic prGO–PDMS macrostructure with the excellent adsorption capacity was fabricated by the surface-coating and partial reduction of hydrophilic GO assemblies through the vapour deposition of PDMS. The adsorption capacity of the prGO–PDMS scaffolds was strongly influenced by the chemical identity of the adsorbates and showed greater efficiency for less polar and chemically similar adsorbates. Furthermore, the cross-linking of prGO walls by the PDMS-coating enabled the prGO–PDMS macrostructures to be reliably regenerated, resolving the inherent brittleness of GO scaffolds. These results offer an effective chemical strategy for the design and fabrication of advanced adsorbents for environmental and energy applications.

Acknowledgements

This work was supported by National Agenda Project (NAP) through National Fusion Research Institute (NFRI), the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2012R1A1A2044094), and the research program of Korea Institute of Energy Research (Project no. B3-2426).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Additional characterization data. See DOI: 10.1039/c3ra45697h

‡ These authors are equally contributed to this work.

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