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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Graphdiyne Sponge for Direct Collection of Oils from Water Jiaofu Li,†,‡ Yanhuan Chen,†,‡ Juan Gao,†,‡ Zicheng Zuo,† Yongjun Li,† Huibiao Liu,*,†,‡ and Yuliang Li†,‡ †
Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Research/Education Center for Excellence in Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *
ABSTRACT: Although several sponge-like sorbents have been developed to treat oil spills and chemical leakages, under harsh conditions (e.g., strong acid or alkali; oils on the sea) their efficiencies can be rather limited. Herein, we provide a graphdiyne sponge that is capable of collecting oil pollution effectively. This graphdiyne sponge exhibits excellent adsorption capacity (up to 160 times its own weight), robust stability (even when immersed in strong acid and alkali for 7 days), and remarkable recyclability (up to 100 times). These features suggest that this new adsorbent material might find applicability in the cleanup of oil spills and many organic pollutants under realistic conditions.
KEYWORDS: 2D material, graphdiyne sponge, collection oils, wettability, hydrophobicity
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INTRODUCTION Rapid industrial development, offshore oil production, and marine transportation have all made great contributions to the advancement of society. Nevertheless, oil spills, accidents, and chemical leakages still occur, potentially harming human health and ecological environments.1−3 Accordingly, a global search remains for effective methods for treating spills of oil and other organic pollutants. Several approaches are widely applied to handle such contaminants, including mechanical collection,4 controlled burning,5 and the use of chemical dispersants6 and filter membranes.7−10 These methods are, however, energyconsuming, of low efficiency, and environmentally unfriendly. The use of adsorbent materials to collect spilled oils and organic solvents from water is gaining increasing attention as a cost-effective, easy-to-operate, ultrafast, and convenient method. Although many common adsorbent materials (e.g., zeolites, activated carbons) have widespread practical applications,11−13 these sorbents have poor selectivity, poor recyclability, and low adsorption capacity and can not be used to treat pollutants effectively. Recently, huge numbers of advanced sponge and sponge-like adsorbent materials have been developed for the selective, direct, and efficient collection of oils and organic solvents from wastewater.14−24 For example, Yu’s group reported a Joule-heated graphene-wrapped sponge for the rapid cleanup of viscous crude oil under an applied voltage,25 while Gao’s group reported a porous, lightweight, nonwoven graphene fiber fabric that exhibited strong adsorption capabilityup to 80 times its own weightfor the collection of organic solvents and, especially, viscous oils.26 Nevertheless, these materials are difficult to scale up and apply in real © XXXX American Chemical Society
environments for several reasons: complicated fabrication, expensive raw materials, high energy consumption, or lack of durability. Thus, while oils and organic solvents remain severe pollutants, it remains challenging to develop new materials for practically handling their spills. Graphdiyne (GDY) is a new carbon form that has attracted great interest since its first synthesis by Li’s group in 2010, because of its excellent physical and chemical properties.27,28 GDY possesses a highly symmetrical chemical structure and high chemical activity, derived from its unique array of sp- and sp2-hybridized carbon atoms, uniform distribution of pores, and high degree of π-conjugation, all of which favor its applications in, for example, energy storage,29−35 catalysis,36−42 field emission devices,43,44 photodetector devices,45 and oil/water separation.46 Previously, GDY has been fabricated only on copper foils and copper foams; its fabrication on other substratesespecially flexible substrates [e.g., carbon paper, poly(ethylene terephthalate) (PET), and sponges]would likely be an enormous breakthrough toward greatly expanding its applications. In this paper, we describe the successful decoration of GDY on a skeleton of melamine sponge (MS)a substrate of high flexibility, low density, high adsorption capacity, and ready Special Issue: Graphdiyne Materials: Preparation, Structure, and Function Received: January 22, 2018 Accepted: February 28, 2018
A
DOI: 10.1021/acsami.8b01207 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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GDY films decorating the skeleton surface of the MS densely and uniformly. To further investigate the compositions of the MS and GDYMS, we performed surface analysis using XPS (Figure S1). Peaks appeared at 284.87, 399.62, 535.87, 63.12, and 168.22 eV representing the C 1s, N 1s, O 1s, Na 1s, and S 2p binding energies, respectively, for both the MS and GDYMS. The high-resolution C 1s peak of GDYMS was stronger than that of the MS, consistent with successful modification with GDY films. Furthermore, we exfoliated the GDY films from GDYMS through ultrasonication and recorded its high-resolution C 1s spectra (Figure 1e). The C 1s peak at 284.8 eV could be deconvoluted into four subpeaks, corresponding to sp-hybridized CC units at 285.1 eV, sp2hybridized CC units at 284.6 eV, CO units at 286.4 eV, and CO units at 287.5 eV. The area ratio of the sp (CC)/ sp2 (CC) units was approximately 2, consistent with the expected chemical composition of GDY. In addition, we used transmission electron microscopy (TEM) to characterize the morphology of the as-prepared GDY films. Figures 1a and b reveal that the films were flat and continuous. The highresolution TEM (HRTEM) image in Figure 1c reveals curved streaks having a lattice parameter of 0.365 nm, which we assigned to the distance between GDY layers. This interlayer spacing is identical to that previously reported experimentally for GDY and from theoretical simulations.47 Because of the high degree of π-conjugation in GDY, UV−Vis spectral characterization revealed a distinct difference between the structures of the MS and GDYMS. The UV−Vis spectrum of MS (Figure S2) featured absorptions only at wavelengths of less than 300 nm. In contrast, the spectrum of GDYMS featured absorptions in the range from 300 to 800 nm, spanning the entire visible region, consistent with the absorption of GDY films.48 Accordingly, we conclude that GDY films had indeed been fabricated on the skeleton of the MS. We used scanning electron microscopy (SEM) to observe the surface morphologies of the MS and GDYMS. The pristine sponge possessed a porous 3D structure with pore diameters on the order of hundreds of micrometers (Figure 2), potentially beneficial for a high adsorption capacity. Figure 2c presents a high-magnification image of the raw sponge, revealing its smooth skeleton surface. The sponge maintained its intact porous structure after the skeleton surface had been completely decorated with GDY films (Figure 2d), implying that the original structure was not damaged. More importantly, a distinct rough skeleton surface of GDYMS is evident in Figure 2f; such a surface is essential for a hydrophobic material. An intermediate state between the Wenzel and Cassie states is always required for a rough foam, with the CA expressed using the equation49,50
scale-upthrough a facile in situ Glaser−Hay coupling. Because of the hydrophobicity of GDY, this GDY-coated MS (GDYMS) selectively adsorbed oils and organic solvents from wastewater with high adsorption capacityup to 160 times its own weight. Furthermore, because of the extraordinary stability of GDY, GDYMS exhibited excellent performance in corrosive liquids (organic solvents, strong acids, alkaline solutions). In addition, GDYMS possessed remarkable recyclability (up to 100 times). Moreover, the water contact angle (CA) of GDYMS was approximately 132°, but it could extract oils and organic solvents from sewage water effectively; accordingly, we did not have to change its wettability from hydrophobic to superhydrophobic. Overall, these surprising characteristics make GDYMS a promising material for recovering spilled oils and organic solvents under realistic environmental conditions.
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RESULTS AND DISCCUSION We prepared GDYMS through cuprous iodide-catalyzed homocoupling of the terminal alkyne units of hexaethynylbenzene (HEB) in the presence of MS (Scheme 1). First, the MS Scheme 1. Schematic Representation and Optical Image of the Preparation of GDYMS
was submerged in a tetrahydrofuran (THF) solution of HEB, resulting in the HEB solution passing into the macropores of the MS. Second, HEB attached noncovalently to the sponge skeleton, stabilized through van der Waals forces. Third, addition of CuI and tetramethylethylenediamine (TMEDA) induced the polymerization of HEB on the surface of the MS skeleton. Finally, the resulting GDY films were anchored firmly to the MS skeleton; the color of the sponge turned brown. To confirm the preparation of GDYMS, both the MS and GDYMS were characterized using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and diffuse reflectance UV−Vis spectroscopy. The Raman spectrum of GDYMS was significantly different from that of the MS, revealing four bands typical of GDY (Figure 1d). The peak at 1383.9 cm−1 represents the breathing vibration of the sp2-hybridized carbon domains in the aromatic rings (D band). The peak at 1575.3 cm−1 was due to the first-order scattering of the E2g mode for in-phase stretching vibration of sp2-hybridized carbon atoms in the aromatic rings (G band). The peaks at 1928.6 and 2182.7 cm−1 correspond to the typical vibrations of conjugated diynes, consistent with the spectra of previously reported GDY samples prepared using other methods.27 In addition, all of the peaks ascribed to the raw MS had disappeared, consistent with the
cos θ* = f (R f cos θ + 1) − 1
where θ* is the static water CA; f is the apparent area fraction of the solid−liquid interface; Rf, which characterizes the surface roughness, is defined as the ratio of the actual area of the rough surface to the geometric projected area; and θ is the water CA of a flat surface prepared from the same material. For GDYMS, optimization of the geometric structural parameter (Rf) would directly improve the water CA and hydrophobicity performance. Experimentally, hydrophobicity can be characterized by a static water CA. Figure 3 reveals that the variation in wettability was correlated with the loading mass ratio of GDY coated on B
DOI: 10.1021/acsami.8b01207 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 1. Characterization of GDYMS and MS using TEM, Raman spectroscopy, and XPS. (a, b) TEM images of GDY films exfoliated from GDYMS through ultrasonication. (c) High-resolution TEM micrograph. (d) Raman spectra of the MS (black line) and GDYMS (red line). (e) High-resolution C 1s spectrum, with deconvolution, of a GDY film.
Figure 2. Morphologies of GDYMS and MS. SEM images of (a−c) the MS and (d−f) GDYMS at various degrees of magnification.
Figure 3. Wettability of GDYMS and MS. (a) Photograph of GDYMS floating on water and the MS immersed in water. Inset: photograph of GDYMS forced to immerse in water. (b−g) Optical images of the wetting behavior of oil and water droplets placed onto (b) the MS and (c−g) GDYMS samples featuring loading ratios of GDY of (c) 11, (d) 15, (e) 17, (f) 19, and (g) 24% (GDYMS-11, GDYMS-15, GDYMS-17, GDYMS-19, and GDYMS-24, respectively). Insets: photographs of the water CAs of the GDYMS samples.
loading ratio = (mGDYMS − mraw )/mraw
the GDYMS; the water CAs increased upon increasing the loading ratio until the water CA reached 145°. We calculated the loading ratio using the equation C
DOI: 10.1021/acsami.8b01207 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 4. Chemical and mechanical stability of GDYMS. (a−c) Optical images of the wetting behavior of water droplets placed on GDYMS-15 immersed in (a) DMF, (b) 1 M NaOH, and (c) 0.5 M H2SO4. (d, e) Stress−strain curves of GDYMS-15 (d) under different amounts of compression (40, 60, or 80%) and (e) after 100 cycles of 50% compression.
Figure 5. Photographs of GDYMS-15 adsorbing oils of different densities. (a) An organic solvent (CHCl3) and (b) an oil (gasoline).
where mGDYMS is the mass of the GDYMS and mraw is the mass of the raw MS. Although the water CA did not reach 150°, GDYMS also exhibited excellent hydrophobicity. As displayed in Figure 3, water droplets (dyed with methylene blue) maintained a spherical shape on the GDYMS and collapsed on the MS. The oils (dyed with oil dyes) wetted both types of sponges. In addition, Figure 3a reveals that GDYMS-15 could float on water and could not be submerged without applying an external force; in contrast, the MS submerged into the water immediately. Furthermore, when we immersed the GDYMS-15 into water under an external force, its surface gave the impression of a silver mirror as a result of air bubbles. These phenomena suggested that GDYMS-15 possessed the ability to selectively adsorb oils. Thermal, chemical, and mechanical stability are all additional indices for evaluating the performance of GDYMS for practical applications. To investigate the thermal stability, we subjected GDYMS-15 and the MS to thermogravimetric analysis (TGA) under a N2 atmosphere. Figure S3 reveals initial weight losses for both GDYMS-15 and the MS at temperatures below 100 °C, attributable to the release of physically adsorbed water. Their major weight losses occurred in the range 200−400 °C, reflecting the decomposition of the raw MS; that is, both materials were stable at temperatures below 200 °C. We assessed the chemical stability by monitoring the water CAs after the samples had been immersed in DMF, 0.5 M H2SO4, and 1 M NaOH for 7 days. To our delight, the water CAs of GDYMS-15 remained at approximately 130° (Figures 4a−c),
suggesting that it was more stable than any previously reported hydrophobic material (Table S1). Because of the extraordinary mechanical stability of the MS as the substrate, GDYMS exhibited excellent mechanical stability. Figure 4d reveals that GDYMS-15 could recover its original shape after compression under a strain of 80%. Notably, this property remained unchanged after 100 cycles of compression under a strain of 50% (Figure 4e). After such treatment, the porous rough skeleton remained undestroyed, as revealed through SEM imaging (Figure S4), suggesting remarkable mechanical stability. Because of its excellent hydrophobicity, porous 3D structure, light weight, and stability, we suspected that GDYMS-15 would be an ideal candidate for use in the adsorption and reclamation of oils and organic solvents from wastewater. Indeed, it adsorbed oils and organic pollutants of various densities (e.g., gasoline, CHCl3), either floating on the water surface or when sunk within it (Figure 5). Most importantly, no water was adsorbed, and the adsorbed oil and organic solvents could be reclaimed through manual squeezing (Figure 6a). Thus, GDYMS-15 appears to be a promising material for handling oil spills and industrial leaks of chemical reagents. To quantify the adsorption capacity of GDYMS-15, we tested its behavior toward various oils and organic solvents, including CHCl3, CH2Cl2, ethyl acetate, petroleum ether, toluene, diesel, gasoline, and silicone oil. We calculated the adsorption capacity using the equation D
DOI: 10.1021/acsami.8b01207 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 6. Performance of GDYMS as an oil collection material. (a) Photographs of GDYMS-15 recovering an organic solvent from an oil/water mixture. (b) Adsorption capacities of GDYMS-15 toward various organic solvents and oils. (c) Adsorption recyclability of GDYMS-15 toward petroleum ether and CHCl3.
Figure 7. Photographs of the use of GDYMS for the separation of oil/water mixtures.
weight. The absorption capacities are considerably higher than those of activated carbon and nanowire membranes and comparable to those of carbon nanotube sponges and graphene foam for similar oils and solvents (Table S1). Furthermore, we examined its recyclability after manual squeezing of the sponge. Figure 6c reveals that the adsorption capacity remained high
mcapacity = (madsorption − mGDYMS)/mGDYMS
where mcapacity is the adsorption capacity of the GDYMS; madsorption is the mass of adsorpted oils and GDYMS; and mGDYMS is the mass of GDYMS. Figure 6b reveals that GDYMS-15 had remarkable adsorption capacity, ranging from 75 to 160 times its own E
DOI: 10.1021/acsami.8b01207 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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voltage of 15 kV. TEM and HRTEM were performed on a JEM-2100F electron microscope with an accelerating voltage of 200 kV. UV−Vis spectra were recorded using a UV-2600 (SHIMADZU) spectrophotometer and scanned across the range from 220 to 800 nm. Raman spectra were recorded at room temperature, using an NT-MDT NTEGRA Spectra system with excitation from an Ar laser at 473 nm. XPS was performed using a VG Scientific ESCALab220i-XL X-ray photoelectron spectrometer, with Al Kα radiation as the excitation source. The binding energies obtained in the XPS analysis were corrected with reference to the C 1s binding energy (284.8 eV). TGA was performed using a Mettler-Toledo TGA/SDTA851e apparatus under a flow of N2 at a heating rate of 10 °C/min from 25 to 500 °C. CAs were measured using an SL200 KB apparatus at ambient temperature; each CA was measured from five positions on a single sponge sample.
after 100 cycles; the water CA of GDYMS-15 remained high at 129.1°, suggesting outstanding recyclability (Figure S5). To verify the practicability of GDYMS in a realistic environment (e.g., oil on the sea, where the bulk water is always in flux), we examined its behavior under turbulent conditions created with intense magnetic stirring. As revealed in the Supporting Information Movie, we poured a large amount of gasoline onto water, stirred the mixture continuously, placed GDYMS-15 on the surface of the mixture, and, after a moment, removed it and squeezed out the adsorbed oil. Importantly, we could reuse the sponge repeatedly until all the oil had been removed from the water surface. Eventually, both the collected oil phase and the remaining water phase were homogeneous, reflecting the selective wettability of GDYMS-15 for oils over water. Therefore, the high adsorption capacity and extraordinary recyclability suggest that GDYMS-15 is a promising material for practical applications in treating oil leakages. In addition, because of the macroporous structure and hydrophobicity of GDYMS, we also investigated its use in the separation of oil/water mixtures in a filtration system. Figure7 displays the moldable GDYMS-15 fixed on a home-built glass apparatus; when we poured an oil/water mixture through the apparatus, the blue water phase was blocked and reserved on top of the GDYMS-15. Meanwhile, the orange organic phase quickly passed through the GDYMS-15 and collected in the storage flask, without the application of any external force (other than gravity). Most importantly, no blue water droplets appeared in the collected organic phase, confirming the excellent hydrophobicity and high separation efficiency of GDYMS-15 (>97%), estimated using reported methods51 (Figure S6).
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S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b01207. The conditions list of TGA curves of MS and GDYMS; Diffuse reflectance UV−Vis spectra; F(R) is a Kubelka− Munk conversion of the raw diffuse reflectance spectrum; XPS survey scan of MS and GDYMS-15; SEM images of GDYMS-15 after mechanical stability test; Optical images of the wetting behavior of water droplets placed on the GDYMS-15 after recycle 100 times; Separation efficiency of GDYMS-15 for oils and organic solvents (PDF) Movie showing a large amount of gasoline being poured onto water, the mixture being continuously stirred, placing GDYMS-15 on the surface of the mixture, and, after a moment, removing it and squeezing out the adsorbed oil.(AVI)
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CONCLUSION In this work, we have fabricated GDY on an MS skeleton through a facile in situ Glaser−Hay coupling. The method of decorating graphdiyne on flexible substrate provides a generic strategy to construct graphdiyne-based hybrid materials. Taking advantage of the properties of both the MS and GDY, the resulting GDYMS exhibited excellent performance, including a high adsorption capacity, extraordinary recyclability (>100 cycles), unprecedented stability under harsh conditions (strong acid and alkali), and good oil/water separation efficiency toward various kinds of oils and organic solvents. Furthermore, this cost-effective, simple, and scalable strategy for the fabrication of GDYMS suggests that it will be amenable to the large-scale production of adsorbents. Finally, the attractive features of GDYMS imply its promising applications in realistic environmental settings.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Zicheng Zuo: 0000-0001-7002-9886 Yongjun Li: 0000-0003-1359-1260 Huibiao Liu: 0000-0002-9017-6872 Yuliang Li: 0000-0001-5279-0399 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by the National Key Research and Development Project of China (2016YFA0200104), the Key Research Program of Frontier Sciences, CAS (Grant NO.QYZDY-SSW-SLH015), the National Nature Science Foundation of China (51573191, 21373235 and Grant Nos. 21790050, 21790053), and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA09020302, XDB12010300).
EXPERIMENTAL METHODS
MS Pretreatment. A commercial MS (thickness: ca. 3 cm) was cleaned successively with acetone and deionized water (sonication for 1 h each), then dried in a vacuum oven at 50 °C. Preparation of GDYMS. HEB was synthesized according to a reported synthetic method.27 The pretreated MS was submerged in a solution of HEB (5, 10, 15, 20, or 25 mg) in THF (40 mL) in a twoneck bottle, and then a solution of copper iodide (2 mg) in TMEDA (10 mL) was added. The mixture was heated at 50 °C under an Ar atmosphere for 72 h while protected from light. The GDY films were successfully decorated on the MS. This sponge was washed sequentially with DMF and acetone to remove any residues and then dried in a vacuum oven at 50 °C to give the target composite. Characterization. Field-emission SEM images were recorded using a Hitachi S-4800 FESEM microscope operated at an accelerating
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DOI: 10.1021/acsami.8b01207 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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