Fabrication of three-dimensional graphene foam with high electrical conductivity and large adsorption capability

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Fabrication of three-dimensional graphene foam with high electrical conductivity and large adsorption capability Guiqiang Chen a,∗ , Yanxia Liu a , Fei Liu b , Xiao Zhang a,∗ a b

College of Information Science and Engineering, Hebei North University, Zhangjiakou 075000, Hebei, China Department of Electrical Engineering, Zhangjiakou Vocational College of Technology, Zhangjiakou 075000, Hebei, China

a r t i c l e

i n f o

Article history: Received 22 March 2014 Received in revised form 24 May 2014 Accepted 24 May 2014 Available online xxx Keywords: Graphene foam Chemical vapor deposition Electrical conductivity Adsorption

a b s t r a c t A three-dimensional (3D), free-standing graphene foam was prepared by plasma-enhanced chemical vapor deposition on nickel-foam. The prepared graphene foam was found to consist of few-layered vertically-aligned graphene sheets with highly graphite structure. Owing to the 3D interconnected porous nanostructures, the graphene foam exhibited a high electrical conductivity of 125 S/cm and a large surface area of 625.4 cm2 /g. For practical application, we prepared the graphene foam/epoxy composites showing a maximum conductivity of 196 S/m at 2.5 vol.% filler loading, and a rather low percolation threshold of 0.18 vol.%. Furthermore, the derived graphene oxide foam exhibited an excellent absorption capability (177.6 mg/g for As(V), 399.3 mg/g for Pb(II)) and recyclability (above 90% removal efficiency after five cycles) for the removal of heavy metal ions. The present study reveals that the multifunctional graphene foam may broaden the graphene-based materials for the applications in electrically conductive composites and environmental cleanup. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Graphene, as a monolayer of sp2 -hybridized carbon atoms arranged in a two-dimensional (2D) lattice, has attracted tremendous attention in recent years owing to its exceptional thermal, mechanical and electrical properties. These extraordinary properties offer a wide range of possibilities to synthesize graphene-based functional materials for various applications [1–5]. One of the most promising applications of graphene is in polymer composites, and such graphene-based fillers have brought about enhanced mechanical–physical properties to polymer composites, which significantly outperform many types of traditional filler materials [6–9]. Despite the great application potential of graphene-based composites, it is worth mentioning that the resulting composites properties still fall far below the theoretical data. This is mainly attributed to the aggregation or restacking of graphene sheets due to the strong van der Waals forces as a consequence of high surface area and high aspect ratio of graphene, resulting in a much decreased performance [10–12]. Recently, a variety of methods including self-assembly, template guided growth, organic sol–gel reaction, have been employed for

∗ Corresponding authors. Tel.: +86 03543652487. E-mail addresses: [email protected] (G. Chen), [email protected] (X. Zhang).

integration of two-dimensional (2D) graphene sheets into 3D graphene macroscopic architectures (3D-GMAs), such as aerogels, sponges and foams [13]. Such 3D network structure has been considered as an effective approach to overcome the aggregation/restacking of graphene sheets and thus to greatly improve the performance of graphene-based composites. For instance, the reported electromechanical properties of the graphene foam/PDMS (poly-dimethylsiloxane) composites under stretching are far superior to those of 2D graphene films, which show mechanical failure over 6% strain on an unstrained PDMS substrate and an order of magnitude increase in resistance at 25% strain on a pre-strained PDMS substrate [14]. On the other hand, benefiting from the unique 3D interconnected porous structure, the 3D-GMAs are always endowed with high surface area. The reported 3D graphene aerogels (3D-GAs) prepared by a sol–gel method exhibited a large pore volume of ∼6 cm3 /g, and an extraordinarily high surface area of ∼1200 m2 /g, approaching the theoretical value of an individual sheet. The high specific areas of 3D-GMAs will enhance the capability of graphene-based materials for loading catalysts, adsorbing organic or inorganic molecules, and fast mass and electron transport, and as such 3D-GMAs may broaden the applications of graphene-based materials in supercapacitors, catalysis, and environmental remediation [13,15,16]. Currently, hazardous heavy metal ions (lead, arsenic, zinc, cadmium, nickel, chromium, copper and mercury) caused water pollution is one of the worldwide environmental

http://dx.doi.org/10.1016/j.apsusc.2014.05.171 0169-4332/© 2014 Elsevier B.V. All rights reserved.

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problems because of their extreme toxicity to human health and other organisms even at a trace level [17]. Therefore, it is very urgent to develop reliable methods for effective removal of these toxic metal ions from the wastewaters before releasing them into the environment. Many methods have been employed to remove heavy metal ions, such as membrane filtration [18], ion exchange [19] and adsorption [20]. Among the above methods, adsorption is recognized as a most effective and economic method for wastewater treatment due to its cost-effectiveness, simplicity, and enabling large-scale application [17]. Because of the 3D porous nanostructure endowing large surface area, we expect that the 3D-GMAs have a large adsorption capability for the removal of heavy metal ions. However, to our knowledge, very few studies have been reported on the investigations of 3D-GMAs for the adsorption of heavy metal ions [21]. Plasma-enhanced CVD (PECVD) is a promising technique that has been used for low-temperature and fast growth of different carbon based nanostructures [22]. The coupling between the carbon source/hydrogen plasma and metal foil in the PECVD process enables a very rapid and localized heating of the metal foil to produce fast graphene growth within a few minutes without any catalysts and supplemental heating. By virtue of these benefits, in this study, we report a high-efficiency synthesis method for preparing free-standing, high-quality 3D graphene foam on nickel foam by PECVD. The graphene foam was successfully demonstrated as filler materials in conductive composites and the resulting graphene foam/epoxy composites showed a much enhanced electrical conductivity and a rather low percolation threshold. It is of particular interest to investigate the use of derived graphene oxide (GO) foam as an absorbent for the removal of heavy metal ions. The adsorption performances of GO foam were systematically studied at various experimental parameters such as pH, adsorption time and solution temperature. The resulting GO foam was found to exhibit an exceptional effectiveness for the removal of Pb(II) and As(V), which significantly outperformed the mostly reported adsorbents.

2. Experimental 3D graphene foam was synthesized by PECVD using nickel foam (20 mm × 20 mm × 2 mm) as the growth substrate. The quartz tube in the PECVD chamber was initially pumped down to a base pressure of 10 Pa. Before introduced to vacuum chamber, the nickel foam was annealed at 950 ◦ C in the presence of Ar (200 sccm) and H2 (20 sccm) for 30 min to remove impurities. After annealing for 20 min, 30 sccm of CH4 was introduced into the tube. After 5 min of deposition at the plasma power of 150 W, the sample was rapidly cooled by pushing the quartz tube to a lower temperature region under Ar atmosphere. In order to obtain the free-standing 3D graphene foam, the Ni–graphene foam covered with graphene sheets was spin-coated with a thin layer of polymethylmethaacrylate (PMMA). Nickel foam was completely etched away by putting the samples into 2 M Fe(NO3 )3 solution for 8 h. Finally the PMMA coating was dissolved in acetone and then blow-dried to obtain the free-standing 3D graphene foam. A series of the graphene foams with different porosities of 97.5–99.6% were prepared by the same PECVD processing based on the various nickel foam substrates with varying porosities. Graphene/epoxy composites with various filler loadings (0.4–2.5 vol.%) were fabricated by a dipping method [23], in which the free-standing graphene foam was dipped into a diluted epoxy solution that is prepared by mixing epoxy resin agent, curing agent (aromatic diamine) and dimetbylformamide (DMF) in a ratio of 5:1:100. The composite films were obtained after removing the solvent in a vacuum oven at 60 ◦ C, followed by thermally curing at 100 ◦ C for 2 h, and at 150 ◦ C for 2 h.

Morphology of the graphene foam was characterized by scanning electron microscopy (SEM, JSM-7401F). Atomic force microscopy (AFM, Digital Instrument D3100) was used to determine the thickness of the graphene sheets. Raman spectra were used to characterize the structure and deformation of graphene foam, which were recorded on a Alpha 300R Raman Microscope with a laser power of 0.28 mW. Surface electronic states of elements in samples were examined on an X-ray photoelectron spectra (XPS) spectrometer (PHI 5000C ESCA) equipped with a hemispherical electron analyzer and an Mg K␣ X-ray source. Electrical conductivity of the bulk graphene foam and graphene foam composites was measured by a two-probe method with an applied voltage ranging from −1.0 to 1.0 V using a source meter. The conductivity values were calculated as the slope of I–V curve. Brunauer–Emmett–Teller (BET) surface area measurements were carried out using a porosimetry system. The batch adsorption experiments were carried out by adding 10 mg of absorbents to 50 mL solution of Pb(II)(As(V)) ions with the desired adsorption time and pH value. The adsorption time was selected as 5–40 min, and the pH of the ions solution was controlled with 0.1 mol/L HCl or NH3 ·H2 O and adjusted from 3 to 10. After the adsorption, the suspension was filtered through a 0.2 ␮m pore size membrane, and the concentrations of Pb(II)(As(V)) ions in the filtrate were determined by atomic absorption spectroscopy (PerkinElmer 3110). Removal efficiency (%) of the metal ions is calculated by dividing the adsorbed amounts of metal ions on adsorbents by the initial amounts of metal ions.

3. Results and discussions 3.1. Characterizations of graphene foam SEM image in Fig. 1(a) clearly shows the typical morphology of a free-standing graphene foam, in which the graphene foam inherits the 3D interconnected structure of the nickel foam, exhibiting a well-defined macroporous structure with the pore diameter 50–300 ␮m. High magnification (Fig. 1(a) inset) shows “petal-like” graphene sheets vertically aligned on the pore walls of graphene foam to form the nest-like porous structure. Each ‘petallike’ graphene sheet is about several nanometres in thickness and ∼100 nm in height. Statistics of the thickness of graphene sheets estimated from the height profiles of the sheets by AFM (Fig. 1(b) inset) are shown in Fig. 1(b). Most of the graphene sheets lie in the 1–4 nm range with an average value of 2.4 nm, suggesting that few-layered graphene sheets are formed. Raman spectra are used to characterize the structure change of the graphene foam, as shown in Fig. 2(a). Raman spectrum of the graphene foam demonstrates the typical graphene characteristic of G band (∼1580 cm−1 , graphite band), D band (∼1360 cm−1 , disorder or defects) and 2D band (∼2700 cm−1 , second-order zoneboundary phonons) [24]. The weak intensity of the disorder D band indicates the large size and low defect concentration. The layer number of the graphene foam (blue curve in Fig. 2(a)) can be determined by the comparison of I(G)/I(2D) intensity ratio [25]. The ratio of I(G)/I(2D) for the obtained graphene foam is 2.1, which approximately corresponds to 3–5 layers. In addition, we can prepare the single-layered or double-layered graphene foam by tuning the PECVD processing conditions such as the deposition time and cooling rate. As shown in the red/black curves of Fig. 2(a), the Raman spectra show the I(G)/I(2D) ratios of 0.5 and 1.1, corresponding to the single-layered and double-layered graphene foam, respectively. However, the single/double layered graphene foam is too fragile to manipulate because its weak building blocks. The collapsed graphene foam composed of mainly double-layered graphene is shown in Fig. 2(b). Therefore, the few-layered graphene

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Fig. 1. (a) SEM image of the morphology of a free-standing GNF (inset for high magnification); (b) statistics of the thickness of the graphene sheets (extruded from the crushed graphene foam) obtained by AFM measurements (inset).

foam exhibiting the robust and flexible structure is more appropriated for practical application relative to single/double layered graphene foam. Fig. 3(a) exhibits the XPS wide spectrum of the graphene foam, and only one C 1s peak appears, indicating that the obtained graphene foam has a high degree of graphitization. Fig. 3(b) shows the high resolution of C 1s XPS spectrum, which is deconvoluted into three distinct curves with the peaks locating at 284.7, 285.1 and 286.6 eV, respectively. The main peak at 284.7 eV originates from the sp2 -hybridized carbon bonding of graphene, the other peaks at 285.1 eV and 286.6 eV are attributed to the sp3 -hybridized C C bonding and C O bonding, respectively [26]. Quantitative analysis indicates about 0.5 wt.% of oxygen for the graphene foam. The presence of trace amount of oxygen may be due to the oxygen adsorption on the graphene foam after the sample is taken from the chamber. 3.2. Electrical conductivity of graphene foam and its composites The electrical conductivity of the graphene foam is determined to be 125 S/cm, which is much higher than those of the graphene films from chemical or thermal reduction of graphene oxide (0.01–80 S/cm) [27,28], doped graphene films (0.02–3 S/cm) [29,30], CNT or graphene papers (2–10 S/cm) [31,32] and graphene aerogels (0.1 S/cm). We believe this remarkable conductivity is due not only to the large size and high crystalline of graphene sheets (Fig. 2(a)) but also to the seamlessly 3D interconnected graphene network of the graphene foam (Fig. 1(a)), since the electrons can

move very quickly through the interconnected network of highquality graphene foam [14]. Therefore, the 3D graphene network with high electrical conductivity will be very helpful for their electrical transport-related applications such as conductive composites, transparent conductive films, and electrode materials for lithium-ion battery and supercapacitor. For practical application in conductive composites, we prepared the graphene foam/epoxy composites using epoxy as insulating polymer matrix. Fig. 4(a) shows the composite conductivity as a function of graphene foam volume fraction at room temperature. As seen, the graphene foam/epoxy composites reveal a dramatic conductivity enhancement in comparison with the pure epoxy. The highest measured conductivity is 196 S/m at 2.5 vol.% graphene foam fraction, which is about twelve orders of magnitudes higher than that of pure epoxy matrix. The electrical conductivity () of polymer composites with conductive filler embedded into the insulating matrix typically obeys a power law as a function of filler fraction [33]. c = 0 (f − fc )

t

(1)

where  0 is a scaling factor that may be comparable to the effective conductivity of the filler [19,20], f is the filler volume fraction, fc is the percolation threshold and t is the critical exponent. The best fit is achieved for fc = 0.18 vol.% and t = 2.8 ± 0.2 (solid line in Fig. 4(b)). Table 1 compares the values of maximum conductivity (filler content below 10 vol. %) and corresponding percolation threshold measured in the current study with those reported in the literature for the various types of 1D carbon nanotubes (CNTs)and 2D

Fig. 2. (a) Raman spectra of the graphene foam; (b) SEM image of the collapsed graphene foam (composed of double-layered graphene sheets). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Electrical conductivity, σ (S/m)

Fig. 3. (a) XPS wide spectrum of the graphene foam; (b) deconvoluted C 1s spectrum.

f=0 .18 vol% c t=2.8 2 R =0.998

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Fig. 4. (a) Electrical conductivity of graphene foam/epoxy composites as a function of graphene foam content; (b) fitting experimental conductivity data of graphene foam/epoxy composites by Eq. (1) (solid line) for determining the percolation threshold.

graphene filled polymer composites. As clear, the present graphene foam composites show a maximum conductivity of 2.0 × 102 S/m, which is significantly higher than the typical 0.1–1.0 × 102 S/m values observed in most CNT-filled or graphene-filled polymer composites [6,34]. The high conductivity in our graphene foam composites can certainly be attributed to the graphene foam network structure. The 3D continuous network structure of graphene foam largely reduces the electrical resistance originating from the graphene–graphene contact resistance and graphene-matrix boundary resistance and thus the overall conductivity of the

composites can be greatly enhanced. However, the conductivity of current composites is still significantly lower than the reported value (1.0 × 103 S/m at 0.22 vol.%) [35] for graphene foam/PDMS composites, which is due possibly to the low relative density (92.4% for maximum value) of the present graphene foam composites [36]. Realizing full relative density is a still challenge because of difficulties in incorporating polymer to such high porosity of the graphene foam (97.5–99.6%), which will be left for our future work. Furthermore, it is observed from Table 1 that the electrical percolation of 0.18 vol.% is significantly lower than those values from the most

Table 1 Comparison of the maximum conductivity (filler content below 10 vol%) and percolation threshold between present graphene foam-filled composites and those CNT, graphene-filled composites from literature. Filler

Matrix

Percolation threshold (vol.%)

Electrical conductivity (S/m) @ filler loadinga

Refs.

CNT

Epoxy Polycarbonate Polyimide PMMA Polypropylene Polystyrene Polyurethane Epoxy Polypropylene Polyvinyl alcohol PMMA Polystyrene Polycarbonate Polyurethane PDMS Epoxy

0.3 1.9 0.3 1.3 0.44 0.8 1.0 1.0 0.7 0.5 0.7 0.1 0.14 0.3 – 0.18

0.5 @ 3 vol.% 0.1 @ 4 vol.% 1.0 @ 3.7 vol.% 1.0 × 10−2 @ 2 vol.% 2.0 @ 4.5 vol.% 0.1 @ 2 vol.% 2.0 @7.5 vol.% 5.0 × 10−2 @ 9.5 vol.% 5.0 × 10−3 @5.0 vol.% 0.1 @ 3.3 vol.% 1.0 @ 5 vol.% 1.0 @ 2.5 vol.% 51.2 @ 2.2 vol.% 30 @ 5 vol.% 1.0 × 103 @ 0.22 vol.% 2 × 102 @ 2.5 vol.%

[57] [58] [59] [60] [61] [62] [63] [64] [65] [66] [67] [12] [68] [69] [35] Present work

Graphene

Graphene foam

a The conversion relations of vol.% = wt.% for single-walled CNT/single-layered graphene and vol.% = 2 wt.% for multi-walled CNT/multi-layered graphene are used independent of the polymer matrix.

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(b) dV/dlog(D) Pore volume (cm3 /g)

2500

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3

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adsorption desorption

500

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0 0.0

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0.6

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1.0

1

10

Relative pressure (P/P0 )

100

Pore diameter (nm)

Fig. 5. (a) Nitrogen adsorption–desorption isotherms and (b) pore size distribution of the graphene foam.

of CNT and graphene composites. This implies that the electrical network can be easily built up in the composites by incorporation of low content of graphene foam, as 3D structure of graphene foam can effectively avoid the occurrence of aggregation/restacking problem that impedes the formation of electrical network when incorporating CNTs or graphene in the polymer composites. 3.3. Adsorption property Nitrogen adsorption–desorption isotherms are used to investigate the surface area and porous structure of graphene foam. As shown in Fig. 5(a), the BET surface area and total pore volume are calculated to be 625.4 m2 /g and 2.18 m3 /g, respectively. Note that the surface area of graphene foam is much higher than that of reported carbon-based adsorbents, such as 1D CNT-based [34] and 2D graphene-based materials [37,38]. The high surface area of GO foam is owing to the particular foam-structure of robust 3D graphene network, which largely preserves the high surface area of graphene. Nevertheless, it is worth noting that even the surface area of 1300–3000 m2 /g [39–41] were reported for 3D graphene assemblies synthesized from the use of sol–gel method, approaching or even exceeding the theoretically maximum value of graphene (2600 m2 /g). Compared to the reported values, the less surface area of present graphene foam is due mainly to the layering or overlapping of graphene sheets within the pore walls of graphene foam. The porous property of the graphene foam can be further confirmed by pore size distribution analysis determined by the Barret–Joyner–Halenda method [42], as shown in Fig. 5(b). The pore size distribution shows that much of the pore volume (1.96 cm3 /g) lies in the 5–50 nm (mesoporous) range, with a mesopore-to-total-pore-volume ratio of 0.9 and a peak pore diameter of 7.1 nm, demonstrating the mesoporous structure of graphene foam.

It is known the graphene oxide (GO) is much more hydrophilic than the graphene due to the hydrophilic functional groups formed on the surface sites of GO [43]. These functional groups containing oxygen atoms possess a lone electron pair, and thus they have strongly complexation capacities with heavy metal ions through sharing an electron pair to form a metal complex [44]. Under such consideration, we transform the graphene foam to GO foam to evaluate its absorption ability for the removal of heavy metal ions. The GO foam can be easily obtained by plasma treatment of the graphene foam under Ar/O2 atmosphere. Fig. 6(a) and (b) presents the XPS spectra of GO foam. The wide scan XPS spectrum (Fig. 6(a)) of GO foam shows a stronger O 1s peak at a binding energy of 531.2 eV compared to that of graphene foam shown in Fig. 6(a), indicating a considerable degree of oxidation. The XPS spectrum of C 1s (Fig. 6(b)) can be deconvoluted into four peaks consisting of the C C bond (284.5 eV) of sp2 carbon, C O bond (286.6 eV), C O group (288.1 eV) of carbonyl carbon, and O C O group (288.4 eV) of carboxylic carbon, showing the typical characteristic of GO containing abundant functional groups [45]. Pb(II) and As(V) are two typical toxic heavy metal ions in water resources and can travel through the food chain via bioaccumulation [20]. Both pose a significant threat to the environment and public health. In this experiment, Pb(II) and As(V) are selected as two representative metal ions for evaluating the removal efficiency of GO foam at different experimental conditions (pH, adsorption time and solution temperature). Solution pH plays a most important role in adsorption of metal ions due to its influence on the solution chemistry of heavy metal ions and the surface charge of the adsorbents [46]. Fig. 7(a) shows the effect of pH on the removal efficiency of Pb(II)/As(V) in the pH range 3–10. It is seen that the removal efficiency of As(V) is higher at low pH range, but a converse trend is observed for Pb(II). This can be understood by their different solution charges of Pb(II) and As(V) resulting in various electrostatic interaction behaviors with GO foam. As(V) presents

(b)

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200

400

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C=O O=C-O

282

284

286

288

290

292

Binding energy (eV)

Fig. 6. (a) XPS wide spectrum of the GO foam; (b) deconvoluted C 1s spectrum.

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(b) 100 Removal efficiency / %

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As(V) Pb(II)

400 300 200 100 295 300 305 310 315 320 325 330 Temperature / K

100 90 80 70 As(V) Pb(II)

60 50 1

2 3 4 5 Adsorption-desorption cycles

Fig. 7. Removal efficiency of the GO foam as a function of pH (a), adsorption time (b), and (d) adsorption–desorption cycles (d), for As(V)/Pb(II) removal; (c) effect of solution temperature on the maximum adsorption capacities of the GO foam for As(V)/Pb(II) removal.

negatively charged ions in the form of H2 AsO4 − [47], while Pb(II) presents positively charged ions in the form of Pb2+ /Pb(OH)+ [48]. At low pH level, GO foam is protonated to be positively charged owing to the functional groups on GO foam [49]. Thus, the negatively charged As(V) tends to bind with positively charged GO foam through electrostatic attractions, leading to the high adsorption capability for As(V) removal. At high pH level, the GO foam is deprotonated and become negatively charged [49], and in this case, GO foam facilitates electrostatic attractions with positively charged Pb(II), resulting in an increased capability for Pb(II) adsorption. Fig. 7(b) shows the effect of adsorption time on the removal efficiency of As(V)/Pb(II) ions at their optimum pH. It is clear that the adsorption processes of both metal ions are very fast during the first 10 min, and 67% for As(V), 82% for Pb(II), are removed from the water after 10 min adsorption. After 30 min, the removal efficiency is almost unchanged and the equilibrium is established, indicating that the availability of large number of binding sites on GO foam is saturated. Therefore, within 30 min adsorption, the maximum adsorption capacity of the GO foam is reached, given as 177.6 and 399.3 mg/g for As(V) and Pb(II), respectively. It is noteworthy that the GO foam exhibits a much higher adsorption capability than mostly reported adsorbents, such as CeO2 hollow nanospheres (22.4 mg/g) [50], graphene–Fe3 O4 hybrids (3.2 mg/g) [51], and ␣Fe2 O3 nanostructures (51.0 mg/g) for As(V) removal, and activated carbon (54.1 mg/g) [52], carbon nanotubes (97.8 mg/g) [53], and carbon aerogels (22.6 mg/g) [54] for Pb(II) removal. The superior adsorption capacity of present GO foam is due mainly to the combined factors of large specific surface area and the abundant functional groups on the surface of GO foam, both of which create numerous binding sites for absorbing metal ions. The adsorption capability of the GO foam can be further enhanced at relatively high solution temperature due to the fact that high temperature induces more activating binding sites of the absorbents [55]. As shown in Fig. 7(c), the adsorption of both As(V) and Pb(II) increases noticeably with the solution temperature, and can be achieved to 232.1 mg/g (As(V)) and 485.9 mg/g (Pb(II)) at 328 K, representing

30.7% and 21.6% higher than their room temperature values, respectively. Furthermore, the regeneration of the adsorbent is a key factor in improving wastewater process economics. In our experiment, the reusability of GO foam for As(V)/Pb(II) adsorption is studied via many cycles of adsorption/desorption. Dilute HCl solutions (∼0.2 mol/L) is used as the good desorption agent to recover metal ions from the adsorbents, since the absorbent becomes unstable in strong acid, and can gradually release the absorbed metal ions after a period of time. Fig. 7(d) shows the effect of adsorption–desorption cycling times on the removal efficiency of Pb(II)/As(V). As seen, with increasing cycles, the removal efficiency decreases very slowly, and still retains nearly 90% after five cycles. Therefore, GO foam offers the possibility for easy recycle and reuse for the removal of heavy metal ions. To better understand the mechanism of metal ions adsorption by GO foam, the XPS spectra of the GO foam before and after adsorption of As(V)/Pb(II) ions are investigated and shown in Fig. 8. Fig. 8(a) shows the wide XPS spectra of GO foam before and after As(V)/Pb(II) adsorption. It is evident that tiny As and Pb related peaks are appeared in the XPS spectra of GO foam after adsorption of As(V)/Pb(II). The high-resolution Pb 4f spectrum after Pb(II) adsorption (Fig. 8(b)) can be deconvoluted into two peaks of Pb 4f7/2 (138.9 eV) and Pb 4f5/2 (143.1 eV), which can be attributed to Pb 4f7/2 O and Pb 4f5/2 O bonding, respectively. The high-resolution As 3d spectrum after As(V) adsorption (Fig. 8(c)) shows a single peak at 45.7 eV, which is assigned to As 3d O bonding. These results indicate that As(V)/Pb(II) are successfully adsorbed on the surface of GO foam. In addition, a high-resolution O 1s XPS spectrum of GO foam before adsorption (Fig. 8(d)) can be deconvoluted into two peaks of OH (531.1 eV) and COOH (533.2 eV) [28,40], where the peak intensity of OH is markedly higher than that of COOH, implying that oxygen-containing groups on GO foam surface consisting of mainly hydroxyl groups. However, the intensity of the OH peak shows significantly decreases after As(V)/Pb(II) absorption, as shown in Fig. 8(e) and (f). The reduction in intensity of the

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(a)

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O1s

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136

Binding energy (eV) O1s

Intensity (a.u.)

-OH

-COOH

526

528

530

532

Binding energy (eV)

534

(e)

GO foam- Pb(II)

536

144

40

42

44

O1s

-OH

526

46

48

50

Binding energy (eV)

(f)

528 530 532 534 Binding energy (eV)

536

O1s

GO foam-As(V)

-COOH

Intensity (a.u.)

GO foam

138 140 142 Binding energy (eV)

-COOH

Intensity (a.u.)

400

As3d

As3d

Intensity (a.u.)

GO foam-As(V)

200

GO foam-As(V)

Pb4f 5/2

As3d

0

(d)

(c)

Intensity (a.u.)

Intensity (a.u.)

GO foam

Pb4f

Pb4f 7/2

7

-OH

526

528 530 532 534 Binding energy (eV)

536

Fig. 8. XPS spectra of GO foam before and after absorption of As(V)/Pb(II): (a) wind-scan; (b) high-resolution Pb 4f spectrum after Pb(II) adsorption; (c) high-resolution As 3d spectrum after As(V) adsorption; (d–e) deconvoluted O 1s spectrum of GO foam before adsorption (d), after Pb(II) adsorption (e), and after As(V) adsorption (f).

OH peak corresponds to the loss of hydroxyl groups after adsorption, suggesting the occurrence of the ion exchange between the hydroxyl groups and As(V)/Pb(II) ions during the adsorption process [56]. Therefore, the above results provide the clear evidences that the combined adsorption mechanisms of electrostatic attraction and ion exchange between hydroxyl groups and metal ions, contribute to the metal ions adsorption by GO foam. 4. Conclusions We developed a high-efficiency synthesis method for preparing the 3D graphene foam by PECVD technique at low temperature. The produced graphene foam consisted of mainly few-layered graphene sheets with high degree of graphitization and low defect concentration. The graphene foam exhibited outstanding electrical properties, and its conductivity reached up to 125 S/cm. The graphene foam was successfully demonstrated as filler materials in conductive composites and the resulting graphene foam/epoxy composites showed a maximum conductivity of 196 S/m at 2.5 vol.% filler loading and a rather low percolation threshold of 0.18 vol.%. The 3D interconnected porous nanostructures made the graphene foam possess large surface area of 625.4 cm2 /g. The GO foam prepared by direct oxidation of graphene foam exhibited an excellence removal efficiency for the adsorption of heavy metal ions, such as As(V) and Pb(II), which was attributed to the combined factors of large specific surface area and the abundant functional groups on the surface of GO foam. The removal efficiency was still above 90% after five cycles, demonstrating a high recyclability. The adsorption mechanism of GO foam is the synergetic contribution of the electrostatic attraction, and ion exchange between hydroxyl groups and metal ions. Therefore, the excellent properties and performance of present 3D graphene foam/GO foam suggest great potential applications in electrically conductive composites and the environmental cleanup. Acknowledgments This work was supported by the Science and Technology Department of Heibei Province Project (No.13210336), financially supported by National Ministry of Science and Technology Project

(No.2012BAJ18B08-6) and Zhangjiakou Science and Technology Bureau Project (No.13110038I-11). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40]

A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183. D.L. Nika, A.A. Balandin, J. Phys.: Condens. Mater. 24 (2012) 233203. A.A. Balandin, Nat. Mater. 10 (2011) 569. A.A. Balandin, S. Ghosh, W. Bao, et al., Nano Lett. 8 (2008) 902. S. Ghosh, I. Calizo, D. Teweldebrhan, et al., Appl. Phys. Lett. 92 (2008) 151911. J.R. Potts, D.R. Dreyer, C.W. Bielawski, R.S. Ruoff, Polymer 52 (2011) 5. V. Goyal, A.A. Balandin, Appl. Phys. Lett. 100 (2012) 073113. K.M. Shahil, A.A. Balandin, Nano Lett. 12 (2012) 861. P. Goli, S. Legedza, A. Dhar, R. Salgado, J. Renteria, A.A. Balandin, J. Power Sources 248 (2014) 37. H. Kim, A.A. Abdala, C.W. Macosko, Macromolecules 43 (2010) 6515. T. Ramanathan, A. Abdala, S. Stankovich, et al., Nat. Nanotechnol. 3 (2008) 327. S. Stankovich, D.A. Dikin, G.H. Dommett, et al., Nature 442 (2006) 282. C. Li, G. Shi, Nanoscale 4 (2012) 5549. Z. Chen, W. Ren, L. Gao, B. Liu, S. Pei, H.M. Cheng, Nat. Mater. 10 (2011) 424. X. Cao, Y. Shi, W. Shi, et al., Small 7 (2011) 3163. W. Chen, S. Li, C. Chen, L. Yan, Adv. Mater. 23 (2011) 5679. A. Liu, K. Hidajat, S. Kawi, D. Zhao, Chem. Commun. (2000) 1145. C. Blöcher, J. Dorda, V. Mavrov, H. Chmiel, N. Lazaridis, K. Matis, Water Res. 37 (2003) 4018. A. Dabrowski, Z. Hubicki, P. Podko´scielny, E. Robens, Chemosphere 56 (2004) 91. H.B. Bradl, J. Colloid Interface Sci. 277 (2004) 1. W. Li, S. Gao, L. Wu, et al., Sci. Rep. 3 (2013) 2125. H. Windischmann, G.F. Epps, Y. Cong, R. Collins, J. Appl. Phys. 69 (1991) 2231. Z. Chen, C. Xu, C. Ma, W. Ren, H.M. Cheng, Adv. Mater. 25 (2013) 1296. A. Ferrari, J. Meyer, V. Scardaci, et al., Phys. Rev. Lett. 97 (2006) 187401. D. Graf, F. Molitor, K. Ensslin, et al., Nano Lett. 7 (2007) 238. D. Yang, A. Velamakanni, G. Bozoklu, et al., Carbon 47 (2009) 145. C. Gómez-Navarro, R.T. Weitz, A.M. Bittner, et al., Nano Lett. 7 (2007) 3499. I. Jung, D.A. Dikin, R.D. Piner, R.S. Ruoff, Nano Lett. 8 (2008) 4283. D. Wei, Y. Liu, Y. Wang, H. Zhang, L. Huang, G. Yu, Nano Lett. 9 (2009) 1752. A.L.M. Reddy, A. Srivastava, S.R. Gowda, H. Gullapalli, M. Dubey, P.M. Ajayan, ACS Nano 4 (2010) 6337. B. Fugetsu, E. Sano, M. Sunada, et al., Carbon 46 (2008) 1256. H. Chen, M.B. Müller, K.J. Gilmore, G.G. Wallace, D. Li, Adv. Mater. 20 (2008) 3557. B. Last, D. Thouless, Phys. Rev. Lett. 27 (1971) 1719. W. Bauhofer, J.Z. Kovacs, Compos. Sci. Technol. 69 (2009) 1486. Z. Chen, W. Ren, L. Gao, B. Liu, S. Pei, H.-M. Cheng, Nat. Mater. 10 (2011) 424. M.A. Worsley, P.J. Pauzauskie, S.O. Kucheyev, et al., Acta Mater. 57 (2009) 5131. D. Dinda, A. Gupta, S.K. Saha, J. Mater. Chem. A 1 (2013) 11221. H. Gao, Y. Sun, J. Zhou, R. Xu, H. Duan, ACS Appl. Mater. Interfaces 5 (2013) 425. M.A. Worsley, S.O. Kucheyev, H.E. Mason, et al., Chem. Commun. 48 (2012) 8428. M.A. Worsley, T.Y. Olson, J.R. Lee, et al., J. Phys. Chem. Lett. 2 (2011) 921.

Please cite this article in press as: G. Chen, et al., Fabrication of three-dimensional graphene foam with high electrical conductivity and large adsorption capability, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.171

G Model APSUSC-27990; No. of Pages 8

ARTICLE IN PRESS G. Chen et al. / Applied Surface Science xxx (2014) xxx–xxx

8 [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54]

J. Biener, S. Dasgupta, L. Shao, et al., Adv. Mater. 24 (2012) 5083. E.P. Barrett, L.G. Joyner, P.P. Halenda, J. Am. Chem. Soc. 73 (1951) 373. S. Stankovich, R.D. Piner, S.T. Nguyen, R.S. Ruoff, Carbon 44 (2006) 3342. G. Zhao, J. Li, X. Ren, C. Chen, X. Wang, Environ. Sci. Technol. 45 (2011) 10454. J. Paredes, S. Villar-Rodil, A. Martinez-Alonso, J. Tascon, Langmuir 24 (2008) 10560. M. Hua, S. Zhang, B. Pan, W. Zhang, L. Lv, Q. Zhang, J. Hazard. Mater. 211 (2012) 317. J.R. Parga, D.L. Cocke, J.L. Valenzuela, et al., J. Hazard. Mater. 124 (2005) 247. B. Yu, Y. Zhang, A. Shukla, S.S. Shukla, K.L. Dorris, J. Hazard. Mater. 84 (2001) 83. G. Cimino, A. Passerini, G. Toscano, Water Res. 34 (2000) 2955. C.Y. Cao, Z.-M. Cui, C.Q. Chen, W.G. Song, W. Cai, J. Phys. Chem. C 114 (2010) 9865. G. Gollavelli, C.-C. Chang, Y.-C. Ling, ACS Sustainable Chem. Eng. 1 (2013) 462. J. Rivera-Utrilla, I. Bautista-Toledo, M.A. Ferro-García, C. Moreno-Castilla, J. Chem. Technol. Biotechnol. 76 (2001) 1209. Y.-H. Li, J. Ding, Z. Luan, et al., Carbon 41 (2003) 2787. J. Goel, K. Kadirvelu, C. Rajagopal, V. Garg, Ind. Eng. Chem. Res. 44 (2005) 1987.

[55] Z. Aksu, Sep. Purif. Technol. 21 (2001) 285. [56] C.Y. Cao, J. Qu, W.S. Yan, J.F. Zhu, Z.Y. Wu, W.G. Song, Langmuir 28 (2012) 4573. [57] F. Du, C. Guthy, T. Kashiwagi, J.E. Fischer, K.I. Winey, J. Polym. Sci. Polym. Phys. 44 (2006) 1513. [58] B. Hornbostel, P. Pötschke, J. Kotz, S. Roth, Phys. Status Solidi B: Basic Solid State Phys. 243 (2006) 3445. [59] X. Jiang, Y. Bin, M. Matsuo, Polymer 46 (2005) 7418. [60] F. Du, J.E. Fischer, K.I. Winey, J. Polym. Sci. Polym. Phys. 41 (2003) 3333. [61] S. Tjong, G. Liang, S. Bao, Scripta Mater. 57 (2007) 461. [62] S.T. Kim, H.J. Choi, S.M. Hong, Colloid. Polym. Sci. 285 (2007) 593. [63] H. Koerner, W. Liu, M. Alexander, P. Mirau, H. Dowty, R.A. Vaia, Polymer 46 (2005) 4405. [64] J. Liang, Y. Wang, Y. Huang, et al., Carbon 47 (2009) 922. [65] K. Kalaitzidou, H. Fukushima, L.T. Drzal, Compos. Sci. Technol. 67 (2007) 2045. [66] H.J. Salavagione, G. Martínez, M.A. Gómez, J. Mater. Chem. 19 (2009) 5027. [67] G. Chen, W. Weng, D. Wu, C. Wu, Eur. Polym. J. 39 (2003) 2329. [68] M. Yoonessi, J.R. Gaier, ACS Nano 4 (2010) 7211. [69] H. He, C. Gao, Chem. Mater. 22 (2010) 5054.

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