Journal ofMaterials Chemistry A

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aSchool of Science, Tianjin University, T

jianpinggao2012@126.com; yanjingls2012@bSchool of Chemical Engineering, Tianjin Un

† Electronic supplementary informa10.1039/c3ta10989e

Cite this: J. Mater. Chem. A, 2013, 1,7612

Received 8th March 2013Accepted 19th April 2013

DOI: 10.1039/c3ta10989e

www.rsc.org/MaterialsA

7612 | J. Mater. Chem. A, 2013, 1, 76

Three-dimensional graphene-based aerogels preparedby a self-assembly process and its excellent catalytic andabsorbing performance†

Tao Wu,a Mingxi Chen,a Lei Zhang,a Xiaoyang Xu,a Yu Liu,a Jing Yan,*a Wei Wangb

and Jianping Gao*a

A simple one-step method for fabricating graphene-based hydrogels (GHs) with interconnected 3D

networks using Cu nanoparticles was developed. During this process, graphene oxide (GO) was reduced

by Cu nanoparticles to form reduced GO (rGO) which then self-assembled to form GHs. Meanwhile, the

Cu nanoparticles were oxidized to form copper(I) oxide which deposited onto the rGO sheets and

became imbedded in the GHs. The GHs were transformed to graphene-based aerogels (GAs) by a green

freeze-drying method. The composition of the GAs can be easily adjusted by simply changing the initial

amount of Cu nanoparticles or the concentration of the GO suspension. The GAs not only possess good

catalytic performance for the catalytic reduction of 4-nitrophenol and the photocatalytic degradation of

methyl orange but also have excellent capacities for removing various oils and dyes from water.

1 Introduction

Graphene has attracted a great deal of attention since it was rstreported in 2004.1 It has excellent electronic,2–7 thermodynamic8

and mechanical properties9–13 that arise from its at monolayerstructure of sp2-bonded carbon atoms which are tightly packedinto two-dimensional (2D) honeycomb lattices.

Two-dimensional macrostructures of graphene, such asmembranes and paper-like thin lms, can be fabricated usinglayer-by-layer assembly,14–17 vacuum-assisted ltration18–21 anddirect chemical vapor deposition.22 However, when graphenenanosheets are assembled into 2D macrostructures, the largeaccessible surface area of those nanosheets is sacriced sincegraphene nanosheets are particularly prone to aggregate andrestack due to strong p–p interactions, hydrophobic interac-tions, and van der Waals forces. In order to solve this problem,three-dimensional (3D) graphene macrostructures, such ashydrogels23–27 and aerogels,27–31 have been developed recently.These 3D graphene-based macrostructures not only retain thelarge accessible surface areas of the graphene nanosheets butalso have highly porous structures with pore sizes of severalmicrometers. As a result, many promising applications, such ascatalysis,24,31,32 energy storage and conversion,23,28 and oilabsorption and water purication27,33 have been developed forthese materials.

ianjin, 300072, P. R. China. E-mail:

163.com; Fax: +86 22 274 034 75

iversity, Tianjin, 300072, P. R. China

tion (ESI) available. See DOI:

12–7621

There are several methods to produce 3D graphene macro-structures, such as cross-linking method,24–26 chemical vapordeposition (CVD),34 and in situ reduction-assemblymethod.23,27,28,31 The cross-linking method generally employs avariety of cross-linkers, like water soluble polymers,35,36 metalions37,38 or DNA39 to promote the assembly of graphene into 3Dmacrostructures by physical or chemical interactions. However,the presence of cross-linkers has been found to lead to adecrease or a complete loss in the electrical conductivity of the3D graphene-based macrostructures. As for the CVD method,3D nickel foams are generally used as substrates, onto whichgraphene is deposited under high temperature conditions.40–42

The self-assembly of graphene assisted by chemical reduc-tants is called in situ reduction-assembly. During this progress,graphene oxide (GO) is reduced to graphene and then it canspontaneously assemble in situ into 3D graphene hydrogelsunder some conditions such as with hydrothermal treatment23

or in the presence of a large amount of reducing agent.27,28,31,43

The prepared 3D graphene hydrogels usually have pore sizesranging from submicrometer to several micrometers, whichmakes the incorporation of nanoparticles into these 3D gra-phene hydrogels possible.

Yu et al. reported a one-step approach to prepare graphene/iron oxide hydrogels using iron(II) ions, and demonstrated theability of the hydrogels to remove large amounts of oil andheavy metal ions from water.45 Yan et al. reported a simplemethod to prepare 3D graphene hydrogels with nanoparticleswrapped inside the network by dispersing nanoparticles in a GOaqueous suspension during an in situ reduction-assembly.44

However, reducing agents, such as NaHSO3, Na2S and HI, wereused in Yan's work and these are unfriendly to the environment.

This journal is ª The Royal Society of Chemistry 2013

Fig. 1 (A) Photographs of the formation process for the graphene hydrogelswith 25 mL of GO suspension (2 mg mL�1) in the presence of Cu (0.1 g); (B) low-and (C) high-magnification SEM images; (D) TEM and (E) high-resolution TEMimages of the graphene hydrogel; (F) XRD pattern of the prepared aerogel; (G)Raman spectra of GO and graphene-based aerogel.

Fig. 2 Photographs of (A) GHs and (B) corresponding aerogels dried from thehydrogels prepared using different amounts of Cu nanoparticles. The amount ofCu used for preparation of GHCu-1–GHCu-3 shown in (A) and (B) was 0.01, 0.05 and0.1 g, respectively. SEM images of the GAs using different amounts of Cu nano-particles: (C and D) 0.01 g; (E and F) 0.05 g; (G and H) 0.1 g. The inset in F is a highresolution SEM image of sample GACu-3. (I) The Raman spectra of GAs with 0.01and 0.1 g of Cu nanoparticles.

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Table 1 Experimental parameters and composition analyses for samplesprepared with different amounts of Cu nanoparticles

SampleamCu

(initial) (g) mhydrogel (g) maerogel (g)Watercontent (%)

Cu-1 0.01 1.6718 0.0191 98.8675Cu-2 0.05 2.9212 0.0640 97.8091Cu-3 0.1 3.7655 0.1324 96.4838

a The initial concentration of GO in the experiments was 2 mg mL�1 (25mL). The hydrogel was prepared in a cylindrical sample vial with avolume of 30 mL.

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Therefore, an efficient and environmentally friendly strategy toprepare 3D graphene-based hydrogels is still desired.

In this paper, a simple one-step method for the fabrication ofgraphene-based hydrogels (GHs) with 3D macrostructures isdemonstrated. During this process, reduced GO (rGO) wasgradually formed by using Cu nanoparticles as the reducingagent and the rGO simultaneously self-assembled to GH. At thesame time, the copper nanoparticles were oxidized to formcopper(I) oxide. Graphene-based aerogels (GAs) were thenobtained by freeze-drying the GHs. In addition, GHs withdifferent compositions were obtained by simply adjusting theinitial amount of Cu nanoparticles or the concentration of GOsuspension. The obtained GAs not only had good catalyticperformance for the reduction of 4-nitrophenol (4-NP) and thephotocatalytic degradation of methyl orange (MO) but alsoexcellent absorption capacities for the removal of various oilsand dyes from water.

2 Experimental section2.1 Materials

The graphite was from Huadong Graphite Co. Potassiumpermanganate, sodium nitrate, concentrated sulfuric acid,hydrogen peroxide (30%), hydrochloric acid, and all otherreagents were all purchased from Tianjin Chemical Reagent Co.All chemicals were analytical grade and used as received.

2.2 Preparation of GO

Graphene oxide was prepared from natural graphite by amodied Hummer's method.46–48

Fig. 3 (A) Photographs of graphene hydrogels prepared using different initialGO concentrations. The concentration of GO used for preparation of the samplesGAGO-1–GAGO-3 was 0.1, 1 and 4 mg mL�1, respectively. SEM images of the gra-phene aerogels prepared from the GO suspensions with concentrations of (B) 0.1,(C) 1 and (D) 4 mg mL�1. (E) The XRD patterns of GAGO-1–GAGO-3. The initialamount of Cu nanoparticles was 0.05 g.

2.3 Preparation of Cu nanoparticles

The copper nanoparticles were prepared by adding a mixedsolution of 2.24 g of KOH (0.04 mol) and 0.27 g of KBH4 (0.005mol) (20 mL, solution A) into 20 mL of a solution containing2.5 g of CuSO4$5H2O (0.01 mol) and 0.8 g of EDTA$2Na$2H2O(solution B) at 30 �C while the solution was stirred vigorously.The prepared Cu nanoparticles were easily isolated from thesolution by vacuum ltration and were then puried by washingwith copious amounts of doubly distilled water and absoluteethanol successively. The nanoparticles were then dried in avacuum at 40 �C. The yield of the copper nanoparticles was98.87%.

7614 | J. Mater. Chem. A, 2013, 1, 7612–7621

2.4 Preparation of GHs and GAs

The desired amount of Cu nanoparticles (0.01–0.1 g) was quicklyadded to 25 mL of a GO aqueous suspension (0.1–4 mg mL�1)

This journal is ª The Royal Society of Chemistry 2013

Table 2 Experimental parameters and composition for the samples preparedwith different initial GO concentrations

Samplea

GOconcentration(mg mL�1)

mhydrogel

(g)maerogel

(g)

Watercontent(%)

Density(mg cm�3)

GO-1 0.1 1.7562 0.0961 94.5279 18.8GO-2 1 1.2520 0.0557 95.5511 24.4GO-3 4 2.9921 0.0637 97.8711 29.6

a The initial amount of Cu nanoparticles was 0.05 g.

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in a 30 mL cylindrical vial. Themixed solution was sonicated for1 h. Then, the vial was placed in a bath for 24 h at 95 �C withoutstirring to form the GH. The GH was then removed, rinsed withdistilled water and freeze-dried into an aerogel for further use.

2.5 Characterization

Ultraviolet-visible (UV-vis) absorption spectra were recordedwith a TU-1901 UV-Vis spectrophotometer. Photos of the GOmaterials were recorded with a digital camera (COOLPIX S260,Nikon, Japan). Transmission electron microscopy (TEM) andenergy dispersive X-ray spectroscopy (EDX) measurements wereperformed using a Philips Tecnai G2F20 microscope at 200 kV.Elemental composition analyses were carried out using X-rayphotoelectron spectroscopy (XPS, PHI1600 ESCA System, PER-KIN ELMER, USA) with Al Ka radiation (hn ¼ 1486.6 eV). TheXPS spectra were tted using the XPS peak 4.1 soware, inwhich a Shirley background was used to perform curve ttingand to calculate the atomic concentrations. Atomic forcemicroscopy (AFM) images were taken with a Nanoscope Multi-mode and Explore atomic force microscope (Veeco Instruments,USA). The X-ray diffraction patterns (XRD) of the samples weremeasured using an X-ray diffractometer (BDX3300) with areference target: Cu Ka radiation (l¼ 1.54 A), voltage: 30 kV andcurrent: 30 mA. The samples were measured from 10 to 30� (2q)with steps of 4� min�1. Raman spectroscopy analysis was per-formed from 500 to 2000 cm�1 at room temperature using anRaman spectrometer (DXR, USA) with a Nd:YAG laser excitationsource (532 nm). The Raman spectrum of graphene is usuallycharacterized by two main features, the G mode arising fromrst order scattering of the E2g phonons of the sp2 C atoms

Scheme 1 Schematic illustration of the formation mechanism of the GHs.

This journal is ª The Royal Society of Chemistry 2013

(usually observed at �1575 cm�1) and the D mode arising froma breathing mode of k-point photons of A1g symmetry (at�1350cm�1). Changes in the relative intensities of the D and G bands(D/G) indicate changes in the electronic conjugation state of theGO during reduction.

2.6 General procedure for the reduction of 4-NP

First, 2.5 mL of aqueous 4-NP solution (1.54 � 10�3 mol L�1)was mixed with 0.5 mL of freshly prepared aqueous NaBH4

solution (0.88 mol L�1) to form a deep yellow solution. Then,0.022 g of catalyst (GA) was added to the solution and when thesolution became colorless it indicated that the reaction wascomplete.

2.7 Oil-uptake experiments

Weighed amounts of the GAs were put into different kinds ofoils for absorption tests. Aer absorption was complete, the GAswere removed from the oil and weighed again aer the surfaceoils has been removed with a lter paper. The absorptioncapacity (Q) was calculated using the following equation:

Q ¼ (M � M0)/M0

where, M0 and M are the weights of the GAs before and aerabsorption, respectively. The weight measurements of the GAswith absorbed oil were done quickly to avoid evaporation of theoils.

2.8 Dye-uptake experiments

The dyes were dissolved in distilled water to form solutions withan initial concentration of 1 mmol L�1. Then the GAs wereadded to the above solutions and the dye-uptake experimentswere performed at room temperature. The concentration of thedye solutions was determined by UV-vis spectroscopy. The dye-uptake of the GAs was expressed as the equilibrated absorptioncapacity of the dye per unit mass of the aerogel and calculatedas follows:

Qeq ¼�Ci � Ceq

�VMd

mga

where, Qeq (mg g�1) is the absorption capacity; Ci (mmol mL�1)and Ceq (mmol mL�1) are the initial and nal concentration of

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Fig. 4 Time-dependent (A) UV-vis absorption spectra of the reduction of 4-NP byNaBH4 in the presence of GA as the catalyst; (B) plot of the logarithm of theabsorbance at 400 nm vs. the reduction time; (C) recycling of GA.

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the dye solution; V (mL) is the volume of the dye solution; Md

(mg mmol�1) is the molecular weight of the dye; mga is theweight of the GA.

7616 | J. Mater. Chem. A, 2013, 1, 7612–7621

3 Results and discussion

Exfoliated GO can be readily dispersed in water with mildultrasonic treatment to form a transparent suspension that isstable for several months (see Fig. S1A†). According to the TEMimage and the atomic force microscopy (AFM) photo (seeFig. S1B and C†), the GO nanosheets consist of one to severallayers. Cu nanoparticles were prepared in advance and the TEMimages and XRD pattern of Cu nanoparticles are shown inFig. S2 and S3.†

When a mixture of 25 mL of GO suspension (2 mgmL�1) andCu nanoparticles (0.1 g) was heated without stirring at 95 �C, ablack monolith of reduced GO (GH) was formed and it oated tothe top of the vial aer 0.5 h as shown in Fig. 1A. As time pro-gressed, the black monolith began to shrink and nally aer24 h, a well-dened black column-shaped hydrogel sank to thebottom of the vial. This indicates that the hydrophobic rGOnanosheets assembled into a 3D macroscopic structure whichoccurs through p–p interactions.

The prepared GH contained about 92.2 wt% water which wasremoved via a freeze-drying method to obtain the correspond-ing aerogel. The ultralight aerogel exhibited uniformly inter-connected pores with sizes of several micrometers (Fig. 1B), anda large number of enwrapped particles were found in themagnied SEM image (Fig. 1C). Fig. S4† shows the EDS analysisof the square area in Fig. 1C. The aerogel contained carbon(75.49%), oxygen (4.86%) and copper (19.65%). The percentageof copper atom is extraordinarily high compared with theamount of oxygen. This indicates that the particles arecomposed of Cu and Cu2O nanoparticles, and that they areanchored onto the graphene sheets (Fig. 1D). The high-resolu-tion TEM image in Fig. 1E clearly indicates a crystal lattice ofcopper, however, it is difficult to distinguish Cu from Cu2Osince the interplanar spacing corresponds to the (111) reectionof Cu which is close to the spacing of the (200) reectionof Cu2O.

However, the XRD diagram of the GA shows the existence ofboth Cu and Cu2O (Fig. 1F), and the Raman spectrum showsthat GO was reduced to rGO (Fig. 1G). So the redox reactionbetween the Cu nanoparticles and GO produced Cu2O nano-particles and RGO. The unreacted Cu and Cu2O nanoparticlesare enwrapped in the GH and both of them should play animportant role in catalytic reactions.

Next the effect of the initial amount of Cu nanoparticles andthe GO concentration on the structure of the GH was investi-gated. First, the impact of the initial amount of Cu nano-particles was investigated using 2 mg mL�1 as the initial GOconcentration. A series of GHs (Fig. 2A) and corresponding GAs(Fig. 2B) (conditions are listed in Table 1) were prepared byadding different amounts of Cu nanoparticles (from 0.01 to0.1 g). They all formed black hydrogels aer heating for 24 hwithout stirring. Aer freeze-drying, GHCu-2 and GHCu-3 formedstable GACu-2 and GACu-3 even though they shrank slightly;however, GACu-1 collapsed upon freeze drying (Fig. 2B). Thisinstability may be attributed to the large pore sizes and theincomplete reduction of GO. The water content of GHCu-1 was98.86% (Table 1) which is the highest among the three GHs; in

This journal is ª The Royal Society of Chemistry 2013

Fig. 5 (A) The structure of methyl orange molecule; (B) UV-vis spectral changesof MO during UV-degradation. Experimental conditions: MO concentration ¼ 1mmol L�1, catalyst dose ¼ 38 mg, H2O2 concentration ¼ 25 mg L�1, UV light; (C)

This journal is ª The Royal Society of Chemistry 2013

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addition, GACu-1 had the largest pore sizes (about ten microm-eters from its SEM images (Fig. 2C and D)) and the most fragilestructure. Second, the Raman spectra of GACu-1 and GACu-3(Fig. 2I) reveal that the GO was not completely reduced in GACu-1since the D/G (1.09) is lower than that for GACu-3 (1.24). There-fore, the incomplete reduction of GO and the large pore sizes inGACu-1 led to the collapse of the GACu-1. In contrast, the smallpore sizes and complete reduction in GACu-2 and GACu-3 lead totheir stability.

In addition, more particles appear on the graphene sheets asthe initial amount of Cu nanoparticles increased (Fig. 2C–F).Interestingly, some of the particles are enwrapped by thegraphene sheets and deposited on the walls of the pores(Fig. 2E and F).

Next the effect of the initial GO concentration was studied.Fig. 3A shows photos of a series of GHs obtained from GOsuspensions with starting GO concentrations ranging from 0.1to 4 mg mL�1. Clearly, all the GO suspensions successfullyformed macroscopic hydrogels; but the volume of the hydrogelsdecreased as the initial GO concentration decreased. All theGHs transformed to stable GAs with porous structures as shownin Fig. 3B–D, but the pore sizes were different. GAGO-1 had anaverage pore size of about 6 mm and a density of 18.8 mg cm�3,the lowest among the three samples (Table 2). The average poresize of GAGO-2 decreased to about 4 mm but its density increasedto 24.4 mg cm�3. As for GAGO-3, it had thick walls of about 2 mmwhich are due to the stacking of the graphene sheets. GAGO-3

had the highest density of 29.6 mg cm�3, so it was the mostsolid and stable. In addition, the intensity of Cu or/and Cu2Onanoparticles was weakening when increasing the initial GOconcentration from GAGO-1 to GAGO-3 according to their XRDpatterns (Fig. 3E).

A possible formationmechanism for the GHs is illustrated inScheme 1. When the GO suspension was mixed with Cu nano-particles under sonication for about 1 h, the Cu nanoparticlesdeposited on the GO nanosheets and some were oxidized toCu2O by the oxygen-containing functional groups on the GOsurface. Meanwhile, the reduced GO nanosheets with Cu andCu2O nanoparticles self-assembled into 3D GHs. The 3D GHshave many interconnected networks owing to enhancedhydrophobic and p–p interactions between the rGO nano-sheets. The Cu and Cu2O nanoparticles act as spacers whicheffectively prevent the aggregation of the rGO nanosheets anddirect the growth of the 3D structure.24 Therefore, rGO reducedfrom GO by Cu nanoparticles can form stable 3D GHs.

The catalytic reduction of 4-nitrophenol (4-NP) to 4-amino-phenol (4-AP) with an excess amount of NaBH4 has oen beenused as a model reaction to evaluate the catalytic performanceof metal nanoparticles.49–51 Themetal particles start the catalytic

degradation of MO at different conditions. Experimental conditions for (1): MOconcentration¼ 1mmol L�1, catalyst dose¼ 38 mg, H2O2 concentration¼ 25 mgL�1, UV light; (2): MO concentration ¼ 1 mmol L�1, catalyst dose ¼ 38 mg, H2O2

concentration ¼ 25 mg L�1, dark; (3): MO concentration ¼ 1 mmol L�1, nocatalyst, H2O2 concentration ¼ 25 mg L�1, UV light; (D) recycling of GA for thedegradation of MO. Experimental conditions: MO concentration ¼ 1 mmol L�1,catalyst dose ¼ 38 mg, H2O2 concentration ¼ 25 mg L�1.

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reduction by relaying electrons from the donor BH4� to the

acceptor 4-NP. Since the prepared GHs consist of Cu and Cu2Onanoparticles, this reaction was used to investigate the appli-cation of the GHs to the catalytic reduction of nitroaromaticcompounds. In order to establish an energy-saving and envi-ronmentally friendly process, room temperature and distilledwater were chosen, instead of high temperature and organicsolvents, as the reaction conditions.

First, 2.5 mL of 1.54 mmol L�1 4-NP solution and 0.5 mL of0.88 mol L�1 NaBH4 solution were mixed in a 3 mL standardquartz cuvette. The UV-vis absorption spectrum of the mixedsolution had an absorption maximum at 400 nm due to thenitro compound. The reduction of 4-NP to 4-AP can be achievedin the presence of NaBH4 without any catalyst, but requiresabout 430 min.

The GA made from 2 mg mL�1 initial GO concentration and0.05 g initial Cu nanoparticles was used to investigate thisreaction. When GA was added to the reaction system, thereduction reaction rate accelerated as seen in Fig. 4A and thereaction was complete in one minute. The reaction kinetics caneasily be monitored from the time-dependent absorptionspectra, which show the decrease of the 400 nmnitro compoundpeak, and the concomitant development of a new 300 nm 4-APpeak. Since the concentration of NaBH4 is very high comparedwith that of 4-NP, the reactions followpseudo-rst order reactionkinetics.51 Thus, the pseudo-rst order rate constant of the

Fig. 6 (A) Photographs of the absorption of diesel oil by GA over time (Sudan III wstate of absorption after 0, 1 and 4 min, respectively); (B) absorption capacities (sat

7618 | J. Mater. Chem. A, 2013, 1, 7612–7621

reaction can be calculated from the equation, ln(At/A0) ¼ kt,where A0 and At are the absorbance values of 4-NP initially and attime t, respectively. The plot of ln(At/A0) vs. time (t) for the reac-tion is linear in the presence of GH as shown in Fig. 4B. Thecorresponding rate constant, 0.084 s�1, was calculated from theslope of the plot. In order to compare this result with reports inthe literature, the ratio of the rate constantKover the total weightof the catalyst, k¼K/m, was calculated. Theactivity factor k forGAwas 0.084 s�1/(0.022 g)¼ 3.81 s�1 g�1, which is higher than thosepreviously reported (0.35 s�1 g�1 and 2.26 s�1 g�1).52,53

The absorption capacity of GA for 4-NP was investigated andthe result is shown in Fig. S6.† Aer 12 h, 44% of 4-NP wasabsorbed on GA. However, the reduction of 4-NP catalyzed byGA was nished within one minute. Therefore, the disappear-ance of the 400 nm nitro compound peak within one minute isthe result of both the weak absorption of 4-NP on GA and thereduction of 4-NP to 4-AP catalyzed by GA.

The temperature of solution also plays an important role inthe reduction of 4-NP. The reaction time decreases when thetemperature of the reaction is increased. Fig. 4C shows therecycling of GA. The reaction time for the reduction of 4-NP to 4-AP was 3 min aer the catalyst was recycled six times (comparedto one minute for the rst use). Based on the TEM images of theGA aer catalyzing the reduction (Fig. S5†), the increase in thereaction time may be caused by the absorption of 4-AP onto theGA. This decreases the active area of the catalyst.

as added to the oil for better observation) (photographs a, b and c represent theurated) of different oils by GA.

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Table 3 Comparison of the absorption of oils by various materials

SorbentsAbsorptioncapacity/g g�1 Reference

Present study 40 —Butyl rubber 23 57Polypropylene 15.7 57Corn stalk 8 58Pith bagasse 25 59Cotton ber 20 60Spongy graphene 87 61Exfoliated graphite 86 62

Fig. 7 (A) Photographs of dye solutions before and after absorption by GA for 5days; (B) absorption capacities of GA for different dyes after 5 days.

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Wastewater released from textile, painting, leather, printingand photography industries oen contains some recalcitrantpollutants, especially synthetic dyes. Dyes limit the subsequentusage of the water and reduce the efficacy of microbial treat-ment because the dyes may be toxic to microorganisms. Methylorange (MO, Fig. 5A) is oen selected as a model dye because ofits environmental signicance, ease of analysis, and relativelygood solubility in water. Cu2O plays an important role in thephotodegradation of MO.54 The photocatalytic activity of the GAfor the degradation of MO and the effects of different variableson the degradation were studied here. The GA was made from2 mg mL�1 initial GO concentration and 0.05 g initial Cunanoparticles. The maximum absorbance wavelength for MO is465 nm and this wavelength was used for evaluating the pho-todegradation of MO. The photodegradation results are shownin Fig. 5B and show that when the GA was used as the catalyst,the peak decreased steadily with time and nally disappearedaer 12 h, indicating the complete decomposition of MO. Thephotocatalytic degradation of MO was also carried out in theabsence of the GA and in the absence of UV irradiation (Fig. 5C).The results show that both GA and UV irradiation are necessaryfor the degradation of MO.

The ability of the GA to be recycled was also tested and theresults are shown in Fig. 5D. Aer each run, the GA was carefullyseparated from the solution and then soaked in distilled waterfor 24 h (the water was changed every 6 h) to remove any reac-tion remnants in the GA. Then, a fresh MO solution along withH2O2 was added to start the next reaction cycle. In the thirdcycle, the peak disappeared aer 20 h, indicating a decrease inthe catalytic ability of the GA.

Since GAs are very hydrophobic and have a high porosity,they should have excellent absorption capacities for oils anddyes. The GA made from 2 mg mL�1 initial GO concentrationand 0.05 g initial Cu nanoparticles was used to investigate theoil-uptake and dye-uptake performance of GA. The GA has ahigh and quick absorption for diesel oil (Fig. 6A). Once the GAwas placed on the surface of the oil–water mixture, the oil layerimmediately started shrinking and was completely gone aer afew minutes. The excellent absorption capability can beattributed to the porous structure and large hydrophobicsurface area of the reduced graphene and to the abundantamount of inorganic nanoparticles on the graphene sheets,which greatly increases the surface roughness of the grapheneabsorbent.55

This journal is ª The Royal Society of Chemistry 2013

The absorption performance of the GA for different oils wascompared and the results are shown in Fig. 6B. The absorptioncapacitieswere28 to40 times theweight of theGA for thedifferentoils. The differences are related to the density of the oils.56

The absorption ability of GA was better than the polymericand natural absorbents shown in Table 3 due to its hydrophobicproperty and high porosity. However, it is weaker than thespongy graphene and exfoliated graphite (Table 3) probablyowing to the existence of nanoparticles. These results show thatGA is a promising adsorbing material for oil–water separation.

Five kinds of dyes, MO, methyl blue (MB), methylene blue(MEB), rhodamine B (RB) and rhodamine 6G (R6G) were used totest the ability of GA to absorb dyes from water. Fig. 7A showsphotographs of dye solutions before and aer absorption by GAfor 5 days. The p–p interactions between the GA and organicdyes driven the dyes molecules adsorbed into the aerogelsgradually decolorize the dyes solutions as shown in Fig. 7A. Theabsorption capacities of GA for the different dyes ranged from150 mg g�1 (0.40 mmol g�1) to 650 mg g�1 (0.81 mmol g�1)(Fig. 7B) which is superior to those previous reported (0.0113 or

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0.6 mmol g�1).63,64 These results indicate that GAs are promisingabsorbents for the removal of dye pollutants from contami-nated water.

4 Conclusion

In summary, this paper demonstrates a simple one-stepmethodinduced by Cu and Cu2O nanoparticles for fabricating GHs andGAs with interconnected 3D networks. During this process, GOwas gradually reduced to form rGO and at the same time Cunanoparticles were oxidized to form copper(I) oxide. Thecomposition of the GHs can easily be changed by simplyadjusting the initial amount of Cu nanoparticles or theconcentration of GO suspension. A possible mechanism for theformation of the GHs was also provided. The GHs and GAs notonly possess good catalytic performance for the catalyticreduction of 4-nitrophenol and the photocatalytic degradationof MO but also have excellent absorption capacities for theremoval of various oils and dyes from water.

Acknowledgements

This work was supported by the National Science Foundation ofChina (21074089, 21276181 and 51202158).

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