Graphene oxide adsorption enhanced by insitu reduction with sodium hydrosulfite
Journal of Hazardous Materials 203–204 (2012) 101–110
Contents lists available at SciVerse ScienceDirect
Journal of Hazardous
Materials
j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /j h a z m a
t
Graphene oxide adsorption enhanced by in situ reduction with sodium hydrosul?te to remove acridine orange from aqueous solution
Ling Sun,Hongwen Yu,Bunshi Fugetsu ?
Laboratory of Environmental Remediation,Graduate School of Environmental Science,Hokkaido University,Sapporo 060-0810,Japan
a r t i c l e
i n f o
Article history:
Received 14October 2011Received in revised form 24November 2011
Accepted 25November 2011
Available online 9 December 2011
Keywords:
Graphene oxide
Sodium hydrosul?te Acridine orange In situ reduction
Hydrogen bonding interaction
a b s t r a c t
Graphene oxide (GO)is a highly effective adsorbent,and its absorbing capability is further enhanced through its in situ reduction with sodium hydrosul?te as the reductant.Acridine orange is the selected target to eliminate with GO as the adsorbent.Under identical conditions,GO without the in situ reduction showed a maximum adsorption capacity of 1.4g g ?1,and GO with the in situ reduction provided a maxi-mum adsorption capacity of 3.3g g ?1.Sodium hydrosul?te converts carbonyl groups on GO into hydroxyl groups,which function as the key sites for the adsorption enhancement.
? 2011 Elsevier B.V. All rights reserved.
1.Introduction
Adsorption is commonly considered to be a fast and rela-tively cost-effective technology for water treatment [1].It normally occurs via both physical and chemical pathways,like activated carbon,which offers the advantage of its porous structure and surface-modi?ed functionalities.Similarly,graphene,consisting of 2D hexagonal lattices of sp 2carbon atoms covalently bonded,has been theorized to have a huge speci?c surface area (over 2600m 2g ?1)[2,3],leading to its potential in the environmental ?eld as an effective choice for pollutant elimination or environ-mental remediation.Many problems nevertheless remain to be solved before the massive production of graphene for scaled use.However,to date,chemically derived graphene oxide (GO)from graphite has revealed a variety of potential uses because of its ?ex-ibility and relatively cheap fabrication.Like a graphene precursor,a subsequent GO reduction could restore the sp 2carbon structure via various methods such as annealing [4],solvothermal/hydrothermal processes [5–7],or different kinds of reductants [8–15].However,compared to mechanical exfoliation,functionalities (as defects)can be meanwhile imperfectly introduced onto graphene sheet surfaces.On the other hand,an oxygenation/reduction process con-fers on GO features that not only are relevant for applications in electrochemistry,such as hybrid materials [16],ultra-capacitors
?Corresponding author.Tel.:+81117062272;fax:+81117062272.E-mail address:hu@ees.hokudai.ac.jp (B.Fugetsu).
[3,17],transparent/conducting ?lms [10,15,18–21],battery elec-trodes [22],and sensors [23–25],but that also carry potential in the bioengineering arena.Hu et al.[26]reported that graphene-based materials,i.e.,GO or reduced GO paper,showed an excellent inhibitory effect on bacterial growth with mild cytotoxicity.Very recently,Gao et al.[27]developed a surface-modi?ed GO mixed with sand for engineered water puri?cation.
Of note,lipophilicity to some extent hinders graphene from being an ideal water-soluble adsorbent.By contrast,however,GO is rich in as-generated oxygen-containing groups,such as hydroxyl and epoxide (mostly located on the top and bottom surfaces),and carboxyl and carbonyl (mostly at the sheet edges),randomly dis-tributed in the graphene structure [28].Previous work has indicated its application for contaminant removal as a GO/composite.Yang et al.[29]demonstrated methylene blue removal by GO with a large adsorption capacity,as high as 714mg g ?1(better than activated carbon,yet less comparable to teak wood bark,with its capac-ity of 914.6mg g ?1).Yang et al.[30]applied the Cu 2+-inducing GO folding/aggregation for ion elimination from aqueous solution,demonstrating an adsorption capacity about 10times better than that for activated carbon.Other inorganic cations,such as Mg 2+,Ca 2+[30]and some organic cations,such as methylene blue and malachite green [31]were also found to be capable of folding and/or aggregating GO,because of the electrostatic interactions.
In most previous investigations,researchers have elected to prepare ?lm-like GO/composites and to use reduction to improve electrical conductance.In contrast,here we have directly added the reductant to the GO solution and found that GO folded/aggregated
0304-3894/$–see front matter ? 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.jhazmat.2011.11.097
102L.Sun et al./Journal of Hazardous Materials 203–204 (2012) 101–
110
Fig.1.Digital image of GO solution and a typical AFM image of exfoliated GO on mica (left)with a cross-sectional pro?le (right)indicating the thickness and width at about 0.9nm and 6?m,respectively.
similarly to Cu 2+,simultaneously with darkening color.With the large adsorptive capacity of GO,a combination with simultane-ous in situ reduction would be another option for environmental application.Recently,Ai et al.[32]reported the removal of methy-lene blue with Fe 3O 4/reduced GO using solvothermal treatment.That work was quite different from the current work in that their simultaneous reduction was performed at the stage of
materials
Fig.2.Images of the GO reduction process with different reductants.Left:l -ascorbic acid;middle:sodium hydrosul?te;right:sodium borohydride.
preparation while here we performed GO reduction during adsorp-tion.Thereafter,we performed competitive comparisons indicating that sodium hydrosul?te [15]was a highly effective reductant as well as being relatively environmentally friendly,so it was chosen as the reductant.We selected acridine orange (AO)as a target contaminant because it is cell permeable and capable of interacting with DNA and RNA.Adsorption tests were conducted with results clearly demonstrating that with the described treat-ment,the adsorption capacity and the removal ef?ciency were dramatically increased compared to as-prepared GO and previous reports.Model ?tting of the isotherms indicated that the adsorption implied a Langmuir monolayer behavior.The maximum adsorp-tion reached an unprecedented 3.3g g ?1,compared to as-prepared GO (1.4g g ?1).We also discussed a possible mechanism for the enhancement,combined with various characterizations,inferring that carbonyl group reduction by sodium hydrosul?te was the key to the relatively increased adsorption.
2.Materials and methods
2.1.Materials and reagents
Puri?ed natural graphite was purchased from Bay Carbon,Inc.,Michigan,USA.Other chemicals unless noted were from Wako Pure Chemical Industries,Ltd.,or Sigma–Aldrich Inc.,Japan.
2.2.Preparation of GO
GO was obtained following a modi?ed Hummers–Offeman method [15,33].Brie?y,graphite (2.0g),sodium nitrate (1.0g),con-centrated sulfuric acid (98%,50mL),and potassium permanganate (6.0g)were consistently mixed in an ice bath for 2h,with the mixture gradually becoming pasty and black-greenish.Next,the mixture was placed in a 35?C water bath and kept at that temper-ature for 30min,followed by the slow addition of distilled water (100mL)to keep the solution from effervescing;the resulting solu-tion was placed at well below 100?C for 3h.With progression of the reaction,the color turned a little yellowish.After further treatment with H 2O 2(5%,100mL),the ?ltered cake was washed with dis-tilled water several times until its supernatant was without SO 42?,as tested using barium chloride (0.1mM).Then the cake was dis-persed in water for further ultra-sonication for 1h.Of note,for facilitating the following experiments,we selected solutions with GO centrifuged at speeds ranging from 2000to 5000rpm,with a concentration and pH value of approximately 0.48%and 3.1,respec-tively.
L.Sun et al./Journal of Hazardous Materials 203–204 (2012) 101–110
103
Fig.3.A series of optical microscopy images showing the aqueous reduction process of GO by the additive Na 2S 2O 4.(A)GO dispersion containing large pieces of GO sheets (scale bar:30?m).(B)Color change of GO the instant Na 2S 2O 4was added.(C)GO aggregated and precipitated after 24h (and inset).
2.3.Adsorption procedure
Time-dependent adsorption tests were conducted in batch mode.Experiments without pH adjustment were carried out at room temperature in tubes containing 50-mL AO at 0.1g L ?1and 1.0mL GO that was either reduced by sodium hydrosul?te (0.2g mL ?1)added at the 15th minute during the adsorption (GO with such in situ reduction,designated as SRGO)or pre-reduced overnight at room temperature (designated as LRGO),respectively.The control (CRTL)groups containing AO and sodium hydrosul?te were set for comparison.The adsorption reaction tubes were agi-tated at 300rpm for 180min to achieve equilibrium.Furthermore,we developed another SRGO control in which GO was pre-reduced for 3h by sodium hydrosul?te,similarly to LRGO (designated as SGRO ).
Concentration-dependent experiments were also carried out at room temperature with different starting AO concentrations.AO was prepared with 1.0g L ?1,and then serially diluted by 10,20,50,80,100,and 200times,designated as ×10,×20,×50,×80,×100,and ×200,respectively.These experiments were performed
with
Fig.4.Tyndall scattering effect of GO (A)and reduced GO (B)with a beam of visible light passing through.(C)GO evenly dispersed in water.(D)Reduced GO in oil phase (hexane).
only SRGO and GO.Both the initial and equilibrium concentrations were recorded for further analysis.
All samples (each 1.5mL)were centrifuged at 14,000rpm for 10min.The supernatant was measured using a UV–Vis spectropho-tometer (JASCO V-570spectrophotometer)at a wavelength of 492nm corresponding to the standard curve.All experiments were performed in triplicate,and data are presented as mean ±SD (stan-dard deviation).The removal ef?ciency and adsorption capacity were calculated with the following equations:
Removal ef?ciency =
(C 0?C e )
C 0
×100%
(1)Adsorption capacity =
(C 0?C e )×V
m
(2)
where C 0(g L ?1)and C e (g L ?1)are the initial concentration and equilibrium concentration for AO,respectively,and V (mL)and m (g)indicate the total volume and mass value of adsorbents used,respectively.
2.4.Characterizations
GO-based samples were characterized using various tech-niques such as atomic force microscopy (AFM,Agilent series 5500AFM instrument using the tapping mode at a scanning rate of 0.5Hz),Raman spectroscopy (Raman,an inVia Raman microscope,Renishaw,with an excitation wavelength at 532nm),Fourier trans-form infrared spectroscopy (FTIR,FT/IR-6100FT-IR
Spectrometer,
Fig.5.UV–vis scanning spectra of AO solution mixed with Na 2S 2O 4.
104L.Sun et al./Journal of Hazardous Materials 203–204 (2012) 101–
110
Fig.6.Digital images of CTRL,GO,SRGO,and LRGO after adsorptions.
JASCO),scanning electronic microscopy (SEM,JSM-6300,JOEL,with acceleration voltage of 30kV),X-ray photoelectron spectroscopy (XPS,JPC-9010MC,JOEL,using Mg K ?radiation,1253.6eV with vac-uum better than 1×10?7Torr),and thermo-gravity analysis (TGA,TG/DTA 6200,SII Exstar6000,with a temperature increment of 5?C/min).
3.Results and discussion
3.1.Graphene oxide and reductants
As mentioned in the experimental section,a modi?ed Hum-mers method [15]was employed to oxidize and exfoliate graphite into GO.Diluted GO was coated onto freshly exfoliated mica to dry in ambient atmosphere for AFM measurement.As shown in Fig.1,overlapped single layered sheets are clearly identi?able with a thickness of about 0.9nm.The heavy oxidative process grafted oxygen-containing groups onto the graphene planar sur-face,enabling development of hydrophilic yet thickened GO sheets (over 0.37nm [34]).As noted,we especially chose sheets cen-trifuged between 2000and 5000rpm so that large-area GO were easily obtained (Fig.1).In contrast to smaller pieces,larger sheets could be more easily precipitated because of their heavier molecu-lar weight [35,36]when undergoing centrifugation,yielding a clean supernatant for sampling.
Considering conventional environmental constraints,we opted for reductants that were highly effective at room temperature.l -ascorbic acid,also known as vitamin C [9],sodium borohydride [37],and sodium hydrosul?te [15]were selected as candidates because of reducing effects comparable to hydrazine [38].Fig.2shows the results of simple tests of these reductants at room temperature.The reaction between sodium borohydride and water produced a
soda
Fig.7.Time-dependent adsorption of AO (100mg L ?1)with GO,SRGO,and LRGO (additive amount of GO:1mL)compared to AO in reducing solution (CTRL),at room temperature.Note:for SRGO,adsorption was performed with addition of the reduc-tant at the 15th minute (removal ef?ciency about 95%);for LRGO,GO was pre-treated with reductant overnight at room temperature with removal ef?ciency of 76%;for GO,removal ef?ciency was about 67%.
water-like effervescence.This hydrolysis of sodium borohydride resulted in decreased reduction ef?ciency [8].For sodium hydro-sul?te,however,a rapid darkening that appeared with its addition demonstrated its high effectiveness.Instantly,GO pieces became more visible and evenly distributed in the sample.As the reaction progressed over 20min,a clear residual liquid was observed.Mean-while,the GO sheets aggregated and precipitated at/around the cell wall.There was less change for sodium borohydride and no observ-able change for l -ascorbic acid,indicating that these reductants would not suit the goals of our applications.
To con?rm the reduction with sodium hydrosul?te,analyses with optical microscopy and of the Tyndall scattering effect were conducted.Fig.3illustrates the morphological changes in GO dur-ing the reduction,and Fig.4provides an indirect representation of the varying GO sizes and hydrophilicity changes.Fewer lay-ered graphene sheets in the GO solution were optically identi?ed because of a rather weak contrast with the surroundings [39].However,multilayered sheets with some crumples/ripples were occasionally found.The results with the Tyndall effect analysis indicated that GO had a colloidal dispersion.Likewise,the added reductant was observed to darken the sheet color.The change made sheet boundaries clearer,even the fewer-layered sheets (Fig.3B).Without much delay,head-to-end branch-like GO sheets inter-twined/aggregated (Fig.3C).The formation of large aggregates ?nally dramatically weakened the Tyndall effect (Fig.4B).Fig.4C and D shows the undoubted hydrophilicity transformation from the reduction,resulting from a decrease in hydrophilic groups on the planar surface and the formation of water-phobic structures at room temperature [28].
Thus,sodium hydrosul?te is inferred to be a highly effective reagent for GO reduction.Furthermore,hydrosul?te is less toxic,less corrosive and,highly environmentally friendly than some other options,e.g.,hydrazine [38]and hydroxylamine hydrochlo-ride [13].Previous studies have suggested possible mechanisms for the effects of sulfur-containing compounds [8,9,15].We
con?rmed
Fig.8.Time-dependent adsorptions with SRGO and SRGO (additive amount of GO:0.5mL).SRGO :GO was pre-reduced for 3h.
L.Sun et al./Journal of Hazardous Materials203–204 (2012) 101–110
105
Fig.9.Adsorption isotherms of GO(A)and SRGO(D),and their results?tted by the Langmuir model(B and E)and the Freundlich model(C and F). that the products contained SO42?using the BaCl2method that
indicates a positive based on a white precipitate,with?ndings con-
sistent with those of Chen et al.[8].A possible mechanism involving
sodium hydrosul?te is shown as follows,with some modi?cations
of the mechanisms described by Zhou et al.[15]:
2S2O42?+H2O→S2O32?+2HSO3?
(3)
(4)
3.2.Adsorption of GO-based materials
Adsorptions were tested using the GO-based materials.Fig.5 shows the adsorption spectra of the control group(CRTL),which contained only AO and sodium hydrosul?te.The absorbance was recorded at492nm,indicating that the reductant did not in?u-ence AO[40].Unless noted speci?cally,subsequent experiments were carried out using this protocol.A set of images shows the time-dependent adsorption results for variously treated GO sam-ples with a starting AO concentration of100mg g?1(Figs.6and7). As seen,the discoloration of varying degrees indicates the diverse capacities of adsorbents.Clearly,SRGO had a better decoloration effect than the other samples.A rapid decrease appeared in the ini-tial10min and then reached the adsorption equilibrium.Based on CTRL,a loss of AO of less than3%indicated no signi?cant in?uence of reductants over AO,ensuring the reliability of the adsorption results.
Regarding SRGO,a further decrease occurred to AO once sodium hydrosul?te was added.It increased the removal ef?ciency to about95%,the highest of all samples(GO,67%;LRGO,76%).This result indicates the generation of more available sites during the in situ reduction.However the?-stacking effect counted very little
106L.Sun et al./Journal of Hazardous Materials 203–204 (2012) 101–
110
Fig.10.SEM images of GO (upper)and reduced GO
(lower).
Fig.11.Histogram of I D /I G ratios from Raman spectra of GO,SRGO,and LRGO.
toward adsorption (from graphite,about 3.0mg g ?1
with an initial
AO concentration of 100mg g ?1).In other words,newly induced
functional groups have functioned for AO sorption.
The adsorption capacities were calculated to be 679.5mg g ?1,976.8mg g ?1,and 781.1mg g ?1for GO,SRGO,and LRGO,respec-tively.Previous work found that methylene blue had a capacity of up to 714mg g ?1[29].Thus,GO de?nitely has proved to have great potential as an adsorbent.In addition,SRGO performed much better than teak wood bark and activated carbon [29].It is worth noting that a longer pre-reduction period lessened the adsorption capac-ity,implying that only a suitable reduction degree would maximize the capacity.
Table 1
The ?tting parameters for AO adsorption on GO/SRGO.
Langmuir isotherm
Freundlich isotherm
K L (L mg ?1)
q m (mg g ?1)R 2K F n
R 2
GO 17.514280.99324011 1.590.9531SRGO
6
3333
0.9959
2906
1.95
0.8818
Other tests were conducted for SRGO and SRGO to con?rm the superiority of the in situ method.Fig.8illustrates that SRGO contin-ued to exhibit a better removal performance as well as adsorption capacity in contrast to SRGO ,a consistent ?nding even though SRGO and SRGO both underwent a 3-h reduction.Analyzing the details for the adsorption progress,given that the reduction degree for GO was no different,required examination of the morphological change in GO sheets.In fact,the pre-reduction partially varied the surface hydrophilicity and resulted in the aggregation of graphene sheets (as similarly reported by Stankovich et al.[38]).In addi-tion,the dissolved Na +and other ions may have produced a salt effect [36].As a result,inadequate exfoliation and inaccessible sur-faces would appear and induce the adsorption capacity decrease for SRGO [41].This scenario could also apply for LRGO.
Here we stress that the amount of the additive GO was halved for SRGO.In other words,GO was diluted twice as much.Nevertheless,its capacity increased to 1382.0mg g ?1,compared to 976mg g ?1,though the removal ef?ciency decreased to 64%(compared to the previous 95%).
The AO adsorption isotherms on GO and SRGO were then deter-mined as presented in Fig.9.Fig.9A and D demonstrates a positive functional relationship between the adsorption capacity and the equilibrium concentration.The as-obtained capacities were found to be 1255mg g ?1for GO and 2158mg g ?1for SRGO,respectively.For further understanding the process,Langmuir and Freundlich models were introduced to evaluate the adsorption data.As seen,the equilibrium concentration (C e ,g L ?1)and equilibrium adsorp-tion capacity values (q e ,mg g ?1)were ?tted by their linearized expressions [29–32,42–44].Accordingly,the maximum adsorption capacity for GO and SRGO were calculated by the Langmuir equa-tion,as follows:
Langmuir :
1q e =1q m +1K L q m C e
(5)Freundlich :ln q e =ln K F +
1
n
ln C e (6)
where,K L is the Langmuir adsorption constant related to the energy of adsorption (L mg ?1);K F is the Freundlich constant (L mg ?1);1/n is the heterogeneity factor;and q m is the Langmuir monolayer adsorption capacity (mg g ?1).
Fig.9and Table 1show the ?tting results.The co-ef?ciency cal-culated based on the Langmuir model were found to be better than 0.99for both GO and SRGO;while the co-ef?ciency calculated based on the Freundlich model were 0.95for GO and 0.88for SRGO.The maximum capacities (q m value)were found to be 1.4g g ?1for GO and 3.3g g ?1for SRGO,respectively.
GO showed a high af?nity for positively charged molecules/ions because of oxygen atoms [29–32,45].AO is positively charged in nature;accordingly,the electrostatic interaction is the primary binding strength [29].Because all adsorptions were conducted in weakly acidic conditions (pH 4–5)without pH adjustment,the main interaction between GO and AO relates to the formation of hydro-gen bonds [43].
C OH(GO)+N C(AO)→
C O NH C
(7)
In our work,the adsorption capacity generally negatively relates to the GO additive amount (e.g.,for GO,from about 700to 900mg g ?1).We consider the dilution be an important factor that
L.Sun et al./Journal of Hazardous Materials203–204 (2012) 101–110
107
Fig.12.XPS general spectra and curve?t of C1s spectra of GO,SRGO,and LRGO.
promotes fully exfoliated GO sheets in the aqueous environment and more access to functional groups as the oxygen functional groups contribute more to adsorption[43,45,46].The SRGO per-formance involved not only the in?uence of the dilution but also the functionality changes resulting from the in situ reduction.
3.3.Characterizations of GO-based adsorbents and a possible mechanism for the enhancement
It is impressive that SRGO showed the far better adsorption. To uncover more about the mechanism involved,characterizations using SEM,XPS,and FTIR were applied to compare the structures and morphologies of GO before and after reduction.The electric resistivity was measured by a four-probe method as one effec-tive indicator of the extent to which GO was reduced.The GO ?lm obtained by?ltration was completely insulating because of the destroyed sp2-carbon network.By contrast,a simple reduc-tion at room temperature could decrease the resistivity to about 104 sq?1(for SRGO and LRGO).This?nding indicates partial recovery of GO.Meanwhile,it is obvious that without stricter con-ditions[7,13,15,46,47](e.g.,higher temperature),the reduction is insuf?cient to maximally improve the conductance.
Fig.10shows the images of GO/reduced GO by SEM,with direct drying at80?C.Reference to the scale shows the presence of large GO sheets.More important,for GO,highly ordered struc-tures formed as a result of the semi-ordered accumulation[47] whenever the evaporation/?ltration progressed.The interactions of the interlayered hydrogen bonds were also important for facil-itating GO sheet adherence to the hydrophilic surfaces[48].The reduced GO morphologically looks quite different in the image,hav-ing become less hydrophilic by reduction.Furthermore,the loss of hydrogen bond donators decreased the interlayered interactions [28]so that the structures became less compact and more random and crumpled.
Raman spectroscopy is widely used to characterize the struc-tural and electronic properties of GO.It usually involves two main features[12]:a D peak at～1349cm?1and a G peak at～1597cm?1, arising from the?rst-order scattering of the E2g phonon of sp2C atoms and a breathing mode of k-point photons of A1g symmetry, respectively.Their intensity ratio(I D/I G)indicates the degree of the disorder,such as defects,ripples,and edges[49].Fig.11re?ects our measurement of the reduction degree of each sample at sev-eral spots.As shown,the ratios gradually decrease from2.17to 1.82.This change could be explained by the average size of the sp2 domain increasing without the massive creation of new smaller graphitic domains[38].Coupled with the negligible shifts of the prominent peaks(not shown),room temperature surely is not the best parameter for conductivity restoration by sodium hydrosul-?te,which is consistent with the conclusion based on the electric method.Such reduction,however,may be favorable to the adsorp-tion.
We also employed XPS to analyze GO before and after reduction to observe the evolution of oxygen functional groups,as shown in Fig.12.The reduction degree of GO is described by taking the ratio of C1s to O1s peak areas.As shown in Fig.13A,the C/O ratio increased from2.67(GO),to3.26(SRGO),and then to4.09(LRGO); the prominence of the carbon peak?nally replaced the oxygen.It is true that GO underwent a deoxygenation at room temperature. Additionally,the four peaks centered at around284.4,286.7,288.5, and290.5eV are assigned to the C(C H/C C),C O(C OH/C O C), C O,and COO groups,respectively(Fig.13B–D).GO is by nature insulating and thus exhibits the binding energy redshift in C1s spec-tra(about+0.5eV here),whereas it does not in?uence the following analysis.As seen in the?gure,the carbon proportion rises from about5%to48%from GO to LRGO.More speci?cally,the intensity
108L.Sun et al./Journal of Hazardous Materials 203–204 (2012) 101–
110
Fig.13.Fourier transform infrared spectra (vs.absorbance)of as-prepared GO,SRGO,and LRGO.
of the C O peak exhibits an increase followed by a decrease,from 48%to 69%and then to 41%;at the same time,the C O dramatically decreases after the 3-h reduction (SRGO)from 39%to 5%.It is clear from the ?gure that the C O still occupies a large proportion over the long reduction process (LRGO),which indicates that the room temperature in situ reduction method could not completely remove the oxygen-containing groups.As the ?gure illustrates,with the dif-ferences between the C O and C O proportions,it can be inferred that the adsorption enhancement is probably related to the C O (carbonyl group).In other words,the increased amount of hydroxyl groups probably results from the reduction of carbonyl groups.
For further evidence,FTIR was also employed,as shown in Fig.13.The carbonyl groups are ascribed to the band at 1720cm ?1.The bands at around 1053,1089,and 1233cm ?1are ascribed to the vibration of C O;the bands at about 2921and 2855cm ?1are ascribed to the vibration of C H.Coupled with XPS ?ndings,a com-mon change is notable before and after the reduction.The intensity of C O is markedly weakened as the C O/C H becomes much stronger after reduction.Meanwhile,the C C peak (at 1625cm ?1)shifted to 1610cm ?1,possibly because of the sp 2structure restora-tion.On the spectrum of LRGO in the ?gure,the C C peak is rather weak in contrast to that of the C O (at 1089cm ?1)and C H.This result demonstrates that the reduction could not restore sp 2struc-tures to the maximum possible when done at room
temperature
Fig.14.Thermo-gravimetric analysis for GO,SRGO,and LRGO.
with sodium hydrosul?te.This result is in agreement with the above conclusions.
As seen in Fig.14,TG-A was also carried out under N 2atmo-sphere.Both LRGO (69.3%)and SRGO (45.5%)are much more stable than GO (36.5%),as deduced by their residues.In general,mass loss throughout the procedure normally consists of two major steps,the small amount of inter-lamellar water evaporation as the ini-tial weight loss and the functional group decomposition into gases between the intercalated layers [8].The former could be used to explain the little peak on the TG-A curve at about 220?C (not shown in Fig.13)[50],but it was not found in SRGO and LRGO.This dis-tinction implies that the ordered layered structures diminished and were replaced by non-compact structures,as seen in the SEM image of the reduced GO.Of interest,there was a considerable mass loss at 810?C for SRGO in the combustion curve.It is inferred that stable oxygen-containing groups would have formed,such as hydroxyl groups.
Based on the various characterizations,the carbonyl group turned out to be the origin of hydroxyl groups after the reduc-tion.Sodium hydrosul?te is known to reduce carbonyl groups,as reported by Devries and Kellogg [51].The reaction here at room temperature is described as
follows:
(8)
Fig.15.Possible mechanism for the AO sorption enhancement.
L.Sun et al./Journal of Hazardous Materials203–204 (2012) 101–110109
The carbonyl is reduced by the hydrosul?te,resulting in the for-mation of C OH and C H bonds.Thus,the reduction/decrease of carbonyl groups increases the hydroxyl and hydrocarbon bonds.As the reduction progresses,the dye molecules are absorbed via the hydrogen bond,as described above(Fig.15).This inference pro-vides a good basis for explaining the change in the characterizations and additional capacity of SRGO.
4.Conclusions
We have developed an enhancement method involving simul-taneous in situ reduction of GO by Na2S2O4,using AO adsorption on GO.An adsorption capacity of GO as high as2158mg g?1was obtained by this means,proving to be the best among other treat-ments in our work.Meanwhile,the as-prepared GO also effectively removed AO in an aqueous environment.The adsorption capac-ity of GO could reach up to1255mg g?1with the mass fraction of GO at about0.048‰.As the GO concentration was increased up to two fold,the removal ef?ciency rose from64%to95%,but the capacity decreased to976mg g?1.The Langmuir model?tted the adsorption data better than the Freundlich model,indicating a maximum capacity as high as3.3g g?1for the in situ method,and 1.4g g?1for GO.Both the adsorptions behave in a monolayer man-ner,and the exfoliation degree of GO is important for its capacity. We also investigated the mechanism involved in the enhancement. With various characterizations using XPS,FTIR,Raman,and TGA,we found that the reduction of carbonyl groups resulted in hydroxyl groups,which played an important role in the development of the extra-large capacity from our method.This phenomenon may open up a route for environmental applications using GO as an effective adsorbent.
References
[1]I.Ali,V.K.Gupta,Advances in water treatment by adsorption technology,Nat.
Protoc.1(2006)2661–2667.
[2]S.Stankovich,D.A.Dikin,G.H.B.Dommett,K.M.Kohlhaas,E.J.Zimney,E.A.Stach,
R.D.Piner,S.T.Nguyen,R.S.Ruoff,Graphene-based composite materials,Nature 442(2006)282–286.
[3]M.D.Stoller,S.J.Park,Y.W.Zhu,J.H.An,R.S.Ruoff,Graphene-based ultracapac-
itors,Nano Lett.8(2008)3498–3502.
[4]C.D.Zangmeister,Preparation and evaluation of graphite oxide reduced at
220?C,Chem.Mater.22(2010)5625–5629.
[5]S.Dubin,S.Gilje,K.Wang,V.C.Tung,K.Cha,A.S.Hall,J.Farrar,R.Varshneya,
Y.Yang,R.B.Kaner,A one-step,solvothermal reduction method for producing reduced graphene oxide dispersions in organic solvents,ACS Nano4(2010) 3845–3852.
[6]D.Long,W.Li,L.Ling,J.Miyawaki,I.Mochida,S.H.Yoon,Preparation of
nitrogen-doped graphene sheets by a combined chemical and hydrothermal reduction of graphene oxide,Langmuir26(2010)16096–16102.
[7]https://www.sodocs.net/doc/2a3095868.html,pton,B.Jain,D.A.Dikin,A.Abouimrane,K.Amine,S.T.Nguyen,Chemi-
cally active reduced graphene oxide with tunable C/O ratios,ACS Nano5(2011) 4380–4391.
[8]W.F.Chen,L.F.Yan,P.R.Bangal,Chemical reduction of graphene oxide to
graphene by sulfur-containing compounds,J.Phys.Chem.C114(2010) 19885–19890.
[9]J.Gao,F.Liu,Y.L.Liu,N.Ma,Z.Q.Wang,X.Zhang,Environment-friendly method
to produce graphene that employs vitamin C and amino acid,Chem.Mater.22 (2010)2213–2218.
[10]S.F.Pei,J.P.Zhao,J.H.Du,W.C.Ren,H.M.Cheng,Direct reduction of graphene
oxide?lms into highly conductive and?exible graphene?lms by hydrohalic acids,Carbon48(2010)4466–4474.
[11]X.F.Gao,J.Jang,S.Nagase,Hydrazine and thermal reduction of graphene oxide:
reaction mechanisms,product structures,and reaction design,J.Phys.Chem.C 114(2010)832–842.
[12]C.Zhu,S.Guo,Y.Fang,S.Dong,Reducing sugar:new functional molecules for
the green synthesis of graphene nanosheets,ACS Nano4(2010)2429–2437.
[13]S.Mao,K.Yu,S.Cui,Z.Bo,G.Lu,J.Chen,A new reducing agent to prepare single-
layer,high-quality reduced graphene oxide for device applications,Nanoscale 3(2011)2849–2853.
[14]T.A Pham,J.S.Kim,J.S.Kim,Y.T.Jeong,One-step reduction of graphene oxide
with l-glutathione,Colloids Surf.A384(2011)543–548.
[15]T.N.Zhou,F.Chen,K.Liu,H.Deng,Q.Zhang,J.W.Feng,Q.A.Fu,A simple and ef?-
cient method to prepare graphene by reduction of graphite oxide with sodium hydrosul?te,Nanotechnology22(2011)045704.[16]J.C.Liu,Y.J.Wang,L.Liu, D.D.Sun,High-quality reduced graphene
oxide-nanocrystalline platinum hybrid materials prepared by simultaneous co-reduction of graphene oxide and chloroplatinic acid,Nanoscale Res.Lett.
6(2011)241.
[17]C.Liu,Z.Yu,D.Neff,A.Zhamu,B.Z.Jang,Graphene-based supercapacitor with
an ultrahigh energy density,Nano Lett.10(2010)4863–4868.
[18]M.Zhou,Y.L.Wang,Y.M.Zhai,J.F.Zhai,W.Ren,F.A.Wang,S.J.Dong,Controlled
synthesis of large-area and patterned electrochemically reduced graphene oxide?lms,Chem.Eur.J.15(2009)6116–6120.
[19]B.Li,X.T.Zhang,X.H.Li,L.Wang,R.Y.Han,B.B.Liu,W.T.Zheng,X.L.Li,Y.C.
Liu,Photo-assisted preparation and patterning of large-area reduced graphene oxide-TiO2conductive thin?lm,https://www.sodocs.net/doc/2a3095868.html,mun.46(2010)3499–3501. [20]Y.W.Zhu,W.W.Cai,R.D.Piner,A.Velamakanni,R.S.Ruoff,Transparent self-
assembled?lms of reduced graphene oxide platelets,Appl.Phys.Lett.95(2009) 103104.
[21]J.Zhao,S.Pei,W.Ren,L.Gao,H.M.Cheng,Ef?cient preparation of large-area
graphene oxide sheets for transparent conductive?lms,ACS Nano4(2010) 5245–5252.
[22]X.Xiao,P.Liu,J.S.Wang,M.W.Verbrugge,M.P.Balogh,Vertically aligned
graphene electrode for lithium ion battery with high rate capability,https://www.sodocs.net/doc/2a3095868.html,mun.13(2010)209–212.
[23]H.Q.Luo,W.T.Huang,Y.Shi,W.Y.Xie,N.B.Li,A reversible?uorescence
nanoswitch based on bifunctional reduced graphene oxide:use for detec-tion of Hg(2+)and molecular logic gate operation,https://www.sodocs.net/doc/2a3095868.html,mun.47(2011) 7800–7802.
[24]J.T.Robinson,F.K.Perkins,E.S.Snow,Z.Q.Wei,P.E.Sheehan,Reduced graphene
oxide molecular sensors,Nano Lett.8(2008)3137–3140.
[25]Y.H.Lin,Y.Y.Shao,J.Wang,H.Wu,J.Liu,I.A.Aksay,Graphene based elec-
trochemical sensors and biosensors:a review,Electroanalysis22(2010) 1027–1036.
[26]W.Hu,C.Peng,W.Luo,M.Lv,X.Li,D.Li,Q.Huang,C.Fan,Graphene-based
antibacterial paper,ACS Nano4(2010)4317–4323.
[27]W.Gao,M.Majumder,L.B.Alemany,T.N.Narayanan,M.A.Ibarra,B.K.Prad-
han,P.M.Ajayan,Engineered graphite oxide materials for application in water puri?cation,ACS Appl.Mater.Interfaces3(2011)1821–1826.
[28]S.Stankovich,R.D.Piner,S.T.Nguyen,R.S.Ruoff,Synthesis and exfolia-
tion of isocyanate-treated graphene oxide nanoplatelets,Carbon44(2006) 3342–3347.
[29]S.T.Yang,S.Chen,Y.Chang,A.Cao,Y.Liu,H.Wang,Removal of methylene blue
from aqueous solution by graphene oxide,J.Colloid Interface Sci.359(2011) 24–29.
[30]S.T.Yang,Y.Chang,H.Wang,G.Liu,S.Chen,Y.Wang,Y.Liu,A.Cao,Fold-
ing/aggregation of graphene oxide and its application in Cu2+removal,J.Colloid Interface Sci.351(2010)122–127.
[31]P.Bradder,S.K.Ling,S.B.Wang,S.M.Liu,Dye adsorption on layered graphite
oxide,J.Chem.Eng.Data56(2011)138–141.
[32]L.Ai,C.Zhang,Z.Chen,Removal of methylene blue from aqueous solution by
a solvothermal-synthesized graphene/magnetite composite,J.Hazard.Mater.
192(2011)1515–1524.
[33]W.S.Hummers,R.E.Offeman,Preparation of graphitic oxide,J.Am.Chem.Soc.
80(1958),1339–1339.
[34]M.Inagaki,Y.A.Kim,M.Endo,Graphene:preparation and structural perfection,
J.Mater.Chem.21(2011)3280–3294.
[35]B.Fugetsu,E.Sano,H.W.Yu,K.Mori,T.Tanaka,Graphene oxide as dyestuffs for
the creation of electrically conductive fabrics,Carbon48(2010)3340–3345.
[36]D.Li,M.B.Muller,S.Gilje,R.B.Kaner,G.G.Wallace,Processable aqueous dis-
persions of graphene nanosheets,Nat.Nanotechnol.3(2008)101–105. [37]H.J.Shin,K.K.Kim,A.Benayad,S.M.Yoon,H.K.Park,I.S.Jung,M.H.Jin,H.K.Jeong,
J.M.Kim,J.Y.Choi,Y.H.Lee,Ef?cient reduction of graphite oxide by sodium borohydrilde and its effect on electrical conductance,Adv.Funct.Mater.19 (2009)1987–1992.
[38]S.Stankovich,D.A.Dikin,R.D.Piner,K.A.Kohlhaas,A.Kleinhammes,Y.Jia,Y.Wu,
S.T.Nguyen,R.S.Ruoff,Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide,Carbon45(2007)1558–1565.
[39]P.Blake,E.W.Hill,https://www.sodocs.net/doc/2a3095868.html,o,K.S.Novoselov,D.Jiang,R.Yang,T.J.Booth,A.K.
Geim,Making graphene visible,Appl.Phys.Lett.91(2007)063124.
[40]H.W.Yu,B.Fugetsu,A novel adsorbent obtained by inserting carbon nanotubes
into cavities of diatomite and applications for organic dye elimination from contaminated water,J.Hazard.Mater.177(2010)138–145.
[41]D.R.Dreyer,S.Park,C.W.Bielawski,R.S.Ruoff,The chemistry of graphene oxide,
Chem.Soc.Rev.39(2010)228–240.
[42]Z.J.Fan,W.Kai,J.Yan,T.Wei,L.J.Zhi,J.Feng,Y.M.Ren,L.P.Song,F.Wei,Facile
synthesis of graphene nanosheets via Fe reduction of exfoliated graphite oxide, ACS Nano5(2011)191–198.
[43]T.Hartono,S.B.Wang,Q.Ma,Z.H.Zhu,Layer structured graphite oxide as a novel
adsorbent for humic acid removal from aqueous solution,J.Colloids Interface Sci.333(2009)114–119.
[44]M.I.Kandah,J.L.Meunier,Removal of nickel ions from water by multi-walled
carbon nanotubes,J.Hazard.Mater.146(2007)283–288.
[45]C.J.Zhou,W.J.Zhang,W.C.Zhou,A.H.Lei,Q.L.Zhang,Q.Wan,B.S.Zou,Fast
and considerable adsorption of methylene blue dye onto graphene oxide,Bull.
Environ.Contam.Toxicol.87(2011)86–90.
[46]N.I.Kovtyukhova,P.J.Ollivier,B.R.Martin,T.E.Mallouk,S.A.Chizhik,E.V.
Buzaneva,A.D.Gorchinskiy,Layer-by-layer assembly of ultrathin composite ?lms from micron-sized graphite oxide sheets and polycations,Chem.Mater.
11(1999)771–778.
110L.Sun et al./Journal of Hazardous Materials203–204 (2012) 101–110
[47]K.W.Putz,https://www.sodocs.net/doc/2a3095868.html,pton,C.Segar,Z.An,S.T.Nguyen,L.C.Brinson,Evolution
of order during vacuum-assisted self-assembly of graphene oxide paper and associated polymer nanocomposites,ACS Nano5(2011)6601–6609.
[48]M.Chhowalla,G.Eda,Chemically derived graphene oxide:towards large-area
thin-?lm electronics and optoelectronics,Adv.Mater.22(2010)2392–2415.
[49]W.Choi,G.H.Moon,Y.Park,W.Kim,Photochemical loading of metal nanopar-
ticles on reduced graphene oxide sheets using phosphotungstate,Carbon49 (2011)3454–3462.[50]X.Tong,H.Wang,G.Wang,L.J.Wan,Z.Y.Ren,J.T.Bai,J.B.Bai,Controllable
synthesis of graphene sheets with different numbers of layers and effect of the number of graphene layers on the speci?c capacity of anode material in lithium-ion batteries,J.Solid State Chem.184(2011)982–989.
[51]J.G.Devries,R.M.Kellogg,Reduction of aldehydes and ketones by sodium
dithionite,https://www.sodocs.net/doc/2a3095868.html,.Chem.45(1980)4126–4129.