Facile room-temperature synthesis of carboxylated graphene oxide copper sulfide nanocomposite

7/24/2019 Facile room-temperature synthesis of carboxylated graphene oxide copper sulfide nanocomposite

1/121SCIENTIFICREPORTS| 5:16369 | DOI: 10.1038/srep16369

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Facile room-temperature synthesis

of carboxylated graphene oxide-

copper sulde nanocomposite

with high photodegradation and

disinfection activities under solarlight irradiationShuyan Yu1, Jincheng Liu1,2, Wenyu Zhu1, Zhong-Ting Hu1, Teik-Thye Lim1,3& Xiaoli Yan1,3,4

Carboxylic acid functionalized graphene oxide-copper (II) sulde nanoparticle composite (GO-COOH-CuS)

was prepared from carboxylated graphene oxide and copper precursor in dimethyl sulfoxide (DMSO)

by a facile synthesis process at room temperature. The high-eective combination, the interaction

between GO-COOH sheets and CuS nanoparticles, and the enhanced visible light absorption were

conrmed by transmission electron microscopy (TEM), eld emission scanning electron microscopy

(FESEM), X-ray powder diraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermo

gravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), UV-vis diuse reectance spectra

(DRS) and Photoluminescence (PL) spectra. The as-synthesized GO-COOH-CuS nanocomposite

exhibited excellent photocatalytic degradation performance of phenol and rhodamine B, high

antibacterial activity toward E. coliand B. subtilis, and good recovery and reusability. The inuence

of CuS content, the synergistic reaction between CuS and GO-COOH, and the charge-transfer

mechanism were systematically investigated. The facile and low-energy synthesis process combined

with the excellent degradation and antibacterial performance signify that the GO-COOH-CuS has a

great potential for water treatment application.

Te ever-growing demand or the clean water has stimulated significant research effort or the huge devel-opment o advanced water treatment technologies. Semiconductor-mediated heterogeneous photocataly-

sis is considered as a promising method to remove pathogens and organic pollutants rom wastewater1

.A great number o materials have been developed and involved, including Cu2-xS (0x ), CdS, ZnOetc. Among those, copper sulfide (CuS) has been reported as one o the most important semiconductorsdue to its excellent chemical and physical properties at room temperature2. Various morphologies o CuSincluding nanopowders, nanotubes, nanorods, nanoribbons and so on have been successully abricatedvia versatile synthetic strategies such as solventless and solution thermolysis, solution-phase reaction,

1School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore

639798, Republic of Singapore. 2Faculty of Chemical Engineering and Light Industry, Guangdong University

of Technology, Guangzhou, China 510009. 3Nanyang Environment and Water Research Institute (NEWRI),

Nanyang Technological University, 1 Cleantech Loop, CleanTech One, Singapore 637141, Republic of Singapore.4Environmental and Water Technology Centre of Innovation, 535 Clementi Road, Singapore 599489, Republic of

Singapore. Correspondence and requests for materials should be addressed to J.L. (email: [email protected]) or

X.Y. (email: [email protected])

Received: 12 June 2015

Accepted: 12 October 2015

Published: 10 November 2015

OPEN
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2SCIENTIFICREPORTS| 5:16369 | DOI: 10.1038/srep16369

ultrasonic and microwave irradiation, etc3,4. However, those methods usually require complicated proce-dures and elevated temperatures, which greatly hinder the process o scale up. Recently, some relativelyacile synthesis approaches have been advocated to prepare CuS-based semiconductor photocatalyst.For example, iO2-CuS was obtained via a acile route using a sol-gel method by Park et al.3. Zhang etal.obtained CuS nanopowder by solvothermal method using Na2S as the sulur source4. Nevertheless,there are still several issues limiting the application o CuS. o the best o our knowledge, there are ewlow-energy approaches developed or the synthesis o CuS-based composite photocatalyst, e.g., at roomtemperature. Moreover, the low photogenerated charge transer rate on the photocatalyst surace andhigh aggregation o CuS nanomaterials, which significantly degrade the photocatalytic perormance. An

effective option or overcoming these drawbacks is to disperse the nanomaterials onto a suitable support,which could increase the surace area, inhibit the recombination o photogenerated charges, and alloweasy recovery o the nanomaterials.

As a single layer o graphite, graphene has a perect sp2-hybridized two-dimensional carbon struc-ture, including perect crystal and microstructure with large surace area5,6. Tose properties can notonly enhance photoactivity o other photocatalysts, but also make it be suitable support7. However, thevan der Waals orces and high - stacking between adjacent layers, acilitate irreversible aggregationo graphene or even restacking to graphite, which greatly impede its application8. Along with a largenumber o different approaches that have been developed to address these obstacles, the most reliableand acile technique is the unctionalization o graphene. Graphene oxide (GO) is an oxygen-containinggraphene derivative with partial breakage o sp2sp2bonds into sp3sp3bonds or the insertion o somependent groups like epoxy, carboxylic acid, and hydroxyl911. For example, with urther carboxylation,GO-COOH has enhanced hydrophilicity and more carboxyl groups as anchoring sites to strongly bindother nanomaterials such as CuS to the surace o GO.

In this work, we put orward a acile yet efficient room-temperature synthesis approach or the ab-rication o GO-COOH-CuS nanocomposite and explored its general environmental applications. Asystematic characterization was carried out or the pristine materials (CuS and GO-COOH) and theircomposite (GO-COOH-CuS). Laboratory-scale batch experiments were conducted to investigate thephotocatalytic degradation o phenol and rhodamine B (RhB), as well as the inactivation o Escherichiacoli(E. coli)and Bacillus subtilis(B. subtilis)under solar light irradiation using the as-synthesized mate-rials as the photocatalysts. A plausible charge-transer mechanism is tentatively proposed or the photo-catalysis process. o the best o our knowledge, the multiunctional GO-COOH-CuS nanocomposite isor the first time rapid synthesized via such acile and scalable room-temperature method and used orwater decontamination and disinection with excellent perormance just during a short operating periodunder solar irradiation.

Results and DiscussionMorphological and structural analysis. EM is employed to characterize the morphology and

microstructure o the graphene-based catalyst, to veriy the effective anchoring o CuS NPs onto thelarge GO-COOH sheets. Figure 1(a) presents that the size o the as-synthesized GO-COOH sheets isaround 12m, which may benefit the recovery o GO-COOH based nanocomposite materials by sim-ple filtration. Te edge o GO-COOH sheets is clearly observed in Fig. 1(ac), indicating no significantrestacking o GO-COOH sheets. It may be due to the insertion o the carboxylic acid unctional groupswith CuS NPs anchored among layers. Moreover, as shown in Fig. 1(b,c), CuS NPs with sizes o 2030 nmare highly monodispersed on the single GO-COOH sheet without any aggregation and no CuS NPs areobserved outside the sheets. Tis can be attributed to the oxygen-containing groups, which provideduniormly anchoring sites on the both sides o GO-COOH sheets or CuS NPs. Tus the CuS NPs couldanchor exactly on GO-COOH sheets and their aggregation was inhibited as well. Te clear 0.28 nmlattice spacing o CuS NPs shown in Fig. 1(d)urther confirms the effective sel-anchoring o CuS NPson the GO-COOH sheets. By the EM analysis, it could be deduced that the carboxylic acid unctionalgroups in the synthesis process have triple unction: (1) increasing the hydrophilicity o GO sheets toacilitate CuS interaction with GO sheets; (2) reducing the van der Waals bond between carbon layers

to promote the delamination o the GO sheets; and (3) providing strong anchoring sites or CuS NPs toinhibit their aggregation12.Figure 2shows the XRD patterns o GO-COOH and GO-COOH-CuS. Te observed diffraction peak

at 2o 11.9 in GO-COOH can be attributed to the stacked GO-COOH sheet, which is consistent withthe [001] interlayer spacing o 7.43 13. In contrast, the [001] reflections o GO are not detected in theXRD patterns o all GO-COOH-CuS samples, indicating the regular stacks o GO were destroyed by theeffective intercalation o CuS NPs. In addition, the obvious diffraction peaks at 29.76, 33.85 and 48.30can be assigned to (102), (103) and (110) acets o cubic CuS (JCPDS No. 011281), which urther val-idate the effective anchoring o CuS NPs on the large GO-COOH sheets14.

XPS is used to analyze the quantity o elements and the surace electronic state o the obtained prod-ucts. Te XPS spectra (Fig. 3) display graphitic C 1s, O 1s, Cu and S 2p peaks or GO-COOH-CuSnanocomposites, and the existence o C and O in the GO-COOH. In addition, the high-resolution C 1sspectra (Fig. 3b) exhibit three main separated peaks at 283.9 eV, 286.2 eV and 288.4 eV or all samples,which are connected to the graphitic C-C, C-O and COOH bonds, respectively. Figure 3c,d show thebinding energies o Cu 2p

3/2

, Cu 2p1/2

peaks at 932.6 and 952.4 eV and double S 2P3/2

, 2p1/2

peaks around
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162 nm. Te analysis o surace elemental composites o GO-COOH-CuS is reported in the SupportingInormation (Supporting able S1). Te XPS results urther confirm the carboxylation o GO and theexistence o CuS in the composites, which is in good agreement with our subsequent EM and XRDanalyses.

Composition and surface properties. Te FIR spectra o GO-COOH and GO-COOH-CuS areshown in Fig. 4. Te bands at 1721 and 1070 cm1are ascribed to the C=O and CO stretching vibrations

Figure 1.EM images o (a) GO-COOH sheet, (bd) GO-COOH-CuS nanocomposite.

Figure 2. XRD patterns of CuS, GO-COOH and GO-COOH-CuS nanocomposites.
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o carboxylic acid group, respectively15. Furthermore, the doublet bands at 1627 and 1373 cm1corre-spond to the COO- symmetric and asymmertric vibrations complexed with surace Cu centers16. Tedoublet bands at 2921 and 2849 cm1 in the GO-COOH-CuS FIR spectrum are assigned to the C-Hstretching vibrations17. Te broad band ranging rom 3000 to 3600 cm1with peak centered at 3423 cm1is attributed to the OH stretching vibrations o adsorbed water molecules. Te FIR analysis confirmedthe coordination between CuS and carboxylic acid groups in GO-COOH, which is essential and decisiveor the controlling growth o CuS on the GO-COOH sheets.

GA was used to evaluate the thermal stability o GO-COOH, CuS and GO-COOH-CuS. In Fig. 5, allmaterials show slight weight losses rom 0 to 200 C, which was due to the evaporation o tightly boundedwater or some organic solvent molecules. In the case o GO-COOH, with the urther increase o tem-perature, a sharp weight loss appears around 200 C. It is resulted rom the thermal decomposition ooxygen-containing unctional groups18. Afer the temperature exceeds 200 C, the weight o CO-COOHalls air steadily. Te grafing percentages o CuS on GO-COOH sheets could be estimated rom the

Figure 3. (a) Full XPS spectra o GO-COOH and o GO-COOH-CuS nanocomposites with different massratios; (bd) high resolution XPS spectrum o C 1s, Cu 2p, S 2p o GO-COOH-CuS.

Figure 4. FIR spectra of GO-COOH and GO-COOH-CuS samples.
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GA thermograms. Te mass percentage o CuS in the as-prepared composites o GO-COOH-CuS-1,5, 10, 20 is about 89%, 47%, 24%, and 20%, respectively.

Optical properties. Figure 6(a) depicts the UV-vis diffuse reflectance spectra (DRS) o the CuS,GO-COOH and GO-COOH-CuS nanocomposite, as well as their Kubelka-Munk (K-M) transormedreflectance spectrum o CuS (inset). A notable broad absorption in the visible-light and near-inrared(NIR) region is observed rom DRS or GO-COOH-CuS compared to GO-COOH, while Ego CuS is2.27 eV obtained by the ormula ((ah)2=A(h Eg) )19. Tis suggests that the GO-COOH-CuS presentsthe enhanced absorbance over pure GO-COOH. Te less amount o coupled CuS can effectively extendthe absorption o GO-COOH onset to the visible light and NIR region, which is similar to the extendedabsorption or rGO-iO2composite caused by the ormation o C-O-i bond20.

Te effect o GO-COOH on the anti-recombination o e/h+was investigated by PL spectroscopy21.

Figure 6(b)displays the as-synthesized nanomaterials with a broad emission peak at =571 nm underan excitation at 400 nm. Te GO-COOH-CuS nanocomposites exhibit a significantly lower PL emissionintensity than pure CuS, which indicates that GO-COOH is effectively suppressing the e/h+ recombi-nation via aster electron transer.

Photocatalytic degradation of phenol and RhB. In order to explore their potential application inwater treatment, phenol and RhB dye were taken as representative organic compounds to investigate thephotodegradation ability o GO-COOH-CuS nanocomposites with various mass ratios (GO-COOH:CuS)under simulated solar light irradiation. Phenol is among the most resistant class o potential pollutantin the wastewater with strong toxicity and excellent solubility. RhB is a basic dye widely used and dis-charged by the paper printing, paint, textile dyeing and leather industries. Figures 7(a) and 8(a) showthe concentration o pollutants as a unction o degradation time. Compared to CuS and GO-COOH,GO-COOH-CuS nanocomposites exhibit higher adsorption or both phenol and RhB. Among the differ-ent mass ratios, GO-COOH-CuS-5 has the strongest adsorption ability, ollowed by GO-COOH-CuS-1,GO-COOH-CuS-10 and GO-COOH-CuS-20, as summarized in able 1. Te acilitation o the organic

Figure 5. Termogravimetric analysis of CuS, GO-COOH and GO-COOH-CuS nanocomposites.

Figure 6. (a) UV-vis diffuse reflectance spectra (DRS) o CuS, GO-COOH and GO-COOH-CuSnanocomposites, and Kubelka-Munk transormed resflectance spectra o CuS (inset); (b) PL spectra(excitation at 400 nm) o CuS, GO-COOH and GO-COOH-CuS nanocomposites.
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pollutants adsorption on nanocomposites may be due to their stronger non-covalent intermolecularorces. For example, the adsorption o phenol by GO-COOH sheets was caused by the interaction o-conjugated and the ion-dipole interactions between GO-COOH and phenol due to the presence osimilar conjugated aromatic cycles. Tis can also confirm the phenomenon that adsorption increaseswith the increasing graphene: CuS ratio, e.g., GO-COOH-CuS-5>GO-COOH-CuS-1. However, it seems

that -conjugated and the ion-dipole interactions are limited or GO-COOH-10 and GO-COOH-20because o excessive unctional groups o GO-COOH, which causes a lower adsorption o phenol andRhB than GO-COOH-CuS-1 and GO-COOH-CuS-5.

As we known, the enhanced adsorption ability o nanomaterials could benefit the subsequent degra-dation process, which is confirmed in our work as well. In Figs 7a and 8a, the degradation ability o thenanocomposites or either phenol or RhB is coherent with their adsorption phenomenon. And in orderto urther investigate the long-term stability o synthesized GO-COOH-CuS composite, the photocata-lytic degradation o phenol and RhB on GO-COOH-CuS-5 under simulated solar light irradiation wasperormed over five cycles. As shown in Figs 7b and 8b, no evident change was observed or the photo-catalytic activity through the 5 cycles o experiments or both phenol and RhB degradation, indicatinggood photocatalytic stability o the GO-COOH-CuS composite.

In addition, Photocatalytic degradations or organic pollutants are ofen described in terms o theLangmuir-Hinshelwood model, which can be simplified as pseudo-first order reaction dC/dt=kC,where kreers to the corresponding reaction rate kinetic constant. Te photocatalytic degradation kineticcurves shown in Fig. 9 apparently ollow the pseudo first-order kinetics in the studied concentration

Figure 7. (a) Photocatalytic perormance o CuS, GO-COOH and GO-COOH-CuS nanocomposites orphenol under solar light irradiation. (b) Four consecutive cycling curves o phenol photodegradation by GO-COOH-CuS-5.

Figure 8. (a) Photocatalytic perormance o CuS, GO-COOH and GO-COOH-CuS nanocomposites or RhBunder solar light, (b) five consecutive cycling curves o RhB photodegradation by GO-COOH-CuS-5.
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range. Te calculated k (min1) constants or all the applied nanomaterials are shown in Fig. 9and thesame order is ound as degradation ability: GO-COOH-CuS-5>GO-COOH-CuS-1>GO-COOH-CuS-10>GO-COOH-CuS-20>GO-COOH>CuS (able 1). Furthermore, the percentage o leached Cu2+afer degradation ollowed the same order as summarized in able 1. All these results indicate that inte-gration o CuS with GO-COOH sheets could enhance the photocatalytic activity, due to the improvedlight absorption, increased pollutants adsorption and inhibition o charge combination, etc. But the opti-mal mass ratio is preerred or the best perormance o nanocomposites, such as GO-COOH-CuS-5 in

this study. Tis could be explained as ollowing: (1) For the GO-COOH-CuS-5, the well combinationo GO-COOH and CuS nanoparticle can motivate more effective charge transer rom conduction band(CB) o CuS to GO-COOH, to achieve low recombination rates o photogenerated electron-hole pairs;(2) the GO-COOH-CuS-10 and GO-COOH-CuS-20 with small amount o CuS has limited capabilitiesto absorb solar light, which results in poor photodegradation abilities; and (3) the GO-COOH-CuS-1with great amount o CuS also shows lower ability than GO-COOH-CuS-5 because the excessive CuSNPs will cover most o the reactive sites on GO-COOH surace and also act as recombination centers,leading to the decreased photocatalytic ability22.

Te total organic content (OC) analysis was monitored as a unction o the rate o mineralizationo phenol and its related intermediates. As the results shown in able 1 and Fig. 10(a), the OC wasdecreasing over time, and the GO-COOH-CuS-5 displayed the highest OC removal ability. Moreover,the main intermediates o phenol degradation were analyzed by HPLC. Afer 3 h solar irradiation, themain intermediate ormed was identified as maleic acid, which is non-toxic and at low concentration.Tose toxic intermediates such as catechol and benzoquinone were not ound, which may due to theirurther degradation during 3 h reaction.

GO-COOH CuS

GO-COOH-CuS

1 5 10 20

Adsorption (%)

Phenol 2.3 1.4 6.3 8.5 4.2 3.7

RhB 6.1 5.9 12.2 15.1 12.8 9.3

Photo degradation (%)

Phenol 16.4 15.7 46.2 61.8 30.2 20.1

TOC removal 15.6 14.7 40.0 55.2 29.3 19.6 RhB 13.8 26.8 72.7 95.4 61.3 41.3

Kinetics (min1)

Phenol 0.0011 0.00095 0.0032 0.0047 0.0019 0.0012

RhB 0.0027 0.0014 0.0013 0.032 0.0095 0.0054

Leached Cu2+afer photodegradation(%)

Phenol / 19.5 6.4 5.8 9.5 10.1

RhB / 15.2 4.5 3.4 7.8 9.3

able 1. Adsorption percentage (%), photo degradation efficiencies (%) and kinetics (min1) of all as-synthesized samples.

Figure 9. Pseudo first order photocatalytic degradation o (a) phenol and (b) RhB over the CuS, GO-COOH and GO-COOH-CuS nanocomposites.
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Disinfection performance. Te antibacterial activity o GO-COOH and GO-COOH-CuS nanocom-posites was evaluated with or without simulated solar light irradiation, using both Gram-negative E.coli and Gram-positive B. subtilis bacteria as the bacterium models. As the results shown in Fig. 11,the nanomaterials could inactive both bacterial strains in the darkness and showed higher antibacterialactivity with CuS anchoring, indicating CuS NPs play an important role in the toxicity. It may due to therelease o Cu2+ions. A possible mechanism is that Cu2+can orm chelates with biomolecules or dislodgein some metalloproteins which may cause their dysunction and urther cell inactivation23. However, itcannot iner that the more CuS content, the higher toxicity o nanocomposites because the excess CuSmay cover the adsorption sites o GO-COOH sheets, leading to the decreased attachment o bacteria

Figure 10. (a) OC concentrations versus time, and (b) HPLC analyses o byproducts o phenoldegradation on CuS, GO-COOH and GO-COOH-CuS nanocomposites under 3 h solar light irradiation.

Figure 11. Antibacterial activities o GO-COOH-CuS nanocomposites with and without solar irradiation(a)E.colisurvival, (b)B. subtilissurvival, and (c) agar plates pictures o E.coliexposed to GO-COOH-CuS-5under solar light with t ime (0 min, 10 min, 30 min, 60 min, 90 min and 120 min).
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on the surace o nanocomposites. In this work, GO-COOH-CuS-5 exhibits the highest toxicity due toits optimal coupling ratio. Afer 60 min exposure to GO-COOH-CuS-5, nearly 40% E. coliand 30% B.subtiliswere inactivated.

Under solar light irradiation, both bacterial strains exposed to the nanocomposites had a much lowersurvival rate contrast to that in darkness. Te improved disinection efficiency is attributable to thesynergistic effect in photocatalysis process. GO-COOH sheets adsorb bacteria on the surace, providingmore biocontact opportunities or CuS NPs. Aside rom the toxicity o the released Cu2+, the pho-toinduced reactive oxygen species (ROS) by CuS NPs can attack the attached bacteria as well, leadingto the enhanced antibacterial activity. Among those nanocomposites, GO-COOH-CuS-5 again showsthe highest photocatalytic disinection efficiency due to its optimal ratio o coupling content. It inacti-vated 90% E. coliand 80% B. subtilisafer 30 min reaction. With the urther increase o contact time to

60 min, the E. coliand B. subtiliswere inactivated nearly by 100%. Te higher inactivation efficiency toB. subtilisindicates that Gram-positive bacteria were more susceptible to the photodynamic effects thanGram-negative bacteria, which might be due to resistance o the outer membrane in Gram-negativebacteria. An additional outer membrane o Gram-negative bacteria can protect their inner layer romradical or chemical attacking24.

2.6 Photocatalytic mechanism. For the enhancement o photocatalytic activity, an efficient chargetranser is crucial25. In the system o GO/PcZn, GO/iO2and rGO/CuS, GO has shown excellent prop-erty to be an effective acceptor and transporter, suppressing the recombination o electron-hole pairs andacilitating charge transer20,26,27. Similarly, a charge transer might be achieved in our GO-COOH-CuSnanocomposite system, where GO-COOH sheets act as the electron acceptor. Based on the experimentalresults and analyses, a possible mechanism or the enhanced photocatalytic activity and good stabilityo synthesized GO-COOH-CuS nanocomposite is proposed in Fig. 12. Under solar light irradiation,the electrons could be generated either rom excited dye (RhB*) molecules or CuS NPs, then transer

to GO-COOH. Tey react with absorbed surace O2on GO-COOH sheets to produce reactive oxygenspecies (ROS) which assist the pollutants degradation. Meanwhile, the photogenerated holes on CuS alsocould oxidize pollutants. In addition to the charge separation, the photocatalytic activity can be influ-enced by the adsorption o pollutants, the light absorption and the surace oxygen adsorption as well. Teenhanced visible light absorption o GO-COOH-CuS is verified by UV-vis absorption analysis (Fig. 5).Te high adsorption ability o GO-GOOH sheets or organic pollutants has been confirmed by previousworks21,28, as well as in our study (Figs 7and 8). All these improved properties o GO-COOH-CuS resultin the higher photocatalytic activity towards our synthesized nano-system.

Materials and MethodsMaterials. Sodium nitrate (NaNO3, 99%), potassium permanganate (KMnO4, 99%), hydrogen per-oxide (H2O2, 35%), concentrated suluric acid (H2SO4, 98%), chloroacetic acid (ClCH2CO2H, 99%),concentrated hydrochloric acid (HCl, 36.5%), and thioacetamide (C2H5NS, 99%) were purchased romSigma-Aldrich (Singapore). Copper (II) nitrate trihydrate (Cu(NO3)2, 99.5%) was purchased rom StremChemicals. Natural graphite (SP1) was purchased rom the Bay Carbon Company (USA). Ethanol, acetone,

Figure 12. Schematic illustration of proposed photocatalytic and antibacterial mechanisms of GO-COOH-CuS nanocomposite.
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dimethyl suloxide (DMSO), and rhodamine B (RhB) were purchased rom Merck Ltd (Singapore). Tedeionized (DI) water was produced rom Millipore Milli-Q water purification system.

Synthesis of GO-COOH sheets and GO-COOH-CuS nanocomposites. GO sheets were exoli-ated rom natural graphite according to the modified Hummers method12,29. GO-COOH was synthe-sized under strongly basic condition to convert hydroxyl groups to carboxylic acid moieties by activatingthe GO sample with chloroacetic acid (ClCH2CO2H)12. In a typical process, 1.2 g o chloroacetic acid,1 g o NaOH and 50 mg o GO were added to 100 mL DI water and then sonicated or 3 h to speed upthe elimination o sodium chloride. Te resulting GO-COOH solution was neutralized by HNO3, andpurified with acetone and water or several times.

For the synthesis o GO-COOH-CuS nanocomposite, we herein developed a acile and scalablemethod to anchor CuS nanoparticles (NPs) on large GO-COOH sheets at room temperature. Briefly,

various amounts (1 mL, 5 mL, 10 mL and 20 mL) o 1 mg mL1

GO-COOH solution were mixed with 1mmol o Cu(NO3)2, 1 mmol o C2H5NS and 10 mL o DMSO. DMSO acted as a solvent and co-reactantto urnish the sulur source in the orm o H2S. Te mixture was stirred or 24 h at ambient temperatureto ensure CuS NPs anchored on GO-COOH sheets. Te products were centriuged and rinsed withacetone, ethanol and DI water several times to remove the residual. Te resulting solid samples werereeze-dried or 2 h under 50 C to obtain the final GO-COOH-CuS catalysts. Here, the samples pre-pared by addition o various amounts GO-COOH solutions (1 mL, 5 mL, 10 mL and 20 mL) were namedas GO-COOH-CuS-1, GO-COOH-CuS-5, GO-COOH-CuS-10 and GO-COOH-CuS-20, respectively.Te synthesis process is illustrated in Fig. 13.

Characterizations. X-ray powder diffraction (XRD) patterns were taken on a D8-AdvanceBruker-AXS diffractometer using Cu K irradiation to determine the crystallinity phase. ransmissionelectron microscopy (EM) images were carried out using a JEOL 2010-H microscope EM operating at200 kV. Te samples were prepared or the analysis by dropping dilute solutions o samples on 400-mesh

carbon-coated copper grids and leading the solvent to dry. Fourier transorm inrared spectra (FIR) werecollected on a Bruker FIR spectroscopy (Nicolet IS10) with solid powder samples. Termogravimetricanalysis (GA) was recorded using a NEESCH SA 409 PC thermgravimeter. Briefly, 56 mg sampleswere heated rom room temperature to 900 C in dry nitrogen at a rate o 10 C min1. X-ray photoelec-tron spectroscopy (XPS) measurements were taken by using a Kratos Axis Ultra Spectrometer with a15 kV and 10 mA monochromic Al Ksource at 1486.7 eV. Te UVvis diffuse reflectance spectra (DRS)o the 0.1 mg mL1dispersion solutions were measured by using an evolution 300 spectrophotometerunder room temperature. Te photoluminescence (PL) emission spectra were recorded on a fluorescencespectrometer (PerkinElmer). Te leaching o Cu2+was checked by quantification o Cu2+concentrationin solution using micro plasma atomic emission spectroscopy (MP 4100 AES). Te total organic carbon(OC) was measured using a OC analyzer (OC-L CPH/CPN; Shimadzu, Kyoto, Japan). Te analysiso intermediates o phenol degradation was conducted by a high perormance liquid chromatography(HPLC, Agilent 1100) with a reverse-phase column (Agilent, Eclipse XDB-C18, 1504.6 mm).

Photocatalytic degradation of phenol and RhB under solar irradiation. In order to investigatethe application potential or wastewater treatment, the photocatalytic degradation o phenol and RhBusing the as-synthesized nanocomposites were conducted in a laboratory-scale batch photoreactor with8 quartz capsules. A 500 W Xenon lamp was used as the simulated solar light source. Cool deionizedwater was circulated through a quartz jacket. And the distance between the light source and the quartztest tubes contained our samples was 15 cm where the measured light intensity was 100 mW cm2. Beorecommencement o irradiation, the reaction suspension containing organic pollutants (40 mg L1RhB or200 mg L1phenol) and photocatalysts (1 g L1) were stirred in the dark or 1 h to achieve equilibrium.Samples o 3 mL solution were withdrawn rom the quartz test tubes and centriuged to collect superna-tants. Te concentrations o RhB and phenol were analyzed by recording their maximum absorbance onthe UV-Vis spectrophotometer at the wavelengths o 550 nm and 270 nm, respectively.

Photocatalytic disinfection under solar irradiation. Gram-negative bacteria E. coli(ACC 8739)and Gram-positive bacteria B. subtilis(ACC 6633) were chosen as standard organisms to evaluate theantibacterial ability o GO-COOH-CuS. Normally, E. coliand B. subtiliswere cultivated in Luria-Bertani

Figure 13. Synthetic route of GO-COOH and GO-COOH-CuS nanocomposites.
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nutrient solution ollowed by incubation at 37 C and 30 C respectively or 18 h to get the exponentialgrowth phase. Te pure E. coli and B. subtilis cells were harvested by centriugation and washed withsaline solution (0.9% NaCl) to remove residual macromolecules. Ten, the cells were re-suspended inthe saline solution to maintain a concentration o ~108colony orming units (cu mL1). All glass appa-ratuses and solutions used in the experiments were sterilized in autoclave at 121 C or 20 min to ensuresterility.

Te photocatalytic disinection experiments were perormed in the photochemical reactor. Te nano-composites (GO-COOH-CuS-1, GO-COOH-CuS-5, GO-COOH-CuS-10 and GO-COOH-CuS-20) weremagnetically mixed with 108cu mL1E. colior B. subtilissaline solution. Afer irradiation under sim-

ulated solar light or 2 h, 100L o solution was daubed uniormly and cultivated on the solidified agarnutrient plates. Te colony orming units were counted and compared with control plates to calculateratio o cell viability (C/C0). As comparison, a contrast experiment under dark condition and a blankcontrol experiment without any nanomaterial were carried out under the same condition. All the exper-iments were conducted in triplicates.

ConclusionsIn summary, with a acile and up scalable synthesis process, multiunctional GO-COOH-CuS nano-composites were successully synthesized. Te uniorm deposition o CuS NPs on GO-COOH sheetscontributes to the superior photocatalytic activities o the nanocomposite, not only towards organic pol-lutants (phenol and RhB) but also waterborne pathogens (E. coli and B. subtilis). Herein, GO-COOHwas demonstrated as an appropriate conductive ligand to support the CuS NPs used in photocatalysis. Inaddition, as the electron acceptor, GO-COOH improved the lietime o electron-hole pairs o CuS result-ing in a better pathway or electron transport. Te GO-COOH-CuS nanocomposite material can over-

come the limitations o the pristine GO-COOH and CuS photocatalysts, and combine the merits o theboth materials to maximum synergistic effects, including (1) perect contact between GO-COOH sheetsand CuS NPs to minimize the photocorrosion. (2) uniormly distributed CuS NPs onto the GO-COOHsurace to improve isolating photogenerated electrons and holes, meanwhile, to enhance the charge trans-er. Moreover, the durability study showed that the photocatalytic activity o the GO-COOH-CuS isstable enough or multiple recycling. Tis study signifies that GO-COOH-CuS nanocomposite can be apromising photocatalyst in the field o water decontamination and disinection.

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AcknowledgementsTe authors appreciate the financial support received rom Nanyang echnological University (M4081044)and Ministry o Education o Singapore (M4011352). We also thank Dr. Bin Cao or his useul discussion.

Author ContributionsS.Y. and J.L. conceived the experiments, S.Y., W.Z. and J.L. conducted the experiments, S.Y., Z..H., ..L.and X.Y. analysed the results. All authors reviewed the manuscript.

Additional InformationSupplementary informationaccompanies this paper at http://www.nature.com/srep

Competing financial interests: Te authors declare no competing financial interests.

How to cite this article: Yu, S. et al.Facile room-temperature synthesis o carboxylated grapheneoxide-copper sulfide nanocomposite with high photodegradation and disinection activities under solarlight irradiation. Sci. Rep.5, 16369; doi: 10.1038/srep16369 (2015).

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Facile room-temperature synthesis of carboxylated graphene oxide copper sulfide nanocomposite