RSC Advances

PAPER

Carbon dot redu

aAdvanced Polymer and Nanomaterial Labo

Tezpur University, Napaam, 784028, Assam

com; Fax: +91-3712-267006bLeibniz-Institut fur Polymerforschung Dresd

Germany. E-mail: voit@ipfdd.de

† Electronic supplementary informationspectra (Fig. S1) of nanohybrid, ECD1.010.1039/c4ra11120f

Cite this: RSC Adv., 2014, 4, 58453

Received 24th September 2014Accepted 20th October 2014

DOI: 10.1039/c4ra11120f

www.rsc.org/advances

This journal is © The Royal Society of C

ced Cu2O nanohybrid/hyperbranched epoxy nanocomposite: mechanical,thermal and photocatalytic activity†

Bibekananda De,a Brigitte Voitb and Niranjan Karak*a

In the present study a highly tough thermostable hyperbranched epoxy nanocomposite was fabricated by

the incorporation of carbon dot reduced Cu2O nanohybrid, which exhibited efficient reusable

photocatalytic activity towards the degradation of pesticide under solar light (light intensity: 800–1000

lux). The catalytic efficacy of the above nanohybrid was compared with that of the parent carbon dot.

Carbon dot reduced Cu2O nanohybrid particles were prepared by the reduction of cupric acetate

solution with carbon dots at 70 �C for 6 h. The formation of nanohybrid (size: 3–4 nm), as well as its

nanocomposites with hyperbranched epoxy, was confirmed by FTIR, XRD, Raman and TEM analyses. The

significant improvement in the performance in terms of tensile strength (20%), elongation at break (2.5

fold), toughness (3.5 fold) and thermal stability (23 �C) of the pristine epoxy thermoset was observed by

the formation of nanocomposite with 1.5 wt% of nanohybrid. The degradation of ethyl paraoxon

pesticide was studied under ambient conditions using normal solar light and the changes of

concentration with respect to initial were monitored by UV study. Thus, the high performance

nanocomposite with photocatalytic attributes has strong potential to be used as a functional thin film

material, as well as thermostable reusable photocatalyst.

Introduction

In recent years, intensive research has been focused on carbondot technology because of its simple synthetic protocol andwide usability in electronic and energy sectors, biomedicalscience, sensor and catalysis, as well as in composites.1–4

Furthermore, it is superior to other zero-dimensional nano-materials as well as semiconductor quantum dots because theformer possesses excellent aqueous solubility, easy functiona-lizability, resistance to photobleaching, and biocompatibility.5,6

Recently it has been intensively used as a photocatalyst becauseof its tunable emissions from the near-infrared to blue wave-length.2 Moreover, the researchers have designed carbon dotswith other metal or metal oxide nanoparticles for enhancingcatalytic efficiency by exploiting its up-conversion luminescenceproperties.2,7,8 Furthermore, carbon dot reduced metal nano-hybrids are gaining importance in different catalytic reactionsbecause carbon dots are capable of reducing metal salts.9

ratory, Department of Chemical Sciences,

, India. E-mail: karakniranjan@yahoo.

en e.V., Hohe Strasse 6, D-01069 Dresden,

(ESI) available: The optical absorptionand ECDCO1.0 are available. See DOI:

hemistry 2014

Attributes like non-toxicity, easy availability, low cost, p-typesemiconductor with a direct band gap 2.2 eV, and photo-catalytic activity under visible light endow Cu2O nanoparticles aunique position in the domain of metal oxide-based photo-catalysts.7,10,11 However, the preparation and stabilization ofCu2O nanoparticles are the major issues because in most of thecases, Cu2O nanoparticles are formed with a mixture of Cu andCuO nanoparticles.12,13 Therefore, a controlled reduction ofCu2+ or oxidation of Cu0 to Cu2O nanoparticles is necessary; theprocess is critical. This controlled reduction or oxidation andstabilization can be performed in the presence of suitablecapping or stabilizing agents. In this milieu, polymer-supportednanoparticles or a polymer nanocomposite is one of the bestoptions. Moreover, the high thermostability, high mechanicalstrength, excellent chemical resistance as well as infusible andinsoluble nature of an epoxy thermoset makes it one of the bestmatrices for this purpose.14,15 In addition, low viscosity, highsolubility, functionality and reactivity, as well as easy process-ing, add to the advantages of a hyperbranched epoxy over itsconventional linear analog.16–19

In addition, photocatalysis is one of the greenest approachesfor the destruction of different hazardous anthropogenicorganic contaminants.7,11,20,21 This is because the photo-degradation of the contaminants is economically favorable andthe process is fast even at very low concentrations and underambient conditions. In this context, semiconductor quantumdots and metal oxide nanoparticles, mainly TiO2 and ZnO, are

RSC Adv., 2014, 4, 58453–58459 | 58453

RSC Advances Paper

widely used.22–25 However, toxicity and low efficiency undervisible light are the major disadvantages of these catalysts.Among the different hazardous anthropogenic organic chem-icals, pesticides are the most common non-degradableanthropogenic chemical contaminants both in water and thesoil.22,26 However, they are extensively used in industry, as wellas in agriculture. Organophosphates like paraoxon are mainlyused as nerve agents, chemical warfare agents, as well aspesticides and insecticides in agriculture.27,28 They enter intothe water and cause convulsions and respiratory paralysis by theprolongation of cholinergic effects in the human beings andanimals.27 However, the degradation of pesticide by a nano-photocatalyst is limited, and the efficiency of such reportedcatalysts is very poor, even under UV irradiation.27

Thus, in the present report, a carbon dot reduced Cu2Onanohybrid/hyperbranched epoxy nanocomposite was used as atough, thermostable and reusable photo-catalyst for the degra-dation of pesticides under solar light. Carbon dots offersimultaneous improvement in the strength, toughness andthermal stability of the thermoset aer the formation of nano-composites, as proved in our earlier study.4 Cu2O will providephotocatalytic activity under solar light with the assistance ofthe up-conversion luminescence property of carbon dot.Therefore, we wish to report the fabrication, characterizationand evaluation of the properties of carbon dot reduced Cu2Onanohybrid/hyperbranched epoxy nanocomposite as an effi-cient reusable photocatalyst.

ExperimentalMaterials

The hyperbranched epoxy resin used in this study was preparedfrom bisphenol-A, triethanol amine and epichlorohydrin by apolycondensation reaction, as previously reported.16 The epoxyequivalents, degree of branching and viscosity of the preparedhyperbranched epoxy were 358 g eq.�1, 0.79 and 19 Pa s (at 25�C), respectively. Banana (Musa acuminate) was collected from alocal market in Assam, India. Cupric acetate [Cu(OAc)2] mono-hydrate was purchased from Rankem, India. Poly(amido-amine)hardener (HY840, amine value 5–7 eq. kg�1) was obtained fromCiba-Geigy, Mumbai, India. Ethanol (Merck, India) (EtOH) wasused aer distillation. Ethyl paraoxon pesticide (Sigma Aldrich,Germany) was directly used without further purication. Allother chemicals used in this study were of reagent grade.

Preparation of carbon dot and carbon dot reduced Cu2Onanohybrid

Carbon dots were prepared by heating banana juice at 150 �C asreported in our previous study.5 Carbon dot reduced Cu2Onanohybrids were prepared by the reduction of cupric acetatesolution in ethanol by carbon dots. In a typical process, 0.5 g ofCu(OAC)2 was dissolved in 25 mL of EtOH with stirring for 15min in a two-necked round bottomed ask. A 20 mL aqueoussolution of NH3 (30%) was added to the above solution andstirred for another 15 min at room temperature to form acopper–ammonium complex. Then, a water-condenser was

58454 | RSC Adv., 2014, 4, 58453–58459

attached to the round bottom ask and the temperature wasraised to 70 �C. At this temperature, a 25 mL ethanolic solutionof carbon dots (0.5 g) was added drop-wise to the mixture. Themixture was continuously stirred for 6 h at the same tempera-ture. Aer the completion of the reaction, the solution wasallowed to cool naturally and the separation of nanoparticleswas done by ultracentrifugation at 5000 rpm for 10 min. Thenanoparticles were washed 2–3 times with EtOH, and thendispersed in 20 mL EtOH by ultrasonication for 5 min.

Preparation of nanocomposites with hyperbranched epoxy

Finally, the carbon dot reduced Cu2O nanohybrid/hyperbranched epoxy nanocomposites were prepared by asolution technique. Briey, the desired amount (0.5, 1.0 and 1.5wt% with respect to hyperbranched epoxy) of EtOH-dispersedcarbon dot reduced Cu2O nanohybrid was added into hyper-branched epoxy resin and magnetically stirred for 2 h at roomtemperature, followed by ultrasonication using an ultrasonicprocessor (UP200S, Hielscher, Germany) with a standardsonotrode (tip diameter 3 mm) for 10 min. An amount of50 wt% poly(amido-amine) with respect to hyperbranchedepoxy was mixed homogeneously with the above mixture andcoated on glass plates. Before curing, the plates were kept undervacuum at room temperature for 24 h to remove all volatiles.Finally, the plates were cured at 100 �C for 1 h into a furnace.The carbon dot reduced Cu2O nanohybrid/hyperbranchedepoxy nanocomposites were designated as ECDCO0.5,ECDCO1.0 and ECDCO1.5. The pristine hyperbranched epoxythermoset and its nanocomposite with 1 wt% carbon dot wereused for comparison purposes and are referred to as ECDCO0and ECD1.0, respectively.

Photocatalytic activity

The photodegradation of ethyl paraoxon organophosphatepesticide was selected for the determination of photocatalyticactivity of the nanocomposites. In a typical procedure, 10� 10 �0.3 mm3 sized (average weight 0.1–0.12 g) lms of both thenanocomposites ECD1.0 and ECDCO1.0 were cut into few smallpieces and separately placed into 25 mL aqueous solution of thepesticide (10 ppm). The solutions were stirred in the presence ofthe small pieces of nanocomposites under normal solar light (notdirectly under sunlight, light intensity: 800–1000 lux) at roomtemperature (25 �C). The experiment was also done for carbondot reduced Cu2O nanohybrid using the same amount of activeagent (1 wt%, 0.001 g) for comparison purposes. The change inthe concentration of the pesticide wasmonitored by detecting UVabsorbance intensity at wavelength of 300 nm for specied timeintervals. The activity of the catalyst was calculated from the rateof change of concentration of pesticide. Aer the completedegradation of pesticide, the pieces of catalyst were removed byltration and weighed aer drying at room temperature. Theused catalyst was further recycled to examine its reusability.

Characterization

FTIR spectra were recorded on a Nicolet FTIR spectrometer(Impact-410) using KBr pellets. The wide angle X-ray diffraction

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Scheme 1 (a) Reduction mechanism for the preparation of carbon dotreduced Cu2O nanohybrid, (b) photocatalytic mechanism for nano-composite film.

Fig. 1 FTIR spectra of carbon dot, carbon dot reduced Cu2O nano-hybrid and ECDCO1.0.

Paper RSC Advances

patterns of carbon dot reduced Cu2O nanohybrid and itsnanocomposite were recorded by an X-ray diffractometer,Miniex (Rigaku Corporation), using CuKa radiation (0.154nm). The morphology of the nanohybrid and nanocompositewere studied by high resolution transmission electron micro-scope, HRTEM (JEOL, JEMCXII, transmission electron micro-scope operating voltage at 200 kV). The tensile strength ofECDCO0, ECDCO0.5, ECDCO1.0 and ECDCO1.5 lms (size: 60� 10� 0.3 mm3) were measured by a Universal testing machine(UTMWDW10) with a 500 N load cell at a crosshead speed of 10mm min�1 using standard test ASTM D822. Scratch hardnesstest (ASTM G171) was done using a scratch hardness tester(Sheen Instrument) on the surface of the glass coated thermosetlms (size: 75 � 25 � 0.3 mm3). Impact test was carried out byan impact tester (S. C. Dey) as per the standard falling ballmethod (ASTM D1709) using the steel plate-coated thermosetlms (size: 150 � 50 � 0.3 mm3). The bending test of thermosetlms was carried out by ASTM D522 method using a mandrelwith a diameter 1–100 mm. All the tests for the measurement ofmechanical properties were repeated ve times and averagevalues were obtained. The thermal stability of the thermosetswas measured by thermogravimetric analysis (PerkinElmerTG4000) under nitrogen ow rate of 30 mL min�1 at a heatingrate of 10 �C min�1 from 30 to 700 �C. UV-visible absorptionspectra were recorded using a UV spectrometer, Hitachi (U2001,Tokyo, Japan). Intensity of the solar light was measured by luxmeter (LX-101, Lutron, Taiwan).

Results and discussionPreparation and characterization of carbon dot reduced Cu2Onanohybrid

The carbon dot reduced Cu2O nanohybrid was prepared by thereduction of Cu2+ by carbon dots. In this reduction processcarbon dots not only act as the reducing agent, but also act asthe capping agent. This is because a large number hydroxyl,carbonyl, carboxylic acid and epoxy groups are present on thesurface of carbon dots, as previously reported.5 Mainly periph-eral polar groups, such as hydroxyl and aldehyde of carbon dots,help to reduce the Cu2+ into Cu+. The hydroxyl groups reduceCu2+ by a polyphenolic mechanism, whereas the aldehydegroups reduce Cu2+ as in the Benedict reaction, as shown inScheme 1a. In the FTIR spectra (Fig. 1) it can be observed thatthe amount of hydroxyl groups (at 3400–3500 cm�1) of carbondot decreased aer the formation of carbon dot reduced Cu2Onanohybrid, whereas the amount of carbonyl groups (at 1650cm�1) increased. This is because the hydroxyl groups are con-verted into keto groups, whereas aldehyde groups are convertedinto carboxylic acid groups. Other functional groups andchemical linkages of carbon dot remained intact aer theformation of the nanohybrid. The Cu–O bonds were found inthe FTIR spectrum of nanohybrid at 465 and 556 cm�1. Theperipheral polar groups on the carbon dot also stabilize theCu2O nanoparticles. In the optical absorption spectrum of thenanohybrid (Fig. S1a†), the p–p* and n–p* transitions ofcarbon dots were found at 240 and 275 nm, whereas theabsorption peak for the Cu2O nanoparticle appeared at around

This journal is © The Royal Society of Chemistry 2014

500–600 nm. The formation of the nanohybrid was conrmedby TEM images (Fig. 2). Fig. 2a reveals that the nanoparticles arenearly spherical in shape with average diameter of around 3–4nm. From Fig. 2a and b, it can be conrmed that Cu2O-attachedcarbon dot and carbon dot embedded Cu2O were formed. Theformation of Cu2O was conrmed from the XRD pattern of thenanohybrid (Fig. 3). In this gure, the peaks at 2q (�) ¼ 36.4,42.3, 61.4 and 73.5 corresponding to crystallographic spacingd111, d200, d220 and d311, respectively, for Cu2O nanoparticleswere found in accordance to other reports and JCPDF #78-2076data.29–31 Bright spots in SAED pattern (Fig. 2d) of the TEM

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Fig. 2 TEM images: (a) higher magnification (inset: the internalstructure of Cu2O), (b) lowermagnification (inset: the internal structureof carbon dot) of carbon dot reduced Cu2O nanohybrid, (c) TEM imageof ECDCO1.0 and (d) selected area electron diffraction (SAED) patternof carbon dot reduced Cu2O nanohybrid.

Fig. 3 XRD patterns for carbon dot reduced Cu2O nanohybrid andECDCO1.0.

Fig. 4 Raman spectrum of carbon dot reduced Cu2O nanohybrid.

RSC Advances Paper

analysis also conrmed the presence of Cu2O crystals. Carbondots are amorphous or have poor crystalline structure, asreported in our earlier study.5 Two types of lattice spacing, d111spacing of Cu2O (0.27 nm) and d002 (0.38 nm) spacing of thecarbon dot, were found, as shown in the inset of the TEMimages (Fig. 2a and b).7 In the Raman spectrum (Fig. 4) ofnanohybrid, the G mode of phonon vibration of Cu2O wasobserved at 650–800 cm�1 (small peak).31 The D-band, G-bandand 2D-band for the carbon dot were observed at 1370, 1550

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and 2900 cm�1, respectively. The high value of the 2D-bandindicates that the carbon dot has a multilayered structure.

Preparation and characterization of nanocomposites

Nanocomposites were prepared by a solution technique andcured with poly(amido-amine) hardener. The hyperbranchedepoxy nanocomposite with 1 wt% of carbon dot (ECD1.0) wasused here only for comparison purposes; it had already beencharacterized in our earlier study.4 The representative carbondot reduced Cu2O nanohybrid/hyperbranched epoxy nano-composite (ECDCO1.0) was characterized by FTIR, XRD andTEM analyses. In FTIR spectra (Fig. 1), the hydroxyl band ofcarbon dot reduced Cu2O nanohybrid shied to 3390 from 3415cm�1 aer the formation of the nanocomposite. This is becauseof the different physico-chemical interactions of carbon dotsand Cu2O nanoparticles with hyperbranched epoxy. Because ofthe same reason, the Cu–O bands of the nanohybrid also shiedto 558 and 468 cm�1 aer the formation of the nanocomposite.The other strong bands around 1500 and 800 cm�1 are becauseof the –NH stretching and bending vibration for poly(amido-amine), respectively. Ether linkage for hyperbranched epoxyand carbon dot is assigned at 1250 cm�1. The strong interactionof nanoparticles with the hyperbranched epoxy matrix was alsorevealed from the XRD patterns (Fig. 3). The crystallographicpeaks of the nanohybrid were completely diminished aer theformation of the nanocomposite, whereas an amorphous poly-meric peak at 2q ¼ 20� was observed. The diminished Cu2Opeaks may also result from the masking effect of the polymermatrix (as the amount of Cu2O is very low compared to thematrix). Good dispersion and interaction of the nanohybridwith the matrices was conrmed form the TEM image (Fig. 2c)of carbon dot reduced Cu2O nanohybrid/hyperbranched epoxynanocomposite. The optical absorption spectra of ECDCO1.0and ECD1.0 lms are shown in Fig. S1b.† From the spectra, itcan be found that ECDCO1.0 lm absorb more visible light thanECD1.0 lm because of the combined absorption of visible lightby carbon dots and Cu2O nanoparticles.

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Table 1 Performance of pristine hyperbranched epoxy and its nanocomposites with carbon dot reduced Cu2O nanohybrid

Parameters ECDCO0 ECDCO0.5 ECDCO1.0 ECDCO1.5

Swelling value (%) 24 24 23 21Tensile strength (MPa) 40 � 1 43 � 1.5 45 � 1 48.5 � 1.5Elongation at break (%) 18.5 � 1 28.5 � 3 40 � 2 49.5 � 2Toughness (MPa)a 540 874 1370 1868.5Scratch hardness (kg)b 9.0 >10.0 >10.0 >10.0Impact resistance (cm)c >100 >100 >100 >100Bending diameter (mm)d <1 <1 <1 <1Initial degradation temperature (�C) 267 284 288 290

a Calculated by integrating the area under stress–strain curves. b Instrument limit of the scratch hardness was 10.0 kg (highest). c Instrument limitof the impact strength was 100 cm (highest). d Instrument limit of the mandrel diameter was 1 mm (lowest).

Paper RSC Advances

Mechanical properties

Mechanical properties, such as tensile strength, elongation atbreak, toughness, impact resistance, scratch hardness andbending values of pristine hyperbranched epoxy and thenanocomposites are given in Table 1. From the results, it wasfound that the tensile strength of pristine epoxy was improvedup to 20% aer the formation of nanocomposite with 1.5 wt%nanohybrid. However, the elongation at break and toughness ofthe hyperbranched epoxy thermoset was dramatically increasedby 2.5- and 3.5-fold aer the formation of the nanocompositewith the same amount of nanohybrid. The stress–strain prolesof the pristine thermoset, as well as those of its nano-composites, are shown in Fig. 5. From the gure, it can be seenthat tensile strength, elongation at break and toughness (areaunder stress–strain curves) of the hyperbranched epoxy ther-moset increased with increasing amounts of nanohybridloading. The simultaneous improvement in strength, elasticityand toughness is because of the strong physico-chemicalinteractions of the matrices with the quantum size carbondots. The presence of the aromatic carbonized structure of thecarbon dot enhances the strength of the nanocomposite, andthe peripheral polar functional groups of the carbon dot provide

Fig. 5 Stress–strain profiles for pristine thermoset and nano-composites with carbon dot and carbon dot reduced Cu2Onanohybrid.

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strong physico-chemical interactions with the hyperbranchedepoxy and poly(amido-amine) matrices; these interactionssimultaneously increase the elasticity and toughness of nano-composites.4 The very small size of the carbon dot and Cu2Onanoparticles also provide a large surface area for stronginteractions with the matrices, which enhances all themechanical properties of the nanocomposites. Because thetoughness of the hyperbranched epoxy thermoset was dramat-ically improved, the other mechanical properties, such asimpact resistance and scratch hardness, which are related totoughness, were also improved by the formation of nano-composites. However, these differences could not be measured,as the values for nanocomposites reached the highest limit ofthe instruments for scratch hardness (10 kg) and impact resis-tance (100 cm). Nanocomposites also exhibited the lowest limitof the instrument for exibility evaluation (1 mm bendingdiameter of mandrel) without damage to the lm.

Thermal stability

The initial degradation temperatures of the pristine hyper-branched epoxy and its nanocomposites are given in Table 1and TGA curves are shown in Fig. 6. From the results, it can beseen that the initial degradation temperature of hyperbranchedepoxy thermoset increased to 23 �C aer the formation of thenanocomposite with 1.5 wt% nanohybrid. The initial degrada-tion temperature increased with increasing amount of nano-hybrid loading. The improvement in thermal stability isbecause of the strong interactions of the quantum-sized carbondots and Cu2O nanoparticles with the hyperbranched epoxy andpoly(amido-amine) matrices. The quantum sizes of carbon dotsand Cu2O nanoparticles provide a large surface area for stronginteractions. Moreover, the presence of the aromatic carbonizedstructure and peripheral polar functional groups of carbon dotprovide strong physico-chemical interactions with the matrices,which enhances the thermal stability of nanocomposites. Inaddition, the nanohybrid acts as a mass transport barrier to thevolatile products generated during decomposition by providinglonger paths for them to travel.32

Photocatalytic activity

The photocatalytic degradation of paraoxon pesticide wasstudied by ECDCO1.0 and ECD1.0 nanocomposite lms under

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Fig. 6 TGA curves for pristine thermoset and its nanocomposites withnanohybrid.

RSC Advances Paper

normal solar light (not directly exposed under the sunlight). Thenanocomposite lm was chopped into small pieces to increasethe surface area, as well as to expose the Cu2O nanoparticles.The concentration changes of the pesticide with time weremonitored by UV absorbance. The plots of optical absorbanceagainst wavelength for the degradation of pesticide at differenttimes for both ECDCO1.0 and ECD1.0 are shown in Fig. 7a andb, respectively. The rate of degradation of pesticide with time forECDCO1.0 (both in dark and solar light), ECD1.0 and nano-hybrid is shown in Fig. 7c. From these gures, it is clear that thetime taken for 90% degradation of the pesticide was 5 h forECDCO1.0 lm, whereas only 50% degradation of the same wasobserved aer 12 h for ECD1.0 lm under solar light; nodegradation was observed under darkness. In the case of pho-tocatalytic activity of nanohybrid, the initial degradation rate of

Fig. 7 Plots of UV absorbance against wavelength at different timesfor the degradation of paraoxon pesticide in the presence of (a)ECDCO1.0 and (b) ECD1.0 film, (c) plots of degradation rate forECDCO1.0 (at dark and solar light), ECD1.0, nanohybrid; and (d) fittingcurves for the ECDCO1.0 and ECD1.0 for pseudo-first order model.

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the nanohybrid is slightly slower than that of the nano-composite lm. This may be because of the different polarfunctional groups of hyperbranched epoxy, which may interactwith the polar organophosphate pesticide and adhere on itssurface, which facilitates the interaction with the active oxygenradicals. However, because the same amount of active agent (1wt% nanohybrid) was used in both the cases, the same extent ofdegradation was observed under the same total period ofexposure. However, the recovery of the nanohybrid catalyst isdifficult because of its nano-level dispersion in water by thestrong interactions of polar functional groups of the carbon dot.The recovery of this catalyst needs a large amount of organicsolvent (acetone), as well as high speed of centrifugation (8000–10 000 rpm) process. However, the nanocomposite lm can beeasily recovered from the reaction medium (water). The activityof pure commercial Cu2O nanoparticles has been reported inthe literature for photocatalytic degradation of dye as <3%.7 Thedegradation of the pesticide using different nanophotocatalystswas also reported in literatures,27 but in all the cases, the pho-tocatalytic activity was studied under UV irradiation. Further-more, the efficiency of these catalysts is inferior compared tothe presently reported catalyst, where there is no need of anyadditional energy other than normal solar light.

The pseudo-rst order kinetic model equation was used todescribe the photo-degradation behavior of nanocompositelms as follows:

�dC/dt ¼ K1t (1)

where C is the concentration of pesticide at any time t and K1 isthe apparent rate constant. Aer integrating eqn (1), thefollowing equation is obtained:

ln(C/C0) ¼ �K1t (2)

where C0 is the initial concentration (at t ¼ 0) of pesticide. Thetting plots of ln(C/C0) versus time are shown in Fig. 7d, whichdemonstrate that the degradation of the paraoxon pesticide iswell described by pseudo-rst order kinetics with the ttingcoefficients over 0.9, indicating a regular photo-degradationbehavior.11

The photodegradation mechanism is schematically illus-trated in Scheme 1b based on other reported carbon dot/metalor metal oxide nanoparticles by exploiting the up-conversionluminescence properties of carbon dot.2,7,8 When ECDCO1.0nanocomposite lm was exposed to the pesticide solutionunder solar light, carbon dot absorbed visible light as well asnear infrared light and emitted shorter wavelengths of light(300–500 nm), as revealed in our previous study.4 This shorterwavelength of light again excites the Cu2O nanoparticles andforms electron/hole (e�/h+) pairs. These electron/hole pairsreact with H2O and O2 to produce active oxygen radicals likecOH and cO2

�, which take part in the degradation of paraoxonpesticide. In case of ECD1.0 nanocomposite, only cO2

� radicalsare formed, and thus the degradation of pesticide takes place ata relatively slow rate.2,7,8

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Table 2 Catalytic activity at different cycles of reuse

CatalystNo. ofcycle

Degradation time(h)

Percentage ofdegradation

Weightloss (%)

ECDCO1.0 1st 5 90 02nd 5 90 03rd 5 90 0

ECD1.0 1st 12 50 02nd 12 50 0.113rd 12.25 50 0.17

Paper RSC Advances

ECDCO1.0 also absorbs more visible light compared toECD1.0, as revealed from their absorption spectra (Fig. S1b†),which helps to increase the catalytic efficiency of ECDCO1.0under solar light. The products that form aer the degradationof the pesticide include H2O, CO2, phosphoric acid, N2, and O2

as reported in literatures.26,27

The catalytic activity of the reused photocatalyst was checkedup to third cycle, as shown in Table 2. The catalytic activity, aswell as the weight, of the nanocomposite lms remained almostconstant. This is because of the strong interaction of thenanoparticles with the hyperbranched epoxy and poly(amido-amine) matrices and the prevented leaching of materials. Inthe case of ECD1.0, a negligible weight loss (0.1–0.2%), and thusloss of catalytic activity of the lms was observed aer 3rd cycle,which may be because of the long exposure time of the lm inthe pesticide solution.

Conclusion

In this study, we have demonstrated a convenient high perfor-mance thermostable hyperbranched epoxy nanocomposite,which exhibits photocatalytic activity towards the degradationof health-hazardous pesticide under solar light. Carbon dotreduced Cu2O nanohybrid was successfully prepared in situ bythe simple reduction of cupric acetate solution by carbon dots.The hyperbranched epoxy thermoset acted as a very goodpolymeric support for efficient recycling and reuse of the cata-lyst by providing strong physico-chemical interactions with thenanohybrid particles.

Acknowledgements

The authors express their gratitude to the NRB for nancialassistance through the grant no. DNRD/05/4003/NRB/251 dated29.02.12. SAIF of NEHU, Shillong is gratefully acknowledged forthe TEM imaging. We also express thanks to Dr PanchananPuzari, Department of Chemical Sciences, Tezpur University forproviding paraoxon pesticide for this study.

Notes and references

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