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Optical Materials 62 (2016) 320e327

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Optical Materials

journal homepage: www.elsevier .com/locate/optmat

Chemically derived luminescent graphene oxide nanosheets and itssunlight driven photocatalytic activity against methylene blue dye

Sumeet Kumar, Ashok Kumar*

Department of Physics, Tezpur University, Assam, 784028, India

a r t i c l e i n f o

Article history:Received 6 September 2016Received in revised form25 September 2016Accepted 6 October 2016

Keywords:Chemically derived graphene oxideWell oxygenatedLarge and transparent GO NSsExcitation wavelength dependentphotoluminescencePhotocatalytic activity

* Corresponding author. Department of Physics,Napaam, 784028, Assam, India.

E-mail address: ask@tezu.ernet.in (A. Kumar).

http://dx.doi.org/10.1016/j.optmat.2016.10.0140925-3467/© 2016 Elsevier B.V. All rights reserved.

a b s t r a c t

In the present work, graphene oxide (GO) nanosheets (NSs) have been synthesized with precise controlover their thickness and molecular structure. The existence of oxygen containing functional groups onGO NSs through chemical treatment confers remarkable optical properties on GO. XRD, TEM, Raman andFTIR techniques were used to confirm the phase and degree of oxidation, morphology, structural in-formation and chemical structure of the synthesized GO NSs. UVeVis. spectroscopy was employed tostudy the optical absorption properties of the synthesized GO NSs. The excitation wavelength dependentPL measurements of the synthesized GO NSs were carried out which could be useful for the design anddevelopment of GO based next generation optoelectronic devices. The most fascinating luminescentproperty of synthesized GO NSs is that its luminescence peak position can be easily tuned by only varyingthe excitation wavelength without significant changes in its size and chemical composition. In order tostudy the photocatalytic degradation of methylene blue (MB) dye using GO NSs as a photocatalyst, asunlight driven photocatalytic activity has been performed. The degradation rate of MB dye becomes fastwhen GO NSs are added to the dye solution. The photodegradation efficiency of GO NSs is calculated tobe 60%. The present results indicate that synthesized GO NSs can be used as sunlight active photocatalyst.The optimistic response to sunlight irradiation validates the potential of GO NSs in solar energyconversion.

© 2016 Elsevier B.V. All rights reserved.

1. Introduction

In the recent times, chemically derived graphene oxide (GO) hasbecome one of the most important graphene derivatives because ofits simple preparation, low cost, high luminescent, low toxicity,high chemical stability and enormous potential for technologicalapplications [1,2]. Chemically derived GO physically preserves twodimensional (2D) atomic level thin structure and looks similar tographene except that the planes and edges of GO are functionalizedwith covalently decorated oxygen containing functional groups. Itis usually accepted that hydroxyl (CeOH) and epoxy (CeOeC) ox-ygen containing functional groups are attached to above and belowof the basal planes, while carboxyl (COOH) and ketones (C]O)groups of oxygen functionalities are bound to the edges of the basalplanes [3]. The covalent addition of oxygen containing functionalgroups imposes a structural disorder by changing the original

Tezpur University, Tezpur,

unsaturated planar sp2 hybridization of carbon atoms into tetra-hedral sp3 carbon atoms hybridization and breaks the continuity ofintrinsic p-conjugated network of GO. This discontinuity of p-network consequently alters the electron density distribution in GOand plays a vital role in the change of electronic properties byintroducing a bandgap through the emergence of new electronicstates from oxygen functionalities. This is because the appearanceof oxidized sites after the adsorption of oxygen functionalitiescreates strong repulsive barrier for p-electron wave functions,which in turn opens the bandgap and changes the electronicstructure of GO from a semimetal to a semiconductor [4]. The newelectronic states provide the tunable emission properties to GOsheets, which are extremely promising for utilizing GO sheets in avariety of optoelectronic applications [5].

Chemically derived GO contains two regions consisting of thehydrophobic p-conjugated sp2 domains and the sp3 domains withhydrophilic oxygen-containing functional groups. These two re-gions coexist and sp2 regions of finite size are cloaked in sp3 matrix.As a result, sp2 domains show quantum confinement effect due tothe confinement of p-electrons, which induces semiconducting

S. Kumar, A. Kumar / Optical Materials 62 (2016) 320e327 321

behavior in GO [6]. In this context, it is clear that chemically derivedGO is a hybrid electronic material in which conducting p and p*electronic states of the sp2 domains that lie within the large energygap of s and s* electronic levels of the sp3 matrix. The size, shapeand fraction of the sp2 domains allow the manipulation of bandgapin GO for tunability of its electronic, electrical and optical propertieswhich could be useful for efficient photocatalytic application [7].The smaller fragment of sp2 domain has large energy gap whichallow photo induced generation of charge carriers. The existence ofband gap and the generation of photo induced electron-hole pairshave been validated through photoconductivity studies [8].

In the present work, chemically derived GO NSs with rich oxy-gen containing functional groups have been prepared throughchemical oxidation of graphite using concentrated sulfuric acid andphosphoric acid with highly oxidizing agents such as potassiumpermanganate. During synthesis, the temperature was also main-tained at a desired level in order to get well oxidized GO NSs. Thefundamental structure and optical properties have been discussedin relation to their prospective applications. The present work hasdemonstrated the strong excitation wavelength dependent lumi-nescence phenomenon in GO NSs without the need of any specialenvironments. Such an intriguing property makes GO a more ver-satile material in order to develop new technologies based on thisproperty. Moreover, the primary objective of this work is to explorethe possibility of utilizing GO NSs in photocatalytic application suchas dye degradation. In order to exploit the possibility of photo-catalytic activity of GO NSs, a sunlight driven photodegradation ofmethylene blue (MB) dye has been done. It should be noted that theuse of GO as individual photocatalytic material is rare. GO NSs havebeen mainly used as supporting material for semiconductornanostructures so far.

2. Experimental

2.1. Materials and synthesis

Graphite flakes, sulfuric acid (H2SO4), phosphoric acid (H3PO4),potassium permanganate (KMnO4), hydrogen peroxide (H2O2) andhydrogen chloride (HCl) were purchased from Sigma e Aldrich(USA) and used without further purification. Graphite flakes is anaturally occurring common source of graphite which is mostlyused for redox reactions. The main advantage of the present syn-thesis method is its simple steps without formation of toxic andexplosive gases such as NO2, N2O4, and ClO2. GO synthesized fromgraphite flakes can be easily dispersed in water and can be pro-duced on a large scale.

GO NSs was synthesized using a procedure based on Improvedmethod [9] reported in my earlier work [10]. In this method, theoxidation of graphite flakes is carried out by treating it with awaterfree mixture of concentrated H2SO4, H3PO4 and KMnO4. First of all,graphite flakes (3 g) was homogeneously mixed up with a mixtureof concentrated H2SO4:H3PO4 (9:1) in a conical flask using a mag-netic stirrer at room temperature. The homogeneous solution ofconcentrated H2SO4/H3PO4 and graphite flakes was made byvigorous stirring for 30 min. Thereafter, the conical flask containinghomogeneous solution was kept in a water bath. After that, 18 g ofKMnO4 was added very slowly under vigorous stirring. Here, it isnoted that the temperature of the resultant solution was main-tained at 35 �C while mixing KMnO4. After mixing of KMnO4properly, the water bath was removed. The obtained solution wasthen heated to 60 �C and left for stirring for 12 h to allow thecomplete oxidation of graphite flakes. During oxidation reaction,the color of the viscous solution turned out to be dark brown (seeFig. 1). After vigorous stirring for 12 h, the viscous solution wascooled to room temperature. Later, the ice is poured to the solution

and H2O2 solution was added gradually to stop the oxidation pro-cess, till the color of the mixed solution turned to bright yellow. Thebright yellow color indicates the high oxidation level of graphiteflakes. The supernatant was then removed through centrifugationat 4000 rpm. The obtained precipitate was washed repeatedly with200 ml of 30% HCl, 200 ml of ethanol and 200 ml of water until thepH became neutral. The remaining solid material was then driedovernight at 40 �C. The obtained dark brown solid powder of GOwas collected for further use.

2.2. Characterization

X-ray diffraction (XRD) pattern was recorded using a RigakuSmart Lab X-ray diffractometer in a 2q range of 5�e30� with CuKaradiation at l ¼ 1.5418 Å. XRD pattern was used to confirm thecrystalline phase of the synthesized sample. Transmission electronmicroscopy (TEM) images were obtained using JEOL JEM-2100 TEMat 200 kV (JEOL Co. Ltd.) to investigate size, shape and morphology.Raman spectrum of the synthesized products was recorded in thespectral range 500e2000 cm�1 using WI-Tec Raman spectrometerat room temperature with Arþ laser at wavelength of 514 nm andpower of 3 mW. The Fourier transform infrared (FTIR) spectrumwas recorded on an NEXUS- 870 FT-IR spectrophotometer.Ultraviolet-visible (UVeVis.) spectrum was recorded on a Perki-nElmer (Lambda35) UVeVis spectrometer in absorbance mode. Allphotoluminescence (PL) spectrawere obtained using a PerkinElmer(LS-55) Fluorescence Spectrometer. The samples were excited atvarious excitation wavelengths and corresponding emissionspectra were recorded.

2.3. Photocatalytic measurements

The photocatalytic activity of GO NSs was evaluated using thephotodegradation of MB dye under sunlight irradiation. In a typicalexperiment, 50 mg of synthesized GO NSs was mixed with 50 mL ofaqueous solution of 10�5 M MB dye. The suspension was stirredvigorously and kept in dark for 30 min in order to achieve theadsorption-desorption equilibration. Then after, the reactionmixture was exposed to sunlight. After a regular interval of sunlightexposure, the supernatant was separated from reaction mixture.The degradation of dye was monitored through recorded absorp-tion spectra of irradiated solution by measuring the absorbance ofMB at 664 nm at different time interval. The photodegradationefficiency % of MB was calculated using Eq. (1).

Photodegradation efficiency % ¼ C0 � CC

� 100 (1)

where C0 is the initial concentration of MB dye and C is the con-centration of MB dye after irradiation.

3. Results and discussion

3.1. XRD studies

XRD pattern provides a conclusive evidence for the degree ofoxidation of raw graphite (graphite flakes) into GO on the basis ofdiffraction peak positions (2q) and interlayer distances (d). Thecharacteristic diffraction peak of raw graphite is usually observed atabout 2q ¼ 26.4� corresponding to (002) plane with d ¼ 0.336 nm.The X-ray diffraction (XRD) pattern of synthesized GO NSs is shownin Fig. 2. The observed diffraction peak at 2q¼ 10.14� correspondingto (002) plane in the XRD spectrum with no diffraction peak at26.4� confirms the formation of GO [10]. The disappearance ofdiffraction peak at 26.4� confirms that the raw material (graphite

Fig. 1. Diagram for the synthesis of GO NSs.

Fig. 2. XRD pattern for GO NSs.

S. Kumar, A. Kumar / Optical Materials 62 (2016) 320e327322

flakes) is not present in the synthesized sample and it has been fullyoxidized into GO. The exfoliation of GO NSs is also validatedthrough longer interlayer spacing of GO obtained from XRD data.The interlayer spacing of prepared GO NSs is calculated to be0.905 nm. The longer value of interlayer spacing for GO NSs(0.905 nm) in comparison to that of graphite flakes (0.336 nm)indicates the development of well exfoliated graphitic layers withhigher degree of oxidation [11]. The longer interlayer distance of GONSs compared to graphite flakes is ascribed to the existence ofinserted water (H2O) molecules and oxygen containing functionalgroups such as; epoxyl, hydroxyl, carbonyl, carboxyl groups at thebasal planes and edges of the structure of GO. The H2O moleculesand oxygen containing functional groups are attached to thestructure of GO during the oxidation process and looses the stacksof graphitic layers effectively [3]. The presence of polar oxygencontaining functional groups strongly changes the vanderWaalsinteractions between GO layers and GO can readily get exfoliated.

The absorbed H2O molecules also reduce the vanderWaals inter-action between GO layers which increase the distance among GOlayers. As a result, GO layers exfoliate effectively [12]. The sharpintensity of diffraction peak of GO NSs suggests that the presentsynthesis method is able to produce crystalline GO with regularcarbon frame work. The broader diffraction peak of GO also in-dicates the presence of fewer exfoliated layers in the disruptedstructure of GO NSs [13].

3.2. TEM analysis

To further investigate the morphology of the as synthesized GONSs, TEMmeasurements were conducted. Fig. 3 shows TEM imagesat different resolutions of chemically derived GO NSs. It is obviousby looking at the TEM images that as synthesized GO NSs areapproximately several micrometers large transparent thin sheetswith noticeable visible wrinkles which suggests that the preparedGO NSs are made up of single or few layers of GO [14]. The lowcontrast of obtained GO NSs also specifies the fact that GO NSs arevery thin and almost transparent, indicating a complete exfoliationof graphite oxide [15]. The GO NSs are not absolutely flat but exhibitcorrugated and curly. It is well reported that corrugated and curlystructures are the intrinsic nature of GOwhich confirm the fact thatthe synthesized GO NSs is thermodynamically stable [16].

3.3. Raman spectroscopy

Raman spectroscopy is a highly sensitive characterizationtechnique to obtain structural information of carbon based mate-rials [17]. The Raman spectrum of as synthesized GO NSs shown inFig. 4 was recorded in the spectral range of 400e2400 cm�1. Ramanspectroscopy of carbon based materials is usually characterized byconsidering twomain features: the graphitic peak (G) and diamondpeak (D). The G peak arises from the first order scattering of theoptical E2g phonon of the sp2 carbon atoms at the Brillouin zonecenter (generally observed at 1575 cm�1) and D peak (1345 cm�1),comes from the breathing mode of k-point photons with A1gsymmetry [13]. The D peak is Raman active only in the presence ofdefects. The intensity of D peak is often used to measure the degreeof lattice distortions [18,19]. In the present study, the G and D peaks

Fig. 3. TEM images at different resolutions of GO NSs.

Fig. 4. Raman spectrum of GO NSs.Fig. 5. FTIR spectrum of GO NSs.

S. Kumar, A. Kumar / Optical Materials 62 (2016) 320e327 323

of GO NSs appear at 1599 and 1369 cm�1, respectively. Theappearance of G and D peaks in higher wavenumber regions in-dicates the formation of defects caused by the destruction of the sp2

network of carbon atoms due to oxidation process [13]. Duringoxidation process, the oxygen containing functional groups areattached to the basal planes and edges of GO NSs which createsdefects in the structure of GO. In addition, the higher intensity of Dpeak than G peak along with their large full width at half maximum(FWHM) also indicates considerable structural disorder in GO NSs[5]. The relative intensity of the D band to the G band (ID/IG) iscalculated to be about 1.14. The higher intensity value of ID/IG ratioconfirms higher disorder density.

3.4. FTIR spectroscopy

The chemical structure of synthesized GO NSs is identified byFTIR spectroscopy. FTIR spectrum of as synthesized GO NSs isshown in Fig. 5. The spectrumwas recorded in the spectral range of4000e400 cm�1. FTIR spectrum also makes possible to investigatethe presence of oxygen containing functional groups formed by theoxidation process. As demonstrated in Fig. 5, the presence of severaloxygen containing functional groups such as; hydroxyl(3436 cm�1), epoxy (1228 cm�1), carbonyl (1701 cm�1), ether

(586 cm�1) groups [20] indicated a high degree of oxidation ofsynthesized GO NSs [ 21]. These oxygen containing functionalgroups are attached on the basal planes and edges of GO sheets[22]. The characteristic peaks of GO NSs are detected at 3436 and1050 cm�1 due to OeH and CeO stretching vibrations, respectively[23]. The peak appeared at 1377 cm�1 corresponds to the eOHdeformation vibration peaks [1]. The peak appeared at 1228 cm�1 isdue to CeO stretching in epoxy group while at 1701 cm�1 is due toC]O stretching vibration in carboxylic acid group [23]. The peakcorresponding to 1609 cm�1 is due to C]C stretching vibrationfrom unoxidized sp2 CC bonds [9]. FTIR results of the present studyare consistent with previous reports [20e23]. The presence of polaroxygen containing functional groups strongly changes the van-derWaals interactions between GO layers and makes it hydrophilic.Due to the hydrophilic nature, GO NSs can easily get dispersed inwater and form a stable colloidal aqueous suspension [12].

3.5. UVeVis. spectroscopy

In order to investigate the optical absorption properties of assynthesized GO NSs, UVeVis. spectroscopy was employed. Theaqueous solution of GO NSs was used to record absorption spec-trum in the optical range of 200e800 nm. The UVeVis. absorptionspectrum is shown in Fig. 6. The absorption spectrum of GO NSs

S. Kumar, A. Kumar / Optical Materials 62 (2016) 320e327324

shows a main absorption peak at 227 nm, attributed to p e p*transition of the CeC and C]C bonds of sp2 conjugated network[24]. Also, a broad shoulder around 305 nm is attributed to the n e

p* transition C]O bonds of the carbonyl groups from sp3 conju-gated network [25]. These obtained characteristic peaks suggestthat the prepared GO sheets have similar structure as XRD, Ramanand FTIR data indicated. The characteristic absorption peaks of GONSs observed in the present study are similar to the previouslyreported for GO [9,26e28].

Based on the above reports, it can be assumed that the opticalabsorption of GO give rise to a characteristic absorption peak in therange of 217e238 nm. The shifting in the absorption peak of GO isarisen due to increasing the degree of oxidation and decreasing thesize of p e conjugation systems. The optical absorption of GO ismainly dominated by p e p* transitions, which gives characteristicabsorption peak [28]. It has been demonstrated by Cushing et al.[26] that after chemical oxidation, the molecular structure of GONSs become enriched with oxygen containing functional groupssuch as; eOH and eCOOH groups. In case of eOH enriched GOnanosheets, the n e p* transition strength is reduced, and the p e

p* transition peak red shifted due to the enhanced number ofelectron donating groups. While, the eCOOH enriched GO NSs blueshifted the p e p* transition and increased the n e p* transitionstrength. The tail of absorption spectra is constant after chemicaloxidation. It is worth noting that in the present work, the UVeVis.absorption spectrum of GO NSs confirms that the optical propertiesare dominated by both the eOH and eCOOH groups. It also in-dicates that as synthesized GONSs arewell oxygenated. The inset ofFig. 6 shows a digital photograph of stable colloidal aqueous sus-pension of GO NSs. The high stability of aqueous suspension of GONSs in air at room temperature offers another advantage for futureapplications. The aqueous stable dispersion of GO NSs is produceddue to the ionization of oxygen containing functional groups andthe weakening of vanderWaals interaction between GO layers. Theionization of oxygen containing functional groups through thetreatment of magnetic stirring and ultrasonicationmakes GO layersnegatively charged and increases electrostatic repulsion amongthem. The absorbed water molecules increase the distance amongGO layers which reduce the vanderWaals interaction betweenthem. As a result, GO NSs easily get exfoliated in water and form astable colloidal aqueous suspension [29].

Fig. 6. UVeVis. spectrum of GO NSs. Inset: Photograph of the aqueous dispersion of GONSs.

3.6. Photoluminescence studies

Fig. 7 shows the excitation wavelength dependent photo-luminescence emission spectra of the aqueous solution of synthe-sized GO NSs. It is clear from the recorded emission spectra that theemission wavelength of GO NSs is significantly dependent on theexcitation wavelength. The emission peak shifts towards higherwavelength from 400 to 450 nm with an increase in the excitationwavelength from 465 to 530 nm. It seems quite interesting that theemission peak position of the synthesized GO NSs can be tuned byjust changing the excitation wavelength without changing its sizeand chemical composition. Hence, discovery of such a strongexcitation wavelength dependent photoluminescence mechanismcould be useful for the development of GO based optoelectronicdevices. Apparently, the excitationwavelength dependent emissionof GO NSs is similar to previous reports on photoluminescencespectra of GO [26,30,31]. The photoluminescence emission ofconventional organic dyes and inorganic nanoparticles is generallyindependent of the excitation wavelength. On the contrary, thepeak position of the photoluminescence emission of GO is pro-foundly dependent on the excitation wavelength [26]. Although,the present work has tried to explore the reasons of strong exci-tation wavelength dependent photoluminescence in GO NSs, theexact mechanisms responsible for excitation wavelength depen-dent PL in GO remain to be elucidated. Considerable research re-sults have shown that GO can emit ultraviolet, visible and infraredcolors depending upon the sp2 structures and oxygen containingfunctional groups [30]. It has been reported that the photo-luminescence in GO NSs is strongly associated with the C]C, CeOHand COOH (or C]O) functional groups that normally exist on theGO NSs [31]. In a previous work [26], it has been reported that in apolar solvent like water, solvation process acts to make the energyof the excited state lower and the emission shifts towards longerwavelength. After excitation, the dipoles of solvent and the excitedstates of GONSs are not in equilibrium. The dipoles of solvent rotateto get aligned with excited dipoles of GO NSs and as a result,decrease the emission energy for GO NSs. The relaxation betweenthe dipoles of solvent and the excited state of GO NSs creates anexcitation wavelength dependent photoluminescence in GO NSs.This phenomenon is called red edge shift which makes the emis-sion peak dependent on the excitation wavelength. The red shift inemission energy is mainly due to the differences in relaxation time

Fig. 7. PL emission spectra of GO NSs excited with light of different wavelengths.

S. Kumar, A. Kumar / Optical Materials 62 (2016) 320e327 325

of solvent and GO NSs. The solvation process gets completed beforeemission and red shifting in emission wavelength is occurred.

The oxidation process makes GO a mixture of sp2 and sp3 hy-bridized carbon atoms. The sp2 hybridized carbon atoms are joinedto the neighboring carbon atoms while the sp3 hybridized carbonatoms are attached to the adjacent oxygen-containing functionalgroups. These oxygen-containing functional groups alter the elec-tronic structure of GO NSs and create an electronic band gap [32].The specific electronic structure of GO NSs arises due to theadsorption of oxygen functional groups which disrupts the p-network symmetry and the oxidized sites create strongly repulsivebarriers for the p-electron wave functions, resulting in the gapopening at the Brillouin Zone boundary [4]. The band gap value ofGO depends on the nature and the atomic ratio of sp2/sp3 hybrid-ized carbon atoms. Tuning the ratio of sp2/sp3 carbon atoms caneffectively transform GO NSs into semiconductor or semimetalwhich vary the PL emission range from visible to near-infraredwavelength [4]. The tunable electronic structure of GO NSs willexploit its application in optical, electronic and biomedical fields[31].

3.7. Photocatalytic activity

GO NSs may become a next generation photocatalytic materialbecause (i) the two dimensional (2D) structure of GO NSs decoratedwith oxygen containing functional groups has large surface areawith a huge number of active sites that provides large reactionspace and reaction centers for adsorbed cationic MB dye moleculesand negatively charged oxygen containing functional groups [33](ii) The 2D thin atomic structure of GO NSs provides fast transferof charge carriers to the surface to perform the desired redox re-action [34] (iii) the presence of polar oxygen functional groupsmakes GO NSs strongly hydrophilic which could be useful for anaqueous photocatalytic system [35]. Thus, GO NSs can be utilized asa photocatalyst due to its own inherent properties.

The application of GO NSs as a photocatalyst depends on thenature of oxygen functional groups, which could lead to the local-ization of electron (e�) � hole (hþ) pairs [5]. Upon the irradiation oflight, the electrons (e�) and holes (hþ) are generated andmigrate tothe surface of GONSs. These photo induced electrons (e�) and holes(hþ) act as oxidizing and reducing sites, respectively and react withthe adsorbed MB dye molecules. The reduction potential of e� inthe conduction band (CB) of GO is around �0.79 V (vs. NHE) andacts as a donor. On the other hand, the oxidation potential of the hþ

in the valance band (VB) of GO is about 4 V (vs. NHE) and serves asan acceptor [7].

Fig. 8. Absorption spectra of aqueous solution of MB dye exposed under direct sunlight at disolution mixed with GO NSs as photocatalysts under direct sunlight irradiation (b).

In order to study the photocatalytic degradation of methyleneblue (MB) dye using synthesized GO NSs nanosheets as a photo-catalyst, a sunlight driven photocatalytic activity has been per-formed. Fig. 8a shows the change in the absorption spectrum of theMB dye solution irradiated by sunlight for different irradiationtimes. The decrease in the intensity of the characteristic absorbanceband at ~664 nm is selected to monitor the degradation of MB dye.Fig. 8b compares the photodegradation rate of pure MB dye solu-tion and dye solution with GO NSs as photocatalyst. The degrada-tion rate of MB dye without catalyst is relatively small, while thedegradation rate becomes fast when GO NSs are added to the dyesolution. The photodegradation efficiency of GO NSs is calculated tobe 60%. The present results indicate that synthesized GO nano-sheets can be used as sunlight active photocatalyst. The optimisticresponse to sunlight irradiation validates the potential of GO NSs insolar energy conversion. One of the great advantages of GO NSs isits tunable band gap that is correlated to the degree of oxidation.The visible light activity (<3.0 eV) of GO upon bandgap modulationhas been confirmed experimentally [36e38].

The schematic diagram of the photocatalytic mechanism of GONSs is shown in Fig. 9. The 2D thin structure of GO NSs decoratedwith negatively charged oxygen containing functional groups offersa large surface area to the attaching cationic MB dye molecules.Therefore, more MB dye molecules can interact with the photo-catalyst, which makes the efficiency of the photodegradation ratehigh. Matsumoto [38] has also investigated themechanisms behindthe photocatalytic activity of GO NSs in water. When light is inci-dent on the MB dye aqueous solution, electron (e�) � hole (hþ)pairs are generated after absorption of photon energy due to a p-p*excitation in the p-conjugated sp2 domains of GO [39]. The holesare allowed to react with OH� ions and creates $OH radicals presentin the aqueous solution of MB dye while electrons may react withdissolved O2 and creates O2

$� radicals into the solution. Finally, theseradicals degrade MB dye molecules into small fragments [40].

4. Conclusion

The present method yields well oxidized hydrophilic GO NSs atlarge scale. The protocol used here for synthesizing GONSs does notproduce large heat and any toxic gases such as; NO2, N2O4, andClO2. The physicochemical properties of GO NSs can be tuned andenhanced by controlling degree of oxidation to facilitate specificoptoelectronic application. The obtained results from XRD, TEM,Raman, FTIR and UVeVis. studies confirm the well exfoliation andoxidation of prepared GO NSs. The observed diffraction peak at2q ¼ 10.14� in the XRD pattern with suppressed diffraction peak at

fferent irradiation times (a) photodegradation rate of pure MB dye solution and MB dye

Fig. 9. Mechanism for photocatalytic degradation of MB dye with GO NSs.

S. Kumar, A. Kumar / Optical Materials 62 (2016) 320e327326

26.4� confirms that the synthesized sample has been oxidized. Theincreased value of interlayer spacing for GO NSs (0.905 nm) ascompared to that of graphite flakes (0.336 nm) indicates the for-mation of well exfoliated graphitic layers with higher degree ofoxidation. TEM images with noticeable visible wrinkles and curlystructures suggest that the prepared GO NSs are thermodynami-cally stable and made up of single or few layers. The higher in-tensity of D peak than that of G peak along with large full width athalf maximum (FWHM) in the Raman spectrum also indicatesconsiderable structural disorder in GO NSs due to oxidation pro-cess. The presence of several oxygen containing functional groupssuch as hydroxyl (3436 cm�1), epoxy (1228 cm�1), carbonyl(1701 cm�1) and ether (586 cm�1) groups in the FTIR spectrumindicates high degree of oxidation of synthesized GO NSs. The ob-tained characteristic absorption peak at 227 nm in UVeVis. spec-trum, attributed to p e p* transition of the CeC and C]C bonds ofsp2 conjugated network, and a broad shoulder around 305 nm,attributed to the n e p* transition C]O bonds of the carbonylgroups from sp3 conjugated network, suggest that the prepared GOsheets are heavily oxidized. The degree of oxidation of GO providesprecise control over the fraction and distribution of sp2 carbonatoms in the sp3 matrix. The sp2 domains exhibit quantumconfinement effect due to the confinement of p-electrons andcreate band gap. Since band gap allows PL to occur, the size, shapeand fraction of the sp2 domains allow excitation wavelengthdependent PL phenomena. The obtained excitation wavelengthdependent PL phenomena of GO NSs confirm that emission prop-erties of GO NSs can be tailored for versatile optoelectronic appli-cations. The photodegradation rate of MB dye in the presence of GONSs depicts that the well controlled structure and morphology willnot only provide desired redox reaction between GO NSs photo-catalyst and reactant molecules but also enhance the lightharvesting.

Acknowledgement

Authors are thankful to University Grants Commission (UGC),India for providing the financial support. SK is thankful to the UGCfor the UGC-Dr. D. S. Kothari Post Doctoral Fellowship Scheme (No.F.4-2/2006(BSR)/PH/15-16/0067).

References

[1] Y. Dong, J. Shao, C. Chen, H. Li, R. Wang, Y. Chi, X. Lin, G. Chen, Blue

luminescent graphene quantum dots and graphene oxide prepared by tuningthe carbonization degree of citric acid, Carbon 50 (2012) 4738e4743.

[2] G. Eda, Y.-Y. Lin, C. Mattevi, H. Yamaguchi, H.-A. Chen, I.-S. Chen, C.-W. Chen,M. Chhowalla, Blue photoluminescence from chemically derived grapheneoxide, Adv. Mater. 22 (2010) 505e509.

[3] G. Wang, B. Wang, J. Park, J. Yang, X. Shen, J. Yao, Synthesis of enhanced hy-drophilic and hydrophobic graphene oxide nanosheets by a solvothermalmethod, Carbon 47 (2009) 68e72.

[4] Z. Luo, P.M. Vora, E.J. Mele, A.T.C. Johnson, J.M. Kikkawa, Photoluminescenceand band gap modulation in graphene oxide, Appl. Phys. Lett. 94 (2009)111909.

[5] G. Eda, Manish Chhowalla, Chemically derived graphene oxide: towards large-area thin-film electronics and optoelectronics, Adv. Mater. 22 (2010)2392e2415.

[6] C.W. Chen, J. Robertson, Nature of disorder and localization in amorphouscarbon, J. Non Cryst. Solids 227e230 (Part 1) (1998) 602e606.

[7] L.K. Putri, L.-L. Tan, W.-J. Ong, W.S. Chang, S.-P. Chai, Graphene oxide:exploiting its unique properties toward visible-light-driven photocatalysis,Appl. Mater. Today 4 (2016) 9e16.

[8] X. Lv, Y. Huang, Z. Liu, J. Tian, Y. Wang, Y. Ma, J. Liang, S. Fu, X. Wan, Y. Chen,Photoconductivity of bulk-film-based graphene sheets, Small 5 (2009)1682e1687.

[9] D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev,L.B. Alemany, W. Lu, J.M. Tour, Improved synthesis of graphene oxide, ACSNano 4 (2010) 4806e4814.

[10] S. Kumar, A.K. Ojha, In-situ synthesis of reduced graphene oxide decoratedwith highly dispersed ferromagnetic CdS nanoparticles for enhanced photo-catalytic activity under UV irradiation, Mater. Chem. Phys. 171 (2016)126e136.

[11] T. Kuila, P. Khanra, S. Bose, N.H. Kim, B.-C. Ku, B. Moon, J.H. Lee, Preparation ofwater-dispersible graphene by facile surface modification of graphite oxide,Nanotechnology 22 (2011) 305710.

[12] T.-Y. Zhang, D. Zhang, Aqueous colloids of graphene oxide nanosheets byexfoliation of graphite oxide without ultrasonication, Bull. Mater. Sci. 34(2011) 25e28.

[13] N.H. Kim, T. Kuila, J.H. Lee, Simultaneous reduction, functionalization andstitching of graphene oxide with ethylenediamine for composites application,J. Mater. Chem. A 1 (2013) 1349e1358.

[14] S. Pang, J.M. Englert, H.N. Tsao, Y. Hernandez, A. Hirsch, X. Feng, K. Müllen,Extrinsic corrugation-assisted mechanical exfoliation of monolayer graphene,Adv. Mater 22 (2010) 5374e5377.

[15] Y. Yang, L. Ren, C. Zhang, S. Huang, T. Liu, Facile Fabrication of functionalizedgraphene sheets (FGS)/ZnO nanocomposites with photocatalytic property,ACS Appl. Mater. Interfaces 3 (2011) 2779e2785.

[16] G.X. Wang, X. Shen, J. Yao, J. Park, Graphene nanosheets for enhanced lithiumstorage in lithium ion batteries, Carbon 47 (2009) 2049e2053.

[17] S. Niyogi, E. Bekyarova, M.E. Itkis, H. Zhang, K. Shepperd, J. Hicks, M. Sprinkle,C. Berger, C.N. Lau, W.A. deHeer, E.H. Conrad, R.C. Haddon, Nano Lett. 10(2010) 4061e4066.

[18] A.C. Ferrari, J. Robertson, Interpretation of Raman spectra of disordered andamorphous carbon, Phys. Rev. B 61 (2000) 14095.

[19] F. Tuinstra, J.L. Koenig, Raman spectrum of graphite, J. Chem. Phys. 53 (1970)1126.

[20] Y. Xue, Y. Liu, F. Lu, J. Qu, H. Chen, L. Dai, Functionalization of graphene oxidewith polyhedral oligomeric silsesquioxane (POSS) for multifunctional appli-cations, J. Phys. Chem. Lett. 3 (2012) 1607e1612.

[21] C.K. Chua, Z. Sofer, P. Simek, O. Jankovsky, K. Klímova, S. Bakardjieva,S.H. Kuckova, M. Pumera, Synthesis of strongly fluorescent graphene quantum

S. Kumar, A. Kumar / Optical Materials 62 (2016) 320e327 327

dots by cage-opening buckminsterfullerene, ACS Nano 9 (2015) 2548e2555.[22] A. Lerf, H. He, M. Forster, J. Klinowski, Structure of graphite oxide revisited,

J. Phys. Chem. B 102 (1998) 4477e4482.[23] S.H. Shim, K.T. Kim, J.U. Lee, W.H. Jo, Facile method to functionalize graphene

oxide and its application to poly (ethyleneterephthalate)/graphene composite,ACS Appl. Mater. Interfaces 4 (2012) 4184e4191.

[24] J. Shang, L. Ma, J. Li, W. Ai, T. Yu, G.G. Gurzadyan, The origin of fluorescencefrom graphene oxide, Sci. Rep. 2 (2012) 1e8.

[25] N.M. Huang, H.N. Lim, C.H. Chia, M.A. Yarmo, M.R. Muhamad, Simple room-temperature preparation of high-yield large-area graphene oxide, Int. J.Nanomed. 6 (2011) 3443e3448.

[26] S.K. Cushing, M. Li, F. Huang, N. Wu, Origin of strong excitation wavelengthdependent fluorescence of graphene oxide, ACS Nano 8 (2014) 1002e1013.

[27] Q. Lai, S. Zhu, X. Luo, M. Zou, S. Huang, Ultraviolet-visible spectroscopy ofgraphene oxides, AIP Adv. 2 (2012) 032146.

[28] L. Zhang, J. Liang, Y. Huang, Y. Ma, Y. Wang, Y. Chen, Size-controlled synthesisof graphene oxide sheets on a large scale using chemical exfoliation, Carbon47 (2009) 3365e3380.

[29] D. Li, M.B. Muller, S. Gilje, R.B. Kaner, G.G. Wallace, Processable aqueous dis-persions of graphene nanosheets, Nat. Nanotechnol. 3 (2008) 101e105.

[30] K.P. Loh, Q. Bao, G. Eda, M. Chhowalla, Graphene oxide as a chemically tunableplatform for optical applications, Nat. Chem. 2 (2010) 1015e1024.

[31] M. Li, S.K. Cushing, X. Zhou, S. Guo, N.Q. Wu, Fingerprinting photo-luminescence of functional groups in graphene oxide, J. Mater. Chem. 22(2012) 23374e23379.

[32] J. Robertson, E.P. O'Reilly, Electronic and atomic structure of amorphouscarbon, Phys. Rev. B 35 (1987) 2946e2957.

[33] S. Thangavel, G. Venugopal, Understanding the adsorption property ofgraphene-oxide with different degrees of oxidation levels, Powder Technol.257 (2014) 141e148.

[34] R. Bissessur, S.F. Scully, Intercalation of solid polymer electrolytes intographite oxide, Solid State Ionics 178 (2007) 877e882.

[35] X.-K. Kong, Q.-W. Chen, Z.-Y. Lun, Probing the influence of different oxygen-ated groups on graphene oxide's catalytic performance, J. Mater. Chem. A 2(2014) 610e613.

[36] T.-F. Yeh, J.-M. Syu, C. Cheng, T.-H. Chang, H. Teng, Graphite oxide as a pho-tocatalyst for hydrogen production from water, Adv. Funct. Mater 20 (2010)2255e2262.

[37] H.-C. Hsu, I. Shown, H.-Y. Wei, Y.-C. Chang, H.-Y. Du, Y.-G. Lin, C.-A. Tseng, C.-H. Wang, L.-C. Chen, Y.-C. Lin, Graphene oxide as a promising photocatalystfor CO2 to methanol conversion, Nanoscale 5 (2013) 262e268.

[38] Y. Matsumoto, M. Koinuma, S. Ida, S. Hayami, T. Taniguchi, K. Hatakeyama,H. Tateishi, Y. Watanabe, S. Amano, Photoreaction of graphene oxide nano-sheets in water, J. Phys. Chem. C 115 (2011) 19280e19286.

[39] K.N. Kudin, Zigzag graphene nanoribbons with saturated edges, ACS Nano 2(2008) 516e522.

[40] S. Kumar, A.K. Ojha, B. Walkenfort, Cadmium oxide nanoparticles grown insitu on reduced graphene oxide for enhanced photocatalytic degradation ofmethylene blue dye under ultraviolet irradiation, J. Photochem. Photobiol. BBiol. 159 (2016) 111e119.