rGO nanocomposite photocatalyst

Materials Chemistry and Physics 223 (2019) 456–465

Contents lists available at ScienceDirect

Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Engineering of ZnO/rGO nanocomposite photocatalyst towards rapid degradation of toxic dyes

T

Sujoy Kumar Mandala, Kajari Duttab, Saptarshi Palc,a, Sumit Mandala, Avigyan Naskard, Pabitra Kumar Pale, T.S. Bhattacharyaf, Achintya Singhaf, Rezaul Saikhg, Sukanta Deg, Debnarayan Janaa,∗ a

Department of Physics, University of Calcutta, 92 A.P.C. Road, Kolkata, 700009, India Department of Physics, Amity University, Major Arterial Road, AA II, Newtown, Rajarhat, Kolkata, 700135, India c Department of Physics, School of Natural Sciences, Shiv Nadar University, NH-91, Thesil Dadri, Gautam Buddha Nagar, Uttar Pradesh, 201314, India d Department of Chemistry, University of Calcutta, 92 A.P.C. Road, Kolkata, 700009, India e Department of Physics, Jadavpur University, 188 Raja S.C. Mallick Road, Kolkata, 700032, India f Department of Physics, Bose Institute, 93/1 Acharya Prafulla Chandra Road, Kolkata, 700009, India g Department of Physics, Presidency University, 86/1 College Street Road, Kolkata, 7000073, India b

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

QD is synthesized by soft che• ZnO mical route followed by ageing pro-

• • •

cess and ZnO/rGO composites by solvothermal method. XRD, RAMAN and TEM measurements confirm the ZnO/rGO composite formation. Electron transfer mechanism is explained from the photoluminescence lifetime measurement. Degradation efficiency is optimized by proper composite formation.

A R T I C LE I N FO

A B S T R A C T

Keywords: ZnO QD Reduced graphene oxide Composite Photocatalyst Toxic dyes

ZnO quantum dot (QD) is prepared in a soft chemical route by ageing process and successfully embedded on reduced graphene oxide (rGO) surface using solvothermal method as confirmed by XRD, TEM and Raman studies. Due to charge transfer from the lowest unoccupied molecular orbital of the rGO layer to the conduction band of ZnO, the luminescence of rGO is also found to be quenched in ZnO/rGO composites. A brief systematic study of photocatalytic behavior towards degradation of various toxic medical and textile dyes, like Safranin O, Methylene blue and Rhodamine 6G by ZnO QD and ZnO/rGO composites are showcased in this work. As a result of high surface to volume ratio, ZnO QD showed good photocatalytic activity towards degradation of different dyes under ultraviolet (UV) irradiation. A composite with ZnO and rGO in 3:1 wt ratio degraded almost 100% of dyes under UV irradiation within 15, 25 and 30 min, showing good efficacy in dye degradation. Photoluminescence lifetime measurement revealed the charge transfer process from rGO to ZnO resulting a Prompt recombination of the UV light induced electron-hole pair in ZnO/rGO composite.

∗

Corresponding author. E-mail address: [email protected] (D. Jana).

https://doi.org/10.1016/j.matchemphys.2018.11.002 Received 18 June 2018; Received in revised form 22 October 2018; Accepted 1 November 2018 0254-0584/ © 2018 Elsevier B.V. All rights reserved.


S.K. Mandal et al.

1. Introduction

quantum efficiency for the degradation of dyes under solar illumination. This provides us the necessary motivation for studying ZnO QD/ rGO composites to acquire efficacious degradation efficiency towards toxic dyes under UV illumination. In this report, ZnO QD has been synthesized by low temperature chemical route followed by ageing process and its composites with rGO by varying amount of ZnO via solvothermal method. A methodical study of the photocatalytic activity towards degradation of medical dye ‘SO’ has been performed in presence of ZnO QD and ZnO/rGO composites under UV light irradiation. Using composite as a photocatalyst, almost 100% degradation of SO dye has been accomplished with just 25 min of irradiation. Further, the photocatalytic efficiency of our synthesized composites has been thoroughly investigated in degrading the MB and Rh6G dyes and the results ensure the next level in terms of efficiency compared to previous reported works. The modern challenge has already been admitted regarding the hazardous release from dyeing industries and that has to be tackled with rapid purification and recycling measures. In this concern our synthesized material would be a potent candidate for waste water treatment. Furthermore, our proposed photocatalyst is non-toxic and environmental friendly; as toxicity of ZnO is very small compared to other oxides. Dr. Andrew Maynard, director of the University of Michigan's Risk Science Centre has reported in council's newsletter on July 2014 that graphene oxide is toxic in the range of 50–300 mg/L. Here, we have used the photocatalyst as 120 mg/L in which rGO concentration is only 25%.

The extensive use of toxic organic dyes in the modern trait of colouring and staining is a frontline issue to the industries like textile, jute, paint etc. as well as medical laboratories [1–3]. The release of the excess dyes into different water bodies not only affect the human life but also the entire biosphere – plants and organisms, living in these water bodies. This eventually extends the boundary of the age old concern of water pollution. Photocatalytic activity is one of the most efficient methods for alleviating the negative environmental impact of hazardous wastes and toxic pollutants in aqueous media. In the last few years different types of materials, like semiconductor oxides and sulfides, metal organic frameworks (MOFs), perovskite materials are emerging as potential photocatalysts [4]. Among these materials, semiconductor oxides (e.g. TiO2, ZnO, NiO, and SnO2) are the most studied materials as photocatalyst [5–8]. Among the members of II-VI semiconductor family, ZnO takes the centre stage in the research arena for superior intrinsic properties. The gifted features of ZnO, such as a direct and wide band gap (∼3.37 eV at room temperature), high exciton binding energy (∼60 meV), UV light sensitivity, and other remarkable optoelectronic properties have made it a sound technologically resourceful material [9]. Previous literature survey reveals that a great deal of work has been done using wide band gap semiconductor oxides as the photocatalyst in degradation of textile industry dyes like methylene blue (MB), rhodamine 6G (Rh6G), methyl red, methyl orange etc. [10,11] but comparatively less effort has been given towards the degradation of medical science dyes. Safranin O (SO) or basic red 2 is a biological stain used routinely in histology and cytology [12]. SO dye above a certain percentage creates a great hazard in environment even it is carcinogenic to human health. Currently, composites of reduced graphene oxide (rGO) and semiconductor oxides are able to exhibit enhanced photocatalytic activity than pure oxide nanostructures [13]. As rGO has narrow band gap compared to wide band oxides, so it absorbs more photons than oxides; as a consequence, a large number of electron and hole pairs are created inside the material. rGO can transfer the photogenerated charge carriers to the conduction band of the semiconductor, resulting a huge population of charge carriers in the composites, which react with surface absorbed O2 and H2O. As a result, a large number of super oxide and hydroxide radicals are formed on the surface of the nanocomposites which degrade the toxic dyes [14]. Graphene, a two dimensional (2D) structure of sp2 hybridized carbon atoms, has unique properties like high surface area, adsorption and good electrical conductivity. Even rGO, after the formation of composite with semiconductors, also possesses the ability to accept the photogenerated electrons to prevent the recombination and provides a favorable adsorption of dye through π - π conjugation between the dye and RGO surface [15]. Due to potentially ballistic electron transport capability of graphene, it can show effectively zero conduction resistance for storing and transporting electrons within its π-rich structure. As a result, its composites naturally reduce the electron-hole pair recombination time and consequently enhance its photocatalytic property towards the degradation of toxic dyes [16,17]. Moussa et al. have reported the effective photocatalysis towards degradation of Orange II under sunlight irradiation by ZnO rods/rGO composites and achieved 100% degradation within 3 h [18]. Zhang and co-workers have synthesized ZnO nanowire/rGO composites for photocatalytic degradation of Rh6G dye under UV light irradiation [19]. Recently, Qin et al. have reported the photocatalytic degradation of MB dye by using ZnO microspheres-rGO nanocomposite by UV irradiation for 20 min [20]. ZnO–graphene composite reported by Xu et al., has showed the photocatalytic efficiency ∼90% in degradation of MB under 40 min UV irradiation [21]. Efficient eco-friendly oxide photocatalyst to degrade the dyes used in medical diagnosis is still being investigated. According to literature survey, there is no report for degradation of SO dye using ZnO/rGO composite as a photocatalyst. It is established that the wide band gap oxide catalysts provide insufficient

2. Experimental section 2.1. Materials Zinc acetate dehydrate (Zn(CH3COO)2·2H2O), methanol, safranin O, methylene blue and rhodamine 6G are purchased from Sigma Aldrich. Sodium nitrate (NaNO3), phosphoric acid (H3PO4), sulfuric acid (H2SO4), potassium permanganate (KMnO4), potassium hydroxide (KOH), hydrogen peroxide (H2O2), N, N-Dimethyl formamide (DMF) and hydrochloric acid (HCl) are from Merck (Germany). Graphite powder (puriss: ≤ 20 μm) is purchased from Fluka. All the chemicals are analytical grade and used without further purification. Deionized water (DI) is used for the synthesis purpose. 2.2. Synthesis of ZnO quantum dots Soft chemical route is followed to synthesize the ZnO quantum dots (QD) [22,23]. 5 mmol of Zn(CH3COO)2·2H2O is dissolved in 60 ml CH3OH and the mixture is stirred for 20 min, labeled as solution-I. Another solution, called solution-II is prepared by dissolving 15 mmol of KOH in 30 ml CH3OH. Solution-II is then added dropwise to solutionI at room temperature (RT) under constant stirring and the solution mixture is placed in an oil bath at 60 °C for 2 h. The product is then kept at RT. for ageing about 90 h. Finally the product is collected by centrifugation at 6000 rpm followed by washing with ethanol and water several times. Final product is collected after overnight vacuum drying. 2.3. Synthesis of graphene oxide, rGO and ZnO/rGO composites Graphene Oxide (GO) is synthesized by standard modified Hummer's method from graphite powder [24,25]. rGO and ZnO/rGO composites are synthesized via solvothermal method using DMF as a solvent [26]. In case of synthesis of rGO, 4.5 ml GO solution from the stock solution (7.0 mg/ml) is dispersed in 40 ml of DMF using ultrasonication for 60 min. After that 8.5 ml DMF is added to the solution and the solution is then autoclaved within a Teflon chamber. This closed chamber is placed in a preheated box furnace and kept at 180 °C for 16 h. For synthesis of ZnO/rGO composites, the amount of GO solution (1.5 ml) is kept fixed. However, the amount of ZnO QD is varied as 10 mg, 20 mg, 30 mg and 40 mg. Here, two separate solutions of DMF 457


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Scheme 1. Schematic representation of the formation mechanism of ZnO/rGO composite.

are made, each of 20 ml; one is GO dispersed at DMF and another is ZnO QD dispersed at DMF. Then, the DMF solution having ZnO is drop wise added to the GO containing solution and after that 11.5 ml DMF is added to reach the solution volume to 53 ml. Solvothermal condition is kept same as rGO synthesis. ZnO-GO suspension in DMF changes color from light brown to dark black, similar to the color change observed during the chemical reduction of graphene oxide [27,28]. Finally, composites are collected by centrifugation and labeled as ZnO/rGO-1, ZnO/rGO-2, ZnO/rGO-3 and ZnO/rGO-4 containing the ZnO: rGO weight ratios as 1:1, 2:1 3:1 and 4:1 respectively. Scheme 1 represents the probable mechanism of the formation of ZnO/rGO composite.

Fig. 1. XRD spectra of ZnO QD, rGO and ZnO/rGO composites.

to upper surface of reaction solution is kept at 10 cm initially. Approximately 2 ml mixture of catalyst and dye solution is collected at different intervals from the photoreactor and then centrifuged at a speed of 10000 rpm for 4 min to remove the photocatalyst from the solution. UV/Vis absorption spectra are recorded after 5 min interval for UV irradiation to monitor the variation of concentration of dye. 3. Results and discussion

2.4. Characterization 3.1. XRD analysis Structural characterization of all samples has been carried out by using X-ray powder diffraction measurements recorded using X-ray diffractometer [Model: X'Pert Pro (PAN Alytical)] operating at 40 kV and 40 mA with Cu-Kα radiation of 1.54 Å. XRD spectra are taken over 2θ range of 20º-80° employing a scanning rate of 0.003° s−1. The size and morphology of ZnO QD and ZnO/rGO samples are characterized by transmission electron microscopy (TEM) [Model: JEOL JEM 2100 HR with EELS] operating at 200-kV accelerating voltage. For TEM observation, the samples are re-dispersed in ethanol by ultrasonic treatment and dropped on carbon–copper grids. Room temperature Raman spectra are recorded by Lab RAM HR Jovin Yvon Raman set-up equipped with Peltier cold CCD detector with an excitation of 488 nm Argon-ion laser light. Perkin Elmer Lambda 25 UV/VIS Spectrometer is used to collect the UV–visible absorption spectra of the concerned samples. Photoluminescence (PL) spectra of all the samples are taken by using Horiba FL 1000 fluorescence spectrometer using 300 nm excitation. Time resolved PL spectra of both ZnO QD and ZnO/rGO-3 are measured using TCSPC Horiba Jobin Yvon Fluocube-01-NL fluorescence life time spectrometer. A diode laser (λ = 292 nm) with FWHM 140 ps is used as the excitation source.

To confirm the formation of ZnO, rGO and their composite, XRD of all synthesized samples are carried out and presented in Fig. 1. The position and broadened nature of all diffraction peaks found in pure ZnO XRD spectra indicate that the synthesized ZnO crystallizes in wurtzite structure [JCPDS 79.0207] and in nano-particle form respectively. In fact, the average crystallite size as calculated by using Scherrer's formula is given by(where D is the average crystallite size, λ is the wavelength of kα radiation, β is the full-width at half-maxima (FMHM) on 2θ scale and θ is the Bragg angle) is estimated to be (6.9 ± 0.5) nm revealing the possibility of quantum dot (QD) formation. Besides it, the presence of a broad diffraction peak (002) at ∼25° and (100) peak at ∼43.5° in pure rGO spectra confirms rGO formation. Further, simultaneous occurrence of characteristic peaks of both ZnO and rGO in all composite samples confirms the composite formation too. Interestingly, in all composites, the broadening of all ZnO peaks is found to be very less compared to that of its pristine one indicating agglomeration of ZnO in rGO sheets. Agglomeration of ZnO in composites is not unexpected as the procedure (solvothermal process) of synthesizing the composites consists of high pressure and high temperature [29]. However, the observation of no change in diffraction peaks of the composites with that of pure ZnO QD indicates that the lattice constant of ZnO remains unchanged because of surface hybridization [30].

2.5. Photocatalytic experiments The photocatalytic activity is measured by using a home-made system with continuous flow of water surrounding the double walled beaker (capacity 50 ml) to cool the reaction solution. The photocatalytic activity of the synthesized products is evaluated by measuring the degradation of SO, MB and Rh6G dyes at RT in aquatic medium of pH 7.0 under UV light exposure. The reaction system containing aqueous solution of dye (1 × 10−5 M, 25 mL) and the photocatalyst (ZnO QD or rGO or ZnO/rGO composite) of 3 mg is magnetically stirred in the dark for 30 min to reach the adsorption equilibrium of dye with the catalyst. UV irradiation is carried out using a 100 W UV lamp with irradiation wavelength 365 nm. For UV irradiation, distance from lamp

3.2. Morphology analysis The morphological characteristics of pure ZnO, rGO sheet and ZnO/ rGO composites are performed by using TEM and shown in Fig. 2(a)–2(e) respectively. Many wrinkles present in rGO sheet confirms the two dimensional structure of rGO (Fig. 2(b)). The average particle size (∼6 nm) as calculated from the TEM image of pure ZnO (Fig. 2(a)) clearly indicates the formation of ZnO QD. TEM image of ZnO/rGO-1, ZnO/rGO-2 and ZnO/rGO-3 composites are shown in 458


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Fig. 2. Low resolution TEM images of (a) ZnO QD (b) rGO (c) ZnO/rGO-1 (d) ZnO/rGO-2 and (e) ZnO/rGO-3 composite. Insets of (a) and (e) show the HRTEM images of the ZnO QD and ZnO/rGO-3 respectively, (f) and (g) show the particle size distribution of ZnO QD and ZnO/rGO-3 composite respectively.

Fig. 2(c), (d) and 2(e) respectively. The agglomeration of ZnO in composites as predicted from XRD results is also confirmed from the TEM image of the composites. Fig. 2(f) and (g) show the particle size distribution of ZnO QD and ZnO/rGO-3 composite. 3.3. Optical Studies To explore the structural information more, a non-destructive technique like Raman spectroscopy is further employed [31]. Moreover, it is an essential tool to confirm the ZnO-rGO composite phase and also it can give the information of lattice defects. Here, Raman spectra of ZnO QD, rGO and ZnO/rGO composites are depicted in Fig. 3. The first order D and G bands of GO arise at 1358 cm−1 and 1600 cm−1 respectively. The G band is attributed to all sp2 carbon forms and provides information on the in-plane vibration of sp2 bonded carbon atom while the D band signifies the sp3 defect, like internal structure defect, dangling bond and edge defect [32]. Compared with GO, the D band of rGO is slightly shifted to higher wave number at 1365 cm−1 and the G band is slightly shifted to lower wave number at 1594 cm−1. The G band shifting towards lower wave number indicates the reduction of GO into rGO while the shift in D band towards higher wavenumber points out the reduction of sp3 defects. ID/IG peak intensity ratio is a measure of disorder degree and average size of the sp2 domains in graphene materials [33]. The intensity ratio between the D band and G band (ID/IG) increases from 0.89 (GO) to 0.93, 0.96, 0.94 and 0.98 for composite ZnO/rGO-1, ZnO/rGO-2, ZnO/rGO-3 and ZnO/ rGO-4 respectively. This implies a decrease in the size of in-plane sp2 domains and formation of the defects and disorders on the graphene surface due to introduction of ZnO QD formation of composite structure [34]. The Raman spectra of pure ZnO QD reveal three main peaks at 101 cm−1, 437 cm−1 and 327 cm−1, corresponding to E2low, E2high and E2high - E2low respectively (Fig. 3). The coexistence of main Raman

Fig. 3. Room temperature Raman spectra of GO, rGO, ZnO QD and ZnO/rGO composites.

peaks of both rGO and ZnO, as found in all composite samples further confirms the composite formation. E2high mode in ZnO is associated to the vibration of the oxygen atoms in ZnO lattice [35,36]. Gradual decrease in intensity and also increase in FWHM in case of E2high mode from ZnO QD to ZnO/rGO-3 clearly indicate the disorder in oxygen lattice. In case of ZnO/rGO-4 the intensity of E2high mode is found to be increased, indicating the less oxygen deficient nature compared to the other composites. Such observation is duly supported by the increase in intensity at ∼575 cm−1, generally ascribed as oxygen deficient disordered state [37]. So, regarding the defects, overall Raman results 459


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Fig. 4. (a) UV–Vis absorption spectra of rGO, ZnO QD and ZnO/rGO composites. (b) Differential absorbance spectra of ZnO QD and ZnO/rGO composites; the inset shows the (αhν)2 versus hν plot of rGO.

reveal that oxygen deficient disorder in ZnO lattice increases with the increase of ZnO concentration upto a certain limit in ZnO/rGO composites which is believed to have direct impact on photocatalytic activity. The UV–Vis absorption spectra of ZnO QD and ZnO/rGO composites are shown in Fig. 4. A closer look to absorption spectra reveals that absorption tailing of ZnO begins at ∼355 nm whereas the same for ZnO/rGO-3 sample starts at ∼380 nm. This clearly manifests that the absorption of pure ZnO is red shifted in composite sample, which is predominantly because of the presence of carbon atoms in the composites [38]. A similar result has been also reported previously in the case of TiO2/rGO [6] and ZnS/rGO [7] nanocomposites. However, the extended tailing of the ZnO/rGO-4 absorption spectrum compared to that of ZnO QD indicates the presence of more disorder induced in the sub band gap defect states, consistent with the Raman spectra analysis. Regarding other two composite samples (ZnO/rGO-1 and ZnO/rGO-2), although the signal is very weak, still a close look confirms the presence of ZnO. Further, band gaps of ZnO and its composites can be determined from differential spectra [39] shown in Fig. 4(b). The band gap (Eg) of ZnO QD is calculated to be 3.63 eV [40]. The decrease in band gap of the composite samples has been observed due to proper composite formation of ZnO with rGO, as rGO possesses band gap energy 1.89 eV obtained from (αhν)2 versus hν plot shown in the inset of Fig. 4(b). A red-shift in the absorption edge of ZnO/rGO composite is clearly obtained ensuring an effective reduction in the band gap energy of ZnO QD in the composites. In case of ZnO/rGO-4 composite, the observed bandgap signifies the bulk nature of ZnO. This is due to the complete agglomeration of ZnO QD because of the excess amount of ZnO used in composite formation. Regarding the photocatalysis study, PL analysis of any material, especially the composite samples is of great concern as it can elucidate the mechanism behind the transfer process of photo-induced electronhole pairs along with the role of defects present in the sample. Here, Fig. 5 represents the PL spectra of ZnO QD and ZnO/rGO composites. The PL spectra of ZnO QD shows typical intense peak around 377 nm, related to the excitonic band-to-band radiative recombination. Additional two peaks appearing near 394 and 470 nm might be due to defect related emissions generally arise in ZnO nanostructures. The PL spectrum of pure rGO is characterized by the presence of blue emission, centered on 425 nm [41]. The introduction of ZnO QD with rGO in different concentration leads to a synergetic effect on the fluorescence property of ZnO. For the sample ZnO/rGO-1, the luminescence peak of the rGO near 425 nm appears with ZnO peaks, as a result the spectrum becomes broaden. Slightly quenching of rGO peak is observed in ZnO/ rGO-2 sample, implying the charge transfer from rGO to ZnO. In case of ZnO/rGO-3 sample, the effect is found to be more prominent. As the concentration of ZnO is high in ZnO/rGO-3 sample, a large number of photo-electrons are elevated from molecular orbital (LUMO) of rGO to the conduction band (CB) and the deep level of ZnO, and eventually decays to the valence band (VB) of ZnO, as a consequence an enhanced photoemission but comparatively lower than ZnO is observed. In ZnO/

Fig. 5. Room temperature PL spectra of rGO, ZnO QD and ZnO/rGO composites with excitation at 300 nm.

rGO-4 composite the defect related emissions are effectively quenched due to the reduction of oxygen deficiency, which is also evident from Raman spectra. Surface defect of rGO plays an important role for charge transfer as well as justification for sufficient dye adsorption in adsorption-desorption process. This is further explained from PL lifetime measurement. The PL decay signal is collected by fixing the emission at 380 nm which corresponds to the band-to-band emission of ZnO. The fluorescence results have been fitted with DAS6 software to bi-exponential curves [Fig. 6] in order to obtain decay time constants, which can be used to calculate the average lifetimes of the fluorescence. The relative contribution of the pre-exponential factors is calculated using the B equation αi = i = 2i , where B1 and B2 denote the bi-exponential fitted ∑i = 1 Bi

amplitudes. Furthermore, the average lifetime < τ > is calculated using the following expression < τ >=(α1τ12 + α2τ22)/(α1τ1+ α2τ2) [42]. The best fitted results are summarized in Table 1. In case of ZnO QD, the faster decay related to band edge emission and the slower component accompanied with defect related emission give lifetimes of 2.23 ns

Fig. 6. Fluorescence lifetime decay curves of ZnO QD (black circle), ZnO/rGO-3 (red circle) in ethanolic solution with excitation 292 nm diode laser measured by TCSPC. Green and magenta lines represent the fitted curves of ZnO QD and ZnO/rGO-3 composite respectively. 460


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decay kinetics, ln (C0/C) = kt or ln(A0/A) = kt, where k, A0, A, C0, C denote surface reaction rate constant, initial absorbance, absorbance after irradiation at various time interval (t), initial concentration of solution and concentration of dye after irradiation at various time interval (t) respectively [44]. On the basis of the above analysis, as shown in Fig. 7, it can be concluded that the enhancement of the photocatalytic activity of the ZnO/rGO composites for SO dye may be mainly attributed to large surface area of rGO, enhanced adsorption ability and superior charge transfer from rGO to ZnO [8]. However, due to hydrophobicity character, the dispersion of rGO in aqueous solution is limited [14]. This type of problem can be cured by proper composite formation with rGO. Out of all synthesized composites, ZnO/rGO-3 possesses the greatest adsorption ability in dark compared to other two composites, which is clear from C/C0 plot (Fig. 7(c)). Hence, the photocatalysis of dye occurs due to synergetic effects of adsorption and photo-degradation. It is easily identified from the C/C0 plot that 50% of SO is adsorption by the composites and remaining 50% is photo-degraded. It has been established that rGO has stronger capacity to capture photons due to its two dimensional (2D) p-conjugation structure [45,46] and can easily transfer the photo-generated electron and holes to the CB of ZnO, due to the smaller work function of rGO compared to ZnO (Scheme 2) [47,48]. As a consequence, the electron-hole pair recombination rate is reduced and photocatalytic activity is efficiently enhanced. Although rGO is beneficial for charge separation, but higher concentration of rGO can effectively shade ZnO QD, thus reducing the efficiency of phototocatalytic activity [49,50]. Therefore, a balance between two is essential during synthesis of proper composite exhibiting the maximum photocatalytic activity. Here, composite ZnO/ rGO-3 synthesized with a certain amount of ZnO has shown the maximum efficiency. The SO molecules are largely adsorbed onto rGO surface and then photo-degraded by ZnO/rGO composites under UV irradiation. Further, the photocatalytic degradation of MB and Rh6G dye is also studied using ZnO QD and ZnO/rGO-3 to check the universal degradation ability. Here, all other conditions have been kept same as that of SO degradation. Like SO dye degradation, here also, ZnO/rGO-3 shows better photocatalytic performance compared to pure ZnO QD as shown in Fig. 8 (a) and (b). The degradation curves of MB and Rh6G dyes in presence of different photocatalysts reveal that both dyes are degraded very rapidly, to be more specific, 100% degradation of MB and Rh6G have been occurred within 15 and 30 min respectively using ZnO/rGO-3 as a catalyst. Fig. 8 (c) represents the C/C0 plot of these two dyes using ZnO and ZnO/rGO-3 as the catalysts. ln(C0/C) vs time plots for the two dyes are represented in Fig. 8 (d). Finally, all the rate constants (k) for SO, MB and Rh6G dye calculated from the slope of linear fitting of ln(C0/C) vs time graph are depicted in Fig. 9 to show the overall scenario in a single frame. In this work, rGO also possesses the degradation efficiency. According to literature, rGO has good adsorption towards dyes [51]. Therefore, to compare the degradation efficiency and adsorption of rGO towards dye (SO), the 80 min dark adsorption along with UV induced degradation after 30 min of dark stirring in case of rGO has been depicted in Fig. 10. In addition, ZnO/rGO-3 is also observed to exhibit good adsorption within 30 min of dark stirring. Hence, we have also performed 80 min dark adsorption of this composite and found that, the dark adsorption is almost saturated within 30 min of dark stirring. This is plotted with photo-degradation curve of the same composite in Fig. 10. Above discussions confirm that, the removal of organic dye is from photodegradation not from adsoption. The cycling performance of the photocatalysts is of utter importance in view of the technological feasibility. Here, the UV irradiated photocatalytic cycle experiments with ZnO/rGO-3 composite is performed upto five cycles for the degradation of SO dye to examine the stability of the photocatalyst. The results, shown in Fig. 11 indicate that there is no substantial variation in the degradation efficiency of ZnO/rGO-3 composite upto the third run. Therefore, our synthesized composite has

Table 1 Kinetic analysis of lifetime decay of ZnO QD and ZnO/rGO composite. Sample

α1

τ1 (ns)

α2

τ2 (ns)

< τ > (ns)

χ2

ZnO QD ZnO/rGO-1 ZnO/rGO-2 ZnO/rGO-3 ZnO/rGO-4

0.67 0.69 0.67 0.98 0.85

2.23 3.03 1.87 1.68 2.37

0.33 0.31 0.33 0.02 0.15

10.37 12.54 7.82 9.74 14.93

7.90 9.21 5.87 2.53 8.98

1.35 1.14 1.17 1.34 1.25

and 10.37 ns respectively [14]. With composite formation, the fast component of ZnO plays the major role (α1 = 0.98) compared to the slow component (α2 = 0.02) in decay process (Table 1). Remarkably, a decrease in average lifetime of ZnO emission is found from 7.9 ns to 2.5 ns (almost 1/3rd) in case of ZnO/rGO-3. On this basis, we can conclude that the band to band transition in ZnO is dominated in composite sample. If electron transfer from rGO to ZnO is only the process accompanying the fast decay component in the ZnO/rGO system, then we can estimate the electron-transfer rate from the emission lifetimes (eq. (1)) [27,43].

ket =

1 1 (ZnO/ rGO) − (ZnO) τ τ

(1)

By substituting the corresponding values of the lifetime (τ1), we have obtained maximum ket of 0.15 × 109 s−1 for ZnO/rGO-3 composite, indicating highest electron transfer rate in the composite. This high rate of charge transfer makes the ZnO/rGO composite as a good photocatalyst under degradation of different toxic dyes, which is discussed in the next section. 3.4. Photocatalytic activity Fig. 7 represents the UV light induced photocatalytic behavior of ZnO and ZnO/rGO samples towards degradation of SO dye. It is evident from Fig. 7(a) that ZnO QD has capability to almost fully degrade the SO dye within 50 min. However, ZnO/rGO-3 is able to fully degrade the same only within 25 min. The activity of other samples (rGO, ZnO/rGO1, ZnO/rGO-2 and ZnO/rGO-4) are found to be in between the above mentioned two samples as evident from the C/C0 vs. time graph (Fig. 7(c)). Further, to understand the kinetics of degradation phenomenon, ln(C0/C) vs. time is plotted and analysis shows that the photocatalytic degradation of SO dye follows the pseudo first- order

Fig. 7. (a), (b) Variation of Safranin O (SO) concentration as a function of irradiation time using 3 mg of ZnO QD and ZnO/rGO-3 respectively as the photocatalyst under UV light. (c) Photocatalytic degradation curve (C/C0 vs. time) and (d) ln(C0/C) versus time plot for SO dye degradation using rGO, ZnO QD and ZnO/rGO composites. 461


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Scheme 2. Schematic diagram showing the mechanism of the photodegradation of dye using ZnO/rGO composites as a catalyst.

Fig. 9. All the surface reaction rate constants (k in min−1) for SO, MB and Rh6G dye in presence of different catalysts.

Fig. 8. (a) Variation of Methylene blue (MB) and (b) Rhodamine 6G (Rh6G) concentration as a function of irradiation time using 3 mg of ZnO/rGO-3 composite as a photocatalyst under UV light. (c) C/C0 versus time plot and (d) ln(C0/C) versus time plot for MB and Rh6G dyes in presence of ZnO QD and ZnO/rGO-3 composite.

considerable stability under ultraviolet light. In a photocatalytic degradation process, the reactive species involved in the photo induced reaction are hole (h+), hydroxyl (·OH) and the superoxide radical (O2·−) [52,53]. To get a greater perception of the degradation mechanism and the role of the reactive species, 3 × 10−4 mol of methanol, p-benzoquinone and sodium oxalate are used as a ·OH, O2·− and h+ scavenger respectively. Fig. 12 shows the relative concentration of SO dye with and without different scavengers. SO dye is found to be degraded 99% within 25 min of UV irradiation without any scavenger. On addition of methanol, sodium oxalate and pbenzoquinone as scavenger, the dye degradation upto 25 min UV irradiation is decreased from 99% to 38%, 82% and 87% respectively. This high quenching of SO degradation (61%) in presence of methanol ensures the major role of ·OH radical in the degradation process. Moreover on addition of other scavengers, the detrimental effect in the dye degradation further confirms the significant role of O2·− and h+ radicals in dye degradation.

Fig. 10. Dark adsorption of rGO and ZnO/rGO-3 composite along with their photocatalytic activity.

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Fig. 11. The recyclability of photocatalytic degradation of SO dye using ZnO/ rGO-3 composite as a catalyst.

Fig. 13. Current-voltage (I–V) characteristics curve of ZnO QD and ZnO/rGO-3 composite.

energy, few electrons are prompted from VB to CB forming an exciton pair on ZnO surface. Due to large surface area of ZnO QD, a large number of exciton pairs are produced on the QD surface. These photogenerated electrons can react with surface adsorbed O2 to form a superoxide radical (.O2−), at the same time the photo-induced holes can react with surface adsorbed hydroxyl to form a highly reactive hydroxyl radical (.OH). These superoxide and hydroxyl radical can directly oxidize the organic dyes, making ZnO QD as a good photocatalyst. As the band gap of rGO is 1.89 eV, which is much smaller than the supplied excitation energy (3.4 eV), a large numbers of electron-hole pairs are generated in the rGO sheet. According to the reported literature the work function of rGO and ZnO are −3.85 eV and −4.05 eV respectively reference to vacuum, resulting the LUMO of rGO places above the CB of ZnO. Hence, a large number of photo-generated electrons can easily be transferred from the LUMO of rGO to the CB of ZnO in ZnO/rGO composite. The charge transfer mechanism is shown in the schematic diagram-2. The charge transfer mechanism is also duly supported by PL and PL lifetime measurement previously. This electron transfer process from rGO to ZnO enhances the photocatalytic activity and reaches its maximum efficiency for the composite ZnO/rGO-3. Therefore, our synthesized composite ZnO/rGO-3 can be considered as an effective material for the nonselective degradation of dyes with UV light irradiation.

Fig. 12. Relative SO concentration as a function of irradiation time using ZnO/ rGO-3 composite catalyst in absence and in presence of different catalyst-scavengers [scavengers: methanol (·OH), sodium oxalate (h+) and p-benzoquinone (O2·-)].

To explore further the mechanism related to photocatalytic degradation, I-V measurements are performed. Fig. 13 shows the I-V characteristic curves of ZnO QDs and its optimized composite ZnO/ rGO-3 under UV light illumination (350 nm). The photocurrent increases 10 times due to composite formation depicted the fact that an accumulation of charge carries in conduction band of ZnO due to charge transfer from rGO to ZnO. The collection of ample of charges in the conduction band of ZnO promotes the photocatalytic efficiency of the composites which have been proved by different systematic studies. Prior to this work, we had made an extensive literature survey and compared our composite with previously reported work of UV light induced photocatalytic degradation of SO dye and tabulated the notable results in Table 2 for comparison. The table ensures that our reported material shows the better efficiency in photocatalytic degradation of not only just SO dye, it is also highly efficient for rapid degradation of other textile dyes (like methylene blue, rhodamine 6G etc.). Scheme 2 represents a schematic diagram of the photodegradation of various dyes using ZnO/rGO composites as a catalyst. The basic mechanism of degradation of dyes is the adsorption-oxidation and finally desorption. In case of pure ZnO QD, when it is illuminated with UV light with maximum at 365 nm (3.4 eV), nearly to its band gap

4. Conclusion ZnO QD and ZnO-rGO composites have been successfully prepared via soft chemical route and solvothermal method respectively. Photocatalytic activities of pure ZnO QD and ZnO/rGO composites in degradation of SO, MB and Rh6G dyes have been studied under UV light irradiation. Pure ZnO QD of size ∼6 nm have demonstrated a superior photocatalytic activity due to its large surface to volume ratio and degraded almost 100% of Rh6G, MB and 90% of SO dye within 45, 30 and 50 min respectively, whereas, synthesized composite ZnO/rGO3 successfully has removed SO, MB and Rh6G dyes from aqueous solution within 25, 15 and 30 min respectively. The degradation efficiency of ZnO/rGO photocatalyst however depends on the amount of ZnO QD in ZnO/rGO composite. The role of rGO in the composite is to limit the agglomeration of nanoparticles by increasing the number of effective active sites. Besides, it has stronger capacity to capture the photon and to transfer the photo-generated electrons to the CB of ZnO. As a consequence of that, the recombination loss is reduced and large number of charges becomes available on the surface of ZnO QD. Thus, 463


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Table 2 Comparison of photocatalytic performance of SO dye with previous reported literature. catalyst used and amount Ag-TiO2, 120 mg/L ZnS QD 5 × 10−4 M Zn0.9Cu0.1S1,100 mg/L WO3, 1000 mg/L sAu/TiO2, 120 mg/L ZnO/rGO Composite, 120 mg/L

concentration and volume of SO dye −5

1 × 10 M, 25 ml 1 × 10−5 M, 5 mg/L, 500 ml 200 mg/L, 100 ml 1 × 10−5 M, 30 ml 1 × 10−5 M, 25 ml

UV light Source

degradation time (min)

degradation %

Reference

Hg UV GL-58 lamp UV(λ = 365 nm) 100 W mercury lamp Nd:YAG laser (λ = 355 nm) 120 W (15 × 8 W) 100 W

70 40 30 10 50 25

96 51 48 94 97 99

Kemary et al. [54] El-Kemary et al. [55] Pouretedal et al. [56] Hayat et al. [57] Bumajdad et al. [58] Our work

the produced huge number of oxide and hydroxide super radicals on the surface of the QD, effectively degrade the dyes under light illumination. So, feasibly our proposed non-toxic, metal free, highly efficient and nonselective photocatalyst is expected to improve the present day waste management of the industries.

[17] S. Mahalingam, Y.H. Ahn, Improved visible light photocatalytic activity of rGOFe3O4-NiO hybrid nanocomposites synthesized by in-situ facile method for industrial wastewater treatment applications, New J. Chem. 42 (2018) 4372–4383. [18] H. Moussa, E. Girot, K. Mozet, H. Alem, G. Medjahdi, R. Schneider, ZnO rods/reduced graphene oxide composites prepared via a solvothermal reaction for efficient sunlight-driven photocatalysis, Appl. Catal. B Environ. 185 (2016) 11–21. [19] C. Zhang, J. Zhang, Y. Su, M. Xu, Z. Yang, Y. Zhang, Hydrothermal preparation of ZnO-reduced graphene oxide hybrid with high performance in photocatalytic degradation, Physica 56 (2014) 251–255. [20] J. Qin, X. Zhang, C. Yang, M. Cao, M. Ma, R. Liu, ZnO microspheres-reduced graphene oxide nanocomposite for photocatalytic degradation of methylene blue dye, Appl. Surf. Sci. 392 (2017) 196–203. [21] T. Xu, L. Zhang, H. Cheng, Y. Zhu, Significantly enhanced photocatalytic performance of ZnO via graphene hybridization and the mechanism study, Appl. Catal. B Environ. 101 (2011) 382–387. [22] M.K. Patra, M. Manoth, V.K. Singh, G.S. Gowd, V.S. Choudhry, S.R. Vadera, N. Kumar, Synthesis of stable dispersion of ZnO quantum dots in aqueous medium showing visible emission from bluish green to yellow, J. Lumin. 129 (2009) 320–324. [23] S. Mahamuni, K. Borgohain, B.S. Bendre, V.J. Leppert, S.H. Risbud, Spectroscopic and structural characterization of electrochemically grown ZnO quantum dots, J. Appl. Phys. 85 (1999) 2861–2865. [24] 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, ACS Nano 4 (2010) 4806–4814. [25] J. Chen, B. Yao, C. Li, G. Shi, An improved Hummers method for eco-friendly synthesis of graphene oxide, Carbon 64 (2013) 225–229. [26] X. Bai, C. Sun, D. Liu, X. Luo, D. Li, J. Wang, N. Wang, X. Chang, R. Zong, Y. Zhu, Performance enhancement of ZnO photocatalyst via synergic effect of surface oxygen defect and graphene hybridization, Appl. Catal. B Environ. 204 (2017) 11–20. [27] G. Williams, P.V. Kamat, Graphene-semiconductor nanocomposites: excited-state interactions between ZnO nanoparticles and graphene oxide, Lamgmuir 25 (2009) 13869–13873. [28] H.A. Becerril, J. Mao, Z. Liu, R.M. Stoltenberg, Z. Bao, Y. Chen, Evaluation of solution-processed reduced graphene oxide films as transparent conductors, ACS Nano 2 (2008) 463–470. [29] M. Azarang, A. Shuhaimi, R. Yousefi, M. Sookhakian, Experimental and theoretical study of enhanced photocatalytic activity of Mg-doped ZnO NPs and ZnO/rGO nanocomposites, J. Appl. Phys. 116 (2014) 84307–84313. [30] X. Zhou, T. Shi, H. Zhou, Hydrothermal preparation of ZnO-reduced graphene oxide hybrid with high performance in photocatalytic degradation, Appl. Surf. Sci. 258 (2012) 6204–6211. [31] A.C. Ferrari, J. Robertson, Interpretation of Raman spectra of disordered and amorphous carbon, Phys. Rev. B 61 (2000) 14095–14107. [32] K.W.R. Gilkes, H.S. Sands, D.N. Batchelder, J. Robertson, W.I. Milne, Direct observation of sp3 bonding in tetrahedral amorphous carbon using ultraviolet Raman spectroscopy, Appl. Phys. Lett. 70 (1997) 1980–1982. [33] I.K. Moon, J. Lee, R.S. Ruoff, H. Lee, Reduced graphene oxide by chemical graphitization, Nat. Commun. 1 (2010) 1–6. [34] A. Khan, V.A. Fonoberov, M. Shamsa, A.A. Balandin, Micro-Raman investigation of optical phonons in ZnO nanocrystals, J. Appl. Phys. 97 (2005) 124313–124318. [35] S. Pal, N. Gogurla, A. Das, S.S. Singha, P. Kumar, D. Kanjilal, A. Singha, S. Chattopadhyay, D. Jana, A. Sarkar, Clustered vacancies in ZnO: chemical aspects and consequences on physical properties, J. Phys. D Appl. Phys. 51 (2018) 105107–105119. [36] V. Russo, M. Ghidelli, P. Gondoni, C.S. Casari, A. Li Bassi, Multi-wavelength Raman scattering of nanostructured Al-doped zinc oxide, J. Appl. Phys. 115 (2014) 73508–73518. [37] T.C. Damen, S.P.S. Porto, B. Tell, Raman effect in zinc oxide, Phys. Rev. 142 (1966) 570–574. [38] Z. Zhan, L. Zheng, Y. Pan, G. Sun, L. Li, Self-powered, visible-light photodetector based on thermally reduced graphene oxide–ZnO (rGO–ZnO) hybrid nanostructure, J. Mater. Chem. 22 (2012) 2589–2595. [39] S. Ghosh, K. Das, G. Sinha, J. Lahtinen, S.K. De, Bright white light emitting Eu and Tb co-doped monodisperse In2O3 nanocrystals, J. Mater. Chem. C 1 (2013) 5557–5566. [40] Y.S. Fu, X.W. Du, S.A. Kulinich, J.S. Qiu, W.J. Qin, R. Li, J. Sun, J. Liu, Stable Aqueous dispersion of ZnO quantum dots with strong blue emission via simple solution route, J. Am. Chem. Soc. 129 (2007) 16029–16033. [41] D. Pan, J. Zhang, Z. Li, M. Wu, Hydrothermal route for cutting graphene sheets into blue‐luminescent graphene quantum dots, Adv. Mater. 22 (2010) 734–738.

Acknowledgements This work is financially supported by the Council of Scientific and Industrial Research, Government of India through CSIR NET fellowship (CSIR JRF Sanction No.09/028/(0976)/2016-EMR-I). Thanks for the reviewers for their critical comments. Dr. De is thankful to department of science and technology (DST), government of India for funding through scheme for young scientists (DST fast track: SB/FTP/PS-190/ 2013) and Presidency University for funding through FRPDF scheme. References [1] C. Pan, Y. Zhu, New type of BiPO4 oxy-acid salt photocatalyst with high photocatalytic activity on degradation of dye, Environ. Sci. Technol. 44 (2010) 5570–5574. [2] S. Yang, J. Kim, J.H. Ryu, H. Oh, C.H. Chun, B.J. Kim, B.H. Min, J.S. Chun, Hypoxiainducible factor-2α is a catabolic regulator of osteoarthritic cartilage destruction, Nat. Med. 16 (2010) 687–694. [3] D. Malwal, P. Gopinath, Fabrication and characterization of poly(ethylene oxide) templated nickel oxide nanofibers for dye degradation, Environ. Sci.: Nano 2 (2015) 78–85. [4] M. Sun, Y. Wang, Y. Shao, Y. He, Q. Zeng, H. Liang, T. Yan, B. Du, Fabrication of a novel Z-scheme g-C3N4/Bi4O7 heterojunction photocatalyst with enhanced visible light-driven activity toward organic pollutants, J. Colloid Interface Sci. 501 (2017) 123–132. [5] P. Xu, X. Shen, L. Luo, Z. Shi, Z. Liu, Z. Chen, M. Zhu, L. Zhang, Preparation of TiO2/ Bi2WO6 nanostructured heterojunctions on carbon fibers as a weaveable visiblelight photocatalyst/photoelectrode, Environ. Sci.: Nano 5 (2018) 720–729. [6] W. Fan, Q. Lai, Q. Zhang, Y. Wang, Nanocomposites of TiO2 and reduced graphene oxide as efficient photocatalysts for hydrogen evolution, J. Phys. Chem. C 115 (2011) 10694–10701. [7] Y. Zhang, N. Zhang, Z. Tang, Y. Xu, Graphene transforms wide band gap ZnS to a visible light photocatalyst. The new role of graphene as a macromolecular photosensitizer, ACS Nano 6 (2012) 9777–9789. [8] B. Xue, Y. Zou, Uniform distribution of ZnO nanoparticles on the surface of grpahene and its enhanced photocatalytic performance, Appl. Surf. Sci. 440 (2018) 1123–1129. [9] S. Dutta, S. Chattopadhyay, A. Sarkar, M. Chakrabarti, D. Sanyal, D. Jana, Role of defects in tailoring structural, electrical and optical properties of ZnO, Prog. Mater. Sci. 54 (2009) 89–136. [10] A. Houas, H. Lachheb, M. Ksibi, E. Elaloui, C. Guillard, J.M. Herrmann, Photocatalytic degradation pathway of methylene blue in water, Appl. Catal. B Environ. 31 (2001) 145–157. [11] H. Fu, C. Pan, W. Yao, Y. Zhu, Visible-light-induced degradation of rhodamine B by nanosized Bi2WO6, J. Phys. Chem. B 109 (2005) 22432–22439. [12] M. Ghaedi, S. Hajjati, Z. Mahmudi, I. Tyagi, S. Agarwal, A. Maity, V.K. Gupta, Modeling of competitive ultrasonic assisted removal of the dyes – methylene blue and Safranin-O using Fe3O4 nanoparticles, Chem. Eng. J. 268 (2015) 28–37. [13] K.M. Lee, C.W. Lai, K.S. Ngai, J.C. Juan, Recent developments of zinc oxide based photocatalyst in water treatment, technology: a review, Water Res. 88 (2016) 428–448. [14] S. Dutta, K. Das, K. Chakrabarti, D. Jana, S.K. De, Sukanta De, Highly efficient photocatalytic activity of CuO quantum dot decorated rGO nanocomposites, J. Phys. D Appl. Phys. 49 (2016) 315107–315115. [15] K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim, J.H. Ahn, P. Kim, J.Y. Choi, B.H. Hong, Large-scale pattern growth of graphene films for stretchable transparent electrodes, Nature Letters 457 (2009) 706–710. [16] B. Wang, W. Feng, L. Zhang, Y. Zhang, X. Huang, Z. Fang, P. Liu, In situ construction of a novel Bi/CdS nanocomposite with enhanced visible light photocatalytic performance, Appl. Catal. B Environ. 206 (2017) 510–519.

464


S.K. Mandal et al.

Appl. Mater. Interfaces 4 (2012) 3944–3950. [51] C.R. Minitha, M. Lalitha, Y.L. Jeyachandran, L. Senthilkumar, R.T.R. Kumar, Adsorption behaviour of reduced graphene oxide towards cationic and anionic dyes:Co-action of electrostatic and π – π interactions, Mater. Chem. Phys. 194 (2017) 243–252. [52] G. Rajender, J. Kumar, P.K. Giri, Interfacial charge transfer in oxygen deficient TiO2-graphene quantum dot hybrid and its influence on the enhanced visible light photocatalysis, Appl. Catal. B Environ. 224 (2018) 960–972. [53] Z. Xin, L. Li, X. Zhang, W. Zhang, Microwave-assisted hydrothermal synthesis of chrysanthemum-like Ag/ZnO prismatic nanorods and their photocatalytic properties with multiple modes for dye degradation and hydrogen production, RSC Adv. 8 (2018) 6027–6038. [54] M.E. Kemary, Y.A. Moneam, M. Madkour, I.E. Mehasseb, Enhanced photocatalytic degradation of Safranin-O by heterogeneous nanoparticles for environmental applications, J. Lumin. 131 (2011) 570–576. [55] M.E. Kemary, H.E. Shamy, Fluorescence modulation and photodegradation characteristics of safranin O dye in the presence of ZnS nanoparticles, J. Photochem. Photobiol. Chem. 205 (2009) 151–155. [56] H.R. Pouretedal, A. Norozib, M.H. Keshavarz, A. Semnani, Nanoparticles of zinc sulfide doped with manganese, nickel and copper as nanophotocatalyst in the degradation of organic dyes, J. Hazard Mater. 162 (2009) 674–681. [57] K. Hayat, M.A. Gondal, M.M. Khaled, Z.H. Yamani, S. Ahmed, Laser induced photocatalytic degradation of hazardous dye (Safranin-O) using self synthesized nanocrystalline WO3, J. Hazard Mater. 186 (2011) 1226–1233. [58] A. Bumajdad, M. Madkour, Y.A. Moneam, M.E. Kemary, Nanostructured mesoporous Au/TiO2 for photocatalytic degradation of a textile dye: the effect of size similarity of the deposited Au with that of TiO2 pores, J. Mater. Sci. 49 (2013) 1743–1754.

[42] Y.C. Chen, K.I. Katsumata, Y.H. Chiu, K. Okada, N. Matsushita, Y.J. Hsu, ZnOgraphene composites as practical photocatalysts for gaseous acetaldehyde degradation and electrolytic water oxidation, Appl. Catal. B Environ. 490 (2015) 1–9. [43] D.W. deQuilettes, S.M. Vorpahl, S.D. Stranks, H. Nagaoka, G.E. Eperon, M.E. Ziffer, H.J. Snaith, D.S. Ginger, Impact of microstructure on local Carrier lifetime in solar cells, Science 348 (2015) 683–686. [44] C. Wang, M. Cao, P. Wang, Y. Ao, J. Hou, J. Qian, Preparation of graphene–carbon nanotube–TiO2 composites with enhanced photocatalytic activity for the removal of dye and Cr (VI), Appl. Catal. Gen. 473 (2014) 83–89. [45] Y. Feng, N. Feng, Y. Wei, G. Zhang, An in situ gelatin-assisted hydrothermal synthesis of ZnO–reduced graphene oxide composites with enhanced photocatalytic performance under ultraviolet and visible light, RSC Adv. 4 (2014) 7933–7943. [46] A. Benayad, H.J. Shin, H.K. Park, S.M. Yoon, K.K. Kim, M.H. Jin, H.K. Jeong, J.C. Lee, J.Y. Choi, Y.H. Lee, Controlling work function of reduced graphite oxide with Au-ion concentration, Chem. Phys. Lett. 475 (2009) 91–95. [47] X. Bai, L. Wang, R. Zong, Y. Lv, Y. Sun, Y. Zhu, Performance enhancement of ZnO photocatalyst via synergic effect of surface oxygen defect and graphene hybridization, Langmuir 29 (2013) 3097–3105. [48] W. Tu, Y. Zhou, Q. Liu, S. Yan, S. Bao, X. Wang, M. Xiao, Z. Zou, An in situ simultaneous reduction-hydrolysis technique for fabrication of TiO2-graphene 2D sandwich-like hybrid nanosheets: graphene-promoted selectivity of photocatalyticdriven hydrogenation and coupling of CO2 into methane and ethane, Adv. Funct. Mater. 23 (2013) 1743–1749. [49] Z. Chen, N. Zhang, Y.J. Xu, Synthesis of graphene–ZnO nanorod nanocomposites with improved photoactivity and anti-photocorrosion, CrystEngComm 15 (2013) 3022–3030. [50] X. Pan, Y. Zhao, S. Liu, C.L. Korzeniewski, S. Wang, Z. Fan, Comparing grapheneTiO2 nanowire and graphene-TiO2 nanoparticle composite photocatalysts, ACS

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