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reduced graphene oxide nanocomposites for degradation of organic pollutants via efficient charge separation pathway
Applied Surface Science 487 (2019) 1279–1288
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Enhanced photocatalytic activities of ZnO dumbbell/reduced graphene oxide nanocomposites for degradation of organic pollutants via efficient charge separation pathway S. Prabhua, S. Megalaa, S. Harishb,c, M. Navaneethanb,d, P. Maadeswarane, S. Sohilaf, R. Ramesha,
T
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a
Department of Physics, Periyar University, Salem 636011, Tamil Nadu, India Center for Materials Science and Nano Devices, Department of Physics and Nanotechnology, SRM Institute of Science and Technology, Kattankulathur, Kancheepuram 603203, Tamil Nadu, India c Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-Ku, Hamamatsu, Shizuoka 432-8011, Japan d Nanotechnology Research Center (NRC), Faculty of Engineering and Technology, SRM Institute of Science and Technology, Kattankulathur, Chennai, 603203, Tamil Nadu, India e Department of Energy Studies, Periyar University, Salem 636011, Tamil Nadu, India f Department of Physics, Shri Sakthikailassh Women's College, Salem 636003, Tamil Nadu, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: ZnO dumbbell Optical property Reduced graphene oxide Photocatalytic activity Charge transport property
In the present work, a well-defined dumbbell shaped ZnO was synthesized by hydrothermal method and ZnO dumbbell/reduced graphene oxide (ZnO/rGO) nanocomposites were prepared by a simple solution mixing method with the different rGO loading amount. The formation of nanocomposites, crystal structure, shape, size and optical properties of the ZnO/rGO nanocomposites were investigated using various analytical techniques. The photocatalytic degradation efficiency of synthesized materials was evaluated by the degradation of aqueous Methyl Orange (MO) and Methylene Blue (MB) under UV–Visible light irradiation. The results revealed that 3 wt % rGO loaded ZnO dumbbell (ZnO-3% rGO) exhibited the higher photocatalytic degradation efficiency than pure ZnO dumbbell and rGO. The enhancement in the photocatalytic dye degradation efficiency of ZnO-3% rGO is attributed to the efficient dye adsorption nature, a red shift in light absorption and inhibition of photo-excited electron-hole pair recombination rate. The photocatalytic dye degradation efficiency also evaluated for various catalyst dosage, dye concentration and pH of the solution. Moreover, it was demonstrated the stability of catalyst over a repeated cycle of dye treatment.
1. Introduction The wastewater from various industries contains toxic organic pollutants which causes severe environmental pollution [1,2]. The organic dyes in the effluents coming from the industries are difficult to degrade due to their chemical stability. Several processes including adsorption, chemical oxidation, ion-exchange, reverse osmosis, precipitation, biological and photocatalytic treatments have developed to treat the organic dye contaminated water [3]. Among them, the semiconductor based photocatalysis is the one of the effective way for degradation organic dyes or organic effluent in the wastewater. Several semiconducting materials have been used for photocatalytic dye degradation process including ZnO, TiO2, Fe2O3, WO3, ZnS, BiVO4, NiO and Co3O4 [4–6]. Among the various semiconductors, ZnO has been attracted considerable attention due to its high electron mobility, suitable
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energy band gap, low cost and nontoxicity [7]. Although, wide bandgap (3.37 eV), photocorrosion and low separation efficiency of electronhole pairs could hinder the extensive applicability of ZnO photocatalyst [8]. In the recent decades, strategies like nanostructuring, doping, and formation of nanocomposites have been adopted to improve the photocatalytic dye degradation efficiency of ZnO [9]. The photocatalytic reaction usually takes place at the interface between catalyst surfaces and organic pollutants. Therefore the photocatalytic dye degradation efficiency of ZnO depends on its morphology [10]. In particular, the dumbbell-shaped ZnO photocatalyst has received much attention due to their superior photocatalytic property. For example, the morphology and facet dependent charge transport property of the ZnO photocatalyst explained by Pawar et al. [11]. Furthermore, the formation of nanocomposite with graphene-based material could improve the photocatalytic property of the ZnO. The extremely high specific surface area,
Corresponding author at: Department of Physics, Periyar University, Salem-11, Tamil Nadu, India. E-mail address: [email protected] (R. Ramesh).
https://doi.org/10.1016/j.apsusc.2019.05.086 Received 9 January 2019; Received in revised form 12 April 2019; Accepted 7 May 2019 Available online 08 May 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 487 (2019) 1279–1288
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temperature. The final product was separated by centrifugation and washed with ethanol and DI water for several times. Finally, the obtained product was dried at 80 °C in a hot air oven.
superior chemical stability and high electrical conductivity of the Graphene (GR) or reduced graphene oxide (rGO), could increase the photoexcited charge carrier separation process of the ZnO photocatalyst [12–14]. Up to date, there are numerous report on the preparation and utilization of rGO/semiconductors for photocatalytic application [15]. A novel hierarchical porous ZnO/Reduced graphene oxide nanocomposite has shown the great interest on the photocatalytic application due to their effective electron-hole separation and its transportation. Recently, Kheirabadi et al., designed Ag/ZnO/3D graphene nanostructure for effective photocatalytic removal of dyes from wastewater [16]. Systematic experimental and theoretical investigations have been performed on ZnO/rGO nanocomposites and it has been proved that enhanced photocatalytic dye degradation mainly due to higher electron transfer to the dye solution [17]. Therefore, it is expected that the formation of rGO based nanocomposites with well-organized ZnO structure can be efficiently utilized for the photocatalytic dye degradation application with superior catalytic performance. In the present work, well defined ZnO dumbbell was synthesized by hydrothermal method using dodecylamine as a surfactant. Furthermore, ZnO/rGO nanocomposites with the varying the loading amount of rGO (1 wt%, 2 wt%,3 wt% and 4 wt%) were synthesized by simple solution mixing method and named as ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposite. The photocatalytic dye degradation performance of the pure ZnO, rGO and ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposites was evaluated using MO and MB dyes as model pollutants. An electrochemical property of the prepared materials was studied to validate the enhanced photocatalytic dye degradation efficiency of the ZnO-3% rGO photocatalyst.
2.4. Preparation of zinc oxide-reduced graphene oxide (ZnO-rGO) nanocomposites A series of ZnO-rGO nanocomposites were synthesized with the varying amount of rGO. For the preparation of ZnO-1% rGO, a 1 wt% of rGO was dispersed in 25 mL of DI water via ultra-sonication (Kerry KC338 kHz) for 1 h to achieve a homogeneous solution followed by 100 mg of as-prepared ZnO was added into the dispersion and then dispersed mixture was continuously stirred by a magnetic stirrer for 12 h. After that the mixture was washed with several times by ethanol and DI water using centrifugation process and then the product was dried at 80 °C in hot air oven for overnight. For comparison, ZnO-rGO nanocomposites were synthesized by the same experimental method under similar conditions with the varying amount of rGO (2 wt%, 3 wt%, and 4 wt%) and the final products were labeled as ZnO-2% rGO, ZnO-3% rGO, and ZnO-4% rGO. 2.5. Characterization techniques The phase purity and crystal structure of the as-prepared samples were characterized by powder X-ray diffraction (XRD) using Rigaku D/ Max Ultima III X-ray diffractometer with a Cu Kα radiation source (λ = 0.154 nm) operated at 40 kV. The optical absorption properties of the prepared samples were characterized by a V-770 diffuse reflectance spectrophotometer (DRS, JASCO). Fourier-transform infrared (FTIR) spectra of KBr powder-pressed pellets were recorded by Bruker a NEXUS 470 in the range of 4000 to 400 cm−1. The surface morphology of obtained samples was visualized by field emission scanning electron microscopy (FESEM, Hitachi S-4800). The crystal phase and internal morphologies of the nanocomposites were obtained by transmission electron microscopy (TEM, Hitachi H-600). X-ray photoelectron spectroscopy (XPS) technique was utilized to identify chemical states of prepared samples at room temperature by using ESCA 3400 spectrometer in ultra-high vacuum with 10–10 mbar base pressure. The measurement chamber is attached with a monochromatic Al (Kα) X-ray source providing photon with hυ = 1486.6 eV and hemispherical analyzer (150 mm diameter) was used for energy analysis. The pass energy was kept 20 eV for the high resolution. Room temperature Raman spectroscopy measurements were carried out on LabRAM HR Evolution Raman microscopes using UV laser as light source with wavelength of 364 nm.
2. Experimental section 2.1. Chemicals All the reagents were purchased commercially and should be used as without further purification. Zinc acetate dihydrate (Zn(O2CCH3)2 99.5%), Dodecylamine (C12H27N - 97.0%), Graphite powder (99.5%), Hydrogen peroxide (H2O2 - 30% purity), Sulfuric acid (H2SO4 - 97%), Potassium permanganate (KMnO4 - 99.0%), Sodium nitrate (NaNO3 99.0%), Hydrochloric acid (HCl - 38.0%), Methyl Orange (MO), Methylene Blue (MB), Benzoquinone (BQ - 98%), Isopropanol (IPA 99.8%), Triethanolamine (TEOA - 99%) and Silver nitrate (AgNO3 99.0%) were purchased from Merck in India. All experiments were performed using deionized (DI) water with an electrical conductivity > 18 MΩ. 2.2. Synthesis of reduced graphite oxide (rGO)
2.6. Photocatalytic measurements Graphene oxide (GO) was prepared by modified Hummer's method and it is reduced by the following procedure to get reduced graphene oxide (rGO) [18]. A 1 g of as-synthesized GO powder was dispersed in 50 mL DI water using bath sonicator for 2 h and then 20 mL of ammonia solution was dropped wise added into the solution. Then the solution was transferred in a 100 mL Teflon-lined autoclave and heated at 160 °C for 12 h in hot air oven. After that, the autoclave was allowed to cool naturally the room temperature. The block precipitate was collected and washed with ethanol and DI water several times. Finally, the collected sample was dried in a hot air oven at 60 °C overnight.
The photocatalytic activity of ZnO dumbbell, rGO, and ZnO-rGO nanocomposites was evaluated through degradation of an aqueous solution of methyl orange (10 mg/L) and methylene blue (10 mg/L) under 500 W Halogen lamp Illumination placed at 15 cm distance from the photoreactor. In the typical experiment, a 50 mg of prepared sample was dispersed in 100 mL aqueous solution of MO and MB with an initial concentration 10 mg/L under ambient condition and continuously stirred under the dark condition for 1 h to reach the adsorption-desorption equilibrium. Then the mixture was exposed by Halogen lamp to start the photocatalytic process. At the regular time interval, 5 mL solution was separated and centrifuged at 10,000 rpm for 1 min to remove the solid catalyst. The concentration of dye molecules in the supernatant were measured from the absorbance spectra recorded on JASCO V-670 double beam spectrophotometer. For comparison, the photocatalytic dye degradation experiment was performed for different amount of catalyst (0.25 g/L, 0.50 g/L, 0.75/L mg and 1.00 g/L), various concentration of dyes (10, 15, 20 and 25 mg/L) and pH contractions (4, 7, 9 and 12) of the solution. The photocatalytic degradation
2.3. Synthesis of ZnO dumbbell In the typical synthesis, 0.1 M of zinc acetate dihydrate (Zn (CO2CH3)2.2H2O) was dissolved in 50 mL of DI water under magnetic stirring and the white precipitate was formed by dropwise addition of 5 ml dodecylamine (DDA). Then the mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 200 °C for 15 h and then the autoclave was naturally cooled down to room 1280
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nanocomposites (f). A broad diffraction peak observed at 2θ = 26.2° in Fig. 1(a), which could be attributed to the (002) plane of the hexagonal phase of rGO (JCPDS no.75-1621). The diffraction peaks from ZnO dumbbell and its composites are similar at 2θ values of 31.9°, 34.3°, 36.2°, 47.6°, 56.6°, 62.8°, 67.8° and 69.2° corresponding to (100), (002), (101), (102), (110), (103), (112) and (201) crystal planes, respectively and it is well consistent with standard diffraction pattern (JCPDS no 361451) for hexagonal wurtzite ZnO structure [18]. In addition to that the relatively low intense broad peak was observed at 2θ = 26.2° from ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO, and ZnO-4% rGO nanocomposites which indicate the presence of rGO in the nanocomposites and peak intensity is increased while increasing the loading content of rGO. The sharp intensity of the diffraction peak from pure ZnO dumbbell and ZnO-rGO nanocomposites indicate that the good crystallinity of the synthesized samples. No other impurities were observed from ZnO dumbbell and ZnO-rGO nanocomposites.
efficiency of dyes was calculated the following equation
A −A D= o × 100% Ao
(1)
where Ao is the initial concentration of dye and A is the concentration of dye after photoirradiation. The reactive superoxide radical (O2%−), hydroxyl radicals (%OH), holes (h+) and electrons (e−) spices were determined in photocatalytic reaction solution by adding 1 mM Benzoquinone, isopropanol (IPA), triethanolamine (TEOA) and silver nitrate, respectively. 2.7. Electrochemical experiments Transient photocurrent and Electrochemical Impedance measurements were performed with SP-150 electrochemical workstation (BioLogic Science Instruments, France) using typical three-electrode system where Pt wire and Ag/AgCl in saturated KCl solution were used a counter and reference electrode respectively. The working electrode was fabricated by the following procedure, a 5 mg of prepared samples were dispersed 0.5 mL ethanol and 20 μL Nafion solution using ultrasonication for 30 min to form the homogeneous ink which was then spin coated on pre-cleaned Fluorine doped tin oxide (FTO) glass substrate with rotation speed 2000 rpm for 2 min. The working electrodes were naturally dried under ambient condition and active surface area was fixed as ca. 1cm2 with the help of Teflon tape. In this experiment, the aqueous Na2SO4 (0.5 M) was employed as an electrolyte solution. The 150 W xenon lamp was used as a light source to illuminate working electrodes with power intensity 100 mW/cm2 and it is calibrated by solar power meter. The transient photocurrent (i-t) response of the prepared electrodes was recorded under the light on and off condition. The electrochemical N-Q plots were recorded in a frequency range of 1 MHz-0.1 Hz with AC amplitude of 10 mV under open circuit potential in absence light.
3.2. Surface morphological study The field-emission scanning electron microscopy (FE-SEM) used to visualize the surface morphology of ZnO and ZnO-3% rGO nanocomposites. As shown in Fig. 2(a, b), ZnO exhibited a uniformly distributed dumbbell shape ZnO morphology consist of two hexagonal crystals in the micrometer size. From the Fig. 2(c, d), it is clearly seen that ZnO dumbbells were uniformly dispersed on rGO sheet. Internal morphology and crystalline property of the prepared samples were analyzed using TEM technique. The well-defined dumbbell shape ZnO was clearly observed in Fig. 3(a). As shown in Fig. 3(b), the HR-TEM image was recorded at the edge of the ZnO dumbbell. The clear lattice fringes were observed with an inter-planar spacing of 0.26 nm for (0001) plane of the hexagonal phase ZnO crystal [19]. Fig. 3(c–d), the TEM and HR-TEM images of ZnO dumbbells were wrapped by rGO, thereby which making the interfacial contact between rGO and ZnO dumbbells. Both FE-SEM and TEM images revealed that the formation of interfacial contact between rGO sheet and ZnO dumbbell and it is expected that which could increase the charge carrier transfer process [20].
3. Results and discussion 3.1. Structural and crystal phase analysis
3.3. FT-IR analysis
Powder X-ray diffraction (XRD) measurements were carried out to identify the phase purity and crystal structure of the prepared samples. Fig. 1 shows the XRD patterns of rGO (a), pure ZnO (b), ZnO-1% rGO (c), ZnO-2% rGO (d), ZnO-3% rGO (e), and ZnO-4% rGO
The chemical structure and vibration modes of the prepared samples were analyzed by FT-IR technique. Fig. 4 shows the FT-IR spectra of (a) rGO, (b) ZnO dumbbell, (c) ZnO-1% rGO (d) ZnO-2% rGO, (e) ZnO-3% rGO and (f) ZnO-4% rGO in range between 4000 and 500 cm−1. The FT-IR spectra of ZnO dumbbell and ZnO-rGO nanocomposites shown in Fig. 4(b–f), the strong peak at 508 cm−1 is corresponding to the ZneO stretching mode of ZnO [21]. On the other hand, the peaks at 3400 cm−1 and 1426 cm−1 attributed to the vibration of the hydroxyl group in nanocomposites. The stretching vibrations of epoxy CeO (1226.92 cm−1) and weak aromatic C]C (1578 cm−1) bonds were observed from pure rGO and it is nanocomposites [22]. Moreover, the full width half maximum (FWHM) of ZneO stretching mode is increased while increasing the rGO weight percentage in the composites. This is due to increase in the peak intensity of CeO and C]C bonds for higher weight percentage of rGO (3% and 4%) in the nanocomposites which could decrease the intensity of ZneO stretching mode thereby the FWHM is increased. And also the peak from ZneO stretching mode was significantly shifted in the lower wavenumber side for ZnO-3% rGO and ZnO-4% rGO nanocomposites which implies that dispersion of ZnO dumbbell on rGO sheet. 3.4. XPS analysis The prepared rGO, ZnO dumbbell and ZnO-3% rGO nanocomposites were further analyzed by XPS technique to identify the chemicals states and compositions. The binding energy scale was calibrated using C 1s
Fig. 1. Powder XRD patterns of (a) rGO (b) ZnO dumbbell, (c) ZnO-1% rGO (d) ZnO-2% rGO, (e) ZnO-3% rGO and (f) ZnO-4% rGO nanocomposites. 1281
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Fig. 2. FE-SEM images of (a, b) ZnO dumbbell and (c, d) ZnO-3% rGO nanocomposites with two deferent magnification.
Carbonyl Carbon (C=C) [23]. The high-resolution spectrum of O1s for rGO, pure ZnO dumbbell and ZnO-3% rGO nanocomposites is further deconvoluted into three peaks which are presented Fig. 5(d). The deconvoluted O1s peaks were shifted to higher binding energy for ZnO3% rGO nanocomposites when compared with pure ZnO dumbbell and rGO due to dispersion of ZnO dumbbell on rGO sheet [24].
peak (284.1 eV) in the XPS spectra. As shown in Fig. 5(a), the survey spectrum of composites exhibited peaks at binding energy of 284.6 eV, 530.7 eV, 1024.3 eV and 1045.7 eV correspond to the C 1s, O 1s, Zn 2p3/2, and Zn 2p1/2 respectively. The peaks at 1024.3 eV and 1045.7 eV in the high-resolution spectrum of ZnO-3% rGO nanocomposites ascribed to the Zn 2p1/2 and Zn 2p2/3, respectively. Besides, the similar peaks were observed for both pure ZnO dumbbell and ZnO3% rGO nanocomposites without changing their intensity which confirms the chemical states of Zn2+ ion in the ZnO-3% rGO nanocomposites is not influenced by rGO in Fig. 5(b). As shown in Fig. 5(c) the C 1s core level spectra of ZnO dumbbell, rGO and ZnO-3% rGO nanocomposites were deconvoluted into three peaks i) non‑oxygenated graphitic CeC bond of Sp2 carbon ii) epoxy and alkoxy (CeO) and iii)
3.5. Micro Raman analysis Micro Raman analysis was performed for the prepared samples to verify the nature of the hybrid materials. Fig. 6 shows the micro Raman spectra of as ZnO dumbbell, ZnO-1% rGO, ZnO-2% rGO, and ZnO-3% rGO nanocomposites. The peaks at 325 cm−1 and 435 cm−1 of ZnO
Fig. 3. (a) TEM image and (b) HR-TEM image of ZnO dumbbell and (c) TEM and (d) HR-TEM images of ZnO-3% rGO nanocomposites. The inset presents the lattice plane of hexagonal ZnO. 1282
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Fig. 4. FT-IR spectra for (a) rGO (b) ZnO dumbbell, (c) ZnO-1% rGO (d) ZnO2% rGO, (e) ZnO-3% rGO and (f) ZnO-4% rGO nanocomposites.
dumbbell are the 2E2 and E1 phonon vibration in the lattice [25]. In addition, the D and G bands were observed at 1335 cm−1 and 1584 cm−1, respectively in the Raman spectra of ZnO-1% rGO, ZnO-2% rGO and ZnO-3% rGO nanocomposites which confirm the formation
Fig. 6. Micro Raman spectra of (a) ZnO dumbbell, (b) ZnO-1% rGO, (b) ZnO2% rGO and (d) ZnO-3% rGO nanocomposites.
Fig. 5. XPS (a) survey, (b) C1s, (c) O1s and (d) Zn 2p spectra of rGO, ZnO, ZnO-3% rGO nanocomposites. 1283
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Fig. 7. UV–VIS DRS spectra of ZnO dumbbell, ZnO/rGO (ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO) nanocomposite. Inset: calculated band gaps from correspond absorption spectra samples.
Fig. 8. Transient photocurrent density of ZnO dumbbell, ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposites under simulated solar irradiation with light on-off condition.
nanocomposites. The G band arises from first-order scattering of E2g mode and denoted stretching vibration of the sp2 hybridization [26]. D band is the defect band due to the presence of sp3 hybridization of carbon atoms [27]. Besides, the intensity of both D and G bands were increased while increasing the loading amount of rGO with ZnO dumbbell which influenced to decrees the intensity of 2E2 and E1 mode from ZnO and it is confirmed the effective formation of nanocomposites.
to the smaller loading amount of rGO in composites. Besides, the ZnO4% rGO-nanocomposites showed the decreased photocurrent density than that of ZnO-3% rGO-nanocomposites. This result may be due to the higher loading amount of rGO in the nanocomposites that could increase the photo-excited charge carriers recombination rates rather than charge carriers separation [33]. Furthermore, the interfacial charge carrier separation and charge transport properties of the ZnO dumbbell, ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposites were investigated from EIS analysis. The Nyquist plots of prepared samples at open circuit potential are depicted in the Fig. 9. It is clearly seen that the arc radius of the N-Q plot for ZnO-3% rGO nanocomposites is much smaller than ZnO dumbbell, ZnO-1% rGO, ZnO-2% rGO and ZnO-4% rGO nanocomposites. This observation implies that the ZnO-3% rGO-nanocomposites possess the efficient charge carrier separation with enhanced interfacial electron transfer performance, which is consistence with the transient photocurrent measurement [34]. Moreover, the current density of pure ZnO dumbbell and ZnO-1% rGO nanocomposites is not stable even when the light is on. The instability of the current density upon the illumination of light is because of the higher surface recombination rate.
3.6. UV–VIS DRS analysis The light absorption nature of the prepared materials was characterized by UV–VIS diffusion reflectance spectroscopy (DRS). The UV–VIS spectra of ZnO dumbbell, ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposites shown in Fig. 7. The light absorption is red shifted by increasing the loading amount of rGO in ZnOrGO nanocomposites compared with pure ZnO dumbbell. The band gaps of prepared materials were calculated from the UV–VIS DRS spectra and shown at inset of Fig. 7. The calculated band gaps were 3.16 eV, 3.04 eV, 2.93 eV, 2.86 eV and 2.74 eV, for ZnO dumbbell, ZnO1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposites, respectively. The reduction in the bandgap of ZnO dumbbell upon varying the loading amount of rGO is mainly due to the formation of Zn-O-C chemical bonds in the nanohybrid [28,29].
3.8. Photocatalytic activities
3.7. Photoelectrochemical properties
The photocatalytic activity of rGO, ZnO dumbbell, ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposites was evaluated by degradation of MO and MB dyes under irradiation of
The photo-induced electron-hole recombination rates and charge separation of prepared materials were characterized by transient photocurrent and electrochemical impedance spectroscopy (EIS) measurements. The transient photocurrent response of ZnO dumbbell, ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO-and ZnO-4% rGO nanocomposites under stimulated solar radiation shown in Fig. 8. The photocurrent density of all samples was quite stable after several on-off cycles. Particularly, the photocurrent density of ZnO-3% rGO nanocomposites is higher than other samples, which implies that ZnO-3% rGO nanocomposites have higher photogenerated electron-hole separation under simulated solar radiation [30]. In general, the higher photocurrent density is mainly attributed to the effective photo-excited charge separation with a minimum of recombination rates [31]. In the charge separation process of the ZnO-3% rGO nanocomposites, the rGO can efficiently separate charge carriers and transport the electrons due to its superior electrical conductivity [32], whereas ZnO-1% rGO-and ZnO2% rGO nanocomposites exhibited the lower photocurrent density compared with that of ZnO-3% rGO nanocomposites may be attributed
Fig. 9. Nyquist plots for ZnO dumbbell, ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposites at open circuit potential. 1284
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Fig. 10. Photocatalytic (a) Methyl Orange dye degradation Efficiency of rGO, ZnO dumbbell, ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposites, (b) Methylene Blue dye degradation Efficiency of rGO, ZnO dumbbell, ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposites.
500 W Halogen lamp. Prior to irradiation, the photocatalytic reaction system was magnetically stirred in the dark for 30 min to attain the adsorption/desorption equilibrium. Supporting Figs. S1&S2 show the UV–Visible absorbance spectra of MO and MB solution in the presence of ZnO-3% rGO nanocomposites under different light illumination time. The photocatalytic MO and MB degradation efficiency of the ZnO dumbbell, ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposites were depicted in Supporting Fig. 1(a&b), respectively. As shown in Fig. 10(a), the rGO, ZnO dumbbell, ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposites have photocatalytic dye degradation efficiency of 55.1%, 67.2%, 78.4%, 80.2%, 89.78% and 81.20% of MO dye respectively after 180 min of light irradiation. Besides, the photocatalytic dye degradation efficiency of 70%, 85%, 90%, 94%, 97%, and 80% was achieved for MB dye at the same illumination time as of MO (Fig. 10(b)). The higher photocatalytic MO and MB dye degradation efficiency were obtained for ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposites than pure rGO and ZnO dumbbell. Fig. 10 represents the remarkable dye degradation efficiency for ZnO dumbbell (ZnO-3% rGO) than other composites which implies that the integration 3w% of rGO with ZnO dumbbell serves as an electron-hole separator and provide the pathway to transfer the electron into the composites [35]. However, the 4 wt% rGO loaded ZnO dumbbell shows the decreased photocatalytic dye degradation activity because of the reason that higher loading of rGO could increase the photoexcited electron-hole recombination rates [36]. It is observably seen that in Fig. 10(a&b), the nanostructures with 3% of rGO (ZnO-3% rGO) exhibits the 89.78% and 97% of photocatalytic MO and MB dye degradation efficiency, respectively which is 1.5 fold higher than the pure ZnO dumbbell. The increased photocatalytic efficiency is mainly due to an effective photoexcited charge carriers separation at the interface of rGO and ZnO [37].
Fig. 11. Active species trapping experiment on ZnO-3% rGO nanostructures during the photocatalytic degradation of MB under UV–VIS. light irradiation.
in this process. Whereas the presence of O2−, h+, and e− radical scavengers in the photocatalytic reaction system, the degradation rate of MB over the ZnO-3% rGO was not dramatically decreased which implies that these radical species play a minor role in the photocatalytic dye degradation reaction system. Based on the aforementioned results, the possible photocatalytic degradation mechanism of MO and MB dye solution over ZnO-3% rGO nanocomposites was explained as follows. The proposed photocatalytic dye degradation mechanism is shown in the Fig. 12. When the ZnO-3% rGO nanocomposites irradiated with UV–VIS. light, the electron excited
3.8.1. Photocatalytic mechanism The hydroxyl radicals (%OH), superoxide anions (O2−%), hole (h+) and electron (e−) are reactive species and they play an important role during the photocatalytic degradation of organic pollutants in aqueous solution [38]. In order to investigate the influence of reactive species generated during the photocatalytic MB dye degradation process on ZnO-3% rGO nanocomposites, a series of trapping experiments were performed and depicted in Fig. 11. In the experiments, Isopropyl alcohol (IPA), Triethanolamine (TEOA), Benzoquinone (BQ), and Silver + − nitrate (Ag(NO)3) were used as the %OH, O2− radical sca, h , and e vengers. As shown in Fig. 11, it is clearly observed that in the presence of %OH radical scavenger, 72% of photocatalytic MB degradation efficiency was depressed compared with radical scavenger free degradation systems which conforming the %OH is the major oxidation species
Fig. 12. Schematic diagram for photocatalytic mechanism during the dye degradation process. 1285
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from valence band (VB) to the conduction band (CB) of ZnO, leaving behind holes in the valence band. These photo-excited electrons could transfer from the CB to the surface of rGO which suppress the electronhole recombination rates during the photocatalytic reaction [39]. Besides, the photo-generated electrons and holes could react with the surface absorbed water and oxygen molecules and produce the reactive species to degrade the dye molecules in the solution [40]. The photocatalytic chemical reactions during was explained as follows
ZnO/ rGO + hv → ZnO/ rGO (e− + h+)
(2)
(e−)
(3)
rGO + e− → rGO (e−)
(4)
O2 + ZnO (e−) → O2• −
(5)
ZnO +
e−
→ ZnO
O2 + rGO
(e−)
ZnO/rGO
(h+)
→
O2• −
+
(OH−)
Fig. 14. Effect of the MB Dye concentration for the ZnO-3% rGO nanocomposites.
(6)
→ ZnO/rGO +
(OH •))
(7)
OH • + pollutant (MO/ MB ) → Degradation product
(8)
O2• − + pollutant (MO/ MB ) → Degradation Product
(9)
3.8.2. Effect of the catalyst dosage The effect catalyst dosage on photocatalytic degradation of MB dye have been investigated for constant dye concentration and pH values and the results shown in the Fig. 13. It can be seen that the photocatalytic dye degradation efficiency was increased from 28.66% to 95.36% while increasing the ZnO-3% rGO nanocomposites catalyst dosage from 0.25 to 1.0 g/L. The highest degradation efficiency was obtained for 1.0 g/L catalyst concentration due to more availability photo-excited electrons and holes in the reaction solution [41].
Fig. 15. Effect of the pH for the ZnO-3% rGO nanocomposites.
3.8.3. Effect of initial concentration of dye The effect of various initial concentration of MB (10 mg/L, 15 mg/L, 20 mg/L and 25 mg/L) on the photocatalytic degradation efficiency of ZnO-3% rGO nanocomposites was examined presented in Fig. 14. As shown in the Fig. 14, the photocatalytic degradation rate of MB dye decreases with an increase in the initial concentration of MB dye. This result could be attributed to the inhibition of catalytic activities of the catalyst by excessive absorption of the MB dye molecules on the surface of the catalyst and diminishing the light penetration in photocatalytic solution resulted from light scattering of highly concentrate MB dyes [42].
were carried out at different pH, ranging from 4 to 12 for constant dye concentration (10 mg/L) and catalyst loading (25 mg/L) and are presented in Fig. 15. As shown in the Fig. 15, the degradation efficiency of MB was increased from 75.83% to 96.11% while increasing the pH from 4 to 12. The maximum photocatalytic dye degradation efficiency of 96.11% was obtained for pH 12. The reason is that the surface of the photocatalyst is negatively charged by adsorbing OH– ions which could favor the formation of hydroxyl radicals (%OH) in the reaction medium [43]. 3.8.5. Recycle test In order to study the stability and reusability of the catalyst, the cyclic photocatalytic degradation experiments were performed for ZnO3% rGO nanocomposites. As shown in the Fig. 16(a) and (b), the degradation rate for both MO and MB dyes show little decreases after four cycles. The stability of the ZnO-3% rGO nanocomposite was further evaluated by XRD patterns recorded after 1st and 4th cycles shown in Fig. 17(a) and (b). It was found that the position of diffracted peaks from XRD pattern was not changed after the photocatalytic reaction. It indicates that the crystal structure of ZnO-3% rGO nanocomposite does not change after 4 cycles of the photocatalytic process [44].
3.8.4. Effect of pH The dye industries are discharging the wastewater at different pH; therefore it is important to study the dye degradation efficiency at different pH. In order to study the effect of pH on MB dye degradation efficiency of ZnO/rGO-3% nanocomposites, the different experiments
4. Conclusions In the present studies, the ZnO dumbbell/reduced graphene oxide (ZnO-rGO) nanocomposites were prepared via simple solution mixing method with varying loading amount of rGO (1 wt%, 2 wt%, 3 wt%, and 4 wt%). Prepared samples were characterized by powder X-ray diffraction, FE-SEM, TEM, FTIR, XPS, micro Raman, UV–VIS DRS, and electrochemical test and the results showed that the effective formation of nanocomposites and the presence of rGO with ZnO dumbbell enhance the light absorption and charge transport properties of ZnO
Fig. 13. Effect of the catalyst dosage for the ZnO-3% rGO nanocomposites. 1286
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Fig. 16. Reusability test for (a) ZnO-3% rGO under MO dye, (b) ZnO-3% rGO nanocomposites under MB dye.
Fig. 17. XRD patterns of reused (a) ZnO-3% rGO photocatalyst after 1st and 4th cycle for MO dye degradation, (b) ZnO-3% rGO photocatalyst after 1st and 4th cycle for MB dye degradation.
dumbbell. The photocatalytic degradation experiments were performed over prepared photocatalyst using MO and MB dyes were chosen as model pollutants under UV–VIS light. The 3 wt% rGO loaded ZnO dumbbell showed the highest photocatalytic degradation activities for both MO and MB dyes. Moreover, the rGO plays an important role in improving the photocatalytic activity of ZnO-3% rGO nanocomposites, owing to the effective charge transportation with minimized photoexcited charge carrier recombination rates. Meanwhile, the ZnO-3% rGO nanocomposites showed excellent photocatalytic stability.
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Enhanced photocatalytic activities of ZnO dumbbell/reduced graphene oxide nanocomposites for degradation of organic pollutants via efficient charge separation pathway S. Prabhua, S. Megalaa, S. Harishb,c, M. Navaneethanb,d, P. Maadeswarane, S. Sohilaf, R. Ramesha,
T
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a
Department of Physics, Periyar University, Salem 636011, Tamil Nadu, India Center for Materials Science and Nano Devices, Department of Physics and Nanotechnology, SRM Institute of Science and Technology, Kattankulathur, Kancheepuram 603203, Tamil Nadu, India c Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-Ku, Hamamatsu, Shizuoka 432-8011, Japan d Nanotechnology Research Center (NRC), Faculty of Engineering and Technology, SRM Institute of Science and Technology, Kattankulathur, Chennai, 603203, Tamil Nadu, India e Department of Energy Studies, Periyar University, Salem 636011, Tamil Nadu, India f Department of Physics, Shri Sakthikailassh Women's College, Salem 636003, Tamil Nadu, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: ZnO dumbbell Optical property Reduced graphene oxide Photocatalytic activity Charge transport property
In the present work, a well-defined dumbbell shaped ZnO was synthesized by hydrothermal method and ZnO dumbbell/reduced graphene oxide (ZnO/rGO) nanocomposites were prepared by a simple solution mixing method with the different rGO loading amount. The formation of nanocomposites, crystal structure, shape, size and optical properties of the ZnO/rGO nanocomposites were investigated using various analytical techniques. The photocatalytic degradation efficiency of synthesized materials was evaluated by the degradation of aqueous Methyl Orange (MO) and Methylene Blue (MB) under UV–Visible light irradiation. The results revealed that 3 wt % rGO loaded ZnO dumbbell (ZnO-3% rGO) exhibited the higher photocatalytic degradation efficiency than pure ZnO dumbbell and rGO. The enhancement in the photocatalytic dye degradation efficiency of ZnO-3% rGO is attributed to the efficient dye adsorption nature, a red shift in light absorption and inhibition of photo-excited electron-hole pair recombination rate. The photocatalytic dye degradation efficiency also evaluated for various catalyst dosage, dye concentration and pH of the solution. Moreover, it was demonstrated the stability of catalyst over a repeated cycle of dye treatment.
1. Introduction The wastewater from various industries contains toxic organic pollutants which causes severe environmental pollution [1,2]. The organic dyes in the effluents coming from the industries are difficult to degrade due to their chemical stability. Several processes including adsorption, chemical oxidation, ion-exchange, reverse osmosis, precipitation, biological and photocatalytic treatments have developed to treat the organic dye contaminated water [3]. Among them, the semiconductor based photocatalysis is the one of the effective way for degradation organic dyes or organic effluent in the wastewater. Several semiconducting materials have been used for photocatalytic dye degradation process including ZnO, TiO2, Fe2O3, WO3, ZnS, BiVO4, NiO and Co3O4 [4–6]. Among the various semiconductors, ZnO has been attracted considerable attention due to its high electron mobility, suitable
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energy band gap, low cost and nontoxicity [7]. Although, wide bandgap (3.37 eV), photocorrosion and low separation efficiency of electronhole pairs could hinder the extensive applicability of ZnO photocatalyst [8]. In the recent decades, strategies like nanostructuring, doping, and formation of nanocomposites have been adopted to improve the photocatalytic dye degradation efficiency of ZnO [9]. The photocatalytic reaction usually takes place at the interface between catalyst surfaces and organic pollutants. Therefore the photocatalytic dye degradation efficiency of ZnO depends on its morphology [10]. In particular, the dumbbell-shaped ZnO photocatalyst has received much attention due to their superior photocatalytic property. For example, the morphology and facet dependent charge transport property of the ZnO photocatalyst explained by Pawar et al. [11]. Furthermore, the formation of nanocomposite with graphene-based material could improve the photocatalytic property of the ZnO. The extremely high specific surface area,
Corresponding author at: Department of Physics, Periyar University, Salem-11, Tamil Nadu, India. E-mail address: [email protected] (R. Ramesh).
https://doi.org/10.1016/j.apsusc.2019.05.086 Received 9 January 2019; Received in revised form 12 April 2019; Accepted 7 May 2019 Available online 08 May 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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temperature. The final product was separated by centrifugation and washed with ethanol and DI water for several times. Finally, the obtained product was dried at 80 °C in a hot air oven.
superior chemical stability and high electrical conductivity of the Graphene (GR) or reduced graphene oxide (rGO), could increase the photoexcited charge carrier separation process of the ZnO photocatalyst [12–14]. Up to date, there are numerous report on the preparation and utilization of rGO/semiconductors for photocatalytic application [15]. A novel hierarchical porous ZnO/Reduced graphene oxide nanocomposite has shown the great interest on the photocatalytic application due to their effective electron-hole separation and its transportation. Recently, Kheirabadi et al., designed Ag/ZnO/3D graphene nanostructure for effective photocatalytic removal of dyes from wastewater [16]. Systematic experimental and theoretical investigations have been performed on ZnO/rGO nanocomposites and it has been proved that enhanced photocatalytic dye degradation mainly due to higher electron transfer to the dye solution [17]. Therefore, it is expected that the formation of rGO based nanocomposites with well-organized ZnO structure can be efficiently utilized for the photocatalytic dye degradation application with superior catalytic performance. In the present work, well defined ZnO dumbbell was synthesized by hydrothermal method using dodecylamine as a surfactant. Furthermore, ZnO/rGO nanocomposites with the varying the loading amount of rGO (1 wt%, 2 wt%,3 wt% and 4 wt%) were synthesized by simple solution mixing method and named as ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposite. The photocatalytic dye degradation performance of the pure ZnO, rGO and ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposites was evaluated using MO and MB dyes as model pollutants. An electrochemical property of the prepared materials was studied to validate the enhanced photocatalytic dye degradation efficiency of the ZnO-3% rGO photocatalyst.
2.4. Preparation of zinc oxide-reduced graphene oxide (ZnO-rGO) nanocomposites A series of ZnO-rGO nanocomposites were synthesized with the varying amount of rGO. For the preparation of ZnO-1% rGO, a 1 wt% of rGO was dispersed in 25 mL of DI water via ultra-sonication (Kerry KC338 kHz) for 1 h to achieve a homogeneous solution followed by 100 mg of as-prepared ZnO was added into the dispersion and then dispersed mixture was continuously stirred by a magnetic stirrer for 12 h. After that the mixture was washed with several times by ethanol and DI water using centrifugation process and then the product was dried at 80 °C in hot air oven for overnight. For comparison, ZnO-rGO nanocomposites were synthesized by the same experimental method under similar conditions with the varying amount of rGO (2 wt%, 3 wt%, and 4 wt%) and the final products were labeled as ZnO-2% rGO, ZnO-3% rGO, and ZnO-4% rGO. 2.5. Characterization techniques The phase purity and crystal structure of the as-prepared samples were characterized by powder X-ray diffraction (XRD) using Rigaku D/ Max Ultima III X-ray diffractometer with a Cu Kα radiation source (λ = 0.154 nm) operated at 40 kV. The optical absorption properties of the prepared samples were characterized by a V-770 diffuse reflectance spectrophotometer (DRS, JASCO). Fourier-transform infrared (FTIR) spectra of KBr powder-pressed pellets were recorded by Bruker a NEXUS 470 in the range of 4000 to 400 cm−1. The surface morphology of obtained samples was visualized by field emission scanning electron microscopy (FESEM, Hitachi S-4800). The crystal phase and internal morphologies of the nanocomposites were obtained by transmission electron microscopy (TEM, Hitachi H-600). X-ray photoelectron spectroscopy (XPS) technique was utilized to identify chemical states of prepared samples at room temperature by using ESCA 3400 spectrometer in ultra-high vacuum with 10–10 mbar base pressure. The measurement chamber is attached with a monochromatic Al (Kα) X-ray source providing photon with hυ = 1486.6 eV and hemispherical analyzer (150 mm diameter) was used for energy analysis. The pass energy was kept 20 eV for the high resolution. Room temperature Raman spectroscopy measurements were carried out on LabRAM HR Evolution Raman microscopes using UV laser as light source with wavelength of 364 nm.
2. Experimental section 2.1. Chemicals All the reagents were purchased commercially and should be used as without further purification. Zinc acetate dihydrate (Zn(O2CCH3)2 99.5%), Dodecylamine (C12H27N - 97.0%), Graphite powder (99.5%), Hydrogen peroxide (H2O2 - 30% purity), Sulfuric acid (H2SO4 - 97%), Potassium permanganate (KMnO4 - 99.0%), Sodium nitrate (NaNO3 99.0%), Hydrochloric acid (HCl - 38.0%), Methyl Orange (MO), Methylene Blue (MB), Benzoquinone (BQ - 98%), Isopropanol (IPA 99.8%), Triethanolamine (TEOA - 99%) and Silver nitrate (AgNO3 99.0%) were purchased from Merck in India. All experiments were performed using deionized (DI) water with an electrical conductivity > 18 MΩ. 2.2. Synthesis of reduced graphite oxide (rGO)
2.6. Photocatalytic measurements Graphene oxide (GO) was prepared by modified Hummer's method and it is reduced by the following procedure to get reduced graphene oxide (rGO) [18]. A 1 g of as-synthesized GO powder was dispersed in 50 mL DI water using bath sonicator for 2 h and then 20 mL of ammonia solution was dropped wise added into the solution. Then the solution was transferred in a 100 mL Teflon-lined autoclave and heated at 160 °C for 12 h in hot air oven. After that, the autoclave was allowed to cool naturally the room temperature. The block precipitate was collected and washed with ethanol and DI water several times. Finally, the collected sample was dried in a hot air oven at 60 °C overnight.
The photocatalytic activity of ZnO dumbbell, rGO, and ZnO-rGO nanocomposites was evaluated through degradation of an aqueous solution of methyl orange (10 mg/L) and methylene blue (10 mg/L) under 500 W Halogen lamp Illumination placed at 15 cm distance from the photoreactor. In the typical experiment, a 50 mg of prepared sample was dispersed in 100 mL aqueous solution of MO and MB with an initial concentration 10 mg/L under ambient condition and continuously stirred under the dark condition for 1 h to reach the adsorption-desorption equilibrium. Then the mixture was exposed by Halogen lamp to start the photocatalytic process. At the regular time interval, 5 mL solution was separated and centrifuged at 10,000 rpm for 1 min to remove the solid catalyst. The concentration of dye molecules in the supernatant were measured from the absorbance spectra recorded on JASCO V-670 double beam spectrophotometer. For comparison, the photocatalytic dye degradation experiment was performed for different amount of catalyst (0.25 g/L, 0.50 g/L, 0.75/L mg and 1.00 g/L), various concentration of dyes (10, 15, 20 and 25 mg/L) and pH contractions (4, 7, 9 and 12) of the solution. The photocatalytic degradation
2.3. Synthesis of ZnO dumbbell In the typical synthesis, 0.1 M of zinc acetate dihydrate (Zn (CO2CH3)2.2H2O) was dissolved in 50 mL of DI water under magnetic stirring and the white precipitate was formed by dropwise addition of 5 ml dodecylamine (DDA). Then the mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 200 °C for 15 h and then the autoclave was naturally cooled down to room 1280
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nanocomposites (f). A broad diffraction peak observed at 2θ = 26.2° in Fig. 1(a), which could be attributed to the (002) plane of the hexagonal phase of rGO (JCPDS no.75-1621). The diffraction peaks from ZnO dumbbell and its composites are similar at 2θ values of 31.9°, 34.3°, 36.2°, 47.6°, 56.6°, 62.8°, 67.8° and 69.2° corresponding to (100), (002), (101), (102), (110), (103), (112) and (201) crystal planes, respectively and it is well consistent with standard diffraction pattern (JCPDS no 361451) for hexagonal wurtzite ZnO structure [18]. In addition to that the relatively low intense broad peak was observed at 2θ = 26.2° from ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO, and ZnO-4% rGO nanocomposites which indicate the presence of rGO in the nanocomposites and peak intensity is increased while increasing the loading content of rGO. The sharp intensity of the diffraction peak from pure ZnO dumbbell and ZnO-rGO nanocomposites indicate that the good crystallinity of the synthesized samples. No other impurities were observed from ZnO dumbbell and ZnO-rGO nanocomposites.
efficiency of dyes was calculated the following equation
A −A D= o × 100% Ao
(1)
where Ao is the initial concentration of dye and A is the concentration of dye after photoirradiation. The reactive superoxide radical (O2%−), hydroxyl radicals (%OH), holes (h+) and electrons (e−) spices were determined in photocatalytic reaction solution by adding 1 mM Benzoquinone, isopropanol (IPA), triethanolamine (TEOA) and silver nitrate, respectively. 2.7. Electrochemical experiments Transient photocurrent and Electrochemical Impedance measurements were performed with SP-150 electrochemical workstation (BioLogic Science Instruments, France) using typical three-electrode system where Pt wire and Ag/AgCl in saturated KCl solution were used a counter and reference electrode respectively. The working electrode was fabricated by the following procedure, a 5 mg of prepared samples were dispersed 0.5 mL ethanol and 20 μL Nafion solution using ultrasonication for 30 min to form the homogeneous ink which was then spin coated on pre-cleaned Fluorine doped tin oxide (FTO) glass substrate with rotation speed 2000 rpm for 2 min. The working electrodes were naturally dried under ambient condition and active surface area was fixed as ca. 1cm2 with the help of Teflon tape. In this experiment, the aqueous Na2SO4 (0.5 M) was employed as an electrolyte solution. The 150 W xenon lamp was used as a light source to illuminate working electrodes with power intensity 100 mW/cm2 and it is calibrated by solar power meter. The transient photocurrent (i-t) response of the prepared electrodes was recorded under the light on and off condition. The electrochemical N-Q plots were recorded in a frequency range of 1 MHz-0.1 Hz with AC amplitude of 10 mV under open circuit potential in absence light.
3.2. Surface morphological study The field-emission scanning electron microscopy (FE-SEM) used to visualize the surface morphology of ZnO and ZnO-3% rGO nanocomposites. As shown in Fig. 2(a, b), ZnO exhibited a uniformly distributed dumbbell shape ZnO morphology consist of two hexagonal crystals in the micrometer size. From the Fig. 2(c, d), it is clearly seen that ZnO dumbbells were uniformly dispersed on rGO sheet. Internal morphology and crystalline property of the prepared samples were analyzed using TEM technique. The well-defined dumbbell shape ZnO was clearly observed in Fig. 3(a). As shown in Fig. 3(b), the HR-TEM image was recorded at the edge of the ZnO dumbbell. The clear lattice fringes were observed with an inter-planar spacing of 0.26 nm for (0001) plane of the hexagonal phase ZnO crystal [19]. Fig. 3(c–d), the TEM and HR-TEM images of ZnO dumbbells were wrapped by rGO, thereby which making the interfacial contact between rGO and ZnO dumbbells. Both FE-SEM and TEM images revealed that the formation of interfacial contact between rGO sheet and ZnO dumbbell and it is expected that which could increase the charge carrier transfer process [20].
3. Results and discussion 3.1. Structural and crystal phase analysis
3.3. FT-IR analysis
Powder X-ray diffraction (XRD) measurements were carried out to identify the phase purity and crystal structure of the prepared samples. Fig. 1 shows the XRD patterns of rGO (a), pure ZnO (b), ZnO-1% rGO (c), ZnO-2% rGO (d), ZnO-3% rGO (e), and ZnO-4% rGO
The chemical structure and vibration modes of the prepared samples were analyzed by FT-IR technique. Fig. 4 shows the FT-IR spectra of (a) rGO, (b) ZnO dumbbell, (c) ZnO-1% rGO (d) ZnO-2% rGO, (e) ZnO-3% rGO and (f) ZnO-4% rGO in range between 4000 and 500 cm−1. The FT-IR spectra of ZnO dumbbell and ZnO-rGO nanocomposites shown in Fig. 4(b–f), the strong peak at 508 cm−1 is corresponding to the ZneO stretching mode of ZnO [21]. On the other hand, the peaks at 3400 cm−1 and 1426 cm−1 attributed to the vibration of the hydroxyl group in nanocomposites. The stretching vibrations of epoxy CeO (1226.92 cm−1) and weak aromatic C]C (1578 cm−1) bonds were observed from pure rGO and it is nanocomposites [22]. Moreover, the full width half maximum (FWHM) of ZneO stretching mode is increased while increasing the rGO weight percentage in the composites. This is due to increase in the peak intensity of CeO and C]C bonds for higher weight percentage of rGO (3% and 4%) in the nanocomposites which could decrease the intensity of ZneO stretching mode thereby the FWHM is increased. And also the peak from ZneO stretching mode was significantly shifted in the lower wavenumber side for ZnO-3% rGO and ZnO-4% rGO nanocomposites which implies that dispersion of ZnO dumbbell on rGO sheet. 3.4. XPS analysis The prepared rGO, ZnO dumbbell and ZnO-3% rGO nanocomposites were further analyzed by XPS technique to identify the chemicals states and compositions. The binding energy scale was calibrated using C 1s
Fig. 1. Powder XRD patterns of (a) rGO (b) ZnO dumbbell, (c) ZnO-1% rGO (d) ZnO-2% rGO, (e) ZnO-3% rGO and (f) ZnO-4% rGO nanocomposites. 1281
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Fig. 2. FE-SEM images of (a, b) ZnO dumbbell and (c, d) ZnO-3% rGO nanocomposites with two deferent magnification.
Carbonyl Carbon (C=C) [23]. The high-resolution spectrum of O1s for rGO, pure ZnO dumbbell and ZnO-3% rGO nanocomposites is further deconvoluted into three peaks which are presented Fig. 5(d). The deconvoluted O1s peaks were shifted to higher binding energy for ZnO3% rGO nanocomposites when compared with pure ZnO dumbbell and rGO due to dispersion of ZnO dumbbell on rGO sheet [24].
peak (284.1 eV) in the XPS spectra. As shown in Fig. 5(a), the survey spectrum of composites exhibited peaks at binding energy of 284.6 eV, 530.7 eV, 1024.3 eV and 1045.7 eV correspond to the C 1s, O 1s, Zn 2p3/2, and Zn 2p1/2 respectively. The peaks at 1024.3 eV and 1045.7 eV in the high-resolution spectrum of ZnO-3% rGO nanocomposites ascribed to the Zn 2p1/2 and Zn 2p2/3, respectively. Besides, the similar peaks were observed for both pure ZnO dumbbell and ZnO3% rGO nanocomposites without changing their intensity which confirms the chemical states of Zn2+ ion in the ZnO-3% rGO nanocomposites is not influenced by rGO in Fig. 5(b). As shown in Fig. 5(c) the C 1s core level spectra of ZnO dumbbell, rGO and ZnO-3% rGO nanocomposites were deconvoluted into three peaks i) non‑oxygenated graphitic CeC bond of Sp2 carbon ii) epoxy and alkoxy (CeO) and iii)
3.5. Micro Raman analysis Micro Raman analysis was performed for the prepared samples to verify the nature of the hybrid materials. Fig. 6 shows the micro Raman spectra of as ZnO dumbbell, ZnO-1% rGO, ZnO-2% rGO, and ZnO-3% rGO nanocomposites. The peaks at 325 cm−1 and 435 cm−1 of ZnO
Fig. 3. (a) TEM image and (b) HR-TEM image of ZnO dumbbell and (c) TEM and (d) HR-TEM images of ZnO-3% rGO nanocomposites. The inset presents the lattice plane of hexagonal ZnO. 1282
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Fig. 4. FT-IR spectra for (a) rGO (b) ZnO dumbbell, (c) ZnO-1% rGO (d) ZnO2% rGO, (e) ZnO-3% rGO and (f) ZnO-4% rGO nanocomposites.
dumbbell are the 2E2 and E1 phonon vibration in the lattice [25]. In addition, the D and G bands were observed at 1335 cm−1 and 1584 cm−1, respectively in the Raman spectra of ZnO-1% rGO, ZnO-2% rGO and ZnO-3% rGO nanocomposites which confirm the formation
Fig. 6. Micro Raman spectra of (a) ZnO dumbbell, (b) ZnO-1% rGO, (b) ZnO2% rGO and (d) ZnO-3% rGO nanocomposites.
Fig. 5. XPS (a) survey, (b) C1s, (c) O1s and (d) Zn 2p spectra of rGO, ZnO, ZnO-3% rGO nanocomposites. 1283
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Fig. 7. UV–VIS DRS spectra of ZnO dumbbell, ZnO/rGO (ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO) nanocomposite. Inset: calculated band gaps from correspond absorption spectra samples.
Fig. 8. Transient photocurrent density of ZnO dumbbell, ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposites under simulated solar irradiation with light on-off condition.
nanocomposites. The G band arises from first-order scattering of E2g mode and denoted stretching vibration of the sp2 hybridization [26]. D band is the defect band due to the presence of sp3 hybridization of carbon atoms [27]. Besides, the intensity of both D and G bands were increased while increasing the loading amount of rGO with ZnO dumbbell which influenced to decrees the intensity of 2E2 and E1 mode from ZnO and it is confirmed the effective formation of nanocomposites.
to the smaller loading amount of rGO in composites. Besides, the ZnO4% rGO-nanocomposites showed the decreased photocurrent density than that of ZnO-3% rGO-nanocomposites. This result may be due to the higher loading amount of rGO in the nanocomposites that could increase the photo-excited charge carriers recombination rates rather than charge carriers separation [33]. Furthermore, the interfacial charge carrier separation and charge transport properties of the ZnO dumbbell, ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposites were investigated from EIS analysis. The Nyquist plots of prepared samples at open circuit potential are depicted in the Fig. 9. It is clearly seen that the arc radius of the N-Q plot for ZnO-3% rGO nanocomposites is much smaller than ZnO dumbbell, ZnO-1% rGO, ZnO-2% rGO and ZnO-4% rGO nanocomposites. This observation implies that the ZnO-3% rGO-nanocomposites possess the efficient charge carrier separation with enhanced interfacial electron transfer performance, which is consistence with the transient photocurrent measurement [34]. Moreover, the current density of pure ZnO dumbbell and ZnO-1% rGO nanocomposites is not stable even when the light is on. The instability of the current density upon the illumination of light is because of the higher surface recombination rate.
3.6. UV–VIS DRS analysis The light absorption nature of the prepared materials was characterized by UV–VIS diffusion reflectance spectroscopy (DRS). The UV–VIS spectra of ZnO dumbbell, ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposites shown in Fig. 7. The light absorption is red shifted by increasing the loading amount of rGO in ZnOrGO nanocomposites compared with pure ZnO dumbbell. The band gaps of prepared materials were calculated from the UV–VIS DRS spectra and shown at inset of Fig. 7. The calculated band gaps were 3.16 eV, 3.04 eV, 2.93 eV, 2.86 eV and 2.74 eV, for ZnO dumbbell, ZnO1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposites, respectively. The reduction in the bandgap of ZnO dumbbell upon varying the loading amount of rGO is mainly due to the formation of Zn-O-C chemical bonds in the nanohybrid [28,29].
3.8. Photocatalytic activities
3.7. Photoelectrochemical properties
The photocatalytic activity of rGO, ZnO dumbbell, ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposites was evaluated by degradation of MO and MB dyes under irradiation of
The photo-induced electron-hole recombination rates and charge separation of prepared materials were characterized by transient photocurrent and electrochemical impedance spectroscopy (EIS) measurements. The transient photocurrent response of ZnO dumbbell, ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO-and ZnO-4% rGO nanocomposites under stimulated solar radiation shown in Fig. 8. The photocurrent density of all samples was quite stable after several on-off cycles. Particularly, the photocurrent density of ZnO-3% rGO nanocomposites is higher than other samples, which implies that ZnO-3% rGO nanocomposites have higher photogenerated electron-hole separation under simulated solar radiation [30]. In general, the higher photocurrent density is mainly attributed to the effective photo-excited charge separation with a minimum of recombination rates [31]. In the charge separation process of the ZnO-3% rGO nanocomposites, the rGO can efficiently separate charge carriers and transport the electrons due to its superior electrical conductivity [32], whereas ZnO-1% rGO-and ZnO2% rGO nanocomposites exhibited the lower photocurrent density compared with that of ZnO-3% rGO nanocomposites may be attributed
Fig. 9. Nyquist plots for ZnO dumbbell, ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposites at open circuit potential. 1284
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Fig. 10. Photocatalytic (a) Methyl Orange dye degradation Efficiency of rGO, ZnO dumbbell, ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposites, (b) Methylene Blue dye degradation Efficiency of rGO, ZnO dumbbell, ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposites.
500 W Halogen lamp. Prior to irradiation, the photocatalytic reaction system was magnetically stirred in the dark for 30 min to attain the adsorption/desorption equilibrium. Supporting Figs. S1&S2 show the UV–Visible absorbance spectra of MO and MB solution in the presence of ZnO-3% rGO nanocomposites under different light illumination time. The photocatalytic MO and MB degradation efficiency of the ZnO dumbbell, ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposites were depicted in Supporting Fig. 1(a&b), respectively. As shown in Fig. 10(a), the rGO, ZnO dumbbell, ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposites have photocatalytic dye degradation efficiency of 55.1%, 67.2%, 78.4%, 80.2%, 89.78% and 81.20% of MO dye respectively after 180 min of light irradiation. Besides, the photocatalytic dye degradation efficiency of 70%, 85%, 90%, 94%, 97%, and 80% was achieved for MB dye at the same illumination time as of MO (Fig. 10(b)). The higher photocatalytic MO and MB dye degradation efficiency were obtained for ZnO-1% rGO, ZnO-2% rGO, ZnO-3% rGO and ZnO-4% rGO nanocomposites than pure rGO and ZnO dumbbell. Fig. 10 represents the remarkable dye degradation efficiency for ZnO dumbbell (ZnO-3% rGO) than other composites which implies that the integration 3w% of rGO with ZnO dumbbell serves as an electron-hole separator and provide the pathway to transfer the electron into the composites [35]. However, the 4 wt% rGO loaded ZnO dumbbell shows the decreased photocatalytic dye degradation activity because of the reason that higher loading of rGO could increase the photoexcited electron-hole recombination rates [36]. It is observably seen that in Fig. 10(a&b), the nanostructures with 3% of rGO (ZnO-3% rGO) exhibits the 89.78% and 97% of photocatalytic MO and MB dye degradation efficiency, respectively which is 1.5 fold higher than the pure ZnO dumbbell. The increased photocatalytic efficiency is mainly due to an effective photoexcited charge carriers separation at the interface of rGO and ZnO [37].
Fig. 11. Active species trapping experiment on ZnO-3% rGO nanostructures during the photocatalytic degradation of MB under UV–VIS. light irradiation.
in this process. Whereas the presence of O2−, h+, and e− radical scavengers in the photocatalytic reaction system, the degradation rate of MB over the ZnO-3% rGO was not dramatically decreased which implies that these radical species play a minor role in the photocatalytic dye degradation reaction system. Based on the aforementioned results, the possible photocatalytic degradation mechanism of MO and MB dye solution over ZnO-3% rGO nanocomposites was explained as follows. The proposed photocatalytic dye degradation mechanism is shown in the Fig. 12. When the ZnO-3% rGO nanocomposites irradiated with UV–VIS. light, the electron excited
3.8.1. Photocatalytic mechanism The hydroxyl radicals (%OH), superoxide anions (O2−%), hole (h+) and electron (e−) are reactive species and they play an important role during the photocatalytic degradation of organic pollutants in aqueous solution [38]. In order to investigate the influence of reactive species generated during the photocatalytic MB dye degradation process on ZnO-3% rGO nanocomposites, a series of trapping experiments were performed and depicted in Fig. 11. In the experiments, Isopropyl alcohol (IPA), Triethanolamine (TEOA), Benzoquinone (BQ), and Silver + − nitrate (Ag(NO)3) were used as the %OH, O2− radical sca, h , and e vengers. As shown in Fig. 11, it is clearly observed that in the presence of %OH radical scavenger, 72% of photocatalytic MB degradation efficiency was depressed compared with radical scavenger free degradation systems which conforming the %OH is the major oxidation species
Fig. 12. Schematic diagram for photocatalytic mechanism during the dye degradation process. 1285
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from valence band (VB) to the conduction band (CB) of ZnO, leaving behind holes in the valence band. These photo-excited electrons could transfer from the CB to the surface of rGO which suppress the electronhole recombination rates during the photocatalytic reaction [39]. Besides, the photo-generated electrons and holes could react with the surface absorbed water and oxygen molecules and produce the reactive species to degrade the dye molecules in the solution [40]. The photocatalytic chemical reactions during was explained as follows
ZnO/ rGO + hv → ZnO/ rGO (e− + h+)
(2)
(e−)
(3)
rGO + e− → rGO (e−)
(4)
O2 + ZnO (e−) → O2• −
(5)
ZnO +
e−
→ ZnO
O2 + rGO
(e−)
ZnO/rGO
(h+)
→
O2• −
+
(OH−)
Fig. 14. Effect of the MB Dye concentration for the ZnO-3% rGO nanocomposites.
(6)
→ ZnO/rGO +
(OH •))
(7)
OH • + pollutant (MO/ MB ) → Degradation product
(8)
O2• − + pollutant (MO/ MB ) → Degradation Product
(9)
3.8.2. Effect of the catalyst dosage The effect catalyst dosage on photocatalytic degradation of MB dye have been investigated for constant dye concentration and pH values and the results shown in the Fig. 13. It can be seen that the photocatalytic dye degradation efficiency was increased from 28.66% to 95.36% while increasing the ZnO-3% rGO nanocomposites catalyst dosage from 0.25 to 1.0 g/L. The highest degradation efficiency was obtained for 1.0 g/L catalyst concentration due to more availability photo-excited electrons and holes in the reaction solution [41].
Fig. 15. Effect of the pH for the ZnO-3% rGO nanocomposites.
3.8.3. Effect of initial concentration of dye The effect of various initial concentration of MB (10 mg/L, 15 mg/L, 20 mg/L and 25 mg/L) on the photocatalytic degradation efficiency of ZnO-3% rGO nanocomposites was examined presented in Fig. 14. As shown in the Fig. 14, the photocatalytic degradation rate of MB dye decreases with an increase in the initial concentration of MB dye. This result could be attributed to the inhibition of catalytic activities of the catalyst by excessive absorption of the MB dye molecules on the surface of the catalyst and diminishing the light penetration in photocatalytic solution resulted from light scattering of highly concentrate MB dyes [42].
were carried out at different pH, ranging from 4 to 12 for constant dye concentration (10 mg/L) and catalyst loading (25 mg/L) and are presented in Fig. 15. As shown in the Fig. 15, the degradation efficiency of MB was increased from 75.83% to 96.11% while increasing the pH from 4 to 12. The maximum photocatalytic dye degradation efficiency of 96.11% was obtained for pH 12. The reason is that the surface of the photocatalyst is negatively charged by adsorbing OH– ions which could favor the formation of hydroxyl radicals (%OH) in the reaction medium [43]. 3.8.5. Recycle test In order to study the stability and reusability of the catalyst, the cyclic photocatalytic degradation experiments were performed for ZnO3% rGO nanocomposites. As shown in the Fig. 16(a) and (b), the degradation rate for both MO and MB dyes show little decreases after four cycles. The stability of the ZnO-3% rGO nanocomposite was further evaluated by XRD patterns recorded after 1st and 4th cycles shown in Fig. 17(a) and (b). It was found that the position of diffracted peaks from XRD pattern was not changed after the photocatalytic reaction. It indicates that the crystal structure of ZnO-3% rGO nanocomposite does not change after 4 cycles of the photocatalytic process [44].
3.8.4. Effect of pH The dye industries are discharging the wastewater at different pH; therefore it is important to study the dye degradation efficiency at different pH. In order to study the effect of pH on MB dye degradation efficiency of ZnO/rGO-3% nanocomposites, the different experiments
4. Conclusions In the present studies, the ZnO dumbbell/reduced graphene oxide (ZnO-rGO) nanocomposites were prepared via simple solution mixing method with varying loading amount of rGO (1 wt%, 2 wt%, 3 wt%, and 4 wt%). Prepared samples were characterized by powder X-ray diffraction, FE-SEM, TEM, FTIR, XPS, micro Raman, UV–VIS DRS, and electrochemical test and the results showed that the effective formation of nanocomposites and the presence of rGO with ZnO dumbbell enhance the light absorption and charge transport properties of ZnO
Fig. 13. Effect of the catalyst dosage for the ZnO-3% rGO nanocomposites. 1286
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Fig. 16. Reusability test for (a) ZnO-3% rGO under MO dye, (b) ZnO-3% rGO nanocomposites under MB dye.
Fig. 17. XRD patterns of reused (a) ZnO-3% rGO photocatalyst after 1st and 4th cycle for MO dye degradation, (b) ZnO-3% rGO photocatalyst after 1st and 4th cycle for MB dye degradation.
dumbbell. The photocatalytic degradation experiments were performed over prepared photocatalyst using MO and MB dyes were chosen as model pollutants under UV–VIS light. The 3 wt% rGO loaded ZnO dumbbell showed the highest photocatalytic degradation activities for both MO and MB dyes. Moreover, the rGO plays an important role in improving the photocatalytic activity of ZnO-3% rGO nanocomposites, owing to the effective charge transportation with minimized photoexcited charge carrier recombination rates. Meanwhile, the ZnO-3% rGO nanocomposites showed excellent photocatalytic stability.
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