Synthesis and characterization of double heterojunction-graphene nano-hybrids for photocatalytic applications

Ceramics International 45 (2019) 17806–17817

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Ceramics International journal homepage: www.elsevier.com/locate/ceramint

Synthesis and characterization of double heterojunction-graphene nanohybrids for photocatalytic applications

T

Sheraz Yousafa,∗, Tehmina Kousara, Muhammad Babar Taja, Philips Olaleye Agboolab, Imran Shakirc, Muhammad Farooq Warsia,∗∗ a

Department of Chemistry, The Islamia University of Bahawalpur, 63100, Bahawalpur, Pakistan College of Engineering Al-Muzahmia Branch, King Saud University, PO-BOX 800, Riyadh, 11421, Saudi Arabia c Sustainable Energy Technologies (SET) Center, College of Engineering, King Saud University, PO-BOX 800, Riyadh, 11421, Saudi Arabia b

A R T I C LE I N FO

A B S T R A C T

Keywords: Double heterojunction Current-voltage measurement Bandgap Photocatalysis Reaction mechanism

Nanoparticles of double heterojunction n-CuO-p-NiO-n-ZnO system and its nano-hybrids with reduced graphene oxide (rGO) were synthesized by simple and easily adoptable co-precipitation method. The prepared n-CuO-pNiO-n-ZnO nanoparticles and their hybrids with rGO were characterized by various characterization techniques. Structural, optical and morphological characterization was done by X-rays diffraction (XRD), Fourier Transform Infrared (FTIR), UV–Visible spectroscopy etc. It was found that the developed hetero-structure is composed by pNiO as cubic, n-CuO as monoclinic and n-ZnO as hexagonal crystal structure. Formation of heterojunction is confirmed by current-voltage (I-V) measurement. The optical bandgap energy of heterojunction was around 1.6 eV which made this system active in both UV and Visible light region. Photocatalytic degradation efficiency was examined using methylene blue and it showed prominent enhancement as compared to pure metal oxides. This excellent efficiency was attributed due to efficient charge carrier separation which leads to inhibit recombination rate at photocatalyst interface. Moreover, photocatalytic activity was increased meaningfully up to 70% by graphene based n-CuO-p-NiO-n-ZnO. This excellent increment in activity was due to large surface area offered by graphene which caused adsorption of dye molecules and helped in transfer of electrons to metal conduction band.

1. Introduction Now-a-days, the increase in environmental pollution is becoming a serious issue along with many other common issues worldwide. Water pollution is one of the major types of environmental pollution. The aquatic pollution contains a variety of pollutants. Each type of pollutant needs a different strategy to overcome. There are three main types of water pollutants. These are pathogens, heavy metals and organic compounds. Researchers have been attracted to develop materials for treatment of various industrial wastes that contain a number of organic pollutants such as dyes etc. These pollutants are damaging our environment and aquatic life badly. Various methods being used to treat industrial and household water such as reverse osmosis, electrochemical, coagulation, ozonolysis, ion exchange and membrane processes etc. [1,2]. Among all of these methods use of semiconductor photocatalyst is considered to be a green and effective way to degrade pollutants. Nickel oxide nanomaterials belong to such class of

∗

semiconductor materials which are extensively used for electro-catalytic oxidation, super-capacitor, energy production, dye sensitized solar cells, fuel cells, hydrogen evolution reactions, in-vitro cytotoxicity, antibacterial and photocatalytic applications [3–6]. Photocatalytic performance of a photocatalyst can be tailored by tuning its band gap energy, superficial region and ability to setup photogenerated electrons and holes. Nickel oxide is a p-type semiconductor and shows extraordinary optical, electrical and photocatalytic applications [7]. Fazlali et al. synthesized nickel oxide nanoparticles and studied effect of morphology on photocatalytic activity. They found that spherical, uniform, homogeneous and highly mono-disperse particles shows > 80% degradation under UV illumination [8]. A number of such metals and metal oxide photocatalysts are available that only show response in UV region due to high bandgap [9–11]. In order to lower the bandgap and electron-hole recombination various strategies were adopted such as metal or nonmetal ion doping and formation of solid solution. Among all of these strategies, the formation of solid solution with phase

Corresponding author. Corresponding author. E-mail addresses: [email protected] (S. Yousaf), [email protected] (M.F. Warsi).

∗∗

https://doi.org/10.1016/j.ceramint.2019.05.352 Received 17 May 2019; Received in revised form 29 May 2019; Accepted 31 May 2019 Available online 31 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

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junction is considered as the best approach. Nickel oxide is not still found as a best catalyst due to high band gap energy and electron hole recombination. Due to high band gap energy nickel oxide will only absorb a narrow region of sunlight spectrum and remaining spectrum of light will remains useless. In addition, improvement in nickel oxide nanoparticles can be made by making its composites along with other materials [12–18]. Zhang et al. synthesize ZnO/NiO nanocomposite and found that, composite effectively completely degraded dye within 3 h with band gap equals to 3.0 eV. Various Cu and Ni based binary heterojunction nanocomposites were also synthesized and characterized [19,20]. High catalytic activity of NiO/ZnO nanocomposite was due to formation of p-n heterojunction [21]. Chen et al. synthesized TiO2MnO2 biphasic nanocomposite with photocatalytic response in UV region [22]. NiO/TiO2 was synthesized by Faisal et al. [23]. Alsheri et al. fabricated NiO/SnO2 nanomaterial that showed promising photocatalytic activity in UV region due to formation of p-n heterojunction with bandgap 3.52 [24]. NiO/γAl2O3 exhibited up to 85% degradation [16]. Although, single heterojunction systems are good, however they suffer for various problems. These problems can be addressed by double heterojunction system approach. There are several reports on double heterojunction systems found in the literature. For example, Wang et al. fabricated and studied photocatalytic activity of ZnO/ZnWO4/WO3. It was found that the proposed double heterojunction system exhibit better performance. As major part of solar radiations coming to earth is visible light therefore we need such a catalyst that is active in whole spectrum of solar light. Juma et al. [25] synthesized and characterized n-CuO-p-NiO-n-ZnO ternary nanocomposite. They found that bandgap energy of synthesized compound was 1.68 eV that made it suitable for degradation activity. Graphene is a 2D material with extremely high surface area and many other extraordinary features. It has been reported for a wide range of applications including tailoring the photocatalytic activity of metal oxide semiconductors. This high surface area of graphene enhances its ability to adsorb targeted molecules and also acts as an electrical contact between semiconductor photocatalyst. Graphene, being highly conducting in nature, is responsible to transfer the photo-excited electrons in conduction band of oxides of metals [26,27]. Pham et al. synthesized nickel incorporated titanium dioxide/ graphene oxide nanocomposite and found an excellent photocatalyst due to narrowing of band gap energy [28]. NiO is a p-type semiconductor and CuO and ZnO is n-type semiconductor. When these are taken in contact with each other, it results in the formation of double pn heterojunction. This heterojunction will create an internal electric field, known as depletion region, along the interface. When this heterojunction is illuminated to light energy, electrons will be transfer to n-region of junction and holes will transfer to p-region. Due to this reason, electron hole recombination will be reduced and in other terms photocatalytic efficiency will be increases. In the present study, synthesis and photocatalytic activity of pure nCuO, p-NiO, n-ZnO, n-CuO-p-NiO-n-ZnO and their hybrids with rGO was studied. Significant enhancement in photocatalytic activity was observed in graphene based n-CuO-p-NiO-n-ZnO heterojunction due to adsorption of dye molecules on the surface of Graphene and easy transfer of electrons to metal conduction band. This study has not been reported earlier according to the best of our knowledge. 2. Experimental work

permanganate (KMnO4, 99.99% Sigma-Aldrich); Sodium nitrate (NaNO3, 99.99% Sigma-Aldrich); Silver nitrate (AgNO3, 99% SigmaAldrich); Ethylenediaminetetraacetic acid ((HO2CCH2)2NCH2CH2N (CH2CO2H)2, 99% Sigma-Aldrich); Sulphuric Acid (H2SO4, 98% Merck); Ethanol (C2H5OH, 98.00% Merck) and 2-propanol ((CH3)2CHOH, 99.5% Merck). We have not carried out any purification of the received chemicals and utilized them as received.

2.2. Synthesis of n-CuO-p-NiO-n-ZnO ternary nanocomposite Stoichiometric quantities of copper chloride (2.4 g), zinc nitrate (3.6 g) and nickel nitrate (5.82 g) were dissolved in 100 cm3 of 4:1 water-ethanol system. The reaction mixture was placed on hotplate and magnetic stirrer. The stirring was carried out using magnetic stirrer at 50 °C for 30 min 5 mM clear solution of sodium hydroxide was added dropwise. The color of reaction mixture turned from aquamarine to dark green. Temperature of reaction mixture was increased up to 100 °C for half an hour and then it was allowed to cool down at room temperature. Precipitates were filtered and their washing using deionized water was done repeatedly until to obtain the neutral pH. Precipitates were dried at 100 °C overnight and grinded to a fine powder. In the last step powdered precipitates were calcined at 500 °C for 2 an hour in muffle furnace [25]. Similar procedure was adopted for synthesis of pure n-type CuO, p-type NiO and n-type ZnO separately.

2.3. Synthesis of graphite oxide Graphite oxide was prepared by modified Hummer's method. Graphite (3 g) and Sodium Nitrate (3 g) were taken in a cleaned, dried beaker and 150 cm3 of concentrated sulphuric acid was added. The black colored suspension was magnetically stirred for half an hour. Black colored suspension was placed in ice bath and KMnO4 (18 g) was added slowly that gave result in the color change from black to green. After addition of KMnO4, green color suspension was magnetically stirred for about 90 min. Then ice bath was removed and green color mixture was stirred for further 48 h. As a result brown colored slurry was obtained. A mixture of warm water (840 cm3) and hydrogen peroxide (60 cm3) was added to brown slurry that gave result in the formation of yellow color suspension. The yellow color suspension was filtered and washed three to four times by using mixture of concentrated sulphuric acid (6 wt%/500 cm3) and Hydrogen peroxide (1 wt %/500 cm3), that results in the formation of brown color suspension. Brown color suspension obtained was further washed with deionized water to a neutral pH that results in the formation of Graphite Oxide [29].

2.4. Synthesis of reduced graphene oxide Aqueous solution of L-Ascorbic acid (10%) was prepared and mixed with 10 cm3 of graphite oxide suspension. Mixture was placed in water bath and continuously stirred for an hour at 95 °C that gave result in the formation of black suspended precipitates. Black precipitated were sonicated for half an hour, filtered, washed with one molar hydrochloric acid solution and with deionized water and then dried [30].

2.1. Chemicals 2.5. Preparation of n-CuO-p-NiO-n-ZnO/rGO nanocomposite Metal oxide nanoparticles and their hybrids with rGO were prepared by utilizing the following precursors. Nickel Nitrate Hexahydrate (Ni (NO3)2.6H2O, 99% Sigma-Aldrich); Zinc nitrate Hexahydrate (Zn (NO3)2.6H2O, 98.00% Sigma-Aldrich); Cupper Chloride Dihydrate (CuCl2.2H2O, 99.99% Sigma-Aldrich); Sodium Hydroxide pellets (NaOH, 98% Sigma-Aldrich); Graphite Powder (C, 99.99% SigmaAldrich); Ascorbic Acid (C6H8O6, 99% Sigma-Aldrich); Potassium

90 mg of nanomaterials and 10 mg of rGo was suspended in deionized water in separate beakers and sonicated for one an hour. After sonication, content of both beakers were mixed with each other and sonicated further for one an hour. After complete sonication, suspension was dried and characterized for further analysis [31]. The overall synthesis process is presented in Fig. 1.

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Fig. 1. Schematic diagram for synthesis of CuO-NiO-ZnO/rGO nanoparticles.

Percent degradation of dye during the course of experiment was calculated by the following equation.

2.6. Characterizations Structural investigation and crystalline nature of synthesized materials were studied by X-ray diffraction (XRD) using Philips X'Pert PRO 3040/60 diffractometer with Cu-Kα radiation source (λ = 0.15402 nm). Fourier transform infrared spectroscopy (FTIR) was used for structural clarification of prepared materials with Tensor 27 spectrometer. Optical characterizations were carried out by using Carry 60 UV–Visible–NIR dual beam spectrophotometer. The electrical properties (I-V) were measured using Keithley 6487 source meter.

2.7. Photocatalytic activity Photocatalytic activity of pure n-CuO, p-NiO, n-ZnO, n-CuO-p-NiOn-ZnO and n-CuO-p-NiO-n-ZnO/rGO composite was carried out by using methylene blue (MB) dye as a model organic pollutant under visible light irradiation. The visible light source used during whole experiment was 60 W incandescent filament bulb with controlling reaction temperature to room temperature by circulating cold water through the jacket of reaction medium in a typical photocatalytic experiment bath. 20 mg of photocatalyst was taken in 50 cm3 of dye solution in a photoreactor. Before the initiation of degradation reaction, the suspension was magnetically stirred under dark for half an hour in order to established total adsorption-desorption equilibrium. After the establishment of equilibrium, light source was switched “ON” and at once 2 cm3 of reaction mixture was taken out, centrifuged in order to remove catalyst and then finally transferred to quartz cuvette to measure the absorption spectra. This is considered as concentration of MB at time zero. Similarly after a desired time interval, 2 cm3 of reaction mixture was taken, centrifuged and absorption spectrum was recorded.

Percent degradation = 1−

Ct ×100 Co

(1)

3. Results and discussion 3.1. Structural analysis Crystalline structure of pure n-CuO, p-NiO, n-ZnO and n-CuO-pNiO-n-ZnO ternary Nano composite and formation of heterojunction was verified by XRD measurements. XRD diffraction patterns are presented in Fig. 2. The peaks at 2θ = 35.6°, 38.93°, 48.51°, 53.44°, 59.2°, 61.74°, 65.48° and 68.32° are assigned to (ī11), (111), (2̅ 02), (020), (202), (ī13), (022) and (220) lattice planes (JCPDS no. 00-001-1117) of monoclinic crystalline structure of n-CuO. Diffraction peaks at 2θ = 37.21°, 43.23° and 62.82° were due to (111), (200) and (220) lattice planes of Cubic crystalline structure of p-NiO (JCPDS no. 01-0780423). Diffraction peaks at 2θ = 31.74°, 34.4°, 36.22°, 47.52°, 56.58°, 62.86°, 66.40°, 67.94° and 69.10° corresponds to (100), (002), (101), (102), (110), (103), (200), (112) and (201) lattice planes of Hexagonal crystalline structure of n-ZnO (JCPDS no. 01-079-2205). In the case nCuO-p-NiO-n-ZnO, peaks at 2θ = 38.73° and 66.27° are due to (111) and (022) lattice planes of CuO, whereas peaks at 2θ = 42.94° and 62.61° correspond to (200) and (220) lattice planes of NiO, while peaks at 2θ = 31.82°, 34.28°, 36.12°, 47.38°, 56.46°, 62.61°, 66.27°, 67.85° and 68.96° relates to the lattice planes of ZnO respectively. The crystallite size of pure n-CuO, p-NiO, n-ZnO and CuO, NiO, ZnO in ternary nanocomposite were calculated by using Debye-Sherrer equation [32]

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Fig. 3. Williamson Hall plot of Pure CuO, NiO and ZnO.

Fig. 2. XRD diffraction pattern of CuO, NiO, ZnO and CuO-NiO-ZnO.

(2).

D=

Kλ βcosθ

(2)

In this equation “D” is crystallite size, “K” is Sherrer's constant and equal to 0.9, “λ” is wavelength of X-rays used during experiment and corresponded to CuKα 1.5406 Å, “θ ” is Bragg's angle while “β” is full width at half maxima (FWHM). According to Debye-Sherrer method, crystallite size of n-CuO was 6.52 nm and 14.64 nm, p-NiO was 14.62 and 7.6 nm and n-ZnO was 23.30 and 30.4 nm in pure and composite respectively. Similarly, the crystallite size of pure n-CuO, p-NiO, n-ZnO and CuO, NiO, ZnO in double heterojunction was also calculated by WilliamsonHall plot method using eq. (3) [33].

βhkl cosθ =

Kλ + 4ε sinθ D

βhkl = βD +βs

(3)

(4)

When a graph was plotted between “4sinθ” on x-axes and “βhklcosθ” on y-axes, a straight line was obtained. Through the intercept of straight line, crystallite size is calculated whereas slope gives the value of microstrain as shown in Figs. 3 and 4. According to Williamson-Hall plot method, crystallite size of pure n-CuO was 5.09 nm, pure p-NiO was 16.49 nm and pure n-ZnO was 28.70 nm. Crystallite size of CuO was 12.29 nm; NiO was 6.18 nm and ZnO 27.42 nm in double heterojunction respectively as given in Table 1. In comparison of crystallite size calculated from two different methods, it was found that both methods give almost the same results while Scherer method gives slightly larger size. It was due to presence of different geometries of particles. On the other hand CuO showed negative microstrain that corresponded to shrinkage of its lattice whereas lattice strain in ZnO crystal lattice was much higher in pure and double heterojunction. Microstrain in CuO crystal lattice was calculated to be −0.00921 and −0.00212 in pure and double heterojunction respectively [34]. The irregularity in the crystal system was calculated in terms of dislocation density by using equation number 5 [35,36].

Fig. 4. Williamson Hall plot of CuO, NiO and ZnO in ternary composite.

δ=

1 D2

(5)

The lowest values of dislocation density represent regularity in crystal system. The values of lattice parameters, unit cell volume, crystallite size, strain and dislocation density were given in Table 1. 3.2. FTIR analysis In the case of metal oxides, FTIR absorption took place usually below 1000 cm−1. In case of pure n-CuO, six absorption bands

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Table 1 X-rays diffraction results of physical parameters of Pure n-CuO, p-NiO, n-ZnO and n-CuO-p-NiO-n-ZnO ternary composite. Nanomaterials

Pure

n-CuO-p-NiO-n-ZnO

Data Card No.

CuO NiO ZnO CuO NiO ZnO

00-001-1117 01-078-0423 01-079-2205 00-001-1117 01-078-0423 01-079-2205

Unit Cell

Monoclinic Cubic Hexagonal Monoclinic Cubic Hexagonal

Cell volume (Å)3

Cell Parameters (Å) a

b

c

4.68 4.18 3.25 4.68 4.20 3.16

3.43 4.18 3.25 3.44 4.20 3.16

5.09 4.18 5.21 5.09 4.20 5.27

80.64 73.06 47.74 81.15 74.22 45.50

Crystallite Size (nm) Sherrer

W-H

6.52 14.62 23.30 14.64 7.60 30.4

5.09 16.49 28.70 12.29 6.18 27.42

Strain

Dislocation Density (nm)−2

−0.00921 0.00024 0.00695 −0.00212 −0.00429 0.00157

0.00528 0.00740 0.00293 0.00467 0.01792 0.00448

Fig. 5. FTIR spectra of (a) CuO, (b) NiO, (c) ZnO and (e) CuO-NiO-ZnO.

appeared below 1000 cm−1 depending upon method of preparation and geometric shape. In present case absorption bands at 464 cm−1, 435 cm−1 and 405 cm−1 corresponded to Cu-O bond vibrations [37]. IR absorption bands at 470 cm−1 and 426 cm−1 were due to Ni-O bond vibrations [38]. The vibration bands of Zn-O bond appeared at 476 cm−1, 456 cm−1, 427 cm−1 and 414 cm−1 in pure n-ZnO [39]. Similarly, IR absorption bands at 464 cm−1, 435 cm−1 and 405 cm−1 relate to CuO, 476 and 426 resembled to NiO and 505 cm−1, 456 cm-1 and 426 cm−1 corresponded to ZnO in ternary composite calcined at 500 °C which in turn supported XRD results for presence of CuO, NiO and ZnO in ternary nanocomposite. FTIR spectra of metal oxides are shown in Fig. 5.

3.3. UV–visible analysis To study optical characteristics of photocatalyst, UV–Visible analysis was performed between 200 and 500 nm. Fig. 6 (a) shows UV–Visible spectrum of prepared powder n-CuO nanoparticles. It was observed that n-CuO showed absorption at 304 nm [40]. Fig. 7 (a) shows

UV–Visible absorption spectrum of p-NiO. It was noticed that p-NiO gives two absorption bands at 208 and 305 nm [41]. Fig. 8 (a) shows UV–Visible absorption spectra of n-ZnO. It was seen that n-ZnO gave sharp absorption peak at 376 nm [42]. Furthermore, n-CuO-p-NiO-nZnO heterojunction also expressed their individual characteristics absorption bands at 208 nm, 306 nm and 375 nm corresponded to NiO, CuO and ZnO as shown in Fig. 9 (a). The UV–Visible absorption spectra of Graphite oxide and reduced Graphene oxide are shown in Fig. 10. Reduction of GO to rGO can simply checked by matching the position of absorption maximum peaks of GO and rGO. Graphite oxide showed maximum absorption peak at 228 nm which arose due to π to π* transition of aromatic carbon frame and a weak shoulder peak at 304 nm is due to n- π* transition of C˭˭˭O bond [43]. UV–Visible spectrum of rGO showed absorption maximum peak at 268 nm, the red shifting of maximum absorption peak confirmed the reduction of GO to rGO [44].

3.3.1. Optical bandgap analysis The bandgap of catalyst is a basic tool to judge whether incident

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Fig. 6. (a) UV–Visible spectra of CuO, (b) Indirect and Direct Bandgap of CuO.

Fig. 7. (a) UV–Visible spectra of NiO, (b) Indirect and Direct Bandgap of NiO.

Fig. 8. (a) UV–Visible spectra of ZnO, (b) Indirect and Direct Bandgap of ZnO.

photon can excite the material and produces electron-hole pairs. The optical bandgap was calculated using following equation [45]. 1

(αhυ) n = A (hυ − Eg )

(6)

In above Tauc's equation, the “α” is molar absorptivity, “A” is absorbance, “h” is Plank's constant, “ʋ” is the frequency of light, “n” is characteristic constant for special type of transition and “Eg” is energy bandgap. Value of “n” is ½ for direct transition and 2 for indirect transition. When a graph is plotted between “hʋ” (on x-axis) and (αhʋ) 1/n (on y-axis) then optical bandgap energy is calculated by extrapolating the straight part of plot to energy axis. The results showed that 2.24 eV and 3.09 eV are direct and indirect bandgap energy of n-CuO [46] as shown in Fig. 6 (b). The direct and indirect bandgap of p-NiO was 2.78 eV and 3.35 eV respectively [47,48] as given in Fig. 7 (b). Similarly, the direct and indirect bandgap of n-ZnO was 2.65 eV and

2.94 eV [49] respectively as given in Fig. 8 (b). In the case of double heterojunction direct and indirect bandgap energy narrowed to 1.6 eV and 2.16 eV respectively as presented in Fig. 9 (b). For these values of optical bandgap, it was concluded that the double heterojunction composed of n-CuO-p-NiO-n-ZnO exhibited the minimum bandgap. 3.4. Thermal study of n-CuO-p-NiO-n-ZnO/rGO hybrids Thermogravimetric analysis was performed to check organic content in graphene based composite and presented in Fig. 11. It was found that first weight loss was due to loss of adsorbed water or moisture content from the composite and second weight loss was due to combustion of carbon content in composite. It was clearly noticed that there was 10% organic content or graphene in the composite. This value exact matched with the amount of rGO used in the synthesis of

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Fig. 9. (a) UV–Visible spectra of CuO-NiO-ZnO, (b) Indirect and Direct Bandgap of CuO-NiO-ZnO.

Fig. 10. UV–Visible spectra of graphite oxide and reduced graphene oxide.

Fig. 12. Current-Voltage characteristics of Cu0-NiO-ZnO.

p-n junction. 3.6. Photocatalytic activity

Fig. 11. Thermogravimetric analysis of CuO-NiO-ZnO/rGO composite.

composite. 3.5. Current-voltage measurement Current-Voltage measurement was done to analyze formation of p-n junction in n-CuO-p-NiO-n-ZnO double heterojunction. I-V curve is shown in Fig. 12. I-V characteristics of n-CuO-p-NiO-n-ZnO showed rectifying behavior that confirmed the formation of p-n junction. In forward bias case, after knee voltage (2.18 V) it showed excellent conductivity whereas in reverse bias condition, its resistance increases so that current is unable to flow through it. After reaching Zener voltage (−3.81 V), avalanche breakdown occurred that resulted in damage of

For evaluating the photocatalytic activity of n-CuO, p-NiO, n-ZnO, n-CuO-p-NiO-n-ZnO and their hybrids with rGO, MB was used as typical compound that exhibit well resolved UV–Visible spectrum. The photocatalytic activity of pure n-CuO, p-NiO and n-ZnO photocatalysts was also conducted under the same conditions. Fig. 13 showed the photocatalytic degradation of methylene blue by prepared different photocatalysts materials. MB did not degrade without photocatalyst either under light or in dark. The photocatalytic degradation of MB was very poor in the presence of p-NiO as compared to that of n-CuO-p-NiO-nZnO/rGO nano-hybrids. This is because the bandgap energy of NiO is quite high i.e. 3.35 eV. This bandgap correspond to UV light [50–52]. Similar case was with CuO nanoparticles [53,54]. ZnO showed very weak photocatalytic response [55]. Photocatalytic activity of n-CuO-pNiO-n-ZnO double heterojunction is better than that of pure n-CuO, pNiO and n-ZnO. This increased in photocatalytic activity was attributed due to lowering of bandgap energy up to 1.6 eV. After addition of rGO, the photocatalytic degradation efficiency increased further. This increased photocatalytic activity by rGO may be attributed to the following reasons. The first reason is that, the rGO provided a larger surface area to get adsorbed the MB molecules by π to π stacking [56]. The rGO sheets have significantly low Fermi levels. Therefore the graphene could play a vital role in controlling the recombination of photoexcited electron-hole pairs. The combined effect of rGO and double hetero-junction increased the photocatalytic activity of the prepared nano-hybrids. Fig. 13(e) showed the progress of photo degradation of MB for n-CuO-p-NiO-n-ZnO/rGO. It can be noticed that the intensity of

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Fig. 13. Absorption spectra of MB dye taken at different photocatalytic degradation time: (a) CuO, (b) NiO, (c) ZnO, (d) CuO-NiO-ZnO, (e) CuO-NiO-ZnO/rGO.

the absorption peaks at 664 nm decreased with irradiation time. It was also observed that the peaks almost disappeared when the photocatalytic degradation for MB was passed out for 60 min. In order to make a supplementary investigation on photocatalytic process, the kinetic performance of photocatalytic degradation was also carried out. The kinetic process of photo-catalysis is described by pseudo-first-order reaction equation [57,58].

ln

Co = Kt Ct

(7)

In this equation, “C0” is concentration of MB at time t = 0 and “Ct” is concentration of MB at time t = t, “k” is the rate constant and is expressed in unit min−1. The appropriate results of photocatalytic MB

degradation are shown in Fig. 14 and Table 2. It can be seen from the results that the n-CuO-p-NiO-n-ZnO/rGO has the fastest rate constant (k = 0.0176 min−1). 3.6.1. Proposed photocatalytic mechanism In order to detect the main reactive species and predict photocatalytic reaction mechanism, the trapping experiments were executed throughout the photocatalytic process for the degradation of methylene blue. In the study, Ethylenediaminetetraacetic acid (EDTA), 2-propanol, silver nitrate and ascorbic acid were used as hole (h+), hydroxyl radical (∗OH), electrons (e−) [59] and superoxide anion radical scavenger (∗O2−2) respectively [60]. The variation of MB degradation by CuONiO-ZnO/rGO composite with different scavenges are presented in

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Fig. 14. (a) Kinetic plot for degradation of MB, (b) –ln (C/Co) vs. time plot for degradation of MB, (c) Histogram showing comparative degradation rate in percentage of MB with the photocatalyst under visible light illumination, (d) Bar graph showing the values of rate constants for all the photocatalysts. Table 2 Percent degradation and rate constants for photocatalytic degradation of MB under visible light illumination by using various photocatalyst. Photocatalyst

Percent degradation (%)

Rate constant (min−1)

CuO NiO ZnO n-CuO-p-NiO-n-ZnO n-CuO-p-NiO-n-ZnO/rGO

2.0 1.4 20.4 48.0 70.6

0.0004 0.0002 0.0033 0.0088 0.0176

Fig. 15. When 0.01 g of EDTA as h+ scavenger was added, the photocatalytic activity of composite was 10%, indicating that h+ was involved in the photocatalytic process. Moreover, the photodegradation

of MB was also reserved to 8.7%, when 0.01 g of silver nitrate was added as e− scavenger, telling that e− was also the noticeable active species for the degradation of MB. In addition, when the 2-propanol as ∗ OH scavenger was added into the system, the degradation of MB was also depressed to 10%, indicating that ∗OH was also involved in the photocatalytic process However, when ascorbic acid as ∗O2−2 scavenger was added into the reaction system, the photocatalytic activity of n-CuO-p-NiO-n-ZnO/rGO composite for degradation of MB was not obviously changed, denoting that .O2−2 had very small or no contribution to the photodegradation of MB. The above results exposed that the photogenerated hole (h+), hydroxyl radical (*OH) and electrons (e−) were the main reactive species for the degradation of MB during the degradation reaction. Scheme 1, shows proposed photocatalytic degradation mechanism

Fig. 15. (a) Kinetic plot for degradation of MB using various scavengers, (b) Histogram showing degradation rate in percentage of MB with the photocatalyst under visible light illumination by using various scavengers. 17814

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Scheme 1. Schematic illustration of degradation of methylene blue by graphene based photocatalyst.

based on experimental results. It is well known fact that rGO behaves as a photosensitizers as well as is liable for the greater absorption in visible region in semiconductor photocatalysis. Due to this universal property of rGO, photocatalytic performance further increases. The conduction band edge and valence band edge energy of CuO, NiO and ZnO is calculated by equations (8) and (9) [61].

ECB =X−Ee −

1 Eg 2

ECB = EVB - Eg

(8)

(9)

In these equations, “EVB” is the valence band edge potential, “ECB” is the conduction band edge potentials, “X” is the electronegativity (CuO is 6.0, NiO is 5.0 and ZnO is 5.9), “Ee” is the energy of free electrons on the basis of Hydrogen scale (4.5 eV) and “Eg” is the optical band gap energy (CuO is 3.09 eV, NiO is 3.35 eV and ZnO is 2.94 eV). By using above stated equatons, “EVB” for CuO, NiO and ZnO was 3.055 eV, 2.89 eV and 2.19 and “ECB” for CuO, NiO and ZnO was −0.003, −0.04 eV and −1.17eV, respectively. The photocatalytic activity is because of creation of double p-n-junction between n-type CuO, p-type NiO and n-type ZnO. Based on the schematic, this p-n hetero-junction forms the electrons and holes in p-NiO and n-ZnO and n-CuO. When photocatalyst was exposed to visible light, photo-excitation of electrons takes place. The interfacial contact between metal oxides is responsible for transfer of photogenerated electrons from more negative conduction band of NiO to less negative conduction band of CuO and ZnO. Similarly, transmission of photogenerated holes from more positive valence band of CuO and ZnO to less positive valence band of NiO takes place. These result in separation of e−-h+ efficiently across the heterojunction. The transmitted electrons from conduction band of NiO to conduction band of CuO and ZnO are not able to produce superoxide anion radicle from dissolved oxygen due to more positive conduction band edge energy of CuO and ZnO than reduction potential of O2 to *O2−2. However, these electrons could yield hydrogen peroxide (H2O2) which on exposure to light and electrons degraded to ∗OH radicals. These highly oxidizing ∗OH radicals degraded dye molecules to CO2 and H2O. In contrast photogenerated holes in valence band of NiO are also unable to create ∗OH radicals from OH− anion and water because of more positive reduction potential of H2O to ∗OH and OH− to ∗OH radical. So these are only responsible for directly degradation of organic pollutant. The overall reaction mechanism is given in equation

(10)–(18) [62].

n− CuO − p− NiO − n− ZnO/rGO + hv → n− CuO − p− NiO − n− ZnO(e−cb + h+vb)

(10)

NiO(e−cb )→ CuO(e−cb)

(11)

NiO(e−cb

(12)

)→

CuO(h+vb)

ZnO(e−cb) NiO(h+vb)

(13)

ZnO(h+vb) → NiO(h+vb)

(14)

CuO(e−cb

)+

O2 → H2 O2

(15)

ZnO(e−cb

)+ H+ + O2 → H2 O2

(16)

→

H+ +

H2 O2 + e− + hv → *O H+ OH− *

(17)

O H+ h+ + organic pollutants → Degradation products(CO2 and H2 O) (18)

Therefore, It could be concluded that ∗OH radical and holes behaves as main active species during photocatalytic degradation process. Furthermore, Table 1 shows the percent degradation and rate constant values. 4. Conclusion In this work, we have fabricated n-CuO-p-NiO-n-ZnO double heterojunction by using co-precipitation approach and exposed them as active photocatalyst for removal of organic pollutant from contaminated water under visible light irradiation. Structural and morphological characterization was done by XRD, FTIR and UV–Visible spectroscopy. XRD results confirmed the formation of CuO (crystallized in monoclinic unit cell), NiO (crystallized in Cubic unit cell) and ZnO (crystalized in hexagonal unit cell) phases in triphase composite. Futher XRD results are supported by FTIR data. UV–Visible spectroscopic analysis expressed the absorption peaks for Zn-O, Ni-O and Cu-O bond in ternary composite. Bandgap energy of heterojunction is red shifted to 1.6 eV that corresponds to visible light. This heterojunction showed superior photocatalytic activity than those of pure n-CuO, p-NiO and nZnO nanoparticles by four folds. n-CuO-p-NiO-n-ZnO heterojunction yielded 40% degradation within 60 min under visible light. However, Graphene based n-CuO-p-NiO-n-ZnO heterojunction gives 70%

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degradation within 60 min under visible light. Various scavengers were used to detect active species in degradation experiment. It was found that electrons, holes and hydroxyl radicles have significant effect on photocatalysis whereas superoxide radicles are not directly or indirectly influencing Photocatalysis. These promising results opened a new avenue for researchers to work in semiconductor based photocatalysts. Acknowledgement Authors from King Saud University (KSA) are thankful to the King Saud University for financial support via grant number RG-1438-068. Other authors are thankful to the Islamia University of Bahawalpur and Higher Education Commission of Pakistan (6276/Punjab/NRPU/R&D/ HEC/2016). References [1] P.V. Nidheesh, J. Khatri, T.S.A. Singh, R. Gandhimathi, S.T. Ramesh, Review of zero-valent aluminium based water and wastewater treatment methods, Chemosphere 200 (2018) 621–631. [2] M. Sillanpää, M. 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