Chemical Physics Letters 719 (2019) 1–7
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Photocatalytic degradation of methylene blue with synthesized rGO/ZnO/ Cu
Maedeh Asghariana, Mohsen Mehdipourghazia, , Behnam Khoshandama, Narjes Keramatib,1 a b
Faculty of Chemical, Petroleum, and Gas Engineering, Semnan University, Semnan 35131-19111, Iran Faculty of Nanotechnology, Semnan University, Semnan 35131-19111, Iran
H I GH L IG H T S
nanostructure was synthesized by combined total reﬂux hydrothermal method. • rGO/ZnO/Cu size was obtained around 3 nm and particle size was reported about 20–100 nm. • Crystal properties is characterized by morphological and structural analysis. • Photocatalyst • Photocatalytic performance was evaluated by methylene blue solution.
A R T I C LE I N FO
A B S T R A C T
Keywords: Photocatalyst Zinc oxide Graphene oxide XRD DRS
In order to observe photocatalysts reactions, rGO/ZnO/Cu nano-structure was synthesized through total reﬂux combined hydrothermal method. The eﬀects of adding rGO and Cu doping on ZnO lattice were assessed. FE-SEM, TEM, EDX analysis, and XRD tests were used to study the morphology and particle size of the catalyst. The XRDSAXS veriﬁed the crystallinity of the catalysts. FTIR and Raman spectroscopy characterized the mineral oxideGO bonds also DRS was applied to study the band gap. Photocatalytic degradation eﬃciency was investigated by methylene blue solution and 25 mg photocatalyst is the best dosage for degradation.
1. Introduction ZnO is a semiconductor has several properties that make it favorable for industrial applications e.g. low toxicity, low cost, high chemical/ physical/thermal stability, broad adsorption spectrum and relatively simple conditions for crystal growth . It is also usually easily obtained. However, direct and wide energy band gap (3.4 eV), high binding energy at room temperature (60 meV) and high electron mobility of about 115–155 cm−2 V−1 S−1 have limited ZnO usage . There has been much research eﬀort to narrow down band gap and prevent recombination of electron-hole pairs by combining ZnO with other functional materials. One approach involves applying carbonbased materials due to their ability in enhancing charge transfer at the interface of metal oxide and carbonic material . Carbon nanotubes, graphite, graphene and graphene oxide (GO) integrate with ZnO and improve its photocatalytic eﬃciency [4–8]. Excellent mechanical properties, electrical conductivity (attributed to its sp2 orbitals) , unique chemical properties such as zero band gap, high speciﬁc surface
area, high transparency of charge carrier (attributed to its one-atomthick layers) and prefect thermal conductivity  make GO a very attractive choice. GO has a very high level of electron conductivity which is of great importance for this purpose. Reduced GO can act as an electron scavenger for ZnO because the potential of the conduction band of ZnO (−0.45 V) is lower than that of GO (−0.08 V) which indicates that photo-excited electrons are more prone to transfer to GO’s surface than going back to the conduction band of ZnO [11–13]. Moreover, the high surface area of GO results in a higher level of contact between the photocatalyst and the substrate such as pollutant molecules leading to an increased rate of reaction . Several methods have been proposed to manufacture ZnO/rGO hybrids such as solvothermal, hydrothermal, chemical vapor deposition, etc [3,15]. ZnO/rGO composition can be produced through three methods: loading ZnO on the rGO sheets, reducing GO on the ZnO lattice and growing ZnO at the same time with GO reduction . Limitation of the ﬁrst two methods is in the optimization of ZnO and GO amounts; limitation of the last one is agglomeration . Although
Corresponding author. E-mail addresses: [email protected]
(M. Mehdipourghazi), [email protected]
(N. Keramati). 1 Co-corresponding author. https://doi.org/10.1016/j.cplett.2019.01.037 Received 20 October 2018; Received in revised form 16 January 2019; Accepted 17 January 2019 Available online 06 February 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
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solid pellet was dried overnight in an oven at 50 °C.
synchronic growth of the ZnO and reducing GO caused non-uniform distribution of ZnO on to the GO surface, it is still a facile route in the presence of certain other materials. Many dopants such as metals and metal oxides have been utilized to improve photocatalytic activity by diminution of the band gap . Cu has a unique advantage in this regard; because of its high level of similarity to Zn in terms of electronic shell structure and atomic diameter, it does not distort the structure or the homogeneity of electron transfer among layers when it is placed in the ZnO lattice [17–21]. Ravichandran et al. used a simple wet chemical method to fabricating rGO/ZnO/Cu as a photocatalytic and antibacterial agent. They found that graphene decreased crystal size and improved antibacterial properties . Hsieh et al. synthesized the same ternary nano-particle through a microwave-assisted hydrothermal method. They reported that adding rGO lowered the recombination rate of the electron-hole and improved visible light absorption . In the present study, the rGO/ZnO/Cu ternary complex was synthesized through a two steps routs include total reﬂux and hydrothermal method. Hydrazine monohydrate (N2H4) was added as a reducing agent to produce rGO from GO and facilitate chemical bond formation between rGO and ZnO. N2H4 changed the media to a suitable neutral alkaline environment that enhanced photocatalytic activity. Cu was added to the nano-composite as an impurity to increase the adsorptive characteristic. Then, the structural and morphological properties of the prepared sample were investigated using Field Emission Scanning Microscopy (FE-SEM), Transmission Electron Microscopy (TEM), Energy Dispersive X-ray (EDX) analysis, X-ray diﬀraction (XRD), XRD-small-angle X-ray Scattering (XRD-SAXS), Fourier Transform Infrared Spectroscopy (FTIR), Raman Spectroscopy, and Diﬀuse Reﬂection Spectroscopy (DRS). At least the eﬃciency of the photocatalyst for degradation of MB was investigated.
2.2.2. Characterization of the photocatalyst The structural and morphological properties of the prepared sample were investigated using Field Emission Scanning Microscopy (FE-SEM), Transmission Electron Microscopy (TEM), Energy Dispersive X-ray (EDX) analysis, X-ray Diﬀraction (XRD), XRD-small-angle X-ray Scattering (XRD-SAXS), Fourier Transform Infrared Spectroscopy (FTIR), Raman Spectroscopy, and Diﬀuse Reﬂection Spectroscopy (DRS). The XRD patterns were recorded using a D5000 device (Siemens AG, Munich, Germany) via Cu-Ka radiation in a range of 2θ = 10–90° at room temperature. The SAXS analysis was carried out using X’Pert PRO MPD (PANalytical, Netherlands) to assess the distribution of size and nano-structure. FT-IR spectra for the GO/ZnO/Cu sample were recorded in a range of 450–4000 cm−1. Raman spectroscopy was used to ensure that GO maintained its original state in the synthesized material (was not converted to graphite, graphene, and other carbon derivatives) and was reduced. Raman analysis is suitable for investigating crystal structures and their defects . 2.2.3. Photocatalytic measurement The photocatalytic activity of nano powder was evaluated by photocatalytic degradation of methylene blue (MB) dye in the water under light source located symmetrical in the water proof tubes. Five diﬀerent concentration of photocatalyst loaded to the aqueous reaction solution of MB and four various type of compound used to ﬁnding out eﬀect of photocatalyst dosage and each element inﬂuence. Two Xe arc lamp 300 W and 500 W (with ultraviolet cut oﬀ ﬁlter) were used as the light source. The MB solution prepared in 1 × 10−5 M for each experiment and signiﬁcant photocatalyst dispersed in 150 ml on to it. Suspension sonicated in dark to reach an adsorption-desorption equilibrium (for 20 min). All tests was done under ambient condition for 60 min and all samples ﬁltered and centrifuged to remove the photocatalyst. UV–vis spectrophotometer was measuring the absorbance of MB at 664 nm to observed degradation amount.
2. Methods and materials 2.1. Materials All the chemicals were purchased and used without any processing or pretreatments as follows: GO: Sigma Aldrich (98% purity), zinc oxide: Sigma Aldrich (99%, 10–30 nm), copper nitrate (CuNO3·5H2O): Merck (Germany), hydrazine monohydrate (N2H4): Sigma Aldrich, methylene blue (C16H18ClN3S), ethanol (C2H5OH) and sodium hydroxide (NaOH): Merck (Germany). During all the experimental steps, deionized water and ethanol (1:1 ratio) were used as the solvent (media).
3. Results and discussion 3.1. XRD and SAXS analysis Fig. 1 presents the results of XRD test for GO, ZnO, ZnO/Cu, rGO/ ZnO and rGO/ZnO/Cu nano-photocatalyst, which illustrated all samples have almost the same pattern. The diﬀraction peaks implies a single phase structure of ZnO without any additive indicating Cu and rGO do not have any eﬀect on the crystallinity . It’s due to the substitution of the Cu2+ ions (cationic radius 0.72 Å) with the Zn2+ ions (cationic radius 0.74 Å) result to Cu peaks are not displayed in the
2.2. Experimental procedures 2.2.1. Preparation of the photocatalyst The rGO/ZnO/Cu nano-structure was synthesized through a twostep ultrasonic-assisted method (the ﬁrst step was a total reﬂux, and the second step was a hydrothermal process) [24,25]. Firstly, two suspensions were prepared separately. For the ﬁrst suspension, GO (0.4 g) was dispersed in a water/alcohol solvent (50/50 v/v: 100 cc) and a few drops of N2H4 (4–6 drops) was added under stirring. This suspension was then mixed by an Ultra-Turrax dispersing machine for 5 min. For the second suspension, ZnO(1 g), Cu(NO)3 solution(10 cc, 0.1 g l−1) and NaOH (2 or 3 drops for pH adjustment) were roughly stirred by the Ultra-Turrax machine. The ﬁrst and second suspensions were mixed and agitated for another 5 min. Then, the suspension was put into an ultrasound device for 15 min to obtain a homogeneous solution. Finally, the materials were transferred to the balloon to conduct total reﬂux at 140 °C under stirring for 12 h. Finally, the synthesis process was conducted in an autoclave at a relatively low temperature (170 °C) for 12 h. As the result, the process created a precipitate containing the synthesized catalyst. To reach a neutral pH, the supernatant was pipetted out and the pellet was washed by water several times (re-suspended in water and centrifuged again). After the autoclave and wash steps, the
Fig. 1. XRD pattern of (I) GO, (II) ZnO, (III) rGO/ZnO, (IV) ZnO/Cu, (V) rGO/ ZnO/Cu nano-particle. 2
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XRD pattern. It’s one of the limitation of XRD. XRD analysis examines samples by their atomic radius; since Zn2+ and Cu2+ have a similar size, signiﬁcant peaks refer to the bulk of the host lattice. Previous studies have explained the interstitial connections with the same results [18,22]. Because of that no signiﬁcant peaks could be observed in XRD pattern. Moreover, peaks related to Cu may be doesn’t detected due to low doped Cu concentration. The ZnO and Cu nano-particles were positioned on the GO sheets, however, due to the disturbance in lattice structure, the GO-related peaks were not recognized [18,22]. This fact discussed previously in literature [23,27,28]. The results of FE-SEM and EDX conﬁrm the presence of all the particles including Cu in the sample (Fig. S3). The rGO/ZnO/Cu pattern veriﬁes the presence of a hexagonal crystal structure and a P63mc space group. The distance between sheets was determined using the Debye-Scherrer Equation, as follows :
Fig. 2. FT-IR spectra of the rGO/ZnO/Cu nano-powder before and after degradation.
where D is the particle size, λ is the X-ray wavelength (0.154 nm), β is the broadening at half of the maximum intensity of the peak, and θ is the diﬀraction angle. The highest peak is located on 36.266°, so the dspacing is 2.47 nm. As shown in Fig. 1, the XRD pattern indicates that the peaks at (2θ = )32°, 34°, 36°, 47°, 56°, 63°, 64.27, 68°, 69°, 73.96, and 77° are parallel with the (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), (2 0 1), (0 0 4) and (2 0 2) crystal planes in ZnO, respectively. All the peaks in the sample structure were indexed by unit cell parameters of a = 3.24982 Å and c = 5.20661 Å. The unit cell volume was calculated using Eq. (2) :
V= √ 3a2c/2
chemical composition of pure ZnO and rGO/ZnO/Cu. The results conﬁrm the presence of rGO (as C atoms) and Cu in the synthesized material (Fig. S3a and b). Atomic and weight percentages of the samples are presented in Table S2a and S2b which indicate diﬀerent compositions in the presence of GO and Cu. Carbon atoms observed in the pure ZnO sample had a small amount of impurity. As shown the morphology of doped particles was investigated using TEM image that is presented in Fig. S2a. As shown in TEM image, the mean grain size was 91 nm. The crystallinity of the sample was obvious which was attributed to the presence of polyhedral particles; this phenomena represents the high degree of crystallinity of the powder.
Pore volume was 46.74042 Å which was calculated using Eq. (2). This ﬁning is consistent with the results reported in the literature . Another property of the lattice, i.e. the length bond of ZnO, was calculated using Eq. (3):
L= √ [a2/3 + (1/2−u)2c2]
3.3. FT-IR spectra Fig. 2 presents the FT-IR spectra for the rGO/ZnO/Cu sample before and after degradation of MB aqueous solution. The sample consist of GO which have covalently oxygen-containing group attachments such as hydroxyl, epoxy, carbonyl and carboxyl groups. FTIR spectrum pattern illustrates the sharp peak at 543.0 cm−1 which assigned to the ZneO bond  and two weak bands at 695.2 and 874.7 cm−1 referred to the vibrational frequencies and seen due to the Cu entrance into ZnO lattice . This ﬁgure presented the oxygen-containing functional groups in domain around 1000–2000 cm−1 which related to presence rGO in compound. These peaks including the alkoxy (CeO) starching vibration at 1023.8 cm−1, the epoxy (CeOeC) starching vibration at 1395.1 cm−1, deformation of carboxylic group (eOH) at 1456.0 cm−1, the aromatic carbon-carbon double bond stretching (C]C) in graphene at 1535.1 cm−1, the other carbon-carbon double bond vibration (C]C) due to un-oxidized graphene parts at 1659.8 cm−1 and the ﬁnal peak at 1742.0 cm−1 related to carbonyl (C]O) reduction in GO [7,34]. The weak shoulders appear at 2858.8, 2922.7 and 2962.3 cm−1 corresponds to CeH stretching vibration caused by epoxide open circle . The famous OeH stretching vibrations at 3452.2 cm−1 belong to the moisture in the rGO/ZnO/Cu sample . The results of FT-IR show that the synthesis method was successfully able to synthesize three materials simultaneously, including the catalyst base (ZnO), the surface multiple layers (GO), and the additive (Cu). As the FTIR spectra revealed there is no observable change in the sample after degradation which indicate nano material possess good repeatability and stability over photocatalytic reactions [24,35].
The wurtzite structure i.e. u was calculated using Eq. (4):
u= a2/3c2 + 0.25
Length bond of the samples showed no signiﬁcant change in the ZnO structure which indicates that incorporating Cu and GO did not alter the ZnO lattice. Moreover, the results showed the agglomeration of ZnO and Cu, which is in line with the results of SEM analysis, as shown in Fig. S2. The sharp slope of the curve (Fig. S1) in the ﬁrst zone, shows an agglomeration in the synthesized powder [30–32]. The cubic diameters measured by SAXS are presented in Table S1. More than 80% of the particles had a diameter of 33.28 nm which indicated a narrow size distribution of nano-particles. Moreover, about 20% of the particles had a diameter of 14.69 nm. 3.2. Surface morphology Fig. S2 presents the FE-SEM images of the rGO/ZnO/Cu nano-particle samples. Fig. S2a and b show the agglomeration in the pure and modiﬁed samples. The shape of ZnO crystals clearly altered after adding the Cu2+ ions and rGO sheets. The size of the ZnO particles, as the precursor, is between 10 and 30 nm, but it changed when coated and stabilized on the rGO sheets. The size of ZnO nano-particles measured by FE-SEM was about 26–108 nm which was perfectly consistent with the results of SAXS analysis. The size of particles and agglomeration show that the growth mechanism of the ZnO crystals is aﬀected by Vander-Waals interactions on the nano-lattice crystals. It provides a driving force for the self-assembly of ZnO particles when rGO is used as the surface developer. Apparently, rGO acts as a ﬂat surface under the crystals and ZnO and Cu grains interact on the top of rGO sheets . As presented in Table S2a and S2b, the results of EDAX analysis show the
3.4. Raman scattering Raman scattering is a basic tool for the characterization of carbonaceous substances and has a good sensitivity in the nano-electronic structures . The Raman-scattering spectra was recorded by a 1064 nm laser light, as a source of excitation. Our nano-composite 3
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absorption edge which is because of enhancing the surface electrical charge and facilitate the electron movements on the rGO hybridization on ZnO [14,46]. This eﬀect on absorptions result in diminution band gap energy. Doping Cu on ZnO narrowing band gap due to strong interaction between 3d shell of cupper ions and 2p shell of oxygen atoms in the Cu/ZnO complex . Also the conduction band of ZnO/Cu has lower level which ascribed to hybridization of 4s-4s band of Cu and Zn . Although rGO aﬀected the ZnO band gap, but this connection is only physically and in compared to Cu, has little eﬀect on the absorption and band gap . According to this fact, band gap in ZnO/Cu is decreasing more than rGO/ZnO/Cu. The band gap energy from Tauc plot was estimated 3.81, 3.27, 3.25 and 3.22 eV for ZnO, rGO/ZnO, ZnO/Cu and rGO/ZnO/Cu, respectively. This could be explain that Cu substitute into ZnO lattice and caused band gap diminish, rGO has like eﬀect but a little less. When Cu and rGO add together to ZnO, the eﬀect of them is counter-currently and results to a band gap in middle of rGO/ ZnO and ZnO/Cu. Xue et al and Pan et al found rGO could reduce band gap and enhance photoresponsive range of adsorption [14,46]. Joshi et al investigated Cu eﬀect on band gap. They discovered ZnO/Cu has smaller band gap in comparison with pure ZnO and describe it with level matching of the 2p and 3d bands in Cu ions and O atoms . In the ZnO/rGO/Cu sample, Cu as an impurity caused defects in ZnO lattice, and rGO as an photosensitizer propagating adsorption range. Hsieh et al found out Cu decrease the band gap but rGO has little or no eﬀect on it .
showed typical D and G bands (Fig. S4). They represent the breathing mode of the defects in graphene sheets (1200 cm−1) and bond scattering of sp2 carbon atoms (1650 cm−1), respectively. Since rGO was blended with other materials, D and G peaks had a slight shift. There is no sign of the 2D peak of pristine GO, which is attributed to the perturbation of graphene layers and its multilayer structure. The intensity ratio of D to G peak (ID/IG) is expected to be 1 in pure rGO [37–39]. Deviation of the intensity ratio from 1 indicates the level of disruption in graphene structure. The ID/IG of our rGO/ZnO/Cu product was equal to 1.8 which was close to 1 and only exhibited slight defects in the graphene structure. The wide peak around 500 cm−1 incorporates several peaks belonging to the ZnO hexagonal crystals [38,40,41]. Reddy et al used ZnO nano particles as photocatalyst and found new peaks which revealed photochemical reaction was done on the surface of ZnO . FTIR spectra of ZnO and ZnO/rGO in Dong et al study revealed no diﬀerent before and after degradation. These results indicate that the prepared photocatalysts have good stability and repeatability . 3.5. DRS analysis Fig. 3 shows the optical properties of the pristine ZnO, ZnO/Cu, rGO/ZnO and rGO/ZnO/Cu that were obtained from UV–vis DRS analysis in absorption mode between 300 and 800 nm. The absorption edge of the synthesized particles was obtained by drawing the tangent line, and band gap calculated based on the Tauc theory, (αhυ)1/ n = A (hυ − Eg ) , where h is Planck’s constant, A is the proportionality constant, ν is the light frequency, and Eg is the band gap energy. For materials with direct and indirect transition mode, n is 1/2 and 2, respectively . The ZnO band gap can be aﬀected by two factors, as follows: (1) the space present in oxygen vacancies can form impurity in the valance band, and (2) high interfacial interaction between ZnO and rGO result in narrowing the band gap . Metal type and synthesis method are the essential key factors in band gap shifting. In a previous study, after adding noble metals like Ag or Pt to ZnO, the results of DRS showed no changes in band gap . Similar results were obtained for Ce-doped ZnO . The absorption spectra of Cudoped ZnO without rGO only had a slight shift toward the visible light region and caused a tiny reduction in band gap . The reduction in band gap was attributed to the eﬀects of quantum size and electrical interactions on the modiﬁed surface . Quantum size itself depends on the crystal nucleus size, surface, and the fabricating procedure. Interfacial electrical interactions depend on rGO which acts as a macromolecular surfactant during the rGO/ZnO organization. This allows rGO to control oxygen vacancy concentration in ZnO through its own abundant oxygenated functional groups [41,44,45]. In this investigation the sharp absorption edge located around 400 nm an all products indicates the ZnO phase. The rGO blended with ZnO and increasing
4. Photocatalytic degradation of methylene blue Photocatalytic degradation of MB with rGO/ZnO/Cu nanomaterial was investigated under visible light irradiation. Fig. 4 illustrate the absorption spectrum of the aqueous MB solution in diﬀerent time for each degradation run. It’s clearly indicated that with increasing light irradiation time, the characteristic MB peaks gradually decreased. As observed in this ﬁgure minimum photocatalytic degradation was reached by 5 mg photocatalyst concentration (Fig. 4a) and maximum dye destruction was in 15 mg rGO/ZnO/Cu (Fig. 4b). Degradation percentage of MB was increased up to 95.14% by increasing rGO/ZnO/ Cu from 5 to 15 mg in the MB solution and light of energy equal to or greater than the band gap energy of ZnO to activated it. However by more increasing in photocatalyst amount, prospecting more adsorption and more degradation, but solution turbidity increased and avoided photo get to the photocatalyst surface. Although adsorption increased, degradation of MB was decreased in 25(Fig. 4c) and 50 mg (Fig. 4d). Fig. 5 illustrated the normalized time dependent photocatalytic degradation of MB solution by using various photocatalyst dosage, diﬀerent photocatalyst type and power lamp. The degradation proﬁle was plotted as C0/Ct versus time, where Ct is a concentration of MB at sampling time(t in min) and C0 is prime concentration of MB. As seen in Fig. 5(a) the visible organic dye degradation eﬃciency is relative to the photocatalyst concentration, which very low in blank reaction without any photocatalyst. Photolysis in the presence of the light shows 17% degradation of MB at the end. Another blank reaction in dark with diﬀerent dosage of photocatalyst was carried out to ﬁnd adsorption capacity, which increased by increasing photocatalyst concentration and has the maximum point in rGO/ZnO/Cu in 50 mg. The maximum degradation value was reported around 97%at 15 mg, and done after 60 min. To identify the eﬀect of each element, photocatalytic degradation of MB evaluated by incorporating rGO, Cu into ZnO matrics. As shown in Fig. 5c pure ZnO has a poor degradation in comparison with the others which due to wide band gap of ZnO. The rGO change the surface electric charge and attach to ZnO lattice physically which caused more adsorption and higher degradation. The photocatalytic properties of ZnO could be more improved when Cu is coming into the ZnO lattice and substitution with Zn ions. This case because of close radius amount of the ZnO and Cu. ZnO and Cu combined chemically and these
Fig. 3. UV–visible diﬀuse reﬂectance spectra (DRS) of pristine ZnO, ZnO/Cu, rGO/ZnO and rGO/ZnO/Cu composite. 4
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Fig. 4. UV–visible absorbance spectra presented the concentration of MB aqueous solution in the presence of (a) 5 mg, (b) 15 mg, (c) 25 mg, (d) 50 mg of rGO/ZnO/ Cu.
intensity on the rate constant, which is acquired 0.0554 and 0.1094 min−1 for 300 W and 500 W, respectively. Along with increasing light intensity, the amount of photon increased and then, more photon bring themselves to the photocatalyst surface. This fact results to more excited electron and not only caused enhancing degradation but also make it faster.
interaction is deeper than rGO/ZnO bands. Whereas, the degradation eﬃciency based on the photocatalyst type are 67, 87, 91 and 97% for ZnO, ZnO/rGO, ZnO/Cu and rGO/ZnO/Cu, respectively. This results is according to previous studies . The light intensity inﬂuence on the degradation of MB was investigated with changing light source from 300 W to 500 W. The complete photocatalytic degradation eﬃciency (∼100%) was observed under 500 W irradiation after 30 min which it was lower (∼97%) for 300 W lamp during 60 min (Fig. 5e). It’s might be due to increasing light intensity could result to enhancing light density and more electrons in active sites get excited. The inﬂuence of initial concentration of MB aqueous solution on the photo-degradation kinetics can be described by pseudo-ﬁrst order kinetics and Langmuir-Hinshelwood model at low concentration of MB [24,27,48]. The pseudo-ﬁrst order reaction kinetics is given by Eq. (5) as:
Ct = C0 e−kt
5. Conclusion In conclusion, using the total reﬂux, a hydrothermal two-step method, and ultrasonication, we introduced a new nano-composite. XRD pattern showed that the obtained product had a wrutzite hexagonal crystallinity. FE-SEM analysis and EDX conﬁrmed the expected agglomeration and atomic content, respectively. A wide range of crystal sizes was observed (with a minimum of around 2.47 nm). The estimated particle size of rGO/ZnO/Cu was about 26 to 208 nm, as measured by XRD and FE-SEM, respectively. FTIR and EDX analyses veriﬁed the partial substitution of Cu ions in the ZnO lattice. Raman spectra scattering was used to ensure the reduction of GO to rGO by hydrazine monohydrate and its stability in the synthesized particle. Optical properties tested with DRS spectroscopy exhibited a red shift in band gap (from 3.37 eV to 3.22 eV). Photocatalyst performance was measuring by MB and maximum value of the degradation obtained in the 25 mg photocatalyst dosage for rGO/ZnO/Cu compound. Because of this improvement, our photocatalyst can be activated by the visible light, instead of UV, which is safer, less expensive, and more easily available. Enhancing light intensity caused increasing MB degradation in short time.
The linear form of this equation written in a form given in Eq. (6):
C ln ⎛ 0 ⎞ = kt ⎝ Ct ⎠ ⎜
where k is the rate constant of pseudo-ﬁrst-order reaction. As observed in Fig. 5(b), (d), (f) the ln(C0/Ct) versus the irradiation time were plotted for varied type of degradation runs and all the experiments have good matched with linear form. The obtained rate constants (k) are 0.0026, 0.023, 0.055, 0.03 and 0.0252 min−1 for photolysis, 5 mg, 15 mg, 25 mg and 50 mg of loaded photocatalyst (Fig. 5(b)). The improved photocatalytic performance (Fig. 5(d)) of ternary complex compared to binary and pure compound and could be attributed to the increase of active sites on the surface of photocatalyst as discussed in previous literature [27,33]. Fig. 5(f) illustrated the eﬀect of light 5
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Fig. 5. Time dependent photocatalytic degradation and kinetic linear simulation curves of MB degradation in (a) and (b) several catalyst dosage, (c) and (d) various photocatalyst, (e) and (f) by using diﬀerent power of lamp.
Declaration of interests 
The authors declare that they have no known competing ﬁnancial interests or personal relationships that could have appeared to inﬂuence the work reported in this paper.
Appendix A. Supplementary material
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cplett.2019.01.037.
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