MoS2-Cu/CuO@graphene Heterogeneous Photocatalysis for Enhanced Photocatalytic Degradation of MB from Water

The industrial revolution resulted in the contamination of natural water resources. Therefore, it is necessary to save and recover the natural water resources. In this regard, polymer-based composites have attracted the scientific community for their application in wastewater treatment. Herein, molybdenum disulfide composites with a mix phase of copper, copper oxide and graphene (MoS2-Cu/CuO@GN) were synthesized through the hydrothermal method. Methylene blue (MB) was degraded by around 93.8% within the 30 min in the presence of MoS2-Cu/CuO@GN under visible light. The degradation efficiency was further enhanced to 98.5% with the addition of H2O2 as a catalyst. The photocatalytic degradation efficiency of pure MoS2, MoS2-Cu/CuO and MoS2-Cu/CuO@GN were also investigated under the same experimental conditions. The structural analysis endorses the presence of the Cu/CuO dual phase in MoS2. The charge recombination ratio and band gap of MoS2-Cu/CuO@GN were also investigated in comparison to pure MoS2 and MoS2-Cu/CuO. The chemical states, the analysis of C1s, O1s, Mo3d and Cu2p3, were also analyzed to explore the possible interaction among the present elements. The surface morphology confirms the existence of Cu/CuO and GN to MoS2.


Introduction
Environmental and energy remediation are two major issues for human beings due to rapid industrialization [1]. The rapid increase in industrialization activities resulted in the contamination of natural water resources. The organic dyes and various kinds of heavy metals are being discharged into water reservoirs on a daily basis from different industrial activities [2]. These contaminations in water are a severe threat for human health and the ecosystem. In this regard, various materials and methods have been deployed to remove the contamination from the water, such as absorption, membrane filtration and photocatalytics [3]. However, the photocatalytic process is being considered an efficient and cost-effective way to remove or neutralize hazardous material from water [4]. Therefore, various materials such as carbon-based metals and metals oxides and, more importantly, polymers are attractive candidates to remove the pollutants through the photocatalytic process [5].
Polymers are among the most versatile materials for wastewater treatment through photocatalysts, membranes and the absorption process owing to chemical stability/versatility, ease of functionalization, high specific surface, etc., [6,7]. Therefore, a two-dimensionallayered structure of molybdenum disulfide (MoS 2 ) has attracted the scientific community because of its extraordinary structure and properties such as strong oxidizing activity, non-toxicity in nature and a low band gap (1.8 eV) that can be further tuned with quantum confinement effects [8]. This low band gap of the MoS 2 is beneficial to absorb the light photons in the visible region which is highly desirable to enhance the photocatalytic process [9,10]. However, the interaction of Mo-S can engender the unsaturated atoms at the crystal edge which may render the photocatalytic activity of MoS 2 [11]. Therefore, the doping of some metals or metal oxides can enhance the photocatalytic activity of MoS 2 .
In this context, p-type semiconductor materials such as copper oxide (CuO) have gained attention due to their photo-conductivity nature, which supports enhancing the photocatalytic activity of n-type MoS 2 by reducing the charge recombination ratio [12]. Moreover, CuO doping can enhance the absorption of light photons which results in the augmentation of catalytic activity [13]. Further the mix phase of Cu/CuO supports the formation of heterojunction which reduces the recombination of charge carriers that can enhance the photocatalytic activity [14]. However, the access amount of CuO in composites can provide the recombination center for charge carriers which will reduce the photocatalytic activity [15]. Therefore, the optimal amount of CuO is also important to enhance photocatalytic activity.
The charge carrier separation can be enhanced by the addition of graphene (GN) which amended the charge separation and absorbed the more visible light which consequently enhanced the photocatalytic activity [16]. Moreover, GN can also create some defects while making the composites with metals and metal oxides. These defects can capture the pollutant during the photocatalytic process [17] which makes it attractive for wastewater treatment. Further, the high specific surface area (2650 m 2 /g), π orbital, π-π interaction, functional groups makes it an attractive dopant with metal, metal oxides and polymers to synthesiss the photocatalysts [18].
Previous studies show that there are various reports on MoS 2 with GN and CuO and other metal oxides as a catalyst [12,19,20]. However, the low catalytic performance is still a major issue for these binary composites. Further, there are few reports on the role of metals and metal oxides in the photocatalytic activity of MoS 2 . Therefore, in this study, the combination of MoS 2 with GN and Cu/CuO mix phases were synthesized through an in-suit hydrothermal process. The role of Cu/CuO and GN to enhance the photocatalytic activity of MoS 2 was explored. Moreover, the change in the functional groups through XPS and the change in optical and structural properties were also studied.

Synthesis of MoS 2 -Cu/CuO@GN
MoS 2 was synthesized by hydrothermal methodology using MoO 3 and thiourea as precursors. In a typical process, 0.1150 g of MoO 3 and 0.2664 g of thiourea was taken in 80 mL of water and the system was put under stirring conditions for 30 min. Thereafter, the whole reaction mixture was transferred to 100 mL of Teflon-lined hydrothermal reactor and subsequently heated at 200 • C for 24 hrs. Thus, obtained the black precipitate of MoS 2 was separated by centrifugation, washed with excess of water and ethanol, dried at 80 • C for 12 h and subsequently stored in desiccator for further experiments.
For the MoS 2 -Cu/CuO@GN, first, binary composite of MoS 2 -Cu/CuO was prepared and further coating of GN over it resulted in MoS 2 -Cu/CuO@GN. The GO (stock solution of 10 mg/mL) and Cu/CuO nanoparticles were prepared separately. The synthesis of GO can be seen elsewhere [21]. The Cu/CuO nanoparticles were synthesized by the reduction of copper (II) sulfate in the presence of CTAB surfactant. In a typical process, 0.1 M copper (II) sulfate solution was dissolved in 100 mL of water and to it, 0.25 g of CTAB was added and the whole system was put under stirring conditions. In another beaker, 50 mL of 0.2 M ascorbic acid solution was prepared. In the second step, the solution of ascorbic acid was slowly added to the copper (II) sulfate solution and, subsequently, 30 mL of 1 M sodium hydroxide solution was also added. The whole system was heated to 80 • C for 2 h and a dark reddish-brown color confirmed the formation of Cu/CuO. Thus, prepared Cu/CuO was separated by centrifugation, washed with excess of water and ethanol and subsequently dried at room temperature [22]. The MoS 2 -Cu/CuO was prepared by mixing 1 g of MoS 2 and 0.1 g of Cu/CuO in 50 mL ethanol and the mixture was put in ultrasonic bath for 1 h, followed by stirring on hot plate until the complete evaporation of ethanol. Further, the fabrication of ternary MoS 2 -Cu/CuO@GN was done by mixing 10 mL of GO with 1 g of MoS 2 -Cu/CuO and the whole mixture was heated at 400 • C for 3 h for the complete reduction of GO into GN and to, subsequently, give MoS 2 -Cu/CuO@GN. The ratios of MoS 2 , Cu/CuO and GN were, respectively, 87.1%, 8.71 and 4.19%.

Photodegradation Measurement
MB was selected as model pollutant to assess the catalytic performance of MoS 2 , MoS 2 -Cu/CuO and MoS 2 -Cu/CuO@GN. In this regard, 25 mg of catalyst (optimized against initial concentration of MoS 2 and pH Figure S1) was added to the 20 ppm aqueous solution of MB. However, the MB solution containing the catalyst was put in the dark under vigorous stirring for 30 min to achieve the adsorption desorption equilibrium. Afterwards, the solution was irradiated with visible light of 2 watt, having a distance of 12 cm from MB solution for 30 min. The intensity was approximately 11.06 watt/meter. During this irradiation, a certain amount (5 mL) of solution was taken to estimate the degradation of MB by measuring the spectrum through UV-Visible spectrometer.
The MB degradation ability of prepared photocatalysts was calculated by applying the following relation [23]: where C o represent the initial taken concentration of MB while C t symbolizes the remaining MB concentration after interval of 10 min. Once the photocatalytic efficiency was calculated, then following giving relation was used to calculate the reaction rate constant during degradation process [24].

Characterizations
The structural and surface compositional analysis of MoS 2 , MoS 2 -Cu/CuO and MoS 2 -Cu/CuO@GN were performed, respectively, with X-ray diffraction (Ultima IV-Rigaku Tokyo Japan) and X-ray photoelectron spectroscopy (PHI-Versa ProbeII Chanhassen USA). The pass energies of 187.85 eV and 47.46.95 eV were used, respectively, to acquire the survey and narrow scan mode. Surface morphology was investigated by field emission scanning electron microscopy (JSM7600-F-Jeol Tokyo Japan). The spectrometer (DR 6000 Hach Loveland USA) was used to calculate the absorption of MB, while charge recombination ratio was investigated through photoluminescence spectrometer (Shimadzu RF 5301PC Kyoto Japan).   20 are the representation of copper (Cu), as revealed in JCPD # 01-085-1326. Therefore, XRD analysis confirms the presence of Cu/CuO with MoS 2 . Moreover, after the addition of GN (MoS 2 -Cu/CuO@GN), no additional peak was observed. The non-observable diffraction peak of GN is attributed due to the exfoliation nature [25]. Further, the functional groups of GN can interact with MoS 2 and Cu/CuO may lead to variations in the structural properties without changing the preferred orientation of the diffraction planes [26]. This could be noticed in our diffraction analysis ( Table 1) that shows the change in the crystal grain size, which also revealed the successful interaction of GN functional groups with MoS 2 and Cu/CuO. The Scherrer relation was used to estimate the crystal size of MoS 2 , MoS 2 -Cu/CuO and MoS 2 -Cu/CuO@GN [27].

Structural Analysis
the tenorite phase of CuO (JCPD # 00-001-1117), while the diffraction peaks at 2θ = 43.43, 50.56 and 74.20 are the representation of copper (Cu), as revealed in JCPD # 01-085-1326. Therefore, XRD analysis confirms the presence of Cu/CuO with MoS2. Moreover, after the addition of GN (MoS2-Cu/CuO@GN), no additional peak was observed. The non-observable diffraction peak of GN is attributed due to the exfoliation nature [25]. Further, the functional groups of GN can interact with MoS2 and Cu/CuO may lead to variations in the structural properties without changing the preferred orientation of the diffraction planes [26]. This could be noticed in our diffraction analysis ( Table 1) that shows the change in the crystal grain size, which also revealed the successful interaction of GN functional groups with MoS2 and Cu/CuO. The Scherrer relation was used to estimate the crystal size of MoS2, MoS2-Cu/CuO and MoS2-Cu/CuO@GN [27].
The crystal grain size of MoS2 was around 2.37 nm, while it was increased to 13.05 and 18.27, respectively, for MoS2-Cu/CuO and MoS2-Cu/CuO@GN. This increment in the crystal grain size has a direct relation to the enhancement of the photocatalytic activity of the material, as reported previously [28]. It could also be noticed that (Section 3.5) MoS2-Cu/CuO@GN enhanced the photocatalytic activity in comparison to MoS2 and MoS2-Cu/CuO. The interaction of Cu/CuO and GN can further lead to a change in the dislocation density of MoS2 and be calculated by the following equation [29].   The crystal grain size of MoS 2 was around 2.37 nm, while it was increased to 13.05 and 18.27, respectively, for MoS 2 -Cu/CuO and MoS 2 -Cu/CuO@GN. This increment in the crystal grain size has a direct relation to the enhancement of the photocatalytic activity of the material, as reported previously [28]. It could also be noticed that (Section 3.5) MoS 2 -Cu/CuO@GN enhanced the photocatalytic activity in comparison to MoS 2 and MoS 2 -Cu/CuO. The interaction of Cu/CuO and GN can further lead to a change in the dislocation density of MoS 2 and be calculated by the following equation [29].
The dislocation density was calculated around 4.43 × 10 −1 , 1.81 × 10 −2 and 1.96 × 10 −2 , respectively, for MoS 2 , MoS 2 -Cu/CuO and MoS 2 -Cu/CuO@GN. This variation in the location density further leads to a change in the lattice strain of MoS 2 after the addition of Cu/CuO and GN. This change was calculated by the following relation [30].
The lattice strain was around 2.01 × 10 −2 , 4.00 × 10 −3 and 3.79 × 10 −3 , respectively, for MoS 2 , MoS 2 -Cu/CuO and MoS 2 -Cu/CuO@GN. In summary, the structural analysis (Table 1) showed the change in the lattice parameter of MoS 2 after the addition of Cu/CuO and GN. However, this addition does not lead to a change in the diffraction orientation or phase of MoS 2 .

Optical Properties
The surface oxygen defects and charge recombination in the photocatalytic material can affect the catalytic efficiency which can be estimated by the PL spectra. Moreover, the peak intensity of PL spectra is attributed to the charge recombination during charge propagation from the valence to conduction band. The PL spectra (Figure 2a [31]. Moreover, the decrease in the PL intensity of MoS 2 -Cu/CuO was also associated to the chemisorption absorption of the oxygen over the surface of the catalyst, which resulted in the enhanced charge separation [32]. PL intensity was further reduced for MoS 2 -Cu/CuO@GN and this lessening is attributed to the interface between GN and MoS 2 -Cu/CuO. Moreover, functional groups attached to the basal planes of GN provided the attractive site to enhance the charge carrier movement by reducing the charge recombination, which ultimately enhanced the catalytic activity of the MoS 2 -Cu/CuO@GN [33]. cation density further leads to a change in the lattice strain of MoS2 after the addition of Cu/CuO and GN. This change was calculated by the following relation [30]. The lattice strain was around 2.01 × 10 −2 , 4.00 × 10 −3 and 3.79 × 10 −3 , respectively, for MoS2, MoS2-Cu/CuO and MoS2-Cu/CuO@GN. In summary, the structural analysis (Table  1) showed the change in the lattice parameter of MoS2 after the addition of Cu/CuO and GN. However, this addition does not lead to a change in the diffraction orientation or phase of MoS2.

Optical Properties
The surface oxygen defects and charge recombination in the photocatalytic material can affect the catalytic efficiency which can be estimated by the PL spectra. Moreover, the peak intensity of PL spectra is attributed to the charge recombination during charge propagation from the valence to conduction band. The PL spectra (Figure 2a) of MoS2, MoS2-Cu/CuO and MoS2-Cu/CuO@GN was recorded from 350 to 650 nm, having the 320 nm excitation wavelength. The PL intensity of the MoS2 is higher in comparison to MoS2-Cu/CuO and MoS2-Cu/CuO@GN, which revealed the higher recombination rate of the charged carrier in MoS2. However, the intensity of MoS2 reduced after the addition of Cu/CuO (i.e., MoS2-Cu/CuO), which indicated the role of Cu/CuO for charge separation and transfer at the heterojunction of MoS2-Cu/CuO [31]. Moreover, the decrease in the PL intensity of MoS2-Cu/CuO was also associated to the chemisorption absorption of the oxygen over the surface of the catalyst, which resulted in the enhanced charge separation [32]. PL intensity was further reduced for MoS2-Cu/CuO@GN and this lessening is attributed to the interface between GN and MoS2-Cu/CuO. Moreover, functional groups attached to the basal planes of GN provided the attractive site to enhance the charge carrier movement by reducing the charge recombination, which ultimately enhanced the catalytic activity of the MoS2-Cu/CuO@GN [33]. The change in the recombination of the charge carrier can change the band gap of the MoS 2 , which ultimately affects the photocatalytic activity. Therefore, the band gap of MoS 2 , MoS 2 -Cu/CuO and MoS 2 -Cu/CuO@GN was estimated by applying the Kubelka-Munk relation [34]. The band gap of MoS 2 (Figure 2b) was approximately 1.7 eV which is consistent with the previous literature [35]. The band gap of MoS 2 reduced to 1.60 eV after the addition of Cu/CuO (MoS 2 -Cu/CuO). This reduction in the band gap is attributed to the absorption ability of Cu/CuO in the visible region, which results in the reduction of the MoS 2 -Cu/CuO band gap [36]. The band gap of MoS 2 -Cu/CuO@GN was approximately 1.50 eV, which enlightens the role of GN to reduce the band gap. Generally, the GN has an sp2 band and other oxygen functional groups in the form of epoxy (C-O-C) and hydroxyl (OH) which enhances the charge carrier mobility with the absorption of light in the GN-based composites [37].  Table 2 shows the detected atomic percentage of each detected element in the prepared catalyst.  The C1s spectra (Figure 3b) [41,42]. However, the Mo, MoS 2 , MoS 3 and MoO 3 contribution was changed with the addition of Cu/CuO and GN, as revealed in (Figure 4b-d). This change in the metal and oxide interaction can lead to changes in the photocatalytic activity of the catalyst [43]. O1s spectra of MoS2 (Figure 5a) revealed the appearance of two peaks at approxi mately 530.9 and 532.5 eV which are attributed to the oxidation of O1s (O − ) and OH [41,44]. However, the small shift in the contribution of O − (87.08 to 87.55%) and OH (12.92  (Figure 5a) revealed the appearance of two peaks at approximately 530.9 and 532.5 eV which are attributed to the oxidation of O1s (O − ) and OH [41,44]. However, the small shift in the contribution of O − (87.08 to 87.55%) and OH (12.92 to 12.45%) were seen in the Cu/CuO counterpart (Figure 5b), which could be due to possible interactions among the constituent elements. The addition of GN to MoS 2 -Cu/CuO resulted in an additional peak (Figure 5c) at approximately 529.02 eV which presents the C-O bonding with the contribution of 15.41% [41].

Surface Morphology
FESEM images of MoS2 (Figure 6a) shows the stacked petal-like structure of MoS2 which is also consistent with previous reports [45]. The surface morphology of MoS2-Cu/CuO (Figure 6b) shows the appearance of some clusters of nanoparticles in addition to the stacked petal-like structure of MoS2 which are the Cu/CuO nanoparticles. Further, with the addition of GN (Figure 6c), flakes were observed with clustered MoS2 which is due to the possible interactions of GN with MoS2 and Cu/CuO [46].

Surface Morphology
FESEM images of MoS 2 (Figure 6a) shows the stacked petal-like structure of MoS 2 which is also consistent with previous reports [45]. The surface morphology of MoS 2 -Cu/CuO (Figure 6b) shows the appearance of some clusters of nanoparticles in addition to the stacked petal-like structure of MoS 2 which are the Cu/CuO nanoparticles. Further, with the addition of GN (Figure 6c), flakes were observed with clustered MoS 2 which is due to the possible interactions of GN with MoS 2 and Cu/CuO [46].

Photocatalytic Activity
The photocatalytic activity of the prepared MoS 2 , MoS 2 -Cu/CuO and MoS 2 -Cu/ CuO@GN was tested against the degradation of MB. The UV abortion peak of MB appeared at 665 nm and was monitored for the purpose of degradation.  Figure S2). This enhanced surface area also provides an ample active site to capture the dye molecules. Moreover, GN provided the support in electron transport and lessened the recombination of the charge carried, which resulted in the enhancement of the catalytic activity of MoS 2 -Cu/CuO@GN [48]. This change in the movement of the charge particle may affect the reaction rate constant (k) which is shown in Figure 7f

Photocatalytic Activity
The photocatalytic activity of the prepared MoS2, MoS2-Cu/CuO and MoS2-Cu/CuO@GN was tested against the degradation of MB. The UV abortion peak of MB appeared at 665 nm and was monitored for the purpose of degradation. Figure 7a  an ample active site to capture the dye molecules. Moreover, GN provided the support in electron transport and lessened the recombination of the charge carried, which resulted in the enhancement of the catalytic activity of MoS2-Cu/CuO@GN [48]. This change in the movement of the charge particle may affect the reaction rate constant (k) which is shown in Figure 7f. The order of the reaction rate constant was MoS2-Cu/CuO@GN > MoS2-Cu/CuO > MoS2 with values, respectively, of 5.01 × 10 −2 , 6.69 × 10 −2 and 9.26 × 10 −2 .

Effect of Hydrogen Peroxide
The catalytic properties of the material can be tuned by adjusting the reaction parameters, such as by introducing the scavenging of free radicals. In this regard, H 2 O 2 is commonly used for the production of reactive oxygen species such as hydroxyl radicals and superoxide during the photocatalytic reaction. These generated radicals can react with MB to enhance the photocatalytic efficiency. Therefore, we decided to investigate the photocatalytic activity of MoS 2 -Cu/CuO@GN at different concentrations of H 2 O 2 , i.e., 0, 2, 4, 6 and 8 mL. Figure 8d-

Effect of Hydrogen Peroxide
The catalytic properties of the material can be tuned by adjusting the reaction parameters, such as by introducing the scavenging of free radicals. In this regard, H2O2 is commonly used for the production of reactive oxygen species such as hydroxyl radicals and superoxide during the photocatalytic reaction. These generated radicals can react with MB to enhance the photocatalytic efficiency. Therefore, we decided to investigate the photocatalytic activity of MoS2-Cu/CuO@GN at different concentrations of H2O2, i.e., 0, 2, 4, 6 and 8 mL. Figure 8d-f show the change in the reaction kinetics of MoS2-Cu/CuO@GN with the addition of H2O2. The degradation efficiency of MoS2-Cu/CuO@GN was approximately 93.8% in the absence of H2O2, having a rate constant of 0.092.min −1 . However, the efficiency increased to 98.5% at 4% of H2O2, with the highest rate constant of 0.141 min −1 . This revealed the generation of maximum free radicals that interacts with MB during the degradation process. However, the catalytic efficiency of MoS2-Cu/CuO@GN was reduced to 96.5% and 93.1 %, respectively, for 6 and 8mL of H2O2. This showed that free radicals react with H2O2 rather than MB which ultimately reduced the photocatalytic efficiency [49].

Reusability of MoS2-Cu/CuO@GN
The reusability of the photocatalysts matters for their potential application. Therefore, the cyclic reusability of MoS2-Cu/CuO@GN was tested. The five consecutive cyclic photocatalytic experiments were performed. A total of 25 mg of MoS2-Cu/CuO@GN was added to 20 ppm of MB solution having 4% of H2O2. The sample was separated through a centrifuge (3000 rpm/min) after completing each cycle. The cyclic results (Figure 9) revealed that the photocatalytic efficiency remained at 96.3% after five consecutive cycles under the same experimental conditions.

Reusability of MoS 2 -Cu/CuO@GN
The reusability of the photocatalysts matters for their potential application. Therefore, the cyclic reusability of MoS 2 -Cu/CuO@GN was tested. The five consecutive cyclic photocatalytic experiments were performed. A total of 25 mg of MoS 2 -Cu/CuO@GN was added to 20 ppm of MB solution having 4% of H 2 O 2 . The sample was separated through a centrifuge (3000 rpm/min) after completing each cycle. The cyclic results ( Figure 9) revealed that the photocatalytic efficiency remained at 96.3% after five consecutive cycles under the same experimental conditions.

Conclusions
In conclusion, the addition of Cu/CuO and GN accelerated the photocatalytic activity of MoS2 under certain optimized experimental conditions. This enhanced the photo-

Conclusions
In conclusion, the addition of Cu/CuO and GN accelerated the photocatalytic activity of MoS 2 under certain optimized experimental conditions. This enhanced the photocatalytic activity of MoS 2 -Cu/CuO@GN, attributed to the change in the charge carrier movement, the alteration in the recombination ratio, the band gap, the interaction of the present element and the structural properties. The band gap of MoS 2 reduced to 1.5 eV from 1.7 eV. Moreover, Cu/CuO and GN supports the chemisorption absorption of the oxygen over the surface of the catalyst by providing the more active sites, which resulted in the enhanced photocatalytic activity. The structural analysis revealed the increase in the grain size of MoS 2 (2.37 nm) with the addition of Cu/CuO (13.05 nm) and GN (18.27 nm) without changing the preferred crystal orientation. The XPS analysis confirms the variation in the C-C, C-OH, C=O and OH functional groups of MoS 2 with the addition of Cu/CuO and GN, which is attributed to the enhanced photocatalytic activity. In short, this study revealed the potential use of polymer-based nanocomposites with metal, metal oxides and graphene for wastewater treatment through a facile photocatalytic process.