Biosynthesis of 3D/2D Ceo2/Mos2 Nanocomposites with Enhanced Photocatalytic Activity to Degrade Organic Dye in Wastewater and Statistical Optimization of Reaction Parameters

Nanocomposites synthesized by alternative approaches like biosynthetic methods are safer than those prepared by traditional chemical techniques. Further, this approach is both economically and environmentally feasible. In this study, we report an eco-friendly methodology for preparing cerium dioxide/molybdenum disulphide (CeO 2 /MoS 2 ) nanocomposites. Moringa oleifera peel was used as the reducing/stabilizing agent for synthesizing CeO 2 nanoparticles. The prepared nanocomposite were characterized using FT-IR analysis, SEM and EDAX analysis, TEM and SAED pattern analysis, X-Ray Diffraction Pattern, Zeta Potential, UV-Visible Diffuse Reectance Spectra, X-Ray Photon Spectroscopy and Photoluminescence spectra. Particle size and morphology were characterized by TEM and SEM. The photocatalytic pursuit of CeO 2 /MoS 2 was explored by the degradation of methyl violet (MV) under visible light irradiation. Our methodology proved to be 96.25% effective in the degradation of MV. Further, we used this Response Surface Methodology for enhancing the process factors like volume of photocatalyst, time for degradation and concentration of MV.


Introduction
Persistent organic pollutants may cause chronic toxicity, which can be potentially carcinogenic and teratogenic [1]. Treatment of organic pollutants in wastewater is an urgent requirement and has drawn extensive attention because of its degradation resistance due to chemical stability [2]. The most common organic pollutants are derivatives of benzene, phenols, halogenated hydrocarbons, and organic dyes [1,3]. These organic molecules are highly toxic and are chemically stable which facilitates their entry into the food chain [4] via aquatic organisms [5]. For the disposal of organic pollutants, many technologies such as biodegradation [6], adsorption [7], ultra-ltration, electrochemical, photochemical, chemical oxidation, electrochemical combustion [8] and other conventional techniques [9] have been used in recent past. The photocatalytic technique is an advanced oxidation process (AOP) that has been effectively used for the treatment of organic pollutants in water bodies. The advantages of this widely accepted technique are that it is cost-e cient, environment friendly and is highly e cient [10].
Photoactive materials facilitate the use of renewable source of energy (from solar light) to provide more effective and cleaner removal of organic pollutants. Extensive studies have been conducted in recent past on various compounds such as TiO 2 [11][12][13], CdS [14], ZnO [15,16], SnO 2 [17], α-Fe 2 O 3 [18], Fe 3 O 4 [19], g-C 3 N 4 [20] and BiVO 4 [21] which have excellent performance as photocatalysts. Several two-dimensional (2D) materials such as graphitic carbon nitrile and graphene have been the centre of attraction for many years [20]. Low bandgap for AOP and antibacterial activity makes 2D materials perfect binding agents for wastewater treatment applications. Recent studies have introduced a 2D structure, molybdenum disulphide (MoS 2 ) which has attracted signi cant heed in the area of photocatalytic degradation [22], catalysis [23], photonics, energy technology [24], material sciences, biosensors [25] and micro-electronics [26]. Good biocompatibility and low toxicity are major factors for the increased use of MoS 2 in environmental applications. Due to the layered structure of MoS 2 , it possesses a large surface area, suitable band gaps, chemically inertness, lubrication, catalysis, and anisotropy. Although, MoS 2 has been used as photocatalyst [27][28][29][30], high recombination rate with narrow band gap of the electron-hole pair restrict its photocatalytic activity. Rare earth oxides of ceria are a widely accepted photocatalyst and has been researched for a long time [31]. Cerium Oxide (CeO 2) nanoparticles can be synthesized in multiple ways, which primarily includes electrochemical [32], chemical reduction [33], microemulsion [34], microwave [35], and hydrothermal [36,37], etc. Some of these methods uses carcinogenic chemical compounds and have complex reaction conditions. CeO 2 seems to be a promising candidate like TiO 2 and ZnO as it has desired properties such as non-toxicity, enhanced chemical stability, cost e ciency and easy synthesis. Although CeO 2 has been a good supporter for catalyst [38], it has some drawbacks as a photocatalyst due to limited light absorption ability, wide band gap, and a higher rate of recombination [39]. As mentioned earlier, CeO 2 is a good supporter of catalyst [38]. Therefore, CeO 2 can be an ideal candidate for combining with MoS 2 because of its larger surface area, proper band edge and suitable lattice match [40][41][42][43][44].
In the present study, we report the preparation ( rst-time) of CeO 2 /MoS 2 composites with the help of methanolic peel extract of the Moringa oleifera (M. oleifera). Various characterization techniques examined the parameters affecting the structure and morphology of the nanocomposites. Photocatalytic activity of the CeO 2 /MoS 2 nanocomposites has been determined using organic pollutants such as methyl violet. Response Surface Methodology was applied to optimize the process parameters like amount of photocatalyst, time for degradation, and concentration of MV dye [45,46].

Experimental
Materials and Methods M. oleifera was procured from Vellore Market, Vellore, Tamil Nadu, India. Botanical Survey of India, Coimbatore performed the peel authentication (BSI/SRC/S/23/2013-14/tech.1116). Thiourea, tartaric acid, ethanol, sodium hydroxide, ceric ammonium nitrate, ammonium molybdate, hydrazine hydrate, methyl violet, and aqueous ammonia were purchased from Sigma Aldrich, India. Analytical grade chemicals were utilized as received without any prior treatment. The solutions were prepared using Milli-Q water.
Peel extract preparation of M. oleifera M. oleifera was collected from marketplace in Vellore, Tamil Nadu, India. The peel separation was performed using the traditional method i.e., peels of drumstick was collected and processed [35]. The peel of M. oleifera was detached from the drumsticks. This peel was cleaned using distilled water numerous times and then powdered using an electrical grinder. In accordance with the maceration protocol, 100 g of dried powder was treated with 500 mL methanol in a glass beaker. Distillation under controlled temperature was done to concentrate the solvent to a syrupy viscosity. This extract was used as a stabilizing agent for the preparation of CeO 2 nanoparticles.

Green synthesis of CeO 2 nanoparticles
The protocol was followed as per the earlier report with little modi cation [35]. Ceric ammonium nitrate solution of 0.1 M was prepared and kept on magnetic stirrer. Methanolic M. oleifera peel extract was subjected. dropwise in the ceric ammonium nitrate solution until a pH in the preferable range of 8 to 9 was obtained. The colour of the solution turned light yellow. This solution was kept in the microwave at 300 W power, 60 °C for 15 min. The CeO 2 particulates formed visibly and were left to settle. After some time, these particulates were washed with water and ethanol. The washing of CeO 2 particles was essential to remove organic and inorganic impurities. Water removes the inorganic impurities such as unreacted ceric ammonium nitrate, whereas ethanol removes the organic impurities which might form during the working of the microwave. Washing with water also helps in achieving the desired pH. After washing the CeO 2 particulates, it was centrifuged at 4000 rpm and dried using a vacuum oven at 60°C for 3 h. This dried product obtained was crushed and kept in a mu e furnace at 450°C for 3 h. This process is known as calcination. The dried product obtained was Cerium oxide (CeO 2 ), which was greenishyellow in colour.

Synthesis of 2D MoS 2 sheet
Synthesis of MoS 2 nanosheets were conducted by following the method reported by Gradkar et al. [47].
Experimental details of "Synthesis of 2D MoS 2 sheet" are presented in Supplementary Information (SI).

Preparation of composites
Composites of CeO 2 /MoS 2 were prepared by xing mass of CeO 2 at 300 mg and MoS 2 mass was varied at 2, 4, 6, and 8% (of 300 mg). MoS 2 and CeO 2 were weighed and added in 50 mL distilled water. This mixture of composites was stirred on a magnetic stirrer for about 30 min. After stirring, ultra-sonication was done at 300 W power, 27000 frequency for 15 min. Mixtures were separated and centrifuged at 6000 rpm. After centrifugation the sample was kept dry for 24 h at 60 o C. The dried product so obtained was CeO 2 /MoS 2 composite. Ratio of composite composition was tabled below.

Characterization
The absorbance was recorded using UV-Visible spectroscopy Hitachi U2910. Functional groups present were examined using FT-IR spectroscopy SHIMAZDU infrared spectrophotometer (4000 to 400 cm −1 ; resolution: 1 cm −1 ). KBr was used in the ratio of 1:100 (sample to KBr weight ratio) to prepare solid thin lm. The crystallinity was analysed using Power XRD, Bruker (Germany, model D8). The XRD data was recorded for 2θ values lying in the range of 10 to 90 degrees. TEM and SAED pattern were recorded on TEM Philips CM-200 with operating voltage of 20-200 kV and 2.4 Å resolution. Surface morphology was analysed on SEM/EDX (JEOL Model JSM -6390LV). Zeta potential was analysed on Horiba Nanoparticle Analyzer (Model No. SZ 100). The XPS was computed using Physical Electronics equipent with model no.
PHI 5000 Versa Probe III. The signal of C 1s was put at 285.2 eV.

Photocatalytic degradation
Experiments involving photocatalytic degradation were carried out in a Haber multi-lamp photochemical reactor. This photochemical reactor can provide a visible light source at 300 W using a cut-off lter in order to eliminate undesirable wavelengths (<420 nm), to make sure that only visible light is used during irradiation. Decomposition of the MV dye in the existence of visible light proved e ciency of the CeO 2 /MoS 2 nanocomposites in photocatalysis. 20 mg of the nanocomposite samples were transferred to 100 ml solution of MV dye at a concentration of 20 mg/L to study the photodegradation e ciency of nanocomposites. The suspension was stirred in darkness to achieve an equilibrium of sorptiondesorption for 30 min. Thereafter, the solutions were irradiated with a visible lamp and the temperature kept constant at 25 °C . The solution samples were collected after every 15 min and kept in dark. The concentrations of MV were monitored by measuring the absorbance against the wavelength via UV−vis spectrometer to record maximum absorbance wavelength (l max ). With the decrease of concentration, the peak height decreased.

Central composite design (CCD)
RSM is an analysis which combines both statistical and mathematical techniques for optimizing, developing, and improving a process and allows to determine and evaluate the relative signi cance and interaction of all variables used. The experiment is especially designed for optimization of affective variables to enhance characteristics performance and reduce the experimental errors [48][49][50][51]. Important variables are selected using minimum number of trial runs. Three variables were used for making the CCD which are-the amount of photocatalyst, time for degradation, and concentration of MV dye. The relation between the independent variables were estimated as: Where y denotes the degradation percentage of MV (response), XiXj denotes the independent variables. The linear coe cient is βi, β 0 is constant for the model and the quadratic coe cients are βii, βij. They are the cross-product coe cients. MV dye concentration of 10.0-30.0 mg/L has been selected. The amount of photocatalyst was in the range of 10 to 30 mg and time was in the range of 75 to 105 min. CCD was used to evaluate the required number of runs for the experiments. The number of iterations used were 20.
In this investigation, the CCD of 3 factors with 4 central points were considered. The alpha axial value was picked as rotatable alpha 6.0 and the levels of the factors were determined as axial points (inscribed). One replicate was used with one response.
Results And Discussion X-Ray Diffraction Pattern (XRD) The powder X-ray diffraction was conducted to study the crystalline structure, phase, purity, and composition of the synthesised nanocomposites. The XRD patterns of CeO 2 , MoS 2 , and CeO 2 /MoS 2 nanocomposites are illustrated in Fig. 1 331) and (420) respectively.
The diffraction pattern revealed the cubic structure of CeO 2 which was in understanding with JCPDS 34-0394 [52]. The distinctive peaks displayed by MoS 2 sheet representing the (002), (100), (103) and (110) planes reveal the hexagonal structure of MoS 2 , which agrees with JCPDS 37-1492 [53]. No other prominent peak implies high purity and good crystallinity of the sample. Transmission Electron Microscopy (TEM) TEM with SAED were employed to study the morphology and particle range of CMS-4 nanocomposite. no. 00-004-0593). Particle size distribution histogram con rms that the average lateral particle size of CeO 2 nanoparticles were 15.4 nm its standard deviation was found out to be 2.99 (Fig. 3f).

X-ray Photoelectron Spectroscopy (XPS)
XPS was used to study the surface chemical compositions and valence state of CeO 2 /MoS 2 nanocomposite. Fig. 4a shows the survey scan for CMS-4 nanocomposite and peaks for Ce, O, Mo, S, C elements can be observed. XPS scan for individual elements are present in Fig. 4b-e.
Due to the mixed valence state of Ce, many peaks were observed as illustrated in Fig. 4b. Two core XPS level groupings, one 3d 5/2 type 880 to 900 eV, and the other 3d 3/2 set 900 to 920 eV were observed. The binding energy of the XPS 914.5 and 886.5 eV peaks of the Ce +4 equals 3d 3/2 and 3d 5/2 respectively [57].
Main XPS peaks of Ce belonged to 3d 3/2 and 3d 5/2 at 903.0 and 883.1 eV, correspondingly. Oxygen peaks in nanocomposite for O 1s can be observed in Fig. 4c. Due to the asymmetry in O 1s region, presences of two types of oxygen species are predicted. Lattice oxygen is attributed to the presence of a strong peak in 530.3 eV and a peak of 527.1 eV to chemisorbed oxygen on the Nanocomposite's [57].

Photocatalytic Activity
Experiments prove that the degradation of dyes (such as MV) was not possible in the absence of photocatalyst obtained under visible light (Fig. 5b). Hence, photodegradation of MV dye was done under visible light using CeO 2 , MoS 2 and CeO 2 /MoS 2 as photocatalysts. In order to monitor the progress of photodegradation reactions, absorbance of irradiated dye solution is measured (Fig. 5). The previous literatures were compared with the present study as shown in Table S1.  Fig. 5a. The shift of the peak ranging from 584 nm to 580 nm for the maximum absorption for MV was due to the cyclo-reversion and azo group removal. As the irradiation time increased, more MV dye molecules degraded, resulting in the decrease in dye concentration; consequently, the adsorption intensity of the light decreased. The more the dye molecules in the solution more will be the absorption intensity.
As illustrated in Fig. 6b, the MV dye degradation kinetics was followed the following equation, ln (C 0 /C) = -kt.
The value of k for CMS-4 nanocomposite (0.0365 min -1 ) was 13.1 times and 7.2 times more as that of Photocatalytic mechanism of CeO 2 /MoS 2 Based on the results of this study, we propose a conceivable photocatalytic mechanism as illustrated in (Fig. 7). The molecules of CeO 2 and MoS 2 get excited instantly during visible light irradiation to yield holes (the characteristic property of photocatalysts) and photogenerated electrons in their respective valence band (VB) and conduction band (CB). The energy gap between conduction band and the valence band for CeO 2 was 2.66 eV, which was higher than that of MoS 2 for which the energy gap was 1.81 eV. The

Explanation of regression analysis
In CCD, the experiments were planned randomly to minimize the effect of uncontrolled variables. As illustrated in the Table S2, 2, three independent variables -the amount of photocatalyst as Z1, the concentration of MV dye as Z2, and time for degradation as Z3 was introduced into the 3 levels (low, basal and high) coded as (-1, 0, +1) respectively. Responses of all 20 experiments obtained were unveiled in Table 2. Analysis of variance (ANOVA) was computed using MINITAB17 for nding important effects and interactions (Table S3). 95 % con dence in statistical signi cance is suggested as per the p-value which was less than 0.05 in the ANOVA. F-test was applied for evaluation of statistical signi cance within con dence interval of 95% Experimental values of degradation percent were compared to the empirical values and plotted (Fig. S1). The outcomes of ANOVA ( Optimization by employing RSM approach In the next unit RSM was developed in order to increase the critical factors for describing the nature of the responding surface in the experiment, taking into account all remarkable interactions in the CCD.  Fig. 8c, S8c. It was observed that, in spite of increasing the of concentration of MV, its degradation e ciency decreases. The ratio of concentration of solute to unoccupied reactive adsorbent sites is low at lower concentration of dye and dye adsorption is accelerated, thus enhancing dye degradation. On the other hand, saturation of adsorption sites leads to low adsorption yield at high concentrations. Conversely, dye degradation percentage was high at lower initial concentration of dye while degradation was low at higher initial percentage. This indicates that initial concentration is important for adsorption of dyes. Reusability and stability of CeO 2 /MoS 2 composites Stability and reusability account as critical factors for photocatalysts in real-world applications. The photocatalytic degradation process was repeated ve times and the results are presented (Fig. 9a). The motive for this repetitive experiment was to check the activity-stability of the nanocomposites prepared. The photocatalytic activity of CMS-4 does not have any observable change even after ve recycles in the degradation of MV dye solution. Since the catalyst prepared is insoluble in water, the mass loss during the recycling process was insigni cant. To notice the stability and reusability, the photocatalyst samples were characterized using XRD before and after the photocatalytic degradation process. It was observed that after the photocatalytic degradation process was completed, the intensity of catalysts remained unchanged as shown in Fig. 9b        Photocatalytic mechanism of CMS-4 nanocomposite sample. Figure 8