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 CeO2, MoS2, and CeO2/MoS2 nanocomposites are illustrated in Fig. 1. The peaks at 2θ value of 28.44, 33.05, 47.46, 56.4, 58.75, 69.37, 76.73 and 79.18˚ represent the planes (110), (200), (220), (311), (222), (400), (331) and (420) respectively. The diffraction pattern revealed the cubic structure of CeO2 which was in understanding with JCPDS 34-0394 [52]. The distinctive peaks displayed by MoS2 sheet representing the (002), (100), (103) and (110) planes reveal the hexagonal structure of MoS2, which agrees with JCPDS 37-1492 [53]. No other prominent peak implies high purity and good crystallinity of the sample.
MoS2 nanosheets comprised of 2D structure which were amorphous in nature, whereas CeO2 nanoparticles which had 3D structure were crystalline in nature (Fig. 1- MS and C). Despite of this, MoS2 characteristic patterns were not obtained in the XRD pattern of CeO2/MoS2 nanocomposites. The reason for this could be the effective intercalation of CeO2 on MoS2 sheets in the CeO2/MoS2 photocatalyst. It was observed that after the introduction of CeO2, the peaks at 2θ angle of 28.81, 33.48, 47.9, 56.84, 59.14, 69.95, 77.16 and 79.61˚ assigned to the (110), (200), (220), (311), (222), (400), (331) and (420) lanes marginally shifted towards a lower Bragg’s 2θ angle which can be presumed due to synergistic interaction between MoS2 and CeO2 [54].
Scanning Electron Microscopy (SEM)
Illustrated in Fig. 2, general morphologies of CeO2, MoS2 and CMS-4 nanocomposite can be observed. The inset images display SEM images under high magnification. CeO2 nanoparticles can be observed in Fig. 2a. Illustrated in Fig. 2b, pure MoS2 comprises of large micron spheres that are tightly combined and are flowerlike along the overlapped or coalesced nanosheet structure. Even when two nanosheets are combined, MoS2 nanosheets are dispersed at the CeO2 surface. Due to this, it tends to exist in the form of smaller nanosheets (Fig. 2c). The reason for the reduced sheets was large surface area and increase in nucleation sites provided by CeO2 which possibly results in the decreased aggregation of MoS2 nanosheets, which is helpful in enhancing the surface-active sites.
Transmission Electron Microscopy (TEM)
TEM with SAED were employed to study the morphology and particle range of CMS-4 nanocomposite. Fig. 3a-c clearly displays the formation of nanocomposite in which nanoparticles (CeO2) are evenly distributed on the surface of nanosheets (MoS2). Fig. 3c displays CeO2 nanoparticles with lateral particle size in the range of 12-22 nm. Well defined fringes with inter planar distance between lattice spacing were calculated to be ~0.303 and 0.281 nm corresponding to the (111) and (200) planes of CeO2 [55]. Another informative view of HR-TEM clear lattice fringe ~0.623 nm is ascribed to the (002) plane of MoS2 nanosheets [56]. The SAED pattern of CMS-4 nanocomposite is displayed in Fig. 3e which exhibits the crystalline nature of the composite. The indicated red dots point measured are SAED d-spacing values which are 0.3203, 0.2795, 0.1938, 0.1674 and 0.1566 nm corresponding to crystalline planes of (111), (200), (220), (311), (222) and (400) CeO2 and was matched well with XRD pattern of CeO2 (PDF-2 card no. 00-004-0593). Particle size distribution histogram confirms that the average lateral particle size of CeO2 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 CeO2/MoS2 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 3d5/2 type 880 to 900 eV, and the other 3d3/2 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 3d3/2 and 3d5/2 respectively [57]. Main XPS peaks of Ce belonged to 3d3/2 and 3d5/2 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]. Fig. 4d indicates the reduction of Mo in the synthesis of MoS2 from Mo as a precursor. The doublet binding energy for 3d3/2 and 3d5/2 were 230.5 eV and 227.4 eV respectively. Reduction of Mo from oxidation state of +6 to +4 was indicated in Mo 3d XPS spectra [58]. In the region of S 2p, two peaks are observed at 161.1 eV and 159.3 eV which corresponded to S 2p1/2 and S 2p3/2 respectively (Fig. 4e) [59].
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 CeO2, MoS2 and CeO2/MoS2 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.
Visible light degradation of MV dye in the presence of photocatalysts, CeO2 and MoS2 exhibit low degradation rate as observed in Fig. 6a. The rate of removal after 90 min of irradiation was only 47.37% and 80.8% for CeO2 and MoS2, respectively. The CeO2/MoS2 nanocomposites sample has more degradation ability than that of pure CeO2 and MoS2. The reason for this could be adequate adsorption and enough reaction sites present on the surfaces of the nanocomposites.
The CMS-4 nanocomposite attains the highest photocatalytic activity among the prepared different percentage (2, 4, 6, and 8%) composites, possessing the degradation ratio of 96.25% within 90 min. The photocatalytic efficiency of CMS-4 nanocomposite in degrading MV dye was about 2.03 times and 1.19 times than that of pure CeO2 and MoS2 correspondingly. This proves that the loading of MoS2 on the CeO2 have a critical effect on the photocatalytic activity. Moreover, it was noted that if the weight percent of MoS2 in the composite was increased beyond 4 %, noticeable reduction in the photocatalytic activity was observed. It was due to loss of heterogeneity on the catalyst surface due to excess of MoS2 in the CeO2/MoS2 nanocomposite it decreased the absorption of visible light. Whereas the MoS2 percentage in 300 mg of CeO2 is as low as 2 %, there were not enough MoS2 reached on CeO2. Further, regulation of bandgap and accelerating the mobility of carriers can be done effectively by the 3D/2D heterostructure due to which longer lifespan of the photoexcited electron-hole pairs and more effective separation can be obtained. Control of grain size of nano ranged CeO2 and its agglomeration was prevented by layering 2D MoS2 and hence more reactive sites and large surface area was attained. To differentiate the photocatalytic activity of the prepared catalyst, kinetic activity of dye degradation was conducted. The UV-Vis spectrum for a fixed concentration of MV dye with CMS-4 catalyst and varying irradiation time is illustrated in 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 (C0/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 MoS2 (0.0183 min-1) and CeO2 (0.00713 min-1) respectively. Thus, CMS-4 will considerably elevate the separation efficacy of the charge carriers, and the MV molecules are almost removed after 90 min under visible light irradiation.
Photocatalytic mechanism of CeO2/MoS2
Based on the results of this study, we propose a conceivable photocatalytic mechanism as illustrated in (Fig. 7). The molecules of CeO2 and MoS2 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 CeO2 was 2.66 eV, which was higher than that of MoS2 for which the energy gap was 1.81 eV. The MoS2 was activated under visible light illumination and the photogenerated electrons are being transferred from the surface of MoS2 to the surface of CeO2. The conduction band’s electrons (e-) generate the ·O2- radicals along with oxygen (O2) molecules. The holes (h+) in the valence band works with OH-group to generate a hydroxyl radical (·OH). Further reaction of ·O2- groups in water, leads to the formation of hydroxyl radicals [60]. Oxidization of MV was done by the hydroxyl radicals to form CO2, H2O along with other small-sized non-polluting molecules. We believe that CMS-4 nanocomposite possesses extraordinary photocatalytic performance due to the 3D/2D heterojunction structure which encourages the separation and transfer of the electron-hole pairs; based on the results
Statistical Optimization of degradation study
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 finding important effects and interactions (Table S3). 95 % confidence in statistical significance is suggested as per the p-value which was less than 0.05 in the ANOVA. F-test was applied for evaluation of statistical significance within confidence interval of 95% [61, 62]. Data analysis of semi-empirical expression for % of MV removal is presented as:
% of MV removal = 95.309 + 2.344 Z1 + 1.217 Z2 + 2.531 Z3 - 3.44 Z1*Z1 - 1.95 Z2*Z2 - 4.94 Z3*Z3 - 0.062 Z1*Z2 - 0.187 Z1*Z3 + 0.570 Z2*Z3
Experimental values of degradation percent were compared to the empirical values and plotted (Fig. S1). The outcomes of ANOVA (Table S3) indicate that the significance of the model (p < 0.01) with F- value of 103.853 [61]. The predicted R2 of 93.24% was in reasonable agreement with the adjusted R2 of 85.72 % [63]. Yet, the Non-significant lack of fit was good. Lack of fit F-value was 6.69 and this implies that the lack of fit was insignificant concerning the pure error and can be used for further analysis [64, 65]. Henceforth, this analysis allowed the use of a response surface for modelling the design space. All factors exert noteworthy effect on light intensity since all of them have a p-value of less than 0.05. 42.01 was the adequate precision which implies a requisite quadratic model. The maximum MV degradation (96.25 %) of the photocatalyst was 20 mg and the MV dye concentration was 20 mg/L for a time period of 90 min.
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. 8, S8 shows the most appropriate design response surfaces and depicts the MV degradation response percentage of the surface compared to major variables [66-68]. Presented graphs were plotted for a given pair of factors at optimum conditions and other variables were fixed. The relation amongst the variables is indicated by the art of the plots. Fig. 8a, S8a shows the degradation percentage of MV with the amount of photocatalyst (Z1) and the concentration of MV dye (Z2). The enhancement in degradation percentage of MV at a higher dose of the amount of photocatalyst (Z1) was noticed which may attribute to increase in reaction sites. The response surfaces plots Fig. 8b, S8b demonstrated the degradation percentage of MV as a function of the amount of photocatalyst (Z1) and time for degradation (Z3) while their interaction is considered. The percentage removal increased with enhancement with the amount of photocatalyst because of its small particle size and high specific surface area. The suggestively increase in rate of adsorption was observed at higher values. At a lower value of the amount of photocatalyst, due to insufficient reactive sites and a lower ratio of dye molecules to vacant sites, percentage degradation significantly decreased. The effect of concentration of MV dye (Z2) versus time for degradation (Z3) on the degradation percentage with its effect on factors are presented in Fig. 8c, S8c. It was observed that, in spite of increasing the of concentration of MV, its degradation efficiency 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 CeO2/MoS2 composites
Stability and reusability account as critical factors for photocatalysts in real-world applications. The photocatalytic degradation process was repeated five 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 five recycles in the degradation of MV dye solution. Since the catalyst prepared is insoluble in water, the mass loss during the recycling process was insignificant. 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. Thus, we can infer that 3D/2D heterojunction of CeO2/MoS2 owns exceptional photocatalytic recyclability and excellent stability in real-world applications.