PEG-200 Assisted Sonochemical Synthesis of Cerium (Ce<sup>3+</sup>) Doped Copper Oxide (CuO) Nano-Composites and Their Photocatalytic Activities

CuO nanocomposites with different Ce 3+ doping concentrations (0, 0.5, 1.0, 1.5, 3.0 and 5.0 mol%) were synthesized by PEG-200 assisted facile sonochemical method. The as-synthesized composites were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and UV-visible absorption spectroscopy. Further, these composites were evaluated for photodegradation activities towards MB dye under sunlight irradiation. The XRD results demonstrated that CuO nanocomposites exhibits monoclinic phase and the crystallinity decreases with increasing Ce 3+ doping concentration. The as-synthesized nanocomposites exhibited vesicular morphology with diameters ranging from 50 to 100nm. UV-visible absorption spectra results demonstrated that these nanocomposites exhibit strong absorption in the visible region and the absorption intensity increases with increasing Ce 3+ doping concentration. The photocatalytic experiments using as-synthesized nanocomposites for degradation of Methylene blue (MB) dye revealed that, compared to undoped CuO; Ce 3+ doped CuO nanocomposites exhibited improved photodegradation ability. The photodegradation rate was maximum for 3.0 mol% Ce 3+ doped CuO which showed 98% degradation within 180 mins under sunlight irradiation. Recycling experiments demonstrated good stability of as-synthesized nanocomposites even after threere cycles.


Introduction
Usual nondestructive nature towards target contaminant associated with the traditional wastewater treatment technologies have led to lose their importance. Additionally, these traditional techniques such as ultra-filtration, precipitation and adsorption generate secondary pollutants by just transforming contaminants from water to other phases [1][2][3][4][5][6]. Recently, advanced oxidation processes (AOP's) have emerged as a potential technique capable of destroying the wide range of organic contaminants [7][8][9][10][11][12]. Comparatively, the greater popularity of this technique is due to its better efficiency and employment of non-toxic, cost-effective and readily available semiconductor photocatalysts.
Among various semiconductor photocatalysts employed in photocatalysis, TiO 2 and ZnO are widely explored and highlighted by various researchers [13]. However, the use of these materials still possesses some drawbacks such as TiO 2 can be utilized under UV light only and the ZnO exhibits low quantum efficiency due to very fast recombination. Since, CuO is a low cost and can be a good alternative to TiO 2 and ZnO; recently researchers are trying to explore possibility of employing CuO based materials for photocatalytic applications. CuO is p-type semiconductor having 1 [19][20][21]. It has been reported that, photoactivity of a semiconductor photocatalyst can be well improved by doping with certain cation [22].  [27]. As can be seen from the extensive literature search, there are very less reports on synthesis of Ce doped CuO nanomaterials and their possible application in photocatalysis.
Considering above discussion, herein present study we are reporting PEG-200 assisted facile sonochemical method for synthesis of undoped and different mol% Cerium (Ce 3+ ) doped CuO nanocomposites. The as-synthesized nanocomposites were characterized by XRD, FESEM and UV-visible absorption spectroscopy and were further tested for sunlight mediated dye degradation of MB dye.
In a typical experiment; 0.02 mol copper acetate and 0-5.0 mol.% cerium nitrate were dissolved in 75 ml double distilled water premixed with 5% PEG-200. Subsequently, 0.2 M NaOH was slowly added into the solution till attainment of pH 10-11. The as-formed grey colored suspensions were then ultrasonically treated for 01h using sonicator probe at amplitude 35% and energy 40 kW. During sonication, the probe was operated 5 sec pulse ON and 2 sec pulse OFF. Post reaction, the precipitate was centrifuged, washed with distilled water and ethanol several times and finally dried in oven at 60°C for 24h.

Characterization
Rigaku Miniflex X-ray diffractometer with CuKα irradiation operated at λ=1.5406Å was employed for X-ray diffraction (XRD) analysis of as-synthesized samples. The surface morphology of as-synthesized samples was analyzed using a JEOL-JSM 6700F field-emission scanning electron microscope. For UV-visible absorption spectra analysis JASCOV-570 spectrophotometer was employed by transferring certain volume of ZnO suspension in a quartz cuvette.

Photocatalytic Activities
Photocatalysis experiments were carried out in 250 ml pyrex beaker under sunlight irradiations using as-synthesized catalysts. Prior to sun-light irradiation, adsorption-desorption equilibrium with MB dye molecules was ensured by agitating the suspension in dark for 30 mins. The extent of MB dye removal was analyzed by measuring changes in absorbance at 663nm.

XRDAnalysis
X-ray diffraction patterns of CuO and Ce 3+ doped CuO with variable cerium contents are shown in Figure 1. All the diffraction peaks match to standard data for monoclinic CuO structure (JCPDS Card. No.05-0661) [28]. The diffraction peaks appearing at 2θ=32.  220) and (004) diffraction planes of monoclinic CuO. Beyond these peaks, no additional peaks related to Ce or cerium oxides were detected within the detection limit of XRD instrument, which further suggests that Ce 3+ ions are entered into the CuO lattices by substituting Cu 2+ or existed in amorphous forms. However, the incorporation of Ce 3+ ions into CuO lattice has no effect on the crystal structure.

Intensity (a.u.)
A close observation of diffraction Ce doped CuO within the diffraction angle ranging from 2θ = 30-40°C (shown in Figure 2), indicates that the diffraction for (002) and (-111) planes of Ce doped CuO samples were slightly shifted to lower angles compared to those of undoped CuO. The shift towards lower angles was increased with increasing concentration of Ce 3+ doping contents.
The observed change in lattice constant by increasing Ce contents can be attributed to difference in ionic radii of dopant Ce 3+ (1.034A°) [29,30] and host Cu 2+ (0.73A°) [31]. These results further confirmed the substitution of Cu 2+ ions by Ce 3+ and successful doping of Ce 3+ ions into the host matrix [32].

FESEM Analysis
The microstructure and morphology of as-synthesized undoped and Ce doped CuO nanocomposites were further analyzed by FESEM analysis. Low and high magnification images of as-synthesized products are shown in Figure 3. As can be seen from the FESEM results, the undoped and Ce doped CuO samples exhibits nearly identical morphology. These consist of vesicular morphology having dimensions in the range of 50-100 nm. The surface of these composites is rough. Additionally, it can be observed that there is no change in size of CuO nanocomposites upto 3.0 mol.% Ce doping concentration, exceeding which agglomerated structures are observed to be formed.

UV-Visible Absorption Spectra Analysis
The UV-visible absorption spectra of as-synthesized undoped and Ce doped CuO nanocomposites are shown in Figure 4. From figure all the as-synthesized composites shows a weak absorption peak in UV region in between 200-250 nm. However, the absorption in the visible region was high for all the as-synthesized nanocomposites. Additionally, it can also be observed that, as the Ce doping concentration increases the intensity of visible light absorption also increases. These results further indicate that as-synthesized nanocomposites can be used for photodegradation making use of sunlight.

Photocatalytic Activity
In present study, the photocatalytic activities of assynthesized CuO and Ce-CuO nanocomposites was studied using MB dye as test contaminant. Figure 5 depicts changes in absorption spectra of MB dye during photodegradation in presence of 3.0 mol.% Ce doped CuO nanocomposites. It can be seen from figure that, the intense absorption maxima at 664 nm corresponding to MB having intensity 1.79 a.u. decreases slowly with the sunlight irradiation time and completely diminished within approximately 180-200 mins.  Figure 6 shows the curves for MB dye degradation with different as-synthesized catalysts. As can be seen from figure, in absence of any catalyst for blank experiment, MB dye washardly degraded. The degradation performance was achieved in presence of catalysts. Undoped CuO hardly degraded 49% of initial MB dye within 180 mins. The MB dye degradation was improved with Ce doped CuO catalysts and 3.0 mol.% Ce-CuO exhibited the highest degradation efficiency capable of degrading 98% MB dye within 180 mins.
Gradual decomposition of MB dye evident from Figure 5 suggests the slow degradation of MB dye molecules over the surface of as-synthesized CuO and Ce-CuO catalysts. The enhanced degradation in presence of Ce doped CuO catalysts can be attributed to the dopant Ce 3+ . During photodegradation, the Ce 3+ ions present in the CuO lattice can get oxidized to Ce 4+ by releasing electrons which can further lead to formation of superoxide radicals (O 2 -) by reacting with adsorbed O 2 . The oxidized Ce 4+ species can act as electron scavenger or acceptor to trap photoexcited electrons from conduction band (CB) of CuO and consequently reduced to Ce 3+ . Thus, the electron (e -)-hole (h + ) recombination can be greatly reduced leading to enhanced photocatalytic activities. The data of photodegradation experiments were further studied for MB dye degradation kinetics. The kinetics of MB dye degradation by as-synthesized catalysts can be described by pseudo first-order kinetics.
Where; K app is the apparent rate constant (min −1 ), C 0 is the initial MB dye concentration and C t is the MB dye concentration at sunlight exposure time t. The values for K app can be calculated by extrapolating a linear fitting to the natural logarithm (C 0 /C t ) versus the corresponding irradiation time and the results are presented in Figure 7.
Repeated use of a catalyst while maintaining a constant activity determines the practical applicability of a catalyst. Therefore, the stability of catalyst to maintain constant activity over repeated use is an important criterion. Herein, present study the stability of photocatalytic activity was also studied by re-use of catalyst (3.0 mol% Ce-CuO) in fresh MB solution under sunlight irradiation. Figure 8 shows the degradation extent for consecutive four cycles using 3.0mol.% Ce-CuO (80 min sunlight irradiation for each cycle). As can be seen from figure, during first reuse, 98.0% of MB was degraded by 3.0 mol.% Ce-CuO. With subsequent reuse, the extent of MB dye degradation decreased to 94.0% and 91.0% for second and third recycles respectively. The decreased photocatalytic activity of 3.0mol.% Ce-CuO to some extent after every recycle can be considered due to handling losses of photocatalyst.

Conclusion
Ce 3+ doped CuO catalysts were prepared by PEG-200 assisted facile sonochemical method. The as-synthesized nanocomposites are having vesicular morphology belonging to monoclinic CuO structures. The absorption intensities were found to increase with the increasing Ce 3+ ion doping concentrations. Ce 3+ doped CuO nanocomposites exhibited improved photocatalytic activities for degradation of MB dye under sunlight irradiation. 3.0 mol.% Ce-CuO exhibited the highest photocatalytic activity for MB degradation leading to 98% degradation within 180 mins, among the tested catalysts. These results further suggest potential candidature of as-synthesized catalysts for degradation of organic contaminants.