Solar Power Light-Driven Improved Photocatalytic Action of Mg- Doped CuO Nanomaterial Modified with Polyvinylalcohol

Department of Chemistry, M.S.S. Wakf Board College, Madurai 625020, Tamil Nadu, India Research Department of Chemistry, Sadakathullah Appa College, Tirunelveli 627011, Tamil Nadu, India Department of Biochemistry, College of Sciences, King Saud University, Riyadh, Saudi Arabia Department of Chemistry, National College (Autonomous), Tiruchirappalli 620001, Tamil Nadu, India Department of Chemistry, Jairams Arts and Science College, Karur 639003, Tamil Nadu, India PG & Research Department of Chemistry, C.P.A. College, Bodinayakanur 625513, Tamil Nadu, India Department of Medicine, University of Missouri, Columbia, Missouri 65201, USA PG Department of Chemistry, Sri S. Ramasamy Naidu Memorial College, Sattur 626203, Tamil Nadu, India Department of Pharmacy, University of Asia Pacific, Dhaka, Bangladesh


Introduction
Nanotechnology has resulted in the creation of a variety of tiny materials. Nanoparticles are a broad category of materials that include particulate substances with a minimum diameter of 100 nm. In recent years, nanotechnology has grown across all fields of science and technology including engineering, medicine, pharmaceuticals, agriculture, and vomiting, and respiratory infections. Water contamination, excretion of harmful organic pollutants from chemicals, coating, paper printing, pharmaceutical, leather, pulp, dyeing, and textile industries are common examples of environmental pollution [3][4][5][6][7]. These organic pollutants can be degraded using semiconductor photocatalysts [8] and transformed to CO 2 , H 2 O, and other tiny molecules [9], which do not change the environmental situation. For example, Abbasi-Asl et al. published a heterostructure photocatalyst for organic pollutant degradation utilizing solar light, while Oliveira et al. described TiO 2 nanoparticles for organic dye degradation. A plethora of studies on the breakdown of organic contaminants have been proposed [10][11][12][13]. Furthermore, standard procedures for the removal of organic contaminants from the water will be developed to ensure that the environment is pollution-free.
Semiconductor-based nanocomposites have recently piqued interest in a variety of scientific and technological domains [14][15][16][17]. CuO and its composites have emerged as a viable candidate in the field of photocatalysis among semiconductor materials. It has a small band gap energy of 1.2-1.6 eV, as well as stable, abundant, nontoxic, and economical physical and chemical features [18][19][20]. It has potential uses in technology such as energy conversion, environmental remediation, sensors, photocatalysts, solar cells, and optical devices [21,22]. Several investigations on CuO as a photocatalyst have been conducted in the previous decade [23]. For example, Sahu et al. [24] described an Au-CuO hybrid material for methylene blue degradation, while Raina et al. [25] discussed the synthesis of Cu nanoparticles for the same. Copper oxide nanoparticles have also been employed in plenty of applications with solar energy conversion, CO 2 reduction, and solar cells [26][27][28].
Furthermore, nanoparticles produced from copper oxide have sparked numerous curiosity in the dissemination of photocatalysis. Sun et al. [29], for example, reported Mg/ CuO/Fe 2 O 3 heterogeneous catalysts for the dissociation of organic pollutants, Mg acting a predominant part in the destruction of organic pollutants in this work. Since then, it has aided in the development of morphological structure, high crystallinity, catalytic ability, oxygen vacancy, and storage. Selvam et al. [30] also explained why Mg-doped ZnO photocatalysts function so well. As a result, a Mg-doping catalyst could be a good way to boost the performance of CuO nanoparticles in photocatalysts. Despite various Mg-doped semiconductors having been described for the damage of organic pollutants, polymer-based nanocomposites for the demolition of organic dyes have attracted a lot of attention [31] because polymers are adjustable in size, shape, chemical structure, ordered nanoparticle assembly, biocompatible, electron promoter, and low in cost [32,33]. To construct the nanocomposites for the dissociation of organic compounds like PVA (polyvinylalcohol) [34,35], chitosan [36,37], and cellulose [33,38,39], a suitable number of natural and synthetic hydrophilic polymeric materials were utilized. Among the hydrophilic material, PVA is a hydrophilic polymer that is nontoxic, inexpensive, readily available, and biocompatible, has excellent thermal strength, is water soluble, can produce fine flexible films, and has been described to have noble photocatalytic activity in wastewater management [32,[40][41][42]. As a result, the PVA-modified CuO-Mg composite has outstanding metal-doped metal oxide and polymer properties and so could be a smart, promising composite for effective photocatalysts.
In this work, CuO-Mg nanoparticles were produced using a sonochemical technique and then modified with PVA. The efficiency of photocatalytic movement of manufactured nanomaterials was investigated under solar light illumination. With remarkable reusability, the CuO/Mg/ PVA composite may achieve effective catalysis and continuous photodegradation of organic dye. Methylene blue (MB) was brought from Merck Chemicals, India. All the chemicals were of analytical grade and employed as received without refinement. Double distilled (DD) water was used during the experiment.

Sonochemical Synthesis of Mg-Doped CuO (MC)
Nanoparticles. The sonochemical approach was used to make magnesium-doped copper oxide nanoparticles. Separately, 2.41 g of Cu(NO 3 ) 2 . 3H 2 O and 1.01 g of MgCl 2 . 6H 2 O were dissolved in 100 mL of DD water, and the solution was well mixed and agitated for 10-15 min using a magnetic stirrer. 0.4 g NaOH was dissolved in 100 mL DD water and dropped into the aforementioned solution drop by drop. The resulting solution was treated with ultrasound for 1 hour at room temperature, using a titanium-made high-intensity ultrasonic horn probe sonicator and modifying the ultrasound frequency (20 kHz and 100 W/cm 2 ). The resultant black precipitate was separated using centrifugation after the irradiation procedure and washed with water and ethanol and dried at 80°C for 1 hour. The final product was calcined for 1 hour at 400°C. In the absence of MgCl. 6H 2 O, copper oxide nanoparticles were also produced using a similar approach.

Preparation of Mg-CuO-PVA (MCP) Nanocomposite.
Mg-CuO-PVA nanocomposite was synthesized by simple chemical impregnation method. 2 g of previous synthesized Mg-CuO nanoparticle in 100 mL of ethanol was blended with 2 g of PVA in 100 mL ethanol by ultrasonication for 1 hr. After 12 hrs of stirring, the solvent was left, and the product was dried at 80°C and heat treatment at 120°C in a hot air oven for 2 hrs.
2.4. Measurement. The functional group was determined using JASCO FT/IR-4200 spectroscopy, and the optical property of produced nanocomposites was measured using a JASCO V-750 spectrophotometer. The Raman spectrum was created by a 532 nm LabRAM HR evolution Raman spectrometer. XPERT PRO X-ray with Cu K radiation of wavelength 1.541 was employed to investigate the crystalline phase of produced nanomaterials. Using a JOEL model JSM 6701FSEM, the SEM-EDX outcome of produced 2 Journal of Nanomaterials nanocomposites was investigated. HR-TEM pictures were captured using a JOEL 3010 microscope. FLUROCUBE-JOBIN YVON spectrofluorometer fluorolog-3 with 450 W xenon lamp is utilized photoluminescence (PL) research. TGA was analyzed using a TGA-50 SHIMADZU analyzer in the range of 1 to 1000°C at 5°C/min in N 2 atmosphere. Micromatrics, ASAP 2020 BET instrument, was employed to find the surface area, and XPS measurement was recorded by AXIS-NOVA system. A JASCO V-530 UV/VIS spectrometer was used to study the absorbance spectrum.
2.5. Photocatalytic Activity Measurement. The photocatalytic actions of manufactured nanomaterials in the decoloration of methylene blue (MB) dye under sunshine were examined. In this experiment, 300 mL of 20 μM MB solution was used to disperse 0.2 g of produced nanomaterials. The resulting mixture was agitated in a slutty type batch reactor for 15 minutes in the dark to accomplish an adsorptiondesorption equilibrium before being illuminated with sunlight. The degradation of MB dye was measured regularly by centrifuging a 5 mL sample of the reaction mixture. The supernatant solution was inspected by a UV-vis spectrometer. The consistent degradation of MB was studied by illumination time, catalyst dosage, and initial concentration of MB dye. The decoloration of the dye from the solution was estimated using the following relation.
where C o is the dye concentration before illumination and C represents the dye concentration after a certain time illumination. Moreover, Tauc's plot is employed to investigate the band gap in prepared nanomaterials by following relation (1).

Results and Discussion
where α, C, hυ, and E g bulk are absorption coefficient, constant, photon energy, and band gap, respectively. Figure 1 shows Tauc's plots of CuO, MC, and MCP. The CuO has an optical band gap of 1.84 eV, which is related to its intrinsic band distance. This means that photocatalytic activity is predominantly restricted to the UV light spectrum. Further-more, the band gap energies for MC and MCP are determined to be 1.23 and 0.67 eV, respectively. As a result, the PVA-incorporated MCP nanocomposite has a small band gap energy and higher optical conductivity than other nanocomposite materials, allowing it to have outstanding photocatalytic activity. Furthermore, the absorption peaks measured at 1144.28, 1465.44, and 2858 cm -1 in the FT-IR spectra of MCP nanocomposite are pertained to the C-O, CH 2 , and C-H groups stretching of PVA, respectively. Furthermore, the wide peak for the O-H stretching mode of PVA exists at 3380.57 cm -1 [44][45][46]. Clearly, the nanocomposite shows a less positive shift in wavenumber in the FT-IR spectrum of MC (Figure 2

Raman Analysis. The Raman spectra of CuO, MC, and MCP nanocomposites were presented in Figures 3(a)-3(c).
In Figure 3(a), the Raman peaks noticed at 296.44, 342.08, and 630.26 cm -1 are related to the Ag, B g 1 , and B g 2 mode of CuO [47]. Then, the Raman peaks of MC (Figure 3 The intensity of all diffraction peaks appears to be reduced when Mg is doped in MC and MCP. The Mg combines with the CuO structure due to a somewhat lower angle shift in the diffraction peaks. In addition, in sample MCP [43], a large peak was discovered at 18.32°, which corresponded to the crystalline phase of PVA. This validated the presence of PVA on the CuO-Mg nanomaterial's surface. The XRD patterns also reveal that PVA intercalation does not affect the monoclinic phase's crystal structure. Scherrer's formula (1) is used to compute the average crystallite size.
where D denotes the average crystallite size, β denotes the peak's FWHM, K is the form of a particle factor (it = 0:89), and θ and λ are the incident angle and wavelength of X-rays, correspondingly.       [29]. The peaks that appeared in the Cu spectrum (Figure 8(a)) at 933. 48 (Figure 8(b)) [50].
On the other hand, the O 1 s region was obtained at 530.96 eV for MCP nanocomposite and illustrated in Figure 8(c). This oxygen energy peak is characteristic of a metal oxide peak [51].
PL spectroscopy is noteworthy to study the segregation performance of the photogenerated electron-hole pair in a semiconductor nanomaterial. Figure 9 shows PL spectra of CuO, MC and MCP nanocomposites. Two intense peaks at 480.1 and 537.4 nm are noticed and PL intensity is feeble. The diminished fluorescence intensity designates that photoelectrons and holes reunion have been effectively reduced, while increased photocatalytic activity is predicted. With Mg and PVA doping, the PL intensity gradually decreased. This suggests a decreasing rate of electron-hole recombination. In other words, the MCP nanocomposite has a greater separation efficiency of photogenerated electrons and holes than other nanomaterials.
The phase purity was employed by thermal gravimetric measurement. The TG curves of CuO, MC, and MCP are disclosed in Figure 9. The weight loss was observed from   is followed by the prepared CuO nanoparticles (Figure 10(a)). The interaction between N 2 gas and CuO (adsorbate-adsorbent) is weak in this case. However, because the tiny pore of CuO does not fill with the adsorbate N 2 gas, this type of isotherm is uncommon. The type-V adsorption isotherm curve of MC is shown in Figure 10(b), indicating a poor contact between the adsorbent and adsorbate on the CuO-Mg surface. The stacking of Mg on copper oxide hints to the development of several porous adsorbents. Accordingly, the surface region was amplified while doping.
Moreover, Figure 10(c) shows type-IV adsorption isotherm curves of MCP. This reveals the individual hysteresis loop and indicates the mesoporous platform of MCP agreeing the IUPAC classification. The structure of nanocomposites has altered from macroporous to mesoporous, as can be noticed. This is because interconnected macroporous allows guest molecules to adsorb by attaching the pore wall locations. CuO, MC, and MCP, respectively, have surface areas of 28.12, 76.67, and 165.43 m 2 /g. The surface area of MCP nanocomposite increases dramatically when PVA is added. This means that PVA aids in the reduction of polarity in CuO-Mg nanoparticles, resulting in better dispersion and smaller clumps. The formation of aggregates of a few nanoparticles eventually resulted in a surface area increase. Among the synthesized nanomaterials, MCP has a brilliant surface area (165.43 m 2 /g) and a more dynamic destination given by PVA. It can shows the brilliant and ameliorate character in photocatalytic action [43].      Journal of Nanomaterials Mg doping, the % of behaviour of the aforesaid catalyst falls in the direction of 95.71%, 80.53%, and 49.72%, respectively. Furthermore, PVA's distinct adsorptive activity and wider intermolecular space were mostly responsible for the significant augmentation of MCP nanocomposite [43]. In comparison to the other samples CuO and MC, the combined impact of Mg and PVA modified with CuO nanoparticles generates an astounding photocatalytic efficiency. Furthermore, the results of the blank studies to assess the effect of photolysis on MB were found to be minimal (8%). The obtained results of degradation of organic dye compared with the already reported values are given in Table 1. Among the reported values, CuO/Mg/PVA nanocomposite shows higher efficiency of degradation of organic dye. The computation of the potential level of band edge for the two semiconductors is essential in the determination of the photoexcited charge transporter in the coupled/doped semiconductor. The valance band (VB) and conduction band (CB) edge position of MCP nanocomposite can be quantified by the accompanying equation.
where X refers to absolute electronegativity, stated as the geometric mean of the absolute electronegativity of the constituent atoms, which is represented by the arithmetic mean of the atomic electron affinity and 1 st I.E. E VB indicates the energy of VB edge potential, E CB is the CB edge potential, E g stands for band gap energy of the semiconductors, and E e is nothing but free electron on the H scale (~4.5 eV). Consequently, the band edge potential of E VB and E CB of MCP nanocomposite area unit is computed to be 3.21 and 0.93 eV, severally.
For the improved photocatalytic movement of MCP nanocomposite, the following mechanism has been proposed. Figure 12 shows a schematic picture of photodegradation of MB in MCP nanocomposite caused by sun radiation. The breakdown of MB under solar light illumination was used to assess MCP's increased photocatalytic activity [63]. Because prior studies [64,65] found that sunlight was more effective than alternative irradiation strategies for dye degradation. When the photocatalyst is illuminated by solar light, PVA modified on the surface of CuO-Mg can be simply excited and generate portable  [29]. This consequences in the creation of numerous reactive oxygen species (O 2 •and • OH), which stimulates the destruction of MB. The benefit of PVA and Mg doping is that it improves electron-hole segregation and increases quantum yield by accelerating electrons from the catalyst to molecular oxygen quickly.
3.8.1. Effect of pH. The pH of the solution is a significant factor that determines dye molecule adsorption. The surface charge of the nanocomposite can influenced by pH changes. The degree of ionization of the adsorbate molecule and the surface behaviour of the adsorbent are affected by the pH solution and variations in pH cause variations in the degree of ionization of the adsorbate molecule and the surface behaviour of the adsorbent [66]. At 30°C, the consequence of pH solution on dye adsorption was investigated by maintaining the dye concentration at 3 μM and the adsorbent dosage at 0.05 g/L. The experiment was carried out with pH levels ranging from 3 to 9. The outcomes are depicted in Figure 13. The percentage of dye elimination increased consistently from 80.23% to 96.18% with a change in pH   Journal of Nanomaterials from 3 to 8 attributable to the better electrostatic interaction between the positively charged dye molecule and contrarily charged nanocomposite. Beyond the pH 9, removal of dye decreased to 90.75%, this is owing to the change in electrostatic fascination or aversion between MB and catalyst, and hence, the optimal pH for degradation dye MB was set to be 8.

Effect of Photocatalyst
Dosage. The effect of photocatalyst dosage was also investigated to find the optimal dosage of MCP nanocomposite by adjusting the catalyst dosage from 0.1 to 0.4 g/L while maintaining the MB concentration at 3 μM. The outcomes are exhibited in Figure 14. When the photocatalyst dosage is enlarged from 0.1 to 0.3 g/L, the photodegradation of MB rises. When more photons are adsorbed and highly reactive oxygen species (O 2 •and •OH) are generated for photodegradation, the accessibility of the absolute surface region and active sites of the catalyst increases. Nevertheless, photodegradation tendency diminished at large photocatalyst dosage (beyond 0.3 g/L) owing to the deficit of surface area and active sites manipulated by agglomeration of nanoparticles as well as an increase in the obscurity of the suspension, which drives infiltration of light via the solution. Thus, only a lesser number of photons will meet the catalyst surface. Hence, the creation of reactive oxygen species is most extreme; therefore, the photocatalytic performance is shriveled.   Figure 15. It is seen that the photodegradation % of MB diminished with an increment in MB concentration. It tends to be clarified dependent on Beer-Lambert's law. As indicated by the law, the pathway length of photons incoming into the sample diminished with expanding of MB concentration, and trivial photons approach the surface of the catalyst. Many MB species are adsorbed on the catalyst surface when the MB dye concentration was higher. As a result, MB molecules completely ring the active places on the catalytic surface. From the outcomes, as the dye concentration grew, the photodegradation efficiency declined. The optimal initial MB concentration of 0.3 μM is achieved for improved photodegradation of MB.
3.8.4. Kinetic of MB Photodegradation. The kinetic of MB degradation under solar light for CuO, MC, and MCP nanocomposites was analyzed. The degradation proficiency of MB dye is quantified by utilizing the Langmuir-Hinshelwood formula.
where C o is the initial concentration of MB at t zero min and C is that the concentration of MB at illumination time "t" is portrayed in Figure 16. From Figure 16, photodegradation reaction implies pseudo 1 st -order kinetics. The corresponding    3.8.5. TOC and COD Measurement. To track the toxicity of the photodegradation dye solution, TOC and COD analyses were conducted. The outcomes are revealed in Figure 17. MCP nanocomposite outperformed all other photocatalysts in terms of removing the most TOC, COD, and degradation. The maximal deterioration, COD, and TOC, respectively, are 88%, 72%, and 64%. The explanation for the modest quantity of TOC and COD removal in organic dyes is that more organic dyes are mineralized during the photocatalytic process.
3.8.6. Repeatability and Reusability. Because of the fundamental characteristic of a brilliant catalyst and any possible saturation influence, the same catalyst is employed by photocatalytic decoloration consistently, and reusability and stability are other important aspects for practical use. Figure 18 illustrates the percentage of MB degradation vs. experimental cycles of CuO, MC, and MCP nanocomposites, which were analyzed for 5 cycles to constant catalyst infinite quality (0.2 g/L) at ideal pH. The photocatalytic activity of M6P did not alter much. Even after five cycles, the degradation percentage of MB is above 80%. Because of the high charge separation efficiency of PVA and Mg on CuO nanoparticles, M6P nanocomposite has superior catalytic activity than other components. The XRD pattern of the M6P nanocomposite ( Figure 19) was examined after the consecutive cycles. The M6P nanocomposite remained untouched. As a result, the M6P photocatalyst can remain stable throughout photocatalytic damage of MB in the existence of solar light.

Conclusion
Novel sun-powered light-driven CuO/Mg/PVA nanocomposite was prepared successfully by combined sonochemical synthesis and chemical impregnated route. Interfacial modifiers such as Mg and PVA have little effect on the crystal structure, as shown by XRD patterns and SEM data. The presence of Cu and Mg elements was confirmed by EDX spectra. Under solar light illumination, the CuO/Mg/PVA nanocomposite exhibits unique photocatalytic characteristics. The photocatalytic degradation of MB has been determined to be 95.71 percent efficient. CuO/Mg/PVA nanocomposite has increased photocatalytic activity owing to the reduction of electron-hole reunion in CuO through PVA and Mg. The photocatalytic destruction of MB under solar light has been presented as a plausible mechanism. The reaction parameters have been tuned, resulting in maximal photocatalytic degradation. Furthermore, the photocatalyst CuO/Mg/PVA nanocomposite is proven by XRD analysis to be steady without dissipating of action even later 5 cycle investigation. The results show that all these properties are improved after doping CuO with Mg and PVA. Among all the three samples, CuO/Mg/PVA nanocomposite has demonstrated an excellent photocatalytic performance in the degradation of MB dye. Hence, this sample can be ideally employed for the purification of water by degrading the methylene blue present in it.

Data Availability
All relevant data are included within the article.

Disclosure
This paper was performed as part of the employment of the authors.

Conflicts of Interest
All authors declare that there is no conflict of interest.