Solvent free fabrication and thermal tuning of copper oxide-zirconium dioxide nanocomposite for enhanced photocatalytic efficacy

Recently, several methods has been used for the synthesis of bimetal oxide nanocomposite, however, very few studies are available on the solvent free mechanochemical synthesis of nanomaterials. In this study, mortar and pestle assisted fabrication of copper oxide-zirconium dioxide nanocomposite (CuO–ZrO2 NC) was carried out and was calcined at 300, 600 and 900 °C. The variation in crystallographic parameters was examined through x-ray diffraction (XRD) and the crystallite size was found to be gradually increased with increasing calcination temperature. The morphological changes with increasing calcination temperature were traced during scanning electron microscopy (SEM) analysis. The percentage elemental composition was verified through energy dispersive x-ray (EDX) spectroscopy whereas the functional group analysis was done through Fourier transform infrared (FTIR) spectroscopy, where the intensity of peaks assigned to hydroxyl moiety decreased with increasing calcination temperature. The CuO–ZrO2 NCs were used as a photocatalysts for the degradation of the Fluorescein in the presence of solar light and highest photodegradation (77.27%) was noticed for the CuO–ZrO2 NC calcined at 900 °C.


Introduction
In the past 30 years, there has been a growing global apprehension regarding public health consequences associated with environmental pollution [1].Water pollution is on the rise as a result of increased environmental pollution.According to the report presented by UNESCO on behalf of UN-Water at the UN 2023 Water Conference in New York, approximately 2 billion individuals worldwide, constituting 26% of the global population, lack access to safe drinking water [2].One of the primary sources of water pollution is heavy industrialization and unmonitored human activities, which result in the release of several hazardous substances, including heavy metals, dyes, pharmaceuticals, pesticides, fluoride, phenols, insecticides, and detergents, into water bodies [3].The widespread utilization of organic dyes in the textile industry generates substantial volumes of heavily contaminated wastewater, and the release of untreated influents is the main reason for water pollution [4][5][6].Water pollution leads to the emergence of various diseases including hepatitis, cholera, dysentery, cryptosporidiosis, giardiasis, diarrhea, typhoid, and even to cancerous conditions [7].Fluorescein (C 20 H 12 O 5 ) is an organic dye that exhibits strong fluorescence when dissolved and exposed to ultraviolet (UV) light.This bright fluorescence makes fluorescein valuable for a variety of applications, including tracing fluids, highlighting markers, creating fluorescent toys, and detecting leaks.Despite this, it's important to note that fluorescein are toxic and may cause an allergic reaction called anaphylaxis marked by low blood pressure, rapid heart rate, wheezing, hives, and itching [8].Various methods have been known to remove organic dyes from wastewater including equalization, sedimentation, biological processes involving bacteria-assisted, algae-assisted, fungiassisted, yeast-assisted, enzyme-assisted biodegradations; chemical processes including advanced oxidation processes (AOPs), coagulation-flocculation, electrochemical treatments including electrocoagulation (EC), electro-fenton (EF), anodic oxidation (AO); and physical processes including adsorption, membrane filtration, microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), ion exchange method, hybrid treatments including PMR and MBR hybrid technologies [3,9].However, these treatment methods may prove ineffective, sometimes, due to the diverse range of dyes, being costly in nature, substantial sludge generation, slow reaction rate, and self-decomposition [10].Beside these techniques, photocatalytic process was first used in 1911, when a German chemist Dr Alexander Eibnor used Zinc Oxide and sunlight to bleach a dark blue pigment [11].This more effective the previously listed methods as it is cost effective, environment friendly, and energy efficient phenomenon [11,12].
The CuO-ZrO 2 NCs exhibits interesting properties due to the synergy between its individual components.As the CuO NPs is a p-type semiconductor with a narrow band gap of 1.2 eV and are known for their high electrical conductivity and catalytic activity [13,14], while ZrO 2 NPs is an n-type semiconductor with wide band gap (5-7 eV) offer excellent chemical stability, thermal resistance, and oxygen storage capacity [15,16].When combined at the nanoscale, CuO-ZrO 2 NCs can possess enhanced electrical conductivity, catalytic performance, thermal stability, and gas sensing properties [17].In environmental remediation, the CuO-ZrO 2 NCs have been utilized for the degradation of organic pollutants, removal of heavy metals from wastewater, and purification of air and water systems [18,19].Various methods have been employed for the preparation of CuO-ZrO 2 NCs, including sol-gel synthesis, co-precipitation, hydrothermal synthesis, and mechanochemical methods [20].Among these, the mechanochemical method stands out as a particularly effective approach, especially in the absence of solvents.This method involves the mechanical milling of precursor materials, leading to intimate mixing and enhanced reactivity between CuO and ZrO 2 .Compared to solvothermal and solgel methods, mechanochemical synthesis offers several advantages, including simplicity, scalability, and environmental friendliness.Moreover, the solvent-free nature of mechanochemical synthesis reduces the risk of solvent contamination and simplifies the purification process.
In this research, CuO-ZrO 2 NCs have been fabricated using a mechanochemical process, a technique that involves the treatment of solids and integrates mechanical and chemical interactions at the molecular level.To the best of our knowledge, for the first time, we introduced the Mortar-Pestle-assisted mechanochemical process for the fabrication of bimetallic nanopowder.Given that this method is more efficient, faster, and more straightforward compared to conventional approaches, it represents a superior strategy for the preparation of bimetallic nanocomposites.

Reagents
To prepare CuO/ZrO 2 mechanochemically, Copper (II) hydroxide and Zirconium (IV) hydroxide were employed.We acquired these chemicals of general-purpose grade from Fisher Scientific.The oxides were ground using a mortar and pestle.Distilled water was employed for cleaning the mortar and pestle.

Synthesis of CuO-ZrO 2 NCs
In order to mechanochemically synthesize CuO-ZrO 2 nanopowder, Copper (II) hydroxide and Zirconium (IV) hydroxide were employed as precursors.Following the standard procedure, 1 gram of each metal hydroxide precursor were ground using a mortar and pestle.The grinding process persisted for a duration of six hours, with careful monitoring of any observable physical changes.The initial product displayed a granular texture.Through prolonged grinding, the product transformed into a powdered state.Following three hours, the sample developed stickiness, a result of hydroxide bond breakdown and the presence of moisture.Approaching the sixhour mark, the product reverted to a powdered state, accompanied by the formation of metal oxides.Subsequently, the resulting powder was securely stored in a tightly sealed sample bottle.The experiment was repeated multiple times to assess the impact of grinding duration on the progress of reaction.Additionally, physical attributes like melting point and solubility were assessed.The final product underwent calcination in a muffle furnace at temperatures of 300, 600, and 900 °C.

Instrumentation
The CuO-ZrO 2 NCs were subjected to various physico-chemical analyses for characterization.The crystalline structure was evaluated using x-ray diffraction (XRD) on a Philips X'Pert model, and the crystallite size was calculated using the Debye-Scherrer equation.Microstructure and surface topography were examined using a scanning electron microscope (SEM), particularly the JEOL JSM-5600LV model from Tokyo, Japan.Elemental composition was confirmed through EDX analysis (model INCA-200 (UK)).Surface functional groups were investigated using the Nicolet 560 FTIR model in the range of 4000-400 cm −1 , and FTIR-ATR analysis was conducted in the range of 4000-500 cm −1 to study surface functional moieties.UV-vis spectra were obtained using a 1601 SHIMADZU UV-Visible spectrophotometer.

Photocatalytic activity assay
The photocatalytic activity of all the prepared samples, including CuO-ZrO 2 in both its uncalcined and calcined forms at 300, 600, and 900 °C, was examined in the process of degrading Fluorescein dye.To begin, a 15 ppm stock solution of Fluorescein dye was created using distilled water.Subsequently, 100 ml of this solution was transferred to a reaction vessel, and 20 mg of the synthesized samples was introduced into it.To achieve adsorption-desorption equilibrium, the reaction mixture was stirred in darkness for 30 min.Following exposure to simulated solar radiation for a specific duration, the sample was meticulously assessed using a double beam spectrophotometer, and changes in the absorbance maxima were recorded over time.

XRD analysis
The XRD analysis of all the CuO-ZrO 2 NCs was carried out and the XRD patterns are shown in the figure 1.All the XRD patterns of the CuO-ZrO 2 NCs (uncalcined and calcined samples) exhibits the diffraction bands at 32.61, 35.58, 38.72, 48.78, 53.44, 61.61, 65.86, 66.24, 68.15, 75.35 correspond to the Miller index of (110), (111̅ ), ( 111), (202̅ ), (020), (−113), ( 022), ( 311), ( 220) and (222̅ ).All these bands are match well with the data listed in JCPDS card (96-901-6106) and are correspond to the monoclinic geometry of CuO (tenorite mineral) of with space group of C12/c1 and space number 15.The length of three coordinates i.e. a, b and c are 4.6833, 3.4208, 5.1294 Å and the interfacial angles i.e. alpha and Gamma are of 90°and beta is equal to 99.57°.The density of the unit cell is 6.51 g cm −3 and volume of 81.00 × 10 6 pm 3 .The crystallite size was calculated through Debye-Scherrer's equation (equation ( 1)), where D is crystallite size, K is Scherer constant (0.98), λ is wavelength (1.54), θ is diffraction angle where β is full length at half maxima (FWHM).The crystallite size with lattice strain for the uncalcined is 28.3(0.402%)whereas for the crystallite size for the samples calcined at 300, 600, and 900 °C are 42.4 nm.The lattice strain for the samples calcined at 300, and 600 °C is 0.268 percent and 0.266 percent lattice strain in the sample calcined at 900 °C.

SEM analysis
The morphological study of the CuO-ZrO 2 NCs was conducted through SEM, the obtained images are posted as figure 2, that provide valuable insights into the structural changes that occurred during different stages of the synthesis process and calcination.The SEM image of the as-prepared CuO-ZrO 2 NC (figure 2(a)) reveals that the after completing the grinding process, the sample are unevenly distributed.No specific morphological shape of the sample is seen in the image and suggests the presence of smaller particles in a loosely packed arrangement.The sample is appeared to be granular in nature.The grain size lies between 35.47 and 52.81 nm, with an average size of 44.43 nm.When the CuO-ZrO 2 NC was calcined at 300 °C (figure 2(b), the size of are seem to be increased and to due to high surface energy are the small grains are likely to fused together led to the formation of larger particles.Through gains have no definite morphological shape, however, some of the grains are seem to have nearly spherical shape.The overall condensation and growth of particles indicate the beginning of structural changes in the sample.The size of the grain ranges between 37.60 nm and 53.11 nm, with an average size of 47.43 nm.Upon further increasing the calcination temperature up to 600 °C, the sample become more condensed and the grain size further increased (figure 2(c)).The increase in size is accompanied by a reduction in empty spaces within the sample as the grains fill in these voids.The morphology of the grains becomes more defined, although still lacking uniformity in size and shape.The size of the grain ranges between 35.50 nm and 72.47 nm, with an average size of 47.43 nm.When the calcination increased up to 900 °C (figure 2(d)), the specific morphological shape of the grains become clearer, where some of the grains exhibit spherical, nearly spherical oval and elongated shapes, indicating a diverse range of structures within the sample.The formation of aggregates is observed, likely due to the fusion of particles, contributing to a significant increase in the overall grain size.The size of the grain ranges between 55.98 nm and 75.3 nm, with an average size of 63.23 nm.The SEM results highlight the dynamic morphological changes in CuO-ZrO 2 NCs during the calcination process.The evolution from unevenly distributed granular particles to well-defined, larger grains with various shapes provides valuable information for understanding the structural development of the CuO-ZrO 2 NCs.

EDX analysis
The EDX spectra of CuO-ZrO 2 NCs uncalcined and those calcined at 300, 600, and 900 °C are shown in figures 3(a)-(d).The EDX spectra of all the fabricated samples show that the peak at 0.5 keV indicates the presence of oxygen in the samples, likely associated with oxides of Cu and Zr.The peaks for Cu appeared at 0.9, 8.05, and 8.9 keV, and those for Zr at 2.1 and 2.5 keV, confirming the desired elemental composition of the samples.In the EDX spectrum, the peak intensities provide information about the relative abundance of a specific element.The decrease in the intensity of the Zr peak suggests a reduction in the weight percent of Zr, whereas the increase in Cu peak intensity indicates a higher weight percent of copper as given in table 1.This suggests that with increasing calcination temperature, the weight percent of Zr and Cu changes in the samples.The observed changes in peak intensities are consistent with the idea that calcination at higher temperatures results in a shift in the elemental composition towards a higher weight percent of copper and a lower weight percent of zirconium.This could be attributed to the thermally-induced transformations, such as phase transitions, crystal growth, or evaporation of volatile components.The homogenization process might be more pronounced at elevated temperatures, leading to a more uniform distribution of elements in the nanocomposites.For the synthesis of Cu-ZrO 2 NCs, 1 g of the respective metal hydroxide was utilized, which is equal to 0.011 moles of Cu(OH) 2 and 0.006 moles of Zr(OH) 2 , respectively.At elevated temperatures, the condensation reaction that occurred led to the evaporation of H 2 O molecules, leading to a loss of mass and potentially influencing the observed weight percent of elements in the nanocomposite.A larger number of hydroxyl groups are present in Cu(OH) 2 (per mole), and their loss would result in an increase in the weight percent of Cu relative to Zr.As these hydroxyl groups dehydrate and the mass loss would be a larger percentage of the starting mass for Cu(OH) 2 compared to Zr(OH) 2 .This can lead to a relatively higher weight percent of Cu in the final product after accounting for the mass loss.
Moreover, the Cu-ZrO 2 NCs undergoes crystal growth during calcination process, the arrangement of atoms within the crystalline lattice may favor the incorporation of certain elements over others.Depending on the growth conditions and crystallographic preferences, Cu with small atomic size may be more readily incorporated into the crystal lattice as compared to Zr, leading to variations in their weight percent in the EDX analysis.Moreover, crystal growth kinetics can influence the final composition.Copper oxide might nucleate and grow faster than zirconium oxide under the applied experimental conditions.As larger CuO crystals form, they can physically hinder the growth of ZrO 2 crystals, leading to a relatively lower abundance of zirconium in the final product [21].

FTIR analysis
The FTIR spectrum shown in figure 4 exhibited peaks at specific wavenumbers (cm −1 ), signifying the positions where the transmission of infrared radiation took place.Broad band at 3462.38 cm −1 was attributed to the stretching collision of H-O-H and hydroxyl absorption [22].The peak observed at 1622.16 cm −1 resulted from   The presence of distinctive peaks at 522 cm −1 and 590 cm −1 signaled the formation of Cu-O stretching vibrations [28].The influence of the calcination temperature was evident in this range, as the increasing temperature had led to the merging of peaks, resulting in the emergence of a broad band in this region.Identical peaks were observed in the same regions across all the samples but their intensity gradually diminished with the increasing calcination temperature.The bands at 3462.38 cm −1 exhibited a significant reduction in intensity in samples subjected to calcination at 300, 600, and 900 °C.Additionally, a pronounced decrease in intensity was observed in the bands at 1622.16 cm −1 in the calcined samples, along with a similar diminishing trend in the peaks at 1329.95 cm −1 and 1169.90 cm −1 .Furthermore, a gradual decline in the peak at 746.38 cm −1 was noted consistently across all samples.This reduction occurred as a result of a condensation reaction, causing the evaporation of water molecules and the conversion of metal hydroxides into metal oxides.

Photocatalytic activity
The photocatalytic efficacy of CuO-ZrO 2 NCs, synthesized through the mechanochemical approach, was assessed for both uncalcined samples and those subjected to various calcination temperatures, using fluorescein dye as the test substrate.The evaluation occurred in an outdoor setting under natural sunlight conditions from June-2023 15 to 30, between 11 a.m. and 3 p.m.The gradual loss of the greenish-yellow color over time signified the decolorization of the reaction mixture.After conducting UV-visible analysis using a double-beam spectrophotometer, there was an initial pronounced decrease observed in the absorbance maxima at 475 nm, indicating the degradation of the chromophore responsible for light absorption at that specific wavelength.The degradation pattern shown in figure 5(a) portrayed a continuous reduction in the absorbance maximum with the rise in calcination temperature.During the first 60 min, there was a noticeable and sharp decline in the absorbance maxima, especially in the sample calcined at 900 °C compared to both, those subjected to different calcination temperatures and the uncalcined sample.Afterward, the rate of decline decelerated in all samples.
The sample calcined at 900 °C exhibited the most significant decrease in the absorbance maxima.
The degradation percentage of Fluorescein was calculated using equation (2), and the outcomes are depicted in figure 5(b) for various CuO-ZrO 2 NCs, encompassing both uncalcined samples and those calcined at different temperatures [29].The uncalcined nanocomposites exhibited a degradation of 50.83%, while those calcined at 300, 600, and 900 °C displayed 58.33%, 67.5%, and 77.27% degradation of fluorescein, respectively.These findings highlighted the comparative ability of each sample to degrade fluorescein under the given experimental conditions.The results revealed that among all the prepared samples, the nanocomposite calcined  3) and indicated the rate of the photochemical reaction [30].According to the results, the rate constants for the photocatalytic reactions conducted in the presence of uncalcined nanocomposites and those calcined at 300, 600, and 900 °C were determined to be 0.00744, 0.0089, 0.01402, and 0.0153 per minute, respectively.These values provided insights into the relative photocatalytic efficacy of all prepared samples in facilitating the degradation of fluorescein under the given reaction conditions.A higher rate constant denotes a faster degradation rate, indicating improved photocatalytic activity in the synthesized CuO-ZrO 2 .The results suggested that among the examined samples, the nanocomposite calcined at 900 °C exhibited the highest rate constant, measuring 0.0153 per min.This also implied that nanocomposites calcined at 900 °C could potentially be more efficient in promoting the degradation of the targeted compound.
The photodegradation mechanism of fluorescein, facilitated by a CuO-ZrO 2 catalyst under solar light, involved a series of steps driven by the interplay between the catalyst, solar light, and the dye molecule (figure 5(d)).Solar light, containing photons, was absorbed by the catalyst, promoting the generation of electron-hole pairs (e − / h + ) within the catalyst material.In this scenario, CuO-ZrO 2 functioned as a semiconductor; the absorbed photons elevated electrons from the valence band (VB) to the conduction band (CB) of the catalyst, creating electron-hole pairs.The excited electrons accumulated in the VB of CuO, while the positive holes gathered in the VB of ZrO 2 .These charge carriers played a pivotal role in subsequent redox reactions.Hydroxyl radicals ( • OH) were produced through the reaction of photogenerated holes with water molecules, whereas the photogenerated electrons reduced oxygen molecules, forming superoxide radicals ( • O 2 − ) that further reacted with hydrogen to generate hydroxyl radicals.Highly reactive hydroxyl radicals ( • OH) then attacked the adsorbed fluorescein molecules, resulting in the breakdown of chemical bonds in the dye and the mineralization of the dye into water and carbon dioxide [28].
3.6.Factors effecting the photocatalytic activity 3.6.1.Effect of pH on % degradation The pH of the fluorescein solution was adjusted using 0.1 M HCl and 0.1 M NaOH.The impact of pH variations on the percentage of dye degradation was closely monitored (figure 6) ZrO 2 exhibits a positive charge under acidic conditions (pH < 6.63) and a negative charge in alkaline environments (pH > 7.0) [31].Same feature is revealed by CuO.Furthermore, within the pH range of 4.3 to 6.4, Fluorescein is present in its monoanionic form, transitioning to its dianionic form at a pH of 6.4.Findings indicated degradation percentages of 54.39% at pH 4, 68.35% at pH 5, 77.27% at pH 6, 72.18% at pH 7, and 63.79% at pH 8.The data indicated that in an acidic solution, attractive forces between dye and catalyst favored the adsorption of the dye onto the photocatalysts surface.This preference was attributed to the presence of the dye in an anionic form in acidic conditions, while the catalyst is in a cationic form.This facilitated a substantial interaction between them, ultimately leading to the adsorption of the dye and consequently causing a higher percentage of degradation which became optimal at 6.Moreover, it was deduced that at elevated pH levels, the ZrO 2 surface acquired a negative charge, causing repulsive forces that reduced adsorption.Consequently, by raising the solution's pH beyond 7, a significant portion of the adsorbed dye molecules did not directly contact the photocatalysts surface, leading to decreased degradation rates.

Effect of dose on % degradation
The degradation of fluorescein was conducted with various catalyst doses (5, 10, 15, 20, and 25 mg) as shown in figure 7. The findings revealed a proportional increase in percent degradation as the catalyst dose rose.The rate of degradation for the decomposition of the dye is discovered to rise with an increase in the concentration of the catalyst [32].The peak activity, observed at a catalyst dose of 20 mg for the degradation of Fluorescein dye in the CuO-ZrO 2 NC, implied an optimal concentration for the photocatalytic process.At higher doses, total surface area of the NC increased which enhanced the active sites that interacted with dye.Upon increasing the dose of CuO-ZrO 2 NC from 5 to 20 mg, % degradation of Fluorescein increased from 44.55% to 77.27% while at 25 mg, it declined to 72.86%.The rise in degradation percentage with an elevation in the catalyst dose can be ascribed to the augmented total surface area of NCs, leading to an increase in active sites that engage with the dye [28].As the number of active sites on NCs increase, more reactive radicals are produce which improve degradation.But the decrease in dye degradation at higher dose i.e. at 25 mg indicated the formation of intermediate byproducts.Moreover, at higher doses, agglomeration of NCs could occur which resulted in possible declined degradation.Less efficient light absorption by the catalyst as well as ineffective interaction of photons with the catalyst surface might be the plausible reasons for this occurrence.

Effect of initial concentration on % degradation
The photocatalytic degradation of Fluorescein was carried out with varying initial concentrations (10, 15, 20, and 25 ppm), resulting in degradation percentages of 70.56, 77.27, 68.95, and 56.21 percent, respectively (figure 8).An increase in the percentage of dye degradation was observed as the initial dye concentration rose up to 15 ppm.However, with a further increase in the initial concentration, a subsequent decrease in the percent degradation was observed.The highest degradation efficiency (77.27%) was attained at an initial concentration of 15 ppm, indicating an optimal condition for enhanced photocatalytic activity.
With higher concentrations, the active sites of CuO-ZrO 2 NCs were fully occupied and saturated, leading to a decrease in the percentage of degradation.Elevated concentrations could potentially disturb the interaction of light with the catalyst surface, resulting in a diminished generation of hydroxyl ions and, consequently, a decline in the efficiency of photocatalytic degradation.Furthermore, the generation of degradation byproducts, which compete with Fluorescein for the limited number of active sites on the surface of the NC, and the presence of reactive radicals could be potential reasons for this decline.

Effect of recycling on % degradation
The endurance of the CuO-ZrO 2 catalyst under prolonged exposure to photocatalytic conditions was a critical consideration.To assess its stability, the CuO-ZrO 2 catalyst underwent four consecutive reuse cycles, with  percent degradation recorded at 77.27%, 71.28%, 62.71%, and 49.68% for cycles 1 through 4, respectively (figure 9).The initially high degradation efficiency (77.27%) was likely attributed to the catalyst's pristine state, featuring optimal available active sites.The subsequent gradual decline in efficiency may be linked to surface contamination, minor structural alterations, or initial catalyst deactivation.
The catalyst's degradation efficiency may experience a slight decline, attributed to both the mass loss of the catalysts and the dye absorbed on the photocatalyst's surface during the recycling process [33].Moreover, during reuse, the catalyst surface could accumulate contaminants from the reaction environment, resulting in a decline in its catalytic activity.Catalyst deactivation could also occur across multiple cycles due to various factors such as agglomeration, leaching of active species, or structural changes, potentially reducing the number of active sites available for the photocatalytic reaction.

Conclusion
An easy, environmental friendly and economical approach was used for the fabrication of CuO-ZrO 2 NCs, where the successful fabrication of CuO-ZrO 2 NCs was confirmed by XRD, EDX and FTIR analysis.The calcination temperature had a profound impact on the crystallinity and morphology of the CuO-ZrO 2 NCs that act as a driving force for the photocatalytic activity.The XRD patterns decrease in broadness and increase in intensity of the Bragg's reflections reveals the increased in the degree of crystallinity and crystallite size.The decreased in the intensity of various FTIR peaks with calcination suggest the condensation/transformation of the metal hydroxide into metal oxide.The photocatalytic efficacy of the CuO-ZrO 2 NCs depends upon the calcination temperature.The photocatalytic activity increases with increasing calcination due to the increasing the crystallinity and homogeneity of the sample.
the bending of H-O-H coordinated water molecule[23].The presence of surface -OH groups in M-OH (either Cu(OH) 2 or Zr(OH) 4 ) was revealed by the absorption peak at the wavelength 1395.63 cm −1[22].Another peak appeared at 1329.95 cm −1 which was accredited to the out of plane O-H bending vibrations.The peak at 1169.90 cm −1 was ascribed to the O-M-O stretching vibrations (either O-Cu-O or O-Zr-O) in the crystal structure while that at 746.38 cm −1 was attributed to the M-O-M stretching vibrations (be it Cu-O-Zr, Cu-O-Cu, or Zr-O-Zr) [24, 25].A broad band appeared in the region 586-423 cm −1 [26].The range from 423 to 586 cm −1 exhibited peaks indicative of the stretching modes of M-O (Zr-O and Cu-O) bonds [27].In the uncalcined sample, a significant peak appeared at 530 cm −1 which was imputed to the presence of Zr-O [27].

Figure 5 .
Figure 5.The photodegradation data of the Fluorescein dye in presence of CuO-ZrO 2 NCs and solar light: (a) degradation profile, (b) percentage degradation, (c) kinetic rate constant and (d) proposed degradation mechanism.

Figure 6 .
Figure 6.Effect of pH on the percentage degradation of Fluorescein in the presence of CuO-ZrO 2 NC calcined at 900 °C.

Figure 7 .
Figure 7. Effect of catalyst dose (CuO-ZrO 2 NC calcined at 900 °C) on the percentage degradation of Fluorescein.

Figure 8 .
Figure 8.Effect of initial concentration on the percentage degradation of fluorescein in the presence of CuO-ZrO 2 NC calcined at 900 °C.

Table 1 .
Weight percent and atomic percent of the Cu, Zr and O in the CuO-ZrO 2 NCs.