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Article

Rapid Photocatalytic Activity of Crystalline CeO2-CuO-Cu(OH)2 Ternary Nanocomposite

by
Govindhasamy Murugadoss
1,*,
Thiruppathi Kannappan
2,
Jothi Ramalingam Rajabathar
3,
Rajesh Kumar Manavalan
4,
Shyju Thankaraj Salammal
1 and
Nachimuthu Venkatesh
5
1
Centre for Nanoscience and Nanotechnology, Sathyabama Institute of Science and Technology, Chennai 600119, Tamil Nadu, India
2
Department of Physics, SRM Valliammai Engineering College, SRM Nagar, Kattankulathur 603203, Tamil Nadu, India
3
Chemistry Department, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia
4
Institute of Natural Science and Mathematics, Ural Federal University, 620002 Yekaterinburg, Russia
5
Department of Nanoscience and Technology, Bharathiar University, Coimbatore 641046, Tamil Nadu, India
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(21), 15601; https://doi.org/10.3390/su152115601
Submission received: 1 September 2023 / Revised: 28 October 2023 / Accepted: 30 October 2023 / Published: 3 November 2023
(This article belongs to the Special Issue Wastewater Treatment and Purification)
Browse Figures
Review Reports Versions Notes

Abstract

:
The development of a heterojunction nanocomposite leads to improved optoelectronic properties. Herein, ceria (CeO2), copper oxide (CuO), and ceria–copper–copper hydroxide (CeO2-CuO-Cu(OH)2) nanocomposites were prepared via a facile chemical method and their structural, morphological, and optical properties were studied using various characteristic techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX), transmission electron microscopy (TEM), ultra-violet visible light absorption (UV-visible), photoluminescence, and thermogravimetry differential thermal analysis (TG-DTA). In the integration of CeO2 and CuO with Cu(OH)2, the band gap is modified to 2.64 eV; this reduced band gap can improve the photocatalytic efficiency of the nanocomposite. The CeO2 can increase light absorption in the nanocomposite, while CuO acts as an electron trap in the composite and this leads to a good enhancement of the optical properties of the CeO2-CuO-Cu(OH)2 nanocomposite. In addition, the heterojunction combination at the interfaces of the CeO2-CuO-Cu(OH)2 nanocomposite facilitates the photo-generated charge separation in the composite, which increases the charge participation in the catalyzed conversion reactions of the prepared composite. The highest photocatalytic degradation efficiencies of 96.4% and 92.7% were achieved for fast green (FG) and bromophenol blue (BP), respectively, using the CeO2-CuO-Cu(OH)2 nanocomposite.

1. Introduction

For a variety of applications, including automotive three-way catalytic conversions, water gas shift reactions [1,2,3], the oxidation of volatile organic compounds, and the development of optical properties [4,5,6], the Cerium oxide nanoparticle (CeO2)-supported CuO nanoparticle with Cu(OH)2 has been regarded as an economically advantageous material. The CeO2-based nanocomposite’s catalytic performance and optical property enhancement have recently been the subject of numerous efforts in various ways. By integrating Cu into the CeO2 lattice and creating well-dispersed copper on the CeO2 surface, Barbato et al. [7] reported that CeO2-supported CuO nanoparticle catalysts are synthesized utilizing a solution combustion approach to improve performance in carbon monoxide. Guo et al. [8] prepared Mn-doped CuO-CeO2 nanoparticles for carbon monoxide conversion, and they were able to increase the O2 vacancy with the formation of more Cu+/Mn4+ species in the solid solution of Cu-Mn-Ce-O metal matrix to improve the conversion of carbon monoxide up to 99% in the range of temperature of 110–140 °C. According to Zabilsky et al.’s [9] investigation on the impact of CeO2 on catalytic efficiency, CeO2 nanorods demonstrated superior catalytic performance in the N2O decomposition reaction by exposing crystal planes on high-energy surfaces. According to Avgouropoulos et al. [10], the method used to manufacture CeO2-CuO catalysts can have an impact on their catalytic activity and optical characteristics when used to selectively oxidize carbon monoxide.
To create CeO2-CuO-nanorod catalysts, Zhou et al. and Liu et al. [11,12] employed the chemical precipitation approach and the wet impregnation method, respectively (thermal breakdown of Cu(NO3)2). They examined the impact of the nanocomposite’s support shape and crystal plane and came to the conclusion that the coordination environment for Cu ions differs on the (1 1 1), (2 2 0), and (3 1 1) planes of CeO2 surfaces. Zheng et al.’s [13] report on the catalytic activity of catalysts for low-temperature carbon monoxide oxidation shows they are significantly impacted by the thermal breakdown temperature of Ce(NO3)3 and the calcination temperature of CeO2-CuO-catalysts. The hydrothermal treatment of citrate precursors, according to Avgouropoulos et al. [14], encourages the mixing of Cu2+ and Ce4+ as well as a close interaction between CuO and CeO2, which increased the structural and thermal stabilization of the composite CeO2-CuO-Cu(OH)2. Numerous studies have highlighted the significance of CuO nanoparticle and CeO2 particle support mixing or interacting during preparation, whether on the Cu(OH)2 surface or in the form of Cu-O-Ce solid solution phases. In the O2-CuO-Cu(OH)2 composite, CeO2 nanoparticles have proven a highly reducible and functional support for CuO catalysts. To better understand the synergistic impact in the CeO2-CuO-Cu(OH)2 composite, considerable surface roughness and flaws were chosen as supports for the CuO catalyst. This allowed for the further exploration of the roles of copper and Ce species distribution on the surface of Cu(OH)2.
The photogenerated holes in the interface nanocomposite combination migrate to the more negative valence band (VB) location, whereas the photogenerated electrons transfer to the more positive conduction band (CB) position [15]. By lowering Eg, which results in light response at lower energies and increases the amount of photogenerated charge carriers, this method efficiently improves the photocatalytic activity and optical property of the material. Due to the heterojunction’s creation, which efficiently promotes charge separation and boosts the rate of electron–hole recombination, charge carriers have longer lives to engage in photocatalytic processes [16]. The best options to increase Cu(OH)2 photocatalytic activity and optical property have been suggested to be semiconductors CeO2 [17,18,19,20,21,22] and CuO [23,24]. CeO2, an n-type semiconductor, combines with CeO2-CuO-Cu(OH)2 nanocomposite to generate a heterojunction at the interface with Eg = 3.04 eV [17] and appropriate band edge positions. During the photocatalytic activity, the capacity to flip between oxidation states, Ce3+ ↔ Ce4+, results in the production of oxygen vacancies that can act as active sites to trap photons. Due to the rapid shift in oxidation states, the insertion of CeO2 in Cu(OH)2 increases light absorption, facilitates charge separation, and suppresses the rate of electron–hole recombination [18,19,20]. Further, metal addition improves the catalytic performance of bare ceria, basically decreasing the temperature corresponding to the maximum soot oxidation rate. The positive effect increases with increasing metal load [21].
Additionally, CeO2 has advantageous photocatalytic qualities such as high thermal stability, strong UV absorption, and a sizable capacity for oxygen storage and release [21,22,23]. Another method to increase optical activity is to employ the p-type semiconductor CuO. CuO is a conductive semiconductor with an Eg of 3.26 eV and is both cheap and readily available [24,25,26]. CuO effectively increased photo conversion efficiency and selectivity when combined with Cu(OH)2 to produce a heterojunction photo catalyst because it reduced the band gap of Cu(OH)2 by introducing defects at d-band states and served as an active electron trap to lessen charge carrier recombination [23]. Additionally, when the surface of the mass transfer catalyst increased, the reduction of Cu2+ → Cu+ during Cu(OH)2 photo conversion also served as the primary adsorption site. In order to maximize photocatalytic activity and improve the optical property, we hypothesized that combining CeO2-CuO-Cu(OH)2 would be a promising strategy. This would utilize the synergy of the excellent photocatalytic properties of the individual components as well as strengthen heterojunctions at the interfaces. As a result, CeO2-CuO-Cu(OH)2 complex heterojunction photo catalysts were made, studied, and their photo conversion efficiency was determined. Hence, the CeO2-CuO-Cu(OH)2 composite was analyzed to generate several reactive species. Photocatalytic dye degradation against the model organic pollutants of fast green (FG) and bromophenol blue (BP) molecules were studied.

2. Materials and Methods

2.1. Materials

The following high-purity chemicals were used to synthesize the CeO2, CuO, and CeO2-CuO-Cu(OH)2 nanocomposite. Hexahydrate of cerium (III) nitrate (Ce(NO3)3), 6H2O, and copper (II) acetate (Cu (CH3COO)2) were all bought from Merck in Bangalore, India. Sodium hydroxide (NaOH) and polyvinylpyrrolidone (PVP) were purchased from the Indian company Nice Chemical Pvt. Ltd. (Kerala, India).

2.2. Synthesis of CeO2-CuO-Cu(OH)2 Nanocomposite

A simple chemical precipitation method was used for the synthesis of CeO2-CuO-Cu(OH)2 nanocomposite. Typically, 0.2 M of cerium nitrate and 1 g of PVP were dissolved in 50 mL deionized water under stirring at 80 °C. Next, 0.2 M copper acetate salt was added into above solution. After confirmation of the above salts dissolving, 4 M of NaOH in 50 mL deionized water was added into the above solution dropwise. After 2 h stirring, the precipitate was collected and washed with acetone and ethanol several times using a centrifuge. Then, the wet sample was dried in a hot air oven at 120 °C for 6 h. The pristine CeO2 and CuO were synthesized using the same chemical precipitation method, using corresponding precursors with same concentration.

2.3. Characterization

The structure and crystalline properties of the synthesized samples were examined via X-ray diffraction (XRD) employing a powder diffractometer (PAN Analytical X’ pert PRO Model X-ray diffractometer) and monochromatic Cu-Kα radiation (λ = 1.5418 Å). The sample was placed on a glass plate with an indentation, and a scan rate of 5°/min was used to record the diffracted signal for the range of 20 to 80°. Using a JEOL JSM-6700F field emission scanning electron microscope (FE-SEM), powder sample morphology was examined. It was manufactured by JEOL GmbH, at Tokyo, Japan. Transmission electron microscopy (TEM, JEM 2100 F, Tokyo, Japan) was used to analyze the morphology, dimensions, and shape of the synthesized powder samples. A UV/VIS-NIR double beam spectrophotometer (VARIAN, Cary 5000, Agilent Technologies, manufactures at Mississauga, Canada) was used to record UV-Vis diffuse reflectance spectra in the wavelength range of 200–900 nm. The photoluminescence (PL) spectra (VARIAN, Cary Eclipse, Agilent Technologies, Mississauga, Canada) with an excitation wavelength of 380 nm were captured using a fluorescence spectrophotometer. The samples were subjected to simultaneous thermal analysis using a thermal analyzer (TG/DTA, SDT Q 600 V20, manufactured at New Castle, DE, USA) in a nitrogen atmosphere, between ambient temperature and 1000 °C.

2.4. Photocatalytic Degradation

The dyes fast green (FG) and bromophenol blue (BP) were used to evaluate the photocatalytic degradation of the produced materials under visible light. The sample concentration was used as 25 mg in 50 mL and the dye solution in aqueous phase was prepared as 20 mg/L. The dye solution was then exposed to direct sunlight for 2 h. During irradiation, 2 mL dye solutions were withdrawn from the system at regular intervals. The greatest absorptions for FG and BP were measured at 620 and 590 nm, respectively. To quantify degradation, the maximum absorbances of FG and BP were measured with a UV-Visible spectrophotometer [24,25].
The formula for calculating deterioration efficiency is as follows:
Degradation efficiency (%) = [(C0 − C)/C0] × 100
where C0 and C are the dye molecules’ initial and fluctuating intensities, respectively. For the catalytic stability experiment, the treated catalyst was centrifuged, cleaned with an acetone/water solution, and dried at 75 °C after the first cycle.
Calculations involving the stability and reusability of the regenerated catalyst were made using the FG and BP dye solutions. In order to determine the precise depollutant rate, the first order kinetic study was also used.
−ln(C/C0) = kt
where k represents the rate of reaction constant and t denotes the duration. The goal of the photocatalytic research was to achieve an average light intensity of 0.85 × 105 lux.

3. Results and Discussion

3.1. Structural Study

Figure 1 shows the XRD patterns of CeO2, CuO and CeO2-CuO-Cu(OH)2 nanocomposite, in the 2θ range between 20–80°. The XRD pattern of CeO2 nanoparticles, as shown in Figure 1, the intense peaks found with 2θ values 28.54°, 44.32°, and 53.21°, and the corresponding planes (1 1 1), (2 2 0), and (3 1 1) were identified from the JCPDS Card No: 48-1548. In the XRD pattern of CuO nanoparticles, the intense peak observed at the 2θ values 32.91°, 35.48°, 38.58°, 48.21°, 58.12°, 63.02°, 67.21°, 68.03°, and 76.31°, and the respective planes (2 0 0), (0 0 2), (1 1 1), (2 2 0), (3 1 0), (1 1 3), (2 2 1), (3 2 1), and (4 0 0) were identified with the JCPDS Card No: 49-0537. The XRD Pattern of CeO2-CuO-Cu(OH)2 nanocomposite shows only one sharp peak in the 2 θ value 28.08° (1 0 0), which was attributed to Cu(OH)2 from the JCPDS Card No: 80-0656. The peak are indicated with asterisk (*).
From the XRD observation, the CeO2 nanoparticles form a fluorite crystal structure with the space group of Fm3m, which is arranged in the form of a face-centered cubic, it is marked with symbol (#). The peaks are slightly broad; this may be due to the smaller particle size of the CeO2 (2.2–3.0 nm). Since it is well known that CeO2’s lattice defect increases with decreasing particle size, smaller CeO2 particles will increase surface lattice defect formation as well as the production of reactive oxygen species. Similarly, the XRD pattern of CuO nanoparticles forms in a tenrite crystallite structure in the lower-symmetry-base-centered monoclinic with a C62n space group. Two peaks indicate the very crystalline nature of the CuO nanoparticle (35.48° and 38.58°), and the remaining peaks are sharp humps, and this may be due to the crystal lattice vibration of the CuO nanoparticle, where the particle size is in the range of 50–59 nm.
The XRD pattern shows the incorporation of CeO2 and CuO nanoparticles in the matrix of CeO2-CuO-Cu(OH)2. Generally, the Cu(OH)2 forms in the base-centered orthorhombic-layered structures, and these structures are linked through hydrogen bonding. The CeO2 and CuO nanoparticle structure is inside the matrix structure of nanocomposite CeO2-CuO-Cu(OH)2, so the peak’s intensity is not visualized clearly, and it can exhibit a super hydrophobic reaction on the surface. Due to its great number of peaks, its intensity is not reflected in the XRD pattern of CeO2-CuO-Cu(OH)2 nanocomposites. The low intensity of the Cu(OH)2 diffraction peaks is due the precursor (copper (II) acetate), which was equally distributed for the formation of the CuO and Cu(OH)2 simultaneously. The particle size can be calculated using the Debye–Scherrer formula [26,27].

3.2. Morphological Study

The morphology of CeO2 is shown in Figure 2a,b. The result shows that the size of the particle was uniform and homogenous in nature. The morphology of CuO is shown in Figure 2c,d, and the result shows the octahedral shape of the CuO nanoparticle. The SEM images of CeO2-CuO-Cu(OH)2 and its EDAX results are presented in Figure 2e–g. The EDAX result shows the distribution of CeO2-CuO-Cu(OH)2 phases throughout the composite. The amount of elemental composition of Ce, Cu, O, and C is shown in Figure 2g (insert); it confirms the existence of CeO2, CuO, and Cu(OH)2 in the nanocomposite. The lesser quantity of the Cu elements in the EDAX may be due to the limited conversion of copper (II) acetate to CuO and Cu(OH)2 via the variation in the dissociation constant for the formation of Ce-O and Cu-O in an alkaline medium. The CeO2-CuO-Cu(OH)2 structure composed of a complex in nature with CeO2, CuO, and Cu(OH)2 exhibits a well-defined cubic structure.
The TEM image of CeO2, CuO, and CeO2-CuO-Cu(OH)2 nanocomposite is shown in Figure 3a–f. The particle size of the element is even and distributed throughout the prepared sample. In the pure materials, the crystalline CeO2 (Figure 3a,b) showed a quasi-spherical shape and a ~5 nm size, and the CuO showed (Figure 2c,d) a mixture of irregular particles of 10 nm coverage size. The major phase of the mixed composite structure was in the octahedral form, and most of the nanoparticle CeO2 and CuO was mixed with Cu(OH)2, as seen in Figure 2e,f. The quantity information of the elements’ presence in the composite is represented using a bar diagram (Figure 2f).

3.3. Optical Study

Figure 4a shows the UV-visible absorption spectra of CeO2, CuO, and CeO2-CuO-Cu(OH)2 nanocomposite. The absorption wavelength was from 200 to 700 nm. The result shows that the absorption wavelength of the CeO2 and CuO shifted to a lower region and that the CeO2-CuO-Cu(OH)2 nanocomposite shifted to a longer wavelength when compared to the CeO2 and CuO nanoparticle.
The absorption onset, i.e., the Eg, of CeO2 is 3.04 eV and for CuO it is 3.20 eV [27,28]. In the composite material, CeO2-CuO-Cu(OH)2 led to a new energy shift to a higher wavelength region with an Eg value of 2.64 eV. Due to this narrowed band gap, the optical property was potentially activated with a lower energy region, which caused the absorption of more light. This results in more photo charge carriers being created when visible light falls on the surface of the nanocomposite [29,30,31,32].
The photoluminescence (PL) spectra of the CeO2, CuO, and CeO2-CuO-Cu(OH)2 nanocomposite are shown Figure 5. The wavelength region is taken from 0 nm to 550 nm, and the PL intensities of the nanoparticle and composite show different trends. Initially, the CeO2 nanoparticle shows a higher intensity and reaches a maximum around 420 nm and then starts decreasing its intensity; this is due to its band gap (Eg = 3.04 eV). Similarly, the nanoparticle CuO shows the same trend, initially increasing the PL intensity and reaching a maximum at 425 nm and then starting to decrease; this result indicates that the charge carriers absorb energy in the lower wavelength region and are excited in the higher wavelength region, but in the higher wavelength region, the charge carriers do not receive sufficient energy, which is why the PL intensity decreases. The nanocomposite CeO2-CuO-Cu(OH)2 shows better PL intensity compared to the nanoparticles CeO2 and CuO. The PL intensity of the nanocomposite CeO2-CuO-Cu(OH)2 is maximum at 420 nm. The composite containing both CeO2 and CuO nanoparticles had a high PL intensity in the combination. Particularly, the prepared nanocomposite contained both Cu2+ and Cu+ species. We hypothesized that the primary cause of Cu+ could be the redox cycle of Cu2+ + Ce3+ to Cu+ + Ce4+, which reduces Cu2+ to Cu+. This result led to a good enhancement of optical properties and greater photocatalytic activity improvement via the separation of charges. This charge separation resulted in multistep charge transfer through the hetero combination of nanocomposite interfaces [31].

3.4. Thermal Study

The thermal stability of the nanocomposite CeO2-CuO-Cu(OH)2 was studied, as shown in Figure 6. The nanocomposite was studied from room temperature to 1000 °C in the nitrogen atmosphere with an increment of 5 °C/min. From the result, it is observed that there are few distinct weight losses in the composite due to the desorption of the water molecule below 200 °C. In the region of 200–450 °C, there are drastic change in the TG curve; this indicates the decomposition of some phases, which are absorbed on the surfaces of the nanocomposites. In this region (200 °C to 450 °C), the complete endothermic and exothermic reactions take place in the nanocomposite; this is clearly reflected in the DTA curve pattern. After this change, there is no change observed in either curve. The decreasing trend is observed from the region of 450 °C to 1000 °C, and this is attributed to the nanocomposite losing all its unwanted species and water molecules in the composite. Finally, the curves TG and DTA clearly show both endothermic and exothermic peaks; this is due to thermal decomposition and oxidative decomposition in the nanocomposite [32,33].

3.5. Photocatalytic Degradation of Textile Dye

Organic pollutants, such as FG and BP dyes, are carcinogenic to the environment. To degrade the dyes, a photocatalytic study was performed for CuO, CeO2, and CeO2-CuO-Cu(OH)2 nanocatalysts under visible light. After being exposed to visible light for two hours, the UV-visible spectra of the FG dye in the presence of the catalysts CuO, CeO2, and CeO2-CuO-Cu(OH)2 are shown in Figure 7a–c. The treatment caused the decolorization of the FG dye to break down since the concentration of the dye solution decreased as the duration of visible light irradiation increased. The organic molecule is degraded by the CuO, CeO2, and CeO2-CuO-Cu(OH)2 nanocatalysts after two hours, with relative efficiencies of around 66.6%, 76.31, and 96.4%. The nanocomposite structure increased the degradation efficiency by speeding up the rate of light absorption, electron–hole pair production, and delayed recombination. The effectiveness of the produced sample’s degradation against the FG dye is shown in Figure 7d.
The FG dye’s initial and final dye concentration changes with respect to time (t) and light irradiation in the presence and absence of the photocatalyst are shown in Figure 8a. On the basis of the sub energy level of the conduction band, significant amounts of reactive oxygen species are produced. The degradation rates of CuO, CeO2, and CeO2-CuO-Cu(OH)2 nanocomposites against FG dye were 0.018 min−1, 0.0267 min−1, and 0.0363 min−1, respectively, in addition to the pseudo-first order kinetics experiments presented in Figure 8b. Similarly, the photocatalytic dye degradation process using the generated catalyst was carried out against another model pollutant of BP dye, as shown in Figure 9a–d. Under identical experimental conditions depicted in Figure 3d, the degradation efficiencies of the CuO, CeO2, and CeO2-CuO-Cu(OH)2 nano-catalysts reached approximately 58.28%, 67.19%, and 92.67%, respectively, within 2 h.
In terms of degradation rates, CeO2-CuO-Cu(OH)2 catalysts were degraded more than CuO and CeO2 catalysts. The variations in initial and final dye concentrations over time (t) for the BP dyes in the presence and absence of a photocatalyst vs. light irradiation are shown in Figure 10a. The sub energy level of the conduction band is the basis for the formation of a significant number of reactive oxygen species (ROS). In addition to the pseudo-first order kinetics experiments reported in Figure 10b, the degradation rates of CuO, CeO2, and CeO2-CuO-Cu(OH)2 nanocatalysts against BP dye were 0.016 min−1, 0.009 min−1, and 0.046 min−1, respectively. Table 1 gives the rate constant and R2 values based on the experiments.
Considering that the CeO2-CuO-Cu(OH)2 nanocomposites had the highest degrading efficiency, they were used and exclusively chosen for the scavenger research to further understand the degradation activity. Particularly, after degradation, organic pollutants were converted into a small number of inorganic ions, non-harmful chemicals, and were mineralized to CO2, H2O, and other substances. Total organic carbon is an index for assessing the degree of organic pollution, as well as an important parameter for water monitoring. The results showed that CeO2-CuO-Cu(OH)2 could efficiently degrade the dye molecule. TOC decreased from 17 mg/L to 3.45 mg/L for FG and 19 mg/L to 5 mg/L for BP after 2 h of degradation. Reactive oxygen species (h+, OH−, and O2−) are generally produced by the photoactive catalyst, and it is crucial to determine which reactive species are crucial for photo degradation. This serves a greater purpose in analyzing the photodegradation of CeO2-CuO-Cu(OH)2 nanocomposites. The resulting accumulation of related scavengers into the reaction solution, such as EDTA (a h+ scavenger), methanol (a •OH scavenger), and 1,4-Benzoquinone (p-BQ) (a •O2− scavenger), removes the h+, OH−, and •O2−. Efficiency with and without scavengers is shown in Figure 11a. Investigations on scavengers show that the efficiency of both dyes in p-BQ and methanol differs noticeably. Because of this, superoxide and hydroxyl radicals are crucial to the overall effectiveness [34]. The following describes the mechanism of FG and BP dye degradation in CeO2-CuO-Cu(OH)2 nanocomposites:
(i) Formation of exciton (electron hole pair)
CeO2-CuO-Cu(OH)2 + hν → h+ (VB) + e− (CB)
(ii) Formation of hydroxyl radicals
OH− + h+ → OH●
H2O + h+ → OH● + H+
(iii) Formation of superoxide radicals
O2 + e− → O2●−
(iv) Photodegradation of FG and BP dye
Dye + OH● + O2●− → Degradation products
The degradation of FG and BP by recycling the catalyst five times indicates the photocatalyst’s stability. The reusability/stability testing of the best CeO2-CuO-Cu(OH)2 nanocomposite sample for the degradation of FG and BP is shown in Figure 11b. Although the renewal of powder-like catalysts is typically troublesome, heterogeneous and doped catalysts are able to compensate for the loss through regeneration. The catalyst is used once again, and then centrifugation, water washing, handling of the ethanol, and air drying are performed.
The findings explain the stability of the catalyst and show that there is no significant decline in degradation efficiency and that it still achieves 90% even after five rounds of the photocatalytic process, albeit efficiency may decrease after five cycles due to catalyst loss during washing. The findings clearly show how well the CeO2-CuO-Cu(OH)2 nanocatalyst can support the actual applications. In order to study the stability of CeO2-CuO-Cu(OH)2, SEM (Figure 12a,b) and TEM (Figure 12c,d) images were recorded for the cycling stability of the samples. The identical morphology of the before- and after-cycling stability of the samples shows high structural stability even after five cycles.

4. Conclusions

The CeO2, CuO and nanocomposite CeO2-CuO-Cu(OH)2 were synthesized using a facile chemical method for photocatalytic application. The nanocomposites were studied for their structural, morphological, optical, and stability properties. The compositions of the compounds and elements were determined by XRD and elemental analysis. Clear information on the crystallite size and crystalline nature of the composite was found using XRD. The well-defined cubic structure of CeO2-CuO-Cu(OH)2 was determined via FESEM and TEM microscopy. The optical absorption of the pristine CeO2 and CuO was tuned into the visible region preparation of CeO2-CuO-Cu(OH)2. The nanocomposites’ robust and wide visible light emission enables them to be used as photocatalysts in a variety of degradation processes. The photocatalytic studies unequivocally demonstrate that dyes such as fast green (FG) and bromophenol blue (BP) degrade in the presence of natural sunlight. Using a CeO2-CuO-Cu(OH)2 nanocomposite, excellent photocatalytic degradation efficiencies of 96.4% and 92.7%, respectively, were achieved for FG and BP. It is interesting to note that the photocatalytic degradation efficiency remained above 90% even after five cycles, proving the catalyst’s durability. The CeO2-CuO-Cu(OH)2 nanocomposites had a desirable band gap location for remediating the textile effluents, which was strongly supported by the examination of the photocatalytic result. This is a big step in the right direction toward solving one of the biggest problems with large-scale reactors for the treatment of industrial wastewater in the future.

Author Contributions

Conceptualization, T.K. and G.M.; Methodology, T.K.; Software, J.R.R.; Validation, G.M., R.K.M. and T.K.; Formal Analysis, S.T.S., N.V. and R.K.M.; Investigation, N.V. and G.M.; Writing—Original Draft Preparation, T.K. and G.M.; Writing—Review and Editing, S.T.S. and G.M.; Supervision, G.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Not applicable.

Acknowledgments

The author G. Murugadoss acknowledges the Vice-Chancellor and the management of the Sathyabama Institute of Science and Technology, Chennai, Tamilnadu, India, for providing lab facilities and support. The author J.R. acknowledges financial support through the Researchers Supporting Project number RSP2023R354, King Saud University, Riyadh 11451, Saudi Arabia. Rajesh Kumar Manavalan thanks contract no. 40/is2 and gratefully acknowledges research funding from the Ministry of Science and Higher Education of the Russian Federation (Ural Federal University project within the Priority 2030 Program).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. XRD pattern of the CeO2, CuO, and CeO2-CuO-Cu(OH)2 nanocomposite.
Figure 1. XRD pattern of the CeO2, CuO, and CeO2-CuO-Cu(OH)2 nanocomposite.
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Figure 2. SEM images with different magnifications of (a,b) CeO2, (c,d) CuO, and (e,f) CeO2-CuO-Cu(OH)2 nanocomposite, and (g) EDX spectrum of the CuO-Cu(OH)2 nanocomposite and bar diagram represent the quantify of the elements.
Figure 2. SEM images with different magnifications of (a,b) CeO2, (c,d) CuO, and (e,f) CeO2-CuO-Cu(OH)2 nanocomposite, and (g) EDX spectrum of the CuO-Cu(OH)2 nanocomposite and bar diagram represent the quantify of the elements.
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Figure 3. TEM images with different (a,b) CeO2, (c,d) CuO, and (e,f) CeO2-CuO-Cu(OH)2 nanocomposite.
Figure 3. TEM images with different (a,b) CeO2, (c,d) CuO, and (e,f) CeO2-CuO-Cu(OH)2 nanocomposite.
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Figure 4. (a) UV-Vis absorption and (b) band-gap curves of the CeO2, CuO, and CeO2-CuO-Cu (OH)2 nanocomposite.
Figure 4. (a) UV-Vis absorption and (b) band-gap curves of the CeO2, CuO, and CeO2-CuO-Cu (OH)2 nanocomposite.
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Figure 5. Photoluminescence spectra of the CeO2, CuO, and CeO2-CuO-Cu(OH)2 nanocomposite.
Figure 5. Photoluminescence spectra of the CeO2, CuO, and CeO2-CuO-Cu(OH)2 nanocomposite.
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Figure 6. TG-DTA spectra of the CeO2-CuO-Cu(OH)2 nanocomposite.
Figure 6. TG-DTA spectra of the CeO2-CuO-Cu(OH)2 nanocomposite.
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Figure 7. Photocatalytic degradation of fast green (FG) by (a) CuO, (b) CeO2, and (c) CeO2-CuO-Cu(OH)2 nanocomposites; (d) photocatalytic degradation efficiency percentage bar diagram of FG.
Figure 7. Photocatalytic degradation of fast green (FG) by (a) CuO, (b) CeO2, and (c) CeO2-CuO-Cu(OH)2 nanocomposites; (d) photocatalytic degradation efficiency percentage bar diagram of FG.
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Figure 8. (a) Photocatalytic degradation efficiency of FG. (b) Rate constant reaction of FG degradation.
Figure 8. (a) Photocatalytic degradation efficiency of FG. (b) Rate constant reaction of FG degradation.
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Figure 9. Photocatalytic degradation of bromophenol blue (BP) by (a) CuO, (b) CeO2, and (c) CeO2-CuO-Cu(OH)2 nanocomposites; (d) photocatalytic degradation efficiency percentage bar diagram of BP.
Figure 9. Photocatalytic degradation of bromophenol blue (BP) by (a) CuO, (b) CeO2, and (c) CeO2-CuO-Cu(OH)2 nanocomposites; (d) photocatalytic degradation efficiency percentage bar diagram of BP.
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Figure 10. (a) Photocatalytic degradation efficiency of BP. (b) Rate constant reaction of BP degradation.
Figure 10. (a) Photocatalytic degradation efficiency of BP. (b) Rate constant reaction of BP degradation.
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Figure 11. (a) Scavenger study; (b) stability test of the CeO2-CuO-Cu(OH)2 nanocomposite; (c) mechanism of photocatalytic degradation of both dyes.
Figure 11. (a) Scavenger study; (b) stability test of the CeO2-CuO-Cu(OH)2 nanocomposite; (c) mechanism of photocatalytic degradation of both dyes.
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Figure 12. (a,b) SEM images with different magnifications and (c,d) TEM images of CeO2-CuO-Cu(OH)2 nanocomposite after five cycles of the treated samples.
Figure 12. (a,b) SEM images with different magnifications and (c,d) TEM images of CeO2-CuO-Cu(OH)2 nanocomposite after five cycles of the treated samples.
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Table 1. Photocatalytic activity studies of CeO2-CuO-Cu(OH)2 nanocatalyst.
Table 1. Photocatalytic activity studies of CeO2-CuO-Cu(OH)2 nanocatalyst.
S. No.CatalystBand Gap
(eV)		Degradation Efficiency
(%) (2 h)		K
(min−1)		R2
Fast green (20 ppm)
1CuO3.0466.60.0180.98
2CeO23.2676.30.0110.99
3CuO-CeO2-Cu(OH)22.6496.40.0590.90
Bromophenol blue (20 ppm)
1CuO3.0458.20.0160.99
2CeO23.2667.10.0200.98
3CuO-CeO2-Cu(OH)22.6492.70.0460.95
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Murugadoss, G.; Kannappan, T.; Rajabathar, J.R.; Manavalan, R.K.; Salammal, S.T.; Venkatesh, N. Rapid Photocatalytic Activity of Crystalline CeO2-CuO-Cu(OH)2 Ternary Nanocomposite. Sustainability 2023, 15, 15601. https://doi.org/10.3390/su152115601

AMA Style

Murugadoss G, Kannappan T, Rajabathar JR, Manavalan RK, Salammal ST, Venkatesh N. Rapid Photocatalytic Activity of Crystalline CeO2-CuO-Cu(OH)2 Ternary Nanocomposite. Sustainability. 2023; 15(21):15601. https://doi.org/10.3390/su152115601

Chicago/Turabian Style

Murugadoss, Govindhasamy, Thiruppathi Kannappan, Jothi Ramalingam Rajabathar, Rajesh Kumar Manavalan, Shyju Thankaraj Salammal, and Nachimuthu Venkatesh. 2023. "Rapid Photocatalytic Activity of Crystalline CeO2-CuO-Cu(OH)2 Ternary Nanocomposite" Sustainability 15, no. 21: 15601. https://doi.org/10.3390/su152115601

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