Enhanced photocatalytic property of Cu doped Ce₂Zr₂O₇ toward photodegradation of methylene blue under visible light

Abstract

Widespread ecosystem degradation from noxious substances like industrial waste, toxic dyes, pesticides, and herbicides poses serious environmental risks. For remediation of these hazardous problems, present study introduces an innovative Cu-doped Ce₂Zr₂O₇ nano-photocatalyst, fabricated via a simple, eco-friendly hydrothermal method, designed to degrade toxic textile dye methylene blue. Harnessing Cu doping for pyrochlore Ce₂Zr₂O₇, structure engineering carried out through a hydrothermal synthesis method to achieve superior photocatalytic performance, addressing limitations of rapid charge carrier recombination in existing photocatalysts. Photoluminescence analysis showed that doped pyrochlore slows charge carrier recombination, boosting dye degradation efficiency. UV–Visible analysis demonstrated an impressive 96 % degradation of methylene blue by Cu-doped Ce₂Zr₂O₇ within 50 min, far exceeding the performance of pristine materials. Trapping experiments clarified the charge transfer mechanism, deepening our understanding of the photocatalytic process. These findings highlight the potential for developing innovative, highly efficient photocatalysts for environmental remediation, offering sustainable solutions to combat pollution. This study not only addresses the limitations of existing photocatalysts but also opens new avenues for enhancing photocatalytic performance through strategic material design.

Graphical abstract

Highlights

• •Cu dopped Ce₂Zr₂O₇ was synthesized via facile hydrothermal approach.
• •The PL study indicate lower Cu dopped Ce₂Zr₂O₇ lower recombination rate.
• •Visible light source employed toward degradation of methylene blue dye.
• •The copper doped Ce₂Zr₂O₇ degrade dye upto 96 % greater than pristine material.
• •Photocatalyst exhibited a stable response after 5 cycle.

1. Introduction

The advent of extensive industrialization, coupled with the discharge of effluents in close proximity to water bodies such as rivers and lakes, has precipitated a global crisis of water contamination, imposing deleterious effects on various life forms. Consequently, imperative are ecologically sound and enduring solutions to address these pervasive global issues. Water tainted by chemical pollutants is fraught with an array of challenges, encompassing diverse hues, pharmacologically active compounds, herbicides, personal care products, industrial chemicals, pesticides, and heavy metals, systematically released into the environment. This continuous discharge poses a menacing threat to human health, aquatic ecosystems, and the overall environment [1,2]. A substantial contributor to water pollution is the textile industry, which emerges as a significant source of water pollution. This industry utilizes an extensive spectrum of over ten thousand different pigments and dyes, resulting in an alarming discharge of more than 0.7 million tons of dye waste annually. The persistence of these pollutants is exacerbated by their non-biodegradable nature and resilience to environmental factors such as light, temperature, detergents, and various chemicals. This resilience underscores the gravity of the issue, often leading to the inadvertent neglect of the profound ecological impact of these persistent pollutants on water bodies [3,4]. Efforts to address these challenges necessitate comprehensive and sustainable strategies that transcend conventional approaches, acknowledging the complexity and persistence of the pollutants involved.

Traditionally, diverse methodologies including biodegradation [5], adsorption [6], membrane process [7], coagulation [8], advanced oxidation process (AOP) [9], activated sludge treatment process (ASTP) and photocatalysis [10] have been harnessed for eradication of pernicious pollutants from industrial effluents. Selection of each strategy is contingent upon situational considerations [[11], [12], [13], [14]]. However, it is imperative to acknowledge that each approach possesses its inherent advantages and drawbacks. Commonly employed physical and chemical treatment methods, such as adsorption and coagulation, demand a substantial quantity of chemicals, rendering them environmentally unfavorable [[15], [16], [17], [18], [19], [20], [21]]. Membrane technologies, including membrane bioreactors, though effective, are often characterized by inefficiencies and operational expenses that are prohibitively high. Amid these methods, heterogeneous photocatalysis emerges as a particularly promising strategy for mitigating organic dye contamination. Noteworthy is its superiority over alternative methods, attributed to cost-effectiveness of semiconductor-based photocatalysts and their enhanced capabilities in degradation of organic dyes [22,23]. Economic viability and superior efficacy of heterogeneous photocatalysis position it as a compelling avenue for addressing challenges posed by organic dye pollutants, offering a potential solution that aligns with both environmental and economic considerations.

The pyrochlore Ce₂Zr₂O₇ stands out among various photocatalysts due to its inherent characteristics, including a small optical band gap (approximately 2.5–2.9 eV) and well-positioned valence and conduction bands at 2.20 and −0.280 eV, respectively, versus normal hydrogen electrode (NHE). Notably, it possesses attributes like efficiency, cost-effectiveness, high oxygen vacancy, chemical stability, and ease of preparation, making it a promising candidate for photo-redox reactions [24]. However, practical application of bare Ce₂Zr₂O₇ is constrained by limitations such as a small specific area, poor hydrophilic nature, low catalytic activity, and rapid recombination of photogenerated charge carriers.

To overcome these limitations, modifications to Ce₂Zr₂O₇ have been explored, involving the incorporation of noble metals (e.g., Au, Ag, Pt) [25] and co-catalysts such as MoP [26], FeP [27], RhP [28], Cu₃P [29], and Ni₂P [30]. These modifications aim to enhance photodegradation outcomes in the reaction system. Moreover, alternative economical materials such as transition metal sulphides and oxides, along with polymeric 2D nanomaterials, have been identified as viable substitutes for noble metals [[31], [32], [33], [34]]. Achieving appropriate band alignment and closely connected surface boundaries between different semiconductors has been demonstrated to impede the recombination process. Consequently, electrons in the conduction band of Ce₂Zr₂O₇ and holes (h⁺) in the valence band of connected semiconductors exhibit heightened oxidizing and reducing abilities [35,36]. These materials have demonstrated superior performance as photocatalysts, owing to increased specific catalytic active sites and greater available surface area [37]. For the enhancement of the catalytic performance of the catalysts researchers have focused on several techniques such as composite formation, heterostructures, alloying and doping, among these doping of semiconductors via conductive Cu proved to be quite viable and effective, as it resulted in enhanced photocatalytic behavior for organic pollutant breakdown. A study demonstrating effect of doping conducted by Rehman et al. achieved a 66 % photocatalytic efficiency using visible light-driven Cu-doped SrTiO₃ nanomaterials fabricated via the sol-gel technique within 2 h of exposure to visible light [38]. Further highlighting the impact of Cu doping on the efficiency of photocatalysts, Heshan Liyanaarachchi conducted research where Cu doping within TiO₂/g-C₃N₄ for methylene blue removal was done. Study achieved a degradation rate of 4.4 × 10⁻³ min⁻¹ [39]. Arzu Ekinci also studied the role of copper in dye removal, finding that CuO nanoparticles removed 96.58 % of methylene blue in 70 min [40]. Additionally, Chayet W. et al. used Chlorophyll-Cu/ZnO to remove Rhodamine B, achieving 99.5 % removal of RhB [41]. Hamid Reza Pouretedal et al. synthesized copper-doped ZnS nanoparticles, investigating the effects of different loading catalysts and pH on the photodegradation of methylene blue, resulting in an impressive 87.3 % photocatalytic efficiency [42]. Zheng Jin et al. reported the hydrothermal synthesis of Cu–P25-graphene, achieving 98 % removal of methylene blue [43]. Halil Demir et al. incorporated copper in different concentrations into the lattice of Bi₂S₃, degrading 40 ppm methylene blue dye to 90.7 % [44]. From these investigations it was observed that the copper doping significantly enhanced the photocatalytic efficiency therefore, we have also choose the copper as dopant in the pyrochlore Ce₂Zr₂O₇. These advancements underscore multifaceted strategies employed to tailor photocatalytic materials for enhanced efficiency in degrading organic pollutants. This study builds on these advancements by overcoming limitations, and prevention of rapid recombination of photogenerated charge carriers.

In this study, Ce₂Zr₂O₇ and Cu-doped Ce₂Zr₂O₇ nanomaterials were synthesized using a facile hydrothermal method, targeting the photocatalytic removal of organic contaminants. Previous attempts have shown that practical application of bare Ce₂Zr₂O₇ is limited by its small specific surface area, poor hydrophilicity, low catalytic activity, and rapid recombination of photogenerated charge carriers. This study addresses these challenges by introducing Cu doping to Ce₂Zr₂O₇, thereby enhancing its photocatalytic properties. A comprehensive analysis, employing diffraction, microscopic, and spectroscopic techniques, was conducted to assess phase, elemental composition, shape, and optical characteristics of synthesized materials. Results revealed robust electronic interactions between Ce₂Zr₂O₇ and Cu, creating favorable conditions for reducing recombination of photogenerated electron-hole pairs (e−/h+) and enhancing absorption efficiency of visible light. To evaluate photocatalytic efficiency, a photodegradation test using methylene blue (MB) dye was conducted. Findings indicated that doped Ce₂Zr₂O₇ exhibited an impressive 96 % degradation, significantly surpassing performance of pristine pyrochlore. These results underscore efficacy of doping strategy in elevating photocatalytic performance of Ce₂Zr₂O₇, highlighting its potential for advanced applications in the remediation of organic contaminants. This study not only overcomes existing limitations of Ce₂Zr₂O₇ but also introduces a novel approach to enhancing its practical applicability, marking a significant advancement in the field of photocatalysis.

2. Materials and methods

2.1. Reagents

All the reagents utilized in this work such as Ce(NO₃)₃.6H₂O (Sigma Aldrich, 99.9 %), Cu(NO₃)₂ (Sigma Adrich, 99.99 %), ZrOCl₂.8H₂O (Sigma Aldrich, 98 %), KOH (Merck, 99.99 %), and methylene blue (Sigma Aldrich, 75 %) were used without further refinement.

2.2. Fabrication of Cu doped Ce₂Zr₂O₇

A straightforward, highly effective, and economical hydrothermal process was employed to synthesize Cu-doped Ce₂Zr₂O₇ nanomaterials. Synthesis procedure involved preparation of an equimolar (0.05 M) solution of Ce(NO₃)₃·6H₂O and ZrOCl₂·8H₂O in 50 mL of deionized water (DI), vigorously stirred for 2 h to ensure complete dissolution of precursors. Subsequently, 3.0 M KOH was added to aqueous solution of cerium nitrate and ZrOCl₂·8H₂O, followed by vigorous stirring on a magnetic hot plate. Next, 5 wt% of Cu(NO₃)₂·6H₂O was introduced into aforementioned mixture and subjected to sonication for 30 min. Resulting reaction solution was then transferred into a hydrothermal reactor and placed in a muffle furnace for 24 h at 180 °C. To remove undesired residues, reaction solution was centrifuged multiple times with ethanol and DI H₂O. Resulting precipitate was subsequently dried overnight at 80 °C using a hot air oven. The dried powder underwent annealing at 850 °C for 4 h in a ceramic crucible. For comparative purposes, Ce₂Zr₂O₇ was synthesized using the same procedure, with the only difference being the omission of Cu(NO₃)₂·6H₂O. This approach ensures a controlled comparison between the doped and pristine materials, elucidating the specific impact of copper doping on the properties of Ce₂Zr₂O₇.

2.3. Characterization

Cu-doped Ce₂Zr₂O₇ sample's crystalline nature and phase purity were assessed using a PANalytical X′-Pert Pro X-Ray diffractometer. Cu Kα radiation source with a wavelength (λ) of 1.5406 was employed for this purpose. Scanning electron microscope (Quanta 200-FEG), coupled with an Energy Dispersive Spectroscopy (EDS) instrument, was utilized to analyze surface morphology and composition of material. Core crystalline nature and morphology of prepared material were investigated using a High-resolution Transmission Electron Microscope (HR-TEM), specifically JEM-2100 model from JEOL, Japan. Structural features of fabricated materials were determined through Raman analysis, conducted using a Bruker Multi RAM instrument. Optical properties of synthesized products were examined via an Ultraviolet Diffused Reflectance Spectrum (UV–Vis DRS), measured with a Cary-60 Agilent instrument. BaSO₄ served as a standard for this analysis. Photoluminescence (PL) properties of fabricated materials were evaluated using Confocal Micro-PL, employing a laser with a suitable excitation wavelength. A 5-ppm solution was prepared in DMSO solvent, followed by sonication to form a clear solution. The PL study encompassed range of 370–650 nm, with an excitation wavelength, observing an excitation band attributable to absorption of photon energy. These comprehensive analyses provide valuable insights into structural, morphological, and optical characteristics of synthesized Cu-doped Ce₂Zr₂O₇ nanomaterial.

2.4. Photocatalytic evaluation

Degradation efficiency of fabricated material as a photocatalyst was assessed using methylene blue (MB) dye as a probe molecule, employing visible light irradiation from a 200 W Newport Xenon lamp. Experimental procedures were carried out within a custom-made cuboidal photoreactor (45 cm × 45 cm), equipped with a stirring plate, magnetic stir bar, and a visible light source to facilitate photocatalysis. To confirm degradation of MB under visible light irradiation, a photolysis experiment was conducted. In a standard photodegradation experiment, 10 mg of synthesized photocatalysts were introduced into a 100 mL dye solution at a concentration of 20 ppm. Dye and synthesized material were agitated in complete darkness for 30 min to establish adsorption-desorption mechanism. This was monitored using a UV–Vis spectrophotometer. Following adsorption phase, dye solution containing photocatalyst was subjected to visible irradiation, and a predetermined volume of the solution (5 mL) was extracted at regular intervals (10 min). Nano-photocatalysts were separated through centrifugation, and UV–Vis spectrophotometer was utilized to analyze photodegradation of the dye at a specific wavelength (667 nm). The removal efficiency of dye was quantified using following expression:

$$\%\text{Degradation}\text{efficiency}=\frac{c_0-ct}{c_0}\mathrm{x}100$$ (1)

In this, C₀ and C_t, corresponds to the concentration when time is equal to zero and concentration of dye at a time (t) during the reaction, accordingly. Radical trapping tests were performed to examine the different reactive species formed throughout the photocatalytic reaction by the elimination process.

3. Result and discussion

3.1. Structural analysis

The crystalline phases and purity of synthesized Ce₂Zr₂O₇ and Cu-doped Ce₂Zr₂O₇ materials were analyzed through Powder X-ray Diffraction (PXRD). Obtained results revealed formation of a pure cubic phase with an Fd-3m space group for pyrochlore Ce₂Zr₂O₇, perfectly matching JCPDS No. 00-052-1104 standard, as depicted in XRD pattern (Fig. 1a). Diffraction peaks at 2 theta values of 29.19°, 34.12°, 48.48°, 57.6° and 60.0°, corresponded to (222), (400), (440), (622) and (444) planes of pyrochlore Ce₂Zr₂O₇ phase, consistent with previous reports [45]. While copper doping did not alter phase, a noticeable shift in peaks indicated strong interactions between copper and dopant [46]. Effect of doping on crystallite size of pure pyrochlore and Cu doped pyrochlore was measured with Debye Scherer formula are 94 nm and 72 nm. The calculated lattice constant and cell volume were found to be 10.609 Å and 1194 Å³, respectively, which increased to 10.633 Å and 1197.4 Å³ after recycling. Change in lattice constant and cell volume suggest that copper has been incorporated in lattices of cubic structure. Shifting of XRD peaks towards lower angles further confirmed effective doping of copper in materials, as illustrated in Fig. 1b. This thorough XRD analysis provides crucial insights into structural characteristics and incorporation of copper in synthesized materials.

Fig. 1.

(a) XRD diffractogram and (b) enlarged view of fabricated pristine and Cu doped Ce₂Zr₂O₇ material.

Raman analysis of both undoped and Cu-doped Ce₂Zr₂O₇ samples given in Fig. 2 reveals distinct spectral features that confirm formation of Ce₂Zr₂O₇ structure and effects of Cu doping. In undoped Ce₂Zr₂O₇, prominent peak observed at 467 cm⁻¹ corresponds to F₂g mode, characteristic of fluorite structure of Ce₂Zr₂O₇ [47]. Additional bands at 138 cm^−¹, 629 cm^−¹, and 828 cm^−¹ can be assigned to O-Zr-O bending, ZrO2 stretching, and Ce-O stretching vibrations, respectively, further validating presence of Ce₂Zr₂O₇. In Cu-doped Ce₂Zr₂O₇ spectrum, notable shifts and intensity changes are observed. Band appearing near the 100 cm⁻¹ is attribution of lattice vibrations. The main intensity peak shifts from 467 cm⁻¹–482 cm⁻¹, indicating an 15 cm⁻¹ shift, which can be attributed to the incorporation of Cu into Ce₂Zr₂O₇ lattice, causing slight distortions and modifying local environment of lattice vibrations. Moreover, there is an increase in intensity of smaller bands at 138 cm^−¹, and 629 cm^−¹. Moreover, there is an increase in intensity of smaller bands at 299 cm⁻¹, 625 cm⁻¹, and 828 cm⁻¹. This increase suggests enhanced vibrational modes due to dopant-induced changes in lattice dynamics and improved structural integrity of Cu-doped material. These spectral changes not only confirm successful doping of Cu into Ce₂Zr₂O₇ structure but also demonstrate the altered vibrational properties due to Cu incorporation, supporting enhanced photocatalytic activity observed in doped samples. Thus, Raman analysis effectively corroborates formation and modification of Ce₂Zr₂O₇ with Cu.

Fig. 2.

Raman analysis of Ce₂Zr₂O₇ and Cu doped Ce₂Zr₂O₇.

3.2. Morphological analysis

Surface morphology of pristine Ce₂Zr₂O₇ and Cu-doped Ce₂Zr₂O₇ was examined through SEM analysis, as illustrated in Fig. 3a&b. SEM micrographs depict a rough sheet-like morphology for Ce₂Zr₂O₇ and well-defined porous-shaped particles for Cu-doped Ce₂Zr₂O₇. Distinct morphology of doped material arises from variations in surface energy of its crystalline planes, resulting in differences in preferred geometry due to copper doping. This well-defined morphology with an uneven surface provides more active sites for catalytic processes. Additionally, heterojunction material is advantageous for charge carrier separation, contributing to improved catalytic performance [48]. TEM micrograph of doped material (Fig. 3c) further illustrated nanoparticle morphology with visible spaces, highlighting porous nature of doped material. This porous structure enhances effective surface area of active substance, providing more active sites for adsorption of dye species. Combination of well-defined morphology and porous structure contributes to the enhanced catalytic properties of the Cu-doped Ce₂Zr₂O₇ material.

Fig. 3.

SEM micrograph of (a) Pristine Ce₂Zr₂O₇ (b) Cu doped Ce₂Zr₂O₇ and (c)TEM micrograph of Cu doped Ce₂Zr₂O₇.

3.3. Textural analysis and compositional analysis

Nitrogen sorption isotherms for developed Ce₂Zr₂O₇ and Cu-doped Ce₂Zr₂O₇ are depicted in Fig. 4a. In high-pressure zone, both materials exhibited an IV-type isotherm with H3-type hysteresis. Desorption process was utilized to assess pore size distribution, revealing a broad dispersion of large pore sizes in all samples. BET analysis indicated that specific surface area of pure Ce₂Zr₂O₇ was 35.51 m²/g, while Cu-doped Ce₂Zr₂O₇ exhibited a higher surface area of 70.21 m²/g, as illustrated in Fig. 4a. This increase in surface area for Cu-doped Ce₂Zr₂O₇ suggests a greater number of active zones on interface, which is expected to enhance photocatalytic activities of these materials. Nitrogen sorption results, combined with BET analysis, provide valuable insights into porous nature and increased surface area, indicating potential for improved photocatalytic performance of Cu-doped Ce₂Zr₂O₇ material [48,49].

Fig. 4.

(a) BET isotherm of the pure and doped material, (b) XPS analysis of Cu doped Ce₂Zr₂O₇ (a) survey, deconvoluted (b) Zr3d spectrum, (c) Ce3d spectrum, (d) O1s spectrum, (e) Cu2p.

X-ray Photoelectron Spectroscopy (XPS) analysis of Cu-doped Ce₂Zr₂O₇ samples reveals detailed insights into chemical states of elements present. Survey spectrum in Fig. 4b displays distinct peaks corresponding to Zr3d, O1s, Ce3d, and dopant Cu2p, indicating successful doping of Cu into Ce₂Zr₂O₇ matrix. In Fig. 4c, high-resolution XPS spectrum of Zr3d is deconvoluted to show peaks at 182.3 eV, attributed to Zr⁴⁺ state in Zr3d5/2 spin state [50], and at 183.7 eV, corresponding to Zr3d3/2 spin state also in 4+ state [51]. Additionally, a smaller hump at 186.0 eV is observed, which is associated with Zr–O bond [52]. Fig. 4d presents deconvoluted Ce3d spectrum with peaks at 882.3, 888.6, 890.2, 898.1, 901.0 and 901.1 eV all associated with +3 and + 4 oxidation states [[53], [54], [55]]. O1s spectrum in Fig. 4e is particularly important as it reveals multiple peaks upon deconvolution: 530.0 eV (Zr–O) [56], 529.7 eV (CeO₂) [57], 534.2 eV (Zr–OH) [58], 535.2 eV (C–OH and CeO₂ species). Dopant Cu is confirmed in Fig. 4f through Cu2p spectrum, which is deconvoluted to show peaks at 932.7 eV and 933.9 eV for Cu2p3/2, and at 953.2 eV and 954.4 eV for Cu2p1/2 confirming the +2 oxidation state of copper [[59], [60], [61]]. These detailed peak assignments confirm the presence and chemical states of the respective elements, indicating successful incorporation of Cu into Ce₂Zr₂O₇ structure and providing insight into the local electronic environments within doped samples.

3.4. Optical analysis

UV–Visible spectroscopy was employed to analyze the optical absorbance behavior and band gap of both pure Ce₂Zr₂O₇ and Cu-doped Ce₂Zr₂O₇, as illustrated in Fig. 5a. In comparison to the bare material, the Cu-doped Ce₂Zr₂O₇ nanomaterial exhibited significantly higher absorption, particularly in the longer wavelength range (red shift). This enhanced absorption in the visible light region is attributed to the successful incorporation of copper into Ce₂Zr₂O₇ nanoparticles. This phenomenon not only broadens the absorption spectrum but also mitigates charge-carrier recombination, a crucial factor contributing to improved catalytic activity [62]. The bandgap of all prepared samples was determined from UV–Vis spectra using Tauc relation, as expressed in following equation [63].

$$\alpha h\upsilon =A{\left(h\upsilon -Eg\right)}^n$$ (2)

Here, $\alpha ,h$, $\upsilon \&\mathrm{A}$ is absorption coefficient, Planck constant, photon frequency, and optical constant, respectively. The optical band gap (E_g) of Ce₂Zr₂O₇ & Cu doped Ce₂Zr₂O₇ are 2.84 eV and 2.38 eV was calculated by plotting (αhυ)² vs. $hʋ$ (Fig. 5b) [64]. The reduction in bandgap in the doped sample indicate that the material can absorb more visible light.

Fig. 5.

(a) UV–Vis. spectra and (b) optical band gap of synthesized material.

The charge carrier migration, oxygen vacancies, and recombination rates induced by photon interactions in the fabricated materials were analyzed through Photoluminescence (PL) spectroscopy at ambient temperature, as illustrated in Fig. 6. A combination of electrons and holes from the valence and conduction bands produces violet emissions. Interstitial vacancies (shallow donors) contribute to the blue transmittance observed in all Cu-doped samples, while deep oxygen vacancies (absent in Ce₂Zr₂O₇) lead to green emission. The presence of oxygen deficiency in the doped sample facilitated the production of reactive species at the catalyst's surface, resulting in improved photocatalytic activity. The recombination of electron-hole pairs significantly influences the photocatalytic behavior of photocatalysts. A decrease in PL intensity indicates a reduction in the recombination rate, resulting in an enhanced photodegradation process. Surprisingly, due to the suppression of electron-hole recombination, Cu-doped samples displayed lower PL excitation intensities than pristine samples. Moreover, the transmittance peak of Cu-doped samples migrated to a longer wavelength than that of Ce₂Zr₂O₇, resulting in a decrease in the energy of the bandgap [65,66]. These PL peaks and the observed blue shift may be attributed to surface oxygen vacancies and interstitial metal ions with Cu dopant nanostructures formed during the growth process. Oxygen vacancies are known to act as excellent electron scavengers, contributing to enhanced photoactivities [67]. Additionally, previous studies by Sriram et al. and Jayaraman et al. provide context for the observed PL characteristics. Sriram et al. reported a high excitation band for Ce₂Zr₂O₇ between 425 and 500 nm, while Jayaraman et al. observed a PL band for Ce₂Zr₂O₇ at 442 nm, excited at 380 nm, attributed to the bandgap of the solid samples [43,44]. These findings underscore the role of PL spectroscopy in elucidating the charge carrier dynamics and defects in the synthesized materials, providing valuable insights into their photocatalytic performance [68].

Fig. 6.

PL spectrum of Ce₂Zr₂O₇ and Cu doped Ce₂Zr₂O₇.

3.5. Photocatalytic activity

The photocatalytic efficacy of the novel nano-catalyst was determined by the photodegradation of Methylene Blue (MB) in the presence of a light source. MB is a common organic pollutant used in the printing, coloring, and textile industries. In the photodegradation investigation, mixtures of MB dye and fabricated materials were exposed to a visible light source, and the changes in MB concentration were measured at intervals of 10 min for all prepared samples as given in Fig. 7a&b and S1a&b. The photocatalytic degradation of MB with an initial concentration of 30 ppm after 50 min of exposure to pristine Ce₂Zr₂O₇ and Cu-doped Ce₂Zr₂O₇ resulted in degradation percentages of 53 % and 96 %, as shown in Fig. 7c. Furthermore, Cu-doped Ce₂Zr₂O₇ exhibited superior photocatalytic degradation activity compared to pure samples, attributed to the significantly enlarged surface area and a more visible light-sensitive band gap of Cu-doped Ce₂Zr₂O₇, contributing to the efficient elimination of the organic pollutant. Generally, nanomaterials with a higher surface area may possess more active sites for degradation than materials with a smaller surface area [69]. During visible light irradiation, electrons from the generated Cu-doped Ce₂Zr₂O₇ composition more readily react with dye molecules, leading to the destruction of the stable MB organic dye. However, a pseudo-first-order reaction model was utilized to explore its photoactive degradation kinetics and calculated using the following expression 3.

$$\ln \left({\mathrm{C}}_{\mathrm{t}}/{\mathrm{C}}_0\right)=\text{kt}$$ (3)

Here, C_t denotes the concentration of dye at any time, C_o indicates initial amount, k is rate constant and t is time and plotted graph shown in Fig. 7 d&e. The Cu doped Ce₂Zr₂O₇ has the 0.045 min⁻¹ higher rate constant than that of pristine material 0.015 min⁻¹ as given in Fig. 7f. The increased rate of organic pollutant degradation attributed to the photocatalyst having unique band alignment and large surface area are responsible for adsorption of greater ions. The interfacial charge transfer among the components in the produced doped material may be the explanation for the improved photodegradation efficiency.

Fig. 7.

Degradation of methylene blue with (a) pristine Ce₂Zr₂O₇ and (b) Cu doped Ce₂Zr₂O₇, (c) Graph of C_t/C_o vs time, (d) Degradation efficiency concerning composition, (e) Plot of ln(C_t/C_o) vs Time and (f) Rate constant with respect to composition. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

3.6. Effect of photocatalyst quantity and dye concentration

In this study, Fig. 8a&b illustrates photodegradation process of MB dye using various doses of Cu-doped Ce₂Zr₂O₇ catalyst, ranging from 10 to 90 mg. Optimal catalyst weight for achieving maximum degradation efficiency was observed to be 60 mg, as compared to alternative catalyst weights of 10 mg, 25 mg, and 75 mg. Catalyst effects are most effectively achieved with weights of 50 mg or greater, as they effectively inhibit activity. However, photocatalyst's efficiency tends to decrease due to particle-particle interactions, resulting in a more pronounced screening effect as catalyst dosage increases. Consequently, system's capacity to produce reactive oxygen species (ROS) is hindered.

Fig. 8.

Effect of (a) Ce₂Zr₂O₇ and (b) Cu doped Ce₂Zr₂O₇ dosage on the photodegradation of MB dye.

Degradation rate of organic pollutants through photocatalytic process is directly proportional to their initial concentration. In Fig. 9a&b, results are presented for organic pollutant concentrations ranging from 20 to 80 parts per million (ppm). The data illustrates a positive correlation between dye concentration and its degradation efficiency. Specifically, an increase in concentration from 10 to 15 ppm resulted in an observed improvement in degradation efficiency. However, when concentration of Methylene Blue (MB) pollutant exceeded 30 ppm, a decline in catalytic effectiveness of photocatalyst was observed. It has been observed that increased amounts of pollutants can be adsorbed onto interface of photocatalyst. Additionally, absence of reactive oxidative substances such as •OH and O_2‾• hinders process of photocatalytic degradation of organic pollutants. Efficiency of photoexcited electron-hole pair in this particular scenario was significantly diminished due to a deficiency in oxidative species, thereby impeding the overall performance of photodegradation.

Fig. 9.

Study of MB dye concentration on photodegradation with (a) Ce₂Zr₂O₇ and (b) Cu doped Ce₂Zr₂O₇.

3.7. The possible degradation mechanism

Irradiation of visible light on pure pyrochlore catalyst resulted in limited photo-induced electron-hole species, leading to poor photodegradation efficiency. However, Cu-doping in Ce₂Zr₂O₇ creates a novel position within the band energy that efficiently traps electrons and prevents the combination of electrons and holes. The photodegradation process is initiated with the formation of electrons (e⁻) and holes (h+) (eq (1)). The photogenerated electrons react with molecular oxygen (O₂) to form superoxide (eq. (2)), and then superoxide reacts with water (H₂O) species to form hydroxyl radicals as well as hydroxyl ions (eq. (3)). The hydroxyl radicals react with molecular O₂ to form H₂O₂ (eq. (4)), and decomposition of H₂O₂ yields hydroxyl radicals, and so on (eq. (5)). The hole species produced in the valence band also react with water species to form hydroxyl radicals (eq. (6)). The hydroxyl radicals are potential species that lead to the photodegradation of MB dye, producing CO₂ as well as water (eq. (7)). The photocatalytic mechanism (eqs. (4), (5), (6), (7), (8), (9), (10))) of photodegradation is described as follows:

$$Cu-C{e}_{2}Z{r}_2{O}_7+h\upsilon \to C{e}_{2}Z{r}_2{O}_7\left(e^{-}+h^{+}\right)$$ (4)

$$e^{-}+O_2\to {O_2}^{-•}$$ (5)

$${O_2}^{-•}+H_{2}O\to {OH}^{-}+H{O}_2^{•}$$ (6)

$$2H{O}_2^{•}\to O_2+H_2{O}_2$$ (7)

$$H_2{O}_2+{O_2}^{-•}\to O_2+{OH}^{-}+{OH}^{•}$$ (8)

$$h^{+}+H_{2}O\to OH+H^{+}$$ (9)

$${OH}^{•}+MB\to C{O}_2+H_{2}O$$ (10)

Enhanced efficiency of nano-catalyst due to Cu-doping may be explained as follows: Red-shift of energy band gap towards lower visible range leads to maximal visible harvesting and stimulation of additional electrons to conduction band, contributing to catalyst surface degradation process. Furthermore, as revealed by Photoluminescence (PL), development of new energy levels near bandgap, as well as the presence of defects in crystal such as oxygen deficiency, obstruct photogenerated electron-hole pair combination. This results in additional electrons as well as holes taking part in degradation process. Photocatalytic mechanism of photoinduced Cu-doped Ce₂Zr₂O₇ is shown in Scheme-I.

Scheme I.

Photocatalytic mechanism of photoinduced Ce₂Zr₂O₇ and Cu doped Ce₂Zr₂O₇.

The scavenger analysis conducted in this study provides a nuanced understanding of the intricate mechanisms underlying the photocatalytic activity of Cu-doped Ce₂Zr₂O₇ as shown in Fig. 10a. Employing various scavengers, each designed to selectively quench specific reactive species, allows for a comprehensive exploration of the roles these species play in the degradation process. Firstly, the utilization of parabenzoquinone (PBQ) as a superoxide scavenger implies the active involvement of superoxide radicals (O₂^•-) in photocatalytic oxidation. Scavenging reaction (4) demonstrates interception of superoxide radicals by PBQ, forming reduced product BQ•- and molecular oxygen. This insight indicates that superoxide radicals play a pivotal role in driving degradation mechanism.

$$\text{PBQ}+{\mathrm{O}}_2^{•-}\to \mathrm{B}{\mathrm{Q}}^{•-}+{\mathrm{O}}_2$$ (11)

Fig. 10.

(a) scavengers and (b) recyclability test profile of Cu doped Ce₂Zr₂O₇.

Additionally, the inclusion of Na₂-EDTA as a hole scavenger suggests participation of photoinduced holes (h⁺) in degradation pathway. Although specific reactions are not detailed, scavenger analysis likely involves processes where holes are captured by EDTA, impeding their involvement in subsequent steps. Use of dimethyl sulfoxide (DMSO) as a hydroxyl radical scavenger indicates generation of hydroxyl radicals (^•OH) during photocatalytic process. Scavenging reaction involves interception of hydroxyl radicals by DMSO, preventing their participation in subsequent reactions. Inclusion of silver nitrate (AgNO₃) as an electron scavenger suggests active role of electrons (e⁻) in overall photodegradation process. Scavenging reaction likely involves reduction of silver ions to metallic silver by captured electrons, illustrating significance of electrons in degradation mechanism. Sustained photocatalytic efficiency observed over five consecutive cycles, as depicted in Fig. 10 (b), underscores stability of Cu-doped Ce₂Zr₂O₇ under visible light illumination. This enduring performance over multiple cycles is crucial for practical applications, indicating material's resilience and effectiveness in repeated photocatalytic processes.

3.8. Post recycle structural analysis

Post-recycle stability analysis of Cu-doped Ce₂Zr₂O₇ photocatalyst reveals that same crystallographic planes, specifically (2 2 2), (4 0 0), (4 4 0) and (6 2 2), were retained at 2θ values of 29.19°, 34.12°, 48.48° and 57.6°, respectively as given in Fig. 11. This retention indicates that fundamental crystal structure remained intact after 5 photocatalytic cycles. However, there was a noticeable decrease in the intensity of (4 0 0) plane, suggesting a reorientation of some crystallites. Additionally (2 2 2) and (4 4 0) planes exhibited widening, along with peak shifted towards lower degree, as well as increased noise in XRD patterns, indicates slight lattice distortions. These distortions likely result from repeated stress and reaction conditions during recycling process. These observations highlight the photocatalyst's structural robustness while also pointing to some degree of degradation, emphasizing need for further optimization to improve long-term stability and performance in practical photocatalytic applications.

Fig. 11.

Pre and Post recycle XRD analysis.

4. Conclusion

This study marks the synthesis and application of Cu-doped Ce₂Zr₂O₇ in photocatalytic removal of methylene blue. By introducing Cu into Ce₂Zr₂O₇, the photocatalytic performance was notably enhanced. Various analytical techniques, including XRD, SEM, UV, and BET analysis, confirmed the phase purity, nano-porous dispersed shape, bandgap (Eg = 2.3 eV), and surface area (70.21 m²/g) of Cu-doped Ce₂Zr₂O₇. Reduction in peak intensity observed in pristine Ce₂Zr₂O₇ with Cu doping, as evidenced by PL analysis confirmed the inhibition of photogenerated charged recombination. The enhanced surface area and the synergistic effects among the components are responsible for the significant improvement in the photocatalytic activity. Cu-doped Ce₂Zr₂O₇ achieved a 96 % degradation efficiency for methylene blue pollutants under visible light within 50 min, outperforming pure photocatalysts. The study explored the effects of dye concentration and catalyst loading on the photodegradation of MB dye. The increased catalytic centers on the expanded surface area significantly improved the composition's effectiveness in removing organic pollutants, leading to a substantial reduction in charge carrier recombination. Superoxide radicals were identified as key agents in the photodegradation process through radical trapping tests. The practical implications of this study are substantial, offering a potential strategy for environmental remediation by developing high-efficiency and stable pyrochlore photocatalysts for the elimination of organic contaminants through photoinduced reactions. This research not only advances the field of environmental science but also provides a promising approach for addressing pollution, highlighting the impact and applicability of Cu-doped Ce₂Zr₂O₇ in environmental cleanup efforts. Future work should focus on scaling up the hydrothermal synthesis process to ensure industrial feasibility, exploring the effectiveness of Cu-doped Ce₂Zr₂O₇ on a broader range of dyes and pollutants, and assessing the long-term stability and reusability of the photocatalyst in continuous environmental remediation processes.

CRediT authorship contribution statement

Razan A. Alshgari: Formal analysis, Data curation, Conceptualization. Muhammad Abdullah: Software, Resources, Formal analysis, Data curation. Syed Imran Abbas Shah: Software, Methodology, Investigation. Abdul Ghafoor Abid: Writing – original draft, Resources, Methodology. Saikh Mohammad: Investigation, Formal analysis, Data curation. Muhammad Fahad Ehsan: Validation, Software, Methodology, Investigation. Muhammad Naeem Ashiq: Writing – original draft, Visualization, Resources, Investigation. Suleyman I. Allakhverdiev: Writing – review & editing, Writing – original draft, Project administration, Investigation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

The following is/are the supplementary data to this article.