Investigating zeolite-infused MWCNTs/ZnO/CeO₂ nanocomposites: photocatalytic efficiency

Abstract

In this study, a range of photocatalysts, including zinc and cerium oxides, were developed using the precipitation technique, while impregnation was used to produce multiwalled CNTs with zinc oxide, multiwalled CNTs with zinc oxide/cerium oxides, and zeolite-based multiwalled CNTs with zinc oxide/cerium oxides. Advanced instruments, such as XRD, UV-Vis, and photoluminescence (PL) devices, were used to analyze the crystal structure, bandgap, and optical properties of each photocatalyst. Methylene blue, an organic contaminant, and sewage from Ethiopia’s Bahir Dar cloth industry were used to assess the photocatalytic efficacy of both assisted and unassisted nanocomposites. Under visible light irradiation, the TZ (zeolite with MWCNTs/ZnO/CeO₂) demonstrated a higher degradation performance of MB dye than the T3 nanocomposite, attaining 93.71% and 83.21%, respectively. This implies that by increasing the adsorption capacity, zeolite greatly improves the composite’s performance. The stability of the TZ in photocatalytic degradation was assessed, showing a 20.8% reduction after four consecutive runs, confirming the nanocomposite’s good stability. Furthermore, the mentioned photocatalyst (TZ) demonstrated greater performance in the degradation of MB with approximately 94.12% compared to the sewage sample efficiency of 76.75%.

Introduction

Industrial effluents containing persistent pollutants significantly contribute to environmental pollution [1]. The increasing application of various synthetic dyes in industries such as fabric dyeing, paper production, textiles, printing, and color photography has resulted in a notable rise in pollution from these effluents, which is becoming increasingly concerning. These industries discharge substantial amounts of organic pollutants [2]. Many of these chemicals pose a health risk because of their environmental persistence, limited biodegradability, and toxicity [3, 4]. Among the numerous organic dye pollutants, methylene blue, a thiazine dye, is particularly worrisome because of its harmful effects on environmental esthetics and human health [5]. Current research is dedicated to developing methods to remove, destroy, or recover these organic dye pollutants from water [6].

ZnO, CeO₂, CdS, Fe₂O₃, TiO₂, and SnO₂ are semiconductor-based photocatalysts commonly employed in the photocatalytic degradation of dyes within various advanced oxidation processes [7]. ZnO stands out among metal oxide semiconductors because of its extensive applications in sensors, electronic devices, photocatalysis, and environmental stability [8]. Compared with other metal oxides, it has recently garnered attention because of its distinctive physical and chemical characteristics, cost-effectiveness, ease of manufacture, and nontoxicity. However, ZnO alone does not fully satisfy the photocatalytic process requirements. This limitation arises from the wide bandgap of ZnO (approximately 3.37 eV, which corresponds to wavelengths below 378 nm, and effectively restricts its photocatalytic activity to UV region irradiation wavelengths [9, 10]. A low quantum yield in the photocatalytic reaction is also caused by the difficulty of ZnO with the photogenerated electrons and holes recombination [11].

Various approaches have been employed to improve the photocatalytic performance of ZnO, such as doping [12], dye sensitization [13], and forming composite photocatalysts through heterojunctions [14], to solve these challenges. Enhancing photocatalytic activity through modification has recently become a popular research topic. One discussed approach involves combining two semiconductors, often resulting in a composite with improved photocatalytic efficiency [15]. When ZnO is combined with other semiconductors, such as CeO₂/ZnO, binary and ternary systems often exhibit higher photocatalytic efficiency than single catalyst systems [16]. MWCNTs and other carbon-based materials possess exceptional properties as dispersants and adsorbents. By preventing charge recombination and changing the optical band gap, they can increase the number of active sites available for photon absorption. Their one-dimensional nanostructures, large surface area, and ability to capture electrons transported from metal oxides to stabilize charge separation have been reported [17].

Owing to its unique properties, CeO₂ has recently attracted significant attention when combined with other metal oxides. CeO₂ has a substantial oxygen storage capacity and strong UV light absorption, enabling it to transition between the Ce⁴⁺ and Ce³⁺ oxidation states, facilitating electron transfer with a comparatively small energy band gap between 2.7 and 3.4 eV [18, 19]. In the ZnO/CeO₂ combination, where ZnO works as a substrate and CeO₂ absorbs visible light as a sensitizer, CeO₂ is the perfect sensitizer to improve the photocatalytic performance because its VB and CB are lower than those of ZnO. The formation of a ternary MWCNTs/ZnO/CeO₂ system could result in a heterojunction that effectively breaks down contaminants in a redox process using both electrons and holes [20, 21].

The synergistic combination of MWCNT, ZnO, and CeO₂ to enhance the visible-spectrum photocatalytic activity is an innovative approach to developing efficient active photocatalysts under visible light [22]. Previous studies have primarily focused on binary and ternary mixed oxide photocatalysts. Despite the advantages of photocatalysts, their application limited by the poor adsorption properties of semiconductors. Various efforts have been made to boost the performance of photocatalysts using suitable supports to tackle this issue. Zeolites stand out among the different supporting materials due to their excellent adsorption properties, high specific surface area, uniform pore structure, and unique ion-exchange capabilities [23, 24]. Zeolite, a microporous aluminosilicate framework with a high cation-exchange capacity and selective adsorption capabilities, is particularly useful for absorbing methylene blue, a cationic dye. Its crystalline structure also facilitates the dispersion of ZnO and CeO₂ nanoparticles, increasing accessibility and total surface area of pollutants. MWCNTs primarily contribute to photocatalysis through van der Waals forces and π–π interactions, offering broad-spectrum adsorption and acting as conductive channels that promote electron mobility and charge separation. To the best of the researchers’ knowledge, no study has been conducted on the photocatalytic efficiency of MWCNTs/ZnO/CeO₂ nanocomposites infused with zeolites for the degradation of M-blue. The freshly synthesized photocatalysts were comprehensively characterized with the use of instruments like photoluminescence, UV-visible, and X-ray diffraction. In this work, researchers are examining the photocatalytic efficiency of zeolite-infused MWCNTs/ZnO/CeO₂ nanocomposites.

Experimental methods

Apparatus and instruments

The as-synthesized nanomaterials’ structures were investigated using powder X-ray diffraction (XRD) using X’Pert Pro PANalytical equipped with an X-ray source of CuKa radiation (wavelength of 0.15406 nm) at a step scan rate of 0.02 (step time: 1 s; 2u range: 5.0–90.4.0.4). UV-Vis spectrophotometers (SANYO, SP65 UK) were used for the determination of the absorption edge of the prepared samples. Optical properties of the as-synthesized supported and unsupported nanomaterials were characterized using photoluminescence spectroscopy (PL) (RF 5301PC Shimadzu). The surface morphology of the synthesized materials was examined using a scanning electron microscope (SEM, Model: JEOL JSM-7600 F, Japan). The instrument was operated at an accelerating voltage of 5–15 kV. Other apparatus like the Analytical balance (OHAUS, made in Switzerland), Oven (Cont herm 260 M), Furnace (BiBBY Stuart, UK), pH meter (MP220), Hot plate (SM6, made in the UK), Centrifuge (HERMLE Z 300, German), Deionizer (B114), orbital shaker S01 (Stuart, UK), Pyrex glass, beakers, Agate and pestle, Ceramic crucibles, Volumetric flasks, Pipettes, and Magnetic stirrer were used.

Chemicals and reagents

All chemicals used in this work are analytical grade (Sigma-Aldrich) and used with no further purification except for the MWCNTs. Zinc acetate dehydrate Zn(CH₃COO)₂.2H₂O (UNI-CHEM, India), oxalic acid (C₂H₂O₄) (LOBACHEM, UK), cerium nitrate hexahydrate Ce(NO₃)₃.6H₂O, and NH₃ (25% Lobachemicals Ltd, India) were used as the precursors of ZnO and CeO₂, respectively, and for the purification of MWCNTs (Sigma Aldrich, USA), conc. H₂SO₄ (98%, laboratory reagent, LOBA, India) and HNO₃ (69% LR, Breckland Scientific Suppliers, UK) were used. NaOH (97.5% BDH Chemicals Ltd., England), HCl (37% BDH Chemicals Ltd., England), NaNO₃, AgNO₃, NaHCO₃, CH₃OH, ethanol (99% Fine Chemicals, Ethiopia), commercially available synthetic Zeolite-Y (CBV400) was used for supporting the as-synthesized ternary nanocomposite, and methylene blue (MB) (C₁₆H₁₈N₃SCl, MW 319.50 g/mol) was used for studying the photocatalytic activity of as-synthesized photocatalysts.

Experimental procedure

Refinement of MWCNTs

The surface properties of MWCNTs were modified to improve dispersion and interaction with metal oxide nanoparticles before the synthesis process [21]. This was achieved by refluxing approximately 300 mg of the nanotubes in a 24 M solution of conc. H₂SO₄ and HNO₃ (1:1, V/V) for two hours at 55 °C, followed by an additional three hours at 80 °C. The NT was then washed away with deionized H₂O through repeated filtration. To obtain purified NTs, they were finally dried for 15 h [22].

Preparation of ZnO nanoparticles

Hexagonal precipitation nanodisks were used to prepare ZnO. A clear solution (solution A) was made by dissolving 2.1948 g of Zn(CH₃COOH)₂.2H₂O in a 50 mL mixture of distilled water and ethanol (1:1 ratio) with stirring. Additionally, 1.2605 g of oxalic acid was dissolved in a 50 milliliter mixture of distilled H₂O and CH₃CH₂OH to create solution B. The precursor solution (A) was progressively added to the oxalic acid solution (B) until slurry formed. This slurry was stirred for five hours. The resulting precipitate was then washed with ethanol and distilled water before being dried in an oven at 60 °C. Subsequently, the dried product was calcined at 500 °C for four hours to produce ZnO nanodisks in a hexagonal shape [23].

Preparation of CeO₂ nanoparticles

Ultrafine CeO₂ NPs were synthesized using a precipitation technique. An aqueous solution was prepared by dissolving Ce(NO₃)₃.6H₂O, specifically 2.342 g of Ce(NO₃)₃, in 20 mL of H₂O following a standard procedure. The pH was adjusted to 10 after 10 min. through gradually adding ammonia solution. After giving the mixture a good stir, a golden cerium hydroxide precipitate formed. The formed precipitate was then washed with water and ethanol, and then dried and heated to 1000 °C for 24 h. To promote the formation of CeO₂ NPs, the dried mixture was ground and then calcined at 700 °C for five hours [24, 25].

Synthesis of MWCNTs/ZnO nanocomposite

To prepare the ZnO precursors for decorating the functionalized MWCNTs, one precursor was gradually added to the other. A suspension of 30 mg of functionalized MWCNTs was created by subjecting them to ultrasonic waves in 50 mL of deionized water for ten minutes. After dissolving zinc acetate (1.65 g) in 50 mL of H₂O, this solution was combined with the purified MWCNTs and stirred for five minutes [26]. The precursors were then agitated for an hour at 80 °C after oxalic acid (3.15 g) dissolved in 100 milliliters of H₂O was added. The dark gray powder that resulted was filtered and repeatedly cleaned. The powder was then collected and allowed to dry overnight at 60 °C. Finally, the powder underwent calcination at 400 °C for 2 h to produce the MWCNTs/ZnO nanocomposite [27].

Synthesis of ZnO/CeO₂ nanocomposite

In accordance with the method outlined in the literature, hexagonal nano-disks of CeO₂/ZnO were synthesized using the precipitation technique. With constant stirring, 2.1949 g of Zn(OAc)₂.2H₂O and 0.1343 g of Ce(NO₃)₃.6H₂O were dissolved in 50 mL of distilled water and ethanol in a 1:1 ratio to create a translucent solution (solution A). To create solution B, 1.2605 g of oxalic acid (Merck) were separately dissolved in fifty milliliters of the identical H₂O and CH₃CH₂OH mixture. Drop by drop, the precursor solution (solution A) was then added to the oxalic acid solution (solution B) until a slurry formed. This slurry was stirred for an additional 5 h. The resulting precipitate was washed with ethanol and distilled water before being dried in an oven at 60 degrees Celsius. The dried product was then calcined at 500 °C for 4 h to get CeO₂/ZnO nanocomposite.

Synthesis of MWCNTs/ZnO/CeO₂ nanocomposite

2.1948 g of zinc acetate and 0.1342 g of Ce(NO₃)₃.6H₂O were dissolved in 50 mL of distilled H₂O and ethanol (1:1 ratio) while stirring to make a transparent solution (B), which served as the basis for the ternary nanocomposite. To create solution A, 75 milligrams of functionalized NTs were dissolved in 50 milliliters of H₂O using ultrasonic equipment for 30 min. After that, solution A was mixed with solution B. To make solution C, oxalic acid (1.2606 g) was also dissolved in 50 mL of H₂O and CH₃CH₂OH (1:1). The precursor solution A was gradually added to the oxalic acid solution (solution C) until a slurry formed, which was stirred for five hours. The resulting precipitate was washed with ethanol and distilled water, then dried in an oven at 60 degrees Celsius. Subsequently, the dried product was calcined at 400 °C for two hours. The samples were labeled T1, T2, and T3, corresponding to the varying amounts of purified MWCNTs: 75, 100, and 125 mg, respectively.

Synthesis of zeolite supported MWCNTs/ZnO/CeO₂ nanocomposite

To create the zeolite-supported material, zeolite Y was combined with the produced T3. Usually, 50 milliliters of distilled water were combined with 2 g of T3 and 20 milliliters of distilled water with 0.2 g of zeolite Y (CBV400), both of which were continuously stirred. After that, the two slurry solutions were mixed and subjected to a 15-minute sonication. At 80 degrees Celsius, the combined oxide and zeolite Y slurry was agitated for an hour. Following the solvent’s evaporation, the formed precipitation was filtered, repeatedly cleaned with H₂O and CH₃CH₂OH, and then dried at 80 °C in an oven. The powder was calcined in air at a temperature of 300 °C for one hour to create the TZ nanocomposite (Fig. 1).

Fig. 1.

Preparation of MWCNTs/ZnO/CeO₂ and TZ nanocomposite steps

Characterization of MWCNTs/ZnO/CeO₂ nanocomposite

Powder XRD was employed to investigate the crystalline phase and structure of the synthesized nanomaterials, utilizing Cu–Ka radiation within the 2θ range of 10°–80° at a scanning speed of 5°/min [28–30]. The solid-state absorption edges of all synthesized samples were identified using a UV/Vis spectrophotometer (SANYO, SP65, UK) that scanned wavelengths from 200 to 800 nm. Furthermore, the optical properties of the synthesized samples were evaluated using PL (RF-5301PC Shimadzu).

Photocatalytic degradation studies

The photocatalytic degradation activity of the newly prepared material was evaluated by the rate at which the MB dye in H₂O decayed when exposed to visible light. 100 mL of a 10 ppm methylene blue (MB) solution was mixed with 140 mg of each photocatalyst to initiate the reaction. A fluorescent lamp, emitting primarily at 480 nanometers with a power rating of 85 W, 240 V, and a frequency of 50–60 Hz, served as the light source for photo-irradiation. The distance between the dye solution and the light source was maintained at 20 centimeters. Before starting the illumination, the reaction mixture was stirred in the dark for an hour to establish adsorption-desorption equilibrium between the dye and the photocatalyst. During the process, air and oxygen were introduced into the solution using a pipette. Subsequently, every 20 min, 10 mL of the solution was extracted, centrifuged at 3500 rpm for 10 min to eliminate catalyst particles, and then analyzed. The absorbance of the MB dye was measured at the λ_max of 665 nm using a UV/visible spectrophotometer. The percentage degradation of the MB dye and the actual sample sewage was calculated using the following formula [31].

1

where A_o is absorbance of dye at initial stage and A_t is absorbance of dye at time t.

Results and discussion

Characterization of the as-synthesized photocatalysts

X-ray diffraction analysis (XRD)

X-ray powder diffraction (XRD) is a rapid analytical technique that determines unit cell dimensions and is primarily employed to identify the phase of a crystalline material [32, 33]. The wurtzite hexagonal phases of ZnO was linked to the lattice planes (100), (002), (101), (012), (110), (013), (200), (112), (201), (004), (202), and (014), as indicated by angles of 2θ = 31.77, 34.42, 36.26, 47.53, 56.58, 62.84, 66.36, 67.93, 69.07, 72.55, and 76.94°, as shown in Fig. 2A. The cubic fluorite phases of CeO₂ correspond to the lattice planes (111), (200), (202), (311), (222), (400), (313), (420), and (422), with diffraction peaks appearing at 2θ = 28.58, 33.24, 47.64, 56.47, 59.23, 69.56, 76.84, 79.26, and 88.52° [34]. Primary representations of the hexagonal wurtzite ZnO structure are the diffraction peaks in the MWCNT/ZnO (B1) nanocomposite at 2θ = 31.63, 34.34, 36.43, 47.58, 56.54, 62.79, 66.44, 68.00, 69.25, 72.49, 76.97, and 81.24°. No additional peaks were observed that would suggest the presence of trace amounts of MWCNTs in the nanocomposite [21, 32]. There are two different phases in the ZnO/CeO₂ (B2) binary system. The wurtzite hexagonal phases of ZnO were responsible for the majority of the diffraction peaks. The wurtzite hexagonal phases of ZnO nanoparticles were the diffraction peaks at 2θ values of 31.84, 34.45, 36.2, 47.58, 56.54, 62.9, 66.44, 67.90, 69.15, and 76.86°. In contrast, the cubic fluorite phases of CeO₂ were linked to the weak diffraction peaks at 2θ values of 28.61, 33.09, 47.58, and 79.26° [JCPDS file No: 96-721-7888]. The diffraction patterns of ZnO/CeO₂ indicate that the host crystal is the predominant component/phase in the binary mixture, as evidenced by the greater number of peaks representing ZnO [35, 36]. The prominent diffraction peaks in the XRD demonstrated the crystallization of the produced catalysts.

Fig. 2.

Powder XRD patterns of single and binary (A), ternary and ternary zeolite supported nanocomposite (B)

In the context of ternary systems, as depicted in Fig. 2B, depending on how many MWCNTs were added to the binary ZnO/CeO₂ system, three different compositions were created. T1 includes 75 mg of MWCNTs within a ZnO and CeO₂ composite. Likewise, in the binary system, T2 comprises 100 mg, and T3 involves 125 mg of MWCNTs. When examining all bare ternary systems T1, T2, and T3, identical diffraction peaks were found at 2θ values of 28.61, 31.74, 33.20, 34.55, 36.32, 47.58, 56.64, 62.90, 66.44, 68.00, 69.15, 76.97, and 89.47. The MWCNTs/ZnO/CeO₂ exhibited two sets of diffraction peaks, signifying the production of nanocomposite materials. Plots at 2θ values of 28.61, 33.20, 47.58, 56.64, 76.97, and 88.47 may be explained by the cubic fluorite phase structure of CeO₂, whereas the hexagonal wurtzite structure of the host crystal ZnO may be responsible for the diffraction peaks at 31.74, 34.45, 36.32, 47.58, 56.64, 62.61, 66.44, and 69.25 [JCPDS file No: 96–230-0113]. Furthermore, the absence of the characteristic MWCNTs in ternary nanocomposites at 2θ values of 26.9 and 43.4° may indicate that the composite catalysts have a uniform coating on ZnO/CeO₂ nanotubes and fewer aggregated pores. Additionally, since MWCNTs are less crystalline than ZnO, their diffraction peaks do not appear in the spectra of the composites. Consequently, during composite formation, the intense ZnO peaks overlap the MWCNT diffraction peaks, masking the MWCNT peaks with those of ZnO/CeO₂ [37, 38]. The inability to detect the MWCNTs band might also be due to the relatively large difference in mass percentages between MWCNTs and ZnO/CeO₂. Thus, the XRD result confirms the synthesis of the ternary MWCNTs/ZnO/CeO₂ nanostructure.

Among the three ternary systems synthesized with different MWCNT contents, T3 was chosen for experimentation depending on its estimated energy gap outcomes from UV-Vis and initial photocatalytic activity. This nanocomposite, supported by zeolite, was chosen for subsequent immobilization onto a Zeolite substrate. The zeolite-supported MWCNTs/ZnO/CeO₂ nanocomposite exhibited diffraction peaks at angles of 6.31, 11.73, 15.79, 20.59, 23.82, 28.51, 31.84, 34.45, 36.32, 47.47, 56.75, 66.55, 67.90, 69.36, 72.59, and 77.07° [JCPDS file no: 43–0168]. The additional weak diffraction peaks at 2θ values of 6.31, 11.73, 15.79, 20.59, and 23.82 are attributed to structural changes in zeolite Y. The patterns of XRD of the TZ show the typical peaks of both Zeolite and T3 (Fig. 2B).

The Debye-Scherrer formula was used to determine the average crystallite sizes of the synthesized photocatalysts [39]:

2

where β is the full width at half maximum in radians, λ is the wave length of the X-ray (0.15406 nm) for Cu target Kα1 radiation, θ is the Bragg’s angle, and D is the crystallite size in nanometer. The good crystalline range between 10 and 50 nm [40] was confirmed by the computed average crystalline sizes of photocatalysts (Table 1).

Table 1.

Crystal size of as-synthesized photocatalysts

As-synthesized photocatalyst	2θ(degree)	β (radians)	D (nm)
(S1) ZnO	36.24	0.007013	20.84
(S2) CeO2	28.58	0.012856	11.12
(B1) MWCNTs/ZnO	36.24	0.005843	25.36
(B2) ZnO/CeO2	36.23	0.009132	15.91
T1 (75 mg MWCNTs/ZnO/CeO2)	36.32	0.006406	22.77
T2 (100 mg MWCNTs/ZnO/CeO2)	36.32	0.006439	22.44
T3 (125 mg MWCNTs/ZnO/CeO2)	36.32	0.006641	21.64
TZ (Zeolite supported MWCNTs/ZnO/CeO2)	36.23	0.006951	20.77

The particle sizes of the synthesized nanomaterials ranged from 11.12 to 22.77 nm, as indicated in Table 1 above. This suggests that the synthesized photocatalysts within the nanometer range exhibit strong photocatalytic activity for breaking down organic pollutants, as they effectively prevent the electron-hole pairs from recombining [41, 42]. Given the preference for higher photocatalytic activity, T3, which has a 21.64 nm crystal size, was chosen and held up by a 20.77 nm crystal size zeolite.

SEM image study

Figure 3a displayed distinct morphology for both zeolite-supported and bare ternary nanocomposites. The morphological study of the ZnO nanoparticles observed in the figure was a road-like structure, which is attributed to the crystal’s growth, which may lead to highly efficient photocatalytic degradation. The cubic-like structures represent CeO₂, and the nanosheet-like structures indicate the MWCNTs. Figure 3b indicates the morphology of the zeolite-supported ternary system. The road, cubic, and nanosheet-like structures show the presence of ZnO, CeO₂, and MWCNTs composites in the zeolite-supported ternary system, respectively however, the shape of zeolite was not observed.

Fig. 3.

SEM image of ST (ternary MWCNTs/ZnO/CeO₂ (a)) and STZ (zeolite-supported MWCNTs/ZnO/CeO₂ (b))

Diffuse absorption of the synthetic photocatalysts in the UV-Vis

The position of the optical absorption maxima for the synthesized photocatalysts was examined using a UV-visible absorption spectrophotometer on the produced powder nanocomposites. The band gap values of the photocatalysts were calculated by evaluating the optical data through the relationship between optical absorbance α and photon energy hv, utilizing Tauc’s plot [43].

3

In this context, v represents the light frequency (in s^{− 1}), h is Planck’s constant (in J.s), n is the electronic inter-band transitions constant, A is the absorption constant, Eg refers to the energy band gap, and α is the absorption coefficient. The resulting band gaps for the ZnO and CeO₂ (S2) single systems were determined to be 3.21 eV and 2.34 eV, respectively. These findings align with previous research on these nanoparticles [44, 45]. The band gaps for the ZnO/CeO₂ (B2) and MWCNTs/ZnO (B1) systems are 2.94 eV and 3.14 eV, respectively. The coupling effect in binary semiconductor systems leads to a red shift, as the interaction between components compensates for individual properties and produces a synergistic outcome. When the two semiconductors were coupled, their band gaps merged, forming new levels of energy with a reduced band gap. The ternary nanocomposites exhibited band gaps of 2.91, 2.88, and 2.85 eV for MWCNTs/ZnO/CeO₂ (T1, T2, and T3). The results showed that ZnO has a comparatively great energy gap (3.21 eV), which allows it to absorb UV light. However, in both its binary and ternary configurations, its absorption edges extend into the visible spectrum. This is likely due to modifications in the electronic levels of individual nanoparticles, forming binary and ternary heterojunctions that promote smaller band gaps [44].

A noticeable red shift results from the formation of ternary systems, which means that the wavelength of light moves farther into the red region of the spectrum, which corresponds to a smaller band gap. This enhances the ability to harvest light in the visible region, facilitating electron transitions. Additionally, it improves conductivity and reduces the energy required for electron excitation, making photodegradation more efficient. The combined influence of CeO₂ and MWCNTs decreases the energy gap of ZnO (Fig. 4A). When MWCNTs adhere to the surface of metal oxide nanoparticles, they further reduce band gap energy, shifting light absorption towards longer wavelengths (Fig. 4B). The resulting TZ nanocomposite supported by zeolites exhibited a band gap of 2.70 eV (Fig. 4C). In this case, the zeolite support pushed the absorption band of T3 deeper into the visible spectrum, enhancing light absorption. Since zeolite Y incorporates a ternary nanocomposite, it is particularly favorable for methylene blue degradation. These results suggest that zeolite-supported nanocomposites hold promise for developing photocatalysts for sunlight-driven applications.

Fig. 4.

Synthesized photocatalysts’ UV-Vis spectra

Photolumenisense (PL) analysis

PL spectra provide insight into various characteristics of nanocomposites, including electron-hole recombination rates, oxygen vacancies, surface defects, and surface states, all of which play a crucial role in photocatalytic reactions [45]. During the electron transitions back to the valence band without engaging in radiative or non-radiative interactions with adsorbed species, they release energy in the form of either heat or light.

A lower PL intensity indicates reduced charge carrier recombination, which enhances photocatalytic activity. ZnO exhibited higher electron-hole recombination due to its greater PL intensity, as shown in Fig. 5. Since Multiwalled CNTs act as conductive pathways, electrons can accumulate along their surface, further decreasing recombination rates and extending charge carrier lifetimes, ultimately improving photocatalytic efficiency. The coupled metal oxide system ZnO/CeO₂ demonstrated efficient inter-particle electron/hole transfer and exhibited the lowest PL emission intensity, and Ce⁴⁺ ions are known to function as trapping sites [46].

Fig. 5.

Photoluminescence of the as synthesized photocatalysts

Ternary nanocomposites typically exhibit lower PL intensities in comparison to single and binary systems. This decrease is due to efficient separation of photogenerated electron-hole pairs by MWCNTs and CeO₂ in ZnO, which reduces e⁻/h⁺ recombination and enhances photocatalytic performance. Additionally, as the MWCNT concentration increases within ternary nanocomposites, PL peak intensity declines significantly. This indicated a further reduction in recombination rates, consistent with previous studies. The electron-accepting and donating properties of the zeolite framework play a vital role in regulating photo-induced charge transfer routes and reducing recombination. As a result, zeolite-supported MWCNTs/ZnO/CeO₂ display lower PL intensities than binary and ternary systems. Furthermore, the robust electrostatic field caused by the zeolite structure efficiently separates e⁻ and h⁺ during ZnO/CeO₂ photo-excitation, leading to an even greater suppression of electron-hole recombination.

Photocatalytic kinetic study

The kinetic analysis of methylene blue (MB) degradation at a catalyst dose of 200 mg/L reveals that the reaction most closely follows pseudo-first-order kinetics, as indicated by the highest correlation coefficient (R² = 0.97) and a rate constant of 0.0050 min⁻¹. This suggests that the degradation rate is primarily dependent on the concentration of MB, which is typical for photocatalytic processes involving reactive species, like hydroxyl radicals. While the zero-order model (R² = 0.86) shows moderate linearity, implying a constant degradation rate possibly due to surface saturation, the second-order model (R² = 0.80) fits poorly, indicating that bimolecular interactions are unlikely to dominate the reaction mechanism. The high removal efficiency of 87.5% further supports the effectiveness of the catalyst under these conditions, with the first-order model providing the most accurate representation of the degradation kinetics (Table 2).

Table 2.

Kinetic parameters for the photocatalytic degradation of methylene blue dyes, comparing zero-order, first-order, and second-order models to determine the most suitable reaction mechanism

		Zero-order		First-order		Second-order
Catalyst dose (mg/L)	%Removal	m0 (µmol/L.min)	R2	m1 (min− 1)	R2	m2 (L µmol min− 1)	R2
MB = 200	87.50	0.0053	0.86	0.0050	0.97	0.05807	0.80

Photocatalytic degradation activity

Methylene blue dye degradation in an aqueous solution was used to assess the photocatalytic activity of the produced nanomaterial [47]. Under UV-visible irradiation, the efficiency of Zinc oxide, Cerium oxide, MWCNTs/ZnO, ZnO/CeO₂, T1, T2, T3, and TZ was tested using an initial dye concentration of 10 ppm and a catalyst load of 140 mg/L. Figure 4 illustrates the percentage of MB dye degradation after 180 min of visible light exposure, with recorded efficiencies of 17.21%, 34.79%, 41.13%, and 50.37% for S1, S2, B1, and B2, respectively. Meanwhile, the degradation efficiency for T1, T2, T3 (pristine ternary nanocomposites), and TZ (the modified ternary system) was measured at 73.37%, 76.38%, 82.22%, and 93.71%, respectively. ZnO showed the lowest photocatalytic efficiency (17.21%) due to its higher electron-hole recombination rate and limited visible light utilization, stemming from its wide band gap. In contrast, CeO₂ achieved higher degradation efficiency (34.78%) because its narrower band gap enhances visible light absorption. When CeO₂ nanoparticles were incorporated into ZnO-based nanocomposites, photocatalytic performance improved due to the reduced band gap energy.

To solve the limitations detected in single-component, multiple semiconductors is an effective approach. Generally, binary and ternary demonstrated superior catalytic effectiveness compared to their single counterparts. This highlights the advantages of heterojunction formation, which fosters an environment conducive to enhanced degradation activity. Under visible light irradiation, MWCNTs/ZnO exhibited significantly improved photocatalytic efficiency compared to ZnO alone. This is attributed to electron trapping, the role of MWCNTs as a photosensitizer, and their ability to suppress electron-hole recombination. The ZnO/CeO₂ heterostructure further promoted charge separation and facilitated inter-particle electron-hole transfer, delaying recombination, as confirmed by PL studies. Moreover, its narrow band gap enabled greater sensitivity to visible light, contributing to enhanced photocatalytic activity.

The degradation efficiencies of pristine ternary nanocomposites (T1, T2, and T3) and the zeolite-supported ternary system (TZ) for methylene blue were recorded as 73.36%, 76.37%, 82.21%, and 93.71%, respectively, as shown in Fig. 6B. Among all tested systems, the TZ nanocomposite showed the highest degradation (93.71%), surpassing single, binary, and ternary counterparts. This enhanced performance is likely due to the effective charge separation of e-h pairs in their excited states, which prolongs recombination time under visible light. Based on these results, ternary and supported systems are recommended for improved photocatalytic performance. Ternary nanocomposites provide a greater number of active sites, increasing their ability to remove dye compared to binary systems [48]. The remarkable photocatalytic efficiency of MWCNT/ZnO/CeO₂ nanocomposites stems from the superior MWCNTs’ delocalized π-electron composition and conductive properties improve electron-hole division and boosts photocatalytic activity.

Fig. 6.

Comparison of (a) single and binary (b) ternary and ternary supported photocatalysts nanocomposite concentration as function of time

In composite photocatalysts, ZnO/CeO₂ acts as an electron donor, facilitating charge transfer and delaying recombination, while MWCNTs function as electron acceptors due to their 1D nanocylindrical structure, which allows free electron movement. Additionally, MWCNTs enhance visible light absorption, serving as an effective light-harvesting material. Its narrow band gap, low PL intensity, and synergistic interactions allowed the ternary MWCNTs/ZnO/CeO₂ nanostructure to degrade dyes and industrial textile effluent with high efficiency. Varying the quantity of MWCNTs from 75 mg to 125 mg while maintaining a constant ZnO/CeO₂ ratio would further boost methylene blue degradation in this study. A higher MWCNT concentration leads to greater electron-hole pair separation and an expanded ternary system surface area, both essential for photodegradation.

An optimal amount of MWCNTs improves photocatalytic efficiency by acting as an adsorbent, photosensitizer, and dispersing agent, effectively concentrating dyes near ZnO/CeO₂, increasing dye availability for photocatalytic reactions, and suppressing recombination of h⁺ in the VB with e⁻ in the CB. However, excessive MWCNT content reduces ZnO’s photocatalytic performance, as its dark color limits visible light absorption. Among all tested systems, the zeolite-supported nanocomposite exhibited the highest photocatalytic activity, attributed to the presence of zeolite. Additionally, incorporating various semiconductor oxides within zeolite cages could further enhance the semiconductor-zeolite photocatalyst’s effectiveness through synergistic interactions. The combination of zeolite’s adsorption properties with the photocatalytic activity of MWCNTs/ZnO/CeO₂ creates a favorable synergy that significantly improves the photocatalytic efficiency of zeolite-supported MWCNTs/ZnO/CeO₂. The increased hydrophobicity of the zeolite Y may be the cause of the higher adsorption capacity in TZ.

To assess the practical performance of the synthesized photocatalyst, a wastewater sample was collected from a sewage tanker at the Bahir Dar textile industry, as depicted in Fig. 7. The degradation efficiency of the zeolite-assisted NTs photocatalyst was tested using a 100 mL wastewater sample with (a catalyst load = 140 milligram/L, pH = 8) under visible light, measured over time. As revealed in Fig. 7, the percentage degradation of the wastewater sample increased with irradiation time, reaching 76.74% after 180 min of exposure. In contrast, the degradation efficiency of the MB solution was greater, at 94.11%. The lower degradation rate in the actual wastewater sample was attributed to its complex composition, which includes various organic dyes and additional chemicals from bleaching processes. These findings confirm that the TZ photocatalyst performed effectively in degrading real wastewater, aligning well with previous studies.

Fig. 7.

Comparison of Methyl blue and real sample sewage under visible light irradiation using TZ nanocomposite as function of time

The synthesized photocatalyst photocatalytic stability

Photocatalysts play a critical role in environmental uses, particularly in treating wastewater containing various dye pollutants [42]. Using a dye (initial conc.: 10 parts per million, catalyst weight: 140 mg/L, pH = 8, irradiation time = 180 min), Fig. 8 illustrates the reusability of zeolite-supported MWCNTs/ZnO/CeO₂ in degrading methylene blue (MB). During the first two cycles, the photocatalyst exhibited a slight decrease in activity, with degradation efficiencies of 93.4% and 88.47%, respectively. However, in cycles C3 and C4, a more noticeable decline was observed, with degradation efficiencies reducing to 83.11% and 73.22%. Despite this gradual decrease, the experiment confirms that the synthesized zeolite-supported MWCNTs/ZnO/CeO₂ photocatalyst remains effective for at least 4 cycles without significant loss of functionality. The decline in degradation efficiency during the final two cycles is attributed to slight mass loss through the recovery and purification procedure, which disturbs its total performance. Nonetheless, under visible light irradiation, the photocatalytic activity of the zeolite-supported MWCNTs/ZnO/CeO₂ remains stable, demonstrating its photostability. While there is a slight reduction in degradation efficiency after multiple cycles, the results indicate that the TZ nanocomposite is a highly stable photocatalyst.

Fig. 8.

MB degradation using TZ Nanocomposites (a catalyst load: 140 mg/L, an initial MB concentration: 10 ppm, and a pH = 8)

Degradation of MB using TZ mechanism

The valence band and conduction band edge potential of ZnO and CeO₂ semiconductors formulas [49–51], obtained using the data on UV-Vis diffuse reflectance spectra (Fig. 4).

4

5

where Ee is the energy of free electrons on the hydrogen scale (roughly 4.5 eV vs. NHE), Eg is the semiconductor’s band gap, ECB is the conduction band edge potential, and EVB is the valence band edge potential. χ is the semiconductor’s electron negativity. ZnO and CeO₂ have electronegativity values of 5.79 eV and 5.57 eV. Their valence band potentials are ZnO (+ 2.89 eV) and CeO₂ (+ 2.24 eV), while their conduction band potentials are ZnO (−0.315 eV) and CeO2 (−0.1 eV), respectively. The band gap of ZnO is 3.21 eV, and that of CeO₂ is 2.34 eV. Since ZnO’s conduction band is less -ve than the ϕ of MWCNT (−4.95 eV), electrons from ZnO’s CB can be captured by MWCNTs when they are present as conductors. Acting as an electron tank, MWCNTs effectively overpower charge carrier recombination and improve photocatalytic activity by enabling electron transfer.

In the MWCNTs/ZnO/CeO₂ ternary system, a stepwise electron transfer mechanism promotes charge carrier separation through a seamless interface, thereby improving photocatalytic efficiency. The strong synergistic interaction between zeolite-MWCNTs/ZnO/CeO₂ photocatalyst, as illustrated in Fig. 9, likely contributes to the superior photocatalytic activity of the zeolite-modified system under visible light compared to MWCNTs/CeO₂. According to prior findings [52], the deposition of MWCNTs/ZnO/CeO₂ onto zeolite Y acts as an electron acceptor. This process effectively inhibits electron-hole pair recombination, making TZ highly desirable for methylene blue (MB) degradation. Similarly, Lewis acids within zeolite facilitate electron transfer from ZnO’s conduction band to electron acceptor sites coordinated with aluminum. Since zeolite Y has a high aluminum content, this reaction is further enhanced. As a result, zeolite-supported MWCNTs/ZnO/CeO₂ nanocomposites exhibit improved visible light absorption and lower PL intensity due to their reduced band gap (Fig. 9).

Fig. 9.

The methylene blue photocatalytic degradation by Zeolite supported NTs under visible light irradiation scheme

Radical scavenging study

Using 140 mg/L of the photocatalyst, 10 ppm of MB, and pH = 8, the effects of active species like superoxide radical (·O²⁻), hole (h⁺), and hydroxyl radical (·OH) in the photodegradation process were investigated to look into the mechanism for the photodegradation of MB over T3. To understand the role of various active species in the photodegradation process based on the change of photocatalytic conversion of MB, different scavengers were used separately to remove the corresponding active species. CH₃OH for ·OH, NaHCO₃ for h⁺, and AgNO₃ for ·O₂ were the scavengers employed in this investigation [53, 54]. Figure 10 demonstrates how the inclusion of scavengers reduced the photodegradation of MB to varying degrees. Without a scavenger, the photocatalytic conversion of MB was 81.62%. Upon the addition of NaHCO₃, CH₃OH, and AgNO₃, it dropped to 71.64%, 62.13%, and 53.77%, respectively.

Fig. 10.

Degradation rate of MB as function of irradiation time in the absence and presence of scavengers at catalyst load of 140 mg/L, initial concentration of MB at 10 ppm and pH at 8)

The findings showed that while all of the scavengers under consideration reduced the effectiveness of photocatalytic degradation, AgNO₃ and CH₃OH had a more noticeable effect. Put another way, it showed that holes and OH do not significantly contribute to dye degradation, whereas O²⁻ is the most active species [55, 56]. The decline in the removal rate when scavengers are present exhibits the following pattern: CH₃OH < NaHCO₃ < AgNO₃. It seems like the holes’ direct involvement was limited. Thus, it was established that the two primary active radical species that cause MB to become decolored when exposed to visible light are photo-generated electrons and OH^•.

Conclusions

In this study, aqueous solutions of Zinc acetate dihydrate, Cerium nitrate hexahydrate, and NH₃ were combined to synthesize single-component (ZnO and CeO₂) and binary (ZnO/CeO₂) nanocomposites. The ZnO/CeO₂ binary nanocomposite was prepared using a precipitation method from their precursor solutions, while ternary nanocomposites with varying MWCNT contents T1 (MWCNTs/ZnO/CeO₂ (75 mg)), T2 (100 mg), and T3 (125 mg) was synthesized via impregnation the use of purified MWCNT precursor solutions, Zn(CH₃COO)₂.2H₂O, Ce(NO₂)₃·6H₂O, and C₂H₂O₄. Additionally, zeolite-supported MWCNTs/ZnO/CeO₂ nanocomposites were synthesized by impregnating the as-synthesized T3 nanocomposite onto synthetic zeolite Y (CVB400) powder. XRD, UV-Vis, and PL analyses were employed to characterize the crystal structure, band gap energy, and optical properties of the photocatalysts. The photocatalytic degradation of methylene blue (MB) dye under visible light irradiation was investigated for all synthesized single, binary, ternary, and supported photocatalysts. The results indicated that, compared to the single, binary, and bare ternary photocatalysts, the supported TZ nanocomposite exhibited the highest adsorption capacity and degradation efficiency (93.71%). These features help capture the released electrons from the semiconductor components, preventing recombination and ultimately improving photocatalytic efficiency. Additionally, MWCNTs and CeO₂ serve as photosensitizers, enabling semiconductors to display photocatalytic activity under both visible and ambient solar radiation. To compare photocatalytic performance, MB and wastewater obtained from the Bahir Dar Textile industry were used. The results showed higher degradation efficiencies for both MB (94.11%) and the real sample (76.74%) under visible light irradiation. Based on these findings, ternary nanocomposites supported by zeolite show strong potential for environmental remediation and wastewater treatment applications.

Author contributions

H.B.T: Conceptualization, Methodology, Software, Visualization, Writing-Original draft preparation. A.T.M: Writing-Reviewing, Methodology, Visualization. D.F.T: Visualization, Writing-Reviewing, Methodology, Visualization, Investigation, and Editing. H.Z.B: Conceptualization, Data curation, Writing-Reviewing, Methodology, Visualization.

Funding

This research received no external funding.

Data availability

The data sets used and analyzed during the current study are available from the corresponding author on reasonable request. We have presented all data in the form of Tables and Figures in the manuscript.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.