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Article

Novel AlCo2O4/MWCNTs Nanocomposites for Efficient Degradation of Reactive Yellow 160 Dye: Characterization, Photocatalytic Efficiency, and Reusability

by
Junaid Ahmad
1,
Amir Ikhlaq
2,
Muhammad Raashid
3,
Uzma Ikhlaq
1,*,
Umair Yaqub Qazi
4,
Hafiz Tariq Masood
1,
Tousif Hussain
5,
Mohsin Kazmi
3,
Naveed Ramzan
6,
Asma Naeem
2,
Ashraf Aly Hassan
7,
Fei Qi
8 and
Rahat Javaid
9,*
1
Department of Physics, University of Sahiwal, Sahiwal 57000, Pakistan
2
Institute of Environmental Engineering and Research (IEER), University of Engineering and Technology (UET) Lahore, GT Road, Lahore 54890, Pakistan
3
Department of Chemical, Polymer and Composite Materials Engineering, University of Engineering and Technology (UET) Lahore, New Campus, Kala Shah Kaku 39021, Pakistan
4
Department of Chemistry, Benedict College, 1600 Harden Street, Columbia, SC 29204, USA
5
Center for Advanced Studies in Physics, Government College University, Lahore 54000, Pakistan
6
Department of Chemical Engineering, University of Engineering and Technology (UET) Lahore, GT Road, Lahore 54890, Pakistan
7
Department of Civil and Environmental Engineering, United Arab Emirates University, Al Ain 15551, United Arab Emirates
8
Beijing Key Laboratory for Source Control Technology of Water Pollution, College of Environmental Science and Engineering, Beijing Forestry University, Beijing 100083, China
9
Molinaroli College of Engineering and Computing, University of South Carolina, Columbia, SC 29208, USA
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(2), 154; https://doi.org/10.3390/catal15020154
Submission received: 8 November 2024 / Revised: 21 December 2024 / Accepted: 5 February 2025 / Published: 7 February 2025
(This article belongs to the Special Issue Photocatalysis towards a Sustainable Future)
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Versions Notes

Abstract

:
The purpose of this work was to consider the decolorization efficiency of reactive yellow 160 (Ry-160) dye utilizing cobalt aluminum oxide (AlCo2O4)-anchored Multi-Walled Carbon Nanotubes (AlCo2O4/MWCNTs) nanocomposites as catalysts for the first time in a photocatalytic process under natural sunlight irradiation. The compositional, morphological, and functional group analyses of AlCo2O4 and AlCo2O4/MWCNTs were performed by utilizing Energy Dispersive Spectroscopy (EDS), Field Emission Scanning Electron Microscopy (FE-SEM), and Fourier Transform Infrared (FTIR) Spectroscopy, respectively. A UV-Vis (UV-Vis) spectrophotometer was used to investigate degradation efficiency. The results exhibited a reduction in the optical bandgap for AlCo2O4/MWCNTs nanocomposites as catalysts from 1.5 to 1.3 eV compared with pure spinel AlCo2O4 nanocomposites. AlCo2O4/MWCNTs nanocomposites showed excellent photocatalytic behavior, and around 96% degradation of Ry-160 dye was observed in just 20 min under natural sunlight, showing first-order kinetics with rate constant of 0.151 min−1. The results exhibited outstanding stability and reusability for AlCo2O4/MWCNTs by maintaining more than 90% photocatalytic efficiency even after seven successive operational cycles. The betterment of the photocatalytic behavior of AlCo2O4/MWCNTs nanocomposites as compared to AlCo2O4 nanocomposites owes to the first-rate storage capacity of electrons in MWCNTs, due to which the catalyst became an excellent electron acceptor. Furthermore, the permeable structure of MWCNTs results in a greater surface area leading to the onset of more active sites, and, in turn, it also boosts conductivity and reduces the formation of agglomerates on the surface of catalysts, which inhibits e−/h+ pair recombination. Concisely, the synthesis of a novel AlCo2O4/MWCNTs catalyst with excellent and fast photocatalytic activity was the aim of this study.

1. Introduction

Due to the ever-increasing population, urban growth, and new industries, pollution on planet Earth is increasing day by day [1]. Although these activities are vital for providing a standard of living to mankind, environmental degradation is an unintended consequence of this progress [2]. Water pollution includes both the intentional as well as unintentional discharge of various contaminants into the surface and underground water bodies including rivers, streams, oceans, seas, wells, etc. [3]. Water pollution is a major health risk for all living organisms, including human beings, causing various diseases, such as a variety of cancers, therefore reducing average life expectancies [4]. Hence, reducing water pollution and treating wastewater to offset the dangerous side effects of human activities is an important need of society and, consequently, an attractive area of study [5]. The textile industry is a chief cause of polluted wastewater all over the globe, owing to the greater demand for clothing driven by the ever-present population growth. As a result, about 7 million tons of different dyes are manufactured every year internationally; of which, around 10% are distributed within the fabric dyeing process, consequently polluting the environment [6]. The World Bank and the US Environmental Protection Agency have confirmed that 100 to 200 L of water are consumed per kilogram of fabric processing. Accordingly, 20% of the worldwide industrial pollution originates solely from the textile segment of the global industry [7,8]. The polluted water may subsequently cause harmful effects on humans, such as damage to the central nervous and reproductive system, owing to their highly unsafe nature, especially to human health as it can cause harmful carcinogenic, mutagenic, and teratogenic effects. Moreover, it stops sunlight from entering the water, which is crucial for freshwater and sea plants [9]. The most dangerous effect of these chemicals in wastewater is their interference with the natural distillation mechanism of wastewater, which retards the transportation of oxygen between air and water [10]. Additionally, even drinking water that has a very small amount of absorbed dye, such as 1 mg/L of drinking water, has been found risky for human consumption [11]. Hence, it is imperative that these harmful dyes be completely eliminated from water as it is a crucial and extremely critical matter. Presently, more than 10 thousand different types of dyes can be found in the global color index [12]. Hence, researchers are very ambitious to develop appropriate methods to resolve this problem.
A range of methods has been utilized to remove these hazardous dyes from wastewater, including, but not limited to, desorption, adsorption, biodegradation, ozonation, and membrane processes [13]. These conventional methods for the purification of wastewater are generally not proficient, as the contaminants cannot be completely removed during decomposition and instead change their phases. Sunlight can be utilized to eliminate these harmful pollutants from wastewater, as it is freely and readily available in nature. Moreover, diverse chemicals work more efficiently for the degradation of synthetic dyes in the existence of sunlight, and this efficient process is known as photocatalysis. It is worthwhile to note that one promising innovation for the decontamination of wastewater is the use of nanocatalysts in the photocatalytic process [14]. It exhibits excellent properties that emerge from nano-dimensions and are suitable for the degradation of harmful pollutants. There are large numbers of studies that utilize transition metal oxides in photocatalysis, including oxides of cobalt, both independently and in combination with other metals [15,16]. Metallic oxides have excellent applications in the field of wastewater treatment for the removal of harmful pollutants such as dyes, for instance, Methylene Blue, Safranin O, and Eosin Yellow [17]. AlCo2O4 nanoparticles have been proven effective for enhanced photocatalytic activity for the degradation of various pollutants [18]. Large surface areas, reusability, and chemical and thermal stability are some of the desired properties of such oxides [19]. The oxides of aluminum and cobalt have been successfully used recently in photocatalysis for the degradation of organic dye-, drug-, and pathogen-containing wastewater [20,21].
MWCNTs are excellent agents that can be utilized to improve catalytic activity because their specific surface areas are higher in addition to better stability, lower weight-to-surface ratio, and higher mechanical strength [22,23,24]; therefore, they can be utilized effectively in the photocatalytic process. These exceptional properties of MWCNTs sparked the author’s interest. In the current investigation, nanostructured spinel AlCo2O4/MWCNTs nanocomposites have been synthesized using the coprecipitation route to enhance the photocatalytic properties that occur when irradiated using visible light. As far as the authors know, the photocatalytic activity of AlCo2O4/MWCNTs for the degradation of Ry-160 dye is being investigated for the first time. The authors believe that this study would help to offer insightful information for the elimination of organic pollutants. The objective of this study is to create an environmentally friendly solution that leads to the removal of Ry-160 and a reduction in its toxicity. To increase the effectiveness of this novel catalyst for photocatalytic treatment, several operational parameters, such as concentration of catalyst, pH effect, temperature effect, and concentration of Ry-160, were evaluated and optimized. Furthermore, photocatalytic activity, kinetics, scavenging effect, and catalyst reusability were also investigated. It was followed by proposing the treatment mechanism. Moreover, the compositional, morphological, functional group analyses and optical properties of synthesized specimens are investigated by utilizing EDX, FESEM, FTIR, and UV–visible spectroscopy, respectively. Additionally, this initiative may contribute to the achievement of the United Nations SDGs No. 3 and 6.

2. Materials and Methods

2.1. Chemicals

Aluminum (II) hexahydrate, ammonia (NH4OH 32% VWR), hydrochloric acid (HCl), sodium hydroxide (NaOH), cobaltus nitrate hexahydrate (Co(NO3)2·6H2O), aluminum nitrate nanohydrate (Al (NO3)·9H2O), and MWCNTs dispersions were obtained from Merck, Darmstadt, Germany.

2.2. Synthesis of Spinal AlCo2O4 Nanocomposites

A simple coprecipitation route was employed for the synthesis of AlCo2O4 nanocomposites. For this purpose, 100 mL of deionized water was used and 1 mL of 0.1 M solutions each of Co(NO3)2·6H2O and Al (NO3)·9H2O were mixed in deionized water. To obtain a homogeneous solution, it was continuously stirred for 30 min utilizing a magnetic stirrer while keeping the pH constant at 7. Then, 1.5 mL of ammonia was poured into the solution dropwise. After that, the mixture was mixed for an additional three hours. The resultant solution was then strained using filter paper that had a 0.2-micron pore size before being thoroughly rinsed a multitude of times with ethanol and then with deionized water. Subsequently, the sample was dried at 75 °C for 24 h and finally annealed at 300 °C for 3 h. Steps involved in the coprecipitation method for the synthesis of spinal AlCo2O4 nanocomposites are mentioned in Figure 1.

2.3. Synthesis of AlCo2O4 Anchored Multiwalled Carbon Nanotubes

For the synthesis of AlCo2O4/MWCNTs, 1 mL of MWCNs dispersions were slowly added to the 1 mL each of 0.1 M solutions of Co(NO3)2·6H2O and Al (NO3)·9H2O. The optimum volumetric ratio of all three solutions, i.e., MWCNs, were Co(NO3)2·6H2O: Al (NO3)·9H2O in a 1:1:1 ratio, which was reached after experimental optimization. To acquire a homogenous solution, the chemical solution containing the beaker was placed on a magnetic stirrer. Then, 1.5 mL ammonia was added in a dropwise pattern to the solution, maintaining the PH at 7. In addition to that, this combination was mixed for an additional three hours. The final prepared solution was then filtered by applying filter paper that had a 0.2-micron pore size before being rinsed multiple times with ethanol and then with deionized water. This was followed by a sample drying at 75 °C for one day and finally annealed at 300 °C for 3 h. The stepwise outline of the co-precipitation method for the synthesis of AlCo2O4/MWCNTs nanocomposites is similar to the one exhibited in Figure 1.

2.4. Instrumentation

The morphology of the formulated catalyst was analyzed by using FE-SEM (JEOL JSM-7401F,) sourced from JEOL, Peabody, MA, USA, whereas the EDSX procured as part of the same package was utilized for compositional analysis. FTIR (IR Prestige-21) from Shimadzu, Tokyo, Japan, was used for the identification of functional groups in the catalyst. UV–visible experiments were conducted at room temperature on a spectrophotometer obtained from Shimadzu, Tokyo, Japan, having model UV-3600. The sample produced was stirred continuously for 20 min in deionized water while being combined to generate UV-vis data for the next step [25]. Dye degradation efficiency and reusability were checked using UV-vis absorption data.

3. Results and Discussion

3.1. Compositional Analysis

As obtained from EDSX, the chemical composition of AlCo2O4 and AlCo2O4/MWCNs was annealed at 300 °C, as shown in Figure 2a,b. It can be observed that oxygen and aluminum are present in substantial amounts in the synthesized specimens. A significant content of Co is also detected. Although EDX is not a reliable method for carbon determination, the higher carbon content in AlCo2O4/MWCNs is observed in EDX data from 3.6% to 4.8%, owing to the addition of MWCNTs and the higher noise level.

3.2. Morphological Analysis

FESEM micrographs are presented in Figure 3a,b. These micrographs reveal the structure of pristine spinal AlCo2O4 and AlCo2O4/MWCNTs specimens, respectively. Pristine AlCo2O4 exhibits a surface morphology comprising clusters of nanocomposites and nanoflakes. Figure 3b exhibits the disappearance of nanoflakes and the formation of nanocomposites dispersed in multiple directions, providing a larger surface area for active sites, thereby increasing photocatalytic activity. The CNTs were instrumental in significantly reducing the size of AlCo2O4/MWCNTs nanocomposites. Moreover, Figure 3b also exhibits that the AlCo2O4/MWCNTs specimen has a porous morphology which may be the reason for better photodegradation. The porous surface morphologies may enable the migration of reactive yellow molecules in the surface pores and hence help in its degradation. The incidence of MWCNTs on the surface of AlCo2O4 could not be proven by any credible piece of evidence. However, we may speculate that MWCNTs could be possibly present in the inner layers of nanocomposites. The magnified images for both samples are presented in Figure 3c,d.

3.3. Fourier Transform Infrared Spectroscopy (FTIR)

FTIR spectra of spinal AlCo2O4 nanocomposites and AlCo2O4/MWCNTs nanocomposites are shown in Figure 4a,b. In both synthesized samples, stretching vibrations of metal–oxygen bonds are observed. The peak at 3450 cm−1 corresponds to the M-OH vibration (metal-substituted oxides), while the peaks at 636 and 560 cm−1 are attributed to the distinctive stretching and bending modes of the vibrations of the Co-O bond in the octahedral site and Al-O sites, respectively. The O-H and H-O-H oscillations are responsible for the peaks at 1630 cm−1, which indicate the occurrence of water [26,27]. Another peak was found at 1023 cm−1, which establishes its link to the C–O bond [28]. Our FTIR results confirm the presence of aluminum cobaltite and the peak at 1023 cm−1 confirms the formation of carbon as a constituent of multiwall nanotubes, which is consistent with our EDX results.

3.4. UV–Visible Spectroscopy

The optical studies of the synthesized specimens have been investigated utilizing a UV–Visible spectrophotometer. Figure 5a,b exhibits the UV–Visible absorption spectra of AlCo2O4 and AlCo2O4/MWCNTs within the wavelength range of 200 to 800 nm. Figure 5b exhibits powerful absorption in the range of visible light regime for AlCo2O4/MWCNTs nanocomposites in comparison to pristine AlCo2O4 nanocomposites. This can be attributed to the stronger absorption properties of MWCNTs. In the current investigation, a Tauc plot is utilized to compute the energy gaps in the optical band of the synthesized specimen. Tauc plot is a graph of photon quantity plotted versus the photon energy (hv) [29]. Calculations were made considering the relation given in Equation (1).
(αhv)1/n = A(hv − Eg)
where h is Plank’s constant and A, α, and v represent absorption, molar absorptivity, and light frequency, respectively. The value of ‘n’ determines the nature of transition, which may be indirect or direct. Here, n = 2 signifies an indirect transition, while n = 1/2 represents the direct transition [30]. The values of hv are plotted versus (αhv)2 to calculate the band gap (Eg) and are presented in Figure 6a,b. It can be easily seen from Figure 6a,b that the bandgap reduces from 1.52 to 1.3 eV with the addition of MWCNTs in AlCo2O4. A possible reason for this observation is the superior absorption ability of MWCNTs compared to pristine spinal AlCo2O4 [31]. The results of our investigation indicate that MWCNTs may cause enhancement in the sunlight harvesting ability of AlCo2O4 nanocomposites.

3.5. Photocatalytic Activity

In the presence of natural sunlight, the synthesized specimens are analyzed for their photocatalytic properties for various irradiation times. The photocatalytic experiment was performed in Sahiwal, Pakistan, on the shiny day of May 2023. Ry-160 was used to study the photodegradation efficiency of AlCo2O4 and AlCo2O4/MWCNs, as shown in Figure 7a,b and Figure 8. For the degradation of (Ry-160), solutions of AlCo2O4 nanoflakes and AlCo2O4/MWCNs nanocomposites were prepared. For this purpose, 0.0125 g of spinel AlCo2O4 nanoflakes were thoroughly mixed in a 20 ppm solution of Ry-160, and the solution was subsequently stirred for 20 min continuously to acquire homogeneity. Afterwards, the brewed solution was subjected to continuous stirring in full darkness for a period of 120 min to attain adsorption–desorption equilibrium. Around 2 mL sample of the treated solution was withdrawn every 4 min at regular intervals. Then, the solution was separated by centrifuge at 4000 rpm for 4 min to isolate the AlCo2O4 nanoflakes nanocomposites from Ry-160 solution. AlCo2O4/MWCNs nanocomposite catalyst was also treated in a similar way. The percentage degradation of dye can be calculated utilizing the formula provided in Equation (2):
Percentage Degradation = (1 − C/C0) × 100
where C and C0 denote the concentrations of the RY160 dye after exposure to natural sunlight and the initial concentration of unexposed specimen, respectively [32,33]. An improved photodegradation efficiency of the AlCo2O4/MWCNs nanocomposites is observed as compared to pristine AlCo2O4 nanocomposites, as seen in Figure 7a,b and Figure 8. It can be observed from Figure 8 that RY-160 is degraded by around 96% when utilizing AlCo2O4/MWCNs as photocatalysts in just 20 min. This may be attributed to the ability of MWCNTs to efficiently store electrons which makes them an outstanding electron acceptor and hence is responsible for the improvement in photocatalytic performance [34]. This may also be related to the fact that photoelectrons generated in the conduction band of AlCo2O4 moved to large electron-accepting MWCNTs, whereas holes remained on the AlCo2O4 surface. The porous structure and a comparatively larger surface area, which help to produce more active sites, may also be responsible for improved catalytic performance [35]. This is in line with our SEM results as given in Figure 3a–d.
A comparison of the degradation of RY-160 with other recent works in the literature performed in Table 1 reveals that photodegradation by AlCo2O4/MWCNs nanocomposites is indeed a fast efficient method for degradation of such dyes, and, moreover, using visible light reduces operational costs and energy requirements as well.
Additionally, a comparison of AlCo2O4/MWCNs nanocomposites with other novel photocatalysts developed in 2024 is given in Table 2. It can be seen that AlCo2O4/MWCNs nanocomposites have superior performance as compared with other photocatalysts used in visible light.

3.6. Effect of pH

pH is an important parameter and operational variable in photocatalysis and may be a crucial factor in the photodegradation of RY-160 using AlCo2O4 and AlCo2O4/MWCNs nanocomposites [45]. Therefore, the effect of pH was evaluated during photodegradation of Y-160 by AlCo2O4 and AlCo2O4/MWCNs nanocomposites by applying varying values of pH, i.e., <=6, ≈7, and >=8 to simulate acidic, neutral, and alkaline situations, respectively. The effects of pH values (pH 2, 4, 6, 8, 10) over time on the effectiveness of color removal are illustrated in Figure 9a,b. The lowest removal efficiency was noted under highly acidic and alkaline conditions at pH 2 and 10, respectively [46]. The greatest removal of pollutants, attained at pH 6, shows that pH affects more than just the surface characteristics of AlCo2O4 and AlCo2O4/MWCNs nanostructures. It also affects the separation of the molecules of the dye and the quantity of hydroxyl radicals produced. The surface of AlCo2O4 and AlCo2O4/MWCNs becomes negatively charged under an alkaline state, which causes an electrostatic attraction between the dye, which is negatively charged due to radicals and the photocatalyst. AlCo2O4 and AlCo2O4/MWCNs display amphoteric behaviors in acidic conditions. As a result, AlCo2O4/MWCNs nanocomposites degrade the pollutants slowly in acidic conditions. It is also suggested that the amphoteric character of the AlCo2O4 and AlCo2O4/MWCNs catalysts may be responsible for the reduced degradation efficiency. Amphoteric oxides are generally more stable and resistant to corrosion than acidic or basic oxides, leading to overall minimal deterioration. The maximum decolorization observed is 82% for AlCo2O4 and 96% for AlCo2O4/MWCNs at pH 6, as the degrading efficacy increases with pH. The degrading efficiency of AlCo2O4 and AlCo2O4/MWCNs are shown in Figure 9a,b, respectively. Moreover, decolorization decreases with a pH increase of over 6. The abundance of negatively charged hydroxyl (OH-) ions and the attractive forces between the negatively charged dye molecules and the anionic surface could be the reason for this [36]. The catalyst has shown good efficiencies at optimum values of pH. Additionally, a lower pH resulted in less decolorization, and this could be happening because anionic dye molecules are attracted to negatively charged surfaces such as OH- ions, of which a plentiful amount is present [36]. The pH range where the catalyst operates most effectively maximizes dye decolorization. For lightly acidic pollutants, the reaction rate increases at lower pH levels, although no appreciable decolorization is achieved. The pH range where the catalyst operates most effectively prevents considerable dye decolorization. For lightly acidic pollutants, where the reaction rates rise at lower pH levels, there was no evident decolorization of the dye molecules. For contaminants that are close to acidic, the rates of response seem to increase at lower pH levels [47].

3.7. Effect of Temperature

The photocatalytic process and, subsequently, the quantity of deterioration or percent degradation of RY-160, are significantly influenced by temperature. Figure 10a,b demonstrates that the removal of RY-160 increases as solution temperature rises under all other experiment settings. The degradation of the dye was investigated at temperatures of 300 K, 340 K, 380 K, and 400 K. Ry-160 had the greatest deterioration at 400 K for both the catalysts, AlCo2O4 and AlCo2O4/MWCNs, with efficiencies of 82% and 96%, respectively. The photodegradation process was found to be most efficient at high temperatures and least efficient at low temperatures. This is because dye molecules are increasingly interacting with the surface of photocatalysts at high temperatures and the rate of reaction increases as well. Consequently, the adsorption increases, favoring dye photodegradation [48].

3.8. Effect of Catalyst Concentration

The synthesized novel nanocomposite AlCo2O4/MWCNTs plays a major role in the removal of RY-160 dye during photocatalytic degradation. The experiments were conducted using varying concentrations of AlCo2O4 and AlCo2O4/MWCNs in the range of 50–150 mg with a constant concentration of Ry-160 dye at 20 ppm. Figure 11a,b shows that 125 mg of AlClo2O4 for 20 ppm solution of the RY-160 resulted in an 82% color loss whereas 125 mg of AlClo2O4/MWCNs in the same 20 ppm solution of the Ry-160 resulted in a 96% color loss, demonstrating a dramatic improvement. Moreover, increasing the catalyst dose provides more active sites on the surface of the AlCo2O4/MWCNTs catalyst, improving the degradation of RY-160. At 25 mg of the AlCo2O4 and AlCo2O4/MWCNs quantities, the color removal became 41% and 49%, respectively. Furthermore, it is concluded from the results that enhanced dye degradation is obtained from AlCo2O4/MWCNTs than AlCo2O4 alone, indicating the superior catalytic performance of AlCo2O4/MWCNTs.

3.9. Effect of Dye Concentration

The effect of different initial concentrations of RY-160 dye, ranging from 20 ppm to 120 ppm, was studied under visible light using 125 mg of catalyst. The greatest degradation rate of RY-160 was seen at a dye concentration of 20 ppm; however, this rate decreased to as low as 120 ppm, as shown in Figure 12a,b. Using the AlCo2O4/MWCNs composites, the solutions having initial dye concentrations of 20 ppm or less were almost entirely degraded. Higher concentrations, such as 100 and 120 ppm, did not, however, substantially deteriorate. For example, after 100 min, the 60 ppm dye solution had dropped the dye content by 65%, but the 80 ppm solution had only degraded by 45% during the same time frame. The presence of active pore sites on the catalyst surface is substantial for effective degradation. The dye solution became more intensely colored, as the concentrations of dye rose. This shorter route length for photons entering the solution may lower the degradation efficiency.
Furthermore, the AlCo2O4/MWCNs have a molecular surface, and only a small number of photons are able to reach the catalyst surface in case of lower concentrations at the surface, meaning that the production of hydroxyl radicals is sufficient. However, when AlCo2O4 and AlCo2O4/MWCNs have a surface that is more fully covered in pollutant molecules, the production of hydroxyl radicals is insufficient to reach the catalyst pores [49]. Furthermore, fewer photons can reach the catalyst’s surface when dye molecules are covering up the surface of both catalysts used, i.e., AlCo2O4 and AlCo2O4/MWCNs, which prevents enough hydroxyl radicals from being produced [50]. AlCo2O4 degrades a maximum of 82% of RY-160 in a 20 ppm dye solution, while AlCo2O4/MWCNs degrades up to 96% of RY-160 in the same concentration as shown in Figure 12a,b.

3.10. Quenching with Hydroxyl Radical Scavenger

To investigate the role of hydroxyl radicals in the degradation of RY-160 dye, 1.5 g of NaHCO3 was poured into the reactor with a 20 ppm solution of RY-160. It was examined that the percentage removal efficiency of dye decreased when quenched with NaHCO3 compared to when it was not quenched as evident from Figure 13. For instance, the removal efficiency at 20 min for AlCo2O4 and AlCo2O4/MWCNs was 60% and 54%, respectively, with sodium bicarbonate, whereas without quenching, it was 82% and 96%, respectively. The reduced removal efficiency upon quenching is attributed to NaHCO3, acting as a radical scavengers, which reduces the availability of free radicals for the degradation of the pollutant [51,52].

3.11. Kinetics

Photocatalysis generally follows pseudo-first order kinetics, given in Equation (3).
C = C0 e−kt
where C and C0 denote the concentrations of the RY160 dye after exposure to natural sunlight and the initial concentration of unexposed specimen, respectively, t refers to time in minutes, and k is the first order rate constant.
To determine the kinetic rate constants for AlCo2O4 and AlCo2O4/MWCNTs, a plot of −lnC/Co was made against time in Figure 14, and a straight line was fitted to the data. The slope of the straight lines in Figure 14 gave the values of rate constants for pseudo-first order kinetics. A 56% increase in the kinetic rate constant upon adding MWCNs is a striking improvement over simple AlCo2O4, i.e., 0.0817 min−1 for AlCo2O4 to 0.151 min−1, for AlCo2O4/MWCNs, with reasonable values of R2, i.e., 0.985 and 0.963 for AlCo2O4 and AlCo2O4/MWCNs, respectively. The determined value is on the higher end of the range found in the literature for dyes and is even close to rate constant values under UV exposure [53].

3.12. Proposed Mechanism

When sunlight strikes the surface of the photocatalyst, it generates a considerable number of electrons and holes. Excited electrons jump to the material’s conduction band (CB) from the valence band (VB), creating a hole in VB. As shown in Figure 15a, the interaction between these electrons and holes leads to a Redox reaction. The oxygen atom in the photocatalyst reacts with the electrons in the CB to form the superoxide radical (O2•−). Meanwhile, the holes in the VB interact with the water and gather electrons to facilitate the formation of hydroxyl radicals (OH•). The resulting O2• and OH• radicals then interact with the RY-160 dye, breaking down the dye molecules into CO2 and water. The following reactions from Equations (4) to (10) illustrate the degradation of RY-160 dye.
AlCo2O4 − MWCNTs + hv → AlCo2O4 − MWCNTs (e− + h+)
AlCo2O4 − MWCNTs (e− + h+) → AlCo2O4 (e−) − MWCNTs (h+)
h+ + H2O → OH• + H+
e− + O2 → O2•−
O2•− + 2H+ → H2O2
H2O2 + O2•− → OH• + OH− + O2
OH• + RY → CO2 + H2O
The ability of metal oxides to trap electrons is evaluated using the work function [54]. The work function value as computed for MWCNTs is 4.60 and 4.80 eV, while AlCo2O4 has a work function of 6.55 eV [55]. The lower work function of MWCNTs relative to AlCo2O4 nanocomposites strongly facilitates the transfer of photogenerated electrons from MWCNTs to AlCo2O4 nanocomposites until Fermi levels are aligned as per Figure 15b. This electron movement enhances the charge density on the AlCo2O4 surface and decreases it on the surface of MWCNTs. Under the conditions of equilibrium as per Figure 15c, a positive charge is induced at the surface of MWCNTs via electrostatic induction, while a negative charge develops on the AlCo2O4 nanocomposite surface. The larger surface area of MWCNTs, along with their increased pore size, high electrical conductivity, and strong optical absorption may also contribute to the enhanced photodegradation of RY-160 [56].

3.13. Reusability

The reusability of AlCo2O4/MWCNs for the degradation of RY160 dye was also tested over seven cycles to assess its longer utilization and is exhibited in Figure 16. RY-160 solution was mixed at a concentration of 20 ppm while taking 150 mg of catalysts dose. The solution was then exposed to natural sunlight for 40 min to complete a cycle. After each cycle, the photocatalyst was entirely removed from the suspension by placing it in the centrifuge. The isolated AlCo2O4/MWCNTs catalyst particles were then cleaned with deionized water multiple times and dried at 700 °C for use in the next cycle. Strikingly, the efficiency was found to be more than 90% after seven cycles, which demonstrated the high stability and reusability of the synthesized catalyst. A little decline in degradation efficiency may be attributed to material weight loss or the presence of persistent pollutants on the surface of the catalyst during each cycle [13].

4. Conclusions

A simple co-precipitation approach was utilized to synthesize nanocomposites of AlCo2O4 and AlCo2O4/MWCNTs. FTIR analysis confirmed the presence of an aluminum cobaltite peak at 1023 cm−1, indicating the formation of carbon as a constituent of multiwall nanotubes. Furthermore, SEM images verified the formation of nanocomposites in both specimens. Our study demonstrated that AlCo2O4/MWCNTs nanocomposites exhibited superior photocatalytic activity compared to AlCo2O4 alone. Specifically, AlCo2O4/MWCNTs nanocomposites achieved over 96% degradation of Ry-160 dye within 20 min, showing enhanced performance. The process followed pseudo-first order kinetic with a rate constant of 0.151 min−1. Moreover, AlCo2O4/MWCNTs displayed excellent stability, and reusability, maintaining over 906% photocatalytic efficiency even after seven successive cycles. This improved performance is attributed to the excellent electron storage capacity of MWCNTs, which enhances the catalyst’s efficiency as an electron acceptor. Thus, AlCo2O4/MWCNTs nanocomposites are promising candidates for the degradation of RY-160 dye and could be utilized as an effective photocatalyst for the treatment of industrial wastewater.

Author Contributions

Conceptualization, J.A., U.I., A.I. and R.J.; methodology, J.A., M.R., U.Y.Q. and H.T.M.; validation, T.H., M.K., N.R., A.N. and A.A.H.; formal analysis, M.R., U.I., A.I., U.Y.Q. and R.J.; data curation, T.H., M.K., N.R., A.N. and F.Q.; writing—original draft preparation, J.A. and U.I.; writing—review and editing, U.I., R.J. and A.I.; visualization, M.R., U.Y.Q., H.T.M. and F.Q. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

All data generated or analyzed during this study are included in this published article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Steps in the coprecipitation method for the synthesis of spinal AlCo2O4 nanocomposites.
Figure 1. Steps in the coprecipitation method for the synthesis of spinal AlCo2O4 nanocomposites.
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Figure 2. EDX analysis of (a) AlCo2O4 and (b) AlCo2O4/MWCNs nanocomposites.
Figure 2. EDX analysis of (a) AlCo2O4 and (b) AlCo2O4/MWCNs nanocomposites.
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Figure 3. FESEM images for (a,c) pristine spinel AlCo2O4 nanocomposite and (b,d) AlCo2O4-anchored MWCNTs nanocomposites.
Figure 3. FESEM images for (a,c) pristine spinel AlCo2O4 nanocomposite and (b,d) AlCo2O4-anchored MWCNTs nanocomposites.
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Figure 4. FTIR spectra for (a) aluminum cobaltite and (b) aluminum cobaltite-anchored MWCNTs.
Figure 4. FTIR spectra for (a) aluminum cobaltite and (b) aluminum cobaltite-anchored MWCNTs.
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Catalysts 15 00154 g005
Figure 5. UV-Vis absorption spectra for (a) AlCo2O4 and (b) AlCo2O4/MWCNTs.
Figure 5. UV-Vis absorption spectra for (a) AlCo2O4 and (b) AlCo2O4/MWCNTs.
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Catalysts 15 00154 g006
Figure 6. Tauc plot for (a) AlCo2O4 nanocomposite and (b) AlCo2O4/MWCNTs nanocomposite.
Figure 6. Tauc plot for (a) AlCo2O4 nanocomposite and (b) AlCo2O4/MWCNTs nanocomposite.
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Catalysts 15 00154 g007
Figure 7. Photodegradation spectra of (a) spinel AlCo2O4 nanocomposite annealed at 300 °C and (b) spinel AlCo2O4/MWCNs nanocomposite annealed at 300 °C.
Figure 7. Photodegradation spectra of (a) spinel AlCo2O4 nanocomposite annealed at 300 °C and (b) spinel AlCo2O4/MWCNs nanocomposite annealed at 300 °C.
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Catalysts 15 00154 g008
Figure 8. Photodegradation results of AlCo2O4 as compared with AlCo2O4/MWCNs nanocomposite.
Figure 8. Photodegradation results of AlCo2O4 as compared with AlCo2O4/MWCNs nanocomposite.
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Figure 9. (a) Effect of different pH values on the color removal efficiency of catalyst (dye concentration: 20 ppm, AlCo2O4 amount: 125 mg) and (b) effect of different pH values on the color removal efficiency of catalyst (dye concentration: 20 ppm, AlCo2O4/MWCNs amount: 125 mg).
Figure 9. (a) Effect of different pH values on the color removal efficiency of catalyst (dye concentration: 20 ppm, AlCo2O4 amount: 125 mg) and (b) effect of different pH values on the color removal efficiency of catalyst (dye concentration: 20 ppm, AlCo2O4/MWCNs amount: 125 mg).
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Figure 10. Effect of temperature on removal efficiency of color for (a) AlCo2O4 and (b) AlCo2O4/MWCNs (dye concentration = 20 ppm, pH = 6).
Figure 10. Effect of temperature on removal efficiency of color for (a) AlCo2O4 and (b) AlCo2O4/MWCNs (dye concentration = 20 ppm, pH = 6).
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Figure 11. (a) Effect of AlCo2O4 dose on color removal efficiency (dye concentration: 20 ppm, pH = 6). (b) Effect of AlCo2O4/MWCNs dose on color removal efficiency (dye concentration: 20 ppm, pH = 6).
Figure 11. (a) Effect of AlCo2O4 dose on color removal efficiency (dye concentration: 20 ppm, pH = 6). (b) Effect of AlCo2O4/MWCNs dose on color removal efficiency (dye concentration: 20 ppm, pH = 6).
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Catalysts 15 00154 g012
Figure 12. Effect of initial dye concentration on degradation of dye using (a) AlCo2O4 (amount: 150 mg, pH = 6) and (b) AlCo2O4/MWCNs (amount: 150 mg, pH = 6).
Figure 12. Effect of initial dye concentration on degradation of dye using (a) AlCo2O4 (amount: 150 mg, pH = 6) and (b) AlCo2O4/MWCNs (amount: 150 mg, pH = 6).
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Catalysts 15 00154 g013
Figure 13. Effect of using NaHCO3 quencher on degradation of dye.
Figure 13. Effect of using NaHCO3 quencher on degradation of dye.
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Figure 14. −ln C/Co vs. time plot for pseudo first order kinetics.
Figure 14. −ln C/Co vs. time plot for pseudo first order kinetics.
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Figure 15. (a) Proposed mechanism of photocatalysis in combined catalyst AlCo2O4/MWCNTs, (b) energy level model of AlCo2O4/MWCNTs for the explanation of better electron transfer, (c) existence and direction of electric field between NiCo2O4 and MWCNTs.
Figure 15. (a) Proposed mechanism of photocatalysis in combined catalyst AlCo2O4/MWCNTs, (b) energy level model of AlCo2O4/MWCNTs for the explanation of better electron transfer, (c) existence and direction of electric field between NiCo2O4 and MWCNTs.
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Figure 16. Cyclic photodegradation of Ry-160 irradiated under sunlight by seven separate-wash-dry cycles for reprocessing the same catalyst.
Figure 16. Cyclic photodegradation of Ry-160 irradiated under sunlight by seven separate-wash-dry cycles for reprocessing the same catalyst.
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Table 1. Comparison of the degradation of RY-160 with other recent works in the literature.
Table 1. Comparison of the degradation of RY-160 with other recent works in the literature.
Technique UsedNovel Material% DegradationReference
BiosorptionNickel nanoparticles91.4% in 40 min[36]
BiosorptionCladosporium cladosporioides≈99.99% in 60 min[37]
ChemisorptionPolymeric ferrite composites94% in 60 min[38]
Photocatalysis (visible light)Bi2O3-ZnO heterostructure91% in 120 min[39]
Photocatalysis (visible light)AlCo2O4/MWCNTs composites>96% in 20 minThis Study
Table 2. Comparison with other novel visible light photocatalysts developed in 2024.
Table 2. Comparison with other novel visible light photocatalysts developed in 2024.
PhotocatalystDye% DegradationReference
Ag/TiO2 nanoparticlesMethylene Blue94% in 40 min[40]
NiAlFe LTH/MoS2 p-n junction heterostructureIndigo≈99.99% in 100 min[41]
Ni1−xCuxCo2−yFeyO4/g-C3N4Congo Red94% in 24 min[42]
10%F-10%B co-doped Carbon Quantum DotsRhodamine B89.13% in 60 min[43]
Silver doped barium titanate nanoparticlesEosin Yellow≈92% in 40 min
99.3% in 60 min		[44]
AlCo2O4/MWCNTs composites Reactive Yellow RY-160>96% in 20 minThis Study
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Ahmad, J.; Ikhlaq, A.; Raashid, M.; Ikhlaq, U.; Qazi, U.Y.; Masood, H.T.; Hussain, T.; Kazmi, M.; Ramzan, N.; Naeem, A.; et al. Novel AlCo2O4/MWCNTs Nanocomposites for Efficient Degradation of Reactive Yellow 160 Dye: Characterization, Photocatalytic Efficiency, and Reusability. Catalysts 2025, 15, 154. https://doi.org/10.3390/catal15020154

AMA Style

Ahmad J, Ikhlaq A, Raashid M, Ikhlaq U, Qazi UY, Masood HT, Hussain T, Kazmi M, Ramzan N, Naeem A, et al. Novel AlCo2O4/MWCNTs Nanocomposites for Efficient Degradation of Reactive Yellow 160 Dye: Characterization, Photocatalytic Efficiency, and Reusability. Catalysts. 2025; 15(2):154. https://doi.org/10.3390/catal15020154

Chicago/Turabian Style

Ahmad, Junaid, Amir Ikhlaq, Muhammad Raashid, Uzma Ikhlaq, Umair Yaqub Qazi, Hafiz Tariq Masood, Tousif Hussain, Mohsin Kazmi, Naveed Ramzan, Asma Naeem, and et al. 2025. "Novel AlCo2O4/MWCNTs Nanocomposites for Efficient Degradation of Reactive Yellow 160 Dye: Characterization, Photocatalytic Efficiency, and Reusability" Catalysts 15, no. 2: 154. https://doi.org/10.3390/catal15020154

APA Style

Ahmad, J., Ikhlaq, A., Raashid, M., Ikhlaq, U., Qazi, U. Y., Masood, H. T., Hussain, T., Kazmi, M., Ramzan, N., Naeem, A., Aly Hassan, A., Qi, F., & Javaid, R. (2025). Novel AlCo2O4/MWCNTs Nanocomposites for Efficient Degradation of Reactive Yellow 160 Dye: Characterization, Photocatalytic Efficiency, and Reusability. Catalysts, 15(2), 154. https://doi.org/10.3390/catal15020154

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