Facile and Low-Cost Fabrication of ZnO/Kaolinite Composites by Modifying the Kaolinite Composition for Efficient Degradation of Methylene Blue Under Sunlight Illumination

Abstract

Zinc oxide (ZnO) photocatalysts are recognized for their ease of synthesis, cost-effectiveness, efficiency, scalability, and environmental compatibility, making them highly suitable for addressing wastewater contamination. In this study, various compositions of kaolinite were used for the hydrothermal deposition of ZnO, including 0.5%, 0.75%, 1%, and 1.25%. The main purpose of this study was to evaluate the effect of kaolinite toward the enhanced performance of ZnO through modification of particle size, morphology and surface functional groups. Several analytical techniques were employed to obtain structural and optical results, including scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and UV–visible spectroscopy, revealing significant changes in particle shape, particle size, surface functional groups, and optical band gap when kaolinite was added. The ZnO/kaolinite composite (sample 4) with 1.25% kaolinite content demonstrated outstanding photocatalytic performance for the degradation of methylene blue in natural sunlight. For sample 4, 15 mg of the dye in a 3.4 × 10⁻⁵ M dye solution exhibited a degradation efficiency of 99%. In contrast, when using 15 mg of catalyst dose and 1.5 × 10⁻⁵ M dye solution, the degradation efficiency was observed to be almost 100%, thus indicating that catalyst dose and dye concentration affect degradation efficiency. The reusability test revealed that sample 4 retained degradation efficiency of 98% after five cycles without showing any morphological changes. By decorating ZnO with kaolinite mineral clay, this study provides exciting findings and insights into the development of low-cost photocatalysts, which could be used to produce solar-powered hydrogen and treat wastewater.

1. Introduction

Due to modern industrial processes and the growing population, various emerging contaminants are posing a growing threat to water quality, including pesticides, synthetic dyes, surfactants, and pharmaceuticals [1,2]. It is common for industrial operations to release a variety of organic dyes into wastewater, including methylene blue dye [3,4]. The removal of organic dyes from wastewater has been achieved using a variety of techniques, including chemical, photochemical, and electrochemical methods [5]; coagulation [6]; filtration [7]; biodegradation [8,9]; ultrafiltration; and adsorption [10,11]. In recent years, photocatalysts have gained considerable attention due to their remarkable efficiency, stability, non-toxicity, and corrosion resistance. Due to this, photocatalysts have emerged as a novel approach to wastewater treatment [12,13,14]. Semiconductors are frequently used as photocatalysts in a wide range of advanced oxidation processes (AOPs) to degrade dyes such as zinc oxide (ZnO) and titanium dioxide (TiO₂) [15,16]. It is important to note that semiconducting metal oxides have a number of potential disadvantages that include a very wide band gap, an extremely high electron-hole recombination rate, poor stability, and limited catalytic activity [10,17,18,19]. In light of its ease of synthesis, low cost, variety of shape structures, and ease of modification of its surface properties, ZnO can serve as an alternative photocatalyst. However, ZnO also has a relatively low quantum efficacy because of the rapid photoinduced hole/electron (h⁺/e⁻) pairs recombination [14,15]. Moreover, photo-corrosion of ZnO during light irradiation significantly reduces its photocatalytic performance and stability [16,17]. Consequently, these issues restrict the widespread use of ZnO.

To overcome these deficiencies, ZnO has been combined with other materials for the development of composites, which has in some way enhanced its performance [10]. It is for this reason that a wide range of materials are used to immobilize photocatalysts, including clay, silica, alumina, and zeolite [18]. Among these materials are clay and clay-based matrices, which offer the advantages of being resistant to deterioration, chemical inertness, as well as a wider market availability [19]. Due to the high cost of fabricating ZnO based composites, as well as the multiple steps involved in their synthesis, ZnO based composites have been limited in their use for degradation of organic dyes. Using aluminosilicate clay minerals as a low-cost and easily processed material, researchers prepared ZnO/clay nanocomposites and observed improved photocatalytic performance [5,20,21]. A strategy that involves incorporating nanomaterials into clay mineral structures has the potential to increase nanomaterials’ photocatalytic activity [22].

The use of clays such as kaolinite [23], montmorillonite [24,25], rectorite [26], and sepolite [27] in wastewater purification has garnered significant attention in recent years. On their exterior surfaces or within their interlaminar regions, natural materials may adsorb organic compounds by contact or substitution. Furthermore, they possess large surface areas, layer structures, and high cation exchange capacities [28]. In combining clays with ZnO and their topologically distinct chemistries (both inside and outside), promising composite photocatalysts can be synthesized [29,30]. Clay minerals combined with ZnO may increase the active area of the composite material, thereby enhancing its ability to effectively degrade pollutants [31]. It has been suggested that the superior performance of ZnO and clay-based composites is due to their ability to absorb light photons in the extended solar spectrum [32]. In spite of kaolinite’s unique structure and functionality and the ease with which it can be functionalized with ZnO, there have only been a few studies that have examined its effect on enhanced photocatalytic activity for the degradation of methylene blue.

A kaolinite mineral is an aluminum silicate mineral with a layered structure; its chemical formula is Si₄Al₄O₁₀(OH)₈ [33]. Among its most appealing characteristics are its low cost, chemical and physical stability, and nontoxicity [34,35]. As presented herein, we have been able to fabricate the next generation of ZnO/kaolinite nanocomposites that are capable of degrading methylene blue at high rates under sunlight irradiation. In this study, ZnO/kaolinite nanocomposites were synthesized by hydrothermal processing using varying amounts of kaolinite. In terms of morphology, crystal arrays, functional groups, and optical perspectives, a successful structure was achieved. In addition to altering the shape structure, reducing optical band gaps, reducing particle size, and modifying surface functional groups, other significant changes were also observed. These factors play a significant role in the efficient degradation of methylene blue.

2. Results and Discussion

2.1. Structure, Morphology, Surface Functional Group and Optical Studies

A powder XRD analysis was conducted on a variety of materials, including pure ZnO and its composites with different amounts of kaolinite (0.5%, 0.75%, 1%, and 1.25%). Figure 1 illustrates the typical diffraction patterns of pure ZnO, including (100), (002), (101), (110), (112), (004), and (201) at 31.68, 34.13, 36.03, 47.44, 56.26, 47.44, and 62.50, respectively. The patterns are in good accordance with the standard JCPDS card no. 96-900-4180, indicating that ZnO is hexagonal. Furthermore, ZnO composites containing kaolinite at varying concentrations (0.5%, 0.75%, 1%, and 1.25%) were analyzed by powder XRD. Figure 1 illustrates the distinctive patterns observed. In addition to ZnO diffraction patterns, typical kaolinite Miller indices were observed, which suggests the presence of ZnO/kaolinite composites. Figure 1 illustrates how the crystal patterns of kaolinite correspond to the standard JCPDS card number 96-101-1046. An XRD study revealed that the typical (101) peak was successively decreased with kaolinite addition during the growth process, which indicates a change in crystal nucleation and alteration of the ZnO shape orientation. A certain percentage of iron oxide, silica, and alluvium phosphate was observed in the ZnO/kaolinite composites as a consequence of XRD analysis, indicating these minerals are present in kaolinite’s chemical composition, which may have accelerated ZnO’s photocatalytic activity. No other compounds were detected in the XRD analysis.

Table 1. The estimated average crystallite size of different prepared materials.

Sample ID	2 Theta (deg)	Peak Position	FWHM	Height	Average Size (nm)
Pristine ZnO	100	31.68	0.31953	439	27
	002	34.32	0.27503	363
	101	36.14	0.31294	686
Sample-1	100	31.54	0.24833	373	33
	002	34.22	0.22486	259
	101	36	0.27881	548
Sample-2	100	31.7	0.13271	293	65
	002	34.32	0.12765	189
	101	36.14	0.12615	460
Sample-3	100	31.68	0.21342	267	35
	002	34.34	0.22506	200
	101	36.12	0.27395	379
Sample-4	100	31.64	0.20004	288	39
	002	34.3	0.2103	192
	101	36.12	0.23353	437

provides the results of the Scherrer equation, which was used to estimate the average crystallite size of the synthesized materials. The average crystallite size of pure ZnO and its composites with different amounts of kaolinite (0.5%, 0.75%, 1%, and 1.25%) were 27 nm, 33 nm, 65 nm, 35 nm, and 39 nm, respectively. These results indicate that during the synthesis of ZnO/kaolinite composites, the bulk kaolinite phase was induced, resulting in increased crystallite sizes.

A detailed examination of the surface morphology of the synthesized materials was necessary to improve their photocatalytic performance; therefore, SEM measurements were conducted, as shown in Figure 2. The SEM micrographs in Figure 2a–e illustrate the distinctive features of pure ZnO and its composites containing different amounts of kaolinite (0.5%, 0.75%, 1%, and 1.25%). Figure 2a shows that pure ZnO is associated with nanorod-like structures with a diameter of less than 200 nm and a length of a few microns. Adding 0.5% kaolinite for ZnO deposition tuned the morphology into a flowerlike structure. A series of nanorods with sharp surfaces on top were used to assemble these flowers. Figure 2b illustrates that the nanorods assembled into flowers could be 1 to 2 microns long and less than 100 nanometers in diameter. The morphology of the ZnO/kaolinite composites synthesized with 0.75% kaolinite was altered, as shown in Figure 2c, indicating the impact of kaolinite on ZnO morphology. In Figure 2d, the nanorods were turned into nanowires using 1% kaolinite for the decoration of ZnO because abundant groups on the surface altered the nucleation kinetics during the growth process, thus altering the morphology. As shown in Figure 2e, the deposition of ZnO using 1.25% clay mineral led to clustered and aggregated structures with a non-uniform orientation. As a possible explanation for the alteration in ZnO morphology in response to adding different amounts of kaolinite, it is possible that the favorable oxide surface of the clay altered the growth kinetics during ZnO deposition, resulting in a variation in ZnO shape and structure.

In Figure 3, FTIR spectra of pure ZnO and its composites with varying levels of kaolinite, such as 0.5%, 0.75%, 1%, and 1.25%, have been collected for the study of different surface functional groups. The typical metal–oxygen stretching vibration occurs between 400 and 600 cm⁻¹. It has been observed that pure ZnO contains a wide range of functional groups, including 431, 517, 701, 794, 917, 1034, 1108, 1342, 1386, 1595, 2846, 2926, and 3444 cm⁻¹. The stretching vibration between 400 to 517 cm⁻¹ could be assigned to Zn–O stretching vibration [35]. The peak at 1595 cm⁻¹ could be attributed to C–C aromatic stretching vibration, and 1342 and 1386 emerged due to the C–H alkenes rocking structure, and 1034 cm⁻¹ to the C–O stretching wings of alcohols/carboxylic. The peaks at 2846 and 2926 cm⁻¹ may be attributed to the stretching vibration of C–H of alkanes. The shoulder peak at 3444 cm⁻¹ could be attributed to adsorbed hydroxide groups [36]. In the case of ZnO/kaolinite composites, there was a slight change in the IR bands, and a few new bands were noticed at 467 and 542 cm⁻¹, indexed to the Zn–O stretching vibrations and absorbed/adsorbed water molecules between 3623 to 3702 cm⁻¹ [37]. The ZnO/kaolinite composites have been confirmed by the FTIR study, which has fully validated the XRD results. Specifically, it was important to investigate the effects of crystallite size and shape orientation on the optical properties of pure ZnO and its composites with different concentrations of kaolinite, such as 0.5%, 0.75%, 1%, and 1.25%.

Figure 4a illustrates the UV–visible absorption spectra used for describing the optical band gap values. A significant decrease in the absorption edge was observed for successive additions of kaolinite around 375 nm compared with pure ZnO, indicating that the kaolinite content strongly influences the absorption edge. Possibly, sample 4 exhibits a different behavior due to the high content of kaolinite, which influences both its particle morphology and particle size. This may be due to a higher kaolinite content, which may affect the optical absorption; therefore, sample 4 shows a different behavior as shown in Figure 4a. Defects such as oxygen vacancies, a reduction in crystallite size, and the shape of the material could contribute to the reduction of the optical band gap [37]. Figure 4b shows the Tauc plots and the corresponding optical values as an inset. For pure ZnO and its composites with different contents of kaolinite, such as 0.5%, 0.75, 1%, and 1.25, the optical band gap was observed to be reduced as compared to pure ZnO. For pure ZnO and its composites with different contents of kaolinite, the estimated values were 3.21 eV, 2.89 eV, 2.73 eV, 2.62 eV, and 2.44 eV, respectively. Several factors may contribute to the variation of the optical band gap, including morphology, crystalline properties, crystal grain size, and facets of crystal growth. Moreover, decreases in the optical band gap could be attributed to variations in particle size, shape structure, defects, and the growth process used [10].

2.2. Photocatalytic Performance of As-Synthesized ZnO/Kaolinite Composites

The as-synthesized ZnO/kaolinite composites were compared with pure ZnO under natural sunlight irradiation for evaluation of their photocatalytic performance. All the different materials were tested for dye degradation under similar conditions. In order to evaluate the performance of catalytic materials under the irradiation of natural sunlight with the same intensity of light, the different catalysts were placed in the different dye degradation vessels, where they were irradiated with almost the same intensity of light at the same time. This allowed us to minimize the effect of variation of light intensity and its impact on the photocatalytic performance of catalytic materials, as presented in the degradation of MB. This means the use of sunlight irradiation on the dye and material should be carried out in different reaction vessels at the same time, so that the effect of light intensity variation on the different testing materials can be minimized. To begin with, we tested pure ZnO with a catalyst dose of 15 mg in a 3.4 × 10⁻⁵ M dye solution. The distinctive UV–visible absorption spectra are shown in Figure 5a. ZnO exhibited a limited degradation rate, as demonstrated by the negligible decrease in absorption over the 240 min period. Figure 5a illustrates a 30 min time interval between each absorption spectrum. These absorption spectra measured at different time intervals were used to evaluate the degradation kinetics and dye degradation rate, as shown in Figure 5b,c. As shown in Figure 5b, the dye degradation followed pseudo-first-order kinetics, and the rate constant value was very low. As shown in Figure 5c, the degradation rate of MB in an aqueous solution under natural sunlight was observed to be limited, indicating the inefficiency of pure ZnO toward MB degradation. It was also estimated that the degradation efficiency would be 49%, as shown in Figure 5d. It is possible that the limited catalytic sites, the wide band gap, and the rapid charge recombination rate of electrons and holes are responsible for the limited performance of pure ZnO. Consequently, researchers are focusing their efforts on the development of the next generation of efficient photocatalysts by coupling ZnO with a variety of materials. With these aspects in mind, different amounts of kaolinite were combined with ZnO for growth, ranging from 0.5% (sample 1) to 0.75% (sample 2), 1% (sample 3), and 1.25% (sample 4), and their photocatalytic performance was evaluated against degradation of MB when exposed to natural sunlight.

As shown in Figure 6a–d, a catalyst dose of 15 mg was employed in a dye concentration of 3.4 × 10⁻⁵ M and irradiated with natural sunlight for 240 min. Using the same catalyst dose and concentration of dye to study the photocatalytic activity of sample 1, sample 2, sample 3, and sample 4, it was evident that the degradation of MB was enhanced linearly from sample 1 to sample 4, as was evident from the linear decrease in the absorption value during the collection of each UV–visible absorption spectrum, as illustrated in Figure 6a–d. Based on these measurements, it appears that kaolinite plays an important role in improving the photocatalytic performance of ZnO. In addition, as shown in Figure 6a–d, the variation in kaolinite content played a significant role in the enhanced degradation of MB under natural sunlight exposure. It is possible that the enhanced photocatalytic activity of ZnO is explained by the unique structure of kaolinite, by attractive surface groups that can be utilized to modify the shape orientation and size of ZnO particles, as well as by the tunable optical band gap of ZnO.

In addition, the kaolinite offered a chemical composition that, when combined with ZnO, achieved a synergistic effect in increasing the degradation rate of MB in an aqueous solution. SEM, XRD, FTIR, and optical band gap studies have shown the excellent role of kaolinite in changing the shape structure, chemical composition, particle size, and surface functional groups, and they together have driven the enhanced photodegradation of MB in an aqueous solution under the irradiation of natural sunlight. Using ZnO in conjunction with kaolinite has decreased the recombination rate of electrons and holes, increased the catalytic sites by providing favorable surfaces to ZnO, reduced the optical band gap, and improved the stability of the composite, demonstrating excellent photocatalytic performance. Similarly, the degradation kinetics of MB with the use of a 15 mg catalyst dose of sample 1, sample 2, sample 3, and sample 4 in the presence of a 3.4 × 10⁻⁵ M dye concentration was examined, as shown in Figure 7a,b. A pseudo-first-order kinetics was evident in the dye degradation kinetics, and the rate constant values were significantly increased, as shown in Figure 7a. Accordingly, the degradation kinetics of MB were dominated only by the MB concentration, and the water concentration showed negligible changes as a result of its excess, so that it could be said the degradation kinetics were pseudo-first-order. According to Figure 7b, the degradation rate of MB was highly increased. In Figure 7b, the degradation enhancement can be explained by the useful surface chemistry of composite materials and how its variability depends on the relative content of kaolinite. In addition, the degradation efficiency was calculated for sample 1, sample 2, sample 3, and sample 4, and the corresponding performance is shown in Figure 7c. Figure 7c illustrates the outstanding performance of sample 4 with regards to the degradation of MB, with a degradation efficiency value of 99%. Again, it was evident that the degradation efficiency of the composites was determined by their relative content of kaolinite, and sample 4, which had the highest percentage of kaolinite, showed superior degradation efficiency over samples 1, 2, and 3. After the evaluation of different ZnO/kaolinite composites toward the degradation of MB, it was found that sample 4 has better photocatalytic activity, hence it was employed for the role of catalyst dose and the initial dye concentration of 1.5 × 10⁻⁵ M. As shown in Figure 8a–c, the catalyst dose of 5 mg, 10 mg, and 15 mg of sample 4 was irradiated with natural sunlight for 120 min. The photocatalytic activity was basically enhanced due to the synergetic effect built between ZnO and kaolinite through influences on the shape, crystalline quality, and particle size that further affected the optical band gap of the composite material.

In addition to observing a drastic decrease in UV–visible absorption when increasing the catalyst dose and using a low concentration of MB, the as-synthesized ZnO/kaolinite composite (sample 4) demonstrated a dose and dye concentration-dependent performance. As shown in Figure 8c, increasing the catalyst dose resulted in a greater decrease in UV–visible absorption. This occurs because there are abundant catalytic sites in the dye solution that promote the degradation of MB, resulting in a drastic decrease in MB concentration. Sample 4 exhibited high performance in lower concentrations as a result of having sufficient surface area to communicate and interact with the surface photocatalysts, leading to enhanced degradation. Moreover, the degradation kinetics under the influence of catalyst dose and dye concentration were investigated, as shown in Figure 8d,e. Figure 8d indicates that the dye degradation followed pseudo-first-order kinetics, and the rate constant value was highly dependent on the dye concentration and catalyst dose due to the enhanced reaction rate. In addition, the degradation rate of MB was primarily determined by the use of a higher catalyst dose and a lower dye concentration, as shown in Figure 8e. It could be attributed to the use of a higher catalyst dose and frequent interactions with dye molecules, which could be fully adsorbable on the surface of the photocatalyst. The degradation efficiency was also calculated by the use of different catalyst doses and a lower concentration of 1.5 × 10⁻⁵ M of dye, as shown in Figure 8f. Figure 8f shows that the degradation efficiency was higher for the catalyst dose of 15 mg of sample 4, with a value of almost 100%. According to the degradation efficiency analysis, the degradation performance of the photocatalyst is highly dependent on the catalyst dose and dye concentration.

In order to demonstrate the pH-dependent photocatalytic performance of the ZnO/kaolinite composite (sample 4), the pH environment on the surface of the photocatalyst played a significant role in dye degradation. The pH of the MB 1.5 × 10⁻⁵ M solution was adjusted to 3, 6, 9, or 12 using 2 M HCl and NaOH aqueous solutions. A catalyst dose of 15 mg was used for each pH value of the dye solution, and the solution was irradiated with natural sunlight for a specific amount of time. Following this, UV–visible absorption spectra were obtained at different intervals of time, as shown in Figure 9a–d. As shown in Figure 9a, the dye degradation rate was limited at an acidic pH of 3, as indicated by a significant decrease in the absorption value. This could be attributed to the cationic nature of MB, enabled by the repulsion induced by the presence of excessive hydrogen ions due to acidic pH, thus reducing the interaction with the surface of the photocatalyst and explaining the limited degradation rate as shown in Figure 9a. However, when the pH of the dye solution was increased, the degradation of MB was found to be more effective, and was the highest at alkaline pH 12, as shown in Figure 9b–d. Therefore, the degradation of cationic dyes like MB is favorable under an enriched negative hydroxide ions environment. As shown in Figure 9d, dye absorption was reduced dynamically and rapidly in a short period of time at an alkaline pH of 12. Interestingly, the kaolinite surface becomes more negatively charged at high alkaline pH, and its higher presence on the surface of the ZnO/kaolinite composite (sample 4) could attract the positively charged MB, thereby increasing degradation rates. Variations in the pH of the dye solution also affect the charge on the surface of the photocatalyst. The pH change in the dye solution could lead to the generation of hydroxyl radicals, the degradation of dye directly with positively charged holes from the valence band, and the reduction of dye directly with electrons from the conduction band. The surface of the photocatalyst receives a positive charge below its isoelectric point, while above its isoelectric point, the surface receives a negative charge. Therefore, the ZnO with kaolinite possesses a higher positive surface charge in acidic pH of the MB solution, while the composite photocatalyst exhibited a negative surface charge in alkaline pH of 12. Thus, an increased degradation rate of MB was observed under this condition since MB is cationic in nature [36,37].

The degradation kinetics and rate of MB under different pH values were examined, and the obtained information is shown in Figure 10a,b. It can be seen that the degradation kinetics well-fit the pseudo-first-order model with large rate constant values. The rate constant value was higher in a pH of 12, confirming the excellent degradation of MB. The degradation rate of MB was highly dependent on the pH of the dye solution and was higher in an alkaline pH of 12 compared to an acidic pH of 3. Additionally, the pH of the dye had a large impact on the time taken for the degradation of MB, and it was seen that MB took a short time to be degraded in an alkaline pH of 12. The degradation efficiency based on the effect of the pH of the dye solution was studied, and the findings are reported in Figure 10c. It was seen that the degradation efficiency was the highest, with a value of 100%, at a pH of 12, revealing that the MB was completely mineralized into harmless products within a short span of 18 min. The pH study has shown that the ZnO/kaolinite composite (sample 4) performed outstandingly in an alkaline pH of 12, owing to its more negative surface charge that favored the interaction with MB due to its positive charge.

2.3. Scavenger and Reusability Tests

For the experimental validation of photocatalytic performance, a variety of scavengers have been used to quench the reactive radicals, offering an important experimental clue about the degradation mechanism. For this purpose, photodegradation of MB in the vicinity of different scavengers such as ethylenediamine tetracetate, silver nitrate (AgNO₃), and ascorbic acid (C₆H₈O₆) with a concentration of 10 ppm were employed in the MB 1.5 × 10⁻⁵ M solution and catalyst dose of 15 mg to harness the reactive species, as shown in Figure 11a. Using ethylenediamine tetracetate, silver nitrate (AgNO₃), and ascorbic acid as scavengers for hydroxide radicals (•OH), superoxide radicals (•O₂⁻), holes (h⁺), and electrons (e⁻), reactive species were monitored during the degradation of MB using sample 4 under the irradiation of natural sunlight, as shown in Figure 11a. Silver nitrate possessed limited performance with holes and electrons from sample 4 during the degradation of MB under the irradiation of natural sunlight for a span of 210 min. Consequently, the holes and electrons emerged as primary reactive species for the degradation of MB, as shown by the decrease in degradation efficiency from 98% to 22%, as shown in Figure 11a.

Furthermore, reusability tests were performed to confirm the stability of the ZnO/kaolinite composite (sample 4) with a catalyst dose of 15 mg in MB 1.5 × 10⁻⁵ M solution during five repeatable degradation cycles of MB, as shown in Figure 11b. For each reusability test, the photocatalytic material was obtained via a strong centrifuge machine operated at 5000 rpm, and there was a slight loss of the material during successive reusability tests. For the controlled experiment, a degradation efficiency of 98% was noticed, and it slightly decreased to 89% during the fifth cycle of reusability, as shown in Figure 11b. To validate the structural and mechanical stability, SEM micrographs of the ZnO/kaolinite composite (sample 4) were collected, as shown in Figure 11c. It was seen that the material retained its shape and structure, confirming excellent structural stability during five consecutive reusability tests.

2.4. Photocatalytic Mechanism

Generally, the dye degradation mechanism on a semiconducting material like ZnO has been illustrated as given previously. The irradiation of light with a wavelength greater than the wavelength of an optical band gap of a photocatalyst like ZnO causes an electron to move from the valence band (VB) to the conduction band (CB). The photons either from sunlight or ultraviolet lamps have been used for these electronic distributions. The excitation of electrons takes place through the applied photonic energy and it causes the generation of electron and hole pairs. The negatively charged electrons are observed in the CB and the positively charged holes are noticed at the VB. The reduction and oxidation reactions can happen on the surface of ZnO due to these generated electron and hole pairs [38]. The absorption of water and hydroxide ions in the vicinity of VB interact with the positively charged holes to produce strong hydroxyl radicals. The interaction of dissolved oxygen with the electrons in the CB results in the formation of hydrogen peroxide, and later it reacts with superoxide radicals and forms hydroxyl radicals. Finally, these hydroxyl radicals interact with the dye molecules like MB adsorbed on the surface of ZnO, thereby short-lived substances like carbon dioxide, water and mineral acids have been formed [39]. Furthermore, ZnO-based photocatalysts exhibit highly reactive species such as electrons and holes at their surface, enabling them to potentially carry out the redox reactions that generate hydroxyl and superoxide radicals. The CB of ZnO/kaolinite (sample 4) showed a reduction potential of  −1.14 eV vs. a normal hydrogen electrode (NHE) for reducing O₂ to ^·O₂⁻(O₂/^·O₂⁻) reactive species. The VB of ZnO has an oxidation potential of 2.44 eV vs. NHE for the oxidation of H₂O and OH⁻ (OH⁻/^·OH) species. But the VB of ZnO has the energy of 1.3 eV, as shown in Scheme 1, which tends to convert water into ^•OH through H⁺; therefore, the organic dyes may be mineralized into non-toxic compounds such as carbon dioxide and water because of their oxidative characteristics [40,41]. Therefore, the possible chemical reactions taking place during the degradation of dye are given as follows:

ZnO + hυ ⟶ ZnO + e⁻ + h⁺

(1)

e⁻ + O₂ ⟶ O₂^•−

(2)

h⁺ + H₂O ⟶ OH^• + H⁺

(3)

O₂^•− + 2H⁺ ⟶ H₂O₂

(4)

H₂O₂ + e⁻ ⟶ OH^• OH⁻

(5)

OH^• or O₂^•− + MB Dye ⟶ CO₂ + H₂O other degraded products

(6)

Scheme 1 describes ZnO, having an optical band gap of 3.21 eV, and its composition with kaolinite possesses an optical band gap of 2.44 eV. It could be seen that the ZnO/kaolinite (sample 4) delivered higher efficiency in removing the MB compared to pure ZnO; thus, the role of the optical band gap for dye removal was described through Scheme 1. Also, it illustrates the role of band potential toward enhanced dye degradation performance by the composite material. The photocatalytic mechanism is described in brief in Scheme 1.

The MB degradation into intermediate products could be described as previously. The MB possesses a maximum absorption band in the visible region around 664 nm and a small shoulder band at 610 nm that is linked to the dye dimer. The coloring property of MB is associated with chromophoric (N–S conjugated system located at a central aromatic heterocycle) and auxochrome groups (N-containing groups having a lone pair of electrons on a benzene ring). The photocatalytic degradation of MB is not only accompanied by adsorption, but it is mainly achieved by the ZnO/kaolinite photocatalytic oxidation process. There are two possible ways to show the intermediate products during the photocatalytic degradation mechanism of MB. Firstly, during the photocatalytic process, sulfhydryl (C–S⁺=C) is converted to a sulfoxide (C–S(=O)–C), thereby opening the central aromatic heterocycle. The ˙OH offers the electrophilic attack on the free doublet of S heteroatom and transforms it from −2 to the 0 oxidation state, while the conversion of C–S⁺˙C to C–S(˙O)–C needs the conversion of double bond conjugation through the opening of the central aromatic ring containing both heteroatoms (S and N). The C–H and N–H bond formation requires H atoms that might originate from proton reduction via photogenerated electrons. Consequently, the intermediate products obtained during the photodegradation of MB are given as 2-amino-5-(N-methyl formamide) benzene sulfonic acid (m/z = 230), 2-amino-5-(methyl amino)-hydroxybenzene sulfonic acid (m/z = 218), benzenesulfonic acid (m/z = 158), and phenol (m/z = 94). These intermediate products are achieved during the degradation of MB at the chromophoric group. Secondly, the degradation of MB via the photocatalytic process at the demethylation group may result in the formation of various intermediate products such as azure A (m/z = 270), azure B (m/z = 256), azure C (m/z = 242), and thionic (m/z = 228). The possible formation of intermediate products through chromophoric and auxochromic groups is described by Scheme 2a,b.

Table 2. The perspectives of the as developed ZnO/kaolinite composite (sample 4) versus recently reported photocatalysts.

Photocatalyst	Synthesis Method	Pollutant	Catalyst Dose	Removal (%)	References
ZnO/KC NCs	Chemical reduction method	MR (100 ppm)	0.02 g	90%	[42]
HC/ZnO NC	Chemical method	MB (25 ppm)	20 mg	90% to 97%	[22]
AAK/ZnO nanocomposites	Wet chemical precipitation method	MB	--------	98%	[43]
ZnO(7.5%)/Bentonite	Impregnation method	SR-3BL (5–75 mg/L)	0.25–1 gL−1	70%	[44]
ZnO/MK30 nanocomposites	Ultrasonication technique	MB	--------	95.76%	[25]
ZnO/sepiolite photolytic composites	Sol–gel method	MB (0.015 g/L)	20 mg	93.5%	[45]
Cu2O/ZnO/Kaolinite-based composite	Co-precipitation method	MB	--------	93%	[46]
ZnO/Kaolinite Sample-4	Hydrothermal method	MB (1.5 × 10−5 M)	15 mg	98%	Herein

shows comparisons of various ZnO composites that were recently developed as photocatalysts in [40,41,42,43,44], and as given in Table 2. It was found that for varying the content of kaolinite during the fabrication of ZnO composites, there is no similar study in the existing literature, and the performance of the proposed material was superior in many aspects such as degradation efficiency, facile and cost-effective nature, and ecofriendly; hence, it can be used as an alternative photocatalyst for real wastewater treatment applications.

3. Materials and Methods

3.1. Chemical Reagents

For the preparation of the materials, various chemicals were used, including zinc acetate dihydrate (Zn(C₄H₆O₄)·2H₂O, ammonia, sodium hydroxide, hydrochloric acid, methylene blue, acetone and ethanol, which were all obtained from Sigma-Aldrich Karachi, Sindh, Pakistan. The clay kaolinite was collected from the desert region of Thar in the southern region of Sindh province. Deionized water was used to prepare the solutions for the synthesis of the materials and the dye degradation process.

3.2. Synthesis of ZnO/Kaolinite Composites Through Hydrothermal Process

Firstly, pure ZnO was prepared using 0.12 M zinc acetate dihydrate and 5 mL ammonia by making a volume of growth solution up to 100 mL using deionized water. A magnetic stirrer was then used to stir the growth solution until the chemical reagents were completely dissolved. A glass beaker containing growth solution was tightly wrapped with aluminum foil and placed in a preheated laboratory oven at 95 °C for five hours. Following the completion of the growth process, the white precipitates with ZnO were collected on the laboratory filter paper and dried at room temperature before further use. To produce the ZnO/kaolinite composites, different amounts of kaolinite were used, namely, 0.5% (sample 1), 0.75% (sample 2), 1% (sample 3) and 1.25% (sample 4). In addition to the successfully synthesized pure ZnO, samples 1, 2, 3, and 4 based on ZnO/kaolinite composites were used to demonstrate structural and methylene blue degradation.

3.3. Structural Analysis

A variety of analytical techniques have been used to evaluate the structure, shape orientation, surface functional groups, and optical properties of the synthesized materials. Among the most important aspects of this study were the verification of the successful formation of ZnO/kaolinite composites, the effect of kaolinite on the morphological and crystalline properties of ZnO, as well as its optical properties. To describe the optical properties of ZnO/kaolinite composites, a UV–visible spectrometer (Double beam, Model: UH-5300, HITACHI) was used. Surface functional groups were identified using Fourier transform infrared (FTIR) spectroscopy of ID7ATR Thermo Scientific (Waltham, MA, USA)/NicoLET IS5 in the range 400–4000 cm⁻¹. The crystal structure was evaluated by using an X-ray diffractometer (Philips PW1729) with a source of X-rays from Cu Kα radiation (λ = 1.5406 Å) and applying 45 kV and 40 mA. Scanning electron microscopy (SEM) with a Carl Zeiss Ultra Plus model was used to reveal the shape structure. We conducted the photocatalytic measurements under the irradiation of natural sunlight between 11:00 a.m. and 3:00 p.m.

3.4. Photocatalytic Measurements

Two different methylene blue (MB) concentrations, 3.4 × 10⁻⁵ M and 1.5 × 10⁻⁵ M, were prepared in deionized water. In the 3.4 × 10⁻⁵ M dye solution, the catalyst doses of pure ZnO, sample 1, sample 2, sample 3, or sample 4 of 5 mg were used in separate solutions. Initially, magnetic stirring was used to achieve equilibrium between adsorption and desorption in the dark. Afterward, the dye solution was irradiated with natural sunlight for different intervals of time and the absorption was measured using a UV–visible absorption spectrometer after being exposed to natural sunlight. The catalyst doses of 5 mg, 10 mg and 15 mg were studied for sample 4 of ZnO/kaolinite composite in 1.5 × 10⁻⁵ M. The effect of the pH of a 1.5 × 10⁻⁵ M dye solution was tested by adjusting the pH with 2 M HCl and NaOH solutions to final pH values of 3, 6, 9 and 12 for a 15 mg catalyst dose of sample 4 of the ZnO/kaolinite composite. The scavenger test was also conducted using 10 ppm of ethylenediamine tetracetate, silver nitrate and ascorbic acid using 1.5 × 10⁻⁵ M dye solution and 15 mg of sample 4. The degradation efficiency was calculated as follows:

Degradation efficiency (%): C_o − C_t × 100/C_o

(7)

where C_o and C_t represent the initial dye concentration and the dye concentration left after a certain interval of time, respectively.

−ln C_t/C_o = −k_app.t

(8)

K_pp describes the rate constant (min⁻¹) and t shows the irradiation time (min) of light during the degradation of MB in aqueous solution.

4. Conclusions

Overall, this study used a facile method by coupling kaolinite clay mineral with ZnO during the hydrothermal method. A variety of kaolinite contents, including 0.5%, 0.75%, 1%, and 1.25%, were used and decorated with ZnO nanostructures. Morphological analysis revealed that the shape orientation changed as successive additions of kaolinite were made during the growth process. Composites were associated with the hexagonal phase of ZnO and the monoclinic phase of kaolinite. A number of surface functional groups were observed on the composite materials. As the ZnO/kaolinite-based composites were exposed to natural sunlight, degradation of MB in the aqueous solution was dominated by the presence of kaolinite. The initial dye concentration, catalyst dose, and pH of the dye solution were investigated, and lower dye concentrations, higher catalyst doses, and an alkaline pH of the dye solution were found to be favorable parameters for the efficient degradation of MB. In the cycling stability test, there was no evidence that the structure of MB had been altered, and the scavenger test demonstrated that electrons and holes are responsible for the degradation of MB into mineralized products. As a result of these findings, it appears that kaolinite clay mineral could be a unique and attractive host material for the development of next-generation photocatalytic materials, both for wastewater treatment and the production of hydrogen driven by solar energy.