Green Synthesis of TiO₂-CeO₂ Nanocomposites Using Plant Extracts for Efficient Organic Dye Photodegradation

Abstract

The growing presence of hazardous organic pollutants in wastewater poses severe environmental and health risks, necessitating sustainable and efficient treatment solutions. Traditional remediation methods have limitations, highlighting the need for innovative approaches. A green synthesis method was developed to produce TiO₂-CeO₂ nanocomposites using Cleistocalyx operculatus leaf extract. The photocatalytic efficiency of the synthesized nanocomposites was evaluated under simulated sunlight by degrading Methylene Blue (MB) dye. Various compositions were tested to determine the optimal performance. The 0.1% TiO₂-CeO₂ nanocomposite achieved the highest degradation efficiency (95.06% in 150 min) with a reaction rate constant (k) of 18.5 × 10⁻² min⁻¹, outperforming commercial TiO₂ (P25, 74.85%, k ≈ 3.7 × 10⁻² min⁻¹). Additionally, the material maintained excellent stability over eight consecutive cycles with only a slight decrease in efficiency from 95.85% to 93.28%. The enhanced photocatalytic activity is attributed to the synergistic effects of CeO₂ incorporation, which enhances charge separation, extends visible light absorption, and promotes reactive oxygen species (ROS) generation. These findings highlight the potential of green-synthesized TiO₂-CeO₂ nanocomposites as a cost-effective and sustainable solution for wastewater treatment.

1. Introduction

In recent years, the rapid development of industrialization has led to the extensive use of organic compounds in water that pose a risk to our health as well as the environment. Out of the organic compounds, Methylene Blue (MB) is one of the most common dyes that is widely used in dying process of cotton fabrics, silk, and wood [1]. Moreover, MB has potential applications in various industrial fields, such as in testing the freshness of milk and dairy products and treating some fish’s diseases [2]. However, MB is regarded as a biologically toxic compound to the environment and life on the earth due to its ability to damage to the nervous system and eyes [1]. Further, like other synthetic dyes that have a complex aromatic molecular structure, MB is stable and hard to degrade naturally. Therefore, MB dye needs to be removed from wastewater.

Many methods have been reported for the removal of MB dye. Although diverse traditional treatment approaches, such as chemical (reduction [3], ozonation [4], precipitation [5], coagulation [6], ion exchange [7]), physical (adsorption [8], selective filters [9], membrane separation [10]), and biological (decomposition by microorganisms [11] such as algae [12] and bacteria [13]) procedures have been effectively applied to clean and treat the wastewater. However, these methods often face several limitations as they operate in specific conditions, produce poisonous byproducts, and require expensive equipment [14]. In addition, due to the presence of aromatic rings in the chemical structure of MB compounds, this dye is highly stable towards sunlight and is also resistant against high temperatures, some oxidizing reagents and microbial disintegration. Thus, it is necessary to develop more useful and advanced techniques to remove or degrade MB dye in water.

Photocatalyst degradation appears to be a promising alternative for dye removal in wastewater due to its simple, cost-effective operation and environment-friendly [15] by quickly converting organic pollutants under light irradiation into simple gases like CO₂, H₂O, or other harmless smaller molecules [15]. In this regard, a nanoparticles-based semiconducting photocatalyst is an attractive candidate for the removal of organic contaminants in wastewater treatment due to their huge surface area, high reactivity, and other dependent features [16]. On account of the photocatalytic activity of semiconductors, the absorbed light with higher energy than the semiconductor band gap will produce electrons and holes, which then can accelerate the redox reactions to degrade the organic pollutants.

Several semiconductor nanomaterials, such as TiO₂ [17], ZnO [18], CuO [19], WO₃ [20], Ag₃PO₄ [21], and CdS [22], have been found to be good photocatalysts for organic dye photodecomposition. In particular, a TiO₂-based photocatalyst is the most popular catalyst for dye degradation owing to its inert chemical properties, high stability, low toxicity and relative availability at low cost [23]. Unfortunately, the optical bandgap of TiO₂ is relatively broad (3.0–3.2 eV), limiting its usage only under UV light with the absorption below 360 nm and affecting its photocatalytic efficiency due to the easy electron–hole recombination [24]. In order to improve its photocatalytic activity, many solutions were proposed. Among them, modification with other materials is an attractive solution because it can improve the lifetime of the photogenerated charges and enhance the spectra response in the visible absorption region. The integration of TiO₂ with various metal oxides such as SnO₂/TiO₂ [25], ZnO/TiO₂ [26], WO₃/TiO₂ [1], Bi₂O₃/TiO₂ [27] and In₂O₃/TiO₂ [28] were reported to have an improved catalytic activity compare to the intrinsic material alone.

Among different metal oxides, cerium (IV) oxide (CeO₂) has attracted much more attention because of its high electrical conductivity and thermal stability, high UV absorption ability, non-toxic, low cost and biocompatibility [29]. Recently, the photocatalytic performance over a CeO₂-based photocatalyst has been widely studied in various applications, such as solar cells, CO₂ conversion and organic pollutant degradation [30]. The most outstanding properties of this metal oxide that facilitate the photoreactions are primarily derived from its high oxygen storage capacity with abundant oxygen vacancies and Ce³⁺ defect sites in the structure of CeO₂ due to its easy transition between the Ce(IV) and Ce(III) oxidation states with the corresponding displacement from CeO₂ and Ce₂O₃ under redox condition that occurs in this particle [31]. The oxygen released by CeO₂ plays an important role in the oxidation reactions. CeO₂ can store oxygen from an aqueous solution and transfer this adsorbed oxygen to TiO₂, these oxygen species are necessary in the oxidation of organic pollutants and also act as electron scavengers which reduce the recombination rate of electron–hole pairs in TiO₂ composites. In addition, various oxidation states of Ce (Ce³⁺ and Ce⁴⁺) could favor electron transfer and enhance the charge separation. Thus, these findings suggest that coupling TiO₂ with CeO₂ could lengthen the lifespan of the photogenerated electron and holes, resulting in an enhancement in the photocatalytic activity.

Recently, cerium dioxide (CeO₂) has attracted considerable attention as a photocatalyst for the degradation of MB, a common dye in wastewater. Due to the face-centered cubic structure of cerium oxide, there are a large number of oxygen vacancies on its surface and body. The existence of these oxygen vacancies makes CeO₂ suitable for applications in adsorption and catalytic remediation of water pollution [32]. For example, Wu et al. prepared CeO₂ for the efficient degradation of volatile organic compounds under vacuum ultraviolet light [33]. In addition, Thanh Tuyen et al. [32] showed that the incorporation of CeO₂ into TiO₂-NTs enhanced the photocatalytic activity in the visible light region. CeO₂/TiO₂-NTs are stable and have potential as visible light photocatalysts for the degradation of organic substances in aqueous solutions.

The photocatalytic performance of TiO₂-CeO₂ nanocomposites is highly dependent on the synthesis method, which significantly influences crystal structure, particle size, and surface characteristics. Various strategies have been employed to enhance their activity [34]. For instance, CeO₂–TiO₂ synthesized via mechanochemical grinding exhibited ~63% degradation of Rhodamine B under visible light after 240 min, attributed to its structural stability and reusability potential [35]. A green synthesis approach using Cleome chelidonii leaf extract resulted in superior MB degradation (95% within 150 min under UV–Vis irradiation), due to reduced band gap, enhanced surface adsorption, and suppressed electron–hole recombination [36]. Moreover, core/shell TiO₂/CeO₂ nanostructures with low CeO₂ content demonstrated remarkable photocatalytic and photoelectrochemical performance, achieving 71% MB degradation and a high photoconversion efficiency of 0.70%, ascribed to efficient charge separation [37]. Notably, a modified sol–gel method produced TiO₂-CeO₂ nanocomposites with a mixed anatase–fluorite heterojunction, ultrasmall particle size (~10 nm), and high BET surface area, leading to enhanced visible light absorption and up to 95% MB degradation under simulated solar irradiation [38]. Among these methods, chemical preparation is the most popular approach to synthesizing nano-photocatalysts, but it requires specific synthetic conditions and toxic chemical precursors that restrict the applications of the products due to serious eco-toxicological concerns.

In this respect, the sustainable green method using natural plant extracts as the precursors to produce nanomaterials is a promising solution because this approach avoids the use of chemicals as well as the toxicity of the products. This process is also more cost-effective because plant materials are abundant in nature. In addition, organic compounds in the plant extract can act as capping or chelating reagents to avoid agglomeration in nanoparticles [39]. To the best of our knowledge, the use of green TiO₂-CeO₂ synthesized from plant extract as a catalyst for dye photodegradation has not been reported before in the literature.

This study aimed to develop a novel green synthesis of TiO₂-CeO₂ nanocomposites using Cleistocalyx operculatus leaf extract and assess their efficacy in removing organic waste, marking a significant advancement in eco-friendly nanomaterial production. The innovative use of ultrasonic solvent extraction—optimized with an ethanol–water ratio of 1/20, a raw material–solvent ratio of 5/30 (g/mL), and a 30 min extraction time—eliminates the need for toxic chemicals, setting a new standard for sustainable synthesis methods. This plant-based approach not only ensures environmentally safe production but also actively supports ecosystem preservation, distinguishing it as a novel, green alternative to conventional chemical-intensive techniques.

The results indicate that TiO₂-CeO₂ nanocomposites are highly effective in decomposing MB under simulated sunlight, achieving up to 95.06% degradation in 150 min—a performance markedly superior to commercial TiO₂ (P25) at 74.85%. The material’s feasibility and stability were further validated through photocatalytic recycling experiments, underscoring its strong potential for water pollution treatment using natural sunlight. These findings highlight the novelty of leveraging green-synthesized semiconductor oxides, offering a sustainable, high-performance solution for environmental remediation and advancing the field of photocatalytic wastewater treatment.

2. Results and Discussion

2.1. Morphological and Microstructural Characterization

FESEM images clearly show the change in morphology of TiO₂-CeO₂ nanocomposite materials through two processing stages. In the SEM images at positions (a–c)—the material after synthesis but not through the calcination process—the TiO₂-CeO₂ particles have a dense cloud structure, tightly linked into a regular network without forming individually.

After calcination at 500 °C for 2 h, the TiO₂-CeO₂ nanoparticles exhibited an ill-defined morphology, as illustrated in Figure 1d–f, in contrast to the more dispersed and spherical structures observed in the as-synthesized samples. The cloud-like architecture was retained, with the formation of tightly bonded and agglomerated clusters, indicating partial structural rearrangement and densification induced by thermal treatment.

Compared to pure TiO₂ nanoparticles (Figure S1, Supporting Information), which exhibit a well-defined spherical morphology, the TiO₂-CeO₂ nanocomposites demonstrated substantial changes in surface texture and particle organization. This morphological evolution underscores the significant role of the Cleistocalyx operculatus leaf extract in the synthesis process. The extract not only serves as a reducing agent but also acts as a stabilizing agent during nanoparticle formation. Specifically, polyphenolic compounds present in the extract are responsible for reducing titanium (Ti⁴⁺) and cerium (Ce³⁺) ions, facilitating the formation of TiO₂-CeO₂ nanostructures. Moreover, these biomolecules likely act as complexing ligands, promoting the formation of interconnected nanoclusters rather than discrete particles.

This bio-assisted synthesis route contributes to enhanced interaction and intimate contact between TiO₂ and CeO₂ domains, which may improve charge separation efficiency and increase photocatalytic performance under light irradiation.

The crystal structures of TiO₂-CeO₂ catalysts were studied via X-ray diffraction (XRD), as shown in Figure 2a. Firstly, all the black-labeled diffraction peaks observed in the XRD pattern align well with the anatase TiO₂ structure (JCPDS card no 21-1272). The pattern displays diffraction peaks corresponding to the (101), (004), (200), (105) and (215) crystal planes of TiO₂ in heterostructure, with a particularly high crystallinity observed on the (101) plane. Secondly, the green labeled in the XRD pattern clearly indicates diffraction on the (111), (200), (220), (311), (222) and (331) crystal planes of CeO₂ in the heterostructure, all of which match the pure cubic structure of CeO₂ (JCPDS card no 34-0394) [40,41]. In addition, no diffraction peaks from other phases were detected, confirming the successful preparation of the TiO₂-CeO₂ heterostructure. Furthermore, the CeO₂ peak in the figure is neither as sharp nor as intense as that of TiO₂, suggesting that CeO₂ has lower crystallinity compared to TiO₂.

The average crystallite size (D) of nanocomposite was calculated based on their most intense diffraction peak using the Debye–Scherrer equation:

$$\begin{array}{c}DB=K·\lambda /\beta cos\theta \end{array}$$

(1)

where K is Scherrer’s constant (0.9), λ is the wavelength of X-ray diffraction (λ = 1.5406 Å), β is full width at half maximum (FWHM), and θ is the Bragg’s angle. The estimated mean grain size of the TiO₂-CeO₂ composite was 0.47 nm, and the diffraction pattern exhibited no obvious impurity peaks.

Energy-dispersive X-ray spectroscopy (EDS) was used to analyze the composition of the TiO₂-CeO₂ composite. As shown in Figure 2b, the spectra confirm the presence of Titanium (Ti), cerium (Ce) and oxygen (O) in the nanocomposite. The atomic ratio of (Ti + Ce)/O was approximately ½, suggesting the formation of TiO₂-CeO₂ composite structure [40].

Figure 3 presents the Raman spectrum of the TiO₂-CeO₂ nanocomposite, where the anatase phase of TiO₂ is identified by characteristic peaks near 148, 195, 395, 514, and 639 cm⁻¹. Additionally, the band at 450 cm⁻¹ corresponds to the F_2g vibrational mode of CeO₂’s cubic fluorite structure [42,43]. The broadening of the spectral peaks in the synthesized samples can be attributed to factors such as small particle size, surface stress, particle size distribution heterogeneity, and non-harmonic effects induced by high temperatures, which collectively cause shifts and variations in the peak positions [44].

The BET analysis of the TiO₂-CeO₂ nanocomposite (Figure 4) revealed a type IV nitrogen adsorption isotherm, characteristic of mesoporous materials, accompanied by a type H3 hysteresis loop as per the IUPAC classification. The measured BET surface area was 29.01 m²/g, with an average pore volume of 0.23 cc/g and a mean pore diameter of 1.90 nm. The presence of the H3 hysteresis loop suggests the formation of a heterogeneous porous structure, attributed to the capillary condensation process. In mesoporous materials, the disparity between adsorption and desorption isotherms typically indicates complex interactions occurring within the capillary network [43,45].

The UV–Vis diffuse reflectance spectra of pure TiO₂ and TiO₂-CeO₂ nanocomposites (Figure 5a,c) reveal significant optical absorption from the UV to the visible region. Pure TiO₂ exhibits an absorption edge around 400 nm, consistent with its intrinsic band gap (~3.15 eV), while the TiO₂-CeO₂ nanocomposite shows a red-shifted absorption edge approaching 450 nm, indicating improved light harvesting in the visible region [14,40].

The Tauc plots (Figure 5b,d) further quantify this red shift, showing a decrease in the direct band gap energy from 3.15 eV for pure TiO₂ to 2.57 eV for the TiO₂-CeO₂ nanocomposite [40]. This notable band gap narrowing suggests a modification of the electronic structure due to CeO₂ incorporation. Specifically, the presence of cerium in mixed valence states (Ce⁴⁺/Ce³⁺) plays a critical role in introducing localized impurity states within the TiO₂ band gap.

From a physicochemical standpoint, the Ce⁴⁺/Ce³⁺ redox couple introduces oxygen vacancies and intermediate energy levels that facilitate electron excitation under lower-energy visible light. The Ce³⁺ species, formed via partial reduction of Ce⁴⁺, generates localized 4f states below the conduction band of TiO₂, thereby creating a cascade of electronic states that effectively bridge the TiO₂ band gap. These defect states act as shallow traps for charge carriers, improving charge separation and extending the photoresponse into the visible spectrum.

Moreover, the intimate interfacial contact between TiO₂ and CeO₂ in the composite enhances charge transfer dynamics. Electrons photoexcited in TiO₂ can be efficiently transferred to the CeO₂ phase, where Ce⁴⁺ can act as an electron sink, being reduced to Ce³⁺. This process not only mitigates charge recombination but also regenerates Ce⁴⁺ sites, enabling continuous redox cycling that is beneficial for photocatalytic activity.

2.2. Evaluation of the Photocatalytic Degradation Efficiency of TiO₂-CeO₂ Nanoparticles on Methylene Blue

Photocatalytic degradation experiments of MB were conducted using TiO₂-CeO₂ nanocomposites with different CeO₂ loadings (0.1%, 0.2%, and 1%) under simulated sunlight and compared with commercial TiO₂ (P25) as a reference. Over time, UV–Vis spectra of the MB solution were recorded to monitor the reduction in absorbance intensity, allowing the estimation of degradation efficiency.

The UV–Vis spectra revealed that MB exhibits strong absorption between 500 and 750 nm, with a peak at 664 nm. As shown in Figure 6, the absorbance peak at 664 nm progressively decreased with irradiation time, indicating effective photocatalytic degradation of MB by TiO₂-CeO₂ nanocomposites. The degradation efficiency (H) was calculated using the formula:

$$\begin{array}{c}H=\left(C0-Ct\right)/C0\times 100\%\end{array}$$

(2)

where C₀ and C_t are the initial and time-dependent concentrations of MB (mg/L), respectively.

Figure 6e clearly demonstrates that the photocatalytic activity of TiO₂-CeO₂ nanocomposites (0.1%, 0.2%, and 1%) was superior to that of P25. After 150 min of irradiation, the MB degradation rates for P25, 0.1% TiO₂-CeO₂, 0.2% TiO₂-CeO₂, and 1% TiO₂-CeO₂ were 74.85%, 95.06%, 83.82%, and 73.01%, respectively. The results indicate that under simulated sunlight, the 0.1% TiO₂-CeO₂ nanocomposite achieved the highest MB degradation of approximately 95.06% after 150 min, with a reaction rate constant k ≈ 18.5 × 10⁻² phút⁻¹. In contrast, P25 showed a degradation efficiency of 74.85% with a lower reaction rate constant k ≈ 3.7 × 10⁻² phút⁻¹. This superior performance of the 0.1% TiO₂-CeO₂ nanocomposite can be attributed to its optimal CeO₂ loading, which enhances light absorption and electron–hole pair separation, thereby improving photocatalytic efficiency.

The photocatalytic performance of TiO₂-CeO₂, as shown in

Table 1. Comparative Analysis of Methylene Blue Photodegradation Efficiency by TiO₂-CeO₂ Nanocomposites.

Photocatalyst	Weight of Catalyst	Volume of MB Solution	Initial MB Solution Concentration	Degr. Time (min)	Light Source	Photo Degr. Effic. (%)	Ref.
CeO2 NPs	60 mg	250 mL	20 mg/L	90	UV light	90.84	[46]
CeO2 NPs	0.10 g	50 mL	20 mg/L	90	Visible light, (Xenon arc lamp, 300 W)	98	[47]
Ag/CeO2	10 mg	100 mL	30 mg/L	120	Visible light (Xenon lamp, 400 nm)	94	[48]
CeO2-NPs/GO	2.5 mg	10 mL	5 mg/L	90	UV-A light	90	[49]
0.1% CeO2–TiO2	3 g	100 mL	10 μmol/L	60	UV light (15 W)	95	[50]
CeO2–TiO2 (5CeTiO)	10 mg		20 mg/L	150	UV–Vis light	95	[36]
TiO2/CeO2	40 mg		40 mg/L	40	UV light	71	[37]
SnO2/CeO2		100 mL	20 mg/L	120	UV light (40 W)	80	[51]
TiO2/SnO2/CeO2	200 mg	50 mL	10 mg/L	120	UV lamp (254 nm)	85.5	[52]
0.1% TiO2-CeO2 nanocomposite	30 mg	20 mL	10 mg/L	150	Xenon lamp (350 W)	95.06	This work

, surpasses that of many other catalyst systems, including pure CeO₂, CeO₂ doped with metal oxides, precious metals, and graphene oxide (GO). This system effectively degrades MB under Xenon lamp irradiation, reducing the required catalyst amount while optimizing reaction volume. Compared to systems reliant on UV lamps, TiO₂-CeO₂ is more environmentally friendly and cost-effective, especially when used with a Xenon lamp or sunlight. UV lamps consume more energy and pose potential health risks, whereas this method lowers operating costs, minimizes environmental impact, and enhances sustainability. However, its performance is influenced by factors such as solution concentration, catalyst dosage, and irradiation time. Despite Table 1 being for reference purposes, TiO₂-CeO₂ remains a promising candidate for environmental remediation due to its high efficiency, low cost, and eco-friendly nature.

Stability is an important factor when evaluating the practicality of photocatalytic materials. The photocatalytic performance of 0.1% TiO₂-CeO₂ nanocomposites was tested over eight consecutive reuse cycles. After each degradation cycle, the material was recovered by centrifugation and washed with ethanol. As shown in Figure 7, under simulated sunlight, the removal efficiency of MB only slightly decreased from 95.85% to 93.28% after eight cycles. This indicates that the TiO₂-CeO₂ nanocomposites maintain stable photocatalytic activity, highlighting their potential for sustainable use in practical applications.

Figure 8 illustrates the effects of reactive species scavengers on the photocatalytic degradation of MB, aiming to elucidate the predominant active species involved in the degradation mechanism over 0.1% TiO₂-CeO₂ nanocomposites. p-Benzoquinone (BQ), isopropyl alcohol (IPA), and ammonium oxalate (AO) were employed as selective scavengers for superoxide radicals (O₂^−•), hydroxyl radicals (OH^●), and photogenerated holes (h⁺), respectively. The introduction of all scavengers led to a significant suppression of MB degradation efficiency, indicating the involvement of multiple reactive species in the photocatalytic process.

Specifically, the degradation efficiency decreased to 4.7%, 55.5%, and 79.8% in the presence of BQ, IPA, and AO, respectively. Notably, the most pronounced inhibition was observed with BQ, highlighting the dominant role of superoxide radicals (O₂^−•) in the photocatalytic mechanism. These findings suggest that (O₂^−•) species primarily drive the oxidative degradation pathway of MB, while OH^● and h⁺ also contribute to a lesser extent. Based on these results, a plausible photocatalytic degradation mechanism involving the generation and participation of these reactive species is proposed for the 0.1% TiO₂-CeO₂ composite system.

Figure 9 illustrates the proposed mechanism of MB photocatalytic reaction over TiO₂-CeO₂ catalyst. Upon light irradiation, both CeO₂ and TiO₂ absorb photon, exciting electrons to their respective conduction bands (CB), leaving behind holes in their valence band (VB). Since the CB energy of CeO₂ is more negative than that of TiO₂, electrons in the CB of CeO₂ can easily jumped to TiO₂’s CB. These electrons interact with O₂ molecules, forming various reactive oxygen radical such as superoxide radicals (O₂*⁻) and hydroxyl radical (OOH^●/OH^●) on the surface of nanocomposite catalyst. Simultaneously, holes are transferred from the VB of TiO₂ to the VB of CeO₂, where they react with H₂O molecules to generate additional hydroxyl radicals. These active species on the surface of catalyst initiate the dye degradation reaction, and then convert the MB molecules into CO₂, H₂O and inorganic anions. The possible photodegradation reactions for TiO₂-CeO₂ are described below [32,37,53]:

$$\begin{array}{c}{\mathrm{TiO}}_2\text{-}{\mathrm{CeO}}_2+\mathrm{h}\mathrm{\lambda } \to {\mathrm{h}}^{+}+{\mathrm{e}}^{-}\end{array}$$

(3)

$$\begin{array}{c}{\mathrm{h}}^{+}+{\mathrm{H}}_2\mathrm{O} \to {\mathrm{H}}^{+}+{\mathrm{OH}}^{●}\end{array}$$

(4)

$$\begin{array}{c}{\mathrm{e}}^{-}+{\mathrm{O}}_2 \to {\mathrm{O}}_2^{-●}\end{array}$$

(5)

$$\begin{array}{c}{\mathrm{O}}_2^{-●}+{\mathrm{H}}_2\mathrm{O} \to {\mathrm{OOH}}^{●}+{\mathrm{OH}}^{-}\end{array}$$

(6)

$$\begin{array}{c}2{\mathrm{OOH}}^{●} \to {\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}}_2\end{array}$$

(7)

$$\begin{array}{c}{\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{e}}^{-} \to {\mathrm{OH}}^{-}+{\mathrm{OH}}^{●}\end{array}$$

(8)

$$\begin{array}{c}{\mathrm{Ce}}^{4+}+{\mathrm{e}}^{-} \to {\mathrm{Ce}}^{3+}\end{array}$$

(9)

$$\begin{array}{c}{\mathrm{Ce}}^{3+}+{\mathrm{O}}_2 \to {\mathrm{Ce}}^{4+}+{\mathrm{O}}_2^{-●}\end{array}$$

(10)

$$\begin{array}{c}({\mathrm{OH}}^{●}, {\mathrm{O}}_2^{-●}+\mathrm{MB} \to \mathrm{products})\end{array}$$

(11)

It has been suggested that the enhanced photocatalytic performance of TiO₂-CeO₂ catalyst could be attributed to the extended charge lifetime due to the presence of Ce³⁺ species within the composites. When the catalyst is exposed to light, the photogenerated electrons are captured by Ce⁴⁺, reducing it to Ce³⁺, which may induce charge imbalance or strain, leading to the formation of oxygen vacancies. The Ce³⁺ was then reoxidized to Ce⁴⁺ by adsorbed oxygen in this system. The coexistence of Ce⁴⁺ and Ce³⁺, along with oxygen vacancies in the composite, causes the 4f band to split into occupied and unoccupied energy levels just below the CB of TiO₂. This inter-band transition from Ce³⁺ promotes interfacial charge transfer, reducing the band gap and thus enhancing photocatalytic degradation.

3. Materials and Methods

3.1. Materials

The synthetic materials, including Xeri nitrat hexahydrat (Ce(NO₃)₃·6H₂O, ≥99.0%) and Ethanol (C₂H₅OH, ≥99.8%) were supplied by Sigma-Aldrich (St. Louis, MO, USA), while Titanium oxide (TiO₂, 99%) was purchased from Xilong (Shantou, China). Deionized (DI) water was used to dissolve the chemicals and remove impurities during the measurement.

3.2. Synthesis of Cleistocalyx Operculatus Leaf Extract

The synthesis process of Cleistocalyx operculatus leaf extract is described in (Figure 10A). Leaves of CE (Cleistocalyx operculatus) were collected in Ha Tay Province, Vietnam, and identified. The CE leaves were dried and ground in powder form. Dried CE leaves (5 g) were heated and constantly stirred in 30 mL of ethanol at a temperature of 50–60 °C for 2 h. Finally, the mixture was filtered using a vacuum filtration system to remove solid residues, yielding a clear, concentrated extract ready for further analysis.

3.3. Synthesis of TiO₂-CeO₂ Nanoparticles

The procedure is schematized in (Figure 10B). The TiO₂-CeO₂ nanocomposite particles were fabricated using a straightforward, template-free sol–gel method as follows: Initially, 0.2%, 0.1%, and 1% molar ratios of cerium nitrate solution (Ce(NO₃)₃·6H₂O) were dissolved in a solution of 25 mL of Cleistocalyx operculatus extract and 75 mL of ethylene glycol, and then continuously stirred with a magnetic stirrer at 600 rpm for 1 h. Then, 0.43 g of TiO₂ was added, and the mixture was heated to 60 °C to promote hydrolysis, resulting in a light yellow-brown gel after 2 h of stirring. After aging the gel for 2 days, the gel was filtered and washed three times with distilled water and twice with ethanol. Finally, the gel was dried at 70 °C for 24 h and calcined in air at 500 °C for 2 h to produce TiO₂-CeO₂ nanocomposites.

3.4. Material Characterization

The nanomaterials were characterized using various techniques: field-emission scanning electron microscopy (JEOL 7600F, Akishima, Japan), powder X-ray diffraction (Advance D8, Bruker, Billerica, MA, USA, with a Cu-K_α1 radiation source, λ = 1.54056 Å), energy-dispersive X-ray spectroscopy, and high-resolution transmission electron microscopy (JEOL 2100F). Optical absorption spectra were collected using a UV−Vis spectrophotometer (Shanghai Yoke Instrument Co., Ltd., Shanghai, China). The bonding nature of the materials was investigated through Raman spectroscopy with a Renishaw inVia confocal micro-Raman system. Fourier transform infrared spectroscopy was performed with a FT/IR-4600 type A spectrometer (JASCO, Hachioji, Japan). Finally, the specific surface area and pore size distribution were determined using Brunauer–Emmett–Teller (BET) analysis (Micromeritics Gemini VII 2390, Norcross, GA, USA) based on nitrogen adsorption/desorption isotherms.

3.5. Photocatalytic Experiments

The photocatalytic performance of the synthesized samples was evaluated by degrading MB dyes under simulated sunlight provided by a 350 W Xenon lamp (Shenzhen Anhongda, Shenzhen, China). For each test, 30 mg of catalyst was stirred in a 10 ppm MB solution (20 mL, pH 7). The solution was initially allowed to reach an adsorption equilibrium in the dark on a magnetic stirrer for 1 h before exposure to the Xenon lamp, positioned 30 cm away, to initiate the photocatalytic process. At specified intervals, aliquots of the solution were withdrawn, centrifuged, and filtered to separate the catalyst. The residual MB concentration was determined using a Cary 5000 UV–Vis–NIR spectrophotometer (Yoke Instrument Co., Ltd., Shanghai, China) at a maximum absorption wavelength of 664 nm. Experiments comparing MB degradation efficiency were conducted with two types of catalysts, TiO₂ and TiO₂-CeO₂, to evaluate and compare the photocatalytic ability of each material.

4. Conclusions

This study successfully demonstrated the green synthesis of TiO₂-CeO₂ nanocomposites using Cleistocalyx operculatus leaf extract, presenting an eco-friendly and highly efficient approach for photocatalytic degradation of MB. The results demonstrated that CeO₂ incorporation significantly enhanced photocatalytic efficiency, achieving 95.06% degradation in 150 min under simulated sunlight. The improved performance is attributed to the efficient charge transfer between CeO₂ and TiO₂, driven by the Ce³⁺/Ce⁴⁺ redox cycle, which promotes interfacial charge separation and suppresses electron–hole recombination. The formation of reactive oxygen species, including OH^●, O₂^−●, and OOH^●, played a crucial role in breaking down MB into CO₂, H₂O, and inorganic ions. Additionally, the coexistence of Ce³⁺/Ce⁴⁺ species and oxygen vacancies facilitated the bandgap reduction, enhancing visible light absorption. The catalyst also exhibited excellent reusability, maintaining over 93% efficiency after eight cycles. These findings suggest that TiO₂-CeO₂ nanocomposites synthesized via green methods highlight industrial relevance for scalability, emphasizing their potential as a sustainable, cost-effective solution for environmental remediation and wastewater treatment.