Ultrasonic-Assisted Fabrication of TiO₂-Based Composite Photocatalysts for Enhanced Photocatalysis of Organic Pollutants: A Review

Abstract

Water contamination and the global energy crisis are two of the most significant challenges in the world. Titanium dioxide (TiO₂) has garnered attention due to its promising photocatalytic performance. However, its wide band gap energy limits its efficiency under visible light irradiation. To address this, TiO₂-based composite photocatalysts have been developed to narrow the band gap energy and suppress the recombination of electron–hole pairs, thereby enhancing photocatalytic performance. The ultrasonic technique, through acoustic cavitation, facilitates a synthetic process involving localized transient high-pressure and high-temperature conditions to produce photocatalysts with superior photocatalytic capabilities. This review focuses on ultrasonication and ultrasonic-assisted fabrication method for modifying TiO₂ into visible light-driven composite heterostructures. It discusses the parameters of ultrasonication that influence the synthesis and modification of these composites, along with the factors affecting photocatalytic performance.

1. Introduction

Water is a crucial resource for all living organisms, yet its scarcity and pollution are escalating global crises. According to the UN World Water Development Report 2024 [1], the global urban population facing water scarcity could double from 930 million in 2016 to 2.4 billion people by 2050, with 82% of wastewater released untreated into the environment [1]. Organic traces such as dyes, pesticides, herbicides, pharmaceuticals, and personal care products can be easily released into water through industrial and agricultural activities, which pose significant threats to aquatic ecosystems, the food chain, and human health due to their recalcitrant and toxic nature. Effective treatment of this contaminated water is therefore essential before its environmental release or reuse.

The textile industry alone consumes over 700,000 tons of dyes annually, with 10–15% lost in effluents [2,3]. These complex, water-soluble compounds are resistant to biological degradation [4,5,6]. Their presence in water blocks sunlight, impairing photosynthesis and biodiversity, while long-term accumulation causes serious health hazards, including damage to the kidney, liver, and central nervous system [4,6,7,8].

Similarly, pharmaceutical pollution is a pervasive threat. A global study found caffeine, metformin, and carbamazepine to be the most common pollutants in rivers, with the highest concentrations in South Asia, South America, and sub-Saharan Africa [9,10]. Introduced via industrial emissions and untreated waste, these chemically stable compounds are non-biodegradable and bio-accumulative, contributing to antibiotic resistance and chronic diseases [9,11,12].

Pesticide usage has also surged to boost crop yields, with India’s production, for example, skyrocketing from 5000 metric tons in 1958 to 102,240 tons in 1998 [13]. These persistent compounds can travel long distances through air and water, causing toxicity to both vertebrates and invertebrates, damaging organs and the central nervous system [13,14,15].

Conventional wastewater treatments like biodegradation [16], adsorption [17], membrane filtration [18], anaerobic fermentation [19], coagulation [20], flocculation [21], and electrolysis [22] suffer from drawbacks including high costs due to extensive energy consumption process, the formation of toxic by-products, the fouling of membranes, regeneration issues of the adsorbent, the production of sludge, the construction of large waste particles, incomplete removal, and a short half-life [4,7]. In contrast, advanced oxidation processes, particularly visible light-driven photocatalysis, offer an efficient, eco-friendly alternative by generating reactive oxygen species (ROS) that mineralize pollutants into harmless residues [6,23].

Titanium dioxide (TiO₂) is a leading photocatalyst prized for its strong oxidation capability, non-toxicity, superior chemical stability and photostability, non-selective degradation, thorough mineralization, low energy consumption, and low cost [6,23,24,25,26]. Upon UV light excitation, it generates electron–hole pairs that form ROS like hydroxyl radicals (^•OH) and superoxide anion (^•O₂⁻), which mineralize organic compounds [23]. However, its wide band gap (3.00–3.30 eV for rutile and anatase form) restricts activation to UV light (~5% of sunlight), and rapid charge carrier recombination further limits its efficiency [6,12,25,26].

To harness visible light, researchers have developed TiO₂-based composites for the photocatalytic degradation of organic pollutants. Examples include TiO₂/g-C₃N₄ decorated with rGO sheet [27], WO₃/TiO₂ [28], TiO₂/SiO₂ [29], Cu₂O/WO₃/TiO₂ [30], Ag₃PO₄/TiO₂-carbon nanofiber [31], and porphyrin polymer/TiO₂ composites [32]. These heterostructures narrow the effective band gap and enhance charge separation, significantly boosting photocatalytic performance [7,33].

While conventional methods like hydrothermal [34], sol–gel [35], wet impregnation [36], and chemical precipitation [37] are common for photocatalystd formation, ultrasonication has emerged as a powerful technique. It reduces reaction time, controls morphology, and produces superior photocatalysts when combined with other methods [12,24,38,39,40]. This review focuses on the ultrasonication-assisted fabrication of visible light-driven TiO₂-based heterostructures for organic pollutant degradation. It examines the governing parameters of the synthesis process, the factors influencing photocatalytic efficiency, and the underlying mechanisms, followed by a discussion of the photoelectrochemical properties of these composites.

2. Basic Photocatalysis Principles

The photocatalytic process is based on the generation of redox-active species to initiate the degradation of organic contaminants [8]. Photocatalysis has been widely demonstrated to effectively eliminate various organic pollutants from wastewater sources, including pesticides [41], pharmaceuticals [42], heavy metals [43], and microplastics [44]. The fundamental mechanism of photocatalysis using titanium dioxide (TiO₂) is illustrated in Figure 1. When TiO₂ absorbs light energy equal to or greater than its band gap energy, electron (e⁻) are photoexcited from the valence band (VB) to empty conduction band (CB), leaving positively charged hole (h⁺) in the VB. The photoexcited e⁻ then react with oxygen molecules (O₂) adsorbed on the photocatalyst surface or dissolved in water, generating superoxide anion (^•O₂⁻), while the hole (h⁺) react with water molecules (H₂O) and hydroxide anion (OH⁻) to produce hydroxyl radicals (^•OH) and protons (H⁺) [3,8], as depicted in Equations (1)–(4) below:

Photocatalyst + light (hv) → e⁻ + h⁺

(1)

O₂ adsorbed + e⁻ → ^•O₂⁻

(2)

H₂O adsorbed + h⁺ → ^•OH + H⁺

(3)

OH⁻ + h⁺ → ^•OH

(4)

These potent oxidizing agents, H⁺, ^•OH, and ^•O₂⁻, react with organic pollutants to produce carbon dioxide (CO₂), H₂O, and mineral acid (s) [3], as demonstrated in Equations (5)–(7) below:

^•O₂⁻ + organic pollutants → intermediates → CO₂ + H₂O + mineral acid (s)

(5)

^•OH + organic pollutants → intermediates → CO₂ + H₂O + mineral acid (s)

(6)

H⁺ + organic pollutants → intermediates → CO₂ + H₂O + mineral acid (s)

(7)

TiO₂

TiO₂, also known as titania [45], is a prominent functional photocatalyst widely utilized as a white pigment in various products such as food coloring, paint, toothpaste, and cosmetics [46]. Researchers have extensively explored TiO₂ since the initial demonstration of photocatalytic water splitting using TiO₂ electrodes under ultraviolet (UV) light in 1972 [46]. TiO₂ exists in three crystalline forms: anatase, rutile, and brookite [47]. While anatase and rutile have been extensively studied in photocatalysis, brookite has received comparatively less attention. Anatase TiO₂ generally exhibits superior photocatalytic activity compared to rutile, which is often attributed to differences in their predominant crystal facets. The key facets for anatase are (101) and (001), whereas for rutile, they are (110), (100), and (101) [47].

Anatase TiO₂ excels as a photocatalyst due to the abundance of under-bonded Ti atoms and the large Ti-O-Ti bond angles on its (001) facet [47]. In its crystal structure, Ti⁴⁺ ions are surrounded by six O²⁻ ions, forming an octahedral geometry. Anatase TiO₂ has a distorted orthorhombic structure in which each octahedron is connected to ten others, whereas rutile TiO₂ has a slightly orthorhombic structure connected to eight neighboring octahedra [47]. These structural differences influence mass density, electronic band structure, and ultimately photocatalytic performance, with anatase TiO₂ exhibiting enhanced photoactivity [47]. Furthermore, the typically smaller particle size and larger surface area of the anatase phase TiO₂ contribute to its enhanced optical activity in photocatalytic applications compared to the rutile phase [12,48].

TiO₂ functions as an n-type semiconductor photocatalyst [23]. Despite its advantages, TiO₂ faces several drawbacks such as large band gap energy, poor solar energy conversion efficiency, high charge carrier recombination rates, and inefficient separation and migration rates of photoexcited electrons [8]. To address these challenges, TiO₂ heterostructure composites have been developed to enhance the separation of photoexcited electron–hole pairs. These composites facilitate the movement of photoexcited electrons from the conduction band (CB) with a more negative potential to one with a less negative potential, while simultaneously transferring holes from the valence band (VB) with a more positive potential to one with a less positive potential [47]. The lower band gap energies of TiO₂ composites also enhance the absorption of visible light, further boosting their photocatalytic efficiency.

3. TiO₂-Based Nanocomposite Photocatalysts

3.1. Formation of Heterojunction

The construction of heterostructures by combining two semiconductors has proven to be an effective strategy for enhancing the photocatalytic performance of titanium dioxide (TiO₂). This approach involves coupling two or more semiconductor materials to facilitate charge carrier separation, reduce recombination rates, and improve redox capabilities [49]. The interface formed between two distinct metal oxides or semiconductors is known as a heterojunction [50]. Heterojunctions are central to this strategy and are typically built using p-type and n-type semiconductors. These configurations can be homojunctions (formed between the same type, e.g., p-p or n-n) or heterojunctions (formed between different types, e.g., p-n), all aimed at significantly improving photocatalytic efficiency.

A p-n heterojunction enhances photocatalytic performance by leveraging the internal electric field at the semiconductor interface to promote the transfer of photoexcited charge carriers, thereby effectively separating electron–hole pairs. For instance, Xiao et al. [51] synthesized a BiOBr/TiO₂ composite with a p-n heterojunction for the photocatalytic degradation of formaldehyde and dyes. High-resolution transmission electron microscopy (HRTEM) analysis revealed lattice fringe spacings corresponding to the (110) plane of BiOBr and the (101) plane of TiO₂, confirming the tight attachment and successful formation of a heterojunction between the two components [51]. Such well-defined lattice fringes indicate a highly crystalline structure, which facilitates rapid charge carrier transfer across the heterointerface and reduces recombination rates [52].

The fermi level (E_F) represents the highest occupied electronic energy level at absolute zero temperature and the electrochemical potential of electrons in a material [49]. In an n-type semiconductor, E_F is situated close to the conduction band (CB) minimum, while in a p-type semiconductor, it lies near the valence band (VB) maximum [49]. When two distinct semiconductors come into contact, the difference in their E_F causes electrons to diffuse from the material with higher E_F to the one with the lower E_F until thermal equilibrium is reached. This charge redistribution creates a built-in electric field at the interface [49]. Under light irradiation, this internal electric field effectively separates the newly generated photoexcited electron–hole pairs, thereby enhancing the photocatalytic performance of the heterostructure composite [50].

The Mott-Schottky (MS) plot is utilized to determine flat band potential ($V_{FB}$) and effective charge carrier density of semiconductors under dark conditions [28]. For instance, Chau et al. [53] investigated heterojunction formation in a Cu₂O/WO₃/TiO₂ composite photocatalyst. Figure 2a illustrates the MS plot for this composite. In such plots, n-type semiconductors like TiO₂ and WO₃ exhibit a positive slope, whereas p-type semiconductors like Cu₂O show a negative slope [54]. The $V_{FB}$ is determined from the intersection point of the tangent to the MS plot with the x-axis (applied potential). A more negative reading indicates stronger reducing power of the photocatalysts [52]. It is important to note that $V_{FB}$ is affected by experimental conditions such as frequency and pH [55]. Chau et al. [53] confirmed the formation of a p-n heterojunction in their Cu₂O/WO₃/TiO₂ composite, as evidenced by the characteristic inverted “V” shape in the MS plot (Figure 2a) [54,56].

Higher charge carrier density generally indicates more efficient charge separation [57]. For example, Tai et al. [56] calculated the acceptor ($N_{A }$) and donor densities ($N_D$) for a GO/TiO₂ composite photocatalyst prepared under varying photoreduction times. They noted that a 12 h photoreduction process yielded the highest charge carrier densities for both $N_A \mathrm{a}\mathrm{n}\mathrm{d} N_D$. The formation of C-O-Ti bonds was identified as a key factor promoting charge carrier transport and increasing charge carrier density. Additionally, the researchers explored the relationship between the built-in potential (${eV}_{bi}$) of the p-n heterojunction in the GO/TiO₂ composite and the charge carrier density, as depicted in Figure 2b. A larger ${eV}_{bi}$ helps suppress the recombination of photoexcited charge carriers by strengthening the internal electric field that drives the separation of electrons in the CB and holes in the VB [56]. It is worth noting that heterojunctions in photocatalysis are generally classified into several types, such as Type-II, Z-scheme, and S-scheme.

3.2. Type II Heterojunction

Figure 3a illustrates the formation of a type II heterojunction between two semiconductors, labeled A and B. In such a heterojunction, a space charge region forms at the interface upon contact. The alignment of Fermi levels (E_F) causes band bending in the conduction band (CB) and valence band (VB) of both semiconductors, establishing a built-in electric field (IEF). Under light illumination, electrons in the VB of both semiconductors are excited to their respective CBs, generating electron–hole pairs. The IEF then facilitates the transfer of photoexcited electrons from the CB of semiconductor B (with a more negative potential) to the CB of semiconductor A. Simultaneously, holes transfer from the VB of semiconductor A (with a more positive potential) to the VB of semiconductor B [58,59]. This spatial separation leads to the accumulation of electrons in semiconductor A and holes in semiconductor B, thereby effectively reducing the recombination rate of charge carriers.

However, conventional Type II heterojunctions have a significant drawback: the useful electrons and holes accumulate in energy bands with weaker redox potentials. This results in a decrease in the overall redox capability of the system. Furthermore, electrostatic repulsion between electrons can hinder their transfer between semiconductor CBs [59]. To overcome these limitations, researchers have developed alternative charge transfer pathways, such as the Z-scheme heterojunction.

3.3. Z-Scheme Heterojunction

Z-scheme photocatalytic system are generally categorized into three types: traditional Z-scheme, all-solid-state Z-scheme, and direct Z-scheme configurations [59]. The foundational principle was introduced by Bard [60] in 1979, who proposed using a redox electron mediator to shuttle charges between two semiconductors for separate reduction and oxidation reactions. However, recent studies have identified challenges in the charge transfer mechanisms of both traditional and all-solid-state Z-schemes, such as inefficiency and light shielding by the mediator [59,61]. Consequently, this review will focus on the direct Z-scheme heterojunction.

In a direct Z-scheme heterojunction, electrons migrate from a photocatalyst with a higher E_F to one with a lower E_F to achieve energy level equilibrium, thereby equalizing the E_F between the two photocatalysts [62]. Figure 3b illustrates a direct Z-scheme heterojunction where photoexcited electrons transfer from the conduction band (CB) of photocatalyst A to the valence band (VB) of photocatalyst B due to electrostatic attraction between the photoexcited charge carriers. This heterojunction structure facilitates efficient separation and reduces the recombination rate of photoexcited charge carriers, thereby enhancing the photocatalytic performance of the composite photocatalyst. The Z-scheme heterojunction preserves the redox potential of the photocatalysts to a significant degree, owing to its unique mode of photoexcited charge carrier transfer. As depicted in Figure 3b, oxidation reactions occur on the photocatalyst A with a higher oxidation potential, while reduction reactions take place on photocatalyst B with a higher reduction potential.

Li et al. [63] demonstrated this concept by fabricating a direct Z-scheme heterojunction of rutile-TiO₂/g-C₃N₄ using a simple solid-state grinding technique. HRTEM confirmed an intimate interface between TiO₂ and g-C₃N₄, indicating successful heterojunction formation. Photoelectrochemical experiments further verified the effective separation and migration of photoexcited charge carriers, confirming the enhanced photocatalytic activity enabled by the direct Z-scheme mechanism. Heba et al. [64] synthesized a SnS₂/SnO₂ nanocomposite that forms a Z-scheme heterojunction. This heterojunction, resulting from the SnO₂–SnS₂ contact, facilitates Z-scheme electron transfer. It leads to a prolonged electron–hole recombination time, thereby improving charge carrier separation.

3.4. S-Scheme Heterojunction

The S-scheme (step-scheme) heterojunction was proposed by Fu et al. [65] to provide a more precise description of the charge transfer mechanism in certain heterojunctions, building upon and refining the direct Z-scheme concept. In an S-scheme system, one semiconductor acts as the reduction photocatalyst (RP) and the other as the oxidation photocatalyst (OP), each with a distinct work function [66]. The RP and OP can be n-type or p-type, but their band structures must be aligned such that the E_F, VB, and CB of RP are all higher than those of the OP. This alignment drives electron transfer from the RP to the OP upon contact until their E_F equilibrate, creating the necessary internal electric field for S-scheme charge transfer [59].

Figure 3c illustrates a typical S-scheme heterojunction. In this configuration, when the RP and OP come into contact, electrons diffuse from the RP (with a higher E_F) to the OP (with a lower E_F) until their E_F equilibrates. This charge transfer leaves the OP negatively charged and the RP positively charged at the interface, forming a space charge region. Consequently, the energy bands of the OP bend downward (electron accumulation), while those of the RP bend upward (electron depletion) [59]. This band alignment creates an IEF directed from the RP to the OP.

Under light irradiation, the combined effects of band bending and the IEF create a selective charge transfer pathway where they prevent useless electrons in the conduction band (CB) of the RP from migrating to the CB of the OP and useless holes in the valence band (VB) of the OP from migrating to the VB of the RP. Instead, electrostatic attraction facilitates the recombination of these less reactive charges—specifically, the holes in the VB of the RP with the electrons in the CB of the OP. This selective recombination effectively leaves the most reactive electrons in the CB of the OP and the most reactive holes in the VB of the RP, thereby enhancing the overall redox capability of the system and suppressing detrimental bulk recombination within the individual semiconductors [59].

The S-scheme heterojunction significantly enhances photocatalytic performance through several key mechanisms. It preserves the strongest redox potentials by accumulating photoexcited electrons with high reduction potential in the CB of the RP and holes with high oxidation potential in the VB of the OP [58,67]. Furthermore, it improves the separation and transfer efficiency of charge carriers and broadens the light absorption range of the composite [68]. These benefits are driven by the synergistic effects of the IEF, band bending, and Coulombic attraction at the heterojunction interface [59].

Consequently, the S-scheme heterojunction effectively combines the efficient charge separation of a Type II system with the preserved redox power of a Z-scheme pathway. This unique combination results in superior charge carrier separation and strong redox potentials. Due to these advantages, researchers have successfully implemented S-scheme heterojunctions in various applications, including water splitting [69], carbon dioxide reduction [70], and photodegradation of organic pollutants [71]. Rezaei et al. [72] synthesized MoS₂ nanoflowers, MoO₃ nanobelts, and MoS₂@MoO₃ heterojunction. The composite demonstrated superior photocatalysis, achieving 96% removal of methylene blue (20 mg/L) in 6 h under visible light. The enhancement is attributed to a synergistic S-scheme mechanism that facilitates charge separation, generating ^•OH, and ^•O₂⁻ radicals for degradation.

Experimental techniques like X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) are vital for confirming S-scheme heterojunction formation. For instance, Salehi et al. [73] observed reduced XRD peak intensities and narrowed peak widths in a TiO₂/ZnIn₂S₄ composite, indicating successful coupling. XPS analysis further confirmed electron transfer from ZnIn₂S₄ to TiO₂, validating the S-scheme mechanism [73,74].

Photoelectrochemical (PEC) studies are crucial for evaluating charge separation. A standard three-electrode setup—with a platinum counter electrode and a reference electrode (e.g., SCE or Ag/AgCl)—is typically used [28]. Electrochemical impedance spectroscopy (EIS) measures charge transfer resistance; a smaller arc radius in the Nyquist plot signifies lower impedance and better charge separation [52]. Feng et al. [75] demonstrated this, where a WO₃/TiO₂/CS-biochar composite showed the smallest arc radius and a longer electron lifetime in Bode plots, confirming enhanced charge dynamics (Figure 4a,b). Similarly, studies on BiVO₄/TiO₂ and CuO/TiO₂ heterojunctions have shown extended electron lifetimes and reduced recombination [76,77]. Wang et al. [78] used photocurrent measurements to show that a ZrO₂/TiO₂ composite had a longer photocurrent decay time (6.001 s) than pure TiO₂ (2.001 s), proving more effective charge separation (Figure 4c,d). These findings underscore how S-scheme heterojunctions enhance photocatalytic processes by optimizing charge carrier separation and utilization. These findings underscore the significant advancements and potential applications of S-scheme heterojunctions in improving photocatalytic processes through efficient charge carrier utilization and separation.

4. Ultrasonic-Assisted Fabrication of TiO₂-Based Nanocomposite Photocatalysts

Numerous methods exist to synthesize TiO₂-based composite photocatalysts with a reduced band gap and lower charge carrier recombination rates, including hydrothermal, sol–gel, and co-precipitation techniques [23,80,81,82,83]. Among these, ultrasonication has recently gained significant attention for its substantial benefits in photocatalyst synthesis [84]. Ultrasonication employs high-frequency sound waves (>20 kHz) to trigger cavitation in a liquid, where the rapid formation and collapse of bubbles generate extreme local conditions—exceeding 10,000 K and 1000 atm [85,86]. This energy facilitates nanoparticle formation, creates surface defects on the photocatalyst, and enhances the dispersion of active species on a support material [87,88].

Also known as the sonochemical process, ultrasonication is a simple and environmentally friendly technique. It provides precise control over nanostructure and material dispersion, and its operation at room temperature makes it ideal for using heat-sensitive precursors without risk of decomposition [12,79,85]. Additionally, operating at room temperature eliminates the need for high-temperature conditions that may decompose precursors used in photocatalyst synthesis, making it suitable for heat-sensitive materials.

This is particularly critical for the synthesis of advanced materials such as Metal–Organic Frameworks (MOFs) and Covalent Organic Frameworks (COFs), whose crystalline porous structures would collapse upon calcination. For instance, MIL-125(Ti) and NH₂-MIL-125(Ti), are effective photocatalysts synthesized solvothermally below 150 °C; exposure to temperatures over 400 °C would destroy their structure, forming amorphous TiO₂ and eliminating their superior adsorption and photocatalytic properties. Similarly, COFs like TFPT-COFs are synthesized at room temperature or with mild heating, and high temperatures would decompose their intricate organic frameworks and active sites [89,90].

Room temperature processing is also essential for photocatalysts using molecular sensitizers, such as Eosin Y-sensitized g-C₃N₄, where heat would carbonize the dye and nullify its function. Systems dependent on specific 2D morphologies, like BiOX nanosheets, likewise require low temperatures to preserve their structural integrity and activity [91,92].

Ultrasonication prevents particle aggregation and enhances photocatalyst performance. Studies demonstrate that it reduces charge transfer resistance (e.g., TiO₂ from 15 kΩ to 7 kΩ) and creates more uniform, dispersed structures with higher surface areas [83].For instance, ultrasonication produced a TiO₂/GO composite with a greater surface area (92 m²/g) and enhanced both methyl orange and congo red dyes degradation [93]. Similarly, an rGO/ZnO/TiO₂ composite synthesized with ultrasonication had a more even element distribution and a 15% higher degradation rate for crystal violet dye than one made with conventional mixing [87].

Govinda Raj et al. [79] synthesized a novel TiO₂/MoS₂/BiVO₄ composite using an ultrasonic-assisted hydrothermal approach. Their study found that ultrasonication significantly enhanced surface area, pore volume, and photocatalytic activity compared to the standard hydrothermal method. SEM and Brunauer–Emmett–Teller (BET) analyses confirmed the formation of well-dispersed, nanosized particles with reduced charge carrier recombination. As shown in Figure 4e,f, the ultrasonicated TiO₂ had a defined nanosized morphology, while the conventional sample was agglomerated. BET analysis measured a higher surface area for both ultrasonicated TiO₂ (52.30 m²/g) and the composite (132.70 m²/g), compared to their conventional counterparts (34.80 and 96.70 m²/g, respectively). This increased surface area suggests reduced charge carrier recombination [79]. Larger pore volumes were also found in the ultrasonicated samples (TiO₂: 0.15 cm³/g; composite: 0.22 cm³/g) versus the conventional ones (0.13 and 0.18 cm³/g). Consequently, the ultrasonicated composite provides more reactive sites for pollutant adsorption, enhancing its overall photocatalytic performance [79].

Figure 4g shows that the ultrasonic-assisted hydrothermal method produces TiO₂ and its composite with lower photoluminescence (PL) intensity, suggesting enhanced charge carrier separation [79]. This is supported by EIS analysis in Figure 4h, where a smaller Nyquist plot arc radius for the sonicated samples indicates improved charge transfer and photocatalytic activity. Additionally, the band gap ($E_g$) was reduced for sonicated TiO₂ (3.02 eV) and the composite (2.84 eV), compared to their conventionally prepared counterparts (3.20 eV and 2.94 eV, respectively) [79].

Researchers have widely explored ultrasonication for synthesizing various photocatalysts. For instance, studies have successfully produced composites like PTA/TiO₂, Fe₂O₃/TiO₂, and ZnO/TiO₂ using ultrasonic-assisted methods such as impregnation and spray pyrolysis [12,94,95]. These methods often yield materials with enhanced properties, including improved visible light absorption and reduced crystallite sizes [95,96]. The benefits are clear in direct comparisons. Ultrasonication increased the surface area of TiO₂ from 59 to 101 m²/g and reduced its particle size from 70–80 nm to 20–40 nm, leading to better performance [86,97]. It also prevents agglomeration, aids in forming complex structures like core–shell microspheres, and improves heterojunction formation in composites like Bi₂WO₆/TiO₂, resulting in a lower band gap energy [98,99,100].

Table 1. TiO₂-based composite photocatalysts prepared through ultrasonication or ultrasonic-assisted method.

Photocatalyst Composite	Preparation Technique	Optimum Ultrasonication Parameters	Band Gap (eV) of Composite	Surface Area (m2/g)	Experimental Conditions	Photocatalysis Performance (%; min−1 for k)	Ref.
rGO/MXene-TiO2/polyaniline	Thermal assisted ultrasonication	Time: 30 min Power: N/A Freq.: N/A	2.99	TiO2: N/A Composite: N/A	Dosage: 0.20 g/L [Methyl orange]: 10 mg/L [Methylene blue]: 10 mg/L [Rhodamine B]: 10 mg/L [Norfloxacin]: 10 mg/L [Tetracycline]: 10 mg/L Time: 60 min pH: N/A Light: Solar Volume: 50 mL	Methyl orange TiO2: N/A Composite: 98.40; Methylene blue TiO2: N/A Composite: 99.10; Rhodamine B TiO2: N/A Composite: 99.80; Norfloxacin TiO2: N/A Composite: 87.80; Tetracycline TiO2: N/A Composite: 95.10	[101]
Ag2O/TiO2/Fe2O3	Impregnation assisted ultrasonication	Time: 45 min Power: N/A Freq.: 28 kHz	2.18	TiO2: N/A Composite: N/A	Dosage: 0.75 g/L [Tetracycline]: 15 mg/L Time: 90 min pH: N/A Light: Visible (60 W) Volume: 300 mL	TiO2: N/A Composite: 90	[102]
WO3/TiO2	Ultrasonic-assisted solvothermal	Time: 30 min Power: N/A Freq.: N/A	2.80 to 3.10	TiO2: 46.49 Composite: 41.73 to 48.20	Dosage: 0.25 g/L [Reactive Black 5]: 30 mg/L Time: 60 min pH: 3 Light: Visible (150 W) Volume: 80 mL	TiO2: N/A Composite: 92; k = 0.044	[28]
Cu2O/WO3/TiO2	Ultrasonic-assisted hydrothermal	Time: 60 min Power: N/A Freq.: N/A	2.70	TiO2: N/A Composite: N/A	Dosage: 0.25 g/L [Acetaminophen]: 1 mg/L Time: 60 min pH: N/A Light: Visible (150 W) Volume: 80 mL	TiO2: k = 0.024 Composite: k = 0.044	[30]
Cu/TiO2	Low temperature chemical process with ultrasonication	Time: 60 min Power: 280 W Freq.: N/A	3.11 to 3.23	TiO2: N/A Composite: N/A	Dosage: 0.30 g/L [Acid orange 7]: N/A [Rhodamine B]: N/A Time: 120 min pH: N/A Light: UV; UV and Visible; Visible Volume: N/A	Acid orange 7 98 Rhodamine B 99	[103]
Cu2O/WO3/TiO2	Ultrasonic-assisted solvothermal	Time: 60 min Power: N/A Freq.: N/A	2.35 to 2.90	TiO2: N/A Composite: 35.77	Dosage: 0.13 g/L [Reactive Black 5]: 30 mg/L Time: 120 min pH: 5 Light: Visible (150 W) Volume: 80 mL	TiO2: N/A Composite: k = 0.023	[53]
BiOBr/TiO2	Ultrasonic-assisted hydrothermal and water bath precipitation	Time: 5 min Power: N/A Freq.: N/A	N/A	TiO2: 46.49 Composite: 42.13	Dosage: 0.20 g/L [Sodium ethyl-xanthate]: 20 mg/L Time: 40 min pH: N/A Light: Visible (400 W) Volume: 50 mL	TiO2: N/A Composite: 93.96; k = 0.065	[104]
BiPO4/TiO2/rGO	Ultrasonic-assisted hydrothermal	Time: N/A Power: N/A Freq.: N/A	1.78	TiO2: N/A Composite: N/A	Dosage: 0.67 g/L [Malachite green]: 20 mg/L [Levofloxacin]: 20 mg/L Time: Until complete degradation of malachite green and levofloxacin pH: N/A Light: Visible (300 W) Volume: 60 mL	Malachite green TiO2: N/A Composite: 100 (45 min); k = 0.091 Levofloxacin TiO2: N/A Composite: 94 (120 min); k = 0.024	[105]
BiVO4/TiO2	Ultrasonic-assisted solvothermal	Time: N/A Power: N/A Freq.: N/A	2.69	TiO2: N/A Composite: N/A	Dosage: 5 g/L [hydrolyzed polyacrylamide]: 20 mg/L Time: Until complete degradation of hydrolyzed polyacrylamide pH: N/A Light: Visible (35 W) Volume: 200 mL	TiO2: 100 (115 min) Composite: 100 (25 min)	[76]
BiVO4/TiO2	Ultrasonication	Time: 120 min Power: N/A Freq.: 80 kHz	2.72	TiO2: 97.70 Composite: 59.30	Dosage: 1 g/L [Rhodamine b]: N/A Time: 120 min pH: N/A Light: Visible (300 W) Volume: 100 mL	TiO2: k = 0.002 Composite: k = 0.021	[106]
chitosan/TiO2	Ultrasonication	Time: 30 min Power: N/A Freq.: 40 kHz	N/A	TiO2: N/A Composite: N/A	Dosage: 1 g/L [Malachite green]: 10 mg/L Time: 240 min pH: 7 Light: Visible (15 W) Volume: 100 mL	TiO2: N/A Composite: 91.94; k = 0.007	[107]
Co3O4/TiO2	Ultrasonic-assisted impregnation	Time: N/A Power: N/A Freq.: N/A	2.35	TiO2: 200 Composite: 184	Dosage: 1 g/L [Ciprofloxacin]: 10 mg/L Time: 60 min pH: N/A Light: Visible (300 W) Volume: 50 mL	TiO2: k = 0.001 Composite: 100; k = 0.016	[108]
CuO/TiO2	Ultrasonication	Time: 15 min Power: N/A Freq.: N/A	2.81	TiO2: N/A Composite: N/A	Dosage: 2.40 g/L [Acid yellow 36]: 30 mg/L Time: Until complete degradation of acid yellow 36 pH: N/A Light: UV-visible (350 W) Volume: 250 mL	TiO2: 100 (100 min) Composite: 100 (45 min)	[77]
Cu2O/TiO2	Ultrasonication	Time: 15 min Power: 100 W Freq.: 20 kHz	2.12	TiO2: N/A Composite: N/A	Dosage: N/A [Methyl orange]: N/A Time: 240 min pH: N/A Light: Visible Volume: N/A	TiO2: N/A Composite: k = 0.223	[109]
α-Fe2O3/TiO2	Ultrasonication followed by wet impregnation	Time: 8 min Power: 70 W Freq.: N/A	2.80	TiO2: N/A Composite: 106.34	Dosage: 0.01 g/L [Cefixime]: 20.50 mg/L Time: 103 min pH: 4.76 Light: Visible Volume: 100 mL	TiO2: N/A Composite: 98.80; k = 0.057	[12]
Fe3O4/SiO2/ZnO/ZnS	Ultrasonic-assisted in situ surface sulfidation	Time: 120 min Power: N/A Freq.: N/A	2.69	TiO2: N/A Composite: 43.50	Dosage: 0.50 g/L [Oxytetracycline]: 10 mg/L [Tetracycline]: 10 mg/L Time: 90 min (UV light); 180 min (visible light) pH: N/A Light: UV (250 W); visible (300 W) Volume: 50 mL	Oxytetracycline (UV light) TiO2: N/A Composite: 99.10 Tetracycline (visible light) TiO2: N/A Composite: 80.90	[110]
Fe3O4/TiO2/MWCNT	Ultrasonic-assisted wet impregnation	Time: 30 min Power: 300 W Freq.: 40 kHz	1.75	TiO2: 28 Composite: 181.42	Dosage: 0.40 g/L [2-chlorophenol]: 2 mg/L Time: 15 min pH: 5 Light: UV-visible (125 W) Volume: 200 mL	TiO2: N/A Composite: 86	[111]
g-C3N4/TiO2	Ultrasonic-assisted hydrothermal	Time: N/A Power: N/A Freq.: N/A	2.88	TiO2: N/A Composite: N/A	Dosage: 0.40 g/L [Methylene blue]: 20 mg/L [Tetracycline]: 20 mg/L [Congo red]: 20 mg/L [Eosin y]: 20 mg/L Time: Different reaction time pH: N/A Light: Visible Volume: 50 mL	Methylene blue TiO2: 68.90; k = 0.019 (60 min) Composite: 96.60; k = 0.055 (60 min) Tetracycline TiO2: 73.60; k = 0.044 (30 min) Composite: 100; k = 0.315 (30 min) Congo red TiO2: N/A Composite: 100 (60 min) Eosin y TiO2: N/A Composite: 87.70 (10 min)	[112]
g-C3N4/TiO2	Ultrasonication	Time: N/A Power: N/A Freq.: N/A	2.98	TiO2: 50.19 Composite: 49.37	Dosage: 1 g/L [Tetracycline]: 50 mg/L Time: 50 min pH: N/A Light: UV (300 W) Volume: 50 mL	TiO2: N/A Composite: 96.53; k = 0.057	[113]
g-C3N4/TiO2	Ultrasonic-assisted wet impregnation	Time: 30 min Power: N/A Freq.: N/A	2.99	TiO2: 125.57 Composite: 80.65	Dosage: N/A [Carbamazepine]: 10 mg/L Time: 360 min pH: N/A Light: UV (24 W) Volume: N/A	TiO2: N/A Composite: 71.41; k = 0.003	[114]
g-C3N4/TiO2	Ultrasonication	Time: 60 min Power: 200 W Freq.: N/A	2.79	TiO2: 156 Composite: 43	Dosage: 1 g/L [Fluorescein dye]: concentration of 5 × 10−5 Time: 120 min pH: N/A Light: Visible (350 W) Volume: 100 mL	TiO2: 45; k = 0.002 Composite: 92; k = 0.025	[115]
GO/TiO2	Ultrasonication	Time: 60 min Power: N/A Freq.: N/A	3.56	TiO2: N/A Composite: N/A	Dosage: N/A [Acid navy blue]: N/A Time: 90 min pH: N/A Light: UV (125 W) Volume: N/A	TiO2: N/A Composite: 95; k = 0.042	[116]
GO/TiO2	Ultrasonic-assisted hydrothermal	Time: 30 min Power: 200 W Freq.: 40 kHz	2.72	TiO2: N/A Composite: N/A	Dosage: 4 g/L [Salicylic acid]: 1 mM Time: 60 min pH: N/A Light: Sunlight Volume: 250 mL	TiO2: N/A Composite: 57; k = 0.001	[117]
Graphene/TiO2	Ultrasonic-assisted hydrothermal	Time: 30 min Power: N/A Freq.: N/A	2.45	TiO2: 29.61 Composite: 35.89	Dosage: N/A [Rhodamine b]: 20 mg/L [Methyl orange]: 30 mg/L Time: 120 min pH: N/A Light: Visible (300 W) Volume: 50 mL	Rhodamine b TiO2: k = 0.003 Composite: k = 0.013 Methyl orange TiO2: k = 0.003 Composite: k = 0.015	[118]
MNP/SiO2/TiO2	Ultrasonic-assisted solvothermal followed by sol–gel	Time: N/A Power: N/A Freq.: N/A	N/A	TiO2: 54 Composite: 167	Dosage: 0.56 g/L [Methylene blue]: 3.50 mg/L Time: 120 min pH: 7 Light: UV (80 W) Volume: 50 mL	TiO2: k = 0.028 Composite: k = 0.060	[119]
rGO/TiO2	Photolysis followed by ultrasonication	Time: 60 min Power: N/A Freq.: N/A	2.70	TiO2: N/A Composite: N/A	Dosage: 0.10 g/L [Congo red]: 10 mg/L [Trichloroacetic acid]: 10 mg/L Time: 100 min pH: N/A Light: Direct sunlight Volume: 100 mL	Congo red TiO2: k = 0.010 Composite: k = 0.026 Trichloroacetic acid TiO2: k = 0.001 Composite: k = 0.026	[120]
rGO/TiO2	Ultrasonication	Time: 30 min Power: N/A Freq.: N/A	N/A	TiO2: N/A Composite: N/A	Dosage: 2 g/L [Methylene blue]: 20 mg/L Time: 30 min pH: 13.20 Light: Sunlight (between 11 am and 3 pm) Volume: 100 mL	TiO2: N/A Composite: 91.30	[121]
rGO/ZnO/TiO2	Ultrasonication	Time: 60 min Power: N/A Freq.: N/A	N/A	TiO2: N/A Composite: N/A	Dosage: 0.10 g/L [Crystal violet]: 50 mg/L Time: 20 min pH: 6.50 Light: UV Volume: 100 mL	TiO2: N/A Composite: 87.06	[87]
rGO/ZnS/TiO2	Ultrasonication	Time: 30 min Power: 240 W Freq.: 22 kHz	Composite showed a red shift of absorbance compared to others	TiO2: N/A Composite: N/A	Dosage: 0.40 g/L [Crystal violet]: 50 mg/L Time: 50 min pH: N/A Light: UV Volume: 100 mL	TiO2: N/A Composite: 97	[86]
Shungite/WO3/TiO2	Ultrasonic-solvothermal	Time: 60 min Power: N/A Freq.: N/A	2.83	TiO2: N/A Composite: N/A	Dosage: 0.20 g/L [Orange II]: 10 mg/L Time: 120 min pH: N/A Light: UV Volume: 50 mL	TiO2: N/A Composite: 93.40; k = 0.036	[122]
SnO2/TiO2	Ultrasonic-assisted impregnation	Time: N/A Power: N/A Freq.: N/A	2.92	TiO2: N/A Composite: N/A	Dosage: 0.20 g/L [Tetracycline]: 10 mg/L [Methylene blue]: N/A [Congo red]: N/A Time: 40 min (tetracycline); 135 min (methylene blue); 100 min (congo red) pH: N/A Light: Visible Volume: 20 mL	Tetracycline TiO2: N/A Composite: 98.76; k = 0.086 Methylene blue TiO2: N/A Composite: 99 Congo red TiO2: N/A Composite: 90	[123]
SnO2/TiO2(A)/TiO2(R)	Ultrasonic-assisted alkaline hydrothermal	Time: 150 min Power: N/A Freq.: N/A	2.72	TiO2: N/A Composite: 78.50	Dosage: N/A [Naphthalene]: 30 mg/L Time: 220 min pH: N/A Light: Visible (300 W) Volume: N/A	TiO2: 24.40; k = 0.001 Composite: 100; k = 0.038	[124]
SnS2/TiO2	Thermosolvent ultrasonication	Time: 240 min Power: N/A Freq.: 15 kHz	Absorption wavelength of 647 nm	TiO2: N/A Composite: 5.65	Dosage: 0.50 g/L [Methyl orange]: 20 mg/L [Methyl blue]: 5 mg/L [Phenol]: 10 mg/L Time: 60 min pH: N/A Light: Visible (300 W) Volume: 400 mL	Methyl orange TiO2: N/A Composite: k = 0.009 Methyl blue TiO2: N/A Composite: k = 0.015 Phenol TiO2: N/A Composite: k = 0.003	[125]
TiO2/BiOI/Ag3PO4	Ultrasonication	Time: 10 min Power: N/A Freq.: N/A	N/A	TiO2: 48.10 Composite: 94.20	Dosage: 0.40 g/L [Rhodamine b]: ~5 mg/L Time: 60 min pH: N/A Light: Visible (50 W) Volume: 250 mL	TiO2: N/A Composite: 99.30; k = 0.061	[126]
TiO2/Bi2O3/CuO/zeolite	Ultrasonic-assisted wet impregnation	Time: 30 min Power: N/A Freq.: 50 kHz	1.73	TiO2: N/A Composite: 16.85	Dosage: 0.40 g/L [Safranin-O]: 16 mg/L Time: 300 min pH: N/A Light: Sunlight Volume: 200 mL	TiO2: N/A Composite: 94.10	[127]
TiO2/Carbon	Ultrasonication	Time: N/A Power: N/A Freq.: N/A	2.77	TiO2: 27.07 Composite: 49.27	Dosage: 0.026 wt% [Rhodamine b]: N/A Time: 75 min pH: N/A Light: UV (400 W) Volume: 100 mL	TiO2: 54 Composite: 94	[128]
TiO2/carbon nanotubes	Ultrasonic-assisted hydrothermal	Time: N/A Power: N/A Freq.: N/A	N/A	TiO2: N/A Composite: N/A	Dosage: 2 g/L [Methylene blue]: 30 mg/L Time: 180 min pH: 6.30 Light: Visible Volume: 100 mL	TiO2: N/A Composite: 84; k = 0.030	[129]
TiO2/C3N4/TCP	Ultrasonication	Time: 30 min Power: 100 W Freq.: 20 kHz	Absorption wavelength between 400 and 700 nm	TiO2: N/A Composite: N/A	Dosage: 1 g/L [Rhodamine b]: 12 mg/L Time: 60 min pH: N/A Light: Visible (300 W) Volume: 10 mL	TiO2: 51.70 Composite: 97.70	[130]
TiO2/Fe2O3	Ultrasonic-assisted impregnation	Time: 60 min Power: N/A Freq.: N/A	2.63	TiO2: N/A Composite: N/A	Dosage: 0.003 g/L [p-nitrophenol]: 50 mg/L Time: Until complete degradation of p-nitrophenol pH: N/A Light: Solar (300 W) Volume: 300 mL	TiO2: 100% (70 min) Composite: 100% (60 min)	[131]
TiO2/g-C3N4	Ultrasonic-assisted calcination	Time: N/A Power: 500 W Freq.: 40 kHz	2.34	TiO2: N/A Composite: N/A	Dosage: 0.50 g/L [Acetaminophen]: 5 mg/L Time: 30 min pH: N/A Light: Visible (300 W) Volume: 100 mL	TiO2: N/A Composite: 99.30	[132]
TiO2/g-C3N4	Ultrasonic-assisted alkali hydrothermal	Time: N/A Power: N/A Freq.: N/A	3.00	TiO2: N/A Composite: 140.73	Dosage: 1 g/L [Methylene blue]: 60 mg/L Time: 240 min pH: N/A Light: Visible Volume: 50 mL	TiO2: N/A Composite: 97	[133]
TiO2/g-C3N4	Ultrasonic-assisted hydrothermal	Time: N/A Power: N/A Freq.: N/A	2.88	TiO2: N/A Composite: N/A	Dosage: 0.20 g/L [2-Chlorophenol]: 25 mg/L Time: Until complete degradation of 2-chlorophenol pH: N/A Light: UV-visible (250 W) Volume: 100 mL	TiO2: 100% (110 min) Composite: 100% (65 min)	[134]
TiO2/g-C3N4	Ultrasonication	Time: 45 min Power: N/A Freq.: N/A	2.97	TiO2: 111 Composite: 155	Dosage: 0.50 g/L [Indigo carmine]: 50 mg/L Time: 60 min pH: N/A Light: Visible Volume: 100 mL	TiO2: 24; k = 0.004 Composite: 98; k = 0.030	[135]
TiO2/g-C3N4	Ultrasonic-assisted hydrothermal	Time: N/A Power: N/A Freq.: N/A	2.68	TiO2: N/A Composite: 58.26	Dosage: 0.20 g/L [Tetracycline]: 50 mg/L Time: 120 min pH: N/A Light: Visible (300 W) Volume: 100 mL	TiO2: N/A Composite: k = 0.008	[74]
P90-TiO2/g-C3N4	Ultrasonication	Time: 480 min Power: 150 W Freq.: 37 kHz	2.53	TiO2: N/A Composite: N/A	Dosage: 0.50 g/L (sunset yellow FCF); 1 g/L (phenol) [Sunset yellow FCF]: 10 mg/L [Phenol]: 30 mg/L Time: 5 min (sunset yellow FCF); 120 min (phenol) pH: N/A Light: Visible Volume: 100 mL	Sunset yellow FCF TiO2: k = 0.089 Composite: 98.80, k = 0.837 Phenol TiO2: k = 0.004 Composite: 99.35; k = 0.036	[96]
TiO2/GO	Ultrasonication	Time: 60 min Power: N/A Freq.: N/A	Overlapping of the absorption edge of TiO2/GO with GO	TiO2: 49 Composite: 92	Dosage: 0.10 g/L [Congo red]: 5 mg/L [Methyl orange]: 5 mg/L Time: 30 min pH: N/A Light: UV Volume: 100 mL	Congo red TiO2: N/A Composite: k = 0.065 Methyl orange TiO2: N/A Composite: k = 0.063	[93]
TiO2/GO	Ultrasonication	Time: N/A Power: N/A Freq.: N/A	3.13	TiO2: N/A Composite: N/A	Dosage: 0.50 g/L [Methylene blue]: 0.01 mM [Crystal violet]: 0.01 mM Time: 10 min pH: N/A Light: Visible (12 W) Volume: 50 mL	Methylene blue TiO2: k = 0.135 Composite: 95; k = 0.325 Crystal violet TiO2: k = 0.162 Composite: 99; k = 0.494	[136]
TiO2/In2O3	Ultrasonic-assisted sol–gel	Time: N/A Power: 50 W Freq.: N/A	2.92	TiO2: N/A Composite: N/A	Dosage: 0.25 g/L [Acid blue]: 10 mg/L Time: 90 min pH: N/A Light: UV-visible (350 W) Volume: 100 mL	TiO2: k = 0.030 Composite: 98; k = 0.040	[137]
TiO2/MoS2/BiVO4	Ultrasonic-assisted hydrothermal	Time: 120 min Power: 750 W Freq.: 20 kHz	2.84	TiO2: 52.30 Composite: 132.70	Dosage: 1 g/L [Tetracycline]: 20 mg/L Time: 90 min pH: N/A Light: Visible (300 W) Volume: 40 mL	TiO2: 52.30 Composite: 90.35	[79]
TiO2/MWCNT	Ultrasonic-assisted sol–gel	Time: 60 min Power: N/A Freq.: 20 kHz	2.75	TiO2: 76.34 Composite: 194.35	Dosage: 0.25 g/L [Methylene blue]: 5 mg/L [Rhodamine b]: 5 mg/L Time: 60 min pH: N/A Light: UV Volume: 200 mL	Methylene blue TiO2: 62.63; k = 0.019 Composite: 92.36; k = 0.051 Rhodamine b TiO2: 71.56; k = 0.022 Composite: 94.13; k = 0.053	[138]
TiO2/MWCNT/Pani	Ultrasonic-assisted in situ polymerization	Time: N/A Power: N/A Freq.: N/A	2.90	TiO2: 120.05 Composite: 78.69	Dosage: 1.50 g/L [Benzene]: 700 mg/L Time: 80 min pH: 6 Light: Visible (500 W) Volume: 10 mL	TiO2: N/A Composite: 84.92	[139]
TiO2/RGO	Ultrasonic-assisted hydrothermal	Time: 60 min Power: N/A Freq.: N/A	3.05	TiO2: 41.20 Composite: 77.20	Dosage: 1 g/L [Methylene blue]: 6.40 mg/L Time: 60 min pH: N/A Light: UV (6 W); sunlight (1pm and 3 pm in Cairo; 30.39w/m2) Volume: 50 mL	UV light TiO2: 28.01 Composite: 85.76 Sunlight TiO2: N/A Composite: 81.00	[140]
TiO2/SiO2	Ultrasonication coupled with alkali leaching	Time: 120 min Power: 160 W Freq.: N/A	Absorption wavelength of 763 nm	TiO2: N/A Composite: N/A	Dosage: 0.20 g/L [2,4-dinitrophenol]: 20 mg/L Time: 120 min pH: N/A Light: Xenon (150 W) Volume: 100 mL	TiO2: N/A Composite: k = 0.080	[98]
TiO2/TPPS	Ultrasonication	Time: 30 min Power: 100 W Freq.: 20 kHz	2.87	TiO2: N/A Composite: N/A	Dosage: 0.10 g/L [Eosin yellow]: 10 mg/L Time: 50 min pH: N/A Light: Visible Volume: 100 mL	TiO2: k = 0.028 Composite: 99; k = 0.090	[24]
TiO2/WO3	Ultrasonication and microwave	Time: 60 min Power: 750 W Freq.: 20 kHz	2.73	TiO2: 260 Composite: 235	Dosage: 0.50 g/L [Ciprofloxacin]: 20 mg/L [Oxytetracycline]: 20 mg/L Time: 45 min (UV light); 120 min (sunlight) for ciprofloxacin 60 min (UV light); 120 min (daylight) for oxytetracycline pH: N/A Light: UVA (20 W); sunlight Volume: 200 mL	Ciprofloxacin TiO2: 100; k = 0.039 (UV light) 45; k = 0.005 (sunlight) Composite: 100; k = 0.133 (UV light) 96; k = 0.034 (sunlight) Oxytetracycline TiO2: 91; k = 0.041 (UV light) 55; k = 0.007 (sunlight) Composite: 100; k = 0.082 (UV light) 97; k = 0.028 (sunlight)	[141]
TiO2/WO3	Ultrasonic-assisted sol–gel	Time: 10 min Power: 22 kHz Freq.: 190 W	2.60	TiO2: N/A Composite: 65.39	Dosage: 0.50 g/L [Congo red]: 20 mg/L [Methyl red]: 20 mg/L Time: 120 min pH: N/A Light: Visible (250 W) Volume: 100 mL	Congo red TiO2: N/A Composite: 95 Methyl red TiO2: N/A Composite: 100	[39]
TiO2/ZnIn2S4	Ultrasonic-assisted hydrothermal	Time: 60 min Power: N/A Freq.: N/A	Absorption wavelength of 436 nm	TiO2: N/A Composite: N/A	Dosage: N/A [Rhodamine b]: N/A Time: 120 min pH: N/A Light: Visible (300 W) Volume: N/A	TiO2: 37.40; k = 0.004 Composite: 90.20; k = 0.033	[73]
TiO2/ZnO	Ultrasonic-assisted precipitation	Time: 10 min Power: N/A Freq.: N/A	Around 2.30	TiO2: N/A Composite: N/A	Dosage: 4 g/L [Methyl orange]: 30 mg/L Time: N/A pH: N/A Light: UV (15 W) Volume: 20 mL	TiO2: 36.20 Composite: 58.70	[142]
Zeolite/CdS/TiO2/CeO2	Ultrasonic-assisted impregnation	Time: N/A Power: N/A Freq.: N/A	2.10	TiO2: N/A Composite: N/A	Dosage: 1.50 g/L [Methylene blue]: 10 mg/L Time: 120 min pH: 8 Light: Visible (85 W) Volume: 100 mL	TiO2: 28 Composite: 99.90	[81]
ZnFe2O4/TiO2	Solvothermal followed by ultrasonication	Time: 90 min Power: N/A Freq.: N/A	2.03	TiO2: 12.73 Composite: 54.81	Dosage: 0.30 g/L [Rhodamine b]: 10 mg/L Time: 60 min pH: N/A Light: UV (500 W) Volume: N/A	TiO2: 94.90; k = 0.045 Composite: 97.30; k = 0.062	[99]
ZnFe2O4/TiO2/Ag2O	Solvothermal, followed by ultrasonication and chemical precipitation	Time: 90 min Power: N/A Freq.: N/A	Absorption wavelength of 702 nm	TiO2: N/A Composite: 120.90	Dosage: 0.30 g/L [Rhodamine b]: 10 mg/L Time: 40 min pH: N/A Light: UV (500 W) Volume: 500 mL	TiO2: N/A Composite: 98.40; k = 0.094	[7]

summarizes various TiO₂-based composite photocatalysts synthesized using ultrasonication. The cited studies consistently demonstrate that this technique enhances material properties and photocatalytic performance by controlling particle size, morphology, and surface characteristics. In conclusion, ultrasonication is a versatile method for producing efficient TiO₂ composites, offering reduced charge carrier recombination and enhanced activity across various applications.

4.1. Parameters of Ultrasonication

In sonochemistry, the effects of ultrasonication parameters including power, frequency, temperature, and duration on photocatalyst size and morphology are actively studied. This review aims to outline the optimization of these variables to achieve materials with highly desirable characteristics.

4.1.1. Time

Ultrasonication duration is a critical parameter in photocatalyst synthesis, directly influencing nanoparticle dispersion and size distribution [143]. While extended times generally promote finer particles and more homogeneous dispersions, excessive exposure can cause aggregation, thermal degradation, and unintended reactions, necessitating careful optimization [143].

The time-dependent effects are evident in specific studies. Rasouli et al. [12] found that 8 min was optimal for dispersing a Fe₂O₃/TiO₂ nanocomposite; longer times introduced excessive energy, promoting agglomeration via processes like Ostwald ripening. Similarly, Karamifar et al. [139] identified 10 min as ideal for dispersing a TiO₂/MWCNT/Pani nanocomposite, with benzene removal efficiency improving from 58.50% to 69.53% as time increased. In the synthesis of ZnO/TiO₂/ZrO₂, Abrinaei and Aghabeygi [144] observed that different durations (5–30 min) yielded platelet-like structures with particle sizes between 30 and 50 nm.

For pure TiO₂, Kumar et al. [145] reported a particle size reduction from 982 nm to 560 nm as sonication time increased from 5 to 25 min, attributing this to cavitation effects. Beyond 25 min, no significant further reduction occurred. Tiple et al. [146] also observed a decrease in particle size (554 nm to 316 nm) with time, selecting 45 min as the optimal balance between effectiveness and cost. Estrada-Monje et al. [147] confirmed that even short durations (5–10 min) at high power (400 W) achieved thorough dispersion.

Conversely, very long durations can alter material structure. Elavarasan et al. [148] sonicated TiO₂ for 3 to 7 h at 90 °C and 500 W, finding that longer times reduced crystallinity, increased interplanar distance due to oxygen vacancies, and decreased the band gap from 2.97 to 2.90 eV. Collectively, these studies underscore that ultrasonication time must be optimized to achieve desired morphological and structural properties, as its effects are both significant and complex.

4.1.2. Power or Intensity

Ultrasonication intensity, defined as the power per unit area of the ultrasonic wave, is a critical parameter influencing nanoparticle dispersion and composite homogeneity [149]. It directly affects nanoparticle size distribution and suspension stability [144]. While higher intensities effectively break down agglomerates for finer, more uniform dispersions [87,150], excessive intensity can cause particle fragmentation or matrix degradation, necessitating careful optimization [150]. This balance is demonstrated in several studies.

Rasouli et al. [12] found that increasing ultrasonication power from 50 W to 70 W improved the photocatalytic degradation of cefixime from 64.91% to 78.80% for a Fe₂O₃/TiO₂ composite, due to more violent cavitation bubble collapse. However, at 90 W, efficiency dropped to 69.53%, likely due to aggregation from increased thermodynamic stability. Karamifar et al. [139] observed a similar trend, with benzene removal efficiency increasing from 64.91% to 69.53% as power rose from 50 W to 90 W for a TiO₂/MWCNT/Pani nanocomposite. Tiple et al. [146] reported a direct correlation between power and size reduction, with TiO₂ particle size decreasing from 780 nm to 352.90 nm as power increased from 80 W to 120 W.

Higher power levels generally enhance performance by improving dispersion. Tian et al. [98] achieved a V₂O₅ separation rate of 99.90% with ultrasonic-assisted alkali leaching, compared to 32.10% without, noting that increased power (20–140 W) significantly boosted separation. Similarly, Abd-Rabboh et al. [106] found that photocatalytic efficiency for rhodamine B degradation increased with ultrasonic intensity up to 300 W for a BiVO₄/TiO₂ composite. Conversely, Qayyum et al. [151] demonstrated that power must be matched to the desired property; lower power (24 W) produced porous TiO₂ with a high surface area (156 m²/g), while higher power (56 W) yielded non-porous, aggregated TiO₂ with a low surface area (11 m²/g).

4.1.3. Frequency

Ultrasonic frequency governs cavitation bubble dynamics, including their number, size, distribution, and chemical activity [152]. Higher frequencies (200–500 kHz) produce numerous small, chemically active bubbles ideal for enhancing reactions. In contrast, lower frequencies (20–100 kHz) generate larger, short-lived bubbles that facilitate the rapid synthesis of nano-photocatalysts without complex agents [152,153].

Bandeira et al. [154] synthesized TiO₂ using audible sound frequencies (174 Hz, 5 kHz, 15 kHz), finding that all frequencies enhanced crystallinity and produced smaller particles compared to non-sonicated samples. Performance improved with frequency, yielding photocatalytic rates of 0.016 min⁻¹ (174 Hz), 0.050 min⁻¹ (5 kHz), and 0.055 min⁻¹ (15 kHz). Oliveira et al. [155] compared frequencies of 274 Hz and 409 Hz, finding that 409 Hz produced larger crystallites (6.50 nm vs. 5.66 nm) and a lower band gap (e.g., 3.14 eV vs. 3.19 eV at 473 °C), attributed to greater amplitude and spectral homogeneity.

Despite its advantages, ultrasonication also poses certain disadvantages, such as the high energy input required during the process, which can potentially damage photocatalysts and influence their properties. As demonstrated by Dong et al. [156] with egg-white protein-curcumin nanoparticles, an optimal duration (12 min) enhances key characteristics like solubility and encapsulation efficiency. Beyond this threshold, however, excessive sonication induces detrimental effects such as aggregation and increased crystallinity, compromising the product’s integrity.

Rasouli et al. [12] investigated the effect of ultrasonication time (3–8 min) on the efficiency, dimension, and structure of the photocatalyst. They found that as the ultrasonic time increased from 5 to 8 min, the continual energy input inhibited the growth of the α-Fe₂O₃@TiO₂ nanocomposites. Furthermore, the photocatalytic removal efficiency increased from 64.91% to 78.8% as the ultrasonic power was raised from 50 W to 70 W. However, a further increase to 90 W reduced the efficiency to 69.53%. This optimal power at 70 W is critical for forming a regular and homogeneous α-Fe₂O₃@TiO₂ nanocomposite, whereas other powers, such as 50 W, yield irregular, agglomerated structures and result in poorer catalytic performance. The initial improvement is attributed to enhanced cavitation bubble collapse at 70 W, which generates stronger shock waves that prevent agglomeration. Conversely, powers exceeding 70 W promote rapid primary nucleation, increasing thermodynamic stability and causing faster particle aggregation due to an excessive energy input.

Karamifar et al. [139] synthesized a TiO₂/MWCNT/Pani nanocomposite via an ultrasonic-assisted method. They determined that a 10 min treatment was optimal, as prolonged ultrasonication caused irregular agglomeration due to the high surface energy of nanostructures driving excessive nucleation. Furthermore, when evaluating benzene degradation, catalyst performance improved from 64.91% to 69.53% as ultrasonic power increased from 50 W to 90 W, establishing 90 W as the optimal power for producing a regular and effective nanocomposite. Thus, while ultrasonication offers significant benefits in enhancing photocatalytic performance, careful parameter optimization is crucial to mitigate potential drawbacks and maximize its effectiveness in material synthesis.

5. Photocatalytic Activity of TiO₂-Based Nanocomposite Photocatalysts

Persistent toxic organic pollutants from agricultural, pharmaceutical, and textile industries are prevalent in various water sources, necessitating effective removal strategies. Studying the photocatalytic degradation of such pollutants, Chau et al. [28] used a WO₃/TiO₂ composite to decompose Reactive Black 5 dye. The degradation was confirmed by the solution’s discoloration and the disappearance of its characteristic UV-Vis absorption peak at 597 nm, which is associated with the dye’s chromophore group. The consistent decrease in this peak indicates that the chromophore was the primary site of oxidation. Similarly, Manivannan et al. [24] observed a gradual reduction in the intensity of the 515 nm peak of Eosin Yellow dye, corresponding to the solution fading over time.

Chemical oxygen demand (COD) is a critical parameter for assessing the complete mineralization of organic compounds and gauging water pollution levels [25]. It measures the oxygen required to chemically oxidize organics in a solution [12]. For instance, Feng et al. [75] used COD to evaluate a WO₃@TiO₂/CS-biochar composite, achieving 97.56% COD removal after two hours of light irradiation. Total organic carbon (TOC) is another key mineralization indicator [157]. In a study on potato wastewater, Li et al. [158] reported COD and TOC removal rates of 82.34% and 63.36%, respectively, using an ultrasonic-assisted TiO₂/Fe₃O₄ photocatalyst.

The photostability and reusability of photocatalysts are crucial for practical application, as photo-corrosion can lead to material leaching, impacting water safety [159]. Rasouli et al. [12] assessed α-Fe₂O₃/TiO₂ reusability over five cycles, noting a performance decline from 98.04% to 94.10%. This was attributed to material loss during recovery and the blocking of active sites by adsorbed dyes or by-products [12,157]. Importantly, the photocatalyst’s morphology and crystal structure remained stable, confirming its suitability for repeated use in wastewater treatment [12].

Photostability is often assessed through photocurrent retention. For example, Debnath et al. [55] demonstrated high photostability with 99% photocurrent retention after two hours under continuous light. Understanding the photocatalytic mechanism requires identifying the primary reactive oxygen species (ROS) through quenching experiments. Key ROS include the superoxide anion (^•O₂⁻), photogenerated hole (h⁺), and hydroxyl radical (•OH). Specific scavengers are used to trap these species: p-benzoquinone (BQ) or a nitrogen gas for ^•O₂⁻; ammonium oxalate, hydrogen (H₂O₂), and disodium ethylene diamine tetraacetate (EDTA-2Na) for h⁺; and tertbutanol or isopropyl alcohol for ^•OH. Silver nitrate (AgNO₃) traps electrons (e⁻) and sodium azide is used for singlet oxygen using [23,28,52,125,160,161,162].

Debnath et al. [55] used electron paramagnetic resonance (EPR) with DMPO as a spin-trapping agent to confirm the generation of both ^•O₂⁻ and ^•OH radicals [139], which exhibit distinct characteristic peaks [139]. Similarly, Hou et al. [160] detected six distinct peaks of ^•O₂⁻ and four characteristic peaks of ^•OH via EPR [75], as shown in Figure 5a,b. Their study compared the formation rates of these radicals for TiO₂/rGO and Ag/TiO₂/rGO against pure TiO₂, correlating the higher radical production in the composites with their enhanced photocatalytic performance. Figure 5c,d show that the EPR signal strength for ^•O₂⁻ and ^•OH was consistently higher for the composites under different illumination periods.

Mahamud et al. [81] investigated photocatalytic performance with and without scavengers. Without scavengers, efficiency was 99.90%, but it decreased to 83.20%, 92.00%, and 78.90% with the addition of methanol, sodium bicarbonate, and AgNO₃, respectively. These results indicate that ^•O₂⁻ and ^•OH play more essential roles in photocatalytic activity than h⁺. Similarly, Karamifar et al. [139] reported 84% benzene removal without scavengers, which dropped to 73.58%, 58.50%, 20.70%, and 29.71% with AgNO₃, EDTA, ethanol, and BQ, respectively. They concluded that ^•OH and ^•O₂ are the primary reactive species.

Wafi et al. [140] utilized photoluminescence (PL) with terephthalic acid to trap ^•OH, observing an increase in PL intensity at 430 nm over time, which confirmed ^•OH generation. Correspondingly, Eskandari et al. [161] found that the PL signal for ^•OH increased proportionally with irradiation time (30–240 min), affirming it as the predominant species in the photodegradation of methylene blue and congo red dyes.

Factors Affecting the Photodegradation Performance

The efficiency of photocatalytic degradation is governed by several parameters, including pH, photocatalyst loading, initial contaminant concentration, light source, and temperature. Identifying optimal conditions is essential for maximizing process efficiency. This section discusses key parameters, starting with catalyst dosage, which is critical for cost-effectiveness. Magdalane et al. [163] observed that the photodegradation efficiency of SnO₂/TiO₂ increased with dosage up to an optimum of 50 mg. Beyond this point, efficiency declined, likely due to light shielding effects that reduce penetration and active radical production [161,163]. Similarly, Yusuff et al. [164] found that methylene blue degradation improved with increased ZnO-TiO₂ dosage up to 1.20 g, enhancing the availability of active sites and oxidizing agents. Further increases beyond this dosage yielded no additional improvement.

The initial dye concentration also significantly influences photodegradation efficiency. Mahamud et al. [81] found maximum efficiency at a lower MB concentration (10 mg/L) using a zeolite-supported CdS/TiO₂/CeO₂ composite, attributing this to enhanced dye adsorption and dispersion on the photocatalyst surface. Conversely, Eskandari et al. [161] observed decreased photodegradation efficiency as the concentrations of Methylene Blue and Congo Red increased from 10 to 30 ppm. This decline is primarily due to increased solution turbidity, which limits light penetration and active site availability on the photocatalyst surface [161], thereby reducing the generation of ROS [81,165].

Photocatalyst activation is dependent on specific light conditions. For instance, TiO₂, with a band gap of 3.20 eV, requires UV light for activation. In contrast, composites like Fe₃O₄@SiO₂@g-C₃N₄/TiO₂ can function efficiently under both UV and visible light [166]. Kalikeri et al. [25] demonstrated that BFO/TiO₂ performs better under solar light than UV light alone due to this dual activation capability. Light intensity and irradiation time are also critical. Huang et al. [167] showed that TiO₂/CoFe₂O₄ degrades pollutants faster under sunlight than UV light.

Eskandari et al. [161] confirmed that longer irradiation times enhance dye degradation by increasing the production of active species. Duta et al. [168]. investigated CuInS₂/TiO₂/SnO₂ and found that higher visible light intensity (32 W/m²) yielded a rate of 0.0028 s⁻¹, compared to 0.0025 s⁻¹ at a lower intensity (9 W/m²). A mixed UV/visible light source achieved even higher rates (0.0032–0.0035 s⁻¹). Similarly, Moradi et al. [169] showed that increasing light intensity from 10 to 40 W/m² raised the dye removal percentage to 85.40%, as more energy is delivered to activate the photocatalyst.

Industrial wastewater pH varies significantly and strongly influences photodegradation rates [165]. A photocatalyst’s acid and alkali resistance is therefore crucial for practical application [23]. Gao et al. [23] found that TiO₂/rGO/TiN maintained stable performance across a broad pH range (3–11). pH affects the surface charge of both the photocatalyst and the pollutant, governing their electrostatic interaction. Kumar et al. [170] observed that at low pH, the positively charged TiO₂ surface effectively adsorbs anionic dyes, enhancing degradation. Conversely, at high pH, the negatively charged surface repels these dyes.

The point of zero charge (pH_pzc) is key to understanding this behavior. For Ag/TiO₂/rGO, the pH_pzc is between 5 and 6 [160]. At a solution pH below this range, the surface becomes positively charged, attracting anions like Cr₂O₇²⁻. Above the pH_pzc, the negatively charged surface causes repulsion. Moradi et al. [169] illustrated this with AB172 dye (pKa = 6), where the dye’s charge shifts from positive (pH < 6) to negative (pH > 6), altering its interaction with the catalyst.

The molecular structure of the contaminant itself directly impacts degradation efficiency. Nguyen et al. [157] found that the mineralization rate of methyl orange (44.2%) after 3 h was lower than that of Rhodamine B (48%) and Methylene Blue (59%). This was attributed to the resilient azo group in methyl orange, which is difficult to cleave, underscoring how molecular composition dictates degradation kinetics [157].

6. Conclusions and Future Perspectives of Titanium Dioxide (TiO₂)-Based Photocatalysts

Researchers continue to develop strategies to overcome the limitations of unmodified TiO₂ photocatalysts, focusing on enhancing their activity and stability. This review has summarized the synthesis and performance of TiO₂-based composite photocatalysts fabricated via ultrasonication techniques. These composites effectively extend the optical absorption of TiO₂ from the UV region to the visible light region by reducing its band gap energy. Furthermore, the heterojunctions formed within these structures significantly suppress the recombination of photoexcited charge carriers, leading to superior photodegradation efficiency.

To ensure the commercial viability of these materials, future research must address key practical challenges. The difficulty in recovering powdered TiO₂ photocatalysts from treated wastewater poses a risk of secondary contamination and limits reusability. Developing composite materials that facilitate easy separation and recovery is therefore essential. Concurrently, issues such as optimizing photocatalyst–contaminant contact and maintaining a high active surface area must be resolved to ensure consistently high performance.

Beyond efficiency, more attention must be paid to the potential environmental impact of the photocatalytic process. The toxicity of transformation products formed from incomplete degradation of organic contaminants is often overlooked. Given their nanoscale dimensions (1–100 nm), these photocatalysts have a high potential for subcellular interaction. Future work should prioritize elucidating detailed degradation pathways and identifying intermediate compounds. Comprehensive assessments using cytotoxicity, phytotoxicity, and genotoxicity assays are crucial to evaluate the safety of treated wastewater, particularly if it is intended for reuse in agricultural applications.