Ti₃C₂T_x MXene-Based Hybrid Photocatalysts in Organic Dye Degradation: A Review

Abstract

This review provides an overview of the fabrication methods for Ti₃C₂T_x MXene-based hybrid photocatalysts and evaluates their role in degrading organic dye pollutants. Ti₃C₂T_x MXene has emerged as a promising material for hybrid photocatalysts due to its high metallic conductivity, excellent hydrophilicity, strong molecular adsorption, and efficient charge transfer. These properties facilitate faster charge separation and minimize electron–hole recombination, leading to exceptional photodegradation performance, long-term stability, and significant attention in dye degradation applications. Ti₃C₂T_x MXene-based hybrid photocatalysts significantly improve dye degradation efficiency, as evidenced by higher percentage degradation and reduced degradation time compared to conventional semiconducting materials. This review also highlights computational techniques employed to assess and enhance the performance of Ti₃C₂T_x MXene-based hybrid photocatalysts for dye degradation. It identifies the challenges associated with Ti₃C₂T_x MXene-based hybrid photocatalyst research and proposes potential solutions, outlining future research directions to address these obstacles effectively.

1. Introduction

Rapid industrialization has escalated the production of toxic and carcinogenic wastewater [1]. The discharge of untreated wastewater from textile, pharmaceutical, dyeing, and paper printing industries significantly increases dye concentrations in water sources, posing a severe environmental threat [2]. These wastewaters contain chromophores and auxochromes, which color water, block sunlight, and disrupt photosynthesis, thereby harming aquatic ecosystems [3]. Discharged dyes are classified as cationic (e.g., methylene blue (MB), crystal violet (CV), rhodamine B (RhB)), which carry positive charges, are prevalent in textiles, and present toxicity risks, or anionic (e.g., acid orange 7 (AO7), acid blue 92 (AB92), Congo red (CR)), which carry negative charges, are water soluble, and are difficult to remove [4]. Various degradation methods, including membrane filtration, oxidation, adsorption, and photocatalysis, have been successfully developed and applied to address this issue [5,6].

Photocatalysis stands out as a highly effective degradation method due to its eco-friendliness and cost-effectiveness in treating organic pollutants [7]. Consequently, extensive research has focused on developing photocatalytic materials with wide energy bandgaps, abundant surface-active sites, and efficient charge separation capabilities to optimize dye photodegradation [8]. While single-component semiconducting photocatalysts like TiO₂, ZnO, ZnS, SnO₂, graphitic carbon nitride (g-C₃N₄), and CdS have been widely employed [9,10], their photocatalytic activity is often limited by poor visible light absorption, rapid charge recombination, and low electron–hole mobility [11,12]. To address these limitations and enhance photocatalytic performance, various strategies have been explored, including heterojunction construction [13], element doping [14,15], and co-catalyst introduction [14].

Modifying semiconducting materials with a co-catalyst is an effective strategy to enhance their photocatalytic activity [16]. A co-catalyst facilitates charge separation by preventing electron–hole recombination, a common limitation in photocatalysis. Materials such as graphene [17], carbon nanotubes [18], and carbon quantum dots [19] have been utilized as co-catalysts to improve dye photodegradation. While significant progress has been achieved using these co-catalysts, they each have certain limitations. Graphene and carbon nanotubes face challenges due to their scarcity of functional groups [20], while carbon quantum dots are hindered by their relatively low conductivity and scalability issues [21]. Challenges such as insufficient surface functional groups, which restrict chemical bonding between the photocatalyst and co-catalyst, and lower electrical conductivity, which impedes charge migration, can hinder photocatalytic efficiency. Consequently, there remains a need for efficient and cost-effective co-catalysts to optimize the photocatalytic degradation of organic dyes.

MXenes, a class of two-dimensional (2D) layered materials composed of transition metal carbides, nitrides, or carbonitrides with the general formula M_n+1X_n (where M is a transition metal like Sc, Ti, Zr, Hf, V, Nb, Ta, or Mo, and X is carbon, nitrogen, or carbonitride), are promising for hybrid photocatalyst fabrication [22,23,24]. Their lower Fermi level compared to most studied semiconductors, abundant terminal functional groups, excellent metallic conductivity, and exposed terminal metal sites contribute to this potential. Specifically, the presence of abundant functional groups such as -OH, -O, and -F on Ti₃C₂T_x MXene facilitates strong interfacial chemical bonding with semiconductors, enabling the formation of a Schottky junction. This junction acts as an electron trap, effectively suppressing photoexcited electron–hole pair recombinations [25]. Furthermore, the excellent metallic conductivity of Ti₃C₂T_x MXene ensures rapid charge carrier migration, promoting efficient separation of photogenerated electrons and holes [26]. Lastly, the exposed terminal metal sites enhance reactivity compared to carbon-based materials, making Ti₃C₂T_x MXene a highly effective co-catalyst.

While Ti₃C₂T_x MXene exhibits limited standalone photocatalytic activity, requiring doping and UV radiation for optimal function [27], it plays a crucial role in composite photocatalysts. Beyond acting as a photogenerated charge acceptor co-catalyst, its large surface area and surface functionalities provide an excellent platform for the uniform growth, size control, and fine dispersion of photocatalysts, thereby exposing more surface-active sites [22,28,29,30]. Moreover, Ti₃C₂T_x MXene possesses distinctive physical and chemical properties, including high electrical conductivity, excellent hydrophilicity, mechanical stability, ion intercalation ability, and tunable surface functionalization [31,32,33]. These attributes render it an ideal component for composite photocatalyst materials. The integration of Ti₃C₂T_x MXene with semiconducting materials has yielded hybrid photocatalysts with micro/nano architectures and multi-junction nanocomposites. These hybrids leverage synergistic interactions between Ti₃C₂T_x MXene and conventional semiconductors or metal nanostructures, leading to enhanced charge separation and reduced recombination rates [23,34]. Consequently, Ti₃C₂T_x MXene-based hybrid photocatalysts demonstrate high effectiveness for the photodegradation of organic pollutants [35].

This review comprehensively explores Ti₃C₂T_x MXene-based hybrid photocatalysts for the degradation of cationic and anionic organic dyes. It details the synthesis of Ti₃C₂T_x MXene, its integration into hybrid photocatalysts, its role and effectiveness in dye degradation, and the underlying mechanisms. Furthermore, it discusses current challenges and emerging opportunities, providing insights into potential future research pathways.

2. Ti₃C₂T_x MXene and Synthesis Methods

2.1. Introduction to Ti₃C₂T_x MXenes

MXenes, first synthesized by Naguib et al. in 2011, are 2D crystals of transition metal carbides, nitrides, or carbonitrides [36]. These 2D materials are produced via top-down approaches, starting with their three-dimensional parent materials, MAX phases. MAX phases, represented by the general formula M_(n+1)AX_n (where n = 1, 2, or 3), consist of an early transition metal M (e.g., titanium, niobium, molybdenum, tantalum, vanadium, chromium), an A-group element A (primarily groups 13 and 14), and carbon, nitrogen, or carbonitride X layers [37]. Chemical etching removes the A layers from the MAX phase, resulting in the formation of 2D MXene crystals. Due to the predominantly aqueous synthesis methods, the M elements on the resulting MXene surfaces are terminated with functional groups, represented by Tx. These terminated MXenes are denoted by the general formula M_(n+1)X_nT_x (where n = 1, 2, or 3), with T_x typically representing -OH, -O, or -F surface terminations [38,39,40,41,42].

MXenes have garnered significant attention due to their exceptional properties, including hydrophilicity, high electrical conductivity, and tunable bandgaps through surface termination modifications [42,43]. Ti₃C₂T_x, the inaugural member of the MXene family, remains the most extensively researched due to its durability, the relatively simple process of etching aluminum layers from its MAX phase precursor, and its remarkable physical and chemical characteristics [36,44,45]. Consequently, leveraging their unique properties, numerous other MXene species have been explored for a wide range of applications.

2.2. Synthesis of Ti₃C₂T_x MXenes

Hydrofluoric acid (HF) etching was the pioneering technique for transforming MAX phases into MXenes [36]. Despite its simplicity and ability to yield high-quality MXenes [33], making it a widely used method, HF’s extreme toxicity and corrosiveness pose significant drawbacks [46]. To mitigate these risks, alternative methods, such as in situ HF generation via a reaction between LiF and HCl, have been developed. This approach also facilitates Li⁺ cation intercalation between MXene layers [47]. A characteristic outcome of HF etching is the formation of MXene layers with surface terminations, primarily -O, -OH, and -F, which enable surface engineering and bandgap tuning through modification of the termination type and quantity [48]. Another modified etching technique is the molten salt method, which involves reacting MAX phases with a Lewis acidic molten salt at elevated temperatures [39]. Similar to HF etching, an intercalant is utilized; however, the molten salt method allows for greater control over surface terminations during synthesis. For instance, -F terminations produced by HF etching can degrade the electrochemical performance of Ti₃C₂T_x electrodes in supercapacitors. To address this, Guo et al. [49] employed a one-step LiCl-KCl-K₂CO₃ molten salt etch and delamination process to replace -F terminations with -O, reducing the -F content from 11.23 to 3.43 atomic percent. This resulted in improved specific capacity, capacitance retention, conductivity, and electrochemical activity-specific surface area.

The hydrothermal etching method was developed as a safer and more environmentally friendly alternative to the highly corrosive and hazardous HF etching process. This technique enables efficient exfoliation and the production of high-quality MXene flakes, offering the advantages of larger interlayer spacing and improved delamination properties. Hydrothermal etching utilizes high-pressure and high-temperature conditions in an aqueous MXene solution to produce high-purity multilayer MXenes [46]. For example, Peng et al. [33] synthesized Ti₃C₂T_x MXenes using this technique by reacting MAX phases with HCl and HCl + NaBF₄ solutions, followed by heating in an autoclave. The resulting MXenes were delaminated using dimethyl sulfoxide (DMSO) and sonication. XRD analysis revealed that hydrothermal etching was significantly more effective at removing aluminum compared to traditional HF etching. Furthermore, dye adsorption studies with cationic MB and anionic methyl orange (MO) demonstrated that hydrothermally etched Ti₃C₂T_x exhibited superior adsorption performance, with lower residual dye concentrations compared to non-hydrothermal counterparts. Some hydrothermal methods incorporate microwaves to excite reagents and reduce reaction temperature and time [50].

The resulting flakes were cleaned, delaminated using tetramethylammonium hydroxide, cleaned again, and then incorporated into a composite with reduced graphene oxide, which effectively degraded dyes via photolysis [51]. Cao et al. [50] utilized microwave hydrothermal etching to rapidly create and oxidize Ti₃C₂T_x MXene nanosheets. After cleaning, this Ti₃C₂T_x was combined with TiO₂ and CdZnS, again using microwave hydrothermal methods, to synthesize a synergistic photocatalytic semiconductive heterojunction. This heterojunction degraded RhB dye molecules by 29.33% within 90 min, a 31.17-fold improvement compared to single Ti₃C₂ [52]. To drastically reduce etching time (from approximately 2–3 days to 45 min) and eliminate -F terminations, Latif et al. [53] applied 5 to 30 M concentrations of NaOH etchant to Ti₃AlC₂ in a microwave hydrothermal reaction. Higher NaOH concentrations resulted in synthesized Ti₃C₂T_x MXene, with a 0.46% aluminum content, as determined by XRD, and a semiconductive bandgap energy of 1.30 to 1.60 eV.

3. Design and Fabrication of Ti₃C₂T_x MXene-Based Hybrid Photocatalysts

A wide range of methods have been employed to fabricate Ti₃C₂T_x MXene-based hybrid photocatalysts, involving the integration of Ti₃C₂T_x MXene nanosheets with diverse nanomaterials. As a result of their chemical synthesis, Ti₃C₂T_x MXene materials naturally acquire surface terminations, and their tunable surface chemistry provides potential pathways for hybridization with various material classes through covalent and non-covalent interactions [54]. The redox behavior of titanium (Ti) has been leveraged to develop TiO₂-based semiconductors on the MXene surface, where MXene serves as a growth platform. The presence of hydrophilic surface functional groups such as -O-, -OH, and -F is theoretically expected to enhance chemical bonding with other semiconductor photocatalysts. However, the development of hybrid photocatalysts utilizing the MXene functional terminations through covalent bonding requires state-of-the-art approaches, and hence, most studies have primarily utilized non-covalent approaches, where charge interactions or hydrophilic groups act as adsorption or anchoring sites for metal salts, semiconductors, and other materials [55,56].

Tran et al. [57] fabricated TiO₂/Ti₃C₂T_x composites through an in situ partial oxidation of Ti₃C₂T_x, resulting in microscale safflower-like structures composed of TiO₂/Ti₃C₂T_x heterostructure nanorods. This transformation involved sequential hydrothermal oxidation, alkalization, ion exchange, and heat treatment, during which layered MXene flakes were fragmented into nanoparticles, from which TiO₂/Ti₃C₂T_x nanorods grew radially. A schematic of this synthesis process is shown in Figure 1.

The resulting TiO₂/Ti₃C₂T_x heterostructures demonstrated exceptional photocatalytic properties. Their photocurrent was ten times greater than that of pristine MXene. Furthermore, the photocatalytic degradation efficiency of rhodamine B (RhB) reached 95%, a significant improvement over the 19% achieved by MXene alone. Even after four cycles, the degradation efficiency remained above 95%, indicating excellent stability. This superior performance was attributed to the rapid generation of TiO₂ carriers, suppressed charge carrier recombination, and enhanced light absorption due to the porous safflower-like structure. Similarly, Quyen et al. [58] employed a novel synthesis approach to create TiO₂@Ti₃C₂T_x nanoflowers with a porous 3D framework derived from 2D Ti₃C₂T_x MXenes via hydrothermal oxidation combined with calcination. This in situ transformation converted the initially conductive Ti₃C₂T_x MXene into a semiconductor, forming a TiO₂@Ti₃C₂T_x heterojunction. Compared to the 36% degradation rate of pure TiO₂ for RhB, the TiO₂@Ti₃C₂T_x composite achieved an impressive 97% degradation within 40 min of light irradiation. This enhancement was attributed to the electronic structure of Ti₃C₂T_x, whose Fermi level is lower than that of TiO₂. Upon light excitation, photoinduced electrons transferred from the valance (CB) of TiO₂ to metallic Ti₃C₂T_x, leaving holes in the valence band (VB) of TiO₂. The resulting Schottky barrier at the interface suppressed electron diffusion back into TiO₂, thereby reducing electron–hole recombination and improving photocatalytic efficiency. In a similar vein, an in situ solvothermal process was utilized to prepare facet-exposed TiO₂/Ti₃C₂T_x [59]. Since the exposed crystal planes of TiO₂ can effectively capture photogenerated holes and facilitate rapid migration to the Ti₃C₂T_x surface, the photocatalytic performance towards methyl orange was significantly superior compared to TiO₂ and Ti₃C₂T_x alone.

Calcination, a simple method for preparing Ti₃C₂T_x MXene composites, involves heating powders of two materials at elevated temperatures. As illustrated in Figure 2, varying amounts of Ti₃C₂T_x MXene powders were thoroughly dispersed in a Zn²⁺-containing solution [60]. The resulting mixture was heated in an oven to evaporate the solvent. Subsequently, the powder was calcined at 550 °C for 4 h, with a heating rate of 5 °C/min in ambient air to synthesize ZnO/Ti₃C₂T_x hybrid structures. Compared to pristine ZnO, the hybrid structure exhibited reduced photoluminescence intensity, an enhanced Brunauer–Emmett–Teller (BET) surface area, and improved photocatalytic degradation efficiency for MO and RhB.

The wet impregnation method, where one material is deposited onto a solid support, can be used to prepare Ti₃C₂T_x MXene composites. Nasri et al. [6] mixed g-C₃N₄ and Ti₃C₂T_x MXene powders in varying weight proportions and sonicated the mixture until a slurry formed. The resulting slurry was dried overnight at 60 °C to obtain the Ti₃C₂T_x MXene/g-C₃N₄ composite photocatalyst. Figure 3 illustrates the fabrication process of this composite. They found that the 1 wt% Ti₃C₂T_x MXene/g-C₃N₄ heterostructure exhibited higher photocatalytic activity for methylene blue degradation compared to pure g-C₃N₄. This enhanced activity was attributed to intimate interfacial contact (observed via field emission scanning electron microscopy (FESEM) analysis), efficient photo-charge carrier transfer, and a larger BET surface area.

Electrostatically driven self-assembly is a facile approach for synthesizing MXene composites, particularly in solutions. Cai et al. [61] synthesized an Ag₃PO₄/Ti₃C₂T_x MXene Schottky catalyst that exhibited prominent photodegradation performance towards various organic dyes, including methyl orange and 2,4-dinitrophenol. In their process, Ti₃C₂T_x sheets were first dispersed in deionized (DI) water using sonication, followed by the addition of an AgNO₃ aqueous solution to the Ti₃C₂T_x suspension under vigorous stirring. Subsequently, a Na₂HPO₄ aqueous solution was added dropwise to the mixture and stirred for 2 h. The resulting precipitate was washed multiple times with DI water and dried in a vacuum oven at 80 °C overnight. The negatively charged surface of Ti₃C₂T_x, due to abundant surface termination groups, facilitated its interaction with Ag₃PO₄. The apparent rate constant for 2,4-dinitrophenol degradation with Ag₃PO₄/Ti₃C₂T_x was 2.5 times higher than that of Ag₃PO₄/RGO and 10 times higher than that of Ag₃PO₄ alone. This enhanced photocatalytic activity of Ag₃PO₄/Ti₃C₂T_x was attributed to the sufficient and close interfacial contact between Ag₃PO₄ and Ti₃C₂T_x, unidirectional electron flow trapped by Ti₃C₂T_x across the Schottky barrier, and the stronger redox reactivity of surface metal Ti sites on Ti₃C₂T_x.

Ultrasonic forces have been employed in the fabrication of Ti₃C₂T_x MXene-based composites to disrupt electrostatic attractions and van der Waals interactions. For example, Lee et al. [62] prepared 2D/2D WO₃/Ti₃C₂T_x heterojunction composites (Figure 4). WO₃ nanosheets were dispersed in deionized (DI) water, followed by the addition of varying amounts of Ti₃C₂T_x nanosheets. The resulting suspension was sonicated, where cavitation bubbles generated localized high-temperature and high-pressure spots, facilitating physical and chemical interactions between WO₃ and Ti₃C₂T_x. This process enabled the effective integration of WO₃ nanosheets with Ti₃C₂T_x structures. The final suspension was dried at 100 °C for 12 h in an electric oven. The WO₃/Ti₃C₂T_x heterojunction demonstrated significantly higher photoexcited carrier transfer and separation efficiency, resulting in exceptional photocatalytic performance for methylene blue (MB) degradation under visible light. Furthermore, photoelectrochemical analysis of WO₃/Ti₃C₂T_x revealed improved charge carrier mobility, effectively reducing the carrier transport barrier between WO₃ and Ti₃C₂T_x. A similar approach has been used to synthesize Ti₃C₂T_x/CuFe₂O₄ nanohybrids [63].

A similar process has been used to prepare ZnS nanoparticle/layered MXene sheets [64]. Likewise, manganese oxide-decorated 2D Ti₃C₂T_x MXene containing MnO₂ nanopetals were also synthesized using the ultrasonic approach [65]. The synergistic effect of these two nanomaterials inhibited electron–hole pair recombination and improved surface activity. The nanocomposite exhibited high photocatalytic ability, degrading approximately 99% of MB within 30 min.

The sol-gel method employs metal alkoxide precursors to form gels via hydrolysis, typically at lower temperatures, followed by calcination. For instance, Iqbal et al. [66] utilized a double-solvent solvothermal method to synthesize BiFeO₃ (BFO)/Ti₃C₂T_x nanohybrid. They separately ultrasonicated Ti₃C₂T_x MXene in deionized (DI) water and BFO nanoparticles in a mixture of acetic acid and ethylene glycol. The resulting solutions were combined and transferred to a Teflon-lined steel autoclave for solvothermal synthesis, where the mixture was heated at 160 °C for 2 h. The final product was washed and dried at 80 °C for 3 h. This nanohybrid exhibited a high BET surface area of 147 m² g⁻¹, a low band gap of 1.96 eV, and a reduced recombination time. These attributes contributed to the material’s superior photocatalytic performance, demonstrated by the degradation of CR dye within a 42 min period under visible light irradiation. A similar process was used to prepare La- and Mn-co-doped BFO nanoparticles embedded in Ti₃C₂T_x sheets [67].

The solvothermal process, which involves reactions in a solvent under elevated temperatures and pressures within a sealed system, has been employed to synthesize MXene nanocomposites. For instance, Zheng et al. [68] synthesized SnO₂/Ti₃C₂T_x composites, and Zhou et al. [69] prepared CeO₂/Ti₃C₂T_x nanocomposites using CeO₂ nanorods on Ti₃C₂T_x sheets. These nanocomposites demonstrated enhanced photocatalytic activity for RhB photodegradation under UV-light irradiation compared to pure CeO₂ semiconductors and Ti₃C₂T_x. This enhancement was attributed to the composite’s narrower bandgap compared to CeO₂, facilitating improved solar energy utilization and resulting in high pollutant degradation efficiency [69]. A similar process was utilized to prepare BiVO₄/Ti₃C₂T_x nanocomposites [70]. This nanohybrid was synthesized by first preparing a 0.005 g/mL Ti₃C₂T_x MXene solution in D.I. water via 15 min ultrasonication. Separately, 0.40 g of BiVO₄ was dispersed in a 1:1 ethanediol–ethanoic acid mixture and ultrasonicated at 80 °C for 30 min. The solutions were then mixed, stirred for 1 h, and subjected to hydrothermal treatment at 150 °C for 3 h in a Teflon-lined autoclave. The product was washed with D.I. water and ethanol, then dried at 80 °C for 6 h, yielding a nanocomposite powder. The fabrication of BiVO₄/Ti₃C₂T_x is illustrated in Figure 5.

Fan et al. [71] synthesized monolayer Ti₃C₂T_x MXene nanosheets through LiF-HCl etching, followed by washing, centrifugation, and ultrasonic exfoliation. MoS₂ nanosheets were produced via lithium-ion etching with n-butyllithium under argon for 48 h, then neutralized, sonicated, and centrifuged. To fabricate MoS₂/MXene/NC composite microspheres, MoS₂ and MXene dispersions were ultrasonicated, nebulized into droplets, and self-assembled in liquid wax at 150 °C. Subsequently, the microspheres were coated with polydopamine (PDA) using dopamine hydrochloride in Tris buffer. Vacuum carbonization at 700 °C yielded composite microspheres with varying PDA coating times (4, 8, 12 h) and MoS₂-to-MXene ratios (1:3, 1:5, 1:7), as detailed in Figure 6.

It was found that the Schottky junction and heterojunction between Ti₃C₂T_x and MoS₂ significantly prolonged the recombination time of electron–hole pairs and broadened the visible light absorption range. Consequently, the nanohybrid exhibited enhanced photocatalytic activity towards MO. It was also found that TiO₂/Ti₃C₂T_x MXene was synthesized via a hydrothermal reaction involving nano-TiO₂ and Ti₃C₂T_x MXene nanosheets [72]. The improved photocatalytic activity was attributed to the suppression of electron–hole recombination, which facilitated electron accumulation and enhanced electron transfer from TiO₂ to MXene. Hydrothermal treatment of Ti₃C₂T_x nanosheets and AgNO₃ at 160 °C for 12 h, with a 2 °C/min heating rate, resulted in the fabrication of AgNPs/TiO₂/Ti₃C₂T_x composite [73]. During this process, silver nanoparticles (AgNPs) were deposited on the MXene surface via the self-reduction of silver salts, where MXene acted as a redox agent, facilitating the nucleation and growth of spherical Ag nanoparticles on the MXene nanosheets. The photocatalytic performance of the oxidized form was significantly improved compared to the pristine form. AgNPs/TiO₂/Ti₃C₂T_x also showed superior degradation efficiency for MB and RhB compared to pristine MXene. Bi₂WO₆/Ti₃C₂T_x was synthesized using a similar process involving heating layered Ti₃C₂T_x MXene and Bi(NO₃)₃·5H₂O at 160 °C for 16 h.

4. Photocatalytic Degradation of Dyes Using Ti₃C₂T_x MXene Hybrids

Photocatalysis presents a relatively safe and cost-effective strategy for degrading hazardous organic pollutants [41,74,75]. Ti₃C₂T_x MXene, with its unique lamellar structure, remarkably high metallic conductivity, and excellent hydrophilicity, has emerged as a promising member of the MXene family. Its distinctive properties have enabled its application as a photocatalyst for environmental remediation [76,77] and as a co-catalyst for enhancing the photocatalytic degradation potential of composite photocatalysts [34,78]. Ti₃C₂T_x MXene-based hybrid photocatalysts offer several advantages, including improved charge separation, efficient atomic utilization, tunable bandgap, optimized morphology, enhanced electron transfer efficiency, and increased photocatalytic activity [79,80,81]. This review assesses the effectiveness of Ti₃C₂T_x MXene-based hybrid photocatalysts by comparing their performance to non-hybrid photocatalysts in the degradation of both cationic and anionic dyes (

Table 1. Selected nonhybrid photocatalysts with photocatalytic degradation performance towards cationic and anionic dyes.

Nonhybrid Photocatalysts	Dyes	Type of Dyes	Degradation Percentage (%)	References
CdS	MO	Anionic	95 (300 min)	[81]
δ-Bi2O3	MO	Anionic	98 (180 min)	[82]
TiO2 NPs	MO	Anionic	~95 (~120 min)	[83]
ZnO	MB	Cationic	40.88 (21 h)	[84]
TiO2 Hollow Nanofiber	MB	Cationic	95.2 (4 h)	[85]
CuO	MB	Cationic	62 (270 min)	[86]
Bi2S2O3	CR	Anionic	82 (75 min)	[87]
ZnO	CR	Anionic	97.6 (75 min)	[88]
MoSe2	CR	Anionic	8.44 (120 min)	[89]
g-C3N4	RhB	Cationic	75 (180 min)	[90]
BiMnO3	RhB	Cationic	68 (75 min)	[91]

).

Table 1 summarizes the results of studies on the photocatalytic degradation of anionic dyes (MO, CR) and cationic dyes (MB, RhB). Dey and Ratan Das [81] achieved 95% degradation of MO within 300 min using CdS as a non-hybrid photocatalyst. In another study, Mohammad Jafri et al. [85] utilized TiO₂ nanofibers, achieving 95.2% degradation of MB within 240 min, and Mary et al. [88] reported 97.6% degradation of CR in 75 min using ZnO. Similarly, Fang et al. [90] prepared g-C₃N₄, achieving 75% degradation of RhB in 180 min. The ZnO standalone photocatalyst required 1260 min (21 h) to degrade 40.88% of MB [84], while MoSe₂ achieved only 8.44% degradation in 120 min (2 h) [89]. These prolonged durations and low efficiencies render such photocatalytic processes economically and temporally inefficient for the practical degradation of organic dye pollutants. Furthermore, pristine MXene photocatalysts exhibit lower dye degradation efficiencies. For example, Qu et al. [92] reported that the alkalized Ti₃C₂T_x demonstrated reduced photocatalytic performance compared to Ti₃C₂T_x MXene-based hybrid photocatalysts. Specifically, the alkalized Ti₃C₂T_x achieved degradation rates of only 17.3% for RhB and 2.8% for MO within 120 min.

Conversely, Ti₃C₂T_x MXene-based hybrid photocatalysts have demonstrated remarkable potential for degrading both cationic and anionic dyes under light irradiation. For example, a Ti₃C₂T_x MXene-based hybrid photocatalyst achieved a degradation efficiency of 99.7% for MO and 100% for RhB, as detailed in Table 2. These photocatalysts effectively decompose dye molecules into less harmful degradation products, showcasing their efficiency in addressing organic dye pollutants. This high efficiency can be attributed to the rich surface chemistry, tunable bandgap structures, high electrical conductivity, hydrophilicity, thermal stability, and large specific surface area with abundant active sites. These properties facilitate efficient dye adsorption and subsequent photocatalytic degradation. For instance, Nasri et al. [6] reported 100% degradation efficiency for MB using a Ti₃C₂T_x/g-C₃N₄ hybrid within 180 min. This exceptional performance was attributed to the effective charge separation and transfer capabilities of Ti₃C₂T_x MXene, which significantly enhanced the generation of reactive species essential for dye degradation.

Ta et al. [60] demonstrated that ZnO/Ti₃C₂T_x achieved a 99.7% degradation efficiency for MO, while Bi₂WO₆/Ti₃C₂T_x effectively degraded RhB, with an impressive 99.9% efficiency within 20 min., Zhao and Cai [93] reported comparable results (see Table 2). These studies highlight the pivotal role of MXene-incorporated heterostructures, characterized by their high conductivity and ability to facilitate rapid electron transfer, which is essential for efficient photocatalysis [94]. Yao and Wang [95] demonstrated that MB achieved a 93.3% degradation effi-ciency within 160 min using a standalone TiO₂ catalyst at a catalyst-to-dye ratio of 1.5:1. However, the MXene-based hybrid AgNPs/TiO₂/Ti₃C₂T_x achieved 99% degrada-tion in just 30 min at a 2.5:1 catalyst-to-dye ratio, demonstrating superior efficiency in both time and degradation rate compared to non-hybrid photocatalysts [73].

Ti₃C₂T_x MXene-based hybrids represent a versatile and highly effective class of photocatalysts for the degradation of both anionic and cationic dyes, making them suitable for various environmental remediation applications. For example, Ti₃C₂T_x MXene-based nanocomposites prepared with Mn₂O₃ were evaluated for photocatalytic dye degradation under light, demonstrating efficient photocatalysis. The one-dimensional (1D) Mn₂O₃-Ti₃C₂T_x (20 wt%) nanocomposite achieved 100% degradation of MB within 25 min, effectively removing the dye [96]. Similarly, NiM-nO3/NiMn₂O₄-Ti₃C₂T_x MXene nanocomposites achieved 100% degradation of MB in 50 min, showcasing excellent dye removal efficiency [97]. Wang et al. [98] synthesized a Ti₃C₂T_x/Bi₄Ti₃O₁₂ heterojunction via a facile in-situ solvothermal method, demonstrating exceptional visible-light-driven photocatalytic performance by achieving 100% degradation of MO and RhB within 60 and 50 min, respectively (see Table 2). These results highlight the potential of Ti₃C₂T_x MXene-based hierarchical composites for water remediation, offering a sustainable approach for the degradation of anionic and cationic organic pollutants. In an independent study, Iqbal et al. [66] reported that the BiFeO₃ (BFO)/Ti₃C₂T_x MXene hybrid achieved 100% degradation of CR in 42 min, highlighting its potential for photocatalysis applications.

It was observed that factors such as the synthesis method, pH, catalyst-to-dye ratio, concentration, and structure significantly influence the functionality and effectiveness of photocatalysts. Based on this evaluation, it is concluded that Ti₃C₂T_x MXene-based hybrid photocatalysts exhibit superior photocatalytic performance and efficiency in degrading organic pollutant dyes compared to non-hybrid photocatalysts.

Table 2. Ti₃C₂T_x MXene-based hybrid photocatalysts with photocatalytic degradation performance towards cationic and anionic dyes.

MXene-Based Hybrid Photocatalysts	Dyes	Type of Dyes (Based on Charge)	Degradation Percentage/(Time)	References
TiO2/Ti3C2Tx	MO	Anionic	92 (50 min)	[59]
MoS2@Ti3C2	MO	Anionic	98 (60 min)	[99]
Ti3C2/TiO2/CuO	MO	Anionic	99 (80 min)	[100]
ZnO/Ti3C2Tx	MO	Anionic	99.7 (50 min)	[60]
Ti3C2Tx/Bi4Ti3O12	MO	Anionic	100 (60 min)	[98]
TiO2/Ti3C2 Mxene	MB	Cationic	96.44 (60 min)	[101]
AgNPs/TiO2/Ti3C2Tx	MB	Cationic	99 (30 min)	[73]
Ti3C2/g-C3N4	MB	Cationic	100 (180 min)	[6]
NiMnO3/NiMn2O4-Ti3C2Tx MXene	MB	Cationic	100 (50 min)	[97]
1D Mn2O3-Ti3C2Tx	MB	Cationic	100 (25 min)	[96]
Mn-codoped bismuth ferrite/Ti3C2	CR	Anionic	93 (30 min)	[67]
CoFe2O4@MXene	CR	Anionic	98.9 (30 min)	[102]
BiVO4/Ti3C2	CR	Anionic	99.5 (60 min)	[70]
BGFO-20Sn/MXene	CR	Anionic	100 (120 min)	[103]
BiFeO3 (BFO)/Ti3C2	CR	Anionic	100 (42min)	[66]
TiO2@Ti3C2	RhB	Cationic	97 (40 min)	[58]
BiOBr/TiO2/ Ti3C2Tx	RhB	Cationic	99.8 (30 min)	[104]
Bi2WO6/Ti3C2	RhB	Cationic	99.9 (20 min)	[93]
ZnS/MXene	RhB	Cationic	100 (100 min)	[64]
Ti3C2Tx/Bi4Ti3O12	RhB	Cationic	100 (50 min)	[98]

Table 2. Ti₃C₂T_x MXene-based hybrid photocatalysts with photocatalytic degradation performance towards cationic and anionic dyes.

MXene-Based Hybrid Photocatalysts	Dyes	Type of Dyes (Based on Charge)	Degradation Percentage/(Time)	References
TiO2/Ti3C2Tx	MO	Anionic	92 (50 min)	[59]
MoS2@Ti3C2	MO	Anionic	98 (60 min)	[99]
Ti3C2/TiO2/CuO	MO	Anionic	99 (80 min)	[100]
ZnO/Ti3C2Tx	MO	Anionic	99.7 (50 min)	[60]
Ti3C2Tx/Bi4Ti3O12	MO	Anionic	100 (60 min)	[98]
TiO2/Ti3C2 Mxene	MB	Cationic	96.44 (60 min)	[101]
AgNPs/TiO2/Ti3C2Tx	MB	Cationic	99 (30 min)	[73]
Ti3C2/g-C3N4	MB	Cationic	100 (180 min)	[6]
NiMnO3/NiMn2O4-Ti3C2Tx MXene	MB	Cationic	100 (50 min)	[97]
1D Mn2O3-Ti3C2Tx	MB	Cationic	100 (25 min)	[96]
Mn-codoped bismuth ferrite/Ti3C2	CR	Anionic	93 (30 min)	[67]
CoFe2O4@MXene	CR	Anionic	98.9 (30 min)	[102]
BiVO4/Ti3C2	CR	Anionic	99.5 (60 min)	[70]
BGFO-20Sn/MXene	CR	Anionic	100 (120 min)	[103]
BiFeO3 (BFO)/Ti3C2	CR	Anionic	100 (42min)	[66]
TiO2@Ti3C2	RhB	Cationic	97 (40 min)	[58]
BiOBr/TiO2/ Ti3C2Tx	RhB	Cationic	99.8 (30 min)	[104]
Bi2WO6/Ti3C2	RhB	Cationic	99.9 (20 min)	[93]
ZnS/MXene	RhB	Cationic	100 (100 min)	[64]
Ti3C2Tx/Bi4Ti3O12	RhB	Cationic	100 (50 min)	[98]

5. Computational Studies and Simulations

The investigation of Ti₃C₂T_x MXene-based hybrids for photocatalytic applications is significantly enhanced by computational tools and techniques, which complement experimental approaches. These methods offer insights into atomic-level phenomena, guide material design, and predict performance under varying conditions.

Chen et al. [101] investigated the electronic and optical properties of -F terminated, -O terminated, and termination-free Ti₃C₂ in an MXene nanosheet/TiO₂ composite using density functional theory (DFT). Their study revealed that surface terminations reduced the density of electronic states, lowered conductivity, and enhanced stability compared to the termination-free MXene. DFT analysis also demonstrated the feasibility of electron transfer from TiO₂ to Ti₃C₂ and identified the Schottky barrier at the interface between the two materials. Furthermore, computational modeling highlighted the synergy between the composite components, showing an extended range of light absorption, suppressed electron–hole recombination, and improved hole oxidation efficiency in the VB of TiO₂. These factors significantly enhanced the photocatalytic performance of the Ti₃C₂/TiO₂ composite, making it a promising candidate for photocatalytic applications, such as treating organic pollutants like dye molecules [101]. Lemos et al. utilized a computational model to evaluate the performance of a Ti₃C₂T_x/TiO₂ nanocomposite hybrid for photocatalyzed dye-sensitized solar cells. Through DFT calculations, they discovered that the anatase potential is reduced at the nanocomposite interface and that the nanocomposite exhibits improved photocarrier separation at the interface between the nanocomposite and the dye [105]. Furthermore, Yang et al. [106] conducted a computational analysis to confirm that a transition metal dichalcogenide/MXene photocatalyst hybrid, MoS₂/Ti₃C₂, functions as a Schottky barrier. A Bader charge analysis revealed that the difference in work functions between MoS₂ and Ti₃C₂, combined with a built-in electric field, facilitates the transfer of photogenerated electrons from MoS₂ to the Ti₃C₂ electron sink. This efficient electron transfer enhances photocarrier separation, resulting in longer lasting photogenerated holes and exceptional photodegradation performance against rhodamine B dye in wastewater [106].

Several computational techniques have been employed to assess and optimize photocatalytic mechanisms in MXene-based composites for dye degradation applications. Liu et al. [107] developed a g-C₃N₄/Ti₃C₂ (CNTC) heterojunction by hybridizing 2D Ti₃C₂ MXene with three-dimensional (3D) g-C₃N₄ for enhanced photodegradation of RhB dye. This photocatalyst exhibited a high specific surface area (85.08 m²/g) and remarkable charge migration capabilities. To evaluate its improved photocatalytic performance, the researchers utilized DFT analysis to examine the differential charge density, electron distribution, and charge transfer dynamics between g-C₃N₄ and the Ti₃C₂ sink. The study revealed that the excellent conductivity of Ti₃C₂ stemmed from the overlap between the Fermi level and CB in the heterojunction. This led to the understanding that these 2D/3D heterojunctions significantly promote charge transfer and separation, which are essential for efficient photocatalysis. Additionally, the combination of a high specific surface area and abundant active sites makes the CNTC particularly effective for dye photodegradation [107]. Cheng et al. [108] synthesized a self-cleaning BiOCl-polypyrrole Ti₃C₂T_x MXene composite membrane with excellent photocatalytic activity and high flux, designed for filtering and degrading pollutants. The membrane’s performance was tested against various dyes. To gain deeper insight into the photocatalytic mechanisms, particularly charge separation and degradation, the researchers conducted DFT calculations. Density of state (DOS) simulations revealed that the chemisorption of oxygen into vacancies on the BiOCl surface generated superoxide radicals. These radicals enhanced the composite’s photocatalytic efficiency through redox interactions with dye molecules and other pollutants [108]. Wang et al. [109] synthesized an S-scheme Pt-MnO₂/TiO₂@Ti₃C₂T_x composite using an electrostatically self-assembled Ti-O-Mn bond and evaluated its oxidative photodegradation performance against MB, MO, and RhB dyes. To investigate the photocatalytic mechanisms, DFT calculations were performed, revealing that the Ti-O-Mn bond induced the formation of metastable Ti atoms and electrostatically adsorbed Mn²⁺ ions. This bond facilitated the separation of photoinduced carriers and optimized their transport pathways. Additionally, the DFT analysis identified the formation of S-scheme heterojunctions between MnO₂ and TiO₂ through the Ti-O-Mn bond, driving the flow of photogenerated carriers. These factors collectively enhanced the composite’s photocatalytic efficiency [109].

DFT has been employed in conjunction with the finite element method to explore ways to enhance the photocatalytic activity and membrane permeability of a novel MXene-based composite membrane, N-doped Bi₂O₂CO₃@Ti₃C₂T_x/Polyethersulfone, designed for oil/water separation and dye degradation [110]. Through DFT analysis of the electron distribution and band structure of doped versus undoped Bi₂O₂CO₃, it was found that N doping improved conductivity, enhanced electron transition activity, and facilitated photogenerated carrier transport (attributed to VB dispersion), thereby making Bi₂O₂CO₃@Ti₃C₂T_x/Polyethersulfone more effective for photocatalytic applications [110].

6. Other Applications of Ti₃C₂T_x MXene-Based Hybrid Photocatalysts

Beyond organic dye degradation, Ti₃C₂T_x MXene-based hybrid photocatalysts have demonstrated potential for various other applications. They have shown considerable promise in wastewater treatment, particularly in degrading organic pollutants such as dyes, pharmaceuticals, and pesticides [111]. Composite materials of Ti₃C₂T_x MXene with semiconductors such as TiO₂ or ZnO have exhibited even greater photocatalytic efficiencies; the synergy between MXenes and these semiconductors results in improved light absorption and charge separation [112,113]. For instance, Ti₃C₂T_x/TiO₂ composites demonstrate enhanced photocatalytic activity under visible light due to the synergistic effects of both materials. These composites can efficiently degrade various dyes under sunlight, including RhB and MO, making them suitable for sustainable and cost-effective wastewater treatment [114]. Furthermore, doping MXenes with other elements or combining them with carbon-based materials like graphene can further enhance their photocatalytic properties. Nitrogen-doped Ti₃C₂T_x MXene shows improved photocatalytic degradation of antibiotics such as tetracycline under visible light, highlighting their potential in treating pharmaceutical contaminants in wastewater [115]. Ti₃C₂T_x MXene-based hybrid photocatalysts are also being explored for air purification applications, particularly in removing volatile organic compounds (VOCs) and other airborne pollutants. Ti₃C₂T_x MXene, when combined with photocatalysts like TiO₂, can effectively degrade formaldehyde and toluene, common indoor air pollutants, under UV and visible light [116]. Incorporating noble metals like Au or Ag onto Ti₃C₂T_x MXene can further enhance their photocatalytic performance by creating localized surface plasmon resonance, increasing light absorption, and improving the degradation rates of VOCs [117]. Additionally, Z-scheme heterostructures involving Ti₃C₂T_x MXene have been developed to mimic natural photosynthesis, achieving efficient separation and transfer of photogenerated charge carriers and enhancing photocatalytic degradation of air pollutants [118]. The most applied technology is the Ti₃C₂T_x MXene-based TiO₂ photocatalyst, which utilizes photocatalysis for glass cleaning, where UV or visible light activates TiO₂, generating reactive oxygen species that decompose organic pollutants on the glass surface. This self-cleaning mechanism maintains transparency, reduces manual cleaning, and prevents pollutant buildup, enhancing the efficiency and durability of glass surfaces.

In addition to the degradation of dyes, the degradation of other organic pollutants, such as pharmaceutical waste and VOCs, has emerged as a challenging task requiring immediate attention [119]. In recent years, extensive studies [120] have highlighted photocatalysis as an attractive method for the efficient degradation of organic pollutants. The photocatalytic performance of a catalyst is defined by properties such as its surface-to-volume ratio, light interaction, and mechanical stability [121]. Among the variety of materials reported thus far, Ti₃C₂T_x MXenes have gained significant attention as photocatalytic materials [122]. Furthermore, the integration of Ti₃C₂T_x MXenes with other nanomaterials can significantly improve photocatalytic performance [123].

Kumar et al. [124] reported a novel photocatalyst composed of g-C₃N₄, Ti₃C₂T_x, and Au nanoparticles for the degradation of cefixime. Figure 7a illustrates the degradation mechanism of cefixime decomposition under light. The composition with 3 wt% Ti₃C₂T_x demonstrated the highest degradation, achieving 64.69% cefixime removal in 105 min under visible light irradiation, as shown in Figure 7b,c. Similarly, Diao et al. [125] reported efficient photocatalytic degradation of tetracycline hydrochloride using MXene-based photocatalysts comprising g-C₃N₄/Ti₃C₂T_x/TiO₂. The reported ternary catalysts exhibited superior performance, chemical and photostability, and recyclability. Another MXene-based ternary photocatalyst, reported by Zhou et al. [126], facilitated photocatalytic degradation of enoxacin under visible light, where Ti₃C₂T_x improved charge separation at the interface, resulting in efficient degradation. Rdewi et al. [127] reported a ZnO-TiO₂-MXene photocatalyst for the decomposition of carbamazepine molecules in wastewater under solar irradiation. The presented results showed 99.6% removal efficiency at pH 7, attributed to improved charge carrier transport and a reduced recombination rate due to the incorporation of TiO₂-MXene with ZnO photocatalysts. The excellent photocatalytic performance also demonstrated reusability across multiple decomposition cycles with exceptional efficiency. Similarly, Abbas et al. [128] reported a ZnO-TiO₂-MXene photocatalyst for the decomposition of ceftriaxone sodium molecules in water using simulated solar light.

Sukidpaneenid et al. [129] reported Ti₃C₂T_x/TiO₂ photocatalysts for the degradation of enrofloxacin antibiotics in water. The precise control over TiO₂ loading on MXene, achieved through variations in hydrothermal processes, played a crucial role in tuning photocatalytic properties. Additionally, the intercalation of sodium ions significantly improved adsorption, and the synergistic effect of TiO₂ loading and NaCl treatment led to the efficient removal of enrofloxacin. Shahzad et al. [130] demonstrated the degradation of carbamazepine (CBZ) under direct sunlight and UV light using Ti₃C₂T_x-based heterostructure photocatalysts with {001} TiO₂. The results showed that the Kapp value under UV irradiation was higher than under sunlight for CBZ degradation. Also, the effects of pH on degradation performance were considered. Mohanty et al. [131] reported a series of SrTiO₃/Ti₃C₂T_x-based photocatalysts decorated with Au nanoparticles for the degradation of colorless organic pollutants such as ciprofloxacin under sunlight. The significant enhancement in photocatalytic degradation for the plasmon-mediated heterostructure catalysts was attributed to the absorption of a broad solar spectrum, charge separation, and charge transport. Du et al. [132] demonstrated the photocatalytic degradation of tetracycline using CeO₂-Ti₃C₂T_x-TiO₂ (CeMXT). The composite exhibited an excellent degradation efficiency of 94.70% for 22.19 mg/L in 104.13 min with 0.65 g/L of catalyst at pH 4.72. This excellent degradation efficiency was attributed to increased radical generation and charge separation.

Sergi et al. [133] reported a series of TiO₂-Ti₃C₂T_x MXene photocatalysts with controlled composition for improved photocatalytic removal of benzene. The incorporation of TiO₂ with Ti₃C₂T_x MXene enhanced optical absorption, leading to improved photocatalytic performance. The report demonstrated the potential of Ti₃C₂T_x MXenes for VOC removal and the ability of heterostructures to enhance photocatalysis. Furthermore, Huang et al. [116] reported the degradation of formaldehyde (HCHO) and acetone (CH₃COCH₃) using Bi₂WO₆/Ti₃C₂ (BT4) under light irradiation. Bi₂WO₆ photocatalysts suffer from a high recombination rate, and combination with Ti₃C₂ can significantly improve charge separation. Ti₃C₂-mediated charge separation caused charge transfer at the interface, significantly improving the photocatalytic performance of BT4. Additionally, DFT simulations indicated higher adsorption of formaldehyde and acetone on the Ti₃C₂ surface compared to the Bi₂WO₆ surface, synergistically tuning photocatalytic performance. Notably, the degradation of formaldehyde and acetone for BT4 was 2 and 6.6 times higher, respectively, than for Bi₂WO₆. Mo et al. [134] demonstrated photocatalytic and photothermal removal of VOCs (phenol) from water using a TiO₂/Ti₃C₂T_x/C₃N₄/PVA (TTCP) hydrogel under sunlight. Figure 7d shows the SEM image of TTCP. The VOC removal efficiency varied from 69.4% to 100% at different phenol concentrations (1–50 mg/L), as shown in Figure 7e,f. The membrane significantly lowered the total dissolved solids (TDSs) and TOC. The TDS level was reduced by more than two orders of magnitude, and the TOC removal efficiency was observed to be 80%.

Figure 7. (a) Schematic of the cefixime degradation mechanism under light. (b) Kinetics of cefixime degradation. (c) Histogram of degradation rate (%). Reproduced with permission from [124], (d) SEM image of the tetracalcium phosphate (TTCP) hydrogel. (e) Total organic carbon (TOC) of source water under different phenol concentrations (orange column), distilled water without photocatalyst (green column), and distilled water with TTCP hydrogel (violet column). (f) TOC of distilled water with different catalysts. Reproduced with permission from [134]. (g) Schematic illustration of the synthesis process of the Ti₃C₂T_x MXene/CeO₂ photocatalysts (inset SEM image). Comparative representation of the production of (h) C₂H₅OH and (i) CH₄ under solar light illumination with different catalysts. Reproduced with permission from [135].

Figure 7. (a) Schematic of the cefixime degradation mechanism under light. (b) Kinetics of cefixime degradation. (c) Histogram of degradation rate (%). Reproduced with permission from [124], (d) SEM image of the tetracalcium phosphate (TTCP) hydrogel. (e) Total organic carbon (TOC) of source water under different phenol concentrations (orange column), distilled water without photocatalyst (green column), and distilled water with TTCP hydrogel (violet column). (f) TOC of distilled water with different catalysts. Reproduced with permission from [134]. (g) Schematic illustration of the synthesis process of the Ti₃C₂T_x MXene/CeO₂ photocatalysts (inset SEM image). Comparative representation of the production of (h) C₂H₅OH and (i) CH₄ under solar light illumination with different catalysts. Reproduced with permission from [135].

The photocatalytic reduction of CO₂ is another aspect of environmental remediation applications. Reducing CO₂ into useful byproducts using light-activated catalysts represents a futuristic pathway towards sustainability [90,136]. The excellent charge separation and slow recombination rate of Ti₃C₂T_x MXene demonstrate its potential as a catalytic material for CO₂ reduction [137]. In this regard, Cao et al. [52] demonstrated 2D/2D ultrathin Ti₃C₂T_x/Bi₂WO₆ nanosheets prepared by in situ growth of Bi₂WO₆ nanosheets over Ti₃C₂T_x nanosheets. The proposed hybrid catalyst exhibited improved efficiency towards CO₂ reduction under light due to reduced charge transfer distance, improved surface-to-volume ratio, and increased adsorption sites. The hybrid catalyst showed improved production of CH₄ and CH₃OH compared to Bi₂WO₆. Quantitatively, the production of CH₄ increased from 0.41 μmol g⁻¹ h⁻¹ to 1.78 μmol g⁻¹ h⁻¹, while the production of CH₃OH increased from 0.07 μmol g⁻¹ h⁻¹ to 0.58 μmol g⁻¹ h⁻¹. Similarly, Mishra et al. [135] investigated a Ti₃C₂T_x-CeO₂ hybrid catalyst with varying Ti₃C₂T_x ratios for the photocatalytic reduction of CO₂. The schematic synthesis process for the hybrid catalysts is shown in Figure 7g. Surprisingly, the hybrid catalyst with 5 wt% Ti₃C₂T_x/CeO₂ produced 6127.04 μmol g⁻¹ of ethanol and 129.5 μmol g⁻¹ of methane within 5 h, significantly higher than CeO₂, as shown in Figure 7h,i. Another report by Li et al. [137] demonstrated the potential of a Ti₃C₂-based hybrid catalyst comprising g-C₃N₄/ZnO/Ti₃C₂T_x for CO₂ reduction into methane (CH₄) and carbon monoxide (CO). The hybrid framework exhibited a notably higher production rate of 1.41 μmol g⁻¹ h⁻¹ towards CO, increased by factors of 2.7 and 1.7 compared to ZnO and g-C₃N₄, respectively.

Ti₃C₂T_x MXene has been extensively studied in recent years for its broad range of photocatalytic applications in environmental remediation, as evidenced by the literature. Reported results highlight the potential of Ti₃C₂T_x MXene-based catalysts for degrading dyes, pharmaceutical wastes, and VOCs, as well as for CO₂ reduction. These photocatalysts benefit from a high surface area, efficient charge separation at the interface, and excellent light absorption, which contribute significantly to their performance.

7. Working Mechanism of Ti₃C₂T_x MXene-Based Hybrid Photocatalyst

The electronic structure of Ti₃C₂T_x MXene-based photocatalysts plays a crucial role in their ability to degrade targeted pollutants through photocatalysis. Generally, an ideal photocatalyst should possess a suitable bandgap, strong absorption in the visible range, prolonged charge separation lifetimes, and adequate redox potential [64]. To enhance photocatalysis efficiency, co-catalysts are employed to separate photogenerated charge carriers. When materials such as ZnO, CdS, TiO₂, and Ti₃C₂ are used as co-catalysts, Schottky junctions formed with Ti₃C₂T_x facilitate the rapid dissociation of these charge carriers [138]. As discussed in the previous section, Ti₃C₂T_x MXene-based photocatalyst composites demonstrate superior photocatalytic performance compared to non-hybrid photocatalysts. This is attributed to the abundant active sites available on Ti₃C₂T_x MXene-based hybrids, their enhanced bandgap, improved light-harvesting capability, reduced charge carrier recombination, and extended photoelectron lifetime [139].

During photocatalytic degradation, Ti₃C₂T_x MXene-based hybrid photocatalysts absorb visible light, exciting photogenerated electrons into the CB and leaving holes in the VB [22]. These excited charge carriers are subsequently transferred to the Ti₃C₂T_x MXene at the interface, primarily due to the higher potential of Ti₃C₂T_x. While the Schottky junction formed between n-type TiO₂ and Ti₃C₂ facilitates hole transfer from TiO₂ to Ti₃C₂, the inherent band bending in n-type TiO₂ creates an energy barrier that inhibits direct electron transfer from Ti₃C₂ to TiO₂ [140]. This limitation means that photogenerated electrons on Ti₃C₂ primarily participate in reactions with adsorbed oxygen, leading to the formation of superoxide radicals (^●O₂⁻). Consequently, the primary role of the Ti₃C₂ in this hybrid system is to act as a hole reservoir, preventing hole recombination at the Ti₃C₂–TiO₂ interface, rather than directly contributing to electron transfer towards reduction reactions on the TiO₂ surface. In this process, photogenerated electrons from the Ti₃C₂T_x MXenes migrate to the surface and react with O₂ to produce ^●O²⁻, and holes from the TiO₂ react with hydroxyl ions (OH⁻) to produce hydroxyl radicals (^●OH). These radicals cause the degradation of cationic and anionic dyes of organic pollutants [22,141,142].

A photocatalytic mechanism for the Ti₃C₂T_x–TiO₂ hybrid, illustrating anionic dye (e.g., MO) degradation, is presented in Figure 8a. Initially, the light source provides high-energy photons, which activate the TiO₂ component, generating photoinduced electrons in its CB and holes in its VB. These photoelectrons rapidly migrate from the CB of TiO₂ to the Ti₃C₂T_x MXene, facilitated by its high electrical conductivity [73,114]. Consequently, Ti₃C₂T_x MXene accumulates a negative charge, while TiO₂ becomes positively charged, leading to the formation of a Schottky barrier at the Ti₃C₂T_x–TiO₂ interface, which functions as a space-charge layer. Subsequently, the photogenerated electrons on Ti₃C₂T_x MXene migrate to its surface, where they react with O₂ molecules to generate superoxide radicals (^●O₂⁻) [114,143]. Simultaneously, the photogenerated holes react with adsorbed OH⁻ to form hydroxyl radicals (^●OH) [58]. These radicals play a crucial role in MO degradation. The photocatalytic reaction mechanism facilitated by the MXene-based hybrid photocatalyst (TiO₂/Ti₃C₂T_x) is illustrated in Figure 8a [114]. The mechanism can be roughly described through the following reactions:

Ti₃C₂–TiO₂ + hϑ → TiO₂ (holes) + Ti₃C₂ (electrons)

(1)

holes + OH⁻ → ^●OH

(2)

electrons + O₂ → ^●O²⁻

(3)

Dye (Anionic or cationic) + ^●O²⁻ → CO₂ + H₂O

(4)

Dye (Anionic or cationic) + ^●OH→ CO₂ + H₂O

(5)

For the degradation of cationic dye like MB, ^●O²⁻ is produced through a reduction reaction between electrons and O₂ molecules, leading to the direct degradation of MB. Additionally, light-induced holes partially oxidize MB and partially react with water to generate ^●OH, ultimately breaking down MB [144]. As evidenced by Figure 8b, ^●OH and holes are the primary reactive species in the TiO₂/Ti₃C₂T_x MXene composite photocatalysis reaction system [72]. Ultimately, these radicals (^●OH, ^●O²⁻), possessing strong oxidizing abilities, degrade both anionic and cationic dyes, such as MO and MB molecules, directly into their oxidation products [145,146]. Therefore, a similar photocatalytic reaction mechanism occurs for both cationic and anionic dyes under light irradiation.

8. Challenges and Future Directions

While Ti₃C₂T_x MXene-based hybrid photocatalysts have demonstrated significant promise for organic dye pollutant degradation and environmental remediation, several challenges must be addressed to further enhance their photocatalytic performance and enable practical application.

One of the most significant challenges in MXene research is the development of green synthesis methods. Current synthesis processes often involve harsh and hazardous chemicals for etching and exfoliation, such as hydrofluoric acid, tetramethylammonium hydroxide, and tetrabutylammonium hydroxide [36]. Eliminating these chemicals is crucial for the wider applicability of MXenes.

Furthermore, the limited yield of high-quality MXene products during synthesis is another major obstacle. Current large-scale synthesis processes are inefficient, hindering commercial use. Therefore, future research efforts should focus on developing mass-production methods for high-quality, uniformly delaminated single- to few-layer MXene nanosheets. This will require a deeper understanding of the reaction kinetics and thermodynamics of the synthesis process to achieve uniform and scalable production.

The long-term stability of MXenes in harsh environmental conditions, such as oxygen, high humidity, and acidic or alkaline media, remains a significant concern [38]. Corrosion and degradation over time can compromise the catalytic efficiency of composite materials and hinder their reuse. Thermal stability is also a considerable challenge for MXene-based photocatalysts, as elevated temperatures can accelerate MXene oxidation [147].

Significant efforts have been made to address the challenges associated with MXene-based hybrid photocatalysts. The development of efficient MXene-based hybrid photocatalysts for practical applications requires a scalable synthesis of high-quality MXenes. However, achieving this remains challenging without a comprehensive understanding of the reaction kinetics and thermodynamics. Currently, the kinetics and thermodynamics of A-layer etching in MAX phases to obtain corresponding MXene layers are still underexplored [148,149,150]. Additionally, the photothermal effect of MXenes is a crucial thermodynamic factor that significantly impacts photocatalytic reactions. Upon irradiation, MXenes generate heat, raising the surrounding temperature and influencing the thermodynamics of charge carriers [151]. Accelerated thermodynamics up to an optimal temperature enhances photocatalytic efficiency; however, excessive heat can negatively impact reactant adsorption [152]. Hence, future research should focus on in-depth studies of reaction kinetics and thermodynamics to achieve uniform and large-scale MXene synthesis and MXene-based hybrid photocatalyst fabrication for practical applications.

Environmentally friendly, large-scale synthesis routes and effective antioxidation strategies for MXenes should be explored to ensure their long-term stability and sustainability. Future Ti₃C₂T_x-based hybrid photocatalyst materials should also utilize their functional terminations, which serve as active sites for reaction centers, promoting covalent interactions with photocatalyst materials. This approach may enhance hybrid materials’ robustness, environmental stability, and efficiency.

To transition Ti₃C₂T_x MXene-based hybrid photocatalysts from laboratory to real-world applications, strategies for integrating them into practical systems, such as water treatment plants or portable water purification devices, should be explored. The development of user-friendly photocatalytic reactors or devices utilizing MXene-based hybrid materials for organic dye and pollutant degradation will advance these materials to the forefront of environmental remediation technologies. Conventional methods for discovering materials with desired properties are often time-consuming and complex. To address these challenges, a machine learning (ML)-driven approach can be employed to predict and understand the properties of functional materials more efficiently [153,154]. When combined with experimental data, data-driven ML models enable researchers to uncover natural correlations between catalyst properties and the degradation rates of contaminants in water resources. Significant progress has already been made in applying ML to study the catalytic activity of both standalone photocatalysts, such as ZnO [155] and TiO₂ [156], as well as nanocomposites, including Bi₂WO₆/MIL-53 [155] and CuWO₄@TiO₂ [157], for degrading various organic dyes. However, the application of such modern artificial intelligence (AI)-based approaches in the field of MXene-based hybrid photocatalysts remains limited. To enhance the efficiency of Ti₃C₂T_x MXene-based hybrid photocatalysts, a predictive design strategy integrating AI-driven modeling with existing databases can be utilized. This methodology enables the identification and screening of optimal material combinations to enhance photocatalytic performance.

9. Conclusions

In summary, this review has examined the fabrication and evaluated the performance of Ti₃C₂T_x MXene-based hybrid photocatalysts for the degradation of organic dye pollutants. Ti₃C₂T_x MXene exhibits exceptional physical and chemical properties, including high electrical conductivity, excellent hydrophilicity, adsorption capability, and efficient charge transfer, while also suppressing electron–hole recombination during photocatalysis. This makes Ti₃C₂T_x MXene a promising, low-cost, and scalable alternative to noble metal catalysts, offering exceptional catalytic performance in hybrid photocatalysts. The evaluation demonstrated that Ti₃C₂T_x MXene-based hybrid photocatalysts significantly enhance dye degradation efficiency, as evidenced by increased percentage degradation and reduced degradation time, compared to non-hybrid or pure semiconducting materials. This review also provided a comprehensive understanding of the dye degradation mechanisms involving Ti₃C₂T_x MXene-based hybrid photocatalysts and highlighted various computational studies and simulations that have advanced research in this field. Furthermore, the application of ML techniques holds significant promise for optimizing the design and performance of these photocatalysts. Integrating ML into future research can accelerate the discovery of novel MXene-based photocatalysts with tailored properties. Finally, the challenges associated with Ti₃C₂T_x MXene-based hybrid photocatalysts were thoroughly identified, and future research directions were suggested to effectively address these challenges.