Ti₃C₂Tₓ MXene-Based Hybrid Photocatalysts in Organic Dye Degradation: A Review

1. Introduction

The fast-growing industrialization has increased the generation of highly toxic and carcinogenic wastewater[1]. Discharging untreated wastewater from textile, pharmaceutical, dying, and paper printing industries leads to increased dyes in the water sources, causing a severe threat to the environment[2]. It contains chromophores for color and auxochromes for fiber affinity, harming aquatic life by coloring water, blocking sunlight, and disrupting photosynthesis, which is crucial for ecosystems [3]. Discharged dyes are categorized into cationic dyes (e.g., methylene blue (MB), crystal violet (CV), rhodamine B (RhB)) that carry positive charges, are widely used in textiles, and pose toxicity risks. Anionic dyes (e.g., acid orange 7 (AO7), acid blue 92 (AB92), Congo red (CR)) carry negative charges, exhibit water solubility, and are challenging to remove [4]. To get rid of these dyes, various degradation approaches like membrane filtration, oxidation, adsorption, and photocatalysis have been successfully developed and demonstrated [5,6].

Photocatalysis is one of the best degradation methods because it is eco-friendly and cost-effective for degrading organic pollutants [7]. Numerous studies have focused on developing photocatalytic materials with a wide energy bandgap, abundant surface active sites, and efficient charge separation capabilities to optimize the photodegradation of dyes. [8]. Single-component semiconducting photocatalysts such as TiO₂, ZnO, ZnS, SnO₂, g-C₃N₄, and CdS have been extensively used[9,10]. However, the photocatalytic activity of these single semiconducting materials is generally limited by poor visible light absorption and utilization, as well as rapid charge recombination and low electron-hole mobility [11,12]. To improve the photocatalytic performance of these single semiconducting materials, various strategies such as the construction of heterojunction[13], doping of elements[14], and introduction of co-catalyst[14] have been proposed and explored.

The photocatalytic activity of semiconducting materials can be effectively enhanced by modifying them with a co-catalyst[15]. The presence of a co-catalyst prevents the recombination between electrons and holes, which is otherwise inevitable by facilitating the separation of the charge. Various materials such as graphene[16], carbon nanotubes[17], and carbon quantum dots[18], have been used as co-catalysts to enhance the efficiency of the photocatalytic degradation of dyes. Although significant advancements have been made with the above-mentioned co-catalysts, they are somehow limited due to the scarcity of functional groups and sufficient conductivity[19]. The lack of sufficient surface functional group restricts the strong chemical bond between photocatalyst and co-catalyst and the lower electrical conductivity impedes charge migration within the material. Both factors play a crucial role in reducing photocatalytic efficiency. Therefore, there is still a need for efficient and inexpensive co-catalysts to increase the photocatalytic efficiency of degradation of the organic dyes.

MXenes, especially Ti₃C₂T_x, have been considered excellent component material for the fabrication of the hybrid photocatalyst due to their lower fermi level than most studied semiconductors, the presence of abundant terminal functional groups, excellent metallic conductivity and exposed terminal metal sites[20,21,22]. MXenes are a class of two-dimensional (2D) layered materials consisting of transition metal carbides, nitrides, or carbonitrides, which are represented by a general formula, M_n+1X_n (n = 1, 2, 3), where M represents a transition metal (such as Sc, Ti, Zr, Hf, V, Nb, Ta, or Mo), and X corresponds to carbon, nitrogen, or carbonitride. The presence of abundance functional groups such as −OH, −O, and −F on Ti₃C₂T_x MXene facilitates the strong interfacial chemical bonding with semiconductors which enables the formation of a Schottky junction that acts as an electron trap, thereby suppressing the recombination of photoexcited electron-hole pairs[23]. The excellent metallic conductivity of Ti₃C₂T_x MXene ensures rapid charge carrier migration, promoting efficient separation of photogenerated electrons and holes[24]. Moreover, exposed terminal metal sites enhance reactivity compared to carbon-based materials, making Ti₃C₂T_x MXene a more effective co-catalyst.

Ti₃C₂Tₓ MXene has limited effectiveness as a standalone photocatalyst, as it requires doping and UV radiation to function optimally [25]. However, when incorporated into composite photocatalysts, its role extends beyond serving as a mere photogenerated charge acceptor co-catalyst. Due to its large surface area and surface functionalities, Ti₃C₂Tₓ MXene provides the superior growing platform for uniform, size-controlled, and fine dispersion of photocatalysts which exposes more surface-active sites of composite photocatalyst [26,27,28,29]. Additionally, Ti₃C₂Tₓ MXene possesses distinctive physical and chemical properties, including high electrical conductivity, excellent hydrophilicity, mechanical stability, ion intercalation ability, and tunable surface functionalization [30,31,32]. These attributes make it an ideal candidate for use in composite photocatalyst materials. The integration of Ti₃C₂T_x MXene with semiconducting materials has resulted in hybrid photocatalysts featuring micro/nano architectures and multi-junction nanocomposites. These hybrid photocatalyst materials leverage synergistic interactions of Ti₃C₂T_x MXene with conventional semiconductors, or metal nanostructures, enabling enhanced charge separation and reduced recombination rates [21,33]. These characteristics make Ti₃C₂T_x MXene-based hybrid photocatalysts highly effective for the photodegradation of cationic and anionic organic pollutants [34].

This review explores Ti₃C₂Tₓ MXene-based hybrid photocatalysts for the degradation of cationic and anionic organic dyes. It covers the synthesis of Ti₃C₂Tₓ MXene, its integration into hybrid photocatalysts, its role and effectiveness in dye degradation, and the associated mechanisms. Additionally, it discusses the challenges and opportunities in this area while providing insights into potential future research directions.

2. Ti₃C₂Tₓ MXene and Synthesis Methods

2.1. Introduction to Ti₃C₂Tₓ MXenes

MXenes, first synthesized by Naguib et al. in 2011, are two-dimensional crystals of transition metal carbides, nitrides, or carbonitrides[35]. These novel 2D materials are synthesized by top-down approaches beginning with their three-dimensional parent material known as MAX. MAX phases are transition metal carbides, nitrides, or carbonitrides with the general formula M_(n+1)AX_n, n $=$1, 2, or 3, where M denotes an early transitional metal such as titanium (Ti), niobium (Nb), molybdenum (Mo), tantalum (Ta), vanadium (V), or chromium (Cr), A refers to an A group element (mainly 13 and 14 group), and X represents a carbon, nitrogen, or carbonitride that intersperses the M layers[36]. When the A layers are removed from the MAX phase through chemical etching, 2D crystals are formed. However, most of the synthesis methods involve an aqueous medium, resulting in the surface termination of the M elements on the obtained 2D MXene crystals. These MXene crystals are represented by the general formula, M_(n+1) X_nT_x , n$=$1, 2, or 3, where T_x refers to surface terminations, mostly -OH, -O, and -F [37,38,39,40,41].

MXenes have gained considerable attention for their exceptional properties, such as hydrophilicity, high electrical conductivity, and the ability to tune their bandgaps through surface termination modifications [41,42]. Ti₃C₂Tₓ, the first member of the MXene family, remains the most widely studied due to its durability, the relatively straightforward process of etching aluminum layers from its MAX phase precursor, and outstanding physical and chemical characteristics [35,43,44]. Owing to their distinct physical and chemical properties, a variety of other MXene species have also been explored for different applications.

2.2. Synthesis of Ti₃C₂Tₓ MXenes

Hydrofluoric acid (HF) etching was the first technique developed to transform MAX phases into MXenes[35]. Due to its simplicity and ability to produce high-quality MXenes [32], it remains one of the most widely used methods. However, this technique has notable drawbacks, primarily that HF is an extremely toxic and highly corrosive acid [45]. To reduce the risks associated with handling HF directly, alternative methods, such as in-situ HF generation via a reaction between LiF and HCl, were introduced. This approach also offers the added advantage of Li⁺ cation intercalation between MXene layers [46]. A distinctive byproduct of the HF etching method is the formation of MXene layers with surface terminations, primarily -O, -OH, and -F. These functional groups provide an opportunity for surface engineering, enabling the electronic structure of MXene layers to be tuned by modifying the type and quantity of surface terminations[47]. Another modified etching method is the molten salt method that involves etching MAX phases by reacting them with a Lewis acidic molten salt at high temperatures [38]. Similar to the HF method, an intercalant is used; however, unlike HF etching, the molten salt method allows for the creation of diverse surface terminations during synthesis. For example, the electrochemical properties of Ti₃C₂T_x electrodes in supercapacitors are degraded by the -F terminations produced by HF etching. To remedy this, Guo et al. [48] applied a one-step LiCl-KCl-K₂CO₃ molten salt etch and delaminate process to replace the -F terminations of Ti₃C₂T_x with -O. Overall, the MXene saw a drop of -F content from 11.23 to 3.43 at% which precipitated an increase in specific capacity, capacitance retention, conductivity, and the 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 added advantages of larger interlayer spacing and improved delamination properties. The hydrothermal etching method utilizes high-pressure and high-temperature conditions in an aqueous MXene solution to produce high-purity multilayer MXenes [45]. For example, Peng et al. [32] employed this technique to synthesize Ti₃C₂Tₓ MXenes by reacting their respective MAX phases with HCl and HCl+NaBF₄ solutions, followed by heating in an autoclave. The resulting MXenes were delaminated using DMSO and sonication. XRD analysis revealed that the hydrothermal method was significantly more effective at removing aluminum compared to the traditional HF etching method. Furthermore, dye adsorption studies with cationic MB and anionic MO showed that hydrothermally etched Ti₃C₂Tₓ exhibited superior adsorption performance, with lower dye concentrations remaining compared to their non-hydrothermal counterparts. Some hydrothermal methods incorporate microwaves to excite reagents and reduce the temperature and time necessary for a reaction [49].

The resulting flakes were cleaned, delaminated using tetramethylammonium hydroxide, cleaned again, and then incorporated into a composite with reduced graphene oxide, which was effective at degrading dyes via photolysis [50]. A microwave hydrothermal ketal was utilized to rapidly create and oxidize Ti₃C₂T_x Mxene nanosheets by Cao et al[49]. After cleaning, this Ti₃C₂T_x was then combined with TiO₂ and CdZnS once more with the aid of a hydrothermal microwave in order to synthesize a synergetic photocatalytic semiconductive heterojunction that could degrade RhB dye molecules by 29.33% in under 90 min, which is 31.17-fold that of single Ti₃C₂ [51]. To significantly reduce etching time (from ~2$-$3 days to just 45 minutes) and eliminate -F terminations, Latif et al. [52] applied 5 to 30 M concentrations of NaOH etchant to Ti₃AlC₂ in a microwave hydrothermal reaction. Higher concentrations of NaOH saw the synthesized Ti₃C₂T_x MXene achieve a 0.46% Al content under XRD and a semiconductive band gap energy of 1.30 to 1.60 eV.

3. Design and Fabrication of Ti₃C₂Tₓ MXene-Based Hybrid Photocatalysts

Ti₃C₂Tₓ MXene-based hybrid photocatalysts have been fabricated using various methods by integrating Ti₃C₂Tₓ MXene nanosheets with a range of other nanomaterials. Some studies have shown that TiO₂/Ti₃C₂Tₓ MXene composite photocatalysts can directly be prepared through the partial oxidation of Ti₃C₂Tₓ. Tran et al. [53] fabricated TiO₂/Ti₃C₂Tₓ composites through an in-situ partial oxidation process, starting from Ti₃C₂T_x. This method led to the formation of microscale safflower-like structures composed of TiO₂/Ti₃C₂Tₓ heterostructure nanorods. The transformation process was achieved via a sequential approach involving hydrothermal oxidation, alkalization, ion exchange, and heat treatment. Throughout these steps, the layered MXene flakes were broken into nanoparticles, from which TiO₂/Ti₃C₂Tₓ nanorods grew radially. The resulting TiO₂/Ti₃C₂Tₓ heterostructures exhibited exceptional photocatalytic properties. Their photocurrent was ten times higher than that of pristine MXene. Moreover, the photocatalytic degradation efficiency of RhB significantly improved, reaching 95%, compared to only 19% for MXene. Even after four cycles, the degradation efficiency remained above 95%, demonstrating excellent stability. The superior photocatalytic performance was attributed to the rapid generation of TiO₂ carriers, the suppressed recombination of charge carriers, and the enhanced light absorption provided by the porous safflower-like structure. Similarly, Quyen et al. [54] used a novel synthesis approach for TiO₂@ Ti₃C₂Tₓ nanoflowers with a porous 3D framework derived from 2D Ti₃C₂Tₓ MXenes via hydrothermal oxidation combined with a calcination process. This in situ transformation converts the initially conductive Ti₃C₂Tₓ MXene into a semiconductor, forming a TiO₂@ Ti₃C₂Tₓ heterojunction. As compared to 36% degradation rate of pure TiO₂ for RhB, the TiO₂@ Ti₃C₂Tₓ composite exhibited an impressive 97% degradation within 40 minutes of light irradiation. This enhancement is attributed to the electronic structure of Ti₃C₂Tₓ, whose Fermi level is lower than that of TiO₂. Upon light excitation, photoinduced electrons transfer from the conduction band (CB) of TiO₂ to metallic Ti₃C₂Tₓ, leaving holes in the valence band (VB) of TiO₂. The resulting Schottky barrier at the interface suppresses electron diffusion back into TiO₂, thereby reducing electron-hole recombination and improving photocatalytic efficiency. Similarly, in situ solvothermal process has been used to prepare facet-exposed TiO₂/Ti₃C₂T_x [55], since the exposed crystal plane of TiO₂ can effectively capture photogenerated holes and rapidly migrate to the surface of Ti₃C₂T_x, the photocatalytic performance towards methyl orange was significantly better as compared to the TiO₂ and Ti₃C₂T_x.

One of the simplest ways of preparing composites of Ti₃C₂T_x MXene is calcination, in which the powders of two materials are heated at higher temperatures. In one example, [56] varying amounts of Ti₃C₂T_x MXene powders were well-dispersed in a Zn²⁺ containing solution. The mixed solution was heated in an oven to evaporate the solvent. The powder was then calcined for 4 h at 550 °C with a heating rate of 5 °C/min in an ambient atmosphere to prepare ZnO/Ti₃C₂T_x hybrid structures. The hybrid structure has reduced photoluminescence intensity, enhanced Brunauer-Emmett-Teller (BET) surface area, and better photocatalytic degradation efficiency for the degradation of MO and RhB as compared to the pristine ZnO.

The wet impregnation method, in which one material is deposited onto a solid support, can be used to prepare Ti₃C₂Tₓ MXene composites. Nasri et al. [57] mixed powders of g-C₃N₄ and Ti₃C₂T_x MXene in different proportions of weight and sonicated until the slurry was formed. The sample was then dried overnight at the temperature of 60ºC to obtain Ti₃C₂Tₓ MXene/g-C₃N₄ composite photocatalyst. Figure 1 summarizes the fabrication process of Ti₃C₂T_x MXene/g-C₃N₄ composite photocatalyst. They found that 1 wt.% Ti₃C₂Tₓ MXene/g-C₃N₄ heterostructure achieved higher photocatalytic activity for the degradation of methylene blue as compared to pure g-C₃N₄. This was attributed to intimate interfacial contact as seen by FESEM analysis, smooth transfer of photo charge carriers and larger BET surface area.

An electrostatically driven self-assembly strategy is one of the facile approaches to synthesizing MXene composites, especially in the solution. Cai et al.[58] have synthesized Ag₃PO₄/Ti₃C₂Tₓ MXene Schottky catalyst with prominent photodegradation performance toward various organic dyes including methyl orange and 2,4-Dinitrophenol. In their work, Ti₃C₂Tₓ sheets were first dispersed into DI water with the help of sonication, and AgNO₃ aqueous solution was added to the above Ti₃C₂Tₓ suspension with vigorous stirring. Na₂HPO₄ aqueous solution was then added dropwise to the mixture with stirring for 2 h. The precipitate was washed with DI water several times and dried in a vacuum (80 °C) overnight before its use. The apparent rate constant of 2,4-Dinitrophenol degradation with Ag₃PO₄/Ti₃C₂Tₓ was 2.5 times that of Ag₃PO₄/RGO and 10 times that of Ag₃PO₄. The enhanced photocatalysis activity of Ag₃PO₄/Ti₃C₂Tₓ was attributed to the sufficient and close interfacial contact between Ag₃PO₄ and Ti₃C₂Tₓ, unidirectional electron flow to be trapped by the Ti₃C₂Tₓ across the Schottky barrier, and the surface metal Ti sites on Ti₃C₂Tₓ with stronger redox reactivity.

Figure 2. Synthesis and fabrication of MoS₂/Ti₃C₂Tₓ MXene/N-doped carbon composite microspheres. Adapted from Ref[59].

Figure 2. Synthesis and fabrication of MoS₂/Ti₃C₂Tₓ MXene/N-doped carbon composite microspheres. Adapted from Ref[59].

Ultrasonic forces have been utilized in the fabrication of Ti₃C₂Tₓ MXene-based composites to disrupt electrostatic attractions and van der Waals interactions. For example, Lee et al.[60] prepared the composites with heterojunctions of 2D/2D WO₃/Ti₃C₂Tₓ. The WO₃ nanosheets were dispersed in DI water, followed by the addition of varying amounts of Ti₃C₂Tₓ nanosheets. The resulting suspension was subjected to sonication, where cavitation bubbles generated localized high-temperature and high-pressure spots, facilitating physical and chemical interactions between WO₃ and Ti₃C₂Tₓ. This process enabled the effective integration of WO₃ nanosheets with Ti₃C₂Tₓ structures. The final suspension was then dried at 100 °C for 12 hours in an electric oven. The WO₃/Ti₃C₂Tₓ heterojunction demonstrated significantly higher photoexcited carrier transfer and separation efficiency, leading to exceptional photocatalytic performance in MB degradation under visible light. Furthermore, the photoelectrochemical analysis of WO₃/Ti₃C₂Tₓ revealed improved charge carrier mobility, effectively reducing the carrier transport barrier between WO₃ and Ti₃C₂Tₓ. A similar approach has been used to synthesize Ti₃C₂Tₓ/CuFe₂O₄ nanohybrids[61]. A similar process has been used to prepare ZnS nanoparticles/layered MXene sheets[62]. Similarly, manganese oxide decorated 2D Ti₃C₂Tₓ MXene containing MnO₂ nanopetals was also synthesized using the ultrasonic approach [63]. The synergistic effect of these two nanomaterials inhibited the electron/hole pair recombination and improved the surface activity. The nanocomposite showed high photocatalytic ability to methylene blue as about 99% of the methylene blue degraded within 30 minutes.

The sol-gel method uses metal alkoxide precursors to form gels through hydrolysis, usually at lower temperatures, followed by calcination. For example, Iqbal et al. [64] used the double-solvent solvothermal method to prepare BiFeO₃ (BFO)/Ti₃C₂Tₓ nanohybrid. In their work, Ti₃C₂Tₓ MXene in DI water and the BFO nanoparticles in a mixture of acetic acid and ethylene glycol were ultrasonicated separately. Both solutions were then mixed and transferred to a Teflon-lined steel autoclave for solvothermal synthesis in which the mixture was heated at 160 ° C for 2 h. The final product was washed and then dried at 80 ° C for 3 h. The nanohybrid was found to have a high BET surface area of 147 m2 g^–1, a low band gap of 1.96 eV, and a low recombination time. These properties led to a superior photocatalytic activity for Congo red as it was able to degrade Congo red only in 42 mins under visible light irradiation. A similar process was used to prepare La- and Mn-co-doped BFO nanoparticles embedded in Ti₃C₂Tₓ sheets [65].

The solvothermal process, in which reactions occur in a solvent under elevated temperature and pressure within a sealed system, has also been used to synthesize nanocomposites of MXene. This method has been used by W Zheng et al.[66] to synthesize SnO₂/Ti₃C₂Tₓ composites and by Zhou et al.[67] to prepare CeO₂/Ti₃C₂Tₓ nanocomposites from CeO₂ nano-rods on Ti₃C₂Tₓ sheets. The nanocomposites exhibited enhanced photocatalytic activity for the photodegradation of Rhodamine B under UV-light irradiation as compared to pure CeO₂ semiconductors and Ti₃C₂Tₓ. The enhancement of photocatalytic activity was attributed to the narrow band energy in the composite as compared to CeO₂, which enhanced utilization of solar energy, both exhibiting high efficiency in pollutant degradation [67]. Similar process has been used to prepare BiVO₄/Ti₃C₂Tₓ nanocomposite [68].

In another study [69], Ti₃C₂Tₓ powder was added to the ultrasonicated precursor solution of MoS₂. The mixture was heated at 200 °C for 10 hours, then cooled, centrifuged, washed with deionized water, and dried to produce the MoS₂@Ti₃C₂Tₓ nanohybrid in Figure 2. The authors found that the Schottky junction and heterojunction between Ti₃C₂Tₓ and MoS₂ were instrumental in prolonging the recombination time of electron-hole pairs and widening the absorption range of visible light. As a result, nanohybrid showed higher photocatalytic activity towards methyl orange. Also, TiO₂/Ti₃C₂Tₓ MXene was prepared through a hydrothermal reaction of nano-TiO₂ and Ti₃C₂Tₓ MXene nanosheets [70]. The higher photocatalytic activity was attributed to the inhibition of electron-hole recombination, which promoted the accumulation of electrons and easier electron transfer from TiO₂ to MXene. Similarly, AgNP_s/TiO₂/Ti₃C₂T_x composite was prepared by hydrothermal treatment of Ti₃C₂T_x nanosheets and AgNO₃ salt at 160 °C for 12 h with the heat ramp-up of 2 °C/min [71]. They found that the photocatalytic performance of oxidized form improved significantly as compared to the pristine form. Also, superior degradation efficiency for MB and RhB was achieved for AgNP_s/TiO₂/Ti₃C₂T_x as compared to pristine MXene. A similar process has been used to prepare Bi₂WO₆/Ti₃C₂Tₓ by heating layered Ti₃C₂Tₓ MXene and Bi(NO₃)₃.5H₂O at 160 ºC for 16 h.

4. Photocatalytic Degradation of Dyes Using Ti₃C₂Tₓ MXene Hybrids

Photocatalysis is a relatively safer and low-cost strategy for degrading hazardous organic pollutants[72,73,74]. Ti₃C₂T_x MXene, with its unique lamellar structure and remarkably high metallic conductivity, and excellent hydrophilicity has emerged as a promising member of MXene family. Its distinctive properties have enabled its use as a photocatalyst for environmental remediation [75,76] and co-catalyst for enhancing the photocatalytic degradation potential of composite photocatalysts[77,78]. Ti₃C₂Tₓ MXene-based hybrid photocatalysts have several advantages, like improved charge separation, sufficient atomic utilization, tunable bandgap, improved morphology, increased electron transfer efficiency, and improved photocatalytic activity[79,80,81]. We assess the effectiveness of Ti₃C₂Tₓ MXene-based hybrid photocatalysts by comparing them 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)	Ref[81].
δ-Bi2O3	MO	’’	98 (180 min)	Ref[82].
TiO2 NPs	MO	’’	~95 (~120 min)	Ref[83].
ZnO	MB	Cationic	40.88 (21h)	Ref[84].
TiO2 Hollow Nanofiber	MB	’’	95.2 (4h)	Ref[85].
CuO	MB	’’	62 (270 min)	Ref[86].
Bi2S2O3	CR	Anionic	82 (75 min)	Ref[87].
ZnO	CR	’’	97.6(75 min)	Ref[88].
MoSe2	CR	’’	8.44(120 min)	Ref[89].
g-C3N4	RhB	Cationic	75 (180 min)	Ref[90].
BiMnO3	RhB	’’	68 (75 min)	Ref[91].

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)	Ref[81].
δ-Bi2O3	MO	’’	98 (180 min)	Ref[82].
TiO2 NPs	MO	’’	~95 (~120 min)	Ref[83].
ZnO	MB	Cationic	40.88 (21h)	Ref[84].
TiO2 Hollow Nanofiber	MB	’’	95.2 (4h)	Ref[85].
CuO	MB	’’	62 (270 min)	Ref[86].
Bi2S2O3	CR	Anionic	82 (75 min)	Ref[87].
ZnO	CR	’’	97.6(75 min)	Ref[88].
MoSe2	CR	’’	8.44(120 min)	Ref[89].
g-C3N4	RhB	Cationic	75 (180 min)	Ref[90].
BiMnO3	RhB	’’	68 (75 min)	Ref[91].

Table 1. Summarizes the results on the studies of photocatalytic degradation of anionic dyes such as MO, and CR and cationic dyes, MB and RhB. Dey and Ratan Das [81] utilized CdS as a nonhybrid photocatalyst, achieving 95% degradation of MO within 300 minutes. Additionally, they employed ZnO to degrade CR, achieving 97.7% degradation in 75 minutes.[88]. In a separate study, N. N. Mohammad Jafri et al. [85] utilized TiO₂ nanofibers, achieving 95.2% degradation of the cationic dye, MB, within 240 minutes. Similarly, S. Fang et al.[90] prepared g-C₃N₄ to degrade RhB, achieving 75% degradation in 180 minutes. Additionally, the ZnO standalone photocatalyst required 21 hours (1260 minutes) to degrade 40.88% of MB [84] and while MoSe₂ took 2 hours (120 minutes) to achieve only 8.44% degradation [89]. Such prolonged durations and low efficiencies make these photocatalytic processes economically and time inefficient for the practical degradation of organic dye pollutants. Furthermore, pristine MXene photocatalysts demonstrate lower dye degradation efficiency. For instance, J. Qu et al. [115] reported that alkalized Ti₃C₂Tₓ exhibited reduced photocatalytic performance compared to Ti₃C₂Tₓ MXene-based hybrid photocatalysts. The alkalized Ti₃C₂Tₓ achieved degradation rates of only 17.3% for the cationic dye, RhB and 2.8% for the anionic dye, MO within 120 minutes.

On the other hand, Ti₃C₂Tₓ MXene-based hybrid photocatalysts have shown remarkable potential for degrading both cationic and anionic dyes under light irradiation. For example, a Ti₃C₂Tₓ 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 break down dye molecules into less harmful degradation products, showcasing their efficiency in addressing organic dye pollutants. Such 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, M.S.I. Nasri et al. [6] reported a 100% degradation efficiency for MB using a Ti₃C₂Tₓ/g-C₃N₄ hybrid within 180 minutes. This remarkable performance was linked to the effective charge separation and transfer capabilities of Ti₃C₂Tₓ MXene, which significantly enhanced the generation of reactive species essential for dye degradation.

Q.T.H. Ta et al. [56] demonstrated that ZnO/Ti₃C₂Tₓ achieved a degradation efficiency of 99.7% for MO, while RhB was effectively degraded by Bi₂WO₆/Ti₃C₂Tₓ with an impressive efficiency of 99.9% within 20 minutes. Similarly, Danxia Zhao and Chun Cai [92] reported comparable results (refer to Table 2 for details). This study highlights the pivotal role of MXene incorporated heterostructure, characterized by its high conductivity and ability to facilitate rapid electron transfer, which are essential for efficient photocatalysis [93]. For instance, Jun Yao and Chaoxia Wang [94] demonstrated that MB achieved a degradation efficiency of 93.3% within 160 minutes using a catalyst-to-dye ratio of 1.5:1 with standalone catalyst, TiO₂. However, the MXene-based hybrid AgNP_s/TiO₂/Ti₃C₂Tₓ achieved 99% degradation in just 30 minutes with a 2.5:1 catalyst-to-dye ratio, demonstrating superior efficiency in both time and degradation rate compared to non-hybrid photocatalysts [95].

Ti₃C₂Tₓ MXene-based hybrid represents 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, Ti₃C₂Tₓ MXene-based nanocomposites prepared with Mn₂O₃ were evaluated for photocatalytic dye degradation under light, demonstrating efficient photocatalysis. The 1D Mn₂O₃-Ti₃C₂Tₓ (20 wt%) nanocomposite achieved 100% degradation of MB within 25 minutes, effectively removing the dye[96], Similarly, NiMnO₃/NiMn₂O₄-Ti₃C₂Tₓ MXene nanocomposites achieved 100% degradation of MB in 50 minutes, showcasing excellent dye removal efficiency[97]. Wang et al. [98] synthesized Ti₃C₂Tₓ/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 minutes, respectively (see details in Table 2). This result highlights the potential of Ti₃C₂Tₓ MXene-based hierarchical composites for water remediation, offering a sustainable approach for the degradation of anionic and cationic organic pollutant applications. In an independent study, Iqbal et al. [64] reported that the BFO/Ti₃C₂Tₓ MXene hybrid achieved 100% degradation of CR in 42 minutes, highlighting its potential for photocatalysis applications.

While we observed that factors such as the synthesis method, pH, catalyst-to-dye ratio, concentration, and structure significantly influence the functionality and effectiveness of photocatalysts, we conclude from this evaluation that Ti₃C₂Tₓ MXene-based hybrid photocatalysts exhibit superior photocatalytic performance and efficiency in the degradation of organic pollutant dyes compared to nonhybrid photocatalysts.

Table 2. Ti₃C₂Tₓ 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)	Ref.[55]
MoS2@Ti3C2	MO	Anionic	98(60 min)	Ref.[69]
Ti3C2/TiO2/CuO	MO	Anionic	99 (80 min)	Ref.[99]
ZnO/Ti3C2Tx	MO	Anionic	99.7(50	Ref.[56]
Ti3C2Tx /Bi4Ti3O12	MO	Anionic	100(60 min)	Ref[98]
TiO2/Ti3C2 Mxene	MB	Cationic	96.44(60)	Ref[100]
AgNPs/TiO2/Ti3C2Tx	MB	Cationic	99 (30 min)	Ref[95]
Ti3C2/g-C3N4	MB	Cationic	100(180 min)	Ref[101]
NiMnO3/NiMn2O4 -Ti3C2Tx MXene	MB	Cationic	100 (50 min)	Ref[97]
1D Mn2O3-Ti3C2Tx	MB	Cationic	100(25 min)	Ref [96]
Mn-codoped bismuth ferrite/Ti3C2	CR	Anionic	93(30 min)	Ref[65]
CoFe2O4 @MXene	CR	Anionic	98.9(30 min)	Ref[102]
BiVO4/Ti3C2	CR	Anionic	99.5(60 min)	Ref[68]
BGFO-20Sn/MXene	CR	Anionic	100(120 min)	Ref[103]
BiFeO3/Ti3C2	CR	Anionic	100(42min)	Ref[64]
TiO2@Ti3C2	RhB	Cationic	97(40 min)	Ref[54]
BiOBr/TiO2/ Ti3C2Tx	RhB	Cationic	99.8(30 min)	Ref[104]
Bi2WO6/Ti3C2	RhB	Cationic	99.9(20 min)	Ref[92]
ZnS/MXene	RhB	Cationic	100(100 min)	Ref [105]
Ti3C2Tx/Bi4Ti3O12	RhB	Cationic	100 (50 min)	Ref [98]

Table 2. Ti₃C₂Tₓ 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)	Ref.[55]
MoS2@Ti3C2	MO	Anionic	98(60 min)	Ref.[69]
Ti3C2/TiO2/CuO	MO	Anionic	99 (80 min)	Ref.[99]
ZnO/Ti3C2Tx	MO	Anionic	99.7(50	Ref.[56]
Ti3C2Tx /Bi4Ti3O12	MO	Anionic	100(60 min)	Ref[98]
TiO2/Ti3C2 Mxene	MB	Cationic	96.44(60)	Ref[100]
AgNPs/TiO2/Ti3C2Tx	MB	Cationic	99 (30 min)	Ref[95]
Ti3C2/g-C3N4	MB	Cationic	100(180 min)	Ref[101]
NiMnO3/NiMn2O4 -Ti3C2Tx MXene	MB	Cationic	100 (50 min)	Ref[97]
1D Mn2O3-Ti3C2Tx	MB	Cationic	100(25 min)	Ref [96]
Mn-codoped bismuth ferrite/Ti3C2	CR	Anionic	93(30 min)	Ref[65]
CoFe2O4 @MXene	CR	Anionic	98.9(30 min)	Ref[102]
BiVO4/Ti3C2	CR	Anionic	99.5(60 min)	Ref[68]
BGFO-20Sn/MXene	CR	Anionic	100(120 min)	Ref[103]
BiFeO3/Ti3C2	CR	Anionic	100(42min)	Ref[64]
TiO2@Ti3C2	RhB	Cationic	97(40 min)	Ref[54]
BiOBr/TiO2/ Ti3C2Tx	RhB	Cationic	99.8(30 min)	Ref[104]
Bi2WO6/Ti3C2	RhB	Cationic	99.9(20 min)	Ref[92]
ZnS/MXene	RhB	Cationic	100(100 min)	Ref [105]
Ti3C2Tx/Bi4Ti3O12	RhB	Cationic	100 (50 min)	Ref [98]

6. Other Applications of Ti₃C₂T_x MXene-Based Hybrid Photocatalysts

Beyond organic dye degradation, Ti₃C₂Tₓ MXene-based hybrid photocatalysts have demonstrated potential for various other applications. They have shown considerable promise in wastewater treatment, especially in degrading organic pollutants such as dyes, pharmaceuticals, and pesticides. Composite materials of Ti₃C₂T_x MXene with semiconductors such as TiO₂ or ZnO have shown even greater photocatalytic efficiencies; the synergy between MXenes and these semiconductors results in improved light absorption and charge separation [113,114]. For instance, Ti₃C₂T_x/TiO₂ composites exhibit enhanced photocatalytic activity under visible light due to the synergistic effects of both materials. These composites can efficiently degrade different dyes under sunlight, including RhB and MO, making them suitable for sustainable and cost-effective wastewater treatment [115]. 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[116]. Ti₃C₂Tₓ 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[117]. Incorporating noble metals like Au or Ag onto Ti₃C₂Tₓ MXene can further enhance their photocatalytic performance by creating localized surface plasmon resonance, increasing light absorption, and improving the degradation rates of VOCs [118]. Additionally, Z-scheme heterostructures involving Ti₃C₂Tₓ MXene have been developed to mimic natural photosynthesis, achieving efficient separation and transfer of photogenerated charge carriers and enhancing photocatalytic degradation of air pollutants [119]. The most applied technology is the Ti₃C₂Tₓ 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.

Beyond the degradation of dyes, the degradation of other organic pollutants, such as pharmaceutical waste and VOCs, has appeared as a challenging task that needs immediate attention[120]. In recent years, extensive studies[121] reported photocatalysis as an attractive method for the efficient degradation of organic pollutants. Properties such as surface-to-volume ratio, light interaction and mechanical stability define the photocatalytic performance of a catalyst [122]. Among the variety of materials reported so far, Ti₃C₂T_x MXenes gained significant attention as a photocatalytic material [123]. Further, the integration of Ti₃C₂T_x MXenes with other nanomaterials can significantly improve photocatalytic performance[124].

Kumar et al. [125] reported a novel photocatalyst with a composition of g-C₃N₄, Ti₃C₂Tₓ and Au nanoparticles for the degradation of cefixime. Figure 3a shows the degradation mechanism of cefixime decomposition under light. The composition with 3 wt% Ti₃C₂Tₓ shows the highest degradation up to 64.69% cefixime in 105 min using visible light irradiation as shown in Figure 3b and c. Similarly, Diao et al. [126] reported an efficient photocatalytic degradation of tetracycline hydrochloride using MXene-based photocatalysts comprising g-C₃N₄/Ti₃C₂Tₓ/TiO₂. The reported ternary catalysts show superior performance, chemical and photostability with recyclability. Another MXene-based ternary photocatalyst reported by Zhou et al.[127] for photocatalytic degradation of enoxacin under visible light where Ti₃C₂Tₓ helps to improve charge separation at the interface and hence efficient degradation. Rdewi et al. [128] reported a ZnO-TiO₂-MXene photocatalyst for the decomposition of carbamazepine molecules in wastewater under solar irradiation. The presented results show 99.6% removal efficiency at a pH of 7 as a result of improved charge carrier transport and reduced recombination rate due to the incorporation of TiO₂-MXene with ZnO photocatalysts. The excellent photocatalytic performance also shows the reusability for multiple decomposition cycles with exceptional efficiency. Similarly, Abbas et al. [129] reported a ZnO-TiO₂-MXene photocatalyst for the decomposition of ceftriaxone sodium molecules in water using simulated solar light.

Furthermore, Sukidpaneenid et al. [130] reported Ti₃C₂Tₓ/TiO₂ photocatalysts for the degradation of enrofloxacin antibiotics in water. The excellent control over TiO₂ amount on MX by varying hydrothermal processes played a crucial role in tuning photocatalytic properties. Further, the intercalation of sodium ions significantly improved the adsorption, and the synergetic effect of TiO₂ amount and NaCl treatment led to the efficient removal of enrofloxacin. Shahzad et al. [131] demonstrated the degradation of carbamazepine (CBZ) under direct sunlight and UV light using Ti₃C₂T_x-based heterostructure photocatalysts with {001} TiO₂. The results show Kapp value under UV irradiation was higher than under sunlight for degradation CBZ also, the effects of pH on degradation performance were taken into account. Mohanty et al.[132] reported a series of SrTiO₃/Ti₃C₂Tₓ-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 is attributed to the absorption of broad solar spectrum, charge separation and charge transport. Du et al. [133] demonstrated the photocatalytic degradation of tetracycline using CeO₂-Ti₃C₂Tₓ-TiO₂ (CeMXT). The composite shows an excellent degradation efficiency of 94.70% for 22.19 mg/L in 104.13 minutes with 0.65 g/L of catalysts with a pH of 4.72. This excellent degradation efficiency is attributed to increased radical generation and charge separation.

Sergi et al. [134] . reported a series of TiO₂-Ti₃C₂T_x MXene photocatalysts with controlled composition for the realization of improved photocatalytic removal of benzene. The incorporation of TiO₂ with Ti₃C₂Tₓ MXene improved the optical absorption to enhance the photocatalytic performance. The report showed the potential of Ti₃C₂Tₓ MXenes for the removal of VOCs and heterostructure to enhance the photocatalysis performance. Further, Huang et al.[117] reported the degradation of HCHO (formaldehyde) and CH₃COCH₃ (acetone) using Bi₂WO₆/Ti₃C₂ (BT4) under light. Bi₂WO₆ as photocatalysts suffers from a high recombination rate, and combination with Ti₃C₂ can significantly improve the charge separation. Ti₃C₂ led charge separation caused charge transfer at the interface, significantly improving the photocatalytic performance of BT4. Further, the higher adsorption of formaldehyde and acetone at the Ti₃C₂ surface than at the surface of Bi₂WO₆ estimated from DFT simulation, synergistically tunes photocatalytic performance. By value, the degradation of formaldehyde and acetone for Bi₂WO₆ was 2 and 6.6 times higher for BT4, respectively. Mo et al.[135] demonstrated photocatalytic and photothermal removal of VOCs (phenol) from water using TiO₂/Ti₃C₂Tₓ/C₃N₄/PVA (TTCP) hydrogel under sunlight. Figure 3d shows the SEM image of TTCP. The VOC removal efficiency varies from 69.4% to 100% at different concentrations of phenol (1-50 mg/L) shown in Figure 3e and f. The membrane significantly lowers the TDS and TOC. The TDS level was reduced by more than 100 times magnitude, and TOC removal efficiency was observed to be 80%.

Figure 3. (a) Schematic representation of cefixime degradation mechanism under light. (b) Degradation kinetics of cefixime degradation (c) Histogram representation of degradation rate (%).[125] (d) SEM image of the TTCP hydrogel (e) 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.[135] (g) Schematic illustration of the synthesis process of the Ti₃C₂Tₓ MXene/CeO₂ photocatalysts (inset SEM image). Comparative representation of the production of (h) C2H5OH and (i) CH₄ under solar light illumination with different catalysts. [136].

Figure 3. (a) Schematic representation of cefixime degradation mechanism under light. (b) Degradation kinetics of cefixime degradation (c) Histogram representation of degradation rate (%).[125] (d) SEM image of the TTCP hydrogel (e) 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.[135] (g) Schematic illustration of the synthesis process of the Ti₃C₂Tₓ MXene/CeO₂ photocatalysts (inset SEM image). Comparative representation of the production of (h) C2H5OH and (i) CH₄ under solar light illumination with different catalysts. [136].

The reduction of CO₂ using photocatalytic activity is another aspect of environmental remediation applications. Reducing CO₂ into other useful byproducts using light-activated catalysts shows the futuristic pathway towards sustainability [90]. The excellent charge separation and slow recombination rate show the potential of Ti₃C₂Tₓ MXene as a catalytic material for CO₂ reduction[137]. In this regard, Cao et al.[138] demonstrated 2D/2D ultrathin Ti₃C₂Tₓ/Bi₂WO₆ nanosheets prepared by in situ growth of Bi₂WO₆ nanosheets over Ti₃C₂Tₓ nanosheets. The proposed hybrid catalyst shows improved efficiency towards reduction of CO₂ under light due to reduced charge transfer distance, improved surface-to-volume ratio, and adsorption sites. The hybrid catalyst shows 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.[136] investigated a Ti₃C₂Tₓ-CeO₂ hybrid catalyst with varying ratios of Ti₃C₂Tₓ for the photocatalytic reduction of CO₂. The schematic synthesis process for hybrid catalysts shown in Figure 3g. Surprisingly, the hybrid catalyst with 5 wt% Ti₃C₂Tₓ/CeO₂ shows a production of 6127.04 μmol g^–1 for ethanol and 129.5 μmol g^–1 for methane, way higher than CeO₂ within 5 hr of time as shown in Figure 3h and i. Another report by Li et al. [139] shows the potential of Ti₃C₂ based hybrid catalyst comprising g-C₃N₄/ZnO/Ti₃C₂Tₓ for the reduction of CO₂ in methane ( CH₄) and carbon monoxide (CO). The hybrid framework shows a notably higher production rate of 1.41 μmol g^-1h^-1 towards CO increased by a factor of 2.7 and 1.7 as that of ZnO and g-C₃N₄.

As the literature suggests, Ti₃C₂T_x MXene has been extensively studied in recent years for its wide range of photocatalytic uses in environmental applications. The results reported so far show the potential of Ti₃C₂T_x MXene-based catalysts for the degradation of dyes, pharmaceutical wastes, VOCs along with CO₂ reduction. The higher surface ratio, charge separation at the interface and excellent absorption of light plays significant roles in the photocatalytic performance of these photocatalysts.

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. In general, an ideal photocatalyst should possess a suitable bandgap, strong absorption in the visible range, prolonged charge separation lifetimes, and adequate redox potential [105]. To enhance the efficiency of photocatalysis, 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 [140]. 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, enhanced bandgap, improved light-harvesting capability, reduced charge carrier recombination, and extended photoelectron lifetime [141].

During photocatalytic degradation, Ti₃C₂T_x MXene-based hybrid photocatalysts absorb visible light, causing photogenerated electrons to become excited into the CB while leaving holes in the VB [20]. The excited charge carriers are then transferred to the Ti₃C₂T_x MXenes at the interface, primarily due to the higher potential of Ti₃C₂T_x MXene. Photogenerated electrons from the Ti₃C₂Tₓ MXenes migrate to the surface and react with O₂ to produce superoxide radicals (^●O^2-), and holes from the TiO₂ react with hydroxyl ions to produce hydroxyl radicals (OH). These radicals cause the degradation of cationic and anionic dyes of organic pollutants [20,142,143].

A photocatalytic mechanism of Ti₃C₂Tₓ–TiO₂ hybrid for anionic dye (e.g., MO) degradation has been presented in Figure 4a as an example. Initially, the light source provides high-energy photons, which activate the TiO₂ component, generating photoinduced electrons (e⁻) in its CB and leaving holes (h⁺) in its VB. The photoelectrons rapidly migrate from the CB of TiO₂ to the Ti₃C₂Tₓ MXene, facilitated by its high electrical conductivity [95,144]. As a result, Ti₃C₂Tₓ MXene accumulates a negative charge, while TiO₂ becomes positively charged, leading to the formation of a Schottky barrier at the Ti₃C₂Tₓ–TiO₂ interface, serving as a space-charge layer. Subsequently, the photogenerated electrons on Ti₃C₂Tₓ MXene migrate to its surface, where they react with O₂ molecules to generate superoxide radicals (^●O²⁻) [115,145]. Meanwhile, the photogenerated holes react with adsorbed hydroxyl ions (OH⁻) to form hydroxyl radicals (^●OH) [54]. These radicals play a crucial role in the degradation of MO. The photocatalytic reaction mechanism as facilitated by the MXene-based hybrid photocatalyst (TiO₂/Ti₃C₂Tₓ) is illustrated in Figure 4a [115]. The mechanism can be roughly described through the following reactions:

Ti₃C₂–TiO₂ + hט → TiO₂ (h+) + Ti₃C₂ (e^-)

(1)

h⁺ + OH− → ^●OH

(2)

e⁻ + O2 → ^●O²⁻

(3)

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

(4)

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

(5)

For the degradation of a cationic dye like Methylene Blue (MB), superoxide radical anions (^●O²⁻ ) are produced through a reduction reaction between electrons and O₂ molecules, leading to the direct degradation of MB. Additionally, the light-induced h⁺ partially oxidizes MB and partially reacts with water to generate hydroxyl radicals, ultimately breaking down MB.[147]. It is evident from Figure 4b that ^●OH and h⁺ are the primary reactive species in the TiO₂/Ti₃C₂Tₓ MXene composite photocatalysis reaction system [146]. Ultimately, the radicals (^●OH, ^●O²⁻) possessing strong oxidizing abilities, degrade both anionic and cationic dyes, such as MO and MB molecules, directly into their oxidation products[148,149]. Therefore, a similar photocatalytic reaction mechanism occurs for both cationic and anionic dyes under light irradiation.

8. Challenges and Future Directions

Ti₃C₂T_x MXene-based hybrid photocatalysts have emerged as promising materials for the degradation of organic dye pollutants, demonstrating significant potential in environmental remediation. However, despite their advantages for applications in organic dye degradation, several challenges remain that must be addressed to further enhance their photocatalytic performance and facilitate their practical application.

While Ti₃C₂T_x MXene-based photocatalysts exhibit impressive photocatalytic activity towards dye degradation, the development of green synthesis methods is one of the biggest challenges in MXene research. Most of the current synthesis processes involves harsh and hazardous chemicals for etching and exfoliation such as hydrofluoric acid and tetramethylammonium hydroxide and tetrabutylammonium hydroxide [35]. The elimination of such chemicals from the synthesis process is highly desirable for the wide applicability of MXenes. Furthermore, the limited yield of the final MXene product during synthesis is another limiting factor. As of this date, the large-scale synthesis processes for high-quality MXenes are known to be inefficient. Hence the development of mass-production methods for high-quality and uniformly delaminated single to few-layer MXenes nanosheets is required for commercial use. Future research efforts should focus on the reaction kinetics and thermodynamics of the synthesis process for uniform and mass production of MXenes.

The long-term stability of MXenes in harsh environmental conditions such as oxygen, high humidity, and acidic or alkaline media is another concern [37].The corrosion and degradation of MXenes over time can compromise the catalytic efficiency of the composite materials and hinder their reuse. The thermal stability is also considerable challenge for MXene-based photocatalysts as the increased temperature can boost the MXene oxidation [150].

Over the years, significant efforts have been put together to address the challenges associated with MXenes-based hybrid photocatalysts. Future research efforts should focus on the reaction kinetics and thermodynamics of the synthesis process for uniform and mass production of MXene photocatalyst. Considering future developments and practical applications, the large-scale synthesis routes without the use of environmentally hazardous chemicals and the antioxidation strategies of MXenes should be explored.

For Ti₃C₂T_x MXene-based hybrid photocatalysts to transition from the laboratory to real-world applications, strategies to integrate them into practical systems, such as water treatment plants or portable water purification devices, should be explored. The development of easy-to-apply photocatalytic reactors or devices that utilize MXene-based hybrid materials for the degradation of organic dyes and other pollutants will help bring these materials to the forefront of environmental remediation technologies. To enhance the efficiency of Ti₃C₂T_x MXene-based hybrid photocatalysts, a predictive design approach leveraging modern AI and existing databases can be utilized to identify and screen suitable materials for improved performance.

9. Conclusion

In conclusion, we reviewed fabrication of Ti₃C₂T_x MXene-based hybrid photocatalysts and evaluated them for their role in the degradation of cationic and anionic organic dyes. Ti₃C₂Tₓ MXene has been found to exhibit exceptional physical and chemical properties, including high electrical conductivity, excellent hydrophilicity, adsorption capability, and efficient charge transfer, while also suppressing electron-hole recombination in the photocatalyst material during photocatalysis. Ti₃C₂Tₓ MXene has emerged as a promising alternative to noble metal catalysts, providing low-cost, scalable synthesis options along with exceptional catalytic performance for hybrid photocatalysts. Our evaluation demonstrated that Ti₃C₂Tₓ MXene-based hybrid photocatalysts significantly enhanced dye degradation efficiency, as reflected in both the percentage degradation and reduced degradation time, compared to nonhybrid or pure semiconducting materials. A comprehensive understanding of the dye degradation mechanisms involving Ti₃C₂Tₓ MXene-based hybrid photocatalysts has also been provided. Additionally, various computational studies and simulations have been conducted to advance research efforts in dye degradation using these photocatalysts. Lastly, the challenges associated with Ti₃C₂Tₓ MXene-based hybrid photocatalysts have been thoroughly identified, and future research directions have been suggested to address these challenges effectively.