Current Scenario of MXene-Based Nanomaterials for Wastewater Remediation: A Review

Abstract

Rapid urban and industrial sectors generate massive amounts of wastewater, creating severe ecological disruption and harming living organisms. The number of harmful pollutants such as dyes, heavy metals, antibiotics, phenolic compounds, and volatile and several organic chemicals discharged into aquatic systems varies depending on the effluent composition of various sectors. MXene-based composites with unique characteristics were spotlighted as newly developed nanomaterials specifically for environmental-related applications. Therefore, this review broadly discusses the properties, basic principles of MXene, and synthesis routes for developing different MXene-based nanomaterials. The most current strategies on the energy and environmental applications of MXene-based nanomaterials, particularly in photocatalysis, adsorption, and water splitting, were deeply explored for the remediation of different pollutants and hydrogen (H₂) evolution from wastewater. The detailed mechanism for H₂ evolution and the remediation of industrial pollutants via photocatalysis and adsorption processes was elaborated. The multi-roles of MXene-based nanomaterials with their regeneration possibilities were emphasized. Several essential aspects, including the economic, toxicity and ecological power of MXene-based nanomaterials, were also discussed regarding their opportunity for industrialization. Finally, the perspectives and challenges behind newly developed MXene and MXene-based nanomaterials for environmental pollution were reviewed.

1. Introduction

Environmental pollutants such as toxic compounds and organic contaminants are discharged into environments alongside rapid industrialization developments and need to presently be at the forefront of global attention. Water resources are deemed contaminated when they contain toxicity and cause harmful effects on human health–the demand for sufficient clean water and energy is critical for the sustainability of the worldwide population growth. Water quality has worsened over the past few decades, mainly because of human activities and inappropriate use of natural water resources. According to the Natural Resources Defense Council (NRDC), approximately 1 billion of the world’s population is affected by water pollution by industrial effluents every year. In particular, people from low-income communities are significantly at risk because their homes are closest to the most polluting industries. Industries discharging effluents should obey the restrictions and regulations established by the Department of Environment.

Due to their high mobility, toxicity, and poor biodegradability, dye effluents from the textile, paper, and printing industries pose a serious risk to the environment and human health worldwide [1]. The colouring process, which uses a large amount of water and generates a large quantity of coloured wastewaters, is the main cause of high levels of dye pollution in wastewater effluent. Due to the discharge of more than 280,000 tonnes of textile manufacturing effluents annually worldwide, the textile industry is recognized as the primary source of aqueous waste and dye effluents [2]. The limits and regulations set forth by the Department of Environmental Protection must be followed by all discharged wastewater and effluents. According to Malaysia’s Fifth Schedule of Environmental Quality (Industrial Effluents) Regulation 2009 [3], the maximum permitted concentration of Color ADMI in wastewater is 100 for Standard A and 200 for Standard B. It has therefore become challenging to effectively remove such persistent organic pollutants from wastewater effluents in order to lower pollution.

Heavy metal ions are another major source of pollution. They are difficult to decompose or metabolize in organisms due to their high toxicity. Persistent organic pollutants are also causing the global environment to deteriorate. As a result, they are continuously condensed to exceed the allowable limit, endangering human health [4]. Organic pollution degradation strategies can purify oil, river water, wastewater, air, and other substances. Biodegradation, chemical oxidation, electrochemical conversion, or combustion can all be used to remove most organic pollutants. However, some low-level harmful organic pollutants are difficult to eliminate using the methods described above, and secondary pollution will occur.

In acknowledgement of safety requirements, researchers introduced numerous approaches to treating the wastewater excreted before the final release: biodegradation, photo/electrocatalysis, ozonation, adsorption, membrane separation, redox, and chemical precipitation. Nevertheless, considering the cost, technical challenges, and methods’ effectiveness, all the treatments mentioned should lace up with their drawbacks. In recent years, among the variety of removal techniques, photocatalytic degradation, also known as the advanced catalysis process, has proven essential advantages such as being environmentally friendly, reusability, and complete degradation. The history of engineering photocatalytic material can be traced back to 1972 when Fujishima and Honda applied the presence of TiO₂ photocatalyst served as the starting point of photocatalytic reaction [5]. In conjunction with the expansion nanoparticle research area, nanomaterials show intriguing features such as a low removal capacity, high specific surface area, and unique environment flexibility [6].

The photocatalytic degradation process has been reported to degrade a broad range of organic matters such as dyes, phenols, and pharmaceutical waste including alkanolamines. Advanced photocatalytic nanomaterials are a promising technology to bridge the gap between global energy challenges and environmental remediation. At present, a significant amount of effort has also been put into developing highly efficient and robust photocatalysts. The photocatalysts were developed with carbon-based or two-dimensional (2D) nanomaterials such as carbon nanotubes, graphene oxides, metal-organic frameworks, and MXene. Remarkably, the research developed mentioned high-tech nanomaterials without needing expensive metal content. Among these photocatalysts, MXene has attracted significant interest in various areas, including environmental pollutants degradation, due to their unique and excellent physicochemical properties. Generally, MXene belongs to the family of transition metals and carbides and nitrides with an uneven distribution of functional groups. Nonetheless, graphene-based nanomaterials are the most well-known 2D material with a single atomic carbon layer, which leads to limited application, whereas MXene may reach 3, 5, or 7 atomic layers thick.

Therefore, MXene materials and their nanocomposite are advanced photocatalysts due to their high surface area, great strength, large interlayer spacing, nontoxicity, hydrophilicity, and environmental flexibility in recognition of the great potential of MXene that has grown to receive increased attention within a shorter period; this review chapter objective to provide a critical overview on the recent studies of MXene in environmental applications. The properties and fundamental principles of MXene were first studied to encourage a better understanding of the photocatalyst system. Then, we focused on several innovative MXene-based photocatalyst developments that mainly analyzed theoretical and experimental aspects, including TiO₂-MXene, g-C₃N₄-MXene, and BiVO₄-MXene. Afterwards, the research progress on the environmental pollution remediation of MXene/MXene-based photocatalyst is explained. The environmental application involved the removal of organic contaminants and concise hydrogen evolution reaction. As a final point, we present a summary of the current perspective and challenges to tackle future opportunities of MXene photocatalysts. The significant features of MXene-based nanomaterials in the current review were summarized in Figure 1.

1.1. Background Study of MXene

In 2011, 2D-layered titanium carbide powder (Ti₃C₂T_x) was the first candidate of the MXene family introduced by Naguib et al. from Drexel University. As shown in Figure 2, MXene has a formula of M_n+1X_nT_x, which is fabricated by etching MAX phase (M_n+1AX_n) in which M corresponds to transition metal, X denotes carbon/nitrogen, T indicates the functional groups such as hydroxyl, oxygen, or fluorine, A represents elements from the group, and n is an integer. MXene materials were initially investigated as a promising alternative for electrochemical energy storage in batteries and super capacitors due to their high electrical conductivity, structural stability, and hydrophilicity [7]. In 2011, 2D-layered titanium carbide powder (Ti₃C₂T_x) was the first candidate of the MXene family introduced by Naguib et al. from Drexel University [8]. MXene materials were broadly examined as vehicles for energy storage/delivery in devices such as Li/Na-ion batteries and supercapacitors, owing to their lamellar structures and exceptional electrical conductivity, in 2012 [9]. MXene was then used to remove various pollutants from the environment, including dye in 2014 [10], Cr (VI) in 2015 [11], U(VI) in 2016 [12], and N₂ reduction in 2018 [13]. In recent developments, MXene has also been recommended for use as a negative electrode in acidic electrolytic solutions to establish higher concentration gradients and increase energy density [14]. Metallic MXenes have the potential to be tuned to match the semiconductor’s band edge by choosing specific compositions and managing their surface terminations, making them promising electrode materials for nanoelectronics devices.

Furthermore, it was demonstrated that in situ Raman spectroscopy could be used to track how the O-termination in a Ti₃C₂T_x electrode protonated to form an OH-termination after being discharged in an H₂SO₄ solution [15]. Hantanasiridakul et al. also studied that this feature is much higher when compared to carbon-based components, which have a lower capacitance and narrowed potential gradient ranges of 1.0 V [15]. Due to its unique structural, physical, chemical, and functional properties, MXene is considered a designer nanomaterial for various applications in various industrial sectors [16]. Throughout the past several decades, the focus of research on this promising material has switched to the use of MXene in nanomembranes for water purification, nanostructures for energy conversion, and also multifunctional nanocomposites for photocatalytic applications, where it was used as a co-catalyst in order to create photocatalytic behavior [17]. MXene-semiconductor composites were also created to improve the performance of pure components by reducing agglomeration, increasing stability, and providing a conductive channel [18]. Furthermore, due to the rapid recombination of photoexcited charge carriers, single material photocatalysts are always insufficient, resulting in reduced quantum effectiveness and poor photocatalytic performance [19]. Thus, to improve the separation efficiency of photogenerated charge carriers, heterostructure photocatalysts were widely assembled. Finally, due to their unique 2D layered structure, MXene-based photocatalysts demonstrated a significant impact for environmental remediation applications.

1.2. Basic Principle of MXene

MXene belongs to transition metal, carbides, and nitrides with an uneven distribution of functional groups. MXene has a formula of M_n+1X_nT_x fabricated by etching MAX phase (M_n+1AX_n) in which M corresponds to transition metal, X denotes carbon/nitrogen, T indicates the functional groups such as hydroxyl, oxygen or fluorine, A represents an element from the group, and n is an integer. Various functional groups of oxygen (-O), fluorine (F), and hydroxyl (-OH) were molded onto MXene surfaces. They were recognized using energy-dispersive X-ray spectroscopy (EDX) to detect the elemental compositions. The intralayer skeleton, interlayer, and surface terminating groups area make up the structure of MXene, Ti₃C₂. As confirmed by neutron diffraction, Ti is bonded to C via an ionic bond in the interlayer skeleton between the Van der Waals forces and hydrogen bonding between -O or F atoms on the surface [21]. MXene materials have gained significant attention for their wide variety of compositions, conductive properties, and unique morphology. The MXene compound possesses superior reflectivity in the ultraviolet (UV) light absorptions with the reported region from a wavelength between 300 and 500 nm, which was a crucial property for photocatalytic reactions with energy gaps of about 0.25–2.0 eV, as shown in Figure 3a,b [22].

Moreover, the lattice layered structure of surface functionalized MXene allows for highly competent and robust photocatalysts with nitride and carbide bonding–this leads to an efficient generation of the reactive species responsible for pollutant degradation. In addition, MXene owns the hydrophilic nature incorporated with the active functional group on its surface to tackle ionic/molecular species, especially for environmental remediation. Furthermore, the morphology of the MXene flake’s structure was identified by some characterization methods, such as transmission electron microscopy (TEM) and scanning electron microscopy (SEM), which displayed accordion-like multilayer structures [23,24,25]. The most well-known 2D materials are graphene-based nanomaterials. Still, they only have a single atomic carbon layer which limits their application, whereas MXene may reach three, five, or seven atomic layers, as shown in Figure 3c,d. The inter-planar distance of MXene sheets was about 0.242 nm with the (1 0 3) plane of MXene.

2. Synthesis Routes of MXene

The introduction of MXene was first synthesized by etching an A layer from a MAX phase with the HF or HCl-LiF method. The enhancement of the hydrophilic compound due to the functionalization of MXene was the result of the etching method. The multilayered MXene is held weakly together by van der Waals bonds and hydrogen. Congruously, HCl and LiF solution’s etching method gives larger interlayer spacing than HF because of the expansion of hydrated Li-ion intercalation and chloride termination. Conversely, HCl–LiF is a lower fluorine content solution that creates a safer and easier method. However, the latter method’s concentration optimization of these two ingredients, LiF and HCl, improved the quality of the produced MXene [28].

Table 1. The recent development of etching treatment using HF and HCl-LiF.

Ti3AlC2 Mass (g)	Etching Concentration	Etching Temp. (°C)	Etching Time (hr)	Reference
5.0	HF-40 wt. %, 100 mL	RT	48	[30]
2.0	HF-49 wt. %, 100 mL	RT	20	[31]
2.0	HF-48 wt. %, 60 mL	RT	5	[32]
3.0	LiF-2 g, HCl-9 M, 40 mL	35	24	[33]
2.0	HF-40 wt. %, 20 mL	35	24	[24]
0.5	HF-30 wt. %, 10 mL	RT	12	[26]
0.5	HF-30 wt. %, 10 mL	RT	24	[34]
1.0	HF-40 wt. %, 20 mL	RT	24	[27]
3.0	HF-49 wt. %, 84 mL	50	24	[35]
5.0	LiF-8 g, HCl-9 M, 100 mL	35	48	[36]
0.5	HF-5 wt. %, 10 mL	RT	24	[37]
2.5	HF-48 wt. %, 40 mL	RT	20	[38]

shows the summarization of the etching treatment using HF and HCl-LiF. It is observed that the reported etching durations are varied from 5 to 48 h to get a well-interspaced multilayer (M-MXene/Ti₃C₂T_x) or single-layer (S-MXene/Ti₃C₂T_x), depending on the etching agent and method used.

Additionally, based on Khatun and Roy’s optimization analysis, 24 h of etching produces well-interspaced accordion-like multilayer MXene, and shorter etching times are insufficient to remove the ideal amount of Al. On the other hand, 48 h of etching causes pits to form and flakes to break, which weakens the MXene’s properties [29]. As a direct consequence, the developed MXene’s morphology and physicochemical characteristics vary depending on the etching duration.

Various fluoride-free etching methods have been developed to fabricate MXene to reduce its adverse effect on the environment, such as alkali-based etching, Lewis acid etching, and electrochemical etching (E-etching). There are a few reports on using alkaline etchants as fluorine-free synthetic methods. In the first work by Li et al. [39], an alkali-assisted hydrothermal method which inspired the Bayer process was used to prepare M-MXene. It was found that the Al from Ti₃AlC₂ was successfully removed at 270 ˚C in a 27.5 M NaOH solution for 12 h. Ideally, Al layers were attacked by OH; thus, Ti and Al were terminated with −OH and −O. As a result, this process yields a 92 wt. % multilayer Ti₃C₂T_x with an interlayer spacing of 1.2 nm. Granting this etching technique prevents the formation of −F terminal groups. Conversely, the high concentrations required of base and harsh reaction conditions are still dangerous and unsuitable for large-scale synthesis [40]. The Lewis acidic etching mechanism is proposed to etch MAX phases by direct redox coupling between the A element and the cation of Lewis acid molten salt [14].

Li et al. [41] reported an etching process of M-Ti₃C₂T_x prepared from Ti₃SiC₂ immersion in CuCl₂ molten salt at 750 °C. The researchers describe that the exposed Si atoms were oxidised by Lewis acid Cu²⁺ resulting in volatile SiCl₄ and associated with the reduction of Cu²⁺ to metallic Cu, then Cl^ˉ anions reacting to form Ti₃C₂Cl₂. Next, the Cu particles were removed by immersing the products in ammonium persulfate ((NH₄)₂S₂O₈), which also marks the O terminal surface groups. In addition, the E-etching technique approach was used for hydrolysis (to obtain MAX phase precursor with reduced lateral size) and a thermos-assisted electrochemical process (to remove the A-layer from the electrode) [42]. The etching process was performed in a two-electrode system with an anode and cathode where bulk Ti₃AlC₂ and chloride-containing electrolytes were chosen as apparent etching effects [43]. They suggested the electrolyte is composed of 1 M NH₄Cl and 0.2 M tetramethylammonium hydroxide (TMA·OH) with a pH value above 9. Cl^ˉ anions enabled rapid anodic Al etching, resulting in −OH termination on M-Ti₃C₂ surfaces, and the intercalation of ammonium species took place simultaneously. Herein, a fluoride-free, facile, and rapid method for MXene preparation was an advantage relative to E-etching.

Several other techniques, such as chemical vapour deposition and hydrothermal and salt templates, are exhibited as functional pathways for fluoride-free MXene preparation. Proper control towards surface functional groups, maximum specific surface distribution, and well-defined chemical stability towards synthesizing MXene should be the target to invent a feasible proposed approach and establish several environmental issues [44]. Although the superior properties of MXene were promising in photocatalysis applications, the performance of MXene is still hindered by its tendency to stack or aggregate, driven by van der Waals, which leads to a lower photocatalytic performance [45]. The high degree of electron-hole pair recombination rate is another factor that restricts the performance of MXene photocatalysts [25]. The photoluminescence (PL) test, which examined the energy released by the photocatalyst, was used to evaluate the separation of photogenerated charge carriers. It is important to note that the recombination of photogenerated charge carriers increases with PL intensity, which influences the overall performance of the photocatalytic system [46]. As more free electrons were available, the separation of photogenerated charge carriers improved and significantly increased the photocatalytic activity.

Heterojunctions were formerly thought to be one of the simplest and most effective ways to modify the internal electrical characteristic structure of MXene and increase its photocatalytic performance [47]. As a result, a new electric field within a space charge region will be developed, resulting in spatial photo charge carrier separation and migration. MXene-based photocatalysts are prepared by replacing the noble metal co-catalyst to enhance the physicochemical properties of the photocatalyst. Generally, the modification aims to improve the charge separation capacity of the photocatalysts [22]. The strategic view is that the best approach to maximize the use of MXene is to use it as a substrate, which facilitates hybridization with other nanomaterials and thus increases catalytic activity.

3. Environmental Applications

The presentation of MXene-based photocatalysts for environmental remediation and resource recovery has garnered maximum attention and has also been applied to developing advanced photocatalyst systems. The profound photocatalytic performance of the MXene-based photocatalyst is mainly governed by its evolution of particular surface termination groups (−OH, −O or −F) [48]. Generally, the pristine MXene (Ti₃C₂) is metallic, and the surface termination groups show semiconducting behaviors. Given the above unique features of the photocatalyst, researchers have attempted strategic experimental performances to reveal their role in an environmental application using the MXene hybridization structure. This section presents a detailed summary of recent prospective pollution control applications through MXene-based catalysts, including antibiotic removal, dye removal, and water splitting.

3.1. Photocatalysis Mechanism

The photocatalysis system mainly depends upon light energy or wavelengths and a photocatalyst. The semiconductors are generally utilized as photocatalysts, and their electronic structures could be helpful to serve as sensitizers for light irradiation [49]. In the degradation mechanism, the light source falls over the photocatalyst surface, and if this energy is equal to or greater than photocatalyst band gap energy, then electrons in the VB are disturbed and transferred towards the CB of the photocatalyst, leaving the holes in the VB. VB holes may oxidize donor molecules and generate hydroxyl radicals after a reaction with water molecules. On the other hand, electrons in CB produce superoxide radicals after a reaction with oxygen species. The produced holes and electrons may cause oxidation and reduction reaction processes that can be adsorbed over the photocatalyst surface to provide necessary outputs [50]. Similar photocatalysis mechanisms have been proposed in several studies. Vishal et al. proposed a photocatalysis mechanism using ZnO/Bi₂WO₆/M-MXene photocatalyst for ciprofloxacin under a visible-light system. Bi₂WO₆ and ZnO are activated under UV and visible-light systems, separating photogenerated electrons and holes. Photoelectrons from ZnO move towards the CB of Bi₂WO_6, and then these electrons are transferred into the CB of MXene. Firstly, MXene carried the photoelectrons towards active sites owing to its good metallic structure and produced H₂O₂ after reacting with adsorbed oxygen molecules. Furthermore, Bi₂WO₆ and ZnO also reacted with adsorbed molecules to produce H₂O₂ species, resulting in the generation of hydroxyl radicals. Secondly, holes in the VB of Bi₂WO₆ transferred towards ZnO VB and produced hydroxyl radicals after reacting with adsorbed water molecules that degrade ciprofloxacin pollutants [51]. In another study, TiO₂/M-MXene photocatalyst was used to propose a photocatalysis mechanism for RhB. The photoelectrons in TiO₂ move from VB towards CB, leaving holes in VB and producing photoelectrons and hole-pairs as shown in Figure 4 [52]. Similar photocatalysis mechanisms have also been investigated in other studies using TiO₂/M-MXene for carbamazepine [53], CuFe₂O₄/M-MXene for sulfonamides [54], BLFMO-5/M-MXene hybrid for cango red [55], g-C₃N₄/S-MXene for RhB [56], CoO@TiO₂/M-MXene, and Fe(Co)/M-MXene/ZSM-5 for phenol [57,58].

3.2. Photocatalysis

The photocatalytic degradation of antibiotics and other different organic pollutants from wastewater has been considered the most effective method, owing to its simplicity, environment-friendliness, and short time-consuming nature [59,60,61]. In recent decades, pharmaceutical antibiotics have drawn increasing utilization in medical therapy and treatment worldwide. The annual consumption was reported to be 1 × 10⁵ to 2 × 10⁶ tons and was discharged to the aquatic environment [62]. Specifically, in 2019, the extensive consumption of antibiotics was accelerated due to the coronavirus disease 2019 (COVID-19) pandemic. Antibiotic pollutants are considered hazardous to human health and the environment to their water solubility, persistence, complex structure, pronounced chemical stability, and active metabolic residues [63].

Consequently, the development of antimicrobial resistance has appeared as a significant health problem for humans and other living creatures and the ecosystem disturbances in a natural water environment. The fast recombination of photo charge carriers’ efficiency and stability of a single photocatalyst minimizes its photo performance. Therefore, MXene assembled a co-catalyst to effectively minimize the photo charge carrier recombination effect and improve the photocatalytic power. Until now, several MXene and MXene-based photocatalysts have shown efficient photocatalytic performance for the remediation of different organic compounds from wastewater. Furthermore, previous studies have found that the combination of MXene and other co-catalysts provides abundant active sites and oxygen-containing functional groups that respond to the magnetic field. The magnetic field could boost the catalyst reactivity and enhance the degradation performance [64]. In literature, MXene-based photocatalysts have been widely tested for the effective degradation of antibiotics from wastewater. Cao et al. studied sulfamethazine photodegradation and compared the CuFe₂O₄, M-MXene, and CuFe₂O₄/M-MXene performances. The research suggested that CuFe₂O₄/M-MXene showed the best degradation performance with a reaction rate of 0.0128 min⁻¹ compared to CuFe₂O₄ (k = 1.44 × 10⁻³ min⁻¹) and M-MXene (k = 2.52 × 10⁻⁴ min⁻¹) [54]. The mechanism of sulfamethazine degradation suggested by Cao et al. was •OH and H⁺ dominated the catalytic process, and dissolved oxygen generated •O₂⁻ to promote photocatalytic degradation. In another study in which sulfadimidine and tetracycline hydrochloride were degraded through the Ag₂WO₄/M-MXene composite photocatalyst and analyzed, it was demonstrated that the conductive properties of MXene can significantly enhance the photocatalytic power up to 88.6 and 62.9%, respectively. MXene may detect and transform into electrons, producing a stabilized Ag₂WO₄/M-MXene photocatalyst [65]. Sukidpaneenid et el. synthesized TiO₂/M-MXene via a microwave-hydrothermal process in HCl/NaCl solutions, encouraging the development of fine TiO₂ over the MXene parent structure. A TiO₂/M-MXene photocatalyst was used to eliminate enrofloxacin antibiotic up to 93.4% in 4 h. The addition of NaCl played a substantial role during the synthesis of MXene-based photocatalysts by introducing sodium ions over the photocatalyst structure, improving photocatalytic power [66]. Sharma et al. reported the synthesis of a ZnO/Bi₂WO₆/M-MXene photocatalyst by a two-step electrostatic assembly process for the degradation of ciprofloxacin antibiotic up to 77% in 160 min under a natural sunlight system [51].

MXene-based photocatalysts have abundantly been employed to eliminate different types of dyes. The development of Z-scheme heterojunction photocatalysts could be an excellent photocatalytic agent. In this regard, Tu et al. prepared g-C₃N₄/S-MXene photocatalyst through a one-step H₂O₂ oxidation process. The development of Z-scheme heterojunction between MXene and g-C₃N₄ hybrid could have a solid power for a separate photogenerated recombination rate and is effective for the degradation of RhB and tetracycline up to 97.22 and 86.34%, respectively, in 60 min [56]. Literature showed that MXene has two-dimensional properties, good electrical performance, and surface hydrophilic functional groups, providing a great chance to build an excellent MXene hybrid. Sun et al. reported M-MXene-bridged-Ag/Ag₃PO₄ hybrid photocatalyst via an electrostatic self-assembly synthesis route. The synergetic impacts from MXene, Ag₃PO₄, and Ag effectively promote the photocatalytic activity for dye and Cr(IV) from wastewater [67]. Zhang et al. synthesized metal/M-MXene/ZSM-5 photocatalysts using a hydrothermal method to improve the visible-light absorption and catalytic power of ZSM-5 material. The incorporation of MXene and metals produced a narrow band gap energy, resulting in a superior photocatalytic activity for phenol up to 99% in just 25 min [57]. Therefore, an MXene-based catalyst is anticipated to be extended to a large variety of nanohybrids for the removal of different pollutants as demonstrated in

Table 2. Photocatalysis studies of different pollutants using MXene-based photocatalysts.

Photocatalyst	Experimental Details	Pollutants	Performance	Reference
Bi2WO6/S-Ti3C2	Dose = 20 mg, Co = 20 ppm, rpm = 400, λ = 300 W Xe lamp	Amoxicillin	~100% in 40 min	[68]
Ag2WO4/M-Ti3C2	Dose = 0.4 g/L, Co = 20 mg/L, λ = 300 W Xe lamp	Tetracyline hydrochloride	~80% in 30 min	[65]
CdS/S-Ti3C2Tx	Dose = 0.5 g/L, Co = 20 mg/L, λ = 300 W Xe arc lamp	4-nitroaniline	~90% in 10 min	[69]
Ag2WO4/M-Ti3C2	Dose = 0.2 g/L, Co = 20 mg/L, λ = 300 W Xe lamp	Sulfadimidine	88.6% in 40 min	[65]
CuFe2O4/M-Ti3C2	Dose = 25 mg, Co = 40 mg/L, λ = 300 W Xe lamp, T = 25 °C	Sulfadimidine	59.4% in 60 min	[54]
TiO2/M-Ti3C2	Dose = 5 mg, Co = 5 mg/L, λ = 100 W medium pressure lamp	Carbamazepine	98.67% in 4 h	[53]
TiO2/Ti3C2/NaCl	Dose = 150 mg, Co 10 mg/L, V = 150 mL, λ = 2.1 mW/cm2 lamp intensity	Enrofloxacin	93.4% in 5 h	[66]
ZnO/Bi2VO6/M-Ti3C2	Dose = 10 mg, Co = 5 × 10−5 M, V = 20 mL, λ = sunlight	Ciprofloxacin	77% in 160 min	[51]
g-C3N4/S-Ti3C2	Dose = 20 mg, Co = 20 mg/L, V = 100 mL, λ = 270 W Xe lamp	TC & RhB	86.34 & 97.87% in 60 min	[56]
In2S3/TiO2@metallic M-Ti3C2Tx	Dose = 0.6 g/L, Co = 20 m/L, λ = 300 W Xenon lamp	MO	92.1% in 60 min	[70]
TiO2@M-Ti3C2/g-C3N4	Dose = 0.1 g/L, Co = 10 g/L, λ = 300 W Xe lamp	RhB	~100% in 60 min	[71]
TiO2-x/M-Ti3C2	Dose = 0.05 g/L, Co = 20 mg/L, λ = 500 W Xe lamp	MB	75% in 150 min	[72]
Microporous-S-MXene/TiO2-x	Dose = Ti-150 ppm, H2O2 =0.7 mM, Co = 30 mg/L	RhB	96% in 10 min	[73]
M-Ti3C2−OH/ln2S3/CdS	Dose = 200 mg/L, Co = 20 mg/L, λ = 300 W Xe lamp	MO & RhB	96% in 30 min & 99.1% in 10 min	[74]
TiO2/M-Ti3C2	Dose = 20 mg, Co = 20 mg/L, V = 50 mL, λ = 300 W Xe lamp	RhB	93.7% in 60 min	[52]
Fe(Co)/M-Ti3C2/ZSM-5	Dose = 0.3 g, Co = 20 mg/L, V = 100 mL, λ = 300 W Xe lamp	Phenol	99% in 25 min	[57]
CoO@TiO2/M-Ti3C2 hybrid	Dose = 20 mg, Co = 20 mg/L, V = 100 mL, λ = 300 W Xe lamp	Phenol	96% in 15 min	[58]
ZnO-ML/M-Ti3C2	Dose = 0.05 g, 4-CP-Co = 20 mg/L, MB-V = 100 mL, λ = 300 W Xe lamp	MB & 4-CP	85.2 in 4 h & 56.1% in 5 h	[75]
BLFMO-5/M-Ti3C2 hybrid	Dose = 100 mg, Co = 100 mg/L, V = 100 mL, λ = 300 W Xe lamp	Congo red	93% in 20 min	[55]
Bi3TaO7/M-Ti3C2	Dose = 0.05 g, Co = 10 mg/L, V = 100 mL, λ = 300 W Xe lamp	MB	99% in 2 h	[76]
Ag/Ag3PO4-M-Ti3C2 hybrid	Dose = 50 mg, metal: Co = 10 mg/L, V = 80 mL, MO: Co = 20 mg/L, V = 100 mL, λ = 300 W Xe lamp	MO & Cr(IV)	93 & 61% in 1 h	[67]
HF-M-Ti3C2	Dose = 200 mg/L, Co = 20 μM, λ = 20 mW/cm2 lamp intensity	PFOA	100% in 16 h	[77]

.

3.3. Adsorption Mechanism

In the adsorption mechanism, the adsorbate is strongly attributed to the surface of employed adsorbent until it has attained the equilibrium stage. The adsorption mechanism contains several significant steps such as (a) physical adsorption: settlement of adsorbate occurred over the surface of applied adsorbents, (b) complexation and precipitation: deposition of adsorbate occurred over the surface of the applied adsorbent, and (c) pore filling: condensation of adsorbate inside the pore of adsorbent. The adsorption mechanism has been defined in literature by employing different kinetic models. From the several kinetics equations, pseudo-first order, pseudo-second order, Elovich, and intraparticle diffusion kinetic model, equations were selected to understand the adsorption mechanism [78]. Ahsan et al. studied the adsorption mechanism of ciprofloxacin over sodium ion-based MXene adsorbent through different kinetic models. They revealed that the Elovich model provided the best understanding of the adsorption mechanism because of its higher R² up to 0.99. Moreover, the chemisorption process is a rate-controlling step as experimental data obeying the Elovich equation [79]. Cai et al. selected pseudo-first-order and pseudo-second-order kinetic models to investigate the adsorption mechanism of MB and RhB dyes over phytic acid-based MXene composite and proposed that pseudo-second-order could be a more admirable model to explain the adsorption mechanism of both dyes providing a higher R² up to 0.999 with a minimum difference between experimental and theoretical outcomes [80]. Similarly, other studies also proposed pseudo-second-order kinetics to understand the adsorption mechanism via MXene-based adsorbents [81,82,83,84].

The adsorption mechanism could be investigated more frequently through adsorption equilibrium studies by employing different isotherm models. The equilibrium relationship between adsorbate and adsorbent can be described using several isotherm equations that quantify the solute amount at room temperature. Langmuir, Freundlich, and Temkin isotherms have been the most commonly used models to produce isotherm profiles based on experimental results [61,85]. Yang et al. selected several isotherms to propose an interaction mechanism between Cr(IV) and S-MXene@IMIZ adsorbent. They revealed that the correlation coefficient R² through Langmuir isotherm was higher up to 0.9949, which is closer to 1 than other isotherms. Therefore, Langmuir is a more suitable isotherm to explain the experimental data [86]. Moon et al. used ultrasound-assisted M-MXene adsorbent to propose the adsorption mechanism for MB dye through Langmuir and Freundlich isotherm equations. Their isotherm profiles described Freundlich isotherm as a more admirable model for explaining experimental data due to the higher R² [87]. XPS analysis was also helpful in characterising the adsorption mechanism of 4-NP over hydrogel adsorbent. Their results revealed that several characteristic peaks, C1s, O1s and N1s, were significantly increased after the adsorption of 4-NP, indicating significant adsorption of 4-NP. The fitting peaks of C-O and C=O in O1s proposed that the adsorption mechanism mainly depends on hydrogen bonding between the adsorbent and the targeted pollutant.

Moreover, N-H could be a proper fitting group to improve the adsorption mechanism. Similarly, Dao et al. also explained the adsorption mechanism of tetracycline using alkalized M-MXene adsorbent, as shown in Figure 5. The weak electrostatic interaction is the main force responsible for the adsorption of tetracycline to an opposing charged adsorbent surface without metal use. The electrostatic interactions between negatively charged alkalized M-Mxene and positively charged metal ions become powerful by adding metal ions to the system. The surface modification of alkalized M-Mxene occurs through metal ions and produces a ternary complex of alkalized M-Mxene-metal ions-tetracycline to enhance the adsorption rate of tetracycline [88].

3.4. Adsorption

The adsorption process provides reasonable compensations such as safe operations, recycling and remediation to the required concentration range. Moreover, the adsorption technique can provide an excellent removal rate for it to be commercially viable [76,86]. Due to the rapid development of industrialization, dye wastewater originating from textile, paper, and printing activities affected the ecological environment and human health. Dye pollutants are a family of organic compounds with high mobility, substantial toxicity, and poor biodegradability [1]. The aromatic molecular makeup of dyes were reported as cancer-causing and mutagenic, whereas the metal-containing dyes can cause severe renal system damage [10]. Many strategies were attempted to address this issue, primarily through adsorption, a potentially promising technique due to the easy operation and economical and energy saving [89]. Therefore, an MXene-based catalyst is a suitable choice to treat dye-polluted wastewater due to the surface functional group structures, effectively absorbing and activating the dye during adsorption activity. In past research, MXene and MXene-based materials have been widely employed to treat different pollutants through adsorption.

Enhancing the effectiveness of MXene, Wei et al. studied varied alkali functionalizations of MXene on dye adsorption performance. They reported that the alkali functionalization assists the expansion of the interlayer spacing of M-MXene and the surface functional group transformation, which enhances the adsorption capability and accelerates the removal rate for MB compared to pristine MXene [90]. The research proposed that NaOH-M-MXene obtained the fastest removal rate with higher methylene removal capacity (189 mg/g) compared to LiOH-M-MXene (121 mg/g), pristine-M-MXene (100 mg/g), and KOH-M-MXene (77 mg/g). The experimental data suggested most MXene-based catalysts’ interaction with dyes were monolayered in form and best described with Langmuir isotherm [91]. A M-MXene layer was composed of concentrated HF and pillared with terephthalate through a rapid direction method and showed the highest adsorption rate of 209 mg/g for MB owing to a higher surface area of 135.7 m²/g and greater interlayer spacing between free carboxylate groups of terephthalate and M-MXene sheets [92]. S-MXene@IMIZ composite was developed through the overlaying of polyimidazole chains over the MXene surface via a multicomponent process using a renewable chitosan reactant. The adsorption process showed that Cr(IV) was transformed into Cr(III) with a lower toxicity effect [86]. MXene nanosheets were incorporated with sodium ions through a fabrication process and used to treat ciprofloxacin antibiotics. Their adsorption results revealed that fast antibiotic and kinetics process removals are obtained through surface termination and spacing between sodium ion-based MXene layers [79]. In another research, tetracycline antibiotic was adsorbed from wastewater through hydroxyl-rich surface alkalized M-MXene adsorbent, prepared through an alkaline intercalation process. The possible influence of Cu(II), Co(II), Ni(II), Cd(II), and Cr(II) metal ions over the removal of antibiotics was investigated through alkalized M-MXene adsorbent. Their adsorption outcomes confirmed that metal ions showed a special removal rate, and the adsorption rate of antibiotics was successfully achieved up to 750% in the presence of Ni(II) metal ions. This highest adsorption happened at the hydroxyl site over alkalized M-MXene surface and showed improvement in surface complexation. Furthermore, increased antibiotic adsorption was also associated with the surface modification of the metal ions mechanism [88]. A summary of various MXene and MXene-based materials applications for different target pollutants removal is presented in

Table 3. Adsorption of different organic pollutants using MXene-based adsorbents.

Adsorbent	Experimental Details	Pollutants	Performance	Reference
Alginate/ M-MXene/CoFe2O4	Dose = 4.5 mg, Co = 200 mg/L, pH = 5.3, rpm = 400	Ciprofloxacin	231.71 mg/g in 48 h	[93]
Co@M-MXenes	Dose = 30 mg, Co = 20 μM, pH = 7.0, rpm = 400	2,4 DCP	~100% in 15 min	[94]
Co2+/M-MXenes membrane	Area = 3.14 cm2, Co= 10 mg/L, rpm = 50 rpm	Tetracycline	97% in 5 min	[64]
M-Ti3C2TX	Dose = 20 mg/L, Co = 5 mg/L, pH = 7, US = 28 kHz	MB	~60% in 30 min	[87]
Fe3O4/M-Ti3C2 magnetic nanoparticles	Dose = 0.5 g/L, Co = 20 mg/L, pH = 3, rpm = 600, T = 25 °C	MB, RhB, MO, Congo red	67.77, 74.82, 53.07, 57.29% in 30 min	[95]
HF/M-MXene	Dose = 0.1g, Co = 50 mg/L, V = 0.2 L, rpm = 300	MB	24 mg/g in 2 h	[96]
HF/M-Ti3C2TX	Dose = 10 mg, Co = 100 mg/L, V = 50 mL, pH = 7, rpm = 150	MB	209 mg/g n 4 h	[92]
AA-Alk-S-Ti3C2	Dose = 100 mg, Co = 90 mg/L, V = 30 mL,	CR, MB	264.46, 193.92 mg/g	[81]
Phytic acid/M-MXene	Dose = 20 mg, MB-Co = 12 mg/L, RhB-Co = 6 mg/L, V = 40 mL	MB, RhB	26.77, 13.44 mg/g in 30 h	[80]
S-MXene@IMIZ	Dose = 10 mg, Co = 30 mg/L, V = 50 mL	Cr(VI)	119.5 mg/g in 80 min	[86]
HF/M-MXene	Dose = 100 mg, Co = 100 mg/L, T = 293 K, V = 100 mL	Cr(VI)	80 mg in 14 h	[23]
M-Ti3C2TX	Dose = 50 mg/L, Co = 2 mg/L, pH = 6, T = 293 K	Pb (II)	36.6 mg/g in 1440 min	[82]
SA-M-MXene spheres	Co = 25 mg/L, V = 30 mL, rpm = 180, T = 293 K, pH = ~ 6	Mercuric ion	932.84 mg/g in 8 h	[97]
M-Ti2CTX/PDDA	Dose = 0.4 g/L, Co = 100 mg/L, pH = 4	Re(VII)	363 mg/g	[98]
M-Ti3C2 composite hydrogels	Dose = known, Co = 2 mg/mL, pH = 7, T = ambient	4-NP	162 mg/g in 48 h	[99]
SI-M-Ti3C2TX	Dose = 10 mg, Co = 10 mg/L, V = 25 mL, T = 25 °C, pH = 3–10	Ciprofloxacin	208.2 mg/g in 120 min	[79]
O2-M-MXene	Dose = 0.01 g, Co = 100 mg/L, pH = 1–6, T = 10–30 °C	Hg(II)	1057.3 mg/g	[83]
M-MXene	Dose = 0–0.1 g, pH = 2–10, Co = 5–50 mg/L	Malachite green	94.1% in 180 min	[84]
M-Ti3C2TX-EHL nanosheets	Dose = 15 & 20 mg, MB-Co = 25 mL of 100 mg/L, Cu2+-Co = 15 mL of 50 mg/L	MB, Cu2+	293.7, 49.96 mg/g in 24 h	[100]
ALK-M-MXene	Dose = 3.6 mg, Co = 40 mg/L, V = 10 mL, pH = 5.5, T = 25 °C	Tetracycline	56.75 in 10 min	[88]

.

3.5. Water Splitting Mechanism

The photocatalytic process is a chemical reaction that can be encouraged through photoirradiation under a suitable photocatalyst material. This kind of photocatalyst will have a solid ability to improve the chemical reaction without being used or converted into another phase. The basic working principle of the photocatalytic mechanism for H₂ production via water splitting is simple, as shown in Figure 6. Initially, when light with energy more significant than the band gap of the photocatalyst is irradiated, the empty conduction band (CB) and the filled valence band (VB) are separated. The electrons in the VB are excited by the CB, and the electrons (e⁻) and hole pairs (h⁺) are generated. These e⁻ and h⁺ reduce and oxidize the chemicals over the photocatalyst surface, respectively, unless they recombine to avoid clean chemical reactions. When the same amount of e⁻ and h⁺ is used for a chemical reaction and recombination, the photocatalyst’s original structure (or chemical composition) does not change. Recently, several MXene-based photocatalysts have been proposed for photochemical reaction mechanisms to produce H₂ under a visible-light activation system. Molybdenum@TiO₂@M-MXene photocatalysts have effectively transformed and separated photoinduced electrons and hole pairs under a visible-light system, as shown in Figure 6. In the photocatalytic mechanism, a maximum number of photogenerated electrons inside the CB of TiO₂ may quickly migrate towards the MXene side MoS₂ impurity bandgap caused by molybdenum vacancies, resulting in an electron-rich environment over the surface planes of MoS₂ and Mxene, via which H₂ is produced through the reduction of water molecules [101]. In another study, ultra-small TiO₂QDs were designed on S-MXene for a photocatalytic H₂ mechanism via the water splitting process. In a proposed mechanism, photogenerated electrons excited from TiO₂QDs transition from the CB to VB of TiO₂QDs. Owing to its low interfacial resistance, e⁻ rush from TiO₂QDs towards S-MXene, facilitating an efficient charge separation effect. The metal-like properties of MXene make it easy to transfer photogenerated electrons to MXene. Consistent with this, the charge transfer path is significantly increased. This accelerates the separation of photogenerated charge carriers and promotes photocatalytic H₂ formation [102]. A similar photocatalytic mechanism for H₂ production via water splitting has been proposed in other research works using MXene-based photocatalysts [103,104,105,106,107].

3.6. Water Splitting

In addition to the versatile photocatalytic decomposition of pollutants, MXene-based catalysts significantly contribute to sustainable clean energy production. Environmental contamination and the energy crisis were global issues due to the rapid consumption of fossil fuels. Fossil fuel reserve depletion times for oil, coal, and gas are approximately 35, 107, and 37 years, respectively, and are computed by the modified formula from the Klass model [108]. Therefore, photocatalytic water-splitting technologies are an excellent way to replace fossil fuels such as hydrogen and oxygen. MXene has the potential as a promising candidate for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) due to its unique features: (1) excellent metal conductivity, (2) higher hydrophilic functionalities, and (3) significant redox reactivity [25].

Wang et al. were one of the earlier researchers on MXene-based catalysts for hydrogen production. They reported that TiO₂/M-Ti₃C₂T_x achieved the highest hydrogen evolution rates with 17.8 μmol g⁻¹ h⁻¹ compared to pure TiO₂ (~5 μmol g⁻¹ h⁻¹) and pristine M-Ti₃C₂T_x (~1 μmol g⁻¹ h⁻¹) [109]. Ti₃C₂T_x in the studies was suggested to provide a 2D platform for intimate interactions with uniformly grown TiO₂ nanoparticles and to promote the separation of photogenerated charge carriers. On the other hand, MXene, as a cocatalyst, helps to accelerate surface redox reactions and enhances charge transfer [110]. As the MXene-based catalysts hybrid technique has been developed and performed in water splitting, these new nanomaterials were proven excellent for HER and low-cost and efficient OER electrocatalysts. According to the literature, MXene–having −O termination–acted as an electron cocatalyst to enhance the water-splitting performance of semiconductors such as CdS, TiO₂, and g-C₃N₄ [103,111].

On the other hand, many researchers believed that MXene with −OH surface adsorption could play a different role in the interfacial transfer of photogenerated electron hole-pairs [104]. Recently, the −OH terminated MXene system has been effectively utilized as a hole moderator for enhancing photo charge carrier and H₂ efficiencies, such as Cu/TiO₂@M-MXene [112] and PTO/TiO₂@M-MXene [113]. In the same outline, Chang et al. have successfully developed anatase/rutile TiO₂ over M-MXene via a hydrothermal process for efficient H₂ generation of 4672 μmol/hrg, owing to optimum synthesis conditions that is 3.76 times greater than pure TiO₂ [114]. In another research, M-MXene/Z_XC_1-XS have been successfully produced to attain excellent H₂ generation. Their experimental results revealed the highest H₂ evolution rate up to 14.17 mmol/hr owing to their collaborative influence of enhanced charge hole-pairs departure performance, oxidation potential, and enhancement of visible-light absorption capability [105]. Li et al. have developed a dynamic and stable MAPbl₃/S-MXene photocatalyst system through the etching and exfoliation synthesis methods and received the highest H2 evaluation rate up to 3124 μmol/hrg6 times greater than that of pure MAPBI₃ [106]. From the above discussions, it can be concluded that MXene is a universal cocatalyst that can improve the generation rate of H₂ by providing different active sites. The application of MXene-based catalysts for water splitting is summarized in

Table 4. Summary of MXene-based catalysts for water splitting.

MXene-Based Catalyst	Experimental Condition	H2 Evolution Rates	Stability	Reference
BiVO4@ZnIn2S4/M-MXene QDs	M = 60 mg, T = 285 K, λ = 300 W Xe	102.67 μmol g−1 h−1	20 h/5 cycles	[38]
BiVO4/S-Ti3C2 nanosheets	M = 10 mg, T = 25 °C, V = 100 mL, λ = 300 W Xe	15.7 μmol	24 h/3 cycles	[115]
M-Ti3C2/TiO2 nanoflowers	M = 20 mg, V = 100 mL, λ = 300 W Xe	783.1 μmol g−1 h−1	30 h/3 cycles	[116]
M-Ti3C2/TiO2 nanoparticles	M = 30 mg, T = 25 °C, V = 40 mL, λ = 200 W Xe	2.65 mmol g−1 h−1	16 h/4 cycles	[117]
BPQDs/M-Ti3C2@TiO2	M = 50 mg, V = 40 mL, λ = 300 W Xe	684.5 µmol g−1 h−1	30 h/6 cycles	[118]
BiOBr/S-Ti3C2	M = 0.1 g, V = 100 mL, λ = 300 W Xe	8.04 mmol g−1	NA	[46]
TiO2/M-Ti3C2@AC composites	M = 40 mg, V = 100 mL, λ = 350 W Xe	33.4 µmol g−1 h−1	NA	[119]
S-Ti3C2/MAPbl3-HI	M =30 mg, V = 15 mL, λ = 300 mW/cm2 Xe	3124 µmol g−1 h−1	5 h/10 cycles	[106]
A/R TiO2/M-Ti3C2TX	M = 20 mg, λ = 300 W Xe	4672 µmol g−1 h−1	25 h/5 cycles	[104]
ZnXCd1-XS/M-MXene	M = 10 mg, V = 50 mL, λ = 300 W Xe	14.17 mmol g−1 h−1	NA	[105]
PCN/M-MXene	M = 10 mg, V = 50 mL, λ = 300 W Xe	2181 mmol g−1 h−1	15 h/5 cycles	[103]
Chl@S-Ti3C2TX composites	M = 3 mg, λ = 300 W Xe	106 μmol g−1 h−1	NA	[107]
NPT-TiO2/C/M-Ti3C2TX	M = 100 mg, V = 100 mL, λ = 300 W Xe	87.2 μmol g−1 h−1	33 h/240 cycles	[120]
C/TiO2/M-Ti3C2TX	M = 50 mg, V = 50 mL, λ = 300 W Xe	69 μmol g−1 h−1	16 h/4 cycles	[121]
C/TiO2/g-C3N4/M-Ti3C2TX	M = 20 mg, V = 100 mL, λ = 300 W Xe	1409 μmol g−1 h−1	10 h/3 cycles	[122]
g-C3N4/M-Ti3C2	M = 50 mg, V = 100 mL, λ = 300 W Xe	116.2 μmol g−1 h−1	15 h/5 cycles	[122]
MoS2/M-Ti3C2	M = 30 mg, V = 50 mL, λ = 300 W Xe	6144.7 μmol g−1 h−1	20 h/4 cycles	[123]
CDs/M-Ti3C2	M = 10 mg, V = 50 mL, λ = 300 W Xe	2407 μmol g−1 h−1	15 h/5 cycles	[124]
Znln2S4/M-Ti3C2	M = 20 mg, V = 40 mL, λ = 300 W Xe	3475 μmol g−1 h−1	NA	[125]
M2S@TiO2@M-Ti3C2	M = 10 mg, T = 25 °C, λ = 300 W Xe	193	24 h/3 cycles	[101]
TiO2QDs/S-Ti3C2	M = 20 mg, V = 30 mL, λ = 300 W Xe	62.50	NA	[102]

NA-Not Available.

.

4. MXene-Based Nanomaterials

MXene-based photocatalysts are created by combining an MXene photocatalyst with another type of photocatalyst or co-catalyst [126]. Improved space charge transfer and separation can be achieved by combining MXene with semiconductors and selecting optimal CB and VB potentials. These nanostructures include 0D/2D, 1D/2D, 2D/2D heterostructures, and other combinations. The band alignment of an MXene-based photocatalyst with a different type of photocatalyst can be tuned to one of three different types. There are three varieties of heterostructure band alignment, as indicated in Figure 7: Type I (straddling gap), Type II (staggered gap), and Type III (broken gap) [127]. The CB and VB are combined in one layer on Type I, resulting in a straddling band alignment. As a result, when the linked photocatalyst absorbs photon energy equal to or greater than its band gap energy, electron-hole pairs in a single component within the heterostructure photocatalyst separate and migrate [128]. As a result, because the CB and VB of component B are at a lower and greater energy than component A, electrons and holes will be transported and accumulate on component A. Two separate CB and VB monolayers contribute to the Type II heterostructure, resulting in upward or downward band bending [70].

Component B’s VB and CB potentials are higher than component A’s in Type II heterostructures due to the staggered band structure seen in Figure 7b [129]. Owing to the more negative CB position of B, photoexcited electrons (e⁻) can flow from B to A, whereas holes (h⁺) can travel oppositely from the more positive VB of A to B, resulting in overall efficient charge separation and improved photocatalytic activity [127]. On the other hand, the Type III heterostructure has no band gap or broken gap. Both the CB and VB of the coupled photocatalyst are not crosslinked in this situation, resulting in a gap between both band alignments in the heterostructure photocatalyst, which provides more substantial driving power for charge transfer [130]. Significant improvements in photocatalytic activity have been experimentally demonstrated using various MXene-based composites such as TiO₂-MXene, g-C₃N₄-MXene, and BiVO₄-MXene. The most widely used practice for synthesized photocatalyst composites is mechanical mixing, self-assembly, and in-situ decoration/oxidation [131]. Mechanical mixing offers electrostatic attraction in which negatively charged MXene is combined with a positively charged photocatalyst. At the same time, in situ decoration/oxidation provides a strong chemical bonding, which was essential in the particular design of applications. In general, the MXene-based heterostructure developed by coupling the MXene with other types of photocatalysts as a co-catalyst improves the electronic and optical properties of MXene [132].

4.1. MXene/TiO₂ Composites

Titania, TiO₂ was the most favourable and environmentally friendly promising photocatalyst, with a high photocatalytic activity that is closely related to its lattice structure. In this regard, this photocatalyst was capable of photoactivity reaction, a solid oxidising power, and high chemical/photo-stability. In the tetragonal unit cell of anatase, each titanium Ti atom is surrounded by six octahedra oxygen, O atoms. Although there are advantages such as a high carrier-forming capacity, toxicity, and good stability, TiO₂ charge carriers tend to show rapid rearrangement and poor performance in visible light owing to their considerable band gap energy, 3.2 eV [133]. Anatase TiO₂ also absorbs very little sunlight in the UV region (approximately 5%) owing to its considerable band gap energy [22]. Zhuang et al. produced a M-MXene/TiO₂ composite using an electrostatic self-assembly technique and tested it for H₂ evolution. During this preparation, Ti₃AlC₂ was etched by hydrofluoric acid (HF) 50 wt. % at room temperature for 2 h, as shown in Figure 8 [17]. MXene/TiO₂ produced good photocatalytic power for H₂ production owing to several factors, such as its enhancement in charge separation performance, reactive facet (101), and extension of light absorption [134]. The increased H₂ evolution could be due to the heterogenous interface between MXene and TiO₂ nanofibers [17]. MXene/TiO₂ composites have also been tested for environmental applications owing to carbon and titanium compounds that greatly benefit environmental remediation. Siwanat et al. used the microwave hydrothermal method to prepare M-MXene/TiO₂ composite in NaCl/HCl solutions and used it for the successful treatment of enrofloxacin from water. The addition of NaCl played a significant role in providing sodium ions, which intercalated over M-MXene/TiO₂ composite to enhance the adsorption and photocatalysis performance of antibiotics from water. Furthermore, the maximum removal of antibiotics over M-MXene/TiO₂ could be formed by hydroxyl radicals [66].

Therefore, a number of strategies were conducted to inhibit the electro-hole pairs recombination, such as metal doping and coupling with other semiconductors, metal oxide, and carbon materials. Superior conductive MXene with known narrow bandgap energies would facilitate receiving electrons generated from other semiconductors [135]. The migration of generated electrons from TiO₂ is favourable to increasing charge carriers’ separation efficiency and inhibiting electron-hole recombination. On the other hand, TiO₂ nanoparticles act as spacers [136].

Table 5. Summary of TiO₂-MXene-based photocatalyst synthesis techniques.

TiO2-MXene	Synthesis Technique	Key Finding	Reference
Safflower-shaped TiO2-M-Ti3C2	Hydrothermal + Ion exchange + Heat treatment	The layered MXene is broken into small pieces and agglomerates uniformly during hydrothermal and ion exchange. Then upon heat treatment, nanorods grow, shaping into microscale safflowers. The safflower-shaped TiO2-M-Ti3C2 decomposes 88% of RhB in 15 min and 95% after 60 min.	[35]
Nanoflower-shaped TiO2-M-Ti3C2	Integrated oxidation	The photocurrent of TiO2-M-Ti3C2 is 700 μA cm−2, which is significantly higher than M-Ti3C2. Nanoflower-shaped TiO2-M-Ti3C2 achieve 97% removal efficiency of RhB within 40 min compared to pure TiO2 and pristine M-Ti3C2, only 36% and 29%, respectively.	[31]
N-doped porous M-MXene/TiO2	Hydrothermal	Nitrogen doping further enlarges the interlayer distance of the porous M-MXene/TiO2 heterogenous film to promote electrolyte penetration and provide more active sites for energy storage. N-doped porous M-MXene/TiO2 demonstrates excellent storage performance with a specific capacitance value of 2194.33 mF cm−2 and 74.39% capacitance performance after 10,000 cycles.	[136]
M-Ti3C2/TiO2@f-MoS2	Solvothermal + Annealing + Hydrothermal	Few layered MoS2 enlarged the interlayer of M-Ti3C2 and prevented restacking beneficial for lithium-ion diffusion, while TiO2 effectively improved the cycling stability. M-Ti3C2/TiO2@f-MoS2 shows high storage capacity of 403.1 mA h g−1 after 120 cycles at 2 A g−1.	[137]
Glycine acid functionalized TiO2-S-Ti3C2 nano complex	In situ decoration	Luminol electrochemiluminescent (ECL) sensor for glucose constructed via covalent immobilization of glucose oxidase on glycine functionalized TiO2-S-Ti3C2. The ECL nano complex thrived and detected the glucose in human serum, fruits, and sweat were also consistent with D-glucose assay kits. The recovery rate of glucose was 98–100% for all samples.	[138]
NC-TiO2/M-MXene	Impregnation + Hydrothermal	Optimum hydrothermal time could attain NC-TiO2/M-MXene with high TiO2 content, whereas a too lengthy hydrothermal time could destroy the MXene substrate. NC-TiO2/M-MXene retains an excellent cycling ability of 157.5 mA h g−1 after 1900 cycles at 2 A g−1 with rate performance in sodium-ion batteries 100.1 mA h g−1 at 10 A g−1.	[139]
M-MXene@TiO2/ CuInS2	Hydrothermal	Photoexcited electrons on CuInS2, ternary sulfide semiconductors flow to M-MXene until their Fermi levels reach equilibrium and suggest a robust interfacial contact between TiO2 and M-MXene. The photocatalytic hydrogen evolution rate was delivered up to 356.27 μmol g−1 h−1, 69 and 636 times higher than MXene@TiO2 and CuInS2, respectively.	[140]

shows TiO₂/MXene composites with various synthesis techniques and their key findings.

4.2. MXene/g-C₃N₄ Composites

Graphitic carbon nitride (GCN), g-C₃N_4, is a metal-free photocatalyst with a medium band-gap energy of 2.7 eV, [141]. The polymeric nature of g-C₃N₄ was structurally analogous to graphene, which was considered the most stable C₃N₄ type at ambient conditions. In addition, this photocatalyst is also highly resistant to thermal and chemical environments, making it a stable material. Furthermore, this nature allows for multiple excitations from a single photon’s absorption, thus effectively producing reactive species for pollutant degradation. Ideally, the compositions of g-C₃N_4, consisting only of carbon and nitrogen atoms were reported with a C/N molar ratio of 0.75 [142]. In addition, g-C₃N₄ has a surface termination as defects and nitrogen atoms for electron localization. It has a unique microstructure in which anchoring inorganic and organic functional groups act as active sites [143].

Conversely, g-C₃N₄ suffers from easy recombination of photo-generated electron-hole pairs, which limits its sorption capacity due to the low specific surface area and quantum efficiency [144]. Moreover, the non-utilization of visible light (450 nm) and low efficiency of charge separation and transfer of g-C₃N₄ itself restrict its further applications. To overcome the bulk individual g-C₃N₄, more light collectors and charge transfer tunnels need to implement in the photocatalyst composites to boost their photocatalytic activity [145]. Yujie et al. reported the preparation of S-MXene/g-C₃N₄ composite using an electrostatic attraction approach and tested it for H₂ production, as shown in Figure 9a. The successful synthesis of S-MXene/g-C₃N₄ composite was confirmed through TEM analysis, as shown in Figure 9b,c. In characteristics, MXene produced a higher surface area of g-C₃N₄ and facilitated the density of active sites.

Moreover, MXene showed good electronic conductivity, which could enhance the composite’s photo charge transfer performance [146]. In another research, Syahmi et al. prepared an M-MXene/g-C₃N₄ heterostructure photocatalyst with different loading of M-MXene from 1 to 12 wt. % using a wet impregnation process. Their photodegradation results revealed that 1 wt. % M-MXene/g-C₃N₄ provided the highest removal of MB under visible light owing to its excellent photo charge carrier between MXene and g-C₃N₄ and higher surface area properties [147]. It is believed that the combination of MXene and g-C₃N₄ could significantly improve the photocatalytic performance for H₂ production and water remediation. Various reports have verified the advantages and excellence of g-C₃N₄-Mxene-based photocatalysts in the wide photocatalytic application, as summarized in

Table 6. Summary of g-C₃N₄-Mxene based photocatalysts synthesis technique.

g-C3N4-Mxene	Synthesis Technique	Key Finding	References
g-C3N4/M-Ti3C2/MoSe2 (CMX)	Calcination treatment	M-Ti3C2 acts as a mediator, and interface electron separation plays a vital role in highly efficient degradation. Photo absorption of CMX broadened to 601 nm and presented better visible light response compared to the absorption edge of pure GCN and MoSe2. CMX achieved effective visible light-induced >90% for removal of Lanoxin within 60 min.	[149]
M-Ti3C2/g-C3N4 nanosheets (TC-CN)	Sonochemical method	Photocatalytic N2 reduction and antibiotic degradation performance of TC-CN were improved by 3.64 times and 3.35 times, respectively. The photocatalytic performance achieved 601 μmol L−1 gcat−1 h−1 for NH4+ production, and levofloxacin successfully removed 72% within 30 min under visible light illumination.	[150]
g-C3N4/TiO2/ M-Ti3C2	In-situ oxidation + Ultrasonication	M-Ti3C2 bonded with TiO2 benefits the transfer and aggregation of photo-induced holes, whereas g-C3N4/TiO2 enhances photo-generated charges’ separation, improving the visible light photocatalytic activity. The g-C3N4/TiO2/M-Ti3C2 possess 66.3%, the best NO degradation compared to g-C3N4 (34.0%) and TiO2/Ti3C2 (28.0%).	[36]
Sm-doped g-C3N4/M-Ti3C2	Polymerization + solid mixture-calcination method	Sm-doping plays a role in transferring the photogenerated electrons to suppress the recombination, and M-Ti3C2 improves the carrier migration efficiency. Sm-doped g-C3N4/M-Ti3C2 presented higher photocatalytic degradation of ciprofloxacin under visible light irradiation with over 99% within 60 min.	[37]
P-doped tubular g-C3N4/S-Ti3C2	Supramolecular + calcination + Electrostatic self-assembly	Photocatalytic H2 evolution rate reached 565 μmol g−1 h−1, which was over 4.3 and 2.0 fold higher than bulk g-C3N4 and P-doped tubular g-C3N4. S-Ti3C2 flake with exposed terminal metal sites as co-catalyst exhibited higher photocatalytic in H2 evolution compared to the carbon materials.	[151]
Mesoporous g-C3N4/M-Ti3C2 (MCT)	Annealing	The large surface area of MCT (adsorption isotherm 43.2 m2/g) compared to M-Ti3C2 (adsorption isotherm 5.7 m2/g) provides abundant adsorption sites for CO2 molecules. The photocatalytic CO2 reduction reactions achieved excellent performance, producing 2.117 μmol g−1 h−1 CH4 and 3.98 μmol g−1 h−1 CO.	[152]
Protonated g-C3N4/S-Ti3C2	Acid treatment + Ultrasonication	Protonated g-C3N4/S-Ti3C2 exhibits narrower band energy of 2.41 eV, which is conducive to capturing visible light photons. As a result, in photocatalytic hydrogen production, the hydrogen evolution rate attained 2181 μmol g−1 h−1 compared to bulk g-C3N4 (393 μmol g−1 h−1) and protonated g-C3N4 (816 μmol g−1 h−1).	[153]

. Generally, hybridization aims to improve photocatalyst-specific surface area and charge separation.

4.3. MXene/BiVO₄ Composites

Bismuth vanadate (BiVO₄) is a photocatalyst material with various intrinsic features such as a suitable band gap, proper band location, excellent stability, and a friendly environment. Photocatalytic properties of BiVO₄ are strongly dependent on its crystal structure, which originates from the mineral pucherite in an orthorhombic structure [154]. Its improved photocatalytic performance was due to the better optical absorption achieved with a monoclinic scheelite structure with a band gap energy of 2.4 eV compared to a tetragonal structure with 2.9–3.1 eV [155]. This is the most significant photocatalyst for visible light water oxidation and photodegradation due to the narrow band gap, which can effectively oxidize water to release oxygen and generate active substances [156]. Primarily, BiVO₄-based materials were extensively used for toxic pigments in the coating and plastic industry and have been shifted into photocatalytic applications due to their intrinsic crystal structure [157].

Nevertheless, the position of the conduction band of BiVO₄ (E_CB = 0.3 eV) in a more favourable spot severely limits the reduction capacity of BiVO₄ and constraints in photocatalytic environmental remediation application [158]. Consequently, modest photocatalytic performance was observed in pristine BiVO₄ due to the low electrical conductivity. On the other hand, slow water oxidation kinetics of BiVO₄ occur due to the photogenerated holes on the photocatalyst surface, promoting the fast recombination of electron-hole pairs [127]. Herein, by modifying BiVO₄ photocatalysts with MXene by in-situ engineered heterojunctions, the hybrid composite system can control the charge transfer process. Co-catalyst modification was also a primary fashion to accelerate surface redox kinetics. Therefore, the photo-generated holes migrated to the surface of MXene and generated an oxidising reaction. Yan et al. prepared M-MXene/BiVO₄ composites through a spin coating of M-MXene over BiVO₄ film surface to improve their kinetics for oxygen evolution purposes, as shown in Figure 10a, and TEM images are shown in Figure 10b,c. The thin coating of M-MXene acts as a co-catalyst to promote photo charge transfer carrier performance and reduce the photogenerated recombination rate to improve water oxidation performance [159]. The synthesis technique and critical findings of various BiVO₄-Mxene-based photocatalysts are summarized in

Table 7. Summary of BiVO₄-MXene based photocatalysts synthesis technique.

BiVO4-MXene	Synthesis Technique	Key Finding	Reference
BiVO4@ZnIn2S4/M MXene QDs	Solvothermal + Hydrothermal + Ultrasonication	M-MXene QDs effectively promote light absorption, and the surface furnished with active sites enhances the redox reaction. BiVO4@ZnIn2S4/M MXene QDs accomplishes visible light-driven pure water splitting into O2 and H2 evolution rates up to 50.83 and 102.67 μmol g−1 h−1 (1:2). High photoactivity also achieved with 96.4% of Bisphenol A removal.	[38]
BiVO4/S-Ti3C2 nanosheets	Electrostatic self-assembly	BiVO4/S-Ti3C2 nanosheets exhibited an increase in H2 production of 15.7 μmol after 8 h, which was 10 times that of pristine BiVO4. At the same time, the O2 production within the same timeframe was 10.28 μmol and two times that of pristine BiVO4. This composite also showed excellent stability and there was no noticeable decrease in gas generation after three cycles.	[115]
M-Ti3C2Tx/BiVO4 electrodes	Spin coating + Annealing	The coating film of M-Ti3C2TX flakes onto the BiVO4 electrodes increased the photocurrent density to 3.45 mA cm−2 at 1.23 V. Therefore, M-Ti3C2TX/BiVO4 electrodes were observed at maximum photoconversion efficiency of 0.78% at 0.87 V and highest surface charge separation which reached 73% at 1.23 V compared to BiVO4 electrodes.	[159]
TiO2-S-MXene/BiVO4	H2O2 induced oxidation	The photoactive hybrid was applied to detect CD44 proteins (photoelectrochemical biosensors). TiO2-S-MXene/BiVO4 can recognise CD44 in wide concentrations of 2.2 × 10−4 ng mL−1 to 3.2 × 10−4 ng mL−1 with a low detection limit of 1.4 × 10−2 pg mL−1.	[160]
BiVO4/S-Ti3C2TX electrodes	Spin coating + Annealing	S-Ti3C2TX flakes coated thinly on the surface of BiVO4 act as co-catalyst to promote the charge transfer of photo-generated carriers and alleviate the charge recombination. BiVO4/S-Ti3C2TX electrodes (photoelectrochemical biosensor) demonstrated a wide linear range from 1 pM to 2 nM of Hg2+ with satisfactory accuracy and repeatability in a practical sample water.	[161]
Ultrathin BiVO4/M-Ti3C2TX	Electrostatic self-assembly	The essential feature of M-Ti3C2TX are large surface hydroxyl groups that can activate CO2 molecules. As a result, ultrathin BiVO4/M-Ti3C2TX reached 20.13 μmol g−1 h−1 of methanol production rate (4.1 times) higher than BiVO4. The recycling tests also exhibit reproducible photoactivity for all three cycles.	[162]

.

5. Multi-Roles of MXene-Based Nanomaterials

5.1. Photocharge Separation and Transfer Role

In photocatalysis, improving the separation efficiency of photogenerated charge carriers has been a hot research topic. MXene, as a co-catalyst, could prevent photogenerated electrons and holes from recombination in the photocatalytic system. The Schottky junction might be experimentally established between semiconductor photocatalysts and MXene [163]. According to theoretical estimates, electrons could move from g-C₃N₄ to MXene in their composites due to the difference in their Fermi levels (EF) and the development of Schottky junctions. Furthermore, because MXene showed strong metallic conductivity and favourable H adsorption-free energy, H⁺ may be converted to H₂, resulting in a significantly higher H₂ production. Ag₃PO₄ has a high carrier recombination rate during the photocatalytic process and is readily converted to elemental silver. In light of this, a Schottky junction was built, and MXene was introduced as a co-catalyst to improve the photocatalytic performance and stability of Ag₃PO₄. Surprisingly, after eight cycles, the photocatalytic clearance rate of tetracycline hydrochloride solution (TC-H) over the Ag₃PO₄/MXene composite marginally decreased, indicating improved anti-photo corrosion efficacy and photocatalytic activity.

5.2. Photocatalytic Active Sites

The surface characteristics of materials are widely recognized to significantly impact their adsorption and photocatalytic activities. MXene and MXene-based materials have 2D heterostructures that increase the contact area with other 0D, 1D, and 2D materials and provide more surface reaction sites for photocatalysis applications. MXene etched using HF solution typically has a large number of −F terminations, but those etched with a mix of HCl and LiF have a large number of −O terminations. Ran et al. studied the influence of surface terminations on the photocatalytic property of MXene composites. They found that changing −F to −O/−OH enhanced the number of active sites, enhancing the photocatalytic H₂ evolution [164]. Sun et al. also showed that MXene with −O terminations facilitated the separation of photogenerated charge carriers, resulting in a significant increase in H₂ generation. Density functional theory (DFT) simulations revealed that MXene with −O terminations had a lower Gibbs free energy and improved H₂ production rate than MXene with −F terminations. Although MXene with homogenous surface terminations has not been synthesized in the lab, the relative concentration of surface terminations can be controlled to improve photocatalytic stability [165].

6. Economic and Eco-Friendly Feasibility

MXene-based nanomaterials are attracting researchers’ interest for use in various disciplines due to their low cost of manufacturing and use and environmental friendliness. Figure 11 depicts MXene-based nanomaterials’ use to remove several contaminants. Different dyes such as methylene blue (MB), saffranine T (ST), neutral red (NR), antibiotics, heavy metals, and phenolic compounds are hazardous to the atmosphere and community. MXene-based nanomaterials’ active surface area, layered structure, and charged surface aid in removing hazardous substances from aqueous solutions. Several heavy metals, such as Pb(II), Cu(II), Cd (II), Cr(IV), and Hg(II), are inorganic and organic chemicals that are difficult to break down and accumulate in living things. MXene-based nanoparticles are used in this application because they include oxygen-containing groups and have a more extensive surface area for removing irons from effluent perfectly. Human respiratory infection and other health problems are caused by wastewater and certain volatile organic chemicals. MXene-based nanomaterials’ features, such as high adsorption energy and selectivity, help to remove toxic organic chemicals and hazardous water pollutants. Pu ions, uranium (II), and Neptune (Np) are radioactive constituents that are radiologically and chemically hazardous. They pollute the environment with nuclear waste. MXene-based nanomaterials are used to eliminate these radioactive materials because of their remarkable ability to resist higher radiation and good chemical compatibility. MXene-based nanomaterials also remove contaminants such as phosphates in the water, phenol, and highly charged cations, owing to their active surface morphology.

7. Regeneration Studies of MXene-Based Nanomaterials

When evaluating an adsorbent’s performance and cost-effectiveness, its ability to be recycled is a significant issue to consider. The adsorption capacities are highest in the early levels due to the accessibility of a greater surface area that gives free binding cores for the sorption of adsorbent materials. However, as the number of adsorption-regeneration rounds grows, pollutant removal and regeneration effectiveness decline. Several studies have investigated the regeneration studies of MXene-based nanomaterials, proving their extraordinary recyclability. Even more surprisingly, high-energy ball milling was discovered to resuscitate a few MXene-based adsorbents that absorb organic pollutants. Organic compounds do mineralize to graphitic and amorphous carbon during high-energy milling. This means that carbon-based components like carbides could be quickly revived using MC techniques. Vakili et al. evaluated the adsorptive characteristics of MXene sheets pillared by terephthalate. They also looked into whether or not the used adsorbent (after adsorption) could be reformatted into the MAX phase. After the colour had been absorbed, the wasted material was mostly titanium and carbon. A large percentage of this is due to the carbide MXene, but terephthalate pillars and methylene blue also play a role. As a result, the wasted adsorbent contained two of the three MAX phase components [92].

The used material was mixed with the appropriate amount of Al powder to resuscitate the MAX phase via MC manufacturing, and the MXene was then MC etched. The results show that the XRD pattern of the revived MXene was identical to that of the MXene produced from the MAX phase formed with graphite, demonstrating that T-MX can be regenerated indefinitely utilizing the MC process (passing through MC etching). As a result, the authors suggest that recyclable materials can be reused endlessly to eliminate organic contaminants from wastewater while ensuring their safe disposal. Furthermore, the recyclability of MXene for the removal of Cs ions was examined using 0.2 mol of HCl to desorb the particular ions. When the HCl solution was added, the number of H ions increased. The target contaminant and H ion adsorbed over MXene fought for exploitable surface sites, causing the pollutant to desorb from the MXene. In this experiment, the adsorbent removal percentage stayed over 90% after 5 days. The regenerations’ characterization remained essentially unchanged. However, when reincarnations increased, the removal efficiencies declined marginally, possibly due to a reduction in available functional sites on the adsorbent’s interface.

Furthermore, Shahzad et al. studied MXene by treating it with HNO₃ or calcium nitrate solution (Ca(NO₃)₂) and regenerating the depleted DL-MXene by the treatment. On the other hand, the adsorbent was used to remove Cu(II) from the water, which was then recycled. The MXene adsorption rate after the initial regeneration set was 80%, but after the second and third studies, the adsorption rate dropped to around 47% and 33%, respectively, which is a considerable decrease compared to the results of the similar study [97]. Furthermore, Dong et al. investigated the regeneration capacities of cross-linked and un-cross-linked MXene-based materials, finding that cross-linked MXene (with a reusability potency of roughly 10 cycles) could achieve a greater adsorption rate than un-cross-linked MXene [166].

8. Conclusions and Future Challenges

MXene-based photocatalysts have emerged as promising candidates in developing advanced photocatalytic systems to address growing global concerns regarding clean water and energy sources, owing to their unique characteristics, composition, and tunable structural properties. The method of synthesizing MXene-based photocatalysts and their environmental remediation applications for uses such as antibiotic removal, adsorbents, photocatalysts for dye removal, and water splitting have been discussed extensively to better understand their characteristics. The photocatalytic performances shown by MXene-based photocatalysts were mainly governed by their fascinating features such as adjustable morphology and bandgap, high hydrophilicity, great thermal and chemical strength, and large specific surface area. Consequently, the broad application of MXene as a co-catalyst was explored, for instance in TiO₂-MXene, g-C₃N₄-MXene, and BiVO₄-MXene. Current research exertions have been dedicated to improving the biocompatibility, stability, and ecological power of MXene in the aquatic organism to boost its effectiveness in water remediation applications. Initial research established that MXene has higher a biocompatibility with no toxicity, even at higher concentrations. It is urgent to comprehensively examine possible threats associated with MXene materials.

Although much recent research and studies of photocatalytic performances were conducted on MXene-based photocatalysts, they have been widely utilized as adsorbents and photocatalysts for the successful remediation of different organic pollutants such as dyes and metal ions from wastewater. Still, this new promising nanomaterial is yet to be fully exploited and less studied compared to other two-dimensional and graphene nanomaterials. Additional improvements on the expansion of MXene-based photocatalysts were desired, such as (1) developing safe synthesis routes with low cost and high MXene yield, (2) introducing spacers to control the interlayer spacing of MXene-based photocatalysts which avoid the aggregation and stacking of MXene sheets, and (3) analyzing the photocatalytic performances of MXene-based photocatalysts using natural industrial wastewater and practical applications. It is incredibly significant to discover newly developed MXene nanomaterials, which could be more environmentally effective in wastewater-related applications. Subsequently, different MXene nanomaterials are already available and have been theoretically projected. More MXene nanomaterials can develop and become accessible for water remediation and other environmental applications. More research is required to cover the gap between newly proposed and experimentally possible MXene nanomaterials, with the hope of signifying the applicant structure with improved thermal, physical, chemical, and catalytic characteristics, which can be more appropriate for water purification and other environmental remediation. Experimental confirmation of modelling estimation is a significant way to facilitate the preparation of structures with improved electrocatalytic, photocatalytic, and other helpful characteristics. These additional efforts will open new routes for commercial applications for MXene.