Recent progress in two dimensional Mxenes for photocatalysis: a critical review

Transition metal carbides and nitrides, generally known as MXenes have emerged as an alternative to improve photocatalytic performance in renewable energy and environmental remediation applications because of their high surface area, tunable chemistry, and easily adjustable elemental compositions. MXenes have many interlayer groups, surface group operations, and a flexible layer spacing that makes them ideal catalysts. Over 30 different members of the MXenes family have been explored and successfully utilized as catalysts. Particularly, MXenes have achieved success as a photocatalyst for carbon dioxide reduction, nitrogen fixation, hydrogen evolution, and photochemical degradation. The structure of MXenes and the presence of hydrophilic functional groups on the surface results in excellent photocatalytic hydrogen evolution. In addition, MXenes’ surface defects provide abundant CO2 adsorption sites. Moreover, their highly efficient catalytic oxidation activity is a result of their excellent two-dimensional nanomaterial structure and high-speed electron transport channels. This article comprehensively discusses the structure, synthesis techniques, photocatalytic applications (i.e. H2 evolution, N2 fixation, CO2 reduction, and degradation of pollutants), and recyclability of MXenes. This review also critically evaluates the MXene-based heterostructure and composites photocatalyst synthesis process and their performance for organic pollutant degradation. Finally, a prospect for further research is presented in environmental and energy sciences.


Introduction
Currently, there is a great focus on developing efficient, stable, and recyclable semiconductor catalysts to counter energy shortage, global warming, and environmental pollution challenges [1,2].There have been many semiconductor catalysts discovered over the past few decades, such as TiO 2 [3], ZnO [4], Fe 2 O 3 [2], graphitic carbon nitride (g-C 3 N 4 ) [5], and BiOX (X = Cl, Br) [6].The photocatalysts are used for the conversion of CO 2 into hydrocarbon fuel, the photocatalytic evolution of hydrogen, and the degradation of pollutants.The photocatalytic activity of these materials is not adequate to make them suitable for industrial applications due to the weak absorption of visible light in semiconductor materials, the rapid recombination rate of photogenerated electron-hole pairs, lack of active sites, inability to regenerate, and instability [7][8][9][10].As a result, a great deal of effort has gone into improving the photocatalytic activity and other desired properties of semiconductor catalysts.These efforts include element doping, surface sensitization, facet control, and assembly of heterojunctions [11].In many different strategies, heterostructure photocatalyst designs have been developed to improve the separation efficiency of photogenerated charge carriers [12].
Owing to the unique properties of MXenes, there is a rapid growth in research on the synthesis and use of MXenes and MXene-based nanomaterials for photocatalytic applications.Up to now, titanium carbide MXene (Ti 3 C 2 T x ) is the most studied MXene member and hence many recent articles have reviewed Ti 3 C 2 MXene's potential for energy conversion and storage applications including supercapacitors and batteries, however, a few have focused on its use in photocatalytic applications [46,59,63,[65][66][67][68].In 2021, Liqun et al reviewed the photocatalytic application of MXene [55,69].However, the author only focused on one member of the MXene family, Ti 3 C 2 .Moreover, the review is short and does not cover all the work published on the concentrated topic, and lacks critical analysis.To develop an innovative future framework, we believe it is necessary to carefully review, discuss, and analyze all the published work in this field.To our knowledge, no comprehensive review has been published covering the structure, synthesis, photocatalytic applications, and recyclability of MXenes.It is imperative to conduct a literature review that describes synthesis techniques and photocatalytic applications of MXenes along with their structure and reusability.Presenting the latest studies and developments in this important field along with challenges and recommendations will surely increase the attention of the scientific community.Thus, in this review, we have focused on the structure, synthesis, and photocatalytic applications of MXenes.Hydrogen (H 2 ) evolution, nitrogen (N 2 ) fixation, carbon dioxide (CO 2 ) reduction, degradation of organic/ inorganic pollutants, and recyclability of MXenes have been explored and discussed in detail.Finally, the challenges and future perspectives in research in MXenes have been discussed in this review.

Structure of MXenes
MXenes are new emerging 2D materials composed of transition metal carbides and nitrides exfoliated from MAX phases (M n+1 AX n ) (M = Mo, Ti, Zr, Cr, A = Al, Ga, Ge, Si, and X = C, N); the structure as well as the composition of MXenes depend on the MAX phases.Generally, MAX phases can be classified into various types based on the initial stoichiometric ratio of selected elements such as 211, 312, 413 etc depending on the value of n in the MAX phase which usually ranges between 1 and 3.The first MXenes, Ti 3 C 2 T x, exfoliated from the Ti 3 AlC 2 (312) MAX phase was reported by Naguib et al [70,71].MXenes have exhibited tremendous structural properties which could make them an excellent potential classe of 2D materials in the field of photocatalysis.The following properties of MXenes could play important roles in photocatalysis: (a) 2D MXenes have a large surface area, which boosts catalytic activity and flexibility of the catalyst.(b) Planar structures of 2D MXenes provide abundant surface atoms and enough space for compounding with semiconductor photocatalysts.(c) The main structural feature of 2D MXenes is that its thickness is extremely low.
MXenes have a hexagonal structure like the ternary nitrides and carbides phase (MAX phase) since they are derived by etching the A atoms of their precursor material.MXene crystals have a hexagonally close-packed (HCP) structure.It has been noted that M 2 X MXenes exhibit an HCP sequence (ABABAB), while M 3 X 2 and M 4 X 3 have a face centered cubic (FCC) sequence (ABCABC) [71].An atomic arrangement like this is important when synthesizing MXenes that involve M elements with HCP ordering in bulk states.For instance, a recent density functional theory (DFT) calculation has revealed greater stability for hexagonal molybdenum carbides (Mo 3 C 2 T x or Mo 4 C 3 T x ) compared with their FCC counterparts [72,73].Furthermore, based on the formation energies of MXenes, it is found that Mo 3 C 2 T x and Mo 4 C 3 T x are unstable because their M atoms are arranged like those of rock salt; however, Mo 2 CT x is stable.The unstable Mo-C bonds are prevented by inserting another transition metal, Ti, into the structure, which bonds with C and prevents them from becoming unstable.By doing so, a collection of ordered double-M element 2D carbides are formed, such as (Mo 2 Ti)C 2 T x and (MoTi 2 )C 3 T x [74,75].In addition, figures 1(a)-(c) shows the three different parts of the structure of Ti 3 C 2 T x : the interlayer skeleton, the interlayer structure, and the surface terminating groups [69,76,77].An intramolecular skeleton is formed when Ti atoms are stacked with C atoms alternately to create ionic bonds, which is the basis for the entire main structure [78].Neutron diffraction studies completed on the interlayer region demonstrated that the interaction between the layers is developed by hydrogen bonds between either the F or O atoms on the surface, as well as van der Waals forces between the atoms [79].It is not only the orientation of -OH in the sheet that affects the strength of the interlayer hydrogen bonding, but also the number and distribution of -OH groups.Whenever water molecule occurs between layers, it is also capable of hydrogen bonding.Additionally, the Ti 3 C 2 T x main structure is covered by a large number of randomly distributed terminal groups [69].The characteristics of the Ti 3 C 2 T x produced are greatly influenced by the surface groups, which may be studied using electron energy loss spectroscopy (TEM), neutron scattering, and nuclear magnetic resonance spectroscopy (NMR) methods [80,81].MXene terminations are generally randomly distributed and mixed, which depends majorly on the synthesized conditions, transition metal elements, and post-synthesis treatments [7].
As far as their morphology is concerned, most MXenes exhibit accordion-like multilayer structures.Nevertheless, MXenes with different structures have also been synthesized [7].As shown in figures 1(d) and (e), Naguib et al [70] produced scroll-like Ti 3 C 2 T x with radii less than 20 nm by using sonication for exfoliation.Additionally, the ethanediaminetreated flower-like Ti 3 C 2 T x was prepared by hydrothermal treatment for 8 h at 80 • C (figure 1(f)) [82].The presence of MXene nanofibers and nanoribbons has been observed in some cases [7].

Synthesis of MXenes
MXenes are emerging 2D nanomaterials derived from MAX phases (M n+1 AX n ) which are ternary nitrides and carbides [83].A MAX phase can be fabricated by combining an element from early d-block transition In comparison to the M-X bonds, the M-A bonds are generally weaker and more reactive, therefore the MXenes are fabricated after removing A-elements from the MAX phases [88].In contrast to the van der Waals interaction found in layered materials such as graphite and transition metal dichalcogenides, the metal bond created between the M n+1 X n layers is much stronger [46].The higher strength and rigidity of MXenes are advantageous to their long-term performance for photocatalytic applications.In 2011, Naguib et al [70], synthesized the first MXene by etching Ti 3 AlC 2 with hydrofluoric acid (HF), while up to now several MXenes have been synthesized via different advanced techniques as shown in figure 2(b) [87].By wet-chemical etching MAX phases, these subsequent MXenes are formed.The most common methods of etching include HF etching, fluoride etching, and a few others.The synthesis methods of  MXenes may affect their efficiency in various applications as shown in table 1.

MAX phase etching by hydrofluoric acid
Most MAX phases can be etched with hydrofluoric acid at various temperatures to produce different MXenes, depending on the different HF parameters such as concentration and environmental conditions like room temperature (figures 3(a)-(d)) table 2 [70,84,[97][98][99][100].HF has very high selectivity as an etchant, allowing it to selectively remove distinct kinds of SiC [101].The etching conditions for the MAX phase containing Al vary based on different factors such as particle size, atomic bond, and structure of transition metal [102].As an example, in the most commonly investigated Ti 3 C 2 T X , 0.5 g Ti 3 AlC 2 MAX phase is progressively introduced to 10 ml etchant and agitated with a Teflon magnetic bar at room temperature [103].The slow addition of Ti it is important to recognize that using high concentrations of HF as an etchant can cause a lot of defects in the Ti 3 C 2 T X nanosheets, and a prolonged etching period can also damage the MXene's layered structure [105,106].The following model equations can be used to illustrate the fabrication of MXenes [83,107]: In the first stage, the MXene layers are formed after the reaction of A layers of MAX phases with HF or fluoride ions, while in the second stage the MAX phases' A atoms are substituted with -OH, -O, and/or -F groups.During the third stage reaction, the interaction between the M n+1 X n layers becomes weak causing loosely packed, graphitelike layered structures to form.In the presence of surface functional groups (-OH, -O, and/or -F), MXene surfaces become negatively charged, resulting in the formation of stable dispersions (flake sizes range from 1 to 10 µm) [83].Several novel MXenes, including Mo 2 C, Ti 2 C, and Ti 3 SiC 2 , have been prepared using the HF etching method [108].Furthermore, because HF is a very corrosive acid, its extensive use as an etchant is not only hazardous for, humans but also pollutes the environment.Moreover, this process cannot synthesize stable MXenes (Ti n+1 N n ) based on nitride-based metals [109].Therefore, it is necessary to develop environmentally friendly and more versatile protocols for MXene synthesis.
HF is highly toxic, reactive, and corrosive, therefore, it is hazardous for regular use in the lab, and hence, its direct use should be avoided.As an alternative to the direct use of HF, MXenes can be produced by an etching agent prepared by combining fluoride salts (such as MF and FeF 3 ) and strong acids (H 2 SO 4 and HCl) [109].Fluoride salts react in situ with the strong acids to produce hydrogen fluoride through the process of etching, the strong acids provide the H + and the fluoride salts release the F − .In the meantime, cations and water molecules can intercalate into MXenes, increasing interlayer distance and weakening interlayer contact, resulting in material delamination during sonication [117].Hence, this approach simplifies the process that involves the use of concentrated HF and a timeconsuming multi-step procedure, allowing a onestep synthesis of MXenes.Yin's group has developed Ti 3 C 2 T x by etching, intercalating, and exfoliating Ti 3 AlC 2 powder with HCl/fluorides in water under ultrasonication as shown in figure 3(e) [110].Similarly, Ghidiu et al [116], effectively produced Ti 3 C 2 T x clay using an HCl/LiF combination as the etching agent.During the etching process, LiF interacts with HCl (equation ( 4)) to generate HF, and the quantity of HF may be precisely controlled without harming the Ti 3 C 2 T x structure.In comparison with pure HF, the LiF and HCl mixture performed better as an etchant, and the produced Ti 3 C 2 exhibited no nanoscale defects [116,118].Monolayered or multilayered MXenes can be obtained immediately by washing away the reaction products, as in situ produced HF etches the Ti 3 AlC 2 [119].Notably, a claylike paste obtained from this process can be repeatedly rolled into flexible and free-standing films, showing the material has good flexibility and strength as shown in figure 3(f) [111].Likewise, MXenes like Ti 2 C [120], Ti 3 CN [121,122], Cr 2 TiC 2 [122], and (Nb 0.8 Zr 0.2 ) 4 C 3 [30] were also synthesized by etching other MAX phases with LiF and HCl mixed solution: In addition, a combination of various fluoride salts such as MF, FeF 3 , CaF 2 , and acids might also be used as etchants to synthesize MXenes.Furthermore, difluorides such as KHF 2 or NH 4 HF 2 are viable etchants for Ti 3 AlC 2 .In Ti 3 AlC 2 , both etching, and intercalation of cations take place concurrently, and the cations can enhance Ti 3 C 2 T x interplanar space, maintaining the 2D flake structure of the material [104].
As reported in related publications, MXenes can also be synthesized by etching MAX phases using a hydrothermal process.Peng et al [123] reported the preparation of two types of 2D MXenes (Nb 2 C and Ti 3 C) without the direct use of HF, and using instead, NaBF 4 and HCl hydrothermal etching as a method of preparation.In the hydrothermal environment, NaBF 4 interacts with HCl to produce HF in situ.
The MAX phase is then etched with HF as a fluorine source, generating the corresponding MXenes, namely Ti 3 C 2 T x and Nb 2 C [87,123].Comparing fluoride etching with HF etching, fluoride etching exhibits more advantages, such as bigger flakes, fewer defects, higher interlayer spacing due to cation intercalation, and simpler operation.As an additional benefit, the sonication treatment can directly create single layers or a few layers of MXenes without using any organic molecular reagent [124].In contrast with MXenes obtained by HF etching, this method is unable to produce the accordion-like morphology of MXenes, which require higher etching temperatures and longer etching times.Furthermore, the etching process is dynamic, and the conditions for conversion for different kinds of MXenes may vary.

MAX phase etching by other methods
Besides etching methods using HF and fluoride salts, several new etching techniques for MXene manufacturing have recently been discovered [125].In a recent report, Li et al [126], described a typical alkali-hydrothermal method for the preparation of Ti 3 C 2 T z (T = OH, _ O) MXene (figure 4(a) [103]).This approach is based on the Bayer process, which is widely utilized in the bauxite refining industry.The entire process is fluorine-free, and multilayer Ti 3 C 2 T z with a purity of approximately 92% (via 27.5 M NaOH, 270 • C) was formed.It was the first time that an alkali etching method was used to synthesize highpurity multilayer MXenes [126].The hydrothermally fabricated Ti 3 C 2 T x has a larger interlayer distance and a higher specific surface area than Ti 3 C 2 T x synthesized through conventional HF etching, because of the hydrothermal slow-release mechanism.Since the hydrothermal etching method avoids the need for high consistency HF, it is also more effective for synthesizing Ti 3 C 2 T x thin films [127].The electrochemical etching technique has been shown to have a promising etching impact on MAX precursors, allowing for the selective removal of layer nano components.Sun et al [128], proved an effective electrochemical etching route for preparing Ti 3 C 2 T x MXene.A layer of Ti 3 C 2 T x MXene on Ti 2 Al was formed after electrochemically etching Al from porous Ti 2 AlC in diluted hydrochloride acid (figure 4(b)).The MXenes only contain -Cl terminal groups, along with the more common -O and -OH groups.Furthermore, the electrochemical etching process can also over-etch the parent MAX phases into carbide-derived carbons (CDC).To produce MXenes without over-etching, a careful balance is required in the etching parameters [128].In addition, electrochemical etching products are hydrophilic in absence of fluorine terminations due to the HF-free etching.This method overcomes the etching energy barrier, gives a flexible range of etching conditions, and allows a variety of etchants to be used.
The electrochemical etching method offers a more environmentally friendly route for the production of 2D MXenes with an accurately controllable surface [127,129].The introduction of an electrochemical etching technique using diluted hydrochloric acid to produce MXenes provides a viable solution to the long-standing problem of excessive HF concentration.Nevertheless, for large-scale production to be successful, issues about low yield need to be resolved.
In a recent study, for the first time, researchers reported a new route of MXene synthesis via the Lewis acidic etching system [130,131].The Ti 3 SiC phase was immersed in Cu Lewis molten salt at 750 • C (figure 4(c)) [132].Ti 3 C 2 Cl 2 MXene in powdered form was obtained after the chemical reaction between Ti 3 SiC 2 and CuCl 2 .Then Ti 3 C 2 Cl 2 was further washed with ammonium persulfate ((NH 4 ) 2 S 2 O 8 ) to obtain the final product MS-Ti 3 C 2 Cl 2 (molten salt-MXene) [131].The chloride ion plays a crucial role in Lewis acidic etching as it does in electrochemical etching.This technique can potentially be used with any Lewis acid salt with a greater redox potential than the corresponding A element [133].Lewis acidic etching system enables the fabrication of new 2D materials that are impossible or extremely difficult to obtain via conventional synthesis methods.This method broadens the variety of MAX-phase precursors that may be employed, and it opens up new possibilities for fine-tuning the surface chemistry and characteristics of MXenes [134].
MAX etching is also reported using halogen as an etchant.In Shi et al's work, the authors used I 2 to etch Ti 3 AlC 2 at 100 • C with anhydrous acetonitrile, resulting in Ti 3 C 2 I x that resembles an accordion-like structure [135].A solution of 1 M HCl was applied afterward to remove any residual AlI 3 formed during the etching process.In the HCl solution, -I groups are transformed to -OH and -O due to the instability of the surface groups.This method enabled the production of fluorine-free Ti 3 C 2 T x (T = O, OH).The recent research of Jawaid and colleagues reported that Ti 3 AlC 2 was etched in organic solvents containing halogens and interhalogens, providing homogeneous Cl, Br, or I terminations on synthesized MXenes (figure 4(d)) [136].The halogen groups on the surface were not converted as the etching process was conducted inside a glove box and the final products were stored in tetrahydrofuran solvent [134].

Delamination of MXenes
The synthesized MXene is produced in multilayers and delamination is required to produce MXene sheets with just one or a few layers.In the etching process, hydrogen bonds and van der Waals forces substitute the strong M-A bonds, so multi-layer MXenes can be layered by adding intercalators.Despite this, MXenes exhibit much stronger interlaminar interactions than graphite, it still has approximately 2-6 times the strength of graphite [138,139].Therefore, the transparent tape method cannot form a single layer of MXene by mechanical peeling.A common method of delaminating MXenes is to intercalate the material before mechanical stirring.The interlayer spacing of Ti 3 C 2 T z flakes can be widened using intercalation compounds, which results in weaker interactions between the 2D layers [140,141].An appropriate solvent for the intercalating agent and material should be carefully chosen during the intercalation and delamination processes.Two steps are involved in introducing the intercalator into the 2D sheet.Using the ultrasonic treatment and centrifugation steps combined in this step, it is possible to separate the layered material from the multilayer (nonlayered).The desired sheet size and density determine the ultrasonic processing stage.The resulting colloidal solution will comprise scattered 2D sheets of electrostatically stabilized MXenes that are processable and functional, as well as stable against aggregation or clumping [142].Furthermore, the steps of ultrasonic processing are adapted according to the type of etching techniques used and the application requirements.To produce smaller flakes with more flaws, it is necessary to increase the ultrasonic power and duration of the ultrasonic process.MXene concentration in the solution is also affected by the technique and type of intercalator that is used during synthesis [87,143].Even though ultrasonic treatment was utilized to delaminate the few-layer thick MXenes in initial reports, the delaminated layers had a low yield because MXenes interact strongly with each other.Therefore, it is crucial to break the prevailing interlayer forces to separate the stacked nanosheets of MXenes.It has proven feasible to weaken interlayer interactions and increase the interlayer spacing between these layers by injecting organic molecules or inorganic ions into them [134].
Various organic solvents such as dimethyl sulfoxide (DMSO), hydrazine monohydrate (HM), and N,N-dimethylformamide (DMF), have been used as intercalates for multilayer MXene exfoliation [142].Even though DMSO exhibited promising results when used as an agent for intercalation of Ti 3 C 2 T x , it does not affect the intercalation of other MXenes, such as V 2 CT x or Mo 2 CT x .In contrast with DMSO, the organic bases with relatively large molecule tetrabutylammonium hydroxide (TBAOH), hydroxyl choline, and n-butylamine, exhibit universal intercalating nature [144].Han et al [145] reported that the bond interaction between the Ti-Ti and Ti-Al inside the Ti 3 C 2 T x layers are the main impediments besides the van der Waals forces.In a hydrothermal process, TMAOH is diffused and intercalated into multilayer MXenes, resulting in their subsequent delamination figure 5(a).Ascorbic acid was utilized as a moderate reductant to prevent MXenes from oxidizing at high temperatures.Figure 5(b) shows the AFM image of bilayered 2D Ti 3 C 2 T x nanosheets with a thickness of 1.7 nm (figure 5(c)), which confirmed the successful fabrication [145].Furthermore, the TMAOH could intercalate and delaminate the MXene with the help of microwaves, although the yield of monolayer MXene nanosheets was relatively low, limiting its preparative applications [146,147].Since TMAOH is one of the main ingredients in commercial etchants for Al.Xuan et al reported that the high reactivity between TMAOH and Al atoms made TMAOH a suitable etchant and intercalator for synthesizing monolayers of Ti 3 C 2 T x from Ti 3 AlC 2 , where other organic intercalators (DMSO, urea, and hydrazine) were not effective [134,148].Currently, delamination of multilayer MXenes is typically performed on products generated by HF etching or other aqueous etching processes.Therefore, further exploration is needed to determine how organic intercalators can be used in non-aqueous systems.

Basic principle of photocatalysis
Photoelectrochemical devices were introduced by Fujishima and Honda in 1972, which decompose water into oxygen and hydrogen when exposed to visible light.Furthermore, they claimed that water splitting could take place regardless of the voltage applied, as long as certain conditions were met [149].In general, photocatalytic reactions can be broken down into the following five main steps (a) light absorption, (b) charge separation, (c) charge migration, (d) interfacial charge transfer, and (e) surface reaction [150].The efficiency of photocatalysis can be influenced by each of these stages.To improve and optimize the photocatalytic reaction, each factor needs to be tuned for practical application.In an aqueous solution, three major active groups participate in the photocatalytic reaction: OH (hydroxyl radical), h + (hole), and •O − 2 (superoxide radical), where OH is the main oxidant [151].There is a notable fact that electrons can only be excited from the valence band (VB) to the conduction band (CB) as long as the energy of the incident photons (hν) exceeds or equals the bandgap (E g ) of the photocatalyst.A negatively charged high-reactive electron (e − ) is generated in the CB, leaving an electropositive hole (h + ) in the VB, thus resulting in the photoexcited electrons and holes.The incident light's wavelength must fulfill the following criteria [7]: In the equation, c stands for velocity of light, λ for wavelength, and h for Planck's constant, while E g denotes the semiconductor bandgap.
TiO 2 and ZnO are the most commonly used photocatalysts, however, their wide band gaps make them less effective in sunlight.Only 9.3% of the solar light consists of ultraviolet light, which can generate electron-hole pairs in these semiconductors.Many advances were made toward the synthesis of visible-light-active TiO 2 and ZnO utilizing various approaches, including surface alteration and bandgap generation [67].Currently, many studies are being carried out using semiconductors having bandgaps suitable for the absorption of visible light of the solar spectrum for photocatalysis [68].However, recent studies on MXenes and MXene-based materials show high electrical conductivity, large specific surfaces, and abundant metal exposure.MXenes and MXenebased have the potential as a cocatalyst to develop photocatalytic activity by enhancing the partitioning of photoexcited charge carriers [152].MXenes are not usually semiconductors and due to their high electrical conductivity, they can provide an excellent platform for semiconducting materials to create composite/heterostructure photocatalysts.So, during photocatalysis, irradiation excites electrons in the semiconductor and photogenerated electrons flow into the conduction band quickly because MXene has a promising electron-trapping property that lets electrons and holes partition efficiently.The electrons from the MXenes layers interact with oxygen to produce O 2 − , while the h + interacts with water to produce OH radicals [153].Carbon dioxide and water are both formed when superoxide radicals and hydroxyl radicals react with pollutant molecules [154].The present review analyzes the use of MXenes in the field of photocatalysis from four research sections: hydrogen evolution reactions (HER), CO 2 reduction reactions (CO 2 RR), N 2 fixation, and pollutant degradation.

Photocatalytic hydrogen evolution
As global warming and energy shortages affect human life globally, they have become worldwide concerns during the last few decades.H 2 gas is a clean combustion fuel and offers a higher energy density compared to fossil fuels, making it an ideal energy source to replace fossil fuels [155,156].The energy density of diesel is 45.5 mega joules per kilogram (MJ kg −1 ), which is slightly lower than the energy density of gasoline, which is 45.8 MJ kg −1 .The energy density of hydrogen is approximately 120 MJ kg −1 , almost three times greater than that of diesel or gasoline [157].Water electrolysis, coal gasification, electrocatalysis, and photocatalysis are four main methods that are used to prepare H 2 to date.Photocatalytic H 2 evolution is considered promising because it is sustainable and does not produce secondary pollutants [155].H 2 evolution process consists of three steps, namely, (a) the initial formation of h  [159].However, the photocatalysts exhibit relatively low light utilization capacity and fast recombination of photogenerated carriers, which limits their practical application [160].In this context, the development of new photocatalysts for H 2 evolution has great significance.In recent years, MXenes have been reongnized as promising candidates for photocatalytic H 2 production [69,140].Based on simulation calculations, it was discovered that ∆G H * = 0.927 eV when all Ti 3 C 2 T x surface groups are -F, and strong adsorption occurs.A catalyst with all surface groups as -O has a |∆G H * | of 0.003 eV, which is superior to the commonly used catalyst Pt (∆G H * -0.090 eV).The most desirable value for |∆G H * | should be zero [161,162].Apart from contributing to the three steps of HER, Ti 3 C 2 T x can also assist in electronhole separation because of the hydrophilic group on its surface, suitable Gibbs adsorption free energies, and excellent electron transfer efficiency [69,163].MXenes anchored onto the monolayer of Ti 3 C 2 T x were used as photocatalysts that enabled the photocatalytic evolution of H 2 over TiO 2 .The effects of Ti 3 C 2 T x on the photocatalytic activity of TiO 2 were evaluated, by irradiating various TiO 2 /Ti 3 C 2 T x nanocomposites with light and using CH 3 OH as a hole scavenger for photocatalytic H 2 evaluation [164].In general, TiO 2 /Ti 3 C 2 T x nanocomposites displayed better photocatalytic performance than pure TiO 2 , because of Ti 3 C 2 T x 's ability to separate photoinduced charge carriers.In the presence of 5% monolayer Ti 3 C 2 T x , the maximum amount of H 2 was produced (2.71 mmol g −1 h −1 ) that was approximately nine times greater than that of pure TiO 2 particles (0.29 mmol g −1 h −1 ).This is due to the enhanced separation of photogenerated e − /h + in TiO 2 , as verified by photoluminescence and transient photocurrent responses, as well as electrochemical impedance spectrum measurements [165].Additionally, quantum dot (QDs) Ti 3 C 2 T x or monolayer Ti 3 C 2 T x are both more active in H 2 evolution.
As a hole scavenger, lactic acid was used to study the effect of MXene on the photocatalytic hydrogen production of ZnCdS under visible light irradiation.Compared with ZnCdS nanoparticles and TM, ZTNM composites improved the efficiency of photocatalytic hydrogen production significantly, and the optimal hydrogen production efficiency of ZTNM-3 was 3.3 times that of ZnCdS nanoparticles [166].
Despite its advantages, monolayer Ti 3 C 2 T x has several disadvantages such as (a) its preparation is complex, (b) the catalyst has low structural stability and is easily oxidized, and (c) monolayer or fewlayer structures are difficult to manipulate.At present, few-layered structures are fabricated electrostatically (self-assembly) or using in situ growth.In situ growth significantly improves the stability of composite catalysts over electrostatic self-assembly [69,155,167].Xiao et al [168] fabricated a 1D/2D CdS/Ti 3 C 2 heterojunction by in situ solvothermal methods for H 2 generation.1D/2D Schottky heterojunctions between CdS and Ti 3 C 2 provide accelerated charge separation as well as a lower Schottky barrier for solardriven hydrogen evolution from water splitting due to their specific interface characteristics.The CdS/Ti 3 C 2 nanosheets are seven times more active than the CdS nanorods in photocatalytic hydrogen evolution suggesting a synergistic response between n-type semiconductor CdS and 2D Ti 3 C 2 MXene.Yang et al [169] found that Ti 3 C 2 MXene@TiO 2 /CuInS 2 (M@T/CIS) exhibited a 356.27 µmol g −1 h −1 rate of H 2 evolution, which is 69 and 636 times higher than that of M@T and CIS, respectively, highlighting its potential as photocatalysts for H 2 evolution.The following synergistic effects were attributed to the enhanced photocatalytic H 2 evolution activity increased lightharvesting ability, enhanced charge separation by Ti 3 C 2 /CIS Schottky junction and large number of active sites on the surface of Ti 3 C 2 MXene, due to its numerous functional groups.
In a recent study, photocatalytic H 2 evolution over hierarchical Ti 3 C 2 T x MXene@In 2 S 3 -NiS is presented in which NiS is coupled with a Schottky heterojunction of Ti 3 C 2 T x MXene@In 2 S 3 using photodepotition technique as sown in the figure 6 [170].Morphology of the synthesized nanocomposites are is depicted in the SEM and TEM images as shown in the figures 6(a)-(c).It was reported that H 2 generation rate over the optimized In 2 S 3 -3%NiS was approximately 29.0 times higher that that of bare In 2 S 3 (figure 6(e)).Coupling of MXebe to form Ti 3 C 2 T x MXene@In 2 S 3 -NiS nanocomposites resulted in further improvement in H 2 generation (figure 6(f)).In another study, Huang et al [171] prepared P-doped tubular g-C 3 N 4 /Ti 3 C 2 (PTCN/TC) MXene by selfassembly approach.The H 2 evolution rate was highest for PTCN/TC (2.20 µmol g −1 h −1 ), as it was 4.3 and 2.0 times higher than that of pure bulk g-C3N4 and PTCN, respectively.The metallic Ti 3 C 2 served as a sink for electrons and a collector for photons in this composite.Furthermore, ultrathin Ti 3 C 2 flakes that showed exposed terminal metal sites as a cocatalyst displayed higher photocatalytic reactivity in H 2 evolution than carbon materials.Very recently, Cao et al [172] reported the successful fabrication of MXene/ZnxCd1-xS photocatalysts for H 2 evolution.The resulting photocatalysts provided excellent photocatalytic performance, with the best rate of H 2 evolution at 14.17 mmol g −1 h −1 .High-efficiency photocatalysis is attributed to the enhanced separation ability of photocarriers and optimal band structure with enhanced oxidation capacity of the valence band of MXene/ZnxCd1-xS photocatalysts.As shown in table 3, Ti 3 C 2 T x results in a greater  yield of H 2 compared to the base catalyst alone.In summary, hydrogen production doubled after loading with Ti 3 C 2 T x .The reason for this incredible promotion is mainly due to the following three aspects of Ti 3 C 2 T x , (a) it provides a high throughput channel as a co-catalyst during which the excited electrons can pass through while holes cannot, (b) its hydrophilicity, and (c) its adsorption on hydrogen has a Gibbs free energy of zero.It is important to dedicate more resources to studying MXene QDs as opposed to their sheet counterparts.Thus, a greater understanding of MXene QDs and their influence on activity is highly desired both from an experimental and computation perspective.

Photocatalytic reduction of CO 2
Photocatalytic CO 2 reduction has received enormous attention in recent decades to simultaneously address the greenhouse effect and the energy shortage.The conversion of CO 2 into CH 4 , CO, CH 3 OH, HCHO, and HCOOH has been studied extensively using a variety of photocatalysts [187][188][189][190][191]. The reduction of CO 2 with photocatalysts is still challenging due to the low utilization rate of carriers, the low thermodynamic stability of CO 2 molecules, and the insufficient adsorption and activation of CO 2 molecules.Therefore, a highly efficient photocatalyst with high CO 2 reduction activity must be developed [60].Several recent studies suggested that MXenes may serve as a co-catalyst in photocatalytic CO 2 reduction due to its remarkable physicochemical properties.The potential of MXene for photocatalysis has been demonstrated by numerous theoretical and experimental studies recently [72,[192][193][194].The Ti 3 C 2 T x MXenebased semiconductor is gaining wide attention as a photocatalytic substrate that is capable of outperforming graphene and noble metals as a co-catalyst in CO 2 reduction.In addition, Ti 3 C 2 T x can allow fast separation of photogenerated charge carriers for light-harvesting materials, which can achieve high photoconversion efficiencies.MXenes offer a large surface area, enriched active adsorption sites, and a good interface contact to allow efficient charge carrier separation [60].Furthermore, the Ti 3 C 2 T x layer exhibits a variety of terminated functional groups, such as O, OH, C, and F, encouraging the need for specific functionalization [72].Photocatalytic CO 2 reduction follows a five-step process: light adsorption, charge separation, CO 2 adsorption, surface redox reaction, and product desorption [195].Charge separation occurs during the recombination of electrons and holes when the photocatalyst's CB is greater than the redox potential of CO 2 [69].There are several factors dictating which of these two competing processes is predominant.As a result of CO 2 adsorption and photogenerated electrons and holes moving from the crystal structure to the surface, the catalyst undergoes a redox reaction.Following the de-attachment of the product, the CO 2 reduction reaction from photocatalysis is completed [196,197].Recently, Li et al [173] fabricated g-C 3 N 4 /MXene (MCT) for photocatalytic CO 2 reduction, which showed excellent performance.As a gray powder, MCT was obtained under an N 2 atmosphere by heating g-C 3 N 4 and Ti 3 C 2 T x mixed in a 10:3 mass ratio respectively at 250 • C for 2 h.After treatment with HF, it is evident that the Al layers in Ti 3 AlC 2 have been stripped away.The mesoporous surface morphology of mesoporous g-C 3 N 4 allows gas molecules to be adsorbed at numerous sites due to its large surface area.Ti 3 C 2 T x has enhanced contact with mesoporous g-C 3 N 4 due to their morphologies, that resulted in remarkable improvements in the electron separation as well as the efficiency of multiple electron reactions including CO 2 reduction.As a result of illumination to excite electrons of the semiconductor into the excitation state, Ti 3 C 2 T x MXene comprises a capacitor that coordinates heterojunction effects.Due to the rapid transfer of electrons, electrons and holes are separated more easily.The steady rise and stable decline of the photocurrent corresponding to the ON and OFF states of the light, respectively, reflect the improved separation of photogenerated electron-hole pairs in Ti 3 C 2 and its electron reservoir capabilities.In this case, Ti 3 C 2 MXene is a cocatalyst that enhances charge separation and provides the active sites necessary for the reduction reaction.As a result of the photocatalytic activity of the MCT sample for CO 2 reduction, several organic products were generated, among which CO and CH 4 were the dominant product.Mesoporous MCT produces 2.4 times more CH 4 than mesoporous g-C 3 N 4 (MC).This evidences the excellent selectivity and activity of MCT.Additionally, a high-speed electronic system in the MXene catalyst enables MCT to also effectively separate charge carriers [173].In another study ZnO loaded Ti 3 C 2 MXene compsites (Ti 3 C 2 -OH/ZnO) was synthesized by a facile electrostatic self-assembly method and used as a co-cocatalyst for CO 2 photoreduction [198].Formation of Ti 3 C 2 -OH/ZnO composite structure due to electrostatic self-assembly between negatively charged Ti 3 C 2 -OH and positively charged ZnO was revealed by the SEM and high resulation TEM images (figures 7(a) and (b)).The surface-alkalinized Ti 3 C 2 -OH/ZnO played an important role in increasing the transfer efficiency of of photoinduced charge carriers and ultimately impoving the CO 2 molecules adsorption, resulting in higher photocatalytic CO 2 reduction as depicted in figure 7(C).It was reported that evolution rates of CO and CH 4 was 30.30µmol g −1 h and 20.33 µmol g −1 h, respectively, which are much higher than the earlier reported values.
In another study, Tahir et al [199] fabricated 2D porous g-C 3 N 4 (PCN) coupled exfoliated 3D Ti 3 C 2 T A MXene (TiC) nanosheets with TiO 2 nanoparticle (NPs) via in-situ growth in a single step through HF treatment approach (figure 8(a)).Figures 8(b) and (c) depict efficacious fabrication of PCN/TiC which can augment the area of interfacial interaction for facilitating the migration of charge carriers.Importantly, the TiO 2 NPs were visible on the Ti 3 C 2 nanosheets.PCN/TiC showed excellent performance for photocatalytic CO 2 reduction by   The CO and CH 4 evolution results were 9.9 and 6.7 folds higher using 10 TiC-48/PCN composite than pristine sample PCN (figures 8(d) and (e)).
The reason for this is the lower separation efficiency of photogenerated charges with pure PCN, so only a smaller number of electrons can combine effectively with CO 2 for activation of the gas.In composite materials, photoluminescence analysis (PL) is an effective method of investigating charge separation and recombination efficiency.In pristine PCN, PL intensity is very high due to the high recombination probability of the single charge carrier, while the PL emission peaks of samples TiC-24, TiC-48, and TiC-96 are significantly smaller than those of PCN.It might be due to the metallic nature of MXenes as well as its dark color.In contrast, PL intensity was significantly reduced when PCN was anchored to TiC-48 to form a TiC-48/PCN heterojunction composite.The composite samples demonstrate efficient charge carrier separation by using Ti 3 C 2 T x MXene decorated with TiO 2 NPs and attached to 2D porous g-C 3 N 4 sheets [199].Moreover, a highly efficient 'storage capacitor' is created when Ti 3 C 2 T x forms a heterojunction with g-C 3 N 4 .Electrons transfer quickly to Ti 3 C 2 T x on the semiconductor surface, but holes cannot.As a result, the material's photocatalytic performance is greatly improved, and the electron-hole recombination is greatly reduced.Meanwhile, abundant defects on the surface of Ti 3 C 2 T x act as excellent adsorption sites for CO 2 [193].Low et al [200] prepared TiO 2 /Ti 3 C 2 composite for photocatalytic reduction of CO 2 .The TiO 2 was tightly bonded to the Ti 3 C 2, and the composites had a rice crust appearance.It was found that the optimized TiO 2 /Ti 3 C 2 composite produced CH 4 at a higher rate than commercial TiO 2 and also had very good photostability.According to the experimental and theoretical results, Ti 3 C 2 's ultrahigh electrical conductivity and light absorption properties allowed carriers to be separated efficiently [200].
The above examples show Ti-MXene's potential for highly selective photocatalytic CO 2 conversion, which is a positive signal that they will have broad applications both theoretically and computationally shortly.Thus, coupling various semiconductive photocatalysts to Ti 3 C 2 T x requires extensive research.In addition, simulation of a structure with such properties will provide a better understanding of Ti 3 C 2 T x 's adsorption, activation, and charge transfer mechanisms.However, the use of Ti 3 C 2 T x is relatively smaller in photocatalytic reduction of CO 2 than in photocatalytic hydrogen evolution.The reason is because of its instability and the fact that it has its carbon resources, which can interfere with the photocatalytic reduction of CO 2 .The results listed in table 4 demonstrate that among limited reports, Ti 3 C 2 T x with both single-layered and multilayered structures displays an obvious production enhancement.

Photocatalytic N 2 fixation
Nitrogen is an essential component of all living organisms in the form of proteins and nucleic acids.Despite the high concentration of nitrogen in the atmosphere, most living organisms are mostly able to use the nitrogen element as ammonia (NH 3 ) or NO − 3 .In the industrial sector, ammonia is a crucial ingredient in fertilizers and is produced through the Haber-Bosch process, which uses N 2 and H 2 as feedstocks.In light of the solid bonding between the NN bonds (945 kJ mol -1 ), such a process requires a high quantity of energy, causing a variety of environmental and energy issues.Recent developments in photocatalytic N 2 fixation utilizing solar energy have been considered a promising option for generating ammonia under mild environmental conditions.In contrast, rapid photogenerated charge carriers and N 2 activation are strongly contested to this technology.Recently, MXene photocatalysts have been used in the photocatalytic N 2 fixation reaction to address these issues.
Recently, Fang et al [209] prepared BiOBr/ MXene-Ti 3 C 2 composite catalysts using electrostatic adsorption and self-assembly.BiOBr/Ti 3 C 2 prepared in 10 wt.% exhibits the best performance for the photocatalytic fixation of N 2 .BiOBr/Ti 3 C 2 showed up to 234.6 µmol g -1 h -1 evolution rate of NH 3 , which is approximately 48.8 and 52.4 times greater than that of pure BiOBr and Ti 3 C 2 , respectively.With the presence of designed double oxygen and titanium vacancies for BiOBr/Ti 3 C 2 , localized electrons are capable of adsorbing and activating N 2 , having the ability to be reduced to NH 3 by the interfacial electrons of BiOBr/Ti 3 C 2 .Furthermore, the in-situ Fourier transform infrared results indicate that continuous protonation processes lead to the generation of N x H y species.Additionally, according to density functional theory calculations, titanium vacancies (VTi) induce high absorption energy for nitrogen atoms on the surface of BiOBr/Ti 3 C 2 .Particularly, the P-electron feedback caused by VTi can elongate the N 2 bond by 31.6% by effectively weakening the N≣N triple bond.Moreover, from the above study, the built-in electric field is demonstrated to drive charge transfer at the local interface after the coupling of BiOBr and Ti 3 C 2 T x .As a result of the electron trap (Schottky junction) between BiOBr and Ti 3 C 2 T x , a special structure is formed.By capturing and accumulating photosensitive electrons, it can assist in promoting multielectron NRR.On top of that, the large specific Brunauer-Emmett-Teller (BET) surface area of MXene [51] in heterojunction materials and their black appearance can provide a high level of 'reaction sites' and improve the light adsorption capacity [209].
In another study, Chang et al [210] employed a controlled solvent-driven approach to synthesize partially reduced layered Ti 3 C 2 T x MXene and integrate it with Au nanospheres in a sandwich-like structure (figure 8).There are many low-valence titanium sites on r-Ti 3 C 2 T x , which serve as active sites for capturing and activating N 2 molecules.By taking advantage of the embedded Au nanospheres, the activated N 2 is reduced with plasmonic hot electrons.It is also important to note that the sandwich architecture prevents the Ti 3 C 2 T x layers from selfstacking, allowing the active sites to be exposed for utilization.The r-Ti 3 C 2 /Au has an incredibly high N 2 photo-fixation activity due to the abundance of Ti (4−x)+ active sites and localized surface plasmon resonance (LSPR) effect working together.To utilize Ti 3 C 2 MXene for N 2 photo-fixation, the Ti 3 C 2 MXene's active sites are important for N 2 adsorption and activation [110].Similarly, an RuO 2 -loaded TiO 2 -MXene was developed by Hao et al [211] for photocatalytic N 2 fixation.Their method uses TiO 2 , Ti 3 C 2 T x , and RuO 2 as light collectors, electron mediators, and N 2 adsorbents, respectively.In the present study, RuO 2 NPs were primarily located on the Ti 3 C 2 T x matrix rather than on TiO 2 .Therefore, upon irradiation with light, photogenerated electrons transferred first from TiO 2 to Ti 3 C 2 T x , and then migrated to RuO 2 for reduction of the adsorbed N 2 on the RuO 2 .It is possible that such intermediate photogenerated charge carrier transfers during the sample preparation could contribute to extending the lifetime of the various photogenerated charge carriers.Consequently, RuO 2 /TiO 2 /Ti 3 C 2 displayed significantly higher photocatalytic N 2 fixation activity than the TiO 2 , RuO 2 , TiO 2 /Ti 3 C 2 , and TiO 2 /RuO 2 , respectively [211].In short, photocatalytic N 2 fixation using Ti 3 C 2 -based photocatalysts is a viable way to improve NH 3 production through photocatalytic N 2 fixation.However, MXene-based photocatalysts have been controversial regarding their photocatalytic N 2 fixation mechanism.The key question is how N 2 molecules adsorb on Ti 3 C 2 T x MXene.In this regard, future experiments, and studies on the N 2 adsorption model for Ti 3 C 2 -based photocatalysts are highly desired.Additionally, in general, very little work has been done on MXenes for photocatalytic N 2 fixation, and the yield rate needs to be increased (table 5).

Photocatalytic degradation of pollutants
Photocatalytic degradation occurs through the generation of oxidizing holes by photocatalytic semiconductor materials.Oxidation of dissolved oxygen can lead to the formation of superoxide radicals, singlet oxygen, and hydroxyl radicals.It is possible for these species to directly oxidize substrates.The Ti 3 C 2 T x material has a wealth of surface groups and active sites, making it a good adsorbent for many substrates.Therefore, Ti 3 C 2 T x has been particularly attractive as a photoactive degradation catalyst and has been investigated by several researchers.For example, the photocatalytic structure of ZnCdS/TiO 2 /Na-MXene composites was prepared by Qin et al [218] to obtain excellent photocatalysts with high photo-corrosion resistance.MXene was wrapped by Na + and ZnCdS nanoparticles under hydrothermal conditions, slowing down further oxidation of Ti 3 C 2 MXene into TiO 2 and improving MXene utilization.A ZnCdS/TiO 2 /Na-MXene composite with an optimal adsorption capacity to methylene blue reached 299.68 mg g −1 , confirming that the MXene content was crucial for the removal of an organic dye.The reported adsorption capacity was achieved by performing a dye adsorption experiment under dark conditions.Further, the degradation capabilities of ZnCdS/TiO 2 /Na-MXene composite for methylene blue were assessed under UV light and the synthesized composites exhibited excellent photocatalytic degradation of dye in a short period.ZnCdS layer intercalation increased the basal distances in ZnCdS/TiO 2 /Na-MXene from 10.59 Å to 15.21 Å, which could encourage the transportation and adsorption of organic pollutants.ZnCdS/TiO 2 /Na-MXene showed 9.3 times higher degradation efficiency than pure ZnCdS in 120 min [218].Another research group preprared Ti 3 C 2 T x (001-T/MX) photocatalyst by hydrothermal treatment of Ti 3 C 2 T x (MXene) and utilsied it for the photodegradation of carbamazepine (figure 9).The adsorption of of carbamazepine onto pristine MXene and 001-T/MX composite wa about 5% and 7%, respectively, which is very lower removal efficiency (figure 4(c)).Whereas, photodegredation removal rate was very higher under UV light irradiation as showed in the figure 9(b).
Besides, Zou et al [219] prepared MXene-Ti 3 C 2 /MoS 2 composites to be used under visible light illumination for the first time in photocatalytic degradation of ranitidine (RAN) and to reduce nitrosamine dimethylamine (NDMA) formation potential (NDMA-FP).These composites were analyzed in terms of the morphology, chemical composition, and structural properties which revealed the generation of a heterojunction between MoS 2 and Ti 3 C 2 T x .This separated electron-hole pairs and facilitated charge transfer, resulting in improved photocatalytic performance.MXene-Ti 3 C 2 /MoS 2 composite showed the highest RAN degradation and mineralization efficiency in 60 min, with an NDMA-FP of 2.01%.The degradation of RAN was greatly aided by radicals, including the radicals •O 2 − , h + , and •OH, but it was the •OH radicals that were responsible for the majority of the photocatalytic activity.
In addition, Sharma et al [5] reported 2D ternary ZnO-B i 2WO 6 -Ti 3 C 2 for photocatalyst degradation of colorless pharmaceutical pollutants.X-ray diffraction confirmed the formation of the individual materials and nanocomposites, while electron microscopy determined the 2D morphology.ZnO-Bi 2 WO 6 -Ti 3 C 2 under sunlight irradiation, exhibited a maximum 77% degradation of ciprofloxacin in 160 min.According to the mechanistic analysis, electrons flow from ZnO and Bi 2 WO 6 to Ti 3 C 2 T x during nanocomposite formation, indicating that the MXene plays a vital role in separating photogenerated charges and improving the photocatalytic activity.In addition to increasing the efficiency of photogenerated charge carrier separation, Fang et al [220] found that Ti 3 C 2 T x could enhance the adsorption capacity of Ag 2 WO 4 for antibiotic pollutants such as tetracycline hydrochloride and sulfadimidine.The adsorption and photocatalytic reactions were simulated using dark and light conditions, respectively, for testing the removal of antibiotic pollutants.Ti 3 C 2 T x showed negligible direct adsorption capacity for pollutants under dark conditions, but it improved the dispersion and prevented the aggregation of Ag 2 WO 4 , thus increasing its adsorption capacity for antibiotics.Combining enhanced adsorption capacity and improved photogenerated carrier separation efficiency, Ag 2 WO 4 /Ti 3 C 2 T x exhibited significantly higher photocatalytic degradation activity compared to both pristine Ti 3 C 2 T x and Ag 2 WO 4 .Table 6 reports a summary of the photocatalytic degradation of contaminants using MXenes.

Recyclability of MXene
The recyclability of MXenes and the ability to retain their properties are among the most important research subjects in terms of efficiency and cost minimization in recent years [221,222].The feasibility of regeneration has been demonstrated using hydrochloric acid, HNO 3 , or Ca(NO 3 ) 2 , alcohol, and thiourea [222][223][224][225].For instance, Khan and Andreescu [222] found that recycling Ti 3 C 2 T x with 0.2 M HCl to remove Cs ions resulted in a sorbent removal rate of more than 90%.According to Shahzad et al's [226] experiment, the authors achieved nearly 100% reusability of MX-SA 4:20 with 8 M hydrochloric acid.In another study, it was found that the MXene/alginate structure is more stable than calcium nitrate crosslinking, which leads to higher absorption rates than non-cross-linked compounds [227].It is crucial to determine whether a photocatalyst can be stable since its photo-corrosion strictly limits its application [228].
A cycling test was performed for the degradation of antibiotics using the Ag 2 WO 4 /Ti 3 C 2 MXene to verify their reusability.After three cycles, a clear  TiO 2 /Ti 3 C 2 T x showed that their photocatalytic H 2 evolution rate dropped from 2.7 to 2.3 mmol g −1 h −1 after four cycles of reaction [229].Although Ti 3 C 2 T x nanoparticles are generally unstable in water containing dissolved oxygen, Ti 3 C 2 T x nanoparticles could be stabilized by removing the dissolved oxygen with Ar gas [230].Therefore, dissolved O 2 was removed through de-aeration with ultrapure Ar gas before photocatalytic H 2 evolution could occur.Ti 3 C 2 T x nanoparticles are less likely to oxidize when exposed to photo-induced e − .In such reaction circumstances, Ti 3 C 2 T X can be fairly constant.An XRD pattern of the used TiO 2 /Ti 3 C 2 T x after photocatalysis showed no notable change in the crystal structure.Nonetheless, as compared to the clean sample, there was a modest decrease in utilized TiO 2 /Ti 3 C 2 T x light absorption, likely due to TiO 2 particle detachment from the nanocomposites [165,229].

Challenges and future perspectives
MXenes have been attracting enormous attention in the fields of photocatalysis and many others such as energy storage, ion batteries, and sensor technology.
The study of MXenes is still in its early stages, and its prospects are primarily as follows: (a) To fabricate Ti 3 C 2 photocatalysts, a multistep process that combines Ti 3 C 2 T x with other composites before hybridizing them is usually utilized, which greatly slows down the fabrication process.There is a need to develop novel fabrication methods to reduce the synthetic time and maximize the feasibility of experiments.(b) Theoretically, there exists a wide variety of MAX precursors, which can be prepared experimentally, and a larger variety of MXene types must be explored to enrich the MXene family.(c) Specifically, the problem lies in the instabilities of the composite material, which result in unstable photocatalytic performances.Several existing solutions employ single-layer or fewlayer structured materials.(d) MXenes band structure must be explored to expand the material chemistry, structural diversity, and potential applications of these materials.MXenes are predicted to be direct band gap semiconductors theoretically, but their experimental synthesis continues to present challenges.(e) Till date, research activities have been focused on hydrogen evolution and pollutant degradation, but little research has been accomplished on oxygen evolution, water splitting total, oxygen reduction, etc. (f) The development of more active composite catalysts should be prioritized, as well as extending their application to other emerging fields.(g) MXenes can be paired with efficient and cheap semiconductors to enhance their photocatalytic performance and commercial viability.
Further applications of Ti 3 C 2 T x MXene in photocatalysis depend upon the material's development.Various methods to enhance the stability of the Ti 3 C 2 T x MXene structure should be investigated, starting with synthetic approaches.Moreover, Ti 3 C 2 T x MXene should be further explored for its hydrophilicity as well as its rich surface groups, particularly in liquid phase photocatalysis.
Since the first report of MXenes in 2011, this research area has been growing rapidly.In recent years, MXenes have received considerable attention for photocatalytic applications for their inherent characteristics, which include superior hydrophilicity, metal-like conductivity, large surface area to volume ratio, excellent microwave absorbing properties, and rich surface chemistry.In this review, we have summarized the recent development of synthesizing Ti 3 C 2 T x MXenes, and Ti 3 C 2 T x -based photocatalysts for different environmental decontamination applications such as hydrogen evolution, MXene has been studied for many years, there is still much to learn about its photocatalytic behavior and applications.Considering its numerous advantages and characteristics, Ti 3 C 2 T x is a compound that needs further investigation, and it is expected to play a very significant role in the field of photocatalysis.

Figure 2 .
Figure 2. (a) Elements known to form MAX Phases.Reprinted from [84], Copyright (2019), with permission from Elsevier.(b) Schematic of the progress in MXene synthesis over the first 10 years.Reprinted from [87], Copyright (2021), with permission from Elsevier.

Figure 3 .
Figure 3. HF etching methods used to obtain MXenes: (a) FESEM image of untreated Ti3AlC2 MAX phase and (b) SEM image of Ti3C2Tx after HF treatment, Floride etching methods used to obtain MXenes.(a), (b) Reprinted from [100], © 2020 Elsevier Ltd and Techna Group S.r.l.All rights reserved.(c) Schematic of synthesis of MXene clay and electrode.(d) Schematic illustration for the preparation of MXene via HCl/fluorides etching, intercalation, and exfoliation under H2O sonication.(c), (d) Reprinted with permission from [110].Copyright (2019) American Chemical Society.

Figure 8 .
Figure 8.(a) Schematic illustrating the synthesis process.(b) SEM image of Ti3C2/Au.(c) SEM image of r-Ti3C2/Au.(d) Reaction rate of ammonia concentrations under white light illumination.(e) Reaction of ammonia under visible light exposure.(a)-(e) Reproduced from [210] with permission from the Royal Society of Chemistry.(f) Ammonia production under both lights conditions.

Table 1 .
Different synthesis methods of MXene and applications.

Table 2 .
Different parameters of HF etching.
[104] 2 MAX is preferred, to inhibit the formation of excessive bubbles during the exothermic reaction.Hydrogen bonds and van der Waals bonds keep the layers of the fabricated MXene powders (2-dimensional) together[104].However, [158]].As of today, a large number of photocatalysts, including metal sulfides, titanium dioxide (TiO 2 ), barium titanate (BaTiO 3 ), g-C 3 N4, and Sr 2 Ta 2 O 7 , have been investigated for H 2 evolution + + e − (b) the production of H * and (c) the formation of 1/2 H 2 .H * adsorption state directly impacts the final hydrogen evolution efficiency in step (c) and is one of the most important factors that can be represented by the Gibbs adsorption free energy |∆G H *

Table 4 .
Summary of the typical photocatalysts for CO2 reduction.CO and CH 4 products.As compared to TiC-24/PCN, TiC-96/PCN, and PCN composite samples, 10TiC-48/PCN produced CO and CH 4 at the maximum rates of 317.3 and 78.55 µmol g −1 h −1 . generating

Table 6 .
Summary of photocatalytic degradation of pollutants.