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Review

Review of Two-Dimensional MXenes (Ti3C2Tx) Materials in Photocatalytic Applications

by
Haidong Yu
1,
Haibing Jiang
1,*,
Shuji Zhang
1,
Xin Feng
1,
Song Yin
1 and
Wenzhi Zhao
2
1
Langfang Natural Resources Comprehensive Survey Center, China Geological Survey, Langfang 065000, China
2
Harbin Natural Resources Comprehensive Survey Center, China Geological Survey, Harbin 150039, China
*
Author to whom correspondence should be addressed.
Processes 2023, 11(5), 1413; https://0-doi-org.brum.beds.ac.uk/10.3390/pr11051413
Submission received: 22 March 2023 / Revised: 22 April 2023 / Accepted: 28 April 2023 / Published: 6 May 2023
(This article belongs to the Special Issue Recent Advances in Functional Materials Manufacturing and Processing)
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Abstract

:
MXenes (Ti3C2Tx) have gotten a lot of interest since their discovery in 2011 because of their distinctive two-dimensional layered structure, high conductivity, and rich surface functional groups. According to the findings, MXenes (Ti3C2Tx) may block photogenerated electron-hole recombination in the photocatalytic system and offer many activation reaction sites, enhancing the photocatalytic performance and demonstrating tremendous promise in the field of photocatalysis. This review discusses current Ti3C2Tx-based photocatalyst preparation techniques, such as ultrasonic mixing, electrostatic self-assembly, hydrothermal preparation, and calcination techniques. We also summarised the advancements in photocatalytic CO2 reduction, photocatalytic nitrogen fixation, photocatalytic hydrogen evolution, and Ti3C2Tx-based photocatalysts in photocatalytic degradation of pollutants. Lastly, the challenges and prospects of Ti3C2Tx in photocatalysis are discussed based on the practical application of Ti3C2Tx.

1. Introduction

The study of 2D nanomaterials has grown in popularity since the discovery of graphene in 2004 [1,2]. Because of graphene’s superior optical and electronic properties, researchers are particularly interested in studying 2D nanomaterials with layered structures and other multifunctional characteristics, such as transition metal disulfides (MoS2, WS2) [3,4], g-C3N4 [5,6], layered double hydroxides (LDHs) [7,8], etc. Additionally, this 2D structure can significantly reduce the migration distance between the carrier and the reaction interface, prevent the recombination of holes and photogenerated electrons, and enhance photocatalytic performance [9]. In recent years, MXenes have recently become one of the most popular 2D nanomaterials due to their diversified element composition, distinctive 2D structure, wide surface area, rich surface terminal groups, and good photoelectron characteristics [10,11,12,13,14,15]. Mn+1XnTx (MXenes) is a 2D layered structure material obtained by etching Mn+1AXn (n = 1–3) phase, where M represents a transition metal (such as Ti, Nb, V, Ta and Mo), A represents the third and fourth main group elements (such as Si, Ga and Al) in the periodic table, X represents carbon or nitrogen, and T represents its surface functional groups, such as -O, -F, -OH, etc. As shown in Figure 1, more than 50 MXenes have been experimentally synthesized since the discovery of Ti3C2Tx MXene in 2011 [16]. Ti3C2Tx has emerged as the most extensively utilized MXenes material out of the several MXenes materials identified due to its reasonably established preparation conditions.
2D Ti3C2Tx MXenes materials demonstrate that they can produce stronger and larger contact between the bulk phases and semiconductors, which often results in low conjunction at the interface, compared to other 2D layer materials (such as graphene and g-C3N4). The separation of charge carriers might be severely limited by the lower and weaker conjunction [18]. Graphene’s oxidation will reduce its electronic conductivity and make carrier transfer less reliable, and the reduction process may interfere with certain of its electronic features [19]. However, Ti3C2Tx MXenes can easily get over these issues.
There are many methods of synthesis of MXene materials. Direct HF etching and indirect in situ HF etching, which combine HCl and LiF, are the processes most frequently employed to create MXenes [20,21]. In addition, high-temperature etching, chemical vapor deposition, molten-salt etching, and delamination with different organic intercalants (such as dimethyl sulfoxide [22], isopropylamine [23], and tetramethylammonium hydroxide [24]) have also been investigated to increase the production of MXenes. For multilayered MXenes, the primary method is HF and sometimes the delamination with intercalants; however, for delaminated MXenes, the most popular method is MILD etching. Figure 2 shows the chronology of MXenes produced using different etching techniques [25]. Ti3C2Tx materials have been widely used in research fields such as supercapacitors, gas sensors and electrocatalysts. In addition to the above applications, in recent years, Ti3C2Tx materials as co-catalysts or precursors have also set off a huge research upsurge in the field of photocatalytic environmental remediation and energy conversion. The single-layer Ti3C2Tx MXenes material offers the following benefits over the multi-layer Ti3C2Tx MXenes material: 1. It might expose more active spots since it has a bigger surface area; 2. The smaller thickness of the single-layer Ti3C2Tx decreases the distance between photoinduced electrons moving to the catalyst’s surface, further lowering the combinative ratio of charge carriers, and enhancing the photocatalytic performance [26]. 3. Since water molecules may easily absorb on the surface of the single-layer Ti3C2Tx material due to its high waterproofness, photocatalytic hydrogen generation can occur more effectively.
Little research has been conducted on the application of Ti3C2Tx-based photocatalysts in photocatalysis. However, a few reviews have been published on the synthesis and use of Ti3C2Tx materials thus far. Therefore, the most recent preparation and application advances of Ti3C2Tx-based photocatalysts in photocatalysis are discussed in this work to better understand the use of Ti3C2Tx materials in the field of photocatalytic environmental remediation and energy conversion. This review first introduces the synthesis methods of Ti3C2Tx-based photocatalysts and then systematically reviews recent research on the environmental and energy applications of Ti3C2Tx-based photocatalysts, such as photocatalytic degradation of organic pollutants, photocatalytic hydrogen evolution, photocatalytic reduction of CO2, and photocatalytic nitrogen fixation. Finally, the issues raised by using Ti3C2Tx materials in photocatalytic environmental remediation and energy science are examined, as are the opportunities for future study.
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Figure 2. Timeline of MXene etching techniques since Gogotsi’s team discovered Ti3C2 for the first time in 2011 [25].
Figure 2. Timeline of MXene etching techniques since Gogotsi’s team discovered Ti3C2 for the first time in 2011 [25].
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2. Synthesis of Ti3C2Tx-Based Photocatalysts

In recent years, one of the most successful approaches to manufacturing high-efficiency photocatalysts has been the fabrication of composite materials. Ti3C2Tx is an attractive study item in photocatalyst preparation due to its unique properties. Mechanical/ultrasonic mixing, electrostatic self-assembly, calcination, and hydrothermal/solvothermal treatment are the most generally utilized procedures for producing Ti3C2Tx-based photocatalysts.

2.1. Mechanical/Ultrasonic Mixing

The mechanical/ultrasonic mixing method is the most basic and widely used method for producing Ti3C2Tx-based photocatalysts. To maintain intimate contact between Ti3C2Tx and the photocatalyst interface, the synthesis method of intense mechanical stirring or high-power ultrasonic vibration is typically used [27]. Liu et al. [28] produced a Ti3C2Tx/g-C3N4 composite by ultrasonic, centrifugation, and drying after combining g-C3N4 aqueous solution with Ti3C2Tx aqueous solution, as shown in Figure 3a. The 2D layered structure of Ti3C2Tx/g-C3N4 did not change appreciably when varied quantities of Ti3C2Tx and g-C3N4 were combined. Similarly, Tahir et al. [29] used an ultrasonic technique to create g-C3N4/Bt/Ti3C2Tx photocatalyst composites. They began by combining the g-C3N4 solution with the Bt suspension. The g-C3N4/Bt mixture was then treated with a small quantity of Ti3C2Tx while vigorously stirred. The precipitate produced after centrifugation was dried at 100 °C for 24 h after being ultrasonically treated for 60 min. The entire procedure is depicted in Figure 3b. The foregoing studies show that following mechanical/ultrasonic mixing treatments, the contact surfaces of 2D g-C3N4 and 2D Ti3C2Tx may be intimately connected.

2.2. Electrostatic Self-Assembly

Electrostatic self-assembly combines two oppositely charged solutes to produce a single or multilayer film on a substrate. The surface of the Ti3C2Tx material is heavily charged with negative charges. Constructing with positively charged semiconductors via electrostatic interaction is simple, resulting in 0D/2D, 1D/2D, or 2D/2D Ti3C2Tx-based photocatalysts. Because monolayer Ti3C2Tx materials are particularly prone to agglomeration, ultrasonic treatment is frequently required prior to mixing. For example, Zheng et al. [30] sonicated 0D CdxZn1−xS (CZS) with 2D Ti3C2Tx, and the potential of CZS was +29.6 mV, whereas Ti3C2Tx nanosheets had a potential of −22 mV. The CZS/Ti3C2Tx composite is created due to the electrostatic attraction between the positively charged CZS nanoparticles and the negatively charged Ti3C2Tx nanoparticles, as illustrated in Figure 4a. The picture also shows that CZS nanoparticles are evenly distributed over the surface of Ti3C2Tx nanosheets. Li et al. [31] recently synthesized the 1D/2D CdS/Ti3C2Tx composites in Figure 4b using an electrostatic self-assembly technique. By electrostatic attraction, Ti3C2Tx nanosheets and CdS nanowires may create a strong link, and the CdS nanowires are evenly dispersed throughout the Ti3C2Tx nanosheets.
The fabrication of 2D/2D structured photocatalysts has gained increasing interest in recent years. 2D/2D structures possess a bigger contact area and tighter interface contact than 0D/2D and 1D/2D structures, which can increase the effective utilization of photogenerated electrons and holes [32]. Employing a two-step electrostatic assembly approach, Sharma et al. [33] created a novel 2D-2D-2D ZnO-Bi2WO6-Ti3C2Tx ternary nanocomposite photocatalyst material made of ZnO, Bi2WO6, and Ti3C2Tx. The interface between the three components contacts and establishes a heterojunction structure, which is favorable to preventing the recombination of photogenerated carriers, as can be shown in Figure 4c.
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Figure 4. Preparation mechanism and TEM pattern of CZS/Ti3C2Tx [30] (a), HRTEM image of the binary CdS/Ti3C2Tx composite and corresponding elemental mapping images [31] (b), SEM and TEM patterns of ZnO-Bi2WO6-Ti3C2Tx [33] (c).
Figure 4. Preparation mechanism and TEM pattern of CZS/Ti3C2Tx [30] (a), HRTEM image of the binary CdS/Ti3C2Tx composite and corresponding elemental mapping images [31] (b), SEM and TEM patterns of ZnO-Bi2WO6-Ti3C2Tx [33] (c).
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2.3. Calcination

For the production of various g-C3N4/Ti3C2Tx-based photocatalysts, calcination is a heat treatment technique carried out in a temperature–controlled environment. It is generally known that MXenes (Ti3C2Tx) materials are quickly oxidized in an atmosphere with high temperatures and oxygen. Therefore, N2 is required as the protective gas for the MXenes-based photocatalyst created via calcination. By calcining a combination of multi-layer Ti3C2Tx and urea, Yang et al. [34] also created Ti3C2Tx/g-C3N4 composites and built a 2D/2D heterostructure. According to the research above, the combination of urea and Ti3C2Tx that is calcined in a single step is advantageous for further stripping the accordion Ti3C2Tx. Recently, Zhou et al. [35] designed low-temperature calcination composites of g-C3N4, Ti3C2Tx, and MoSe2. They effectively construct CXM heterojunction by grinding, combining, and calcining ultra-thin Ti3C2Tx, g-C3N4, and MoSe2 at 400 °C for two hours (Figure 5c,d). Strong interface contact between two-dimensional materials may be established by this heterojunction, increasing the separation effect of photogenerated carriers [36].
In addition, other non-Ti3C2Tx MXenes photocatalysts have also been obtained through calcination. For example, Wan et al. [37] prepared g-C3N4/Mo2CTx photocatalyst composite by calcining the mixture of Mo2CTx and urea in one step. During the calcination process, urea can release ammonia gas to peel off the multi-layer Mo2CTx further and generate g-C3N4 on the surface in situ (Figure 5a,b). This calcination method avoids the tedious ultrasonic stripping process of Mo2CTx and greatly improves the yield of ultra-thin Mo2CTx.
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Figure 5. Preparation mechanism (a), SEM pattern of g-C3N4/Mo2CTx [37] (b), TEM pattern (c), preparation mechanism of CXM [35] (d).
Figure 5. Preparation mechanism (a), SEM pattern of g-C3N4/Mo2CTx [37] (b), TEM pattern (c), preparation mechanism of CXM [35] (d).
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2.4. Hydrothermal/Solvothermal

The hydrothermal/solvothermal process allows synthesising composites with high crystallinity and predictable shape since it is carried out in a confined container at high temperature and pressure. Recently, the hydrothermal approach has been used to create several Ti3C2Tx-based photocatalysts. For instance, the hydrothermal technique was used by Zou et al. [38] to synthesize MoS2/Ti3C2Tx composite. It is evident from Figure 6a that the Schottky heterojunction is formed by the uniform distribution of the crystal MoS2 phase on the Ti3C2Tx layer. Creating 2D/2D MXene matrix composites has also often employed the hydrothermal/solvothermal process. For example, Chen et al. [39] adopted the solvothermal approach to creating 2D/2D CdS/Ti3C2Tx composites. The creation of a 2D/2D heterojunction is clearly shown in Figure 6b by the distinguishable interface between Ti3C2Tx and CdS.
The hydrothermal/solvothermal method has also been used to obtain other non-Ti3C2Tx MXenes photocatalytic compounds. Using an in-situ metal ion derivation method, Cui et al. [40] synthesized 2D/2D Bi2WO6/Nb2CTx composites. To create 2D/2D Bi2WO6/Nb2CTx composites, Bi2+ was adsorbed onto the surface of Nb2CTx during the synthesis and then combined with an aqueous solution of Na2WO4·2H2O containing a tiny quantity of CTAB. This reaction took place for 24 h at 120 °C. Figure 6c depicts the mechanism of synthesis as can be observed in Figure 6c, a 2D Bi2WO6 and a 2D Nb2CTx nano slice successfully combined to produce a Schottky circuit, which effectively prevents the recombination of electrons and holes.
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Figure 6. SEM and TEM patterns of MoS2/Ti3C2Tx photocatalysts [38] (a), TEM patterns of CdS/Ti3C2Tx photocatalysts [39] (b), Synthesis mechanism, SEM and TEM patterns of Bi2WO6/Nb2CTx photocatalyst [40] (c).
Figure 6. SEM and TEM patterns of MoS2/Ti3C2Tx photocatalysts [38] (a), TEM patterns of CdS/Ti3C2Tx photocatalysts [39] (b), Synthesis mechanism, SEM and TEM patterns of Bi2WO6/Nb2CTx photocatalyst [40] (c).
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2.5. Other Methods

High-energy ball milling and wet chemical oxidation are also employed to create Ti3C2Tx-based photocatalysts in addition to calcination oxidation and hydrothermal oxidation. For instance, Li et al. [41] generated TiO2-C composites by high-energy ball milling using Ti3C2Tx as a precursor (Figure 7a). TiO2-C nanoflakes have strong photocatalytic activity because titanium dioxide nanoparticles are uniformly and firmly scattered over amorphous carbon, resulting in intimate contact between titanium dioxide and carbon. However, the dotted line in the illustration illustrates that titanium dioxide nanoparticles also have some surface flaws. Some of these structural flaws are brought on by the process of stripping Ti2AlC using high-energy ball milling [42], and another portion is brought on by the fracture of the original structure brought on by the activation of a few electrons in the titanium dioxide lattice by the mechanical force of ball milling. These flaws enable the development and nucleation of titanium dioxide when Ti2CTx is oxidized to TiO2 during high-energy ball milling [43]. In conclusion, the high-energy ball milling technique has a tight relationship to the structure and characteristics of the TiO2-C composites that have been created.
It is generally recognized that a photocatalyst’s micro-morphology can affect how effectively it performs as a photocatalyst. Through a series of processes, including hydrothermal oxidation, ion exchange, and heat treatment, Tran et al. [44] created the safflower-shaped TiO2/Ti3C2Tx heterostructure (Figure 7b). A 2D Ti3C2Tx thin sheet’s layered structure is broken down into nanoparticles after hydrothermal treatment and ion exchange. They discovered that after heating the agglomerated nanoparticles, nanorods began to grow radially from the nanoparticles, eventually forming a composite that appeared like a bit of safflower. The recombination of photogenerated carriers may be successfully prevented by this special nano-flower structure, which promotes charge transfer.
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Figure 7. Synthesis mechanism and TEM pattern of TiO2-C by high-energy ball milling [41] (a), Synthesis mechanism and SEM pattern of safflower-like TiO2/Ti3C2Tx photocatalyst [44] (b).
Figure 7. Synthesis mechanism and TEM pattern of TiO2-C by high-energy ball milling [41] (a), Synthesis mechanism and SEM pattern of safflower-like TiO2/Ti3C2Tx photocatalyst [44] (b).
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3. MXenes (Ti3C2Tx) Materials for Photocatalytic Applications

3.1. Photocatalytic Degradation of Pollutants

Numerous water resources are being contaminated by organic chemicals due to the quick expansion of contemporary industry and the acceleration of urbanization, with irreparable harm to human health and the environment. As a result, there is a lot of worry about the remediation of water pollution. Adsorption, biological treatment, peroxymonosulfate activation, Fenton oxidation, photocatalysis, and other techniques have all been used in recent years to reduce contaminants in water [45,46,47]. Among them, photocatalysis is a simple and evident method of photooxidation and photoreduction technology degradation, with carbon dioxide and water as the end products [48,49].
Among the different photocatalysts that may efficiently degrade organic pollutants, Ti3C2Tx-based photocatalysts have recently emerged as a research hotspot. Furthermore, several investigations have demonstrated the potential of composite materials made of metallic Ti3C2Tx and other semiconductor photocatalysts to degrade various contaminants [50]. Wang et al. [51], for instance. Using a hydrothermal technique aided by ultrasound, 2D/2D Ti3C2Tx/SnNb2O6 composites with interfacial Schottky connections were created. When used to photo-catalytically degrade rhodamine B (RhB), Ti3C2Tx/SnNb2O6 showed good results (Figure 8a).
Regarding the shape and structure of composite materials, 2D/2D heterostructures exhibit greater photocatalytic activity than 0D/2D and 1D/2D heterostructures under the same circumstances. This is because 2D/2D heterostructures maximize the area of contact between the two materials while simultaneously offering additional surface reaction activation sites for photocatalysis. Furthermore, since Ti3C2Tx has a lower escape work than SnNb2O6, electrons will flow from it to that material until equilibrium, establishing a potential barrier at the interface that considerably aids in separating photogenerated electrons and holes. Recently, Shao et al. [52] discovered that 2D/2D CoAl-LDHs/Ti3C2Tx nanocomposites had a favorable impact on the breakdown of tetracycline hydrochloride (TCH) (Figure 8b). Furthermore, the photocatalytic activity is outstanding due to the synergistic interaction between CoAl-LDHs and Ti3C2Tx and the Schottky junction generated on its contact surface (Figure 8c).
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Figure 8. Degradation effect and degradation mechanism of Ti3C2Tx/SnNb2O6 photocatalyst on RhB [51] (a), Degradation effect of CoAl-LDHs/Ti3C2Tx photocatalyst on TCH [52] (b), Degradation mechanism of TCH by CoAl-LDHs/Ti3C2Tx photocatalysts [52] (c).
Figure 8. Degradation effect and degradation mechanism of Ti3C2Tx/SnNb2O6 photocatalyst on RhB [51] (a), Degradation effect of CoAl-LDHs/Ti3C2Tx photocatalyst on TCH [52] (b), Degradation mechanism of TCH by CoAl-LDHs/Ti3C2Tx photocatalysts [52] (c).
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3.2. Photocatalytic Hydrogen Evolution

Today, the most significant energy source used worldwide is still fossil fuels. However, our energy use is rising together with the quick growth of human society. Therefore, finding alternative renewable energy sources is so crucial. Because it has cleaner combustion byproducts and a greater energy density than fossil fuels, hydrogen is well known to be a suitable energy source to replace them [53]. Water electrolysis, coal gasification, electrocatalysis, and photocatalysis are now the most widely utilized processes for producing hydrogen [54,55,56,57,58,59,60,61,62,63,64,65,66]. However, due to its sustainability and absence of secondary emissions, photocatalytic hydrogen generation is the most promising of these preparation techniques. Numerous types of photocatalysts, including metal sulfide, titanium dioxide, barium titanate, and g-C3N4, have been investigated thus far for hydrogen production. However, these photocatalysts still show quick photogenerated carrier recombination and limited light consumption. Therefore, creating novel photocatalysts for hydrogen production by photocatalysis is crucial.
Ti3C2Tx has recently been discovered to efficiently separate photogenerated electrons and holes in the field of photocatalytic hydrogen synthesis because of its acceptable Fermi energy level and strong conductivity [67,68]. Therefore, there is a lot of interest in employing Ti3C2Tx as electron acceptors and transporters in producing hydrogen via photocatalysis. Using the solvothermal technique, Cao et al. [69] created composites of Ti3C2Tx and ZnxCd1−xS photocatalysts. The experimental results demonstrate that Ti3C2Tx/ZnxCd1−xS has a very good photocatalytic hydrogen evolution effect, up to 14.17 mmol/(h*g), and that this effect is due to the promotion of carrier separation and the enhancement of the oxidation ability of the valence band with a suitable energy band structure (Figure 9a). Meanwhile, it can be seen that Ti3C2Tx/ZnxCd1−xS has good photocatalytic stability, which is due to the introduction of Ti3C2Tx to inhibit the photo-corrosion of metal sulfides [70].
It is generally recognized that the photocatalyst’s shape significantly impacts how effectively it functions as a catalyst. Due to its two-dimensional ultra-thin structure, two dimensions, and huge surface area, 2D/2D heterostructure significantly impacts photocatalysis. Based on this assumption, Chen et al. [39] created and produced a special 2D/2D CdS/Ti3C2Tx composite photocatalyst with a good photocatalytic hydrogen evolution effect (1.73 mmol/(h*g)) that is higher than that of pure CdS nanoflakes (0.37 mmol/(h*g)) (Figure 9b). This is due to the heterojunction created by the composite of 2D/2D CdS and Ti3C2Tx, which prevents the quick recombination of photogenerated carriers and encourages the transmission of photogenerated electrons. The above research showed that a Ti3C2Tx-based photocatalyst could significantly enhance the yield of photocatalytic hydrogen evolution compared with a single semiconductor photocatalyst.
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Figure 9. Photocatalytic mechanism, hydrogen evolution and cycle experiment of Ti3C2Tx/ZnxCd1−xS [69] (a), 2D/2D CdS/Ti3C2Tx photocatalytic mechanism, band structure and photocatalyst hydrogen evolution [39] (b).
Figure 9. Photocatalytic mechanism, hydrogen evolution and cycle experiment of Ti3C2Tx/ZnxCd1−xS [69] (a), 2D/2D CdS/Ti3C2Tx photocatalytic mechanism, band structure and photocatalyst hydrogen evolution [39] (b).
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3.3. Photocatalytic Reduction of CO2

In recent decades, individuals have made significant efforts in photocatalytic CO2 reduction to address both the global warming trend and the energy dilemma. The reduction of CO2 to hydrocarbons (methane, methanol, formaldehyde, formic acid, etc.) and the release of oxygen during photocatalytic CO2 reduction is a process that mimics the photosynthesis of natural plants. As a result, numerous photocatalysts have been developed and manufactured. However, reducing carbon dioxide by photocatalysts is still difficult because of the interaction of photogenerated carriers, the low thermodynamic stability of carbon dioxide molecules, and the weak adsorption and activation abilities of carbon dioxide molecules [71,72,73,74]. Therefore, the development of photocatalysts with strong carbon dioxide reduction activity is urgently needed.
Recently, Li et al. [75] shown that Ti3C2Tx may be utilized to construct Ti3C2Tx/ZnO photocatalyst, which can increase the reduction performance of ZnO photocatalyst to carbon dioxide, and ZnO photocatalyst as a co-catalyst without noble metals. The conversion rate of CO and methane is about seven times and 35 times that of pure ZnO, respectively. The good metal-like conductivity and electron-rich environment of the surface-alkalized Ti3C2Tx, which favor the separation and transport of photogenerated electrons and holes, are responsible for its higher photocatalytic activity. In addition, the surface terminal -OH group may provide many sites for the adsorption and activation of carbon dioxide molecules. Similarly, Tang et al. [76] employed g-C3N4 and alkalized Ti3C2Tx as a cocatalyst to make Ti3C2Tx-OH/g-C3N4. Compared to pure g-C3N4, its carbon dioxide photocatalytic reduction increased 5.9 times. According to the study, the -OH terminal groups on the surface of Ti3C2Tx may effectively promote the photo-induced transfer of electrons from semiconductors to Ti3C2Tx-OH (Figure 10). The photocatalytic activity of CO2 reduction can also be improved by the substantial number of active sites that -OH terminal groups can provide for the adsorption and activation of acidic CO2 molecules.

3.4. Photocatalytic Nitrogen Fixation

As a necessary component of all living things, nitrogen (N) plays a significant role in forming proteins and nucleic acids. The traditional Haber-Bosch method, which currently relies on the reaction of nitrogen and hydrogen to form ammonia at high temperatures and pressures (500–600 °C, 20–50 MPa), is still used to artificially fix nitrogen. However, this process uses a lot of energy and produces a lot of greenhouse gases, which are extremely harmful to the environment. Therefore, the need for a sustainable artificial nitrogen fixation method is important. Photocatalytic technology is one of the most successful ways of artificial nitrogen fixation [77,78,79,80,81,82]. Ti3C2Tx materials have been the subject of much investigation in photocatalytic nitrogen fixation.
Ti3C2Tx has enormous promise as a cocatalyst in applying photocatalytic nitrogen fixation. Through experimental investigation and theoretical calculation, Chen et al. [83] recently demonstrated the photocatalytic nitrogen fixation capability of a 2D/2D Bi4O5Br2/Ti3C2Tx composite. As shown in Figure 11a,b, without using a sacrificial agent, the photocatalyst with a mass ratio of 15% Ti3C2Tx has a maximum photocatalytic nitrogen fixation rate of 277.74 µmol/(g*h). Bi4O5Br2 and the Ti3C2Tx contact form a special ohmic heterojunction that allows electrons to move without a potential barrier. Through an internal electric field, photogenerated electrons may be readily transported from Bi4O5Br2 to Ti3C2Tx.
Furthermore, creating a Bi4O5Br2/Ti3C2Tx 2D/2D heterojunction enhances the exposure of active edge sites, shortens the electron transfer path, and increases the efficiency of nitrogen fixation. Similarly, Hou et al. [84] created an in situ Ti3C2Tx/TiO2 photocatalyst using a one-step calcination technique and successfully fixed nitrogen by photocatalysis under full-spectrum illumination. As a result, Ti3C2Tx-based photocatalysts appear to be a potential photocatalytic nitrogen-fixing material because they can create heterojunction via in-situ growth and boost photogenerated carrier separation efficiency.
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Figure 11. Bi4O5Br2/Ti3C2Tx photocatalytic nitrogen fixation curve, nitrogen fixation rate and mechanism diagram [83] (a), Experimental diagram of Ti3C2Tx/TiO2/Co photocatalytic nitrogen fixation NH4+ [85] (b).
Figure 11. Bi4O5Br2/Ti3C2Tx photocatalytic nitrogen fixation curve, nitrogen fixation rate and mechanism diagram [83] (a), Experimental diagram of Ti3C2Tx/TiO2/Co photocatalytic nitrogen fixation NH4+ [85] (b).
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3.5. Photocatalytic Applications of Ti3C2Tx-Derived Materials

Two-dimensional metal carbonates are regarded as good precursors of synthetic composites, whereas the use of Ti3C2Tx derivative photocatalysts in the field of photocatalysts has recently gained significant interest from academics. Recent research has demonstrated that the photocatalytic activity of the system may be increased by mixing two-dimensional Ti3C2Tx materials with TiO2 nanoparticles. For instance, Peng et al. [86] produced a photocatalyst (TiO2(001)/Ti3C2Tx) generated from Ti3C2Tx that exposed the uranium dioxide cut (Figure 12a). Under the same conditions, the removal rate of methyl orange (MO) is significantly higher than that of the particle TiO2/Ti3C2Tx, which can effectively inhibit the composition of photogenic electrons and holes, improving the photocatalyst degradation effect of the methyl orange (MO). TiO2(001)/Ti3C2Tx exhibits good photocatalyst degradation properties for methyl orange (MO) under ultraviolet light. However, in TiO2/Ti3C2Tx, the photogenic electron and hole oxidation restore voltage is lower than before the electron transfer, which somewhat restricts the generation of photo-catalytically active compounds [87]. To address this issue more effectively, a Z-type heterogeneous catalytic system has been investigated. This system fixes the shortcomings of the conventional heterogeneous catalytic system, makes it easier to separate the photon electron from the hole, and maximizes the oxidation-restoration power level of the heterogeneous system [88]. Wu et al. [89] created a triple composite photocatalyst of TiO2/Ti3C2Tx/AgI with Z-type heterogeneous nodes using straightforward solvent, thermal procedures, and co-deposition techniques. As a result, the AgI-TiO2 Z-type heterogeneous bond may be formed using the Ti3C2Tx layer as a load transfer bridge (Figure 12b), and the TiO2/Ti3C2Tx/AgI composite material shows effective photocatalytic capabilities for the destruction of TCH.
The Ti3C2Tx-derived photocatalysts have also sparked a huge research boom in hydrogen evolution. The separation efficiency of photoinduced carriers has been markedly enhanced by the in-situ growth of TiO2 on the surface of Ti3C2Tx when combined with other semiconductors, thus improving photocatalytic hydrogen efficiency [90]. Yang et al. [91] prepared a PtO/TiO2/Ti3C2Tx composite photocatalyst by depositing PtO nanoparticles on the in-situ synthesized TiO2/Ti3C2Tx, as shown in Figure 13a, after calcination and oxidation at different time, Ti3C2Tx formed TiO2/Ti3C2Tx composites with varying degrees of oxidation. The photocatalytic hydrogen evolution rate was TiO2/Ti3C2Tx-12 h > TiO2/Ti3C2Tx-20 h > TiO2/Ti3C2Tx-4 h, which showed that the increase in titanium dioxide content 12 h before the heating reaction could improve the hydrogen production efficiency. After 12 h, the Ti3C2Tx content decreased, resulting in a decrease in hole transfer efficiency, thereby reducing hydrogen evolution efficiency. When PtO nanoparticles are deposited on the surface of TiO2/Ti3C2Tx, the hydrogen production efficiency of photocatalysts is significantly increased to 2.54 mmol/(h*g), and Ti3C2Tx serves as a hole acceptor for titanium dioxide and PtO in this photocatalyst system, facilitating the separation of photogenerated electrons and holes. Yang et al. [92] used the two-step water thermal location growth method to synthesize the three-dimensional heterogeneous photocatalyst Ti3C2Tx/TiO2/CuInS2 (Figure 13b), and the following factors are primarily responsible for the good photocatalytic effect: 1. Increase the narrow band gap semiconductor CuInS2′s ability to absorb light; 2. Improve charge separation by the synergy of S-type heterojunctions between CuInS2 and TiO2 and the interface Schottky junction of Ti3C2Tx/CuInS2; 3. There are several active areas on the Ti3C2Tx surface, which could enhance charge carries separation. Table 1 summarises the photocatalytic H2 evolution performances of the common Ti3C2Tx-derived photocatalysts.
Hou et al. [84] synthesized Ti3C2Tx/TiO2 photocatalyst in situ by one-step calcination technology and achieved a good photocatalytic nitrogen fixation effect under full-spectrum radiation. It can be seen that Ti3C2Tx-derived photocatalyst can form heterojunctions through in-situ growth, improve the separation efficiency of photogenerated carriers, and is a promising photocatalytic nitrogen fixing material. On this basis, Gao et al. [85] synthesized Ti3C2Tx/TiO2/Co by introducing Co into Ti3C2Tx-derived photocatalysts using a two-step calcination method, which achieved high efficiency and stable photocatalytic nitrogen fixation. As shown in Figure 11b, the NH3 yield without any pore sacrifice was as high as 110.45 µmol/(g*h) under an N2 environment and UV-visible light. This is attributed to the improvement of carrier transfer and separation by in-situ growth of Ti3C2Tx/TiO2 heterojunction by calcination, Co doping effectively regulates the chemical adsorption equilibrium of reactant N2 and product ammonia on the catalyst surface, and Ti3C2Tx/TiO2/Co photocatalysis shows certain cyclic stability due to the presence of Ti3C2Tx. From the above experiments, it can be seen that Ti3C2Tx has great application prospects in photocatalytic nitrogen fixation.

3.6. Photocatalytic Applications of Non-Ti3C2Tx MXenes

Aside from the widespread use of Ti3C2Tx materials in photocatalytic applications, some non-Ti3C2Tx materials have significantly advanced in photocatalysis in recent years. For example, the new MXene 2D niobium carbide (Nb2CTx), which has a lower Fermi level than Ti3C2Tx, was used as an excellent co-catalyst. Recently, Makola et al. [99] used an in-situ calcination method to prepare a 2D/2D Nb2CTx@g-C3N4 metal-free Schottky junction photocatalyst with different loading percentages of Nb2CTx. The varying quantities of Nb2CTx loaded onto the g-C3N4 had no discernible effect on the chemical composition and structure. However, due to alterations in band edge locations, the produced heterostructures had narrower energy band gaps than g-C3N4.
Furthermore, the development of the Schottky junction, where Nb2CTx functions as an electron sink, considerably reduced electron recombination rates. The electrochemical tests revealed that the samples had increased photocatalytic activity following the creation of the heterostructure. The band edge diagram demonstrates good band locations for the composites to be employed in various photocatalytic applications, including CO2 reduction, photooxidation, and species reduction. To further specific applications, Huang et al. [100] recently demonstrated the photocatalytic hydrogen generation capability of a 1D/2D CdS/Nb2CTx MXene composite. The photocatalyst’s maximal photocatalytic hydrogen production rate with a purity of 60 mg Nb2CTx is 5.3 mmol/(g*h), which is 1.7 times greater than that of pristine CdS. The findings above are due to the special properties of Nb2CTX MXene (high conductivity, 2D structure, and enough active sites) and the close interface contact. In the meantime, the CdS/Nb2CTX combination can efficiently increase the number of free radicals and the separation and transmission of photoexcited electron-hole pairs. The studies mentioned above can serve as references for the design of non-Ti3C2Tx as a co-catalyst for better photocatalytic applications. However, there are currently limited studies on non-Ti3C2Tx materials in photocatalysis.

4. Major Properties of Ti3C2Tx MXenes Materials in Photocatalysis

4.1. Facilitating the Separation of Photogenerated Electrons and Holes

To address the poor efficiency of photocatalysis, the primary method is to increase the separation efficiency of photogenerated carriers. Using Ti3C2Tx as a co-catalyst may generally successfully prevent the recombination of photogenerated electrons and holes in the photocatalytic system because of the material’s suitable Fermi energy level and strong electrical conductivity. In addition, numerous experimental findings demonstrate that Ti3C2Tx with a semiconductor photocatalyst can produce a heterojunction (ohmic or Schottky contact). The metal-semiconductor Schottky contact will form on the contact surface of the two when the work function of the metal is greater than that of the semiconductor in the n-type semiconductor, and a Schottky barrier can form inside it that can be used as an electron absorber to stop electron backflow [101]. On the other hand, a metal-semiconductor ohmic contact will form on the contact surface of the two when the work function of the metal in an n-type semiconductor is lower than that of the semiconductor [102].
In contrast to Schottky contact, Ohmic contact is not constrained by an energy barrier, allowing photogenerated electrons to move spontaneously from Ti3C2Tx to semiconductors until they achieve equilibrium [103]. Furthermore, the kinetics of the photocatalytic reaction may be efficiently accelerated by ohmic contact, increasing the photocatalytic efficiency. In conclusion, the heterojunction structure created by combining semiconductors with Ti3C2Tx materials may effectively prevent the recombination of photogenerated carriers, enhancing photocatalytic activity.

4.2. Providing a Large Number of Modifiable Active Sites

The structure and surface characteristics of the material, in addition to the material’s inherent features, have a significant influence on the catalytic activity. The two-dimensional planar structure of the Ti3C2Tx material not only reduces the transmission distance of the widely produced charge carriers and offers additional surface-active areas for photocatalysis. There are currently two techniques that are often used to produce Ti3C2Tx materials. One involves using HF to peel directly, while the other involves using a solution of HCL and LiF to peel indirectly. The two strategies result in distinct terminal groupings. The latter has more -O/-OH terminals than the Ti3C2Tx etched using hydrochloric acid or lithium fluoride, which mostly has -F terminals. The photocatalytic process is somewhat influenced by the various end groups. For instance, Ran et al. [104] showed that the photocatalytic hydrogen precipitation rate rose with a decrease in the terminal group -F to -O ionization ratio. However, the result was not particularly satisfactory. The -F exposed to the Ti3C2Tx surface renders the external atomic charge saturated since the halogen elements are in group VIIA. From a thermodynamic perspective, the Ti3C2Tx surface is highly stable, making it challenging for the externally photoexcited electrons to reach it.
The Ti3C2Tx surface is negatively charged due to the -F and -OH terminals from a kinetic perspective, making Coulomb gravity difficult to attract foreign electrons with the same negative charge. At the same time, Li et al. [105] demonstrated through theory and experimentation that Ti3C2Tx at the -F and -OH terminals covered up their active surface sites, resulting in their catalytic ammonia production and efficiency for nitrogen fixation. In conclusion, Ti3C2Tx, functionalized with a non-active group, is unsuitable for a cocatalyst in the photocatalytic reaction. This significantly restricts the application of Ti3C2Tx materials in the area of photocatalysis. To address the issues mentioned above, Zhong et al. [106] replaced the passivation groups (-F, -O/-OH) on the Ti3C2Tx surface with diamino ethanethiol (AET) using a straightforward sonication process. As a result, they produced surface-modified CdZnS/Ti3C2Tx-AET nanosheets with high surface activity. The surface-modified CdZnS/Ti3C2Tx-AET had much better catalytic activity than the original CdZnS and CdZnS/Ti3C2Tx and outperformed most of the reported Ti3C2Tx-based catalysts. Therefore, adding the right functional groups to Ti3C2Tx surfaces may significantly increase the interfacial carrier transfer activity, create many active sites, and broaden the use of Ti3C2Tx materials in photocatalysis.

4.3. Enhancing Photocatalytic Stability by Inhibiting Anti-Photocorrosion

Various semiconductor photocatalysts for environmental cleaning have been produced, such as AgNO3 and CdS. However, aside from the combination of photoinduced electrons and holes substantially restricting photoactivity, the issues caused by photogenerated holes-induced anti-photocorrosion greatly restrict its practical applicability.
Cai et al. [107] prepared the Ag3PO4/Ti3C2 MXene Schottky catalyst to solve the above issues via a driven self-assembly approach. It not only has great light degradation capabilities for dye, but it also has superior photocatalytic stability compared to pure Ag3PO4. After eight cycles (Figure 14a), the photocatalytic capabilities of pure Ag3PO4 reduced by approximately 92.2%, suggesting a larger quantity of breakdown induced by photocorrosion. In contrast, the Ag3PO4/Ti3C2 composite has a relatively small loss of after-light catalysis performance (about 31.6% in eight cycles), indicating that its anti-photocorrosion is inhibited, which is attributed to a full and tight interface contact between Ag3PO4 and Ti3C2, a single-directional electron flow captured by Ti3C2 through the Schottky barrier, and Ti sites with higher redox reactivity on the surface of Ti3C2. Similarly, Xie et al. [70] used Ti3C2 as a precursor to create 2D/2D CdS/Ti3C2 heterostructures using a simple electrostatic self-assembly technique. CdS/Ti3C2 showed high photoactivity for 4-nitroaniline (4-NA) degradation. Meanwhile, Ti3C2 inhibited CdS photocorrosion. As shown in Figure 14b, the degradation of 4-NA over pure CdS nanosheets gradually declines, implying that the use of ammonium formate as hole scavengers cannot entirely prevent photocorrosion of CdS. CdS-0.5% MXene, on the other hand, has better stability for 4-NA conversion. The close connection between CdS nanosheets and Ti3C2Tx is advantageous in exerting the “Cd2+ confinement effect” of Ti3C2Tx, hence improving the stability of CdS-based photocatalysts. In summary, introducing Ti can effectively enhance the stability of photocatalyst composite materials.

5. Summary and Outlook

Since Ti3C2Tx material initially appeared in 2011, its use in photocatalysis has garnered much interest. The preparation techniques for Ti3C2Tx-based photocatalysts that are often utilized include mechanical or ultrasonic mixing, electrostatic self-assembly, the water/solvothermal approach, and the calcination method, as discussed in this study. Ti3C2Tx is typically used as a cocatalyst in the field of photocatalytic environmental remediation and energy conversion, including organic pollutant degradation, water decomposition for hydrogen production, carbon dioxide reduction, and photocatalytic nitrogen fixation, due to its adjustable element composition, unique 2D layered structure, large surface area, rich surface ends, and good photoelectron properties. Furthermore, theoretical research and numerous tests demonstrate that the semiconductor photocatalyst and Ti3C2Tx can create a heterojunction (ohmic contact or Schottky contact), which can successfully facilitate the separation of photogenerated carriers. Additionally, by appropriately altering the many terminal sites on its surface, the photocatalytic efficacy may increase further. However, contrary to studies on other 2D materials, Ti3C2Tx is still in its infancy, and several problems need to be resolved before it can be applied broadly.
  • Ti3C2Tx is currently synthesized using procedures that are still mostly difficult, time-consuming, and yield-poor. To synthesize Ti3C2Tx on a wide scale, it is important to develop an affordable, effective, and ecologically friendly process.
  • MXenes materials have not been thoroughly studied in comparison to Ti3C2Tx. For instance, there aren’t many studies on V2CTx, Nb2CTx, and the composites they’re formed from, such as V2O5/V2CTx, Nb2O5/Nb2CTx, etc. Therefore, future research into non-Ti3C2Tx MXenes photocatalysts is quite promising. Additionally, V2CTx and Nb2CTx materials perform better than Ti3C2Tx MXenes, which may result in discoveries in the field of photocatalysis.
  • The ease with which Ti3C2Tx may oxidize in oxygen-containing environments is widely recognized. However, the Ti3C2Tx structure’s decomposition will impact the photocatalytic activity. Numerous methods have been suggested thus far to increase the stability of Ti3C2Tx, such as enhancing the preparation conditions, introducing antioxidants, etc. Photocatalysts based on Ti3C2Tx will perform at a higher level in energy and environmental applications if the problem of oxidation resistance can be resolved.

Author Contributions

Conceptualization, H.J.; methodology, H.Y.; software, S.Y.; validation, S.Z.; formal analysis, W.Z.; investigation, H.Y.; resources, X.F.; data curation, S.Y.; writing—original draft preparation, H.J.; writing—review and editing, H.Y.; visualization, X.F.; supervision, S.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the study’s design; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. 2D MXenes material that has been reported [17].
Figure 1. 2D MXenes material that has been reported [17].
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Figure 3. Synthesis process of Ti3C2Tx/g-C3N4 photocatalyst and TEM pattern of Ti3C2Tx/g-C3N4 [28] (a), Synthesis process of g-C3N4/Bt/Ti3C2Tx photocatalyst [29] (b).
Figure 3. Synthesis process of Ti3C2Tx/g-C3N4 photocatalyst and TEM pattern of Ti3C2Tx/g-C3N4 [28] (a), Synthesis process of g-C3N4/Bt/Ti3C2Tx photocatalyst [29] (b).
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Figure 10. Comparison and mechanism diagram of Ti3C2Tx-OH/g-C3N4 photocatalytic reduction of CO2 [76].
Figure 10. Comparison and mechanism diagram of Ti3C2Tx-OH/g-C3N4 photocatalytic reduction of CO2 [76].
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Figure 12. TiO2/Ti3C2Tx band mechanism diagram [86] (a), TiO2/Ti3C2Tx/AgI band degradation mechanism diagram and degradation curve [89] (b).
Figure 12. TiO2/Ti3C2Tx band mechanism diagram [86] (a), TiO2/Ti3C2Tx/AgI band degradation mechanism diagram and degradation curve [89] (b).
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Figure 13. PtO/TiO2/Ti3C2Tx and different calcination times TiO2/Ti3C2Tx hydrogen evolution curves, photoluminescence patterns and mechanism diagrams [91] (a) Data and mechanism diagram of hydrogen evolution effect of Ti3C2Tx/TiO2/CuInS2 [92] (b).
Figure 13. PtO/TiO2/Ti3C2Tx and different calcination times TiO2/Ti3C2Tx hydrogen evolution curves, photoluminescence patterns and mechanism diagrams [91] (a) Data and mechanism diagram of hydrogen evolution effect of Ti3C2Tx/TiO2/CuInS2 [92] (b).
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Figure 14. The photocatalytic stability of Ag3PO4/Ti3C2 composite [107] (a), The photocatalytic stability of CdS/Ti3C2 composite [70] (b).
Figure 14. The photocatalytic stability of Ag3PO4/Ti3C2 composite [107] (a), The photocatalytic stability of CdS/Ti3C2 composite [70] (b).
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Table
Table 1. Summary of photocatalytic activity of Ti3C2/TiO2 composites for H2 evolution.
Table 1. Summary of photocatalytic activity of Ti3C2/TiO2 composites for H2 evolution.
PhotocatalystsSacrificial RegentH2 Production Rate
(µmol/(h*g))		Light Source
Ti3C2/TiO2 [93]Methanol2650200 W
Hg lamp
Cu2O/Ti3C2/TiO2 [48]Methanol1496300 W
Xenon lamp
Ti3C2/TiO2/UiO-66-NH [90]Na2SO31980300 W
Xenon lamp
Ti3C2/TiO2/Ru [94]Methanol235.3300 W
Xenon lamp
WS2/Ti3C2/TiO2 [95]Triethanolamine3409.8300 W
Xenon lamp
LDS-S-Ti3C2/TiO2 [96]Methanol333300 W
Xenon lamp
Ti3C2/TiO2
(Nanoflowers) [97]		TEOA526300 W
Xenon lamp
MoS2/Ti3C2/TiO2 [98]TEOA6425.297300 W
Xenon lamp
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Yu, H.; Jiang, H.; Zhang, S.; Feng, X.; Yin, S.; Zhao, W. Review of Two-Dimensional MXenes (Ti3C2Tx) Materials in Photocatalytic Applications. Processes 2023, 11, 1413. https://0-doi-org.brum.beds.ac.uk/10.3390/pr11051413

AMA Style

Yu H, Jiang H, Zhang S, Feng X, Yin S, Zhao W. Review of Two-Dimensional MXenes (Ti3C2Tx) Materials in Photocatalytic Applications. Processes. 2023; 11(5):1413. https://0-doi-org.brum.beds.ac.uk/10.3390/pr11051413

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Yu, Haidong, Haibing Jiang, Shuji Zhang, Xin Feng, Song Yin, and Wenzhi Zhao. 2023. "Review of Two-Dimensional MXenes (Ti3C2Tx) Materials in Photocatalytic Applications" Processes 11, no. 5: 1413. https://0-doi-org.brum.beds.ac.uk/10.3390/pr11051413

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