1 Introduction

Energy shortage and environmental pollution have become the two major issues faced by humanity due to limited fossil fuel resources and increasing consumption. Developing sustainable and clean energy is the key to addressing these two problems [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]. In being clean and inexhaustible, solar energy shows great potential to be one of the most promising future energy sources. Solar energy can be exploited in photovoltaic technologies [16], CO2 photoreduction [17, 18], N2 photo-fixation [19], degradation of organic compounds [20,21,22,23,24,25,26], and photocatalytic water splitting [27]. In renewable hydrogen fuel-based photocatalytic water-splitting systems [28,29,30], photocatalysts play a critical role [31, 32]. Photo-catalyzed solar energy conversion can be divided into three steps: (1) light absorption, (2) charge separation and transfer, and (3) surface reaction. Any improvement on each of these steps will contribute to enhancing the total conversion efficiency. Conventional photocatalysts such as TiO2, g-C3N4, and CdS demonstrate low photocatalytic efficiency due to rapid charge recombination in these materials. Using noble metals such as Pt, Ru, and Pd as co-catalysts will increase cost, although such materials can enhance charge separation ability and suppress recombination of charges. A co-catalyst that is both efficient and cheap is thus urgently needed to promote the development of photocatalysis.

MXenes, comprising transition metal carbides, nitrides, and carbonitrides, are a new family of two-dimensional (2D) materials that have attracted much attention in recent years [2]. The general formula of MXene is Mn+1Xn (n = 1, 2, 3), where M represents a transition metal, such as Sc, Ti, Zr, Hf, V, Nb, Ta, and Mo, while X represents C and/or N. Owing to their unique structure and superior photoelectronic properties, layered structure MXenes show various potential applications in different areas, such as energy storage [3, 33,34,35,36,37,38], electromagnetic interference shielding [39, 40], gas sensors [41], wireless communication [42], water treatment [43, 44], solar cells [45,46,47], and catalysis [41, 48,49,50,51]. 2D MXenes are being increasingly studied in the past few years, as evidenced by the rapidly increasing number of scientific articles published per year (Fig. 1a). MXenes are usually synthesized by selectively etching the A layer from MAX phases, which constitute a family of tertiary ductile ceramics, where the A layer is made of an element such as Al, Ga [52], or Si [53]. After selective etching of the A layer, 2D MX layers with surface functional groups (–O, –OH, –F, or a mixture of several groups denoted as Tx) are left. The most widely used methods for selective etching are wet chemical HF etching and in situ HF etching (using a mixture of acids and fluoride salts), although other routes using tetramethylammonium hydroxide (TMAOH) [54, 55], electrochemical [56, 57], or etching with NaOH [58], and ZnCl2 [49]) have also been explored. Generally, multilayered MXenes are produced by HF etching, whereas single or few-layered MXene flakes are obtained by in situ HF etching or through delamination of a multilayered MXene by intercalation of large organic molecules (Fig. 1b). The etching methods of Ti3C2Tx MXene, which is the first discovered and the most studied MXene, have been reviewed elsewhere [59, 60].

Fig. 1
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a The rapid expansion of 2D MXenes materials and b the most widely used methods to synthesize MXenes

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In view of the rapid development in the application of 2D MXenes, several reviews on their synthesis [59,60,61], and application in energy storage [33, 48, 62] and catalysis [51] have been reported. MXenes are promising for application in photocatalysis [63] because of their large surface area, good conductivity, presence of a sufficient number of active sites, and containing suitable elements for effective photocatalysis, but they cannot be directly used as photocatalysts since MXenes are generally not semiconductors [51, 62]. Although there are some MXene semiconductors that have been predicted theoretically [64,65,66,67,68], these have not yet been experimentally synthesized. In this review, we give a detailed discussion on MXene as a co-catalyst in photocatalysis and describe the different methods used for the synthesis of MXene-derived photocatalysts, along with problems encountered in this system and a prospective outlook on future research in this field.

2 Synthetic Methods for MXenes as Co-catalysts in Photocatalysis

In view of their good conductivity and large surface area, MXenes have been applied in photocatalysis both to replace noble metal co-catalysts and to enhance the charge separation ability of the photocatalyst (Fig. 2). The most common methods used for the preparation of photocatalyst composites include mechanical mixing, self-assembly, in situ decoration and oxidation, or a combination of the three methods.

Fig. 2
figure 2

Schematic showing charge separation between MXene co-catalyst and a photocatalyst taken from a Ye et al. Reprinted with permission from Ref. [69]. Copyright 2018 John Wiley & Sons. b Ran et al. Reprinted with permission from Ref. [70]. Copyright 2017 Nature Publishing Group. c, d Peng et al. Reprinted with permission from Ref. [71]. Copyright 2016 American Chemical Society

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2.1 Mechanical Mixing and Self-assembly

Mechanical mixing is the easiest method to form photocatalyst composites. Stirring the two components in the liquid phase or grinding of powders can be used for sample preparation. Interestingly, due to electrostatic attraction, photocatalysts with positive charge are easily combined with MXenes whose surfaces are enriched with negative charges, leading to self-assembled photocatalyst composites. In addition, the self-assembling property could be further improved by using other induced techniques simultaneously, where the photocatalysts and co-catalysts are prepared in advance [44].

An et al. [72] demonstrated that synergetic effects of Ti3C2 MXene and Pt when used as dual co-catalysts enhanced the photoactivity of g-C3N4 for hydrogen evolution (Fig. 3a), where HF-etched exfoliated Ti3C2 and g-C3N4 were mixed in liquid by stirring followed by photodeposition of Pt on the composites. The photoactivity of the dual co-catalysts-modified photocatalysts (g-C3N4/Ti3C2/Pt) was much better than that of Pt- or Ti3C2-only systems, reaching 5.1 mmol h−1 g−1 in hydrogen production (Fig. 4a). This enhanced performance was due to the presence of Ti3C2 MXene that facilitated interfacial charge separation and carrier transport from the conduction band (CB) of g-C3N4 to Pt. Our group prepared g-C3N4/Ti3C2Tx composites by grinding g-C3N4 and Ti3C2Tx powders together followed by annealing in different gas atmospheres, to tune the surface termination groups (Fig. 4b) [74]. X-ray photoelectron spectroscopy data showed an increase in –O termination groups accompanied by a decrease in –F termination groups on the surface of Ti3C2. Ti3C2 with –O termination groups had better photoactivity, revealing that the presence of such groups in Ti3C2 had a positive effect on hydrogen production by increasing the number of active sites. Moreover, this finding was consistent with density functional theory (DFT) simulation results. The |ΔGH| of Ti3C2 with –O terminations was found to be as low as 0.01 eV, which is lower than that of Pt (111). In a similar study, Ye et al. [69] treated HF-etched Ti3C2 with KOH to convert –F groups into –OH groups, and then combined the KOH-treated Ti3C2 with TiO2 (P25) powder by stirring in water (Fig. 3c). DFT calculations demonstrated that –OH groups played the role of active sites for the adsorption and activation of CO2 reduction [69]. Experimentally, the photoactivities for CO2 reduction were increased 3 times and 277 times after KOH treatment, for CO and CH4, respectively (Fig. 4d). Interestingly, increasing the number of –OH groups not only improved the photo-conversion efficiency but also changed the nature of the products. The –OH groups resulting from KOH treatment provided more active sites for CO2 adsorption and enabled greater electron transfer to CO2 and facilitated its reduction to CH4. Though the surface termination groups can be changed through annealing and KOH treatments, –F groups could not be completely exchanged. More studies to precisely tailor the termination groups need to be carried out in the future.

Fig. 3
figure 3

TEM images of photocatalysts combined with a MXene by mechanical mixing taken from a An et al. Reprinted with permission from Ref. [72]. Copyright 2018 The Royal Society of Chemistry. b Xie et al. Reprinted with permission from Ref. [73]. Copyright 2018 Elsevier. c Ye et al. Reprinted with permission from Ref. [69]. Copyright 2018 John Wiley & Sons. d Liu et al. Reprinted with permission from Ref. [44]. Copyright 2018 Elsevier

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Fig. 4
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Hydrogen production of different samples taken from a An et al. Reprinted with permission from Ref. [72]. Copyright 2018 The Royal Society of Chemistry. b Sun et al. Reprinted with permission from Ref. [74]. Copyright 2018 The Royal Society of Chemistry. c Photo-degradation of 4-nitroaniline (4-NA) over different samples from Xie et al. Reprinted with permission from Ref. [73]. Copyright 2018 Elsevier. d Rates of evolution of CO and CH4 over different samples from Ye et al. Reprinted with permission from Ref. [69]. Copyright 2018 John Wiley & Sons. e Ciprofloxacin degradation from Liu et al. Reprinted with permission from Ref. [44]. Copyright 2018 Elsevier

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Xie et al. [73] used an electrostatic self-assembly process to combine positively charged CdS nanosheets and Ti3C2 nanosheets (possessing negative charge) (Fig. 3b) for CO2 reduction (Fig. 4c). Cai et al. [75] synthesized Ag3PO4/Ti3C2 by electrostatically driven self-assembly method, which had the advantage of being a mild method that prevented Ti3C2 from oxidation. The composites showed better performance than reduced graphene oxide (rGO), and this preparation procedure provided a new direction to the preparation of semiconductor-MXene composites. Liu et al. [44] fabricated a 2D layered and stacked g-C3N4/Ti3C2 composite by evaporation-induced self-assembly and used it to degrade organic pollutants (ciprofloxacin) (Fig. 3d). Both photogenerated holes and superoxide radicals (·O2−) resulting from photogenerated electrons played important roles in ciprofloxacin decomposition (Fig. 4f); in this process, self-assembly was an efficient method that allowed intimate mixing of the components in the composite. The sample was also more homogeneous than mechanically mixed ones because of the electrostatic attraction between the charged entities. However, opposite charges on each surface were required for self-assembly, which limited wider application of this process. Therefore, other techniques to induce self-assembly such as evaporation-induced self-assembly were developed to widen the range of application of products [44].

The above-mentioned MXene-based composites prepared by mechanical mixing and self-assembly methods for photocatalysis application are summarized in Table 1. Results from all these works prove that 2D MXene is an efficient additive material to enhance charge separation and charge transfer during photocatalysis. In these two methods, the properties of MXenes are retained by avoiding high temperature and use of other solvents or surfactant. No change in oxidation or surface termination groups occurs in these synthesis methods. Therefore, these two are the easiest and allow synthesis under the mildest conditions.

Table 1 MXene-based composites prepared by different synthetic methods for photocatalysis applications
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2.2 In Situ Decoration of Semiconductors onto the Surface of MXenes

In contrast to composites prepared by mechanical mixing of materials, in situ decoration methods consist in synthesizing a different material directly onto the MXene surface. As a result, in situ synthetized materials and MXenes are chemically bonded, which could be an important advantage in some designs. However, the range of viable synthetic conditions for in situ decoration is limited, because MXenes are easily oxidized in solution, especially at high temperatures [107]. It is therefore necessary to use mild conditions to protect MXenes from oxidation, especially when mono- and few-layered MXenes are used. So far, g-C3N4, TiO2, CdS, and bismuth compounds have been bonded to various MXenes using this strategy.

g-C3N4 is one 2D semiconductor material that is combined with MXenes used as a co-catalyst in the photocatalysis process (Fig. 5). MXene can be added during the calcination of a precursor, such as melamine and thiourea, but the high calcination temperature (around 550 °C) may cause the oxidation of MXene into TiO2. The high photoactivity of g-C3N4/MXene is attributed to the efficient charge separation; moreover, the heterojunction formed by TiO2/g-C3N4 also plays an important role in charge separation [108]. Shao et al. [81] synthesized Ti2C/g-C3N4 by melamine calcination and used it in hydrogen production (Fig. 5a, d). Though the ratio of Ti2C in the composite was as low as 0.4 wt%, a peak due to TiO2 resulting from the oxidation of Ti2C could be seen in the XRD pattern. Liu et al. [19] synthesized TiO2@C/g-C3N4 heterojunction by melamine calcination (Fig. 5b), where Ti3C2 was oxidized to TiO2@C during the calcination process. This composite was highly effective in the reaction of nitrogen reduction to ammonia, with the best performance reaching as high as 250.6 μmol h−1 g−1, which was better than that of TiO2@C and g-C3N4 (Fig. 5e). Xu et al. [82] synthesized Ti3+-rich Ti3C2/g-C3N4 by calcination of thiourea and employed it as an electrode for CO2 reduction in a photoelectrocatalytic (PEC) system (Fig. 5c, f), achieving a total CO2 reduction rate of 25.1 mmol h−1 g−1. The Ti3+ species suppressed charge recombination at the Ti3C2/g-C3N4 heterojunctions, leading to a corresponding increase in CO2 conversion efficiency.

Fig. 5
figure 5

Photocatalytic composites based on MXene in combination with g-C3N4 formed by in situ decoration. a–c TEM images, d XRD patterns, e hydrogen production, and f mechanism for PEC reduction of CO2 from Shao et al. a, d Reprinted with permission from Ref. [81]. Copyright 2017 The Royal Society of Chemistry; b, e with permission from Ref. [19], Copyright 2018 The Royal Society of Chemistry; and c, f with permission from Ref. [82]. Copyright 2018 The Royal Society of Chemistry

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Apart from the above-mentioned synthesis methods, composite photocatalysts can also be synthesized by combining TiO2, a metal sulfide, or a bismuthide with MXene under hydrothermal conditions (Fig. 6). Gao et al. [83] synthesized TiO2/Ti3C2 nanocomposites by a hydrothermal method using TiSO4 as a precursor for methyl orange (MO) degradation (Fig. 6a), where small TiO2 particles could be observed on the surface of multilayered Ti3C2. Wang et al. [84] employed TiCl4 as the precursor in the hydrothermal synthesis of rutile TiO2/Ti3C2Tx for hydrogen production by water splitting (Fig. 6d). The photocatalytic activity of TiO2 when combined with other MXenes (Ti2CTx and Nb2CTx flakes) as co-catalysts was also explored; results proved that in general, MXenes could be used as effective co-catalysts for solar hydrogen production. Ran et al. [70] combined CdS and Ti3C2 particles by a one-step hydrothermal reaction (Fig. 6b). A hydrogen production rate of 14,342 μmol h−1 g−1 was achieved when using Ti3C2 as the co-catalyst; this performance is 136.6 times higher than that of the pure CdS photocatalyst. The effectivity and versatility of Ti3C2 MXene as a co-catalyst for photocatalytic hydrogen production was demonstrated by other metal sulfides (ZnS) [91] photocatalysts as well. Xie et al. [73] showed that Ti3C2 flakes enabled the local confinement of Cd2+ released during photo-corrosion and thus enhanced the stability of the metal sulfide. Besides CdS, In2S3/Ti3C2Tx hybrids synthesized by hydrothermal method have been used for methyl orange degradation as reported by Wang et al. [90]. Among the hybrids based on other additives (carbon nanotubes (CNT), rGO, MoS2, and TiO2), Ti3C2-based composites showed the best photocatalytic activity, which is attributed to their high electrical conductivity. Shi et al. [85] synthesized TiO2/C/BiVO4 composites by hydrothermal method for the degradation of Rhodamine B, where Ti3C2 was employed both as a support for the growth of BiVO4 nanoparticles and as a precursor for the generation of 2D-carbon upon oxidation. The electron transfer process was accelerated by the presence of Ti3C2-derived 2D-carbon layers, thus improving the photocatalytic performance for Rhodamine B degradation. Ultrathin 2D/2D heterojunction of MXene/Bi2WO6 prepared by the in situ growth of ultrathin Bi2WO6 nanosheets on the surface of ultrathin Ti3C2 nanosheets for photocatalytic CO2 reduction was reported by Cao et al. [88] (Fig. 6c). The CH4 and CH3OH yield were 4.6 times higher than those obtained with pristine Bi2WO6, which was ascribed to the enhanced CO2 adsorption arising from the increased specific surface area and improved pore structure of the layered heterojunction. The different composites/hybrids containing MXene or MXene-derived products prepared by hydrothermal methods and used in photocatalysis are listed in Table 1.

Fig. 6
figure 6

Photocatalysts based on in situ decoration of MXenes. SEM images from a Gao et al. Reprinted with permission from Ref. [83]. Copyright 2015 Elsevier. b Ran et al. Reprinted with permission from Ref. [70]. Copyright 2017 Nature Publishing Group. c TEM images from Cao et al. Reprinted with permission from Ref. [88]. Copyright 2018 John Wiley & Sons. d Hydrogen production from Wang et al. Reprinted with permission from Ref. [84]. Copyright 2016 John Wiley & Sons. e Degradation of methyl orange (MO) from Wang et al. Reprinted with permission from Ref. [90]. Copyright 2018 Elsevier

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The synthetic process for MXenes-based composites includes doping into the photocatalysts or using MXene as a support for in situ decoration of the semiconductor photocatalyst. The chemical reactions taking place during photocatalyst formation led to increased interfacial area, thus providing greater possibilities for the transfer of photogenerated electrons. However, one disadvantage of this method is the oxidation of MXenes during photocatalyst synthesis. Although difficult to precisely characterize, conditions of formation of the photocatalysts may be too harsh and cause structural degradation of MXenes, especially in the case of single-layered MXenes, due to their lower stability toward oxidation.

2.3 MXene-Derived Photocatalysts

Different from mechanical mixing, self-assembly, and decoration methods, the in situ oxidation method using MXene (Ti3C2 is the most studied example) as a precursor for the synthesis of photocatalysts has also been explored (Fig. 7). Peng’s group tuned the facet of TiO2/Ti3C2 using a hydrothermal method without using an additional TiO2 precursor (Fig. 7a, b) [71, 93]. NaBF4 and NH4F were used as reagents to, respectively, control morphology in the synthesis of (001) TiO2/Ti3C2 and (111) TiO2/Ti3C2, which were then applied in methyl orange degradation. Both the facet type of TiO2 and the ratio of TiO2 to Ti3C2 could be controlled by changing the duration of the hydrothermal reaction. Jia et al. [94] obtained closely aggregated TiO2 nanorods with high carbon doping starting from Ti3C2 flakes and demonstrated a better photoactivity than commercially available P25 for hydrogen production (Fig. 7c). The carbon doping also changed the electron structure of TiO2 and enhanced its light absorption ability. Peng et al. [95] also used Ti3C2 as a hole trap and Cu as an electron trap to separate the charges through a dual-carrier-separation mechanism, showing the potential of MXene as an efficient functional material for photocatalysis (Fig. 7d).

Fig. 7
figure 7

In situ oxidized MXenes by hydrothermal method for photocatalysis. SEM images taken from a Peng et al. Reprinted with permission from Ref. [71]. Copyright 2016 American Chemical Society. b Peng et al. Reprinted with permission from Ref. [93]. Copyright 2017 Elsevier. c TEM image from Jia et al. Reprinted with permission from Ref. [94]. Copyright 2018 American Chemical Society. d Charge transfer in Cu/TiO2@Ti3C2(OH)x from Peng et al. Reprinted with permission from Ref. [95]. Copyright 2018 Elsevier

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Calcination under atmosphere containing gases such as CO2 and O2 is another method used for the controlled oxidation of MXenes (Fig. 8). Lu et al. [96] obtained Ti3C2/TiO2/CuO by annealing Cu(NO3)2 and Ti3C2 together under argon atmosphere (Fig. 8a). Because of its good electronic conductivity, the incorporation of Ti3C2 improved electron/hole separation and led to better methyl orange degradation. Yuan et al. [97] annealed Ti3C2 in CO2 to prepare 2D-layered C/TiO2 hybrids used in hydrogen production, in which the presence of 2D carbon layers increased electron transport channels and enhanced charge separation efficiency (Fig. 8b). In addition, the effects of oxidation temperature and CO2 on the grain size and crystal structure of TiO2 were also investigated, revealing that increasing oxidation temperature and CO2 gas flux led to larger grain sizes and more rutile TiO2 formation. Low et al. [98] calcined Ti3C2 at different temperatures, enabling the in situ growth of TiO2 nanoparticles on Ti3C2 nanosheets, thus forming TiO2/Ti3C2 composites with different loading amounts of TiO2 with the aim to improve performance in CO2 reduction reaction (Fig. 8c). Interestingly, three main products were obtained during the photocatalytic CO2 reduction process due to the sufficiently high intrinsic reduction potential of TiO2. Results of the study also pointed out that excess of Ti3C2 in the composite could have an adverse effect on photocatalytic performance. Su et al. [99] used CO2 to partially oxidize Nb2C to form Nb2O5/Nb2C composites for hydrogen production, where Nb2O5 and metallic Nb2C served, respectively, as the semiconductor photocatalyst and co-catalyst (Fig. 8d). The easily formed junction at the interface served as an electron sink to efficiently capture photogenerated electrons and suppress recombination of photogenerated electron–hole pairs, thus enhancing the efficiency of charge separation and contributing to improved photocatalytic activity [71, 93, 99, 102].

Fig. 8
figure 8

Photocatalysts containing in situ oxidized MXenes formed by calcination. SEM images from a Lu et al. Reprinted with permission from Ref. [96]. Copyright 2017 Hindawi. b Yuan et al. Reprinted with permission from Ref. [97]. Copyright 2017 John Wiley & Sons. c Gaseous products of CO2 reduction from Low et al. Reprinted with permission from Ref. [98]. Copyright 2018 Elsevier. d TEM image from Su et al. Reprinted with permission from Ref. [99]. Copyright 2018 John Wiley & Sons

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Besides the hydrothermal method and calcination, other routes such as chemical oxidization and high-energy ball milling were also used to oxidize MXenes (Fig. 9). Cheng et al. [100] oxidized Ti3C2 flakes with 30% H2O2 to form microporous-MXene/TiO2−x nanodots (Fig. 9a). This composite worked as a photo-Fenton bifunctional catalyst for Rhodamine B degradation under both dark and illumination conditions. Li et al. [101] synthesized TiO2@C nanosheets from Ti2C by high-energy ball milling and used it for methylene blue degradation (Fig. 9b). Shortly thereafter, our group used water to oxidize Ti3C2 to be applied in hydrogen production using Eosin Y as a sensitizer [102]. Similar to other oxidized MXenes, amorphous carbon and TiO2 were formed after oxidation (Fig. 9c, d). The various MXene-derived composites obtained by in situ oxidation to be used as photocatalysts are listed in Table 1.

Fig. 9
figure 9

MXene-derived photocatalysts synthesized by other in situ oxidation methods. a Mechanisms of degradation over mp-MXene/TiO2-x from Cheng et al. Reprinted with permission from Ref. [100]. Copyright 2018 The Royal Society of Chemistry. b TEM image from Li et al. Reprinted with permission from Ref. [101]. Copyright 2018 Elsevier. c Raman spectra and d TGA from Sun et al. Reprinted with permission from Ref. [102]. Copyright 2018 The Royal Society of Chemistry

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The MXenes oxidation is different from other methods because of the residual presence of carbon (mostly amorphous carbon) after oxidation, and the M element is oxidized into metal oxide on the carbon layer. Thus, the composite obtained is of the form metal oxide/MXenes/C. Both MXenes and C can be used as co-catalysts in the photocatalysis process. However, in this method, the ratio of the photocatalyst to MXenes varies within a certain range since no precursor is introduced. The limitation of this method is that only a few semiconductors (depending on M element) can be used as the photocatalyst.

3 Mechanism of MXenes as Co-catalysts

Since MXenes are conductors and serve as co-catalysts, the mechanism of action of a MXenes-based photocatalytic system is through accelerated charge separation and suppression of carrier recombination [69,70,71]. The photocatalysts absorb visible light and photogenerated electrons are excited to the CB, while holes are left in the valence band (VB). The excited charge carriers are transferred to MXenes at the interface mainly because of the higher potential of MXenes. Electrons transfer to MXenes without recombination and react on the MXene surface to generate H2 by reducing H+ [74, 78, 81, 91, 94, 102, 103], CH4 and CO by reducing CO2 [88, 98], or NH3 by reducing N2 [19], as shown in Fig. 10 process (a). In process (b), holes transfer to MXenes and react to produce OH· that can be utilized for degradation of organics [71, 93, 95]; electrons can also produce OH· for organic degradation [71, 93]. The charge transfer process from the photocatalyst to MXenes improves electron–hole pair separation and suppresses charge recombination in photocatalysts, thus enhancing the photoactivity.

Fig. 10
figure 10

Schematic of the working mechanism of MXenes applied in photocatalysis

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Another advantage of using MXenes in photocatalysis is due to their termination groups. For example, –O termination groups show the best potential for hydrogen production because of their low |ΔGH| and the availability of active sites for the adsorption of hydrogen atoms [70, 74]. Though termination groups are important in photocatalysis, currently, it has not been possible to precisely control the relative concentrations of the different termination groups. Using presently available synthetic methods, changing the different reaction conditions can partially modify the termination groups on MXenes surface and thereby affect their performance in photocatalysis.

4 Conclusion and Outlook

In summary, the application of MXenes in photocatalysis has shown rapid development since 2015. Among the MXenes family, Ti3C2 has been the most studied MXene. Mechanical mixing and self-assembly are mild and easy methods of synthesis, where the ratio of MXenes to the photocatalyst can be controlled. In addition, MXenes can also be doped into the photocatalysts by in situ decoration of a semiconductor photocatalyst. The large interfacial area afforded by the doping process improves electron transfer. However, the MXenes oxidation method has the advantage of obtaining both carbon and MXenes as co-catalysts by forming a metal oxide/MXenes/C structure. Though the above-mentioned four synthetic methods are generally used for photocatalysts, with further development in the field of MXenes, new processes may be discovered.

Besides developing improved synthetic methods, the other aspects that need to be focused on in the future are as follows:

  1. 1.

    Controlling the morphologies of MXenes. MXene flakes show larger surface area than multilayered MXenes, since mono- or few-layered MXenes provide a greater number of active sites for photocatalytic reactions. The flakes are also convenient for building structures, such as quantum dots, spheres, and nanorods. However, the instability of MXenes should be taken into account during heat treatment [107].

  2. 2.

    MXenes combine with efficient photocatalysts. MXenes can be used as co-catalysts to combine with many semiconductor photocatalysts due to their excellent electronic conductivity and the presence of numerous hydrophilic groups on the surface. Hundreds of semiconductor photocatalysts have been reported for photocatalysis so far. Attention should be paid to combining the efficient and cheap photocatalysts with MXenes to achieve better photocatalytic performance. So far, only g-C3N4, CdS, ZnS, TiO2, CuO, Nb2O5, BiVO4, Ag3PO4, α-Fe2O3, In2S3, Bi2WO6, Bi0.90Gd0.10Fe0.80Sn0.20O3, and BiOBr have been explored, with TiO2 and g-C3N4 attracting the most attention.

  3. 3.

    Surface modification of MXenes. Surface termination groups significantly affect the properties of MXenes, and thus, tuning the surface termination groups and modifying the MXenes surface are expected to greatly influence its potential as co-catalyst.

  4. 4.

    Synthesis of new MXenes. To date, only a small fraction of the different possible MXenes has been synthesized in laboratories. Some MXenes showing semiconducting properties have been reported based on theoretical calculations. Theoretical predictions help in the synthesis of semiconductor MXenes and applied in photocatalysis. Once obtained experimentally, potential MXenes can be applied as photocatalysts, thus widening the application range of MXenes. Moreover, new types of transition metal borides (MBenes) have also been predicted [34, 109] and have shown potential for photocatalysis applications. More work needs to be done in this direction.

  5. 5.

    Developing new synthesis methods for MXenes. HF and in situ HF wet chemical treatment are by far the most used methods in MXenes synthesis. Other HF-free methods are emerging and leading to MXenes with different properties. Yet, these have not been investigated in photocatalytic applications, and thus, the effect of the type of synthesis process used on the final performance of the MXene is currently not understood.

In short, due to tremendous effort of scientists worldwide, the great potential of MXenes in photocatalysis has been revealed. With the fast-growing development in this area, it is expected that more and more studies will focus on the applications of MXenes photocatalysis and pave the way to the commercialization of photocatalytic technologies based on these materials.