Novel Architecture Titanium Carbide (Ti₃C₂T_x) MXene Cocatalysts toward Photocatalytic Hydrogen Production: A Mini-Review

Abstract

Low dimensional transition metal carbide and nitride (MXenes) have been emerging as frontier materials for energy storage and conversion. Ti₃C₂T_x was the first MXenes that discovered and soon become the most widely investigated among the MXenes family. Interestingly, Ti₃C₂T_x exhibits ultrahigh catalytic activity towards the hydrogen evolution reaction. In addition, Ti₃C₂T_x is electronically conductive, and its optical bandgap is tunable in the visible region, making it become one of the most promising candidates for the photocatalytic hydrogen evolution reaction (HER). In this review, we provide comprehensive strategies for the utilization of Ti₃C₂T_x as a catalyst for improving solar-driven HER, including surface functional groups engineering, structural modification, and cocatalyst coupling. In addition, the reaming obstacle for using these materials in a practical system is evaluated. Finally, the direction for the future development of these materials featuring high photocatalytic activity toward HER is discussed.

1. Introduction

To date, sustainable solar hydrogen (H₂) production, which directly produces by utilizing semiconductor photocatalysts, could provide a promising and environmental-friendly approach to solve the worldwide energy issues and reduce the dependence on fossil fuels [1,2]. Particularly, enormous progress has been made in developing a new system of photocatalysts such as transition metal dichalcogenides [3,4,5,6,7,8,9,10,11,12,13,14,15], transition metal oxide (TMOs) [16,17], transient metal sulfides (TMSs), graphitic carbon nitride (g–C₃N₄) [18,19,20,21,22], metal–organic framework (MOFs) [23,24,25], transition metal nitride (TMNs) [26], and transition metal carbide (TMCs) [27,28,29,30] that could efficiently enhance the H₂ production, and readily scale up for commercialization [2].

As an advanced and broad group of novel nanostructured materials, MXenes has been discovered and synthesized from the parent layered solids MAX phases (as shown in Figure 1) [31].

Figure 1.

The schematic diagram is representing the process of synthesizing MXenes from MAX phases. Reproduced with permission from [31]. Copyright Wiley-VCH, 2014 and [32] Copyright American Chemical Society, 2012.

In essence, the chemical formula of MAX phases is M_n+1AX_n, which is defined by Barsoum [33,34,35,36,37]. In detail, the M element stands for transition metals from groups 3 (Sc), 4 (Ti, Zr, and Hf), 5 (V, Nb, and Ta), and 6 (Cr and Mo), the A element represents from groups 12 (Cd), 13 (Al, Ga, In, and Tl), 14 (Si, Ge, Sn, and Pb), 15 (P and As), or 16 (S), and the X element is C and/or N [33,38,39]. MXenes are generally prepared by selectively getting rid of the element of A from the parent MAX phase to form M_n+1X_nT_x (n = 1–3), where T_x is the surface termination groups ((–O), (–F), and (–OH)) [31]. MXenes materials, which offer many advantages electronic, optical, plasmonic, and thermoelectric properties [36], have attracted much interest recently. They are currently explored for a variety of applications, including energy, environment, catalysis, photocatalysis, optical devices, electronics, biomedicals, sensors, electromagnetic, others, etc. (Figure 2) [40,41,42,43]. Among MXenes, many efforts have been devoted to promoting titanium carbide (Ti₃C₂T_x) as the most promising candidate of cocatalysts [44,45,46]. Based on the published literature dealing with MXenes, which was taken from 2011–2019 on the Web of Science, there was about 70% of researches on MXenes associated with Ti₃C₂T_x, as seen in the third ring of the pie chart in Figure 2 [40]. It also notes that the Ti₃C₂T_x also shows a high potential to replace the expensive Pt cocatalyst in photocatalysis. In 2017, Alhabeb et al. have provided an excellent report to give step-by-step guidance to preparing of Ti₃C₂T_x by using different etchants (HF and in situ HF) and delamination methods (Figure 3a) [38]. Their corresponded scanning electron microscopy (SEM) images were obtained and shown in Figure 3b–g. For detail, Ti₃AlC₂ sample show compactly layered morphology (Figure 3b), while the morphology of the multilayered Ti₃C₂T_x samples was influenced by weight percent (wt %) of HF (Figure 3c–e). On the other hand, the morphology of the multilayered NH₄–Ti₃C₂T_x sample (Figure 3f) and MILD-Ti₃C₂T_x sample (Figure 3g) are structurally similar to that of 5F–Ti₃C₂T_x multilayered powders (Figure 3e). Theoretically, the Ti₃C₂Tx fulfill the prerequisite requirement condition for applications as catalysts for HER. It has been reported that the O and F terminated Ti₃C₂ are metallic based semiconductors with a conductivity up to 9880 S·cm⁻¹, which is higher than that of graphene [47]. This indicates that the charge transfer between Ti₃C₂ to the active site is superior to most of the reported semiconducting catalysts. Furthermore, the H* adsorption energy on the surface of Ti₃C₂ is close to 0, making it the best among noble metal free catalysts for application in HER [48]. However, most MXenes including Ti₃C₂T_x are semiconductors with indirect bandgaps [49]. To apply as photocatalysts, T₃C₂T_x needs to pair with other photoactive materials such as TiO₂, CdS, g–C₃N₄ and metal organic frameworks (MOFs). Although, the development of MXenes for wide-range application in recent years have been thoroughly summarized and discussed [34,35,36,49,50,51], a review that focuses on Ti₃C₂T_x for photocatalytic HER has not been reported yet.

Figure 2.

The general applications and properties of MXenes. The center pie chart explored the applications and properties of MXenes. The starting year in the middle pie chart ring indicates the exploration time of each application/property. The outer ring shows the ratio of publications, which were taken from 2011 to 2019 on the Web of Science, with the term of Ti₃C₂T_x versus the publications deal with all MXene compositions (M₂XT_x, M₃X₂T_x, and M₄X₃T_x). Reproduced with permission from [40]. Copyright Springer Nature, 2019.

Figure 3.

(a) The schematic diagram representing the process to prepare Ti₃C₂T_x by using different etchants (HF and in situ HF) and delamination methods and (b–g) their corresponded scanning electron microscopy (SEM) images. Reproduced with permission from [37]. Copyright Royal Society of Chemistry, 2019.

In this review, we present the use of Ti₃C₂T_x as the most potential and promising cocatalysts toward photocatalytic hydrogen production. Based on the recent research works, the influence on different morphology (nanotubes, nanoscrolls, quantum dots, etc.), surface termination groups (–F, –OH, and –O), and photocatalyst systems (titania (TiO₂), graphitic carbon nitride (g–C₃N₄) coupled Ti₃C₂ photocatalysts, etc.) are reviewed and intensified. Additionally, attention and outlook on critical challenges, prospects, and potential applications for Ti₃C₂T_x cocatalysts toward sustainable solar hydrogen production are also highlighted.

2. Coupled Morphological and Structural Ti₃C₂T_x Cocatalysts

Since the morphology of photocatalysts could directly influence the photocatalytic process, active sites, and charge transfer, various nanostructures of Ti₃C₂T_x photocatalysts have been explored to improve the efficiency of H₂ production. However, it has not been shown yet, which types of morphology and structure of Ti₃C₂T_x cocatalysts perform the best photocatalytic H₂ production rate. In this section, the photocatalytic activity over different morphological and structural Ti₃C₂T_x cocatalysts was adequately highlighted and critically evaluated in terms of the H₂ production rate (μmol·g_cat⁻¹·h⁻¹) for convenient comparative purposes.

Su et al. prepared a series of Ti₃C₂T_x/TiO₂ composite photocatalysts with a monolayer and multilayers Ti₃C₂T_x as the cocatalyst (as shown in Figure 4a) [52]. The result showed that a monolayer Ti₃C₂T_x/TiO₂ composite exhibited the superior H₂ production rate (2650 μmol·g_cat⁻¹·h⁻¹) under a 200 W Hg lamp integrated with a cutoff filter of 285–325 nm, which had more than nine-fold and two-fold higher, compared to the pure TiO₂ (290 μmol·g_cat⁻¹·h⁻¹) and multilayer counterpart (920 μmol·g_cat⁻¹·h⁻¹), respectively. The enhancement of performance is possible due to the advanced electrical conductivity of a monolayer Ti₃C₂T_x and the effective charge-carrier separation at the Ti₃C₂T_x/TiO₂ interface. To propose a new morphology, Li et al. designed Ti₃C₂T_x/TiO₂ nanoflowers, which performed an outstanding H₂ production rate, compared with that of pure TiO₂ (as shown in Figure 4b) [53]. In detail, the Ti₃C₂T_x/TiO₂ nanoflower could reach to 526 μmol·g_cat⁻¹·h⁻¹ in the H₂ production rate under a 300 W Xe arc lamp, which was more than four-fold higher than that of the TiO₂ nanobelts (121.82 μmol·g_cat⁻¹·h⁻¹). It notes that under the same experimental conditions, the H₂ production rate was 371.17 μmol·g_cat⁻¹·h⁻¹ over the Pt/TiO₂ nanosheet. It suggests that the noble metal-free Ti₃C₂T_x was considered as an alternative cocatalyst to replace the expensive and precious noble metals, such as Pt, Au, etc. To further boost the H₂ production activity, Yuan et al. prepared the Ti₃C₂T_x nanofibers (NFs) structure by hydrolyzation and selective etching of Ti₃AlC₂ MAX ceramics (Figure 4c) [54]. Compared with traditional Ti₃C₂ flakes, the Ti₃C₂ NFs could provide a much higher BET (Brunauer–Emmett–Teller) surface area and expose more catalytic active sites, leading to enhanced H₂ production activity, high cycling stability, and long-term viability. Very recently, Li et al. had successfully designed Ti₃C₂T_x quantum dots (QDs) by a self-assembly method, which their schematic synthesis of g–C₃N₄@Ti₃C₂T_x QDs composites was shown in Figure 4d [55]. As expected, g–C₃N₄@Ti₃C₂T_x QDs composites performed the best photocatalytic activity (5111.8 μmol·g_cat⁻¹·h⁻¹) under artificial sunlight (300 W Xe arc lamp integrated with an AM-1.5 filter), which was nearly ten-fold higher than that of g–C₃N₄/Ti₃C₂T_x sheets (524.3 μmol·g_cat⁻¹·h⁻¹). Compared to the traditional Ti₃C₂T_x sheets, Ti₃C₂T_x QDs offered more abundant active edge sites, and excellent electronic conductivity. Additionally, the photoexcited carriers in g–C₃N₄@Ti₃C₂T_x QDs composites could be effectively separated to rapidly take part in photocatalytic H₂ production activity, leading to enhanced photocatalytic performance efficiently. Therefore, owing to excellent physical properties, g–C₃N₄@Ti₃C₂T_x QDs composites performed a remarkable enhancement in the photocatalytic H₂ production rate of 5111.8 μmol·g_cat⁻¹·h⁻¹, indicating its high potential to scale up and accelerate the H₂ production via the green photocatalysis approach.

Figure 4.

(a) Monolayer and multilayers Ti₃C₂T_x as the cocatalysts. Reproduced with permission from reference [52]. Copyright American Chemical Society, 2019; (b) the preparation of Ti₃C₂T_x/TiO₂ nanoflowers and their corresponding SEM images. Reproduced with permission from ref. [53]. Copyright Nature Publishing Group, 2018; (c) the preparation of Ti₃C₂T_x nanofibers and their corresponding SEM, TEM images. Reproduced with permission from ref. [54]. Copyright American Chemical Society, 2018; and (d) Schematic diagram for preparing of g–C₃N₄@Ti₃C₂T_x quantum dots composites. Reproduced with permission from ref. [55]. Copyright American Chemical Society, 2019.

The synthesis of different morphologies of Ti₃C₂T_x cocatalysts was successfully proposed. Based on the recent studies, morphologies of Ti₃C₂T_x (nanotubes, nanoscrolls, quantum dots, etc.), which might provide more BET surface area to enrich the active adsorption sites, and inhibit the recombination of e⁻–h⁺ pairs, resulting in effective influence to the photocatalytic activity, high cycling stability, and long-term viability.

3. Modified Ti₃C₂T_x Cocatalysts with Surface Termination Groups

In general, surface termination groups (–F, –OH, and –O) of Ti₃C₂T_x, which are predominantly dependent on the synthesis methods, have profoundly altered their physicochemical properties [56]. Based on theoretical calculations, many studies suggested that surface termination groups strongly influence the stability, electronic, optical, and transport properties of Ti₃C₂T_x [57,58,59,60]. Due to improving the photocatalytic activity toward sustainable solar hydrogen production, there has been motivation to enhance and control the physicochemical properties of Ti₃C₂T_x through surface termination groups. Li et al. found that the Ti₃C₂T_x/TiO₂ hybrids, which synthesized through simple calcination of F-Ti₃C₂T_x, exhibited potential photocatalytic activity. Its performance was two-fold higher than that of the Ti₃C₂T_x/TiO₂ hybrids with calcining OH-Ti₃C₂T_x [37]. On the other hand, Ran et al. used density functional theory (DFT) calculations for designing and exploring the potential of novel Ti₃C₂T_x nanoparticles as a promising H₂ production cocatalyst [61]. They replaced the (–F) terminations by (–O)/(–OH) terminations by the hydrothermal treatment, and found that (–O)/(–OH) terminations play a notable role for photocatalytic activity. This result was consistent with the previous finding by Sun et al. [56], who observed significant enhancement of H₂ production (88 μmol·g_cat⁻¹·h⁻¹) over O-Ti₃C₂T_x, compared to control samples. To further modify the surface termination groups of Ti₃C₂T_x, Yang et al. successfully prepared O-Ti₃C₂T_x/CdS hybrids through the radiofrequency oxygen plasma method (O₂/N₂, 2.2 Pa, 500 °C, 1400 W, 2.45 GHz, and 30 min), providing (a) sufficient catching water molecules and hydrogen ions on the surface of the catalyst, and (b) stable transfer channel for electrons to repress the recombination of e⁻–h⁺ pairs [62]. In another approach, Xu et al. carried out a plasma treatment (N₂/H₂, atmosphere, 500 °C, 1400 W, and 30 min) for preparing layered g–C₃N₄/plasma-treated Ti₃C₂T_x photocatalyst [63]. Based on analyzed results by Raman, FTIR, and XPS, Xu et al. observed an increase of Ti–O with a decrease of Ti–C, Ti–F, and Ti–OH. Additionally, the plasma-treated Ti₃C₂T_x photocatalyst worked as an excellent acceptor of photogenerated electrons, leading to substantially reinforce the photocatalytic activity. Though the surface termination groups could be modified by several methods, such as hydrothermal treatment, simple calcination, plasma treatment, etc., more studies that elucidate the modification mechanism of surface termination groups need to be paid attention in the future.

4. The Design of Ti₃C₂T_x Composite Photocatalysts

4.1. Couple with Transition Metal Oxide (TMOs)

Transition metal oxide, such as titanium dioxide (TiO₂), coupled photocatalysts have attracted dramatically increasing interest in the area of photocatalytic hydrogen generation [64,65,66]. Their photocatalytic activities have been markedly improved through the efforts of many research groups. However, its large bandgap and fast charge recombination limit its efficiency. To overcome this limitation, Ti₃C₂T_x has been considered as promising cocatalysts for hydrogen production with TiO₂ as the photocatalyst. Zhuang has successfully prepared TiO₂/Ti₃C₂T_x nanocomposites by the electrostatic self-assembly technique (Figure 5) [67]. Owing to the highly efficient separation of photogenerated carriers, which derived from the intense interfacial contact between TiO₂ nanofibers and Ti₃C₂T_x nanosheets, the photocatalytic performance over TiO₂/Ti₃C₂T_x nanocomposites was significantly improved. The H₂ production rate was up to 6979 μmol·g_cat⁻¹·h⁻¹ using a 10% methanol solution as the sacrificial electron donors under a 300 W Xe lamp, which was 3.8 times higher than that of pure TiO₂ nanofibers. There was no hydrogen production capacity over Ti₃C₂T_x nanosheets due to its metallic character.

Figure 5.

Schematic illustration displaying procedure for fabrication of TiO₂/Ti₃C₂T_x composite. Reproduced with permission from reference [67]. Copyright Elsevier B.V., 2019.

To simplify the synthesis method, simple calcination was first proposed by Li et al. to prepare truncated octahedral bipyramidal TiO₂ (TOB-T)/Ti₃C₂T_x hybrids [37]. The resultant TiO₂/Ti₃C₂T_x hybrids retained the multilayer structure, and TiO₂ exhibited a truncated octahedral bipyramidal structure with exposed (001) and (101) facets. A surface heterojunction between (101) and (001) facets was established, and it could prevent the recombination of photogenerated carriers in TiO₂. Moreover, the remaining Ti₃C₂T_x could act as a cocatalyst to accelerate the migration of photoinduced electrons because of its high electronic conductivity. Meanwhile, the concentration of fluorine sharply decreased during calcination, thereby reducing the toxicity and increasing the conductivity of the samples. They pointed out that Ti₃C₂T_x could enhance the photocatalytic activity of those composite photocatalysts due to the Schottky junction between Ti₃C₂T_x and TiO₂ and its excellent electronic conductivity. Besides TiO₂, ZnO has also been investigated for hydrogen production [68]. It was experimentally demonstrated that the ZnO nanorods (NRs)/Ti₃C₂T_x hybrids exhibited the inferior photocatalytic H₂ production activity (456 μmol·h⁻¹), while pure ZnO NRs displayed no performance [68]. However, the photocatalytic activity of the ZnO/Ti₃C₂T_x composite was still much lower compared to the TiO₂/Ti₃C₂T_x, thus, more investigation is necessary.

4.2. Couple with Transient Metal Sulfides (TMSs)

Transition metal surface such as CdS [69,70,71], CdSe [72], MoS₂ [73,74,75], and WS₂ [76,77,78] has been demonstrated as potential catalysts for electrocatalytic and photocatalytic HER. Therefore, the coupling of these materials with Ti₃C₂T_x might produce the composite with unprecedented performance in photocatalytic HER. As expected, Ran et al. coupled O–Ti₃C₂T_x with cadmium sulfide (CdS) via a hydrothermal method to yield a composite catalyst for HER with very high performance [61]. In specific, the catalysts with the optimized composition (2.5 wt % Ti₃C₂T_x) can produce up to 14,342 μmol·g_cat⁻¹·h⁻¹, which was higher than that of Pt-CdS (10,978 μmol·g_cat⁻¹·h⁻¹). The HR-TEM and SEM images of the O–Ti₃C₂T_x coupled CdS nanoparticles are shown in Figure 6a–b. The high photocatalytic HER performance of the O-Ti₃C₂T_x/CdS composite attributed to very low free energy for atomic H adsorption on the surface of O–Ti₃C₂T_x (Figure 6c) and efficient charge generation and separation upon light at the interface of the composites (Figure 6d–e). Similarly, Xiao et al. coupled Ti₃C₂T_x with CdS nanorod to construct a Schottky heterojunction for photocatalytic HER [79]. As a result, the CdS nanorod/Ti₃C₂T_x nanosheet exhibited a performance 7-fold higher than that of pristine CdS [79]. The improvement was postulated to originate from the synergistic effect between the CdS nanorod and Ti₃C₂T_x nanosheets that improves light absorption, charge separation, and conductivity of the composite catalysts. Tie et al. decorated ZnS nanoparticles with Ti₃C₂T_x nanosheets to yield photocatalytic HER with a production rate of 502.6 μmol·g_cat⁻¹·h⁻¹ under optimal conditions, is almost 4-fold higher than pure ZnS (124.6 μmol·g_cat⁻¹·h⁻¹) [80]. Besides, the alloy transition metal sulfide/Ti₃C₂T_x was also investigated. For example, Cheng et al. demonstrated a high-performance composite for photocatalytic HER composed of CdLa₂S₄ and Ti₃C₂T_x nanocomposite [81]. In specific, these composite nanomaterials yield photocatalytic HER with the H₂ production rate of 11,182.4 μmol·g_cat⁻¹·h⁻¹, and apparent quantum efficiency reached 15.6% at 420 nm. The performance of CdLa₂S₄/Ti₃C₂T_x nanocomposite, therefore, improves the production rate up to 13.4 times compared to that of pristine CdLa₂S₄ and even higher than that of Pt/CdLa₂S₄. To sum up, Ti₃C₂T_x couple with TMSs could reach to a desirable level. In detail, 2.5 wt % Ti₃C₂T_x/CdS and ZnS/Ti₃C₂T_x nanosheets exhibited very attractive photocatalytic activity, making them good candidates for photocatalytic HER.

Figure 6.

(a,b), TEM and SEM images of Ti₃C₂T_x/CdS composite structure; (c) the calculated free-energy band diagram of HER with different catalysts including MoS₂, WS₂, and O-Ti₃C₂T_x; (d) band diagram of Ti₃C₂T_x/CdS showing the charge separation and transferring from CdS to Ti₃C₂T_x for HER; and (e) the proposed mechanism of HER over Ti₃C₂T_x/CdS composite. Reproduced with permission from reference [61]. Copyright Nature Publishing Group, 2017.

4.3. Couple with the Metal–Organic Framework

MOFs and their derivative have been emerging as efficient catalysts for photo electrocatalytic HER. The first combination of Ti₃C₂T_x/MOFs composite was reported by Tian et al. in 2019 [82]. The TEM images in Figure 7a,b indicated that the MOFs were well connected with the MOFs. As a result, the Ti₃C₂T_x/MOFs composite displays photocatalytic activity better than the Pt decorated MOFs (2 wt % Pt/UiO-66-NH₂). The performance of Ti₃C₂T_x/MOFs can be observed in Figure 7c. The schematic illustration of energy band alignment between Ti₃C₂T_x and MOFs is shown in Figure 7d. Under sunlight irradiation, the electron-hole pairs were generated in MOFs. Owing to the good contact and conductivity, the photo-induced electron can be easily transferred to the Ti₃C₂T_x surface to participate in the HER, thus, improving the overall performance of the composite catalysts.

Figure 7.

(a,b)TEM images presented the formation of Ti₃C₂T_x and Zr-MOFs heterostructure; (c) Hydrogen production rates of Ti₃C₂T_x/Zr-MOF with different concentrations of Ti₃C₂T_x; (d) Energy band diagram of Ti₃C₂T_x/Zr-MOF for photocatalytic HER. Reproduced with permission from reference [82]. Copyright Elsevier B.V., 2019.

4.4. Coupled with Graphitic Carbon Nitride (g–C₃N₄)

Graphitic carbon nitride (g–C₃N₄) coupled photocatalysts have attracted dramatically increasing interest in the area of visible-light-induced photocatalytic hydrogen generation due to the unique electronic band structure and high thermal and chemical stability of g–C₃N₄ [83,84,85]. Besides, the work had been done by Li et al. in the previous section, g–C₃N₄@Ti₃C₂T_x QDs [55], another study that couples Ti₃C₂T_x/g–C₃N₄ has also been reported. Typically, Su et al. constructed a heterojunction using Ti₃C₂T_x and g–C₃N₄ nanosheets via the electrostatic self-assembly method [86]. A small amount of Ti₃C₂T_x was loaded onto g–C₃N₄, with a concentration that ranged from 1% to 5%. Interestingly, the Ti₃C₂T_x/g–C₃N₄ exhibits significantly improved photocatalytic activity towards HER compared to that of pristine g–C₃N₄ [86]. Instead of using pristine g–C₃N₄, Lin et al. used O-doped g–C₃N₄ to form the heterostructure with Ti₃C₂T_x to improve the H₂ production rate of catalysts two-fold [87]. The fabrication process for constructing Ti₃C₂T_x/O-doped g–C₃N₄ is shown in Figure 8a. The SEM and TEM images in Figure 8b–d indicates that well interspersed Ti₃C₂T_x/O-doped g–C₃N₄ heterostructure was obtained. As a result, the Ti₃C₂T_x/O-doped g–C₃N₄ yield H₂ with a production rate of 25,124 μmol·g_cat⁻¹·h⁻¹, whereas, pristine O-doped g–C₃N₄ and Ti₃C₂T_x/pristine g–C₃N₄ exhibit a lower H₂ generation rate of 13,745 and 15,573 μmol·g_cat⁻¹·h⁻¹, respectively. Figure 8e indicates that the electron from O-doped g–C₃N₄ can be easily transferred to Ti₃C₂T_x for the HER. These results suggested that g–C₃N₄ is a very good photoactive material to pair with Ti₃C₂T_x to yield efficient photocatalytic HER. However, the research related to this topic is still very limited, thus it needs more investigation in the near future.

Figure 8.

(a) Fabrication process of the Ti₃C₂T_x/O-doped g–C₃N₄ heterostructure. (b–d) SEM images, TEM images, and EDS spectra of Ti₃C₂T_x/O-doped g–C₃N₄. (e) The working mechanism of Ti₃C₂T_x/O-doped g–C₃N₄ photocatalyst. Reproduced with permission from reference [87]. Copyright Elsevier B.V., 2019.

4.5. Ternary Composites

Apart from binary composites, ternary composites of Ti₃C₂T_x have also been rationally developed. To obtain the ternary composite catalyst, Tial et al. first introduced TiO₂ onto the surface of Ti₃C₂T_x via thermal annealing at 600 °C under N₂ atmosphere [88]. After that, the Zr–MOF (UiO-66-NH₂) was growth on Ti₃C₂T_x/TiO₂ using a facile hydrothermal approach. The schematic illustration of the synthesis procedure is shown in Figure 9a. The TEM displaying the ternary phase of the composite is presented in Figure 9b. It can be observed that the ternary structure was well established. As a consequence, the ternary composite (Ti₃C₂T_x/TiO₂/UIO-66-NH₂) exhibited a performance two times higher than that of the binary composite (Ti₃C₂T_x/UIO-66-NH₂). The improvement in the catalytic activity of the Ti₃C₂T_x/TiO₂/UIO-66-NH₂) not only comes from the improvement of the light absorption by using a double light absorber (TiO₂/UIO-66-NH₂) but also the enhancement of the charge separation of collection efficiency. The working mechanism of the binary and ternary composite was clearly illustrated in Figure 9c. Additionally, by taking advantage of the ternary composites with the composition of Mo_xS/TiO₂/Ti₃C₂T_x, Li et al. improved the H₂ production rate up to 10,505.8 μmol·g_cat⁻¹·h⁻¹, which was 193 times compared to that of pristine TiO₂ [46]. Similarly, many other ternary composites have been constructed with excellent photocatalytic activity towards HER such as Mo_xS@TiO₂@Ti₃C₂T_x [46], Cu/TiO₂@Ti₃C₂T_x [89], 1T–MoS₂/Ti₃C₂T_x/TiO₂ [90], 1T–WS₂@TiO₂@Ti₃C₂T_x [91], Cu₂O/(001)/TiO₂/Ti₃C₂T_x [92], Ti₃C₂T_x/TiO₂/g–C₃N₄ [93], g–C₃N₄/Ti₃C₂T_x/Pt [45], CdS/MoS₂/Ti₃C₂T_x [94], and TiO₂/Ti₃C₂T_x/CoS_x [95]. However, it is noted that a multicomponent photocatalytic hybrid composed of MXene with other cocatalysts are still in an early stage and requires further efforts.

Figure 9.

(a) Route for the synthesis of Ti₃C₂T_x/TiO₂/UiO-66-NH₂ ternary composite, (b) TEM image of the Ti₃C₂T_x/TiO₂/UiO-66-NH₂ ternary composite, and (c) working mechanism of ternary composite photocatalyst for HER. Reproduced with permission from reference [88]. Copyright Elsevier B.V., 2019.

5. Comparison of the Photocatalytic Hydrogen Production

To sum up, a detailed summary and comparison of recently reported Ti₃C₂T_x cocatalysts toward photocatalytic hydrogen production are given in Table 1. Although the experimental reaction conditions were different, we compared the photocatalytic activity in terms of the H₂ evolution rate. Then, all the evolution rate of H₂ were obtained and transformed into a logical unit (μmol·g_cat⁻¹·h⁻¹) for acceptable comparative purposes. We found that Ti₃C₂T_x/O-doped g–C₃N₄ achieved interest in the H₂ evolution rate (25,124 μmol·g_cat⁻¹·h⁻¹). To further understand the photocatalytic activity of MXenes, a broad comparison was collected for different types of MXenes (as shown in Table 2). In addition to Ti₃C₂T_x, only a few studies using other types of MXenes cocatalysts, such as Nb₂CT_x [96] and Ti₂C [97], for hydrogen production. Interestingly, the hybrid composite of Zn_0.5Cd_0.5S and Ti₂C/TiO₂ exhibited an attractive H₂ production rate (32,560 μmol·g_cat⁻¹·h⁻¹) [97]. This photocatalytic enhancement might be contributed by the effective light absorption and the efficient separation of electron-hole pairs.

Table 1.

Photocatalytic hydrogen production over Ti₃C₂T_x cocatalysts.

No.	Photocatalysts	Light Source	Reaction Temp.	Scavenger	Reactant Medium	H2 Production Rate (μmol·gcat−1·h−1)	Ref/(Year)
1	TiO2 nanofibers/ Ti3C2Tx nanosheets (3 wt %)	300 W Xe lamp	Room temperature (RT)	Methanol	CH3OH/H2O (l, 1:9)	6979	[67]/2019
2	TiO2 nanofibers					1831
3	Ti3C2Tx nanosheets					ND
4	F–Ti3C2Tx/TiO2 hybrids	350 W Xe arc lamp	RT	Glycerin	C3H8O3/H2O (l, 1:9)	127.1	[37]/2019
5	OH–Ti3C2Tx/TiO2 hybrids					61.4
6	CdS (CT0)	300 W Xe arc lamp: λ ≥ 420 nm; 80 mW·cm−2	RT	Lactic acid	C3H6O3/H2O (l, 17.6:62.4)	105	[61]/2017
7	Ti3C2Tx nanoparticles					ND
8	0.05 wt % Ti3C2Tx nanoparticles/CdS (CT0.05)					993
9	0.1 wt % Ti3C2Tx nanoparticles/CdS (CT0.1)					1278
10	2.5 wt % Ti3C2Tx nanoparticles/CdS (CT2.5)					14,342
11	5 wt %Ti3C2Tx nanoparticles/CdS (CT5)					3377
12	Pt/CdS					10,978
13	NiS/CdS					12,953
14	Ni/CdS					8649
15	MoS2/CdS					6183
16	Ti3C2Tx nanosheets modified Zr–MOFs (UiO-66-NH2)	350 W Xe lamp	RT	S2−/SO32−	0.1 M Na2S and 0.1 M Na2SO3	204	[82]/2019
17	2 wt % Pt/UiO-66-NH2					123
18	UiO-66-NH2					25.6
19	Zn2In2S5/Ti3C2Tx hybrids	300 W Xe arc lamp: λ ≥ 420 nm;	RT	S2−/SO32−	0.35 M Na2S and 0.25 M Na2SO3	2596.8	[92]/2019
20	Ti3C2Tx/TiO2/UiO-66-NH2 hybrid	300 W Xe lamp (PerkinElmer): 350 < λ < 780 nm	5 °C	S2−/SO32−	0.1 M Na2S and 0.1 M Na2SO3	1980	[88]/2019
21	Ti3C2Tx/UiO-66-NH2					1320
22	UiO-66-NH2					942.9
23	MoxS@TiO2@Ti3C2Tx composite	300 W Xe arc lamp: an AM1.5 filter; 180 mW·cm−2 within a range of 200–1200 nm.	25 °C	Triethanolamine (TEOA)	TEOA in aqueous acetone	10505.8	[46]/2020
24	Cu/TiO2@Ti3C2Tx	300W Xe lamp (CEL-HXF 300E)	RT	Methanol	CH3OH/H2O (l, 1:14)	764	[89]/ 2018
25	TiO2@Ti3C2Tx					65
26	1T–MoS2 nanopatch/Ti3C2Tx/TiO2 nanosheet	300 W Xe arc lamp: an AM1.5 filter; 180 mW·cm−2 within a range of 200–1200 nm.	25 °C	TEOA	TEOA/Acetone/H2O (l, 1:3:16)	9738	[90]/2019
27	Ti3C2Tx/TiO2 nanosheet					898
28	TiO2 nanosheet					74
29	1T–WS2@TiO2@ Ti3C2Tx	300 W Xe arc lamp: an AM-1.5 filter	25 °C	TEOA	TEOA/Acetone/H2O (l, 1:3:16)	3409.8	[91]/2019
30	TiO2					67.8
31	ternary Cu2O/(001) TiO2@Ti3C2Tx	300 W Xe lamp (CEL-HXF 300E)	RT	Methanol	CH3OH/H2O (l, 1:14)	1496	[92]/2019
32	(001) TiO2@ Ti3C2Tx					165
33	Ti3C2Tx@TiO2@MoS2 composites	300 W Xe arc lamp (CELHXF300): an AM1.5 filter	25 °C	TEOA	TEOA in aqueous acetone	6425.3	[95]/2019
34	Ti3C2Tx@TiO2					898.1
35	TiO2/Ti3C2Tx/CoS	300 W Xe arc lamp	RT	Methanol	CH3OH/H2O (l, 1:4)	950	[95]/2019
36	TiO2					140
37	CoS					10
38	TiO2/Ti3C2Tx					330
39	TiO2/CoS					540
40	g–C3N4/Ti3C2Tx/Pt	300 W Xe arc lamp	RT	TEOA	TEOA/H2O (l, 1:9)	5100	[45]/2018
41	g–C3N4/Ti3C2Tx					1700
42	g–C3N4/Pt					1275
43	g–C3N4@Ti3C2Tx quantum dots	300 W Xe arc lamp (CELHXF300): an AM-1.5 filter	RT	TEOA	TEOA/H2O (l, 3:17)	5111.8	[55]/2019
44	g–C3N4					196.8
45	Pt/g–C3N4					1896.4
46	Ti3C2Tx/O-doped g–C3N4	300 W Xe lamp	RT	TEOA	TEOA (l)	25,124	[87]/2019
47	O-doped g–C3N4					13,745
48	Ti3C2Tx/g–C3N4					15,573
49	Ti3C2Tx/TiO2/g–C3N4 nanocomposites	300 W Xe lamp: λ > 420 nm	25 °C	TEOA	TEOA/H2O (l, 2:17)	1620	[93]/2018
50	g–C3N4					670
51	CdLa2S4/Ti3C2Tx nanocomposite	300 W Xe lamp: a high-pass filter (λ > 420 nm)	RT	S2−/SO32−	0.35 M Na2S and 0.25 M Na2SO3	11,182.4	[81]/2019
52	Pt/CdLa2S4					1734.7
53	CdLa2S4					832
54	Ti3C2Tx					ND
55	CdS nanorod/ Ti3C2Tx nanosheet	300 W Xe lamp (PerkinElmer): a cut-off filter (λ > 420 nm)	6 °C	Lactic acid	C3H6O3/H2O (l, 1:9)	2407	[79]/2019
56	CdS nanorod					360
57	ZnS nanoparticles/Ti3C2Tx nanosheets	300 W Xe lamp	RT	Lactic acid	C3H6O3/H2O (l, 1:4)	502.6	[80]/2019
58	ZnS nanoparticles					124.6
59	ZnO nanorods /Ti3C2Tx hybrids	300 W Xe lamp: λ > 420 nm	RT	Ethanol	C2H5OH/H2O (l, 3:16)	456	[68]/2020
60	ZnO nanorods					ND
61	CdS/MoS2/Ti3C2Tx composites	300 W Xe lamp (CELHXF300): a cut-off filter (λ > 420 nm)	RT	S2−/SO32−	0.25 M Na2S and 0.35 M Na2SO3	9679	[94]/2019
62	plasma-Ti3C2Tx/CdS hybrids	300 W arc Xe lamp (PLSSXE300): a UV cut-off filter (λ > 420 nm);	RT	Lactic acid	C3H6O3/H2O (l, 1:9)	825	[62]/2019
63	Ti3C2Tx/CdS hybrids					473
64	g–C3N4/plasma-Ti3C2Tx	350 W Xe lamp: a UV cut-off filter (λ > 400 nm); 70 mW·cm−2	RT	TEOA	TEOA/H2O (l, 1:9)	17.8	[63]/2020
65	g–C3N4/Ti3C2Tx					7.5
66	g–C3N4					0.7
67	TiO2/Ti3C2Tx@AC-48 h composite	350 W Xe lamp (AHD 350): a cut-off filter (λ > 400 nm)	RT	Ascorbic acid (AA)	29 mg·mL−1 AA with the sensitization of 1 mM EY in aqueous solution	33.4	[98]/2019
68	1% Pt/TiO2					0.7
69	TiO2/Ti3C2Tx@AC-48 h composite				29 mg·mL−1 AA in aqueous solution	0.3

Table 2.

Photocatalytic hydrogen production over selected MXenes cocatalysts.

No.	Photocatalysts	Light Source	Reaction Temp.	Scavenger	Reactant Medium	H2 Production Rate (μmol·gcat−1·h−1)	Ref./(Year)
1	Ti3C2Tx/O-doped g–C3N4	300 W Xe lamp	RT	TEOA	TEOA (l)	25,124	[87]/2019
2	CdLa2S4/Ti3C2Tx nanocomposite	300 W Xe lamp: a high-pass filter (λ > 420 nm)	RT	S2−/SO32−	0.35 M Na2S and 0.25 M Na2SO3	11,182.4	[81]/2019
3	2.5 wt % Ti3C2Tx nanoparticles/CdS (CT2.5)	300 W Xe arc lamp: λ ≥ 420 nm; 80 mW·cm−2	RT	Lactic acid	C3H6O3/H2O (l, 17.6:62.4)	14,342	[61]/2017
4	Nb2O5/C/Nb2CTx Composites	200 W Hg lamp: λ = 285–325 nm; 120 mW·cm−2	25 °C	Methanol	CH3OH/H2O (l, 1:3)	7.81	[96]/2018
5	Zn0.5Cd0.5S/Ti2C/TiO2	300 W Xe lamp: λ ≥ 400 nm;	RT	S2−/SO32−	0.3 M Na2S and 0.3 M Na2SO3	32,560	[97]/2020

6. Summary and Perspectives

In conclusion, Ti₃C₂T_x exhibited excellent catalytic properties toward photocatalytic HER. However, the property of Ti₃C₂T_x was strongly affected by its surface functional groups and coupled materials. Specifically, the O terminated Ti₃C₂T_x offered the best catalytic activity. The performance of Ti₃C₃T_x could also be improved by paring with other photoactive materials such as TiO₂, ZnO, MoS₂, WS₂, CdS, and graphitic carbon nitride. The composite materials not only improved light absorption but also enhanced the charge separation and active sites. Thus improving the overall performance Ti₃C₂T_x under UV-vis light irradiation. Nonetheless, there were still limitations that hinder the application of Ti₃C₂T_x for practical applications such as scalability and stability. The future development of Ti₃C₂T_x as photocatalysts can be extended into the following directions: (1) developing a novel method for production of Ti₃C₂T_x in large scale at a mild condition such as a lower temperature, less toxic etchant, and solution-processable; (2) constructing novel functional groups on the surface of Ti₃C₂T_x for improving the catalytic properties; (3) designing novel materials to couple with Ti₃C₂T_x for further enhancing the photocatalytic activity such as oxide perovskite and halide perovskite can be considered; and (4) improving the stability of Ti₃C₂T_x for improving the lifetime of catalysts under working through structural engineering or passivation.

Author Contributions

V.-H.N, S.Y.K, and Q.V.L conceived the idea and supervised the project. All authors wrote and approved the final version of the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Creative Materials Discovery Program through the NRF funded by the Ministry of Science and ICT (grant number 2017M3D1A1039379) and the Basic Research Laboratory of the NRF funded by the Korean government (grant number 2018R1A4A1022647). The authors gratefully acknowledge Lac Hong University, Vietnam for the financial and equipment support under grant number LHU-RF-TE-18-01-09.

Conflicts of Interest

The authors declare no conflict of interest