Applications of MXene (Ti₃C₂T_x) in photocatalysis: a review

Abstract

MXenes are two-dimensional nanomaterials, which can be constructed from different elements. The rich interlayer groups, surface groups, and the flexible layer spacing of MXenes make them ideal catalysts. Among these, Ti₃C₂T_x has gained particular attention as a photocatalyst for photocatalytic CO₂ reduction reactions (CO₂RR), hydrogen evolution reactions (HER), and photocatalytic degradation reactions. The structure of Ti₃C₂T_x, hydrophilic surface functional groups, and the Gibbs free energy for hydrogen adsorption lead to the excellent photocatalytic HER performance of this material. Numerous surface defects on Ti₃C₂T_x also provide plentiful CO₂ adsorption sites for CO₂RR. It is the structure of two-dimensional nanomaterials and their high-speed electron transport channels that enable their excellent catalytic oxidation activity. However, at present, there are still challenges that limit their further application, the most significant of which is the material stability. In order to overcome this, the synthetic routes to prepare these photocatalysts need to be adapted.

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1 Introduction

Photocatalysis is an environment-friendly technology developed in the 20th century. When light is absorbed by some special semiconductors, namely “photocatalysts”, electrons (e⁻) originally in the valence band (VB) are excited to the conduction band (CB) and holes (h⁺) are formed in the initial position. Free electrons with strong reducibility can reduce the valence state of some elements in compounds (such as carbon in CO₂ and nitrogen in N₂) when regular methods are not useful or cost too much. Thus far, photocatalysis has shown its huge application prospects in clean energy, environmental remediation, and many other fields; currently, researchers are heading towards new photocatalyst development and reaction mechanisms. Photocatalytic hydrogen evolution reaction (HER), CO₂ reduction reaction (CO₂RR), and photocatalytic degradation reaction represent the three main aspects of photocatalysis.

Photocatalytic nanomaterials have an extensive number of potential applications. When their particle size is below a certain value, the Fermi level of the electronic energy levels morphs from continuous to discrete levels and the energy gap grows wider. These semiconductors are, therefore, more susceptible to photon excitation, which improves their photocatalytic activity.¹

Nanomaterials can be divided into four categories, according to the dimensions of their structural scale: (1) zero-dimensional materials, e.g., groups of nanostructure clusters; (2) one-dimensional nanomaterials, e.g., fibrous nanotubes, nanowires, nanoribbons, or other related structures; (3) two-dimensional nanomaterials, e.g., layered nanomaterials, quantum wells, superlattices, and other structures; (4) three-dimensional nanomaterials, e.g., composite structures consisting of one or more zero-dimensional, one-dimensional, or two-dimensional nanomaterials. The first three are collectively known as low Vannami materials. In low Vannami materials, two-dimensional nanomaterials show significant changes in the surface, electron energy levels, state density, and other aspects compared with three-dimensional materials. This due to the fact that their thickness is greatly reduced compared to other two-dimensional materials; thus, these possess unique optical and electronic characteristics, which make them a hot topic in catalysis.^2,3

MXenes are two-dimensional nanomaterials and have a general material formula of M_n+1X_nT_x. In this formula, M represents nitrogen or carbon, X is generally a transition metal element, and T represents the functional groups. MXenes typically consist of transition metal carbides, nitrides, or carbides that are several atomic layers thick. It was first reported in 2011⁴ that MXene materials have comparable conductivity towards transition metal carbides due to the presence of hydroxy groups or terminal oxygen species on their surfaces. The most important feature of this range of materials is that, unlike conventional battery materials, they provide more channels for ions to move through, thus dramatically increasing their speed.

Ti₃C₂T_x was the first discovered MXene material and is also the most widely used MXene material in the field of photocatalysis.^5–7 It was first obtained by etching the Al layer of Ti₃AlC₂ with hydrofluoric acid. In this paper, the application of Ti₃C₂T_x as a photocatalyst and approaches to improve its catalytic performance are summarized.

2 MXene

2.1 From MAX to MXene. MXenes are a type of two-dimensional nanomaterials with covalent bonds formed between the early transition elements and nitrogen or carbon elements. This furnishes an intramolecular skeleton in which the layers are connected by hydrogen bonds. Since the first Mxene, Ti₃C₂T_x, was discovered in 2011, more than 20 kinds of MXene materials including M₂X, M₃X₂, and M₄X₃ have been successively prepared.^7,8 Due to their unique structure and properties, MXenes have been widely favored for use in battery applications.

The preparation of MXenes can be divided into two approaches, namely, bottom-up and top-down. Presently, the top-down etching method is commonly employed. This is due to the MAX phase^9,10 (commonly, M means early transition metal elements such as Ti and Nb; A represents the Al or Si layer; X represents C or N elements) as M is mainly composed of metallic bonds between the atoms, which are connected to A. The chemical properties are largely dictated by A. By using certain concentrations of hydrofluoric acid or LiF/HCl¹¹ to etch the MAX phase of Ti₃AlC₂, ternary carbides within the titanium carbon layers become closer to each other. In this process, the Al layer is etched away gradually, resulting in a greater carbon–titanium interlayer spacing in the Ti₃C₂T_x product. In order to obtain Ti₃AlC₂ with a graphene-like structure consisting of only a few or single layers, mechanical or chemical intercalation dissection is required. However, when chemical intercalation is used for stripping, some organic molecules may occupy the active sites exposed on the surface, which is unfavorable for photocatalytic reactions.

Etching is a slow process, as shown in Fig. 1(a). In this process, Al layers are gradually peeled off, while the Ti–C skeleton layers are not damaged because of their strong ionic bonding.¹² Free groups such as –OH and some H₂O molecules enter into the framework of Ti–C and become inter-connected by hydrogen bonds, which expands the layer spacing of Ti₃C₂T_x. This permits ions with a large radius to enter the layer spacing,¹³ providing an operating space for the ion intercalation method to peel-off few layers of Ti₃C₂T_x. The number of –OH groups and H₂O molecules within the interlayer space accounts for the large electrical capacity of Ti₃C₂T_x.

Fig. 1 The process of etching Ti₃AlC₂ to yield Ti₃C₂.

Ti₃C₂T_x obtained by direct etching with hydrofluoric acid possesses a different morphology to Ti₃C₂T_x obtained by etching with LiF/HCl. Furthermore, NMR spectroscopy revealed a greater number of –OH and –F functional groups on the surface of Ti₃C₂T_x etched by hydrofluoric acid, while LiF/HCl etching furnished a material with predominantly –O functional groups.

2.2 Structure and properties of Ti₃C₂T_x

2.2.1 Structure of Ti₃C₂T_x. As shown in Fig. 2(a), the structure of Ti₃C₂T_x is comprised of three parts: the intralayer skeleton region, the interlayer region, and the surface terminating groups. In the intramolecular skeleton region, Ti atoms and C atoms are stacked alternately to form ionic bonds, and the skeleton of the entire main structure is formed on this basis. In the interlayer region, it was found through neutron diffraction that the interaction between the layers is established through hydrogen bonding between either O or F atoms on the surface and van der Waals forces between these atoms. The strength of interlayer hydrogen bonding depends not only on the orientation of –OH relative to the entire sheet but also on the number and distribution of the –OH groups. When there is water between the layers, it can also participate in hydrogen bonding. In addition, a large number of terminal groups are randomly distributed on the surface of the Ti₃C₂T_x main structure. These surface groups are directly bonded to the exposed surface and Ti₃C₂T_x is obtained by top-down etching, which mainly includes surface groups such as –O, –OH, and –F. After a period of placement, the –F groups can be replaced by –O groups. The surface groups have a great influence on the properties of the Ti₃C₂T_x formed, which can be analyzed by electron energy loss spectroscopy (TEM),^14,15 neutron scattering,¹⁶ and NMR techniques.^17,18 These experiments confirm that the surface functional groups of Ti₃C₂T_x are randomly distributed with –O, –OH, and –F all directly bonded to the surface of the exposed MXene plane. Furthermore, there are no adjacent –OH functional groups found between the layers. The presence of cations between the layers causes the MXene lamellae to slide easily relative to each other, changing their rheological properties and resulting in their clay-like properties.

Fig. 2 Schematic diagram showing the crystal structure of Ti₃C₂ (a)¹⁹ and its monolayer with (b) top and (c) side view. The large blue balls represent Ti atoms and the small brown balls represent C atoms. The highlighted unit cell indicates the high symmetry A, B, and C adatom sites.²⁰

2.2.2 Properties of Ti₃C₂T_x. The structure of Ti₃C₂T_x determines the electric storage performance. As Ti₃C₂T_x has a wide spacing between the layers, the ions can travel through the layers at a high speed, whilst the hydroxyl or terminal oxygen groups on the surface furnish a material with excellent conductivity. Both factors are important for the use of these MXene materials in batteries.^20–25

Various surface groups (such as –O, –F, and –OH) have supplied abundant anchored sites for the base photocatalyst to form efficient heterojunction structures, which are ideal for photocatalytic activities.²⁶ There is also a large number of exposed metal sites on the surface, which can be used as active sites for reactions.

The surface chemical state of MXene materials has a large influence on the regulation of its physical properties. When –F on the surface is replaced by an –O group, the electrochemical performance is improved. For example, when Ti₃C₂T_x is treated with a KOH and CH₃OOK solution, the –O groups on the surface increase, along with the electric capacity. Under an atmosphere of N₂, Ar, or other inert gases, the number of –F groups on the surface of Ti₃C₂T_x is reduced, after which the electrical capacity is greatly increased.

Ti₃C₂T_x shows an excellent absorption of light between 300 nm and 500 nm.²⁷ Recently, researchers have even found that the absorption can be broadened to the near-infrared (NIR) region. According to a further study, this may be related to its surface plasmon resonance (SPR), and the thinner the material, the stronger the SPR.²⁸ Such a peculiarity makes Ti₃C₂T_x an ideal photothermal co-catalyst.

2.2.3 Instability of Ti₃C₂T_x. MXenes typically have poor stability. Ti₃C₂T_x is rapidly oxidized when heated under CO₂, air, and other environments, and when the surface groups are all –O, Ti₃C₂O₂ exhibits metallic properties.^29,30 Ti₃C₂T_x is also slowly oxidized when exposed to air under atmospheric conditions. After oxidation, Ti₃C₂T_x is called oxidized MXene (denoted as MO). When Ti₃C₂T_x is oxidized, it is only the Ti atoms that are oxidized into the corresponding oxide (TiO₂), while the C atoms remain unchanged. In other words, MO comprises a sandwich structure of layered carbon layers and titanium oxide.³¹Fig. 3 shows the structural evolution of Ti₃C₂T_x oxidation to MXene (MO). As can be seen from Fig. 3, MO maintains a layered structure, whilst the TiO₂ nanoparticles formed by oxidation are coated within the carbon layer structure. Due to the photoresponse capacity of TiO₂, this resulting structure is photocatalytically active. An example of this is MO/g-C₃N₄, which can photocatalytically split water with a relatively high efficiency.³²

Fig. 3 Oxidation process of Ti₃C₂T_x.

The crystal structure of Ti₃C₂T_x contains Ti defects, which appear to contribute significantly to the instability of this material.^14,33 High angle annular dark field (HAADF)-STEM imaging is an important tool in 2D materials’ characterization and is used to unambiguously resolve the crystal structure and defect configurations.^34,35 As shown in Fig. 4, widespread Ti defects were directly detected through the HAADF-STEM imaging of the single-layer Ti₃C₂T_x flakes. Single-layered Ti₃C₂T_x obtained by HF etching was observed through HAADF-STEM images in Fig. 4(a)–(c). Fig. 4(d) was obtained by calculating tens of such images, and it reveals that the relationship between HF concentration and defect formation. It was found that vacancy clusters are rarely observed after etching with 2.7 wt% HF concentration but are relatively common after etching with 7 wt% HF.³³ Generally speaking, the average concentration of V_Ti (Ti vacancies) is positively related to that of HF.

Fig. 4 HAADF-STEM images from single-layer Ti₃C₂T_x MXene flakes prepared using etchants with different HF concentrations: (a) 2.7 wt% HF, (b) 5.3 wt% HF, and (c) 7 wt% HF. Single V_Ti vacancies are indicated by the red circles, while vacancy clusters V^C_Ti are shown by the blue circles. (d) Scatter plot of the defect concentration from the images acquired from samples produced using different HF concentrations. The red line shows the error plot with the average and standard deviation for different HF concentrations.³³

3. Application in photocatalysis

Due to the excellent structural properties of Ti₃C₂T_x, there are many cases in which Ti₃C₂T_x is used as a co-catalyst in photocatalytic systems or is directly involved in photocatalytic reaction systems. This paper summarizes the application of Ti₃C₂T_x in the field of photocatalysis from three aspects: photocatalytic hydrogen evolution reactions (HER), photocatalytic CO₂ reduction reactions (CO₂RR), and photocatalytic degradation reactions.

3.1 Application in HER

Ti₃C₂T_x is the most widely used photocatalytic agent in hydrogen evolution reactions^36–39 (HER). Ti₃C₂T_x has the following advantages that make it ideal for use in photolysis: (a) hydrophilic surface functional groups are conducive for the adsorption of water molecules and promote the reaction; and (b) the Gibbs free energy of Ti₃C₂T_x adsorption on hydrogen approaches zero infinitely, which is conducive for the reduction of H⁺.

There are three important steps in the HER process, which are:^30,40,41 (a) initial h⁺ + e⁻ formation; (b) generation of H* (the intermediate adsorption state); and (c) formation of the 1/2 H₂ product. The adsorption state of H* in process (b) directly affects the final hydrogen evolution efficiency and is an extremely important factor, which can be represented by the Gibbs adsorption free energy |ΔG_H*|. Through simulation calculations, it was found that when all the Ti₃C₂T_x surface groups are –F, ΔG_H* = −0.927 eV, and the adsorption is too strong. When all the surface groups are –O, |ΔG_H*| is 0.003 eV, which is even better than the commonly used catalyst Pt (ΔG_H* ≈ − 0.090 eV).^42,43 Therefore, Ti₃C₂T_x is a good HER co-catalyst. Examples of Ti₃C₂T_x used for photolysis in recent years are summarized below (Table 1).

Table 1 Comparison of photocatalysts including Ti₃C₂T_x in HER

Name	H2 production (μmol h^−1 gcatalyst^−1)	AQY (%)	Activity improvement factor	Sacrificial reagent	Preparation methods	Monolayer or multilayer	Light source	Morphology	Year	Ref.
CdS/Ti3C2Tx	14 342	40.1% (420 nm)	135.59 times	Lactic acid (17.6%)	One-step hydrothermal method	Ti3C2Tx NPs	300 W Xe lamp (λ > 420 nm)	Cauliflower-structure by self-assembly of many NPs	2017	30
2D-Layered Carbon/TiO2	480.8	1.98% (400 nm)		TEOA (10%)	Ti3C2Tx oxidation	Multilayer	300 W Xe lamp (λ > 400 nm)	Nanosheets	2017	44
Ti3C2Tx/rutile TiO2	17.8	0.3%	Approximately 4 times	Methanol (25%)	Hydrothermal method	Monolayer	200 W Hg lamp (λ > 400 nm)	2D sheetswith TiO2 attached on the surfaces and between the sheets	2016	45
Ti3C2/Pt/g-C3N4	5000	3.1% (420 nm)	15 times than pristine g-C3N4	TEOA (10%)	Hydrothermal and photodeposition method	Monolayer	300 W Xe lamp	Nanosheets with porous nanoparticles	2018	46
Sulfur-doped Carbon/TiO2	333	7.36%		Methanol (10%)	Ti3C2Tx oxidation	Multilayer	300 W Xe lamp (λ > 400 nm)	Nanosheets	2018	47
Ti3C2Tx/TiO2 nanoflowers	783.11	5.86 (350 nm)	6 times	Methanol (20%)	Hydrothermal and calcination	Multilayer	300 W Xe lamp	Nanoflowers	2018	48
Zn2In2S5/Ti3C2Tx	2596.76	8.96% (420 nm)	1.97 times	0.25 M Na2SO3/0.35 M Na2S/H2PtCl6	Hydrothermal	Multilayer	300 W Xe lamp (λ > 420 nm)	Flower-like microspheres	2018	49
d-Ti3C2/TiO2/g-C3N4	1620	4.16% (420 nm)	12.15 times than pure g-C3N4	TEOA (10%)	Calcination	Monolayer	300 W Xe lamp (λ > 420 nm)	2D–2D heterostructure	2018	50
ZnS/Ti3C2	502.6		4 times	Lactic acid (20%)	Hydrothermal	Multilayer	300 W Xe lamp	Sphere-like structure	2019	51
1D CdS nanorod/2D Ti3C2 MXene nanosheet	2407	35.6% (429 nm)	6.68 times	Lactic acid (10%)	Electrostatic self-assembly	Monolayer	300 W Xe lamp (λ > 420 nm)	1D/2D nanosheets	2019	52
TiO2 nanofibers/MXene Ti3C2	6979		3.8 times than TiO2 nanofibers	Methanol (10%)	Electrostatic self-assembly technique	Monolayer	300 W Xe lamp	Nanofibers/nanosheets	2019	53
TiO2 nanoparticale/monolayer Ti3C2	2650	15.8% (305 nm)	2.88 times than TiO2 nanoparticles/multilayer Ti3C2	Methanol (25%)	Electrostatic self-assembly technique	Monolayer	200 W Hg lamp (285–325 nm)	Nanosheets	2019	54
MoS2/Ti3C2	6144.7		2.33 times	Methanol (30%)	Hydrothermal	Multilayer	300 W Xe lamp (λ > 400 nm)	Spheres-like structure	2019	55
Ti3C2 MXene/O-doped g-C3N4	25124	17.59% (405 nm)	1.8 times than O-doped g-C3N4	TEOA	Electrostatic self-assembly technique	Multilayer	300 W Xe lamp	2D nanosheets structure	2019	56
CdLa2S4/Ti3C2	11182.4	15.60% (420 nm)	13.4 times	0.25 M Na2SO3 and 0.35 M Na2S	Hydrothermal	Monolayer	300 W Xe lamp (λ > 420 nm)	Particle-like	2019	57
Ti3C2 MXene quantum dots/g-C3N4	5111.8	3.654%	25.97 times	TEOA (15%)	Deposition	Ti3C2 MXene quantum dots	300 W Xe lamp	Nanosheets	2019	58
MoxS@TiO2@Ti3C2	10505.8	7.535%	5.99 times than Mo2S@TiO2@Ti3C2	TEOA	In situ growth and hydrothermal	Multilayer	300 W Xe lamp	Nanosheets	2019	59
Ti3C2/porous MOFs (UiO-66-NH2)	204		Approximately 8 times	0.1 M Na2S and 0.1 M Na2SO3	Hydrothermal	Monolayer	350 W Xe lamp	3D structure	2019	60
C-TiO2/g-C3N4	1409		8 times than C-TiO2	TEOA (10%)	Calcination	Multilayer	300 W Xe lamp (λ > 420 nm)	Smooth sheet-like structure	2019	32
CdS@Ti3C2@CoO	134.46		1.75 times than CdS@CoO		Calcination	Monolayer	300 W Xe lamp (λ > 420 nm)	Spheres-like structure	2019	61
TiO2–Ti3C2–CoSx	950		5.8 times than TiO2	Methanol (20%)	Hydrothermal	Multilayer	300 W Xe lamp	Smooth round block morphology	2019	62
Ti3C2(TiO2)@CdS/MoS2	8470		3.76 times than CdS/MoS2	lactic acid (20%)	Hydrothermal	Multilayer	300 W Xe lamp (λ > 420 nm)	Nanospheres	2019	63
Ti3C2 MXene/MoS2 nanosheets/TiO2 nanosheets	6425.297	4.61%	7.15 times than TiO2/Ti3C2	TEOA	Ti3C2Tx oxidation	Multilayer	300 W Xe lamp	Ti3C2 nanosheets with MoS2 nanoparticales	2019	64
2D/3D g-C3N4/Ti3C2 (MXene) heterojunction	116.2		6.64 times	TEOA (10%)	Calcination	Multilayer	300 W Xe lamp (λ > 420 nm)	Nanosheets	2020	65
Au/MoS2/Ti3C2	12000			Methanol (30%)	Electrostatic self-assembly technique	Multilayer		Nanosphere-like	2020	66
2D/2D Ti3C2/g-C3N4	72.3	0.81% (400 nm)	10.18 times than pure g-C3N4	TEOA (10%)	Electrostatic selfassembly approach	Monolayer	200 W Hg lamp	Flat irregularly shaped nanosheets of 2D/2D structures	2019	67
MXene@Au@CdS	17070.43		1.85 times than pure CdS	0.35 mol L^−1 Na2S and 0.25 mol L^−1 Na2SO3 solution	Hydrothermal	Monolayer	300 W Xe lamp (λ > 420 nm)	Nanosheets	2020	68
Black phosphorus quantum dots/Ti3C2@TiO2	684.5		11.35 times	TEOA (25%)	Solvent-heatmethod	Multilayer	300 W Xe lamp (λ > 420 nm)	Nanosheets	2020	69

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Ti₃C₂T_x plays a significant role in HER, whether as a co-catalyst or as a part of the overall catalyst, as it greatly improves the performance of the base catalyst. As shown in Table 1, the presence of Ti₃C₂T_x increases the yield of H₂ compared to solely the base catalyst by more than 2 times. The quantum efficiency is also significantly improved to 40.1%, whilst the maximum value of hydrogen production is 14.34 mmol g⁻¹ h⁻¹.

Monolayer Ti₃C₂T_x or quantum dot Ti₃C₂T_x displays better activity in HER. However, the use of monolayer Ti₃C₂T_x as a photocatalyst has several disadvantages: (a) the preparation of monolayer Ti₃C₂T_x is complex; (b) the structural stability is low and the catalyst is easily oxidized in water; (c) manipulation of the mono-layer or few-layer structures is not easy to carry out. Few-layer structures are presently prepared by electrostatic self-assembly or by in situ growth. The stability of the composite catalyst obtained by in situ growth is significantly greater than that obtained by electrostatic self-assembly.

Due to the surface hydrophilic groups,⁷⁰ suitable Gibbs adsorption free energies |ΔG_H*|, and excellent electron transfer efficiency, Ti₃C₂T_x not only plays an important role in the three-step process of HER but can participate in electron hole separation.

Xiao et al. successfully synthesized the Schottky junction of 1D CdS nanorod/2D Ti₃C₂ MXene nanosheet in 2019.⁵² As shown in Fig. 5, Xiao et al. anchored Cd²⁺ using the deficiency of Ti on the Ti₃C₂ surface and the electrostatic interaction of free Cd²⁺ to prepare the 1D CdS nanorods. The composite material demonstrated excellent hydrogen production performance (2407 μmol h⁻¹ g_catalyst⁻¹), producing 6.68 times as much H₂ as pure CdS (Fig. 6).

Fig. 5 TEM images of (a–c) CdS, (d–f) exfoliated Ti₃C₂ MXene nanosheets, (g–i) the composite CM-20, (j) the corresponding elemental mapping results of CM-20, and (k) the oxidation process of Ti₃C₂T_x.⁵²

Fig. 6 (a and b) Photocatalytic H₂ evolution performance of different samples, (c) the recycled photocatalytic H₂ evolution experiments of CM-20, (d) AQY values and the wavelength dependence of photocatalytic H₂ evolution in the composite CM-20.⁵²

Theoretically, the surface negative value (zeta potential value: ∼18 mV) of Ti₃C₂T_x is sufficient to adsorb positively charged Cd²⁺. Ti₃C₂T_x treated with DMSO forms a low-layered structure, on which Cd²⁺ can be anchored and one-dimensional CdS nanorods can be grown. As shown in Fig. 5, due to the constraint effect of Ti₃C₂T_x, the length of 1D CdS nanorods in the 1D CdS nanorods/2D Ti₃C₂T_x heterojunctions is smaller than that of the 1D CdS nanorods alone.

CdS equipped with Ti₃C₂T_x displays excellent electrochemical properties. As shown in Fig. 7, the photocurrent of 1D CdS nanorods/2D Ti₃C₂T_x was significantly better than that of one-dimensional CdS nanorods and the optical resistance was significantly lower than that of one-dimensional CdS nanorods. ESR tests show that the hydroxyl radical and superoxide radical signals of 1D CdS nanorods/2D Ti₃C₂T_x were significantly enhanced after the addition of Ti₃C₂T_x. In conclusion, under the same illumination conditions, 1D CdS nanorods/2D Ti₃C₂T_x generate more photogenic carriers. These produce oxygen-containing groups with oxidizing reductivity, which can participate in photocatalytic hydrogenation reactions.

Fig. 7 (a) Photocurrent density curves and (b) EIS Nyquist plots of CdS and CM-20.⁵²

1D CdS nanorods/2D Ti₃C₂T_x typically exhibit better visible light response, electron hole separation efficiency, and more effective carrier transport efficiency after the formation of multi-dimensional heterojunctions. This accounts for their excellent photocatalytic hydrogen evolution capability.

Using Ti₃C₂T_x as the co-catalyst, Wang et al. synthesized a TiO₂/Ti₃C₂T_x complex photocatalyst,⁴⁵ which was 4 times more efficient than pure phase TiO₂ in photohydrolyzing aquatic hydrogen. This is attributed to the Schottky barrier formed between TiO₂ and Ti₃C₂T_x, which effectively improves the separation efficiency of the electron holes. As shown in Scheme 1, excited electrons can be wired to Ti₃C₂T_x from the conduction band of TiO₂ owing to the close contact between Ti₃C₂T_x and TiO₂; thus, negative charge is accumulated in Ti₃C₂T_x and a depletion layer formed at the metal–semiconductor interface, which is the Schottky barrier.⁴⁵

Scheme 1 Formation of Schottky barrier at the MXene/TiO₂ interface.⁴⁵

In this work, Wang et al. treated Ti₃C₂T_x with DMSO to form low-layer structures. Amorphous TiO₂ was formed from TiCl₄ hydrolysis and then on the surface of Ti₃C₂T_x, amorphous TiO₂ was coated. After hydrothermal treatment, anatase TiO₂/Ti₃C₂T_x material was formed, as shown in Fig. 8. Amorphous TiO₂ is micro-spherical and is coated on the surface of Ti₃C₂T_x, displaying a low-layered structure (Fig. 8). After water heat treatment, the whole structure forms into a brittle cake structure (Fig. 9).

Fig. 8 SEM images of (a) TiO₂ (50 wt%), (d) Ti₃C₂T_x, (b) TiO₂/Ti₃C₂T_x (5 wt%), and (c) TiO₂/Ti₃C₂T_x.⁴⁵

Fig. 9 (a) Photocatalytic hydrogen production rates and (b) recycling studies over the TiO₂/Ti₃C₂T_x (5 wt%) sample.⁴⁵

The TiO₂/Ti₃C₂T_x material displays excellent photocatalytic hydrogen evolution capability with good cycling stability. The hydrogen production efficiency of TiO₂/Ti₃C₂T_x-5% is about 4 times as high as that of pure phase TiO₂, reaching 17.8 μmol h⁻¹ g_catalyst⁻¹. The hydrogen production efficiency of the 10% and 50% samples decreased slightly, which may be related to light energy absorption, as shown in Fig. 10(a). With the increase in Ti₃C₂T_x addition, the light absorption capacity of the samples in the 250–380 nm region is gradually decreased. This significant improvement in the hydrogen production efficiency is closely related to the smooth carriage of Ti₃C₂T_x. As shown in Fig. 10(b), after the formation of the Schottky barrier, the carrier separation efficiency is improved, thus improving its photocatalytic capacity.

Fig. 10 (a) PL spectra and (b) DRS spectra of TiO₂, TiO₂/Ti₃C₂T_x (5 wt%), TiO₂/Ti₃C₂T_x (10 wt%), and TiO₂/Ti₃C₂T_x (50 wt%).⁴⁵

Thus, in conclusion, after loading with Ti₃C₂T_x, H₂ production increased at least twice. Such an amazing promotion is mainly related with 3 aspects of Ti₃C₂T_x: (a) it supplies a high throughput channel as a co-catalyst for the excited electrons while the holes cannot pass the boundaries; (b) its hydrophilcity; and (c) the Gibbs free energy of Ti₃C₂T_x adsorption on hydrogen approaches zero infinitely.

3.2 Application in CO₂RR

The photocatalytic CO₂ reduction reaction (CO₂RR) consists of five steps:^71–74 light absorption, charge separation, CO₂ adsorption, surface redox reaction, and product desorption. As shown in Fig. 11, when the CB of the photocatalyst is greater than the redox potential of CO₂, and charge separation occurs whilst the electrons and holes recombine. Several complex factors dictate which of these two competing processes predominantly occurs. After the adsorption of CO₂ and the migration of photogenerated electrons and holes from the inside of the crystal structure to the surface, the redox reaction is carried out on the surface of the catalyst. The product then de-attaches, which completes the entire photocatalytic CO₂ reduction reaction.

Fig. 11 CO₂RR process.

Ti₃C₂T_x is also widely used in the photocatalytic CO₂ reduction reaction. However, due to its own carbon source and instability, further research is needed to understand the mechanism of photocatalytic CO₂ reduction of Ti₃C₂T_x.

In 2017, Zhang et al. summarized the CO₂ reduction capacity of three MXene materials with surface groups, which terminate with –O through theoretical calculations.⁷⁵ Among the three materials, Ti₂CO₂, V₂CO₂, and Ti₃C₂O₂, Ti₂CO₂ showed the best photocatalytic CO₂ reduction capacity. Of the two reduction paths^76–78 shown in Fig. 12, the pathway of “CO₂–HCOO–HCOOH” has a favorable energy barrier of about 0.53 eV.

Fig. 12 Two pathways in CO₂RR.

Through DFT calculations, it was revealed that in the first step reaction of CO₂ adsorption in CO₂RR, the O atom of the CO₂ molecule occupies an O defect position on the MXene. This mode in adsorption requires the lowest energy. The adsorption energies of the three materials were Ti₃C₂O₂ (−0.73 eV), Ti₂CO₂ (−0.67 eV), and V₂CO₂ (−0.35 eV). Ti₂CO₂ has a lower adsorption energy compared to V₂CO₂ as the Ti atoms are more likely to lose electrons than the V atoms.

If the reaction proceeds via pathway 1 (Fig. 11), one of the oxygen atoms of the CO₂ molecule is captured by the oxygen defect. This results in the breaking of the C–O bond, while CO is produced. In this step, Ti₃C₂O₂ would lower the energy barrier of the C–O bond to about 0.86 eV. Pathway 2 (Fig. 11) has an energy barrier greater than 1 eV. In this pathway, the CO₂ molecules are captured by an oxygen defect on the surface of MXene and are hydrogenated to form COOH. This is further hydrogenated and converted into the products CO and H₂O. CO, which is produced, can further react to form HCOOH, HCOH, CH₂OH, CH₄, and other products.

Studies into the application of Ti₃C₂T_x in CO₂RR is summarized in Table 2.

Table 2 Comparison of photocatalysts including Ti₃C₂T_x in CO₂RR

Photocatalyst	Products and yield (μmol g^−1 h^−1)	Activity improvement factor	Reaction conditions	Light source	Preparation method	Morphology	Monolayer or Multilayer	Year	Ref.
2D/2D Ti3C2 MXene/g-C3N4 nanosheet	CO (5.19)	8.37 (CO)	20 mg	300 W Xe lamp (λ > 420 nm)	Calcination under N2 atmosphere	2D/2D nanosheets	Monolayer	2020	79
	CH4 (0.044)	2.09 (CH4)	catalyst gas–solid
Alklinized Ti3C2/decorating g-C3N4	CO (11.21 μmol g^−1)	5.96 (CO)	40 mg catalyst gas–solid	300 W Xe lamp (λ > 420 nm)	Alkali etching	3D	Multilayer	2019	80
	CH4 (0.044 μmol g^−1)	5.6 (CH4)
TiO2/Ti3C2	CO	—	50 mg	300 W Xe lamp	Calcination	Nanoparticles	Multilayer	2018	81
	CH4 (0.22)		catalyst liquid–solid
2D/2D ultrathin Ti3C2/Bi2WO6	CO	4.34 (CH4)	100 mg	Xe lamp	Hydrothermal	Flat shape 2D structure	Monolayer	2018	82
	CH4 (1.78)	6.28 (CH3OH)	catalyst liquid–solid
	CH3OH (0.44)
2D/2D/0D TiO2/C3N4/Ti3C2	CO (4.39)	1.39 (CO) (than TiO2/C3N4)	30 mg catalyst liquid–solid	300 W Xe lamp	Electrostatic self-assembly	2D/2D structure	Ti3C2 quantum dots	2020	83
	CH4 (1.20)

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In 2018, Cao et al. prepared a 2D/2D heterogeneous junction of Ti₃C₂T_x/Bi₂WO₆ and the composite showed excellent photocatalytic CO₂ reduction performance.⁸² As shown in Fig. 13, multi-layer structure Ti₃C₂T_x was tested with DMSO. After the formation of low-layer structure Ti₃C₂T_x, the oxygen-rich surface was negatively charged, which permitted Bi³⁺ to be adsorbed from hydrolyzed Bi(NO₃)₃₅5H₂O.⁸⁴ After the addition of a tungsten source, a 2D/2D Ti₃C₂T_x/Bi₂WO₆ heterojunction was formed. The concurrent addition of CTAB furthermore ensures the ultrathin structure of both Bi₂WO₆⁸⁵ and Ti₃C₂T_x.⁸⁶

Fig. 13 (a–c) Typical FESEM, AFM images, and height cutaway view of Bi₂WO₆, (d–f) Ti₃C₂ nanosheets, (g–i) TB2 (Ti₃C₂T_x/Bi₂WO₆), and (j) schematic illustration of the synthetic process.⁸²

The successful preparation of heterojunctions greatly enhances the ability of Bi₂WO₆ to reduce CO₂. The CH₄ production of the sample TB2 reached 1.78 μmol h⁻¹ g⁻¹, while the yield of CH₃OH reached 0.44 μmol h⁻¹ g⁻¹. The isotopic spectra of Fig. 14(b) and (c) indicates that the produced CH₄ and CH₃OH are formed from the photocatalytic reduction of CO₂.

Fig. 14 (a) Photocatalytic activity of TB0 to TB5; (b) GC-MS spectra over TB2 after irradiation for several hours with different carbon sources; (c) GC-MS analysis of the reaction products with ¹²C and ¹³C as carbon sources.⁸²

As shown in Fig. 15(a), Ti₃C₂T_x exhibits excellent light absorption performance between 200–800 nm. The light absorption capacity of Bi₂WO₆ was also significantly improved by carrying Ti₃C₂T_x. To be noted, as shown in Fig. 14(b), the fluorescence lifetime decreased after loading with Ti₃C₂T_x. This is because TC supplies a more efficient non-radiative decay pathway. In electrochemical tests, the photocurrent photoelectric impedance spectrum further revealed that the carriage of Ti₃C₂T_x greatly promoted the carrier strength of Bi₂WO₆. This further confirmed the successful construction of the Ti₃C₂T_x/Bi₂WO₆ heterojunction.

Fig. 15 (a) UV-Vis DRS of all the as-prepared samples; (b) TRPL spectra of TB0 and TB2; (c) EIS plots and (d) transient photocurrent of the prepared samples.⁸²

Yang et al. prepared 2D/2D Ti₃C₂ MXene/g-C₃N₄ heterojunctions in 2020.⁷⁹ As shown in Fig. 16(g), Ti₃AlC₂ was successfully etched to form Ti₃C₂, as indicated by the XRD patterns.^87,88 2D g-C₃N₄ was found to grow on the surface of Ti₃C₂ under an atmosphere of N₂. The formed 2D/2D Ti₃C₂ MXene/g-C₃N₄ demonstrated excellent photocatalytic CO₂ reduction capability. As shown in Fig. 17, the photocatalytic performance of pure phase g-C₃N₄ for the production of CO and CH₄ is only 0.62 μmol h⁻¹ g⁻¹ and 0.021 μmol h⁻¹ g⁻¹, respectively, in contrast to Ti₃C₂, wherein the production of CO and CH₄ is 5.19 μmol h⁻¹ g⁻¹, 0.044 μmol h⁻¹ g⁻¹, respectively. The isotopic experiments confirm that the product is produced by the photocatalytic reduction of CO₂.

Fig. 16 FESEM images of UCN (a) and 10TC (b) samples, AFM images and the corresponding height profiles of UCN (c and e), 10TC (d and f) samples, and (g) schematic illustration for the fabrication process.⁷⁹

Fig. 17 Photocatalytic CO₂ reduction performance of the as-prepared samples (a); cycling tests over the 10TC sample (b); GC-MS analysis of the products from the photoreduction of CO₂ over 10TC using labelled ¹²CO₂ and ¹³CO₂ as the carbon sources (c and d).⁷⁹

The tests of PL and TRPL showed that the composite rate of electron holes^45,89 decreased significantly after carrying Ti₃C₂. As shown in Fig. 18(b), the fitted pure phase C₃N₄ had a lifetime of only 4.14 ns, while 10TC had a lifetime of 4.51 ns, which represents a significant increase in the lifetime of the carriers. This is closely related to the smooth carrying of Ti₃C₂. An excellent “storage capacitor” is produced when Ti₃C₂ forms a heterojunction with g-C₃N₄. When the electrons are transmitted to the semiconductor surface, they transfer to Ti₃C₂ quickly while the holes cannot. This greatly reduces the electron hole composite and improves the photocatalytic performance of the material. On the other hand, abundant defects on the Ti₃C₂ surface provide excellent sites for CO₂ adsorption.

Fig. 18 PL spectra, EIS, and TPR plots of UCN, Ti₃C₂, and 10TC samples (a, c, and d); TR-PL spectra of UCN and 10TC (b).⁷⁹

In conclusion, the application of Ti₃C₂T_x in CO₂RR is relatively less than that of photocatalytic water splitting. This is because of its instability and its own carbon resources, which can cause interferences during the photocatalytic CO₂ reduction reaction. As shown in Table 2, among limited reports, Ti₃C₂T_x with both single-layered structures and multi-layered structures shows an obvious production promotion. It is to be noted that there are no new products (such as C₂ products, formaldehyde, and methyl ether) after loading with Ti₃C₂T_x compared to the base photocatalyst. This phenomenon confirms that Ti₃C₂T_x cannot change the energy barrier of the base photocatalyst for CO₂ reduction. Thus, in general, the obvious promotion during CO₂RR may be related to the two features of Ti₃C₂T_x: (a) abundant surface vacancies for CO₂ adsorption and (b) promoting the separation of carriers.

3.3 Applications in degradation

The main principle of photocatalytic degradation by photocatalytic semiconductor materials is that light stimulates the generation of oxidizing holes.^90–93 These can oxidize dissolved oxygen into efficient oxygen-active species such as superoxide radicals (˙O₂⁻), singlet oxygen (˙O), and hydroxyl radicals (˙OH). These species can directly oxidize the substrate.^94–96 Ti₃C₂ has a wealth of surface groups and active sites, which are conducive for the adsorption of substrates. Accordingly, Ti₃C₂ has been of particular interest as a photoactive degradation catalyst. A summary of the previous studies investigating the application of Ti₃C₂ in photocatalytic degradation reactions is shown in Table 3.

Table 3 Application of Ti₃C₂T_x in photocatalytic degradation reactions

Photocatalyst		Substrate of degradation				Removal rate (%)/rate constants (min^−1)			Reaction conditions			Light source		Oxygenic species			Morphology	Monolayer or multilayer			Year			Ref.
Ag3PO4/Ti3C2		Methyl orange (MO)				(rate constants) 0.094 (MO)			20 mg catalyst + 50 mL 20 mg L^−1 substrate, 30 min dark adsorption			300 W Xe lamp (λ > 420 nm)		h^+ (main)			2D Ti3C2/Ag3PO4 particles	Monolayer			2018			87
		2,4-Dinitrophenol (2,4-DNP)				2,4-DNP (0.005)								˙OH
		Tetracycline hydrochloride (TC–H)				TC–H (0.32)
		Thiamphenicol (TPL)				TPL (0.0042)
		Chloramphenicol (CPL)				CPL (0.025)
Ti3C2/SrTiO3 composites		UO2^2+				(Removal rate)			20 mg catalyst + 60 mL 50 ppm substrate, 8 hours dark adsorption			300 W Xe lamp (λ = 320–2500 nm)		˙OH			2D Ti3C2/SrTiO3 particles	Multilayer			2019			97
						77% in 180 min
Ti3C2–OH/Bi2WO6 composites	Rhodamine B			(Rate constants) 0.0596			10 mg catalyst + 50 mL 2 × 10^−5 mol L^−1 substrate, 30 min dark adsorption				300 W Xe lamp (λ = 400–2500 nm)				h^+		Porous spherical structure			Ti3C2–OH			2019			98
MoS2@Ti3C2 Nanohybrid	Liquid paraffin (LP)			(Rate constants) 0.0476			A certain amount of sample + 2.0 g of deionized water and 1.0 g of LP + 10 mL dichloromethane as the sacrifice reagent, 30 min dark adsorption				1000 W high-pressure mercury lamp						MoS2 nanosheets/Ti3C2 sheets			Multilayer			2019			99
0D/2D Bi3TaO7/Ti3C2			Methylene blue		(Rate constants) 0.032			50 mg catalyst + 100 mL 10 mg L^−1		300 W Xe lamp (λ > 420 nm)			˙OH			Bi3TaO7 nanoparticles/Ti3C2 nanosheets			Multilayer			2020			100
								Substrate, 60 min dark adsorption
2D/2D Ti3C2/Porous g-C3N4			Phenol		(Rate constants) 0.022			20 mg catalyst + 50 mL 10 mg L^−1		500 W Xe lamp (λ > 400 nm)						2D/2D Ti3C2/PCN nanocomposite			Multilayer with ultra-sonication			2020			101
								Substrate, 30 min dark adsorption
CdS@Ti3C2@TiO2			Sulfachloropyridazine (SCP)		(Removal rate) SCP (about 95% in 60 min)			50 mg catalyst + 200 mL 20 mg L^−1		Light intensity 300 mW cm^−2 (λ = 400–1050 nm)			˙O2^−			CdS nanoparticles/Ti3C2@TiO2 bulk			Bulk Ti3C2@TiO2			2019			102
			Methylene blue (MB)		MB (about 80% in 60 min)
			Rhodamine B (RhB)		RhB (about 99% in 60 min)			substrate, 30 min dark adsorption
			Phenol		Phenol (about 50% in 60 min)
(111) TiO2-x/Ti3C2			Mmethylene blue (MB)		(Removal rate) MB (75% in 150 min)			10 mg catalyst + 200 mL 20 mg L^−1		500 W Xe lamp (λ > 400 nm)			˙OH (main)			TiO2 nanoparticles/Ti3C2 nanosheets			Multilayer			2017			103
(001) TiO2/Ti3C2			Methyl orange (MO)		(Rate constants) 0.018			10 mg catalyst + 200 mL 20 mg L^−1 substrate, 60 min dark adsorption		300 W Xe lamp			˙OH (main)			TiO2 square nanosheets/Ti3C2 nanosheets			Multilayer			2016			104
Graphene layers anchored TiO2/g-C3N4			Rhodamine B (RhB)		(Rate constants) 0.0559 (RhB)			10 mg catalyst + 200 mL (RhB 20 mg L^−1, TC 10 mg L^−1, CIP 3 mg L^−1, BPA 5 mg L^−1), 60 min dark adsorption		300 W Xe lamp (λ > 400 nm)			˙OH			3D bulk			Bulk Ti3C2@TiO2			2020			105
			Tetracycline (TC)		0.0244 (TC)								˙O2^−
			Ciprofloxacin (CIP)		0.0168 (CIP)								h^+
			Bisphenol A (BPA)		0.0194 (BPA)
2D/2D Ti3C2/MoS2			Methylene orange (MO)		(Rate constants) 0.00836			50 mg catalyst + 50 mL 20, 30, 50 mg L^−1 substrate, 30 min dark adsorption, 60 min dark adsorption		400 W metal halide lamp			h^+			Flower-like nanosphere			Multilayer			2020			106
													˙OH
α-Fe2O3/ZnFe2O4@Ti3C2			Rhodamine B (RhB)		(Rate constants) 0.02686 (RhB) (Removal rate)			20 mg catalyst + 100 mL 10 mg L^−1 substrate, 30 min dark adsorption		300 W Xe lamp (λ > 400 nm)			˙O2^−			α-Fe2O3/ZnFe2O4 nanoparticles/Ti3C2 nanosheets			Multilayer			2019			107
			Cr(VI)		Cr(VI)								˙OH
					Light off: about 70% in 90 min								h^+
					Light on: about 90% in 90 min

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In 2018, Cai et al. produced a Ag₃PO₄/Ti₃C₂ composite photocatalyst, which possessed excellent photocatalytic degradation performance.⁸⁷

As shown in Fig. 19, after DMSO and sonication treatment, Ti₃C₂ with a low-layer structure was formed. After the addition of silver nitrate, Ag⁺ was adsorbed due to the negative charge on the surface of Ti₃C₂. Ag₃PO₄ nanoparticles were grown in situ, forming a heterojunction between the Ag₃PO₄ nanoparticles and the Ti₃C₂ nanosheets.

Fig. 19 TEM images of (a) bulk Ti₃C₂, (b) single Ti₃C₂ sheet, (c) Ag₃PO₄/Ti₃C₂ composite. (d) EDX spectra of the Ag₃PO₄/Ti₃C₂ composite and (e) the schematic representation of single 2D Ti₃C₂ sheets and Ag₃PO₄/Ti₃C₂ synthesis.⁸⁷

As shown in Fig. 20, the heterojunction of the Ag₃PO₄ nanoparticles/Ti₃C₂ nanosheets shows a photocatalytic degradation stage rate K of 0.094, 0.005, 0.32, and 0.0042 min⁻¹ for methyl orange (MO), 2,4-dinitrophenol (2,4-DNP), tetracycline (TC–H), thiamphenicol (TPL), and chloramphenicol (CPL), respectively. According to EPR analysis, the hydroxyl radical (˙OH) plays an important role in the oxidation system, as shown in Fig. 20(f). This may be related to abundant Ti defects on the Ti₃C₂ surface. Ti sites exposed on the surface of Ti₃C₂ have strong redox reactivity, which promotes multiple electron reduction reactions (O₂ → H₂O₂ → ˙OH).

Fig. 20 Photocatalytic degradation of various pollutants by the as-prepared catalysts. (a) CPL, (b) TPL, and (c) TC-H degradation efficiency in the presence of the as-prepared catalysts under visible light irradiation (λ > 420 nm). HPLC chromatogram of (d) CPL and (e) TPL under different degradation times using the as-prepared catalysts. (f) UV-vis absorption spectra of TC–H under different degradation times using different catalysts.⁸⁷

As shown in Fig. 21, PL, TRPL, and the electrochemical characterization spectra indicate that the carrier separation efficiency of the material is significantly improved after carrying Ti₃C₂. This may be attributed to (i) the abundant surface hydrophilic functional groups of the Ti₃C₂ construct, which have strong interfacial contact with Ag₃PO₄, facilitating the separation of carriers; (ii) the strong redox reactivity of the surface Ti sites, which promote multiple electron reduction reactions to induce more ˙OH production; and (iii) a Schottky junction formed at the Ag₃PO₄/Ti₃C₂ interface enabling efficient transfer electrons to the Ti₃C₂ surface. This inhibits the photocossion of Ag₃PO₄ caused by photogeneration electrons.

Fig. 21 UV-vis diffuse reflectance spectra (a), PL spectra (b), time-resolved PL decay spectra (c), EIS Nyquist plots (d), transient photocurrent responses (e) of the as-prepared catalysts, and DMPO spin-trapping ESR spectra for DMPO–˙OH in the Ag₃PO₄/Ti₃C₂ system in the presence or absence of HA.⁸⁷

Under high temperature conditions, Ti in the Ti₃C₂ skeleton layer is oxidized into TiO₂, while C still exists in the form of a graphene-like layer. Therefore, under high temperature conditions, Ti₃C₂ can be converted into amorphous TiO₂ anchored within the graphene-like layer. In 2020, Wu et al. took advantage of this material, which displayed excellent photocatalytic degradation performance.¹⁰⁵

As shown in Fig. 22(e) and (f), high temperature treated Ti₃C₂ still retains its morphology and a 3D block-shaped morphology is formed after carrying g-C₃N₄ (Fig. 23).

Fig. 22 SEM images of C₃N₄ (a and b), Ti₃C₂ (c and d), heated Ti₃C₂ (e and f), and graphene layers anchored TiO₂/g-C₃N₄ (g and h).¹⁰⁵

Fig. 23 The photocatalytic degradation performance of TC (a), CIP (b), BPA (c), and RhB (d) by photocatalysts under visible light irradiation.¹⁰⁵

Graphene layers anchored to TiO₂/g-C₃N₄ show first-order kinetic constants for the degradation of rhodamine B (RhB), tetracycline (TC), ciprofloxacin (CIP), and bisphenol A (BPA) of 0.0559, 0.0244, 0.0168, and 0.0194 min⁻¹, respectively. According to the EPR test results (Fig. 24(a)–(d)), the oxygen active species that play a role in the oxidation process mainly include ˙O₂⁻ and ˙OH. Furthermore, signals corresponding to the holes (h⁺) were also detected. The contribution to the degradation of these test molecules appears to be in the order of ˙O₂⁻ > h⁺ > ˙OH.

Fig. 24 Trapping experiment for the photocatalytic degradation of TC over GTOCN3 (a); ESR spectra of CNTOC3 for (b) DMPO–˙O₂⁻, (c) TEMPO–h⁺, (d) DMPO–˙OH in the dark and under visible light irradiation, I–T curves under visible light irradiation (e) and the EIS response (f) of the samples.¹⁰⁵

High-temperature treated Ti₃C₂ has a greatly enhanced light absorption capacity, whilst the carrier separation efficiency and transmission efficiency are also improved. The improvement of the photocurrent (Fig. 24(e) and (f)) also verified that the graphene layers anchoring TiO₂ lead to the formation of a heterogeneous junction. This is due to the change in the electric field between g-C₃N₄.

In conclusion, as a co-catalyst, the application of Ti₃C₂T_x in photocatalytic degradation is mainly due to its three characteristics: (a) in a liquid–solid phase reaction, its hydrophilicity makes it easy for the adsorption or contact between the pollutants and photocatalysts; (b) high throughput electron transfer makes it easier to generate concentrated holes (h⁺); and (c) Ti sites exposed on the surface of Ti₃C₂ have strong redox reactivity, which promotes multiple electron reduction reactions, such as the reaction of activating molecular oxygen (O₂→H₂O₂→˙OH).

4. Challenges

The application of Ti₃C₂T_x in photocatalysis is worthy of further investigation, despite the many problems that need to be solved. The main issue lies in the instability of the composite material, resulting in unstable photocatalytic performance. This contributes to the difficulty in determining the photocatalytic process mechanisms of Ti₃C₂T_x-based photocatalysts. Many of the existing solutions use either few- or single-layer structured materials. However, the preparation process of these is complex. Despite this, Ti₃C₂T_x has an excellent optical response ability and displays broad catalytic activity.

5. Summary and outlook

5.1 Summary

In recent years, Ti₃C₂T_x has attracted wide interest as a photocatalytic material due to its rich surface space and surface defects, hydrophilic properties, large interlayer spacing, and excellent microwave absorbing properties. Ti₃C₂T_x-based photocatalysts are widely used in hydrogen evolution reactions (HER), CO₂ reduction reactions (CO₂RR), photocatalytic degradation reactions, and show excellent catalytic performance. The application of Ti₃C₂T_x in photocatalysis still warrants further investigation.

The further application of Ti₃C₂T_x in photocatalysis depends on the development of the material itself. Methods to improve the stability of the Ti₃C₂T_x structure need to be explored, starting from synthetic methods. In addition, the rich groups on the surface of Ti₃C₂T_x and its hydrophilicity should be further explored, particularly in photocatalytic liquid phase reactions.

5.2 Outlook

5.2.1 Mechanism. During the photocatalytic reaction, especially the reaction including liquid phase, the mechanism needs to be explored. As is known to all, the structure of pure Ti₃C₂T_x is not stable in both air and water. In air, the fresh-etched surface groups (such as –OH and –F) can be replaced by oxygen termination after being exposed to air for a period of time; in water, Ti₃C₂T_x can even be oxidized after being replaced for 21 days in room temperature.¹⁰⁸ Thus, it needs to be explored more whether the structure of Ti₃C₂T_x is changed during the photocatalytic reaction and if it does, how it changes.

During photocatalytic CO₂RR, although both monolayer-structured and multilayer-structured Ti₃C₂T_x exhibit high performance, isotope detection shows that some carbon resources come from CO₂ molecular; thus, there are still some ambiguities and other possibilities. For example, the valence state of “C” in Ti₃C₂T_x is mostly “−4”, which makes it possible for CO₂ to react with Ti₃C₂T_x in order to form the CO as product; this pathway involves redox reaction rather than catalysis.

This, in all, the mechanism needs to be explored more, both during the photocatalytic reaction and the oxidation of Ti₃C₂T_x itself.

5.2.2 Development direction. The application of Ti₃C₂T_x in photocatalysis is meaningful not just because it obviously promotes the reaction but also due to its applications in other new two-dimensional materials. For further applications, the following directions are necessary: to explore new methods of preparation to get structurally-stable Ti₃C₂T_x; to explore easier methods of preparation to get monolayered Ti₃C₂T_x; to explore more effective combination between Ti₃C₂T_x and the base photocatalyst; to explore methods of regulating the surface groups and interlayer groups; to explore other application in the MXene family; to explore the mechanism.

Ti₃C₂T_x is the earliest material in the MXene family; thus, the improvement of its application in photocatalysis represents a great significance for the application of the whole family.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51702270 and 51872147), the 111 Project (D20015), the Program for Innovative Research Team of Science and Technology in the University of Henan Province (19IRTSTHN025), PetroChina Innovation Foundation (No. 2018D-5007-0604), and Sichuan Science and Technology Program (No. 2020JDJQ0057).

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