Advances in Titanium Carbide (Ti₃C₂T_x) MXenes and Their Metal–Organic Framework (MOF)-Based Nanotextures for Solar Energy Applications: A Review

Abstract

Introducing new materials with low cost and superior solar harvesting efficiency requires urgent attention to solve energy and environmental challenges. Titanium carbide (Ti₃C₂T_x) MXene, a 2D layered material, is a promising solution to solve the issues of existing materials due to their promising conductivity with low cost to function as a cocatalyst/support. On the other hand, metal–organic frameworks (MOFs) are emerging materials due to their high surface area and semiconducting characteristics. Therefore, coupling them would be promising to form composites with higher solar harvesting efficiency. Thus, the main objective of this work to disclose recent development in Ti₃C₂T_x-based MOF nanocomposites for energy conversion applications to produce renewable fuels. MOFs can generate photoinduced electron/hole pairs, followed by transfer of electrons to MXenes through Schottky junctions for photoredox reactions. Currently, the principles, fundamentals, and mechanism of photocatalytic systems with construction of Schottky junctions are critically discussed. Then the basics of MOFs are discussed thoroughly in terms of their physical properties, morphologies, optical properties, and derivatives. The synthesis of Ti₃C₂T_x MXenes and their composites with the formation of surface functionals is systematically illustrated. Next, critical discussions are conducted on design considerations and strategies to engineer the morphology of Ti₃C₂T_x MXenes and MOFs. The interfacial/heterojunction modification strategies of Ti₃C₂T_x MXenes and MOFs are then deeply discussed to understand the roles of both materials. Following that, the applications of MXene-mediated MOF nanotextures in view of CO₂ reduction and water splitting for solar fuel production are critically analyzed. Finally, the challenges and a perspective toward the future research of MXene-based MOF composites are disclosed.

1. Introduction

In the modern age, the major issues faced by mankind is the energy crisis and the rapid depletion of fossil fuels. There is a growing urgency to look for alternative fuels and energy sources to partially fulfill the increasing energy demand. Thus, researchers are devoted to discovering sustainable technologies to overcome the energy crisis.^1,2 For instance, renewable energy production such as steam reforming for H₂ energy production³ and CO₂ hydrogenation reactions⁴ to produce fuels such as hydrocarbons or alcohols are mature technologies that have already reached commercial scale operations. However, these approaches are highly energy-intensive as they require high thermal energy input to initiate the reactions. In turn, photocatalytic H₂ production and CO₂ reduction are emerging as promising advances in generating fuels and energy as they rely solely on naturally available solar energy to drive the reactions.⁵⁻⁷ Not only that, abundantly available CO₂ and H₂O function as low-cost feed stocks for the photocatalytic reactions. Hence, solar-driven CO₂ reduction and water splitting is an up-and-coming approach to generate renewable fuels due to their inexpensiveness and environmentally benign process.⁸⁻¹⁰

In 2011, MXenes were a class of novel materials first discovered by Gogotsi and co-workers.¹¹ MXenes are promising photocatalytic materials that are derived from MAX via acid etching to remove the Al layers, leaving behind the M (transition metals) and X (C or N) with a general formula of M_n+1X_n. MXenes exist in either 0D quantum dots (QDs) or as 2D layered structures, which provides a higher specific surface area, reaction sites, ameliorated light harvesting, and electron acceptance capability as well as the prolonged lifetime of charge carriers.^12,13 Additionally, they have good mechanical and chemical stability as well as easily tunable valence electrons via preparation using lighter and heavier transition metals.¹⁴ Due to its metallic properties, MXenes are able to act as a cocatalyst for photocatalytic reactions due to their ability to form a Schottky junction, which acts as an electron trap and mediator.¹⁵ By pairing the MXenes with other photoresponsive materials, a built-in internal electric field is induced, thus electron separation and migration are promoted.¹⁶ Also, several studies reported that some MXenes exhibit semiconducting properties with band gaps from 0.05 to 2.87 eV.¹⁷ Thus, MXenes are promising cocatalysts and can be coupled with semiconductors to boost reactivity.

Among the semiconductors, metal–organic frameworks (MOF) are a promising class of materials that have different morphologies and dimensionalities, ranging from 1D rods to 2D layered sheets and 3D network structures.¹⁸ Yaghi’s group pioneered the discovery and successful preparation of MOFs in 1999.¹⁹ The existence of highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) separated by an energy band gap (E_bg) makes them suitable candidates as photocatalysts due to their ability to harvest solar energy. Not only that, MOFs also have various beneficial properties such as extremely large specific surface area,²⁰ high porosity,²¹ and a tunable structure. The light harvesting capability of MOFs is augmented due to the large surface area, which increases the exposed surface area of the material to light irradiation. Moreover, the desirable porosity makes it an excellent CO₂ adsorbent that eases the subsequent conversion to solar fuels.²²

Researchers all around the world are devoted to discovering highly active catalysts for solar-energy-driven renewable energy evolution. Pristine Ti₃C₂T_x MXene alone cannot be utilized as a photocatalyst for either solar-driven H₂ production or CO₂ reduction due to its inability to generate electron/hole pairs for photoredox reactions. Conversely, they are effective as cocatalysts due to their blackbody, which facilitates light absorption and harvesting.²³ Not only that, Ti₃C₂T_x MXenes are able to function as electron reservoirs, in which they can efficiently trap photogenerated electrons.²⁴ Hence, Ti₃C₂T_x MXenes must be coupled with other photoresponsive materials to unleash their full potential as a cocatalyst. In the past, Ti₃C₂T_x-based composite photocatalysts have been prepared, in which Ti₃C₂-based semiconductor composites were widely employed. For instance, Li et al. paired Ti₃C₂ QDs with a g-C₃N₄ semiconductor as a photocatalyst for H₂ production, where the MXene effectively trapped the photogenerated electrons from the g-C₃N₄ nanosheets.²⁵ Similarly, a ternary semiconductor-based TiO₂/C₃N₄/Ti₃C₂ composite was utilized for solar-driven CO₂ reduction. An S-scheme heterojunction was first obtained between the TiO₂/C₃N₄ interface due to formation of an internal electric field, band bending, and Coulomb force, where the electrons in TiO₂ combined with the holes in the valence band (VB) of C₃N₄. Subsequently, the electrons produced in C₃N₄ were trapped by Ti₃C₂T_x MXenes.²⁶ However, the composite pairing with semiconductors presents a glaring disadvantage, which is the fast recombination of charge carriers due to large energy band gaps. In efforts to overcome this shortcoming, Ti₃C₂T_x-based MOF nanotextures are gaining attention as photocatalysts for solar fuel production. To date, 90,000 different MOF structures have been discovered, each with their own unique properties and energy band gap. This means that the potential combinations of Ti₃C₂T_x/MOF composites are limitless, in turn presenting a huge potential and research interest in maximizing solar fuel generation.

Herein, we reviewed the various applications of Ti₃C₂T_x MXene-based MOF composites on the photocatalytic production of solar fuels. First, an in-depth discussion on the principles and mechanism of photocatalysis is conducted. Then the fundamentals of photocatalytic CO₂ reduction and water splitting are discussed by understanding the various possible oxidation and reduction reactions involved. The various preparation method of Ti₃C₂T_x MXenes are thoroughly reviewed, which includes HF etching, acid-containing fluoride ions, water-free etching, alkali etching, electrochemical etching, and molten salt substitution. Then a comprehensive discussion on the preparation of Ti₃C₂T_x MXene-based MOF composites is conducted. Numerous applications of Ti₃C₂T_x MXene-based MOF composites are reviewed and discussed for the sustainable production of solar fuels. Finally, the challenges and comparative analysis of Ti₃C₂T_x MXenes are conducted over various materials. Despite limited studies conducted on the Ti₃C₂T_x–MOF composites, they are definitely promising photocatalysts for solar fuel production and should be further explored for various applications.

2. Fundamentals and Principles

2.1. Principles of Photocatalysis

Photocatalysis is an up-and-coming sustainable approach to induce catalysis as it only utilizes naturally available solar energy as energy input to the reaction system. The photocatalysis pathway consists of four crucial steps, as shown in Figure 1. They are light harvesting, charge excitation, occurrence of redox reaction, and the recombination of charge carriers.²⁷ Initially, the photoresponsive materials are exposed to light irradiation. There are three main radiations from solar energy, each with varying energy levels, with UV radiations having the highest energy, followed by visible light and infrared radiation. Hence, UV radiations show the highest efficiency in generating electron/hole pairs, whereas the infrared radiations only function to provide a heating effect.²⁸ The light harvesting step can be further enhanced through multiple reflections and a light scattering effect over materials with rough surfaces as a result of meso- and macropores.²⁹ Upon successful light harvesting, electrons are excited with sufficient energy to transverse the E_bg from the VB or HOMO to the conduction band (CB) or LUMO. On the other hand, the photogenerated holes are then left behind in the VB or HOMO.³⁰ Following the charge excitation step where electron/hole pairs are formed, there are two possibilities that may occur, which is the favorable utilization of electron/hole pairs for redox reactions or the unfavorable recombination of charge carriers leading to loss of energy. The CO₂ reduction reaction occurs on the CB/LUMO of the material, where the electrons are utilized to convert CO₂ into solar fuels such as CO, HCOOH, CH₄, and CH₃OH, whereas the oxidation half-reaction occurs over the VB/HOMO.³¹ Similarly, photocatalytic water splitting reactions consist of two half-reactions, which is the reduction and oxidation of water to produce H₂ and O₂ gas, respectively. There are two types of recombinations that occur, namely, volume recombination where the charge carriers undergo recombination at the bulk of the material or surface recombination where the electron/hole pairs recombine on the surface of the material.³² The recombination of charge carriers will then lead to the loss of energy in the form of heat. Hence, the recombination should be prevented at all costs via different approaches, such as the formation of heterojunctions and surface sensitization, which can trap electrons and provide spatial separation, which prolongs the lifetime of the photogenerated electron/hole pairs.

Figure 1.

Schematic illustrating the overall mechanism for photocatalysis of CO₂ reduction and hydrogen production. Adapted with permission from ref (30). Copyright 2022 Elsevier.

2.2. Fundamentals of Photocatalytic Solar Fuel Production

Recently, photocatalytic solar fuel production has been gaining traction due to its simplicity, cost-effectiveness, and environmentally benign process. There are two widely known and sustainable technologies to produce renewable fuels, namely, photocatalytic CO₂ reduction and water splitting to produce H₂. For photocatalytic reactions to occur, there are two main processes occurring, which is the reduction and oxidation half-reactions. For a particular reduction half-reaction to occur, the CB/LUMO of the material must be more negative compared to the redox potential. Conversely, the VB/HOMO of the photoresponsive material should be greater than that of the redox potential of the oxidation half-reaction for it to be feasible.³³Equation 1 shows one electron reduction of CO₂ to the CO₂^– radical.³⁴ However, the reaction is deemed unfeasible due to it possessing a high redox potential. Not only that, it is impossible for a photocatalyst to supply enough potential to transfer a single electron to a CO₂ molecule.³⁵ Hence, reactions with low redox potentials are proton-assisted multielectron reactions, making them achievable over a wide range of materials.³⁶ As shown in eqs 2–5, CO₂ can be reduced to a wide variety of products, namely, CO, HCOOH, CH₃OH, and CH₄, respectively. The product selectivity depends on the amount of the number of electrons supplied to the CO₂ molecule. For instance, photocatalytic CO₂ reduction usually favors the production of CO and HCOOH due to it only requiring two electrons to initiate the reaction. On the other hand, the formation of CH₃OH and CH₄ requires more electrons and protons (6 and 8, respectively) to facilitate the reaction. For the photocatalytic water splitting to produce H₂, it involves two crucial steps. The first step is shown in eq 6, where two holes are utilized to oxidize the water molecules to produce oxygen and two protons. Subsequently, the protons and electrons proceed via eq 7 to produce H₂.^37,38

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2.3. Overview, Fundamentals, and Properties of MXenes

MXenes are a type of two-dimensional (2D) nanomaterial obtained through selective etching of MAX phases. The general formula of MXene is M_n+1X_nT_X, in which the M stands for the early transitional metal elements such as Sc, Ti, Hf, Mo, Ta, Nb, Cr, and so on. The acronym A represents the Al or Si layer, while X constitutes either C or N elements. The chemical exfoliation of the A element of MAX results in the termination of the M surface with abundant of functional groups, T_x such as −F, −OH, and −O. The first MXene material ever synthesized was in 2011 which is titanium carbide MXene (Ti₃C₂T_x) by Naguib and co-workers via chemical etching with hydrofluoric acid (HF).¹¹ It was known that the mechanical method was not able to exfoliate the M_n+1X_n layers due to the strong metallic M–A bonds in MAX phase. Therefore, the selective etching is considered a preferable method to break the M–A bonds owing to the different chemical activities of M–A and M–X bonds in MAX.³⁹ The n in the formula of MXene represents the atomic layer in a unit cell. A different compositional formula of MXene with different atomic layers can be obtained such as M₂XT_x, M₃X₂T_x, and M₄X₃T_x, as presented in Figure 2a. The typical structure of MXenes could be categorized as mono-M MXenes and double-M MXenes based on their compositional configurations.⁴⁰ In mono-M MXenes, the compositional arrangement is distinguished by the position of an early transitional metal covering (M) the C or N (X) elements. On the other hand, double-M MXenes are subcategorized into solid solution and ordered type. In solid solution of double-M MXenes, two early transitional metals, M1 and M2, are randomly distributed into the MXenes, while for ordered type, one of the M metals is positioned between the second transition M metal.

Figure 2.

(a) Different compositional formula of Ti₃C₂T_x MXene with different atomic layers. Reprinted with permission from ref (40). Copyright 2019 Wiley. (b) Different electronic properties of Ti₃C₂T_x MXene. Reprinted from ref (15). Copyright 2021 American Chemical Society.

Typically, Ti₃C₂T_x MXenes are metallic materials that exhibit excellent electrical conductivity of 6000–8000 S cm^–1, comparable to noble metal and graphene materials. This makes available as significant cocatalysts to promote the photocatalysis process. Additionally, the electrical properties of Ti₃C₂T_x MXenes can also be tailored through different M metal compositions, morphological modulation, and tuning the termination properties.¹⁵ For instance, delaminating the multilayer Ti₃C₂T_x MXene layers into their individual layer is reported to exhibit higher electrical conductivity up to 15000 S cm^–1.⁴¹ The findings also suggested that the thickness of the Ti₃C₂T_x MXenes flakes is significantly linked to the electrical conductivity by which the monolayer Ti₃C₂T_x MXenes exhibited higher electrical conductivity compared to that of the bilayers and trilayers.⁴²

In the perspective of electronic conductivity, Ti₃C₂T_x MXenes are metallic in nature and thus unable to generate electrons and holes. Therefore, they are typically used as a cocatalyst to assist photocatalytic activity due to the Schottky barrier effects. A higher work function is one of the distinct properties of Ti₃C₂T_x MXenes that give them a significant role as a photocatalytic enhancer and favorable as a noble metal substitution. A work function as high as 6.25 eV could be achieved, which is as high as other renowned metal cocatalysts such as Ag (∼4.5 eV), Au (∼5.38 eV), and Pt (∼6.10 eV).⁴³⁻⁴⁵ In the photocatalysis process, incorporating metal with a higher work function could improve the carrier dynamics and induce a stronger redox reaction.

Nevertheless, the work function of Ti₃C₂T_x MXenes can be varied depending on the tailored surface termination groups. This is because their work function is mainly controlled by the induced dipole moments arising from the transfer of charges between the termination group of Ti₃C₂T_x MXenes and the changes in the total surface dipole moments due to surface relaxation. In this context, the findings suggested that Ti₃C₂T_x MXenes with an −OH terminal group were observed to exhibit a work function of 1.6–2.8 eV. On the other hand, −O-terminated Ti₃C₂T_x MXenes displayed a higher work function between 5.75 and 6.25 eV.⁴⁶ Furthermore, the termination of −OH groups caused the decrease of the work function compared to that with −O and −F termination, where the work function was shown to either increase or decrease depending on the M metal constituted.⁴⁷ However, theoretical analysis through first-principles calculations indicated that some MXenes can exhibit semiconducting properties depending on the type of M metal constitution, surface terminating group, and the number of the MXene layers, such as that presented in Figure 2b. It was suggested that Ti₃C₂T_x manifests metallic properties regardless of any surface functionalization. The same findings were observed on the MXenes with thicker sheets and a large number of transition metals. Therefore, the probability of MXenes to undergo the shifting of the electronic properties into semiconducting is high in thinner MXenes, such as those with n = 2. Therefore, not all MXenes are metallic in nature, and some can exhibit semiconducting properties by configuring their termination groups, morphological structure, and types of metal composition. The feasibility of tailoring the electronic and electrical properties is one of the distinct properties of MXenes that favors them in a wide range of scientific applications.

2.4. Fundamental and Principles of Metal–Organic Frameworks

MOFs are a class of crystalline materials with metal nodes held together by organic linkers, forming highly sophisticated porous structures.⁴⁸ Initially, MOFs were mainly used in the field of gas storage, but slowly their role expanded into various applications such as gas/liquid adsorption and separation,⁴⁹ electrode development for supercapacitors,^50,51 and batteries⁵² as well as heterogeneous catalysis.⁵³⁻⁵⁵ MOFs are able to be used in broad applications mainly due to their unique properties and characteristics, such as an unprecedented specific surface area, high porosity, and easily tunable chemistry.⁵⁶ MOFs exists in a variety of morphology and dimensionality, from 1D rod structures to 2D layers and finally 3D network structures. Additionally, MOFs can also act as sacrificial templates to produce a different material such as porous metal oxides and layered double hydroxides. Similar to semiconductor materials, MOFs are also bestowed with impressive optical properties, making them able to induce electron/hole pairs upon irradiation with a light source. Table 1 summarizes the various MOFs discovered over the years with their respective metal nodes, linkers, and physical and optical properties.

Table 1. Various Classification of MOFs and Their Properties.

MOF	metal node	linkers	dimension	surface area (m2/g)	HOMO (eV)	LUMO (eV)	Ebg (eV)	refs
ZIF-67	Co	2-methylimidazole	3D	1139.00	1.18	–0.74	1.92	(95,96)
Co-MOF-74	Co	2,5-dihydroxyterephthalic acid (H4DOBDC)	1D	1025.00				(97)
MOF-71	Co	terephthalate acid (H2BDC)	2D	56.50				(98)
MOF-5	Zn	terephthalic acid (H2BDC)	3D	1101.00			3.82	(87)
ZIF-7	Zn	benzimidazole	3D	167.00				(99)
ZIF-7-NH2	Zn	2-aminobenzimidazole	3D	417.00				(99)
ZIF-8	Zn	dimethylimidazole	3D	1123.00	2.10	–1.20	3.30	(20,100)
HKUST-1	Cu	1,3,5-tricarboxylic acid (H3BTC)	3D	1143.06	2.01	–0.58	2.59	(101)
Cu-MOF	Cu	1,3,5-trimesic acid	3D	19.00	1.64	–0.80	2.44	(102)
Cu3(L)2(4,4′-bipy)	Cu	4,4′-bipyridine (4,4′-bipy)	3D	83.86			2.82	(103)
UiO-66	Zr	terephthalic acid (H2BDC)	3D	835.00	3.19	–0.72	3.91	(86)
UiO-66-NH2	Zr	2-aminoterephthalic acid (H2BDC-NH2)	3D	917.00	1.82	–1.01	2.83	(86)
UiO-67	Zr	biphenyl-4,4′-dicarboxylate (H2BPDC)	3D	2500.00	2.43	–1.20	3.63	(63)
MIL-101	Fe	1,4-benzenedicarboxylic acid (H2BDC)	3D	1413.00			2.59	(54,104)
NH2-MIL-101	Fe	2-aminoterephthalic acid (NH2-BDC)	3D	1237.40	0.97	–1.75	2.72	(105)
PCN-250	Fe	3,3′5,5′-azobenzene tetracarboxylic acid (H4abtc)	3D	1240.00	1.19	–0.89	2.08	(88)
MOF-901	Ti	4-aminobenzoid acid	2D	550.0			2.65	(106)
NH2-MIL-125	Ti	2-aminoterephthalic acid (H2BDC-NH2)	3D	405.40	2.10	–0.40	2.50	(53)
MIL-125	Ti	terephthalic acid	3D	1510.00	3.43	0.16	3.27	(66,107)
NTU-9	Ti	2,5-dihydroxyterephthalic acid (H4DOBDC)	2D	1205.00	0.32	–1.58	1.90	(108,109)
PCN-415	Ti	terephthalic acid	3D	1050.00			3.31	(110)

2.4.1. Chemistry of Metal–Organic Frameworks

MOFs are composed of two main building blocks, including metal nodes and the bridging organic linkers. More than 60,000 types of MOFs can be discovered due to the endless possibilities of combination between different metal precursors with organic linkers. Figure 3 summarizes the general chemistry of MOFs. One of the more straightforward ways to classify and identify MOFs is through their metal nodes. For instance, Co, Zn, Cu, Zr, Fe, and Ti are some of the typical metal nodes present in monometallic MOFs. Co-based MOFs are highly utilized due to their relatively low cost and simple preparation methods.⁵⁷ In the field of catalysis, Co metals were reported to show impressive activity in renewable fuel production.⁵⁸ Zn-based MOFs such as ZIF-7 and ZIF-8 are referred to as noble-metal-free systems due to Zn being a cheaper and abundant alternative to noble metals, as well as showing promising performance under light irradiation.⁵⁹ Cu is another transition metal that has shown beneficial properties in promoting photocatalytic performance. A study reported that the doping of Cu can enhance the reception of visible light by 35% as well as reduce the energy band gap significantly.⁶⁰ Zr-based MOFs possess great structural stability with a high specific surface area due to Zr-carboxylate coordination bonds.⁶¹ UiO-66⁶² and UiO-67⁶³ are two examples of well-known Zr-based MOFs that have been utilized in various photocatalytic reactions. In the case of Fe-MOFs, oxo-iron clusters are present which makes them potential candidates as photocatalysts as they exhibit good visible light harvesting ability.⁶⁴ Finally, Ti-based MOFs consist of tetravalent Ti⁴⁺ cations which can form tough frameworks due to strong metal–ligand bonding. The presence of Ti-oxo clusters contributes to the photocatalytic activity by enhancing the redox reactions.⁶⁵

Figure 3.

Illustration of the chemistry of MOFs in terms of metal node classification and organic linkers.

The second building blocks that are used to prepare MOFs are organic linkers. The linkers are the bridges between metal nodes that originate from organic acids such as terephthalic acid or trimesic acid. Usually, functionality can be introduced to MOFs via the use of functionalized linkers or via postsynthetic modifications. Most notably, the Ti-based MIL-125 and NH₂-MIL-125 have a relatively similar synthesis method with similar metal precursors and solvents used, in which only the organic acids deployed are different. In the case of preparing MIL-125, terephthalic acid is utilized, whereas amine-functionalized terephthalic acid (2-amino terephthalic acid) is used to prepare NH₂-MIL-125.⁶⁶ Hence, NH₂-MIL-125 will have amine functional groups on the linkers of the MOF. Another way to induce functionalization is through the postsynthetic modification (PSM) of the linkers.⁶⁷ Su’s group reported an improvement in water resistance of Mg-MOF-74 via PSM by functionalizing it with tetraethylenepentamine (TEPA). Hence, amine functional groups were introduced to Mg-MOF-74, which inhibits the damaging effects of water entering into the framework.⁶⁸

2.4.2. Physical Property of Metal–Organic Frameworks

MOFs are materials that have been widely researched in various fields and applications due to their desirable and unique physical properties. Here, the physical properties of MOFs such as their surface area, porosity, and various synthesis methods are critically discussed, which is then summarized, as shown in Figure 4. First, MOFs tend to boast incredibly high specific surface area, usually in the range of hundreds if not thousands. Titanium-based MIL-125 has a surface area of 1510 m²/g,⁶⁶ whereas its metal oxide counterpart TiO₂ only has a mere surface area of 35 m²/g.⁶⁹ Moreover, MOFs are also bestowed with numerous pores with large openings. These large pores are beneficial for enabling them to function as effective materials for gas adsorption and capture. A chromium-based MIL-101 features a high pore volume of 1.63 cm³/g, in turn promoting a huge N₂ uptake capacity of 1003 cm³/g.⁷⁰ Interestingly, the porosity of MOFs not only is limited to gas adsorption but also enables them to capture large heavy metal molecules, as well. Nimbalkar et al. noticed that UiO-66 with a free carboxylic group is riddled with micropores of 0.4–0.7 nm, in turn providing a high lead and methylene blue adsorption capacity of 100 and 169 mg/g, respectively.⁷¹ Hence, a large surface area and pore volume is a highly sought-after property as it opens doors to various applications such as catalysis and adsorption.

Figure 4.

Illustration of the physical properties and preparation method of MOFs.

Even though most of the MOFs exist as three-dimensional (3D) polyhedrons, there are also some MOFs exhibiting 1D rods and 2D sheet structures. Recently, a novel 1D chiral imine Zr-MOF (DUT-136) was synthesized via the use of Schiff base of (R,R)-1,2-diphenylethylenediamine and 4-formylbenzoic acid linkers.⁷² Similarly, a number of 2D MOFs have been discovered by researchers, such as NUS-1⁷³ and MIL-169.⁷⁴ In the past, various preparation methods for MOFs have been discovered. The most common approach to prepare MOFs is through the solvothermal method. Here, the metal precursors and linkers were first dissolved in solvent, such as methanol and N,N-dimethylformamide (DMF). Then the mixture was placed in a Teflon-lined stainless-steel autoclave where heat was introduced to the system to initiate the chemical reaction. Finally, the obtained slurry was washed and dried to obtain the MOF crystals.⁷⁵ The solvothermal synthesis step usually utilizes a reaction temperature of not more than 200 °C, with a reaction time ranging from 12 to 72 h. A titanium-based NH₂-MIL-125 was prepared via a conventional solvothermal method with reaction conditions of 150 °C and 72 h.⁷⁶ Correspondingly, an Fe-MOF was prepared by first dissolving FeCl₃·6H₂O and H₂BDC precursors in DMF solvent, followed by reaction for 24 h at 110 °C.⁷⁷ Despite the easy process of the solvothermal synthesis method, it is limited by various factors such as a long synthesis time and inhomogeneous heating of reactants.⁷⁸

MOFs can also be prepared via a straightforward co-precipitation method where the reactants are mixed until precipitates are formed without involving external energy sources.⁷⁹ Co-precipitation provides a uniform distribution of components. ZIF-67 is a prime example of a MOF prepared via the co-precipitation method. The synthesis step is relatively straightforward: Co precursors were mixed with linkers (2-methylimidazole) in methanol and stirred for 6 h. Then the resulting solution was washed and dried to obtain 3D purple dodecahedral crystals.⁸⁰ However, to date, there is only a limited number of MOFs prepared using this method. MOFs are also prepared using an energy-free approach, which is the slow evaporation method. In this method, the solvent is gradually evaporated at room conditions, leaving behind the dried MOF powders. Even though this process does not consume any energy, the synthesis requires a long period of time. Ghosh and co-workers reported a preparation period of 1 week for the synthesis of an Fe-MOF ([C₁₈H₂₂FeN₂O₆]NO₃) via the slow evaporation method.⁸¹ In another study, 2 weeks were consumed to prepare purple Co-MOF crystals via the slow evaporation approach.⁸² Finally, microwave-assisted synthesis was also employed to achieve rapid synthesis of MOFs. This technique utilizes microwaves to provide a heating effect to initiate the chemical reaction, in turn producing nanosized crystals.⁸³ The microwave-assisted approach is almost similar to the conventional solvothermal method but provides additional benefits such as fast crystallization time and facile morphology control and produces MOFs with narrow particle size distribution.⁶⁴ Most notably, Reza et al. discovered that using a microwave-assisted method to prepare UiO-67 required a much shorter preparation time (2.5 h) compared to that with the conventional solvothermal method (24 h). This is because the microwave heating couples directly with the solvent molecules, causing a rapid rise in temperature at the reaction media, which induces localized superheating. This provides a much more efficient heat transfer compared to conventional heating, resulting in a much shorter reaction time.⁸⁴

2.4.3. Optical Property of Metal–Organic Frameworks

Solar energy is a renewable source of energy that can be easily accessed. To fully utilize this abundant source of energy, researchers have been devoted to discovering materials that can effectively harvest and adsorb solar energy. Solar energy mainly comprises three main irradiations, namely, infrared, visible light, and ultraviolet (UV) radiations. Here, UV radiation has the highest energy content (shortest wavelength), followed by visible light and infrared radiation. Only high energy UV and visible light can generate electron/hole pairs over photoresponsive materials. However, despite the highest energy content of UV radiation, it only accounts for less than 4% of the solar spectrum, whereas visible light and infrared radiation constitutes 43 and 53% of solar energy, respectively.⁸⁵ Hence, to effectively utilize solar energy for photocatalysis, suitable materials must be explored to efficiently harvest visible light radiations.

In this context, MOFs are highly sought-after materials due to their desirable optical properties, which makes them photoresponsive materials. To address the energy crisis, MOFs are employed as photocatalysts to harvest solar energy to generate renewable fuels. First, MOFs have a narrow energy band gap (E_bg) which allows electron to migrate over from the VB/HOMO to the CB/LUMO. The migration of charge carriers is only possible when photons with energy equal to or greater than E_bg strikes the surface of the MOF.⁶⁴ Second, there has been a lot of MOFs discovered over the years, each with their own unique E_bg, as shown in Figure 5a. Some MOFs tend to have a wider band gap, such as UiO-66⁸⁶ and MOF-5⁸⁷ with E_bg of 3.91 and 3.82 eV, respectively. However, there are also MOFs with narrow band gaps, such as PCN-250⁸⁸ (2.08 eV) and NTU-9¹⁰² (1.90 eV). A narrow E_bg means that photons with lower energy are required to induce charge carrier formation. This means that visible light irradiation is sufficient to facilitate the photocatalytic reaction. Inversely, a wide E_bg requires more energy to generate electron/hole pairs, implying that UV irradiation will be more effective compared to visible light to drive photocatalysis. The relationship between the E_bg, the solar spectrum, and light wavelengths is illustrated in Figure 5b.

Figure 5.

(a) Band structures of various metal–organic frameworks. (b) Relationship between E_bg of photoresponsive materials (eV), solar spectrum, and light wavelength. Reprinted with permission from ref (64). Copyright 2021 Elsevier.

2.4.4. Metal–Organic Frameworks as Sacrificial Templates

MOFs in their original form are proper candidates as photocatalysts but can also serve as sacrificial templates to produce other materials. One of the more common MOF derivatives is metal oxides, which are obtainable by simple thermal annealing of MOFs. By calcinating the MOF at elevated temperature, the organic linkers will be removed, leaving behind the metal nodes which are then oxidized, producing metal oxides which inherit the morphology, porosity, and surface area of the parent MOF.⁸⁹ An Fe-MOF was synthesized via a solvothermal method with FeCl₃·6H₂O as the precursor and 1,4-benzenedicarboxylic acid linkers. The as-prepared MOF showed a well-defined nanospindle structure with a smooth surface, as illustrated in Figure 6a. Upon annealing the Fe-MOF in a N₂ atmosphere, Fe₂O₃ was obtained while retaining the spindle structure of the parent MOF. From Figure 6b, it was noticed that the surface of the Fe-MOF-derived Fe₂O₃ was rougher with nanoparticles on the surface.⁹⁰ Similarly, a Co-based MOF ZIF-67 was successfully converted into Co₃O₄ hollow polyhedrons via calcination treatment at 300 °C for 3 h. From the SEM image in Figure 6c, ZIF-67 particles have well-defined polyhedral shapes with a width of around 200 nm. After thermal treatment, the Co₃O₄ retained a similar polyhedral morphology with a specific surface area of 44.28 m²/g but had a hollow structure which is clearly observed in the TEM image in Figure 6d.⁹¹ Recently, there have been numerous reports on MOFs as templates to produce layered double hydroxides (LDHs). One study conducted by Chen et al. successfully converted 3D ZIF-9 into 2D Co LDH by varying the water/ethanol solvent volume ratio. According to the scheme in Figure 6e, when a 45:0 water/ethanol ratio was used, pure ZIF-9 with a 3D structure was obtained. However, as the ethanol amount increased from 15:30 to 10:35, ZIF-9 changed to ZIF-9/Co LDH and finally to pure 2D Co LDH.⁹² A bimetallic Ni–Co LDH-grown reduced graphene oxide (rGO) was also prepared via the etching of ZIF-67 by H⁺ upon the hydrolysis of nickel nitrate. As depicted in Figure 6f, ZIF-67 was first grown on GO. After successfully growing the ZIF-67 particles, the addition of nickel nitrate caused the framework of ZIF-67 to collapse into LDH platelets, resulting in the platelets inheriting a 3D structure of the parent MOF, forming standing and lying LDH on the GO.⁹³ Another study reported an almost similar procedure to prepare Ni–Co LDH nanosheets, in which Co-MOF nanosheets were gradually decomposed and transformed into LDH via a hydrolyzed etching step.⁹⁴ Thus, MOF templating is a useful way to produce other materials such as metal oxides and LDHs as the derivatives tend to retain the beneficial properties of the parent MOF, in turn promoting an enhanced photocatalytic activity.

Figure 6.

SEM images of (a) Fe-MOF and (b) Fe₂O₃ derived from Fe-MOF. Reprinted from ref (90). Copyright 2020 American Chemical Society. (c) SEM image of ZIF-67 and (d) TEM image of Co₃O₄ hollow polyhedrons. Reprinted with permission from ref (91). Copyright 2018 Elsevier. (e) Scheme of solvent ratio influence on formation of ZIF-9, ZIF-9/Co LDH, and Co LDH. Reprinted with permission from ref (92). Copyright 2021 Elsevier. (f) Preparation scheme of Ni–Co LDH on reduced graphene oxide. Reprinted with permission from ref (93). Copyright 2017 Elsevier.

2.5. Fabrication of a MXene-Based Schottky Junction

The performance efficiency of single semiconductor materials such as MOFs in photocatalytic solar fuel conversion can be enhanced through engineering modifications such as hybridization with other semiconductor materials, forming heterojunction composites. Other modifications included engineering the morphology of the semiconductor into different dimensions, defect engineering, and creation of vacancies.²⁷ However, one of the techniques that has been proven effective in creating higher-performance semiconductor materials is by the addition of metallic material. In this regard, the performance efficiency can be enhanced through an improvement in the carriers’ dynamics. This indicates that Ti₃C₂T_x MXene possesses a higher metallic electrical conductivity, which provides them with the ability to form a potential barrier at the metal–semiconductor interfaces, known as the Schottky junction.¹¹¹ Ti₃C₂T_x MXenes are, in general, unable to generate electrons and holes as they are naturally a metallic material. Therefore, they are mostly employed as a cocatalyst and paired with other semiconductor materials such as MOFs to promote the photoactivity through the synergy of a metal–semiconductor junction. The role of adding Ti₃C₂T_x MXene material is to improve the separation efficiency and promote the transfer of electrons in single semiconductor materials.

Fundamentally, the charge transfer mechanism in Ti₃C₂T_x MXene-based composites was achieved through the difference in the work function between Ti₃C₂T_x MXene and the semiconductor materials.¹¹² The built-in internal electric field at the metal–semiconductor junction called Schottky barrier was formed through the band alignment when both the semiconductor and Ti₃C₂T_x MXene are in contact with each other. The Ti₃C₂T_x MXenes normally serve as electron acceptors due to their large metal work function. Typically, the electrons will migrate from higher Fermi level to lower Fermi level materials, which is the case for MOF-Ti₃C₂T_x MXene composites, where the electrons will migrate from a higher Fermi level of the MOF to a lower Fermi level for Ti₃C₂T_x MXene.¹¹³ Note that the metal work function value (ϕ) is defined as the energy required to transfer an electron from the Fermi level to the vacuum level, where the vacuum level is the reference level at which the electrons possess an energy level outside of the metal with zero kinetic energy. It is worth mentioning that the larger the difference of the work function between the two hybrid materials, the stronger the Schottky barrier that offers excellent separation of the photocarriers. Therefore, metal with a higher work function value such as noble metals and Ti₃C₂T_x MXene are highly sought after to optimize the solar to fuel conversion.¹¹⁴Figure 7 illustrates the electron transfer mechanism in Schottky junction composite materials. It can be observed that the electrons migrated to the surfaces of the Ti₃C₂T_x MXene are trapped and unable to return back to the semiconductor MOF; therefore, they reserve more electrons to undergo the redox reaction. The creation of a Schottky junction between MOF-Ti₃C₂T_x MXene interfaces prevents the reverse injection of electrons, hastens the electrons transfer to the Ti₃C₂T_x MXene surfaces, and inhibits the recombination of photogenerated charges.¹¹⁵

Figure 7.

Construction of a Schottky junction and electron transfer mechanism.

3. Different Synthesis Techniques of MXene

In general, several different techniques are available to synthesize Ti₃C₂T_x MXene. One of the common and most applied techniques for synthesizing Ti₃C₂T_x MXene is HF etching, which utilizes hydrofluoric acid as a primary etchant. The utilization of an etchant is significant, especially in breaking the Ti–Al bond in Ti₃AlC₂ MAX, as the mechanical method cannot exfoliate the Al layer. Since the pioneering study of Ti₃C₂T_x MXene synthesis through HF etching, several other techniques have been developed to combat the utilization of concentrated HF acid, which is non-ecological, especially in wide-scale industrial production. Therefore, safer routes such as acid-containing fluoride etchant, alkali etching, electrochemical etching, halogen etching, water-free etching, and molten salt substitution techniques are among the alternatives to synthesize Ti₃C₂T_x MXene. Among them, the etching process through the utilization of acid-containing fluoride etchant, known as the in situ HF technique, has been broadly employed after the HF etching technique, unlike other newly developed techniques that have not been deeply researched.¹¹⁶Figure 8 summarizes the various preparation methods together with their reaction conditions for the synthesis of Ti₃C₂T_x MXenes, and Table 2 provides the reaction parameters for the different Ti₃C₂T_x synthesis methods.

Figure 8.

Different synthesis techniques of Ti₃C₂T_x MXenes. Reprinted from ref (117). Copyright 2022 American Chemical Society.

Table 2. Summary of Preparation Methods and Parameters of Ti₃C₂ MXenes.

synthesis method	catalyst	etching agents	termination group	synthesis parameters	comments	refs
HF etching	TiO2/Ti3C2	HF	O, OH, F	20 mL of 48% HF, 15 h etching time	multilayer MXene produced with lower photocatalytic efficiency compared to monolayer counterpart	(138)
	2D/2D HCN/MXene	HF		39% HF, 24 h etching time	high purity of MXene was produced with well-defined multilayer structure	(139)
				39% HF, 48 h etching time	the growth of anatase TiO2 on exfoliated multilayers MXene
				39% HF, 96 h etching time	the growth of anatase and rutile TiO2 was observed on MXene layer; more exfoliated layer and increase in the growth of TiO2 NP with increasing etching time
	TiO2/Ti3C2	HF	O, OH, F	49% HF, 24 h etching time (35 °C); ethyl alcohol as a washing agent; hydrothermally treated at 450 °C	the formation of TiO2/Ti3C2 safflower-like morphology with ∼1 μm safflower size and shorter nanorods formation	(140)
				49% HF, 24 h etching time (35 °C); ethyl alcohol as a washing agent; hydrothermally treated at 550 °C	average safflower size ∼2 μm upon heat increase heat treatment; well-formed safflower morphology
				49% HF, 24 h etching time (35 °C); ethyl alcohol as a washing agent; hydrothermally treated at 650 °C	average safflower size ∼2 μm with lower spatial density of TiO2/Ti3C2 nanorods compared to at 550 °C heat treatment; the safflower morphology slightly deformed at higher temperature
	BQ/TiC/UCN	HF		40% HF, 72 h etching time, 20 mL DMSO, 24 h intercalation time	ultrathin MXene sheet is produced, effectively function as the electron mediator	(141)
	Ti3C2-QD/Ni-MOF	HF		20 mL HF, 24 h etching time, DMF intercalating agent	MXene QD constructed with enhance charge transfer and higher separation capabilities of the photocarriers	(142)
	2D/2D g-C3N4/Ti3C2TA/R (CN/TCT) MXene	HF		39% HF, 24 h etching time	less distribution of TiO2 NPs on the surface of MXene	(121)
				39% HF, 96 h etching time	higher amount of TiO2 growth/conversion with more of rutile phase compared to anatase
				49% HF, 24 h etching time	higher distribution of TiO2 on the MXene surface when employing more concentrated etchants
				49% HF, 96 h etching time	both TiO2 (rutile) and TiO2 (anatase) having higher concentration due to more oxidation
acid-containing fluoride ions	TiO2/Ti3C2	LiF + HCl	O, OH, F	LiF with 6 M HCl, 24 h etching time	MXene flakes exhibit excellent hydrophilicity and dispersity with flakes size range of 10–100 nm	(138)
	ZnCdS/TiO2/Na-MXene	LiF + HCl		LiF with 9 M of HCl, 24 h etching time (35 °C)	the preintercalation of Na+ and attachment of ZnCdS nanoparticles on the MXene flakes improve the oxidation stability and slowing the oxidation to TiO2	(143)
	2D-Bi2MoO6@2D MXene	LiF + HCl	O, OH	LiF with 9 M of 20 mL HCl, 24 h etching time (35 °C)	2D MXene serves as effective platform for impeding the agglomeration and support the growth of Bi2MoO6 constructing a hierarchical composite structure	(144)
	Ti3AlC2/Ti3C2Tx	LiF + HCl		3.08 M LiF/HCl, 6 h etching time; 1, 3, 6, 24, and 36 h	delaminated MXene with more conversion to Ti3C2Tx than 1 h etched	(145)
				3.08 M LiF/HCl, 24 h etching time	delaminated MXene are formed with residual of Ti3AlC2 was observed
				3.08 M LiF/HCl, 36 h etching time	delaminated MXene with residual of Ti3AlC2 was observed; partial delamination occurs even at 36 h etched time
	Ti3C2Tx	LiF + HCl	O, OH, F	LiF with 9 M HCl, 24 h etching time (30 °C)	c-lattice parameter of 25.75 Å	(146)
				LiF with 9 M HCl, 24 h etching time (35 °C), ethanol as washing agent	the highest c-lattice parameter of 30.99 Å obtained with ethanol as washing agent with improved delamination ratio
alkaline etching	Ti3C2Tx	KOH	O, OH	5 M KOH, 120 °C hydrothermal treatment	less developed of accordion-like structure of MXene; more exposed of Ti-OH sites with with alkali etching	(147)
	Ti3C2Tx	NaOH	O, OH	27.5 M NaOH treatment at 270 °C	higher purity (92%) of multilayer MXene produced with removal of −F termination group	(129)
				27.5 M NaOH treatment at 250 °C	Ti3C2Tx yield decrease with reaction temperature
				5–10 M NaOH treatment at 270 °C	formation of Na/K–Ti–O compounds (NTOs) due to lower concentration of NaOH, increasing the water content and facilitate oxidation
				NaOH treatment at 100–220 °C	no formation of MXene as reation cannot happen at lower temperature regardless of any concentration
electrochemical etching	Ti3C2Tx	NH4Cl, TMAOH	O, OH	1 M NH4Cl, 0.2 M TMAOH	higher yield of single or bilayer (>90%) with larger average dimension	(126)
	Ti2C	HCl	–Cl, −O, −OH	1 M HCl, 0.6 V, 1 day etching	less conversion of MXene sheets.	(128)
				2 M HCl, 0.6 V, 5 days etching	the formation of carbon-derived-carbide (CDC) as the results of overetching
molten salt substitution	Ti3C2Tx	CuCl2	–Cl, −O	Ti3SiC2 in CuCl2 molten salt (750 °C)	delamination of the layer proceed with the aid of SiCl4 gas molecules produced through etching process	(136)
	Ti3C2Tx	ZnCl2	–Cl, −O	Ti3ZnC2 + ZnCl2 (annealed time: 0.5 h, 1 h, 1.5 h, 3.0 h)	the increasing of the ratio of molten salt, ZnCl2 in the starting precursor, a gradual conversion to Ti3C2Cl2 can be observed	(134)
	Ti3C2Tx	NaCl2, KCl, CuCl2	–I, −Br, −Cl	NaCl2, KCl, CuCl2 (mixed ratio of 1:2:2:3), 10 h annealed time in Ar (700 °C)	well-configured Lewis acidic etching route could tailored the surface chemistry of MXene	(148)

3.1. HF Etching

The HF etching technique is considered to be a pioneering technique and is an extensively used method to synthesize Ti₃C₂T_x MXene. HF was initially chosen as the primary etching agent due to its ability to break the large particle of MAX into smaller grains and its distinct properties to be one of the few selected etchants that can etch the titanium oxide layer typically present as a protective layer of Ti₃C₂T_x MAX.¹¹⁸ It was suggested that Ti₃C₂T_x MXene could be formed by treating the Ti₃AlC₂ MAX with 50% of HF acid under continuous stirring at room temperature.^11,119 From the morphological perspectives, as presented in Figure 9a, the SEM image reveals an accordion-like structure, suggesting that the 2D structures of Ti₃C₂T_x MXene are formed after 2 h etching with HF. Studies also suggested that different morphologies of Ti₃C₂T_x MXene such as rolls, nanotubes, and multilayers were obtained after sonication. Moreover, the X-ray diffraction (XRD) pattern in Figure 9b shows that other parameters such as etching time could affect the removal of the Al layer of MAX. In this regard, the (104) peak at 2θ = 39, subjected to the MAX phase, was observed to slowly diminish with increasing etching time and was shown to completely disappear after 15 h of etching. This further revealed that increasing the etching time could assist the removal of the Al layer and the formation of Ti₃C₂T_x MXene. Further details on the mechanism of the Ti–Al bond breaking and the formation of Ti₃C₂T_x MXene is presented in eqs 8–10.¹²⁰

8

9

10

Figure 9.

(a) SEM image of the 2D structure of Ti₃C₂T_x MXene after 2 h etching with HF. Reprinted from ref (119). Copyright 2012 American Chemical Society. (b) XRD analysis on Ti₃C₂T_x MXene at different etching times. Reprinted with permission from ref (125). Copyright 2013 Elsevier. (c) Formation of TiO₂ on the Ti₃C₂T_x MXene sheets at different etching times. Reprinted with permission from ref (121). Copyright 2021 Elsevier. (d) XRD analysis on the formation of Ti₃C₂T_x MXene through reaction with LiF with HCl solution. Reprinted with permission from ref (122). Copyright 2014 Springer.

Generally, Ti₃C₂T_x MXene is formed by removing the Al layer sandwiched between the individual layer of Ti₃C₂T_x. As presented in eq 8, Ti₃AlC₂ MAX will react with three molecules of HF to form white small solid particles of AlF₃ with the generation of H₂ gas. The loss of the metallic bonding leads to the formation of layered Ti₃C₂T_x MXene, where the surface of Ti is terminated with functional groups such as −F and −OH such as that shown in eqs 9 and 10. It has been noted that the termination groups present are dependent on the type of etchants utilized. Therefore, the utilization of HF as a primary etching agent successfully etched away the Al layer of MAX. This technique has been favored due to the cost-effective and straightforward procedures. However, our research group found that the etching time could affect the creation of TiO₂ on the Ti₃C₂T_x MXene surfaces.¹²¹ In this context, the etching of Ti₃C₂T_x MXene was carried out by utilizing 39 and 49% HF concentration, and both were etched at 24 and 96 h. It was suggested that increasing etching time with HF assisted the formation of TiO₂ nanoparticles (NPs) on the surfaces of Ti₃C₂T_x MXene, as represented in Figure 9c. In this regard, the increasing number of active TiO₂ semiconductor formation suggests a robust photocatalytic activity due to metal–semiconductor synergy. Even though the HF etching technique is highly utilized, the synthesis parameters, such as the concentration of HF and the etching time, are significant to ensure the purity of the produced Ti₃C₂T_x MXene. Moreover, controlling the synthesis parameters could essentially affect the photocatalytic activity of the Ti₃C₂T_x MXene-based materials.

3.2. Acid-Containing Fluoride Ions

Another synthesis technique that garnered scientific attention for the synthesis of Ti₃C₂T_x MXene is the acid-containing fluoride ion method. In order to minimize the use of concentrated HF acid, researchers have sought a new environmentally safe technique to synthesize Ti₃C₂T_x MXene. In this context, acid-containing fluoride ion etchants have been utilized to etch Al layers of Ti₃AlC₂ MAX, which at the same time delaminates their multilayer structure into the individual compartment.¹²² In this regard, the synthesis and delamination of Ti₃C₂T_x MXene can be achieved in a single step, unlike those by HF etching which requires intercalating agents to delaminate the multilayered structure of Ti₃C₂T_x MXene. The pioneering study reported that the single-step preparation method could be achieved by reacting Ti₃C₂T_x MXene with 6 M HCl with LiF at 45 °C for 45 h. Further analysis by powder XRD observed the removal of the Al layer from the Ti₃AlC₂ MAX, where the disappearance of Ti₃AlC₂ can be seen from Figure 9d. In this method, different types of acid and fluoride ions can be utilized, such as using sulfuric acid (H₂SO₄) containing NaF, KF, NH₃F, CsF, and tetrabutylammonium fluoride.¹²³ However, each combination might affect the properties of the synthesized Ti₃C₂T_x MXene. For instance, a study found that utilizing HCl might improve the electrochemical capacity of Ti₃C₂T_x MXene compared to those by HF.¹²⁴ Additionally, each fluoride salt utilized might require different synthesis parameters such as the reaction temperatures and etching time. It has been suggested that utilizing fluoride salts such as NaF and KF might require 48 h etching time at 40 °C to ensure a complete etching process. However, acid-containing NH₄F shows complete delamination after 24 h at 30 °C. A similar morphology can be observed in all synthesized Ti₃C₂T_x MXenes regardless of any types of fluoride ions. In this regard, the accordion-like structure is present, suggesting that different acid-containing fluoride ions successfully form the Ti₃C₂T_x MXene.

3.3. Electrochemical Etching

One of the newly developed techniques in synthesizing Ti₃C₂T_x MXene is the electrochemical etching by which the fluoride-based etchant can be eliminated. In this synthesized process, the electrode and electrolytes are required for the reaction to occur. The Al layer will be selectively etched with the assistance of electrolytes through anodic etching.¹²⁶ In general, the Ti₃AlC₂ MAX will serve as a working electrode where the etching reaction will take place. Particularly, chemical and electrochemical etching is part of the surface reaction involving electron transfer, suggesting the possibility of forming Ti₃C₂T_x MXene by electrochemically etching the MAX phase. A study found that the Ti₂AlC MAX can be electrochemically etched to form Ti₂CT_x MXene by utilizing an aqueous HCl (2 M) electrolyte. The absence of fluoride ions significantly yields fluoride-free terminated Ti₂CT_x MXene with the presence of −Cl, −O, and −OH terminal groups. In this approach, a three-layered structure of Ti₂CT_x MXene is formed, consisting of carbon-derived carbide (CDC), Ti₂CT_x MXene, and unetched MAX. Parameter study such as voltage, etching time, and electrolyte concentration was observed to affect the morphology and properties of Ti₂CT_x MXene, such as that shown in Figure 10a–d. In particular, increasing the concentration of the HCl and etching time shows pronounced delamination surfaces. However, overetching of Ti₂CT_x MXene might lead to the formation of CDC. Even though the three-layered structures consisting of CDC, MXene, and MAX are formed, bath sonication could separate the layered structure to obtain Ti₂CT_x MXene. The synthesis process of Ti₂CT_x MXene through this electrochemical etching is demonstrated in Figure 10e.

Figure 10.

(a) SEM images MAX, (b) Ti₂CT_x MXene etched with 1 M of HCl at 0.6 V for 1 day, (c) 2 M of HCl at 0.6 V for 5 days, (d) 2 M of HCl at 0.6 V for 14 days. (e) Synthesis illustration of the Al removal from MAX phase and formation of three-layered Ti₂CT_x MXene structure. Reprinted with permission from ref (128). Copyright 2017 Royal Society of Chemistry. (f) Synthesis process of electrochemical etching of Ti₂CT_x MXene in two-electrode system. Reprinted with permission from ref (126). Copyright 2018 Wiley.

Another study revealed that Ti₃C₂T_x MXene was successfully formed by utilizing binary aqueous electrolytes in a two-electrode system, such as that present in Figure 6f.¹²⁶ In this method, the electrolyte consisting of 1 M NH₄Cl and 0.2 M tetramethylammonium hydroxide (TMAOH) at pH >9 was employed for a 5 h reaction time to etch the Al layer and undergo intercalation to extract the carbide flakes. The presence of a Cl ion in the electrolyte expedites the anodic Al etching and breaks the Ti–Al bond. Intercalation by the ammonium hydroxide (NH₄OH) promotes the etching underlying the surfaces. This approach successfully yields 90% of single and double Ti₃C₂T_x MXene layers with flakes larger than those in the conventional HF technique. On the other hand, the fluoride-free Ti₃C₂T_x MXene offers better capacitance (220 mF cm^–2) than those conventionally prepared. Even though this synthesis method is shown to offer Ti₃C₂T_x MXene with Cl termination and successfully eliminate the −F functional, the formation of CDC required much attention. As mentioned, overetching the MAX phase in dilute HF, HCl, and NaCl electrolytes might lead to the removal of both Al and Ti layers.¹²⁷ Moreover, the newly developed technique requires in-depth scientific study to ensure a successful formation of Ti₃C₂T_x MXene with excellent catalytic properties.

3.4. Alkali Etching

Aside from acid utilization as the etching agent, the study has found that alkali can also be employed to eliminate the Al layer of Ti₃AlC₂ MAX.¹²⁹ A study on alkali etching revealed that the hydrothermal-assisted technique utilizing NaOH as the etching agent successfully etched the Al layer of Ti₃AlC₂ MAX to form Ti₃C₂T_x MXene. Alkali etching is one of the techniques to synthesize a fluorine-free Ti₃C₂T_x MXene as it does not utilize any fluoride-containing chemicals. Additionally, this newly developed preparation method was also revealed to yield high-purity (∼92%) Ti₃C₂T_x MXene with a multilayered structure. The strong binding ability of the NaOH with the Al element assists the etching process, thus making it possible. It has been noted that the synthesis parameters such as the reaction temperature and etching concentration are significant to ensure successful elimination of the Al layer. This is because, as demonstrated in Figure 11a, the low temperature might prevent the Al extraction process by the Al (oxide) hydroxides. Moreover, the inability to form Al₄(OH)⁻ due to lattice confinement from the Ti layer, known as the “jamming effect”, blocks formation of Ti₃C₂T_x MXene. On the other hand, utilizing a higher concentration of NaOH at a higher temperature leads to the oxidation of Ti₃C₂T_x MXene and the formation of NTOs such as Na₂Ti₃O₇ and Na₂Ti₅O₁₁. Higher water content promotes the oxidation of Ti to form NTOs. Therefore, a suitable reaction temperature and alkali concentration are required to ensure a complete formation of Ti₃C₂T_x MXene. The hydrothermal reaction at 270 °C with 27.5 M NaOH concentration successfully yielded Ti₃C₂T_x MXene with high purity compared to HF-etched MXene. In this hydrothermal-assisted NaOH etching method, controlling the temperature is essential to ensure a complete formation of Ti₃C₂T_x MXene, while the optimum concentration of Ti₃C₂T_x MXene controlled the purity of the produced Ti₃C₂T_x MXene. Even though the alkali-assisted, hydrothermal synthesis method has been shown to successfully generate Ti₃C₂T_x MXene, preventing the Ti₃C₂T_x MXene from overetching to form NTOs is one of the challenges and requires extra attention. In addition, this method needs in-depth research to confirm the successful formation of fluoride-free Ti₃C₂T_x MXene. Through this technique, −O and −OH terminal groups can be formed, and their role in stimulating photocatalytic activity and fuel conversion can be further extended.

Figure 11.

(a) Synthesis process of Ti₃C₂T_x MXene through alkali etching. Reproduced with permission from ref (129). Copyright 2018 Wiley. (b) Schematic representation of water-free etching of MXene by utilizing propylene carbonate. Reproduced with permission from ref (137). Copyright 2020 Elsevier. (c) SEM image of Ti₃C₂T_x MXene prepared through molten salt substitution. Reproduced from ref (134). Copyright 2019 American Chemical Society. (d–f) SEM image of Ti₃C₂T_x MXene by utilizing different types of MAX with chloride molten salts. Reproduced with permission from ref (136). Copyright 2020 Springer.

3.5. Water-Free Etching

Water-free etching is another alternative in producing Ti₃C₂T_x MXene with outstanding oxidation stability.¹³⁰ In this synthesis process, the employment of water as a primary solvent is replaced by the polar organic solvent to ensure no presence of water. The exceptional potential of Ti₃C₂T_x MXenes in a wide array of applications indicates that they are one of the most favored materials. However, their hydrophilicity and tendency to oxidize limit their potential in applications requiring a longer storage period. Therefore, water-free etching was found to improve oxidation stability and broadened their role in any water-sensitive applications. In this preparation method, ammonium dihydrogen fluoride was utilized as the etching agent due to its capability to dissociate into NH₄F and HF when dissolved in the polar solvents. Three reaction steps are required: etching, washing, and delamination. Specifically, as presented in Figure 11b, the etching process occurred at which Ti₃AlC₂ MAX underwent a reaction with NH₄HF₂, where the Al layer was etched out in the form of AlF₃ and (NH₄)₃Al₆. Reaction with propylene carbonate (PC) promotes delamination into single-layer Ti₃C₂T_x MXene. It has also been suggested that higher interlayer spacing of Ti₃C₂T_x MXene is obtained compared to water-etched Ti₃C₂T_x MXene.¹³¹ Controlling the termination groups are favorable as it has been revealed that, through this technique, approximately 70% of −F and −30% of −O/–OH containing MXene was obtained.¹³² In short, this method utilizes polar organic solvent, PC to replace water which was found to reduce the presence of the −OH terminating group. The control of the terminating group significantly enhances the oxidation stability, which raises Ti₃C₂T_x MXene potential in the water-sensitive applications as the finding suggested that more exposure of the Ti–OH species of Ti₃C₂T_x MXene expedites the oxidation process.¹³³

3.6. Molten Salt Substitution

Another alternative in producing −Cl-terminated Ti₃C₂T_x MXene is through molten salt substitution.¹³⁴ This method utilizes molten salts such as ZnCl₂ and CuCl₂ to undergo a substitution reaction with Ti₃AlC₂ MAX for the removal of the Al layer. This reaction process significantly produces nanolaminated Ti₃C₂T_x MXene with −Cl termination, which was found to be more stable than −F-terminated Ti₃C₂T_x MXene.¹³⁵ The reaction mechanism is presented in eqs 11–13, which generally involves the replacement of the A element of MAX with the metal salts.

11

12

13

In eqs 11–13, the Zn²⁺ from the molten salts undergoes elemental replacement with the Al³⁺ from the Ti₃AlC₂ MAX. Stronger Lewis acidity by ZnCl₂ promotes the substitution of Zn²⁺ with Al³⁺, forming Ti₃ZnC₂ MAX. Additionally, Zn²⁺ acts as a Lewis acid in the molten salt as it is a stronger Cl^– and electron acceptor. Specifically, molten salt of ZnCl₂ was ionized into Zn²⁺ and ZnCl₄^2– in its molten state, while Al from Ti₃AlC₂ MAX undergoes redox conversion into Al³⁺. The formation of AlCl₃ occurred through the bonding of ionic Al³⁺ with Cl^–, where it will be evaporated. Rapid evaporation of AlCl₃ facilitates the outward diffusion of the Al atom to undergo a substitution reaction. The elemental replacement further takes place when the reduced Zn atoms intercalate into the Al layer in the MAX phase, forming Ti₃ZnC₂, which is further exfoliated into Ti₃C₂–Cl₂ MXene. The SEM image in Figure 11c reveals the morphology of the Cl-terminated MXene, where it shows a more compact structure with a less distinct accordion sheet compared to other conventional preparation techniques. Another study by Li et al. revealed that the synthesis of Ti₃C₂T_x MXene from different types of MAX phases could be achieved by employing different types of chloride molten salts such as CuCl₂, FeCl₂, NiCl₂, and AgCl.¹³⁶ Different morphology of MXenes etched by different chloride molten salts can be observed in Figure 11d–f, by which most of them show a prominent accordion-like structure of MXene.

4. Design Principles and Considerations for Ti₃C₂T_x MXene-Based MOF Composite

4.1. Morphological Design and Engineering of Ti₃C₂T_x MXenes and MOFs

In the context of morphological development, Ti₃C₂T_x MXenes intrinsically manifest a unique accordion-like multilayer structure. However, their morphology can be modified into different dimensional structures, such as 0D quantum dots, 2D delaminated flakes, and 3D hierarchical/multidimensional structures. Delaminating Ti₃C₂T_x MXenes into single flakes was shown to supply more exposed catalytic active sites and was observed to be highly efficient in driving the redox reaction compared to their multilayer morphology.¹⁴⁹ Typically, delaminating of Ti₃C₂T_x MXene can be mainly performed through direct delamination or a two-step delaminating process. It has to depend on the preparation method of the Ti₃C₂T_x MXene itself. Direct delamination can be achieved via an acid-containing fluoride ion etching technique.¹⁵⁰ In this regard, no intercalating agents are required to separate the layer as the fluoride ions can serve as intercalants to aid in the delamination process. On the other hand, the HF etching technique is regarded as a two-step process, requiring intercalating agents to assist the layer separation. Commonly used intercalating agents include dimethyl sulfoxide (DMSO), dimethylformamide (DMF), tetrabutylammonium hydroxide (TBAOH), and tetramethylammonium hydroxide (TMAOH).^151−153 Interestingly, a comparison study by Su et al. shows that monolayer Ti₃C₂T_x MXene is more reactive toward the photoredox reaction due to its thinner sheet structure.¹³⁸ This is because the size and structure of Ti₃C₂T_x MXenes distinctly affect their photocatalytic property and are correlated with their Fermi level energy. Hydrogen yield with 2.5 times enhancement was disclosed by employing monolayer Ti₃C₂T_x MXene compared to their multilayer counterpart. Even though the precise location of active sites in Ti₃C₂T_x MXenes is yet to be confirmed, through structural analysis, it was inferred to be positioned at the edge of the monolayer flakes. This corroborates with the experimental study where Ti₃C₂T_x MXenes with smaller flake sizes were observed to have an exceptional catalytic activity compared to that of a denser multilayer structure. More exposure of the catalytic active sites due to size effects and exposed flake edges supports the conjecture. Correspondingly, delaminated Ti₃C₂T_x MXenes through intercalation with TMAOH were revealed to have an apparent effect on the photocatalytic activity with 1.49 times activity improvement over that of the standard HF-etched multilayered Ti₃C₂T_x MXene.¹⁵⁴ Moreover, the 2D geometry of monolayer Ti₃C₂T_x MXene offers excellent support for other semiconductor photocatalysts, especially for nanoparticle materials from agglomeration, providing uniform dispersion. Therefore, the solar absorption performance can be augmented owing to a larger light receiving area.¹⁵⁵

Additionally, the tunability of their morphology indicates that they are a highly selective cocatalyst for photocatalytic enhancement. For instance, quantum dot Ti₃C₂T_x MXene can be constructed through layer cutting by DMSO intercalant and thermal treatment with PEI.^25,156 Another study reported the preparation of Ti₃C₂T_x MXene QD through an ammonia-assisted hydrothermal reaction.¹⁵⁷ Moreover, hydrothermal intercalation with DMF was also found to convert the nanosheets into QD structures.¹⁴² A benefit in converting into smaller nanosize QDs is to offer better solvent solubility and to expose more active edge sites. As mentioned, dimensional size has a great impact on the photoreaction activity, which highlights the significance of well-controlled morphology. The smaller dimensional size of the Ti₃C₂T_x MXene QD is shown to provide an effective coating for stabilizing the oxidation of other semiconductor materials. For instance, the Ti₃C₂T_x MXene QD was found to cover the porous surface of Cu₂O NWs by coating them and hindering the easy oxidation of the NWs.²⁵ In this regard, the photoreaction stability was observed to maintain up to six long cycles and retain as high as 86% methanol yield. The relevance of morphologically tuning the Ti₃C₂T_x MXene into a QD structure can prevail with their outstanding photoreaction activity for hydrogen generation, successfully obtaining their 2D nanosheet structure. A study found that the Ti₃C₂T_x MXene QD successfully produced solar hydrogen generation with as high as 10 times enhancement than that of Ti₃C₂T_x MXene nanosheets and 3 times higher than that incorporated with Pt, disclosing the novelty of the Ti₃C₂T_x MXene QD as a noble metal replacement.²⁵

Aside from morphologically constructing Ti₃C₂T_x MXene into single flakes and quantum dots, hierarchical and multidimensional Ti₃C₂T_x MXene constructed through the combination of different dimensional domains was observed to benefit them through a supportive platform. Typically, multidimensional Ti₃C₂T_x MXene can be constructed by hybridizing them with other dimensional photocatalysts. For instance, the Ti₃C₂T_x MXene safflower shape was successfully generated by hydrothermal oxidation followed by ion exchange to form TiO₂/Ti₃C₂T_x MXene with a 3D porous framework.¹⁴⁰ Thermally oxidized Ti₃C₂T_x MXene could support the formation of TiO₂ nanoparticles and, through the ionic exchange process, form the safflower-like-shaped TiO₂–Ti₃C₂T_x. The merit of having a 3D structure framework could hinder the agglomeration of the nanoparticles and sheet aggregation, which could reduce the absorption efficiency. Thinner and smaller sizes of Ti₃C₂T_x MXene are highly beneficial for the carrier’s transmission where shorter distances are required to carry out the redox reaction, but the implication of agglomerated sheets is one of the serious drawbacks.¹⁵⁸ The creation of a multidimensional structure of a Ti₃C₂T_x MXene-based composite with a 1D/2D structure highly benefits the carrier dynamics as different dimensional domains have an individual contribution for photocatalytic enhancement. For instance, 1D/2D CdS/Ti₃C₂T_x MXene constructed through in situ assembly and solvothermal treatment was found to give an eminent improvement in the photocatalytic activity compared to that of their intrinsic counterpart.¹⁴⁹ The effective platform of 2D Ti₃C₂T_x MXene sheets for CdS nanorod growth provides intimate contact and aids in the separation of charges. For instance, the longitudinal oriented charge transmission with high length-to-diameter ratio semiconductors could improve the separation efficiency as a longer time is needed for the charges to travel back and recombine. On the other hand, hierarchical nanoflower-like core–shell morphology is constructed by in situ hydrothermal LDH on Ti₃C₂T_x MXene.¹⁵⁹ The growth of smaller LDH nanoflakes creates a Ti₃C₂T_x MXene with abundant active sites for photocatalytic CO₂ reduction. The well-dispersed LDH nanoflakes covering the Ti₃C₂T_x MXene surfaces in the core–shell-like shape improve the light absorption and hinder the serious agglomeration typically observed in LDH nanoflakes. Therefore, designing Ti₃C₂T_x MXene into different morphological structures is one of the promising engineering considerations for producing a very efficient photocatalyst for solar fuel conversion. Figure 12 summarizes the development of Ti₃C₂T_x MXene into different morphological structures.

Figure 12.

Morphological development of Ti₃C₂T_x MXene into monolayer, quantum dots, and hierarchical/multidimensional structure.

As mentioned previously, MOFs tend to naturally occur in 1D rods, 2D sheets, or 3D polyhedrons, in which the latter tend to be the more common morphology. 1D MOFs are usually identified by their nanometric diameter together with an extended length, adopting chains or rod structures. On the other hand, 2D MOFs exist as sheets or layers that have a nanoscale thickness. 2D MOFs have a few beneficial properties as an efficient photocatalyst, such as high exposed surface area to light irradiation and reactants, as well as excellent electron transferability.^160,161 Finally, 3D MOFs are held together by strong chemical bonds, which come together to form a highly complicated network structure. These polyhedrons boast an extremely high surface area, with uniform pore size distributed all over the surface of the MOF.³⁰ Nevertheless, the morphology and dimensions of MOFs can be further regulated via two main approaches which is (1) modification of synthesis parameters and (2) introduction of additives. First, various synthesis parameters were reported to modify the morphology of MOFs, such as temperature, reaction time, and solvent ratio. Yuan et al. noticed that the structure of Ni-MOF alters according to the solvothermal synthesis temperature. At room temperature and 60 °C, the Ni-MOF exhibits an urchin-like structure with a diameter of approximately 200 nm. As the temperature increased to 100 °C, one-dimensional nanowires with 10 nm diameter were obtained. The change in morphology was mainly due to the dynamic viscosity of 2-methylimidazole linkers, which limits the mass transfer through the temperature change.¹⁶² In another study, the effects of reaction temperature and time were studied with a Co-MOF. The original 2D nanoplates of Co-MOF were transformed into 1D layered microrods as both the reaction temperature (120 to 160 °C) and time (5 to 30 min) increased.¹⁶³ The solvent ratio can also effectively alter the structure of the MOF due to the role of solvent as a directing agent as well as ligands, where they can be incorporated onto the lattice of the MOF. It was previously reported that by manipulating the DMF/ethanol solvent ratio from 3 to 1, the 3D structure of the MOF was converted into 2D nanosheets.¹⁶⁴

Second, the use of additives such as modulators and surfactants is effective at tuning the morphology of MOFs. For example, a 3D hollow porous concave octahedral bimetallic Fe–Zn MOF was prepared via a conventional solvothermal method using FeCl₃·6H₂O, Zn(NO₃)₂·6H₂O, and terephthalic acid as linkers, with DMF and C₂H₅OH alcohol as solvent. Upon introduction of polyvinylpyrrolidone (PVP), the morphology of the MOF showed a drastic change, from 3D octahedrons to 1D hollow porous nanorods.¹⁶⁵ Additionally, Sarawade et al. investigated the effect of various surfactants on the morphology on a Co-MOF with linkers of 2,6-naphthalenedicarboxylic acid (H₂ndc) and trans-1,2-bis(4-pyridyl)ethylene (bipyen). Without the use of surfactant, the Co-MOF exhibited a spindle-like microrod structure. However, upon the use of cetyltrimethylammonium bromide, sodium dodecyl sulfate (SDS), and Pluronic triblock copolymer (P123) surfactants, the original morphology was converted into 2D nanoplates, nanosheets, and 1D nanorods, respectively.¹⁶³ The morphology engineering of MOFs was then summarized, as shown in Figure 13. MOFs tend to exist mainly in 3D structures, with some MOFs naturally existing in 1D rods or 2D sheets. However, the morphology and dimensions of MOFs can be further engineered either by varying the synthesis parameters or by using additives.

Figure 13.

Scheme illustrating the general morphology and morphological engineering of MOFs.

4.2. Active Sites over Ti₃C₂T_x MXene-Based MOF Composite

Active sites are the locations or specific sites where a certain reaction has the highest activity. It is of utmost importance to identify the active sites over photocatalysts to gain a deeper understanding of how to quantify and optimize the active sites with respect to a specific chemical reaction. This is because the types of active sites present on the surface of the catalyst can influence the overall selectivity of the products. The role of active sites is especially evident in thermal and photothermal catalysis. For example, the utilization of Ni¹⁶⁶ and Co⁸⁰ metals tends to yield a higher production of CH₄ from the CO₂ hydrogenation reaction. However, the use of Fe promotes the Fischer–Tropsch reaction, in turn exhibiting a higher C₂₊ hydrocarbon evolution.^167,168 From these examples, we can get a general idea on the active sites of MOFs. MOFs are made up of a variety of metal nodes, such as Co, Zr, and Ti and can be regarded as isolated quantum dots that can be excited upon light irradiation.¹⁶⁹ Second, the organic linkers can also function as active sites or be functionalized to anchor active sites. Most notably, UiO-66 linkers modified with Cu(II) porphyrins not only enhanced the overall photocatalytic activity but also boosted the light adsorption of the photocatalyst.¹⁷⁰ Similarly, MOF-525, which consists of porphyrin linkers anchored Co metal, successfully incorporated the Co active sites onto the framework of the MOF. The introduction of CO active sites to the framework prolonged the lifetime of charge carriers and improved the CH₄ production from photocatalytic CO₂ reduction.¹⁷¹ Lastly, the huge cavity of MOFs is a suitable host for encapsulating photoactive metal nanoparticles. Zhao’s group successfully encapsulated Ni nanoparticles within the pores of UiO-66, which limited the growth of Ni via agglomeration.¹⁷² Interestingly, the encapsulation is not limited to only metals. Li et al. reportedly boosted the photocatalytic CO₂ reduction by embedding carbon dots inside NH₂–UiO-66 particles. These carbon dots function as electron receptors and photosensitizers, which promote charge separation and transfer.¹⁷³ The discussion on the active sites of MOFs are summarized in Figure 14, where they exist either at the metal nodes, organic linkers, or within the cavity of MOFs.

Figure 14.

Overview of the active sites present over metal–organic frameworks.

Up to the current development, the exact active sites of Ti₃C₂T_x MXene are still under exploration. Some studies inferring the position of the active sites might be located at the edges of the Ti₃C₂T_x MXene sheet. This has been experimentally hypothesized by which delaminating MXene into thinner flakes exposed more of the active sites and induced higher reaction activity compared to that with the multilayer structure.¹³⁸ Therefore, engineering the morphology Ti₃C₂T_x MXene was observed to affect the photoreaction activity which is highly linked to the degree of exposure of the catalytic active sites. Nevertheless, despite the vagueness in specifying the active site of MXene, an analysis study revealed that the termination group has a prominent role in tailoring the stability and catalytic properties of Ti₃C₂T_x MXene. The type of the termination group can be qualitatively controlled through parameter regulation. Designing Ti₃C₂T_x MXene with suitable termination groups is significant to fit the application requirement and control its environmental stability. For instance, the formation of −F termination has been linked with poor ambient stability in a colloidal dispersion, which led to the formation of −F free terminated Ti₃C₂ MXene by Shi et al.¹⁷⁴ On the other hand, Ti₃C₂T_x MXene can easily be oxidized under ambient environments, and the exposure of the Ti atom with oxygen could lead to the formation of TiO₂. Worth mentioning is that the functional group is the active oxidative site which could affect the oxidation stability of Ti₃C₂T_x MXene. For instance, computational analysis on each type of termination group revealed that the termination group is linked with the defect formation energy. In this regard, the −OH termination group relating to lower oxidation stability is due to lower energy formation of the vacancy. However, =O termination is oxidatively stable due to higher energy formation of the vacancy. Moreover, it was confirmed by the study conducted indicating that the Ti–OH group is highly reactive owing to the lower work function.¹⁷⁵ The abundance of reactive sites in Ti–OH further facilitates the oxidation of Ti₃C₂T_x MXene. The removal of −OH through hydrogen annealing was found to improve the oxidation stability of Ti₃C₂T_x MXene.¹³³ In additional, preparing Ti₃C₂T_x MXene with −Cl termination was observed to possess higher oxidation stability compared to that of −F- and =O-terminated Ti₃C₂T_x MXene.¹⁷⁶ Therefore, perfect control of the surface termination is significant, especially for ensuring a highly stable Ti₃C₂T_x MXene in an ambient environment. This is because it was noticed that Ti₃C₂T_x MXene could easily be oxidized during the thermal treatment even with no support of oxidizing agents due to the presence of surface terminating groups.¹⁷⁷

4.3. Modification Strategies through Interfacial/Heterojunction Engineering

4.3.1. Ti₃C₂T_x MXene

Another design consideration to increase the potential of Ti₃C₂T_x MXene is through interfacial/heterojunction engineering. The metallic nature and excellent electrical conductivity in Ti₃C₂T_x MXene make it the most influential cocatalyst to assist the solar fuel conversion.^178−181 Ti₃C₂T_x MXene could serve as an electron trapper through the creation of an electrostatic potential barrier at the Ti₃C₂T_x MXene/semiconductor junction. This phenomenon is termed the Schottky barrier, where electrons that transfer from the semiconductor to Ti₃C₂T_x MXene are trapped and unable to transfer back to the semiconductor counterpart and recombine with holes. The Schottky barrier can be constructed through the contact formation between Ti₃C₂T_x MXene with other semiconductor photocatalysts. Ti₃C₂T_x MXene with a large work function could highly benefit from the carrier separation. This is because the higher the difference between the metal work function and the semiconductor, the stronger the potential barrier created at the metal–semiconductor junction, making the electrons from the Ti₃C₂T_x MXene unable to travel back. The Schottky barrier induced by Ti₃C₂T_x MXene is a distinction that aids photocatalytic enhancement.

In addition to the creation of the Schottky barrier, Ti₃C₂T_x MXene could also serve as a mediator to assist electron transmission and is prominent in the construction of the Z-scheme heterojunction system.¹⁸² For the first time, Ti₃C₂T_x MXene was employed as a mediator in the three-phase Z-scheme system catalyst of CdS@Ti₃C₂@TiO₂ by Liu et al.¹⁸³ The electron sinking effects by Ti₃C₂T_x MXene with a well-matched band configuration give a substantial improvement in the photocatalytic activity through the creation of a Z-scheme electron transfer system. In this system process, Ti₃C₂T_x MXene serves as an electron-trapping center and as the mediator. The role of Ti₃C₂T_x MXene as the electron mediator facilitates the electrons from the CB of TiO₂ to transfer to the VB of CdS, hastening the electron migration. Another study disclosed the significance of Ti₃C₂T_x MXene in mediating the electron transfer between g-C₃N₄ and MoSe₂, constructing a Z-scheme heterojunction system.¹⁸⁴ Separation of photocarriers was enhanced, and the electrons effectively transferred from one semiconductor to another semiconductor to undergo a redox reaction. The vital role of Ti₃C₂T_x MXene as a trapping center and a mediator provides greater reductive/oxidative ability as more electrons are available and less recombination occurs. Nevertheless, the employment of Ti₃C₂T_x MXene as an electron mediator in the Z-scheme system is still not widely explored compared to other cocatalysts such as reduced graphene oxide (rGO), which might be due to their thicker sheet reducing the contact formation with multiple semiconductor photocatalysts.¹⁸³

In general, thermally oxidized Ti₃C₂T_x MXene could generate a Ti₃C₂T_x MXene–TiO₂ hybrid.¹⁸⁵ Moreover, our group also investigated the construction of anatase/rutile TiO₂ formation by regulating the etching time with HF.¹²¹ Therefore, employing a Ti₃C₂T_x MXene–TiO₂ hybrid with other semiconductor photocatalysts such as g-C₃N₄, CdS, and MoS₂ could induce a hybrid Schottky–Z-scheme/Schottky–type II heterojunction system. In this context, the electrons will migrate following the Z-scheme/type II configuration to the TiO₂-induced MXene and will then migrate to the reductive Ti₃C₂T_x MXene to undergo a reduction reaction.^{121,139,186,187} The Schottky barrier will be formed at the TiO₂–Ti₃C₂T_x junction and thus suppress the backward reversal of electrons back to TiO₂. Therefore, the combination of Schottky–Z-scheme/Schottky–type II heterojunction is highly propitious for boosting the photocatalytic performance of Ti₃C₂T_x MXene in the solar fuel conversion. Figure 15 presents the electron transfer mechanism of MXene as an electron mediator, in a Schottky barrier and a hybrid Schottky heterojunction system.

Figure 15.

Overall representation of the electron transfer mechanism in MXene as a cocatalyst.

4.3.2. MOFs

Since MOFs are photoresponsive materials that can generate electron/hole pairs when irradiated with solar energy, their photocatalytic activity can be enhanced via two main approaches, which is the prolongation of the lifetime of charge carriers and narrowing of band gap energy (E_bg). The lifetime of charge carriers can be prolonged by adopting various strategies to provide spatial separation of the electron/hole pairs. Additionally, the light harvesting ability of MOFs in the visible light region can be enhanced by modulating the E_bg of MOFs. Here, several approaches to promote the photocatalytic activity of MOFs will be discussed, such as the formation of heterojunctions, surface sensitization, MOF functionalization, doping of MOFs, and utilization of photosensitizers.

Heterojunctions are formed upon contact of two photoresponsive materials, in which different heterojunction systems have been discovered such as type I, type II, type III, Z-scheme, and step-scheme heterojunctions. The formation of heterojunctions not only promotes the adsorption of visible light but also provides spatial separation of charge carriers.¹⁸⁸Figure 16a shows a type II heterojunction between Co-MOF and Cu₂O (x-CMC). Here, electrons migrate from the CB of Cu₂O to the LUMO of Co-MOF, whereas the holes from HOMO of Co-MOF transfers to the VB of Cu₂O, achieving good spatial separation of charge carriers. Additionally, the formation of heterojunctions drastically improved the visible light adsorption of pristine Cu₂O, as shown in the UV–vis diffuse reflectance spectra in Figure 16b.¹⁸⁹ In another study, a Z-scheme heterojunction system (Figure 16c) was achieved over PCN-224(Cu)/TiO₂ photocatalysts which improved the overall photocatalytic reaction.¹⁹⁰ Surface sensitization is achieved when the Schottky junction is formed upon introduction of metals. Xiao’s group successfully encapsulated Cu nanoparticles within the pores of UiO-66 through an advanced double-solvent approach. Due to the intimate contact between the metals and MOF, a Schottky junction was generated, in which solar energy utilization and separation of charge carriers were ameliorated.¹⁹¹ By combining MOFs with MXenes, Schottky junction together with heterojunctions can be generated as reported by Wu et al. A Ti₃C₂-modulated MIL-125-NH₂ nanohybrid exhibited both type II heterojunctions and Schottky junctions. This is because Ti₃C₂T_x MXenes tend to oxidize into the TiO₂ semiconductor, allowing the heterojunction between TiO₂ and MIL-125-NH₂, while the metallic Ti₃C₂T_x MXenes induce a Schottky junction. Hence, the photogenerated electron is efficiently separated via migration to Ti₃C₂T_x, while the holes are left behind in the HOMO of the MOF.¹⁹² Functionalization is another promising strategy to enhance visible light adsorption as it can reduce the energy band gap of MOF. For instance, pristine UiO-66 has an E_bg of 3.91 eV, which decreased to 2.83 eV upon functionalization with an amine group.⁸⁶ Dao’s group reported an efficient type II heterojunction system over NH₂-MIL-125/g-C₃N₄ for photocatalytic CO₂ reduction, as shown in Figure 16d. The −NH₂ functional group not only enhanced CO₂ adsorption but also promoted light adsorption in the visible light range.¹⁰⁵

Figure 16.

(a) Type II heterojunction system between Co-MOF and Cu₂O. (b) UV–vis diffuse reflectance spectra of Co-MOF and Cu₂O composites (x-CMC). Reprinted with permission from ref (189). Copyright 2022 Elsevier. (c) Z-scheme heterojunction system on PCN-224(Cu)/TiO₂ photocatalysts. Reprinted from ref (190). Copyright 2019 American Chemical Society. (d) Type II heterojunction scheme over NH₂-MIL-125/g-C₃N₄. Reprinted from ref (105). Copyright 2020 American Chemical Society. Influence of different metal dopant on the E_bg of (e) MIL-125-NH₂ and (f) UiO-66-NH₂. Reprinted from ref (194). Copyright 2019 American Chemical Society.

Doping is a type of point defect where impurities or foreign atoms are introduced, which disrupts the original lattice structure of the material.¹⁹³ A study conducted by Syzgantseva et al. reported that the doping of metals into the lattice of MOFs is able to modulate the energy band gap of MOFs. Here, two MOFs, namely, MIL-125-NH₂ and UiO-66-NH₂, were doped with different types of metals, and their E_bg were observed. From Figure 16e, the doping of MIL-125-NH₂ with Sn increased the E_bg, but doping with Nb, W, and V narrowed the E_bg to 2.3, 2.2, and 1.6 eV, respectively. Similarly, UiO-66-NH₂ (Figure 16f) doped with Y element caused a slight increase in E_bg, whereas Ta, Nb, and W dopants further reduced the E_bg to 2.6, 2.3, and 1.7 eV, respectively.¹⁹⁴ A narrower band gap is able to promote a broader light adsorption spectrum. Finally, utilizing photosensitizers can promote light harvesting especially in the visible region due to them possessing a strong optical absorption over a wide range of wavelengths.¹⁹⁵ When photosensitizers are used together with MOFs, MOFs benefit by obtaining energy from the incident light that is absorbed by the photosensitizer.¹⁹⁶ Mu’s group coated CdS nanorods with Co(BDC) MOF and applied a [Co(bpy)₃]²⁺ photosensitizer. The MOF layer successfully bridges the heterogeneous photosensitizer and molecular cocatalyst, which enhanced the CO₂ reduction reaction and prevented photocorrosion of the photosensitizer.¹⁹⁷ In another study, the Ru(bpy)₂]Cl₂ photosensitizer and Re(CO)₃Cl molecular catalyst were grafted into the framework of MOF-808. It was noted that the covalent grafting of the photosensitizer not only enhanced the light harvesting ability of the photocatalyst but also improved the lifetime of electron/hole pairs and shortened the transport distance of charge carriers.¹⁹⁸ Therefore, it is clear that the photocatalytic activity of MOFs in terms of spatial separation of charge carriers and light adsorption ability can be further regulated with various modification strategies.

5. Synthesis of MXene-Based MOF Composites and Their Characterization

In the past, there have been various ways to prepare Ti₃C₂ MXene-based MOF composites for various applications. The composite can be prepared by two main ways, which is either through physical mixing of both MOF and MXene together or by growing the MOF particles on the surface of MXenes. Also, MOFs can be used as sacrificial templates to prepare various materials such as carbon materials and layered double hydroxides, which is then dispersed on the surface of the MXenes. Herein, a thorough discussion on the various synthesis approaches and their characterization are explored, where the overview of the approaches is summarized as shown in Figure 17.

Figure 17.

Overview of approaches to prepare Ti₃C₂ MXene-based MOF composites.

One of the more common approaches to prepare the Ti₃C₂ MXene-based MOF composite is by physically mixing both Ti₃C₂ and MOF together. By mixing them, the MOFs and Ti₃C₂ MXenes will be randomly bound and be in contact with each other. Ultrasonication is a promising way to successfully synthesize MXene/MOF composites. This is because, under ultrasonication, the Ti₃C₂ sheets can be exfoliated into multilayers.¹⁹⁹ For instance, Qin et al. prepared a Ti₃C₂ MXene-based MOF composite (Ti₃C₂-QD/Ni-MOF) through a facile ultrasonic method, as illustrated in Figure 18a. Ti₃C₂ quantum dots were prepared in two steps, which is the etching of the MAX phase to Ti₃C₂ multilayers, followed by hydrothermal treatment to obtain the Ti₃C₂ QDs. Then the prepared Ni-MOF was mixed with Ti₃C₂ QDs under continuous ultrasonication for 4 h. High-resolution transmission electron microscopy (HRTEM) images in Figure 18b,c show the successful preparation of the composite, in which a uniform dispersion of quantum dots on the 2D Ni-MOF was observed. Also, point defects are observed over the surface of Ti₃C₂, supplying coordination-unsaturated sites for catalytic reactions.¹⁴² Similarly, Liu and co-workers adopted the sonication method to prepare the MXene/MOF composite. First, NiCo-MOF nanosheets were prepared via a precipitation method. Upon obtaining the MOF powders, they were dispersed into a Ti₃C₂ aqueous solution under sonication. Due to the two-dimensional structure of both MOF and MXene, the ultrasonication promotes the interlacing of both materials, as studied over the field emission scanning electron microscopy (FE-SEM) image in Figure 18d. The interconnected porous networks which are developed between the MXene/MOF composite restricts the self-restacking of the NiCo MOF and Ti₃C₂ sheets.²⁰⁰

Figure 18.

(a) Illustration of Ti₃C₂-QD/Ni-MOF synthesis via ultrasonication approach. (b,c) HRTEM images of Ti₃C₂-QD/Ni-MOF. Reprinted from ref (142). Copyright 2020 American Chemical Society. (d) FE-SEM image of layered Ti₃C₂/NiCo-MOF. Reprinted with permission from ref (200). Copyright 2020 MDPI.

Another way to prepare the composite is through in situ growth of the MOF on the surface of MXenes. By growing the MOF particles on the MXenes, a more intimate contact between the two materials can be achieved. Zheng et al. anchored a 3D pillared Ni-MOF on the surface of 2D layered Ti₃C₂ MXenes, as illustrated in Figure 19a. The 4,4′-bipyridine (Bpy) and thiophene-2,5-dicarboxylate (Tdc) linkers were first introduced to the Ti₃C₂ sheets, followed by subsequent addition of NiCl₂·6H₂O into the solution. The addition of the linkers to the MXene bound the organic ligands to the interlayer functional groups of MXene. Then the addition of Ni^II coordinated the organic ligands over the Ti₃C₂ surface to produce a Ti₃C₂@Ni-MOF composite. The HRTEM images in Figure 19b display the successful growth of Ni-MOF on the surface of the 2D Ti₃C₂ sheets. The growth of Ni-MOF on MXene can prevent the oxidation of MXene due to the ample interlayer functional groups which diminish the exposure of surface atoms on MXenes.²⁰¹ In another study, dual heterojunction NH₂-MIL-125 modulated with a Ti₃C₂ hybrid was prepared using an in situ construction approach. Here, Ti₃C₂ nanosheets were added dropwise into the precursor aqueous solution for NH₂-MIL-125, followed by subsequent hydrothermal synthesis at 150 °C for 1 day. Interestingly, as shown in the TEM images in Figure 19c, TiO₂ is formed in the process, which is due to the hydrolysis of unreacted tetrabutyltitanate. Thus, Figure 19d shows that a type II heterojunction was formed between NH₂-MIL-125 and TiO₂, whereas a Schottky junction is formed where Ti₃C₂ acts as cocatalyst to trap photogenerated electrons.¹⁹² Long’s group also grew NH₂-MIL-88B particles on the surface of Ti₃C₂ MXenes. Figure 19e,f depicts the accordion layered structure of Ti₃C₂ MXene and hexagonal prism structure of NH₂-MIL-88B, respectively. Upon successful in situ growth of MOF on MXene, it can be observed that an optimal dispersion of NH₂-MIL-88B is achieved over the Ti₃C₂ surface, where the MXene provides growth platforms for the MOF particles, as shown in Figure 19g.²⁰²

Figure 19.

(a) Scheme of in situ growth for Ti₃C₂ MXene@Ni-MOF composite. (b) HRTEM image of MXene@Ni-MOF. Reprinted with permission from ref (201). Copyright 2022 Elsevier. (c) HRTEM image and (d) energy level diagram for NH₂-MIL-125/TiO₂/Ti₃C₂ composite. Reprinted with permission from ref (192). Copyright 2020 Elsevier. SEM images of (e) Ti₃C₂ MXene, (f) NH₂-MIL-88B, and (g) MXene/NH₂-MIL-88B composites. Adapted with permission from ref (202). Copyright 2021 Elsevier.

Finally, MXene-based MOF composites can also be prepared by using MOFs as sacrificial templates to prepare various materials. For example, MOFs can be used to derive various carbon materials that are usually used for electrochemical applications such as batteries, supercapacitors, and electrochemical detection.²⁰³ Nitrogen-doped porous carbon (N-PC) was prepared by carbonizing a Zn-based MOF-5-NH₂ in an Ar atmosphere at 900 °C over 2 h. Then N-PC (Figure 20a) was introduced to the Ti₃C₂ MXene (Figure 20b) via mixing and ultrasound treatment, obtaining the final MOF-derived composite denoted as Ti₃C₂/N-PC. From Figure 20c, it is noticed that the interlamellar spacing of the Ti₃C₂ MXene was enlarged, due to the successful insertion of N-PC between the gaps of the sheets.²⁰⁴ Likewise, Jiang and co-workers reported the in situ decoration of MOF-derived carbon on N-doped Ti₃C₂ nanosheets for applications in Li–S batteries. Ti₃C₂ solution and PVP solution were first mixed together for 10 min, followed by the addition of Co(NO₃)₂·6H₂O and 2-methylimidazole for the growth of ZIF-67 on the surface of the MXene. Figure 20d illustrates the SEM image of Ti₃C₂/ZIF-67, indicating the effective growth of MOF on the MXene surface. Upon carbonizing the composite at 800 °C for 2 h, the 2D nanosheet morphology of N–Ti₃C₂/C inherited from the parent Ti₃C₂/ZIF-67 composite can be seen in Figure 20e. Also, in situ growth of ZIF-67-derived porous carbon mitigated the restacking of Ti₃C₂ sheets, as observed in the weaker XRD peak intensity of N–Ti₃C₂/C compared to that of Ti₃C₂ (Figure 20f).²⁰⁵ Not only that, MOFs can also act as precursors to derived LDHs.²⁰⁶ Hu’s work reported the growth of ZIF-67 on Ti₃C₂/nickel foam (Ti₃C₂/NF), followed by an etching–doping process, where Fe(SO₄)₂·7H₂O and urea were added. The etching–doping process then yielded a MOF-derived LDH/MXene composite denoted as CoFe MLDH/NF. The LDH was obtained due to the etching of the Co ions due to the addition of urea, in which Fe salt was gradually co-precipitated with Co ions and some anions, producing CoFe LDH.²⁰⁷

Figure 20.

SEM images of (a) N-PC, (b) Ti₃C₂ MXene, and (c) Ti₃C₂/N-PC. Reprinted with permission from ref (204). Copyright 2020 Elsevier. (d) Ti₃C₂/ZIF-67 and (e) N–Ti₃C₂/C, and (f) XRD plot of N–Ti₃C₂/C and Ti₃C₂. Reprinted with permission from ref (205). Copyright 2019 Elsevier.

6. Ti₃C₂T_x MXene-Based MOF Composites for Solar Fuel Production

In the past, there have been numerous studies conducted on Ti₃C₂T_x MXene-based composites and MOF-based composites for the photocatalytic production of solar fuels.²⁰⁸ For instance, MOFs are materials that have HOMO and LUMO separated by an energy band gap, indicating their capability to generate charge carriers upon light irradiation. Even though pristine MOFs are reported to show photocatalytic activity for solar fuel production,^209,210 their efficiency is very low due to the fast recombination of charge carriers. On the other hand, due to the metallic properties of Ti₃C₂T_x MXene, they can act as a cocatalyst to form a Schottky junction with other photoactive materials to effectively trap photogenerated electrons. It was observed that the Schottky junction between the CdZnS/Ti₃C₂ nanocomposite (Figure 21a) successfully ameliorated the photocatalytic reduction of CO₂ to form CH₄ and CO due to a highly efficient charge carrier separation.²¹¹ Ti₃C₂T_x MXene-based MOF composites present a number of advantageous synergies. First, MOFs which possess high surface area and porosity are able to host MXenes and prevent the restacking and agglomeration of MXene layers. The composites are also able to show improved stability due to the synergistic effects between the surface terminal groups of MXenes and functionalities of MOFs.²¹² Not only that, a built-in electric field between the MXene and MOF known as the Schottky junction is induced, where efficient separation, rapid mobility, and transportation of charge carriers are promoted.²¹³ Ti₃C₂T_x MXene-based MOF composites can function as attractive photocatalysts for CO₂ reduction and H₂ production via water splitting. However, there are only a handful of studies conducted on the composite for solar fuel production.

Figure 21.

(a) Scheme of Schottky junction between CdZnS@Ti₃C₂ nanocomposite for CO₂ reduction. Reprinted with permission from ref (211). Copyright 2021 Elsevier. (b) Schematic diagram of the preparation of MOF-derived Co–Co LDH/Ti₃C₂T_x nanosheets. (c) Photocatalytic stability over Co–Co LDH/Ti₃C₂T_x for CO₂ reduction reaction. (d) Mechanism of the CO₂ reduction over the nanocomposite with [Ru(bpy)₃]Cl₂ photosensitizer and TEOA sacrificial agent. Reprinted with permission from ref (206). Copyright 2020 Elsevier.

Chen and co-workers prepared a Ti₃C₂T_x composite with MOF-derived Co–Co LDH nanosheets, as shown in Figure 21b, for visible light CO₂ reduction in the presence of a [Ru(bpy)₃]Cl₂ photosensitizer. The bulk Ti₃AlC₂ MAX was subjected to etching to remove the Al layers, followed by ultrasonic exfoliation to obtain Ti₃C₂T_x nanosheets (TNS). Then in situ growth of ZIF-67 was conducted on the Ti₃C₂T_x nanosheets. Upon successful loading of ZIF-67, the nanocomposite was subjected to solvothermal treatment, in which Co–Co LDH/TNS nanosheets were obtained. Pristine Ti₃C₂T_x nanosheets did not show any photocatalytic CO₂ reduction activity. However, upon optimal loading of 15 mg of TNS on the MOF-derived Co–Co LDH, maximum CO generation rate of 1.25 × 10⁴ μmol h^–1 g^–1 was achieved. The nanocomposite also exhibited an excellent stability, maintaining good photocatalytic performance for up to five cycles, as illustrated in Figure 21c. Figure 21d depicts the photocatalytic mechanism of the Co–Co LDH/TNS nanocomposite in the presence of a photosensitizer. Upon visible light irradiation, the photosensitizer is excited but is subsequently quenched by the TEOA electron donor to form a [Ru(bpy)₃]Cl₂^– reduced state. The electrons are then transmitted to the nanocomposite, where the electrons are rapidly migrated to the Co active sites for the reduction of CO₂ to CO.²⁰⁶

Ti₃C₂T_x MXene-based composites are also widely used as catalysts for photocatalytic water splitting to produce H₂. Ti₃C₂T_x MXenes are two-dimensional sheets that boast a large surface area/volume ratio as well as strong hydrophilicity, which improves the interaction between the photocatalyst and water molecules.²¹⁴ Tian et al. modified Ti₃C₂T_x nanosheets by introducing a water-stable and porous Zr-based UiO-66-NH₂ for efficient photocatalytic hydrogen evolution reaction. It was observed that the pristine UiO-66-NH₂ MOF displayed limited H₂ production at only 25.6 μmol h^–1 g^–1. Upon introduction of Ti₃C₂ nanosheets, the photocatalytic activity of the composite (denoted as TU10) increased 8-fold, achieving H₂ formation of 204 μmol h^–1 g^–1. This is obviously due to the formation of a Schottky junction between Ti₃C₂T_x and UiO-66-NH₂. As shown in the photoluminescence plot in Figure 22a, the Ti₃C₂T_x-modified UiO-66-NH₂ sample showed an intensity lower than that of unmodified UiO-66-NH₂, elucidating a lower recombination rate over the composite. Figure 22b illustrates the energy level diagram of TU10, in which the electrons are first excited from the VB to CB of the UiO-66-NH₂, followed by migration of electrons from CB of the MOF to Ti₃C₂. The efficient migration and spatial separation of electrons was mainly attributed to the O-terminated Ti₃C₂ having a low Gibbs free energy and highly positive Fermi level, which makes it trap electrons from the MOF for H₂ production.²¹⁵

Figure 22.

(a) Photoluminescence plot of pristine UiO-66-NH₂ and TU10. (b) Mechanism of the Schottky junction between UiO-66-NH₂ and Ti₃C₂T_x MXenes. Reprinted with permission from ref (215). Copyright 2019 Elsevier. (c) Photocatalytic H₂ production over different catalysts, (d) Proposed mechanism of the hydrogen evolution reaction over Ti₃C₂/TiO₂/UiO-66-NH₂. Reprinted with permission from ref (216). Copyright 2019 Elsevier. (e) SEM image of Ti₃C₂@MIL-NH₂. (f) Stability test of photocatalytic H₂ formation over Ti₃C₂@MIL-NH₂. Reprinted with permission from ref (217). Copyright 2021 Elsevier.

Similarly, Ti₃C₂ MXenes were first annealed in a N₂ atmosphere to obtain TiO₂ layers, followed by coating UiO-66-NH₂ on the surface of Ti₃C₂T_x layers, obtaining a Ti₃C₂/TiO₂/UiO-66-NH₂ composite. From the activity test shown in Figure 22c, the annealing of Ti₃C₂T_x to produce TiO₂ increased the photocatalytic production by 1.5 times compared to that of its Ti₃C₂/UiO-66-NH₂ counterpart. This is because the formation of TiO₂ induced a negative shift in the Mott–Schottky plots, elucidating an enhanced electron/hole pair separation as well as stronger reducibility. Not only that, the intimate contact as well as the favorable Fermi level and band gaps of Ti₃C₂, UiO-66-NH₂, and TiO₂ constructed various pathways for accelerating the transfer of electron/hole pairs. As illustrated in Figure 22d, the first and second pathway is via the Schottky junction between Ti₃C₂ nanosheets with UiO-66-NH₂ and TiO₂, respectively. The third pathway is the type II heterojunction system between UiO-66-NH₂ and TiO₂, followed by the transfer of electrons from the CB of TiO₂ to Ti₃C₂ MXene.²¹⁶ Fascinatingly, Li and co-workers successfully coordinated the Ti atom in Ti₃C₂ MXenes with MIL-NH₂ via in situ growth, producing a Ti₃C₂@MIL-NH₂ composite. Compared to conventional physical mixing of Ti₃C₂ with MIL-NH₂, the in situ grown Ti₃C₂@MIL-NH₂ boasted a 5-fold H₂ production rate compared to that of the former. This is due to the intimate contact between the Ti₃C₂ and MIL-NH₂, as shown in the SEM image in Figure 22e. Also, the self-aggregation of the MIL-NH₂ is inhibited due to the even immobilization of MIL-NH₂ on the surface of the MXene. Figure 22f displays that Ti₃C₂@MIL-NH₂ also exhibited excellent stability, where the H₂ production rate showed insignificant decrease even after four cycles. The excellent photoactivity and stability are attributed to the rapid photoinduced electrons from the MOF to Ti₃C₂ via the Ti–N path. This in turn enriches the Ti₃C₂ with electrons, where H⁺ is effectively adsorbed and reduced to H₂.²¹⁷ Nevertheless, despite limited studies conducted, 2D Ti₃C₂ sheets coupled with MOF composites show great promise as a photocatalyst for efficient solar fuel production and should be studied intensively in future works. Table 3 presents the summary of Ti₃C₂ MXene-based MOF photocatalysts for the production of solar fuels.

Table 3. Summary of Ti₃C₂ MXene-Based MOF Photocatalyst for Production of Solar Fuels.

catalysts	operating conditions	results	ref
MOF-derived Co–Co LDH/Ti3C2Tx	•0.5 mg of catalyst	•CO = 6.248 μmol h–1	(206)
	•[Ru(bpy)3]Cl2·6H2O photosensitizer	•CO = 1.25 × 104 μmol h–1 g–1
	•MeCN/H2O/TEOA = 3/2/1 mL	•AQE = 0.92%
	•5 W LED lamp
	•light intensity = 32.3 mW cm–2
Ti3C2/UiO-66-NH2	•20 mg of catalyst	•formation of Schottky junction	(215)
	•0.1 M Na2S and 0.1 M Na2SO3 (50 mL)	•H2 = 204 μmol h–1 g–1
	•350 W Xe lamp
	•atmospheric temperature and pressure
	•light intensity = 10 mW cm–2
Ti3C2/TiO2/UiO-66-NH2	•20 mg of catalyst	•formation of Schottky junction and heterojunctions	(216)
	•0.1 M Na2S and 0.1 M Na2SO3 (50 mL)
	•300 W Xe lamp	•H2 = 1980 μmol h–1 g–1
	•T = 5 °C
	•light intensity = 10 mW cm–2
in situ grown Ti3C2@MIL-NH2	•70 mg of catalyst	•H2 = 4383.1 μmol h–1 g–1	(217)
	•20 mL of CH3OH and 0.3 mL of TEOA
	•300 W Xe arc lamp
	•light intensity = 552 mW cm–2
TiO2–Ti3C2–CoSx	•20 mg of catalyst	•CoSx derived from ZIF-67	(218)
	•40 mL of distilled water and 10 mL of methanol
	•300 W Xe arc lamp	•H2 = 0.95 mmol h–1 g–1
	•UV–visible irradiation
UiO-66-NH2(Zr/Ti)/ carboxyl-functionalized MXene (UZR/CFMX)	•10 mg of catalyst	•decorated carboxyl group on Ti3C2 MXene	(219)
	•10 vol % triethanolamine and 400 μL of H2PtCl6
	•300 W Xe lamp	•H2 = 2187 μmol g–1 h–1

7. Challenges and Comparative Analysis of Ti₃C₂T_x MXene

Here, the challenges and comparative analysis of Ti₃C₂ MXenes with other materials are discussed and summarized, as shown in Figure 23. Higher work function in Ti₃C₂T_x MXene and their role as an electron trapper gave them potential to maximize the solar fuel production. However, some challenges that require attention for employing Ti₃C₂T_x MXene as photoactivity enhancer include the control of their termination functional groups. The tunable termination groups, T_x, have a significant influence in the electronic and optical properties. Studies suggested that MXene materials could either exhibit semiconducting or metallic properties depending on the type of termination groups present.⁴⁷ Different termination properties might affect their photocatalytic efficiency due to differences in the work function value. The difference in the work function based on terminating groups was theoretically explicated due to changes in the total surface dipole moments and the transfer of charge between active sites of Ti₃C₂T_x MXene with the termination groups.⁴⁶ Theoretical studies divulged that the −O-terminated Ti₃C₂T_x MXene exhibits a work function of 5.75–6.25 eV, whereas −OH-terminated Ti₃C₂T_x MXene was found to possess a work function of 1.6–2.8 eV.²²⁰ In photocatalysis processes, such as that in hydrogen production, studies suggested that a large difference in the metal work function between the main catalyst and the metal cocatalyst could optimally stimulate the solar to hydrogen conversion.¹¹⁴ However, experimental evaluation revealed that a perfect single termination group is not possible to achieve, causing a great disparity with the theoretical analysis. Moreover, it is difficult to control the distribution of the termination groups in the MXene materials.¹¹ Therefore, the synthesis of mixed terminating Ti₃C₂T_x MXene might offer different results and catalytic efficiency. Other challenges can be observed in the thicker layer of Ti₃C₂T_x MXene, which is found to be difficult to employ as an electron mediator, resulting in weak contact formation between two photocatalysts.¹⁸³ In the context for commercializing Ti₃C₂T_x MXene in high-scale production, it was noticed that the yield is still low, and up to the current development, many preparation methods are still unable to develop high-yield Ti₃C₂T_x MXene which can par up to high scalability industrial production.

Figure 23.

Drawbacks of other cocatalysts and advantages of MXenes.

Nevertheless, in the regards of their beneficial attribution, most of the studies suggested that employing Ti₃C₂T_x MXene as a cocatalyst offers an indisputable and positive result in ameliorating the performances of the semiconductors.^{121,139,199,221} Higher metal electrical conductivity and unique properties prevailed them as one of the potentially influence materials to substitute non-economic and low efficiency cocatalysts. Moreover, a tailorable work function to fit specific application requirements is one of the distinct properties of Ti₃C₂T_x MXene compared to other metallic materials such as noble metals. Not to mention, their economical synthesis cost and easily accessible precursor is one of the positive attributes which leads the way for commercialization. On the other hand, Ti₃C₂T_x MXene material is new and is still under exploration where the their physical, catalytic, and chemical properties is still undergoing thorough research for more improvement which may facilitate more promising discoveries. Their unique morphology and possible conversion to different morphological dimension is one of the compelling characteristics of Ti₃C₂T_x MXene. This is due to the ability of their multilayer structure to delaminate into monolayer flakes, convert to quantum dots, and form a hierarchical structure, extending their performance in the field of photocatalysis.

Other materials that have similar properties of Ti₃C₂T_x MXene include noble metals such as Ag, Au, and Pt. The compelling characteristics of noble metal in the energy conversion field such as surface plasmon resonance effects and their potential to create the Schottky barrier enable them to be one of the leading metal cocatalysts. However, the main drawback in employing noble metals despite their positive influence on the photocatalytic activity is due to the exorbitant prices.²²² Comparatively, Ti₃C₂T_x MXene is regarded as the best substitution for noble metals with similar functionality and affordable material cost, which is commercially viable. Therefore, a high-scale production of semiconductor materials is more economic by employing Ti₃C₂T_x MXene compared to noble metals. Moreover, the work function of Ti₃C₂T_x MXene is highly comparable to those of noble metals ranging from 3.9 to 6.25 eV.²²³ In parallel, functional properties similar to those of noble metal including the creation of a potential energy barrier between the metal–semiconductor enhance the carrier dynamics and efficiently mediate the electrons transfer.

In terms of the morphological structure, Ti₃C₂T_x MXenes are known to be morphologically layered with a 2D structure, which is often referred to as an accordion-like shape. The accordion-like structure of Ti₃C₂T_x MXene offers a large surface area compared to that of the Ti₃AlC₂ MAX precursor, which is bulky, compact, and has dense layers.²²⁴ Loose layers with a bigger space interval of Ti₃C₂T_x MXene provide efficient site attachment for other semiconductors compared to MAX material, which exhibits more packed layers. Stronger interfacial contact between the semiconductor and Ti₃C₂T_x MXene supports the charge transmission and expedites the redox reaction. Moreover, Ti₃C₂T_x MXene offers a stable platform for a uniform distribution of the particulate semiconductors and inhibits the agglomeration. In addition, the well-defined layers of Ti₃C₂T_x MXene with a space interval support the redox reaction taking place at the inner and outer layers.¹³⁹ Even though numerous studies suggested that Ti₃AlC₂ MAX could promote the solar to fuel conversion, their catalytic efficiency is still not comparable to that of Ti₃C₂T_x MXene.²²⁵ As previously mentioned, the removal of an Al layer through chemical etching could terminate the surfaces of Ti with functional groups such as −OH, −F, and −O. These functional groups known as surface terminations provide excellent contact with water molecules and exhibit stronger hydrophilicity. Compared to Ti₃AlC₂ MAX, the presence of functional group such as −OH stimulate the hydrogen generation through the reduction into hydrogen through water capture.

In different material studies, similar layered structures often utilized as a cocatalyst known as layered double hydroxide exhibit the flexibility and diversity in their compositional matrix. Similar to MXene materials, LDH existed in a multilayered structure form, and their electronic properties are easily tuned according to the types of metal cations and anions present in their matrix structures.²⁷ LDH consisted of layers that stacked onto each other, where they exhibit negatively and positively charged layer. Generally, the positively charged layer of LDH consisted of metal cations while the negatively charged layers, known as interlamellar spaces, are composed of the intercalated anions. Therefore, the unique structure of LDHs attracts research attention to further explore their potential in photocatalytic fuel conversion field. However, the LDH structure, the metal coordination, and their nature need to be meticulously studied. In addition, their complicated structure and design criterion are some of the great challenges in employing LDH as a cocatalyst. Some criteria that require extra attention in designing LDH material include the nature of metal cations incorporated into the host layer of LDH, the compositional control of each metal cations, and their valence states and the anion intercalation. The mentioned design parameters could affect the fabrication of the LDH materials and their photocatalytic efficiency.²²⁶ For instance, mismatched metal cation pairing will lower the photocatalytic performances, and some might be inactive. Even though their energy gap can be tuned to meet specific application requirements, their complicated synthesis process and design criteria limit their role as compelling semiconductors. Therefore, the best alternative is to employ Ti₃C₂T_x MXene, which is economically affordable, is easily fabricated, and exhibits tunable electronic properties, unique structural properties, and excellent photocatalytic efficiency.

8. Conclusion

In conclusion, Ti₃C₂T_x MXenes are a class of materials that has seen widespread use in the field of photocatalysis. MXenes tend to exist in 0D quantum dots but usually adopt 2D layered morphologies that are able to provide a high surface area/volume ratio. However, pristine MXenes are unable to initiate any photocatalytic activity for solar fuel production as they do not have energy band gaps. The absence of a band gap means that there will not be any formation of electron/hole pairs despite light irradiation. Nevertheless, Ti₃C₂T_x is an excellent cocatalyst for photocatalytic fuel production due to its metallic properties, which allows it to function as an efficient electron sink and trap. Ti₃C₂T_x MXenes are usually hybridized with other materials containing energy band gaps such as semiconductors. This is due to the photogenerated electrons from the CB of the semiconductor to the Ti₃C₂T_x MXene, which has a more positive Fermi level. The efficient transfer of electrons will then ameliorate and prolong the lifetime of charge carriers, enhancing the photocatalytic formation of solar fuels.

The employment of Ti₃C₂T_x MXene in the solar fuel conversion field offers a sustainable approach for producing a clean and renewable energy source. Excellent metallic electrical conductivity of Ti₃C₂T_x MXene endows them with the ability to construct an electrostatic potential barrier which aids in the improvement of carrier dynamics and elevates their potential as highly sought-after cocatalysts. Mainly, HF etching is the pioneering technique for synthesizing Ti₃C₂T_x MXene with a −F termination group. However, their prominent role as a photocatalytic driver led to the development of various synthesis techniques, which facilitate tailoring their surface terminating groups. It is worth mentioning the blossoming of the synthesis techniques favoring the morphological development of Ti₃C₂T_x MXene into different structural growth. Tailorable functional groups with modifiable structure boosted their photoredox activity and improved the overall photocatalytic efficiency.

Metal–organic frameworks are another class of materials that possess a wide range of unique properties that make them promising candidates as photocatalysts. First, they have an extremely high surface area, usually in the range of a few thousand square meters per gram. Pores with large volume and uniform size are riddled all over the surface of the MOF, allowing them to effectively adsorb small gas molecules as well as large dye molecules. Interestingly, despite the usual 3D polyhedrons of MOFs, they can also naturally exist in 1D rods or 2D sheets, as well. These multidimensional MOFs can be obtained via different synthesis steps, namely, the solvothermal synthesis, the co-precipitation method, the slow evaporation method, and the microwave-assisted approach. Under solar energy irradiation, MOFs are able to generate photoinduced charge carriers. This is because they have a narrow energy band gap, where electrons can be excited from the HOMO to LUMO when photons with sufficient energy strike the surface of the MOF. Finally, MOFs can function as a sacrificial template to form other derivatives. Metal oxides are derived when MOFs are subjected to thermal treatment, whereas LDHs can be obtained when the MOFs undergo the hydrolyzed etching step.

In an effort to further improve the photocatalytic activity of both Ti₃C₂T_x MXenes and MOFs, various design principles and considerations were taken into account. In this context, the design considerations were discussed in three main criteria, namely, the morphological engineering of Ti₃C₂T_x and MOFs, the active sites over Ti₃C₂T_x-based MOF nanocomposites, and the interfacial/heterojunction engineering. In recent years, huge strides have been made in modifying the conventional 2D accordion-like multilayer of Ti₃C₂T_x, such as the formation of 0D quantum dots, 2D delaminated flakes, and 3D multidimensional structures. These can be achieved via various approaches such as delamination with or without intercalating agents and combining with various domains of different dimensions. Similarly, the morphology and dimensions of MOFs can be tuned between 1D rods, 2D sheets, or 3D polyhedrons by adjusting the synthesis parameters such as reaction time, temperature, and solvent ratio. Also, the use of surfactants such as polyvinylpyrrolidone and cetyltrimethylammonium bromide can effectively modulate the structure of MOFs. The active sites of MOFs were also discussed, where active sites can be found naturally on the MOF or by further introduction of the MOF by anchoring them on the organic ligands or cavity of the MOF.

The metallic nature of Ti₃C₂T_x MXenes make them highly promising cocatalysts due to their ability to trap electrons via the formation of a Schottky barrier with other photoresponsive materials. Interestingly, a Ti₃C₂T_x MXene–TiO₂ hybrid could be generated as a result of oxidation, opening the door to hybrid Schottky–Z-scheme/Schottky–type II heterojunction systems. Also, there are reports that Ti₃C₂T_x can also serve as an electron mediator for Z-scheme heterojunction systems. Similar to semiconductors, MOFs are able to generate electron/hole pairs upon light irradiation, making them serviceable photocatalysts. However, their efficiency can be further improved via a series of strategies, such as formation of heterojunctions with other photoresponsive materials, surface sensitization with materials with metallic properties, functionalization of MOFs, doping, and use of photosensitizers. By deploying these strategies, the photocatalytic activity of MOFs can be enhanced as the light harvesting especially in the visible spectrum is ameliorated, and a narrower E_bg is achieved, and spatial charge carrier separation is realized. By preparing Ti₃C₂T_x MXene-based MOF photocatalysts, the photocatalytic activity can be effectively enhanced due to various synergistic effects such as boosted stability and formation of a built-in electric field known as the Schottky junction. Despite promising findings from Ti₃C₂-based MOF composites for solar fuel production, there is still very limited studies conducted. Hence, it is recommended to further explore Ti₃C₂ with various MOFs for photocatalytic CO₂ reduction and water splitting reactions.

9. Future Perspective

In short, Ti₃C₂T_x and MOFs have made huge progress over the years. For instance, various new synthesis approaches were discovered, in turn producing a wide range of structures for both Ti₃C₂T_x MXene and MOFs. Similarly, both materials have seen an increasing role as photocatalysts in the production of renewable solar fuels. Hence, Ti₃C₂T_x MXene-based MOF nanotextures are highly promising photocatalysts for the production of green and renewable solar fuels. This is due to the synergistic effect between the two materials, in which MOFs are able to generate electron/hole pairs, whereas MXenes can effectively trap the electrons. This in turn will prolong the lifetime of charge carriers and ameliorate the production of solar-driven fuels. However, up to now, there is only a handful of research done on Ti₃C₂ MXene-based MOF composite on the applications of CO₂ reduction and solar-driven H₂ production. Thus, the future prospects are as follows:

• 1.Different type of MXenes as well as morphologies should be utilized for the photocatalytic production of solar fuels. For example, different MXenes such as Nb₂C, V₂C, and Mo₂C should be explored as cocatalysts for renewable fuel production. Other than 2D accordion layered structures of MXenes, facile synthesis approaches for 0D MXene quantum dots and exfoliated monolayer MXenes should also be researched intensively.
• 2.The exact active sites of Ti₃C₂T_x MXene are yet to be confirmed. It is suggested for more studies to be conducted to identify, quantify, and gain a deeper understanding on the active sites over MXenes. Also, the study on the active sites can provide more insights into ways to further boost the role of Ti₃C₂T_x MXene as a cocatalyst for solar fuel production.
• 3.Over the years, MXenes are mainly prepared using HF etching, which is detrimental to the environment due to the use of harsh chemicals. Also, formation of QDs of monolayered MXenes utilizes various intercalating agents like DMSO TMAOH, which may pose environmental threats. Therefore, more research must be conducted on the green synthesis of MXenes to reduce the negative impact on the environment.
• 4.A solvothermal synthesis method is most commonly used to prepare a wide range of MOFs. However, they are severely limited by their long reaction time, inhomogeneous heating, and huge heat loss. Thus, it is recommended that more studies be conducted on other synthesis methods such as the microwave-assisted method to not only speed up the preparation period but also improve the efficiency and homogeneity of heating.
• 5.More combinations of MOFs must be explored by either experimenting with different metal nodes, metal clusters, or linkers to obtain MOFs with enhanced photocatalytic activity for solar fuel production. Also, energy band gap engineering should also be conducted to further improve the light harvesting and utilization of MOFs especially in the visible light spectrum.
• 6.Lastly, more research is needed on Ti₃C₂T_x MXene-based MOF nanotextures to produce renewable solar fuels. Despite its promising properties and characteristics, there are still limited studies on MXene/MOF for CO₂ reduction and production of H₂ solar fuels. Different types of MOFs can be paired together with Ti₃C₂T_x MXene to further unveil and gain a better understanding on the interaction and mechanism of the overall photocatalytic process.

The authors declare no competing financial interest.