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Introduction to Titanium Carbide (Ti3C2) MXenes for Energy and Environmental Applications

Muhammad Tahir

United Arab Emirates University, Chemical and Petroleum Engineering Department, Sheik Khalifa Bin Zayed Street, Asharej, P.O. Box 15551, Al Ain, United Arab Emirates

1.1 Introduction

Transition metal carbide- and nitride-based materials were explored in the twentieth century as having high thermal resistance, chemically stable, hard, water resistance, and conductive characteristics. Since the 1970s, several designs of nanotextures have been proposed; however, there was a small increase in the surface area and the performance of these metal carbides and nitrides. A breakthrough in this field happened in 2011, with the discovery of 2D transition metal carbide and nitride nanotexture compounds, named titanium carbide (Ti3C2Tx) MXenes.

The general formula of MXene is Mn+1XnTX, which is derived from MAX materials. M stands for the early transitional metal elements such as Sc, Ti, Hf, Mo, Ta, V, Zr, Nb, and Cr. The "X" constitutes either carbon or nitrogen elements, and the acronym "A" represents the Al or Si layer and n ranges from 1 to 3. The chemical exfoliation of the A element of MAX results in the termination of the M surface with abundant functional groups, Tx, such as -F, -Cl, -OH, and -O [1, 2]. The first MXene material (titanium carbide MXene [Ti3C2Tx]) was synthesized in 2011 by Naguib et al. via chemical etching with hydrofluoric acid (HF) [3]. In general, 2D sheets have a thickness in the range of 1 nm; however, it can be altered by varying the number n in the MXene formula such as M2XT, M3X2Tx, and M4X3Tx. Most of the MXenes (25 out of 29) are made of MAX materials by selective etching of the Al layer. A recent study showed that instead of Al and Si, transition metals of groups 8-12 of the periodic table (Fe, Cu, Zn, Cd, Ir, and Au) can also become the A layer of MAX phases.

Ti3C2 MXene, due to its 2D layered structure, has drawn attention worldwide in the area of energy storage and conversion applications [4-6]. This is due to its promising properties such as its higher specific area and terminal functional groups (OH, O, and -F). In photocatalytic applications, the Ti3C2 functional group aids in the formation of Schottky junctions for the trapping of electrons through a strong chemical connection at the semiconductor photocatalyst's interface and prevent the recombination of charge carriers produced by light. Due to Ti3C2 MXenes' better electron conductivity and increased reactivity, the exposed metallic active sites have an advantage. This causes efficient movement of charge carriers and separation of photogenerated electrons from holes [7-9]. Additionally, because of its diverse morphologies in terms of forms and dimensions, its application and photocatalyst activity capabilities have considerably changed and improved, which has expanded its use in photocatalytic energy conversions [10, 11]. The detailed information about the fundamental properties and characteristics of Ti3C2 MXenes has been discussed in Chapter 2.

The synthesis of Ti3C2 Mxenes is possible by using an etching agent under different operating conditions. Different etching agents are used for the removal of the Al layer in order to produce Ti3C2 in a variety of ways. The most often used techniques include electrochemical etching, molten salt etching, halogen etching, hydrofluoric acid etching, and acid F-salt etching. The detailed information related to synthesis and characterization of MXenes has been discussed in Chapter 3. The kind and concentration of the etching agent, reaction duration, intercalation agents, and synthesis conditions are only a few of the variables that have a significant impact on Ti3C2 etching [12]. These factors have the ability to change the structure, termination groups, defect morphology, conducting and semiconducting characteristics, and other features of Ti3C2 MXene. The features of the formed Ti3C2 can affect its quality, environmental stability, and other qualities [13, 14]. The conductive metal cores in the layered structure endow MXene with excellent metallic conductivities, and the modification of properties can lead to the formation of Ti3C2 MXene with the required properties for various applications in the field of energy conversion applications.

The single-Ti3C2 MXenes have less photocatalytic activity due to conductive characteristics; thus, they are commonly used as a cocatalysts with semiconductor materials. Ti3C2 MXenes can be combined with a variety of materials for use in energy conversion applications because of the several benefits such as (i) numerous functional beneficial characteristics for the construction of an intimate contact interface with other semiconductors; and (ii) the bandgap alignment of Ti3C2 modulated for tuning the surface chemistry [15-17]. A detailed discussion about the synthesis and characterization of MXenes based heterojunction formation has been given in Chapter 4. The layered Ti3C2 MXene structure, which resembles an accordion, may be transformed into different dimensions, including a three-dimensional structure, very thin two-dimensional nanosheets, one-dimensional (1D) nanorods and nanowires, and zero-dimensional (0D) quantum dots [18-21]. A range of 2D, 0D, and 1D photocatalyst materials may also be loaded onto Ti3C2 MXene to provide efficient, renewable solar fuel [22, 23]. For the production of Ti3C2 MXene-based composite photocatalysts, a number of techniques have been published in the literature. The mechanical mixing, self-assembly, in situ semiconductor decorating over the MXene surface, and the in situ oxidation approach are the most often utilized techniques [24-27].

Titanium dioxide material is considered the most extensively investigated photocatalysis material owing to its photocatalytic activity, good chemical stability, abundancy, and low cost. However, because of its wide bandgap energy (3.2 eV), the TiO2 photocatalyst is UV-active only and functions poorly under visible light, thus lowering its photoactivity [28, 29]. Recently, titanium carbides (MXenes) have gained significant concern in improving the photocatalytic activity of semiconductors, in particular TiO2, due to their exceptional chemical and physical properties. In Chapter 5, the properties and characteristics of the benchmark TiO2 photocatalysts and the highly conductive 2D MXene (Ti3C2) materials are discussed. The role of MXene in enhancing the photocatalytic performance of TiO2 photocatalysts is summarized in three main sections; the formation of Schottky heterojunctions, enhancing light harvesting; and enhancing reactant adsorption. Finally, Chapter 5 discloses the recent developments of MXene/TiO2 nanocomposites for environmental remediation and energy production applications such as photocatalytic CO2 reduction, photocatalytic H2 production, and pollutant photodegradation.

Due to the appropriate construction, non-toxicity, and high reduction capacity, graphitic carbon nitride (g-C3N4) is regarded as an ideal element for hydrogen production in the visible light spectrum. However, they have a higher recombination of charged carriers, which significantly limits g-C3N4 photocatalytic efficacy. The performance of g-C3N4 with fast charge carrier separation can be effectively achieved by constructing composites with MXenes. In this perspective, 2D/2D heterojunction of g-C3N4 with Ti3C2 can be achieved. Due to its tunable terminal groups, metallic conductivity, layered structure, and simple morphological arrangement, Ti3C2 MXene as a cocatalyst could boost photocatalytic performance [16, 17]. The detailed information about the Ti3C2 coupled g-C3N4 composite has been discussed in Chapter 6.

In recent years, metal-organic frameworks (s) have been explored for photocatalytic applications because of their high specific surface area, tunable bandgap, changeable crystalline structures, adjustable chemistry, and functionality. Furthermore, MOFs have outstanding physical qualities which can be employed for adsorption processes in environmental applications [30, 31]. However, the crystalline architecture of MOFs inherently introduces structural flaws such as electron-hole recombination centers and low electrical conductivity, limiting their efficiency in photocatalytic water splitting [32, 33]. Ti3C2 MXenes are permissive materials that can be used with MOFs for energy and environmental applications. Notably, conductive TiC MXenes may improve charge carrier transport efficiency and allow for fine-tuning of catalytic performance in MXene/MOF multicomponent catalyst systems [34]. Chapter 7 discloses detailed information about the Ti3C2-based MOF composites for energy and environmental application.

Currently, several two-dimensional (2D) materials have been employed to improve the catalytic performance by virtue of their non-toxic behavior, high durability, and huge surface area. Layered double hydroxides (LDHs) have been widely utilized in energy storage and conversion application due to their unique properties. For example, the Fe-, Co-,...