1
Two-Dimensional MXenes: Fundamentals, Characteristics, Synthesis Methods, Processing, Compositions, Structure, and Applications

Sudipta Goswami and Chandan Kumar Ghosh*

School of Materials Science and Nanotechnology, Jadavpur University, Jadavpur, Kolkata, India

Abstract

During the last decade, among various two-dimensional materials, the carbides, nitrides or carbonitrides of transition metal ions (MXenes), defined by Mn+1XnTx (n = 1 – 4), where M, X, and Tx stand for, respectively, the transition metal ions (e.g. Ti, V, Nb, Mo, etc.), carbon and/or nitrogen, and the surface terminated groups of different population, have drawn a lot of interest of the scientists as they exhibit unique features such as large conductivity, possibility of processing in the form of solution, large aspect ratio of the structure, and tunability of the properties. In this chapter, fundamental properties and classification of MXenes are discussed in detail along with different synthesis strategies and applications. Emphasis is given on discussing MXene hydrogel as they are widely being used in flexible electronics. Since surface functionalization plays a prominent role in this class of materials, controlling surface functionalization is discussed thoroughly. Correlation between applications of MXene and their structure is also discussed here.

Keywords: MXene, MAX phase, 2D materials, surface termination, exfoliation, etching, energy storage, biomedical application

1.1 Introduction

In this chapter we discuss certain salient features of a class of two-dimensional compounds which are known by the acronym MXenes. They are composed of two-dimensional transition metal ion layers interleaved with carbon and nitrogen layers (thereby forming carbide or nitrides). The hydroxyl or oxy-halide groups are attached onto the top layers. During the last fifteen years or so, intense research is being pursued on this class of low-dimensional systems for their tremendous use in storage and harvesting of energy as well as in biomedical and sensor sectors. Among a wide range of oxide/non-oxide compounds including the lower-dimensional van der Waals solids, the MXenes (classified based on the type of transition metal ion and the number of layers) have carved a niche for themselves for their many unique features. We shall highlight those features while discussing the crystallographic and electronic structures, their synthesis and properties, and, finally, use in a score of applications. We first describe the fundamentals of these compounds along with their classification. Next, we discuss the electronic, physical, and chemical properties. The different techniques being used for synthesizing the MXenes are presented then and, different application potential of MXenes in various sectors have been discussed.

1.2 Fundamentals

The carbides or nitrides of transition metal ions in two-dimensional (2D) form are described by the general chemical formula Mn+1XnTx where M, X, and T, respectively, designate the transition metal ion, carbon or nitrogen, and the hydroxyl (OH) or oxy-halide (OCl, OF, OBretc) ions which terminate the surface; ‘n’ defines the number of layers [1]. There are, primarily, three types of MXenes – M2XTx, M3X2Tx, and M4X3Tx. (Figure 1.1) [2] Interest has been generated due to their application potential in areas such as harvesting and/or storage of energy (electrochemical supercapacitors, Li-ion, Na-ion batteries), catalysis (hydrogen or oxygen evolution reaction, CO2 reduction), electronics/spintronics (memories, sensors), environment (membranes for clean air or water), structural, biomedical (biosensors, cancer treatment), sensors (gas, humidity, strain) etc. Unique features such as large electrical and thermal conductivity (multilayered MXenes exhibit higher conductivity than multilayered graphene), tunability of the band gap via surface terminated ions (from metallic to semiconducting), mechanical strength, etc., have made this class of compounds quite attractive for a variety of applications [3]. Here, we shall first examine their crystallographic and electronic structures for understanding the origin of the uniqueness of the properties of MXenes.

Figure 1.1 The crystallographic structure of single-, double-, and triple-layered MXene compounds Mn+1XnTx, (n = 1 to 4) where MX, and T represent, respectively, early transition metals, C or N, and F, O, or OH ions. More than one transition metal atoms may occupy the M sites forming solid solutions or ordered structures. Reproduced with permission from Ref. [34]. Copyright 2021, Wiley-VCH.

1.2.1 Crystallographic Structure

MXenes are derivatives of MAX phases where the A ions belong to the III–IV groups of the periodic table (i.e., the Al, Ga, In, Tl, Si, Ge, Sn, Pb, P, Bi, As, B, Te, S, etc.). The crystallographic structure of the MAX phase is shown in Figure 1.2 [4]. They assume hexagonal P63/mmm (No. 194) structure [5]. By selectively leaching out the ‘A’ ions, it is possible to synthesize the MXenes. Therefore, the two-dimensional layers of MXenes to assume hexagonal structure (space group P63/mmm). Apart from the pure systems, alloy MXenes can also be synthesized where two different M ions – M′ and M″ – coexist and form a solid solution. In such alloy systems, ordering of the M′ and M″ ions have been noticed. The M′ and M″ ions could be ordered along the outer direction or within the planar structure [6]. In the case of ordering along the outer direction, the M′(M″) ions could be on the outermost layers while the M″(M′) ions occupy the inner layers. In the case of in-plane ordering, the M′ and M″ ions are ordered within each layer– inner and outer. However, the in-plane ordering of M′ and M″ ions can be different from the hexagonal structure and close to the Kagome structure [7] when the size of M′ and M″ ions differ and the size of M′ ion turns out to be smaller than that of M″ ions. The space group in such cases becomes C2/c (monoclinic). The derivation of MXenes from the MAX phases requires mechanical or chemical exfoliation. The HF treatment, for example, yields formation of AF2 and H2 as the longer M-A bonds are relatively weak than the shortest M-X bonds. Theoretical simulation [8, 9] of the bond strengths (Figure 1.3) and the exfoliation processes offers insights behind the exfoliation processes. The MXenes also contain the F, OH, O surface ions. During the exfoliation process, vacancies are generated at the X-sites below the M layers. As a result, two sites – one with an X ion and another without (i.e., with a vacancy) – are generated. The F, OH, O groups are adsorbed at the vacancy sites and, thereby, form the stable structure with octahedral field of transition metal M ions.

Figure 1.2 The crystal structures of the (a) MAX [M3AX2] phase and (b) out-of- and (c) in-plane ordered M′2M″AX2 phases. Reproduced with permission from Ref. [83]. Copyright 2019, Elsevier.

Figure 1.3 Variation of the force constant with bond length in various MAX phases. Reproduced with permission from Ref. [9]. Copyright 2018, The Royal Society of Chemistry.

1.2.2 Electronic Structure

Most of the MXenes are either metals or semimetals or semiconductors where spin-orbit coupling (SOC) does not have any significant effect on the electronic bands. In fact, when the functionalization of the surface is absent, the pristine MXenes are metallic. In these cases, the Fermi energy lies on the d-bands of the M ions. Surface functionalization by F, OH, O ions leads to the formation of new bands with hybridization of M d bands. The Fermi energy then shifts to the gap between M d bands and X p bands and the compound becomes semiconducting [10]. Such an observation has been made in the cases of Sc2CT2 (T = O, OH, F) and M2CO2 (M = Ti, Zr, Hf). In the cases of (M′, M″)XTx systems, Fermi energy shifts to the gap generated from d band splitting due to octahedral crystal field and, as a result, the compounds exhibit semiconducting behavior with finite energy gap. In general, single layer MXenes could exhibit semiconducting property whereas the double- or triple-layer ones are primarily metallic. However, in some cases, the SOC plays a significant role. When SOC is not present, the valence and the conduction bands (comprising of the d levels of the M ions) touch at the Γ point and give rise to semimetallic behavior. SOC splits the bands and opens a gap at the Γ point (Figure 1.4) [11]. The compounds M2CO2 (M = W, Mo) and M2′M″CO2 (M′ = Mo, W; M″ = Ti, Zr, Hf) exhibit such two-dimensional topological semimetallic or insulator behavior with topologically protected states with conducting edges which remain robust against nonmagnetic impurities and disorder [11].

Figure 1.4 The electronic band structure of Mo2HfC2O2 with and without spin-orbit coupling (SOC). Reproduced with permission from Ref. [11]. Copyright 2016, The American Chemical Society.

1.2.3 Magnetic Structure

The MXenes exhibit finite magnetism and magnetic order depending on the electron states, band splitting, spin-orbit coupling, etc. For example, most of the compounds containing Cr and/or Mn exhibit magnetic order [12]. If the...