Roles of Metal Ions in MXene Synthesis, Processing and Applications: A Perspective

Abstract

With a decade of effort, significant progress has been achieved in the synthesis, processing, and applications of MXenes. Metal ions play many crucial roles, such as in MXene delamination, structure regulation, surface modification, MXene composite construction, and even some unique applications. The different roles of metal ions are attributed to their many interactions with MXenes and the unique nature of MXenes, including their layered structure, surface chemistry, and the existence of multi-valent transition metals. Interactions with metal ions are crucial for the energy storage of MXene electrodes, especially in metal ion batteries and supercapacitors with neutral electrolytes. This review aims to provide a good understanding of the interactions between metal ions and MXenes, including the classification and fundamental chemistry of their interactions, in order to achieve their more effective utilization and rational design. It also provides new perspectives on MXene evolution and exfoliation, which may suggest optimized synthesis strategies. In this respect, the different effects of metal ions on MXene synthesis and processing are clarified, and the corresponding mechanisms are elaborated. Research progress on the roles metal ions have in MXene applications is also introduced.

Keywords: MXenes; application; metal ion; processing; synthesis.

Figure 1

Interactions between MXene and metal ions in MXene synthesis, processing, and applications.

Figure 2

The effect of metal ion intercalation in MXene synthesis. a) Schematic of metal ion intercalation in MXene synthesis. Route 1: the molar ratio of LiF and Ti₃AlC₂ is 5:1; Route 2: the molar ratio of LiF and Ti₃AlC₂ is 7.5:1. The inset SEM image is multilayer Ti₃C₂T _x prepared by HF etching. The inset TEM images are Ti₃C₂T _x flakes produced by Route 1 and 2. Left‐hand side: Reproduced with permission.^{[ 7 ]} Copyright 2011, Wiley‐VCH. Right‐hand side: Reproduced with permission.^{[ 40 ]} Copyright 2016, Wiley‐VCH. b) XRD patterns of M@Ti₃C₂‐I and M@Ti₃C₂‐II. Reproduced with permission.^{[ 39 ]} Copyright 2020, Elsevier. c) XRD patterns of Ti₃C₂T _x MXene prepared by Route1 and Route 2. d) AFM height profiles of Ti₃C₂T _x flakes produced by Route 1 and 2. Those produced by Route 2 have the same height of ≈2.7 nm and are identified as monolayers. Reproduced with permission.^{[ 40 ]} Copyright 2016, Wiley‐VCH.

Figure 3

The effect of metal ion intercalation in MXene processing. a) EDS element mapping of Al³⁺‐MXene, Mg²⁺‐MXene, Ca²⁺‐MXene, and Ho³⁺‐MXene. The MXene here is Ti₃C₂T _x . b) XRD patterns of neat MXene, Ca²⁺‐MXene, Mg²⁺‐MXene, and Al³⁺‐MXene under 75% relative humidity at room temperature. c) XRD patterns of Be²⁺‐MXene, In³⁺‐MXene, Ho³⁺‐MXene, and neat MXene. Metal ion intercalation enlarges the interlayer spacing. Reproduced with permission.^{[ 42 ]} Copyright 2020, Wiley‐VCH.

Figure 4

The mechanism of metal ion intercalation. a) Illustration of ion exchange in a clay‐like MXene. The ion exchange is generally thought to occur between the solution and ions located between the MXene layers, accompanied by H⁺ release. Reproduced with permission.^{[ 58 ]} Copyright 2017, American Chemical Society. b) Correlation of the interlayer spacing of M ^{x +}‐MXene (including Ca²⁺, Mg²⁺, Ho³⁺, In³⁺, Be²⁺, and Al³⁺) electrodes and the hydrated radius of intercalated metal ions (obtained from Marcus's group). Reproduced with permission.^{[ 42 ]} Copyright 2020, Wiley‐VCH. c) XRD patterns of ion‐intercalated Ti₃C₂T _x MXene with humidity control. Reproduced with permission.^{[ 38 ]} Copyright 2016, American Chemical Society. d) Li⁺, Cs⁺ and Mg²⁺ representative atomic arrangements around different cations confined in two Ti₃C₂T _x layers. Reproduced with permission.^{[ 60 ]} Copyright 2020, Royal Society of Chemistry. e) ²³Na NMR spectrum to investigate the mechanism of Na⁺ intercalation into Ti₃C₂T _x , during electrochemical cycling. Reproduced with permission.^{[ 67 ]} Copyright 2016, American Chemical Society.

Figure 5

Metal ions as crosslinking agents to initiate the gelation of Ti₃C₂T _x MXene. a) Schematic of Ti₃C₂T _x MXene hydrogel formation initiated by Fe²⁺ ion interaction. b) Photos of a Ti₃C₂T _x MXene hydrogel with crosslinking agents of Mg²⁺, Co²⁺, Ni²⁺, Al³⁺ and K⁺. Reproduced with permission.^{[ 28 ]} Copyright 2019, Wiley‐VCH. c) Schematic of Mg²⁺‐MXene aerogels produced from the CT‐MXene platform and Mg²⁺‐induced gelation. d) N₂ adsorption‐desorption isotherms of the original MXene film, the Mg²⁺‐MXene aerogel, and the MXene aerogel after acid washing. The inset Figure in (d) shows the corresponding pore size distributions. e) Photos of a MXene aerogel (without Mg²⁺ intercalation) and an Mg²⁺‐MXene aerogel after ultrasonication for 10 min. f) Electrical conductivities of pristine MXene and Mg²⁺‐MXene aerogels. Reproduced with permission.^{[ 68 ]} Copyright 2021, Wiley‐VCH.

Figure 6

The mechanism of ion crosslinking. a,b) O 1s and F 1s XPS spectra of pristine Ti₃C₂T _x MXene and a MXene monolith. Reproduced with permission.^{[ 28 ]} Copyright 2019, Wiley‐VCH. c) The normalized XANES at the Ti K‐edge of Zn²⁺ Ti₃C₂T _x MXene foam. d) Fourier transform extended X‐ray absorption fine structure spectra of Zn²⁺ MXene foam. The gelation process did not cause obvious degradation of the MXene sheets, and the Zn²⁺ ionic state of the Zn sites remained. Reproduced with permission.^{[ 74 ]} Copyright 2020, American Chemical Society.

Figure 7

The etching of metal ions in MXene during Ti₃C₂T _x MXene synthesis and processing. a) Schematic of Ti vacancies generated in the synthesis of Ti₃C₂T _x MXene in molten ZnCl₂ salts. At the same time, single zinc atoms are produced and immobilized on the MXene layers (Zn‐MXene). Reproduced with permission.^{[ 81 ]} Copyright 2020, American Chemical Society. b) SEM image and HAADF‐STEM image of a single atom Cu catalyst synthesized by etching Ti₃AlNC in molten CuCl₂. Reproduced with permission.^{[ 82 ]} Copyright 2021, Elsevier. c) Schematic of the Cu‐SA/Ti₃C₂T _x synthesis procedure, by adding a certain amount of CuCl₂·2H₂O to a Ti₃C₂T _x suspension. (d) HAADF‐STEM images of Cu‐SA/Ti₃C₂T _x and Cu‐NC/Ti₃C₂T _x . Here, 0.67 mL CuCl₂·2H₂O (1 mg mL^–1) for Cu‐SA/Ti₃C₂T _x and 2.01 mL for Cu‐NC/Ti₃C₂T _x were used. Reproduced with permission.^{[ 84 ]} Copyright 2021, Springer Nature. e) Diagram of the porous Ti₃C₂T _x MXene structure produced by the chemical etching of Cu²⁺ ions. f) High‐resolution TEM images of a Ti₃C₂T _x flake after soaking in a 0.2 m CuSO₄ solution at RT. The left image is before acid washing and shows TiO₂ nanoparticles on the surface, and the right one is after acid washing showing pores. Reproduced with permission.^{[ 85 ]} Copyright 2016, Wiley‐VCH.

Figure 8

The mechanism of metal ion etching. a) A diagram of the anchoring of single metal atoms on Ti vacancies during molten salt etching. Reproduced with permission.^{[ 82 ]} Copyright 2021, Elsevier. b,c) High‐resolution Cr 2p and Ti 2p XPS spectra of Ti₃C₂T _x ‐based films before and after HCrO₄ ⁻ removal. The valence state of Cr in HCrO₄ ⁻ is reduced from Cr(VI) to Cr(III), accompanied by a valence state increase of Ti in Ti₃C₂T _x . d) Differences in charge density of HCrO₄ ⁻ on O‐ and OH‐terminated Ti₃C₂. The turquoise and yellow regions indicate depletion and accumulation of electrons, respectively. Reproduced with permission.^{[ 89 ]} Copyright 2019, Springer Nature. e) Schematic illustration of U(VI) removal at different pH values. At low pH, the reduced U(IV) species is identified as mononuclear with bidentate binding to the MXene substrate. At near‐neutral or high pH, nanoparticles of the UO_{2+ x} phase adsorb on the substrate with some Ti₂CT _x being transformed to amorphous TiO₂. Reproduced with permission.^{[ 88 ]} Copyright 2018, American Chemical Society.

Figure 9

Applications in supercapacitors. a) Bader charge on Ti oxidation state changes with different H coverages. Insets show the side and top views of partially protonated Ti₃C₂O₂. Reproduced with permission.^{[ 107 ]} Copyright 2020, Royal Society of Chemistry. b) Comparison of the rate performance for LiF/HCl‐produced MXene and HF‐produced MXene. Reproduced with permission.^{[ 37 ]} Copyright 2014, Springer Nature. c) The volumetric‐specific capacitance of pure Ti₃C₂T _x MXene, Be²⁺‐MXene, Al³⁺‐MXene, In³⁺‐MXene, Ho³⁺‐MXene electrodes in 1.0 Ｍ Li₂SO₄ electrolyte at scan rates ranging from 20 to 200 mV s⁻¹. Reproduced with permission.^{[ 42 ]} Copyright 2020, Wiley‐VCH. d) Cyclic voltammetry profiles of MXene powder and hydrogel at different scan rates. Reproduced with permission.^{[ 28 ]} Copyright 2019, Wiley‐VCH. e) Schematic of MnO₂/MXene composite synthesis using the interaction between Mn ions and MXene. Reproduced with permission.^{[ 108 ]} Copyright 2019, American Chemical Society.

Figure 10

Applications in metal‐ion batteries, with interlayer structure design, 3D structure design, and MXene composite strategy. a) Bonding charge density for Li, Na, K, and Ca ions in the Ti₃C₂T _x system. These bonding charge distributions clearly show charge transfer from the atoms to the Ti₃C₂ monolayer. Reproduced with permission.^{[ 130 ]} Copyright 2014, American Chemical Society. b) XRD patterns and TEM images of Ti₃C₂T _x upon sodiation and de‐sodiation. The scale bars in the TEM images indicates 5 nm. Reproduced with permission.^{[ 67 ]} Copyright 2016, American Chemical Society. c) Schematic of the fabrication process of (polyvinylpyrrolidone, PVP)‐Sn(IV)@Ti₃C₂. With the PVP surfactant, the adsorbed Sn(IV) can be uniformly distributed in the alk‐Ti₃C₂ matrix. Reproduced with permission.^{[ 131 ]} Copyright 2016, American Chemical Society. d) Illustration of the microwave‐assisted approach for the N (V)‐modified Ti₃C₂T _x . V_Ti represents a Ti vacancy. Reproduced with permission.^{[ 132 ]} Copyright 2020, American Chemical Society. e) Cycling performance at 100 mA g⁻¹ for K‐PMM, PMM, and MXene films. The inset Figure is a diagram of the preparation process of K‐PMM and PMM. Reproduced with permission.^{[ 133 ]} Copyright 2021, Elsevier. f) Cycling performance of Ti₃C₂, Ti₃C₂/Ni₂P, and Ti₃C₂/NiCoP electrodes. The inset Figure in (f) is an illustration of the synthesis of the Ti₃C₂/NiCoP composite. Reproduced with permission.^{[ 134 ]} Copyright 2019, Royal Society of Chemistry.

Figure 11

Applications of MXenes in catalysis. a) Illustration of the simultaneous self‐reduction stabilization process for preparing of Pt₁/Ti_{3‐ x} C₂T_y. b) Catalytic performance of the N‐formylation of aniline using different catalysts. c) HAADF‐STEM images and corresponding elemental maps of M₁/Ti_{3‐ x} C₂T_y (M: Ru, Ir, Rh, and Pd). Reproduced with permission.^{[ 179 ]} Copyright 2019, American Chemical Society. d) Capillary‐forced assembly of MXene into a 3D structure and related composite systems by spray drying the aerosol droplets of colloids containing MXene. e) SEM and TEM images of the CoP@3D Ti₃C₂‐MXene structure. f) A comparison of the onset overpotential and overpotential of different catalysts at applied current density (j = 10 mA cm⁻²). Reproduced with permission.^{[ 184 ]} Copyright 2018, American Chemical Society.

Figure 12

Applications of MXenes in water treatment, especially heavy metal ion removal and water purification membranes. a) Schematic of Pb²⁺ adsorption to form Ti₃C₂(O₂H_2‐2mPb_m). Reproduced with permission.^{[ 198 ]} Copyright 2015, American Chemical Society. b) The removal efficiency of solute elements in simulated nuclear wastewater due to adsorption by Alk‐Ti₃C₂T _x and Ti₃C₂T _x . Reproduced with permission.^{[ 199 ]} Copyright 2018, Royal Society of Chemistry. c) Illustration of the removal mechanism of Cr(VI) from water by Ti₃C₂T _x sheets. Reproduced with permission.^{[ 200 ]} Copyright 2015, American Chemical Society. d) Time‐online profiles for removal of HCrO₄ ⁻ from water. Error bars represent systematic errors in the measurements. Reproduced with permission.^{[ 89 ]} Copyright 2019, Springer Nature. e) Schematic of Al³⁺ intercalated MXene membranes (MXMs) for effective ion sieving. The strong interaction between Al³⁺ ions and MXene layers determines the d‐spacing. Hydrated cations, such as Na⁺, are rejected, while the water molecules permeate the membrane. f) Time‐dependent Na⁺ permeation through untreated MXMs and Al³⁺‐intercalated MXMs, indicating good long‐term stability of the Al³⁺‐intercalated MXMs with a much lower ion permeation rate. Na⁺ permeation rates (inset) through untreated MXMs and Al³⁺‐intercalated MXMs as a function of time. Reproduced with permission.^{[ 49 ]} Copyright 2020, Springer Nature.

Figure 13

Applications of MXenes in other fields. a) Graphical representation of in‐situ one‐step solution processing synthesis of Ag, Au, and Pd@MXene hybrids by soft‐solution processing. The inset is TEM images of MXene, Ag@MXene, Au@MXene, and Pd@MXene. b) Raman spectra of Ti₃C₂T _x after soaking in MB dispersed in ethanol and subsequent drying. SERS spectra of MB with (red) Ag@, (purple) Au@, and (yellow) Pd@MXene. Reproduced with permission.^{[ 93 ]} Copyright 2016, Springer Nature. c) Schematic of theranostic functions of MnO _x /Ti₃C₂‐SP composite sheets, i.e., MR/PA imaging‐guided efficient tumor ablation of cancer. d) In vivo PA Imaging of MnO_x/Ti₃C₂ nanosheets. Reproduced with permission.^{[ 214 ]} Copyright 2017, American Chemical Society.