Synthesis of Two-Dimensional Nb_1.33C (MXene) with Randomly Distributed Vacancies by Etching of the Quaternary Solid Solution (Nb_2/3Sc_1/3)₂AlC MAX Phase

Abstract

Introducing point defects in two dimensional, 2D, materials can alter or enhance their properties. Here, we demonstrate how etching a laminated (Nb2/3Sc1/3)2AlC MAX phase (solid solution) of both the Sc and Al atoms, results in a 2D Nb1.33C material (MXene) with a large number of vacancies and vacancy clusters. This method is applicable to any quaternary, or higher, MAX phase wherein one of the transition metals is more reactive than the other and could be of vital importance in applications such as catalysis and energy storage. We also report, for the first time, on the existence of (Nb2/3Sc1/3)3AlC2 and (Nb2/3Sc1/3)4AlC3 phases. 3 Two-dimensional (2D) materials have shown great promise for many applications. 1-6 The reduced dimension leads to an increase in the surface to volume ratio, and can fundamentally alter the chemical, optical and electronic properties of a material. The properties can be altered further, either chemically via surface functionalization, 7 intercalation 8 or structurally, by introducing defects. 8-9 About 7 years ago, a new class of 2D materials based on transition metal carbides and/or nitrides (MXenes) was discovered. 10-11 MXenes are mainly produced by etching the Mn+1AXn (MAX) phases or related ternary phases. 12 The MAX phases are a family of hexagonal, layered ternary transition metal carbides and/or nitrides where M stands for an early transition metal, A stands for group 13 and 14 elements, X stands for carbon and/or nitrogen and n = 1, 2, or 3. 13 Various acidic solutions, containing fluoride ions are used to selectively etch the A layers (either Al or Ga) and convert MAX to MXene. 10, 14-17 The A layers are replaced with oxygen, hydroxyl and/or fluoride surface terminating (T) groups. 18 MXenes show promise for a large host of applications including batteries, supercapacitors, transparent conducting electrodes, catalytic and photocatalytic applications, water treatment, electromagnetic shielding, gas sensors and biosensors. 19-25 MXene properties can be tuned in at least three ways that involve either altering their: i) composition, ii) surface terminations, Tx and/or, iii) structure/morphology. The composition can be changed by e.g. forming solid solutions though alloying on the M-, 26 and/or X-27 sites in the parent MAX phase. The quaternaries, (Nb0.8,Ti0.2)4C3Tx and (Nb0.8,Zr0.2)4C3Tx 28 are examples

The Role of Cation Vacancies in Electrode Materials for Enhanced Electrochemical Energy Storage: Synthesis, Advanced Characterization, and Fundamentals

“…However, when using a milder etching procedure (etching in 10% HF for 72 h, right bottom in Figure 5d), both Y and Al atoms were selectively removed, leading to a cation‐deficient Mo 1.33 C phase. Similar results have also been obtained by Halim et al [ 101 ] on cation‐deficient Nb 1.33 C and by Tao et al [ 134 ] on cation‐deficient Mo 1.33 CT x . Thus, selective removal of cations in acid/base solution is also a promising and facile approach for intentionally incorporating cation vacancies into the metal oxides/carbides with at least two constituent cations.…”

“…Single V Ti , two/three adjacent V Ti , and V Ti cluster have been clearly observed experimentally, which are consistent with the simulated images. Similar results have also been observed by Halim et al [ 101 ] They synthesized 2D Nb 1.33 C material (MXene) with a large number of vacancies and vacancy clusters, which were directly characterized by STEM. Figure 6g shows typical morphology of a Nb 1.33 CT x single flake.…”

“…It is also more favorable for the electrochemical intercalation of electrolyte cations near the vacancies, which leads to enhanced rate capability in the defective MnO 2 for electrochemical intercalation/deintercalation. Following this pioneering study, more research works have been conducted to intentionally introduce cation vacancies into different transition metal oxides/carbides, such as MnO 2 , [ 89–93 ] TiO 2 , [ 94–96 ] Fe 2 O 3 , [ 77,78,97,98 ] ZnCo 2 O 4 , [ 99 ] ZnMn 2 O 4 , [ 76 ] and MXenes, [ 79,100–102 ] providing better and more fundamental understanding of the role of cation vacancies in boosting electrochemical performance ( Figure 2 ). Several density functional theory (DFT) calculations have also indicated that the presence of cation vacancies results in a decrease of energy barrier for ion diffusion, [ 95,103 ] and an increase of the materials' electronic conductivity, [ 80 ] thus benefits the charge storage process.…”

“…There are several ways for incorporating cation vacancies and tailoring its content in transition metal oxides/carbides through synthetic approaches, such as replacing the constituent cations or anions with aliovalent substitutes, [ 94–96,104,105 ] equilibration at solutions with different pH values, [ 89,90,99,106,107 ] selective removal of constituent cations, [ 101,102,108 ] thermal annealing in defects inducing atmospheres, [ 109,110 ] and plasma etching. [ 111 ] The characterization of cation vacancies, however, is usually challenging.…”

On the Structural Stability of MXene and the Role of Transition Metal Adatoms

“…12,[15][16][17][18][19][20][21][22][23][24] Among MXenes, Nb 2 C is a highly promising candidate as an electrode material in Li-ion batteries, maintaining the high capacity and handling high charge-discharge rates. [25][26][27] Furthermore, Nb 2 CT x have shown promising performance for photothermal cell ablation and as a photocatalyst for hydrogen evolution. [28][29][30] Regardless of which MXene is investigated, these are typically oxidized with time under ambient conditions.…”