Synthesis of Ti 3 AlC 2 and electrochemical performance of Ti 3 C 2 T x nanosheet electrode

Abstract

Two-dimensional (2D) transition metal carbides, carbonitrides, and nitrides (MXenes) were discovered in 2011. Since the original discovery, more than 20 different compositions have been synthesized by the selective etching of MAX phase and other precursors and many more theoretically predicted. They offer a variety of different properties, making the family promising candidates in a wide range of applications, such as energy storage, electromagnetic interference shielding, water purification, electrocatalysis, and medicine. These solution-processable materials have the potential to be highly scalable, deposited by spin, spray, or dip coating, painted or printed, or fabricated in a variety of ways. Due to this promise, the amount of research on MXenes has been increasing, and methods of synthesis and processing are expanding quickly. The fast evolution of the material can also be noticed in the wide range of synthesis and processing protocols that determine the yield of delamination, as well as the quality of the 2D flakes produced. Here we describe the experimental methods and best practices we use to synthesize the most studied MXene, titanium carbide (Ti 3 C 2 T x), using different etchants and delamination methods. We also explain effects of synthesis parameters on the size and quality of Ti 3 C 2 T x and suggest the optimal processes for the desired application. ■ INTRODUCTION Since the isolation of single layer graphene in 2004, 1 two-dimensional (2D) materials have gained tremendous attention because of their distinctive properties relative to their bulk form. The isolation of graphene has become a reference for all 2D materials and opened the possibility to discover even more. Today, there are dozens of new 2D materials, including hexagonal boron nitride, transition metal dichalcogenides, transition metal oxides, clays, etc. 2 In 2011, a new family of 2D materials, named MXenes, were discovered by Drexel University scientists. 3 The MXene family is comprised of transition metal carbides, carbonitrides, and nitrides with a general formula of M n+1 X n , where M represents transition metals (such as Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, etc.) and X is carbon and/or nitrogen. 3,4 The name "MXene" was given to describe the similarities between this 2D material family and graphene but also to recognize the parent ternary carbide and nitrides, MAX phases, which MXenes are synthesized from. 5 MAX phases are layered ternary carbides and nitrides with a general formula M n+1 AX n , where A represents elements from the group 13 and 14 of the periodic table. 5,6 MXenes reported to date are synthesized by wet-chemical etching in hydrofluoric acid (HF) or HF-containing or HF-forming etchants, 7−9 which add surface functionalities such as −O, −F, or −OH, represented by T x in this formula as M n+1 X n T x. Etching is required because of strong chemical bonds between A and M elements in MAX phases that make mechanical exfoliation hardly possible. To date, more than 20 members of the MXene family have been synthesized, and dozens more are predicted, making it one of the fastest growing 2D material families. 7 Additionally, the MXene family comes in three atomic structures, ranging from M 2 X to M 3 X 2 and M 4 X 3 , yielding tunability and opportunity to discover and mold materials based on necessary demands. 7,10,11 In contrast to most other 2D materials, including graphene, MXenes possess hydrophilic surfaces 7 and high metallic conductivities (∼6000−8000 S/cm), 12,13 showing promising performance in energy storage devices, 14−17 water desalination , 18 catalysis, 19 electromagnetic interference shielding, 20 transparent, conducting thin films, 12,21,22 and many other applications. 7 Since their discovery, production of multilayered MXene flakes was possible through wet-chemical etching with HF, and different MXene compositions were synthesized in this form, such as Ti 2 CT x , Ti 3 CNT x , Nb 2 CT x , and V 2 CT x (Figure 1). 3,23,24 However, it was not until 2013 when single-layer MXene flakes were isolated by intercalating large organic molecules and delaminating the sheets from each other, opening the door to exploration of the truly 2D nature of MXenes. 25 In 2014, Ti 3 C 2 T x MXene was produced using a HF-containing etchant such as ammonium bifluoride (NH 4 HF 2) salt, 21 and later in the