High-Temperature Behavior and Surface Chemistry of Carbide MXenes Studied by Thermal Analysis

Abstract

Two-dimensional (2D) transition-metal carbides and nitrides (MXenes) have attracted significant attention due to their electronic, electrochemical, chemical, and optical properties. However, understanding of their thermal stability is still lacking. To date, MXenes are synthesized via top-down wet chemical etching, which intrinsically results in surface terminations. Here, we provide detailed insight into the surface terminations of three carbide MXenes (Ti 3 C 2 T x , Mo 2 CT x , and Nb 2 CT x ) by performing thermal gravimetric analysis with mass spectrometry analysis (TA−MS) up to 1500 °C under a He atmosphere. This specific technique enables probing surface terminations including hydroxyl (−OH), oxy (O), and fluoride (−F) and intercalated species, such as salts and structural water. The MXene hydrophilicity depends on the type of etching (hydrofluoric acid concentration and/or mixed acid composition) and subsequent delamination conditions. We show that the amount of structural water in Ti 3 C 2 T x increases with decreasing O-containing surface terminations. The thermal stability of Ti 3 C 2 T x is improved by employing a low HF concentration or using a mixture of etchant acids, such as H 2 SO 4 /HF or HCl/HF instead of only HF, due to the reduced defect density. When tetramethylammonium hydroxide (TMAOH) is used for delamination, new N-containing species appear on the MXene surface. Moreover, free-standing films produced from Ti 3 C 2 T x etched with different HF concentrations and delaminated using TMAOH have similar TA−MS profiles, indicating that the post-treatment of Ti 3 C 2 T x controls its surface chemistry. The thermal stability of MXenes strongly depends on their chemical composition and structure; Ti 3 C 2 T x is more thermally stable than the fewer-atomic-layered Mo 2 CT x or Nb 2 CT x , and Mo 2 CT x is more/less thermally stable than Nb 2 CT x .

Two-dimensional metal carbides and nitrides (MXenes): preparation, property, and applications in cancer therapy

“…There was no observation of the atoms such as C, F, and O in the dark-field image. However, they used the annular brightfield image to find their existence and relative position with the Ti atom successfully, which was consistent with previous research [21,31].…”

“…Generally, the crystal with large negative lattice energy has a stable structure. Seredych et al found that the lattice energy of MXene is negative and stable through the calculation of the first principle [21]. They also studied the stability of MXene phase from two aspects of binding energy and formation energy.…”

Environmental Stability of MXenes as Energy Storage Materials

“…The layered structure was almost constant except that the sheet was thinned, and a small amount of invisible titanium dioxide was formed therein. Also, Yury's group revealed the non-negligible effect of surface terminations on the thermal stability of Ti 3 C 2 T X , Nb 2 CT X , Mo 2 CT X MXenes in the He atmosphere (Seredych et al, 2019). At 1,500 • C, Ti 3 C 2 T X was confirmed to converted to the TiC phase.…”

“…At 1,000 • C, Ti 3 C 2 T X was no longer present, leaving only a high temperature stable phase of rutile. Furthermore, with the protection of inert atmospheres, such as Ar, N 2 , or He, MXenes were able to retain their original layered structure over a range of temperatures, but still produced a small amount of oxide (Li et al, 2015;Rakhi et al, 2015;Han et al, 2016;Seredych et al, 2019;Xu et al, 2019). At the same time, the inert carbon layers were kept.…”

“…However, in terms of MXenes themselves, natural oxidation phenomenon is an inevitable drawback for electrodes pursuing the stability during cycling (Zhang et al, 2017;Liang et al, 2018;Li H. et al, 2018;Liu et al, 2019a,b;Mo et al, 2019;Seredych et al, 2019;Yang et al, 2019b). This problem is also one of the essential reasons why MXenes-based electrodes are mainly employed in organic electrolytes, in which the absence of water or/and oxygen can effectively avoid their spontaneous oxidation.…”