pH, Nanosheet Concentration, and Antioxidant Affect the Oxidation of Ti₃C₂T_x and Ti₂CT_x MXene Dispersions

Abstract

groups (OH, O, F), n usually ranges from 1 to 3, and x reflects the number of terminal groups. [1-5] MXenes have drawn much attention for their potential use in energy storage, [1,6] sensing technology, [7,8] functional coatings, [9-11] plasmonics, [12] and catalytic applications [13-15] due to their high electrical conductivity, hydrophilicity, and surface charge. Most of those properties can be traced back to their metallic-like 2D structure and functional groups attached during the etching and delamination processes. [4,5,16-18] However, recent studies suggest that MXenes are prone to react with dissolved oxygen and water molecules, which results in the formation of transition metal oxides and carbon residues. [3,19,20] Initially, Zhang et al. claimed that MXene oxidizes due to the contact with dissolved oxygen in water. [20] However, Huang et al. and our group also demonstrated that water molecules, rather than oxygen molecules, play a critical role in MXene degradation. [21,22] MXenes were reported to oxidize and degrade more rapidly in water rather than in organic solvents, air, or polymer matrixes. [3,23] Zhang et al., Chae et al., and Habib et al. (our group) also found that temperature and humidity have an influence on MXene oxidation. [3,19,20] They proposed that low temperatures and low humidity can mitigate the oxidation of MXene nanosheets due to the slower reaction kinetics and reduced exposure to water molecules, respectively. In addition, MXene nanosheets that are single-to few-layered or have smaller lateral size oxidize faster than multilayered MXene clay particles or larger-size nanosheets. MXenes oxidize rapidly when exposed to oxidizers such as hydrogen peroxide or treated by flash-annealing at high temperatures. [24,25] In addition, elemental composition may influence the oxidation kinetics. [26] VahidMohammadi et al. and Huang et al. reported that M 2 XT x MXenes, such as V 2 CT x and Ti 2 CT x , oxidize and degrade much faster than the more common M 3 X 2 T x , such as Ti 3 C 2 T x. [21,27] Other aspects may also contribute to the oxidation of MXenes, such as the amount and types of terminal groups, etching conditions, ultraviolet exposure, and the number of defects on the MXene nanosheets.

In‐Plane Assembled Single‐Crystalline T‐Nb2O5 Nanorods Derived from Few‐Layered Nb2CTx MXene Nanosheets for Advanced Li‐Ion Capacitors

“…The underlying reasons for this can be reasonably ascribed to the fact that MXene NSs in the aqueous phase can interact with hydroxyl anions rapidly, then become more prone to oxidation due to the unstable terminal group –O − . [ 49 ] When hydrochloric acid is added dropwise to make the pH value 6.0 (Figure S6d, Supporting Information), there are a number of NRs growing on the surface of NSs. Furthermore, when the pH value is adjusted to 2.0 (Figure S6e, Supporting Information), not only NSs, but also few of ungrown NRs still exist.…”

“…This is also consistent with the previous report. [ 49 ] As a result, the importance of pH value in the formation of Nb 2 O 5 NRs morphology is evident. Simply, under the alkaline conditions, it tends to directly oxidize MXene into Nb 2 O 5 NSs, while, under the acidic conditions, it will slow down the oxidation process of MXene.…”

Improving oxidation stability of 2D MXenes: synthesis, storage media, and conditions

“…Zhao et al investigated the role of particle–particle interactions in the oxidation kinetics [ 42 ]. Ti 3 C 2 T x MXene at a fixed concentration (3.6 mg mL −1 ) was magnetically stirred in the dark for 20 days at 500 rpm.…”

“…These factors directly affect the oxidation kinetics of the synthesized MXenes. Therefore, oxidative degradation of MXenes is controlled not only by synthesis parameters including the quality of the parent MAX phase [ 31 , 35 ], chemical etching conditions such as the type and concentration of acid etchants [ 36 ], and ultrasonication and post-etching treatments [ 11 , 37 ], but also by storage environments such as storage media [ 38 – 40 ], temperature [ 41 ], pH [ 42 ], and dispersion concentrations [ 41 – 43 ].

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“…The formation of CO 2 and H 2 CO 3 was confirmed by the decreased pH values (shifting toward acidic values) of the aqueous dispersions of Ti 2 CT x MXene. However, the non-proportional relationship between the decreased pH value and increased atomic percentage of Ti(IV) does not prove this hypothesis [ 42 ]. Huang and co-worker later claimed that only CH 4 is released during degradation of carbide MXenes, whereas carbonitride MXene (Ti 3 CNT x ) released NH 3 as well [ 53 ].…”