The Role of Antioxidant Structure in Mitigating Oxidation in Ti₃C₂T_x and Ti₂CT_x MXenes

Abstract

carbide MXenes (e.g., Ti 3 C 2 T x , Ti 2 CT x , V 2 CT x , Nb 2 CT x , etc.) are synthesized from their ceramic MAX phase precursor (with a formula of M n+1 AX n , where A is a group 13 or 14 element) by selectively etching the A phase using hydrofluoric acid (HF) or fluoride-containing aqueous solutions. [6][7][8][9][10] MXenes are known for their variety of surface terminal groups, excellent aqueous processability, and high electrical conductivity. [6][7][8][9][10] Extensive studies have focused on practical applications of MXenes for energy storage, [11][12][13][14][15][16][17][18] electromagnetic wave shielding, [19][20][21][22] layer-by-layer functional coatings, [23][24][25] nanocomposites, [26][27][28] hydrogels with 3D networks, [17,29] optoelectronic devices, [30,31] sensors, [23,32] catalysis, [33] and biomedical applications. [34,35] One limitation to the widespread use of MXenes is that they are prone to oxidation, especially in their commonly used aqueous colloidal state. Prior studies have demonstrated that MXenes undergo chemical reaction with water and strong oxidants, resulting in their structural and chemical degradation over time, most commonly leading to formation of transition metal oxides. [36,37] This phenomenon of rapid MXene oxidation and degradation is currently not wellunderstood and is rarely discussed in reports focused on MXene applications, even though oxidation significantly changes performance-related properties. Indeed, low oxidation stability threatens to restrict both the shelf life of MXene dispersions and the product lifetime for MXene-based devices. Multiple studies have confirmed that oxidation results in the degradation of the MXene's 2D structure and loss of functional properties. Ti 3 C 2 T x and Ti 2 CT x nanosheets in their aqueous colloidal state and in functional films can react with water molecules, resulting in the formation of titanium dioxides (TiO 2 ) and amorphous carbon. [36][37][38][39] Oxidation rates of MXene nanosheets dispersed in water were found to be a function of temperature, solution pH, exposure to 800 nm laser and ultraviolet light, and the presence of radical-forming chemical oxidants such as hydrogen peroxide and ozone. [36,[40][41][42] In addition to dispersed nanosheets, dried MXene nanosheet powders and MXene-based devices are also prone to gradual degradation and worsened electrical properties. [36,43,44] Naguib et al. and others reported that MXene oxidation starts from nanosheet edges; thus, large nanosheets may have better oxidation stability than smaller ones. [45,46] MXene oxidation is also dependent on intrinsic nanosheet properties,

Progression in the Oxidation Stability of MXenes

“…Here, converting MXenes into solid forms, such as powder or film [ 42 ], is one efficient way to extend their shelf-life; however, regaining their colloidal state necessitates an extra step of sonication, which poses a risk of rapid oxidation or flake shattering into smaller sizes. Among the reported strategies, MAX phase alteration [ 43 ], deaerated atmospheres [ 22 ], organic solvents [ 44 ], and antioxidants [ 45 ] have also proven effective in delaying the oxidation of MXenes. However, an effective method that could completely halt the spontaneous oxidation of MXenes in an aqueous environment is yet to be established.…”

“… Copyright 2021, American Chemical Society. c Various antioxidant molecules and their antioxidation ability and corresponding digital photographs of Ti 3 C 2 T x dispersion during 14 days of storage, d the variation of Ti (IV) content of MXenes after 31 h of storage in aqueous media containing different antioxidants [ 45 ]. Copyright 2022, Wiley–VCH.…”

“…The reactive molecular dynamics simulation supported the edge-capping mechanism involving the loss of Na + cation from L-sodium ascorbate and the association of the corresponding anions at the edges of the Ti 3 C 2 T x , preventing its interaction with water molecules. Zhao et al [ 45 ] later expanded the study to systematically examine the interaction of three classes of organic anti-oxidants with Ti n +1 C n T x MXenes. Here, three classes of molecules (-hydroxy acids, polycarboxylic acids and phenolic compounds) were examined for their antioxidation activity for MXenes (Ti 3 C 2 T x and Ti 2 CT x ) during a storage period of 14 days.…”

“…At present, the addition of anti-oxidants to stabilize MXene is a relatively easy and efficient route to prolong the shelf-life storage of MXenes in their aqueous form [ 17 ]. However, the overall stabilization efficiency depends on the nature of the anti-oxidants (structure) and their mode of interaction with MXenes [ 45 ]. Though effective, the use of anti-oxidants presents a concern in terms of quantity since their content is significantly greater than MXenes concentration.…”

Effect of terminal groups on the degradation stability of Ti3C2Tz MXenes

“…28 Several approaches have been proposed to mitigate the degradation of MXenes including addition of an antioxidant like sodium L-ascorbate, low temperature storage, and dispersion in polymers or in organic solvents. 26,[29][30][31] Our group has demonstrated that a higher 'n' composition helps in increasing the shelf lives of MXenes. 26 Additionally, our group has also showed that molten salt etched MXenes are less susceptible to degradation than acid-etched MXenes due to reduced content of oxygen-containing terminal groups.…”