Two-dimensional transition metal carbide and nitride nanomaterials, known as MXenes, exhibit low chemical stability in aqueous environments; they tend to oxidize and react with water molecules, resulting in structural degradation and decreased electrical conductivity. This significantly limits their storage lifetime and potential use in the presence of water, particularly in nanosheet-assembled films for battery electrodes and functional coatings. Here we demonstrate that thermal annealing of Ti3C2T z films at elevated temperatures (∼600 °C) causes changes in the termination distribution as well as the formation of a protective layer of TiO2 on the outermost layer of films. The induced chemical and structural changes during thermal treatment arrest MXene oxidation and enable the MXene films to be stable in aqueous solutions for over 10 months.

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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,

MXenes, 2D nanomaterials derived from ceramic MAX phases, have drawn considerable interest in a wide variety of fields including energy storage, catalysis, and sensing. There are many possible MXene compositions due to the chemical and structural diversity of parent MAX phases, which can bear different possible metal atoms “M”, number of layers, and carbon or nitrogen “X” constituents. Despite the potential variety in MXene types, the bulk of MXene research focuses upon the first MXene discovered, Ti3C2T. With the recent discovery of polymer/MXene multilayer assemblies as thin films and coatings, there is a need to broaden the accessible types of multilayers by including MXenes other than Ti3C2T z ; however, it is not clear how altering the MXene type influences the resulting multilayer growth and properties. Here, we report on the first use of MXenes other than Ti3C2T z , specifically Ti2CT z and Nb2CT z , for the layer-by-layer (LbL) assembly of polycation/MXene multilayers. By comparing these MXenes, we evaluate both how changing M (Ti vs Nb) and “n” (Ti3C2T zvs Ti2CT z ) affect the growth and properties of the resulting multilayer. Specifically, the aqueous LbL assembly of each MXene with poly­(diallyldimethylammonium) into films and coatings is examined. Further, we compare the oxidative stability, optoelectronic properties (refractive index, absorption coefficient, optical conductivity, and direct and indirect optical band gaps), and the radio frequency heating response of each multilayer. We observe that MXene multilayers with higher “n” are more electrically conductive and oxidatively stable. We also demonstrate that Nb2CT z containing films have lower optical band gaps and refractive indices at the cost of lower electrical conductivities as compared to their Ti2CT z counterparts. Our work demonstrates that the properties of MXene/polycation multilayers are highly dependent on the choice of constituent MXene and that the MXene type can be altered to suit specific applications.

Despite numerous prior reports of molten salt etching of MAX phases, few of these reports achieved water-dispersible MXene nanosheets, and none for Nb-based MXenes. Here we demonstrate the synthesis and...