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.

As the demand for wearable electronic devices increases, interest in small, light, and deformable energy storage devices follows suit. Among these devices, wire-shaped supercapacitors (WSCs) are considered key components of wearable technology due to their geometric similarity to woven fiber. One potential method for creating WSC devices is the layer-by-layer (LbL) assembly technique, which is a "bottom-up" method for electrode fabrication. WSCs require conformal and adhesive coatings of the functional material to the wire-shaped substrate, which is difficult to obtain with other processing techniques such as vacuum filtration or spray-coating. However, the LbL assembly technique produces conformal and robust coatings that can be deposited onto a variety of substrates and shapes, including wires. In this study, we report WSCs made using the LbL assembly of alternating layers of positively charged reduced graphene oxide functionalized with poly-(diallyldimethylammonium chloride) and negatively charged Ti 3 C 2 T x MXene nanosheets conformally deposited on activated carbon yarns. In this construct, the added LbL film enhances capacitance, energy density, and power density by 240, 227, and 109%, respectively, relative to the uncoated activated carbon yarn, yielding high specific and volumetric capacitances (237 F g −1 , 2193 F cm −3 ). In addition, the WSC possesses good mechanical stability, retaining 90% of its initial capacity after 200 bending cycles. This study demonstrates that LbL coatings on carbon yarns are promising as linear energy storage devices for fibrous electronics.

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.

No abstract

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.