Direct synthesis and chemical vapor deposition of 2D carbide and nitride MXenes (2212.08922v2)

Abstract: Two-dimensional (2D) transition metal carbides and nitrides (MXenes) are a large family of materials actively studied for various applications, especially in the field of energy storage. To date, MXenes are commonly synthesized by etching the layered ternary compounds, MAX phases. Here we demonstrate a direct synthetic route for scalable and atom-economic synthesis of MXenes, including phases that have not been synthesized from MAX phases, by the reactions of metals and metal halides with graphite, methane or nitrogen. These directly synthesized MXenes showed excellent energy storage capacity for Li-ion intercalation. The direct synthesis enables chemical vapor deposition (CVD) growth of MXene carpets and complex spherulite-like morphologies. The latter form in a process resembling the evolution of cellular membranes during endocytosis.

• The paper presents novel direct synthesis and chemical vapor deposition (CVD) methods for 2D MXenes, offering scalable and atom-economical alternatives to traditional etching from MAX phases.
• Directly synthesized Ti2CCl2 MXene shows high structural fidelity (lattice a=3.2284 Å, c=8.6969 Å) and excellent Li-ion storage capabilities (341 F g⁻¹ specific capacitance, 286 mAh g⁻¹ capacity at 0.1 A g⁻¹).
• These new synthesis techniques enable access to previously inaccessible MXene phases and morphologies, significantly broadening potential applications in energy storage and catalysis.

Overview of Direct Synthesis and Chemical Vapor Deposition of 2D Carbide and Nitride MXenes

The article "Direct synthesis and chemical vapor deposition of 2D carbide and nitride MXenes" examines novel methodologies for synthesizing two-dimensional (2D) transition metal carbides and nitrides, known as MXenes. These materials hold significant promise in fields such as energy storage, electromagnetic interference shielding, and catalysis. The research contrasts traditional etching-derived synthesis of MXenes from MAX phases with innovative solid-state reactions and chemical vapor deposition (CVD), offering scalable and atom-economical alternatives.

Main Contributions and Techniques

The traditional preparation of MXenes through selective etching of MAX phases is resource-intensive and hazardous due to the use of hydrofluoric acid or Lewis acidic salts. The research presents an alternative direct synthetic approach that bypasses MAX phases, introducing the possibility of solid-state reactions involving metals, metal halides, and gases like nitrogen or methane. This method facilitates the synthesis of previously inaccessible MXene phases, expanding their potential applications.

• The direct synthesis method allows the scalable production of MXenes. For instance, the reaction of titanium metal and chlorides with carbon or nitrogen sources results in Ti2CCl2 and Ti2NCl2 MXenes. These materials demonstrate notable Li-ion intercalation capacities, suitable for high-performance energy storage applications.
• The CVD growth process enables new MXene morphologies, such as vertically aligned "MXene carpets" and unique microsphere formations. These morphologies enhance ion intercalation capabilities by exposing edge sites with high catalytic activities.

Results and Findings

The paper reports several key structural and electrochemical characteristics of the synthesized MXenes:

• Directly synthesized MXenes (ds-MXenes) exhibit a Ti2CCl2 MXene phase with a lattice parameter a = 3.2284(2) Å and c = 8.6969(1) Å, reflecting high structural fidelity.
• Delaminated ds-Ti2CCl2 yields a specific capacitance of 341 F g^-1, with a maximum storage capacity of 286 mAh g^-1 at a specific current of 0.1 A g^-1, suggesting a robust energy storage mechanism.
• The interlayer distance of ds-Ti2CCl2 calculated from high-resolution imaging is approximately 0.88 nm, consistent with its expected ideal stoichiometry and full Cl surface coverage.

Implications and Future Directions

The implications of this work are significant on both practical and theoretical fronts. The advent of direct synthesis and CVD methods not only broadens the range of accessible MXene phases but also suggests methodologies for tuning MXene properties through morphological control. The enhanced ion intercalation and conductive properties in non-etched MXenes could drive the development of new energy storage solutions with improved efficiencies.

The theoretical understanding of the CVD growth mechanism, particularly the self-limiting and "vesicle" formation processes, could illuminate broader material science concepts related to non-equilibrium growth in layered materials. The potential parallels between MXene growth and biological membrane mechanisms highlight rich areas for future research.

In conclusion, the innovations in synthesizing MXenes present in this paper open new avenues for material design and application, challenging previous limitations in MXene synthesis. The continued evolution of these methods will likely yield further unforeseen benefits in the domain of advanced materials for energy and catalysis.