Understanding the oxidation reaction of aqueous Ti3C2T x MXene suspensions is very important for fostering fundamental academic studies as well as widespread industrial applications. Herein, we investigated the mechanism and kinetics of the oxidation reaction of aqueous Ti3C2T x suspensions at various pH and temperature conditions. Through comprehensive analysis, the mechanism of the chemical oxidative degradation of aqueous Ti3C2T x colloids was established. Chemical oxidation produces solid products such as TiO2 and amorphous carbon as well as various gaseous species including CH4, CO, CO2, and HF. Additionally, our comprehensive kinetic study proposes that aqueous Ti3C2T x dispersions are degraded via an acid-catalyzed oxidation reaction, where, under acidic conditions, the protonation of the hydroxyl terminal groups on the Ti3C2T x flakes induces electron localization on titanium atoms and accelerates their oxidation reaction. In contrast, under basic conditions, the electrostatically alkali-metalized hydroxyl intermediates forming a bulky solvent cage results in less electron localization on titanium atoms, and thus retards their oxidative degradation.
Ligands can control the surface chemistry, physicochemical properties, processing, and applications of nanomaterials. MXenes are the fastest growing family of two-dimensional (2D) nanomaterials, showing promise for energy, electronic, and environmental applications. However, complex oxidation states, surface terminal groups, and interaction with the environment have hindered the development of organic ligands suitable for MXenes. Here, we demonstrate a simple, fast, scalable, and universally applicable ligand chemistry for MXenes using alkylated 3,4-dihydroxy-l-phenylalanine (ADOPA). Due to the strong hydrogen-bonding and π-electron interactions between the catechol head and surface terminal groups of MXenes and the presence of a hydrophobic fluorinated alkyl tail compatible with organic solvents, the ADOPA ligands functionalize MXene surfaces under mild reaction conditions without sacrificing their properties. Stable colloidal solutions and highly concentrated liquid crystals of various MXenes, including Ti2CT x , Nb2CT x , V2CT x , Mo2CT x , Ti3C2T x , Ti3CNT x , Mo2TiC2T x , Mo2Ti2C3T x , and Ti4N3T x , have been produced in various organic solvents. Such products offer excellent electrical conductivity, improved oxidation stability, and excellent processability, enabling applications in flexible electrodes and electromagnetic interference shielding.
While two-dimensional (2D) Ti 3 C 2 T x MXene in aqueous dispersions spontaneously oxidizes into titanium dioxide (TiO 2 ) nanocrystals, the crystallization mechanism has not been comprehensively understood and the resultant crystal structures are not controlled among three representative polymorphs: anatase, rutile, and brookite. In this study, such control on the lattice structures and domain sizes of the MXene-derived TiO 2 crystallites is demonstrated by means of the oxidation conditions, pH, and temperature (3.0−11.0 and 20−100 °C, respectively). It is observed that the formation of anatase phase is preferred against rutile phase in more basic and hotter oxidizing solutions, and even 100% anatase can be obtained at pH 11.0 and 100 °C. At lower pH and temperature, the portion of rutile phase increases such that it reaches ∼70% at pH 3 and 20 °C. Under certain circumstances, small portion of brookite phase is also observed. Smaller domain sizes of both anatase and rutile phases are observed in more basic oxidizing solutions and at lower temperatures. Based on these experimental results, we propose the crystallization mechanism in which the oxidative dissociation of Ti 3 C 2 T x first produces Ti ions as the intermediate state, and they bind to abundant oxygen in the aqueous dispersions, and nucleate and crystallize into TiO 2 .
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