Tunable Ti3C2Tx MXene-Derived TiO2 Nanocrystals at Controlled pH and Temperature

While two-dimensional (2D) Ti3C2Tx MXene in aqueous dispersions spontaneously oxidizes into titanium dioxide (TiO2) nano -crystals, 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 TiO2 crystallites is demonstrated by means of the oxidation conditions, pH, and temperature (3.0-11.0 and 20-100 degrees 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 degrees C. At lower pH and temperature, the portion of rutile phase increases such that it reaches similar to 70% at pH 3 and 20 degrees 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 Ti3C2Tx first produces Ti ions as the intermediate state, and they bind to abundant oxygen in the aqueous dispersions, and nucleate and crystallize into TiO2.