Tunable Ti₃C₂T_x MXene-Derived TiO₂ Nanocrystals at Controlled pH and Temperature

Abstract

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 .

Biocompatible and Oxidation‐Resistant Ti3C2Tx MXene with Halogen‐Free Surface Terminations

“…In many cases, fluorinated Ti 3 C 2 T x spontaneously degrades by oxidation in aqueous media and forms nanoscale TiO 2 , amorphous carbon, and derivatives from the surface moieties, including F − ions. [28][29][30][31] To examine the effect of such oxidative degradation of Ti 3 C 2 T x , aged NaOH-Ti 3 C 2 T x and MILD-Ti 3 C 2 T x were also tested similarly after 48 h, 72 h, 1 week, and 2 weeks of incubation at 37 °C in phosphate buffer solution (PBS) with gentle agitation (Figure 5b,c, Figure S10 and Table S3, Supporting Information). The results indicated that the 1-week-aged MILD-Ti 3 C 2 T x became more cytotoxic, as demonstrated by cell viabilities of 84.3 ± 9.2% and 47.3 ± 4.7% for 1000 and 2000 μg mL −1 , respectively.…”

Ligand chemistry for surface functionalization in MXenes: A review

“…After that, they adhere to the plentiful oxygen present in a medium, nucleates, and forms anatase or rutile crystals of TiO 2 depending on the pH conditions of the aqueous solution. This assertion was substantiated by the detection of titanium ions during the oxidative breakdown of water‐based dispersion using inductively coupled plasma mass spectrometry (ICP‐MS) 41 . 4.38 ppm of [Ti] ions were detected in the fully oxidized water based dispersion, whereas 1.01 ppm of [Ti] ions were observed in pristine aqueous dispersions.…”

“…(J) Mechanism of TiO 2 crystallization from Ti 3 C 2 T x suspensions in basic and acidic environments. Reproduced with permission: Copyright 2022, American Chemical Society 41 …”

“…When a certain quantity of carbon dioxide and hydrofluoric acid is dissolved in aqueous solution, it causes increment in the concentration of H + ions. Furthermore, the crystallization mechanism for the oxidized TiO 2 was comprehensively studied under different conditions, pH, and temperature (20–100°C) 41 . The findings show that titanium ions are initially produced as an intermediary state during disintegration of MXene caused by oxidation.…”

Modulation of Surface Ti–O Species in 2D-Ti3C2TX MXene for Developing a Highly Efficient Electrocatalyst for Hydrogen Evolution and Methanol Oxidation Reactions

“…Jia et al 44 have used carbon-doped MXene to delay the TiO 2 formation. Besides, Chae et al 45 have also perceived the formation of TiO 2 from MXene and studied different phases of TiO 2 by varying the pH (∼3, 5.4, 8.5, and 11) and temperatures (20−100 °C). The authors have claimed that high acidic conditions (pH ∼ 3) as well as high basic conditions (pH ∼ 11) result in TiO 2 formation on the MXene surface even at low temperatures.…”