Abstract
AbstractTuning and tailoring of surface terminating functional species hold the key to unlock unprecedented properties for a wide range of applications of the largest 2D family known as MXenes. However, a few routes for surface tailoring are explored and little is known about the extent to which the terminating species can saturate the MXene surfaces. Among available terminations, atomic oxygen is of interest for electrochemical energy storage, hydrogen evolution reaction, photocatalysis, etc. However, controlled oxidation of the surfaces is not trivial due to the favored formation of oxides. In the present contribution, single sheets of Ti3C2Tx MXene, inherently terminated by F and O, are defluorinated by heating in vacuum and subsequentially exposed to O2 gas at temperatures up to 450 °C in situ, in an environmental transmission electron microscope. Results include exclusive termination by O on the MXene surfaces and eventual supersaturation (x > 2) with a retained MXene sheet structure. Upon extended O exposure, the MXene structure transforms into TiO2 and desorbs surface bound H2O and CO2 reaction products. These results are fundamental for understanding the oxidation, the presence of water on MXene surfaces, and the degradation of MXenes, and pave way for further tailoring of MXene surfaces.
Highlights
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The O-saturation process was followed in situ by high-resolution TEM (HRTEM) imaging, electron energy-loss spectroscopy (EELS), and electron diffraction (ED) in high vacuum after sequential 2 mbar O2 exposures from room temperature (RT) and up to 450 °C
A gradual increase in disorder is observed from RT to 400 °C, which is visible through the fast Fourier transform (FFT) insets, where a diffuse centrosymmetric background represent disorder, e.g., at 400 °C, the MXene lattice is barely visible in the image, but a periodicity is registered through the FFT
Summary
Single Ti3C2Tx flakes, inherently terminated by F and O, were subjected to an initial high-temperature treatment to remove the F-terminations and leaving parts of the MXene surface nonterminated, see Figure S1 in the Supporting Information.[22]. A detailed inspection shows that these reflections exhibits lattice spacings that corresponds to TiO2 rutile, brookite and/or anatase nanoparticle formation (see Figure S6 and Table S1 in the Supporting Information), in line with previous reports.[31,34] In contrast to the ED patterns for lower temperatures, the ED pattern at 450 °C exhibit a diffuse centrosymmetric background that indicates amorphization of the sheet. The present results show that oxygen binds to the MXene surface, when energy is supplied to the surface (through heating) that lead to increasing supersaturation and final breakdown of the structural integrity at 450 °C Following this result, an intrinsically less stable structure such as Ti2C[36] that does not exhibit a stabilizing core M layer would presumably degrade at even lower temperatures during oxygen exposure. Attempts have been made to increase the O content on MXene surfaces,[30,31] this is the first experimental investigation to verify supersaturation of O on the MXene surface, that is not associated with a structural transformation into carbon supported titania, owing to the initial deflourination process
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