Two-dimensional (2D) transition metal carbides and nitrides (MXenes) have shown outstanding performances in electrochemical energy storage and many other applications. Delamination of MXene flakes in water produces colloidal solutions that are used to manufacture all kinds of products (thin films, coatings, and electrodes, etc.). However, the stability of MXene colloidal solutions, which is of critical importance to their application, remains largely unexplored. Here we report on the degradation of delaminated-Ti3C2T x colloidal solutions (T represents the surface functionalities) and outline protocols to improve their stability. Ti3C2T x MXene solutions in open vials degraded by 42%, 85%, and 100% after 5, 10, and 15 days, respectively, leading to the formation of cloudy-white colloidal solutionss containing primarily anatase (TiO2). On the other hand, the solution could be well-preserved when Ti3C2T x MXene colloidal solutionss were stored in hermetic Ar-filled bottles at 5 °C, because dissolved oxygen, the main oxidant of the MXene flakes, was eliminated. Under such a recipe, the time constant of the solution was dramatically increased. We have found that the degradation starts at the edges and its kinetics follows the single-exponential decay quite well. Moreover, we performed size selection of the MXene solution via a cascade technique and showed that the degradation process is also size-dependent, with the small flakes being the least stable. Furthermore, a dependence between the degradation time constants and the flake size allows us to determine the size of the nanosheets in situ from UV–vis spectra and vice versa. Finally, the proposed method of storing the MXene colloidal solution in Ar-filled vials was applied to Ti2CT x to improve its stability and time constant, demonstrating the validity of this protocol in improving the lifetime of different MXene solutions.

Atomic force microscopy (AFM) has been used to study the effect of pressure on the structural and frictional properties of self-assembled monolayers of n-octadecanethiol on Au(111). Sharp microfabricated silicon nitride tips (tip radii 100-300 Å) were used. At low load, the periodicity of the thiol layer is imaged. At higher load, the layer is observed to become disordered. At a critical contact pressure of ∼2.3 GPa, a transition from the thiol overlayer to the Au(111) substrate periodicity is observed in the lattice resolution images. This transition is gradual and reversible. During the transition, frictional forces first increase and then decrease as the tip-sample separation decreases by a distance approximately equivalent to the thickness of the thiol layer.

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The oxidation kinetics of polycrystalline hafnium (Hf) at room temperature a d low oxygen pressure (Poz -lo-' Ton) has been studied by x-ray photoelectron spectroscopy (XPS). After a chemisorption stage for exposures < 5 L, Hf suboxides are initially formed and are dominant until -25 L. HfO, appears at -10 L. Above 25 L, HfO, grows by oxidation of the suboxides, whereas the oxide film thickness remains constant. Above 500 L, a saturation region is obse~ed that corresponds to an oxide layer of 12 hi thick with an average composition HfO,., .

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