Stabilization and strengthening effects of functional groups in two-dimensional titanium carbide

Two-dimensional (2D) materials have attracted considerable interest due to their remarkable properties and potential applications for nanoelectronics, electrodes, energy storage devices, among others. However, many well-studied 2D materials lack appreciable conductivity and tunable mechanical strength, limiting their applications in flexible devices. Newly developed MXenes open up the opportunity to design novel flexible conductive electronic materials. Here, using density functional theory (DFT), we investigate systematically the effects of several functional groups on the stabilization, mechanical properties, and electronic structures of a representative MXene. It is found that oxygen possesses the largest adsorption energy as compared to other functional groups, indicating its good thermodynamic stabilization. In comparison with bare and other functionalized titanium carbides, the oxygen functionalized one exhibits the most superior ideal strength; however, the premature softening of the long-wave phonon modes might limit the intrinsic strength for $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{\mathrm{O}}_2$. Furthermore, the introduction of functional groups can induce a strong anisotropy under tensile loading. By analyzing the deformation paths and the electronic instability under various loadings, we demonstrate that the unique strengthening by oxygen functional groups is attributed to a significant charge transfer from inner bonds to outer surface ones after functionalization. Our results shed a novel view into exploring a variety of MXenes for their potential applications in flexible electronic and energy storage devices.

Figure 1

Schematic of the exfoliation process for (c) MXenes from the corresponding (a) MAX phases. Red circles show two different configurations (I and II) of the surface group adsorption in vertical views.

Figure 2

The structural topology and bond lengths for (a) and (g) bare $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2$ and $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{T}_2$ ($T=\mathrm{Cl}$, F, H, O, OH) in (b)–(f) Configuration I and (h)–(l) Configuration II.

Figure 3

(a) The stress-strain curves in the uniaxial tension $Y$ direction. (b) The schematic of tensile directions and stacking orders for $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2$ and $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{\mathrm{O}}_2$. (c) The stress-strain curves in the uniaxial tension $X$ direction, and the VCDDs of ${\mathrm{S}}_1$ and ${\mathrm{S}}_2$ for $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2$ and ${\mathrm{P}}_1$ and ${\mathrm{P}}_2$ for $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{\mathrm{O}}_2$. (d) The stress-strain curves in biaxial tension, and the VCDDs of ${\mathrm{R}}_1$ and ${\mathrm{R}}_2$ for $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2$ and ${\mathrm{T}}_1$ and ${\mathrm{T}}_2$ for $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{\mathrm{O}}_2$. All the vertical lines mention the maximum stress values. The isosurface maps of the VCDD correspond to $\pm 0.012$ electrons/$\mathrm{Boh}{\mathrm{r}}^3$.

Figure 4

Electronic partial density of states (PDOS) for (a) bare $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2$ and (b) $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{\mathrm{O}}_2$.

Figure 5

Cross-sectional plots of the VCDDs in the vertical plane including the Ti2-C bond for (a) bare $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2$ and (b) $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{\mathrm{O}}_2$. The thick solid and thin dotted contours represent positive and negative values, respectively. Charge unit is electron/$\mathrm{Boh}{\mathrm{r}}^3$.