Ti2CO2 Nanotubes with Negative Strain Energies and Tunable Band Gaps Predicted from First-Principles Calculations.

MXenes, a series of two-dimensional (2D) layered early transition metal carbide, nitride, and carbonitride, have been prepared by exfoliating MAX phases recently. In addition to 2D planar MXene, one-dimensional tubular forms-MXene nanotubes-are also expected to form. Herein, we design atomic models for Ti2C as well as Ti2CO2 nanotubes in the 1-4 nm diameter range and investigate their basic properties through density functional theory (DFT). It is shown that though the strain energy of Ti2C nanotubes are always positive, Ti2CO2 nanotubes have negative strain energies when diameter beyond 2.5 nm, indicating that they could possibly folded from 2D Ti2CO2 nanosheets. Moreover, the band gap of Ti2CO2 nanotubes decrease with the growing diameter and the maximum band gap can reach up to 1.1 eV, over 3 times that of their planar form. Thus, tunable band gaps provide strong evidence for the effectiveness of nanostructuring on the electronic properties of Ti2CO2 nanotubes.