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.