Chemical activity, oxidation resistance and infrared transparency of (Ti3C2)n quantum dots induced by quantum confinement effect

MXene-derived quantum dots (QDs) not only inherit excellent physicochemical properties from its parement bulk phase, but also exhibit a unique set of optical and electronic properties. Herein, the electronic and optical properties of (Ti3C2)n QDs are systematically investigated using density function theory. It is found that binding energy and hardness gradually go up with the decrease of the size of (Ti3C2)n QDs, indicating that (Ti3C2)n QDs are unstable. The origin of the reduced stability is the increase of the proportion of unsaturated atoms. In contrast, the activity of (Ti3C2)n QDs, i.e., averaged energy of d-states of Ti atom (〈εd〉) and averaged energy of p-states of C atom (〈εp〉), is linearly correlated with their coordination numbers. However, VIP and VEA analyses show that (Ti3C2)n QDs are more easier to obtain electrons, suggesting that the oxidation properties of (Ti3C2)n QDs are gradually improved. It is found that HOMO-LUMO gap gradually increases as the size of (Ti3C2)n QDs decreases, indicating strong quantum confinement effect. Finally, it is observed that quantum confinement effect has significant influence on the optical properties of (Ti3C2)n QDs. For example, the spectra exhibit discrete sharp spectral peaks, absorption threshold blue shift, low reflectivity and low absorption coefficient etc., as the size decreases. The spectrum peaks in high energy region are mainly determined by the inter-band transition. These findings will be useful to get an insight into designing functionalized quantum dots based on Ti3C2.