Ti2C is one of the thinnest layers in MXene family with high potential for applications. In the present study, the adsorption of NH3, H2, CH4, CO, CO2, N2, NO2, and O2 on monolayer Ti2CO2 was investigated by using first-principles simulations to exploit its potential applications as gas sensor or capturer. Among all the gas molecules, only NH3 could be chemisorbed on Ti2CO2 with apparent charge transfer of 0.174 e. We further calculated the current-voltage (I-V) relation using the nonequilibrium Green's function (NEGF) method. The transport feature exhibits distinct responses with a dramatic change of I-V relation before and after NH3 adsorption on Ti2CO2. Thus, we predict that Ti2CO2 could be a promising candidate for the NH3 sensor with high selectivity and sensitivity. On the other hand, the adsorption of NH3 on Ti2CO2 could be further strengthened with the increase of applied strain on Ti2CO2, while the adsorption of other gases on Ti2CO2 is still weak under the same strain, indicating that the capture of NH3 on Ti2CO2 under the strain is highly preferred over other gas molecules. Moreover, the adsorbed NH3 on Ti2CO2 could be escapable by releasing the applied strain, which indicates the capture process is reversible. Our study widens the application of monolayer Ti2CO2 not only as the battery material, but also as the potential gas sensor or capturer of NH3 with high sensitivity and selectivity.

Monolayer Ti2CO2: indirect–direct band gap transition under biaxial strain of ∼4% and uniaxial strain of ∼6%.

Xu et al.’s recent experimental work (Adv. Mater. 2017, 29, 1702007) suggested that C3N is a potential candidate as Li-ion battery with unusual electrochemical characteristics. However, the obvious capacity loss (from 787.3 to 383.3 mA h·g–1) occurs after several cycles, which restricts its high performance. To understand and further solve this issue, in the present study, we have studied the intercalation processes of Li ions into C3N via first-principle simulations. The results reveal that the Li-ion theoretical capacity in pure C3N is only 133.94 mA h·g–1, the value is obviously lower than experimental one. After examining the experimental results in detail, it is found that the chemical component of the as-generated C x N structure is actually C2.67N with N excess. In this case, the calculated theoretical capacity is 837.06 mA h·g–1, while part of Li ions are irreversibly trapped in C2.67N, resulting in the capacity loss. This phenomenon is consistent with the experimental results. Accordingly, we suggest that N excess C3N, but not pure C3N, is the proposed Li-ion battery material in Xu et al.’s experiment. To solve the capacity loss issue and maintain the excellent performance of C3N-based anode material, the C3N with slightly excess C (C3.33N), which has been successfully fabricated in the experiment, is considered in view of its relatively low chemical activity as compared with N excess C3N. Our results reveal that the C excess C3N is a potential Li-ion battery material, which exhibits the low open circle voltage (0.12 V), high reversible capacity (840.35 mA h·g–1), fast charging/discharging rate, and good electronic conductivity.

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