Bandgap engineering of strained S-terminated MXene and its promising application as NOx gas sensor

Very recently, Kamysbayev et al.'s study (Science, 2020, 369, 979) reported the successful synthesis of S-terminated MXenes, while the metallic properties of these materials suppress their further applications. In the present study, by taking M2CS2 (M = Ti, Hf or Zr) as the examples, our theoretical results revealed that the M2CS2 exhibits the high selectivity and poor sensitivity towards NOx gases. To solve this issue, the electronic property of M2CS2 was considered to modulate by applying strains. It is found that the band gap of Hf2CS2 is opened at 5% biaxial strain under the GGA-PBE functional, while the structures of Ti2CS2 and Zr2CS2 become disordered during the modulation processes. Interestingly, the Hf2CS2 at 5% biaxial strain changes from semiconductor into conductor after the adsorption of NOx, and the sensitivity of Hf2CS2 towards NO and NO2 increases from 15% and 21% (without strain) to 106% and 329% (at 5% strain), respectively. Together with the moderate adsorption energies (-0.33 to-0.17 eV), we proposed that the Hf2CS2 is a promising reusable NOx gas sensor with both high selectivity and sensitivity. More importantly, our results provided an effective strategy to modulate the electronic properties of S-terminated MXenes, which will further extend their applications.