Excited-State Dynamics in Two-Dimensional Heterostructures: SiR/TiO₂ and GeR/TiO₂ (R = H, Me) as Promising Photocatalysts

The electronic structure and excited-state dynamics of heterojunctions composed of 2D silicane and germanane-based materials (SiR, GeR; R = H, Me) and anatase or rutile isomorphs of TiO₂ are investigated. Our calculations reveal a high tunability of the band gaps of these heterostructures: 2.28 eV in SiH/a-TiO₂ to 0.16 eV in GeMe/r-TiO₂. Nonadiabatic molecular dynamic (NA-MD) simulations suggest that electron-hole recombination in SiH/a-TiO₂, SiH/r-TiO₂, and GeH/a-TiO₂ occurs within 46.0, 3.6, and 1.2 ns, respectively, which is notably slower than in other analogous materials. The methylation of Si or Ge monolayers and the use of rutile polymorphs increase the nonadiabatic coupling and accelerate the recombination. A simple accelerated NA-MD method is devised in this work to evaluate the time scales for extremely slow dynamics of excited states. On the contrary to the electron-hole recombination, "hot" electrons are found to thermalize within a picosecond time scale, whereas some hot holes thermalize notably slower, on the order of 20.5 ps in SiH/a-TiO₂ and 65.3 ps in GeH/a-TiO₂. High tunability of the band gaps, suitable electron and hole localization, and long recombination time scales suggest that SiH/a-TiO₂, SiH/r-TiO₂, and GeH/a-TiO₂ heterostructures may be promising candidates for photocatalytic applications.