Promising multifunctional van der waals heterostructure Ti2CO2/HfSi2N4 for photovoltaics and photocatalytic OER

This report presents an in-depth investigation of four stacking configurations of the van der Waals heterostructure (vdW-HS) of Ti2CO2 and HfSi2N4, conducted to explore their potential for green energy applications. The vdW-HS Ti2CO2/HfSi2N4 has a negligible lattice mismatch of 0.23% between the constituent monolayers, guaranteeing high structural compatibility. The dynamic stability has been confirmed by the phonon band structure, which has no imaginary frequencies throughout the full Brillouin zone. From the electronic band structure analyses, it has been confirmed that all stacking configurations yield identical band characteristics along with an indirect band gap of 0.88 eV calculated by using the Heyd−Scuseria−Ernzerhof (HSE06) functional with spin-orbit coupling (SOC). Remarkably, this vdW-HS displayed type-I band alignment, the electrons tunnel directly from the VBM to CBM of Ti2CO2 monolayer, allowing efficient carrier confinement and recombination, which is advantageous for advanced optoelectronic applications. Moreover, the electronic band edges of the vdW-HS Ti2CO2/HfSi2N4 demonstrate its high suitability for photocatalytic oxygen evolution reaction (OER), but not for hydrogen evolution reaction (HER). The latter unsuitability of the considered vdW-HS is also confirmed by ΔGH > 0.2 or ΔGH < −0.2 eV for all possible sites at the surface of the heterostructure. The considered vdW-HS has a significant static dielectric constant of 4.72, along with noticeable optical absorption in the visible spectrum and intense absorption of 1.50 × 106 cm-1 in the ultraviolet region. The spectroscopic limited maximum efficiency (SLME) of ∼32% is higher than other highly appreciated thin-film photo-responsive absorber materials such as CuInSe2 (∼28%) and CdTe (∼31.5%). The n-type carriers have a higher value of Seebeck coefficient as compared to p-type carriers, which confirms that n-type doping will be more beneficial than p-type. The lattice thermal conductivity κph of the vdW-HS Ti2CO2/HfSi2N4 is 8.41 W/mK at room temperature, which is at least 2.5 and 4.6 times lower than the lattice thermal conductivity of Ti2CO2 and HfSi2N4, respectively. These results highlight the potential of the vdW-HS Ti2CO2/HfSi2N4 as a highly suitable candidate for next-generation optical absorbers and thermoelectric materials for green energy technologies.