Strain-tunable ZrS$$_2$$-1 H/ZrS$$_2$$-1T van der Waals heterostructures with quasi Type-II band alignment for solar oxygen evolution and optoelectronic applications

Abstract We report a first-principles study of ZrS $$_2$$ 2 -1 H and ZrS $$_2$$ 2 -1T monolayers, and their compositionally identical ZrS $$_2$$ 2 -1 H/ZrS $$_2$$ 2 -1T van der Waals heterostructure. This platform allows phase, stacking, and strain effects to be isolated without altering composition. The 1 H and 1T monolayers display HSE06 bandgaps of 1.32 and 2.16 eV. The most stable heterostructure has a lattice constant of 3.60 Å and remains semiconducting, with GGA, HSE06, and quasiparticle bandgaps of 1.10, 1.83, and 1.96 eV. Orbital analysis shows quasi type-II band alignment, with the CBM from Zr- d states of 1 H and VBM from S- p states of 1T, facilitating charge separation. Biaxial strain tunes band edges and charge distribution while preserving the semiconducting nature. Tensile strain shifts the 1T CBM to –4.79 eV at +2 $$\%$$ % . Calculated electron and hole effective masses are 1.03 $$m_0$$ m 0 and 0.26 $$m_0$$ m 0 , with mobilities of 9.1 and 120 cm $$^2$$ 2 V $$^{-1}$$ - 1 s $$^{-1}$$ - 1 , respectively. The heterostructure exhibits strong, tunable optical absorption from visible to UV, peaking at 0.65 $$\times $$ × 10 $$^6$$ 6 cm $$^{-1}$$ - 1 near 4.3 eV for 1T. These findings indicate that ZrS $$_2$$ 2 -1 H/ZrS