Enhancing the electronic and photocatalytic properties of (SnO₂)_<i>n</i>/(TiO₂)_<i>m</i> oxide superlattices for efficient hydrogen production: a first-principles study

The effects of biaxial tensile and compressive strain on the structural, electronic, and photocatalytic properties of tetragonal [001] (SnO₂)_<i>n</i>/(TiO₂)_<i>m</i> superlattices have been theoretically explored using density functional theory (DFT) calculations. Various stacking periodicities between <i>n</i> SnO₂ layers and <i>m</i> TiO₂ layers, including (<i>n</i> = <i>m</i>), (<i>n</i>, 1), and (1, <i>m</i>) were studied in the context of water splitting for hydrogen production. The results reveal that the (1, <i>m</i>) stacking periodicity exhibit the highest bulk modulus, Poisson's ratio, and Debye temperature values. Phonon dispersion analysis showed excellent stability for the (SnO₂)₃/(TiO₂)₁ superlattice under both tensile and compressive strains ranging from -5% to 5%, while other superlattices remain stable within the range of -3% to 4%. Furthermore, the electronic analysis revealed a decreasing trend in the band gap for all structures as the tensile strain increases. A tunable band gap from 3.34 eV to 2.54 eV under tensile strains for (SnO₂)₁/(TiO₂)₃ was found using the HSE06 functional, exhibiting high carrier mobility. Favorable band edge alignments at pH 0, 7, and 14 highlight the potential of these superlattices as efficient photocatalysts. The results demonstrate that applying tensile strain to (SnO₂)_<i>n</i>/(TiO₂)_<i>m</i> superlattices with high TiO₂ thickness can result in optimal band gap values and favorable band edge alignments with the water redox potentials. This makes these superlattices promising for hydrogen production from water splitting.