Mixed Valence Tin Oxides as Novel van der Waals Materials: Theoretical Predictions and Potential Applications

Van der Waals (vdW) heterostructures, which can be assembled by combining 2D atomic crystals in a precisely chosen sequence, enable a wide range of potential applications in optoelectronics, photovoltaics, and photocatalysis. However, the difficulty of peeling isolated atomic planes and the lattice mismatch between different materials is the main obstacle to hinder vdW materials from more practical applications. In this work, the mixed valence tin oxides, Sn_xO_y (0.5 < x/y < 1), are proposed as a new member of vdW materials and these mixed valence tin oxides show promise to overcome the above-mentioned obstacle. Density-functional theory calculations are combined with an evolutionary algorithm to predict the crystal structures of a series of previously reported tin oxides (Sn₂O₃, Sn₃O₄, Sn₄O₅, and Sn₅O₆), unreported compositions (Sn₇O₈, Sn₉O₁₀, and Sn₁₁O₁₂), and a new β-SnO phase. These structures consist of β-SnO, Sn₂O₃, and Sn₃O₄ monolayers. Their band gaps can be engineered in the 1.56-3.25 eV range by stacking the monolayers appropriately. The band gap depends linearly on the interlayer distance, as understood from interlayer Sn²⁺-Sn²⁺ and intralayer Sn²⁺-O interactions. Sn_xO_y structures exhibit high photoabsorption coefficients and suitable band-edge positions for photoexcited H₂ evolution; this indicates potential for environmentally benign solar energy conversion in photovoltaic and photocatalytic applications.