Sn(IV) Polyanionic Materials as Efficient Visible-Light-Driven Water-Splitting Photocatalysts

Developing low-cost, eco-friendly, and viable photocatalysts for visible-light-driven water splitting is essential for generating clean hydrogen energy to assuage the global energy crisis. Using first-principle calculations, we show that newly synthesized Rb₃NaSn₃Se₈, K₂Na₂Sn₃S₈, and their solid solutions spontaneously split water under visible-light illumination. Based on our chemical and bond analysis, the oxidation state IV of Sn, which has no 5s² lone pair anymore, plays a key role in the photocatalytic activity enhancement. Strong covalent bonding formed among Sn⁴⁺ and S/Se improves the structural stability, optics, and water reduction, while weak-ionically bonded alkali-metal cations facilitate the water oxidation reaction on the surface, known as the bottleneck of the water-splitting reaction. The Sn⁴⁺ empties the 5s states, places it in the conduction band, and widens the band gap up to 2.6 eV, beyond that of Sn²⁺ compounds. Moreover, Sn⁴⁺ uses its sp³ and sp³d hybridization to build robust covalent polyanions that allow the construction of full-range solid solutions. As such, the high-entropy engineering reduces the band gap of K₂Na₂Sn₃S_8–xSe_x (x = 0–8), switches its band’s nature from indirect to direct, and adjusts the conduction band minimum. Here, our findings provide a roadmap for developing nontoxic, earth-abundant photocatalysts.