Revisiting Polytypism in Hexagonal Ternary Sulfide ZnIn₂S₄ for Photocatalytic Hydrogen Production within the Z-Scheme

The ternary chalcogenide, ZnIn₂S₄, is known to exhibit various polymorphic expressions: from the cubic spinel phase to various polytypic layered hexagonal structures, commonly known as α, β, II_a, and II_b. Notwithstanding numerous recent studies on the superior photocatalytic activities of hexagonal ZnIn₂S₄, it remains unclear how the polymorphic expressions in this material may influence its physiochemical properties (and thus their performance in actual photodevices). Thus, revisiting and addressing the open questions on their intrinsic atomic and electronic structure properties in relation to their photoactivity is the main intention of this work. Here, we systematically perform first-principles density functional theory calculations to examine the atomic and crystal structures of layered hexagonal ZnIn₂S₄. Based on the compelling evidence from lattice dynamics, chemical bonding, and optoelectronic structure analysis, we propose a revised atomic structure for the II_a polymorph and explain why the previously acclaimed structure proposed in experiments must be disputed. Furthermore, from band alignment calculations, we rationalize that all hexagonal polymorphs of ZnIn₂S₄ are indeed potentially superior photocathode materials, adding to a new class of high-performance photocatalysts within the Z-scheme.