Visible Light-Driven Z‑Scheme Water Splitting Using Oxysulfide H₂ Evolution Photocatalysts

A Sm₂Ti₂S₂O₅ (STSO) oxysulfide photocatalyst prepared by a novel flux method showed a higher degree of crystallinity and greater photocatalytic activity than that prepared by conventional polymerized complex and sulfurization processes. Co-loading with both IrO₂, as an oxidative cocatalyst, and Pt, as a reductive cocatalyst, was found to be essential for promoting the photocatalytic activity of the STSO. Visible light-driven Z-scheme water splitting into H₂ and O₂ was realized by utilizing the STSO photocatalyst for H₂ evolution in conjunction with a WO₃ photocatalyst treated with H⁺ and Cs⁺ and loaded with PtO_<i>x</i> for O₂ evolution, and a triiodide/iodide (I₃^–/I^–) redox couple as a shuttle electron mediator. Various other narrow band gap oxysulfide photocatalysts with H₂ evolution activity, such as La₅Ti₂CuS₅O₇ and La₆Ti₂S₈O₅, were also shown to be applicable as H₂ evolution photocatalysts in the present Z-scheme water splitting system

Plate-like Sm₂Ti₂S₂O₅ Particles Prepared by a Flux-Assisted One-Step Synthesis for the Evolution of O₂ from Aqueous Solutions by Both Photocatalytic and Photoelectrochemical Reactions

Sm₂Ti₂S₂O₅ (STSO) is a visible-light-responsive oxysulfide semiconductor photocatalyst with applications to water splitting. In this work, plate-like STSO particles were synthesized through a flux-assisted one-step method at various temperatures. The activities of these materials during photocatalytic and photoelectrochemical O₂ evolution from aqueous solutions were investigated. Single-phase STSO with a single crystal habit was produced at 923 K, which is approximately 200 K lower than the temperatures required for previously reported methods, such as solid-state reactions and thermal sulfurization under a H₂S flow. The STSO sample synthesized at the optimal temperature exhibited an AQE of 1.3 ± 0.2% at 420 nm during photocatalytic sacrificial O₂ evolution. This efficiency is twice the values reported for specimens prepared using conventional methods. An STSO/Ti/Sn electrode fabricated by the particle transfer method generated a photoanodic current and evolved O₂ by water oxidation with a Faradaic efficiency of approximately 70 ± 7%. The synthesis temperature yielding the highest activity was lower for photocatalytic O₂ evolution than for photoelectrochemical O₂ evolution. This work demonstrates the applicability of the flux method to the synthesis of well-crystallized oxysulfides having various particle sizes and intended for different uses