Surface-Modified Metal Sulfides As Stable H2 Evolving Photocatalyst in Z-Scheme Water Splitting System with [Fe(CN)6]3–/4– Redox Mediator Under Visible Light Irradiation

Abstract

Photocatalytic water splitting using semiconductor materials has attracted considerable attention due to the potential for the clean production of H2 from water by utilizing solar energy. Since nearly half of the solar energy incident on the Earth’s surface lies in the visible region, an efficient harvest of wide range of visible light is essential for realizing practically high efficiency in the photocatalytic H2 production under natural sunlight. Most of metal sulfide semiconductors, such as CdS, possess appropriate band levels for water reduction and oxidation as well as a narrow band gap allowing visible light absorption because the valence bands of them are mainly formed by S 3p orbital. However, the most of sulfides have no capability of O2 evolution from water due to the occurrence of self-oxidative deactivation by photogenerated holes. Even for the sacrificial H2 evolution, they require particular electron donor such as S2– and SO3 2– to generate H2 stably. In the present study, we introduce a new and versatile way that can stabilize metal sulfides, not only CdS but also others such as ZnIn2S4 and CdIn2S4, as H2-evolving photocatalyst in Z-scheme water splitting with [Fe(CN)6]3–/4– redox. Pt/CdS was found to generate considerable amount of H2 from water in the presence of [Fe(CN)6]4– under visible light, while the H2 evolution was soon terminated at ca. 28 µmol in the unbuffered aqueous solution (Fig. 1). However, the production of nearly the twice amount (ca. 53.6 µmol) of oxidant, i.e., [Fe(CN)6]3–, was confirmed in the solution after the reaction. This result indicated that photocatalytic H2 evolution over Pt/CdS proceeded under visible light irradiation, accompanied by the oxidation of [Fe(CN)6]4– to [Fe(CN)6]3–. The cease of H2 evolution was due to increased pH value (6.7 → 10.7). Then, it was found that the use of borate buffer (BB) was effective to suppress such decrease in H2evolution rate. Photocatalytic H2 evolutions over other metal sulfides (ZnIn2S4 and CdIn2S4) were also examined in the presence of [Fe(CN)6]4–. Pt/ZnIn2S4 showed appreciable H2 evolution rate (4.8 µmol/h). On the other hand, Pt/CdIn2S4 showed quite low activity (0.8 µmol/h). ATR-FTIR analysis revealed that hexacyano complexes, such as M x [CdFe(CN)6]1) and M x [ZnFe(CN)6]2) were formed on Pt/CdS and Pt/ZnIn2S4, respectively, after each reaction, both of which showed appreciable H2 evolution. These hexacyano complexes would be generated from the reaction of [Fe(CN)6]3–/4– and Cd2+ or Zn2+ ion derived from photocorrosion during photocatalytic reaction. Rubin et al. have reported that such hexacyano complex formed on CdS electrode efficiently scavenged photogenerated holes, leading to increased rate of oxidation of [Fe(CN)6]4– and also suppression of photocorrosion of CdS. To improve the activity for H2 evolution, the hexacyano complex K2[CdFe(CN)6] prepared in advance, was loaded onto ZnIn2S4 and CdIn2S4 particles. The loading indeed improved the H2 evolution rate in both cases (ZnIn2S4: 4.8 →15 µmol/h, CdIn2S4: 0.8 → 6 µmol/h). Finally, we demonstrated that combination of metal sulfides as H2-evolving photocatalyst and O2 evolution system (TaON photoanode3)) could split water stably into H2 and O2 under visible light irradiation (Fig. 2).