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 S^2– 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).

References

1. Rubin, H. D.; Arent, D. J.; Humphrey, B. D.; Bocarsly, A. B. Electrochem. Soc. 1987, 134, 93-101.

2. Reguera, E.; Gomez, A.; Balmaseda, J.; Contreras, G..; Escamilla, A. Struct. Chem., 2001, 12. 59-66.

3. Higashi, M.; Domen, K.; Abe, R. J. Am. Chem. Soc. 2012, 134, 6968–6971.