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
Authors: Higashi Masanobu , Shirakawa Takashi , Matsuoka Hikaru , Tomita Osamu , Nakada Akinobu , Abe Ryu 
Published: February 2020

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. K2Cd[Fe(CN)6], which is one of metal hexacyanoferrates (MHCF), was formed on the CdS surface via photocorrosion and suppressed further photocorrosion. Additionally, influence of other MHCFs (M = In, Zn, Cu, Ag) on photocatalytic H2 evolution over ZnIn2S4 was investigated.
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 mmol in the unbuffered aqueous solution. However, the production of nearly the twice amount (ca. 53.6 mmol) 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, 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 H2 evolution 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 mmol/h). On the other hand, Pt/CdIn2S4 showed quite low activity (0.8 mmol/h). ATR-FTIR analysis revealed that MHCF, such as K2Cd[Fe(CN)6]1 and K2Zn[Fe(CN)6]2 were formed on Pt/CdS and Pt/ZnIn2S4, respectively, after each reaction, both of which showed appreciable H2 evolution. These hexa-cyanometallates would be generated from the reaction of [Fe(CN)6]4– and Cd2+ or Zn2+ ion derived from photocorrosion during photo-catalytic reaction. Rubin et al. have reported that such MHCF 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 hexa-cyanoferrate 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 mmol/h, CdIn2S4: 0.8 → 6 mmol/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.
We investigated influence of other MHCFs (M = In, Zn, Cu, Ag) on photocatalytic H2 evolution from water over ZnIn2S4 in the presence of [Fe(CN)6]4­–­­ as an electron donor. The modification with ZnHCF or InHCF was found to improve the activity as well as that of CdHCF, as shown in Fig. 1. In contrast, the modification with AgHCF or CuHCF showed no photocatalytic H2 evolutoin. We found that their activities strongly depend the redox properties of MHCFs. As for ZnHCF, InHCF, and CdHCF, which show higher activity, stable redox currents derived from Fe3+/Fe2+ in MHCFs were observed around 0.72 ~ 0.99 VSHE. These MHCFs effectively scavenged photogenerated holes and thus improved H2 evolution. On the other hand, AgHCF showed irreversible behavior. The intensity of peaks corresponding to both the oxidation and reduction drastically decreased after the 2nd cycle, indicating some irreversible reactions occurred. Such irreversible species were formed on the surface, resulting in suppression of the oxidation reaction. As for CuHCF, a reversible redox wave for Fe3+/Fe2+ was observed at 0.94V, whereas an irreversible peak, which probably corresponds to the reduction of Cu2+ included in CuHCF, also appeared at –0.2 VSHE. Photoexcited electrons in ZnIn2S4 were consumed for reduction of Cu2+, instead of H2 evolution.

References

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Figure 1