NiFe Prussian blue analog cocatalyzed TiO2/In2S3 type-II heterojunction for solar water splitting

Due to the excellent stability of titanium dioxide (TiO2), there is still value in improving its solar-to-hydrogen conversion efficiency through tremendous attempts. Metal sulfides with a narrow bandgap are good candidates to broaden the ultraviolet light absorption of TiO2 into the visible light region. However, sulfides suffer from the photocorrosion issue, leading to poor stability. Herein, a type-II heterojunction of TiO2/In2S3 is fabricated by a hydrothermal method, and a NiFe Prussian blue analog (NFP) overlayer is deposited on the surface of TiO2/In2S3 through a chemical bath deposition technique. Under AM1.5G illumination, a photocurrent density of 1.81 mA cm-2 can be obtained with NFP coated TiO2/In2S3 at 1.23 V vs. reversible hydrogen electrode, which is six folds of the photocurrent of TiO2. This photocurrent value can reach up to about 90% of its theoretical photocurrent. During a 12 h stability test, the TiO2/In2S3/NFP photoanode exhibits a high photocurrent retention of 95.17% after an initial transient decrease. The type-II heterojunction of TiO2/In2S3 can efficiently boost the charge separation because of the built-in electric field and enhance the visible-light absorption because of the narrow bandgap of In2S3. A NFP overlayer serves as the cocatalyst for water oxidation reaction due to its valence changes of nickel and iron elements. NFP cocatalyst can rapidly extract the photogenerated holes from In2S3 and then improve the charge separation/injection efficiencies. Thanks to chemical stability of NFP, its coating can also make In2S3 resistant to photocorrosion by physically separating the photoanode from the electrolyte. Therefore, there is a good synergistic effect between the TiO2/In2S3 heterojunction and NFP cocatalyst. This work provides some crucial insights for the interface engineering and material design in photoelectrochemical systems.