(Invited) All-Solid-State Z-Scheme Overall Water-Splitting Photocatalysts for Effective Utilization of Solar Energy
Authors: Irie Hiroshi 
Published: December 2024

Since Fujishima and Honda reported photoelectrochemical water splitting using a TiO2 electrode in 1972 [1], solar hydrogen (H2) from water using a photocatalyst has attracted attention as a clean energy resource. Most recent research has been focusing on the visible-light sensitization of catalysts to effectively utilize solar energy. Our group has investigated water-splitting photocatalysts that are able to utilize the whole range of visible light and even infrared light on the basis of material designs and mechanistic approaches. We successfully created a solid-state hetero-junction photocatalyst, following a conventional well-known Z-scheme mechanism [2], but the present photocatalyst did not require any chemicals as a redox mediator. The solid-state hetero-junction photocatalyst is silver (Ag) inserted zinc rhodium oxide (ZnRh2O4 (ZRO), bandgap energy (E
g) of 1.2 eV) and bismuth vanadate (Bi4V2O11 (BVO), E
g of 1.7 eV), abbreviated to ZRO/Ag/BVO, where Ag, ZRO, and BVO act as an electron mediator, H2-evolution photocatalyst, and oxygen (O2)-evolution photocatalyst, respectively [3,4]. Overall water splitting, liberating H2 and O2 simultaneously from pure water at a stoichiometric ratio 2 to 1, was accomplished using ZRO/Ag/BVO irradiated with red light at wavelengths of up to 740 nm. This system functioned via the inserted Ag which could transfer photoexcited electrons from the conduction band (CB) of BVO to the valence band (VB) of ZRO. The photoexcited electrons in the CB of ZRO and photogenerated holes in the VB of BVO can effectively reduce and oxidize water, respectively, producing H2 and O2 at a molar ratio of 2 to 1.
In place of BVO, silver vanadium oxide (Ag2V4O11 (AVO), E
g of 1.4 eV) was utilized to directly connect to ZRO, forming ZRO/BVO. ZRO/BVO accomplished overall pure-water splitting under irradiation with near-infrared light at wavelengths of up to 910 nm. The key point without an electron mediator is that the CB potential of AVO is almost the same as the VB top potential of ZRO [5]. Thus, photoexcited electrons transfer smoothly from the CB of AVO to the VB of ZRO.
Notably, both ZRO/Ag/BVO and ZRO/AVO systems did not include a cocatalyst, which is inevitably required for the water splitting reaction due to the increase in the lifetime of electron-and-hole pairs and storing either electrons or holes. Thus, to enhance the water splitting activity, we tried to load a cocatalyst, platinum (Pt), Ag or copper (Cu), selectively onto the H2-evolution photocatalyst, ZRO, in ZRO/Ag/BVO to enhance the photosensitivity of this system. The loading of Pt, Ag, or Cu was performed by a photo-deposition method under light irradiation at wavelength longer than 850 nm. Under the irradiated light condition, only ZRO was photo-excited because the energy of irradiated light was higher than the E
g of ZRO (1.2 eV) but smaller than that of BVO (1.7 eV). Due to the specific photoexcitation of ZRO, Pt, Ag, or Cu was expected to be only photo-deposited onto this material, generating Pt/ZRO/Ag/BVO, Ag/ZRO/Ag/BVO, or Cu/ZRO/Ag/BVO. The deposition of Pt, Ag, or Cu 3-fold enhanced the H2 and O2 evolution rates compared to those of bare ZRO/Ag/BVO, demonstrating that Pt, Ag and Cu functioned as cocatalysts for the overall water-splitting reaction [6,7]. In addition, we discuss on the selective loading of a cocatalyst, cobalt oxide (CoOx), onto the O2-evolution photocatalyst, BVO, in ZRO/Ag/BVO and co-loading of Cu and CoOx on ZRO and BVO, respectively, at the conference.
This work was performed by Assoc. Prof. T. Takashima, Mr. M Yoda, and Mr. J. Osaki. I am deeply grateful to JSPS KAKENHI (Grant-in-Aid for Scientific Research) (B), Grant Number JP21H02043 for its financial support.
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