Overall Water-Splitting Photocatalyst, Silver-Inserted Zinc Rhodium Oxide and Bismuth Vanadium Oxide, Sensitive to Red Light
Authors: Irie Hiroshi , Kobayashi Ryoya , Takashima Toshihiro , Ohtani Bunsho 
Published: February 2020

Since Fujishima and Honda reported photoelectrochemical water splitting using a TiO2 electrode in 1972 [1], solar hydrogen (H2) production from water using a photocatalyst has attracted attention as a clean energy resource. Most recent researches have been focusing on the visible-light sensitization of catalysts to effectively utilize solar energy. Very recently, we have developed a novel solid-state photocatalyst by inserting silver (Ag) as an electron mediator between zinc rhodium oxide (ZnRh2O4, Eg of 1.2 eV) and silver antimonite (Ag1-xSbO3-y, Eg of 2.7 eV) as H2 and O2 photocatalysts, respectively (ZnRh2O4/Ag/Ag1-x
SbO3-y
) [2]. In this system, we achieved overall pure-water splitting photocatalyzed via the inserted Ag, which could transfer photoexcited electrons from the conduction band (CB) of Ag1-x
SbO3-y
to the valence band (VB) of ZnRh2O4. The ZnRh2O4/Ag/Ag1-x
SbO3-y
utilized visible light up to 545 nm depending on the photoabsorption capability of the Ag1-x
SbO3-y
. Therefore, to improve the photosensitivity of this system at longer wavelengths of light, we searched for a new, smaller-bandgap O2 photocatalyst to replace Ag1-x
SbO3-y
, and we found that bismuth vanadium oxide (Bi4V2O11), which has an Eg of 1.6–2.2 eV, is a promising material [3, 4].
ZnRh2O4 and Bi4V2O11 powders were synthesized using a solid-state reaction method. Commercial available zinc oxide (ZnO) and rhodium oxide (Rh2O3) powders were used for ZnRh2O4, and bismuth oxide (Bi2O3) and vanadium oxide (V2O5) powders were used for Bi4V2O11 as the starting materials. Stoichiometric amounts of the starting materials for both materials were calcined at 1000°C for 24 h and 850°C for 8 h to obtain ZnRh2O4 and Bi4V2O11 powders, respectively. Then, the obtained powders were thoroughly ground. We also prepared Bi4V2O11 powders obtained by pulverizing Bi4V2O11 single crystals, for which a melting-slow cooling method was applied to grow them. The stoichiometric mixtures of Bi2O3 and V2O5 were melted at 940oC and slow-cooled at the rate of 4oC/h to 740oC [5]. Single crystals obtained by this method were thin plate-like sheets. Such thin sheets were pulverized to provide experiments (denoted by s-Bi4V2O11, and the powder obtained by the conventional solid-state reaction is denoted by p-Bi4V2O11).
Powdered photocatalysts composed of Ag, ZnRh2O4 and s-Bi4V2O11 or p-Bi4V2O11 (ZnRh2O4/Ag/s-Bi4V2O11 or ZnRh2O4/Ag/s-Bi4V2O11, respectively) were prepared using the following method. Ag2O, ZnRh2O4, and s-Bi4V2O11 or p-Bi4V2O11 powders were mixed and the mixed powders were pressed into pellets, and then the pellets were heated at 750°C for 2 h. After grinding the pellets into fine powders, the powders were soaked in nitric acid aqueous solution. The powders were subsequently filtered then washed with a sufficient amount of distilled water and dried at 65°C for 12 h.
To examine the activity of s-Bi4V2O11and compare its activity with p-Bi4V2O11 as the O2 evolution photocatalyst, O2 evolution originated from the half reaction of water over s-Bi4V2O11 and p-Bi4V2O11was evaluated using Ce4+ as the sacrificial agent. The apparent quantum efficiency (AQE) values of s-Bi4V2O11 were ~2–3 times larger than those of p-Bi4V2O11. It was presumably considered that s-Bi4V2O11 had higher crystallinity and anisotropy than p-Bi4V2O11, causing the enhanced mobility and separation of photogenerated electrons and holes.
ZnRh2O4/Ag/s-Bi4V2O11 was able to photocatalyze overall pure-water splitting under visible light with a wavelength of up to 740 nm, whereas ZnRh2O4/Ag/p-Bi4V2O11 was with the wavelength of up to 700 nm. Through the use of s-Bi4V2O11 in place of p-Bi4V2O11, the AQE for overall water splitting was increased and the sensitivity of the system was increased to wavelengths up to 740 nm. On the basis of the analogy with a solid-state heterojunction photocatalyst composed of ZnRh2O4, Ag1-x
SbO3-y
, and Ag [2], we consider that the overall water-splitting performance of the ZnRh2O4/Ag/s-, p-Bi4V2O11 photocatalyst was derived from the photoproduced holes that were generated in the VB of s-, p-Bi4V2O11 contributing to O2 liberation and the photoexcited electrons that were generated in the CB of ZnRh2O4 contributing to H2 liberation. Importantly, Ag acts as a solid-state electron mediator for the transfer of electrons from the CB of s-, p-Bi4V2O11 to the VB of ZnRh2O4. We also achieved the overall pure-water splitting under visible light over gold (Au)-inserted photocatalyst instead of Ag, ZnRh2O4/Au/p-Bi4V2O11, which will be introduced at the conference.
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