Selective Loading of Platinum or Silver Cocatalyst on a Hydrogen-Evolution Photocatalyst in an Overall Water-Splitting System, Silver-Inserted Zinc Rhodium Oxide and Bismuth Vanadium Oxide

Abstract

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. Recently, we have developed a novel solid-state photocatalyst by inserting silver (Ag) or gold (Au) as an electron mediator between zinc rhodium oxide (ZnRh2O4, Eg of 1.2 eV) and bismuth vanadate (Bi4V2O11, Eg of 1.7 eV) as H2- and oxygen (O2)-photocatalysts, respectively (ZnRh2O4/Ag/Bi4V2O11, ZnRh2O4/Au/Bi4V2O11) [2-4]. In this system, we achieved overall pure-water splitting, liberating H2 and O2 simultaneously from pure water at a stoichiometric ratio 2 to 1, photocatalyzed via the inserted Ag or Au which could transfer photo-excited electrons from the conduction band (CB) of Bi4V2O11 to the valence band (VB) of ZnRh2O4. The ZnRh2O4/Ag (Au)/Bi4V2O11 utilized visible light up to 740 nm. Notably, the ZnRh2O4/Ag/Bi4V2O11 system did not include a cocatalyst, which is inevitably required for the water-splitting reaction due to increasing the lifetime of electron-and-hole pairs and storing either electrons or holes. Thus, in the present study, we tried to load a cocatalyst, Ag or Pt, selectively onto the H2-evolution photocatalyst, ZnRh2O4, in ZnRh2O4/Ag/Bi4V2O11 to enhance the photosensitivity of this system.Powdered ZnRh2O4/Ag/Bi4V2O11 was prepared following the previously reported procedures [2]. The photo-deposition of Pt or Ag was performed under light irradiation at wavelength longer than 850 nm using hexachloroplatinic acid (H2PtCl6∙6H2O) or silver nitrate (AgNO3) solution as the source of Pt or Ag. Under these irradiated light conditions, only ZnRh2O4 was photo-excited because the energy of irradiated light was higher than the band-gap energy of ZnRh2O4 (1.2 eV) but smaller than that of Bi4V2O11 (1.7 eV). Due to the specific photoexcitation of ZnRh2O4, Pt or Ag was expected to be only photo-deposited onto this material, generating Pt/ZnRh2O4/Ag/Bi4V2O11 and Ag/ZnRh2O4/Ag/Bi4V2O11.To quantitatively determine the valencies of Pt and Ag, and analyze the amounts of Pt and Ag in the photocatalysts (bare ZnRh2O4/Ag/Bi4V2O11, Pt/ZnRh2O4/Ag/Bi4V2O11, and Ag/ZnRh2O4/Ag/Bi4V2O11), a peak deconvolution method was applied to the Bi 4f, Ag 3d, and Pt 4f spectra. Ag 3d peaks for the three photocatalysts were deconvoluted by one pair of Ag 3d peaks, Ag 3d5/2 and Ag 3d3/2, which were assigned to metallic Ag (Ag0). These results indicate that Ag was included as a metallic particle in the three photocatalysts. In contrast, the Pt 4f peaks for Pt/ZnRh2O4/Ag/Bi4V2O11 were deconvoluted by three pairs of Pt 4f peaks, attributable for metallic Pt (Pt0), Pt2+, and Pt3+ The atomic ratios of Pt0, Pt2+, and Pt3+ vs. total Pt were calculated to be 0.63, 0.25, and 0.12, respectively. Moreover, as we considered that the Bi4V2O11 surface remained unchanged even after the Pt or Ag deposition, we estimated the amount of Pt or Ag in the photocatalysts based on the amount of Bi. The weight percentage (wt%) of Ag in the three photocatalysts was calculated. As expected, the amount of Ag in ZnRh2O4/Ag/Bi4V2O11 and Pt/ZnRh2O4/Ag/Bi4V2O11 was identical at 2.1 wt%. The total amount of Ag in Ag/ZnRh2O4/Ag/Bi4V2O11 was calculated to be 3.2 wt%, thus the Ag cocatalyst deposited in Ag/ZnRh2O4/Ag/Bi4V2O11 was estimated to be 1.1 wt% based on the assumption that the amount of Ag inserted in Ag/ZnRh2O4/Ag/Bi4V2O11 was 2.1 wt%, identical to those in ZnRh2O4/Ag/Bi4V2O11 and Pt/ZnRh2O4/Ag/Bi4V2O11. Similarly, the amount of Pt cocatalyst in Pt/ZnRh2O4/Ag/Bi4V2O11 was calculated to be 1.2 wt%.The bare ZnRh2O4/Ag/Bi4V2O11, Pt/ZnRh2O4/Ag/Bi4V2O11, and Ag/ZnRh2O4/Ag/Bi4V2O11 photocatalysts evolved H2 and O2 from water at a molar ratio of ca. 2 to 1, indicating that the overall water splitting reaction proceeded. The deposition of Pt or Ag 3-fold enhanced the H2 and O2 evolution rates compared to those of bare ZnRh2O4/Ag/Bi4V2O11, demonstrating that Pt and Ag functioned as cocatalysts for the overall water-splitting reaction [5].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 17H03126 for its financial support.