Gold Amount Dependence of Red Light Responsive Z-Scheme Photocatalyst on Water-Splitting Activity Using Gold Prepared By Sputtering in Ionic Liquid
Authors: Yoda Masaomi , Takashima Toshihiro , Akiyoshi Kazutaka , Torimoto Tsukasa , Irie Hiroshi 
Published: December 2020

Since the Fujishima effect was reported in 1972 [1], researches on photocatalytic water splitting using sunlight have been progressing as a method for producing hydrogen (H2), which attracts attention as an alternative clean energy source for fossil fuels. For the effective use of sunlight, we have investigated an all-solid-state Z-scheme photocatalyst that can be sensitive to long-wavelength, low-energy light and can decompose pure water (without chemicals as a sacrificial agent or pH adjuster). In our previous study, we fabricated the Z-scheme photocatalyst, consisting of zinc rhodium oxide (ZnRh2O4) and bismuth vanadium oxide (Bi4V2O11) as a H2- and oxygen (O2)-evolution photocatalyst, respectively, and gold (Au) as a solid redox mediator (ZnRh2O4/Au/Bi4V2O11). The photocatalyst has successfully achieved the overall pure water splitting irradiated with red light (wavelength 700 nm) via Au, which mediated the transfer of photoexcited electrons from the conduction band (CB) of Bi4V2O11 to the valence band (VB) of ZnRh2O4. Then, photogenerated holes in the VB of Bi4V2O11 and photoexcited electrons in the CB of ZnRh2O4 effectively oxidized and reduced water, respectively, to achieve overall water splitting [2].

One of the co-authors (T.T.) reported a simple technique using a sputtering method to synthesize Au nanoparticles (Ausp), much smaller than commercially available Au [3]. In the synthesis procedures described below, some part of Ausp, supported only on the surface of ZnRh2O4, functions as a co-catalyst for H2 evolution. Thus, it was expected that Ausp could be homogeneously distributed as the redox mediator and co-catalyst, resulting in the improvement of the water splitting reaction. In this study, various concentrations of Ausp in ZnRh2O4/Ausp/Bi4V2O11 were prepared, and the dependence of Ausp concentration on water-splitting activity was evaluated.

Ausp were evenly deposited by sputtering onto an ionic liquid (PP13-TFSI; N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide). Before performing the process, treatments of heating and stirring were performed in a vacuum atmosphere so as to extract air and water from the ionic liquid. ZnRh2O4 and Bi4V2O11 powders were prepared by a solid-state reaction method. ZnO and Rh2O3 powders, and Bi2O3 and V2O5 powders as starting materials were uniformly mixed in a stoichiometric ratio to prepare ZnRh2O4 and Bi4V2O11, respectively. The mixture was pelletized and calcined at 1150 °C for ZnRh2O4 for 24 h and 850 °C for Bi4V2O11 for 8 h, then pulverized to obtain ZnRh2O4 and Bi4V2O11 powders. ZnRh2O4 were added to the prepared Ausp containing in PP13-TFSI and the mixture was heated and stirred in a nitrogen atmosphere in a test tube for 280 °C for 1 h (the expected molar percentage of Au to ZnRh2O4 was 2, 10, 12, and 17 mol%). Next, PP13-TFSI was removed by centrifugation and drying overnight to produce ZnRh2O4+Ausp. ZnRh2O4/Ausp was obtained by calcining the ZnRh2O4+Ausp at 850 ° C for 2 h to strengthen the connection. ZnRh2O4/Ausp/Bi4V2O11 was produced by mixing Bi4V2O11 with ZnRh2O4/Ausp at a ratio of 20 mol% with respect to ZnRh2O4 and following the same procedure as for producing Bi4V2O11.

As-prepared Ausp size distribution was determined by measuring the size of individual particles in the TEM image, and the average particle size was 2.3 nm. The average particle size of Ausp containing in ZnRh2O4+Ausp, ZnRh2O4/Ausp and ZnRh2O4/Ausp/Bi4V2O11 after heat treatment was 9.0 nm, 27 nm, and 42 nm. Thus, it was confirmed that Ausp could be used in the Z-scheme photocatalyst with the particle size much smaller than that of the commercial Au (300-500 nm) [2]. The amount of Au containing in ZnRh2O4/Ausp/Bi4V2O11 was estimated from XPS. Since Ausp is mainly bonded to the surface of ZnRh2O4, we estimated the Ausp amount based on the Bi 4f peak of Bi4V2O11, which is unlikely to be affected by the change of Ausp amount when measured by XPS. Then, the estimated Ausp amounts were 12, 18, 20, and 29 mol% (vs. 1.0 mol of ZnRh2O4 + 0.2 mol of Bi4V2O11). All photocatalysts were confirmed that H2 and O2 were generated from pure water at a ratio of approximately 2 : 1 by irradiation of red light with a wavelength of 700 nm. The water splitting activity tended to be proportional to the amount of Ausp estimated from XPS. These will be discussed in detail at the conference.

[1] A. Fujishima and K. Honda, Nature, 238, 37 (1972)

[2] K. Kamijyo, T. Takashima, M. Yoda, J. Osaki, and H. Irie, Chem. Commun., 54, 7999 (2018)

[3] T. Torimoto, K. Okazaki, T. Kiyama, K. Hirahara, N. Tanaka, and S. Kuwabata, Appl. Phys. Lett., 89, 243117 (2006)