Chromia Photodeposition on a Gold-Inserted Iron Disilicide and Rutile Titanium Dioxide Heterojunction Photocatalyst and Its Water-Splitting Reaction

A solid-state Z-scheme system is constructed using a sputtering method in which rutile titania (TiO2) and beta-iron disilicide (β-FeSi2), which act as oxygen (O2)- and hydrogen (H2)-evolution photocatalysts, respectively, are combined with gold (Au), which acts as a solid electron mediator (β-FeSi2/Au/TiO2, described as BAT). Platinum (Pt) is selectively deposited onto β-FeSi2 in BAT using the photoexcitation of only β-FeSi2 to prepare Pt-loaded β-FeSi2/Au/TiO2 (Pt/BAT), followed by selective loading of chromium oxide (CrOx) onto Pt (K. Akiyama et al., ACS Omega 2022, 7, 38744–38751). In this study, we attempted that CrOx was photodeposited onto both TiO2 and Pt on β-FeSi2 in Pt/BAT by the photoexcitation of both TiO2 and β-FeSi2 to prepare Pt/BAT/CrOx and that it was only onto Pt on β-FeSi2 in Pt/BAT by the photoexcitation of only β-FeSi2 to prepare CrOx/Pt/BAT. We confirmed that CrOx was precipitated on the surface of not only the β-FeSi2 grain but also TiO2 around the β-FeSi2 grain in Pt/BAT/CrOx and that it was only on the surface of the β-FeSi2 grain in CrOx/Pt/BAT. The Pt/BAT/CrOx with 2 wt % CrOx photocatalyst greatly enhanced the overall water-splitting activity (more than 10 times) compared with CrOx/Pt/BAT with 0.4 wt % CrOx. In addition, the overall water-splitting activity over Pt/BAT/CrOx with 0.8 wt % CrOx was demonstrated to be approximately twice as high as that of CrOx/Pt/BAT with 0.8 wt % CrOx. These observations indicated that CrOx on the surface of TiO2 around the β-FeSi2 grain attributed to the enhancement of the overall water-splitting activity. This is possibly owing to the enhancement of the combination of the hole of the valence band in β-FeSi2 and the electron of the conduction band in TiO2 at the heterojunction interfaces by suppressing the contact of photoexcited carriers with water via β-FeSi2–Au and Au–TiO2 interface and by decreasing the recombination of them in β-FeSi2 and TiO2.