Cs2co3

Graphite (SP-1) was purchased from BAY CARBON Inc. KMnO₄, H₂SO₄, H₂O_2, Cs₂CO₃, hydrazine hydrate, and K₂Cr₂O₇ were purchased from Wako Pure Chemical Industries, Ltd. Benzylalcohol and 1,2-dichloroethane were purchased from Tokyo Chemical Industry Co. All reagents were used directly without further purification.

Raw materials Cs₂CO₃ and WO₃ of 99.9% purity were purchased from Fujifilm Wako Pure Chemical Corporation (Tokyo, Japan), and methyl isobutyl ketone (MIBK) of 99.5% purity was purchased from Kanto Chemical Co., Inc. (Tokyo, Japan).
As the starting material, Cs₄W₁₁O₃₅ was selected because its Cs/W ratio was close to that of Cs_0.32WO_3−y. Cs₄W₁₁O₃₅ was prepared by heating the Cs₂CO₃ and WO₃ raw materials in air at 800 °C for 30 hours or at 850 °C for 5 hours, respectively. The X-ray diffraction (XRD) powder pattern revealed the primary Cs₄W₁₁O₃₅ phase with a minutely mixed Cs₆W₁₁O₃₆ phase.

Surface-treated WO₃ for Z-scheme water splitting was prepared in the following procedure^{42 (link),43 (link)}. PtO_x (0.5 wt% as Pt) was loaded on WO₃ (99.99%, Kojundo Chemical Laboratory Co., Ltd.) by immersing WO₃ powder in an aqueous H₂PtCl₆ solution, followed by calcination in air at 823 K for 0.5 h. The obtained PtO_x-loaded WO₃ was then impregnated in a Cs₂CO₃ (97%, FUJIFILM Wako Pure Chemical Industries, Ltd.) solution with a molar ratio of Cs to W of 1%. After calcination in air at 773 K for 10 min, the Cs⁺-treated and PtO_x-loaded WO₃ were soaked in 1 M H₂SO₄ solution with vigorous stirring for 1 h in order to adequately induce ion-exchange reactions. Finally, the resulting powder was collected by filtration and dried in air at room temperature overnight.