Srco3

All chemicals were used directly without any further purification. SrCO₃ (AR, ≥99%) and isopropanol (AR, ≥99.7%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Dimethyl sulfoxide (DMSO, ≥99.9%) was purchased from Shanghai Macklin Biochemical Co., Ltd. CuO (AR, 99%) and WO₃ (AR, 99.8%) were purchased from Shanghai Aladdin Biochemical Technology. Nafion 117 solution (5 wt%) and D₂O (99.9 atom% D) were purchased from Sigma-Aldrich Biochemical Technology. HNO₃ (AR, 65–68%), H₂O₂ (AR, 30%), HF (AR, ≥40%), and HCl (AR, 36–38%) were purchased from Xilong Scientific. High-purity CO₂ gas (99.999%), Ar gas (99.999%), and 10 vol% H₂–Ar gas (99.999%) were purchased from Qingdao Dehaiweiye Technology Co., Ltd.

Ta foils (10 mm × 15 mm × 0.2 mm, ZhongNuo Advanced Material Co., 99.95%) were cleaned in ethanol and acetone. SrTaO₂N films were prepared on a Ta foil substrate by a reactive inorganic vapor deposition method under a flow of NH₃. Briefly, 0.2 g of SrCl₂/SrCO₃ eutectic salt, which consisted of 26 mol% SrCO₃ (Sinopharm Chemical Reagent Co., 99.5%) and 74 mol% SrCl₂·6H₂O (Sinopharm Chemical Reagent Co., 99.5%), was placed in an alumina combustion boat. Ta foils were put on the top of the alumina combustion boat in a sealed quartz tube furnace. The Ta foils were then annealed at 950°C for 2 hours under a flow of ammonia gas (flow rate: 250 sccm) at a rate of 10°C/min. The samples were then cooled to room temperature with the flow of ammonia gas. After the ammonolysis process, the obtained SrTaO₂N photoanodes were washed with sufficient deionized water and dried overnight in air before testing. BaTaO₂N and CaTaO₂N films were synthesized by using the corresponding flux agents of BaCl₂ (74%)/BaCO₃ (26%) and CaCl₂ (74%)/CaCO₃ (26%), respectively. SrNbO₂N and BaNbO₂N films were synthesized at the same condition by using Nb foil as a precursor and corresponding flux of SrCl₂ (74%)/SrCO₃ (26%) and BaCl₂ (74%)/BaCO₃ (26%), respectively.

The solid state method was applied to synthesize Sr_1−1.5xTb_xWO₄ (0 ≤ x ≤ 0.2) bulk samples. The starting materials, SrCO₃ (99.9%, Sinopharm Chemical Reagent Co., Ltd.), Tb₄O₇ (99.99%, Alfa Aesar), and WO₃ (99.5%, Sinopharm Chemical Reagent Co., Ltd.), were used after a pre-calcination at 800 °C to remove possible absorbed moisture or CO₂. Typically, for their synthesis, Sr_0.82Tb_0.12WO₄, SrCO₃ (2.62 mmol, 0.3870 g), Tb₄O₇ (0.10 mmol, 0.0717 g), and WO₃ (3.20 mmol, 0.7412 g) were homogenized using an agate mortar, followed by preheating at 800 °C for 10 h. The resultant powder was re-ground thoroughly by hand. Finally, it was heated at 1000 °C for another 15 h in air.