1：Experimental Design and Method Selection:

A flux growth method was used to synthesize Ba5Ta4O15 precursor crystals with various morphologies and sizes by employing different fluxes (BaCl2, KCl, RbCl, CsCl, KCl+BaCl2, K2SO4) and solute concentrations (1-50 mol%). These precursors were then nitrided at 950°C for 20 h under NH3 flow with and without KCl flux to produce BaTaO2N crystal structures. The study aimed to compare the effects on water oxidation and photoelectrochemical performance.

2：Sample Selection and Data Sources:

Samples were prepared based on specific flux types and solute concentrations as detailed in Table 1 of the paper. Characterization included XRD, SEM, TEM, UV-vis spectroscopy, and performance tests for photocatalytic water oxidation and photoelectrochemical measurements.

3：List of Experimental Equipment and Materials:

Equipment included a MiniflexII XRD diffractometer (Rigaku), JSM-7600F SEM (JEOL), EM-002B HR-TEM (TOPCON), V-670 UV-vis-NIR spectrophotometer (JASCO), JEM-2800 TEM, gas chromatograph (GC-8A, Shimadzu), potentiostat (HSV-120, Hokuto), RF magnetron sputtering system, and light sources (300 W Xe lamp, AM 1.5G solar simulator). Materials were purchased from Wako Pure Chemical Industries, Ltd., including BaCO3, Ta2O5, various fluxes, Co(NO3)2·6H2O, AgNO3, La2O3, and others.

4：5G solar simulator). Materials were purchased from Wako Pure Chemical Industries, Ltd., including BaCO3, Ta2O5, various fluxes, Co(NO3)2·6H2O, AgNO3, La2O3, and others. Experimental Procedures and Operational Workflow:

4. Experimental Procedures and Operational Workflow: Precursor synthesis involved mixing solutes and fluxes, heating in a platinum crucible at 1000°C for 10 h, cooling, washing, and drying. Nitridation was performed at 950°C for 20 h under NH3 flow, with optional KCl flux. Photocatalytic tests used a side-irradiation reactor with CoOx-loaded samples, AgNO3 sacrificial agent, and La2O3 pH buffer. Photoelectrochemical measurements involved preparing photoanodes by particle transfer, sputtering Ti/Ta layers, and using a three-electrode system with linear sweep voltammetry.

5：Data Analysis Methods:

XRD patterns were compared with ICDD database, SEM and TEM images were analyzed for morphology and size using Photo Measure software, UV-vis data were converted using Kubelka-Munk method, photocurrent and O2 evolution rates were measured and compared statistically.