1：Experimental Design and Method Selection:

A template-assisted cation-exchange method was used, involving the synthesis of CdSe and CdSe1-xSx nanoplatelets as templates, followed by cation exchange with various metal halide precursors to produce heavy-metal-free nanoplatelets.

2：Sample Selection and Data Sources:

CdSe and CdSe1-xSx nanoplatelets were synthesized and used as templates; metal halide precursors included SnBr2, GeCl2, PbBr2, InCl3, BiI3, and HgCl

3：List of Experimental Equipment and Materials:

Equipment included a Philips X'pert diffractometer for XRD, JEOL 2100F TEM for microscopy, VG Escalab 220i-XL for XPS, Agilent ICP-OES, Shimadzu UV-2600 spectrophotometer, Edinburgh FLS980 for fluorescence, Metrohm Autolab PGSTAT302N for PEC measurements, Oriel LCS-100 solar simulator, Shimadzu GC-8A gas chromatographer. Materials included oleylamine, oleic acid, hexane, toluene, ethanol, fluorine doped tin oxide (FTO) glass, TiO2 paste (18 NR-AO, Dyesol), CTAB, ZnS for SILAR coating.

4：Experimental Procedures and Operational Workflow:

Synthesis involved degassing precursors, heating, adding template NPLs, cooling, and purification. PEC anode preparation included spin-coating TiO2, annealing, electrophoretic deposition of NPLs, CTAB treatment, ZnS coating. PEC measurements used a three-electrode system with specific electrolytes and illumination conditions.

5：Data Analysis Methods:

XRD and TEM for structural analysis, XPS and ICP-OES for compositional analysis, UV-vis and fluorescence spectroscopy for optical properties, PEC data analysis for photocurrent and hydrogen generation rates, EIS for charge transfer resistance using NOVA software.