1：Experimental Design and Method Selection:

The study involved doping Nb into TiO2 to reduce band gap energy and combining it with g-C3N4 to form a direct Z-scheme system for enhanced photocatalytic activity. Methods included sol-gel synthesis, calcination, and characterization techniques like XRD, XPS, UV-Vis, TEM, and photoelectrochemical analysis.

2：Sample Selection and Data Sources:

Samples included pristine TiO2, Nb-TiO2, g-C3N4, and various compositions of Nb-TiO2/g-C3N4 (e.g., 25%, 50%, 75% Nb-TiO2). Data were obtained from laboratory experiments and instrumental analyses.

3：2). Data were obtained from laboratory experiments and instrumental analyses. List of Experimental Equipment and Materials:

3. List of Experimental Equipment and Materials: Equipment included Bruker AXN XRD, Thermo Fisher ESCALAB 250Xi XPS, Hitachi U-4100 spectrophotometer, FEI Tecnai G2-F20 TEM, photoelectrochemical cell with Ag/AgCl and Pt electrodes, 1000W Xe lamp, gas chromatography unit (Agilent 7890B). Materials included tetra isopropyl orthotitanate, niobium ethoxide, melamine, ethanol, 2,4-pentanedione, dimethyl formamide, FTO conductive glass, CO2 gas.

4：Experimental Procedures and Operational Workflow:

Nb-TiO2 was prepared by sol-gel method with calcination at 500°C. g-C3N4 was synthesized from melamine calcination. Composites were made by grinding and calcining mixtures. Characterization involved XRD, XPS, UV-Vis, TEM. Photocatalytic reduction experiments used a reactor with CO2/H2O vapor under visible light, with product analysis by GC.

5：Data Analysis Methods:

Data were analyzed using Tauc plot for band gap estimation, Gaussian fitting for XPS, and calculation of electron/hole usage rates based on product generation.