1：Experimental Design and Method Selection:

The study employed sol–gel synthesis for Fe-doped TiO2 nanoparticles with varying Fe:Ti molar ratios (

2：

1, 1:1, 5:1, 10:1). An acid-treatment step using HCl was introduced to remove iron oxide contamination. Photocatalytic degradation of phenol under visible light was assessed using a Langmuir–Hinshelwood kinetic model.

3：1). An acid-treatment step using HCl was introduced to remove iron oxide contamination. Photocatalytic degradation of phenol under visible light was assessed using a Langmuir–Hinshelwood kinetic model. Sample Selection and Data Sources:

2. Sample Selection and Data Sources: Phenol was used as the pollutant representative. Nanoparticles were synthesized with titanium isopropoxide and iron(III) nitrate nonahydrate as precursors. Deionized water was used for synthesis.

4：List of Experimental Equipment and Materials:

Equipment included XRD (Empyrean, Panalytical), XPS (Omicron XPS, Scienta Omicron), EDX (S-4800, Hitachi), HRTEM (HF-3300v, Hitachi), UV–Vis spectrometers (LAMBDA 1050, PerkinElmer; Cary 100, Agilent), and a 300 W xenon lamp (Cermax? Xenon, Excelitas). Materials included TTIP, Fe(NO3)3·9H2O, HCl, NaOH, and deionized water.

5：Experimental Procedures and Operational Workflow:

Nanoparticles were synthesized via sol–gel, calcined at 400°C, and acid-treated with HCl. Photocatalytic reactions were conducted in a Pyrex reactor with phenol solution, adjusted pH, and visible light irradiation. Adsorption equilibrium was achieved in the dark before irradiation.

6：Data Analysis Methods:

Degradation efficiency was calculated from UV–Vis absorption. Kinetics were modeled using the Langmuir–Hinshelwood equation. Particle size was determined using the Scherrer equation from XRD data.