1：Experimental Design and Method Selection:

A two-step anodization method was used. First, TiO2 nanotubes were fabricated by conventional anodization in an ethylene glycol electrolyte with ammonium fluoride and de-ionized water at temperatures from 2°C to 60°C and 50 V for 30 minutes. Second, doping was performed by applying a negative voltage of 50 V for 20 seconds to the nanotubes in a 0.5 M aluminium sulphate electrolyte.

2：5 M aluminium sulphate electrolyte. Sample Selection and Data Sources:

2. Sample Selection and Data Sources: Ti foil was used as the anode and cathode. Samples were prepared at various bath temperatures (2°C to 60°C).

3：List of Experimental Equipment and Materials:

Equipment included a Carl Zeiss Sigma FESEM for morphology, RBS for compositional analysis, Bruker D8 Advance XRD for structural properties, Joel JEM 2100 HR-TEM, WITec Alpha 300 RA Raman spectrometer, diffuse reflectance spectrometer for optical measurements, and Keithley 2450 source meter for electrical measurements. Materials included Ti foil, ammonium fluoride, ethylene glycol, de-ionized water, and aluminium sulphate.

4：Experimental Procedures and Operational Workflow:

Anodization was conducted at specified temperatures and voltages, followed by doping. Samples were characterized using FESEM, RBS, XRD, TEM, Raman spectroscopy, reflectance spectrometry, and conductivity measurements.

5：Data Analysis Methods:

Data were analyzed using SIMNRA for RBS fitting, XRD patterns for crystallinity, TEM and SAED for structural confirmation, Raman spectra for phase identification, Kubelka-Munk function for band gap calculation, and conductivity measurements from voltage-current data.