## 3 Materials and Methods
### Photocatalytic Efficiency (PE) Measurement
#### 3.4.3 UV-Vis by Immersing Pollution  
UV-Vis analysis was performed to evaluate the degradation rate of RhB dye in aqueous dispersion by measuring the decolorization of the solution. This was achieved by absorbance spectroscopy, specifically by monitoring the maximum absorbance at a wavelength of 554 nm (the absorbance peak of RhB). The selection of 554 nm as the reference wavelength was based on its stability and high intensity, ensuring a reliable evaluation of RhB degradation [38].  
The process was divided into two phases. In the first, each sample was immersed in the RhB solution in the dark to achieve adsorption/desorption equilibrium. At intervals of 1, 2, 4, and 6 h and at 24 h, 3 mL of each dispersion was collected, and the absorbance was measured by spectrophotometry UV-3101 PC (Shimadzu, Germany, Europe), wavelengthrange 380-800 nm). The solution was returned to the beaker after each measurement (Figure 12a).  
Before measurement, the spectrophotometer was calibrated using distilled water as a reference to establish a baseline correction, ensuring accurate and reliable absorbance readings. In the second phase (Figure 12b), the samples were irradiated in the same irradiation system used for the colorimetry analysis, maintaining a distance of 29 cm between the sample surfaces and the light source, with a light power density equal to 7.3 W/m\({}^{2}\), measured by a Quantum Photo Radiometer HD9021 (Delta-Ohm, Padova, Italy).  
During irradiation, a transparent plastic film covered the containers with RhB solution and samples to prevent evaporation. The film, transmitting over 90% of light in the 292-900 nm range, minimally affected the light wavelength and intensity required for photocatalysis.  
At 60, 180, and 540 min of irradiation, 3 mL of RhB solution of each beaker was collected and their absorbance was measured in the wavelength range of 380-800 nm with a Shimadzu UV-310 PC spectrophotometer. Subsequently, the PE was determined using the maximum absorbance value at a wavelength of 554 nm, applying the Beer-Lambert law. According to the Beer-Lambert law, at low concentrations of a dye in solution, its absorbance is directly proportional to its concentration [38]. Therefore, the dye absorption values at 554 nm were used to calculate the PE of the photocatalytic coatings according to Equation (2):  
\[\Phi[\%]\ =\ \left(\frac{\mathrm{A_{0}-A}}{\mathrm{A_{0}}}\right)\times 100 \tag{2}\]  
where \(\Phi\) is the photocatalytic efficiency [%], and \(\mathrm{A_{0}}\) and \(\mathrm{A}\) correspond to the maximum absorbance of RhB in aqueous dispersion at time zero and at times T = 60, 180, and 540 min. Time zero (\(\mathrm{A_{0}}\)) was considered as the moment when light irradiation began (end of dark condition). The change in RhB concentration could be evaluated by measuring the change in the intensity of its main absorption peak. A decrease in the absorbance of the solution indicated the dye decomposition rate photocatalyzed by TiO\({}_{2}\) nanoparticles as measured at the wavelength of 554 nm.  
Finally, after calculating the PE using the three different techniques, the linear relationship between them was calculated and discussed.