## 4 Conclusions  
In this study, a photocatalytic protocol testing TiO\({}_{2}\) nano-powder activity under visible light irradiation is proposed by using a batch, horizontal, rectangular, vis-LED equipped reactor with reflective walls. For this, N-doped TiO\({}_{2}\) powder was synthesized, fully characterized by XRD, micro-Raman, XPS, FESEM, TEM, and UV-Vis, and its photocatalytic  
Figure 13: Experimental results when following the proposed photocatalytic protocol in (**a**) degradation rate and (**b**) photocatalytic kinetics.  
behavior was studied. The results showed the powder consists mainly of crystalline anatase phase (62.1%) with the presence of a smaller percentage of rutile (37.9%). The mean nanocrystalline size is \(\sim\)14 nm and the powder is morphologically homogeneous, with a particle size ranging between 5-15 nm. Furthermore, the presence of the N dopant is verified and there was a significant reduction in the energy gap from 3.2 eV to 2.7 eV, which allows the catalyst to be active in visible light.  
The chosen basic factors affecting photocatalysis were examined in order to create an optimal protocol of photocatalytic testing. Nine experiments (I9) were conducted by applying the Taguchi statistical method, for the control of four factors (pollutant, photocatalyst/pollutant load (g/L), distance from the irradiation source (cm), irradiation time (total min/measuring gap time)), with three levels (options) each.  
The results revealed that the type of pollutant is the most important factor affecting photocatalysis. Among the three pollutants studied, Rhodamine B shows the greatest stability under visible light irradiation, focusing on the intrinsic catalytic performance excluding the dye-sensitized phenomena. Regarding the amount of catalyst, that is the second factor mostly affecting the photocatalytic process; 1 g catalyst per L pollutant was the most preferable for photocatalytic testing, taking into account the cost effect apart from the highest degradation rate. For the distance of the cell from the radiation source, affecting the photocatalysis by 11%, a 10 cm distance is preferred. For the irradiation time, 150 min total time with absorbance testing every 30 min was chosen. Radiation time affects the final degradation by only 8%.  
The proposed photocatalytic protocol led to the nano-sized N-TiO\({}_{2}\) catalyst degrading the RhB pollutant at 60% after 150 min of visible light irradiation.  
Conceptualization, M.-E.K. and E.A.P.; methodology, M.-E.K.; software, M.-E.K.; validation, M.-E.K. and E.A.P.; formal analysis, M.-E.K. and Z.N.; investigation, Z.N.; resources, E.A.P.; data curation, M.-E.K.; writing--original draft preparation, M.-E.K.; writing--review and editing, E.A.P.; visualization, M.-E.K.; supervision, E.A.P.; project administration, E.A.P. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable.  
For the XPS measurements, the authors would like to acknowledge [PERSON] from Surface Science Laboratory, Foundation of Research and Technology Hellas, Institute of Chemical Engineering and High Temperature Chemical Processes (FORTH/ICE-HT). The authors would also like to acknowledge [PERSON] and [PERSON] from the Institute of Nanoscience and Nanotechnology, National Center for Scientific Research \"Demokritos\", Agia Paraskevi, Greece, for the TEM analysis. [PERSON] would like to acknowledge Research Committee of N.T.U.A. for supporting her with a doctoral scholarship during the realization of this research. The authors declare no conflict of interest.