## 3 Results and Discussion
### Photocatalytic Activity Evaluation  
The photocatalytic activity of the prepared photocatalyst samples was evaluated via measuring the photodegradation efficiency of a simulated MB dye wastewater under light irradiation using a xenon lamp as a visible light source, over a period of 120 min (Figure 10). MB dye photodegradation was also tested by using pure TiO\({}_{2}\) as a reference sample. It can be seen from Figure 10 that pure TiO\({}_{2}\) sample under light irradiation shows negligible MB dye degradation (6.46%), while for NT samples the MB dye degradation performance follows the trend NT-3 > NT-2 > NT-1. Hence the N doping of pure TiO\({}_{2}\) by urea addition as well as anatase phase enrichment enhances the photocatalytic performance and increases the photodegradation efficiency from 6.46% for pure TiO\({}_{2}\) to greater than 99% for NT-3 samples. Such an enhanced performance can be attributed to the extension of light absorption towards the visible region induced by the energy states created by  
Figure 8: High resolution XPS spectra with peak fittings for pure TiO\({}_{2}\) sample showing: (**a**) Ti 2p, (**b**) O 1s, (**c**) N 1s, and (**d**) C 1s, regions. nitrogen doping. Such energy states created as a result of doping, narrow down the band gap of the photocatalysts, resulting in generation of increased photo-excited charges under light irradiation. In addition to nitrogen doping, enrichment of the anatase phase with increased urea amount also contributes to the photocatalytic performance. As a control test, photocatalytic dye degradation employing a standard TiO\({}_{2}\) nanopowder (Degussa P25) was performed. The dye degradation absorption spectra and relative concentration profile are shown in Figure S2. It can be observed that P25 samples showed better performance (approximately 24% degradation after 120 min.) as compared to the pure TiO\({}_{2}\) sample but less efficient than the N-doped TiO\({}_{2}\) samples. Figure 10: Photocatalytic degradation of MB dye simulated wastewater by pure TiO\({}_{2}\), NT-1, NT-2 and NT-3 samples. Figure 9: High resolution XPS spectra with peak fittings for pure NT-3 sample showing: (**a**) Ti 2p, (**b**) O 1s, (**c**) N 1s, and (**d**) C 1s, regions. Furthermore, as is well-established, the photodegradation of MB dye is influenced by various parameters, such as nanoparticle size, crystallinity, phase composition, porous nature and surface area. It can be observed that amongst all samples, NT-3 sample exhibited the best performance with the lowest surface area, highest nitrogen doping and highest anatase phase. Such an increased performance might be attributed to these two effects i.e., more anatase phase in the NT samples and increased light absorption due to N doping of TiO\({}_{2}\). It is well established that MB dye is considered a basic dye due to presence of cationic groups on its surface such as -NH3\({}^{+}\). The affinity for adsorption for MB dye is enhanced towards anatase TiO\({}_{2}\) phase, which is rich in surface OH groups [43; 44]. Therefore, when the amount of urea used as a dopant is increased, the anatase phase is enriched along with N doping, which leads to enhanced and better MB dye adsorption (in dark) as compared to a pure TiO\({}_{2}\) sample. Hence, an optimal combination of the anatase phase and N doping might lead to the formation of photocatalysts with best performance. The proposed mechanism of MB dye photodegradation by the prepared samples is shown in Figure 11. Upon light irradiation, the photogenerated electrons jump to the conduction band whereas holes remain in the valence band. The photogenerated charges (electrons and holes) generate hydroxyl radicals by reacting with oxygen and water, respectively, which in turn acts as a strong oxidizing agent, degrading MB dye to other products such as CO\({}_{2}\), SO\({}_{4}\)\({}^{2-}\), NH\({}_{4}\)\({}^{+}\) and NO\({}_{3}\)\({}^{-}\). The possible reactions involved in the photodegradation of the MB dye simulated textile wastewater are presented in Equations (3)-(7).