## 3 Results and Discussion
### Optical Characterization
For 1.0 wt.% dopant concentration, the spectrum shows a large band centered at 689 cm\({}^{-1}\), which also may be assigned to the symmetric stretching mode of Nb-O polyhedra [15; 16] characteristic of the orthorhombic phase. Other modes present in the Mn-doped Nb\({}_{2}\)O\({}_{5}\) 1.0 wt.% sample are at 230 and 318 cm\({}^{-1}\), corresponding also to the T-Nb\({}_{2}\)O\({}_{5}\) phase (orthorhombic structure) [17]. In Figure 6b-d the Raman band around 881 cm\({}^{-1}\) can be related to the Mn ions vibration corresponding to the MnNb\({}_{2}\)O\({}_{6}\) phase, since its peak intensity increases with the increase of Mn concentration. The peak located at 264 cm\({}^{-1}\) in Figure 6d can be associated to T\({}_{2g}\) mode of vibration [13]. Peaks with relatively lower intensity shown in Figure 6b,c may respond to phonon scattering caused by small grains and point defects [13]. The Raman study reveals that an increase of Mn content causes a decrease in the crystallinity of material, which is evidenced in the change of peaks sharpness. This fact is supported by the results of Rietveld refinement. Figure 7 shows the evolution of reflectance by varying Mn concentration. A decrease in the reflectance is observed with an increase of Mn concentration. This can be explained in terms of lattice distortion due to the incorporation of the larger Mn ions inside the Nb\({}_{2}\)O\({}_{5}\) lattice. A considerable reduction in reflectance starts at about 600 nm, which suggests the formation of band tail due to incorporation of impurity states. A shoulder at about 400 nm for the Nb\({}_{2}\)O\({}_{5}\) is not distinguished for the doped samples. Through DRS it is possible to obtain the optical energy gap (\(Eg\)) values. [PERSON] and [PERSON] [18] proposed the theory that makes it possible to use DRS spectra. The Kubelka-Munk transformation relates the reflectance with the absorption coefficient, \(\alpha\). Thus, the band gap Eg and absorption coefficient of a direct band gap semiconductor are related in the parabolic band structure. The materials optical band gap, e.g., was extracted out of Equation (1). \[\alpha=\frac{B(h\
u-Eg)^{n}}{h\
u} \tag{1}\]  
Here, \(h\
u\) is the incident photon energy, \(\alpha\) is the absorption coefficient, B is a proportionality constant, and \(Eg\) is the optical band gap [19]. The n value depends on the nature of the transition. Direct transition allowed n = 1/2. The optical band gap was obtained by plotting \((ah\
u)^{2}\) vs. \(h\
u\) (Figure 8). The intersection between the linear fit and the photon energy axis results in \(Eg\)[18]. The band gap energy values for the samples annealed at 700 \({}^{\circ}\)C, are lower than the values obtained in [20; 21]. For un-doped Nb\({}_{2}\)O\({}_{5}\) the band gap value lies in the range of 3.32 to 3.40 eV. Whereas,  
Figure 7: Diffuse reflectance spectra as a function of wavelength of samples submitted at 700 \({}^{\circ}\)C, by varying Mn concentration. \(E_{g}\) for Nb\({}_{2}\)O\({}_{5}\):Mn decreases as Mn concentration increases. The Eg values obtained are suitable for photocatalytic purposes and they can be attributed to the spin forbidden transition on Mn\({}^{2+}\) in a tetrahedral environment [2]. Figure 9 shows the PL spectra of the Mn-doped Nb\({}_{2}\)O\({}_{5}\) nanostructured particles. As evidenced, all particles exhibit similar excitonic PL signals, demonstrating that Mn does not give rise to new PL phenomena. In this spectrum, the peaks position belong to emission bands at 3.63, 3.23, 2.01 and 1.73 eV. The highest intensity is located in the 384 nm peak position and it can be related to dissipation of energy during the recombination process of photo-induced charge carriers. The peak at 341 nm of UV emission may be attributed to the free exciton recombination in the niobium pentoxide. Peaks at 616 and 717 nm may be related to excitonic PL processes that lead to higher probabilities of exciton occurrence, oxygen vacancy presence and defects content. The thermal treatment and the synthesis method could explain this process.