## 4 Conclusions  
Mn-doped Nb\({}_{2}\)O\({}_{5}\) nanostructured particles were successfully synthesized by the polymeric precursor method. Mn incorporation had a strong effect on the structural, vibrational, morphological, surface, and optical properties of Nb\({}_{2}\)O\({}_{5}\). The XRD studies of the samples modified with the highest Mn content and subjected to heat treatments at 700 \({}^{\circ}\)C revealed the formation of a secondary phase corresponding to MnNb\({}_{2}\)O\({}_{6}\). Results from the XRD analysis indicated that Mn\({}^{2+}\) ions may replace Nb\({}^{5+}\) ions without distortions of the crystalline structure, with Mn content to 1.0 wt.%. The vibrational study showed that an increase of Mn content causes a decrease in the material crystallinity due to the presence of less sharp peaks. In general, the nanostructured particles at 700 \({}^{\circ}\)C showed the best crystallized quality, which suggests that this temperature not only raised its crystalline quality but also improved its optical properties. FE-SEM analysis contributed to determining a well-defined morphology with the presence of nanometric structure grains. The determined \(Eg\) values for the samples annealing at 700 \({}^{\circ}\)C, at different Mn concentrations, were minor compared to the values of Nb\({}_{2}\)O\({}_{5}\), and are suitable for photocatalytic purposes.  
In turn, the Nb\({}_{2}\)O\({}_{5}\):Mn (5.0 wt.%) sample showed good photocatalytic performance. This sample showed appropriate morphological, surface and optical properties for the Rhodamine B photodegradation. The Mn segregation in MnNb\({}_{2}\)O\({}_{6}\) secondary phase may be responsible for the properties improvement and the reduction of band gap energy of the Nb\({}_{2}\)O\({}_{5}\):Mn (5.0 wt.%) sample. The Nb\({}_{2}\)O\({}_{5}\):Mn (1.0 wt.%) sample could be a good photocatalyst due its orthorhombic structure; however, for this manganese concentration, the element did not act as an appropriate morphological and surface modifier. Doping allowed obtaining smaller particles and gap energy values lower than pure niobium pentoxide. We believe that these properties may be desirable in other applications, such as bactericidal activity. We attribute the low photocatalytic response to the loss of crystalline properties with the increase in Mn content. At 5% and 10% of Mn content, the crystalline phase of Nb\({}_{2}\)O\({}_{5}\) was progressively amorphized.  
**Author Contributions:** Conceptualization, A.M.R.-P. and C.F.-G.; methodology, M.R.J., and A.M.R.-P.; investigation, A.M.R.-P.; data curation, A.M.R.-P., C.F.-G., and I.S.-G.; first draft preparation, A.M.R.-P.; writing--review and editing, M.R.J., and A.M.R.-P.; supervision, M.R.J.; All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Acknowledgments:** We would like to thank Universidad Francisco de Paula Santander UFPS, Cacuta Colombia, Universidad Nacional de Colombia, Universidad Pedagogica y Tecnologica de Colombia UFTC, CINVESTAV Mexico D.F., and COLCIENCIAS (757-2016) for providing us with the financial support to conduct this study.  
**Conflicts of Interest:** The authors declare that they have no conflict of interest.