## 1 Introduction  
Pure or doped metal oxide semiconductors are commonly used as photocatalysts in reactions to remove contaminants from water [1]. In these reactions, electron-hole pairs need energy to overcome the band gap between the valence and the conduction band; these pairs are created in bulk by irradiation and circulate on the semiconductor surface. The photodegradation of contaminants occurs when there is a charge transfer between electron-hole pairs and adsorbed species onto the semiconductor surface [1]. Furthermore, the surface area and the surface defects play an important role in the photocatalytic activities of metal oxides. Doping with transition metals increases surface defects. In addition, it affects the optical and electronic properties and most likely could shift the optical absorption towards the visible region [2]. Niobium pentoxide (Nb\({}_{2}\)O\({}_{5}\)) is an n-type semiconductor with unique properties to develop highly efficient solar-cells, electrochromic devices, and sensors [3; 4; 5]. This oxide draws attention due to its presence as a constituent of different compounds such as barium and lithium niobate, all used in optical waveguides and modulators [6; 7]. Furthermore, Nb\({}_{2}\)O\({}_{5}\) can be used in the photodegradation of contaminants as it exhibits a low band gap value (\(\sim\)3.4 eV), and is easily recycled and environmentally friendly. Niobium pentoxide is one of the highly active species in liquid phase oxidation, reduction and polymerization, therefore medium catalyst separation is easily achieved from its reaction [8]. In addition, Nb\({}_{2}\)O\({}_{5}\) can absorb UV light with a wavelength equal or less than 385 nm-although it is desirable for it to absorb not only UV but also visible light for higher photocatalytic efficiency. In order to do so, the Nb\({}_{2}\)O\({}_{5}\) band gap must be narrowed or split into several sub-gaps by implanting transition metal ions. Rare-earth doped Nb\({}_{2}\)O\({}_{5}\) materials have been obtained with an orthorhombic structure and average crystallite size of around 25 nm [9]. Additionally, the photocatalytic and antibacterial activity of manganese doped ZnO nanoparticles has been evaluated [10] showing the effectiveness of this dopant agent. The inclusion of manganese is meant to modify properties such as surface area and band gap energy. It is therefore expected that Mn in Nb\({}_{2}\)O\({}_{5}\) crystalline matrix influences aspects like photo-reactivity, charge carrier recombination rates and interfacial electron-transfer rates [2]. In the modified Nb\({}_{2}\)O\({}_{5}\), the location of the modifying ion in the lattice is not obvious when this replaces the Nb ion, due to size and/or charge mismatches. Notwithstanding, it is a matter of debate whether the Mn\({}^{2+}\) can substitute the Nb\({}^{5+}\) cation in a crystalline lattice. One possible contribution to the solution of this problem is verifying if it is feasible to dope Mn ions in the Nb\({}_{2}\)O\({}_{5}\) lattice. For this reason, we found it interesting to study the Nb\({}_{2}\)O\({}_{5}\) synthesis and the properties doped with Mn. Moreover, although niobium pentoxide has demonstrated to be a valuable material and is used as a catalyst, very little information is available on the preparation and spectroscopic research of nanocrystalline Mn-doped Nb\({}_{2}\)O\({}_{5}\). In this paper, we report the properties of Mn-doped Nb\({}_{2}\)O\({}_{5}\) synthesized by the polymeric precursor method. Mn concentration values in the range of 1.0-10.0 wt.% were selected as to understand the effect of metal dopant on Nb\({}_{2}\)O\({}_{5}\) properties and annealing temperatures ranging between 400 \({}^{\circ}\)C and 700 \({}^{\circ}\)C were chosen to analyze the effect of heat treatment on the structural characteristics and physical properties of Mn-doped Nb\({}_{2}\)O\({}_{5}\). Subsequently, structural, vibrational, morphological, surface, and optical characteristics as well as photocatalytic performance were also studied.