## 4 Conclusions  
A simple hydrothermal method has been developed for the fast synthesis of well-defined crystalline \(\alpha\)-MnO\({}_{2}\) nanofibers. The results showed that the crystalline \(\alpha\)-MnO\({}_{2}\)-type manganese oxide was formed and the autoclaving time and temperature had a great influence on the process of obtaining the desired structure. The as-obtained nanowires were several micrometers long and 20-30 nm in diameter, with narrow distributions in their dimensions.  
Figure 14: Rh110 absorbance vs. time for catalytic (\(\lambda\) = 498 nm) and photocatalytic (\(\lambda\) = 494 nm) reactions over \(\alpha\)-MnO\({}_{2}\).  
Figure 13: Hypsochromic shift and intensity decrease of the RhB characteristic peak during (**a**) catalytic and (**b**) photocatalytic reactions.  
The \(\alpha\)-MnO\({}_{2}\) material resulting from the synthesis displayed two colors: black and brown. However, X-ray powder diffraction analysis shows that in both cases the material is well-crystallized and can be indexed in the \(\alpha\)-MnO\({}_{2}\) structure. Still, the two types of \(\alpha\)-MnO\({}_{2}\) showed different degrees of crystallization and morphology, differences that may have been caused by the effect of heat transfer in the autoclave during the heat treatment applied in the MnO\({}_{2}\) synthesis.  
TG analysis indicates mass losses and phase transformations between 30 and 150 \({}^{\circ}\)C, 200-260 \({}^{\circ}\)C, and 340-500 \({}^{\circ}\)C.  
SEM images of the distinctly colored materials showed particles with nanowire morphology.  
The electrochemical study conducted on graphite supports modified with compositions containing \(\alpha\)-MnO\({}_{2}\) showed that the electrode with the highest double-layer capacitance was the one manufactured from a suspension in ethanol containing 3 mg \(\alpha\)-MnO\({}_{2}\) and 2 mg Vulcan carbon. The values of the double-layer capacitance, roughness factor, and electrochemically active surface area obtained for this electrode were 3.441 mF cm\({}^{-2}\), 57.4, and 16.072 cm\({}^{2}\), respectively.  
The experiments carried out for investigating the catalytic and photocatalytic RhB degradation revealed that \(\alpha\)-MnO\({}_{2}\) showed improved results during photocatalysis considering that the RhB degradation reached 97.2%; whereas, in the case of the catalytic reaction, the RhB degradation reached 74%. Additionally, the identification of multiple bands overlapped in the visible spectra that simultaneously increased and decreased in intensity and the enhanced hypsochromic shift together with a higher absorbance decrease led to the conclusion that \(\alpha\)-MnO\({}_{2}\) is an efficient photocatalyst, for which RhB underwent a more highly selective deethylation degradation process.  
Conceptualization, F.S.R. and B.-O.T.; methodology, F.S.R.; software and visualization J.N.G.; validation, S.D.N. and F.S.R.; formal analysis, M.I.; writing--original draft preparation, review and editing, F.S.R.; supervision, S.D.N. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Ministry of Research and Innovation from PN 19 22 01 01 projects, with contract number 40N/2019. The ab initio calculations were performed under the project HPC-EUROPA3 (INFRAIA-2016-1-730897), with the support of the EC Research Innovation Action under the H2020 Programme; in particular, the authors gratefully acknowledge the support of [PERSON] and the hospitality from CNR-SPIN c/o Department of Physical and Chemical Sciences, University of L'Aquila, Via Vetoio, I-67100, Coppito, L'Aquila, Italy. We are grateful for the computer resources and technical support provided by CINECA.  
Not applicable.  
We would like to thank [PERSON] for the support in obtaining the XRD and TGA characterizations of the samples.  
The authors declare no conflict of interest.