Characterization, surface morphology and microstructure of water soluble colloidal MnO 2 nanoflakes

In the present work, characterization of water soluble colloidal MnO 2 nanoflakes which act as an oxidizing agent was carried out using UV–visible spectroscopy. Transmission electron microscopy microstructure of colloidal MnO 2 nanoflakes confirms the shape and nature of these particles. Selected area electron diffraction ring indicated that colloidal nanoflakes were amorphous in nature. Surface morphology of synthesized colloidal MnO 2 nanostructure was determined by field emission scanning electron microscopy indicating a crumpled net like arrangement.


Introduction
Transition nonmetal oxides have attracted the attention of chemists and environmentalists due to their broad application in various areas such as antimicrobial activity [1], water treatment [2], food packaging [3], medical devices [4], textile industries [5,6] and antibacterial activities [7][8][9]. Manganese dioxide nanostructure materials exhibit novel physical and chemical properties and are widely used in biomedical field as bactericide [10][11][12][13] and applied as a coat onto an ultrafiltration membrane to destroy toxins present in drinking water [14,15]. The performance of nanostructure materials is greatly affected by their morphologies and crystallographic forms. A variety of methods like sol-gel, hydrothermal, electro deposition, combustion, and micro emulsion are applied to synthesize different morphology nanowires, nanoplates, and nanoparticles [16][17][18][19][20].
Water soluble nanoparticles of MnO 2 are good substitute over the insoluble forms due to increased catalytic and oxidizing activities. The adsorption properties of MnO 2 makes it an appropriate choice as a catalyst for redox reactions. Oxidation of formic and oxalic acid in aqueous medium [21,22], and in micellar medium [23] are noteworthy. Our group is currently engaged in oxidation reactions using water soluble nanoparticles of colloidal MnO 2 in both micellar and aqueous media [24][25][26][27][28][29] not only because of its broad usage in catalysis, ion-exchange, molecular adsorption, biosensor but also due to its low economical price and eco-friendliness. Solution based base synthesis and use of metal oxide nanostructures is popular due to economically cheap, mild, and viable conditions. It occurs under environmentally safe settings without additional templates and sophisticated apparatus. Synthesis of nanomaterial in the form of colloidal solution provides the possibility of separate nucleation avoiding inter-particle aggregation and controlled growth.
In this paper, we report the characterization, surface morphology and microstructure of water soluble MnO 2 nanoflakes which has not been reported so far in literature.

Characterization of water soluble MnO 2 nanoflakes
The

Surface morphology and microstructure
Field emission scanning electron microscopy image of colloidal MnO 2 was taken in a Ziss Supra 40 FESEM operated under 5 kV accelerating voltage. The colloidal solution of MnO 2 was drop casted on a carbon tape and dried at 60 °C for 6 h. Further, the sample deposited carbon tape was mounted on the sample holder of FESEM and coated with a thin film of gold. TEM image was captured in a FEI Techni G2 TEM operated at 200 kV accelerating voltage. The colloidal solution was drop casted on a carbon coated Cu TEM grid and allowed to stand at room temperature for overnight. Finally, the grid was used for TEM study. Surface morphology of synthesized colloidal MnO 2 nanostructure was determined by FESEM analysis as shown in Fig. 1. Low magnification micrograph showed aggregated nano structures with a net like morphology.
Magnified image clearly revealed that netlike aggregates were composed with very thin flakes of MnO 2 . Thickness of each flake was found to be about 2-4 nm. TEM was used to analyze the morphology and particle size of colloidal nanoflakes as shown in Fig. 2.
The image shows that nanoflakes are found in aggregates and are stacked one over the other, crumpled and assembled to form net like arrangement. Selected area electron diffraction (SAED image 2 (Fig. 2) reveals a diffused ring pattern. This diffused SEAD ring indicated that colloidal nanoflakes were amorphous in nature. Elemental analysis was done with the help of energy dispersive X-ray (EDX). Figure 3 shows the constituent element of colloidal nanostructures. Presence of manganese and oxygen confirms the formation of manganese dioxide which was further supported by UV-VIS spectra of colloidal MnO 2 [24][25][26][27][28][29][30]. The UV-visible spectra of water soluble colloidal MnO 2 nanoflakes showed λ max at 390 nm. For this wavelength (λ max ) Kabir Ud-Din et al. [24][25][26][27][28] and Kabir Ud-Din and Iqubal [29,30] had worked and suitable for kinetic observations.

Conclusion
In summary, the author has successfully reported the surface morphology and microstructure of water soluble colloidal MnO 2 nanoflakes. TEM microstructure of colloidal MnO 2 nanoflakes confirm that the particles are spherical and amorphous in nature. It is conducted with economically cheap and readily available reagents. The reaction occurs under mild and under environmentally safe conditions. Thus, it is believed that the present work is a major breakthrough in the area of nanomaterials. College  Data availability statement Data will be made available upon request.

Conflict of interest No conflict of interest.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.