Hydrothermal decomposition of KMnO₄ in the acidic environment in the presence of Fe³⁺(aq) ions leads to the formation of α-MnO₂ nanotubes. The morphology evolution from the sponge-like spheres to the nanotubes as a function of reaction time was imaged by FE-SEM and TEM. The presence of iron in nanotubes was unambiguously confirmed by STEM-EELS measurement across the nanotube diameter.

Introduction

In ancient times different minerals of the manganese dioxide (MnO₂) were used as pigment and to clarify glass. Nowadays its properties are being primarily exploited in catalysis, ion-exchange processes, various magnetic applications, and as cathode material in lithium ion batteries [1].
Physical properties of MnO₂ depend not only on the adopted crystals structure [2] (MnO₂ can grow in several polymorphic forms), and redox flexibility of manganese ions (oxidation states 4+ and 3+), but also greatly on the morphology, specific surface area, pore volume and pore dimensions [3]. Lately intensive research efforts have been directed into the controlled growth of different polymorphic forms of MnO₂ adopting different morphologies.
There are several established synthetic approaches for the synthesis of MnO₂ nanostructures that base on the redox reactions of MnO^4- and/or Mn²⁺ [4] but the most simple and successful one is the hydrothermal decomposition of KMnO₄(aq) in acidic media which typically yield in α- MnO₂ nanorods [5] (term hydrothermal refers that reaction takes place at constant volume that means in closed reaction vessel). However, we have shown that when we add Fe³⁺(aq) ions to the reaction mixture α-MnO₂ nanotubes are formed with a narrow diameter distribution[6].
In order to determine the optimal reaction time for the formation of α-MnO₂ nanotubes we decided to vary reaction time and then study the reaction products with SEM and TEM techniques.

Synthesis and Imaging

α-MnO₂ nanotubes were synthesized under hydrothermal conditions (closed reaction vessel) at 150 °C from an aqueous solution of KMnO₄ in acidic media in the presence of Fe³⁺ ions.

Reaction times were varied from 1, 2, 3, 6, and 17 hours. Detailed procedure is described in the reference [6].
A FEG-SEM, Carl Zeiss, Supra 35LV, equipped with a field emission gun, secondary and Inlens detectors, and TEM, Jeol 2100 equipped with CCD camera were used for samples imaging. All SEM micrographs were recorded at accelerating voltage of 1 or 1.5 kV using secondary or combination of secondary and InLens detectors. STEM-HAADF and STEM-EELS measurements were preformed with a dedicated STEM (Vacuum Generators HB 501) equipped with a home-modified Gatan spectrometer. All spectra were recorded in STEM mode with 100 keV incident electrons focused on the specimen. EELS mapping was obtained by rastering a 1 nm probe on the surface of the single nanotube and collecting spectra with a spectral domain from 200 to 900 eV. Local compositions was then be obtained by extractions of O 1s, Mn 2p and Fe 2p excitations.

SEM Imaging

FEG-SEM micrographs of the products isolated from the reaction mixture after 1, 2 and 17 hours are presented in figure 1. During the first hour (fig. 1A) already round microstructures are formed with a spongy like surface (fig. 1B). In most cases these microstructures grow from one another, what is clearly seen in fig. 1A although also individual ones are observed.
After the two hours the surface of these microstructures is changed in sense that becomes more structured (fig. 1C). On the edges of microstructures edges nanorods starts to form (fig. 1D). We stres that at this point of the reaction the interior of microstructure's is solid as it is demonstrated from a cross section of one of these microstructures (right bottom side in fig. 1C).
The product collected after 17 hours of reaction shows further development of the micro- and nanostructure. Microstructures are now hollow with the walls that are self-assembled from α-MnO₂ nanotubes (fig. 1E). The nanotube cavities with the inner diameter of ~15 nm can be clearly seen in the fig. 1F.
SEM examination of reaction products collected after 3-6 hours show, that the transformation from the microstructures composed of nanorods (Figure 1B) to the ones composed from nanotubes (Figure 1E) happens gradually and that the coexistence of both types of microstructures takes place. With longer reaction times the microstructures self-assembled from nanotubes prevail.

TEM Imaging

Evolution of α-MnO₂ nanotubes, for the products isolated from the reaction mixture after 1, 2 and 17 hours, was in parallel investigated with TEM (fig. 2). The microstructures formed in the early stage of the reaction (1 h) have the solid interior (fig. 2A) while their spongy like surface as observed from SEM (fig. 1B) is composed from single layers (fig. 2B) which already curl at the edges.
TEM micrographs of product isolated from reaction mixture after 2 hours show that the interior of the microstructures is still full (fig. 2C and E). In some case is the surface of microstructures completely composed from nanorods (fig. 2C and D) while in other cases nanorods are not yet formed (fig. 2E). Microstructures composed of nanorods at this stage of reaction represent the minor part of the product. Nanorods shown in fig. 2D correspond to the microstructures shown in fig. 2C have a typical diameter between 25 to 40 nm. At this stage of reaction we did not observe any nanotubes. We thus conclude that the cavities are formed in the later stage of reaction (3-6 hours).
When the reaction takes place for 17 hours only nanotubes are found in the isolated product, as mentioned above. The typical nanotube is shown in Fig. 2F. Their inner cavity is not uniform along the nanotube and it gradually reduces away from one end to the other. In fact, the inner cavities rarely extend through entire nanotube. The outer diameter of the nanotubes ranges between 25-40 nm and is very uniform along the individual nanotube. It is essentially the same as for nanorods thus implying that nanotubes develop straight from the nanorods through the chemical reaction that takes place only in the core of the nanoparticle. The length of the nanotube in figure 2F is ~370 nm and agrees well with the shell thickens of the microstructures (340-390 nm).
HRTEM microgrpah of the end of the α-MnO₂ nanotube (fig 3) reveals that nanotubes are crystalline. The interplanar distances of TEM fringes parallel to the rod axis are 0.5 nm, which agrees well with the d value of (200) planes of α-MnO₂ (fig. 3). Therefore, the direction of nanotubes growth is along the [001] direction.