Synthesis of MnO₂ Microfiber with Secondary Nanostructure by Cotton Template

Abstract

Hierarchical MnO₂ microfibers were prepared by using cotton as the template and KMnO₄ as the precursor via an ultrasonic assistance route. The results of scanning electron microscope characterization showed that the concentration of KMnO₄ had a significant effect on the morphology of MnO₂ microfiber. At low concentration of KMnO₄, the microfiber was composed of MnO₂ nanorods with single crystal structure. With increasing the concentration of KMnO₄, the secondary nanostructure of MnO₂ microfibers had a transformation from nanorod to nanoparticle. The results of N₂ adsorption-desorption analysis indicated that MnO₂ microfibers had BET surface area of about 25 m²/g. This synthesis provides a new way to control the secondary nanostructure of MnO₂ microfiber by adjusting the concentration of precursor. Furthermore, the mechanism for the replication was proposed and discussed.

1. Introduction

In recent years, many templates, such as mesoporous silica [1, 2], AAO [3–5], and surfactant micelle [6, 7], have been used for the preparation of nanostructured oxides. Biological materials have also been used as templates or scaffolds for the preparation of nanostructured oxides because of their interesting structures [8, 9]. In comparison with other templates, biological materials are “green” and easy to obtain. In the process of replication, the structure of biological materials provides a stable and controllable condition to guide the assembly of nanostructured oxides. The obtained nanostructured oxides can replicate the characteristic structures or even the functionalities of biological materials [10, 11]. Different species of biological materials, such as wood [12–14], filter paper [15, 16], and sisal [17], were used for the fabrication of various inorganic nanomaterials. In comparison with other kinds of plant fibers, the morphology of cotton fiber is more uniform.

In the past few years, MnO₂ nanomaterials have attracted considerable attention due to their potential applications in the areas of catalyst, ion-sieves, and electrode materials [18]. Recently, Zhu et al. prepared a series of manganese oxides via a “green” biotemplating method [19]. Herein, we fabricated MnO₂microfibers with secondary nanostructure by using cotton as the template and KMnO₄ as the precursor. The effect of the concentration of KMnO₄ on the morphology of MnO₂ microfibers was studied and the possible formation mechanism was also discussed.

2. Experimental Section

Medical absorbent cotton was used as the template. It was pretreated before use. The cotton was immersed in 5 wt% HCl aqueous solution for 3 hours, washed with distilled water, and dried at 80°C in air. Then the sample was immersed in 6 wt% NaOH aqueous solution for 3 hours at 60°C, washed with distilled water, and dried at 80°C in air.

1.0 g of the pretreated cotton was dispersed in KMnO₄ aqueous solution in a beaker. The beaker was exposed to ultrasonic irradiation (100 W) for 6 hours. The sample was taken out, washed ten times with distilled water, and dried at 80°C in air. Then the sample was calcined at 500°C for 3 hours in air. Finally, the sample was washed with distilled water and alcohol to remove the byproduct, and then dried at 80°C in air. The concentrations of KMnO₄ aqueous solution were 5, 10, and 20 g/mL, respectively. The as-prepared samples were denoted as sample 1, sample 2, and sample 3, respectively. Furthermore, the sample without calcination process was denoted as the precursor/cotton.

The crystal structure of the sample was characterized by X-ray diffraction (XRD) (Bruker D8 advance). The morphology of the sample was examined by scanning electron microscopy (SEM Gemini LEO1530) and transmission electron microscopy (TEM JEOL JEM-2100). The N₂ adsorption-desorption analysis was measured on a Micromeritics ASAP 2010 instrument.

3. Results and Discussion

The XRD pattern of the precursor/cotton of sample 1 is shown in Figure 1a. Two peaks can be seen at about 15° and 23°. It is difficult to judge the phase of the sample. Then the precursor/cotton was calcined to remove the cotton template. XRD patterns of sample 1 and sample 3 are shown in Figures 1(b), and 1(c), respectively. The results indicate that the samples are MnO₂ with a tetragonal structure (JCPDS 44-0141).

Figure 1 

XRD patterns of the precursor/cotton of sample 1 (a), sample 1 (b), and sample 3 (c).

Figures 2(a) and 2(b) show the SEM images of the precursor/cotton of sample 1. Figures 2(c) and 2(d) shows the SEM images of the sample 1. Compared with the precursor/cotton sample, the obtained MnO₂ sample replicates the fiber structure of the cotton, but with some shrinkage in size. Figures 2(b) and 2(d) indicated that the surface morphology of MnO₂ microfiber is obviously different from the precursor/cotton sample. MnO₂ microfiber is composed of abundant nanorods. The TEM image (Figure 2(f)) also indicates that the microfiber is composed of nanorods. The corresponding selected-area electron diffraction (SAED) pattern (the inset in Figure 2(f)) indicates that the nanorods possess single crystal structure. The HRTEM images of the nanorods are shown in Figures 2(g) and 2(h). A layer separation of about 0.69 nm can be observed, which is corresponding to the 𝑑 value of the (110) planes of MnO₂.

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Figure 2 

SEM images of the precursor/cotton of sample 1 (a), (b) and sample 1 (c)–(e) and TEM images of sample 1 (f)–(h); the inset in (f) is SAED pattern; the rectangle areas in panel (f) correspond to panels (g) and (h).

Figure 3 shows the SEM images of sample 2 and sample 3. From the pictures, it can be seen that sample 2 is composed of nanoparticles and nanorods. Sample 3 is composed of nanoparticles. Therefore, it can be concluded that the secondary structure of the microfibers has a transformation from nanorod to nanoparticle with increasing the concentration of KMnO₄. In addition, the nanostructure of the microfibers becomes dense with increasing the concentration of KMnO₄.

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Figure 3 

SEM images of sample 2 (a), (b) and sample 3 (c), (d).

The N₂ adsorption-desorption isotherms and the corresponding BJH pore-size distributions of MnO₂ microfibers are shown in Figure 4. All the samples have a sorption isotherm with a clear capillary condensation step in a relative pressure range of 0.9–1.0 (Figure 4(a)), implying the porous structure with large pore size. The porous structure was mainly produced from the stack of the nanomaterials. The BJH pore-size distributions indicate that all the samples possess large pore-size and broad pore distributions. The Brunauer-Emmett-Teller (BET) surface areas for sample 1, sample 2, and sample 3 are 24.8, 26.1, and 23.6 m²/g, respectively. The pore volumes for sample 1, sample 2, and sample 3 are 0.12, 0.14, and 0.14 cm³/g, respectively.

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Figure 4 

N2 adsorption-desorption isotherms (a) and BJH pore-size distributions from adsorption branches (b) for the samples. The isotherms for sample 2 and sample 3 are offset vertically by 40 and 80 cm3/g.

The formation mechanism of the samples is studied. The purpose of the pretreatment for the cotton is to remove the lignin and form R-CH₂ONa, which make MnO⁻₄ access the hierarchical pore channels more easily because of the resulting high porosity and the ionic environment [19]. The hydroxyl groups of the cotton facilitate the complexation of MnO⁻₄ ions with the cotton. In order to investigate the effect of ultrasonic irradiation, comparative experiment was performed. It is found that the precursor adsorbed on the cotton is very little if without the ultrasonic irradiation. The high-intensity ultrasound through water can lead to acoustic cavitation, which is the formation, growth, and violent collapse of gas microbubbles in the solution. The near adiabatic compression of the gas molecules inside the microbubbles in the liquid leads to a dramatic rise in the temperature and pressure within the bubbles [20], and this can have the effect of generating reactive free-radical species (MnO⁻₄ ions) in solution. The flow of solution made from the gas microbubbles leads to more MnO⁻₄ into the channels of cotton. The cotton fiber not only serves as the template for the formation of microfiber, but also provides the location for the nucleation and the growth of MnO₂ nanostructure during the heat treatment. Moreover, the hydroxyl groups of the cotton act as passivation contacts for the stabilization of the nanoparticles formed inside the cotton. In addition, the concentration of the precursor plays an important role in the morphology of MnO₂ nanostructure. The SEM results (Figures 2(e) and 3(b)) indicated that the nanostructure becomes dense with increasing the concentration of KMnO₄. Thus, we think that the growth void of MnO₂crystals determines the final morphology of the nanostructure. The nanorod structure can be obtained only when the growth void is enough.

4. Conclusions

In conclusion, hierarchical MnO₂ microfibers with secondary nanostructure were successfully prepared by using cotton as the template and KMnO₄as the precursor via an ultrasound route. The MnO2microfibers with secondary nanostructure may be found to potential applications in the areas of catalyst, ion-sieve, and electrode material. It is expected that this biotemplating method can be extended to other metal oxide materials.

Acknowledgment

This work was supported by Natural Science Foundation of Jiangsu Province (BK2006195).