Facile synthesis and magnetic properties of manganese dioxide nanowires

Abstract

A large number of MnO₂ nanowires were fabricated by a facile hydrothermal method. The nanowires have a tetragonal pyrolusite structure and a smooth surface. The common bulk defects such as dislocations, twinnings and stacking faults are not detected by HRTEM measurement. The magnetisation dependence of temperature indicates that the magnetisation, linearly and monotonically, increases with decreasing temperature in the range 300–80 K, revealing the paramagnetic properties of the nanowires. The first discharge capacity reaches 223.5 mA h g⁻¹, and the value of capacity steadily decreases during the following cycles, down to an acceptable 122.3 mA h g⁻¹ after 25 cycles. The high surface ratio of nanowires is the main reason for the excellent discharge cycle property of the MnO₂ nanowires.

Keywords:

• MnO₂
• nanowires
• hydrothermal synthesis
• magnetic properties

1. Introduction

In the past decades, one-dimensional (1-D) nanostructures including nanowires, nanoribbons and nanotubes have attracted considerable attention due to their promising applications in numerous areas such as nanoelectronics, nano-optoelectronics, nanosensors and solar energy storages 1–14. As an important functional inorganic material, manganese dioxide (MnO₂) has numerous applications in catalysis, ion exchange, molecular adsorption, rechargeable batteries and supercapacitors 15–23. Recently, much attention has focused on the synthesis of 1D MnO₂ nanostructures because of its promising applications in catalysis, rechargeable batteries, ion sieves and supercapacitors 24–30. Up to now, various methods including sol-gel route, wet chemical method, template-assisted method and hydrothermal route have been developed to synthesise 1-D MnO₂ nanostructures 31–36. Among these methods, the hydrothermal route is the most useful and efficient because the method can control the nanostructures by choosing the reaction temperature, concentration and time. Moreover, the method is a low cost one and the yield is high enough to meet the industrial demand. In this study, a facile hydrothermal route was developed to fabricate a large number of MnO₂ nanowires. The chemical composition, microstructure and magnetic properties were investigated, which present some new features on the MnO₂ nanowires.

2. Experiment

All the chemicals were of analytic grade and used without further purification. In the experiment, 0.002 mol manganese chloride tetrahydrate (MnCl₂ · 4H₂O), 0.001 mol potassium permanganate (KMnO₄) and 30 mL distilled water were added into 50 mL Teflon-lined stainless steel autoclaves and stirred for half an hour. Then, 10 mL H₂SO₄ (2 M) was slowly added into the mixed solution under vigorous stirring. Finally, the autoclave was sealed and maintained at 150°C for 24 h. After the autoclave was cooled to room temperature, precipitates were taken out and washed with distilled water for several times and dried in air for further characterisation. The chemical reaction involves two steps:

1.	Mn2+ + 2H2O → MnO2 + 4 H+
2.	+ 4 H+ → MnO2 + 2H2O

The two-step chemical reaction process can be generally expressed as follows:

The black precipitates are the MnO₂ products, which can be confirmed by the following characterisation. Phase identifications were performed on X-ray diffractometer (PANalytical X’PERT MPD) with a Cu-Kα radiation (λ = 1.5406 Å) operated at 40 kV and 40 mA. Morphology and microstructure of the products were characterised by field-emission scanning electron microscopy (SEM; FEI XL30 S-FEG) and transmission electron microscopy (TEM; Philips CM200) equipped with energy-dispersive X-ray spectroscopy (EDS). The magnetic properties were measured by Physical Property Measurement System (PPMS, Quantum Design). The as-synthesised MnO₂ nanowires were employed as cathode active materials for rechargeable lithium-ion cells. The detailed measurement process is in accordance with the literature 37.

3. Results and discussion

Figure 1(a) shows the XRD patterns of the as-prepared precipitates. All the diffraction peaks in Figure 1(a) can be indexed to pure tetragonal β-MnO₂ with lattice constants a = 4.392 Å and c = 2.871 Å, agreeing well with the standard card (ICDD-PDF No: 24-0735). Impurities such as α- and γ-structures are not detected within the instrumental resolution. The broad diffraction peaks are maybe due to the nano-size effects on the products. The corresponding SEM image indicates that the precipitates consist of a large number of nanowires, as shown in Figure 1(b). The average diameter and length of these nanowires are ca. 30 nm and 1 µm, respectively, further confirming that the broad diffraction peaks are induced by the nano-size effects. The corresponding TEM image further indicates regular and dispersed nanowire formation, as shown in Figure 2(a). Figure 2(b) shows the TEM image of an individual MnO₂ nanowire. The nanowire has a smooth surface and it is straight along the growth direction. The corresponding EDS spectrum (Figure 2c) indicates that the nanowire is mainly composed of Mn and O elements. The atomic ratio of Mn and O is about 1:1.926, further confirming the MnO₂ formation of the nanowires. The HRTEM image indicates that the clear interplanar spacing of the lattice planes is ca. 0.312 nm, agreeing well with the distance value of (1 1 0) lattice planes of the tetragonal MnO₂, as shown in Figure 2(d). The surface of the nanowire is smooth and the common thin amorphous layer cannot be detected. Moreover, the common bulk defects such as dislocations, twinnings and stacking faults are not detected, revealing that the nanowires are largely free of the bulk defects.

Figure 1. (a) XRD patterns of the MnO₂ nanowires and (b) SEM image of the MnO₂ nanowires.

Figure 2. (a) TEM image of a large area of MnO₂ nanowires; (b) TEM image of a individual MnO₂ nanowire; (c) EDS spectrum of the MnO₂ nanowires; and (d) HRTEM image of the MnO₂ nanowire.

Magnetisation dependence of temperature was measured under a field-cooled mode in the magnetic field of 3000 Oe at temperature range 80–300 K, as shown in Figure 3(a). The magnetisation of MnO₂ nanowires was weak and its values were ca. 4.44, 5.85, 7.14 memu g⁻¹ measured at 300, 150 and 80 K, respectively. The magnetisation, linearly and monotonically, increases with decreasing temperature, which means that the MnO₂ nanowires are paramagnetic. Figure 3(b) shows the magnetic hysteresis loops measured at 300, 150 and 80 K, respectively. Linear magnetic hysteresis loops at 300, 150 and 80 K are clearly observed, further demonstrating the paramagnetic properties of MnO₂ nanowires.

Figure 3. (a) Temperature dependence of magnetisation measured under a field-cooled mode at the temperature range from 80 to 300 K and (b) (colour online) magnetic hysteresis loop of MnO₂ nanowires measured at 300, 150, and 80 K, respectively.

Figure 4 shows the discharge capacities of Li–MnO₂ cells over the first 25 cycles. The first discharge capacity reaches 223.5 mA h g⁻¹. The value is slightly higher than that observed in the data of commercial β-MnO₂ powders and reported 1-D nanostructures, and much higher than that of bulk materials 37,38. The value of discharge capacity is relatively decreased rapidly with cycling from 1 to 10 times, and is steadily decreased in the following cycles, down to an acceptable capacity of 122.3 mA h g⁻¹ after 25 cycles. These results indicate that the reversibility during Li insertion/extraction reactions can easily occur in the Li–MnO₂ nanowire cells, and the reactions have a steady loss of capacity on cycling. Moreover, because of the high surface ratio nature of the nanowires, the capacities of the nanowires are much higher than that of bulk materials and the aggregated nanostructures.

Figure 4. Discharge capacities of the MnO₂ nanowires over the first 25 cycles.

4. Conclusions

Tetragonal β-MnO₂ nanowires were obtained by a facile hydrothermal method. The average length and diameter of the nanowires are about 1 µm and 30 nm, respectively. The magnetisation dependence of temperature indicates that the magnetisation values of MnO₂ nanowires are ca. 4.44, 5.85, 7.14 memu g⁻¹ at 300, 150 and 80 K, respectively. The magnetisation, linearly and monotonically, increases with decreasing temperature, indicating the paramagnetic properties of the MnO₂ nanowires. The first discharge capacity reaches 223.5 mA h g⁻¹, much higher than that of bulk materials. The value of capacity is steadily decreased during the following cycles, down to an acceptable capacity of 122.3 mA h g⁻¹ after 25 cycles. We deduce that high surface ratio of nanowires facilitates the Li insertion/extraction reactions in the Li–MnO₂ nanowire cell, resulting in an excellent cycle property.

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (nos 51002143, 51072184) and the Natural Science Foundation of Henan Provincial Educational Department (no. 2010B140015).