Facile and large-scale synthesis of single-crystalline

manganese oxyhydroxide/oxide nanostructures

Zhen Fang a, Kaibin Tang b,c,*, Lisheng Gao c, Dong Wang b,c,Suyuan Zeng b,c, Qiangchun Liu b,c

a College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, PR Chinab Nanomaterial and Nanochemistry, Hefei National Laboratory for Physical Sciences at Microscale,

University of Science and Technology of China, Hefei 230026, PR Chinac Department of Chemistry, University of Science and Technology of China, Hefei 230026, PR China

Received 25 January 2006; received in revised form 17 August 2006; accepted 4 November 2006

Available online 3 January 2007

Abstract

On basis of the redox reaction between MnO4� and Mn2+, a facile hydrothermal method combined with the adjustment of pH

value in the reaction system has been employed to synthesize single-crystal g-MnOOH nanorods, and square-like Mn3O4

nanoparticles. Additionally, single-crystalline b-MnO2 and Mn2O3 nanorods can be obtained after subsequent calcinations of the

as-prepared g-MnOOH nanostructures at 300 8C and 600 8C, respectively. Furthermore, electrochemical property of as-obtained

b-MnO2 is also studied.

# 2006 Elsevier Ltd. All rights reserved.

Keywords: A. Nanostructures; B. Crystal growth; C. Electrochemical measurement; C. Thermogravimetric analysis; C. Electron microscopy

1. Introduction

Manganese oxides and oxyhydroxides have attracted considerable attentions in the technological applications

including electrochemical reaction, catalyst, molecular adsorption, high-density magnetic recording and others,

because these materials have the outstanding structural flexibility combined with novel chemical and physical

properties [1–5]. MnO2, Mn2O3 and g-MnOOH have been used as important substrates for Li–Mn–O spinel cathode

materials in rechargeable lithium ion batteries [4–15]. Mn3O4 is known to be an active catalyst for chemical reaction

[16–18], such as the oxidation of methane and carbon monoxide, selective reduction of nitrobenzene and combustion

of organic compounds at temperatures of the range of 373–773 K.

With the development of one-dimensional nanostructures, the dimensionality and size of the materials have been

regarded as the important factors that may address some novel properties, so the control of manganese oxide and

oxyhydroxide materials with well-controlled dimensionality, size and crystal structure is still of great significance.

Usually, oxidize of Mn2+ or deoxidize of MnO4� can be used to prepare nanostructure of MnO2 or MnOOH, in which

a-, b-, g- and d-MnO2 nanowires and nanorods have been synthesized [19–21]. The transformation of MnO2 under

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Materials Research Bulletin 42 (2007) 1761–1768

* Corresponding author at: Department of Chemistry, University of Science and Technology of China, Hefei 230026, PR China.

Tel.: +86 551 3601791; fax: +86 551 3601791.

E-mail addresses: fangzhen@ustc.edu (Z. Fang), kbtang@ustc.edu.cn (K. Tang).

0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2006.11.025

hydrothermal conditions has also been utilized to obtain MnOOH and Mn2O3 nanowires [22–24]. And Mn3O4

nanowires could be prepared in NaCl flux by solid-state reaction [25]. Since pH is generally believed to have great

affect on the crystal forms of the final products [19,26], it is interesting to control the structural forms of manganese

oxyhydroide/oxide through simply adjusting the pH value with the combination of the redox reaction in aqueous

solution. In this work, we present a facile hydrothermal method, utilizing the redox reactions between MnO4� and

Mn2+ and the adjustment of the pH, to synthesize single-crystal g-MnOOH nanorods and quadrangle-shaped Mn3O4

nanoparticles. Here, g-MnOOH can be obtained under strong acid condition, which has not been reported by other

researcher, to our best knowledge. Furthermore, single-crystalline MnO2 and Mn2O3 nanorods can be obtained after

subsequent calcinations of the as-prepared g-MnOOH nanostructures at 300 8C and 600 8C, respectively. This method

does not need any catalyst or template, which is capable for large-scale production.

2. Experimental

All the reagents used in the experiments were analytically pure and were purchased from Shanghai Chemical

Reagent Company and were used without further purification. The synthetic procedures are as follows.

2.1. Preparation of g-MnOOH nanorods and Mn3O4 nanoparticles

Typically, 0.044 g KMnO4 (0.3 mmol) and 0.421 g Mn(CH3COO)2�4H2O (1.7 mmol) was added into 40 mL

distilled water, and its pH value was adjusted to �1 or �14 with diluted hydrochloric acid or NaOH solution.

The resulting solution was transferred into a Teflon-lined stainless steel autoclave of 50 mL capacity, and then the

autoclave was maintained at 180–210 8C for 12 h after the autoclave was sealed, then cooled to room temperature

naturally. The products were filtered off, and washed with de-ionized water and absolute ethanol for several times to

remove other ions and then dried at ambient air at 60 8C.

2.2. Preparation of MnO2 and Mn2O3 nanorods

For calcinations, the as-obtained products prepared at pH 1 were loaded into horizontal tube furnace in quartz boats,

then heated at 1 8C min�1 to 300 8C or 600 8C and kept at that temperature for 1 h. Finally, the final products were

cooled to room temperature naturally for further characterization.

2.3. Characterization

The composition and the phase of these as-prepared samples were determined by XRD analysis, using a Philips

X’Pert Pro Super diffractometer with Cu Ka radiation (l = 1.54187 A). TEM images were taken with a Hitachi H-800

transmission electron microscope. The selected-area electron diffraction (SAED) patterns and HRTEM images were

performed with a JEOL-2010 transmission electron microscope. The TGA measurement was carried out on a

Shimadzu TA-50WS analyzer in air at a heating rate of 10 8C min�1 in the temperature range from 30 8C to 800 8C.

The positive electrodes were fabricated by pasting slurries of the as-prepared b-MnO2 (85 wt%), acetylene black

(10 wt%) and PVDF (5 wt%) dissolved in N-methyl-pyrrolidone (NMP) on Al foil strips by the doctor blade

technique. Metallic lithium electrodes (lithium foil pressed onto a stainless steel gauze) were used as anodes.

3. Results and discussion

Fig. 1a–c shows the XRD patterns of the products obtained at the same reaction temperature of 210 8C but the

different pH values of 1, 7 and 14, respectively. All the reflection peaks in Fig. 1a and c can be indexed as monoclinic-

phase g-MnOOH (JCPDS File No. 41-1379, a = 5.30 A, b = 5.28 A, c = 5.31 A) and tetragonal-phase Mn3O4 (JCPDS

File No. 89-4837, a = 5.76 A, c = 9.45 A), respectively. And the mixture of g-MnOOH and Mn3O4 would be obtained

at pH 7, as shown in Fig. 1b. Fig. 2a and b shows the XRD patterns of the two samples originated from the calcinations

of the g-MnOOH at 300 8C and 600 8C, respectively, in which Fig. 2a is the typical XRD pattern of cubic-phase

Mn2O3 (JCPDS File No. 41-1442, a = 9.41 A) and Fig. 2b is that of tetragonal-phase b-MnO2 (JCPDS File, No.

24-0735, a = 4.40 A, b = 2.88 A).

Z. Fang et al. / Materials Research Bulletin 42 (2007) 1761–17681762

Fig. 3a and b presents the TEM images of the g-MnOOH obtained at 210 8C and 180 8C, respectively, clearly

revealing the presence of a large quantity of nanorods. The diameters of g-MnOOH nanorods obtained at 180 8C are in

the range of 40–60 nm and the lengths are between 300 nm and 1.4 mm (Fig. 3b), while the diameters of the g-

MnOOH nanorods heated at 210 8C get thicker and the lengths become longer (Fig. 3a). Obviously, it can be found that

the aspect ratio of the g-MnOOH nanorods decreases with the increase of the reaction temperature, indicating the

influence of the temperature on the morphology of the products. HRTEM studies of the g-MnOOH nanorods provide

further insight into the structures of these materials and Fig. 3c is a HRTEM image of g-MnOOH obtained at 180 8C,

Z. Fang et al. / Materials Research Bulletin 42 (2007) 1761–1768 1763

Fig. 1. XRD patterns of the products obtained at 210 8C while under different pH value: (a) pH 1, g-MnOOH; (b) pH 7, the mixture of g-MnOOH

and Mn3O4; (c) pH 14, Mn3O4.

Fig. 2. XRD patterns of the products resulted from the calcinations of the g-MnOOH at different temperature: (a) 600 8C, Mn2O3 and (b) 300 8C, b-

MnO2.

showing that the nanorods have the uniform structures. In Fig. 3c, the interlayer distance is ca. 0.34 nm and 0.25 nm,

which is consistent with the lattice space of ð1 1 1Þ and (1 1 1) plane of bulk g-MnOOH, respectively. The single-

crystal nature and the growth direction of the g-MnOOH nanorods can be further confirmed by the SAED pattern (the

inset of Fig. 3c) recorded perpendicular to the long axis of a singe nanorod, which can be indexed to the ½0 1 1� zone

axis of the monoclinic g-MnOOH and thus suggests that the nanorods grow along [1 1 1] crystal direction. While

Z. Fang et al. / Materials Research Bulletin 42 (2007) 1761–17681764

Fig. 3. TEM images of the g-MnOOH nanorods obtained at pH 1 and the different temperature: (a) 210 8C and (b) 180 8C. (c) HRTEM image of

single g-MnOOH nanorod obtained at 180 8C and the inset is its SAED pattern and (d) TEM image of the square-like Mn3O4 nanoparticles obtained

at 210 8C and pH 14, and the inset is its SAED pattern.

Fig. 3d is a TEM image of the as-prepared Mn3O4 obtained at 210 8C and pH value of 14, which shows that the

products mainly consist of quadrangle-shaped nanoparticles with the size of 30–50 nm. And the inset of Fig. 3d is the

corresponding SAED pattern. Fig. 4a and b is the typical TEM images of b-MnO2 and Mn2O3 originated from the

calcinations of the g-MnOOH at 300 8C and 600 8C, respectively, clearly indicating that they reserve the similar

morphologies of g-MnOOH nanorods. And the insets are their corresponding SAED patterns.

Fig. 5 is the TGA curve for the as-prepared g-MnOOH nanorods, presenting several weight losses at the

temperature range of 25–800 8C. The significant weight loss of ca. 1.88% at 200–320 8C is due to the dehydroxylation

Z. Fang et al. / Materials Research Bulletin 42 (2007) 1761–1768 1765

Fig. 4. TEM images of: (a) b-MnO2 nanorods and (b) Mn2O3 nanorods and the insets are their corresponding SAED patterns.

[22,27], accompanying with the nucleation of b-Mn2O3 (4MnOOH + O2! 4MnO2 + 2H2O). Another sharp weight

loss of 8.33 wt% at 500–600 8C corresponds to the oxygen release for the reduction of b-MnO2 to a-Mn2O3 [28]. No

further weight loss can be observed over 600 8C, which indicates the completed conversion of the g-MnOOH under

heat conditions. This is in good agreement with the XRD results of Fig. 2a and b.

The electrochemical behaviors of the b-MnO2 were characterized by cyclic voltammograms (CV) with test cells,

and Fig. 6 is cyclic voltammograms of the first five cycles of b-MnO2 versus metallic Li, which indicates the redox

occurs at 2.68 Vand the corresponding oxidation peaks around 3.15 V (versus Li/Li+). Fig. 7 shows the voltage versus

capacity curves for the cells between 1.0 Vand 4.0 Vat a current density of 0.1 mA/cm2. In the case of the obtained b-

MnO2 nanorods, the initial discharge and charge capacities are 36 mAh/g and 33 mAh/g. The relative low capacity

might be attribute to its structural property and the availability of only narrow one-dimensional channels for lithium-

ion diffusion. [1]

Z. Fang et al. / Materials Research Bulletin 42 (2007) 1761–17681766

Fig. 5. TGA curve of the as-prepared g-MnOOH nanorods.

Fig. 6. Cyclic voltammograms of the first five cycles of b-MnO2 vs. metallic Li.

The chemical reaction that we employed in the process could be formulated as:

2MnO4� þ 3Mn2þ þ 2H2O ! 5MnO2þ 4Hþ

On the basis of the value of E0, the standard Gibbs free energy change DG0 of reaction could be estimated to be

�56.14 kJ mol�1, implying that the reaction is thermodynamically favourable. Additionally, MnO2 might react with

water and transform into g-MnOOH [19]. With the increase of the pH value (pH � 4), g-MnOOH would become

unstable in the system and some could dehydrate to produce Mn3O4, especially at higher temperature. Therefore, the

mixture of g-MnOOH and Mn3O4 would be obtained if the pH value were in the range of 4–11, as demonstrated in

Fig. 1b. If continuing to increase the pH value using alkali, the phase of Mn3O4 would be dominant and the pure phase

of Mn3O4 could be achieved at pH value of 14 and the temperature of 210 8C. However, when the reaction is conducted

at 180 8C, the mixture of g-MnOOH and Mn3O4 rather than pure Mn3O4 would be obtained even at pH 14. If

continuing to reduce the reaction temperature to 150 8C, g-MnOOH nanoparticles were produced no matter what the

pH was. It is apparent that both the reaction temperature and the pH value play an important role in influencing the

crystallographic form of the final products (Table 1).

4. Conclusion

In summary, a facile hydrothermal method, utilizing the redox reactions between MnO4� and Mn2+ and the

adjustment of the pH, has been used to synthesize single-crystal g-MnOOH nanorods and quadrangle-shaped Mn3O4

Z. Fang et al. / Materials Research Bulletin 42 (2007) 1761–1768 1767

Fig. 7. Voltage vs. capacity curves for the cells between 2.0 V and 4.0 V at a current density of 0.1 mA/cm2.

Table 1

Influence of the temperature and the pH value on the final product

Reactant Temperature (8C) pH Product

MnO4� + Mn2+ 210 1–4 g-MnOOH nanorods

4–13 g-MnOOH and Mn3O4

14 Square-like Mn3O4 nanoparticles

180 1–4 g-MnOOH nanorods

4–14 g-MnOOH and Mn3O4

150 1–14 MnOOH nanoparticles

nanoparticles. And single-crystalline MnO2 and Mn2O3 nanorods can be formed after subsequent calcinations of the

as-prepared g-MnOOH nanostructures at 300 8C and 600 8C, respectively.

Acknowledgements

Financial supports from National Natural Science Foundation of China and from Program for New Century

Excellent Talents in University are gratefully acknowledged. We thank funds from Anhui province Natural Science

Foundation.

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