- Email: [email protected]
Synthesis, characterization and magnetic properties of β-MnO2 nanorods
Powder Technology 154 (2005) 120 – 124 www.elsevier.com/locate/powtec
Synthesis, characterization and magnetic properties of h-MnO2 nanorods Xian-Ming Liua,b, Shao-Yun Fua,*, Chuan-Jun Huanga a
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China b Graduate School, Chinese Academy of Sciences, Beijing 100039, PR China Received 19 November 2004; received in revised form 13 March 2005; accepted 2 May 2005 Available online 23 June 2005
Abstract h-MnO2 nanorods were successfully prepared by a simple refluxing route using manganese sulfate (MnSO4IH2O), sodium persulfate (Na2S2O8) and sodium hydroxide (NaOH) as the raw materials. The product was characterized by XRD, SEM, TEM, ED, EDX and TGDTG. The results showed that the nanostructured materials were exactly manganese dioxides (pyrolusite) with rutile crystal structures and the diameter of h-MnO2 nanorods ranged from 50 to 80 nm and the length ranged from 1.0 to 2.0 Am. Meanwhile, the high yield (¨70%) of high-quality h-MnO2 nanorods can be attained. The magnetic properties of the products have been evaluated using a vibrating magnetometer, which showed that the h-MnO2 nanorods exhibited ferromagnetic characteristics at room temperature. D 2005 Elsevier B.V. All rights reserved. Keywords: Manganese dioxide; Nanorods; Formation mechanism; Ferromagnetic characteristics
1. Introduction MnO2, an important substance used widely as cathodic materials, catalysts or magnetic materials [1], is a nonstoichiometric compound and has many crystalline forms such as a-, h-, g- and y-type, etc [2]. Great attention has been paid to synthesis of manganese dioxides with different crystallographic structure. On the basis of the redox reactions of MnO4 and/or Mn2+, several methods have been evolved in the preparation of manganese dioxides with controlled morphologies and crystalline structures. These methods include thermal [3], refluxing [4 – 6], hydrothermal [7], sol-gel [8,9], electrochemical [10], as well as solid-state reaction [11]. The influences of pH, counteractions, temperature, concentrations, and anions on the crystallinity and crystal forms of the final products have been extensively studied. In most of the previous reported synthetic routes, the formation of different tunnel structures have been controlled through the adjustment of the pH with H2SO4 or NaOH solution [5], for example, a- and E-MnO2 tend to be favored in aqueous concentrated acid [4– 6], whereas y* Corresponding author. Tel./fax: +86 10 62659040/62564049. E-mail address: [email protected] (S.-Y. Fu). 0032-5910/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2005.05.004
MnO2 forms preferentially in aqueous concentrated base [12]. Luo and Suib [12] have found that increasing the basicity of the system is beneficial for the increase of the crystallization rate and the high yield of y-MnO2 layer structures. Recently, much effort has been made to preparation of low-dimensional nanostructured MnO2 because dimensionality is a crucial factor in determining the properties of nanomaterials. For example, Li and co-workers [13,14] reported the hydrothermal preparation of a-MnO2 nanowires by oxidizing MnSO4 in KMnO4 and (NH4)2S2O8, respectively. They concluded that the cation concentrations were vital to the formation of these tunnel structures. It was reported that g-MnO2 nanowires can be synthesized through a coordination-polymer-precursor route [15]. Also, g-MnO2 nanowires can be prepared by facile hydrothermal treatment of commercial granular g-MnO2 crystals [16]. Al-Sagheer and Zaki [17] reported the preparation of pure y-MnO2 nanorods by sol-gel method. a-, g- and y-MnO2 contain significant amounts of small molecules or ions as integral parts of the structure, while h-MnO2 (pyrolusite) is relatively pure MnO2 with a rutile crystal structure [18]. h-MnO2 nanorods have been prepared generally under strict conditions. Wang and Li [14] reported the hydrothermal
X.-M. Liu et al. / Powder Technology 154 (2005) 120 – 124
preparation of h-MnO2 nanorods by strictly controlling the cation concentrations. The yield of h-MnO2 nanorods from this method is very low. Xi and co-workers [19] reported the synthesis of h-MnO2 nanorods with high aspect ratios by calcinating E-MnOOH nanorods precursor but very strict synthetic conditions are required for production of highquality h-MnO2 nanorods. It is thus of significance to design a facile route to synthesize high-quality h-MnO2 nanorods with a high yield. Although much work has been done on nanostructured h-MnO2 as cathodic and catalytic materials, there are few studies on nanostructured h-MnO2 as magnetic materials. Furthermore, there is no report on the h-MnO2 nanorods prepared by the refluxing route in the literature. In this work, a simple refluxing route was reported for preparing h-MnO2 nanorods without any physical template and surfactant. This method is quite simple and facile, without any catalyst or any template to direct the growth of nanorods. The structure and morphology of the products were characterized by XRD, SEM, TEM, ED, EDX and TG-DTG. Magnetic properties of such morphologically designed MnO2 structure were examined by VSM.
2. Experiment All the reagents were of analytical grade and used as received without further purification. Manganese sulfate (MnSO4IH2O), sodium persulfate (Na2S2O8) and sodium hydroxide (NaOH) were purchased from Beijing Chemical Reagent Co. (China). A typical synthesis was as follows: MnSO4IH2O (2 g) and Na2S2O8 (2.82 g) were placed in deionized water at room temperature to form a homogeneous solution completely. A certain volume of NaOH aqueous solution (the molar fraction of OH/Mn2+ was 2) was added to the above solution. The pink solution changed to brown and was stirred at room temperature for 30 min until a suspending liquid was formed. All experimental processes were under magnetic stirring. First, the suspending liquid was refluxed at 50 -C for 18 h, then refluxed at 80 -C for 5 h, finally at 100 -C for 3 h. After the reaction was complete, the resulting brownish-black solid product was filtered off, then rinsed with deionized water and absolute ethyl alcohol several times to remove ions possibly remaining in the final product, and finally dried at 80 -C in air. According to
121
the practical and theoretical weight, the product yield can be calculated. The products were characterized by X-ray power diffraction (XRD) using a M18XCE X-ray power diffractometer equipped with graphite-monochromated Cu-Ka ˚ ), employing a scanning rate of radiation (k = 1.54178 A 0.02- s 1 in the 2h ranging from 10- to 70-. The TEM photographs and the electron diffraction (ED) pattern were recorded on a Hitachi H-800 transmission electron microscope, using an accelerating voltage of 200 kV. The scanning electron microscopy (SEM) images and EDX were obtained using a HITACHI S-4300 microscope and EMAX Horiba, respectively. Thermogravimetric (TG) and differential thermogravimetric (DTG) analysis was performed on a NETZSCH STA 409 PC instrument in flowing air with a temperature-increasing rate of 10 -C/min. Their magnetic properties at room temperature were investigated using a vibrating sample magnetometer (VSM, Lakeshore 7307, USA).
3. Results and discussion Wang et al. had proposed a wet chemical method using manganese sulfate and sodium persulfate to get the high pure battery-grade g-MnO2 [20]. We extended this method here for preparation of h-MnO2 nanorods. First, the Mn2+ reacted with NaOH to produce amorphous Mn(OH)2 at room temperature, and then the mixed system was stirred and heated according to temperature rise procedure, which led to the formation of MnO2 with regular shapes. The whole reaction can be represented as follows: MnSO4 þ Na2 S2 O8 þ 4NaOH Y MnO2 þ 3Na2 SO4 þ 2H2 O
ð1Þ
It was reported that one-dimensional nanostructures can be obtained from the rolling of a natural or artificial lamellar structure [21]. Under certain conditions, a layer structure would begin to curl, and the thus-obtained tubular structure could serve as the original driving force for the growth of one-dimensional nanostructures. Among the several crystallographic forms of MnO2, y-MnO2 alone has a layer structure, which is indispensable in the formation of h-MnO2 nanorods. A schematic representation of the steps involved in the synthesis of h-MnO2
Fig. 1. Schematic representation of the synthesis process of h-MnO2 nanorods.
X.-M. Liu et al. / Powder Technology 154 (2005) 120 – 124
10
20
30
40
50
60
(002)
(310)
(220)
(200) (111)
(211)
Intensity / (a.u.)
(101)
(110)
122
70
2 / (˚ ) Fig. 2. XRD pattern of h-MnO2 nanorods.
nanorods was depicted in Fig. 1. It could be suggested that the formation of h-MnO2 nanorods by the refluxing process consist of three parts: (A) Mn(OH)2 was oxidized to amorphous MnO2 by Na2S2O8, (B) the amorphous MnO2 was transformed into layered structural y-MnO2, and (C) the h-MnO2 nanorods was formed. X-ray powder diffraction (XRD) pattern could reveal the phase and purity of the products. The typical XRD pattern of the h-MnO2 nanorods is shown in Fig. 2. The feature peaks were assigned around the 2h angles of 28.8-(110), 37.5-(101), 43.1-(111), 56.8-(211) and 65.0-(002). All of the reflections of the XRD pattern can be readily indexed to the pure pyrolusite (h-MnO2) with ˚ and c = 1.61 T 0.01 A ˚, the lattice constants a = 3.10 T 0.01 A which agree well with the values reported in the literature (JCPDS Card. No. 24-0735). Thus it could be determined that the crystals are of tetragonal structure. No characteristic peaks are observed for other impurities such as a-, E-, and y-MnO2. The panoramic morphology of the products was obtained by SEM, in which the solid sample was mounted on conductive resin with dispersion treatment. SEM images of the products are shown in Fig. 3, indicating that all the products (Fig. 3A) have almost identical morphology and the proportion of the nanorods in the whole sample is above 95%. Careful observation (Fig. 3B) shows that nanostructured h-MnO2 consists of nanorods with high aspect ratios.
Fig. 4. TEM photographs of h-MnO2 nanorods (Inset in Fig. 4(B) shows ED pattern of h-MnO2 nanorods).
The morphology and microstructure of the h-MnO2 nanorods were further investigated with TEM and ED. Typical TEM images (Fig. 4A and B) showed that the samples of h-MnO2 consisted of straight and smooth nanorods with diameters in the range of 50 – 80 nm and lengths up to several micrometers. The ED pattern of hMnO2 nanorods (inset in Fig. 4B), which was taken from a single rod, indicated that these crystalline h-MnO2 nanorods were of high quality. We also found that the yield of hMnO2 nanorods was above 70% of theoretical weight, which was higher than 50% by hydrothermal method. Elemental analysis by EDX (Fig. 5A) confirms the presence of Mn and O elemental signatures. The elemental analysis showed that the products have over 98% MnO2 by mass, with a small amount of carbon (< 2%). The presence of a small amount of C could be anticipated from the conducting resin during measurements. The ratio of Mn to O in the products is 1 / 2 in agreement with theoretical analysis. Fig. 5B depicts typical thermogravimetric (TG) behavior of the synthesized product in air. The slight weight loss (¨0.9%) in the TG curve at 20– 400 -C resulted from the removal of physically adsorbed water and the release of tightly bound water in the products [22]. The sharp weight loss (¨9%) at the temperature higher than 580 -C is due to the oxygen release corresponding to the formation of Mn2O3 [22]. The TG profile and the range of weight loss
Fig. 3. SEM images of h-MnO2 nanorods. A) low magnification; B) high magnification.
X.-M. Liu et al. / Powder Technology 154 (2005) 120 – 124
B
100
0
Weight loss (%)
98 96
TG
94
-1 -2
92 -3
90
Peak:587.5 oC
88 0
Energy (keV)
DTG
DTG (%/min)
A
123
-4 100 200 300 400 500 600 700 800 o Temperature ( C)
Fig. 5. EDX profile (A) and TG-DTG curves (B) of h-MnO2 nanorods.
are different from those reported for a-MnO2, E-MnO2, yMnO2 and Na-birnessite [11,16,17,23], which suggests that the hydrous manganese oxides don’t exist in the products. Elemental and TG analysis indicated that the reaction was rather complete and the product consisted of h-MnO2 nanorods. Magnetic properties of the h-MnO2 nanorods were investigated using a vibrating sample magnetometer with an applied field 3000 Oe H 3000 Oe at room temperature. Plot of magnetization vs. applied magnetic field for the sample is given in Fig. 6. At low external field, the hysteresis loop exhibits high coercivity (Hc = 144 Oe), which is characteristic of ferromagnetism. At high external field, the dependence of magnetization on applied field is markedly linear and saturation magnetization is not reached even at the maximum applied field of 3000 Oe. From the viewpoint of magnetic interactions, only Mn4+ – O2 – Mn4+ is in ferromagnetic coupling, while all other interactions are in antiferromagnetic coupling. Therefore, the magnetic response of nanostructured h-MnO2 could be ascribed to the ferromagnetic coupling of Mn4+ – O2 – Mn4+, because the magnetic properties are determined by interactions between the Mn ions, which in turn depend on the Mn valence distribution in the [Mn2]O4 framework [24].
4. Conclusions In this study, h-MnO2 nanorods with diameters in the range of 50 – 80 nm and lengths of several micrometers were synthesized by a simple refluxing route under certain conditions. This synthetic method for manganese dioxide nanorods is quite simple and facile, without any catalyst to serve as the energetically favorable site for the absorption of reactant molecules, and without any template to direct the growth of nanorods. Meanwhile, the high yield (¨70%) of h-MnO2 nanorods can be attained. Magnetic properties of h-MnO2 nanorods prepared by the refluxing route were investigated using a vibrating sample magnetometer at room temperature, which showed that the nanostructured h-MnO 2 exhibited the characteristics of ferromagnetism.
Acknowledgements We appreciate the financial support of the National High Technical Research and Development Program of China (No 2003AA305890).
References 0.2
M / (emu•g-1)
0.1
0.0
-0.1
-0.2 -3000
-2000
-1000
0
1000
2000
3000
H / (Oe) Fig. 6. Plot of magnetization vs. applied field for h-MnO2 nanorods measured at room temperature.
[1] H.M. Huang, S. Mao, H. Feick, et al., Science 292 (2001) 1897. [2] K.V. Kordesch, Batteries, Manganese Dioxide, vol. 1, New York, 1974. [3] Y. Muraoka, H. Chiba, T. Atou, et al., J. Solid State Chem. 144 (1999) 136. [4] N. Kijima, H. Yasuda, T. Sato, et al., J. Solid State Chem. 159 (2001) 94. [5] T.D. Xiao, P.R. Strutt, M. Benaissa, et al., Nanostruct. Mater. 10 (1998) 1051. [6] R.N. DeGuzman, Y.F. Shen, E.J. Neth, et al., Chem. Mater. 6 (1994) 815. [7] Y.F. Shen, R.P. Zerger, S.L. Suib, et al., Science 5 (1993) 260. [8] S. Ching, D.J. Petrovay, M.L. Jorgensen, Inorg. Chem. 36 (1997) 883. [9] S. Ching, J.L. Roark, N.G. Duan, et al., Chem. Mater. 9 (1997) 750. [10] L.I. Hill, A. Verbaere, D. Guyomard, J. Power Sources 119 – 121 (2003) 226. [11] Q. Li, G. Luo, J. Li, X. Xia, J. Mater. Process. Technol. 137 (2003) 25 – 29. [12] J. Luo, S.L. Suib, J. Phys. Chem., B 101 (1997) 10403.
124 [13] [14] [15] [16]
X.-M. Liu et al. / Powder Technology 154 (2005) 120 – 124
X. Wang, Y. Li, Chem. Commun. (2002) 764. X. Wang, Y. Li, J. Am. Chem. Soc. 124 (2002) 2880. Y.J. Xiong, Y. Xie, Z.Q. Li, et al., Chem. Eur. J. 9 (2003) 1645. Z.Y. Yuan, Z.L. Zhang, G.H. Du, et al., Chem. Phys. Lett. 378 (2003) 349. [17] F.A. Al-Sagheer, M.I. Zaki, Colloids Surf., A Physicochem. Eng. Asp. 173 (2000) 193. [18] S.A. Chambers, Y. Liang, Surf. Sci. 420 (1999) 123.
[19] [20] [21] [22] [23]
G. Xi, Y.Y. Peng, Y.C. Zhu, et al., Mater. Res. Bull. 39 (2004) 1641. E.I. Wang, L.F. Lin, W.L. Bowden, US Patent, 5,482,796. X. Wang, Y. Li, Chem. Eur. J. 9 (2003) 300. R. Giovanoli, Thermochim. Acta 234 (1994) 303. Y. Omomo, T. Sasaki, M. Watanabe, Solid State Ionics 151 (2002) 243. [24] C. Masquelier, M. Tabuchi, K. Ado, et al., J. Solid State Chem. 123 (1996) 255.
Synthesis, characterization and magnetic properties of h-MnO2 nanorods Xian-Ming Liua,b, Shao-Yun Fua,*, Chuan-Jun Huanga a
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China b Graduate School, Chinese Academy of Sciences, Beijing 100039, PR China Received 19 November 2004; received in revised form 13 March 2005; accepted 2 May 2005 Available online 23 June 2005
Abstract h-MnO2 nanorods were successfully prepared by a simple refluxing route using manganese sulfate (MnSO4IH2O), sodium persulfate (Na2S2O8) and sodium hydroxide (NaOH) as the raw materials. The product was characterized by XRD, SEM, TEM, ED, EDX and TGDTG. The results showed that the nanostructured materials were exactly manganese dioxides (pyrolusite) with rutile crystal structures and the diameter of h-MnO2 nanorods ranged from 50 to 80 nm and the length ranged from 1.0 to 2.0 Am. Meanwhile, the high yield (¨70%) of high-quality h-MnO2 nanorods can be attained. The magnetic properties of the products have been evaluated using a vibrating magnetometer, which showed that the h-MnO2 nanorods exhibited ferromagnetic characteristics at room temperature. D 2005 Elsevier B.V. All rights reserved. Keywords: Manganese dioxide; Nanorods; Formation mechanism; Ferromagnetic characteristics
1. Introduction MnO2, an important substance used widely as cathodic materials, catalysts or magnetic materials [1], is a nonstoichiometric compound and has many crystalline forms such as a-, h-, g- and y-type, etc [2]. Great attention has been paid to synthesis of manganese dioxides with different crystallographic structure. On the basis of the redox reactions of MnO4 and/or Mn2+, several methods have been evolved in the preparation of manganese dioxides with controlled morphologies and crystalline structures. These methods include thermal [3], refluxing [4 – 6], hydrothermal [7], sol-gel [8,9], electrochemical [10], as well as solid-state reaction [11]. The influences of pH, counteractions, temperature, concentrations, and anions on the crystallinity and crystal forms of the final products have been extensively studied. In most of the previous reported synthetic routes, the formation of different tunnel structures have been controlled through the adjustment of the pH with H2SO4 or NaOH solution [5], for example, a- and E-MnO2 tend to be favored in aqueous concentrated acid [4– 6], whereas y* Corresponding author. Tel./fax: +86 10 62659040/62564049. E-mail address: [email protected] (S.-Y. Fu). 0032-5910/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2005.05.004
MnO2 forms preferentially in aqueous concentrated base [12]. Luo and Suib [12] have found that increasing the basicity of the system is beneficial for the increase of the crystallization rate and the high yield of y-MnO2 layer structures. Recently, much effort has been made to preparation of low-dimensional nanostructured MnO2 because dimensionality is a crucial factor in determining the properties of nanomaterials. For example, Li and co-workers [13,14] reported the hydrothermal preparation of a-MnO2 nanowires by oxidizing MnSO4 in KMnO4 and (NH4)2S2O8, respectively. They concluded that the cation concentrations were vital to the formation of these tunnel structures. It was reported that g-MnO2 nanowires can be synthesized through a coordination-polymer-precursor route [15]. Also, g-MnO2 nanowires can be prepared by facile hydrothermal treatment of commercial granular g-MnO2 crystals [16]. Al-Sagheer and Zaki [17] reported the preparation of pure y-MnO2 nanorods by sol-gel method. a-, g- and y-MnO2 contain significant amounts of small molecules or ions as integral parts of the structure, while h-MnO2 (pyrolusite) is relatively pure MnO2 with a rutile crystal structure [18]. h-MnO2 nanorods have been prepared generally under strict conditions. Wang and Li [14] reported the hydrothermal
X.-M. Liu et al. / Powder Technology 154 (2005) 120 – 124
preparation of h-MnO2 nanorods by strictly controlling the cation concentrations. The yield of h-MnO2 nanorods from this method is very low. Xi and co-workers [19] reported the synthesis of h-MnO2 nanorods with high aspect ratios by calcinating E-MnOOH nanorods precursor but very strict synthetic conditions are required for production of highquality h-MnO2 nanorods. It is thus of significance to design a facile route to synthesize high-quality h-MnO2 nanorods with a high yield. Although much work has been done on nanostructured h-MnO2 as cathodic and catalytic materials, there are few studies on nanostructured h-MnO2 as magnetic materials. Furthermore, there is no report on the h-MnO2 nanorods prepared by the refluxing route in the literature. In this work, a simple refluxing route was reported for preparing h-MnO2 nanorods without any physical template and surfactant. This method is quite simple and facile, without any catalyst or any template to direct the growth of nanorods. The structure and morphology of the products were characterized by XRD, SEM, TEM, ED, EDX and TG-DTG. Magnetic properties of such morphologically designed MnO2 structure were examined by VSM.
2. Experiment All the reagents were of analytical grade and used as received without further purification. Manganese sulfate (MnSO4IH2O), sodium persulfate (Na2S2O8) and sodium hydroxide (NaOH) were purchased from Beijing Chemical Reagent Co. (China). A typical synthesis was as follows: MnSO4IH2O (2 g) and Na2S2O8 (2.82 g) were placed in deionized water at room temperature to form a homogeneous solution completely. A certain volume of NaOH aqueous solution (the molar fraction of OH/Mn2+ was 2) was added to the above solution. The pink solution changed to brown and was stirred at room temperature for 30 min until a suspending liquid was formed. All experimental processes were under magnetic stirring. First, the suspending liquid was refluxed at 50 -C for 18 h, then refluxed at 80 -C for 5 h, finally at 100 -C for 3 h. After the reaction was complete, the resulting brownish-black solid product was filtered off, then rinsed with deionized water and absolute ethyl alcohol several times to remove ions possibly remaining in the final product, and finally dried at 80 -C in air. According to
121
the practical and theoretical weight, the product yield can be calculated. The products were characterized by X-ray power diffraction (XRD) using a M18XCE X-ray power diffractometer equipped with graphite-monochromated Cu-Ka ˚ ), employing a scanning rate of radiation (k = 1.54178 A 0.02- s 1 in the 2h ranging from 10- to 70-. The TEM photographs and the electron diffraction (ED) pattern were recorded on a Hitachi H-800 transmission electron microscope, using an accelerating voltage of 200 kV. The scanning electron microscopy (SEM) images and EDX were obtained using a HITACHI S-4300 microscope and EMAX Horiba, respectively. Thermogravimetric (TG) and differential thermogravimetric (DTG) analysis was performed on a NETZSCH STA 409 PC instrument in flowing air with a temperature-increasing rate of 10 -C/min. Their magnetic properties at room temperature were investigated using a vibrating sample magnetometer (VSM, Lakeshore 7307, USA).
3. Results and discussion Wang et al. had proposed a wet chemical method using manganese sulfate and sodium persulfate to get the high pure battery-grade g-MnO2 [20]. We extended this method here for preparation of h-MnO2 nanorods. First, the Mn2+ reacted with NaOH to produce amorphous Mn(OH)2 at room temperature, and then the mixed system was stirred and heated according to temperature rise procedure, which led to the formation of MnO2 with regular shapes. The whole reaction can be represented as follows: MnSO4 þ Na2 S2 O8 þ 4NaOH Y MnO2 þ 3Na2 SO4 þ 2H2 O
ð1Þ
It was reported that one-dimensional nanostructures can be obtained from the rolling of a natural or artificial lamellar structure [21]. Under certain conditions, a layer structure would begin to curl, and the thus-obtained tubular structure could serve as the original driving force for the growth of one-dimensional nanostructures. Among the several crystallographic forms of MnO2, y-MnO2 alone has a layer structure, which is indispensable in the formation of h-MnO2 nanorods. A schematic representation of the steps involved in the synthesis of h-MnO2
Fig. 1. Schematic representation of the synthesis process of h-MnO2 nanorods.
X.-M. Liu et al. / Powder Technology 154 (2005) 120 – 124
10
20
30
40
50
60
(002)
(310)
(220)
(200) (111)
(211)
Intensity / (a.u.)
(101)
(110)
122
70
2 / (˚ ) Fig. 2. XRD pattern of h-MnO2 nanorods.
nanorods was depicted in Fig. 1. It could be suggested that the formation of h-MnO2 nanorods by the refluxing process consist of three parts: (A) Mn(OH)2 was oxidized to amorphous MnO2 by Na2S2O8, (B) the amorphous MnO2 was transformed into layered structural y-MnO2, and (C) the h-MnO2 nanorods was formed. X-ray powder diffraction (XRD) pattern could reveal the phase and purity of the products. The typical XRD pattern of the h-MnO2 nanorods is shown in Fig. 2. The feature peaks were assigned around the 2h angles of 28.8-(110), 37.5-(101), 43.1-(111), 56.8-(211) and 65.0-(002). All of the reflections of the XRD pattern can be readily indexed to the pure pyrolusite (h-MnO2) with ˚ and c = 1.61 T 0.01 A ˚, the lattice constants a = 3.10 T 0.01 A which agree well with the values reported in the literature (JCPDS Card. No. 24-0735). Thus it could be determined that the crystals are of tetragonal structure. No characteristic peaks are observed for other impurities such as a-, E-, and y-MnO2. The panoramic morphology of the products was obtained by SEM, in which the solid sample was mounted on conductive resin with dispersion treatment. SEM images of the products are shown in Fig. 3, indicating that all the products (Fig. 3A) have almost identical morphology and the proportion of the nanorods in the whole sample is above 95%. Careful observation (Fig. 3B) shows that nanostructured h-MnO2 consists of nanorods with high aspect ratios.
Fig. 4. TEM photographs of h-MnO2 nanorods (Inset in Fig. 4(B) shows ED pattern of h-MnO2 nanorods).
The morphology and microstructure of the h-MnO2 nanorods were further investigated with TEM and ED. Typical TEM images (Fig. 4A and B) showed that the samples of h-MnO2 consisted of straight and smooth nanorods with diameters in the range of 50 – 80 nm and lengths up to several micrometers. The ED pattern of hMnO2 nanorods (inset in Fig. 4B), which was taken from a single rod, indicated that these crystalline h-MnO2 nanorods were of high quality. We also found that the yield of hMnO2 nanorods was above 70% of theoretical weight, which was higher than 50% by hydrothermal method. Elemental analysis by EDX (Fig. 5A) confirms the presence of Mn and O elemental signatures. The elemental analysis showed that the products have over 98% MnO2 by mass, with a small amount of carbon (< 2%). The presence of a small amount of C could be anticipated from the conducting resin during measurements. The ratio of Mn to O in the products is 1 / 2 in agreement with theoretical analysis. Fig. 5B depicts typical thermogravimetric (TG) behavior of the synthesized product in air. The slight weight loss (¨0.9%) in the TG curve at 20– 400 -C resulted from the removal of physically adsorbed water and the release of tightly bound water in the products [22]. The sharp weight loss (¨9%) at the temperature higher than 580 -C is due to the oxygen release corresponding to the formation of Mn2O3 [22]. The TG profile and the range of weight loss
Fig. 3. SEM images of h-MnO2 nanorods. A) low magnification; B) high magnification.
X.-M. Liu et al. / Powder Technology 154 (2005) 120 – 124
B
100
0
Weight loss (%)
98 96
TG
94
-1 -2
92 -3
90
Peak:587.5 oC
88 0
Energy (keV)
DTG
DTG (%/min)
A
123
-4 100 200 300 400 500 600 700 800 o Temperature ( C)
Fig. 5. EDX profile (A) and TG-DTG curves (B) of h-MnO2 nanorods.
are different from those reported for a-MnO2, E-MnO2, yMnO2 and Na-birnessite [11,16,17,23], which suggests that the hydrous manganese oxides don’t exist in the products. Elemental and TG analysis indicated that the reaction was rather complete and the product consisted of h-MnO2 nanorods. Magnetic properties of the h-MnO2 nanorods were investigated using a vibrating sample magnetometer with an applied field 3000 Oe H 3000 Oe at room temperature. Plot of magnetization vs. applied magnetic field for the sample is given in Fig. 6. At low external field, the hysteresis loop exhibits high coercivity (Hc = 144 Oe), which is characteristic of ferromagnetism. At high external field, the dependence of magnetization on applied field is markedly linear and saturation magnetization is not reached even at the maximum applied field of 3000 Oe. From the viewpoint of magnetic interactions, only Mn4+ – O2 – Mn4+ is in ferromagnetic coupling, while all other interactions are in antiferromagnetic coupling. Therefore, the magnetic response of nanostructured h-MnO2 could be ascribed to the ferromagnetic coupling of Mn4+ – O2 – Mn4+, because the magnetic properties are determined by interactions between the Mn ions, which in turn depend on the Mn valence distribution in the [Mn2]O4 framework [24].
4. Conclusions In this study, h-MnO2 nanorods with diameters in the range of 50 – 80 nm and lengths of several micrometers were synthesized by a simple refluxing route under certain conditions. This synthetic method for manganese dioxide nanorods is quite simple and facile, without any catalyst to serve as the energetically favorable site for the absorption of reactant molecules, and without any template to direct the growth of nanorods. Meanwhile, the high yield (¨70%) of h-MnO2 nanorods can be attained. Magnetic properties of h-MnO2 nanorods prepared by the refluxing route were investigated using a vibrating sample magnetometer at room temperature, which showed that the nanostructured h-MnO 2 exhibited the characteristics of ferromagnetism.
Acknowledgements We appreciate the financial support of the National High Technical Research and Development Program of China (No 2003AA305890).
References 0.2
M / (emu•g-1)
0.1
0.0
-0.1
-0.2 -3000
-2000
-1000
0
1000
2000
3000
H / (Oe) Fig. 6. Plot of magnetization vs. applied field for h-MnO2 nanorods measured at room temperature.
[1] H.M. Huang, S. Mao, H. Feick, et al., Science 292 (2001) 1897. [2] K.V. Kordesch, Batteries, Manganese Dioxide, vol. 1, New York, 1974. [3] Y. Muraoka, H. Chiba, T. Atou, et al., J. Solid State Chem. 144 (1999) 136. [4] N. Kijima, H. Yasuda, T. Sato, et al., J. Solid State Chem. 159 (2001) 94. [5] T.D. Xiao, P.R. Strutt, M. Benaissa, et al., Nanostruct. Mater. 10 (1998) 1051. [6] R.N. DeGuzman, Y.F. Shen, E.J. Neth, et al., Chem. Mater. 6 (1994) 815. [7] Y.F. Shen, R.P. Zerger, S.L. Suib, et al., Science 5 (1993) 260. [8] S. Ching, D.J. Petrovay, M.L. Jorgensen, Inorg. Chem. 36 (1997) 883. [9] S. Ching, J.L. Roark, N.G. Duan, et al., Chem. Mater. 9 (1997) 750. [10] L.I. Hill, A. Verbaere, D. Guyomard, J. Power Sources 119 – 121 (2003) 226. [11] Q. Li, G. Luo, J. Li, X. Xia, J. Mater. Process. Technol. 137 (2003) 25 – 29. [12] J. Luo, S.L. Suib, J. Phys. Chem., B 101 (1997) 10403.
124 [13] [14] [15] [16]
X.-M. Liu et al. / Powder Technology 154 (2005) 120 – 124
X. Wang, Y. Li, Chem. Commun. (2002) 764. X. Wang, Y. Li, J. Am. Chem. Soc. 124 (2002) 2880. Y.J. Xiong, Y. Xie, Z.Q. Li, et al., Chem. Eur. J. 9 (2003) 1645. Z.Y. Yuan, Z.L. Zhang, G.H. Du, et al., Chem. Phys. Lett. 378 (2003) 349. [17] F.A. Al-Sagheer, M.I. Zaki, Colloids Surf., A Physicochem. Eng. Asp. 173 (2000) 193. [18] S.A. Chambers, Y. Liang, Surf. Sci. 420 (1999) 123.
[19] [20] [21] [22] [23]
G. Xi, Y.Y. Peng, Y.C. Zhu, et al., Mater. Res. Bull. 39 (2004) 1641. E.I. Wang, L.F. Lin, W.L. Bowden, US Patent, 5,482,796. X. Wang, Y. Li, Chem. Eur. J. 9 (2003) 300. R. Giovanoli, Thermochim. Acta 234 (1994) 303. Y. Omomo, T. Sasaki, M. Watanabe, Solid State Ionics 151 (2002) 243. [24] C. Masquelier, M. Tabuchi, K. Ado, et al., J. Solid State Chem. 123 (1996) 255.