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Formation of manganite fibers under the directing of cationic surfactant
Materials Science and Engineering C 26 (2006) 653 – 656 www.elsevier.com/locate/msec
Formation of manganite fibers under the directing of cationic surfactant Xiaodan Sun *, Xiangdong Kong, Yude Wang, Chunlai Ma, Fuzhai Cui, Hengde Li Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Received 1 November 2004; received in revised form 26 May 2005; accepted 26 June 2005 Available online 2 November 2005
Abstract The effects of organic molecules on the morphology control of inorganic materials in the process of biomineralization have long been realized. Nowadays, these effects have been utilized to prepare inorganic materials with desired morphologies in different systems. In this paper, manganite (MnOOH) fibers are chemically synthesized under extremely low surfactant (cetyltrimethylammonium bromide CTAB) concentrations at basic conditions. Powder X-ray diffraction (XRD) and transmission electron microscopy (TEM) are used to characterize the products. Characterization of samples aged for different time shows that the formation of MnOOH fibers is intimately related to a layered structured manganese oxide. A corresponding transformation mechanism is proposed based on the experiment results, and it could be inferred that CTAB plays an important directing role in this process. D 2005 Elsevier B.V. All rights reserved. Keywords: Manganese oxide; Fiber; Layered structure; Surfactant; Biomimetic process
M
hausmannite manganite M feitknechtite layered mesostructure
relative intensity (a.u.)
M
(e) 2.5 days
M M
(f) 3 days
M
(d) 2 days
M
10
20
(c) 1 day (b) 13 hours (a) 6 hours
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M
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Manganese oxides have long been studied for their use in battery applications and as catalytic materials. Many reports have been given on the preparation and characterization of MnO2 that have potential applications as cathodic materials for high energy density lithium batteries [1 –5], in which MnOOH is produced during the electrochemical processes [6]. Manganese (III) compounds belong to the class of newer and interesting oxidants [7]. MnOOH has also been used as precursors to prepare Li– Mn – spinel [8]. Recently, many researches have been evolved in the preparation of manganese oxides with special morphologies and crystalline structures because the effects of powder morphology, crystalline structure, or bulk density on the properties of materials have attracted much attention of the scientists [1]. On the other hand, layered phases with birnessite-type structure have attracted the interest of many scientists because of their relatively high surface area and transition metal oxide content. Preparations of layered phases with birnessite-type structure [9– 11], MnOOH with elongated shape [12] or fiber-like
morphology [13] and MnO2 nanowires [14,15] have been reported. We have also reported some facile novel ways to prepare layered manganese oxides with large interplanar spacings [16] and manganese oxides whiskers [17]. In our recent work, we find that formation of the MnOOH fibers is
M
1. Introduction
30
40
50
60
70
2 (degree) * Corresponding author. Tel.: +86 10 62772977. E-mail address: [email protected] (X. Sun). 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.06.059
Fig. 1. XRD patterns of the samples aged for different time: (a) 6 h; (b) 13 h; (c) 1 day; (d) 2 days; (e) 2.5 days; (f ) 3 days.
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X. Sun et al. / Materials Science and Engineering C 26 (2006) 653 – 656
intimately related to the layered manganese oxides. A transformation mechanism about the formation of the fibers is proposed based on the experimental results. The cationic surfactant CTAB is supposed to have played an important role in the formation process of the fibers. This work may be inspiring for preparation of fibers of other compositions. 2. Experimental All the chemical reagents used in the experiments were obtained from commercial sources as guaranteed-grade reagents and used without further purification and treatment. The synthetic procedures were as follows: 7.3 g CTAB was mixed with 600 ml distilled deionized water to get a
homogenous solution before 300 ml MnSO4IH2O (1.13 g) solution was introduced into it with stirring. After a short time for the mixing of CTAB and MnSO4, 50 ml ethylamine (65 – 70 wt.% water solution) was added. 500 ml distilled deionized water was added finally and the whole mixture was stirred for 40 min before it was aged at 120 -C in an autoclave for different time from 6 h to 3 days. Then, the aged products were filtered, washed with distilled water and dried at ambient temperature in air to get the as-synthesized samples. XRD patterns of the samples were recorded using a Rigaku ˚ ). D/max-RB diffractometer with Cu Ka radiation (k = 1.5418A TEM micrographs were obtained on a 200 CX transmission electron microscope operated at 200 kV. The samples for TEM B
(a) (b)
100nm
60nm
A C 200
220 020
C
B
(c)
(d) A
3nm
400nm
(e)
Fig. 2. TEM images of the samples aged for different time: (a) 6 h, crystalline Mn3O4 particles; (b) 13 h, crystalline Mn3O4 particles, h-MnOOH thin plates and fibers. A, B and C beside the arrows in the image point to crystalline Mn3O4 particles,h-MnOOH thin plates and fibers respectively; (c) 13 h, layered mesostructure; (d) 1 day, h-MnOOH thin plates, fibers and layered mesostructure. A, B and C beside the arrows in the images point to h-MnOOH thin plates, fibers and layered mesostructure respectively; (e) 3 days, single crystalline MnOOH fiber.
X. Sun et al. / Materials Science and Engineering C 26 (2006) 653 – 656
were prepared by suspending powder in ethanol ultrasonically and pipetting the solution onto holy carbon-coated copper grids.
655
(a)
3. Results and discussion According to the XRD pattern (Fig. 1f ) and TEM image (Fig. 2e), it can be shown that single crystalline MnOOH (manganite, JCPDS 41-1379) fibers can be synthesized under a CTAB concentration of 0.5 wt.% when the sample is aged for 3 days [18]. As a comparison, a control experiment is carried out in the absence of CTAB, and only g-Mn3O4 (hausmannite, JCPDS 24-734) particles are obtained after aging for 3 days (Fig. 3). This suggests that the cationic surfactant CTAB plays an important role in the formation of MnOOH. A phase transition process can be reflected by the XRD patterns of samples aged for different time: Only crystalline g-Mn3O4 can be found in the sample aged for 6 h (Fig. 1a). With the increase of the aging time (Fig. 1b), the intensity of the diffraction peaks of g-Mn3O4 becomes weaker, while some ordered low-angle diffraction peaks caused by a mesolayered birnessite-type manganese oxide [16] and peaks
(b)
(a) relative intensity (a.u.)
γ-M n3O4
Fig. 4. TEM images of the transitional structures in the sample aged for 1 day: (a) fiber out of h-MnOOH thin plate; (b) fiber out of layered mesostructure. The arrows in (b) point to the interconnection part of the fiber and the large thin plate corresponding to the layered mesostructure.
10
20
30
40
50
60
70
2θ (degree)
(b)
caused by h-MnOOH (feitknechtite, JCPDS 18-804, a layered manganese oxyhydroxide that is free of interlayer cations [19]) turn to appear. When the aging time increases to 1 day (Fig. 1c), diffraction peaks of the layered mesostructure and h-MnOOH become sharper and stronger while the peaks of g-Mn3O4 disappear entirely. When the aging time increases further (Fig. 1d, e), diffraction peaks of the layered mesostructure and h-MnOOH become weaker and weaker until they fade away, while those of MnOOH, which appear in the sample aged for 13 h, become stronger and stronger until the peaks turn to be of pure MnOOH for the sample aged for 3 days (Fig. 1f ). Depending on the changes reflected by XRD patterns, a transformation process during the aging is assumed, which is: c-Mn3 O4 Y layered mesostructure þ feitknechtite Y manganite:
100nm
Fig. 3. XRD pattern (a) and TEM image (b) of the sample synthesized in the absence of CTAB, showing crystalline g-Mn3O4.
In order to confirm the samples’ structures and to clarify the possible formation mechanism, TEM is used to examine the samples further. As shown in Fig. 2, TEM images of the samples demonstrate results that are consistent with those of XRD: Only crystalline Mn3O4 particles can be seen in the sample aged for 6 h (Fig. 2a), while in the sample aged for 13 h (Fig. 2b, c), a mixture of crystalline Mn3O4 paticles, h-
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X. Sun et al. / Materials Science and Engineering C 26 (2006) 653 – 656
MnOOH thin plates, layered mesostructure and MnOOH fibers [13,17] can be seen. Similar mixed compositions can be found in the sample aged for 1 day except that crystalline Mn3O4 particles are absent (Fig. 2d). The sample aged for 3 days turns to be well crystallized pure manganite fibers (Fig. 2e). Interestingly, some transitional structures are also observed in the sample aged for 1 day (Fig. 4). Selected area electron diffraction pattern of the thin plate in Fig. 4a demonstrates that this platy part is h-MnOOH (not shown), while the stripy and curly morphology of the large thin plate in Fig. 4b indicates that this platy part corresponds to the layered mesostructure. Li and colleagues have reported several systems with nanotube structures that are thought to be rolled up from lamellar mesostructures [20 –22]. The coexistence of the thin plates and the final products, which are nanotubes in their work, is similar to what we observed in our experiments. However, the product in our study is fiber instead of nanotube, so the rolling mechanism seems not to be applicable for our work. But, it gives us an inspiration that the final product may be transformed from a layered phase. On the other hand, different nanowires have been synthesized under the directing of certain surfactants [13,23 – 25], demonstrating the importance of the directing effect of the templates in controlling the morphology and structure of the final products. Based on these reports and the results of our work, a transformation mechanism can be supposed like this: The polar heads of CTAB (CTA+) can interact with the inorganic ions (e.g. O2) of manganese oxides, and the absorption and the binding of surfactant molecules on certain crystal face of manganese oxides may lead to the formation of 1D structures [23,24]. Thus, the growth of manganite is limited in one direction by the localization effect of CTAB molecules so that preferred orientation is obtained to form a fiber-like morphology. 4. Conclusion Pure manganite (MnOOH) fibers are chemically synthesized under the directing of cationic surfactant CTAB. A transformation process is proposed to be g-Mn3O4Ylayered mesostructure + feitknechtiteYmanganite. The localization effect of CTAB is believed to be the controlling force for the formation of manganite fibers. This facile way of using surfactant as directing templates may be applicable for preparation of fibers
of other compositions, although it still needs further studies to validate the mechanism. Acknowledgement The authors would like to thank the National Natural Science Foundation of China for financial support (No. 50402001). References [1] S. Bach, M. Henry, N. Baffier, J. Livage, J. Solid State Chem. 88 (1990) 325. [2] S. Bach, J.P. Pereira-Ramos, N. Baffier, R. Messina, Electrochim. Acta 36 (1991) 1595. [3] J. Hunter, J. Solid State Chem. 39 (1981) 142. [4] S. Bach, J.P. Pereira-Ramos, N. Baffier, J. Solid State Chem. 120 (1995) 70. [5] F.A. Al-Sagheer, M.I. Zaki, Colloids Surf., A Physicochem. Eng. Asp. 173 (2000) 193. [6] S.I. Cordoba de Torresi, A. Gorenstein, Electrochim. Acta 37 (1992) 2015. [7] T.J. Pastor, F.T. Pastor, Talanta 52 (2000) 959. [8] T. Kanasaku, K. Amezawa, N. Yamamoto, Solid State Ion. 133 (2000) 51. [9] M. Nitta, Appl. Catal. 9 (1984) 151. [10] S.-T. Wong, S. Cheng, Inorg. Chem. 31 (1992) 1165. [11] M.E. Landis, B.A. Aufdembrink, P. Chu, I.D. Johnson, G.W. Kirker, M.K. Rubin, J. Am. Chem. Soc. 113 (1991) 3189. [12] M. Ocana, Colloid Polym. Sci. 278 (2000) 443. [13] P.K. Sharma, M.S. Whittingham, Mater. Lett. 48 (2001) 319. [14] X. Wang, Y.D. Li, J. Am. Chem. Soc. 124 (2002) 2880. [15] X. Wang, Y.D. Li, Chem. Commun. 7 (2002) 764. [16] X.D. Sun, C.L. Ma, L. Zeng, Y.D. Wang, H.D. Li, Mater. Res. Bull. 37 (2002) 331. [17] X.D. Sun, C.L. Ma, L. Zeng, Y.D. Wang, H.D. Li, Inorg. Chem. Commun. 10 (2002) 747. [18] XRD pattern of the sample aged for 3 days that is reported here is a little different from that we have reported before in reference [17] because the samples were prepared in different batches, while the formation of MnOOH fibers were affected by many preparation parameters (e.g. the time to add ethylene or water), which may change in different batches. [19] S. Ching, K.S. Krukowska, S.L. Suib, Inorg. Chim. Acta 294 (1999) 123. [20] Y.D. Li, X.L. Li, R.R. He, J. Zhu, Z.X. Deng, J. Am. Chem. Soc. 124 (2002) 1411. [21] Y.D. Li, J.W. Wang, Z.X. Deng, Y.Y. Wu, X.M. Sun, D.P. Yu, P.D. Yang, J. Am. Chem. Soc. 123 (2001) 9904. [22] X. Chen, X.M. Sun, Y.D. Li, Inorg. Chem. 41 (2002) 4524. [23] Y. Li, J.H. Wan, Z.N. Gu, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 286 (2000) 106. [24] L. Yan, Y.D. Li, Z.X. Deng, J. Zhuang, X.M. Sun, Int. J. Inorg. Mater. 3 (2001) 633. [25] X.C. Song, Z.D. Xu, W.X. Chen, G. Han, B. Liu, Y.F. Zheng, Chin. J. Inorg. Chem. 20 (2004) 186.
Formation of manganite fibers under the directing of cationic surfactant Xiaodan Sun *, Xiangdong Kong, Yude Wang, Chunlai Ma, Fuzhai Cui, Hengde Li Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Received 1 November 2004; received in revised form 26 May 2005; accepted 26 June 2005 Available online 2 November 2005
Abstract The effects of organic molecules on the morphology control of inorganic materials in the process of biomineralization have long been realized. Nowadays, these effects have been utilized to prepare inorganic materials with desired morphologies in different systems. In this paper, manganite (MnOOH) fibers are chemically synthesized under extremely low surfactant (cetyltrimethylammonium bromide CTAB) concentrations at basic conditions. Powder X-ray diffraction (XRD) and transmission electron microscopy (TEM) are used to characterize the products. Characterization of samples aged for different time shows that the formation of MnOOH fibers is intimately related to a layered structured manganese oxide. A corresponding transformation mechanism is proposed based on the experiment results, and it could be inferred that CTAB plays an important directing role in this process. D 2005 Elsevier B.V. All rights reserved. Keywords: Manganese oxide; Fiber; Layered structure; Surfactant; Biomimetic process
M
hausmannite manganite M feitknechtite layered mesostructure
relative intensity (a.u.)
M
(e) 2.5 days
M M
(f) 3 days
M
(d) 2 days
M
10
20
(c) 1 day (b) 13 hours (a) 6 hours
M
M
M
Manganese oxides have long been studied for their use in battery applications and as catalytic materials. Many reports have been given on the preparation and characterization of MnO2 that have potential applications as cathodic materials for high energy density lithium batteries [1 –5], in which MnOOH is produced during the electrochemical processes [6]. Manganese (III) compounds belong to the class of newer and interesting oxidants [7]. MnOOH has also been used as precursors to prepare Li– Mn – spinel [8]. Recently, many researches have been evolved in the preparation of manganese oxides with special morphologies and crystalline structures because the effects of powder morphology, crystalline structure, or bulk density on the properties of materials have attracted much attention of the scientists [1]. On the other hand, layered phases with birnessite-type structure have attracted the interest of many scientists because of their relatively high surface area and transition metal oxide content. Preparations of layered phases with birnessite-type structure [9– 11], MnOOH with elongated shape [12] or fiber-like
morphology [13] and MnO2 nanowires [14,15] have been reported. We have also reported some facile novel ways to prepare layered manganese oxides with large interplanar spacings [16] and manganese oxides whiskers [17]. In our recent work, we find that formation of the MnOOH fibers is
M
1. Introduction
30
40
50
60
70
2 (degree) * Corresponding author. Tel.: +86 10 62772977. E-mail address: [email protected] (X. Sun). 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.06.059
Fig. 1. XRD patterns of the samples aged for different time: (a) 6 h; (b) 13 h; (c) 1 day; (d) 2 days; (e) 2.5 days; (f ) 3 days.
654
X. Sun et al. / Materials Science and Engineering C 26 (2006) 653 – 656
intimately related to the layered manganese oxides. A transformation mechanism about the formation of the fibers is proposed based on the experimental results. The cationic surfactant CTAB is supposed to have played an important role in the formation process of the fibers. This work may be inspiring for preparation of fibers of other compositions. 2. Experimental All the chemical reagents used in the experiments were obtained from commercial sources as guaranteed-grade reagents and used without further purification and treatment. The synthetic procedures were as follows: 7.3 g CTAB was mixed with 600 ml distilled deionized water to get a
homogenous solution before 300 ml MnSO4IH2O (1.13 g) solution was introduced into it with stirring. After a short time for the mixing of CTAB and MnSO4, 50 ml ethylamine (65 – 70 wt.% water solution) was added. 500 ml distilled deionized water was added finally and the whole mixture was stirred for 40 min before it was aged at 120 -C in an autoclave for different time from 6 h to 3 days. Then, the aged products were filtered, washed with distilled water and dried at ambient temperature in air to get the as-synthesized samples. XRD patterns of the samples were recorded using a Rigaku ˚ ). D/max-RB diffractometer with Cu Ka radiation (k = 1.5418A TEM micrographs were obtained on a 200 CX transmission electron microscope operated at 200 kV. The samples for TEM B
(a) (b)
100nm
60nm
A C 200
220 020
C
B
(c)
(d) A
3nm
400nm
(e)
Fig. 2. TEM images of the samples aged for different time: (a) 6 h, crystalline Mn3O4 particles; (b) 13 h, crystalline Mn3O4 particles, h-MnOOH thin plates and fibers. A, B and C beside the arrows in the image point to crystalline Mn3O4 particles,h-MnOOH thin plates and fibers respectively; (c) 13 h, layered mesostructure; (d) 1 day, h-MnOOH thin plates, fibers and layered mesostructure. A, B and C beside the arrows in the images point to h-MnOOH thin plates, fibers and layered mesostructure respectively; (e) 3 days, single crystalline MnOOH fiber.
X. Sun et al. / Materials Science and Engineering C 26 (2006) 653 – 656
were prepared by suspending powder in ethanol ultrasonically and pipetting the solution onto holy carbon-coated copper grids.
655
(a)
3. Results and discussion According to the XRD pattern (Fig. 1f ) and TEM image (Fig. 2e), it can be shown that single crystalline MnOOH (manganite, JCPDS 41-1379) fibers can be synthesized under a CTAB concentration of 0.5 wt.% when the sample is aged for 3 days [18]. As a comparison, a control experiment is carried out in the absence of CTAB, and only g-Mn3O4 (hausmannite, JCPDS 24-734) particles are obtained after aging for 3 days (Fig. 3). This suggests that the cationic surfactant CTAB plays an important role in the formation of MnOOH. A phase transition process can be reflected by the XRD patterns of samples aged for different time: Only crystalline g-Mn3O4 can be found in the sample aged for 6 h (Fig. 1a). With the increase of the aging time (Fig. 1b), the intensity of the diffraction peaks of g-Mn3O4 becomes weaker, while some ordered low-angle diffraction peaks caused by a mesolayered birnessite-type manganese oxide [16] and peaks
(b)
(a) relative intensity (a.u.)
γ-M n3O4
Fig. 4. TEM images of the transitional structures in the sample aged for 1 day: (a) fiber out of h-MnOOH thin plate; (b) fiber out of layered mesostructure. The arrows in (b) point to the interconnection part of the fiber and the large thin plate corresponding to the layered mesostructure.
10
20
30
40
50
60
70
2θ (degree)
(b)
caused by h-MnOOH (feitknechtite, JCPDS 18-804, a layered manganese oxyhydroxide that is free of interlayer cations [19]) turn to appear. When the aging time increases to 1 day (Fig. 1c), diffraction peaks of the layered mesostructure and h-MnOOH become sharper and stronger while the peaks of g-Mn3O4 disappear entirely. When the aging time increases further (Fig. 1d, e), diffraction peaks of the layered mesostructure and h-MnOOH become weaker and weaker until they fade away, while those of MnOOH, which appear in the sample aged for 13 h, become stronger and stronger until the peaks turn to be of pure MnOOH for the sample aged for 3 days (Fig. 1f ). Depending on the changes reflected by XRD patterns, a transformation process during the aging is assumed, which is: c-Mn3 O4 Y layered mesostructure þ feitknechtite Y manganite:
100nm
Fig. 3. XRD pattern (a) and TEM image (b) of the sample synthesized in the absence of CTAB, showing crystalline g-Mn3O4.
In order to confirm the samples’ structures and to clarify the possible formation mechanism, TEM is used to examine the samples further. As shown in Fig. 2, TEM images of the samples demonstrate results that are consistent with those of XRD: Only crystalline Mn3O4 particles can be seen in the sample aged for 6 h (Fig. 2a), while in the sample aged for 13 h (Fig. 2b, c), a mixture of crystalline Mn3O4 paticles, h-
656
X. Sun et al. / Materials Science and Engineering C 26 (2006) 653 – 656
MnOOH thin plates, layered mesostructure and MnOOH fibers [13,17] can be seen. Similar mixed compositions can be found in the sample aged for 1 day except that crystalline Mn3O4 particles are absent (Fig. 2d). The sample aged for 3 days turns to be well crystallized pure manganite fibers (Fig. 2e). Interestingly, some transitional structures are also observed in the sample aged for 1 day (Fig. 4). Selected area electron diffraction pattern of the thin plate in Fig. 4a demonstrates that this platy part is h-MnOOH (not shown), while the stripy and curly morphology of the large thin plate in Fig. 4b indicates that this platy part corresponds to the layered mesostructure. Li and colleagues have reported several systems with nanotube structures that are thought to be rolled up from lamellar mesostructures [20 –22]. The coexistence of the thin plates and the final products, which are nanotubes in their work, is similar to what we observed in our experiments. However, the product in our study is fiber instead of nanotube, so the rolling mechanism seems not to be applicable for our work. But, it gives us an inspiration that the final product may be transformed from a layered phase. On the other hand, different nanowires have been synthesized under the directing of certain surfactants [13,23 – 25], demonstrating the importance of the directing effect of the templates in controlling the morphology and structure of the final products. Based on these reports and the results of our work, a transformation mechanism can be supposed like this: The polar heads of CTAB (CTA+) can interact with the inorganic ions (e.g. O2) of manganese oxides, and the absorption and the binding of surfactant molecules on certain crystal face of manganese oxides may lead to the formation of 1D structures [23,24]. Thus, the growth of manganite is limited in one direction by the localization effect of CTAB molecules so that preferred orientation is obtained to form a fiber-like morphology. 4. Conclusion Pure manganite (MnOOH) fibers are chemically synthesized under the directing of cationic surfactant CTAB. A transformation process is proposed to be g-Mn3O4Ylayered mesostructure + feitknechtiteYmanganite. The localization effect of CTAB is believed to be the controlling force for the formation of manganite fibers. This facile way of using surfactant as directing templates may be applicable for preparation of fibers
of other compositions, although it still needs further studies to validate the mechanism. Acknowledgement The authors would like to thank the National Natural Science Foundation of China for financial support (No. 50402001). References [1] S. Bach, M. Henry, N. Baffier, J. Livage, J. Solid State Chem. 88 (1990) 325. [2] S. Bach, J.P. Pereira-Ramos, N. Baffier, R. Messina, Electrochim. Acta 36 (1991) 1595. [3] J. Hunter, J. Solid State Chem. 39 (1981) 142. [4] S. Bach, J.P. Pereira-Ramos, N. Baffier, J. Solid State Chem. 120 (1995) 70. [5] F.A. Al-Sagheer, M.I. Zaki, Colloids Surf., A Physicochem. Eng. Asp. 173 (2000) 193. [6] S.I. Cordoba de Torresi, A. Gorenstein, Electrochim. Acta 37 (1992) 2015. [7] T.J. Pastor, F.T. Pastor, Talanta 52 (2000) 959. [8] T. Kanasaku, K. Amezawa, N. Yamamoto, Solid State Ion. 133 (2000) 51. [9] M. Nitta, Appl. Catal. 9 (1984) 151. [10] S.-T. Wong, S. Cheng, Inorg. Chem. 31 (1992) 1165. [11] M.E. Landis, B.A. Aufdembrink, P. Chu, I.D. Johnson, G.W. Kirker, M.K. Rubin, J. Am. Chem. Soc. 113 (1991) 3189. [12] M. Ocana, Colloid Polym. Sci. 278 (2000) 443. [13] P.K. Sharma, M.S. Whittingham, Mater. Lett. 48 (2001) 319. [14] X. Wang, Y.D. Li, J. Am. Chem. Soc. 124 (2002) 2880. [15] X. Wang, Y.D. Li, Chem. Commun. 7 (2002) 764. [16] X.D. Sun, C.L. Ma, L. Zeng, Y.D. Wang, H.D. Li, Mater. Res. Bull. 37 (2002) 331. [17] X.D. Sun, C.L. Ma, L. Zeng, Y.D. Wang, H.D. Li, Inorg. Chem. Commun. 10 (2002) 747. [18] XRD pattern of the sample aged for 3 days that is reported here is a little different from that we have reported before in reference [17] because the samples were prepared in different batches, while the formation of MnOOH fibers were affected by many preparation parameters (e.g. the time to add ethylene or water), which may change in different batches. [19] S. Ching, K.S. Krukowska, S.L. Suib, Inorg. Chim. Acta 294 (1999) 123. [20] Y.D. Li, X.L. Li, R.R. He, J. Zhu, Z.X. Deng, J. Am. Chem. Soc. 124 (2002) 1411. [21] Y.D. Li, J.W. Wang, Z.X. Deng, Y.Y. Wu, X.M. Sun, D.P. Yu, P.D. Yang, J. Am. Chem. Soc. 123 (2001) 9904. [22] X. Chen, X.M. Sun, Y.D. Li, Inorg. Chem. 41 (2002) 4524. [23] Y. Li, J.H. Wan, Z.N. Gu, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 286 (2000) 106. [24] L. Yan, Y.D. Li, Z.X. Deng, J. Zhuang, X.M. Sun, Int. J. Inorg. Mater. 3 (2001) 633. [25] X.C. Song, Z.D. Xu, W.X. Chen, G. Han, B. Liu, Y.F. Zheng, Chin. J. Inorg. Chem. 20 (2004) 186.