- Email: [email protected]
A facile hydrothermal synthesis and growth mechanism of novel hollow β-MnO2 polyhedral nanorods
Materials Chemistry and Physics 136 (2012) 831e836
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
A facile hydrothermal synthesis and growth mechanism of novel hollow b-MnO2 polyhedral nanorods Zhong-Hai Ji a, Bin Dong a, c, *, Hai-Ling Guo a, b, Yong-Ming Chai a, b, Yan-Peng Li a, b, Yun-Qi Liu a, b, Chen-Guang Liu a, b, * a b c
State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266555, PR China Key Laboratory of Catalysis, China National Petroleum Corp. (CNPC), China University of Petroleum (East China), Qingdao 266555, PR China College of Science, China University of Petroleum (East China), Qingdao 266555, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
< Hollow polyhedra of b-MnO2 were hydrothermally synthesized. < The formation process of hollow bpolyhedra has been MnO2 investigated. < The growth mechanism of hollow bMnO2 polyhedra was discussed for the first time. < Hollow b-MnO2 polyhedra form by oriented attachment and Ostwald ripening mechanism.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 October 2011 Received in revised form 13 April 2012 Accepted 25 May 2012
Novel hollow b-MnO2 polyhedral nanorods have been successfully synthesized by a simple hydrothermal process. The morphology and structure of the samples obtained at different reaction time have been characterized by XRD, TEM and SEM. The results indicate that b-MnO2 nanorods prepared at reaction time 12 h have hollow, polyhedral structure and uniform size with the length of about 2e3 mm. At longer reaction time, the shells of the hollow b-MnO2 polyhedral nanorods were broken. After analyzing the above results, it’s obvious that the reaction time has a great effect on the structure and morphology of the products. The morphology evolution of MnO2 products demonstrates that the growth mechanism of hollow b-MnO2 polyhedral nanorods could be assigned to the cooperation of oriented attachment and Ostwald ripening mechanism. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Oxides Chemical synthesis Crystal growth Microstructure
1. Introduction Manganese dioxides have attracted great attention due to their unique physical and chemical properties and extensive applications in catalysts [1], molecular-sieves [2], ion-exchangers [3] and
* Corresponding authors. State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266555, PR China. Tel.: þ86 532 86981716; fax: þ86 532 86981787. E-mail addresses: [email protected] (B. Dong), [email protected] (C.-G. Liu). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.05.087
electrochemical supercapacitor [4,5]. Manganese dioxides also have many different structural forms, such as a-, b-, g- and other types possessing the same basic unit (Octahedral MnO6) [6]. It is well known that the properties of MnO2 have close relationship with the crystalline structure and morphology [7]. Thus, development of convenient synthetic methods that could manufacture novel nanostructures will be urgently important. Among conventional synthetic methods, hydrothermal method has proved promising in developing simple avenues for generating different kinds of nanomaterials with different structures, such as onedimensional nanostructures of ZnO, SnO2 and MoO3 [8e10], and
832
Z.-H. Ji et al. / Materials Chemistry and Physics 136 (2012) 831e836
nanoflowers of TiO2, SnS and ZnSe [11e13], Recently, various unique morphologies of MnO2 have also been hydrothermally fabricated including nanospheres, nanowires, nanoplates and nanorods etc. [14e20], which greatly enriched the fundamental research and provided a novel approach to bring forth new properties for MnO2. For example, Cao et al. [21] prepared solid a- and bMnO2 nanorods through a facile hydrothermal process and the solid a- and b-MnO2 nanorods showed the excellent catalytic effects in the Fenton-like reaction. The nanomaterials with hollow interiors are recently receiving increasing commercial interests due to their low density, high surface area, large amount of nanopores and their potential application in delivery and controlled release of drugs, catalysis, acoustic insulation, and low dielectric constant materials [22,23]. To date, despite the great progress that has been made, one-step and template-free synthesis of hollow polyhedral nanomaterials based on growth mechanisms such as Ostwald ripening, Kirkendall effect and oriented attachment mechanism remains a challenge [24e28]. MnO2 with hollow polyhedral nanostructure should have more potential in applications than those with bulk or solid structure. However, reports on MnO2 with hollow polyhedral structures are still limited. Zhang et al. [29] first reported the production of bMnO2 with hollow octahedral morphology and as-prepared samples possessed good catalytic performance on the oxidation of MB dye. However, in Zhang’s work the morphology and size of MnO2 hollow polyhedron was not very uniform and the growth mechanism of MnO2 was not investigated. Hollow polyhedral microrods were also fabricated in the work of Portehault’s and Shen’s respectively [30,31], but both through tedious and complicated procedures. In the latest research, Chen et al. [32] synthesized hollow b-MnO2 polyhedra with the assistance of ionic liquid. However, the introduction of ionic liquid was obviously high cost and further purification was required to get pure products. Therefore, a comprehensive interpretation of the formation mechanism and facile template-free routes to synthesize b-MnO2 hollow polyhedral structures are still lacking. In the present work, a simple hydrothermal method has been developed to prepare novel hollow b-MnO2 polyhedral nanorods without the assistance of templates, surfactant or ionic solvents. MnO2 products synthesized at the different reaction time have been investigated systemically. The growth mechanism of hollow b-MnO2 polyhedral nanorods was also discussed in detail for the first time. It has been found that hollow b-MnO2 nanorods with the polyhedral morphology are formed through the coorporation of oriented attachment and Ostwald ripening. To the best of our knowledge, there has been no report on hollow b-MnO2 polyhedral nanorods obtained in solution based on the combination of the two basic mechanisms. In addition, the results of the present work may provide a new insight into the template-free synthesis of other polyhedral hollow nanostructures.
and then dried at 80 C for 10 h. The influence of the different reaction time on the morphology and crystallinity of MnO2 products was investigated under the same conditions. 2.2. Characterization of the samples The structures and phase compositions of as-synthesized MnO2 products were characterized by X-ray diffraction (XRD) using Panalytical X’Pert Pro MPD diffractometer, equipped with a graphite monochromator using Cu Ka radiation (k ¼ 1.54060 Å) operating at 45 kV and 40 mA. The scan rate was 5 min1 in the 2q range from 10 to 75 . SEM images were taken by a Hitachi S-4800 scanning electron microscope at an accelerating voltage of 5 kV. Transmission electron micrographs (TEM) and High resolution transmission electron micrographs (HRTEM) were performed with JEM-2100 UHR transmission electron microscope at an accelerating voltage of 200 kV. Samples for TEM observation were ultrasonically dispersed in absolute ethanol and then dropped onto a carboncoated copper grid before performance. 3. Results and discussion Fig. 1 shows XRD patterns of samples obtained at 180 C for 12 h. All the diffraction peaks can be readily indexed to the tetragonal phase of b-MnO2 (space group: P42/mnm (136); a ¼ 4.3999 Å and c ¼ 2.8740 Å; JCPDS. 024-0735). No peaks for other polymorphic forms of amorphous MnO2 are detected in the XRD pattern, indicating the high purity of b-MnO2 products. Fig. 2 shows the SEM and TEM morphology of prepared b-MnO2 products at 180 C for 12 h. It can be seen from Fig. 2a that b-MnO2 products consist of large amount of uniform nanorods with the length of about 2e3 mm. With higher magnification, Fig. 2b shows that there is a central opening with side lengths of 200 nm at the end of every nanorod, suggesting they might have hollow structures. It can be also seen from Fig. 2b and c that every b-MnO2 nanorod has a pseudooctahedral appearance. Moreover, the surface of every b-MnO2 nanorod is composed of planes of distinct textures, with part of the relatively rougher ones dissolved. Further information on the structure of as-synthesized samples was conducted by TEM, as shown in Fig. 2d, which confirms the obvious hollow interior of b-MnO2 polyhedral nanorods. In order to shed light on the growth mechanism of the novel hollow b-MnO2 polyhedral nanorods, systematic investigation of
2. Experimental 2.1. Hydrothermal synthesis of the samples All chemicals were of analytical grade and were used as received without further purification. Deionized water was used throughout. In a typical procedure, 4 mmol KMnO4 and an equal amount of (NH4)2SO4 were firstly dissolved into 80 ml deionized water under magnetic stirring, then 1.7 ml of HCl (37 wt%) was added to above the mixture. After stirring for 15 min, the solution was transferred to a 100 ml Teflon-lined autoclave, sealed and maintained at 180 C for 12 h. After the autoclave cooling to room temperature naturally, the resulting products were collected, washed with deionized water and absolute alcohol several times,
Fig. 1. XRD pattern of MnO2 obtained at 180 C for 12 h.
Z.-H. Ji et al. / Materials Chemistry and Physics 136 (2012) 831e836
833
Fig. 2. SEM and TEM images of MnO2 synthesized at 180 C for 12 h.
MnO2 samples synthesized at different reaction time was conducted. The XRD patterns of MnO2 products obtained at different growth stages are presented in Fig. 3. The XRD pattern of MnO2 after hydrothermal process for 3 h can be well assigned to pure tetragonal phase of a-MnO2 (space group I4/m (87), JCPDS 440141). With the longer reaction time of 3.5 h, the XRD diffraction peaks of b-MnO2 have also been observed, which indicates that the products become a mixture of a-MnO2 and b-MnO2. The main diffraction peaks (12, 18, 42, 50 and 60) of a-MnO2 quickly become weaker, suggesting the phase transformation from a-MnO2 to bMnO2 was very rapid over the over the time span of 3e3.5 h. Similar but much slower phase transformation process ranging from 8 h to 48 h was reported by Zhang et al. [18]. We proposed that the higher concentration of Hþ in our method could facilitate the rapid formation of b-MnO2 phase [33]. When the reaction time is
Fig. 3. Powder XRD patterns of nanoproducts synthesized at different aging times. Filled diamonds: a-MnO2 (space group I4/m (87), JCPDS 44-0141), circles: b-MnO2 (space group P42/mnm (136), JCPDS 024-0735).
prolonged to 6 h, only the reflection peaks of b-MnO2 can be detected. This implies the longer hydrothermal process favors the formation of b-MnO2, which is consistent with Portehault’s report [30]. The corresponding morphology evolution of MnO2 samples at different reaction time is shown in Fig. 4. It can be seen from Fig. 4a that a-MnO2 synthesized at reaction time of 3 h have slim rod-like structures with diameters in the range of 20e50 nm and length up to 1 mm. The twin boundaries shown in the inset of Fig. 4b illustrate an obvious growth of the primary nanorods along the longitudinal axis through lateral aggregation. HRTEM image shows that (521) lattice fringes along different directions are connected at the boundaries (see the white arrow in Fig. 4b). We assume that primary nanorods once possessed the same crystalline planes (521) before coalescence, for effective collision cannot be achieved by particles with distinct crystallographic orientation [41]. Moreover, it has been proven that a self-recrystallization process usually follows after the agglomeration of primary particles [42]. Thus, the secondary nanorods are believed to undergo a self-recrystallization process and prefer growing to (110) due to its lower energy. However, such surface rearrangements are obviously not completed at this stage, and therefore two different crystalline planes are observed for the secondary nanorod made of oriented attachment. Defects usually related to the oriented attachment mechanism are also detected (see white lines in Fig. 4b), implying that this mechanism may play an important role in such selfassembly of nanomaterials [34,40]. Wang and Li [35] believed that d-MnO2 was a crucial intermediate in the formation process of a-or b-MnO2 one-dimensional structures, but d-MnO2 is not detected in our work. Moreover, in the light of other growth mechanisms, such as nucleation-dissolution-recrystallization growth mechanism [36] and oriented attachment [37], which are also deemed to be responsible for the growth of one-dimensional nanostructures, the precise formation of primary a-MnO2 nanorods under current conditions deserves further research. Fig. 4c shows SEM morphology of the mixture of a-MnO2 and b-MnO2 at reaction time 3.5 h. It gives a description of well-shaped solid polyhedral b-MnO2 nanorod with pyramid top deriving from the
834
Z.-H. Ji et al. / Materials Chemistry and Physics 136 (2012) 831e836
Fig. 4. TEM, HRTEM, and SEM images of the products prepared after reaction at 180 C for: (a, b) 3 h; (c, d) 3.5 h; (e, f) 6 h.
Fig. 5. SEM micrographs (a, b) and HRTEM (c) of hollow b-MnO2 polyhedral nanorods obtained at 180 C for 24 h.
Z.-H. Ji et al. / Materials Chemistry and Physics 136 (2012) 831e836
aggregates of a-MnO2 nanorods, which may be corresponding to the phase transformation from a-MnO2 to b-MnO2. There are smooth surfaces on the b-MnO2 polyhedral rods, which means that Ostwald ripening process should occur at this stage [28]. While the presence of tough surfaces and defects on b-MnO2 polyhedral rods indicates that the polyhedra might be formed via oriented attachment growth [27,28]. TEM was conducted in order to obtain more distinct structure of the samples. The loose aggregates of slim aMnO2 nanorods with pseudo-polyhedral configuration are clearly demonstrated in Fig. 4d. The pseudo-polyhedral configuration (as stated in Fig. 4d) is corresponding to the polyhedral structure of bMnO2 nanorods. Further extending the reaction time to 6 h, as shown in Fig. 4e, more b-MnO2 prismatic rods were formed with fewer a-MnO2 nanorods coexisting, which can not be detected by XRD due to the too low content of a-MnO2 (as shown in Fig. 3). It also can be seen from Fig. 4e that there is an opening at the top of bMnO2 nanorod, indicating that the solid evacuation firstly occurs at reaction time 6 h. TEM image confirms that more b-MnO2 nanorods occurs and the solid evacuation starts from the end of b-MnO2 nanorod (as shown in Fig. 4f). However, most of hollow b-MnO2 polyhedral nanorods are broken at reaction time 24 h (Fig. 5a). It can be seen from the magnified image (as shown in Fig. 5b) that the shell of b-MnO2 nanorods has been obviously dissolved. TEM image (Fig. 5c) also confirms the damage of hollow b-MnO2 nanorods. The above results show the reaction time has a great influence on the structure of hollow b-MnO2 nanorods. It was reported that thinner shells of hollow structures formed through Ostwald Ripening mechanism will be produced via longer reactions, which implies that the solid evacuation of novel b-MnO2 nanorods in our work can be assigned to the Ostwald ripening mechanism [38,39]. On the other hand, although Cl2 is generated in the reaction, no evidence is found that favors self-generated soft-template mechanism, in which hollow particles are formed by “attachment of fine nanoparticles on the gas/liquid interface” [23]. On the basis of the above results and the mechanisms previously proposed [18,27,28], the growth of hollow b-MnO2 polyhedral nanorods can be briefly elucidated in the schematic illustration as shown in Fig. 6. First, primary a-MnO2 nanorods were fabricated. Due to the high surface energy resulting from defects at primary nanorods, lateral aggregation of these primary nanorods occurred by oriented attachment mechanism, leading to the formation of secondary nanorods [40]. As-produced secondary nanorods could still aggregate by oriented attachment mechanism to further reduce surface energy. Secondly, solid b-MnO2 polyhedral nanorods were obtained from the oriented aggregations of a-MnO2 nanorods through dissolution-recrystallization process because of the high solubility of Mn species in a highly acidic medium [40]. As the dissolution of a-MnO2 proceeded inside-out, smaller b-MnO2 crystallites were initially generated in the cores due to the high supersaturation conditions, and relatively large b-MnO2 crystallites formed on or near the surface due to the relatively low supersaturation conditions. At the same time, the oriented attachment on MnO2 particles’ high energy planes accelerated the phase
Fig. 6. Schematic illustration of the possible formation mechanism of hollow b-MnO2 polyhedral nanorods.
835
transformation from a-MnO2 to b-MnO2 [43]. The rapid phase transformation may lead to size difference of crystallites on the surface. In the third stage, adjacent nanorods could be attached and fused into solid b-MnO2 polyhedral nanorods until the total consumption of a-MnO2 nanorods. In the fourth stage, due to their higher surface energy than larger ones, smaller crystallites in the central part would dissolve and regrow on larger ones to lower the total energy of the system [38,39]. Such solid evacuation in the central part via Ostwald ripening results in novel hollow b-MnO2 polyhedral nanorods. The slight dissolution of relatively small crystallites in tough surfaces of b-MnO2 nanorods also happened coincidently via Ostwald ripening, which might enventually destroy hollow structures in the case of longer reaction. 4. Conclusions In summary, we have developed a novel and facile hydrothermal route to synthesize hollow polyhedron of b-MnO2 without the assistance of catalysts, templates or ionic liquid, etc. Based on experimental results, the growth mechanism of hollow b-MnO2 polyhedral nanorods was discussed in detail for the first time, which could be attributed to the combination of oriented attachment and Ostwald ripening mechanism. This study may provide a new insight into the growth behaviors of MnO2 crystals and developing template-free avenues for the synthesis of other polyhedral hollow nanostructures. Furthermore, as-prepared samples may find potential applications in catalysis, lithium batteries and magnetics due to their unique structure and morphology. Acknowledgments This work was financially supported by the Major State Basic Research Development Program of China (973 Program, 2010CB226905) and the Fundamental Research Funds for the Central Universities (10CX04019A) and the National Natural Science Foundation of China (No. 21006128). References [1] Y. Dong, H. Yang, K. He, S. Song, A. Zhang, Appl. Catal. B 85 (2009) 155e161. [2] S.L. Suib, Acc. Chem. Res. 41 (2008) 479e487. [3] J.K. Yuan, W.N. Li, S. Gomez, S. Suib, J. Am. Chem. Soc. 127 (2005) 14184e 14185. [4] X.K. Huang, D.P. Lv, Q.S. Zhang, H.T. Chang, J.L. Gan, Y. Yang, Electrochim. Acta 55 (2010) 4915e4920. [5] P.K. Nayak, N. Munichandraiah, Microporous Mesoporous Mater. 143 (2011) 206e214. [6] M.M. Thackeray, Prog. Solid State Chem. 25 (1997) 1e4. [7] L.C. Zhang, Z.H. Liu, H. Lv, X.H. Tang, K. Ooi, J. Phys. Chem. C 111 (2007) 8418e 8423. [8] O. Lupan, L. Chow, G.Y. Chai, B. Roldan, A. Naitabdi, A. Schulte, H. Heinrich, Mater. Sci. Eng. B 145 (2007) 57e66. [9] O. Lupana, L. Chow, G.Y. Chai, A. Schulte, S. Park, H. Heinrich, Mater. Sci. Eng. B 157 (2009) 101e104. [10] X.W. Lou, H.C. Zeng, Chem. Mater. 14 (2002) 4781e4789. [11] A.K. Sinha, S. Jana, S. Pande, S. Sarkar, M. Pradhan, M. Basu, S. Saha, A. Pal, T. Pal, CrystEngComm 11 (2009) 1210e1212. [12] H.L. Zhu, D. Yang, H. Zhang, Mater. Lett. 60 (2006) 2686e2689. [13] F. Cao, W.D. Shi, L.J. Zhao, S.Y. Song, J.H. Yang, Y.Q. Lei, H.J. Zhang, J. Phys. Chem. C 112 (2008) 17095e17101. [14] Y. Liu, M. Zhang, J. Zhang, Y. Qian, J. Solid State Chem. 179 (2006) 1757e1761. [15] L. Li, Y. Chu, Y. Liu, L. Dong, Mater. Lett. 61 (2007) 1609e1613. [16] H.E. Wang, D. Qian, Mater. Chem. Phys. 109 (2008) 399e403. [17] J. Yan, T. Wei, J. Cheng, Z.J. Fan, M.L. Zhang, Mater. Res. Bull. 45 (2010) 210e215. [18] X. Zhang, W.S. Yang, J.J. Yang, D.G. Evans, J. Cryst. Growth 310 (2008) 716e722. [19] L.C. Zhang, Z.H. Liu, X.H. Tang, J.F. Wang, K. Ooi, Mater. Res. Bull. 42 (2007) 1432e1439. [20] N. Wang, H.T. Pang, H.R. Peng, G.C. Li, X.G. Chen, Cryst. Res. Technol. 44 (2009) 1230e1234. [21] G.S. Cao, L. Su, X.J. Zhang, H. Li, Mater. Res. Bull. 45 (2010) 425e428. [22] F. Caruso, Chem. Eur. J. 6 (2000) 413e419. [23] X.W. Lou, L.A. Archer, Z.C. Yang, Adv. Mater. 20 (2008) 3987e4019.
836 [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
Z.-H. Ji et al. / Materials Chemistry and Physics 136 (2012) 831e836 L.N. Ye, C.Z. Wu, W. Guo, Y. Xie, Chem. Commun. (2006) 4738e4740. H.G. Yang, H.C. Zeng, Angew. Chem. Int. Ed. 43 (2004) 5930e5933. L.N. Ye, W. Guo, Y. Yang, Y.F. Du, Y. Xie, Chem. Mater. 19 (2007) 6331e6337. J.J. Teo, Y. Chang, H.C. Zeng, Langmuir 22 (2006) 7369e7377. B.P. Jia, L. Gao, Cryst. Growth Des. 8 (2008) 1372e1376. Y.G. Zhang, L.Y. Chen, Z. Zheng, F.L. Yang, Solid State Sci. 11 (2009) 1265e1269. D. Portehault, S. Cassaignon, E. Baudrinc, J.P. Jolivet, J. Mater. Chem. 19 (2009) 2407e2416. X.F. Shen, Y.S. Ding, J. Liu, J. Cai, K. Laubernds, R.P. Zerger, A. Vasiliev, M. Aindow, S.L. Suib, Adv. Mater. 17 (2005) 805e809. N. Chen, K. Wang, X. Zhanga, X.P. Chang, L.P. Kang, Z.H. Liu, Colloids Surf. A: Physicochem. Eng. Asp. 387 (2011) 10e16. X.K. Huang, D.P. Lv, H.J. Yue, A. Attia, Y. Yang, Nanotechnology 19 (2008) 225606e225612.
[34] [35] [36] [37] [38] [39] [40] [41] [42] [43]
R.L. Penn, A.T. Stone, D.R. Veblen, J. Phys. Chem. B 105 (2001) 4690e4697. X. Wang, Y.D. Li, Chem. Eur. J. 9 (2003) 300e306. G.C. Xi, Y.Y. Peng, W.C. Yu, Y.T. Qian, Cryst. Growth Des. 5 (2005) 325e328. K.S. Cho, D.V. Talapin, W. Gaschler, C.B. Murray, J. Am. Chem. Soc. 127 (2005) 7140e7147. H.C. Zeng, Curr. Nanosci. 3 (2007) 177e181. C.K. King’ondu, A. Iyer, E.C. Njagi, N. Opembe, H. Genuino, H. Huang, R.A. Ristau, S.L. Suib, J. Am. Chem. Soc. 133 (2011) 4186e4189. D. Portehault, S. Cassaignon, E. Baudrin, J.P. Jolive, Chem. Mater. 19 (2007) 5410e5417. C.J. Dalmaschio, C. Ribeirob, E.R. Leite, Nanoscale 2 (2010) 2336e2345. J.M. Yuk, J. Park, P. Ercius, K. Kim, D.J. Hellebusch, M.F. Crommie, J.Y. Lee, A. Zettl, A.P. Alivisatos, Science 336 (2012) 61e64. C. Ribeiro, C. Vila, J.M.E. Matos, J. Bettini, E. Longo, E.R. Leite, Chem. Eur. J. 13 (2007) 5798e5803.
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
A facile hydrothermal synthesis and growth mechanism of novel hollow b-MnO2 polyhedral nanorods Zhong-Hai Ji a, Bin Dong a, c, *, Hai-Ling Guo a, b, Yong-Ming Chai a, b, Yan-Peng Li a, b, Yun-Qi Liu a, b, Chen-Guang Liu a, b, * a b c
State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266555, PR China Key Laboratory of Catalysis, China National Petroleum Corp. (CNPC), China University of Petroleum (East China), Qingdao 266555, PR China College of Science, China University of Petroleum (East China), Qingdao 266555, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
< Hollow polyhedra of b-MnO2 were hydrothermally synthesized. < The formation process of hollow bpolyhedra has been MnO2 investigated. < The growth mechanism of hollow bMnO2 polyhedra was discussed for the first time. < Hollow b-MnO2 polyhedra form by oriented attachment and Ostwald ripening mechanism.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 October 2011 Received in revised form 13 April 2012 Accepted 25 May 2012
Novel hollow b-MnO2 polyhedral nanorods have been successfully synthesized by a simple hydrothermal process. The morphology and structure of the samples obtained at different reaction time have been characterized by XRD, TEM and SEM. The results indicate that b-MnO2 nanorods prepared at reaction time 12 h have hollow, polyhedral structure and uniform size with the length of about 2e3 mm. At longer reaction time, the shells of the hollow b-MnO2 polyhedral nanorods were broken. After analyzing the above results, it’s obvious that the reaction time has a great effect on the structure and morphology of the products. The morphology evolution of MnO2 products demonstrates that the growth mechanism of hollow b-MnO2 polyhedral nanorods could be assigned to the cooperation of oriented attachment and Ostwald ripening mechanism. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Oxides Chemical synthesis Crystal growth Microstructure
1. Introduction Manganese dioxides have attracted great attention due to their unique physical and chemical properties and extensive applications in catalysts [1], molecular-sieves [2], ion-exchangers [3] and
* Corresponding authors. State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266555, PR China. Tel.: þ86 532 86981716; fax: þ86 532 86981787. E-mail addresses: [email protected] (B. Dong), [email protected] (C.-G. Liu). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.05.087
electrochemical supercapacitor [4,5]. Manganese dioxides also have many different structural forms, such as a-, b-, g- and other types possessing the same basic unit (Octahedral MnO6) [6]. It is well known that the properties of MnO2 have close relationship with the crystalline structure and morphology [7]. Thus, development of convenient synthetic methods that could manufacture novel nanostructures will be urgently important. Among conventional synthetic methods, hydrothermal method has proved promising in developing simple avenues for generating different kinds of nanomaterials with different structures, such as onedimensional nanostructures of ZnO, SnO2 and MoO3 [8e10], and
832
Z.-H. Ji et al. / Materials Chemistry and Physics 136 (2012) 831e836
nanoflowers of TiO2, SnS and ZnSe [11e13], Recently, various unique morphologies of MnO2 have also been hydrothermally fabricated including nanospheres, nanowires, nanoplates and nanorods etc. [14e20], which greatly enriched the fundamental research and provided a novel approach to bring forth new properties for MnO2. For example, Cao et al. [21] prepared solid a- and bMnO2 nanorods through a facile hydrothermal process and the solid a- and b-MnO2 nanorods showed the excellent catalytic effects in the Fenton-like reaction. The nanomaterials with hollow interiors are recently receiving increasing commercial interests due to their low density, high surface area, large amount of nanopores and their potential application in delivery and controlled release of drugs, catalysis, acoustic insulation, and low dielectric constant materials [22,23]. To date, despite the great progress that has been made, one-step and template-free synthesis of hollow polyhedral nanomaterials based on growth mechanisms such as Ostwald ripening, Kirkendall effect and oriented attachment mechanism remains a challenge [24e28]. MnO2 with hollow polyhedral nanostructure should have more potential in applications than those with bulk or solid structure. However, reports on MnO2 with hollow polyhedral structures are still limited. Zhang et al. [29] first reported the production of bMnO2 with hollow octahedral morphology and as-prepared samples possessed good catalytic performance on the oxidation of MB dye. However, in Zhang’s work the morphology and size of MnO2 hollow polyhedron was not very uniform and the growth mechanism of MnO2 was not investigated. Hollow polyhedral microrods were also fabricated in the work of Portehault’s and Shen’s respectively [30,31], but both through tedious and complicated procedures. In the latest research, Chen et al. [32] synthesized hollow b-MnO2 polyhedra with the assistance of ionic liquid. However, the introduction of ionic liquid was obviously high cost and further purification was required to get pure products. Therefore, a comprehensive interpretation of the formation mechanism and facile template-free routes to synthesize b-MnO2 hollow polyhedral structures are still lacking. In the present work, a simple hydrothermal method has been developed to prepare novel hollow b-MnO2 polyhedral nanorods without the assistance of templates, surfactant or ionic solvents. MnO2 products synthesized at the different reaction time have been investigated systemically. The growth mechanism of hollow b-MnO2 polyhedral nanorods was also discussed in detail for the first time. It has been found that hollow b-MnO2 nanorods with the polyhedral morphology are formed through the coorporation of oriented attachment and Ostwald ripening. To the best of our knowledge, there has been no report on hollow b-MnO2 polyhedral nanorods obtained in solution based on the combination of the two basic mechanisms. In addition, the results of the present work may provide a new insight into the template-free synthesis of other polyhedral hollow nanostructures.
and then dried at 80 C for 10 h. The influence of the different reaction time on the morphology and crystallinity of MnO2 products was investigated under the same conditions. 2.2. Characterization of the samples The structures and phase compositions of as-synthesized MnO2 products were characterized by X-ray diffraction (XRD) using Panalytical X’Pert Pro MPD diffractometer, equipped with a graphite monochromator using Cu Ka radiation (k ¼ 1.54060 Å) operating at 45 kV and 40 mA. The scan rate was 5 min1 in the 2q range from 10 to 75 . SEM images were taken by a Hitachi S-4800 scanning electron microscope at an accelerating voltage of 5 kV. Transmission electron micrographs (TEM) and High resolution transmission electron micrographs (HRTEM) were performed with JEM-2100 UHR transmission electron microscope at an accelerating voltage of 200 kV. Samples for TEM observation were ultrasonically dispersed in absolute ethanol and then dropped onto a carboncoated copper grid before performance. 3. Results and discussion Fig. 1 shows XRD patterns of samples obtained at 180 C for 12 h. All the diffraction peaks can be readily indexed to the tetragonal phase of b-MnO2 (space group: P42/mnm (136); a ¼ 4.3999 Å and c ¼ 2.8740 Å; JCPDS. 024-0735). No peaks for other polymorphic forms of amorphous MnO2 are detected in the XRD pattern, indicating the high purity of b-MnO2 products. Fig. 2 shows the SEM and TEM morphology of prepared b-MnO2 products at 180 C for 12 h. It can be seen from Fig. 2a that b-MnO2 products consist of large amount of uniform nanorods with the length of about 2e3 mm. With higher magnification, Fig. 2b shows that there is a central opening with side lengths of 200 nm at the end of every nanorod, suggesting they might have hollow structures. It can be also seen from Fig. 2b and c that every b-MnO2 nanorod has a pseudooctahedral appearance. Moreover, the surface of every b-MnO2 nanorod is composed of planes of distinct textures, with part of the relatively rougher ones dissolved. Further information on the structure of as-synthesized samples was conducted by TEM, as shown in Fig. 2d, which confirms the obvious hollow interior of b-MnO2 polyhedral nanorods. In order to shed light on the growth mechanism of the novel hollow b-MnO2 polyhedral nanorods, systematic investigation of
2. Experimental 2.1. Hydrothermal synthesis of the samples All chemicals were of analytical grade and were used as received without further purification. Deionized water was used throughout. In a typical procedure, 4 mmol KMnO4 and an equal amount of (NH4)2SO4 were firstly dissolved into 80 ml deionized water under magnetic stirring, then 1.7 ml of HCl (37 wt%) was added to above the mixture. After stirring for 15 min, the solution was transferred to a 100 ml Teflon-lined autoclave, sealed and maintained at 180 C for 12 h. After the autoclave cooling to room temperature naturally, the resulting products were collected, washed with deionized water and absolute alcohol several times,
Fig. 1. XRD pattern of MnO2 obtained at 180 C for 12 h.
Z.-H. Ji et al. / Materials Chemistry and Physics 136 (2012) 831e836
833
Fig. 2. SEM and TEM images of MnO2 synthesized at 180 C for 12 h.
MnO2 samples synthesized at different reaction time was conducted. The XRD patterns of MnO2 products obtained at different growth stages are presented in Fig. 3. The XRD pattern of MnO2 after hydrothermal process for 3 h can be well assigned to pure tetragonal phase of a-MnO2 (space group I4/m (87), JCPDS 440141). With the longer reaction time of 3.5 h, the XRD diffraction peaks of b-MnO2 have also been observed, which indicates that the products become a mixture of a-MnO2 and b-MnO2. The main diffraction peaks (12, 18, 42, 50 and 60) of a-MnO2 quickly become weaker, suggesting the phase transformation from a-MnO2 to bMnO2 was very rapid over the over the time span of 3e3.5 h. Similar but much slower phase transformation process ranging from 8 h to 48 h was reported by Zhang et al. [18]. We proposed that the higher concentration of Hþ in our method could facilitate the rapid formation of b-MnO2 phase [33]. When the reaction time is
Fig. 3. Powder XRD patterns of nanoproducts synthesized at different aging times. Filled diamonds: a-MnO2 (space group I4/m (87), JCPDS 44-0141), circles: b-MnO2 (space group P42/mnm (136), JCPDS 024-0735).
prolonged to 6 h, only the reflection peaks of b-MnO2 can be detected. This implies the longer hydrothermal process favors the formation of b-MnO2, which is consistent with Portehault’s report [30]. The corresponding morphology evolution of MnO2 samples at different reaction time is shown in Fig. 4. It can be seen from Fig. 4a that a-MnO2 synthesized at reaction time of 3 h have slim rod-like structures with diameters in the range of 20e50 nm and length up to 1 mm. The twin boundaries shown in the inset of Fig. 4b illustrate an obvious growth of the primary nanorods along the longitudinal axis through lateral aggregation. HRTEM image shows that (521) lattice fringes along different directions are connected at the boundaries (see the white arrow in Fig. 4b). We assume that primary nanorods once possessed the same crystalline planes (521) before coalescence, for effective collision cannot be achieved by particles with distinct crystallographic orientation [41]. Moreover, it has been proven that a self-recrystallization process usually follows after the agglomeration of primary particles [42]. Thus, the secondary nanorods are believed to undergo a self-recrystallization process and prefer growing to (110) due to its lower energy. However, such surface rearrangements are obviously not completed at this stage, and therefore two different crystalline planes are observed for the secondary nanorod made of oriented attachment. Defects usually related to the oriented attachment mechanism are also detected (see white lines in Fig. 4b), implying that this mechanism may play an important role in such selfassembly of nanomaterials [34,40]. Wang and Li [35] believed that d-MnO2 was a crucial intermediate in the formation process of a-or b-MnO2 one-dimensional structures, but d-MnO2 is not detected in our work. Moreover, in the light of other growth mechanisms, such as nucleation-dissolution-recrystallization growth mechanism [36] and oriented attachment [37], which are also deemed to be responsible for the growth of one-dimensional nanostructures, the precise formation of primary a-MnO2 nanorods under current conditions deserves further research. Fig. 4c shows SEM morphology of the mixture of a-MnO2 and b-MnO2 at reaction time 3.5 h. It gives a description of well-shaped solid polyhedral b-MnO2 nanorod with pyramid top deriving from the
834
Z.-H. Ji et al. / Materials Chemistry and Physics 136 (2012) 831e836
Fig. 4. TEM, HRTEM, and SEM images of the products prepared after reaction at 180 C for: (a, b) 3 h; (c, d) 3.5 h; (e, f) 6 h.
Fig. 5. SEM micrographs (a, b) and HRTEM (c) of hollow b-MnO2 polyhedral nanorods obtained at 180 C for 24 h.
Z.-H. Ji et al. / Materials Chemistry and Physics 136 (2012) 831e836
aggregates of a-MnO2 nanorods, which may be corresponding to the phase transformation from a-MnO2 to b-MnO2. There are smooth surfaces on the b-MnO2 polyhedral rods, which means that Ostwald ripening process should occur at this stage [28]. While the presence of tough surfaces and defects on b-MnO2 polyhedral rods indicates that the polyhedra might be formed via oriented attachment growth [27,28]. TEM was conducted in order to obtain more distinct structure of the samples. The loose aggregates of slim aMnO2 nanorods with pseudo-polyhedral configuration are clearly demonstrated in Fig. 4d. The pseudo-polyhedral configuration (as stated in Fig. 4d) is corresponding to the polyhedral structure of bMnO2 nanorods. Further extending the reaction time to 6 h, as shown in Fig. 4e, more b-MnO2 prismatic rods were formed with fewer a-MnO2 nanorods coexisting, which can not be detected by XRD due to the too low content of a-MnO2 (as shown in Fig. 3). It also can be seen from Fig. 4e that there is an opening at the top of bMnO2 nanorod, indicating that the solid evacuation firstly occurs at reaction time 6 h. TEM image confirms that more b-MnO2 nanorods occurs and the solid evacuation starts from the end of b-MnO2 nanorod (as shown in Fig. 4f). However, most of hollow b-MnO2 polyhedral nanorods are broken at reaction time 24 h (Fig. 5a). It can be seen from the magnified image (as shown in Fig. 5b) that the shell of b-MnO2 nanorods has been obviously dissolved. TEM image (Fig. 5c) also confirms the damage of hollow b-MnO2 nanorods. The above results show the reaction time has a great influence on the structure of hollow b-MnO2 nanorods. It was reported that thinner shells of hollow structures formed through Ostwald Ripening mechanism will be produced via longer reactions, which implies that the solid evacuation of novel b-MnO2 nanorods in our work can be assigned to the Ostwald ripening mechanism [38,39]. On the other hand, although Cl2 is generated in the reaction, no evidence is found that favors self-generated soft-template mechanism, in which hollow particles are formed by “attachment of fine nanoparticles on the gas/liquid interface” [23]. On the basis of the above results and the mechanisms previously proposed [18,27,28], the growth of hollow b-MnO2 polyhedral nanorods can be briefly elucidated in the schematic illustration as shown in Fig. 6. First, primary a-MnO2 nanorods were fabricated. Due to the high surface energy resulting from defects at primary nanorods, lateral aggregation of these primary nanorods occurred by oriented attachment mechanism, leading to the formation of secondary nanorods [40]. As-produced secondary nanorods could still aggregate by oriented attachment mechanism to further reduce surface energy. Secondly, solid b-MnO2 polyhedral nanorods were obtained from the oriented aggregations of a-MnO2 nanorods through dissolution-recrystallization process because of the high solubility of Mn species in a highly acidic medium [40]. As the dissolution of a-MnO2 proceeded inside-out, smaller b-MnO2 crystallites were initially generated in the cores due to the high supersaturation conditions, and relatively large b-MnO2 crystallites formed on or near the surface due to the relatively low supersaturation conditions. At the same time, the oriented attachment on MnO2 particles’ high energy planes accelerated the phase
Fig. 6. Schematic illustration of the possible formation mechanism of hollow b-MnO2 polyhedral nanorods.
835
transformation from a-MnO2 to b-MnO2 [43]. The rapid phase transformation may lead to size difference of crystallites on the surface. In the third stage, adjacent nanorods could be attached and fused into solid b-MnO2 polyhedral nanorods until the total consumption of a-MnO2 nanorods. In the fourth stage, due to their higher surface energy than larger ones, smaller crystallites in the central part would dissolve and regrow on larger ones to lower the total energy of the system [38,39]. Such solid evacuation in the central part via Ostwald ripening results in novel hollow b-MnO2 polyhedral nanorods. The slight dissolution of relatively small crystallites in tough surfaces of b-MnO2 nanorods also happened coincidently via Ostwald ripening, which might enventually destroy hollow structures in the case of longer reaction. 4. Conclusions In summary, we have developed a novel and facile hydrothermal route to synthesize hollow polyhedron of b-MnO2 without the assistance of catalysts, templates or ionic liquid, etc. Based on experimental results, the growth mechanism of hollow b-MnO2 polyhedral nanorods was discussed in detail for the first time, which could be attributed to the combination of oriented attachment and Ostwald ripening mechanism. This study may provide a new insight into the growth behaviors of MnO2 crystals and developing template-free avenues for the synthesis of other polyhedral hollow nanostructures. Furthermore, as-prepared samples may find potential applications in catalysis, lithium batteries and magnetics due to their unique structure and morphology. Acknowledgments This work was financially supported by the Major State Basic Research Development Program of China (973 Program, 2010CB226905) and the Fundamental Research Funds for the Central Universities (10CX04019A) and the National Natural Science Foundation of China (No. 21006128). References [1] Y. Dong, H. Yang, K. He, S. Song, A. Zhang, Appl. Catal. B 85 (2009) 155e161. [2] S.L. Suib, Acc. Chem. Res. 41 (2008) 479e487. [3] J.K. Yuan, W.N. Li, S. Gomez, S. Suib, J. Am. Chem. Soc. 127 (2005) 14184e 14185. [4] X.K. Huang, D.P. Lv, Q.S. Zhang, H.T. Chang, J.L. Gan, Y. Yang, Electrochim. Acta 55 (2010) 4915e4920. [5] P.K. Nayak, N. Munichandraiah, Microporous Mesoporous Mater. 143 (2011) 206e214. [6] M.M. Thackeray, Prog. Solid State Chem. 25 (1997) 1e4. [7] L.C. Zhang, Z.H. Liu, H. Lv, X.H. Tang, K. Ooi, J. Phys. Chem. C 111 (2007) 8418e 8423. [8] O. Lupan, L. Chow, G.Y. Chai, B. Roldan, A. Naitabdi, A. Schulte, H. Heinrich, Mater. Sci. Eng. B 145 (2007) 57e66. [9] O. Lupana, L. Chow, G.Y. Chai, A. Schulte, S. Park, H. Heinrich, Mater. Sci. Eng. B 157 (2009) 101e104. [10] X.W. Lou, H.C. Zeng, Chem. Mater. 14 (2002) 4781e4789. [11] A.K. Sinha, S. Jana, S. Pande, S. Sarkar, M. Pradhan, M. Basu, S. Saha, A. Pal, T. Pal, CrystEngComm 11 (2009) 1210e1212. [12] H.L. Zhu, D. Yang, H. Zhang, Mater. Lett. 60 (2006) 2686e2689. [13] F. Cao, W.D. Shi, L.J. Zhao, S.Y. Song, J.H. Yang, Y.Q. Lei, H.J. Zhang, J. Phys. Chem. C 112 (2008) 17095e17101. [14] Y. Liu, M. Zhang, J. Zhang, Y. Qian, J. Solid State Chem. 179 (2006) 1757e1761. [15] L. Li, Y. Chu, Y. Liu, L. Dong, Mater. Lett. 61 (2007) 1609e1613. [16] H.E. Wang, D. Qian, Mater. Chem. Phys. 109 (2008) 399e403. [17] J. Yan, T. Wei, J. Cheng, Z.J. Fan, M.L. Zhang, Mater. Res. Bull. 45 (2010) 210e215. [18] X. Zhang, W.S. Yang, J.J. Yang, D.G. Evans, J. Cryst. Growth 310 (2008) 716e722. [19] L.C. Zhang, Z.H. Liu, X.H. Tang, J.F. Wang, K. Ooi, Mater. Res. Bull. 42 (2007) 1432e1439. [20] N. Wang, H.T. Pang, H.R. Peng, G.C. Li, X.G. Chen, Cryst. Res. Technol. 44 (2009) 1230e1234. [21] G.S. Cao, L. Su, X.J. Zhang, H. Li, Mater. Res. Bull. 45 (2010) 425e428. [22] F. Caruso, Chem. Eur. J. 6 (2000) 413e419. [23] X.W. Lou, L.A. Archer, Z.C. Yang, Adv. Mater. 20 (2008) 3987e4019.
836 [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
Z.-H. Ji et al. / Materials Chemistry and Physics 136 (2012) 831e836 L.N. Ye, C.Z. Wu, W. Guo, Y. Xie, Chem. Commun. (2006) 4738e4740. H.G. Yang, H.C. Zeng, Angew. Chem. Int. Ed. 43 (2004) 5930e5933. L.N. Ye, W. Guo, Y. Yang, Y.F. Du, Y. Xie, Chem. Mater. 19 (2007) 6331e6337. J.J. Teo, Y. Chang, H.C. Zeng, Langmuir 22 (2006) 7369e7377. B.P. Jia, L. Gao, Cryst. Growth Des. 8 (2008) 1372e1376. Y.G. Zhang, L.Y. Chen, Z. Zheng, F.L. Yang, Solid State Sci. 11 (2009) 1265e1269. D. Portehault, S. Cassaignon, E. Baudrinc, J.P. Jolivet, J. Mater. Chem. 19 (2009) 2407e2416. X.F. Shen, Y.S. Ding, J. Liu, J. Cai, K. Laubernds, R.P. Zerger, A. Vasiliev, M. Aindow, S.L. Suib, Adv. Mater. 17 (2005) 805e809. N. Chen, K. Wang, X. Zhanga, X.P. Chang, L.P. Kang, Z.H. Liu, Colloids Surf. A: Physicochem. Eng. Asp. 387 (2011) 10e16. X.K. Huang, D.P. Lv, H.J. Yue, A. Attia, Y. Yang, Nanotechnology 19 (2008) 225606e225612.
[34] [35] [36] [37] [38] [39] [40] [41] [42] [43]
R.L. Penn, A.T. Stone, D.R. Veblen, J. Phys. Chem. B 105 (2001) 4690e4697. X. Wang, Y.D. Li, Chem. Eur. J. 9 (2003) 300e306. G.C. Xi, Y.Y. Peng, W.C. Yu, Y.T. Qian, Cryst. Growth Des. 5 (2005) 325e328. K.S. Cho, D.V. Talapin, W. Gaschler, C.B. Murray, J. Am. Chem. Soc. 127 (2005) 7140e7147. H.C. Zeng, Curr. Nanosci. 3 (2007) 177e181. C.K. King’ondu, A. Iyer, E.C. Njagi, N. Opembe, H. Genuino, H. Huang, R.A. Ristau, S.L. Suib, J. Am. Chem. Soc. 133 (2011) 4186e4189. D. Portehault, S. Cassaignon, E. Baudrin, J.P. Jolive, Chem. Mater. 19 (2007) 5410e5417. C.J. Dalmaschio, C. Ribeirob, E.R. Leite, Nanoscale 2 (2010) 2336e2345. J.M. Yuk, J. Park, P. Ercius, K. Kim, D.J. Hellebusch, M.F. Crommie, J.Y. Lee, A. Zettl, A.P. Alivisatos, Science 336 (2012) 61e64. C. Ribeiro, C. Vila, J.M.E. Matos, J. Bettini, E. Longo, E.R. Leite, Chem. Eur. J. 13 (2007) 5798e5803.