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The synthesis of ultra-long cobalt chains and its outstanding catalytic performance on the thermal decomposition of ammonium perchlorate
Materials Chemistry and Physics 201 (2017) 235e240
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
The synthesis of ultra-long cobalt chains and its outstanding catalytic performance on the thermal decomposition of ammonium perchlorate Yongjie Zhao a, *, Xiaowei Zhang a, Xiangming Xu a, Yuzhen Zhao b, Heping Zhou b, Jingbo Li a, Haibo Jin a a Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications, School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, China b State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, 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
Ultra-long cobalt chains were synthetized. The growth mechanism was explored by controlling the reaction time. Elevated ferromagnetic property relies on the preferred orientation and shape anisotropy of cobalt chains. A synergistic effect mechanism for excellent catalytic performance was proposed.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 December 2016 Received in revised form 21 July 2017 Accepted 21 August 2017 Available online 24 August 2017
A simple method without assisted of external magnetic field or surfactant soft templates was utilized to fabricate ultra-long cobalt chains. The formation mechanism was investigated by time dependent experiment, and it was considered that the link of adjacent spherical particles were through dipolar interaction. Due to the preferred orientation and shape anisotropy of the chain structure, the coercively (Hc) value was about 84.5 Oe and saturation magnetization value was up to 149 emu/g at room temperature. To develop its further practical application, the products with regard to accelerate the thermal decomposition of ammonium perchlorate revealed that it exhibited an outstanding performance of reducing the thermal decomposition temperature. This excellent performance has great practical significance on the development of solid rocket fuels. The mechanism of the excellent performance probably ascribed to the synergistic effect of cobalt and cobalt oxide was proposed. © 2017 Elsevier B.V. All rights reserved.
Keywords: Ultra-long cobalt chains Hydrothermal Magnetization Thermal decomposition of AP
1. Introduction
* Corresponding author. E-mail address: [email protected] (Y. Zhao). http://dx.doi.org/10.1016/j.matchemphys.2017.08.057 0254-0584/© 2017 Elsevier B.V. All rights reserved.
The synthesis of magnetic nanomaterials was significantly due to their applications in magnetic storage devices, magnetic
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refrigeration systems and magnetic carriers for drug targeting [1e3]. Magnetic products including Fe, Co and Ni metals have been studied for many years. Strong magnetic interactions in these particle systems make it difficult to form stable colloids [4]. Uncontrolled agglomeration of the magnetic particles often makes it impossible to separate, and thus could not meet the requirement of many applications, such as magnetic recording [5]. By far, there were two feasible approaches to increase the magnetic anisotropy. One was to modify the shape of the nanoparticles, and the other was to assemble nanocrystals into multidimensional morphologies [6e9]. It has been found that magnetic properties of nanomaterials depending on the size and morphology widely. Based on the excellent physical and chemical properties, the relevant research activities about one-dimensional (1D) structures have increased dramatically [1e3,10]. Compared with other 1D structure such as wires, rods, and tubes, the chain-like structure was especially special. The coupling interactions between the adjacent particles could give rise to some novel properties. Virtually all types of methods have been employed to synthesize 1D chain, such as linear templates and physical methods like magnetic-field alignment. However, there were several intrinsic disadvantages [4,11]. When the templates were removed through physical or chemical method, the morphological of the products would be varied. In addition, the large-scale magnetic field could not avoid intrinsic disorder in the chains, and consequently the efficiency of the devices would be decreased obviously [12,13]. In addition, One-dimensional (1D) structures present excellent performance in catalytic field as well. Compared with other structures, 1D structure would facilitate the formation of 3D net so as to increase the reaction active sites [14,15]. Ammonium perchlorate (AP) is the most common oxidant in composite solid propellants since its thermal decomposition characteristics directly influence the combustion behavior of the propellant. In the past decades, metal oxides acting as catalysts have a significant role in promoting the thermal decomposition of AP [16e20]. However in comparison to metal oxide nanoparticles, the corresponding metals exhibited superior properties on thermal decomposition of AP and the employ of rocket propellants [21,22]. The metal nanoparticles contain many defects in their crystal lattice, and atoms on the defects tend to be saturated by combining with surplus electrons on its surface. Coincidentally, the nitrogen atoms of AP contain surplus electrons, thus the N-X bond would be easily broken due to the absorption of metal atom. This is favorable for the decomposition of AP. Especially, the cobalt metal revealed excellent catalytic performance compared with other kinds of metals [17]. In current work, a simply and environmentally method was employed to prepare the ultra-long cobalt chains. The growth mechanism was proposed through investigating the SEM images and Raman spectra of the products at different reaction time. The obtained one-dimensional cobalt chains with preferred orientation and shape anisotropy exhibited elevated ferromagnetic property. In addition, the decomposition temperature region of ammonium perchlorate (AP) could be significantly decreased by the presence of chain-like cobalt. The ultra-long cobalt chains showed a promising potential application in the field of high-energy fuel. 2. Experimental 2.1. Synthesis method In a typical procedure, 20 mL of CoCl2$6H2O ethylene glycol solution (0.714 g) was added to 20 mL of NaOH ethylene glycol solution (3.75 g) under vigorous stirring at room temperature. After 30 min of reaction, the final products were put into a 50 mL Teflon-
lined stainless steel autoclave. The sealed tank was incubated at 200 C for 10 h, and then cooled to room temperature naturally. Finally, the products were washed with alcohol several times and dried at 60 C for 10 h. 2.2. Characterization The phase structure of the as-synthesized products were examined by X-ray diffraction (XRD, D8/ADVANCE diffractometer, Cu Ka l ¼ 1.5418 Å). The morphology analysis was characterized by scanning electron microscopy (SEM, LEO-1530, Oberkochen, Germany) and transmission electron microscope (TEM, JEM-2100F). The room temperature magnetic characterization of the products was performed by a vibrating sample magnetometer (VSM, BHV-50 HTI). The properties of products concerning on reducing the thermal decomposition of AP were tested with a Thermo Gravimetric Analyzer (N33-TG 209F3). The chemical valence of the products was measured by XPS (PHI QUANTERA-II SXM) with an Al Ka radiation source (1486.6 eV). Raman scattering was excited using the 633 nm radiation from He-Ne laser and was collected by a microRaman spectrometer in the 100e2000 cm1 range at room temperature. 3. Results and discussion The SEM images in Fig. 1 displayed disparate magnifications morphologies and assembled structures of the product. Fig. 1a and b indicated that the length for the large proportion of products ranged from 100 to 200 mm and the diameter was about 5 mm. There were also some short-chains and spherical particles spreading in the products. A closer observation in Fig. 1c displayed that some small pompon-like particles with different size attached on the product surface. X-ray powder diffraction pattern (Fig. 1d) displayed that two phases were formed in the product including hexagonal close-packed (hcp) and face-centered cubic (fcc). The reported data of the two phases were PDF#05e0727 (a ¼ 2.514 Å, c ¼ 4.105 Å) and PDF#15e0806 (a ¼ b ¼ c ¼ 3.545 Å) respectively. The two phases were both close-packed structures but differed on the stacking sequence of (111) direction atomic planes. Low activation energy on stacking faults often led to the formation of two phases under high-temperature crystallization techniques [5,23]. However, the relative intensities of diffraction peaks did not agree well with the PDF card (JCPDS 05e0727). In the PDF card, the diffraction peak of (101) was the strongest while the (002) peak was weaker. However, the relative intensity of the (002) peak significantly increased in the product. This phenomenon was closely related with the anisotropic of the products. Transmission electron microscopy (TEM) was employed to figure out the microstructure of the cobalt chains. From the bright field TEM image (Fig. 2a), a small amount of nanosheets could be found. The crystallinity and the corresponding SAED pattern of an isolated nanosheet were illustrated at Fig. 2b and c. The nanosheet was in the state of polycrystalline, and the planar spacing was 0.2169 nm and 0.2050 nm which corresponded well to the H (100) and F (111) plane. The planar spacing obtained from the pattern were 2.030, 1.930, 1.477, 1.250 and 1.150 nm which corresponded to the (002), (101), (102), (110), and (103) planes of hexagonal cobalt respectively and the 1.750 nm corresponded to (200) plane of cubic cobalt phase. To investigate the growth process, the shape evolution of the product at various stages was investigated by SEM. Fig. 3 exhibited the SEM images of the products which obtained from different reaction times after reaching 200 C. Seen from Fig. 3a, the reaction solution was comprised of many uniformly petals-shaped particles. As the reaction time was extended to 1 h, the flowery crystals
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Fig. 1. (a, b) SEM images of the cobalt chains. (c) Detailed morphology of the cobalt chains and nanosheets. (d) XRD pattern of the material.
Fig. 2. (a) TEM image. (b) HRTEM image of nanosheet. (c) SAED pattern of an isolated nanosheet.
tended to be aggregated. The petals-shaped particles almost disappeared after reaction for 2 h. Meanwhile, these primary nanoparticles aggregated to form spherical particles with size from several hundred nanometers to micrometer. The formation of spherical morphology was beneficial for the reduction of surface energy [4]. Consequently, these spherical particles assembled into short chains (Fig. 3c). As the reaction time increased to 6 h (Fig. 3d), the rest of spherical particles were attracted by short chains through the dipolar interaction along arbitrary directions. Finally, the self-assembly process was ongoing and most of the short chains connected together to form the ultra-long cobalt chains (Fig. 3e). Fig. 3f showed the Raman spectra of the products obtained at different times. It was observed that five Raman peaks centered at about 193, 474, 515, 610, and 679 cm1, which corresponded well to five Raman-active modes F2g, Eg, F2g, F2g, and A1g, suggesting the formation of Co3O4 [24]. Compared with the bulk Co3O4, the observed Raman spectra shift towards lower wavenumbers attributing to the enhanced strain in nano-sized primary crystallites [25,26]. With the prolonged reaction time, the crystallinity of the product was increased as well. The whole growth process was illustrated in Fig. 4. The initial stage was the formation of cobalt nanoparticles. Then the nanosheets underwent the shape evolution under the high temperature and pressure environment. The above
conduction was well interpreted by the SEM results. X-ray photoelectron spectra (XPS) had been adopted to investigate electronic structure and chemical bonding of the cobalt chains. XPS spectrum of the products was provided in Fig. 5. As we can see, the peak of Co 2p revealed two prominent absorption bands. Indeed, the intensity ratio and binding energy separation of the satellite signals from the Co 2p3/2 and Co 2p1/2, as well as spin orbital splitting, were dependent on the chemical state of cobalt. There were two-pairs of peaks at about 780.8 and 786.2 eV as well as 796.5 and 802.5 eV basing on Gauss fitting. The former pair of peaks was assigned to the Co 2p3/2 and the later was related with the satellites of the Co 2p1/2 electrons in the Co3O4 bond [27]. Compared to the values of bulk cobalt (778.5 and 794.7 eV), the Co 2p3/2 and Co 2p1/2 peaks of the current product were shifted to higher binding energies. This shift might attribute to both the quantum size effect and the interaction of cobalt with oxygen. In addition, the core electrons which were strongly restricted by the atomic nucleus could also give rise to the shift [28]. The peak of O 1s located at 53.7 eV. The C 1s spectrum had a mainly peak at 284.9 eV with the maximum intensity and another peak at 288.8 eV that represented the CeC and the CeO bonds respectively [29]. From these observations, it could demonstrate that the cobalt oxide existed in the product. However, the XRD displayed that there was
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Fig. 3. (aed) SEM images of the products collected at different reaction times after the reaction temperature reached 200 C: (a) 30 min, (b) 1 h, (c) 2 h, (d) 6 h, (e) 10 h. (f) Raman spectra of the products collected at different reaction times after the reaction temperature reached 200 C.
Fig. 4. Illustration for the morphology evolution of the long chains.
no detection of cobalt oxide, indicating the ultra-long cobalt chains contained a small amount of cobalt oxide which could be easily introduced in the product during the preparation process. The hysteresis hoop (Fig. 6) of the products measured at room temperature displayed a ferromagnetic behavior. The saturation
magnetization (Ms) was 149 emu/g and coercivity (Hc) value was about 84.5 Oe. The saturation magnetization was lower than the single-hcp cobalt (168 emu/g). This difference might be resulted from the formation of oxides on the surface of particles [30]. It knows that magnetic properties of nanomaterials highly depended on the shape, crystallinity, magnetization direction, and so on [31]. Ultra-long cobalt chains had the relative high coercively owing to its preferred orientation and shape anisotropy [24]. This may be associated with the oriented growth of nanocrystallites and linear array structure of the chains formed. The DSC and TG curves for thermal decomposition of AP in the presence of cobalt products were plotted in Fig. 7. Thermal decomposition of pure AP showed three stages according to DSC and TG data [32]. The endothermic peak appeared at the first stage which was assigned to the transition from orthorhombic to cubic AP [14,33]. The weak exothermic peak corresponding to the second stage at 434 C was attributed to the partial decomposition of AP and the formation of intermediate product [31]. Fig. 7a demonstrated DSC curves for the decomposition of AP by different content of cobalt product. The product additives occupied 2 wt%, 4 wt%, 6 wt%, and 8 wt%, respectively. Endothermic peaks in all of the mixtures were observed at 240 C. Therefore, different contents of cobalt chains had little effect on the temperature of crystallographic phase transition. However, as found in the DSC curves, the additive could significantly reduce the temperature of exothermic
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Fig. 5. (a) XPS survey spectrum of the products. (b) XPS spectra of Co 2p. (c) XPS spectra of O 1s. (d) XPS spectra of C 1s.
long cobalt chains possessed the superior catalytic properties compared with the other morphologies of cobalt metal [34]. There were lots of pompon-like particles attached on the surface of chain structure so as to increase the reaction active sites for thermal decomposition of AP. The above results indicated that the ultralong cobalt chains had a significant role in accelerating the decomposition of AP. Up to now, the thermal decomposition mechanism of AP was not fully understood because of the decomposition process had been a complex hetero phase process involving coupled reactions in the solid, adsorbed and gaseous phases. Further research concerning the mechanism of AP thermal decomposition was indeed necessary. Jacobs proposed the mechanism for the first time [35]. And the details are as follows:
Fig. 6. Room temperature magnetic hysteresis loop for the as-synthesized cobalt chains.
peak. With the increase content of the additive, the temperature of main exothermic peak gradually decreased. The minimum content of the additive (the product amount 2 wt% of the mixture) also had the decomposition temperature about 329 C which was much lower than in pure AP (434 C). Furthermore, the addition of catalyst could improve decomposition heat enormously. Specifically, adding 4% of the cobalt chains promoted the decomposition heat to rise from 0.236 to 1.304 kJ g1 which was favorable for propellant applications. Furthermore at Fig. 7b, the initial thermal decomposition of pure AP occurred at 294 C, while the mixture occurred at 269 C. Clearly, the final decomposition temperature was gradually decreased with the increase content of the products additive. These were in agreement well with the DSC curves. Ultra-
NH4 ClO 4 ⇔NH3 H ClO4 ⇔NH3 HClO4 ⇔NH3ðaÞ HClO4ðaÞ
(1)
NH3ðaÞ þ HClO4ðaÞ ⇔NH3ðgÞ þ HClO4ðgÞ
(2)
The process (1) was confirmed to be a heterogeneous process which included the formation of melting eutectics between oxide additives and ammonium perchlorate or intermediate amine compounds. During the procedure, large amount of N2O, O2, Cl2, H2O, HCl, NO gas would be produced. Afterward, the second step concerning the decomposition of AP was gas-phase reaction and there were some intermediate products which could be formed such as NH3 and HClO4 in the process [36]. Firstly, in most case both of O2 and Cl2 were active and would be preferably absorbed by the metallic species, resulting in the rapid exothermic reaction. Notably, the interaction of metallic cobalt with active gas would release a large amount of heat, finally further accelerating the decomposition of AP [21]. There were also plenty of pompon-like particles attached on the products surface so as to increase the
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Fig. 7. (a) DSC curves for the decomposition of AP mixed with different weight content of cobalt products. (b) TG curves for the decomposition of AP mixed with different content of cobalt products.
reaction active sites. Furthermore, metal cobalt could donate electrons to initiate the decomposition of ClO 4 , while cobalt oxide accepted the produced electrons, pushing the decomposition reaction (2) towards the right-hand side [27]. It is reasonable that the reduction of decomposition heat is related to the increase of intermediate eutectics, while the decreased decomposition temperature of AP might be related to the oxidized ammonium radicals by oxygen which released from the cobalt oxide thin layers on product surface [37]. Consequently, it reached that the mechanism of the excellent catalytic performance was mainly ascribed to the electron transfer effect, and the synergistic effect of cobalt and the surface cobalt oxide also had a promotion effect on catalytic process. 4. Conclusions In summary, ultra-long cobalt chains assembled by petals-like nanosheets had been prepared from CoCl2$6H2O via a hydrothermal approach. The shape evolution of the ultra-long chain structure was investigated and a possible growth mechanism had been proposed. Owing to the chain structure with anisotropy, the products revealed excellent ferromagnetic property at room temperature. Basing on the interaction of cobalt and cobalt oxide, the obtained products could lower the thermal decomposition temperature of AP from 434 C to 303 C, exhibiting excellent performance in the thermal sensitization. Given versatile application of cobalt, the as prepared cobalt chains are also likely to exhibit outstanding performance in other applications. Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant No. 51602023), and the opening project of State Key Laboratory of Explosion Science and Technology (Beijing Institute of Technology, Grant No. ZDKT17-01). References [1] R.Z. Liu, Y.Z. Zhao, R.X. Huang, Y.J. Zhao, H.P. Zhou, CrystEngComm 12 (2010) 4091e4094. [2] Y.N. Xia, P.D. Yang, Y.G. Sun, Y.Y. Wu, B. Mayers, B. Gates, Y.D. Yin, F. Kim, H.Q. Yan, Adv. Mater. 15 (2003) 353e389. [3] G.R. Patzke, F. Krumeich, R. Nesper, Angew. Chem. Int. Ed. 41 (2002)
2446e2461. [4] H.L. Niu, Q.W. Chen, H.F. Zhu, Y.S. Lin, X. Zhang, J. Mater. Chem. 13 (2003) 1803e1805. [5] S.H. Sun, C.B. Murray, J. Appl. Phys. 85 (1999) 4325e4330. [6] V.F. Puntes, D. Zanchet, C.K. Erdonmez, A.P. Alivisatos, J. Am. Chem. Soc. 124 (2002) 12874e12880. [7] V.F. Puntes, K.M. Krishnan, A.P. Alivisatos, Science 291 (2001) 2115e2117. [8] R. Xu, T. Xie, Y.G. Zhao, Y.D. Li, Cryst. Growth Des. 7 (2007) 1904e1911. [9] Y.L. Hou, H. Kondoh, T. Ohta, Chem. Mater. 17 (2005) 3994e3996. [10] Z.L. Wang, Adv. Mater. 15 (2003) 432e436. [11] S. Minko, A. Kiriy, G. Gorodyska, M. Stamm, J. Am. Chem. Soc. 124 (2002) 10192e10197. [12] L.R. Meng, W.M. Chen, C.P. Chen, H.P. Zhou, Q. Peng, Y.D. Li, Cryst. Growth Des. 10 (2010) 479e482. [13] Z.Y. Tang, N.A. K, Adv. Mater. 17 (2005) 951e962. [14] D. Su, Y. Zhao, R. Zhang, M. Ning, Y. Zhao, H. Zhou, J. Li, H. Jin, Appl. Surf. Sci. 389 (2016) 112e117. [15] J. Wei, D. Zhang, Q. Yang, S. Chen, S. Gao, Inorg. Chem. Commun. 30 (2013) 13e16. [16] G. Singh, I.P.S. Kapoor, S. Dubey, P.F. Siril, J. Sci. Conf. Proc. 1 (2009) 7e14. [17] G. Singh, S.K. Sengupta, I.P.S. Kapoor, S. Dubey, R. Dubey, S. Singh, J. Energ. Mater. 31 (2013) 165e177. [18] X.M. Xu, Y.J. Zhao, Y.Z. Zhao, H.P. Zhou, F. Rehman, J.B. Li, H.B. Jin, CrystEngComm 17 (2015) 8248e8255. [19] M. Zou, X.H. Jiang, L.D. Lu, X. Wang, J. Hazard. Mater. 225e226 (2012) 124e130. [20] W.J. Zhang, P. Li, H.B. Xu, R.D. Sun, P.H. Qing, Y. Zhang, J. Hazard. Mater. 268 (2012) 273e280. [21] H. Duan, X. Lin, G. Liu, L. Xu, F. Li, J. Mater. Process. Technol. 208 (2008) 494e498. [22] I.P.S. Kapoor, P. Srivastava, G. Singh, Propellants Explos. Pyrotech. 34 (2009) 351e356. [23] D.P. Dmitry, M.G. Bawendi, Angew. Chem. Int. Ed. 38 (1999) 1788e1791. [24] D. Gallant, M. Pezolet, S. Simard, J. Phys. Chem. B 110 (2006) 6871e6880. [25] Y. Zhao, X. Xu, Y. Zhao, H. Zhou, J. Li, H. Jin, J. Alloys Compd. 654 (2016) 523e528. [26] V.G. Hadjievl, M.N. Ilievl, I.V. Vergilov, J. Phys. C. Solid State Phys. 21 (1988) 199e201. [27] S.J. Wang, B.P. Zhang, C.H. Zhao, S.J. Li, M.X. Zhang, L.P. Yan, Appl. Surf. Sci. 257 (2011) 3358e3362. [28] M. Rakap, S. Ozkar, Appl. Catal. B 91 (2009) 21e29. [29] A.A. Khassin, T.M. Yurieva, V.V. Kaichev, V.I. Bukhtiyarov, A.A. Budneva, E.A. Paukshtis, V.N. Parmon, J. Mol. Catal. A Chem. 175 (2001) 189e204. [30] S.L. Wen, Y. Liu, X.C. Zhao, Adv. Powder Technol. 26 (2015) 1520e1528. [31] X.M. Xu, Y.J. Zhao, J.B. Li, H.B. Jin, Y.Z. Zhao, H.P. Zhou, Mater. Res. Bull. 72 (2015) 7e12. [32] C. Yang, F. Xiao, J. Wang, X.T. Su, J. Colloid Interface Sci. 435 (2014) 34e42. [33] Y. Zhang, N. Wang, Y. Huang, W. Wu, C. Huang, C. Meng, Ceram. Int. 40 (2014) 11393e11398. [34] Z.T. Liu, X. Li, Z.W. Liu, J. Lu, Powder Technol. 189 (2009) 514e519. [35] P.W.M. Jacobs, A. Russell-Jones, J. Phys. Chem. 72 (1968) 202e207. [36] L.L. Bircumshaw, B.H. Newman, Proc. Roy. Soc. A227 (1954) 115e132. [37] L. Li, X. Sun, X. Qiu, J. Xu, G. Li, Inorg. Chem. 47 (2008) 8839e8846.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
The synthesis of ultra-long cobalt chains and its outstanding catalytic performance on the thermal decomposition of ammonium perchlorate Yongjie Zhao a, *, Xiaowei Zhang a, Xiangming Xu a, Yuzhen Zhao b, Heping Zhou b, Jingbo Li a, Haibo Jin a a Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications, School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, China b State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, 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
Ultra-long cobalt chains were synthetized. The growth mechanism was explored by controlling the reaction time. Elevated ferromagnetic property relies on the preferred orientation and shape anisotropy of cobalt chains. A synergistic effect mechanism for excellent catalytic performance was proposed.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 December 2016 Received in revised form 21 July 2017 Accepted 21 August 2017 Available online 24 August 2017
A simple method without assisted of external magnetic field or surfactant soft templates was utilized to fabricate ultra-long cobalt chains. The formation mechanism was investigated by time dependent experiment, and it was considered that the link of adjacent spherical particles were through dipolar interaction. Due to the preferred orientation and shape anisotropy of the chain structure, the coercively (Hc) value was about 84.5 Oe and saturation magnetization value was up to 149 emu/g at room temperature. To develop its further practical application, the products with regard to accelerate the thermal decomposition of ammonium perchlorate revealed that it exhibited an outstanding performance of reducing the thermal decomposition temperature. This excellent performance has great practical significance on the development of solid rocket fuels. The mechanism of the excellent performance probably ascribed to the synergistic effect of cobalt and cobalt oxide was proposed. © 2017 Elsevier B.V. All rights reserved.
Keywords: Ultra-long cobalt chains Hydrothermal Magnetization Thermal decomposition of AP
1. Introduction
* Corresponding author. E-mail address: [email protected] (Y. Zhao). http://dx.doi.org/10.1016/j.matchemphys.2017.08.057 0254-0584/© 2017 Elsevier B.V. All rights reserved.
The synthesis of magnetic nanomaterials was significantly due to their applications in magnetic storage devices, magnetic
236
Y. Zhao et al. / Materials Chemistry and Physics 201 (2017) 235e240
refrigeration systems and magnetic carriers for drug targeting [1e3]. Magnetic products including Fe, Co and Ni metals have been studied for many years. Strong magnetic interactions in these particle systems make it difficult to form stable colloids [4]. Uncontrolled agglomeration of the magnetic particles often makes it impossible to separate, and thus could not meet the requirement of many applications, such as magnetic recording [5]. By far, there were two feasible approaches to increase the magnetic anisotropy. One was to modify the shape of the nanoparticles, and the other was to assemble nanocrystals into multidimensional morphologies [6e9]. It has been found that magnetic properties of nanomaterials depending on the size and morphology widely. Based on the excellent physical and chemical properties, the relevant research activities about one-dimensional (1D) structures have increased dramatically [1e3,10]. Compared with other 1D structure such as wires, rods, and tubes, the chain-like structure was especially special. The coupling interactions between the adjacent particles could give rise to some novel properties. Virtually all types of methods have been employed to synthesize 1D chain, such as linear templates and physical methods like magnetic-field alignment. However, there were several intrinsic disadvantages [4,11]. When the templates were removed through physical or chemical method, the morphological of the products would be varied. In addition, the large-scale magnetic field could not avoid intrinsic disorder in the chains, and consequently the efficiency of the devices would be decreased obviously [12,13]. In addition, One-dimensional (1D) structures present excellent performance in catalytic field as well. Compared with other structures, 1D structure would facilitate the formation of 3D net so as to increase the reaction active sites [14,15]. Ammonium perchlorate (AP) is the most common oxidant in composite solid propellants since its thermal decomposition characteristics directly influence the combustion behavior of the propellant. In the past decades, metal oxides acting as catalysts have a significant role in promoting the thermal decomposition of AP [16e20]. However in comparison to metal oxide nanoparticles, the corresponding metals exhibited superior properties on thermal decomposition of AP and the employ of rocket propellants [21,22]. The metal nanoparticles contain many defects in their crystal lattice, and atoms on the defects tend to be saturated by combining with surplus electrons on its surface. Coincidentally, the nitrogen atoms of AP contain surplus electrons, thus the N-X bond would be easily broken due to the absorption of metal atom. This is favorable for the decomposition of AP. Especially, the cobalt metal revealed excellent catalytic performance compared with other kinds of metals [17]. In current work, a simply and environmentally method was employed to prepare the ultra-long cobalt chains. The growth mechanism was proposed through investigating the SEM images and Raman spectra of the products at different reaction time. The obtained one-dimensional cobalt chains with preferred orientation and shape anisotropy exhibited elevated ferromagnetic property. In addition, the decomposition temperature region of ammonium perchlorate (AP) could be significantly decreased by the presence of chain-like cobalt. The ultra-long cobalt chains showed a promising potential application in the field of high-energy fuel. 2. Experimental 2.1. Synthesis method In a typical procedure, 20 mL of CoCl2$6H2O ethylene glycol solution (0.714 g) was added to 20 mL of NaOH ethylene glycol solution (3.75 g) under vigorous stirring at room temperature. After 30 min of reaction, the final products were put into a 50 mL Teflon-
lined stainless steel autoclave. The sealed tank was incubated at 200 C for 10 h, and then cooled to room temperature naturally. Finally, the products were washed with alcohol several times and dried at 60 C for 10 h. 2.2. Characterization The phase structure of the as-synthesized products were examined by X-ray diffraction (XRD, D8/ADVANCE diffractometer, Cu Ka l ¼ 1.5418 Å). The morphology analysis was characterized by scanning electron microscopy (SEM, LEO-1530, Oberkochen, Germany) and transmission electron microscope (TEM, JEM-2100F). The room temperature magnetic characterization of the products was performed by a vibrating sample magnetometer (VSM, BHV-50 HTI). The properties of products concerning on reducing the thermal decomposition of AP were tested with a Thermo Gravimetric Analyzer (N33-TG 209F3). The chemical valence of the products was measured by XPS (PHI QUANTERA-II SXM) with an Al Ka radiation source (1486.6 eV). Raman scattering was excited using the 633 nm radiation from He-Ne laser and was collected by a microRaman spectrometer in the 100e2000 cm1 range at room temperature. 3. Results and discussion The SEM images in Fig. 1 displayed disparate magnifications morphologies and assembled structures of the product. Fig. 1a and b indicated that the length for the large proportion of products ranged from 100 to 200 mm and the diameter was about 5 mm. There were also some short-chains and spherical particles spreading in the products. A closer observation in Fig. 1c displayed that some small pompon-like particles with different size attached on the product surface. X-ray powder diffraction pattern (Fig. 1d) displayed that two phases were formed in the product including hexagonal close-packed (hcp) and face-centered cubic (fcc). The reported data of the two phases were PDF#05e0727 (a ¼ 2.514 Å, c ¼ 4.105 Å) and PDF#15e0806 (a ¼ b ¼ c ¼ 3.545 Å) respectively. The two phases were both close-packed structures but differed on the stacking sequence of (111) direction atomic planes. Low activation energy on stacking faults often led to the formation of two phases under high-temperature crystallization techniques [5,23]. However, the relative intensities of diffraction peaks did not agree well with the PDF card (JCPDS 05e0727). In the PDF card, the diffraction peak of (101) was the strongest while the (002) peak was weaker. However, the relative intensity of the (002) peak significantly increased in the product. This phenomenon was closely related with the anisotropic of the products. Transmission electron microscopy (TEM) was employed to figure out the microstructure of the cobalt chains. From the bright field TEM image (Fig. 2a), a small amount of nanosheets could be found. The crystallinity and the corresponding SAED pattern of an isolated nanosheet were illustrated at Fig. 2b and c. The nanosheet was in the state of polycrystalline, and the planar spacing was 0.2169 nm and 0.2050 nm which corresponded well to the H (100) and F (111) plane. The planar spacing obtained from the pattern were 2.030, 1.930, 1.477, 1.250 and 1.150 nm which corresponded to the (002), (101), (102), (110), and (103) planes of hexagonal cobalt respectively and the 1.750 nm corresponded to (200) plane of cubic cobalt phase. To investigate the growth process, the shape evolution of the product at various stages was investigated by SEM. Fig. 3 exhibited the SEM images of the products which obtained from different reaction times after reaching 200 C. Seen from Fig. 3a, the reaction solution was comprised of many uniformly petals-shaped particles. As the reaction time was extended to 1 h, the flowery crystals
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Fig. 1. (a, b) SEM images of the cobalt chains. (c) Detailed morphology of the cobalt chains and nanosheets. (d) XRD pattern of the material.
Fig. 2. (a) TEM image. (b) HRTEM image of nanosheet. (c) SAED pattern of an isolated nanosheet.
tended to be aggregated. The petals-shaped particles almost disappeared after reaction for 2 h. Meanwhile, these primary nanoparticles aggregated to form spherical particles with size from several hundred nanometers to micrometer. The formation of spherical morphology was beneficial for the reduction of surface energy [4]. Consequently, these spherical particles assembled into short chains (Fig. 3c). As the reaction time increased to 6 h (Fig. 3d), the rest of spherical particles were attracted by short chains through the dipolar interaction along arbitrary directions. Finally, the self-assembly process was ongoing and most of the short chains connected together to form the ultra-long cobalt chains (Fig. 3e). Fig. 3f showed the Raman spectra of the products obtained at different times. It was observed that five Raman peaks centered at about 193, 474, 515, 610, and 679 cm1, which corresponded well to five Raman-active modes F2g, Eg, F2g, F2g, and A1g, suggesting the formation of Co3O4 [24]. Compared with the bulk Co3O4, the observed Raman spectra shift towards lower wavenumbers attributing to the enhanced strain in nano-sized primary crystallites [25,26]. With the prolonged reaction time, the crystallinity of the product was increased as well. The whole growth process was illustrated in Fig. 4. The initial stage was the formation of cobalt nanoparticles. Then the nanosheets underwent the shape evolution under the high temperature and pressure environment. The above
conduction was well interpreted by the SEM results. X-ray photoelectron spectra (XPS) had been adopted to investigate electronic structure and chemical bonding of the cobalt chains. XPS spectrum of the products was provided in Fig. 5. As we can see, the peak of Co 2p revealed two prominent absorption bands. Indeed, the intensity ratio and binding energy separation of the satellite signals from the Co 2p3/2 and Co 2p1/2, as well as spin orbital splitting, were dependent on the chemical state of cobalt. There were two-pairs of peaks at about 780.8 and 786.2 eV as well as 796.5 and 802.5 eV basing on Gauss fitting. The former pair of peaks was assigned to the Co 2p3/2 and the later was related with the satellites of the Co 2p1/2 electrons in the Co3O4 bond [27]. Compared to the values of bulk cobalt (778.5 and 794.7 eV), the Co 2p3/2 and Co 2p1/2 peaks of the current product were shifted to higher binding energies. This shift might attribute to both the quantum size effect and the interaction of cobalt with oxygen. In addition, the core electrons which were strongly restricted by the atomic nucleus could also give rise to the shift [28]. The peak of O 1s located at 53.7 eV. The C 1s spectrum had a mainly peak at 284.9 eV with the maximum intensity and another peak at 288.8 eV that represented the CeC and the CeO bonds respectively [29]. From these observations, it could demonstrate that the cobalt oxide existed in the product. However, the XRD displayed that there was
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Fig. 3. (aed) SEM images of the products collected at different reaction times after the reaction temperature reached 200 C: (a) 30 min, (b) 1 h, (c) 2 h, (d) 6 h, (e) 10 h. (f) Raman spectra of the products collected at different reaction times after the reaction temperature reached 200 C.
Fig. 4. Illustration for the morphology evolution of the long chains.
no detection of cobalt oxide, indicating the ultra-long cobalt chains contained a small amount of cobalt oxide which could be easily introduced in the product during the preparation process. The hysteresis hoop (Fig. 6) of the products measured at room temperature displayed a ferromagnetic behavior. The saturation
magnetization (Ms) was 149 emu/g and coercivity (Hc) value was about 84.5 Oe. The saturation magnetization was lower than the single-hcp cobalt (168 emu/g). This difference might be resulted from the formation of oxides on the surface of particles [30]. It knows that magnetic properties of nanomaterials highly depended on the shape, crystallinity, magnetization direction, and so on [31]. Ultra-long cobalt chains had the relative high coercively owing to its preferred orientation and shape anisotropy [24]. This may be associated with the oriented growth of nanocrystallites and linear array structure of the chains formed. The DSC and TG curves for thermal decomposition of AP in the presence of cobalt products were plotted in Fig. 7. Thermal decomposition of pure AP showed three stages according to DSC and TG data [32]. The endothermic peak appeared at the first stage which was assigned to the transition from orthorhombic to cubic AP [14,33]. The weak exothermic peak corresponding to the second stage at 434 C was attributed to the partial decomposition of AP and the formation of intermediate product [31]. Fig. 7a demonstrated DSC curves for the decomposition of AP by different content of cobalt product. The product additives occupied 2 wt%, 4 wt%, 6 wt%, and 8 wt%, respectively. Endothermic peaks in all of the mixtures were observed at 240 C. Therefore, different contents of cobalt chains had little effect on the temperature of crystallographic phase transition. However, as found in the DSC curves, the additive could significantly reduce the temperature of exothermic
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Fig. 5. (a) XPS survey spectrum of the products. (b) XPS spectra of Co 2p. (c) XPS spectra of O 1s. (d) XPS spectra of C 1s.
long cobalt chains possessed the superior catalytic properties compared with the other morphologies of cobalt metal [34]. There were lots of pompon-like particles attached on the surface of chain structure so as to increase the reaction active sites for thermal decomposition of AP. The above results indicated that the ultralong cobalt chains had a significant role in accelerating the decomposition of AP. Up to now, the thermal decomposition mechanism of AP was not fully understood because of the decomposition process had been a complex hetero phase process involving coupled reactions in the solid, adsorbed and gaseous phases. Further research concerning the mechanism of AP thermal decomposition was indeed necessary. Jacobs proposed the mechanism for the first time [35]. And the details are as follows:
Fig. 6. Room temperature magnetic hysteresis loop for the as-synthesized cobalt chains.
peak. With the increase content of the additive, the temperature of main exothermic peak gradually decreased. The minimum content of the additive (the product amount 2 wt% of the mixture) also had the decomposition temperature about 329 C which was much lower than in pure AP (434 C). Furthermore, the addition of catalyst could improve decomposition heat enormously. Specifically, adding 4% of the cobalt chains promoted the decomposition heat to rise from 0.236 to 1.304 kJ g1 which was favorable for propellant applications. Furthermore at Fig. 7b, the initial thermal decomposition of pure AP occurred at 294 C, while the mixture occurred at 269 C. Clearly, the final decomposition temperature was gradually decreased with the increase content of the products additive. These were in agreement well with the DSC curves. Ultra-
NH4 ClO 4 ⇔NH3 H ClO4 ⇔NH3 HClO4 ⇔NH3ðaÞ HClO4ðaÞ
(1)
NH3ðaÞ þ HClO4ðaÞ ⇔NH3ðgÞ þ HClO4ðgÞ
(2)
The process (1) was confirmed to be a heterogeneous process which included the formation of melting eutectics between oxide additives and ammonium perchlorate or intermediate amine compounds. During the procedure, large amount of N2O, O2, Cl2, H2O, HCl, NO gas would be produced. Afterward, the second step concerning the decomposition of AP was gas-phase reaction and there were some intermediate products which could be formed such as NH3 and HClO4 in the process [36]. Firstly, in most case both of O2 and Cl2 were active and would be preferably absorbed by the metallic species, resulting in the rapid exothermic reaction. Notably, the interaction of metallic cobalt with active gas would release a large amount of heat, finally further accelerating the decomposition of AP [21]. There were also plenty of pompon-like particles attached on the products surface so as to increase the
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Fig. 7. (a) DSC curves for the decomposition of AP mixed with different weight content of cobalt products. (b) TG curves for the decomposition of AP mixed with different content of cobalt products.
reaction active sites. Furthermore, metal cobalt could donate electrons to initiate the decomposition of ClO 4 , while cobalt oxide accepted the produced electrons, pushing the decomposition reaction (2) towards the right-hand side [27]. It is reasonable that the reduction of decomposition heat is related to the increase of intermediate eutectics, while the decreased decomposition temperature of AP might be related to the oxidized ammonium radicals by oxygen which released from the cobalt oxide thin layers on product surface [37]. Consequently, it reached that the mechanism of the excellent catalytic performance was mainly ascribed to the electron transfer effect, and the synergistic effect of cobalt and the surface cobalt oxide also had a promotion effect on catalytic process. 4. Conclusions In summary, ultra-long cobalt chains assembled by petals-like nanosheets had been prepared from CoCl2$6H2O via a hydrothermal approach. The shape evolution of the ultra-long chain structure was investigated and a possible growth mechanism had been proposed. Owing to the chain structure with anisotropy, the products revealed excellent ferromagnetic property at room temperature. Basing on the interaction of cobalt and cobalt oxide, the obtained products could lower the thermal decomposition temperature of AP from 434 C to 303 C, exhibiting excellent performance in the thermal sensitization. Given versatile application of cobalt, the as prepared cobalt chains are also likely to exhibit outstanding performance in other applications. Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant No. 51602023), and the opening project of State Key Laboratory of Explosion Science and Technology (Beijing Institute of Technology, Grant No. ZDKT17-01). References [1] R.Z. Liu, Y.Z. Zhao, R.X. Huang, Y.J. Zhao, H.P. Zhou, CrystEngComm 12 (2010) 4091e4094. [2] Y.N. Xia, P.D. Yang, Y.G. Sun, Y.Y. Wu, B. Mayers, B. Gates, Y.D. Yin, F. Kim, H.Q. Yan, Adv. Mater. 15 (2003) 353e389. [3] G.R. Patzke, F. Krumeich, R. Nesper, Angew. Chem. Int. Ed. 41 (2002)
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