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nanorods
Materials Research Bulletin 47 (2012) 3650–3653
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Synthesis and Raman properties of magnesium borate micro/nanorods Shuang Li a,b,*, Dapeng Xu c, Hongzhi Shen c, Jing Zhou c, Ya Fan a,b a
Department of Physics, Changchun University of Science and Technology, Changchun, PR China International Joint Research Centre for Nanophotonics and Biophotonics, CUST, Changchun, PR China c College of Physics, Jilin University, Changchun, PR China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 November 2011 Received in revised form 24 May 2012 Accepted 14 June 2012 Available online 23 June 2012
With cetyltrimethylammonium bromide (CTAB) as a soft template, magnesium borate (Mg2B2O5) onedimensional micro/nanorods were synthesized. The products prepared in the absence of CTAB were Mg2B2O5 nanoparticals and needles. However, using CTAB as a soft template the products were Mg2B2O5 whiskers (diameter: 200 10 nm, length: 1–2 mm). The formation mechanism was discussed. In addition, the experimental and theoretical Raman spectra of Mg2B2O5 were reported for the first time, and the possible vibrations modes of Mg2B2O5 crystals were assigned based on the calculation results. Crown Copyright ß 2012 Published by Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures B. Chemical synthesis C. Raman spectroscopy D. Crystal structure
1. Introduction Magnesium borates, with many chemical compositions such as Mg2B2O5, MgB4O7 and Mg3B2O6, are remarkable ceramic materials. Among them, Mg2B2O5 has been paid much attention in recent years due to its excellent properties. As an excellent antiwear and anticorrosion material, Mg2B2O5 has been widely used to reduce friction coefficient [1,2]. Mg2B2O5 also has other potential applications, such as catalysts for the conversion of the hydrocarbons [3,4], luminescent materials for cathode ray tube screens [5] and thermo-luminescence application [6–8]. Now, onedimensional (1D) micro-/nanomaterials have become the research focus for their unique structures, novel properties and potential applications [9]. Magnesium borates with 1D structure, such as nanorods [10–12], nanowires [13–15], nanobelts [16], nanotubes [17] and whiskers [18,19] have been reported. Varieties of synthetic methods, for example, CVD method [13], catalyst-free method [15], high temperature solid-state synthesis method [10,14], solvothermal method [11], and mechano-chemical method [12] have been widely used in synthesis of 1D micro-/ nanostructure magnesium borate. Nevertheless, these traditional methods expose some disadvantages such as multistep, high energy consumption as well as byproducts. Thus, developing a simple, cost-effective and sustainable synthesis method is an important aspect to obtain high crystalline, high purity and well dispersed production for further properties study and wide
* Corresponding author. Tel.: +86 043185582741. E-mail address: [email protected] (S. Li).
applications. As a straightforward route to obtain 1D structure, template technique has been used as a powerful tool for the fabrication of 1D nanomaterials [20,21]. However, no report has been found on the synthesis of 1D nanostructure magnesium borate with this method. In this paper, a CTAB-assisted soft template method for the synthesis of Mg2B2O5 micro-/nanorods from MgBr26H2O and NaBH4 is reported. Besides XRD and SEM characterization methods, further evidence for the structure of Mg2B2O5 was obtained from the Raman spectra in this work. Nowadays, Raman spectroscopy has been widely used to study local atomic arrangement in borate glasses. But there were few reports about Raman research on borates crystals, and no information on the Raman spectrum of the Mg2B2O5 crystal is available yet. The vibration modes may be different between borate glasses and borates crystals [22,23]. Herein the room temperature experimental Raman spectrum of Mg2B2O5 crystal was obtained, and its theoretical Raman spectrum was also calculated by quantum chemical methods. The possible vibrations modes in Mg2B2O5 were assigned based on the calculation results and previous Raman study reports of the similar system. 2. Experimental Firstly, Mg2B2O5 precursor solution was prepared by the following process. 3 mmol of MgBr26H2O and 6 mmol of NaBH4 were dissolved in 10 ml and 20 ml of ethanol, respectively. Secondly, 0.5 mmol of cetyltrimethylammonium bromide (CTAB) was added to MgBr26H2O solution and the mixed-solution under vigorous magnetic stirring until dissolved completely. Thirdly, the NaBH4 solution was added dropwise to the MgBr2/CTAB
0025-5408/$ – see front matter . Crown Copyright ß 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.06.046
S. Li et al. / Materials Research Bulletin 47 (2012) 3650–3653
mixed-solution. Large evolution of gas observed and the solution turned turbid. This reaction mixture thickened over time and formed a gel when left open to the atmosphere for several hours. The as-prepared precursors were transferred into a furnace for heat treatment. The furnace temperature was ramped to 800 8C with a heating rate of 2 8C min1, and held at 800 8C for 2 h, then the furnace was cooled down to room temperature as the natural process. A gray, powdery product was obtained after washing with deionized water and centrifugated for three times, and dried at 80 8C for 8 h. For comparison, the similar experimental was carried out in the absence of CTAB. In addition, in order to investigate the effect of the molar ratio of raw materials on the compositions, the reaction was also conducted with the molar ratio of MgBr2:NaBH4 as 1:4 in the presence of CTAB. The crystal structure and the morphology of the products were examined by X-ray diffraction (XRD, RigakuD/max-2500), Raman spectrometer (Jobin Yvon-HR800), selected-area electron diffraction, scanning electron microscopy (SEM, JSM-6480F), and energy dispersive X-ray spectroscopy (EDS, GENE SIS 2000 XMS 60 S, EDAX, Inc.), respectively. 3. Results and discussion Fig. 1 shows the XRD patterns of the as-prepared products with different molar ratios of Mg:B:CTAB. All of the diffraction peaks of the three products are indexed to the monoclinic phase Mg2B2O5 (PCPDF 860531) and no other crystalline phases are detected. It means that CTAB dose not affect the formation of monoclinic phase Mg2B2O5. However, It is worth noting that the product, which was prepared with the molar ratio of Mg:B as 1:4 in the presence of CTAB, is well-crystallized.
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Fig. 1. XRD patterns of the as-prepared products.
The formation of Mg2B2O5 can be express by the following reactions: NaBH4 þ H2 O ! BO2 H þ NaOH þ H2
CTAB
MgBr2 þ NaOH þ BO2 H! NaBr þ Mg2 B2 O5 þ H2 O
(1)
(2)
The morphologies of the products were observed from SEM images (Fig. 2A–C). It can be seen that the product synthesized
Fig. 2. SEM images of as-prepared products with the molar ratio of Mg:B:CTAB as (A) 3:6:0; (B)3:6:0.5; (C)3:12:0.5; (D) EDS and SAED image of the Mg2B2O5 micron-crystal rod.
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S. Li et al. / Materials Research Bulletin 47 (2012) 3650–3653
Fig. 3. The scheme for the formation of one-dimensional Mg2B2O5 nanostructure.
without using soft template exhibits clustered granules and needle-like materials (Fig. 2A). The products synthesized with CTAB as a soft-template have rods-like morphologies (Fig. 2B). According to XRD and SEM analysis results, it can be seen that the different molar ratio of Mg:B dose not affect the chemical compositions and the phase structure, but lead to different morphologies of the final products although they were prepared through the same calcinations process. The product obtained with the Mg:B molar ratio of 1:2 exhibits well dispersed rods-like morphology with uniform diameters of about 200 nm and the lengths of about 1–2 mm (Fig. 2B). The products obtained with the Mg:B molar ratio of 1:4 were larger size micron-rods with the average diameter of about 3 mm and the length of tens of micrometers. It means that excessive introduction of NaBH4 should accelerate the growth of crystals and the reason will be given in the following discussion for formation mechanism. In order to investigate the structure properties of Mg2B2O5, the well-crystallized Mg2B2O5 micron-crystal rods were chosen as samples for further characterized. Fig. 2D shows the typical energy dispersive spectrum (EDS) of the Mg2B2O5 micron-crystal rods. It confirms that the products are composed of Mg, B and O. The selected area electron diffraction (SAED) pattern of the individual Mg2B2O5 micron-crystal rods (the inset of Fig. 2D) confirms that they are single crystals and are indexed as monoclinic phase Mg2B2O5, which is consistent with XRD results. Based on the results mentioned above, we propose the formation mechanism of the one dimensional Mg2B2O5 by using CTAB as a soft template (Fig. 3). First, CTAB can dissolve in the solution to give CTA+ and Br ions, and one-dimensional CTAB micelles formed when the concentration of CTAB is in the range of critical micelles concentration (CMC) [20]. Then the negatively charged MgBr42 interacts with CTA+ micelle through electrostatic interaction to form CTA+–MgBr42 ions pairs. When adding the NaBH4, Mg2B2O5 formed and grew along one-dimension direction due to the existence of CTAB micelles. With the increasing of
NaBH4 concentration the morphology of Mg2B2O5 crystalline rods has no change but the size of that obviously increases. Welldispersed Mg2B2O5 nanorods were formed when using 3 mmol of MgBr2 and 6 mmol of NaBH4 in the reaction system. However, as the concentration of NaBH4 was 12 mmol, an obvious larger rodlike Mg2B2O5 was apparent. A possible mechanism is that the precursors affect the surfactant assembly. Excessive NaBH4 provides more OH, which is adsorbed on the surface of the CTAB micelle. On one hand, it can accelerate the aggregate size of CTAB micelle, on the other hand, it can make the electrostatic interaction between CTA+–MgBr42 ion pairs stronger, and more Mg2B2O5 crystal nucleus will aggregate and grow into large-size crystalline rods during the calcinations. In addition, the room-temperature Raman spectrum of the Mg2B2O5 micron-crystal rods was shown in Fig. 4A. To our knowledge, no experimental Raman and theoretical Raman studies of the Mg2B2O5 crystalline have been reported. In order to give assignment for the observed Raman vibrations, the Raman spectrum of Mg2B2O5 was calculated using quantum chemical methods. The structure optimization and Raman spectrum calculations of Mg2B2O5 were carried out with the Gaussian09 software. The computations were performed at the Hartree–Fock (HF) 3-21G level of theory to get the optimized structure. The optimized parameters are listed in Table 1, which is consistent with that of previously public data [24]. Based on the optimized structure, the Raman spectrum was calculated at HF/STO-3G levels of theory. Table 2 lists the experimental and calculation Raman vibrations and tentative assignments. As can be seen from Fig. 4, the calculated Raman spectrum is in good agreement with the observed Raman spectrum in the range of 400–1000 cm1. However, the difference appeared in low wavenumber (200– 400 cm1) and the high wavenumber (1000–2000 cm1) regions, respectively. It can be explained based on the previous Raman analysis of other borate systems [25–27]. The spectrum contains two distinct regions: (i) n < 600 cm1 contains the external mode vibrations corresponding to translational motion of the metal ions; (ii) 600 cm1 < n < 1400 cm1 contains the internal modes corresponding to stretching vibration and bending vibration of BxOy groups, which have nothing to do with metal ion. In Ref. [25], the authors show that a signification TO–LO splitting exists for these internal vibration modes, and the TO–LO splitting leads to large change in the dipole moment within the BO33 plane. The intensity of majority of these internal modes is very weak and it is difficult to observe clearly the polarization behavior of these modes in the Raman scatting study. So in our study the observed modes are less than the calculated modes in the high frequency region (1000–2000 cm1). As for the low wavenumber region
Table 1 The parameters of bond and angle in the optimized structure.
Fig. 4. The experimental and theoretical Raman spectra of Mg2B2O5 micron-crystal rod.
Mg1–O Mg2–O BO3 B2O5
Bond (nm)
Angle (8)
0.1984 0.2050 0.1332/0.1375/0.1415 0.1332/0.1375/0.1415/0.1435/0.1352
– – 116.895/117.917/125.159 135.112 (B–O–B)
S. Li et al. / Materials Research Bulletin 47 (2012) 3650–3653 Table 2 Raman shift (cm1) and possible assignment of the vibrations in Mg2B2O5. Raman (cm1) Obs.
shift
Assignment
Calc.
233 279 305 330 341 365 409 422 477 547 633
233 283 308 333 342 368 409 419 460 542 634
684 706 713 – – 846 1024 1124 1284
684 708 718 749 754 844 1039 1135 1289
Lattice Lattice Lattice Lattice Lattice Lattice Lattice Lattice Lattice Lattice Scissor bending of O–B–O bond in B2O5 and in-plane torsion vibration of BO3 unit In-plane torsion vibration of B2O5 unit In-plane torsion vibration of BO3 unit Out-plane torsion bending vibration of B2O5 unit O–B–O scissor bending vibration Out-plane torsion bending vibration in B2O5 unit Scissor bending vibration in B2O5 unit Stretching vibration of B–O bond in BO3 and B2O5 units Stretching vibration of B–O bond in B2O5 Asymmetric stretching vibration of B–O bond in BO3
(200–400 cm1) more observed modes exist than the calculated result may be due to the result of crystal field interaction, which we do not consider in the Raman calculation, strongly affects the vibrations modes of MgO6 groups. As to our knowledge, there are no experimental and theoretical Raman reports about Mg2B2O5 crystalline, and the possible assignment of the observed Raman modes for Mg2B2O5 were given on the basis of quantum chemical calculations method and previous Raman study reports of the similar system. In fact more precise assignment of the Raman modes for Mg2B2O5 should be proposed on the basis of lattice dynamics calculations. 4. Conclusions With CTAB as a soft template, one-dimensional Mg2B2O5 crystal rods were synthesized. The results indicated that no changes on the chemical compositions and the phase structure of the asproducts using different concentration of CTAB and NaBH4, The formation of the gel in the initial reaction mixture may be the key to the formation of the 1D nanostructured products. A gel is typically composed of ID CTAB micelle and ultrafine particles. This prearrangement of the precursor particles with the participation of
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the CTAB surfactant could form the one-dimensional nanostructure. In the concentration of NaBH4 greatly affect the size of the final products although in the same calcination process. The possible assignment of the observed Raman modes for Mg2B2O5 was given on the basis of quantum chemical calculations method and previous Raman study reports of the similar system. However, more precise assignment of the Raman modes for Mg2B2O5 should be proposed on the basis of lattice dynamics calculations. Acknowledgements Financial support from the National Natural Science Foundation of China (61006065) and the Natural Science Foundation of Jilin Province (20101546) are greatly appreciated. References [1] Z.S. Hu, R. Lai, F. Lou, L.G. Wang, Z.L. Chen, G.X. Chen, J.X. Dong, Wear 252 (2002) 370. [2] Zeng, Y. Yang, H. Fu, W. Qiao, L. Chang, L. Chen, J. Zhu, H. Zou, Guangtian, Mater. Res. Bull. 43 (August (8–9)) (2008) 2239–2247. [3] A.F. Qasrawi, T.S. Kayed, A. Mergen, M. Gurn, Mater. Res. Bull. 40 (2005) 583. [4] E.G. Baker, Boron zinc oxide and boron magnesium oxide catalysts for conversion hydrocarbons, US Patent 2,889,266 (1959). [5] P.W. Ranby, Titanium activated magnesium borate, US Patent 2,758,094 (1956). [6] D.I. Shahare, S.J. Dhoble, S.V. Moharil, J. Mater. Sci. Lett. 12 (1993) 1873. [7] M. Prokic, Nucl. Instrum. Methods 175 (1980) 83. [8] C. Furetta, G. Kitis, P.S. Weng, T.C. Chu, Nucl. Instrum. Methods Phys. Res., Sect. A: Accel. Spectrom. Detect. Assoc. Equip. 420 (1999) 441. [9] Y.N. Xia, P.D. Yang, Y.G. Sun, Adv. Mater. 15 (2003) 353–389. [10] E.M. Elssfah, A. Elsanousi, J. Zhang, Mater. Lett. 61 (2007) 4358–4361. [11] B.S. Xu, T.B. Li, Y. Zhang, Z.X. Zhang, X.G. Liu, Cryst. Growth Des. 8 (4) (2008) 1218–1222. [12] S. Li, X. Fang, J. Leng, H.Z. Shen, Y. Fan, D.P. Xu, Mater. Lett. 64 (2010) 151–153. [13] Y. Li, Z.Y. Fan, J.G. Lu, R.P.H. Chang, Chem. Mater. 16 (2004) 2512–2514. [14] Y. Zeng, H.B. Yang, W.Y. Fu, L. Qiao, Mater. Res. Bull. 43 (2008) 2239–2247. [15] X.Y. Tao, X.D. Li, Nano Lett. 8 (2) (2008) 505–510. [16] J. Zhang, Y.M. Zhao, Acta Phys. Chim. Sin. 22 (2006) 110–113. [17] R.Z. Ma, Y. Bando, D. Golberg, T. Sato, Angew. Chem. Int. Ed. 42 (2003) 1836–1838. [18] W. Zhu, Q. Zhang, L. Xiang, S. Zhu, CrystEngComm 13 (2011) 1654–1663. [19] L. Cong Wang, Y. Shan Zhang, Y. Qi Wang, D. Mei Cao, X. Ping Huang, Y. Liu, Adv. Mater. Res. 287–290 (2011) 683–687. [20] Y. Yu, F. Du, C. Jimmy, Y. Zhuang, P. Wong, J. Solid State Chem. 177 (2004) 4640–4647. [21] N. Asim, S. Radiman, M.S. Mohd Ambar Yarmo, B. Golriz, Micropor. Mesopor. Mater. 120 (2009) 397–401. [22] V.P. Solntsev, A.V. Davydov, V.K. Malinovsky, N.V. Surovtsev, J. Cryst. Growth 312 (20) (2010) 2962–2966. [23] Y. Tetsuji Kunimine, N. Shibata, S. Yamane Masayuki, J. Non-Cryst. Solids 321 (3) (2003) 137–146. [24] G-C. Guo, W-D. Cheng, J-T. Chen, H-H. Zhuang, J-S. Huang, Q-E. Zhang, Acta Crystallogr., Sect. C 51 (1995) 2469–2471. [25] M. Maczka, J. Hanuza, A. Pajaczkowska, J. Raman Spectrosc. 35 (2004) 266–273. [26] G. Barros, E.N. Silva, A.P. Ayala, I. Guedes, C.-K. Loong, J. Wang, X. Hu, H. Zhang, Vib. Spectrosc. 46 (2008) 100–106. [27] R.L. Frost, J. Raman Spectrosc. 42 (2011) 540–543.
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Synthesis and Raman properties of magnesium borate micro/nanorods Shuang Li a,b,*, Dapeng Xu c, Hongzhi Shen c, Jing Zhou c, Ya Fan a,b a
Department of Physics, Changchun University of Science and Technology, Changchun, PR China International Joint Research Centre for Nanophotonics and Biophotonics, CUST, Changchun, PR China c College of Physics, Jilin University, Changchun, PR China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 November 2011 Received in revised form 24 May 2012 Accepted 14 June 2012 Available online 23 June 2012
With cetyltrimethylammonium bromide (CTAB) as a soft template, magnesium borate (Mg2B2O5) onedimensional micro/nanorods were synthesized. The products prepared in the absence of CTAB were Mg2B2O5 nanoparticals and needles. However, using CTAB as a soft template the products were Mg2B2O5 whiskers (diameter: 200 10 nm, length: 1–2 mm). The formation mechanism was discussed. In addition, the experimental and theoretical Raman spectra of Mg2B2O5 were reported for the first time, and the possible vibrations modes of Mg2B2O5 crystals were assigned based on the calculation results. Crown Copyright ß 2012 Published by Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures B. Chemical synthesis C. Raman spectroscopy D. Crystal structure
1. Introduction Magnesium borates, with many chemical compositions such as Mg2B2O5, MgB4O7 and Mg3B2O6, are remarkable ceramic materials. Among them, Mg2B2O5 has been paid much attention in recent years due to its excellent properties. As an excellent antiwear and anticorrosion material, Mg2B2O5 has been widely used to reduce friction coefficient [1,2]. Mg2B2O5 also has other potential applications, such as catalysts for the conversion of the hydrocarbons [3,4], luminescent materials for cathode ray tube screens [5] and thermo-luminescence application [6–8]. Now, onedimensional (1D) micro-/nanomaterials have become the research focus for their unique structures, novel properties and potential applications [9]. Magnesium borates with 1D structure, such as nanorods [10–12], nanowires [13–15], nanobelts [16], nanotubes [17] and whiskers [18,19] have been reported. Varieties of synthetic methods, for example, CVD method [13], catalyst-free method [15], high temperature solid-state synthesis method [10,14], solvothermal method [11], and mechano-chemical method [12] have been widely used in synthesis of 1D micro-/ nanostructure magnesium borate. Nevertheless, these traditional methods expose some disadvantages such as multistep, high energy consumption as well as byproducts. Thus, developing a simple, cost-effective and sustainable synthesis method is an important aspect to obtain high crystalline, high purity and well dispersed production for further properties study and wide
* Corresponding author. Tel.: +86 043185582741. E-mail address: [email protected] (S. Li).
applications. As a straightforward route to obtain 1D structure, template technique has been used as a powerful tool for the fabrication of 1D nanomaterials [20,21]. However, no report has been found on the synthesis of 1D nanostructure magnesium borate with this method. In this paper, a CTAB-assisted soft template method for the synthesis of Mg2B2O5 micro-/nanorods from MgBr26H2O and NaBH4 is reported. Besides XRD and SEM characterization methods, further evidence for the structure of Mg2B2O5 was obtained from the Raman spectra in this work. Nowadays, Raman spectroscopy has been widely used to study local atomic arrangement in borate glasses. But there were few reports about Raman research on borates crystals, and no information on the Raman spectrum of the Mg2B2O5 crystal is available yet. The vibration modes may be different between borate glasses and borates crystals [22,23]. Herein the room temperature experimental Raman spectrum of Mg2B2O5 crystal was obtained, and its theoretical Raman spectrum was also calculated by quantum chemical methods. The possible vibrations modes in Mg2B2O5 were assigned based on the calculation results and previous Raman study reports of the similar system. 2. Experimental Firstly, Mg2B2O5 precursor solution was prepared by the following process. 3 mmol of MgBr26H2O and 6 mmol of NaBH4 were dissolved in 10 ml and 20 ml of ethanol, respectively. Secondly, 0.5 mmol of cetyltrimethylammonium bromide (CTAB) was added to MgBr26H2O solution and the mixed-solution under vigorous magnetic stirring until dissolved completely. Thirdly, the NaBH4 solution was added dropwise to the MgBr2/CTAB
0025-5408/$ – see front matter . Crown Copyright ß 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.06.046
S. Li et al. / Materials Research Bulletin 47 (2012) 3650–3653
mixed-solution. Large evolution of gas observed and the solution turned turbid. This reaction mixture thickened over time and formed a gel when left open to the atmosphere for several hours. The as-prepared precursors were transferred into a furnace for heat treatment. The furnace temperature was ramped to 800 8C with a heating rate of 2 8C min1, and held at 800 8C for 2 h, then the furnace was cooled down to room temperature as the natural process. A gray, powdery product was obtained after washing with deionized water and centrifugated for three times, and dried at 80 8C for 8 h. For comparison, the similar experimental was carried out in the absence of CTAB. In addition, in order to investigate the effect of the molar ratio of raw materials on the compositions, the reaction was also conducted with the molar ratio of MgBr2:NaBH4 as 1:4 in the presence of CTAB. The crystal structure and the morphology of the products were examined by X-ray diffraction (XRD, RigakuD/max-2500), Raman spectrometer (Jobin Yvon-HR800), selected-area electron diffraction, scanning electron microscopy (SEM, JSM-6480F), and energy dispersive X-ray spectroscopy (EDS, GENE SIS 2000 XMS 60 S, EDAX, Inc.), respectively. 3. Results and discussion Fig. 1 shows the XRD patterns of the as-prepared products with different molar ratios of Mg:B:CTAB. All of the diffraction peaks of the three products are indexed to the monoclinic phase Mg2B2O5 (PCPDF 860531) and no other crystalline phases are detected. It means that CTAB dose not affect the formation of monoclinic phase Mg2B2O5. However, It is worth noting that the product, which was prepared with the molar ratio of Mg:B as 1:4 in the presence of CTAB, is well-crystallized.
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Fig. 1. XRD patterns of the as-prepared products.
The formation of Mg2B2O5 can be express by the following reactions: NaBH4 þ H2 O ! BO2 H þ NaOH þ H2
CTAB
MgBr2 þ NaOH þ BO2 H! NaBr þ Mg2 B2 O5 þ H2 O
(1)
(2)
The morphologies of the products were observed from SEM images (Fig. 2A–C). It can be seen that the product synthesized
Fig. 2. SEM images of as-prepared products with the molar ratio of Mg:B:CTAB as (A) 3:6:0; (B)3:6:0.5; (C)3:12:0.5; (D) EDS and SAED image of the Mg2B2O5 micron-crystal rod.
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S. Li et al. / Materials Research Bulletin 47 (2012) 3650–3653
Fig. 3. The scheme for the formation of one-dimensional Mg2B2O5 nanostructure.
without using soft template exhibits clustered granules and needle-like materials (Fig. 2A). The products synthesized with CTAB as a soft-template have rods-like morphologies (Fig. 2B). According to XRD and SEM analysis results, it can be seen that the different molar ratio of Mg:B dose not affect the chemical compositions and the phase structure, but lead to different morphologies of the final products although they were prepared through the same calcinations process. The product obtained with the Mg:B molar ratio of 1:2 exhibits well dispersed rods-like morphology with uniform diameters of about 200 nm and the lengths of about 1–2 mm (Fig. 2B). The products obtained with the Mg:B molar ratio of 1:4 were larger size micron-rods with the average diameter of about 3 mm and the length of tens of micrometers. It means that excessive introduction of NaBH4 should accelerate the growth of crystals and the reason will be given in the following discussion for formation mechanism. In order to investigate the structure properties of Mg2B2O5, the well-crystallized Mg2B2O5 micron-crystal rods were chosen as samples for further characterized. Fig. 2D shows the typical energy dispersive spectrum (EDS) of the Mg2B2O5 micron-crystal rods. It confirms that the products are composed of Mg, B and O. The selected area electron diffraction (SAED) pattern of the individual Mg2B2O5 micron-crystal rods (the inset of Fig. 2D) confirms that they are single crystals and are indexed as monoclinic phase Mg2B2O5, which is consistent with XRD results. Based on the results mentioned above, we propose the formation mechanism of the one dimensional Mg2B2O5 by using CTAB as a soft template (Fig. 3). First, CTAB can dissolve in the solution to give CTA+ and Br ions, and one-dimensional CTAB micelles formed when the concentration of CTAB is in the range of critical micelles concentration (CMC) [20]. Then the negatively charged MgBr42 interacts with CTA+ micelle through electrostatic interaction to form CTA+–MgBr42 ions pairs. When adding the NaBH4, Mg2B2O5 formed and grew along one-dimension direction due to the existence of CTAB micelles. With the increasing of
NaBH4 concentration the morphology of Mg2B2O5 crystalline rods has no change but the size of that obviously increases. Welldispersed Mg2B2O5 nanorods were formed when using 3 mmol of MgBr2 and 6 mmol of NaBH4 in the reaction system. However, as the concentration of NaBH4 was 12 mmol, an obvious larger rodlike Mg2B2O5 was apparent. A possible mechanism is that the precursors affect the surfactant assembly. Excessive NaBH4 provides more OH, which is adsorbed on the surface of the CTAB micelle. On one hand, it can accelerate the aggregate size of CTAB micelle, on the other hand, it can make the electrostatic interaction between CTA+–MgBr42 ion pairs stronger, and more Mg2B2O5 crystal nucleus will aggregate and grow into large-size crystalline rods during the calcinations. In addition, the room-temperature Raman spectrum of the Mg2B2O5 micron-crystal rods was shown in Fig. 4A. To our knowledge, no experimental Raman and theoretical Raman studies of the Mg2B2O5 crystalline have been reported. In order to give assignment for the observed Raman vibrations, the Raman spectrum of Mg2B2O5 was calculated using quantum chemical methods. The structure optimization and Raman spectrum calculations of Mg2B2O5 were carried out with the Gaussian09 software. The computations were performed at the Hartree–Fock (HF) 3-21G level of theory to get the optimized structure. The optimized parameters are listed in Table 1, which is consistent with that of previously public data [24]. Based on the optimized structure, the Raman spectrum was calculated at HF/STO-3G levels of theory. Table 2 lists the experimental and calculation Raman vibrations and tentative assignments. As can be seen from Fig. 4, the calculated Raman spectrum is in good agreement with the observed Raman spectrum in the range of 400–1000 cm1. However, the difference appeared in low wavenumber (200– 400 cm1) and the high wavenumber (1000–2000 cm1) regions, respectively. It can be explained based on the previous Raman analysis of other borate systems [25–27]. The spectrum contains two distinct regions: (i) n < 600 cm1 contains the external mode vibrations corresponding to translational motion of the metal ions; (ii) 600 cm1 < n < 1400 cm1 contains the internal modes corresponding to stretching vibration and bending vibration of BxOy groups, which have nothing to do with metal ion. In Ref. [25], the authors show that a signification TO–LO splitting exists for these internal vibration modes, and the TO–LO splitting leads to large change in the dipole moment within the BO33 plane. The intensity of majority of these internal modes is very weak and it is difficult to observe clearly the polarization behavior of these modes in the Raman scatting study. So in our study the observed modes are less than the calculated modes in the high frequency region (1000–2000 cm1). As for the low wavenumber region
Table 1 The parameters of bond and angle in the optimized structure.
Fig. 4. The experimental and theoretical Raman spectra of Mg2B2O5 micron-crystal rod.
Mg1–O Mg2–O BO3 B2O5
Bond (nm)
Angle (8)
0.1984 0.2050 0.1332/0.1375/0.1415 0.1332/0.1375/0.1415/0.1435/0.1352
– – 116.895/117.917/125.159 135.112 (B–O–B)
S. Li et al. / Materials Research Bulletin 47 (2012) 3650–3653 Table 2 Raman shift (cm1) and possible assignment of the vibrations in Mg2B2O5. Raman (cm1) Obs.
shift
Assignment
Calc.
233 279 305 330 341 365 409 422 477 547 633
233 283 308 333 342 368 409 419 460 542 634
684 706 713 – – 846 1024 1124 1284
684 708 718 749 754 844 1039 1135 1289
Lattice Lattice Lattice Lattice Lattice Lattice Lattice Lattice Lattice Lattice Scissor bending of O–B–O bond in B2O5 and in-plane torsion vibration of BO3 unit In-plane torsion vibration of B2O5 unit In-plane torsion vibration of BO3 unit Out-plane torsion bending vibration of B2O5 unit O–B–O scissor bending vibration Out-plane torsion bending vibration in B2O5 unit Scissor bending vibration in B2O5 unit Stretching vibration of B–O bond in BO3 and B2O5 units Stretching vibration of B–O bond in B2O5 Asymmetric stretching vibration of B–O bond in BO3
(200–400 cm1) more observed modes exist than the calculated result may be due to the result of crystal field interaction, which we do not consider in the Raman calculation, strongly affects the vibrations modes of MgO6 groups. As to our knowledge, there are no experimental and theoretical Raman reports about Mg2B2O5 crystalline, and the possible assignment of the observed Raman modes for Mg2B2O5 were given on the basis of quantum chemical calculations method and previous Raman study reports of the similar system. In fact more precise assignment of the Raman modes for Mg2B2O5 should be proposed on the basis of lattice dynamics calculations. 4. Conclusions With CTAB as a soft template, one-dimensional Mg2B2O5 crystal rods were synthesized. The results indicated that no changes on the chemical compositions and the phase structure of the asproducts using different concentration of CTAB and NaBH4, The formation of the gel in the initial reaction mixture may be the key to the formation of the 1D nanostructured products. A gel is typically composed of ID CTAB micelle and ultrafine particles. This prearrangement of the precursor particles with the participation of
3653
the CTAB surfactant could form the one-dimensional nanostructure. In the concentration of NaBH4 greatly affect the size of the final products although in the same calcination process. The possible assignment of the observed Raman modes for Mg2B2O5 was given on the basis of quantum chemical calculations method and previous Raman study reports of the similar system. However, more precise assignment of the Raman modes for Mg2B2O5 should be proposed on the basis of lattice dynamics calculations. Acknowledgements Financial support from the National Natural Science Foundation of China (61006065) and the Natural Science Foundation of Jilin Province (20101546) are greatly appreciated. References [1] Z.S. Hu, R. Lai, F. Lou, L.G. Wang, Z.L. Chen, G.X. Chen, J.X. Dong, Wear 252 (2002) 370. [2] Zeng, Y. Yang, H. Fu, W. Qiao, L. Chang, L. Chen, J. Zhu, H. Zou, Guangtian, Mater. Res. Bull. 43 (August (8–9)) (2008) 2239–2247. [3] A.F. Qasrawi, T.S. Kayed, A. Mergen, M. Gurn, Mater. Res. Bull. 40 (2005) 583. [4] E.G. Baker, Boron zinc oxide and boron magnesium oxide catalysts for conversion hydrocarbons, US Patent 2,889,266 (1959). [5] P.W. Ranby, Titanium activated magnesium borate, US Patent 2,758,094 (1956). [6] D.I. Shahare, S.J. Dhoble, S.V. Moharil, J. Mater. Sci. Lett. 12 (1993) 1873. [7] M. Prokic, Nucl. Instrum. Methods 175 (1980) 83. [8] C. Furetta, G. Kitis, P.S. Weng, T.C. Chu, Nucl. Instrum. Methods Phys. Res., Sect. A: Accel. Spectrom. Detect. Assoc. Equip. 420 (1999) 441. [9] Y.N. Xia, P.D. Yang, Y.G. Sun, Adv. Mater. 15 (2003) 353–389. [10] E.M. Elssfah, A. Elsanousi, J. Zhang, Mater. Lett. 61 (2007) 4358–4361. [11] B.S. Xu, T.B. Li, Y. Zhang, Z.X. Zhang, X.G. Liu, Cryst. Growth Des. 8 (4) (2008) 1218–1222. [12] S. Li, X. Fang, J. Leng, H.Z. Shen, Y. Fan, D.P. Xu, Mater. Lett. 64 (2010) 151–153. [13] Y. Li, Z.Y. Fan, J.G. Lu, R.P.H. Chang, Chem. Mater. 16 (2004) 2512–2514. [14] Y. Zeng, H.B. Yang, W.Y. Fu, L. Qiao, Mater. Res. Bull. 43 (2008) 2239–2247. [15] X.Y. Tao, X.D. Li, Nano Lett. 8 (2) (2008) 505–510. [16] J. Zhang, Y.M. Zhao, Acta Phys. Chim. Sin. 22 (2006) 110–113. [17] R.Z. Ma, Y. Bando, D. Golberg, T. Sato, Angew. Chem. Int. Ed. 42 (2003) 1836–1838. [18] W. Zhu, Q. Zhang, L. Xiang, S. Zhu, CrystEngComm 13 (2011) 1654–1663. [19] L. Cong Wang, Y. Shan Zhang, Y. Qi Wang, D. Mei Cao, X. Ping Huang, Y. Liu, Adv. Mater. Res. 287–290 (2011) 683–687. [20] Y. Yu, F. Du, C. Jimmy, Y. Zhuang, P. Wong, J. Solid State Chem. 177 (2004) 4640–4647. [21] N. Asim, S. Radiman, M.S. Mohd Ambar Yarmo, B. Golriz, Micropor. Mesopor. Mater. 120 (2009) 397–401. [22] V.P. Solntsev, A.V. Davydov, V.K. Malinovsky, N.V. Surovtsev, J. Cryst. Growth 312 (20) (2010) 2962–2966. [23] Y. Tetsuji Kunimine, N. Shibata, S. Yamane Masayuki, J. Non-Cryst. Solids 321 (3) (2003) 137–146. [24] G-C. Guo, W-D. Cheng, J-T. Chen, H-H. Zhuang, J-S. Huang, Q-E. Zhang, Acta Crystallogr., Sect. C 51 (1995) 2469–2471. [25] M. Maczka, J. Hanuza, A. Pajaczkowska, J. Raman Spectrosc. 35 (2004) 266–273. [26] G. Barros, E.N. Silva, A.P. Ayala, I. Guedes, C.-K. Loong, J. Wang, X. Hu, H. Zhang, Vib. Spectrosc. 46 (2008) 100–106. [27] R.L. Frost, J. Raman Spectrosc. 42 (2011) 540–543.