- Email: [email protected]
Phase and morphology controllable synthesis of Sb2O3 microcrystals
ARTICLE IN PRESS Journal of Crystal Growth 311 (2009) 3948–3953
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Phase and morphology controllable synthesis of Sb2O3 microcrystals Debao Wang a,, Yanhong Zhou a, Caixia Song b, Mingqi Shao a a b
College of Chemistry and Molecular Engineering, Qingdao University of Science & Technology, Qingdao 266042, PR China College of Materials Science & Engineering, Qingdao University of Science & Technology, Qingdao 266042, PR China
a r t i c l e in f o
a b s t r a c t
Article history: Received 13 February 2009 Received in revised form 3 May 2009 Accepted 10 June 2009 Communicated by S. Uda Available online 18 June 2009
A simple and facile solution route has been developed for phase and morphology controllable synthesis of antimony trioxide (Sb2O3) microcrystals. Orthorhombic phase and cubic phase Sb2O3 microcrystals have been selectively synthesized in high yield. The products were characterized by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). The as-obtained microcrystals exhibited a variety of morphologies and structures, such as microspindles, nanoplates, and octahedra. Several experimental parameters have been investigated to gain morphology control of Sb2O3 microcrystals. Based on the time-dependent experimental results, an aggregation, and recrystallization mechanism was proposed to describe the formation process of these novel microstructures. & 2009 Elsevier B.V. All rights reserved.
PACS: 81.10.Dn 81.16.Be Keywords: A1. Crystal morphology A2. Growth from solutions B1. Oxides
1. Introduction It is now widely accepted that many fundamental properties and applications of nanocrystals depend strongly on their shapes, sizes, and microstructures. Over the past decades, driven by the development of next generation low-cost and advanced micro- or nanodevices, researchers have been exploring strategies for controlling shapes of nano-(micro) building blocks. Antimony trioxide (Sb2O3) is an important member of main group metal oxides and has wide industrial applications, such as conductive material, functional filler, retardant, and highefficiency flame retardant synergist in plastics, paints, adhesives, and textile back coating [1]. In addition, Sb2O3 is also used as a catalyst in the polyester industry and as a clarifying agent in optical glass, TV tubes, and crystal production [2]. Recently, Sb2O3 microscale wires have been reported to be used as the effective pH electrode with characteristics of long-term stability, fast response, and reproducibility [3]. Similar to the other well-studied metal oxide semiconductors, antimony trioxide is also expected to be an interesting semi-conducting material. For Sb2O3 nanomaterials, most efforts have been focused on the synthesis of onedimensional nanocrystals, such as Sb2O3 nanorods, nanowires, nanobelts, and nanotubes [4–7]. Electrochemical preparation of Sb2O3 was carried out by employing an antimony anode and a Corresponding author. Tel.: +86 532 4022787.
E-mail address: [email protected] (D. Wang). 0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.06.020
stainless steel cathode [8]. Sb2O3 thin films have been prepared by a low energy cluster beam deposition technique [9]. Sb2O3 nanoparticles were synthesized via hydrolysis-precipitation method [10]. Comparing with the science and technology demands for Sb2O3, phase and morphology controllable synthesis of Sb2O3 microcrystals remains a challenge. In the present work, we have developed a convenient method for phase and morphology controllable preparation of Sb2O3 microcrystals. Cubic phase Sb2O3 octahedra and orthorhombic phase Sb2O3 microspindles with a variety of shapes and structures have been selectively prepared. It provides an opportunity to further explore the shape dependence of their properties and application potentials of Sb2O3.
2. Experimental procedure The synthesis was carried out in a 50 mL Teflon-lined stainless steel autoclave. For the growth of spindles, typically, a total of 0.5 g poly-(vinylpyrrolidone) (PVP, K30), 0.2 g SbCl3, and 0.2 g NaOH were dissolved in 40 mL of water–ethanol solution (1:1, v/v) in a Teflon liner. Afterwards, the liner was sealed in a stainless steel autoclave. The autoclave was placed in a preheated oven and maintained at 170 1C for 24 h, then cooled to room temperature naturally. The white powders were collected, washed several times with distilled water and ethanol, and then dried in vacuum at 60 1C for 2 h.
ARTICLE IN PRESS D. Wang et al. / Journal of Crystal Growth 311 (2009) 3948–3953
Cubic phase Sb2O3 octahedra were prepared by the same experimental procedure as described above except that 2 mmol of potassium antimony tartrate, 0.42 g of sodium dodecyl benzene sulfonate (SDBS), and 1 mmol NaOH were dissolved in 40 mL of de-ionized water. The experiments were carried out at 150 1C for 24 h. The phase structure and phase purity of the as-prepared samples were identified by powder X-ray diffraction (XRD) using a Philips X’pert X-ray diffractometer. The morphologies, micro/nanostructure of the samples were investigated on a JSM-6700F field emission scanning electron microscope (SEM).
3. Results and discussion 3.1. Synthesis of Sb2O3 microspindles Fig. 1a shows the XRD pattern of Sb2O3 sample obtained in water–ethanol solutions. From the XRD pattern, all labeled reflection peaks could be readily indexed to the orthorhombic phase of Sb2O3 according to the JCPDS cards (No. 11-689). The calculated lattice constants of a ¼ 0.491 nm, b ¼ 1.234 nm, and c ¼ 0.534 nm are in good agreement with the standard values for bulk Sb2O3. No obvious impurities could be detected in the pattern, and the sharp and strong characteristic peaks suggest that the resulting products were well crystallized. The morphologies of the resulting products were examined using SEM. Figs. 1b–d exhibit SEM images of the as-prepared Sb2O3 sample. It is clearly demonstrated that the majority of the crystals have a uniform spindle-like shape with diameter of about 800 nm at the center and length of 4 mm on average (Fig. 1b). Fig. 1c shows the high-magnification SEM image, and a few partly cracked spindles can be observed. The highly magnified SEM image of an individual spindle in Fig. 1d reveals the configuration and structure of the spindle. It is evident that the spindle is constructed with loosely attached subunits, and the size of these nanoparticles is about 100 nm.
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To explore the shape developing process of Sb2O3 microspindles, samples subjected to different reaction durations were characterized through SEM observation. Fig. 2a displays SEM image of the Sb2O3 sample obtained after the reaction was conducted for 0.5 h. It can be seen that spindle-like nanoparticles are the main products. These nanospindles have diameters of about 100 nm at the center and lengths in the range 600–800 nm. Some of them have been tending to aggregate into bigger ones. XRD pattern in Fig. 2b indicates that orthorhombic phase of Sb2O3 can be obtained in the initial stage. When the reaction time was increased to 5 h, solid-structured spindles of ca. 2 mm in length and ca. 500 nm in diameter were formed. Fig. 2d displays the SEM image of an individual Sb2O3 microspindle. The spindle shape is uniform, and the relatively smooth surface is becoming rough. Upon further prolonging the reaction time to 8 h, Sb2O3 microspindles have developed to 3 mm in length (Fig. 2e). Fig. 2f displays the SEM image of several microspindles in high magnification. It reveals that the surfaces of the microspindles have become much rougher together with the growing size when the reaction time was prolonged. The cracked spindle in the image clearly indicates that the microspindles were constructed with smaller subunits less than 100 nm in size. When the reaction was conducted for 24 h, the aggregated subunits become distinguishable, and loosely constructed microspindles were obtained (Fig. 1d). A series of experiments were conducted to further understand the influence of other parameters on the morphology control of Sb2O3 microcrystals. Fig. 3a shows a SEM image of Sb2O3 sample obtained in the absence of PVP. It reveals that microspindles could also be obtained, but their shapes are not so uniform and some branched nanorods grew out of the spindles. If cetyltrimethylammonium bromide (CTAB) is used as capping reagent instead of PVP, as shown in Fig. 3b, microspindles can also be obtained, which means that it does not play a key role in the formation of microspindles. When the experiments are carried out with higher concentration of NaOH, the spindles exhibit much loosely stacked architectures constructed with Sb2O3
Fig. 1. (a) XRD pattern, and (b–d) SEM images of Sb2O3 spindles obtained in water–alcohol, at 170 1C for 24 h with 0.2 g SbCl3 and 0.5 g PVP.
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Fig. 2. SEM images of Sb2O3 samples obtained in water–alcohol at 170 1C for (a) 0.5 h, (c,d) 5 h, and (e,f) 8 h, and (b) XRD pattern of Sb2O3 sample for 0.5 h.
nanoparticles (Fig. 3c). Upon using pure water as solvent, the hydrothermal process results in the formation of irregular aggregates of Sb2O3 nanoparticles, and no spindle-like aggregates were formed (Fig. 3d), which hints that solvents play an important role for the oriented aggregation of Sb2O3 nanoparticles.
3.2. Synthesis of Sb2O3 nanoplates Interestingly, when the reaction was conducted at higher concentration of SbCl3 in water–alcohol solvent (for example, 0.4 g SbCl3 was added) with other reaction parameters remained unchanged, Sb2O3 nanoplates were obtained. Fig. 4a exhibits that these nanoplates have good dispersibility and well-confined shapes, and their edge length is in the range 1.5–2.0 mm. Highmagnification SEM image in Fig. 4b reveals that the plate-like crystals are actually polyhedron shapes, and are about 150 nm in thickness at the center. The bottom and top facets of the polyhedra are rough, and the four side facets are smooth. Their edge width is about 1.0 mm and length ranges in 1.5–2.0 mm. XRD pattern in Fig. 4c1 confirms the formation of orthorhombic phase of Sb2O3 (JCPDS cards No. 11-689) and hints well crystallization. Comparing SEM images in Fig. 4 with that in Fig. 3c, it is
concluded that the mole ratio of PVP to SbCl3 plays a key role for the morphology control of orthorhombic phase Sb2O3. The shape evolution of Sb2O3 plate-like nanocrystals has been investigated by recording SEM images of Sb2O3 samples obtained at different reaction stages. Fig. 4d shows the SEM image of Sb2O3 sample prepared after reacting 0.5 h. It is notable that quasispherical aggregated microparticles were obtained at this reaction stage, and these microparticles are constructed with oriented subunits. XRD pattern in Fig. 4c2 indicates that orthorhombic phase of Sb2O3 has formed at the early reaction stage. When the reaction proceeded for another period of time, as shown in Fig. 4e, the aggregated particles have been developing into faceted microcrystals at this reaction stage. SEM image in the inset of Fig. 4e reveals the definite shape of an individual microparticle exhibiting well-defined facets.
3.3. Synthesis of Sb2O3 octahedra When the reaction was conducted using potassium antimony tartrate as precursor, a series of Sb2O3 octahedron related crystals could be obtained. Fig. 5a shows XRD pattern of an as-prepared Sb2O3 sample obtained in the presence of 0.42 g SDBS at 150 1C for 24 h. All the diffraction peaks are labeled in Fig. 5a and can be
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Fig. 3. SEM images of Sb2O3 samples obtained in water–alcohol: (a) no PVP, (b) 0.5 g CTAB, and (c) 0.4 g NaOH, and (d) Sb2O3 sample obtained in the absence of alcohol.
readily indexed to cubic phase of Sb2O3, with calculated cell parameter a ¼ 1.12 nm, consistent with the data reported in the literature (JCPDS cards No 5-534). The pattern exhibits stronger intensity of the (111) peaks of the cubic phase. Other relatively strong reflection peaks are becoming weak in this pattern comparing with the standard data, which suggests that Sb2O3 crystals mainly show their {111} facets. Fig. 5b and c exhibit the SEM images of the corresponding Sb2O3 sample. Based on the SEM observation in Fig. 5b, a large amount of Sb2O3 microcrystals are produced, and they are relatively monodisperse. It is interesting to note from high-magnification SEM image in Fig. 5c that Sb2O3 crystals obviously exhibit an octahedron shape (ratio 80%) and have diameters in the range 800 nm–1.5 mm. It could be deduced that each of the octahedral particles is build up with eight {111} facets of cubic lattice of Sb2O3 and has smooth surfaces. This anisotropic growth is well consistent with the XRD results showing that the peaks of (111) planes largely enhanced, while others are substantially weakened as comparing with the standard XRD pattern. To study the growth mechanism of the Sb2O3 octahedra, the time-dependent crystal morphology of the sample was reported in Fig. 5d. It is clear that Sb2O3 microparticles with six-horns were obtained when the reaction was carried out for 11 h. Most of particles have a size of about 1.5 mm and are partially inclined to transform into octahedral-like frames at this reaction time. Careful examination of the particles indicates that surfaces of these particles are particularly rough, suggesting that they might be made from the aggregation of smaller Sb2O3 nanoparticles.
3.4. Discussion Generally, the coarsening and morphology evolution of nanostructure features after fast nucleation in solution can be described in terms of two primary mechanisms: [11] the
aggregation growth process and the Ostwald ripening process. In the present work, time-dependent experimental results yield important clues to the formation mechanisms of Sb2O3 microcrystals. It is suggested that an aggregation and recrystallization process could be used to describe the formation progress of Sb2O3 microspindles, and three consecutive steps may be followed: (i) formation of Sb2O3 primary particles or subunits in the early stage, (ii) aggregation of Sb2O3 primary particles into microspindles driven by the minimization of surface energy, and (iii) conversion to loosely stacked Sb2O3 microspindles via crystal aging and recrystallization. At the early reaction stage, primary Sb2O3 nanoparticles were formed through conventional nucleation and a subsequent crystal growth process, relating to the PVP-mediated water–alcohol mixed solution conditions, and orthorhombic phase Sb2O3 were obtained. Although the formation of smaller crystallites is kinetically favored during the initial agglomeration, larger crystallites are thermodynamically favored, for that an aggregation process involving the formation of larger crystals by greatly reducing the interfacial energy of small primary nanocrystals is energetically favored. The freshly formed Sb2O3 primary nanoparticles would act as building units to self-assemble and produce bigger microspindles through linear aggregation with the assistance of PVP. The driving force may be that the elimination of the pairs of high-energy surfaces of subunits will lead to a substantial reduction on surface free energy from the thermodynamic viewpoint [12]. As a result, aggregated spindle-like superstructures were obtained. This kind of spontaneous aggregation has been observed in many systems. In the previous report [13], ZnO ellipsoidal structures were also combined by the PEG mediated oriented aggregation mechanism to reduce the free energies of the surface and grain boundaries and generate thermodynamically stable configurations. Zhang et al. reported that Ln3+-doped YF3 nanospindles formed through the aggregation of multiple nanoparticles in a linear aggregate that developed gradually into one spindle under the effect of EDTA [14].
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Fig. 4. (a,b) SEM images and (c1) XRD pattern of Sb2O3 crystals obtained in water-alcohol with 0.4 g SbCl3, at 170 1C for 24 h, SEM image (d) and XRD pattern (c2) of Sb2O3 crystals obtained for 0.5 h, and SEM image (e) of Sb2O3 crystals obtained for 8 h.
When higher concentration of SbCl3 was used in water–alcohol solvent, faster reaction rate would result in more primary Sb2O3 nanocrystallites in the nucleation stage, and the linear aggregation of these subunits would be limited considering lower mole ratio of PVP to Sb(III). Three-dimensional aggregates were the main products in the early stage (Fig. 4d). In the following crystal aging stage, Sb2O3 polyhedra were formed through Ostwald ripening of the aggregated nanocrystallites, which involves the dissolution of fine particles and growth of larger particles [15,16]. Similar formation mechanism for the polyhedra has been drawn to explain the developing process of icositetrahedral morphology from self-aggregated microspheres [17]. When the reaction took place using potassium antimony tartrate as precursor, cubic phase of Sb2O3 was obtained. Previous reports for cubic phase PbS and NiSe2 showed that the intrinsic surface energy of the charged {111} faces should be higher than that of the uncharged {1 0 0} faces [18,19]. After cubic phase Sb2O3 nuclei or primary particles emerged from the solution, similarly, tartrate ions would be more prone to adsorb on their {111} facets due to their higher surface energy and high coordination ability of tartrate ions. As a result, the {111} facets chelated more tartrate ions and adsorbed more SDBS molecules, and aggregation along /111S directions would be greatly limited considering the steric
hindrance caused by the chelating tartrate ions and the capping SDBS molecules. During the following crystal growth process, the primary building units would preferentially aggregate along six /1 0 0S directions along with their own growth. And it is reasonably speculated that six-horn shaped Sb2O3 aggregates were obtained (Fig. 5c), which might be similar to that of PbS and NiSe2 [18,19]. Subsequently, the self-aggregated primary particles would further fuse and recrystallize into octahedron through Ostwald ripening. An octahedron enclosed with eight {111} planes is then obtained. Previously, the formation of zeolite polyhedra has been confirmed to follow a route based on oriented aggregation of nanoparticles and then recrystallization of these nanoparticles at the surface of the aggregated microspheres [17]. In addition, microsized Sb2O3 octahedra were fabricated via polymer-assisted route with tartrate as chelating reagent [20]. It has been reported that the final shape of nanocrystals was dominated by the inherent crystal structure during the initial nucleation stage and the subsequent growth stage through the delicate control of external experimental parameters, such as surfactants, temperature, and reaction time [21]. Our experimental results just well reflected the great influence of different reaction parameters on the final products, such as, the capping capability of surfactant, the coordination effects between Sb(III)
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Fig. 5. (a) XRD pattern and (b,c) SEM images of Sb2O3 sample using potassium antimony tartrate as precursor for 24 h, and (d) for 11 h.
and surfactant, as well as solvent. Of course, the exact formation mechanism of the Sb2O3 microcrystals needs to be further investigated.
4. Conclusion In summary, orthorhombic phase Sb2O3 microspindles, polyhedral nanoplates, and cubic phase Sb2O3 octahedra have been successfully synthesized by the one-pot solvothermal (hydrothermal) method. Time-dependent experiments have been conducted and an aggregation and recrystallization mechanism has been deduced to describe the crystal growth process. The experimental results suggest that metal oxides crystals with desired architecture can be consistently synthesized by programming the growth parameters in the initial synthetic scheme.
Acknowledgement This work was financially supported by the Found for Excellent Young and Middle Aged Scientist (2007BS04029) in Shandong Province in China. References [1] [a] H. Sato, K. Kondo, S. Tsuge, H. Ohtani, N. Sato, Polym. Degrad. Stab. 62 (1998) 41; [b] M. Brebua, E. Jakabb, Y. Sakataa, J. Anal. Appl. Pyrolysis 79 (2007) 346; [c] B. Pillep, P. Behrens, U.A. Schuber, J. Spengler, H. Konezinger, J. Phys. Chem. B 103 (1999) 9595; [d] H.H. Liu, Y. Iwasawa, J. Phys. Chem. B 106 (2002) 2319.
[2] M. Legouera, P. Kostka, M. Poulain, J. Phys. Chem. Solids 65 (2004) 901. [3] Y. Ha, M. Wang, Electroanalysis 18 (2006) 1121. [4] [a] L. Guo, Z.H. Wu, T. Liu, W.D. Wang, H.S. Zhu, Chem. Phys. Lett. 318 (2000) 49; [b] X.Y. Chen, X. Wang, C.H. An, J.W. Liu, Y.T. Qian, Mater. Res. Bull. 40 (2005) 469; [c] S. Friedrichs, R.R. Meyer, J. Sloan, A.I. Kirkland, J.L. Hutchison, M.L. Green, Chem. Commun. (2001) 929; [d] Y.X. Zhang, G.H. Li, J. Zhang, L.D. Zhang, Nanotechnology 15 (2004) 762; [e] D. Sendor, T. Weirich, U. Simon, Chem. Commun. 1 (2005) 5790. [5] Z.T. Deng, D. Chen, F.Q. Tang, X.W. Meng, J. Ren, L. Zhang, J. Phys. Chem. C 111 (2007) 5325. [6] Q.R. Zhao, X.J. Zhang, Q. Yang, Y. Xie, Can. J. Chem. 83 (2005) 1093. [7] Y.X. Zhang, G.H. Li, L.D. Zhang, Chem. Lett. 33 (2004) 334. [8] S. Mohan, S. Pushpavanam, S. Vasudevan, Ind. Eng. Chem. Res. 46 (2007) 7870. [9] J. Nayak, S.N. Sahu, Mater. Lett. 61 (2007) 1388. [10] Y.H. Hu, H.H. Zhang, H.M. Yang, J. Alloys Compds. 428 (2007) 327. [11] [a] Y.D. Yin, A.P. Alivisatos, Nature 437 (2005) 664; [b] A.P. Alivisatos, Science 271 (1996) 933; [c] S. Sun, C.B. Murray, D. Weller, L. Folks, A. Moser, Science 287 (2000) 1989. [12] A.P. Alivisatos, Science 289 (2000) 736. [13] J. Liu, X. Huang, K.M. Sulieman, F. Sun, X. He, J. Phys. Chem. B 110 (2006) 10612. [14] M. Zhang, H. Fan, B. Xi, X. Wang, C. Dong, Y.T. Qian, J. Phys. Chem. C 111 (2007) 6652. [15] J. Geng, J.J. Zhu, D.J. Lu, H.Y. Chen, Inorg. Chem. 45 (2006) 8403. [16] Y. Chang, J.J. Teo, H.C. Zeng, Langmuir 21 (2005) 1074. [17] X.Y. Chen, M.H. Qiao, S.H. Xie, K.N. Fan, W.Z. Zhou, H.Y. He, J. Am. Chem. Soc. 129 (2007) 13305. [18] Y.W. Jun, J.H. Lee, J.S. Choi, J.W. Cheon, J. Phys. Chem. B 109 (2005) 14795. [19] W. Du, X. Qian, X. Niu, Q. Gong, Cryst. Growth Des. 7 (2007) 2733. [20] X.H. Ma, Z.D. Zhang, X.B. Li, Y. Du, F. Xu, Y.T. Qian, J. Solid State Chem. 177 (2004) 3824. [21] [a] S.-M. Lee, S.-N. Cho, J. Cheon, Adv. Mater. 15 (2003) 441; [b] X.G. Peng, Adv. Mater. 15 (2003) 459.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Phase and morphology controllable synthesis of Sb2O3 microcrystals Debao Wang a,, Yanhong Zhou a, Caixia Song b, Mingqi Shao a a b
College of Chemistry and Molecular Engineering, Qingdao University of Science & Technology, Qingdao 266042, PR China College of Materials Science & Engineering, Qingdao University of Science & Technology, Qingdao 266042, PR China
a r t i c l e in f o
a b s t r a c t
Article history: Received 13 February 2009 Received in revised form 3 May 2009 Accepted 10 June 2009 Communicated by S. Uda Available online 18 June 2009
A simple and facile solution route has been developed for phase and morphology controllable synthesis of antimony trioxide (Sb2O3) microcrystals. Orthorhombic phase and cubic phase Sb2O3 microcrystals have been selectively synthesized in high yield. The products were characterized by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). The as-obtained microcrystals exhibited a variety of morphologies and structures, such as microspindles, nanoplates, and octahedra. Several experimental parameters have been investigated to gain morphology control of Sb2O3 microcrystals. Based on the time-dependent experimental results, an aggregation, and recrystallization mechanism was proposed to describe the formation process of these novel microstructures. & 2009 Elsevier B.V. All rights reserved.
PACS: 81.10.Dn 81.16.Be Keywords: A1. Crystal morphology A2. Growth from solutions B1. Oxides
1. Introduction It is now widely accepted that many fundamental properties and applications of nanocrystals depend strongly on their shapes, sizes, and microstructures. Over the past decades, driven by the development of next generation low-cost and advanced micro- or nanodevices, researchers have been exploring strategies for controlling shapes of nano-(micro) building blocks. Antimony trioxide (Sb2O3) is an important member of main group metal oxides and has wide industrial applications, such as conductive material, functional filler, retardant, and highefficiency flame retardant synergist in plastics, paints, adhesives, and textile back coating [1]. In addition, Sb2O3 is also used as a catalyst in the polyester industry and as a clarifying agent in optical glass, TV tubes, and crystal production [2]. Recently, Sb2O3 microscale wires have been reported to be used as the effective pH electrode with characteristics of long-term stability, fast response, and reproducibility [3]. Similar to the other well-studied metal oxide semiconductors, antimony trioxide is also expected to be an interesting semi-conducting material. For Sb2O3 nanomaterials, most efforts have been focused on the synthesis of onedimensional nanocrystals, such as Sb2O3 nanorods, nanowires, nanobelts, and nanotubes [4–7]. Electrochemical preparation of Sb2O3 was carried out by employing an antimony anode and a Corresponding author. Tel.: +86 532 4022787.
E-mail address: [email protected] (D. Wang). 0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.06.020
stainless steel cathode [8]. Sb2O3 thin films have been prepared by a low energy cluster beam deposition technique [9]. Sb2O3 nanoparticles were synthesized via hydrolysis-precipitation method [10]. Comparing with the science and technology demands for Sb2O3, phase and morphology controllable synthesis of Sb2O3 microcrystals remains a challenge. In the present work, we have developed a convenient method for phase and morphology controllable preparation of Sb2O3 microcrystals. Cubic phase Sb2O3 octahedra and orthorhombic phase Sb2O3 microspindles with a variety of shapes and structures have been selectively prepared. It provides an opportunity to further explore the shape dependence of their properties and application potentials of Sb2O3.
2. Experimental procedure The synthesis was carried out in a 50 mL Teflon-lined stainless steel autoclave. For the growth of spindles, typically, a total of 0.5 g poly-(vinylpyrrolidone) (PVP, K30), 0.2 g SbCl3, and 0.2 g NaOH were dissolved in 40 mL of water–ethanol solution (1:1, v/v) in a Teflon liner. Afterwards, the liner was sealed in a stainless steel autoclave. The autoclave was placed in a preheated oven and maintained at 170 1C for 24 h, then cooled to room temperature naturally. The white powders were collected, washed several times with distilled water and ethanol, and then dried in vacuum at 60 1C for 2 h.
ARTICLE IN PRESS D. Wang et al. / Journal of Crystal Growth 311 (2009) 3948–3953
Cubic phase Sb2O3 octahedra were prepared by the same experimental procedure as described above except that 2 mmol of potassium antimony tartrate, 0.42 g of sodium dodecyl benzene sulfonate (SDBS), and 1 mmol NaOH were dissolved in 40 mL of de-ionized water. The experiments were carried out at 150 1C for 24 h. The phase structure and phase purity of the as-prepared samples were identified by powder X-ray diffraction (XRD) using a Philips X’pert X-ray diffractometer. The morphologies, micro/nanostructure of the samples were investigated on a JSM-6700F field emission scanning electron microscope (SEM).
3. Results and discussion 3.1. Synthesis of Sb2O3 microspindles Fig. 1a shows the XRD pattern of Sb2O3 sample obtained in water–ethanol solutions. From the XRD pattern, all labeled reflection peaks could be readily indexed to the orthorhombic phase of Sb2O3 according to the JCPDS cards (No. 11-689). The calculated lattice constants of a ¼ 0.491 nm, b ¼ 1.234 nm, and c ¼ 0.534 nm are in good agreement with the standard values for bulk Sb2O3. No obvious impurities could be detected in the pattern, and the sharp and strong characteristic peaks suggest that the resulting products were well crystallized. The morphologies of the resulting products were examined using SEM. Figs. 1b–d exhibit SEM images of the as-prepared Sb2O3 sample. It is clearly demonstrated that the majority of the crystals have a uniform spindle-like shape with diameter of about 800 nm at the center and length of 4 mm on average (Fig. 1b). Fig. 1c shows the high-magnification SEM image, and a few partly cracked spindles can be observed. The highly magnified SEM image of an individual spindle in Fig. 1d reveals the configuration and structure of the spindle. It is evident that the spindle is constructed with loosely attached subunits, and the size of these nanoparticles is about 100 nm.
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To explore the shape developing process of Sb2O3 microspindles, samples subjected to different reaction durations were characterized through SEM observation. Fig. 2a displays SEM image of the Sb2O3 sample obtained after the reaction was conducted for 0.5 h. It can be seen that spindle-like nanoparticles are the main products. These nanospindles have diameters of about 100 nm at the center and lengths in the range 600–800 nm. Some of them have been tending to aggregate into bigger ones. XRD pattern in Fig. 2b indicates that orthorhombic phase of Sb2O3 can be obtained in the initial stage. When the reaction time was increased to 5 h, solid-structured spindles of ca. 2 mm in length and ca. 500 nm in diameter were formed. Fig. 2d displays the SEM image of an individual Sb2O3 microspindle. The spindle shape is uniform, and the relatively smooth surface is becoming rough. Upon further prolonging the reaction time to 8 h, Sb2O3 microspindles have developed to 3 mm in length (Fig. 2e). Fig. 2f displays the SEM image of several microspindles in high magnification. It reveals that the surfaces of the microspindles have become much rougher together with the growing size when the reaction time was prolonged. The cracked spindle in the image clearly indicates that the microspindles were constructed with smaller subunits less than 100 nm in size. When the reaction was conducted for 24 h, the aggregated subunits become distinguishable, and loosely constructed microspindles were obtained (Fig. 1d). A series of experiments were conducted to further understand the influence of other parameters on the morphology control of Sb2O3 microcrystals. Fig. 3a shows a SEM image of Sb2O3 sample obtained in the absence of PVP. It reveals that microspindles could also be obtained, but their shapes are not so uniform and some branched nanorods grew out of the spindles. If cetyltrimethylammonium bromide (CTAB) is used as capping reagent instead of PVP, as shown in Fig. 3b, microspindles can also be obtained, which means that it does not play a key role in the formation of microspindles. When the experiments are carried out with higher concentration of NaOH, the spindles exhibit much loosely stacked architectures constructed with Sb2O3
Fig. 1. (a) XRD pattern, and (b–d) SEM images of Sb2O3 spindles obtained in water–alcohol, at 170 1C for 24 h with 0.2 g SbCl3 and 0.5 g PVP.
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Fig. 2. SEM images of Sb2O3 samples obtained in water–alcohol at 170 1C for (a) 0.5 h, (c,d) 5 h, and (e,f) 8 h, and (b) XRD pattern of Sb2O3 sample for 0.5 h.
nanoparticles (Fig. 3c). Upon using pure water as solvent, the hydrothermal process results in the formation of irregular aggregates of Sb2O3 nanoparticles, and no spindle-like aggregates were formed (Fig. 3d), which hints that solvents play an important role for the oriented aggregation of Sb2O3 nanoparticles.
3.2. Synthesis of Sb2O3 nanoplates Interestingly, when the reaction was conducted at higher concentration of SbCl3 in water–alcohol solvent (for example, 0.4 g SbCl3 was added) with other reaction parameters remained unchanged, Sb2O3 nanoplates were obtained. Fig. 4a exhibits that these nanoplates have good dispersibility and well-confined shapes, and their edge length is in the range 1.5–2.0 mm. Highmagnification SEM image in Fig. 4b reveals that the plate-like crystals are actually polyhedron shapes, and are about 150 nm in thickness at the center. The bottom and top facets of the polyhedra are rough, and the four side facets are smooth. Their edge width is about 1.0 mm and length ranges in 1.5–2.0 mm. XRD pattern in Fig. 4c1 confirms the formation of orthorhombic phase of Sb2O3 (JCPDS cards No. 11-689) and hints well crystallization. Comparing SEM images in Fig. 4 with that in Fig. 3c, it is
concluded that the mole ratio of PVP to SbCl3 plays a key role for the morphology control of orthorhombic phase Sb2O3. The shape evolution of Sb2O3 plate-like nanocrystals has been investigated by recording SEM images of Sb2O3 samples obtained at different reaction stages. Fig. 4d shows the SEM image of Sb2O3 sample prepared after reacting 0.5 h. It is notable that quasispherical aggregated microparticles were obtained at this reaction stage, and these microparticles are constructed with oriented subunits. XRD pattern in Fig. 4c2 indicates that orthorhombic phase of Sb2O3 has formed at the early reaction stage. When the reaction proceeded for another period of time, as shown in Fig. 4e, the aggregated particles have been developing into faceted microcrystals at this reaction stage. SEM image in the inset of Fig. 4e reveals the definite shape of an individual microparticle exhibiting well-defined facets.
3.3. Synthesis of Sb2O3 octahedra When the reaction was conducted using potassium antimony tartrate as precursor, a series of Sb2O3 octahedron related crystals could be obtained. Fig. 5a shows XRD pattern of an as-prepared Sb2O3 sample obtained in the presence of 0.42 g SDBS at 150 1C for 24 h. All the diffraction peaks are labeled in Fig. 5a and can be
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Fig. 3. SEM images of Sb2O3 samples obtained in water–alcohol: (a) no PVP, (b) 0.5 g CTAB, and (c) 0.4 g NaOH, and (d) Sb2O3 sample obtained in the absence of alcohol.
readily indexed to cubic phase of Sb2O3, with calculated cell parameter a ¼ 1.12 nm, consistent with the data reported in the literature (JCPDS cards No 5-534). The pattern exhibits stronger intensity of the (111) peaks of the cubic phase. Other relatively strong reflection peaks are becoming weak in this pattern comparing with the standard data, which suggests that Sb2O3 crystals mainly show their {111} facets. Fig. 5b and c exhibit the SEM images of the corresponding Sb2O3 sample. Based on the SEM observation in Fig. 5b, a large amount of Sb2O3 microcrystals are produced, and they are relatively monodisperse. It is interesting to note from high-magnification SEM image in Fig. 5c that Sb2O3 crystals obviously exhibit an octahedron shape (ratio 80%) and have diameters in the range 800 nm–1.5 mm. It could be deduced that each of the octahedral particles is build up with eight {111} facets of cubic lattice of Sb2O3 and has smooth surfaces. This anisotropic growth is well consistent with the XRD results showing that the peaks of (111) planes largely enhanced, while others are substantially weakened as comparing with the standard XRD pattern. To study the growth mechanism of the Sb2O3 octahedra, the time-dependent crystal morphology of the sample was reported in Fig. 5d. It is clear that Sb2O3 microparticles with six-horns were obtained when the reaction was carried out for 11 h. Most of particles have a size of about 1.5 mm and are partially inclined to transform into octahedral-like frames at this reaction time. Careful examination of the particles indicates that surfaces of these particles are particularly rough, suggesting that they might be made from the aggregation of smaller Sb2O3 nanoparticles.
3.4. Discussion Generally, the coarsening and morphology evolution of nanostructure features after fast nucleation in solution can be described in terms of two primary mechanisms: [11] the
aggregation growth process and the Ostwald ripening process. In the present work, time-dependent experimental results yield important clues to the formation mechanisms of Sb2O3 microcrystals. It is suggested that an aggregation and recrystallization process could be used to describe the formation progress of Sb2O3 microspindles, and three consecutive steps may be followed: (i) formation of Sb2O3 primary particles or subunits in the early stage, (ii) aggregation of Sb2O3 primary particles into microspindles driven by the minimization of surface energy, and (iii) conversion to loosely stacked Sb2O3 microspindles via crystal aging and recrystallization. At the early reaction stage, primary Sb2O3 nanoparticles were formed through conventional nucleation and a subsequent crystal growth process, relating to the PVP-mediated water–alcohol mixed solution conditions, and orthorhombic phase Sb2O3 were obtained. Although the formation of smaller crystallites is kinetically favored during the initial agglomeration, larger crystallites are thermodynamically favored, for that an aggregation process involving the formation of larger crystals by greatly reducing the interfacial energy of small primary nanocrystals is energetically favored. The freshly formed Sb2O3 primary nanoparticles would act as building units to self-assemble and produce bigger microspindles through linear aggregation with the assistance of PVP. The driving force may be that the elimination of the pairs of high-energy surfaces of subunits will lead to a substantial reduction on surface free energy from the thermodynamic viewpoint [12]. As a result, aggregated spindle-like superstructures were obtained. This kind of spontaneous aggregation has been observed in many systems. In the previous report [13], ZnO ellipsoidal structures were also combined by the PEG mediated oriented aggregation mechanism to reduce the free energies of the surface and grain boundaries and generate thermodynamically stable configurations. Zhang et al. reported that Ln3+-doped YF3 nanospindles formed through the aggregation of multiple nanoparticles in a linear aggregate that developed gradually into one spindle under the effect of EDTA [14].
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Fig. 4. (a,b) SEM images and (c1) XRD pattern of Sb2O3 crystals obtained in water-alcohol with 0.4 g SbCl3, at 170 1C for 24 h, SEM image (d) and XRD pattern (c2) of Sb2O3 crystals obtained for 0.5 h, and SEM image (e) of Sb2O3 crystals obtained for 8 h.
When higher concentration of SbCl3 was used in water–alcohol solvent, faster reaction rate would result in more primary Sb2O3 nanocrystallites in the nucleation stage, and the linear aggregation of these subunits would be limited considering lower mole ratio of PVP to Sb(III). Three-dimensional aggregates were the main products in the early stage (Fig. 4d). In the following crystal aging stage, Sb2O3 polyhedra were formed through Ostwald ripening of the aggregated nanocrystallites, which involves the dissolution of fine particles and growth of larger particles [15,16]. Similar formation mechanism for the polyhedra has been drawn to explain the developing process of icositetrahedral morphology from self-aggregated microspheres [17]. When the reaction took place using potassium antimony tartrate as precursor, cubic phase of Sb2O3 was obtained. Previous reports for cubic phase PbS and NiSe2 showed that the intrinsic surface energy of the charged {111} faces should be higher than that of the uncharged {1 0 0} faces [18,19]. After cubic phase Sb2O3 nuclei or primary particles emerged from the solution, similarly, tartrate ions would be more prone to adsorb on their {111} facets due to their higher surface energy and high coordination ability of tartrate ions. As a result, the {111} facets chelated more tartrate ions and adsorbed more SDBS molecules, and aggregation along /111S directions would be greatly limited considering the steric
hindrance caused by the chelating tartrate ions and the capping SDBS molecules. During the following crystal growth process, the primary building units would preferentially aggregate along six /1 0 0S directions along with their own growth. And it is reasonably speculated that six-horn shaped Sb2O3 aggregates were obtained (Fig. 5c), which might be similar to that of PbS and NiSe2 [18,19]. Subsequently, the self-aggregated primary particles would further fuse and recrystallize into octahedron through Ostwald ripening. An octahedron enclosed with eight {111} planes is then obtained. Previously, the formation of zeolite polyhedra has been confirmed to follow a route based on oriented aggregation of nanoparticles and then recrystallization of these nanoparticles at the surface of the aggregated microspheres [17]. In addition, microsized Sb2O3 octahedra were fabricated via polymer-assisted route with tartrate as chelating reagent [20]. It has been reported that the final shape of nanocrystals was dominated by the inherent crystal structure during the initial nucleation stage and the subsequent growth stage through the delicate control of external experimental parameters, such as surfactants, temperature, and reaction time [21]. Our experimental results just well reflected the great influence of different reaction parameters on the final products, such as, the capping capability of surfactant, the coordination effects between Sb(III)
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Fig. 5. (a) XRD pattern and (b,c) SEM images of Sb2O3 sample using potassium antimony tartrate as precursor for 24 h, and (d) for 11 h.
and surfactant, as well as solvent. Of course, the exact formation mechanism of the Sb2O3 microcrystals needs to be further investigated.
4. Conclusion In summary, orthorhombic phase Sb2O3 microspindles, polyhedral nanoplates, and cubic phase Sb2O3 octahedra have been successfully synthesized by the one-pot solvothermal (hydrothermal) method. Time-dependent experiments have been conducted and an aggregation and recrystallization mechanism has been deduced to describe the crystal growth process. The experimental results suggest that metal oxides crystals with desired architecture can be consistently synthesized by programming the growth parameters in the initial synthetic scheme.
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