Symmetrical Six-horn Nickel Diselenide Nanostars Growth from Oriented Attachment Mechanism Weimin Du,† Xuefeng Qian,*,† Xinshu Niu,‡ and Qiang Gong† School of Chemistry and Chemical Technology, Shanghai Jiao Tong UniVersity, Shanghai 200240, P. R. China, and College of Chemistry and EnVironmental Science, Henan Normal UniVersity, Xinxiang, Henan 453002, P.R. China
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 12 2733–2737
ReceiVed January 25, 2007; ReVised Manuscript ReceiVed July 25, 2007
ABSTRACT: Nickel diselenide nanocrystals in well-defined star shape have been successfully synthesized via an improved solvothermal route with oleic acid as a capping ligand. The obtained products were characterized by powder X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM), and energy-dispersive X-ray analysis (EDS). Results revealed that cubic NiSe2 nanocrystals were in a star-like shape and each nanostar consisted of a central core and six symmetrical horns in which the length of each horn was about 85 nm, the diameter of central core was about 90 nm, and the overall length between two most distance vertices was about 260 nm. Based on the HR-TEM analysis and the reaction process, an oriented attachment growth mechanism was suggested for the formation of six-horn star-like NiSe2, which resulted from the attachment growth of primary building particles along six 〈100〉 directions. Controlled experiments further demonstrated that the star-like shape of NiSe2 nanocrystals was controlled by the cooperative effects of precursor, solvent, and capping regents. Introduction Shape-controlled synthesis of nanomaterials has attracted more and more attention due to their fundamental shape- and size-dependent properties and technological applications.1 Control of their chemical composition, crystal structure, size, and shape allows people to observe the unique properties of nanocrystals and to tune their chemical and physical properties as desired.2 So far, an organic solution-phase synthetic route and its alternatives in diverse surfactants or solvents have been demonstrated as one of the most effective approaches for the controlled synthesis of nanocrystals with various shape and size. Moreover, this route could provide some models for understanding their precipitation and growth mechanisms, in which the growth of nanocrystals generally occurs through two primary mechanisms after fast nucleation in solution: the Ostwald ripening process and the aggregation growth process.2 The former refers to the growth of larger crystals at the expense of smaller crystals, and this mode may result in the formation of faceted particles if the difference of surface energy sufficiently existed in different crystallographic facets.3 Otherwise, crystal growth by aggregation can occur by two means: random aggregation4 and the oriented attachment mechanism,5 which provide routes for the incorporation of defects (e.g., edge and screw dislocations) in stress-free and initially defect-free nanocrsytalline materials.5a Therefore, microstructural features (e.g., defects, porosity, and particle morphology) may yield important clues to the formation mechanisms by which primary building units self-assemble to produce larger particles.5c As one of the special modes or new means for crystal growth, the oriented attachment mechanism has attracted increasing interest in recent years to fabricate or self-organize nanocrystals, for example, high-quality cubic ZnS nanorods formed through oriented attachment of quasi-spherical nanoparticles.6a Moreover, onedimensional nanorods or nanoribbons can further self-attach by * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +86-21-54743262. Fax: +86-21-54741297. † Shanghai Jiao Tong University. ‡ Henan Normal University.
planar van der Waals interactions or lateral “lattice fusion” to generate either length-multiplied one-dimensional nanostructures or two-dimensional crystal sheets and walls.6b With deeper understanding of crystal growth, crystals with complex forms from butterfly-like shapes,6c dendrites,6d or star-shapes,6e to hollow dandelion-like6f or octahedral6g nanostructures have been successfully created via this size-amplifying mechanism. Meanwhile, bioinspired morphosynthesis by organic templates offers another route to generate some inorganic crystals with unusual structures.6h Among these advanced three-dimensional architectures, star-shaped materials have attracted much attention due to their highly symmetrical shape and the shape-dependent physicochemical properties, and some materials (e.g., PbS, Cu2O, CdS, Au)7 with star shape have also been reported. As a good electrical conductor and Pauli paramagnetic metal compound, whose magnetic susceptibility is weakly paramagnetic (=1 × 10-6 emu/g) and increases very weakly with temperature,8 nickel diselenide (NiSe2) has attracted considerable research attention over several decades because of its interesting electronic and magnetic properties and important applications in the field of materials science.9 To date, though great efforts have been focused on it, aggregated particles or polycrystalline materials have been often obtained in most works through the hydrothermal or solvothermal process,9a–9c except the nanofilament and octahedral crystal in micrometer scale with ethylenediamine as solvent9d–9f and polycrystalline nanotube by using Se nanorods as sacrificing-template.9g Therefore, there still exist difficulties in shape-controlled synthesis of nickel diselenide. In this study, star-like NiSe2 nanocrystals have been successfully synthesized through an improved solvothermal process with oleic acid as a capping ligand, and the formation mechanism was investigated. To the best of our knowledge, this is the first report on star-shaped NiSe2 nanocrystals. Experimental Procedures Nickel acetylacetone (95% mass fraction) was purchased from Aldrich Corp. Oleic acid, anisole, and selenium powder were chemically pure reagents (Shanghai Chemical Reagent Corp.). All the reagents were used as received without further purification. In a typical process to prepare
10.1021/cg070088t CCC: $37.00 2007 American Chemical Society Published on Web 11/15/2007
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Figure 1. (a, b) FE-SEM and (c) TEM images, (d) XRD pattern, and (e) EDS pattern of the star-shaped NiSe2 nanocrystals synthesized in anisole solution at 180 °C for 24 h. The inset in panel b is a single NiSe2 nanostar, and the inset in panel c shows the electron diffraction (ED) pattern obtained from a horn of an individual NiSe2 nanostar. NiSe2 nanostars, 0.4 mmol of nickel acetylacetone and 2 mL of oleic acid were added to 38 mL of anisole in a Teflon-lined stainless steel autoclave with a capacity of 50 mL at room temperature. The mixed solution was stirred for 30 min before 0.8 mmol of selenium powder was added. After the autoclave was sealed and maintained at 180 °C for 24 h in a preheated oven, it was moved and cooled to room temperature naturally. The final products were obtained after being treated with absolute ethanol and dried in a vacuum oven. The crystal phase of as-prepared products was characterized in a Rigaku D/Max-2200 diffractometer equipped with a rotating anode and a Cu KR radiation source (λ ) 0.154 18 nm). The morphology, crystal lattice, and composition of the obtained samples were characterized by field-emission scanning electron microscopy (FE-SEM, FEI SIRION 200, with an accelerating voltage of 5 kV), transmission electron microscopy (TEM, JEOL JEM-100CXII, with an accelerating voltage of 100 kV, and JEM2010, with an accelerating voltage of 200 kV), high-resolution transmission electron microscopy (HR-TEM, JEOL JEM-2100F, with an accelerating voltage of 200 kV), and energy-dispersive X-ray analysis (EDS, JEOL JSM- 6460, with an accelerating voltage of 5 kV).
Results and Discussion Figure 1a shows that uniform star-like nanocrystals are the exclusive products when the reaction was carried out at 180 °C
for 24 h via the anisole solvothermal process with oleic acid as a capping ligand, which means NiSe2 nanostars can be produced in large scale by this method. An enlarged FE-SEM image (Figure 1b) clearly reveals that the NiSe2 nanostars exhibit a well-defined star-shaped geometry with a central core and six symmetrical horns in which the length of each horn is about 85 nm, the diameter of central core is about 90 nm, and the overall length between the two most distance vertices is about 260 nm. The two-dimensional (2D) projection of the three-dimensional (3D) six-horn stars shown in the TEM image (Figure 1c) also reveals that the products are star shaped and the size is consistent with that in the FE-SEM image. The regular diffraction spots (due to the [100] zone axis of cubic NiSe2) shown in electron diffraction (ED) pattern of a random single horn indicate that the six-horn star is actually a single-crystalline structure. All the diffraction peaks in the XRD pattern (shown in Figure 1d) can be indexed to the cubic phase of NiSe2 (JCPDS No. 881711), which indicates that the obtained products are in pure cubic phase. Other samples prepared in this work presented similar profiles. The atom content ratio between Ni and Se measured by EDS is 1:2.2, demonstrating that the obtained
Symmetrical Six-Horn NiSe2 Nanostars
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Figure 3. XRD patterns of NiSe2 synthesized at 180 °C for different reaction time; # stands for the crystal phase of Se powder.
Figure 4. TEM images of NiSe2 synthesized at 180 °C for (a) 4 h, (b) 8 h, and (c) 24 h, respectively.
Figure 2. (a, c) Individual nanostars of NiSe2, (b) HR-TEM image of the circled zone in panel a, and (d, e) HR-TEM images of the top (d) and down (e) horn in panel c, respectively.
product is consistent with its stoichiometric proportion. Similar EDS profiles for samples prepared at different temperature suggest that reaction temperature does not have a great effect on the chemical composition of the final products. In order to understand the crystallographic orientation of NiSe2 nanostars, HR-TEM analysis (Figure 2) was carried out on two random nanostars. Clear lattice fringes (shown in Figure 2b from the circled zone in Figure 2a) corresponding to the [200] planes of cubic NiSe2 with 2.99 Å lattice space indicate that each horn grow along the 〈100〉 direction according to the symmetrical star-like shape and equivalent crystallographic facet principle. Meanwhile, the 2.95 Å lattice space shown in Figure 2d from the top horn in Figure 2c is also consistent with the [200] d spacing of cubic NiSe2, which further indicates that the six horns of the star-shaped NiSe2 represent preferential growth along this direction, that is, 〈100〉 direction. Additionally, the “dimples” and “creases” shown in the Figure 2d suggest that the six-horn NiSe2 is constructed from many primary building blocks. The HR-TEM image (Figure 2e) of the down horn in Figure 2c with a gap (highlighted by black arrow) further demonstrates that the formation of larger NiSe2 nanocrystals resulted from the oriented aggregation growth of the primary building units, and the parallel fringes in the selected zone imply that the obtained nanocrystals are single crystalline structures and the jointed primary building blocks have parallel crystallographic orientations between primary building block neighbors. As previously described in the Introduction, microstructural
features, such as defects (e.g., edge dislocations), porosity, and particle morphology yield important clues to the aggregation– growth mechanism.5c Therefore, the observation of microstructural features (e.g., gap, “dimples”, and “creases”) retained in NiSe2 nanostars implies that the oriented attachment growth is the dominant mechanism for the formation of the star-shaped NiSe2 nanocrystals. Controlled experiments over different reaction times demonstrated the phase evolution and the corresponding oriented attachment growth process. From the XRD patterns of NiSe2 synthesized at 180 °C for different reaction times (Figure 3), it was found that pure cubic NiSe2 could be obtained only when the reaction time exceed 8 h due to the complete reduction limit of the Se resource. If the reaction time was not enough, a mixture of NiSe2 and Se was obtained. When the reaction was carried out for 4 h, some smaller nanoparticles and looser nanostars were obtained (see Figure 4a). Subsequently, NiSe2 nanostars in a looser stacked shape (shown in Figure 4b) were the main products when the reaction was carried out for 8 h. It could also be found from Figure 4a,b that these looser nanostars were constructed from smaller primary particles with the size 20–30 nm, which were highlighted by black arrows. When the reaction time was prolonged to 24 h, close-grained nanostars were obtained (Figure 4c). These results demonstrated that the formation of the star-shaped NiSe2 nanocrystals resulted from the aggregation growth of primary particles. During this process, the oleic acid molecule also played an important role on the aggregation of NiSe2 primary particles. When the primary particles or nuclei were formed, it would cap on the surface of nanocrystals and help the capped particles to orderly selfassemble into larger particles. Hence, primary nanoparticles were stabilized, and loose self-assembling particles were obtained. With reaction time further prolonged, fusion or a recrystallizing process happened, and crystalline nanostars were formed by excluding surfactant molecule among the interfaces between primary particles. However, from Figure 2, some defects (e.g.,
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Figure 6. A schematic illustration of the formation process of the sixhorn star-shaped NiSe2 nanocrystals.
Figure 5. (a) The unit cell of NiSe2, (b) the supercell structure of NiSe2 with range of A ) 3a, B ) 3b, C ) 3c, and (c, d) the side elevation images of NiSe2 perpendicular to the [111] direction (c) and [100] direction (d), respectively.
gap, “dimples”, and “creases”) are also existed in local areas, which reveal that the recrystallizing process is not perfect. Generally, 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 factors, for example, surfactants, temperature, and time.10 Therefore, the final crystalline shape is the cooperative result of internal crystal factors and external controlled factors. According to Gibbs–Wulff’s theorem,11 γ1 γ2 γ3 ) ) ) · · · ) constant h1 h2 h3 where γn is the surface tension of crystal face n, and hn is the distance of that face from the Wulff’s point in the crystal, higher surface tension faces tend to grow along their normal direction and eventually disappear from the final appearance. For cubic phase, a sequence of γ{111} < γ{100} < γ{110} can be easily deduced from the distances between these three faces and the central Wulff’s point.11,12 Another variation from Wulff’s theorem uses additives to tune the surface energies of specific crystallographic faces. Furthermore, the intrinsic energy sequences of different surfaces are also affected by the array of atoms or ions in different compounds. Visualization of the cubic NiSe2 crystal structure helps to illustrate the energy sequences of {100} and {111} facets in cubic NiSe2. It is well known that NiSe2 belongs to the pyrite structure (cubic) with space group Th6 (Pa3), and can be considered a NaCl-like group of metal atoms and selenium atom pairs, Se2.13 Based on the spatial location of Ni and Se atoms, Ni at 0 0 0 and Se at 0.383 0.383 0.383, modeled through the XRD patterns of bulk NiSe2,14 the modeled unit cell and the supercell of cubic NiSe2 can be obtained (shown in Figure 5a,b, respectively). From the side elevation images of [111] and [100] facets in cubic NiSe2 (Figure 5c,d), it is found that the [111] faces of cubic NiSe2 only contain
Ni or Se ions and the [100] faces contain mixed Ni/Se ions. This indicates that the intrinsic surface energy of the charged [111] faces should be higher than that of the uncharged [100] faces, which is similar to that of PbS.15 Therefore, the surface of NiSe2 nuclei formed from reaction solution would be circled by [100] and [111] facets, and the oleic acid molecules are more prone to absorb on the [111] facets. As a result, the [111] facets absorbed more surfactant molecules, and the aggregation along this direction was greatly prevented due to the steric hindrance. During the subsequent crystal growth process, the primary building units would preferentially “coalesce” along six 〈100〉 directions along with their own growth. Subsequently, the aggregated primary particles would further fuse, recrystallize, and eliminate the interface to produce larger single crystals. Such a spontaneous “coalescence” of the small nanoparticles along the specific faces and then undergoing fusion and recrystallization is consistent with the proposed “orientated attachment” mechanism5 and the single-crystal formation mechanism through the mesocrystal intermediate formed by nanoparticle selfassembly.16 From these analyses, it is reasonably speculated that the formation of the six-horn star-shaped NiSe2 nanocrystals resulted from the oriented attachment growth of the primary building units along six 〈100〉 directions. The formation process of the star-shaped NiSe2 was illustrated in Figure 6. Further experiments revealed that the precursor and capping ligand played important roles for the formation of the star-like shape of NiSe2. If the nickel acetylacetone was replaced by nickel acetate, irregular aggregated nanoparticles were obtained. When oleic acid was substituted with hexadecylamine and other reaction parameters remained unchanged, mixtures of nanoparticles and nanorods were the main products. These phenomena just well reflected the great influence of different reaction factors on the final products, for example, dissolvability of precursors in anisole solvent, the capping capability and alkalinity–acidity of the surfactants, and the coordinating effects between nickel ions and surfactants. Corresponding TEM images are shown in Figure 7. The effect of higher temperature, 200 °C, was also investigated. Results show that reaction temperature above 180 °C will produce larger size NiSe2 nanostars. Therefore, a reaction temperature of 180 °C is sufficient to obtain the well-defined star-like NiSe2 nanocrystals. The optimal conditions for the formation of star-shaped NiSe2 were with nickel acetylacetone as precursor and oleic acid as capping ligand at 180 °C for 8–24 h. Longer reaction times, up to 48 h, will not bring about evident structural and morphological modifications. On the other hand, anisole also has a great effect on the formation of NiSe2 because of its reducibility. If other solvents, for example, benzene or toluene, were used, a mixture of NiSe2 and Se was obtained because of the incomplete reaction of Se. Of course, oleic acid also has some redox properties. However, it does not play a
Symmetrical Six-Horn NiSe2 Nanostars
Figure 7. TEM image of (a) the NiSe2 synthesized with Ni(Ac)2 as precursor and oleic acid as capping ligand at 180 °C for 24 h and (b) NiSe2 synthesized with Ni(acac)2 as precursor and hexadecylamine as capping ligand at 180 °C for 24 h.
key role because NiSe2 can still be obtained if oleic acid is substituted with hexadecylamine. Conclusion In summary, NiSe2 nanocrystals in well-defined star shape with six symmetrical horns have been successfully synthesized through the anisole solvothermal route with oleic acid as a capping ligand. The oriented attachment growth mechanism was supposed for its formation in which the six-horn shape resulted from the attachment growth of primary building particles along six 〈100〉 directions. Further results showed that the precursor, capping ligand, and solvents also had great effect on the formation or the morphologies of NiSe2. On the other hand, the present anisole solvothermal route might be generalized to synthesize other metal diselenide nanocrystals with more complicated structures. Acknowledgment. The work described here was supported by the National Science Foundation of China (No. 20671061) and the Program for New Century Excellent Talents of Education Ministry of China.
References (1) (a) Burda, C. B.; Chen, X. B.; Narayanan, R.; EI-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (b) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, H. Q. AdV. Mater. 2003, 15, 353. (c) Tessler, N.; Medvedev, V.; Kazes, M.; Kan, S.; Banin, U. Science 2002, 295, 1506.
Crystal Growth & Design, Vol. 7, No. 12, 2007 2737 (2) (a) Yin, Y. D.; Alivisatos, A. P. Nature 2005, 437, 664. (b) Alivisatos, A. P. Science 1996, 271, 933. (c) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (3) Banfield, J. F.; Welch, S. A.; Zhang, H.; Ebert, T. T.; Penn, R. L. Science 2000, 289, 751. (4) (a) Penn, R. L.; Banfield, J. F. Geochim. Cosmochim. Acta 1999, 63, 1549. (b) Privman, V.; Goia, D. V.; Park, J.; Matijevic, E. J. Colloid Interface Sci. 1999, 213, 36. (c) Ocana, M.; Morales, M. P.; Serna, C. J. J. Colloid Interface Sci. 1995, 171, 85. (5) (a) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969. (b) Alivisatos, A. P. Science 2000, 289, 736. (c) Penn, R. L.; Oskam, G.; Strathmann, T. J.; Searson, P. C.; Stone, A. T.; Veblen, D. R. J. Phys. Chem. B 2001, 105, 2177. (6) (a) Yu, J. H.; Joo, J.; Park, H. M.; Baik, S.; Kim, Y. W.; Kim, S. C.; Hyeon, T. J. Am. Chem. Soc. 2005, 127, 5662. (b) Lou, X. W.; Zeng, H. C. J. Am. Chem. Soc. 2003, 125, 2697. (c) Jun, Y.; Casula, M. F.; Sim, J. H.; Kim, S. Y.; Cheon, J. W.; Alivisatos, A. P. J. Am. Chem. Soc. 2003, 125, 15981. (d) Liu, B.; Yu, S.; Li, L.; Zhang, Q.; Zhang, F.; Jiang, K. Angew. Chem., Int. Ed. 2004, 43, 4745. (e) Geng, J.; Lv, Y. N.; Lu, D. J.; Zhu, J. J. Nanotechnology 2006, 17, 2614. (f) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 8124. (g) Yang, H. G.; Zeng, H. C. Angew. Chem., Int. Ed. 2004, 43, 5930. (h) Yu, S. H.; Clfen, H. J. Mater. Chem. 2004, 14, 2124. (7) (a) Zhao, N. N.; Qi, L. M. AdV. Mater. 2006, 18, 359. (b) Zhou, G. J.; Lu, M. K.; Xiu, Z. L.; Wang, S. F.; Zhang, H. P.; Zhou, Y. Y.; Wang, S. M. J. Phys. Chem. B 2006, 110, 6543. (c) Wang, D. B.; Mo, M. S.; Yu, D. B.; Xu, L. Q.; Li, F. Q.; Qian, Y. T. Cryst. Growth Des. 2003, 3, 717. (d) Chae, W. S.; Shin, H. W.; Lee, E. S.; Shin, E. J.; Jung, J. S.; Kim, Y. R. J. Phys. Chem. B 2005, 109, 6204. (e) Nehl, C. L.; Liao, H. W.; Hafner, J. H. Nano Lett. 2006, 6, 683. (8) (a) Ogawa, S. J. Appl. Phys. 1979, 50, 2308. (b) Inoue, N.; Yasuoka, H.; Ogawa, S. J. Phys. Soc. Jpn. 1980, 48, 850. (9) (a) Chen, X. H.; Fan, R. Chem. Mater. 2001, 13, 802. (b) Zhang, W. X.; Hui, Z. H.; Cheng, Y. W.; Zhang, L.; Xie, Y.; Qian, Y. T. J. Cryst. Growth 2000, 209, 213. (c) de las Heras, C.; Agullo-Rueda, F. J. Phys.: Condens. Matter 2000, 12, 5317. (d) Zhuang, Z. B.; Peng, Q.; Zhuang, J.; Wang, X.; Li, Y. D. Chem.sEur. J. 2006, 12, 211. (e) Yang, J.; Cheng, G. H.; Zeng, J. H.; Yu, S. H.; Liu, X. M.; Qian, Y. T. Chem. Mater. 2001, 13, 848. (f) Han, Z. H.; Li, Y. P.; Lu, J.; Yu, S. H.; Zhao, H. Q.; Qian, Y. T. Mater. Res. Bull. 2000, 35, 1825. (g) Zhao, A. W.; Xu, L. Q.; Luo, T.; Qian, Y. T. Chem. Lett. 2005, 34, 1136. (10) Lee, S. M.; Cho, S. N.; Cheon, J. W. AdV. Mater. 2003, 15–441. (11) Wulff, G. Z. Kristallogr. 1901, 34, 449. (12) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153. (13) Bither, T. A.; Bouchard, R. J.; Cloud, W. H.; Donohue, P. C.; Siemons, W. J. Inorg. Chem. 1968, 7, 2208. (14) Furuseth, S.; Kjekhus, A. Acta Chem. Scand. 1969, 23, 2325. (15) Jun, Y. W.; Lee, J. H.; Choi, J. S.; Cheon, J. W. J. Phys. Chem. B 2005, 109, 14795. (16) Cölfen, H.; Mann, S. Angew. Chem., Int. Ed. 2003, 42–2350.
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