J. Phys. Chem. C 2007, 111, 12677-12680
12677
Ligand-free Self-Assembly of Ceria Nanocrystals into Nanorods by Oriented Attachment at Low Temperature Ning Du, Hui Zhang, Bingdi Chen, Xiangyang Ma, and Deren Yang* State Key Lab of Silicon Materials and Department of Material Science and Engineering, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China ReceiVed: May 24, 2007; In Final Form: July 3, 2007
Single-crystalline ceria nanorods have been fabricated by self-assembly of ceria nanocrystals via a simple, low-temperature, and ligand-free approach. Detailed high-resolution transmission electron microscopy shows that the nanorods are formed along the [211] or [110] direction by self-organization of truncated octahedral ceria nanocrystals, sharing the {111} or {200} planes with each other, whereas the previous report shows that only the [110] direction exists in ceria nanorods. The nucleation temperature and molar ratio of Ce3+ and OH- also play key roles in the formation of ceria nanorods. Moreover, the formation mechanism was explained in detail.
Introduction In recent years, the research interests on nanomaterials have been greatly spurred due to their essential significance in science and potential application in technology.1 The chemical solution approach has been acting as an important synthesis route for synthesizing nanocrystals with specific shape, size, and orientation, which play a key role in tailoring the properties of nanomaterials.2 Among chemical solution routes, the wellknown Ostwald ripening mechanism, in which the growth of large particles is at the expense of smaller particles, is generally believed to be the main one.3 However, during the intense research on coarsening behavior and morphology evolution of nanocrystalline titania under hydrothermal condition, Lee and Banfield presented a new crystal growth mechanism involving the attachment between two or more nanocrystals followed by sharing a common crystallographic orientation and joining at the planar interfaces, which is highlighted and termed as “oriented attachment” by Alivisatos.4 Recently, oriented attachment has attracted increasing interest as a new means for fabrication and self-organization of nanostructured materials, especially, one-dimensional (1D) nanostructures.5 For example, Pradhan et al. reported the synthesis of CdSe quantum wires by reacting cadmium acetate and selenourea in amines in the temperature range between 100 and 180 °C via oriented attachment.6 Cho et al. reported the solution-based synthesis of crystalline PbSe nanowires by oriented attachment of nanocrystals in the mixture of lead acetate, oleic acid, diphenyl ether, n-tetradecylphosphonic acid, and TOPSe at 190 to ∼250 °C.7 However, relatively high temperature (>100 °C) and toxic or expensive nonaqueous solvents and ligands were employed in most of the previous reports. Only a few works have been reported to prepare the nanostructures via oriented attachment under the conditions of relatively low-temperature and ligandfree solution. For example, Pacholski et al. convincingly showed that ZnO nanorods could be formed by the 1D oriented attachment of spherical nanodots along the c-axis in an alcohol solution at 60 °C.8 Overall, self-assembly of nanocrystals into * Corresponding author. E-mail:
[email protected].
Figure 1. Morphological and structural characterization of the ceria nanorods prepared by dropwise adding NaOH aqueous solution into Ce(NO3)3 aqueous solution at 100 °C in 4 min and subsequent refluxing for 8 min: (a) TEM image; (b-d) HRTEM images.
nanorods by oriented attachment in a ligand-free environment at low temperature still remains a tremendous challenge. Ceria, one of the most important rare earth metal oxides, has been extensively studied and applied in the field of catalysts, UV blockers, fuel cells, oxygen sensors, and chemicalmechanical planarization materials.9 Recently, ceria nanostructured materials have attracted intensive attention due to their size, shape, and orientation-dependent properties.10 Moreover, several ceria nanostructures such as nanoparticles, nanowires, and nanotubes have been synthesized by various methods including room-temperature solution precipitation,11 high-temperature chemical reaction,12 spray pyrolysis,9e a hydrothermal/ solvothermal process,13 microemulsion,14 and a sonochemical route.15 For instance, Yang and Gao reported the shape- and
10.1021/jp074011r CCC: $37.00 © 2007 American Chemical Society Published on Web 08/09/2007
12678 J. Phys. Chem. C, Vol. 111, No. 34, 2007
Du et al.
Figure 2. Morphological and structural characterization of the pearl-chain-like ceria nanostructures prepared by dropwise adding NaOH aqueous solution into Ce(NO3)3 aqueous solution at room temperature in 4 min and subsequent refluxing for 4 min: (a) TEM image; (b-e) HRTEM images.
size-controlled synthesis of ceria nanocubes by reaction of cerium nitrate aqueous solution and tert-butylmine in toluene and oleic acid at 180 °C via a solvothermal process.16 Moreover, ceria nanocrystals with spherical, wire, and tadpole shapes were controllably fabricated via a nonhydrolytic sol-gel process using oleic acid and oleylamine as cosurfactant.17 Very recently, Si et al. synthesized ceria nanocrystals via an alcohothermal process at 180 °C in ethanol, using alkylamine as the base and polyvinylpyrrolidone (PVP) as the stabilizer, and furthermore, the ceria nanocrystals were self-organized into chainlike and dendritic nanostructures by oriented attachment.18 However, it was necessary for the oriented aggregation by the addition of PVP. Herein, we report on a simple and essentially ligand-free self-assembly of ceria nanocrystals into nanorods via oriented attachment, in which only the Ce(NO3)3 and NaOH were used as the reactants and the reaction temperature was only 100 °C. Experimental Section All the chemicals are analytical grade without further purification. Ceria nanorods were fabricated by the reaction of Ce(NO3)3 and NaOH at 100 °C in aqueous solution and subsequent reflux. First, 50 mL of 5 M NaOH aqueous solution was added into 50 mL of 2 M Ce(NO3)3 aqueous solution at 100 °C in 4 min. Subsequently, the mixed solution was refluxed at 100 °C for different times of 4 and 8 min. The effect of molar ratio of Ce3+ and OH- was investigated by varying the concentration of NaOH from 1 to 10 M, while keeping other condition unchanged. Moreover, a similar experiment was carried out by directly mixing 50 mL of 5 M NaOH aqueous solution and 50 mL of 2 M Ce(NO3)3 aqueous solution at room temperature and subsequently refluxing at 100 °C for 8 min. After the reaction was complete, the resulting solid products were centrifuged, washed with distilled water and ethanol to remove the ions possibly remaining in the final products, and finally dried at 60 °C in air. The obtained samples were characterized by X-ray powder diffraction (XRD) using a Rigaku D/max-ga X-ray diffracto-
Figure 3. XRD patterns of the ceria nanorods (a) and pearl-chain-like ceria nanostructures (b).
meter with graphite-monochromatized Cu KR radiation (λ ) 1.54178 Å). The morphology of the sample was obtained from transmission electron microscopy (TEM, JEM 2010 200 kV). High-resolution transmission electron microscopy (HRTEM) observation was performed on JEM 2010F at 300 kV. Results and Discussion As shown in Figure 1a, the ceria nanorods could be achieved through dropping NaOH into Ce(NO3)3 aqueous solution at 100 °C and subsequently refluxing for 8 min. As can be seen, the nanorods are 5 to ∼15 nm in lateral size and 100 nm in longitudinal size. Moreover, the structure of ceria nanorods is further characterized by HRTEM, as shown in Figure 1b-d. It is shown that the ceria nanorods are cubic and single-crystalline in nature. In addition, there are two kinds of ceria nanorods with different growth directions. Most of the ceria nanorods grow along the [211] direction with exposed {111} planes, as shown in Figure 1, parts b and c. Only a minority of ceria nanorods are observed along the [110] orientation with {200} terminated surface, as shown in Figure 1d. By contrast, in the previous report, the growth direction of ceria nanorods was usually along the [110] orientation.19
Self-Assembly of Ceria Nanocrystals into Nanorods
Figure 4. TEM image of the ceria nanostructures prepared by dropwise adding NaOH aqueous solution into Ce(NO3)3 aqueous solution at 100 °C for 4 min and subsequent refluxing for 8 min at different molar ratios of Ce3+ and OH-: (a) 2.5:1; (b) 1:1; (c) 1:2.5; (d) 1:5.
In order to clarify the formation mechanism of the ceria nanorods, the detailed morphological and structural analysis for the early stage of nanorod formation is shown in Figure 2. As can be seen from Figure 2a, the ceria nanocrystals with a lateral size of about 5 nm are formed and most of the ceria nanocrystals are self-organized into pearl-chain-like structures by 4 min of refluxing. The shape and orientation of the ceria nanocrystals are further characterized by HRTEM, as shown in Figure 2be. In these images, the dominate fringes of the ceria nanocrystal are the {111} and {200} along the [110] observation, indicating that the main shape of the ceria nanocrystals is the truncated octahedral enclosed by eight {111} and six {200} planes, which is similar to the previous results20 (a typical individual ceria nanocrystal can be seen in Supporting Information, Figure S1). Moreover, it can be seen that most of the ceria nanocrystals are perfectly aligned each other by the {111} or {200} plane and are epitaxially fused together. However, the bottlenecks between the adjacent nanocrystals are still visible. In addition, two kinds of attachment pathways are observed for the alignment of ceria
J. Phys. Chem. C, Vol. 111, No. 34, 2007 12679 nanocrystals. One is the attachment aligned along the [211] direction with enclosure by {111} planes, as shown in Figure 2b-d, the other is the attachment aligned along [110] direction with enclosure by {200} planes, as shown in Figure 2e. With the extension of chemical reaction, the single-crystalline ceria nanorods are formed due to the Ostwald ripening process. However, the defects are usually observed within the ceria nanorods, which are marked by the black arrows, as shown in Figure 1. The XRD patterns of the pearl-chain-like ceria nanostructures and the nanorods prepared by the reaction of cerium nitrate and sodium hydroxide for 4 and 8 min, respectively, are shown in Figure 3. All the diffraction peaks can be indexed as the cubic ceria with a lattice constant a ) 5.412 Å, consistent with the value in the standard card (JCPDS 81-0792). Moreover, in view of the fact that the full width at half-maximum (fwhm) of the diffraction peaks is decreased with the extension of reaction time, it is derived that the crystallinity of ceria nanostructures is improved, while the size of them increases from nanoparticles to nanorods. In order to further clarify the formation mechanism of the ceria nanorods, some additional experiments were carried out. For example, only the pearl-chain-like ceria nanostructures were obtained (Supporting Information, Figure S3) when first forming ceria nanocrystals by mixing Ce(NO3)3 and NaOH aqueous solution at room temperature, followed by concentration and refluxing, which was similar to the synthesis strategy for the ZnO nanorods reported by Pacholski et al.8 Further experiments show that only ceria pearl-chain-like nanostructures were obtained when the nucleation temperature is less than 100 °C, which indicates that the higher nucleation temperature (100 °C) is beneficial to the formation of nanorods. When the nucleation temperature is lower than 70 °C, no ceria pearl-chain-like nanostructures could be obtained. It is believed that the lower solubility of ceria with respect to that of ZnO in the base medium results in the different results as mentioned above.23 Moreover, the molar ratio of Ce3+ and OH- is also a key factor for the formation of ceria nanorods. When the molar ratio of Ce3+ and OH- was smaller than 2.5, only the pearl-chain-like ceria nanostructures were formed, as shown in Figure 4, parts a and b. When the molar ratio of Ce3+ and OH- was larger than 2.5, the ceria nanorods are achieved, as shown in Figure 4, parts c and d. Herein, the increase of OH- concentration might promote the formation of ceria nanorods. The result is similar to the high-temperature synthesis of CdSe nanocrystals, in
Figure 5. Schematic diagram for the self-assembly of ceria nanocrystals into ceria nanorods by oriented attachment and subsequent Ostwald ripening.
12680 J. Phys. Chem. C, Vol. 111, No. 34, 2007 which, Peng and Peng considered that the high monomer concentration is beneficial to the 1D growth.24 On the basis of the above discussion, the schematic diagram for the mechanism of the self-assembly of ceria nanocrystals into ceria nanorods by oriented attachment is shown in Figure 5. According to the theoretical calculation, as for ceria crystal, the surface energies of the crystal planes increase in a sequence of {111}, {200}, and {110}; consequently, the {111} and {200} planes are easily exposed and the truncated octahedral shape is therefore formed.21 As a result, the morphology of ceria nanocrystals with the truncated octahedral shape is a hexagon enclosed by four {111} planes and two {200} planes aligned in the [110] direction, as shown in Figure 5. In general, the aggregation of nanocrystals is often the case because the nanocrystals tend to shrink the exposed surface in order to reduce the surface energy, whereas the 1D oriented aggregation proceeds through the dipole-dipole interaction or van der Waals forces. Moreover, it was revealed that the very short ligand greatly decreased the distance between the primary particles and, thereby, enhanced the dipole interaction between them.22 The low reaction temperature reduced the thermal energy of the primary particles, allowing them to align their dipole moments during the attachment process. Therefore, it is believed that the low reaction temperature and ligand-free environment can improve the 1D oriented attachment of the ceria nanocrystals in this work. There are two kinds of oriented attachment modes between the hexagons enclosed by four {111} and two {200} planes, as shown in Figure 5. One is well alignment along the [211] direction with exposed {111} planes; the other is perfectly aligned along the [110] direction and enclosed by {200} planes. Therefore, the pearl-chain-like ceria nanostructures enclosed by {111} or {200} planes are formed by self-assembly of ceria nanocrystals, sharing {111} or {200} planes with each other. Moreover, the lateral oriented attachment parallel to the [211] or [110] direction results in the formation of ceria nanorods with different diameters (Supporting Information, Figure S2). Finally, two kinds of single-crystalline ceria nanorods are formed through the Ostwald ripening process, as shown in Figure 5. Indeed, the above explanations for the formation of ceria nanorods are somewhat conjectural and phenomenological, and extensive work is underway toward its further clarification. In summary, we have developed a simple, low-temperature, and ligand-free approach to self-assembly of ceria nanocrystals into nanorods by oriented attachment in aqueous solution. Ceria nanorods are formed along the [211] or [110] direction by selforganization of truncated octahedral ceria nanocrystals, sharing the {111} or {200} planes with each other. Moreover, the nucleation temperature and molar ratio of Ce3+ and OH- play key roles in the formation of ceria nanorods. The approach presented herein can be extended to synthesize other metal oxide nanorods by oriented attachment. Acknowledgment. The authors appreciate the financial support from the Program for Changjiang Scholar and Innovative Team in University and the Program for New Century Excellent Talents in University. Thanks are given to Professor Youwen Wang and Yuewu Zeng for the TEM measurements. Supporting Information Available: Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Huang, M.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (b) Pan, W. Z.;
Du et al. Dai, R. Z.; Wang, Z. L. Science 2001, 291, 1947. (c) Duan, X.; Huang, Y.; Agarwal, R.; Lieber, C. M. Nature 2003, 421, 241. (d) Fuhrer, M. S.; Nygard, J.; Shih, L.; Forero, M.; Yoon, Y. G.; Mazzoni, M. S. C.; Choi, H. J. Science 2000, 288, 494. (2) (a) Yin, Y. D.; Alivisatos, P. A. Nature 2005, 437, 664. (b) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J. T.; Scher, E.; Kadavanich, A.; Alivisators, A. P. Nature 2000, 404, 59. (c) Park, J.; An, K.; Hwang, Y.; Park, J.; Noh, H.; Kim, J.; Park, J. Hwang, N.; Hyeon, T. Nat. Mater. 2004, 3, 891. (d) Pileni, M. P. Nat. Mater. 2003 2, 145. (e) Kan, S.; Mokari, T.; Rothenberg, E.; Banin, U. Nat. Mater. 2003, 2, 155. (f) El-sayed, M. A. Acc. Chem. Res. 2004, 37, 326. (g) Burda, C.; Chen, X. B.; Narayanan, R.; El-sayed, M. A. Chem. ReV. 2005, 105, 1025. (3) (a) Sugimoto, T. AdV. Colloid Interface Sci. 1987, 28, 65. (b) Wong, E. M.; Bonevich, J. E.; Searson, P. C. J. Phys. Chem. B 1998, 102, 7770. (4) (a) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969. (b) Alivisatos, A. P. Science 2000, 289, 5480. (5) (a) Banfield, J. F.; Welch, S. A.; Zhang, H. Z.; Ebert, T. T.; Penn, R. L. Science 2000, 289, 751. (b) Alexandrou, I.; Ang, D. K. H.; Mathur, N. D.; Haq, S.; Amaratunga, G. A. J. Nano Lett. 2004, 4, 2299. (c) Tang, Z. Y.; Wang, Y.; Shanbhag, S.; Giersig, M.; Kotov, N. A. J. Am. Chem. Soc. 2006, 128, 6730. (d) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2005, 127, 18262. (e) Zitoun, D.; Pinna, N.; Frolet, N.; Belin, C. J. Am. Chem. Soc. 2005, 127, 15034. (f) Tang, Z. Y.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237. (g) Niederberger, M.; Garnweitner, G.; Buha, J.; Polleux, J.; Ba, J. H.; Pinna, N. J. Sol.-Gel Sci. Technol. 2006, 40, 259. (6) Pradhan, N.; Xu, H. F.; Peng, X. G. Nano Lett. 2006, 6, 720. (7) Cho, K. S.; Talapin, D. V.; Gaschler, W.; Murray, C. B. J. Am. Chem. Soc. 2005, 127, 7140. (8) Pacholski, C.; Kornowski, A.; Weller, H. Angew. Chem., Int. Ed. 2002, 41, 1188. (9) (a) Masui, T.; Fujiwara, K.; Machida, K. I.; Adachi, G. Y. Chem. Mater. 1997, 9, 2197. (b) Shao, Z. P.; Haile, S. M.; Ahn, J. M.; Ronney, P. D.; Zhan, Z. L.; Barnett, S. A. Nature 2005, 435, 795. (c) Jasinski, P.; Suzuki, T.; Anderson, H. U. Sen. Actuators, B 2003, 95, 73. (d) Yamashita, M.; Yabe, K. S.; Yoshida, S.; Fujisiro, Y.; Kawai, T.; Sato, T. J. Mater. Sci. 2002, 37, 683. (e) Feng, X. D.; Sayle, D. C.; Wang, Z. L.; Paras, M. S.; Santora, B.; Sutorik, A. C.; Sayle, T. X. T.; Yang, Y.; Ding, Y.; Wang, X. D.; Her, Y. S. Science 2006, 312, 1504. (10) Zhou, K. B.; Wang, X.; Sun, X. M.; Peng, Q.; Li, Y. D. J. Catal. 2005, 229, 206. (11) (a) Zhang, F.; Chan, S. W.; Spanier, J. E.; Apak, E.; Jin, Q.; Robinson, R. D.; Herman, I. P. Appl. Phys. Lett. 2002, 80, 127. (b) Hsu, W. P.; Ronnquist, L.; Matijevic, E. Langmuir 1988, 4, 31. (c) Zhou, X. D.; Huebner, W.; Anderson, H. U. Appl. Phys. Lett. 2002, 80, 3814. (d) Deshpande, A. S.; Pinna, N.; Beato, P.; Antonietti, M.; Niederberger, M. Chem. Mater. 2004, 16, 2599. (12) Si, R.; Zhang, Y. W.; You, L. P.; Yan, C. Y. Angew. Chem., Int. Ed. 2005, 44, 3256. (13) (a) Han, W. Q.; Wu, L. J.; Zhu, Y. M. J. Am. Chem. Soc. 2005, 127, 12814. (b) Inoue, M.; Kimura, M.; Inui, T. Chem. Commun. 1999, 11, 957. (c) Tang, B.; Zhuo, L. H.; Ge, J. C.; Wang, G. L.; Shi, Z. Q.; Niu, J. Y. Chem. Commun. 2005, 28, 3565. (d) Tang, C. C.; Bando, Y.; Liu, B. D.; Golberg, D. AdV. Mater. 2005, 17, 3005. (14) (a) Kuiry, S. C.; Patil, S. D.; Deshpande, S.; Seal, S. J. Phys. Chem. B 2005, 109, 6936. (b) Zhang, J.; Ju, X.; Wu, Z. Y.; Liu, T.; Hu, T. D.; Xie, Y. N.; Zhang, Z. L. Chem. Mater. 2001, 13, 4192. (15) (a) Liao, X. H.; Zhu, J. M.; Zhu, J. J.; Xu, J. Z.; Chen, H. Y. Chem. Commun. 2001, 10, 937. (b) Wang, H.; Zhu, J. J.; Zhu, J. M.; Liao, X. H.; Xu, S.; Ding, T.; Chen, H. Y. Phys. Chem. Chem. Phys. 2002, 4, 3794. (16) Yang, S. W.; Gao, L. J. Am. Chem. Soc. 2006, 128, 9390. (17) Yu, T.; Joo, J.; Park, Y.; Hyeon, T. Angew. Chem., Int. Ed. 2005, 44, 7411. (18) Si, R.; Zhang, Y. W.; You, L. P.; Yan, C. H. J. Phys. Chem. B 2006, 110, 5994. (19) (a) Mai, H. X.; Sun, L. D.; Zhang, Y. W.; Si, R.; Feng, W.; Zhang, H. P.; Liu, H. C.; Yan, C. H. J. Phys. Chem. B 2005, 109, 24380. (b) Huang, P. X.; Wu, F.; Zhu, B. L.; Gao, X. P.; Zhu, H. Y.; Yan, T. Y.; Huang, W. P.; Wu, S. H.; Song, D. Y. J. Phys. Chem. B 2005, 109, 19169. (20) Wang, Z. L.; Feng, X. D. J. Phys. Chem. B 2003, 107, 13567. (21) (a) Baudin, M.; Wojcik, M.; Hermansson, K. Surf. Sci. 2000, 468, 51. (b) Sayle, D. C.; Maicaneanu, S. A.; Watson, G. W. J. Am. Chem. Soc. 2002, 124, 11429. (22) (a) Narayanaswamy, A.; Xu, H. F.; Pradhan, N.; Kim, M.; Peng, X. G. J. Am. Chem. Soc. 2006, 128, 10310. (b) Narayanaswamy, A.; Xu, H. F.; Pradhan, N.; Kim, M.; Peng, X. G. Angew. Chem., Int. Ed. 2006, 45, 5361. (23) Ribeiro, C.; Lee, J. H. E.; Giraldi, T. R.; Longo, E.; Varela, J. A.; Leite, E. R. J. Phys. Chem. B 2004, 108, 15612. (24) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2001, 123, 1389.
