Surface-binding-mediated growth of monodisperse cobalt-doped ceria nanocrystals

Abstract

In this study, we explore a new strategy to tailor the growth process of monodisperse nanocrystals. By means of X-ray diffraction, high-resolution transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray fluorescence spectrometry, and X-ray absorption spectroscopy, it is found that the growth of ceria followed Ostwald ripening, and polydispersed ceria nanocrystals were obtained in a hydrothermal condition. With the addition of cobalt to the growing ceria nanocrystals, cobalt acted as a surface dopant and preferred to bind on the surface of large-sized ceria nanocrystals via Co–O–Ce bonds. The binding could modulate the Ostwald ripening process and narrow the size distribution. With the competition between cobalt dopants and Ce monomers onto the surface of nanocrystals, monodisperse cobalt-doped nanocrystals were formed. This route is expected to be applicable to a variety of monodisperse doped nanocrystals.

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Introduction

For the last few decades, the synthesis of monodisperse nanocrystals (size variation < 5%) has been intensively pursued for their fundamental scientific interest and technological applications.¹ Several colloidal chemical methods have been developed to synthesize monodisperse nanocrystals of various materials. Exploring new strategies to delicately tailor the growth process of monodisperse nanocrystals is important to control the desired sizes of the nanocrystals and to manipulate their properties. However, it is still very challenging.²

As nanocrystals of a well-known metal oxide, ceria (CeO₂) nanocrystals have extensive applications in optics, catalysis and magnetics. Metal doping of CeO₂ nanocrystals is receiving significant attention, which offers a convenient and cost-effective solution for the modulation of nanocrystal properties.³ Remarkable changes in the optical properties of ceria nanocrystals are induced by lanthanide ion doping.⁴ Cobalt-doped ceria nanocrystals show significantly enhanced catalytic activity.⁵ Within Ni-doped ceria nanosystems, Ce–O–Ni interactions can induce the formation of oxygen vacancies and tune the catalytic performance.⁶ Room-temperature ferromagnetism has been discovered in ceria nanocrystals by nickel or cobalt doping.⁷ For future applications, the synthesis of monodisperse metal-doped nanocrystals is critical because their chemical and physical properties are strongly dependent on their dimensions. The fine-tuning of local structure around a cerium atom and the associated bonding behaviors are essential for optimizing the process of monodisperse nanocrystal growth.⁸ Cobalt-doped ceria nanocrystals is one exciting example of these materials. We demonstrated that the cobalt dopant modifies the local structure of a cerium atom and modulates the surface bonding behaviors of ceria nanocrystals. A new strategy for controlling the growth of monodisperse nanocrystals was proposed, which can promote the assembly of nanocrystals with a novel structure and function in nanotechnology.

Results and discussion

Crystal structure and morphology characterization

Crystal structures of the samples were characterized by XRD analysis. As shown in Fig. 1, all patterns are identified on the basis of the standard ceria database (Joint Committee for Power Diffraction Studies (JCPDS) file no. 75-0151). The synthesized samples maintain a long-range pure fluorite cubic structure. The crystallite size has been estimated by the broadening of the corresponding (111) and (200) diffraction peaks using the Scherrer formula. The calculated size is 5.5 nm, 3.6 nm, and 3.3 nm for CeCo0, CeCo6, and CeCo14, respectively. The error in the theoretical values is 0.1 nm.

Fig. 1 XRD spectra for the synthesized samples.

The morphology of the synthesized samples was analyzed by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). TEM images (Fig. 2(a)) of the undoped ceria nanocrystals show sphere-like morphology with an average diameter of 3.8 nm (see Fig. S2, ESI†). The single crystallinity of samples was confirmed by using HRTEM (Fig. 2(b)). The exposed faces are the (111) and (200) facets, with interplanar spacings of 0.31 and 0.27 nm, respectively.⁹ For the cobalt-doped ceria nanocrystals prepared via adding Co precursors in the hydrothermal process, the images in Fig. 2(b)–(f) show two obvious features induced by cobalt doping: (1) the mean size of Co-doped nanocrystals, consistent with the result of XRD analysis, is dramatically decreased, and (2) the relative standard deviation (the standard deviation divided by the mean value) of the size distribution is diminished. As shown in Fig. S2 (see Fig. S2, ESI†), the size distribution was narrowed by adding Co precursors to the reagent. Further, the self-assembly structure was present in the TEM images of Co-doped ceria nanocrystals.

Fig. 2 (a) TEM image and (b) HRTEM image of CeCo0. (c) TEM image and (d) HRTEM image of CeCo6. (e) TEM image and (f) HRTEM image of CeCo14.

FTIR study

To understand the effect of the cobalt dopant on the surface binding behaviors of nanocrystals, the synthesized nanocrystals and Co(OA)₂ were investigated by using the FTIR spectra. As shown in Fig. 3, two characteristic bands of pure ceria nanocrystals centered at 1538 and 1440 cm⁻¹ originate, respectively, from the antisymmetric and symmetric stretching vibrations of carboxylate anions bound to the ceria surface.¹⁰ One Ce atom associates with two oxygen atoms in the carboxylate group, like a chelating bidentate configuration.¹⁰ However, based on previous reports, the type of interaction between a carboxylate group and the Co atoms is similar to a bridging bidentate configuration.¹¹ Two oxygen atoms in the carboxylate are coordinated to the two Co atoms. The wavenumber separation between ν_as(COO⁻) and ν_s(COO⁻) bands in the bridging bidentate configuration (see the FTIR spectrum of Co(OA)₂) is larger than that in the chelating bidentate configuration. Here, with cobalt doping, the antisymmetric stretching vibration band was blue shifted to 1550 cm⁻¹, indicating that the band could arise from the overlap of two subbands corresponding to the carboxylate group bound to the Ce and Co atoms on the surface of the nanocrystals, as shown in Fig. 4. It was also revealed that the surface binding behaviors of ceria nanocrystals can be fine-tuned by Co doping.

Fig. 3 FTIR spectra of the synthesized samples and Co(OA)₂.

Fig. 4 Impact of cobalt doping on the surface bonding property of ceria nanocrystals.

XANES and EXAFS analysis

To further investigate the coordination environments of Co atoms in doped nanocrystals, we measured X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) data at the Co K edge in the synthesized samples.¹² As shown in Fig. 5(a), the XANES structure features in Co-doped ceria nanocrystals are distinctly different from the features of the standard CoO and Co₃O₄.¹³ Moreover, the IFEFFIT package¹⁴ was employed to analyze the EXAFS data with a theoretical model generated by FEFF 8.4.¹⁵ The EXAFS analysis (Fig. 6 and Table 1) shows that the first shell for CeCo6 and CeCo14 corresponded to Co–O bonding with 6.3 and 6.6 oxygens around Co at 2.03 Å and 2.04 Å, respectively. It is quite different from Ce–O bonding (2.33 Å) in CeO₂ and Co–O bonding (2.13 Å) in CoO, indicating that the cobalt ion neither substitutionally doped into the CeO₂ host (similar to Ce_1−xCo_xO₂ solid solution) nor did it form a mixture (similar to CeO₂/CoO). The best EXAFS fitting shows that the second peak can be attributed to the 1.9 cerium at 3.31 Å. A comparison of the above results with the results of Co(OA)₂ EXAFS fitting suggests that the doped nanocrystals contain Co–O–Ce bonding rather than adsorption of Co(OA)₂ on the surface of ceria nanocrystals.¹⁶

Fig. 5 (a) XANES spectra at the Co K edge of the reference compounds and synthesized samples. (b) Fourier transform of the corresponding EXAFS data.

Fig. 6 Fit of the EXAFS spectra at the Co K edge of (a) Co(OA)₂ and (b) the synthesized samples (solid: experimental data, dashed line: fit).

Table 1 Co structural parameters for Co(OA)₂ and the synthesized samples as obtained by EXAFS

	Shells	CN^a	R^b (Å)	σ^2 (×10^−3 Å^2)
a Errors were estimated to be 20% for the coordination numbers (CN).b Errors were estimated to be 0.03 Å for the bond distances (R).
Co(OA)2	Co–O	9.3	1.95	12.4
	Co–Co	1.9	3.03	12.7
	Co–Co	1.7	3.34	12.7
CeCo6	Co–O	6.3	2.03	6.7
	Co–Ce	1.9	3.31	14.5
CeCo14	Co–O	6.6	2.04	7.9
	Co–Ce	1.7	3.31	15.3

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To understand the cerium configuration in the synthesized samples, Ce L₃ edge XAS measurements were also performed. The XANES (Fig. 7(a)) and EXAFS (Fig. 7(b)) spectra show that the line shape of cerium in the synthesized samples is quite similar to that of the standard CeO₂. The best EXAFS fitting (Fig. 8 and Table 2) shows that the first peak of all the samples is obtained in the case of 6.0 oxygen atoms and an average bond length of about 2.33 Å.¹⁷

Fig. 7 (a) XANES spectra at the Ce L₃ edge of the reference compounds and synthesized samples. (b) Fourier transform of the corresponding EXAFS data.

Fig. 8 Fit of the EXAFS spectra at the Ce L₃ edge of the synthesized samples and of the reference compounds (solid: experimental data, dashed line: fit).

Table 2 Ce structural parameters for the synthesized samples as obtained by EXAFS

	Shells	CN^a	R^b (Å)	σ^2 (×10^−3 Å^2)
a Errors were estimated to be 20% for the coordination numbers (CN).b Errors were estimated to be 0.02 Å for the bond distances (R).c The content of Ce or Co in components of the samples of CeCo6 and CeCo14.
CeCo0	Ce–O1	6.3	2.33	6.7
	Ce–Ce	8.7	3.82	6.5
	Ce–O2	9.4	4.49	4.3
CeCo6	Ce–O1	5.9	2.32	7.2
	Ce–Ce (93.7%)^c	7.2	3.83	5.1
	Ce–Co (6.3%)^c	1.9	3.31	14.5
	Ce–O2	5.9	4.46	6.8
CeCo14	Ce–O1	6.2	2.33	8.3
	Ce–Ce (85.7%)^c	7.4	3.83	5.3
	Ce–Co (14.3%)^c	1.7	3.31	15.3
	Ce–O2	5.4	4.46	7.0
CeCo0	Ce–O1	6.3	2.33	6.7
	Ce–Ce	8.7	3.82	6.5
	Ce–O2	9.4	4.49	4.3

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Although doping did not significantly affect the local structure of the first shell, the higher coordination shell was tuned by the cobalt dopant.¹⁸ The Ce-metal shell has two subshells, one with 7.2 Ce at 3.83 Å and the other with 1.9 Co at 3.31 Å, which is consistent with the analysis results of the Co K edge EXAFS data. The interior structural configuration of the doped ceria nanocrystals was not affected by the dopant; this suggests that the ceria nanocrystals were preferably formed in the hydrothermal process. Cobalt was doped only at the surface producing a ceria nanocrystal core that was stabilized by the Ce–O–Co–(oleate)_x complex (similar to surfactants). These results also corresponded to the results of the XRD and FTIR analyses. Consistent with the results of the XAFS and FTIR analyses, the energy dispersive X-ray spectroscopy (EDS) analysis (see Fig. S1, ESI†) confirmed the uniform distribution of cerium and cobalt in the analysed sample.

Based on the abovementioned and previous works,^1a,2,19 the formation of oleate-stabilized ceria nuclei upon hydrolytic sol–gel condensation of the Ce–oleate complex with a base at ambient temperature, and growth by the dissolution/reprecipitation process yields polydisperse ceria nanocrystals.²⁰ When the Co–oleate complex and the Ce–oleate complex coexist in the hydrothermal process, as shown in Fig. 9, the Ce intermediate species are first generated from the hydrolysis of the Ce–oleate complex. These intermediate species act as monomers for cerium oxide nanocrystals. As the Ce monomer concentration increases to critical supersaturation, stable ceria nuclei form and accumulate. Since the rate of Ce monomer consumption resulting from the nucleation and growth processes exceeds the rate of the Ce monomer supply, the Ce monomer concentration decreases until it reaches the critical level at which nucleation is effectively stopped and the particles keep growing. In a process known as Ostwald ripening (see Fig. S3, ESI†), smaller particles dissolve and larger particles grow by receiving the monomers from the dissolving particles. Meanwhile, cobalt dopants are generated from the Co–oleate complex. The cobalt dopants prefer to bind on the surface of large ceria nanocrystals via the Co–O–Ce bond. This binding could terminate Ostwald ripening and tune the growth of polydispersed nanocrystals. In this situation, small nanocrystals grow faster than larger ones, and as a result, the size distribution becomes nearly monodisperse.²¹

Fig. 9 Scheme of the synthesis of monodisperse cobalt-doped ceria.

Conclusions

In this paper, we explore a new strategy to tailor the growth process of monodisperse nanocrystals. By means of various analysis methods, it is found that the growth of ceria followed Ostwald ripening and polydispersed ceria nanocrystals were obtained under a hydrothermal condition. With the addition of cobalt to the growing ceria nanocrystals, cobalt acted as a surface dopant and preferred to bind on the surface of large ceria nanocrystals via Co–O–Ce bonds. This binding could terminate the Ostwald ripening process and narrow the size distribution. With the competition between cobalt dopants and Ce monomers on the surface of the nanocrystals, monodisperse cobalt-doped nanocrystals were formed. This route is expected to be applicable to a variety of monodisperse doped nanocrystals. It will trigger new applications in various fields including information technology, biomedicine, and environmental technology.

Experimental section

Synthesis of samples

15 mL of 50 mmol L⁻¹ cerium(III) nitrate aqueous solution was mixed with 1.5 mL ethanol and 15 mL toluene. 15 mL of 150 mmol L⁻¹ sodium oleate aqueous solution was dropped into the above solution under magnetic stirring. The upper layer of the toluene phase with a cerium oleate precursor was transferred to a 50 mL Teflon-lined stainless-steel autoclave containing 15 mL deionized water and 0.15 mL tert-butylamine. A cobalt oleate precursor was added to the initiator for the Co-doped sample. The Co/(Co + Ce) atomic ratio in the reagent was 0%, 10% or 20. The sealed autoclave was transferred to a 180 °C oven, held there for 12 hours and then cooled to room temperature under natural conditions. The upper brown supernatant was precipitated with an adequate volume of ethanol. The samples were centrifuged at 8000 rpm for 10 min. The obtained nanocrystals could be easily re-dispersed in the non-polar solvent cyclohexane. The product was then repeatedly washed with cyclohexane and ethanol 4 times. Finally, the product was dispersed in cyclohexane. For convenience, as the Co/(Co + Ce) atom ratio in the synthesized samples was 0%, 6.3% and 14.3% (see Fig. S1, ESI†), the corresponding samples are denoted by CeCo0, CeCo6 and CeCo14, respectively.¹⁰ The cobalt dopant percentage in the ceria nanocrystals was measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES). The ICP-AES result was consistent with the XRF result (see Fig. S1, ESI†).

Characterization

X-ray fluorescence (XRF) analysis was conducted with an Eagle III μProbe operating at 40 kV and 250 μA. XRD patterns of the samples were recorded with a Bruker D8 ADVANCE X-ray diffractometer using filtered Cu Kα radiation at 40 kV and 40 mA. HRTEM images and EDS spectra were obtained using a JEOL JEM-2100 transmission electron microscope operating at 200 kV. Fourier transform infrared (FTIR) spectra were recorded with a Spectrum GX. ICP-AES spectra were recorded with SPS5100. XAFS spectra at the Ce L₃ edge were measured in the transmission mode at the X-ray absorption station (beam line 1W1B and 1W2B) of the Beijing Synchrotron Radiation Facility (BSRF) using a double crystal Si (111) monochromator. XAFS spectra at the Co K edge were measured in the fluorescence mode by using a Lytle detector and Fe filter. EXAFS functions were Fourier transformed to the R space with a k² weight in the range of 3.2–10.5 Å⁻¹ and 2.5–9.6 Å⁻¹ for Co and Ce, respectively. Back Fourier transformations were performed in the range of 1.0–3.5 Å and 1.3–3.5 Å with a Hanning window for Co and Ce, respectively. The amplitude factors S₀² were obtained using the best fit values found in the standard compounds.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (10734070, 11375229, 10979054, 11135008 and 11005145), the Science Fund for Creative Research Groups of the NSFC (11321503), and the Knowledge Innovation Program of the Chinese Academy of Sciences (KJCX2-YW-N42). We gratefully acknowledge the assistance in FTIR measurements from Dr Takeyuki Suzuki. We are grateful to the referees for their valuable and very helpful suggestions.

Notes and references

1. (a) J. Park, J. Joo, S. G. Kwon, Y. Jang and T. Hyeon, Angew. Chem., Int. Ed., 2007, 46, 4630–4660; (b) J. Park, K. An, Y. Hwang, J.-G. Park, H.-J. Noh, J.-Y. Kim, J.-H. Park, N.-M. Hwang and T. Hyeon, Nat. Mater., 2004, 3, 891–895; (c) S. Xu, Z. Nie, M. Seo, P. Lewis, E. Kumacheva, H. A. Stone, P. Garstecki, D. B. Weibel, I. Gitlin and G. M. Whitesides, Angew. Chem., 2005, 117, 3865; (d) M. A. El-Sayed, Acc. Chem. Res., 2004, 37, 326–333.
2. S. G. Kwon and T. Hyeon, Acc. Chem. Res., 2008, 41, 1696–1709.
3. (a) G. Zhou, L. Barrio, S. Agnoli, S. D. Senanayake, J. Evans, A. Kubacka, M. Estrella, J. C. Hanson, A. Martínez-Arias, M. Fernández-García and J. A. Rodriguez, Angew. Chem., 2010, 122, 9874–9878; (b) X. Wang, J. A. Rodriguez, J. C. Hanson, D. Gamarra, A. Martínez-Arias and M. Fernández-García, J. Phys. Chem. B, 2005, 109, 19595–19603; (c) A. B. Kehoe, D. O. Scanlon and G. W. Watson, Chem. Mater., 2011, 23, 4464–4468; (d) P. Singh and M. S. Hegde, J. Solid State Chem., 2008, 181, 3248–3256.
4. Z. Wang, Z. Quan and J. Lin, Inorg. Chem., 2007, 46, 5237–5242.
5. X.-H. Guo, C.-C. Mao, J. Zhang, J. Huang, W.-N. Wang, Y.-H. Deng, Y.-Y. Wang, Y. Cao, W.-X. Huang and S.-H. Yu, Small, 2012, 8, 1515–1520.
6. C. Pirez, M. Capron, H. Jobic, F. Dumeignil and L. Jalowiecki-Duhamel, Angew. Chem., Int. Ed., 2011, 50, 10193–10197.
7. (a) A. Thurber, K. M. Reddy, V. Shutthanandan, M. H. Engelhard, C. Wang, J. Hays and A. Punnoose, Phys. Rev. B: Condens. Matter Mater. Phys., 2007, 76, 165206; (b) A. Bouaine, R. J. Green, S. Colis, P. Bazylewski, G. S. Chang, A. Moewes, E. Z. Kurmaev and A. Dinia, J. Phys. Chem. C, 2010, 115, 1556–1560.
8. (a) T. Yao, Z. Sun, Y. Li, Z. Pan, H. Wei, Y. Xie, M. Nomura, Y. Niwa, W. Yan, Z. Wu, Y. Jiang, Q. Liu and S. Wei, J. Am. Chem. Soc., 2010, 132, 7696–7701; (b) T. Yao, S. Liu, Z. Sun, Y. Li, S. He, H. Cheng, Y. Xie, Q. Liu, Y. Jiang, Z. Wu, Z. Pan, W. Yan and S. Wei, J. Am. Chem. Soc., 2012, 134, 9410–9416.
9. Z. L. Wang, J. Phys. Chem. B, 2000, 104, 1153–1175.
10. N. Qiu, J. Zhang, Z. Wu, T. Hu and P. Liu, Cryst. Growth Des., 2012, 12, 629–634.
11. (a) D. Pan, G. Xu, J. Wan, Z. Shi, M. Han and G. Wang, Langmuir, 2006, 22, 5537–5540; (b) N. Wu, L. Fu, M. Su, M. Aslam, K. C. Wong and V. P. Dravid, Nano Lett., 2004, 4, 383–386; (c) K. Fa, T. Jiang, J. Nalaskowski and J. D. Miller, Langmuir, 2004, 20, 5311–5321; (d) N. Bao, L. Shen, W. An, P. Padhan, C. Heath Turner and A. Gupta, Chem. Mater., 2009, 21, 3458–3468.
12. (a) P. A. Lee and J. B. Pendry, Phys. Rev. B: Solid State, 1975, 11, 2795–2811; (b) P. A. Lee, P. H. Citrin, P. Eisenberger and B. M. Kincaid, Rev. Mod. Phys., 1981, 53, 769–806.
13. T. Liu, H. Xu, W. S. Chin, P. Yang, Z. Yong and A. T. S. Wee, J. Phys. Chem. C, 2008, 112, 13410–13418.
14. (a) M. Newville, J. Synchrotron Radiat., 2001, 8, 322–324; (b) S. M. Webb, Phys. Scr., 2005, 2005, 1011.
15. (a) A. L. Ankudinov, B. Ravel, J. J. Rehr and S. D. Conradson, Phys. Rev. B: Condens. Matter Mater. Phys., 1998, 58, 7565; (b) A. L. Ankudinov and J. J. Rehr, Phys. Rev. B: Condens. Matter Mater. Phys., 2000, 62, 2437.
16. Y. Bao, W. An, C. H. Turner and K. M. Krishnan, Langmuir, 2009, 26, 478–483.
17. N. J. Lawrence, J. R. Brewer, L. Wang, T. Wu, J. WellsKingsbury, M. M. Ihrig, G. Wang, Y. Soo, W. Mei and C. L. Cheung, Nano Lett., 2011, 11, 2666–2671.
18. Z. Wu, J. Zhang, R. E. Benfield, Y. Ding, D. Grandjean, Z. Zhang and X. Ju, J. Phys. Chem. B, 2002, 106, 4569–4577.
19. (a) S. G. Kwon, Y. Piao, J. Park, S. Angappane, Y. Jo, N.-M. Hwang, J.-G. Park and T. Hyeon, J. Am. Chem. Soc., 2007, 129, 12571–12584; (b) L. Zu, D. J. Norris, T. A. Kennedy, S. C. Erwin and A. L. Efros, Nano Lett., 2005, 6, 334–340.
20. T. Taniguchi, T. Watanabe, N. Sakamoto, N. Matsushita and M. Yoshimura, Cryst. Growth Des., 2008, 8, 3725–3730.
21. Y. Yin and A. P. Alivisatos, Nature, 2005, 437, 664–670.

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Footnote

† Electronic supplementary information (ESI) available: XRF, EDS, TEM analysis and particle size distribution of synthesized samples. See DOI: 10.1039/c3ra47661h

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