Tuning Ceria Nanocrystals Morphology and Structure by Copper Doping

Article pubs.acs.org/crystal

Tuning Ceria Nanocrystals Morphology and Structure by Copper Doping Nan Qiu,† Jing Zhang,*,† Ziyu Wu,*,‡,† Tiandou Hu,† and Peng Liu† †

Beijing Synchrotron Radiation Laboratory, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, 100049, P. R. China ‡ National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026, P. R. China S Supporting Information *

ABSTRACT: Ceria (CeO2) nanocrystals have unique and highly attractive properties that depend on the morphology and structure. In this paper, we demonstrate that by controlling the addition of copper to growing ceria nanocrystals it is possible to modulate the resulting structure properties. We show also by means of X-ray diffraction, high-resolution transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray fluorescence spectrometry, and X-ray absorption spectroscopy that copper configuration affects both the morphology and the electronic structure of cerium oxide nanocrystals. Indeed, external and internal copper doping on ceria nanocrystals leads to structural transitions, including a morphological transition from cubic to truncated octahedron, along with cerium valence changes. The results of this study, optimizing the structural properties of such nanocrystals, may certainly trigger new opportunities and, ultimately, new applications. effect of the location, or environment, of the dopants16 on the resulting structure in terms of uniform size at sub 10 nm, welldefined crystal shape, valence, and of course oxygen vacancies. Here, we present a simple method to synthesize ceria nanocrystals of 3−5 nm, with a specific tailor-made structure via controlled copper doping in a hydrothermal process. The environment of copper, for example, “internal” and “external” doping on ceria nanocrystals, could modulate the resulting properties including the valence state from tetra- to trivalent and morphological changes from cubic to truncated octahedron. These findings have implications for our understanding of the nonhomogeneous doping engineered nanocrystal morphology and structural transitions.

1. INTRODUCTION Tailoring the morphology and structure of nanocrystals is interesting because it allows modification of surface area and physical properties.1−3 As a well-known metal oxide, ceria (CeO2) nanocrystals have extensively applications in catalysis, electrochemistry, and optics.4−7 For example, to improve the ceria catalytic activity, it is desirable to decrease the size of ceria nanocrystals by increasing the ratio of surface area to volume. Along with this research, experimental and theoretical studies have shown that nanocube ceria with exposed {100} crystal planes exhibits the highest reactivity among nanosphere-like ceria with exposed {110} and {111} planes.8,9 In addition, oxygen vacancies play an important role in catalysis. In a carbon monoxide oxidation reaction, high concentration of oxygen vacancies make ceria nanorods exposed with the less active surface (111) much more reducible than one with a theoretically higher reactive surface such as the (110).10 Metal doping on ceria nanocrystals is known as a unique approach to alter the nanocrystal structure and lead to their novel properties. Gd3+-doped CeO2 nanocrystals can decrease crystallite size and change nanocrystal morphology.11 Introducing Cu dopants in ceria nanocrystals demonstrates interesting physical and chemical properties and enhances the catalytic activity.12,13 Within Ni doping ceria nanosystems, Ce−O−Ni interactions can induce the formation of oxygen vacancies and tune the catalytic performance.14,15 For practical and academic reasons, it is very important to gain a better understanding of doping induced nanocrystal morphology and structure evolution. However, there has been limited research into the © 2011 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Synthesis of Samples. The ceria nanocrystal was synthesized by a convenient surfactant-assisted hydrothermal approach. All chemical reagents are commercial analytical reagents used without further purification. Fifteen milliliters of 116.7 mmol/L cerium(III) nitrate aqueous solution was mixed with 15 mL of toluene. 22.5 mL of 233.3 mmol/L sodium oleate aqueous solution was dropped into the above mixture solution with magnetic stirring. The upper layer of the toluene phase with cerium oleate precursor was transferred to a 50 mL Teflon-lined stainless-steel autoclave with 15 mL of deionized water and 0.35 mL of t-butylamine. The sealed autoclave was transferred to a 180 °C oven, held there for 12 h, and then cooled to room Received: June 15, 2011 Revised: October 21, 2011 Published: December 12, 2011 629

dx.doi.org/10.1021/cg200757j | Cryst. Growth Des. 2012, 12, 629−634

Crystal Growth & Design

Article

temperature under natural conditions. The upper brown supernatant was precipitated with an adequate volume of ethanol. The obtained nanocrystals could be easily redispersed in the nonpolar solvent cyclohexane. As for the copper-doped ceria nanocrystals, the procedure was similar, but a different amount of 116.7 mmol/L copper(II) nitrate aqueous solution was added to the cerium nitrate aqueous solution, and the volume of mixed aqueous solution was maintained at 15 mL. The Cu/ (Cu + Ce) atomic ratio in the reagent is 0%, 10% and 60%, respectively. A total of 22.5 mL of a 233.3 mmol/L sodium oleate aqueous solution was dropped into the above mixture solution with magnetic stirring. The upper layer of the toluene phase with cerium− copper oleate precursor was transferred to a 50 mL Teflon-lined stainless-steel autoclave with 15 mL of deionized water and 0.35 mL of t-butylamine. The sealed autoclave was transferred to a 180 °C oven, held there for 12 h, and then cooled to room temperature. The upper supernatant was precipitated with an adequate volume of ethanol. The obtained copper-doped ceria nanocrystals could be also easily redispersed in the nonpolar solvent cyclohexane. The copper precursor content changed as 0%, 10% and 60%; however, the Cu and Ce precursors have different solubilities in hydrothermal conditions. The proportion of copper in the synthesized samples is 0%, 5% and 6%, respectively (see Supporting Information). For convenience, the corresponding samples are denoted as CeCu0, CeCu5, and CeCu6. 2.2. Characterization. To determine the molar ratios of Ce and Cu in the synthesized samples, X-ray fluorescence (XRF) analysis was performed with an Eagle III μProbe operating at 40 kV and 250 μA. Xray diffraction (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. High-resolution transmission electron microscopy (HRTEM) images were obtained using a JEOL JEM-2100 transmission electron microscope operating at 120 kV. X-ray absorption spectroscopy (XAS) spectra at the Ce L3 edge were measured in transmission mode at the X-ray absorption station (beamline 1W2B) of the Beijing Synchrotron Radiation Facility (BSRF) using a double crystal Si (111) monochromator. The storage ring was working at the energy of 2.5 GeV with an average electron current of 150 mA. The incident and output beam intensities were monitored and recorded using ionization chambers filled by pure nitrogen. To suppress the higher harmonics content in the low energy range, double crystals were detuned to 30% of the incident beam intensity. The Cu K edge XAS spectra were recorded in fluorescence mode by using a Lytle detector and Ni filter. Samples for XAS experiments were prepared using a cell sealed by Kapton tapes. Fourier transform infrared spectroscopy (FTIR) was recorded with a Spectrum GX.

Figure 1. XRD results of the synthesized samples. The center of the (111) peak is at 28.68°, 28.62°, and 28.55°, for CeCu0, CeCu5, and CeCu6, respectively.

planes of cubic ceria, which implies the synthesis of ceria nanocubes with (001) surfaces. For the copper-doping ceria nanocrystals (CeCu5) prepared via adding 10% Cu/(Cu + Ce) precursors in the hydrothermal process, the images in Figure 2b,e show that the dominant morphology is still cubic with the exposed (100) planes. Further, we change the growth environment by increasing the Cu/(Cu + Ce) atomic ratio up to 60% in the initial reagents. It can be found that the decreased nanocrystal size with polyhedron shape occurred in CeCu6. The HRTEM image in Figure 2f displays a cross-lattice pattern with an interplanar spacing of 0.31 and 0.27 nm, indicating the nanocrystals with exposed (100) and (111) faces. For the face center cubic ceria crystal, as illustrated by Wang,18 the shape of nanocrystals was mainly determined by the ratio (R) between the growth rates along ⟨100⟩ and ⟨111⟩ directions. In CeCu0, The organic ligand molecules are likely to interact preferentially with the (001) surface: this greatly reduces the growth rate of the crystals in the ⟨001⟩ direction, while crystal growth in the ⟨111⟩ direction becomes comparatively predominant. This change leads to the formation of nanocubes with exposed (001) surfaces. The growth process of ceria nanocrystals was not much affected by adding 10% Cu/(Cu + Ce) precursor in the hydrothermal process. However, with the copper precursor up to 60%, the growth rate of the crystals in the ⟨001⟩ and ⟨111⟩ directions can be modified and both two directions can be suppressed.11 The CeCu6 nanocrystals were exposed (100) and (111) faces, corresponding to a morphological transition from cubic to truncated octahedron.18 3.2. XANES and EXAFS Analysis. To understand the local structure and electronic structure in ceria nanocrystals with copper doping, we investigated X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) data in the synthesized samples. XANES reveals structural information and electronic information about the local and partially empty states above the Fermi level.19,20 The valence state of cerium in the synthesized samples was evaluated by Ce L3 edge XANES. Figure 3 shows XANES spectra at the Ce L3 edge for the synthesized samples and two reference compounds. The assignments of Ce L3-edge spectra were based on refs 21−24. For the Ce L3-edge spectrum of standard cerium oleate (Ce(OA)3), the strongest peak A at

3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Morphology Characterization. Crystal structures of the samples were characterized by the XRD analysis. All patterns are identified on the basis of standard ceria database (Joint Committee for Power Diffraction Studies (JCPDS) file No. 34-0394). As shown in Figure 1, the synthesized samples maintain a long-range pure fluorite cubic structure. The crystallite size has been estimated by the broadening of the corresponding (111) diffraction peaks using the Scherrer formula. This estimated value is 4.4 ± 0.1 nm, 4.2 ± 0.1 nm, and 3.3 ± 0.1 nm for CeCu0, CeCu5, and CeCu6, respectively. The fits show a slight peak shift toward lower angles, indicating lattice expansion in the ceria nanocrystals by copper doping.17 The morphology of the synthesized samples was analyzed by TEM and HRTEM. Figure 2a shows that the majority of ceria nanocrystals without doping have a cubic-like shape. Figure 2d shows a lattice spacing of 0.27 nm, corresponding to the interplanar separation between the {002} or {020} lattice 630



Article

Figure 2. HRTEM images of the synthesized samples. Nondoped CeCu0 (a, d), Cu-doped CeCu5 (b, e), and Cu-doped CeCu6 (c, f).

which the intensity of peak A is enhanced and the peak D vanishes. As noted before, peak A probes the Ce 3+ configuration and its intensity is proportional to the content of trivalent cerium present in the CeCu5 sample. To estimate the amount of the trivalent configuration in these samples, XANES spectra have been fit by a combination of Lorentzians and an arctangent function. As shown in Figure 4, the fitting areas (intensity) of the peaks are, respectively, denoted as IA, IB, and IC. The amount of the trivalent cerium R is defined as R = IA/( IA + IB + IC). The analysis shows that the trivalent configuration is 17.8% ± 0.3% and 21.5% ± 0.4% for the CeCu5 and CeCu6, respectively. Compared to the values of bulk CeO2 (10.7% ± 0.4%) and CeCu0 (10.8% ± 0.3%), the results demonstrate that the cerium valence in the nanocrystals can be modified by copper doping. Moreover, the IFEFFIT25,26 package was employed to analyze EXAFS data with a theoretical model generated by FEFF 8.4.27,28 EXAFS functions were Fourier transformed to R space with k2-weight in the range 2.2−10.0 Å−1. Back Fourier

about 5725.5 eV, due to the dipole-allowed transition of Ce 2p to Ce 4f15d final states, characterizes the Ce in the trivalent state.24 However, the standard CeO2 XANES spectrum from 5710 to 5750 eV contains four peaks (D, A, B, C). The preedge structure, labeled D, is assigned to final states with delocalized d character at the bottom of the conduction band. Because of the cubic crystal-field splitting of Ce 5d states, features A and B are associated with the transitions of Ce 2p to the Ce 4f15d eg L and Ce 4f15d t2g L states, where L denotes an oxygen ligand 2p hole and 4f1 refers to an electron going from an oxygen 2p orbital to a cerium 4f one (charge-transfer-like). The energy separation between peak A and B is about 3.6 eV, which is in agreement with previous works.22 The feature C is attributed to the contribution of a different final state configuration 4f05d. Generally, the Ce spectrum has a distinct double-peaked structure (B, C) with the higher lying peak corresponding to the Ce4+ valence state.24 Spectrum of nondoped synthesized CeCu0 shows the same characteristic features of bulk ceria. However, a clear difference arises in the spectrum of CeCu5, in 631



Article

Figure 3. XANES spectra at the Ce L3 edge of the synthesized samples and two reference compounds.

Figure 5. Fit of the EXAFS spectra at the Ce L3 edge of the synthesized samples and of the reference compounds (solid line: experimental data, dashed line: fit).

Table 1. Ce Structural Parameters for the Synthesized Samples and the Reference Compounds As Obtained by EXAFSa CeCu6 CeCu5 CeCu0 Ce(OA)3 CeO2 bulk

shells

CN

R (Å)

σ2 (10−3 Å2)

Ce−O1 Ce−O2 Ce−O1 Ce−O2 Ce−O1 Ce−O2 Ce−O Ce−O

7.2 2.2 7.1 1.8 5.4 3.4 13.5 8.0

2.34 2.74 2.30 2.78 2.30 2.31 2.55 2.31

10.0 7.5 11.3 28.7 8 8 15.2 4

a

Errors were estimated to be 20% for the coordination numbers (CN) and 0.03 Å for the bond distances (R).

Figure 4. Fittings of the experimental XANES spectra of the Ce L3 edge of the synthesized samples and reference compounds.

window. We have six independent parameters in the fitting processes. From the EXAFS analysis (see Figure 7 and Table 2), the copper in CeCu5 is surrounded by 5.6 oxygen atoms with an average bond length of about 1.94 Å. Furthermore, FTIR could explore the interaction between organic ligand and ceria nanocrystals. FTIR (see Supporting Information) showed two characteristic bands at 1545 and 1443 cm−1 assigned to the asymmetric and symmetric stretching modes of the carboxylate group, respectively.31,32 These IR bands indicated that carboxylate groups from organic ligands were bonded to the surface of ceria nanocrystals.31 The FTIR spectrum of the CeCu5 showed that the intensity of the peak of the asymmetric stretching was clearly enhanced. These data indicate that copper can act as external dopant and modulate the interface interaction between organic ligands and ceria nanocrystals, further tuning the valence and local structure of cerium in the nanocrystals. However, the XANES spectrum of CeCu6 showed a distinct feature, the absorption energy shifted toward low energy by 2.4 eV, suggesting the Cu valence was lower than those of the cations in CuO. EXAFS analysis also showed that the first shell of CeCu6 had two Cu−O subshells, one with 4.4 oxygen at 1.93 Å and the other with 0.9 oxygen at 2.47 Å, which mainly consisted of the results of the first principles density functional calculations for bulk Ce0.75Cu0.25O2 and Ce0.875Cu0.125O2.12This scenario indicated that some copper atoms could enter into the

transformations were performed in the range 1.2−2.5 Å with a Hanning window. We have six independent parameters in the fitting processes. The EXAFS analysis (Figure 5 and Table 1) shows that a single shell mode can be chosen to fit the Ce−O bond in the case of the bulk ceria. On the contrary, the double shells mode is suitable to describe copper-doped samples.29 Two Ce−O bonds are present in these samples with one bond length around 2.30 Å, the same as the standard Ce−O bond in the ceria characterized by the fluorite cubic structure, and the other Ce−O bond around 2.78 Å, which could result from the organic molecular binding on the surface of ceria nanocrystals.30 It also suggests that copper doping modulates the nanoceria structure. In the inner regions, nanoceria preserves the fluorite cubic structure, while in the outer regions the coordination of surface ions changes with the proportion of the cerium trivalent configuration increases.30 To understand the copper configuration in the synthesized samples, Cu K edge XAS measurements were also performed. XANES (Figure 6) spectra show that the line shape of copper in CeCu5 is quite similar to that of the Cu-precursor. EXAFS functions were Fourier transformed to the R space with k2weight in the range 2.2−12.7 Å−1. Back Fourier transformations were performed in the range 1.2−2.2 Å with a Hanning 632



Article

Figure 6. Cu K edge spectra for the Cu-oleate precursor, synthesized CeCu5, CeCu6, and Cu2O and CuO (left). Compared to the CeCu6, the spectra of CeCu5 show similar features with respect to those of the Cu-precursor (right).

Figure 8. Concept of tuning ceria nanocrystals morphology and structure by copper doping.

stabilized via binding of the organic molecular.30 Both of them could determine the copper configuration, and affect the valence of cerium. Indeed, the obtained CeCu6 ceria nanocrystals with internal copper doping have a higher O deficiency. In ongoing work of our research group, doping effects of ceria nanocrystal on the valence and morphology, thermal stability, and the catalysis properties are investigated in detail. These results will be presented in the next paper.

Figure 7. Fit of the EXAFS spectra at the Cu K edge of the synthesized samples and of the reference compound (solid line: experimental data; dashed line: fit).

Table 2. Cu Structural Parameters for the Synthesized Samples and the Bulk Cu2O as Obtained by EXAFSa bulk Cu2O CeCu5 CeCu6

shells

CN

R (Å)

σ2 (10−3 Å2)

Cu−O Cu−O Cu−O1 Cu−O2

2.0 5.6 4.4 0.9

1.87 1.94 1.93 2.47

1 3 5 1

4. CONCLUSIONS In summary, through accurate synthesis we investigated in a controlled way the effect of copper doping in ceria nanocrystals, especially the dependence on the doping environment. With spectroscopic techniques we are able to investigate the local geometry and identify the copper site in ceria nanocrystals, allowing us to distinguish between “internal” and “external” doping. We also find that the geometry of copper doping affects the morphology and the structure of ceria nanocrystals. The results of this study, optimizing the structural properties of such nanocrystals, may certainly trigger new opportunities and, ultimately, new applications.

a

Errors were estimated to be 20% for the coordination numbers (CN) and 0.02 Å for the bond distances (R).

ceria nanocrystal. However, the low coordination number at 2.47 Å suggested the distortion in CeCu6 (Ce1−xCuxO2−y solid solution-like), compared with that in theoretical Ce0.75Cu0.25O2 and Ce0.875Cu0.125O2. The results of copper-doped ceria nanocrystals are far from those in ref 12, which could originate from the different synthesis conditions. Here, as the ratio of Cu/(Cu + Ce) changed from 10 to 60% in the hydrothermal process, the large change in the amount of initial reagent could provide a different reaction environment. Copper can act as internal dopant and embed in ceria nanocrystals, which can induce a very distorted local structure of nanoceria with defects and oxygen vacancies, On the other hand, an oxygen vacancy near the surface can be

■

ASSOCIATED CONTENT

S Supporting Information *

XRF and FTIR spectra of the samples. This material is available free of charge via the Internet at http://pubs.acs.org. 633



■

Article

(31) Zhang, J.; Ohara, S.; Umetsu, M.; Naka, T.; Hatakeyama, Y.; Adschiri, T. Adv. Mater. 2007, 19, 203. (32) Bolis, V.; Magnacca, G.; Cerrato, G.; Morterra, C. Thermochim. Acta 2001, 379, 147.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.Z.); [email protected] (Z.W.).

■

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (NSFC) (Grant No. 10979054).

■

REFERENCES

(1) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (2) Park, J.; An, K.; Hwang, Y.; Park, J.-G.; Noh, H.-J.; Kim, J.-Y.; Park, J.-H.; Hwang, N.-M.; Hyeon, T. Nat. Mater. 2004, 3, 891. (3) Guo, L.; Ji, Y. L.; Xu, H.; Simon, P.; Wu, Z. J. Am. Chem. Soc. 2002, 124, 14864. (4) Campbell, C. T.; Peden, C. H. F. Science 2005, 309, 713. (5) Banerjee, S.; Devi, P. S.; Topwal, D.; Mandal, S.; Menon, K. Adv. Funct. Mater. 2007, 17, 2847. (6) Yu, T.; Lim, B.; Xia, Y. Angew. Chem., Int. Ed. 2010, 49, 4484. (7) Silva, G. A. Nat. Nanotechnol. 2006, 1, 92. (8) Zhang, J.; Kumagai, H.; Yamamura, K.; Ohara, S.; Takami, S.; Morikawa, A.; Shinjoh, H.; Kaneko, K.; Adschiri, T.; Suda, A. Nano Lett. 2011, 11, 361. (9) Skorodumova, N. V.; Baudin, M.; Hermansson, K. Phys. Rev. B 2004, 69, 075401. (10) Liu, X.; Zhou, K.; Wang, L.; Wang, B.; Li, Y. J. Am. Chem. Soc. 2009, 131, 3140. (11) Taniguchi, T.; Watanabe, T.; Sakamoto, N.; Matsushita, N.; Yoshimura, M. Cryst. Growth Des. 2008, 8, 3725. (12) Wang, X.; Rodriguez, J. A.; Hanson, J. C.; Gamarra, D.; Martínez-Arias, A.; Fernández-García, M. J. Phys. Chem. B 2005, 109, 19595. (13) Wang, X.; Rodriguez, J. A.; Hanson, J. C.; Gamarra, D.; Martínez-Arias, A.; Fernández-García, M. J. Phys. Chem. B 2005, 110, 428. (14) Barrio, L.; Kubacka, A.; Zhou, G.; Estrella, M.; Martínez-Arias, A.; Hanson, J. C.; Fernández-García, M.; Rodriguez, J. A. J. Phys. Chem. C 2010, 114, 12689. (15) Zhou, G.; Barrio, L.; Agnoli, S.; Senanayake, S. D.; Evans, J.; Kubacka, A.; Estrella, M.; Hanson, J. C.; Martínez-Arias, A.; Fernández-García, M.; Rodriguez. Angew. Chem., Int. Ed. 2010, 49, 9680. (16) Bohle, D. C.; Spina, C. J. J. Phys. Chem. C 2010, 114, 18139. (17) Tsunekawa, S.; Ishikawa, K.; Li, Z. Q.; Kawazoe, Y.; Kasuya, A. Phys. Rev. Lett. 2000, 85, 3440. (18) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153. (19) Wu, Z. Y.; Jollet, F.; Gota, S.; Thromat, N.; Gautier-Soyer, M.; Petit, T. J. Phys.: Condens. Matter 1999, 11, 7185. (20) Bianconi, A.; Koningsberger, D. X-Ray Absorption: Principles, Applications,Techniques of EXAFS, SEXAFS, XANES; John Wiley & Sons: New York, 573−662, 1988. (21) Kaindl, G.; Schmiester, G.; Sampathkumaran, E. V. Phys. Rev. B 1988, 38, 10174. (22) Soldatov, A. V.; Ivanchenko, T. S.; Longa, S. D.; Kotani, A.; Iwamoto, Y.; Bianconi, A. Phys. Rev. B 1994, 50, 5074. (23) Douillard, L.; Gautier, M.; Thromat, N.; Duraud, J. P. Nucl. Instr. Methods Phys. Res. B 1995, 97, 133. (24) Capehart, T. W.; Mishra, R. K.; Herbst, J. F. J. Appl. Phys. 1992, 72 (2), 676. (25) Newville, M. J. Synchrotron Radiat. 2001, 8, 322. (26) Webb, S. M. Phys. Scr. 2005, 2005, 1011. (27) Ankudinov, A. L.; Ravel, B.; Rehr, J. J.; Conradson, S. D. Phys. Rev. B 1998, 58, 7565. (28) Ankudinov, A. L.; Rehr, J. J. Phys. Rev. B 2000, 62, 2437. (29) Wu, Z.; Zhang, J.; Benfield, R. E.; Ding, Y.; Grandjean, D.; Zhang, Z.; Ju, X. J. Phys. Chem. B 2002, 106, 4569. (30) Zhang, J.; Naka, T.; Ohara, S.; Kaneko, K.; Trevethan, T.; Shluger, A.; Adschiri, T. Phys. Rev. B 2011, 84, 045411. 634


Tuning Ceria Nanocrystals Morphology and Structure by Copper Doping

Recommend Documents