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Microstructural Evolution of CeO2 from Porous Structures to Clusters of Nanosheet Arrays Assisted by Gas Bubbles via Electrodeposition Gao-Ren Li,*,†,‡ Dun-Lin Qu,† Xiao-Lan Yu,† and Ye-Xiang Tong*,† MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry and Chemical Engineering, Institute of Optoelectronic and Functional Composite Materials, Sun Yat-Sen UniVersity, Guangzhou 510275, PR China, and State Key Lab of Rare Earth Materials Chemistry and Applications, Beijing 100871, PR China ReceiVed NoVember 30, 2007. In Final Form: January 20, 2008 Here we report the preparation of porous CeO2 and clusters of CeO2 nanosheet arrays via a simple, efficient electrochemical approach. Gas bubbles functioning as a dynamic template were utilized in our research for the synthesis of nanosheet array clusters. The Hc and Mr values of porous CeO2 are almost the same as those of CeO2 nanosheet array clusters at 5 K, and they are about 5916 Oe and 8.83 × 10-4 emu, respectively. However, the saturation magnetization of CeO2 nanosheet array clusters is much larger than that of porous CeO2 structures. The magnetic property of the prepared CeO2 deposits may be caused by the existence of Ce(III), indicating potential interest in the nanodevices because of their electronic and magnetic properties.
Introduction Ceria (CeO2) is an important functional materials with many attractive properties that make it highly promising in a wide range of applications such as chemical-mechanical polishing for microelectronics, solid electrolytes in solid oxide fuel cells, phosphorescence/luminescence, catalysts for three-way automobile exhaust systems, additives in ceramics, ultraviolet absorbers, and oxygen sensors.1-10 At present, many different methods are used to synthesize CeO2, including a hydrothermal route,11-13 a microemulsion method,14 mechanochemical processing,15 thermal hydrolysis,16 thermal decomposition,17 solution precipitation,18 aerosol pyrolysis,19 and sol-gel.20 However, high * Corresponding authors. E-mail:
[email protected] (G.-R.L.),
[email protected] (Y.-X.T.). † Sun Yat-Sen University. ‡ State Key Lab of Rare Earth Materials Chemistry and Applications. (1) Feng, X.; 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, 1504-1508. (2) Steele, B. C. H. Solid State Ionics 2000, 129, 95. (3) Morshed, A. H.; Moussa, M. E.; Bedair, S. M.; Leonard, R.; Liu, S. X.; El-Masry, N. Appl. Phys. Lett. 1997, 70, 1647. (4) Laha, S. C.; Ryoo, R. Chem. Commun. 2003, 2138. (5) Messing, G. L.; Zhang, S. C.; Jayanthi, G. V. J. Am. Ceram. Soc. 1993, 76, 2707. (6) Masui, T.; Fujiwara, K.; Machida, K. I.; Adachi, G. Y. Chem. Mater. 1997, 9, 2197. (7) Li, R. X.; Yabe, S.; Yamashita, M.; Momose, S.; Yoshida, S.; Yin, S.; Sato, T. Solid State Ionics 2002, 151, 235. (8) Beie, H. J.; Gnoerich, A. Sens. Actuators, B 1991, 4, 393. (9) Jasinski, P.; Suzuki, T.; Anderson, H. U. Sens. Actuators, B 2003, 95, 73. (10) Ho, C.; Yu, J. C.; Kwong, T.; Mak, A. C.; Lai, S. Chem. Mater. 2005, 17, 4514-4522. (11) Bondioli, F.; Ferrari, A. M.; Lusvarghi, L.; Manfredini, T.; Nannarone, S.; Pasquali, L.; Selvaggi, G. J. Mater. Chem. 2005, 15, 1061. (12) Shuk, P.; Greenblatt, M.; Croft, M. Chem. Mater. 1999, 11, 473-479. (13) Shuk, P.; Greenblatt, M. Solid State Ionics 1999, 116, 217. (14) Bumajdad, A.; Zaki, M. I.; Eastoe, J.; Pasupulety, L. Langmuir 2004, 20, 11223. (15) Tsuzuki, T.; Robinson, J. S.; McCormick, P. G. J. J. Aust. Ceram. Soc. 2002, 38, 15. (16) Hirano, M.; Fukuda, Y.; Iwata, H.; Hotta, Y.; Inagaki, M. J. Am. Ceram. Soc. 2000, 83, 1287. (17) Wang, Y.; Mori, T.; Li, J.; Ikegami, T. J. Am. Ceram. Soc. 2002, 85, 3105. (18) Sakai, N.; Tatsuta, M.; Yano, H.; Iishi, H.; Ishiguro, S. Gastrointest. Endosc. 2000, 51, 69. (19) Lopez-Navarrete, E.; Caballero, A.; Ocana, Gonzalez-Elipe, A. R. M. J. Mater. Res. 2002, 17, 797. (20) Hartridge, A.; Bhattacharya, A. K. J. Phys. Chem. Solids 2002, 63, 441.
temperature, high pressure, and surface capping agents are often relied on in these methods. The electrodeposition route represents a low-cost, simple preparation method,21-28 but the control of CeO2 nanostructures still needs to be well investigated to tap the full range of potential applications of CeO2. As we all know, the nanostructured materials exhibit unique optical, electrical, magnetic, mechanical, and thermal properties that are obviously different from those of bulk materials, and they have become a hot research field because of the great number of potential applications in manufacturing nanoscale electronic or optoelectronic devices. The shapes and sizes of nanostructures are two crucial factors in determining the properties of nanomaterials, and thus the control of shape and size is of great interest. For example, the synthesis of 1D or 2D nanostructures and the assembly of these nanometer-scale building blocks to form ordered superstructures or complex functional architetures offer great opportunities for exploring their novel properties and the fabrication of nanodevices.29 It is even more intriguing that the arrangement of the nanofeatures is responsible for much of the interesting behavior. Surface patterning with nanoscale features is of great interest for applications in optoelectronics, photonics, biosensor arrays, and biochip detection. Here we report the synthesis of CeO2 porous structures and clusters of nanosheet arrays via an electrochemical route. Gas bubbles as a dynamic template are used in our research for the synthesis of nanosheet array clusters. Patterning based on electrochemical processes offers a number of advantages. For example, the growth rate and film thickness can be well controlled (21) Kamada, K.; Higashikawa, K.; Inada, M.; Enomoto, N.; Hojo, J. J. Phys. Chem. C 2007, 111, 14508-14513. (22) Elbelghiti, H.; Lair, V.; Ringuede´, A.; Cassir, M. ECS Trans. 2007, 7, 2391. (23) Kulp, E. A.; Limmer, S. J.; Bohannan, E. W.; Switzer, J .A. Solid State Ionics 2007, 178, 749-757. (24) Li, G.-R.; Bu, Q.; Dawa, C.-R.; Lu, X.-H.; Qu, D.-L.; Yao, C.-Z.; Zheng, F.-L.; Tong, Y.-X. J. Phys. Chem. C 2007, 111, 6678-6683. (25) Phok, S.; Bhattacharya, R. N. Phys. Status Solidi A 2006, 203, 37343742. (26) Zhitomirsky, I.; Petric, A. Ceram. Int. 2001, 27, 149-155. (27) Zhitomirsky, I. Surf. Eng. 2004, 20, 43-47. (28) Stefanov, P.; Atanasova, G.; Stoychev, D.; Marinova, T. Surf. Coat. Technol. 2004, 180, 446-449. (29) Zhou, J.; Ding, Y.; Deng, S. Z.; Gong, L.; Xu, N. S.; Wang, Z. L. AdV. Mater. 2005, 17, 2107-2110.
10.1021/la7037526 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/01/2008
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by deposition potentials or salt concentrations, including the ability to pattern nanoscale features over large areas simultaneously. The prepared CeO2 nanosheet array clusters show great interest for the progress of nanoscience, nanotechnology, and nanodevices because of their special electronic, magnetic, and catalytic properties,30,31 and this may also be desirable for economic reasons. Experimental Section A simple three-electrode cell was used in our experiments. During the measuring of cyclic voltammograms, a Pt foil (99.99%, 1.10 cm2) was used as the auxiliary electrode, and a Pt wire (99.99%, 0.12 cm2) was used as the working electrode. A saturated calomel electrode (SCE) was used as the reference electrode that was connected to the cell with a double salt bridge system. During electrochemical deposition, a spectral-grade graphite rod of about 4.0 cm2 was used as the auxiliary electrode, and a pure copper foil (99.99%) of about 1.0 cm2 was used as the working electrode. The electrochemical deposition of CeO2 was carried out in an aqueous solution of 0.02 mol/L Ce(NO3)3 + 0.1 mol/L LiClO4 under galvanostatic conditions with cathodic current densities of 0.1-1.0 mA/cm2. Before electrodeposition, the Cu substrate was cleaned ultrasonically in 0.1 M HCl, distilled water, and acetone and then rinsed in distilled water again. All electrochemical deposition experiments were carried out in a configured glass cell at room temperature. The deposits were characterized by energy-dispersive spectroscopy (EDS, FEI/Quanta 400), X-ray diffraction (XRD, D/MAX 2200 VPC), and field-emission scanning electron microscopy (FE-SEM, JSM-6330F). X-ray photoelectron spectroscopy (XPS, ESCALAB 250) was used to assess the chemical state and surface composition of the deposits. A magnetic property measurement system (MPMS XL-7) was used to investigate the magnetic behavior of the deposits.
Results and Discussion The electrodeposition of CeO2 was carried out on Cu substrates in a solution of 0.005 mol/L Ce(NO3)3 + 0.1 mol/L LiClO4 with a cathodic current density of 0.5 mA/cm2 for 60 min at room temperature. The surface morphologies of the deposits were examined with SEM. Typical SEM images with different magnifications are shown in Figure 1, which shows that porous CeO2 was successfully prepared. The size of the pores is about 2 µm, and the thickness of the walls is about 600 nm. When the electrodeposition of CeO2 was carried out in solution of 0.02 M Ce(NO3)3 + 0.1 M LiClO4 with a cathodic current density of 0.5 mA/cm2, the obtained CeO2 deposits are composed of clusters of CeO2 nanosheet arrays as shown in Figure 2a. Highermagnification SEM images are shown in Figure 2b,c. The average thickness of these nanosheets is about 50-60 nm, and most of distances between the nanosheets are about 50 nm. Figure 3a shows a low-magnification TEM image of a nanosheet in the clusters of CeO2 nanosheet arrays. It can be seen that the thickness of the nanosheet is about 50 nm. Figure 3b shows a high-resolution TEM (HRTEM) image of the marked area in Figure 3a. The HRTEM image clearly indicates that the nanosheet is composed of many tiny grains of different orientations (average grain size of ∼6 nm). The selected-area electron diffraction pattern (SAED, inset in Figure 3b) further indicates that the structure of the nanosheet has a polycrystalline structure and is face-centered cubic CeO2. The chemical composition of the deposit was determined by EDS, and the EDS measurements were carried out at a number
of locations throughout the prepared deposits. The representative EDS pattern was shown in Figure 4a. Besides elemental Cu from substrate, the peaks of element Ce and O were detected in the EDS pattern. The composition analysis results demonstrated that the ratio of Ce to O was almost 1:2 in the prepared film. The EDS pattern of a reference CeO2 sample was shown in Figure 4b, which further indicated the ratio of Ce to O was almost 1:2 in the deposited samples. It is well known that there are two different oxidation states for elemental Ce, namely, Ce(III) and Ce(IV). However, the Ce(IV) oxidation state is very stable compared with the Ce(III) oxidation state in the presence of air. To investigate the oxidation state of Ce in the obtained deposits, XPS analyses were carried out. It is clear that cerium exists as the Ce(IV) oxidation state in the deposits from the Ce 3d corelevel peak in the XPS spectra shown in Figure 5a, but a small quantity of impurity of the Ce(III) oxidation state in the deposits was also detected. In addition, the XPS peak centered at 529.3 eV in Figure 5b can be attributed to the O2- contribution.32,33 The XPS-detected binding energies of Ce 3d and O 1s are in good agreement with standard CeO2, so the XPS results demonstrate that the obtained deposits are CeO2 with a small quantity of impurity of the Ce(III) oxidation state.
(30) Tian, Z.; Voigt, J.; Liu, J.; Mckenzie, B.; Mcdermott, M. J.; Rodriguez, M. A.; Konishi, H.; Xu, H. Nat. Mater. 2003, 2, 821. (31) Chen, S. J.; Liu, Y. C.; Shao, C. G.; Mu, R.; Lu, Y. M.; Zhang, J. Y.; Shen, D. Z.; Fan, X. W. AdV. Mater. 2005, 17, 586.
(32) Salvi, A. M.; Decker, F.; Varsano, F.; Speranza, G. Surf. Interface Anal. 2001, 31, 255. (33) 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, 19169-19174.
Figure 1. SEM images of porous CeO2 deposits prepared in a solution of 0.005 mol/L Ce(NO3)3 + 0.1 mol/L LiClO4 with a cathodic current density of 0.5 mA/cm2 at different magnifications: (a) ×650 and (b) ×13 000.
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Figure 3. (a) TEM and (b) HRTEM images and (inset) the electron diffraction pattern of the CeO2 nanosheet.
Figure 2. SEM images of the clusters of CeO2 nanosheet arrays prepared in a solution of 0.02 mol/L Ce(NO3)3 + 0.1 mol/L LiClO4 with a cathodic current density of 0.5 mA/cm2 at different magnifications: (a) ×3300, (b) ×5500, and (c) ×22 000.
The phases of the prepared CeO2 samples were investigated by XRD analysis. Figure 6 shows the XRD pattern of the prepared clusters of CeO2 nanosheet arrays, and peaks corresponding to the CeO2 (111), (200), (220), and (311) planes were observed. All of these diffraction peaks of CeO2 can be indexed as a facecentered cubic phase as identified using the standard data JCPDS 34-0394. No any other impurities were detected besides Cu peaks
that come from the substrate. Here, we should notice that the XRD peaks in the 2θ range from 25 to 80° exhibit broadened peaks with a little shift toward smaller angles. On the basis of the Scherrer equation, the crystallite size of a sample is inversely proportional to the full width at half-maximum (fwhm), indicating that a broader peak represents a smaller crystallite size. According to the Scherrer equation (i.e., D ) Kλ/(β cos θ), where λ is the wavelength of the X-ray radiation, K is a constant taken to be 0.89, θ is the diffraction angle, and β is the full width at halfmaximum), the strongest peak (111) at 2θ ) 28.7° and peak (220) at 2θ ) 47.1° were used to calculate the average crystallite size of CeO2 nanocrystals, determined to be around 5.0 nm, which is consistent with the results of HRTEM. The calculated cell parameter (a) is equal to 0.5518 nm, which is a little larger than that of bulk CeO2 (0.5411 nm). This can be attributed to the lattice expansion effect resulting from the increased oxygen vacancies and Ce(III) ions.34-38 The electrochemical formation process of CeO2 in this deposition system was investigated here. Electroreduction of the nitrate ion (NO3- to NO2-) in the neutral solution of Ce3+, which results in an increase of the concentration of OH- near the cathodic (34) (a) Wang, Z.; Quan, Z.; Lin, J. Inorg. Chem. 2007, 46, 5237-5242. (b) Tsunekawa, S.; Sahara, R.; Kawazoe, Y.; Ishikawa, K. Appl. Surf. Sci. 1999, 152, 53. (35) Tsunekawa, S.; Ishikawa, K.; Li, Z. Q.; Kawazoe, Y.; Kasuya, A. Phys. ReV. Lett. 2000, 85, 3440. (36) Zhou, X.-D.; Huebner, W. Appl. Phys. Lett. 2001, 79, 3512. (37) Wu, L. J.; Wiesmann, H. J.; Moodenbaugh, A. R.; Klie, R. F.; Zhu, Y.; Welch, D. O.; Suenaga, M. Phys. ReV. B 2004, 69, 125415. (38) Skorodumova, N. V.; Baudin, M.; Hermansson, K. Phys. ReV. B 2004, 69, 075401.
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Figure 4. (a) EDS pattern of the clusters of hierarchical CeO2 nanosheet arrays (where elemental Cu comes from substrate). (b) EDS pattern of a reference CeO2 sample.
electrode surface, is crucial (reaction 1).39 Furthermore, water and dissolved O2 may also be reduced simultaneously by the reactions 2 and 3.40 These produced OH- ions will result in the formation of CeO2 by reaction 4.40
NO3- + H2O + 2e f NO2- + 2OH-
(1)
2H2O + 2e f H2v + 2OH-
(2)
O2 + 2H2O + 4e f 4OH-
(3)
4Ce3++ 12OH- + O2 f 4CeO2 + 6H2O
(4)
The formation mechanism of the clusters of CeO2 nanosheet arrays can be explained by the H2 gas bubbles functioning as a dynamic template during electrodeposition. The illustrations for the formation of CeO2 porous structures and the clusters of nanosheet arrays during electrodeposition are shown in Figure 7. When the electrodeposition of CeO2 was carried out in solution of 0.005 M Ce(NO3)3 + 0.1 M LiClO4 with a cathodic current density of 0.5 mA/cm2, the gas bubbles will be produced as shown in eq 2. The deposition rate of CeO2 is slow because the concentration of Ce(NO3)3 and the cathodic current density of deposition both are small, which favors the produced gas bubbles integrating together to form large bubbles. These produced gas bubbles will move toward the electrolyte/air interface during the (39) (a) Peulon, S.; et al. AdV. Mater. 1996, 8, 166. (b) Izaki, M. Appl. Phys. Lett. 1996, 68, 2439. (40) Arurault, L.; Monsang, P.; Salley, J.; Bes, R. S. Thin Solid Films 2004, 466, 75-80.
electrodeposition process. Thus, metal growth toward the gas bubble will be prohibited simply because there are no metal ions available there,41 which will lead to the electrodeposition happening only between gas bubbles and, accordingly, the formation of CeO2 porous structures as illustrated in Figure 7a. When the concentration of Ce(NO3)3 was increased to 0.02 M, the deposition rate of CeO2 was also increased, which will lead to the formation of smaller gas bubbles. Most of the produced gas bubbles cannot integrate together to form large bubbles because these produced deposits will hold back the integration of gas bubbles. Therefore, only a few large gas bubbles and many small gas bubbles will form during electrodeposition as illustrated in Figure 7b,c, respectively. However, these small gas bubbles possibly move easily and can often be pushed aside by the deposits, and finally, many small gas bubble arrays lying between the deposits will form as shown in Figure 7c, which will lead to the formation of nanosheet arrays of CeO2. The larger gas bubbles will lead to detachment between the CeO2 nanosheet arrays. Therefore, the formation of clusters of CeO2 nanosheet arrays is the result of competition between electrodeposition and gas evolution. The magnetic properties of the prepared CeO2 nanosheet array clusters and porous CeO2 were studied by the magnetic property measurement system (MPMS XL-7), and Figure 8 shows their magnetic hysteresis loops at room temperature and 5 K. For all of the samples, the coercivity field (Hc) and the remnant magnetization (Mr) were almost zero at room temperature, as shown in Figure 8a. The Hc and Mr values of the porous CeO2 structures were almost the same as those of CeO2 nanosheet (41) Li, G.-R.; Ke, Q,-F.; Zhang, Z.-S.; Dawa, C.-R.; Liu, P.; Liu, G.-K.; Tong, Y.-X. Chem. Mater. 2007, 19, 2283-2287.
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Figure 7. (a) Illustration of the formation of (a) the porous structures, (b) the nanosheet array clusters, and (c) the nanosheet arrays.
Figure 5. XPS spectra of the clusters of hierarchical CeO2 nanosheet arrays: (a) Ce 3d and (b) O 1s.
Figure 8. Magnetic hysteresis loops of the clusters of CeO2 nanosheet arrays at (a) (1) room temperature and (b) (1) 5 K. Magnetic hysteresis loop of porous CeO2 at (a) (2) room temperature and (b) (2) 5 K.
Figure 6. XRD diffraction pattern of the clusters of CeO2 nanosheet arrays.
array clusters at 5 K, and they were about 5916 Oe and 8.83 × 10-4 emu, respectively. Therefore, the values of Hc and Mr at 5 K were much larger than those measured at room temperature for the CeO2 nanosheet array clusters and porous CeO2. In addition, the saturation magnetizations of CeO2 nanosheet array clusters were larger than those of porous CeO2 structures at room temperature and 5 K, and this may be attributed to the different nanostructures. It is well known that stoichiometric CeO2 should be nonmagnetic. What leads to the magnetic property of the prepared CeO2 nanosheet array clusters? It has been proven that the introduction of the nanostructures of stoichiometric CeO2
could affect the electronic structure in the surface layers.42 In this study, the oxygen defects and Ce(III) existed in CeO2 deposits as proven by XRD and XPS, so here we believe that the magnetic properties of the CeO2 nanosheet array clusters and porous CeO2 may be due to the special electronic structures, which are aroused by the Ce(III) ions in deposits. In summary, here we report that clusters of CeO2 nanosheet arrays and porous CeO2 can be successfully synthesized via a simple and efficient electrochemical approach. In addition, gas bubbles as a dynamic template are utilized in our research for the synthesis of nanosheet array clusters. The magnetic property of the prepared CeO2 deposits can be caused by the Ce(III) ions, indicating potential interest in the nanodevices because of their magnetic properties. The electrochemical synthesis route assisted (42) Ho, C.; Yu, J.C.; Kwong, T.; Mak, A. C.; Lai, S. Chem. Mater. 2005, 17, 4514-4522.
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by bubbles could be expected to prepare other nanosheet array clusters or porous structures of rare earth oxides. Acknowledgment. This work was supported by the Natural Science Foundations of China (grant nos. 20603048 and
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20573136), the Natural Science Foundations of Guangdong Province (grant nos. 06300070, 06023099, and 04205405), and the Foundation of Potentially Important Natural Science Research. LA7037526
