pubs.acs.org/Langmuir © 2010 American Chemical Society
Facile Electrochemical Synthesis of Single Crystalline CeO2 Octahedrons and Their Optical Properties Xi-hong Lu,† Xi Huang,† Shi-lei Xie,† De-zhou Zheng,† Zhao-qing Liu,†,‡ Chao-lun Liang,§ 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, China, ‡School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, China, and §Instrumental Analysis and Research Centre, Sun Yat-Sen University, Guangzhou 510275, China Received November 19, 2009
We developed a simple electrochemical process for the large-scale fabrication of single crystalline CeO2 octahedrons and nanospheres from DMSO aqueous solution. The octahedrons with some structural defects have a size ranging from 200 to 300 nm. Moreover, highly crystalline CeO2 nanospheres were also obtained via this electrochemical process based on the oriented attachment mechanism. The absorption edge of octahedrons and spheres shows a red-shift, and that of the octahedrons was near the visible region.
Introduction Nanomaterials are of great interest due to their unique shape/ size-dependent properties.1 CeO2, an important technological material, has been extensively applied in catalysts, polishing materials, UV blocks, gas sensors, fuel cells, and solar cells due to its high mechanical strength, oxygen ion conductivity, optical property, thermal stability, and oxygen storage capacity.2 Over the past few years, CeO2 nanomaterials have attracted intensive attention because they can dramatically enhance catalytic ability and redox properties and produce some novel properties such as high ionic conductivity and photovoltaic response.3 The controllable growth of nanostructures is difficult but crucial for nanoscience and nanotechnology. In recent years, various synthetic strategies have been developed to fabricate nanoceria, and a remarkable process has been procured in the shape/size-controlled synthesis of nanoceria.4 Among these methods, a solution-based strategy has proved to be a powerful *Corresponding author: Tel þ86 20 84110071; Fax þ86 20 84112245; e-mail
[email protected]. (1) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. Adv. Mater. 2003, 15, 353. (2) (a) 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. (b) Fu, X. Q.; Wang, C.; Yu, Y. G.; Wang, T. H. Nanotechnology 2007, 18, 145503. (c) Liao, L.; Mai, H. X.; Yuan, Q.; Lu, H. B.; Li, J. C.; Liu, C.; Yan, C. H.; Shen, Z. X.; Yu, T. J. Phys. Chem. C 2008, 112, 9061. (d) Liu, W. W.; Zhou, K. B.; Wang, L.; Wang, B. Y.; Li, Y. D. J. Am. Chem. Soc. 2009, 131, 3140. (3) (a) Wang, X.; Sun, X. M.; Peng, Q.; Li, Y. D. J. Catal. 2005, 229, 206. (b) Sathyamurthy, S.; Leonard, K. J.; Dabestani, R. T.; Paranthaman, M. P. Nanotechnology 2005, 16, 1960. (c) Kim, S.; Lee, J. S.; Mitterbauer, C.; Ramasse, Q. M.; Sarahan, M. C.; Browning, N. D.; Park, H. J. Chem. Mater. 2009, 21, 1182. (4) (a) Wu, G. S.; Xie, T.; Yuan, X. Y.; Cheng, B. C.; Zhang, L. D. Mater. Res. Bull. 2004, 39, 1023. (b) Chen, G. Z.; Xu, C. X.; Song, X. Y.; Xu, S. L.; Ding, Y.; Sun, S. X. Cryst. Growth Des. 2008, 8, 4449. (c) Taniguchi, T.; Watanabe, T.; Sakamoto, N.; Matsushita, N.; Yoshimura, M. Cryst. Growth Des. 2008, 8, 3725. (d) Zhou, H. P.; Zhang, Y. W.; Mai, H. X.; Sun, X.; Liu, Q.; Song, W. G.; Yan, C. H. Chem.;Eur. J. 2008, 14, 3380. (5) (a) Ho, C.; Yu, J. C.; wong, T. K.; Mak, A. C.; Lai, S. Chem. Mater. 2005, 17, 4514. (b) 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. (c) Zhou, K. B.; Yang, Z. Q.; Yang, S. Chem. Mater. 2007, 19, 1215–1217. (d) Pan, C. S.; Zhang, D. S.; Shi, L. Y.; Fang, J. H. Eur. J. Inorg. Chem. 2008, 15, 2429. (e) Yu, R.; Yan, L.; Zheng, P.; Chen, J.; Xing, X. J. Phys. Chem. C 2008, 112, 19896.
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approach to fabricate CeO2 nanostructures. Recently, well-defined CeO2 nanostructures, such as nanoparticles, nanorods, nanotubes, and nanocubes, have been synthesized via the hydrothermal process.5 However, this method generally needs high reaction temperature, long-time treatments, surfactants, or templates, and calcination has usually been necessary to obtain crystalline nanoceria. Thus, the development of facile, mild, and effective methods for large-scale preparation of nanoceria with high crystalline is still a tremendous challenge. Electrochemical deposition offers a promising method to prepare the nanometerials because of its simplicity, ease of scale-up, low cost, and environmental friendliness.6 Up to now, electrochemical synthesis of ceria nanostructures has been reported a lot.7 Nevertheless, to the best of our knowledge, no studies have been reported on the preparation of CeO2 octahedrons via the electrochemical process, even though the synthesis of CeO2 octahedrons by a hydrothermal process has been previously reported.8 In this paper, we describe a facile electrochemical reaction to synthesize single crystalline CeO2 octahedrons on F-doped SnO2-coated glass (FTO) from DMSO aqueous solution. The absorption edge of CeO2 octahedrons was near the visible region, disclosing that these octahedrons may be used as the photovoltaic material. Furthermore, highly crystalline CeO2 nanospheres were also obtained via this electrochemical process, which is based on the oriented attachment mechanism. And the influence of DMSO concentration on the morphology was investigated as well. The welldefined CeO2 nanostructures grown directly on FTO substrates will also enhance their applications in electronic and optical devices. (6) (a) Therese, G. H. A.; Kamath, V. P. Chem. Mater. 2000, 12, 1195. (b) She, G. W.; Zhang, X. H.; Shi, W. S.; Fan, X.; Chang, J. C.; Lee, C. S.; Lee, S. T.; Liu, C. H. Appl. Phys. Lett. 2008, 92, 053111. (c) Pradhan, D.; Leung, K. T. Langmuir 2008, 24, 9707. (7) (a) Arurault, L.; Monsang, P.; Salley, J.; Bes, R. S. Thin Solid Films 2004, 466, 75. (b) Wang, A. Q.; D'Souza, N.; Golden, T. D. J. Mater. Chem. 2006, 16, 481. (c) Kamada, K.; Higashikawa, K.; Inada, M.; Enomoto, N.; Hojo, J. J. Phys. Chem. C 2007, 111, 14508. (d) Inguanta, R.; Piazza, S.; Sunseri, C. Nanotechnology 2007, 18, 485605. (e) Li, G. R.; Qu, D. L.; Tong, Y. X. Electrochem. Commun. 2008, 10, 80. (8) Yan, L.; Yu, R. B.; Chen, J.; Xing, X. R. Cryst. Growth Des. 2008, 8, 1474.
Published on Web 01/27/2010
DOI: 10.1021/la904882t
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Figure 1. XRD pattern of CeO2 octahedrons.
Figure 3. (a) TEM image, (b) HRTEM image, (c) SAED pattern, and (d) schematic diagram of CeO2 octahedrons.
Figure 4. TEM images of CeO2 octahedrons observed with different incident electron beam.
Figure 2. SEM images of CeO2 octahedrons.
Experimental Section Electrochemical preparation was carried out in a conventional three-electrode cell via galvanostatic electrodeposition. The working electrode was an F-doped SnO2-coated glass (FTO) with a sheet resistance of 14 Ω/0. A graphite rod of about 4.0 cm2 was used as the auxiliary electrode. A saturated Ag/AgCl electrode was used as the reference electrode, which was connected to the cell with a double salt bridge. The FTO glass was cleaned ultrasonically in distilled water, ethanol, and acetone and then rinsed in distilled water again before electrodeposition. The electrolytic solution contained 10 mM Ce(NO3)3 and DMSO at different concentration (the volume ratio of DSMO to H2O). All reagents used were analytical grade and were used directly without any purification, and the reaction temperature was kept at 70-90 °C. The surface morphology and the composition of the samples were analyzed by scanning electron microscope (SEM, Quanta 400). The structure of the samples were investigated via X-ray diffraction (XRD, Bruker, D8 ADVANCE) with Cu KR radiation (λ = 1.5418 A˚) and transmission electron microscopy (TEM, JEM2010-HR). The optical properties of the ZnO nanostructures were measured with a UV-vis-NIR spectrophotometer (UV, Shimadzu UV-3150) and a combined fluorescence lifetime and steady state spectrometer (PL, EDINBURGH). Raman spectroscopy was performed on a laser micro-Raman spectrometer (Renishaw inVia) using visible laser (λ = 514.5 nm) with an output laser power of 50 mW as the excitation wavelength at room temperature.
Results and Discussion The crystalline phase of the products was identified by X-ray powder diffraction (XRD), and the XRD pattern of the sample was prepared via cathodic electrdeposition in 10 mM Ce(NO3)3 þ 30% DMSO (30 vol % DMSO:70 vol % H2O) with a current density of 2 mA cm-2 for 60 min at 90 °C and is presented in 7570 DOI: 10.1021/la904882t
Figure 1. All of the peaks in the XRD pattern can be perfectly indexed to a fluorite cubic structure of CeO2 (JCPDF # 65-2975) with lattice constants a = 0.5411 nm. No any other peaks are detected besides SnO2 peaks that come from the substrate. This implies that the deposits are highly pure. The SEM images of the sample are shown in Figure 2. From Figure 2a,b, it can be seen that large-scale CeO2 octahedrons were successfully synthesized on the FTO substrate. The prepared CeO2 octahedrons are uniform with an average size ranging from 200 to 300 nm. The high-magnification SEM images, illustrated in Figure 2c,d, reveal that the synthesized octahedrons are an octahedral morphology with some defects. More structural details of the sample were further investigated by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and a select area electron diffraction (SAED) pattern. Figure 3a shows a typical TEM image of CeO2 octahedron with (110) facet perpendicular to the electron beam. The image is made up by two parallelograms with an acute angle equal to 68°, which indicates the crystal is not a perfect octahedral morphology. Interestingly, it is noted that the morphology changed while observed by TEM with different incident electron beam, as shown in Figure 4. This phenomenon further confirms the octahedron has some structural defects. The amplificatory HRTEM image (Figure 3c,d) of the selected area marked by white squares in Figure 3b reveals the lattice fringe spacing of 0.27 and 0.32 nm, which matches well with the (200) and (111) lattice spacing of face-centered-cubic (fcc) CeO2. The corresponding select area electron diffraction (SAED) pattern further confirms that the CeO2 octahedron is single crystalline with fcc structure. On the basis of above results, the prepared octahedron is high crystalline with {111} facets as the main exposed surface, and the geometrical structure of an individual CeO2 octahedron is schematically illustrated in Figure 3d. Figure 5a shows the representative SEM image of a large quantity of nanospheres synthesized under similar conditions using a lower DMSO concentration of 10% at 70 °C. The highmagnification SEM image in Figure 5b clearly discloses that the Langmuir 2010, 26(10), 7569–7573
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Figure 5. (a, b) SEM images and (c) size distribution of CeO2 nanospheres.
Figure 6. (a) TEM and (b, c) the corresponding HRTEM images and (d) SAED pattern (recorded with the whole sphere) of CeO2 sphere.
nanospheres have a narrow size distribution with the mean diameter 180 ( 40 nm (Figure 5c). Figure 6a-c is the TEM and HRTEM images of single CeO2 nanosphere with diameter of 140 nm, clearly indicating the sphere is constructed by many nanocrystallites (as marked by white circles). The HRTEM images in Figure 6a taken from the selected areas marked by the boxes b and c exhibit well-resolved two-dimensional lattice fringes with the spacings of 0.27 and 0.32 nm, corresponding to the interplanar spacings of (200) and (111) planes, respectively, as shown in Figure 6b,c. Moreover, the HRTEM observation further demonstrates that the oriented nanoparticles are uniformly distributed and textured about [011] axis. Figure 6d is the SAED pattern recorded with the whole sphere. The extra pairs of spots (as shown by white arrows) presented in high-order diffractions suggest that the sphere is composed of highly organized nanoparticles with slight misorientation. The diffraction pattern indicates that the crystallographic axes of the nanoparticles from different parts of the sphere are parallel or nearly parallel to each other. According to the HRTEM observation and SAED analysis, the sphere is a textured polycrystal composed of a large number of highly oriented nanoparticles, which is based on the oriented attachment mechanism.9 The UV-vis absorption spectra of CeO2 octahedrons and nanospheres are shown in Figure 7a. All the samples exhibit two strong bands in the UV region that originated from the chargetransfer transitions from O 2p to Ce 4f.5a Comparing to CeO2 nanospheres, a clear red-shift of the absorption edge can be observed for octahedrons. Most interestingly, the absorption edge of CeO2 octahedrons has shifted toward the visible region, (9) (a) Yang, H. G.; Zheng, H. C. Angew. Chem., Int. Ed. 2004, 43, 5930. (b) Zhang, Q.; Liu, S. J.; Yu, S. H. J. Mater. Chem. 2009, 19, 191. (c) Fu, Y. S.; Song, Y. F.; Kulinichc, S. A.; Sun, J.; Liu, J. X.; Du, W. J. Phys. Chem. Solids 2008, 69, 880.
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Figure 7. (a) Optical absorption spectrum and (b) (Rhν)2 vs hν curves for the CeO2 samples.
which is great importance for application in photovoltaic devices. The optical band gap (Eg) of a semiconductor material can be calculated from the equation of (Rhν)n = A(hν - Eg), where hν is the photon energy, R is the absorption coefficient, A is a constant for the material, and n is 2 for a direct transition or 1/2 for an indirect transition.10 From Figure 7b, the calculated direct Eg for octahedrons and spheres is about 2.90 and 3.05 eV, which are fairly smaller than values for bulk of CeO2 (3.19 eV). The red-shift phenomenon of the UV absorption spectra of CeO2 nanostructures has been observed by some research scholars.11 However, the true mechanism of the red-shift of CeO2 nanostructures is still not clear so far. In recent years, two kinds of explanations have been proposed for the red-shift of the optical band gap of CeO2 nanorstructures. One is the shape effect. It is known that the optical band gap will be blue-shifted with the reduction of particle (10) Tauc, J.; Menth, A. J. Non-Cryst. Solids 1972, 8-10, 569. (11) (a) Chen, H. Y.; Chang, H. Y. Solid State Commun. 2005, 133, 593. (b) Corma, A.; Atienzar, P.; Garcia, H.; Chane-Ching, J.-Y. Nat. Mater. 2004, 3, 394. (c) Sun, C.; Li, H.; Zhang, H.; Wang, Z.; Chen, L. Nanotechnology 2005, 16, 1454. (d) Patsalas, P.; Logothetidis, S.; Metaxa, C. Appl. Phys. Lett. 2002, 81, 466. (e) Patsalas, P.; Logothetidis, S.; Sygellou, L.; Kennou, S. Phys. Rev. B 2003, 68, 035104. (f) Chen, M. Y.; Zu, X. T.; Xiang, X.; Zhang, H. L. Physica B 2007, 389, 263.
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Figure 8. Room temperature Raman spectra of (a) FTO substrate, (b) CeO2 octahedrons, and (c) CeO2 nanospheres.
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Figure 9. Room temperature photoluminescence spectrum of CeO2 octahedrons and nanospheres with the excitation wavelength of 290 nm.
size due to the quantum confinement effect,12 and the quantum confinement effect may be ruled out when the size is up to 10 nm. Chen et al.11a have observed a red-shift phenomenon for the CeO2 nanoneedles, which is attributed to the shape effect. The other is the presence of defects arising from the charge transmission between Ce3þ and Ce4þ ion, which is generally accepted for the explanation with the red-shift of CeO2 films.11b-f Patsalas et al.11d,e found that the red-shift of optical band gap of CeO2 film is correlated with the increase of Ce3þ content, and the defects are directly proportional to the concentration of Ce3þ. The localized states of band gap resulting from the defects will advance with the increase of Ce3þ, leading to a red-shift. Similar conclusions have been made by Chen et al.11f They investigated the effects of ion irradiation and annealing on optical properties of CeO2 films and found the band gap shifted toward short wavelength (blue-shift) after annealing in rich O2 ambient, which is caused by the valence transition of Ce3þ f Ce4þ. In our case, the size of octahedrons is in the range of 200-300 nm while the spheres are about 140-220 nm. Furthermore, the Raman and PL results (discussed in the next two paragraphs) indicate the octahedrons and spheres possess high oxygen vacancies. Thus, on the basis of the above discussion, it is reasonably concluded that the red-shift of the absorption edge is ascribed to shape effect and the surface defects. Compared with octahedrons, the spheres have less defects and quantum confinement effect because it is composed of closely packed nanocrystals, resulting in the adsorption at shorter wavelength. In order to better understand the defects of samples, Raman scattering was carried out. Figure 8 shows the typical Raman spectrum of the CeO2 octahedrons and spheres. One strong Raman peak centered at about 462 nm was observed for both of samples, which is originated from the F2g Raman-active mode of CeO2 cube structure.13 In addition, two weak second-order peaks were also detected. The peak at 1047 nm is attributed to the second-order Raman mode feature of peroxide adspecies (O22-), and the weak peak at about 1172 nm is assigned to the secondorder Raman mode feature of surface superoxide species (O2-).13a Moreover, the relative intensity of the above dioxygen adspecies (vs the strongest peak) of octahedrons is higher
than spheres, implying the CeO2 octahedrons has more oxygen vacancies. More evidence can be observed from the room temperature photoluminescence (PL) spectrum, as shown in Figure 9. It is obviously that one strong emission band at ∼435 nm were observed for the two samples. It is known that CeO2 is a wideband semiconductor, and the forbidden gap is about 5.5 eV.14 The electron transition from the valence band to Ce 4f level is facile due to the Ce 4f energy level with 1.2 eV width, which localizes at the forbidden band and lies about 3 eV above the valence band (O 2p). Generally, the broad PL emission peaks (
