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J. Phys. Chem. C 2008, 112, 17076–17080
Great Influence of Anions for Controllable Synthesis of CeO2 Nanostructures: From Nanorods to Nanocubes Qiang Wu,* Fan Zhang, Pei Xiao, Haisheng Tao, Xizhang Wang, and Zheng Hu* Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, China
Yinong Lu¨ College of Materials Science and Engineering, Nanjing UniVersity of Technology, Nanjing 210009, China ReceiVed: May 10, 2008; ReVised Manuscript ReceiVed: August 20, 2008
Controllable synthesis of well-shaped nanocrystals is of significant importance for understanding the surfacerelated properties as well as for the exploration of potential applications. Herein, CeO2 nanorods and nanocubes were selectively synthesized using cerium(III) chloride and cerium(III) nitrate as precursor, respectively. Counter anions of the cerium source were crucial to the shapes of the resulting products. Intriguingly, the as-synthesized nanorods could be converted into nanocubes by the addition of an appropriate amount of NO3- ions into the hydrothermal reaction. The NO3- ions are considered as both a capping agent and an oxidizer during the formation of CeO2 nanocubes. Moreover, the influences of several others anions are investigated. Br-, I-, and SO42- ions have similar roles to Cl- ions, which lead to the formation of nanorods. The introduction of BrO3- ions can bring on the generation of irregular nanoparticles because they can function as an oxidizer but not a capping agent. The anion-induced controllable growth process is simple and low cost, which makes this strategy potentially useful for the preparation of other faceted nanostructures. 1. Introduction The diverse shapes and sizes of nanomaterials may largely influence their physical and chemical properties because different exposed crystal surfaces show much difference in surface atom densities, electronic structures, and chemical reactivities.1 Well-shaped nanomaterials exhibit superior properties in fields such as catalysis, optics, and sensing to the corresponding unshaped counterparts.2 Many advanced prototype devices such as biosensors, lasers, and field emitters have been fabricated on the basis of these nanostructures.3 Therefore, the controllable synthesis of nanocrystals with specific shapes and exposed surfaces is a very important topic for understanding the surfacerelated properties as well as for the exploration of potential applications. In the past decade, various synthetic routes have been developed for the preparation of different faceted nanostructures such as nanocubes, triangular nanoprisms, nanorods (or nanowires), and nanotubes from inorganic precursors.1-7 In principle, the intrinsic crystal anisotropy is the driving force for the formation of these highly faceted geometries. In some cases, the shape of crystals could also be regulated by introducing appropriate capping agents to act as inhibitors or promoters for the anisotropic growth.4 The capping agents were selectively absorbed on a certain plane to form the adlayer, which could change the surface free energies, thus kinetically control the growth rates of various facets, giving rise to the faceted nanostructures.5 For instance, by controlling the molar ratio of poly(vinylpyrrolidone) (PVP) to bismuth species in a polyol process, bismuth nanocubes, triangular nanoplates, and nanospheres could be respectively synthesized because of the selective interaction between PVP and various crystallographic * To whom correspondence should be addressed. E-mail: wqchem@ nju.edu.cn (Q.W.);
[email protected] (Z.H.).
planes of bismuth.6 The capping agents could be adscititious additives (organic molecules, polymers, or surfactants) or anions such as counterions of precursors themselves.7 The simplicity, convenience, and low cost of the latter make this strategy very potential for the preparation of highly faceted nanostructures. As one of the most reactive rare earth oxides, ceria (cerium oxide, CeO2) has attracted increasing attention owing to its wide applications as catalysts, catalyst supports, oxygen gas sensors, solid electrolytes for fuel cells, polishing materials, and so on.8 Both experimental9 and theoretical10 studies showed that the properties of CeO2 nanocrystals exhibited surface structure dependent characteristics. As revealed by Sayle et al.,10a the (100) terminated surface is inherently more reactive and catalytically important as compared with (111) and (110) surfaces; therefore, controllable growth of CeO2 nanostructures with defined exposed surface is of significant importance. CeO2 nanostructures such as nanowires (nanorods),11 nanotubes,12 and tadpole-shaped nanostructures13 have been successfully prepared through the hydrothermal process. In the synthesis, the surfactants are crucial to modify the surfaces and induce the anisotropic growth of these nanocrystals. Recently, CeO2 nanocubes that have the only exposed surfaces of (100) were synthesized using oleic acid as capping agents in a toluene-water system14 or under high temperature (180 °C) and high base concentration (CNaOH ) 6-9 mol L-1).11b In spite of these achievements, the facile synthesis of CeO2 nanocubes under a mild condition is still a challenge to date. Herein, a convenient route has been developed to synthesize CeO2 nanocubes simply by changing the counteranions of the cerium source. Cerium hydroxide nanorods were first formed via a hydrothermal process using cerium(III) chloride as precursor, and the subsequent addition of nitrate ions into the reaction mixture could result in the conversion of Ce(OH)3 nanorods to CeO2 nanocubes.
10.1021/jp804140e CCC: $40.75 2008 American Chemical Society Published on Web 10/11/2008
Controllable Synthesis of CeO2 Nanostructures
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The conversion process was studied in detail and the great influence of anions, that is, the formation mechanism, was accordingly elucidated. 2. Experimental Section 2.1. Synthesis of CeO2 Nanorods. Typically, 0.67 g of cerium(III) chloride (CeCl3 · 7H2O) was dissolved in 30 mL of NaOH solution (CNaOH ) 9 mol L-1) under vigorous stirring. The suspension was transferred to a 50 mL Teflon-lined stainless-steel autoclave and held in 140 °C for 48 h. After the autoclave was cooled to room temperature naturally, fresh precipitates were separated by centrifugation, washed with deionized water to neutrality, and with ethanol several times. The CeO2 nanorods were obtained by drying the precipitates at 60 °C in air overnight. 2.2. Synthesis of CeO2 Nanocubes. Cerium(III) nitrate [Ce(NO3)3 · 6H2O, 0.78 g] was dissolved in 30 mL of NaOH solution (CNaOH ) 9 mol L-1) under vigorous stirring. The rest of the synthetic procedure was similar to that used for the synthesis of the nanorods. Using CeCl3 · 7H2O as the cerium source and adding an appropriate NaNO3 into the hydrothermal solution, CeO2 nanocubes could also be synthesized via a similar synthetic procedure. To clarify the influence of counteranions in the reaction, the precursors with different molar ratios of [NO3-]:[Ce(III)] were used, to observe the changes of product. If there is no special claim, the treatment time is 48 h. 2.3. Conversion of the Nanorods into Nanocubes. The hydrothermal reaction after the addition of nitrate ions into the as-synthesized nanorods could result in the conversion from nanorods to nanocubes. To obtain the information about the conversion mechanism, the conversion process was studied as follows. After the synthesis of nanorods was conducted via the hydrothermal treatment of CeCl3 · 7H2O in NaOH solution for 24 h, the autoclave was cooled to room temperature and an appropriate amount of NaNO3 (with various [NO3-]:[Ce(III)] molar ratio) was added into the solution under vigorous stirring. The autoclave was then placed in 140 °C for different time to study the time-dependent conversion process. For comparison, the experiments using NaBrO3, NaBr, NaI, and Na2SO4 substituting NaNO3 were also conducted with a similar procedure to investigate the influence of different anions. 2.4. Characterization. The as-prepared products were characterized by X-ray diffraction (XRD), recorded on a Philips X’pert Pro X-ray diffractometer with Cu KR radiation of 1.5418 Å. Transmission electron microscopy (TEM) observations were conducted on a JEOL-JEM-1005 microscope operated at 80 kV or on a JEM 2010 microscope operated at 200 kV. Scanning electron microscopy (SEM, Hitachi, S-4800) was used to observe the morphologies of the products. 3. Results The fresh precipitates synthesized through the hydrothermal reaction of CeCl3 · 7H2O with NaOH at 140 °C are composed of nanorods when carefully dried at low temperature (∼5 °C) under the protection of nitrogen. These nanorods have typical diameters of about 15-25 nm and lengths up to a few micrometers (part a of Figure 1). The corresponding XRD pattern (part f of Figure 1, curve a) indicates that these nanorods are highly pure Ce(OH)3 with hexagonal phase (JCPDF #74-0665). By drying the precipitates at 60 °C in air overnight, CeO2 nanorods were obtained because Ce(OH)3 species is ready for dehydration and oxidation during the drying process. TEM image reveals the long rodlike nanostructures with the diameter and length similar to those of Ce(OH)3 nanorods (part b of
Figure 1. (a) Typical TEM image of the nanorods synthesized at 140 °C and dried in nitrogen. (b) Typical TEM image of the nanorods synthesized at 140 °C and dried in air. (c) HRTEM image of a CeO2 nanorod. Inset is the corresponding FFT pattern. (d and e) Typical TEM images of nanorods synthesized at 160 and 180 °C, and dried in air. (f) XRD patterns corresponding to the samples in (a), (b), (d) and (e), respectively. All of the samples are synthesized from cerium(III) chloride precursor.
Figure 1), illustrating that the morphologies of these nanorods are retained during the dehydration and oxidation process. Part c of Figure 1 shows the high resolution TEM (HRTEM) image of a typical CeO2 nanorod combined with a fast Fourier transform (FFT) analysis (inset). Three kinds of lattice fringes of (111), (200), and (220) could be identified with a respective interplanar spacing of 0.314, 0.276, and 0.193 nm, respectively, indicating the cubic structure of the CeO2 nanorod with [110] growth direction. The products hold the nanorod-like morphology when the hydrothermal temperature was elevated up to 160 and 180 °C, except that the diameters increased to 25-45 nm and 50-125 nm, respectively (parts d and e of Figure 1). The hydrothermal reaction for longer time (even longer than 96 h) did not show obvious change in morphology (although a few polyhedra formed in the product), indicating that the nanorodlike geometry is the intrinsic formation for the case only with Cl- counteranions. XRD patterns of CeO2 nanorods corresponding to the above three products (part f of Figure 1, curves b, d, and e) indicate their cubic fluorite structure (JCPDF #81-0792), in agreement with the HRTEM characterization. Interestingly, when nitrate salt (either Ce(NO3)3 or NaNO3) was introduced into the hydrothermal reaction system while keeping the other condition, the morphology of the product gradually changed from nanorods to nanocubes, and uniform CeO2 nanocubes could be obtained by the addition of enough amount of nitrate salt. Part a of Figure 2 shows the typical TEM image of the product obtained by 48 h hydrothermal treatment
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Figure 2. TEM images of the product obtained (a) by adding enough NO3- ions into the hydrothermal system of cerium (III) chloride and NaOH solution, (b) using cerium (III) nitrate as precursor. (c and d) Typical HRTEM images of CeO2 nanocubes with or without truncations. The inset shows the corresponding FFT pattern.
of CeCl3 · 7H2O (60 mmol L-1) in NaOH solution (CNaOH ) 9 mol L-1) in the presence of NO3- ions (60 mmol L-1) at 140 °C. No nanorod could be observed, indicating that the introduction of nitrate ions dramatically changed the nucleation and growth of the product. The critical concentration of NO3- ions needed for initiating the morphological change is about 2 mmol L-1, in other words, the [NO3-]:[Ce(III)] molar ratio of about 1:30 is high enough to induce the formation of CeO2 nanocubes. The concentration of NO3- needed for the complete conversion of nanorods to nanocubes is dependent on the hydrothermal temperature, that is, 20 mmol L-1 for 180 °C, 36 mmol L-1 for 160 °C, and 60 mmol L-1 for 140 °C. From the preceding experimental results, it is learned that nitrate ions facilitate the formation of CeO2 nanocubes. As expected, morphologically uniform CeO2 nanocubes with the sizes ranged from 8 to 30 nm (part b of Figure 2) were synthesized at the temperature of 140 °C from the direct hydrothermal reaction of Ce(NO3)3 · 6H2O and NaOH for 48 h. The HRTEM image in part c of Figure 2 combined with FFT analysis (inset) displays the clear (200) and (220) lattice fringes with the interplanar spacings of 0.271 and 0.189 nm, respectively, implying that the CeO2 nanocubes are only enclosed by {200} planes. The truncations of this nanocube at {220} facets are occasionally observed. Actually, most of the CeO2 nanocubes have sharp corners as typically shown in parts b and d of Figure 2. The time-dependent evolution of the product using Ce(NO3)3 as the cerium source indicates that Ce(OH)3 nanorods formed at first and subsequent hydrothermal treatment for longer time led to the conversion from nanorods to nanocubes (Figure S1 of the Supporting Information). It is worth noting that CeO2 nanocubes directly formed in the reaction system after the hydrothermal treatment for 48 h (Figure S2 of the Supporting Information), different from the case for the formation of CeO2 nanorods via dehydration and oxidation of preformed Ce(OH)3 counterparts. This surfactant-free process could perform at the temperature as low as 140 °C and has great advantage for mass production of CeO2 nanocubes. More intriguingly, it is found that even though the Ce(OH)3 nanorods were preformed through the hydrothermal reaction of
Figure 3. (a) TEM images of the nanostructures obtained from autoclave 1# without NO3- ions. (b-f) TEM images of the nanostructures obtained from autoclave 2-6# by further hydrothermal treatment with NO3- ions for 7, 16, 26, 36, and 48 h respectively. The molar ratio of [NO3-] to [Ce(III)] is 3:1.
CeCl3 and NaOH, the addition of nitrate ions into the reaction system could convert the nanorods to nanocubes through a sequent hydrothermal process. To understand the influence of NO3- ions more clearly, a set of experiments were designed as follows. Six Teflon-lined autoclaves containing 30 mL mixture solution of CeCl3 · 7H2O (60 mmol L-1) and NaOH (CNaOH ) 9 mol L-1) were held at 140 °C for 24 h. Then, autoclave 1# was cooled to room temperature, while autoclaves 2-6# were added with sodium nitrate of 180 mmol L-1 to reach the [NO3-]: [Ce(III)] molar ratio of 3:1 and further held at 140 °C for different hours. TEM images of the final products from autoclaves 1-6# are grouped in Figure 3 for comparison. It is seen that, without the addition of sodium nitrate, nanorods with diameter of ∼20 nm were obtained and no nanocube was observed (1#, part a of Figure 3). After additional 7 h hydrothermal treatment with NO3- ions, some nanocubes with size of about 8 nm appeared, in addition to the predominant nanorods (2#, part b of Figure 3). Upon increasing the treatment time, the nanocubes gradually largened and the relative proportion of nanocubes to nanorods increased correspondingly (parts c-e of Figure 3). When the hydrothermal treatment lasted for 48 h, only nanocubes with size of ∼20 nm were obtained (part f of Figure 3). This process clearly indicates that NO3- ions suppressed the growth of nanorods and facilitated the formation of nanocubes. At the lower concentration of NO3- ions with the [NO3-]:[Ce(III)] molar ratio of 1:1, only about half of nanorods could be converted into nanocubes after the additional 48 h treatment due to the insufficient NO3- ions (Figure S3 of the Supporting Information). 4. Discussion It is generally accepted that the structural anisotropy of crystals is the main driving force for the formation of faceted
Controllable Synthesis of CeO2 Nanostructures
Figure 4. Schematic illustration for the conversion from nanorods to nanocubes.
nanostructures in template-free synthesis. As an isotropic crystal,15 CeO2 was preferred to form polyhedral nanoparticles and adscititious surfactants were required to tune the surface free energies and induce the anisotropic growth of well-shaped nanostructures.14,16 In the present work, neither template nor adscititious surfactant is used in the hydrothermal process, which is really convenient and hence suitable for mass production and potential applications. The shape-selective growth of CeO2 nanostructures (nanorods and nanocubes) could be understood as schematically illustrated in Figure 4. As known, cerium(III) could easily combine with OH- to form Ce(OH)3 nuclei in the basic hydrothermal condition. The anisotropic growth of these nuclei resulted in the formation of Ce(OH)3 nanorods for the cases either with cerium chloride precursor or with cerium nitrate precursor within a short reaction time (Figure 1 and Figure S1 of the Supporting Information). As presented in the preceding part, our experimental results revealed that the longtime hydrothermal treatment would generate nanorods together with a few polyhedra for Cl- anions case, while producing nanocubes in the presence of NO3- ions. In addition, we found that the strong basic condition is necessary during the morphological conversion from nanorods to nanocubes. If nitrate salts solution was added into the newly formed Ce(OH)3 nanorods while keeping pH ∼7, no nanocube could be observed. It is believed that the conversion from nanorods to nanocubes involved a dissolution-recrystallization process under the strong basic condition (Figure S4 of the Supporting Information). For the case with Cl- anions, Ce(OH)3 nanorods did not change their morphology with time, except that few CeO2 polyhedron was produced (Figure S5 of the Supporting Information) through the reaction of cerium(III) species and trace oxygen in the solution. This suggests that the dissolution-recrystallization equilibrium on the nanorods surface was not broken probably due to the very weak adsorption of Cl- anions on the nanorods surface. Thus, CeO2 nanorods were obtained through the dehydration and oxidation of the Ce(OH)3 nanorods during drying in air (Line I in Figure 4). When NO3- ions were present in the solution, the dissolution-recrystallization process was largely influenced. During this process, CeO2 nuclei also formed owing to the oxidation by the trace oxygen in the solution. The subsequent growth of isotropic CeO2 nuclei should lead to the formation of polyhedra, whereas nanocubes with well-defined shape formed in this case, which strongly suggested that NO3counteranions played a key role in the growth of nanocubes. The CeO2 polyhedron has various exposed surfaces, and NO3ions were selectively absorbed on the {100} crystalline planes of CeO2 nuclei. The formed adlayer changed the surface free energies of CeO2 crystals and thus kinetically controlled the
J. Phys. Chem. C, Vol. 112, No. 44, 2008 17079
Figure 5. (a) TEM and (b) SEM images of nanostructures obtained by another 48 h hydrothermal treatment after the addition of BrO3ions into the newly formed nanorods. The molar ratio of [BrO3-] to [Ce(III)] is 6.6:1.
growth rates of different facets, leading to the formation of CeO2 nanocubes enclosed by {100} planes (Line II in Figure 4). With respect to the reason for the completely conversion from nanorods to nanocubes in the presence of NO3- ions, we believe the NO3- ions acted as an oxidizer simultaneously in this conversion. The cerium(III) ions dissociated from Ce(OH)3 species could be oxidized to CeO2 species by NO3- ions; meanwhile the selective adsorption of NO3- ions on the CeO2 nuclei could kinetically control the locatable deposition of these CeO2 species, resulting in the nanocube-like products. With the continued oxidation of cerium(III) ions into CeO2 species by NO3- ions, Ce(OH)3 nanorods were slowly diminished and CeO2 nanocubes gradually formed with time. There is a circumstantial evidence to support this speculation. During the morphological conversion process, NaNO3 was replaced by sodium bromate (NaBrO3) and the other procedure was unaltered, and the nanorods were entirely converted into CeO2 nanoparticles owing to the oxidation of bromate ions. Most of these nanoparticles (Figure 5) show irregular shapes rather than the nanocube-like geometries. This may arise from the fact that bromate ions could not be selectively absorbed on certain planes of the CeO2 crystals, and therefore it could not induce the anisotropic growth to produce well-shaped nanocubes. The influence of other anions has been systematically studied, and the morphological conversion from nanorods to CeO2 nanocubes (or nanoparticles) did not occur for the cases using Br-, I-, and SO42- ions (Figure S6 of the Supporting Information). The products still have the rodlike morphologies, which indicated that the shape-controlled growth of CeO2 nanostructures is anion-sensitive. From the preceding results, it is concluded that the controllable synthesis of CeO2 nanostructures is compactly relevant to the counteranions in the hydrothermal reaction, much different with the reference results where the reaction temperature and NaOH concentration were thought of playing key roles in the shape evolution.11b This approach by modulating the anions in the reaction will be an effective and convenient strategy for the controllable synthesis of other faceted nanomaterials. 5. Conclusions CeO2 nanorods and nanocubes were selectively synthesized using cerium(III) chloride and nitrate as precursor respectively. Intriguingly, the as-synthesized nanorods could be converted into nanocubes by the addition of an appropriate amount of NO3- ions into the hydrothermal reaction. The counteranions of the cerium source were crucial to the shapes of the resulting products. It is revealed that nanorods could be obtained when the counteranions were Cl-, Br-, I-, or SO42- ions, whereas nanocubes formed in the presence of NO3- ions, and irregular
17080 J. Phys. Chem. C, Vol. 112, No. 44, 2008 nanoparticles were produced in the presence of BrO3- ions. The anion-induced controllable growth of CeO2 nanomaterials is simple and low cost, which makes this strategy very potential for the preparation of other faceted nanostructures. Acknowledgment. This work was financially supported by the National Basic Research Program of China (2007CB935503), the NSF of China (20601013 and 20525312), and the Foundation of Jiangsu Province (BK2005416). Supporting Information Available: Time-dependent evolution of the product obtained using cerium(III) nitrate as precursor, XRD patterns of the products dried in nitrogen, TEM image of the product obtained with the lower concentration of NO3- ions, TEM evidence for the dissolution-recrystallization process, TEM images of the product with the coexistence of CeO2 nanorods and polyhedra, TEM images of the products obtained in the presence of Br-, I-, or SO42- ions. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153–1175. (b) PerezJuste, J.; Pastoriza-Santos, I.; Liz-Marzan, L. M.; Mulvaney, P. Coord. Chem. ReV. 2005, 249, 1870–1901. (2) (a) Lee, H.; Habas, S. E.; Kweskin, S.; Butcher, D.; Somorjai, G. A.; Yang, P. D. Angew. Chem., Int. Ed. 2006, 45, 7824–7828. (b) Glaspell, G.; Hassan, H. M. A.; Elzatahry, A.; Fuoco, L.; Radwan, N. R. E.; El-Shall, M. S. J. Phys. Chem. B 2006, 110, 21387–21393. (c) Hu, J. T.; Li, L. S.; Yang, W. D.; Manna, L.; Wang, L. W.; Alivisatos, A. P. Science 2001, 292, 2060–2063. (d) Chen, J. Y.; Wiley, B.; Li, Z. Y.; Campbell, D.; Saeki, F.; Cang, H.; Au, L.; Lee, J.; Li, X. D.; Xia, Y. N. AdV. Mater. 2005, 17, 2255–2261. (e) Wang, J. F.; Gudiksen, M. S.; Duan, X. F.; Cui, Y.; Lieber, C. M. Science 2001, 293, 1455–1457. (f) Wang, C.; Daimon, H.; Lee, Y.; Kim, J.; Sun, S. J. Am. Chem. Soc. 2007, 129, 6974–6975. (3) (a) Patolsky, F.; Zheng, G.; Lieber, C. M. Anal. Chem. 2006, 78, 4260–4269. (b) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897– 1899. (c) Wang, Z. L. Annu. ReV. Phys. Chem. 2004, 55, 159–196. (d) Lee, C. J.; Lee, T. J.; Lyu, S. C.; Zhang, Y.; Ruh, H.; Lee, H. J. Appl. Phys. Lett. 2002, 81, 3648–3650. (e) Yin, Y.; Alivisatos, A. P. Nature 2005, 437, 664–670. (4) (a) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700–12706. (b) Ha, T. H.; Koo, H. J.; Chung, B. H. J. Phys.
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JP804140E
