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Preparation and Characterization of Monodisperse Cerium Oxide Nanoparticles in Hydrocarbon Solvents Hua Gu and Mark D. Soucek* Department of Polymer Engineering, UniVersity of Akron, Akron, Ohio 44325 ReceiVed June 7, 2006. ReVised Manuscript ReceiVed January 11, 2007
Uniform-sized monodisperse cerium oxide nanoparticles were prepared in a one-step reaction. A ceriumoleate complex was heated in an organic solvent with high boiling point to decompose and form nanocrystals. The products were characterized by Raman spectroscopy, XRD, UV-visible spectroscopy, room-temperature photoluminescence spectroscopy, and TEM. The TEM images and UV-visible spectra show that the CeO2 particles prepared are uniform nanosized and absorb UV light in the range of 250-400 nm. The size of the nanoparticles prepared can be controlled from 5 to 20 nm by choice of solvent, reaction time, and reactant concentration without any size sorting. The nanoparticles show a strong violet/blue emission at 400 nm in photoluminescence spectrum (PL). The PL emission was found to be sensitive to the particle size.
Background In recent years, nanocrystalline cerium oxide (CeO2) particles have been extensively studied. Cerium oxide has potential uses in many applications, such as UV-blockers and filters,1,2 buffer layers with silicon wafer,3 gas sensors,4 and high refractive index material5 as well as luminescent material.6 Cerium oxide also has application as catalysts in fuel cell technology,7 catalytic wet oxidation,8 engine exhaust catalysts,9 NO removal,10 and photocatalytic oxidation of water.11 Numerous techniques have been developed to synthesize nanosized cerium oxide particles. The previous synthesis strategies of cerium oxide mainly involved use of hightemperature approaches. Ultrafine CeO2 powder has been prepared and used to decrease the sintering temperature from 1500 to 1200 °C.12 CeO2 nanoparticles within 3-12 nm range have been formed by directly mixing equal volumes of Ce(NO3)3 solutions and hexamethylenetetramine at room temperature.13 A hydrothermal process has been employed to produce CeO2 nanoparticles.14,15 Colloidal solutions of (1) Tsunekawa, S.; Fukuda, T.; Kasuya, A. J. Appl. Phys. 2000, 87, 1318. (2) Yamashita, M.; Kameyama, K.; Yabe, S.; Yoshida, S.; Fujishiro, Y.; Kawai, T.; Sato, T. J. Mater. Sci. 2002, 37, 683. (3) Tashiro, J.; Sasaki, A.; Akiba, S.; Satoh, S.; Watanabe, T.; Funakubo, H.; Yoshimoto, M. Thin Solid Films 2002, 415, 272. (4) Garzon, F. H.; Mukundan, R.; Brosha, E. L. Solid State Ionics 2000, 136/137, 633. (5) Mogensen, M.; Sammes, N. M.; Tompsett, G. A. Solid State Ionics 2000, 129, 63. (6) Yu, X. J.; Xie, P. B.; Su, Q. D. Phys. Chem. Chem. Phys. 2001, 3, 5266. (7) Logothetidis, S.; Patsalas, O.; Charitidis, C. Mater. Sci. Eng. C 2003, 23, 803. (8) Larachi, F.; Pierre, J.; Adnot, A.; Bernis, A. Appl. Surf. Sci. 2002, 195, 236. (9) Dario, M. T.; Bachiorrini, A. Ceram. Int. 1999, 25, 511. (10) DiMonte, R.; Fornasiero, P.; Graziani, M.; Kaspar, J. J. Alloys Compd. 1998, 277, 877. (11) Bamwenda, G. R.; Arakawa, H. J. Mol. Catal., A. 2000, 161, 105. (12) Chen, P. L.; Chen, I. W. J. Am. Ceram. Soc. 1993, 76, 1577. (13) Zhang, F.; Chan, S. W.; Spanier, J. E.; Apak, E.; Jin, Q.; Robinson, R. D.; Herman, I. P. Appl. Phys. Lett. 2002, 80, 127.
ultrafine ceria particles have been synthesized by the reaction of cerium metal in 2-methoxyethanol at 200-300 °C which is called solvothermal process.16 Solvothermal synthesis utilizes a solvent under pressures and temperatures above its critical point to increase the solubility of a solid and to accelerate reactions between solids. In addition, homogeneous solution precipitation in the presence of hexamethylenetetramine12 or in alcohol/water mixed solvents17 or urea was used to synthesize CeO2 nanoparticles.18 The sol-gel process has also been used to produce CeO2 nanoparticles, which was prepared via the sol-gel route from aqueous solutions of inorganic precursors of the metal salts Ce(NH4)2(NO3)6.19 Sathyamurthy et al. reported the synthesis of CeO2 nanoparticles using reverse micelles which is also called microemulsion. The microemulsion system consisted of a continuous oil phase like n-octane, a surfactant, and an aqueous solution as the dispersed phase.20 Yin et al. reported the sonochemical synthesis of cerium oxide nanoparticles using cerium nitrate and azodicarbonamide as starting materials with ethylenediamine or tetraalkylammonium hydroxide as additives,21 or in the presence of poly(ethylene glycol) and NaOAc.22 A semibatch reactor method was developed for synthesizing CeO2 nanoparticles at room temperature.23 (14) Hirano, M.; KInagaki, M. J. Mater. Chem. 2000, 10, 473. (15) Masui, T.; Hirai, H.; Imanaka, N.; Adachi, G. J. Mater. Sci. Lett. 2002, 21, 489. (16) Inoue, M.; Kimura, M.; Inui, T. Chem. Commun. 1999, 957. (17) Chen, H.; Chang, H. Colloids Surf., A 2004, 243, 49. (18) Chu, X.; Chung, W.; Schmidt, L. D. J. Am. Ceram. Soc. 1993, 76, 2115. (19) Makishima, A.; Kubo, H.; Wada, K.; Kitami, Y.; Shimohira, T. J. Am. Ceram. Soc. 1986, 69, C127. (20) Sathyamurthy, S.; Leonard, K. J.; Dabestani, R. T.; Paranthaman, M. P. Nanotechnology 2005, 16, 1960. (21) Yin, L.; Wang, Y.; Pang, G.; Koltypin, Y.; Gedanken, A. J. Colloid Interface Sci. 2002, 246, 78. (22) Liao, X. H.; Zhu, J. M.; Zhu, J. J.; Xu, J. Z.; Chen, H. Y. Chem. Commun. 2001, 937. (23) Zhou, X. D.; Huebner, W.; Anderson, H. U. Chem. Mater. 2003, 15, 378.
10.1021/cm061332r CCC: $37.00 © 2007 American Chemical Society Published on Web 02/13/2007
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Hyeon and co-workers have developed a new procedure for the synthesis of monodisperse nanocrystals of metals, metal oxides, and metal sulfides without going through a size-selecting process.24-25 In these syntheses, metal-surfactant complexes generated in situ are thermally decomposed to form monodisperse nanoparticles such as 6-13 nm iron oxide nanoparticles. In addition, they reported that large quantities 40 g batches of uniform-sized magnetite (Fe3O4) nanoparticles and other materials such as MnO and CoO nanoparticles can be prepared in a single reaction without a size-sorting step.26 Instead of using toxic and expensive organometallic compounds, the metal-oleate complex can be prepared by reacting inexpensive and environmentally friendly compounds, namely, metal chlorides and sodium oleate. Then when the material is heated slowly in a solvent with high boiling point, the complexes decompose and form nanocrystals. Even though there are many reports12-16 on the synthesis of CeO2 nanoparticles, it is still difficult to obtain a highquality and stabilized homogeneous cerium oxide nanoparticle dispersion. Microemulsion is an efficient way for producing highly monodispersed CeO2 nanoparticles, but is difficult to scale up commercially. Most reports13-15,17-19 focused on producing CeO2 nanoparticles in aqueous solutions. There are only a few reports about synthesizing stable monodispersed CeO2 nanoparticles in hydrocarbon solvents such as microemulsion,20 and the synthesis of CeO2 nanoparticles in ethylene glycol using Ce(NH4)2(NO3)6 as starting material.27 Herein, we present a method to produce stable monodispersed cerium oxide nanoparticles in hydrocarbon solvents using oleic acid as surfactant. A cerium-oleate complex was decomposed in the solvent with high boiling point to form cerium oxides nanoparticles. Raman spectrum, XRD, TEM, UV-vis absorption spectrum, and photoluminescence spectrum were used to characterize the as-prepared CeO2 nanoparticles. The effects of the temperature, reaction time, and solid concentration on the size of CeO2 particles were studied. Experimental Section Materials. Sodium oleate was purchased from VWR Co. Cerium(III) chloride heptahydrate (CeCl3‚7H2O) and oleic acid were purchased from Aldrich Co. High boiling point solvents octyl ether, hexyl ether, 1-tetradecene, and decalin were purchase from Aldrich Co. The cosolvents dipropylene glycol n-butyl ether and dipropylene glycol monomethyl ether are manufactured by Van Waters & Rogers Inc. The cosolvent 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate is manufactured by Eastman Chemical Co. All chemicals were used as received. Instrumentations. FTIR spectrum was collected on a Nicolet 5SXC Fourier transform infrared spectrophotometer. The spectrum was collected at a 4 cm-1 resolution. The NMR spectrum was recorded on a Gemini 200 spectrometer. The sample was dissolved in chloroform-d. (24) Hyeon, T. S.; Lee, S.; Park, J.; Chung, Y.; Na, H. B. J. Am. Chem. Soc. 2001, 123, 12798. (25) Joo, J.; Na, H. B.; Yu, T.; Yu, J. H.; Kim, Y.-W.; Wu, F.; Zhang, J. Z.; Hyeon, T. J. Am. Chem. Soc. 2003, 125, 11100. (26) Park, J.; Hyeon, T. Nat. Mater. 2004, 3, 891. (27) Ho, C.; Yu, J. C.; Kwong, T.; Mak, A. C.; Lai, S. Chem. Mater. 2005, 17, 4514.
Gu and Soucek Transmission electron microscope (TEM) images were recorded on a FEI Tacnai scanning transmission electron microscope. The resultant cerium oxide nanoparticle solution is too concentrated for TEM; thus, it was diluted with solvent. The diluted solutions were deposited on carbon-coated copper grids for measurements. The solvents were removed in a vacuum oven overnight. UV-vis spectra were recorded on a Perkin-Elmer Model LS-5 spectrometer. Raman spectrum was recorded on a Jobin Yvon T64000 spectrometer equipped with multichannel detector and photomultiplier. XRD was performed on an X-ray diffractometer (Rigaku 12kW Rotaflex) using Cu KR radiation. Room-temperature photoluminescence spectra were collected on a Shimadzu UV2401Fluorolog-3 spectrometer. XPS spectrum was collected on a Physical Electronics 5600 X-ray photoelectron spectrometer (ESCA). ESEM-EDS measurements were performed on a FEI Quanta 200 environmental scanning electron microscope with energy dispersive spectroscope attachment. Synthesis of Cerium-Oleate Complex (1). The cerium-oleate complex was prepared by reacting sodium oleate (12.2 g, 40.5 mmol) with cerium chloride heptahydrate (5.0 g, 13.5 mmol) in mixture solvent (40 mL of ethanol + 30 mL of water + 70 mL of hexane) at 70 °C for 4 h. When the reaction was completed, the upper organic layer containing the cerium-oleate complex was washed three times with 20 mL of distilled water in a separatory funnel. After washing, hexane was removed by evaporation; light yellow waxy solids were obtained. It was characterized by FTIR and NMR spectrum. Synthesis of Cerium Oxide Nanoparticles in Octyl Ether Using Oleic Acid as Surfactant at the Reflux Temperature. The cerium-oleate complex (0.5 g, 0.5 mmol) and oleic acid (0.08 mL, 0.25 mmol) were dissolved in 40 mL of octyl ether (bp ) 290 °C). The solution was heated slowly to 290 °C under Ar gas. Then, the solution was kept at this temperature for 4 h. The reaction system was cooled to room temperature. The resultant solution was dark brown and transparent and contained 1 wt % solids. After the reaction, ethanol was added to the cooled solution to induce the precipitation of the nanoparticles, which were separated from the solvent by centrifugation at 5000 rpm for 10 min. A brown powder (2) was obtained. It was characterized by Raman spectrum and wide-angle X-ray diffraction spectrum (XRD). Synthesis of Cerium Oxide Nanoparticles in 1-Tetradecene Using Oleic Acid as Surfactant at the Reflux Temperature. The cerium-oleate complex (0.2 g, 0.2 mmol) and oleic acid (0.04 mL, 0.1 mmol) were dissolved in 40 mL of 1-tetradecene (bp ) 251 °C). The solution was heated slowly to 250 °C under Ar gas. Then, the solution was kept at this temperature for 2 h. The reaction system was cooled to room temperature. The resultant solution was colorless and transparent and contained 1 wt % solids. Synthesis of Cerium Oxide Nanoparticles in Decalin Using Oleic Acid as Surfactant at the Reflux Temperature. The cerium-oleate complex (0.2 g, 0.2 mmol) and oleic acid (0.04 mL, 0.1 mmol) were dissolved in 40 mL of decalin (bp ) 189-191 °C). The solution was heated slowly to 180 °C under Ar gas. Then, the solution was kept at this temperature for 5 h. The reaction system was cooled to room temperature. The resultant solution was pale yellow and transparent and contained 1 wt % solids. Synthesis of Cerium Oxide Nanoparticles in Cosolvent Dipropylene Glycol Monomethyl Ether Using Oleic Acid as Surfactant at the Reflux Temperature. The cerium-oleate complex (0.2 g, 0.2 mmol) and oleic acid (0.04 mL, 0.1 mmol) were dissolved in 40 mL of dipropylene glycol monomethyl ether (bp ) 185 °C). The solution was heated slowly to 185 °C under Ar gas. Then, the solution was kept at this temperature for 10 h.
CeO2 Nanoparticles in Hydrocarbon SolVents
Figure 1. FTIR spectrum of Ce-oleate.
The reaction system was cooled to room temperature. The resultant solution was dark brown and transparent and contained 1 wt % solids. 5 wt % solids solution was also synthesized using the same procedure but with the shorter reaction time of 1 h. More ceriumoleate complex (0.8 g, 0.8 mmol) was added in the same amount of solvent. Synthesis of Cerium Oxide Nanoparticles in Cosolvent Dipropylene Glycol n-Butyl Ether Using Oleic Acid as Surfactant at the Reflux Temperature. The cerium-oleate complex (0.2 g, 0.2 mmol) and oleic acid (0.04 mL, 0.1 mmol) were dissolved in 40 mL of dipropylene glycol n-butyl ether (bp ) 230 °C). The solution was heated slowly to 225 °C under Ar gas. Then, the solution was kept at this temperature for 1 h. The reaction system was cooled to room temperature. The resultant solution was dark brown and transparent and contained 1 wt % solids. Synthesis of Cerium Oxide Nanoparticles in Cosolvent 2,2,4Trimethyl-1,3-pentanediol Monoisobutyrate Using Oleic Acid as Surfactant at the Reflux Temperature. The cerium-oleate complex (0.2 g, 0.2 mmol) and oleic acid (0.04 mL, 0.1 mmol) were dissolved in 40 mL of 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate (bp ) 255 °C). The solution was heated slowly to 255 °C under Ar gas. Then, the solution was kept at this temperature for 1 h. The reaction system was cooled to room temperature. The resultant solution was yellow and transparent and contained 1 wt % solids. 5 wt % solids solution was also synthesized using the same procedure. The resultant solution was dark brown.
Results and Discussion Cerium oxide nanoparticles were prepared by decomposing cerium-oleate complex in hydrocarbon solvents with high boiling point. In the previous studies, the syntheses of iron oxide, manganese oxide, and cobalt oxide nanoparticles by this method were reported.26 There are many reports on preparing cerium oxide nanoparticles in aqueous solutions.13,16,18,21 The objective of this study is to prepare stable uniform-sized cerium oxide nanoparticles in hydrocarbon solvents. The effects of different solvents on the size of the nanoparticles were also investigated to control particle size. Characterization of Synthesized Compounds 1 and 2. Cerium-oleate complex (1) was prepared by reacting sodium-oleate with cerium chloride heptahydrate. The product was characterized by FTIR (Figure 1), 1H NMR
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(Figure 2), and 13C NMR (Figure 3) spectra. In the FTIR spectrum, the peak at 1550 cm-1 belongs to carboxylate vibration (CO2) stretching. And the peak at 676 cm-1 can be assigned to the Ce-O-C vibrational stretch. From the 13 C NMR spectrum, it can be seen that there is a small resonance at about 185 ppm attributed to carboxylate group in the cerium-oleate complex. The cerium oxide powder (2) was separated from solvent octyl ether by centrifugation and characterized by FTIR spectrum, wide-angle XRD (Figure 4), and Raman spectrum (Figure 5). The FTIR spectrum shows a peak around 400 cm-1 which is assigned to Ce-O stretching band.27 The XRD spectrum is consistent with the spectrum of pure CeO2 in the literature,28 which indicates the fluorite structure29 of prepared CeO2 nanoparticles. The Raman spectrum also agrees well with that of the pure CeO2 nanoparticles in the literature.26 The broad peaks in both spectra indicate that the size of the particles is small. The EDS spectrum shown in Figure 6 confirms there are cerium and oxygen in the sample. These results show that the cerium oxide as-prepared is CeO2. However, pure CeO2 is white powder. It may be explained that the composition of cerium oxide is CeO2-x. As the particle size gets smaller, the larger that x becomes; thereby, the larger Ce3+ ions replace the smaller Ce4+ ions as x increases.30 Therefore, the product as-prepared consists of not only cerium(IV) oxide but also cerium(III) oxide which makes the powder a brown color. To further confirm this result, XPS spectrum (Figure 7) was recorded. Two strong peaks at around 917 and 899 eV in the XPS spectrum were assigned to Ce 3d5/2 for the Ce4+ state. The spectrum also shows two peaks located at around 903 and 885 eV, which were assigned to Ce 3d3/2 for the Ce3+ state.31 The typical satellite peaks of the “shape-down” type indicate that the as-prepared nanoparticles contain Ce4+ and Ce3+ two oxidation states.32 TEM, UV-Vis, and Photoluminescence Characterization of Cerium Oxide Nanoparticle Solutions. Figure 8 presents the TEM images of cerium oxide nanoparticles dispersed in different high boiling point solvents. It can be observed that the particles are well-dispersed in the solvents (see Figure 8d). In hydrothermal process, it is difficult to obtain well-dispersed particles because agglomeration and subsequent precipitation occurred as soon as the particles formed. In this method, 1 wt % oleic acid was added as surfactant to help stabilize particles and prevent further particle growth. Figure 8a is a TEM image of cerium oxide particles dispersed in solvent octyl ether (bp 290 °C), Figure 8b is the cerium oxide nanoparticles in 1-tetradecene (bp 252 °C), and Figure 8c is in decalin (bp 191 °C). In these micrographs, the particle size is 10-20 nm in octyl ether and 1-tetradecene and is 5-8 nm for decalin. This indicates (28) Orel, Z. Internet J. Vib. Spectrosc. 1999, 3, 4. (29) X-ray Powder Diffraction Standards; ASTM: Philadelphia, PA; Card 34-394 (CeO2). (30) Zhang, F.; Chan, S.; Spanier, J. E.; Apak, E.; Jin, Q.; Robinson, R. D.; Herman, I. P. Appl. Phys. Lett. 2002, 80, 127. (31) Mullins, R. D. Surf. Sci. 1998, 409, 307. (32) Izaki, M.; Sato, T.; Chigane, M.; Ishikawa, M.; Katayama, J.; Inoue, M.; Yamashita, M. J. Mater. Chem. 2001, 11, 1972.
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Figure 2.
1H
Figure 3.
13C
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NMR spectrum of Ce-oleate.
NMR spectrum of Ce-oleate.
that uniform-sized particles can be formed by this method, and more importantly the particle size can be controlled by using different solvents. It was also observed that the reaction was slower with lower boiling point solvents. The cerium oxide nanoparticles were also produced in amphiphilic cosolvents such as dipropylene glycol monomethyl ether, dipropylene glycol n-butyl ether, and 2,2,4trimethyl-1,3-pentanediol monoisobutyrate which have miscibility in polar and hydrogen bonding solvents. The TEM images of cerium oxide nanoparticles in different cosolvents are shown in Figure 9. Figure 9a is the TEM image of the particles in dipropylene glycol monomethyl ether, Figure 9b
is the image of the particles in dipropylene glycol n-butyl ether, and Figure 9c is the particles in 2,2,4-trimethyl-1,3pentanediol monoisobutyrate. The solids percentages are all ∼1 wt %. It can be found that the size of particles in dipropylene glycol monomethyl ether and 2,2,4-trimethyl1,3-pentanediol monoisobutyrate is larger, which is about 30-60 nm; the size of particles in dipropylene glycol n-butyl ether is smaller, which is about 10 nm. The difference in size can be attributed to the experimental conditions. The reaction in dipropylene glycol monomethyl ether was heated at reflux temperature of 185 °C for 10 h. This is much longer than the reaction in 2,2,4-trimethyl-1,3-pentanediol monoisobu-
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Figure 4. XRD spectrum of cerium oxide solid powder. Inset picture: The top spectrum is from pure CeO2.26
Figure 5. Raman spectrum of cerium oxide solid powder. Inset: The top spectrum is from pure CeO2.26
Figure 6. EDS spectrum of cerium oxide solid powder.
tyrate and dipropylene glycol n-butyl ether, which were heated at reflux temperatures 255 and 225 °C for 1 h,
respectively. It can be concluded from this result that either longer heating time or higher temperature will produce larger
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Figure 7. XPS spectrum of cerium oxide solid powder.
Figure 8. (a) TEM image of cerium oxide particles in octyl ether; (b) TEM image of cerium oxide particles in 1-tetradecene; (c) TEM image of cerium oxide particles in decalin; (d) enlarged TEM image of cerium oxide particles in decalin.
size particles. 0.1 wt % oleic acid was used as surfactant in these solutions, which are pretty stable. The solutions stay clear for more than 1 month. Higher solids concentration solutions were also synthesized. 5 wt % solids in 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate and 5 and 10 wt % solids in dipropylene glycol monomethyl ether were produced. Figure 9d shows the TEM image of the particles in 10 wt % solids dipropylene glycol monomethyl ether solution. The size of particles is about 60-90 nm, which is larger than the particles in 1 wt % solids dipropylene glycol monomethyl ether solution (Figure 9a). This result indicates that higher solids concentration will also result in larger particles. Above ∼5 wt % solids nanoparticle solution, the particles are not stable and precipitate within 3 days.
In Figures 10 and 11, the UV-visible spectra of the cerium oxide nanoparticles in different solvents and cosolvents are shown, respectively. It can be observed that, in each spectrum, most of the UV light (200-350 nm) is blocked. As expected, with different solvents, there is a little shift in UV absorption. The solutions have different colors, which may represent the different components in cerium oxide nanoparticles. The solution of octyl ether is dark brown, 1-tetradecene is colorless, and decalin is pale yellow. The darker the color, the more Ce(III) in the particles, making the UV absorption shift to higher wavelength. Since CeO2 is a direct band gap semiconductor, a decrease in particle size is expected to be manifested by a blue shift of the absorption edge.33 According to TEM results, the particles produced in decalin are smaller than those in octyl ether. It
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Figure 9. TEM images of nanoparticles in different cosolvents: (a) in DPM, 1 wt % solids; (b) in dipropylene glycol n-butyl ether, 1 wt % solids; (c) in 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate, 1 wt % solids; (d) in dipropylene glycol monomethyl ether, 10 wt % solids.
Figure 10. UV-vis spectra of cerium oxide nanoparticles in different solvents: (a) octyl ether; (b) 1-tetradecene; (c) decalin.
can be observed that there is a blue shift from the UV spectrum of particles in octyl ether to the spectrum in decalin. Figure 11 presents the UV-vis spectra of cerium oxide nanoparticles produced in cosolvents. Most of the UV light (250-370 nm) is absorbed. It also can be observed that UV absorption edge shifts to lower wavelength due to the decrease of the nanoparticle size. Comparing the TEM images of the particles in solvent such as octyl ether (Figure 8) and cosolvents (Figure 9), the size of particles in solvent such as octyl ether is smaller than that of particles in cosolvents and is more uniform. It may be due to the solubility of cerium-oleate in these solvents. Cerium-oleate is more soluble in those less polar solvents (like octyl ether) than cosolvents. An attempt was made to synthesize cerium oxide nanoparticles in cosolvents ethylene (33) Brus, L. E. J. Phys. Chem. 1986, 90, 2555.
Figure 11. UV-vis spectra of cerium oxide nanoparticles in different cosolvents: (a) in dipropylene glycol monomethyl ether; (b) in 2,2,4trimethyl-1,3-pentanediol monoisobutyrate; (c) in dipropylene glycol n-butyl ether.
glycol and propylene glycol. The starting material ceriumoleate did not dissolve in these two solvents even at higher temperature. Ethylene glycol and propylene glycol are more polar solvents than dipropylene glycol monomethyl ether, dipropylene glycol n-butyl ether, and 2,2,4-trimethyl-1,3pentanediol monoisobutyrate. It confirms that the polarity of solvent may affect the formation of nanoparticles. Figure 12b-e shows the room-temperature photoluminescence (PL) spectra of the cerium oxide nanoparticle dispersion in different solvents. The concentrations of these dispersions were all diluted to 1.0 × 10-3 M. Figure 12a presents the PL spectrum of the cerium oxide microparticles. The particle size decreases from dispersion (a) to (e); the emission peaks were found to gradually shift toward short wavelength located at 430, 395, 369, 364, and 355 nm, respectively. The violet/blue light emission peaks at 395, 369,
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advantage of producing well-dispersed CeO2 nanoparticles in organic solvents is that the particles can be easily incorporated into organic polymeric matrixes for use as UV blockers, high refractive index materials, or photoluminescent materials. Another advantage is that the method can also be performed in large scale. The size of nanoparticles can be controlled by choice of hydrocarbon solvent. As a consequence, the PL emission of CeO2 nanoparticles can be controlled. Conclusions
Figure 12. (a) Room-temperature photoluminescence spectra of the CeO2 microparticles with an excitation wavelength of 290 nm; (b)-(e): roomtemperature photoluminescence spectra of the diluted CeO2 nanoparticle dispersions in (b) 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate, (c) dipropylene glycol monomethyl ether, (d) 1-tetradecene, and (e) decalin.
364, and 355 nm observed with decreased particle size could be explained by charge transitions from the 4f band to the valence band of CeO2.34 Cerium oxide is a wide band gap compound semiconductor, whose gap is about 5.5 eV. It is easy to observe the hopping from Ce 4f to O 2p (>3 eV). In addition, the defect levels localized between the Ce 4f band and the O 2p band can result in wider emission bands.35,36 This is the first report of stabilized cerium oxide nanoparticles that are well-dispersed in hydrocarbon solvents. The (34) Gao, F.; Li, G. H.; Zhang, J. H.; Qin, F. G.; Yao, Z. Y.; Liu, Z. K.; Wang, Z. G.; Lin, L. Y. Chem. Phys. Lett. 2001, 18, 443.
Cerium oxide nanoparticles were successfully synthesized by decomposing cerium-oleate complex in solvents with high boiling point at higher temperature. The XPS spectrum indicates that the cerium oxide solid powder as-prepared is a mixture of cerium(IV) oxide and cerium(III) oxide. The TEM images and UV-vis spectra show that the uniformsized nanoparticles were obtained and are capable of blocking most UV spectra. It also has been found that the particle size can be controlled by reaction conditions, especially choice of solvent. However, larger particle size was obtained with longer reaction time and higher solids concentration. The nanoparticles exhibited room-temperature photoluminescence (PL). The PL emission was found to exhibit particle-size-dependent blue shift (hypsochromic shift) toward shorter wavelengths as the particle size decreases. CM061332R (35) Marabeli, F.; Wachter, P. Phys. ReV. B. 1987, 36, 1238. (36) Cai, C. L.; Yang, S. Y.; Liu, Z. K.; Liao, M. Y.; Chen, N. F. Chin. Sci. Bull. 2003, 48, 780.
