Precipitation of Nanocrystalline CeO2 Using Triethanolamine

Langmuir 2009, 25, 67-70

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Precipitation of Nanocrystalline CeO2 Using Triethanolamine Ranjan K. Pati, Ivan C. Lee, Karen J. Gaskell, and Sheryl H. Ehrman* Department of Chemical and Biomolecular Engineering, UniVersity of Maryland, College Park, Maryland 20742, U.S. Army Research Laboratory, Sensors and Electron DeVices Directorate, Adelphi, Maryland 20783, and Department of Chemistry and Biochemistry, UniVersity of Maryland, College Park, Maryland 20742 ReceiVed September 24, 2008. ReVised Manuscript ReceiVed NoVember 21, 2008 Synthesis of cerium oxide nanocrystallites via precipitation using triethanolamine is reported. The molecular water associated with the cerium nitrate precursor is exploited to generate hydroxyl ions with the help of triethanolamine, facilitating precipitation. The small crystallite diameter (3 nm) in the as prepared powder is believed to result from the limited amount of water present. Solvent type has no effect on the final crystallite size or structure; however, it plays an important role in the dispersion of the nanoparticles with dispersity of the particles increasing with increasing carbon chain length of the solvent alcohol used.

Introduction In recent years, cerium oxide (CeO2) nanoparticles have attracted considerable attention because of their vast technological applications, such as electrolytes for solid oxide fuel cells,1,2 catalysts for the water gas shift reaction,3 abrasives for chemical-mechanical planarization,4,5 gas sensors,6 ultraviolet absorbents for sunscreens,7 hybrid solar cells,8 and sorbents for H2S removal.9 These applications mainly exploit the high oxygen storage capacity and high ionic conductivity of ceria when doped with other materials.10 These properties are strongly dependent on the size and size distribution of ceria,11 among other variables. Numerous solution-based chemical methods have been reported for the preparation of nanocrystalline ceria, including precipitation using ammonium hydroxide12 and urea,13hexamethylenetetramine,12 hydrothermal,14 alkoxide,15 combustion,16 and electrochemical methods.17 In all these methods, water was used as the solvent. One main disadvantage of these methods is the formation of hard agglomerates, mainly due to the bridging of adjacent particles with water by hydrogen bonds and the * To whom correspondence should be addressed. Telephone: +1 301 405 1917. Fax: +1 301 405 0523. E-mail: [email protected]. (1) Murray, E. P.; Tsai, T.; Barnett, S. A. Nature 1999, 400, 649–651. (2) Steele, B. C. H. Solid State Ionics 2000, 129, 95–110. (3) Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Science 2003, 301, 935–938. (4) Lu, Z. Y.; Lee, S. H.; Gorantla, V. R. K.; Babu, S. V.; Matijevic, E. J. Mater. Res. 2003, 18, 2323–2330. (5) Shchukin, D. G.; Caruso, R. A. Chem. Mater. 2004, 16, 2287–2292. (6) Garzon, F. H.; Mukundan, R.; Brosha, E. L. Solid State Ionics 2000, 136, 633–638. (7) Imanaka, N.; Masui, T.; Hirai, H.; Adachi, G. Chem. Mater. 2003, 15, 2289–2291. (8) Lira-Cantu, M.; Krebs, F. C. Solar Energy Mater. Solar Cells 2006, 90, 2076–2086. (9) Flytzani-Stephanopoulos, M.; Sakbodin, M.; Wang, Z. Science 2006, 312, 1508–1510. (10) Zhang, X.; Klabunde, K. J. Inorg. Chem. 1992, 31, 1706–1709. (11) Spanier, J. E.; Robinson, R. D.; Zheng, F.; Chan, S. W.; Herman, I. P. Phys. ReV. B 2001, 64, 245407. (12) Chen, P. L.; Chen, I. W. J. Am. Ceram. Soc. 1993, 76, 1577–1583. (13) Aiken, B.; Hsu, W. P.; Matijevic, E. J. Am. Ceram. Soc. 1988, 71, 845– 853. (14) Hirano, M.; Kato, E. J. Am. Ceram. Soc. 1996, 79, 777–780. (15) Nakagawa, K.; Murata, Y.; Kishida, M.; Adachi, M.; Hiro, M.; Susa, K. Mater. Chem. Phys. 2007, 104, 30–39. (16) Bera, P.; Gayen, A.; Hegde, M. S.; Lalla, N. P.; Spadaro, L.; Frusteri, F.; Arena, F. J. Phys. Chem. B 2003, 107, 6122–6130. (17) Zhou, Y. C.; Phillips, R. J.; Switzer, J. A. J. Am. Ceram. Soc. 1995, 78, 981–985.

subsequently large capillary forces produced during drying.18 As a result, nanocrystalline powders produced by the alkoxide method generally show much higher reactivity toward densification than those produced via aqueous routes. Li et al. developed a mimic alkoxide method using diethylamine for the preparation of nanocrystalline CeO2. In this method, hydroxyl ions are produced by the reaction of diethylamine and molecular water associated with the precursor.19 Pure cubic phase CeO2 was detected at a calcination temperature of 400 °C/2 h having the crystallite size of ∼8 nm. Use of triethanolamine in place of diethylamine would be a better choice, because triethanolamine not only releases the OH- ion for precipitation from molecular water, but also helps to reduce the oxide formation temperature,20 resulting in smaller crystallites. Both diethylamine and triethanolamine act in the same manner, releasing OH- ion from the molecular water associated with the precursor material. Additionally, triethanolamine can form a polymeric complex with cerium.21 In the present study, we report a precipitation method for the preparation of CeO2 nanoparticles consisting of small aggregates of crystallites that are highly uniform in size. In this process, triethanolamine is used as a precipitating agent. The molecular water associated with the precursor material (cerium nitrate hexahydrate) plays an important role in making the oxide material.

Experimental Section The cerium source and precipitant for powder synthesis were cerium nitrate hexahydrate (Ce(NO3)3 · 6H2O, 99%, Aldrich Chemicals, USA) and triethanolamine ((C2H5OH)3N, 98%, Aldrich Chemicals, USA), respectively. Anhydrous ethanol (99.9%, Fisher Chemicals), 1-propanol (99.9%, Fisher Chemicals), and 1-butanol (99.9%, Fisher Chemicals) were used as solvents. Stock solutions were made by dissolving the cerium nitrate hexahydrate and triethanolamine into each solvent, and the final concentrations were 0.1 M for cerium nitrate solution and 0.4 M for triethanolamine solution. Precipitation was performed at room temperature. 100 mL of 0.1 M cerium nitrate solution was dripped at a rate of 3-4 mL/min from (18) Kaliszewski, M. S.; Heuer, A. H. J. Am. Ceram. Soc. 1990, 73, 1504– 1509. (19) Li, J. G.; Ikegami, T.; Lee, J. H.; Mori, T. Acta Mater. 2001, 49, 419–426. (20) Pati, R. K.; Pramanik, P. J. Am. Ceram. Soc. 2000, 83, 1822–1824. (21) Guran, C.; Jitaru, I.; Meghea, A.; Popescu, M.; Popescu, V. ReVue Roumaine De Chimie 1997, 42, 289–295.

10.1021/la8031286 CCC: $40.75  2009 American Chemical Society Published on Web 12/05/2008

68 Langmuir, Vol. 25, No. 1, 2009 a burette into 100 mL of 0.4 M triethanolamine precipitant solution contained in an Erlenmeyer flask under mild stirring. The stirring was continued for 1 h after the completion of precipitation, and the solution was filtered by a suction filter using Whatman 42 (pore diameter 2.5 µm) filter paper. The completion of precipitation was confirmed by adding 10 mL of filtrate with ammonium hydroxide solution. The precipitate was washed with the solvent alcohol before drying at room temperature for 24 h. The resulting powders were then washed with acetone to remove the remaining alcohol, unreacted triethanolamine, and other byproduct. The as-prepared materials were then heated at a rate of 10 °C/min and calcined at 200 °C for 3 h. The as-prepared and calcined materials were characterized by powder X-ray diffraction (XRD, Bruker AXS D8 Advance) using Cu KR (λ ) 1.5408 Å) incident radiation for crystal structure determination. The crystallite diameter of the CeO2 powder was estimated from the Scherrer equation.22 Surface area of the samples was measured using the BrunauerEmmett-Teller gas absorption method (BET, Micromeritics ASAP 2010C). High-resolution transmission electron microscopy (HRTEM, JEOL 2100) was used for determination of morphology including crystallite size analysis. Thermogravimetric (TG, Perkin-Elmer thermogravimetric analyzer TGA-7) analysis was performed in air at a heating rate of 10 °C/min to identify the presence of impurities, and the thermal behavior was examined by differential scanning calorimetric (DSC, 2920 Modulated DSC, TA Instruments) analysis in the as-prepared materials. The oxidation states of cerium in the as-prepared and calcined samples were detected using X-ray photoelectron spectroscopy (XPS, Kratos AXIS 165) using Al KR radiation (1486.6 eV). The instrument was run in hybrid mode with a pass energy of 160 eV for survey spectra and a pass energy of 20 eV for high-resolution spectra. The size distribution of the particles in solution was determined via dynamic light scattering (DLS, He-Ne laser, 05-LHP-121, Melles Griot and FEU-79 photon counter). A rough estimate of the packing density was made by hand-packing the powder into a preweighed 1 mL pipet and weighing the pipet and powder.

Results In the preparation of nanocrystalline CeO2 powder, the solution exhibited several color changes during the precipitation process. The color changes are due to the light absorption by the ligand field of the corresponding complex formed during each step of the reaction. Initially, the colorless solution turned yellow, which is probably due to the formation of cerium hydroxide,12 and then the color changed from yellow to light khaki yellow at the end of the reaction, which is due to the formation of CeO2. Figure 1a,b,c shows the HRTEM images of the ceria crystallites made by using ethanol, 1-propanol, and 1-butanol as the solvents, respectively. Here, we define crystallites to be the individual grains, whereas we use particles to denote the aggregates of crystallites. The crystallites appear aggregated in each case having an average crystallite diameter of 3 nm with a standard deviation of 1 nm. This result is consistent for all the as-prepared CeO2 samples, which indicates that there is not a significant effect of solvent alcohol on crystallite size. The selected area electron diffraction (SAED) patterns indicate the presence of {200}, {220}, and {311} planes for all the as-prepared samples. The interfringe distances of the dominant 2D lattice fringes of the HRTEM images are 0.267 nm, which are close to the {200} lattice spacing of the cubic phase of CeO2 at 0.271 nm. The dispersion of the particles made using different solvents is confirmed by dynamic light scattering (DLS). The average hydrodynamic radii of the particles prepared by ethanol are 72 nm with a standard deviation of 20 nm, 1-propanol 50 nm with (22) Klug, P.; Alexander, L. E. Diffraction Procedures for Polycrystalline and Amorphous Materials, 1st ed.; Wiley: New York, 1954.

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Figure 1. HRTEM images and selected area electron diffraction (SAED) patterns of the CeO2 crystallites prepared using (a) ethanol, (b) 1-propanol, (c) 1-butanol, and (d) 1-butanol with calcination at 200 °C/3 h.

a standard deviation of 18 nm, and 1-butanol 14 nm with a standard deviation of 8 nm. This result indicates the agglomeration of particles with small crystallites. However, the extent of agglomeration is smaller compared to the larger yttria-doped ceria agglomerate size of 1.5 µm having crystallite size of 6 nm prepared by Chavan et al.23 The DLS result also indicates that, as the carbon chain length of the solvent alcohol increases, the dispersion of the particles increases. A similar result was found by Ikegami et al., who studied the dispersion of Al(OH)3 precipitated in alcohols.24 This is because ethanol, 1-propanol, and 1-butanol all have some affinity to water and can be dissolved at low concentrations in water to form homogeneous solutions.18 However, as the carbon chain length increases, steric effects increase, which prevents the close approach of individual crystallites and increases the dispersion of the resultant powders. Figure 1d shows the HRTEM and SAED images of CeO2 crystallites calcined at 200 °C/3 h, made using 1-butanol as the solvent. It is clear from the images that the material is crystalline with the presence of cubic phase of CeO2, which is also confirmed by X-ray diffraction study (Figure 2b). Figure 2a shows the X-ray diffraction results for the as-prepared powder. The material is crystalline, but the diffraction lines do not match with any known cerium compound in the database. However, the diffraction pattern of the as-prepared material is identical to that of [La(TEA)2(NO3)](NO3)2,25 indicating the possible formation of Ce(TEA)2(NO3)](NO3)2. Therefore, it is possible that the asprepared material is a mixture of [Ce(TEA)2(NO3)](NO3)2, Ce(OH)4, and CeO2 · nH2O. Calcination at 200 °C for 3 h helps the material to crystallize to pure CeO2 (Figure 2b) with the (23) Chavan, S. V.; Tyagi, A. K. J. Mater. Res. 2004, 19, 474–480. (24) Ikegami, T.; Saito, N. J. Ceram. Soc. Jpn. 1996, 104, 469–470. (25) Fowkes, A.; Harrison, W. T. A. Crystallogr. Acta, Sect. C: Cryst. Struct. Commun. 2006, C62, m232–m233.

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Figure 2. X-ray diffraction study of the CeO2 powders (a) as-prepared and (b) calcined at 200 °C/3 h. * indicates peaks associated with the cubic phase of CeO2.

presence of all the characteristic peaks of the cubic phase. The lower formation temperature is because of the heat released during calcination of the as-prepared powder containing triethanolamine.20 The crystallite diameter, estimated from the Scherrer equation, was approximately 5 nm, which is similar to the crystallite size obtained from the HRTEM image of the calcined sample (Figure 1d). The crystallite diameter could not be estimated for the as-prepared samples, since there is no distinct peak observed in the XRD pattern (Figure 2a). The BET surface areas of the calcined CeO2 powders were 78 m2/g, 80 m2/g, and 84 m2/g with ethanol, 1-propanol, and 1-butanol used as solvents, respectively. The particle diameter, calculated using the BET surface areas of the calcined materials assuming spherical particles, are 10, 9.8, and 9.3 nm for ethanol, 1-propanol, and 1-butanol used as solvents, respectively. This again indicates that the solvent type has little effect on the particle diameter and surface area of the materials. This diameter, slightly greater than the crystallite diameters obtained using TEM and XRD, reflects the aggregated state of the crystallites, as not all crystallite surfaces in the aggregate are available for nitrogen absorption. Figure 3a shows the thermogravimetric analysis (TGA) of the material prepared using 1-butanol as the solvent. There are twostep weight losses observed in the analysis. The first weight loss between 200 and 350 °C may be due to the loss of water, nitrate ions, and triethanolamine, and the second weight loss between 350 and 600 °C is attributed to the loss of carbon. After 600 °C, no significant weight loss was observed, which confirms the formation of impurity-free CeO2. The calculated weight loss from [Ce(TEA)2(NO3)](NO3)2 to CeO2 was about 72% and Ce(OH)4 to CeO2 was 18%. However, the total weight loss observed from TGA was ∼58%, which indicates the presence of a mixture of [Ce(TEA)2(NO3)](NO3)2, Ce(OH)4, and hydrated CeO2 in the as-prepared samples. The DSC result (Figure 3b) indicates that there was an endothermic peak at 173 °C when the sample was heated from 40 to 250 °C. As comparison in XRD, the annealed sample did not show any endothermic peak (Figure 3c). As shown in Figure 2b, the CeO2 phase was formed when the as-prepared sample was annealed at 200 °C. Therefore, the XRD and DSC results (Figure 3c) suggest that an irreversible phase transition occurred between 150 and 180 °C in which the CeO2 is formed. XPS survey revealed that the as-prepared sample contained cerium, oxygen, nitrogen, and some carbon due to adventitious hydrocarbon species (XPS spectra and details regarding peak fitting can be found in the Supporting Information). After calcination at 200 °C, the nitrogen signal disappeared, and only cerium, oxygen, and a small amount of carbon remained. Cerium 3d spectra were examined in detail to determine the oxidation

Figure 3. (a) Thermogravimetric analysis (TGA) and (b) differential scanning calorimetric (DSC) analysis of the as-prepared sample made with 1-butanol as the solvent. (c) DSC analysis of the calcined (200 °C) sample.

state of cerium. The determination of oxidation state is complicated by the photoreduction of cerium26 during the XPS experiment, so to try to minimize this reduction, oxidation states were determined from spectra collected after the samples had been under X-radiation for less than 15 min. Both the as-prepared and the calcined samples were found to contain a mixture of Ce3+ and Ce4+ oxidation states, with the as-prepared sample having 81% Ce3+ and 19% Ce4+ and the calcined sample 33% Ce3+ and 67% Ce4+. This result again confirms that the asprepared sample may be a mixture of [Ce(TEA)2(NO3)](NO3)2, Ce(OH)4, and CeO2 · nH2O.

Discussion Generally, in the formation of CeO2 via precipitation processes using ammonium hydroxide or hexamethylenetetramine, a series of chemical reactions take place: oxidation of Ce3+ to Ce4+, hydration of Ce4+ followed by the formation of a [Ce(H2O)x(OH)y](4-y)+ complex (color change from colorless to pinkish purple), deprotonation of the hydrated complex to form hydrated CeO2 (color change from pinkish purple to khaki yellow), and finally dehydration of the hydrated CeO2 to form CeO2 powder (color change from khaki yellow to whitish yellow).12 It is reported that, during precipitation, the deprotonation process occurs very slowly,12 and consequently, a high temperature of 70-85 °C is needed to complete the deprotonation reaction depending on the precipitation agent used. Again, because of the strong basicity and higher charge, Ce4+ ions usually undergo strong hydration resulting in the formation of [Ce(H2O)x(OH)y](4-y)+ complex, where (x + y) is the coordination number of Ce4+. In the present study, since there was no pinkish purple color noticed, it is expected that Ce4+ did not form any complex with H2O and OH-, although both H2O and OH- were present in the (26) Park, P. W.; Ledford, J. S. Langmuir 1996, 12, 1794.

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solution. Additionally, XRD confirms the possible formation of [Ce(TEA)2(NO3)](NO3)2 complex during precipitation. Therefore, it can be suggested that the formation of [Ce(H2O)x(OH)y](4-y)+ and [Ce(TEA)2(NO3)](NO3)2 complexes occur simultaneously. It should be mentioned here that, following oxidation, Ce4+ needs only four molecules of hydroxyl ions for precipitation, which are available as six water molecules associated with each cerium nitrate molecule. Therefore, it is expected that a slight excess of water was present to facilitate the precipitation. At the beginning, as noted previously, the color changes observed in the synthesis were the colorless precipitant solution immediately changing to yellow. The precipitate gradually thickened with the continuous addition of cerium nitrate solution and became light khaki yellow at the end of the reaction. This may be due to complex formation and deprotonation, which were occurring simultaneously. Solvent alcohols are not likely to participate in this fast deprotonation because of their weaker molecular polarity compared to water. A similar result was observed by Li et al. for the preparation of CeO2 using diethylamine.19 However, deprotonation by triethanolamine and diethylamine is possible, as it easily forms hydrogen bonds with water and can take away protons from the [Ce(H2O)x(OH)y](4-y)+ complex by the following reaction27

[Ce(H2O)x(OH)y](4-y)+ + C2H5OH)3N f Ce(OH)4/CeO2·nH2O + (C2H5OH)3NH+ Thus, Ce(OH)4/CeO2 · nH2O forms even at room temperature in the solution. Rapid deprotonation of [Ce(H2O)x(OH)y](4-y)+ has a significant effect on the morphology of the nanocrystalline CeO2 powders because of the polymeric nature of [Ce(H2O)x(OH)y](4-y)+, resulting in volume shrinkage and agglomeration during drying. In the present work, the precipitates that were dried at room temperature are quite porous, which is supported by the low packing density of ∼0.6 g/cm3 compared to the bulk (28) Samsonov, G. V. The Oxide Handbook; IFI/Plenum: New York, 1982; p 463.

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density of 7.65 g/cm328 and the small aggregate diameters obtained from DLS.

Conclusions In conclusion, we present the synthesis of nanocrystalline CeO2 particles by precipitation using triethanolamine. The as-prepared crystallites have an average size of 3 nm with standard deviation of 1 nm. There was no additional water used in the precipitation process other than molecular water associated with the cerium nitrate precursor. The OH- ions, participated in the precipitation process, were produced by the reaction between molecular water in cerium nitrate and triethanolamine. The solvent type has no effect on the crystallite size of the synthesized particles, but does affect agglomerate size dispersion. In the present study, materials synthesized using 1 butanol (14 ( 8 nm aggregate hydrodynamic diameter) as the solvent showed better dispersion compared to 1-propanol (50 ( 18 nm) and ethanol (72 ( 20 nm). This is because of the decrease in interparticle attraction force with an increase in carbon chain length of the alcohol, which sterically inhibits the close approach of individual crystallites. Using this method, doped CeO2 for the use as electrode materials in solid oxide fuel cell and transition metal supported CeO2 as water gas shift (WGS) catalysts for fuel reformation can be prepared. Acknowledgment. The authors would like to thank Kirtland Linegar for assistance with DLS measurements and Peter Zavalij for assistance with X-ray diffraction. This work was partially supported by U.S. Army Research Laboratory, Contract Number: W911QX-04-C-0105. Support from the National Science Foundation under Grant No. DMR-0080008 for the purchase of the X-ray photoelectron spectrometer is acknowledged. Support of the Maryland NanoCenter and its NispLab is also acknowledged. The NispLab is supported in part by the National Science Foundation as a MRSEC Shared Experimental Facility. Supporting Information Available: XPS spectra and detailed peak fitting procedures. This material is available free of charge via the Internet at http://pubs.acs.org. LA8031286