Synthesis of Nanocrystalline CeO2âZrO2 Solid Solutions by a Citrate

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J. Phys. Chem. C 2009, 113, 914–924

Synthesis of Nanocrystalline CeO2-ZrO2 Solid Solutions by a Citrate Complexation Route: A Thermochemical and Structural Study Rodolfo O. Fuentes†,‡ and Richard T. Baker*,‡ CINSO (Centro de InVestigaciones en So´lidos), CITEFA-CONICET, J.B. de La Salle 4397, 1603 Villa Martelli, Buenos Aires, Argentina, and School of Chemistry, UniVersity of St. Andrews, North Haugh, St. Andrews, Fife, KY16 9ST, United Kingdom ReceiVed: October 6, 2008; ReVised Manuscript ReceiVed: NoVember 26, 2008

The synthesis of nanocrystalline solid solutions of general formula CexZr1-xO2 (x ) 0.1, 0.25, 0.5, 0.75, and 0.9) using a citrate complexation technique was followed with thermochemical methods, and the resulting powders were characterized using X-ray diffraction (XRD) and electron microscopy (scanning electron microscopy and high-resolution transmission electron microscopy (HRTEM)). Qualitative analysis of XRD data indicated that the samples exhibited either a cubic (Fm3m) or a tetragonal (P42/nmc) phase, depending on CeO2 content. Electron diffraction results were consistent with the XRD findings. In HRTEM, the internal crystal structure of the nanoparticles was observed to be highly ordered and primary particle size data were collected. Average crystallite size, obtained from the XRD data, was 4.8-8.3 nm, with the larger values at the compositional extremes. A similar trend was observed in HRTEM, although values were generally slightly higher. The Ce0.5Zr0.5O2 solid solution had the smallest crystallites (4.8 nm) and the highest specific surface area (45.8 m2 · g-1). Introduction Substitutional solid solutions of CeO2 and ZrO2 have attracted much interest in recent years because of their extensive use in a number of different fields, for example, as active supports or “oxygen buffers” in three-way catalysts. Since CeO2-ZrO2 solid solutions have been demonstrated to have better catalytic properties than pure CeO2,1,2 they may also have properties that make them attractive for application as hydrocarbon reforming or hydrocarbon activation catalysts in solid oxide fuel cells (SOFCs). The properties of CeO2-ZrO2 mixed oxides are strongly related to their crystal structure and local order.3-7 In particular, the metastable forms of the tetragonal phase have been widely investigated since they are the most suitable for many applications. The crystal structure of compositionally homogeneous CeO2-ZrO2 solid solutions has been studied by Yashima et al.8,9 They reported the existence of three forms of the tetragonal phase, all belonging to the P42/nmc space group. The stable form of the tetragonal phase is called the t form, which is restricted to the solubility limit predicted by the equilibrium phase diagram. The t′ form has a wider solubility range, but it is thermodynamically unstable, the stable phase in this compositional range being a mixture of the t form and the cubic phase. Finally, the t′′ form has an axial ratio c/a equal to unity, but with the oxygen ions displaced from their ideal sites in the cubic structure (8c sites of the Fm3m space group) along the c axis. Other new metastable phases have been recently reported and characterized for the 50 mol % CeO2-ZrO2.4 These phases, κ and t*, are obtained by reoxidation of the pyrochlore, Ce2Zr2O7+y, and differ in that the cations are ordered. The structure of this phase (Fd3m space group) is related to the fluorite-type structure but with 1/8 of the oxygens missing.10 * Corresponding author, [email protected]. † CINSO (Centro de Investigaciones en So´lidos), CITEFA-CONICET. ‡ School of Chemistry, University of St. Andrews.

Several routes have been used for the synthesis of nanocrystalline CeO2-ZrO2 solid solutions including the sol-gel method,11 hydrothermal synthesis,12 the polymerized complex method,13,14 the amorphous citrate process,15 and gel combustion.16 All of these produce compositionally homogeneous materials. Lamas et al. reported a study of the tetragonal-cubic phase transition on nanocrystalline powders obtained by a pHcontrolled nitrate-glycine gel-combustion method.17 The average crystallite sizes were in the range of 8-20 nm. The t′/t′′ and t′′/c compositional boundaries were 68 and 85 mol % CeO2, respectively. The characterization of CeO2-ZrO2 mixed oxides synthesized by two different chemical methods (citrate complexation and sol-gel) was reported by Alifanti et al.18 For low ceria contents, they reported that their sol-gel route gave rise to homogeneous solid solutions. However, an increase in ceria content (to >20%) led to a segregation of phases. They reported that the citrate method gave rise to a lower specific surface area (SSA) than the sol-gel procedure but offered the possibility of obtaining solid solutions over a wider compositional range. However, it is important to emphasize that the determination of phase homogeneities in nanostructured Ce-Zr solid solution is very difficult when conventional characterization techniques are employed. On the nanoscale, the possibility of the existence of ceria- and zirconia-rich nanodomains cannot be ruled out. Reliable synthetic routes for producing these Ce-Zr mixed oxides as nanoparticular powders over a wide compositional range and preferably via low-energy chemical steps are particularly sought after. Nanopowders are of especial interest for catalyst applications because of their high SSA. Sample purity and crystallinity are also important factors. In this work, a variant of the sol-gel technique known as cation complexation was employed to obtain nanocrystalline Ce-Zr solid solutions over the whole compositional range. This method has the advantage of low cost and relative simplicity. Moreover, this technique generates fewer carbon residues than other similar synthesis

10.1021/jp808825c CCC: $40.75  2009 American Chemical Society Published on Web 12/30/2008

Synthesis of Nanocrystalline CeO2-ZrO2 Solid Solutions

J. Phys. Chem. C, Vol. 113, No. 3, 2009 915

techniques and it has proved to be quite effective for producing highly sinterable ceramic powders. A detailed study of the structural and physical properties of the Ce-Zr mixed oxides was carried out employing X-ray diffraction (XRD) and scanning and high-resolution transmission electron microscopy (SEM and HRTEM) with elemental analysis in order to evaluate the quality of the final nanopowders over a broad compositional range. It was of particular interest to determine the best compositions for application as catalysts in SOFCs. The formation of the mixed oxide products during synthesis was followed using thermogravimetric analysis coupled with differential scanning calorimetry (TGA/DSC) and by XRD. These nanopowders are currently being assessed by the authors as active support materials for SOFC catalysts. 2. Experimental Procedure Ce-Zr mixed oxides were synthesized from nitrate precursors by complexing the metal cations with the citrate ion. Compositions containing 10, 25, 50, 75, and 90 equivalent mol % of CeO2 were prepared and will be referred to as CZ10, CZ25, CZ50, CZ75, and CZ90, respectively. Cerium nitrate hexahydrate (99.99%, Aldrich) and zirconium dinitrate oxide (99.9%, Alfa Aesar) were employed as precursors. Each nitrate was dissolved in deionized (DI) water individually, and the solutions were then mixed. Anhydrous citric acid (99.5%, Riedel-deHaen) was dissolved in DI water and was added to the cation solution. The molar ratio of total oxide (TO) to citric acid (CA) was 1:2 for all preparations. This solution was homogenized by mixing at ambient temperature. The solution was heated to 80 °C and maintained at this temperature under stirring to remove excess water and to convert the solution to a transparent gel. During the increase in temperature, the solution became more viscous with the evolution of foam, and finally it gelled without any visible precipitation or turbidity. As the solution was maintained at 80 °C, there was an increase in viscosity and simultaneous elimination of water and NO2. The initial thermal decomposition of the precursor was carried out at 250 °C for 1 h in air. The resulting ashlike material was calcined in a muffle furnace at 500 °C for 1 h in air. The heating rate was 5 °C/min. Thermogravimetric analysis (TGA/DTA) of gel precursors of the Ce-Zr mixed oxides was carried out in a TA Instruments SDT 2960 using a Pt crucible in flowing air with a heating rate of 5 °C/min and a maximum temperature of 800 °C. Further TGA/DSC studies with analysis of the off-gases by mass spectrometer were carried out in flowing O2 with a similar setup and temperature program using a Netzsch STA 499 C Jupiter instrument. The residual carbon content of the samples after calcination at 500 °C for 1 h in air was determined by elemental analysis using a Carlo Erba CHNS analyzer. Brunauer, Emmett, and Teller (BET) analysis by nitrogen adsorption (ASAP 2010, Micromeritics) was used to obtain SSA measurements. Calcined powders were studied by X-ray diffraction (XRD) using a Philips PW 1710 diffractometer (Cu KR radiation). Data in the range 2θ ) 20°-100° were collected in step-scanning mode (0.02° steps with a step-counting time of 10 s). Highgrade silicon powder was used as a standard to allow for the instrument broadening correction. SEM images were obtained using a Philips XL30 E-SEM instrument with a field emission gun. TEM images, elemental analyses and selected area electron diffraction (SAED) patterns of the samples were obtained using a JEOL 2011 TEM instrument equipped with a LaB6 filament, a Gatan digital

Figure 1. TGA/DSC of metal nitrate precursor mixtures for (a) CZ10, (b) CZ50, and (c) CZ90.

camera, and an energy dispersive X-ray spectroscopy (EDS) analysis system. Samples were prepared for study in the TEM by dipping a 3 mm copper grid coated with holey carbon film into an ultrasonicated dispersion of the sample powder in hexane and allowing this to dry overnight before use. 3. Results and Discussion Figure 1 shows the TGA/DSC plot for the Ce-Zr mixed oxide gel precursors, employing O2 as a carrier gas and with analysis of the off-gas using mass spectrometry (MS). For all samples, the DSC curve exhibits two large exothermic peaks. The first one started at about 110 °C, regardless of the gel

916 J. Phys. Chem. C, Vol. 113, No. 3, 2009

Fuentes and Baker

Figure 2. XRD pattern of CZ50 gel precursor calcined at (a) 100 °C and (b) 250 °C for 1 h.

Figure 3. XRD pattern of nanocrystalline solid solutions of (a) CZ90, (b) CZ75, (c) CZ50, (d) CZ25, and (e) CZ10 calcined at 500 °C for 1 h.

composition, and can be ascribed to H2O release. The second one is strongly dependent on the metal content of the gel and exhibited starting temperatures of 293, 266, and 240 °C for the CZ10, CZ50, and CZ90 gel precursors, respectively. These second, higher temperature, exothermic peaks can be related to the burn out of organic materials, liberating NOx, CO, and CO2 and to the crystallization of the Ce-Zr solid solution. The amorphous to crystalline transition of the Ce-Zr mixed oxides was clearly strongly affected by sample composition since the transition temperature increased as the CeO2 content decreased. This is in agreement with Suresh Kumar et al.19 The MS data confirmed that H2O is the main product released in the first exothermic peak with only traces of NOx, while NOx, CO, and CO2 are the main gases liberated during the second peak. It is possible to clearly identify three steps in the TGA curves. For the two samples with lower Ce contentsCZ10 and CZ50sthese steps are clearly defined. However, for the CZ90 gel precursor these steps are not clearly exhibited. The “intermediate” step relates to a weak endothermic peak, observed in Figure 1 and

starting at approximately 152 °C, and could be attributed to the release of absorbed water. XRD measurements performed on gel precursors calcined at 100 °C for 1 h exhibited an amorphous pattern, while crystalline and single-phase patterns were observed in samples treated at temperatures higher than 250 °C. As an example, XRD patterns corresponding to the CZ50 gel precursor calcined at different temperatures are shown in Figure 2. In XRD patterns obtained at room temperature for Ce-Zr solid solutions calcined at 500 °C for 1 h in air (Figure 3) relatively broad peaks, ascribable to the presence of small crystallites, were resolved. The average crystallite size, DXRD, was determined by applying the Scherrer formula,20 D ) 0.9λ/β cos θ, where λ is the wavelength of the radiation, β is the corrected peak width at half-maximum intensity (fwhm in radians), and θ is the peak position. Since the line shape was approximately Lorentzian for all the samples, the value of β was corrected using the formula β ) βm - βins, where βm is the measured peak width and βins is the instrumental broadening.

Synthesis of Nanocrystalline CeO2-ZrO2 Solid Solutions TABLE 1: Specific Surface Area (SSA) and Average Crystallite Size Data As Calculated by Scherrer’s Equation and from BET Data for CeO2-ZrO2 Powders Calcined at 500 °C for 1 h in Air sample

formula

CZ90 CZ75 CZ50 CZ25 CZ10

Ce0.9Zr0.1O2 Ce0.75Zr0.25O2 Ce0.5Zr0.5O2 Ce0.25Zr0.75O2 Ce0.1Zr0.9O2

DXRD/nm SSA/m2 · g-1 dBET/nm d/D ratio 7.2(1) 5.4(4) 4.8(2) 5.9(3) 8.3(1)

31.6(6) 30.3(9) 45.8(6) 25.1(3) 32.0(2)

26.6 28.4 19.6 37.7 30.1

3.72 5.26 4.07 6.41 3.63

Errors in crystallite size were derived by estimating the error in the fwhm to be equal to the 2θ step size. In Table 1, results of average crystallite size, SSA, and the primary particle size, dBET, are summarized. The average particle size, calculated from the BET data, was estimated by means of the equation: dBET ∼ 6/FA, where F is the theoretical density of the material and A is the SSA of the powder. The theoretical density was calculated from XRD results for all the compositions using the formula F ∼ ZM/NAV, where Z is the number of formula units in the unit cell, M is the formula weight, NA is Avogadro‘s number, and V is the unit cell volume. The dBET/ DXRD ratio in the Ce-Zr mixed oxide solid solutions ranged from 3.6 to 6.5, indicating that the crystallites in all the samples were agglomerated. A splitting of 400 and 004 reflections is expected for the tetragonal phase, indicating c/a > 1. This is clearly observed in the CZ10 and CZ25 solid solutions (Figure 4). However, this splitting is not evident in the CZ50 powder, but a marked asymmetry is observed in this peak. In addition, by assuming the presence of two peaks, the better fit of this profile is obtained. In contrast, only one peak was observed in CZ75 and CZ90 solid solutions, indicating that the cations are situated on ideal fluorite sites (c/a ) 1). For the CZ90 composition, which has the cubic structure, the 112 reflection is forbidden, while in compositions close to the tetragonal-cubic phase boundary, i.e., CZ75, the detection of this peak is extremely difficult because it is rather weak and, in this case, is very broad (due to the small particle size of ∼5.4 nm).

Figure 4. XRD pattern in the vicinity of the 004 and 400 peaks for solid solutions of (a) CZ90, (b) CZ75, (c) CZ50, (d) CZ25, and (e) CZ10 calcined at 500 °C for 1 h.

J. Phys. Chem. C, Vol. 113, No. 3, 2009 917 The structural study was performed by Rietveld refinement employing the program FullProf.21 For the tetragonal phase, the P42/nmc space group was assumed, with (Zr4+, Ce4+) cations and O2- anions in 2a and 4d positions, respectively. For the cubic phase, the Fm3m space group was assumed, with (Zr4+, Ce4+) cations and O2- anions in 4a and 8c positions, respectively. The results of these refinements were given in terms of the usual pseudofluorite unit cell. The peak shape was assumed to be a pseudo-Voigt function. The background of each profile was fitted using a six-parameter polynomial function in (2q)n, n ) 0-5. The thermal parameters corresponding to Zr and Ce atoms were assumed to be equal. The results of Rietveld refinements of the XRD data for Ce-Zr solid solutions are summarized in Table 2. Solid solutions with CeO2 content up to 75 mol % exhibited a tetragonal structure (P42/nmc space group), whereas the CZ90 exhibited a cubic structure (Fm3m space group). The lattice parameters determined in this work for all samples (Table 2 and Figure 5a) are in agreement with those reported in previous work on nanocrystalline Ce-Zr solid solutions prepared by the gel-combustion method which had average crystallite sizes of about 8-20 nm.17 Figure 5b displays the pseudocubic lattice parameter, defined as a* ) (2a + c)/3, as a function of CeO2 content. A clear linearity is apparent, indicating that compositionally homogeneous solid solutions have been formed.2 The pseudocubic lattice parameter for the CZE50 sample does exhibit a slight deviation from linearity, however, which might be caused by possible nonhomogeneity in this sample. According to Di Monte et al., a simple test to check the sample homogeneity can be performed by calcining the material at 1000 °C for 5 h.4 In Figure 6, the XRD pattern of the CZ50 sample calcined at 1000 °C for 5 h does not exhibit extra peaks. This indicates that, in fact, no phase segregation had occurred. An alternative explanation for the deviation in Figure 5b in the case of the CZ50 is that a cell expansion may have taken place because of the reduction of some Ce4+ to Ce3+(with formation of O2- defects), since the ionic radius of Ce3+(1.14 Å) is larger than that of Ce4+ (1.01 Å). A strong inverse relationship between lattice parameter and particle size has been reported by Tsunekawa et al. for ceria nanocrystals.24,25 The authors proposed that these nanoparticles consisted of a stable core-shell structure in which O2- defects and Ce3+ cations were located only at the surface of the particles. The importance of this surface layer therefore decreased as crystallite size increased, and the lattice parameter tended toward the value expected for bulk ceria. In the present work, this might explain the slight deviation above the trend line in Figure 5b for CZ50, which had the smallest average particle size. Conversely, the extremes of the compositional range, which had larger average particle sizes, fall slightly below the trend line, as would be expected according to this explanation. InFigure 7, the variation of the axial ratio, c/a, with CeO2 content is plotted. It can be observed that the axial ratio decreased as CeO2 content increased. For CZ75 and CZ90, the axial ratio is 1, indicating that the cations are placed in a cubic structure. However, it is not possible to confirm whether the overall crystal structure of the CZ75 sample is cubic or tetragonal. Values of residual carbon content in the Ce-Zr solid solutions calcined at 500 °C for 1 h are shown in Table 3 and confirm that the citrate complexation method is an excellent route for the preparation of compounds with low levels of carbon contamination. Taking the CZ50 composition as an example, carbon content was seen to decrease with increasing calcination


Fuentes and Baker

TABLE 2: Structural Parameters and Standard Rietveld Agreement Factors for Nanocrystalline CeO2-ZrO2 Solid Solutions Calcined at 500°C for 1 h in Air sample

CZ10

CZ25

CZ50

CZ75

CZ90

formula space group a (Å) c (Å) c/a z(O) Rp Rwp Re χ2

Ce0.1Zr0.9O2 P42/nmc 5.1250(4) 5.2001(3) 1 0.198(2) 7.01 7.23 5.82 1.54

Ce0.25Zr0.75O2 P42/nmc 5.1872(4) 5.2321(8) 1.0087(2) 0.199(3) 7.41 8.18 6.65 1.51

Ce0.5Zr0.5O2 P42/nmc 5.2767(6) 5.297(2) 1.0038(4) 0.201(3) 7.33 8.57 6.71 1.63

Ce0.75Zr0.25O2 P42/nmc 5.346(1) 5.348(1) 1.0004(3) 0.214(7) 6.24 7.91 6.42 1.52

Ce0.9Zr0.1O2 Fm3m 5.3860(2)

temperature. Samples calcined at 250 and 800 °C exhibited values of residual carbon content (%) of 0.79 and 0.17, respectively.22 Similar values of carbon content have been reported for nanopowders synthesized by a gel-combustion process.23 However, the calcination temperatures used in that study were higher than those employed here. SEM images of powders calcined at 500 °C are presented in Figure 8 and show the typical morphology of the Ce-Zr mixed oxides of different compositions synthesized by the citrate complexation route. There is a large difference between the morphologies of powders with low CeO2 content (Figure 8,

1 0.25 5.03 7.08 5.8 1.49

panels a and b) and those with high CeO2 content (Figure 8, panels e and f). In the case of CZ10, the aggregates are characterized by an angular shape, while CZ90 consists of rounded aggregates. In both cases, there is a high degree of agglomeration. For CZ50 (Figure 8, panels c and d), a spongy morphology with pores of 20-30 µm diameter was observed. The higher magnification images show that all three samples consisted of bubbles or voids with walls of material between them. These walls appeared to be thickest in the CZ10 and thinnest in the CZ50, where the aggregates were composed of

Figure 6. XRD pattern of CZ50 calcined first at (a) 500 °C for 1 h and then at (b) 1000 °C for 5 h.

Figure 5. (a) Lattice parameters and (b) the pseudocubic lattice parameter (a*) vs CeO2 content: (a) at and ct correspond to the tetragonal phase, while ac corresponds to the cubic phase; (b) a* ) (2a + c)/3, and the solid line is a linear fit.

Figure 7. Axial ratio, c/a, as a function of CeO2 content. Solid line is a quadratic fit.



TABLE 3: Carbon Content of CeO2-ZrO2 Powders Calcined at 500 °C for 1 h in Air sample

C content (%)


0.18 0.21 0.35 0.50 0.37

thin, paper-like sheets in egg-shell-like structures. This is in agreement with the high values of SSA determined in the BET analyses (Table 1). TEM and associated techniques were used to study samples of the Ce-Zr mixed oxides across the whole compositional range after calcination at 500 °C for 1 h. EDX spectra obtained in the TEM from relatively large regions (typically ∼1 µm2) of samples of the five different compositions, after calcination at 500 °C, are presented in Figure 9. The Cu peaks are artifacts commonly seen in such spectra when Cu sample grids are used in the TEM instrument and can be ignored. The spectra clearly show the expected variation in the ratio of Ce and Zr peak intensities. They also demonstrate the purity of these materials since only Ce, Zr, O, and C were detected. The intensity of the C peak can be expected to include a contribution from the holey C film present on the TEM grid and therefore should not be attributed entirely to the samples. Indeed, the elemental analyses summarized in Table 3 show that these samples had very low C contents. Electron diffraction was used in the TEM to study the crystal structure of the samples as the Ce:Zr ratio changed. SAED patterns for the five different compositions are presented in Figure 10 together with corresponding low-magnification TEM images. These images clearly show that sheetlike aggregates composed of the mixed oxide nanoparticles are typical of all five sample compositions. These sheets may be interpreted as Figure 9. EDS spectra obtained in the TEM of (a) CZ90, (b) CZ75, (c) CZ50, (d) CZ25, and (e) CZ10 calcined at 500 °C for 1 h.

Figure 8. SEM images of nanocrystalline solid solutions calcined at 500 °C for 1 h of (a and b) CZ10, (c and d) CZ50, and (e and f) CZ90.

fragments of the “shell” of the egg-shell-like structures seen in the SEM images. The SAED patterns were obtained from such regions. The concentric rings in these patterns are essentially continuous, indicating that a very large number of crystals contributed to each diffraction pattern, and suggesting, therefore, that the individual crystals are nanosized. Interplanar spacings, in angstroms, were calculated from the SAED patterns, and these are included in the figure. The patterns are also labeled with the corresponding Miller indices, referred to the fluorite crystal structure. As the Ce content of the samples decreased, these interplanar spacings show a general decrease, in agreement with the more accurate data from the XRD patterns which are summarized in Table 2. For high Ce contents (Figure 10a-c), the patterns can be indexed to the fluorite structure. However, for CZ25, and more clearly for CZ10, an additional ring is observed which corresponds to interplanar spacing of 2.07 and 2.04 Å, respectively. This corresponds to reflection from the {112} planes in the fluorite structure, which are forbidden, or the {102} planes in the tetragonal crystal structure, which are allowed. Therefore, the electron diffraction results confirm a move from a cubic to a tetragonal crystal structure on increasing the Ce content of these Ce-Zr mixed oxides. By measurement of the dimensions of a large number of nanocrystals from the TEM images, average particle sizes and particle size distribution data were obtained. Histograms showing


Fuentes and Baker

Figure 11. Histograms of nanoparticle size distribution (left) and typical intermediate magnification TEM images (right) of (a) CZ90, (b) CZ75, (c) CZ50, (d) CZ25, and (e) CZ10 calcined at 500 °C for 1 h. Figure 10. SAED patterns (left) and typical low-magnification TEM images (right) of (a) CZ90, (b) CZ75, (c) CZ50, (d) CZ25, and (e) CZ10 calcined at 500 °C for 1 h.

primary particle size distributions for samples of the five Ce-Zr compositions are presented in Figure 11 together with corresponding typical intermediate magnification images. Particle size statistics are also collected together in Table 4. The particle size distributions appear to be unimodal and approximately Gaussian. There is also a clear trend, apparent in Figure 11 and in the values given in Table 4, for the samples of intermediate compositions to have the smaller particles, while at both extremes of the compositional range they are larger. Average particle size is expressed as a simple numerical average, dTEM,

TABLE 4: Average Crystallite Size Data Obtained from TEM Images of Nanocrystalline CeO2-ZrO2 Solid Solutions Calcined at 500 °C for 1 h in Air sample

dTEM/nm

σ/nm

dVTEM/nm

sample size


6.07 5.51 5.07 5.23 6.55

2.41 1.82 1.55 1.81 1.75

10.0 7.60 6.55 7.11 7.93

191 90 111 62 70

and as a volume-corrected average, dVTEM. Assuming that the density of each material is independent of particle size, dVTEM can also be thought of as being mass corrected. It is this value which should be compared with the particle size data estimated


Figure 12. HRTEM images of a sample of CZ90 calcined at 500 °C for 1 h showing nanocrystals aligned in the (a) [011], (b), [001], (c) [011], and (d) [112] zone axes. DDPs are inset, and the diffraction spots are indexed to the fluorite structure.

from the degree of line-broadening in the XRD patterns (DXRD in Table 1). Both sets of data show the same trend, although the TEM-derived values are consistently slightly higher. This difference may be explained by two possible effects. First, it may be the result of the presence of some nanodomains within individual particles which would affect the XRD data but may not be obvious in the HRTEM images. Care was taken in the TEM measurements to establish that each particle measured appeared to be a single crystal, that is, that it has a continuous internal crystal structure. However, these were often onedimensional, rather than two-dimensional, lattice images, and so it is possible that some were twinned but that this was not apparent in the image. Second, there may be a morphological effect. CeO2-based materials often have disk-like crystal morphology.26,27 There is therefore a tendency for them to lie flat on the grid and so for their larger dimensions to be preferentially visible in the TEM images. That this would occur in the sheet-like agglomerate structures observed in the current work, however, would imply that there were preferential crystallite growth directions in the plane of these sheets. HRTEM images showing the internal crystal structure of individual nanocrystals of each of the five chemical compositions, after calcination at 500 °C for 1 h, were recorded. Selected images, together with the corresponding digital diffraction patterns (DDPs), are presented in Figures 12-16 for CZ90, CZ75, CZ50, CZ25, and CZ10, respectively. DDPs are diffraction patterns generated by performing a mathematical Fourier transform on a selected area of the lattice image of a single nanocrystal in the HRTEM images. Here, they are labeled with the appropriate zone axis of the fluorite structure, unless otherwise indicated. When more than one particle is resolved in the field of view, the particle used to calculate the DDP is circled. In the HRTEM images of the CZ90, CZ75, and CZ50 samples (Figures 12-14), no evidence was found for any crystal structure other than the cubic fluorite structure. Clear lattice images were obtained which could be indexed to the fluorite structure viewed, in order of decreasing occurrence, along the [011], [001], [112],


Figure 13. HRTEM images of a sample of CZ75 calcined at 500 °C for 1 h showing nanocrystals aligned in the (a) [011], (b) [001], (c) [111], and (d) [112] zone axes. DDPs are inset, and the diffraction spots are indexed to the fluorite structure.

Figure 14. HRTEM images of a sample of CZ50 calcined at 500 °C for 1 h showing nanocrystals aligned in the (a and b) [011], (c) [001], and (d) [112] zone axes. DDPs are inset, and the diffraction spots are indexed to the fluorite structure.

and [111] zone axes. It should be remembered, however, that both the tetragonal and monoclinic phases of Ce-Zr mixed oxides may give DDPs which are identical with those given by the fluorite structure, as well as giving rise to a number of patterns, viewed in certain zone axes, which are characteristic of the tetragonal or monoclinic phases and cannot be given by the fluorite structure. The CZ25 sample exhibited especially densely agglomerated particles, and because of this it was not possible to obtain a large number of lattice images of individual particles. The images in Figure 15 are typical of those that were obtained in that they also could be indexed to the fluorite structure. The figure shows nanocrystals aligned in the [011]


Fuentes and Baker TABLE 5: Density and Crystal Structure of CeO2-ZrO2 Mixed Oxides Sintered at 1450 °C for 8 h in Air

Figure 15. HRTEM images of a sample of CZ25 calcined at 500 °C for 1 h showing nanocrystals aligned in the (a) [011] and (b) [001] zone axes. DDPs are inset, and the diffraction spots are indexed to the fluorite structure.

Figure 16. HRTEM images of a sample of CZ10 calcined at 500 °C for 1 h showing nanocrystals aligned in the (a) [011] and (b) [112] zone axes of the fluorite structure and (c and d) in the [010] zone axis of the tetragonal phase. DDPs are inset, and the diffraction spots are labeled with their Miller indices.

sample

Fm/FT/%

crystal structure


90 80 74 70 68

cubic tetragonal 2 phases (t+c) 2 phases (t+c) monoclinic

and [001] zone axes. In the CZ10 sample (Figure 16), the lattice images of the majority of the nanocrystals imaged could again be indexed to the fluorite structure, viewed in the [011] and [001] zone axes in Figure 16, panels a and b, respectively. However, there was also evidence for the presence of a tetragonal phase. The nanocrystals in panels c and d of Figure 16 cannot be indexed to the fluorite structure but coincide well with the tetragonal phase viewed along the [010] zone axis. The DDPs in both cases show spots which are forbidden in the cubic fluorite structure. The 001 spots correspond to planes separated by ∼5.2 Å and the 102j spot in the DDP of Figure 16d corresponds to planes with an interplanar spacing of ∼2.1 Å. Electron diffraction from these planes gave rise to the additional line in the SAED patterns of the CZ25 and CZ10 samples shown in Figure 10. This line was more intense in the CZ10 sample. Therefore, there was good overall agreement between the SAED and the HRTEM findings and also between these and the results of the XRD study, regarding the transition from cubic to tetragonal structure with decreasing CeO2 content. Figure 17 shows the XRD patterns of Ce-Zr mixed oxides sintered at 1450 °C for 8 h. The crystal structure of the CZ10 sample is monoclinic with very small traces of tetragonal phase, while the CZ75 and CZ90 compositions exhibited single phases of the tetragonal and cubic structures, respectively. In the cases of CZ25 and CZ50, two phases (cubic and tetragonal) were clearly observed. No monoclinic traces were detected in these samples. All results, summarized in Table 5, are in agreement with the metastable-stable phase diagram reported by Yashima et al.13 The density of sintered samples was seen to increase with increasing CeO2 content (Table 5), showing poor densification in samples with CeO2 contents lower than 75 mol %.

Figure 17. XRD patterns of (a) CZ90, (b) CZ75, (c) CZ50, (d) CZ25, and (e) CZ10 powders sintered at 1450 °C for 8 h.

Synthesis of Nanocrystalline CeO2-ZrO2 Solid Solutions Finally, it is worth discussing the possible existence of compositional heterogeneities at a nanoscale level in these nanoparticulate CeO2-ZrO2 materials. Unfortunately, conventional techniques, such as laboratory X-ray diffraction, are unable to detect any evidence of heterogeneities or mixture of phases in high surface area nanoparticles because of the inherent broadening of the diffraction peaks caused by the small crystallite size, even if Rietveld analysis is used.4 Mamontov et al. studied Ce0.5Zr0.5O2 powders synthesized by spray pyrolysis and coprecipitation processes by neutron diffraction, and by means of a detailed pair-distribution function analysis, they established the existence of nanodomains with a composition of Ce0.4Zr0.6O2, 2.5-3.0 nm in size, embedded in a matrix of Ce0.7Zr0.3O2.28 These authors also pointed out that a conventional Rietveld analysis of neutron diffraction data did not reveal these heterogeneities. This illustrates the difficulty generally encountered for the clear assessment of the compositional homogeneity of nanocrystalline CeO2-ZrO2 solid solutions. A practical criterion, proposed by Fornasiero et al.,29 to verify the compositional homogeneity of HSA materials consists of performing XRD analysis of powders calcined at 1000 °C for 5 h in order to reveal compositional inhomogeneities, This treatment reduces XRD peak broadening, because of the growth of the crystallites and does not induce phase segregation if cation distribution is homogeneous. However, it must be taken into account that the criterion of Fornasiero et al. may not be sensitive enough to detect nanoscale heterogeneities.4 The use of a high-intensity synchrotron diffractometer makes possible a fine study of the compositional homogeneity of CeO2-ZrO2 nanopowders.17 If nanodomains of 2.5-3.0 nm with a different composition to that of the matrix exist in the samples, they could be detected by the existence of very broad and weak peaks at 2θ positions displaced from those of the matrix. If both compositions are very different, as reported by Mamontov et al.,28 these broad peaks should be clearly observed at high angles. In addition, they could also modify the profile of the low-angle peaks, even for a smaller difference in composition. In the previous work cited, Lamas et al. analyzed very carefully the profile of the peaks in order to detect these effects and no evidence was observed in any sample. It must be mentioned that the low-resolution configuration used in that work produced a small distortion of the XRD pattern at very low 2θ angles (as determined by analyzing the background of XRD patterns of reference materials), but it was found to be negligible for 2θ > 30, so it was possible to carry out the above analysis properly.17 4. Conclusion 1. Compositionally homogeneous Ce-Zr mixed oxides were successfully synthesized by a citrate cation complexation route. TG/DSC and XRD investigations revealed the appearance of a solid solution at temperatures of around 250 °C, depending on the Ce:Zr ratio. Low carbon contents were found in powders of all compositions after calcination at 500 °C. 2. SEM and TEM images indicated an arrangement of thin sheets of nanoparticles in an egg-shell-like morphology. 3. Average crystallite size was estimated from the degree of peak broadening in the XRD patterns and by direct measurement in the HRTEM images. Both sets of data showed maxima in average particle size for the extremes of the compositional range. The CZ50 powder exhibited the smallest average crystallite size (4.8 nm from XRD data) and the largest value of SSA (45.8 m2 · g-1). 4. Average particle sizes obtained from the HRTEM data were consistently slightly higher than values obtained from the XRD

J. Phys. Chem. C, Vol. 113, No. 3, 2009 923 data. It is suggested either that some of the nanoparticles contained several crystal nanodomains or that the particles were disk shaped and lay preferentially with their long dimensions in the plane of the sheets which contained them. 5. The crystal structure was investigated by XRD, electron diffraction, and in HRTEM images. Metastable forms of the tetragonal phase were identified in solid solutions with CeO2 content lower than 75 mol % by fitting the XRD data. The CZ90 sample exhibited the cubic phase. The variation of pseudocubic lattice parameters was in good agreement with that reported in the literature for Ce-Zr solid solutions with relatively large crystallite sizes (8-20 nm). The SAED and DDP patterns obtained in the TEM were in agreement with the findings of the XRD study. In the high-resolution images, the nanoparticles appeared to be single nanocrystals with highly ordered lattice structures. 6. Finally, it is important to note that a more detailed study, employing synchrotron radiation X-ray diffraction and neutron diffraction, must be performed to confirm the apparent compositional homogeneity of the solid solutions obtained here. At the nanoscale, the possibility of the existence of ceria- and zirconia- rich nanodomains cannot be ruled out completely. Acknowledgment. This work was carried out within the UK EPSRC SUPERGEN Fuel Cell consortium. Electron microscopy was carried out at the Electron Microscopy Facility, University of St. Andrews (TEM) and at the CHIPS Facility, University of Dundee (SEM). References and Notes (1) Gonzalez-Velasco, J. R.; Gutierrez-Ortiz, M. A.; Marc, J. L.; Botas, J. A.; Gonzalez-Marcos, M. P.; Blanchard, G. Appl. Catal., B 1999, 22, 167. (2) Beckers, J.; Clerc, F.; Blank, J. H.; Rothenberg, G. AdV. Synth. Catal. 2008, 350, 2237. (3) Catalysis by Ceria and Related Materials; Trovarelli, A., Ed.; Imperial College Press: London, 2002. (4) Di Monte, R.; Kaspar, J. J. Mater. Chem. 2005, 15, 633. (5) Vidal, H.; Bernal, S.; Kaspar, J.; Pijolat, M.; Perrichon, V.; Blanco, G.; Pintado, J. M.; Baker, R. T.; Colon, G.; Fally, F. Catal. Today 1999, 54, 1–93. (6) Vidal, H.; Kaspar, J.; Pijolat, M.; Colon, G.; Bernal, S.; Cordon, A.; Perrichon, V.; Fally, F. Appl. Catal., B 2000, 1, 49. (7) Bernal, S.; Blanco, G.; Calvino, J. J.; Hernandez, J. C.; Perez-Omil, J. A.; Pintado, J. M.; Yeste, M. R. J. Alloy Compd. 2008, 451 (1-2), 521. (8) Yashima, M.; Morimoto, K.; Ishizawa, N.; Yoshimura, M. J. Am. Ceram. Soc. 1993, 76, 1745. (9) Yashima, M.; Sasaki, S.; Yamaguchi, Y.; Kakihana, M.; Yoshimura, M.; Mori, T. Appl. Phys. Lett. 1998, 2, 182. (10) Thomson, J. B.; Amstrong, A. R.; Bruce, P. G. J. Am. Chem. Soc. 1996, 118, 11129. (11) Rossignol, S.; Gerard, F.; Duprez, D. J. Mater. Chem. 1999, 9, 1615. (12) Cabanãs, A.; Darr, J. A.; Lester, E.; Poliakoff, M. J. Mater. Chem. 2001, 11, 561. (13) Yashima, M.; Ohtake, K.; Kakihana, M.; Yoshimura, M. J. Am. Ceram. Soc. 1994, 77, 2773. (14) Luo, M. F.; Lu, G. L.; Zheng, X. M.; Zhong, Y. L.; Wu, T. H. J. Mater. Sci. Lett. 1998, 17, 1553. (15) Kaspar, J.; Fornasiero, P.; Balducci, G.; Di Monte, R.; Hickey, N.; Sergo, V. Inorg. Chim. Acta 2003, 349, 217. (16) Lamas, D. G.; Lascalea, G. E.; Jua´rez, R. E.; Djurado, E.; Pe´rez, L.; Walsoë de Reca, N. E. J. Mater. Chem. 2003, 13, 904. (17) Lamas, D. G.; Fuentes, R. O.; Fa´bregas, I. O.; Fernańdez de Rapp, M. E.; Lascalea, G. E.; Casanova, J. R.; Walsoë de Reca, N. E.; Craievich, A. F. J. Appl. Crystallogr. 2005, 38, 867. (18) Alifanti, M.; Baps, B.; Blangenois, N.; Naud, J.; Grange, P.; Delmon, B. Chem. Mater. 2003, 15, 395. (19) Suresh Kumar, K.; Mathews, T. J. Alloys Compd. 2005, 391, 177. (20) Klug, H.; Alexander, L. In X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials: John Wiley: New York, 1974; pp 618.

924 J. Phys. Chem. C, Vol. 113, No. 3, 2009 (21) Rodrı´guez-Carvajal, J., FullProf98, Version 02; Laboratoire Leoń Brillouin (CEA-CNRS), Saclay, France, 1998. (22) Fuentes R. O.,; Baker R. T., Submitted to J. Alloys Compd., 2008. (23) Larrondo, S.; Vidal, M.; Irigoyen, B.; Craievich, A.; Lamas, D.; Faı´bregas, I.; Lascalea, G.; de Reca, N.; Amadeo., N. Catal. Today 2005, 107-108, 53. (24) Tsunekawa, S.; Ito, S.; Kawazoe, Y. Appl. Phys. Lett. 2004, 85, 17–3845. (25) Tsunekawa, S.; Sivamohan, R.; Ito, S.; Fukuda, T. Nanostruct. Mater. 1999, 11, 141.

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