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Chemical Engineering Journal 178 (2011) 436– 442

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Chemical Engineering Journal

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Synthesis of Fe-doped CeO2 nanorods by a widely applicable coprecipitationroute

Zizi Wanga, Ying Xina, Zhaoliang Zhanga,∗, Qian Lia, Yihe Zhanga,b,∗, Limin Zhouc

a Shandong Provincial Key Laboratory of Fluorine Chemistry and Chemical Materials, School of Chemistry and Chemical Engineering, University of Jinan,106 Jiwei Road, Jinan 250022, Chinab National Laboratory of Mineral Materials, China University of Geosciences, 29 Xueyuan Road, Beijing 100083, Chinac Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong

a r t i c l e i n f o

Article history:Received 27 April 2011Received in revised form29 September 2011Accepted 5 October 2011

Keywords:Cerium oxideNanorodIronCoprecipitationChromium

a b s t r a c t

Fe-doped CeO2 solid solution nanorods were successfully prepared via a simple coprecipitation methodwithout templates/surfactants at ambient temperature and pressure. The single crystalline nanorodstructure was preserved after calcination at higher temperatures. The formation of the nanorod mor-phology strongly depends on Fe contents. Beyond the solid solution limit, only nanoparticles wereobserved. The morphology was also affected by precipitation agents, temperatures and aging times.The formation of the nanorods originates from the intrinsic anisotropy of Ce hydroxide. The dissolu-tion/recrystallization mechanism was thus proposed, in which the coexistent ammonia and/or Fe act asthe capping agents for the formation of nanorods. Simultaneously, the enrichment of Fe on the surfaceresults in the nanorods with round shape and sharp tips. The element-capped concept for the formationof the nanorod morphology can be extended to Cr-doped CeO2.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Ceria-based oxides play an important role in various applica-tions as heterogeneous catalysis [1], solid oxide fuel cells [2], hightemperature ceramics [3], gas sensors [4] and UV absorbents [5]due to their unique properties in (i) the oxygen storage capacity(OSC) based on the redox behavior between Ce4+ and Ce3+; (ii) theease of formation of labile oxygen vacancies and the relatively highmobility of bulk oxygen species [6]. Generally, two strategies wereadopted to improve the above properties. One approach is dopingwith another metal ion into the ceria lattice, which offers an oppor-tunity not only to improve the performance of the involved metaloxide but also to form new stable compounds that lead to totally dif-ferent physical and chemical properties. Therein, low-valence tran-sition metal ions such as Fe are mostly used ones [7,8]. It is demon-strated that the doping of these ions can produce large amounts ofoxygen vacancies, which facilitates the activation and transporta-tion of active oxygen species [7,9]. Fe-doped CeO2 has been usedas catalysts for Fischer-Tropsch synthesis [10,11], N2O decomposi-tion [12] and diesel soot combustion [13]. The other is morphologycontrol, for instance, one dimensional (nanorods [14], nanowires[15] and nanocubes [16]) and two dimensional (nanobelts [17],

∗ Corresponding authors. Tel.: +86 531 89736032; fax: +86 531 89736032.E-mail addresses: chm zhangzl@ujn.edu.cn (Z. Zhang), zyh@cugb.edu.cn

(Y. Zhang).

nanorings [18], nanoplates and nanodisks [19]) nanostructureshave been reported, in which nanorods have currently attractedconsiderable attention because of the selective exposure of thehighly reactive crystal planes [9,14,20]. So far, various methodshave been developed for the synthesis of CeO2 nanorods.

The hydrothermal method at the higher temperature and pres-sure with [21–23] or without [24–26] surfactants has been widelyused for the preparation of crystalline CeO2 nanorods. Ultra-sonication method has also been proposed [27]. Ho et al. [28]reported a microsized, rod-shaped and spindle-like CeO2 usinga polyol method. But the as-prepared sample is cerium for-mate, which converts to the CeO2 structure after calcination at600 ◦C for 4 h. These methods always make use of surfactants,templates and/or hydrothermal treatment at a higher tempera-ture and pressure. Recently, the CeO2 nanorods were obtainedwithout templates by precipitation [29] or reflux [30] methodsusing Ce(NO3)3·6H2O and NaOH. However, the synthesis of Fe-doped CeO2 solid solution nanorods has hardly been reported[3,7,8,10–12].

In this paper, we report a simple template-free coprecipi-tation approach for the synthesis of Fe-doped CeO2 nanorods.The nanorod structure was almost preserved after calcination athigher temperatures (650 ◦C, 6 h). The formation of the nanorodmorphology strongly depends on Fe contents. Meanwhile, the mor-phology was also affected by precipitation agents, temperatures,aging times and precursors. A plausible mechanism was suggested.This element-capped concept for the formation of the nanorod

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morphology might be extended to all CeO2 mixed oxides dopedwith similar transition metals, for instance, chromium (Cr).

2. Experimental

2.1. Catalyst preparation

All of the chemicals used in this experiment were of analyti-cal grade and used without further purification. A series of Ce–Femixed oxides with 1, 5, 10, 20 and 30 at.% Fe metal (Ce balance) wereprepared by a coprecipitation method. Hereafter, they are denotedas CxFy, in which x (=100Ce/(Fe + Ce)) and y (=100Fe/(Fe + Ce)) arethe atom percentages of Ce and Fe, respectively. A stoichiomet-ric solution (100 ml) of the nitrates of Ce and Fe was dropped into150 ml NH3·H2O solution (25%) under vigorous agitation and thenthe resultant precipitate was aged in contact with air for 48 h (96 hfor C99F1) at ambient temperature and pressure. The resultant pre-cipitates were dried at 100 ◦C overnight and calcined at 650 ◦C for6 h in static air. For comparison, pure Ce and Fe oxides were alsoprepared using a similar procedure, and these were determined tobe CeO2 and Fe2O3, respectively.

2.2. Catalyst characterization

X-ray powder diffraction (XRD) patterns were recorded ona Rigaku D/max–rc diffractometer employing Cu K� radiation.Transmission electron microscopy (TEM) and high-resolutiontransmission electron microscopy (HRTEM) equipped with energydispersive spectroscopy (EDS) and selective area electron diffrac-tion (SAED) were conducted on a JEOL JEM–2010 at an acceleratingvoltage of 200 kV. The morphology was also characterized usinga Hitachi SU–70 field emission scanning electron microscopy(FESEM).

3. Results and discussion

3.1. Characterization

Our previous XRD results show that the �-Fe2O3 phase with ahexagonal hematite structure was detected only when y ≥ 30 [13],

which suggests that the doping content of 20 at.% is the solid solu-bility limit of Fe in CeO2.

Figs. 1 and 2 show the morphologies of CxFy before and aftercalcination at 650 ◦C for 6 h. Although irregular nanoparticles wereobserved for pure Ce (Fig. 1a and Fig. 2a and b) and pure Fe (Fig. 1gand Fig. 2l) samples, the nanorods were obtained for C99F1, C95F5,C90F10 and C80F20. Furthermore, no nanorods but nanoparticlesas the sole product were detected for C70F30 (Fig. 1f and Fig. 2k).This suggests that the nanorod morphology seems to be inducedby doping with Fe in the solid solution range. The result is excit-ing considering that the morphology was mostly controlled bystructure-directing agents (soft or hard template) and/or reac-tion conditions. Unfortunately, the morphology consisting of onlynanorods without any nanoparticles was not obtained until nowunder the present procedure.

The TEM images clearly indicate the nanorod structure of theproducts with round shape. Sayle et al. [20] also reported a cylin-drical shape rod for Ti-doped CeO2. This is not like the pure CeO2[14] or Co3O4 [31] nanorods, which shows a rectangular cross sec-tion. The nanorods with sharp tips (spindles) were also observedfor C95F5 (Fig. 1c), C90F10 (Fig. 1d) and C80F20 (Fig. 1e and Fig. 2n).

After calcinations, the nanorods shrink in dimensions andnanoparticles are coexistent. Fig. 2a shows the TEM image of CeO2nanoparticles in the size of 20 nm. The HRTEM image in Fig. 2bshows the clear (1 1 1) and (2 0 0) lattice fringes with the interplanarspacing of 0.31 and 0.27 nm, respectively, revealing that the CeO2particles were dominated by an octahedral or truncated octahedralshape [32]. Fig. 2l shows the TEM image of Fe2O3 nanoparticlesin the size of 50–100 nm. The HRTEM image in Fig. 2l shows theclear (1 1 0) and (0 1 2) lattice fringes with the interplanar spacingof 0.25 and 0.36 nm, respectively, belonging to �-Fe2O3. Fig. 2c,e, g and i shows TEM images of the CxFy nanorods. C99F1, C95F5and C90F10 show the diameter about 10 nm and the length within60 nm. However, the diameter and length for C80F20 are about20 nm and 100 nm, respectively. This suggests that the increase inFe-doping content promotes the growth of the rods. Fig. 2d, f, hand j shows the corresponding HRTEM images combined with afast Fourier transform (FFT) analysis (inset), which indicates thatthe CxFy nanorods are single crystalline. According to FFT analysis,three kinds of lattice fringe directions attributed to (1 1 1), (0 0 2)

Fig. 1. TEM images of pure Ce sample (a), C99F1 (b), C95F5 (c), C90F10 (d), C80F20 (e), C70F30 (f) and pure Fe sample (g) after drying (before calcinations).

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Fig. 2. TEM and HRTEM images of pure Ce sample (a, b), C99F1 (c, d), C95F5 (e, f), C90F10 (g, h), C80F20 (i, j), C70F30 (k) and pure Fe sample (l) after calcination at 650 ◦C for6 h. Insets are the corresponding FFT patterns. FESEM images of C95F5 (m) and C80F20 (n).

and (2 2 0) were observed for the nanorods, which have a respectiveinterplanar spacing of 0.31, 0.27 and 0.19 nm [14,26]. This suggestsa preferred growth direction along [1 1 0].

As an example, the average atomic ratio between Fe and Ce wasanalyzed by EDS for C90F10 after calcination [13], which equalsto about 0.1, almost the same value as in the starting materials.This suggests the incorporation of Fe ions into the CeO2 structure.Fig. S1 shows the SAED patterns of an area (a circle with a diam-eter of 100 nm) containing several nanorods (nanoparticles) forC99F1, C95F5, C90F10 and C80F20. Only a set of rings instead spots

were obtained due to the random orientation of the nanocrystal-lites. The locations of planes corresponding to (1 1 1), (2 0 0), (2 2 0),(3 1 1), (4 0 0), (3 3 1) and (4 2 2) are in good agreement with thecubic CeO2 phase (JCPDS No. 34–0394). No additional iron oxidephase was detected suggesting that the doped Fe ions are incor-porated into the ceria lattice forming CeO2 solid solutions. Theseresults are consistent with that of XRD. Moreover, the calculatedinterplanar distances dh k l of the samples from the SAED patternsare slightly reduced because of the incorporation of the smallerFe ions (0.64 A for Fe3+ vs. 0.97 A for Ce4+). The formation of solid

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Fig. 3. TEM images of C99F1 aging for different times (after drying). (a) 0 h; (b) 24 h; (c) 96 h; (d) 144 h; (e) 192 h; (f) 240 h.

solutions for CxFy is further demonstrated by XRD and Raman spec-tra [13]. Furthermore, all CxFy samples were studied as catalysts insoot combustion, which show significant activity compared to thatof pure CeO2 and Fe2O3 [13].

3.2. Possible mechanism of nanorod formation

The formation of the nanorod morphology was reproduced withseveral preparative batches of CxFy. An example is shown in Fig. S2.To reveal the possible mechanism, Fig. 3 shows the evolution of themorphology of C99F1 with aging time. The yield of the nanorodsincreases in proportion with time, but the nanorods dissolved com-pletely after aging for 240 h. XRD results show that all samples afterdrying were CeO2 structure (results not shown here) except forpure Fe samples (Fig. 4), which shows the amorphous Fe(OH)3 pre-cipitate with some poor crystallites distinguished as orthorhombic�-FeOOH (JCPDS 29–0713) [33,34]. The FeOOH phase is reportedto come from Fe(OH)3 by the loss of a water molecule [35]. In addi-tion, the Ce(OH)3 phase was detected for the wet C95F5 precipitate(Fig. 4, JCPDS 19–0284). Therefore, one would expect the formationof Ce(OH)3 and Fe(OH)3 phases during coprecipitation. The hexag-onal crystal structure of Ce(OH)3 suggests that the anisotropicgrowth of the nanorods is derived from the inherent crystal struc-ture of Ce(OH)3 [26,36]. Because Ce(OH)3 is unstable and extremelysensitive to oxygen, it can even be oxidized to CeO2 by NO3

− [25]or dissolved oxygen [37] in solutions during the precipitation pro-cess [16], while CeO2 has a cubic structure without anisotropy. Theanisotropic growth was limited, as indicated above, the octahe-dral or truncated octahedral shaped CeO2 nanoparticles were thusobtained for the pure Ce sample.

The anisotropic growth is affected by chemical potential, whichis mainly determined by the pH value [36]. Fig. 5 shows TEM imagesof C95F5 after aging at 45 ◦C and 80 ◦C for 48 h (after drying).Nanorods were observed for the former while not for the latter due

Fig. 4. XRD patterns of pure Fe and C95F5 precipitate in wet state (before drying).

to the strong NH3 volatilization at the higher temperature whichresults in the decrease of the pH value. As shown in Fig. 3f, theelongation of aging time can bring the same effect. At the low OH−

concentration, the chemical potential for the anisotropic growthof Ce(OH)3 nuclei is inadequate. As a result, nanoparticles wereproduced. However, as shown in Fig. 6, if NaOH was used as the pre-cipitate agent, nanoparticles were obtained as well. This suggeststhat the formation of CxFy nanorods depends on the co-existenceof ammonia. As was proposed by Li et al. for CuO nanowires [38],ammonia might be a capping agent. The adsorption of NH4

+ onthe surface of the hydroxides was supported by other work [39](and references therein). This assumption can also be proved bythe following evidence. If the beaker for the precipitation reaction

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Fig. 5. TEM images of C95F5 aging at 45 ◦C (a) and 80 ◦C (b) for 48 h and after drying.

of pure Ce sample was sealed before aging (that is, the evaporationof ammonia was prevented), CeO2 shows the nanorod morphologyafter drying (Fig. 7). However, even if ammonia evaporates slowlyat room temperature during aging as for the CxFy solid solutions,in this case Fe(OH)3 can substitute ammonia as a capping agent toprohibit Ce(OH)3 from oxidation into CeO2. This can be deducedfrom the facts that (1) the dimensions (length and diameter) ofnanorods increase with the increase in Fe; (2) the high proportionof nanorods was obtained after aging for 96 h for C99F1, while only48 h was needed for C95F5, C90F10 and C80F20; and (3) an enrich-ment of Fe on the CxFy solid solution surfaces has been detected byXPS [13]. The absence of any peaks corresponding to Fe(OH)3 in thewet C95F5 precipitate (Fig. 4) could be due to poor crystallinity ofthis compound, as mentioned above. The capping function of thedopant is reported by many researchers. Feng et al. [40] suggestedthat the doping element Ti changed the shape of CeO2 nanocrystalsfrom polyhedron to sphere by the encapsulating effect of amor-phous TiO2. Therefore, as indicated above, the nanorods exhibit around rather than a rectangular shape, or even a spindle shape forC80F20 due to its high Fe content.

However, when the doping amount of Fe is higher than thesolid solubility limit, as for C70F30, Fe would separate from solidsolutions as the second phase. In this case, no capping effect was

Fig. 6. TEM image of C95F5 aging for 96 h using NaOH as the precipitate agent andafter drying.

present. Therefore, only nanoparticles were observed for C70F30.Furthermore, the presence of nanoparticles in addition to nanorodsis due to the inhomogeneity of the precipitates during the aging,which is also evidence of the anisotropic growth.

Summarily, the dissolution/recrystallization mechanism can beproposed, in which the larger particles grow at the expense of thesmaller particles driven by the decreasing of surface energy [41],which is described in Scheme 1.

Fig. 7. TEM image of CeO2 aging for 48 h in a sealed condition using ammonia as theprecipitate agent and after drying.

Scheme 1. The formation mechanism of the CxFy nanorods.

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3.3. Cr doping

The same strategy was extended to the synthesis of the Cr-dopedCeO2, and 1D nanostructure was also obtained. Our previous resultsfor Cr-doped CeO2 show that Cr is highly dispersed on the sur-face of CeO2 (no Cr2O3 phase is observed) when the Cr/(Cr + Ce)ratio is below 20 at.% [42]. The evolution of the morphology ofthe Cr-doped CeO2 with Cr doping contents is shown in Fig. S3.As expected, nanorods were observed when the Cr/(Cr + Ce) ratiois below 10 at.%. While only nanoparticles were obtained for the20 at.% Cr-doped CeO2. This suggests that the nanorod morphologyof the Cr-doped CeO2 is dependent on doping with Cr in the con-centration range of high dispersion. As an example, Fig. S3f showsthe TEM (HRTEM) images of the 5 at.% Cr-doped CeO2 after cal-cination at 650 ◦C for 6 h. Although the nanorod morphology wasobserved, some of the nanorods collapsed into shorter nanorodsand the particles became irregularly shaped.

The enrichment of Cr on the surface confirmed the above-suggested mechanism. The presence of Cr-doped CeO2 nanorodstestified that this method is suitable for the synthesis of the dopedCeO2 with similar transition metals.

4. Conclusions

Fe-doped CeO2 solid solution nanorods have been prepared viaa simple coprecipitation process without templates/surfactants atambient temperature and pressure. The formation of the nanorodmorphology strongly depends on Fe contents. Beyond the solidsolution limit, only nanoparticles were observed. The morphologywas also affected by precipitation agents, temperatures and agingtimes. The single crystalline nanorod structure was preserved aftercalcination at higher temperatures.

The formation of nanorods originates from the intrinsicanisotropy of Ce hydroxides. The dissolution/recrystallizationmechanism was proposed, in which the coexistent ammonia and/orFe act as the capping agents for the formation of nanorods. Simulta-neously, the enrichment of Fe on the surface results in the nanorodswith round shape and sharp tips.

The element-capped concept for the formation of the nanorodmorphology is applicable to Cr-doped CeO2 mixed oxides.

Acknowledgements

This work was supported by the 863 program of the Ministryof Science and Technology of the People’s Republic of China (no.2008AA06Z320), the National Natural Science Foundation of China(no. 20777028, 21077043) and the Program of the Development ofScience and Technology of Shandong Province (no. 2011GSF11702).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.cej.2011.10.006.

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