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Facile Synthesis under Near-Atmospheric Conditions and Physicochemical Properties of Hairy CeO2 Nanocrystallines Xiaodong Li,† Ji-Guang Li,*,†,‡ Di Huo,† Zhimeng Xiu,† and Xudong Sun† Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials and Metallurgy, Northeastern UniVersity, Shenyang 110004, China, and Nano Ceramics Center, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan ReceiVed: NoVember 3, 2008; ReVised Manuscript ReceiVed: December 2, 2008

Treating freshly prepared cerous hydroxide with hydrogen peroxide (H2O2) under ambient conditions provides a facile and efficient way for converting CeO2 nanoparticles (∼22 nm, 37 m2/g) into nanowires of fine diameters (down to ∼5 nm) and outstanding specific surface areas (∼184 m2/g). The CeO2 nanomaterials were examined by XRD, BET, FT-IR, TG, HR-TEM, Raman scattering, and UV-vis absorption spectroscopy. The results show that the nanowires are single crystalline and of high purity and low defect (oxygen vacancy) concentration and are growing perpendicular to {220} planes. The CeO2 nanowires show indirect and direct band gaps of ∼2.37 and 2.91 eV, respectively, which are both ∼0.15 eV lower than those observed from the nanoparticle form (∼22 nm) for the respective interband transitions. The CeO2 nanowires were proposed to grow via a dissolution-reprecipitation mechanism, with an unstable Ce(OH)3O · OH hyperoxide intermediate as the template. 1. Introduction Cerium dioxide (ceria, CeO2) exhibits diverse interesting physicochemical properties owing to its excellent ability to accept a wide range of dopants while keeping its fluorite crystal structure and its strong redox capability (oxygen storage ability). Ceria and its derivatives have been finding a wide range of applications in intermediate-temperature solid oxide fuel cells (IT-SOFCs), heterogeneous catalysis, luminescent materials, polishing materials, gas sensors, UV blockers, and especially automotive three-way catalysts (TWC).1 As the underlying physicochemical reactions essential to most of the above applications take place on surfaces/interfaces, nanocrystalline CeO2 has been a focus of studies during recent years due to its unique properties arising from its low dimensionality and its high specific surface area. A handful of methodologies have been developed for nanocrystalline CeO2 synthesis, either by direct crystallization or via thermal decomposition of its specific precursors. Thanks to these extensive efforts, CeO2 is now available in various morphologies, including discrete nanocrystallites with a variety of crystal shapes,2 spherical aggregates of nanocrystallites,3 nanotubes,4 nanowires/nanorods,5 and mesostructured powders.6 Subsequent property investigations indicate that the performances (such as catalytic properties) of these low-dimensional CeO2 nanomaterials may heavily depend on their particle shape and the specific crystal planes exposed outside. For example, the experimentally observed excellent oxygen-storage capability and high reducibility of CeO2 nanorods and nanotubes were postulated to arise from the surfaces terminated by (110) and (100) planes.2g,7 Due to their high specific ratio, CeO2 nanorods/nanowires have been drawing much attention among the various morpho-

logical forms reported up to date, and they might be especially desirable for sensing, heterogeneous catalysis, and TWC applications. This is due to the facts that (1) the surfaces of nanorods/nanowires are readily accessible by gases due to the porous structure formed by the bridging of individual nanorods/ nanowires and (2) sintering, which has been a major cause of surface area reduction at elevated temperatures, may be significantly retarded for an assembly of nanowires/nanorods when compared with the nanoparticle form,2b,c,8c due to the significantly porous structure. Several techniques, mostly templated growth, have been available in the literature for CeO2 nanorod/nanowire synthesis. These mainly include polyethylene glycol (PEG) directed growth via ultrasonication (5-10 nm diameter),5a sodium bis(2-ethylhexyl) sulfosuccinate templated synthesis via precipitation and thermostatic treatments (30-120 nm diameter),5b cetyltrimethylammonium bromide (CTAB) and octadecylamine directed growth (10-25 nm diameter),5c,d anodic alumina membranes (AAM) templated synthesis (60-70 nm diameter),5e and chemical precipitation combined with hydrothermal treatments (20-70 nm diameter).5f The above techniques generally need high temperature, high pressure, or lengthy processing, and their low efficiency imposes severe limitations to the practical application of the nanomaterials. Furthermore, the resultant nanorod/nanowire rarely exhibits a diameter thinner than about 10 nm. We report in this work a straightforward way to synthesize hairy CeO2 nanocrystallines (nanowires) of high specific surface area (184 m2/g) and fine diameters (down to 5 nm) under near-atmospheric conditions. Structural features, thermal behavior, and optical properties of the nanowires were characterized in depth, and a formation mechanism was also proposed based upon comparative studies. In the following sections, we report the synthesis, characterization, and properties of the CeO2 nanowires.

* Corresponding author. Tel: +81-29-860-4394. Fax: +81-29-860-4701. E-mail: [email protected]. † Northeastern University. ‡ National Institute for Materials Science.

2. Experimental Section 2.1. CeO2 Nanowire Synthesis. The generation of CeO2 nanowires mainly consists of two steps: (1) reacting an aqueous

10.1021/jp809703h CCC: $40.75  2009 American Chemical Society Published on Web 01/12/2009

Hairy CeO2 Nanocrystallines

Figure 1. XRD patterns of the nano-CeO2 samples synthesized under various conditions. R denotes the H2O2/Ce molar ratio.

solution (0.2 M, 250 mL) of cerous nitrate (Ce(NO3)3 · 6H2O, reagent grade, Wako Pure Chemical Industries, Osaka, Japan) with an ammonia solution (Wako, diluted to 1.0 M) for precipitation, and (2) treating the fresh precipitate obtained in step 1 with a 30% hydrogen peroxide solution (H2O2, reagent grade, Junsei Chemical Co., Ltd., Tokyo, Japan). All the reactions were performed under atmospheric conditions. In step 1, ammonia solution was dripped at a speed of 5 mL/min into the nitrate solution under magnetic stirring until a final pH of 9 was reached, and the resultant suspension was divided into several equal parts. One part of the suspension was directly washed repeatedly with distilled water via ultrasonication and suction filtration till pH ) 7 and finally rinsed with anhydrous ethanol before drying at 250 °C for 24 h. To other parts, H2O2 was added at 1 mL/min under magnetic stirring of the suspensions and the reaction was allowed to continue for 30 min before performing the washing and drying procedures mentioned above. The H2O2/Ce molar ratio (R) was varied to investigate its effects on properties of the final products. 2.2. Characterization Techniques. Phase identification was performed via X-ray diffractometry (XRD) in conjunction with Raman spectroscopy. XRD analysis was conducted on a Philips PW1800 diffractometer (Philips Research Laboratories, Eindhoven, The Netherlands) operating at 40 kV/50 mA using nickelfiltered Cu KR radiation and a scanning speed of 1.0° 2θ/min. Raman spectroscopy was made using Ar+ laser excitation (514.5 nm) with an incident power of 50 mW and a resolution of 1 cm-1 (Model NR-1800, JASCO, Tokyo). Fourier transform infrared (FT-IR) spectroscopy (Model FTS-65, Bio-RAD Laboratories, Tokyo) of the nanomaterials was performed by the standard KBr method. Thermogravimetry (TG, Model TAS200, Rigaku, Tokyo) was made in stagnant air with a heating rate of 10 °C/min. Specific surface area of the nano-CeO2 was determined by Brunauer-Emmett-Teller (BET) analysis (Model Belsorp 18, Bell Japan Inc., Tokyo) via nitrogen adsorption at 77 K, after pretreatment at 200 °C for 2 h. Morphologies of the products were observed via transmission electron microscopy (TEM, Model JEOL FEM-3000F, Tokyo) operating at 300 kV. Optical properties of the CeO2 nanomaterials were studied via UV-vis absorption spectroscopy (Model V-570, JASCO Co., Tokyo). 3. Results and Discussion Figure 1 shows XRD patterns of the powders synthesized under various intended conditions. It can be seen that, in each case, the direct product has been crystallized and almost all the

J. Phys. Chem. C, Vol. 113, No. 5, 2009 1807 characteristic diffractions corresponding to CeO2 with the fluorite crystal structure (JCPDS No. 34-394, space group Fm3jm) have appeared. H2O2 treatment produces appreciably broadened XRD peaks, suggesting decreased dimensions of the crystallites. X-ray line broadening analysis via the Scherrer formula performed on the (111) diffractions yielded average crystallite sizes of ∼21.5, 11.4, 7.1, and 8.3 nm for the CeO2 nanocrystallines processed at R ) 0, 5, 10, and 20, respectively. Lattice parameters of the as-made CeO2 nanomaterials were determined by fitting the observed reflections with a least-squares computer program based on the Rietveld method to be a ) 0.5416 ( 0.0009 nm and 0.5419 ( 0.0011 nm for the powders made at R ) 0 and 5, respectively, while similar values of 0.5421 ( 0.0014 nm for those synthesized at R ) 10 and 20. These lattice edge constants are slightly higher than that (a ) 0.541 09 nm) reported in the data file (JCPDS No. 34-394). A similar lattice relaxation phenomenon was previously observed from CeO2 nanocrystallites at decreased sizes.8 It was understood that the chemical bonds have a directional character in oxide particles and that at the outer surface of each particle there would be unpaired electronic orbitals that would repel each other.9 This contribution from the surface layer increases with decreasing crystallite size and thus leads to a larger value of the lattice parameter.9 For nanocrystalline CeO2, which generally possesses high redox capabilities, another potential reason for the observed lattice expansion is the presence of oxygen vacancies in the crystal lattice, which would create larger Ce3+ ions for charge compensation (for 8-fold coordination in the fluorite structure, Ce3+ and Ce4+ have their respective ionic radii of 0.1143 and 0.097 nm).10 Our results of Raman spectroscopy shown below, however, did not reveal the existence of an appreciable amount of oxygen vacancies, which may imply that the observed lattice relaxation is largely owing to size effect. BET analysis found specific surface areas of ∼37, 108, 184, and 167 m2/g for the nano-CeO2 prepared with R ) 0, 5, 10, and 20, respectively. The drastically increased specific surface areas by H2O2 treatment are in accordance with the finer crystallite sizes revealed via XRD analysis. Figure 2 shows TEM images of the CeO2 nanomaterials synthesized under some typical conditions, from which the pronounced effects of H2O2 treatment on particle morphology can be clearly seen. The inserted electron diffraction (ED) patterns comply with the crystalline nature of the samples revealed by XRD. The CeO2 powder made at R ) 0 is obviously composed of rounded particles with primary sizes ranging from ∼10 to 100 nm, with the presence of some crystallites cemented together to form hard aggregates. Treating the freshly prepared precipitate with H2O2 at R ) 5 largely converts “nanoparticles” into “nanowires” with diameters of 5-20 nm and lengths of up to ∼200 nm (Figure 2b). It is noticeable that at this R value a small portion of nanoparticles still remains in the final product but with much finer average sizes (sub-10 nm) when compared with those in Figure 2a, indicating an eroding away of the nanoparticles by H2O2. Increasing the R value to 10 almost completely eliminates the nanoparticles, yielding a mass of “nano-hair” with diameters down to ∼5 nm (Figure 2c). Inset in Figure 2c is the HR-TEM image of a single-crystalline CeO2 nanowire, where the well-resolved lattice fringes indicate its excellent crystallinity while the spacing of ∼0.19 nm corresponds well to the {220} planes of the CeO2 lattice. Further increasing the R value to 20 does not alter the overall hairy morphology of the product, but the nanowires tend to be less uniform in diameter, with the appearances of rodlike ones with bigger diameters of up to ∼40 nm.

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Figure 4. TG traces showing thermal decomposition behaviors of the CeO2 nanoparticle (R ) 0) and nanowire (R ) 10). Data were taken in stagnant air at a constant heating rate of 10 °C/min.

Figure 2. TEM micrographs showing morphologies of the CeO2 nanomaterials synthesized in this work. Panels a-d correspond to R values of 0, 5, 10, and 20, respectively. The bottom inset in (c) is the lattice fringe of a single CeO2 nanowire with a diameter of ∼5 nm.

Figure 5. Raman spectra of the nanocrystalline CeO2 obtained under some typical synthetic conditions. The sharp lines indicated with stars (/) are noise signals.

Figure 3. FT-IR spectra of the CeO2 nanoparticle (R ) 0) and nanowire (R ) 10).

Figure 3 exhibits FT-IR spectra for two typical samples. The existence of crystal water was confirmed for both the nanoparticle and nanowire forms, as evidenced by the appearances of absorption bands centered at ∼1640 and 3430 cm-1. No other IR absorption band was clearly resolved. The 1640 cm-1 absorption is associated with the H-O-H bending mode (υ2) of molecular water while that at 3430 cm-1 may correspond to the O-H stretching vibrations in molecular water (symmetric υ1 and antisymmetric υ3, at approximately 3200-3500 cm-1).11 Hydroperoxyl (-OOH) groups would exhibit typical IR absorptions at ∼1400 cm-1,12 which were not found for the H2O2 treated samples, implying high purity of the CeO2 nanowires. TG analysis revealed negligible weight losses for both the

nanoparticle (∼1.1 wt %) and the nanowire (∼1.8 wt %) up to ∼1000 °C (Figure 4). Assuming that all the weight losses arise from crystal water, compositions of the nanoparticle and nanowire might be expressed as CeO2 · 0.107H2O and CeO2 · 0.175H2O, respectively. The slightly higher weight loss of the nanowire form is understandable from its much higher specific surface area, which allows enhanced surface absorptions in air. Figure 5 exhibits Raman spectra of the nanostructured CeO2 samples. The strong scattering peak observed at around 464 cm-1 for the R ) 0 sample (nanoparticle) corresponds to the triply degenerate F2g mode of the fluorite crystal structure of CeO2, which is the only one allowed in first order.8a,13 For the H2O2-treated samples, this main Raman band red-shifted to ∼461 cm-1 with appreciably increased line width and asymmetry. Assuming that the asymmetry is defined as the ratio lw/ rw between the left-hand and right-hand half-widths of the band, values of ∼0.763, 0.844, and 0.986 were determined for the samples prepared at R ) 10, 5, and 0, respectively. The red shift of this main Raman band is attributable to the lattice relaxation revealed by XRD,8a,13a while several other factors, such as phonon confinement, increased lattice strain, nonuniform strain and variations in phonon relaxation may account for the broadening and the loss of symmetry of the F2g band at

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Figure 6. UV-vis absorption spectra of the CeO2 nanoparticle and the nanowire.

decreased crystallite/particle sizes.8a,13a Another important observation is the absence of a ∼595 cm-1 peak from these Raman spectra. The occurrence of this 595 cm-1 scattering is known to be associated to a local mode centered on oxygen vacancies8a,13a,14 and has been widely observed in CeO2-x and rareearth-doped CeO2.8a,13a,15 The lack of this peak may thus suggest negligible amounts of vacancies in the crystal lattice of our CeO2 nanomaterials. Nano-CeO2 with particle/crystallite sizes of below 10 nm tends to possess spontaneously formed oxygen vacancies due to its enhanced redox ability at the nanoscale. The low contents of oxygen vacancies revealed from our CeO2 samples might be understood by considering that (1) the CeO2 nanoparticles made at R ) 0 has a mean crystallite size (∼22 nm) well above 10 nm and accordingly a low vacancy concentration, and (2) the high oxidation power of H2O2 diminishes Ce3+ ions and therefore oxygen vacancies in the CeO2 lattice despite the sub-10 nm diameter of the nanowires. Optical responses of the CeO2 nanomaterials have been investigated via UV-vis absorption spectroscopy, and the results are presented in Figure 6 for two typical samples. The absorption edges extend to ∼500 nm for both the nanoparticle and the nanowire, which are in accordance with the vivid yellow colors of the samples. The nanowire, however, was observed to have higher absorptions of the blue and UV lights than the nanoparticle form, allowing it to be used as a better UV blocker. CeO2 has been treated as a semiconductor,8a,16 and the estimation of its band gap energy can be made from the absorption spectra (Figure 6), where the steep increases of absorption are observed due to the charge transfer transitions from O 2p to Ce 4f, which overrun the well-known f to f spin-orbit splitting of the Ce 4f state.16b Semiconductors are classified as either direct or indirect according to the lowest allowed electronic transition. Direct semiconductors are characterized by the minimum of the lowest conduction band positioned in k space directly under the maximum of the highest valence band, while for indirect semiconductors the minimum of the lowest conduction band is shifted relative to the maximum of the highest valence band and the lowest-energy interband transition must then be accompanied by phonon excitation.17 Both direct and indirect interband transitions may occur simultaneously under irradiations with phonons of sufficient energy17 and can be distinguished by their energy dependence of the optical absorption edge. The relation between absorption coefficient (R) and

Figure 7. Determination of indirect (a) and direct (b) interband transition energies for the CeO2 nanoparticle and nanowire. The A in the Y axis title in part (b) represents absorbance, which is proportional to the absorption coefficient R.

incident photon energy (hν) can be written as R ) Bd(hν Eg)1/2/hν and R ) Bi(hν - Eg)2/hν for allowed direct and indirect transitions, respectively,18 where Bd and Bi denote absorption constants for direct and indirect transitions, respectively. Figure 7 shows indirect (a) and direct (b) absorptions of the CeO2 nanoparticle and nanowire as a function of phonon energy, from which the indirect and direct interband transition energies of ∼2.50 and 3.07 eV, respectively, were determined for the nanoparticle form by extrapolating the linear parts of the curves. The nanowire has its respective indirect and direct band gaps of ∼2.37 and 2.91 eV, as determined similarly. For both the direct and indirect transitions, Eg is lowered by ∼0.15 eV along with the crystallite size decreasing from ∼22 nm for the CeO2 nanoparticle to ∼7 nm for the nanowire. Though there have been reports showing that the Eg of nano-CeO2 would increase at a decreased crystallite size due to the quantum confinement effect,16a,b a detailed study by Suzuki et al.16c on pure CeO2 and Gd-doped ones revealed two regions of the size dependence of band-gap energy: it increases along with decreasing grain size to ∼50 nm and then decreases progressively at even smaller crystallite sizes. Such a tendency conforms to our observations. In addition, both the band-gap values and the size-dependent

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Eg variation observed in this work are in good agreements with the recent findings of Herna´ndez-Alonso et al.8a for quasistoichiometric CeO2 nanoparticles of ∼6-9 nm. It should be noted that the optical properties of nano-CeO2 seems rather sensitive to synthetic routes, or in other words surface states/ surface impurities of the nanomaterials. A detailed characterization of the photochemical processes taking place within the surface layers would provide further insights to the optical responses of nano-CeO2. It is now the time to address the formation processes of CeO2 nanoparticles and nanowires. The generation of Ce(IV) oxide from a Ce(III) precursor involves an oxidation process, which has been well documented in the literature.2b,19 The direct precipitate produced by titrating cerous nitrate solution with ammonia-water is primarily of cerous hydroxide (Ce(OH)3), which is unstable in air and readily undergoes oxidation, especially at elevated temperatures, to yield Ce(OH)4. Dehydration of this cerium (IV) hydroxide produces ceria with a general formula of CeO2 · nH2O, where the n value depends upon the dehydration temperature (the extent of dehydration). This would account for the formation of CeO2 nanoparticles at R ) 0. The formation of CeO2 nanowires via H2O2 treatment involves another scenario;20 that is, H2O2 oxidizes the freshly precipitated Ce(OH)3 to form unstable Ce(OH)3O · OH (or CeO3 · 2H2O hyperoxide) compound, which then yields CeO2 · nH2O via decomposition. The reactions involved in this latter case may be expressed as follows:

4. Conclusions CeO2 nanowires have been synthesized by treating freshly prepared cerous hydroxide with hydrogen peroxide (H2O2) under ambient conditions followed by drying at 250 °C. The resultant nanowires are single crystalline and possess high purity, low contents of oxygen vacancies, ultrathin diameters (down to ∼5 nm), and a high specific surface (∼184 m2/g). Growth of the nanowires was found to be perpendicular to {220} planes. Energies for indirect and direct interband transitions of the CeO2 nanowires were determined from UV-vis absorption spectra to be ∼2.37 and 2.91 eV, respectively; both are ∼0.15 eV lower than those of the nanoparticles (∼22 nm) for the respective transitions. The CeO2 nanowires were proposed to be templated from unstable Ce(OH)3O · OH via a dissolution-reprecipitation mechanism. Acknowledgment. This work has been partially supported by the National Science Fund for Distinguished Young Scholars (50425413), the Program for New Century Excellent Talents in University (NCET-25-0290), the National Natural Science Foundation of China (50672014, 50772020), and the Program for Changjiang Scholars and Innovative Research Teams in University (PCSIRT, IRT0713). References and Notes

2Ce(OH)3 + 2H2O2 f H2 + 2Ce(OH)3O · OH

words the slow introduction of solute species via solid/liquid interfacial reaction is indispensible to nanowire building-up.

(that is, CeO3 · 2H2O) (1)

2CeO3 · 2H2O f 2CeO2 · nH2O + O2

(2)

It was indeed observed that the freshly prepared precipitate gradually turns from pale purple to orange yellow with heat and gas release upon H2O2 treatment (eq 1), while after drying the product turns bright yellow (eq 2), exhibiting a color typically observed for nanocrystalline CeO2. TEM observations clearly indicate that H2O2 acts as not only an oxidant for Ce(OH)3 but also an effective particle morphology modifier. The cubic fluorite structure of CeO2 generally prevents its anisotropic growth into nanowire/nanorod shapes, and this is the main reason why CeO2 nanowires/nanorods have been mostly synthesized via templated growth strategies. Based upon the above observations, it is believed that in this work the CeO2 nanowires are formed via a dissolutionreprecipitation mechanism (eq 1) and that Ce(OH)3O · OH templates nanowire growth. That is, the newly precipitated cerous hydroxide is gradually etched away upon hydrogen peroxide addition and the cerium hyperoxide nucleates when its solubility product is reached. This hyperoxide compound, however, has rarely been characterized in terms of crystal structure and so on due to its poor stability even under ambient conditions.21 The H2O2/Ce molar ratio needed for a complete conversion of nanoparticles to nanowires (R ) ∼10) is much higher than the stoichiometric value (R ) 1, eq 1), and this is due to the exothermic nature of eq 1, which causes H2O2 to decompose. It is noteworthy that preoxidizing Ce3+ to Ce4+ ions with H2O2 followed by titration with ammonia-water yields nanoparticles rather than nanowires.19b This was thought to be due to the high nucleation density of liquid phase reactions, which hinders anisotropic growth of the nuclei into nanowires. In this context, the solid-liquid interfacial reactions between Ce(OH)3 and H2O2 are crucial to the development of nanowire morphologies, or in other

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