Ceria mesostructures
Controlled Assembly of Preformed Ceria Nanocrystals into Highly Ordered 3D Nanostructures** Atul S. Deshpande, Nicola Pinna, Bernd Smarsly, Markus Antonietti, and Markus Niederberger*
Design and fabrication of mesostructured materials has been a pivotal issue in the development of materials science.[1–3] Various strategies have been developed during the last decade to obtain materials with ordered mesoporous structures.[4, 5] Most of the methods require templating techniques that involve the use of a molecular moiety or a supramolecular assembly as a structure directing agent, and a molecular precursor that can be transformed by physicochemical processes into the final inorganic material. The precursor-based methods have proven useful in obtaining silica as well as non-silica materials. In this context, mesoporous crystalline CeO2 is a highly promising candidate for applications in catalysis.[6, 7] Accordingly, several studies report the fabrication of mesoporous crystalline ceria.[8–10] However, the ceria mesostructure often undergoes a severe breakdown throughout the final crystallization step, which leads to rather ill-defined porosity without controlled nanocrystallinity in the pore walls, in terms of the spatial distribution and the size of the oxide nanocrystals. Although partially crystalline mesoporous materials have been reported a few years ago,[11] the preparation of fully crystalline frameworks is still the major issue in the synthesis of these materials, and only recently the preparation of almost fully crystalline mesoporous metal oxides has been reported.[12] In order to avoid these difficulties, the use of preformed, crystalline nanoparticles with controlled shape and size for the fabrication of organized structures is highly advantageous.[13] Even though this approach has been successfully employed in obtaining macroporous materials,[14, 15] reports about the preparation of ordered mesoporous materials based on these ideas remain scarce. Ying and co-workers demonstrated the synthesis of thermally stable, mesoporous tungstated zirconium oxide, using zirconia nanoparticles as building blocks and a tungstate salt as the “binding agent” to achieve good
[*] A. S. Deshpande, N. Pinna, B. Smarsly, M. Antonietti, M. Niederberger Max Planck Institute of Colloids and Interfaces Research Campus Golm 14424 Potsdam (Germany) Fax: (+ 49) 331-5679502 E-mail:
[email protected] [**] Financial support by the Max Planck Society is gratefully acknowledged. We thank the Fritz-Haber-Institute and Prof. R. Schlçgl for the use of the electron microscope and Klaus Weiss for technical assistance. We also thank Dr. H. Schlaad and I. Below for supplying the block copolymers and J. Polleux for the low-resolution TEM images. small 2005, 1, No. 3, 313 –316
DOI: 10.1002/smll.200400060
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communications connectivity between the particles.[16] A recent report presented highly ordered mesoporous TiO2 thin films using anatase sols,[17] whereas functionalized ceria nanoparticles were used for the synthesis of mesoporous CeO2 powders with a hexagonal pore structure.[18] The latter work highlights a novel application of nanostructured CeO2 as a potential solar-cell material. In this work, we report the assembly of nearly monodisperse and highly crystalline ceria nanoparticles (with a diameter of about 3 nm) into highly ordered 3D mesostructures. Compared to previous studies in the literature, the present approach shows several particularities and novel aspects. For the first time, we demonstrate the use of sols of pure (that is, without any organic functionalization to stabilize the particles in solution) CeO2 nanoparticles as building blocks in a block-copolymer-assisted assembly process to obtain mesoporous materials. Nanoparticle sols with the composition Ce1 xZrxO2 (x = 0–1) have been synthesized and fully characterized before.[19] Furthermore, the mesopores are particularly large and show an exceptionally high degree of ordering in terms of pore shape and 3D arrangement. A hydrogenated polybutadiene–poly(ethylene oxide) block copolymer (PHB–PEO; H[CH2CH2CH2CH(CH2 CH3)]x(OCH2CH2)yOH; MW = 4400 g mol 1 for the PHB block and MW = 3920 g mol 1 for the PEO block) was used as structure-directing agent. These types of block copolymers have been used to obtain highly ordered cubic mesoporous silica and titania structures with large ( 13–14 nm) spherical pores.[12, 20] In comparison to the commercially available family of Pluronic block copolymers, the micelles of these polymers have higher hydrophobic contrast between the two blocks, low sensitivity towards alcohols, and higher temperature stability. These features make PHB– PEO block copolymers highly suitable for assembly processes. As an important feature, this polymer can form aggregates even in ethanol and THF. This study comprises a complete and thorough quantitative structural investigation by various methods such as SAXS, WAXS, TEM, HRTEM, and physisorption. The synthesis protocol involves the addition of the CeO2 nanoparticle sol in a mixture of ethanol and water to an alcoholic solution of the PHB–PEO block copolymer. Evaporation of the solvent induces the cooperative assembly of the nanoparticles and the PHB–PEO block copolymer micelles.[21] The amount of water present in the system plays a crucial role in this process. It was observed that the use of a purely aqueous sol instead of an ethanol/water mixture resulted in disordered materials. This might be due to the fact that gelation of the nanoparticles interferes with the ordering of the micelles, since both are driven by an increase in concentration upon solvent evaporation. In highly aqueous media, gelation of the nanoparticles occurs faster than the ordering process. The water content of the nanoparticle sol was reduced by dialyzing it against ethanol. As the stability of the sol is reduced by decreasing the water content,[19] the solvent composition was optimized to keep a stable sol upon addition of the block copolymer solution. As seen from Figure 1 a, the SAXS patterns of the asprepared as well as the calcined sample show a sharp and
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Figure 1. a) SAXS patterns for the as-prepared sample and the sample calcined at 500 8C for 2 h. Arrows indicate the position of the reflections; b) SAXS pattern of the as-prepared sample (&) in comparison to a pattern generated based on numerical calculation assuming a distorted closed-packed fcc lattice of spherical voids (c).
well-resolved high-intensity first-order reflection, followed by two low-intensity higher order reflections. Figure 1 b shows the simulated pattern assuming a distorted lattice of closed-packed spheres of 14 nm in diameter. For the structure factor, we applied both a face-centered cubic (fcc) lattice and the Percus–Yevick approach.[22] The simulated pattern fits well with the experimental SAXS pattern of the asprepared material in terms of peak positions and intensities of the reflections. Similar fittings were observed for the calcined samples, which provided a pore size of about 12 nm in size and an fcc lattice parameter of a = 18 nm. The TEM images of the as-prepared sample show large domains of highly ordered spherical pores. The pore size was determined from these images to be approximately 11 nm. Figure 2 a and b show the cubic arrangement of the mesopores, viewed along different directions. The pore walls have a thickness of about 6–7 nm, which suggests that the walls mainly consist of a double layer of nanoparticles, as the average size for these nanoparticles was reported to be 2.5 nm.[19] Figure 3 a shows the TEM image of the calcined sample, which proves that the mesostructure was well retained. Only minor shrinkage due to the consolidation of the inor-
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Figure 2. TEM images of the as-prepared samples: a) [100] direction; b) same sample viewed along the [110] direction.
the walls was further confirmed by HRTEM investigations (Figure 3 c), which showed well-defined pore walls consisting of nanoparticles with sizes of around 6–7 nm. The lattice fringes are randomly oriented, which indicates that the pore walls are completely crystalline, but there exists no specific orientation of the nanoparticles with respect to each other. The power spectrum of the same image also displays a ring pattern (Figure 3 d). The increase in the particle size was induced by the heat treatment. The XRD pattern of the calcined sample (Figure 4) shows all the characteristic reflections of cubic CeO2. The increase in crystallite size is evident from the decrease in peak width as compared to the initial nanoparticles.[19] The average crystallite size calculated from the Scherrer equation was found to be 6.8 nm.
Figure 4. Wide-angle XRD pattern of the sample calcined at 500 8C for 2 h.
ganic matrix was found, which resulted in a decreased pore size of about 10 nm. Selected-area electron diffraction of such a zone (Figure 3 b) shows diffraction rings characteristic of a structure composed of small domains with their crystallographic axes randomly oriented with respect to each other. The d spacing measured from the diffraction rings are in good agreement with the cubic CeO2 structure (JCPDS: 34-394). The random orientation of the nanoparticles within
Nitrogen sorption studies carried out on the calcined sample showed a type IV adsorption–desorption isotherm with an H2-type hysteresis, thus confirming that the sample is mesoporous (Figure 5). The pronounced hysteresis suggests that the spherical mesopores are connected through smaller pores. The BET surface area obtained for the calcined sample was 87 m2 g 1. The relatively low specific surface area value is partly due to the high density of CeO2 (7.132 cm3 g 1). While the determination of the BET surface area and also the mesopore size distribution suffers from the unknown interaction potential between nitrogen and the CeO2 material, the pore volume (0.12 mL g 1) allows a direct comparison with the SAXS analysis. Based on a fcc arrangement of the mesopores, and taking into account the pore size (12 nm) and the lattice parameter (a = 18 nm) as determined by SAXS, the pore volume of the mesopores amounts to 0.13 mL g 1, with an internal surface area of 61 m2 g 1. This is in reasonable agreement with the data obtained from the nitrogen sorption study. Hence, these results point to an almost complete accessibility of the mesopores. In conclusion, the present study successfully demonstrates the block-copolymer-assisted assembly of crystalline CeO2 nanoparticles into crystalline mesoporous structures
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Figure 3. a) TEM image of the sample calcined at 500 8C for 2 h; b) selected-area electron diffraction of a zone of 250 nm in diameter; c) HRTEM image of a zone of 32 32 nm2 ; d) the associated power spectrum.
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communications 500 8C for 2 h. A special calcination procedure was used in which the temperature was raised to 250 8C with a ramp of 2 8C min 1. After reaching 250 8C, the temperature was kept constant for 1 h, raised to 500 8C, and held constant at 500 8C for 2 h. Upon calcination, powdered yellow products were obtained. Characterization: A Kratky camera setup (Anton-Paar, Austria) was used to record the SAXS patterns of all the samples. The samples were analyzed in transmission mode under vacuum with an X-ray source of CuKa radiation. The XRD studies of the sample were carried out in reflection mode (CuKa radiation) on a Bruker D8 diffractometer equipped with a scintillation counter. TEM images were taken using Zeiss EM-912Omega instrument operating at an acceleration voltage of 120 kV, and HRTEM images and SAED were recorded on a Philips CM200 FEG microscope operated at 200 kV. Figure 5. Nitrogen adsorption–desorption isotherm for the mesoporous CeO2 sample obtained after calcination at 500 8C for 2 h.
Keywords: block copolymers · cerium · mesoporous materials · nanostructures · oxides
without the aid of external agents such as surface functionalization or binding agents, to yield bulk mesostructured powders with a high degree of pore ordering. The mesostructure was stable upon calcination at 500 8C and consists of relatively large mesopores of about 10–12 nm, which exhibit a significantly better mutual arrangement compared to those prepared in previous studies.
Experimental Section Materials: Aqueous CeO2 sol containing 0.005 mol of CeO2 was prepared as reported elsewhere.[19] PHB–PEO block copolymers (H[CH2CH2CH2CH(CH2CH3)]79(OCH2CH2)89OH; referred to also as “KLE”) were prepared by coupling “Kraton Liquid” (w-hydroxypoly(ethylene-co-butylene), Exxon) with ethylene oxide using an anionic polymerization procedure. Both blocks are narrowly distributed, with Mw = 4400 g mol 1 for the polyethylene-co-butylene block and Mw = 3920 g mol 1 for the poly (ethylene oxide) block. For further details, see ref. [20] Synthesis: Absolute ethanol (10 mL) was added to the aqueous sol. The ethanol/water sol was dialyzed against absolute ethanol to remove water using Spectra/Por membranes (MWCO: 6-8000). Prior to the dialysis, weights of the dialysis membrane, clips, and the sol were determined separately. From the oxide content, the weight of ethanol added, and the weight of the sol, the amount of water in the sol was calculated. The sol was then transferred to the dialysis membrane and dialyzed against absolute ethanol. The weight of the membrane tube containing the sol was checked every few minutes and absolute ethanol was replaced intermittently. The dialysis was carried out until approximately 5 mL of water remained in the sol. 0.125 g of the PHB– PEO block copolymer was dissolved in absolute ethanol (2.5 mL), and then half of the ethanol/water sol was added dropwise. The resultant solution was transferred to a petri dish, and the solvent was allowed to evaporate under ambient conditions for 2–3 days to obtain a transparent yellow gel. The gel was aged at 100 8C for one day. The dry gel obtained is termed “as-prepared”. The as-prepared gel was calcined in flowing air at
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