Langmuir 2006, 22, 8639-8641
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Toward the Innovative Synthesis of Columnar CeO2 Nanostructures Davide Barreca,*,† Alberto Gasparotto,‡ Chiara Maccato,‡ Cinzia Maragno,‡ and Eugenio Tondello‡ ISTM-CNR and INSTM-Department of Chemistry, PadoVa UniVersity, Via Marzolo, 1-35131 PadoVa, Italy, and Department of Chemistry, PadoVa UniVersity and INSTM, Via Marzolo, 1-35131 PadoVa, Italy ReceiVed April 18, 2006. In Final Form: July 18, 2006 We report on the preparation of supported columnar CeO2 nanostructures by a simple catalyst-free chemical vapor deposition process at temperatures as low as 623 K. A suitable choice of experimental parameters enables us to control the structural and morphological features of the resulting ceria nanosystems.
Recently, 1D oxide-based nanosystems such as rods, belts, and tubes have become the focus of intensive research activity owing to their applications in mesoscopic physics and in the fabrication of nanoscale devices.1,2 This ever-increasing interest is motivated by their anisotropic architecture, allowing us, in turn, to exploit the dependence of their properties on dimensionality and size reduction.3 As a result, columnar 1D structures are expected to have a key impact in several fields, such as catalysis, nanoelectronics, optoelectronics, biotechnology, and gas sensing.2,4 Actually, these applications rely on the synthesis of columnar nanostructures on suitable substrates. Consequently, advancements in these fields are dependent on the availability of suitable synthetic strategies enabling control of the structure, morphology, and composition of the obtained systems. In this context, notable efforts have been devoted to the synthesis of columnar metal oxide nanostructures by several techniques.2,5,6 In particular, cerium(IV) oxide (CeO2), or ceria, with a fluorite-type crystal structure, has been widely employed in three-way catalysts (TWCs) for the control of automobile exhaust emissions, besides being an appealing candidate for the preparation of oxygen sensors and solid electrolytes for fuel cells.2,3,7-9 The functional performances of CeO2-based systems can be suitably altered by controlling both their composition and structure. As concerns the former, the co-presence of Ce(III) and Ce(IV) centers, resulting from oxygen deficiencies, and its synergy with the peculiar nanosystem features have been the objects of recent investigations.2,9 Another tool consists of doping ceria with suitable metal centers (Y, Zr, Hf, Pr, Sm, Gd,...) in order to tailor its conduction properties.10-15 However, the morphological control of CeO2 nanosystems for these applications is still an open challenge. Until now, most syntheses of 1D CeO2
materials have adopted solution-phase techniques such as solgel4,7,16 and solvothermal methods.2,3,17 Nevertheless, none of the above investigations has succeeded in obtaining supported columnar CeO2 nanostructures, as required for technological utilization. Herein, we report the synthesis of ceria nanosystems by chemical vapor deposition (CVD) and their structural and morphological characterization. To the best of our knowledge, no previous studies focusing on the use of CVD routes for the preparation of columnar CeO2 nanostructures have ever been performed. CVD experiments were performed using Ce(hfa)3‚diglyme (Hhfa ) 1,1,1,5,5,5-hexafluoro-2,4-pentanedione; diglyme ) bis(2-metoxyethyl)ether) as the cerium source. This compound has already been used as a molecular precursor in the CVD of CeO2 and CeF3 thin films, and its inherent advantages have already been discussed.18-21 Deposition experiments were carried out on HF-etched Si(100) in the temperature range of 623-773 K under a N2 + O2 reaction atmosphere using a total pressure of 10 mbar and a precursor vaporization temperature of 353 K. Correlations between substrate temperature and the chemicophysical properties of the obtained samples were investigated by glancing incidence X-ray diffraction (GIXRD), X-ray photoelectron spectroscopy (XPS), and field emission-scanning electron microscopy (FE-SEM). Figure 1 reports representative diffraction patterns for selected specimens. As a general trend, well-developed reflections attributable to cubic ceria22 were observed, but their relative intensity was strongly influenced by the deposition temperature, thus indicating a concomitant evolution of the system texturing. In fact, for T e 648 K, the 200 reflection intensity was higher
* Corresponding author. E-mail:
[email protected]. Phone: +390498275170. Fax: +39-0498275161. † ISTM-CNR and INSTM-Department of Chemistry, Padova University. ‡ Department of Chemistry, Padova University and INSTM.
(10) Izu, N.; Shin, W.; Matsubara, I.; Murayama, N.; Oh-hori, N.; Itou, M. Sens. Actuators, B 2005, 108, 216. (11) Izu, N.; Shin, W.; Matsubara, I.; Murayama, N. Electrochemistry 2005, 73, 478. (12) Duncan, H.; Lasia, A. Solid State Ionics 2005, 176, 1429. (13) Mori, T.; Wang, Y. R.; Drennan, J.; Auchterlonie, G.; Li, J. G.; Ikegami, T. Solid State Ionics 2004, 175, 641. (14) Elyassi, B.; Rajabbeigi, N.; Khodadadi, A.; Mohajerzadeh, S. S.; Sahimi, M. Sens. Actuators, B 2004, 103, 178. (15) Stefanik, T. S.; Tuller, H. L. J. Eur. Ceram. Soc. 2001, 21, 1967. (16) Wu, G. S.; Xie, T.; Yuan, X. Y.; Cheng, B. C.; Zhang, L. D. Mater. Res. Bull. 2004, 39, 1023. (17) Tang, B.; Zhuo, L.; Ge, J.; Wang, G.; Shi, Z.; Niu, J. Chem. Commun. 2005, 3565. (18) Pollard, K. D.; Jenkins, H. A.; Puddephatt, R. J. Chem. Mater. 2000, 12, 701. (19) Lo Nigro, R.; Malandrino, G.; Fragala`, I. L.; Bettinelli, M.; Speghini, A. J. Mater. Chem. 2002, 12, 2816. (20) Lo Nigro, R.; Malandrino, G.; Fragala`, I. L. Chem. Mater. 2001, 13, 4402. (21) Lo Nigro, R.; Toro, R. G.; Malandrino, G.; Fragala`, I. L. J. Mater. Chem. 2005, 15, 2328. (22) Pattern no. 34-394, JCPDS 2000.
(1) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (2) Vantomme, A.; Yuan, Z.-Y.; Du, G.; Su, B.-L. Langmuir 2005, 21, 1132. (3) Sun, C.; Li, H.; Zhang, H.; Wang, Z.; Chen, L. Nanotechnology 2005, 16, 1454. (4) Yada, M.; Sakai, S.; Torikai, T.; Watari, T.; Furata, S.; Katsuki, H. AdV. Mater. 2004, 16, 1222. (5) Huang, P. X.; Wu, F.; Zhu, B. L.; Gao, X. P.; Zhu, H. Y.; Yan, T. Y.; Huang, W. P.; Wu, S. H.; Song, D. Y. J. Phys. Chem. B 2005, 109, 19169. (6) Shantha Shankar, K.; Raychaudhuri, A. K. Mater. Sci. Eng., C 2005, 25, 738. (7) Yu, T. Y.; Joo, J.; Park, Y. I.; Hyeon, T. Angew. Chem., Int. Ed. 2005, 44, 7411. (8) Sun, C.; Li, H.; Wang, Z. X.; Chen, L.; Huang, X. Chem. Lett. 2004, 33, 662. (9) Barreca, D.; Gasparotto, A.; Tondello, E.; Sada, C.; Polizzi, S.; Benedetti, A. Chem. Vap. Deposition 2003, 9, 199.
10.1021/la061052q CCC: $33.50 © 2006 American Chemical Society Published on Web 09/13/2006
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Figure 1. GIXRD patterns (angle of incidence ) 0.5°) for CeO2 systems supported on Si(100) as a function of the substrate temperature. The reflections reported for cubic CeO222 are indexed.
than the 111 one, and a different situation was observed at higher T. In particular, at T ) 723 K the intensity ratio I111/I200 was greater than that of the powder spectrum,22 and the value expected for cubic ceria was recovered at T ) 773 K. As a whole, these data point to the presence of a 100 preferred orientation for T e 673 K, whose entity undergoes a progressive decrease on increasing the substrate temperature. Finally, when T ) 723 K, the appreciable 111 relative intensity with respect to 200 suggests the occurrence of 111 texturing. The system chemical composition was investigated by XPS analyses. Regarding Ce 3d photoelectron peaks, their position and shape allowed us to confirm unequivocally the preponderance of Ce(IV) species over Ce(III) ones, irrespective of the synthesis conditions.9 Moreover, all as-grown samples were characterized by the presence of fluorine. In particular, the F 1s signal always presented a double-peak structure, with contributing bands centered at binding energys (BEs) of 684.3 and 688.5 eV, respectively. Although the former was attributed to the presence of Ce(III)-F bonds,23 the latter could be ascribed to the presence of undecomposed precursor residuals.24 Unfortunately, a detailed analysis of the associated C 1s contribution was hampered by the presence of the overlapping Ce 4s signal. The F/Ce atomic ratio underwent a progressive decrease from 0.6, at T ) 623 K, to 0.3, at T ) 773 K. It is worthwhile to observe that fluorine signals disappeared to the noise level after thermal treatment in wet air at 873 K for 2 h, thus suggesting that Ce-F species underwent a hydrolitic transformation to Ce-O species upon annealing. Representative cross-section FE-SEM images are displayed in Figure 2. For T < 773 K, all specimens were characterized by columnar CeO2 nanostructures with morphological features directly dependent on the substrate temperature. For T ) 623 K, a peculiar wheat-ear shape was clearly observable. Under these conditions, each column was formed by the agglomeration of (23) Daniele, S.; Hubert-Pfalzgraf, L. G.; Perrin, M. Polyhedron 2002, 21, 1985. (24) Chadwick, D.; McAleese, J.; Senkiw, K.; Steele, B. C. H. Appl. Surf. Sci. 1996, 99, 417.
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several nanoparticles and presented an average aspect ratio (length/diameter) of 22 ( 3. Correspondingly, their density was as high as 2 × 1010 column/cm2. In a different way, an increase in the substrate temperature resulted in columnar CeO2 nanostructures with less evident granular morphology and more densely packed on the growth surface. The corresponding aspect ratio decreased with T, down to 12 ( 1 at T ) 723 K. A detailed data inspection revealed that this trend was essentially related to a lowering of the growth rate with temperature. As a matter of fact, a thickness reduction is usually encountered in CVD of thin films on increasing the substrate temperature.25 In the present case, it is believed that the lowered column length might be ascribed to a temperature-dependent nucleation modality. In fact, an increase in the thermal energy supply is likely to produce “less-selective” 1D growth, favoring the progressive formation of additional nucleation sites on the substrate with respect to the anisotropic growth over preformed structures. Such an observation is corroborated by the fact that for T ) 773 K columnar morphology was not observed and a ceria continuous film was obtained. Correspondingly, the diffraction pattern (Figure 1) presented the intensity ratio expected for cubic ceria powders.22 This observation indicates that a suitable choice of substrate temperature is necessary for the selective synthesis of columnar CeO2 nanostructures by the adopted CVD route. The dependence of the system morphology on temperature can be rationalized in the framework of the zone model, originally proposed by Movchan and Demchishin25 and already applied to CeO2 films.20,21 An evaluation of the homologous temperature (Th ) Tdeposition/Tmelting(CeO2), where Tmelting(CeO2) ) 2223 K) yields 0.28 e Th e 0.35 for 623 e T e 773 K. Accordingly, a smooth transition from a porous cauliflower columnar structure (I) to a film with coalesced columnar grains (II) is expected, with a transition zone around Th ≈ 0.3. This prediction agrees to a large extent with the present findings because the morphologies observed in Figure 2 lie between those reported for (I) and (II) and evolve toward continuous films at the highest deposition temperatures. The correlations between XRD and SEM results displayed in Figures 1 and 2 point to a relationship between the system structural features and its morphology. In particular, the formation of wheat-ear columnar CeO2 nanostructures was associated with the occurrence of 100 texturing arising from preferential growth along the fastest crystallographic directions.26-28 Because in a cubic system the crystal growth rates perpendicular to different planes are proportional to their surface energies γ (γ111 < γ100), the occurrence of 100 texturing in conjunction with the observed morphology is justified in the present case, indicating that most of the columns grow along the 100 crystallographic direction. In particular, the formation of pseudoconic column tips with spiral-like morphology (Figure 3) occurs by a spiral dislocation mechanism (Frank-Read source),29 where the spiral plane perpendicular to the screw dislocation line provides a continuous low-energy site for growth. The growth rate along the dislocation line is much faster than that in the radial direction, thus resulting in anisotropic structures. These observations hold for T < 723 K because this temperature corresponded to a change in sample (25) Jensen, K. F. In Chemical Vapor Deposition: Principles and Applications; Hitchman, M. L., Jensen, K. F., Eds.; Academic Press: London, 1993. (26) Wu, J.-J.; Liu, S.-C. J. Phys. Chem. B 2002, 106, 9546. (27) Chen, R.-S.; Huang, Y.-S.; Tsai, D.-S.; Chattopadhyay, S.; Wu, C. H.; Lan, Z.-H.; Chen, K.-H. Chem. Mater. 2004, 16, 2457. (28) Greene, L. E.; Law, M.; Tan, D. H.; Montano, M.; Goldberger, J.; Somorjai, G.; Yang, P. Nano Lett. 2005, 5, 1231. (29) Laudise, R. A. In Treatise on Solid State Chemistry; Hannay, N. B., Ed.; Plenum Press: New York, 1975; Vol. 5, Chapter 8, p 413.
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Figure 2. Selected cross-section FE-SEM micrographs of CeO2-based specimens obtained at different substrate temperatures.
Figure 3. Plane-view SEM image for a columnar CeO2 nanosystem grown at T ) 623 K.
texture, as revealed by GIXRD (cf. Figure 1). Correspondingly, most of the columns were oriented along the 111 direction. In conclusion, we have demonstrated an amenable CVD approach to the synthesis of columnar CeO2 nanostructures at temperatures as low as 623 K. A suitable choice of experimental parameters enables us to control the structural and morphological features of the resulting ceria nanosystems. Key points of the proposed route are the absence of catalyst particles and templating matrixes, making the above strategy an amenable alternative for large-scale applications. Studies are currently in progress to investigate the gas-sensing properties of the obtained nanosystems as a function of their preparative conditions. Experimental Section The deposition of ceria-based specimens was performed by a cold-wall low-pressure horizontal chemical vapor deposition (CVD) reactor equipped with a quartz tubular chamber (tube length ) 20 cm; inner diameter ) 3 cm) and a resistively heated susceptor. The pressure and gas flow were measured by capacitance manometers
and mass-flow controllers (MFCs), respectively. The total pressure, flow rates, and substrate temperature were adjusted independently. p-type Si(100) (MEMC, Merano, Italy) wafers were used as substrates and etched in a 2% HF solution to remove the native oxide layer prior to each deposition. CVD experiments were performed by employing Ce(hfa)3‚diglyme (Hhfa ) 1,1,1,5,5,5-hexafluoro-2,4-pentanedione; diglyme ) bis(2metoxyethyl)ether) as the cerium molecular source. Deposition experiments were carried out in the temperature range of 623-773 K under a N2 + O2 reaction atmosphere (1:1) using a total pressure of 10 mbar. The precursor compound, prepared according to the literature,30 was placed in a vaporization vessel that was directly connected to the reactor tube and maintained at 353 K throughout each film deposition. The gas line and valves between the vessels and the reaction chamber were heated to ∼383 K to avoid vapor condensation. Glancing incidence X-ray diffraction (GIXRD) patterns were recorded using a Bruker D8 Advance instrument equipped with a Go¨bel mirror and a Cu KR source (40 kV, 40 mA) at a fixed angle of incidence of 0.5°. X-ray photoelectron spectroscopy (XPS) analyses were performed on a Perkin-Elmer Φ 5600ci spectrometer with a nonmonochromatized Al KR (1486.6 eV) source at a working pressure lower than 2 × 10-9 mbar. The reported binding energies (BEs; standard deviation ) (0.2 eV) were corrected for charging by assigning to the C 1s line of adventitious carbon a BE of 284.8 eV. The atomic compositions were evaluated using sensitivity factors provided by Φ V5.4A software. Field emission-scanning electron microscopy (FE-SEM) measurements were carried out by means of a Zeiss SUPRA 40VP instrument.
Acknowledgment. This work was financially supported by the Research Programs FISR-MIUR “Inorganic and hybrid nanosystems for the development and innovation of fuel cells” and INSTM-PRISMA “Oxide films with high dielectric constant from liquid and vapor phase routes”. LA061052Q (30) Malandrino, G.; Lo Nigro, R.; Benelli, C.; Castelli, F.; Fragala`, I. L. Chem. Vap. Deposition 2000, 6, 233.
