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Innovative Approaches to Oxide Nanosystems: CeO2±ZrO2 Nanocomposites by a Combined PE-CVD/Sol±Gel Route** By Lidia Armelao,* Davide Barreca, Gregorio Bottaro, Alberto Gasparotto, Eugenio Tondello, Matteo Ferroni, and Stefano Polizzi A novel combined PE-CVD/sol±gel approach to oxide-based nanosystems is proposed and employed for the synthesis of CeO2±ZrO2 nanocomposites. The method consists of the plasma-enhanced (PE) CVD of ceria (guest) on a zirconia xerogel ZrOx(OH)y(OR)z (host) prepared by sol±gel (SG) at a temperature of 200 C. The system evolution under annealing in air (600 C and 900 C, 1 h) and its mutual relationships with the synthesis conditions were investigated in detail by a multi-technique characterization. The proposed synthetic pathway enables the synthesis of CeO2±ZrO2 nanostructured systems ranging from composites to solid solutions, as a function of thermal treatment and Ce/Zr ratio as well. In this framework, particular attention is devoted to achieving an intimate Ce/Zr intermixing by exploiting the synergy between the SG xerogel properties (porosity, ±OH content) and the peculiar actions exerted by plasma (ablation/deposition, infiltration power). Such features enable an in-depth penetration of ceria into the inner zirconia xerogel layers already in the as-grown systems. Keywords: CeO2±ZrO2, Nanosystems, PE-CVD, Sol±gel.
1. Introduction CeO2±ZrO2-based materials are widely employed as catalysts for the conversion of emissions from automobiles and industrial plants into non-toxic compounds.[1] The application of such systems is based on the facile CeIII « CeIV switching, influencing the oxygen storage capacity (OSC) of ceria,[2,3] and on the improvement of thermal stability and redox properties featured by ceria±zirconia mixed oxides with respect to pure CeO2.[4,5] The functional properties of Ce-Zr-O materials might be further enhanced by exploiting the synergy between oxygen deficiency and the peculiar characteristics of nanostructured systems,[6] that can be tailored by an adequate choice of the synthetic approach and processing conditions.[7,8] As a matter of fact, two main pathways to the ªnanoº dimensional scale can be ± [*] Dr. L. Armelao, Dr. D. Barreca, Dr. G. Bottaro Molecular Sciences and Technologies Institute CNR and INSTM Department of Chemistry, Padova University Via Marzolo, 1, I-35131, Padova (Italy) E-mail:
[email protected] Dr. A. Gasparotto, Dr. E. Tondello Department of Chemistry and INSTM, Padova University Via Marzolo, 1, I-35131, Padova (Italy) Dr. M. Ferroni Physical Chemistry Department and INFM UdR Venezia Ca' Foscari Venice University Via Torino, 155/B Venezia-Mestre (Italy) Dr. S. Polizzi Physical Chemistry Department, Ca' Foscari Venice University Via Torino, 155/B Venezia-Mestre (Italy) [**] Consorzio OPTEL-PNR Art. 10 legge 46/1982 and AGENZIA 2000 funded by CNR, Italy, are gratefully acknowledged for financial support.
Chem. Vap. Deposition 2004, 10, No. 5
DOI: 10.1002/cvde.200306296
identified. In particular, methods based on a progressive size reduction of bulk materials are classified as top±down, while controlled assembly processes of small building blocks like atoms, molecules, or clusters are termed bottom±up.[9±11] Among the chemical bottom±up techniques, CVD[12±14] and SG[15±17] have gained outstanding importance for the synthesis of both inorganic and hybrid nanosystems under soft and controlled conditions. In particular, the CVD method permits good conformal coverage of the substrate surface, yielding layers with tuneable crystallinity, chemical composition, and morphology. Further interesting perspectives arise from the possibility of achieving a vapor infiltration and subsequent nucleation into porous or fibrous structures,[13,18] which has been applied to the preparation of semiconductor clusters in the ªinternal surfacesº of zeolites[19±21] and macroporous metal oxides.[22] The SG route enables the tailoring of oxo-hydroxo macromolecular networks (from gels to fully densified materials in the form of powders, composites, fibers, ¼) starting from suitable precursor solutions. As a consequence, high surface area materials with controlled pore size and spatial distribution can be obtained, thus enabling the preparation of systems for a wide variety of applications. The common features of CVD and SG methods are based on the following points: i) the possibility of operating under non-equilibrium conditions, where nucleation predominates over the subsequent particle growth, thus favoring the preparation of nanosystems; ii) the ability to exert a chemical control of the process by the suitable choice of precursor compounds, in the framework of a molecule-tonanosystem preparation strategy. 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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The aim of the present work was to develop a novel synthetic approach to CeO2±ZrO2-based nanosystems by exploiting the synergy and the peculiar advantages of PE-CVD and SG techniques. In particular, attention was focused on the dispersion of nanoparticles of the guest phase (CeO2) on a porous zirconia xerogel (host matrix) and on the subsequent thermal treatments of the obtained composite systems (Fig. 1a). As a matter of fact, the as-prepared SG films (xerogels) can be regarded as active substrates, being characterized by a porous structure in which a controlled amount of nonbridging bonds (±OH and ±OR) are present. These groups can provide reaction sites for successive chemical modifications on both the surface and sub-surface layers.[17] In fact, the presence of ±OH and ±OR moieties on the substrate in CVD processes enhances the precursor decomposition from the vapor phase, especially in the case of metal b-diketonates.[23,24] Moreover, the plasma action in PE-CVD permits the achievement of nanosystems thanks to the competition between deposition/ablation phenomena characterizing glow discharges.[6] In these processes, plasma promotes and activates chemical reactions both in the gas phase and on the substrate (Fig. 1b), producing an intimate (a)
Step 1 SG MOx(OH)y (OR)z matrix (xerogel)
Step 2 PE-CVD of M’Ox
Step 3 Annealing
intermixing between the SG host matrix and the PE-CVD guest phase. Such effects can be further enhanced by exsitu thermal treatments, allowing the production of materials ranging from composites, where host nanoparticles are dispersed by PE-CVD in/on a SG matrix, to solid solutions, where a single and homogeneous phase is obtained. Thanks to the above characteristics, the use of a combined PE-CVD/SG route is expected to be more advantageous than a simple CVD (or SG) process for the possibility of obtaining nanosystems with optimized guest/host dispersion and well-tailored particle size. To the best of our knowledge, very few papers concerning similar joint approaches to CeO2±ZrO2 nanosystems have appeared in the literature to date,[25±27] and none of them has reported a hybrid PE-CVD/SG methodology. In the present work, PE-CVD of CeO2 was performed on SG zirconia xerogels and the obtained composites were subsequently annealed in air at 600 C and 900 C.[28] Depositions were accomplished starting from Ce(dpm)4 (Hdpm = 2,2,6,6-tetramethyl-3,5-heptanedione) and Zr(OtBu)4 (OtBu = tert-butoxy) in the PE-CVD and SG routes, respectively. The system nanostructure, composition, and morphology were investigated as functions of processing conditions using glancing-incidence X-ray diffraction (GIXRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Preliminary results have already been reported.[29]
2. Results and Discussion substrate
M’Ox (guest)
In the present section, the most relevant results concerning the chemico-physical characterization of the synthesized CeO2±ZrO2 nanosystems are presented. Particular attention is devoted to highlighting analogies and differences in the system properties as functions of thermal treatment (600 C and 900 C in air, 1 h) and ceria overall content (increasing from set A, 10 min CeO2 deposition time, to set C, 60 min CeO2 deposition time; see Table 1).
MOx(OH)y (OR)z (host)
(b)
2.1. Microstructural Characterization
Fig. 1. a) Schematic representation of the PE-CVD/SG synthetic approach adopted in the present work for the preparation of CeO2±ZrO2 systems. b) Schematic representation of step 2. The ±OH and ±OR groups are shown in the boxes.
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The phases detected by GIXRD and their relevant crystallite sizes are summarized in Table 1 and described in detail below. It is worthwhile noticing that the obtained size values represent only an upper limit, since part of the line broadening of the GIXRD peak may be due to lattice disorder and/or defects, not accounted for by using the Scherrer method. Furthermore, sizes in Table 1 must be regarded as approximate values, due to the very large width of the reflections and of the grazing incidence geometry.
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Table 1. Preparative conditions for CeO2±ZrO2 nanosystems and phases with relevant crystallite sizes (in parentheses) obtained by GIXRD. In all cases, annealing was performed in air for 1 h. Typical film thickness was » 20 nm for all the heated specimens.
As-prepared zirconia
Amorphous t'±ZrO2 (12 nm) Amorphous ZrO2 t'±ZrO2 (12 nm) t'±ZrO2 (12 nm) Amorphous ZrO2 + CeO2 (
