Template-directed synthesis and characterization of microstructured ceramic Ce/ZrO2@SiO2 composite tubes.

Template-directed synthesis and characterization of microstructured ceramic Ce/ZrO2@SiO2 composite tubes Jörg J. Schneider* and Meike Naumann Full Research Paper Address: Eduard-Zintl Institut für Anorganische und Physikalische Chemie, Technische Universität Darmstadt, Alarich-Weiss-Str.12, 64287 Darmstadt, Germany Email: Jörg J. Schneider* - Open Access Beilstein J. Nanotechnol. 2014, 5, 1152–1159. doi:10.3762/bjnano.5.126 Received: 30 January 2014 Accepted: 01 July 2014 Published: 25 July 2014 Associate Editor: J. Sagiv * Corresponding author Keywords: electrospinning; exotemplating; nanostructured solid solution; sol–gel chemistry; Stoeber process; ternary oxide © 2014 Schneider and Naumann; licensee Beilstein-Institut. License and terms: see end of document. Abstract An exo-templating synthesis process using polymeric fibers and inorganic sol particles deposited onto structured one-dimensional objects is presented. In particular, CeO2/ZrO2@SiO2 composite tubes were synthesized in a two-step procedure by using electrospun polystyrene fibers as fiber template. First, a sol–gel approach based on an exo-templating technique was employed to obtain polystyrene(PS)/SiO2 composite fibers. These composite fibers were subsequently covered by spray-coating with ceria and zirconia sol solutions. After drying and final calcination of the green body composites, the PS polymer template was removed, and composite tubes of the composition CeO2/ZrO2@SiO2 were obtained. The SiO2/ZrO2/CeO2 microtubes, which consist of interconnected silica particles, are held together by ceria and zirconia deposits formed during the thermal treatment process. These microtubes are mainly located in the pendentive connecting the individual spherical silica particles and glue them together. The composition and crystallinity of this material connecting the individual silica particles contains the elements Ce and Zr and O as mixed oxide solid solution identified by XRD, Raman and high-resolution TEM and EFTEM. High-resolution microscopy techniques allowed for an elemental mapping on the surface of the silica host structure and determination of the O, Zr and Ce elemental distribution with nm precision. Introduction Ceria, CeO2, is well-known for its unique acid–base and redox properties, which has led to numerous applications in catalysis, energy related studies (e.g., for solid fuel cells), in gas sensor technologies and in biochemistry [1-3]. Its high oxygen storage/ release capacity is a result of the high reducibility of Ce4+ to Ce3+, which relies on the high mobility of oxygen ions inside the ceria lattice [2,3]. Pure ceria, however, has a low thermal stability and is prone to sintering at high temperatures, which 1152 Beilstein J. Nanotechnol. 2014, 5, 1152–1159. leads to its deactivation as a catalyst. The addition of a defined amount of zirconia enhances its active surface area, thermal stability, and oxygen storage capacity [3,4]. Such ceria/zirconia solid composite materials represent solid solutions in which the Ce/Zr ratio can be adjusted over a wide range. It has been shown that CexZr1−xO2 solid solutions have enhanced structural and textural properties, improved thermal stability as well as redox properties [5-8]. The transformation into ordered crystalline mesoporous structures of composition CexZr1−xO2 (x = 0.4–0.8) has also been shown [9]. In these solid solutions ultrafine zirconia allows for a better mechanical behavior, which facilitates enhanced fracture toughness with critical stress intensities as high as 20 MPa m1/2 [10]. With respect to catalysis, solid solutions of the composition Ce x Zr 1−x O 2 have been studied for the generation of hydrogen (e.g., in fuel cell applications), CO2 reforming (e.g., for production of synthesis gas), direct methane oxidation (e.g., for solid oxide fuel cells), SOFCs [3,5] and electrochromic smart window applications [11]. Adding silica as a support enhances the oxygen storage capacity (OSC) of such ceria–zirconia composite materials [2,4]. Besides synthetic methods such as the thermal decomposition of precursors [12], co-precipitation for the preparation of powders [13], impregnation [9], dip-coating [5], or hydrothermal synthesis [4], sol–gel synthesis routes have been widely employed for the preparation of CexZr1−xO2 solid solutions [14]. Pure aqueous sols or sols stabilized by the addition of organics, e.g., surfactants, allow for the stabilization of reactive pre-ceramic compositions in solution. High-temperature annealing steps have to be employed to obtain the solid ceramic solutions from such a solution processing route of the precursors. studies may investigate the effect of this phase on the hardness, viz. the mechanical properties. Herein, we report on a newly developed synthetic process based on a combination of electrospinning and exotemplating leading to hollow CeO 2 /ZrO 2 @SiO 2 composite tubes. Firstly, after electrospinning of polystyrene fibers, the fibers were covered by an exotemplating step with a sol solution containing monodisperse silica particles obtained from a Stoeber process. This is followed by the addition of ceria and zirconia nanoparticles, both of which are obtained from stable sol solutions by spray-coating onto the former material. The overall process yields ceramic microtubes of the composition Ce 0.13 / Zr 0.87 O 2 @SiO 2 after calcination when a 1:7 molar ratio of Ce:Zr was employed. The obtained tubes are strengthened by the defined zirconia/ceria composition compared to pure SiO2 tubes obtained under the identical exo-templating conditions. It is shown that a stable and intimate ceramic nanocrystalline interface exists between the Stoeber particles composing the silica tubes and the mixed ceria/zirconia solid solution of the composition Ce 0.13 /Zr 0.87 O 2 which is connecting the SiO 2 particles as a ceramic binder. Based on these findings future Synthesis and characterization of ternary ceria/zirconia@silica composite microtubes Results and Discussion Synthesis of hollow SiO2 microtubes by exotemplating Tubular structures of ternary oxide CeO2/ZrO2@SiO2 were synthesized by using a multistep synthesis route. In the first step, dense fibrous mats of polystyrene (PS) fibers were obtained by an electrospinning process [15]. An inorganic/ polymer composite, PS/silica, was synthesized by using these PS template fibers as substrate by depositing a Stoeber particle sol solution on the surface of this polymeric fibrous material. This solution contains spherical silica particles in a narrow size range of 150 ± 10 nm. For an effective tethering of these uniform silica particles on the PS fiber surface, the as-obtained PS fibers were surface-functionalized in a reactive oxygen plasma atmosphere (rf plasma, 60 W, 20% O2) prior to the addition and anchoring of the silica particles to these surface-functionalized groups. This ensures a dense and covalent linking of the particles to the PS surface. Calcinat (...truncated)