We present herein a new concept for the preparation of nanofibrous metal oxides based on the simultaneous electrospinning of metal oxide precursors and silica nanoparticles. Precursor fibers are prepared by electrospinning silica nanoparticles (20 nm in diameter) dispersed in an aqueous solution of poly(acrylic acid) and metal salts. Upon calcination in air, the poly(acrylic acid) matrix is removed, the silica nanoparticles are cemented, and nanocrystalline metal oxide particles of 4−14 nm are nucleated at the surface of the silica nanoparticles. The obtained continuous silica fibers act as a structural framework for metal oxide nanoparticles and show improved mechanical integrity compared to the neat metal oxide fibers. The hierarchically nanostructured materials are promising for catalysis applications, as demonstrated by the successful degradation of a model dye in the presence of the fibers. KEYWORDS: catalysis, ceria, electrospinning, lithium cobalt oxide, metal oxide, nanoparticles
■ INTRODUCTIONBecause of their large ratio of surface area to volume and interconnected porosity, metal oxide meshes have been proposed for applications in catalysis, energy storage, and sensors.1−3 Various bottom-up approachessuch as vapor− liquid−solid (VLS) 4 and vapor−solid (VS) 5 growthand topdown techniquessuch as nanocarving 6 and electrospinning 7 have been described. Among these techniques, electrospinning is probably the most versatile, because it allows the fabrication of fibrous mats from a wide variety of both organic and/or inorganic materials. 8,9 Moreover, it allows for control over the diameter, morphology, porosity, alignment, and composition of the resulting fibers.
10,11Calcination of electrospun mats obtained from metal oxide precursors is a commonly used approach because of its facility and potential for upscaling. The calcination process is usually applied to mixtures of polymers/metal oxide precursors and it is based on an oxidative conversion of the polymeric component by heat treatment. There are many successful examples in literature for the fabrication of inorganic fibers by calcination of precursor materials prepared by sol−gel, 12−18 polymer-based sol−gel processes, 19−22 or from electrospinning of polymer dispersions in the presence of ex situ formed colloids.23−27 Upon increase of the temperature, metal oxide crystals nucleate and grow while the polymeric component undergoes degradation. 28,29 In the electrospinning of inorganic precursor, one has to take into account that the calcination leads almost automatically to a shrinkage of the fibers because the polymer template is removed. After calcination, the nanofibers are usually more brittle 30 because of their thinner section and the internal stress generated by the shrinkage. Therefore, an additional material that is not mechanically or chemically affected by the calcination process needs to be incorporated in the fibers during the electrospinning.The recently reviewed electrospinning of colloids 31 has been explored for the ...