Inorganic Nanostructures Prepared by Electrospinning and Atomic Layer Deposition

Nanostructures are structures where at least one dimension is in nanoscale which ranges typically from 1 to 100 nm. 1D nanostructure is an object where two dimensions are in the nanometer scale and one dimension in a larger scale, for example carbon nanotubes and electrospun fibers. Due to a very small size, nanostructured materials have different properties than what they have in bulk form, for example chemical reactivity is increased when the size comes smaller.

Electrospinning is a very simple but versatile and scalable method for preparing micro- and nanosized fibers. In an electrospinning process an electrical charge is used to spin very fine fibers from a polymer solution or melt. By changing electrospinning parameters, for example voltage and spinneret-collector distance, fibers of different diameters can be obtained. With different electrospinning setups it is also possible to prepare hollow fibers, and even macroscopic objects with fiber walls can be obtained.

This work was concentrated on A) constructing different electrospinning setups and verifying their operation by electrospinning various materials, and B) preparing 1D nanostructures like inorganic nanofibers directly by electrospinning and nanotubes by combining electrospinning and atomic layer deposition, ALD. This is so called Tubes by Fiber Template (TUFT) –process.

The electrospinning setup was constructed successfully, and its operation was verified. Several materials were electrospun. Polymers (PVP, PVA, PVAc, PEO, PMMA and PVB, Chitosan) were electrospun directly from polymer/solvent solution, and ceramic materials like TiO2, BaTiO3, SnO2, CuO, IrO2, ZnO, Fe2O3, NiFe2O4, CoFe2O4, SiO2 and Al2O3 were electrospun from polymer solutions containing the corresponding metal precursor(s). In the case of the ceramic fibers, the electrospinning was followed by calcination to remove the polymer part of the fibers. Metallic fibers were obtained by a reduction treatment of the corresponding oxides, for example Ir fibers were prepared by reducing IrO2 fibers.

Combination of electrospinning and ALD was used for TUFT processing of ceramic nanotubes. In the TUFT process, electrospun template fibers were coated with the desired material (Al2O3, TiO2, IrO2, Ir, PtOx and Pt) and after coating the template fibers were removed by calcination. The inner diameter of the resulting tubes was determined by the template fiber and the tube wall thickness by the thickness of the ALD deposited film.

Promising results were obtained in searching for new applications for electrospun fibers. For the first time, by combining electrospinning and ALD, the TUFT process was used to prepare reusable magnetic photocatalyst fibers. These fibers have a magnetic core fiber and a photocatalytic shell around it. After a photocatalyst purification was completed, the fibers could be collected from the solution by a strong magnet and reused in cleaning the next solution.

In this study, the most commercially and environmentally valuable application invented was to use electrospun ion selective sodium titanate nanofibers for purification of radioactive wastewater. These fibers were found to be more efficient than commercial granular products, and they need much less space in final disposal.