REPOSITORY > RESULTS

Abstract

One-dimensional hydrothermally synthesized nanostructures such as nanotubes and nanobelts composed of layered titanates from A2TinO2n+1 family have attracted great interest due to their high specific surface area, ion-exchange ability and semiconducting properties, which make them promising candidates for potential applications in the field of catalysis, gas sensors, dye-sensitized solar cells, lithium batteries and photocatalysis.
Titanate-based one-dimensional (1D) nanostructures are characterized as a layered, lamellar structure with mesoporous morphology, which may facilitate the transport of ions to the surface of the elongated nanostructures. The ability of ion-exchange of titanate-based 1D nanostructures has been utilized for the formation of Ca-doped titanate nanotubes and nanostructured CaTiO3. Implementation of titanate nanotubes as a template material ensures formation of modified or new materials of nano-dimensions with elongated structure inherited from the parent nanotubes.
In the field of photocatalysis several approaches that modify electron band structure of titanates have been successfully introduced to enhance the photosensitivity of titanate-based 1D nanostructures in the UV and visible regime. Recently, the increased photoactivity in the visible range was observed with the metallic nanoparticles/titanate nanocomposites.
Polyelectrolyte multilayer (PEM) film of desired thickness formed by the layer-by-layer self assembly of appositively charged polyions assembled on the surface of the one-dimensional titanate-based nanostructures can be utilized as nanoreactor for the in-situ synthesis of metallic nanoparticles. By this approach the surrounding polymer prevents aggregation of formed nanoparticles within PEM film and thus enables a control over the size and density of metallic nanoparticles in the nanocomposite.
The weak PEMs of polyallylamine (PAH) and polyacrylic acid (PAA) possessing ion-exchangeable carboxylic groups were assembled on hydrophilic surface of titanate-based nanobelts by the sequential deposition of polyions from an aqueous solution. PEM films fabricated by Layer by Layer (LbL) electrostatic assembly of weak appositively charged polyions were used to bind metal cations within the PEM film. The free carboxylic acid groups that do not electrostatically bind to the appositively charged groups of PAH during the film assembly are later used to bind the metal ions as the PEM is exposed to the metal salt solution at the nominally neutral pH. By subsequent chemical reaction of the metal ions, metallic (Ag, Cu) nanoparticles are formed within the PEMs. Due to the regeneration of the carboxylic acid groups upon nanoparticle formation, the control over the volume fraction, the size and the density of the particles within the polymer film was obtained by repeatedly cycling the synthesis process. By subsequent thermal treatment PEM is removed resulting in the metallic Ag(Cu) nanoparticles-titanate nanobelts composite formation.
Under photo-excitation the metallic nanoparticles in the composite act as electron traps and hence suppress the electron-hole recombination rate in the semiconductor (titanate nanobelts) and thus enhance photo-response of such hybrid material in UV region of solar spectrum. Additionally, visible photo-activity of metal-1D titanate nanostructures nanocomposites is obtained by the electromagnetic near field induced by the surface plasmon resonance (SPR) of metallic (Ag, Cu) nanoparticles.
Furthermore, the PEM films formed by the layer-by-layer self assembly on the surface of titanate nanobelts were utilized for the fabrication procedure of titanate nanobelts thin films. Organic-inorganic PEM-titanate nanobelts composite was assembled in sequential steps with fully charged polyelectrolyte Poly(styrenesulfonic acid) (PSS) to fabricate 10 and 20 bilayered film assemblies. By applying PEM onto the surface of titanate nanobelts the negative surface charge of hydrophilic titanate nanobelts become positive which then enables electrostatic coupling with strongly negatively charged PSS polyelectrolyte yielding inorganic-organic multilayered thin film with linear film thickness increment as the number of layers was increased. After annealing at 450°C the inorganic films are composed solely of crystalline titanate nanobelts that are not oriented parallel with the substrate. Random orientation of nanostructures building as-formed thin film induces nanoscale porosity and hence renders these multilayers potentially effective photo-sensitive materials for the degradation of organic pollutants.