Synthesis of amorphous titania nanostructures by pulsed-laser decomposition of liquid metal-organic precursor with post-annealing transformation into crystalline-layered TiO2 nanorods and nanospheres

Abstract

Amorphous and non-stoichiometric titania nanoparticles and films are produced by pulsed-laser decomposition of a liquid titanium tetra-isopropoxide (TTIP) precursor bath. Nanoparticles are formed in the TTIP solution, where the submerged laser-induced plasma generates vaporized species that are rapidly quenched by the surrounding (chilled) liquid precursor. Additionally, relatively-dense films are formed on a glass substrate above the surface of the TTIP bath by vapor transport and condensation of pyrolyzed species. Interestingly, upon post-annealing at 400 °C (∼ 0.3 TM) in ambient air of the collected nanoparticles in the bath, a transformation of the amorphous non-stoichiometric titania nanoparticles occurs, producing novel anatase-TiO2 morphologies, such as crystalline-layered nanorods and onion-like nanospheres.Chemical analysis of the as-synthesized particles and films shows they are rich in oxygen and carbon relative to TiO2. As a result, an amorphous-to-crystalline phase transformation during heat treatment in air starts at the surfaces of the nanoparticles, irrespective of their morphologies, and propagates into the interior. During annealing, the in-situ reaction of trapped-in species likely yields gaseous products (e.g., CO, CH4, H2O) that diffuse out of the particles, leaving sufficient Ti and O to enable crystallization of anatase-TiO2.Preliminarily, enhanced photocatalytic activity is found for the novel TiO2 nanostructures for hydrogen production from water reduction with methanol as a sacrificial agent. Decreased band-gap energy and increased absorbance in both the high-energy (i.e., 200 nm – 250 nm) and low-energy (i.e., 340 nm – 440 nm) UV regions are measured compared to that of commercial anatase.